Carbon films for the protection of magnetic recording disks

ELSEVIER

Surface and Coatings

Technology

72 (1995) 152-156

Carbon films for the protection of magnetic recording disks* A. Kobayashi a, D. Yoshitomi a, 0. Yoshihara a, T. Imayoshi a, A. Kinbara b, T. Fumoto ‘, M. Ueno ’ a Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan b Department of Mechanical Engineering, Kanazawa Institute of Technology, Kanazawa 921, Japan ’ Fuji Electric Corporate Research and Development Ltd., Yokosuka, Kanagawa 240-01, Japan

Received 21 March 1994; accepted in final form 20 July 1994

Abstract

Hydrogenated amorphous carbon (a-C:H) thin films were deposited by d.c. magnetron sputtering a graphite target in a mixeddischarge Ar-CH, gas. Raman spectra and other optical measurements revealed the growth of a-C:H films. CH, derivatives are shown to contribute to the film formation together with sputtering of the target. Both the friction coefficient and the adhesion strength of the films were obtained by scratch testing and the former is shown to be almost independent of the CH4 concentration in the mixed-discharge gas; the latter depends on the concentration of CH,. An optimum composition for the discharge gas is proposed. Keywords: Carbon

films; Adhesion

strength

1. Introduction In response to the increasing demand for protective coating materials in the electronic industry, thin films of hydrogenated amorphous carbon (a-C:H) have been intensively investigated. The desirable properties of the films are as follows: anti-abrasion, low friction, strong adhesion to substrate material, high electric resistivity, non-magnetic response, good optical transparency and so on. These properties are also suitable for protective film on magnetic recording disks. The films can be deposited by ion beam sputtering, magnetron sputtering, and plasma-assisted chemical vapor deposition (PACVD). The introduction of hydrogen into the film has been demonstrated in terminating sp2-bonded clusters and sp3-bonded networks and causes a decrease in electron mobility and an increase in the number and extent of atomic scale voids in the films [1,2]. A decrease in the wear resistance of the films has also been shown for the films containing higher hydrogen concentration, although other hydrogenated films have shown the

* Paper

at the 21st presented Metallurgical Coatings and Third 25-29, 1994.

International Films, San

Conference on Diego, CA, April

0257-8972/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(94)02346-S

highest wear resistivity in three other films: a-Si:H, a-SiN:H, and a-Ge:H [ 31. Although the recent development of scratch measurements now allow direct assessment of the mechanical properties of the films [6-91, few papers have been reported the tribology and adhesion of the films. Above all it has not yet been made clear how preparation condition or techniques can effect the adhesion and tribology of the films prepared. In the case of PACVD, H radicals have been shown to be important species for hard film deposition from a CH,-H2 mixture[4], and it has been shown that carbon deposition predominates in a certain CH, concentration range of the mixed gas of Ar-CH, and below a certain d.c. power range [ 51. a-C:H films were also prepared by d.c. magnetron sputtering a graphite target with an Ar-H2 mixture

PI. In this study, a systematic approach has been taken to investigate how the adhesion strength and tribology of the film vary as a function of deposition condition such as the CH, content in Ar and CH, mixture when films are grown by d.c. magnetron sputtering. Raman spectroscopy and Fourier-transform IR spectroscopy (FTIR) are used to characterize the microstructure of the films. The electrical resistivity and optical

A. Kobayashi

transmittance microstructure.

are

measured

and

et al.lSwface

related

to

and Coatings Technology, 72 (1995) 152-06

the

2. Experimental

153

The film thickness obtained ranged from 60 to 568 nm, and the deposition rate determined by dividing the thickness by the sputtering time varied from 1.4 to 8 nm min-’ depending on the sputtering conditions such as CH, concentration in the discharge gas.

2.1. Film preparation Thin films of a-C:H were prepared by dc. magnetron sputtering a graphite target of 99.999% purity. The target of 8 cm diameter was attached to the cathode of a sputtering system. The discharge chamber had a diameter of 20 cm and a height of 15 cm and was evacuated with an oil diffusion pump and a mechanical pump. The distance between the cathode and substrate surface was about 4.5 cm. The constant-voltage mode (450 V) was used to ignite the discharge gas. The pressure was monitored with a vacuum gauge attached to the discharge chamber. The films were deposited under the condition of a constant total gas flow (5.0 standard cm3 min- ‘) and a constant total pressure (6.65 Pa). The gases (Ar, 4.0-5.0 standard cm3 mini; CH,, O-l.0 standard cm3 min ’) were introduced through separate mass flow controllers and mixed in the chamber. Species in the gas were detected by a detecter of quadraupole mass spectroscopy (QMS) attached to the vacuum chamber, and ionized species in the plasma were monitored by optical emission spectroscopy (OES). The strong spectral lines for CH at 430 nm, for C, at 516.5 nm and for H, at 486.1 nm were selected as the analytical lines. A linear relationship was obtained between the ratio of CH, to Ar obtained from the QMS signals and the flow rate ratio defined by the flow rates of CH4 divided by the total flow rate of the gases; so the flow rate ratio was used as the variables. Substrates of fused silica glass and silicon wafer were employed. Before setting them in the chamber they were washed with detergent in an ultrasonic bath, rinsed in methanol and dried in dry air. The substrates were mounted on the grounded electrode, facing the plasma. No heat was applied to the substrates during deposition. Films grown on the silica were used for the scratch testing and for the measurements of electrical resistivity and optical transmission in the visible and near-IR range. Raman spectra and FTIR were carried out on the films prepared on the Si substrates. Prior to the deposition run, the discharge chamber was evacuated to about 1.33 x lo- 3 Pa and was heated at about 100°C for 1 h. After cooling, the target was sputtered by Ar at the pressure of 6.65 Pa for several minutes in order to obtain a fresh target surface. At least six samples were prepared under the same deposition conditions in order to confirm reproducibility of the measurements explained in Section 2.2; so the total number of the samples prepared was 30 or more. The data indicated in the all figures of this text show the average value of the every measurement. The statistical error of the data was distributed within about 10%.

2.2. Film characterization Raman spectra were obtained from the films in a backscattering geometry using a 488 nm Ar+ laser. The spectra obtained were over wavenumbers from 900 to 1800 nm-‘. FTIR was made for the purpose of evaluating the relative amount of C-H radicals in the films. An integrated absorption intensity was taken into consideration over the wavenumbers from 2700 to 3200cm-’ for evaluating the amount of C-H radicals. Optical transmittance was also obtained in the range from UV to near IR. Electrical measurements were made by evaporating silver onto the film surface as electrode. The space between the electrodes was about 0.5 mm and the resistivity p was derived from the current-voltage characteristics of the films. Scratch testing was carried out to measure both friction coefficient ,u and adhesion strength F of the films using a microtribometer developed by Baba et al. [6]. The details of this method have been described elsewhere [S-S]. The radius of the diamond stylus used was about 0.1 mm. The loading rate was 51.6 mN min- ’ and the stylus amplitude was about 0.01 mm. All the measurements were carried out in air at room temperature.

3. Results The growth rate of about 1.5 nm min-’ was obtained for the films deposited with only Ar discharge gas, while the growth of the films deposited by the mixed gas was distributed in the range from about 2 to 8 nm min-’ depending on the CH, content in the gas. However, the relationship between these rates and the flow rate ratios was not linear. Fig. 1 shows Raman spectra obtained from the films. The spectra mainly consist of a peak at around 1550 cm-’ with a shoulder at the smaller wavenumbers than the peak and fluorescent luminescence over all wavenumbers for films deposited by only Ar discharge gas or below a flow rate ratio of about 3%. The peak and the shoulder can be thought to be from the G- band and D- band in graphite [2]. The intensity of the fluorescence becomes higher while both the peak and the shoulder disappear in accordance with increasing CH, content in the discharge gas. On the other hand the amount of CH radicals obtained by FTIR increased with increasing CH, concentration in the mixture. A

A. Kobayashi et al. ISurface and Coatings Technology 72 (1995) 152-156

154

together with the relative amount of C-H radical per unit film thickness as a function of the flow rate ratio. One can see a rapid decrease in the graphite intensity on increasing the CH, concentration, but the number of C-H radicals increases within the flow rate ratio of about 3% and becomes saturated over a concentration of 4%. The adhesion strength and friction coefficient of the films were estimated from the scratching traces obtained. Fig. 3 shows one of the typical traces of the scratch testing. The ordinate represents the frictional force [7] and the applied load is shown on the abscissa. The friction coefficient was estimated from a linear slope of the friction force vs load. At a point indicated by A on the load the film is found to have peeled off completely from the substrate, and the adhesion strength F is estimated by the load WC at that point and by the following equation proposed by Benjamin and Weaver c91:

----------_II I

I

(4

F = Wc”2H,(7w2H,- wc)-l’2 1800

1500

900

1200

Wavenumber

[cm-r]

Fig. 1. Raman spectra of a-C:H films deposited ratios: (a) 0%; (b) 2%; (c) 6%; (d) 10%.

by different

flow rate

value l,/lr, is introduced for the purpose of estimating the changes in the graphitic phase in the films, where I, is deduced from the Raman intensity at the peak over and above by the fluorescence intensity lf at the same wavenumber. In Fig. 2, the values of l&r are shown

L

0

(1)

We used 5.7 GPa for the Brine11 hardness H, of fused silica in the estimation. r is the radius of the stylus used (0.1 mm). The friction coefficients obtained are shown in Fig. 4(a). The adhesion strength is shown in Fig. 4(b). The strength obtained will be discussed later in relation to the adheasion energy of carbon films obtained by Gille and Rau [lo]. Different optical transmittances were obtained when the films were deposited under different CH, concentrations. The transmittance per unit film thickness obtained at around 850 nm is shown as function of the flow rate ratio in Fig. 5. The transparency increases abruptly for the films grown by the discharge gas containing about 2% CH, or more. Fig. 5 also shows the electrical resistivity p obtained. One can see p increases drastically for the films grown by the discharge gas containing almost same amount as has been shown for the transmittance of the films.

0

5

10

15

20

25

Flow rate ratio (%)

I

50

100

Load [mN] Fig. 2. Is/If (x, left-hand ordinate) and the relative amount of CH radicals per unit thickness of the films (0, right-hand ordinate) vs. the flow rate ratio.

Fig. 3. A typical trace of scratch testing, where the adhesion strength is estimated from the load at a point denoted by A (a.u., arbitrary units).

A. Kohuyashi et aL/Surface and Coatings Technology 72 (1495) 152-156

Flow

rate ratio (%)

Fig. 4. (a) Friction coefficient vs. the flow rate ratio adhesion strength vs. the flow rate ratio.

X

X

0

5

(b)

x

X

10

are shown;

15

20

25

Flow rate ratio (%) Fig. 5. Optical transmittance (x, right-hand ordinate) and resistivity (Cl, left-hand ordinate) vs. the flow rate ratio.

electrical

4.Discusion The CH, intensity obtained by QMS was reduced to about one seventh after ignition of the discharge gas,

155

indicating decomposition of CH, in plasma. A higher growth rate of the films resulted in the deposition process not only caused by sputtering of the graphite target but also by carbon atoms or molecules caused by decomposition of CH,. Hydrogen caused by the decomposition of CH, is also thought to contribute to increases in both the film resistivity and the optical transmittance because hydrogen incorporated in carbon thin films is known to be a terminator of carbon bonds [l] and to stop the growth of crystalline phases of carbon such as graphite and diamond [2]. These characteristics can be seen in the films grown by the present study. as indicated in Figs. 2 and 5. The friction coefficients obtained are smaller than 0.36 and have a small dependence on the CH, content. The coefficients are shown to be distributed around 0.3 and are slightly larger than coefficients which have been reported to be about 0.2 [ll-131. These friction coefficients were obtained on the polished surface of diamondrich films, and it has been shown that as-deposited films result in a higher coefficient [ 111. The tribological properties of the diamond film have significant difference depending on their morphology, grain size and roughness of the surface [ 121, but smaller coefficients are obtained for the films containing much more crystalline diamond [lo]. No diamond exists in the present films as shown by the higher friction coefficient and the results of the Raman spectroscopy. The introduction of CH, into the discharge gas clearly effects the adhesion of the films. The adhesion strength obtained was of the order of lo--’ GPa or smaller. We also tried to estimate the strength by using the adhesion energy obtained by Gille and Rau [lo]. The estimation was carried out by dividing the energy by the atomic space between the film and the substrate surface. The space was supposed to be 1 nm. The estimated strength was l-10 GPa and much larger than the strength obtained in the present work. There are two reasons that could explain this great difference in the adhesion. One is that the estimation is unsuitable because, even if the value of the energy is correct, there is an uncertainty in the spacing, and also the estimation method may be incorrect. The other is the smaller adhesion energy of the present samples because of contamination on the substrate surface. Apart from these, it seems that there has been few reports of adhesion of carbon film on glass substrate by scratch testing. So it is unsuitable to compare simply the values cited above, and the present work shows the adhesion strength of a-C:H films prepared by sputtering graphite with an Ar-CH, mixture, and that the strength can be affected by the CH, concentration in the discharge gas. As is shown in Fig. 4(b), the adhesion strength increases for a flow rate ratio below 6% and increases slowly for a ratio above 6%. It is reasonable that there is a relationship between the decreasing characteristics

A. Kobayashi et al./Surface and Coatings Technology 72 (1995) 1S2-I56

5. Summary

cl

I3

cl

0

I

1

5

10

I

I

1.5

20

2.5

Flow rate ratio (%) Fig. 6. FOM

vs. the flow rate ratio.

of the strength and the decrease in the graphitic structure. The adhesion of diamond film has been shown to increase with increasing diamond crystalline content [ 133. The strength is basically caused by the interaction between the film and the substrate; so it is natural to take into consideration that the same physical-chemical changes occurred in the films because of the incorporated hydrogen, but it can also be speculated that the surface composition of the substrate is changed by chemical reaction with the species in the plasma. In the case of Si substrates, the growth of Sic has been shown to be the result of an increased amount of CH4 in the Ar-CH, mixture [S]. The growth of Sic on the Si substrate has also been reported by Wong et al. [ 123. Finally, we define a figure of merit (FOM) CIof the films by F Lx=- logp

P

(2)

because a high adhesion F, high electric resistivity p and low friction coefficient p cause a higher FOM and a higher FOM is desirable for the present films. Fig. 6 shows the relationship between c( and the flow rate ratio, and one can see that the highest FOM can be attained at a flow rate ratio of 10%.

a-C:H thin films have been prepared with a d.c. magnetron sputtering graphite target in Ar-CH, discharge gas. The films are desired to have good characteristics for use as a protecting material. Changes in the CH, content in the sputtering gas mixture resulted in systematic changes in the microstructure of the films and in the adhesion strength. Raman spectroscopy and FTIR measurements revealed a decrease in crystalline structure and increase in hydrocarbon radicals in the films. The electric resistivity and the optical transmittance of the film were related to the structural changes in the films. Although the CH, content has a small effect on the friction coefficient, the adhesion strength was a minimum. More precise study is needed to understand the adhesion mechanism of the films. It appears that there is an optimum condition for depositing a-C:H films by d.c. magnetron sputtering in Ar-CH, gas, at a flow rate ratio of around 10%.

References [l] [2] [3] [4] [ 51 [6] [7] [S] [9] [lo] [ 111 [ 121 [ 131

J.C. Angus and F. Jansen, .I. Vat. Sci. Technol. A,6 (1988) 1778. N.-H. Cho, K.M. Krishnan, D.V. Veins, M.D. Rubin, C.B. Hopper, B. Bushan and D.B. Bogy, J. Mater. Res., 5 (1990) 2543. F. Jansen and Machonkin, Thin Solid Films, 140 (1986) 227. K. Kobayashi, N. Matsukura and Y. Machi, .I. Appl. Phys., 59 (1986) 910. K. Tsuji and K. Hirokawa, Appl. Surf. Sci., 59 (1992) 31. S. Baba, A. Kikuchi and A. Kinbara, J. Vat. Sci. Technol. A,5 (1987) 1860. A. Kinbara and S. Baba, Thin Solid Films, 163 (1988) 67. A. Kinbara, S. Baba and A. Kikuchi, J. Adhes. Sci. Technol., 2 (1988) 1. P. Benjamin and C. Weaver, Proc. R. Sot. London, Ser.A, 254 (1960) 163. G Gille and B. Rau, Thin Solid Films, 120 (1984) 109. M. Kohzaki, K. Higuchi, S. Noda and K. Uchida, J. Mater. Res., 7 (1992) 1769. MS. Wong, R. Meilunas, T.P. Ong and R.P.H. Chang, Appl. Phys. Lett., 54 (1989) 2006. S.S. Perry, J.W. Ager III and G.A. Somorjai, J. Mater. Res., 8 (1993) 2577.