Langasite wafer and method of producing same

Details Langasite wafer and method of producing same Langasite wafer and method of producing same

1999-11-08

2001-10-16

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from liquid or supercritical state

Having pulling during growth

C117S019000, C117S020000

Reexamination Certificate

active

06302956

ABSTRACT:

TECHNICAL FIELD
The present invention relates to radioelectronics and can find application in acoustoelectronic frequency-selective devices operating on surface acoustic waves (SAW) and volume acoustic waves (VAW).
BACKGROUND ART
SAW- and OAW-devices possess great potentialities whose practical realization involves developing new materials and cheaper techniques of growing piezoelectric crystals having preset quality and dimensions.
Single crystals of langasite (La
3
Ga
5
SiO
14
) prove to be a promising piezoelectric material useful for providing modern radioelectronic equipment with small-size selecting devices as possessing a set of definite properties, i.e., langasite crystals are superior to berlinite as to dielectric constant and to quartz as to Q-factor. Their triclinic symmetry allows for the existence of cuts having a low or even zero temperature coefficient of frequency with an adequate value of the electromechanical-coupling coefficient.
Implementation of the new material, i.e., langasite will allow of reducing the cost of crystals, developing technology of their industrial production, and increasing the dimensions of grown crystals.
The closest to the proposed wafer and method of its production as to the technical essence and attainable result is a disk-shaped langasite wafer for SAW-devices (cf. K. Shimamura et al. “Growth and characterization of lanthanum gallium silicate La
3
Ga
5
SiO
14
single crystals for piezoelectric applications”. J. of Crystal Growth, 1996, v.163, pp.388-392). The diameter of langasite wafers is not in excess of 50 mm because the known techniques of growing langasite crystals fail to produce quality crystals of a larger diameter. However, production process equipment used by the majority of domestic and foreign firms is designed for a minimum disk diameter of 76 mm (3 in), since said size minimizes losses in the devices along the disk periphery with respect to a-total number of devices on the entire surface of the disks.
One prior-art technique of growing single crystals of langasite by the Czochralski method is known to comprise high-frequency induction melting of a charge in a platinum crucible, said charge having been synthesized from a mixture of oxides of lanthanum, gallium, and silicon using the method of solid-phase synthesis, followed by air-pulling of the crystal from the melt onto an oriented seed (M. F. Dubovik et al., “Langasite (La
3
Ga
5
SiO
14
), an optical piezoelectric growth and properties”, 1994, IEEE International frequency control symposium, 1994, pp.43-47). The method allows of growing langasite crystals having a diameter of 60 to 70 mm and a weight of 1 kg, using a cylinder-shaped crucible 100 mm in diameter which is nearly equal to its height. The thus-grown crystals are then annealed at a temperature of 1623 K.
One more prior-art technique of growing single crystals by the Czochralski method is known to comprise high-frequency induction melting of presynthesized charge in a platinum crucible and pulling the crystals from the melt onto an oriented seed in the atmosphere of nitrogen doped with O
2
(3 vol. %) (A. A. Kaminski et al. “Investigation of trigonal (La
1-x
Na
x
)
3
Ga
3
SiO
14
crystals”. Phys. stat. Sol. (a), 1983, v.80, pp.387-398). However, crystal growing in the atmosphere of pure nitrogen is accompanied by substantial evaporation of gallium oxide Ga
2
O
3
, while adding oxygen increases platinum content of the melt.
The inventors of another known technique of growing langasite crystals were to solve the problem of developing industrial-scale technique of growing langasite crystals by improving the construction arrangement of the thermal unit of the crystallization chamber (A. N. Gotalskaya et al. “Aspects of growing langasite crystals and their properties”. Journal de physique IV, 1994, v.4, pp.201-210). The result was the grown crystals 62 mm in diameter and up to 2 kg in weight. Crystals are grown from a charge prepared by the method of solid-phase synthesis.
The closest to the proposed method as to the technical essence and the attainable result is a method involving the Czochralski technique of growing a langasite crystal, comprising charging a crucible with a presynthesized material corresponding to La
3
Ga
5
SiO
14
as for its composition, creating a shielding atmosphere followed by melting the feed material, bringing the rotating oriented seed crystal in contact with the melt surface, and pulling the oriented crystal from the melt (cf. K. Shimamura et al. “Growth and characterization of lanthanum gallium silicate La
3
Ga
5
SiO
14
single crystals for piezoelectric applications”. Journal of Crystal Growth, 1996, v.163, pp.388-392). According to the known method, crystals are grown in an induction-heated growing apparatus. Once the feed material has been charged in a platinum or iridium crucible, crystals are grown in the stream of a gaseous mixture consisting of argon and oxygen (1-2 vol. %).
However, crystals grown by the known technique exhibit the onset of second phases, the properties of crystals are unreproducible from process to process, and numerous optically visible scattering centers appear. The situation is aggravated when attempt is made to grow crystals having a diameter exceeding 70 mm. Moreover, use of crystals grown by the known technique involves a heavy loss in material during manufacture of wafers because the latter are cut off at a large angle to the crystal growth axis.
The closest to the proposed method of preparing a synthesized material (charge) for growing single crystals of langasite is a method of preparing the charge by solid-phase synthesis by sintering the starting oxides of lanthanum, gallium, and silicon (B. V. Mil et al. “Modified rare-earth gallates having the structure of Ca
3
Ga
2
GeO
14
”. Transactions of the USSR Academy of Sciences, 1982, v.264, # 6, pp.1385-1389).
According to the known method, the oxides are mixed in a stoichiometric ratio, then sintered in an oxygen-containing atmosphere at 1300° C. However, the thus-prepared charge cannot be used for growing single crystals of langasite having a stoichiometric-ratio composition, since sintering of the metal oxides is accompanied with loss of the volatile component thereof. Lowering the sintering temperature below 1300° C. allows of reducing the loss of the volatile component which, however, results in a lower yield of lanthanum gallosilicate due to an incomplete proceeding of the reaction as to volume. To render the synthesis process more complete necessitates repeating the comminution of the synthesis product and mixing the oxides followed by their heating. This in turn results in fouling the end product, i.e., the langasite charge and hence in higher manufacturing cost thereof.
DISCLOSURE OF THE INVENTION
The present invention has for its principal object to provide an ecologically pure industrial-scale method of growing stoichiometric-composition single crystals of langasite having a minimum diameter of 76 mm (3 in) (of an inscribed circle) and a weight exceeding 3.5 kg, since said dimension minimizes the loss in devices along the disk periphery with respect to the entire surface of the disks. The resultant crystals should be free from optically detectable scattering centers and have such an orientation that wafers produced therefrom be so oriented that the value of the temperature coefficient of frequency approximates zero and loss in material be minimized.
The foregoing object is accomplished due to the fact that a disk-shaped langasite wafer for use in devices operating on SAW has a diameter of 75 to 80 mm and the disk has a base cut.
Furthermore, the foregoing object is accomplished due to the fact that in a known method of growing single crystals of langasite, using the Czochralski technique, comprising loading a crucible with a presynthesized material (charge) corresponding to La
3
Ga
5
SiO
14
as for its composition, creating a shielding atmosphere followed by melting the feed material, bringing the rotating oriented seed crystal in contact with the melt surface, and pulling the oriented c

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