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Surface morphology and sputtering yield of SiO2 with oblique-incidence gas cluster ion beam
Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Surface morphology and sputtering yield of SiO2 with oblique-incidence gas cluster ion beam K. Sumie a,⇑, N. Toyoda b, I. Yamada b a b
Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo, Japan Incubation Center, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2012 Received in revised form 25 December 2012 Accepted 30 January 2013 Available online 22 March 2013 Keywords: Gas cluster ion beam Ripple Sputtering yield
a b s t r a c t The dependence of surface morphology and sputtering yield of SiO2 thin films on the incident angle of gas cluster ion beams (GCIBs) was studied. When the incident angles (h) were either 0° or 30° ripples did not form on the surface of SiO2, and the sputtering depth increased linearly with increasing ion fluence. When h = 45°, ripples formed in the direction perpendicular to that of the incident beam. The ripple pattern became shaper and wider with increasing h–60°. When h = 60°, the ripple wavelength and amplitude increased linearly with increasing ion fluence. However when h = 80°, ripples formed in the direction parallel to that of the GCIB. When h = 60°, the etching depth decreased with increasing ion fluence. This change in the sputtering rate can be associated with the change in the structure of the ripples. Ó 2013 Published by Elsevier B.V.
1. Introduction Ion beam sputtering of solid surfaces is a very effective method of producing self-organized nanostructures without a requiring the use of a mask [1]. The ion-induced formation of surface nanopatterns is currently a topic of considerable interest [2]. Various patterns can be fabricated depending on the irradiation conditions [3]. Since the nanostructure is formed through self-organization, maskless pattern formation can be achieved using various methods of ion beam irradiation. The formation of ripple patterns during ion sputtering has previously been explained with the Bradly and Harper (BH) model [4], which is based on the theory of sputtering proposed by Sigmund [5]. Bradly and Harper considered the formation of ripple patterns as a competition between the local curvature dependence of the sputtering yield and the thermal diffusion. This theory is consistent with numerous experimental results. It has previously been reported that gas cluster ion beam (GCIB) can be used to form ripple or dot structures at oblique-incidences [8,9]. Gas cluster ions are aggregates of several thousands of gaseous atoms or molecules. Therefore, thousands of very-low-energy atoms (energy as low as several eV per atom) simultaneously bombard the same position on the surface of a target. Hence, the atoms collide numerous times near the surface of the target. In contrast to when atomic or molecular ion beams bombard the surface of a ⇑ Corresponding author. Address: Incubation center, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan. Tel.: +81 79 267 4973; fax: +81 79 267 4972. E-mail address: [email protected] (K. Sumie). 0168-583X/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nimb.2013.01.087
target, when a GCIB does so at an incidence normal to the surface of a target, it induces lateral motion of surface atoms, e.g. Angular distribution of sputtered atom of monomer-ion beam, followed the cosine law, which indicated an isotropic ejection of atoms from the surface. This shows agreement with the linear cascade collision theory. However, the angular distribution with cluster ion shows it to be under-cosine by lateral motion of surface atoms. As a result, sputtering effects by GCIB are completely different from those of atomic or molecular (monomer) ion beam [6,7]. Since the collision process with GCIB is quite different from those during monomerion-beam, it is difficult to apply the Bradly Harper model, which is based on the Sigmund theory of sputtering, to the formation of ripples on surfaces with GCIB. Furthermore, although sidewallsmoothing using grazing incidence GCIB was previously demonstrated for three-dimensional structures such as micro electro mechanical systems (MEMS) for photonic devices [10], the previous studies did not describe the relation between the surface morphology of and sputtering yield. Therefore, the relation between the sputtering yield and the surface morphology was determined in this study by varying the incident angle of the GCIB.
2. Experiment The details of the GCIB system have been already reported in the previous paper [11]. The neutral cluster beam was formed using a nozzle. The inlet gas pressure was 1.8 MPa. Neutral clusters were subsequently ionized using electron bombardment at 300 eV. After the neutral clusters were ionized, GCIB irradiation was electrostatically accelerated at 20 kV. Atomic ions were subsequently
K. Sumie et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293
291
Fig. 1. AFM images (top) of SiO2 surfaces after irradiation at incident angles of 0°, 30°, 45°, 60°, or 80°. Corresponding cross-sectional SEM images (bottom). Applied dose was 2 1016 ions/cm2 at 20-keV Ar-GCIB. Direction of incident GCIB is indicated by arrows.
Fig. 2. AFM images (top) of SiO2 surfaces after irradiation at ion doses from 1 1016 to 5 1016 ions/cm2 at incident angle of 60°. Corresponding cross-sectional SEM images (bottom). Direction of incident GCIB is indicated by arrows.
removed using a permanent magnet, and only cluster ions were transported into the target chamber. The targets (500-nm-thick thermally grown SiO2 films) were irradiated with an Ar-GCIB through a 5-mm-diameter aperture. Each target was mounted on a tilted sample holder. The average cluster size was about 3000 atoms/cluster. The ion fluence was varied between 1 1016 and 8 1016 ions/cm2. The incident angles of the Ar-GCIB were varied between 0° and 80° from the normal to the surface of the target. After the SiO2 surfaces were irradiated, they were observed using atomic force microscopy (AFM, Veeco Instruments, Dimension 3100) in the tapping mode over a scanning area of 10 10 lm. The surface morphology, ripple wavelength, amplitude, and average roughness were analyzed. The etching depth of the SiO2 films was measured using cross-sectional scanning electron microscopy (SEM). Since 500-nm-thick SiO2 was used as the targets, the etching depth was defined by subtracting the residual thickness of the target from the initial one. When there was a ripple on the SiO2 surface, the etching depth was defined by the distance from the initial surface to the average height of the ripple. 3. Results and discussion Fig. 1 shows the AFM images of the thermally grown 500nm-thick SiO2 films irradiated using an Ar-GCIB at incident angles
(h) 0°, 30°, 45°, 60°, or 80°. The corresponding cross-sectional SEM images of the films under each condition are also shown. The acceleration voltage and ion fluence were 20 kV and 2 1016 ions/cm2. The observation areas used for AFM were 5 lm 5 lm for h between 0° and 60°, and 1 lm 1 lm at 80°. Ripples did not form when h was either 0° or 30°. Small ripples formed perpendicular to the direction of the GCIB when h was 45°. Larger ripples formed when h was 60°. However, when the incident angle was 80°, the ripples formed in the direction parallel to that of the GCIB. This is the same tendency for the formation of ripples as those that form using monomer ion beam irradiation [12], which is explained by the BH model. When ripples formed, the average surface roughness of the films increased for h between 0° and 60°, and it rapidly decreased for h = 80°. From the cross-sectional SEM images, the etching depth increased with increasing h between 0° and 60°, and it also rapidly decreased for h = 80°. The dependence of the formation of ripples on the SiO2 surface on the incident angle of the Ar-GCIB is similar to that for the Ar-monomer ion beam [12]. The dependence of ion fluence on ripple formation and etching depth was studied at the incident angle of 60°. Fig. 2 shows the AFM images of the SiO2 surfaces before and after Ar-GCIB irradiation at h = 60°. The ion fluence was varied between 1 1016 and 5 1016 ions/cm2. In addition, the corresponding
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Fig. 3. Dependences of ripple amplitude and wavelength on ion dose from 20-keV Ar-GCIB at incident angle of 60°.
Fig. 4. Relation between etching depth of ripples and ion dose for incident angles of 0°, 30°, 45°, 60°, or 80°.
Fig. 3 shows the amplitude and wavelength of the ripples plotted as functions of ion fluence from the AFM images. The ripple wavelength was determined from the peak wavelength in the power spectrum of the AFM images. The ripple wavelength and amplitude increased roughly in proportion to the ion fluence. These tendencies have also been reported for monomer ion beam [12]. Ion fluence is an important factor in the growth of ripples for both GCIB irradiation and monomer ion irradiation at grazing incidence. The dependence of etching depth on ion fluence was plotted for each incident angle to study effect of surface morphology on etching rate. Fig. 4 shows the relation between the etching depth and the ion fluence measured at h = 0°, 30°, 45°, 60°, or 80°. For h = 0°, 30°, or 80°, the etching depth increased linearly. However, when h = 45°, the slope of the etching depth slightly decreased with increasing ion fluence. When h = 60°, the decrease in the slope of the etching depth became more apparent with increasing ion fluence. For h = 0° or 30°, ripples did not form on the SiO2 surface. Further, although ripples did form when h = 80°, the amplitude and wavelength of the ripples were small (amplitude = 24 nm; wavelength = 25 nm). Therefore, the etching depth increased linearly with increasing ion fluence. Ripples formed when h = 45°, resulting in a change in the etching rate. Since the formation of ripples became more apparent when h = 60°, the etching rate increased. Fig. 5 shows relation between the sputtering yield and the average surface roughness when h = 0°, 30°, 45°, 60°, or 80°. When h = 0°, 30°, or 80°, both the sputtering yield and the average surface roughness remained almost the same, even when the ion fluence was varied. This result means that etching does not alter the surface morphology of the films. The plotted points became slightly scattered, on the other hand, when h = 45°. The plotted points became very scattered when h = 60°. The sputtering yield significantly decreased because large ripples had formed. In addition, the sputtering yield also decreased with increasing average surface roughness. This result indicates that the change in surface morphology significantly affects the sputtering yield of GCIB. The dependence of the sputtering yield on change in the surface morphology has previously been reported by M.-A. Makeev and A.-L. Barabási [13]. They described that each wavelength and amplitude of the ripples significantly affects the sputtering yield. However, GCIB sputtering is different from monomer ion beam sputtering. Therefore, future work should focus on developing a model to describe the formation of ripples in GCIB. 4. Summary
Fig. 5. Relation between sputtering yield and average surface roughness for incident angles of 0°, 30°, 45°, 60°, or 80°.
AFM and cross-sectional SEM observations were combined to study the relation between the formation of ripples and the sputtering yield for GCIB. Clear ripples formed when the incident angle was 60°, which is consistent with previous results. Further, both the wavelength and amplitude of the ripples increased with increasing ion fluence. The sputtering yield increased for surfaces that had ripples. The sputtering yield of GCIB is strongly affected by the formation of ripples. A new model is needed to describe GCIB. References
cross-sectional SEM images are shown for each ion fluence. The average surface roughness increased and larger ripples formed with increasing ion fluence. From the results for the incident angles for which ripples formed, the ripples grew with increasing surface irradiation.
[1] B. Ziberi, F. Frost, T. Höche, B. Rauschenbach, Vacuum 81 (2006) 155–159. [2] M. Batzill, F. Bardou, K.J. Snowdon, Phys. Rev. B 63 (2001) 2334081–2334084. [3] B. Ziberi, F. Frost, T. Höche, B. Rauschenbach, Phys. Rev. B 72 (2005) 2353101– 2353107. [4] R. Bradley, J. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390–2395. [5] P. Sigmund, J. Mater. Sci. 8 (1973) 1545–1553. [6] T. Aoki, J. Matsuo, G. Takaoka, Nucl. Instr. Meth. B 202 (2003) 278–282. [7] T. Aoki, J. Matsuo, Nucl. Instr. Meth. B 242 (2006) 517–519.
K. Sumie et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293 [8] H. Kitani, N. Toyoda, J. Matsuo, I. Yamada, Nucl. Instr. Meth. B 121 (1997) 489– 492. [9] N. Toyoda, T. Mashita, I. Yamada, Nucl. Instr. Meth. B 232 (2005) 212–216. [10] E. Bourelle, A. Suzuki, A. Sato, T. Seki, J. Matsuo, Nucl. Instr. Meth. B 241 (2005) 622–625.
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[11] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 234–240. [12] A. Keller, S. Facsko, W. Möller, J. Phys. 21 (2009) 495305–495312. [13] M.-A. Makeev, A.-L. Barabási, Nucl. Instr. Meth. B 222 (2004) 335–354.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Surface morphology and sputtering yield of SiO2 with oblique-incidence gas cluster ion beam K. Sumie a,⇑, N. Toyoda b, I. Yamada b a b
Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo, Japan Incubation Center, Graduate School of Engineering, University of Hyogo, Himeji, Hyogo, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2012 Received in revised form 25 December 2012 Accepted 30 January 2013 Available online 22 March 2013 Keywords: Gas cluster ion beam Ripple Sputtering yield
a b s t r a c t The dependence of surface morphology and sputtering yield of SiO2 thin films on the incident angle of gas cluster ion beams (GCIBs) was studied. When the incident angles (h) were either 0° or 30° ripples did not form on the surface of SiO2, and the sputtering depth increased linearly with increasing ion fluence. When h = 45°, ripples formed in the direction perpendicular to that of the incident beam. The ripple pattern became shaper and wider with increasing h–60°. When h = 60°, the ripple wavelength and amplitude increased linearly with increasing ion fluence. However when h = 80°, ripples formed in the direction parallel to that of the GCIB. When h = 60°, the etching depth decreased with increasing ion fluence. This change in the sputtering rate can be associated with the change in the structure of the ripples. Ó 2013 Published by Elsevier B.V.
1. Introduction Ion beam sputtering of solid surfaces is a very effective method of producing self-organized nanostructures without a requiring the use of a mask [1]. The ion-induced formation of surface nanopatterns is currently a topic of considerable interest [2]. Various patterns can be fabricated depending on the irradiation conditions [3]. Since the nanostructure is formed through self-organization, maskless pattern formation can be achieved using various methods of ion beam irradiation. The formation of ripple patterns during ion sputtering has previously been explained with the Bradly and Harper (BH) model [4], which is based on the theory of sputtering proposed by Sigmund [5]. Bradly and Harper considered the formation of ripple patterns as a competition between the local curvature dependence of the sputtering yield and the thermal diffusion. This theory is consistent with numerous experimental results. It has previously been reported that gas cluster ion beam (GCIB) can be used to form ripple or dot structures at oblique-incidences [8,9]. Gas cluster ions are aggregates of several thousands of gaseous atoms or molecules. Therefore, thousands of very-low-energy atoms (energy as low as several eV per atom) simultaneously bombard the same position on the surface of a target. Hence, the atoms collide numerous times near the surface of the target. In contrast to when atomic or molecular ion beams bombard the surface of a ⇑ Corresponding author. Address: Incubation center, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan. Tel.: +81 79 267 4973; fax: +81 79 267 4972. E-mail address: [email protected] (K. Sumie). 0168-583X/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nimb.2013.01.087
target, when a GCIB does so at an incidence normal to the surface of a target, it induces lateral motion of surface atoms, e.g. Angular distribution of sputtered atom of monomer-ion beam, followed the cosine law, which indicated an isotropic ejection of atoms from the surface. This shows agreement with the linear cascade collision theory. However, the angular distribution with cluster ion shows it to be under-cosine by lateral motion of surface atoms. As a result, sputtering effects by GCIB are completely different from those of atomic or molecular (monomer) ion beam [6,7]. Since the collision process with GCIB is quite different from those during monomerion-beam, it is difficult to apply the Bradly Harper model, which is based on the Sigmund theory of sputtering, to the formation of ripples on surfaces with GCIB. Furthermore, although sidewallsmoothing using grazing incidence GCIB was previously demonstrated for three-dimensional structures such as micro electro mechanical systems (MEMS) for photonic devices [10], the previous studies did not describe the relation between the surface morphology of and sputtering yield. Therefore, the relation between the sputtering yield and the surface morphology was determined in this study by varying the incident angle of the GCIB.
2. Experiment The details of the GCIB system have been already reported in the previous paper [11]. The neutral cluster beam was formed using a nozzle. The inlet gas pressure was 1.8 MPa. Neutral clusters were subsequently ionized using electron bombardment at 300 eV. After the neutral clusters were ionized, GCIB irradiation was electrostatically accelerated at 20 kV. Atomic ions were subsequently
K. Sumie et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293
291
Fig. 1. AFM images (top) of SiO2 surfaces after irradiation at incident angles of 0°, 30°, 45°, 60°, or 80°. Corresponding cross-sectional SEM images (bottom). Applied dose was 2 1016 ions/cm2 at 20-keV Ar-GCIB. Direction of incident GCIB is indicated by arrows.
Fig. 2. AFM images (top) of SiO2 surfaces after irradiation at ion doses from 1 1016 to 5 1016 ions/cm2 at incident angle of 60°. Corresponding cross-sectional SEM images (bottom). Direction of incident GCIB is indicated by arrows.
removed using a permanent magnet, and only cluster ions were transported into the target chamber. The targets (500-nm-thick thermally grown SiO2 films) were irradiated with an Ar-GCIB through a 5-mm-diameter aperture. Each target was mounted on a tilted sample holder. The average cluster size was about 3000 atoms/cluster. The ion fluence was varied between 1 1016 and 8 1016 ions/cm2. The incident angles of the Ar-GCIB were varied between 0° and 80° from the normal to the surface of the target. After the SiO2 surfaces were irradiated, they were observed using atomic force microscopy (AFM, Veeco Instruments, Dimension 3100) in the tapping mode over a scanning area of 10 10 lm. The surface morphology, ripple wavelength, amplitude, and average roughness were analyzed. The etching depth of the SiO2 films was measured using cross-sectional scanning electron microscopy (SEM). Since 500-nm-thick SiO2 was used as the targets, the etching depth was defined by subtracting the residual thickness of the target from the initial one. When there was a ripple on the SiO2 surface, the etching depth was defined by the distance from the initial surface to the average height of the ripple. 3. Results and discussion Fig. 1 shows the AFM images of the thermally grown 500nm-thick SiO2 films irradiated using an Ar-GCIB at incident angles
(h) 0°, 30°, 45°, 60°, or 80°. The corresponding cross-sectional SEM images of the films under each condition are also shown. The acceleration voltage and ion fluence were 20 kV and 2 1016 ions/cm2. The observation areas used for AFM were 5 lm 5 lm for h between 0° and 60°, and 1 lm 1 lm at 80°. Ripples did not form when h was either 0° or 30°. Small ripples formed perpendicular to the direction of the GCIB when h was 45°. Larger ripples formed when h was 60°. However, when the incident angle was 80°, the ripples formed in the direction parallel to that of the GCIB. This is the same tendency for the formation of ripples as those that form using monomer ion beam irradiation [12], which is explained by the BH model. When ripples formed, the average surface roughness of the films increased for h between 0° and 60°, and it rapidly decreased for h = 80°. From the cross-sectional SEM images, the etching depth increased with increasing h between 0° and 60°, and it also rapidly decreased for h = 80°. The dependence of the formation of ripples on the SiO2 surface on the incident angle of the Ar-GCIB is similar to that for the Ar-monomer ion beam [12]. The dependence of ion fluence on ripple formation and etching depth was studied at the incident angle of 60°. Fig. 2 shows the AFM images of the SiO2 surfaces before and after Ar-GCIB irradiation at h = 60°. The ion fluence was varied between 1 1016 and 5 1016 ions/cm2. In addition, the corresponding
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K. Sumie et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293
Fig. 3. Dependences of ripple amplitude and wavelength on ion dose from 20-keV Ar-GCIB at incident angle of 60°.
Fig. 4. Relation between etching depth of ripples and ion dose for incident angles of 0°, 30°, 45°, 60°, or 80°.
Fig. 3 shows the amplitude and wavelength of the ripples plotted as functions of ion fluence from the AFM images. The ripple wavelength was determined from the peak wavelength in the power spectrum of the AFM images. The ripple wavelength and amplitude increased roughly in proportion to the ion fluence. These tendencies have also been reported for monomer ion beam [12]. Ion fluence is an important factor in the growth of ripples for both GCIB irradiation and monomer ion irradiation at grazing incidence. The dependence of etching depth on ion fluence was plotted for each incident angle to study effect of surface morphology on etching rate. Fig. 4 shows the relation between the etching depth and the ion fluence measured at h = 0°, 30°, 45°, 60°, or 80°. For h = 0°, 30°, or 80°, the etching depth increased linearly. However, when h = 45°, the slope of the etching depth slightly decreased with increasing ion fluence. When h = 60°, the decrease in the slope of the etching depth became more apparent with increasing ion fluence. For h = 0° or 30°, ripples did not form on the SiO2 surface. Further, although ripples did form when h = 80°, the amplitude and wavelength of the ripples were small (amplitude = 24 nm; wavelength = 25 nm). Therefore, the etching depth increased linearly with increasing ion fluence. Ripples formed when h = 45°, resulting in a change in the etching rate. Since the formation of ripples became more apparent when h = 60°, the etching rate increased. Fig. 5 shows relation between the sputtering yield and the average surface roughness when h = 0°, 30°, 45°, 60°, or 80°. When h = 0°, 30°, or 80°, both the sputtering yield and the average surface roughness remained almost the same, even when the ion fluence was varied. This result means that etching does not alter the surface morphology of the films. The plotted points became slightly scattered, on the other hand, when h = 45°. The plotted points became very scattered when h = 60°. The sputtering yield significantly decreased because large ripples had formed. In addition, the sputtering yield also decreased with increasing average surface roughness. This result indicates that the change in surface morphology significantly affects the sputtering yield of GCIB. The dependence of the sputtering yield on change in the surface morphology has previously been reported by M.-A. Makeev and A.-L. Barabási [13]. They described that each wavelength and amplitude of the ripples significantly affects the sputtering yield. However, GCIB sputtering is different from monomer ion beam sputtering. Therefore, future work should focus on developing a model to describe the formation of ripples in GCIB. 4. Summary
Fig. 5. Relation between sputtering yield and average surface roughness for incident angles of 0°, 30°, 45°, 60°, or 80°.
AFM and cross-sectional SEM observations were combined to study the relation between the formation of ripples and the sputtering yield for GCIB. Clear ripples formed when the incident angle was 60°, which is consistent with previous results. Further, both the wavelength and amplitude of the ripples increased with increasing ion fluence. The sputtering yield increased for surfaces that had ripples. The sputtering yield of GCIB is strongly affected by the formation of ripples. A new model is needed to describe GCIB. References
cross-sectional SEM images are shown for each ion fluence. The average surface roughness increased and larger ripples formed with increasing ion fluence. From the results for the incident angles for which ripples formed, the ripples grew with increasing surface irradiation.
[1] B. Ziberi, F. Frost, T. Höche, B. Rauschenbach, Vacuum 81 (2006) 155–159. [2] M. Batzill, F. Bardou, K.J. Snowdon, Phys. Rev. B 63 (2001) 2334081–2334084. [3] B. Ziberi, F. Frost, T. Höche, B. Rauschenbach, Phys. Rev. B 72 (2005) 2353101– 2353107. [4] R. Bradley, J. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390–2395. [5] P. Sigmund, J. Mater. Sci. 8 (1973) 1545–1553. [6] T. Aoki, J. Matsuo, G. Takaoka, Nucl. Instr. Meth. B 202 (2003) 278–282. [7] T. Aoki, J. Matsuo, Nucl. Instr. Meth. B 242 (2006) 517–519.
K. Sumie et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 290–293 [8] H. Kitani, N. Toyoda, J. Matsuo, I. Yamada, Nucl. Instr. Meth. B 121 (1997) 489– 492. [9] N. Toyoda, T. Mashita, I. Yamada, Nucl. Instr. Meth. B 232 (2005) 212–216. [10] E. Bourelle, A. Suzuki, A. Sato, T. Seki, J. Matsuo, Nucl. Instr. Meth. B 241 (2005) 622–625.
293
[11] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 234–240. [12] A. Keller, S. Facsko, W. Möller, J. Phys. 21 (2009) 495305–495312. [13] M.-A. Makeev, A.-L. Barabási, Nucl. Instr. Meth. B 222 (2004) 335–354.