APPROVED: Oliver M.R. Chyan, Major Professor Guido Verbeck, Committee Member Michael G. Richmond, Chair of the Department of

Chemistry Sandra L. Terrell, Dean of the Robert B. Toulouse

School of Graduate Studies

FTIR-ATR CHARACTERIZATION OF HYDROGEL, POLYMER FILMS,

PROTEIN IMMOBILIZATION AND BENZOTRIAZOLE

ADSORPTION ON COPPER SURFACE

Karthikeyan Pillai, B.Tech

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

December 2007

Pillai, Karthikeyan. FTIR-ATR Characterization of Hydrogel, Polymer Films, Protein

Immobilization and Benzotriazole Adsorption on Copper Surface. Master of Science (Analytical

Chemistry), December 2007, 62 pages, 3 tables, 26 figures, 81 references.

Plasma polymerization techniques were used to synthesize and deposit hydrogel on

silicon (Si) substrate. Hydrogel is a network of polymer chains that are water-insoluble and has

a high degree of flexibility. The various fields of applications of hydrogel include drug release,

biosensors and tissue engineering etc. Hydrogel synthesized from different monomers possess a

common property of moisture absorption. In this work two monomers were used namely 1-

amino-2-propanol (1A2P) and 2(ethylamino)ethanol (2EAE) to produce polymer films deposited

on Si ATR crystal. Their moisture uptake property was tested using FTIR-ATR technique. This

was evident by the decrease in –OH band in increasing N2 purging time of the films.

Secondly, two monomer compounds namely vinyl acetic acid and glycidyl methacrylate

which have both amine and carboxylic groups are used as solid surface for the immobilization of

bovine serum albumin (BSA). Pulsed plasma polymerization was used to polymerize these

monomers with different duty cycles. Initial works in this field were all about protein surface

adsorption. But more recently, the emphasis is on covalent bonding of protein on to the surface.

This immobilization of protein on solid surface has a lot of applications in the field of

biochemical studies. The polymerization of vinyl acetic acid and glycidyl methacrylate were

shown as successful method to attach protein on them.

Chemical mechanical polishing (CMP) of Cu is one of the processes in the integrated

chips manufacturing industry. Benzotriazole is one of the constituents of this CMP slurry used

as corrosion inhibitor for Cu. Benzotriazole (C6H5N3) is a nitrogen heterocyclic derivative

having three nitrogen atoms, each with an unshared pair of electrons, forming five-membered

ring structure. This molecule coordinates with Cu atoms by loosing a proton from one of its

nitrogen atom and thereby forming a film which is polymeric in nature that prevents further

oxidation of Cu. In this work, aqueous solution of BTA was used to study the adsorption

characteristics on Cu when a Cu electrodeposited Si was immersed in to it. This study was

performed using ATR-FTIR technique. The adsorption of BTA on Cu surface was evident by

the presence of aryl C-H stretching band at 3061 cm-1.

ii

Copyright 2007

by

Karthikeyan Pillai

iii

ACKNOWLEDGEMENTS

First of all, I would like to express my gratitude to my advisor Dr. Oliver Chyan for his

effective and encouraging guidance during my course of graduate study. I greatly appreciate his

immeasurable patience in teaching me about all aspects of doing research. His sincerity,

intelligence, patience and industriousness are those which I hope to emulate in my professional

and personal life. Special thanks are to Dhiman Bhattacharyya whose contributions are very

significant in my thesis. I have learnt a quite about plasma polymerization by working with him

and thanks to him we have a paper published in Chemistry of Materials Journal. I greatly thank

Dr. Praveen Nalla for his sincere efforts in teaching me about FTIR. My innumerable thanks are

to my ever friend Shyam S. Venkataraman for his great support in difficult situations and for his

help in conducting my research work. I would specially like to thank Kyle Yu for being a great

friend, and for his encouraging philosophical words with which I was able to keep up my spirit to

complete my thesis work. I also thank my past and present group members Sarah Flores and Fan

Yang for their useful discussions.

I greatly thank my committee member Dr. Guido Verbeck for his help in finishing up my

thesis. I would like to express my admiration towards his great enthusiasm and optimistic spirit

in professional life. I would like to thank Dr. Teresa D. Golden for being a good mentor of my

graduate courses. I thank my family members for their love and support that encouraged me a

lot to accomplish my degree.

iv

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS........................................................................................................... iii LIST OF TABLES......................................................................................................................... vi LIST OF ILLUSTRATIONS........................................................................................................ vii Chapters

1. INTRODUCTION ...................................................................................................1

1.1 Silicon ..........................................................................................................2

1.1.1 Properties of Semiconductors ..........................................................2

1.1.2 Structure of Silicon ..........................................................................4

1.2 Silicon ATR Preparation..............................................................................6

1.3 Surface Preparation of Si .............................................................................6

1.3.1 Cleaning of Silicon Surface .............................................................7

1.3.2 Metal Deposition on Silicon ............................................................9

1.4 FTIR-ATR Spectroscopy...........................................................................10

1.4.1 Total Internal Reflection ................................................................10

1.4.2 Attenuated Total Reflection...........................................................11

1.5 Plasma Polymerization...............................................................................14

1.6 Chapter References ....................................................................................17 2. SYNTHESIS OF HYDROGEL VIA PLASMA POLYMERIZATION AND

IMMOBILIZATION OF BIOMOLECULES ON POLYMER SURFACES .......19

2.1 Introduction................................................................................................19

2.2 Experimental ..............................................................................................21

2.2.1 Cleaning of Silicon ATR Crystal...................................................21

2.2.2 Rotating Pulsed Radio Frequency (RF) Plasma Reactor ...............22

2.2.3 Hydrogel Preparation by Plasma Polymerization ..........................23

2.2.4 Poly Vinyl Acetic Acid Film Preparation......................................24

2.2.5 ATR-FTIR Instrumentation ...........................................................25

2.2.6 Plasma Polymerization...................................................................25

2.3 Results and Discussion ..............................................................................26

v

2.3.1 FTIR Characterization of Plasma Polymerized 1A2P ...................26

2.3.2 FTIR Characterization of Plasma Polymerized 2EAE...................29

2.3.3 Immobilization of BSA on to Vinyl Acetic Acid (VAA) Polymer Film................................................................................................32

2.3.4 EDC Chemistry..............................................................................33

2.3.5 Immobilization of BSA on Plasma Polymerized Glycidyl Methacrylate ..................................................................................37

2.4 Conclusion .................................................................................................40

2.5 Chapter References ....................................................................................42 3. STUDY OF BENZOTRIAZOLE ADSORPTION ON COPPER SURFACE BY

FTIR-ATR SEPCTROSCOPY ..............................................................................46

3.1 Introduction................................................................................................46

3.2 Experimental ..............................................................................................48

3.3 Results and Discussion ..............................................................................49

3.3.1 Microscopic and FTIR Results of Si-H .........................................49

3.3.2 Optimization of Electroless Cu Deposition ...................................51

3.3.3 Contact Angle Measurement..........................................................54

3.3.4 FTIR results of Cu/Si and BTA/Cu/Si ...........................................55

3.3.5 Concentration Effect of BTA on Adsorption on Cu ......................56

3.4 Conclusions and Future Investigations ......................................................59

3.5 Chapter References ....................................................................................61

vi

LIST OF TABLES

Page

1.1 List of commonly used IRE ...............................................................................................14

3.1 Series of experiments to produce uniform Cu coverage on Si...........................................51

3.2 Contact angle measurements of Cu on Si surface with different times of deposition .......53

vii

LIST OF ILLUSTRATIONS

Page

1.1 Energy bands for (a) metals (b) semiconductors (c) insulators ...........................................3

1.2 Miller indices structure for Si (100) and Si (111) planes.....................................................4

1.3 The lattice structure of a) (100) and b) (111) orientation planes .........................................5

1.4 Schematic representation of ATR spectroscopy ................................................................12

1.5 Schematic of the bell-shaped RF plasma polymerization reactor......................................16

2.1 Chemical structures of 1-amino-2-propanol and 2(ethylamino)-ethanol...........................20

2.2 360° rotating pulsed RF plasma reactor.............................................................................23

2.3 FTIR spectra of 1A2P films; a) 1800 Å, 2 side coated; b) 1800 Å, 1 side coated; c) 180 Å, 2 side coated; d) 180 Å, 1 side coated ..............................................................................27

2.4 FTIR spectra of 1A2P films synthesized under different duty cycles a) 10/30 b) 10/10 and c) CW for different purging times .....................................................................................29

2.5 FTIR spectra of 2EAE synthesized under different duty cycles a) 10/30 b) 10/10 and c) CW with different purging times .......................................................................................31

2.6 Schematic representation of the process of BSA immobilization on surface....................32

2.7 Mechanism of EDC reaction with carboxylic group to produce amide.............................34

2.8 IR spectrum subtracted from that of VAA showing amide carbonyl peaks after 6-aminohexonoic acid has reacted with –COOH group of VAA..........................................35

2.9 FTIR spectra of PP VAA, spacer reacted PP VAA and BSA reacted PP VAA................35

2.10 PP VAA spectrum was subtracted from that of BSA showing only the BSA spectrum with different interval of 1% sodium dodecyl sulfate (SDS) washing ..............................36

2.11 Structure of glycidyl methacrylate.....................................................................................37

2.12 (a) FTIR spectra of 50nm (A), 150nm (B), 300nm (C) thick GM polymer films (b) FTIR spectra of PP GM, rinsed PP GM film and BSA reacted PP GM film..............................38

2.13 FTIR subtracted spectra of BSA after covalent attachment to the GM film .....................39

3.1 Structure of benzotriazole ..................................................................................................47

viii

3.2 Microscopic image of hydrogen terminated silicon in bright field, 100x magnification ............................................................................................................................................50

3.3 FTIR spectrum of silicon hydride sample..........................................................................50

3.4 Microscopic dark filed images of copper deposited on silicon substrate for different times of immersion ......................................................................................................................53

3.5 FTIR spectra of three samples with different durations of Cu deposition in NH4F electrolyte A) 60 minutes B) 25 minutes C) 5 minutes .....................................................55

3.6 FTIR spectra of BTA adsorbed on Cu for 5 minutes in the region 4000 – 2000 cm-1 showing aryl C-H stretching at 3061 cm-1 .........................................................................56

3.7 FTIR spectra of Cu sample immersed in different concentrations of BTA (a) 0.01 M BTA; (b) 0.05 M BTA; (c) 0.5 M BTA .............................................................................58

3.8 Structures of BTA derivatives ...........................................................................................59

CHAPTER 1

INTRODUCTION

This introductory chapter is written with an objective of explaining the purposes,

analytical techniques, and methods etc. for a better understanding for the reader. The

synergetic effect of attenuated total reflection (ATR) in sampling with Fourier transform

infrared (FTIR) spectroscopy forms the foundation for my research work. For ATR

analysis, the element silicon (Si) was chosen as internal reflection element (IRE) due to

its diverse applications [1]. Chapter 1 begins with an overview of preparing, cleaning

and surface modifying of silicon ATR crystals and explaining about different techniques

used to characterize the silicon surface modifications. The core of this research work can

be broadly divided in to two chapters. Chapter 2 discusses about the method of formation

of thin film polymers on silicon substrate using plasma polymerization and its properties

characterized by FTIR-ATR technique. This plasma polymerization technique [2] was

used for two purposes; to produce thin film of hydrogel on Si substrate with the starting

monomer 1-amino-2-propanol; to produce a vinyl acetic acid polymer film on Si leaving

the surface with carboxylic group and this functionalized surface was then used to

immobilize the bio-molecule bovine serum albumin (BSA) studied by FTIR-ATR.

Chapter 3 deals with the modification of Si ATR crystal surface with copper (Cu)

by electroless deposition technique [3] and various parameters followed for the Cu

deposition on Si. The Cu deposited substrate was then used for the study of adsorption of

benzotriazole ( BTA) molecule which is an aromatic heterocyclic molecule with five

membered triazole ring is a well known corrosion inhibitor for Cu and its alloys. The

adsorption of BTA on Cu surface was characterized by FTIR-ATR. The primary

1

substrate which binds these studies together is the use of silicon as the foundation from

which the research is conducted.

1.1 Silicon

Since the past several years, it can be said that silicon is one of the most important

elements whose application has a tremendous effect in the improvement of technological

world. Silicon and silicon based materials form backbone of the integrated circuit (IC)

chips. With its semi-conducting characteristics upon doping, silicon has attracted a

colossal interest in various research fields like optoelectronics, sensors, microelectronics.

1.1.1 Properties of Semiconductors The materials involved in this work fall into one of the categories of metals,

semiconductors, and insulators. The discussion of their properties is necessary in order to

better understand their behavior as a whole. Metals are conductors of electricity because

of their possession of high density charge carriers. Insulators are non-conductors of

electricity which refers to their zero charge density. Semiconductors become highly

useful and important because of their conducting nature that lies between metals and

conductors.

All these three types of materials have different properties in terms of charge

conduction which is attributed to the energy band difference. The energy band structure

of these materials is illustrated in figure 1.1. Crucial for conduction process is whether or

not there are electrons in the conduction band. In insulators, the electrons in the valence

band are separated by a large gap from the conduction band which is attributed to non-

2

conduction in them. In metals or conductors, the valence and the conduction bands are

overlapped. But in semiconductors, the energy gap between the valence and conduction

band is small enough so that any thermal or other excitations can bridge the gap. An

important parameter is the Fermi level, the top of the available electron energy levels at

low temperatures. The position of the Fermi level with respect to the conduction band is

a critical factor in determining the electrical properties.

Conduction Band

Valence Band Valence Band

Conduction Band

Valence Band

Conduction Band

E

EfEg Ef

(a)

(c) (b)

Fig. 1.1 Energy bands for (a) metals (b) semiconductors (c) insulators.

For non-doped, intrinsic semiconductors like silicon and germanium, the Fermi

level is essentially in the halfway between valance and conduction band. There will not

be any conduction at 0 K, but at elevated temperatures, the electrons in the valence band

get excited in to the conduction band. The structure of Si plays an important role in

having different properties.

3

1.1.2 Structure of Silicon

Since all the work done described in this thesis have foundation on silicon as the

primary substrate, it is necessary to discuss about the properties of silicon based on its

structure. Si forms four covalent bonds with other Si atoms resulting in tetrahedral shape.

It has diamond structure that has fcc Bravais lattice where each silicon atom is sp3

hybridized [4]. Single crystal is a crystalline solid where the crystal lattice of the entire

sample is continuous. Polycrystalline solid is made up of number of small crystals

known as crystallites. Single crystal Si wafers are commercially available and are widely

used in microelectronic industry [1]. The (100) and the (111) planes are the most

common and popular orientations used in single crystal silicon wafers for IC processing.

Fig. 1.2 Miller indices structure for Si (100) and Si (111) planes.

Fig 1.2 illustrates the structure of Si (100) and Si (111) orientation. The set of integers

used in the bracket are called Miller indexes that define the crystal orientations. These

integers are obtained from the reciprocal of the intercepts made by the atomic plane to a

three dimensional x, y and z axes at corresponding distances of the unit vectors a, b and c.

4

Note that the (100) plane has a square shape and (111) plane is in triangular shape. See

Fig. 1.3 which illustrates the lattice structures of (100) and (111) orientation planes.

Fig.1.3 The lattice structure of a) (100) and b) (111) orientation planes [5].

The (100) oriented crystals are generally used for making metal-oxide

semiconductor integrated circuit (MOS IC) chips and the (111) crystals for making

bipolar transistors. The reason that (111) crystals are used to make bipolar transistors is

that the surface of (111) crystal is stronger which is because the atom surface density is

higher and is a better fit for higher power devices. The fragments of the (100) crystals

normally have right angles and that of (111) crystals will have 60 triangles [5]. In

chapter 2, Si (100) ATR crystal was used merely as a substrate for plasma polymerization

but in chapter 3, Si (111) ATR crystal was used to develop smooth hydride surface that

serves as a source of electrons for Cu ions reduction on Si surface.

a)

Basic lattice Silicon atom Basic lattice cell Silicon atom

b)

5

1.2 Silicon ATR Preparation

Double polished silicon N-type phosphorus doped (100) (Wacker Co.) wafers

with a resistivity of 0.6 – 1.0 Ohm cm and N-type (111) wafers (from PCA) with a

resistivity of 1.0 – 10.0 Ohm cm were used to make ATR crystals. The silicon ATR

crystals were cut from those wafers with 5.8 cm × 1 cm × 0.7 cm. The two ends of the

crystal were ground and polished to a fineness of 1 micron with the help of a polishing

machine from High Tech Allied Products. The silicon ATR crystal was covered all

around with Teflon tape in order to protect any scratch/damage to the surface and was

clamped by sample holder at an angle of 45 to the plane of the rotating platen of the

polishing machine. The platen was rotated with a speed of 100 rpm in the direction along

the bevel face. Continuous flow of water was provided to flush away any particles that

were grinded by the polishing pad. At first, 30 micron grit size pad was used to coarse

polish the bevel face. Subsequently, 15, 9, 3, and 1 micron pads were used to further

polish the bevel face to have fineness. Blue lube solution was used in lieu of water when

3 and 1 micron grit size polishing pads were used. All polishing pads were from Allied

High Tech Products (www.alliedhightechproducts.com). Distilled water rinsing and

observation under an optical microscope was used to ensure the quality of polishing

1.3 Surface Preparation of Silicon

Since all the results obtained from this work are dependent on the nature of the Si

surface, it requires that the surface has to be clean of metal and organic contaminants.

Silicon ATR crystal after polishing at both the ends at 45 angle is subjected to wet

cleaning in order to remove the contaminants on the surface. A renowned cleaning

6

method called RCA cleaning was employed to clean the Si surface to remove the metal

and organic contaminants present on the surface. Similar standard procedures are

followed in preparing the Si surface for further adsorption or metallization studies. The

RCA cleaning was developed by Radio Corporation of America that includes many

chemical steps. The following are the different cleaning steps;

1.3.1 Cleaning of Silicon Surface 1.3.1. A. Standard Cleaning - I (SC1) The SC1 cleaning is done to remove the organic contaminants present on the Si

surface. This is a basic cleaning method required for Si surface before using it for further

treatments. The SC1 solution is a mixture of ammonium hydroxide (NH4OH), hydrogen

peroxide (H2O2) and ultra pure water (UPW) in a ratio of 1:1:5. The silicon crystal after

polishing is sequentially rinsed and sonicated with UPW, isopropyl alcohol (IPA) and

immersed in the SC1 solution for 10min at a temperature of 70-80 C. The hydrogen

peroxide is a strong oxidizing agent which oxidizes the silicon and forms a layer of

surface oxide. The ammonium hydroxide etches away the oxide layer formed by

hydrogen peroxide. This process of oxide formation and etching removes all the organic

contaminants and particles from the surface. This process leaves the silicon surface with

chemically formed clean oxide surface. The SC1 cleaned Si (100) crystal was used as a

substrate for polymer deposition which is described in chapter 2.

7

1.3.1. B. Standard Cleaning - II (SC2) The SC2 cleaning is done to remove the metallic impurities present on the Si

surface. This solution is a mixture of hydrochloric acid (HCl), H2O2 and UPW in the

ratio of 1:1:5. The silicon substrate is immersed in this solution for about 10 minutes at a

temperature of 70-80 C. HCl acid present in the solution dissolves all the metal particles

present on the surface. Simultaneously, the silicon surface is oxidized by the presence of

H2O2. This process removes alkali ions and residual metals that are left after SC1

cleaning.

1.3.1. C. Piranha Cleaning The Piranha solution is a mixture of sulfuric acid (conc.) and hydrogen peroxide

used to clean organic residues off the substrate. This treatment can be done to Si in place

of SC1, in order to minimize the surface roughness which will be high in SC1 treatment.

The mixture is a strong oxidizer which helps in removing organic matters and it will also

makes the surface more hydrophilic. Many different mixture ratios like 4:1 or even

7:1are commonly used, but a typical mixture is 3:1 conc. sulfuric acid and hydrogen

peroxide (30%). The following reaction equation explains the ions formed in the Piranha

solution.

H2SO4 + H2O2 -------> H3O+ + HSO4- + O ---------------- (1)

The atomic oxygen species formed in the solution allows the Piranha solution to dissolve

the elemental carbon. The sulfate ions help in removing metals as soluble complexes.

The substrate is exposed for about 10-15 minutes at a temperature of 100 C.

8

1.3.1. D. Diluted Hydrofluoric Acid Etching (DHF) HF is used to remove the oxide from the silicon surface and contamination in the

oxide from the wafer surface [6,7]. HF is extremely dangerous and should be handled

with great care. The concentration of HF typically used is 4.9%. It reacts with silicon

dioxide and the proposed mechanism for the etching mechanism is given by

4HF + SiO2 ------> SiF4 + 2H2O ---------------- (2)

(OR)

SiO2 + 6HF ------> 2H+ + SiF62- + 2H2O ---------------- (3)

The SiF4 is formed by the interaction of SiO2 with HF. As a result, the Si-F bond

polarizes the Si-Si bond in the second layer of the substrate. Hence the surface silicon is

removed as SiF4 which is basically induced by these polarizations. This process renders

the silicon surface to be hydrogen terminated that is extremely hydrophobic. This Si-H

surface behaves as a source of electrons for reduction of Cu ions which is described in

chapter 3.

1.3.2 Metal Deposition on Silicon by Electroless Method Various techniques such as electron beam evaporation, electrochemical deposition

physical and chemical vapor deposition, laser ablation are available for depositing a

metal film on a substrate. Electroless deposition is one of the techniques widely used in

the semiconductor-devices industries. It is useful to discuss some of the important

concepts of metal deposition on silicon because in chapter 3, the electroless deposition of

Cu on silicon substrate is an important experiment carried out for making Cu/Si substrate

9

for benzotriazole adsorption studies. Metal deposition on hydrogen terminated silicon

surface has been studied extensively by researchers [8,9]. The metal that is reduced on

H-terminated silicon has to have more positive reduction potential (0.34 V vs. SHE) with

respect to hydrogen (0.00 V vs. SHE) and be noble. Metals such as Cu2+, Ag2+, Au2+, and

Pt2+ will fall under this category. The deposition of metal on H-silicon surface without an

external applied potential is known as immersion or electroless deposition. Such kinds of

depositions are essentially based on the electrochemical phenomenon. This involves

reduction of metal ions on silicon leading to deposition and simultaneous oxidation of

silicon resulting in its dissolution [8]. The metal ion reduction reaction can be written as

Mn+ + ne- ------> M ---------------- (4)

In electroless deposition process, a metal is deposited on the surface of a substrate

through the transfer of electron between a metal ion and a reducing agent (e.g.

formaldehyde) in the solution. In this work, HF is used instead of a reducing agent in the

electroless deposition of Cu on Si. Under such conditions, the metal is deposited at the

expense of etching the substrate material.

1.4 Background and Theory of FTIR-ATR Spectroscopy

1.4.1 Total Internal Reflection A phenomenon of refraction can be explained as when a ray of light passes from a

denser to a rarer medium, it bends away from the normal in to the rarer medium. But

above a certain angle of incidence which is called as critical angle, if the light is

completely reflected back in to the denser medium, it refers to the phenomenon of total

internal reflection.

10

1.4.2 Attenuated Total Reflection

Attenuated total reflection is a sampling technique used in conjunction with

infrared spectroscopy which enables samples to be examined directly in the solid or

liquid state without further preparation [9]. Since FTIR-ATR spectroscopy has been

extensively used in this work that I have undertaken, it is worth to know about the theory

and background of this technique. ATR uses a property of total internal reflection called

the evanescent wave, the phenomenon which was observed by Newton in the early

1700s. Extensive research has been done after 1970 in this field with the arrival of

ameliorated instrumentations like FTIR etc. The advantages that make this technique

unique refer to its simplicity in sampling and the amount of information derived from it.

Fig. 1.4 depicts the basic principle of ATR spectroscopy.

In FTIR-ATR spectroscopy technique, the IR radiation gets propagated through

the internal reflection element and produces an evanescent electrical field that decays

exponentially with distance Z. For the internal reflection to occur, two mediums must be

in contact with each other; a denser medium with high refractive index (Si, 3.42) and a

rarer medium which is the thin film of sample on the surface of the silicon. The angle of

incidence of IR radiation must be greater than the critical angle θc, which is a function of

refractive index of both the sample and IRE (Si crystal) [10,11]

11

dp Depth of few microns

IR in

IR Out Internal Reflection Element

Evanescent Wave

Sample

Fig. 1.4 Schematic representation of ATR spectroscopy.

θc = Sin-1(n2/n1) ---------------- (5)

where n2 = refractive index of sample and n1 = refractive index of ATR element. The

number of reflections of IR radiation depends on the dimensions of ATR element.

Typically, an ATR element has dimensions of 6×1×0.7 cm with the bevel edge polished

to an angle of 45. Normally, the number of reflections that can be achieved in our lab

will be up to 100 reflections. The total number of reflections (N) of a given length (L)

and width (W) of an ATR element is given by;

N = (L / W) Cot θc ---------------- (6)

The evanescent wave which is basically an electrical field (E), is produced normal to the

reflecting surface. The exponential decay of the evanescent wave is in the order of

microns and depends on the energy of the IR radiation. If Eo is considered to be the

amplitude of the incident energy, then E, the amplitude of the evanescent wave can be

expressed as [12]

12

E = Eo exp [-(2π/λ1) (sin2θ – (n2/n1)) 1/2Z ---------------- (7)

Where λ1 = λ/n1 is the wavelength of radiation in Si ATR, λ is the wavelength of

radiation in air and Z is the distance from the surface. Harrick [10] modified this equation

by taking in to account the energy absorption in the rarer medium as,

E = Eo exp [-γZ] ---------------- (8)

Decay Constant,

1/2π (sin2θ – (n2/n1)2) 2 ---------------- (9)

γ = λ1

The penetration depth of the evanescent wave is expressed as dp = 1/γ . That is,

2π (sin2θ – (n2/n1)2) 1/2

λ1 dp = ---------------- (10)

where λ1 is the wavelength of IR radiation, n1 is the refractive index of ATR element and

n2 is that of sample medium. A sample calculation of penetration depth is shown.

At wavenumber 400 cm-1 (25 µm) with angle of incidence 45,

dp = 25

2 * 3.14 * [(sin2 45 – (1/3.42)2 ] 1/2

= 6.18 µm

13

Hence the penetration depth can be controlled by varying angle of incidence or by

choosing IRE element. Various IRE elements are there with different refractive index. A

list of elements with their refractive indexes is provided in the following table.

IRE Refractive Index

KRS 5 (Thallium Bromide/Thallium Iodide)

2.35

AgCl 1.98

AgBr 2.16

Silicon 3.42

Germanium 4

Table 1.1 List of commonly used internal reflection elements.

In ATR spectroscopy, the bevels of ATR element crystals were shaped in such a way that

the angle of its surface be 45. The main advantage of using this ATR crystal is that it

undergoes multiple reflections with IR radiation which enables amplification of weak

signals as a result of which the sensitivity is increased. It can detect up to sub-monolayer

of substance.

1.5 Plasma Polymerization

Since chapter 2 deals with the synthesis of hydrogel and other polymer films

through plasma polymerization, it is important to discuss the functions of plasma

polymerization. Plasma polymerization is a unique technique used to fabricate thin

polymer films from a variety of organic and organometallic starting materials [13-20].

14

The principle behind this technique is that it uses plasma sources to generate a gas

discharge which provides energy to activate or fragment gaseous or liquid monomer

mostly that contains vinyl group and initiates polymerization. It is a procedure wherein

the gaseous monomers are stimulated through plasma and condensed on selective

substrates as highly cross-linked layers.

The functionalization of polymer surfaces with specific reactive groups has

attracted a lot of research interest into it. Plasma polymerization is a specific type of

chemistry between plasma species, between plasma and surface species or between

surface species [13]. Two types of reaction mechanism may be postulated 1) plasma-

induced polymerization 2) plasma-state polymerization. Former one is the conventional

polymerization of molecules containing carbon-carbon bonds and the latter depends on

the presence of electrons and other species in plasma, energetic enough to break bonds.

Unlike conventional polymers, plasma polymers do not consist of regular repeated units,

but they tend to form an irregular three dimensional cross-linked network.

Plasma polymers generally are chemically inert, stable, insoluble, mechanistically

tough, and have been used in wide variety of applications protective coatings, electrical,

optical and biomedical films. The several advantages of plasma polymerization over

conventional one are;

1. The starting gases may not contain functional groups that are normally associated

with conventional polymerization

2. Such films are highly coherent and adherent to various substrates

3. This process can be used to produce films of thickness 500 Å to 1µm easily

4. An ultrathin, pin-hole free films can be produced by plasma polymerization

15

Pressure Transducer

Monomer Inlet

Vacuum System Matching Unit

Substrate Bidirectional

Coupler

Function Generator

Pulse Generator

Oscilloscope Amplifier

Fig.1.5 Schematic of the bell-shaped radio frequency plasma polymerization reactor.

5. It is possible to tailor the films with respect to specific chemical functionality,

thickness and other physical and chemical properties by carefully controlling the

polymerization parameters

In this work 1-amino-2-propanol and vinyl acetic acid were used as starting monomers

and introduced into the plasma chamber to get polymerized and deposited on silicon ATR

crystal. These films were further characterized by FTIR-ATR spectroscopy.

16

1.6 Chapter References

1. Runyan, W.R.; Bean K.E; Semiconductor Integrated Circuit Processing

Technology; Addison-Wesley Publishing Company: Reading, 1990.

2. Y. Vickie Pan, Roger A. Wesley, Reto Luginbuhl, Denice D. Denton, Buddy D.

Ratner, Biomacromolecules 2001, 2, 32-36.

3. L.A. Nagahara, T. Ohmori, K. Hashimoto, and A. Fujishima; J. Vac. Sci.

Technology A 1993,11,4, Jul/Aug.

4. Cotton, F.A.; Murillo, C.; Advanced Inorganic Chemistry, John Wiley & Sons;

New York, 1999.

5. Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Prentice-

Hall Inc., Upper Saddle River, New Jersey, 2001.

6. V.A. Burrows, Y.J. Chabal, G.S. Higashi, K. Raghavachari and S.B. Christman,

Appl. Phys. Lett., 1988, 53, 998.

7. Y.J. Chabal, G.S. Higashi, K. Raghavachari and S.B. Christman J. Vac. Sci

Technol., 1989, A7, 2104.

8. In-Churl Lee, Sang-Eun Bae, Moon-Bong Song, Jong-Soon Lee, Se-Hwan Paek,

and Chi-Woo J. Lee* Bull. Korean Chem. Soc. 2004, 25, 2.

9. Shen Ye, Taro Ichihara, and Kohei Uosaki, Journal of the Electrochemical

Society, 2001,148, 6, C421-C426.

10. N.J.Harrick, Internal Reflection Spectroscopy, Interscience Publishers, NY, 1967.

11. FT-IR Spectroscopy—Attenuated Total Reflection (ATR). Perkin Elmer Life and

Analytical Sciences, 2005.

12. F.M. Mirabella, N.J. Harrick, Internal Reflection Spectroscopy; Review and

17

Supplement, Harrick Scientific Corporation, N.Y. 1985.

13. N. Morosoff, An introduction to Plasma Polymerization, in R. d’iigostino (ed.),

Plasma Deposition, Treatment, and Etching of Polymers, Academic Press, 1990.

14. Y. Takahashi, M. Iijima, K. Inagawa and A. Ito, J. Vac. Sci. Technol. A, 1987, 5,

2253.

15. Y. Kagami, T. Yamauchi and Y. Osada, J. Appl. Phys., 1990, 68, 610.

16. R. Hollahan and A.T. Bell, (eds.), Techniques and Applications of Plasma

Chemistry, Wiley, New York, 1974.

17. A.T. Bell and M. Shen (eds.), Plasma Polymerization, American Chemical

Society, Washington, DC, 1979.

18. W. Gombotz and A. Hoffman, J. Appl. Polym. Sci.: Appl. Polym. Symp. 1988, 2

285.

19. W. Gombotz and A. Hoffman, J. Appl. Polym. Sci., 1989, 37, 91.

20. H. Yasuda, Glow DischargePolymerization, in J. Vossen and W. Kern (eds.),

Thin Film Processes, Academic Press, New York, 1978.

18

CHAPTER 2

SYNTHESIS OF HYDROGEL VIA PLASMA POLYMERIZATION AND

IMMOBILIZATION OF BIOMOLECULES ON POLYMER SURFACES

2.1 Introduction

Hydrogels are defined as a network of polymer chains that are water-insoluble

and possess a degree of flexibility very similar to natural tissue due to their significant

water content. The swelling and de-swelling property of these polymeric materials is

dependent on nature of both intermolecular and intramolecular cross linking and also the

extent of hydrogen bonding in the polymer network [1]. The applications of hydrogel

are varied including drug release [2-4], biosensors [5-7], tissue engineering [8-10] and pH

sensors [11,12]. Different monomers are nowadays in utilization for the synthesis of

hydrogel. Some of the monomers used for this hydrogel synthesis are

N-isopropylacrylamide(NIPAM)[13-16], vinyl alcohol [8,17], ethylene glycol [18,19]

and N-vinylpyrrolidone [20]. The techniques that have been employed to immobilize

these thin films on solid surfaces are electron beam radiation [24,25], photo-initiated

grafting [8,26,27], use of activated and functionalized substrates [28,29] and plasma

polymerization [13,14,30-32].

This work was done in collaboration work with Mr. Dhiman Bhattacharyya from

Dr. Richard B. Timmons Lab, University of Texas, Arlington (UTA); to prepare surface-

immobilized hydrogel films using plasma polymerization technique and this forms the

first part of this chapter. The monomers used here to synthesize hydrogel film are 1-

amino-2-propanol (1A2P) and 2(ethylamino)-ethanol (2EAE) which are low-molecular-

weight, volatile and contain both amine and hydroxyl functional groups.

19

H3CNH2

OH

1-amino-2-propanol

OHNH

H3C

2-(ethylamino)ethanol

Fig. 2.1 Chemical structures of 1-amino-2-propanol and 2(ethylamino)-ethanol.

These monomers are plasma polymerized and deposited as thin films on

substrates placed inside the plasma reactor chamber. The synthesis of these polymer

films were carried out under variable duty cycle pulsed plasma conditions to control film

compositions obtained during plasma polymerization [33]. This method of

polymerization under variable duty cycle conditions will not only yield hydrogel but are

also energetic to produce films that are strongly grafted on to the surface. What is

reported here is the study of rapid moisture uptake property of these films which in turn

concerns about the type of substrates used. It is known that the amount of O-H functional

group present on these films relates to the moisture uptake property. Specifically, FTIR

technique is used to analyze quantitatively the O-H functional group present on the film.

Hence, ATR crystals made from Si wafers serve as a suitable substrate for depositing

these polymer films and the technique ATR-FTIR spectroscopy [34,35] is used to study

the moisture uptake properties. It describes the single-step, plasma polymerization

synthetic route to prepare these uniform and conformal thin films of thermoresponsive

hydrogel using 1A2P and 2EAE monomers.

20

The second part of this chapter deals with plasma polymerization of vinyl acetic

acid (VAA) and glycidyl methacrylate (GM) monomers, and the immobilization of

proteins on these polymer surfaces. FTIR-ATR technique was used to characterize these

films. Such biochemical studies include probing cell-surface interactions [36-38], and

inflammatory responses to implants [39,40]. Initial work in this area dealt primarily

about the simple protein surface adsorption. Since protein denaturation occurs readily

upon adsorption [41,42], the emphasis has shifted to studies in which the proteins are

covalently bound to the surfaces and retain their inherent structures and functions.

Plasma enhanced chemical vapor deposition (PECVD) was used to synthesize thin

polymeric films on substrates. Certain plasma conditions were followed under which the

polymeric films formed will retain their original functional groups. These functional

groups are available for the attachment of bio-molecules. The polymer film made from

vinyl acetic acid had –COOH group on the surface and subsequent attachment of BSA

was achieved using a spacer molecule, 6-aminohexanoic acid and carbodiimide

chemistry.

2.2 Experimental

2.2.1 Cleaning of Silicon ATR Crystal

The silicon ATR crystals after polishing contain organic, inorganic and metallic

contaminants on the surface. In order to have a clean surface, the crystals had to undergo

certain cleaning steps. The lab wares used here for all the experiments were made of

polyfluoroalkoxy material. These lab wares were cleaned many times in boiling

solutions of 10% HNO3 before had they employed in cleaning of silicon crystals.

21

Ultra pure water with resistance > 18.2 MΩ/cm was supplied via a Millipore Milli-Q®

Elix5® ultra pure water purification system (Millipore Corp., Bedford, MA,

www.millipore.com) for rinsing purposes as well as for any dilution of chemicals. The

silicon crystal was initially rinsed and sonicated in UPW followed by rinsing with

isopropyl alcohol and dichloromethane. High purity electronic grade HF, H2O2, NH4OH

and HCl (the chemicals were from Air Liquide, Dallas Chemical Center, 13456 N.

Central Expwy, Dallas, Texas, 75243, www.airliquide.com) were used in cleaning of

silicon crystals.

All silicon ATR crystals used were cleaned in hot solutions (70-80 C) of SC1

(Standard Cleaning-I; 1:1:5; H2O2 : NH4OH : H2O) and SC2 (Standard Cleaning-I; 1:1:5;

H2O2 : HCl : H2O) for 10 minutes. The crystals after cleaning from SC1 and SC2 were

rinsed with UPW and dried with a stream of N2 gas.

2.2.2 Rotating Pulsed Radio Frequency (RF) Plasma Reactor

The rotating pulsed RF plasma reactor shown in the fig 2.2 was used to synthesize

the hydrogel and other polymer films that were described in this chapter. A simple

working principle of this plasma reactor was described. The AC signal from the power

outlet is converted in to radio frequency by function generator. The radio frequency

generated is equivalent to 13.56 MHz. This RF signal is amplified by using RF

Amplifier. This instrument has the following components; a cylindrical reactor made of

Pyrex in which polymerization reaction occurs, a hot electrode connected to RF

amplifier, a ground electrode connected to ground, a low pressure monomer inlet which

is used to introduce low volatile monomers in to the reactor, a high pressure monomer

22

inlet which is used to introduce high volatile monomers, gas cylinders of oxygen,

hydrogen, argon which are used to produce plasma inside the reactor.

vv

vvv

To Vacuum

Monomer Input Port IIFor Low Vapor Pressure

Monomer

Flow Direction

Pyrex PlasmaReactor

360 de

gree ro

tating

Ferro-Fluid

ic Valve

Hot electrode

Pressure

Controller

MassFlowmeter

MKS BaratronPressure Transducer

Butterfly Valve

Monomer Input Port IFor High Vapor

Pressure Monomer &Oxygeb, Hydrogen or

Argon

Fig 2.2 360 rotating pulsed RF plasma reactor.

2.2.3 Hydrogel Film Preparation by Plasma Polymerization

This preparation of hydrogel film was done in Dr. Timmons lab in UTA. The

1A2P and 2EAE monomers were obtained from Sigma Aldrich, St Louis, MO and had a

stated purity of +98%. The Si ATR crystals were sonicated in acetone, methanol and

hexane to clean the atmospheric adsorbed organic contaminants from the crystal surface

prior to use. The crystals were placed inside the reactor to have the monomer deposited

23

on to the surface. All the substrates inside the reactor chamber were subjected to oxygen

plasma at 100W average power input to remove any carbonaceous residue left on the

surface. A background pressure of 6 mtorr was achieved for each run. Before

deposition, these monomers were freeze-thawed to remove any dissolved gases.

Monomer vapors were subjected to a radio-frequency plasma glow discharge, at room

temperature, 130 mtorr pressures and at 150W peak power input. Two different duty

cycles namely 10/30, 10/10 (ton/toff ms) and a continuous wave (CW) operational mode

were employed.

2.2.4 Preparation of Polymer Thin Films by Pulsed Plasma Polymerization

The chemicals required for this experiment such as vinyl acetic acid, glycidyl

methacrylate, 6-aminohexanoic acid and BSA were obtained from Sigma-Aldrich, St.

Louis, MO. The chemicals 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide

hydrochloride (EDC) and 2-(N-morpholino ethanesulphonic acid (MES) buffer were

from Pierce, Rockford, IL. The cleaned Si ATR crystal was placed inside the reactor

chamber. The reactor was evacuated to a background pressure of 2 mtorr. After

introducing the vinyl acetic acid monomer in to the reaction chamber, plasma was ignited

at a constant monomer pressure of 80 mtorr. VAA was plasma polymerized under pulsed

conditions. The optimum plasma duty cycle was determined to be 0.75 ms/20 ms (ton/toff

ms) and 200 W power input. Typically, 50 nm thick films were developed.

24

2.2.5 ATR-FTIR Instrumentation

A Bruker Equinox 55 FTIR instrument (Bruker Optics Inc., 19 Fortune Drive,

Manning Park Billerica, MA 01821 – 3991, www.brukeroptics.com) having ATR

accessory was used to characterize the all the silicon samples and their surface

modifications. The silicon ATR crystal was placed in a variable angle ATR accessory

from Pike Technologies such that the angle of IR light incidence is normal to the ATR

bevel face. Before taking the spectrum, the sample was adjusted for its correct position

to get maximum throughput of IR beam. A liquid nitrogen cooled mercury cadmium

telluride (MCT, high D*, narrow band, EG&G Inc. 200 Orchard Ridge Drive, Suite 100,

Gaithersburg, MD 20878, www.egginc.com) detector was used to detect the IR signal.

2.2.6 Plasma Polymerization

Plasma polymerization technique is used to synthesize novel materials in the

form of thin films and the surface modifications of those films have become increasingly

active research area in recent years. Those polymeric films possess different properties

that have many applications [43,44]. The primary approach of this plasma

polymerization technique is focused on the film chemistry controllability during

deposition. The goal here is to produce surface active coatings and/or molecularly

tailored surfaces. The advantages of plasma technique include all-dry, single step,

relatively rapid process which provides pin-hole free, conformal and adhesive films of

uniform thickness [43]. The important feature of this approach is that it offers an

opportunity to use low total power input. This tells the fact that the power is turned off

25

during polymerization process in the film. The average power given under pulsed

conditions is calculated from the equation:

<P> = [ton / (ton + toff)] * peak power.

Where ton – time duration when power is on

toff – time duration when power is off

peak power – power input during the plasma on periods.

2.3 Results and Discussion

2.3.1 Characterization of Plasma Polymerized 1A2P by FTIR

The plasma polymerized 1A2P thin film was deposited on Si ATR crystal and

characterized by FTIR-ATR. As an initial step, four samples of 1A2P were prepared

with two different thick films (1800 Å and 180 Å) each with one and two side coatings

on ATR crystal. Fig.2.3 shows the IR spectra of these films from which the peak

assignments are as follows: (a) N-H and O-H stretching modes at the region from 3600

cm-1 to ~3000 cm-1; (b) –CH3 asymmetric stretch at 2970 cm-1; (c) –CH2 asymmetric

stretch at 2930 cm-1; (d) –CH3 symmetric stretch at 2866 cm-1; (e) –C≡N stretch at 2240

cm-1; (f) -N≡C stretch at 2180 cm-1; (g) amide C=O stretch at ~1650 cm-1. It was

observed that the intensity of the each stretching band is proportional to the amount of

film present on the sample.

26

4000 3500 3000 2500 2000 1500 1000-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

2240

2866

29302970

a b c d

1650

2180

3330

Abs

orba

nce

Wavenumber cm-1

Fig. 2.3 FTIR spectra of 1A2P films; a) 1800 Å, 2 side coated; b) 1800 Å, 1 side coated;

c) 180 Å, 2 side coated; d) 180 Å, 1 side coated.

The 1A2P polymer films are prone to absorb large amounts of moisture from the

atmosphere. These polymer films synthesized under 10/30, 10/10 and CW conditions

were characterized by ATR-FTIR as a function of atmospheric exposure to purging time

with dry stream of N2 gas. The FTIR spectra were recorded after the films had been

exposed sufficiently to atmosphere so that the film will be in equilibrium with the

atmospheric moisture. From the spectra (fig 2.4), it was observed that the thickness of

the films under 10/30 and 10/10 duty cycle conditions, varied linearly with time of

deposition except that the film thickness formed by CW run indicates, the rate of

formation of the film was decreased with time of deposition .

27

Secondly, for each of the film the spectra were recorded for 0, 2, 5, and 10

minutes of dry nitrogen gas purging that refers to spectra A, B, C and D respectively.

28

Fig. 2.4 Reproduced with permission from American Chemical Society. FTIR spectra of

1A2P films synthesized under different duty cycles a) 10/30; b) 10/10 and c) CW for

different purging times.

It was observed that there was a substantial decrease in –OH band intensity for all the

films as the purging time was increased. The absorptions of atmospheric CO2 and H2O

were eliminated by subtracting a background spectrum of uncoated ATR crystal from

each of the polymer film absorptions.

2.3.2 FTIR Characterization of Hydrogel Synthesized from the Monomer

2(ethlyamino)ethanol (2EAE)

One of the general properties of hydrogel films is moisture absorption. In order to

test that generality, a hydrogel from another monomer 2EAE was synthesized via plasma

29

polymerization and characterized by FTIR. Figure 2.5 shows the IR spectra of hydrogel

films synthesized from 2EAE with three different deposition conditions namely 10/30,

10/10 and CW. The 2EAE monomer contains structural features (N-H and O-H groups)

in common with 1A2P.

4000 3500 3000 2500 2000 1500 1000

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

W avenum ber cm -1

A B C D

(a)

4000 3500 3000 2500 2000 1500 1000

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

W avenum ber cm -1

A B C D

(b)

30

4000 3500 3000 2500 2000 1500 1000

-0.2

0.0

0.2

0.4

0.6

0.8

1.0 A B C D

Abs

orba

nce

W avenumber cm -1

(c)

Fig. 2.5 FTIR spectra of 2EAE synthesized under different duty cycles a) 10/30

b) 10/10 and c) CW with different purging times.

The depositions were carried out at 150 W peak RF powers, which were the same

for 1A2P. It turns out that the IR spectra were essentially identical to those of 1A2P,

with absorption bands. Like 1A2P, 2EAE polymer films also posses the water absorption

property when exposed to atmosphere. This was evidenced by the decrease in the peak

intensity at the region from 3600 cm-1 to ~3000 cm-1, which corresponds to –OH band as

the nitrogen gas purging time was increased. In spite of the fact that the absorption bands

were same as the 1A2P, it was observed that there was a decrease in the peak intensity at

the region between 3500 cm-1 and 3100 cm-1 relative to the peak centered around 1600

cm-1. Comparing to 1A2P, this decrease was more in 2EAE.

31

2.3.3 Immobilization of Bovine Serum Albumin (BSA) on to Vinyl Acetic Acid (VAA)

Polymer Film

The use of plasma enhanced CVD is unique, to deposit thin polymeric films on

the surfaces in order to have the functional groups available for protein attachment onto

it. With this motto, VAA was coated on Si ATR crystal for a film thickness of 50 nm.

CONH

CONH

CONH

CONH

COOH

OH

OH

OH

CO

CO

CO

PP VAA CO

CO

CO

CO

OH

OH

OH

OH

6-aminohexanoic acid H2NCH2(CH2)3CH2-COOH

EDC, in MES at pH 4.7

EDC, in MES at pH 4.7

BSA Solution BSA

CONH

CONH

CONH

CONH

CO

CO

CO

CO

NH

NH

NH

NH

Fig.2.6 Schematic representation of the process of BSA immobilization on solid surface.

The plasma polymerized VAA (PP VAA) film was characterized using FTIR, and

the spectra were observed to have peaks at ~1700 cm-1 and at the region between 3500

cm-1 and 3000 cm-1, indicating the presence of C=O and O-H functional groups

respectively. The PP VAA coated Si ATR crystal was then washed with 2-(N-

32

morpholino)ethanesulphonic acid (MES) buffer solution at pH 4.7, to remove any

physisorbed oligomers/monomers.

Fig 2.6 shows the steps followed in the process of BSA immobilization on VAA

polymer film. The film was then dipped in the 6-aminohexanoic acid that acts as spacer

and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) chemistry

was followed for reaction between spacer and VAA. EDC chemistry is known to activate

–COOH group of VAA to react with –NH2 terminal of spacer to form amide carbonyl (-

CONH) [43]. After dipping into the spacer, the film was left with amide carbonyl

functional groups by the reaction of –NH2 with –COOH of VAA polymer. This was

confirmed by the presence of amide carbonyl peaks at the regions of 1654 cm-1 and 1530

cm-1 in the (see fig. 2.8)

2.3.4 EDC (Carbodiimide) Chemistry

The carboxylic acid (1) will react with carbodiimide to produce the key

intermediate O-acylisourea (2) which can be defined as a carboxylic ester with an

activated leaving group. The O-acylisourea will react with amines to produce the desired

product amide (3) and undesired side-product urea (4). The key intermediate reacts with

an additional carboxylic acid to produce acid anhydride (5) which in turn reacts with

amines to produce amide (3) and urea (4). The main undesired reaction involves the

rearrangement of O-acylisourea to the stable N-acylurea (6).

33

OH

O

R1

N C N

R R

O

O

R1

N C

R R

N

H

O

O

R1

C

HNR

NR

2

1

O

O

R1

O

R1

OH

O

R1

NH

NH

O

R RNH

NH

O

R R

H2N R2

H2N R2

4 4

NH

O

R1

R2

3

NH

O

R1

R2

3OH

O

R1 1

+

5

NH

N

O OR

R1

R6

Ref: http://en.wikipedia.org/wiki/Carbodiimide

Fig 2.7 Mechanism of EDC reaction with carboxylic group to produce amide.

34

1750 1700 1650 1600 1550 1500

0.025

0.030

0.035

0.040

0.045

0.0501530

1654A

bsor

banc

e

W avenumber cm -1

Fig. 2.8 IR spectrum subtracted from that of VAA showing amide carbonyl peaks after 6-

aminohexonoic acid has reacted with –COOH group of VAA.

Fig. 2.9 Reproduced with permission from American Chemical Society. FTIR spectra of

PP VAA, spacer reacted PP VAA and BSA reacted PP VAA. Inset showing the amide

carbonyl peaks after reaction with spacer molecule.

35

The spacer reacted film was then tested for the attachment of protein onto it by

dipping it in to BSA solution (1 mg/ml BSA in MES buffer solution). The presence of

BSA on the surface was confirmed by the appearance of strong amide carbonyl (>C=O)

peaks at 1654 cm-1 and 1528 cm-1. The characteristic N-H stretching frequency due to

the presence of amide bonds in BSA was observed at 3305 cm-1 (fig. 2.9).

Fig. 2.10 Reproduced with permission from American Chemical Society. PP VAA

spectrum was subtracted from that of BSA showing only the BSA spectrum with

different interval of 1% sodium dodecyl sulfate (SDS) washing.

The covalent bonding of BSA was checked by further washing the BSA attached

film with 1% (SDS) for 1 hr, 2 hr and 4 hrs of duration and the FTIR spectra were taken

after each washing step. It turns out that the >C=O peak intensity remains essentially

constant even after 4 hours of washing with SDS. See fig 2.10.

36

2.3.5 Immobilization of BSA on Plasma Polymerized Glycidyl Methacrylate (GM)

To test the generality of BSA covalent bonding on to the polymer film surfaces,

this work was extended to include one more monomer, glycidyl methacrylate (C7H10O3).

See fig. 2.11 for the structure of GM.

H3C

CH2

O

O

O

Fig 2.11 Structure of glycidyl methacrylate.

Three films were synthesized through plasma polymerization with different thicknesses,

50nm, 150nm, 300nm. Fig 2.12 shows the ATR-FTIR spectra of these three polymer

films; it was observed that the absorbance is proportional to the thickness of the film and

that the spectra were essentially identical with that of VAA. The peaks are assigned as

(a) C=O at the region of 1710 cm-1; (b) –CH3 asymmetric stretch at 2976 cm-1; (c) –CH2

asymmetric stretch at 2930 cm-1; (d) –CH3 symmetric stretch at 2866 cm-1; The C=O

functional group present on the film surface was observed to react with BSA.

37

4000 3500 3000 2500 2000 1500 1000-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.41728 A

B C

Abs

orba

nce

Wavenumber cm-1

(a)

4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8

1.0

2972

33011656

1727

1539

(b) PP GM Water rinsed B SA attached

Abs

orba

nce

Wavenumber cm-1

Fig. 2.12 (a) FTIR spectra of 50nm (A), 150nm (B), 300nm (C) thick GM polymer films

(b) FTIR spectra of PP GM, rinsed PP GM film and BSA reacted PP GM film.

38

.

4000 3500 3000 2500 2000 1500 1000-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

3313

1539

1656 BSA 1 hr SDS washing 2 hr SDS washing 4 hr SDS washing

Abs

orba

nce

Wavenumber cm-1

Fig 2.13 FTIR subtracted spectra of BSA after covalent attachment to the GM film.

In order to test the protein attachment to the surface, the films were dipped in to the BSA

solution and IR spectra were taken. It was observed that the amine functional group

present in protein reacts with the carbonyl terminal of the film and forms amide carbonyl

functional group. This was detected through FTIR and their respective stretching bands

are at 1645 cm-1 and 1528 cm-1. The characteristic N-H peak at 3313 cm-1 was observed

(see fig. 2.13)

39

2.4 Conclusion

A thin film hydrogel was prepared using plasma polymerization of new class of

monomers. The monomers included in the study were 1A2P and 2EAE and the films

formed by the plasma polymerization of those monomers were characterized using ATR-

FTIR technique for the molecular structures and for the moisture uptake property of these

films. It was observed that the hydrogel films formed possess moisture retention property

which was revealed in FTIR spectra. The increase and decrease in O-H absorption band

with changing the films purging time tells about the moisture absorption from the

atmosphere. Variable duty cycle in plasma polymerization was useful to study the

different characteristics of hydrogel film produced under different duty cycle conditions.

From the hydrogel films, it was noted that the thickness of the films under of 10/30 and

10/10 conditions varied linearly with deposition time which was not true in films

produced under continuous wave run, instead, the rate of film formation was decreased

with increase in deposition time. The reason for this presumably arises from an

increasing contribution from film extirpation at relatively high power conditions, an

observation that has been made in prior plasma-polymerization studies.

As said in this work, plasma polymerization technique is a simple approach for

the functionalization of the surfaces with reactive functional groups. These functional

groups can further be used for the attachment of target molecules like protein. This

process involves single, solventless coating method and provides a conformal, pin-hole

free surface. The film can be coated on any solid surface. The polymerization of vinyl

acetic acid and glycidyl methacrylate were shown to have reactive surface species, and

thus it is convenient for the covalent coupling of target biomolecule via simple

40

derivatization reaction. This is a general approach where not only protein but, a host of

other compounds can also be used for covalent attachment. Finally, it should be noted

that this method of plasma polymerization can be used for a range of other monomers and

the technique ATR-FTIR can be very well used to characterize the attached

biomolecules.

41

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2. Zhang, X. Z.; Wu, D. Q.; Chu, C. C. Biomaterials 2004, 25, 3793.

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45

CHAPTER 3

STUDY OF BENZOTRIAZOLE ADSORPTION ON COPPER SURFACE BY

FTIR-ATR SPECTROSCOPY

3.1 Introduction

In integrated circuit fabrication industry one of the processes is “removing” which

means removing material from the silicon wafer either chemically, physically or a

combination of both. The process chemical mechanical polishing (CMP) is the one that

has been followed to remove materials from wafer surface by both chemical reaction and

mechanical grinding to achieve planarization [1]. The choice of process left for the

copper metallization is dual damascene process as copper is very difficult to dry etch.

One of the applications of CMP is the copper interconnection. The components involved

in CMP process are polishing pad, a rotating wafer carrier that holds the wafer face down

against the polishing pad and a slurry dispenser unit. Slurry plays a very important role

in CMP process. The chemicals in the slurry solution react with surface particulates and

dissolve the materials or form chemical compounds that can be removed by abrasive

particles.

Two main kinds of CMP slurries are there for oxide removal and metals removal.

Additives in the slurry control the pH, which affects the chemical reactions in the CMP

processes and helps to achieve optimal process results. During CMP process, the Cu

surface is exposed to different kinds of solutions and corrosion happens. To eliminate

this problem, one of the additives called benzotriazole (BTA) is added in the slurry which

acts as the corrosion inhibitor for copper during CMP process. Benzotriazole (C6H5N3) is

a nitrogen heterocyclic derivative having three nitrogen atoms, each with an unshared

46

lone pair of electrons, forming a five-membered ring structure and widely known as a

good corrosion inhibitor for Cu and Cu-based alloys in different environments [2].

HN

N

N

Fig. 3.1 Structure of benzotriazole.

Extensive studies have been conducted on the adsorption of BTA on Cu surfaces

using various techniques like XPS, XRD, and FTIR etc. Cotton’s studies indicated that

the BTA reacts with Cu and forms protective layers that are inert, thermally stable and

long-lasting on the metal surfaces. The protective action of BTA is because of its ability

to react at Cu surfaces to form a chemisorbed, two dimensional polymeric barrier layers

[3]. He discovered that the triazole ring plays an important role in the formation of

complex with Cu. The Cu(I)BTA complex is formed from cuprous salts with a basic

formula C6H4N3Cu. Cotton suggested that the bonding between cuprous ion and BTA

molecule was achieved by the formation of coordination complex. The proton from the

NH group is replaced by the Cu(I) ion forming a covalent link. A coordination bond

from the lone pair of electrons is also formed with one of the nitrogen atoms and thus it

forms a polymeric chain like structure. In this chapter, the adsorption of benzotriazole on

Cu surface has been characterized using ATR-FTIR technique. In continuance of this

work, the corrosion inhibition effect of BTA on Cu surface will be studied.

47

3.2 Experimental

N-type (111) Si single crystal double polished wafers from polishing corporation

of America, 430 Martin Ave. Santa Clara, CA 95050 (www.pcaSi.com/ - PCA#7493)

were cut in to pieces with dimensions of 6×1 cm and polished at the ends with an angle of

45 using allied high tech products’ techprep/multiprep polishing machine. These Si

crystals are used as attenuated total reflection (ATR) crystals which undergo multiple

internal reflections with IR light. These ATR crystals were used as substrates for copper

deposition. They were cleaned by the procedure described in chapter 2 and after cleaning

they were immersed in 4.9% HF solution for 5 minutes. HF etching of cleaned silicon

substrates removes oxide and terminates the surface with hydride. The hydride

terminated silicon substrate was used for copper deposition through electro-less plating

technique. The IR spectra were taken after each experimental step.

After the IR measurements of Si-hydride crystal were taken, the bevel plane of the

ATR crystal was covered with a tape before immersing it into the copper containing

solution. This was done to prevent the bevel plane from copper deposition on it so that

IR light can pass through in to the crystal and get reflections. Then the silicon hydride

substrate was immersed in 50 ml of 4.9% HF solution containing cupric ions of

concentration 1.5mM for a time period of 20 sec. Cu ions present in the solution were

reduced to metallic Cu at the silicon surface. The crystal was rinsed well with ultra pure

water after removing it from the copper solution. The electroless plated copper film on

silicon surface was observed through optical microscope (Nikon, Japan) to study the

morphology of the surface and then the IR spectrum of Cu/Si crystal was taken which

serves as a reference for BTA adsorption. The crystal was immersed in an aqueous

48

solution of benzotriazole of concentration 0.05M for 5 minutes, rinsed and dried with a

stream of dry air. ATR-FTIR measurements were made after each successive step using

the same equipment and parameters as described in chapter 2. All the experiments were

conducted using PFA lab wares that were cleaned in hot solution of 10% HNO3. The

ultra pure water (UPW) was supplied via a Millipore Milli-Q® Elix 5® ultrapure water

(Millipore Corporation, Bedford, MA, www.millipore.com/ ) purification system

(R>18.2 MΩ/cm) and was used to rinse the samples as well as any dilution of chemicals.

3.3 Results and Discussion

3.3.1 Microscopic and FTIR Results of Hydrogen Terminated Silicon Surface The silicon (111) ATR crystal after SC1 treatment was terminated with hydrogen

using 4.9% HF solution. The oxide for about a thickness of 30 Å developed on the

surface due to SC1 cleaning has been etched away in the process of hydrogen

termination. The hydrogen terminated silicon surface was microscopically smooth as can

be seen from images below at 100x magnification, bright field, and fig 3.2. It has been

reported in the literature that the mean roughness of HF etched silicon surface will be

<0.15 nm [4]. A silicon wafer exposed to HF reacts with the surface silicon oxide, SiO2

as per the following reactions [5],

SiO2 (s) + 6 HF (aq) H2SiF6 (aq) + 2H2O --------------- (1)

H2SiF6 (aq) SiF4 (g) + 2 HF --------------- (2)

49

Fig.3.2 Microscopic image of hydrogen terminated silicon in bright field, 100 x

magnifications.

2300 2250 2200 2150 2100 2050 2000 1950 1900-0.005

0.000

0.005

0.010

2072

2138

2104

2085

Abs

orba

nce

W avenumber cm -1

Fig. 3.3 FTIR spectrum of silicon hydride sample.

The ATR-FTIR characterization of hydrofluoric acid passivated silicon surface

exhibits a complex vibrational spectrum as shown in Figure 3.3. The Si-H vibrational

50

stretching frequency was observed in the region 2050 cm-1 – 2150 cm-1. The

monohydride stretching modes are at 2085 cm-1 and 2072 cm-1 corresponding to the

coupled monohydride (H-Si-Si-H) symmetric and asymmetric vibrational stretching

frequencies. The silicon dihydride band exists at 2104 cm-1 and the trihydride at 2138

cm-1. After the hydrogen termination process, the surface is highly hydrophobic such that

the water contact angle is ~70

3.3.2 Optimization of Electroless Copper Deposition Parameters

Metal deposition on substrates by electroless technique is more advantageous over

the other techniques in terms of parameters like uniformity, super-filling, simplicity and

cost. The application of electroless deposition is vital in semiconductor-devices

industries. A series of experiments were done for the electroless Cu deposition in order

to obtain smooth, fully covered and small nuclei Cu particles on the Si surface. See table

3.1 for the different parameters followed to optimize the deposition. For the optimization

of Cu deposition, a number of small Si chips (1cm×1cm) were used.

The parameters that were under optimization include a) electrolyte for Cu

deposition viz. 4.9% HF and 40% NH4F solutions, b) concentration of Cu ions, c) time of

deposition d) surface pretreatments of Si. In the event of optimizing these parameters, it

was observed through optical microscope that the Si crystal being treated with SC1

solution, annealed at 850C for 30 minutes that undergone Cu deposition for 20s using

4.9% HF containing 1.5mM Cu (highlighted in the table) had smooth and fully covered

Cu particles on the surface as can be seen from microscope images in fig. 3.4. The main

concern for this optimization of Cu deposition parameters is towards the goal of

51

achieving good “IR throughput”. The IR throughput will be diminished totally if the

deposited Cu film is too thick on the Si ATR crystal.

Cleaning Method Hydrogen Termination Cu Deposition

Time of Cu Deposition

SC1 4.9% HF 0.6mM Cu/NH4F 30 min, 60 min, 90 min

Piranha Solution 10 min 40% HN4F 0.6mM Cu/NH4F 5 min

Piranha Solution 10 min 4.9% HF 0.6mM Cu/NH4F 5 min

Piranha Solution 15min 4.9% HF 0.6mM Cu/NH4F 5 min

Cu Deposition Using HF in lieu of NH4F

Piranha Solution 10 min 4.9% HF 0.6mM Cu/HF 5 min

Hot Piranha Solution 10min 4.9% HF 0.6mM Cu/HF 5 min

Piranha Solution (2:1) 4.9% HF 0.6mM Cu/HF 5 min

Piranha Solution (2:1) - Hot 4.9% HF 0.6mM Cu/HF 5 min

Growing Thick Oxide on Si before HF Dipping

SC1, Annealing 850C, 30 min 4.9% HF 0.6mM Cu/HF 5 min

SC1, Annealing 850C, 30 min 4.9% NH4F 0.6mM Cu/NH4F 5min

SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 90 sec

SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 40 sec

SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 30 sec

SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 20 sec

SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 10 sec Table 3.1 Series of experiments conducted to produce uniform Cu coverage on Si.

52

The process of electroless deposition indicates the mechanism of direct

displacement reaction that results in the dissolution of Si by fluoride ions present in the

solution with simultaneous reduction of Cu ions on the Si surface by obtaining the

electrons released by Si-H. The possible oxidation/reduction reaction for the Cu

deposition on HF-treated silicon surfaces can be represented as,

2Cu2+ + Si + 6HF 2Cu(s) + SiF62- + 6H+ --------------- (3)

a) Immersion time: 90s b) Immersion time: 40s

c) Immersion time: 30s d) Immersion time: 20s

Fig. 3.4 Microscopic dark filed images of copper deposited on silicon substrate for

different times of immersion.

53

3.3.3 Contact Angle Measurements

The selection of annealing process as an initial treatment, to grow thermal oxide

good coverage of Cu particles as the surface

ecome

d.

.

Si

easurements of Cu on Si surface with different times of

eposition.

on Si surface was found suitable for a

b s smooth and homogeneous after the HF etching. Along with the next step of

optimizing the Cu deposition time, contact angles of the Cu/Si surface were measure

Contact angles were measured using a contact angle goniometer which is a very useful

tool in studying the nature of the surface. Table 3.2 gives the contact angles of Cu/Si

surfaces having different times of Cu deposition. It was observed that the contact angle

of Cu deposited for 90s was 95 which is pretty close to that of a pure Cu metal surface

From this information, it can be said that the Si surface was fully covered with Cu

particles which was evidenced from microscopic results. The contact angle was

decreased as the deposition time was decreased indicating that the Cu coverage on

surface was decreasing.

Table 3.2 Contact angle m

Time of deposition in 1.5mM Cu

Nature of Cu film from Microscope observation

Contact angle with water

90 sec Uniform, Dense particles 95

40 sec Uniform, Less dense 68-75

30 sec Uniform 50-55

20 sec Uniform and Good Coverage 40

10 sec Dispersed particles 30

d

54

3.3.4 FTIR-ATR Characterization of Cu/Si and BTA/Cu/Si Samples

Fig. 3.5 depicts the FTIR spectra of Si ATR crystals that had Cu deposition for 5,

d that the increase

in the t

4F

lectrolyte A) 60 minutes B) 25 minutes C) 5 minutes

0.05M of BTA for 5 min and IR

pectra were taken. Fig. 3.6 shows the FTIR spectrum of BTA adsorbed on Cu/Si

substrate. It has been reported that the BTA molecule loses the hydrogen atom from its

4000 3500 3000 2500 2000 1500

0.0

0.2

0.4

0.6

0.8

1.0

25 and 60 minutes in 40% NH4F solution. From the spectra, it is note

ime of Cu deposition results in the increase in the absorbance of the spectra. The

Cu film deposited using 4.9% HF was found to be uniform, smaller sized particles and

having good IR signal. The signal was diminished for all the copper deposited Si ATR

crystals except for the one that had 20s Cu using 4.9% HF.

1.2 A B

C

Abs

orba

nce

W avenumber cm-1

Fig. 3.5 FTIR spectra of three samples with different durations of Cu deposition in NH

e

The Cu/Si sample after rinsing was immersed in

s

55

triazole

on

m-1 showing aryl C-H stretching.

the corrosion protection on Cu was due to the

rmation of a complex polymeric surface film on Cu. The bonding structure and

ied by many investigators

using various techniques [9-14]. However, the effect of concentration of BTA plays an

4000 3500 3000 2500 2000

0.00

0.01

0.02

0.03

ring and the Cu is bonded to the nitrogen atom. The spectrum is in agreement

with Poling’s [5] work in that the signature IR band for the BTA molecule adsorbed

the Cu surface in IR spectrum is aryl C-H stretching at 3061 cm-1. The evidence that the

BTA molecule was chemisorbed on the Cu surface is the appearance of aryl C-H

stretching band at 3061cm-1.

0.04

3061

Abs

orba

nce

Wavenumber cm-1

Fig. 3.6 FTIR spectra of BTA adsorbed on Cu for 5 minutes in the region 4000 – 2000

c

3.3.5 Concentration Effect of BTA on the Adsorption on Cu

Many papers indicated that

fo

adsorption behavior of Cu and BTA has been extensively stud

56

0.0156

0.0158

0.0160

3100 3075 3050 3025 30000.0146

0.0148

0.0150

0.0152

0.0154

30611mAbs

(a)

im nt role in corrosion inhibition of Cu. The concentrations of BTA used in m

cases were about 1-50 mM, and people have studied the properties of the treated

solutions such as pH, corrosion anion concentrations like Cl

ceso

rban

Ab

Wavenumber cm-1

porta any

.

. With

and

.6

- and SO4- etc. J-H Chen et al

have studied the growth kinetics and structure of CuBTA film as a function of BTA

concentration and in further the inhibition mechanism. They found that the inhibition

effect is poor in low concentration of BTA. The critical concentration is 0.17 mM

this concept, the adsorption of BTA on Cu surface was studied through FTIR-ATR as a

function of BTA concentration. Three concentrations were used viz. 0.01 M, 0.05 M

0.5 M. The time of immersion of Cu sample in BTA aqueous solution is 5 min. Fig. 3

shows the result. It was observed that the intensity of aryl C-H peak at 3061 cm-1 is

increased as the BTA concentration is increased.

57

1mAbs

3100 3050 30000.015

0.016

0.017

0.018

0.019

Abso

rban

ce

Wavenumber cm -1

(b)

ig. 3.7 FTIR spectra of Cu sample immersed in different concentrations of BTA

) 0.01 M BTA; (b) 0.05 M BTA; (c) 0.5 M BTA.

3200 3150 3100 3050 3000 29500 .030

0 .032

0 .034

0 .036

0 .038

W avenum ber cm -1

3061

Abs

orba

nce

1 mAbs

(c)

F

(a

58

3.4 Conclusions and Future Investigations

Benzotriazole adsorption on Cu surface was investigated with FTIR-ATR

ing at 3061 cm-1. BTA concentration

ace have been conducted. It was observed

.

Benzotriazole-5-carboxylic acid

1-(2-Pyridylcarbonyl)benzotriazole

technique. It is known that the BTA forms bond with Cu surface through its triazole ring

and IR results show the presence of aryl C-H stretch

dependent studies for adsorption on Cu surf

that the absorbance of aryl C-H band was proportional to the concentration of BTA

My future investigations are going to be on the study of corrosion inhibition effect

of BTA on Cu surface in low concentrations and various pH conditions. The corrosion

inhibition effect of many BTA derivatives such as in the figure 3.8

Benzotriazole-1-carboxamide

Fig. 3.8 Structures of BTA derivatives.

59

will also be studied. It was reported that the bu tached to the BTA lky group at molecule

eric effect and thereby prevents further oxidation of Cu [13-15].

Sec ruc re, th surface of Cu is left with

organic impurities like BTA which is used as one of the constituents in the slurry. These

inhibitors and other organic impurities have to be removed from the Cu surface before the

successive processes. Hence, my focus would be finding a method to remove BTA from

Cu surface without changing the nature of Cu. Thirdly, my aim is to establish a

methodology to deposit ruthenium (Ru) on Si as Ru can be used as a barrier layer for Cu

diffusion into Si at higher temperatures. The effect of BTA on Ru will also be studied.

causes st

ondly, in the process of dual damascene Cu st tu e

60

3.5 Chapter References

. Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Prentice-

Hall Inc., Upper Saddle River, New Jersey, 2001

. J.B.Cotton, Proc, 2nd Int. Congr. Metallic Corrosion, NACE, New York,1963.

3. J.B. Cotton and I.R. Scholes, Br. Corrosion Jnl., 1967, 2, 1.

4. Oliver M. R. Chyan, Jin-Jian Chen, and Hsu Y. Chien; J Electrochem. Soc., 1996,

143, 1.

5. Fred Meyer and John B White, Semiconductor International, Silicon Engineering

Technology Center Laboratory, 1999, July, 137.

6. G. W. Poling, Corros. Sci. 1970, 10, 359.

7. S. Thibault, Corros. Sci. 1977, 17, 701.

8. F. Manfeld and T. Smith, Corrosion, 1973, 29, 105.

9. D. Chadwick and T. Hashemi, Corros. Sci., 1978, 18, 39.

10. S. L. Cohen, V. A. Brusic, F. B. Caufman, G. S. Frankel. S. Motakef and B. Rush.

J. Vac. Sci. Technol., 1990, A8, 2317.

11. R. R. Thomas, V. A. Brusic and B. M. Rush, J.Electrochem. Soc., 1992, 139, 678.

12. Chen Jin-Hua, Lin Zhi-Cheng, Chen Shu, Nie Li-Hua and Yao Shou-Zhuo*

Electrochemica Acta, 1998, 43, 3-4, 265-274.

13. C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg and C. Leygraf; J.

Electrochem. Soc., 1989, 136, 1.

14. L. Tommesani, G. Brunoro, A. Frignani, c. Monticelli and M. Dal Cole Corrosion

Science, 1997, 39, 7, 1221-1237.

1

2

61

15. N. Huynh, S.E. Bottle, T. Notoya, D.P.; Schweinsberg Corrosion Science, 2000,

42, 259-274.

62