JPH09208380A - Production of oxide single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an oxide single crystal capable of readily forming a crack by a thermal strain in good productivity and yield by growing specified growing edges under specific conditions when growing the shoulder part of the oxide single crystal from a melt by a pulling up method. SOLUTION: The shoulder part of a oxide single crystal is grown so that the dispersion in width of growing edges may be the maximum one of the growing edges of <4 times based on the minimum one thereof formed in the (012) face group at the time of forming the shoulder part of the single crystal when producing the single crystal of lithium niobate or lithium tantalate. For example, a doughnut platy reflector 6 made of iridium is arranged in a crucible 3 made of iridium and a cylindrical after-heater 7 made of iridium is further arranged on the reflector 6. Raw materials are placed in the crucible 3 so that the composition ratio may be Li/Ta=0.943 (molar ratio), melted, then pulled up so that the dispersion in width of growing edges may be the maximum width of growing edges of <4 times based on the minimum one thereof formed in the (012) face group by using a Czochralski method (CZ method). Thereby, the lithium tantalate having 20 deg. angle θ of a conical portion of the shoulder part is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing an oxide single crystal using the Czochralski method, and particularly to a lithium tantalate single crystal and a lithium niobate single crystal used in a surface acoustic wave device. The present invention relates to a manufacturing method of.

[0002]

2. Description of the Related Art Lithium tantalate and lithium niobate oxide single crystals are generally manufactured by the Czochralski method, which is a method for manufacturing a noble metal crucible. Surrounded by refractories,
After heating and melting the raw material, the seed crystal is immersed in the melt, and the seed crystal is pulled while rotating.

In this case, as shown in FIG. 1, the shape of the shoulder of the crystal A is a cone, and the angle θ of the cone B is 45 °.
Since it is said that a high-quality crystal with the least distortion and few defects such as dislocations can be formed, in the past, θ was usually made in the range of 30 to 60 °.

Further, in order to improve the productivity in crystal production, Japanese Patent No. 1406486 (Japanese Patent Publication No. 60-23560).
In order to prevent cracks from occurring even when the conical portion is reduced in number, the angle θ of the conical portion of the crystal shoulder should be 30 ° or less. A method of growing a single crystal so that the width is 10 to 30% of the crystal diameter is disclosed.

However, in the method of Japanese Patent No. 1406486 in which the growth edge is 10 to 30% of the crystal diameter, the temperature of the melt in the crucible is too low, resulting in a small temperature gradient.
The pulled crystal cannot be made longer. For this reason, there is a drawback that productivity in crystal production is poor.

The present invention provides a method for solving the above-mentioned problems during growth of an oxide single crystal, and in particular, cracks due to thermal strain during crystal growth of a lithium tantalate single crystal or a lithium niobate single crystal. An object of the present invention is to provide a method for producing an oxide single crystal for producing easily produced crystals with high productivity and high yield.

[0007]

Means for Solving the Problems and Embodiments of the Invention The inventors of the present invention have made various studies on the shape of the growth ridge and the incidence of cracks during the formation of the shoulder, and have investigated the width of the growth ridge and the crystal defect. By clarifying the relationship with the above, a method for producing an oxide single crystal having both good productivity and yield in a state where a temperature gradient capable of lengthening is large has been reached.

That is, when the present inventors manufacture oxide single crystals, particularly lithium niobate single crystals and lithium tantalate single crystals by the pulling method, when forming shoulders of these oxide single crystals, The variation in the width of the growth edge is (01
2) It was found that the growth should be performed so that the maximum width is less than 4 times the minimum width of the growth edge formed in the surface group,
The present invention has been completed. The width of the growth edge in the present invention is the length in the direction perpendicular to the crystal growth direction and is also called the thickness.

More specifically, crystal growth proceeds at about the same rate when a certain degree of supercooling is applied to the melt. For this reason, when the output of the heating unit is reduced to a certain value such that the melt is in a supercooled state after the end of the neck portion, the crystal diameter is maintained at a constant rate after a lapse of time depending on the heat capacity around the crucible. Begins to spread. The temperature gradient in the radial direction of the melt monotonically increases in the crucible direction from the center of the melt. Reduce the output. In order to make the spread of the crystal diameter constant, it is necessary to control the degree of supercooling of the crystal growth portion to be constant.

However, it is difficult to directly measure the melt temperature during crystal growth, and if the supercooling degree changes greatly, the spread of the crystal diameter becomes non-uniform, and thermal strain remains in the crystal.
The crystals are likely to crack.

The present invention provides a solution to this problem. That is, in the supercooled state of the melt, a peculiar surface of the crystal appears as a facet in a characteristic direction. With lithium niobate single crystal, lithium tantalate single crystal, etc.,
Within that plane, the crystal growth rate is the fastest, the slowest in the normal direction of the plane (012), and its equivalent plane is crystallized. Since the facet surface such as the growth edge changes depending on the degree of supercooling, it serves as a barometer of the degree of supercooling during shoulder growth. Therefore, the output may be controlled so that the variation in the width of the growth edge is within a certain range.

On the other hand, in the case of the growth edge width as disclosed in Japanese Patent No. 1406486, since the melt temperature in the crucible is too low, the temperature gradient in the pulling axis direction becomes small and the pulling crystal becomes long. Can not. Therefore, when increasing the temperature gradient to increase the pulling length of the pulled crystal,
The growth edge width is 10% or less of the crystal diameter.

In order to prevent residual strain from being applied during crystal growth even when the growth edge width is 10% or less of the crystal diameter due to such a large temperature gradient, this growth edge variation It was found that it is important to make the maximum width less than 4 times the minimum width of the growth edge.

Therefore, in the present invention, when the shoulder portion of the oxide single crystal is grown from the melt by the pulling method, (01
2) To provide a method for producing an oxide single crystal, which is characterized in that the growth is formed so that the variation in the width of the growth ridge formed in the surface group is less than 4 times the maximum width of the minimum width of the growth ridge. .

The method for producing an oxide single crystal of the present invention is to pull up and grow an oxide single crystal by the Czochralski method as described above, and particularly, to produce a lithium tantalate single crystal and a lithium niobate single crystal. In the present invention, when forming the shoulder portion of these oxide single crystals, the width of the growth edge formed in the (012) plane group crystallized in the shoulder portion The output is controlled so that the maximum width is less than 4 times, preferably 1.5 times or less with respect to the minimum width of the growth ridge, whereby the output fluctuation of the heating portion can be suppressed, and It becomes possible to suppress the occurrence of cracks.

As described above, in the shoulder forming process of the present invention,
Output control is performed to suppress the variation in the width of the growth edge,
It is only necessary to form a growth edge having a width as uniform as possible. Also,
Forming a growth edge with a uniform width means that the degree of supercooling of the crystal growth portion becomes constant, and as a result, the output fluctuation of the heating portion is suppressed, so that the step difference during shoulder formation, Also, cracks due to thermal strain can be prevented.

The minimum width of the growth edge is appropriately selected, but in the method of the present invention, the temperature gradient of the melt is increased in order to increase the pulling length of the crystal, and the (012) plane group is formed. It is preferably used when the minimum width of the growth layer is 10% or less of the crystal diameter.

In the present invention, the raw material for obtaining the oxide single crystal, the pulling conditions other than those mentioned above, the apparatus and the like can be carried out by using known methods and apparatuses.

[0019]

According to the present invention, as described above, when a seed crystal is brought into contact with the melt to form an oxide single crystal by the Czochralski method, it is formed on the shoulder (012).
By controlling the output so that the width of the growth edge formed in the surface group is less than 4 times the maximum width with respect to the minimum width, a single crystal with few crystal defects can be obtained with a high yield.

[0020]

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

A iridium crucible having a diameter of 150 mm and a height of 150 mm is provided with a donut plate-shaped reflector having an outer diameter of 160 mm and an inner diameter of 105 mm, and a cylindrical after-heater made of iridium having a diameter of 150 mm and a height of 180 mm. Was placed.

In this crucible, the composition ratio is Li / Ta = 0.
10000g of raw material prepared to be 943 (molar ratio)
After melting, the maximum width is less than 4 times the minimum width of the growth ridge formed in the (012) plane group by the Czochralski method, and tantalic acid having a diameter of 80 mm and a weight of 6500 g. The lithium single crystal was pulled up. In this case, the angle θ of the conical portion of the shoulder was 20 °.

For comparison, the lithium tantalate single crystal was pulled up in the same manner as above except that the maximum value was set to 4 times or more the minimum width of the growth edge.

Table 1 shows the results of evaluation of the occurrence of cracks in the obtained single crystal.

[0026]

[Table 1]

As described above, when the maximum width was less than 4 times the minimum width of the growth ridge formed on the (012) plane group, no crack was generated. On the other hand, when the maximum width was 4 times or more, the result was that all cracks occurred.

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

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a pulled state of an oxide single crystal.

FIG. 2 is a schematic sectional view of a single crystal pulling apparatus used for carrying out the present invention.

[Explanation of symbols]

 A crystal B conical section 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 (2)

[Claims] 1. When growing a shoulder portion of an oxide single crystal from a melt by a pulling method, the variation in the width of the growth edge formed in the (012) plane group is greater than the minimum width of the growth edge. A method for producing an oxide single crystal, which comprises growing the oxide single crystal to be less than 4 times. 2. The method according to claim 1, wherein the oxide single crystal is lithium tantalate or lithium niobate.