JPH06247796A - Lithium tetraborate single crystal - Google Patents

Abstract

PURPOSE:To form a high quality lithium tetraborate single crystal having an extremely reduced bubble content. CONSTITUTION:Lithium tetraborate is melted by heating in an atmosphere of gaseous He or hydrogen and a crystal is grown from the resulting melt to produce the objective lithium tetraborate single crystal having an extremely reduced bubble content.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium tetraborate single crystal used as a substrate material for a piezoelectric device such as a surface acoustic wave element and a method for producing the same.

[0002]

2. Description of the Related Art A surface acoustic wave device using a lithium tetraborate single crystal as a substrate can realize a zero temperature coefficient by cutting the substrate of the lithium tetraborate single crystal by selecting an appropriate cut angle with respect to a crystal axis. It has been known. This lithium tetraborate single crystal is produced by the Czochralski method (CZ method) in which a seed crystal is slowly and continuously pulled up from a lithium tetraborate melt in a rotated crucible, or in a furnace having a temperature gradient. The crucible containing the melt was moved, and it was generally produced by the Bridgman method in which the lithium tetraborate melt was solidified from the tip of the crucible containing the seed crystal.

However, the melt of lithium tetraborate has a relatively high viscosity, and when the lithium tetraborate pieces as the raw material are melted, the bubbles mixed in the melt cannot easily move, so that the bubbles remain in the crystal. However, there is a problem that when a substrate is cut out from a crystal containing bubbles, the bubbles appear as holes and thus cannot be used as a substrate of a piezoelectric device.

Further, the bubble also causes a linear defect and has a drawback that the quality of the grown crystal is extremely deteriorated. Therefore, in the conventional crystal growth method as described above, the crystal growth was carried out by extremely slowing down the growth rate, but of course the productivity is deteriorated and it is still possible to sufficiently remove the bubbles. However, it was expensive because only a small part of the grown crystal columns could be used.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to eliminate the drawbacks of the conventional lithium tetraborate single crystal as described above, and provides a good quality lithium tetraborate single crystal in which the content of bubbles is limited. With the goal.

[0006]

SUMMARY OF THE INVENTION To achieve the above object, the present invention produces a lithium tetraborate single crystal in which bubbles in the crystal are limited by heating and melting lithium tetraborate in a helium atmosphere and then growing the crystal. It is a thing.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. FIG. 1 shows an embodiment of a Bridgman furnace for growing a lithium tetraborate single crystal according to the present invention. The Bridgman furnace 1 includes a platinum crucible 4 fixed to the tip of a wire 3 extending from a wire winding device 2, a heater 5 arranged so as to surround the crucible 4, and a tube 6 for shielding these from the outside world. It consists of and. still,
The crucible 4 can be moved in the vertical direction in the figure by adjusting the feeding length of the wire 3 by the wire winding device 2, and the heater 5 has a temperature gradient so that the lower side in the figure has a low temperature. .

First, helium gas was introduced into the tube 6 from the gas inlet 7 at a rate of 1 (Nl / min) to replace the inside of the tube 6 with a helium atmosphere, and then a lithium tetraborate piece 8 as a raw material was filled. The crucible 4 is heated by the heater 5
In the figure, the raw material is melted by heating it to a temperature of 917 ° C. or higher, which is the melting point of lithium tetraborate. Then, by moving the crucible 4 downward in the figure at a predetermined speed, the melt of lithium tetraborate is gradually cooled from the seed crystal 9 side previously inserted in the small diameter portion at the lower end of the crucible, and then cooled. The crystal grows. The growing speed is adjusted by the moving speed of the crucible 4, that is, the feeding of the wire 3. In addition, helium gas was continuously supplied from the start of heating to the raw material to after the crystal growth.

FIG. 2 shows the relationship between the state of bubbles remaining in the lithium tetraborate single crystal grown in the above-mentioned Bridgman furnace and the growth rate when the helium gas was introduced into the furnace and the atmosphere was introduced. The remaining state of the bubbles is what was observed visually by shining bright light on the grown single crystal. As described above, when a lithium tetraborate single crystal is grown in the atmosphere, the bubbles remaining in the crystal decrease as the growth rate is slowed down. On the other hand, it was found that when growing in a helium atmosphere, bubbles remaining in the crystal were not observed regardless of the growth rate of the crystal.

It is speculated that this is because helium molecules are extremely small, and therefore helium molecules diffuse into the melt or single crystal of lithium tetraborate. In other words, the molecules of nitrogen or oxygen in the atmosphere are relatively large and cannot diffuse in lithium tetraborate, so it is considered that they will remain in the crystal as bubbles, whereas they will be grown in a helium atmosphere. The obtained lithium tetraborate single crystal contains diffused helium molecules, but the content is at most 1000 ppm, and as is well known, helium is a stable inert atom. No deterioration of the characteristics was observed even when used as a substrate. Therefore, as described above, only by performing the steps from heating and melting to crystallization in a helium atmosphere,
It becomes possible to obtain a lithium tetraborate single crystal in which bubbles remain in the crystal to the limit without slowing the growth rate.

Although the present invention has been described above by taking as an example a crystal grown by introducing helium gas into a Bridgman furnace, the present invention is not limited to this, and melting and crystallization in a helium atmosphere are performed. Any crystal growth method may be used as long as it can be converted into, for example, the Czochralski method or the like. Further, if a single crystal is grown using a lithium tetraborate ingot obtained by melting and cooling lithium tetraborate pieces in a helium atmosphere in advance, it becomes possible to more efficiently produce a lithium tetraborate single crystal. .

Further, although there is a restriction that facilities such as explosion-proof are required and a carbon crucible must be used instead of a platinum crucible because a reducing atmosphere is created, hydrogen gas having a small molecule like helium gas is used. It is easily conceivable that a bubble-free crystal can be obtained even when the helium gas is introduced into the furnace instead of the helium gas.

[0013]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, it is possible to limit the inclusion of air bubbles in the crystal without any particular difficulty, and to obtain a high-quality lithium tetraborate simple substance at a relatively high growth rate. It has a remarkable effect in producing crystals.

[0014]

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a Bridgman furnace for growing a lithium tetraborate single crystal according to the present invention.

FIG. 2 is a diagram showing the relationship between the state of bubbles remaining in a lithium tetraborate single crystal and the growth rate.

[Explanation of symbols]

 1 ... Bridgman furnace 3 ... Wire 4 ... Crucible 5 ... Heater 6 ... Tube 7 ... Gas inlet 8 ... Lithium tetraborate piece 9 ... Seed crystal

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H03H 9/25 C 7259-5J

Claims (1)

[Claims] 1. A lithium tetraborate single crystal produced by heating and melting lithium tetraborate, followed by gradual cooling and crystal growth, which contains fine molecules such as helium gas or hydrogen gas as a component. A lithium tetraborate single crystal, characterized in that the gas is diffused into the crystal by performing the above steps in a gas atmosphere.