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Diamond deposition procedure from microwave plasmas using a mixture of CO2–CH4 as carbon source
Thin Solid Films 316 Ž1998. 18]23
Diamond deposition procedure from microwave plasmas using a mixture of CO 2 ]CH 4 as carbon source Ken-ichi Itoh, Osamu MatsumotoU Department of Chemistry, Aoyama Gakuin Uni¨ersity, Chitosedai, Setagaya-ku, Tokyo, Tokyo 157, Japan
Abstract Diamond was deposited on an Mo substrate placed in CO 2 ]CH 4 ]Ar and CO 2 ]CH 4 microwave plasmas. In these plasmas, larger amounts of CO and OH radicals were identified than those of CH and C 2 radicals by OES. Strong peaks due to CO and H 2 were identified by means of QMA. The surface temperature of the substrate was about 1150" 20 K. Particles exhibiting cubo-octahedral habit planes were observed in SEM images. The sharp lines due to diamond were identified in X-ray diffraction patterns and Raman spectra. The adsorption of CO on the surface of the deposit from the CO 2 ]CH 4 microwave plasma was identified by XPS at the beginning of the deposition. Diamond could be deposited from the CO 2 ]CH 4 microwave plasma through the adsorption of CO molecule as precursors. Q 1998 Elsevier Science S.A. Keywords: Diamond deposition; CO 2 ]CH 4 gas mixture; Microwave plasma; Adsorbed CO
1. Introduction Diamond particles and films are generally deposited from the plasmas including hydrocarbons diluted with large amounts of hydrogen that were prepared using microwave discharge w1,2x or dc plasma jet w3,4x. Here, CH x species such as methyl radicals prepared in the plasma would be considered to be the precursors of diamond deposition w5,6x. Moreover, hydrogen atoms prepared in the plasma play important roles in diamond deposition w7x. Recently, diamond deposition from the CO 2 ]CH 4 and the Ar]CO 2 ]CH 4 microwave plasmas, into which hydrogen gas was not added, was identified w8]10x. An optical emission spectrum from the CO 2 ]CH 4 microwave plasma was similar to one from the CH 4-H 2 plasma w9x. On the other hand, the existence of large amounts of CO and H 2 O were identified in the
U
Corresponding author. Tel.: q81 3 5384 1111; fax: q81 3 5384 6200; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00381-2
Ar]CO 2 ]CH 4 microwave plasma by means of quadrupole mass analysis ŽQMA. w10x. Therefore, it is scientifically interesting to study the diamond deposition procedure from the CO 2 ]CH 4 plasma. Diamond deposition from Ar]CO 2 ]CH 4 plasmas which were prepared using three types of discharges, such as the DC plasma jet, the microwave discharge, and the inductive rf discharge was examined w11x. Deposits from the microwave plasma exhibited only crystalline habit planes and were identified as diamond by XRD and Raman spectroscopy. In the microwave plasma, the dissociation of molecules was suppressed and large amounts of CO and OH radicals were identified in the plasma by optical emission spectroscopy ŽOES.. Therefore, we performed a diamond deposition in the CO 2 ]CH 4 microwave plasma to examine the diamond deposition procedure. In the present paper, the results obtained are reported comparing with those from the Ar]CO 2 ]CH 4 microwave plasma. Moreover, the reaction scheme is formulated from the identification of both species in the plasma and on the
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Table 1 Diamond deposition conditions Plasma gas CO2rCH4 ratio Substrate Gas flow rate Ždm3 hy1 . Pressure ŽkPa. Discharge power ŽW. Discharge time Žh.
CO2 Ž11.5%. ]CH4 Ž8.5%. ]Ar CO2 Ž57%. ]CH4 Ž43%. 1.3 Mo 1.2 2 100 ;5
Fig. 2. X-Ray diffraction patterns of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
mately the same as that reported in a previous paper w2,10x. After the substrate sample was placed in the fused silica discharge tube and the discharge tube had been evacuated below 0.1 Pa, a mixture of argon, carbon dioxide, and methane in the desired ratio was
Fig. 1. Scanning electron micrographs of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
surface of the deposits comparing with those speculated in the CH 4 ]H 2 ]O 2 and the CO]H 2 microwave plasmas w12,13x. 2. Experimental The specimens used as substrates in the diamond deposition were 10 = 10 = 1 mm of molybdenum Žpurity 99.9%. sheets which were polished, degreased, and dried in vacuum. Purified argon, carbon dioxide, and methane Žeach purity G 99.9%. were used as the plasma gases. The diamond deposition apparatus was approxi-
Fig. 3. Raman spectra of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Table 2 Relative intensities of band heads and line of identified species in both plasmas by OES. The value within a parenthesis is a value of relative intensity divided byFranck]Condon factor, A(¨ 9 ] ¨ 0 ) w18]20x Plasma
CO2 ]CH4 ]Ar CO2 -CH4
Relative intensity CO a
OH b
CH c
C2
45 Ž774. 33 Ž588.
16 Ž25. 12 Ž18.
3 Ž4. 2 Ž2.
4 Ž5. 3 Ž4.
d
Hb 3 1
b 3 S-a3 P Ž0]0. ACO Ž0 ] 0. s 0.0581 w18x. A2 Sq-X 2 P Ž0]0. AOH Ž0 ] 0. s 0.65 w19x. c A2D-X 2 P Ž0]0. ACH Ž0 ] 0. s 0.9914 w20x. d A3 P g-X 93 P uŽ0]0. A C2Ž0 ] 0. s 0.7352 w18x.
a
the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 3. The sharp line due to diamond at 1333 cmy1 w16,17x was observed. Small amounts of graphitic andror amorphous carbon were also identified in the deposits w17x. Approximately the same results were obtained using three characterization methods for each sample in the plasma. Therefore, the deposits were identified as diamond including small amounts of graphitic andror amorphous carbon.
b
introduced into the discharge tube. The discharge conditions are given in Table 1. During the discharge, the species in the plasmas were identified by means of optical emission spectroscopy ŽOES. and quadrupole mass analysis ŽQMA.. The detailed procedures are given in Ref. w2x. The surface temperature of the substrate in the plasma was measured with thermocrayons. The deposits were characterized by scanning electron microscopy ŽSEM. observation, Xray diffraction ŽXRD., and Raman spectroscopy. The adsorbed species on the deposits were identified by X-ray photoelectron spectroscopy ŽXPS.. By the measuring the amount of weight gain of the substrate with varying discharge time, the kinetics of diamond deposition was studied together with measurements of changes in the deposits with varying discharge time. 3. Results and discussion 3.1. Characterization of the deposits on the Mo substrates During the discharge, the surface temperature of the substrate was about 1150 " 20 K. After the discharge for a period of 5 h, the deposits obtained on the Mo substrate were characterized by means of SEM observation, XRD, and Raman spectroscopy. Scanning electron micrographs of the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 1. The particles that exhibited cubo-octahedral habit planes were observed in the deposits from both plasmas. XRD patterns of the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 2. The lines due to Ž111., Ž220., and Ž311. planes of diamond were observed in both diffraction patterns. The lines due to molybdenum carbides ŽMo 2 C and MoC., which were probably oxygen-stabilized phases w14,15x, were also observed with those of diamond. Raman spectra of
3.2. Identification of the species in the plasmas Since diamond was deposited on the Mo substrate from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas, species in the plasmas were identified by means of OES and QMA in order to clarify the relationship between diamond deposition and the plasma. The excited species identified by OES in each plasma were approximately the same: they were CO, CH, C 2 , and OH radicals and H atoms. The relative intensities of band heads and line of these species identified in both plasmas are given in Table 2. In Table 2, the values which were band head intensities divided by Franck]Condon factor w18]20x are also given. Larger amounts of CO and OH radicals were observed than those of CH and C 2 radicals. By means of QMA, increases in CO and H 2 were observed in the gas phase by the plasma formation as shown in Fig. 4. During discharge, the peaks due to CH X species, which were observed in the CH 4 ]H 2 microwave plasma w2x and considered as precursors of the diamond deposition from the CH 4 ]H 2 plasmas, were very weak in both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas. Therefore, CO and OH radicals and H 2 molecules may be precursors to diamond formation in the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas. 3.3. Diamond deposition procedure from CO2 ]CH4 microwa¨e plasma The diamond deposition on the Mo substrate was observed from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas, in which large amounts of CO and OH radicals and H 2 molecules were identified. Therefore, we performed experiments according to the time dependence of the diamond deposition to clarify how CO and OH radicals and H 2 molecules, which were identified in the plasma, contribute to the diamond deposition from microwave plasmas using a mixture of CO 2 ]CH 4 as carbon source. Since the deposits from both plasmas were
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Fig. 4. Mass spectra of the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas.
approximately the same, the following investigation is carried out only the deposit from CO 2 ]CH 4 microwave plasma. The gradient of the line in the logarithmic plot of weight gain vs. discharge time changed from 1:2 to 1:1 after 1 h of discharge time as shown in Fig. 5. This indicated that the diamond deposition followed carbide formation on the surface of the substrate during the discharge w4x. The variation of deposits with increasing deposition time was also identified by means of SEM observation, Raman spectroscopy, and XRD. At the beginning of the discharge, no deposit was observed on the Mo substrate by SEM observation and Raman spectroscopy as shown in Figs. 6 and 7, respectively. Only the formation of molybdenum carbides was identified by XRD as given in Table 3. After 1 h of discharge time, diamond was identified on the substrate by means of the above three characterization methods. Diamond deposition was clearly observed with an increase in discharge time. This corresponded to the change in weight gain as shown in Fig. 5. After diamond deposition for the desired time, the surface of the deposit was investigated by means of Table 3 Identified species in the deposits obtained from the CO 2 ]CH 4 microwave plasma with varying discharge time by XRD Discharge time Žh.
Identified species
0.25 1 5
Mo 2 C, MoC Mo 2 C, MoC, Diamond Mo 2 C, MoC, Diamond
Fig. 5. Weight gain vs. discharge time in the diamond deposition from the CO 2 ]CH 4 microwave plasma.
X-ray photoelectron spectroscopy in order to identify the adsorbed species on the surface. The XPS spectra of the C1s and the O1s electrons of the deposits obtained by varying the deposition time were measured. XPS spectra of samples as deposited and after 1 min of Arq ion etching are shown, respectively, in Fig. 8. As the C]O bond was identified on the surface of sample, we assumed that the adsorption of CO molecules in the form of b ]CO w21x on the surface of the deposit had occurred. In the adsorbed b ]CO, oxygen atoms directly bonded with the surface metal atoms of the substrate and the C]O bonds were weakened w21x. Moreover, the binding energy in the C1s core level of b ]CO was approximately the same
22
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
Fig. 7. Raman spectra of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time.
the surface of the deposits after 1 h of discharge time as like in the CH 4 ]H 2 plasma w2,5,6x. The adsorption of CO molecules would contribute to the diamond deposition from the CO 2 ]CH 4 plasma. Summarizing the results that were shown in Figs. 5 and 8, the overall diamond deposition scheme in the CO 2 ]CH 4 microwave plasma is speculated as shown in Fig. 9. At the beginning of the discharge, CO molecules in the plasma were adsorbed onto the surface of the Mo substrate in the form of b ]CO. Then adsorbed CO molecules diffused into the Mo Fig. 6. Scanning electron micrographs of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time.
as that of Mo]C bond in the Mo 2 C w22x. Therefore, some of CO molecules in the plasma were dissociatively adsorbed onto the surface of the deposits and reacted with the Mo to form Mo 2 C at the beginning of the discharge. The intensities of XPS peaks of the C1s and the O1s electrons due to b ]CO decreased with an increase in the discharge time. On the other hand, the intensity of XPS peak of C1s electron due to CH x increased with an increase in the discharge time and the surface of the deposits would be covered with adsorbed CH x after 1 h of discharge time. Moreover, XPS spectrum of C1s electron after 1 h of discharge time is similar to that of the deposit from CH 4 ]H 2 microwave plasma w2x andror the carbon film after exposure to the hydrogen microwave plasma w23x. These results indicate that adsorbed CO molecules would react with hydrogen atoms andror molecules and adsorbed CH X would be formed on
Fig. 8. XPS spectra of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time. ŽA. As-deposited, ŽB. After 1 min of Arq ion etching.
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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CO 2 ]CH 4 microwave plasmas by means of SEM observation, XRD, and Raman spectroscopy. In those plasmas, which were used in this study, large amounts of CO and OH radicals and H 2 molecules were identified by means of OES and QMA. The adsorption of CO molecules in the form of b ]CO was identified on the surface of the deposits obtained from the CO 2 ]CH 4 microwave plasma by XPS. Since the adsorption of CO molecules onto the surface of the Mo substrate andror the deposits from the CO 2 ]CH 4 plasma was identified and the deposition of diamond was observed after the carburization of the substrate, diamond could be deposited from the CO 2 ]CH 4 microwave plasma through the adsorption of CO molecules as precursors. References
Fig. 9. Possible diamond deposition scheme in the CO 2 ]CH 4 microwave plasma.
substrate and reacted with Mo metals to form molybdenum carbides. After the carbon concentration in the surface layer was saturated Žthat is to say after 1 h of discharge time at the present work., adsorbed CO molecules reacted with hydrogen molecules andror hydrogen atoms in the plasma and the following reaction would occur. CO Ž ad. q H 2 Ž g. ª CH X Ž ad. q OH Ž g. Then diamond would be formed through the adsorbed CHX which were formed by the surface reaction of adsorbed CO molecules with hydrogen molecules andror atoms. As a result, diamond could be deposited through the adsorption of b ]CO followed by a formation of CH X like in the process in the CH 4 ]H 2 plasma w5,6x. The result obtained verify the speculation of the diamond deposition process in the CH 4 ]H 2 ]O 2 andror the CO]H 2 microwave plasmas w12,13x. 4. Conclusions Diamond deposition on the Mo substrate was observed from both the CO 2 ] CH 4 ]Ar and the
w1x M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 Ž1983. 642. w2x O. Matsumoto, H. Toshima, Y. Kanzaki, Thin Solid Films 128 Ž1985. 341. w3x K. Kurihara, K. Sasaki, M. Kawarada, N. Koshino, Appl. Phys. Lett. 52 Ž1988. 437. w4x R. Furukawa, H. Uyama, O. Matsumoto, IEEE Trans. Plasma Sci. 18 Ž1990. 930. w5x M. Tsuda, M. Nakajima, S. Oikawa, Jpn. J. Appl. Phys. 26 Ž1987. L527. w6x S.J. Harris, Appl. Phys. Lett. 56 Ž1990. 2298. w7x M. Frenklach, J. Appl. Phys. 65 Ž1989. 5142. w8x C.-F. Chen, C.-L. Lin, T.-M. Hong, Surf. Coat. Technol. 52 Ž1992. 205. w9x G. Balestrino, M. Marinelli, E. Milani, et al., Diamond Relat. Mater. 2 Ž1993. 389. w10x P. Joeris, C. Benndorf, R. Kroger, Proceedings of the 3rd International Symposium on Diamond Materials, 1993, p. 536. w11x K. Itoh, O. Matsumoto, Proceedings of the 12th International Symposium on Plasma Chemistry, 1995, p. 2227. w12x J.A. Mucha, D.L. Flamm, D.E. Ibbotson, J. Appl. Phys. 65 Ž1989. 3448. w13x Y. Muranaka, H. Yamashita, K. Sato, H. Miyadera, J. Appl. Phys. 67 Ž1990. 6247. w14x K. Kuo, G. Hagg, Nature 170 Ž1952. 245. w15x L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York and London, 1971, p. 22. w16x D.S. Knight, W.B. White, J. Mater. Res. 4 Ž1989. 385. w17x P. Bou, L. Vandenbulcke, J. Electrochem. Soc. 138 Ž1991. 2991. w18x F.S. Ortenberg, Opt. Spectrscr. 16 Ž1964. 398. w19x H.P. Broida, K.E. Shuler, J. Chem. Phys. 20 Ž1952. 168. w20x Z. Bembenek, R. Kepa, A. Para, et al., J. Mol. Spectrosc. 139 Ž1990. 1. w21x J.H. Yates, Jr., T.E. Madey, N.E. Erickson, Surf. Sci. 43 Ž1974. 257. w22x L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nordling, J. Phys. Chem. Solids 30 Ž1969. 1835. w23x O. Matsumoto, T. Katagiri, Thin Solid Films 146 Ž1987. 283.
Diamond deposition procedure from microwave plasmas using a mixture of CO 2 ]CH 4 as carbon source Ken-ichi Itoh, Osamu MatsumotoU Department of Chemistry, Aoyama Gakuin Uni¨ersity, Chitosedai, Setagaya-ku, Tokyo, Tokyo 157, Japan
Abstract Diamond was deposited on an Mo substrate placed in CO 2 ]CH 4 ]Ar and CO 2 ]CH 4 microwave plasmas. In these plasmas, larger amounts of CO and OH radicals were identified than those of CH and C 2 radicals by OES. Strong peaks due to CO and H 2 were identified by means of QMA. The surface temperature of the substrate was about 1150" 20 K. Particles exhibiting cubo-octahedral habit planes were observed in SEM images. The sharp lines due to diamond were identified in X-ray diffraction patterns and Raman spectra. The adsorption of CO on the surface of the deposit from the CO 2 ]CH 4 microwave plasma was identified by XPS at the beginning of the deposition. Diamond could be deposited from the CO 2 ]CH 4 microwave plasma through the adsorption of CO molecule as precursors. Q 1998 Elsevier Science S.A. Keywords: Diamond deposition; CO 2 ]CH 4 gas mixture; Microwave plasma; Adsorbed CO
1. Introduction Diamond particles and films are generally deposited from the plasmas including hydrocarbons diluted with large amounts of hydrogen that were prepared using microwave discharge w1,2x or dc plasma jet w3,4x. Here, CH x species such as methyl radicals prepared in the plasma would be considered to be the precursors of diamond deposition w5,6x. Moreover, hydrogen atoms prepared in the plasma play important roles in diamond deposition w7x. Recently, diamond deposition from the CO 2 ]CH 4 and the Ar]CO 2 ]CH 4 microwave plasmas, into which hydrogen gas was not added, was identified w8]10x. An optical emission spectrum from the CO 2 ]CH 4 microwave plasma was similar to one from the CH 4-H 2 plasma w9x. On the other hand, the existence of large amounts of CO and H 2 O were identified in the
U
Corresponding author. Tel.: q81 3 5384 1111; fax: q81 3 5384 6200; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00381-2
Ar]CO 2 ]CH 4 microwave plasma by means of quadrupole mass analysis ŽQMA. w10x. Therefore, it is scientifically interesting to study the diamond deposition procedure from the CO 2 ]CH 4 plasma. Diamond deposition from Ar]CO 2 ]CH 4 plasmas which were prepared using three types of discharges, such as the DC plasma jet, the microwave discharge, and the inductive rf discharge was examined w11x. Deposits from the microwave plasma exhibited only crystalline habit planes and were identified as diamond by XRD and Raman spectroscopy. In the microwave plasma, the dissociation of molecules was suppressed and large amounts of CO and OH radicals were identified in the plasma by optical emission spectroscopy ŽOES.. Therefore, we performed a diamond deposition in the CO 2 ]CH 4 microwave plasma to examine the diamond deposition procedure. In the present paper, the results obtained are reported comparing with those from the Ar]CO 2 ]CH 4 microwave plasma. Moreover, the reaction scheme is formulated from the identification of both species in the plasma and on the
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Table 1 Diamond deposition conditions Plasma gas CO2rCH4 ratio Substrate Gas flow rate Ždm3 hy1 . Pressure ŽkPa. Discharge power ŽW. Discharge time Žh.
CO2 Ž11.5%. ]CH4 Ž8.5%. ]Ar CO2 Ž57%. ]CH4 Ž43%. 1.3 Mo 1.2 2 100 ;5
Fig. 2. X-Ray diffraction patterns of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
mately the same as that reported in a previous paper w2,10x. After the substrate sample was placed in the fused silica discharge tube and the discharge tube had been evacuated below 0.1 Pa, a mixture of argon, carbon dioxide, and methane in the desired ratio was
Fig. 1. Scanning electron micrographs of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
surface of the deposits comparing with those speculated in the CH 4 ]H 2 ]O 2 and the CO]H 2 microwave plasmas w12,13x. 2. Experimental The specimens used as substrates in the diamond deposition were 10 = 10 = 1 mm of molybdenum Žpurity 99.9%. sheets which were polished, degreased, and dried in vacuum. Purified argon, carbon dioxide, and methane Žeach purity G 99.9%. were used as the plasma gases. The diamond deposition apparatus was approxi-
Fig. 3. Raman spectra of the deposits from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas Ždischarge time: 5 h..
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Table 2 Relative intensities of band heads and line of identified species in both plasmas by OES. The value within a parenthesis is a value of relative intensity divided byFranck]Condon factor, A(¨ 9 ] ¨ 0 ) w18]20x Plasma
CO2 ]CH4 ]Ar CO2 -CH4
Relative intensity CO a
OH b
CH c
C2
45 Ž774. 33 Ž588.
16 Ž25. 12 Ž18.
3 Ž4. 2 Ž2.
4 Ž5. 3 Ž4.
d
Hb 3 1
b 3 S-a3 P Ž0]0. ACO Ž0 ] 0. s 0.0581 w18x. A2 Sq-X 2 P Ž0]0. AOH Ž0 ] 0. s 0.65 w19x. c A2D-X 2 P Ž0]0. ACH Ž0 ] 0. s 0.9914 w20x. d A3 P g-X 93 P uŽ0]0. A C2Ž0 ] 0. s 0.7352 w18x.
a
the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 3. The sharp line due to diamond at 1333 cmy1 w16,17x was observed. Small amounts of graphitic andror amorphous carbon were also identified in the deposits w17x. Approximately the same results were obtained using three characterization methods for each sample in the plasma. Therefore, the deposits were identified as diamond including small amounts of graphitic andror amorphous carbon.
b
introduced into the discharge tube. The discharge conditions are given in Table 1. During the discharge, the species in the plasmas were identified by means of optical emission spectroscopy ŽOES. and quadrupole mass analysis ŽQMA.. The detailed procedures are given in Ref. w2x. The surface temperature of the substrate in the plasma was measured with thermocrayons. The deposits were characterized by scanning electron microscopy ŽSEM. observation, Xray diffraction ŽXRD., and Raman spectroscopy. The adsorbed species on the deposits were identified by X-ray photoelectron spectroscopy ŽXPS.. By the measuring the amount of weight gain of the substrate with varying discharge time, the kinetics of diamond deposition was studied together with measurements of changes in the deposits with varying discharge time. 3. Results and discussion 3.1. Characterization of the deposits on the Mo substrates During the discharge, the surface temperature of the substrate was about 1150 " 20 K. After the discharge for a period of 5 h, the deposits obtained on the Mo substrate were characterized by means of SEM observation, XRD, and Raman spectroscopy. Scanning electron micrographs of the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 1. The particles that exhibited cubo-octahedral habit planes were observed in the deposits from both plasmas. XRD patterns of the deposits on the Mo substrates obtained from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas are shown in Fig. 2. The lines due to Ž111., Ž220., and Ž311. planes of diamond were observed in both diffraction patterns. The lines due to molybdenum carbides ŽMo 2 C and MoC., which were probably oxygen-stabilized phases w14,15x, were also observed with those of diamond. Raman spectra of
3.2. Identification of the species in the plasmas Since diamond was deposited on the Mo substrate from the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas, species in the plasmas were identified by means of OES and QMA in order to clarify the relationship between diamond deposition and the plasma. The excited species identified by OES in each plasma were approximately the same: they were CO, CH, C 2 , and OH radicals and H atoms. The relative intensities of band heads and line of these species identified in both plasmas are given in Table 2. In Table 2, the values which were band head intensities divided by Franck]Condon factor w18]20x are also given. Larger amounts of CO and OH radicals were observed than those of CH and C 2 radicals. By means of QMA, increases in CO and H 2 were observed in the gas phase by the plasma formation as shown in Fig. 4. During discharge, the peaks due to CH X species, which were observed in the CH 4 ]H 2 microwave plasma w2x and considered as precursors of the diamond deposition from the CH 4 ]H 2 plasmas, were very weak in both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas. Therefore, CO and OH radicals and H 2 molecules may be precursors to diamond formation in the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 plasmas. 3.3. Diamond deposition procedure from CO2 ]CH4 microwa¨e plasma The diamond deposition on the Mo substrate was observed from both the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas, in which large amounts of CO and OH radicals and H 2 molecules were identified. Therefore, we performed experiments according to the time dependence of the diamond deposition to clarify how CO and OH radicals and H 2 molecules, which were identified in the plasma, contribute to the diamond deposition from microwave plasmas using a mixture of CO 2 ]CH 4 as carbon source. Since the deposits from both plasmas were
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
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Fig. 4. Mass spectra of the CO 2 ]CH 4 ]Ar and the CO 2 ]CH 4 microwave plasmas.
approximately the same, the following investigation is carried out only the deposit from CO 2 ]CH 4 microwave plasma. The gradient of the line in the logarithmic plot of weight gain vs. discharge time changed from 1:2 to 1:1 after 1 h of discharge time as shown in Fig. 5. This indicated that the diamond deposition followed carbide formation on the surface of the substrate during the discharge w4x. The variation of deposits with increasing deposition time was also identified by means of SEM observation, Raman spectroscopy, and XRD. At the beginning of the discharge, no deposit was observed on the Mo substrate by SEM observation and Raman spectroscopy as shown in Figs. 6 and 7, respectively. Only the formation of molybdenum carbides was identified by XRD as given in Table 3. After 1 h of discharge time, diamond was identified on the substrate by means of the above three characterization methods. Diamond deposition was clearly observed with an increase in discharge time. This corresponded to the change in weight gain as shown in Fig. 5. After diamond deposition for the desired time, the surface of the deposit was investigated by means of Table 3 Identified species in the deposits obtained from the CO 2 ]CH 4 microwave plasma with varying discharge time by XRD Discharge time Žh.
Identified species
0.25 1 5
Mo 2 C, MoC Mo 2 C, MoC, Diamond Mo 2 C, MoC, Diamond
Fig. 5. Weight gain vs. discharge time in the diamond deposition from the CO 2 ]CH 4 microwave plasma.
X-ray photoelectron spectroscopy in order to identify the adsorbed species on the surface. The XPS spectra of the C1s and the O1s electrons of the deposits obtained by varying the deposition time were measured. XPS spectra of samples as deposited and after 1 min of Arq ion etching are shown, respectively, in Fig. 8. As the C]O bond was identified on the surface of sample, we assumed that the adsorption of CO molecules in the form of b ]CO w21x on the surface of the deposit had occurred. In the adsorbed b ]CO, oxygen atoms directly bonded with the surface metal atoms of the substrate and the C]O bonds were weakened w21x. Moreover, the binding energy in the C1s core level of b ]CO was approximately the same
22
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
Fig. 7. Raman spectra of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time.
the surface of the deposits after 1 h of discharge time as like in the CH 4 ]H 2 plasma w2,5,6x. The adsorption of CO molecules would contribute to the diamond deposition from the CO 2 ]CH 4 plasma. Summarizing the results that were shown in Figs. 5 and 8, the overall diamond deposition scheme in the CO 2 ]CH 4 microwave plasma is speculated as shown in Fig. 9. At the beginning of the discharge, CO molecules in the plasma were adsorbed onto the surface of the Mo substrate in the form of b ]CO. Then adsorbed CO molecules diffused into the Mo Fig. 6. Scanning electron micrographs of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time.
as that of Mo]C bond in the Mo 2 C w22x. Therefore, some of CO molecules in the plasma were dissociatively adsorbed onto the surface of the deposits and reacted with the Mo to form Mo 2 C at the beginning of the discharge. The intensities of XPS peaks of the C1s and the O1s electrons due to b ]CO decreased with an increase in the discharge time. On the other hand, the intensity of XPS peak of C1s electron due to CH x increased with an increase in the discharge time and the surface of the deposits would be covered with adsorbed CH x after 1 h of discharge time. Moreover, XPS spectrum of C1s electron after 1 h of discharge time is similar to that of the deposit from CH 4 ]H 2 microwave plasma w2x andror the carbon film after exposure to the hydrogen microwave plasma w23x. These results indicate that adsorbed CO molecules would react with hydrogen atoms andror molecules and adsorbed CH X would be formed on
Fig. 8. XPS spectra of the deposits from the CO 2 ]CH 4 microwave plasma with varying discharge time. ŽA. As-deposited, ŽB. After 1 min of Arq ion etching.
K. Itoh, O. Matsumoto r Thin Solid Films 316 (1998) 18]23
23
CO 2 ]CH 4 microwave plasmas by means of SEM observation, XRD, and Raman spectroscopy. In those plasmas, which were used in this study, large amounts of CO and OH radicals and H 2 molecules were identified by means of OES and QMA. The adsorption of CO molecules in the form of b ]CO was identified on the surface of the deposits obtained from the CO 2 ]CH 4 microwave plasma by XPS. Since the adsorption of CO molecules onto the surface of the Mo substrate andror the deposits from the CO 2 ]CH 4 plasma was identified and the deposition of diamond was observed after the carburization of the substrate, diamond could be deposited from the CO 2 ]CH 4 microwave plasma through the adsorption of CO molecules as precursors. References
Fig. 9. Possible diamond deposition scheme in the CO 2 ]CH 4 microwave plasma.
substrate and reacted with Mo metals to form molybdenum carbides. After the carbon concentration in the surface layer was saturated Žthat is to say after 1 h of discharge time at the present work., adsorbed CO molecules reacted with hydrogen molecules andror hydrogen atoms in the plasma and the following reaction would occur. CO Ž ad. q H 2 Ž g. ª CH X Ž ad. q OH Ž g. Then diamond would be formed through the adsorbed CHX which were formed by the surface reaction of adsorbed CO molecules with hydrogen molecules andror atoms. As a result, diamond could be deposited through the adsorption of b ]CO followed by a formation of CH X like in the process in the CH 4 ]H 2 plasma w5,6x. The result obtained verify the speculation of the diamond deposition process in the CH 4 ]H 2 ]O 2 andror the CO]H 2 microwave plasmas w12,13x. 4. Conclusions Diamond deposition on the Mo substrate was observed from both the CO 2 ] CH 4 ]Ar and the
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