JP2019156655A - Method for manufacturing lithium tantalate substrate - Google Patents

Abstract

To provide a method for manufacturing a lithium tantalate (LT) substrate excellent in electrical properties, capable of suppressing the occurrence of color unevenness (reduction unevenness).SOLUTION: A method for manufacturing a LT substrate using a LT crystal grown by the Czochralski method comprises: burying a LT crystal 3 processed in a substrate shape into a mixed powder 2 of aluminum powder (Al powder) and aluminum oxide powder (AlOpowder) in a vessel 1; arranging the vessel 1 in a heating furnace; and heating the LT crystal at a temperature of less than the curie temperature of the LT crystal to manufacture the LT substrate. The ratio of the aluminum powder in the mixed powder 2 is set to 20 wt.% or less; inert gas is continuously supplied and exhausted into the heating furnace in the atmospheric pressure atmosphere; and when the average particle size of the aluminum powder is S (μm), the average particle size of the AlOpowder is set to a range of 0.9S-1.2S (μm).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a lithium tantalate substrate using a lithium tantalate crystal grown by the Czochralski method, and in particular, a lithium tantalate substrate excellent in electrical characteristics free from color unevenness (reduction unevenness). It relates to a manufacturing method.

  The lithium tantalate (hereinafter sometimes abbreviated as LT) crystal is a ferroelectric having a melting point of about 1650 ° C. and a Curie temperature of about 600 ° C., and a lithium tantalate substrate manufactured using this crystal is It is mainly applied as a surface acoustic wave (SAW) filter material used in a mobile phone transmission / reception device.

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

  The SAW filter has a structure in which a pair of comb electrodes are formed of a metal thin film such as Al or Cu on a substrate made of a piezoelectric material such as LT. It plays an important role. The comb electrode is formed by depositing a metal thin film on a piezoelectric material by sputtering, leaving a pair of comb patterns, and removing unnecessary portions by etching using a photolithographic technique.

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

The grown LT crystal is colorless and transparent or has a light yellow color with high transparency. 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, poling treatment for making a single polarization, that is, the LT crystal is heated from room temperature to a predetermined temperature above the Curie temperature. The temperature is raised, a voltage is applied to the crystal, a temperature is lowered to a predetermined temperature below the Curie temperature while the voltage is being applied, and then a series of processes of stopping the voltage application and cooling to room temperature is performed. After the poling process, the LT crystal (referred to as an ingot) that has been peripherally ground to adjust the outer diameter of the crystal becomes a substrate through mechanical processing such as slicing, lapping, and polishing. The finally obtained substrate is almost colorless and transparent, and its volume resistivity is about 10 ¹⁴ to 10 ¹⁵ Ω · cm.

  By the way, in the substrate obtained by such a conventional method, in the surface acoustic wave device (SAW filter) manufacturing process, electric charges are transferred to the surface of the substrate due to the temperature change received in the process because of the pyroelectric property which is a characteristic of the LT crystal. The comb-shaped electrode formed on the surface of the substrate is destroyed due to the charge-up caused by this, and further, the substrate is cracked and the yield in the element manufacturing process is reduced.

Therefore, several techniques for increasing the electrical conductivity have been proposed in order to eliminate defects due to pyroelectricity of the LT crystal. For example, in Patent Document 1, the conductivity is increased by heat treating the LT substrate in a reducing atmosphere of a gas selected from argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen and combinations thereof. A method is proposed, and Patent Document 2 proposes a method of increasing the conductivity by heat-treating the LT substrate in a reduced pressure atmosphere of 20 Pa or less. Patent Document 3 discloses a method in which LT crystals processed into a substrate state are embedded in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder) and heat-treated (reduction treatment). Proposed. Note that the LT substrate with increased conductivity causes light absorption due to the introduction of oxygen vacancies. The observed color tone of the LT substrate appears reddish brown in the transmitted light and black in the reflected light. Therefore, the reduction process for increasing the conductivity is also called a blackening process, and such a color tone change phenomenon. Is called blackening.

Japanese Patent Laid-Open No. 11-92147 (see Claims, paragraph 0027) JP 2004-152870 A (see claims 4 and 8, paragraph 0014) Japanese Patent No. 4063191 (see Examples 3 and 8)

  When the methods of Patent Document 1 and Patent Document 2 are applied to a lithium tantalate substrate having a high melting point of about 1650 ° C., unlike the lithium niobate substrate having a relatively low melting point of about 1250 ° C., the conductivity of the LT substrate However, there is a problem that the effect of improving the defect due to pyroelectricity is not sufficient.

In recent years, in order to improve the yield in the surface acoustic wave device (SAW filter) manufacturing process, there is a demand for lowering the volume resistivity which is a characteristic of the LT crystal. For example, the volume resistivity of the LT substrate is reduced to 1 × 10. There is a demand to make it ⁹ (Ω · cm) or less.

And in the method of patent document 3 which embeds the LT crystal processed into the state of the substrate in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder), the ratio of Al powder As a result, an LT substrate having a volume resistivity of about 1 × 10 ⁹ (Ω · cm) is obtained (see Examples 3 and 8). However, as the Al powder ratio in the mixed powder increases, black spots (color unevenness, that is, reduction unevenness) with a diameter of about 1 to 5 mm are likely to occur, and the generation rate increases as the Al powder ratio increases. There was a problem that worsened the sex.

  The present invention has been made paying attention to such problems, and the problem is that the effect of improving defects due to pyroelectricity is uniform, the occurrence of uneven color defects can be suppressed, and low An object of the present invention is to provide an LT substrate manufacturing method that is excellent in reproducibility and production efficiency at a low cost.

  Therefore, in order to solve the above problems, the present inventors tried to improve the method according to Patent Document 3.

First, in the method of Patent Document 3, an aluminum powder (Al powder) and an aluminum oxide powder (Al ₂ O ₃ powder) are filled in a container 1 shown in FIG. The lithium tantalate crystal 3 processed into the state of the substrate is embedded, and a plurality of containers 1 embedded with the lithium tantalate crystal 3 are accommodated in a large container 4 made of aluminum, and the large container 4 is After being placed in a heating furnace (not shown), a lithium tantalate substrate is manufactured by heat treatment at a temperature lower than the Curie temperature of the lithium tantalate crystal.

  Although the container 1 and the large container 4 are covered with a lid, they are not sealed containers.

  In the method of Patent Document 3, since the atmosphere in the heating furnace is set to a vacuum condition (see Example 8) or an inert gas sealing condition (see Examples 1 and 3), It is expected that heat will accumulate in one place and the above color unevenness (reduction unevenness) is likely to occur, especially when the proportion of aluminum powder in the mixed powder exceeds 20% weight. It was confirmed that the rate became remarkable. Furthermore, since the average particle diameters of the aluminum powder and the aluminum oxide powder used are not uniform, it is difficult to form a gap between the aluminum powder and the aluminum oxide powder. It was thought that this was a cause of the occurrence. In other words, it has been found that the average particle size of the aluminum powder is such that the smaller the particle size, the stronger the reducing power. However, the average particle diameter of aluminum powder generally used from the viewpoint of avoiding the danger of dust explosion is set to about 100 μm. On the other hand, since the average particle diameter of conventionally used aluminum oxide powder is about 50 μm, it is difficult to form a gap space between the aluminum powder and the aluminum oxide powder. it was thought.

Therefore, the atmosphere in the heating furnace set to the vacuum condition or the inert gas sealing condition is changed to the atmospheric pressure condition, and the inert gas is continuously supplied to and discharged from the heating furnace (to the heating furnace). The inert gas is supplied from the air supply port provided and the inert gas is discharged from the exhaust port of the heating furnace), and the ratio of the aluminum powder in the mixed powder is set to 20% by weight or less. When an aluminum oxide powder having an average particle size of 90 μm or more is applied in accordance with the average particle size (about 100 μm), the effect of improving defects due to pyroelectricity is uniform, and the occurrence of color unevenness defects can be suppressed. It was confirmed that a lithium tantalate substrate having a volume resistivity of 1 × 10 ⁹ Ω · cm or less can be produced at low cost and with good reproducibility. The present invention has been completed through such technical analysis and technical confirmation.

That is, the first invention according to the present invention is:
A method of manufacturing a lithium tantalate substrate using a lithium tantalate crystal 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 manufacturing the lithium tantalate substrate by embedding the lithium tantalate crystal and placing the container in a heating furnace, and then heat-treating the lithium tantalate crystal at a temperature lower than the Curie temperature of the lithium tantalate crystal,
The ratio of the aluminum powder in the mixed powder is set to 20% by weight or less, and the inert gas is continuously supplied and discharged into the heating furnace under an atmospheric pressure atmosphere, and the average particle size of the aluminum powder is set to S ( μm), the average particle diameter of the aluminum oxide powder is set in the range of 0.9 S (μm) to 1.2 S (μm).

Next, the second invention is:
In the method for producing a lithium tantalate substrate according to the first invention,
The average particle size S of the aluminum powder is 100 μm,
The third invention is
In the method for producing a lithium tantalate substrate according to the first invention or the second invention,
The ratio of aluminum powder in the mixed powder is 10% by weight,
In addition, the fourth invention is
In the method for producing a lithium tantalate substrate according to any one of the first to third inventions,
The inert gas is an argon gas, and the flow rate of the argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5 L / min.

  According to the method of the present invention, the ratio of the aluminum powder in the mixed powder is set to 20% by weight or less, the inert gas is continuously fed into and discharged from the heating furnace under the atmospheric pressure atmosphere, and the average of the aluminum powder When the particle size is S (μm), the average particle size of the aluminum oxide powder is set in the range of 0.9 S (μm) to 1.2 S (μm), so that there is a gap space between the aluminum powder and the aluminum oxide powder. Is easily formed.

  Then, by continuously supplying and discharging the inert gas into the heating furnace under the atmospheric pressure atmosphere, it becomes difficult for heat in the heating furnace to accumulate in one place, and further, formed between the aluminum powder and the aluminum oxide powder. Since the air permeability is also improved by the gap space, it is possible to manufacture a lithium tantalate substrate having excellent electrical characteristics by suppressing the occurrence of uneven color (uneven reduction).

Explanatory drawing which shows the state with which the several stainless steel container which embedded the lithium tantalate crystal processed into the state of the board | substrate in the mixed powder of the aluminum powder and aluminum oxide powder which concern on patent document 3 was accommodated in the aluminum large container.

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

A lithium tantalate (LT) crystal changes in electrical conductivity and color depending on the concentration of oxygen vacancies present in the crystal. When oxygen vacancies are introduced into the LT crystal, the valence of some Ta ions changes from 5+ to 4+ due to the need for charge balance, resulting in electrical conductivity and light absorption. Electric conduction is considered to occur because electrons as carriers move between Ta ⁵⁺ ions and Ta ⁴⁺ ions. The electrical conductivity of a crystal is determined by the product of the number of carriers per unit volume and the mobility of carriers. If the mobility is the same, the electrical conductivity is proportional to the number of oxygen vacancies. The color change due to light absorption is considered to be due to the level of electrons introduced by the oxygen vacancies.

By the way, when increasing the conductivity of the LT substrate, Patent Document 1 in which oxygen vacancies are introduced into the LT substrate by heat-treating the LT substrate at a temperature lower than the Curie temperature in an inert gas having a sufficiently low oxygen partial pressure. Can be considered. However, generally commercially available argon gas (usually oxygen partial pressure of about 1 × 10 ⁻⁶ atm) contains oxygen of several ppm or less as an impurity. It is not possible to sufficiently increase the conductivity of the LT substrate by the heat treatment using.

In contrast, a lithium tantalate (LT) crystal processed into a substrate state is embedded in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder), for example, sealing of inert gas. In the method according to Patent Document 3 in which the heat treatment is performed under the stop condition (see Examples 1 and 3), the oxygen partial pressure of the inert gas existing around the LT crystal is reduced by the oxidation reaction of Al, and the oxygen crystal is freed of oxygen in the LT crystal. The conditions under which holes can be introduced are obtained.

However, when an LT substrate having a volume resistivity of about 1 × 10 ⁹ (Ω · cm) is manufactured by the method according to Patent Document 3, the atmosphere in the heating furnace is in a vacuum condition (see Example 8) or the above inert gas. Therefore, the heat in the heating furnace accumulates in one place, and color unevenness (reduction unevenness) is likely to occur. In particular, the Al powder ratio is 20 weight. When the percentage exceeds 50%, there is a problem that black spots (color unevenness, that is, reduction unevenness) having a diameter of about 1 to 5 mm occur on the surface of the LT substrate.

  In order to solve this problem, the present inventors examined a method of reducing the oxygen partial pressure of the inert gas existing around the LT crystal by an oxidation reaction of Al without increasing the ratio of Al powder.

  First, in order to reduce the oxygen partial pressure of the inert gas existing around the LT crystal by the oxidation reaction of Al, oxygen desorbed from the LT crystal by the oxidation reaction of Al can be quickly diffused on the surface of the Al powder. It becomes important. This is because when the desorbed oxygen stays around the LT crystal, the oxygen partial pressure of the inert gas around the LT crystal increases and the reduction reaction of the LT crystal becomes difficult to proceed. Therefore, in order to prevent the desorbed oxygen from staying around the LT crystal, an attempt was made to improve the formation of a gap space for ventilation between the aluminum powder and the aluminum oxide powder. This is because by forming the gap space, oxygen desorbed from the LT crystal by the oxidation reaction of Al can be quickly diffused to the surface of the Al powder. Therefore, when an attempt was made to apply an aluminum oxide powder having an average particle size of 90 μm to 120 μm in accordance with the average particle size (about 100 μm) of the aluminum oxide powder that is generally used, the average particle size was about 50 μm. Compared with the case of using a conventional aluminum oxide powder, the bulk density was reduced by about 8%, and it was confirmed that a gap space was formed between the aluminum powder and the aluminum oxide powder. Since the air permeability around the LT crystal is improved by the gap space, oxygen desorbed from the LT crystal due to the oxidation reaction of Al does not stay around the LT crystal. As a result, the reduction reaction of the LT crystal is prevented. It becomes possible to form a state that makes it easier to proceed.

Furthermore, the following improvement was also tried. That is, instead of these conditions of Patent Document 3 in which the atmosphere in the heating furnace is a vacuum condition (see Example 8) or an inert gas sealing condition (see Examples 1 and 3), The atmosphere is changed to atmospheric pressure conditions, and the inert gas is continuously supplied to and discharged from the heating furnace (the inert gas is supplied from the air supply port provided in the heating furnace, and the inert gas is supplied from the exhaust port of the heating furnace. Attempt to improve (release active gas). As a result, it is possible to produce a lithium tantalate substrate having a volume resistivity of 1 × 10 ⁹ Ω · cm or less even when the Al powder ratio is set to 20% by weight or less, and further to the surface of the lithium tantalate substrate. It has been confirmed that black spots (color unevenness) having a diameter of about 1 to 5 mm can be avoided.

  That is, the present invention is a method of manufacturing an LT substrate using an LT crystal grown by the Czochralski method, wherein the substrate is in a mixed powder of aluminum powder and aluminum oxide powder filled in a container. In the method of manufacturing an LT substrate by embedding a processed LT crystal and placing the container in a heating furnace and then heat-treating it at a temperature lower than the Curie temperature of the LT crystal, the ratio of the aluminum powder in the mixed powder is determined. When the inert gas is continuously supplied and discharged into a heating furnace under an atmospheric pressure atmosphere and the average particle size of the aluminum powder is S (μm), the average particle size of the aluminum oxide powder is It is characterized by being set in the range of 0.9S (μm) to 1.2S (μm).

In the method for producing an LT substrate according to the present invention, the LT crystal processed into a substrate state is embedded in a mixed powder of aluminum powder and aluminum oxide powder and processed at a temperature of 350 ° C. to less than the Curie temperature of the LT crystal (about Less than 600 ° C.). The mixed powder of aluminum powder and aluminum oxide powder affects the volume resistivity of the LT substrate after processing. By increasing the ratio of the aluminum powder, the oxidation reaction of Al is promoted and the volume resistivity can be reduced. For example, when the volume resistivity is 1 × 10 ⁹ (Ω · cm) or less, in the conventional method according to Patent Document 3, it is necessary to set the aluminum powder ratio in the mixed powder to an amount exceeding 20% by weight. . However, when the aluminum powder ratio in the mixed powder exceeds 20% by weight, the above-described black spots (color unevenness) having a diameter of about 1 to 5 mm become remarkable, and this color unevenness affects the ratio of the aluminum powder. As the ratio of the aluminum powder increases, the rate of occurrence of color unevenness increases. In the method for producing an LT substrate according to the present invention, in order to reliably suppress the occurrence of the color unevenness, the Al powder ratio is set to 20% by weight or less, preferably 15% by weight or less, more preferably 10% by weight or less. Good.

Moreover, generally available argon gas (oxygen partial pressure is about 1 * 10 < ^-6 > atm), nitrogen gas, etc. can be applied to the inert gas supplied and discharged into the heating furnace.

  In addition, when the inert gas is argon gas, it is preferable that the flow volume of the inert gas continuously supplied / exhausted in a heating furnace is 0.5-5 L / min.

  And since the manufacturing method which concerns on this invention does not make the inside of a heating furnace pressure reduction or vacuum, and does not require an airtight container and a pressure reduction processing apparatus, reduction of installation cost can also be aimed at.

  Examples of the present invention will be specifically described below with reference to comparative examples. However, 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 comparative examples is provided with an air supply port and an exhaust port. Also, a stainless steel container placed in the heating furnace is filled with a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder), and is generally commercially available argon gas. (The oxygen partial pressure is about 1 × 10 ⁻⁶ atm) is continuously supplied into the heating furnace through the air supply port, and argon gas (inert gas) is continuously supplied to the outside of the heating furnace through the exhaust port. The inside of the heating furnace is adjusted to an atmospheric pressure atmosphere (not under argon gas sealing conditions). Note that the flow rate of the argon gas supplied and discharged into the heating furnace is set to 2 L / min.

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

  An LT crystal ingot is subjected to heat treatment for removing thermal strain and poling treatment for single polarization, and then peripheral grinding, slicing and polishing are performed to form a 42 ° RY (Rotated Y axis) substrate. The LT crystal was processed into a state.

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

[Example 1]
An LT crystal processed into a substrate state is embedded in a mixed powder of 10 wt% aluminum powder (Al powder) and 90 wt% aluminum oxide powder (Al ₂ O ₃ powder) filled in a stainless steel container. And after arrange | positioning the stainless steel container in which the LT crystal was embedded in the said heating furnace, the said argon gas was supplied in the heating furnace via the air supply port.

The average particle size of the Al powder was 100 μm, and the average particle size of the Al ₂ O ₃ powder was 95 μm. The average particle size was a value obtained by measuring each powder with a laser diffraction particle size distribution meter.

  Then, argon gas was continuously supplied and discharged at a flow rate of 2 L / min into a heating furnace in an atmospheric pressure atmosphere, and heat treatment (blackening treatment) was performed at 580 ° C. for 20 hours.

  A similar heat treatment was performed on a total of 200 LT crystals processed into the substrate state, the volume resistivity of the LT substrate after the treatment 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 heat treatment (blackening treatment) is about 4.0 × 10 ⁸ Ω · cm (average value of 200 substrates), and the occurrence rate of color unevenness on the substrate surface is 2%. there were.

  These results are shown in Table 1 below.

[Comparative Example 1]
Heat treatment (blackening treatment) was performed under the same conditions as in Example 1 except that Al ₂ O ₃ powder having an average particle size of 52 μm was used.

The volume resistivity of the LT substrate after heat treatment (blackening treatment) is about 1.5 × 10 ⁹ Ω · cm (average value of 200 substrates), and the color unevenness occurrence rate on the substrate surface is 2.5. %Met.

  These results are also shown in Table 1.

[Comparative Example 2]
Other than using Al ₂ O ₃ powder having an average particle diameter of 52 μm and the ratio of Al powder to Al ₂ O ₃ powder in the mixed powder being Al powder: 20 wt%, Al ₂ O ₃ powder: 80 wt% Were subjected to heat treatment (blackening treatment) under the same conditions as in Example 1.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) is about 7.0 × 10 ⁸ Ω · cm (average value of 200 substrates), and the occurrence rate of color unevenness on the substrate surface is 17.0. %Met.

  These results are also shown in Table 1.

[Example 2]
Heat treatment (blackening treatment) was performed under the same conditions as in Example 1 except that Al ₂ O ₃ powder having an average particle size of 110 μm was used.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) is about 4.5 × 10 ⁸ Ω · cm (average value of 200 substrates), and the occurrence rate of color unevenness on the substrate surface is 1.5. %Met.

[Comparative Example 3]
Heat treatment (blackening treatment) was performed under the same conditions as in Example 1 except that Al ₂ O ₃ powder having an average particle size of 8 μm was used.

The occurrence rate of color unevenness on the substrate surface after the heat treatment (blackening treatment) was 2.0%, but the volume resistivity of the LT substrate was about 7.0 × 10 ⁹ Ω · cm (200 substrates) Average value).

[Example 3]
Other than using Al ₂ O ₃ powder having an average particle size of 95 μm and the ratio of Al powder to Al ₂ O ₃ powder in the mixed powder being Al powder: 15 wt%, Al ₂ O ₃ powder: 85 wt% Were subjected to heat treatment (blackening treatment) under the same conditions as in Example 1.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) is about 3.5 × 10 ⁸ Ω · cm (average value of 200 substrates), and the occurrence rate of color unevenness on the substrate surface is 4.0. %Met.

[Example 4]
Other than using Al ₂ O ₃ powder with an average particle size of 95 μm and the ratio of Al powder and Al ₂ O ₃ powder in the mixed powder to Al powder: 20 wt%, Al ₂ O ₃ powder: 80 wt% Were subjected to heat treatment (blackening treatment) under the same conditions as in Example 1.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) is about 3.0 × 10 ⁸ Ω · cm (average value of 200 substrates), and the occurrence rate of color unevenness on the substrate surface is 12.0. %Met.

[Result]
(1) Aluminum oxide powder (Al ₂ O ₃ powder) having an average particle diameter of 95 μm to 110 μm is applied to aluminum powder (Al powder) having an average particle diameter of 100 μm, and the ratio of Al powder in the mixed powder is In Examples 1 to 4, in which argon gas is continuously supplied and discharged into a heating furnace under an atmospheric pressure atmosphere, the color unevenness generation rate is 1.5% to 12.0%. It is confirmed that the volume resistivity of the LT substrate is 1.0 × 10 ⁹ (Ω · cm) or less.

(2) On the other hand, for Al powder having an average particle diameter of 100 μm, Al ₂ O ₃ powder having an average particle diameter of 8 μm and 52 μm is applied, and the ratio of Al powder in the mixed powder is set to 10% by weight. In Comparative Example 1 and Comparative Example 3 in which argon gas is continuously supplied to and discharged from a heating furnace under an atmospheric pressure atmosphere, the occurrence rate of color unevenness is suppressed to 2.0% to 2.5%, The volume resistivity of the LT substrate exceeds 1.0 × 10 ⁹ (Ω · cm), confirming that the conductivity is not sufficiently improved.

(3) In addition, an Al ₂ O ₃ powder having an average particle diameter of 52 μm is applied to an Al powder having an average particle diameter of 100 μm, and the ratio of the Al powder in the mixed powder is set to 20% by weight. In Comparative Example 2 in which argon gas is continuously supplied to and discharged from a heating furnace under atmospheric pressure, the volume resistivity of the LT substrate is 1.0 × 10 ⁹ (Ω · cm) or less, but the conductivity is excellent. The occurrence rate of color unevenness is as high as 17.0%, confirming that the productivity is deteriorated.

  According to the present invention, it is possible to produce a lithium tantalate substrate that suppresses the occurrence of color unevenness (reduction unevenness) and has excellent electrical characteristics, and is therefore used as a substrate material for a surface acoustic wave device (SAW filter). Has industrial applicability.

DESCRIPTION OF SYMBOLS 1 Container 2 Mixed powder 3 Lithium tantalate crystal processed into the state of a board | substrate 4 Large container

Claims (4)

A method of manufacturing a lithium tantalate substrate using a lithium tantalate crystal 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 manufacturing the lithium tantalate substrate by embedding the lithium tantalate crystal and placing the container in a heating furnace, and then heat-treating the lithium tantalate crystal at a temperature lower than the Curie temperature of the lithium tantalate crystal,
The ratio of the aluminum powder in the mixed powder is set to 20% by weight or less, and the inert gas is continuously supplied and discharged into the heating furnace under an atmospheric pressure atmosphere, and the average particle size of the aluminum powder is set to S ( μm), the average particle size of the aluminum oxide powder is set in the range of 0.9 S (μm) to 1.2 S (μm).   The method for producing a lithium tantalate substrate according to claim 1, wherein the aluminum powder has an average particle size S of 100 μm.   The method for producing a lithium tantalate substrate according to claim 1 or 2, wherein a ratio of the aluminum powder in the mixed powder is 10% by weight.   The inert gas is argon gas, and the flow rate of argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5 L / min. The manufacturing method of the lithium tantalate board | substrate of description.

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