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The formation of ridges in various directions on (111), (100) and (110) Gd3Ga5O12 substrates
,o . . . . . o, C R Y S T A L G R O W T H
Journal of Crystal Growth 130 (1993) 123-131 North-Holland
The formation of ridges in various directions on (111), (100) and (110) Gd3Ga50 2 substrates Yujiro Katoh, Naoto Sugimoto and Atsushi Shibukawa NTT Opto-Electronics Laboratories, Tokai, Ibaraki 319-11, Japan Received 27 June 1992; manuscript received in final form 13 November 1992
Straight ridge patterns, about 4/xm high, are formed in various directions on (111), (100) and (110) Gd3GasOl2 single crystal substrates by argon ion-beam etching. This is followed by soaking in phosphoric acid solution in order to optimize the garnet waveguide formation process. The substrate orientation and the ridge direction both have considerable influence on the anisotropic etching of the ion-beam etched ridges in the phosphoric acid solution. The ridges parallel to the [211] direction on (111) substrate are symmetrical; however, their faces are slightly different from those reported for YaFesO~2 or Y3AI5Ot2. The ridges on the (100) and (110) substrates have smooth side walls dependent on the symmetrical garnet crystal structure. Both the intermediate and the final dissolved ridge shapes are similar to the results that have been reported for a garnet single crystal sphere.
I. I n t r o d u c t i o n
Much attention has been focused on the optimization of the preparation process for garnet waveguide devices [1-5] such as waveguide optical isolators. This is because of the attractive optical and magneto-optical functions of garnet single crystals, their potential low cost and their capacity for optical integration. The demand is increasing for buried channel garnet waveguides with a core cross section which is ~,bout the same size as the diameter of a single-mode fiber core. This is because such waveguides could be easily coupled to fiber endfaces or to other waveguide devices. However, garnet is an extremely refractory material, so there have been few reports on the nronnratinn nf garnet waveeuides with a cross section that is of the order of a fiber-core diameter. Important requirements for optimizing buried channel garnet waveguide preparation include the following: (1) the surface of the waveguide side wall should be smooth in order to minimize the process induced transmission loss;
(2) it must be possible to have good control of both the width and the height of the ridge to enable good reproducibility. On the basis of these requirements, we examined the shape of ridges in various directions on (111), (100) and (110) Gd3Ga5012 (GGG) substrates using an argon ion-beam etching technique [6]. Ar ion-beam etching provided a good control of both the ridge width (about 3 ~m) and the ridge height (about 5/zm).The side wall obtained using only ion-beam etching was rather jagged butsubsequent soaking in a phosphoric acid solution was successful in smoothing it [7]. The faces of the dissolved ridges were identified by comparison with the excellent reports on the dissolution forms of garnet crystal spheres in acid or flux [8-10]. During growth, the synthetic garnet exhibits {110}, {211}, {321}, {210}, {332} and {100} facets [11,12]. {720} and {221} faces were reported for the dissolution forms in phosphoric acid by Hartmann and co-workers [8,9]. The same behavior was reported for the GGG sphere as well as for the Y3Fe5Ol2 (YIG) sphere [8,9]. {210} and {331} faces were reported for garnet crystals grown
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124
E Katoh et al. / Formation of ridges on Gd3GasOt2 substrates
from PbO/BzO 3 flux by Heimann and Tolksdorf [10]. They also reported the appearance of (hkO) and (hhl) faces in the intermediate stage. The possibility of the appearance of the {321} face was also predicted [12]. It is apparent that a buried waveguide structure with cladding is necessary for practical devices [1,13]. The substrate orientation should be selected d e p e n d e n t on the properties of the epitaxially grown garnet film. For example, a (111) GGG substrate is the best choice for YIG film growth by liquid phase epitaxy (LPE)[14]. Nevertheless, these studies may be significant with regard to the garnet etching process independent of garnet epitaxial growth.
Fig. 1. Typical ridge viewed from the upper right direction formed on a G G G substrate by Ar ion-beam etching.
2. Procedure and methods
Two-inch diameter 400 ~m thick G G G single crystal substrates parallel to (111), (100) and (110) planes were processed in this study. The double layered etching mask consisted of an approximately 8 t~m thick organic layer (Shipley AZ4620), which was baked at 200°C, and an approximately 200 nm thick ion-beam deposited Ta film as an overlayer. The substrate orientations and the corresponding ridge directions are li,:ted in table 1. For the (111) substrate, the line patterns were parallel to the [112] and [110] directions. The ridge patterns of the (100) substrate were parallel to the [001] and [011] directions. For the (110) substrate, the ridges were parallel to the [110] and [001] directions.
A four-inch Kaufman-type ion source apparatus was used to etch the G G G substrate. The cathode voltage, ion-beam current density, and argon pressure werc 500 V, 0.6 m A / c m 2 and 260 mPa, respectively. After the ion-beam etching, each sample were soaked in a diluted phosphoric acid solution at 125, 130 and 140°C for 2-5 min. The ridge shapes were observed directly with a scanning-electron microscope (SEM). The crystal faces of the ridge side walls were indexed simply based on the angles between the faces and the substrate surface observed in the ×7000 SEM micrographs, with reference to the stereogram [11], refs. [8-10], and the theoretical interplanar angles for cubic cry.stals [15].
3. Results and discussion Table 1 Substrate orientations and corresponding ridge directions Substrate orientation
Ridge directions
(l t l)
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Fig. 1 shows a typical view of a ridge formed on a GGG substrate by Ar ion-beam etching. The c r o s s s e c t i o n w a s u c i m v c u uy ~nnp~c c l e a v a g e ,
l
ridge height and width are about 3.9 and 4.8/.,m, respectively. The inclined region at the ridge foot was caused by the shadow effect [16]. A slightly jagged and striped structure at the side wall, whi:h is perpendicular to the bottom, coincided with the structure of the double layered mask side wall.
Y. Katoh et al. / Formation of ridges on GdjGasOtz substrates
19_5
(112)
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Fig. 3. SEM micrographs of a []~2] direction ridge on a (111) substrate which was soaked in phosphoric acid solution at 140°C for 2 min.
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Fig. 2. Stereographic projection of the (111) garnet substrate with the 11 possible kinds of crystal faces.
3.1. The dissolution ridge forms on the (111) substrate
Fig. 2 shows a stereographic projection of a (111) garnet substrate with the 11 possible kinds of garnet crystal face that have appeared in the dissolution forms [8-10].
3.I.1. Results for the [7-[2] direction on the (1111 substrate m
~
A SEM micrograph of the [112] direction ridge which was soaked in a phosphoric acid solution at 140°C for 2 min is shown in fig. 3. The cross section corresponds to the (112) plane. The [112] ,~.,~,.,,, . . . . . . . ~,.o was . . . . . . . . Pross et a!. [ ! ] because it is the only direction parallel to a mirror plane on the (111) garnet substrate.They employed wet etching using hot phosphoric acid for the substituted YIG and their results were completely symmetrical with the {321} and the {211} faces [1]. In this study, other crystal faces were obtained by soaking in phosphoric acid sub-
sequent to the ion-beam etching. G G G can be expected to exhibit the similar behavior as the YIG in the acid [8,9]. Two kinds of face were reported by Huo and Hou for the dissolution forms on YAG in hot phosphoric acid along the [112] direction [17]. Their results are very instructive. From the angles between the faces and the substrate surface, the two YAG faces may be the {110} and the {321} faces. In fig. 3, there are three regions in both ridge side walls. A slightly tilted or alternate structure can be seen in their intermediate region. It may be composed of at least two crystal faces. From the angle of about 39 ° between the (111) plane and the faces, the regions next to the ridge top are the (201) and (021) faces in the stereogram. One of the theoretical interplanar angles between {210} and {111} is 39o14 ' which is nearest to the observed one [15]. In fig. 3, it can be seen that the {210} faces are slightly tilted and have a striped texture. It seems that they include some fraction of other faces such as the (110} or {720} faces 31 The (312) a n d - ( 1 3 2 ) faces, which have an interplanar angle of 22012, against the (l 1l) face,cannot be seen in the ridge side wall from the angles between the substrate surface. The
#~ The theoretical interplanar angles for a cubic crystal are quoted from ref. [15].
Y. Katoh et al. / Formation @ridges on Gd3GasOt2 substrates
126
fraction of terraces, because the interplanar angle is about 80° while they should be completely perpendicular to the substrate surface. The fraction of the {210} faces, that is the upper region of the side wall, increased with both higher soaking temperature and longer soaking time. The final dissolution forms of the []]2] direction ridge may yield {210} faces [7] contrary to the results of Pross et al. or the results for Y3AlsOtz (YAG).
Fig. 4. SEM micrographs of a [110] direction ridge on a (111) substrate which was subsequently soaked at 130°C for 5 min.
respective faces at each ridge foot have angles of about 60°. They may be the (311) and (]31) faces because they seem perpendicular to the cross section (112) face, as seen in fig. 3. They have the theoretical interplanar angle of 58031' against the substrate surface [15]. However, the {311} face has never been reported for synthetic garnet dissolution forms. The faces, that appear in the intermediate region of each ridge side wall, may be composed of alternate structures of the {321} and the {110} faces in the equatorial line, it can be described as the (321), (l'10) and (231) faces appearing one after another. They may have some
3.1.2 Results for a I170] direction ridge on a ( l i d substrate Fig. 4 shows a SEM micrograph of a [110] direction ridge cross section, which was subsequently soaked at 130°C for 5 min. The ridge form is asymmetrical. The cross section corresponds to the (110) face. Huo and Hou reported that the dissolution forms on YAG in a hot phosphoric acid along the [110] direction were the {110} face and the {211} face [17]. A YAG ridge, which was prepared using only the wet etching technique, was asymmetrical but the faces were parallel to the [110] direction. Figs. 5a and 5b show SEM micrographs of the right and left sides, respectively, of the [110] direction ridge cross section which was subsequently soaked at 140°C for 5 rain. The main difference between fig. 4 and fig. 5 is the existence of the intermediate region in both ridge
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Y. Katoh et ai. /Fomzation of ridges o n Gd ~GasOt2 substrates
side walls in the former. The intermediate regions for the two sides seen in fig. 4 have angles of about 73 ° and 66 °, respectively. The 73 ° face on the right may correspond to the (113) face, which has the theoretical angle of 79°58'. The 66" face on the left may correspond to the (111) face, which has the theoretical angle of 70032 ' . The upper right parts, which have an angle of 20 °, is the appearance of the (112) face. The (112) face has the theoretical angle of 19°28 ', which is nearest to the observed angle. The angle between the upper region of the left side wall and the (111) substrate plane is about 45 °. This may be the (331) face, which has the theoretical interplanar angle of 48032 ' against the substrate surface. The lower part of the right side wall in fig. 4 cannot be identified as a single face from the crossing angle of about 38 ° to the substrate surface. The same kind of face should appear in the lower part of the right side wall in fig. 5a. However, the angle in fig. 5a is about 52 ° which is the (001) face. The (001) face has the theoretical angle of 54044 ' The lower part of the right side wall in fig. 4 might be caused by the alternate structure of the {510} or {720} faces. If it is composed of only one kind of crystal face, the (115) face is the most likely candidate, which has the theoretical angle of 38°57'. The lower part of the left side wall, which has an angle of about 20 °, may be the (331) or (221) face. The (331) and
127
Fig. 7. SEM micrographs of a [001] direction ridge on a (100) substrate which was subsequently soaked at 140°C for 5 min.
(221) faces have theoretical angles of 22°0 ' and 15°48 ', respectively. The final form may be the (211) and the (331)/(221) faces for the [170] direction ridge, because figs. 5a and Sb show a tendency for the fraction of these faces to become larger than those for the ridge which was soaked at 130°C. The final dissolution form of YAG in phosphoric acid was characterized as the {321} face [17] #2. Thus, GGG exhibited different behavior from YAG. The final dissolution form tends to appear in the upper and the lower part of the ridge side wall. Thus, the {210}, the {211}, the {321} and the {331} faces are considered to be the final form for the ridge on the (111) substrate.
3.2. The dissolution fol~ns of ridges on the (100) substrate The standard (001) stereographic projection for cubic crystals after Wood [18] is employed in this section. The straight ridges are parallel to the [001] and [011] directions.
Fig. 6. SEM micrographs of a [001] direction ridge on a (100) substrate which was subsequently soaked at 125°C for 5 rain.
#2 Huo and Hou described the final form of YAG, which had the interplanar angle of 20°50 ' against the (I 1I) plane, as in the case of the {211} face. However, it is apparent from the stereogram that it is the {321} face.
128
Y Katoh et aL
/ Formation of ridges on Gd3GasOte substrates
Fig. 8. SEM micrographs of a [011] direction ridge on a (100) substrate which was subsequently soaked at 125°C for 5 min and 140°C for 5 rain: (a) cross section of the ridge soaked at 125°C; (b) cross section of the ridge left side wall soaked at 140°C.
3.2.1. Results for a 10011 direction ridge on a (100) substrate Fig. 6 is a SEM micrograph of the [001] direction ridge, which was soaked in phosphoric acid at 125°C for 5 rain after the ion-beam etching. The cross section is the (001) face. The angle between the side wall and the top surface is 45 °. The face was easily identified as the ( l l 0 ) face. Fig. 7 shows the cross section of a ridge which was soaked at 140° for 5 min. There are three kinds of facem in the side wall. The lowest part is clearly the (110) face. The angles between the top surface, and the first and second faces are about 12° and 31 °, respectively. The face nearest the top is possibly the (5]0) face. The (5]0) face has the theoretical interplanar angle of 11°18 ' against the (100) surface. The most likely candidate in the stere•gram for the second face is the (2]0) face. The theoretical angle between the (2]0) face and the (100) surface is 26034 '. The final form may be the {510} face because it appeared at the higher soaking temperature in the upper region.
surface is about 57 °. This face was identified as the (1]1) face, which has the theoretical interplanar angle of 54044 ' The lower side wall which has an angle of 35 ° could be identified as the
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Fig. 8a is a SEM micrograph of the [011] direction ridge, which was soaked in phosphoric acid at 125°C for 5 min after ion-beam etching. The side wall is composed of two kinds of face. The angle between the upper side wall and the
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Y. Katoh et al. / Formation of ridges on Gd ~GasOt2 substrates
129
(211) face, which has the theoretical angle of 35016 ' , in the right side wall. Fig. 8b shows a cross scction of the left side wall of a ridge which was soaked at 140°C for 5 min. The lowest part, which covers most of the side wall is clearly the {211} face. The {211} face is slightly curved as seen in fig. 8b. The intermediate faces in the side walls are also easily identified as the (111) and (111) faces. The angle between the upper face and the top surface is about 9 °. This face should exist on the line which runs from the center in the [011] direction in the stereogram. However, no low index face in the stereogram corresponds to the angle. This face may be a {117} face which is not any of the 11 kinds of face that have ever been reported. Nevertheless, the final form would be the {211} face because the fraction of the {211} face increases at higher temperatures. 3.3. The dissolution forms of ridges on a (I10) substrate Fig. 9 is a stereographic projection of the (110) garnet substrate with the 11 possible kinds of face. The ridges were parallel to the [110] and [001 ] directions. 3.3.1. Results for a [I10] direction ridge on a (11 O) substrate From fig. 9, it can be see~ that there are 6 possible faces that can appear in the [110] direction ridge side wall. They are the (331), (221), (332), (111), (112) and (001) faces. Figs. 10a and 10b are SEM micrographs of the [110] direction ridge, which was soaked in phosphoric acid at 125°C for 5 min after the ion-beam etching. Four kinds of face appeared in the side walls. The angles between the substrate surface and the four faces in the side wail are about I3 °, 35 °, m
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Fig. 10. SEM micrographs of a [1i0] direction ridge on a (I 10) substrate which was subsequently soaked at 125°C for 5 min and 140°C for 5 rain, respectively: (a) cross section of the ridge soaked at 125°C; (b) ridge which was soaked at 125°C from the upper right; (c) cross section of the ridge soaked at 140°C.
130
Y. Katoh et al. / Formation of ridges on GdaGasO~2 substrates
56 °, and 84 °, respectively. They are considered to be the (331), ( l i D , (112) and (001) faces, respectively, for the right side wall. These four faces have the theoretical angles of 13016 ', 35°16 ', 54044 ' and 90 °, respectively. Fig. 10c is a SEM micrograph of a ridge, which was soaked in phosphoric acid at 140°C for 5 min. The jagged and steep face, which could be the (001) face, disappeared. Three other smooth faces still remained, however, the fraction of the intermediate (112) face became small. The final form of the [1]0] direction ridge on the (110) G G G substrate may be the lower {111} a n d / o r upper {331} faces.
3.3.2. Results for a [001] direction ridge on a (110) substrate It can be seen in fig. 9 that there are 8 possible kinds of face that can appear in the [001] direction ridge side wall. They are the (210), (720), (510), (100), (5]0), (72.0), (2]0) and (110) faces on the line from the center in the [110] direction. Fig. 11 is a SEM micrograph of the [001] direc--..i..~n .;,. the phosphoric tion ridge, which was ~u,~,,.,, acid at 140°C for 5 min after the ion-beam etching. The cross section is the (001) face. The ridge shape is also largely symmetrical. From fig. 11 it can be seen that the side wall is composed of four kinds of face. The respective angles between these four faces and the substrate surface are about 14°, 20 °, 35 ° and 55 °. The latter three faces were m
easily identified as the (210), (510) and (510) faces from the stereogram. These three faces have the theoretical angles of 18026 ' , 33030 ' and 56030 ' , respectively. The face of the 14° angle may be the terrace structure which is composed of the (210) and the (110) faces.
4. Conclusions The dissolution forms of ion-beam etched ridges on G G G substrates in diluted phosphoric acid have been investigated. Substrate orientation and ridge direction have considerable influence on the anisotropic etching of the ion-beam etched ridges. The crystal faces, which appeared in the ridge side-walls, have been successfully speculated by comparing the observed and the theoretical interplanar angles, and using a stereogram. In addition, the {311} face was identified, which is consistent with the appearance of the (Lhl) faces. The ridge shapes were different from those reported for ridges prepared using only wet etching. Ridges on the (111) substrate were sometimes asymmetrical and had a slightly jagged side-wall. On the other hand, the ridges on the (100) and (110) substrates were symmetrical and had smooth side-walls. Future studies should focus on the preparation of a ridge with smooth side walls and a rectangular cross section on a (111) substrate. It is, of course, important that crystal growth on the (100) and (110) is refined. A theoretical discussion based on a PBC analysis [12] may also be an interesting area for study.
Acknowledgements The authors would like to express their thanks to Dr. Akiyuki Tate, Dr. Yasuhiro Nagai, Mr. 'ITS" . . . . . . .
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References Fig. 11. SEM micrographs of a [001] direction ridge on a (110) substrate which was subsequently soaked at 140°C for 5 rain.
[l] E. Pross, H. Dammann and W. Tolksdorf, J. Appl. Phys. 68 (1990) 3849.
Y. Katoh et al. / Formation of ridges on Gd3Gas012 substrate~"
[2] R. Wolfe, R.A. Lieberman, V.J. Fratello, R.E. Scotti and N. Kopylov, Appl. Phys. Letters 56 (I 990) 426. [3] Y. Okamura and S. Yamamoto, SPIE Proc. 1177 (1989) 354. [4] K. Ando, T. Okoshi and N. Koshizuka, Appl. Phys. Letters 53 (1988) 4. [5] S. Kaewsuriyathumrong, T. Mizumoto, H. Mak and Y. Naito, J. Lightwave Technol. 8 (1990)177. [6] Y. Katoh, N. Sugimoto, A. Tate and A. Shibukawa, Japan. J. Appl. Phys. 31 (1992)141. [7] Y. Katoh, N. Sugimoto, A. Tate and A. Shibukawa, Japa,,. J. Appl. Phys. 31 (1992) L591. [8] E. Hartmann, E. Beregi and J. Labar, J. Crystal Growth 71 (1985) 191. [9] E. Beregi, E. Sterk, F. Tanos, E. Hartmann and J. Labar, J. Crystal Growth 65 (1983) 562. [10] R.B. Heimann and W. Tolksdorf, J. Crystal Growth 62 (1983) 75.
131
[11] W. Tolksdorf and I. Barrels, J. Crystal Growth 54 (1981) 417.
[121 P. Bennema, E.A. Giess and J.E. Weidenborner. J. CD'stai Growth 62 (1983) 41.
[131 W. Tolksdolf, H. Dammann, E. Pross, B. Strocka, F. Welz and P. Willich, J. Crystal Growth 99 (1990) 616.
[141 W. van Erk, H.J.G.J. van Hoek-Martens and G. Bartels, J. Crystal Growth 48 (1980) 621.
[151 International Tables for X-Ray Crystallography, Vol. 11, Eds. J.S. Kasper and K. Lonsdale (Kynoch, Birmingham, 1972). [161 H. Gokan and S. Esho, J. Vacuum Sci. Technol. 16 (1981) 28. [171 D.T.C. Huo and T.W. Hou, J. Electrochem. Soc. 133 (1986)1492. [181 E.A. Wood, Crystal Orientation Manual (Columbia Univ. Press, New York, 1963).
Journal of Crystal Growth 130 (1993) 123-131 North-Holland
The formation of ridges in various directions on (111), (100) and (110) Gd3Ga50 2 substrates Yujiro Katoh, Naoto Sugimoto and Atsushi Shibukawa NTT Opto-Electronics Laboratories, Tokai, Ibaraki 319-11, Japan Received 27 June 1992; manuscript received in final form 13 November 1992
Straight ridge patterns, about 4/xm high, are formed in various directions on (111), (100) and (110) Gd3GasOl2 single crystal substrates by argon ion-beam etching. This is followed by soaking in phosphoric acid solution in order to optimize the garnet waveguide formation process. The substrate orientation and the ridge direction both have considerable influence on the anisotropic etching of the ion-beam etched ridges in the phosphoric acid solution. The ridges parallel to the [211] direction on (111) substrate are symmetrical; however, their faces are slightly different from those reported for YaFesO~2 or Y3AI5Ot2. The ridges on the (100) and (110) substrates have smooth side walls dependent on the symmetrical garnet crystal structure. Both the intermediate and the final dissolved ridge shapes are similar to the results that have been reported for a garnet single crystal sphere.
I. I n t r o d u c t i o n
Much attention has been focused on the optimization of the preparation process for garnet waveguide devices [1-5] such as waveguide optical isolators. This is because of the attractive optical and magneto-optical functions of garnet single crystals, their potential low cost and their capacity for optical integration. The demand is increasing for buried channel garnet waveguides with a core cross section which is ~,bout the same size as the diameter of a single-mode fiber core. This is because such waveguides could be easily coupled to fiber endfaces or to other waveguide devices. However, garnet is an extremely refractory material, so there have been few reports on the nronnratinn nf garnet waveeuides with a cross section that is of the order of a fiber-core diameter. Important requirements for optimizing buried channel garnet waveguide preparation include the following: (1) the surface of the waveguide side wall should be smooth in order to minimize the process induced transmission loss;
(2) it must be possible to have good control of both the width and the height of the ridge to enable good reproducibility. On the basis of these requirements, we examined the shape of ridges in various directions on (111), (100) and (110) Gd3Ga5012 (GGG) substrates using an argon ion-beam etching technique [6]. Ar ion-beam etching provided a good control of both the ridge width (about 3 ~m) and the ridge height (about 5/zm).The side wall obtained using only ion-beam etching was rather jagged butsubsequent soaking in a phosphoric acid solution was successful in smoothing it [7]. The faces of the dissolved ridges were identified by comparison with the excellent reports on the dissolution forms of garnet crystal spheres in acid or flux [8-10]. During growth, the synthetic garnet exhibits {110}, {211}, {321}, {210}, {332} and {100} facets [11,12]. {720} and {221} faces were reported for the dissolution forms in phosphoric acid by Hartmann and co-workers [8,9]. The same behavior was reported for the GGG sphere as well as for the Y3Fe5Ol2 (YIG) sphere [8,9]. {210} and {331} faces were reported for garnet crystals grown
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All right~; reserved
124
E Katoh et al. / Formation of ridges on Gd3GasOt2 substrates
from PbO/BzO 3 flux by Heimann and Tolksdorf [10]. They also reported the appearance of (hkO) and (hhl) faces in the intermediate stage. The possibility of the appearance of the {321} face was also predicted [12]. It is apparent that a buried waveguide structure with cladding is necessary for practical devices [1,13]. The substrate orientation should be selected d e p e n d e n t on the properties of the epitaxially grown garnet film. For example, a (111) GGG substrate is the best choice for YIG film growth by liquid phase epitaxy (LPE)[14]. Nevertheless, these studies may be significant with regard to the garnet etching process independent of garnet epitaxial growth.
Fig. 1. Typical ridge viewed from the upper right direction formed on a G G G substrate by Ar ion-beam etching.
2. Procedure and methods
Two-inch diameter 400 ~m thick G G G single crystal substrates parallel to (111), (100) and (110) planes were processed in this study. The double layered etching mask consisted of an approximately 8 t~m thick organic layer (Shipley AZ4620), which was baked at 200°C, and an approximately 200 nm thick ion-beam deposited Ta film as an overlayer. The substrate orientations and the corresponding ridge directions are li,:ted in table 1. For the (111) substrate, the line patterns were parallel to the [112] and [110] directions. The ridge patterns of the (100) substrate were parallel to the [001] and [011] directions. For the (110) substrate, the ridges were parallel to the [110] and [001] directions.
A four-inch Kaufman-type ion source apparatus was used to etch the G G G substrate. The cathode voltage, ion-beam current density, and argon pressure werc 500 V, 0.6 m A / c m 2 and 260 mPa, respectively. After the ion-beam etching, each sample were soaked in a diluted phosphoric acid solution at 125, 130 and 140°C for 2-5 min. The ridge shapes were observed directly with a scanning-electron microscope (SEM). The crystal faces of the ridge side walls were indexed simply based on the angles between the faces and the substrate surface observed in the ×7000 SEM micrographs, with reference to the stereogram [11], refs. [8-10], and the theoretical interplanar angles for cubic cry.stals [15].
3. Results and discussion Table 1 Substrate orientations and corresponding ridge directions Substrate orientation
Ridge directions
(l t l)
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l
ridge height and width are about 3.9 and 4.8/.,m, respectively. The inclined region at the ridge foot was caused by the shadow effect [16]. A slightly jagged and striped structure at the side wall, whi:h is perpendicular to the bottom, coincided with the structure of the double layered mask side wall.
Y. Katoh et al. / Formation of ridges on GdjGasOtz substrates
19_5
(112)
( i]01
(TIO)
Fig. 3. SEM micrographs of a []~2] direction ridge on a (111) substrate which was soaked in phosphoric acid solution at 140°C for 2 min.
~lfL
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Fig. 2. Stereographic projection of the (111) garnet substrate with the 11 possible kinds of crystal faces.
3.1. The dissolution ridge forms on the (111) substrate
Fig. 2 shows a stereographic projection of a (111) garnet substrate with the 11 possible kinds of garnet crystal face that have appeared in the dissolution forms [8-10].
3.I.1. Results for the [7-[2] direction on the (1111 substrate m
~
A SEM micrograph of the [112] direction ridge which was soaked in a phosphoric acid solution at 140°C for 2 min is shown in fig. 3. The cross section corresponds to the (112) plane. The [112] ,~.,~,.,,, . . . . . . . ~,.o was . . . . . . . . Pross et a!. [ ! ] because it is the only direction parallel to a mirror plane on the (111) garnet substrate.They employed wet etching using hot phosphoric acid for the substituted YIG and their results were completely symmetrical with the {321} and the {211} faces [1]. In this study, other crystal faces were obtained by soaking in phosphoric acid sub-
sequent to the ion-beam etching. G G G can be expected to exhibit the similar behavior as the YIG in the acid [8,9]. Two kinds of face were reported by Huo and Hou for the dissolution forms on YAG in hot phosphoric acid along the [112] direction [17]. Their results are very instructive. From the angles between the faces and the substrate surface, the two YAG faces may be the {110} and the {321} faces. In fig. 3, there are three regions in both ridge side walls. A slightly tilted or alternate structure can be seen in their intermediate region. It may be composed of at least two crystal faces. From the angle of about 39 ° between the (111) plane and the faces, the regions next to the ridge top are the (201) and (021) faces in the stereogram. One of the theoretical interplanar angles between {210} and {111} is 39o14 ' which is nearest to the observed one [15]. In fig. 3, it can be seen that the {210} faces are slightly tilted and have a striped texture. It seems that they include some fraction of other faces such as the (110} or {720} faces 31 The (312) a n d - ( 1 3 2 ) faces, which have an interplanar angle of 22012, against the (l 1l) face,cannot be seen in the ridge side wall from the angles between the substrate surface. The
#~ The theoretical interplanar angles for a cubic crystal are quoted from ref. [15].
Y. Katoh et al. / Formation @ridges on Gd3GasOt2 substrates
126
fraction of terraces, because the interplanar angle is about 80° while they should be completely perpendicular to the substrate surface. The fraction of the {210} faces, that is the upper region of the side wall, increased with both higher soaking temperature and longer soaking time. The final dissolution forms of the []]2] direction ridge may yield {210} faces [7] contrary to the results of Pross et al. or the results for Y3AlsOtz (YAG).
Fig. 4. SEM micrographs of a [110] direction ridge on a (111) substrate which was subsequently soaked at 130°C for 5 min.
respective faces at each ridge foot have angles of about 60°. They may be the (311) and (]31) faces because they seem perpendicular to the cross section (112) face, as seen in fig. 3. They have the theoretical interplanar angle of 58031' against the substrate surface [15]. However, the {311} face has never been reported for synthetic garnet dissolution forms. The faces, that appear in the intermediate region of each ridge side wall, may be composed of alternate structures of the {321} and the {110} faces in the equatorial line, it can be described as the (321), (l'10) and (231) faces appearing one after another. They may have some
3.1.2 Results for a I170] direction ridge on a ( l i d substrate Fig. 4 shows a SEM micrograph of a [110] direction ridge cross section, which was subsequently soaked at 130°C for 5 min. The ridge form is asymmetrical. The cross section corresponds to the (110) face. Huo and Hou reported that the dissolution forms on YAG in a hot phosphoric acid along the [110] direction were the {110} face and the {211} face [17]. A YAG ridge, which was prepared using only the wet etching technique, was asymmetrical but the faces were parallel to the [110] direction. Figs. 5a and 5b show SEM micrographs of the right and left sides, respectively, of the [110] direction ridge cross section which was subsequently soaked at 140°C for 5 rain. The main difference between fig. 4 and fig. 5 is the existence of the intermediate region in both ridge
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Y. Katoh et ai. /Fomzation of ridges o n Gd ~GasOt2 substrates
side walls in the former. The intermediate regions for the two sides seen in fig. 4 have angles of about 73 ° and 66 °, respectively. The 73 ° face on the right may correspond to the (113) face, which has the theoretical angle of 79°58'. The 66" face on the left may correspond to the (111) face, which has the theoretical angle of 70032 ' . The upper right parts, which have an angle of 20 °, is the appearance of the (112) face. The (112) face has the theoretical angle of 19°28 ', which is nearest to the observed angle. The angle between the upper region of the left side wall and the (111) substrate plane is about 45 °. This may be the (331) face, which has the theoretical interplanar angle of 48032 ' against the substrate surface. The lower part of the right side wall in fig. 4 cannot be identified as a single face from the crossing angle of about 38 ° to the substrate surface. The same kind of face should appear in the lower part of the right side wall in fig. 5a. However, the angle in fig. 5a is about 52 ° which is the (001) face. The (001) face has the theoretical angle of 54044 ' The lower part of the right side wall in fig. 4 might be caused by the alternate structure of the {510} or {720} faces. If it is composed of only one kind of crystal face, the (115) face is the most likely candidate, which has the theoretical angle of 38°57'. The lower part of the left side wall, which has an angle of about 20 °, may be the (331) or (221) face. The (331) and
127
Fig. 7. SEM micrographs of a [001] direction ridge on a (100) substrate which was subsequently soaked at 140°C for 5 min.
(221) faces have theoretical angles of 22°0 ' and 15°48 ', respectively. The final form may be the (211) and the (331)/(221) faces for the [170] direction ridge, because figs. 5a and Sb show a tendency for the fraction of these faces to become larger than those for the ridge which was soaked at 130°C. The final dissolution form of YAG in phosphoric acid was characterized as the {321} face [17] #2. Thus, GGG exhibited different behavior from YAG. The final dissolution form tends to appear in the upper and the lower part of the ridge side wall. Thus, the {210}, the {211}, the {321} and the {331} faces are considered to be the final form for the ridge on the (111) substrate.
3.2. The dissolution fol~ns of ridges on the (100) substrate The standard (001) stereographic projection for cubic crystals after Wood [18] is employed in this section. The straight ridges are parallel to the [001] and [011] directions.
Fig. 6. SEM micrographs of a [001] direction ridge on a (100) substrate which was subsequently soaked at 125°C for 5 rain.
#2 Huo and Hou described the final form of YAG, which had the interplanar angle of 20°50 ' against the (I 1I) plane, as in the case of the {211} face. However, it is apparent from the stereogram that it is the {321} face.
128
Y Katoh et aL
/ Formation of ridges on Gd3GasOte substrates
Fig. 8. SEM micrographs of a [011] direction ridge on a (100) substrate which was subsequently soaked at 125°C for 5 min and 140°C for 5 rain: (a) cross section of the ridge soaked at 125°C; (b) cross section of the ridge left side wall soaked at 140°C.
3.2.1. Results for a 10011 direction ridge on a (100) substrate Fig. 6 is a SEM micrograph of the [001] direction ridge, which was soaked in phosphoric acid at 125°C for 5 rain after the ion-beam etching. The cross section is the (001) face. The angle between the side wall and the top surface is 45 °. The face was easily identified as the ( l l 0 ) face. Fig. 7 shows the cross section of a ridge which was soaked at 140° for 5 min. There are three kinds of facem in the side wall. The lowest part is clearly the (110) face. The angles between the top surface, and the first and second faces are about 12° and 31 °, respectively. The face nearest the top is possibly the (5]0) face. The (5]0) face has the theoretical interplanar angle of 11°18 ' against the (100) surface. The most likely candidate in the stere•gram for the second face is the (2]0) face. The theoretical angle between the (2]0) face and the (100) surface is 26034 '. The final form may be the {510} face because it appeared at the higher soaking temperature in the upper region.
surface is about 57 °. This face was identified as the (1]1) face, which has the theoretical interplanar angle of 54044 ' The lower side wall which has an angle of 35 ° could be identified as the
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Fig. 8a is a SEM micrograph of the [011] direction ridge, which was soaked in phosphoric acid at 125°C for 5 min after ion-beam etching. The side wall is composed of two kinds of face. The angle between the upper side wall and the
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Y. Katoh et al. / Formation of ridges on Gd ~GasOt2 substrates
129
(211) face, which has the theoretical angle of 35016 ' , in the right side wall. Fig. 8b shows a cross scction of the left side wall of a ridge which was soaked at 140°C for 5 min. The lowest part, which covers most of the side wall is clearly the {211} face. The {211} face is slightly curved as seen in fig. 8b. The intermediate faces in the side walls are also easily identified as the (111) and (111) faces. The angle between the upper face and the top surface is about 9 °. This face should exist on the line which runs from the center in the [011] direction in the stereogram. However, no low index face in the stereogram corresponds to the angle. This face may be a {117} face which is not any of the 11 kinds of face that have ever been reported. Nevertheless, the final form would be the {211} face because the fraction of the {211} face increases at higher temperatures. 3.3. The dissolution forms of ridges on a (I10) substrate Fig. 9 is a stereographic projection of the (110) garnet substrate with the 11 possible kinds of face. The ridges were parallel to the [110] and [001 ] directions. 3.3.1. Results for a [I10] direction ridge on a (11 O) substrate From fig. 9, it can be see~ that there are 6 possible faces that can appear in the [110] direction ridge side wall. They are the (331), (221), (332), (111), (112) and (001) faces. Figs. 10a and 10b are SEM micrographs of the [110] direction ridge, which was soaked in phosphoric acid at 125°C for 5 min after the ion-beam etching. Four kinds of face appeared in the side walls. The angles between the substrate surface and the four faces in the side wail are about I3 °, 35 °, m
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Fig. 10. SEM micrographs of a [1i0] direction ridge on a (I 10) substrate which was subsequently soaked at 125°C for 5 min and 140°C for 5 rain, respectively: (a) cross section of the ridge soaked at 125°C; (b) ridge which was soaked at 125°C from the upper right; (c) cross section of the ridge soaked at 140°C.
130
Y. Katoh et al. / Formation of ridges on GdaGasO~2 substrates
56 °, and 84 °, respectively. They are considered to be the (331), ( l i D , (112) and (001) faces, respectively, for the right side wall. These four faces have the theoretical angles of 13016 ', 35°16 ', 54044 ' and 90 °, respectively. Fig. 10c is a SEM micrograph of a ridge, which was soaked in phosphoric acid at 140°C for 5 min. The jagged and steep face, which could be the (001) face, disappeared. Three other smooth faces still remained, however, the fraction of the intermediate (112) face became small. The final form of the [1]0] direction ridge on the (110) G G G substrate may be the lower {111} a n d / o r upper {331} faces.
3.3.2. Results for a [001] direction ridge on a (110) substrate It can be seen in fig. 9 that there are 8 possible kinds of face that can appear in the [001] direction ridge side wall. They are the (210), (720), (510), (100), (5]0), (72.0), (2]0) and (110) faces on the line from the center in the [110] direction. Fig. 11 is a SEM micrograph of the [001] direc--..i..~n .;,. the phosphoric tion ridge, which was ~u,~,,.,, acid at 140°C for 5 min after the ion-beam etching. The cross section is the (001) face. The ridge shape is also largely symmetrical. From fig. 11 it can be seen that the side wall is composed of four kinds of face. The respective angles between these four faces and the substrate surface are about 14°, 20 °, 35 ° and 55 °. The latter three faces were m
easily identified as the (210), (510) and (510) faces from the stereogram. These three faces have the theoretical angles of 18026 ' , 33030 ' and 56030 ' , respectively. The face of the 14° angle may be the terrace structure which is composed of the (210) and the (110) faces.
4. Conclusions The dissolution forms of ion-beam etched ridges on G G G substrates in diluted phosphoric acid have been investigated. Substrate orientation and ridge direction have considerable influence on the anisotropic etching of the ion-beam etched ridges. The crystal faces, which appeared in the ridge side-walls, have been successfully speculated by comparing the observed and the theoretical interplanar angles, and using a stereogram. In addition, the {311} face was identified, which is consistent with the appearance of the (Lhl) faces. The ridge shapes were different from those reported for ridges prepared using only wet etching. Ridges on the (111) substrate were sometimes asymmetrical and had a slightly jagged side-wall. On the other hand, the ridges on the (100) and (110) substrates were symmetrical and had smooth side-walls. Future studies should focus on the preparation of a ridge with smooth side walls and a rectangular cross section on a (111) substrate. It is, of course, important that crystal growth on the (100) and (110) is refined. A theoretical discussion based on a PBC analysis [12] may also be an interesting area for study.
Acknowledgements The authors would like to express their thanks to Dr. Akiyuki Tate, Dr. Yasuhiro Nagai, Mr. 'ITS" . . . . . . .
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References Fig. 11. SEM micrographs of a [001] direction ridge on a (110) substrate which was subsequently soaked at 140°C for 5 rain.
[l] E. Pross, H. Dammann and W. Tolksdorf, J. Appl. Phys. 68 (1990) 3849.
Y. Katoh et al. / Formation of ridges on Gd3Gas012 substrate~"
[2] R. Wolfe, R.A. Lieberman, V.J. Fratello, R.E. Scotti and N. Kopylov, Appl. Phys. Letters 56 (I 990) 426. [3] Y. Okamura and S. Yamamoto, SPIE Proc. 1177 (1989) 354. [4] K. Ando, T. Okoshi and N. Koshizuka, Appl. Phys. Letters 53 (1988) 4. [5] S. Kaewsuriyathumrong, T. Mizumoto, H. Mak and Y. Naito, J. Lightwave Technol. 8 (1990)177. [6] Y. Katoh, N. Sugimoto, A. Tate and A. Shibukawa, Japan. J. Appl. Phys. 31 (1992)141. [7] Y. Katoh, N. Sugimoto, A. Tate and A. Shibukawa, Japa,,. J. Appl. Phys. 31 (1992) L591. [8] E. Hartmann, E. Beregi and J. Labar, J. Crystal Growth 71 (1985) 191. [9] E. Beregi, E. Sterk, F. Tanos, E. Hartmann and J. Labar, J. Crystal Growth 65 (1983) 562. [10] R.B. Heimann and W. Tolksdorf, J. Crystal Growth 62 (1983) 75.
131
[11] W. Tolksdorf and I. Barrels, J. Crystal Growth 54 (1981) 417.
[121 P. Bennema, E.A. Giess and J.E. Weidenborner. J. CD'stai Growth 62 (1983) 41.
[131 W. Tolksdolf, H. Dammann, E. Pross, B. Strocka, F. Welz and P. Willich, J. Crystal Growth 99 (1990) 616.
[141 W. van Erk, H.J.G.J. van Hoek-Martens and G. Bartels, J. Crystal Growth 48 (1980) 621.
[151 International Tables for X-Ray Crystallography, Vol. 11, Eds. J.S. Kasper and K. Lonsdale (Kynoch, Birmingham, 1972). [161 H. Gokan and S. Esho, J. Vacuum Sci. Technol. 16 (1981) 28. [171 D.T.C. Huo and T.W. Hou, J. Electrochem. Soc. 133 (1986)1492. [181 E.A. Wood, Crystal Orientation Manual (Columbia Univ. Press, New York, 1963).