JP2005317822A - Manufacturing method of singly polarized lithium tantalate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of effectively manufacturing a lithium tantalate crystal in a short time, the crystal having high conductivity and being singly polarized. <P>SOLUTION: The method is to manufacture a singly polarized lithium tantalate crystal, and performs at least a heat treatment of raising conductivity to a lithium tantalate crystal under a reducing atmosphere, and thereafter performs a singly polarizing treatment to the lithium tantalate crystal at a temperature above a Curie temperature under a reducing atmosphere or under a non-oxidizing atmosphere. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a unipolarized lithium tantalate crystal used for an application in which an electric signal is processed by forming a pattern with a metal electrode on a wafer such as a surface acoustic wave device.

  Lithium tantalate is used for applications that utilize electrical characteristics, such as SAW devices that perform signal processing using surface acoustic waves (SAW). In this application, lithium tantalate crystals are monopolarized. Is used. Lithium tantalate crystals suitable for this application show the piezoelectric response (piezoelectricity) required for SAW devices due to their crystal structure, but lithium tantalate crystals that are available by conventional methods are piezoelectric. In addition, pyroelectric response (pyroelectricity) occurs.

  The piezoelectricity of the lithium tantalate crystal is an indispensable characteristic when the lithium tantalate crystal is used as a SAW device. On the other hand, pyroelectricity is observed as a surface charge generated on the outer surface of the crystal by applying a temperature change to the lithium tantalate crystal, and charges the crystal. This surface charge is considered to cause a spark discharge between metal electrodes formed on a wafer made of lithium tantalate crystals when using the lithium tantalate crystals as a SAW device, causing a significant performance defect of the SAW device. ing. For this reason, in designing a SAW device using a lithium tantalate crystal, a device that does not generate surface charge, a device that releases the generated surface charge, or a device that widens the interval between metal electrodes is required. Therefore, there is a disadvantage that the design of the SAW device itself is restricted.

  In the manufacturing process of a SAW device using a lithium tantalate crystal, there is a process of applying heat to the lithium tantalate crystal in processes such as deposition of a metal film and removal of a resist. When given to lithium acid crystals, charge is generated on the outer surface due to the pyroelectric nature of the lithium tantalate crystals. As described above, this surface charge causes a spark discharge between the metal electrodes, which causes destruction of the electrode pattern. Therefore, the SAW device manufacturing process is devised so as not to change the temperature as much as possible. These measures have been devised, and this contrivance has a disadvantage that the throughput of the manufacturing process is reduced or the margin for guaranteeing the performance of the SAW device is narrowed.

  In unipolarized lithium tantalate crystals produced by conventional methods, the external surface charge generated by pyroelectricity is neutralized by free charge from the surrounding environment and disappears over time. Since the disappearance time is as long as several hours or more, it is not industrial to eliminate the generated surface charges by such natural neutralization in the SAW device manufacturing process.

  From the above background, for applications such as SAW devices, a piezoelectric crystal in which no charge generation or accumulation is observed on the outer surface of the crystal while maintaining the piezoelectricity required to exhibit device characteristics. Therefore, there is a need for unipolarized lithium tantalate crystals with no surface charge accumulation for such applications.

  As a method for producing a lithium tantalate crystal with reduced surface charge, the lithium tantalate crystal is exposed to a reducing atmosphere of 500 ° C. or higher to increase the conductivity, and the surface charge generated by pyroelectricity can be quickly neutralized or A method of extinction is disclosed (see Patent Document 1).

JP-A-11-92147

  However, when the lithium tantalate crystal is reduced by the above-described method, the unipolarization required for the SAW device application is required when the reducing atmosphere temperature is 610 ° C. or more, which is the Curie temperature of lithium tantalate. When the structure is lost and the temperature is 610 ° C. or lower, the surface resistance is still as high as 1.2E + 14Ω even when heated at 500 ° C. As is apparent, the rate of reduction treatment is extremely slow, and as a result, the electrical conductivity of industrially unipolarized lithium tantalate crystals cannot be improved.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for efficiently producing a single-polarized lithium tantalate crystal having high conductivity in a short time. It is in.

  In order to solve the above-mentioned problem, the present invention is a method for producing a unipolarized lithium tantalate crystal, and at least a lithium tantalate crystal is subjected to a heat treatment for increasing conductivity in a reducing atmosphere, A method for producing a single-polarized lithium tantalate crystal is then provided, wherein the single-polarization treatment is performed at a temperature equal to or higher than the Curie temperature in a reducing atmosphere or a non-oxidizing atmosphere (claim 1). ).

  In this way, the lithium tantalate crystal is subjected to a heat treatment for increasing the conductivity in a reducing atmosphere, and then subjected to a single polarization treatment (single component) at a temperature equal to or higher than the Curie temperature in a reducing atmosphere or a non-oxidizing atmosphere. If domain treatment is performed, a single-polarized lithium tantalate crystal having a high conductivity and a single domain structure can be effectively produced in a short time.

In this case, it is preferable that the heat treatment for increasing the conductivity and the single polarization treatment are performed continuously or separately (claim 2).
As described above, if the heat treatment for increasing the conductivity and the single polarization treatment are continuously performed, the process can be further simplified and the time can be shortened. Can be performed under more optimal conditions.

Moreover, it is preferable that the heat processing temperature which raises the said electrical conductivity shall be 750 degreeC or more and less than 1400 degreeC (Claim 3).
Thus, if the heat treatment temperature for increasing the conductivity is 750 ° C. or higher, the heat treatment can be carried out more quickly and effectively, and if it is less than 1400 ° C., the crystal structure of lithium tantalate is surely obtained. Thus maintaining the piezoelectricity.

Further, the single polarization treatment can be performed at a temperature lower than the heat treatment temperature for increasing the conductivity.
As described above, when the single polarization treatment is performed, the single polarization treatment can be performed sufficiently and effectively at a temperature lower than the heat treatment temperature for increasing the electrical conductivity if the temperature is equal to or higher than the Curie temperature.

Further, when performing the single polarization treatment, it is preferable that voltage application is started at a temperature equal to or higher than the Curie temperature with respect to the lithium tantalate crystal and voltage application is ended at a temperature equal to or lower than the Curie temperature. 5).
Thus, when performing single polarization treatment, it is more reliable and effective if voltage application is started at a temperature equal to or higher than the Curie temperature with respect to the lithium tantalate crystal and voltage application is terminated at a temperature equal to or lower than the Curie temperature. Thus, a single polarization process can be performed.

Further, it is preferable to use, as the lithium tantalate crystal, a wafer subjected to at least slice processing, a wafer subjected to lapping processing, or a wafer subjected to mirror processing on at least one side.
Since the method of the present invention can be applied regardless of the form of the lithium tantalate crystal, for example, a crystal before slicing may be used. Thus, at least a sliced wafer, or a lapped wafer, Alternatively, if a wafer having at least one side mirror-finished is used, a reduction heat treatment or a single polarization process can be quickly performed to increase the conductivity.

Further, the reducing atmosphere of hydrogen, deuterium, or carbon monoxide and NO X _(X <2.5), or be a mixed gas of two or more of these preferred (claim 7) .
If such a reducing gas or a mixed gas thereof is appropriately selected and used as a reducing atmosphere, heat treatment and unipolarization treatment in the reducing atmosphere can be easily and effectively performed.

At this time, one or more gases of rare gas, nitrogen, and carbon dioxide may be further added to the reducing atmosphere.
Thus, by adding these non-oxidizing gases to the reducing atmosphere, it is possible to control the treatment time and the effect by controlling the reaction rate of the treatment in the reducing atmosphere.

The non-oxidizing atmosphere is preferably a rare gas, nitrogen, carbon dioxide, or a mixed gas composed of two or more of these.
As described above, by appropriately selecting the non-oxidizing gas and using it as a non-oxidizing atmosphere, a single domain structure can be obtained without reducing the high conductivity obtained by the reduction heat treatment. In addition, a wide range of electrode and container materials necessary for the single polarization process can be selected.

  In accordance with the present invention, the lithium tantalate crystal is subjected to a heat treatment for increasing the conductivity in a reducing atmosphere, and then subjected to a single polarization treatment at a temperature equal to or higher than the Curie temperature in a reducing atmosphere or a non-oxidizing atmosphere. A single-polarized lithium tantalate crystal having high conductivity and a single domain structure can be produced effectively in a short time. The unipolarized lithium tantalate crystal thus obtained has improved crystal conductivity, so that no charge is accumulated on the crystal surface while maintaining the piezoelectricity. Therefore, it is a material that is extremely advantageous for the manufacture of SAW devices. Furthermore, the method of the present invention is industrially advantageous because the above-described lithium tantalate crystal can be obtained in a very short time.

Hereinafter, the present invention will be described in detail.
As described above, since the lithium tantalate crystal has both piezoelectricity and pyroelectricity, surface charge is generated when the temperature is changed in the manufacturing process of the SAW device using the crystal, and spark discharge is generated between the metal electrodes of the SAW device. Or the like may occur, leading to deterioration or damage of device characteristics. In order to avoid this, the design and manufacturing process of the SAW device may be restricted, which is a factor that limits the performance and productivity of the device.

  In addition, as described above, a method is disclosed in which lithium tantalate crystals are exposed to a reducing atmosphere at 500 ° C. or higher to increase the conductivity and to quickly neutralize or eliminate the surface charges generated by pyroelectricity. In the case of heat treatment at 610 ° C. or higher, which is the Curie temperature of lithium tantalate, the single domain structure required for SAW device application is lost, and heating at a temperature of 610 ° C. or lower, for example, 500 ° C. Even so, the speed of the reduction treatment is very slow, so the surface resistance remains as high as, for example, 1.2E + 14Ω, and as a result, it is found that the conductivity of industrially unipolarized lithium tantalate crystals cannot be improved. It was.

  Therefore, as a result of extensive research, the inventor first heat-treated the lithium tantalate crystal in a reducing atmosphere to increase the conductivity, and then perform a single polarization treatment to have a single domain structure, In addition, the inventors have found that lithium tantalate crystals having high conductivity can be effectively produced in a short time, and have completed the present invention.

Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto.
The lithium tantalate crystal used in the present invention can be obtained, for example, by the following method. First, lithium carbonate and tantalum pentoxide are weighed and mixed, and the lithium tantalate polycrystal obtained by heating to 1000 ° C. or higher in an electric furnace is put into a crucible made of noble metal such as iridium. The polycrystal is heated and melted under an atmospheric gas in which a slight amount of oxygen is mixed with nitrogen, and then the seed crystal is immersed in the melt and pulled up (so-called Czochralski method) to grow the crystal. By performing the above, for example, a lithium tantalate crystal in a multi-domain state (a state in which the crystal is composed of a large number of polarization domains polarized in an arbitrary direction) having a diameter of 4 inches (100 mm) is obtained.

  The thus obtained multi-domain lithium tantalate crystal having a diameter of 4 inches (100 mm) is sliced by using, for example, a wire saw, so that the slice processing has a diameter of 4 inches (100 mm) and a thickness of, for example, 0.5 mm. Then, a wafer with a diameter of 4 inches (100 mm) and a thickness of 0.4 mm is obtained by processing both surfaces of the wafer with a lapping machine.

  In the following description, a case where a wrap wafer obtained by lapping a lithium tantalate crystal is used as an example, but the present invention is not limited to this, and a slice wafer subjected to a slicing process may be used, or You may use the mirror surface processing wafer which further mirror-processed the single side | surface or both surfaces of the lapping wafer. However, it is preferable to perform heat treatment on at least the sliced wafer because a reduction reaction can be performed more quickly than heat treatment in a state before the grown single crystal is sliced.

  Next, the tantalum tantalate crystal lapped wafer is subjected to a heat treatment for increasing the conductivity in a reducing atmosphere (hereinafter sometimes referred to as a high conductivity heat treatment), and then in a reducing atmosphere or a non-oxidizing atmosphere. The single polarization treatment is performed at a temperature equal to or higher than the Curie temperature. First, the case where the high conductivity heat treatment and the single polarization treatment are successively performed will be described.

  In this case, for example, the wrap wafer obtained as described above is placed in an alumina container laid with lithium tantalate polycrystal (powder), and carbon electrodes are installed on both sides of the wrap wafer, and a single polarization treatment is performed. In order to make this effective, lithium tantalate polycrystals are filled in the gaps in the container. The alumina container containing the lithium tantalate crystal is placed in a stainless steel container, and the stainless steel container is placed in an electric furnace, and hydrogen gas as a reducing gas is introduced into the container at a rate of 1.5 liters per minute. The temperature of the furnace is raised from room temperature at a rate of about 6.7 ° C. per minute, for example, maintained at 1000 ° C. for 24 hours, and heat treatment for increasing conductivity is performed. Thereafter, the furnace is cooled to 700 ° C. at a rate of about 6.7 ° C. per minute, and held at 700 ° C. for 5 hours while applying a voltage of 200 volts between the electrodes. A one-domain structure is formed, and then the temperature is lowered at a rate of 6.7 ° C. per minute. When the temperature reaches 500 ° C., the voltage application is finished, and when the temperature falls to 30 ° C. or less, the alumina container is taken out from the stainless steel container, and the lap wafer is taken out from the alumina container.

  In this case, the temperature of the heat treatment for increasing the conductivity is not limited to 1000 ° C. as described above as long as the reduction treatment can be performed. However, the temperature is preferably set to a temperature equal to or higher than the Curie temperature. The effect of increasing it appears quickly, and if it is lower than 1400 ° C., the conductivity can be increased while the crystal structure of lithium tantalate is reliably maintained. In particular, when the temperature is set to 900 ° C. or higher, the heat treatment time is shortened and the high conductivity heat treatment can be performed more quickly. What is necessary is just to adjust suitably about heat processing time so that it may become desired electrical conductivity. The optimum heat treatment time may differ between crystal production batches, but if the heat treatment temperature is 1150 ° C or less, the heat treatment time is adjusted so as to reduce the variation of the final conductivity obtained between production batches. It is suitable for doing. About heat processing time, what is necessary is just to be 1 to 100 hours, for example, Preferably it is 1 to 50 hours.

Further, the reducing atmosphere is not limited to hydrogen as described above, and any of reducing gases such as deuterium, carbon monoxide and NO _x (X <2.5), or a mixed gas thereof may be used. Alternatively, a non-oxidizing gas such as a rare gas, nitrogen and carbon dioxide, or a mixed gas thereof may be added thereto. The composition of these atmospheric gases can be appropriately selected according to the characteristics of the heat-treated lithium tantalate, the heat treatment temperature, and the like. Moreover, the heat treatment time and the effect of heat treatment can be adjusted by adjusting these compositions.

  The atmosphere at the time of performing the single polarization treatment may be a reducing atmosphere using a reducing gas as described above, or any of rare gases such as helium, argon, neon, nitrogen, and carbon dioxide, Alternatively, a non-oxidizing atmosphere using such a mixed gas is preferable because the unipolarization process can be performed without reducing the conductivity of the lap wafer that has been increased by the high conductivity heat treatment. When the single polarization treatment is performed in a non-oxidizing atmosphere, the gas flowing in the container after the heat treatment for increasing conductivity in the reducing atmosphere and before the single polarization treatment is performed as described above. Switch to gas.

  Further, the temperature at which the single polarization treatment is performed is not limited to 700 ° C. as described above as long as it is 610 ° C. which is the Curie temperature of lithium tantalate. Even if the single polarization treatment is performed after the heat treatment and then the temperature is lowered to a lower temperature, a sufficient and effective single polarization treatment can be performed. The processing time at this time can be 1 to 10 hours, for example.

  Further, as described above, when performing a single polarization process, voltage application is started at a predetermined temperature equal to or higher than the Curie temperature for the lapping wafer, and the single polarization process is performed for a predetermined time while applying the voltage, If the voltage application is then terminated after the temperature is lowered to a temperature equal to or lower than the Curie temperature, the voltage application becomes effective, and the single domain structure can be reliably maintained even after the voltage application is completed.

Note that if the high conductivity heat treatment and the single polarization treatment are continuously performed as in the above embodiment, the process can be simplified and shortened, but the present invention is not limited to this, and these treatments are performed. It may be done individually.
In particular, when performing these treatments individually, when performing a single polarization treatment in a non-oxidizing atmosphere, not only the carbon electrode as described above but also platinum, gold, Metals such as silver, tantalum, tungsten, and molybdenum can also be used. Electrodes such as platinum cannot be used continuously due to embrittlement in a reducing atmosphere, but electrodes and containers of various materials including those described above can be used in a non-oxidizing atmosphere.

The conductivity of a high conductivity, single-polarized lithium tantalate crystal wafer manufactured in accordance with the present invention as described above can be measured as follows. That is, the conductivity is the reciprocal of the volume resistivity, but the volume resistivity is obtained by the following equation from the resistance value measured using a measuring instrument such as Hewlett Packard, 4329A High Resistance Meter, 16008A Resistivity Cell, etc. be able to.
ρ = (πd ² / 4t) · R
ρ: Volume resistivity (Ω · cm)
π: Pi ratio
d: Center electrode diameter (cm)
t: Lithium tantalate wafer thickness (cm)
R: Resistance value (Ω)
In this case, for example, a voltage of 500 volts is applied to the wafer, and in order to obtain a stable measurement value, the resistance value one minute after applying the voltage may be measured.

  The conductivity of the wafer obtained by the above measurement is sufficiently high, and the surface charge caused by pyroelectricity disappears quickly. Therefore, this single-polarized wafer does not accumulate charges on the wafer surface while maintaining piezoelectricity, and is a very advantageous material for SAW device manufacturing.

Examples of the present invention will be described in more detail below, but the present invention is not limited thereto.
(Examples 1-8)
Lithium carbonate and tantalum pentoxide were weighed and mixed, and the lithium tantalate polycrystal obtained by heating to 1000 ° C. or higher in an electric furnace was put into an iridium crucible. A nitrogen tantalate crystal having a diameter of 4 inches (100 mm) rotated by 40 ° in the y direction and a length of 50 mm was grown by the Czochralski method using nitrogen containing 1% oxygen as an atmospheric gas. Thereafter, the grown lithium tantalate crystal was sliced with a wire saw and then lapped to obtain a double-sided lapped wafer having a thickness of 0.4 mm.

  Next, as shown in FIG. 1, the double-sided wrap wafer 11 was placed in an alumina container 12 together with a carbon electrode 13. Specifically, first, a powdered lithium tantalate polycrystal 14 was spread on the bottom of a hollow alumina container 12 having an outer dimension of 150 mm and a thickness of 10 mm, and 10 double-sided lapping wafers 11 were stacked thereon. Next, two carbon electrodes 13 having an outer dimension of 120 mm × 120 mm and a thickness of 5 mm were placed so as to face each other with the double-sided wrap wafer 11 in between. Thereafter, lithium tantalate polycrystal 14 was filled in the gap. Then, the alumina container 12 with the double-sided wrap wafer 11 installed in this manner was placed in a stainless steel container (not shown) having an outer diameter of 260 mm and an inner diameter of 240 mm with a gas pipe, and the lid was capped to form a sealed space. Although not shown, the carbon electrode 13 is connected to an external power source for single polarization processing.

  Then, this stainless steel container is inserted into a tubular furnace, and 100% hydrogen gas (or a mixed gas of 50% hydrogen gas and 50% nitrogen gas) is supplied into the stainless steel container at 1.5 liters per minute. However, the temperature of the furnace was increased from room temperature at a rate of about 6.7 ° C. per minute. Then, after heating up to the predetermined temperature shown in Table 1 and holding for a predetermined time and heat-treating, the furnace was cooled to 700 ° C. at a rate of about 6.7 ° C. per minute, and a voltage of 200 volts was applied between the electrodes. Was started and maintained at 700 ° C. for 5 hours while applying a voltage, and then the temperature was lowered at a rate of 6.7 ° C. per minute. On the way, the voltage application was terminated when the temperature reached 500 ° C. When the temperature became 30 ° C. or lower, the stainless steel container, the alumina container, and the double-sided lapped wafer were taken out to obtain a double-sided lapped wafer of lithium tantalate crystal that had been subjected to heat treatment and single polarization treatment.

This wafer is further lapped on both sides with a lapping machine, and then a single-sided lithium tantalate product wafer is obtained by mirror-polishing one side with a polishing machine in order to obtain a standard single-sided mirror surface as a substrate for a SAW device. Obtained.
For this product wafer, the volume resistivity was calculated, and the conductivity was obtained from the reciprocal thereof. The volume resistivity was calculated as described above from the resistance values measured using Hewlett Packard's 4329A High Resistance Meter and 16008A Resistivity Cell. The resistance value was measured in a stable state 1 minute after applying a voltage of 500 volts.

(Examples 9 to 11)
Except for changing the stainless steel container to one made of alumina and raising the predetermined temperature, the double-sided lapped wafer of lithium tantalate crystal was heat-treated and subjected to a single polarization treatment as in Examples 1 to 8, and one side was mirror-finished A product wafer was prepared by polishing, and its conductivity was determined.

Tables 1 to 11 show the heat treatment temperature, heat treatment time, atmospheric gas composition, and conductivity values of product wafers finished to a single-sided mirror surface for Examples 1 to 11. In Table 1, a description such as “1.1E-11” of conductivity means “1.1 × 10 ⁻¹¹ ”. Moreover, the electrical conductivity of Table 1 is an average value of a total of 50 batches of 5 batches.

As shown in Table 1, in Examples 1 to 7, 9, and 10 according to the present invention, the product wafer has a black coloration, and the conductivity is 9.4E-13 to 7.8E-11Ω− ¹ · cm ⁻¹ . A high value was obtained. Moreover, all became a single domain structure. However, when the heat treatment temperature is 700 ° C. as in Example 8, the color is not seen in the treatment batch and the conductivity is not so high, that is, the conductivity is not so high compared to before the treatment. Therefore, the average value of the conductivity was low. Further, as in Example 11, when the temperature was 1400 ° C., although there was a black coloration, there was a wafer that had cracks. The cracked sample was collected and analyzed by X-ray diffraction. As a result, there was a portion where the crystal structure was other than lithium tantalate (LiTaO ₃ ), and the desired crystal structure required to have piezoelectricity Could not be maintained. That is, the heat treatment temperature is preferably 750 ° C. or more and less than 1400 ° C. in order to effectively perform the heat treatment for increasing the conductivity in a short time and to reliably maintain the crystal structure of lithium tantalate.

(Example 12, Comparative Example 1)
The high conductivity heat treatment and the single domain treatment are performed separately, the single polarization treatment is performed in a non-oxidizing atmosphere, and heat treatment is performed in the same manner as in Examples 1 to 8 except that platinum is used for the electrodes. A single polarization treatment was performed. Specifically, a lithium tantalate double-sided wrap wafer was prepared, and a powdered lithium tantalate polycrystal was spread on the bottom of an alumina container, and 10 double-sided lap wafers were placed thereon. This alumina container was placed in a stainless steel container and covered with a lid to form a sealed space. The stainless steel container was inserted into a tubular furnace, and while supplying hydrogen gas into the stainless steel container at 1.5 liters per minute, the furnace temperature was increased from room temperature to 1000 ° C. at a rate of about 6.7 ° C. per minute. The temperature rose. After holding for 24 hours, the furnace was cooled down at a rate of about 6.7 ° C. per minute. When the temperature dropped below 30 ° C., the stainless steel container and the alumina container and the double-sided wrap wafer were taken out and black-colored multi-domain tantalum. A double-sided lapped wafer of lithium acid crystal was obtained.

  This black colored double-sided double-sided wrap wafer was again placed in an alumina container together with a platinum electrode. Specifically, a powdered lithium tantalate polycrystal was spread on the bottom of an alumina container, and 10 double-sided lap wafers in a multi-domain state were placed thereon. Next, two platinum electrodes having an outer dimension of 120 mm × 120 mm and a thickness of 0.5 mm were placed so as to face each other with a double-sided wrap wafer in a possibly divided state. Thereafter, the gap was filled with powdered lithium tantalate polycrystal. This alumina container was placed in a stainless steel container, and the lid was capped to form a sealed space. The platinum electrode is connected to an external power source for single polarization processing. Then, this stainless steel container was inserted into a tubular furnace, and in Example 12, nitrogen gas was supplied into the stainless steel container at 1.5 liters per minute, while in Comparative Example 1, there was no supply gas (that is, air as the atmosphere). The furnace temperature was increased from room temperature at a rate of about 6.7 ° C. per minute. When the temperature reached 700 ° C., a voltage of 200 volts was applied and held for 5 hours, and then the temperature was lowered at a rate of 6.7 ° C. per minute. On the way, the voltage application was finished at 500 ° C. When the temperature became 30 ° C. or lower, the stainless steel container and the alumina container and the wafer were taken out to obtain a single-polarized double-sided lapped wafer.

In Example 12 in which the single polarization treatment was performed in a non-oxidizing atmosphere using nitrogen gas, the black coloration due to the increase in conductivity was maintained, and the conductivity was 1.1E-11Ω ^−1. A high value of cm ⁻¹ was obtained. Moreover, embrittlement of the platinum electrode did not occur. On the other hand, when the single polarization treatment is performed in the air atmosphere as in Comparative Example 1, the black color disappears and the conductivity is a very small value of 9.1E-15Ω ⁻¹ · cm ^−1. Yes, the value was the same as before heat treatment.

  The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

It is explanatory drawing explaining the heat processing and unipolarization process of the lithium tantalate wafer which concern on the Example of this invention.

Explanation of symbols

11 ... Double-sided wrap wafer, 12 ... Alumina container, 13 ... Carbon electrode,
14: Lithium tantalate polycrystal.

Claims (9)

  A method for producing a unipolarized lithium tantalate crystal, wherein at least the lithium tantalate crystal is subjected to a heat treatment for increasing conductivity in a reducing atmosphere, and then in a reducing atmosphere or a non-oxidizing atmosphere A method for producing a single-polarized lithium tantalate crystal, wherein the single-polarization treatment is performed at a temperature equal to or higher than the Curie temperature.   The method for producing a single-polarized lithium tantalate crystal according to claim 1, wherein the heat treatment for increasing the conductivity and the single-polarization treatment are performed continuously or individually.   The method for producing a single-polarized lithium tantalate crystal according to claim 1 or 2, wherein a heat treatment temperature for increasing the conductivity is set to 750 ° C or higher and lower than 1400 ° C.   The unipolarized lithium tantalate according to any one of claims 1 to 3, wherein the unipolarization treatment is performed at a temperature lower than a heat treatment temperature for increasing the conductivity. Crystal production method.   2. The voltage application to the lithium tantalate crystal is started at a temperature equal to or higher than the Curie temperature and the voltage application is terminated at a temperature equal to or lower than the Curie temperature when the single polarization treatment is performed. The method for producing a unipolarized lithium tantalate crystal according to any one of claims 4 to 5.   6. The lithium tantalate crystal according to claim 1, wherein at least a sliced wafer, a lapped wafer, or a wafer having at least one side mirror-finished is used. Process for the preparation of the described single polarized lithium tantalate crystals. The reducing atmosphere is any one of hydrogen, deuterium, carbon monoxide and NO _x (X <2.5), or a mixed gas composed of two or more thereof. Item 7. A method for producing a unipolarized lithium tantalate crystal according to any one of Items 6 to 6.   The single-polarized structure according to any one of claims 1 to 7, wherein one or more gases selected from a rare gas, nitrogen, and carbon dioxide are added to the reducing atmosphere. A method for producing a lithium tantalate crystal.   The non-oxidizing atmosphere is any one of a rare gas, nitrogen, and carbon dioxide, or a mixed gas composed of two or more of these, and is described in any one of claims 1 to 8. A method for producing a single-polarized lithium tantalate crystal.

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