JP6598378B2 - Method for producing lithium tantalate single crystal substrate - Google Patents

Description

The present invention relates to a method for producing a lithium tantalate single crystal substrate pyroelectric is suppressed.

  In recent years, a communication system such as a mobile phone supports a plurality of communication standards, and each communication standard is composed of a plurality of frequency bands. In such a communication system, for example, a surface acoustic wave (SAW) device in which a comb electrode for exciting a surface acoustic wave is formed on a piezoelectric substrate is used as a component for frequency adjustment or selection. It has been.

This surface acoustic wave device is required to have a small size, a small insertion loss, and a performance that does not pass unnecessary waves. Lithium tantalate (LiTaO ₃ ; LT) and lithium niobate (LiNbO ₃ ; LN) are used as piezoelectric substrate materials. It is done. On the other hand, further improvement in the performance of surface acoustic wave devices has been demanded, and improvement in the performance of these piezoelectric substrates has been studied.

  Non-Patent Document 1 shows that LT with a stoichiometric composition produced by a double crucible method has an electromechanical coupling coefficient that is 20% higher than that of an LT with a normal congruent melting (congruent) composition. .

  Patent Document 1 shows that in an acoustic wave device including an IDT electrode containing copper, the breakdown mode can be suppressed by using the stoichiometric composition LT as a piezoelectric body.

  Further, Patent Document 2 discloses a method of adjusting the lithium oxide concentration in the LT substrate by a vapor phase method to obtain a stoichiometric composition.

JP2011-135245A US Pat. No. 6,652,644 Patent No. 4220997 JP2015-98410A

Science and Technology Promotion Coordination Results Report Promotion of Industry-Academia-Government Collaboration Joint Research Ex-post Evaluation “Practical Development of Optomedia Crystals that Support IT” pages 25-27 [online] [searched November 1, 2016], Internet <URL : http: //scfdb.tokyo.jst.jp/pdf/20021220/2004/200212202004rr.pdf> Yan Tao et al. “Formation mechanism of black LiTaO3 single crystals through chemical reduction.” J. Appl. Cryst. 44 (2011), 158-162.

  By the way, in recent years, pyroelectricity suppression processing has been performed on LT substrates used in surface acoustic wave devices in order to prevent electrode breakdown due to electrostatic spark. This is to reduce pyroelectricity by reducing the LT substrate and reducing the volume resistivity. In addition to the method described in Patent Document 3, several such reduction treatment techniques have been studied.

  For example, Non-Patent Document 2 discloses a reduction treatment method for an LT substrate in which heat treatment is performed with a mixed powder of iron and lithium carbonate in a nitrogen gas atmosphere. It was found that color unevenness was generated on the LT substrate surface and the in-plane direction uniformity was poor.

  Further, in the reduction treatment method of Non-Patent Document 2, it is considered that lithium carbonate decomposes at a temperature of 948K or lower to generate carbon monoxide, and this carbon monoxide reduces Ta5 + to Ta4 +. On the other hand, iron acts as a catalyst, and it is thought that carbon dioxide generated by the decomposition of lithium carbonate is further decomposed into carbon monoxide, so the color unevenness of the substrate surface by this reduction treatment method is unevenly distributed with iron acting as a catalyst, This is thought to be due to a difference in the degree of reduction.

  In order to avoid such substrate color unevenness and in-plane inhomogeneity, it is conceivable to use a single reducing agent. However, when the reduction treatment is performed only with lithium carbonate, the reduction is sufficient. There is a problem of not progressing.

  Further, Patent Document 4 describes that the LT substrate subjected to the Li diffusion treatment is subjected to heat treatment in a reducing atmosphere in a state where the LT substrate and the reduced ceramic are brought into contact with each other. Yes.

  However, in this method, there is a case where the reduction process is not uniformly performed by one process, and two processes are necessary to surely perform the reduction process, so that there is a problem that the work process increases. In addition, since it is necessary to prepare a reducing substance such as reduced ceramics, the work process becomes complicated and the cost increases.

  Therefore, as a result of further investigation on this reduction treatment method, it was found that even when the treatment was performed twice, the uniformity of the reduction treatment was not always sufficient, and there was room for improvement. As a cause of this, in order to perform the reduction treatment uniformly by this method, it is necessary to bring the reducing substance into uniform contact with the LT substrate. In the LT substrate subjected to the Li diffusion treatment, the lattice difference is caused by the difference in the composition in the substrate. Since the constant and the coefficient of thermal expansion differ, it is considered that the substrate is distorted and it is difficult to bring the reducing substance into uniform contact.

  If the uniformity of the reduction treatment is not sufficient, unevenness occurs in the volume resistivity and color of the LT substrate, and such unevenness may induce discharge due to temperature changes and exposure unevenness in the photolithography process in device manufacturing. There is.

  As a result of further investigation, the present inventors have found that the LT substrate subjected to the Li diffusion treatment tends to have a higher volume resistivity than the LT substrate having the congruent composition. In the reduction treatment method, it has been found that the desired volume resistivity may not be reached even if treatment is performed a plurality of times.

Accordingly, an object of the present invention is to develop a useful reduction treatment method for an LT substrate subjected to Li diffusion treatment, a lithium tantalate single crystal whose volume resistivity and color unevenness are small and whose substrate surface has a pseudo stoichiometric composition It is to provide a method for manufacturing a substrate.

That is, in the production method of the present invention, Li diffusion treatment is performed on a lithium tantalate single crystal substrate having a congruent composition so that the Li concentration on the surface of at least one substrate is higher than the Li concentration in the congruent composition. In the diffusion treatment step and the Li diffusion treatment step, a Li diffusion layer having a pseudo stoichiometric composition is formed from at least one substrate surface to a specific depth, and the thickness of the Li diffusion layer is 200 μm or more and less than 500 μm. Lithium carbonate powder having a maximum particle size of 180 μm or more and 500 μm or less and impurities of 1000 ppm or less of a lithium tantalate single crystal substrate that is processed so that the ratio to the thickness of the entire substrate is 15% or less embedded in, the concentration of reducing gas than 0.2 vol% 30 vol% or less of the reducing gas Under囲気, 350 ° C. or higher, heat treatment is carried out at normal pressure of not higher than the Curie temperature temperatures, the volume resistivity in the thickness direction of the substrate is 1.0 × ^{10 11} Ω · cm or more, 2.0 × ^{10 13} Ω · and a pyroelectric suppression treatment step that is performed so that the ratio between the maximum value and the minimum value of the volume resistivity in the substrate is 4.0 or less .

  In the Li diffusion treatment step of the present invention, it is preferable that at least one substrate surface has a pseudo stoichiometric composition and is processed so that the substrate has a lower Li concentration range than the substrate surface.

  In the pyroelectric suppression treatment process of the present invention, the lightness L * of at least one substrate surface is 30 or more and 85 or less, and the ratio of the maximum value and the minimum value of the lightness L * in the substrate is 1.5 or less. It is preferable to process so that it may become.

  The production method of the present invention may include a step of subjecting the lithium tantalate single crystal substrate subjected to the Li diffusion treatment to a single polarization treatment, and the reducing gas contains hydrogen or carbon monoxide. be able to.

According to the method for producing a lithium tantalate single crystal substrate of the present invention, the diffusion treatment can be uniformly performed even on the LT substrate subjected to the Li diffusion treatment. In addition, the lithium tantalate single crystal substrate obtained by the production method of the present invention is superior in various characteristics and has a uniform suppression of pyroelectricity as compared with a normal congruent composition LT. This is useful as a piezoelectric substrate.

1 is a schematic view showing a lithium tantalate single crystal substrate having a Li diffusion layer having a pseudo stoichiometric composition from a substrate surface to a specific depth according to an embodiment of the present invention. It is the schematic diagram which showed the measurement location of the volume resistivity and the lightness L * in an Example. It is the figure which showed the profile in the depth direction of Li density | concentration about the lithium tantalate single crystal substrate to which the Li diffusion treatment of Example 1 was performed.

  Hereinafter, although one embodiment of the present invention is described in detail based on a drawing, the present invention is not limited to this at all.

In the method for producing a lithium tantalate single crystal substrate of the present invention, a lithium tantalate single crystal substrate having a congruent composition is subjected to Li diffusion treatment, and the Li concentration on at least one substrate surface is higher than the Li concentration in the congruent composition. A Li diffusion treatment step is performed.
The congruent composition of the LT single crystal is approximately Li / (Li + Ta) = 0.485, and here, a composition of Li / (Li + Ta) = 0.480 to 0.490 is referred to.

  The method for performing the Li diffusion treatment is not particularly limited, and Li can be diffused by bringing the Li diffusion source into contact with the piezoelectric layer. At this time, the state of the Li diffusion source may be solid, liquid, or gas.

As a specific method for the Li diffusion treatment, a composition containing Li, Ta, and O, which are constituent elements of the LT substrate, is used. For example, Li can be diffused by embedding an LT substrate in a powder mainly containing Li ₃ TaO ₄ and heating. Li can also be diffused by impregnating the LT substrate into a melt obtained by mixing LiNO ₃ , NaNO ₃ , and KNO ₃ in an equimolar ratio.

  This Li diffusion treatment is performed so that the Li concentration on at least one substrate surface is higher than the Li concentration in the congruent composition. The Li diffusion treatment may be performed from only one side of the substrate, but may be performed from both sides, and the Li concentration on both substrate surfaces may be higher than the Li concentration in the congruent composition. However, it should be noted that if only the Li concentration on one side of the substrate is increased, the substrate may be warped.

  In this Li diffusion treatment step, it is preferable that at least one substrate surface has a pseudo stoichiometric composition. This is because if the substrate surface has a pseudo stoichiometric composition, various characteristics such as an electromechanical coupling coefficient and temperature characteristics can be improved.

  Moreover, it is preferable to process so that it may have the range where Li density | concentration is lower than a pseudo stoichiometric composition inside a board | substrate. Further, when the Li diffusion layer having a pseudo stoichiometric composition is formed from the substrate surface to a specific depth, the ratio of the thickness of the Li diffusion layer to the thickness of the entire substrate (the thickness of the Li diffusion layer / the entire substrate) The thickness is preferably 15% or less, and more preferably 13% or less. If it does in this way, Li diffusion processing can be performed in a short time, and the curvature and crack of a board | substrate can be suppressed. If the ratio of the thickness of the Li diffusion layer to the thickness of the entire substrate exceeds 15%, the distortion of the substrate may increase and break.

  Here, the pseudo stoichiometric composition refers to a composition of Li / (Li + Ta) = 0.498 to 0.502. The thickness of the Li diffusion layer is the thickness of the layer in which the Li concentration is in the above range, and refers to the thickness of only one side even when the Li diffusion layer is formed on both sides of the substrate. Further, the ratio of the thickness of the Li diffusion layer to the thickness of the entire substrate is, for example, T2 / T1 in FIG.

  Note that the Li concentration of the LT substrate may be measured by a known method, but can be evaluated by, for example, Raman spectroscopy. Regarding the LT substrate, it is known that there is an approximately linear relationship between the half-value width of the Raman shift peak and the Li concentration (the value of Li / (Li + Ta)). Therefore, the composition at an arbitrary position of the LT substrate can be evaluated by using an expression representing such a relationship.

  The relational expression between the half-width of the Raman shift peak and the Li concentration is obtained by measuring the Raman half-width of several samples having known compositions and different Li concentrations, but the Raman measurement conditions are the same. If so, a relational expression that has already been clarified in literature may be used. For example, for the LT single crystal, the following formula can be used (see 2012 IEEE International Ultrasonics Symposium Proceedings, Page (s) 1252-1255).

  Li / (Li + Ta) = (53.15-0.5FWHM1) / 100

Here, “FWHM1” is the full width at half maximum of the Raman shift peak near 600 cm ⁻¹ , and refer to the literature for details of the measurement conditions.

  Furthermore, in the method for producing a lithium tantalate single crystal substrate of the present invention, a lithium tantalate single crystal substrate subjected to Li diffusion treatment is embedded in lithium carbonate powder, and is heated to 350 ° C. or higher in a reducing gas atmosphere. It includes a pyroelectric suppression treatment process in which heat treatment is performed at a temperature lower than the temperature. This pyroelectric suppression treatment step is preferably performed under normal pressure that does not require a special device or the like.

  Here, it is preferable to use a lithium carbonate powder having a maximum particle size of 500 μm or less, preferably 300 μm or less. Lithium carbonate powder having a maximum particle size of 500 μm or less is obtained by passing a commercially available lithium carbonate powder through a sieve of 32 mesh (aperture 500 μm), and lithium carbonate powder having a maximum particle size of 300 μm or less is 48 mesh (mesh It is obtained by passing through a sieve having an opening of 300 μm. By removing the bulk lithium carbonate powder through such a sieve, it is possible to suppress unevenness in volume resistivity and color unevenness due to the degree of reduction.

  Further, the sieve opening may be further reduced, but when it becomes about 80 mesh (opening 180 μm), the lithium carbonate powder is easily clogged and the workability is deteriorated, so the maximum particle size of the lithium carbonate powder is 180 μm or more. It is preferable to do.

  Since the lithium carbonate powder can be used repeatedly, it is excellent in cost. However, since the lithium carbonate powder after use may be in a lump shape, it is preferably reused after sieving again and removing the lump lithium carbonate powder.

  In order to suppress unevenness due to the degree of reduction, it is preferable that the lithium carbonate powder does not contain any substance other than lithium carbonate, but impurities may be contained in a small amount of 1000 ppm or less.

  The LT substrate embedded in the lithium carbonate powder is heat-treated at a temperature of 350 ° C. or higher and a Curie temperature or lower in a reducing gas atmosphere. If the heat treatment is performed at a temperature lower than 350 ° C., the reduction does not proceed sufficiently. On the other hand, if the heat treatment is performed at a temperature higher than the Curie temperature, a multi-domain structure is formed, which is not preferable.

  The Curie temperature of the LT single crystal varies depending on the composition, and the Curie temperature of the LT single crystal having a congruent composition of Li / (Li + Ta) = 0.485 is about 604 ° C., and the pseudo stoichiometric composition of Li / (Li + Ta) = 0.500. The LT single crystal has a Curie temperature of about 695 ° C. Therefore, it is preferable to perform the heat treatment at a temperature of 600 ° C. or lower here.

  The reducing gas may be arbitrarily selected from hydrogen, carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen monoxide and the like, but it is preferable to use hydrogen or carbon monoxide from the viewpoint of ease of handling. By setting it as such a reducing gas atmosphere, it becomes possible to fully advance reduction | restoration only with lithium carbonate powder, without using catalysts, such as iron.

  Here, an inert gas may be introduced to form a mixed gas atmosphere of a reducing gas and an inert gas. As the inert gas, a rare gas such as nitrogen, argon, or helium can be used. Among these, it is preferable to use relatively inexpensive nitrogen.

  If the mixed gas atmosphere of the reducing gas and the inert gas is used, the volume resistivity can be adjusted by controlling the reduction degree of the LT substrate by controlling the concentration of the reducing gas. Also, when an explosive gas such as hydrogen is used as the reducing gas, the danger of explosion can be suppressed by diluting with an inert gas.

  The concentration of the reducing gas is preferably 0.2 vol% or more and 30 vol% or less, and more preferably 0.4 vol% or more and 20 vol% or less. The concentration of the reducing gas may be appropriately controlled according to the target volume resistivity. However, the lower the concentration of the reducing gas, the more the unevenness due to the degree of reduction tends to be suppressed. Therefore, from the viewpoint of unevenness suppression, the concentration of the reducing gas is preferably 10 vol% or less, more preferably 5 vol% or less, and even more preferably 3 vol% or less.

  If the concentration of the reducing gas is too high, unevenness may become large, or reduction may progress too much and the substrate may become brittle. On the other hand, if the concentration of the reducing gas is too low, the reduction does not proceed sufficiently and the pyroelectricity may not be suppressed.

  When the reduction does not proceed sufficiently, it is necessary to perform a plurality of reduction treatments in order to suppress pyroelectricity. If the reduction treatment is required a plurality of times, the number of manufacturing steps is increased and the cost is increased, and the total heat treatment time is also increased, so that the risk of warping and cracking of the substrate is increased.

In the pyroelectric suppression treatment step, the volume resistivity in the thickness direction of the substrate is 1.0 × 10 ¹¹ Ω · cm or more and 2.0 × 10 ¹³ Ω · cm or less, and the volume resistivity in the substrate is The ratio between the maximum value and the minimum value (maximum value / minimum value) is preferably 4.0 or less.

When the volume resistivity is less than 1.0 × 10 ¹¹ Ω · cm, an electrical loss is likely to occur when a SAW device is manufactured, which is not preferable. Moreover, when the volume resistivity exceeds 2.0 × 10 ¹³ Ω · cm, pyroelectricity may not be sufficiently suppressed. A more preferable range of the volume resistivity is 1.0 × 10 ¹² Ω · cm or more and 1.0 × 10 ¹³ Ω · cm or less.

  The ratio of the maximum value and the minimum value of volume resistivity is preferably small, more preferably 2.5 or less, and further preferably 2.0 or less. If the ratio between the maximum value and the minimum value of the volume resistivity exceeds 4.0, there may be a problem in the manufacturing process of the SAW device or the characteristics of the final product may not be stable.

In the pyroelectric suppression treatment step, the lightness L * of at least one substrate surface is 30 or more and 85 or less, and the ratio between the maximum value and the minimum value of the lightness L * in the substrate (maximum value / minimum value) is It is preferable to set it to 1.5 or less.
Here, the lightness L * refers to the CIE 1976 lightness index defined in JIS Z 8781-4: 2013.

  If the lightness L * is less than 30, color unevenness may occur on the substrate. On the other hand, if the lightness L * exceeds 85, pyroelectricity may not be sufficiently suppressed. In the photolithography process when manufacturing the SAW device, it may be preferable that the lightness L * is small. A more preferable range of the lightness L * is 35 or more and 80 or less.

  The ratio of the maximum value and the minimum value of the lightness L * is preferably small, and more preferably 1.2 or less. If the ratio between the maximum value and the minimum value of the volume resistivity exceeds 1.5, there may be a problem in the manufacturing process of the SAW device or the characteristics of the final product may not be stable.

  The method for producing a lithium tantalate single crystal substrate of the present invention may include a step of subjecting the lithium tantalate single crystal substrate subjected to the Li diffusion treatment to a single polarization treatment. The single polarization process may be performed by a known method. In general, the LT substrate is heated to a temperature equal to or higher than the Curie temperature and lowered to a temperature equal to or lower than the Curie temperature while an electric field is applied. However, the method is not limited to this method.

  The single polarization treatment step is preferably performed between the Li diffusion treatment step and the pyroelectric suppression treatment step, and may be performed simultaneously with the Li diffusion treatment step. That is, in the Li diffusion treatment step, the temperature can be increased by heating and then decreasing the temperature with an electric field applied.

  In the present invention, the thickness of the entire LT substrate is preferably 200 μm or more and less than 500 μm. If the thickness of the entire substrate is less than 200 μm, the warpage of the substrate due to distortion becomes large. In addition, since a substrate having a thickness of 500 μm or more is rarely used in the manufacture of SAW devices, a substrate having a thickness greater than this may cause trouble in the transport system of the apparatus, resulting in a cost reduction. Will also be high.

According to the method for producing a lithium tantalate single crystal substrate of the present invention, at least one of the substrate surfaces has a pseudo stoichiometric composition, the substrate has a lower Li concentration range than the substrate surface, and the volume in the thickness direction of the substrate. The resistivity is 1 × 10 ¹¹ Ω · cm or more and 2 × 10 ¹³ Ω · cm or less, and the ratio of the maximum value and the minimum value of the volume resistivity in the substrate is less than 4.0. It is possible to manufacture a lithium tantalate single crystal substrate in which the ratio of the maximum value and the minimum value of the lightness L * in the substrate is 1.5 or less. Further, this LT substrate is useful as a piezoelectric substrate used for a surface acoustic wave device because it has various characteristics and LT is uniformly suppressed as compared with an LT having a normal congruent composition.

<Example 1>
In Example 1, first, a 4-inch (10 cm) diameter lithium tantalate single crystal having a congruent composition (Li / (Li + Ta) = 0.485) was produced by a pulling method. Next, this LT single crystal was sliced and lapped on both sides to prepare a 42 ° rotated Y-cut LT substrate having a thickness of 350 μm.

On the other hand, as a Li diffusion source used in the Li diffusion treatment step, a powder mixed at a molar ratio of Li ₂ CO ₃ : Ta ₂ O ₅ = 7: 3 was baked at 1350 ° C. for 10 hours to obtain Li ₃ TaO _4. The powder which has as a main component was prepared.

  Next, an LT substrate having a congruent composition and a powder serving as a Li diffusion source were placed in a platinum container, and this container was heated at 1000 ° C. for 50 hours to perform a Li diffusion treatment.

With respect to the LT substrate subjected to this Li diffusion treatment, using a laser Raman spectroscopic measurement apparatus (LabRam HR series manufactured by HORIBA Scientific, He-Ne ion laser, spot size 1 μm, room temperature), 600 cm ^{− serving as} an index of Li amount ^{The full} width at half maximum (FWHM1) of the Raman shift peak near ¹ was measured, and the Li concentration was calculated from the measured full width at half maximum using Equation 1 above.

  The result is shown in FIG. The LT substrate surface had a pseudo stoichiometric composition in the range of Li / (Li + Ta) = 0.498 to 0.502. Further, a Li diffusion layer having a pseudo stoichiometric composition was formed from the substrate surface to a depth of 30 μm.

  Subsequently, an electrode was attached to a surface corresponding to the Z-axis direction of the LT substrate via a paste mainly composed of LT polycrystal. Thereafter, the temperature was raised to 750 ° C., which is equal to or higher than the Curie temperature, and the temperature was lowered in a state where a voltage of 400 V was applied, thereby performing a single polarization treatment.

The volume resistivity of the LT substrate subjected to single polarization treatment was measured for 1 minute at a voltage of 500 V using a Hiresta UP MCP-HT450 manufactured by Mitsubishi Chemical Corporation. ¹⁴ Ω · cm or more. Further, when this LT substrate was heated to 100 ° C. with a hot plate and the surface potential at that time was measured using SK-030 manufactured by Keyence Corporation, the result showed a value of 10 kV or more. Sex was confirmed.

  Next, lithium carbonate powder (Honjo Chemical Co., Ltd.) used in the pyroelectric suppression treatment process was prepared. This lithium carbonate powder is prepared by passing through a sieve of 48 mesh (aperture 300 μm) so that the maximum particle size of the lithium carbonate powder is 300 μm or less.

  Subsequently, an LT substrate that has been subjected to a Li diffusion treatment is embedded in the lithium carbonate powder, and under a normal pressure, nitrogen gas is supplied at 6 L / min and hydrogen gas is supplied at 100 cc / min to form a reducing gas atmosphere at 570 ° C. For 8 hours. At this time, the concentration of the reducing gas (hydrogen gas) is 1.7 vol%.

The volume resistivity of the LT substrate thus obtained was measured for a total of five points, ie, the center and the outer periphery of the substrate shown in FIG. 2, and a maximum value of 4.1 × 10 ¹² Ω · cm and a minimum value of 2. 3 × 10 ¹² Ω · cm, and the ratio of the maximum value to the minimum value at this time was 1.8, indicating a relatively small value.

  Further, the brightness L * value of the substrate surface was measured using a NF555 spectral color difference meter manufactured by Nippon Denshoku Industries Co., Ltd. for a total of 5 points including the center and the outer periphery 4 points shown in FIG. The minimum value is 65, and the ratio of the maximum value to the minimum value at this time is 1.0, indicating a relatively small value.

  And when this LT board | substrate was heated to 100 degreeC and the surface potential was measured, the value is 0-0.1 kV, and it was confirmed that pyroelectricity is fully suppressed.

<Examples 2 to 7 and Reference Examples 1 and 2>
Here, in the pyroelectric suppression treatment process of Example 1, LT substrates were prepared and evaluated by variously changing the reducing gas species and the reducing gas concentration. Table 1 shows the evaluation results for each of these substrates.
Here, as for the presence or absence of pyroelectricity, the one having a surface potential of 1 kV or higher when heated to 100 ° C. is regarded as having pyroelectricity.

  The LT substrates of Examples 1 to 7 had no pyroelectricity, no warpage, and good appearance. However, the LT substrate of Reference Example 1 had a reducing gas concentration as low as 0.1 vol%, so that the reduction did not proceed sufficiently, resulting in pyroelectricity. Further, the LT substrate of Reference Example 2 had a non-uniform color because the reducing gas concentration was too high at 33.3 vol%.

<Examples 8 to 12 and Reference Examples 3 to 5>
Here, the LT substrate of Example 1 was fabricated and evaluated by changing the thickness of the LT substrate and the Li diffusion treatment time in various ways. Table 2 shows the evaluation results of each of these substrates.
Here again, regarding the presence or absence of pyroelectricity, the one having a surface potential of 1 kV or higher when heated to 100 ° C. was regarded as having pyroelectricity.

  The LT substrates of Examples 8 to 12 had no pyroelectricity, no warpage, and good appearance. However, since the LT substrate of Reference Example 3 has a thickness of 180 μm, which is thinner than 200 μm or less, the warpage of the substrate due to distortion is as large as 250 μm, and there is a concern that troubles may occur in suction conveyance in the device manufacturing process. Is done. Reference Example 4 has no pyroelectricity and good appearance, but since the entire substrate is as thick as 500 μm, trouble may occur in the transport system of the apparatus and the cost increases. There is an inconvenience. Furthermore, since the ratio of the thickness of the Li diffusion layer to the thickness of the entire substrate was large, the LT substrate of Reference Example 5 was greatly distorted and cracked after the Li diffusion treatment step.

<Comparative example 1>
In Comparative Example 1, the Li diffusion treatment step and the single polarization treatment were performed in the same manner as in Example 1. Next, the LT substrate subjected to the Li diffusion treatment and the LT substrate having the congruent composition subjected to the reduction treatment were brought into contact with each other, and heat treatment was performed at 570 ° C. for 10 hours in a hydrogen gas atmosphere. Thereafter, a heat treatment was performed in the atmosphere at 40 ° C. for 5 hours, and the LT substrate having a congruent composition subjected to the reduction treatment was again brought into contact therewith, and a heat treatment was performed at 570 ° C. for 10 hours in a hydrogen gas atmosphere.

The LT substrate thus obtained was evaluated in the same manner as in Example 1. As a result, the maximum value of the volume resistivity was 1.7 × 10 ¹³ Ω · cm, and the minimum value was 1.8 × 10 ¹² Ω. -It was cm, and the ratio of the maximum value and the minimum value at this time was 9.4, indicating a relatively large value. Further, the maximum value of the lightness L * value was 81 and the minimum value was 58, and the ratio of the maximum value to the minimum value at this time was 1.4, indicating a relatively large value.

  Next, when this LT substrate was heated to 100 ° C. and the surface potential was measured, the value was 0 to 0.1 kV, and it was confirmed that pyroelectricity was suppressed.

<Comparative example 2>
In Comparative Example 2, the Li diffusion treatment step and the single polarization treatment were performed in the same manner as in Example 1. Next, a mixed powder of lithium carbonate powder (manufactured by Honjo Chemical Co., Ltd.) and iron used in the pyroelectric suppression treatment step was prepared. This mixed powder is prepared by passing through a sieve of 48 mesh (aperture 300 μm) so that the maximum particle size of the lithium carbonate powder is 300 μm or less, and the mass ratio of lithium carbonate and iron is Fe: Li ₂ CO _3. = 5: 100.

  Subsequently, the LT substrate subjected to the Li diffusion treatment was embedded in the mixed powder, and a heat treatment was performed at 570 ° C. for 8 hours under a normal pressure and a nitrogen gas atmosphere.

The LT substrate thus obtained was evaluated in the same manner as in Example 1. As a result, the maximum value of volume resistivity was 1.1 × 10 ¹³ Ω · cm, and the minimum value was 7.4 × 10 ¹¹ Ω. -It was cm, and the ratio of the maximum value to the minimum value at this time was 14.9, indicating a relatively large value. Further, the maximum value of the lightness L * value was 80, and the minimum value was 35. The ratio between the maximum value and the minimum value at this time was 2.3, indicating a relatively large value.

  Next, when this LT substrate was heated to 100 ° C. and the surface potential was measured, the value was 0 to 0.1 kV, and it was confirmed that pyroelectricity was suppressed.

<Comparative Example 3>
In Comparative Example 3, in the pyroelectric suppression treatment process of Example 1, an LT substrate was prepared and evaluated under a nitrogen gas atmosphere without using a reducing gas.

The LT substrate thus obtained was evaluated in the same manner as in Example 1. As a result, both the maximum value and the minimum value of the volume resistivity showed 1.0 × 10 ¹⁴ Ω · cm or more. Further, the maximum value of the lightness L * value was 87 and the minimum value was 86, and the ratio of the maximum value to the minimum value at this time was 1.0.

  Next, when this LT substrate was heated to 100 ° C. and the surface potential was measured, the value was 6 to 7 kV, and pyroelectricity was confirmed.

<Example 13>
In Example 13, the LT substrate manufactured in Example 1 was subjected to sputtering treatment to form an Al film having a thickness of 0.2 μm. Subsequently, a photoresist was applied to the substrate, and a resonator electrode pattern was exposed and developed using a stepper. Thereafter, heat treatment was performed at 120 ° C., and RIE (Reactive Ion Etching) was further performed to form an electrode of the SAW device. One wavelength of the patterned electrode was 2.50 μm. After RIE, when each pattern of the formed electrode was observed with an optical microscope, no problem was found in the electrode pattern of 95% or more.

  For comparison, the electrode of the SAW device was similarly formed on the LT substrate manufactured in Comparative Example 1, and no problem was found with the electrode pattern of about 60%, but the electrode pattern of about 40% The electrode pattern was partially cut or the line width varied. This is considered to be due to unevenness in exposure due to color unevenness of the LT substrate, or a discharge phenomenon partially resulting from the pyroelectric effect during the heat treatment.

  Next, the LT substrate of Example 1 on which the electrode of the SAW device was formed was placed on the stage, the temperature of the stage was changed from about 20 ° C. to 80 ° C., and the temperature coefficient of the anti-resonance frequency and the resonance frequency was confirmed. Since the temperature coefficient of the resonance frequency was −25 ppm / ° C. and the temperature coefficient of the anti-resonance frequency was −35 ppm / ° C., it was confirmed that the average frequency temperature coefficient was −30 ppm / ° C.

  For comparison, the temperature coefficient of the 42 ° rotated Y-cut lithium tantalate single crystal substrate that was subjected to pyroelectric suppression treatment without performing Li diffusion treatment was also confirmed. The temperature coefficient of resonance frequency was −33 ppm. Since the temperature coefficient of the anti-resonance frequency was −43 ppm / ° C., the average frequency temperature coefficient was −38 ppm / ° C., and the LT substrate of Example 1 was superior in temperature characteristics. Was confirmed.

Further, when the electromechanical coupling coefficient k ² is calculated from the anti-resonance frequency and the resonance frequency value based on the following equation 1, the electromechanical coupling coefficient k ² of the LT substrate of Example 1 is 7.7. %, A value about 1.2 times that of the LT substrate subjected to the pyroelectricity suppression treatment without performing the Li diffusion treatment.

DESCRIPTION OF SYMBOLS 1 Lithium tantalate single crystal substrate 2 Li diffusion layer T1 of pseudo stoichiometric composition Thickness of whole substrate T2 Thickness of Li diffusion layer of pseudo stoichiometric composition

Claims (5)

Li diffusion treatment step for performing Li diffusion treatment on a lithium tantalate single crystal substrate having a congruent composition so that the Li concentration on at least one substrate surface is higher than the Li concentration in the congruent composition, and the Li diffusion treatment In the process, a Li diffusion layer having a pseudo stoichiometric composition is formed from at least one substrate surface to a specific depth, and the ratio of the thickness of the Li diffusion layer to the total thickness of the substrate of 200 μm or more and less than 500 μm is 15 The lithium tantalate single crystal substrate subjected to Li diffusion treatment is embedded in lithium carbonate powder having a maximum particle size of 180 μm or more and 500 μm or less and impurities of 1000 ppm or less . in concentration of 0.2 vol% or more 30 vol% or less of the reducing gas atmosphere, 350 ° C. or higher Subjected to heat treatment at atmospheric pressure the temperature of not higher than the Curie temperature, the volume resistivity in the thickness direction of the substrate is 1.0 × ^{10 11} Ω · cm or more, or less 2.0 × ^{10 13} Ω · cm, and the substrate And a pyroelectric suppression treatment step applied so that the ratio of the maximum value and the minimum value of the volume resistivity is 4.0 or less .   2. The process according to claim 1, wherein in the Li diffusion treatment step, at least one substrate surface is processed so as to have a pseudo stoichiometric composition and a Li concentration in the substrate is lower than that of the substrate surface. A method for producing a lithium tantalate single crystal substrate. In the pyroelectric suppression treatment step, the lightness L * of at least one substrate surface is 30 or more and 85 or less, and the ratio between the maximum value and the minimum value of the lightness L * in the substrate is 1.5 or less. The method for producing a lithium tantalate single crystal substrate according to claim 1 or 2 , wherein the treatment is carried out in the following manner. The method for producing a lithium tantalate single crystal substrate according to any one of claims 1 to 3 , further comprising a step of subjecting the lithium tantalate single crystal substrate subjected to Li diffusion treatment to a single polarization treatment. The method for producing a lithium tantalate single crystal substrate according to any one of claims 1 to 4 , wherein the reducing gas contains hydrogen or carbon monoxide.

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