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Annealing behavior of tin implanted silica
Nuclear Instruments and Methods in Physics Research B 141 (1998) 279±283
Annealing behavior of tin implanted silica Y.-S. Tung a, A. Ueda a, R. Mu a, M. Wu a, J. Chen a, Z. Gu a, D.O. Henderson W.E. Collins a, C.W. White b, R. Zhur b a
a,*
,
Chemical Physics Laboratory, Physics Department, Fisk University, 1000 17th Avenue North, Nashville, TN 37208, USA b Oak Ridge National Laboratory, P.O. Box 2009, Oak Ridge, TN 37831, USA
Abstract Tin ions were implanted into silica substrates at 275 keV. The samples were annealed at 300±1000°C. The as-implanted and annealed samples were studied by the infrared specular re¯ectance technique. Kramers±Kronig transformation (KKT) was carried out on the re¯ectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra. Based on these spectra, we can understand the eects of ion implantation and annealing on the Si±O±Si bond angle, Si±O±Si bond breaking, and density change of implanted silica. Ó 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 41.75.A; 68.55.I; 78.30 Keywords: Ion implantation; Infrared spectroscopy; Krainers±Kronig transformation
1. Introduction Ion implantation was used extensively by this group to fabricate metal nanocrystals and semiconductor quantum dots [1]. One of the important issue of the ion implantation technique is the defects generated by ion implantation. Another is the reduction of defects by thermal annealing. The eects of ion implantation in silica substrate have been studied by using infrared spectroscopy and ellipsometry [2]. It was observed that the implantation causes the substrate to densify and the densi®cation changes the Si±O±Si bond angle. The
* Corresponding author. Tel.: +1 615 329 8622; fax: +1 615 329 8634; e-mail: hendersn@dubois.®sk.edu.
other eect of the implantation is the generation of Si±O± dangling bonds in the substrate, which has an unique absorption band at around 1050 cmÿ1 [3]. Infrared re¯ectance analysis has been performed to study the eects of ion implantation and thermal annealing on the Si±O±Si bonding. Similar to the previous studies, the ion implantation greatly reduces the 1100 cmÿ1 band intensity due to breaking of the Si±O±Si bonding and a band at 1000 cmÿ1 is generated due to the Si±O dangling bonds. The new approach in this paper extends the infrared re¯ectance analysis by performing Kramers±Kronig transformation (KKT) on the re¯ectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra [4]. The new results are the following: (1) The k spectra of samples annealed at dierent tempera-
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 3 7 - 2
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Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
tures can indicate the temperature range where most of the Si±O±Si bonds are recovered. (2) Based on the calculated spectra it is apparent that the k spectra of as-implanted and annealed samples divided by the k spectrum of the virgin silica are directly related to the vibrations of dangling bonds present in each sample. Therefore, these ratios can indicate the amount of dangling bonds and provide the peak positions of the dangling bonds. (3) The changes in peak positions of LO and TO spectra are analyzed in terms of the relationship derived by Devine [2]. Densi®cation alone cannot explain the peak position shifts and the shift must be related to the stoichiometry changes as well. 2. Experimental Tin (275 keV) was implanted into glass substrates at room temperature with doses of 1, 3, 6 and 10 ´ 1016 ions/cm2 . Thermal annealing was conducted in a tube furnace between 300 and 1100°C under a reducing (10%H2 + 90% Ar) environment. The standard annealing time was 30 min at each chosen temperature. Infrared re¯ectance measurements were made with BOMEM MB-102 Fourier Transform infrared spectrometers coupled with a 7° incidence angle re¯ectance accessory. Typically 300 scans at 4 cmÿ1 were used to collect the spectra. 3. Results Fig. 1 shows the re¯ectance spectra of tin implanted in silica at a dose of 6 ´ 1016 ions/cm2 , the same sample annealed at 300°C, 500°C, 600°C, 800°C, 900°C, and 1100°C under hydrogen. Two aspects are clear: ®rst the intensities of the 1100 cmÿ1 band increase with annealing temperature and the shoulder at around 1000 cmÿ1 gradually disappears with the annealing. The general explanation of this phenomena is that the Si±O± Si bond of the glass structure is broken during the ion implantation and the reduced concentration of Si±O±Si bonds causes the reduction of the intensity of the 1100 cmÿ1 band. The breakage
Fig. 1. Re¯ectance spectra of tin implanted in silica at a ¯uence of 6 ´ 1016 ions/cm2 ; the same sample annealed at 300°C, 500°C, 600°C, 800°C, 900°C, and 1100°C under hydrogen.
of the Si±O±Si bond forms Si±O dangling bonds that are responsible for the shoulder at 1000 cmÿ1 . The annealing obviously recovers some of the broken bonds which results in the increase of the intensity of the 1100 cmÿ1 band and the decrease of the 1000 cmÿ1 shoulder. The re¯ectance measurements generally have contributions from both n and k spectra. In order to separate n and k spectra, the KKT was performed on all spectra. Fig. 2 shows the n spectra and Fig. 3 shows the k spectra of the as-implanted, annealed samples, and unimplanted silica. It is clear that both the n and k spectra experience a blue shift with the annealing. The k spectra show a clear increase of intensity with annealing at 600°C. This increase of intensity is correlated with the recovery of the Si±O±Si bonds in the glass network structure. Figs. 4 and 5 show the n and k spectra of the as-implanted and annealed sample ratioed with the n and k spectra of the virgin silica. The ratioed n, and k spectra should have contributions mostly from the dangling bonds. To verify this assumption, we calculated the re¯ectance spectrum based on the ratioed n, k spectra of the 6 ´ 1016 ions/cm2 sample by r n ÿ 12 k 2 =n 12 k 2 . We compare this calculated re¯ectance spectrum with another
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
281
Fig. 2. The n spectra of the as-implanted, annealed samples, and unimplanted silica.
Fig. 4. The n spectra of the as-implanted and annealed sample ratioed with the n spectrum of the virgin silica.
Fig. 3. The k spectra of the as-implanted, annealed samples, and unimplanted silica.
Fig. 5. The k spectra of the as-implanted and annealed sample ratioed with the n spectrum of the virgin silica.
spectrum calculated by dividing the re¯ectance spectrum of the as-implanted, 6 1016 ions/cm2 , sample with the re¯ectance spectrum of the virgin silica. Fig. 6 plots the two spectra. Both spectra show the 1000 cmÿ1 peak which we attribute to
the Si±O± dangling bond. The 1100 cmÿ1 peak does not appear in the spectrum calculated from ratioed n and k spectra, indicating the calculated spectrum has major contributions from the dangling bond. Therefore, the ratioed n, k spectra will
282
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
Fig. 6. Top: re¯ectance spectrum of the as-implanted (6 ´ 1016 ions/cm2 ) sample ratioed with the re¯ectance spectrum of the virgin silica. Bottom: re¯ectance spectrum calculated from ratioed n and k.
provide us the information of how annealing is reducing the dangling bond. From the ratioed n, k spectra, it is clear that most of the defects are annealed out in the 600±800°C range. In fact, further annealing at higher temperature does not have any eect on the implanted sample. Figs. 7 and 8 plot the TO and LO spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica. There are some controversies about the assignments of the peaks near 1100 cmÿ1 and we follow the assignment of Kamitsos et al. [5]. The TO band at 457 cmÿ1 and the LO band at 507 cmÿ1 are associated with rocking of the Si±O±Si bridge and TO band at 1080 cmÿ1 , LO at 1250 cmÿ1 , are associated with the AS1 (Asymmetric stretching of the Si± O±Si bridge). Around 600°C, the peak positions blue shift to a position close to that of the virgin silica. It is interesting to notice that for the as-implanted and samples annealed lower than 600°C the TO±LO splitting of the AS1 mode is around 211 cmÿ1 and samples annealed above 600°C the TO±LO splitting is about 182 cmÿ1 . For virgin silica, the splitting is about 177 cmÿ1 . In terms of the rocking mode, the as-implanted samples and samples annealed lower than 600°C, the TO±LO split-
Fig. 7. The transverse optical (TO) spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica.
Fig. 8. The longitudinal optical (LO) spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica.
ting is about 67 cmÿ1 and in samples annealed above 600°C the TO±LO splitting is around 60 cmÿ1 . The splitting is about 52 cmÿ1 for virgin silica. Clearly, the reduced splitting is related with more Si±O±Si bond formation.
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
4. Discussion From the results section, we observed that the TO and LO mode of the AS1 mode red shifted from the virgin silica after implantation and it gradually blue shifts back to the virgin silica position during annealing. On possible explanation of the red shift of the peak position for the implanted sample is the impact densi®cation caused by ion implantation. The densi®cation reduced the bond angle of the Si±O±Si bridge and the reduction of bond angle causes the red shift. The peak position of the AS1 TO and LO mode have been given by s H 2H 2 b cos m; 1 xTO 2 a sin 2 2 xLO
s H H m; 2 a sin2 b cos2 cSS 2 2
283
plantation. This is especially true since we observed the formation of the dangling bond and observed its disappearance with annealing. In a previous paper, Pai studied the TO mode of the a-SiOx ®lm and established a relationship of dxTO =dx 67:5 cmÿ1 [7]. According to that relationship, we will have dx of 0.59 which suggests we have SiO1:4 in our implanted layer. This stoichiometry would suggest a 30% reduction in the k value of the peak at 1100 cmÿ1 due to implantation which is not the case, Fig. 3. Therefore, the most possible scenario is that both bond breaking and densi®cation contribute to the red shift of the peak positions. 5. Conclusions
2
where H is the Si±O±Si bonding angle, m is the atomic mass of oxygen, a is central force constant, and b is the noncentral force constant. cSS is de®ned as cSS fZ 2 =e1 e0 2m Mgq, where e1 is the relative permittivity at in®nite frequency and the e0 , absolute permittivity of free space, M is the atomic mass of Si, Z is an electrical charge related to the movement of O's, and q is the mass density of the a-SiO2 network. Based on experimental results it has been proposed that dxTO =dq ÿ93 cmÿ1 and dxLO =dq ÿ10 cmÿ1 [6]. In other words, the change of the peak position of the LO mode is approximately 1/9 of the change of the TO mode because of density eects. After implantation, the TO mode red shifted 40 cmÿ1 while the LO mode red shifted only 2 cmÿ1 . Clearly, density changes alone can not explain the blue shift of the TO and LO mode. The other explanation is that the stoichiometry changed during im-
The KKT analysis provides us the n, k, LO, and TO spectra of implanted silica after annealing at various temperatures. Both densi®cation and stoichiometry changes are given as the reasons for the shifts of AS1, LO and TO peak positions. References [1] D.O. Henderson et al., J. Vac. Sci. Technol. B 13 (1995) 1198; D.O. Henderson et al., J. Non-Cryst. Solids 205±207 (1996) 788; D.O. Henderson et al., J. Vac. Sci. Technol. A 14 (1996) 1199; R. Mu et al., J. Vac. Sci. Technol. A 14 (1996) 1482; C.W. White et al., Mat. Sci. Rep. 4 (1989) 43; C.W. White et al., J. Appl. Phys. 79 (1996) 1876. [2] R.A.B. Devine, J. Non-Cryst. Solids 152 (1993) 50. [3] D.O. Henderson et al., Mat. Res. Soc. Symp. Proc. 316 (1994) 451. [4] G. Anderrnann et al., J. Opt. Soc. Amer. 55 (1965) 1210. [5] F.T. Kamitsos et al., Phys. Rev. B 48 (1993) 12499. [6] R.A.B. Devine, Appl. Phys. Lett. 68 (1996) 3108. [7] P.G. Pal, J. Vac. Sci. Technol. A 4 (1986) 689.
Annealing behavior of tin implanted silica Y.-S. Tung a, A. Ueda a, R. Mu a, M. Wu a, J. Chen a, Z. Gu a, D.O. Henderson W.E. Collins a, C.W. White b, R. Zhur b a
a,*
,
Chemical Physics Laboratory, Physics Department, Fisk University, 1000 17th Avenue North, Nashville, TN 37208, USA b Oak Ridge National Laboratory, P.O. Box 2009, Oak Ridge, TN 37831, USA
Abstract Tin ions were implanted into silica substrates at 275 keV. The samples were annealed at 300±1000°C. The as-implanted and annealed samples were studied by the infrared specular re¯ectance technique. Kramers±Kronig transformation (KKT) was carried out on the re¯ectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra. Based on these spectra, we can understand the eects of ion implantation and annealing on the Si±O±Si bond angle, Si±O±Si bond breaking, and density change of implanted silica. Ó 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 41.75.A; 68.55.I; 78.30 Keywords: Ion implantation; Infrared spectroscopy; Krainers±Kronig transformation
1. Introduction Ion implantation was used extensively by this group to fabricate metal nanocrystals and semiconductor quantum dots [1]. One of the important issue of the ion implantation technique is the defects generated by ion implantation. Another is the reduction of defects by thermal annealing. The eects of ion implantation in silica substrate have been studied by using infrared spectroscopy and ellipsometry [2]. It was observed that the implantation causes the substrate to densify and the densi®cation changes the Si±O±Si bond angle. The
* Corresponding author. Tel.: +1 615 329 8622; fax: +1 615 329 8634; e-mail: hendersn@dubois.®sk.edu.
other eect of the implantation is the generation of Si±O± dangling bonds in the substrate, which has an unique absorption band at around 1050 cmÿ1 [3]. Infrared re¯ectance analysis has been performed to study the eects of ion implantation and thermal annealing on the Si±O±Si bonding. Similar to the previous studies, the ion implantation greatly reduces the 1100 cmÿ1 band intensity due to breaking of the Si±O±Si bonding and a band at 1000 cmÿ1 is generated due to the Si±O dangling bonds. The new approach in this paper extends the infrared re¯ectance analysis by performing Kramers±Kronig transformation (KKT) on the re¯ectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra [4]. The new results are the following: (1) The k spectra of samples annealed at dierent tempera-
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 3 7 - 2
280
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
tures can indicate the temperature range where most of the Si±O±Si bonds are recovered. (2) Based on the calculated spectra it is apparent that the k spectra of as-implanted and annealed samples divided by the k spectrum of the virgin silica are directly related to the vibrations of dangling bonds present in each sample. Therefore, these ratios can indicate the amount of dangling bonds and provide the peak positions of the dangling bonds. (3) The changes in peak positions of LO and TO spectra are analyzed in terms of the relationship derived by Devine [2]. Densi®cation alone cannot explain the peak position shifts and the shift must be related to the stoichiometry changes as well. 2. Experimental Tin (275 keV) was implanted into glass substrates at room temperature with doses of 1, 3, 6 and 10 ´ 1016 ions/cm2 . Thermal annealing was conducted in a tube furnace between 300 and 1100°C under a reducing (10%H2 + 90% Ar) environment. The standard annealing time was 30 min at each chosen temperature. Infrared re¯ectance measurements were made with BOMEM MB-102 Fourier Transform infrared spectrometers coupled with a 7° incidence angle re¯ectance accessory. Typically 300 scans at 4 cmÿ1 were used to collect the spectra. 3. Results Fig. 1 shows the re¯ectance spectra of tin implanted in silica at a dose of 6 ´ 1016 ions/cm2 , the same sample annealed at 300°C, 500°C, 600°C, 800°C, 900°C, and 1100°C under hydrogen. Two aspects are clear: ®rst the intensities of the 1100 cmÿ1 band increase with annealing temperature and the shoulder at around 1000 cmÿ1 gradually disappears with the annealing. The general explanation of this phenomena is that the Si±O± Si bond of the glass structure is broken during the ion implantation and the reduced concentration of Si±O±Si bonds causes the reduction of the intensity of the 1100 cmÿ1 band. The breakage
Fig. 1. Re¯ectance spectra of tin implanted in silica at a ¯uence of 6 ´ 1016 ions/cm2 ; the same sample annealed at 300°C, 500°C, 600°C, 800°C, 900°C, and 1100°C under hydrogen.
of the Si±O±Si bond forms Si±O dangling bonds that are responsible for the shoulder at 1000 cmÿ1 . The annealing obviously recovers some of the broken bonds which results in the increase of the intensity of the 1100 cmÿ1 band and the decrease of the 1000 cmÿ1 shoulder. The re¯ectance measurements generally have contributions from both n and k spectra. In order to separate n and k spectra, the KKT was performed on all spectra. Fig. 2 shows the n spectra and Fig. 3 shows the k spectra of the as-implanted, annealed samples, and unimplanted silica. It is clear that both the n and k spectra experience a blue shift with the annealing. The k spectra show a clear increase of intensity with annealing at 600°C. This increase of intensity is correlated with the recovery of the Si±O±Si bonds in the glass network structure. Figs. 4 and 5 show the n and k spectra of the as-implanted and annealed sample ratioed with the n and k spectra of the virgin silica. The ratioed n, and k spectra should have contributions mostly from the dangling bonds. To verify this assumption, we calculated the re¯ectance spectrum based on the ratioed n, k spectra of the 6 ´ 1016 ions/cm2 sample by r n ÿ 12 k 2 =n 12 k 2 . We compare this calculated re¯ectance spectrum with another
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
281
Fig. 2. The n spectra of the as-implanted, annealed samples, and unimplanted silica.
Fig. 4. The n spectra of the as-implanted and annealed sample ratioed with the n spectrum of the virgin silica.
Fig. 3. The k spectra of the as-implanted, annealed samples, and unimplanted silica.
Fig. 5. The k spectra of the as-implanted and annealed sample ratioed with the n spectrum of the virgin silica.
spectrum calculated by dividing the re¯ectance spectrum of the as-implanted, 6 1016 ions/cm2 , sample with the re¯ectance spectrum of the virgin silica. Fig. 6 plots the two spectra. Both spectra show the 1000 cmÿ1 peak which we attribute to
the Si±O± dangling bond. The 1100 cmÿ1 peak does not appear in the spectrum calculated from ratioed n and k spectra, indicating the calculated spectrum has major contributions from the dangling bond. Therefore, the ratioed n, k spectra will
282
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
Fig. 6. Top: re¯ectance spectrum of the as-implanted (6 ´ 1016 ions/cm2 ) sample ratioed with the re¯ectance spectrum of the virgin silica. Bottom: re¯ectance spectrum calculated from ratioed n and k.
provide us the information of how annealing is reducing the dangling bond. From the ratioed n, k spectra, it is clear that most of the defects are annealed out in the 600±800°C range. In fact, further annealing at higher temperature does not have any eect on the implanted sample. Figs. 7 and 8 plot the TO and LO spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica. There are some controversies about the assignments of the peaks near 1100 cmÿ1 and we follow the assignment of Kamitsos et al. [5]. The TO band at 457 cmÿ1 and the LO band at 507 cmÿ1 are associated with rocking of the Si±O±Si bridge and TO band at 1080 cmÿ1 , LO at 1250 cmÿ1 , are associated with the AS1 (Asymmetric stretching of the Si± O±Si bridge). Around 600°C, the peak positions blue shift to a position close to that of the virgin silica. It is interesting to notice that for the as-implanted and samples annealed lower than 600°C the TO±LO splitting of the AS1 mode is around 211 cmÿ1 and samples annealed above 600°C the TO±LO splitting is about 182 cmÿ1 . For virgin silica, the splitting is about 177 cmÿ1 . In terms of the rocking mode, the as-implanted samples and samples annealed lower than 600°C, the TO±LO split-
Fig. 7. The transverse optical (TO) spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica.
Fig. 8. The longitudinal optical (LO) spectra of the as-implanted, 300°C, 1000°C annealed and virgin silica.
ting is about 67 cmÿ1 and in samples annealed above 600°C the TO±LO splitting is around 60 cmÿ1 . The splitting is about 52 cmÿ1 for virgin silica. Clearly, the reduced splitting is related with more Si±O±Si bond formation.
Y.-S. Tung et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 279±283
4. Discussion From the results section, we observed that the TO and LO mode of the AS1 mode red shifted from the virgin silica after implantation and it gradually blue shifts back to the virgin silica position during annealing. On possible explanation of the red shift of the peak position for the implanted sample is the impact densi®cation caused by ion implantation. The densi®cation reduced the bond angle of the Si±O±Si bridge and the reduction of bond angle causes the red shift. The peak position of the AS1 TO and LO mode have been given by s H 2H 2 b cos m; 1 xTO 2 a sin 2 2 xLO
s H H m; 2 a sin2 b cos2 cSS 2 2
283
plantation. This is especially true since we observed the formation of the dangling bond and observed its disappearance with annealing. In a previous paper, Pai studied the TO mode of the a-SiOx ®lm and established a relationship of dxTO =dx 67:5 cmÿ1 [7]. According to that relationship, we will have dx of 0.59 which suggests we have SiO1:4 in our implanted layer. This stoichiometry would suggest a 30% reduction in the k value of the peak at 1100 cmÿ1 due to implantation which is not the case, Fig. 3. Therefore, the most possible scenario is that both bond breaking and densi®cation contribute to the red shift of the peak positions. 5. Conclusions
2
where H is the Si±O±Si bonding angle, m is the atomic mass of oxygen, a is central force constant, and b is the noncentral force constant. cSS is de®ned as cSS fZ 2 =e1 e0 2m Mgq, where e1 is the relative permittivity at in®nite frequency and the e0 , absolute permittivity of free space, M is the atomic mass of Si, Z is an electrical charge related to the movement of O's, and q is the mass density of the a-SiO2 network. Based on experimental results it has been proposed that dxTO =dq ÿ93 cmÿ1 and dxLO =dq ÿ10 cmÿ1 [6]. In other words, the change of the peak position of the LO mode is approximately 1/9 of the change of the TO mode because of density eects. After implantation, the TO mode red shifted 40 cmÿ1 while the LO mode red shifted only 2 cmÿ1 . Clearly, density changes alone can not explain the blue shift of the TO and LO mode. The other explanation is that the stoichiometry changed during im-
The KKT analysis provides us the n, k, LO, and TO spectra of implanted silica after annealing at various temperatures. Both densi®cation and stoichiometry changes are given as the reasons for the shifts of AS1, LO and TO peak positions. References [1] D.O. Henderson et al., J. Vac. Sci. Technol. B 13 (1995) 1198; D.O. Henderson et al., J. Non-Cryst. Solids 205±207 (1996) 788; D.O. Henderson et al., J. Vac. Sci. Technol. A 14 (1996) 1199; R. Mu et al., J. Vac. Sci. Technol. A 14 (1996) 1482; C.W. White et al., Mat. Sci. Rep. 4 (1989) 43; C.W. White et al., J. Appl. Phys. 79 (1996) 1876. [2] R.A.B. Devine, J. Non-Cryst. Solids 152 (1993) 50. [3] D.O. Henderson et al., Mat. Res. Soc. Symp. Proc. 316 (1994) 451. [4] G. Anderrnann et al., J. Opt. Soc. Amer. 55 (1965) 1210. [5] F.T. Kamitsos et al., Phys. Rev. B 48 (1993) 12499. [6] R.A.B. Devine, Appl. Phys. Lett. 68 (1996) 3108. [7] P.G. Pal, J. Vac. Sci. Technol. A 4 (1986) 689.