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Fabrication of micromechanical structures on substrates selectively etched using a micropatterned ion-implantation method
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 777±781
www.elsevier.nl/locate/nimb
Fabrication of micromechanical structures on substrates selectively etched using a micropatterned ion-implantation method Shizuka Nakano
a,*
, Sachiko Nakagawa b, Haruo Ishikawa c, Hisato Ogiso
a,d
a
d
Nanotechnology Division, Mechanical Engineering Laboratory, Agency of Industrial Science and Technology (AIST), Ministry of International Trade and Industry (MITI), 1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan b Graduate School of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan c University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan National Institute for Advanced Interdisciplinary Research, AIST, MITI, 1-1-4 Higashi, Tsukuba, Ibaraki 305-8562, Japan
Abstract An advanced micromachining technique using ion implantation to modify materials was studied. Gold ion implantation into silicon decreased the etching rate when the silicon was etched in potassium hydroxide solution after the ion implantation; the implanted region remained, thus forming the microstructure. Observation of the cross-section of the resulting etched structure by transmission electron microscopy showed that the structure was made only from the ion-implanted region, and that gold was precipitated on the surface. To clarify the mechanism involved in the decrease in the etching rate, we varied the etching conditions. Our results show that precipitation of implanted gold on the surface decreased the etching rate, because solubility of gold is lower. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.40.H; 07.10.C; 61.72.T; 81.65.C Keywords: Micromachine; Microelectro-mechanical systems; Micromachining; Ion implantation; Etch rate; Transmission electron microscopy
1. Introduction When ion implantation, which is a well-known surface treatment technology, is applied to the fabrication of micromachine devices, it is actually a material technology, because the dimensions of the ion modi®ed thickness and area and dimensions of the device are in the same order [1,2]. Ion
*
Tel.: +81-298-61-7163; fax: +81-298-61-7007. E-mail address: [email protected] (S. Nakano).
implantation used as micromaterial technology has three advantages. One is that modi®ed material of a wide variety of characteristics can be made, and by using various species of ions, those characteristics can be chosen. The second is that devices can be made from combined material characteristics, because ion implantation modi®es the character of each minute part of the devices. The third is that combination with other micromachining techniques is possible, because both ion implantation and micromachining are semiconductor manufacturing techniques. Then, because
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 6 0 5 - 4
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S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
Fig. 1. Microcantilever was composed only of an ion-implanted layer fabricated by etching a silicon substrate after gold ion implantation with a micropattern.
the ion-implantation technique utilizes the ionimplanted layer of a substrate for the microdevice, a technique that combines micropattern implantation with selective etching of the substrate was previously developed, and then used to fabricate structures such as the microcantilever shown in Fig. 1 [3,4]. Furthermore, the elastic modulus and electrical properties of the microcantilever dier from the substrate, and both can be controlled by adjusting the ion dose [5]. In this study, using the ion implantation, we fabricated a microcantilever beam to evaluate the resulting structure and then fabricated microcantilevers with dierent patterns under the same conditions to evaluate the etching conditions for gold ion implantation. The experimental conditions are the same as those previously reported [4,5]. Using cross-section transmission electron microscopy (XTEM) and energy dispersive X-ray analysis (EDX) to examine the resulting microstructures, we found that the etching rate is decreased by the implanted-ion deposition. This ®nding can be used in future device design. 2. Experimental In the fabrication of the microcantilevers, we used an undoped (1 0 0) silicon wafer for the substrate, where 3.1 MeV gold ions were implanted in the substrate. In the ion implantation, the sample was installed on a temperature-controlled sample holder, and low-temperature implantation at 95 K was done. After ion implantation, the silicon
substrate was etched by using 30% potassium hydroxide (KOH) solution at 345±350 K. Two types of samples were fabricated, one for evaluating the structure of the fabricated microcantilever, and one for evaluating the mechanism that hinders the etching process. Both types have a microcantilever pattern; however, the substrate under the cantilever of the ®rst sample was completely removed, where that for the second was not. For all samples, the ion dose was 1 1017 cm 2 . For the micropatterning of the microcantilever for evaluating the microstructure, we used a tantalum stencil, because of its durability and long life. In the fabrication of this microcantilever, the base pattern was also implanted into the back surface of the substrate. Then, the substrate under the cantilever was removed by single-side etching from the back using the stainless steel holder. The result was a microcantilever beam. For the microcantilevers used in evaluating the etching conditions, we made four types of samples: three for evaluating the etching time (each one at an etching time of 10, 20 and 30 min), and the fourth for evaluating the contamination from stainless steel etching holder, it was etched for 20 min with the stainless steel etching holder that was used to fabricate the cantilever. For these samples, we used a stainless steel mask, because a tantalum mask that was used for the above was broken and it is more dicult to fabricate than a stainless steel mask. For all samples, the microstructures were then studied using XTEM observation and EDX analysis. 3. Results 3.1. Cross-sectional structure of the microcantilever Fig. 2 shows a XTEM photograph of the center part of the microcantilever. The structure of the cantilever shows three layers; a black center layer and two gray surface layers that contain black particles. The cantilever was 0.9 lm thick, with the black layer being 0.6 lm thick. The EDX analysis results in Table 1 show that the black layer was silicon with implanted gold atom distribution. The
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
779
diameter) on the interface, and large grains (few tens of nanometers) on the boundary. Among the three samples, the 10 min etched sample (Fig. 3(a)) had a larger number of large grains on the boundary than the 20 min or 30 min etched samples (Figs. 3(b) and (c), respectively). The EDX analysis results show that the small grains on the interface were between 80 and 95 at.% gold with silicon and that the larger grains were between 3 and 50 at.% iron with gold, silicon, nickel and chromium. The sample that was etched with the stainless steel holder (Fig. 3(d)) diered from the other samples that it had grains in which the golden grains were enclosed by an iron-rich structure on the boundary. All samples also contained iron, nickel and chromium in the gold implanted bulk as Fig. 2. XTEM image of the microcantilever showing a threelayer structure and nano-size grains.
gray surface layers were SiO2 and the black particles were gold and iron. Furthermore, a small recrystallized region appeared on the interface of the center gold distribution layer and SiO2 layer; however, its composition could not be identi®ed. 3.2. Evaluation of etching conditions In the samples prepared for evaluating the etching conditions in which various etching times were used, the stainless steel stencil mask was used during the ion implantation. Fig. 3 shows the XTEM photographs of these samples. In all the samples, independent of etching time, two types of grains were observed; small grains (5 nm in
Fig. 3. XTEM image of the gold ion-implanted silicon after (a) 10, (b) 20, (c) 30 min of etching without using the stainless steel holder and (d) 20 min of etching with stainless steel etching holder. Small arrow on (d) indicates the golden grains that were enclosed by iron-rich structure on the boundary.
Table 1 EDX analysis results for dierent locations in the cross-section of the microcantilevera
a
Analyzed point
Silicon (at.%)
Gold (at.%)
Oxygen (at.%)
Iron (at.%)
1, 2, 4, 20, 21 3, 19 5±18 22 23 24
35.3±38.3 38.27, 36.7 90.7±99.8 5.48 39.32 35.76
0 0.03 9.3±0.2 9.0 4.12 7.77
64.6±61.7 61.7, 63.7 0 3.12 56.55 55.44
0 0 0 82.41 0 1.03
Locations are indicated by the numbers in Fig. 3.
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S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
Fig. 4. EDX analysis of gold ion-implanted silicon fabricated by using a stainless steel stencil mask. Iron, chromium and nickel peaks appear along with a gold peak.
shown in representative results in Fig. 4. The etching rates were calculated by dividing the maximum thickness of the gold layer (determined from lower magni®cation XTEM photographs) by the etching time. Fig. 5 shows the calculated etching rates and the density of the gold ions as a function of silicon depth [5]. Etching rate decreased with etched depth, namely, etching time.
Fig. 5. Average etch rate () calculated by dividing the etch depth by the etch time, and density of the implanted gold ions (+).
4. Discussion Comparison of the 0.9 lm thick microcantilever with the 1.9 lm thick gold distribution layer before the etching (Fig. 2) shows that the microcantilever was composed only of an ion-implanted layer. This shows that ion implantation is an eective material fabrication technique for microstructures. By evaluating the material properties for various ion species, this technique may prove eective as a micromachining technique. The iron particles in the oxidized layer of the cantilever came from the vacuum devices of the ion-implantation system and from the stainless steel holder during the etching. No contamination was detected in the substrate before etching, suggesting that the etching dissolved iron from the holder. The deep SiO2 layer of the cantilever beam was oxidized during fabrication and XTEM evaluation, because the cantilever was used for other experiments, e.g., elastic property measurements [5]. Based on our results, the growth of the surface silicon oxidation of the cantilever structure caused the gold and iron grains on the surface (Fig. 4) to migrate to the center of the oxidized layer. Therefore, oxidation of the surface of silicon microdevices must be considered when applying this technique. Moreover, the mechanism involved in the recrystallization region in the SiO2 boundary needs further study.
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
In the ion implantation of the samples used in evaluating the etching conditions, the stencil mask was made from stainless steel. The use of this stencil caused the contamination from sputter of the mask by chromium, nickel and iron (Fig. 4), which are components of stainless steel. Therefore the ion beam spatters the stencil, when stainless stencil mask was used, and the contamination occurred. This means that, in actual processing, tantalum is a better material for the stencil. When the stainless steel holder was placed in the etching bath, a signi®cant amount of iron deposited on the surface, and enclosed the gold. It means that the stainless steel holder was etched, and much iron was deposited on the surface. In the case of the microcantilever, only the iron grains exist on the surface layer. Then, we regard it as not contaminated during the ion implantation but that it is produced deposition during the etching. In the samples used in evaluating the etching conditions, small black grains (about 5 nm in diameter) composed of gold and iron were observed in a single row in the silicon interface boundary. On the boundary, the gold and iron grains were grown up to several tens of nanometers. This means that when etching dissolved the silicon atom, the gold atoms having lower solubility were precipitated as grain. When etching was progressing, the grains were grown. Furthermore, the etching rate was inversely proportional to the gold distribution, as shown in Fig. 5. The gold grains that have lower solubility protected the silicon dissolvent, thus etching rate was decreased. However, the number of larger gold grains on the boundary decreased with increasing etch time. This suggests that the grains on the boundary were separated from the surface by etching. On the other hand, the number of the interface grains is increased with increasing the etch time though the grains on the interface relate to the etching rate. The samples used in evaluating the etching conditions showed no SiO2 layer on the surface,
781
because the samples were evaluated by the TEM immediately after the etching. Furthermore, small grains were precipitated only on the interface; the SiO2 layer in the cantilever was formed after the cantilever was prepared, and therefore was not due to the etching. 5. Conclusion 1. The micromachining process, ion implantation, and selective etching were successfully used to form microstructures composed only of an ion-implanted layer. 2. On the surface of the microstructures, grains of implanted gold and iron that were entrapped during the etching, crystallized. 3. Iron, nickel and chromium contaminated the microstructures due to the sputtering process when stainless steel was used for the stencil mask of the micropattern implantation. 4. The etching rate was inversely proportional to the gold distribution. 5. Decrease in the etching rate of the ion-implanted microstructures was partly due to the precipitation of the implanted gold on the interface.
References [1] S. Nakano, K. Yamanaka, H. Ogiso, T. Koda, in: Proceedings of the Third International Symposium on Micromachines and Human Science, Nagoya, 1992, p. 51. [2] K. Yamanaka, S. Nakano, H. Ogiso, O.V. Kolosov, T. Koda, in: Proceedings of the Third International Symposium on Micromachines and Human Science, Nagoya, 1992, p. 59. [3] B. Schmidt, L. Bischo, J. Teichert, Sensors Actuat. A 61 (1997) 369. [4] S. Nakano, H. Ogiso, A. Yabe, Nucl. Instr. and Meth. B 155 (1999) 79. [5] S. Nakano, H. Ogiso, H. Sato, S. Nakagawa, Surf. Coat. Technol. 128±129 (2000) 71.
www.elsevier.nl/locate/nimb
Fabrication of micromechanical structures on substrates selectively etched using a micropatterned ion-implantation method Shizuka Nakano
a,*
, Sachiko Nakagawa b, Haruo Ishikawa c, Hisato Ogiso
a,d
a
d
Nanotechnology Division, Mechanical Engineering Laboratory, Agency of Industrial Science and Technology (AIST), Ministry of International Trade and Industry (MITI), 1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan b Graduate School of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan c University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan National Institute for Advanced Interdisciplinary Research, AIST, MITI, 1-1-4 Higashi, Tsukuba, Ibaraki 305-8562, Japan
Abstract An advanced micromachining technique using ion implantation to modify materials was studied. Gold ion implantation into silicon decreased the etching rate when the silicon was etched in potassium hydroxide solution after the ion implantation; the implanted region remained, thus forming the microstructure. Observation of the cross-section of the resulting etched structure by transmission electron microscopy showed that the structure was made only from the ion-implanted region, and that gold was precipitated on the surface. To clarify the mechanism involved in the decrease in the etching rate, we varied the etching conditions. Our results show that precipitation of implanted gold on the surface decreased the etching rate, because solubility of gold is lower. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.40.H; 07.10.C; 61.72.T; 81.65.C Keywords: Micromachine; Microelectro-mechanical systems; Micromachining; Ion implantation; Etch rate; Transmission electron microscopy
1. Introduction When ion implantation, which is a well-known surface treatment technology, is applied to the fabrication of micromachine devices, it is actually a material technology, because the dimensions of the ion modi®ed thickness and area and dimensions of the device are in the same order [1,2]. Ion
*
Tel.: +81-298-61-7163; fax: +81-298-61-7007. E-mail address: [email protected] (S. Nakano).
implantation used as micromaterial technology has three advantages. One is that modi®ed material of a wide variety of characteristics can be made, and by using various species of ions, those characteristics can be chosen. The second is that devices can be made from combined material characteristics, because ion implantation modi®es the character of each minute part of the devices. The third is that combination with other micromachining techniques is possible, because both ion implantation and micromachining are semiconductor manufacturing techniques. Then, because
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 6 0 5 - 4
778
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
Fig. 1. Microcantilever was composed only of an ion-implanted layer fabricated by etching a silicon substrate after gold ion implantation with a micropattern.
the ion-implantation technique utilizes the ionimplanted layer of a substrate for the microdevice, a technique that combines micropattern implantation with selective etching of the substrate was previously developed, and then used to fabricate structures such as the microcantilever shown in Fig. 1 [3,4]. Furthermore, the elastic modulus and electrical properties of the microcantilever dier from the substrate, and both can be controlled by adjusting the ion dose [5]. In this study, using the ion implantation, we fabricated a microcantilever beam to evaluate the resulting structure and then fabricated microcantilevers with dierent patterns under the same conditions to evaluate the etching conditions for gold ion implantation. The experimental conditions are the same as those previously reported [4,5]. Using cross-section transmission electron microscopy (XTEM) and energy dispersive X-ray analysis (EDX) to examine the resulting microstructures, we found that the etching rate is decreased by the implanted-ion deposition. This ®nding can be used in future device design. 2. Experimental In the fabrication of the microcantilevers, we used an undoped (1 0 0) silicon wafer for the substrate, where 3.1 MeV gold ions were implanted in the substrate. In the ion implantation, the sample was installed on a temperature-controlled sample holder, and low-temperature implantation at 95 K was done. After ion implantation, the silicon
substrate was etched by using 30% potassium hydroxide (KOH) solution at 345±350 K. Two types of samples were fabricated, one for evaluating the structure of the fabricated microcantilever, and one for evaluating the mechanism that hinders the etching process. Both types have a microcantilever pattern; however, the substrate under the cantilever of the ®rst sample was completely removed, where that for the second was not. For all samples, the ion dose was 1 1017 cm 2 . For the micropatterning of the microcantilever for evaluating the microstructure, we used a tantalum stencil, because of its durability and long life. In the fabrication of this microcantilever, the base pattern was also implanted into the back surface of the substrate. Then, the substrate under the cantilever was removed by single-side etching from the back using the stainless steel holder. The result was a microcantilever beam. For the microcantilevers used in evaluating the etching conditions, we made four types of samples: three for evaluating the etching time (each one at an etching time of 10, 20 and 30 min), and the fourth for evaluating the contamination from stainless steel etching holder, it was etched for 20 min with the stainless steel etching holder that was used to fabricate the cantilever. For these samples, we used a stainless steel mask, because a tantalum mask that was used for the above was broken and it is more dicult to fabricate than a stainless steel mask. For all samples, the microstructures were then studied using XTEM observation and EDX analysis. 3. Results 3.1. Cross-sectional structure of the microcantilever Fig. 2 shows a XTEM photograph of the center part of the microcantilever. The structure of the cantilever shows three layers; a black center layer and two gray surface layers that contain black particles. The cantilever was 0.9 lm thick, with the black layer being 0.6 lm thick. The EDX analysis results in Table 1 show that the black layer was silicon with implanted gold atom distribution. The
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
779
diameter) on the interface, and large grains (few tens of nanometers) on the boundary. Among the three samples, the 10 min etched sample (Fig. 3(a)) had a larger number of large grains on the boundary than the 20 min or 30 min etched samples (Figs. 3(b) and (c), respectively). The EDX analysis results show that the small grains on the interface were between 80 and 95 at.% gold with silicon and that the larger grains were between 3 and 50 at.% iron with gold, silicon, nickel and chromium. The sample that was etched with the stainless steel holder (Fig. 3(d)) diered from the other samples that it had grains in which the golden grains were enclosed by an iron-rich structure on the boundary. All samples also contained iron, nickel and chromium in the gold implanted bulk as Fig. 2. XTEM image of the microcantilever showing a threelayer structure and nano-size grains.
gray surface layers were SiO2 and the black particles were gold and iron. Furthermore, a small recrystallized region appeared on the interface of the center gold distribution layer and SiO2 layer; however, its composition could not be identi®ed. 3.2. Evaluation of etching conditions In the samples prepared for evaluating the etching conditions in which various etching times were used, the stainless steel stencil mask was used during the ion implantation. Fig. 3 shows the XTEM photographs of these samples. In all the samples, independent of etching time, two types of grains were observed; small grains (5 nm in
Fig. 3. XTEM image of the gold ion-implanted silicon after (a) 10, (b) 20, (c) 30 min of etching without using the stainless steel holder and (d) 20 min of etching with stainless steel etching holder. Small arrow on (d) indicates the golden grains that were enclosed by iron-rich structure on the boundary.
Table 1 EDX analysis results for dierent locations in the cross-section of the microcantilevera
a
Analyzed point
Silicon (at.%)
Gold (at.%)
Oxygen (at.%)
Iron (at.%)
1, 2, 4, 20, 21 3, 19 5±18 22 23 24
35.3±38.3 38.27, 36.7 90.7±99.8 5.48 39.32 35.76
0 0.03 9.3±0.2 9.0 4.12 7.77
64.6±61.7 61.7, 63.7 0 3.12 56.55 55.44
0 0 0 82.41 0 1.03
Locations are indicated by the numbers in Fig. 3.
780
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
Fig. 4. EDX analysis of gold ion-implanted silicon fabricated by using a stainless steel stencil mask. Iron, chromium and nickel peaks appear along with a gold peak.
shown in representative results in Fig. 4. The etching rates were calculated by dividing the maximum thickness of the gold layer (determined from lower magni®cation XTEM photographs) by the etching time. Fig. 5 shows the calculated etching rates and the density of the gold ions as a function of silicon depth [5]. Etching rate decreased with etched depth, namely, etching time.
Fig. 5. Average etch rate () calculated by dividing the etch depth by the etch time, and density of the implanted gold ions (+).
4. Discussion Comparison of the 0.9 lm thick microcantilever with the 1.9 lm thick gold distribution layer before the etching (Fig. 2) shows that the microcantilever was composed only of an ion-implanted layer. This shows that ion implantation is an eective material fabrication technique for microstructures. By evaluating the material properties for various ion species, this technique may prove eective as a micromachining technique. The iron particles in the oxidized layer of the cantilever came from the vacuum devices of the ion-implantation system and from the stainless steel holder during the etching. No contamination was detected in the substrate before etching, suggesting that the etching dissolved iron from the holder. The deep SiO2 layer of the cantilever beam was oxidized during fabrication and XTEM evaluation, because the cantilever was used for other experiments, e.g., elastic property measurements [5]. Based on our results, the growth of the surface silicon oxidation of the cantilever structure caused the gold and iron grains on the surface (Fig. 4) to migrate to the center of the oxidized layer. Therefore, oxidation of the surface of silicon microdevices must be considered when applying this technique. Moreover, the mechanism involved in the recrystallization region in the SiO2 boundary needs further study.
S. Nakano et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 777±781
In the ion implantation of the samples used in evaluating the etching conditions, the stencil mask was made from stainless steel. The use of this stencil caused the contamination from sputter of the mask by chromium, nickel and iron (Fig. 4), which are components of stainless steel. Therefore the ion beam spatters the stencil, when stainless stencil mask was used, and the contamination occurred. This means that, in actual processing, tantalum is a better material for the stencil. When the stainless steel holder was placed in the etching bath, a signi®cant amount of iron deposited on the surface, and enclosed the gold. It means that the stainless steel holder was etched, and much iron was deposited on the surface. In the case of the microcantilever, only the iron grains exist on the surface layer. Then, we regard it as not contaminated during the ion implantation but that it is produced deposition during the etching. In the samples used in evaluating the etching conditions, small black grains (about 5 nm in diameter) composed of gold and iron were observed in a single row in the silicon interface boundary. On the boundary, the gold and iron grains were grown up to several tens of nanometers. This means that when etching dissolved the silicon atom, the gold atoms having lower solubility were precipitated as grain. When etching was progressing, the grains were grown. Furthermore, the etching rate was inversely proportional to the gold distribution, as shown in Fig. 5. The gold grains that have lower solubility protected the silicon dissolvent, thus etching rate was decreased. However, the number of larger gold grains on the boundary decreased with increasing etch time. This suggests that the grains on the boundary were separated from the surface by etching. On the other hand, the number of the interface grains is increased with increasing the etch time though the grains on the interface relate to the etching rate. The samples used in evaluating the etching conditions showed no SiO2 layer on the surface,
781
because the samples were evaluated by the TEM immediately after the etching. Furthermore, small grains were precipitated only on the interface; the SiO2 layer in the cantilever was formed after the cantilever was prepared, and therefore was not due to the etching. 5. Conclusion 1. The micromachining process, ion implantation, and selective etching were successfully used to form microstructures composed only of an ion-implanted layer. 2. On the surface of the microstructures, grains of implanted gold and iron that were entrapped during the etching, crystallized. 3. Iron, nickel and chromium contaminated the microstructures due to the sputtering process when stainless steel was used for the stencil mask of the micropattern implantation. 4. The etching rate was inversely proportional to the gold distribution. 5. Decrease in the etching rate of the ion-implanted microstructures was partly due to the precipitation of the implanted gold on the interface.
References [1] S. Nakano, K. Yamanaka, H. Ogiso, T. Koda, in: Proceedings of the Third International Symposium on Micromachines and Human Science, Nagoya, 1992, p. 51. [2] K. Yamanaka, S. Nakano, H. Ogiso, O.V. Kolosov, T. Koda, in: Proceedings of the Third International Symposium on Micromachines and Human Science, Nagoya, 1992, p. 59. [3] B. Schmidt, L. Bischo, J. Teichert, Sensors Actuat. A 61 (1997) 369. [4] S. Nakano, H. Ogiso, A. Yabe, Nucl. Instr. and Meth. B 155 (1999) 79. [5] S. Nakano, H. Ogiso, H. Sato, S. Nakagawa, Surf. Coat. Technol. 128±129 (2000) 71.