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Synchrotron radiation etching of diamond surfaces in the low pressure atmosphere of O2 and SF6
Journal of Electron Spectroscopy and Related Phenomena 80 (1996) 77-80
S y n c h r o t r o n radiation etching of d i a m o n d surfaces in the low pressure a t m o s p h e r e of Oz and SF 6 Eiji Ishiguro a, Haruhiko Ohashi b, Tomohiko Sasano c and Kosuke Shobatake d a
College of Education, University of the Ryukyus, Senbaru 1, Nishihara-cho, Okinawa 903-01, Japan
b Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558, Japan d Department of Materials Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan We report that etching of diamond has been successfully performed for the first time with the aid of synchrotron radiation excitation in the atmosphere of 02 even at a low temperature of -140 °C. 1. I N T R O D U C T I O N Development of microfabrication techniques for diamond is important from the viewpoint of applications of diamond to electronic devices and micromachines. Etching of diamond however is difficult because of its high hardness and chemical inertness. Photoexcited etching is one of the promising techniques for microfabrication of semiconductor devices, since it is a low temperature process without damaging the substrate. As far as the authors know direct patterning on diamond surface has been done only by means of excimer laser excitation [1], the etching mechanism of whic.h however involves both photochemical and thermal processes. In the present paper we report that etching of diamonds has been successfully performed for the first time with the aid of synchrotron radiation(SR) excitation even at a temperature as low as -140 °C in the atmosphere of 02. The SR excitation etching was successfully performed for three types of diamond; chemical vapor deposition (CVD) diamond, high pressure synthesized, and natural diamonds. Here, we confine ourselves to reporting the results of CVD diamonds. The 0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0368-2048 (96) 02926-X
results of the other types of diamonds will be reported elsewhere[2]. 2. E X P E R I M E N T A L Undispersed synchrotron radiation emitted from a bending magnet field of the 750 MeV UVSOR ring at Institute for Molecular Science was used as an excitation source for the etching experiment. The critical energy of the SR is 425 eV. The SR was focused on a sample in a reaction chamber by a toroidal mirror which was located 2.5 m downstream from the source. The incident angle to the mirror was 88 ° and the angular divergence accepted in the horizontal direction was 7.5 mrad. The distance between the mirror center and the sample was 2.8 m. The spot size of SR on the sample was about 2(vert.)mm × 4(hori.) mm. A nickel mesh with 60 wires/inch and a 90 % transmittance (a line spacing of 400 t t m and a wire width of 40/1 m) was used as a mask. It was located 5 mm above the sample surface. The reaction chamber was connected to the beam line through three stages of differential pumping system to make it
78 possible to fill it with an etchant gas of 02 or S F 6 up to 0.5 Torr. The t e m p e r a t u r e of the sample was varied from 400°C to -140°Cby using a heater or by flowing liquid nitrogen. The CVD d i a m o n d s used in the present experiment were supplied by Osaka Diamond Industrial Co., Ltd which made them by depositing onto a Si or a S i 3 N 4 substrate. The accumulated beam current in the storage ring was used as a measure of the dose.
3. R E S U L T S AND D I S C U S S I O N It is known that oxidation of d i a m o n d starts at about 500°C in the air. In the present study, experiments were p e r f o r m e d below 400°C to avoid thermal oxidation. SR excited etching of d i a m o n d surface was f o u n d to occur even at a low temperature of -140°C. A micrograph of a surface etched at -140 °C is shown in Figure 1. The pressure of 02 was 0.1 Torr and the dose was 50 A • min. One can clearly see the mask pattern f o r m e d on the surface. Figure 2 shows a cross sectional profile of the etched surface which was measured with a stylus gauge with a 12.5 /~ m radius. We can see the line spacing of 400,u m and the line width ( F W H M ) of 38 /1 m. The slope of the line edges was fairly sharp and the height at the center of irradiated area was about 1.1 ,um. From the results shown above, it is obvious that only the surface irradiated by SR can be etched and therefore surfac.e excitation plays an important role in SR excited etching of diamond in the 02 atmosphere. At a surface temperature of 370 °C and an O z pressure of 0.10 Torr the etch rates were f o u n d to be 0.42 and 0.65 A / m A / m i n (see Figure 3), the unit of which is d e f i n e d as a depth at the center of the irradiated area in A per unit dose. As the temperature was lowered from 370°Cthe etch rate was found to decrease. The rates were found to be from 0.2 to 0.3 A / m A / m i n from - 140 to 150 °C, although the error bars for these values were large. From these f i n d i n g s it is c o n c l u d e d that there is no strong temperature
d e p e n d e n c e of the etch rate in the low t e m p e r a t u r e range - 140 to 150 °C. To confirm this point experiments are under way. Irradiation of SR on a d i a m o n d surface gives rise to not only etching but also graphitization of the irradiated surface, as is j u d g e d from the dark area seen in Figure 1. Probably an amorphous carbon layer might have been formed in this area. The carbon layer was easily scraped away from the surface with a tip of tweezers. In other words, upon irradiation of SR not all carbon atoms of the d i a m o n d surface are removed as volatile compounds such as CO and CO 2 when etched in the oxygen atmosphere, but a change to an amorphous carbon seems to have occurred. G r a p h i t i z a t i o n of d i a m o n d surface is reported to occur in ArF excimer laser etching in vacuo[1 ], in which case a thermal heating effect is considered to play an important role. On the other hand, the thermal effect can be neglected in our SR excited etching, since the photon power is much smaller than that of the excimer laser. In fact no change was observed in the d i a m o n d surface irradiated by SR in vacuo. This suggests that the presence of oxygen gas is essential to SR excited etching of diamond surface and may play some role in graphitization process.
Figure 1. Micrograph of the diamond surface etched with the aid of SR excitation in O 2 atmosphere. The line spacing in the mesh pattern is 400/1 m. The dark area around the center is due to graphitization.
79
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2000
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atoms and ii) carbon atoms in a carbon film are removed as CO molecules after the reaction with the O atoms. Since physical and chemical properties of diamond differ enormously from those of carbon films used by Kyuragi[3] the mechanisms of etching reactions for two types of samples must be quite different from each other. Si[4], SIO214,5,6 ], SIC[6,7] and Si3N418] samples have been found to be etched by SR excitation in the SF 6 atmosphere. Even carbon
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Figure 2. Profiles of the etched diamond surface measured by a stylus gauge of 12.5 ,um radius; (a) the whole profile of the etched surface and (b) a line profile in the center of the etched area. The line width (FWHM) is 38/1 m and the height 1.2 F m. Figure 3 shows the etch rate plotted versus pressure of the oxygen gas at 370"C. The etch rate increases as the oxygen pressure is increased. However it is difficult to give an expression for the dependence of the etch rate on the oxygen pressure, since the observed etch rates are considered to have an error of :L:20 %. Nevertheless, the etch rate does not seem to increase linearly to the oxygen pressure. It has been reported that carbon films which are made in rf sputtering can be easily etched by SR in an oxygen atmosphere [3]. From the observed square root oxygen pressure dependence of the etch rate an etching mechanism has been proposed that i) the oxygen molecules adsorbed on the surface are photodissociated by SR into oxygen
0.1 •
. .i
0.01
0.1
02 Pressure (ton')
Figure 3. Etch rate of diamond surface vs. 02 pressure. The temperature of the sample was 3700C.
Figure 4. Micrograph of the surface irradiated by SR in the S F 6 atmosphere. The temperature of the sample was -90 *C. A mask pattern is obscurely seen on the surface.
80
atoms in SiC seem to have been etched and thus SF 6 gas was also used as an etchant for diamond etching. The CVD diamond samples irradiated in the S F 6 atmosphere however did not exhibit any distinct evidence that etching does occur at any temperature. A few samples showed obscurely the mask pattern on the surface, as shown in Figure 4 which illustrates a micrograph of the diamond surface etched at a surface temperature of-90°C and a n S F 6 pressure of 5 X 10 -4 Torr and for a dose level of 26.5 A • min. At -90°CSF6 molecules are considered to be frozen on the diamond surface as a thin film, since the melting point of SF 6 is -50.8 °C. The lines were not observed when the surface profile was measured with a stylus gauge and therefore we did not find an evidence that the diamond was etched in the S F 6 atmosphere. Namely this means that diamond is not substantially etched with SF 6, or the etch rate must be quite small, if any. The irradiated surface must be examined in detail by various methods such as SEM and STM to make clear what has happened to the surface. 4. S U M M A R Y Undispersed SR radiation from a bending magnet field of the UVSOR ring was irradiated on CVD-diamond surfaces in the oxygen atmosphere. We find that the diamond can be etched by shining the SR even at a low temperature o f - 1 4 0 ~ . A mask pattern with a line width of 4 0 # m was successfully transferred to the diamond surface with a line profile having sharp edges, which indicates that surface excitation processes play an important role in the etching of diamond. A structural change of the diamond to an
amorphous carbon was also observed on the irradiated surface. Conspicuous evidence of SR excited etching of diamond surface in S F 6 atmosphere however was not found. ACKNOWLEDGMENTS The authors would like to thank the staff of UVSOR for their kind support of the present experiment. We also thank Mr.T.Nagano (Shimadzu Corporation) for taking micro-graphs of diamond surfaces. REFERENCES 1. M.Rothschild,C.Arnone and D.J.Ehrlich, J.Vac.Sci.Technol.B4(1) (1988) 310. 2. H.Ohashi, E.Ishiguro, T.Sasano and K.Shobatake, to be published. 3. H. Kyuragi and T. Urisu Phys. Lett. 50 (1987) 1254. 4. T. Urisu and H. Kyuragi, J.V.Sci.Technol. B5 (1987) 1436. 5. K. Shobatake, H. Ohashi, K. Fukui, A. Hiraya, N. Hayasaka, H. Okano, A. Yoshida and H. Kume, Appl. Phys. Lett. 56 (1990) 2189. 6. H. Ohashi, A. Yoshida, K. Tabayashi and K. Shobatake, Appl. Surf. Sci. 69 (1993) 20. 7. E. Ishiguro, K. Yamashita, H. Ohashi, M. Sakurai, O. Aita, M. Watanabe, K. Sano, M. Koeda and T. Nagano, Rev. Sci. Instrum. 63 (1992) 1439. 8. O. Kitamura, T. Goto, S. Terakado, S. Suzuki, T. Sekitani and K. Tanaka, Appl. Surf. Sci. 79/80 (1994) 122.
S y n c h r o t r o n radiation etching of d i a m o n d surfaces in the low pressure a t m o s p h e r e of Oz and SF 6 Eiji Ishiguro a, Haruhiko Ohashi b, Tomohiko Sasano c and Kosuke Shobatake d a
College of Education, University of the Ryukyus, Senbaru 1, Nishihara-cho, Okinawa 903-01, Japan
b Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558, Japan d Department of Materials Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan We report that etching of diamond has been successfully performed for the first time with the aid of synchrotron radiation excitation in the atmosphere of 02 even at a low temperature of -140 °C. 1. I N T R O D U C T I O N Development of microfabrication techniques for diamond is important from the viewpoint of applications of diamond to electronic devices and micromachines. Etching of diamond however is difficult because of its high hardness and chemical inertness. Photoexcited etching is one of the promising techniques for microfabrication of semiconductor devices, since it is a low temperature process without damaging the substrate. As far as the authors know direct patterning on diamond surface has been done only by means of excimer laser excitation [1], the etching mechanism of whic.h however involves both photochemical and thermal processes. In the present paper we report that etching of diamonds has been successfully performed for the first time with the aid of synchrotron radiation(SR) excitation even at a temperature as low as -140 °C in the atmosphere of 02. The SR excitation etching was successfully performed for three types of diamond; chemical vapor deposition (CVD) diamond, high pressure synthesized, and natural diamonds. Here, we confine ourselves to reporting the results of CVD diamonds. The 0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0368-2048 (96) 02926-X
results of the other types of diamonds will be reported elsewhere[2]. 2. E X P E R I M E N T A L Undispersed synchrotron radiation emitted from a bending magnet field of the 750 MeV UVSOR ring at Institute for Molecular Science was used as an excitation source for the etching experiment. The critical energy of the SR is 425 eV. The SR was focused on a sample in a reaction chamber by a toroidal mirror which was located 2.5 m downstream from the source. The incident angle to the mirror was 88 ° and the angular divergence accepted in the horizontal direction was 7.5 mrad. The distance between the mirror center and the sample was 2.8 m. The spot size of SR on the sample was about 2(vert.)mm × 4(hori.) mm. A nickel mesh with 60 wires/inch and a 90 % transmittance (a line spacing of 400 t t m and a wire width of 40/1 m) was used as a mask. It was located 5 mm above the sample surface. The reaction chamber was connected to the beam line through three stages of differential pumping system to make it
78 possible to fill it with an etchant gas of 02 or S F 6 up to 0.5 Torr. The t e m p e r a t u r e of the sample was varied from 400°C to -140°Cby using a heater or by flowing liquid nitrogen. The CVD d i a m o n d s used in the present experiment were supplied by Osaka Diamond Industrial Co., Ltd which made them by depositing onto a Si or a S i 3 N 4 substrate. The accumulated beam current in the storage ring was used as a measure of the dose.
3. R E S U L T S AND D I S C U S S I O N It is known that oxidation of d i a m o n d starts at about 500°C in the air. In the present study, experiments were p e r f o r m e d below 400°C to avoid thermal oxidation. SR excited etching of d i a m o n d surface was f o u n d to occur even at a low temperature of -140°C. A micrograph of a surface etched at -140 °C is shown in Figure 1. The pressure of 02 was 0.1 Torr and the dose was 50 A • min. One can clearly see the mask pattern f o r m e d on the surface. Figure 2 shows a cross sectional profile of the etched surface which was measured with a stylus gauge with a 12.5 /~ m radius. We can see the line spacing of 400,u m and the line width ( F W H M ) of 38 /1 m. The slope of the line edges was fairly sharp and the height at the center of irradiated area was about 1.1 ,um. From the results shown above, it is obvious that only the surface irradiated by SR can be etched and therefore surfac.e excitation plays an important role in SR excited etching of diamond in the 02 atmosphere. At a surface temperature of 370 °C and an O z pressure of 0.10 Torr the etch rates were f o u n d to be 0.42 and 0.65 A / m A / m i n (see Figure 3), the unit of which is d e f i n e d as a depth at the center of the irradiated area in A per unit dose. As the temperature was lowered from 370°Cthe etch rate was found to decrease. The rates were found to be from 0.2 to 0.3 A / m A / m i n from - 140 to 150 °C, although the error bars for these values were large. From these f i n d i n g s it is c o n c l u d e d that there is no strong temperature
d e p e n d e n c e of the etch rate in the low t e m p e r a t u r e range - 140 to 150 °C. To confirm this point experiments are under way. Irradiation of SR on a d i a m o n d surface gives rise to not only etching but also graphitization of the irradiated surface, as is j u d g e d from the dark area seen in Figure 1. Probably an amorphous carbon layer might have been formed in this area. The carbon layer was easily scraped away from the surface with a tip of tweezers. In other words, upon irradiation of SR not all carbon atoms of the d i a m o n d surface are removed as volatile compounds such as CO and CO 2 when etched in the oxygen atmosphere, but a change to an amorphous carbon seems to have occurred. G r a p h i t i z a t i o n of d i a m o n d surface is reported to occur in ArF excimer laser etching in vacuo[1 ], in which case a thermal heating effect is considered to play an important role. On the other hand, the thermal effect can be neglected in our SR excited etching, since the photon power is much smaller than that of the excimer laser. In fact no change was observed in the d i a m o n d surface irradiated by SR in vacuo. This suggests that the presence of oxygen gas is essential to SR excited etching of diamond surface and may play some role in graphitization process.
Figure 1. Micrograph of the diamond surface etched with the aid of SR excitation in O 2 atmosphere. The line spacing in the mesh pattern is 400/1 m. The dark area around the center is due to graphitization.
79
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1
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(a) A
E ::t -0.5
Q -1 I
T
0
=
1000 x
-
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I
I
2000
3000
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atoms and ii) carbon atoms in a carbon film are removed as CO molecules after the reaction with the O atoms. Since physical and chemical properties of diamond differ enormously from those of carbon films used by Kyuragi[3] the mechanisms of etching reactions for two types of samples must be quite different from each other. Si[4], SIO214,5,6 ], SIC[6,7] and Si3N418] samples have been found to be etched by SR excitation in the SF 6 atmosphere. Even carbon
;
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I
.
.
.
.
.
.
.
.
I
E -0.5 :::t
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1
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I
1 O0 x (/Jm)
~
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=
200
=_o ILl
Figure 2. Profiles of the etched diamond surface measured by a stylus gauge of 12.5 ,um radius; (a) the whole profile of the etched surface and (b) a line profile in the center of the etched area. The line width (FWHM) is 38/1 m and the height 1.2 F m. Figure 3 shows the etch rate plotted versus pressure of the oxygen gas at 370"C. The etch rate increases as the oxygen pressure is increased. However it is difficult to give an expression for the dependence of the etch rate on the oxygen pressure, since the observed etch rates are considered to have an error of :L:20 %. Nevertheless, the etch rate does not seem to increase linearly to the oxygen pressure. It has been reported that carbon films which are made in rf sputtering can be easily etched by SR in an oxygen atmosphere [3]. From the observed square root oxygen pressure dependence of the etch rate an etching mechanism has been proposed that i) the oxygen molecules adsorbed on the surface are photodissociated by SR into oxygen
0.1 •
. .i
0.01
0.1
02 Pressure (ton')
Figure 3. Etch rate of diamond surface vs. 02 pressure. The temperature of the sample was 3700C.
Figure 4. Micrograph of the surface irradiated by SR in the S F 6 atmosphere. The temperature of the sample was -90 *C. A mask pattern is obscurely seen on the surface.
80
atoms in SiC seem to have been etched and thus SF 6 gas was also used as an etchant for diamond etching. The CVD diamond samples irradiated in the S F 6 atmosphere however did not exhibit any distinct evidence that etching does occur at any temperature. A few samples showed obscurely the mask pattern on the surface, as shown in Figure 4 which illustrates a micrograph of the diamond surface etched at a surface temperature of-90°C and a n S F 6 pressure of 5 X 10 -4 Torr and for a dose level of 26.5 A • min. At -90°CSF6 molecules are considered to be frozen on the diamond surface as a thin film, since the melting point of SF 6 is -50.8 °C. The lines were not observed when the surface profile was measured with a stylus gauge and therefore we did not find an evidence that the diamond was etched in the S F 6 atmosphere. Namely this means that diamond is not substantially etched with SF 6, or the etch rate must be quite small, if any. The irradiated surface must be examined in detail by various methods such as SEM and STM to make clear what has happened to the surface. 4. S U M M A R Y Undispersed SR radiation from a bending magnet field of the UVSOR ring was irradiated on CVD-diamond surfaces in the oxygen atmosphere. We find that the diamond can be etched by shining the SR even at a low temperature o f - 1 4 0 ~ . A mask pattern with a line width of 4 0 # m was successfully transferred to the diamond surface with a line profile having sharp edges, which indicates that surface excitation processes play an important role in the etching of diamond. A structural change of the diamond to an
amorphous carbon was also observed on the irradiated surface. Conspicuous evidence of SR excited etching of diamond surface in S F 6 atmosphere however was not found. ACKNOWLEDGMENTS The authors would like to thank the staff of UVSOR for their kind support of the present experiment. We also thank Mr.T.Nagano (Shimadzu Corporation) for taking micro-graphs of diamond surfaces. REFERENCES 1. M.Rothschild,C.Arnone and D.J.Ehrlich, J.Vac.Sci.Technol.B4(1) (1988) 310. 2. H.Ohashi, E.Ishiguro, T.Sasano and K.Shobatake, to be published. 3. H. Kyuragi and T. Urisu Phys. Lett. 50 (1987) 1254. 4. T. Urisu and H. Kyuragi, J.V.Sci.Technol. B5 (1987) 1436. 5. K. Shobatake, H. Ohashi, K. Fukui, A. Hiraya, N. Hayasaka, H. Okano, A. Yoshida and H. Kume, Appl. Phys. Lett. 56 (1990) 2189. 6. H. Ohashi, A. Yoshida, K. Tabayashi and K. Shobatake, Appl. Surf. Sci. 69 (1993) 20. 7. E. Ishiguro, K. Yamashita, H. Ohashi, M. Sakurai, O. Aita, M. Watanabe, K. Sano, M. Koeda and T. Nagano, Rev. Sci. Instrum. 63 (1992) 1439. 8. O. Kitamura, T. Goto, S. Terakado, S. Suzuki, T. Sekitani and K. Tanaka, Appl. Surf. Sci. 79/80 (1994) 122.