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Applied Surface Science 253 (2007) 3722–3726

The effect of applied negative bias voltage on the structure of

Ti-doped a-C:H films deposited by FCVA

Peng Wang a,b, Xia Wang a,b, Youming Chen a,b, Guangan Zhang c,Weimin Liu a,*, Junyan Zhang a,*

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China

c School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China

Received 25 April 2006; accepted 2 August 2006

Available online 7 September 2006

Abstract

Ti-doped hydrogenated diamond-like carbon (DLC) films were deposited on Si(1 0 0) substrates by a filtered cathodic vacuum arc (FCVA)

method using Ar and CH4 as the feedstock. The composition and microstructure of the films were investigated by Raman spectroscopy, X-ray

photoelectron spectroscopy and IR spectroscopy. The internal stress was determined by the radius of curvature technique. The influence of the bias

voltage on the microstructure of the as-deposited films was investigated. It was found that the graphite-like bonds was dominated in the Ti-doped

DLC film deposited at 0 V bias voltage. When bias voltage was increased to�150 V, more diamond-like bond were produced and the sp3 content in

film reached the maximum value, after which it decreased and more graphite-like bonds feature produced with further increase of the negative bias

voltage. The compressive internal in the Ti-doped DLC films also exhibited a maximum value at�150 V bias voltage. IR results indicated that C–H

bonded intensity reduced, and H atoms bonded with C atoms were substituted for the Ti atoms as the negative bias voltage increasing. All the

composition and microstructure change can be explained by considering the plasma conditions and the effect of negative bias voltage applied to the

substrate.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Hydrogenated amorphous carbon (a-C:H) films; Applied bias voltage; Filtered cathodic vacuum arc (FCVA)

1. Introduction

Diamond-like carbon (DLC) films have been attracting

considerable interest owing to their unique properties of high

hardness, optical transparency, low friction coefficient,

chemical inertness and high electrical resistivity [1]. These

properties are promising for a wide range of applications such

as low-friction and wear-resistant coatings, protective optical

and biomedical coatings, electroluminescence materials, and

field emission devices [2–6]. However, successful preparation

and application of DLC films have been restricted by a large

internal compressive stress as high as 10 GPa in the films,

which greatly limits the adhesive strength between the film and

* Corresponding authors. Tel.: +86 931 4968166 (W. Liu), +86 931 4968295

(J. Zhang).

E-mail addresses: wmliu@lzb.ac.cn (W. Liu), junyanzh@yahoo.com

(J. Zhang).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.08.003

substrate, and leads to the peeling-off of the films from the

substrate during the duration of work [7,8].

Recently, many methods have been used to release the

internal stress in DLC films. Previous studies have shown that

the internal stress in the DLC films can be markedly decreased

by doping additional elements in the films. Ti, Cr, W, N, B,

and Si are often used as doping elements [9–12]. However, the

metal incorporation into the DLC matrix has been usually

achieved by sputtering the metal target. Following dis-

advantageous effects should be considered in such a sputter-

based deposition system. The films with a relatively high

graphitic bond can grow by sputtering due to the low energy

of the sputtered atoms, resulting in less-dense films.

Amorphous hydrocarbon films can be also deposited onto

the metal target especially at a high hydrocarbon to inert gas

ration because the target is usually attached to the bias

cathode. This so-called target poisoning could result in the

reduction of metal content in the films with an increase of

process time [13].

P. Wang et al. / Applied Surface Science 253 (2007) 3722–3726 3723

Filtered cathodic vacuum arc (FCVA) deposition is a

promising technique for the production of high quality hard thin

films. The main feature of the FCVA technique is to employ a

curved magnetic field to guide the plasma generated from the

cathodic vacuum arc to deposit on an out-of-sight substrate.

Through this special magnetic filter, most of the unwanted

macro-particles and neutral atoms will be removed. Only ions

within a defined energy range can be reached the substrate, this

technique produced films with good controllability and

reproducibility [14].

In the recent work, the filtered cathodic vacuum arc (FCVA)

technique was general employed to deposit the ta-C films. In

this paper, Ti target was used as cathode to deposit Ti-doped

hydrogenated DLC films at the methane and Argon atmosphere,

as the depositing species in the FCVA process are fully ionized,

the kinetic energy of these ions can be precisely controlled by

adjusting the bias voltage applied to the substrate, so the

influence of ions kinetic energy on the structure of Ti-doped

DLC films was investigated with different negative bias voltage

applied to substrate.

2. Experimental details

The Ti doped hydrogen DLC films were deposited by the

FCVA technique. It employs a 908 curved filter to remove the

macro-particles. Pure titanium cylindrical target was used to

generate plasma with 65 mm in diameter. A rotated substrate

holder was placed normal to the arc plasma beam at a

distance of 15 cm from the exit of the FCVA source. The

deposition conditions are detailed in Table 1. The vacuum

chamber with 900 mm in diameter was evacuated to the base

pressure less 4.0 � 10�3 Pa using a turbomolecular pumping

system, the film was deposited at a pressure of 4.0 � 10�1 Pa

using methane and argon reactive gases. A Si(1 0 0) wafer

was used for the substrate. Prior to deposition, the silicon

substrates were sputtered by an Ar ion beam for 10 min in

order to remove the native oxide layer. During deposition, a

negative bias voltage was applied to the substrate ranging

from 0 to �400 V.

The film thickness was measured using a surface

profilometre. The surface morphology of films was observed

on an SPM-9500 atomic force microscope (AFM). The

chemical states of carbon and Ti elements were analyzed on a

PHI-5702 X-ray photoelectron spectroscope (XPS) operating

with monochromated Al K irradiation at pass energy of

29.4 eV. Micro-Raman backscattering spectra of the DLC

films were recorded on a Jobin Yvon T64000 spectrometer

Table 1

Summary of Ti-doped DLC films deposition conditions

Item Parameter

CH4 gas flow rate (sccm) 100

Ar gas flow rate (sccm) 100

Deposition pressure (Pa) 0.4

Thickness (nm) 300

Arc current (A) 70

operating with 514.5 nm Ar laser as the excitation source. The

Raman spectra were fitted based on two Gaussian curve shapes

with a curve-fitting software to identify peak positions and

intensity.

The internal stress of the DLC films was measured by the

stress-induced bending on an interferometric surface profiler.

The curvature radii of the substrate before and after film

deposition were measured by the observation of Newton’s rings

using an optical interferometry system, and the internal stress

was calculated by the Stoney equation [15]:

s ¼ Es

6ð1� nsÞ

�t2s

tf

��1

R2

� 1

R1

�

where s is the internal stress, R1 and R2 the curvature of the

substrate before and after deposition, ns the Poisson’s ration of

the substrate, Es the Young’s modulus of the substrate, and tsand tf are the thickness of the substrate and film, respectively.

The values of Es of 115 GPa and ns of 0.2 were adopted for Si

substrate, respectively.

3. Results and discussion

3.1. Deposition rate

Fig. 1 shows the relationship between the deposition rate of

Ti-doped a-C:H films and the applied negative bias voltage. It

is seen that the deposition rate decreased from 27 to 3 nm/min

and the temperature of substrate increased as the bias voltage

increasing from 0 to –400 V. This trend can be rationally

understood if one notices the mechanism of a-C:H deposition,

which highly depends on the ion energy [16]. Firstly, the

sputtering of the film by Ar ion is stronger when the bias

voltage increases. Secondly, the etching of the film by atom

hydrogen plays a critical role in the deposition of a-C:H films,

and the etching rate increases with temperature which

increases with the bias voltage increasing [17]. So, the

growth rate decreases with the applied bias voltage

increasing.

Fig. 1. Deposition rate of Ti-doped a-C:H films and temperature of substrate as

a function of applied negative bias voltage.

P. Wang et al. / Applied Surface Science 253 (2007) 3722–37263724

Fig. 2. Four typical AFM images of the Ti-doped nanocomposite DLC films deposited at different negative bias voltage: (a) 0 V, (b) 150 V, (c) 200 V, and (d) 400 V.

3.2. Surface morphology

AFM images shown in Fig. 2 illustrate the effect of the bias

voltage on the surface roughness and feature. The films

deposited at all conditions show very smooth and flat surface,

benefit from the FCVA method, in which most of the unwanted

macro-particles and neutral atoms are removed through a

curved magnetic field. While the surface of the Ti-doped DLC

films became smoother with the increased voltage from 0 to

400 V, resulting in an obvious reduction in surface roughness

from 4.21 to 0.28 nm (Table 2), and exhibited a flat and shiny

surface. This decreasing tendency of surface roughness with

increasing the negative bias voltage was reported before. This

might be related with the energy of impinging ions to the

Table 2

Deposition parameters, composition, and characterization of Ti-doped DLC

films deposited at different bias voltage

Films no. Bias (V) Composition (at%) ID/IG rms

(nm)

Deposition rate

(nm/min)C Ti O

1 0 88.25 3.54 8.21 2.2 4.21 27

2 �100 87.52 5.18 7.30 1.7 3.32 23

3 �150 86.03 5.35 8.59 1.2 1.54 20

4 �200 81.81 7.33 10.86 2.2 0.85 10

5 �400 79.21 10.52 10.27 4.5 0.28 5

growing film that help improve the roughness by the surface

diffusion [18].

3.3. Composition of films

Seen the result of Table 2, an increase in bias voltage leads to

a significant increase in titanium content of Ti-doped DLC

films. Fig. 3 displays the C1s XPS spectra of the Ti-doped DLC

Fig. 3. XPS C1s spectra of the Ti-doped DLC films deposited at different bias

voltage.

P. Wang et al. / Applied Surface Science 253 (2007) 3722–3726 3725

Fig. 5. Visible Raman spectra of the Ti-doped DLC films deposited at different

negative bias voltage.

films deposited at different negative bias voltage. The C1s

spectra of Ti-doped DLC films can be fitted into three

components around 281.7, 284.5, and 286 eV, respectively. The

one around 281.7 eV corresponds to Ti–C bonds, and that

around 284.5 eV corresponds to C–C bonds. The third peak

near 286 eV is assigned to some C–O contamination formed on

the film surface due to air exposure [14]. It can be seen that the

intensity of C–Ti bond at 281.7 ev was built up as bias voltage

increasing. It means that the amount of Ti–C bond in the films

increase with increasing the bias voltage.

3.4. Internal stress

Fig. 4 shows the internal stress in the DLC films deposited on

Si(1 0 0) substrates as a function of the bias voltage. It indicates

that all films have compressive stress. It can be seen that the

compressive stress in the DLC films deposited on Si substrate

increases with increasing the bias voltage, and reaches to a

maximum value of 4.49 GPa at a negative bias voltage of 150 V,

followed by decreasing with further increase of bias voltage.

Interestingly, the minimum ratio of the intensity of D peak to G

peak in the Raman spectra of DLC films, i.e. ID/IG, was

obtained at the bias voltage of �150 V (in Table 2), which is

good agreement with the highest compressive stress in the

corresponding DLC film. This trendy can be explained by

considered the Raman structure changing.

3.5. Raman analysis

Fig. 5 shows the visible Raman spectra of the Ti-doped DLC

films at different bias voltage range from 0 to�400 V. The peak

positions of the D and G were determined by fitting the Raman

spectra using two Gaussian distributions. From the spectra, it

can be seen that the G peak position shifted from 1595 to

1566 cm�1 and ID/IG decreases from 2.1 to 1.2 as the bias

voltage changed from 0 to 150 V, this tendency implies the

increase of diamond-like bonds and sp3 content in films [19–

21]. Then, G-peak shifted from 1566 to 1592 cm�1 and ID/IG

Fig. 4. The internal stress in the Ti-doped DLC films deposited on Si substrates

as a function of the negative bias voltage.

increased from 1.2 to 4 as the bias voltage increasing from 150

to 400 V. D peak intensity obviously increased meaning more

graphite-like bonds in films. This variation trend can be

rationally understood if one notices that the growth process of

DLC films is consisted of the chemical adsorption processes of

species at the growing surface and the physical process of

implantation at the subsurface, which mainly depend on the ion

energy [22–25]. Accordingly, without bias voltage applied to

substrate, the ion energy is too weak to penetrate into the

growing surface and most of the ions are only trapped on the

growing surface, resulting in the formation of the loose cross-

linking films and graphite-like bond, with low internal stress

and sp3 content. Increasing the negative bias voltage to 150 V,

the ion species will have sufficient energy to penetrate into the

subsurface. Under this deposition condition, the ion implanta-

tion is associated with a compressive stress and densification,

which in turn lead to the sp3 bond increase, thus, the film has the

most diamond-like character with high internal stress and sp3

content. However, at higher bias voltage (higher than 150 V),

the dissipation of the excess heat generated by the impinging of

the energetic ions can relax the high compressive stress or

excess density phase that leads to loose carbon networks, and

hence, the sp3 content decreases and graphite-like bonds

formation in this case.

3.6. IR absorption

IR spectroscopy is widely used to characterize the C–H

bonding in a-C:H films. The IR absorption characteristics of the

films as a function of applied dc bias voltage are shown in

Fig. 6. It can be seen that the C–H absorption peaks centered at

2920 cm�1, which is associated with the majority presence of

hydrogen bonds in the form of sp3-CH and sp3-CH2 asymmetric

stretching mode, and two smaller shoulder peaks occur at 2950

and 2860 cm�1 corresponding to sp3-CH3 asymmetric stretch-

ing mode and sp3-CH3 symmetric model, respectively [16]. The

C–H absorption peak in the IR spectra is an indication of the

bonded hydrogen content in the DLC films. This suggests that

most of the hydrogen is bonded to sp3-hybridized C atoms.

P. Wang et al. / Applied Surface Science 253 (2007) 3722–37263726

Fig. 6. IR spectra of Ti-doped DLC films deposited at different bias voltage.

Obviously, an overall reduction in the intensity of these modes

is seen in the films with an increasing in applied bias voltage.

The reduction in the intensity of C–H bonds may be the result of

a more severe breakage of the C–H bonds in the presence of an

enhanced substrate bombardment by ion species with higher

energy at an increased applied bias voltage. This can be

explained by considering plasma conditions and the effect of

bias voltage. Corresponding, the energy (E) of the deposition

ions can be approximately expressed as E = ne(Vp�Vs) + E0,

where Vp � �13 V, is the plasma potential, Vs the negative bias

voltage, E0 the original ion energy in the plasma (�28 V), e the

electron charge, and n is the charge state of the ions in the

vacuum arc plasma [14]. According to the experimental results

of Anders [26], CH4 plasma consists of

H+ + CH+ + CH22+ + C2H4

2+ with an average charge state of

1.82, while the titanium arc plasma is composed of

11%Ti+ + 75%Ti2+ + 14%Ti3+ with an average charge state

of 2.03. So, the energy of titanium ion was higher than CH ion

when the negative bias voltage applied on substrate increased

because its ion average charge state is higher than Ti ion. At low

deposition bias voltage, CH ion and titanium ion reached

substrate with low energy and formed C–H bonds and little

carbide. As bias voltage increasing, the ion energy of the

titanium was higher than CH ion, it bombards the films and

breaks C–H bond, then substitutes for H atoms, the formation of

TiC phase reduced the co-ordination numbers of C network by

binding carbon atoms into carbide, and led to the reduction of

the H content and the increase of the titanium content in films.

4. Conclusion

Ti-doped hydrogenated diamond-like carbon films were

deposited on Si(1 0 0) substrates by a filtered cathodic vacuum

arc (FCVA) method using Ar and CH4 as the feedstock. The

effects of the applied negative bias voltage on the structure of

the DLC films were investigated. IR results indicates that

increasing negative bias voltage applied to substrate leads to the

reduction of the bonded H content, and the increase of the

titanium content in films. XPS results show that the formation

of TiC phase reduced the co-ordination numbers of C network

by binding carbon atoms into carbide. Changing negative bias

voltage with methane flow rate at 100 sccm, the sp3 content

reached the maximum value at �150 V bias voltage, it records

the maximum internal stress value in the film, beyond which it

decreased and more graphite-like bonds feature produced with

further increase of the negative bias voltage. Similarly, the ion

energy and plasma condition have a great influence on the

structure of the Ti-doped hydrogenated DLC films, and the ions

kinetic energy produced from FCVA can be precisely

controlled by adjusting the bias voltage applied to the substrate,

so high quality Ti-doped hydrogenated DLC films can be

produced by FCVA methods through using suitable bias voltage

applied to substrate.

Acknowledgments

The authors are grateful to the National Natural Science

Foundation of China (Grant Nos. 50323007 and 50572108), the

Innovative Group Foundation from NSFC (Grant No.

50421502), and the ‘‘Hundred Talents Program’’ of the

Chinese Academy of Science for financial support.

References

[1] J. Robertson, Mater. Sci. Eng. R. 37 (1998) 129.

[2] H. Alan Littington, Carbon 36 (1998) 555.

[3] R. Hauert, Diamond Relat. Mater. 12 (2003) 583.

[4] H. Li, T. Xu, C. Wang, J. Chen, et al. J. Phys. D: Appl. Phys. 38 (2005) 62.

[5] C. Donnet, A. Erdemir, Surf. Coat. Technol. 180–181 (2004) 76.

[6] H.Y. Ueng, C.T. Guo, Appl. Surf. Sci. 249 (2005) 246.

[7] Q. Wei, J. Sankar, J. Narayan, Surf. Coat. Technol. 146–147 (2001) 250.

[8] Q. Wei, R.J. Narayan, A.K. Sankar, J. Vac. Sci. Technol. A 17 (6) (1999)

3406.

[9] M.C. Chiu, W.P. Hsieh, W.Y. Ho, et al. Thin Solid Films 476 (2005) 258.

[10] K.R. Lee, K.Y. Eun, et al. Thin Solid Films 377 (2000) 261.

[11] Y. Pauleau, F. Thiery, Surf. Coat. Technol. 180–181 (2004) 313.

[12] D.Y. Wang, K.W. Weng, S.Y. Hwang, Diamond Relat. Mater. 9 (2000)

1762.

[13] W.J. Yang, T. SeKino, K.B. Shim, et al. Thin Solid Films 473 (2005) 252.

[14] X. Ding, B.K. Tay, H.S. Tan, et al. Surf. Coat. Technol. 138 (2001) 301.

[15] M. Ban, T. Hasegawa, Surf. Coat. Technol. 162 (2002) 1.

[16] H. Li, T. Xu, J. Chen, et al. J. Phys. D: Appl. Phys. 36 (2003) 3183.

[17] A. Won Keudell, W. Jacob, J. Appl. Phys. 81 (1997) 1531.

[18] M.S. Leu, S.Y. Chen, J.J. Chang, et al. Surf. Coat. Technol. 177–178

(2004) 566.

[19] A.A. Voevodin, M.A. Capano, S.J.P. Laube, et al. Thin Solid Films 298

(1998) 107.

[20] T. Mikami, H. Nakazawa, M. Kudo, et al. Thin Solid Films 488 (2005) 87.

[21] D.R. Mckenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991)

773.

[22] E. Liu, L. Li, B. Blanpain, et al. J. Appl. Phys. 98 (2005).

[23] L.Y. Kasi, S.R. Rabalais, et al. Phys. Rev. Lett. 62 (1989) 1290.

[24] M. Ban, T. Hasegawa, Diamond Relat. Mater. 12 (2003) 47.

[25] J. Robertson, Diamond Relat. Mater. 3 (1994) 361.

[26] A. Ander, Phys. Rev. E 55 (1997) 969.