WO2006051610A1

WO2006051610A1 - Method for producing lithium tantalate crystal

Info

Publication number: WO2006051610A1
Application number: PCT/JP2004/016924
Authority: WO
Inventors: Shoji Akiyama; Yoshiyuki Shiono
Original assignee: Shin-Etsu Chemical Co., Ltd.
Priority date: 2004-11-15
Filing date: 2004-11-15
Publication date: 2006-05-18
Also published as: JP4488249B2; JPWO2006051610A1

Abstract

Disclosed is a method for producing a lithium tantalate crystal having improved conductivity which is characterized in that a lithium tantalate crystal to be processed is arranged close to but not in contact with a material which has been reduced at a temperature (T1) and then the lithium tantalate crystal is exposed to a reducing atmosphere at a temperature (T2) which is lower than the temperature (T1).

Description

Specification

Method for producing lithium tantalate crystals

Technical field

TECHNICAL FIELD [0001] The present invention relates to a method for producing a lithium tantalate crystal used for a purpose of processing an electric signal by forming a pattern with a metal electrode on a wafer such as a surface acoustic wave element. Background art

[0002] Lithium tantalate is used for applications that utilize electrical characteristics in combination with surface acoustic wave (SAW) signal processing. Lithium tantalate crystals suitable for this application show the piezoelectric response (piezoelectricity) required for SAW devices due to their crystal structure, but the lithium tantalate crystals available by conventional methods are in addition to piezoelectricity. Cause a pyroelectric response (pyroelectricity).

[0003] The piezoelectricity of lithium tantalate crystals is an indispensable characteristic when using lithium tantalate crystals as SAW devices. On the other hand, pyroelectricity is obtained by changing the temperature of lithium tantalate crystals. It is observed as a surface charge generated on the outer surface of the crystal and charges the crystal. This surface charge can cause a spark discharge between metal electrodes formed on a wafer made of lithium tantalate crystals when using lithium tantalate crystals as SAW devices, causing significant performance defects in SAW devices. It has been. For this reason, in designing SAW devices that use lithium tantalate crystals, it is necessary to devise measures that do not generate surface charges, evacuate generated surface charges, or widen the spacing between metal electrodes. There were disadvantages if the design of the SAW device itself was constrained to incorporate!

[0004] Further, in the SAW device manufacturing process using lithium tantalate crystals, there are processes in which heat is applied to the lithium tantalate crystals in processes such as vapor deposition of the metal film and removal of the resist layer. When such a temperature change is applied to the lithium tantalate crystal, a charge is generated on the outer surface due to the pyroelectric property of the lithium tantalate crystal. This surface charge causes a spark discharge between the metal electrodes, resulting in destruction of the electrode pattern. Therefore, the SAW device manufacturing process should be devised so as not to change the temperature as much as possible. When the change is moderated, the device is devised at times, which causes disadvantages such as a decrease in the throughput of the manufacturing process or a narrow margin for guaranteeing the performance of the SAW device. .

[0005] In the lithium tantalate crystal produced by the usual method, the charge on the outer surface generated by pyroelectricity is neutralized by the free charge from the surrounding environment, and the power disappears over time. In the SAW device manufacturing process, which lasts several hours or more, this spontaneous pyroelectricity loss cannot be expected.

[0006] For applications such as surface acoustic wave (SAW) devices, electric charges are generated on the outer surface of the crystal due to the above background while maintaining the piezoelectricity required to exhibit device characteristics. There is an increasing demand for piezoelectric crystals that do not show any of these, and there is a need for lithium tantalate crystals without surface charge accumulation for such applications.

[0007] As a method for producing a lithium tantalate crystal with improved conductivity, there is a force disclosed in JP-A-11 92147 (Patent Document 1). Here, a lithium tantalate crystal is reduced in a reducing atmosphere of 500 ° C or higher. Is disclosed. However, when the lithium tantalate crystal is reduced by the method disclosed in Japanese Patent Laid-Open No. 11-92147 (Patent Document 1), it is used for SAW devices when the treatment temperature in the reducing atmosphere is 610 ° C or higher, which is the Curie point. The required single-domain structure is lost, and at a temperature of 610 ° C or lower, which is the Curie point, the speed of the reduction process becomes extremely slow. As a result, JP-A-11 92147 (Patent Document 1) It was found that the electrical conductivity of the lithium tantalate crystal cannot be industrially improved by the method disclosed in 1).

[0008] In view of the above, the present inventor previously made contact with a tantalum lithium crystal in a reducing atmosphere at a temperature lower than the temperature T1 by contacting the substance previously reduced at the temperature T1. A method has been proposed in which the surface charge generated in the lithium tantalate crystal is eliminated without accumulating by improving the conductivity of the lithium oxalate (Patent Documents 2 and 3: JP 2004-26930 A, International Publication No. 2004Z079061) (See pamphlet), but further development of methods to improve the conductivity of lithium tantalate crystals is desired.

Patent Document 1: Japanese Patent Application Laid-Open No. 11 92147

Patent Document 2: JP 2004-269300 A Patent Document 3: International Publication No. 2004Z079061 Pamphlet

Disclosure of the invention

Problems to be solved by the invention

[0010] The present invention meets the above-mentioned demand, and a method for producing a lithium tantalate crystal capable of performing uniform conductivity improvement treatment, having uniform and high conductivity, and preventing accumulation of surface charges. The purpose is to provide.

Means for solving the problem

[0011] As a result of intensive investigations to achieve the above object, the present inventor has been subjected to reduction treatment at a temperature T1, as disclosed in Japanese Patent Application Laid-Open No. 2004-269300 and International Publication No. 2004Z079061. Lithium tantalate crystals are placed in close contact with a substance that has been reduced at a temperature T1, which is not the case where the reduction treatment is performed while the lithium tantalate crystal to be treated is in contact with the substance (hereinafter referred to as the contact method). In addition, a sufficient reduction treatment is possible, and the reduction heat treatment is performed in a non-contact state in this way, and compared with the above contact method, the contact of the substance reduced at temperature T1 with respect to the lithium tantalate crystal is achieved. Because the reducing gas spreads uniformly on the surface of the lithium tantalate crystal to be treated without being dependent on the degree of variation, the uniform reduction is performed at the same time as high reduction. It has been found that a lithium tantalate crystal can be maintained, which has a uniform and high conductivity, and in which surface charge accumulation is reliably prevented, and has led to the present invention. It was.

[0012] Therefore, the present invention provides a lithium tantalite crystal to be treated in proximity to a substance reduced at temperature T1 in a non-contact state, and reduces the lithium tantalate crystal at a temperature T2 lower than the temperature T1. Provided is a method for producing a lithium tantalate crystal with increased conductivity, characterized by being exposed to an atmosphere.

[0013] In this case, it is preferable that the reduction-treated substance and the lithium tantalate crystal to be treated are arranged at a distance (d) 0.1 to 20 mm apart. In addition, the reduction treatment at the temperature T1, which is preferably 700 ° C or higher, is a reducing property containing one or more mixed gases selected from hydrogen, carbon monoxide, and dinitrogen monoxide. In this case, it is preferable to carry out in a gas, the reducing gas is further selected from one or more kinds selected from a rare gas, nitrogen, diacid-carbon power. It may contain a mixed gas. As the substance reduced at the temperature T1, any one of inorganic crystals, ceramics, and metals, particularly composite oxides having a non-stoichiometric composition, particularly lithium tantalate, lithium niobate, and hydrogen storage alloys are used. It is preferable.

[0014] Further, as the lithium tantalate crystal to be treated, the wafer is pre-sliced before being sliced and Z or lapped as a single polarized crystal, particularly as a single polarized crystal. It is preferable to use a crystal of Furthermore, it is preferable to introduce the air at a temperature of 250 ° C or lower after the treatment at the temperature T2 where the temperature T2 is preferably in the range of 400-600 ° C. The reduction treatment at temperature T2 is preferably performed in a reducing gas containing one or more mixed gases selected from hydrogen, carbon monoxide, and dinitrogen monoxide. The gas may further contain a rare gas, nitrogen, one or more mixed gases selected from carbon dioxide and carbon dioxide.

The invention's effect

[0015] According to the present invention, a highly conductive lithium tantalate crystal having no unevenness can be easily and reliably produced by uniform reduction heat treatment.

Brief Description of Drawings

FIG. 1 is a side view of a state in which substances treated at a temperature T1 and lithium tantalate crystals to be treated are arranged horizontally.

FIG. 2 is a side view showing a state where substances treated at temperature T1 and lithium tantalate crystals to be treated are arranged vertically.

FIG. 3 is a schematic diagram of the furnace used in Example 1.

[Fig.4] Conductivity (Ω ¹ · cm " ¹ ) of lithium tantalate crystals when a substance treated at temperature T1 and a lithium tantalate crystal to be treated are placed close to each other at a distance of dmm and subjected to a reduction heat treatment. It is a graph to show.

Explanation of symbols

[0017] 1 Substance treated at temperature T1

2 Lithium tantalate crystals to be treated

11 Alumina treatment tube 12 Gas supply port

13 Gas outlet

14 cap

15 Alumina carrier

16 LT Wrap Ueno

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] According to the method for producing a lithium tantalate crystal with increased conductivity according to the present invention, a lithium tantalate crystal to be treated is placed close to the material reduced at temperature T1 in a non-contact state, and this treatment is performed in this state. The lithium tantalate crystal to be heat-treated in a reducing atmosphere at a temperature T2 lower than the above temperature T1 can increase the conductivity of the lithium tantalate crystal, resulting in a temperature change in the lithium tantalate crystal. It is possible to suppress the pyroelectricity generated when applying

Thus, the surface charge generated by changing the temperature of the lithium tantalate crystal is eliminated without accumulating the generated surface charge by improving the conductivity of the lithium tantalate crystal. It is a life that can be done.

[0020] Here, examples of the substance to be reduced at the temperature T1 include inorganic crystals, ceramics, and metals. In this case, examples of the crystal include lithium tantalate crystal and lithium niobate crystal, and examples of the ceramic include amorphous lithium tantalate and lithium niobate. It is preferable to use ceramics that have composite acidity and strength. In the complex oxides with this non-stoichiometric composition, there is a cation deficiency, and this deficiency is thought to be closely related to the reduction treatment. Examples of the composite oxide having this non-stoichiometric composition include ceramics having a lithium tantalate force or ceramics having a lithium niobate force. Furthermore, lithium tantalate crystals or lithium niobate crystals can also be used. In this case, when lithium tantalate crystals or lithium niobate crystals are used, the stoichiometric composition is low in cation deficiency. It is preferable to use crystals with a composition of V, which has a stoichiometric composition of V, which is called a congruent composition.

[0021] The metal treated at the temperature T1 is preferably a hydrogen storage (occlusion) alloy. [0022] The temperature Tl for reducing the above-mentioned substance is preferably 700 ° C or higher, particularly 800 ° C or higher in order to perform the reduction reaction quickly. The upper limit temperature is suitably selected at 1,200 ° C or less, particularly preferably 1,150 ° C or less.

[0023] In addition, the atmosphere in which the substance is reduced at a temperature T1 may be a generally known reducing gas atmosphere. For example, hydrogen, carbon monoxide, dinitrogen monoxide, or a mixture thereof A reduction-treated substance can be obtained by carrying out in a reducing gas with a gas power. In this case, the reducing treatment can be performed in an atmosphere in which a rare gas such as He, Ne, Ar, nitrogen, carbon dioxide, or an inert gas having a mixed gas force is added to the reducing gas. . These inert gases preferably have a content of 0 to 95% by volume, particularly 20 to 80% by volume, in the reducing gas for reducing the above substances.

[0024] Although the time for reducing the above-mentioned substance is appropriately selected, it is usually 0.5 to 40 hours, more preferably 0.5 to 20 hours, and the substance to be treated is reduced and treated. The reduction process can be completed when the surface of the target substance is uniformly black.

[0025] In addition, as the reducing gas atmosphere, it is preferable to use a substance that can treat the substance to be reduced in as short a time as possible. For example, it is preferable to use hydrogen.

[0026] Here, in the present invention, ceramics made of lithium tantalate can be cited as an example of the substance reduced at temperature T1, which includes lithium carbonate and tantalum pentoxide. Is weighed, mixed, and heated to 1,000 ° C or higher in an electric furnace. In addition, for ceramics that are capable of lithium niobate, they can be obtained by using pentoxide instead of tantalum pentoxide. The ceramics thus obtained are placed in a container such as stainless steel or quartz, placed in a sealed furnace, and the reducing gas is introduced at 0.1-20 liters per minute, especially 0.1-10. Circulate in a sealed furnace at a rate of 1 liter, raise the furnace temperature from room temperature to the above treatment temperature (above 700 ° C), hold for 0.5 to 40 hours, especially 0.5 to 20 hours, The temperature can be reduced at a rate of 20 ° C per minute, especially 10 ° C per minute, and the vessel can be taken out of the furnace to obtain a reduced material.

[0027] Further, in the present invention, as an example of the substance reduced at the temperature T1, the force includes a lithium tantalate crystal. This is a ceramic made of the above-described lithium tantalate before the reduction treatment. Is put into a crucible made of precious metal, heated and melted, and then rotated using a seed crystal. It is possible to grow lithium tantalate crystals by raising (so-called Chiyoklarsky method).

[0028] The lithium tantalate crystal thus obtained is placed on a quartz stage, placed in a sealed furnace, subjected to reduction heat treatment under the same conditions as described above, and then subjected to reduction treatment by lowering the temperature. It can be obtained as a material. When the temperature falls, the atmosphere can be introduced into the furnace at 250 ° C or below, and the treated material can be taken out when the temperature falls below 30 ° C.

[0029] Here, a noble metal electrode is placed on a lithium tantalate crystal having a diameter of, for example, 4 inches before the reduction treatment obtained by the above-mentioned Tjokralski method, and the voltage is applied at a temperature above the Curie point, for example, 650 ° C. A single-domain process can be performed by applying this, and a wafer that has been subjected to a slicing process with a diameter of 4 inches and a thickness of 0.5 mm by slicing the crystal subjected to the single-domain process using, for example, a wire saw. Further, this wafer is processed with a lapping machine to obtain a lapped wafer with a diameter of 4 inches and a thickness of 0.4 mm.

[0030] In the present invention, as an example of the substance subjected to the reduction treatment at the temperature T1, the force includes a lapped wafer that also has a lithium tantalate crystal. This is because the lithium tantalate crystal obtained in the above step is used. It can be obtained by reducing and heat-treating the lap wafer under the above conditions, lowering the temperature, introducing air if necessary, and removing the wafer from the furnace when the temperature falls below 30 ° C. This treatment changes the color of the lithium tantalate wafer from white before processing to black, so that it has the ability to absorb light. However, since the temperature T1 is higher than the Curie point of the lithium tantalate crystal, the resulting lithium tantalate has a multi-domain structure that is unsuitable for SAW devices.

[0031] Next, in the present invention, the lithium tantalate crystal to be treated is placed in close contact with the substance treated at the temperature T1 in a non-contact state, and the temperature T2, lower than the temperature T1, preferably 400 or higher. 1 600 ^o C, special 500 1 600 ^o C [trowel reduction heat treatment.

[0032] Here, as the lithium tantalate crystal to be treated at the temperature T2, a single-polarized crystal obtained as described above can be used. When lithium tantalate crystals are used, the lithium tantalate crystals obtained in the present invention do not require a single domain treatment after the reduction treatment at temperature T2. In this case, the form of the single polarized crystal was the crystal before the slicing or the slicing process was performed. A wafer or a lapped wafer can be used.

[0033] It should be noted that the normal single domain treatment is performed in the atmosphere at a temperature higher than the Curie point (about 610 ° C) of the lithium tantalate crystal. On the other hand, the lithium tantalate crystal with improved conductivity obtained in the present invention loses the improved conductivity when the temperature is set to 400 ° C or higher in the atmosphere. As a result, the present invention The lithium tantalate crystal that has been subjected to the treatment by the above has a property of returning to the state before the reduction heat treatment when the single domain treatment is performed thereafter. However, in the present invention, the temperature T2 for reducing heat treatment of lithium tantalate is set to a temperature lower than the Curie point (610 ° C.) of the lithium tantalate crystal, and the processing atmosphere is in a reducing atmosphere. If the conductivity is lost, the problem does not arise.

[0034] In this case, the distance d (see Figs. 1 and 2) for bringing the lithium tantalate crystal to be treated in close contact with the substance treated at the temperature T1 is 0.1-20 mm, especially It is preferably about 0.2 to 3 mm. FIGS. 1 and 2 show the arrangement of the substance 1 treated at temperature T1 and the lithium tantalate crystal 2 to be treated. FIG. 1 shows a wafer made of lithium tantalate crystal 2 placed horizontally. For example, Fig. 2 shows an example of vertical installation, in which substance 1 treated at temperature T1 and lithium tantalate crystal 2 to be treated are alternately arranged at a distance d.

[0035] In the present invention, the atmosphere in which the lithium tantalate crystal is subjected to a reduction heat treatment at a temperature T2 may be a generally known reducing gas atmosphere. For example, hydrogen, carbon monoxide, or monoxide By carrying out in a reducing gas such as dinitrogen or a mixed gas of these, a reduced material can be obtained. In this case, the reduction treatment can be performed in an atmosphere in which a rare gas such as He, Ne, Ar, nitrogen, carbon dioxide, or an inert gas such as a mixed gas thereof is added to the reducing gas. It is preferable that these inert gases have a content of 0 to 95% by volume, particularly 20 to 80% by volume, in the reducing gas for reducing the above substances.

[0036] The temperature at which the lithium tantalate crystal is treated is preferably 400-600 ° C, as described above, and the reduction treatment time is appropriately selected, but is usually 0.5-20 hours, more preferably 0. The reduction treatment can be completed when the lithium tantalate crystal to be treated is in a state of uniformly black or light black in the surface for 5 to 10 hours. [0037] As described above, after the reduction heat treatment, the temperature may be lowered. In this case, when the temperature falls below 250 ° C, introduction of air prevents cracking due to heat shrinkage or the like at the time of removal. Or point to prevent reaction with atmospheric components recommended.

[0038] As a preferred method for obtaining a lithium tantalate crystal having a single domain structure and improved conductivity, which is the object of the present invention, for example, reduction heat treatment should be performed at the temperature T2 of the present invention. Lithium tantalate crystal lapped wafers that have undergone single-domain treatment and lithium tantalate wafers that have turned black by treatment at temperature T1 are placed in close proximity (0.1-2 Omm) in a non-contact state. Installed in the furnace, flowing reducing gas such as hydrogen gas at a rate of 0.1-20 liters per minute, especially 0.1-10 liters per minute. In particular, the temperature is raised at a rate of 10 ° C and the furnace is kept at the required temperature T2 for 0.5–20 hours, especially 0.5–10 hours, then the furnace is 400–600 ° C per minute, especially 500–600 °. The temperature is lowered at a rate of C, the air is introduced into the furnace at 250 ° C or below, and the wafer is taken out of the furnace when it reaches 30 ° C or below. Masui.

[0039] Here, by preventing the lithium tantalate crystal to be treated from coming into contact with the substance treated at the temperature T1, the reducing gas is spread over the entire surface of the lithium tantalate crystal to achieve a uniform in-plane distribution. It is possible to maximize the influence of the substance reduced at the temperature T1 on the crystal of lithium tantalate to be processed.

[0040] The lithium tantalate crystal obtained in the present invention has a feature that the surface charge accumulation caused by the temperature change is substantially not observed due to the improved conductivity of the crystal. This indicates that the lithium tantalate crystal obtained in the present invention does not accumulate charges on the outer surface of the crystal while maintaining the piezoelectricity, and is an extremely advantageous material for SAW device manufacturing. is there. Further, in the method of the present invention, the above-described lithium tantalate crystal can be obtained in a very short time, which is an industrially advantageous production method.

[0041] Here, the measurement of the conductivity for lithium tantalate crystal is as described in Example, the conductivity of the lithium-tantalate crystal before thermal reduction according to the present invention is usually 10 ¹⁴ one 10 ¹⁵ whereas a omega ¹ · cm ^1, the conductivity of the tantalate lithium © beam crystals after the reduction heat treatment of the present invention is usually ^{^{^{10- 9 - 10- 13 Ω- 1 ·}}} cm , especially 10- ^omega - the 10- ¹² Ω- ^¹ · cm ¹ Example

[0042] Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples.

[0043] [Production example of LT wafer]

LT wafer fabrication was performed as follows. A lithium tantalate crystal with a diameter of 100 mm and a length of 50 mm oriented by rotating 36 ° in the y direction with respect to the surface normal was obtained by using the Tyoklalsky method and the conventional secondary processing method ( Hereinafter referred to as LT crystal). This LT crystal was cut and subjected to lapping force to obtain a double-sided lapped wafer having a thickness of 0.4 mm (hereinafter, this wafer is referred to as “LT lapped wafer”). One side of the LT wrap wafer was polished to obtain a wafer having a thickness of 0.35 mm (hereinafter, this wafer is referred to as an LT polished wafer). This wafer is colorless and translucent.

[0044] [Example 1]

The LT wrap wafer was placed in a sealed furnace where each gas in Table 1 circulated at a rate of about 1.5 liters per minute. Figure 3 shows an overview of the furnace.

In the furnace 10, an alumina treatment tube 11 having a diameter of 200 mm is disposed along the horizontal direction, and three zones A, B, and C are formed in the treatment tube 11, and Are provided with thermocouples a, b, and c corresponding to these bands A, B, and C, respectively. Both ends of the alumina treatment tube 11 are extended to the outside from the furnace 10, and one extended end face is hermetically closed, and a gas supply port 12 is formed on the closed face, and the other end face is closed. A cap 14 is detachably attached to the extended end face, and a gas discharge port 13 is formed in the vicinity of the other extended end face. Although not shown in the drawing, both extended base ends of the alumina processing tube 11 serving as a wafer outlet are hermetically sealed by O-rings. The means for heating the inside of the furnace is based on resistance heating.

First, the cap 14 is removed, the alumina carrier 15 supporting the LT wrap wafer 16 is placed in the alumina treatment tube 11, and a gas flow is introduced into the alumina treatment tube 11 from the gas supply port 12, Heating of the furnace was started with the gas discharged from the gas outlet 13. The furnace temperature was raised from room temperature to the temperature T1 in Table 1 at a rate of about 6.7 ° C per minute. After holding for 1 hour at temperature T1, the furnace was cooled at a rate of about 6.7 ° C per minute. Introduce air into the furnace at 250 ° C or below The wafer was removed from the furnace when the temperature dropped below 30 ° C (hereinafter referred to as T1-treated LT wafer).

Next, the LT wrap wafer and the T1 processed LT wafer are alternately arranged close to each other as shown in FIG. 1 so that the distance (d) between the LT wrap wafer and the T1 processed LT wafer is 0.25 mm. Then, it was placed in a sealed furnace in which reducing gas such as hydrogen gas circulated at a rate of about 1.5 liters per minute. In this embodiment, this furnace is the same as the temperature T1 reduction treatment. The furnace temperature was raised from room temperature at a rate of about 6.7 ° C per minute. After holding at the temperature T2 in Table 1 for 1 hour, the furnace was cooled at a rate of about 6.7 ° C per minute. Air was introduced into the furnace at 250 ° C or lower, and the wafer was removed from the furnace when the temperature dropped to 30 ° C or lower (hereinafter referred to as T2-treated LT wafer).

[0048] The conductivity was measured as follows. The conductivity is the reciprocal of the volume resistivity, and the volume resistivity was measured using 4329A High Resistance Meter and 16008A Resistivity manufactured by Hewlett Packard. The volume resistivity can be obtained by the following formula.

Ρ: Volume resistivity (Ω -cm)

π: Pi ratio

d: Center electrode diameter (cm)

t: T2 treatment LT wafer thickness (cm)

R: Resistance value (Ω)

The resistance was measured by applying a voltage of 500 volts to the sample and measuring the resistance one minute after the voltage was applied.

[0049] The surface potential was measured as follows. Pyroelectricity is the amount of charge that accumulates on the surface when a temperature difference occurs. This is similar to static electricity, and surface potential measurement is known as a quantitative measurement. T2 treatment LT wafer was heated from 30 ° C to 70 ° C on a hot plate in 1 minute, and Ion Systems' SFM775 was used, and the difference in surface potential during that time was taken as the measured value. .

[0050] Table 1 shows the conductivity and surface potential. In all the tables and figures, conductivity, such as "9. 3E-14" according as that is the sense of "9. 3 X 10- ^14".

[0051] [Table 1] Temperature T2 Temperature Tl Temperature T2 Surface potential Spacing d Mixed gas (.c) CO (Ω-cm- "(V) (mm) (Volume%)

100% Η ₂ 1,100 600 2.9E-11 0.1 0.25

100% Η ₂ 1,100 550 7.IE-12 0.1 0.25

100% Η ₂ 1,100 500 4.8E-12 0.1 0.25

100% Η ₂ 1,100 400 2.6E-12 0 0.1 0.25

100% Η ₂ 1,100 350 1.7E-13 123 0.25

100% Η ₂ 700 600 2.9E-12 0 0.1 0.25

100% Η ₂ 650 600 1.4E-13 138 0.25

100% CO 700 600 2.2E-12 <0.1 0.25

100% Ν ₂ 0 700 600 1.3E-12 0 0.1 0.25

90% Η ₂ 700 600 1.5E-12 0 0.1 0.25 10% Ν ₂

10% Η ₂ 700 600 2.6E-12 0.1 0.25 90% He

10% Η ₂ 700 600 1.8E-12 0 0.1 0.25 90% Ar

[0052] [Example 2]

Temperature T2 reduction treatment-Lithium tantalate crystal

The LT wrap wafer was placed in a sealed furnace with hydrogen gas flowing at a rate of about 1.5 liters per minute. The temperature was maintained at Tl (l, 100 ° C) for 1 hour to obtain a Tl-treated LT wafer. As shown in Figure 1, the LT wrap wafer and the T1 treated LT wafer are alternately placed close to each other in a non-contact state (d = 0.1–50mm), and a mixed gas of 90vol% N + 10vol% H is about 1.5 liters per minute. Speed

twenty two

Placed in a sealed oven circulating at a temperature. Here, the interval between the wafers is set to 0.1 mm or more because if it is shorter than this, it is difficult to fill the wafer. After holding at temperature T2 (600 ° C) for 1 hour, the furnace was cooled down at a rate of about 6.7 ° C per minute. Atmosphere was introduced into the furnace at 250 ° C or below, and when the temperature was below 30 ° C, the wafer was taken out from the furnace to obtain a T2 treated LT wafer. Figure 4 shows the dependence of the conductivity on the spacing d. It can be seen that the conductivity increases as the distance d decreases, and that there is almost no effect above 20 mm. In addition, it does not interfere with wafer filling, and the viewpoint power of increasing conductivity. The preferable interval is in the range of about 0.2-3.Omm.

Further, when the wafer obtained by the contact method (Japanese Patent Laid-Open No. 2004-269300) and the wafer obtained by this example were visually inspected, both of the wafers produced by this example were contacted. It was confirmed that the discoloration was more uniform in the wafer surface than that obtained in the above.

Claims

The scope of the claims

[I] Lithium tantalate crystals to be treated are placed in close contact with the substance reduced at temperature Tl in a non-contact state, and the lithium tantalate crystals are exposed to the reducing atmosphere at a temperature T2 lower than temperature T1. A process for producing lithium tantalate crystals with increased conductivity characterized by

[2] The method for producing a lithium tantalate crystal according to claim 1, wherein the reduction-treated substance and the lithium tantalate crystal to be treated are arranged at a distance (d) O. 1—20 mm apart.

[3] The method for producing a lithium tantalate crystal according to claim 1 or 2, wherein the temperature T1 is 700 ° C or higher.

[4] The reduction treatment at a temperature T1 is performed in a reducing gas containing one or more mixed gases selected from hydrogen, carbon monoxide, and nitrogen monoxide. The method for producing a lithium tantalate crystal as described in 1, 2 or 3.

5. The method for producing a lithium tantalate crystal according to claim 4, wherein the reducing gas further contains one kind or two or more kinds of mixed gases in which a rare gas, nitrogen, and diacid-carbon power are also selected.

6. The method for producing a lithium tantalate crystal according to any one of claims 1 to 5, wherein any one of inorganic crystals, ceramics, and metals is used as the substance reduced at temperature T1.

7. The method for producing a lithium tantalate crystal according to claim 6, wherein the inorganic crystal or the ceramic is a composite oxide having a non-stoichiometric composition.

8. The method for producing a lithium tantalate crystal according to claim 6 or 7, wherein lithium tantalate or lithium niobate is used as the inorganic crystal or the ceramic.

9. The method for producing a lithium tantalate crystal according to claim 6, wherein a hydrogen storage alloy is used as the metal.

10. The method for producing a lithium tantalate crystal according to any one of claims 1 to 9, wherein a single-polarized crystal is used as the lithium tantalate crystal to be treated.

[II] Slice processing and Z or lapping processing were performed as the single polarized crystal. 11. The method for producing a lithium tantalate crystal according to claim 10, wherein:

12. The method for producing a lithium tantalate crystal according to claim 10, wherein a crystal before slicing is used as the single polarized crystal.

[13] The method for producing a lithium tantalate crystal according to any one of [1] to [12], wherein the temperature T2 is in a range of 400 to 600 ° C.

14. The method for producing a lithium tantalate crystal according to claim 13, wherein the atmosphere is introduced at a temperature of 250 ° C. or lower after the treatment at the temperature T2.

[15] The reduction treatment at temperature T2 is performed in a reducing gas containing one or more mixed gases selected from hydrogen, carbon monoxide, and dinitrogen monoxide. 15. The method for producing a lithium tantalate crystal according to any one of items 14 to 14.

16. The method for producing a lithium tantalate crystal according to claim 15, wherein the reducing gas further contains one kind or two or more kinds of mixed gases selected from rare gas, nitrogen and diacid-carbon power.