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Mechanical properties of DLC films prepared in acetylene and methane plasmas using electron cyclotron resonance microwave plasma chemical vapor deposition
Diamond and Related Materials 10 Ž2001. 1862᎐1867
Mechanical properties of DLC films prepared in acetylene and methane plasmas using electron cyclotron resonance microwave plasma chemical vapor deposition K.H. Lai, C.Y. Chan, M.K. Fung, I. Bello, C.S. LeeU , S.T. Lee Center of Super-Diamond and Ad¨ anced Films (COSDAF) and Department of Physics and Materials Science, City Uni¨ ersity of Hong Kong, Tat Chee A¨ enue, Kowloon, Hong Kong, PR China
Abstract Diamond-like carbon ŽDLC. films were deposited on silicon using methane and acetylene plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition ŽECR-MPCVD.. The mechanical properties of DLC films were characterized by micro-Raman system, atomic force microscope, tribometer, nano-indenter used for both hardness and nano-scratch test measurements. The mechanical properties of both DLC films, prepared in methane and acetylene plasmas, respectively, strongly depended on the kinetic energy of impinging particles. The deposition at y120 V substrate bias gave rise to DLC films with the best mechanical properties for both methane and acetylene plasmas. The hardness measurements with variable indentation depth showed the characteristic changes in hardness values implying elastic deformations of supporting substrates. The maximum hardness value of DLC M films was 20 GPa while that of DLC A films was 28 GPa. However, the hardness dropped when DLC films were prepared at substrate biases more negative than y120 V due to the thermal graphitization. The improvement in DLC properties usually provided the films with smaller hydrogen content and higher density of sp 3 bondings. These parameters were engineered through controlling the deposition parameters. Particularly, the bombardment of growing DLC films by energetic ions showed to be extremely important to yield films with lower internal stress. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon films; Electron cyclotron resonance; Mechanical properties; Methane and acetylene plasma
1. Introduction Over the past two decades, the research works and industrial applications of diamond-like carbon ŽDLC. have been widely expanded. This trend is the result of the unique combination of many desirable characteristic and material properties, including high optical transparency, high dielectric strength, high thermal conductivity, high chemical inertness, biocompatibility, high surface smoothness and high hardness combined
U
Corresponding author. Tel.: q852-2788-7826; fax: q852-27887830. E-mail address: [email protected] ŽC.S. Lee..
with a low coefficient of friction in ambient w1᎐3x. Such extraordinary properties of DLC films suggest many practical applications immediately. One of the many examples of industrial applications is the use of DLC films for the coating of hard disks. In this particular application, very thin DLC films provide both mechanical and chemical protection of sensitive magnetic layers which are functional layers of storage media. Although many researchers have put a lot of effort on measuring various properties of DLC films prepared by plasma enhanced CVD w4x, magnetron sputtering w5᎐7x and cathode arc discharge method w8x, there have been a few systematic comparisons between the films prepared by different gas sources under the same conditions. In this work, we present study of the mechani-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 6 0 - 5
K.H. Lai et al. r Diamond and Related Materials 10 (2001) 1862᎐1867
cal properties of DLC films prepared on silicon by both methane and acetylene plasmas using an electron cyclotron resonance microwave plasma chemical vapor deposition ŽECR-MPCVD. technique.
2. Experimental An ECR-MPCVD apparatus, ASTeX AX2040, was used for the deposition of thin DLC films. A radiofrequency Žr.f.. generator, RFX-600, operating at 13.56 MHz was coupled to the substrate holder to induce a negative self-bias voltage on substrates. This arrangement allowed independent controlling, the production of reactive plasma constituents and kinetic energy of ions impinging to substrates. The DLC films were de˚ posited with a thickness of 3000 A. The DLC deposition was carried out in either a mixture of methanerargon or a mixture of acetylenerargon plasma. In the following text, the former and latter plasmas are referred to methane and acetylene plasmas, respectively. The DLC films prepared in methane plasma are assigned as DLC M films while those synthesized in acetylene plasma are assigned as DLC A films. The methane and argon were fed into the deposition system at flow rates of 6 and 60 sccm. In order to provide the same atomic carbon to argon atomic ratio in the case of acetylene plasma, acetylene and argon were supplied at flow rates of 3 and 60 sccm. Both DLC M and DLC A films were prepared using variable substrate bias voltage. The bias voltage varied from y60 to y150 V. All silicon substrates were ultrasonically cleaned in acetone and ethanol baths for 15 min, respectively. Native oxide of silicon was stripped off in a hydrofluoric acid solution. The etched and hydrogen passivated silicon were loaded through a fast entry chamber into the deposition chamber. Prior to deposition, all samples were sputter cleaned in argon plasma at a substrate bias of y120 V for 5 min. The phase structural changes in DLC films was studied using a Reinshaw Raman system 2000 operating at the 514.5-nm Ar laser excitation and at a resolution of 1 cmy1 . The surface roughness was measured by an atomic force microscope ŽAFM. operating in a contact mode for acquiring root mean square Žrms. roughness over a fixed sampling area of 1 = 1 m2 . Hardness of DLC films was measured by a nanoindenter ŽNano Indenter XP. with a maximum indentation depth of 300 nm. Nano-scratch resistance was measured using the same instrument with a special Berkovich tip. During the scratch test, the instrument continuously ramp the load from 20 to the 60 000 N as the indenter travels across the surface over 500 m. Tribological behaviors of DLC films were studied in terms of the
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time stability of friction coefficient during film wearing. The film wearing and measurements of friction coefficients were performed by a CSEM ball-on-disk ŽBOD. tribometer in ambient at a relative humidity and temperature of 56% and 22⬚C, respectively. The wearing ball was SiC with a diameter of 6 mm. A 5-N dead weight was used as applied loading. Wearing was carried out with a speed of 3 cmrs over 380 m corresponding to 20 000 laps on the tribometer disk.
3. Results and discussion 3.1. Raman analysis Raman is commonly used for characterization of DLC films using the graphitic band ŽG. at approximately 1550 cmy1 and disordered band ŽD. at approximately 1360 cmy1 in the Raman spectrum. These bands have been associated with several macroscopic and microscopic quantities of DLC films. In this experiment, the Raman spectra were fitted by combined Gaussian and Lorentzian functions. Fig. 1 shows the shift of the G band positions with changing the substrate bias for DLC films prepared in both methane and acetylene plasmas. The shift of G band position towards higher wavenumbers with the increase in substrate bias resulted from the structural changes towards graphite-like structure. The increase in substrate temperature induced by higher energetic ion bombardment promoted the film graphitization. Hence, such a graphitization process was thermally driven. The DLC A film was characterized with a wider full width at half maximum ŽFWHM. of G band ranging from approximately 180.1 to 189.7 cmy1 and the position of G band of DLC A film was centered at higher wavenumber when compared with the DLC M film ŽFWHM approx. 163.2 to 170.9.. Schwan et al. w9x reported that the FWHM of
Fig. 1. Variation of the Raman spectrum G band position with substrate bias voltage.
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G band may be related to the graphitic cluster size and internal stress of carbon films, the G band position may be associated with the hydrogen content in amorphous films. The higher G band position in the Raman spectrum and the lower background slope indicate the lower hydrogen content in the DLC A film. 3.2. Surface roughness The variation of root mean square Žrms. roughness with the substrate bias for DLC films is shown in Fig. 2. For both the DLC M and DLC A films, the trends of roughness variation with the bias voltage were quite similar. The rms roughness generally decreased with increasing negative bias voltage. At higher biases, the kinetic energy of the ions efficiently eliminates voids, asperities and rough particles by their high surface atomic mobility during film growth and preferential sputtering and etching topographically higher surface w10x. Hence, smoother surfaces can be achieved at higher bias. The unusual high rms roughness of DLC A prepared at a bias of y60 V is due to the formation of small clusters on the surface ŽFig. 3b.. The kinetic energy of the ions at such a low bias is insufficient to remove certain asperities and hence, unable to smooth the film surface w5x. Roughness of the DLC M films prepared at y60 V were found to be lower than that of the DLC A films when prepared under the same bias. The higher smoothness of DLC M is attributed to the difference in CrH ratios of methane and acetylene plasmas. Both Rusli et al. w11x and Hammer et al. w12x reported that synergetic effect of ion bombardment and formation of volatile hydrocarbon compounds is responsible for smoothing DLC films. Since hydrogen acts as an efficient etching agent and promotes smoothing surfaces of DLC films, methane plasma, richer in hydrogen, always yields smoother surfaces than those resulting from acetylene plasma.
Fig. 2. RMS roughness of DLC films against different substrate bias.
Fig. 3. AFM image of DLC A prepared in Ža. y120 V and Žb. y60 V.
3.3. Film hardness
The hardness of the DLC films measured against the indentation depth is plotted in Fig. 4 and summarized in Fig. 5. For the DLC A , the highest hardness value is always recorded at the indentation depth of one third of the film thickness and then it decreases with the increase in indentation depth. In contrast, in the case of DLC M films, the hardness values is almost independent on the indentation depth. Such contradictory results are associated with the substrate deformation in different portions along the penetration depth affecting the recorded hardness value. Silicon substrate has a hardness value of approximately 10 GPa. The increase in indentation depth will record hardness towards this value. Accordingly, harder DLC A films gave rise to greater reduction of recorded hardness with increasing the indentation depth. The hardness of DLC M films prepared at y60 V bias is relatively small and reaches a value approximately 10 GPa. As bias voltage increases in its absolute value, the hardness does as well and it acquires a maximum value of approximately 20 GPa at y120 V bias. Then it slightly drops at a higher bias y150 V bias due to thermally driven graphitization process. The hardness values of DLC A films vs. bias voltage showed a similar trend as that described for DLC M films, however, the hardness variation was rather mod-
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and subsequent hydrogen incorporation into carbon films. The films deposited at lower substrate bias and grown from plasmas richer in hydrogen results in higher hydrogen incorporation w13x. Since monovalent hydrogen can serve as the terminating atoms in a carbon network, lower hardness of films can be induced by hydrogen incorporation, most likely by developing softer and polymeric-like regions. Based on the difference in atomic structures of methane and acetylene, the number of monovalent hydrogen was obviously reduced by using acetylene in DLC deposition. Hence, high hardness of DLC A films is still maintained at low substrate bias. DLC A films are not polymer-like even at such a low bias as y60 V. The increase in substrate bias leads to the intense impingement distorting the carbon planes and then reinstating by cross-linked sp 3 carbon bonding as described by Pool et al. w14x. Correspondingly, the cross-linkages reduce the space in the network and increase the packing density of atoms in films. This increases the fraction of sp 3 sites and the local bonding environment becomes more diamond-like. As a result, the hardness of both DLC M and DLC A increases with increasing the substrate bias. However, an ion energy corresponding to greater bias than y120 V can induce film graphitization and thus alternation of carbon film properties as suggested above. 3.4. Scratch resistance
Fig. 4. Hardness against depth of Ža. DLC A and Žb. DLC M .
erate. In addition, the hardness value of DLC A films prepared at y60 V was already higher Ž24 GPa. than the maximum hardness value Ž20 GPa. of DLC M films. These differences are interrelated with kinetic energy of plasma constituents impinging on substrate surface
Fig. 5. Maximum hardness of DLC films at different substrate bias.
Fig. 6a shows surface scratching profiles along 700 m long tracks in DLC M films prepared at y90 V bias. The pre-scan profile illustrates that the initial surface, before its scratching, is smooth and horizontal. For the scratch-scan profile, as the loading continues increases along the scratching direction, the penetration depth first increases smoothly until reaching a critical load. At the critical load, the film fractures and the tip makes multi-contacts with the substrate resulting in a zigzag profile. The critical load is identified by the first abrupt change in friction force. After scratching, the film and substrates partially recover from the plastic deformation, therefore, the unloaded post-scan is normally above that of scan scratch. The post-scan usually retains the shape of scratch-scan that contains smooth and fractured zigzag regions. However, the scratching of DLC M film prepared at y150 V substrate bias ŽFig. 6b. shows the extension of the zigzag region towards non-fracture region. The initial zigzag region of the post-scan profile drops below scratch-scan profile indicating that the considered region was not fractured during the scratch process. Propagation of zigzag region indicates that the internal stress in the film is so high that film tends to release the stress by delamination. The initial cracking, induced on the substrate-film interface by external force, promotes the
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films under y150 V bias, other than a decrease in hardness, is probably due to the great increase in internal stress. The high energy bombardment of plasma ions induce a larger internal compress stress inside the film. Such high stress promotes film delamination at a smaller loading. 3.5. Tribological measurements
Fig. 6. Scratch profile of Ža. DLC M prepared at y90 V Žwhich is a typical scratch profile among most of DLC films. and Žb. scratch profile of DLC M prepared at y150 V.
film delamination with a lower consumed energy. The presence of such effect indicates that the deposition condition favors the formation of films with the highest internal stress. A plot of critical loading for film fracture of DLC films is shown in Fig. 7. Such comparison shows the film resistance against plastic deformation. It has been reported that hardness is an important parameter and that H 3rE 2 Žwhere H is the hardness and E is the elastic modulus. can give a quantity measurement of resistivity against fracture w8x. But the film internal stress and the adhesion between the film and the substrate also affect the critical load of film fracture. Both DLC A and DLC M give an almost identical trend in resistance to fracture. Under the same bias, however, DLC A films always provide the higher resistance than DLC M films do. The trend of the resistance to fracture is consistent with the film hardness when films are prepared within a range of y60 V bias to y120 V substrate bias. The higher the hardness of the film, the greater load the films can endure. The great decrease in critical load in both DLC M and DLC A
Fig. 8 shows the change of friction coefficients for DLC films deposited at different bias voltages and different gases with the wearing distance. Normally, friction coefficients of DLC films range from 0.1 to 0.4 at ambient w15x. Such variation is usually due to different structures, compositions and wettability of the DLC films. Report of Liu et al. w15x and Donnet w16x demonstrates that the frictions and wear rates of DLC films are very sensitive to the hydrogen, water and oxygen species. All friction coefficients of the investigated DLC films shown in Fig. 8 were relatively very small and did not exceed a value of 0.25. The exception is only DLC M prepared at y60 V bias voltage. This film cracked after 10 000 laps and friction coefficients raised to 0.6 being the uncoated silicon worn by a SiC ball. This film which is easily worn, results from the film structure itself being soft and polymer-like. The friction coefficients largely depend on the substrate bias as well. Both DLC A and DLC M films prepared at y120 V bias have the lowest friction coefficients. The films prepared at y60, y90 and y150 V have similar friction coefficients though they are deposited in different plasma environments. Most of DLC films show similar trends in friction coefficients plotted vs. the number of laps or traveled distance. First, the friction coefficients are almost steady for the first 6000 laps corresponding to a travel distance of approximately 120 m. Then the friction coefficients slowly decrease with the traveled dis-
Fig. 7. Critical loading for film fracture of DLC films against substrate bias.
K.H. Lai et al. r Diamond and Related Materials 10 (2001) 1862᎐1867
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4. Conclusions In most of the measurements presented above, the DLC A prepared in acetylene plasma showed higher quality of mechanical properties and performed much better in wear and scratch tests when compared with the DLC M films prepared in methane plasma under the same deposition conditions. Although the surface of DLC A films was slightly rougher than that of DLC M films, the DLC A films consistently provided the higher hardness values and better scratch resistance. The highest hardness values for both DLC A and DLC M were measured for the films prepared at y120 V substrate bias. The maximum hardness values were 20 and 29 GPa for DLC M and DLC A films, respectively. The improvement of film properties usually is normally accompanied with reduction of hydrogen content and higher yield of carbon ᎏ carbon sp 3 bonding states which are normally achieved using suitable ion bombardment giving rise to low internal stress and or less thermally-induced graphization. References
Fig. 8. Friction coefficient of Ža. DLC A and Žb. DLC M vs. wear distance.
tance before stabilizing at their lower values. Such behavior is caused by wearing the ball surface. The DLC derby on the wear track forming a thin DLC layer on the ball surface slowly implying that the friction process switched from SiC against DLC to DLC on ball surface against DLC film. Such processes are stable and have low friction coefficients. The graphitization of DLC films by the released heat during friction process is another reason for low friction coefficients and their steady values. Since graphite is a common lubricant in ambient, due to sorption processes, the friction coefficients can achieve low values when graphitization reaches certain marginal values. It can be easily demonstrated that the G band of Raman peak collected from a wear track gives rise to a shift in approximately 2 cmy1 towards the higher wavenumber.
w1x P.W. Pastel, W.J. Varhue, J. Vac. Sci. Technol. A 9 Ž1991. 1129. w2x S.C. Kao, E.E. Kunhardt, A.R. Srivatsa, Appl. Phys. Lett. 59 Ž1991. 2532. w3x I. Nagai, A. Ishitani, H. Kuroda, M. Yoshikawa, N. Nagai, J. Appl. Phys. 67 Ž1990. 2890. w4x A. Grill, V. Patel, B.S. Meyerson, in: Y. Tzeng, M. Youhikawa, A. Feldman ŽEds.., Applications of Diamond Films and Related Materials, , 1991, p. 683. w5x W.-C. Chan, B. Zhou, Y.-W. Chung, C.S. Lee, S.T. Lee, J. Vac. Sci. Technol. A 16 Ž1998. 1970. w6x B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 Ž1998. 2491. w7x E.V. Anoikin, M.M. Yang, J.L. Chao, J.R. Elings, D.W. Brown, J. Vac. Sci Technol. A 16 Ž1998. 1741. w8x T.Y. Tsui, G.M. Pharr, W.C. Oliver, C.S. Bhatia, R.L. White, S. Anders, A. Anders, I.G. Brown, Mater. Res. Soc. Symp. Proc. 383 Ž1995. 447. w9x J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 Ž1996. 440. w10x D. Li, Y.W. Chung, M.S. Wong, W.D. Sproul, Tribol. Lett. 1 Ž1995. 87. w11x P.S. Andry, P.W. Pastel, W.J. Varhue, J. Mater. Res. 11 Ž1996. 221. w12x A. Rusli, S.F. Yoon, H. Yang, Q. Zhang, J. Ahn, Y.L. Fu, J. Val. Sci. Technol. A 16 Ž1998. 572. w13x Wai-Chung Chan, Man-Keung Fung, Igor Bello, Chun-Sing Lee and Shuit-Tong Lee, Diamond Relat. Mater., in press. w14x F.S. Pool, Y.H. Shing, J. Appl. Phys. 68 Ž1990. 62. w15x C. Donnet, Condens. Matter News 4 Ž1995. 9. w16x E. Liu, B. Blanpain, X. Shi, J.-P. Celis, H.-S. Tan, B.-K. Tay, L.-K Cheah, J.R. Roos, Surf. Coat Technol. 106 Ž1998. 72.
Mechanical properties of DLC films prepared in acetylene and methane plasmas using electron cyclotron resonance microwave plasma chemical vapor deposition K.H. Lai, C.Y. Chan, M.K. Fung, I. Bello, C.S. LeeU , S.T. Lee Center of Super-Diamond and Ad¨ anced Films (COSDAF) and Department of Physics and Materials Science, City Uni¨ ersity of Hong Kong, Tat Chee A¨ enue, Kowloon, Hong Kong, PR China
Abstract Diamond-like carbon ŽDLC. films were deposited on silicon using methane and acetylene plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition ŽECR-MPCVD.. The mechanical properties of DLC films were characterized by micro-Raman system, atomic force microscope, tribometer, nano-indenter used for both hardness and nano-scratch test measurements. The mechanical properties of both DLC films, prepared in methane and acetylene plasmas, respectively, strongly depended on the kinetic energy of impinging particles. The deposition at y120 V substrate bias gave rise to DLC films with the best mechanical properties for both methane and acetylene plasmas. The hardness measurements with variable indentation depth showed the characteristic changes in hardness values implying elastic deformations of supporting substrates. The maximum hardness value of DLC M films was 20 GPa while that of DLC A films was 28 GPa. However, the hardness dropped when DLC films were prepared at substrate biases more negative than y120 V due to the thermal graphitization. The improvement in DLC properties usually provided the films with smaller hydrogen content and higher density of sp 3 bondings. These parameters were engineered through controlling the deposition parameters. Particularly, the bombardment of growing DLC films by energetic ions showed to be extremely important to yield films with lower internal stress. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon films; Electron cyclotron resonance; Mechanical properties; Methane and acetylene plasma
1. Introduction Over the past two decades, the research works and industrial applications of diamond-like carbon ŽDLC. have been widely expanded. This trend is the result of the unique combination of many desirable characteristic and material properties, including high optical transparency, high dielectric strength, high thermal conductivity, high chemical inertness, biocompatibility, high surface smoothness and high hardness combined
U
Corresponding author. Tel.: q852-2788-7826; fax: q852-27887830. E-mail address: [email protected] ŽC.S. Lee..
with a low coefficient of friction in ambient w1᎐3x. Such extraordinary properties of DLC films suggest many practical applications immediately. One of the many examples of industrial applications is the use of DLC films for the coating of hard disks. In this particular application, very thin DLC films provide both mechanical and chemical protection of sensitive magnetic layers which are functional layers of storage media. Although many researchers have put a lot of effort on measuring various properties of DLC films prepared by plasma enhanced CVD w4x, magnetron sputtering w5᎐7x and cathode arc discharge method w8x, there have been a few systematic comparisons between the films prepared by different gas sources under the same conditions. In this work, we present study of the mechani-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 6 0 - 5
K.H. Lai et al. r Diamond and Related Materials 10 (2001) 1862᎐1867
cal properties of DLC films prepared on silicon by both methane and acetylene plasmas using an electron cyclotron resonance microwave plasma chemical vapor deposition ŽECR-MPCVD. technique.
2. Experimental An ECR-MPCVD apparatus, ASTeX AX2040, was used for the deposition of thin DLC films. A radiofrequency Žr.f.. generator, RFX-600, operating at 13.56 MHz was coupled to the substrate holder to induce a negative self-bias voltage on substrates. This arrangement allowed independent controlling, the production of reactive plasma constituents and kinetic energy of ions impinging to substrates. The DLC films were de˚ posited with a thickness of 3000 A. The DLC deposition was carried out in either a mixture of methanerargon or a mixture of acetylenerargon plasma. In the following text, the former and latter plasmas are referred to methane and acetylene plasmas, respectively. The DLC films prepared in methane plasma are assigned as DLC M films while those synthesized in acetylene plasma are assigned as DLC A films. The methane and argon were fed into the deposition system at flow rates of 6 and 60 sccm. In order to provide the same atomic carbon to argon atomic ratio in the case of acetylene plasma, acetylene and argon were supplied at flow rates of 3 and 60 sccm. Both DLC M and DLC A films were prepared using variable substrate bias voltage. The bias voltage varied from y60 to y150 V. All silicon substrates were ultrasonically cleaned in acetone and ethanol baths for 15 min, respectively. Native oxide of silicon was stripped off in a hydrofluoric acid solution. The etched and hydrogen passivated silicon were loaded through a fast entry chamber into the deposition chamber. Prior to deposition, all samples were sputter cleaned in argon plasma at a substrate bias of y120 V for 5 min. The phase structural changes in DLC films was studied using a Reinshaw Raman system 2000 operating at the 514.5-nm Ar laser excitation and at a resolution of 1 cmy1 . The surface roughness was measured by an atomic force microscope ŽAFM. operating in a contact mode for acquiring root mean square Žrms. roughness over a fixed sampling area of 1 = 1 m2 . Hardness of DLC films was measured by a nanoindenter ŽNano Indenter XP. with a maximum indentation depth of 300 nm. Nano-scratch resistance was measured using the same instrument with a special Berkovich tip. During the scratch test, the instrument continuously ramp the load from 20 to the 60 000 N as the indenter travels across the surface over 500 m. Tribological behaviors of DLC films were studied in terms of the
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time stability of friction coefficient during film wearing. The film wearing and measurements of friction coefficients were performed by a CSEM ball-on-disk ŽBOD. tribometer in ambient at a relative humidity and temperature of 56% and 22⬚C, respectively. The wearing ball was SiC with a diameter of 6 mm. A 5-N dead weight was used as applied loading. Wearing was carried out with a speed of 3 cmrs over 380 m corresponding to 20 000 laps on the tribometer disk.
3. Results and discussion 3.1. Raman analysis Raman is commonly used for characterization of DLC films using the graphitic band ŽG. at approximately 1550 cmy1 and disordered band ŽD. at approximately 1360 cmy1 in the Raman spectrum. These bands have been associated with several macroscopic and microscopic quantities of DLC films. In this experiment, the Raman spectra were fitted by combined Gaussian and Lorentzian functions. Fig. 1 shows the shift of the G band positions with changing the substrate bias for DLC films prepared in both methane and acetylene plasmas. The shift of G band position towards higher wavenumbers with the increase in substrate bias resulted from the structural changes towards graphite-like structure. The increase in substrate temperature induced by higher energetic ion bombardment promoted the film graphitization. Hence, such a graphitization process was thermally driven. The DLC A film was characterized with a wider full width at half maximum ŽFWHM. of G band ranging from approximately 180.1 to 189.7 cmy1 and the position of G band of DLC A film was centered at higher wavenumber when compared with the DLC M film ŽFWHM approx. 163.2 to 170.9.. Schwan et al. w9x reported that the FWHM of
Fig. 1. Variation of the Raman spectrum G band position with substrate bias voltage.
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G band may be related to the graphitic cluster size and internal stress of carbon films, the G band position may be associated with the hydrogen content in amorphous films. The higher G band position in the Raman spectrum and the lower background slope indicate the lower hydrogen content in the DLC A film. 3.2. Surface roughness The variation of root mean square Žrms. roughness with the substrate bias for DLC films is shown in Fig. 2. For both the DLC M and DLC A films, the trends of roughness variation with the bias voltage were quite similar. The rms roughness generally decreased with increasing negative bias voltage. At higher biases, the kinetic energy of the ions efficiently eliminates voids, asperities and rough particles by their high surface atomic mobility during film growth and preferential sputtering and etching topographically higher surface w10x. Hence, smoother surfaces can be achieved at higher bias. The unusual high rms roughness of DLC A prepared at a bias of y60 V is due to the formation of small clusters on the surface ŽFig. 3b.. The kinetic energy of the ions at such a low bias is insufficient to remove certain asperities and hence, unable to smooth the film surface w5x. Roughness of the DLC M films prepared at y60 V were found to be lower than that of the DLC A films when prepared under the same bias. The higher smoothness of DLC M is attributed to the difference in CrH ratios of methane and acetylene plasmas. Both Rusli et al. w11x and Hammer et al. w12x reported that synergetic effect of ion bombardment and formation of volatile hydrocarbon compounds is responsible for smoothing DLC films. Since hydrogen acts as an efficient etching agent and promotes smoothing surfaces of DLC films, methane plasma, richer in hydrogen, always yields smoother surfaces than those resulting from acetylene plasma.
Fig. 2. RMS roughness of DLC films against different substrate bias.
Fig. 3. AFM image of DLC A prepared in Ža. y120 V and Žb. y60 V.
3.3. Film hardness
The hardness of the DLC films measured against the indentation depth is plotted in Fig. 4 and summarized in Fig. 5. For the DLC A , the highest hardness value is always recorded at the indentation depth of one third of the film thickness and then it decreases with the increase in indentation depth. In contrast, in the case of DLC M films, the hardness values is almost independent on the indentation depth. Such contradictory results are associated with the substrate deformation in different portions along the penetration depth affecting the recorded hardness value. Silicon substrate has a hardness value of approximately 10 GPa. The increase in indentation depth will record hardness towards this value. Accordingly, harder DLC A films gave rise to greater reduction of recorded hardness with increasing the indentation depth. The hardness of DLC M films prepared at y60 V bias is relatively small and reaches a value approximately 10 GPa. As bias voltage increases in its absolute value, the hardness does as well and it acquires a maximum value of approximately 20 GPa at y120 V bias. Then it slightly drops at a higher bias y150 V bias due to thermally driven graphitization process. The hardness values of DLC A films vs. bias voltage showed a similar trend as that described for DLC M films, however, the hardness variation was rather mod-
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and subsequent hydrogen incorporation into carbon films. The films deposited at lower substrate bias and grown from plasmas richer in hydrogen results in higher hydrogen incorporation w13x. Since monovalent hydrogen can serve as the terminating atoms in a carbon network, lower hardness of films can be induced by hydrogen incorporation, most likely by developing softer and polymeric-like regions. Based on the difference in atomic structures of methane and acetylene, the number of monovalent hydrogen was obviously reduced by using acetylene in DLC deposition. Hence, high hardness of DLC A films is still maintained at low substrate bias. DLC A films are not polymer-like even at such a low bias as y60 V. The increase in substrate bias leads to the intense impingement distorting the carbon planes and then reinstating by cross-linked sp 3 carbon bonding as described by Pool et al. w14x. Correspondingly, the cross-linkages reduce the space in the network and increase the packing density of atoms in films. This increases the fraction of sp 3 sites and the local bonding environment becomes more diamond-like. As a result, the hardness of both DLC M and DLC A increases with increasing the substrate bias. However, an ion energy corresponding to greater bias than y120 V can induce film graphitization and thus alternation of carbon film properties as suggested above. 3.4. Scratch resistance
Fig. 4. Hardness against depth of Ža. DLC A and Žb. DLC M .
erate. In addition, the hardness value of DLC A films prepared at y60 V was already higher Ž24 GPa. than the maximum hardness value Ž20 GPa. of DLC M films. These differences are interrelated with kinetic energy of plasma constituents impinging on substrate surface
Fig. 5. Maximum hardness of DLC films at different substrate bias.
Fig. 6a shows surface scratching profiles along 700 m long tracks in DLC M films prepared at y90 V bias. The pre-scan profile illustrates that the initial surface, before its scratching, is smooth and horizontal. For the scratch-scan profile, as the loading continues increases along the scratching direction, the penetration depth first increases smoothly until reaching a critical load. At the critical load, the film fractures and the tip makes multi-contacts with the substrate resulting in a zigzag profile. The critical load is identified by the first abrupt change in friction force. After scratching, the film and substrates partially recover from the plastic deformation, therefore, the unloaded post-scan is normally above that of scan scratch. The post-scan usually retains the shape of scratch-scan that contains smooth and fractured zigzag regions. However, the scratching of DLC M film prepared at y150 V substrate bias ŽFig. 6b. shows the extension of the zigzag region towards non-fracture region. The initial zigzag region of the post-scan profile drops below scratch-scan profile indicating that the considered region was not fractured during the scratch process. Propagation of zigzag region indicates that the internal stress in the film is so high that film tends to release the stress by delamination. The initial cracking, induced on the substrate-film interface by external force, promotes the
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films under y150 V bias, other than a decrease in hardness, is probably due to the great increase in internal stress. The high energy bombardment of plasma ions induce a larger internal compress stress inside the film. Such high stress promotes film delamination at a smaller loading. 3.5. Tribological measurements
Fig. 6. Scratch profile of Ža. DLC M prepared at y90 V Žwhich is a typical scratch profile among most of DLC films. and Žb. scratch profile of DLC M prepared at y150 V.
film delamination with a lower consumed energy. The presence of such effect indicates that the deposition condition favors the formation of films with the highest internal stress. A plot of critical loading for film fracture of DLC films is shown in Fig. 7. Such comparison shows the film resistance against plastic deformation. It has been reported that hardness is an important parameter and that H 3rE 2 Žwhere H is the hardness and E is the elastic modulus. can give a quantity measurement of resistivity against fracture w8x. But the film internal stress and the adhesion between the film and the substrate also affect the critical load of film fracture. Both DLC A and DLC M give an almost identical trend in resistance to fracture. Under the same bias, however, DLC A films always provide the higher resistance than DLC M films do. The trend of the resistance to fracture is consistent with the film hardness when films are prepared within a range of y60 V bias to y120 V substrate bias. The higher the hardness of the film, the greater load the films can endure. The great decrease in critical load in both DLC M and DLC A
Fig. 8 shows the change of friction coefficients for DLC films deposited at different bias voltages and different gases with the wearing distance. Normally, friction coefficients of DLC films range from 0.1 to 0.4 at ambient w15x. Such variation is usually due to different structures, compositions and wettability of the DLC films. Report of Liu et al. w15x and Donnet w16x demonstrates that the frictions and wear rates of DLC films are very sensitive to the hydrogen, water and oxygen species. All friction coefficients of the investigated DLC films shown in Fig. 8 were relatively very small and did not exceed a value of 0.25. The exception is only DLC M prepared at y60 V bias voltage. This film cracked after 10 000 laps and friction coefficients raised to 0.6 being the uncoated silicon worn by a SiC ball. This film which is easily worn, results from the film structure itself being soft and polymer-like. The friction coefficients largely depend on the substrate bias as well. Both DLC A and DLC M films prepared at y120 V bias have the lowest friction coefficients. The films prepared at y60, y90 and y150 V have similar friction coefficients though they are deposited in different plasma environments. Most of DLC films show similar trends in friction coefficients plotted vs. the number of laps or traveled distance. First, the friction coefficients are almost steady for the first 6000 laps corresponding to a travel distance of approximately 120 m. Then the friction coefficients slowly decrease with the traveled dis-
Fig. 7. Critical loading for film fracture of DLC films against substrate bias.
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4. Conclusions In most of the measurements presented above, the DLC A prepared in acetylene plasma showed higher quality of mechanical properties and performed much better in wear and scratch tests when compared with the DLC M films prepared in methane plasma under the same deposition conditions. Although the surface of DLC A films was slightly rougher than that of DLC M films, the DLC A films consistently provided the higher hardness values and better scratch resistance. The highest hardness values for both DLC A and DLC M were measured for the films prepared at y120 V substrate bias. The maximum hardness values were 20 and 29 GPa for DLC M and DLC A films, respectively. The improvement of film properties usually is normally accompanied with reduction of hydrogen content and higher yield of carbon ᎏ carbon sp 3 bonding states which are normally achieved using suitable ion bombardment giving rise to low internal stress and or less thermally-induced graphization. References
Fig. 8. Friction coefficient of Ža. DLC A and Žb. DLC M vs. wear distance.
tance before stabilizing at their lower values. Such behavior is caused by wearing the ball surface. The DLC derby on the wear track forming a thin DLC layer on the ball surface slowly implying that the friction process switched from SiC against DLC to DLC on ball surface against DLC film. Such processes are stable and have low friction coefficients. The graphitization of DLC films by the released heat during friction process is another reason for low friction coefficients and their steady values. Since graphite is a common lubricant in ambient, due to sorption processes, the friction coefficients can achieve low values when graphitization reaches certain marginal values. It can be easily demonstrated that the G band of Raman peak collected from a wear track gives rise to a shift in approximately 2 cmy1 towards the higher wavenumber.
w1x P.W. Pastel, W.J. Varhue, J. Vac. Sci. Technol. A 9 Ž1991. 1129. w2x S.C. Kao, E.E. Kunhardt, A.R. Srivatsa, Appl. Phys. Lett. 59 Ž1991. 2532. w3x I. Nagai, A. Ishitani, H. Kuroda, M. Yoshikawa, N. Nagai, J. Appl. Phys. 67 Ž1990. 2890. w4x A. Grill, V. Patel, B.S. Meyerson, in: Y. Tzeng, M. Youhikawa, A. Feldman ŽEds.., Applications of Diamond Films and Related Materials, , 1991, p. 683. w5x W.-C. Chan, B. Zhou, Y.-W. Chung, C.S. Lee, S.T. Lee, J. Vac. Sci. Technol. A 16 Ž1998. 1970. w6x B. Wei, B. Zhang, K.E. Johnson, J. Appl. Phys. 83 Ž1998. 2491. w7x E.V. Anoikin, M.M. Yang, J.L. Chao, J.R. Elings, D.W. Brown, J. Vac. Sci Technol. A 16 Ž1998. 1741. w8x T.Y. Tsui, G.M. Pharr, W.C. Oliver, C.S. Bhatia, R.L. White, S. Anders, A. Anders, I.G. Brown, Mater. Res. Soc. Symp. Proc. 383 Ž1995. 447. w9x J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 Ž1996. 440. w10x D. Li, Y.W. Chung, M.S. Wong, W.D. Sproul, Tribol. Lett. 1 Ž1995. 87. w11x P.S. Andry, P.W. Pastel, W.J. Varhue, J. Mater. Res. 11 Ž1996. 221. w12x A. Rusli, S.F. Yoon, H. Yang, Q. Zhang, J. Ahn, Y.L. Fu, J. Val. Sci. Technol. A 16 Ž1998. 572. w13x Wai-Chung Chan, Man-Keung Fung, Igor Bello, Chun-Sing Lee and Shuit-Tong Lee, Diamond Relat. Mater., in press. w14x F.S. Pool, Y.H. Shing, J. Appl. Phys. 68 Ž1990. 62. w15x C. Donnet, Condens. Matter News 4 Ž1995. 9. w16x E. Liu, B. Blanpain, X. Shi, J.-P. Celis, H.-S. Tan, B.-K. Tay, L.-K Cheah, J.R. Roos, Surf. Coat Technol. 106 Ž1998. 72.