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Recent advances in vacuum techniques in Japan
Vacuum/volume43/number 11/pages 1089 to 1090/1992 Printed in Great Britain
0042-207X/92$5.00+.00 © 1992 Pergamon Press Ltd
Recent advances in vacuum techniques in Japan Souji Komiya, UL VAC Corporation, Nippon Typewriter Building 1- 11-2, Kyobashi, Chuo-Ku, Tokyo 104, Japan
Extreme high vacuum (xhv) down to the 10 11 Pa range has become realistic in several independent groups. In parallel, efforts to investigate the achievement of xhv from many aspects have been cooperated as a national scientific project of Special Coordination Funds for Promotion of Science and Technology in Japan. In this paper, some developments in the recent few years in Japan are reviewed, including: (i) critical evaluation about outgassing rate measurement techniques; (ii) the conductance-modulation technique used to determine the pumping speed and outgassing rate in the uhv region; (iii) low outgassing vacuum wall materials and the origin of desorbed species; and (iv) investigation of wall materials for xhv systems. 1. Introduction
No one could neglect to evaluate the contribution of the ultra high vacuum (uhv) technique, which started in 1950, to the progress of modern science and technologies. Research in new fields of surface science and space simulation experiments could be realized in the initial stage of the development. It also contributed greatly to the development of the semiconductor integrated circuit (]C) industry. Surface analysis and the molecular epitaxial growth technique receives direct benefits from the uhv technique. In the mid-70s, installation of the Tokamak, a big experimental machine for thermonuclear fusion, had been started as a national project of Japan. Feasibility studies to make extreme clean plasma in a huge plasma chamber have been started. At the same period, utilization of controlled plasma has become a major fraction in the IC manufacturing processes, and has resulted in drawing attention to the development of a clean, dust-free vacuum environment. Development for uhv systems made from low-Z materials is required to create high energy particle accelerators. The development was carried out by groups who have joined to construct accelerators and supports in Japanese vacuum industries. The vacuum technique in Japan was well organized in these periods. What were the special interesting items for the research and development area of vacuum technique in the duration bridged between the 80s and 90s in Japan? Table 1 shows the typical area appearing in contributed papers for the Japan Vacuum Society Symposium in 1989 and 1990. It tells us that the field ofxhv, especially the development ofvacuum wall materials that we are interested in. One of the stimulating matters is the big project of the Special Coordination Funds for Promoting Science and Technology, supported by the Science and Technology Agency of Japan. The project is named as research on the development of technology of producing, measuring and utilizing uhv and xhv. Recent progress of the development related to the xhv technique is reviewed and shown in this table. 2. Background of outgassing rate measurement
In our memory, the middle of the 1970s, installation of the big Tokamak machine had been started as the national project of Japan. Revaluation for the outgassing rate of various candidates of the vacuum wall materials and the first wall materials has been mandatory. Measurements of the outgassing rates of vacuum
wall materials have been carried out by the author's affiliate with the request of JAERI ~. After finishing the series of the experiments, it was felt necessary to provide another experimental system for the short sample, which is capable of measuring the outgassing rate in situ in the uhv chamber, before and after plasma cleaning, ion or electron bombardment with a defined flux density and energy. In cooperation with the Institute of Industrial Research, University of Tokyo, a molecular beam type experimental system for the outgassing rate measurement for a short sample has been made 2. The measured outgassing rate by this experimental system is surprisingly higher than expected with the previous data. It was not concluded that this is due to systematic experimental error. It might include the essential nature of molecular beam type measurement. 3. Conductance-modulation method for outgassing rate measurement
In the recent years, the group of the Institute for Industrial Research, University of Tokyo, has successfully carried out measurement of the pumping speed and outgassing rate in the uhv and xhv regions 3. Pressure is measured by total pressure gages surrounding the equilibrium vacuum wall. The conductance-modulation method by Terada, Okano and Tuzi 3 is a revival of that introduced by Oatley 4 in 1954. The special feature of this method is that no requirement for an absolute value of the total pressure reading is needed for pumping speed measurement, the linearity of the pressure reading in the measuring range can be assumed. Moreover, this method can be utilized to calibrate the true pressure by introducing a defined throughput. The Oatley method has been forgotten for a long time because of the difficulty of moving orifices in situ in vacuum. Terada et al introduced the modulation of conductance into an xhv system. More recently, Okano 5 has succeeded in measuring the pumping speed of his electron-bombarded tantalum getter pump in the xhv region by a conductance-modulation method. Tuzi 6 also measured the pumping speed of a cryopump in the uhv and xhv regions. The conductance modulation method is also useful to determine the outgassing rate in the range of uhv and xhv. Fujita and Homma 7 of the Institute of Industrial Research, have utilized this method for their outgassing rate measurement of hexagonal boron-nitride coated stainless steel in the uhv and xhv regions.. 1089
S Komiya . Vacuum techniques in Japan
Table I. Some typical area in papers appeared in Japan Vacuum Society Symposiums since 1980 I. x h v
I.I Pump : Turbomolecular pump Nb evaporated lihn on Ti substrate Cryopump with GM engine 1.2 Component AI alloy flange Stainless steel (SS) tlange with knife edge seal 1.3 Material: SS, low temperature bakcout SS, temperature dependence of outgassing rate SS for xhv AI alloy, outgassing rate AI alloy finished/EAP, surface analysis 1.4 Coated metals : BN on SS, outgassing rate TiN on SS
Mitsubishi Heavy Industry National Research Institute of Metals ANELVA RIKEN-JAERI, I-It Heavy lndustr? Naka Fusion Res Estbl JAERI Hilachi, KEK Natl Lab Hitachi ETL, ANELVA Hokkaido Univ Kobe Steel Lid Hitachi. NKK U LVAC
2. Dry roughing 2.1 Dry pump, ranging from arm to HV 2.2 Molecular drag dynamics in free molecular flow 2.3 Transient flow of radial flow impeller of turbomolecular pumps Dust-free components for clean vacuum 3.1 Heat radiation of stepping motor 3.2 Axial gap stepping motor 3.3 Axial gap stepping motor with solid lubricants 3.4 Magnetic levitation transport system
Musashi Institute of Technology ULVAC ULVAC ULVA("
4. Vacuum gages using laser beam 4.1 Medium pressure measurement by Rayleigh scattering 4.2 Vacuum photo-thermal vibration of a thin plate, dependence of resonance frequency on laser diode power
4. Vacuum wall materials for extreme high vacuum
Stainless steel has been used frequently as a constructing material for an uhv system instead of glass since the 1950s. The stainless steel surface is usually covered with a dense native oxide layer. Occluded gases in the oxide layer is in a relatively small a m o u n t a n d is desorbed easily by a b a k i n g - o u t process. Stainless steel has been used for a long time as a constructing material for uhv systems. The limiting factors for the lowest outgassing rate have been estimated as follows. A t o m i c hydrogen occluded in the stainless steel v a c u u m will diffuse t h r o u g h bulk stainless steel and will be released by c o m b i n a t i o n to form molecular hydrogen or react with oxygen in the oxide layer to form water molecules. However, c a r b o n m o n o x i d e also very frequently appears in residual gas mass spectra o f an uhv system. Both h y d r o g e n a n d c a r b o n m o n o x i d e are the gas molecules finally remaining in an uhv system. T h e source o f c a r b o n m o n o x i d e is still not clear. Bulk diffusion of c a r b o n is far lower t h a n that o f hydrogen. One can estimate a plausible source as being grain b o u n d a r y diffusion of atomic c a r b o n at a m b i e n t temperature. These diffused c a r b o n a t o m s t h r o u g h the grain b o u n d a r y can react with oxygen in the oxide layer to form c a r b o n m o n o x i d e which can be released from the stainless steel surface. A t t e m p t s to neglect these oxide layers as the gas sink and to cover the artificial coated layer to obtain m u c h lower outgassing rates are very interesting. Specially treated a l u m i n u m alloy is one of the materials used for xhv systems, a n d has been developed by I s h i m a r u a n d the K E K G r o u p ~. They have controlled carefully a 1090
Mitsubishi tleavy Industry Beijing Lab of Vacuum Physics Hitachi
Kyusyu University, ULVAC Gifu National College, Nagoya University
native oxide layer of the a l u m i n u m alloy, and found that this material is suitable lbr an xhv constructing material. Komiya el al '~ have tried to provide a TiN coated layer on stainless steel and a l u m i n u m alloy. U n f o r t u n a t e l y , this a t t e m p t to prove it as an xhv material was not continued. A d v a n c e d research to measure the outgassing rate and depositing conditions of TiN has been investigated precisely by T s u k a h a r a a n d the U L V A C g r o u p ' " The hexagonal b o r o n intruded layer formed on b o r o n impregnated stainless steel has been promisingly examined as thc wtcuum wall mate,ial for p r o d u c t i o n of xhv. This investigation has been carried out by Fujita and H o m m a ~. A few of the other groups have l\~llowed them to prove its high accessibility.
References
H Yoshikawa, Y Gomay, Y Sugiyama, M Mizuno, S Komiya and 1' lazima, Proc 7th hTt Vac ('ongr and 3rd Inl Conf Solid Surlace (Vienna, 1977). Vol 1,p367. S Komiya, Y Sugiyama. M Kobayashi and Y Tuzi, J t'ac Sci Techmd, 16, 689 (1979). K Tcrada, T Okano and Y Tuzi, J Vac Sci Technol. A7, 2397 (1989). 4(- W Oatley. Br J AppI Phys. $, 358 (1954). • T Okano, private communication. r, y Tuzi, private comnmnication. 7D Fujita and T Homma, Report of the Institute o1" Industrial Science. The University of Tokyo, Vol 36, No 3, Ser 232 (1991). ~M Miyanoto, Y Sumi, S Komaki, K Narushima and H Ishimaru, .I b ac Sci TechnoL A4, 2515 (1986), H Ishimaru. J Vac Sci Technol, A7, 2439 (1989). ~S Kmniya, N Uemzu and C Hayashi, 7?~inSolM Films, 63, 341 (1979). '" S Tsukahara, priw~te communication.
0042-207X/92$5.00+.00 © 1992 Pergamon Press Ltd
Recent advances in vacuum techniques in Japan Souji Komiya, UL VAC Corporation, Nippon Typewriter Building 1- 11-2, Kyobashi, Chuo-Ku, Tokyo 104, Japan
Extreme high vacuum (xhv) down to the 10 11 Pa range has become realistic in several independent groups. In parallel, efforts to investigate the achievement of xhv from many aspects have been cooperated as a national scientific project of Special Coordination Funds for Promotion of Science and Technology in Japan. In this paper, some developments in the recent few years in Japan are reviewed, including: (i) critical evaluation about outgassing rate measurement techniques; (ii) the conductance-modulation technique used to determine the pumping speed and outgassing rate in the uhv region; (iii) low outgassing vacuum wall materials and the origin of desorbed species; and (iv) investigation of wall materials for xhv systems. 1. Introduction
No one could neglect to evaluate the contribution of the ultra high vacuum (uhv) technique, which started in 1950, to the progress of modern science and technologies. Research in new fields of surface science and space simulation experiments could be realized in the initial stage of the development. It also contributed greatly to the development of the semiconductor integrated circuit (]C) industry. Surface analysis and the molecular epitaxial growth technique receives direct benefits from the uhv technique. In the mid-70s, installation of the Tokamak, a big experimental machine for thermonuclear fusion, had been started as a national project of Japan. Feasibility studies to make extreme clean plasma in a huge plasma chamber have been started. At the same period, utilization of controlled plasma has become a major fraction in the IC manufacturing processes, and has resulted in drawing attention to the development of a clean, dust-free vacuum environment. Development for uhv systems made from low-Z materials is required to create high energy particle accelerators. The development was carried out by groups who have joined to construct accelerators and supports in Japanese vacuum industries. The vacuum technique in Japan was well organized in these periods. What were the special interesting items for the research and development area of vacuum technique in the duration bridged between the 80s and 90s in Japan? Table 1 shows the typical area appearing in contributed papers for the Japan Vacuum Society Symposium in 1989 and 1990. It tells us that the field ofxhv, especially the development ofvacuum wall materials that we are interested in. One of the stimulating matters is the big project of the Special Coordination Funds for Promoting Science and Technology, supported by the Science and Technology Agency of Japan. The project is named as research on the development of technology of producing, measuring and utilizing uhv and xhv. Recent progress of the development related to the xhv technique is reviewed and shown in this table. 2. Background of outgassing rate measurement
In our memory, the middle of the 1970s, installation of the big Tokamak machine had been started as the national project of Japan. Revaluation for the outgassing rate of various candidates of the vacuum wall materials and the first wall materials has been mandatory. Measurements of the outgassing rates of vacuum
wall materials have been carried out by the author's affiliate with the request of JAERI ~. After finishing the series of the experiments, it was felt necessary to provide another experimental system for the short sample, which is capable of measuring the outgassing rate in situ in the uhv chamber, before and after plasma cleaning, ion or electron bombardment with a defined flux density and energy. In cooperation with the Institute of Industrial Research, University of Tokyo, a molecular beam type experimental system for the outgassing rate measurement for a short sample has been made 2. The measured outgassing rate by this experimental system is surprisingly higher than expected with the previous data. It was not concluded that this is due to systematic experimental error. It might include the essential nature of molecular beam type measurement. 3. Conductance-modulation method for outgassing rate measurement
In the recent years, the group of the Institute for Industrial Research, University of Tokyo, has successfully carried out measurement of the pumping speed and outgassing rate in the uhv and xhv regions 3. Pressure is measured by total pressure gages surrounding the equilibrium vacuum wall. The conductance-modulation method by Terada, Okano and Tuzi 3 is a revival of that introduced by Oatley 4 in 1954. The special feature of this method is that no requirement for an absolute value of the total pressure reading is needed for pumping speed measurement, the linearity of the pressure reading in the measuring range can be assumed. Moreover, this method can be utilized to calibrate the true pressure by introducing a defined throughput. The Oatley method has been forgotten for a long time because of the difficulty of moving orifices in situ in vacuum. Terada et al introduced the modulation of conductance into an xhv system. More recently, Okano 5 has succeeded in measuring the pumping speed of his electron-bombarded tantalum getter pump in the xhv region by a conductance-modulation method. Tuzi 6 also measured the pumping speed of a cryopump in the uhv and xhv regions. The conductance modulation method is also useful to determine the outgassing rate in the range of uhv and xhv. Fujita and Homma 7 of the Institute of Industrial Research, have utilized this method for their outgassing rate measurement of hexagonal boron-nitride coated stainless steel in the uhv and xhv regions.. 1089
S Komiya . Vacuum techniques in Japan
Table I. Some typical area in papers appeared in Japan Vacuum Society Symposiums since 1980 I. x h v
I.I Pump : Turbomolecular pump Nb evaporated lihn on Ti substrate Cryopump with GM engine 1.2 Component AI alloy flange Stainless steel (SS) tlange with knife edge seal 1.3 Material: SS, low temperature bakcout SS, temperature dependence of outgassing rate SS for xhv AI alloy, outgassing rate AI alloy finished/EAP, surface analysis 1.4 Coated metals : BN on SS, outgassing rate TiN on SS
Mitsubishi Heavy Industry National Research Institute of Metals ANELVA RIKEN-JAERI, I-It Heavy lndustr? Naka Fusion Res Estbl JAERI Hilachi, KEK Natl Lab Hitachi ETL, ANELVA Hokkaido Univ Kobe Steel Lid Hitachi. NKK U LVAC
2. Dry roughing 2.1 Dry pump, ranging from arm to HV 2.2 Molecular drag dynamics in free molecular flow 2.3 Transient flow of radial flow impeller of turbomolecular pumps Dust-free components for clean vacuum 3.1 Heat radiation of stepping motor 3.2 Axial gap stepping motor 3.3 Axial gap stepping motor with solid lubricants 3.4 Magnetic levitation transport system
Musashi Institute of Technology ULVAC ULVAC ULVA("
4. Vacuum gages using laser beam 4.1 Medium pressure measurement by Rayleigh scattering 4.2 Vacuum photo-thermal vibration of a thin plate, dependence of resonance frequency on laser diode power
4. Vacuum wall materials for extreme high vacuum
Stainless steel has been used frequently as a constructing material for an uhv system instead of glass since the 1950s. The stainless steel surface is usually covered with a dense native oxide layer. Occluded gases in the oxide layer is in a relatively small a m o u n t a n d is desorbed easily by a b a k i n g - o u t process. Stainless steel has been used for a long time as a constructing material for uhv systems. The limiting factors for the lowest outgassing rate have been estimated as follows. A t o m i c hydrogen occluded in the stainless steel v a c u u m will diffuse t h r o u g h bulk stainless steel and will be released by c o m b i n a t i o n to form molecular hydrogen or react with oxygen in the oxide layer to form water molecules. However, c a r b o n m o n o x i d e also very frequently appears in residual gas mass spectra o f an uhv system. Both h y d r o g e n a n d c a r b o n m o n o x i d e are the gas molecules finally remaining in an uhv system. T h e source o f c a r b o n m o n o x i d e is still not clear. Bulk diffusion of c a r b o n is far lower t h a n that o f hydrogen. One can estimate a plausible source as being grain b o u n d a r y diffusion of atomic c a r b o n at a m b i e n t temperature. These diffused c a r b o n a t o m s t h r o u g h the grain b o u n d a r y can react with oxygen in the oxide layer to form c a r b o n m o n o x i d e which can be released from the stainless steel surface. A t t e m p t s to neglect these oxide layers as the gas sink and to cover the artificial coated layer to obtain m u c h lower outgassing rates are very interesting. Specially treated a l u m i n u m alloy is one of the materials used for xhv systems, a n d has been developed by I s h i m a r u a n d the K E K G r o u p ~. They have controlled carefully a 1090
Mitsubishi tleavy Industry Beijing Lab of Vacuum Physics Hitachi
Kyusyu University, ULVAC Gifu National College, Nagoya University
native oxide layer of the a l u m i n u m alloy, and found that this material is suitable lbr an xhv constructing material. Komiya el al '~ have tried to provide a TiN coated layer on stainless steel and a l u m i n u m alloy. U n f o r t u n a t e l y , this a t t e m p t to prove it as an xhv material was not continued. A d v a n c e d research to measure the outgassing rate and depositing conditions of TiN has been investigated precisely by T s u k a h a r a a n d the U L V A C g r o u p ' " The hexagonal b o r o n intruded layer formed on b o r o n impregnated stainless steel has been promisingly examined as thc wtcuum wall mate,ial for p r o d u c t i o n of xhv. This investigation has been carried out by Fujita and H o m m a ~. A few of the other groups have l\~llowed them to prove its high accessibility.
References
H Yoshikawa, Y Gomay, Y Sugiyama, M Mizuno, S Komiya and 1' lazima, Proc 7th hTt Vac ('ongr and 3rd Inl Conf Solid Surlace (Vienna, 1977). Vol 1,p367. S Komiya, Y Sugiyama. M Kobayashi and Y Tuzi, J t'ac Sci Techmd, 16, 689 (1979). K Tcrada, T Okano and Y Tuzi, J Vac Sci Technol. A7, 2397 (1989). 4(- W Oatley. Br J AppI Phys. $, 358 (1954). • T Okano, private communication. r, y Tuzi, private comnmnication. 7D Fujita and T Homma, Report of the Institute o1" Industrial Science. The University of Tokyo, Vol 36, No 3, Ser 232 (1991). ~M Miyanoto, Y Sumi, S Komaki, K Narushima and H Ishimaru, .I b ac Sci TechnoL A4, 2515 (1986), H Ishimaru. J Vac Sci Technol, A7, 2439 (1989). ~S Kmniya, N Uemzu and C Hayashi, 7?~inSolM Films, 63, 341 (1979). '" S Tsukahara, priw~te communication.