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Deposition of large area high quality diamond wafers with high growth rate by DC arc plasma jet
Diamond and Related Materials 9 Ž2000. 1673᎐1677
Deposition of large area high quality diamond wafers with high growth rate by DC arc plasma jet U
H. Guoa, , Z.L. Suna , Q.Y. Hea , S.M. Dua, X.B. Wua , Z.N. Wanga , X.J. Liua , Y.H. Caia , X.G. Diaoa, G.H. Lia , W.Z. Tangb, G.F. Zhongb, T.B. Huangb, J.M. Liub, Z. Jiangb, F.X. Lub a
b
Hebei Pro¨ ince Academy of Science, Shijiazhuang 050081, PR China Department of Materials Science, Uni¨ ersity of Science and Technology Beijing, Beijing 100083, PR China
Abstract We have successfully developed a system for deposition of large area diamond films by a DC arc plasma jet operated in gas recycling mode. In the present paper, the influence of substrate temperature, methane concentration, flow rate of feeding gas and the input power of the jet for diamond film deposition is presented. Deposition of a large area of uniform thickness high quality diamond wafer of ⌽65 mm in diameter at a growth rate of 15 mrh is reported. The thickness of the wafer is 0.7 mm and the thermal conductivity can be 18.1 Wrcm K. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: DC arc plasma jet; Chemical vapor deposition; Diamond wafer
1. Introduction The DC arc plasma jet is generally considered to be one of the most promising chemical vapor deposition ŽCVD. techniques for high quality diamond films growth w1x. However, the conventional design of the plasma torch based on industrial plasma spraying or plasma cutting suffers the disadvantages of high gas consumption, low heat efficiency, the complicity and high investment of the equipment, and most of all, the difficulties for large area uniformity of diamond deposition. In our previous work, we have successfully developed a 100-kW high power DC arc plasma jet CVD system operated in gas cycling mode for deposition of large area diamond films w2x, by which diamond wafers over a diameter of ⌽110 mm can be deposited at a growth rate of 40᎐50
U
Corresponding author. Tel.: q86-311-3020717; fax: q86-3113026250. E-mail address: [email protected] ŽH. Guo..
mrh, with thickness uniformity better than 90% w3,4x. However, detailed studies about the dependence of diamond quality on process parameters by this system were few. In the present paper, the effects of substrate temperature, methane concentration, input power of the jet, especially the flow rate of feeding gas on diamond deposition will be discussed, and the deposition of large area high quality uniform thickness diamond wafers will be reported.
2. Experimental 2.1. Experimental apparatus Diamond wafers were deposited in a 30-kW DC arc plasma jet CVD system developed on the experience of our 100 kW high power jet. Proper control of magnetic field and the characteristics of fluid dynamics in this jet make it possible that the jet steadily discharges over a large area. Fig. 1 shows the working state of the arc
0925-9635r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 2 8 9 - 2
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H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
Fig. 1. Photo of a working arc plasma. The region indicated by the arrow is a so-called mustache.
plasma. As shown, a large area of uniformly-distributed arc plasma region between the substrate and outlet orifice is achieved. From the uniformity of thickness and crystalline grain size of deposited diamond wafers, we can conclude that the temperature of plasma above substrate should be uniform. In this system diamond wafers of ⌽65 mm in diameter can be deposited with a growth rate as high as 60 mrh. For economical running, the jet is operated in gas recycling mode in which consumption of feed gas is decreased by 90%. Generally, for steady combustion of the arc, the flow rate of gas must not less than 50᎐100 supplying gas flow ŽSLM. in a conventional jet working at the blow down mode, and in our system it was reduced to 5᎐10 SLM when operating at gas recycling mode. Fig. 2
shows a schematic diagram of the gas recycling system. Supplying gases are fed into the torch through mass flow controllers ŽMFC.. After use, it is cooled down to room temperature and is fed back for recycling by pump 1 through gas valve 1. In this way, most of the gas is recycled while only a small part is vented from valve 2 by pump 2. After a few recycling periods, the system will be in a steady state in which the gas flow of supplying and venting should be in balance. By adjusting the valves the gas cycling system will turn into a new balance and the gas recycling flow will vary also. Gas recycling flow ŽGRF. is indicated by the difference Ž ⌬ P . in chamber pressure P1 and pump 2 outlet pressure P 2. An increase in ⌬ P will result in an increase of GRF, contrarily, GRF should decrease.
3. Experimental conditions Process parameters are given in Table 1. Prior to deposition the substrate was abraded with diamond powder of approximately 14 m in size and cleaned in an ultrasonic cleaner. Substrate temperature was measured by an infrared optical pyrometer with an emissivity setting at 0.4. Deposition time was more than 10 h. In the present paper thermal conductivity was used as the primary technique for characterization of diamond quality. This is because thermal conductivity is strongly affected by even a small amount of nondiamond bonded carbon, therefore it can be taken as an indicator of the quality of diamond films. We employed photothermal deflection technique w5,6x to measure the thermal conductivity of the diamond wafers at room temperature. The measure system is PTDS-II mode made by University of Science and Technology of China ŽUSTC. w7x. The minimum area of measurable sample is approximately 2 = 5 mm2 , this means that the result is an average value in this area. It is apparent that the result is not only affected by the defects in crytallinity, other factors, such as micro-cracks, impurities and cavity, etc., are also important. For a better thermal property the grain size of diamond wafers was larger than 50 m in the present work w8x. In addition, Table 1 Processing parameters employed for diamond film deposition Supplying gas flow ŽSLM.
Fig. 2. Schematic diagram of gas cycling mode.
Pressure of chamber ŽkPa. Difference in cycling pressure ŽkPa. Substrate Žmm. Substrate temperature Ž⬚C. Input power of jet ŽkW.
Ar H2 CH 4
3᎐6 3᎐6 0.05᎐0.2 1᎐10 1.5᎐12
Mo
⌽60 700᎐1000 17
H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
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diamond films were also studied by scanning electron microscopy ŽSEM., X-ray diffraction and Raman spectroscopy.
4. Results and discussion The growth rate and thermal conductivity of diamond will increase with the increase of input power to the plasma jet when other deposition parameters remain constant. This is the reason why the design of high power jets is being attempted. In the present investigation, the input power was kept at 17 kW. 4.1. Effect of methane concentration Fig. 3 shows the dependence of growth rate and thermal conductivity on methane concentration while other deposition conditions were as follows: substrate temperature Ts was 900⬚C, chamber pressure P was 3 kPa and the difference of cycling pressure ⌬ P was 9.5 kPa. It can be seen from Fig. 3 that the growth rate still remained as high as 9 mrh while methane concentration decreased to a relative lower value of 0.7% CH4rH2 , and at the same time thermal conductivity increased to 18.3 Wrcm K. This is in general agreement with that reported in literature with other CVD techniques. However, it is worth mentioning that the high thermal conductivity obtained in the present investigation was due to the high concentration of atomic hydrogen provided by the high temperature arc plasma jet. Further more, it can be seen from Fig. 3 that thermal conductivity remained as high as 10 Wrcm K even when the growth rate increased to 30 mrh, which confirmed the advantages of DC arc plasma jet CVD and its commercial prospect is evident.
Fig. 4. Graph of rate and thermal conductivity as a function of substrate temperature while CH4 rH2 is 1.0%, chamber pressure is 3 kPa and difference of cycling pressure is 11 kPa.
function of substrate temperature in Fig. 4 while the methane concentration was 1.0% CH4rH2 , chamber pressure was 3 kPa and difference in cycling pressure was 11 kPa. It can be seen that growth rate was nearly constant when the temperature increased from 750 to 950⬚C, however, a further increase in temperature would result in an increase in the growth rate. Whilst the thermal conductivity increased with increasing temperature from 750 to 900⬚C, reached a maximum at 900⬚C, and then decreased again. It can be concluded that the optimum temperature for the deposition of high quality diamond wafer is approximately 900⬚C. 4.3. Effects of recycling gas flow
Growth rate and thermal conductivity is plotted as a
The flow rate of gas recycling is one of the most important parameters for our DC arc plasma jet CVD system. As discussed above, ⌬ P is the indicator of flow rate of gas recycling. Fig. 5 shows the dependence of growth rate and thermal conductivity on ⌬ P when Ts s 910⬚C, CH4rH2 s 1.3%, Ps 3.6 kPa. From Fig. 5, it can be seen that the growth rate decreased sharply and the thermal conductivity increased linearly with increasing ⌬ P. It can be explained as follows. As shown in Fig. 1, while the jet is working steadily,
Fig. 3. Dependence of growth rate and thermal conductivity on methane concentration at a substrate temperature of 900⬚C, chamber pressure of 3 kPa and difference of cycling ⌬ P of 9.5 kPa.
Fig. 5. Dependence of growth rate and thermal conductivity on ⌬ P when Ts s 910⬚C, CH4 rH2 s 1.3% and Ps 3.6 kPa.
4.2. Effect of substrate temperature
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H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
Fig. 8. X-Ray diffraction pattern of high quality diamond films.
Fig. 6. Photograph of a high quality diamond wafer. Ža. ⌽60 mm unpolished sample Ž0.5 mm thick.; Žb. polished sample Ž0.4 mm thick..
the main part of the hot gas stream coming out of the anode nozzle is being driven directly onto the substrate, which provides chemically active radicals for diamond deposition. However, a small part of the hot gas stream flows from the gap between the nozzle and the substrate, which contributes nothing to diamond growth. There is also a green zone around the arc column, whose grime droops down along substrate, looks like a ‘mustache’ of the substrate. A decrease in ⌬ P will result in rising up of the mustache, so the ratio of hot gas driven to the substrate increases, which in turn results in an increase in growth rate. In contrary, the mustache would droop down, the ratio of chemically active radicals driven to the substrate would decrease, thus results in the decrease of growth rate. In addition, it appears that the growth rate should increase continually with the further decrease of GRF. However, further decrease of GRF will cause radial
Fig. 7. SEM micrograph of high quality diamond films: Ža. surface, Žb. cross-section.
inhomogeneous thickness and sharply decrease heat efficiency of the jet. Furthermore, the methane concentration in the reactant gas mixture will decrease with the increase of GRF, because of the consumption of methane. Consequently, a decrease of growth rate and improvement of thermal conductivity will be observed. Based on the above discussions, with optimal process parameters 0.7-mm-thick high quality diamond wafers up to 65 mm in diameter were successfully deposited, Thermal conductivity of diamond wafer were as high as 18 Wrcm K. High quality diamond wafers appear light yellow Žsee Fig. 6a.. Words in paper can be seen clearly through polished samples without backup light Žshown in Fig. 6b.. Fig. 7 is a SEM micrograph of high quality diamond wafers. Fig. 8 shows the X-ray diffraction spectroscopy. Raman spectroscopy of high quality diamond film is shown in Fig. 9. It can be seen that the spectrum is dominated by a sharp characteristic diamond peak at 1332rcm without any sign of a nondiamond carbon peak.
5. Conclusions The effects of process parameters on deposition of diamond wafers by DC arc plasma jet operating in gas recycling mode were investigated. It was found that substrate temperature of 900⬚C and a lower methane concentration is helpful for depositing high quality
Fig. 9. Raman spectroscopy of high quality diamond.
H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
diamond wafers. The system should be operated at a higher flow of recycled gas for not only achieving higher quality of diamond, but also the stable operation of the arc jet. With optimal process parameters it is possible to deposit high quality diamond wafers up to ⌽65 mm in diameter by the DC arc plasma jet CVD system.
Acknowledgements The authors gratefully acknowledge the financial support of the NAMCC ŽNational Advanced Materials Committee of China. and Hebei Province Natural Science foundation.
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References w1x J.V. Busch, J.P. Dismukes, Diamond Relat. Mater. 3 Ž1994. 295. w2x G.H. Li, F.X. Lu, et al. Chinese Patent: CN1099547A, 1995. w3x F.X. Lu, G.F. Zhong, J.G. Sun et al., Diamond Relat. Mater. 7r6 Ž1998. 737. w4x F.X. Lu, G.F. Zhong, Y.L. Fu et al., Proceedings of the International Diamond Symposium Seoul, Ž1996. 115. w5x A. Salazar, A. Sancher-Lavega, J. Fernander, J. Appl. Phys. 65 Ž1989. 4150. w6x A. Salazar, A. Sancher-Lavega, J. Fernander, J. Appl. Phys. 69 Ž1991. 1216. w7x J. Cui, H. Chen, J. Zhang, D. Ban, R. Fang, High Technol. Lett. 1 Ž1995. 91. w8x E.A. Burgemeister, J. Mater Sci. Technol. 16 Ž1981. 1730.
Deposition of large area high quality diamond wafers with high growth rate by DC arc plasma jet U
H. Guoa, , Z.L. Suna , Q.Y. Hea , S.M. Dua, X.B. Wua , Z.N. Wanga , X.J. Liua , Y.H. Caia , X.G. Diaoa, G.H. Lia , W.Z. Tangb, G.F. Zhongb, T.B. Huangb, J.M. Liub, Z. Jiangb, F.X. Lub a
b
Hebei Pro¨ ince Academy of Science, Shijiazhuang 050081, PR China Department of Materials Science, Uni¨ ersity of Science and Technology Beijing, Beijing 100083, PR China
Abstract We have successfully developed a system for deposition of large area diamond films by a DC arc plasma jet operated in gas recycling mode. In the present paper, the influence of substrate temperature, methane concentration, flow rate of feeding gas and the input power of the jet for diamond film deposition is presented. Deposition of a large area of uniform thickness high quality diamond wafer of ⌽65 mm in diameter at a growth rate of 15 mrh is reported. The thickness of the wafer is 0.7 mm and the thermal conductivity can be 18.1 Wrcm K. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: DC arc plasma jet; Chemical vapor deposition; Diamond wafer
1. Introduction The DC arc plasma jet is generally considered to be one of the most promising chemical vapor deposition ŽCVD. techniques for high quality diamond films growth w1x. However, the conventional design of the plasma torch based on industrial plasma spraying or plasma cutting suffers the disadvantages of high gas consumption, low heat efficiency, the complicity and high investment of the equipment, and most of all, the difficulties for large area uniformity of diamond deposition. In our previous work, we have successfully developed a 100-kW high power DC arc plasma jet CVD system operated in gas cycling mode for deposition of large area diamond films w2x, by which diamond wafers over a diameter of ⌽110 mm can be deposited at a growth rate of 40᎐50
U
Corresponding author. Tel.: q86-311-3020717; fax: q86-3113026250. E-mail address: [email protected] ŽH. Guo..
mrh, with thickness uniformity better than 90% w3,4x. However, detailed studies about the dependence of diamond quality on process parameters by this system were few. In the present paper, the effects of substrate temperature, methane concentration, input power of the jet, especially the flow rate of feeding gas on diamond deposition will be discussed, and the deposition of large area high quality uniform thickness diamond wafers will be reported.
2. Experimental 2.1. Experimental apparatus Diamond wafers were deposited in a 30-kW DC arc plasma jet CVD system developed on the experience of our 100 kW high power jet. Proper control of magnetic field and the characteristics of fluid dynamics in this jet make it possible that the jet steadily discharges over a large area. Fig. 1 shows the working state of the arc
0925-9635r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 2 8 9 - 2
1674
H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
Fig. 1. Photo of a working arc plasma. The region indicated by the arrow is a so-called mustache.
plasma. As shown, a large area of uniformly-distributed arc plasma region between the substrate and outlet orifice is achieved. From the uniformity of thickness and crystalline grain size of deposited diamond wafers, we can conclude that the temperature of plasma above substrate should be uniform. In this system diamond wafers of ⌽65 mm in diameter can be deposited with a growth rate as high as 60 mrh. For economical running, the jet is operated in gas recycling mode in which consumption of feed gas is decreased by 90%. Generally, for steady combustion of the arc, the flow rate of gas must not less than 50᎐100 supplying gas flow ŽSLM. in a conventional jet working at the blow down mode, and in our system it was reduced to 5᎐10 SLM when operating at gas recycling mode. Fig. 2
shows a schematic diagram of the gas recycling system. Supplying gases are fed into the torch through mass flow controllers ŽMFC.. After use, it is cooled down to room temperature and is fed back for recycling by pump 1 through gas valve 1. In this way, most of the gas is recycled while only a small part is vented from valve 2 by pump 2. After a few recycling periods, the system will be in a steady state in which the gas flow of supplying and venting should be in balance. By adjusting the valves the gas cycling system will turn into a new balance and the gas recycling flow will vary also. Gas recycling flow ŽGRF. is indicated by the difference Ž ⌬ P . in chamber pressure P1 and pump 2 outlet pressure P 2. An increase in ⌬ P will result in an increase of GRF, contrarily, GRF should decrease.
3. Experimental conditions Process parameters are given in Table 1. Prior to deposition the substrate was abraded with diamond powder of approximately 14 m in size and cleaned in an ultrasonic cleaner. Substrate temperature was measured by an infrared optical pyrometer with an emissivity setting at 0.4. Deposition time was more than 10 h. In the present paper thermal conductivity was used as the primary technique for characterization of diamond quality. This is because thermal conductivity is strongly affected by even a small amount of nondiamond bonded carbon, therefore it can be taken as an indicator of the quality of diamond films. We employed photothermal deflection technique w5,6x to measure the thermal conductivity of the diamond wafers at room temperature. The measure system is PTDS-II mode made by University of Science and Technology of China ŽUSTC. w7x. The minimum area of measurable sample is approximately 2 = 5 mm2 , this means that the result is an average value in this area. It is apparent that the result is not only affected by the defects in crytallinity, other factors, such as micro-cracks, impurities and cavity, etc., are also important. For a better thermal property the grain size of diamond wafers was larger than 50 m in the present work w8x. In addition, Table 1 Processing parameters employed for diamond film deposition Supplying gas flow ŽSLM.
Fig. 2. Schematic diagram of gas cycling mode.
Pressure of chamber ŽkPa. Difference in cycling pressure ŽkPa. Substrate Žmm. Substrate temperature Ž⬚C. Input power of jet ŽkW.
Ar H2 CH 4
3᎐6 3᎐6 0.05᎐0.2 1᎐10 1.5᎐12
Mo
⌽60 700᎐1000 17
H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
1675
diamond films were also studied by scanning electron microscopy ŽSEM., X-ray diffraction and Raman spectroscopy.
4. Results and discussion The growth rate and thermal conductivity of diamond will increase with the increase of input power to the plasma jet when other deposition parameters remain constant. This is the reason why the design of high power jets is being attempted. In the present investigation, the input power was kept at 17 kW. 4.1. Effect of methane concentration Fig. 3 shows the dependence of growth rate and thermal conductivity on methane concentration while other deposition conditions were as follows: substrate temperature Ts was 900⬚C, chamber pressure P was 3 kPa and the difference of cycling pressure ⌬ P was 9.5 kPa. It can be seen from Fig. 3 that the growth rate still remained as high as 9 mrh while methane concentration decreased to a relative lower value of 0.7% CH4rH2 , and at the same time thermal conductivity increased to 18.3 Wrcm K. This is in general agreement with that reported in literature with other CVD techniques. However, it is worth mentioning that the high thermal conductivity obtained in the present investigation was due to the high concentration of atomic hydrogen provided by the high temperature arc plasma jet. Further more, it can be seen from Fig. 3 that thermal conductivity remained as high as 10 Wrcm K even when the growth rate increased to 30 mrh, which confirmed the advantages of DC arc plasma jet CVD and its commercial prospect is evident.
Fig. 4. Graph of rate and thermal conductivity as a function of substrate temperature while CH4 rH2 is 1.0%, chamber pressure is 3 kPa and difference of cycling pressure is 11 kPa.
function of substrate temperature in Fig. 4 while the methane concentration was 1.0% CH4rH2 , chamber pressure was 3 kPa and difference in cycling pressure was 11 kPa. It can be seen that growth rate was nearly constant when the temperature increased from 750 to 950⬚C, however, a further increase in temperature would result in an increase in the growth rate. Whilst the thermal conductivity increased with increasing temperature from 750 to 900⬚C, reached a maximum at 900⬚C, and then decreased again. It can be concluded that the optimum temperature for the deposition of high quality diamond wafer is approximately 900⬚C. 4.3. Effects of recycling gas flow
Growth rate and thermal conductivity is plotted as a
The flow rate of gas recycling is one of the most important parameters for our DC arc plasma jet CVD system. As discussed above, ⌬ P is the indicator of flow rate of gas recycling. Fig. 5 shows the dependence of growth rate and thermal conductivity on ⌬ P when Ts s 910⬚C, CH4rH2 s 1.3%, Ps 3.6 kPa. From Fig. 5, it can be seen that the growth rate decreased sharply and the thermal conductivity increased linearly with increasing ⌬ P. It can be explained as follows. As shown in Fig. 1, while the jet is working steadily,
Fig. 3. Dependence of growth rate and thermal conductivity on methane concentration at a substrate temperature of 900⬚C, chamber pressure of 3 kPa and difference of cycling ⌬ P of 9.5 kPa.
Fig. 5. Dependence of growth rate and thermal conductivity on ⌬ P when Ts s 910⬚C, CH4 rH2 s 1.3% and Ps 3.6 kPa.
4.2. Effect of substrate temperature
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H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
Fig. 8. X-Ray diffraction pattern of high quality diamond films.
Fig. 6. Photograph of a high quality diamond wafer. Ža. ⌽60 mm unpolished sample Ž0.5 mm thick.; Žb. polished sample Ž0.4 mm thick..
the main part of the hot gas stream coming out of the anode nozzle is being driven directly onto the substrate, which provides chemically active radicals for diamond deposition. However, a small part of the hot gas stream flows from the gap between the nozzle and the substrate, which contributes nothing to diamond growth. There is also a green zone around the arc column, whose grime droops down along substrate, looks like a ‘mustache’ of the substrate. A decrease in ⌬ P will result in rising up of the mustache, so the ratio of hot gas driven to the substrate increases, which in turn results in an increase in growth rate. In contrary, the mustache would droop down, the ratio of chemically active radicals driven to the substrate would decrease, thus results in the decrease of growth rate. In addition, it appears that the growth rate should increase continually with the further decrease of GRF. However, further decrease of GRF will cause radial
Fig. 7. SEM micrograph of high quality diamond films: Ža. surface, Žb. cross-section.
inhomogeneous thickness and sharply decrease heat efficiency of the jet. Furthermore, the methane concentration in the reactant gas mixture will decrease with the increase of GRF, because of the consumption of methane. Consequently, a decrease of growth rate and improvement of thermal conductivity will be observed. Based on the above discussions, with optimal process parameters 0.7-mm-thick high quality diamond wafers up to 65 mm in diameter were successfully deposited, Thermal conductivity of diamond wafer were as high as 18 Wrcm K. High quality diamond wafers appear light yellow Žsee Fig. 6a.. Words in paper can be seen clearly through polished samples without backup light Žshown in Fig. 6b.. Fig. 7 is a SEM micrograph of high quality diamond wafers. Fig. 8 shows the X-ray diffraction spectroscopy. Raman spectroscopy of high quality diamond film is shown in Fig. 9. It can be seen that the spectrum is dominated by a sharp characteristic diamond peak at 1332rcm without any sign of a nondiamond carbon peak.
5. Conclusions The effects of process parameters on deposition of diamond wafers by DC arc plasma jet operating in gas recycling mode were investigated. It was found that substrate temperature of 900⬚C and a lower methane concentration is helpful for depositing high quality
Fig. 9. Raman spectroscopy of high quality diamond.
H. Guo et al. r Diamond and Related Materials 9 (2000) 1673᎐1677
diamond wafers. The system should be operated at a higher flow of recycled gas for not only achieving higher quality of diamond, but also the stable operation of the arc jet. With optimal process parameters it is possible to deposit high quality diamond wafers up to ⌽65 mm in diameter by the DC arc plasma jet CVD system.
Acknowledgements The authors gratefully acknowledge the financial support of the NAMCC ŽNational Advanced Materials Committee of China. and Hebei Province Natural Science foundation.
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References w1x J.V. Busch, J.P. Dismukes, Diamond Relat. Mater. 3 Ž1994. 295. w2x G.H. Li, F.X. Lu, et al. Chinese Patent: CN1099547A, 1995. w3x F.X. Lu, G.F. Zhong, J.G. Sun et al., Diamond Relat. Mater. 7r6 Ž1998. 737. w4x F.X. Lu, G.F. Zhong, Y.L. Fu et al., Proceedings of the International Diamond Symposium Seoul, Ž1996. 115. w5x A. Salazar, A. Sancher-Lavega, J. Fernander, J. Appl. Phys. 65 Ž1989. 4150. w6x A. Salazar, A. Sancher-Lavega, J. Fernander, J. Appl. Phys. 69 Ž1991. 1216. w7x J. Cui, H. Chen, J. Zhang, D. Ban, R. Fang, High Technol. Lett. 1 Ž1995. 91. w8x E.A. Burgemeister, J. Mater Sci. Technol. 16 Ž1981. 1730.