JP2020147459A - Method for growing lithium niobate single crystal - Google Patents

Abstract

To provide a method for stably growing a high quality lithium niobate (LN) single crystal by the Czochralski method even when growing a large-sized crystal having a diameter of 4 inches or more.SOLUTION: A method for growing a LN single crystal by the Czochralski method, capable of contacting a seed crystal 1 to a raw material melt in a crucible 12 arranged in the chamber 11 of a single crystal growth apparatus 10 and pulling the seed crystal by a pulling shaft (a seed rod) 16 while rotating the seed crystal to grow a crystal shoulder part and a crystal straight body part followed thereby comprises: controlling a temperature atmosphere in a space above the crucible so that the average temperature gradient on the pulling shaft 16 from a position above 1 mm from the surface of the raw material melt to a position above 20 mm is 2-10°C/cm; and contacting the seed crystal to the raw material melt to grow a lithium niobate single crystal having 4 inches or more of the crystal diameter of the crystal straight body part.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for growing a lithium niobate single crystal used as a substrate material for a surface acoustic wave device, and in particular, grows a high-yield, stable and high-quality lithium niobate single crystal by the chocolate ralsky method. It is about improving the method.

Lithium niobate (LiNbO ₃ ; sometimes abbreviated as LN) single crystal is an artificial ferroelectric crystal having a melting point of about 1250 ° C. and a Curie temperature of about 1140 ° C. The LN single crystal substrate cut out from the LN single crystal and obtained by polishing is mainly used as a material for a surface acoustic wave element (SAW filter) mounted on a mobile communication device.

Industrially, the LN single crystal is a nitrogen-oxygen mixed gas in an atmospheric atmosphere or having an oxygen concentration of about 20%, mainly by the chocolate ralsky (sometimes abbreviated as Cz) method, usually using a platinum crucible. It is cultivated in an atmosphere. The grown LN single crystal is colorless and transparent or has a highly transparent pale yellow color. The grown LN single crystal undergoes an "annealing process" to remove residual strain due to thermal stress during growth and cooling, and a "polling process" to align the electrical polarity of the entire crystal to make it a single polarization. After that, it is handed over to the substrate processing process.

Here, the Cz method is a method of obtaining a single crystal having the same crystal orientation as the seed crystal by bringing the seed crystal into contact with the surface of the raw material melt in the crucible and continuously pulling the seed crystal while rotating it. is there. After growing the crystal to the desired size (crystal diameter x crystal length), the grown crystal is separated from the raw material melt by adjusting the crystal pulling speed and melt temperature, and then cooled to near room temperature to grow. Remove the crystals from the furnace. In crystal growth by the Cz method, it is important to efficiently conduct the latent heat generated by the solidification of the raw material melt at the crystal growth interface upward through the seed crystal, so the temperature is upward from the surface of the raw material melt. It is carried out under a temperature gradient adjusted so that In the Cz method, it is generally possible to obtain a single crystal having a diameter several times to several tens of times the diameter of the seed crystal.

By the way, in crystal growth by the Cz method, if the temperature gradient of the growth environment is increased, the shape controllability of the growth crystal can be improved and the growth speed can be increased, but on the other hand, polycrystallization or intracrystal by deterioration of crystallinity The rate of occurrence of cracks and the like due to thermal stress caused by the temperature difference between the two increases. On the contrary, if the temperature gradient of the growing environment is reduced, the shape controllability of the growing crystal deteriorates (for example, the crystal bends or twists), and the growing speed also needs to be slowed down, so that the productivity decreases. .. Therefore, in order to efficiently obtain a single crystal having a desired shape, it is necessary to adjust the temperature gradient of the growing environment within an appropriate range. In Patent Documents 1 to 5, the "LN single crystal is grown by the Cz method". Various proposals have been made regarding "temperature gradient".

However, the "temperature gradient" proposed in Patent Documents 1 to 4 is intended for small crystals having a crystal diameter of up to about φ3 in (inch) to be grown. For example, Patent Document 1 covers a crystal having an average vertical temperature gradient of 40 ° C./cm or less up to 5 mm directly above the melt and a crystal diameter of φ2.6 inches (= 66 mm). The temperature gradient at the melt interface is set to 20 ° C to 60 ° C / cm, and a small crystal having a crystal diameter of about φ1.26 inch (= 32 mm) is targeted. In Patent Document 4, the average temperature gradient above the melt is 15. The target is a crystal having a crystal diameter of about φ3.15 inch (= 80 mm) at ° C. to 20 ° C./cm. Further, Patent Document 5 targets a large crystal having a crystal diameter of φ5.1 inch (= 130 mm), but a special growing furnace in which an expensive precious metal (platinum) reflector is attached above the seed crystal is used. LN crystals were grown using this.

In recent years, the diameter and price of LN single crystal substrates have been increasing and the prices have been reduced, and the mainstream φ4in (inch) substrate is shifting to φ6in (inch) substrates, and it is expected that larger diameter substrates will be required in the future. To. Further, the grown crystal for obtaining a large-diameter substrate has also become large in size, and it is difficult to grow a large single crystal under the "temperature gradient" condition proposed in Patent Documents 1 to 4. In addition, the special growing furnace described in Patent Document 5 cannot meet the demand for price reduction.

JP-A-57-067099 (Claims, see pages 7-19, lower left column of page 2) Japanese Patent Application Laid-Open No. 03-20882 (Claims, see Example 2) Japanese Patent Application Laid-Open No. 04-08939 (Claims, see Example 1) Japanese Patent Application Laid-Open No. 06-183896 (Refer to Claims, Examples) Japanese Unexamined Patent Publication No. 2003-221299 (see paragraph No. 0044, FIG. 1)

The present invention has been made by paying attention to such a problem, and the subject thereof is stable and high quality in high yield even when a large crystal of φ4 in (inch) or more is targeted for growth. It is an object of the present invention to provide a method for growing a lithium niobate single crystal by a chocolate ralsky method.

That is, the present invention
A chocolate that brings the seed crystal into contact with the raw material melt of the 坩 堝 placed in the chamber of the single crystal growing device, and pulls the seed crystal by the pulling shaft while rotating it to grow the crystal shoulder and the subsequent crystal straight body. In the method for growing a lithium niobate single crystal by the Rusky method,
After adjusting the temperature atmosphere of the space above the crystal so that the average temperature gradient on the pulling shaft from the position 1 mm above the surface of the raw material melt to the position 20 mm above is 2 ° C / cm to 10 ° C / cm, the above species It is characterized in that a single crystal of lithium niobate having a crystal diameter of 4 inches or more is grown by bringing the crystal into contact with a raw material melt.

According to the method for growing a lithium niobate single crystal according to the present invention.
Lithium niobate after adjusting the temperature atmosphere of the space above the crucible so that the average temperature gradient on the pull-up axis from the position 1 mm above the surface of the raw material melt to the position 20 mm above is 2 ° C / cm to 10 ° C / cm. Since the single crystal is grown, it is possible to stably grow a large-diameter lithium niobate single crystal without twisting or cracking.

Therefore, it is possible to manufacture a lithium niobate single crystal substrate using a high-quality, large-diameter lithium niobate single crystal substrate, so that the productivity of the lithium niobate single crystal substrate can be improved and the cost can be reduced. It becomes.

The cross-sectional view which shows typically the schematic structure of the single crystal growth apparatus by the Chocolat ski method. FIG. 5 is a cross-sectional view schematically showing a schematic configuration of a single crystal growing apparatus by the chocolateal ski method in which a thermocouple is attached to the tip of a pulling shaft (seed rod) to measure the temperature distribution of the growing environment. It is a graph showing the relationship (temperature distribution) between "distance from the melt surface (mm)" and "temperature difference (° C) from the melt surface". In FIG. 3, "conditions 1 to 3" are workpieces. Three types of temperature distribution curves adjusted by the shape of the coil, the shape of the crucible, the composition and material of the refractory material, and the relative position between the work coil and the crucible are shown. FIG. 4A relates to Comparative Example 1 grown under the condition that the “average temperature gradient is 1.5 ° C./cm” on the pulling shaft from the position 1 mm above the surface of the raw material melt to the position 20 mm above. A photographic diagram showing a state in which the LN crystal is twisted, FIG. 4B shows an "average temperature gradient of 2 ° C./cm-10" on the pull-up axis from a position 1 mm above the surface of the raw material melt to a position 20 mm above. The photograph which shows the untwisted LN crystal grown under the proper condition of "° C./cm".

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In order to facilitate the understanding of the description, the same components are designated by the same reference numerals as possible in each drawing, and duplicate description will be omitted.

[Overview of single crystal growth device and single crystal growth method]
First, with reference to FIG. 1, a configuration example of the single crystal growing apparatus 10 by the Cz method and an outline of the single crystal growing method will be described.

FIG. 1 is a cross-sectional view schematically showing a schematic configuration of a high-frequency induction heating type single crystal growing device 10, but a resistance heating type single crystal growing device is also used for growing an LN single crystal. The difference between the high frequency induction heating type single crystal growth device and the resistance heating type single crystal growth device is that in the case of the high frequency induction heating type, the metal crucible installed in the work coil 15 by the high frequency magnetic field formed by the work coil 15. An eddy current is generated on the side wall of the twelve, and the crucible 12 itself becomes a heating element due to the eddy current, and a temperature environment necessary for melting the raw material and growing crystals in the crucible 12 is formed. In the case of the resistance heating type, the heat generated by the resistance heating heater installed on the outer periphery of the crucible is used to form the temperature environment necessary for melting the raw material and growing crystals. Since the essence of the Cz method does not change regardless of which heating method is used, the single crystal growing method using the high frequency induction heating type single crystal growing device will be described below.

As shown in FIG. 1, the high-frequency induction heating type single crystal growing device 10 arranges the crucible 12 in the chamber 11. The crucible 12 has a diameter of about 150 mm to 300 mm and a depth of about 150 mm to 300 mm, and is placed on the crucible stand 13 as shown in FIG. A refractory material 14 is arranged in the chamber 11 so as to surround the crucible 12. The work coil 15 is arranged so as to surround the crucible 12, and an eddy current flows through the wall of the crucible 12 due to the high frequency magnetic field formed by the work coil 15, and the crucible 12 itself becomes a heating element. A pull-up shaft (seed rod) 16 is provided on the upper portion of the chamber 11 so as to be rotatable and vertically movable. A seed holder 17 for holding the seed crystal 1 is attached to the tip of the lower end of the pulling shaft (seed rod) 16.

In the Cz method, a single crystal piece to be the seed crystal 1 is brought into contact with the melt surface of the single crystal raw material 18 in the pit 12, and the seed crystal 1 is pulled upward while being rotated by the pulling shaft (seed rod) 16. As a result, a cylindrical single crystal having the same orientation as the seed crystal 1 is grown.

The rotation speed and pulling speed of the seed crystal 1 depend on the type of crystal to be grown and the temperature environment at the time of growth, and need to be appropriately selected according to these conditions. In addition, when growing crystals, it is necessary to release the latent heat of solidification generated by the crystallization of the melt at the growth interface upward through the seed crystal, so it is necessary to carry out under a temperature gradient in which the temperature drops upward from the growth interface. is there. In addition, in order to prevent the shape of the grown crystal from bending or twisting, even in the raw material melt, the horizontal direction from the growth interface toward the crucible wall and the vertical direction from the growth interface toward the crucible bottom. It is necessary to carry out under a temperature gradient where the temperature becomes high.

When growing an LN single crystal, the melting point of the LN crystal is 1250 ° C., and oxygen is required in the growing atmosphere. Therefore, the crucible 12 made of platinum (Pt) having a melting point of about 1760 ° C. and is chemically stable. Is used. The pulling speed at the time of growing is generally about several mm / H, and the rotation speed is about several to several tens of rpm (for example, 3 rpm to 10 rpm). In addition, the inside of the furnace at the time of growing is generally an atmosphere or a mixed gas atmosphere of nitrogen and oxygen having an oxygen concentration of about 20%. Under such conditions, after growing the crystals to the desired size, the grown crystals are separated from the melt by performing operations such as changing the pulling speed and gradually increasing the melt temperature, and then the growing furnace. The crystals are slowly cooled by reducing the power of the above at a predetermined rate, and the crystals are taken out from the growing furnace after the temperature in the furnace becomes close to room temperature.

The crystals grown by such a method and taken out from the furnace were subjected to an annealing treatment for removing residual strain caused by a temperature difference in the crystals and a polling treatment for aligning the directions of spontaneous polarization in the crystals. Later, it is handed over to a substrate processing process for slicing, polishing, and the like.

[Adjustment of temperature distribution in the growing environment]
In the high-frequency induction heating type single crystal growing device, the temperature distribution of the growing environment (that is, the temperature atmosphere in the space above the crucible) is adjusted by adjusting the shape of the work coil 15 forming a high-frequency magnetic field, the shape of the crucible 12 serving as a heating element, and the crucible. This is done by changing the configuration and material of the refractory material 14 surrounding the 12 and the relative positions of the work coil 15 and the crucible 12.

Therefore, in order to investigate the relationship between the shape of the work coil 15, the shape of the crucible 12 as a heating element, the configuration of the refractory material 14 surrounding the crucible 12, the conditions such as the material, and the "temperature distribution of the growing environment", FIG. As shown in, a thermocouple 20 is attached to the tip of the pulling shaft (seed rod) 16 of the high-frequency induction heating type single crystal growing device, and the above conditions are changed to measure the temperature distribution in the space above the surface of the melt. went. The measurement range is a range from the surface of the melt to a position 200 mm above. FIG. 3 shows the measurement results of the temperature distribution under the three changed conditions (described as conditions 1 to 3).

As can be confirmed from the graph of FIG. 3, the temperature gradient in the space above the melt surface varies greatly depending on the conditions 1 to 3 from the melt surface to 20 mm. It can be seen that there is no significant difference in the temperature gradient in the space further above 20 mm. In addition, the temperature difference between the melt surface and the position 1 mm above the melt surface is large, but the measured value of the melt surface temperature fluctuates greatly due to the melt convection, and is 1 mm above the melt surface and the melt surface. The reproducibility was poor with respect to the temperature difference at the position.

From these results, the average temperature gradient on the pulling shaft (seed rod) from the position 1 mm above the melt surface to the position 20 mm above was used as an index to investigate the relationship with the growth result of the LN single crystal. It was found that a high single crystallization rate can be obtained by growing in a temperature gradient range of 2 ° C./cm to 10 ° C./cm. Further, when the growth was carried out under the condition that the average temperature gradient was less than 2 ° C./cm, it became difficult to control the crystal shape and it became necessary to reduce the pulling speed. On the other hand, when the growth was carried out under the condition that the average temperature gradient exceeded 10 ° C./cm, the probability that the grown crystals would crack during cooling became very high.

That is, the shape of the work coil, the shape of the crucible, the composition of the refractory material surrounding the crucible, the material, and other conditions are individually set, and the pulling shaft (seed) from a position 1 mm above the surface of the raw material melt to a position 20 mm above. By adjusting the average temperature gradient on the rod) to be 2 ° C./cm to 10 ° C./cm, it is possible to stably grow a large-diameter LN single crystal without twisting or cracking.

Hereinafter, examples of the present invention will be specifically described with reference to comparative examples.

[Example 1]
Using the high-frequency induction heating type single crystal growing device shown in FIG. 1, the average temperature gradient on the pulling shaft (seed rod) from a position 1 mm above the surface of the raw material melt to a position 20 mm above is adjusted to 6 ° C./cm. Under these conditions, 128 ° RY-LN crystals were grown by the Cz method. The 128 ° RY is the crystal growth orientation (crystal orientation of the seed crystal).

First, LN powder is charged as a raw material 18 in the Pt crucible 12, the raw material 18 is melted, and then the tip of the seed crystal 1 is immersed in the raw material melt in the crucible 12 and pulled at 2 mm / H while rotating. By raising it, an LN single crystal having a diameter of 6 inches (= 152 mm) and a straight body length of 100 mm was grown. The grown LN single crystal was separated from the raw material melt, the crystal was cooled to near room temperature, and then the crystal was taken out.

Then, the growth was carried out 50 times under the same conditions to obtain 48 LN single crystals. The single crystallization rate was 96%. Further, the grown crystal was a high-quality LN single crystal without twisting or the like as shown in FIG. 4 (B).

[Example 2]
Φ6 under the same conditions as in Example 1 except that the average temperature gradient on the pull-up shaft (seed rod) from a position 1 mm above the surface of the raw material melt to a position 20 mm above is adjusted to 2 ° C./cm. A 128 ° RY-LN crystal having an inch length and a straight body length of 100 mm was grown.

Then, the growth was carried out 50 times under the same conditions to obtain 45 LN single crystals. The single crystallization rate was 90%. The five defective crystals were polycrystallized from the crystal habit line portion.

[Example 3]
Φ6 under the same conditions as in Example 1 except that the average temperature gradient on the pulling shaft (seed rod) from a position 1 mm above the surface of the raw material melt to a position 20 mm above is adjusted to 10 ° C./cm. A 128 ° RY-LN crystal having an inch length and a straight body length of 100 mm was grown.

Then, the growth was carried out 50 times under the same conditions to obtain 43 LN single crystals. The single crystallization rate was 86%. All of the seven defective crystals had cracks during cooling after the grown crystals were separated from the melt.

[Comparative Example 1]
Under the same conditions as in Example 1 except that the average temperature gradient on the pull-up shaft (seed rod) from a position 1 mm above the surface of the raw material melt to a position 20 mm above is adjusted to 1.5 ° C./cm. A 128 ° RY-LN crystal having a diameter of 6 inches and a straight body length of 100 mm was grown.

Then, the growth was carried out 50 times under the same conditions to obtain 39 LN single crystals. The single crystallization rate was 78%, but as shown in FIG. 4 (A), the shape of the latter half of the straight body of the crystal was twisted, and the average effective length (the length that a single crystal substrate of φ150 mm can be processed) was It became about 50 mm. In addition, all 11 defective crystals were polycrystallized from the crystal habit line portion.

[Comparative Example 2]
Φ6 under the same conditions as in Example 1 except that the average temperature gradient on the pull-up shaft (seed rod) from a position 1 mm above the surface of the raw material melt to a position 20 mm above is adjusted to 12 ° C./cm. A 128 ° RY-LN crystal having an inch length and a straight body length of 100 mm was grown.

Then, the growth was carried out 50 times under the same conditions to obtain 36 LN single crystals. The single crystallization rate was 72%. All of the 14 defective crystals had cracks during cooling after the grown crystals were separated from the melt.

According to the method for growing a lithium niobate single crystal according to the present invention, a large-diameter lithium niobate single crystal without twisting or cracking can be stably grown, and thus niobate used as a substrate material for a surface elastic wave filter. It has industrial potential for the production of lithium niobate single crystals.

1 Seed crystal 10 Single crystal growing device 11 Chamber 12 Crucible 13 Crucible stand 14, 19 Refractory 15 Work coil 16 Pulling shaft (seed rod)
17 Seed holder 18 Single crystal growth raw material 20 Thermocouple

Claims (1)

A chocolate that brings the seed crystal into contact with the raw material melt of the 坩 堝 placed in the chamber of the single crystal growing device, and pulls the seed crystal by the pulling shaft while rotating it to grow the crystal shoulder and the subsequent crystal straight body. In the method for growing a lithium niobate single crystal by the Rusky method,
After adjusting the temperature atmosphere of the space above the crystal so that the average temperature gradient on the pulling shaft from the position 1 mm above the surface of the raw material melt to the position 20 mm above is 2 ° C / cm to 10 ° C / cm, the above species A method for growing a lithium niobate single crystal, which comprises contacting a crystal with a raw material melt to grow a lithium niobate single crystal having a crystal diameter of 4 inches or more in the straight body of the crystal.