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Surface roughness behaviour of ion irradiated industrial steel
Surface & Coatings Technology 196 (2005) 358 – 363 www.elsevier.com/locate/surfcoat
Surface roughness behaviour of ion irradiated industrial steel Jae S. Lee*, Jae H. Lee Korea Atomic Energy Research Institute, P.O. BOX 105, Yusong, Taejon 305-600, Republic of Korea Available online 2 November 2004
Abstract We have studied the sputtering yield and surface roughness changes of Starvax steels (Cr:13.6, C: 0.38%, Si:0.8%, Mn: 0.5%, V: 0.3%, Mold and Die Steel) used for the manufacture of optical back-light polymer. For this study, we have built a dedicated high current low energy ion source(15 keV, 5 mA). After irradiation of various ion species onto the specimen at various beam incidence with respect to the specimen species, sputtering rates and roughness characteristics of the irradiated surface were analyzed by Atomic Force Microscope (AFM) and a large area optical profiler system. After irradiation with a low energy ion beam (15 keV), the sputtering yield increased and the surface roughness decreased with increasing ion mass and incident angle below a critical incident angle. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion source; Ion beam sputtering; Mold and die; Sputtering yield
1. Introduction Ion beams have been used for over 30 years to modify materials in manufacturing of integrated circuits, and improving the tribological and corrosion properties of surfaces [1]. Recently, the requirements for ion beam processes are becoming especially challenging in the following areas: (i) ultra shallow junction formation for LSI fabrication, (ii) low damage high rate ion beam sputtering and smoothing, (iii) high quality functional surface treatment for electrical and optical properties [2]. These trends require high current low energy ion beam technology and equipment. The low energy beams are very difficult to produce at high current density due to the space charge limited current. We manufactured the high current low energy ion irradiation system for ion beam sputtering process. Ion beam sputtering is the erosion of the sample by energetic particle bombardment. Ion beam sputtering technique is to reduce surface roughness of materials with selective detaching atoms and micro-particles from the surface by bombarding energetic ions of a few tens keV onto the material surfaces [3]. This technique can be applied for * Corresponding author. Tel.: +82 42 868 2794; fax: +82 42 868 8131. E-mail address: [email protected] (J.S. Lee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.175
the surfaces that need to have surface roughness below sub micrometer. Although this is relatively high-cost process, it shall be widely demanded in the industries for developing the high quality products. Generally, the mechanical polishing technique for mold and die is relatively expensive and does not produce the required surface finish. In this study, we studied in relation to the ion beam machining process condition (incident ion, incident angle) the sputtering rate and surface roughness change on Starvax steel (Cr:13.6, C: 0.38%, Si:0.8%, Mn: 0.5%, V: 0.3%, Mold and Die Steel) materials, which is used for the manufacture of optical back-light polymer, using the ion beam sputtering technology. It is important that the surface roughness of mold and die materials decreases because the surface roughness of mold and die is proportional to that of manufactured optical product.
2. Experimental The ion beam sputtering system is designed and manufactured to make an experiment on the surface sputtering and smoothing of the mold and die materials, as shown in Fig. 1. It consists of an DuoPIGatron ion source [4], vacuum system, diagnostic system, power supply, irradiator chamber (base pressure: approx. 105 Pa, and work
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
359
Fig. 1. The Schematic of ion beam sputtering system (consists of ion source, vacuum system, diagnostic system, power supply, irradiator chamber and target system).
pressure: approx. 104 Pa) and target system. DuoPIGatron ion source is possible to make the high current low energy ion beam up to 30 keV, 20 mA for ion beam sputtering process, as shown in Fig. 2. The typical beam profile is measured by a linear scanning system based on a Faraday cup with 5-mm diameter as shown in Fig. 3. The range of beam size was about 10 cm with Gaussian distribution. The current density of ion beam was up to 5 AA/m2 at 10 mA Xe ion beam. The secondary electrons produced inside the Faraday cup by the primary incident ion beams are suppressed by a magnetic field generated by the toroidal permanent magnet with 250 Gauss static field strength. We studied the ion beam sputtering process for mold and die materials (Stavax, R a=450 nm), which carried out the process as a function of incident ion and angle at 15 keV, 5 mA. The distance between the ion beam aperture and specimen is 20 cm. Sputtering rate and roughness characteristics of the irradiated surface were analyzed by Atomic Force Microscope (AFM, AutoProbe CP manufactured by ThermoMicroscopes) and a large area non-contact surface measurement system (NT2000 Optical Profiler manufactured by WYKO).
3. Results and discussions Theoretical sputtering yield of Stavax alloy was calculated by the Lindhard model [5]. The sputtering yield, Y, which accounts for both heavy-ion and light-ion sputtering, is given by h i2:8 as Q s Sn ð E Þ Y ð EÞ ¼ 0:42 1 ðEth =E Þ0:5 U0 ½1 þ 0:35U0 Se ðeÞ where Y(E) is the sputtering yield for ions with energy E at normal incidence, a s and Q s are empirical parameters, E th is the sputtering threshold energy, S e(e) is the reduced Lindhard electronic cross-section, S n(E) is the nuclear cross-section, and U 0 is the surface binding energy. When the bombarding ion is incident at glancing angles, it differs from the normal incidence yield. The sputtering yield for incident angle b, Y(b), is related to the normal incidence sputtering yield, Y, according to Y ðbÞ ¼ ðcosbÞfs Y where f s is a function of M target/M ion.
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J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
Fig. 2. DuoPIGatron ion source (Max. ion energy: 30 kV, Max. beam current: 20 mA).
Table 1 shows the calculated theoretical sputtering yield ( Y th) of Stavax alloy as a function of the incident angle and ion at the energy of 15 keV, which we take
into account each element. Y th of Stavax alloy increased with increasing ion mass and incident angle. The increase of Y th by ion mass is due to the increase of nuclear stopping power in near surface. The increase of Y th by incident angle is due to the longer path the bombarding ion travels closer to the surface, which permits more excited atoms to escape from the surface [6]. For the measurement of the etch rate, the features of Stavax alloy surface after ion beam sputter processing at various incident angle (08, 458, 608) and incident ion (H+, Ar+, Xe+) for 2 h are shown in Fig. 4. The top-left side in
Table 1 The theoretical sputtering yield (Y th) of Stavax alloy as a function of the incident angle and incident ion at the incident energy of 15 keV
Fig. 3. The beam profile measured by a linear scanning system based on a Faraday cup.
Incident angle (8)
Incident ion H
Ar
Xe
0 45 60
0.039 0.055 0.078
3.477 4.938 7.020
7.809 13.914 24.792
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
361
Fig. 4. The features of Stavax alloy surface after ion beam sputtering process at various incident angle and incident ion for 2 h (08, 458, 608); (a) after Xe ion beam sputtering (incident angle: 08); (b) after Xe ion beam sputtering (incident angle: 458); (c) after Xe ion beam sputtering (incident angle: 608); (d) after H ion beam sputtering (incident angle: 458); (e) after Ar ion beam sputtering (incident angle: 458).
every figure was shielded against the ion beam with stainless steel mask. As a result, the step between irradiation area and non-irradiation area increased with increasing ion mass and incident angle. This tendency
matched up to the theoretical sputtering yield. After we measured the step between irradiation area and nonirradiation area, etch rates were calculated at various condition as shown in Table 2. The experimental
Table 2 The etch rates (Am/h) measured after ion beam sputtering process(15 kV, 5 mA) at various processing condition
Table 3 The experimental sputtering yield ( Yex) of Stavax alloy as a function of the incident angle and incident ion at the incident energy of 15 keV, the beam current of 5 mA
Incident angle (8) 0 45 60
Incident ion H – 0.35 –
Ar
Xe
Incident angle (8)
– 0.75 –
0.4 1.25 1.25
0 45 60
Incident ion H – 4.763 –
Ar
Xe
– 10.206 –
3.849 17.009 24.055
362
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
The average surface roughness in ion-bombarded area in Fig. 4 was enhanced with increasing the ion mass and incident angle below critical incident angle. This is because the displacement energy transfer in the side of valley on surface was concentrated in closer surface with increasing the ion mass and incident angle [7]. Fig. 5 was the AFM image before and after ion beam sputtering. After ion beam sputtering, the average surface roughness (R a) was decreased from 450 to 235 nm. This result was caused by the reason above mentioned. Also, the maximum surface roughness (R max) was decreased from 3.756 to 1.765 Am. The reason was that sputtering at the top of the valley has the priority.
4. Conclusions
Fig. 5. The AFM image before and after ion beam sputtering process (Xe ion sputtering, incident angle: 608, incident energy: 15 kV, for 2 h); (a) before the ion beam sputtering process; (b) after the ion beam sputtering process.
sputtering yield, Yex, can readily be derived from the etch rate Vex Vex ¼ 9:6 1025
Sex cosh n
A˚ =min ; mA=cm2
where n is the atomic density of the target materials in atom/ cm3 and the cos term accounts for the reduced current density at angles off the normal [6]. The experimental sputtering yield Yex is shown in Table 3. The tendency of Yex changes as a function of the ion mass and incident angle is similar to that of Y th changes. Comparing with Y th and Yex, the Y th is larger than the Yex at normal incident angle (08). While Y th is smaller than Yex at the incident angle of 458. The reason for these results is because the theoretical sputtering yield, Y th, was calculated on the assumption that the surface roughness is perfect (R a=0) while the actual surface roughness of Stavax alloy was about 450 nm. When the incident angle is 08, incident ion beam was bombarded to the top of valley on surface. The decrease of actual sputtering yield is caused by selective sputtering. As the incident angle increased, incident ion beam was bombarded to the side of valley on surface. Therefore, the actual sputtering yield was larger than the theoretical sputtering yield above normal incident angle due to the edge effect. However, the difference between Y th and Yex above the critical angle was reduced due to the ion reflection and shadow effect [6]. Further research is necessary. Also, the difference between Y th and Yex at 458 was reduced with increasing the ion radius because the bombarding ion amount decreased by shadow effect.
We have designed and manufactured the ion beam sputtering system for the surface sputtering and smoothing of the mold and die materials. Irradiating a low energy ion beam (15 keV) to the mold and die materials, specially Stavax, the sputtering yield of Stavax alloy increased with increasing ion mass and incident angle. It is assumed that the increase of sputtering yield by increase of ion mass is due to the increase of nuclear stopping power in near surface. The increase of sputtering yield by increase of incident angle is due to the longer path the bombarding ion travels closer to the surface. The theoretical sputtering yield is smaller than the experimental sputtering, at the incident angle of 458 due to the edge effect. However, the difference between Y th and Yex above the critical angle was reduced due to the ion reflection and shadow effect. The average surface roughness after ion bombardment was enhanced with increasing the ion mass and incident angle below critical incident angle. After ion beam sputtering, the average surface roughness (R a ) was decreased from 450 to 235 nm. As a result of these features, it will be possible to apply the ion beam smoothing of mold and die materials. Also, these results are important to application in the optical industry field.
Acknowledgements This work was sponsored by the Ministry of Science and Technology, Republic of Korea, and I.S High Tech. in Korea.
References [1] D.F. Downey, M. Farley, K.S. Jones, G. Ryding (Eds.), Proc. Int. Conf. Ion Impl. Technol.-92, Gainsville, FL, 1992, North Holland Amsterdam, 1993. [2] I. Yamada, Nucl. Instrum. Methods, B 148 (1999) 1.
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363 [3] I. Yanada, J. Matsuo, Z. Insepov, D. Takeuchi, M. Akizuki, N. Toyoda, J. Vac. Sci. Technol., A 14 (1996) 781. [4] Bernhard Wolf, Handbook of Ion Sources, CRC Press, New York, 1995, p. 51. [5] N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, et al., At. Data Nucl. Data Tables 31 (1984) 1.
363
[6] S. Somekh, J. Vac. Sci. Technol. 13 (5) (1976) 1003. [7] T.W. Jolly, R. Clampitt, P. Reader, in: Ion Beam Machines and Applications, Proceedings of the 7th International Symposium on Electromachining, Birmingham, 12–14, p. 201.
Surface roughness behaviour of ion irradiated industrial steel Jae S. Lee*, Jae H. Lee Korea Atomic Energy Research Institute, P.O. BOX 105, Yusong, Taejon 305-600, Republic of Korea Available online 2 November 2004
Abstract We have studied the sputtering yield and surface roughness changes of Starvax steels (Cr:13.6, C: 0.38%, Si:0.8%, Mn: 0.5%, V: 0.3%, Mold and Die Steel) used for the manufacture of optical back-light polymer. For this study, we have built a dedicated high current low energy ion source(15 keV, 5 mA). After irradiation of various ion species onto the specimen at various beam incidence with respect to the specimen species, sputtering rates and roughness characteristics of the irradiated surface were analyzed by Atomic Force Microscope (AFM) and a large area optical profiler system. After irradiation with a low energy ion beam (15 keV), the sputtering yield increased and the surface roughness decreased with increasing ion mass and incident angle below a critical incident angle. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion source; Ion beam sputtering; Mold and die; Sputtering yield
1. Introduction Ion beams have been used for over 30 years to modify materials in manufacturing of integrated circuits, and improving the tribological and corrosion properties of surfaces [1]. Recently, the requirements for ion beam processes are becoming especially challenging in the following areas: (i) ultra shallow junction formation for LSI fabrication, (ii) low damage high rate ion beam sputtering and smoothing, (iii) high quality functional surface treatment for electrical and optical properties [2]. These trends require high current low energy ion beam technology and equipment. The low energy beams are very difficult to produce at high current density due to the space charge limited current. We manufactured the high current low energy ion irradiation system for ion beam sputtering process. Ion beam sputtering is the erosion of the sample by energetic particle bombardment. Ion beam sputtering technique is to reduce surface roughness of materials with selective detaching atoms and micro-particles from the surface by bombarding energetic ions of a few tens keV onto the material surfaces [3]. This technique can be applied for * Corresponding author. Tel.: +82 42 868 2794; fax: +82 42 868 8131. E-mail address: [email protected] (J.S. Lee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.175
the surfaces that need to have surface roughness below sub micrometer. Although this is relatively high-cost process, it shall be widely demanded in the industries for developing the high quality products. Generally, the mechanical polishing technique for mold and die is relatively expensive and does not produce the required surface finish. In this study, we studied in relation to the ion beam machining process condition (incident ion, incident angle) the sputtering rate and surface roughness change on Starvax steel (Cr:13.6, C: 0.38%, Si:0.8%, Mn: 0.5%, V: 0.3%, Mold and Die Steel) materials, which is used for the manufacture of optical back-light polymer, using the ion beam sputtering technology. It is important that the surface roughness of mold and die materials decreases because the surface roughness of mold and die is proportional to that of manufactured optical product.
2. Experimental The ion beam sputtering system is designed and manufactured to make an experiment on the surface sputtering and smoothing of the mold and die materials, as shown in Fig. 1. It consists of an DuoPIGatron ion source [4], vacuum system, diagnostic system, power supply, irradiator chamber (base pressure: approx. 105 Pa, and work
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
359
Fig. 1. The Schematic of ion beam sputtering system (consists of ion source, vacuum system, diagnostic system, power supply, irradiator chamber and target system).
pressure: approx. 104 Pa) and target system. DuoPIGatron ion source is possible to make the high current low energy ion beam up to 30 keV, 20 mA for ion beam sputtering process, as shown in Fig. 2. The typical beam profile is measured by a linear scanning system based on a Faraday cup with 5-mm diameter as shown in Fig. 3. The range of beam size was about 10 cm with Gaussian distribution. The current density of ion beam was up to 5 AA/m2 at 10 mA Xe ion beam. The secondary electrons produced inside the Faraday cup by the primary incident ion beams are suppressed by a magnetic field generated by the toroidal permanent magnet with 250 Gauss static field strength. We studied the ion beam sputtering process for mold and die materials (Stavax, R a=450 nm), which carried out the process as a function of incident ion and angle at 15 keV, 5 mA. The distance between the ion beam aperture and specimen is 20 cm. Sputtering rate and roughness characteristics of the irradiated surface were analyzed by Atomic Force Microscope (AFM, AutoProbe CP manufactured by ThermoMicroscopes) and a large area non-contact surface measurement system (NT2000 Optical Profiler manufactured by WYKO).
3. Results and discussions Theoretical sputtering yield of Stavax alloy was calculated by the Lindhard model [5]. The sputtering yield, Y, which accounts for both heavy-ion and light-ion sputtering, is given by h i2:8 as Q s Sn ð E Þ Y ð EÞ ¼ 0:42 1 ðEth =E Þ0:5 U0 ½1 þ 0:35U0 Se ðeÞ where Y(E) is the sputtering yield for ions with energy E at normal incidence, a s and Q s are empirical parameters, E th is the sputtering threshold energy, S e(e) is the reduced Lindhard electronic cross-section, S n(E) is the nuclear cross-section, and U 0 is the surface binding energy. When the bombarding ion is incident at glancing angles, it differs from the normal incidence yield. The sputtering yield for incident angle b, Y(b), is related to the normal incidence sputtering yield, Y, according to Y ðbÞ ¼ ðcosbÞfs Y where f s is a function of M target/M ion.
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J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
Fig. 2. DuoPIGatron ion source (Max. ion energy: 30 kV, Max. beam current: 20 mA).
Table 1 shows the calculated theoretical sputtering yield ( Y th) of Stavax alloy as a function of the incident angle and ion at the energy of 15 keV, which we take
into account each element. Y th of Stavax alloy increased with increasing ion mass and incident angle. The increase of Y th by ion mass is due to the increase of nuclear stopping power in near surface. The increase of Y th by incident angle is due to the longer path the bombarding ion travels closer to the surface, which permits more excited atoms to escape from the surface [6]. For the measurement of the etch rate, the features of Stavax alloy surface after ion beam sputter processing at various incident angle (08, 458, 608) and incident ion (H+, Ar+, Xe+) for 2 h are shown in Fig. 4. The top-left side in
Table 1 The theoretical sputtering yield (Y th) of Stavax alloy as a function of the incident angle and incident ion at the incident energy of 15 keV
Fig. 3. The beam profile measured by a linear scanning system based on a Faraday cup.
Incident angle (8)
Incident ion H
Ar
Xe
0 45 60
0.039 0.055 0.078
3.477 4.938 7.020
7.809 13.914 24.792
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
361
Fig. 4. The features of Stavax alloy surface after ion beam sputtering process at various incident angle and incident ion for 2 h (08, 458, 608); (a) after Xe ion beam sputtering (incident angle: 08); (b) after Xe ion beam sputtering (incident angle: 458); (c) after Xe ion beam sputtering (incident angle: 608); (d) after H ion beam sputtering (incident angle: 458); (e) after Ar ion beam sputtering (incident angle: 458).
every figure was shielded against the ion beam with stainless steel mask. As a result, the step between irradiation area and non-irradiation area increased with increasing ion mass and incident angle. This tendency
matched up to the theoretical sputtering yield. After we measured the step between irradiation area and nonirradiation area, etch rates were calculated at various condition as shown in Table 2. The experimental
Table 2 The etch rates (Am/h) measured after ion beam sputtering process(15 kV, 5 mA) at various processing condition
Table 3 The experimental sputtering yield ( Yex) of Stavax alloy as a function of the incident angle and incident ion at the incident energy of 15 keV, the beam current of 5 mA
Incident angle (8) 0 45 60
Incident ion H – 0.35 –
Ar
Xe
Incident angle (8)
– 0.75 –
0.4 1.25 1.25
0 45 60
Incident ion H – 4.763 –
Ar
Xe
– 10.206 –
3.849 17.009 24.055
362
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363
The average surface roughness in ion-bombarded area in Fig. 4 was enhanced with increasing the ion mass and incident angle below critical incident angle. This is because the displacement energy transfer in the side of valley on surface was concentrated in closer surface with increasing the ion mass and incident angle [7]. Fig. 5 was the AFM image before and after ion beam sputtering. After ion beam sputtering, the average surface roughness (R a) was decreased from 450 to 235 nm. This result was caused by the reason above mentioned. Also, the maximum surface roughness (R max) was decreased from 3.756 to 1.765 Am. The reason was that sputtering at the top of the valley has the priority.
4. Conclusions
Fig. 5. The AFM image before and after ion beam sputtering process (Xe ion sputtering, incident angle: 608, incident energy: 15 kV, for 2 h); (a) before the ion beam sputtering process; (b) after the ion beam sputtering process.
sputtering yield, Yex, can readily be derived from the etch rate Vex Vex ¼ 9:6 1025
Sex cosh n
A˚ =min ; mA=cm2
where n is the atomic density of the target materials in atom/ cm3 and the cos term accounts for the reduced current density at angles off the normal [6]. The experimental sputtering yield Yex is shown in Table 3. The tendency of Yex changes as a function of the ion mass and incident angle is similar to that of Y th changes. Comparing with Y th and Yex, the Y th is larger than the Yex at normal incident angle (08). While Y th is smaller than Yex at the incident angle of 458. The reason for these results is because the theoretical sputtering yield, Y th, was calculated on the assumption that the surface roughness is perfect (R a=0) while the actual surface roughness of Stavax alloy was about 450 nm. When the incident angle is 08, incident ion beam was bombarded to the top of valley on surface. The decrease of actual sputtering yield is caused by selective sputtering. As the incident angle increased, incident ion beam was bombarded to the side of valley on surface. Therefore, the actual sputtering yield was larger than the theoretical sputtering yield above normal incident angle due to the edge effect. However, the difference between Y th and Yex above the critical angle was reduced due to the ion reflection and shadow effect [6]. Further research is necessary. Also, the difference between Y th and Yex at 458 was reduced with increasing the ion radius because the bombarding ion amount decreased by shadow effect.
We have designed and manufactured the ion beam sputtering system for the surface sputtering and smoothing of the mold and die materials. Irradiating a low energy ion beam (15 keV) to the mold and die materials, specially Stavax, the sputtering yield of Stavax alloy increased with increasing ion mass and incident angle. It is assumed that the increase of sputtering yield by increase of ion mass is due to the increase of nuclear stopping power in near surface. The increase of sputtering yield by increase of incident angle is due to the longer path the bombarding ion travels closer to the surface. The theoretical sputtering yield is smaller than the experimental sputtering, at the incident angle of 458 due to the edge effect. However, the difference between Y th and Yex above the critical angle was reduced due to the ion reflection and shadow effect. The average surface roughness after ion bombardment was enhanced with increasing the ion mass and incident angle below critical incident angle. After ion beam sputtering, the average surface roughness (R a ) was decreased from 450 to 235 nm. As a result of these features, it will be possible to apply the ion beam smoothing of mold and die materials. Also, these results are important to application in the optical industry field.
Acknowledgements This work was sponsored by the Ministry of Science and Technology, Republic of Korea, and I.S High Tech. in Korea.
References [1] D.F. Downey, M. Farley, K.S. Jones, G. Ryding (Eds.), Proc. Int. Conf. Ion Impl. Technol.-92, Gainsville, FL, 1992, North Holland Amsterdam, 1993. [2] I. Yamada, Nucl. Instrum. Methods, B 148 (1999) 1.
J.S. Lee, J.H. Lee / Surface & Coatings Technology 196 (2005) 358–363 [3] I. Yanada, J. Matsuo, Z. Insepov, D. Takeuchi, M. Akizuki, N. Toyoda, J. Vac. Sci. Technol., A 14 (1996) 781. [4] Bernhard Wolf, Handbook of Ion Sources, CRC Press, New York, 1995, p. 51. [5] N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, et al., At. Data Nucl. Data Tables 31 (1984) 1.
363
[6] S. Somekh, J. Vac. Sci. Technol. 13 (5) (1976) 1003. [7] T.W. Jolly, R. Clampitt, P. Reader, in: Ion Beam Machines and Applications, Proceedings of the 7th International Symposium on Electromachining, Birmingham, 12–14, p. 201.