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Study on spatial resolution of three-dimensional analysis by full count TOF-RBS with beryllium nanoprobe
Nuclear Instruments and Methods in Physics Research B 273 (2012) 266–269
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study on spatial resolution of three-dimensional analysis by full count TOF-RBS with beryllium nanoprobe Satoshi Abo ⇑, Takayuki Azuma, Tivadar Lohner, Fujio Wakaya, Mikio Takai Center for Quantum Science and Technology Under Extreme Conditions, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan
a r t i c l e
i n f o
Article history: Available online 27 July 2011 Keywords: Nuclear nanoprobe RBS Time-of-flight (TOF) FIB Three dimensional analysis
a b s t r a c t Three dimensional nanostructures fabricated by electron beam (EB) induced deposition were measured by time of flight (TOF) Rutherford backscattering spectrometry (RBS) for checking the spatial resolution of a three dimensional analysis. Pt peaks in TOF-RBS spectra for Pt stripes under a SiO2 layer on a Si substrate samples were slightly shifted to slower position with increasing the thicknesses of the SiO2 layer. The atomic thicknesses for SiO2 fabricated by EB induced deposition measured by TOF-RBS were approximately 25% of the physical thicknesses obtained by a scanning electron microscope. Because SiO2 fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution in the three dimensional analysis with the TOF-RBS was approximately 10 nm for SiO2 layer. Embedded Ga stripes under a SiO2 layer and Pt stripes on a Si substrate were observed by TOF-RBS. The in-plane resolution under the SiO2 layer was at least less than 70 nm in the three dimensional analysis with the TOF-RBS. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Three dimensional nanostructures were easily fabricated by electron beam (EB) and ion beam deposition and etching [1–4]. Many techniques, for example secondary ion mass spectroscopy (SIMS), auger electron spectroscopy (AES) and so on, can analyze such a small structure. However, most of these require a sputtering process or a special sample preparation. Non-destructive analysis technique was required with a short time measurement. In our recent studies, non-destructive analysis technique for three dimensional micro-structures using Rutherford backscattering spectrometry (RBS) [5] and time of flight (TOF) RBS with a start trigger from a high speed pulse generator [6–10] were developed with medium energy focused beryllium beam. The non-destructive analysis technique for three dimensional nanostructures using TOF-RBS has been developed [11–13] with a target resolution of less than 10 nm. For short time analysis, a secondary electron, generated by ion incidence to the sample, was used for a start trigger in TOF-RBS measurement. The flight path of the secondary electron from the sample to the secondary electron detector (SED) affected the time resolution of TOF-RBS measurement. The time resolution of full count TOF-RBS system was improved to 4.4 ns by redesigning the SED structure [12,13]. In this study, three dimensional test structures fabricated by EB induced deposition were measured using full-count three dimen-
⇑ Corresponding author. E-mail address: [email protected] (S. Abo). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.07.091
sional TOF-RBS for checking the time and the spatial resolution on three dimensional analysis. 2. Experimental Be ions were provided by a liquid metal ion source (LMIS) from a Au–Si–Be eutectic alloy and accelerated by a 150 kV focused ion beam (FIB) column for three dimensional TOF-RBS measurement. The typical beam current was 0.2–3 pA with a beam spot size less than 10 nm [11]. The FIB system was described in details elsewhere [11,12]. Four micro channel plates (MCPs) with a diameter of 42 mm were installed for TOF-RBS with a flight path of 140 mm and a scattering angle of 125o. The start signal for the TOF-RBS measurement was a secondary electron signal from a high speed SED located at a scattering angle of 100o [12]. The SED consisted of a short decay time scintillator and a high-speed photo multiplier tube (PMT). The diameter, the decay time and the applied voltage of the scintillator for the SED were 15 mm, 2.3 ns and 10 kV, respectively. A full width at half maximum (FWHM) of the output signal from the PMT was 3.5 ns. The sample was scanned by the probe beam with steps of 10 nm– 1 lm in synchronization with TOF-RBS measurement at each of 64 64–128 128 pixels. The total dose for each pixel was controlled with the count of start signals from the SED. Three dimensional test structures were fabricated by EB induced deposition. Two types of samples were measured in this study. The first samples were Pt stripes under SiO2 layer samples shown in Fig. 1. The Pt stripes with width, pitch and thickness of 500, 1500 and 43 nm, respectively, were fabricated on a Si substrate fabricated by EB induced deposition with and without SiO2
S. Abo et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 266–269
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57.4 at.%, which were observed by energy dispersive X-ray spectrometry (EDX). The other sample consisted of SiO2 top layer, Pt stripes middle layer and implanted Ga stripes bottom layer fabricated by EB induced deposition on a Si substrate shown in Fig. 2. The thickness of the top SiO2 layer was 60 nm. The width, pitch and thickness of the middle Pt stripes were 70 nm, 1.0 lm and 78 nm, respectively. The width, pitch, dose and energy of the bottom Ga implanted stripes were 100 nm, 1.0 lm, 5 1016 ions/cm2 and 30 keV, respectively. The middle Pt stripes were deposited rectangular to the Ga stripes.
3. Results and discussion
Fig. 1. SEM images and schematic diagrams of Pt stripes under SiO2 layer samples.
top layer. The sample 1 is Pt stripes alone. The thicknesses of the SiO2 top layers for samples 2 and 3 were 40 and 80 nm, respectively, which was cross-sectionally observed after SiO2 deposition by a scanning electron microscope (SEM). The elemental compositions of the Pt and SiO2 fabricated by EB induced deposition were Pt: 12 at.%, C: 85 at.%, O: 3 at.% and Si: 9.8 at.%, O: 32.8 at.%, C:
Fig. 3 shows the TOF-RBS spectra for SiO2/Pt stripes samples 1– 3. Pt stripes under the SiO2 layer were observed by TOF-RBS. Total dose of the TOF-RBS measurements were 2.4–3.0 1016/cm2, which were lower than that for destructing the Si crystal by the ion bombardment [14]. The Pt peaks for samples 2 and 3 were slightly shifted to slower position than that for the sample 1 with increasing the thickness of the SiO2 layer. These time delays were from the energy lost in the top surface SiO2 layers for the samples 2 and 3. The SiO2 thicknesses calculated from the stopping power of SiO2 and the time delay of the TOF-RBS spectra for samples 2 and 3 were 9.5 and 18.4 nm, respectively. The SiO2 thicknesses obtained by TOF-RBS were approximately 25% of the designed thicknesses, because the SiO2 layer fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution of the three dimensional analysis with TOF-RBS was approximately 10 nm for SiO2 layer, since the difference of the SiO2 thicknesses for samples 2 and 3 were observed. Fig. 4 shows the tomography images of samples 2 and 3 obtained by three dimensional TOF-RBS with a time window of Pt signal, which were reconstructed from the three
Fig. 2. SEM image and schematic diagram of SiO2/Pt stripes/implanted Ga stripes sample.
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Fig. 3. TOF-RBS spectra for Pt stripes under SiO2 layer samples.
Fig. 4. Tomography image obtained by TOF-RBS measurement for Pt stripes under SiO2 layer samples with a time window of Pt signal.
dimensional TOF-RBS data rectangular to the Pt stripes. Both tomography images of samples 2 and 3 clearly show the embedded Pt stripes under the SiO2 top layer. The pitch of the Pt spot was 1.5 lm, which was same as the designed one. This indicates that the three dimensional analysis with TOF-RBS was enough to observe the three dimensional structures. Figs. 5 and 6 show the TOF-RBS spectrum and tomography image obtained by three dimensional TOF-RBS measurement for SiO2/ Pt stripe/implanted Ga stripes sample. The scanned area was 4.2 4.2 lm2. The beam current and total dose were 0.8 pA and 1.5 1017/cm2, which were slightly lower than that for destructing the Si crystal by the ion beam bombardment [14]. The tomography image shown in Fig. 6 was reconstructed from the three dimensional TOF-RBS data along the dashed arrows shown in Fig. 2 with time windows of Pt and Ga signals. It was difficult to separate the Pt and Ga stripes in the TOF-RBS spectrum shown in Fig. 5. However, the embedded Pt and Ga stripes were clearly resolved in the tomography images shown in Fig. 6. The in-plane resolution
in this measurement was less than 70 nm, since Pt stripes of 70 nm width were resolved. Smaller nanostructure was required for checking the in-plane resolution. 4. Conclusion Three dimensional nanostructures were fabricated by EB induced deposition and FIB implantation for checking the depth and spatial resolution in the three dimensional analysis with TOF-RBS. Pt peaks in the Pt stripes under the SiO2 layer on a Si substrate were shifted to slower position with increasing the thicknesses of the SiO2 layer. The SiO2 thicknesses obtained by TOFRBS were approximately 25% of the physical thicknesses obtained by the SEM. Because SiO2 fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution in the three dimensional analysis with TOF-RBS was approximately 10 nm. The embedded Pt and Ga stripes under the SiO2 layer were clearly observed by the three dimensional analysis with TOF-RBS. The
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in-plane resolution was less than 70 nm in the three dimensional analysis with TOF-RBS. Acknowledgements This work was partially supported by the System of Joint Research with Industry in 1998–2002 (the Ministry of Education, Science, Sport and Culture, and STARC) and the Development of System and Technology for Advanced Measurement and Analysis in 2004–2007 (The Japan Science and Technology Agency). References
Fig. 5. TOF-RBS spectrum for SiO2/Pt stripes/implanted Ga stripes sample.
Fig. 6. Tomography image obtained by TOF-RBS measurement for SiO2/Pt stripes/ implanted Ga stripes sample with time windows of Pt and Ga signals.
[1] H.W.P. Koops, O.E. Hoinkis, M.E.W. Honsberg, R. Schmidt, R. Blum, G. Böttger, A. Kuligk, C. Liguda, M. Eich, Microelectron. Eng. 57/58 (2001) 995. [2] K. Murakami, W. Jarupoonphol, K. Sakata, M. Takai, Jpn. J. Appl. Phys. 42 (2003) 4037. [3] R. Kometani, T. Morita, K. Watanabe, T. Hoshino, K. Kondo, K. Kanda, Y. Haruyama, T. Kaito, J. Fujita, M. Ishidam, Y. Ochiai, S. Matsui, J. Vac. Sci. Technol. B 22 (2004) 257. [4] K. Murakami, M. Takai, J. Vac. Sci. Technol. B 22 (2004) 1266. [5] H. Ryo Mimura, Hirosuke Takayama, Mikio Takai, Nucl. Instr. Meth. B 210 (2003) 104. [6] J. Tajima, S. Kado, Y.K. Park, R. Mimura, M. Takai, Nucl. Instr. Meth. B 181 (2001) 44. [7] K. Iwasaki, J. Tajima, H. Takayama, F. Paszti, M. Takai, Nucl. Instr. Meth. B 190 (2002) 296. [8] H. Takayama, K. Iwasaki, K. Hayashi, M. Takai, Nucl. Instr. Meth. B 210 (2003) 108. [9] Kei Hayashi, Hirosuke Takayama, Masao Ishikawa, Satoshi Abo, Tivadar Lohner, Mikio Takai, Nucl. Instr. Meth. B 220 (2004) 589. [10] N.V. Nguyen, S. Abo, T. Lohner, H. Sawaragi, F. Wakaya, M. Takai, Nucl. Instr. Meth. B 260 (2007) 105. [11] M. Takai, S. Abo, F. Wakaya, T. Kikuchi, H. Sawaragi, Nucl. Instr. Meth. B 261 (2007) 466. [12] Satoshi Abo, Shunya Kumano, Katsuhisa Murakami, Fujio Wakaya, Tivadara Lohner, Mikio Takai, Nucl. Instr. Meth. B 268 (2010) 2019. [13] Satoshi Abo, Shunya Kumano, Takayuki Azuma, Ryota Sugimoto, Kohei Koresawa, Katsuhisa Murakami, Fujio Wakaya, Mikio Takai, Nucl. Instr. Meth. B 269 (2011) 2233. [14] K. Inoue, M. Takai, K. Ishibashi, K. Hirai, Y. Kawata, S. Namba, Nucl. Instr. Meth. B 54 (1991) 231.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Study on spatial resolution of three-dimensional analysis by full count TOF-RBS with beryllium nanoprobe Satoshi Abo ⇑, Takayuki Azuma, Tivadar Lohner, Fujio Wakaya, Mikio Takai Center for Quantum Science and Technology Under Extreme Conditions, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan
a r t i c l e
i n f o
Article history: Available online 27 July 2011 Keywords: Nuclear nanoprobe RBS Time-of-flight (TOF) FIB Three dimensional analysis
a b s t r a c t Three dimensional nanostructures fabricated by electron beam (EB) induced deposition were measured by time of flight (TOF) Rutherford backscattering spectrometry (RBS) for checking the spatial resolution of a three dimensional analysis. Pt peaks in TOF-RBS spectra for Pt stripes under a SiO2 layer on a Si substrate samples were slightly shifted to slower position with increasing the thicknesses of the SiO2 layer. The atomic thicknesses for SiO2 fabricated by EB induced deposition measured by TOF-RBS were approximately 25% of the physical thicknesses obtained by a scanning electron microscope. Because SiO2 fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution in the three dimensional analysis with the TOF-RBS was approximately 10 nm for SiO2 layer. Embedded Ga stripes under a SiO2 layer and Pt stripes on a Si substrate were observed by TOF-RBS. The in-plane resolution under the SiO2 layer was at least less than 70 nm in the three dimensional analysis with the TOF-RBS. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Three dimensional nanostructures were easily fabricated by electron beam (EB) and ion beam deposition and etching [1–4]. Many techniques, for example secondary ion mass spectroscopy (SIMS), auger electron spectroscopy (AES) and so on, can analyze such a small structure. However, most of these require a sputtering process or a special sample preparation. Non-destructive analysis technique was required with a short time measurement. In our recent studies, non-destructive analysis technique for three dimensional micro-structures using Rutherford backscattering spectrometry (RBS) [5] and time of flight (TOF) RBS with a start trigger from a high speed pulse generator [6–10] were developed with medium energy focused beryllium beam. The non-destructive analysis technique for three dimensional nanostructures using TOF-RBS has been developed [11–13] with a target resolution of less than 10 nm. For short time analysis, a secondary electron, generated by ion incidence to the sample, was used for a start trigger in TOF-RBS measurement. The flight path of the secondary electron from the sample to the secondary electron detector (SED) affected the time resolution of TOF-RBS measurement. The time resolution of full count TOF-RBS system was improved to 4.4 ns by redesigning the SED structure [12,13]. In this study, three dimensional test structures fabricated by EB induced deposition were measured using full-count three dimen-
⇑ Corresponding author. E-mail address: [email protected] (S. Abo). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.07.091
sional TOF-RBS for checking the time and the spatial resolution on three dimensional analysis. 2. Experimental Be ions were provided by a liquid metal ion source (LMIS) from a Au–Si–Be eutectic alloy and accelerated by a 150 kV focused ion beam (FIB) column for three dimensional TOF-RBS measurement. The typical beam current was 0.2–3 pA with a beam spot size less than 10 nm [11]. The FIB system was described in details elsewhere [11,12]. Four micro channel plates (MCPs) with a diameter of 42 mm were installed for TOF-RBS with a flight path of 140 mm and a scattering angle of 125o. The start signal for the TOF-RBS measurement was a secondary electron signal from a high speed SED located at a scattering angle of 100o [12]. The SED consisted of a short decay time scintillator and a high-speed photo multiplier tube (PMT). The diameter, the decay time and the applied voltage of the scintillator for the SED were 15 mm, 2.3 ns and 10 kV, respectively. A full width at half maximum (FWHM) of the output signal from the PMT was 3.5 ns. The sample was scanned by the probe beam with steps of 10 nm– 1 lm in synchronization with TOF-RBS measurement at each of 64 64–128 128 pixels. The total dose for each pixel was controlled with the count of start signals from the SED. Three dimensional test structures were fabricated by EB induced deposition. Two types of samples were measured in this study. The first samples were Pt stripes under SiO2 layer samples shown in Fig. 1. The Pt stripes with width, pitch and thickness of 500, 1500 and 43 nm, respectively, were fabricated on a Si substrate fabricated by EB induced deposition with and without SiO2
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57.4 at.%, which were observed by energy dispersive X-ray spectrometry (EDX). The other sample consisted of SiO2 top layer, Pt stripes middle layer and implanted Ga stripes bottom layer fabricated by EB induced deposition on a Si substrate shown in Fig. 2. The thickness of the top SiO2 layer was 60 nm. The width, pitch and thickness of the middle Pt stripes were 70 nm, 1.0 lm and 78 nm, respectively. The width, pitch, dose and energy of the bottom Ga implanted stripes were 100 nm, 1.0 lm, 5 1016 ions/cm2 and 30 keV, respectively. The middle Pt stripes were deposited rectangular to the Ga stripes.
3. Results and discussion
Fig. 1. SEM images and schematic diagrams of Pt stripes under SiO2 layer samples.
top layer. The sample 1 is Pt stripes alone. The thicknesses of the SiO2 top layers for samples 2 and 3 were 40 and 80 nm, respectively, which was cross-sectionally observed after SiO2 deposition by a scanning electron microscope (SEM). The elemental compositions of the Pt and SiO2 fabricated by EB induced deposition were Pt: 12 at.%, C: 85 at.%, O: 3 at.% and Si: 9.8 at.%, O: 32.8 at.%, C:
Fig. 3 shows the TOF-RBS spectra for SiO2/Pt stripes samples 1– 3. Pt stripes under the SiO2 layer were observed by TOF-RBS. Total dose of the TOF-RBS measurements were 2.4–3.0 1016/cm2, which were lower than that for destructing the Si crystal by the ion bombardment [14]. The Pt peaks for samples 2 and 3 were slightly shifted to slower position than that for the sample 1 with increasing the thickness of the SiO2 layer. These time delays were from the energy lost in the top surface SiO2 layers for the samples 2 and 3. The SiO2 thicknesses calculated from the stopping power of SiO2 and the time delay of the TOF-RBS spectra for samples 2 and 3 were 9.5 and 18.4 nm, respectively. The SiO2 thicknesses obtained by TOF-RBS were approximately 25% of the designed thicknesses, because the SiO2 layer fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution of the three dimensional analysis with TOF-RBS was approximately 10 nm for SiO2 layer, since the difference of the SiO2 thicknesses for samples 2 and 3 were observed. Fig. 4 shows the tomography images of samples 2 and 3 obtained by three dimensional TOF-RBS with a time window of Pt signal, which were reconstructed from the three
Fig. 2. SEM image and schematic diagram of SiO2/Pt stripes/implanted Ga stripes sample.
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Fig. 3. TOF-RBS spectra for Pt stripes under SiO2 layer samples.
Fig. 4. Tomography image obtained by TOF-RBS measurement for Pt stripes under SiO2 layer samples with a time window of Pt signal.
dimensional TOF-RBS data rectangular to the Pt stripes. Both tomography images of samples 2 and 3 clearly show the embedded Pt stripes under the SiO2 top layer. The pitch of the Pt spot was 1.5 lm, which was same as the designed one. This indicates that the three dimensional analysis with TOF-RBS was enough to observe the three dimensional structures. Figs. 5 and 6 show the TOF-RBS spectrum and tomography image obtained by three dimensional TOF-RBS measurement for SiO2/ Pt stripe/implanted Ga stripes sample. The scanned area was 4.2 4.2 lm2. The beam current and total dose were 0.8 pA and 1.5 1017/cm2, which were slightly lower than that for destructing the Si crystal by the ion beam bombardment [14]. The tomography image shown in Fig. 6 was reconstructed from the three dimensional TOF-RBS data along the dashed arrows shown in Fig. 2 with time windows of Pt and Ga signals. It was difficult to separate the Pt and Ga stripes in the TOF-RBS spectrum shown in Fig. 5. However, the embedded Pt and Ga stripes were clearly resolved in the tomography images shown in Fig. 6. The in-plane resolution
in this measurement was less than 70 nm, since Pt stripes of 70 nm width were resolved. Smaller nanostructure was required for checking the in-plane resolution. 4. Conclusion Three dimensional nanostructures were fabricated by EB induced deposition and FIB implantation for checking the depth and spatial resolution in the three dimensional analysis with TOF-RBS. Pt peaks in the Pt stripes under the SiO2 layer on a Si substrate were shifted to slower position with increasing the thicknesses of the SiO2 layer. The SiO2 thicknesses obtained by TOFRBS were approximately 25% of the physical thicknesses obtained by the SEM. Because SiO2 fabricated by EB induced deposition was not a pure SiO2 layer. The depth resolution in the three dimensional analysis with TOF-RBS was approximately 10 nm. The embedded Pt and Ga stripes under the SiO2 layer were clearly observed by the three dimensional analysis with TOF-RBS. The
S. Abo et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 266–269
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in-plane resolution was less than 70 nm in the three dimensional analysis with TOF-RBS. Acknowledgements This work was partially supported by the System of Joint Research with Industry in 1998–2002 (the Ministry of Education, Science, Sport and Culture, and STARC) and the Development of System and Technology for Advanced Measurement and Analysis in 2004–2007 (The Japan Science and Technology Agency). References
Fig. 5. TOF-RBS spectrum for SiO2/Pt stripes/implanted Ga stripes sample.
Fig. 6. Tomography image obtained by TOF-RBS measurement for SiO2/Pt stripes/ implanted Ga stripes sample with time windows of Pt and Ga signals.
[1] H.W.P. Koops, O.E. Hoinkis, M.E.W. Honsberg, R. Schmidt, R. Blum, G. Böttger, A. Kuligk, C. Liguda, M. Eich, Microelectron. Eng. 57/58 (2001) 995. [2] K. Murakami, W. Jarupoonphol, K. Sakata, M. Takai, Jpn. J. Appl. Phys. 42 (2003) 4037. [3] R. Kometani, T. Morita, K. Watanabe, T. Hoshino, K. Kondo, K. Kanda, Y. Haruyama, T. Kaito, J. Fujita, M. Ishidam, Y. Ochiai, S. Matsui, J. Vac. Sci. Technol. B 22 (2004) 257. [4] K. Murakami, M. Takai, J. Vac. Sci. Technol. B 22 (2004) 1266. [5] H. Ryo Mimura, Hirosuke Takayama, Mikio Takai, Nucl. Instr. Meth. B 210 (2003) 104. [6] J. Tajima, S. Kado, Y.K. Park, R. Mimura, M. Takai, Nucl. Instr. Meth. B 181 (2001) 44. [7] K. Iwasaki, J. Tajima, H. Takayama, F. Paszti, M. Takai, Nucl. Instr. Meth. B 190 (2002) 296. [8] H. Takayama, K. Iwasaki, K. Hayashi, M. Takai, Nucl. Instr. Meth. B 210 (2003) 108. [9] Kei Hayashi, Hirosuke Takayama, Masao Ishikawa, Satoshi Abo, Tivadar Lohner, Mikio Takai, Nucl. Instr. Meth. B 220 (2004) 589. [10] N.V. Nguyen, S. Abo, T. Lohner, H. Sawaragi, F. Wakaya, M. Takai, Nucl. Instr. Meth. B 260 (2007) 105. [11] M. Takai, S. Abo, F. Wakaya, T. Kikuchi, H. Sawaragi, Nucl. Instr. Meth. B 261 (2007) 466. [12] Satoshi Abo, Shunya Kumano, Katsuhisa Murakami, Fujio Wakaya, Tivadara Lohner, Mikio Takai, Nucl. Instr. Meth. B 268 (2010) 2019. [13] Satoshi Abo, Shunya Kumano, Takayuki Azuma, Ryota Sugimoto, Kohei Koresawa, Katsuhisa Murakami, Fujio Wakaya, Mikio Takai, Nucl. Instr. Meth. B 269 (2011) 2233. [14] K. Inoue, M. Takai, K. Ishibashi, K. Hirai, Y. Kawata, S. Namba, Nucl. Instr. Meth. B 54 (1991) 231.