JPH09241092A - Production of oxide single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good-quality oxide single crystal cbouing reduced crystal live defects by specifying the relation between the crystal growing rate and crystal breeding rate in growing an oxide single crystal by the Czochralski process. SOLUTION: A seed crystal is brought into contact with the melt in a crucible to give an oxide single crystal. e.g. lithium tantalate single crystal and lithium niobate single crystal by the Czochralski process. In this case, the crystal growing rate is controlled to 1/4 to 1/2 times the breeding rate for cell growth. Consequently, a solid-liq. equilibrium is stabilized at the breeding interface, the crystal face is not shifted, and a crystal defect is suppressed. Accordingly, a good quality oxide single crystal minimized in crystal defect is produced by the method, and an optical element such as LT and LNG substrates excellent in optical characteristic is obtained from the crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an oxide single crystal using the Czochralski method, and in particular, tantalic acid useful as a substrate for optical elements such as surface acoustic wave elements and optical waveguides. The present invention relates to a method for producing a lithium single crystal or a lithium niobate single crystal.

[0002]

2. Description of the Related Art Since oxide single crystals of lithium tantalate, lithium niobate, etc. have a high melting point, a precious metal crucible is used to melt the raw material contained therein by high frequency heating or resistance heating to melt it. It is generally performed by the Czochralski method in which a seed crystal is brought into contact with the hot water surface to pull up. Usually, the crystal growth rate is as fast as cell growth occurs to increase its productivity, and the amount of crystal growth per unit time is large, so the controllability of the straight body part is good and the crystal is eccentric. Nothing.

[0003]

However, single crystals of lithium tantalate, lithium niobate, and the like are grown in a congruent composition. For example, in the case of lithium tantalate, Ta is higher than the stoichiometric value in this congruent composition. Since the amount is large, the crystal contains some defects, and crystal defects are likely to occur. Especially,
When the growth rate is increased, a minute surface shift occurs in the grown crystal, which becomes a starting point and induces a large crystal defect. Regarding the shape, if the straight body portion has a large variation in diameter or if there is a large amount of eccentricity such as twisting, crystal strain accumulates in that portion, which causes crystal defects and the like. It is said that the device pattern of the surface acoustic wave device will be further miniaturized in the future, and in that case, crystal defects, which have not been a problem in the conventional SAW grade, become a problem during patterning. Further, in the case of an optical element such as an optical waveguide, there occurs a problem that the refractive index changes at a portion where a crystal defect exists, and the performance of the element is significantly deteriorated. Further, the seed crystals necessary for growing these crystals are produced by processing these crystals, defects in the crystals propagate to the growing crystals and induce large crystal defects, and cracks occur. Causes a problem that occurs.

[0004] The present invention has been made in view of the above circumstances,
An object of the present invention is to provide a method for producing an oxide single crystal, which can obtain a high-quality oxide single crystal such as lithium tantalate or lithium niobate with few crystal defects.

[0005]

Means for Solving the Problems and Modes for Carrying Out the Invention As a result of intensive studies for achieving the above-mentioned object, the present inventor has brought seed crystals into contact with a melt in a crucible to obtain lithium tantalate and niobium. When growing an oxide single crystal such as lithium oxide by the Czochralski method, by increasing the crystal growth rate to 1/4 to 1/2 times the growth rate that causes cell growth, oxidation with few crystal defects is performed. A single crystal of the crystal can be obtained, and in this case, particularly, in a portion where the deviation of the radius from the center of the crystal is within ± 7 mm and the diameter of the crystal is almost constant, the minor axis of the crystal diameter is X.
The present invention has been made based on the finding that a portion having a diameter variation of X / 16 mm or less when having a thickness of mm is a crystal for an optical element with few crystal defects and a small change in refractive index.

Therefore, according to the present invention, (1) when a seed crystal is brought into contact with a melt in a crucible to grow an oxide single crystal by the Czochralski method, the crystal growth rate is set to a growth rate that causes cell growth. A method for producing an oxide single crystal, which is characterized by being ¼ to ½ times, (2) A deviation of a radius from a crystal center in the oxide single crystal obtained by the production method of (1) above. Is within ± 7 mm, and the diameter of the crystal is almost constant, and when the minor axis of the crystal diameter is X mm, the diameter variation should be within X / 16 mm. And (3) the method for producing an oxide single crystal according to the above (1) or (2), wherein the oxide single crystal is a lithium tantalate single crystal or a lithium niobate single crystal.

The present invention will be described in more detail below.

The method for producing an oxide single crystal of the present invention is carried out by a pulling method based on the Czochralski method, and it can be effectively used particularly for producing a lithium tantalate or lithium niobate single crystal.

In this case, the raw material composition, the single crystal pulling conditions,
As the device and the like, known methods and devices can be used,
In the present invention, the crystal growth rate is 1/4 to 1/2 times, preferably 1/3 to, the cell growth rate.
Crystal growth is performed in a range of ½ times, which slows down the growth rate, stabilizes the solid-liquid equilibrium at the growth interface, and prevents the occurrence of crystal defects without causing crystal plane misalignment. be able to. On the other hand, when the crystal growth rate is less than 1/4 times the cell growth rate, the density of crystal defects is not decreased and only the productivity is deteriorated. Further, if the growth rate is larger than 1/2 times the minimum rate for generating cell growth, the solid-liquid equilibrium becomes unstable,
The density of crystal defects increases sharply. The minimum growth rate at which cell growth occurs is about 8 mm / H for large-diameter crystals of 3 inches and 4 inches, for example.

In the present invention, the growth rate is set to 1/4 to 1/2 times the cell growth rate in this way. By lowering the growth rate, the amount of crystal growth per unit time is increased. In the automatic control method using the weight change, the controllability of the straight body may be deteriorated, and the shape may deviate from a predetermined shape, resulting in distortion and crystal defects.

In this case, in forming the oxide single crystal obtained at the above growth rate, the deviation of the radius from the crystal center is ±.
If the diameter is within 7 mm, the shape is not twisted, and when the minor axis of the crystal diameter is X mm, the diameter fluctuation of the diameter is X.
If it exceeds / 16 mm, thermal strain is accumulated due to the diameter variation and the solid-liquid interface becomes unstable, so that the density of crystal defects increases. Therefore, from such a point, the deviation of the radius from the center of the crystal is ± 7 mm, preferably within ± 4 mm, and the diameter variation of the diameter is X / 16 mm or less, preferably X / 18 mm.
It is preferable to collect crystals within. By using this collected single crystal, it is possible to obtain a crystal of an optical element with a small change in refractive index.

[0012]

According to the present invention, it is possible to obtain a good-quality single crystal with extremely few crystal defects, and from this crystal, an LT or LN substrate having excellent optical characteristics can be provided.

[0013]

EXAMPLES The present invention will be described below in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[Examples 1 to 3 and Comparative Examples 1 and 2] A lithium tantalate single crystal was pulled using the apparatus shown in FIG. Here, in FIG. 1, 1 is a refractory crucible stand, 2
Is an alumina base, 3 is a crucible, 4 is a heat insulating material, 5 is a heat insulating material 4.
A reflector support made of alumina disposed on the upper end of the, 6 is a reflector, 7 is an after-heater, 8 is a heating coil, 9 is a melt, 10 is a seed holder, 11 is a seed crystal, 12 is a growing crystal, Reference numeral 13 indicates a lid.

In an iridium crucible having a diameter of 150 mm and a height of 150 mm, 10000 g of raw material (Li / Ta = 0.
943 (molar ratio)) and charged by the Czochralski method with a diameter of 80 mm, a straight body length of 130 mm, and a weight of 6500 g.
The single crystal of lithium tantalate was pulled up. First,
It was confirmed that cell growth occurred at the bottom of the crystal by pulling up at a growth rate of 8 mm / H. Next, the crystal was grown at the growth rate shown in Table 1, cut into both end faces and polished to perform single domainization, and the presence or absence of defects and the number of defects were examined by visible light and polarized light.
The results are shown in Table 1.

[0016]

[Table 1]

From the results shown in Table 1, it was confirmed that a lithium tantalate single crystal with few defects can be obtained by setting the crystal growth rate to a range of 1/4 to 1/2 times the growth rate that causes cell growth. Was done.

Next, crystal growth (diameter 80 mm) was carried out under the conditions of Example 2 above, and the shape of the obtained single crystal (diameter diameter variation, radius deviation from crystal center) and defects were examined. . Table 2 shows the result of the diameter variation of the diameter, and Table 3 shows the result of the deviation of the radius from the crystal center.

[0019]

[Table 2]

[0020]

[Table 3]

From the results of Tables 2 and 3, No. 1-3 and N
o. It was confirmed that the defects of 5 and 6 had few defects.

The same result was obtained when the lithium niobate single crystal was pulled up.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a single crystal pulling apparatus used for carrying out the present invention.

[Explanation of symbols]

 1 crucible table 2 alumina table 3 crucible 4 heat insulating material 5 support 6 reflector 7 after-heater 8 heating coil 9 melt 10 seed holder 11 seed crystal 12 growing crystal 13 lid

Claims (3)

[Claims] 1. When a seed crystal is brought into contact with the melt in a crucible to grow an oxide single crystal by the Czochralski method, the crystal growth rate is 1/4 to 1/2 of the growth rate that causes cell growth. A method for producing an oxide single crystal, which is characterized by doubling. 2. An oxide single crystal obtained by the method according to claim 1, which has a radius deviation from the crystal center of ± 7 mm or less and a diameter of the crystal which is substantially constant. A method for producing an oxide single crystal, characterized in that, when a minor axis of a crystal diameter is X mm, a portion whose diameter variation is within X / 16 mm is sampled. 3. The method according to claim 1, wherein the oxide single crystal is a lithium tantalate single crystal or a lithium niobate single crystal.