CVD of hard DLC films in a radio frequency inductively coupled plasma source

Thin Solid Films 390 Ž2001. 98᎐103

CVD of hard DLC films in a radio frequency inductively coupled plasma source S.J. YuU , Z.F. Ding, J. Xu, J.L. Zhang, T.C. Ma State Key Laboratory of Material Modification by Laser Electron and Ion Beams, Dalian Uni¨ ersity of Technology, Dalian 116024, PR China

Abstract Chemical vapor deposition ŽCVD. of hard diamond-like carbon ŽDLC. films on silicon Ž100. substrates from methane was successfully carried out using a radio frequency Žr.f.. inductively coupled plasma source ŽICPS.. Different deposition parameters such as bias voltage, r.f. power, gas flow and pressure were involved. The structures of the films were characterized by Fourier transform infrared ŽFTIR. spectroscopy and Raman spectroscopy. The hardness of the DLC films was measured by a Knoop microhardness tester. The surface morphology of the films was characterized by atomic force microscope ŽAFM. and the surface roughness Ž R a . was derived from the AFM data. The films are smooth with roughness less than 1.007 nm. Raman spectra shows that the films have typical diamond-like characteristics with a D line peak at ; 1331 cmy1 and a G line peak at ; 1544 cmy1, and the low intensity ratio of I DrIG indicate that the DLC films have a high ratio of sp 3 to sp 2 bonding, which is also in accordance with the results of FTIR spectra. The films hardness can reach approximately 42 GPa at a comparatively low substrate bias voltage, which is much greater than that of DLC films deposited in a conventional r.f. capacitively coupled parallel-plate system. It is suggested that the high plasma density and the suitable deposition environment Žsuch as the amount and ratio of hydrocarbon radicals to atomic or ionic hydrogen. obtained in the ICPS are important for depositing hard and high quality DLC films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon ŽDLC. film; Inductively coupled plasma ŽICP.; Chemical vapor deposition ŽCVD.; Radio frequency Žr.f..

1. Introduction Diamond-like carbon ŽDLC. films have attracted considerable attention recently due to their properties of high mechanical hardness, high electrical resistivity, low friction coefficient, optical transparency and chemical inertness w1x. These properties make the DLC films suitable for numerous potential applications in hard and wear-resistant coating, lithography, protective optical and biomedical coating, electroluminescence materials and field-emission devices w2,3x. Various tech-

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Corresponding author. Tel.: q86-411-4708389; fax: q86-4114708389. E-mail address: [email protected] ŽS.J. Yu..

niques have been used for preparing the DLC films, including ion-beam deposition, reactive magnetron sputtering, pulsed laser deposition ŽPLD. and filtered cathodic vacuum arc ŽFCVA., direct current plasma CVD, radio frequency plasma CVD, electron cyclotron resonance ŽECR. microwave plasma CVD w4᎐7x. Recently a radio frequency inductively coupled plasma source ŽICPS. has been developed as a new plasma excitation method since the early 1990s w8x. The ICPS is very attractive from the plasma processing point of view, since it can provide a high-density Ž) 10 11 rcm3 ., low capacitively coupling, large-area uniformity and low-pressure plasma by a relatively simple method w9x. These characteristics of the ICPS are favorable for chemical vapor deposition of various high quality films like DLC and SiO 2 . In this paper, we report about how

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hard diamond-like carbon films have been prepared successfully in an ICPS system and study the results of parameter effects on RF-ICPECVD on DLC films. 2. Experimental DLC films were deposited on single crystal siliconŽ100. substrates using high purity CH 4 gas Ž99.95%. as the source in a 13.56-MHz radio frequency inductively coupled plasma enhanced chemical vapor deposition system. Operation of this system may be summarized as follows: a single circle coupled coil was wrapped around a heat-resistant glass chamber Ždiameter of the chamber is approx. 270 mm.. The power was supplied by an RF generator and a matching network, and generated a RF magnetic field through the coupled coil and then creates an induced electrical field inside the chamber. CH 4 gas was decomposed into various radicals or ions and electrons with the r.f. power, and the electrons accelerated by the induced electrical field, gained high energy to ionize other neutral molecules and radicals, finally as a result it formed an inductively coupled plasma with relatively high ionization efficiency. Negative bias voltage applied on to the substrates was controlled by another r.f. bias power source independently. It became easy to regulate the deposition power and bias voltage during the CVD process. A shield case was used for prevention from r.f. power radiation. The base pressure of the deposition chamber was less than 2 = 10y3 Pa. Fig. 1 shows the schematic diagram of the RF-ICPECVD system. Prior to deposition the substrates were cleaned with methanol and acetone, and a 2᎐4-min argon cleaning was carried out to remove the native oxide from the silicon surface. Due to the plasma self-heating effect, the temperature of substrate measured by a thermocouple was approximately 50⬚C. The details of other parameter ranges are listed in Table 1. The thickness of deposited DLC films is in the scope of 1᎐8 ␮m with different deposition parameters. The surface morphology of the films was characterized by a Nanoscope IIIa atomic force microscope ŽDigital Instruments Inc.. which can operate in ambient air. The AFM data were used to derive a surface roughness Ž R a .. Hardness of the films was determined by a SHIMAZU DHS-2 micro hardness tester, and

Fig. 1. Schematic diagram of the RF-ICPECVD system.

knoop hardness was used for characterization of the DLC films. The Raman spectra were obtained using a Spex-1430 Raman spectrometer, and frequency in the range of 1100᎐1800 cmy1 at 2 cmy1 intervals were recorded. The spectra are fitted by two Gaussian line shapes using curve-fitting software. Fourier transform infrared ŽFTIR. spectroscopy ŽNicholet, avator 360. was used to detect the changes in the hydrogen bonding of the DLC films. Because not all of carbon bonded to hydrogen, the values of C᎐H sp 2rsp 3 ratio determined from FTIR spectroscopy are qualitative, not quantitative w10x. 3. Results and discussion Fig. 2 shows the AFM images of the DLC film deposited in ICPS. The surface morphology suggests that the DLC film was made of lots of amorphous carbon clusters. Fig. 3 shows the variation of surface roughness of the DLC films deposited at different bias voltage and r.f. power levels. The films are smooth and

Table 1 Parameter range of DLC films deposited in RF-ICPECVD system RF power ŽW. CH4 flow Žsccm. Bias voltage ŽV. Pressure ŽPa. Time Žmin.

100᎐600 20᎐80 y60 to y240 0.7᎐10 20᎐180

Fig. 2. AFM images of hard DLC films Ž300 W, 42 sccm, y140 V, 2.5 Pa, 150 min..

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S.J. Yu et al. r Thin Solid Films 390 (2001) 98᎐103

Fig. 3. Variation of roughness Ž R a . with bias voltage and r.f. power Ž42 sccm, 2.5 Pa..

the maximum roughness is less than 1.007 nm, and the roughness increases as the r.f. power and bias voltage increase. No film is deposited when bias voltage and power are too high. Fig. 4 shows the relationship between the knoop hardness of the DLC films and bias voltage. The hardness increases first as the bias voltage increases, and then decrease when the bias voltage exceeds y140 V. Compared with the conventional r.f. capacitively coupled parallel-plate system, harder DLC films can deposit at relatively low bias voltage. The maximum hardness can reach 42 GPa at bias voltage of approximately ᎐140 V, it is much greater than that in the 10᎐30 GPa range of the r.f. capatively coupled parallel-plate system while bias voltage is not less than ᎐400 V w11x. We

think that this difference is attributed to the high plasma density in the ICPS during DLC films deposition. Raman spectra can provide much information about the DLC films structure due to its ability to distinguish sp 3 and sp 2 bonding type w12x. In general, the Raman spectra of amorphous carbon film consists of a broad peak at ; 1550 cmy1 ŽG line. and a shoulder at ; 1350 cmy1 ŽD line. w13x. The D line will shift towards a lower wavenumber in the Raman spectra as the sp 3 bonding in the films increases, and the integral intensity ratio I D r IG deduced from Gaussian curve fitting of the Raman spectra decreases with increase of sp 3rsp 2 bond in DLC films w14x. Fig. 5 shows a typical Raman spectra of DLC films. By Gaussian curve fitting

Fig. 4. Hardness as a function of bias voltage and r.f. power Ž42 sccm, 2.5 Pa..

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Fig. 5. Raman spectra of DLC film Ž300 W, 42 sccm, y140 V, 2.5 Pa, 150 min..

the G line and D line peak position is at 1544 and 1331 cmy1 , respectively. The downward shift of D line and G line peak can be attributed to the decrease of a graphite-like sp 2-bonded phase, and indicates a much higher percentage of sp 3 bonding in the films, and the films are more diamond-like. The variations of I D r IG with bias voltage and r.f. power are shown in Figs. 6 and 7, respectively. The I D r IG ratio decrease as the r.f. power increases, and with the increase of bias voltage the ratio of I D r IG decrease at first and than increase. The results are in good agreement with the variation of hardness shown in Fig. 4, and it is consistent with Robertson’s conclusion that the sp 3 bonding carbon clusters mainly control the mechanical properties such as hardness and Young’s modulus w15x.

The C᎐H absorption bands ranging from 2750 to 3200 cmy1 in FTIR spectrum are often used to analyze the quality of amorphous carbon films, so the FTIR spectrum is deconvoluted into subbands to estimate the sp 2rsp 3 ratio of the DLC films qualitatively w16x. Fig. 8 shows the variation of sp 2rsp 3 ratio with the bias voltage. From Fig. 8 it can be seen that there exists the same tendency as in Fig. 6. According to the chemical vapor deposition mechanism of DLC films from CH 4 , it is generally well known that the CH 4 molecules are decomposed into some hydrocarbon neutral radicals ŽCH, CH 2 , CH 3 ., ionic q. radicals ŽCHq,CHq 3 , C 2 H 5 , and atomic or ionic hydroq gen ŽH, H . , the hydrocarbon ions are accelerated by substrate bias voltage, impinge into the substrate sur-

Fig. 6. Variation of I D r IG ratio with bias voltage Ž42 sccm, 2.5 Pa..

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S.J. Yu et al. r Thin Solid Films 390 (2001) 98᎐103

Fig. 7. Variation of I D r IG ratio with r.f. power Ž42 sccm, 2.5 Pa..

face and create the dangling bonds on it, the neutral hydrocarbon radicals are adsorbed at the dangling bonds, at the same time more of graphite-like sp 2bonded phases are etched by atomic or ionic hydrogen and the hard DLC films are formed w17,18x. So the hydrocarbon and hydrogen ions flux density and the ion energy have great influence on the structure and characteristics of the DLC films. High plasma density with the ICPS in our experiment can greatly promote the decomposition of CH 4 , more hydrogen is peeled off from hydrocarbon ions, and the etching effect of atomic hydrogen is also enhanced due to the plasma density increase. The DLC films deposited in the ICPS have a high ratio of sp 3-bonded hydrocarbon and less hydrogen at comparatively low bias voltage. But on the other hand an excessive bias voltage will cause excessive

peeling of hydrogen from the hydrocarbon radical and cause the films to be graphite-like. This explanation is verified by the analysis of the result shown in Figs. 6 and 7. Under the conditions of high r.f. power or bias voltage the etching of atomic hydrogen dominates the process and the phenomenon of no film being deposited will arise. The relationship between surface roughness and r.f. power or bias voltage shown in Fig. 3 is due to the bombardment of ions in plasma on the substrate surface increasing as the bias voltage or r.f. power increases. 4. Conclusion Our results show that chemical vapor deposition of hard diamond-like carbon films was successfully carried

Fig. 8. Variation of sp 2rsp 3 ratio with the bias voltage and r.f. power Ž42 sccm, 2.5 Pa..

S.J. Yu et al. r Thin Solid Films 390 (2001) 98᎐103

out using a radio frequency Žr.f.. inductively coupled plasma source ŽICPS.. The films are smooth and the maximum film hardness can reach approximately 42 GPa at a comparatively low substrate bias voltage. It shows that the films deposited by RF-ICPECVD under the proper conditions can be very hard. The films have improved diamond-like characteristics with a D line peak at ; 1331 cmy1 and a G line peak at ; 1544 cmy1 . The process parameters greatly affect the structure and characteristics of DLC films and choosing proper parameters can achieve better quality DLC films. It is suggested that depositing high quality hard DLC films is attributed to high plasma density and the suitable deposition environment obtained in the ICPS source. Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China ŽNSFC. for financial support of the project ŽNo. 19835030.

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