JPS5933559B2 - Single crystal manufacturing method - Google Patents

Description

Detailed Description of the Invention (1) Technical field of the invention This invention relates to lithium niobate (LiNbO3) single crystal,
This invention relates to improvements in the manufacturing method of single crystals such as lithium tantalate (LiTa03).

(2) Technical background of the invention A piezoelectric single crystal is produced by placing a single crystal material in a noble metal crucible.
Melt the crystalline material by indirect or direct heating to form a molten material,
It is grown as a single crystal by dipping a seed crystal into the melt and crystallizing it during the pulling process.

In order to process the single crystal pulled from this melt and use it as an element of a PIF surface wave filter, a vibrator, or various piezoelectric elements, the single crystal must be polarized. The polarization treatment is performed by heating the single crystal to a temperature near or above its Curie temperature and applying a voltage to an electrode attached to the surface of the single crystal. Electrodes for polarization are sometimes attached to the surface of a single crystal. After the polarization process has been completed, the single crystal is cooled and taken out of the furnace, where it is cut and processed into the required size according to the intended use and manufactured into products.

(3) Problems with the Background Art A single crystal pulled from a melt is polarized through the process outlined above, but the single crystal often cracks during this process.

The cracks occur after the pulled single crystal is slowly cooled and before or during the next process, that is, the cutting process for attaching electrodes.
It can occur during any process such as during electrode attachment work or when left unattended while waiting for the process. The degree of cracking is not uniform and varies from vertical cracking to horizontal cracking, and if it is even slight, the single crystal cannot be used. Moreover, the occurrence rate is approximately 90% for lithium niobate and approximately 60% for lithium tantalate, based on the number of single crystals pulled.

Such cracks not only increase production costs but also cause deterioration in quality characteristics and reliability, and must be reduced as much as possible. (4) Purpose of the Invention The object of the present invention is to provide a method for producing a single crystal of good quality and high reliability by minimizing cracks that occur after pulling the single crystal.

(5) Summary of the Invention The inventors conducted various experimental investigations in order to reduce cracks in lithium niobate single crystals.

First, we performed heat treatment on the pulled single crystal to remove strain. After pulling, the single crystal is slowly cooled to near room temperature (25° C.), and then the single crystal is slowly cooled again, maintained at about 1200° C. for 10 hours, and then slowly cooled again. At this time, the heating rate is 0.
5℃/min ~ 5℃/min, slow cooling rate 0.5℃/min ~ 5℃/min
Tests and evaluations were carried out using various combinations up to 10 minutes, but although the cracks were reduced by 2 to 3% compared to those without heat treatment, no significant improvement could be achieved. As a result of further detailed investigation into the crack generation process, we found that if the temperature of the single crystal was pulled and slowly cooled to less than 40°C and then left or processed, many cracks would occur, but if the single crystal was kept at a temperature above 40°C, cracks would occur. It was found that the temperature decreases significantly, especially at 50°C.

Furthermore, it was found that the rate of cracking in single crystals was significantly different before and after polarization, and that once polarized crystals hardly cracked even when left at room temperature. The solid line in Figure 1 shows the relationship between the lithium niobate single crystal maintenance temperature and the crack occurrence rate when the single crystal was pulled and allowed to stand for 7 days after being slowly cooled. An example of lithium tantalate evaluated in the same manner is shown by the broken line in FIG. The present invention was made based on the results obtained from the above tests and investigations, and includes a step of pulling a lithium niobate single crystal or a lithium tantalate single crystal by a pulling method, and then slowly cooling it, if necessary. A method for producing a single crystal comprising at least a step of heating the single crystal through a processing step to a temperature equal to or higher than the temperature of the single crystal, and performing a polarization treatment by applying a predetermined voltage, The temperature of the single crystal from the end of the pulling to the end of the polarization treatment through the predetermined processing step was set at 40°C.
This is a method for producing a single crystal characterized by maintaining the temperature at or above ℃.

Further, the single crystal is characterized in that the temperature of the single crystal is maintained at 50° C. or higher from the end of pulling to the end of the polarization treatment through the predetermined processing steps. (6) Embodiments of the invention Example 1 Lithium niobate material in a platinum crucible is melted at 1300°C, a 128-beam Y-axis seed crystal is immersed, and the lithium niobate material is melted at a predetermined speed.
282 A LiNbO3 single crystal is grown by pulling in the Y-axis direction.

The pulled single crystal is slowly cooled at a rate of 1° C./min, and when the temperature reaches about 60° C., the z-axis direction of the single crystal is quickly confirmed using X-rays. Next, the surface of the single crystal is cut to make it easier to attach the electrodes, but at this time the temperature of the cutting oil and tools is around 6.
It is 0°C. Next, platinum electrode plates are provided on both sides of the single crystal in the z-axis direction. Use an electrode plate that has been preheated to 60°C and apply it quickly. If the waiting time for the process is long, keep the temperature at 6°C in a small electric furnace. Thereafter, it is placed in an electric furnace preheated to 60°C. The temperature of the single crystal from the X-ray measurement until before being placed in the electric furnace was maintained at approximately 60°C and at the lowest temperature of 50°C.

Start heating again in the electric furnace, raise the temperature until it reaches around 1200℃, and then apply electricity to the lead wire connected to the electrode plate to increase the voltage to about 2V.
While the voltage of /CTrL is applied, the X temperature is lowered at a rate of 1° C./min to polarize, and the power source voltage is cut off at 100° C., and the crystal is cooled to room temperature to obtain a polarized LlNbO3 single crystal. When 50 single crystals were manufactured using the above method, the crack occurrence rate was 2%, which was confirmed to be significantly improved compared to the conventional rate of 90%.

Furthermore, the crack occurrence rate was similarly low when the single crystal was left at 60° C. for 7 days before being placed in the electric furnace for polarization treatment, and when it was cut after being left standing. Example 2 A lithium tantalate material in a platinum rhodium crucible is melted at 1700° C., the X-axis seed crystal is immersed therein, and the material is pulled up at a predetermined speed to grow a lithium tantalate single crystal.

The pulled single crystal is slowly cooled at a rate of 1° C./min, and when the temperature reaches about 55° C., the z-axis direction of the single crystal is quickly confirmed using X-rays. Next, the surface of the single crystal is cut to make it easier to attach the electrodes, but at this time the temperature of the cutting oil and tools is around 5-5.
The temperature is 5°C. Next, silver-palladium paste is applied as electrodes to both sides of the single crystal in the Z-axis direction. Use electrode paste that has been preheated to 55°C and apply it quickly. Thereafter, it is placed in an electric furnace preheated to 55°C. The temperature of the single crystal from the X-ray measurement to before being placed in the electric furnace was maintained at approximately 55°C and at least 45°C.

Start heating again in the electric furnace, raise the temperature until it reaches around 640℃, apply electricity to the lead wire connected to the electrode paste, apply a voltage of about 10V/(V7l), and lower the X temperature at a rate of 1℃/min. LiTa was polarized at 500°C, then cooled to room temperature without turning off the power supply voltage, and polarized.
Obtain an O3 single crystal. When 50 single crystals were manufactured using the above method, the crack occurrence rate was 1%, which was 60% compared to the conventional method.
It was confirmed that it was significantly improved compared to . (7
) Effects of the Invention As described above, according to the present invention, the crack occurrence rate of ferroelectric single crystals such as lithium niobate and lithium tantalate pulled by the pulling method can be significantly reduced, and as a result, single crystal products can be improved. This has the remarkable effect of dramatically improving yields and significantly improving quality and reliability.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the temperature at which pulled lithium niobate and lithium tantalate single crystals are left and the crack occurrence rate.

Claims (1)

[Claims] 1. After pulling a lithium niobate (LiNbO_3) single crystal by a pulling method, the single crystal is heated to approximately its Curie temperature or higher through a step of slow cooling and, if necessary, a predetermined processing step. In a single crystal production method comprising at least a step of performing polarization treatment by heating to a temperature and applying a predetermined voltage, the single crystal is temperature 4
A method for producing a single crystal, characterized by maintaining the temperature at 0°C or higher. 2. The method for producing a single crystal according to claim 1, characterized in that the temperature of the single crystal is maintained at 50° C. or higher from the end of pulling until the end of polarization treatment through the predetermined processing steps. 3 After pulling a lithium tantalate (LiTaO_3) single crystal by a pulling method, the single crystal is heated to approximately its Curie temperature or higher through a slow cooling process and, if necessary, a predetermined processing process, to a predetermined temperature. A single crystal manufacturing method comprising at least a step of performing polarization treatment by applying a voltage of A method for producing a single crystal characterized by maintaining. 4. The method for producing a single crystal according to claim 3, characterized in that the temperature of the single crystal is maintained at 50° C. or higher from the end of pulling to the end of the polarization treatment through the predetermined processing steps.