Method for manufacturing lithium tantalate single crystal substrate

Abstract

This invention provides a method for manufacturing a lithium tantalate single-crystal substrate, which can suppress the increase in volume resistivity caused by insufficient reduction of the lithium tantalate single-crystal substrate even when lithium carbonate powder is repeatedly used in the heat treatment of the lithium tantalate single-crystal substrate. The volume resistivity of this invention is 1×10⁻⁶. 10 Ω・cm or more and less than 1×10 12 A method for manufacturing a lithium tantalate single crystal substrate with a volume resistivity of 1×10 Ω·cm, comprising: ... 12 A lithium tantalate single-crystal substrate with an Ω·cm or higher and a single-crystal domain structure has a BET specific surface area of ​​0.13 m². 2 In the process of heat treatment of lithium carbonate powder of g or more at a temperature of 350°C or higher and Curie temperature under normal pressure, the lithium carbonate powder is lithium carbonate powder used to fill the lithium tantalate single crystal substrate during the heat treatment of the lithium tantalate single crystal substrate at a temperature of 350°C or higher and Curie temperature. At the beginning of the heat treatment, the heat treatment is carried out in a mixed gas atmosphere of inactive gas and reducing gas. After the heat treatment is carried out in the mixed gas atmosphere, the heat treatment is carried out in a single gas atmosphere of inactive gas.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a lithium tantalate single-crystal substrate for surface elastic wave elements. Background Technology

[0002] Lithium tantalate (LiTaO3; LT) single crystals possess piezoelectric properties and are used as piezoelectric substrates for elastic surface wave elements. Furthermore, lithium tantalate single crystals also exhibit pyroelectric properties, generating surface charges due to temperature changes. This pyroelectricity is sometimes utilized in the form of sensors, but it can become problematic when using lithium tantalate crystals as piezoelectric substrates for elastic surface wave elements.

[0003] For example, when the piezoelectric substrate becomes charged due to temperature changes, electrostatic discharge can occur within the substrate, potentially causing cracks or breakage. Furthermore, the electrodes formed on the surface of the piezoelectric substrate can short-circuit due to static electricity.

[0004] Therefore, in order to suppress the charging of lithium tantalate substrates, a method of reducing lithium tantalate substrates at temperatures below the Curie temperature has been considered and widely implemented (for example, see Patent Document 1). Patent Document 1 describes a method for manufacturing lithium tantalate single crystal substrates with high homogeneity and volume resistivity by embedding lithium tantalate single crystal substrates into lithium carbonate powder and performing a reduction treatment.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent document 1: Japanese Patent Application Publication No. 2017-165611. Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] Since lithium carbonate powder is not reduced by the reduction process of the lithium tantalate substrate, it can be considered that lithium carbonate powder can be reused in the reduction process of the lithium tantalate substrate. However, if lithium carbonate powder that has been used in the reduction process of the lithium tantalate substrate is reused, there is a risk that the lithium tantalate substrate cannot be fully reduced, and the volume resistivity of the lithium tantalate substrate will be greater than 1×10⁻⁶. 12 The case of Ω・cm. In addition, the volume resistivity of the lithium tantalate substrate deviates.

[0010] Therefore, the object of the present invention is to provide a method for manufacturing a lithium tantalate single crystal substrate, which can suppress the increase in volume resistivity caused by insufficient reduction of the lithium tantalate single crystal substrate even when lithium carbonate powder is repeatedly used in the heat treatment of the lithium tantalate single crystal substrate.

[0011] Methods for solving problems

[0012] To achieve the above-mentioned objective, the inventors conducted in-depth research and discovered that if lithium carbonate powder is used in the reduction treatment of lithium tantalate substrates, the BET specific surface area of ​​the lithium carbonate powder decreases. Therefore, if lithium carbonate powder is used again to reduce the lithium tantalate substrate, the volume resistivity of the lithium tantalate substrate increases, thus completing the following invention. The invention is as follows.

[0013] [1] A method for manufacturing a lithium tantalate single-crystal substrate, wherein the substrate has a volume resistivity of 1×10⁻⁶. 10 Ω・cm or more and less than 1×10 12 A method for manufacturing a lithium tantalate single crystal substrate with a volume resistivity of 1×10 Ω·cm, comprising: ... 12 A lithium tantalate single-crystal substrate with an Ω·cm or higher and a single-crystal domain structure has a BET specific surface area of ​​0.13 m². 2 The process involves heat treatment of lithium carbonate powder at a temperature of 350°C or higher and below the Curie temperature under normal pressure. The lithium carbonate powder is lithium carbonate powder used for burying lithium tantalate single crystal substrates during heat treatment of lithium tantalate single crystal substrates at a temperature of 350°C or higher and below the Curie temperature under normal pressure. In the heat treatment process, the heat treatment is performed in a mixed gas atmosphere of inactive gas and reducing gas at the beginning of the heat treatment, and after the heat treatment in the mixed gas atmosphere, the heat treatment is performed in a single gas atmosphere of inactive gas.

[0014] [2] According to the manufacturing method of lithium tantalate single crystal substrate described in [1] above, wherein the horizontal axis obtained by measuring the BET specific surface area of ​​the aforementioned lithium carbonate powder is the relative pressure (adsorption equilibrium pressure (P) / saturated vapor pressure (P0)) and the vertical axis is the adsorption amount (cm). 3 In the adsorption isotherm of ( / g), the slope of the aforementioned adsorption isotherm is 0.04 or higher in the range of relative pressure (P / P0) of 0.2 or higher and 0.8 or lower.

[0015] [3] According to the method for manufacturing a lithium tantalate single crystal substrate described in [1] or [2] above, wherein the aforementioned lithium carbonate powder is obtained by exposing the aforementioned used lithium carbonate powder to an atmospheric atmosphere, and the product of the absolute humidity of the aforementioned atmospheric atmosphere and the time during which the aforementioned used lithium carbonate powder is exposed to the aforementioned atmospheric atmosphere is 200 (g / m²). 3 )・hr or more.

[0016] [4] The method for manufacturing a lithium tantalate single crystal substrate according to any one of [1] to [3] above, wherein the aforementioned lithium carbonate powder is obtained by exposing the aforementioned used lithium carbonate powder to an atmospheric atmosphere, and the product of the absolute humidity of the aforementioned atmospheric atmosphere and the time during which the aforementioned used lithium carbonate powder is exposed to the aforementioned atmospheric atmosphere is 300 (g / m²). 3 )・hr or more.

[0017] [5] The method for manufacturing a lithium tantalate single crystal substrate according to any one of [1] to [4] above, wherein the aforementioned lithium carbonate powder is obtained by exposing the aforementioned used lithium carbonate powder to an atmospheric atmosphere, and the absolute humidity of the aforementioned atmospheric atmosphere is 40 g / m³. 3 The product of the absolute humidity of the aforementioned atmospheric atmosphere and the exposure time of the previously used lithium carbonate powder in the aforementioned atmospheric atmosphere is 300 (g / m²). 3 )・hr or more.

[0018] The effects of the invention

[0019] According to the present invention, a method for manufacturing a lithium tantalate single crystal substrate is provided, which can suppress the increase in volume resistivity caused by insufficient reduction of the lithium tantalate single crystal substrate even when lithium carbonate powder is repeatedly used in the heat treatment of the lithium tantalate single crystal substrate. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the adsorption isotherm of lithium carbonate powder used in the fabrication of the lithium tantalate single crystal substrate in the examples and comparative examples.

[0021] Figure 2 This is a schematic diagram showing the relationship between the absolute humidity of the atmospheric atmosphere in which lithium carbonate powder was exposed and the product of the exposure time of the lithium carbonate powder in the atmospheric atmosphere, and the BET specific surface area of ​​the lithium carbonate powder obtained from exposure to the atmospheric atmosphere.

[0022] Figure 3 This is a schematic diagram showing the relationship between the product of the absolute humidity of the atmospheric atmosphere in which lithium carbonate powder was exposed and the exposure time of the lithium carbonate powder in the atmosphere, and the slope of the adsorption isotherm of the lithium carbonate powder obtained after exposure to the atmosphere.

[0023] Figure 4 This is a schematic diagram showing the relationship between the L value and the volume resistivity of a lithium tantalate single-crystal substrate. Detailed Implementation

[0024] [Manufacturing method of lithium tantalate single crystal substrate]

[0025] The following is a detailed description of one embodiment of the present invention, but the present invention is not limited thereto.

[0026] One embodiment of the present invention describes a method for manufacturing a lithium tantalate single-crystal substrate with a volume resistivity of 1×10⁻⁶. 10 Ω・cm or more and less than 1×10 12 A method for developing a lithium tantalate single-crystal substrate with a volume resistivity of 1×10 Ω·cm includes: 12 A lithium tantalate single-crystal substrate with an Ω·cm or higher and a single-crystal domain structure has a BET specific surface area of ​​0.13 m². 2 The process involves heat-treating lithium carbonate powder at a temperature of 350°C or higher and below the Curie temperature under normal pressure. The lithium carbonate powder in question is used lithium carbonate powder previously used to fill lithium tantalate single-crystal substrates during heat treatment at a temperature of 350°C or higher and below the Curie temperature under normal pressure. Furthermore, in the heat treatment process described above, the initial heat treatment is performed in a mixed gas atmosphere of an inactive gas and a reducing gas, followed by heat treatment in a single gas atmosphere of an inactive gas after the initial heat treatment in the mixed gas atmosphere.

[0027] In one embodiment of the manufacturing method of a lithium tantalate single-crystal substrate of the present invention, firstly, a substrate with a volume resistivity of 1×10⁻⁶ is prepared. 12 A lithium tantalate single-crystal substrate with an Ω·cm or higher and a single-crystal domain structure. Such a lithium tantalate single-crystal substrate is obtained by, for example, growing lithium tantalate single crystals using the Jochraski method, subjecting the resulting crystal block to polarization treatment, and processing it into a substrate shape.

[0028] Next, the prepared lithium tantalate single-crystal substrate is embedded in lithium carbonate powder. When the lithium carbonate powder is unused, it is preferably lithium carbonate powder with a maximum particle size of 500 μm or less, more preferably lithium carbonate powder with a maximum particle size of 300 μm or less. Lithium carbonate powder with a maximum particle size of 500 μm or less is obtained, for example, by sieving commercially available lithium carbonate powder through a 32-mesh sieve (500 μm mesh). Furthermore, lithium carbonate powder with a maximum particle size of 300 μm or less is obtained, for example, by sieving commercially available lithium carbonate powder through a 48-mesh sieve (300 μm mesh). By sieving with such a sieve to remove lumpy lithium carbonate powder, a lithium tantalate single-crystal substrate with high homogeneity in the in-plane direction and no color inhomogeneity on the substrate surface can be obtained. In addition, the mesh size of the sieve can be further reduced, but when it reaches about 80 mesh (180 μm), the lithium carbonate powder is prone to clogging and the workability deteriorates. Therefore, the maximum particle size of the lithium carbonate powder is preferably set to 180 μm or more.

[0029] On the other hand, when lithium carbonate powder is used to fill lithium tantalate single-crystal substrates during heat treatment at ambient pressure, temperatures above 350°C and below the Curie temperature, the BET specific surface area of ​​the lithium carbonate powder must be 0.13 m². 2 / g or more. If the BET specific surface area of ​​the used lithium carbonate powder is less than 0.13m². 2 When lithium tantalate single crystal substrates are embedded in used lithium carbonate powder and subjected to heat treatment, sometimes the volume resistivity of the lithium tantalate single crystal substrate reaches 1×10⁻⁶ g due to insufficient reduction of the lithium tantalate single crystal substrate. 12 Ω·cm or higher. From this perspective, the BET specific surface area of ​​the used lithium carbonate powder is preferably 0.14 m². 2 / g or more, more preferably 0.15m 2 / g or more. There is no particular upper limit to the range of BET specific surface area of ​​used lithium carbonate powder, for example, 0.20m². 2 / g or less.

[0030] The BET specific surface area of ​​the used lithium carbonate powder is less than 0.13 m². 2 At a density of / g, as long as the BET specific surface area of ​​lithium carbonate powder is 0.13m² through the prescribed treatment, 2 / g or higher is acceptable. For example, it is preferable to use materials with a BET specific surface area of ​​less than 0.13m² due to heat treatment for use in lithium tantalate single crystal substrates. 2 / g of used lithium carbonate powder is exposed to the atmospheric atmosphere. At this time, the product of the absolute humidity of the atmospheric atmosphere and the exposure time of the used lithium carbonate powder to the atmospheric atmosphere preferably reaches 200 (g / m²). 3 )・hr or higher. Therefore, it is possible to more reliably achieve a BET specific surface area of ​​0.13 m² for the used lithium carbonate powder. 2 / g or more. Furthermore, used lithium carbonate powder that cannot be used in the heat treatment of lithium tantalate single-crystal substrates can be easily converted into a material that can be used in the heat treatment of lithium tantalate single-crystal substrates. From this point of view, the product of the absolute humidity of the atmospheric atmosphere and the time the used lithium carbonate powder is exposed to the atmospheric atmosphere is more preferably 300 (g / m²). 3 The product of the absolute humidity of the atmospheric atmosphere and the exposure time of the used lithium carbonate powder in the atmospheric atmosphere is further preferably 400 (g / m³) or more. 3 • hr or higher. Furthermore, it is even more preferable that the absolute humidity of the atmospheric atmosphere is 40 g / m³. 3 The product of the absolute humidity of the atmosphere and the exposure time of the used lithium carbonate powder in the atmosphere is 300 (g / m²). 3 )・hr or higher, and even more preferably: the absolute humidity of the atmospheric atmosphere is 40 g / m3 The product of the absolute humidity of the atmosphere and the exposure time of the used lithium carbonate powder in the atmosphere is 400 (g / m²). 3 )・hr or more.

[0031] From the viewpoint of shortening the exposure time of used lithium carbonate powder to atmospheric atmosphere and stabilizing the volume resistivity of lithium tantalate single crystal substrate, the absolute humidity of the atmospheric atmosphere when the used lithium carbonate powder is exposed to atmospheric atmosphere is preferably 40 g / m³. 3 The above, more preferably 50g / m 3 The above is further preferred to be 60g / m 3 The above is further preferred to be 70g / m 3 above.

[0032] From the viewpoint of shortening the exposure time of used lithium carbonate powder in the atmosphere and stabilizing the volume resistivity of lithium tantalate single crystal substrate, the temperature of the atmosphere when the used lithium carbonate powder is exposed to the atmosphere is preferably 10~80°C, more preferably 20~75°C, even more preferably 30~70°C, and even more preferably 40~65°C.

[0033] The atmospheric pressure at which used lithium carbonate powder is exposed can be atmospheric pressure. Furthermore, pressurization can be applied to shorten the exposure time of used lithium carbonate powder in the atmosphere.

[0034] Next, for the lithium tantalate single-crystal substrate embedded in lithium carbonate powder, heat treatment is performed at a temperature above 350°C and below the Curie temperature in a mixed gas atmosphere of inactive and reducing gases under normal pressure. Here, if heat treatment is performed at a temperature below 350°C, reduction may sometimes be insufficient. On the other hand, if heat treatment is performed at a temperature above the Curie temperature, the lithium tantalate single-crystal substrate may sometimes exhibit a polycrystalline structure.

[0035] At the start of heat treatment, the heat treatment is carried out in a mixed atmosphere of inert and reducing gases. As the inert gas, rare gases such as nitrogen, argon, and helium can be used, with nitrogen being preferred due to its lower cost. Furthermore, as the reducing gas, any of the following can be selected: hydrogen (H2), carbon monoxide (CO), hydrogen sulfide (H2S), sulfur dioxide, and nitric oxide (N2O). From the perspective of ease of processing, hydrogen (H2) or carbon monoxide (CO) is preferred.

[0036] In one embodiment of the present invention, a method for manufacturing a lithium tantalate single-crystal substrate involves creating a mixed gas atmosphere of inactive gas and reducing gas, thereby enabling sufficient reduction of the lithium tantalate single-crystal substrate even with only lithium carbonate powder. If the same reduction process is performed in an inactive gas atmosphere lacking reducing gas, the reduction will not be sufficient, requiring multiple reduction processes. Multiple reduction processes not only increase manufacturing costs but also lengthen the total heat treatment time, thus increasing the likelihood of substrate warping and cracking, which is undesirable.

[0037] Furthermore, the concentration of the reducing gas is preferably 20.0% by volume or less, more preferably 10.0% by volume or less, and even more preferably 5.0% by volume or less. This is because if the concentration of the reducing gas is too high, the reduction may be over-induced. Over-reduced lithium tantalate single-crystal substrates may become brittle or excessively dark, potentially causing problems in the equipment manufacturing process. It should be noted that the lower limit of the reducing gas concentration range is not particularly limited, for example, it may be 0.5% by volume or more.

[0038] After heat treatment in a mixed atmosphere of inert and reducing gases, heat treatment is then performed in a single atmosphere of inert gas. This process helps to suppress deviations in the volume resistivity of the lithium tantalate single-crystal substrate.

[0039] During the heat treatment, a mixed gas atmosphere is preferred. Furthermore, by adjusting the duration of the mixed gas atmosphere after heating and the concentration of the reducing gas, the degree of reduction of the lithium tantalate single-crystal substrate can be controlled, and the volume resistivity of the lithium tantalate single-crystal substrate can be adjusted. The heat treatment time in the mixed gas atmosphere after heating is preferably 10 minutes to 8 hours, and the treatment time varies depending on the desired volume resistivity of the lithium tantalate single-crystal substrate. Furthermore, from the viewpoint of improving the in-plane uniformity of the lithium tantalate single-crystal substrate, a longer heat treatment time in a single-gas atmosphere of inactive gas is preferable. The heat treatment time in a single-gas atmosphere of inactive gas is preferably, for example, 1 to 24 hours, more preferably 6 to 22 hours, and even more preferably 7 to 20 hours.

[0040] When used lithium carbonate powder is used for the heat treatment of lithium tantalate single-crystal substrates, the horizontal axis (adsorption equilibrium pressure (P) / saturated vapor pressure (P0)) and the vertical axis (adsorption capacity (cm²)) are obtained by measuring the BET specific surface area of ​​the lithium carbonate powder. 3 In the adsorption isotherm of / g), within the range of relative pressure (P / P0) of 0.2 or higher and 0.8 or lower, the slope of the adsorption isotherm is preferably 0.04 or higher. By using the lithium carbonate powder that displays this adsorption isotherm for heat treatment of lithium tantalate single crystal substrates, the volume resistivity of the lithium tantalate single crystal substrates can be reliably maintained at 1×10⁻⁶. 10Ω・cm or more and less than 1×10 12 Within the range of Ω·cm. From this point of view, the slope of the above-mentioned adsorption isotherm is more preferably 0.05 or more, and even more preferably 0.06 or more. It should be noted that the upper limit of the slope range of the above-mentioned adsorption isotherm is not particularly limited, for example, it is 0.1 or less.

[0041] The BET specific surface area of ​​unused lithium carbonate powder is also preferably 0.13 m², similar to that of used lithium carbonate powder. 2 / g or more, preferably 0.14m 2 / g or more, further preferably 0.15m 2 / g or more. Furthermore, there is no particular upper limit to the range of BET specific surface area of ​​unused lithium carbonate powder, for example, 0.20m². 2 / g or less. By performing the above treatment on used lithium carbonate powder, the BET specific surface area of ​​the used lithium carbonate powder can be easily reduced to 0.13m². 2 / g or more, preferably 0.14m 2 / g or more, more preferably 0.15m 2 / g or more.

[0042] The slope of the adsorption isotherm of unused lithium carbonate powder is also preferably 0.04 or higher, more preferably 0.05 or higher, and even more preferably 0.06 or higher, similar to that of used lithium carbonate powder. Furthermore, the upper limit of the slope range of the adsorption isotherm of unused lithium carbonate powder is not particularly limited, for example, it is 0.1 or lower. By performing the above treatment after using such unused lithium carbonate powder, it is possible to easily make the slope of the adsorption isotherm of the used lithium carbonate powder preferably 0.04 or higher, more preferably 0.05 or higher, and even more preferably 0.06 or higher. Example

[0043] The following examples illustrate the present invention described above, but the present invention is not limited to the examples.

[0044] (Fabrication of lithium tantalate single crystal substrate)

[0045] First, lithium tantalate single crystals were grown using the Jochraski method. The resulting crystal blocks were then subjected to a state-transformation treatment to achieve single-crystal localization, and subsequently cut to obtain multiple substrates (raw material substrates). At this point, the volume resistivity of the lithium tantalate single crystal substrate was 3.0 × 10⁻⁶. 14 Ω・cm.

[0046] (Preparation of used lithium carbonate powder)

[0047] Lithium carbonate powder (manufactured by Honjo Chemical Co., Ltd.) was sieved through a 48-mesh sieve (300 μm mesh) to ensure that the maximum particle size of the lithium carbonate powder was below 300 μm.

[0048] Next, for the lithium tantalate single-crystal substrate with a single-crystal domain structure embedded in lithium carbonate powder, nitrogen gas was passed through at a rate of 6 L / min and hydrogen gas (hydrogen concentration of 2.0 vol%) was passed through at a rate of 120 cc / min under normal pressure, and the temperature was raised to 570°C for a further heat treatment of 1 hour. After that, the hydrogen gas was stopped, and the heat treatment was carried out at 570°C for 8 hours.

[0049] (Preparation of lithium carbonate powder 1~6)

[0050] Lithium carbonate powder used for heat treatment of lithium tantalate single crystal substrates was removed at room temperature and sieved through a 48-mesh sieve (300 μm mesh) to ensure a maximum particle size of 300 μm or less, thus producing lithium carbonate powder 1. Furthermore, using a constant temperature and humidity bath (manufactured by Especk Corporation, trade name: PL3J), lithium carbonate powder 1 was exposed to atmospheric atmosphere under the conditions shown in Table 1 to produce lithium carbonate powders 2 to 6.

[0051] (Preparation of heat-treated lithium tantalate single crystal substrates in Examples 1-4 and Comparative Examples 1 and 2)

[0052] Lithium tantalate single-crystal substrates of Examples 1-4 and Comparative Examples 1 and 2 were fabricated by heat treatment using lithium carbonate powders 1-6. It should be noted that the heat treatment conditions were the same as those described in the fabrication project using the lithium carbonate powders mentioned above.

[0053] The following evaluations were performed on lithium carbonate powders 1 to 6.

[0054] (Adsorption isotherm, BET specific surface area, and slope of the adsorption isotherm)

[0055] The adsorption isotherms, BET specific surface area, and slope of the adsorption isotherms for lithium carbonate powders 1-6 were determined using a constant volumetric gas adsorption method manufactured by Microtrac Bell (BELSORP-miniII). First, the samples were heated at 200°C for 2 hours under reduced pressure of 0.01 Pa. Using the weight of the resulting samples, the relative pressure was increased from 0.0015 at the start of the measurement in increments of 0.025 to 0.3, and measurements were taken between 0.3 and 0.9 in increments of 0.05. At each relative pressure, the point where the pressure variation over 5 minutes reached less than 1% of the target pressure was taken as the adsorption amount at that relative pressure. The relative pressure was gradually increased to 0.9, and the relationship between relative pressure (P / P0) and adsorption amount (adsorption isotherm) was determined. Furthermore, the BET specific surface area and the slope of the adsorption isotherm were determined from these adsorption isotherms. It should be noted that the slope of the adsorption isotherm is derived from the adsorption amount (cm³) at a relative pressure (P / P0) of 0.8. 3 / g) minus the adsorption amount (cm) when the relative pressure (P / P0) is 0.2. 3 The value obtained by dividing by 0.6 (=0.8-0.2) is used for calculation. The results are shown in Table 1 and... Figure 1 In addition, the absolute humidity of the atmosphere (g / m³) 3 (A) Multiplied by the time (hr) the lithium carbonate powder was exposed to atmospheric atmosphere (B) (g / m 3 The relationships between (A×B) and the BET specific surface area and the slope of the adsorption isotherm are shown in the figures. Figure 2 and Figure 3 .

[0056] The following evaluations were performed on the heat-treated lithium tantalate single-crystal substrates of Examples 1-4 and Comparative Examples 1 and 2.

[0057] (Volume resistivity)

[0058] The volume resistivity of a lithium tantalate single-crystal substrate was measured using a digital insulation resistance meter (Agilent Technologies, product name: Agilent 4339B High Reliance Spectrometer) and a measuring fixture (Agilent Technologies, product name: 16008B Reliance Spectrometer). First, the lithium tantalate single-crystal substrate was placed in the measuring fixture, and a voltage of 500V was applied for 1 minute to measure the volume resistivity. The volume resistivity of the lithium tantalate single-crystal substrate was then calculated from the obtained volume resistivity values. The results are shown in Table 2.

[0059] (L value)

[0060] The L-values ​​(L=0 for black, L=100 for white) of heat-treated lithium tantalate single-crystal substrates were measured using a spectrophotometer (NF555, manufactured by Nippon Denshoku Kogyo Co., Ltd.). The results are shown in Table 2. It should be noted that if the reduction of the lithium tantalate single-crystal substrate is insufficient, the substrate will appear white, thus increasing the L-value. Furthermore, the relationship between the L-value and volume resistivity of the lithium tantalate single-crystal substrate is shown in Table 2. Figure 4 .

[0061]

[0062] The BET specific surface area of ​​lithium carbonate powders 1 and 2 is smaller than that of lithium carbonate powders 3-6, even from the perspective of... Figure 1 The adsorption isotherms also indicate that nitrogen is difficult to adsorb. On the other hand, the BET specific surface area and adsorption capacity of lithium carbonate powder 3, lithium carbonate powder 5, and lithium carbonate powder 6 are approximately constant. Therefore, it can be concluded that the BET specific surface area of ​​lithium carbonate powder 4 is smaller compared to lithium carbonate powder 5 and lithium carbonate powder 6.

[0063] Depend on Figure 2 It can be seen that: if the product of absolute humidity and time of exposure to the atmosphere (exposure humidity) is 200 (g / m³), then... 3 If the exposure humidity is above 400 (g / m³), the BET specific surface area can be fully restored. Furthermore, if the exposure humidity value is 400 (g / m³), the BET specific surface area can be fully restored. 3 If the specific surface area of ​​BET is above hr, it can be restored to the saturated specific surface area, and as a result, the amount of gas adsorbed during the reduction treatment can also remain constant. It can be considered that the amount is less than 200 (g / m³), as with lithium carbonate powder 1 and lithium carbonate powder 2. 3 When the temperature reaches 1000 rpm, the BET surface area is not fully recovered, which is the cause of the deviation in reducing power.

[0064] Depend on Figure 2 and Figure 3 It can be seen that by adjusting the product of absolute humidity and exposure time in the atmosphere (exposure humidity), the BET specific surface area and the slope of the adsorption isotherm of lithium carbonate powder can be controlled. Furthermore, it can be seen that by setting the product of absolute humidity and exposure time in the atmosphere (exposure humidity) to 200 (g / m²), the desired effect can be achieved. 3 By increasing the exposure humidity to 400 (g / m³), the specific surface area of ​​BET and the slope of the adsorption isotherm can be significantly increased. Furthermore, it is known that by setting the exposure humidity to 400 (g / m³), the specific surface area of ​​BET can be significantly increased. 3 ) ・hr or more, thereby enabling the BET specific surface area and the slope of the adsorption isotherm to increase to around the value at which saturation occurs.

[0065]

[0066] The landfill has a specific surface area of ​​0.13 m². 2The volume resistivity of the lithium tantalate single-crystal substrates of Examples 1-4, which were heat-treated in used lithium carbonate powder at a temperature above 350°C and below the Curie temperature under normal pressure, has a volume resistivity of 1×10⁻⁶ g / g. 10 Ω・cm or more and less than 1×10 12 Ω・cm. Furthermore, although the volume resistivity of the lithium tantalate single-crystal substrate of Example 4 is lower than that of the lithium tantalate single-crystal substrates of Examples 1-3, the L value of the lithium tantalate single-crystal substrate of Example 4 is still higher than that of the lithium tantalate single-crystal substrates of Examples 1-3. This is likely because the reduction deviation of the lithium tantalate single-crystal substrate of Example 4 is greater than that of the lithium tantalate single-crystal substrates of Examples 1-3. Therefore, it is preferable to restore the BET specific surface area of ​​the used lithium carbonate powder to its saturated specific surface area. On the other hand, the filling should be done until the BET specific surface area is less than 0.13 m². 2 The volume resistivity of the lithium tantalate single-crystal substrates of Comparative Examples 1 and 2, which were subjected to heat treatment at ambient pressure, above 350°C and below the Curie temperature, in / g of used lithium carbonate powder, was greater than 1×10⁻⁶. 12 Ω・cm.

[0067] Depend on Figure 4 It can be seen that if the reduction of the lithium tantalate single crystal substrate is insufficient, the volume resistivity of the lithium tantalate single crystal substrate will increase.

Claims

1. A method for manufacturing a lithium tantalate single-crystal substrate, wherein the substrate has a volume resistivity of 1×10⁻⁶. 10 Ω·cm or higher and less than 1×10 12 A method for using a lithium tantalate single-crystal substrate with an Ω·cm density, comprising: With a volume resistivity of 1×10 12 A lithium tantalate single-crystal substrate with an Ω·cm or higher and a single-crystal domain structure has a BET specific surface area of ​​0.13 m². 2 The process involves heat treatment of lithium carbonate powder at a concentration of 1 g or higher, under normal pressure, at a temperature of 350°C or higher and below the Curie temperature. The aforementioned lithium carbonate powder is used lithium carbonate powder that was used to fill lithium tantalate single crystal substrates during heat treatment at ambient pressure, temperatures above 350°C and below the Curie temperature. The aforementioned lithium carbonate powder is obtained by exposing the previously used lithium carbonate powder to an atmospheric atmosphere. The product of the absolute humidity of the aforementioned atmospheric atmosphere and the exposure time of the previously used lithium carbonate powder in the aforementioned atmospheric atmosphere is 200 (g / m²). 3 ·hr or above, In the aforementioned heat treatment process, the heat treatment is initially performed in a mixed gas atmosphere of inactive gas and reducing gas, and after the heat treatment is performed in the mixed gas atmosphere, the heat treatment is then performed in a single gas atmosphere of inactive gas.

2. The method of producing a lithium tantalate single crystal substrate according to claim 1, wherein In the adsorption isotherms obtained by measuring the BET specific surface area of ​​the aforementioned lithium carbonate powder, with the horizontal axis representing relative pressure (adsorption equilibrium pressure P / saturated vapor pressure P0) and the vertical axis representing adsorption capacity, the slope of the aforementioned adsorption isotherms is 0.04 or higher within the range of relative pressure (P / P0) of 0.2 or higher and 0.8 or lower. The unit of adsorption capacity is cm³. 3 / g.

3. The method of producing a lithium tantalate single crystal substrate according to claim 1 or 2, wherein The aforementioned lithium carbonate powder is obtained by exposing the previously used lithium carbonate powder to an atmospheric atmosphere. The product of the absolute humidity of the aforementioned atmosphere and the time during which the aforementioned used lithium carbonate powder is exposed to the aforementioned atmosphere is 300 (g / m 3 ) · hr or more.

4. The method of producing a lithium tantalate single crystal substrate according to claim 1 or 2, wherein The aforementioned lithium carbonate powder is obtained by exposing the previously used lithium carbonate powder to an atmospheric atmosphere. The absolute humidity of the aforementioned atmosphere is 40 g / m 3 The above, The product of the absolute humidity of the aforementioned atmosphere and the time for which the aforementioned used lithium carbonate powder is exposed to the aforementioned atmosphere is 300 (g / m 3 ) · hr or more.

Images