Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2000-01-31
2002-06-11
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S033000, C117S034000, C117S910000, C117S948000
Reexamination Certificate
active
06402834
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a monocrystal producing technique, and in particular to a producing device and a producing process for producing a monocrystal by a pulling-down method, and a monocrystal.
BACKGROUND ART
In recent years, monocrystals of oxides such as lithium tantalate LiTaO
3
(referred to as LT hereinafter), lithium niobate LiNbO
3
(referred to as LN hereinafter), lithium tetraborate Li
2
B
4
O
7
(referred to as LBO hereinafter), and langasite La
3
Ga
5
SiO
14
(referred to as LGS hereinafter) have been used to produce various surface acoustic wave devices. These monocrystals are piezoelectric crystals having a large electromechanical coupling coefficient than that of a quartz crystal substrate. LBO and LGS have a cut angle of a zero temperature coefficient. Therefore, if these monocrystals are used for a surface acoustic wave device, terminals such as a portable telephone become small-sized and come to have high functions. In crystals of LT and LN, the ratio of Li to Ta or the ratio of Li to Nb stoichiometrically becomes 1:1. Crystals having such a composition are suitable for optical materials since they have in their lattices no defect or gap to become an ideal crystal structure and they have a constant refractive index in the crystals thereof to generate no diffuse reflection.
The methods for growing the above-mentioned monocrystals are roughly classified into the following three methods. That is, the methods are the Czochralski method (CZ method), the vertical Bridgman method (VB method) and a pulling-down method.
As shown in
FIG. 6
, the Czochralski method (CZ method, rotation pulling method) is the method of putting a raw material to be crystallized into a platinum crucible
41
, heating the raw material to the melting point thereof or higher in an electric furnace
42
to be melted, immersing the lower end of a seed crystal
44
in a rod form into the resultant
43
, and pulling the crystal with slow rotation so as to grow a crystal
45
from the lower end of the seed crystal
44
.
As shown in
FIG. 7
, the vertical Bridgman method is the method of putting a raw material to be crystallized into a platinum crucible
51
, heating the raw material to the melting point thereof or higher in an electric furnace
52
to be melted, putting a plate-form seed crystal
53
into the platinum crucible
51
from one end thereof, and slowly moving the platinum crucible
51
, with keeping the side of the seed crystal
53
ahead, from the side of high temperature to the side of low temperature in the state that temperature gradient is generated inside the electric furnace
52
, so as to grow a crystal successively from the side of the seed crystal
53
.
The pulling-down method is a monocrystal growing method published in a document (Journal of the Ceramic Society of Japan 105[7] 1997) by one of the inventors of the present application. As shown in
FIG. 8
, this method is the method of putting a polycrystal raw material into a platinum crucible
61
having a fine hole
610
a
in its bottom, arranging this platinum crucible
61
at the position where temperature gradient is steepest inside an electrical furnace
62
whose upper side is kept over the melting point of the raw material and whose lower side is kept below the melting point thereof to melt the raw material, and pulling down a seed crystal
63
with rotation in the state that the upper end of the seed crystal
63
in a rod form is brought into contact with the raw material melt that has flown out from the fine hole
61
a
of the platinum crucible
61
by gravity. According to this pulling-down method, the raw material melt is kept between the platinum crucible
61
and the seed crystal
63
, using both of wettability of the raw material melt to the crucible
61
, this melt being a melt that has leaked out from the fine hole
61
a
of the bottom of the platinum crucible
61
, and surface tensile thereof, so as to grow a crystal.
In general, monocrystals such as LN, LT and LGS are grown by the rotation pulling method (CZ method), and LBO is mainly grown by the vertical Bridgman method (VB method). LBO can be however grown by the CZ method.
However, conventional monocrystal growing methods, which are represented by the CZ method, have the following problems.
In connection with the melting points of raw materials, it is general that a platinum crucible is necessary for the growth of LN and an iridium crucible is necessary for the growth of LT and LGS. In connection with crystal size, a crucible of about 4 kg and a crucible of about 5 kg are necessary for the growth of, e.g., a crystal of 3 inches in diameter and a crystal of 4 inches in diameter, respectively. If an after-heater is used to keep growing temperature constant, a noble metal, such as platinum and iridium, of 1-2 kg becomes necessary. Since such a noble metal is used in a large amount, a large burden is imposed from the viewpoint of costs.
The conventional methods are in a so-called batch manner, in which it is necessary that not only a monocrystal to be pulled from a crucible and grown but also a considerably excess monocrystal are melted in the crucible and the total amount thereof is kept over the melting point. Therefore, there are limitations in enlarging the diameter of a pulled crystal and making the crystal long. Furthermore, electric power consumed with the heater and the like increases largely as the crystal becomes larger.
Crystals such as LN and LT have a wide solid solution area. Since their chemical composition is different from its congruent melt composition, their composition easily changes between the initial and final periods of the growing of the crystals. If, for example, temperature and the molar ratio (%) of lithium oxide (LiO
2
) are expressed by a vertical axis and a horizontal axis, respectively, the state diagram (phase diagram) of LT is as shown in FIG.
9
. When the monocrystal having varied compositions are used to make SAW devices, their propagation speeds and piezoelectric constants are scattered, so as to result in a drop in the yield of products.
In the CZ method, reacting treatments and high temperature treatments such as mixing of raw materials of tantalum pentaoxide Ta
2
O
5
and lithium carbonate Li
2
CO
3
, sintering, pulverizing and press are conducted as a treatment before a raw material is charged into a crucible. Therefore, the composition changes in the step of preparing the raw material by evaporation of lithium oxide Li
2
O or the like, which has a high vapor pressure. Besides, the composition of the resultant crystal changes by evaporation of a specific substance in the crystal growing step.
In the CZ method, a monocrystal rod is grown by the steps of sowing seeds, producing a shoulder portion, and then growing a body portion. However, it takes a long time to grow the body portion. Moreover, in order to obtain the body portions having a little dispersion of their diameters, a high-priced ADC (Automatic Diameter Control) device is necessary, so that costs for the production thereof become high.
As can also be. understood in
FIG. 9
, crystals such as LN and LT are crystallized into their congruent melt composition at T
1
, which is the highest temperature in the case that the liquid phase is changed into the solid phase. When the melt of Li and Nb is put into a crucible and a seed crystal is pulled upon the growth of a crystal by the CZ method, the initial growth of the crystal (the crystal containing Nb whose amount is larger than that of Li) advances in the state of its congruent composition, which causes easy crystallization. However, Li and Nb are beforehand mixed at a ratio of 1 to 1 in the crucible, so that the amount of Nb becomes smaller than that of Li in the melt as the growth of the crystal advances. As a result, a crystal having a content of Li larger than the Li content in the congruent composition is grown when crystallization advances slowly. That is, regions having different compositions are generated in a monocrystal.
Accordingly, in the CZ method a melt is bef
Kodaira Kohei
Nagai Kunihiko
Sakamoto Hideki
Tanaka Hiroyuki
Koda & Androlia
Kunemund Robert
Toyo Communication Equipment Co., Ltd.
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