JP2020040866A - 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 without reduction unevenness even when a desired volume resistivity is low.SOLUTION: The method for manufacturing a LT substrate using a LT crystal comprises: burying the LT crystal treated in the state of a substrate into powder mixed with Al powder and AlOpowder charged in a vessel; 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. The AlOpowder used as powder mixed with the Al powder adsorbs alcohol; the alcohol adsorbed on the AlOpowder is decomposed when heating the LT crystal to form carbon; and the addition of the reduction of the carbon to the reduction of the Al powder can set an Al powder ratio to less than 20 mass% to prevent reduction unevenness.SELECTED DRAWING: None

Description

  The present invention relates to a method for manufacturing a lithium tantalate substrate using a lithium tantalate crystal, and particularly to a method for stably manufacturing a lithium tantalate substrate excellent in electrical characteristics without color unevenness (reduction unevenness). 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, in the method of Patent Document 4 in which a substrate-shaped LT crystal is embedded in a mixed powder of aluminum powder and aluminum oxide powder and heat-treated (reduction treatment), a desired volume resistivity (conductivity) can be obtained, In addition, since there is no need to install an expensive oxygen pump, there is an advantage that the production cost is excellent. However, as the desired volume resistivity becomes lower (about 7.0 × 10 ⁸ Ω · cm exemplified in Reference Example 1 below), it is necessary to set the ratio of the aluminum powder in the mixed powder to be higher, When the ratio of the aluminum powder is 20% by mass or more, black dots (color unevenness, that is, reduction unevenness) having a diameter of about 1 to 5 mm are easily generated, and there is a problem that productivity is deteriorated.

The present invention has been made in view of such a problem, and the subject thereof is that even when a desired volume resistivity is low (for example, about 10 ⁸ Ω · cm), color unevenness (reduction unevenness) is caused. It is an object of the present invention to provide a method for manufacturing a lithium tantalate substrate capable of stably manufacturing a lithium tantalate substrate excellent in electrical characteristics without any problem.

Therefore, the present inventor completes a method capable of adjusting the volume resistivity of the lithium tantalate substrate to be low (for example, about 10 ⁸ Ω · cm) after the heat treatment even if the ratio of the aluminum powder is set to less than 20% by mass. Therefore, as a result of diligent research, they have found that this can be achieved by adsorbing alcohol on aluminum oxide powder used as a mixed powder with aluminum powder. The present invention has been completed based on such technical findings.

That is, the first invention according to the present invention is:
A method for producing a lithium tantalate substrate using lithium tantalate crystals, wherein a lithium tantalate crystal processed into a substrate state is embedded in a mixed powder of aluminum powder and aluminum oxide powder filled in a container. And, after disposing the container in a heating furnace, a method for producing a lithium tantalate substrate by heat treatment at a temperature lower than the Curie temperature of lithium tantalate crystals,
The mixed powder is characterized by comprising an aluminum powder and an aluminum oxide powder adsorbing alcohol.

The second invention is
In the method for producing a lithium tantalate substrate according to the first invention,
The amount of carbon contained in the aluminum oxide powder having adsorbed alcohol is 0.1% by mass to 2.0% by mass.

The third invention is
In the method for producing a lithium tantalate substrate according to the first invention or the second invention,
It is characterized in that the alcohol is composed of an alcohol having 2 to 18 carbon atoms.

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 alcohol is adsorbed on the aluminum oxide powder used for the powder mixture with the aluminum powder, when the lithium tantalate crystal embedded in the powder mixture is heat-treated, the alcohol adsorbed on the aluminum oxide powder is decomposed. Generates carbon.

Then, since the reducing action of the generated carbon is added together with the reducing action of the aluminum powder, the volume resistivity of the lithium tantalate substrate is low (for example, 10 ⁸ Ω · cm) even when the ratio of the aluminum powder is set to less than 20% by mass. It is possible to stably produce a lithium tantalate substrate which can be adjusted and has excellent electrical characteristics without color unevenness (reduction unevenness).

  Hereinafter, embodiments of the present invention will be described in detail.

The present invention provides a method for 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. Alcohol is adsorbed on the aluminum oxide powder so that black spots (reduction unevenness) having a diameter of about 1 to 5 mm do not occur even when the resistivity is low (for example, about 10 ⁸ Ω · cm). It is.

  Hereinafter, a method for manufacturing an LT substrate according to the present invention will be described for each process.

(A) Alcohol adsorption step of aluminum oxide powder This step is a step of adsorbing alcohol on aluminum oxide powder. In this step, the type of alcohol adsorbed on the aluminum oxide powder is arbitrary, and methanol, ethanol, propanol (n-propyl alcohol, isopropyl alcohol), butanol (n-butyl alcohol, isobutyl alcohol, secondary butyl alcohol, tertiary butyl alcohol) are used. Examples thereof include alcohols having 2 to 18 carbon atoms, such as monohydric alcohols such as butyl alcohol) and oleyl alcohol, and dihydric alcohols such as ethylene glycol.

  The method of adsorbing the alcohol on the aluminum oxide powder is also optional. For example, a method of spraying the alcohol onto the aluminum oxide powder stored in the tray using a sprayer or the like and adsorbing the alcohol on the aluminum oxide powder is exemplified. Further, as the spray amount of alcohol, the amount of carbon contained in the aluminum oxide powder to which the alcohol is adsorbed corresponds to 0.1% by mass to 2.0% by mass, preferably 0.1% by mass to 1.0% by mass. Quantity. When no alcohol was adsorbed on the aluminum oxide powder, the amount of carbon contained in the aluminum oxide powder measured using a thermogravimetric / mass spectrometer was 0.02% by mass or less.

(B) 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, the aluminum oxide powder (Al ₂ O ₃ powder) and the aluminum powder (Al powder), to which the alcohol has been adsorbed in the alcohol adsorption step (A) of aluminum oxide powder, are mixed.

  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 volume resistivity of the LT substrate is adjusted by appropriately adjusting the amount of alcohol adsorbed on the aluminum oxide powder (Al ₂ O ₃ powder). It becomes possible to reduce to a desired value.

(C) 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 LT crystal in the form of a substrate is subjected to heat treatment, the alcohol adsorbed on the aluminum oxide powder (Al ₂ O ₃ powder) in the mixed powder is decomposed by the heat treatment to generate carbon, which becomes a strong reducing agent. .

  Then, as the amount of alcohol (that is, the amount of carbon) adsorbed on the aluminum oxide increases, the volume resistivity of the LT substrate after the heat treatment tends to decrease. Therefore, by appropriately adjusting the amount of alcohol adsorbed in the alcohol adsorption step of (A) aluminum oxide, the ratio of Al powder in the mixed powder can be set lower than before, whereby the ratio of Al powder in the mixed powder can be reduced to 20 mass%. %, It is possible to suppress the occurrence of black spots (defective color unevenness) having a diameter of about 1 to 5 mm.

  Hereinafter, examples of the present invention will be specifically described with reference to reference 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 the examples and the reference examples is provided with an air supply port and an exhaust port, and a commercially available argon gas (oxygen partial pressure is about 1 × 10 ⁻⁶ atm) is supplied through the air supply port. In addition to being continuously supplied into the heating furnace, argon gas (inert gas) is continuously exhausted outside the heating furnace through an exhaust port, and the inside of the heating furnace is adjusted to an atmospheric pressure atmosphere. 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]
Ethanol was sprayed toward the Al ₂ O ₃ powder (aluminum oxide powder) having an average particle size of 52 μm stored in the tray using a sprayer, and the ethanol was adsorbed on the Al ₂ O ₃ powder. The amount of ethanol sprayed by the atomizer is an amount corresponding to 0.1% by mass of carbon contained in the Al ₂ O ₃ powder having ethanol adsorbed thereon. The average particle size is a value obtained by measuring the Al ₂ O ₃ powder with a laser diffraction type particle size distribution meter.

90 mass% of Al ₂ O ₃ powder to which ethanol is adsorbed and 10 mass% of Al powder (aluminum powder) having an average particle size of 100 μm are mixed to obtain a mixed powder of Al powder and Al ₂ O ₃ powder. 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.

The volume resistivity of the LT substrate after the heat treatment (reduction treatment, blackening treatment) is 9.0 × 10 ⁸ 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]
Heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed under the same conditions as in Example 1 except that the amount of carbon contained in the Al ₂ O ₃ powder to which ethanol had been adsorbed was set to an amount corresponding to 1.0% by mass. went.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 5.0 × 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]
Butanol was sprayed toward the Al ₂ O ₃ powder having an average particle size of 52 μm contained in the tray, except that the amount of carbon contained in the Al ₂ O ₃ powder having adsorbed butanol was adjusted to an amount corresponding to 0.1% by mass. Heat-treated (reduction treatment, blackening treatment) of the LT crystal under the same conditions as in Example 1.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 9.0 × 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 4]
Heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed under the same conditions as in Example 3 except that the amount of carbon contained in the Al ₂ O ₃ powder to which butanol was adsorbed was set to an amount corresponding to 1.0% by mass. went.

After heat treatment (reduction treatment, blackening treatment), the LT substrate has a volume resistivity of 4.8 × 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 5]
Oleyl alcohol is sprayed toward the Al ₂ O ₃ powder having an average particle size of 52 μm stored in the tray, and the amount of carbon contained in the Al ₂ O ₃ powder having adsorbed oleyl alcohol is equivalent to 0.1% by mass. Heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed 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 8.9 × 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 6]
Heat treatment of LT crystal (reduction treatment, blackening treatment) under the same conditions as in Example 5 except that the amount of carbon contained in Al ₂ O ₃ powder to which oleyl alcohol was adsorbed was set to an amount corresponding to 1.0% by mass. Was done.

After heat treatment (reduction treatment, blackening treatment), the LT substrate has a volume resistivity of 4.8 × 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 7]
The same conditions as in Example 1 except that 95% by mass of Al ₂ O ₃ powder having adsorbed ethanol in an amount corresponding to 1.0% by mass of carbon and 5% by mass of Al powder were mixed. Heat treatment (reduction treatment, blackening treatment) of the LT crystal.

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 8
The same conditions as in Example 1 except that 85% by mass of Al ₂ O ₃ powder having adsorbed ethanol in an amount corresponding to 1.0% by mass of carbon and 15% by mass of Al powder were mixed. Heat treatment (reduction treatment, blackening treatment) of the LT crystal.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 1.0 × 10 ⁸ Ω · cm, the color unevenness occurrence rate is 5%, and an LT substrate having excellent electrical characteristics is obtained. Was completed.

  The results are shown in Table 1.

[Example 9]
The same conditions as in Example 1 except that 90% by mass of Al ₂ O ₃ powder having adsorbed ethanol corresponding to 2.0% by mass of carbon and 90% by mass of Al powder were mixed. Heat treatment (reduction treatment, blackening treatment) of the LT crystal.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity of the LT substrate is 1.0 × 10 ⁸ Ω · cm, the color unevenness occurrence rate is 2%, and an LT substrate having excellent electrical characteristics is obtained. Was completed.

  However, as the amount of alcohol increased (2.0% by mass), the production cost became higher. The results are shown in Table 1.

[Reference Example 1]
Al ₂ O ₃ without adsorb ethanol flour, and heat treatment of the LT crystal in the same conditions as in Example 1 except that the Al ₂ O ₃ 80 wt% powder and Al powder were mixed at a ratio of 20 mass% ( Reduction treatment, blackening treatment).

The carbon content of the Al ₂ O ₃ powder on which ethanol was not adsorbed was measured by a thermogravimetric / mass spectrometer, and as a result, the carbon content was 0.02% by mass.

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 heat treatment (reduction treatment, blackening treatment) of the LT crystal was performed under the same conditions as in Example 1 except that ethanol was not adsorbed on the Al ₂ O ₃ powder.

The carbon amount of the Al ₂ O ₃ powder on which ethanol was not adsorbed was measured by a thermogravimetric / mass spectrometer in the same manner as in Reference Example 1, and as a result, the carbon amount was 0.02% by mass.

The volume resistivity of the LT substrate after the heat treatment (reduction treatment, blackening treatment) is 1.5 × 10 ⁹ Ω · cm, and has the same electrical characteristics as in Example 7 in which the Al powder ratio is 5% by mass. However, the occurrence rate of color unevenness was 2.5%, which was inferior to that of Example 7. The results are shown in Table 1.

[Verification]
(1) In Examples 1, 2 and 9 in which Al powder was used at 10% by mass and ethanol adsorbed Al ₂ O ₃ powder was used, Example 1 (carbon content of Al ₂ O ₃ powder: 0.1% by mass, volume resistivity: 9.0 × 10 ⁸ Ω · cm),
Example 2 (carbon content of Al ₂ O ₃ powder: 1.0% by mass, volume resistivity: 5.0 × 10 ⁸ Ω · cm),
Example 9 (Carbon content of Al ₂ O ₃ powder: 2.0% by mass, volume resistivity: 1.0 × 10 ⁸ Ω · cm)
According to the data, the larger the carbon content of the Al ₂ O ₃ powder, the smaller the volume resistivity.

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

(1-1) Examples 3 and 4 of Al powder 10 mass%, butanol adsorption Al ₂ O ₃ powder application, and
(1-2) The same applies to Examples 5 to 6 in which Al powder is used at 10% by mass and oleyl alcohol-adsorbed Al ₂ O ₃ powder is used.

(2) Example 1 (volume resistivity: 9.0) having substantially the same volume resistivity as Reference Example 1 (20 mass% of Al powder, volume resistivity: 7.0 × 10 ⁸ Ω · cm). × 10 ⁸ Ω · cm), Example 2 (volume resistivity: 5.0 × 10 ⁸ Ω · cm), Example 3 (volume resistivity: 9.0 × 10 ⁸ Ω · cm), Example 4 ( Volume resistivity: 4.8 × 10 ⁸ Ω · cm, Example 5 (volume resistivity: 8.9 × 10 ⁸ Ω · cm), and Example 6 (volume resistivity: 4.8 × 10 ⁸⁾ Ω · cm), the volume resistivity equivalent to that of Reference Example 1 can be obtained even when the ratio of the Al powder is less than 20% by mass (Examples 1 to 6: 10% by mass).

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

(3) On the other hand, in Reference Example 2 in which the amount of carbon contained in the Al ₂ O ₃ powder was 0.02% by mass and the ratio of the Al powder was 10% by mass, the volume resistivity of the LT substrate was 1.5 × 10 ⁹ It is a high value of Ω · cm, and it is necessary to set the ratio of the aluminum powder to 20% by mass or more (see Reference Example 1) in order to lower the volume resistivity (for example, about 10 ⁸ Ω · cm). Is also confirmed.

According to the method of the present invention, even if the desired volume resistivity is low (for example, about 10 ⁸ Ω · cm), a lithium tantalate substrate excellent in electrical characteristics without color unevenness (reduction unevenness) can be manufactured. It has industrial applicability to provide a stable lithium tantalate substrate used as a substrate material for a surface acoustic wave device.

Claims (6)

A method for producing a lithium tantalate substrate using lithium tantalate crystals, wherein a lithium tantalate crystal processed into a substrate state is embedded in a mixed powder of aluminum powder and aluminum oxide powder filled in a container. And, after disposing the container in a heating furnace, a method for producing a lithium tantalate substrate by heat treatment at a temperature lower than the Curie temperature of lithium tantalate crystals,
A method for producing a lithium tantalate substrate, wherein the mixed powder is composed of aluminum powder and aluminum oxide powder adsorbing alcohol.   The method for producing a lithium tantalate substrate according to claim 1, wherein the amount of carbon contained in the aluminum oxide powder to which the alcohol is adsorbed is 0.1% by mass to 2.0% by mass.   The method for producing a lithium tantalate substrate according to claim 1, wherein the alcohol is an alcohol having 2 to 18 carbon atoms.   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.