Atmospheric Pressure Plasmas for Surface Modifications

UNIVERSITA DEGLI STUDI DI MILANO-BICOCCA

SCUOLA DI DOTTORATO DI SCIENZE

CORSO DI DOTTORATO DI RICERCA IN FISICA E ASTRONOMIA

Riccardo A. Siliprandi

Atmospheric PressurePlasmas for

Surface Modifications

Relatore: Prof. Claudia Riccardi

Coordinatore: Prof. Claudio Destri

Ciclo XX 2004-2007

to my family

Avro piacere d’esser illuminato e tratto d’errore

Simplicio in Dialogo sopra i due massimi sistemi del mondo

Galileo Galilei, 1632

Contents

1 Introduction 1

1.1 Cold Atmospheric pressure plasmas . . . . . . . . . . . . . . . 1

1.2 Surface modifications with atmospheric plasmas . . . . . . . . 2

1.3 Motivations and thesis outline . . . . . . . . . . . . . . . . . . 3

2 Atmospheric pressure discharges and surface processes 5

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Electrical Breakdown of Gases . . . . . . . . . . . . . . . . . 6

2.2.1 Townsend breakdown mechanism . . . . . . . . . . . . 7

2.2.2 Streamer breakdown mechanism . . . . . . . . . . . . 10

2.3 Dielectric Barrier Discharges . . . . . . . . . . . . . . . . . . 15

2.3.1 Overview and properties of dielectric barrier discharges 16

2.3.2 Dielectric barrier discharge regimes . . . . . . . . . . . 16

2.3.3 Streamer Discharge Regimes . . . . . . . . . . . . . . 18

2.3.4 Micro-discharge interaction and pattern formation . . 20

2.4 Plasma-surface interactions . . . . . . . . . . . . . . . . . . . 21

2.4.1 Gas-phase chemistry and processes . . . . . . . . . . . 21

2.4.2 Surface kinetics and processes . . . . . . . . . . . . . . 24

VII

VIII CONTENTS

3 Dielectric barrier discharge devices 29

3.1 DBD device for surface modifications . . . . . . . . . . . . . . 29

3.1.1 Plasma reactor . . . . . . . . . . . . . . . . . . . . . . 30

3.1.2 Pumping system and gas distribution . . . . . . . . . 30

3.1.3 Electric power supply and configuration . . . . . . . . 33

3.1.4 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . 33

3.2 DBD device for streamer regime characterization . . . . . . . 34

3.2.1 Plasma reactor . . . . . . . . . . . . . . . . . . . . . . 34

3.2.2 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . 35

4 Plasma and material diagnostics 37

4.1 Optical emission spectroscopy . . . . . . . . . . . . . . . . . . 37

4.1.1 Determination of molecular vibrational temperature . 38

4.2 Voltage Current measurements . . . . . . . . . . . . . . . . . 38

4.2.1 Implementation of Rogowski coils for measurements

nanoseconds current pulses . . . . . . . . . . . . . . . 38

4.3 Characterization of the materials surfaces . . . . . . . . . . . 43

4.3.1 Infrared spectroscopy (FTIR/ATR-FTIR) . . . . . . . 44

4.3.2 Atomic force microscopy (AFM) . . . . . . . . . . . . 45

4.3.3 Contact angle measurements and surface energy de-

termination . . . . . . . . . . . . . . . . . . . . . . . . 46

5 Statistical characterization of a streamer discharge regime 51

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.2 Statistical characterization of current signal . . . . . . . . . . 53

5.2.1 Structure of the discharge current: bumps, bursts and

streamers. . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.2.2 Discharge current regimes . . . . . . . . . . . . . . . . 56

5.3 Statistical analysis of temporal behavior . . . . . . . . . . . . 62

5.3.1 Inter- and intra-bump correlations: surrogate model

and Hurst exponents . . . . . . . . . . . . . . . . . . . 62

5.3.2 Temporal correlations between streamers . . . . . . . 68

5.4 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 75

CONTENTS IX

6 Characterization of the DBD device in nitrogen atmosphere 79

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

6.2 Experimental setup and methods . . . . . . . . . . . . . . . . 80

6.3 Discharge regimes in Nitrogen Atmosphere . . . . . . . . . . . 80

6.3.1 Characterization of the discharge as a function of in-

jected power . . . . . . . . . . . . . . . . . . . . . . . 83

6.3.2 Characterization of the discharge as a function of pres-

sure and gas fluxes . . . . . . . . . . . . . . . . . . . . 85


7 Deposition process of organosilicon thin films 91

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

7.2 Materials and methodology . . . . . . . . . . . . . . . . . . . 92

7.3 Characterization of the deposition process . . . . . . . . . . . 94

7.3.1 Plasma characterization . . . . . . . . . . . . . . . . . 95

7.3.2 Thin film characterization . . . . . . . . . . . . . . . . 97


8 Fluorination of polymer surfaces 107

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

8.2 Experimental, diagnostics and methods . . . . . . . . . . . . 109

8.3 Characterization of the fluorine grafting process . . . . . . . . 110

8.3.1 Plasma-phase characterization . . . . . . . . . . . . . 110

8.3.2 Material surface characterization . . . . . . . . . . . . 113


9 Plasma Application for modification of paper surfaces 121

9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

9.1.1 Cellulose and paper . . . . . . . . . . . . . . . . . . . 121

9.2 Deposition of organic silicon compounds for hydrophobicity . 123

9.2.1 Experimental setup and diagnostics . . . . . . . . . . 124

9.2.2 Hydrophobicity of treated paper surface . . . . . . . . 125

9.3 Fluorination process for oil repellency . . . . . . . . . . . . . 128

9.3.1 Experimental setup and diagnostics . . . . . . . . . . 129

X CONTENTS

9.3.2 Oil repellency of paper surfaces . . . . . . . . . . . . . 129

10 Conclusions 135

Bibliography 139

CHAPTER1

Introduction

1.1 Cold Atmospheric pressure plasmas

Atmospheric pressure plasmas are growing as an interesting alternative to

low pressure plasmas for several applications. The possibility to develop con-

tinuous processing without the costs of vacuum technologies has attracted in

the last decade the efforts of several industries and research groups all over

the world. Within the different types of atmospheric pressure non-thermal

plasmas, dielectric barrier discharges (DBDs) are the most interesting so-

lution. DBDs are a well known type of gas discharge. They have been

widely used in industrial applications like ozone generators, plasma display

panels, excimer lamps, volatile organic compounds destruction and surface

modifications [1, 2, 3, 4, 5, 6].

DBDs are low temperature, non-equilibrium, transient gas discharges

operating in a quasi-continuous discharge regime. They usually consist of

two electrodes (at least one of them covered with a dielectric material) to

1

2 INTRODUCTION

which an AC high voltage is applied for frequencies generally varying in the

range of 102Hz to 102kHz. At low pressures DBDs operate in a Townsend

breakdown regime [5] generating a diffuse glow discharge. At atmospheric

pressure, the realization of a diffuse discharge is restricted to limited condi-

tions of geometry, electrical parameters and gas composition [7, 8, 9, 10, 11],

and DBDs operate usually in a streamer discharge in which several narrow

discharge filaments are typically formed. The streamer regime constitutes

a strongly interacting system of discharges exhibiting cooperative behavior.

This leads, under specific conditions, to the formation of coherent spatial

configurations that have been observed in different types of experimental

setups [12, 13, 14, 6]. However, micro-discharges seem, to some extent, to

occur at random within the discharge gap for most applications of DBDs.

Despite of the existence of several industrial applications and intense study

during decades, DBDs still lack a clear physical interpretation of the dis-

charge regimes and of the complex chemistry involved in the processes.

1.2 Surface modifications with atmospheric plasmas

Plasma-surface interaction is a rather complicate process which involves

several complex chemical and physical mechanisms [15, 16]. For this rea-

son plasma processing is the subject of study in many research areas like

plasma physics, surface science, gas-phase chemistry and atomic and molec-

ular physics. The common theme is the creation and use of plasmas to ac-

tivate a chain of chemical reactions at a substrate surface. At low pressure

the behavior of many chemical processes in presence of a plasma have been

the subject of intense research in the recent years and is now a well estab-

lished industrial process[17, 15, 18, 19, 2]. An example is the semiconductor

industry which successfully employs plasma processes for the production of

integrated circuits.

DBDs are already employed in industry for modification of material sur-

faces [1]. This application has regarded mainly the processes in atmospheric

Air for treatments of polymer surfaces to attain wettability, printability and

adhesion properties [1, 20, 21]. Its use in different reactive atmospheres

has proved more difficult because of the strong dependence of the discharge

regime on the atmosphere composition and the absence of an environment

1.3 MOTIVATIONS AND THESIS OUTLINE 3

sealed from contaminations [1, 22]. Moreover, the behavior of several reac-

tive atmospheres, well known at low pressure, is different to some degree

when used at atmospheric pressure. This is due to the great difference be-

tween the discharge regimes that can be realized at high pressure and the

substantial change in chemical and physical processes both in gas-phase and

on the surface [23, 24, 25, 26].

1.3 Motivations and thesis outline

This thesis concerns the study of atmospheric pressure plasmas realized in

dielectric barrier discharges and their applications to surface modifications

of materials.

The first part of the work is dedicated to the study and the charac-

terization of the streamer regime. Streamer development is still subject of

intense study and several theoretical models and few experiment describe

the development of the single phenomena. However, due to the complex-

ity of the interaction between micro-discharges, a clear physical picture of

their behavior as a whole system is not presently available. In this study a

novel approach to the problem is used, and the tools of statistical analysis

are used to investigate the temporal behavior of micro-discharges through

the measurement of current signal. Several interesting feature regarding

the discharge dynamics and the temporal propagation of correlations are

discussed.

The second part of this work is dedicated to the study of plasma pro-

cessing of material surfaces. These studies are performed using a newly

built plasma reactor which gives the possibility to study both the discharge

physics and the plasma-surface interaction during continuous processing in

a wide range of pressures and compositions of the atmosphere. Two specific

processes are studied: a deposition process of thin organosilicon films for the

creation of hydro-repellent coatings and a grafting of fluorine atoms process

to produce hydro- and oil-repellent properties on the surfaces of organic soft

matter. The applications of these processes to the modification of paper

surfaces is then studied.

The thesis is organized as follows. In Chapter 2 a brief review of the

physical foundations of plasma discharges and plasma-surface interactions

4 INTRODUCTION

is given. Chapter 3 describes the plasma devices that have been realized

and used in the experiments. Chapter 4 describes the diagnostics both

for the plasma and materials. The description of study and realization of

high bandwidth Rogowski coils for the measurement of fast current pulses is

given. Chapter 5 involves the study of the streamer regime and its charac-

terization by means of statistical analysis of current signals. Chapter 6 gives

a characterization of plasma discharges in nitrogen atmosphere. Chapters 7

and 8 describe two atmospheric pressure processes of surface modifications.

A deposition process of thin organosilicon films and a grafting process of

fluorine radicals. Chapter 9 is finally devoted to the presentation of some

results concerning the application of studied processes for modification of

paper surfaces.

CHAPTER2

Atmospheric pressure discharges

and surface processes

In this chapter the general concepts of and features of electrical discharges

in gases the the plasma-surface processes are briefly reviewed. Attention is

focused on physical phenomena interesting for the arguments of the present

research. For a more complete insight of the problems reader is advised to

refer to literature [5, 2, 15, 1].

2.1 Introduction

One of the simplest way to produce a plasma is applying an electric field to

a neutral gas. These artificially generated plasmas can be classified into two

main categories: thermal and non-thermal ones.

In a plasma, as in any gas, the temperature is determined by the average

kinetic energy of its components. However, a plasma can exhibit multiple

5

6 ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES

temperatures, usually one for the heavy particles and one for the electrons

(Ti and Te respectively) unless sufficient collision occur between them. Be-

cause of the large difference in mass between electrons and other particles

the temperatures of these two species remains different in many conditions.

When Te≃Ti the plasma is considered in local thermodynamic equilibrium ,

LTE, (which requires also the absence of chemical gradients) and termed as

thermal plasma. These discharges are characterized by high temperature of

the gas. Examples of thermal plasma are the Plasma Torches or the fusion

plasma devices. Otherwise, when large deviations from LTE are present (i.e.

Te > Ti) the plasma is not thermalized an is called non-equilibrium or non-

thermal plasma. The main feature of non-thermal plasmas is that the most

part of the electrical energy injected in the system is used for the production

of energetic electrons rather than heating the gas, while the neutral species

and ions remain relatively cold because of the low energy exchange with light

particles. The electrons have enough energy to ionize other molecules and

atoms generating excited species, other electrons and free radicals. They

can achieve sufficient energy to initiate chemical reactions usually forbidden

to standard chemistry in the same condition. Plasma can initiate several

physical and chemical processes on material surfaces which can provide an

efficiency increase in processing methods and very often can reduce environ-

mental impact in comparison to more conventional processes.

2.2 Electrical Breakdown of Gases

Electric breakdown is referred to the process that transforms a non-conducting

material to a conducting one when a sufficient strong electric field is applied.

Although the breakdown is a rather complicate process that strongly de-

pends on the system conditions, it begins always with an electron avalanche,

i.e. the multiplication of some primary seed electrons in cascade ionization

when accelerated by the electric field. After this initial stage the following

development of the discharge depends on several parameters as gas compo-

sition, pressure, distance between electrodes, frequency of the applied field

and geometry of the system. For sufficient low pressure the mean free path

of the electrons is high and the initial avalanche proceeds until the plasma is

generated in the whole discharge gap. For relative High pressure the mean

2.2 ELECTRICAL BREAKDOWN OF GASES 7

Figure 2.1: Townsend electrical breakdown in a gap d with constant electric fieldE = V/d. Secondary electrons emitted by the cathode generate the multiplicationof avalanches [2].

free path of the electrons is drastically reduced and the avalanche ionization

can generate a great number of electrons giving rise to a localized space

charge which propagates in the discharge gap creating a thin conductive

channel named streamer. If no means are taken to limit the current in

the system, the the streamer is only the initial stage of the so called arc

discharge.

2.2.1 Townsend breakdown mechanism

The discharge process at low pressure or for low values of pd products,

where p is the pressure and d is the inter-electrode gap distance, is called

Townsend1. For the sake of simplicity consider a system of parallel plate

electrodes at a distance d to which is applied a DC Voltage V that provides

a constant field E = V/d. The seed electrons generated from an external

source (for example cosmic rays or natural radioactivity) are accelerated by

the electric field in the gap and reach the anode unless they are lost in the

way by ion recombination or interaction with the chamber wall. The greater

the external electric field (i.e. faster electrons), the smaller the the fraction

1From the name of John Sealy Townsend who first introduced this model to explainelectrical breakdown in gases.


of the electrons lost before they reach the anode. As a result, the elec-

tric current i in the circuit, which is proportional to the number of charged

species which reach the electrodes, initially increases with increasing voltage

V . Beginning at a certain voltage, quite all the charged particles (electron

and ions) reach the electrodes and the current reaches a saturation i0 and

ceases to depend on V . At this point the discharge is non self-sustaining,

i.e. the discharge depends on the presence of an external sources (point A

in Figure 2.2). At still higher voltage, the electron impact ionization on

neutral gas molecules starts initiating the avalanche process (Figure 2.1)

and amplifying the initial current i0 due to the external source. It is conve-

nient to describe the ionization in an avalanche by the Townsend ionization

coefficient α that express the electron production per unit length:

dne

dx= αne −→ ne(x) = n0

eeαx (2.1)

where x is the distance from the cathode, ne the electron density and n0e is

the initial electron density created by the external sources. For simplicity

here the electron losses due to recombination and attachment to electroneg-

ative molecules are neglected. The current at the anode (and so the current

in the closed circuit) is equal to: i = i0eαd where i0 = en0

e and e is the

charge of the electron. The primary process of electron impact ionization

generates n0e[e

αd − 1] ions during the avalanche propagation which become

important when the voltage is furthermore raised and hitting the cathode

they can generate γn0e[e

αd − 1] electrons in the process of secondary electron

emission. γ is the secondary electron emission coefficient and it depends on

cathode material, state of the surface and electric field (which define ion en-

ergy). Taking into account this secondary process the current in the circuit

is:

i =i0e

αd

1 − γ[eαd − 1](2.2)

which is called Townsend formula and was first derived in 1902 to describe

the breakdown process in electric discharges. The transition from non self-

sustaining to self-sustaining discharge is controlled by the denominator in

Equation (2.2). If µ = γ[eαd−1] < 1 the discharge is still non self-sustaining,

but when µ approaches to unity the current grows to infinity and the dis-

charge becomes self-sustaining, i.e. the breakdown occurs. The simplest


Figure 2.2: Voltage-current characteristic of low temperature discharge betweenelectrodes for a wide range of currents. A: region of non-self-sustaining discharge,(BC) Townsend discharge, (CD) subnormal glow discharge, (DE) normal glow dis-charge, (EF) abnormal glow discharge, (GH) arc discharge [5].

breakdown condition can be expressed as:

µ = γ[eαd − 1] −→ αd = ln(1

γ+ 1) (2.3)

which means that each primary electron generated in the initial avalanche

and lost at the anode is replaced by another electron generated by secondary

emission at the cathode. This represents a steady self-sustained current in

the homogeneous field Et = Vt/d (point B in Figure 2.2), where Vt is the

breakdown voltage and is determined from Equation (2.3) as a function of

d an in terms of γ and the known function α(E).

In the presented ideal framework the current i for a voltage V = Vt

would increase to infinity. Any real circuit of the type described above has

an ohmic resistance Ω which sets a limit to the current achievable for a

given electromotive force E . In the case Ω is so great that only a really

small current can flow through the gap and the electrode gap is small in

comparison to electrode dimensions, the field is constant and equal to the

field in absence of discharge. The potential will be equal to the breakdown

voltage Vt. This stable self-sustained discharge with extremely low current is

called Townsend dark discharge (segment BC in Figure 2.2). Let us gradually

increase the current. This can be realized by reducing the load resistance Ω


or by increasing the electromotive force E . The voltage across the electrodes

begins to decrease after a certain current is reached. The fall then stops

and the current remains almost constant over a fairly wide range of values

(sometimes of several orders of magnitude). This is the so-called normal glow

discharge (segment DE in Figure 2.2). The lower part of the transition region

(segment CD in Figure 2.2) corresponds to a sub-normal glow discharge. The

normal discharge has one remarkable property. As the discharge current is

varied, its density at the cathode remains unchanged while changes the area

through which the current flows. When Ω or E is varied, the luminous

current spot on the cathode surface expands or contracts. When no more

free surface is left on the cathode, the current is increased by increasing the

voltage, hence extracting more electrons from unit surface area. Indeed, the

cathode current density must grow. This discharge is said to be abnormal

(segment CD in Figure 2.2). The glow first covers the entire cathode surface

facing the anode, then reaches every spot unprotected by dielectric on the

lateral and inner surfaces and on the support pin, and only having exhausted

these possibilities does it become more extended and intense to a degree

typical of the abnormal discharge. When i ∼ 1A, the glow discharge cascades

down to an arc discharge which is characterized by high current and low

voltage. The segment FG in Figure 2.2 describes the transition, and GH

represents the arc discharge.

2.2.2 Streamer breakdown mechanism

When the pressure is high an the pd > 100 Torr·cm, the Townsend break-

down cannot describe the discharge development. This mechanism is based

on the emission of secondary electrons from the cathode and develops in

time of the order 10−5−10−3. For high values of pd the breakdown develops

much faster and the independence of the breakdown voltage on the mate-

rial of the cathode, established by very accurate measurements, is evidence

against the participation of cathode processes in the breakdown mechanism.

This different breakdown is called streamer breakdown for the thin localized

plasma channels that are generated in the process. The concept of streamer

was originally developed by Raether [27], Loeb [28] and Meek [29].

Also at high pressure an individual avalanche is a primary and com-

pulsory element of the breakdown mechanism. Consider an avalanche in a


uniform external field E0 between plane electrodes. Let it be initiated by

a single electron that leaves the cathode at the time t = 0. The x axis is

directed from a point on the cathode to the anode. The radial distance from

the x axis is denoted by r. Taking into account the possible formation of

negative ions, we find the total numbers of electrons and ions increasing as

the avalanche moves forward:

dNe

dx= (α − η)Ne,

dN+

dx= αNe,

dN−

dx= αNe, (2.4)

Ne = e(α−η)x, N+ =α

α − η(Ne − 1), N− =

α

α − η(Ne − 1), (2.5)

where α and η are the ionization and attachment coefficients. All the new

electrons fly to the anode in a group at the drift velocity vd = µeE0 where

mueE0 is the electron mobility. However, free diffusion (De)makes the elec-

tron cloud spread around the central point x0 = vdt, r = 0. Taking into

account both the effects the electron density can be expressed as:

ne = (4πDet)−3/2 exp

[

− (x − vdt)2 + r2

4Det+ (α − η)vdt

]

(2.6)

The density ne decreases with distance from the moving center following a

Gaussian law. The radius of the sphere on which the density is exactly e

times less than that at the centre, ne(x0, 0, t), increases with time (during

the progress of the avalanche) by the characteristic diffusion law:

rD =√

4Det =

√

4De

µe

x0

E0=

√

4Tex0

eE0. (2.7)

The ions remain practically fixed during the time of flight of the avalanche

to the anode. (see Figure 2.3). Thus, they accumulate at each point. The

positive ion density is

n+(x, r, t) =

∫ t

0αvdne(x, r, t′)dt′, (2.8)

In the absence of attachment in the limit t → ∞ and for regions not too far

from the axis, an approximate calculation of the integral using Equations


Figure 2.3: Formation of a streamer[5].

(2.6) and (2.8) gives

n+(x, r) =α

πr2a(x)

exp[

αx − r2

r2a(x)

]

(2.9)

where r2a(x) is the avalanche radius defined by Equation (2.7). The ion

concentration in the trail of the avalanche is growing up along the axis

in accordance with exponential (2.4) increase of number of electrons. The

qualitative change in avalanche behavior takes place when the charge am-

plification exp(αx) is high. In this case the production of a space charge

with its own significant electric field E′ takes place. This local electric field

E′ should be added to the external field E0. Because the electrons are

much faster than ions the electrons always run at the head of avalanche

leaving the ions behind and thus creating a dipole with the characteristic

length 1/α (mean distance for an electron before creating an ion) and charge

Ne ∼ exp (αx). The fields E′ and E0 in front of the avalanche head add up

to give a field stronger than E0. The fields E′ and E0 in the zone between

the centers of the space charges of opposite signs point in opposite directions

and the resultant field is weaker than E0. When the avalanche reaches the

anode, the electrons sink into the metal and only the positive space charge

of the ionic trail remains in the gap (Figure 2.3). The field is formed by the

ionic charge and by its ”image” in the anode. The image in the relatively


distant cathode plays a rather insignificant role. The field close to the anode

is less than E0, but exceeds it farther off. The field reaches its maximum at

the axial distance from the anode of the order of one ionization length α.

When the number of charges Ne is high, the diffusional spreading of

the electron cloud is replaced by their electrostatic repulsion. The law of

expansion R(x), is given by:

R =( 3e

αE0

)1/3exp

(αx

3

)

=3E′

αE0. (2.10)

The fast growth of the transverse avalanche size restricts the electron density

in the avalanche by the value: ne = (3Ne)/(4πR3) = (αE0)/(4πe). When

the transverse avalanche size reaches the characteristic ionization length

1/α (about 0.1 cm at atmospheric pressure in Air), the broadening of the

avalanche head slows down dramatically. Obviously, the avalanche electric

field is about the external one in this case (see eq. 2.16). The typical values

of maximum electron density in an avalanche are about 1012 − 1013cm3.

When the avalanche head reaches the anode, the electrons sink into the

electrode leaving the ions occupy the discharge gap. At the electron absence,

the total electric field is due to the external field, the ionic trail and also

the ionic charge image in the anode. The resulting electric field in the ionic

trail near the anode is less than the external electric field, but farther off

the electrode it exceeds E0. The total electric field reaches the maximum

value on the characteristic ionization distance (about 1 mm from the anode).

A strong primary avalanche amplifies the external electric field leading to

formation of thin weakly ionized plasma channel, so-called streamer. The

avalanche-to-streamer transformation takes place, when the internal field of

an avalanche becomes comparable with the external one, that is when the

amplification parameter αd is big enough. At a relatively small discharge

gaps, the transformation occurs only when the avalanche reaches the anode.

Such a streamer is known as the cathode-directed or positive streamer. If the

discharge gap and over-voltage are big enough, the avalanche-to-streamer

transformation can take place quite far from anode. In this case the so-

called anode-directed or negative streamer is able to grow toward the both

electrodes.

The cathode-directed streamer starts near the anode. It looks like and


operates as a thin conductive needle growing from the anode. The electric

field at the tip of the anode needle is very high, which stimulates the fast

streamer propagation in direction of cathode. Usually the streamer propa-

gation is limited by neutralization of the ionic trail near the tip of the needle.

The electric field there is so high, that provides electron drift with velocity

about 108 cm/s. One hypothesis states that the decisive role is played by

energetic photons that are emitted by atoms excited in the avalanche and

produce photoionization in the vicinity of the primary avalanche. (Events

of production of electrons at the cathode or far from the trail are unimpor-

tant in this context because they result in avalanches similar to the primary

one.) Electrons produced by photons initiate secondary avalanches that are

pulled into the trail due to the direction of the resulting field. Secondary-

avalanche electrons intermix with primary-avalanche ions and form a quasi-

neutral plasma. They also excite atoms, so that new photons are emitted.

Secondary-avalanche ions en- enhance the positive charge at the cathode

end of the evolved plasma channel. This charge attracts the electrons of

the next generation of secondary avalanches, etc. This is how the streamer

grows. The process of ionization along the ion trail of the primary avalanche

begins at the spot where the positive charge and the field are the highest,

that is, at the anode, provided the degeneration condition E′ ∼ E0 has been

reached there. This is the situation shown in Figure 2.3.

The anode-directed streamer occurs between electrodes, if the primary

avalanche becomes strong enough even before reaching the anode. The

streamer propagates in direction of cathode in the same way as cathode-

directed streamer. Mechanism of the streamer growth in direction of anode

is also similar, but in this case the electrons from primary avalanche head

neutralize the ionic trail of secondary avalanches. However, the secondary

avalanches could be initiated here not only by photons, but also by some

electrons moving in front of the primary avalanche

When the streamer channel connects the electrodes, the current may be

significantly increased to form the spark or arc discharge which are charac-

terized by high current and low voltage. This would lead to Joule heating

of the gas and the generation of a thermal plasma.

2.3 DIELECTRIC BARRIER DISCHARGES 15

Figure 2.4: Examples of dielectric barrier discharge systems[2].

2.3 Dielectric Barrier Discharges

As it has been shown in Section 2.2.2 once the conducting streamer channel

is established electrons can flow trough it and sink at the anode until current

rises and the streamer converts to a spark. If no means are used to limit the

current, the temperature of the gas will rise rapidly due to Joule heating

(thermal plasma). The simplest solution to the problem is to place a di-

electric barrier between the electrodes which prevents the electrons to reach

the electrodes and sink. At this point after the streamer channel is created

only a limited current for a short time can flow and the temperature of the

gas remains quite low while the electrons posses temperatures of the order

of electronvolts. This solution establishes a transient discharge which must

be reactivate by the external circuit using an alternating or pulsed current

power supply. With such system it is possible to obtain a quasi-continuous

regime. These are called dielectric barrier discharges (DBDs). Example of

DBD system are shown in Figure 2.4


2.3.1 Overview and properties of dielectric barrier discharges

Dielectric barrier discharges have a high number of industrial applications

[19, 18, 1, 2] because they operate at strongly non-equilibrium conditions

at atmospheric pressure and at reasonably high power levels, without using

sophisticated pulse power supplies. This discharge is industrially applied

in ozone generation [30], CO2 lasers, and as a UV-source in excimer lamps

[31, 32]. In addition, the DBD in air is commonly used to treat polymer

surfaces in order to promote wettability, printability, and adhesion [1, 22].

DBD application for pollution control is quite promising, but the largest

expected DBD application is related to plasma display panels for large-area

flat television screens. Strong thermodynamic non-equilibrium and simple

design these distinctive properties of DBD allow hoping on expansion of

its applications in low temperature atmospheric pressure plasma chemistry.

DBD has a big potential to be a prospective technology of exhaust cleaning

from CO, NOx and volatile organic compounds [33, 34]. Successful use of

DBD reported in recent research on plasma-assisted combustion may result

in new applications [35].

2.3.2 Dielectric barrier discharge regimes

Usually at atmospheric pressure for values of product pd > 100Torr·cm the

breakdown is the streamer breakdown (see Section 2.2.2) which leads to the

formation of several narrow micro-discharges. The origin of the streamer

is a large electronic avalanche creating enough ions to localize the electrical

field. It is observed when the gas gap becomes large compared with the elec-

tron mean free path. However, yet in 1968, Bartnikas found that helium ac

discharges between closely spaced plane-parallel electrodes, metallic or cov-

ered with a dielectric layer, can exhibit diffuse glow discharge characteristics

[36]. After this first observation several research groups studied this partic-

ular regime finding that stable diffuse discharges could be obtained in gases

including helium, neon, argon, nitrogen, oxygen, and air [7, 8, 37, 9, 10, 11].

However, this diffuse regimes remains extremely unstable and tends to con-

vert to the more stable streamer regime.

A detailed explanation of the operation of diffuse discharges is not known.

It is clear, however, that streamers can be avoided by using an applied elec-


tric field below the Meek criterion. The requirement for establishing a stable

diffuse discharge is that the number of seed electrons is large enough to cause

appreciable overlap and merging of the primary avalanches. This results in

a smoothing of the field gradients due to space-charge at the stage when

streamer formation would otherwise occur. The governing parameters of

this transition are the effective first ionization coefficient α (which is de-

fined as α = α − η where η is the electron attachment coefficient) and the

secondary electron emission from the cathode γ. The coefficient α, or bet-

ter the quantity ∂(α/n)/∂(E/n) (evaluated at the breakdown) is bound to

the radius of the propagating streamer channel [4, 6, 38]. A low value of

this quantity results in a wider streamer channels that overlap more eas-

ily to form a diffuse discharge. The increase of streamer radius has also

been observed experimentally [39]. However, the fundamental mechanism

that ensures the presence of enough seed electrons is the so called Penning

ionization [40, 41]. A Penning mixture usually consists of a gas with small

admixture of impurities. If the components of the impurity B have a ion-

ization potential lower than the metastable potential of the gas A, then the

metastable atoms of the main gas can ionize the molecules of the admixture

according to

A ∗ +B−→A + B+ + e−. (2.11)

Usually, the probability of this process is so high that very small admixtures

may have considerable influence on the discharge development. For exam-

ple, in Helium which possess highly energetic metastable levels (e.g. He[23S]

and [21S]), for the creation of a Penning mixture the background impuri-

ties may be enough. The presence of seed electrons lowers the breakdown

voltage allowing the discharge to develop without the creation of intense

field gradients due to space-charge. However this condition requires that

the slope of the voltage versus time is limited. Thus, in presence of a sinu-

soidal excitation voltage, its amplitude and frequency that allows to obtain

a diffuse discharge are limited. Very high values of these parameters induce

instabilities which lead to a pure filamentary discharge and limits the dis-

charge power in this regime. Another problem of the diffuse discharge is

that the presence of electronegative gases in the mixture can rapidly quench

the seed electrons reducing their number. This leads again to the streamer


regime, limiting the reactive atmosphere that can be eventually employed

in diffuse plasma processes.

2.3.3 Streamer Discharge Regimes

The dielectric barrier discharge gap usually includes one or more dielectric

layers, which are located in the current path between metal electrodes. Two

specific DBD configurations, planar and cylindrical are illustrated in Fig-

ure 2.4 . Typical discharge gaps varies from 0.1 mm to several centimeters.

Breakdown voltages of these gaps with dielectric barriers are practically the

same as those between metal electrodes. If the dielectric-barrier discharge

gap is a few millimeters, the required AC driving voltage with frequency 500

Hz to 500 kHz is typically about 10 kV at atmospheric pressure. The di-

electric barrier can be made from glass, quartz, ceramics or other materials

of low dielectric loss and high breakdown strength. Then a metal electrode

coating can be applied to the dielectric barrier. The barrier-electrode com-

bination also can be arranged in the opposite manner, e.g. metal electrodes

can be coated by a dielectric. As an example, steel tubes coated by an

enamel layer can be effectively used in the dielectric-barrier discharge. In

most cases, dielectric barrier discharges are not uniform and consist of nu-

merous micro-discharges distributed in the discharge gap as can be seen from

figure 2.5. The physics of micro-discharges is based on an understanding of

the formation and propagation of streamers, and consequent plasma channel

degradation. The electrons in the conducting plasma channel established by

the streamers dissipate from the gap in about 40 ns, while the heavy and

slowly drifting ions remain in the discharge gap for several microseconds.

Deposition of electrons from the conducting channel onto the anode dielec-

tric barrier results in charge accumulation and prevents new avalanches and

streamers nearby until the cathode and anode are reversed (if applied voltage

is not much higher than voltage necessary for breakdown). The usual oper-

ation frequency used in the dielectric barrier discharges is around 20 kHz,

therefore the voltage polarity reversal occurs within 25 µs. After the volt-

age polarity reverses, the deposited negative charge facilitates the formation

of new avalanches and streamers in the same spot. As a result, a many-

generation family of streamers is formed that is macroscopically observed

as a bright filament that appears to be spatially localized. It is important


to clarify and to distinguish terms streamer and micro-discharge. An initial

electron starting from some point in the discharge gap (or from cathode

or dielectric that cover the cathode in the case of well developed DBD)

produces secondary electrons by direct ionization and develops an electron

avalanche. If avalanche is big enough the cathode directed streamer is ini-

tiated (usually from the anode region). Streamer bridges the gap in few

nanoseconds and forms a conducting channel of weakly ionized plasma. In-

tensive electron current will flow through this plasma channel until local

electric field is collapsed. Collapse of the local electric field is caused by

the charges accumulated on dielectric surface and ionic space charge (ions

are too slow to leave the gap for the duration of this current peak). Group

of local processes in the discharge gap initiated by avalanche and developed

until electron current termination usually called micro-discharge. After elec-

tron current termination there is no more electron-ion plasma in the main

part of micro-discharge channel, but high level of vibrational and electronic

excitation in channel volume along with charges deposited on the surface

and ionic charges in the volume allow us to separate this region from the

rest of the volume and call it micro-discharge remnant. Positive ions (or

positive and negative ions in the case of electronegative gas) of the remnant

slowly move to electrodes resulting in low and very long ( 10 µs for 1 mm

gap) falling ion current. Micro-discharge remnant will facilitate formation of

new micro-discharge in the same spot as the polarity of the applied voltage

changes. That is why it is possible to see single filaments in DBD. If micro-

discharges would form at a new spot each time the polarity changes, the

discharge would appear uniform. Thus filament in DBD is a group of micro-

discharges that form on the same spot each time polarity is changed. The

fact that micro-discharge remnant is not fully dissipated before formation of

next micro-discharge is called memory effect. The principal micro-discharge

properties for most of the frequencies do not depend on the characteristics

of the external circuit, but only on the gas composition, pressure and the

electrode configuration. An increase of power just leads to generation of

a larger number of micro-discharges per unit time, which simplifies scal-

ing of the dielectric barrier discharges. Modeling of the micro-discharges is

closely related to the analysis of the avalanche-to-streamer transition and

streamer propagation. Detailed 2D-modeling of formation and propagation


Figure 2.5: Example of stable pattern formation in a one dimensional DBD in airat a pressure of 500 mbar.

of streamers can be found literature [42, 43] where also the mutual influence

of micro-discharges is considered [43].

2.3.4 Micro-discharge interaction and pattern formation

Although the DBDs in a streamer regimes have been studied and utilized

in industries for several decades, the interaction between micro-discharges is

still subject of intense studies and a clear physical picture is yet to be found.

In the past decades several experiments have been performed showing that

under specific conditions regular pattern can be obtained [44, 14, 13, 12].

These patterns have been modeled using methods that apply generally to

pattern formation in nonlinear dynamical systems [45, 46]. Thus, the dy-

namical interactions between filaments, as well as the chemical and elec-

tronic interactions within filaments, needs yet a clear explanation. The

development and propagation of a single streamer have been studied both

from a theoretical point of view [42, 47, 48] and in few experiments [49, 50].

Also some efforts have been performed to describe the interaction between

streamers during their initial stage and propagation [43] but up until now,

the only possibility to investigate 3D patterns on the time scale of many

breakdowns was Monte Carlo simulation of the micro-discharges distribu-

tion [51, 52]. An example of pattern formation in a dielectric barrier dis-

charge is shown in Figure 2.5. What is still to be explained is the role of

the interaction between developing discharges and the interaction between

the micro-discharge remnants on the dielectric surface and discharge volume

(a sort of interaction-through memory effect). In Chapter 5 a different ap-

proach based on the temporal analysis is proposed to explain some features

and limitations of the memory effect.

2.4 PLASMA-SURFACE INTERACTIONS 21

2.4 Plasma-surface interactions

Plasmas are largely employed for the modification of surface properties of

materials. Plasma technologies have a great importance in several industrial

fields for the optical, physical, and chemical modifications of materials sur-

face. For example about one-third of the processes needed to make a modern

semiconductor chip involve a plasma-based process. Indeed, Materials and

surface structures can be fabricated that are not attainable by any other

method, and the surface properties modifications are unique.

In a typical reactive plasma the gas phase chemistry is extremely complex

because the highly energetic electrons can activate a great number of reac-

tions. In a plasma the species include neutral atoms and molecules, positive

and negative ions, radicals, electrons and photons. These species interact

with the surface of materials activating a number of processes which can

be reassumed as: reaction of atom or chemical groups insertion (grafting),

generation of free radicals on the surface (activation), deposition of a thin

layers adherent to the surface (film deposition), chemical or physical ablation

of the material surface (etching). Often in reactive plasmas all of the cited

processes are present, thus the knowledge both of the gas-phase and surface

chemistry is fundamental for the development of plasma applications.

2.4.1 Gas-phase chemistry and processes

Describing the complexity of the processes and reaction occurring in a

plasma is not a simple task and is far beyond the scope of this introduction

(see Ref. [15, 2]). Here are briefly introduced the fundamental processes of

a reactive plasma.

Ionization processes

The fundamental process in a plasma is ionization because it is responsible

of its generation and sustainment. There are different kind of such processes.

Direct ionization by electron impact is the basic plasma reaction and

include the ionizations of non-excited atoms, molecules and radicals. It

involves the interaction of an enough energetic electron hitting the other

neutral species when its energy is high enough to create an ion-electron

pair.


Preliminary exited neutral species can undergo further ionization in a

stepwise ionization by electron impact. This kind of process is important in

thermal or highly energetic discharge when the degree of ionization (ratio of

electron and ion density) is high.

Ionization by collision with heavy particles can generate electrons during

ion-molecular or ion-atomic collisions involving also vibrationally or elec-

tronically excited species. Chemical reactions are involved too.

Photoionization processes generate electron in the collision process be-

tween an heavy particle and a photon. Photoionization is important in ther-

mal plasma and in the propagation process of a streamer channel (Section

2.2.2).

Surface ionization with electron emission can be provided by ion, elec-

tron or photon collisions or just by surface heating (thermoionic electron

emission). One of the most important processes is the secondary electron

emission (or Auger emission) involving the neutralization of ions at the sur-

face.

Electron and charged particles losses

Many processes bring to the loss of a free electron and charged particles. The

balance between theses processes and the ionization processes determines the

degree of ionization and plasma density.

Electron-ion recombination processes involve the neutralization of a pos-

itive ion with an electron. It is a highly exothermic reaction which need a

channel for accumulation of the energy released during the process. This can

lead to molecular dissociation, creation of excited species, photon emission,

etc.

Especially in presence of an electronegative gas (O2, CO2, SF6, CF4,

etc.) the electron attachment processes are extremely important and are

often responsible for the balance of charged particles. An attachment pro-

cess typically take place in electronegative gases when a molecular fragment

(dissociation products) has a positive electron affinity.

When the electron attachment processes are involved in the balance of

electrons and ions (electronegative gases), the actual losses of charged parti-

cles are mostly due to ion-ion recombination processes which are the mutual

neutralization of positive and negative ions in binary or three-body colli-


sions.These processes can proceed by many different mechanisms and have

very high rate coefficients.

Finally, as for ionization, must be considered in the balance of charged

particle losses the surface recombination processes. These processes are the

most important in low pressure discharges because they are usually kineti-

cally limited by the diffusion of charged particles to the walls.

Gas-phase chemical reactions

Along with the processes described above, in a typical reactive gas a wide

variety of chemical reactions are to be considered in the gas-phase chem-

ical equilibrium. The radical production processes are responsible for the

creation of extremely reactive species that can interact with other elements

of the atmosphere or on the surface. These species are usually extremely

important for plasma processing. A wide variety of gas phase chemical reac-

tions involving all the active species in the plasma are also to be considered.

Usually the number of these reactions is very high and the complete de-

scription of the chemical equilibrium of a reactive plasma can become an

overwhelming task.

Excited atoms and molecules in plasma

Excited species are extremely important in plasma chemical kinetics. High

electron temperatures and thus highly energetic electrons, can provide a

high excitation rates of different electronically excited state of atoms and

molecules by electron impact. If the radiative transition to the ground state

is not forbidden by quantum selection rules, such a state is called resonant

excited state. It has typically a short lifetime (10−8 ÷ 10−6 sec.) and does

not interfere with chemical kinetics. Otherwise, if the radiative transition

is forbidden, this state is called metastable excited state and because its

lifetime can be very long (10−2 ÷ 102 sec.), it can significantly contribute to

the chemical kinetics.

In presence of molecules in the plasma an extremely important pro-

cess is the vibrational excitation of molecules by electron impact. Indeed,

in a molecular gas, most of the electron energy can be transferred to the

vibrational excitation. For this reason, the vibrational excitation, relax-


ation, and reaction of vibrationally excited molecules strongly influence the

chemical kinetics of the plasma. Several relaxation processes are impor-

tant: vibrational-translational (VT) processes convert vibrational energy in

kinetic energy of the whole particle and it is the loss mechanism of vibra-

tional energies. Vibrational-vibrational (VV) processes rearrange the energy

between vibrational levels and are responsible for the creation of highly vi-

brationally excited molecules which are extremely influent in the chemical

equilibrium. In fact, these molecules can posses enough energy for dissocia-

tion and/or other endothermic chemical reactions. The vibrational levels are

usually thermalized and a vibrational temperature Tv can be defined. How-

ever, because the VT processes are often weakly efficient in non-thermal

discharges, molecular vibrations ”trap” the electron energy, and Tv > T0

where T0 is the ion and gas temperature.

Also the rotational levels of molecules can be excited by rotational ex-

citation of molecules by electron impact processes. Similarly to vibrational

levels, the relaxation of rotational levels can happen through rotational-

rotational (RR) or rotational-translational (RT) relaxation processes. How-

ever, the probability of RT (and RR) processes in very high because of the

smallness of rotational quanta, and, in many non-thermal plasmas, the rates

of rotational relaxation processes are comparable with the rate of transla-

tional thermalization (TT processes). In this cases the defined rotational

temperature Tr ∼ T0.

2.4.2 Surface kinetics and processes

Physical and chemical surface processes are central to plasma processing.

Some of these processes, which are important for sustainment of the dis-

charge and its chemical equilibrium, have been described in Section 2.4.1.

Indeed, the surface and gas-phase reactions sets are strongly coupled and

cannot be considered separately. Here attention is concentrated on those

surface processes which are fundamental in plasma processing of materials

and on the kinetics of surfaces.


Figure 2.6: Typical reaction set for a surface process.

Surface kinetics

Surface reaction mechanism for most plasma processes are still not well un-

derstood and characterized. However, adsorption and desorption of reactive

species on the surface are usually part of the complex surface processes.

Adsorption is the mechanism that brings an atom (or molecule) to form

a stable bond with the surface. There are two kind of adsorption processes:

physiosorption, which is the creation of a bound due to the weak attractive

Van der Waals forces between the atom and the surface, and chemiosorption

which is due to the formation of a chemical bond between the atom (or

molecule) and the surface. These two kind of adsorption mechanism are

often found in the same system with different regimes favored depending on

surface temperature and chemical environment. Desorption is the reverse

reaction to adsorption and, in thermal equilibrium, the two reaction must

be balanced. In Figure 2.6 is illustrated a typical reaction set for a surface

process. Reactive species diffuse or flow to the surface with rate constant

K1, where they are adsorbed (K2) and react (K3). If the process generates

by-products (for example in chemical etching) they can desorb (K4) and

diffuse or flow into the gas phase (K5). In addition, must be considered the

desorption without reaction of the reactive species (K6) and the backward

adsorption of the eventual by-products. This is the most simple reaction

scheme that must be considered for the description of a surface process.


Etching processes

Plasma Etching is a fundamental process for the removal of material from a

surfaces. The process can be chemically selective (i.e. can remove a specific

type of material leaving others unaltered) and is the only commercially vi-

able technology for anisotropic removal of material (i.e. can remove material

at the bottom of a trench while leaving the same material on the side walls

unaffected). Most application of plasma etching are in the field of integrated

circuit fabrications, but other applications exist (in association with other

processes) for polishing, cleaning or sterilization of surfaces.

Etch process are typical at lower pressure and comprise three main pro-

cesses. Sputtering is the ejection of atoms from surface due to energetic ion

bombardment. It is an unselective (i.e. sputtering yields do not change too

much with material), highly anisotropic process, and is the only one which

can remove in-volatile products from a surface. In pure chemical etching

the discharge supplies gas-phase etchant atoms or molecules that chemically

react with the surface to form gas-phase product following a the scheme

shown in Figure 2.6. This process is highly chemical selective. Ion-enhanced

etching is a process in which the discharge supplies both etchants and ener-

getic ions to the surface. The energetic ions increase the etching rate and

anisotropy but reduce the selectiveness of pure chemical etching.

Deposition processes

Plasma assisted deposition, implantation and surface modification processes

are extremely important for the creation of thin films on surfaces and for

the modification of surface properties. Plasma-generated thin films can

have unique chemical composition and morphology that are not attainable

with conventional chemical vapour deposition (CVD) and other processes.

Plasma enhanced chemical vapour deposition (PECVD) consists of a plasma

activated set of gas-phase and surface reactions that produce a solid prod-

uct at the surface. Reaction scheme is rather complicate and can involve

also polymerization processes both in the gas-phase and on the surface. A

deposition process of this kind is studied in Chapter 7.


Plasma grafting

Plasma grafting is the insertion through chemical bonding of a specific func-

tional group on the surface. It can be considered a deposition processes

which follows the reaction scheme illustrated in Figure 2.6. It starts like a

deposition with the generations in the gas-phase of a reactive species which

is adsorbed on the surface where form a stable chemical bond. However, are

not present reactive species that can start a polymerization process neither

in gas-phase nor on surface. This results is the creation of a single molecu-

lar (or atomic) layer on the original surface. A grafting process of fluorine

atoms is studied in Chapter 8.

Plasma Activation

What is called plasma activation is usually a combination of an etching and a

grafting process that in intended to modify or improve surface properties in

order to attain, for example, better adhesion of polymeric webs to coatings,

painting, gluing, wetting, etc. Plasma activation can also promote cross-

linking and is always present also during deposition processes and increases

the bonding of reactive species to surfaces. Grafting of specific functional

groups can be promoted (for example polar groups in air plasma treatment of

polymer surfaces to attain wettability), and the removal of weakly bounded

layers through etching can be attained.


CHAPTER3

Dielectric barrier discharge devices

This chapter is devoted to the description of the experimental setups utilized

in the research. The plasma device developed for the study of surface process

in a controlled atmosphere is described in details as long as the motivations

of the choices. The diagnostics are mentioned here only when needed in the

description and are discussed more deeply in Chapter 4

3.1 DBD device for surface modifications

The main advantage of the atmospheric pressure DBD is its easy adapt-

ability to continuos material processing [1, 2, 3, 20]. The idea behind the

realization of this experimental setup is the possibility to study both the

discharge physics and the plasma-surface interaction during continuos pro-

cessing in a wide range of pressures and compositions of the atmosphere.

Continuous treatment of web material is a central feature of industrial ap-

plications and the roll-to-roll configuration is a compulsory characteristic of

29

30 DIELECTRIC BARRIER DISCHARGE DEVICES

the plasma discharge system.

3.1.1 Plasma reactor

The dielectric barrier discharge (DBD) device used for the experiments is

mostly similar to the typical corona treater already used at industrial level

for adhesion improvement. The choice of this configuration is motivated by

the ease of development and scaling of this type of configuration.

A schematic representation of the experimental setup is given in Figure

3.1. The electrode system consists of two parallel high voltage electrodes

and a rotating cylindrical grounded electrode. The high voltage electrodes

are two cylindrical rods 230 mm long with a 8 mm diameter, coated with

pure (> 99.7%) Al2O3 sintered ceramic dielectric of 2 mm thickness. The

grounded rotating electrode is a void steel cylinder coated with ceramic

dielectric of 5 mm thickness. Distance between electrodes can be varied

between 0.5 and 5 mm. An electric motor with a controller and a motion

vacuum feed-through can rotate the grounded cylinder with tangent speeds

between 0.1 and 100 m/min.

The electrodes are enclosed in a vacuum chamber (Copra Cube by CCR

Technologies Gmbh) 40x40x40 cm where particular gaskets have been em-

ployed to avoid leakage both in vacuum and in over-pressure. These par-

ticular solution allows to perform experiments without contamination not

only in under-pressure, but also in slightly over-pressure. The affordable

working pressure range is between 10−1 and 1300 mbar, limited below by

the evacuation of impurities and above by the leakage of gaskets.

3.1.2 Pumping system and gas distribution

The control of the reactive atmosphere is a key feature of this plasma device.

To ensure a minimal concentration of uncontrolled contaminations during

the experiments, a double-stage rotary pump (SD-301 by Varian) is used to

evacuate the atmosphere to a limiting pressure of 5 · 10−3 mbar. Because

this pump cannot work efficiently at high pressure without a considerable

overheating, a second dry pump (ZA60 by Rial) is used at higher pressure

experiments. The desired pressure is maintained constant balancing the

inlet fluxes through the regulation of dosing valve V2 (see Figure 3.1).

3.1 DBD DEVICE FOR SURFACE MODIFICATIONS 31

Figure 3.1: Schematic representation of the DBD reactor. The electrodes areenclosed in a vacuum chamber. A motor with a rotation control (RC) rotates thegrounded electrode. A rotary pump (P1) is used to evacuate the chamber and apiston pump (P2) with a dosing valve (V2) are used to stabilize the desired workingpressure balancing the inlet fluxes. A Pirani pressure gauge (PG1) and a capacitivegauge (PG2) measure the pressure in the chamber. Three mass flow controllers(MFC1,2,3) with different capacity can mix gases and a controlled evaporator andmixer (CEM) can also mix liquid (MFCL) as vapours in a carrier gas (MFC). Anamplified signal generator and a current transformer provide the high voltage to theelectrodes. Current and voltage are acquired in a oscilloscope with a specificallydesigned Rogowski coil (ROG) and a high voltage probe (HVP). Optical emissionis acquired through an optical fibre with an UV-VIS spectrometer.


Figure 3.2: Layout of a section of the discharge region. The two high voltageelectrodes are in front of a rotating grounded electrode. A polycarbonate injectionsystem guarantee the uniformity of gas and vapour flow over the width of theelectrodes.

Two pressure gauge are used to measure low and high pressure ranges: a

Pirani pressure gauge (Ttr 91 by Leybold) measures the pressure respectively

in the range 10−3 ÷ 5 mbar and a capacitive gauge (DI2000 by Leybold)

measures the pressure 1 ÷ 2000 mbar.

The inlet fluxes are controlled by a gas and vapour mixing system (Fig-

ure 3.1). Process gases with high purity level are regulated and mixed using

a system of 3 mass f low controllers (El-flow by Bronkhorst) with different

capacity. To use liquid precursors at atmospheric pressure a controlled evap-

orator and mixer (Bronkhorst CEM System) is also attached to the inlet al-

lowing the mixing of vapours with concentration up to the saturation at the

given conditions of temperature and pressure in the vacuum chamber. The

inlet fluxes generated by the gas and vapour mixing system are injected into

the vacuum chamber directly between the high voltage electrodes through an

injection nozzle. The nozzle is a polycarbonate shower specifically designed

and realized to ensure uniform fluxes on the whole width of the electrodes

up to 50 ln/min.. Polycarbonate have an upper limit working temperature

around 80 C and good chemical resistance. A sectional view of the injection

nozzle arrangement is shown in Figure 3.2.

3.1 DBD DEVICE FOR SURFACE MODIFICATIONS 33

3.1.3 Electric power supply and configuration

The AC is applied by an amplified signal generator with frequencies between

10 KHz and 50 KHz and through a high voltage transformer. The power

supply is composed by a current rectifier which brings the line current from

230V AC to a 310 V DC. A transistor switching system create the AC current

which is connected to the primary winding of the high voltage transformer.

The secondary of the transformer is then directly connected to the electrodes

and the whole system consist of a resonant circuit. The voltage applied to

the primary winding of the transformer is constant and the voltage applied

to the electrodes is controlled by the resonance between the proper frequency

of the system and the applied frequency. Usually the complete voltage (and

power) range of the device is within a span of few kilohertz.

3.1.4 Diagnostics

We characterize the plasma discharges principally by means of optical and

electrical diagnostics. The emission spectra of the discharges have been mea-

sured with a wide band spectrometer A complete description of instrument

and measuring techniques is given in Section 4.1.

Both the current and the voltage are acquired in a Nicolet 450 oscillo-

scope respectively with a specific designed Rogowski coil (see Section 4.2.1)

and a high voltage probe (Tektronix P6015A). In Figure 3.3 are plotted the

amplitude-frequency and phase shift-frequency response of the Rogowski coil

used for current measurements in the experiments with the present setup.

Current measurement are performed with two different Rogowski probes.

The first probe has a lower bandwidth (5 kHz-25 MHz) and is used to mea-

sure the displacement current and longer current pulses without introducing

dephasing. The second probe is used to record the shape of the fast current

pulses due to micro-discharge formation and has an higher bandwidth in

the range 400 kHz-120 MHz. Probe type have been selected and adjusted

to necessity of different experiment. A complete description of development

and calibration of Rogowski coils is given in Section 4.2.1.


10-3

10-2

10-1

100

101

102

-12-9-6-303

Atte

nuat

ion

[dB

]

10-3

10-2

10-1

100

101

102

Frequency [MHz]

-0.4-0.2

00.20.4

Phas

e [π

]

Figure 3.3: Amplitude vs. frequency (upper panel) and phase shift vs. frequency(lower panel) response of the two Rogowski coils. Circles represent the values forthe ferrite core coil used for displacement current measurements with a 5 kHz-25MHz bandwidth. Diamonds represent the values for the NiZn core coil used asa reference for the fast current pulses, with a bandwidth in the range 400 kHz-120 MHz. The dotted lines in the upper panel represent the usual 3 dB limit todetermine the probe bandwidth.

3.2 DBD device for streamer regime characterization

In order to use the simplest configuration for the characterization of the

streamer regime it has been used a different configuration with respect to

the one described in Section 3.1. The parallel rod electrodes allow a better

understanding of the discharge properties from the analysis of the current-

voltage signals, however the same amplified signal generator as in Section

3.1 have been used in nearly the same frequency range.

3.2.1 Plasma reactor

The DBD device used here consists of two high voltage (center grounded)

electrodes working under atmospheric pressure conditions in air. They are

constituted by two rod electrodes 290 mm long with a 9 mm square section,

coated with pure (> 99.7%) Al2O3 sintered ceramic dielectric, with an ex-

ternal 15 mm square section and 3 mm thickness. A schematic diagram of

the experimental setup is shown in Fig. 3.1.

The steel electrodes are cave and a cooling gas flow pass through them

3.2 DBD DEVICE FOR STREAMER REGIME CHARACTERIZATION 35

ROG

HV

T

OSCILLOSCOPE

HV

Figure 3.4: The DBD device is made up of two rod electrodes of square sec-tion coated with pure (> 99.7%) Al2O3 ceramic dielectric. The distance betweenelectrodes is 4 mm. An amplified signal generator and a current transformer (T)provide the high voltage to the electrodes. Current and voltage are acquired witha specifically designed Rogowski coil (ROG) and a high voltage probe (HV).

to keep the temperature low. The length of the discharge gap is fixed at

4 mm. The AC voltage is applied using the same amplified signal generator

used for the device described in Section 3.1. The applied voltage to the

primary winding of the high voltage transformer is constant and the one

applied to the electrodes is controlled by the resonance between the proper

frequency of the system and the applied frequency. The frequency range

spans between 31 kHz and 36 kHz in the affordable voltage (and power)

range of the device.

3.2.2 Diagnostics

As for the other device both the current and the voltage are acquired with

a Nicolet-Multipro oscilloscope. Using a high voltage (Tektronix P6015A)

probe, voltage is acquired ( on one of the hot wire and doubled to consider

the symmetry of the electric circuit. The current signal is acquired using

a specific designed Rogowski coil with a ferrite magnetic core coil and a

bandwidth of 50 kHz-70 MHz) and a , respectively. A second NiZn core

coil with a bandwidth of 400 kHz-120 MHz have been used (see Fig. 3.5)

in order to control the response of the ferrite coil to short current pulses

due to streamers. The response of the two coils to the shortest current

pulses measured in the experiments were almost undistinguishable from each

other. Because of the low frequency range of the generator, lying below the


probe bandwidth, the displacement current response is underestimated and

dephased. However, this does not affect our analysis of fast current pulses

associated to the discharge current. Probe type have been selected and

adjusted to necessity of different experiment. A complete description of

development and calibration of Rogowski coils is given in Section 4.2.1.

10-2

10-1

100

101

102

-12-9-6-303

Atte

nuat

ion

[dB

]

10-2

10-1

100

101

102

Frequency [MHz]

-0.4-0.2

00.20.4

Phas

e [π

]

Figure 3.5: Amplitude vs. frequency (upper panel) and phase shift vs. frequency(lower panel) response of the two Rogowski coils. Circles represent the values forthe ferrite core coil used for measurements with a 50 kHz-70 MHz bandwidth.Diamonds represent the values for the NiZn core coil used as a reference for thefast current pulses, with a bandwidth in the range 400 kHz-120 MHz. The dottedlines in the upper panel represent the usual 3 dB limit to determine the probebandwidth.

CHAPTER4

Plasma and material diagnostics

In this Chapter are described the diagnostics used to characterize the plasma

discharges and the modifications induced on material surfaces. Both the

instruments and the analysis methods are described.

4.1 Optical emission spectroscopy

The emission spectra of the plasma discharges have been measured by means

of a wide band, low resolution spectrometer (PS2000 by Ocean Optics).

The spectrometer, equipped with a 10 µm slit, a holographic grating (600

lines/mm, blazed at 400 nm) and a 1024 pixels CCD, has a resolution of

1.02 nm and a spectral band extending from 200 to 850 nm. Integration

time is changed depending on the discharge brightness. Emission spectra of

the discharges have been recorded through an UV enhanced optical fiber,

connected to the device by a vacuum feed-through. Depending on the gas

composition, intensities of the emission lines can allow the calculation of

37

38 PLASMA AND MATERIAL DIAGNOSTICS

several properties of plasma discharge and gas-phase chemistry like concen-

trations, vibrational and electron temperature.

4.1.1 Determination of molecular vibrational temperature

When a molecular gas is present in the discharge atmosphere the excitation

of its vibrational and rotational levels becomes a dominant process and in

some cases the most part of the electron energy is spent in these processes

(see Section 2.4.1). Because the vibrational levels are in thermal equilib-

rium, supposing a Boltzmann distribution it is possible to determine the

vibrational temperature Tv. The method depends on the molecule and gas-

phase chemistry and will be explained for mixtures containing nitrogen in

Chapter 6

4.2 Voltage Current measurements

Voltage and current measurement are fundamental to plasma discharge un-

derstanding. Both signals are digitally acquired in a Nicolet MultiPro (3

channels, 200 MHz, 8 bit, 1 GS/s) or a Nicolet 450 Oscilloscope (4 channels,

200 MHz, 2 GS/s) or a Tektronix TDS 4020 (2 channels, 60 MHz, 1 GS/s)

and analyzed with the aid of the computer.

Voltage is usually acquired with a wide bandwidth, high voltage probe

(Tektronix P6015A, 40 kV, 75 MHz) which allows to recognize eventual

fluctuations of the applied sinusoidal voltage.

Because of the presence in the current signal of very fast processes due

to micro-discharges, particular attention must be paid to the current mea-

surement. The development and calibration of home-made Rogowski coil

sensor will be described in the subsequent Paragraphs.

4.2.1 Implementation of Rogowski coils for measurements nanoseconds cur-

rent pulses

Detailed measurements of the current response of a dielectric barrier dis-

charge require particular attention to the bandwidth of the probe. The

displacement current response will be at the same frequency of the applied

voltage, but the discharge current is generally in pulses with duration from

4.2 VOLTAGE CURRENT MEASUREMENTS 39

microseconds down to tenth of nanoseconds depending on the discharge

regime [4, 6, 10, 9]. An easy way to measure the current is to introduce

a shunt in series with the electrical circuit. The current shunt has a good

response bandwidth but the protective circuit needed to avoid damage to

measuring instruments can cause distortion of the read waveform. The Ro-

gowski coil is galvanically separated from the main circuit and can be de-

signed for a precise measurement of the nanosecond current pulses typical

of the streamer discharge regime[53, 54].

Theory and principle of Rogowski coils

A Rogowski coil is a conducting wire that is wound in a spiral around a

magnetic or non magnetic core and then returns to the original point. The

coil is placed around the conductor to couple the pulse signals. The operat-

ing principle was formulated by Rogowski and Steinhaus in 1912 [55]. In the

original design Rogowski coils were air cored to avoid saturation of magnetic

core when measuring high currents. In the present measurements, currents

are constantly far below saturation and it will be shown how the choice

of the magnetic core influences the bandwidth of the coils. A schematic

representation of a Rogowski coil is shown in Figure 4.1. The current I

flowing in a cable generate an electromotive force (emf) E at the output of

the coil which is proportional (following Faraday law) to the rate of change

of the current: ∂I/∂t. The signal E must be integrated with a passive or

active circuit. The high frequency behaviour of the coil, in particular its

bandwidth and susceptibility to high frequency oscillations, is significantly

influenced by the integration circuit impedance. With the right choice of the

configuration of the coil the integrating circuit can be reduced in a simple

resistance [56, 57, 58, 53, 54]. A lumped parameter model can be introduced

to describe the circuit behavior (Figure 4.2). The variable current i1(t) pro-

duce a magnetic field and the rate of change in current produce a voltage in

the coil equal to

Ui(t) = Mdi1(t)

dt, (4.1)


Figure 4.1: Schematic representation of a Rogowski coil. The current I flowingin a cable generate a emf E at the output of the coil which is proportional to ∂I/∂tand must be integrated with a passive or active circuit.

Figure 4.2: The equivalent circuit diagram (lumped parameter) (M , mutual in-ductance; Ls, self-inductance; Cs, stray capacitance; Cp, turn-to-turn capacitance,which can be ignored in the spaced winding in the design because it is very small;Rs, equivalent resistance of coil; R, integral resistance larger than Rs in the design;U0(t), voltage of the integral resistance; Ui(t), the induced voltage) [54].

4.2 VOLTAGE CURRENT MEASUREMENTS 41

where M is the mutual inductance between the measured circuit and the

coil. The transfer function for the lumped parameter model of Figure 4.2 is:

U0(t) =R

LsCsRs2 + (Ls + RsCsR)s + Rs + RUi(t), (4.2)

where the generic impedance of the integrating resistance have been consid-

ered a pure resistive load. If the integrating resistance R is chosen in order to

have R ≪√

Ls/Cs the pole of the transfer function 4.2 move along the real

axis so the system does not oscillate and the Rogowski coil is self-integrating

between the two poles [53].

For toroidal coils with rectangular square section the lumped parameters

can be calculated as:

Ls =µN2h

2πlog

d2

d1= µN2 A

l(4.3)

Cs =4π2ǫK1

log K1/K2(4.4)

where l is the length of the effective magnetic path, µ is the magnetic per-

mittivity of the core, A is the cross-sectional area of the core, d2 and d1 are

the outer and inner diameters of the coil respectively, h is the height of the

coil, N is the number of turns in the coil, ǫ is the dielectric constant of the

core, K1 = (d2 + d1)/2 and K2 = (d2 − d1)/2. From the lumped parameter

model represented in Figure 4.2 can be deduced the following equations:

fl =1

2π

R + Rs

Ls + RRsCs≈ 1

2π

R

Ls(4.5)

fh =1

2π

Ls + RRsCs

RLsCs≈ 1

2π

1

RCs(4.6)

where fl and fh are the lower and upper frequency limits respectively. From

the above equations is possible to determine which construction parameters

must be changed in order to obtain the desired bandwidth. Thou, In order

to increase the frequency band, fl should be as low as possible, while fh

should be as high as possible. According to equations (4.4), (4.5) and (4.6),

it can be seen that to obtain the wider bandwidth Ls should be as large

as possible while R should be as small as possible. A bigger inductance Ls

can be obtained increasing µ or, more efficiently, increasing N which gives a


quadratic dependency. With increasing N and decreasing R, the bandwidth

will become wider, but the sensitivity will become lower. A balance in these

parameters must be attained in order to have the wider bandwidth and

ensure a good sensitiveness.

me ? m0

N—–2

Although bandwidth of the current transducer can reach the desired

range by controlling R and Ls in theoretical analysis, the frequency range

and configuration parameters of the magnetic core play important roles in

determining bandwidth of the current transducer. First, the frequency range

of the magnetic core should include the desired frequency range. Two kind

of magnetic core have been used to build the Rogowski coils used in the ex-

periments. For the lower bandwidth (and in order to measure displacement

currents of the order of kilohertz) a ferrite material with initial permeabil-

ity µ = 4300 NA−2, coercive field strength Hc = 0.19 Oe and saturation

flux density Bs = 3900 Gauss, have been used which gives an higher sensi-

tiveness but cannot resolve higher frequencies. For the detection of fastest

current pulses due to micro-discharges a Nickel-Zinc core have been used

which guarantees an higher bandwidth at the cost of sensitiveness. Its pa-

rameters are: initial permeability µ = 1500 NA−2, saturation flux density

Bs = 2800 Gauss, coercive field strength Hc = 15 Oe. Also the geometrical

parameters of the core can have some influence the final bandwidth [54],

but here they are not considered because in the experiments they are con-

strained by dimensions of the cables and cannot be varied. From the value

of Bs the maximum measured current Imax can be calculated according t

[54]:

Imax =0.8Bsl

µ. (4.7)

In all the experiments and for all the build current sensor the measured

current is always under the maximum value (I < Imax).

Calibration of the Rogowski coils

The calibration circuit (Figure 4.3) consists of a wide bandwidth signal

generator which is connected to a 50Ω non inductive resistor through an

RG 58 BNC cable. The cable is split on the resistor side with the hot wire

4.3 CHARACTERIZATION OF THE MATERIALS SURFACES 43

Figure 4.3: The calibration system used to evaluate the frequency and amplituderesponse of the Rogowski coils.

passing through the Rogowski probe to be tested. Acquiring the voltage

drop across the resistor and the probe signal it is possible to determine the

amplitude response, the phase shift and the sensitiveness of the Rogowki

Coil.

Several combination of magnetic core and number of turns N have been

tested while the integrating impedance have been kept constant to a 50 Ω

non-inductive resistance. In Figure 4.4 are showed the Bode plots for some

built Rogowski coils. The parameters are the type of magnetic core are

indicated in the figure legend.

The Bode plots for the specific coils used in the single experiments are

showed in Sections 3.1 and 3.2 in the description of the experimental setups.

We utilized two different Rogowski probes. The first one with a ferrite

magnetic core and a bandwidth of 5KHz-25MHz to detect the displacement

current and longer current pulses. The second one with an high frequency

NiZn magnetic core and a bandwidth of 250KHz-120MHz to detect the cur-

rent pulses of single streamers.

4.3 Characterization of the materials surfaces

In this section are briefly described the diagnostics used for the charac-

terization of the material surfaces after the plasma treatments. At a mi-

croscopic level both the chemistry and the morphology are characterized


Figure 4.4: Amplitude vs. frequency (upper panel) and phase shift vs. frequency(lower panel) response of some Rogowski coils with different construction parame-ters and magnetic core.

with infrared spectroscopy and AFM/SEM/FIB measurements respectively.

Modified macroscopic properties are evaluated with contact angle measure-

ments.

4.3.1 Infrared spectroscopy (FTIR/ATR-FTIR)

Fourier transform infrared spectroscopy (FTIR) can be used to identify

chemical composition of the realized coatings. FTIR is perhaps the most

powerful tool for identifying types of chemical bonds (functional groups).

Molecular bonds vibrate at various frequencies depending on the elements

and the type of bonds. For any given bond, there are several specific fre-

quencies at which it can vibrate. The wavelength of light absorbed is char-

acteristic of the chemical bond as can be seen and identified in the spectrum.

Sometimes transmission measurements cannot be performed on several

specimens and a surface analysis must be used. An attenuated total reflec-

tion (ATR-FTIR) technique operates by measuring the changes that occur

in a totally internally reflected infrared beam when the beam comes into

contact with a sample. An infrared beam is directed onto an optically dense

crystal with a high refractive index at a certain angle. This internal re-

flectance creates an evanescent wave that extends beyond the surface of


the crystal into the sample held in contact with the crystal. This evanescent

wave protrudes only a few microns (0.5 µm- 5 µm) beyond the crystal surface

and into the sample. Consequently, there must be good contact between the

sample and the crystal surface. In regions of the infrared spectrum where the

sample absorbs energy, the evanescent wave will be attenuated or altered.

The attenuated energy from each evanescent wave is passed back to the IR

beam, which then exits the opposite end of the crystal and is passed to the

detector in the IR spectrometer. The system then generates an infrared

spectrum. The measurement have been performed with a Nicolet Avatar

360 with a resolution of 4 cm−1 in the range 400÷4000 cm−1, and equipped

with an ATR accessory (PIKE-Technology).

4.3.2 Atomic force microscopy (AFM)

The atomic force microscopy (AFM)is a rather recent technique to mea-

sure the morphology of surfaces down to nanometer scale resolution. The

functional scheme of an AFM is represented in Figure 4.5. The AFM head

uses a beam deflection scheme to monitor the cantilever displacement. This

scheme is quite simple and permits registration of both normal deflection of

the cantilever with sub-angstrom resolution and its twisting angle, so normal

and lateral force can be measured simultaneously. A laser beam is focused

onto the back surface of cantilever close to tip position, and reflected beam

falls onto the quadrant photodiode. Cantilever deflection causes displace-

ment of the reflected beam over sections of the photodiode. An amplified

differential signal from the quadrant photodiode permits measurement of

angular deviation with the accuracy of less than 0.1 degrees, that corre-

sponds to normal cantilever deflection of the order of 0.05 nm. Among the

several techniques used to measure the morphology of surfaces, have been

used contact mode and semi-contact (tapping) mode. In contact mode can-

tilevers touches the surface while scanning in repulsive mode (like a needle

of gramophone), but can scratches softer surfaces. The semi-contact mode

is a special modulation technique for non-destructive imaging of soft sam-

ples as well as of hard. It measures topography by tapping the surface with

an oscillating probe tip. The measurements in the present work have been

performed with a NT-MDT Solver P47H-pro.


Figure 4.5: Functional scheme of an AFM.

4.3.3 Contact angle measurements and surface energy determination

Contact angle measurements have been performed to evaluate macroscopic

properties of material surface such as surface energy and wettability. Static

and dynamic measurements have been performed using a video-supported

contact angle measuring instrument Dataphysics OCA 20.

Wettability and surface energy

Wetting [59]describes the ability of a liquid deposited on a solid substrate to

spread out or remain confined. When the surface energy of a dry substrate

is higher than the energy of the wetted one (by some liquid), the liquid

spreads completely on the surface in order to lower its energy (for example,

the behaviour of water on a clean glass surface). On the contrary, when the

surface energy of a dry substrate is lower, the liquid partially wets the surface

forming drops (for example, water on a plastic surface). At the contact line

between the three phases (liquid, solid, gas or vapour) the contact angle

between the liquid drop and the surface is determined by the equilibrium of

the surface tensionsσ (or surface energies) of the interfaces according to the


Figure 4.6: Young equilibrium between surface tension determining contact angle.

Young’s equilibrium [60] (see Figure 4.6):

σlv cos θe = σsv − σsl, (4.8)

where the subscripts indicate the inter-phase between liquid (l), solid (s)

and vapour (v). The equilibrium contact angle θe is a physical constant

depending only on the materials, and in no other way on the particular

configuration considered.

According to the Owens-Wendt two-parameter model the surface ten-

sions of the solid-vapor and liquid-vapor inter-phases consist of two com-

ponents: a dispersive one accounting for van der Waals and other non-site-

specific interactions and a polar one accounting for dipole-dipole, dipole-

induced dipole, hydrogen bonding and other site-specific interactions[61].

The surface tensions of the liquid and the solid (in contact with vapour) can

be expressed as:

σsv = σdsv + σp

sv, (4.9)

σlv = σdlv + σp

lv. (4.10)

The solid-liquid interfacial tension can be expressed as [62]:

σsl = σsv + σlv − 2[

√

σdsvσ

dlv +

√

σpsvσ

plv

]

(4.11)


Figure 4.7: Contact Angles as a function of drop volume when it is increased(right-hand arrow) and decreased (left-hand arrow) whit a syringe. Advancing andreceding contact angle are determined as indicated.

Combining equations (4.8) and (4.11) yields:

(1 + cos θe)σd

lv + σplv

2√

σdlv

=√

σdsv +

√

σpsv

√

σplv

σdlv

. (4.12)

The two unknown components of the surface tension σdsv and σp

sv in equation

(4.12) can be determined from the measured contact angles against at least

two test fluids with known values of surface tension components σdlv and

σplv. A plot of left hand side of equation (4.12) versus

√

σp

lv

σdlv

for different

liquids yields the dispersive component (square of the y-intercept), the polar

component (square of the slope) and consequently the surface tension of the

solid-vapor interface σsv from equation (4.10).


Contact angle hysteresis

According to Young’s equation (4.8) the static equilibrium contact angle θe

is related to the surface tension of the solid-vapor and solid-liquid interfaces,

and it is ideally a unique property of the material system being considered,

but practically a hysteresis often arises depending on how the interfaces

form. If a liquid droplet is quietly settled on a solid surface (or if its volume is

slightly increased after it settling) thus measured contact angle is larger, even

up to several tens of degree, than the angle measured for the same droplet

after reducing its volume. The advancing angle θa is the largest contact angle

achievable before the wetting line begins to move in the direction of the gas

phase and the receding angle θr is the smallest contact angle achievable

before the wetting line begins to move in the direction of the liquid phase.

Many theories of the contact angle hysteresis have been proposed [63, 64,

65, 66] even if a clear interpretation of this effect still lacks. Hysteresis

is usually connected with changes of roughness and chemical heterogeneity

of the surface [59]. The measurements of advancing and receding contact

angles can give interesting informations on the solid-liquid interactions.

With the OCA20 instrument the advancing and receding angles have

been measured by modifying the volume of a drop by inflating and deflating

liquid with a computer controlled syringe. Recording a movie of the dynamic

contact angle it is possible to obtain the estimate of advancing (receding)

angle (see Figure 4.7) as the maximum (minimum) value before the drop

base diameter increases (decrease).


CHAPTER5

Statistical characterization of a

streamer discharge regime

5.1 Introduction

In this chapter, we investigate the temporal behavior of current pulses for a

streamer regime in a DBD at atmospheric pressure. As explained in Chapter

2, at low pressures DBDs operate in a Townsend breakdown regime [5] gen-

erating a diffuse glow discharge. At atmospheric pressure, the realization of

a diffuse discharge is restricted to limited conditions of geometry, electrical

parameters and gas composition, and DBDs operate usually in a streamer

discharge in which several narrow discharge filaments are typically formed

(see Section 2.3.2). The streamer regime constitutes a strongly interact-

ing system of discharges exhibiting cooperative behavior. This leads, under

specific conditions, to the formation of coherent spatial configurations that

have been observed in different types of experimental setups [12, 13, 14, 6].

51

52 STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME

However, micro-discharges seem, to some extent, to occur at random within

the discharge gap for most applications of DBDs. To our knowledge, the

statistical properties of such discharges in air have not been discussed with

sufficient insight in literature so far, even if similar studies have been per-

formed in dealing with the so-called partial discharges [67, 68]. In partic-

ular, we find the existence of two different streamer regimes as a function

of applied voltage, separated by a value V spp (here V s

pp ≃ 23.6 kV). The two

different regimes can be characterized by the first moments of the discharge

distributions, suggesting a way for determining the separatrix voltage V spp.

The peculiar feature of DBDs (see Chapter 2) is that the charge trans-

ported by the micro-discharges to the dielectric cannot reach the conducting

electrode and accumulates near the surface until the change in the local elec-

tric field extinguishes the filament. Because of the slow diffusion of charges

on the surface, in the subsequent half-cycle of the driving voltage the locally

modified field promotes the formation of a streamer in the same spot. This

so called ‘memory effect’ is a dominant feature in DBDs (see Section 2.3.3).

A typical streamer has a lateral spatial extension of about 0.1 mm and a

duration of the order of nanoseconds depending on the device configuration

and type of gas. The presence of the memory effect suggests temporal corre-

lations may be found in the discharge current signal. In particular it is found

that in the studied streamer discharge regime the existing residual correla-

tions propagating between the discharge processes (half-cycles) are only an

effect of the non-stationariety of the discharge current response, thus, cor-

relations vanish outside the single discharge process. On the contrary, by

analyzing the current signal inside the half-cycle, it is found that on time

scales of the order of hundreds of nanoseconds (i.e., within a single current

burst, in which the streamers develop sufficiently close in time), strong cor-

relations exist which also reveal a peculiar ordered temporal structure of the

discharge current signal. The experimental setup and the diagnostics used

here are described in Section 3.2 and Section 4.2, respectively. In Figure

5.1(a) is shown a schematic representation. The current signal is recorded

at constant intervals of τ0 = 5 ns for a total of 3 × 105 steps, for different

applied peak-to-peak external voltages in the range (22÷26) kV.

5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL 53

ROG

HV

T

OSCILLOSCOPE

HV

(a) Experimental setup. (b) Simplified electrical equivalent circuit.

Figure 5.1: The DBD device is made up of two rod electrodes of square sec-tion coated with pure (> 99.7%) Al2O3 ceramic dielectric. The distance betweenelectrodes is 4 mm. An amplified signal generator and a current transformer (T)provide the high voltage to the electrodes. Current and voltage are acquired witha specifically designed Rogowski coil (ROG) and a high voltage probe (HV).

0 10 20 30 40 50 60 70 80-200

-100

0

100

200

Cur

rent

[m

A]

0 10 20 30 40 50 60 70 80time [µs]

-400

-200

0

200

400

Cur

rent

[m

A]

Figure 5.2: Typical current signal of a DBD device. The upper and lower panelsrefer respectively to a low voltage (23 kV) and high voltage (25 kV) current sig-nal, representing the two typical discharge patterns observed in the device. Thecontinuous sinusoidal line is the displacement current of the system.

5.2 Statistical characterization of current signal

Because the interest is in a detailed analysis of fast current pulses due to

micro-discharges, the displacement current of the system must be deter-

mined and subtracted. Because the ionization of the gas is very low, it is

suitable to assume that the capacitance of the gas does not change during

the discharge process and use a simplified electrical equivalent circuit of the

discharge system (Figure 5.1(b)). Thus, the current measured in the system


can be considered to be made of two components: a sinusoidal displacement

current which does not depend on the presence of the plasma and the re-

sponse current due to the discharge process [9, 11, 5]. The displacement and

discharge currents can be then calculated as

Idisplace(t) = CxdV (t)

dt(5.1)

Idisch(t) = Itot(t) − Idisplace(t), (5.2)

where Cx is a capacitance including both dielectric layers and gas gap, V

is the applied voltage, Itot the total measured current, Idisplace the displace-

ment current and Idisch the discharge current. An example of the separation

of the two components is given in Figure 5.2.

5.2.1 Structure of the discharge current: bumps, bursts and streamers.

Once the displacement current has been subtracted from the signal we are

left with a series of discharge patterns of alternating sign. Since we are inter-

ested in the discharge amplitudes, we have first checked that both positive

and negative discharge values occur with a similar distribution, indicating

that we can treat them on the same foot. Then, we change the sign to

those negative discharge patterns. However, small negative current values

still occur in the series which are due to errors introduced by the sinusoidal

fit and the intrinsic errors of the current probe. To this end, we introduce a

cut-off threshold for the current, below which it is set to zero. The cut-off

value Icut is taken as the minimum value of the current measured within the

full time steps, Icut = |min I0(t)|. Then, the discharge current is taken as

I(t) =

I0(t), if I0(t) ≥ Icut,

0 , if I0(t) < Icut.(5.3)

For the present results, we find Icut ≃ 10 mA.

An example of the resulting signal within a single half-cycle, which we

denote as a discharge bump is shown in Figure 5.3. It can be noted that

a bump is composed of several well separated discharge bunches, which we

call bursts. The bursts are made of a sequence of single streamers (their

structure and temporal behavior will be discussed in detail in Section 5.3.2)


0 2 4 6 8 10time [µs]

0

100

200

300

400

500I B

[m

A]

0 100 200 300 400IB [mA]

10-5

10-3

10-1

PDF

[mA

-1]

λ=50 mA

Figure 5.3: Discharge current signal IB(t) within a half-cycle as a function oftime t [µs], for a high voltage situation (25.3 kV, see also lower panel of Figure5.2). The continuous line is the mean current response

⟨

IB

⟩

, Eq. (5.4). The insetrepresents the probability distribution function (PDF) where the straight line is anexponential fit using Eq. (5.5) with λ = 50 mA.

which are clustered together. This clustering is a result of the presence of

strong short-time correlations in the discharge patterns. We are going to

analyse these correlations below in Sect. 5.3.1. In what follows, we perform

a statistical analysis of bumps.

In our analysis, we consider from Eq. (5.3) values of I(t) only within

an effective time interval tmin < t < tmax, for a fixed applied voltage Vpp.

To stress this fact, the discharge current within a bump is indicated as

IB(t). The lower bound tmin is defined as the lowest time, within a half-

cycle, at which I(t) > 0 for the first time, calculated among all bumps.

The upper bound tmax is defined as the largest time, within a half-cycle,

at which I(t) > 0 for the last time. The total number of bumps, NB, is

typically NB ≃ 100 in the recorded interval, while ∆t = tmax − tmin varies

in the range 10 [µs] < ∆t < 20 [µs], depending on Vpp. We denote as T the

half-cycle period.

The continuous line in Figure 5.3 represents the mean discharge current


response of the DBD device within a half-cycle. It is calculated as

⟨

IB(t)⟩

=1

NB

NB∑

i=1

I(i)B (t), (5.4)

for tmin < t < tmax, and⟨

IB

⟩

= 0 otherwise, where i denotes the bump

index. Note that max⟨

IB(t)⟩

≪ maxIB(t), where maxIB(t) is the

maximum value of IB(t) within a bump. In the case of the bump shown in

Figure 5.3 we find maxIB(t) ≃ 400 mA, while max⟨

IB(t)⟩

≃ 70 mA.

The inset to Figure 5.3 displays the probability distribution function of

IB. It can be approximately fitted by the exponential form (see also Figure

5.6 and related explanation) valid for IB ≥ 0,

P (IB) = P0 δ(IB) +PA

λexp(−IB/λ), (5.5)

where P0 represents the fraction of zero current values inside the bump,

indicating the existence of a characteristic current intensity. Exponentially

decaying functions are typical of random systems displaying uncorrelated

fluctuations. Using the normalization condition,∫ ∞

0 dIB P (IB) = 1, with

the convention that∫ ∞

0 dIB δ(IB) = 1, one has PA = 1 − P0, representing

the fraction of positive current events, denoted also as activity ratio, inside

a bump. We have verified that the PDF is only weakly dependent on the

current cut-off.

5.2.2 Discharge current regimes

The evolution of the mean discharge current is plotted in Figure 5.4 (lower

panel) for different applied voltages. The maximum of⟨

IB(t)⟩

tends to

occur at early times, t ≃(2-3) µs, while only at large voltages the mean

response spans the whole bump width (see e.g. curve (e) in Figure 5.4). The

upper panel in Figure 5.4 displays the corresponding PDFs, which seem to

attain a limiting shape, independent of Vpp, for Vpp > 23.5 kV. This is a

first indication that discharge currents may be organized into two different

discharge regimes. We explore this possibility further in the following.

An important quantity assessing the efficiency of the discharge device is

the total charge,⟨

Qtot

⟩

, transferred during each half-cycle of the system.


0 2 4 6 8 10time [µs]

0

20

40

60

80

100

<I B

> [

mA

]

(a) 22.9 kV(b) 23.0 kV(c) 23.5 kV(d) 23.7 kV(e) 24.7 kV

0 200 400IB [mA]

10-7

10-5

10-3

10-1

P(I B

) [m

A-1

]22.9 kV23.0 kV23.5 kV23.7 kV24.7 kVλ=55 mA

23 24 25

Vpp [kV]0

20

40

60

80

λ [m

A]

ab

cde

Figure 5.4: (Upper panel) PDF’s P (IB) [mA−1] vs IB [mA], for different appliedpotentials Vpp indicated in the plot. The straight line displays an exponentialfunction with decay constant λ = 55 mA (see Eq. (5.5)) and is shown as a guide.(Lower panel) Mean discharge currents

⟨

IB(t)⟩

versus time [µs], for the same valuesof Vpp considered in the upper panel.

The total charge can be calculated from the mean discharge current⟨

IB(t)⟩

as,⟨

Qtot

⟩

=

∫ tmax

tmin

dt⟨

IB(t)⟩

, (5.6)

where tmin and tmax are the temporal bounds for bumps. The total charge⟨

Qtot

⟩

is plotted in Figure 5.5, where one can see the emergence of two

distinct regimes separated by a threshold value Vpp ≃ 23.55 kV. The latter

is consistent with a similar behavior obtained from the shape of the PDF’s


100

101

102

103

<Q

tot>

[nC

]

23 24 25V

pp [kV]

0

0.1

0.2

0.3

0.4

0.5

0.6

Act

ivity

Rat

io

I II

Figure 5.5: Mean total charge⟨

Qtot

⟩

[nC] vs applied voltage Vpp [kV], transferredby the discharge currents within a half-cycle (open circles, left-scale). The dashedand continuous lines are quadratic fits to the numerical data and are shown as aguide to stress the presence of a two-regime discharge pattern in the DBD, sepa-rated by a value Vpp ≃ 23.55 kV. Also shown, the activity ratio PA (open squares,right-scale) plotted vs applied voltage. The dashed (continuous) line is a linear(quadratic) fit plotted as a guide to the eye.

displayed in Figure 5.4.

In support to these findings, we can add that visual inspection of actual

discharge patterns show also two qualitatively different behaviors at low

and high applied voltages. In the low voltage regime, few moving discharges

occur at random along the electrodes. At higher voltages, many discharges

covering essentially the full electrode length occur in a fully random fashion.

The activity ratio of the DBD, that is the fraction of time within a

half-cycle in which a positive current is measured, is also plotted in Figure

5.5. It does not show such a clear change of behavior as⟨

Qtot

⟩

does. Yet,

we have found that to obtain accurate fits the data need to be separated

into two parts, one below Vpp ≃ 23.55 kV, where just a linear dependence

occurs, and one above it, in which a quadratic function is required. In this

sense, also the activity ratio reflects the presence of two different regimes.


We can also mention that an accurate quadratic fit to the data for⟨

Qtot

⟩

can not be obtained for the whole interval of voltages considered. This

difficulty reflects to some extent also the presence of an underlying two-

regime discharge process.

Using the approximate form for the discharge PDF, Eq. (5.5), we com-

ment on the two observed discharge regimes. For low voltage values, Vpp <

23.55 kV, the decay parameter λ increases rapidly with applied voltage (see

upper panel in Figure 5.4), i.e. the larger the value of λ the more likely the

higher current values are. Similarly, the activity parameter PA (Figure 5.5)

increases also, and it does it linearly in regime I. Based on evidence that

the charge transported by a single streamer does not seem to depend on

the applied voltage [6, 5], we suggest that the increase in height and dura-

tion of bursts we observe is due to the increasing number of simultaneous or

close-in-time streamers as a function of the applied voltage. In this plausible

scenario, new streamers can occur spanning the largely available space on

the dielectric without strongly experiencing the repulsive interaction with

residual charges deposited from previous micro-discharges within the same

half-cycle (bump).

For higher voltages, Vpp > 23.55 kV, λ stops growing, indicating that

a limiting shape of the discharge PDF has been reached. Thus, an upper

number of simultaneous streamers seems to occur, as suggested by the shape

of the mean discharge current shown in the lower panel of Figure 5.4, shape

which becomes broader in time but reaching a limiting upper value as the

voltage is increased. In other words, for low values of Vpp bursts are made

up of few streamers and⟨

IB

⟩

remains low, spreading in time. By rising the

applied voltage,⟨

IB

⟩

first increases in height up to a limiting value, then its

temporal duration starts to grow.

The two regimes identified previously can be further characterized by

looking at selected moments of order n of the discharge current IB(t), which

can be calculated according to,

⟨

In

⟩

=1

∆t

∫ tmax

tmin

dt⟨

InB(t)

⟩

, (5.7)

where here 1 ≤ n ≤ 4 and ∆t = tmax − tmin. The symbol⟨ ⟩

denotes an

average over different bumps. In addition to the mean,⟨

I1

⟩

, and standard


23 24 25Vpp [kV]

20

40

60

80

100

<I 1>

[m

A]

23 24 25Vpp [kV]

20

40

60

80

100

σ I [m

A]

23 24 25Vpp [kV]

0

1

2

3

SI

23 24 25Vpp [kV]

2

4

6

8

10

FI

Figure 5.6: First moments of the current signal IB(t) versus applied voltage

Vpp [kV]. Shown are: the mean value⟨

I1

⟩

, standard deviation σI , skewness SI

and flatness FI . The horizontal line represents the values of SI and FI for anexponential PDF, Eq. (5.9). The vertical dashed line indicates the separatrix valueVpp ≃ 23.55 kV.

deviation, σI , of IB(t), we consider, in order to characterize the discharge

current, also the skewness, SI =⟨

I3

⟩

/σ3I and the flatness, FI =

⟨

I4

⟩

/σ4I .

It is worth noticing that the total charge and mean current are related

to each other according to⟨

Qtot

⟩

=⟨

I1

⟩

∆t. The results are shown in

Figure 5.6. As one can see from the figure, the mean and standard deviation

display two different regimes clearly separated by the value Vpp ≃ 23.55 kV,

consistent with our previous results (see Sect. 5.2.1). The first two moments

strongly increase with applied voltage, and tend to stabilize above the value

Vpp ≃ 23.55 kV.

Higher moments, such as skewness and flatness of the distributions can

be compared with the values expected from an exponential PDF,

P (I) =1

λexp(−I/λ), I ≥ 0, (5.8)

yielding the moment of order n,

⟨

In

⟩

= λn

∫ ∞

0dy yn exp(−y) = λnΓ(n + 1), (5.9)


23 24 25Vpp [kV]

40

80

120

160<

τ δ> [

ns]

23 24 25Vpp [kV]

40

80

120

160

σ δ [ns

]

23 24 25Vpp [kV]

0

1

2

3

4

Sδ

23 24 25Vpp [kV]

0

5

10

15

Fδ

Figure 5.7: First moments of the burst lengths τδ versus applied voltage Vpp [kV].

Shown are: the mean⟨

τδ

⟩

, standard deviation σδ, skewness Sδ and flatness Fδ.The horizontal line represents the values of Sδ and Fδ for an exponential PDF,Eq. (5.9). The vertical dashed line indicates the separatrix value Vpp ≃ 23.55 kV.

where Γ(n) is the Gamma function. According to Eq. (5.9), the skewness

and flatness take the values SI = 3/√

2 and FI = 6, respectively. The latter

are displayed in Figure 5.6 by the horizontal lines. The good agreement of

the higher moments confirms that the choice of the exponential distribution

as an approximation was adequate. The slightly deviation of flatness from

the predicted value is due to the poor statistic for high current values.

A similar analysis can be performed for the burst duration, denoted here

as τδ. The result for the corresponding moments are displayed in Figure 5.7.

Again in this case, the change of discharge regime becomes apparent around

Vpp ≃ 23.55 kV, consistent with our previous findings. The mean value⟨

τδ

⟩

first increases with applied voltage, while above 23.55 kV the burst

length stops growing, suggesting that a saturation number of simultaneous

streamers has been reached. A similar behavior is displayed by the stan-

dard deviation σδ, telling us that also fluctuations around mean values are

bound when Vpp > 23.55 kV. The further increase of the first two moments

for higher voltages may be due to the apparent overlap of nearby bursts not

resolved with the present diagnostic resolution. Higher moments of distribu-

tions are finally compared with the values expected for an exponential PDF,


Eq. (5.8), suggesting that the actual PDF deviates a bit from an exponential

shape.

5.3 Statistical analysis of temporal behavior

The peculiar presence of memory effect in DBDs (see Introduction 5.1 and

Section 2.3.2) suggests that temporal correlations may exist also in the ap-

parently random behavior of micro-discharges in the streamer regimes an-

alyzed here and patterns (not visible with the eye) are formed. In this

section it will be studied the existence and propagation of temporal cor-

relations both between discharge processes (bumps) and inside the single

discharge process.

5.3.1 Inter- and intra-bump correlations: surrogate model and Hurst expo-

nents

Inter-bump correlations

In the following we deal with the question of correlations between discharges.

We consider first correlations between bumps, or inter-bump correlations.

To this end, we study the quantity CBiBj(τ), representing the correlations

between bump Bi and Bj, separated by a time lag τ = kT , where k = |i− j|and T is the half-cycle period,

CBiBj(τ) =

⟨(

IBi(t) −

⟨

IBi

⟩)

·(

IBj(t) −

⟨

IBj

⟩)⟩

∆t

σBiσBj

(5.10)

where the symbol⟨⟩

∆tindicates the average over the N∆ = ∆t/τ0 values

of IB(t) present inside each bump. We find that CBiBj(τ) ≃ const (i.e.

independent of τ) for k ≥ 1, indicating that a residual correlation is present

between any pair of bumps. By averaging over the total number of bump

pairs in the signal we obtain the mean residual correlation between bumps

as,⟨

CBB

⟩

=1

NB(NB − 1)

∑

i6=j

CBiBj|k≥1. (5.11)

The mean correlation⟨

CBB

⟩

is shown in Figure 5.10 as a function of

applied voltage Vpp, where⟨

CBB

⟩

≃ (0.2 − 0.3) for Vpp > 23.5 kV. Note

5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR 63

0 2 4 6 8 10time [µs]

0

100

200

300

400

500

I B [

mA

]SurrogateReal<I

B>

2.2 2.4 2.6 2.8 3 3.2

time [µs]0

100

200

300

400

I B [

mA

]

Figure 5.8: Surrogate uncorrelated time series generated with the model comparedwith the original source data.

that there occurs a maximum of⟨

CBB

⟩

around Vpp = 23.55 kV, suggesting

another way of determining the separatrix voltage between regimes I and II.

In order to understand the origin of such correlations, we implement a

surrogate model in which fully uncorrelated discharges (streamers), IS(t),

occur inside a bump. Several methods exist for the generation of an uncor-

related time series with a specific PDF [69, 70]. A simple rejection method

can be used to obtain a stationary time series with a PDF like those rep-

resented in Figure 5.4. To make the model more realistic, we take into

account the intrinsic non-stationarity of the process, that is represented by

the time dependence of the mean discharge response. To do this, we calcu-

late N∆ local PDFs, PL(t), one for each time step inside a bump, obtained

from the total number of bumps in the signal, NB. Then, for each point

inside a bump, we generate an uncorrelated surrogate current signal IS(t)

according to the local PDF, PL(t). In Figure 5.8 the original signal is com-

pared with the surrogate one and it can be observed that the original shape

of the bump is well reproduced. In Figure 5.9 are compared the P (IB) of the

surrogate generated time series and it can be observed that the two PDFs

overlap very well although a little underestimation of the zero component

of the original signal is present. This is probably due to the limited number

of data available for the calculation of local PDFs PL(t). A consequence


0 100 200 300 400 500IB [mA]

10-6

10-4

10-2

100

P(I B

) [m

A-1

]

SurrogateReal

Figure 5.9: Comparison between the PDF of the original source data with thePDF of the surrogate uncorrelated time series generated with the model.

is a slightly overestimation of lower current values, which brings, for exam-

ple, to an increase of about 15% of the calculated⟨

Qtot

⟩

values. However,

this little differences do not affect the correlation analysis performed in the

following. As is apparent from Figure 5.8, the surrogate signal displays a

similar shape as the real current, but it looks more uniformly distributed as

the discharge clustering typical of bursts is not implemented in the model.

Yet, the clustering is not important for determining the residual correlations

between bumps as shown by the correlation values⟨

CSS

⟩

(open diamonds in

the Figure 5.10). We conclude that the residual correlation between bumps

we observe in the discharge patterns is due to the non-stationarity of the

signal. This conclusion is further supported by the calculation of⟨

CBB

⟩

for

the detrended signal, that is the discharge current normalized by its mean

value, i.e. ID(t) = IB(t)/⟨

IB(t)⟩

. A similar definition is applied to the surro-

gate signal. As clearly seen from Figure 5.10, the cross-correlations between

bumps vanish for the detrended signals, indicating that residual correlations

are a result of the non-stationarity of the discharge process. Similar behavior


23 24 25V

pp[kV]

0

0.1

0.2

0.3

0.4

0.5

<C

BB>

Real DataDetrended Real DataSurrogateDetrended Surrogate

2 4 6 8time [µs]

100

200

300

I [m

A]

Figure 5.10: Mean value of the residual cross-correlation⟨

CBB

⟩

between dischargeprocesses as a function of the applied voltage Vpp [kV]. Both real and surrogatesignals are shown in comparison with their respective detrended signals. The insetshows an example of the original signal, a surrogate and the mean response functionfor Vpp = 25 kV.

is displayed by the surrogate current.

Intra-bump correlations

Verified that the memory effect between half-cycle has no influence on the

temporal behaviour of the discharge it is interesting to see if correlations

survive inside the single discharge process. The role of clustering (bursts)

becomes apparent when studying intra-bumps correlations, as we do next.

To study intra-bump correlations, or autocorrelations in the discharge signal

IB, we apply the method known in literature as the fluctuation analysis (FA)

based on Haar wavelets (HW) [71, 72]. We briefly summarize the FAHW

method in the following.

The FA approach is based on random walk concepts. One regards the

fluctuation of the signal,

∆IB(t) = IB(t) −⟨

IB(t)⟩

, (5.12)

as a jump performed by a random walker at time step t (in units of τ0),


where tmin ≤ t ≤ tmax. Then, one calculates the position W (ti) of the RW

at time ti = iτ0, with 1 ≤ i ≤ N∆, as the sum over all previous jumps

∆IB(tj), tj ≤ ti,

W (ti) =

i∑

j=1

(IB(tj) −⟨

IB(tj)⟩

), (5.13)

which is also denoted as the ‘profile’ of the random walk. Once the profile

has been obtained, we study the scaling behavior of W (t) on the time scale

τ . To do this, we divide the total number of points inside the bump, N∆,

into consecutive non-overlapping segments of length ℓ ≥ 1, corresponding to

the time scale τ = ℓτ0. Inside each segment m, 1 ≤ m ≤ N∆/ℓ, we evaluate

the average of W according to,

Bm(ℓ) =1

ℓ

ℓ∑

j=1

W (t(m−1)ℓ+j). (5.14)

The FAHW approach consists in studying the fluctuations of the profile on

the ‘time scale’ ℓ, defined as

F 21 (ℓ) =

⟨[

Bm+1(ℓ) − Bm(ℓ)]2⟩

, (5.15)

where the subindex 1 in F1(ℓ) refers to the first-order Haar wavelet, and the

average is performed over all consecutive boxes m and m + 1. Higher-order

wavelets can be introduced [71], allowing for eliminating possible higher-

order trends in the profile. The dependence of F1(ℓ) on ℓ is expected to

obey a scaling behavior of the form,

F1(ℓ) ∼ ℓH , (5.16)

which defines the Hurst exponent H. The value H = 1/2 indicates un-

correlated fluctuations, or standard random walk behavior. Cases in which

H 6= 1/2 correspond to signals in which autocorrelations are present. If

this occurs for ℓ → ∞, one says that the signal features long-time correla-

tions. Cases in which H > 1/2 denote persistence, and cases with H < 1/2

anti-persistence. More common situations are those in which a power-law

exponent H 6= 1/2 occurs only on finite time scales, typically at short time

scales. These methods have been also employed for the analysis of turbulent


100

101

102

103

104

Time [ns]

100

101

102

<F 1(l

)>B

H=0.50H=0.88

100

101

102

<F 1(l

)>B

H=0.50H=0.85

Figure 5.11: (color online) Fluctuation analysis of intra-bump correlations.

Shown is the quantity⟨

F1(ℓ)⟩

versus time scale τ = ℓτ0 [ns]. The average ofF1 has been performed over all bumps in the signal. The straight lines have slopesH , yielding the Hurst exponents. The time series analysed corresponds to a highvoltage case, Vpp = 25.5 kV. The open circles represent the original signals, whilethe open squares the detrended ones. Upper panel : Real data. The diamonds wereobtained by excluding the zero current values, yielding H ≃ 0.5. The vertical lineindicates the time scale τ = 165 ns. Lower panel : Surrogate model. The detrendedsurrogate signal displays uncorrelated fluctuations H ≃ 0.5.

behaviors in magneto-plasma devices [73, 74].

Results of the FAHW analysis performed for real discharge currents and

for the surrogate model are displayed in Figure 5.11. We observe that within

a time scale of the order of 160 ns, the real data display strong autocorre-

lations with H ≃ 0.85. These correlations reflect the discharge clustering

within bursts. The detrended signal behaves similarly as the original one for

time scales within bursts, suggesting that bursts clustering is a robust type

of correlation, even when non-stationarity of the signal is eliminated. For

time scales larger than mean bursts width, i.e. 160 ns, we observe different


behaviors between the original and the detrended signal: The original signal

seems to display persistent correlations at such long time scales, while for

the detrended one fluctuations become flat. The latter behavior may suggest

the presence of long-time anti-correlations. These two unexpected behaviors

can be shown to be an artefact of zero-current events in the calculation of

F1(ℓ). To show this, we have studied the scaling behavior of F1(ℓ) for the

case of positive current values, by excluding zero-current events from the

analysis. The corresponding points are displayed by the open diamonds in

the upper panel of Figure 5.11. As expected, current fluctuations between

bursts are uncorrelated yielding the standard behavior H = 1/2 (dashed

line). The surrogate signal displays an effective Hurst exponent H ≃ 0.88,

suggesting strong autocorrelations inside a bump. These correlations are

shown to be an artefact of the non-stationarity of the model and vanish for

the detrended signal. This correlation analysis suggests that inside the sin-

gle discharge process (bump) exists a cooperative behaviour of the streamers

occurring sufficiently close in time (i.e. inside a burst), however the eventual

pattern formed by discharge remnants on the dielectric is destroyed by the

subsequent burst which does not retain memory of the previous one.

5.3.2 Temporal correlations between streamers

The presence of strong correlations inside the single burst found with the

Hurst analysis (Section 5.3.1) suggests that a deeper insight into the dis-

charge process is required in order to understand the nature of these cor-

relations. To this end, a specific, short time scale analysis of the burst

structure has been performed. The lower panel of Fig. 5.12 displays the

short-time scale of a single typical burst. One can anticipate the existence

of an internal structure of the burst by the presence of several emerging

peaks, aside from few significant ones, that represent the micro-discharges

or the temporal superposition of more micro-discharges. From now on they

will be all referred to as streamers. The aim is to extract information about

the temporal streamer distribution by performing an accurate fit to the full

burst shape using Gaussian functions as the basis set. Thus, the single burst


Figure 5.12: Upper panel : A typical discharge pattern [mA] within a half-cycle vstime [µs] for Vpp=25 kV. The continuous line represents the mean (absolute value)discharge response of the system, averaged over all half-cycles in the time series.Lower panel : The internal structure of a single burst taken from the upper panel(zoomed around 4 µs). The Gaussian fits are physically identified as streamers.The fit virtually coincides with the discharge pattern. Here, τa represents the timeinterval between two adjacent streamers, and τb the time separation between twoadjacent bursts.

shape IB(t) is written according to,

IB(t) =

NB∑

i=1

ρi1√2πσi

exp

[

−(t − ti)2

2σ2i

]

, (5.17)

where ρi is the streamer charge, σi the standard deviation and ti the tempo-

ral location of the streamer. First it is required that the number of Gaussians

to be used be reduced to a minimum. This is done by searching for their

possible locations using information from the (numerically evaluated) first


101

102

103

104

τ [ns]

10-4

10-3

10-2

10-1

100

σ P(

τ)

intra-B (52 ns)inter-B (293ns)

F=τ3.7,τ-6

F=log-normal

F=τ4.8Gauss

3.7 -6

Figure 5.13: The scaled PDF, σP (τ), of intra-burst (open circles) and inter-burst(open squares) times vs τ [ns]. The lines are different types of fits to the numericaldata. Intra-burst times: power-law fits ∼ τ3.7 for τ < 50 ns and ∼ τ−6 for τ > 50ns (continuous line); ∼ τ4.8×Gaussian (dashed line). Inter-burst times: log-normalfit (continuous line). The mean intra-burst and inter-burst times are indicated inparenthesis. The corresponding standard deviations are: σa = 17 ns and σb = 250ns, respectively. Averages over different applied Vpp ranging from 24.5 kV to 25.5kV have been performed.

derivative of IB(t). The obtained initial locations t(0)i are used to initiate the

search. The fit parameters for all Gaussians inside a burst are then deter-

mined using a recursive least-square method. The latter is implemented by a

random search of the parameter values using a simulated annealing [75] type

of strategy. The final fit yields global absolute error of the order of 10−4 mA

in most cases, and the fit is generally indistinguishable from the experimen-

tal data, as illustrated in the lower panel of Fig. 5.12. The reconstruction

algorithm described above allows to perform a detailed study of the statistic

of time intervals between discharge processes, which are represented by the

quantities τa which is the time interval between two adjacent streamers, and

τb which represents the time separation between two adjacent bursts. The

aim here is to identify the nature of the correlations individuated in Section


5.3.1.

A first insight can be observed in the behavior of PDFs of waiting times.

In Figure 5.13 are represented the separated distributions of waiting times

within bursts τa and inter-burst times τb. Data have been accumulated

for applied Vpp voltages ranging from 24.5 to 25.5 kV after having verified

the statistical properties do not depend significantly on voltage. It can be

observed that the distributions of waiting times inside a single burst and

waiting times between bursts follow different distribution laws. While the

inter-burst times τb show a fast-decaying typically uncorrelated behavior,

which in Figure 5.13 is fitted with a log-normal distribution, the intra-burst

waiting times τa show a non-trivial power law decaying character. Although

the PDF analysis in not enough to recognize a correlated behavior, the

presence of power laws in distributions have already been connected with

anomalous behaviors and presence of correlations (for example see literature

on fluctuation analysis in magnetized plasmas [76, 77, 78, 79, 80, 81, 73, 74]).

This anomalous behavior of τa distribution suggests the presence of an intra-

burst structure. To show this, two correlation functions are constructed to

analyze the time series of the occurrence of streamers (or superposition of

contemporary streamers) and the charge transported by them. The first is

the temporal pair distribution function g(τ) and is defined by:

ρτ =

∫ ∆τ

0dτg(τ) = Ns − 1 (5.18)

where Ns is the total number of streamers pairs within ∆τ , ∆τ is the time

lag limit and ρτ = Ns/∆τ . g(τ) can be calculated as a discrete quantity:

g(τ) =∆τ/dτ

Ns(Ns − 1)

Ns∑

j=1

Ns∑

i=1,i6=j

δdτ ((ti − tj) − τ) (5.19)

where dτ is the discrete time interval and δdτ is a Dirac function. The func-

tion g(τ) is constructed identically to the pair distribution function which is

used to recognize spatial correlations in liquid or solid state matter [82, 83].

g(τ) is its temporal equivalent and expresses the probability to find two

streamers occurring at a distance τ in time. Note that g(τ) tends to unity

for large values of τ .


0 100 200 300 400 500τ [ns]

0.00

1.00

2.00

3.00

g(τ) 0 50 100 150 200

τ [ns]

0.0

0.5

1.0

Cqq

(τ)

Figure 5.14: The streamers temporal pair correlation function, g(τ) (open circles),vs time lag τ [ns], from the data shown in Fig. 5.13. The dashed line is the fity = 1 + 4.6 exp(−τ/

⟨

τa

⟩

), with⟨

τa

⟩

= 52 ns. The inset shows the autocorrelationfunction, Cqq(τ) (open circles), of streamer charge transfer Q vs time lag τ [ns].The horizontal line is a guide.

In Figure 5.14 it is represented g(τ) for the time series of occurrence of

Gaussian functions (5.17) obtained with the reconstruction algorithm de-

scribed above. It is evident a persistent characteristic time interval for the

occurrence of streamers. This suggests the presence of a characteristic ”fre-

quency” of occurrence which disappears outside the single discharge burst.

The presence of this strong correlation between streamers and its vanish-

ing for time intervals longer than typical burst duration, confirms what was

found with the analysis of Section 5.3.1. Furthermore, the presence of a

characteristic time interval can be connected to the discharge development

in which the occurrence of a streamer is somehow produced by a previous

one. A possible interpretation can be given with the following mechanism.

When a streamer occurs in some point on the dielectric surface, in its sur-

roundings exist several other ”seed” micro-discharge remnants that may

not have yet reached the breakdown conditions. At this point the pho-

tons emitted by the excited atoms in the first streamers may ”induce” the


other remnants to attain the breakdown condition. This photo-ionization

induced mechanism is influenced by many parameters such as the time for

the streamer to reach an adequate emission intensity, remnants on the di-

electric surface and others, but finally shows a characteristic time which is

evident in the temporal structure shown in Figure 5.14. The influence of

radiation on micro-discharges has been reported in the past [84, 85]. If one

assumes that this excitation mechanism exists, then the⟨

τa

⟩

of Figure 5.14

is its characteristic time. The nature of this interaction, however, cannot

be determined by the present analysis as it requires a different diagnostic

approach.

Another interesting quantity is the charge associated to every Gaussian

streamer which is proportional to the number of real micro-dischargers oc-

curring at the same time. To evaluate the presence of temporal correlations

in this quantity it is introduced the function:

Cqq(τ) =1

Nτ

∑

j

(qj −⟨

q⟩

) · (qj−τ −⟨

q⟩

)

σ2q

, (5.20)

where the sum over j indicates the sum on the total number of time steps

Nτ and⟨

q⟩

and σ2q are the mean value and variance of the distribution of

charges ρ in the time series. The function (5.20) is normalized to unity

by definition and indicates whether or not correlations between transfered

charge are present at a certain distance τ in the time series of Gaussian

streamers. In the inset of Figure 5.14 is plotted the function Cqq for the

data shown in Figure 5.13. It can be seen that a correlation persists on

shorter time scales than the g(τ). This suggests that the number of micro-

discharges activated by the hypothetical excitation mechanism stated above

has wider fluctuations and de-correlates faster within the time length of

the single current burst. It is possible to interpret the two functions (5.19)

and (5.20) as the description of two aspects of the discharge development.

The former describes the temporal connections in micro-discharges forma-

tion which is independent on their number and thus it is more independent

on geometrical constraints like the electrode dimensions (but still depends

on the gap distance and atmosphere composition and pressure). That is, it

describes a more general property of the discharge process: i.e. the temporal

aspect of a possible reaction mechanism in which the occurrence of a micro-


discharge in a certain position in the gap promotes the formation of other

micro-discharges within a well defined time interval. Obviously, this can be

verified only with a spatiotemporal analysis which is beyond the capabili-

ties of the present diagnostics. The function (5.20), on the contrary, takes

into account the number of micro-discharges occurring in the gap and thus

can be considered more dependent on electrode dimensions which limit the

intensity of current pulses (see Section 5.2.1). With this interpretation g(τ)

should be found independent of the streamer regimes described in Section

5.2.2 while Cqq should depend on them. Unfortunately, the non-interacting

(low-voltages) regime has a poor statistical basis to perform the analysis

described here and a diagnostic improvement is needed. It is interesting to

note a similar behavior of the current signal (called multi-peaks which have

been observed in diffuse DBDs [86, 87, 88]). Even if discharge conditions,

gas compositions and time scales are completely different, similarly, a prop-

agation mechanism (in this case of the ionization front) has been proposed

to explain the effect [89, 87, 90].

The temporal analysis performed above requires to prove that the pe-

culiar structure is due to correlations between streamers. To this end, an

uncorrelated model time series is generated and analyzed with the function

(5.19). To make the comparison possible the surrogate time series are gen-

erated with a rejection method [69, 70] starting from the P (τ) fit function

which is defined as:

P (τ)R = 0.061(τ/52)−3.75

1 + (τ/52)9.7. (5.21)

Also a Gaussian distribution with same mean value and standard deviation

is considered. In Figure 5.15 are compared the generating functions with

the PDFs of the obtained series. The time series of streamer occurrence are

then obtained by simple integration of the waiting time series.

In Figure 5.16 the function g(τ) is calculated for the surrogate time se-

ries and compared with the experimental data. It is evident that in absence

of temporal correlations the oscillation of g(τ) vanishes almost immediately,

revealing that the temporal structure of the experimental data is effectively

due to the presence of temporal correlations between streamers in the dis-

charge process. This is also confirmed by the behavior of the power spectra

5.4 CONCLUDING REMARKS 75

100

101

102

103

104

τ[ns]

10-4

10-3

10-2

10-1

P(τ)

[ns

-1]

Uncorrelated Process<τ>= 51.9 στ = 17.0

Gaussian FunctionUncorrelated process <τ>=53.0 στ=18.3

τa Fit Function

Figure 5.15: The PDF, P (τ), of surrogate waiting time series compared with thegenerating functions. Circles refers to time series generated with equation (5.21).Diamonds refer to time series generated with a Gaussian function with same meanvalue and standard deviation. (Top-right legend) Because the time series refers towaiting times between streamers, the negative values are ignored.

of g(τ) (inset of Figure 5.16) where the presence of a characteristic time for

the real data is well evidenced.

5.4 Concluding remarks

The streamer regime of a DBD in air has been characterized by means

of the statistical analysis of the discharge current. The presence of two

different discharge regimes has been observed in several quantities both re-

garding the statistical properties of the current intensity and its temporal

behavior. These regimes have been found to be dependent on the applied

voltage. It has been shown that below a threshold value of the applied volt-

age, the streamers generated in the discharge process can span the largely

available space on the dielectric without being affected by the repulsive in-

teraction with residual charges deposited from previous micro-discharges.

This brings a rapid growth of the charge transferred by the system within a


0 100 200 300 400 500τ[ns]

0

1

2

3

g(τ)

Exp. <τ>=52.0 ns στ=17.0 ns

Gauss. <τ>=52.0 ns στ=17.0 ns

P(τ)R <τ>=53.0 ns στ=18.3 ns

0 20 40 60 80f [MHz]

10-2

10-1

F[g(

τ)]

19 MHz

Figure 5.16: The streamers temporal pair correlation function, g(τ) (open circles),vs time lag τ [ns], from the data shown in Figure 5.13. The squares and diamondsrepresent g(τ) for the streamer occurrence time series calculated from the waitingtime time series represented in Figure 5.15. (Inset) The power spectrum of the g(τ)functions

single discharge process. For higher voltages, a limited number of simultane-

ous streamers seem to occur, as suggested by the behavior of the discharge

current shape and temporal properties. In this discharge regime, where the

streamers strongly interact, the rate at which energy is transferred by the

system to the plasma discharge gets slower with increasing voltages.

The presence of correlations between discharge processes and within the

single discharge process have been studied. With the help of a surrogate

model it has been shown that the observed residual cross-correlations be-

tween half-cycles are only an effect of the intrinsic non-stationarity of the

signal, indicating that no memory persistence is present in the temporal

structure of the discharge. Also it has been shown that, within the dis-

charge process, strong correlations are present in the current signal within a

short time scale of the order of the mean value of the burst duration. This

suggests that the interaction between streamers can act only when they oc-


cur close in time and the eventual memory left as discharge patterns on

the dielectric is destroyed by the subsequent burst. Decorrelation between

bursts and bumps promotes uniformity of energy pattern deposition over

time.

Using newly defined correlation functions, the temporal structure of

bursts have been revealed to be extremely correlated and the existence of

a characteristic frequency in the occurrence of streamers have been found.

This frequency is possibly related to the propagation of discharge in the gap.


CHAPTER6

Characterization of the DBD device

in nitrogen atmosphere

6.1 Introduction

In this Chapter the newly developed plasma device described in Section 3.1

is characterized in an atmosphere of pure nitrogen. The aim is to find the

device capabilities by exploring the control parameters and give a description

of the plasma discharge device in nitrogen atmosphere, which is often chosen

as carrier gas for the development of plasma processes for applications. The

controlled parameter for such device are: the power injected into the system,

flux of the nitrogen gas through the injection nozzle and pressure. Plasma

discharges in the DBD will be characterized as a function of these three

parameters using voltage and current measurements and optical emission

spectroscopy.

Nitrogen discharges have been the subject of studies already for many

79

80 CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE

decades [91, 92, 42]. It can form stable atmospheric pressure discharges and,

being an electropositive gas, it does not have the tendency to quench electron

activity in the plasma. Moreover, nitrogen is mostly chemically neutral and,

for example, does not alter chemical composition of thin film deposited with

plasma processes (see Chapter 7), even if active species are formed during the

discharge. These species, principally molecules in metastable states, due to

their long lifetimes and elevated potential energy can transfer energy to other

species, for example a reactive compound added to the mixture. Last, but

not least, nitrogen is extremely cheap with respect to rare gases like helium

or argon. For all this reasons nitrogen, as a basis of plasma discharges, is

often chosen to develop plasma processes for applications [93, 94, 95, 96].

6.2 Experimental setup and methods

The experimental setup and the diagnostic utilized are described in Section

3.1 and Section 4.2, respectively. The experiments have been performed as

follows. The discharge chamber has been evacuated with the rotary pump

P1 (Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then a

calibrated flux from the injection system fill the chamber up to the desired

working pressure. After the working pressure is reached the dry pump P2

is used to balance the inlet flux and keep the pressure stable. The current

and voltage signals are acquired with a time step of 5 ns for a total length

of 0.5 ms. The inter-electrode gap has been kept fixed at 2.5 mm.

6.3 Discharge regimes in Nitrogen Atmosphere

Usually, in atmospheric pressure DBDs in nitrogen, the discharge regime is

a filamentary one even if, under specific conditions, a homogeneous diffuse

discharge may be obtained [86, 9, 97]. However, it is generally difficult

to obtain and reliably control such homogeneous discharges at atmospheric

pressure. For example, minor changes in the electrode configuration or small

variations of the amplitude or repetition frequency of the applied voltage

can cause a transition from the relatively unstable diffuse mode to that of

a much more stable filamentary discharge. For many potential industrial

applications, the diffuse behaviour is a severe disadvantage compared to the

6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE 81

(a) Discharge in Nitrogen: flux of 2 ln/min.,injected power 220W.

(b) Discharge in ambient Air: flux of2 ln/min., injected power 200W.

Figure 6.1: Typical current voltage characteristics for discharges in nitrogen at-mosphere and ambient air at high injected power. Lower panel: applied voltageand total current. Upper panel: applied voltage and discharge current according toequation (5.2).

easier implementation of filamentary DBDs. Moreover, ways can be found

for ensuring that the intrinsic instantaneous inhomogeneity of this random

filamentary DBD does not lead to global inhomogeneity.

Important informations can be obtained by the analysis of the current-

voltage characteristics. For all the discharges in the present setup, because

the ionization of the gas is very low, it is possible to consider that the

capacitance of the gas does not change during the discharge process [9, 11].

The discharge current is thus calculated with equations (5.2) following the

procedure described in Section 5.2. In Figure 6.1 are compared the typical

voltage and current waveforms for discharges in nitrogen and ambient Air. It

can be observed that both show the typical current pulses due to streamers

(which are also visible by the eye) but substantial differences are present.

In air, current bursts are typically short and more intense (see Chapter 5

for a complete description of the streamer regime in air) while in nitrogen

they seem to be lower in height and longer in time with a peculiar slowly

decaying current tail. This behavior can be connected to the dimension

and duration of single micro-discharges. As already discussed in Chapter

5, current bursts are the temporal superposition of more micro-discharges.

It has been shown that oxygen admixtures to nitrogen can lead to plasma

channel reduction [98, 4, 38]. Thus, the lower intensity of the current bursts

observed in nitrogen can be possibly explained by the presence of wider


Figure 6.2: Typical emission spectra for discharges in nitrogen atmosphere. Fluxof 2 ln/min., injected power 100W, pressure 500 mbar.

streamer and, thus, less contemporary streamers developing close in time.

Also the different charge transported by the single discharge process must

be considered. Moreover, the longer duration of current bursts in nitrogen,

characterized by the slowly decaying current tail, suggests the presence of

active species with longer lifetime like N2(A3Σ+

u ) metastable molecules [99]

that maintain active the discharge. The presence of electronegative oxygen

gas quenches more rapidly this activity.

Useful information on the plasma phase can be achieved from the anal-

ysis of emission spectra. Nitrogen is a very active species which has a com-

plex reaction scheme involving electronic, vibrational and rotational excited

states along with ionized species. Particularly important is the role of vi-

brational excitations. The creation of excited vibrational state by electron

impact is highly favorable while the relaxation processes (see Section 2.4.1)

are less effective. Thus, vibrational states adsorb a large part of the energy

and act as a sort of reservoir. These energies are typically high enough to

activate chemical reactions with other species [2]. In Figure 6.2 is shown the

typical emission spectra of a discharge in nitrogen. The spectrum is shown

between 300 nm and 500 nm because outside this region the emission lines

are absent or too weak . The spectrum is dominated by the second posi-

tive system (SPS) of N2 (C3Πu→B3Πg) [100, 101]. Vibrational levels are

usually thermalized because vibrational-vibrational transition processes are


very effective. Thus, a vibrational temperature Tv can be calculated from

the SPS structure by determining the populations of the vibrational levels

of N2 [102, 103, 104, 105, 106] and according to the formula:

nN2(C,ν) =∑

ν′

IN2(C,ν)→N2(B,ν′)

ν4N2(C,ν)→N2(B,ν′)

∝ e−EN2(C,ν)

kTv (6.1)

where ν and ν ′ are the vibrational level index, IN2(C,ν)→N2(B,ν′) is the inten-

sity and ν the frequency of the electronic transition between N2(C3Πu) and

N2(B3Πg) levels of nitrogen. It is interesting to recall that also the electron

temperature could be determined by the vibrational population levels [107]

and, generally, it increases in the same way as Tv. An emission line from the

first negative system (FNS) of N+2 (B2Σ+

u →X2Σ+g ∆ν = 0) is also visible at

391.3 nm. This emission line is usually connected to the electron energy and

electron energy distribution function because the ionization threshold, from

a neutral N2 molecule, of the N+2 (B2Σ+

u ) is higher than N2(C3Πu). Thus, to

a first approximation, the ratio between the intensities of two characteristic

lines of FSN and SPS is a monotone function of the electron temperature

[108, 109]. The ratio between 391 nm line of FSN and 357 nm line of SPS will

be used in the following to estimate the variation of electron temperature

as a function of discharge parameters.

6.3.1 Characterization of the discharge as a function of injected power

The DBD discharge device is powered by a simple transistor switching sys-

tem which does not allow to control separately frequency and voltage (see

Section 3.1.3). The voltage applied to the electrode is varied by varying

the difference of the frequency with respect to the resonance frequency of

the system. The generator also provides a measure of the power injected.

The generator can support powers up to 600W but in the present experi-

ments the power levels have been kept under 250W to avoid overheating and

damage to the polycarbonate injection nozzle. As a first characterization, a

current-voltage plot is obtained (Figure 6.3(a)) spanning the available power

range of the system. It can be observed that the behavior is not unique in

the above range and two regions are evident. By comparing the root mean

square (rms) value of displacement and total current as a function of the rms


(a) Current-voltage characteristic of theDBD device.

(b) Mean value of total transferred charge ina half period

˙

Qtot

¸

.

Figure 6.3: Discharge regimes: Voltage-Current characteristic and total trans-ferred charge. Flux of 2 ln/min., pressure 900 mbar.

applied voltage V rms, it is evident that, while Irmsdisplace has a rough linear

dependence on V rms, Irmstot shows a two-stage behavior. This is the same

effect that has been characterized for discharges in Air in Chapter 5. In the

lower voltage regime new streamers can occur spanning the largely available

space on the dielectric without strongly experiencing the repulsive interac-

tion with residual charges deposited from previous micro-discharges within

the same half-cycle. In the high voltage regime the micro-discharges are

forced to a strong repulsive interaction which limits the possibility to add

more streamers to the electrodes. In the same way and similarly to equation

(5.6) the two discharge region can be well recognized by measuring the mean

value of the total charge transferred by the discharge process which can be

calculated as:⟨

Qtot

⟩

=⟨

∫ T/2

0

∣

∣Idisch(t)∣

∣dt⟩

, (6.2)

where the integration is over a half period and the mean⟨⟩

is calculated

over all the half periods recorded in the time series. It is worth to mention

that, even if the electrode system is not symmetric, no asymmetries have

been found in the measure of Qtot.⟨

Qtot

⟩

has been calculated according to

equation (6.2) and the results are shown in Figure 6.3(b). A separatrix rms

voltage V rmss ≃ 5.1kV is found.

In Figure 6.4(a) the vibrational temperature Tv is shown, calculated

according to equation (6.1). It can be noted that the vibrational temperature


(a) Vibrational temperature as a function ofpower. The solid red line is a linear relationplotted as guideline.

(b) 391nm/357nm line intensity ratio as afunction of injected power.

Figure 6.4: Variation of vibrational temperature and 391nm/357nm line intensityratio as a function of the injected power.

is of the order of 2500 K, which means that in such discharges also the

neutral molecules have thermal energy sufficient to influence directly the

chemical kinetics evolution of the gas-phase. A slow decrease of Tv with

increasing injected power is evident which means that at higher power levels

it is allowed a more pronounced quenching of the excited vibrational levels

respect to its ground state. In Figure 6.4(b) the 391nm/357nm line intensity

ratio is plotted as a function of the injected power. It is evident that no

clear trends can be recognized in the plot because all variations seem to be

within the error of the measure. This means that no substantial variations

are present for the electron temperature.

6.3.2 Characterization of the discharge as a function of pressure and gas

fluxes

One of the most interesting capabilities of this DBD device is the possibility

to work in a completely controlled atmosphere being the electrodes inside a

vacuum chamber. This characteristic allows to work also at lower pressure.

In the following the electrical and optical behaviour of the discharge is ob-

served as a function of pressure in the range 50÷900 mbar. It has not been

possible to perform experiments below 50 mbar, because the system tends

to realize a diffuse discharge that is not confined in the electrode gap.

In Figure 6.5 are shown the current and voltage waveforms for exper-


(a) Discharge at 50 mbar, 75W (b) Discharge at 200 mbar, 100W

(c) Discharge at 500 mbar, 100W (d) Discharge at 900 mbar, 100W

Figure 6.5: Current-voltage characteristic for discharges at various pressure.Lower panel: applied voltage and total current. Upper panel: applied voltage anddischarge current according to equation (5.2).

iments at various pressures. At first glance, it is evident that for lower

pressure the discharge seems to occur in a sort of continuous mode while

at higher pressure the identification of current bursts is possible. This can

be explained by the fact the duration of current pulses due to streamers

have been measured to be proportional to inverse square of the pressure [4].

As it is known, when the pressure (number density) is low the ionization

by direct electron impact α is lower and a localized space charge (which is

the origin of the streamer channel) is not created. Moreover, a fundamental

role is played by metastable species which (by Penning ionization, equation

2.11) keep the number of seed electrons high and lower the breakdown volt-

age. The requirement for establishing a stable diffuse discharge (dominated

by Townsend breakdown mechanism) is that the number of seed electrons

is large enough to cause appreciable overlap and merging of the primary

avalanches. With a better analysis of the discharge current for the 50 mbar


(a) Rms voltage and total charge accordingto equation (6.2).

(b) Mean value of total transferred charge ina half period

˙

Qtot

¸

.

Figure 6.6: Rms voltage, total charge and absolute intensity of emission as afunction of pressure. Flux of 2 ln/min., power 100 W (75W for 50 mbar).

discharge (Figure 6.5(a), top panel) it is possible to see that, even if the

discharge seems a single process, several well separated current peaks can

be observed. This kind of behaviour is different from the current character-

istic of a well developed diffuse discharge [110, 108]. This can be possibly

explained by the fact that, even if the lower pressure would allow the devel-

opment of a diffuse discharge, does not exist an effective process to maintain

the number of secondary electrons high enough. Thus, the discharge pro-

cesses last on smaller time scales giving origin to the observed current peaks.

The transition between these two regimes is not clear even if they have been

studied as a function of gas composition [111], electrical and geometrical

parameters [10, 112] and pressure [108]. A simple visual observation sug-

gests that a diffuse discharge exists up to 300 mbar, but this is due to the

superposition of thousands discharge processes. It has been shown that the

use of fast cameras can reveal the presence of the streamers [113, 110, 114].

An interesting observation is that a substantial reduction of the duration of

the discharge process occurs with increasing pressure. Moreover, the same

occurs for the slowly decaying current tail.

In Figure 6.6 are shown the behaviours of the rms voltage and total

charge transferred according to equation (6.2) as a function of pressure and

for a constant injected power of 100 W. As it can be expected [5, 1], the rms

voltage increases with pressure because the breakdown voltage increases.

The interesting aspect is the behaviour of⟨

Qtot

⟩

which decreases with pres-


Figure 6.7: Left scale: Vibrational temperature as a function of pressure. Rightscale: 391nm/357nm line intensity ratio as a function of pressure.

sure. This means that for the same power level the system at lower pressure

transfers more efficiently energy to the plasma. This is also confirmed by the

behaviour of the absolute intensity of the discharge (Figure 6.6(b)) which

is higher for lower pressure. In Figure 6.7 are shown the behavior of the

optical emission spectra by measuring vibrational temperature and mean

electron energy variations. The grow of the mean electron energy with the

decrease of pressure has been observed elsewhere [108] and it is also evidence

that the discharge regime moves toward a diffuse one [114, 99]. The similar

behaviour of the vibrational temperature is consistent with the general re-

lation between these two quantities [107]. What is more surprising is that

Tv shows a minimum at 300 mbar and increases for increasing pressures

while the 391nm/357nm ratio seems to reach a plateau with only a slightly

increase with pressure. The behaviour of the latter quantity has already

been observed elsewhere [108] for air and has been connected to the transi-

tion to filamentary discharge. The observed increase of Tv above 300 mbar

has no simple explanation. Assuming that electron temperature does not

change too much (as indicated by the 391nm/357nm line intensity ratio),

some change in the kinetic equilibrium favours the excitation of vibrational

states, moreover, a role of the quenching processes of the vibrational state

should be considered. However, a deeper study of the phenomena also with


Figure 6.8: Left scale: Vibrational temperature as a function of nitrogen flux.Right scale: 391nm/337nm line intensity ratio as a function of nitrogen flux.

other diagnostics should be performed in order to give a complete explana-

tion.

Finally, it has been observed also the behaviour of the discharge when the

inlet fluxes are changed. In Figure 6.8 are shown the behavior of the optical

emission spectra by measuring vibrational temperature and mean electron

energy variations. It is evident that the adjoint of gas convection does

not influence the discharge in a measurable way. Also electrical discharge

behavior remains unchanged.


The capabilities of the developed DBD device have been verified by explo-

ration of parameter space. A nitrogen atmosphere has been chosen because

of its capability to transfer energy to other species and generate reactive

environments without influencing too much the chemistry of the processes

and being often the best basis for the study and development of plasma

processes for applications. It has been observed that the system shows the

presence of two discharge regimes as a function of the applied voltage as

already observed for the system described in Chapter 5. For higher volt-

ages, because of the limitation in the number of simultaneous streamers, the


rate at which energy is transferred by the system to the plasma discharge

gets slower with increasing voltages. It has been observed also that a pos-

sible change in the quenching mechanism of the vibrational state generates

a slightly decrease in the vibrational temperature with increasing injected

power. An interesting behaviour of electrical and optical measurements has

been observed when the pressure is varied even if a complete transition to a

diffuse discharge regime cannot be reached.

CHAPTER7

Deposition process of organosilicon

thin films

7.1 Introduction

In this chapter a deposition process of thin organosilicon films at atmospheric

pressure is investigated as a method to obtain and control hydrophobicity

of materials surface.

Recently, plasma deposition at atmospheric pressure has become a promis-

ing alternative to low pressure plasma enhanced chemical vapour deposition

(PECVD) [25, 24, 115, 116, 117, 23, 118]. The main advantages are the

possibility to avoid the expensive vacuum systems, to decrease the time of

treatment, and to simplify the technological transfer where the processes of

production are making in continuous mode.

The use of organosilicon compounds as precursors for deposition pro-

cesses of thin films of silicon compound has been studied for several pur-

91

92 DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS

poses like vapour and gas barrier creation [119, 120, 121], wear and friction

reduction [122, 123], anti-corrosion protection [124, 125], biocompatibility

[126, 127], hydrophobicity of surfaces [128].

Modification of hydrophobicity properties of surfaces with plasma treat-

ment can be obtained with fluorination processes, coating processes with

fluorocarbon of hydrocarbon films. However, these processes may become

unstable and show aging by oxidation [129] or other more complex aging

processes depending on substrate (see for example Chapter 9). Low surface

energies, which mean also hydrophobicity, can be attained with high reten-

tion in the coating of methyl groups (CH3) which have a non-polar character

and tend to repel highly polar water molecules. Starting from an organosil-

icon precursor like hexamethyl-disiloxane (HMDSO, see Figure 7.1) it it

possible to obtain a highly organic deposit taking advantage of the elevated

intrinsic stability due to their partly cross-linked Si-O chains (backbone)

which results in an enhanced long-term durability (See Chapter 9).

Here are presented results regarding the deposition process of thin or-

ganosilicon films generated at atmospheric pressure in nitrogen with small

admixtures of HMDSO vapours. The plasma source is the DBD described

in Section 3.1 which works in a roller configuration and is able to simulate

continuos treatments of material surfaces and operates in controlled atmo-

sphere.

7.2 Materials and methodology

The liquid HMDSO is introduced as a vapour in small quantities using the

evaporator system described in Section 3.1.2. The carrier gas which dilute

the vapor is nitrogen and concentrations of HMDSO are varied up to 1.2%.

According to data from [130], at a pressure of 1 bar and at room temperature

(25 C), the maximum concentration of HMDSO before condensation occurs

is 5.5%.

The vacuum chamber is initially evacuated with the rotary pump P1

(Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated

fluxes from the injection system fill the chamber up to a working pres-

sure of 900 mbar. Although the chamber is provided with gaskets both

for under- or over-pressure, a slightly lower pressure ensure a better insu-

7.2 MATERIALS AND METHODOLOGY 93

(a) Chemical representation. (b) Graphic representation.

Figure 7.1: Representation of the hexamethyl-disiloxane (HMDSO). Formula:C6H18OSi2. Molecular weight: 168.38 amu. Boiling point: 373± 2 K.

lation from possible contaminations. Experiments have been performed to

verify that the hundred millibar difference does not affects the deposition

process. After the working pressure is reached the dry pump P2 is used to

balance the inlet fluxes and keep the pressure stable. The carrier nitrogen

gas is maintained at 2 ln/min, while the liquid flux is regulated as needed

to obtain the desired concentration of HMDSO. In the present study the

experiments are implemented at constant power of 170 W injected in the

system. The specimens undergoing the treatment are exposed to the plasma

at the tangent speed of 1 m/min for 15 times. For this kind of geometric

configuration is convenient to use the so called corona dose which is defined

as D = Powerelectrode width×tangent speed

which has the dimensions of energy

on surface. Thus the experiments are performed with D = 728.6 kJ/m2.

The rather high energy dose has been chosen to reduce errors on experimen-

tal measurements (weighting, FTIR, thickness). To estimate the residence

time it can be considered the diameter of the rod electrodes (12 mm) as the

discharge length1 as a rough estimate of its dimension. Thus, for these ex-

periments, the total residence time is around 21 seconds which is extremely

low with respect to low pressure plasma processes to obtain the same results.

The plasma discharge is characterized by current-voltage measurements

and the acquisition of optical emission spectra (see Sections 3.1, 4.2 and

1This is actually an over estimate because the typical length of the discharge is less.Thus, the actual value cannot be measured independently on power and other parametersin such DBD devices


4.1).

The deposits are characterized with several methods. KBr salt pellets

of 8 mm diameter are prepared by compression starting from the powder

(Fluka) and exposed to the plasma. The transmission infrared spectra of

the pellets are measured with the FTIR spectrometer (see Section 4.3.1) be-

fore and after the treatment and their difference is considered. Small sheets

(10x4 cm2) of low density polyethylene (PE) 0.2 mm thick, is washed in ace-

tone and attached to the grounded rotating electrode. It is used to evaluate

the morphology of the deposits using the atomic force microscope (AFM)

(see Section 4.3.2). Mass deposition rates were evaluated by weighting larger

sheets with an analytical balance before and after the exposition to the

plasma with an appropriate mask (15x15 cm2). Small pieces (∼10x5 mm2)

cut from (100) silicon wafer where also exposed to the treatment. Where

needed, the specimens were attached to the grounded electrode with tape.

7.3 Characterization of the deposition process

As already mentioned in Section 7.1 HMDSO possesses some features that

make it extremely effective for the realization of hydrophobic coatings. The

main polymerization process is through the creation of Si-O bonds with

the creation of a highly cross-linked inorganic backbone. This kind of re-

actions are chemically favorable once in the plasma are generated radicals

by fragmentation of the original compound. For this reason organosilicon

compounds can produce better deposition rate than simply organic precur-

sors. From Figure 7.1 it is evident that the monomer is initially highly

organic. The retention of initial methyl groups is controlled by discharge

conditions. Higher levels of power injected into the system usually promote

a higher grade of fragmentation which induce a loss in organic character

of the deposit. When a completely inorganic coating is needed (silica-like),

for example for the realization of barrier effects, usually oxygen is added

to the gas mixture in order to promote the oxidation of the organic com-

pounds. Under specific conditions a nearly complete elimination of organic

character can be achieved [122, 119, 120]. In the present experiments, on

the opposite, it is searched the highest retention of methyl group in order to

obtain the best achievable hydrophobicity. In the following, the best degree

7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS 95

Figure 7.2: Current-voltage characteristic of a discharge in nitrogen with 0.45%of HMDSO vapour.

of organic retention is searched by analyzing both the plasma and deposit

characteristics.

7.3.1 Plasma characterization

Important informations can be obtained by the analysis of the current volt-

age characteristic. The typical current and voltage waveform of the discharge

process is plotted in Figure (7.2). From the presence of fast current pulses

it is possible to recognize the typical behaviour of the streamer regime al-

ready described in the preceding Chapters. In comparison with nitrogen

atmosphere (Figure 6.1(a)), it is evident the streamer have a shorter du-

ration. Moreover, a visual observation reveals thinner plasma channels.

Possibly this effect can be similar to that observed in air (see Chapter 6)

where the presence of oxygen leads to plasma channel reduction [98, 4, 38].

The uniformity of the treatment is then guaranteed by the mean effect due

to the large difference between the time scales of the treatment (seconds),

of the discharge process (microseconds) and of the typical duration of the

streamers (nanoseconds). It is well proved that uniformity is achieved down

to the microscopic scale as it is evident from the roughness analysis (see

Figure 7.5). Thus, the presence of a streamer regime dose not undermine

uniformity issues usually fundamental in plasma applications.

Useful information on the plasma phase can be achieved from the analysis

of emission spectra. In Figure (7.3) is depicted the typical emission spectra of


the discharge. The spectrum is shown between 300 nm and 500 nm because

outside this region the emission lines are absent or too weak . The spectrum

is dominated by the second positive system (SPS) of N2 (C3Πu→B3Πg)

[100, 101].

From the SPS structure it is possible to determine the populations of

the vibrational levels of N2 molecules and calculate the vibrational temper-

ature Tv [102, 103, 104, 105, 106] which is an interesting plasma parameter

because processes such as vibrational relaxation and excitation can strongly

influence plasma chemistry [2]. This is because the vibrational levels are

mostly excited by direct electron impact and vibrational-translational re-

laxation processes are not efficient in converting vibrational energy into ki-

netic energy (heating of the gas). Thus, energy remains ”trapped” in the

vibrational levels which give to molecules a reservoir of energy to activate

several chemical reactions (see also Section 2.4.1). The determined values

are 2000± 100 K for all concentrations of HMDSO explored. These tem-

peratures are lower than temperature achieved in pure nitrogen atmosphere

(∼2700±40 K). This finding suggests that, being Tv a monotone function

of the electron temperature [131, 107], also the latter is lower in these dis-

charges.

In Figure (7.3) an emission line from the first negative system (FNS) of

N+2 (B2Σ+

u →X2Σ+g ∆ν = 0) is also visible at 391.3 nm. This emission line

is usually connected to the electron energy [108, 109], but here is not easily

observed because of the presence of the CN bands.

The most interesting feature of the spectra is the presence of the CN

violet system at 388 nm and 422 nm (B2Σ+u →X2Σ+

g ) which is a consequence

of the chemistry of N2+HMDSO vapour mixtures in the plasma state. A

complete evaluation of the concentration of active chemical species, ions or

radicals from OES diagnostics requires a detailed modeling of the excitation

and quenching processes for each light emitting energy level observed in the

spectra. However relative information could be inferred by normalizing the

emission intensities of different emitting molecules. The CN line intensity

at 387.1 nm has been normalized to that of one of the brightest band of the

SPS of nitrogen at 357 nm. In this way the dependency of absolute intensity

from high energy electrons density in the discharge region is factored out.

Moreover, assuming that electron temperature does not change too much


Figure 7.3: Emission spectrum from the plasma of a mixture of nitrogen and0.15% HMDSO vapour.Inset: CN(387.1)/N2(357.7) line intensity ratio as a functionof HMDSO concentration.

and, since dissociation level is usually very low, that the absolute density

of nitrogen is constant, the the intensity ratio should be proportional to

the relative concentration of CN during the discharge[132]). We can see

in the inset of Figure 7.3 that this quantity shows a stepwise behaviour

evidencing a threshold value of concentration around 0.3% after which the

CN line intensity abruptly decreases. The presence of cyano radical CN is

connected to the fragmentation of initial HMDSO monomer in plasma. The

formation of CN requires carbon atoms which can come only by the monomer

and are created by consecutive fragmentation of the organic components of

HMDSO. The vanishing of the CN emission band possibly means others

reaction channels are preferred at higher concentrations of HMDSO and the

monomer retains more of the initial organic character. From the analysis of

the emission spectra it is possible to suggest the presence of two different

discharge regimes in which the chemistry of the plasma changes in some way.

This behaviour will be observed in other quantities further in the following.

7.3.2 Thin film characterization

Although the plasma phase analysis can give useful information on the de-

velopment of the deposition process, it cannot give too much hints on the


plasma-surface interactions and surface reactions complexity. Usually at at-

mospheric pressure the role of energetic ions less important and the typical

reaction scheme is more like the one described in Section 2.4.2, where the

radicals, compounds and ions created in the plasma phase are adsorbed to

the surface where chemical reactions take place.

The change in the morphology of the depositions have been measured

on PE substrates with an atomic force microscope as a function of HMDSO

concentration. In Figure (7.4) are compared images of the deposits at differ-

ent concentration of HMDSO in comparison with the untreated PE surface

(a). At a concentration of 0.05% (b) we can see that the deposition pro-

cess is generate a ”dust” like film with evident nanoscale structures. At a

concentration of 0.15% (c) and 0.3% (d) it is possible to see that still some

structure is present which is embedded in a structureless deposit. At higher

concentrations (e and f) the formation of nanoscale structures is no longer

visible and the deposition is extremely smooth. The change in the mor-

phology is evident also in the roughness of the surface which is evaluated

from the root mean square (RMS) of the heights of the surfaces defined as

RMS =

√

⟨

h2⟩

−⟨

h⟩2

, which is the standard deviation of probability distri-

bution function of heights.

The change in the morphology can be explained with the presence of

two mechanism of deposition. When the fragmentation of monomer is high,

a plasma-phase polymerization with subsequent adsorption and reaction on

the surface is predominant with the result of a higher grade of cross-linking

and the creation of nanoscale structures. These structures are often observed

in other experiments both at atmospheric pressure [24] and low pressure (in

this case usually with smaller characteristic dimensions [133]). On the other

end, when the concentration of monomer is high a different deposition pro-

cess takes place. As mentioned before, the residence time of specimens in the

discharge region is of 21 seconds while the treatment times lasts for around

10 minutes. Thus, the specimens are exposed, for most of the time, to the

neutral atmosphere containing the monomer vapour. Possibly, if the concen-

tration is high enough, the monomer is absorbed on the substrate surface

and reacts with the radicals created before by the plasma or is activated

when exposed to the plasma in a sort of mechanism of adsorption/reaction

polymerization. This creates a smoother, softer, structureless deposit which


Figure 7.4: AFM images of thin films deposited on PE substrate at differentconcentrations of HMDSO. a=untreated, b=0.05%, c=0.15%, d=0.3%, e=0.45%,f=1.2%.


0 0.2 0.4 0.6 0.8 1 1.2HMDSO [%]

0

10

20

30

40

50

RM

S [n

m]

Untreated PE

Figure 7.5: Roughness of treated PE substrates estimate with RMS as a functionof HMDSO concentration.

embeds the morphological structure of the plasma phase polymerization.

This second kind of deposit is usually really soft and flexible.

Large differences can be found also in the chemical composition of the

deposited thin film which have been characterized measuring the infrared

absorption spectra. Figure (7.6) shows the spectra of the deposits at dif-

ferent HMDSO concentrations. The spectra show the typical bands already

recognized in the literature [134, 135, 124] and indicated in the Figure (7.6).

According to the literature the stronger absorption band in the range 1000-

1150 cm−1 can be assigned to the Si-O-Si asymmetric stretching mode.

Other typical absorption band can be assigned: the CH3 symmetric bending

in Si-CH3 at 1260 cm−1, the CHx symmetric and asymmetric stretching at

2900-2960 cm−1, the CH3 rocking in Si-(CH3)2 at 800 cm−1 and the Si-CH3

rocking vibration in Si-(CH3)3 at 840 cm−1. Bands at 800 cm−1, 840 cm−1,

1260 cm−1 and 2900-2960 cm−1 indicate retention of methyl group in the

plasma deposit, which brings the condition for the creation of hydrophobic

surfaces with HMDSO plasma. It is interesting to observe the two peaks at

800 cm−1 and 840 cm−1. It can be seen a rapid growth of the Si-(CH3)3 peak

against the Si-(CH3)2 as the concentration of HMDSO grows. Si-(CH3)3

groups are termination sites in the network structure of the deposited films.

The abundance of such groups at higher monomer concentration indicates

the films are composed of shorter chains having a less cross-linked structure.

Observing the spectra it is evident an increase of all the bands relative to


Figure 7.6: FTIR spectra o the thin films at different HMDSO concentrations.a=0.05%, b=0.15%, c=0.3%, d=0.45%, e=1.2%. The spectra are normalized onthe Si-O-Si peak intensity.

organic compounds which are responsible for the hydrophobic character of

the resulting surface.

In order to evaluate in a more quantitative way the increase of the organic

character of the deposits, the ratios of the areas of the peaks of interest in

the spectrum have been analyzed. To this end, a deconvolution process of

the spectra has been performed using Lorentzian function as basis [136]. In

Figure 7.7 is represented the resulting fit for a 0.05% HMDSO deposit. Not

all the peaks are considered: the areas of interest are marked with pattern

fill and named out. A quantitative analysis from an infrared spectra can only

be performed by calculating ratios of the areas internal to a single measure

[136].

To this end two ratios have been evaluated as a function of HMDSO


Figure 7.7: peaks areas recognition with Lorentzian functions from the FTIRspectra of 0.05% HMDSO concentration. The areas of interest are marked withpattern fills and named.

concentration. The first is the ratio between the areas at 1260 cm−1 rel-

ative to Si-CH3 symmetric bending and the area of the band at 1000-

1150 cm−1 relative to Si-O-Si asymmetric stretching mode [137]. This ra-

tio gives some information on the organic character of the deposits and

in particular on the methyl group retention. The second ratio considered

is the CH3 rocking in Si-(CH3)2 at 800 cm−1 and the Si-CH3 rocking in

Si-(CH3)3 at 840 cm−1. The presence of three methyl groups attached to

silicon means a termination of the polymeric chain or cross-linked structure.

Two methyl groups attached to silicon are related to compounds of the type

Me − (Me2SiO)n − SiMe3 (or ramifications of them). Compounds of this

kind have been observed with gas chromatography analysis of the exhaust

of process gases [23, 138]. In Figure (7.8) (left scale) it is plotted the value

of the first ratio as a function of the HMDSO concentration. We can observe

that the organic part of the thin film grows rapidly up to a saturation value

of ∼0.24. This means the retention of methyl groups cannot grow beyond a

certain value which possibly represents the limit of stability of the deposit.

This is confirmed by the saturation behaviour of the second ratio which

represents somehow the degree of polymerization and, thus, the stability of

the deposit. This chemical analysis confirms the observation of a softer and


Figure 7.8: Ratio of the area at 1260 cm−1 relative to Si-CH3 and the area at1000-1150 cm−1 relative to Si-O-Si asymmetric stretching mode(left scale). CH3

rocking in Si-(CH3)2 and the Si-CH3 rocking in Si-(CH3)3 at 840 cm−1 (right scale).

Figure 7.9: Mass deposition rate as a function of the HMDSO concentration.

smoother deposit at higher concentrations.

The same behaviour is visible in the mass deposition rate plotted in

Figure 7.9. The reaching of a saturation value of ∼0.8 µg/mm2 suggests

the growth of the polymer is not simply limited by the quantity of monomer

present in the gas phase but it depends on the complex chemistry both in the

plasma phase and on the surface. Indeed, this saturation is reached with


Figure 7.10: Advancing and receding contact angle with water as a function ofHMDSO concentration. The error bars indicate the statistical errors on 5 indepen-dent measures. Horizontal dashed lines are, in the same colour the advancing andreceding contact angle of untreated PE surface.

the increase in monomer concentration while the power is kept constant,

and could be interpreted as that the deposition process reaches a power

deficient regime in which not enough fragmentation is achieved in plasma

phase [139, 140]. However the deposition process here seems much more

complicated by the presence of two phases (plasma and neutral atmosphere)

alternate during the treatment because of the electrode configuration.

The retention of methyl groups can be evaluated by a macroscopic mea-

surement of water contact angles. Using the technique described in Section

4.3.3 it is possible to gather information on the microscopic chemical het-

erogeneity of the deposit and evaluate the achieve degree of hydrophobicity

of the surfaces. In Figure 7.10 are showed the advancing and receding water

contact angle measured on treated PE surfaces. Advancing angle, which is

connected to the presence of non-polar groups on the surface have a satura-

tion behaviour similar to other quantities observed before, indicating that

a limiting retention degree of the initial methyl groups has been reached.

The observed receding angles lay always below the value of untreated PE.

This behaviour can be connected, on one side to the presence of inorganic

SiOx compounds in the deposit which are characterized by a high wetta-


bility, on the other side can be connected to the presence on the surface of

polar compounds affine to water [141] which are due to a plasma activation

of the surface and the subsequent reaction with atmospheric oxygen and

water [16] (which brings to the creation of polar compounds like hydroxyl,

carboxyl or carbonyl groups). This surface effect suggests that together with

the deposition process an activation process due to nitrogen is also present.


The deposition process of organosilicon thin films with plasma of nitrogen

with small admixtures of HMDSO vapour has been characterized. Analyzing

the behaviour of several quantities as a function of the HMDSO concentra-

tion we have found the deposition mode changes with increasing concentra-

tion. For lower values the deposition strongly depends on the concentration

itself, while after some threshold value it remains most independent. This

behaviour has been observed in several quantities relative both to the plasma

phase and to the resulting deposits. The retention of organic compounds in

the deposits have been studied at a microscopic and macroscopic level. It

has been found that the retention of initial monomer methyl groups saturate

with concentration and so does the hydrophobic character of the resulting

surface. Stability issues of the resulting deposit will be discussed in Chapter

9 dealing with the application of this process to cellulosic materials (paper).


CHAPTER8

Fluorination of polymer surfaces

8.1 Introduction

In this chapter a grafting process of fluorine atoms on polyethylene (PE)

surface is investigated at atmospheric pressure as a method to obtain and

control hydrophobicity and oil-repellency of materials surface.

Plasma induced modifications of materials surface with fluorination (graft-

ing of fluorine atoms) processes and deposition of fluorocarbon thin films

have been studied for several applications because of the unique properties

that can be attained. For example, fluorination of the polymer surfaces pro-

duces hydrophobic surfaces, at the same time, preserving the bulk properties

of the materials [142, 143, 144, 145]. Fluorocarbon thin films have been stud-

ied for the creation of barrier layer against solvent and fuel permeation [146],

adhesion of carbon nano-tubes in composite material [147], bio-compatible

materials production [148]. Most of the processes are performed at low

pressure even if few reports of atmospheric pressure processes are present

107

108 FLUORINATION OF POLYMER SURFACES

[149, 26, 7]. However, at atmospheric pressure, the process is rather far from

being well understood both when dealing with the plasma-phase properties

and the induced surface modifications. Moreover, experiments on fluorina-

tion at atmospheric pressure have never been performed in continuous-mode

plasma reactors such as the one used here (see Section 3.1).

Plasma processes based on Sulfur hexafluoride (SF6) are an effective

source of fluorine radicals and fluorination of materials surface can be suc-

cessfully realized [26, 142, 144, 143]. Fluorination process is a grafting pro-

cess (see Section 2.4.2) which involves the substitution of an hydrogen atom,

bound to carbon, with a fluorine. The basic reaction scheme is the following:

−CHhν,e−

−−−−→Ion,F•

−C• F•−−→ −CF, (8.1)

where the surface can be activated by ions, electrons, photons and other ac-

tive species generated in the plasma (see Section 2.4.2) and, then, a fluorine

radical adsorbed to the surface reacts with a carbon radical forming a sta-

ble chemical bond. SF6 is a highly electronegative gas which posses a high

dielectric strength (i.e. the minimum electric field strength for breakdown)

and is usually used as electrical insulator in high-voltage circuit breaker

[150]. For this reason, in the present experiment SF6 is only added in small

quantities to argon to obtain stable discharges. Thus, materials surface is

exposed to both the effect of argon carrier and SF6. It must be recalled

that when polymer surfaces are brought into contact with chemically inert

plasmas (like argon), activation (hydrogen abstraction) and etching of low

molecular weight molecules are the main processes occurring at the polymer-

plasma interface, leading to the formation of radical species on the treated

surface. In this case a chemical modification of the treated surface is a con-

sequence to the exposure of the treated specimens to the atmosphere [16],

mainly because reacts with oxygen and water forming peroxide and hydro-

peroxide radicals which then form hydroxyl, carboxyl or carbonyl groups

[151]. When polymer surfaces are brought into contact with reactive plas-

mas (like Ar/SF6 mixtures), grafting of chemical species, simultaneous to

surface activation or etching occur leading to surface functionalization. This

means that the balance of these two competitive processes will determine

the final surface properties. To this end, the key parameter that controls

8.2 EXPERIMENTAL, DIAGNOSTICS AND METHODS 109

this equilibrium is the concentration of SF6 in the gas mixture. In the fol-

lowing, both the plasma-phase characteristic and surface properties will be

studied as a function of SF6 concentration.

8.2 Experimental, diagnostics and methods

The plasma source used for the experiments is the DBD described in Section

3.1 which works in a roller configuration and is able to simulate continuous

treatments of material surfaces and operates in a controlled atmosphere.

Small sheets (10x10 cm2) of low density polyethylene (PE) 0.2 mm thick,

are washed in acetone and attached to the grounded rotating electrode with

tape. The inter-electrode gap distance is kept fixed at 2.5 mm. Gas mixture

of Ar and SF6 are created with the mixing unit described in Section 3.1.2.

The vacuum chamber is initially evacuated with the rotary pump P1 (Figure

3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated fluxes

from the injection system fill the chamber up to a working pressure of 900

mbar. After the working pressure is reached the dry pump P2 is used to

balance the inlet fluxes and keeps the pressure stable. The total gas flux

is maintained at 10 ln/min. The experiments are implemented at constant

power of 155 W injected in the system. The specimens undergoing the

treatment are exposed to the plasma at the tangent speed of 1 m/min. for

8 times. The experiments are performed with a corona dose (see Section

7.2) D = 354.3 kJ/m2. An estimate of the residence time (see Section 7.2)

is around 11.2 seconds which is lower with respect to low pressure plasma

processes to obtain the same results. Treatment times have been chosen

rather higher than those needed to obtain a good fluorination effect in order

to allow a more meaningful comparison of the experiments at different SF6

concentrations.

The plasma discharge is characterized by current-voltage measurements

and by the acquisition of optical emission spectra (see Sections 3.1, 4.1 and

4.2). The materials surface properties are characterized at a microscopic

level by measuring the morphology of the treated PE using the atomic force

microscope (AFM) (see Section 4.3.2), and at a macroscopic level by mea-

suring the dynamical contact angle with water (bi-distilled, de-ionized) and

α-bromonaphthalene (from Aldrich) and by calculating the resulting surface


energy with its polar and dispersive components (see Section 4.3.3).

8.3 Characterization of the fluorine grafting process

The substitution of hydrogen atoms with fluorine can give unique properties

to material surfaces. The CF groups show a strong repulsion of hydrogen-

bond forming molecules like water and other compounds containing hy-

droxyl, carboxyl or carbonyl groups. This characteristic gives a high hy-

drophobic property to the surface along with the resistance to organic polar

compounds (some oils and solvents). Moreover, the polarity of the CF group

also shows repulsion with non-polar compounds like organic molecules and

some other oils. This characteristics are the basis of the high chemical stabil-

ity, and unique properties of fluorocarbon-based polymers. The attainment

of these properties on polymer surfaces is the aim of the present research. It

is worth to mention that the fluorination process of equation (8.1) involves

only few atomic layers of the substrate. This means that a little quantity of

fluorine on the surface is needed to obtain the desired properties. For this

reason the fluorination process is somehow preferred to deposition processes

of fluorocarbon thin films which require longer times and greater quantities

of reactive fluorocarbon gases.

8.3.1 Plasma-phase characterization

Important information on plasma discharges can be obtained by the anal-

ysis of current-voltage (I-V) characteristic. Figure 8.1 shows the current

and voltage waveforms for discharges in Ar/SF6 mixtures at various concen-

trations in comparison with pure Ar discharge1. In Figure 8.1(a) the pure

argon discharge I-V characteristic is shown. By observing the discharge

current Idisch, calculated according to equation (5.2), it seems that a dif-

fuse discharge mode is achieved [152]. However, a visual observation reveals

the presence of really bright diffuse zones in proximity of the dielectrics con-

nected by wide plasma channels. Anyway, the discharge is rather uniform on

the electrode surfaces as also confirmed by the uniformity of surface proper-

1The pure argon discharge has a slightly lower power level because the low breakdownvoltage may cause the discharge to happen outside the discharge gap. This is a limitationof the present experimental setup.

8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS 111

(a) Discharge in pure argon, 88W (b) Discharge in argon and 0.5% SF6, 155W

(c) Discharge in argon and 3% SF6, 155W (d) Discharge in argon and 13.4% SF6, 155W

Figure 8.1: Current-voltage characteristic for discharges at various concentrationsof SF6 in comparison with a pure argon discharge. Lower panel: applied voltageand total current. Upper panel: applied voltage and discharge current according toequation (5.2).

ties of the treated PE. Even small admixtures of SF6 (Figure 8.1(b)) changes

drastically the discharge which goes into a fully developed streamer regime

composed by numerous and thin plasma channels. This is in agreement

with the increase of the derivative of reduced effective ionization coefficient

with respect to reduced field in Ar/ SF6 mixtures [153] which brings to the

reduction of streamer radius (see Section 2.3.2). In comparison with air and

nitrogen atmosphere (Figure 6.1), it is evident the streamer have a shorter

duration.

It is interesting to observe the behaviour of root mean square (rms) quan-

tities (Figure 8.2). V rms and Irmstot increase in the same way because the Irms

tot

is dominated by the displacement component. However, by observing the

two components of current and considering their rms values Irmsdisplacement and

Irmsdischarge, it is evident a decrease of the latter in comparison to the former.


(a) RMS Voltage (left scale) and total cur-rent (right scale) as a function of SF6 con-centration.

(b) RMS displacement current (left scale)and discharge current (right scale) as a func-tion of SF6 concentration.

Figure 8.2: Behaviour of rms quantities as a function of SF6 concentration atconstant injected power.

This means that for the same power level, at higher concentrations of SF6 a

lesser part of the current is effectively due to the plasma discharge. Possibly,

the SF6 tends to shorten the lifetime of the plasma channel (streamer) reduc-

ing the current flow and so the total charge transferred during the discharge

process which has the same behaviour as Irmsdischarge varying from 230 nC for

the pure argon discharge to 56 nC for the Ar/SF6 13.4% discharge.

Concomitant with the decrease of the discharge current is the decrease

of the brightness of the discharge as it can be observed by the absolute in-

tensity recorded with the spectrometer. In Figure 8.3 the emission spectra

of a discharge in argon with 1% of SF6 between 600 nm and 860 nm because

outside this region the emission lines are absent or too weak. The spec-

trum is dominated by emission lines of the argon and no contributions of

the brightest emission lines of the fluorine are visible. In the inset of Figure

8.3 are compared the spectra for various SF6 concentrations normalized on

the 772.5 nm line of the argon. A drastic change in the distribution of the

intensities between all the argon emission lines is evident (as it is also in

other parts of the spectra which are not shown). It is known the relative

intensities of the lines of the argon emission can be connected through a

collisional-radiative model to the electron temperature in the plasma dis-

charge [154, 155, 156] even if a complete validation of these models for the

streamer regime has to be found. It is possible to suggest that the quench-

ing of electrons in the presence of SF6 leads to a decrease of the electron


Figure 8.3: Emission spectrum of a discharge in argon with 1% of SF6. Emissionis dominated by argon lines and no emission line of fluorine are present. Inset:Comparison between spectra for various SF6 concentrations normalized on the 772.4nm emission line of the Argon.

temperature with increase of SF6 concentration. A lower electron tempera-

ture means less effectiveness of the plasma creating radicals both in plasma

phase and on the surface. This is possibly a key effect for the less effective-

ness of the process at higher SF6 concentrations, as it will be shown in the

following.

8.3.2 Material surface characterization

A complete characterization of the materials surface treated with a fluori-

nation process is more difficult than the characterization of the deposition

process described in Chapter 7. This is because, even if other modifications

can be introduced by concurrent etching process due to argon activity, a

grafting process involves only few atomic layers on the surface and the pres-

ence of fluorine cannot be easily detected. Here the characterization has

been performed at a microscopic level with the AFM to measure eventual

modification of the morphology of PE surfaces, while the fluorination has


a

c d

b

Figure 8.4: AFM images of PE surfaces exposed to plasma treatment for variousconcentrations of SF6 in argon and compared with the untreated PE. (a): untreatedPE. (b): pure argon plasma. (c):argon with 1% SF6. (d): argon with 6.7% SF6

been evaluated by indirect measurements of the macroscopic properties of

the surface with the techniques described in Section 4.3.3.

In Figure 8.4 are compared images of the deposits at different concen-

tration of SF6 in comparison with the untreated PE surface (a). It can be

observed in Figure 8.4 that the exposure to pure argon treatments changes

the surface morphology to some extent. However, the measurements of

roughness does not show significant changes. The root mean square (RMS)

of the heights of the surfaces has been measured for different image sizes

down to 1x1 µm2 but appreciable changes has not been found. Possibly, the

morphological intrinsic roughness of the PE covers other eventual effects.

Moreover, the ionic activity is low and the effect of the argon treatment


[

mNm

]

Surface Energy Polar Dispersive

Water 72.1 52.2 19.9

α-bromonaphthalene 44.0 0.0 44.0

Table 8.1: Surface tension with polar and dispersive components according toRef. [157].

is at most an activation of the surface that reacts with oxygen and water

when exposed to the ambient atmosphere. Another interesting observation

is the presence on the surfaces treated with Ar/SF6 plasma, of spot-like

structures. These structures tend to reduce in dimension and vanish with

increasing SF6 concentration.

The presence of fluorine bounded to carbon on the surface has been eval-

uated indirectly by measuring the macroscopic effect that the presence of

fluorocarbons or polar groups generates. These effect are the repellency or

affinity of the surface to water and non polar liquid compounds. By mea-

suring the advancing and receding contact angles (see Section 4.3.3) with

liquids of different polar and dispersive character it is possible to evaluate

the presence of fluorine or other polar compounds bound to the surface. In

fact, Figure 8.4 and the measure of RMS guarantees that roughness does

not change significantly between the different experiments and the change

in advancing and receding angles is then only connected to the chemical

heterogeneity of the surface. The two chosen liquids are water for its high

polar components and α-bromonaphthalene which is completely lacking of

polar groups and posses only a dispersive component. The use of pure liq-

uids also avoids complications with adsorption kinetics which can influence

the dynamic of wetting and de-wetting phenomena [59]. In Table 8.1 are

reassumed the surface tensions of the used liquids according to Ref. [157].

The measurement of advancing and receding contact angles can give in-

formations on the presence of affinity or repellency to a liquid deposited

on a surface [59, 158, 159, 160]. It is possible to interpret the wetting or

de-wetting process as an irreversible process in which some surface energy

is dissipated as heat to the environment [161], i.e. some potential energy

between atoms or molecules is dissipated as vibration (heat) as bonds are

formed or snapped in the process. With this assumption a phenomeno-


Figure 8.5: Advancing and receding contact angle of PE surfaces with water as afunction of SF6 concentration in argon. Dotted lines represent in the same coloursthe advancing and receding angles of untreated PE.

logical interpretation of the advancing and receding contact angles can be

stressed. When a contact line is de-pinned from the surface (receding), sta-

ble bonds between liquid and solid must be broken, so receding angle (θr)

can be connected to the affinity of components between the solid and liquid

phases (for example, a low receding angle with water means a high presence

of polar groups on the surface forming hydrogen-bonds with water). On the

contrary, when the contact line tries to advance, it remains pinned to the

surface (advancing) because the liquid must overcome the energy barriers

due to repulsion (for example, a high advancing angle θa with water means

an high presence of non-polar groups on the surface which repel the highly

polar water molecules). A similar kinetic interpretation is given for water on

hydro-repellent surfaces in [141], here the concept is extended to a general

interaction scheme between liquid and solid phase.

Figure 8.5 shows the measured advancing and receding water contact

angle of treated and untreated PE surfaces as a function of SF6 concentra-

tion. It can be observed that a pure argon treatment lowers both θa and θr

indicating that an activation process brings, after exposure to oxygen and

water of ambient air, to the grafting of polar groups on the originally non-

polar surface. For higher concentrations the effect of fluorination is evident


Figure 8.6: Advancing and receding contact angle of PE surfaces with α-bromo-naphthalene as a function of SF6 concentration in argon. Dotted lines represent inthe same colours the advancing and receding angles of untreated PE.

the increase of θa which, as explained above, is sensible to repulsive interac-

tions (in this case between water and fluorocarbon groups). It is interesting

to observe that for SF6 concentrations below 1% θr remains under the un-

treated PE value indicating that probably the activation process is more

effective than the fluorination, thus, the surface still undergoes grafting of

polar groups when exposed to ambient air. Another interesting aspect is the

decrease of θr for high SF6 concentrations. Possibly, this can be explained

with the reduced activity of the discharge due to the SF6 (see Section 8.3.1).

In Figure 8.6 are shown the measured advancing and receding α-bromo-

naphthalene contact angle of treated and untreated PE surfaces as a function

of SF6 concentration. As for water contact angles, a pure argon treatment

slightly lowers the repelling properties of the surface. The presence of fluoro-

carbon groups on the surface can be seen even at low concentrations of SF6

as both the advancing and receding contact angles are abruptly increased.

This is because both untreated PE and α-bromonaphthalene have only a

non-polar character and this affinity cause the liquid to wet very well the

surface while the presence of fluorocarbon groups introduce immediately a

strong repulsive effect. A far more glaring dependence on SF6 concentration


Figure 8.7: Calculated surface energy of PE surfaces as a function of SF6 concen-tration in argon. Total, polar and dispersive components are plotted. Dotted linesrepresent in the same colours the surface energy and its components of untreatedPE.

of the treatment effectiveness is evident in comparison with water contact

angles (Figure 8.5). In particular, the receding θr angle, which is bound to

the presence of affine (non-polar) groups on the surface, most return to the

value of untreated PE for high concentration. This confirms that an effec-

tive fluorination of surface is not reached for concentrations of SF6 too high.

The analysis of advancing and receding contact angles for the chosen liquids

shows clearly that there exists an optimal concentration of SF6 reactive gas

for the maximum effectiveness of fluorination process with respect to the

competitive activation process.

From the contact angle measurements with two different liquids it is

possible to determine the surface energy components of the treated PE sub-

strates (see Section 4.3.3). The determination of surface energy is based on

the measure of the equilibrium contact angle of the liquid with the surface.

However, an equilibrium contact angle is even hard to define and a generally

accepted definition still lacks [162, 163, 164]. It must be stressed that the

system can be prepared with an apparent stable contact angle θs with the

restriction θr < θs < θa. Usually the advancing contact angle is chosen for

the determination of the surface energy [162] as it is done here. In Figure


8.7 are shown the calculated surface energies of PE surfaces as a function of

SF6 concentration in comparison with the untreated PE. Total, polar and

dispersive components are plotted. It can be seen that the surface energy

remains higher than untreated PE for concentrations below 0.5% SF6 most

because a high polar character is created by the activation process. At con-

centrations of 1% SF6 the polar component most vanishes indicating a high

hydro-repellency has been reached. After experiencing a minimum value

around 3% SF6 the surface energy increases indicating that the reduction of

the dispersive component is less effective at high concentrations.


The fluorination process of polymer surfaces with mixtures of Ar/SF6 at

atmospheric pressure has been studied. The presence of SF6 strongly modi-

fies the discharge properties converting the diffuse discharge regime of pure

argon to a streamer regime. The increase of SF6 concentration also reduces

the effective current flowing through the plasma possibly quenching the elec-

trons in th discharge. By analyzing the surface morphology it is found that

modifications are introduced by the treatment on the surface on the hun-

dred nanometer scale but these alterations do not affect substantially the

roughness of the substrates. The effectiveness of the fluorination process

has been evaluated through the analysis of the macroscopic surface proper-

ties with dynamical and static measurements of contact angles with water

and a non-polar liquid. It is found the effective existence of two competi-

tive processes: an activation process which brings to the grafting of polar

groups from the atmosphere and a fluorination process. The former process

is more important at low concentrations of SF6 and tends to disappear for

higher concentrations. The fluorination process is more effective in a con-

centration interval also reducing it effectiveness at higher concentrations.

The combination of these two effects determine the presence of an optimal

SF6 concentration where the surface energy is lower. This process is then

interesting for the modification of the properties of organic materials like

polymers, fabrics, paper, leather and others in order to obtain resistance to

water and oils.


CHAPTER9

Plasma Application for modification

of paper surfaces

9.1 Introduction

In this chapter some applications of the process studied in the previous

chapters are discussed. Some results obtained with treatments of cellulosic

surfaces (paper) are showed.

9.1.1 Cellulose and paper

Cellulose is a natural polymer of vegetal origin which is found in wood

and plants (for example cotton). Cellulose is a polysaccharide with formula

(C6H10O5)n (Figure 9.1 (a)) and it is usually found in plants as microfibrils

2-20 nm diameter and 100-40000 nm long (Figure 9.1 (c)). Cellulose is a

linear polymer stabilized by intra- and inter-molecular hydrogen bonding

121

122 PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES

(Figure 9.1 (b)) which minimizes its flexibility. Cellulose tends to form

crystal structures allowing the more hydrophobic ribbon faces to stack. Each

residue is oriented 180 to the next with the chain synthesized two residues

at a time. Although individual strand of cellulose are intrinsically no less

hydrophilic, or no more hydrophobic, this tendency to form crystals utilizing

extensive intra- and inter-molecular hydrogen bonding makes it completely

insoluble in normal aqueous solutions.

Figure 9.1: Cellulose: (C6H10O5)n monomer structure (a), polymer structure andhydrogen bond linkage (b), SEM image [165] of cellulose fibres (c).

Paper is the most important utilization of cellulose which is typically ob-

tained from wood after removing lignin. The applications of paper products

are numerous and do not include only the production of printing paper, but

also applications in packaging, filtering, biomedical, construction and more.

Paper of pure cellulose is rarely used and, besides the fibres, it may contain

fillers such as chalk or china clay, which improve the characteristics of the

paper for printing or writing. Also coatings may be applied to the paper web

later in the manufacturing process in order to attain, for example, water or

oil and grease resistance.

Plasma based technologies are an interesting alternative for cellulose and

paper modification to the standard chemical treatments. The main advan-

tages of cold plasma technologies are several. Without heating which would

result in damage of the soft cellulosic materials, the typical energies of active

species are comparable with the values of most common bond energies of or-

ganic molecules. Consequently gas-phase and surface-phase plasma-induced

9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY 123

reaction mechanisms can conveniently be developed. The plasma-generated

surface modification reactions involve only a thin layer (tenth of nanome-

ters) of the substrates leaving the bulk properties mostly unchanged. Due to

the ubiquitous thin-layer nature of the modifications, very small amounts of

starting materials are required for the surface modification processes. This

is also extremely important for the environmental issues of the modern re-

strictive regulations which limit the use of some reagents and require the

treatment of waste-products. These characteristics allow to give the desired

properties to surface layers of paper, depending on the nature of plasma

gases and plasma parameters.

In the following some applicative results are presented focusing on the

achievement of the desired properties rather than discussing the processes

in detail. In Section 9.2 the deposition of thin organosilicon films on paper

surfaces is used to achieve hydrophobicity. In Section 9.3 a fluorination

process is used to obtain oil-repellent paper surfaces.

9.2 Deposition of organic silicon compounds for hydrophobic-

ity

In most part of the applications of paper a certain grade of hydrophobicity

is required. The hydrophilic character of cellulose is then a problem for

applications like liquid recipients, printing and packaging. As a matter of

fact, in environments with 50% relative humidity, cellulose adsorbs about

5% of its own weight of water [166]. Due to its fibre network structure,

paper is a porous material, and can be covered by polymer films in order

to make it impermeable to water. In some applications however, it is desir-

able to combine permeability to air and water repellency. Currently, water

repellency is obtained using solvents and organic reagents which can cause

environmental problems. Plasma-based technologies have all the character-

istics to solve many of these problems and are, thus, an extremely interesting

alternative to conventional methods. The application of plasma to modify

cellulosic materials have been studied starting from different active precur-

sors. Plasma polymerization of hydrocarbon monomers [167], fluorocarbon

compounds [168, 165, 169] and organic silicon compounds [166, 170]. How-

ever, all these experiments have been conducted in low pressure plasma


reactors and vacuum technology is unadaptable to paper processing that is

usually continuous. The use of atmospheric pressure plasmas is indeed a

new challenge in the field of paper surface treatment. The aim is to explore

the possibility to transfer processes already known in a low pressure envi-

ronment the high pressure plasma regime and to the continuous processing

mode which is typical for web materials treatment. Comparison with low

pressure treatment will be shown also in the following.

9.2.1 Experimental setup and diagnostics

The experimental setup is described in Section 3.1 and a discussion on the

deposition process can be found in Chapter 7. Here the discussion will

be mostly on the applicative results on paper surfaces. The process used

to modify the properties of paper surfaces is a plasma deposition of thin

organosilicon films using nitrogen gas with small admixtures of hexamethyl-

disiloxane (HMDSO) with the aim of achieve a high retention of the initial

organic component of HMDSO and obtain hydro-repellent surfaces. Meth-

ods are also similar to those described in Section 7.2 even if the treatment

times are greatly reduced. The specimens utilized cover a good range of

kind of papers. Have been chosen: a collated low weight paper for food

packaging (Paper Type A), a medium weight printing paper (Paper Type

B), a low weight filter paper (Filter Paper) and a high weight packaging

paper (Packaging paper A). Hydrophobicity is tested directly on the treated

surfaces using different methods. The water static contact angle is measured

with the optical goniometer (Dataphysics OCA20) described in Section 4.3.3.

This instrument is also used to determine water adsorption rate of a water

droplet (3 µl) by measuring its dynamical behaviour on the surface. From

the geometrical properties it is possible to determine the volume lessening

of the drop over time. It has been verified that the adsorption rate is con-

stant over the measurement time and the interface base diameter remains

constant without changing too much for different kinds of paper surfaces

analyzed. Also the evaporation effect of water is negligible and does not

influence the measurement. Thus, this method can give information on the

adsorption degree of different paper surfaces. Cobb60 measurement methods

is anyway used because it is a more standardized value for paper industries.

It consists of exposing the paper surface to a 1 cm thick layer of water for


60 seconds and measuring by weighting, before and after the exposure, the

mass of water that has been adsorbed by the paper surface. It is expressed

in g/m2.

9.2.2 Hydrophobicity of treated paper surface

As already shown in Chapter 7 the concentration of hexamethyldisiloxane

is a key parameter in determining the retention of the organic character

of HMDSO. The non-polar character of the methyl groups (CH3) present

in the original monomer is fundamental for the repellency of water which

possesses, instead, a high polar character. In Figure 9.2(a) the behaviour

(a) Static contact angle. (b) Water drop adsorption rate.

Figure 9.2: Effect of variation of HMDSO concentration for various type of papersurfaces and comparison with a low pressure deposition process. The 0 concentra-tion is the value for untreated surfaces. When the untreated value is not indicatedthe adsorption is mostly instantaneous and measurements cannot be performed.

of water contact angle and water adsorption is showed as a function of the

HMDSO concentration. It is evident that even at very low concentrations

the contact angle grows to very high values and it is much independent

on the concentration. This is possibly due to the roughness of the paper

surface which greatly increases the water contact angle1 to a sort of satu-

ration value and, thus, make it dependent on the surface roughness more

than on its chemical heterogeneity. On the contrary, the behaviour of water

adsorption rate (showed in Figure 9.2(b)) is more sensible to the variation

1It is known that the roughness of a surface will increase the apparent contact angle ifthe equilibrium contact angle θe > 90, while it is decreased if θe < 90 [59]


in HMDSO concentrations. It is interesting to observe that the hydropho-

bic properties of surfaces are dependent on the kind of paper only below

a certain saturation value of the HMDSO concentration. In Figure 9.3 the

(a) Static contact angle (b) Water drop adsorption

Figure 9.3: Effect of tangent speed variation for various type of paper surfacesand comparison with a low pressure deposition process [137].

behaviour of water contact angle and water adsorption is showed as a func-

tion of the tangent speed of the web treated which is inversely proportional

to the residence time of the specimens in the plasma discharge area. The

results show that the measure of the contact angle is mostly independent

on speed while the adsorption measure can detect the different behaviour

for different speeds. Also for this parameter exists a saturation value which

cancels the differences between different papers. It is interesting to com-

pare the behaviour of the papers versus HMDSO concentration and speed.

For example, Paper Type B properties strongly depend on HMDSO con-

centration while is most independent on speed (residence time) variations.

The Packaging paper A shows, instead, an opposite behavior being more

dependent on speed and less dependent on concentration. This suggests

that exists a parameter region in which the behavior of the different sub-

strates is dissimilar. Developing processes in this region would require the

optimization of parameter for each specimen. However, exist a region of

the parameters in which the behaviour of the different substrates is almost

the same. The possibility to find a parameter combination which makes

the treatment independent on the paper specimen type is confirmed also by

the adsorption measurements with Cobb60 methods showed in Figure 9.4,


Figure 9.4: Water adsorption of different kind of paper surfaces measured withCobb60 method. A comparison with a low pressure deposition process [137].

where the specimens have been treated ”beyond” the saturation discussed

before. it is evident that conditions exist where the water-repellency effect is

really similar and does not depend too much on the kind of paper surfaces.

Another interesting result showed in Figure 9.4 is the comparison with a

treatment with HMDSO plasma in a low pressure device [137]. It is possible

to see that almost the same adsorption value are achieved even if the typical

treatment times are greatly reduced at atmospheric pressure.

In the applications of plasma treatment to industrial processes, of great

importance is the stability of the properties of treated surfaces. To this

end, have been performed aging experiments on the treated paper surfaces.

Specimens have been exposed after a plasma treatment to an atmosphere

at 80 C and 65% relative humidity for 7 days. The results are showed in

Table 9.1. It is evident that the thin film deposited is extremely stable and

has a good adhesion to paper surfaces. Multiple specimens have been tested

also to verify uniformity of plasma treatments in the continuous mode.

In conclusion, results for a deposition process of organosilicon thin films

on paper surfaces have been shown. There exist regions of the treatment

parameters where the water-repellency effect strongly depend on the paper


Specimen After treatment After aging

Paper Type A 13.6±1 11.5±1

Packaging paper A 4.5±1 5.1±1

Table 9.1: Cobb60 measurements [g/m2] of paper foils after plasma treatmentand after the aging process. The errors are statistical standard deviations on fivedifferent specimens.

type but it is possible to find a region where this dependency vanishes.

Also, the best results achieved with low pressure treatment have been well

reproduced with atmospheric pressure treatments. It has also been shown

that the resulting water-repellency is extremely stable and is not affected by

aging. These results show that atmospheric pressure plasmas are extremely

interesting for the development of new industrial applications.

9.3 Fluorination process for oil repellency

In many applications of paper water repellency is not sufficient. Specifically,

in food packaging applications, also a great resistance to oil and grease is

required. Usually these properties are achieved using expensive fluorine

based coatings and standard wet chemistry processes. Costs in terms of

reagents and dangerous waste by-products disposal are really high and even

more restrictive directives of the governments make the use of these tradi-

tional methods everyday more difficult. The search for different methods

to achieve the same property becomes then a necessity. Plasma treatments

have potentially the lowest environmental impact due to the little quanti-

ties of reagents needed and the nanometer scale character of the processes.

Obtain oil-repellency is not a simple task. While water, which possesses a

high surface tension due to strong polar component, can be easily repelled

by surfaces with non polar character, oils have usually a low surface tension

and cannot be easily repelled. Moreover, oils can be both polar or non-

polar and a choice at a microscopic level for the surface modification is not

enough. It is known that fluorine containing compound are able to give oil

and grease repellency to surfaces, together with a water repellency due to the

repulsion of fluorocarbon to form hydrogen bonds with water. Experiments

for the modification of paper surfaces with compounds containing fluorine

9.3 FLUORINATION PROCESS FOR OIL REPELLENCY 129

have been performed using CF4 [171, 168, 172], perfluoro-methylcyclohexane

[165] or fluorotrimethylsilane [169]. However, as for water repellency (see

Section 9.2), all these experiments have been performed with low pressure

plasma devices which are not suited for applications in paper industry.

9.3.1 Experimental setup and diagnostics

The experimental setup is the same described in preceding Section 9.2. The

methods are similar to those described in Chapter 8 and the treatments have

been optimized for different kind of papers. The fluorination is obtained in a

mixture of argon and sulfur hexafluoride (SF6). Here are shown the results

regarding only a single kind of paper. and compared with a different kind

of low pressure plasma process process [172]. Oil repellency is evaluated

measuring the surface energy of paper surfaces. This is achieved with the

method described in Section subsec:advrec. A typical method to estimate

the oil repellency is the so called Kit Test [173]. It consist of exposing the

surface to a drop of a mixture of oils an solvents for determined amount of

time. There exist two kinds of Kit Test: non-polar Kit Test which is made

up of mixtures of castor oil, toluene and heptane which are all non-polar

compounds. Polar Kit Test is made up of mixture of water and isopropyl

alcohol which are polar compounds. For each mixture is assigned a number

and increasing numbers indicate lower surface tensions. The test is passed

if the mixture is not adsorbed by the analyzed surface.

9.3.2 Oil repellency of paper surfaces

Fluorination process is a grafting process (see Section 2.4.2) which is typical

for plasma discharges in mixtures containing fluorinated gases and involves

the substitution of an hydrogen atom, bound to carbon, with a fluorine atom

following the reaction scheme:

−CHhν,e−

−−−−→Ion,F•

−C• F•−−→ −CF, (9.1)

where a fluorine radical adsorbed to the surface reacts with a carbon radical

activated by the plasma on the surfaces. X-ray photoelectron spectroscopy

(XPS) can be used to determine the degree of fluorination achieved in the

process. The F/C ratio increases from 0 to 0.45 while the O/C ratio de-


(a) Untreated paper surface

(b) Fluorinated paper surface

(c) Comparison of the area relative to carbon carbonbonds for the two energy spectra above

(d) Deconvolution of area for treated surface.

Figure 9.5: XPS analysis of treated and untreated paper surfaces. The graftingof fluorine atoms to carbon is evidenced.

creases from 0.53 to 0.46 indicating that fluorine not only replace hydrogen

atoms but also oxygen containing groups which are possibly removed from


the surface. In Figure 9.6 is shown the Kit Test values for a paper surface

Figure 9.6: Kit Test value as a function of time for an atmospheric pressurefluorination plasma process based on SF6 containing gas mixture and a low pressureplasma process with CF4 gas.

as a function of time. Two processes are compared: an atmospheric pres-

sure fluorination plasma process based on SF6 containing gas mixture and

a low pressure plasma process with CF4 gas. The main difference in the

two processes is that the CF4 gas can activate both the fluorination mecha-

nism of equation (9.1) and a deposition process of fluorocarbon compounds.

By adjusting the plasma parameters the process will create a thin film of

teflon-like polymer deposited on the paper surface. It is evident that, if the

fluorine is replaced directly on the cellulose fibres, a fast aging effect cancels

completely the oleo-phobic properties of the surface. However, if a depo-

sition process is added the aging effect is not present. Possibly the aging

effect is due to the presence of extensive intra- and intermolecular hydrogen

bonding which form the cellulose structure and fibres. When fluorine atoms

replace the hydrogen atoms the possibility to realize hydrogen bonds no

longer exist and the cellulose structure near the surface becomes extremely

unstable. This new mobility allows the fibres to move more freely to find

the configuration which minimize surface energy. Possibly, thus, because of

the repulsion of fluorocarbon to form hydrogen bonds with water, the fluo-

rine containing fibres rotate to the inside of the surface leaving conventional


cellulose to be exposed to the atmospheric humidity. As shown in Figure 9.6

Figure 9.7: Kit Test value as a function of time for an atmospheric pressurefluorination plasma process based on SF6 containing gas mixture and the sameprocess used on papers previously treated with a deposition process of organosiliconfilms.

for the low pressure CF4 process, the presence of a deposition mechanism

can stabilize the treatment. Thus, a solution to the problem can be found

in the utilization of a deposition process which is able to create a stable,

well adherent thin film on paper surfaces. To this end, the fluorination pro-

cess has been used on paper surfaces previously treated with the deposition

process described in preceding Section 9.2, which has shown (see Table 9.1)

to be extremely stable to aging effects. The result are shown in Figure 9.7.

The deposition of organosilicon films has two advantages: it is extremely

stable, blocking the cellulose fibres from turning and has a high number of

CH groups exposed to the atmosphere (because of the methyl groups re-

tention) being a good basis for the grafting process of fluorine atoms. It is

evident that the double treatment is stable and removes the aging effect.

In conclusion, it has been shown that an atmospheric pressure plasma

can produce oil-repellent paper surfaces using a fluorination process based

on SF6 containing gas mixtures. A specific problem with the molecular

structure of cellulosic materials generate a fast aging effect which removes

completely the attained properties. It has been shown that by combining a


deposition process of thin organosilicon films with the fluorination process

can remove the aging effect giving stable properties over time. These results

show that atmospheric pressure plasmas are extremely interesting for the

development of new industrial applications for the substitution of traditional

coating processes.


CHAPTER10

Conclusions

The great interest in cold atmospheric pressure plasmas, in dielectric barrier

discharges and their potential for the development of plasma processes is one

of the motivations of this thesis. DBDs are rapidly growing as one of the

the best choice for atmospheric plasma applications, but the field is still new

and unexplored in many aspects both regarding the discharge processes and

their applications, for example, in material surface modifications. Another

motivation is the lack of a clear understanding of the discharge regimes that

can develop in DBDs. The streamer regime, which was the first observed

(and utilized) back in 1857 Siemens’s DBD ozonizer, still presents some un-

clear phenomena particularly when dealing with the interactions between

the micro-discharges. In this thesis (Chapter 5) the streamer regime of a

DBD in air has been characterized by means of the statistical analysis of

the discharge current. The typical time scales of micro-discharges (tens of

nanoseconds in this experimental setup) made compulsory the development

of suitable diagnostics based on home-made current probes (Rogowski coils)

135

136 CONCLUSIONS

able to catch the fast current pulses due to streamers (Section 4.2.1). It

has been found that the interaction between micro-discharges determines

the presence of two different discharge regimes, depending on the applied

voltage, which has been observed in several quantities both regarding the

statistical properties of the current intensity and its temporal behavior. One

of the great issues of DBDs is the so called memory effect which is due to

the presence of the dielectric layers and tends to promote the formation of

a streamer in the same spot of the preceding half-cycle. This effect could

bring to pattern formation which could affect performance of DBD treat-

ment in application where spatial uniformity is required. To understand if

this ”memory” is present in the apparently random streamer regime, the

presence of correlations between discharge processes and within the single

discharge process has been studied. With the help of a surrogate model it has

been shown that the observed residual cross-correlations between half-cycles

are only an effect of the intrinsic non-stationarity of the signal, indicating

that no memory persistence is present in the temporal structure of the dis-

charge. However, by analyzing the current signal inside the half-cycle, it

is found that on time scales of the order of hundreds of nanoseconds (i.e.,

within a single current burst, in which the streamers develop sufficiently

close in time), strong correlations exist which also reveal a peculiar ordered

temporal structure of the discharge current signal. This temporal structure

has been studied using newly defined correlation functions, which reveal

the existence of a characteristic frequency in the occurrence of streamers.

This frequency suggests the existence of an excitation mechanism between

the streamers which connects their development in the gap. These findings

reveal very interesting aspect of the cooperative behaviour of the streamer

regime and suggest the possibility to carry on the research maybe using also

fast optical diagnostics in order to obtain a clearer picture of the spatiotem-

poral behaviour of streamers.

Atmospheric pressure plasma processing is a leading thematic in develop-

ment of plasma applications. In the last decades great efforts have been done

by many research groups in this field, however still lacks a clear knowledge

of the plasma discharge properties and of their interaction with surfaces. In

this work a new DBD device has been developed (Chapter 3) which is able to

operate continuous treatment of web materials in a wide range of pressures

137

and compositions of the gas mixture. The plasma discharge has been ini-

tially characterized in nitrogen (Chapter 6) which is often chosen as a basis

of discharges for the development of plasma processes for applications. The

capabilities of the new DBD device have been explored varying the control

parameters and finding that it can work in a wide range of conditions.

Another motivation of this work is the study and development of previ-

ously known low pressure processes at atmospheric pressure. The deposition

process of organosilicon thin films with plasma of nitrogen with small ad-

mixtures of HMDSO vapour has been characterized (Chapter 7). It has

been found that concentration is a key parameter in controlling the or-

ganic/inorganic character of the resulting deposit. Analyzing the behavior

of several quantities as a function of the HMDSO concentration we have

found the deposition mode changes with increasing concentration. It has

been found that the retention of initial monomer methyl groups saturate

with concentration and so does the hydrophobic character of the resulting

surface. It has also been found that the DBD device is able to create uniform

and smooth deposits even if working in a full developed streamer regime.

This kind of process is able to create highly hydrophobic surfaces with lower

treatment times in comparison with similar low pressure processes.

The fluorination process of polymer surfaces with mixtures of Ar/SF6 at

atmospheric pressure has been studied (Chapter 8) and it has been found

that the presence of SF6 strongly modifies the discharge properties con-

verting the diffuse discharge regime of pure argon to a streamer regime.

It is found the effective existence of two competitive processes: an etch-

ing/activation process which brings, after the treatment, to the grafting of

polar groups from the ambient air, and a fluorination process in which graft-

ing of fluorine atoms to carbon in the backbone of polymers is realized. The

balance of these two processes is controlled by the SF6 concentration. By

analyzing the surface morphology it is found that modifications are intro-

duced by the treatment on the surface on the hundred nanometer scale but

these alterations do not affect substantially the roughness of the substrates.

The effectiveness of the fluorination process has been evaluated through the

analysis of the macroscopic surface properties with dynamical measurements

of contact angles with water and a non-polar liquid. The result is that this

process is effective in the modification of the properties of organic materials

138 CONCLUSIONS

making the surface highly hydrophobic and oleophobic. These properties

are needed in order to obtain resistance to water and oils.

Finally, the studied atmospheric plasma processes have been employed

for the modification of cellulosic materials (paper). Some aspects of this

research are discussed in Chapter 9. It is found the deposition process of

thin organosilicon films is able to produce effective hydro-repellent paper

surface and that exists a deposition condition in which very different sub-

strate kinds assume the same surface properties. It has also been found

that the resulting water-repellency is extremely stable and is not affected by

aging. These results show that atmospheric pressure plasmas are extremely

interesting for the development of new industrial applications. It has been

found that an atmospheric pressure plasma can produce oil-repellent paper

surfaces using a fluorination process based on SF6 containing gas mixtures.

A disturbance of the fluorination process with the molecular structure of

cellulosic materials generate a fast aging effect which removes completely

the attained properties. It has been found that by combining a deposition

process of thin organosilicon films with the fluorination process it is pos-

sible to remove the aging effect giving stable properties over time. These

results show that atmospheric pressure plasmas are extremely interesting

for the development of new industrial applications for the substitution of

traditional oil-repellent coating processes.

In conclusion, it has been shown that suitable diagnostics and experi-

mental and statistical characterization can lead to unveil part of the puzzling

aspects concerning dielectric barrier discharges. Also it has been shown, us-

ing a newly developed DBD device, that a better understanding of discharge

conditions allows to investigate suitable plasma processes to give new surface

properties to natural and artificial polymers. The results of this thesis work

can be the basis for researches focused on the study of dielectric barrier

discharge dynamics and on the development of new atmospheric pressure

plasma processes for surface modifications.

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Atmospheric Pressure Plasmas for Surface Modifications