JP2020040840A - Method for manufacturing lithium tantalate substrate - Google Patents

Abstract

To provide a method for manufacturing a lithium tantalate substrate, capable of stably manufacturing a lithium tantalate (LT) substrate excellent in electrical properties even when production lots of aluminum oxide powder (AlOpowder) are different.SOLUTION: The method for manufacturing a LT substrate using a LT crystal grown by the Czochralski method comprises: burying the LT crystal treated in the state of a substrate into powder mixed with aluminum powder (Al powder) charged in a vessel and AlOpowder subjected to humidification treatment; arranging the vessel in a heating furnace; and heating the LT crystal at a temperature less than the Curie temperature of the LT crystal to manufacture the LT substrate having a desired volume resistivity. An appropriate water included in the AlOpowder by the humidification treatment becomes water vapor during the heat treatment, and hydrogen obtained by reacting the water vapor with the aluminum powder can contribute to the reduction of the LT crystal.SELECTED DRAWING: None

Description

  The present invention relates to a method for manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method, and particularly to a method for stably manufacturing a lithium tantalate substrate having excellent electrical characteristics. The present invention relates to a method for manufacturing a lithium substrate.

  A lithium tantalate (hereinafter sometimes abbreviated as LT) crystal is a ferroelectric substance having a melting point of about 1650 ° C. and a Curie temperature of about 600 ° C. A lithium tantalate substrate manufactured using this crystal has It is mainly applied as a surface acoustic wave (SAW) filter material used in transmitting and receiving devices of mobile phones.

  It is predicted that the number of SAW filters in the frequency range around 2 GHz will increase rapidly in the future due to the increase in the frequency of mobile phones and the spread of Bluetooth (registered trademark) (2.45 GHz), which is a wireless LAN for various electronic devices.

  The SAW filter has a structure in which a pair of comb-shaped electrodes is formed of a metal thin film of Al, Cu, or the like on a substrate made of a piezoelectric material such as LT. It has an important role to play. The above-mentioned comb-shaped electrode is formed by forming a metal thin film on a piezoelectric material by sputtering, leaving a pair of comb-shaped patterns, and removing unnecessary portions by etching using a photolithographic technique.

  In addition, the LT single crystal is industrially grown mainly by the Czochralski method in an electric furnace having a nitrogen-oxygen mixed gas atmosphere having an oxygen concentration of about several% to 20%. An iridium crucible having a melting point is used, and the grown LT single crystal is obtained by being cooled at a predetermined cooling rate in an electric furnace and then taken out of the electric furnace.

The grown LT crystal has a colorless and transparent color or a highly transparent pale yellow color. After the growth, in order to remove the residual strain due to the thermal stress of the crystal, heat treatment is performed under a soaking temperature close to the melting point, and further, a poling treatment for making the single polarization, that is, the LT crystal is heated from room temperature to a predetermined temperature equal to or higher than the Curie temperature. After the temperature is raised, a voltage is applied to the crystal, and the temperature is lowered to a predetermined temperature equal to or lower than the Curie temperature while the voltage is applied, a series of processes of stopping the voltage application and cooling to room temperature are performed. After the poling process, the LT crystal (referred to as an ingot) that has been subjected to outer peripheral grinding to adjust the outer diameter of the crystal becomes a substrate through mechanical processing such as slicing, wrapping, and polishing. The substrate finally obtained is almost colorless and transparent, and has a volume resistivity of about 10 ¹⁴ to 10 ¹⁵ Ω · cm.

  By the way, in a substrate obtained by such a conventional method, in a surface acoustic wave device (SAW filter) manufacturing process, due to pyroelectricity, which is a characteristic of LT crystal, electric charges are deposited on the substrate surface by a temperature change received in the process. The comb-shaped electrodes formed on the substrate surface are destroyed due to the charge-up and the resulting discharge, and furthermore, the substrate is cracked or the like, and the yield in the element manufacturing process is reduced.

In order to solve the problem caused by the pyroelectricity of the LT crystal, several techniques for increasing the conductivity have been proposed. For example, in Patent Document 1, an LT crystal (hereinafter, referred to as a “substrate”) processed into a substrate in a reducing atmosphere of a gas selected from argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen and a combination thereof. A method of increasing the conductivity by heat treatment (reduction treatment) of the “LT crystal having a shape” and distinguishing it from the LT substrate after the reduction treatment) has been proposed. A method of increasing the conductivity by heat treatment in a reduced-pressure atmosphere has been proposed. Patent Document 3 proposes a method in which a heat treatment is performed while supplying an inert gas having an oxygen partial pressure of 1 × 10 ⁻²² atm or less using a oxygen pump into a processing chamber in which a substrate-shaped LT crystal is accommodated. Patent Document 4 proposes a method in which a substrate-shaped LT crystal is embedded in a mixed powder of an aluminum powder (Al powder) and an aluminum oxide powder (Al ₂ O ₃ powder) and heat-treated. The LT substrate having the increased conductivity causes light absorption due to the introduction of oxygen vacancies. Since the color tone of the LT substrate observed is reddish brown in transmitted light and black in reflected light, the reduction treatment for increasing conductivity is also referred to as blackening treatment. Is called blackening.

JP-A-11-92147 JP 2004-152870 A Japanese Patent No. 6001261 (see FIG. 3) Japanese Patent No. 4063191

By the way, unlike the lithium niobate crystal having a relatively low melting point of about 1250 ° C. and applying the methods of Patent Documents 1 and 2 to a lithium tantalate crystal having a high melting point of about 1650 ° C., the conductivity of the LT substrate is low. However, there is a problem that the problem due to pyroelectricity is not sufficiently improved because the property is not sufficiently increased. Further, in the method of Patent Document 3 in which heat treatment is performed by using an oxygen pump to supply an inert gas having an oxygen partial pressure of 1 × 10 ⁻²² atm or less into a processing chamber in which LT crystals are accommodated, 10 ⁻¹² to 10 ⁻ An electrical conductivity (volume resistivity of about 10 ¹¹ to 10 ¹² Ω · cm) of about ¹¹ Ω ⁻¹ · cm ⁻¹ (see the graph of FIG. 3 in Patent Document 3) is obtained, but inertness supplied into the processing chamber. In order to make the oxygen partial pressure of the gas 1 × 10 ⁻²² atm, it is necessary to install an expensive oxygen pump composed of a solid electrolyte such as zirconium oxide (ZrO ₂ ), which is prepared by the oxygen pump. Since the amount of the inert gas is small, only a small number of LT crystals can be processed, and there is a problem that production cost and productivity are poor.

  On the other hand, the method of Patent Document 4 in which a substrate-shaped LT crystal is embedded in a mixed powder of an aluminum powder and an aluminum oxide powder and heat-treated (reduction treatment) is obtained, while obtaining a desired volume resistivity (conductivity). Since there is no need to install an expensive oxygen pump, it has an advantage in production cost. However, when the production lot of the aluminum oxide powder used is different, the volume resistivity of the LT substrate after the heat treatment may fluctuate. For this reason, it is necessary to change the ratio of the aluminum powder in the mixed powder in accordance with the production lot of the aluminum oxide powder to be used, and to adjust to obtain a desired volume resistivity. Had an inferior problem. Further, when the ratio of the aluminum powder in the mixed powder is 20% by mass or more, black spots having a diameter of about 1 to 5 mm (color unevenness, that is, reduction unevenness) are easily generated, and there is a problem that productivity is deteriorated. Was.

  The present invention has been made in view of such problems, and the problem is that even when the production lot of the aluminum oxide powder to be used is different, the electrical characteristics are excellent without deteriorating the productivity. An object of the present invention is to provide a method for manufacturing a lithium tantalate substrate that can stably manufacture a lithium tantalate substrate.

  Then, in order to solve the above-mentioned problem, the present inventor conducted an in-depth analysis on the cause of the fluctuation of the volume resistivity of the LT substrate after the heat treatment when the production lot of the aluminum oxide powder used is different.

as a result,
(1) that the water contained in the aluminum oxide powder is involved in the reduction of the LT crystal in addition to the reducing action by the aluminum powder;
(2) the amount of water contained in the aluminum oxide powder is different for each production lot;
(3) When the production lot of the aluminum oxide powder to be used is different, the volume resistivity of the LT substrate fluctuates due to the difference in the amount of water involved in the reduction of the LT crystal;
We have obtained some technical knowledge.

  The present invention has been completed based on such technical analysis and knowledge.

That is, the first invention according to the present invention is:
A method of manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method, which is processed into a substrate state in a mixed powder of aluminum powder and aluminum oxide powder filled in a container. In the method of embedding the lithium tantalate crystals, and, after placing the container in a heating furnace, a heat treatment at a temperature lower than the Curie temperature of the lithium tantalate crystals to produce a lithium tantalate substrate,
The aluminum oxide powder is characterized by being humidified.

The second invention is
In the method for producing a lithium tantalate substrate according to the first invention,
The aluminum oxide powder is characterized by being dried before the humidification.

The third invention is
In the method for producing a lithium tantalate substrate according to the first invention or the second invention,
The humidified aluminum oxide powder has a water content of 0.08% by mass to 0.4% by mass.

The fourth invention is
In the method for producing a lithium tantalate substrate according to any one of the first to third inventions,
The ratio of the aluminum powder in the mixed powder is less than 20% by mass.

The fifth invention is
The method for producing a lithium tantalate substrate according to any one of the first to fourth inventions,
An inert gas is continuously supplied and discharged into the heating furnace under an atmospheric pressure atmosphere.

In a sixth aspect,
In the method for producing a lithium tantalate substrate according to the fifth invention,
The inert gas is composed of argon gas, and the flow rate of argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5 L / min.

According to the method for producing a lithium tantalate substrate according to the present invention,
Since the aluminum oxide powder is humidified and the amount of water contained in the aluminum oxide powder is appropriately adjusted, it is possible to prevent a change in the volume resistivity of the lithium tantalate substrate even when the production lot of the aluminum oxide powder used is different. It becomes possible. Therefore, even when the production lot of the aluminum oxide powder is different, there is an effect that a lithium tantalate substrate having excellent electric characteristics can be stably produced without deteriorating the productivity.

  Furthermore, the proportion of the aluminum powder in the mixed powder can be set lower than before due to the appropriately adjusted reducing action of the water contained in the aluminum oxide powder. Therefore, since it is not necessary to set the ratio of the aluminum powder to 20% by mass or more, there is an effect that occurrence of black spots (defective color unevenness) having a diameter of about 1 to 5 mm can be suppressed.

  Hereinafter, embodiments of the present invention will be specifically described.

  The present invention relates to an aluminum oxide used in a method of manufacturing an LT substrate in which a lithium tantalate crystal (LT crystal having a substrate shape) processed into a substrate state is embedded in a mixed powder of an aluminum powder and an aluminum oxide powder and heat-treated. The present invention is characterized in that the aluminum oxide powder is humidified so that the volume resistivity of the LT substrate after the treatment does not fluctuate even when the production lot of the powder is different.

  Further, by performing a drying treatment before the humidifying treatment of the aluminum oxide powder, moisture present in the aluminum oxide powder which differs for each production lot is removed, and a moisture amount present in the aluminum oxide powder after the drying step is obtained. , The humidification process can be performed stably so that the desired volume resistivity is obtained.

Hereinafter, a method of manufacturing an LT substrate according to the present invention will be described for each process.
(A) Step of drying aluminum oxide powder This step is a step of removing water present in the aluminum oxide powder. The aluminum oxide powder is placed in a container made of alumina, and heated at 300 ° C. to 1100 ° C. in an electric furnace. By treating for 10 to 40 hours, an aluminum oxide powder from which water has been removed is prepared.

  When the temperature of the electric furnace is 300 ° C. or higher, moisture present in the aluminum oxide powder can be removed. However, by performing the heating at about 1000 ° C., impurities and the like contained in the aluminum oxide powder can be removed, which is more preferable. .

  The heating time depends on the amount of the aluminum oxide powder to be heat-treated, but is appropriately set so that moisture can be uniformly removed.

  The atmosphere for the drying treatment is air or an inert gas atmosphere, and argon gas or nitrogen gas can be used as the inert gas.

  By performing the drying treatment, it is possible to make the amount of water present in the aluminum oxide powder different for each production lot substantially the same.

  By this drying treatment, the water content of the aluminum oxide powder can be reduced to 0.04% by mass or less. The water content is a value measured by a Karl Fischer moisture meter.

(B) Step of humidifying aluminum oxide powder This step is intended to intentionally adsorb moisture to the aluminum oxide powder from which the moisture has been removed by the drying treatment, or to treat the aluminum oxide powder that has not been subjected to the drying treatment. This is a step of intentionally adsorbing moisture. In this step, the aluminum oxide powder that has been subjected to the drying treatment or has not been subjected to the drying treatment is placed in a stainless steel container, and adjusted to have a predetermined moisture content in a humidity chamber or the like. For example, in a constant-humidity chamber, the treatment is performed at a temperature of 30 to 60 ° C. and a humidity of 70 to 90% RH for 12 to 60 hours to reduce the amount of water contained in the aluminum oxide powder to 0.08 to 0.4 mass%. adjust. The temperature, humidity, holding time, and the like may be appropriately set so that the water content of the aluminum oxide powder falls within a predetermined range.

When the humidification treatment was not performed on the aluminum oxide powder from which water was removed by the drying treatment, the water content of the aluminum oxide powder was 0.04% by mass or less. Even if the LT crystal is heat-treated by applying aluminum oxide powder having a water content of 0.04% by mass or less to the mixed powder, the volume resistivity of the LT substrate is only about 7.0 × 10 ¹² Ω · cm, It was insufficient (see Comparative Example 1 below).

(C) Step of mixing aluminum powder and aluminum oxide powder This step is a step of mixing aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder). In this step, (A) humidification treatment is performed on (A) aluminum oxide powder from which moisture has been removed in the drying treatment step or on aluminum oxide powder not subjected to the above-mentioned drying treatment for removing moisture. The aluminum oxide powder (Al ₂ O ₃ powder) to which water has been adsorbed in the step is mixed with the aluminum powder (Al powder).

  The ratio of the Al powder to be mixed is preferably less than 20% by mass, more preferably 15% by mass or less, and further preferably 5 to 15% by mass.

  When the ratio of the Al powder is set to 20% by mass or more, sufficient conductivity (volume resistivity) of the LT substrate can be achieved, but a black dot (color unevenness) of about 1 to 5 mm in diameter increases as the ratio of the Al powder increases. It is confirmed that the rate of occurrence of "defective" increases, and the productivity is deteriorated.

In the present invention, even when the ratio of the Al powder is less than 20% by mass, the conductivity (volume) of the LT substrate is adjusted by appropriately adjusting the amount of water contained in the aluminum oxide powder (Al ₂ O ₃ powder). Resistivity) can be improved.

(D) Heat Treatment Step of Lithium Tantalate Substrate (LT Substrate) In this step, heat treatment is performed by embedding LT crystal in the form of a substrate in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder). It is a process.

  The heat treatment temperature is lower than the Curie temperature of the LT crystal (less than about 600 ° C.), and the heat treatment atmosphere may be a vacuum condition or an inert gas sealing condition, or the inert gas may be introduced into a heating furnace under an atmospheric pressure atmosphere. The atmosphere may be a continuous supply and discharge. In the case of vacuum conditions or inert gas sealing conditions, the heat in the heating furnace may accumulate at one location and cause reduction unevenness (color unevenness). It is preferable to use an atmosphere in which an inert gas is continuously supplied and discharged into the heating furnace. By continuously supplying and discharging the inert gas into the heating furnace, heat can be made uniform in the furnace. As the inert gas, an argon gas, a nitrogen gas, or the like can be used. When the inert gas is an argon gas, the flow rate of the inert gas continuously supplied and discharged into the heating furnace is preferably 0.5 to 5 L / min.

By the way, when the substrate-shaped LT crystal is heat-treated, moisture contained in the aluminum oxide powder (Al ₂ O ₃ powder) contained in the mixed powder in an appropriate amount becomes water vapor by this heat treatment. It reacts as shown in the reaction formula to generate hydrogen, and becomes a strong reducing agent.
2Al + 6H ₂ O → 2Al (OH) ₃ + 3H ₂

  Then, as the amount of water contained in aluminum oxide increases, the volume resistivity of the LT substrate after the heat treatment tends to decrease. Therefore, by appropriately adjusting the amount of water contained in the aluminum oxide in the (B) humidifying process of the aluminum oxide powder, the ratio of the Al powder in the mixed powder can be set lower than the conventional ratio of the Al powder. Therefore, since it is not necessary to set the Al powder ratio to 20% by mass or more, it is possible to suppress the occurrence of black spots (defective color unevenness) having a diameter of about 1 to 5 mm.

  In addition, in the present invention, the amount of water contained in the aluminum oxide powder is appropriately adjusted by humidifying the aluminum oxide powder. Therefore, even when the production lot of the aluminum oxide powder to be used is different, the variation in the volume resistivity of the LT substrate is not affected. Can be prevented.

  Further, by performing a drying treatment before the humidifying treatment of the aluminum oxide powder, moisture present in the aluminum oxide powder which differs for each production lot is removed, and a moisture amount present in the aluminum oxide powder after the drying step is obtained. Makes it possible to stably perform a desired humidification process thereafter, and to stably produce an LT substrate having a desired volume resistivity even when the production lot is different. .

  Hereinafter, Examples of the present invention will be specifically described with reference to Comparative Examples, but the technical scope of the present invention is not limited by the following Examples.

[Configuration of heating furnace]
The heating furnace used in Examples, Comparative Examples and Reference Examples is provided with an air supply port and an exhaust port, and is supplied with a commercially available argon gas (oxygen partial pressure is about 1 × 10 ⁻⁶ atm). The gas is continuously supplied to the heating furnace through the port, and argon gas (inert gas) is continuously exhausted to the outside of the heating furnace through the exhaust port. ing. The flow rate of the argon gas supplied and discharged into the heating furnace is set at 2 L / min.

[Growth of LT crystal and processing of ingot, etc.]
Using a raw material having a congruent composition, an LT single crystal having a diameter of 4 inches was grown by the Czochralski method. The growth atmosphere is a nitrogen-oxygen mixed gas having an oxygen concentration of about 3%. The obtained LT crystal ingot was transparent and pale yellow.

  The LT crystal ingot is subjected to heat treatment for removing thermal strain and poling treatment for forming a single polarization, and then is subjected to outer circumference grinding, slicing, and polishing to obtain a substrate shape of 42 ゜ RY (Rotated Y axis). LT crystal processed into

The obtained 42 ° RY crystal was colorless and transparent, had a volume resistivity of 1 × 10 ¹⁵ Ω · cm, and a Curie temperature of 603 ° C.

[Example 1]
1 kg of aluminum oxide powder (Al ₂ O ₃ powder) having an average particle size of 52 μm was dried at 1000 ° C. for 20 hours to obtain an Al ₂ O ₃ powder from which water contained in the powder was removed. In addition, the said average particle diameter is the value which measured the aluminum oxide powder with the laser diffraction type particle size distribution analyzer.

The dried Al ₂ O ₃ powder was subjected to a humidification treatment at a constant humidity bath temperature of 40 ° C. and a humidity of 85% RH for 12 hours to adsorb moisture to the Al ₂ O ₃ powder. The moisture content of the Al ₂ O ₃ powder to which the moisture was adsorbed was 0.08% by mass in Karl Fischer moisture content.

A mixture of Al powder and Al ₂ O ₃ powder is obtained by mixing 90% by weight of Al ₂ O ₃ powder to which moisture has been adsorbed and 10% by weight of aluminum powder (Al powder) having an average particle size of 100 μm. Was.

  Next, the mixed powder is filled in a stainless steel container, the LT crystal processed into a substrate state is embedded in the mixed powder, and the stainless steel container in which the LT crystal is embedded has an air supply port and an exhaust port. It was placed in the heating furnace.

  Then, a commercially available argon gas is supplied and discharged into the heating furnace under an atmospheric pressure atmosphere at a flow rate of 2 L / min, and heat treatment (reduction treatment, blackening treatment) is performed at 580 ° C. for 20 hours. Was.

  The same heat treatment was performed on 200 LT crystals processed into a substrate state, the volume resistivity of the processed LT substrate was measured, and the occurrence rate of color unevenness was investigated.

  The volume resistivity is measured by a three-terminal method based on JIS K-6911.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 1.6 × 10 ⁹ Ω · cm (average value of 200 substrates, the same applies hereinafter), and the color unevenness occurrence rate is 2%. Thus, an LT substrate having excellent electrical characteristics could be obtained. The results are shown in Table 1.

[Example 2]
The dried Al ₂ O ₃ powder is subjected to a humidification treatment at a constant humidity of 40 ° C. and a humidity of 85% RH for 24 hours to reduce the water content of the adsorbed Al ₂ O ₃ powder to 0.15 mass%. The LT crystal was subjected to a heat treatment (reduction treatment, blackening treatment) under the same conditions as in Example 1 except for the above.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 8.3 × 10 ⁸ Ω · cm, the color unevenness occurrence rate is 2%, and an LT substrate having excellent electrical characteristics is obtained. Was completed.

  The results are shown in Table 1.

[Example 3]
The dried Al ₂ O ₃ powder is subjected to a humidification treatment at a constant humidity bath temperature of 40 ° C. and a humidity of 85% RH for 48 hours, and the moisture content of the Al ₂ O ₃ powder having adsorbed moisture is 0.25% by mass. The LT crystal was subjected to a heat treatment (reduction treatment, blackening treatment) under the same conditions as in Example 1 except for the above.

After heat treatment (reduction treatment, blackening treatment), the LT substrate has a volume resistivity of 6.3 × 10 ⁸ Ω · cm, a color unevenness occurrence rate of 2%, and an LT substrate having excellent electrical characteristics. Was completed.

  The results are shown in Table 1.

[Example 4]
The dried Al ₂ O ₃ powder is subjected to a humidification treatment at a constant humidity bath temperature of 40 ° C. and a humidity of 85% RH for 60 hours, and the moisture content of the adsorbed Al ₂ O ₃ powder is 0.4% by mass. The LT crystal was subjected to a heat treatment (reduction treatment, blackening treatment) under the same conditions as in Example 1 except for the above.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 3.2 × 10 ⁸ Ω · cm, the color unevenness occurrence rate is 2%, and an LT substrate having excellent electrical characteristics is obtained. Was completed. However, the time (60 hours) of the humidification treatment was prolonged, and the production efficiency was accordingly slightly reduced.

  The results are shown in Table 1.

[Example 5]
The heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed under the same conditions as in Example 2 except that 95% by mass of Al ₂ O ₃ powder adsorbed with water and 5% by mass of Al powder were mixed. Was.

After heat treatment (reduction treatment, blackening treatment), the LT substrate has a volume resistivity of 1.3 × 10 ⁹ Ω · cm, a color unevenness occurrence rate of 2%, and an LT substrate having excellent electrical characteristics. Was completed.

  The results are shown in Table 1.

[Example 6]
The heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed under the same conditions as in Example 2 except that 85% by mass of Al ₂ O ₃ powder to which moisture was adsorbed and 15% by mass of Al powder were mixed. Was.

After heat treatment (reduction treatment, blackening treatment), the LT substrate has a volume resistivity of 6.0 × 10 ⁸ Ω · cm, a color unevenness occurrence rate of 5%, and an LT substrate having excellent electrical characteristics. Was completed.

  The results are shown in Table 1.

[Comparative Example 1]
The heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed on the dried Al ₂ O ₃ powder under the same conditions as in Example 1 except that the humidification treatment was not performed. The moisture content of the dried Al ₂ O ₃ powder was 0.04% by mass in Karl Fischer moisture content.

After the heat treatment (reduction treatment, blackening treatment), the color unevenness occurrence rate was 2%, but the volume resistivity of the LT substrate was about 7.0 × 10 ¹² Ω · cm, and the LT substrate had excellent electrical characteristics. Substrate could not be obtained. The results are shown in Table 1.

[Reference Example 1]
Al ₂ O ₃ without performing drying powder and the humidification process and a 80% by mass of Al ₂ O ₃ powder, except that a mixture of Al powder in a proportion of 20 mass% in the same conditions as in Example 1 Heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed.

The water content of the Al ₂ O ₃ powder not subjected to the drying treatment and the humidification treatment was 0.09% by mass in Karl Fischer moisture content.

The volume resistivity of the LT substrate after the heat treatment (reduction treatment, blackening treatment) was 7.0 × 10 ⁸ Ω · cm, which was excellent in electrical characteristics, but the color unevenness occurrence rate was 17%, and the diameter was 1 to 1. Black spots (improper color unevenness) of about 5 mm occurred. The results are shown in Table 1.

[Reference Example 2]
The LT crystal was subjected to a heat treatment (reduction treatment, blackening treatment) under the same conditions as in Example 1 except that the drying treatment and the humidification treatment of the Al ₂ O ₃ powder were not carried out.

The water content of the Al ₂ O ₃ powder not subjected to the drying treatment and the humidification treatment was 0.09% by mass in Karl Fischer moisture content.

The volume resistivity of the LT substrate after the heat treatment (reduction treatment, blackening treatment) was 1.5 × 10 ⁹ Ω · cm, and had the same electrical characteristics as in Example 1. Was 2.5%, which was inferior to Example 1. The results are shown in Table 1.

[Verification]
(1) aluminum powder (Al powder) of the powder mixture is Example in Examples 1-4 is 10% by mass 1 (Al ₂ O ₃ powder moisture content: 0.08 wt%, the volume resistivity: 1.6 × 10 ⁹ Ω · cm),
Example 2 (moisture content of Al ₂ O ₃ powder: 0.15 mass%, volume resistivity: 8.3 × 10 ⁸ Ω · cm),
Example 3 (moisture content of Al ₂ O ₃ powder: 0.25% by mass, volume resistivity: 6.3 × 10 ⁸ Ω · cm),
From the data of Example 4 (moisture content of Al ₂ O ₃ powder: 0.40 mass%, volume resistivity: 3.2 × 10 ⁸ Ω · cm), the greater the moisture content adsorbed on Al ₂ O ₃ powder, the greater the volume resistivity Is getting smaller.

From this, it is confirmed that the water contained in the aluminum oxide powder (Al ₂ O ₃ powder) in the mixed powder is involved in the reduction of the LT crystal.

(2) Example 3 (volume resistivity: 6.3 × 10 ⁸ Ω) having substantially the same volume resistivity as Reference Example 1 (20 mass% of Al powder, volume resistivity: 7.0 × 10 ⁸ Ω · cm). · Cm), Example 4 (volume resistivity: 3.2 × 10 ⁸ Ω · cm) and Example 6 (volume resistivity: 6.0 × 10 ⁸ Ω · cm), the proportion of Al powder was less than 20% by mass (implemented). (Example 3: 10% by mass, Example 4: 10% by mass, Example 6: 15% by mass), the same volume resistivity as Reference Example 1 is obtained.

  Therefore, since it is not necessary to set the ratio of the aluminum powder to 20% by mass or more, it is confirmed that the occurrence of black spots (defective color unevenness) having a diameter of about 1 to 5 mm can be suppressed.

(3) On the other hand, in Comparative Example 1 (0.04% by mass) in which the amount of water contained in the Al ₂ O ₃ powder is smaller than that in Reference Example 1 and Reference Example 2 (0.09% by mass), water involved in the reduction of LT crystals It is also confirmed that the volume resistivity of the LT substrate after the heat treatment is lower than 1 × 10 ¹⁵ Ω · cm before the heat treatment, but is about 7.0 × 10 ¹² Ω · cm because of the small amount.

  According to the method of the present invention, even when the production lot of the aluminum oxide powder is different, a change in the volume resistivity of the lithium tantalate substrate after the heat treatment can be prevented, so that the lithium tantalate used as the substrate material for the surface acoustic wave device can be prevented. It has industrial applicability to provide stable substrates.

Claims (6)

A method of manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method, which is processed into a substrate state in a mixed powder of aluminum powder and aluminum oxide powder filled in a container. In the method of embedding the lithium tantalate crystals, and, after placing the container in a heating furnace, a heat treatment at a temperature lower than the Curie temperature of the lithium tantalate crystals to produce a lithium tantalate substrate,
A method for producing a lithium tantalate substrate, wherein the aluminum oxide powder is subjected to a humidifying treatment.   The method for producing a lithium tantalate substrate according to claim 1, wherein the aluminum oxide powder is subjected to a drying treatment before the humidification treatment.   The method for producing a lithium tantalate substrate according to claim 1 or 2, wherein the moisture content of the humidified aluminum oxide powder is 0.08% by mass to 0.4% by mass.   The method for producing a lithium tantalate substrate according to any one of claims 1 to 3, wherein a ratio of the aluminum powder in the mixed powder is less than 20% by mass.   The method for producing a lithium tantalate substrate according to any one of claims 1 to 4, wherein an inert gas is continuously supplied and discharged into the heating furnace under an atmospheric pressure atmosphere.   6. The lithium tantalate according to claim 5, wherein the inert gas is composed of argon gas, and a flow rate of the argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5 L / min. Substrate manufacturing method.