JP7279395B2 - Manufacturing method of lithium niobate substrate - Google Patents

Description

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

A lithium niobate (hereinafter sometimes abbreviated as LN) crystal is a ferroelectric substance with a melting point of about 1250°C and a Curie temperature of about 1140°C. 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 increasing frequency of mobile phones and the spread of Bluetooth (registered trademark) (2.45 GHz) through wireless LAN in various electronic devices.

The SAW filter has a structure in which a pair of comb-shaped electrodes are formed of a metal thin film such as Al or Cu on a substrate made of a piezoelectric material such as LN. plays an important role in influencing The comb-shaped electrodes are formed by forming a metal thin film on the piezoelectric material by sputtering, leaving a pair of comb-shaped patterns, and removing unnecessary portions by etching using photolithography.

Industrially, the LN single crystal is mainly grown by the Czochralski method in a nitrogen-oxygen mixed gas atmosphere with an oxygen concentration of about 20% or in an electric furnace in an air atmosphere. A platinum crucible having a melting point is used, and the grown LN single crystal is cooled in an electric furnace at a predetermined cooling rate and then removed from the electric furnace.

The grown LN crystal is colorless and transparent or pale yellow with high transparency. After growth, in order to remove residual strain due to thermal stress of the crystal, heat treatment is performed under soaking near the melting point, and further poling treatment for single polarization, that is, the LN crystal is heated from room temperature to a predetermined temperature above the Curie temperature. A series of processes are performed in which the temperature is raised, a voltage is applied to the crystal, the temperature is lowered to a predetermined temperature below the Curie temperature while the voltage is applied, the voltage application is stopped, and the crystal is cooled to room temperature. After the poling treatment, the LN crystal (hereinafter referred to as an ingot) whose outer circumference is ground to adjust the crystal shape becomes a substrate after undergoing machining such as slicing, lapping and polishing. The finally obtained substrate is almost colorless and transparent, and has a volume resistivity of about 10 ¹⁴ to 10 ¹⁵ Ω·cm.

By the way, in the substrate manufactured by such a conventional method, in the surface acoustic wave device (SAW filter) manufacturing process, due to the pyroelectricity that is a characteristic of the LN crystal, charges are generated on the substrate surface due to the temperature change received in the process. The comb-shaped electrodes formed on the surface of the substrate are broken due to the electric discharge caused by charge-up, and cracks of the substrate occur, resulting in a decrease in the yield in the device manufacturing process.

Therefore, in order to solve the problems caused by the pyroelectricity of LN crystals, several techniques have been proposed for increasing the electrical conductivity.

For example, in Patent Document 1, an LN crystal processed into a substrate shape (hereinafter referred to as “substrate-shaped LN crystal” and distinguished from the LN substrate after heat treatment) is mixed with 85% nitrogen gas and 15% hydrogen gas. A method of increasing conductivity by heat treatment in a gas atmosphere (reducing atmosphere) is disclosed in Examples, and in Patent Document 2, aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O ₃ powder) are disclosed. A method has been proposed in which substrate-shaped LN crystals are embedded in a mixed powder of and heat-treated (reduction treatment). The LN substrate with increased conductivity absorbs light due to the introduction of oxygen vacancies. And the color tone of the observed LN substrate is reddish brown in transmitted light and black in reflected light, so the reduction treatment that increases conductivity is also called blackening treatment, and such a color tone change phenomenon is called blackening.

JP-A-11-92147 (see paragraph 0028) Japanese Patent No. 4492291 (see paragraph 0025)

However, the method of Patent Document 1, in which the LN crystal is heat-treated in a mixed gas atmosphere (reducing atmosphere) of nitrogen gas and hydrogen gas, has a problem in workability because it uses combustible hydrogen gas.

In addition, carbon monoxide, carbon dioxide, water, and argon listed in claim 5 of Patent Document 1 (however, examples in which the above gases other than the mixed gas of nitrogen gas and hydrogen gas were specifically used) Not described in Document 1), etc.), when a plurality of LN crystals are superimposed and heat treated (reduction treatment), the LN crystals arranged in the uppermost and lowermost stages and the LN crystals arranged in the center It was confirmed that there was a difference in the degree of reduction (volume resistivity) of the LN crystal, and the method of Patent Document 1 also had the problem of being unsuitable for mass production.

On the other hand, the method of Patent Document 2, in which a substrate-shaped LN crystal is embedded in a mixed powder of Al powder and Al ₂ O ₃ powder and heat-treated (reduction treatment), depends on the mixing ratio of Al powder, but it produces a dot-like shape. Reduction unevenness (black dot-like color unevenness) sometimes occurred. Floating dust such as fibers that are inevitably mixed in the mixed powder of the Al powder and the Al ₂ O ₃ powder is considered to be the cause of the dot-like reduction unevenness.

That is, the main component of the fiber is cellulose [molecular formula (C ₆ H ₁₀ O ₅ ) _n ], but cellulose self-decomposes at high temperature during the reduction treatment, and as shown in the following reaction formula, carbon gas (C), Water vapor (H ₂ O) and the like are generated.
_{C6H10O5 → 6C} + _5H2O

Then, the generated water vapor reacts with the Al powder contained in the mixed powder, and the Al powder is rapidly oxidized, causing local heat generation. It is considered that the dots are locally reduced and the dot-like reduction unevenness (black dot-like color unevenness) is generated.

Further, in the method of Patent Document 2, since the substrate-shaped LN crystal is embedded in the mixed powder of Al powder and Al ₂ O ₃ powder and heat-treated, the Al powder is uniformly dispersed in the mixed powder and the mixed powder is Since it is necessary to bury the LN crystal while leveling it, workability is difficult, and there is a problem that pattern-like reduction unevenness (pattern-like color unevenness) is generated due to the leveling unevenness.

Furthermore, since a mixed powder of Al powder and Al ₂ O ₃ powder is used, exhaust equipment and protective equipment for dust countermeasures are required, and used Al powder and Al ₂ O ₃ powder are treated as industrial waste. Worker health and global environmental concerns also existed due to the need to dispose.

The present invention has been made with a focus on such problems, and its object is to uniformly improve the effect of improving defects due to pyroelectricity, suppress the occurrence of color unevenness, and improve the health of workers. To provide a method for producing a lithium niobate substrate which has a small safety risk, is free from global environmental problems, and is excellent in reproducibility and production efficiency.

In order to solve the above problems, the present inventors focused on the method of Patent Document 1 that does not use the mixed powder of Al powder and Al ₂ O ₃ powder, and used the combustible hydrogen gas etc. disclosed in Patent Document 1. Intensive studies have been made on a novel method that can simultaneously reduce multiple LN crystals without having to do so.

First, inert gases such as argon (Ar) and nitrogen (N ₂ ), which are easier to handle than combustible hydrogen gas, were examined. General industrial liquefied Ar gas and N ₂ gas contain about 1 ppm of oxygen impurity, which is 1×10 ⁻⁶ atm in terms of oxygen partial pressure.

On the other hand, regardless of whether it is a metal, a non-metal, or a ceramic, the higher the temperature, the higher the equilibrium oxygen partial pressure and the easier the reduction of the substance, and the same is true for LN crystals.

That is, under high temperature conditions, the equilibrium oxygen partial pressure of the LN crystal exceeds the above 1 × 10 ^-6 atm, so even in an inert gas atmosphere such as Ar, it becomes a chemically reducing atmosphere, and the LN crystal Reducing treatment becomes possible. The final degree of reduction is determined by the difference between the equilibrium oxygen partial pressure of the LN crystal and the oxygen partial pressure of the inert gas atmosphere.

Then, the equilibrium oxygen partial pressure of the LN crystal increases as the treatment temperature increases, whereas the oxygen partial pressure of the commercially available inert gas such as Ar is approximately constant at 1 × 10 ^-6 atm. can control the degree of reduction in the LN crystal.

Furthermore, when a plurality of LN crystals are superimposed and reduced at the same time, the superimposed LN crystals (laminated structure of LN crystals) are housed in a porous container and subjected to reduction treatment. It has also been found that the degree of reduction (volume resistivity) of the LN crystals arranged in the middle and the LN crystal arranged in the center can be made the same. The present invention has been completed through such technical studies.

That is, the first invention according to the present invention is
In a method for producing a lithium niobate substrate using lithium niobate crystals grown by the Czochralski method,
A laminated structure of lithium niobate crystals is formed by laminating a plurality of lithium niobate crystals processed into a substrate shape, and the laminated structure is housed in an air-permeable porous container. After the porous container containing the body is placed in a heating furnace, the curie of the lithium niobate crystal is heated at 350° C. or higher in an inert gas atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of the lithium niobate crystal. The lithium niobate substrate is manufactured by heat-treating at a temperature below the temperature.

The second invention is
In the method for producing a lithium niobate substrate according to the first invention,
The porous container is made of graphite or alumina.

Moreover, the third invention is
In the method for producing a lithium niobate substrate according to the first invention or the second invention,
The inert gas is composed of argon gas, the heating furnace has an air supply port and an exhaust port, and the flow rate of argon gas continuously supplied to and discharged from the heating furnace is 0.5 to 5.0 L / min. It is characterized by

A method for producing a lithium niobate substrate according to the present invention comprises laminating a plurality of lithium niobate crystals processed into a substrate shape to form a laminated structure, and placing the laminated structure in a porous container having air permeability. After placing the porous container containing the laminated structure in a heating furnace, the niobium is heated at 350° C. or higher in an inert gas atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of the lithium niobate crystal. The lithium niobate substrate is manufactured by heat-treating at a temperature lower than the Curie temperature of the lithium oxide crystal.

In addition, according to the method for producing a lithium niobate substrate according to the present invention, the mixed powder of Al powder and Al ₂ O ₃ powder used in Patent Document 2 is not used. It is possible to suppress the occurrence of dot-like reduction unevenness (black dot-like color unevenness) caused by accumulated floating dust and pattern-like reduction unevenness (pattern-like color unevenness) caused by leveling unevenness. It does not require the work of embedding substrate-shaped LN crystals in powder, exhaust equipment or protective equipment for dust countermeasures, and does not cause problems in terms of worker's health and global environment.

In addition, since the combustible hydrogen gas used in Patent Document 1 is not used, the lithium niobate substrate can be manufactured safely, and a laminated structure in which a plurality of lithium niobate crystals are laminated can be obtained. Since reduction treatment is performed in a porous container, it is possible to match the reduction degree (volume resistivity) of the lithium niobate crystals arranged in the top and bottom rows and the lithium niobate crystal arranged in the center. As a result, it becomes possible to efficiently manufacture a lithium niobate substrate which is uniformly improved in defects caused by pyroelectricity.

FIG. 2 is an explanatory diagram showing a state in which a laminated structure 10 in which a plurality of lithium niobate crystals 1 processed into a substrate shape are laminated is accommodated in a porous container 2 having air permeability.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail.

First, the LN crystal changes its electrical conductivity and color depending on the concentration of oxygen vacancies present in the crystal. When oxygen vacancies are introduced into the LN crystal, the valence of some Nb ions changes from 5+ to 4+ due to the need to maintain charge balance, resulting in electrical conductivity and light absorption. Electric conduction is considered to occur because electrons, which are carriers, move between Nb ⁵⁺ ions and Nb ⁴⁺ ions. The electrical conductivity of a crystal is determined by the product of the number of carriers per unit volume and the carrier mobility. If the mobility is the same, the electrical conductivity is proportional to the number of oxygen vacancies. Color change due to light absorption is believed to be due to electron levels introduced by oxygen vacancies.

By the way, as a conventional method for increasing the conductivity of the LN crystal, as described above, the method of heat-treating the LN crystal in a mixed gas atmosphere (reducing atmosphere) of nitrogen gas and hydrogen gas (Patent Document 1), and Al powder A method (Patent Document 2) is known in which LN crystals are embedded in a mixed powder of Al ₂ O 3 and Al 2 O ₃ powder and heat treatment (reduction treatment) is performed. existed.

Therefore, in order to solve each problem of Patent Document 1 and Patent Document 2, the method of the present invention is to laminate a plurality of LN crystals processed into a substrate shape to form a laminated structure, and the laminated structure is formed into a porous container. and placing the porous container containing the laminated structure of the LN crystal in a heating furnace, in an inert gas atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of the LN crystal , 350 ° C. As described above, the LN substrate is manufactured by heat-treating at a temperature lower than the Curie temperature of the LN crystal.

The method of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a plurality of LN crystals 1 processed into a substrate shape are laminated to form a laminated structure 10, and after the laminated structure 10 is accommodated in a porous container 2, the laminated structure 10 is accommodated. The porous container 2 thus prepared is placed in a heating furnace (not shown). The open portion of the porous container 2 shown in FIG. 1 is covered with a lid member 20 having air permeability.

Then, in an inert gas atmosphere such as argon (Ar) or nitrogen (N ₂ ), heat treatment is performed on the laminated structure 10 housed in the porous container 2 at a temperature of 350° C. or more and less than the Curie temperature of the LN crystal. , a plurality of LN crystals 1 are reduced at the same time. The degree of reduction (volume resistivity) of the LN crystal is determined by the difference between the "equilibrium oxygen partial pressure" of the heated LN crystal 1 and the "oxygen partial pressure" of the inert gas atmosphere, as described above.

In addition, when the laminated structure 10 is directly placed in the heating furnace and the reduction treatment is performed (that is, the reduction treatment is performed without the laminated structure 10 of the LN crystal 1 being accommodated in the porous container 2), the laminated structure 10 The LN crystals 1 located in the upper and lowermost stages (because a gap is likely to be formed between the mounting surface of the heating furnace and the laminated structure) are directly exposed to the inert gas during the reduction treatment, Since the LN crystals 1 located at and near the center of the laminated structure 10 are less likely to be exposed to the inert gas, the laminated structure 10 is less likely to be exposed to the inert gas than the LN crystals 1 arranged at the top and bottom of the laminated structure 10. It has been confirmed that the LN crystals 1 arranged at and near the center of the are under-reduced.

Therefore, in the method of the present invention, the laminate structure 10 is housed in the porous container 2 having air permeability, and the reduction conditions at the top and bottom stages of the laminate structure 10 and at the center and near the center of the laminate structure 10 are approximately adjusted to be uniform. That is, by suppressing the amount of inert gas supplied into the porous container 2 covered with the lid member 20, the reduction conditions are accordingly relaxed (reduction rate is slowed down, etc.), so the laminated structure 10 It is possible to match the degree of reduction (volume resistivity) of the LN crystals 1 arranged in the uppermost and lowermost stages and the LN crystals 1 arranged in the center and near the center.

The configuration of the method of the present invention will be described in detail below.

(1) Air permeable porous container In the method of the present invention, a permeable porous container is used. That is, by using a porous container covered with a lid material, the amount of inert gas supplied into the porous container is suppressed, and the reduction conditions are relaxed accordingly, so that the The degree of reduction (volume resistivity) of the LN crystals arranged in the uppermost and lowermost stages of the laminated structure and the LN crystals arranged in the center and near the center can be made uniform.

Any material can be used for the porous container provided that it has heat resistance and is stable in an inert gas, and its porosity is preferably 10% or more. For example, a porous graphite container (20% porosity) and a porous alumina container (30% porosity) can be used.

As for the porosity of the porous container, the higher the porosity, the more difficult it is to relax the reducing conditions because the permeability of the inert gas is improved. For this reason, as in the case where the laminated structure is directly arranged in the heating furnace and subjected to reduction treatment, the LN crystals arranged in the uppermost and lowermost stages of the laminated structure and the LN crystals arranged in the center and near the center are reduced. Variation in degree (volume resistivity) increases. Then, depending on the diameter of the LN crystal, the number of layers of the laminated structure, etc., when the diameter of the LN crystal is 4 inches to 6 inches and the number of layers of the laminated structure is 20 to 50, A porosity of 10% to 30% is exemplified as the porosity of the porous container.

The size of the porous container is set larger than the substrate-shaped LN crystal to be accommodated, preferably about 5 mm larger than the diameter of the LN crystal. By setting the size larger than the substrate-shaped LN crystal, the entire LN crystal can be accommodated in the porous container, so that the reduction treatment can be performed uniformly under the same conditions.

Further, the thickness of the plate constituting the porous container is not particularly limited, and it is preferably set to be thin provided that it does not crack during handling, and is exemplified by 1 mm to 5 mm.

(2) Heat treatment conditions A laminated structure is formed by laminating a plurality of LN crystals processed into a substrate shape, and the laminated structure is placed in a heating furnace while being housed in a porous container . In an inert gas atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure , heat treatment is performed at a temperature of 350° C. or higher and lower than the Curie temperature of the LN crystal to reduce the LN crystal.

As the inert gas, commercially available argon gas (oxygen partial pressure is about 1×10 ⁻⁶ atm), nitrogen gas, or the like can be applied. By not applying the combustible hydrogen gas (highly reducing gas) used in the examples of Patent Document 1, although the reducing ability is low, variations in the degree of reduction (volume resistivity) in the LN crystal can be suppressed. .

In addition, the atmosphere in the heating furnace has an air supply port and an exhaust port, and the inert gas is continuously supplied to and discharged from the heating furnace, and the pressure in the heating furnace is set to an atmospheric pressure atmosphere. exemplified.

When the inert gas is argon gas, the flow rate of the inert gas continuously supplied to and discharged from the heating furnace is preferably 0.5 to 5 L/min. In addition, since a heating furnace that continuously supplies and discharges inert gas is applied, there is no need to reduce the pressure or set the inside of the heating furnace to a vacuum, so a closed container or a decompression processing device is not required, which reduces the equipment cost. can be reduced.

By the method of the present invention, the volume resistivity of the LN substrate can be set to about 2.0×10 ⁹ to 1.5×10 ¹¹ (Ω·cm).

EXAMPLES 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 furnaces used in Examples 1 to 4 and Comparative Examples 1 to 2 and 4 were provided with an air supply port and ^an exhaust port. degree) is continuously supplied into the heating furnace through the air supply port, and argon gas (inert gas) is continuously exhausted out of the heating furnace through the exhaust port, and the inside of the heating furnace is at atmospheric pressure. Adjusted to the atmosphere. The flow rate of argon gas supplied to and exhausted from the heating furnace is set at 2 L/min.

[LN crystal growth and ingot processing, etc.]
An LN single crystal with a diameter of 4 inches was grown by the Czochralski method using a raw material with a congruent composition. The growth atmosphere is a nitrogen-oxygen mixed gas with an oxygen concentration of about 20%. The obtained LN crystal ingot was colorless and transparent.

The LN crystal ingot is subjected to heat treatment to remove thermal strain and poling treatment to achieve single polarization, followed by peripheral grinding, slicing, and polishing to obtain a substrate shape of 42 ° RY (Rotated Y axis). It was an LN crystal processed to.

The obtained 42° RY LN crystal was colorless and transparent, and had a volume resistivity of 1×10 ¹⁵ Ω·cm and a Curie temperature of 1140°C.

[Example 1]
Twenty LN crystals 1 processed into a substrate shape were laminated to form a laminated structure 10, and the laminated structure 10 was placed in a porous container 2 made of porous graphite (porosity of 20%).

Then, after placing the porous container 2 containing the laminated structure 10 of the LN crystal 1 in a heating furnace (not shown), commercially available argon gas was supplied into the heating furnace through the inlet. . The oxygen partial pressure of argon gas in the heating furnace was 5.0×10 ⁻⁷ atm.

Next, the argon gas was continuously drawn into and discharged from the heating furnace under atmospheric pressure at a flow rate of 2 L/min, and heat treatment (reduction treatment, blackening treatment) was performed at 500° C. for 20 hours. Moreover, the same treatment was repeated 10 times.

For a total of 200 heat-treated LN crystals, the volume resistivity of the LN substrate after treatment was measured, and the incidence of "dot-like color unevenness" and "pattern-like color unevenness" was visually investigated. bottom. The volume resistivity is measured by the three-probe method according to JIS K-6911.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) is about 1.5 × 10 ¹¹ Ω cm (average value of 200 substrates), and "dotted color unevenness" on the LN substrate surface ” and “patterned color unevenness” were both 0.0%.

These results are shown in Table 1.

[Example 2]
Heat treatment (reduction treatment, blackening treatment) of the LN crystal was performed under the same conditions as in Example 1, except that the treatment temperature was changed to 550°C.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) is about 1.5 × 10 ¹⁰ Ω cm (average value of 200 substrates), and "dotted color unevenness" on the LN substrate surface ” and “patterned color unevenness” were both 0.0%.

These results are shown in Table 1.

[Example 3]
Heat treatment (reduction treatment, blackening treatment) of the LN crystal was performed under the same conditions as in Example 1, except that the treatment temperature was changed to 600°C.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) is about 2.0 × 10 ⁹ Ω cm (average value of 200 substrates), and "dotted color unevenness" on the LN substrate surface ” and “patterned color unevenness” were both 0.0%.

These results are shown in Table 1.

[Example 4]
Under the same conditions as in Example 1, except that the porous container 2 made of porous alumina (30% porosity) was used instead of the porous container 2 made of porous graphite (20% porosity). A heat treatment (reduction treatment, blackening treatment) was performed on the LN crystal.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) is about 1.5 × 10 ¹¹ Ω cm (average value of 200 substrates), and "dotted color unevenness" on the LN substrate surface ” and “patterned color unevenness” were both 0.0%.

These results are shown in Table 1.

[Comparative Example 1]
Reduction treatment was performed by the method of Patent Document 2, in which LN crystals are embedded in a mixed powder of Al powder and Al ₂ O ₃ powder and heat treated. The mixing ratio of Al powder was 0.5%, and argon gas was continuously sucked into and exhausted from the heating furnace under atmospheric pressure at a flow rate of 2 L/min during the heat treatment. Moreover, the same treatment was repeated 10 times.

After heat treatment (reduction treatment, blackening treatment), the volume resistivity was measured by the same method as in Example 1, and the occurrence rate of "dot-like color unevenness" and "pattern-like color unevenness" on the LN substrate surface investigated.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) is about 1.0 × 10 ¹⁰ Ω cm (average value of 200 substrates), and "dotted color unevenness" on the LN substrate surface and "pattern-like color unevenness" were 5.0%, which were higher than those of Examples 1-4.

[Comparative Example 2]
Heat treatment (reduction treatment, blackening treatment) of the LN crystal was performed under the same conditions as in Example 1, except that the treatment temperature was changed to 300°C. Moreover, the same treatment was repeated 10 times.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) was about 1.0 × 10 ¹² Ω cm (average value of 200 substrates). Couldn't get rate.

The occurrence rate of "dot-like color unevenness" and "pattern-like color unevenness" on the LN substrate surface was both 0.0%.

[Comparative Example 3]
The LN crystal was heat treated (reduction treatment, blackening treatment) under the same conditions as in Example 1 except that the product inlet of the heating furnace was opened and argon gas was not sucked into and discharged from the heating furnace. The atmospheric oxygen partial pressure in the heating furnace during the heat treatment was 2.0×10 ⁻¹ atm. Moreover, the same treatment was repeated 10 times.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) was about 1.0×10 ¹⁵ Ω·cm (average value of 200 substrates), and was not reduced.

It should be noted that the rate of occurrence of "dot-like color unevenness" and "pattern-like color unevenness" on the surface of the LN substrate could not be determined because the LN crystal was not blackened.

[Comparative Example 4]
Twenty LN crystals 1 processed into a substrate shape are laminated to form a laminated structure 10, and the laminated structure 10 is directly placed in a heating furnace (not shown) without being housed in a porous container 2. was subjected to heat treatment (reduction treatment, blackening treatment) of the LN crystal under the same conditions as in Example 1. Moreover, the same treatment was repeated 10 times.

The volume resistivity of the LN substrate after heat treatment (reduction treatment, blackening treatment) was about 1.0×10 ¹⁰ Ω·cm (average value of 10 treatments) at the top and bottom of the laminated structure 10 . However, the volume resistivity of the LN substrate arranged in the center of the laminated structure 10 was about 1.0×10 ¹² Ω·cm (average value of 10 treatments) and varied.

The occurrence rate of "dot-like color unevenness" and "pattern-like color unevenness" on the LN substrate surface was both 0.0%.

According to the method of the present invention, dot-like reduction unevenness (black dot-like color unevenness) and the like can be suppressed, and a lithium niobate substrate having excellent electrical characteristics can be efficiently manufactured. It has industrial applicability as a substrate material for filters).

1 substrate-shaped LN crystal 2 porous container 10 laminated structure 20 lid material

Claims (3)

In a method for producing a lithium niobate substrate using lithium niobate crystals grown by the Czochralski method,
A laminated structure of lithium niobate crystals is formed by laminating a plurality of lithium niobate crystals processed into a substrate shape, and the laminated structure is housed in an air-permeable porous container. After the porous container containing the body is placed in a heating furnace, the curie of the lithium niobate crystal is heated at 350° C. or higher in an inert gas atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of the lithium niobate crystal. A method for producing a lithium niobate substrate, characterized in that the lithium niobate substrate is produced by heat-treating at a temperature lower than a temperature. 2. The method for producing a lithium niobate substrate according to claim 1, wherein the porous container is made of graphite or alumina. The inert gas is composed of argon gas, the heating furnace has an air supply port and an exhaust port, and the flow rate of argon gas continuously supplied to and discharged from the heating furnace is 0.5 to 5.0 L / min. The method for producing a lithium niobate substrate according to claim 1 or 2, characterized in that:

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