JPH05339094A - Apparatus for producing oxide single crystal - Google Patents

Abstract

PURPOSE:To provide the apparatus for producing an oxide single crystal which is suitable for growing the oxide single crystal of a relatively low melting temp., such as lithium tetraborate single crystal, provides a sufficiently high in-furnace temp. gradient and enables stable growth free from temp. fluctuation. CONSTITUTION:The apparatus for producing the oxide single crystal by a Czochralski method includes (a) a cylindrical crucible 1 with a bottom of an inside diameter D which houses a raw material melt 7, (b) a heater 4 for heating this crucible 1, (d) a pulling up shaft 2 which holds and vertically moves a seed crystal on the raw material melt, (d) a heat insulator 5 which has an aperture penetrated with this pulling up shaft 2 and hermetically seals the above-mentioned melt surface exclusive of this aperture. (e) The volume hermetically sealed by this heat insulator 5 is <=(0.45D)<3>pi and (f) the diameter of above- mentioned opening is 0.20D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for producing an oxide single crystal, particularly a lithium tetraborate single crystal used as a substrate material for a surface acoustic wave device.

[0002]

2. Description of the Related Art When growing an oxide single crystal, the Czochralski method (crystal pulling method) in which a seed crystal is brought into contact with a raw material melt and pulled while rotating, is often used. In the Czochralski method, it is important to properly arrange the reactor internal structure so that an appropriate temperature gradient (temperature gradient in the furnace) can be obtained in the vertical direction on the raw material melt. Particularly, when an oxide single crystal having a relatively low melting point (1000 ° C. or less) such as a lithium tetraborate single crystal is grown in a state where the temperature gradient in the furnace is low, it is difficult to grow the crystal satisfactorily due to the generation of polycrystals. It was

A manufacturing apparatus as shown in FIG. 1 is known for increasing the temperature gradient in the furnace. In this apparatus, a reflecting plate 8 having an opening through which a grown crystal passes is provided on the opening surface of the crucible 1 filled with the raw material melt 7. Since only the vicinity of the raw material melt 7 is kept warm by the reflecting plate 8, the temperature difference from the upper part thereof is increased, and a sufficiently high temperature gradient in the furnace can be obtained.

[0004]

However, when such a reflector is used, the temperature gradient in the furnace can be made sufficiently high, but there is a drawback that the temperature fluctuation of the melt on the surface of the raw material melt becomes large. It was The expansion of the temperature fluctuation has a great adverse effect on the crystal growth, such as changing the growth rate at the crystal growth interface and inducing the generation of polycrystals.

An object of the present invention is to provide an apparatus for producing an oxide single crystal, which can obtain a sufficiently high temperature gradient in the furnace and can perform stable growth without temperature fluctuation.

[0006]

Means for Solving the Problems and Actions The present inventors have made the following inventions as a result of extensive studies. The present invention is, in an apparatus for producing an oxide single crystal by the Czochralski method, (a) a cylindrical crucible with a bottom having an inner diameter D for containing a raw material melt, and (b) a heater for heating the crucible. , (C) has a pulling shaft that holds a seed crystal on the raw material melt and moves up and down, and (d) has an opening through which the pulling shaft penetrates,
A heat insulator for sealing the melt surface is included except for the opening, and (e) the volume sealed by the heat insulator is (0.4
5D) and the ³ [pi less, and is intended at (f) the diameter of the opening is 0.20D or more.

According to the present invention, the melt surface is sealed in a narrow space (volume is (0.45D) ³ π or less) by the heat insulator, and a large opening (diameter is 0.20D) is formed in the heat insulator.
Since the above is provided, it is possible to suppress temperature fluctuation due to convection of the atmospheric gas, and at the same time, it is possible to obtain a high temperature gradient in the furnace due to radiation from the opening in the central portion. The volume of the space sealed by the heat retaining body is (0.
45D) When it exceeds ³ π, the temperature fluctuation increases, and when the diameter of the opening in the upper part of the heat retaining body is less than 0.20D,
The temperature gradient in the furnace cannot be increased.

[0008]

EXAMPLES The present invention will be described below with reference to examples. Figure 2
FIG. 3 shows a cross-sectional view of an oxide single crystal manufacturing apparatus that is an embodiment of the present invention. On a platinum crucible 1 (diameter 200 mm, depth 200 mm) having a cylindrical and flat bottom with a diameter D, a pulling shaft 2 having a seed crystal attached to the lower end and moving up and down while rotating is provided. The crucible 1 is placed on a crucible stand 3 and is surrounded by a cylindrical heater 4 made of a Fe—Cr—Al-based metal heating element. The heater 4 is feedback-controlled by the temperature of the thermocouple installed inside. A heat insulator 5 made of quartz glass having a thickness of 3 mm is installed on the crucible 1, and the heat insulator 5 is kept warm by an in-furnace heat insulator 6. The atmosphere in the furnace is air at atmospheric pressure, and the heater 4 and the crucible stand 3 are kept warm by a heat retaining material (not shown).

As shown in the cross-sectional view of FIG. 3, the heat retaining body 5 is connected to the ring-shaped portion 5a having an outer diameter Da of 230 mm and an opening of the ring-shaped portion 5a which face each other in parallel with the melt surface. It is composed of a cylindrical portion 5b, a frustoconical portion 5c is connected to the upper portion of the cylindrical portion 5b, and the opening is narrowed upward. Each dimension is the diameter D of the cylindrical portion 5b.
b is 120 mm (0.6D) and its height Hb is 50 m
m, the diameter Dc of the upper end of the truncated cone portion 5c is 40 mm
(0.2D) and its height Hc is 50 mm. When the raw material was put in such a manner that the melt surface came 20 mm below the upper surface of the crucible 1, the volume when the upper opening of the truncated cone part 5c was closed by one plane (the heat insulator 5 and the melt surface were closed) Volume) is 1725 cm ³ ((0.41D) ³ π)
Becomes In addition, D represents the diameter of the crucible 1.
The heat retaining body 5 may be made of sintered ceramics such as alumina as a material, but the thickness thereof is sufficient as long as the convection of the atmosphere can be suppressed, and it is desirable to make the thickness as thin as 5 mm or less.

Next, single crystal growth using this apparatus will be described. A high-purity lithium tetraborate raw material was placed in the crucible 1 and the heater 4 was heated to about 1100 ° C. to obtain a raw material melt 7. At this time, the distance between the lower end of the heat retaining body 5 and the surface of the raw material melt 7 is 20 mm, and the temperature gradient in the furnace (that is, the average temperature gradient between the temperature of the melt surface at the center of the melt surface and 20 mm directly above it) Was 25 ° C./cm, and the temperature fluctuation (that is, the temporal temperature fluctuation between the melt surface at the center of the melt surface and 100 mm directly above it) was 8 ° C. or less. Seeding is continued and pulling speed is 0.7mm
/ H, the growing direction was <110>. Under these conditions, a lithium tetraborate single crystal having a diameter of 100 mm and a length of 100 mm can be grown with good reproducibility. In order to increase the temperature gradient in the furnace, the distance between the lower end of the heat retaining body 5 and the surface of the raw material melt 7 is 0.05D to 0.2D.
Is desirable.

[0011]

[Comparative Example] Crystals were grown under the same conditions as in Example except that the dimensions of the heat retaining body 5 were changed. In the first comparative example, the diameter Db of the cylindrical portion 5b is 160 mm (0.8D), its height Hb is 70 mm, and the frustoconical portion 5c has an upper end diameter Dc of 70 mm (0.35 D) and a height Hc. Is 70 mm, and the volume sealed by the heat insulator 5 and the melt surface is 2840.
cm ³ ((0.48D) ³ π). Further, in the second comparative example, the diameter Db of the cylindrical portion 5b is 110 mm (0.5
5D), the height Hb is 30 mm, the frustoconical portion 5c has an upper end diameter Dc of 30 mm (0.15 D), and a height Hc.
Is 30 mm, and the volume sealed by the heat retaining body 5 and the melt surface is 1398 cm ³ ((0.38D) ³ π).

In the first comparative example, the temperature gradient in the furnace is 32 ° C.
/ Cm, and the maximum temperature fluctuation was 20 ° C. Although it is possible to obtain a temperature gradient in the furnace of 20 ° C./cm or more, which allows the lithium tetraborate single crystal to be grown with good reproducibility,
Since the temperature fluctuation was large, the crystal growth became unstable, and polycrystals were generated during the crystal growth, and a good crystal could not be obtained.

On the other hand, in the second comparative example, the temperature gradient in the furnace was 16 ° C./cm, and the temperature fluctuation was 5 ° C. at maximum. Although the temperature fluctuation was sufficiently low at 5 ° C. or less, the temperature gradient in the furnace was too low, so that sufficient crystal growth could not be performed and good crystals could not be obtained.

As described above, according to this embodiment, the volume sealed by the heat retaining body is (0.40D) ³ π or more,
(0.45D) ³ π or less and the diameter of the opening is 0.2
Since it is set to 0D or more and 0.30D or less, the temperature gradient in the furnace can be set to 20 ° C / cm or more and the temperature fluctuation can be set to 8 ° C or less. Therefore, it is possible to prevent the generation of polycrystals during the growth, and it is possible to grow the lithium tetraborate single crystal, which is the substrate material for the surface acoustic wave device, with good reproducibility by the Czochralski method. In addition, when the inner diameter of the cylindrical portion of the heat retaining body is less than 0.5D, the outer diameter of the grown crystal is small and the productivity is poor. On the other hand, when it exceeds 0.7D, the heat retaining body 5 and the melt surface When the sealed volume is set within the range of the present invention, the length of the grown crystal cannot be increased, resulting in poor productivity.

[0015]

As described above, according to the present invention, in an apparatus for producing an oxide single crystal by the Czochralski method, (a) a cylindrical crucible with a bottom having an inner diameter D for containing a raw material melt, b) a heater for heating the crucible, (c) a pulling shaft that holds a seed crystal on the raw material melt and moves up and down, and (d) an opening through which the pulling shaft passes,
A heat insulator for sealing the melt surface is included except for the opening, and (e) the volume sealed by the heat insulator is (0.4
5D) and the ³ [pi less, and is intended at (f) the diameter of the opening is 0.20D or more.

According to the present invention, since the melt surface is sealed in a narrow space by the heat retaining body and the heat retaining body is provided with a large opening, temperature fluctuation due to convection of atmospheric gas can be suppressed. At the same time, a high temperature gradient in the furnace can be obtained by radiation from the opening in the center. Therefore, a good crystal of an oxide single crystal such as a lithium tetraborate single crystal can be obtained with high productivity.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating a conventional oxide single crystal manufacturing apparatus.

FIG. 2 is a cross-sectional view for explaining an oxide single crystal manufacturing apparatus that is an embodiment of the present invention.

FIG. 3 is a cross-sectional view showing details of a heat retaining body used in an example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Crucible, 2 ... Pull-up shaft, 3 ... Crucible stand, 4 ... Heater, 5 ... Insulator, 6 ... In-furnace heat insulating material, 7 ... Raw material melt, 8 ... Reflector.

Claims (1)

[Claims] 1. An apparatus for producing an oxide single crystal by the Czochralski method, comprising: (a) a cylindrical crucible with a bottom having an inner diameter D for containing a raw material melt, and (b) a heater for heating the crucible. And (c) a pulling shaft that holds a seed crystal on the raw material melt and moves up and down, and (d) has an opening through which the pulling shaft penetrates, and seals the melt surface other than the opening. (E) The volume sealed by the heat retaining body is (0.45
D) ³ π or less, and (f) the diameter of the opening is 0.20 D or more, an apparatus for producing an oxide single crystal.