JP2020011874A - Manufacturing method of lithium tantalate substrate - Google Patents

Abstract

To provide a manufacturing method of a lithium tantalate (LT) substrate excellent in electrical characteristics that suppresses generation of punctiform irregularities (reduction irregularities).SOLUTION: A LT substrate is manufactured from a LT crystal grown by a Czochralski method. Porous alumina sheets 2 are laminated on both surfaces of the LT crystal 4 processed to a substrate state to compose a laminate structure 10 and a plurality of the laminate structures are laminated with aluminum plates 3 in between to compose a laminate assembly 100. After a container 1 containing the laminate assembly 100 is put in a furnace, the laminate assembly is subjected to heat treatment under an inert gas atmosphere at 350°C or higher and a temperature lower than Curie temperature of the LT crystal to manufacture the LT substrate. Because the porous alumina plate 2 prevents contact of the LT crystal 4 and the aluminum plate 3, generation of punctiform irregularities (reduction irregularities) is suppressed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method, and in particular, to a lithium tantalate substrate excellent in electrical characteristics without color unevenness (reduction unevenness). And a method for producing the same.

  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, the spread of Bluetooth (registered trademark) (2.45 GHz) by various types of electronic devices via wireless LAN, and the like.

  The SAW filter has a structure in which a pair of comb-shaped electrodes is formed of a metal thin film such as Al or Cu 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.

  The LT single crystal is industrially grown mainly by the Czochralski method in an electric furnace in a nitrogen-oxygen mixed gas atmosphere having an oxygen concentration of several percent to 20%, and usually has a high 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 of the electric furnace to obtain.

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, a 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, from a 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.

Japanese Patent No. 4063191 (refer to claim 1) Japanese Patent No. 4220997 (see page 4, line 30 to page 5, line 37) Japanese Patent No. 5133279 (see paragraphs 0013 to 0016)

  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 due to the pyroelectricity of the LT crystal, several techniques for increasing the conductivity have been proposed. For example, Patent Literature 1 discloses that an LT crystal processed into a substrate state (hereinafter referred to as a “substrate-shaped LT crystal” and distinguished from an LT substrate after heat treatment) is made of aluminum powder (Al powder) and aluminum oxide powder (Al A method of heat treatment (reduction treatment) by embedding in a mixed powder with ₂ O ₃ powder) is disclosed. 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.

However, the method of Patent Document 1 in which a substrate-shaped LT crystal is embedded in a mixed powder of an Al powder and an Al ₂ O ₃ powder and heat-treated is used, although it depends on the mixing ratio of the Al powder, dot-like reduction unevenness (black) Point) may occur. In addition, since the heat treatment is performed by embedding the substrate-shaped LT crystal in the mixed powder, it is necessary to uniformly disperse the Al powder in the mixed powder and embed the LT crystal while leveling the mixed powder evenly. There was a difficulty in workability.

  On the other hand, a method using no powder has been developed. For example, Patent Document 2 discloses that a base material (a slice wafer made of LT crystal) reduced at a temperature T1 (700 ° C. or more) is treated at a temperature T2 (300 to 600 ° C.) lower than the temperature T1 and in a reducing atmosphere. There is disclosed a method of increasing the conductivity of an LT crystal material by bringing the LT crystal material into contact with a single-polarized LT crystal material (a target of blackening treatment). Further, Patent Document 3 discloses that after immersing an LT crystal material (a target of blackening treatment) in a solution containing sodium chloride or potassium chloride (metal halide), the LT crystal material is heated at a temperature lower than the Curie temperature and in a reducing atmosphere. And a method of superposing a reducing agent (polypolarized LT obtained by heat-treating an LT crystal at a temperature not lower than the Curie temperature and not higher than 950 ° C. under a reducing atmosphere) and the LT crystal material, and performing a heat treatment. I have. The methods disclosed in Patent Literature 2 and Patent Literature 3 do not use a powder and use the LT crystal material (a multi-polarized LT) as a reducing material (a slice wafer composed of an LT crystal) or a reducing agent (polypolarized LT). Since the same LT crystal as that of the blackening treatment is applied, the LT crystal to be a product is not contaminated, the workability is good, and the productivity is improved. However, in these methods, since the LT crystal material (the subject of the blackening treatment) is brought into contact with the base material or the reducing agent to perform the reduction treatment, depending on the degree of contact of the LT crystal material, striped or annual ring-shaped color unevenness ( (A variation in conductivity occurs in the substrate). Further, in these methods, it is necessary to prepare in advance a base material (sliced wafer made of LT crystal) and a reducing agent (polypolarized LT) which have been subjected to a reduction treatment, so that the production efficiency is correspondingly low.

  The present invention has been made in view of such a problem, and it is an object of the present invention that the effect of improving the problem due to pyroelectricity is uniform, the occurrence of color unevenness can be suppressed, and the reproducibility is low. And a method for producing a lithium tantalate substrate excellent in production efficiency.

In order to solve the above-mentioned problems, the present inventor has disclosed a method described in Patent Literature 1 in which it is not necessary to prepare a reduction-treated base material (a slice wafer made of LT crystal) or a reducing agent (polypolarized LT) in advance. In this method, a detailed analysis was performed on the cause of the occurrence of black dot-like reduction unevenness (dot-like color unevenness) having a diameter of about 1 to 5 mm in this method. As a result, when the method of Patent Document 1 is carried out, clothing fibers or the like inevitably mixed into a mixed powder composed of Al powder and Al ₂ O ₃ powder, or inevitably adsorbed on the substrate-shaped LT crystal surface. It has been found that the floating refuse is the cause of the reduction unevenness (dotted color unevenness).

That is, the main component of the garment fiber is cellulose [molecular formula (C ₆ H ₁₀ O ₅ ) _n ], but the cellulose is self-decomposed at a high temperature during the reduction treatment, and carbon gas ( C), water vapor (H ₂ O) and the like are generated.
C ₆ H ₁₀ O ₅ → 6C + 5H ₂ O

  Then, the generated water vapor reacts with the Al powder contained in the mixed powder, and the Al powder is rapidly oxidized to generate local heat. This reaction occurs in the vicinity of the LT crystal in the form of a substrate, and the portion is reduced. It is considered that the reduction is locally reduced and the above-mentioned black dot-like reduction unevenness (dot-like color unevenness) occurs.

The present invention has been completed by such technical analysis and discovery, and no aluminum comes into contact with the substrate-shaped LT crystal, and the LT powder is mixed in the mixed powder of Al powder and Al ₂ O ₃ powder. A lithium tantalate substrate (which does not require a complicated operation of embedding the crystal and obtains a volume resistivity equivalent to that of a conventional processing method of embedding the LT crystal having a substrate shape in the mixed powder (that is, the method of Patent Document 1)) (LT substrate).

That is, the first invention according to the present invention is:
In a method of manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method,
A laminated structure is formed by laminating a porous alumina plate on both sides of a lithium tantalate crystal processed into a substrate state, and a plurality of the laminated structures are laminated via an aluminum plate to form a laminated assembly. And a heat treatment at 350 ° C. or higher and a temperature lower than the Curie temperature of the lithium tantalate crystal in an inert gas atmosphere after placing the container in which the laminated assembly is housed in a heating furnace. Is manufactured.

The second invention is
In the method for producing a lithium tantalate substrate according to the first invention,
The thickness of the porous alumina plate superposed on both surfaces of the lithium tantalate crystal is 0.5 mm or more and 5 mm or less.

The third invention is
In the method for producing a lithium tantalate substrate according to the first invention or the second invention,
The porosity of the porous alumina plate superimposed on both surfaces of the lithium tantalate crystal is 10% or more and 80% or less.

The fourth invention is
In the method for producing a lithium tantalate substrate according to any one of the first to third inventions,
The aluminum plate is characterized by being constituted by an etched aluminum plate whose surface area is increased by etching.

The fifth invention is
In the method for producing a lithium tantalate substrate according to the first 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 the argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5.0 L / min. It is characterized by being.

The sixth invention is
In the method for producing a lithium tantalate substrate according to the first invention,
The inert gas is composed of argon gas, the heating furnace is sealed, and the pressure in the furnace is set to an atmospheric pressure by the argon gas in the heating furnace.

According to the method of the present invention, aluminum does not come into contact with the lithium tantalate crystal processed into a substrate state, so that the occurrence of the above-mentioned reduction unevenness (dot-like color unevenness) is suppressed, and the Al powder disclosed in Patent Document 1 and Al ₂ O is not necessary to perform complicated operations _{to 3} embedding the mixed powder lithium-tantalate crystal which is processed to the state of the substrate during the powder.

  Therefore, it is possible to efficiently manufacture a lithium tantalate substrate having a uniform effect of improving the problem caused by pyroelectricity.

The porous alumina plates 2 and 2 are superposed on both surfaces of the substrate-shaped LT crystal 4 to form a laminated structure 10, and a plurality of laminated structures 10 are laminated via the aluminum plate 3 to form a laminated assembly 100. FIG. 3 is an explanatory view showing a state in which one container (in FIG. 1, one stainless steel container is shown) or a plurality of containers 1 in which the stacked aggregate 100 is stored in a large container 5.

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

The LT 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 LT crystal, the valency of some Ta ions changes from 5+ to 4+ due to the need to balance the charge, which causes electrical conductivity and light absorption. It is considered that electric conduction occurs because electrons serving 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 the carriers. For the same mobility, 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 oxygen vacancies.

Meanwhile, when increasing the conductivity of the LT crystal, the LT crystal having a substrate shape is heat-treated at a temperature lower than the Curie temperature in an inert gas having a sufficiently low oxygen partial pressure to introduce oxygen vacancies into the LT crystal. A method is conceivable. However, even a commercially available inert gas having a low oxygen concentration contains a few ppm or less of oxygen as an impurity. The conductivity of the crystal cannot be increased. Patent Document 1 discloses a method in which a substrate-shaped LT crystal is embedded in a mixed powder of Al powder and Al ₂ O ₃ powder and heat-treated. However, as the ratio of Al powder in the mixed powder increases, unevenness of reduction (dot-like color unevenness) of black dots having a diameter of about 1 to 5 mm tends to occur. This reduction unevenness is inevitably mixed into the mixed powder of Al powder and Al ₂ O ₃ powder as described above, or is caused by floating dust such as clothing fibers inevitably adsorbed on the LT crystal surface. Conceivable.

  Therefore, in the method of the present invention, a porous alumina plate is superposed on both surfaces of the LT crystal in the form of a substrate to form a laminated structure, and a plurality of the above laminated structures are laminated via an aluminum plate to form a laminated structure. Thus, a condition for introducing oxygen vacancies into the LT crystal by reducing the oxygen partial pressure of the inert gas existing around the LT crystal by the oxidation reaction of the aluminum plate is obtained, and further, the porous alumina plate is interposed. Since the LT crystal in the form of a substrate does not come into contact with the aluminum plate, the occurrence of the reduction unevenness (dot-like color unevenness) is suppressed.

  That is, as shown in FIG. 1, porous alumina plates 2 and 2 are superimposed on both surfaces of a substrate-shaped LT crystal 4 to form a laminated structure 10, and a plurality of laminated structures 10 are interposed via an aluminum plate 3. The stacked assembly 100 is formed by overlapping. Then, the laminated assembly 100 is accommodated in the stainless steel container 1, and one (in FIG. 1, one stainless steel container 1 is shown) or a plurality of stainless steel containers 1 is a large container 5 made of aluminum. The large container 5 is placed in a heating furnace (not shown), and then heat-treated in an inert gas atmosphere at a temperature of 350 ° C. or higher and lower than the Curie temperature of the lithium tantalate crystal to form a substrate LT. Crystal 4 is being reduced.

  The container 1 and the large container 5 shown in FIG. 1 are covered with a cover material, but are not closed containers. Further, the large container 5 may be omitted and the container 1 may be directly disposed in the heating furnace.

  In the method of the present invention, as shown in FIG. 1, the porous alumina plate 2 intervenes and the LT crystal 4 in the form of a substrate does not come into contact with the aluminum plate 3, so that the exothermic reaction between water vapor and aluminum caused by the above-mentioned floating dust and the like causes It can be prevented from occurring near the shaped LT crystal 4, and it is possible to suppress the occurrence of the above-described reduction unevenness (dot-like color unevenness).

  Further, by using the porous alumina plate 2, the convection velocity of the argon gas (inert gas) near the substrate-shaped LT crystal 4 is increased, and the diffusion of oxygen molecules desorbed from the LT crystal 4 is reduced, thereby reducing Processing can be promoted.

  Furthermore, the volume resistivity of the LT crystal 4 can be reduced by increasing the surface area of the aluminum plate 3 to be superimposed on the porous alumina plate 2. Increasing the surface area of the aluminum plate 3 increases the probability of contact between oxygen in the argon gas (inert gas) and the aluminum plate, and facilitates the oxidation reaction of aluminum. ) Can be reduced to a low oxygen partial pressure, and as a result, it is possible to promote the reduction treatment of the LT crystal 4 having a substrate shape. As a method of increasing the surface area of the aluminum plate 3, a method of forming a hole by punching or a method of using an etched aluminum plate having fine irregularities formed on the surface by etching is used.

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

(1) Porous alumina plate In the method of the present invention, a porous alumina plate is used. By using a porous alumina plate, air permeability is improved, and the reducibility of the aluminum plate is promoted.

  When a dense alumina plate or an alumina plate having a porosity of less than 10% is used, the air permeability with the aluminum plate is deteriorated. Therefore, similar to the method of Patent Documents 2 and 3 depending on the contact condition with the LT crystal having a substrate shape. Striped or annual ring-shaped reduction unevenness may occur. The porosity of the alumina plate is preferably 10% or more and 80% or less, and more preferably 60% or more. By increasing the porosity of the alumina plate, the air permeability is improved, and the reducibility of the aluminum plate is promoted. Further, when the porosity of the alumina plate exceeds 80%, the strength of the alumina plate is weakened, and the alumina plate is frequently broken or chipped during handling of the alumina plate. Therefore, the porosity of the alumina plate is preferably 10% or more and 80% or less as described above.

The size of the alumina plate is set to be larger than the size of the LT crystal.
Preferably, the outer peripheral side of the alumina plate protruding outward from the outer peripheral end side of the LT crystal is set to be larger by the thickness of the LT crystal and equal to or smaller than the thickness of the alumina plate. By setting the outer peripheral side of the alumina plate that protrudes outward from the outer peripheral end side of the LT crystal to be equal to or less than the thickness of the alumina plate, the outer peripheral edge of the LT crystal becomes dark black. Blackening caused by the peripheral portion being strongly reduced) can be prevented. By setting the outer peripheral side of the alumina plate protruding outward from the outer peripheral end side of the LT crystal by the thickness of the LT crystal and not more than the thickness of the alumina plate, the outer peripheral edge of the LT crystal is set. Can be uniformly reduced.

  Further, the thickness of the alumina plate is preferably 5 mm or less, more preferably 2 mm or less. If the thickness exceeds 5 mm, the distance between the substrate-shaped LT crystal and the aluminum plate becomes large, so that the reducing action is reduced. In addition, the thickness of the laminated structure 10 becomes large and is accommodated in the container 1. As the number of processed sheets decreases, productivity deteriorates. When the thickness is less than 0.5 mm, the strength of the alumina plate is weakened, and the alumina plate is often broken or chipped during handling of the alumina plate. Therefore, the thickness of the alumina plate is preferably 0.5 mm or more and 5 mm or less.

(2) Aluminum plate As the aluminum plate, an aluminum plate with a smooth surface or an aluminum plate with roughened front and back surfaces can be used. As for the size of the aluminum plate, similarly to the above-mentioned alumina plate, the outer peripheral side of the aluminum plate protruding outward from the outer peripheral end side of the LT crystal is larger by the thickness of the LT crystal and not more than the thickness of the alumina plate. Should be set to. The thickness of the aluminum plate is not particularly limited, and may be 0.005 mm to 1 mm.

  When a plurality of laminated structures 10 are superposed to form the laminated assembly 100 shown in FIG. 1, both sides of the LT crystal 4 located at the uppermost stage and the lowermost stage are also placed on the aluminum plate via the porous alumina plate 2. Is arranged, the uppermost surface and the lowermost surface of the laminated assembly 100 are formed of the aluminum plate 3 as shown in FIG. When the thickness of the aluminum plate (foil) 3 constituting the uppermost surface of the laminated assembly 100 is small, undulations and the like occur on the surface of the aluminum plate (foil) 3 and the aluminum plate (foil) is formed on the porous alumina plate 2. 3 may not adhere uniformly. In such a case, the porous alumina plate 2 may be further disposed outside the aluminum plate (foil) 3 in order to enhance the adhesion of the aluminum plate (foil) 3.

Further, as described above, the surface area of the aluminum plate may be increased to promote its reducibility. For example, an aluminum plate formed with a plurality of through-holes of φ0.5 mm to φ1 mm by punching may be used. Good. Also, fine irregularities may be formed on the front and back surfaces of the aluminum plate instead of the method of forming the through holes. For example, fine irregularities may be formed by etching and roughening the front and back surfaces of an aluminum plate. The size of the fine irregularities may be represented by a specific surface area. Incidentally, the specific surface area of the aluminum plate having a smooth surface is 0.025 m ² / g as shown in the following examples. Further, the specific surface area of the aluminum (etched aluminum) plate having fine irregularities formed on the surface by etching is 0.694 m ² / g (see Example 13). When the specific surface area of the aluminum plate increases, the reduction treatment of the LT crystal is further promoted. The specific surface area is calculated by measuring with a specific surface measuring device by a BET flow method.

(3) Heat treatment conditions As shown in FIG. 1, porous alumina plates 2 and 2 are superposed on both surfaces of a substrate-shaped LT crystal 4 to form a laminated structure 10, and a plurality of laminated structures are interposed via an aluminum plate 3. The stacked bodies 100 are stacked to form a stacked assembly 100, and the stainless steel container 1 in which the stacked assembly 100 is housed is accommodated in a large aluminum container 5, and then placed in a heating furnace (not shown). Then, a heat treatment is performed in an inert gas atmosphere at a temperature of 350 ° C. or higher and lower than the Curie temperature of the lithium tantalate crystal (about 600 ° C.) to manufacture a lithium tantalate substrate. Further, as described above, the stainless steel container 1 may be directly arranged in the heating furnace, omitting the large container 5 made of aluminum.

As the inert gas, commercially available argon gas (oxygen partial pressure is about 1 × 10 ⁻⁶ atm), nitrogen gas, and the like can be used. Further, the atmosphere in the heating furnace has an air supply port and an exhaust port, an inert gas is continuously supplied and discharged into the heating furnace, and the pressure in the heating furnace is set to an atmospheric pressure atmosphere, or An example is a condition in which the heating furnace is sealed and the pressure in the heating furnace is set to an atmospheric pressure atmosphere by an inert gas sealed in the heating furnace.

  In the former case (that is, a heating furnace in which the inert gas is continuously supplied and discharged), the inert gas is an argon gas with respect to the flow rate of the inert gas continuously supplied and discharged into the heating furnace. In this case, it is preferably 0.5 to 5 L / min. In addition, when the above-mentioned heating furnace in which the inert gas is continuously supplied and discharged is applied, the inside of the heating furnace is not set to a reduced pressure or a vacuum, and a closed vessel and a decompression device are not required, thereby reducing equipment costs. Can be achieved.

According to the method of the present invention, the volume resistivity of the LT substrate can be set to about 3 × 10 ^{9 to} 5 × 10 ¹² (Ω · cm). The volume resistivity of the LT substrate can be appropriately adjusted by the porosity, plate thickness, specific surface area of the aluminum plate, and the like of the alumina plate.

  Further, in the method of the present invention, since the porous alumina plate intervenes and the LT crystal in the form of the substrate does not come into contact with the aluminum plate, an exothermic reaction between water vapor and aluminum due to floating dust occurs near the LT crystal. This can prevent the occurrence of reduction unevenness (dot-like color unevenness).

Further, in the method of the present invention, it is not necessary to perform a complicated operation of embedding the LT crystal having the substrate shape in the mixed powder of the Al powder and the Al ₂ O ₃ powder in Patent Document 1, so that the productivity of the LT substrate is significantly improved. It is possible to do.

  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 and Comparative Examples are provided with an air inlet and an air outlet, and generally commercially available argon gas (oxygen partial pressure is about 1 × 10 ⁻⁶ atm) is supplied through the air inlet. 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 peripheral 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]
The laminated structure 10 is formed by laminating the porous alumina plates 2 and 2 on both sides of the LT crystal 4 processed into a substrate shape, and the four laminated structures 10 are laminated via the aluminum plate 3 to obtain the laminated structure shown in FIG. After constructing the laminated assembly 100 shown, the laminated assembly 100 was housed in a stainless steel container. Note that the uppermost surface and the lowermost surface of the stacked assembly 100 are formed of an aluminum plate 3 as shown in FIG. The porous alumina plate 2 had a diameter of 102 mm, a porosity of 70%, and a thickness of 1 mm. The smooth aluminum plate (foil) 3 had a diameter of 102 mm, a thickness of 15 μm, and a specific surface area of 0.025 m ² / g measured by a BET flow method.

  Then, after placing the stainless steel container accommodating the laminated assembly 100 in the heating furnace, a commercially available argon gas was supplied into the heating furnace through the air inlet.

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

  For a total of 200 heat-treated LT crystals, the volume resistivity of the treated LT substrate was measured, and the occurrence rate of spot-like color unevenness (reduction unevenness) was examined by visual inspection. 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 (blackening treatment) is about 3.0 × 10 ⁹ Ω · cm (average value of 200 LT substrates), and the occurrence rate of spot-like unevenness (color unevenness) on the surface of the LT substrate Was 0.0%. The results are shown in Table 1.

[Example 2]
The LT crystal was heat-treated (blackened) under the same conditions as in Example 1 except that the porosity of the porous alumina plate was 60%.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) was about 5.0 × 10 ⁹ Ω · cm, and the occurrence rate of spot-like unevenness (uneven color) was 0.0%.

  Table 1 shows the results.

[Examples 3 to 12]
The LT crystal was heat-treated (blackened) under the same conditions as in Example 1 except that the porosity and thickness of the porous alumina plate were changed to the conditions described in Table 1.

The volume resistivity of the LT substrates according to Examples 3 to 12 after the heat treatment (blackening treatment) is 17.0 × 10 ^{9 to} 5.0 × 10 ¹² Ω · cm, and the rate of occurrence of dot-like unevenness (color unevenness) Was 0.0%.

  The results are shown in Table 1.

[Comparative Example 1]
The LT crystal was heat-treated (blackened) under the same conditions as in Example 1 except that the porous alumina plate was changed to a dense alumina plate having a thickness of 1 mm.

The occurrence rate of spot-like unevenness (uneven color) on the LT substrate surface after the heat treatment (blackening treatment) was 0.0% as in Example 1, but the volume resistivity was equal to or higher than the upper limit of measurement (10 ¹⁵ Ω · cm). The color tone change was confirmed to be slightly blackened, but hardly blackened.

  Table 1 shows the results.

Example 13
A heat treatment (blackening treatment) of the LT crystal was performed under the same conditions as in Example 1 except that an etched aluminum plate (foil) having fine irregularities formed on the surface by etching was used instead of the smooth aluminum plate (foil). went. The etched Al plate (foil) had a thickness of about 100 μm and a specific surface area of 0.694 m ² / g measured by the BET flow method.

The volume resistivity of the LT substrate after the heat treatment (blackening treatment) was about 1.5 × 10 ⁹ Ω · cm, and the occurrence rate of color unevenness failure was 0.0% (similar to Example 1).

  Table 1 shows the results.

[Comparative Example 2]
The reduction treatment was performed by a method according to Patent Document 1 in which LT crystals were embedded in a mixed powder of Al powder and Al ₂ O ₃ powder and heat-treated. The mixing ratio of the Al powder was 20%, and argon gas was continuously supplied and discharged into the heating furnace under an atmospheric pressure atmosphere at a flow rate of 2 L / min during the heat treatment.

  After the heat treatment (blackening treatment), the volume resistivity was measured and the occurrence rate of color unevenness failure was investigated by the same method as in Example 1.

The volume resistivity of the LT substrate according to Comparative Example 2 after the heat treatment (blackening treatment) was as good as 0.7 × 10 ⁹ Ω · cm, but the occurrence rate of color unevenness was 15.0%. It was higher than Example and Comparative Example 1. Table 2 shows the results.

[Verification]
(1) The volume resistivity of the obtained LT substrate was compared between Examples 1 to 5 and Comparative Example 1 in which the thickness of the alumina plate was all 1 mm and the porosity of the alumina plate was different.

Example 1 having a porosity of 70% (3.0 × 10 ⁹ Ω · cm),
Example 2 having a porosity of 60% (5.0 × 10 ⁹ Ω · cm),
Example 3 having a porosity of 50% (7.0 × 10 ⁹ Ω · cm),
Example 4 having a porosity of 30% (1.0 × 10 ¹⁰ Ω · cm),
Example 5 having a porosity of 10% (1.5 × 10 ¹⁰ Ω · cm)
Comparative Example 1 in which a dense alumina plate was applied (measurement upper limit),
From these comparisons, it was confirmed that the porosity of the alumina plate was important, and that the volume resistivity of the LT substrate after the heat treatment was further reduced as the porosity of the alumina plate was increased.

  On the other hand, it was confirmed that the dense alumina plate was not blackened because the low oxygen partial pressure Ar gas did not reach the vicinity of the LT crystal.

(2) The volume resistivity of the obtained LT substrate was compared for Examples 1, 6, and 11 in which the porosity of the alumina plate was all 70% and the thickness of the alumina plate was different.

Example 1 in which the thickness of the alumina plate was 1 mm (3.0 × 10 ⁹ Ω · cm),
Example 6 (2.2 × 10 ¹⁰ Ω · cm) in which the thickness of the alumina plate was 2 mm,
Example 11 in which the thickness of the alumina plate is 5 mm (3.0 × 10 ¹¹ Ω · cm),
From these comparisons, it was confirmed that as the thickness of the alumina plate becomes larger, the Ar gas having a low oxygen partial pressure becomes more difficult to reach the vicinity of the LT crystal, so that the volume resistivity of the LT substrate becomes more difficult to decrease. Was.

(3) The volume resistivity of the obtained LT substrate was compared between Example 1 and Example 13 where the alumina plate (porosity was 70% and the thickness was 1 mm) was the same and the applied aluminum plate was different.

Example 1 (3.0 × 10 ⁹ Ω · cm) using a smooth aluminum plate (foil),
Example 13 using an etched aluminum plate (foil) (1.5 × 10 ⁹ Ω · cm),
From the comparison with Example 1 in which a smooth aluminum plate (foil) is applied, the volume resistivity of the LT substrate is further reduced by increasing the surface area of the aluminum plate using the etched aluminum plate (foil). Was confirmed.

(4) Comparison between Examples 1 to 13 to which the method of the present invention is applied and Comparative Example 2 to which the method of Patent Document 1 is applied in which LT crystals are embedded in a mixed powder of Al powder and Al ₂ O ₃ powder and heat treatment is performed. Therefore, the method of the present invention can reduce the volume resistivity of the LT substrate to the same level as the method of Patent Document 1, and can prevent the occurrence of dot-like unevenness (color unevenness) which is a problem in Patent Document 1. Was also confirmed.

  According to the method of the present invention, the occurrence of point-like unevenness (reduction unevenness) is suppressed, and a lithium tantalate substrate having excellent electrical characteristics can be manufactured. Therefore, the method can be used as a substrate material for a surface acoustic wave device (SAW filter). It has the industrial applicability used.

DESCRIPTION OF SYMBOLS 1 Stainless steel container 2 Porous alumina plate 3 Aluminum plate 4 Substrate-shaped lithium tantalate crystal 5 Aluminum large container 10 Laminated structure 100 Laminated assembly

Claims (6)

In a method of manufacturing a lithium tantalate substrate using lithium tantalate crystals grown by the Czochralski method,
A laminated structure is formed by laminating a porous alumina plate on both sides of a lithium tantalate crystal processed into a substrate state, and a plurality of the laminated structures are laminated via an aluminum plate to form a laminated assembly. And a heat treatment at 350 ° C. or higher and a temperature lower than the Curie temperature of the lithium tantalate crystal in an inert gas atmosphere after placing the container in which the laminated assembly is accommodated in a heating furnace. And a method for producing a lithium tantalate substrate.   The method for producing a lithium tantalate substrate according to claim 1, wherein the thickness of the porous alumina plate superimposed on both surfaces of the lithium tantalate crystal is 0.5 mm or more and 5 mm or less.   The method for producing a lithium tantalate substrate according to claim 1 or 2, wherein the porosity of the porous alumina plate superimposed on both surfaces of the lithium tantalate crystal is 10% or more and 80% or less.   The method for producing a lithium tantalate substrate according to any one of claims 1 to 3, wherein the aluminum plate is constituted by an etched aluminum plate whose surface area is increased by etching.   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 the argon gas continuously supplied and discharged into the heating furnace is 0.5 to 5.0 L / min. The method for producing a lithium tantalate substrate according to claim 1, wherein:   2. The method according to claim 1, wherein the inert gas is formed of argon gas, the heating furnace is sealed, and the pressure in the furnace is set to an atmospheric pressure by the argon gas in the heating furnace. Of producing a lithium tantalate substrate.