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Ridge structure etching of LiNbO3 crystal for optical waveguide applications
Optical Materials 28 (2006) 216–220 www.elsevier.com/locate/optmat
Ridge structure etching of LiNbO3 crystal for optical waveguide applications W.J. Park a, W.S. Yang a,b,*, W.K. Kim b, H.Y. Lee b, J.-W. Lim c, M. Isshiki c, D.H. Yoon a b
a Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon GyungGi-Do 440-746, South Korea Optical Telecommunication Components Laboratory, Korea Electronics Technology Institute, Pyungtaek GyungGi-Do 451-865, South Korea c Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received 9 April 2004; accepted 31 July 2004 Available online 8 February 2005
Abstract A plasma dry etching technique has been applied to the fabrication of LiNbO3 optical waveguide with a ridge structure for broadband operation. The etching characteristics of a LiNbO3 single crystal have been investigated according to various ratios of Ar/C3F8 gas mixture. A Ni metal was used as a dry etching mask. The effects of a gas mixture ratio on etching profile angle, sidewall roughness and etching rate were also studied. The etching surface roughness was evaluated by atomic force microscopy (AFM). The etch rate and profile was observed by scanning electron microscopy (SEM). The optimum etching conditions, considering etch rate, profile and surface roughness, were obtained at the 20 sccm C3F8 gas flow. 2005 Elsevier B.V. All rights reserved. Keywords: Neutral loop plasma etching; LiNbO3; Optical waveguide; C3F8 gas flow ratio
1. Introduction LiNbO3 exhibits a high electro-optical effect and has been applied widely for high-density memory, optical modulators, electric field sensors [1] and so on. In order to meet simultaneously the RF characteristic impedance matching and optic/RF phase velocity matching, i.e., to expand the modulation bandwidth of LiNbO3 based optical modulators, the plasma dry etching technique is utilized for the fabrication of a trench structure on a LiNbO3 wafer, in which about 3 4 lm of a LiNbO3 substrate except the optical waveguide is removed in depth. Such a structure reduces effectively the permittivity of the substrate surface around modulation electrodes, resulting in the broadening of the bandwidth
*
Corresponding author. Tel.: +82 31 6104 398; fax: +82 31 6104 126. E-mail address: [email protected] (W.S. Yang).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.07.023
and on the other hand in the reduction of the driving voltage [2]. Although LiNbO3 wet etching has been processed by using an aqueous solution of hydrofluoric acid, the etching results were not enough for optical waveguides, thus improvement in a etching technique or a dry etching technique should be introduced. High density and low-energy ions allow an ion assisted surface chemical reaction and also ensure a high etching rate [3,4]. However, uniform plasma with high density and low electron temperature is difficult to be generated in a large diameter chamber, owing to electron consumption on the etched wall surface. The magnetic neutral loop discharge (NLD) plasma is a useful tool for deep dry etching in the above respect since the NLD has a high plasma density with low electron temperature characteristics which is suitable for large area etching and further the plasma position can be controlled even during a etching process, which is important to realize uniform etching [5–9].
W.J. Park et al. / Optical Materials 28 (2006) 216–220
In this study, the etching characteristics of a LiNbO3 single crystal have been investigated under various mixture ratios of the Ar/C3F8 gas. Their effects on etch profile angle, sidewall roughness and etch rate were also studied. The etched surface roughness was evaluated by atomic force microscopy (AFM). The etch rate and profile was observed by scanning electron microscopy (SEM).
217
Table 1 The etching system conditions for z-cut LiNbO3 etching Parameters
Range
Etch time Bias power Antenna power Operational pressure [C3F8]/[Ar + C3F8] Circulation temperature He pressure
20 min 300 W 600 W 0.33 Pa 10 50 sccm 20 C 17 Torr
2. Experimental The NLD plasma etcher used in this study is shown schematically in Fig. 1. Three electromagnetic coils were set co-axially in a metal vacuum chamber. The coil produces a magnetic neutral loop inside the vacuum chamber. The vertical position of a neutral loop (NL) is usually set in the same plane as the middle coil. The current of the second coil can change the radius of the NL interior the coils. A quartz vessel with a 300 mm inner diameter was used as the vacuum chamber. A parallel double turn RF antenna coil was wound around the vessel whose vertical position was usually in the same plane as the center of the middle coil. The vertical distance between the substrate electrode and the RF antenna was about 120 mm. An RF power and a bias power at 13.56 MHz was supplied to the antenna and the substrate, respectively. A z-cut LiNbO3 single crystal wafer polished to a mirror finish with a thickness of 1 mm is used for a NLD etching. The used gas mixture was C3F8/Ar, and a total gas flow rate was fixed at 100 sccm. The full range of process parameters for the system is summarized in Table 1. The Ni metal with 1.2 lm thickness deposited on a LiNbO3 wafer was etched as a dry etching mask to have patterns with 15 lm width by the photolithography method. The LiNbO3 etching rate, sidewall roughness
and mask etch rate were measured by SEM (Philips, XL-30 FEG/ESEM). Also etched surface roughness was evaluated by AFM (THERO-Microscopy, CP Research). The average surface roughness is calculated by Z LZ L 1 f ðx; yÞdx dy; Ra ¼ 2 L 0 0 where, f(x, y) is the height at the position (x, y) on the etched surface and L is the scan distance of 3 lm.
3. Results and discussions For general NLD etching processes, the Antenna power, the bias power, the operating pressure and the etching time were fixed as 600 W, 300 W, 0.33 Pa and 20 min, respectively, and the C3F8 gas flow was only varied in the range of 5–50 sccm while the total amount of Ar/C3F8 was fixed at 100 sccm. Fig. 2 shows the LiNbO3 etch rate and the mask etch rate as a function of the C3F8 gas flow. As the amount of the C3F8 gas flow increased from 5 to 20 sccm, the LiNbO3 etch rate increased from 128 to 184 nm/min. Also, the etch rate slightly decreased as the C3F8 gas flow increased above 25 sccm. The Ni mask showed the lowest etch rate at the C3F8 gas flow of 20 and 35 sccm, but the Ni mask etch rate rapidly increased at
100
300
Etch Rate [nm]
80 200 60
40 100
Mask Etch Rate [nm]
Etch rate Mask etch rate
20 0
0
10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)]
Fig. 1. Diagram of a NLD etching equipment.
Fig. 2. LiNbO3 etching rate and mask etching rate as a function of the C3F8 gas flow.
218
W.J. Park et al. / Optical Materials 28 (2006) 216–220 90
Profile [deg.]
80
73.07° 71.49°
70
60
10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)] Fig. 3. The C3F8 gas flow dependence of the etching profile.
the C3F8 gas flow higher than 35 sccm. It was confirmed that the LiNbO3 etch rate was a little affected but the Ni mask etch rate was strongly affected by the carbon fluorine gas reaction. Usually, the reaction between LiNbO3 and C3F8 is made by formation of thermally stable compounds of Li ions and F radicals. Further, they need to mix Ar into the chemical etching gases to physically remove the Li and fluorine compounds [10]. Therefore, it is suggested that LiNbO3 etching rate due to the amount of F radicals generated by the discharge increases with the amount of C3F8 etching gas, but the etch rate is reduced at the gas flow higher than 35 sccm due to quenching of the C3F8 reaction gas. The Ni mask is also etched during dry etching of LiNbO3. The etching rate of the Ni mask depended on the total energy of ions such as Ar+ and CFþ xðx¼1–3Þ because the Ni metal film was attacked only by physical ion bombardment. Fig. 3 shows an etch profile as an etching angle at various C3F8 gas flow. As the C3F8 gas flow increased from 5 to 20 sccm, the etch profile also increased from 67.83 to 73.07. Especially, at 20 sccm C3F8 gas flow the maximum profile was observed. Figs. 4 and 5 show SEM micrographs of LiNbO3 waveguides, including cross sectional views, etched at the 20 sccm and 50 sccm C3F8 gas flow, respectively. The etched pattern shape and sidewall roughness depended on the C3F8 gas ratio. The higher C3F8 gas flow decreased the aspect ratio of the ridge structure of waveguides, that is, resulted in a ridge with narrower top and wider bottom. Increase of the C3F8 gas flow caused the etched side wall rougher. It is regarded because more amount of chemically reactive fluorine was adsorbed in the sidewall and particularly in the bottom edge of LiNbO3. The rough surface is induced by the accumulation of the grain particles formed by both the ion bombardment by Ar+ ions and the chemical reaction by CFþ x . It is supposed that such particles, which are in
Fig. 4. SEM micrographs of LiNbO3 waveguides etched at 20 sccm C3F8 gas flow (a) etched Cross-section, (b) shape of etched LiNbO3 waveguide, (c) sidewall.
the liquid state during an etching process due to the high substrate temperature, should be adhered to the etching surface, which is governed by the free energy of the surface. Then, the liquid state particles can be solidified during a cooling process, keeping the same shape as the liquid. This phenomena should be slightly intensified in the bottom edge area than the bottom surface or the sidewall, considering the surface energy dynamics. Fig. 6 shows the variation of root-mean-square (RMS) roughness of etched LiNbO3 as a function of the C3F8 gas flow ratio. The surface RMS roughness (Ra) gradually decreased with the increase of the C3F8 gas flow in the range of 5–50 sccm. The minimum value of RMS roughness was obtained to be 20.56 nm at 50 sccm of the C3F8 gas flow. Fig. 7 shows the AFM images (3 · 3 lm) of the etched surface morphology at various etching conditions. The value of Ra for the non-etched surface was ˚ . The surfaces became rougher after approximately 5.5 A
W.J. Park et al. / Optical Materials 28 (2006) 216–220
219
40
RMS Roughness [nm]
35
30
25
20
15 10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)] Fig. 6. RMS roughness of etched LiNbO3 at various C3F8 gas flow.
etching, and the surface got smoother with increase of the C3F8 gas flow from 5 to 50 sccm. It is suggested that contamination layer can be formed on a etched surface and that the surface morphology is strongly affected by the chemical reaction.
4. Conclusions
Fig. 5. SEM micrographs of LiNbO3 waveguides etched at 50 sccm C3F8 gas flow (a) etched Cross-section, (b) shape of etched LiNbO3 waveguide, (c) sidewall.
The etching process of LiNbO3 by neutral loop plasma etching using 100 sccm Ar/C3F8 as a etching gas has been analyzed at various gas mixture ratios. As the C3F8 gas flow increased from 5 to 20 sccm the LiNbO3 etch rate increased from 128 to184 nm/min, and then the etch rate decreased at the C3F8 gas flow above 25 sccm, which resulted in the maximum etching rate at 20 sccm of at the C3F8 gas flow. Increase of the C3F8 gas flow led to rougher etched surface and sidewall probably
˚ ), (b) C3F8 gas 10 sccm (31 nm), (c) C3F8 Fig. 7. AFM images of the etched surface morphology at several etching conditions. (a) Non-etched (5.5 A gas 20 sccm (28.68 nm), (d) C3F8 gas 50 sccm (20.56 nm).
220
W.J. Park et al. / Optical Materials 28 (2006) 216–220
due to chemically reactive fluorine adsorbed in the sidewall and particularly in the bottom edge of the LiNbO3 ridge structure. The surface RMS roughness gradually decreased with the increase of the C3F8 gas flow in the range of 5–50 sccm. The minimum value of RMS roughness was 20.56 nm with at 50 sccm C3F8.
References [1] K. Tajima, N. Kuwabara, F. Amemiya, R. Kobayashi, Trans. ICICE 76B (1993) 538.
[2] M. Minakata, Electron. Commun. Jpn. 77 (1994) 37. [3] K. Noguchi, O. Mitomi, H. Miyazawa, S. Seki, J. Lightwave Technol. 13 (1995) 1164. [4] M. Tamura, S. Yoshikado, Sci. Technol. Adv. Mater. 2 (2001) 563. [5] K. Shima, N. Mitsugi, H. Nagata, J. Mater. Res. 13 (1998) 527. [6] H. Nagata, N. Mitrugi, K. Shima, M. Tamai, E.M. Haga, J. Crystal Growth 87 (1998) 573. [7] T. Uchida, J. Vac. Technol. A 16 (1998) 1529. [8] W. Chen, Y. Morikawa, M. Itoh, T. Hayashi, K. Sugita, H. Shindo, T. Uchida, J. Vac. Technol. A 17 (1999) 2546. [9] Y.M. Sung, K. Uchino, K. Muraoka, J. Vac. Technol. A 18 (2000) 2149. [10] K. Shima, J. Mater. Res. 13 (1998) 527.
Ridge structure etching of LiNbO3 crystal for optical waveguide applications W.J. Park a, W.S. Yang a,b,*, W.K. Kim b, H.Y. Lee b, J.-W. Lim c, M. Isshiki c, D.H. Yoon a b
a Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon GyungGi-Do 440-746, South Korea Optical Telecommunication Components Laboratory, Korea Electronics Technology Institute, Pyungtaek GyungGi-Do 451-865, South Korea c Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received 9 April 2004; accepted 31 July 2004 Available online 8 February 2005
Abstract A plasma dry etching technique has been applied to the fabrication of LiNbO3 optical waveguide with a ridge structure for broadband operation. The etching characteristics of a LiNbO3 single crystal have been investigated according to various ratios of Ar/C3F8 gas mixture. A Ni metal was used as a dry etching mask. The effects of a gas mixture ratio on etching profile angle, sidewall roughness and etching rate were also studied. The etching surface roughness was evaluated by atomic force microscopy (AFM). The etch rate and profile was observed by scanning electron microscopy (SEM). The optimum etching conditions, considering etch rate, profile and surface roughness, were obtained at the 20 sccm C3F8 gas flow. 2005 Elsevier B.V. All rights reserved. Keywords: Neutral loop plasma etching; LiNbO3; Optical waveguide; C3F8 gas flow ratio
1. Introduction LiNbO3 exhibits a high electro-optical effect and has been applied widely for high-density memory, optical modulators, electric field sensors [1] and so on. In order to meet simultaneously the RF characteristic impedance matching and optic/RF phase velocity matching, i.e., to expand the modulation bandwidth of LiNbO3 based optical modulators, the plasma dry etching technique is utilized for the fabrication of a trench structure on a LiNbO3 wafer, in which about 3 4 lm of a LiNbO3 substrate except the optical waveguide is removed in depth. Such a structure reduces effectively the permittivity of the substrate surface around modulation electrodes, resulting in the broadening of the bandwidth
*
Corresponding author. Tel.: +82 31 6104 398; fax: +82 31 6104 126. E-mail address: [email protected] (W.S. Yang).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.07.023
and on the other hand in the reduction of the driving voltage [2]. Although LiNbO3 wet etching has been processed by using an aqueous solution of hydrofluoric acid, the etching results were not enough for optical waveguides, thus improvement in a etching technique or a dry etching technique should be introduced. High density and low-energy ions allow an ion assisted surface chemical reaction and also ensure a high etching rate [3,4]. However, uniform plasma with high density and low electron temperature is difficult to be generated in a large diameter chamber, owing to electron consumption on the etched wall surface. The magnetic neutral loop discharge (NLD) plasma is a useful tool for deep dry etching in the above respect since the NLD has a high plasma density with low electron temperature characteristics which is suitable for large area etching and further the plasma position can be controlled even during a etching process, which is important to realize uniform etching [5–9].
W.J. Park et al. / Optical Materials 28 (2006) 216–220
In this study, the etching characteristics of a LiNbO3 single crystal have been investigated under various mixture ratios of the Ar/C3F8 gas. Their effects on etch profile angle, sidewall roughness and etch rate were also studied. The etched surface roughness was evaluated by atomic force microscopy (AFM). The etch rate and profile was observed by scanning electron microscopy (SEM).
217
Table 1 The etching system conditions for z-cut LiNbO3 etching Parameters
Range
Etch time Bias power Antenna power Operational pressure [C3F8]/[Ar + C3F8] Circulation temperature He pressure
20 min 300 W 600 W 0.33 Pa 10 50 sccm 20 C 17 Torr
2. Experimental The NLD plasma etcher used in this study is shown schematically in Fig. 1. Three electromagnetic coils were set co-axially in a metal vacuum chamber. The coil produces a magnetic neutral loop inside the vacuum chamber. The vertical position of a neutral loop (NL) is usually set in the same plane as the middle coil. The current of the second coil can change the radius of the NL interior the coils. A quartz vessel with a 300 mm inner diameter was used as the vacuum chamber. A parallel double turn RF antenna coil was wound around the vessel whose vertical position was usually in the same plane as the center of the middle coil. The vertical distance between the substrate electrode and the RF antenna was about 120 mm. An RF power and a bias power at 13.56 MHz was supplied to the antenna and the substrate, respectively. A z-cut LiNbO3 single crystal wafer polished to a mirror finish with a thickness of 1 mm is used for a NLD etching. The used gas mixture was C3F8/Ar, and a total gas flow rate was fixed at 100 sccm. The full range of process parameters for the system is summarized in Table 1. The Ni metal with 1.2 lm thickness deposited on a LiNbO3 wafer was etched as a dry etching mask to have patterns with 15 lm width by the photolithography method. The LiNbO3 etching rate, sidewall roughness
and mask etch rate were measured by SEM (Philips, XL-30 FEG/ESEM). Also etched surface roughness was evaluated by AFM (THERO-Microscopy, CP Research). The average surface roughness is calculated by Z LZ L 1 f ðx; yÞdx dy; Ra ¼ 2 L 0 0 where, f(x, y) is the height at the position (x, y) on the etched surface and L is the scan distance of 3 lm.
3. Results and discussions For general NLD etching processes, the Antenna power, the bias power, the operating pressure and the etching time were fixed as 600 W, 300 W, 0.33 Pa and 20 min, respectively, and the C3F8 gas flow was only varied in the range of 5–50 sccm while the total amount of Ar/C3F8 was fixed at 100 sccm. Fig. 2 shows the LiNbO3 etch rate and the mask etch rate as a function of the C3F8 gas flow. As the amount of the C3F8 gas flow increased from 5 to 20 sccm, the LiNbO3 etch rate increased from 128 to 184 nm/min. Also, the etch rate slightly decreased as the C3F8 gas flow increased above 25 sccm. The Ni mask showed the lowest etch rate at the C3F8 gas flow of 20 and 35 sccm, but the Ni mask etch rate rapidly increased at
100
300
Etch Rate [nm]
80 200 60
40 100
Mask Etch Rate [nm]
Etch rate Mask etch rate
20 0
0
10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)]
Fig. 1. Diagram of a NLD etching equipment.
Fig. 2. LiNbO3 etching rate and mask etching rate as a function of the C3F8 gas flow.
218
W.J. Park et al. / Optical Materials 28 (2006) 216–220 90
Profile [deg.]
80
73.07° 71.49°
70
60
10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)] Fig. 3. The C3F8 gas flow dependence of the etching profile.
the C3F8 gas flow higher than 35 sccm. It was confirmed that the LiNbO3 etch rate was a little affected but the Ni mask etch rate was strongly affected by the carbon fluorine gas reaction. Usually, the reaction between LiNbO3 and C3F8 is made by formation of thermally stable compounds of Li ions and F radicals. Further, they need to mix Ar into the chemical etching gases to physically remove the Li and fluorine compounds [10]. Therefore, it is suggested that LiNbO3 etching rate due to the amount of F radicals generated by the discharge increases with the amount of C3F8 etching gas, but the etch rate is reduced at the gas flow higher than 35 sccm due to quenching of the C3F8 reaction gas. The Ni mask is also etched during dry etching of LiNbO3. The etching rate of the Ni mask depended on the total energy of ions such as Ar+ and CFþ xðx¼1–3Þ because the Ni metal film was attacked only by physical ion bombardment. Fig. 3 shows an etch profile as an etching angle at various C3F8 gas flow. As the C3F8 gas flow increased from 5 to 20 sccm, the etch profile also increased from 67.83 to 73.07. Especially, at 20 sccm C3F8 gas flow the maximum profile was observed. Figs. 4 and 5 show SEM micrographs of LiNbO3 waveguides, including cross sectional views, etched at the 20 sccm and 50 sccm C3F8 gas flow, respectively. The etched pattern shape and sidewall roughness depended on the C3F8 gas ratio. The higher C3F8 gas flow decreased the aspect ratio of the ridge structure of waveguides, that is, resulted in a ridge with narrower top and wider bottom. Increase of the C3F8 gas flow caused the etched side wall rougher. It is regarded because more amount of chemically reactive fluorine was adsorbed in the sidewall and particularly in the bottom edge of LiNbO3. The rough surface is induced by the accumulation of the grain particles formed by both the ion bombardment by Ar+ ions and the chemical reaction by CFþ x . It is supposed that such particles, which are in
Fig. 4. SEM micrographs of LiNbO3 waveguides etched at 20 sccm C3F8 gas flow (a) etched Cross-section, (b) shape of etched LiNbO3 waveguide, (c) sidewall.
the liquid state during an etching process due to the high substrate temperature, should be adhered to the etching surface, which is governed by the free energy of the surface. Then, the liquid state particles can be solidified during a cooling process, keeping the same shape as the liquid. This phenomena should be slightly intensified in the bottom edge area than the bottom surface or the sidewall, considering the surface energy dynamics. Fig. 6 shows the variation of root-mean-square (RMS) roughness of etched LiNbO3 as a function of the C3F8 gas flow ratio. The surface RMS roughness (Ra) gradually decreased with the increase of the C3F8 gas flow in the range of 5–50 sccm. The minimum value of RMS roughness was obtained to be 20.56 nm at 50 sccm of the C3F8 gas flow. Fig. 7 shows the AFM images (3 · 3 lm) of the etched surface morphology at various etching conditions. The value of Ra for the non-etched surface was ˚ . The surfaces became rougher after approximately 5.5 A
W.J. Park et al. / Optical Materials 28 (2006) 216–220
219
40
RMS Roughness [nm]
35
30
25
20
15 10
20
30
40
50
Gas Flow Ratio [C3F8/(Ar+C3F8)] Fig. 6. RMS roughness of etched LiNbO3 at various C3F8 gas flow.
etching, and the surface got smoother with increase of the C3F8 gas flow from 5 to 50 sccm. It is suggested that contamination layer can be formed on a etched surface and that the surface morphology is strongly affected by the chemical reaction.
4. Conclusions
Fig. 5. SEM micrographs of LiNbO3 waveguides etched at 50 sccm C3F8 gas flow (a) etched Cross-section, (b) shape of etched LiNbO3 waveguide, (c) sidewall.
The etching process of LiNbO3 by neutral loop plasma etching using 100 sccm Ar/C3F8 as a etching gas has been analyzed at various gas mixture ratios. As the C3F8 gas flow increased from 5 to 20 sccm the LiNbO3 etch rate increased from 128 to184 nm/min, and then the etch rate decreased at the C3F8 gas flow above 25 sccm, which resulted in the maximum etching rate at 20 sccm of at the C3F8 gas flow. Increase of the C3F8 gas flow led to rougher etched surface and sidewall probably
˚ ), (b) C3F8 gas 10 sccm (31 nm), (c) C3F8 Fig. 7. AFM images of the etched surface morphology at several etching conditions. (a) Non-etched (5.5 A gas 20 sccm (28.68 nm), (d) C3F8 gas 50 sccm (20.56 nm).
220
W.J. Park et al. / Optical Materials 28 (2006) 216–220
due to chemically reactive fluorine adsorbed in the sidewall and particularly in the bottom edge of the LiNbO3 ridge structure. The surface RMS roughness gradually decreased with the increase of the C3F8 gas flow in the range of 5–50 sccm. The minimum value of RMS roughness was 20.56 nm with at 50 sccm C3F8.
References [1] K. Tajima, N. Kuwabara, F. Amemiya, R. Kobayashi, Trans. ICICE 76B (1993) 538.
[2] M. Minakata, Electron. Commun. Jpn. 77 (1994) 37. [3] K. Noguchi, O. Mitomi, H. Miyazawa, S. Seki, J. Lightwave Technol. 13 (1995) 1164. [4] M. Tamura, S. Yoshikado, Sci. Technol. Adv. Mater. 2 (2001) 563. [5] K. Shima, N. Mitsugi, H. Nagata, J. Mater. Res. 13 (1998) 527. [6] H. Nagata, N. Mitrugi, K. Shima, M. Tamai, E.M. Haga, J. Crystal Growth 87 (1998) 573. [7] T. Uchida, J. Vac. Technol. A 16 (1998) 1529. [8] W. Chen, Y. Morikawa, M. Itoh, T. Hayashi, K. Sugita, H. Shindo, T. Uchida, J. Vac. Technol. A 17 (1999) 2546. [9] Y.M. Sung, K. Uchino, K. Muraoka, J. Vac. Technol. A 18 (2000) 2149. [10] K. Shima, J. Mater. Res. 13 (1998) 527.