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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 8, AUGUST 2013 1819

DLC Coating by HiPIMS: The Influence of

Substrate Bias VoltageSetsuo Nakao, Ken Yukimura, Shizuka Nakano, and Hisato Ogiso

Abstract— Carbon-related materials are prepared using var-ious physical vapor deposition (PVD) methods. High-powerimpulse magnetron sputtering is a PVD method that uses glowplasma and this method is employed to prepare diamond-likecarbon (DLC) films. The densities of the glow current and theconsumed power for an effective area of plasma generation are

1.4 A/cm2 and 1.2 kW/cm2, respectively. A pulsed bias is appliedto the substrate (subsequently called substrate bias voltage).The pressures of the background gas and the substrate biasvoltages influence the surface morphology and the roughnessof the deposited films. It is discovered that a critical pressureof 0.3 Pa and a critical bias voltage of −100 V is needed tochange the DLC film characterization. A drastic change in thesecharacteristics is seen for pressures <0.3 Pa; a bumping surface

and dumpling-like aggregations are produced. In addition, thereis a bias-voltage dependency on these films, as smoother surfacesare seen in a bias-voltage range that is higher than −100 V.This phenomenon may be related to the DLC structure, whichis evaluated by Raman parameters of the deposited films. It isfound that the position and the full width at half maximum of thegraphite peak show a minimum and a maximum, respectively,at a bias voltage of −100 V. The results of X-ray photoemission

spectroscopy reveal that the sp3 bond ratio indicates a maximumat a pressure of 0.3 Pa and at a bias voltage of −100 V. Thus,it is clear that the gas pressure of 0.3 Pa and the bias voltageof −100 V are critical values that change the pressure and biasdependence of the film characteristics.

Index Terms— Ar gas pressure, atomic force microscopy,diamond-like carbon films, high-power pulsed magnetron sput-

tering, Raman spectroscopy, substrate bias voltage, X-rayphotoemission spectroscopy.

I. INTRODUCTION

PHYSICAL vapor deposition (PVD) is a film-deposition

method that uses metallic species, atoms, molecules and

ions, and this method is widely used in many areas such

as automotive industries and biomedical applications [1], [2].

In addition, plasma enhanced chemical vapor deposition

(PECVD) methods are used to prepare a variety of films that

consist of metals and submetals, and their compounds. Various

Manuscript received December 4, 2012; revised March 14, 2013; accepted

March 26, 2013. Date of publication June 6, 2013; date of current versionAugust 7, 2013.

S. Nakao is with the Materials Research Institute for Sustainable Devel-opment, National Institute of Advanced Industrial Science and Technology,Nagoya 462-8560, Japan (e-mail: [email protected]).

K. Yukimura is with the Nanoelectronics Research Institute, NationalInstitute of Advanced Industrial Science and Technology, Tsukuba 305-8568,Japan (e-mail: [email protected]).

S. Nakano and H. Ogiso are with the Advanced Manufacturing ResearchInstitute, National Institute of Advanced Industrial Science and Technology,Tsukuba 305-8564, Japan.

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2013.2256800

sources are used for PVD deposition, namely, electron beam,

vacuum arcs, and magnetron sputtering glows. In vacuum arcs,

high-hardness diamond-like carbon (DLC) films are prepared

and several types of filtering systems are developed because

generally there is a drawback on generating macroparticles in

that the film characteristics deteriorate [3], [4]. In magnetron

sputtering, glow plasmas are advantageous because they avoid

macroparticles during the coating process. However, many of

the sputtering carbon species stay in a neutral state as they

travel in space, and as a result, they cannot be controlled by

electric and magnetic fields.

Recently, high-power impulse magnetron sputtering

(HiPIMS) technologies have been developed, and they

have great potential for material processing and the surface

modification of substrates. The reasons why HiPIMS

technology is promising for material processing are based

on the utilization of charged particles for deposition:

1) there is easy control of film structure and stoichiometric

film preparation; 2) the charged particles that are conducted

to the substrate are appropriate for a conformal deposition

into a 3-D substrate, or in other words, good coverage is

attained; and 3) selective deposition is possible [1], [5]. In

the generation of HiPIMS plasma, the peak power density is

higher than 1 kW/cm2, which is more than a factor of 100

greater than those densities that are obtained by conventional

direct-current magnetron sputtering (dc-MS) systems. The

current density is over 1 A/cm2. In addition, HiPIMS

glow plasma technology for material processing has the

following potential advantages: 1) a high adhesion ability [6];

2) anti-corrosion [6]; 3) a uniform coating [7]; and 4) a

dense structure with a relatively flat surface [8]. In contrast, a

major disadvantage of HiPIMS is a low deposition rate that is

caused by the high ionization rate of the plasma species [1].

However, it is also reported that there is a possibility of

improving the deposition rate to explore both the experimental

setup [9] and the deposition conditions [10].

Carbon-related materials have promising applications to

different fields of industries from electronics, and mechan-ics, to bioindustries because there are different allotropes

of carbon-related materials such as graphite, graphene, dia-

mond, and diamond-like carbon; these material differences

are based on their atomic combinations [11]. When CVD is

used in carbon-related film preparation, hydrocarbon gases

are decomposed in both inductively coupled plasma (ICP)

sources [12] and microwave plasma sources [13] to deposit

carbon species onto substrates. DLC films attracted a great

deal of attention as a hard coating because of their excellent

properties, such as a high degree of hardness, a high elastic

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1820 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 8, AUGUST 2013

modulus, low friction, optical transparency, and chemical

inertness [14], [15]. Therefore, DLC films are prepared by

many methods, such as sputtering [15], [16], vacuum arc

deposition [17], PECVD [18], pulsed laser deposition [19],

[20], ion beam deposition [21], radio frequency (RF) plasma

deposition [22], and dielectric barrier discharges [23]. Addi-

tionally, hybrid systems of these techniques with a plasma-

based ion implantation (PBII) are proposed [2], [24]. A hollow

cathode’s discharges are combined with PBII technology to

increase the deposition rate [2]. In addition, there is an

example of a hybrid system of RF-ICP that is combined with

PBII [12]. In this case, DLC is used to pretreat polyethylene

terephthalate films to reduce oxygen permeation. A critical

advantage of hybrid systems is that they may result in the

relaxation of the residual stress in the deposited films [25],

where the relationship between the stress relaxation and ther-

mal spike [26] is closely related to an energetic ion bom-

bardment. The combination of cathodic arcs and a sputtering

process was also proposed [27].

In CVD cases, a significant quantity of hydrogen may be

incorporated into the deposited films. In contrast, in PVDcases, carbon is supplied directly from the solid carbon

(graphite) materials. Therefore, it is advantageous that, in PVD

cases, less contamination is included in the deposited films.

In addition, it is also reported that the electron-field-emission

characteristics of DLC films could be improved by doping

metal species [28].

It is a promising way that DLC films are closely fabri-

cated by controlling the behavior of the carbon ions when

denser carbon ions are contained in the process to realize

a higher hardness and a smoother film surface. HiPIMS

technologies have great potential to obtain DLC films with

these properties [29]–[31], because plasma generated through

the HiPIMS technique features high rates for the ionizationof the background gas and the sputtering of the metallic

species [32]–[36].

Even in HiPIMS, the carbon ionization rate is <5% [1],

[30] and it is worthwhile to use HiPIMS technology to prepare

carbon films on a substrate that is near the plasma source.

Partly high-density carbon films are deposited, and they have

a density of 2.7 g/cm3 [26]. In addition, although there are

few results in carbon films that used high-power-consumed

glow plasma, some results include a high deposition rate [37].

Indeed, a hybrid-plasma-generation system is possible to con-

trol the film microstructures and the deposition rate [27].

In DLC film deposition, the microstructure of the films is

affected by the preparation conditions, which are related tothe amount of ion species and ion energy [38]. The pressure

of the background gas, which is typically argon (Ar), also

influences both the sputtering characteristics and the deposi-

tion characteristics, because the sputtering pressure has a great

influence on the stages of the ejection of sputtered carbon, the

transport of the species and their arrival and condensation onto

the growing film surface [39]. The substrate bias voltage has

a direct influence on the ion energy of ion species that should

also affect the microstructure of the films. Few parameter

surveys of DLC deposition are carried out, which is most likely

because of a low ionization rate of sputtered carbon species.

Fig. 1. Experimental apparatus.

Hence, the pressure dependence is not always clarified in DLC

film deposition in an HiPIMS process. In [40], the influence

of the background gas pressure on the microstructure of DLC

films prepared by the HiPIMS system was examined by Raman

spectroscopy. It is found that the pressure ∼0.3 Pa may be

suitable for sp2 phase disordering and less sp2 clustering,

relating to the increase of sp3 C–C configuration in the films.

However, the effects of the pressure on the film characteristics,

such as surface morphology and sp3 ratio, are not reported.

In this paper, DLC films are prepared by HiPIMS under the

different conditions of the pressures and the bias voltages, andin particular, the influence of the bias voltage that is applied to

the substrate is examined by Raman spectroscopy. The changes

in not only the surface morphology but also the sp3 ratio are

also examined as a function of the Ar gas pressure as well

as the substrate bias voltage. Therefore, the surface becomes

smoother with increases in the pressure and the bias voltage.

The results of Raman analysis suggest that the sp3 phase that

is related to the disordered sp2 phase may reach its maximum

quantity at a bias voltage of −100 V. In addition, the results

of X-ray photoemission spectroscopy (XPS) analysis reveal

that a pressure of 0.3 Pa and a bias voltage of −100 V are

more favorable to sp3 phase formation under the current exper-

imental conditions. Hence, there are notable critical values for

the ambient gas pressure and the substrate bias voltage that

determine the film properties. In addition, the relationships

between the electrical characteristics of the HiPIMS system

and the pressure and the bias voltage dependences on the

microstructure of the films are also discussed.

I I . EXPERIMENTAL A RRANGEMENT

Fig. 1 shows a schematic diagram of the film deposition sys-

tem [41], which includes plasma generation and ion extraction

at a substrate holder electrode (SHE). The signal flows that are

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NAKAO et al.: DLC COATING BY HiPIMS 1821

used to control the plasma generation and the ion extraction are

also shown in the diagram. An unbalanced magnetron (UBM)

sputtering plasma source is located in the upper side of a

vacuum chamber with the dimensions of 340 × 550 × 380

(W × L × H) mm3, where carbon (C) is the target material

and the effective target diameter is 150 mm. A pulsed negative

voltage of 1.2 kV with a duration of 50 µs is applied to

generate plasma in an Ar gas environment.

The pulse duration of the UBM glow plasma is determined

by turning on and off the timing of the insulated gate bipolar

transistor (IGBT) as a closing switch; that is, the IGBT is

turned on by applying a trigger signal (shown as TS1) from the

pulse generator, which is abbreviated as PuG, and the IGBT

is turned off by applying a signal that is generated in the

dc power-source unit. After the glow initiation, the plasma is

stably generated until the IGBT is turned off. A series resistor,

R1, with a resistance of 5 is provided to limit the current

through the plasma generation circuit.

A pulse modulator, which is abbreviated as PuM, is used

to apply a series of pulsed voltages to the SHE. The pulsed

voltage is applied to the SHE at a designated time that isdelayed from the TS1 signal and TS2 is a delayed signal that

is set in the PuG. The time duration of the pulse applied to

the SHE is set in the PuM, and in addition, the PuM is used

to adjust the output voltage. Thus, the PuM could set both the

output voltage and its time duration, although the start of the

pulse output is determined by the TS2 signal. By applying a

pulsed negative voltage to the SHE, ions are extracted from

the glow plasma.

The Ar ions produced in the UBM glow plasma are accel-

erated toward the C target, which is sputtered by an energetic

Ar ion bombardment. When the electrode is immersed in the

glow plasma, a pulse-modulator-output voltage on the order

of several tens to hundreds of volts is enough to extract ionsfrom the glow plasma.

The evacuation system consists of a turbo molecular pump

(TMP) as the main evacuator, a mechanical booster pump, and

a rotary pump that functions as a back-up system of the TMP.

The inlet gas pressure is controlled by a mass flow controller.

In addition, the current through the circuit is observed using

a current monitor (Pearson, model 411), the target voltage is

observed using a voltage probe (Tektronix, model P5100), and

waveforms are monitored by an oscilloscope (Tektronix, model

303 4B).

DLC films are deposited on silicon (100) substrates. Typical

deposition conditions are as follows: a negative pulse voltage

of −1 kV is initially applied at the target for 5 min, then thevoltage is increased to −1.2 kV and maintained for 20 min.

Thus, the whole deposition process-time is 25 min for all

samples. The repetition frequency of the target pulse voltage is

1 kHz. Ar gas is used as a background gas to sputter the target.

The pulse durations of the target voltage and the substrate

bias voltage are 50 and 10 µs, respectively. In addition, the

delay time of the substrate-bias-voltage application is set at

105 µs from the target voltage application to avoid the transit

of an arc discharge on the substrate from a magnetron sputter

discharge. Therefore, the substrate bias voltage is applied in an

after-glow region where the stable operation of the deposition

process is enabled. The distance L between the target and the

SHE is 175 mm; the deposition is carried out under different

conditions of Ar gas pressure and substrate bias voltage. The

pressure is varied from 0.11 to 0.52 Pa by changing the flow

rate of the Ar gas at a constant substrate bias voltage of −60 V.

In addition, the bias voltage is varied from −30 to −180 V

under a constant Ar gas pressure of 0.18 Pa.

The thickness of the film is measured by observing a cross-

sectional image with a scanning electron microscope. The

Raman spectra are measured by a Renishaw inVia micro-

Raman spectrometer and an Ar ion laser with a line of

514.5 nm is used and focused in a spot 1 µm in diameter

on the surface of the films. The laser power and typical

measurement time are 10 mW and 10 s, respectively. Atomic

force microscopy (AFM) images are taken by KEYENCE

VN-8000, and the average surface roughness ( Ra ) is obtained

in a square area that is 2 × 2µm. XPS spectra are measured

by Thermo VG Scientific Sigma Probe using an Al Kα line

of 1486.6 eV.

III. RESULTS AND D ISCUSSION

A. Electrical Characteristics

Fig. 2(a)–(f) shows the temporal behaviors of the target volt-

age, the target current through the circuit (the glow current),

the power and energy that are consumed in the plasma, and

the ion current that is extracted by the substrate bias voltage

(the substrate current), respectively. The Ar gas pressure is

varied from 0.11 to 0.52 Pa for Fig. 2(a)–(f) and the substrate

bias voltages are −30 and −180 V with durations of 10 µs

for Fig. 2(f). The bias voltage is applied 105 µs after the

target’s voltage application (= TS2–TS1). In addition, the

source voltage applied to the target is −1.2 kV and its pulse

has a duration of 50 µs.The target voltage and the glow current are denoted by

V T and i , respectively, and the consumed electrical power P is

calculated by P = V T × i . The energy E D that is consumed

in the glow plasma is calculated by

E D =

τ0

P(t )dt (1)

where τ is the pulse duration of the source voltage (= 50 µs).

The average power consumed in the plasma is calculated

by product of E D and the repetition rate of the pulse. We

confirmed that the source voltage remained constant during the

pulse. The average power consumed in the plasma is calculated

by the product of E D and the repetition rate of the pulse.The target voltage shows the source voltage at the moment

of the voltage application because no glow plasma can be

ignited in < 2 µs. It is observed that the plasma ignition

time is not altered for various ambient pressures under the

experimental conditions. However, the current rise time after

the plasma ignition is altered by the gas pressure in this

way: the ignition time becomes shorter with a lowering gas

pressure because the electron’s mean free path at 0.11 Pa is

over 300 mm long, and as a result, the ionization rate of the

ambient gas is less than the same rate at 0.52 Pa. After 10 µs

have passed as the voltage is first applied, the ionization is

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-1200

-800

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0

T a r g e t v o l t a g e [ V ]

(a)

1

6

0

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40

60

80

T a r g e t c u r r e n t [ A ]

12

3

456

(b)

0

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40

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80

P o w e r [ k W ]

(c)1

6

0 20 40 60 80

0

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E n e r g y [ J ] (d)

1

6

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0

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S u b s t r a t e c u r r e n t [ A ]

1 2 3 4 5 6

(e)

100 104 108 112 116 120 124

0

0.1

0.2

0.3

0.4

0.5

0.6

S u b s t r a t e c u r r e n t [ A ]

Time [ s]

1

(f)

2

1: -30 V2: -180 V

Fig. 2. (a) Waveforms of the target voltage. (b) Current through the circuit,(c) consumed power and (d) consumed energy in the plasma, and (e) and(f) extracted substrate current at 105 µs after the target voltage’s applicationunder a variety of Ar gas pressures that ranged from 0.11 to 0.52 Pa. Inaddition, the substrate bias voltage ranged from −30 to −180 V (the insertedseries resistance is 5 , the pulse repetition rate is 1 kHz, and the sourcevoltage applied to the carbon target is −1.2 kV). In Fig. 2(a)–(e), 1: 0.11 Pa,2: 0.18 Pa, 3: 0.24 Pa, 4: 0.30 Pa, 4: 0.40 Pa, and 5: 0.52 Pa.

facilitated by lowering the Ar gas pressure. At the end of the

pulse, the glow current becomes greater with a decreased Ar

gas pressure. Thus, both the consumed peak power and the

average power are the greatest at 0.11 Pa. It is shown that the

substrate current depends on the Ar gas pressure. In addition,

0.1 0.2 0.3 0.4 0.5 0.60

0.4

0.8

1.2

1.6

2.0

Argon gas pressure [Pa]

C o n s u m e d e n e r g y [ J ] (a)

20 40 60 80

20

40

60

80

100

120

0

0.2

0.4

0.6

0.8

1.0

1.2

0

Current at 105 s [A]

I o n c u r r e n t a t 1 0 5 s

[ m A ]

(b)

I o n

d e n s i t y a t 1 0 5 s

[ x 1 0 1 7 m - 3 ]

Fig. 3. (a) Consumed energy as a function of the Ar gas pressure.(b) Extracted ion current 105 µs after the target voltage’s application as afunction of the glow current at the end of the pulse. The applied substratepulse voltage is −60 V with a duration of 10 µs. The ion density is calculatedon the basis of the ion saturation current.

a greater glow current causes a greater extracted ion current

through the SHE, which suggests that the energy of the ions

should be changed by the substrate bias voltage but the amount

of extracted ions is almost the same at a constant pressure.

Therefore, the consumed energy increases when the Ar gaspressure is decreased, as shown in Fig. 3(a).

The ion density in the postplasma state at 105 µs is

∼1 × 1017 m−3 at 0.11 Pa, which is calculated under an

assumption of an ion saturation current at an electron temper-

ature of 2 eV. Thus, Fig. 3(b) shows that the plasma density

is on the order of 1016 m−3 at 105 µs under the experimental

conditions.

The instantaneous peak power that is consumed in the

plasma and the peak current at 0.11 Pa are ∼54 kW and

65 A, respectively. The consumed energy is ∼1.7 J for every

application of the voltage. The average power is varied with

the pressure in a range of 1.0 to 1.7 kW. Whereas the power

corresponds to a power density of ∼1.2 kW/cm2, the currentdensity is ∼1.4 A/cm2 at the target surface. These densities are

calculated for an effective target area of ring-shaped plasma

with an area of ∼46 cm2.

B. DLC Film Deposition and Characterization

1) Deposition Rate: Fig. 4 shows the deposition rates

of deposited DLC films for various substrate bias voltages.

The deposition rate is obtained by dividing the film thickness

by the total processing time of 25 min. It was reported [40]

that the thickness decreased with an increased Ar gas pressure,

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0

4

8

12

16

20

(1) (2) (3) (4) (5)

D e p o s i

o n r a t e [ n m / m i n ]

Substrate bias voltage [V]

(1) -30 V, (2) -50 V, (3) -100 V,

(4) -140 V, (5) -180 V

Fig. 4. Deposition rate as a function of the substrate bias voltage (the processtime is 25 min because of 5 min of preparation at a source voltage of 1.0 kVand 20 min of preparation at a source voltage of 1.2 kV).

which resulted in a decreased deposition rate for the pressure

in a range of 0.11 to 0.30 Pa. However, the deposition rate

tended to be constant for the pressures over 0.30 Pa. Incontrast, although the deposition rate does not have a clear

dependence on the bias voltage being in the range of −30 to

−180 V, this rate is likely to be at its minimum at −100 V,

as shown in Fig. 4.

2) Atomic Force Microscopy: The influence of the pressure

on the surface morphology of prepared DLC films is discussed.

Fig. 5(a)–(f) shows the AFM images of the surface morphol-

ogy of prepared DLC films at different pressures and bias

voltages. A bumpy surface is observed at 0.11 Pa, where many

aggregations have the shape of a dumpling and an average

diameter of 250 nm. The surface becomes smoother when the

pressure is increased, which results in a smaller grain size

that is ∼50 nm in average diameter at a pressure of 0.52 Pa,as shown in Fig. 5(c). Concerning the influence of the bias

voltage on the surface morphology, a similar trend on the

surface morphology is observed. Whereas a bumpy surface is

observed at −50 V, as shown in Fig. 5(d), a smoother surface

appears when the bias voltage is increased. Fig. 6(a) and (b)

shows the average surface roughness ( Ra ) of the prepared

DLC films at different pressures and bias voltages. The Ra

is extraordinary high at ∼ 3.1 nm under a pressure of 0.11 Pa

and the Ra is in a range that is smaller than 0.7 for pressures

that are higher than 0.24 Pa. In a detailed observation, the

Ra decreases with an increase in the Ar gas pressure that

raises it to 0.3 Pa. Upon further increases of the pressure, the

Ra almost remains constant at ∼0.5 nm. Concerning the biasvoltage dependence on the Ra , although the Ra is 0.7 nm

under a bias voltage of −50 V, it decreases with increases

in the bias voltage that raise it to −100 V. Upon a further

increase of the voltage, the Ra does not change significantly.

Thus, it can be mentioned that the pressure of 0.30 Pa and the

bias voltage of −100 V are both critical values for showing

the change in the surface’s roughness.

3) Raman Spectroscopy: Fig. 7 shows the Raman spectra

of the DLC films that are prepared at different bias voltages.

For all of the samples, there are two broad peaks of disorder-

bands and graphite-bands in the range of 800–2000 cm−1. It is

well known that these peaks are referred to as the D peak and

G peak, respectively. In the previous paper [40], we reported

that the intensity of these peaks decreased when the Ar gas

pressure was increased to 0.3 Pa and the intensity adversely

increased upon a further increase of the pressure. A similar

trend is also shown in Fig. 7 when the substrate bias voltage

is changed: the intensity decreases when the substrate bias

voltage is increased to −100 V, and the intensity increases

upon a further increase of the voltage. The deconvolution of

the spectra into these two bands is carried out by a curve-fitting

procedure using Gaussian functions, and then, the intensity

ratio of the D peak to G peak, which is denoted I ( D)/ I (G),

and the G peak’s position and full width at half maxima

(FWHM) are obtained. An ambient pressure was varied at

a bias voltage of −60 V in the previous paper [40]. It is

found that I ( D)/ I (G) linearly decreased from ∼3.8 to 3.3

with increases in the Ar gas pressure. In a pressure range up to

0.3 Pa, the G peak’s position shifted the Raman wavenumbers

from 1584 to 1571 cm−1 and the width at the G peak increased

from 125 to 144 cm−1. In contrast, in a pressure range over

0.3 Pa, the G peak’s position tended to slightly increase to1578 cm−1, and its width tended to peak at 0.3 Pa, which is

followed by a significant decrease for higher pressures.

Fig. 8(a)–(c) shows I ( D)/ I (G), the G peak’s position and

the FWHM of the G peak, respectively, when a bias voltage

is varied at a pressure of 0.18 Pa. Concerning the influence of

the bias voltage on the DLC film’s characteristics, I ( D)/ I (G)

does not change significantly relative to the substrate bias

voltage. In addition, the changes in the G peak’s position

are not always clear against the substrate bias voltage. How-

ever, this position is likely to be at a minimum at a bias

voltage of −100 V. The FWHM of the G peak apparently

indicates a maximum of 149 cm−1 at −100 V. Thus, it

can be mentioned that the pressure of 0.30 Pa and the biasvoltage of −100 V are critical to representing the DLC film

characteristics.

4) X-ray Photoemission Spectroscopy: Fig. 9 shows a typ-

ical XPS spectrum of DLC films that are prepared under

pressure of 0.11 Pa. The peaks caused by C1s and O1s

are observed. In addition, the same spectra are commonly

observed for all samples. The O1s signal originated from

the absorbed oxygen at the sample surface, because of their

exposure to air after the film preparation. Fig. 10 shows an

XPS spectrum with a higher resolution that is near the binding

energy of the C1s peak for the film of Fig. 9, where curve

fitting by the Lorenz (L) and Gaussian (G) mix function is

carried out under the assumption of an L/G ratio of 10%that peaks at sp2–C, sp3–C, and C–O. The background is

fitted by the Shirley equation and the spectrum is standardized

using a peak of sp2–C with a binding energy of 284.4 eV.

A shoulder peak at the energy of 285.8 eV and a broad peak

with a small intensity at the approximately energy of 288 eV

are also observed. Whereas the former peak is ascribed to

an sp3–C bonding state, the latter peak is ascribed to C–O

bonds. Fig. 11(a) and (b) shows the intensity ratio of sp3–C to

(sp2–C + sp3–C) at different pressures and bias voltages,

respectively. The proportion of sp3–C is ∼20% at 0.11 Pa and

it increases to ∼28% with increases in the Ar gas pressure

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0 [nm]

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10

20

(a)

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1

3

2

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1500

1000

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500

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12

8

(c)

1500

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500

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8

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500

1000

1500

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4

8

(e)

1500

1000

500

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500

1000

1500

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8

(f)

Fig. 5. AFM images on the surfaces of DLC films that are prepared at: (a) 0.11 Pa, (b) 0.3 Pa, (c) 0.52 Pa, (d) −50 V, (e) −100 V, and (f) −180 V.The samples (a)–(c) and (d)–(f) are prepared at −60 V and 0.18 Pa, respectively.

that raise it to 0.3 Pa. However, the proportion of sp3–C

appears to decrease if there are more increases in the Ar

pressure. The trend is similar in the case of a substrate

bias voltage. Although the proportion of sp3–C increases to

∼27% when the bias voltage is increased to −100 V, it

decreases upon further increases of the voltage. Thus, the

maximum sp3–C content can be obtained at 0.30 Pa and at

−100 V.

C. Discussion

The deposition rate, which is defined as the film thickness

divided by the processing time (thickness/time) of DLC films,

decreased with increases in the Ar gas pressure, especially

if the pressure was <0.3 Pa, as previously reported in [40].

As shown in Fig. 2(b), the target current decreases when

the pressure is increased to 0.3 Pa, but the current tends to

saturate upon a further increase of the pressure. Thus, the flux

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0

1

2

3

4

(1) (2) (3) (4) (5)

S u r f a c e r o u g h n e s s [ n m ]


(1) 0.11 Pa, (2) 0.24 Pa, (3) 0.3 Pa,

(4) 0.4 Pa, (5) 0.52 Pa

(a)

0

0.5

1

1.5

(1) (2) (3) (4) (5)

S u r f a c e r o u g h n e s s [ n m ]


(1) -30 V, (2) -50 V, (3) -100 V,

(4) -140 V, (5) -180 V

(b)

Fig. 6. (a) Average surface roughness of the prepared DLC films versus theAr gas pressure. (b) Substrate bias voltage.

0

2000

4000

6000

8000

800 1000 1200 1400 1600 1800 2000

I n t e n s i t y [ a . u . ]

Raman shi [cm-1]

1: -30 V

2: -100 V

3: -180 V

1

2

3

Fig. 7. Raman spectra of the DLC films that are prepared for differentsubstrate bias voltages.

of energetic Ar ions that are required to bombard to the C

target decreases when the pressure is increased to 0.3 Pa,

then it becomes saturated when the pressure is >0.3 Pa.

Therefore, we suggest that the decrease of the deposition rate

in this pressure range is caused by a decrease in the particle

density of the sputtered C species that occurs when the Ar

gas pressure is increased. In other words, the sputtered C

species decreases when the pressure increases because of the

reduction of both the consumed energy and the plasma density,

as shown in Fig. 3. In addition, the sp3–C ratio increases

in the film as the pressure increases to 0.3 Pa, as shown in

Fig. 11(a). The increase in the sp3–C ratio results in moredensification of DLC films, which reduces the film thickness

for an identical process time. In contrast, it appears that the

deposition rate does not depend on the substrate bias voltage,

as shown in Fig. 4. The sputter yield of the C species from

the C target that is caused by energetic ion bombardment

may be unchanged under a constant pressure of 0.18 Pa

because of the intrinsic low sputter yield in a low-voltage

region. Hence, the deposition rate (or thickness) does not vary

significantly relative to the substrate bias voltage. However, the

deposition rate slightly decreases and shows a minimum value

at a substrate bias voltage of −100 V. As shown in Fig. 2(e)

0

1

2

3

4

5

6

(1) (2) (3) (4) (5)

I ( D ) / I ( G )


(a)

(1) -30 V, (2) -50 V, (3) -100 V, (4) -140 V, (5) -180 V

1540

1560

1580

1600

(1) (2) (3) (4) (5) G p e a k p o s i o n [ c m - 1 ]


(b)

110

120

130

140

150

160

(1) (2) (3) (4) (5) F W H M o f G p e a k [ c m - 1 ]


(c)

Fig. 8. Bias voltage dependences of: (a) intensity ratio of the D peak to theG peak, or I(D)/I(G), (b) G peak’s position, and (c) FWHM of the G peak.

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000 1200

I n t e n s i t y [ c o u n t s ]

Binding energy [eV]

C1s

O1s

Fig. 9. Typical XPS spectrum of the prepared DLC film at 0.11 Pa.

and (f), the substrate current that corresponds to the amount of

extracted ions mainly depends on the Ar gas pressure and is

not significantly influenced by the substrate bias voltage. This

result suggests that the amount of extracted ions is almost the

same regardless of the substrate bias voltages. However, the

mean free path is roughly 64 and 14 mm for pressures of

0.11 and 0.52 Pa, respectively, which are sufficiently larger

than the sheath length that the extracted ions reach the growing

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0

500

1000

1500

2000

2500

3000

280 282 284 286 288 290

I n

t e n s i t y [ c o u n t s ]

Binding energy [eV]

sp3C

sp2C

C-O

C1s

Fig. 10. Typical C1s spectrum of the DLC film prepared at 0.11 Pa. Thecurve fit results are also indicated.

surface without a collision in the sheath region. Thus, the

energy of the extracted ions should be controlled by the

substrate bias voltage under the same target voltage. It is

expected that the ion energy is ∼

100 eV at a substrate biasvoltage of −100 V. According to the subplantation model [42],

[43] and the results of computer simulation [38], the ion energy

of 100 eV is the critical value that is needed to create an sp 3 C

bond by means of ion bombardment. It is observed that the

sp3–C increases at −100 V, as shown in Fig. 11(b). Therefore,

the decrease of the thickness that is due to densification may

be responsible for the observed reduction of the deposition

rate at the substrate bias voltage of −100 V.

The surface roughness of DLC films is changed by vary-

ing the preparation conditions of the Ar gas pressure and

the substrate bias voltage, as shown in Fig. 5(a)–(f). The

surface diffusion of C atoms that is because of both the

substrate surface temperature and the ion energy has a sig-nificant effect on the surface’s roughening and smoothing.

Peng et al. [44] reported that whereas a surface temperature

of 350 °C during a deposition created an apparently rough

surface, an estimated ion energy of more than 50 eV made

the surface smooth in DLC films that are prepared using three

different techniques, namely, RF discharge deposition, dc-MS,

and ion beam deposition. According to their report, we believe

that the increase of the surface temperature may be responsible

for the rough surface. In contrast, the ion bombardment may be

responsible for the surface’s smoothing. As shown in Fig. 6(a),

the surface roughness increases with a reduction of the Ar

gas pressure. Therefore, it is assumed that the temperature

increases as the pressure decreases, although the temperature isnot measurable in our system. The reason why the temperature

increases at a low pressure is not entirely clear. However,

a tentative explanation is as follows: in HiPIMS, a high

negative pulse voltage is applied to the target and the ions are

accelerated toward the target. Simultaneoulsy, the electrons

are accelerated toward the substrate. At a low pressure, the

electrons with high energies rapidly reach the substrate, and

at that point, these impinging electrons heat up the substrate,

which results in the enhancement of the surface diffusion.

With increasing Ar pressure, the electrons frequently collide

with the Ar gas species and lose their energy, and as a result,

0

10

20

30

40

(1) (2) (3) (4) (5)

s p 3 / ( s p 2 + s p 3 ) [ % ]


(1) 0.11 Pa, (2) 0.24 Pa, (3) 0.3 Pa,

(4) 0.4 Pa, (5) 0.52 Pa(a)

0

10

20

30

40

(1) (2) (3) (4) (5)

s p 3 / ( s p 2 + s p 3 ) [ % ]


(1) -30 V, (2) -50 V, (3) -100 V,

(4) -140 V, (5 ) -180 V(b)

Fig. 11. (a) Ratio of sp3 components: sp3 /(sp2 + sp3) versus the Ar gaspressure. (b) Substrate bias voltage.

the substrate temperature because of these impinging electrons

decreases when there is a higher pressure. At a substrate bias

voltage of −60 V, the ion energy is expected to be more

than 60 eV. Thus, an ion bombardment under high pressure

also causes surface smoothing. An ion bombardment’s effect

is more apparent when the substrate bias voltage is varied.

As shown in Fig. 6(b), the surface roughness decreases with

an increasing substrate bias voltage. It is believed that the

temperature should not change very much under a constant

pressure of 0.18 Pa despite the different substrate bias voltages.

An increase in the substrate bias voltage causes an increase of the ion energy, and as a result, the surface roughness decreases

because of a high-energy ion bombardment.

Raman spectrum analysis is useful for an examination of

the microstructure of DLC films. According to Ferrari and

Robertson [42], [43], the changes of the Raman parameters

of the I ( D) / I (G) ratio and the position and FWHM of the

G peak reflect the disordered sp2 phase, which is related to the

connection of sp3-bonded carbon in DLC films. In the previous

paper [40], the ratio I ( D) / I (G) decreased, the G peak’s

position shifted to lower wavenumbers and the FWHM of the

G peak increased when the Ar gas pressure was increased to

0.3 Pa. These results showed that the sp2 phase was decreased

with increasing the pressure. In fact, we confirmed that thesp3–C ratio increases when the pressure is increased to 0.3 Pa,

as shown in Fig. 11(a). However, the G-peak’s position slightly

shifted to higher wavenumbers and the FWHM of the G

peak decreased upon a further increase of the pressure. In

addition, the Raman spectrum notably decreased one time

in intensity when the pressure was increased to 0.3 Pa, but

this spectrum increased in intensity upon a further increase

of the pressure, as reported in the previous paper [40]. This

occurrence suggested that the sp2 phase increased in quantity

in the films under higher pressures than 0.3 Pa. Actually,

we also confirmed that the sp3–C ratio decreases gradually

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under higher pressures than 0.3 Pa, as shown in Fig. 11(a).

In Fig. 8(a)–(c), the trends of the Raman parameters due to

the substrate bias voltage are similar to those trends that were

due to the Ar gas pressure. The position and FWHM of the G

peak show a minimum and a maximum at the substrate bias

voltage of −100 V, respectively, although the I ( D) / I (G) does

not always significantly change. These results suggest that the

amount of sp3–C increases at the substrate bias voltage of

−100 V, which is confirmed by the results of XPS analysis,

as shown in Fig. 11(b).

There is a question about why the peak intensity of the

sample that is prepared at 0.52 Pa is higher by a factor of

over two compared with the intensities of samples prepared

under lower pressures, as reported in the previous study [40].

It is found that hydrogen (H) content is ∼15 at% in DLC

films under a high Ar pressure. The H in the films may come

from contaminated water that is contained within the chamber

wall. This water may be activated during the HiPIMS glow

discharge [30], [31]. It is well known that the hydrogenation

of carbon films affects the optical properties of the refractive

index and the extinction coefficient, and especially, that thishydrogenation results in a decrease of the extinction coeffi-

cients of the films. This decrease of the extinction coefficients

causes an increase in the penetration depth of the laser light

that is used for Raman measurement, and as a result, the

effective volume for Raman scattering increases. Thus, the

hydrogen incorporation is responsible for the higher intensity

of the Raman signal for the sample that is prepared at 0.52 Pa.

The increase in the H concentration that is due to a significant

activation of the water may also decrease the density of the

films [40]. This possibility is consistent with the results of

XPS analysis, which indicate that there is a decrease in the

sp3 ratio for Ar gas pressures higher than 0.4 Pa, as shown in

Fig. 11(a).

IV. CONCLUSION

DLC films were prepared using an HiPIMS with a pulsed

negative source voltage of 1–1.2 kV that was applied to

a carbon target. The target size was 150 mm in diameter

and the substrate was 175 mm away from the target. The

pulsed bias voltage was applied to the substrate in the range

of −30 to −180 V and the peak values for the effective

plasma area of the glow current- and the consumed power-

densities were 1.4 A/cm2 and 1.2 kW/cm2, respectively. These

values decreased when the Ar gas pressure was increased from

0.11 to 0.52 Pa. The average power was within the range of

1.0 to 1.7 kW and the ion density in the postdischarge regionwas on the order of 1017 m−3.

The Ar gas pressure and the substrate bias voltage influ-

enced the film characteristics. The deposition rate decreased

when the pressure was increased from 0.11 to 0.52 Pa, as pre-

viously reported in [40], which may be caused by the decrease

of the carbon sputter yield because of the decrease of the flux

of Ar ions. In addition, the densification of the deposited films

was also responsible for the decrease of the deposition rate

as the pressure was raised to 0.3 Pa. In addition, there were

pressure dependences that affected the surface morphology and

the surface roughness of the prepared DLC films: not only

a bumpy surface but also a dumpling-like aggregation were

seen at a pressure of 0.11 Pa. However, the film morphology

became smoother when the pressure was increased, which

may result from the suppression of surface diffusion that was

achieved by lowering the surface temperature of the substrate,

because of the decrease of the impinging electron energy at

higher pressure. The ion bombardment that was achieved by

applying a substrate bias voltage of −60 V contributed to

the surface smoothing. With the XPS spectra, we discovered

that there were three types of peaks, sp2–C, sp3–C, and C–O,

which were obtained by curve-fitting using the mix function

(10% Lorentzian–Gaussian). Concerning the Raman spectra,

it was previously found [40] that the intensity ratio of the

D and G bands, or I ( D) / I (G), the G-peak’s position and

the FWHM of the G peak changed with the gas pressure;

I ( D) / I (G) decreased monotonically and the G peak’s position

and its FWHM reached a minimum and maximum at 0.3 Pa,

respectively, which was followed by an increase and a decrease

under higher pressures than 0.3 Pa, respectively. From these

results, it was assumed that the pressure of 0.3 Pa favored

more sp

3

configuration due to sp

2

phase disordering and lesssp2 domain clustering. The assumption was confirmed by the

fact that the maximum sp3 content of 28% was observed at

0.3 Pa in the XPS measurements.

The deposition rate indicated a minimum value at the

substrate bias voltage of −100 V. The flux of the sputtered C

from the target would be the same because the target current

(the flux of Ar ions at the target) is almost the same under

a constant pressure of 0.18 Pa. Thus, the decrease of the

deposition rate should mainly be caused by the densification

of the film. The maximum sp3 content of 27% was also

observed at −100 V. The surface morphology and roughness

were influenced by the bias voltage that was applied to the

substrate. It was observed that the surface of the depositedfilms became smooth and that the average roughness decreased

when the bias voltage was increased to make it higher than

−100 V. In the Raman spectra, the G peak’s position and its

FWHM, which were also related to the sp3 content in the film,

showed a minimum and a maximum at −100 V, respectively.

However, the ratio I ( D) / I (G) was not changed significantly.

Currently these characteristics for a variety of pressures

and bias voltages are not sufficiently clear, but there may be

an appropriate value of an ambient gas pressure and a bias

voltage that can be used to realize a smoothed surface of the

deposited films with a high content of sp3 components. These

are important subjects to investigate in the near future.

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Shizuka Nakano received the B.Eng, M.Eng, andPh.D. (Eng.) degrees in mechanical engineeringfrom the University of Electro-Communications,Tokyo, Japan, in 1987, 1989, and 2003, respectively.

He was with the Mechanical Engineering Lab-oratory (MEL), Agency of Industrial Science andTechnology (AIST), MITI, Tsukuba, Japan, in 1989,and a Senior Research Scientist since 1997. He waswith the New Energy and Industrial TechnologyDevelopment Organization in 2001. He worked for

National Institute of Industrial Science and Technol-ogy (AIST) in 2002, established by unifying with MEL and 14 other researchinstitutes in METI. He has worked for surface modification technology by ionbeam and its applications, such as micro electro-mechanical systems. He isinterested in “Minimal Manufacturing system” which is a manufacturing styleinclude smallest environmental load, minimum energy and materials charge,and high performance products concept. The demonstrational fabricationsystem named “on-demand manufacturing system” for MEMS device usingmetal press, aerosol-deposition method and other some process was developedin 2007. He joined the “Minimal Fab system consortium,” developing minimalplasma processing equipments for semiconductor and MEMS Lithography.

Hisato Ogiso received the B.Sci. degree in astro-physics from Kyoto University, Kyoto, Japan, in1985, and the Ph.D. degree in material processengineering from Tohoku University, Sendai, Japan,in 2000.

He was with the Mechanical Engineering Lab-oratory (MEL), Ministry of International Tradeand Industry (MITI), Tsukuba, Japan, in 1985. Heworked for National Institute for Advanced Inter-disciplinary Research (NAIR), MITI, in 1993, andhas been a Senior Research Scientist since 1994. He

worked for National Institute of Advanced Industrial Science and Technologyin 2001, established by unifying with NAIR and 14 research institutes in MITI,where he presently works. He is also a Visiting Professor with the KanazawaInstitute of Technology, Kanazawa, Japan, since 2001. His current researchinterests include the surface-improvement technology using ion beam andplasma, and also the characterization technology of materials in the micro andnano-scale region. He was engaged in the several national projects, “Advancedmaterial processing and machining technology (FY1986–FY1994)”, “UltimateManipulation of Atoms and Molecules (FY1989–FY1998)” as a ResearchScientist, and “Nano Structure Forming for Advanced Ceramic IntegrationTechnology (FY2002-2006) as a Vice Project Leader.

Ken Yukimura received the Dr.Eng. degree in elec-trical engineering from Doshisha University, Kyoto,Japan, in 1977.

He was a Faculty of Engineering in 1977 and hasbeen a Professor of Doshisha University since 1992.He has been an Invited Research Scientist of theNational Institute of Advanced Industrial Scienceand Technology since 2010, and a Guest Professor of Iwate University, Morioka, Japan, since April 2013.He is also an Advisory Professor of Southwest Jiao-

tong University, Sichuan, China, since June 2006,and a Contract Professor with the Harbin Institute of Technology, Harbin,China, since June 2010. He was a Chairperson of Technical Committeeof Plasma Science and Technology, The Institute of Electrical Engineersof Japan, and the NPS-05 in IEEE Japan Chapter from April 2008 toMarch 2011. He was a Co-Chairperson of the Fifth International Workshopon Plasma-based Ion Implantation (Kyoto, Japan, December 1999). He hasresearch experiences of exploding wire phenomena, excimer laser and vacuumultraviolet emission technologies, ion technology and surface modificationtechnology by plasmas at atmospheric pressure. His current research interestsinclude pulsed plasma technologies, such as metal plasma generation andtheir applications. Typically, he has experienced studies of shunting arc- andglow-discharges and high-power pulsed sputtering glow discharges, pulsedion technology, such as ion implantation into 3-D components, and exhaustgas processing in atmospheric pressure such as decomposition of nitric oxidegases.

Dr. Yukimura was a Guest Editor of a special issue relevant to Plasma-Based

Surface Modification and Treatment Technologies in the IEEE TRANSAC-TIONS ON P LASMA SCIENCE in 2005–2006 and 2008–2009, to HIPIMS andHigh Power Glow Discharges in 2009–2010 and to Carbon-Related MaterialsProcessing by Plasma Technologies in 2011–2012.

Setsuo Nakao received the Dr.Eng. degree in elec-trical and computer engineering from the NagoyaInstitute of Technology, Nagoya, Japan, in 1991.

He was with the Government Industrial ResearchInstitute, Nagoya (GIRIN), Ministry of InternationalTrade and Industry (MITI), in 1991. He was withthe National Industrial Research Institute of Nagoya(NIRIN), MITI, in 1994. He has been a SeniorResearch Scientist since 1995. He was with the

National Institute of Advanced Industrial Scienceand Technology (AIST), in 2001, established by

unifying with NIRIN and 14 research institutes in MITI which was reorganizedto Ministry of Economy, Trade and Industry (METI), and where he presentlyworks. His current research interests include the surface modification andthin film deposition by ions using ion beam technology and pulsed plasmatechnology. Typically, he has experienced studies on the surface modificationby ion implantation and the deposition of diamond-like carbon films usingthe bipolar-type plasma based ion implantation (PBII) technique developedby AIST. He also has research experience with the characterization of thinfilms by ion beam analysis.

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