JP2021134097A - Method for manufacturing lithium tantalate substrate, and lithium tantalate substrate - Google Patents

Abstract

To provide a method for manufacturing a lithium tantalate (LT) substrate, capable of suppressing yield reduction due to pyroelectricity in an element manufacturing process without affecting element quality in the exposure step of the element manufacturing process, and the LT substrate.SOLUTION: A method for manufacturing a LT substrate using a LT crystal comprises: burying a substrate-shaped LT crystal 3 in the mixed powder 2 of aluminum powder and aluminum oxide powder charged in a vessel 1; arranging the vessel in a heating furnace in an atmospheric pressure atmosphere; and heating the LT crystal at a temperature less than the curie temperature of the LT crystal while continuously supplying and exhausting an inert gas into the heating furnace. When setting a distance between a plurality of substrate-shaped LT crystals and buried into the mixed powder 2 to L (mm) and setting a distance between the outer peripheral edge part of the substrate-shaped LT crystal and the inner wall of the vessel to L1 (mm), L1/L satisfies a range of more than 1.5 and 3.5 or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for producing a lithium tantalate substrate using lithium tantalate crystals, and in particular, a decrease in yield in an element manufacturing process due to charcoalism is suppressed, and an exposure step in the element manufacturing process, etc. The present invention relates to a method for manufacturing a lithium tantalate substrate and a lithium tantalate substrate that do not affect the element quality.

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. It is mainly applied as a surface acoustic wave (SAW) filter material used in mobile phone transmission / reception devices.

It is predicted that the number of SAW filters in the frequency range of 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), which is a wireless LAN for 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 and Cu on a substrate made of a piezoelectric material such as LT, and the comb-shaped electrodes enhance the characteristics of the device. It plays an important role in influence. Further, the comb-shaped electrode is formed by forming a metal thin film on a piezoelectric material by sputtering, and then removing unnecessary parts by leaving a pair of comb-shaped patterns by a photolithographic technique including an exposure step and an etching step. It is formed.

Further, the above LT single crystal is industrially grown mainly by the Czochralski method in an electric furnace having a nitrogen-oxygen mixed gas atmosphere having an oxygen concentration of about several% to 20%, and is usually high. An iridium crucible having a melting point is used, and the grown LT single crystal is cooled in an electric furnace at a predetermined cooling rate and then taken out from the electric furnace. The method for growing the LT single crystal is not limited to the above-mentioned Czochralski method, and other known growing methods, for example, a pulling method may be used.

The grown LT crystal is colorless and transparent or has a highly transparent pale yellow color. After growing, in order to remove residual strain due to thermal stress of the crystal, heat treatment is performed under soaking temperature close to the melting point, and further polling treatment for unipolarization, that is, 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 equal to or lower than the Curie temperature while the voltage is applied, and then the voltage application is stopped and the temperature is cooled to room temperature. After the polling process, the LT crystal (referred to as an ingot) whose outer diameter is ground to adjust the outer diameter of the crystal becomes a substrate through machining 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.

In the substrate obtained by such a conventional method, in the surface acoustic wave element (SAW filter) manufacturing process, due to the pyroelectricity characteristic of the LT crystal, the charge is charged to the surface of the substrate due to the temperature change received in the process. However, the electric charge generated by this causes the comb-shaped electrode formed on the surface of the substrate to be destroyed, and further, the substrate is cracked or the like, resulting in a decrease in the yield in the element manufacturing process.

Therefore, some techniques for increasing the conductivity have been proposed in order to eliminate the problems caused by the pyroelectricity of the LT crystal. For example, in Patent Document 1, an LT crystal processed into a substrate state in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O _{3 powder) filled in a container (hereinafter, “substrate”.} A method has been proposed in which a "LT crystal having a shape" is used to distinguish it from a "LT substrate" after heat treatment) and heat treatment (reduction treatment) is performed. In Patent Document 2, Al powder filled in a container and Al ₂ O _{3 are proposed.} After embedding a substrate-shaped LT crystal in a mixed powder with powder and arranging the above container in a heating furnace under an atmospheric pressure atmosphere, heat treatment (reduction treatment) is performed while continuously supplying and discharging inert gas into the heating furnace. ), An improved method of Patent Document 1 has been proposed. The LT substrate with increased conductivity will absorb light due to the introduction of oxygen vacancies. Since the observed color tone of the LT substrate looks reddish-brown in transmitted light and black in reflected light, the reduction treatment for increasing conductivity is also called blackening treatment, and such a color tone change phenomenon. Is called blackening.

Japanese Patent No. 4063191 Japanese Unexamined Patent Publication No. 2019-112267

By the way, in the field of manufacturing surface acoustic wave elements (SAW filters), the volume resistivity of the LT substrate is set to 1 × 10 ¹⁰ Ω. There is a request to set it to about cm to 1 × 10 ¹¹ _{Ω · cm, and patent documents 1 to 2 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 (reduction treatment) are performed. It is possible to manufacture an LT substrate having a volume resistivity of about 1 × 10 ^{10 Ω · cm by the above method.} However, in the LT substrate having increased conductivity, oxygen vacancies are introduced to cause light absorption, so that the transmittance of the LT substrate decreases as the conductivity increases.

On the other hand, the transmittance of the LT substrate needs to be adjusted within a predetermined range because the transmittance of the LT substrate affects the device quality in the device manufacturing process.

Therefore, it is not preferable to reduce the volume resistivity of the LT substrate to dark clouds with the aim of preventing cracking of the LT substrate due to pyroelectricity.

The present invention has been made by paying attention to such a problem, and the object thereof is that a decrease in yield in the device manufacturing process due to charcoalism is suppressed, and an exposure process in the device manufacturing process, etc. It is an object of the present invention to provide an LT substrate manufacturing method and an LT substrate that do not affect the element quality.

In order to solve the above problems, the present inventor conducted the following technical analysis.

First, in the case of manufacturing a surface acoustic wave element (SAW filter), a portion (region) about 5 mm inside from the outer peripheral edge of the LT substrate is not normally used as an element material, so that the inner region is about 5 mm from the outer peripheral edge. It was found that even if the transmittance is set lower than the others, the element quality is not affected in the exposure process or the like. Further, in the manufacturing process of the surface acoustic wave element (SAW filter), a portion where cracking of the LT substrate is likely to occur due to pyroelectricity was investigated. It was confirmed that it is in the vicinity thereof), and it was also found that it is effective to increase the conductivity of the outer peripheral portion of the LT substrate in order to prevent cracking of the LT substrate due to pyroelectricity.

Further, in the method of Patent Documents 1 and 2 in which a plurality of substrate-shaped LT crystals are embedded in a mixed powder of Al powder and Al ₂ O _{3 powder and heat-treated (reduction treatment), the outer peripheral edge of the LT crystal is formed.} In addition to the Al powder in the mixed powder intervening between the LT crystals, the Al powder in the mixed powder intervening between the outer peripheral edge of the LT crystal and the inner wall of the container also contributes, so that it is outside compared to the central part of the LT crystal. It was confirmed that the peripheral portion was reduced, and the volume resistance of the outer peripheral edge portion of the LT substrate and the region in the vicinity thereof after the reduction treatment was lower than that of the central portion of the LT substrate.

Therefore, in consideration of the above contribution of the Al powder in the mixed powder to the outer peripheral edge of the LT crystal, cracking of the LT substrate due to pyroelectricity is suppressed, and the LT does not affect the element quality in the exposure process or the like. As a result of investigating the embedding conditions of the LT crystal capable of manufacturing the substrate, the following technical discoveries were made. That is, the distance between a plurality of LT crystals embedded in the mixed powder of Al powder and Al ₂ O ₃ powder is L (mm), and the distance between the outer peripheral edge of the LT crystal and the inner wall of the container is L1 (mm). If this is the case, by setting (L1 / L) to the range of "more than 1.5 and 3.5 or less", cracking of the LT substrate due to pyroelectricity is suppressed, which affects the element quality in the exposure process and the like. It has been found that an LT substrate that does not give the same amount of material can be obtained. The present invention has been completed by such technical analysis.

That is, the first invention according to the present invention is
A method for producing a lithium tantalate substrate using lithium tantalate crystals, which is a lithium tantalate crystal processed into a substrate state in a mixed powder of aluminum powder and aluminum oxide powder filled in a container (hereinafter referred to as “lithium tantalate crystal”). , "Lithium tantalate crystal in the shape of a substrate" to distinguish it from the "lithium tantalate substrate" after heat treatment), and after arranging the container in a heating furnace under atmospheric pressure atmosphere, the inside of the heating furnace In a method for producing a lithium tantalate substrate, heat treatment is performed at a temperature lower than the Curie temperature of lithium tantalate crystals while continuously supplying and discharging an inert gas.
The distance between a plurality of substrate-shaped lithium tantalate crystals embedded in the mixed powder is L (mm), and the distance between the outer peripheral edge of the substrate-shaped lithium tantalate crystal and the inner wall of the container is L1 (mm). , L1 / L satisfies the range of more than 1.5 and 3.5 or less.

Moreover, the second invention is
In the method for producing a lithium tantalate substrate according to the first invention.
It is characterized in that the above L1 / L satisfies the range of 2 or more and 3 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 distance L between a plurality of substrate-shaped lithium tantalate substrates embedded in the mixed powder satisfies the range of 0.5 mm to 10 mm.

The fourth invention is
In the method for producing a lithium tantalate substrate according to any one of the first invention to the third invention.
The inert gas is argon gas, and the flow rate of the argon gas continuously supplied and discharged into the heating furnace satisfies the range of 0.5 to 5 L / min.

Next, the fifth invention is
In the lithium tantalate substrate
The transmittance of the central portion used as the element material at a wavelength of 600 nm is in the range of 40% to 60%, and the transmittance of the outer peripheral portion at a wavelength of 600 nm is 5% or more lower than the transmittance of the central portion.

According to the method of the present invention
The distance between a plurality of substrate-shaped lithium tantalate crystals embedded in a mixed powder of aluminum powder and aluminum oxide powder is L (mm), and the distance between the outer peripheral edge of the substrate-shaped lithium tantalate crystal and the inner wall of the container is When the distance is L1 (mm), lithium tantalate crystals are embedded in the mixed powder under the condition that L1 / L satisfies the range of more than 1.5 and 3.5 or less.

Therefore, the volume resistivity of the outer peripheral edge of the lithium tantalate substrate and its vicinity can be set to a low level (that is, a high level of conductivity) in which cracking of the substrate due to pyroelectricity is suppressed, and the device can be set. The transmittance of the central portion used as a material can also be adjusted to a level that does not affect the device quality in the exposure process of the device manufacturing process or the like.

Therefore, there is an effect that a decrease in yield in the element manufacturing process due to pyroelectricity can be suppressed, and a lithium tantalate substrate that does not affect the element quality in the exposure process of the element manufacturing process can be provided.

Explanatory drawing which shows the state in which a plurality of containers in which substrate-shaped lithium tantalate crystals are embedded in a mixed powder of aluminum powder and aluminum oxide powder are housed in a large container. The explanatory view which shows the state which a plurality of lithium tantalate crystals were embedded in the mixed powder of the aluminum powder and the aluminum oxide powder filled in the said container.

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

The present invention relates to an improvement of an LT substrate that suppresses a decrease in yield in the surface elastic element manufacturing process due to pyroelectricity and does not affect the element quality in the exposure process of the surface elastic element manufacturing process. Is.

That is, the volume resistance of the outer peripheral edge of the LT substrate that is not used as a surface elastic element and its vicinity can be adjusted to a low level (that is, a high level of conductivity) in which cracking of the substrate due to charcoalism is suppressed. At the same time, the transmittance of the central part of the LT substrate used as the surface elastic element material is at a level that does not affect the element quality in the exposure process of the surface elastic element manufacturing process (for example, the transmittance at a wavelength of 600 nm is 40% or more). It relates to an LT substrate that can be adjusted to 60%) and a method for manufacturing the same.

Hereinafter, a method for manufacturing an LT substrate according to the present invention will be described with reference to the drawings.

FIG. 1 is an explanatory view showing a state in which a plurality of containers in which substrate-shaped LT crystals are embedded in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al ₂ O _{3 powder) are housed in a large container.} FIG. 2 is an explanatory diagram showing a state in which a plurality of the above LT crystals are embedded in a mixed powder of _{Al powder and Al 2} O _{3 powder filled in a container.}

As shown in FIG. 1, a _{mixed powder 2 of Al powder and Al 2} O ₃ powder is filled in a stainless steel container 1, and a plurality of substrate-shaped LT crystals 3 are embedded in the mixed powder 2 at equal intervals. , A plurality of containers 1 configured in the same manner are collectively housed in the large container 4, and the large container 4 is placed in a heating furnace (not shown), and then the following heat treatment (reduction treatment) is performed. ..

The inside of the heating furnace during heat treatment (reduction treatment) is set to a vacuum atmosphere, a sealing atmosphere filled with an inert gas, an atmospheric pressure atmosphere in which the inert gas is continuously supplied and discharged, etc. When the sealing atmosphere is filled with an inert gas, the heat distribution in the heating furnace may accumulate in one place and cause uneven reduction in the LT substrate. Therefore, it is preferable that the inside of the heating furnace during the heat treatment (reduction treatment) has an atmospheric pressure atmosphere in which the inert gas is continuously supplied and discharged. In the case of an atmospheric pressure atmosphere, it is not necessary to provide a sealing means, a decompression treatment device, or the like in the heating furnace, so that the equipment cost can be reduced. Further, as the inert gas, argon, nitrogen gas or the like can be used. When the inert gas is argon, the flow rate of the inert gas continuously supplied to and discharged from the heating furnace is preferably 0.5 to 5 L / min.

Then, while continuously supplying and discharging an inert gas such as argon into the heating furnace in an atmospheric pressure atmosphere, the temperature of the heating furnace is raised from 350 ° C. to a temperature lower than the Curie temperature (about 600 ° C.) of the LT crystal, and a predetermined value is determined. After that, the temperature is lowered to room temperature to complete the reduction treatment.

By the above heat treatment (reduction treatment) ^{, an LT substrate having a volume resistivity of about 1 × 10 10} Ω · cm to 1 × 10 ¹¹ Ω · cm and a transmittance of 40% to 60% at a wavelength of 600 nm can be obtained.

[Inner diameter size of stainless steel container 1]
By the way, by changing the inner diameter size of the stainless steel container 1 shown in FIG. 2, the reduction conditions at the outer peripheral edge portion of the LT crystal 3 can be appropriately adjusted.

That is, the distance between the substrate-shaped LT crystals 3 embedded in the mixed powder 2 of the Al powder and the Al ₂ O ₃ powder is L (mm), the outer peripheral edge of the substrate-shaped LT crystals 3 and the stainless steel container 1 When the distance between the inner walls of the LT crystal 3 is L1 (mm), by changing the inner diameter size of the stainless steel container 1, the outer peripheral edge of the LT crystal 3 and the inner wall of the stainless steel container 1 with respect to the above distance L between the LT crystals 3 The ratio of the distance L1 to the space, that is, (L1 / L) can be arbitrarily adjusted, whereby the reduction conditions at the outer peripheral edge of the LT crystal 3 can be appropriately adjusted.

In the present invention, the inner diameter size of the stainless steel container 1 and the embedding conditions of the LT crystal 3 are set so that the ratio of the distance L1 to the distance L, that is, (L1 / L) exceeds 1.5 and satisfies the range of 3.5 or less. It is necessary to make appropriate adjustments.

Here, in the reduction of the LT crystal 3, the partial pressure of oxygen around the LT crystal 3 is greatly reduced by the reaction between the trace amount of oxygen contained in the inert gas around the LT crystal 3 and the Al powder in the mixed powder 2. It is believed that this happens. Then, at the time of heat treatment (reduction treatment), the outer peripheral edge portion of the LT crystal 3 is added to the Al powder in the mixed powder 2 interposed between the LT crystals 3, and as described above, the outer peripheral edge portion of the LT crystal 3 and the container 1 Since the Al powder in the mixed powder 2 interposed between the inner walls of the above is also contributed, the decrease in oxygen partial pressure becomes larger than that in the central part of the LT crystal 3, and as a result, the reduction of the outer peripheral part further progresses to the outside. It is possible to manufacture an LT substrate in which the volume resistivity of the peripheral portion and the peripheral region thereof is lower than that of the central portion (that is, the conductivity is high).

When manufacturing a surface acoustic wave element (SAW filter), as described above, a portion (region) about 5 mm inside from the outer peripheral edge of the LT substrate is not used as an element material, so that the area is about 5 mm inside from the outer peripheral edge. Even if the transmittance of the device is set lower than the others (since the volume resistivity is lower than that of other parts, the transmittance from the outer peripheral edge to the inside by about 5 mm is low), the element quality is not affected.

Further, since the portion where the LT substrate is likely to crack due to pyroelectricity is the outer peripheral portion of the LT substrate (the outer peripheral portion of the LT substrate and its vicinity region) as described above, the outer peripheral edge portion and the peripheral portion thereof. By lowering the volume resistivity in the vicinity region (that is, increasing the conductivity), it is possible to suppress cracking of the LT substrate due to pyroelectricity.

However, when the ratio of the distance L1 to the distance L, that is, (L1 / L) is 1.5 or less, the volume resistivity of the outer peripheral portion and the vicinity region thereof is lower (high conductivity) than that of the central portion. It becomes difficult to manufacture the substrate. On the other hand, when the ratio (L1 / L) exceeds 3.5, the contribution of the Al powder in the mixed powder 2 interposed between the outer peripheral edge of the LT crystal 3 and the inner wall of the container 1 becomes stronger, and as a result, the outside Since the volume resistivity of the portion beyond the inner region of about 5 mm from the peripheral edge portion also becomes low, there is a problem that the area that can be used as the element material becomes small. Therefore, the ratio of the distance L1 to the distance L, that is, (L1 / L) needs to satisfy the range of more than 1.5 and 3.5 or less, and more preferably (L1 / L) is 2 or more and 3 or less. Is desirable.

Further, the outer peripheral edge portion of the LT substrate and the region in the vicinity thereof according to the present invention are adjusted to have a lower volume resistivity (higher conductivity) than the central portion, and accordingly, with the outer peripheral edge portion of the LT substrate. The transmittance in the vicinity region at a wavelength of 600 nm is also lower than that in the central region by 5% or more. However, since the outer peripheral edge portion of the LT substrate and the region in the vicinity thereof are not used as the device material as described above, the device quality is not affected.

[Distance L (mm) between the substrate-shaped LT crystals 3]
Next, regarding the distance L (mm) between the substrate-shaped LT crystals 3 embedded in the mixed powder 2 of the Al powder and the Al ₂ O _{3 powder, the uniformity of the mixed powder filled between the LT crystals} From the viewpoint of productivity, 0.5 mm to 10 mm is preferable. When the distance L between the LT crystals exceeds 10 mm, the number of LT crystals embedded in the mixed powder is reduced, which is inconvenient in terms of productivity, and the distance L between the LT crystals is less than 0.5 mm. In this case, when the LT crystals are embedded in the mixed powder, the mixed powder cannot be uniformly evened, which causes a disadvantage that the conditions vary. Therefore, the distance L (mm) between the LT crystals embedded in the mixed powder is preferably 0.5 mm to 10 mm, preferably 1.5 mm to 5 mm, and more preferably 2 to 3 mm.

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 to the following examples.

[Structure of heating furnace]
The heating furnaces used in Examples 1 and 2 and Comparative Examples are provided with an air supply port and an exhaust port. Further, the stainless steel container arranged 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. (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. The flow rate of the argon gas supplied and discharged into the heating furnace is set to 2 L / min.

[Growth of LT crystals and processing of ingots, etc.]
Using a raw material having a congluent composition, an LT single crystal having a diameter of 4 inches was grown by the Czochralski method. The growing atmosphere is a nitrogen-oxygen mixed gas having an oxygen concentration of about 3%. The obtained LT crystal ingot was transparent pale yellow. The method for growing the LT crystal used in the method for producing the LT substrate according to the present embodiment is not limited to the above-mentioned Czochralski method, and other known growing methods, for example, a pulling method may be used.

The LT crystal ingot is heat-treated to remove thermal strain and polled to achieve single polarization, and then peripheral grinding, slicing, and polishing are performed to obtain a 42 ° RY (Rotated Y axis) substrate. It was made into an LT crystal 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 had a Curie temperature of 603 ° C.

[Example 1]
It is processed into a substrate state in a mixed powder of 10% by weight aluminum powder (Al powder) and 90% by weight aluminum oxide powder (Al ₂ O _{3 powder) filled in a stainless steel cylindrical container with an inner diameter of 112 mmφ.} Twenty-five pieces of LT crystals having a diameter of 100 mmφ and a thickness of 280 μm are embedded at intervals of 3 mm, and a stainless steel cylindrical container in which the LT crystals are embedded is placed in the heating furnace and then marketed through an air supply port. The above-mentioned argon gas (oxygen partial pressure is ^{about 1 × 10 -6} atm) was supplied into the heating furnace.

From the above conditions, the distance L (mm) between the LT crystals is 3.0 mm, the distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is (112 mm-100 mm) / 2 = 6.0 mm, and the ratio (L1 / L). Is 2.0 (see Tables 1 and 2).

Then, the 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 (reduction treatment) was performed at 580 ° C. for 20 hours.

For a total of 25 heat-treated LT crystals, the volume resistivity and transmittance of the central portion and the outer peripheral portion of the treated LT substrate were measured, respectively.

Further, each LT substrate whose volume resistivity and transmittance have been measured is polished on one side to make the thickness of each LT substrate 200 μm, and then the cracking rate of the LT substrate when rapidly heated to 270 ° C. is determined. Examined.

The volume resistivity was measured by a three-terminal method based on JIS K-6911, and the transmittance was measured by a transmittance measuring device.

The volume resistivity and transmittance are the average values of the measured 25 LT substrates, and the cracking rate due to heating is [(the number of cracked LT substrates / 25 LT substrates) x 100 (%)]. It was calculated as. Further, the heating test was an accelerated test for determining cracking of the LT substrate due to pyroelectricity.

The measurement results are shown in Tables 1 and 2.

[Example 2]
The LT crystal was heat-treated (reduced) under the same conditions as in Example 1 except that the inner diameter of the stainless steel cylindrical container was 118 mmφ.

The distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is (118 mm-100 mm) / 2 = 9.0 mm, and the ratio (L1 / L) is 3.0 (see Tables 1 and 2).

The volume resistivity, transmittance, and heating test results are shown in Tables 1 and 2.

[Comparison example]
The LT crystal was heat-treated (reduced) under the same conditions as in Example 1 except that the inner diameter of the stainless steel cylindrical container was 106 mmφ.

The distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is (106 mm-100 mm) / 2 = 3.0 mm, and the ratio (L1 / L) is 1.0 (see Tables 1 and 2).

The volume resistivity, transmittance, and heating test results are shown in Tables 1 and 2.

[Verification]
(1) Example 1
(1-1) In Example 1 in which the distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is set to 6.0 mm, the ratio (L1 / L) = 6.0 / 3.0 = 2. It becomes 0 and satisfies the condition of "more than 1.5 and 3.5 or less" of the present invention.

(1-2) Therefore, the volume resistivity of the central part of the LT substrate used as the element material ^{is adjusted to 2.5 × 10 10} (Ω · cm), and the transmittance at a wavelength of 600 nm is adjusted to 48%, while the element material. The volume resistivity of the outer peripheral portion of the LT substrate not used is 1.8 × 10 ¹⁰ (Ω · cm), and the transmittance at a wavelength of 600 nm is 39%.

(1-3) That is, the volume resistivity of the outer peripheral portion of the LT substrate is set to a low level (1.8 × 10 ¹⁰ Ω · cm) in which cracking of the substrate due to charcoalism is suppressed, and the device is set. The transmittance of the central portion used as a material is also adjusted to a level that does not affect the device quality (the transmittance at a wavelength of 600 nm is 48%) in the exposure process of the device manufacturing process.

(1-4) As a result, the cracking rate during heating is as low as 2% (see Table 2), the decrease in yield in the device manufacturing process due to charcoalism is suppressed, and the device is used in the exposure process of the device manufacturing process. It is confirmed that the LT substrate does not affect the quality.

(2) Example 2
(2-1) Even in Example 2 in which the distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is set to 9.0 mm, the ratio (L1 / L) = 9.0 / 3.0 = 3. It becomes 0 and satisfies the condition of "more than 1.5 and 3.5 or less" of the present invention.

(2-2) Therefore, the volume resistivity of the central part of the LT substrate used as the element material ^{is adjusted to 2.4 × 10 10} (Ω · cm), and the transmittance at a wavelength of 600 nm is adjusted to 47%, while the element material. The volume resistivity of the outer peripheral portion of the LT substrate not used is 0.8 × 10 ¹⁰ (Ω · cm), and the transmittance at a wavelength of 600 nm is 13%.

(2-3) That is, the volume resistivity of the outer peripheral portion of the LT substrate is set to a low level (0.8 × 10 ¹⁰ Ω · cm) in which cracking of the substrate due to charcoalism is suppressed, and the element is set. The transmittance of the central portion used as a material is also adjusted to a level (transmittance at a wavelength of 600 nm is 47%) that does not affect the device quality in the exposure process of the device manufacturing process.

(2-4) As a result, the cracking rate during heating is as low as 1% (see Table 2), the decrease in yield in the device manufacturing process due to pyroelectricity is suppressed, and the device is used in the exposure process of the device manufacturing process. It is confirmed that the LT substrate does not affect the quality.

(3) Comparative Example (3-1) In the comparative example in which the distance L1 (mm) between the outer peripheral edge of the LT crystal and the inner wall of the container is set to 3.0 mm, the ratio (L1 / L) = 3.0 / 3. 0 = 1.0, which does not satisfy the condition of "more than 1.5 and 3.5 or less" of the present invention.

(3-2) Therefore, the volume resistivity of the central part of the LT substrate used as the element material ^{is adjusted to 2.3 × 10 10} (Ω · cm), and the transmittance at a wavelength of 600 nm is adjusted to 47%, while the element material. The volume resistivity of the outer peripheral portion of the LT substrate that is not used is 2.5 × 10 ¹⁰ (Ω · cm), and the transmittance at a wavelength of 600 nm is 48%.

(3-3) That is, the volume resistivity of the outer peripheral portion of the LT substrate ^{is not set to a low level (2.5 × 10 10} Ω · cm) in which cracking of the substrate due to pyroelectricity is suppressed.

(3-4) As a result, it is confirmed that the cracking rate during heating is as high as 5% (see Table 2), and the decrease in yield in the device manufacturing process due to pyroelectricity cannot be suppressed.

According to the present invention, a decrease in yield in the element manufacturing process due to charcoalism is suppressed, and a lithium tantalate substrate that does not affect the element quality in the exposure process of the element manufacturing process can be produced. It has industrial potential for use as a substrate material for filters.

1 Stainless steel container 2 Mixed powder 3 Substrate-shaped LT crystal 4 Large container

Claims (5)

A method for producing a lithium tantalate substrate using lithium tantalate crystals, which is a lithium tantalate crystal processed into a substrate state in a mixed powder of aluminum powder and aluminum oxide powder filled in a container (hereinafter referred to as “lithium tantalate crystal”). , "Lithium tantalate crystal in the shape of a substrate" to distinguish it from the "lithium tantalate substrate" after heat treatment), and after arranging the container in a heating furnace under atmospheric pressure atmosphere, the inside of the heating furnace In a method for producing a lithium tantalate substrate, heat treatment is performed at a temperature lower than the Curie temperature of lithium tantalate crystals while continuously supplying and discharging an inert gas.
The distance between a plurality of substrate-shaped lithium tantalate crystals embedded in the mixed powder is L (mm), and the distance between the outer peripheral edge of the substrate-shaped lithium tantalate crystal and the inner wall of the container is L1 (mm). A method for producing a lithium tantalate substrate, wherein L1 / L satisfies a range of more than 1.5 and 3.5 or less. The method for producing a lithium tantalate substrate according to claim 1, wherein L1 / L satisfies the range of 2 to 3. The production of a lithium tantalate substrate according to claim 1 or 2, wherein a distance L between a plurality of substrate-shaped lithium tantalate substrates embedded in the mixed powder satisfies a range of 0.5 mm to 10 mm. Method. Any of claims 1 to 3, wherein the inert gas is argon gas, and the flow rate of the argon gas continuously supplied and discharged into the heating furnace satisfies the range of 0.5 to 5 L / min. The method for manufacturing a lithium tantalate substrate according to the above. Tantalate used as a device material has a transmittance in the range of 40% to 60% at a wavelength of 600 nm in the central portion, and a transmittance at a wavelength of 600 nm in the outer peripheral portion is 5% or more lower than the above-mentioned transmittance in the central portion. Lithium acid acid substrate.

Images