JP2739547B2 - Method for producing lithium borate 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 borate single crystal, and more particularly to a technique useful for producing a lithium tetraborate single crystal used as a substrate material for a surface acoustic wave device.

[0002]

2. Description of the Related Art In recent years, lithium borate single crystals have attracted attention as substrate materials for surface acoustic wave devices because of their excellent properties such as having a zero temperature coefficient and a crystal orientation having a high electromechanical coupling coefficient. The Bridgman method is known as one method for producing such a lithium borate single crystal, but has a drawback that bubbles are easily generated in the obtained lithium borate single crystal.

[0003] Here, the cause of the generation of bubbles is that the raw material melt contains water exceeding the saturated water content of the lithium borate single crystal, and crystal growth that can be employed when industrially producing a single crystal. At a speed (0.2 to 0.5 mm / hr), the diffusion of water in the raw material melt is not sufficiently performed. That is, when crystals are crystallized from the raw material melt, water 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 near the solid-liquid interface is taken into the growing crystal due to compositional supercooling, and remains in the crystal as bubbles during crystallization.

Therefore, in order to overcome the above-mentioned drawbacks, a lithium borate single crystal is used in a growth atmosphere having a water content of 1% (10000 ppm) or less using a lithium borate material having a water content of 0.2% (2000 ppm) or less. Has been proposed (Japanese Patent Laid-Open No. Hei 5-201797). According to the proposal, it is said that a lithium borate single crystal with a small water content of 200 ppm or less and few bubbles can be obtained.

[0005]

However, in one embodiment described in Japanese Patent Application Laid-Open No. Hei 5-201797, under a growth atmosphere having a water content of 0.07% (700 ppm), a water content of 1200 ppm was used. The water content in the lithium borate single crystal obtained from the lithium borate raw material is 12
It is 0 ppm, and although there are few bubbles present in the single crystal, it is not none at all. That is, even with the above proposal, it is extremely difficult to completely eliminate bubbles. Therefore, in order to provide a high-quality substrate for a surface acoustic wave device, it is necessary to completely eliminate bubbles in a 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 such circumstances, and an object of the present invention is to provide a method for producing a lithium borate single crystal containing no bubbles.

[0007]

Means for Solving the Problems To achieve the above object, the inventors of the present invention used the Bridgman method to vary the water content C, crystal growth rate v, and temperature gradient ΔT in a lithium borate raw material melt. A lithium borate single crystal was manufactured by changing the above conditions, and the obtained single crystal was examined for the presence or absence of air bubbles. The inventors have found that there is a correlation between the temperature gradients ΔT near the melting point, and have completed the present invention.

That is, according to the present invention, when a lithium borate raw material is heated and melted to grow a lithium borate single crystal from the molten lithium borate raw material melt, the water content C (unit: ppm), crystal growth rate v (unit: mm / hour) and temperature gradient ΔT (unit: ° C / c)
m) and the following formula: C ≦ (2.8 × ΔT−5.4) / v (1) is proposed to grow the lithium borate single crystal. .

The derivation of the above relational expression will be described.

In various lithium borate single crystal growth experiments (Examples 1 and 2 described later) performed by the present inventors, for example, the temperature gradient ΔT was set to 20 ° C./cm,
When a lithium borate raw material melt having a water content C of 250 ppm or 70 ppm was used, the crystal growth rate v was about 0.2 mm / hour or less at 250 ppm, and about 0.2 mm / hour at 70 ppm. If it is 7 mm / hour or less, it was found that there was no bubble in the obtained single crystal. When a single crystal was grown in the same manner with the temperature gradient ΔT set to 7 ° C./cm, no bubbles were generated. The crystal growth rate v was about 0.06 mm / hour or less at 250 ppm and 70 ppm. In this case, the crystal growth rate v was found to be about 0.2 mm / hour or less.

FIG. 1 shows the critical characteristics of the relationship between the water content C in the lithium borate raw material melt and the crystal growth rate v when a single crystal without bubbles is grown at each of the temperature gradients ΔT. From the figure, at the critical point where bubbles do not occur,
It is considered that the water content C in the lithium borate raw material melt and the crystal growth rate v have an opposite relationship.

That is, if the crystal growth rate v is low, excess water discharged from the growing single crystal diffuses sufficiently in the raw material melt and becomes difficult to be taken into the crystal. Is less likely to occur. On the other hand, if the crystal growth rate v is high, the excessive water that has been discharged is not sufficiently diffused, and the water is easily taken into the crystal, so that bubbles are easily generated in the single crystal.

In other words, when producing a lithium borate single crystal without bubbles, the upper limit of the allowable range of the water content C in the lithium borate raw material melt becomes lower as the crystal growth rate v becomes higher.

FIG. 2 shows the relationship between the water content C in the lithium borate raw material melt and the temperature gradient ΔT when a single crystal without bubbles is grown when the crystal growth rate v is 0.2 mm / hour. Shows critical properties. From the figure, it is considered that the water content C in the lithium borate raw material melt and the temperature gradient ΔT have a proportional relationship at the critical point where no bubbles are generated.

That is, when the temperature gradient ΔT is small, when irregular solidification occurs due to the influence of heat fluctuation at the solid-liquid interface, the solidification is instantaneously solidified to a range relatively far from the solid-liquid interface. As a result, a large amount of water is taken into the crystal to generate bubbles. However, if the temperature gradient ΔT is large, the solidification range at the solid-liquid interface is extremely limited to the vicinity of the interface, and the influence of heat fluctuation at the solid-liquid interface becomes relatively small, so that moisture is taken into the crystal. It becomes difficult.

That is, when producing a lithium borate single crystal without bubbles, as the temperature gradient ΔT becomes smaller,
The upper limit of the allowable range of the water content C in the lithium borate raw material melt becomes lower.

As described above, the relationship between the water content C in the lithium borate raw material melt, the crystal growth rate v, and the temperature gradient ΔT is collectively determined, and the coefficient is obtained based on the data obtained in each of the above experiments. And the above equation (1) is derived. In the formula (1), it does not matter whether the water content C in the lithium borate raw material melt is higher, lower, or equal to the saturated water content of the lithium borate single crystal.

Here, the selection range of the temperature gradient ΔT is 5 ° C.
/ Cm or more and 25 ° C / cm or less. The reason is that if the temperature gradient ΔT is less than the lower limit, the crystal growth becomes too slow, which is not suitable for industrial implementation. On the other hand, if the temperature gradient ΔT exceeds the upper limit, cracks may occur in the grown crystal. Because there is.

The selection range of the crystal growth rate v is 0.
It is not less than 2 mm / hour and not more than 1.0 mm / hour. The reason is,
If the crystal growth rate v is less than the lower limit, the growing cost is too high, which is not suitable for industrial implementation. On the other hand, if it exceeds the upper limit, polycrystallization may occur.

In order to maintain the water content C in the lithium borate raw material melt so as to satisfy the above formula (1) from the start to the end of the growth of the lithium borate single crystal, first, the crystal growth rate v and the temperature A lithium borate raw material having the same water content as the water content C in the raw material melt determined according to the gradient ΔT is prepared. Then, the raw material is placed in a growth vessel and heated and melted, and a crystal is grown under a dry atmosphere (including a vacuum state) so that moisture is not dissolved in the raw material melt from the growth atmosphere. However, when the water content of the prepared lithium borate raw material is close to the saturated water content in the lithium borate raw material melt, or compared with the upper limit of the water content C allowable at the set crystal growth rate v and temperature gradient ΔT. If the temperature is extremely low, the atmosphere may not be a dry atmosphere.

[0021]

According to the above-described means, the growth of a lithium borate single crystal can be performed under such conditions that the water content C, the crystal growth rate v, and the temperature gradient ΔT in the lithium borate raw material melt satisfy the above equation (1). As a result, excessive moisture discharged from the single crystal at the solid-liquid interface during crystal growth is prevented from being taken into the crystal, and a lithium borate single crystal without bubbles can be obtained.

[0022]

The following is a description of the growth conditions satisfying the above equation (1).
An example in which a lithium borate single crystal is grown by the well-known Bridgman method will be described.

Example 1 A growth vessel containing a lithium borate raw material having a water content of 250 ppm was set in a crystal growth furnace, and the lithium borate raw material was heated and melted in a dry atmosphere. Then, the temperature gradient ΔT is set to 20 ° C./cm, the crystal growth rate v is set to 0.2 mm / hour, and while keeping the water content C in the lithium borate raw material melt at 250 ppm, the raw material melt is gradually cooled from one end. Thus, a lithium borate single crystal was grown. No bubbles were observed in the obtained single crystal.

Example 2 A lithium borate raw material having a water content of 70 ppm was used. Then, the temperature gradient ΔT is set at 7 ° C./c.
m, the crystal growth rate v was 0.2 mm / hour, and while maintaining the water content C in the lithium borate raw material melt at 70 ppm,
A lithium borate single crystal was grown in the same manner as in Example 1 except that the other growth conditions were the same as in Example 1. No bubbles were observed in the obtained lithium borate single crystal.

It is needless to say that the present invention is not limited by the above embodiments. That is, the water content C, the crystal growth rate v, and the temperature gradient ΔT in the lithium borate raw material melt can be arbitrarily selected as long as the above formula (1) is satisfied.

[0026]

According to the method for producing a lithium borate single crystal according to the present invention, the water content C in the lithium borate raw material melt is adjusted.
In order to grow a single crystal of lithium borate under such conditions that the crystal growth rate v and the temperature gradient ΔT satisfy the above formula (1), excessive water discharged from the single crystal at the solid-liquid interface during the crystal growth causes the crystal to grow. It is prevented from being taken in,
A bubble-free lithium borate single crystal can be produced.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a criticality of a relationship between a water content C in a lithium borate raw material melt and a crystal growth rate v.

FIG. 2 is a characteristic diagram showing a criticality of a relationship between a water content C in a lithium borate raw material melt and a temperature gradient ΔT.

Claims (1)

(57) [Claims] 1. A method for heating and melting a lithium borate raw material and growing a lithium borate single crystal from the molten lithium borate raw material melt, the water content C in the lithium borate raw material melt, the crystal growth rate v, and the temperature. A method for producing a lithium borate single crystal, characterized in that the gradient ΔT satisfies the following expression: C ≦ (2.8 × ΔT−5.4) / v.