JPH09188596A - Processing of lithium niobate single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of crack in the cutting work of a lithium niobate single crystal grown by a pull-up method and having X-axis as the growing direction and to improve the processing yield of the crystal by heat- treating the lithium niobate single crystal at a specific temperature before processing. SOLUTION: A lithium niobate single crystal grown by a pull-up method and having X-axis as the growing direction is processed after the heat-treatment at a temperature higher than the Curie point of lithium niobate by >=10 deg.C and lower than the melting point (1260 deg.C), preferably higher than the Curie point by >=40 deg.C and below 1240 deg.C. Preferably, the polarization structure of the lithium niobate single crystal grown in the X-axis direction is converted from a binary divided structure into a multiple polarized structure or a pseudo-single polarization structure by the heat treatment. An intense electric field generated by the binary divided polarization structure causes the cracking. The polarization structure is converted into a multiple polarization structure or a pseudo-single polarization structure by the heat treatment to suppress the generation of cracks in processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for processing a high quality lithium niobate single crystal used for an optical material such as a waveguide type optical device.

[0002]

2. Description of the Related Art Lithium niobate has a high electro-optical effect and is widely used in waveguide type optical devices such as optical modulators and optical switches. A waveguide type optical device is manufactured by diffusing titanium, protons or the like into a lithium niobate substrate. Conventionally, when a lithium niobate single crystal is used as a waveguide type optical device, it is mainly the Z axis, that is, <0.
A Z-cut substrate has been used in which a single crystal grown in the 01> direction is cut and polished perpendicularly to the growth direction. However, Z
The cut substrate exhibits its characteristics only for light with a specific polarization direction, that is, it has polarization dependence, and can be used by inserting it arbitrarily in the middle of the existing single-mode optical fiber transmission line. I can't. Since the device configuration is a directional coupler, it is easily affected by environmental conditions such as temperature. (Reference: Ishikawa, Furuta The Institute of Electronics, Information and Communication Engineers Technical Research Report vol.91, No.511, OCS91 75 85 Page 51-58, 1992) and other disadvantages, especially external modulators for bidirectional optical transmission, etc. Was difficult to use.

On the other hand, an optical waveguide is constructed so as to propagate light in the Z-axis direction by using an X-cut substrate obtained by cutting and polishing a lithium niobate single crystal grown in the X-axis, that is, the <100> direction, perpendicularly to the growing direction. However, when an electric field is applied in the Y-axis direction, polarization independence can be easily obtained, and it is possible to manufacture a waveguide type optical device with less characteristic variation due to environmental conditions such as temperature, compared to a Z-cut substrate. There is a lot of attention, and research has been advanced recently. This X-axis grown lithium niobate single crystal has problems that the temperature conditions during growth are more severe and cracks are likely to occur during processing, as compared with the conventional Z-axis grown lithium niobate single crystal. Yes, there is a need to resolve this.

A lithium niobate single crystal is grown from a melt in a platinum crucible by a pulling method called Czochralski method (abbreviated as CZ method). The composition of the melt is 48.
A composition called a congruent composition in which the composition of the melt and the composition of the crystal match in the vicinity of 4 to 48.6 Li ₂ O% is used. This congruent composition changes depending on the crystal growth conditions and is determined by the growth orientation (X axis, Z axis pulling up) and the temperature distribution during growth.

Since the size of the crystal increases the number of devices taken from a single wafer, crystals with a diameter of 3 inches or 4 inches are usually grown. After annealing this lithium niobate single crystal, cutting, single polarization, cylindrical grinding, slicing and polishing are performed to obtain a lithium niobate substrate. However, since the crystal has a large diameter, internal strain caused by non-uniform temperature distribution in the crystal during slow cooling causes cracks during crystal growth and subsequent processing, resulting in a reduced yield. It is important to improve this and make a lithium niobate single crystal with less cracks in order to provide a low cost lithium niobate substrate. Z-axis grown crystals are described, for example, in the inventions described in JP-A Nos. 57-11896 and 63-285197, and X-axis grown lithium niobate single crystals are described in, for example, JP-A-7-144997. In the invention, the cracks are reduced by changing the growing conditions. After the growth, annealing is usually performed at a temperature of 900 to 1100 ° C. to remove the internal strain. However, although the Z-axis grown lithium niobate single crystal has some effect, it is an X-axis grown lithium niobate single crystal. It is not an effective countermeasure against cracking.

[0006]

When a lithium niobate single crystal, especially a single crystal grown in the X-axis direction, is cut and processed, cracks tend to occur and the processing yield cannot be increased.
It has been desired to improve the yield by a processing method that does not cause cracks.

[0007]

The inventors of the present invention have found that many cracks of lithium niobate during processing occur in an X-axis grown lithium niobate single crystal, which is greatly affected by the polarization structure of the single crystal. I found a thing. The Z-axis grown lithium niobate single crystal has a multi-polarized structure finely polarized in opposite directions (FIG. 1). In this multi-polarized structure, even if charges are generated due to the pyroelectric effect, the polarization axes are opposite to each other, so that they cancel each other out and the surface charge becomes zero. Even in the same Z-axis grown lithium niobate single crystal, a quasi-single-polarized structure with strong polarization is observed depending on the growth conditions (Fig. 2). In this case, the charges are not completely canceled but The electric charges accumulated in the electric field are slowly discharged in the air or through the crystal surface. On the other hand, it was found that in the X-axis grown lithium niobate single crystal, the center of the crystal had a (−) pole and the outside had a (+) pole, which was a two-divided polarization structure (FIG. 3). In this two-divided polarization structure, when a charge is generated by the piezoelectric or pyroelectric effect, the internal resistance of the crystal is high, so that (-) charge is accumulated in the center of the crystal, which induces a high electric field between the crystal surface and the crystal surface. It was found that the high electric field caused dielectric breakdown of the crystal at a portion having low insulation resistance, which triggered the crystal to break. The part with a low insulation resistance may be a part where subgrains or holes are gathered.

As a result of various investigations, the high electric field generated by this two-divided polarization structure causes cracking. Therefore, by eliminating this two-divided polarization structure into a multi-polarization or quasi-single-polarization structure, cracking during processing occurs. It was found that the heat treatment can be prevented, and that the heat treatment for changing the polarization structure is the most effective. Normally, annealing of a Z-axis grown lithium niobate single crystal is 90
Although it is performed at a temperature of 0 to 1100 ° C., in the X-axis grown lithium niobate single crystal, annealing in this temperature range slightly reduces thermal strain, but it is not sufficient to prevent cracking during processing, and the crystal curie It is possible to sufficiently suppress cracking during processing of an X-axis oriented lithium niobate single crystal by changing the polarization structure from a bipartite polarization structure to a multi-polarization structure or a pseudo-single polarization structure by heat treatment at a temperature determined by the temperature. Was found.

Therefore, according to the present invention, a lithium niobate single crystal grown by the pulling method and having a growth orientation on the X-axis is heat-treated at a temperature of at least 10 ° C. above the Curie temperature of the lithium niobate and below its melting point. The present invention provides a method for processing a lithium niobate single crystal, which is characterized in that the processing is carried out.

According to the studies and experiments conducted by the present inventors, it is necessary to perform heat treatment at a temperature at least 10 ° C. higher than the Curie temperature in order to change the polarization structure of the X-axis grown lithium niobate single crystal. Heat treatment at a temperature near or below the temperature cannot prevent cracking during processing that occurs during X-axis growth. The upper limit temperature of the heat treatment needs to be the melting point of the crystal (1260 ° C. = 1533 ° K.) or less, but 1240 ° C. or less is preferable. As a more preferable heat treatment temperature range, it is desirable to perform the heat treatment in a temperature range of at least 40 ° C. or higher above the Curie temperature to 1220 ° C. or lower. Since the Curie temperature changes by 40 ° C. or more per 1% of change in crystal composition, it is necessary to set the heat treatment temperature according to each crystal composition in order to obtain good heat treatment results. In addition, even if the raw material composition is the same, the out-diffusion rate of lithium oxide in lithium niobate differs depending on the growth conditions and the slow cooling environment, and the composition of the crystals changes, so heat treatment under the same composition and the same growth conditions It is necessary to determine the temperature.

As is well known, the Curie temperature is a transition temperature of a ferroelectric substance to a paraelectric substance, and by accurately measuring this, a Curie temperature of an oxide piezoelectric material such as lithium niobate can be measured. It is possible to know the composition and physical properties with high accuracy. In that case, the Curie temperature is L which constitutes the crystal.
It changes depending on the ratio of i and Nb, and if the amount of Li is large, the Curie temperature becomes high. Curie temperature is usually measured by differential thermal analysis (D
TA) or differential scanning calorimetry (DSC). The measurement of the Curie temperature of lithium niobate is described in detail in JP-A-6-201618 or JP-A-6-265495, but simply speaking, a reference substance such as α-alumina is put in one container. , Put a sample of lithium niobate in the other container and heat in a furnace, gradually raise the temperature and measure the difference in temperature rise between the two. When the temperature of the sample exceeds a Curie temperature at a certain temperature, an endothermic reaction is exhibited, so the position of the endothermic peak is defined as the Curie temperature. At that time, the measurement accuracy can be improved by adjusting, for example, the particle size of the powder sample or the distribution thereof.

Further, until now, the melt composition of the grown crystal and the Curie temperature were compared, but the composition of the melt and the composition of the crystal did not match depending on the composition, so the crystal composition can be known from the Curie temperature. Therefore, it is necessary to correctly obtain the relationship between the composition of lithium niobate and the Curie temperature, and a method of preparing a lithium niobate standard sample for that purpose is described in JP-A-7-97252 or JP-A-7-53253. . For example, the accuracy can be improved by pressing the raw material and firing the container in a sealed state.

In the present invention, the heat treatment temperature is determined from the Curie temperature of the lithium niobate crystal thus obtained, and by performing the heat treatment at that temperature, the X-axis grown niobate which could not be conventionally achieved. It is possible to sufficiently prevent cracking during processing of a lithium single crystal.

As described above, the heat treatment temperature in the present invention is a temperature of at least 10 ° C. higher than the Curie temperature and not higher than the melting point (1260 ° C.), preferably the Curie temperature plus 40 ° C. or higher and 1240 ° C. or lower, and the upper limit is 122.
It is more preferable that the temperature is 0 ° C. or lower. After this heat treatment,
Various processes such as cutting process, slicing process, and polishing process are performed according to a conventional method, and cracks at that time can be sufficiently prevented.

This heat treatment is conventionally performed at 1000 ° C.
It can be performed independently of or combined with the annealing before and after. When independently performed, it is preferable to perform annealing before the heat treatment of the present invention. As described above, the thermal strain can be reduced by annealing, but it cannot sufficiently suppress cracks during processing.

[0016]

EXAMPLES Examples and comparative examples will be given below. (Example 1) A lithium niobate single crystal was grown in a growth furnace of the CZ (Czochralski) method. 125 mm in diameter,
About 6.2 kg of lithium niobate raw material (48.5 Li2 O%) was dissolved in a platinum crucible having a depth of 125 mm, and a seed crystal of lithium niobate was immersed in the melt and the seed crystal was fed at a rate of 15 to 24 per minute. It was grown at a pulling rate of 2 mm per hour while being rotated. The growing direction was such that the pulling direction was the X axis, and a lithium niobate single crystal having a diameter of 80 mm and a length of 100 mm was obtained. Under this growth condition, this lithium niobate single crystal has a Curie temperature of 1142 ° C. both at the upper part (early growth stage) and at the lower part (end stage of growth) of the crystal, and the lithium niobate single crystal taken out from the growth furnace has a Curie temperature plus 58. The heat treatment was performed at 20 ° C., that is, 1200 ° C. for 20 hours. 20 minutes for heating and 20 hours for cooling
Time. Next, the shoulder and bottom of the crystal were cut off with a slicing machine. Further, in order to examine the polarization structure of the crystal, a part of the crystal was cut out so that the Z plane was exposed and the polarization orientation was examined, and it was confirmed that the crystal had a multi-polarization structure.

After that, in order to align the polarization directions of the crystals,
Single-domain processing (boring) was performed at 1160 ° C. for 6 hours by applying an electric field of 1.5 V / cm to the crystal. This crystal was rounded to a diameter of 3 inches (76.2 mm), an orientation flat serving as a mark of the crystal orientation was attached by polishing, and this was made into a wafer having a thickness of 1 mm with a wire saw. By the processing up to this point, no cracks occurred in the grown three lithium niobate single crystals. (Example 2) The same procedure as in Example 1 was carried out except that the heat treatment temperature after the growth was set to the Curie temperature plus 38 ° C, that is, 1180 ° C, but no crack was generated in the three grown lithium niobate single crystals. . Further, in order to investigate the polarization structure of the crystal, a part of the crystal after the heat treatment was cut out so that the Z plane was exposed and the polarization orientation was examined, and it was confirmed that the crystal had a multi-polarization structure. (Comparative example) Curing temperature plus 8 after heat treatment
When the same procedure as in Example 1 was carried out except that the temperature was set to 1 ° C, that is, 1150 ° C, cracks due to discharge occurred in all 17 crystals mainly during slicing. In addition, in order to investigate the polarization structure of the crystal, a part of the crystal after the heat treatment was cut out so that the Z plane was exposed and the polarization orientation was examined, and it was confirmed that the crystal had a two-divided polarization structure.

[0018]

According to the present invention, the two-divided structure that causes cracking during processing of lithium niobate can be eliminated, whereby cracking can be reduced, and a lithium niobate substrate can be efficiently manufactured.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a multi-polarized structure of a lithium niobate single crystal.

FIG. 2 is an explanatory view showing a pseudo monopolar structure of the above crystal.

FIG. 3 is an explanatory diagram showing a two-divided polarization structure of the above crystal.

Claims (3)

[Claims] 1. After heat treatment of a lithium niobate single crystal grown by a pulling method and having a growth orientation on the X axis, at a temperature of at least 10 ° C. above the Curie temperature of the lithium niobate and below its melting point. A method of processing a lithium niobate single crystal, which comprises processing. 2. The heat treatment temperature is higher than the Curie temperature of the lithium niobate single crystal by at least 40 ° C. or more.
The method for processing a lithium niobate single crystal according to claim 1, which is 40 ° C. or lower. 3. The lithium niobate single crystal according to claim 1, wherein the polarization structure of the lithium niobate single crystal grown on the X axis by the heat treatment is changed from a bipartite structure to a multipolarization structure or a pseudo-monopolarization structure. Crystal processing method.