JPH09328394A - Production of oxide single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an oxide single crystal by raising the oxide single crystal by Czochralski method and subsequently cooling the raised single crystal from the raising temperature to room temperature, enabling to prevent the generation of cracks by cooling the single crystal so that a temperature difference between both the upper and lower ends of the single crystal is a specific value or smaller. SOLUTION: This method for producing an oxide crystal comprises setting heat- insulating alumina 5 on a heat-insulating material 4 surrounding a crucible 3, attaching a doughnut-like reflector 6 to the upper surface of the crucible 3 or the heat-insulating alumina 5, attaching a cylindrical afterheater 7 to the upper surface of the reflector 6, heating a raw material in the crucible 3 with a heating coil 8 to produce a molten liquid 9, bringing a seed crystal 11 into contact with the molten liquid 9, raising the oxide single crystal 12 by Czochralski method, cutting off the raised crystal 12 from the molten liquid 9, moving the whole body of the raised crystal 12 to a place above the reflector 6, starting to cool the crystal, and cooling the crystal to room temperature. Thereby, a difference between temperatures at both the upper and lower ends of the raised crystal 12 on the cooling treatment can be reduced to 50 deg.C or lower to produce the good crystal slight in defects.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide single crystal produced by using the Czochralski method, and particularly to a substrate for a surface acoustic wave element such as lithium niobate or lithium tantalate single crystal in which a crystal easily cracks. The present invention relates to a method for producing an oxide single crystal that is useful as

[0002]

2. Description of the Related Art Single crystal oxides of lithium tantalate, lithium niobate, etc. are generally manufactured by the Czochralski method, which is a method for manufacturing a noble metal crucible. It is a common practice to enclose a refractory with a refractory and to install a precious metal structure on the crucible or refractory with a function of heat reflection to keep an appropriate temperature gradient in the upper part of the crucible. .

Here, in order to pull this single crystal straight long, it is necessary to take a large temperature gradient in the axial direction, but a large temperature is required for oxide single crystals such as lithium niobate single crystal and lithium tantalate single crystal. There is a problem that cracks are likely to occur due to thermal strain due to the gradient.

Therefore, for the purpose of strengthening the heat retention in the crucible, relaxing the internal strain of the crystal to prevent the generation of cracks, and improving the yield, the present inventors have an outer diameter larger than the crucible diameter at the upper end of the crucible, A donut plate-shaped precious metal reflector larger than the crystal whose inner diameter is pulled up is placed, and a cylindrical noble metal after-heater is installed on it to pull up lithium niobate and lithium tantalate single crystals. Suggested to do. As this noble metal, iridium, platinum, platinum rhodium, etc. may be selected according to the melting point of the crystal to be pulled, the crucible diameter, the crucible height, the melt amount, etc. are appropriately set, and the crystal diameter becomes constant in the latter half of pulling. If the upper end of the curved part is exposed above the reflector, the heat of the crystal itself can be used to dissipate the heat near the solid-liquid interface in the crucible to the upper part of the reflector through the cone part of the crystal. It was found that the temperature gradient was maintained and the crystal could be lengthened without being twisted, and it was confirmed that a crystal having a body portion with a diameter of 3 inches and a length of 130 mm was obtained (JP-A-7-33586).

However, when a single crystal of lithium tantalate or lithium niobate is pulled by the above method or the like and then cooled to room temperature, it has a large diameter of 3 inches or more and a length of 70 mm or more. A problem with a long single crystal is that there are relatively many cracks during cooling, and therefore a solution to this problem has been desired.

The present invention has been made to solve the above problems, and provides a method for producing an oxide single crystal having a large yield and a good yield with few cracks during cooling even in the case of a large diameter and long single crystal. The purpose is to

[0007]

Means for Solving the Problems and Modes for Carrying Out the Invention In order to achieve the above-mentioned object, the present inventors have made an oxidation particularly prone to cracks such as lithium niobate or lithium tantalate single crystal by the Czochralski method. As a result of various studies on a method for obtaining a high-quality single crystal having a large diameter of 3 inches or more and a length of 70 mm or more, after growing an oxide single crystal from the melt by the Czochralski method, To reduce the temperature difference in the vertical direction of the crystal when the crystal is cooled from the growth temperature to room temperature, specifically, to cool the crystal so that the temperature difference between the upper and lower ends of the crystal is 50 ° C. or less. As a result, it was found that cracks are less likely to occur during cooling and a high quality oxide single crystal can be obtained with good yield.

That is, in the case of obtaining an oxide single crystal of lithium niobate, lithium tantalate, or the like by the Czochralski method, a donut plate-shaped reflector is installed on the upper end of a precious metal crucible, and a cylindrical shape is formed on the reflector. If the after-heater is installed, it is possible to obtain a long crystal of good quality with little twist. However, in this method, after growing a single crystal, when it is cooled, cracks due to thermal strain due to the temperature difference between the upper and lower parts of the crystal accompanying the installation of the reflector occur relatively often, but the crystal grown after the crystal growth Is located so that the temperature difference between the upper and lower parts of the crystal is small, and the temperature is lowered and allowed to cool down to reduce the thermal strain generated in the crystal, and even for crystals with a large diameter of 3 inches or more. The present inventors have completed the present invention by finding that it is possible to obtain a good quality crystal free from defects such as cracks with a high yield, as well as realizing a longer crystal.

Therefore, according to the present invention, (1) when a seed crystal is brought into contact with the melt in the crucible to grow an oxide single crystal by the Czochralski method, and then cooled from the growth temperature of the crystal to room temperature, A method for producing an oxide single crystal, which comprises cooling the crystal so that the temperature difference between the upper and lower ends of the crystal is 50 ° C. or less, (2) contacting a seed crystal with a melt in a crucible to form the oxide single crystal. After the crystal is grown by the Czochralski method, when the crystal is cooled from the growth temperature to room temperature, the temperature difference between the upper and lower ends of the crystal is 50 ° C.
A method for producing an oxide single crystal, which comprises cooling to a position as described below, and (3) bringing the seed crystal into contact with the melt in the crucible to make the oxide single crystal a Czochralski method. Upon growing the oxide single crystal by using the furnace internal structure for crystal growth in which a donut plate-shaped reflector is installed at the upper end of the precious metal crucible in which the melt is stored, from the growth temperature of the crystal When cooling to room temperature,
A method for producing an oxide single crystal, wherein the crystal is moved upward to position the entire crystal above the reflector, and then cooled, and (4) the oxide single crystal is lithium niobate. Alternatively, there is provided the production method according to the above (1), (2) or (3), which is a lithium tantalate single crystal.

Hereinafter, the present invention will be described in more detail.
The oxide single crystal of the present invention is particularly effectively used in the production of lithium niobate single crystal and lithium tantalate single crystal, which are likely to cause cracks. A usual method of the Czochralski method, in which seed crystals are brought into contact with the melt and pulled up to grow, can be adopted.

In this case, the apparatus shown in FIG. 1 is preferably used as the apparatus for producing the oxide single crystal. That is, FIG. 1 shows a vertical cross-sectional view of this oxide single crystal production apparatus. It shows a crucible 3 installed on an alumina base 2 installed on the bottom of a ceramic crucible base 1. A refractory alumina 5 is installed on the surrounding heat insulating material 4, a donut plate-shaped reflector 6 is mounted on the crucible 3 or the refractory alumina 5, and a cylindrical after-heater 7 is mounted above the reflector 6. This is an induction heating of the crucible 3 with a heating coil 8 to melt the raw material to form a melt 9 into which a seed crystal 11 attached to a seed rotating shaft 10 is dipped and pulled up. The single crystal 12 is grown, and after obtaining a predetermined crystal weight, it is separated from the melt 9.

In the present invention, after the oxide single crystal is grown as described above, when the crystal is cooled from the growth temperature to room temperature, the strain generated inside the crystal during temperature lowering / cooling is relaxed, and the crack of the crystal cracks. For the purpose of preventing the occurrence of the above and improving the yield, the temperature difference between the upper and lower ends of the crystal when the crystal is cooled from the growth temperature to room temperature is 50 ° C. or less,
It is preferably cooled to 25 ° C. or less.

The method for ensuring such a temperature difference of 50 ° C. or less is not particularly limited as long as such a temperature difference is achieved, but the grown crystal is moved above the crucible to obtain the above-mentioned temperature difference. A method of moving to a position where the temperature difference between the upper and lower ends is 50 ° C. or less, preferably 25 ° C. or less, can be adopted, and it is preferable to cool after moving to a position where the temperature difference is 50 ° C. or less. Is.

In this case, when a single crystal is grown using an apparatus having a donut plate-shaped reflector on the upper end of a noble metal crucible as an in-furnace structure for growing a crystal as shown in FIG. And cooling is performed in a state where the entire crystal is positioned above the reflector. Specifically, the grown crystal 12 is separated from the melt 9, and the reflector 6 is further separated.
After that, the electric power applied to the heating coil 8 is gradually reduced, the output is lowered to zero, and then the crystal is allowed to cool until the temperature of the crystal falls to about room temperature. You can

Here, based on the conventional technique, 10 to 40 mm above the melt where the lower end of the tail portion of the crystal is the cut-off position.
When the cooling is performed as described in (1), the crystal tail portion is heated by the solidification heat generated when the melt solidifies, and in the case of a long crystal having a crystal length of more than 70 mm, the upper end of the crystal is located near the reflector. However, sometimes the height of the crystal becomes higher than the position of the reflector, so that the upper end of the crystal is easily cooled, and as a result, a temperature difference occurs in the vertical direction of the crystal, and this temperature difference causes a large stress on the crystal.

By the way, oxide single crystals, such as lithium niobate and lithium tantalate single crystals, which are prone to cracks, are liable to cause a defect of a small tilt angle boundary. When a large stress due to the above-mentioned temperature difference is applied to a crystal containing this defect, a stress in the compressive direction acts on the upper part of the crystal and a stress in the tensile direction on the lower part of the crystal. A minute crack enters in the direction of. When the magnitude of stress is relatively small, this crack is a crack only in the lower part of the crystal, but when the stress is large, it is considered that the crack occurs in the entire crystal.

On the other hand, in addition to optimizing the conditions for growing the crystal, the temperature difference of the crystal when it is cooled from the growth temperature to room temperature is 50 ° C. or less, preferably 25, as described above. 1 ° C or lower, or when the crystal is cooled from the growth temperature to room temperature, the crystal is moved to a position where the temperature difference of the crystal is 50 ° C or lower, preferably 25 ° C or lower, and then cooled. As shown in Fig. 5, the in-furnace structure for growing crystals is placed on the upper end of the precious metal crucible after the donut plate-shaped reflector is grown to grow the crystals, and when the crystals are cooled from the growth temperature to room temperature, the crystals are moved upward. By moving and positioning the entire crystal above the reflector, a lithium niobate single crystal or a lithium tantalate single crystal can be produced with a high yield.

According to the present invention, a large diameter of 3 inches or more,
A long oxide single crystal having a length of 70 mm or more, such as lithium niobate or lithium tantalate, can be obtained with a good yield while minimizing the occurrence of cracks. In addition, the growth of lithium niobate or lithium tantalate single crystal is 140
It is carried out at a temperature of 0 ° C. or higher, and after crystal growth, the electric power applied to the heating coil is gradually reduced to lower the temperature.
The temperature difference between the upper and lower ends of the crystal after growth during cooling is 50 ° C. or less,
In particular, if the temperature is set to 25 ° C or lower, this temperature decreasing rate is
In addition to the advantage that the crystal can be produced with a high yield even if it is r or more, and the cooling time after cooling is relatively short, that is, 4 hours or less, in this case, the crystal is cooled and released. Since the thermal strain generated when cooling is small,
The advantage is that it is possible to easily obtain a high-quality crystal that has extremely small internal strain and does not have a non-uniform refractive index portion in the crystal such as striae and defects, and is particularly suitable for optical applications.

[0019]

EXAMPLES The present invention will be described below in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

Example 1 Using the apparatus shown in FIG. 1, an iridium crucible having a diameter of 150 mmφ and a height of 150 mm was provided with a donut plate-shaped reflector having an outer diameter of 155 mmφ, an inner diameter of 120 mmφ and a thickness of 2 mm. A cylindrical after-heater made of iridium having a diameter of 150 mmφ and a height of 200 mm was arranged on this.

When 10 kg of the raw material melt was charged in the crucible, the temperature gradient in the axial direction was measured using a thermocouple after melting and before pulling the crystal, the results shown in FIG. 2 were obtained. In FIG. 2, the origin of height is the melt surface position, and R is the position of the reflector.

Next, the Czochralski method is used to set the X-axis direction to 3
A single lithium tantalate single crystal with an inch diameter of 70 mm is pulled up, and its tail is 20 m above the reflector.
After moving to the position of m, the temperature was lowered over 12 hours (the temperature lowering rate was 140 ° C./hr on average), and the crystal was taken out 8 hours after the end of the temperature reduction, no local crack was observed in the latter half of the crystal. It was When 19 single crystals were pulled up by the same method, one cracked due to the occurrence of cracks during cooling, but 18 were good quality crystals with few defects, and this yield was as high as 95%. there were.

[Embodiment 2] With the same reactor internal structure as that used in Embodiment 1, one Lithium tantalate single crystal having a diameter of 3 inches in the X-axis direction and a length of 70 mm is pulled by the Czochralski method. After moving the tail part to a position 20 mm above the reflector in the same manner as in Example 1, the temperature was decreased over 2 hours (the temperature decrease rate was 800 ° C./hr on average), and the crystals were taken out 4 hours after the completion of the temperature reduction. No local crack was observed in the latter half. When 19 single crystals were pulled up by the same method, 2 cracked due to the occurrence of cracks during cooling, but 17 were good quality crystals with few defects, and this yield was as high as 90%. there were.

[Embodiment 3] With the same furnace structure as that used in Embodiment 1, one lithium tantalate single crystal having a length of 3 mm and a diameter of 3 inches in a Y-axis direction of 36 ° is produced by the Czochralski method. After pulling up and moving the tail part to a position 20 mm above the reflector, the temperature was lowered over 2 hours (the temperature lowering rate was 800 ° C./hr on average), and after the temperature was lowered, 4
When the crystal was taken out after a lapse of time, no local crack was observed in the latter half of the crystal. When 9 single crystals were pulled by the same method, one cracked due to the occurrence of cracks during cooling, but 8 were good quality crystals with few defects, and this yield was as high as 90%. there were.

Example 4 A lithium niobate single crystal having a length of 140 mm and a diameter of 3 inches in the Y-axis direction of 127.8 ° was produced by the Czochralski method with the same furnace structure as that used in Example 1. After pulling up one and moving the tail part to a position 20 mm above the reflector, the temperature was lowered over 1.5 hours (the temperature reduction rate was 800 ° C./hr on average), and the crystals were taken out 4 hours after the end of the temperature reduction, No local crack was observed in the latter half of the crystal. When 9 single crystals were pulled by the same method, one cracked due to the occurrence of cracks during cooling, but 8 were good quality crystals with few defects, and this yield was as high as 90%. there were.

[Comparative Example 1] With the same furnace structure as that used in Example 1, one 70% lithium tantalate single crystal having a diameter of 3 inches in the X-axis direction was pulled by the Czochralski method to form a crystal. After the growth, the tail was separated from the melt, the tail part was moved 30 mm above the melt, and the temperature was lowered over 12 hours (the temperature reduction rate was 140 ° C./hr on average), and the crystals were taken out 8 hours after the completion of the temperature reduction. However, no local crack was observed in the latter half of the crystal. When 19 single crystals were pulled by the same method, 9 cracked due to the occurrence of cracks during cooling, but 10 were good quality crystals with few defects, and the yield was 55%. It was

[Embodiment 5] Diameter 180 mmφ, height 18
A donut plate-shaped reflector made of iridium having an outer diameter of 185 mmφ, an inner diameter of 130 mmφ, and a thickness of 2 mm is placed in a 0 mm iridium crucible, and a diameter of 180 mm is further placed on the reflector.
A cylindrical iridium after-heater having a φ and a height of 200 mm was arranged.

When 15 kg of the raw material melt was charged in the crucible and the temperature gradient in the axial direction was measured using a thermocouple after melting and before pulling the crystal, the results shown in FIG. 3 were obtained (FIG. 3). In 3, the origin of height is 10 from the melt surface.
mm is the position above, and R indicates the arrangement position of the reflector). After that, a Czochralski method was used to pull up one 150 mm long lithium tantalate single crystal having a diameter of 4 inches in the X-axis direction, and the tail portion thereof was 20 m above the reflector.
After moving to the position of m, the temperature was lowered over 12 hours (the temperature lowering rate was 140 ° C./hr on average), and the crystal was taken out 8 hours after the end of the temperature reduction, no local crack was observed in the latter half of the crystal. It was When 9 single crystals were pulled by the same method, one cracked due to the occurrence of cracks during cooling, but 8 were good quality crystals with few defects, and this yield was as high as 90%. there were.

[0029]

According to the manufacturing method of the present invention, it is possible to obtain a high-quality oxide single crystal having no cracks, few defects, no twist, a large diameter and a long length and a high yield.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an oxide single crystal production apparatus used for carrying out the present invention.

FIG. 2 is a graph showing temperature distributions of devices used in Examples 1 to 3 and Comparative Example 1.

FIG. 3 is a graph showing the temperature distribution of the device used in Example 4.

[Explanation of symbols]

 1 Refractory Crucible Table 2 Alumina Table 3 Crucible 4 Heat Insulation Material 5 Refractory Alumina 6 Reflector 7 After-heater 8 Heating Coil 9 Melt 10 Seed Rotation Axis 11 Seed Crystal 12 Growth Crystal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kunio Nakazato Kunio Nakazato 2-13-1, Isobe, Annaka City, Gunma Prefecture Shin-Etsu Kagaku Kogyo Co., Ltd., Institute for Precision Materials (72) Inventor Yoshinori Kuwahara Annaka City, Gunma Prefecture 2-13-1 Isobe Shinetsu Kagaku Kogyo Co., Ltd. Precision Materials Research Laboratory (72) Inventor Toshihiko Nagao 2-13-1 Isobe, Naka, Gunma Prefecture Shin-Etsu Kagaku Kogyo Co. Ltd.

Claims (4)

[Claims] 1. A seed crystal is brought into contact with the melt in a crucible to grow an oxide single crystal by the Czochralski method, and when the crystal is cooled from the growth temperature to room temperature, the upper and lower ends of the crystal are A method for producing an oxide single crystal, which comprises cooling so that the temperature difference is 50 ° C. or less. 2. A seed crystal is brought into contact with the melt in the crucible to grow an oxide single crystal by the Czochralski method, and when the crystal is cooled from the growth temperature to room temperature, the crystal is grown at its upper and lower end portions. The method for producing an oxide single crystal is characterized in that cooling is carried out after the temperature difference is moved to a position where the temperature difference is 50 ° C or less. 3. When a seed crystal is brought into contact with the melt in the crucible to grow an oxide single crystal by the Czochralski method, a donut plate-shaped reflector is installed at the upper end of the precious metal crucible containing the melt. After growing an oxide single crystal by using the in-furnace structure for crystal growth described above, when cooling from the growth temperature of the crystal to room temperature, the crystal is moved upward and the entire crystal is positioned above the reflector. A method for producing an oxide single crystal, which comprises cooling after the heating. 4. The method according to claim 1, wherein the oxide single crystal is lithium niobate or lithium tantalate single crystal.