JPS63210100A - Production of single-domain lithium tantalate single crystal - Google Patents

Abstract

PURPOSE:To remarkably improve the quality of the title crystal without oxygen deficiency and strained parts by heat-treating the crystal grown by the Czochralski method at specified oxygen partial pressure and temp., cutting both ends of the crystal, and then forming a single domain in the crystal. CONSTITUTION:A lithium tantalate single crystal is grown in the atmosphere having <=10% oxygen partial pressure by using the Czochralski method. The single crystal is heat-treated at 900-1,200 deg.C for >=5hr in the atmosphere having >=20% oxygen partial pressure. Both ends of the heat-treated single crystal are cut, and then a single domain is formed in the crystal.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for producing single-domain lithium tantalate single crystals used as surface acoustic wave devices, pyroelectric devices, and optical devices. The present invention relates to a method for producing single crystals with high yield.

[Conventional technology]

Since the lithium tantalate single crystal has a high melting point of about 1630° C., the crucibles that can be used are generally limited to those made of iridium or platinum-rhodium alloy. Among these, iridium is easily oxidized at high temperatures, so it is used in an inert gas atmosphere. Platinum-rhodium alloys do not have any oxidation problems, but in order to prevent rhodium from being mixed into the crystal,
44202, is often used in an inert gas atmosphere. For this reason, no matter which crucible is used, the occurrence of oxygen defects is unavoidable. In order to reduce this defect, Japanese Patent Application Laid-Open Nos. 59-69490 and 55-42
238, a method of growing in a mixed gas atmosphere containing an inert gas and a small amount of oxygen has been proposed, but the effect was not necessarily sufficient.

On the other hand, regarding single-area processing,
How to make it into a single domain while maintaining the raised shape as shown in 3.
There are many methods, such as a method of embedding it in powder to form a single domain as in JP-A-57-140400, a method of processing both ends after growing and inspecting the inside, and then forming a single-domain as in JP-A-61-141699. There are several methods proposed, but if you try to make it into a single domain without applying sufficient heat treatment,
During the single-domain processing, the crystal may have a surface crank, or because the crystal is made into a single-domain with inherent distortion, the entire crystal is not uniformly made into a single-domain, and this may occur during or after the single-domain processing. There were problems such as the crank fitting during the machining process. In order to solve these problems, there are methods that combine heat treatment and single-segment processing as in JP-A-57-67100, and methods that perform heat treatment after single-sector processing as in JP-A-58-48519. However, the effect was not sufficient because single-domain processing was performed without sufficiently removing internal distortions.

[Problem that the invention seeks to solve]

As mentioned above, in the conventional technology, ■ Because it is difficult to remove oxygen defects caused by the atmosphere during growth, coloring and internal distortions occur, when processing the crystal to create various devices. The performance and reliability of the device is tf.
■ Crank tends to occur in the crystal because it is attempted to make it into a single domain while the strain is still present, and it is difficult to make it into a single domain uniformly throughout the crystal.
There were two problems.

The purpose of the present invention is to provide an optimal treatment process to simultaneously solve these two problems, and in particular to provide optimal conditions for the heat treatment process, thereby increasing the yield of uniform single-domain lithium tantalate single crystals. The purpose is to provide a method for manufacturing well.

[Means for solving problems]

A feature of the present invention is that, prior to the single-domain treatment, heat treatment and cutting of both ends of the crystal are performed in advance. Heat treatment is performed at a temperature of 900°C or higher in an atmosphere with an oxygen partial pressure of 20% or higher.
By performing the heating at 200° C. or lower for 5 hours or more, it was effective in reducing oxygen defects, removing coloring, and alleviating internal strain. When the temperature was less than 900° C., oxygen did not diffuse and the above effect could not be achieved. On the other hand, 1200
At high temperatures exceeding ℃, surface cranks were sometimes formed in the crystals.

Therefore, the optimal heat treatment temperature is 900°C or more and 1200°C or less, more preferably 1000°C to 1150°C. Cutting both ends of the crystal is important in the sense that it removes the part of the crystal where the strain is most accumulated. The upper end of the crystal is strongly distorted due to the rapid increase in crystal diameter during growth, and the lower end of the crystal is subjected to large temperature changes when the melt and crystal are switched at the end of growth. Such strong distortion cannot be removed by heat treatment, and if single-domain processing is performed with this distortion still present, cranks will occur from this distorted part during single-domain processing.
In some cases, cracks spread to the entire crystal. For this reason, when both ends of the crystal were cut before the single-domain processing, the occurrence of cracks in the single-domain processing completely disappeared.

To summarize the above, prior to the single-domain treatment, there are two steps: (1) heat treatment at an oxygen partial pressure of 20% or more and a temperature of 900°C or more and 1200°C or less for more than 5 hours, and (2) cutting both ends of the crystal after the heat treatment. Through these operations, it became possible to produce uniform single-domain lithium tantalate single crystals with a high yield.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to specific examples.

Example 1 Using an iridium crucible with a diameter of 150 mm,
A lithium tantalate single crystal with a length of 120 am was pulled in the X-axis direction. Nitrogen gas was flowed into the furnace at a rate of 101/min during growth and cooling. The grown crystals were colored dark brown.

This crystal was divided into two parts, and the upper half was subjected to single-domain processing as it is, and the lower half was heat-treated and then subjected to single-domain processing.

For the upper half, single-domain processing was performed using the usual method of forming electrodes on both sides of the crystal in the Z-axis direction, heating the crystal to 700 °C, applying a DC voltage between both electrodes, and then cooling it. I did it. After the single-domain treatment, several surface cracks were observed at the top of the crystal, but these surface cracks expanded into the interior of the crystal during the process of cutting off the top of the crystal for the purpose of internal observation. The coloring of the crystals had faded in some areas, and the shading was uneven. When two faces were cut out from this crystal and examined by etching, it was confirmed that they were made into a single domain. However, as shown in FIG. 1-a, it was found that many etch pits were present and distortion remained in the crystal.

The remaining lower half was heat treated in the air at 1100° C. for 8 hours in a box electric furnace. Heat treatment was performed by placing lithium tantalate powder on an alumina tray and placing the crystals on top of it. After heat treatment, the lower end of the crystal was cut off, both ends were polished to a mirror finish, and the internal coloring was examined. It was confirmed that the crystal was colorless, although it was slightly opaque. In addition, this time, even when cutting the lower end of the crystal, there was no occurrence of two or more cranks.

Next, the lower half of the crystal was subjected to a single domain treatment under the same conditions as the upper half of the crystal. When we examined the crystals after single-domain treatment, we found that
It remained colorless, and the opacity before the single-domain treatment disappeared, and it became completely colorless and transparent. Also, like the upper half, 2
When the surface was cut out and etched, it was found that it was formed into a single domain, and as shown in FIG. 1-b, the etch focus density was low and the distortion was relaxed.

Example 2 Using an iridium crucible with a diameter of 150 mm, a diameter of 85 mm x
A lithium tantalate single crystal with a length of 120 degrees was pulled up in the Y direction with a rotation of 36 degrees. During growth and cooling, a mixed gas of oxygen and nitrogen was flowed into the furnace at a rate of 101/min so that the oxygen partial pressure was 3%. The grown crystals were yellowish.

After this crystal was heat treated at 1100° C. for 5 hours in the air, both ends of the crystal were cut. No cracks occurred at all during cutting. Next, electrodes are formed on the top and bottom ends of the crystal, and
Single domain processing was performed by heating to 00°C, applying a DC voltage between both electrodes, and cooling to room temperature.

No cranking occurred during the single-area processing stage. These crystals are processed into wafers with a diameter of 75 m1 and a thickness of 0.35 m, and each wafer is inspected for coloring at a rate of 1 in 10.
A strain test using orthogonal polarized light and an etching test confirmed that single domain formation was performed over the entire crystal, that there was no coloring or uneven coloring, and that there was no local strain.

〔Effect of the invention〕

As described above, according to the present invention, in order to remove oxygen defects and cut both ends of a strongly distorted crystal before the single-domain treatment, (1) cranking of the crystal during an intermediate process is eliminated; A colorless and transparent crystal is obtained; ■ Uniform single-domain formation is possible without remaining partially multi-domain or opposite-direction domains; ■ Local distortion detected under orthogonal polarization is eliminated; ■Etch pit density was also lowered, and crystal quality was greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a photograph of the crystal structure of a lithium tantalate single crystal after the single-domain treatment described in Example 1. A shows the case where no heat treatment was performed, and b shows the case where the single domain processing was performed after heat treatment. Procedural amendment

Claims (1)

[Claims] A lithium tantalate single crystal grown using the Czochralski method in an atmosphere with an oxygen partial pressure of 10% or less is grown at an oxygen partial pressure of 20% or less.
% or more at a temperature of 900°C to 1200°C for 5 hours or more, and cutting both ends of the heat-treated single crystal to form a single domain. Method of manufacturing crystals.