JPH07138095A - Method of growing single crystal of lithium borate - Google Patents

Abstract

PURPOSE:To obtain bubble-free single crystal of lithium borate. CONSTITUTION:The starting lithium borate is melted in a dry atmosphere and the single crystal of lithium borate is allowed to grow from the starting borate, as the moisture content is kept less than 25ppm in the molten material. At this time, the moisture content is controlled to less than 100ppm in the dry atmosphere. This process removes the limitations in the crystal growth rate, temperature gradient and the like which have been applied to conventional processes and enables the new process to select the growth conditions such as growth rate or temperature gradient so that increased productivity and improved quality are attained.

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 borate single crystal, for example, a technique useful when applied to the production of a lithium tetraborate single crystal used as a substrate material of a surface acoustic wave device.

[0002]

2. Description of the Related Art Lithium borate single crystals have recently attracted attention as a substrate material for surface acoustic wave devices due to their excellent properties such as having a zero temperature coefficient and a crystal orientation with a high electromechanical coupling coefficient. The production of such a lithium borate single crystal is generally carried out by the Czochralski method or the Bridgman method, but there is a drawback that bubbles are easily generated in the obtained lithium borate single crystal.

The cause of the bubbles is that the raw material melt contains water in excess of the saturated water content of the lithium borate single crystal, and the normal crystal growth rate (0.2 to 0.5 mm).
/ Hour) means that the water in the raw material melt is not sufficiently diffused. That is, when the crystal is crystallized from the raw material melt, the water content exceeding the saturated water content of the single crystal is discharged to the liquid phase side of the solid-liquid interface and remains near the solid-liquid interface. Then, the water in the vicinity of the solid-liquid interface is taken into the growing crystal by compositional supercooling, and remains in the crystal as bubbles during crystallization.

In order to overcome the above drawbacks, it has been proposed to grow a lithium borate single crystal with a water content of 1% (10000 ppm) or less in the growth atmosphere (JP-A-5-201797). . Moreover, in the proposal, the water content in the lithium borate raw material is 0.2%.
It has also been proposed to make it (2000 ppm) or less.

According to the above proposal, the dissolution of water in the growth atmosphere into the raw material melt is reduced, and the water content is 20%.
It is said that a lithium borate single crystal having less than 0 ppm of bubbles can be obtained.

[0006]

However, in one embodiment described in the above-mentioned Japanese Patent Application Laid-Open No. 5-201797, a water content of 1200 ppm is obtained in a growth atmosphere having a water content of 0.07% (700 ppm). The water content in the lithium borate single crystal obtained from the lithium borate raw material was 12
It is 0 ppm, and although the bubbles present in the single crystal are few, they are not completely nonexistent. That is, it is extremely difficult to completely eliminate bubbles even with the above proposal. Therefore, in order to provide a high-quality substrate for a surface acoustic wave device, it is necessary to completely eliminate the bubbles in the lithium borate single crystal, and there has been an urgent need to develop a manufacturing method for realizing it.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a lithium borate single crystal containing no bubbles.

[0008]

In order to achieve the above object, the present inventors measured the water content by infrared absorption method for the transparent portion of lithium borate single crystal grown under various conditions. As a result, it was found that the water content was 25 ppm under any growth condition. Based on this finding, the present inventors have found that the saturated water content of a lithium borate single crystal is 25 ppm. Therefore, if the water content in the raw material melt is kept at 25 ppm or less during the growth process of the single crystal, It was thought that it is possible to prevent the amount of water in the vicinity of the solid-liquid interface of the liquid phase from becoming excessive during crystal growth, and the present invention has been completed.

That is, according to the present invention, the lithium borate raw material is melted in a dry atmosphere, and the molten lithium borate raw material melt is maintained at a water content of 25 ppm or less, while the lithium borate raw material melt is melted with boric acid. It is proposed to grow a lithium single crystal. The water content in the dry atmosphere is preferably 100 ppm or less.

The upper limit of the water content in the lithium borate raw material melt held during the production of the lithium borate single crystal is 25 ppm, which is obtained from the experiment conducted by the inventors as described above. It is based on the fact that the saturated water content of the obtained lithium borate single crystal is 25 ppm. That is, if the water content in the raw material melt is lower than the saturated water content, the water in the vicinity of the solid-liquid interface of the liquid phase does not become excessive during crystal growth.

Examples of the lithium borate raw material include lithium borate powder, lithium borate glass (glassy lithium borate), and lithium borate crystals (polycrystal or single crystal) obtained by crystallizing them.

Further, the water content in the dry atmosphere is 1
According to an experiment conducted by the present inventors (Example 2 described later), the content of 00 ppm or less is a raw material obtained by melting a lithium borate raw material containing 25 ppm of water in a growth atmosphere having a water content of 100 ppm. 25ppm of water content in the melt throughout
The reason is that if the water content in the atmosphere is lower than 100 ppm, the water content in the raw material melt can be kept even lower.

Further, in the dry atmosphere, not only a dry air flow in which an inert gas such as nitrogen gas or argon having a water content of 100 ppm or less or air is flown, but also in a dry airtight state in which no gas is flown, in a vacuum state, It also includes a reduced pressure state.

[0014]

According to the above-mentioned means, a lithium borate raw material melt having a water content of 25 ppm or less is grown in a dry atmosphere having a water content of 100 ppm or less to grow a lithium borate single crystal. The dissolution of water in the melt is suppressed, and the amount of water contained in the raw material melt is maintained at 25 ppm or less throughout the growth of the crystal. Therefore, the water content in the vicinity of the solid-liquid interface of the liquid phase is kept below the saturated water content of the single crystal, and excessive water is prevented from being taken into the single crystal to obtain a single crystal having no bubbles.

[0015]

EXAMPLES The features of the present invention will be clarified below with reference to Examples and Comparative Examples. In Examples and Comparative Examples, a lithium borate raw material was placed in a growth vessel, the raw material was heated and melted, and a lithium borate single crystal was grown by the well-known Bridgman method.

Example 1 A raw material of lithium borate having a water content of 200 ppm was placed in a carbon crucible, which was vacuum-sealed in a quartz ampoule at a pressure of 3 × 10 ⁻³ Torr, and the quartz ampoule was heated in a heating furnace. It was placed inside and the starting material was heated and melted. After holding in that state for 10 hours, the quartz ampoule was taken out from the heating furnace, and the melt in the crucible was rapidly cooled and solidified to obtain lithium borate glass. When the amount of water contained in the lithium borate glass was measured by the infrared absorption method, the amount of water was 6 ppm. The carbon crucible has a function of removing water in the melt by reacting with carbon.

Next, a lithium borate single crystal was produced using the above lithium borate glass having a water content of 6 ppm as a raw material. The above lithium borate glass was placed in a carbon growth vessel, and the growth vessel was placed in a crystal growth furnace in a dry nitrogen gas stream (flow rate 1000 cc / min) with a water content of 25 ppm to heat and melt the lithium borate glass. After that, the raw material melt was gradually cooled from one end to grow a lithium borate single crystal. At that time, the growth rate was 0.3 mm / hour, and the temperature gradient near the melting point measured near the growth vessel in the furnace was 16 ° C./cm.

When the amount of water contained in the obtained lithium borate single crystal was measured by an infrared absorption method, the water content was 6 ppm, and no bubbles were observed.

Example 2 A raw material of lithium borate having a water content of 200 ppm was placed in a carbon crucible, and a dry nitrogen gas stream having a water content of 100 ppm (flow rate 100
(0 cc / min) and placed in a heating furnace to heat and melt the starting material. After holding in that state for 10 hours, the crucible was taken out from the heating furnace, and the melt in the crucible was rapidly cooled and solidified to obtain a lithium borate glass. When measured by an infrared absorption method, the water content of the lithium borate glass was 25 ppm.

Next, using the above-mentioned lithium borate glass having a water content of 25 ppm as a raw material, a lithium borate single crystal was manufactured in the same manner as in Example 1. The water content of the nitrogen gas flown into the crystal growth furnace was 100 ppm, and the flow rate was 1000 cc / min. The growth rate was 0.3 mm / hour and the temperature gradient was 16 ° C./cm. Other growth conditions and the like were the same as in Example 1 above.

When the amount of water contained in the obtained lithium borate single crystal was measured by an infrared absorption method, the water content was 20 ppm and no bubbles were observed.

(Comparative Example) A lithium borate single crystal was produced using lithium borate glass having a water content of 70 ppm as a raw material and a platinum crucible under the same growth conditions as in Example 2 above. Many bubbles were observed in the single crystal.

From the above Examples 1, 2 and Comparative Example, a bubble-free lithium borate single crystal was produced by using a lithium borate raw material having a water content of 25 ppm or less in a dry atmosphere having a water content of 100 ppm or less. It was confirmed. As a result, the restrictions such as growth rate and temperature gradient that were conventionally specified to prevent bubble generation are eliminated, and growth conditions such as growth rate and temperature gradient are set so as to achieve higher productivity and higher quality. It becomes possible to select.

Needless to say, the present invention is not limited to the above embodiments. For example, the method of setting the water content of the lithium borate raw material melt to 25 ppm or less is
The present invention is not limited to the above-mentioned examples, and may be a crystallized lithium borate crystal or may be kept in a dry atmosphere having a water content of 100 ppm or less for a long time. The crucible is not limited to carbon, but may be made of other material such as platinum. Further, the growth atmosphere of the single crystal is not limited to the nitrogen gas flow or the vacuum state, and as long as the water content is 100 ppm or less, an inert gas or air may be flown, or a dry airtight state in which no gas is flown. It may be under reduced pressure. Furthermore, other than the Bridgman method, another method such as the Czochralski method may be used.

[0025]

According to the method for producing a lithium borate single crystal according to the present invention, the amount of water contained in the raw material melt can be kept at 25 ppm or less throughout the growth process of the lithium borate single crystal. The water content in the vicinity of the solid-liquid interface of the liquid phase becomes equal to or less than the saturated water content of the single crystal, excessive water is prevented from being taken into the single crystal, and a single crystal without bubbles can be manufactured.

Claims (2)

[Claims] 1. A lithium borate raw material is melted under a dry atmosphere, and a lithium borate single crystal is produced from the lithium borate raw material melt while keeping the water content in the molten lithium borate raw material melt at 25 ppm or less. A method for producing a lithium borate single crystal, which comprises growing the single crystal. 2. The water content in the dry atmosphere is 10
It is 0 ppm or less, The manufacturing method of the lithium borate single crystal of Claim 1 characterized by the above-mentioned.