Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2000-10-12
2003-02-04
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S023000, C117S937000
Reexamination Certificate
active
06514336
ABSTRACT:
FIELD
The present invention relates to a method of growing large diameter piezoelectric Ln
3
Ga
5.5
Me
0.5
O
14
(Ln=La, Pr, Nd and Me=Nb,Ta) and Ln
3
Ga
5
M′O
14
(Ln=La, Pr, Nd M′=Si, Ti, Zr, Hf) single crystals and solid solutions on this basis and more particularly to a method of growing such crystals for use as wafers in bulk acoustic wave (BAW), surface acoustic wave (SAW), and pseudo surface acoustic wave (PSAW) devices, having an excellent temperature characteristic and a large electromechanical coupling factor. The SAW devices are currently used, for example, as bandpass filters, resonators, delay lines and converters, in a broad range of wireless applications, cellular communication devices and cable TV.
DESCRIPTION OF THE PRIOR ART
Piezoelectric material based on lanthanide gallium crystals, including the langasite family of crystals, i.e. La
3
Ga
3
SiO
14
, referred to as langasite (LGS), La
3
Ga
5.5
Nb
0.5
O
14
, referred to as langanite (LGN), and langatate La
3
Ga
5.5
Ta
0.5
O
14
(LGT) are known to be useful for piezoelectric applications. A SAW device having a LGS single crystal substrate is disclosed in U.S. Pat. No. 5,821,673, U.K. Pat. No 2,328,815, RU Pat. No 2073952, and U.S. Pat. No. 5,917,265. A LGT single crystal substrate having a prescribed range of Euler angles for substrate and crystal orientation to improve signal processing in a SAWdevice, is disclosed in U.S. Pat. No. 6,097,131.
A SAW device comprising a wafer constructed of a trigonal langasite crystal cut at predominated cut angles is disclosed in U.S. Pat. No. 5,981,673. An optimal cut for SAW devices made from langatate crystals is disclosed in U.S. Pat. No. 6,097,131. A substrate for piezoelectric device and SAW devices composed of a single crystal of langanite is proposed in JP Pat. No 11106294A.
The method of oriented crystallization from a melt, eliminating any contact with the side faces of a growing crystal with solid walls, are now well known and increasingly becoming widespread. These are the Czochralski (conventional Czochralski), Stepanov, Verneuil and floating zone techniques, characterized by a fixed orientation of crystallization at a fixed position of the solid—liquid interface. Each of these techniques has a predominant field of application with the common feature of the elimination of contact between the crystal and a solid wall. As a result, both the shape and the size of a growing crystal are essentially determined by capillary forces, which form a meniscus in the interface boundary zone. In addition, the crystallization process also depends on the conditions of heat and mass exchange in the crystal-melt system, which is generally described in the publication “Some aspects of the macroscopic theory of oriented crystallization from the melt”, E. A. Brener and V. A. Tatarchenko, Acta Physica Academias scientiarum Hungaricas, 47 (1-3) (1979) 133-138. A Stepanov crystal growth method based on a capillary formed pole of melt by means of a special mould and the crystallization of the pole outside the container is generally described in “Capillary shaping in crystal growth from melts” V. A. Tatarchenko, Journal of Crystal Growth 37 (1977) 272-284 and “Crystallization stability during capillary shaping” G, I. Babkin, E. A. Brener and V. A. Tatarchenko, Journal Crystal Growth 50 (1980) 45-50. Another crystal growth method, the EFG crystal growth method, is a method of profiled crystal growth or a method of edge defined film fed growth. The capillary action shaping technique, CAST differs from the EFG technique in the construction of the forming mould and a presence of forced inert gas cooling. The Stepanov, EFG and CAST methods are described in the publication “Growth the profiled single crystals by Stepanov technique” P. I. Antonov, L. M. Zatulovskii, A. C. Kostyugov, (eds) Leningrad “Nauka” (1981) 280 pp.
The method proposed for growing a piezoelectric material based on lanthanide gallium crystals is the conventional Czochralski crystal growth method. The growth of the langasite family of crystals by the Czochralski method is accurately described in “Investigation of trigonal (La
1-x
Nd
x
)
3
Ga
2
SiO
14
crystals” A. A. Kaminskii, B. V. Mill, G. G. Khodzhabagyan, A. F. Konstantinova, A. I. Okorochkov, and I. M. Silvestrova, Physics Status Solid (A) 80 (1983) 387-398 and “Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La
3
Ga
5
SiO
14
(LGS), La
3
Ga
5.5
Nb
0.5
O
14
(LGN) and La
3
Ga
5.5
Ta
0.5
O
14
(LGT), Part I.” J. Bohm, R. B. Heimann, M. Hengst, R. Roewer, J. Schindler, Journal of Crystal Growth 204 (1999) 128-136 and “Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La
3
Ga
5
SiO
14
(LGS), La
3
Ga
5.5
Nb
0.5
O
14
(LGN) and La
3
Ga
5.5
Ta
0.5
O
14
(LGT), Part II. Piezoelectric and elastic propeties” J. Bohm, E. Chilla, C. Flannery, H.-J. Frohlich, T. Hauke, R. B. Heimann, M. Hengst, U. Strauber, J. Schindler, Journal of Crystal Growth 216 (2000) 293-298. The Czochralski growth method of solid solutions of piezoelectric lanthanum gallium silicate single crystals is presented in the publication “Czochralski growth of RE
3
Ga
5
SiO
14
(RE=La, Pr, Nd) single crystals for the analysis of the influence of rare earth substitution on piezoelectricity” J. Sato, H. Takeda, H. Morikoshi, K. Shimamura, P. Rudolph, T. Fukuda, Journal Crystal Growth 191 (1998) 746-753.
The successful growth of 1 or 2 inch diameter and 130 mm length La
3
Ga
5
SiO
14
single crystal using the conventional Czochralski technique is described in “Growth and characterization of lanthanum gallium silicate La
3
Ga
5
SiO
14
single crystals for piezoelectric applications” Kiyoshi Shimamura, Hiroaki Takeda, Tsuguo Fukuda, Journal of Crystal Growth 163 (1996) 388-392. The Czochralski method for growth of a langasite crystal of 3-inch diameter with a 90 mm length cylindrical part along the Z-axis is described in the publications “Growth of a 3” langasite crystal with clear faceting” Satoshi Uda, O. Buzanov, Journal of Crystal Growth, 211 (2000) p. 318-324, “Growth of 3-inch langasite single crystal and its application to substrate for surface acoustic wave filters”, Satoshi Uda, Akihiro Bungo, Chunyun Jian, Japan Journal Applied Physics, 38 (1999) 5516-5519. Growth of langatate La
3
Ta
0.5
Ga
5.5
O
14
crystals by the Czochralski method is described in “Growth and characterization of La
3
Ta
0.5
Ga
5.5
O
14
single crystals” Hiroyuki Kawanaka, Hiroaki Takeda, Kiyoshi Shimamura, Tsuguo Fukuda, Journal of Crystal Growth 183 (1998) 274-277.
A method of growing single crystals of lanthanum-gallium silicate is disclosed in RU Pat. No 2126064, RU Pat. No 2143015, RU Pat. No 2126063, RU Pat. No 2108417, RU Pat. No 2108418, and W.O. Pat. No 9961686A1. The essence of the method consists in the selection of the orientation of the seed crystal ensuring growth by the Czochralski method of single crystals of lanthanum-gallium silicate along the directions <01.1>, <02.1>, <02.3>, <03.2> or at 54 degrees to the “Y” axis. A method of growing of lanthanum gallium tantalum single crystal (LGT) is disclosed in JP Pat. No 11322495A and JP Pat. No 11199392A in which langatate crystals are doped with Pr, Nd, Ce, Sm and Eu impurities are grown by the Czochralski method.
While these methods can be used to make piezoelectric crystals, which are useful in certain applications, there is a variability of the piezoelectric behavior of different wafers cut from the same boule of lanthanide gallium crystal. The growth of langasite is characterized by distinct faceting along the (0001), (01{overscore (1)}0) and (01{overscore (1)}1) planes. Facet growth requires a greater supercooling than growth with a rough surface and hence the interface advances periodically rather than continuously.
Further, greater supercooling often leads to the presence of secondary or polycrystalline phases, resulting in scattering and/or cracking which is another problem of the conventional Czochra
Koutovoi Serguei A.
Loveland Damien G.
Studenikin Pavel A.
Zagoumenny Alexander I.
Zavartsev Iouri D.
Hall Priddy Myers & Vande Sande
Hiteshew Felisa
Utar Scientific, Inc.
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