JP2809363B2 - Method for producing lithium tetraborate single crystal - Google Patents

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 a lithium tetraborate single crystal, and more particularly to a method for producing a lithium tetraborate single crystal which is useful for producing a SAW device.

[0002]

2. Description of the Related Art Conventionally, as a single crystal material for a SAW device, lithium tantalate, lithium niobate, quartz,
Lithium tetraborate and the like have been put into practical use. Among these single crystals, lithium tetraborate has a relatively large electromechanical coupling coefficient and has very small characteristic change due to fluctuations in the peripheral temperature of the device. , Cordless phones,
It is considered to be a useful material for filters used in automobile telephones.

[0003] This lithium tetraborate single crystal is
In general, it is grown from the melt by the Czochralski method or the Bridgman method, and this single crystal is processed into a cylindrical shape.
Furthermore, it is cut into a wafer with a certain crystal plane orientation,
It is provided for various devices in the form of a polished substrate,
For example, a SAW device is manufactured by forming an electrode mainly made of Al on a wafer substrate manufactured in this manner and then cutting out a square chip. The growth of a lithium tetraborate single crystal by the Bridgman method is performed. As an example, a method has been reported in which a platinum crucible with a small diameter seed tube is used to grow at a very low growth rate of 0.3 mm / hour or less (J. C.).
rytal Growth, 99 (1990), 811).

[0004]

However, according to the Bridgman method, a single crystal of lithium tetraborate is prepared from the melt raw material by 0.5 mm.
If the single crystal is grown at a rate higher than / h, there is a problem in that inclusions in the form of bubbles are mixed into the single crystal and crystal defects are likely to occur. Therefore, for the purpose of preventing the generation of the cellular inclusions, it has been proposed that the raw material should have a loss on ignition of 0.01% by weight or less (see Japanese Patent Application Laid-Open No. 58-55398). This method alone was insufficient to suppress cell growth.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a lithium tetraborate single crystal which has solved the above disadvantages, and a method for growing a lithium tetraborate single crystal from a melt by the Bridgman method. In the above, the temperature gradient at the growth point of the crystal was 35 to 70 ° C./cm, and the moving speed of the crucible was 0.5.
1.21.2 mm / hour.

That is, the present inventors have studied various methods for growing a lithium tetraborate single crystal for a SAW device, and changed the temperature gradient of the crystal growth point and the moving speed of the crucible to obtain these values and the difference in the crystal. The relationship between the observed abundant inclusions and the abundance was examined in detail. Assuming that the temperature gradient of the crystal growth point was X ° C / cm and the moving speed of the crucible was Ymm / hour, the values of X and Y were determined. Found that a lithium tetraborate single crystal in which the generation of cellular inclusions was suppressed was obtained when was within the range of the hatched portion in FIG. 1 and completed the present invention.
This will be described in more detail below.

[0007]

The present invention relates to a method for producing a single crystal of lithium tetraborate, which comprises a method of growing a single crystal of lithium tetraborate from a melt by the Bridgman method. C./cm, and the crucible moving speed is 0.5 to 1.2 mm / hour, but according to this, it is possible to prevent the inclusion of bubbles in the crystal and the generation of cracks. Advantages are given.

The production of the lithium tetraborate single crystal according to the present invention is carried out by the Bridgman method as described above. This Bridgman method is known, and is performed, for example, by the apparatus shown in FIG. FIG. 2 is a longitudinal sectional view of a single crystal growth method by the Bridgman method. The electric furnace 1 is made of a wool-like heat insulating material 2 made of alumina silicate, which is a refractory, and a kantalum wire or a silicon knit. A heater section 3 composed of a heating element is housed in a high-purity alumina pipe 4, and a platinum crucible 6 is stored on a crucible table 5 made of silicon carbide or quartz.

In the production of a single crystal of lithium tetraborate by this apparatus, a predetermined amount of a polycrystal of lithium tetraborate as a raw material is charged into a platinum crucible 6, and this is heated by a heater unit 3.
It is heated to 920 to 980 ° C and melted. However, since the lithium tetraborate polycrystal is preferably as pure as possible, it is usually made to have a purity of 99.95% or more.
This may be a single crystal grown in advance as a raw material.

The melt melted in the platinum crucible 6 is converted into a single crystal in the crucible 6 in accordance with the orientation of the seed crystal 8 stored in the seed tube 7. Is carried out by lowering the crucible table 5 supporting the cradle with the vertical drive unit 9. However, in this case, it is necessary to reduce the thickness of the boundary layer between the melt and the crystal as much as possible in order to suppress the generation of bubble-like inclusions. When the moving state of the crucible was variously changed and the state of generation of the cellular inclusions at this time was examined, the result shown in FIG. 1 was obtained.

That is, as shown in FIG. 1, when a lithium tetraborate single crystal is grown from a melt by the Bridgman method, the temperature gradient at the growth point of this crystal is changed from 20 ° C. to 80 ° C. Movement speed from 0.5mm / hour to 2.0mm /
The results as shown in FIG. 1 were obtained by examining the occurrence of bubbles and the presence or absence of cracks at that time. The temperature gradient was as follows. When the temperature is 70 ° C / cm or higher, thermal strain remains in the crystal and cracks occur. When the crucible moving speed is slower than 0.5 mm / h, productivity is reduced. If the speed is higher than 1.2 mm / h, bubbles will not be able to escape sufficiently, so the temperature gradient should be 35-70 ° C / c.
It has been confirmed that the crucible moving speed is preferably in the range of 0.5 to 1.2 mm / hour.

[0012]

Next, examples of the present invention will be described. Examples 1 to 6, Comparative Examples 1 to 5 A 4N-purity lithium tetraborate polycrystal prepared at a predetermined molar ratio was weighed and calcined at about 500 ° C., for a Bridgman diameter of 50 mm and a length of 100 mm. 500g in a platinum crucible
Insert a lithium tetraborate single crystal with a diameter of 5mm, a length of 50mm and an orientation of <110> into the seed tube so as not to dissolve the seed.
The raw materials were dissolved at 920 ° C or higher.

Next, the temperature of the bias of the multi-zone single crystal growing furnace is changed to change the temperature gradient at the crystal growth point within the range shown in FIG. 1, and also change the crucible descending speed within the range shown in FIG. The crucible descending speed was also changed within the range shown in FIG. 1. After moving the crucible by 100 mm to grow a single crystal, the temperature was lowered and the platinum crucible was broken to take out the lithium tetraborate single crystal. When the presence or absence of cellular inclusions and the presence or absence of cracks were examined, the results shown in Table 1 were obtained. In addition,
When the crucible was moved at 0.3 mm / hour, the productivity was poor because it was too slow, and it was not practical.

[0014]

[Table 1]

[0015]

The present invention relates to a method for producing a lithium tetraborate single crystal, which comprises a method for growing a lithium tetraborate single crystal from a melt by the Bridgman method as described above. 35 ° -70 ° C temperature gradient
/ Cm, and a crucible moving speed of 0.5 to 1.2 mm / hour, which makes it possible to obtain a lithium tetraborate single crystal in which inclusion of cellular inclusions is prevented. Therefore, there is provided an industrial advantage that a tetraborate single crystal useful for producing a SAW device can be easily obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the operating temperature range of a crystal, the moving speed of a crucible, and the abundance of cellular inclusions during the production of a lithium tetraborate single crystal by the Bridgman method.

FIG. 2 is a longitudinal sectional view of a lithium tetraborate single crystal manufacturing apparatus according to the Bridgman method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electric furnace, 2 ... Insulation material, 3 ... Heater part, 4 ... High-purity alumina pipe, 5 ... Crucible stand, 6 ... Platinum crucible, 7 ... Seed tube part, 8 ... Seed crystal , 9 ... vertical drive.

Claims (1)

(57) [Claims] 1. A method for growing a lithium tetraborate single crystal from a melt by the Bridgman method, wherein the temperature gradient of the crystal growth point is 35 ° -70 ° C./cm, and the moving speed of the crucible is
A method for producing a lithium tetraborate single crystal, wherein the thickness is 0.5 to 1.2 mm / hour.