Method for producing oxide single crystal substrate

Abstract

To provide a method for manufacturing an LT substrate capable of suppressing color unevenness of the LT substrate.SOLUTION: In the method for manufacturing the LT substrate, a plurality of planar LT crystals 2 having substrate surfaces 21, 22 are embedded in a mixed powder 1 of an aluminum powder (Al powder) and an aluminum oxide powder (Al2O3 powder) filled in a heat-resistant vessel 10 made of a stainless steel material through the mixed powder 1, and heat-treated at a temperature lower than the Currie temperature of the LT crystal 2. A dummy substrate 40 is installed between the substrate surface 21 on the inner-wall surface 10a side of the LT crystal 2a in which the substrate surface 21 is opposed to the inner-wall surface 10a of the heat-resistant vessel 10 via the mixed powder 1 and the inner-wall surface 10a, and the color unevenness generated in the LT substrate is suppressed by the action of the dummy substrate 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing an oxide single crystal substrate. [Background technology]

[0002] Oxide single crystals such as lithium tantalate (hereinafter sometimes abbreviated as LT) single crystals and lithium niobate (hereinafter sometimes abbreviated as LN) single crystals have piezoelectric properties, and there is a great demand for SAW (surface acoustic wave) devices that utilize this piezoelectricity, and the market is showing explosive growth, especially in mobile communications applications such as mobile phones.

[0003] The LT and LN single crystals that form the basis of SAW devices are often grown by a pulling method known as the Czochralski method (CZ method). A disk-shaped substrate is cut from the grown LT or LN single crystal, polished, electrodes are formed, cut into chips, and packaged to complete the SAW device. Unless otherwise specified, the term "crystal" used in this specification refers to a single crystal. The following explanation will use an LT single crystal as an example.

[0004] In the CZ method, single crystals are typically grown in an electric furnace using a high-melting-point noble metal crucible, and after growth, the crystal is cooled at a predetermined cooling rate and then removed from the electric furnace.

[0005] The grown crystal undergoes annealing to remove distortion and single polarization before being delivered to the substrate processing process. In the substrate processing process, the shoulders and tails of the grown single crystal are cut off, and it is then processed by cylindrical grinding into a cylindrical shape slightly thicker than the final product, and is often cut into substrates using a multi-wire saw. The cut substrates are finally finished to the product diameter by peripheral beveling, and the edges are chamfered. The substrate surface is then finished by lapping and mirror polishing to create the completed single crystal substrate. LT substrates are available in sizes of 4 to 6 inches in diameter.

[0006] In the surface acoustic wave element (SAW filter) manufacturing process of substrates obtained by such conventional methods, the pyroelectricity characteristic of LT crystal causes electric charges to build up on the substrate surface due to temperature changes during the process, resulting in discharges that destroy the comb-shaped electrodes formed on the substrate surface and even cause cracks in the substrate, resulting in a decrease in yield in the element manufacturing process.

[0007] Therefore, in order to solve the problems caused by the pyroelectricity of LT crystals, several techniques for increasing electrical conductivity have been proposed. For example, Patent Document 1 proposes a method in which LT crystals processed into a substrate state are embedded in a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al2O3 powder) and then heat-treated (reduced), and Patent Document 2 describes a method in which LT crystals are embedded in lithium carbonate powder and then heat-treated (reduced). Furthermore, Patent Document 3 describes how controlling the positions of the LT crystals when embedding them in the mixed powder can suppress color unevenness defects (blackening of the periphery) that occur during the reduction process. This reduction process reduces the volume resistivity of the LT substrate to approximately 10 10 (Ω·cm)~10 11 (Ω·cm). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 4063191 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-165611 [Patent Document 3] JP 2019-202919 A Summary of the Invention [Problem to be solved by the invention]

[0009] Incidentally, the reduction treatments described in Patent Documents 1 to 3 involve embedding multiple LT crystals processed into a substrate state in a powdered reducing agent such as aluminum powder or lithium carbonate powder filled in a heat-resistant container and then heat-treating them. Therefore, in addition to the color unevenness defect (blackening of the outer periphery) described in Patent Document 3, defects in the appearance of the substrate are likely to occur; for example, color unevenness has been confirmed in the center of the LT substrate.

[0010] This color unevenness in the center of the LT substrate is likely to occur on the LT crystals facing the inner wall of the heat-resistant container when multiple LT crystals are embedded in a powdered reducing agent and subjected to reduction treatment, resulting in a color unevenness defect in which the center of the LT substrate turns black. Although the occurrence rate of color unevenness in the center of the LT substrate is low, it is a problem that occurs only on specific substrates facing the inner wall of the heat-resistant container, and improvement is therefore desired.

[0011] The present invention has been made in view of these problems, and an object of the present invention is to provide a method for producing an oxide single crystal substrate that can suppress color unevenness that occurs on the substrate. [Means for solving the problem]

[0012] That is, the method for producing an oxide single crystal substrate according to the present invention comprises the steps of: A method for producing an oxide single crystal substrate, comprising embedding a plurality of flat oxide single crystals having substrate surfaces in a powder containing a powdered reducing agent filled in a heat-resistant container with the powder interposed therebetween, and heat-treating the resulting oxide single crystals at a temperature lower than the Curie temperature of the oxide single crystals, The method is characterized in that a dummy substrate is placed between the inner wall surface and the substrate surface of the oxide single crystal, the substrate surface facing the inner wall surface of the heat-resistant container via the powder. [Effects of the Invention]

[0013] According to the method of the present invention, it is possible to suppress color unevenness that occurs on an oxide single crystal substrate. [Brief explanation of the drawings]

[0014] [Figure 1]1A to 1C are explanatory views showing a method for manufacturing an oxide single crystal substrate according to a first embodiment. [Figure 2] The figures show the peripheral bevel processing of an oxide single crystal processed into a substrate state, with Figure 2(A) being a cross-sectional view of the outer edge of the oxide single crystal before peripheral bevel processing, Figure 2(B) being a cross-sectional view of the upper side of the outer edge being peripheral bevel processed, and Figure 2(C) being a cross-sectional view of the outer edge of the oxide single crystal after peripheral bevel processing. [Figure 3] 5A to 5C are explanatory diagrams showing a method for producing an oxide single crystal substrate according to a second embodiment. [Figure 4] A plan view of an oxide single crystal processed into a substrate. [Figure 5] FIG. [Figure 6] FIG. 10 is an explanatory diagram showing a conventional method for manufacturing an oxide single crystal substrate. [Figure 7] FIG. 10 is an explanatory diagram showing another conventional method for manufacturing an oxide single crystal substrate. DETAILED DESCRIPTION OF THE INVENTION

[0015] The method for manufacturing an oxide single crystal substrate according to an embodiment of the present invention will be described together with a conventional method for manufacturing an oxide single crystal substrate.

[0016] The oxide single crystal substrate is fabricated from a piezoelectric oxide single crystal such as lithium niobate (LiNbO3:LN) single crystal or lithium tantalate (LiTaO3:LT) single crystal.

[0017] Hereinafter, the LT single crystal will be described as a representative example of an oxide single crystal.

[0018] [1] Conventional method for manufacturing an oxide single crystal substrate (1) Growth of oxide single crystals LT single crystals are grown by single crystal growth methods such as the Czochralski method (CZ method), in which a seed crystal is immersed in a melt obtained by melting raw material powder and then pulled up to grow a single crystal. For example, by using a high-frequency induction heating device, large single crystals can be stably produced.

[0019] The grown LT single crystal is then subjected to an annealing treatment to remove strain and a single polarization treatment (poling). This poling treatment involves heating the grown LT single crystal to a temperature above the Curie point, for example, 600 to 700°C, and applying a voltage of, for example, 200 to 500 V in the Z-axis direction for approximately 0.5 to 2 hours to polarize the single crystal.

[0020] After the poling process, the LT single crystal is delivered to the substrate processing process.

[0021] (2) Substrate processing process The substrate processing process includes a facing process in which the shoulders and tails of the grown LT single crystal are cut off; a cylindrical processing process in which the side of the straight body from which the shoulders and tails have been cut is cylindrically ground to form a cylindrical shape slightly thicker than the final product; a slicing process in which the resulting cylindrical single crystal is cut with a multi-wire saw to obtain flat (disk-shaped) substrates; a beveling process in which the outer edge surfaces of the resulting substrates are chamfered; a lapping process in which both the front and back surfaces of the substrate are smoothed; and a polishing process in which one or both surfaces of the substrate are mirror-polished.The reduction treatment of the substrate is performed after the slicing process and before the lapping process.

[0022] (3) Reduction treatment process The volume resistivity of the LT substrate is 10 10 (Ω·cm)~10 11 There are various known reduction treatment methods for adjusting the resistivity to about (Ω·cm). In the present invention, however, a method is used in which multiple LT crystals processed into a substrate state (i.e., processed into a flat plate having a substrate surface) are embedded in a powder containing a powdery reducing agent and then heat-treated (reduced).

[0023] Hereinafter, a specific description will be given of an example in which the powder containing the powdery reducing agent is made up of a mixed powder of aluminum powder (Al powder) and aluminum oxide powder (Al2O3 powder).

[0024] First, a mixed powder 1 of Al powder and Al2O3 powder and LT crystals 2 processed into a substrate state are alternately placed into a heat-resistant container 10 made of stainless steel or the like, and, for example, 20 to 30 LT crystals 2 are embedded in the mixed powder 1 with the mixed powder 1 interposed between the LT crystals 2 as shown in Figure 6. After that, the open part of the heat-resistant container 10 is closed with an appropriate lid (not shown). Next, the multiple heat-resistant containers 10 with the multiple LT crystals 2 embedded in the mixed powder 1 are placed in a heating furnace (not shown), and, for example, the LT crystals are reduced by heat treatment at a temperature below the Curie temperature of the LT crystals while continuously flowing an inert gas into the heating furnace under atmospheric pressure.

[0025] In FIG. 6, multiple LT crystals 2 are embedded in the mixed powder 1 of Al powder and Al2O3 powder so as to be aligned in the vertical direction of the heat-resistant container 10 (the up-and-down direction in the figure) and then heat-treated. However, as shown in FIG. 7, multiple LT crystals 2 may be embedded in the mixed powder 1 of Al powder and Al2O3 powder so as to be aligned in the horizontal direction of the heat-resistant container 10 (the left-right direction in the figure) and then heat-treated.

[0026] The volume resistivity of the LT substrate can be adjusted by the mixing ratio of Al powder and Al2O3 powder. By increasing the ratio of Al powder, the volume resistivity can be increased to 10 9 It is possible to reduce the resistance to approximately (Ω·cm).

[0027] However, as the ratio of Al powder in the mixed powder increases, black dots (color unevenness, i.e., reduction unevenness) with a diameter of approximately 1 to 5 mm tend to occur. Furthermore, the outer periphery of the substrate may be reduced more strongly than the center of the substrate. For this reason, as described in Patent Document 3, it is important to uniformize the positions of the LT crystals when embedding them in the mixed powder, as well as the distance between the edges of the embedded LT crystals and the inner wall surface of the heat-resistant container. Such control can improve color unevenness.

[0028] (4) Problems with the conventional manufacturing method However, although it is rare, color unevenness may occur in the center of the LT board.

[0029] As a result of the investigation, it was confirmed that the color unevenness in the center of the LT substrate occurred on a specific LT substrate facing the inner wall surface of the heat-resistant container in which the reduction treatment was performed (the LT crystal 2a located at the bottom of the embedding order in Figure 6, where the LT crystals 2 are embedded lined up in the vertical direction of the container 10, and the LT crystals 2a and 2b located on both ends of the horizontal direction of the container 10 in Figure 7, where the LT crystals 2 are embedded lined up in the horizontal direction of the container 10).

[0030] That is, the conventional manufacturing method has a problem in that color unevenness occurs in the center of the specific substrate facing the inner wall surface of the heat-resistant container.

[0031] [2] The method for manufacturing an oxide single crystal substrate according to the present invention (1) Analysis of the issue The present inventors have conducted research into heat-resistant containers used in reduction treatment.

[0032] The heat-resistant container is heated with 20 to 30 LT crystals embedded in a mixture of Al powder and Al2O3 powder, and after the LT crystals are reduced, the heat-resistant container is reused repeatedly. However, it was confirmed that the repeated use causes the components of the container to dissolve, and the above components adhere to the inner wall surface of the heat-resistant container in powder form.

[0033] Furthermore, when a heat-resistant container with the above-mentioned components adhered thereto is used and a reduction treatment is performed with LT crystals arranged vertically (see FIG. 6), black discoloration and color irregularities tend to occur in the center of the substrate surface 21 of the LT crystal 2a closest to the inner wall surface 10a on the bottom side of the heat-resistant container 10, and it has been confirmed that the shape of the color irregularities is similar to the traces of the components adhered to the inner wall surface 10a.

[0034] Furthermore, when a heat-resistant container with the above-mentioned components adhered thereto is used and a reduction treatment is carried out with the LT crystals arranged horizontally (see FIG. 7), black discoloration tends to occur on the substrate surface 21 of the LT crystal 2a closest to the inner wall surface 10a on both ends of the heat-resistant container 10, and on the center of the substrate surface 22 of the LT crystal 2b, and it has been confirmed that the shape of the color unevenness is similar to the traces of the components adhered to these inner wall surfaces 10a.

[0035] From these analyses, it was speculated that the color unevenness in the center of the LT substrate was caused by components of the heat-resistant container 10 that had adhered to the inner wall surface 10a of the container.

[0036] Therefore, when the inner wall surface of the heat-resistant container was periodically cleaned, the color unevenness was suppressed, but the additional cleaning work resulted in a problem of reduced productivity.

[0037] (2) The method for producing an oxide single crystal substrate according to the present invention In this invention, a dummy substrate is placed between the inner wall surface of an LT crystal, whose substrate surface faces the inner wall surface of a heat-resistant container via a mixed powder of Al powder and Al2O3 powder, and the inner wall surface, blocking the contribution of the above-mentioned components adhering to the inner wall surface of the heat-resistant container with the dummy substrate. In particular, in the embodiment described below, a dummy substrate that is not subject to reduction and has been processed into a substrate state via a mixed powder of Al powder and Al2O3 powder is placed on the underside of the LT crystal located at the bottom of the embedding order, blocking the contribution of the above-mentioned components adhering to the inner wall surface on the bottom side of the heat-resistant container with the dummy substrate, thereby suppressing color unevenness in the center of the LT substrate.

[0038] (2-1) First embodiment The method for manufacturing the LT substrate according to the first embodiment will be specifically described below.

[0039] First, a dummy substrate 40 that is not to be reduced is placed on the inner wall surface 10a on the bottom side of a heat-resistant container 10 made of stainless steel or the like, as shown in FIG.

[0040] Next, a mixed powder 1 of Al powder and Al2O3 powder and LT crystals 2 to be reduced that have been processed into a substrate state (i.e., processed into a flat plate shape having substrate surfaces 21 and 22) are alternately placed into a heat-resistant container 10 in which a dummy substrate 40 has been placed, and with the mixed powder 1 interposed between the LT crystals 2, for example, about 20 to 30 LT crystals 2 are embedded in the mixed powder 1, and then, as in the conventional method, the open part of the heat-resistant container 10 is closed with an appropriate lid material (not shown).

[0041] Each LT crystal 2 to be reduced is subjected to outer periphery bevel processing as shown in Fig. 1. That is, as shown in Figs. 2(A) to (B), outer periphery edge 30 of LT crystal 2 held on crystal holder 31 is pressed against the upper inclined surface of rotating chamfering grindstone 32 to polish the upper edge corner of outer periphery edge 30, and then crystal holder 31 is lowered by a predetermined amount to polish the lower edge corner of outer periphery edge 30 of LT crystal 2, thereby performing outer periphery bevel processing as shown in Fig. 2(C).

[0042] Then, the heat-resistant containers 10 in which the LT crystals 2 are embedded in the mixed powder 1 are placed in a heating furnace (not shown), and the LT crystals 2 are reduced by heat-treating them at a temperature below the Curie temperature of the LT crystals while continuously flowing an inert gas into the heating furnace under atmospheric pressure.

[0043] According to the method for manufacturing an LT substrate of the first embodiment, the dummy substrate 40, which is not the target of reduction, is placed on the inner wall surface 10a on the bottom side of the heat-resistant container 10. This blocks the involvement of eluted components that have adhered near the center of the inner wall surface 10a on the bottom side of the heat-resistant container 10, making it possible to suppress color unevenness in the center of the substrate surface 21 of the LT crystal 2a that is closest to the inner wall surface 10a on the bottom side of the heat-resistant container 10.

[0044] In the first embodiment, the dummy substrate 40 is directly installed on the inner wall surface 10a on the bottom side of the heat-resistant container 10 as shown in FIG. 1, but the dummy substrate 40 may be installed on the inner wall surface 10a on the bottom side of the heat-resistant container 10 via a mixed powder 1 of Al powder and Al2O3 powder.

[0045] That is, in each embodiment, a dummy substrate 40 is placed between the inner wall surface 10a on the bottom side of the heat-resistant container 10 and the substrate surface 21 of the LT crystals 2a to be reduced. At this time, the dummy substrate 40 is placed so as to be along the inner wall surface 10a on the bottom side of the heat-resistant container 10 or the LT crystals 2a to be reduced.

[0046] Furthermore, when the dummy substrate 40 is directly placed on the inner wall surface 10a on the bottom side of the heat-resistant container 10, the dummy substrate 40 may become misaligned during placement. Furthermore, when the size of the dummy substrate 40 is equal to the diameter of the LT crystal 2, the positional misalignment of the dummy substrate 40 may impair the blocking effect of the dummy substrate 40. Therefore, it is preferable to set the size of the dummy substrate 40 larger than the diameter of the LT crystal 2. Furthermore, to avoid the above-mentioned positional misalignment of the dummy substrate 40, it is more preferable to set the size of the dummy substrate 40 equal to or slightly smaller than the inner diameter of the heat-resistant container 10. For example, it is preferable to set the size of the dummy substrate 40 approximately 1 to 4 mm smaller than the inner diameter of the heat-resistant container 10. This makes it possible to avoid positional misalignment of the dummy substrate during placement.

[0047] Incidentally, when the lid material (not shown) that closes the open portion of the heat-resistant container 10 is made of a heat-resistant material and is used repeatedly in the same manner as the heat-resistant container 10, it is preferable to also place a dummy substrate between the inner wall surface of the lid material and the substrate surface 22 of the LT crystal 2b (the LT crystal located at the top of the embedding order) that faces the inner wall surface of the lid material in order to suppress color unevenness on the substrate surface 22 of the LT crystal 2b that faces the inner wall surface of the lid material.

[0048] Furthermore, when multiple LT crystals 2 are arranged horizontally (see Figure 7) and heat-treated, in order to suppress color unevenness on the substrate surfaces 21 of the LT crystals 2a and 22 of the LT crystals 2b located at both ends of the heat-resistant container 10 in the horizontal direction, dummy substrates are placed between the inner wall surface 10a of the heat-resistant container 10 and the substrate surface 21 of the LT crystal 2a facing the inner wall surface 10a, and between the inner wall surface 10a of the heat-resistant container 10 and the substrate surface 22 of the LT crystal 2b facing the inner wall surface 10a.

[0049] (2-2) Second embodiment Next, a method for manufacturing an LT substrate according to the second embodiment will be specifically described.

[0050] An appropriate amount of mixed powder 1 of Al powder and Al2O3 powder is placed on the inner wall surface 10a on the bottom side of a heat-resistant container 10 made of stainless steel or the like, and a dummy substrate 50 shown in Figure 3, which is not to be reduced, is placed on this mixed powder 1. The size of the dummy substrate 50 is set to be approximately equal to the diameter of the LT crystal 2.

[0051] Next, a mixed powder 1 of Al powder and Al2O3 powder and LT crystals 2 to be reduced that have been processed into a substrate state (i.e., processed into a flat plate shape having substrate surfaces 21 and 22) are alternately placed into a heat-resistant container 10 in which a dummy substrate 50 has been placed, and with the mixed powder 1 interposed between the LT crystals 2, for example, about 20 to 30 LT crystals 2 are embedded in the mixed powder 1, and then, as in the conventional method, the open part of the heat-resistant container 10 is closed with an appropriate lid material (not shown).

[0052] Each of the LT crystals 2 to be reduced is beveled as shown in FIG. 3, but the dummy substrate 50 is not beveled to differentiate it from the LT crystals 2.

[0053] Then, the heat-resistant containers 10 in which the LT crystals 2 are embedded in the mixed powder 1 are placed in a heating furnace (not shown), and the LT crystals 2 are reduced by heat-treating them at a temperature below the Curie temperature of the LT crystals while continuously flowing an inert gas into the heating furnace under atmospheric pressure.

[0054] In the method for manufacturing an LT substrate according to the second embodiment, a dummy substrate 50 that is not to be reduced is placed on the inner wall surface 10a on the bottom side of the heat-resistant container 10 via the mixed powder 1. This blocks the involvement of eluted components that have adhered near the center of the inner wall surface 10a on the bottom side of the heat-resistant container 10, thereby making it possible to suppress color unevenness in the center of the substrate surface 21 of the LT crystal 2a that is closest to the inner wall surface 10a on the bottom side of the heat-resistant container 10.

[0055] (3) Dummy board The dummy substrate is not particularly limited as long as it is made of a material that does not undergo chemical change during the reduction treatment, and is preferably made of, for example, stainless steel or the same material as the LT substrate or LN substrate. In particular, if the dummy substrate is made of the same material as the LT substrate or LN substrate to be reduced, it is more preferable because the dummy substrate undergoes little chemical change during the reduction treatment even when used repeatedly.

[0056] When the dummy substrate is to be used repeatedly, it is desirable to clean the dummy substrate before reusing it.

[0057] Furthermore, it is advisable to differentiate the dummy substrate so that it can be distinguished from the LT or LN substrates to be reduced. To differentiate the dummy substrate from the LT or LN substrates to be reduced, it is desirable to make the shape of the dummy substrate visually distinguishable by changing the number of oriental flats (OF) or notches, changing the positions of these, or by not subjecting the dummy substrate to peripheral bevel processing.

[0058] For example, as shown in Fig. 1, if the diameter of the dummy substrate is made larger than that of the LT substrate 2, or if the size of the dummy substrate is approximately the same as that of the LT substrate 2, while the LT substrate 2 to be reduced has one oriental flat (OF) as shown in Fig. 4, the dummy substrate 50 can have two oriental flats (OF) as shown in Fig. 5, and the dummy substrate can be differentiated from the LT substrate or LN substrate by changing the installation position of the OF or by whether or not it is chamfered. Such differentiation can be achieved by using shape, color, tactile or other marks that can be distinguished by the worker, as appropriate. [Example]

[0059] Examples of the present invention will be specifically described below with reference to comparative examples, but the technical scope of the present invention is not limited to the following examples in any way.

[0060] [Heating furnace configuration] The heating furnaces used in the examples and comparative examples were provided with an air inlet and an exhaust port.

[0061] The stainless steel heat-resistant container placed in the heating furnace was filled with a mixture of Al powder and Al2O3 powder, and was also filled with commercially available argon gas (oxygen partial pressure 1×10 -6 At the same time, argon gas (an inert gas) is continuously exhausted from the furnace through the exhaust port, maintaining the interior of the furnace at atmospheric pressure. The flow rate of argon gas supplied to and exhausted from the furnace is set to 2 L / min.

[0062] [LT crystal growth and ingot processing, etc.] Using raw materials with a congruent composition, LT single crystals with a diameter of 4 inches were grown by the Czochralski method. The growth atmosphere was a nitrogen-oxygen mixed gas with an oxygen concentration of approximately 3%. The resulting LT crystal ingot was transparent and pale yellow in color.

[0063] The LT crystal ingot was subjected to heat treatment to remove thermal distortion and poling to create a single polarization, and then the substrate was processed by surface grinding, cylindrical machining, slicing, and beveling to create a 42°RY (Rotated Y axis) substrate.

[0064] The obtained 42°RY LT crystal was colorless and transparent, and the volume resistivity was 1×10 15 (Ω·cm), and the Curie temperature was 603°C.

[0065] [Example 1] A heat-resistant container (cylindrical container) 10 made of stainless steel and having an inner diameter of φ110 mm as shown in Fig. 1 was prepared, and a single dummy substrate 40 having a size of φ108 mm and made of LT was placed directly on the inner wall surface 10a on the bottom side of this heat-resistant container 10. Note that the dummy substrate 40 was not beveled on the outer periphery in order to differentiate it from the LT substrate.

[0066] Then, into the heat-resistant container 10 with the dummy substrate 40 placed therein, a mixed powder 1 of 10 wt % Al powder and 90 wt % Al2O3 powder, and LT crystals 2 with a diameter of φ100 mm and beveled outer periphery, were alternately placed with a 5 mm gap between the LT crystals 2, and after 25 LT crystals 2 were embedded in the mixed powder 1, the open part of the heat-resistant container 10 was closed with a lid.

[0067] Next, the heat-resistant container 10 in which 25 LT crystals 2 were embedded in the mixed powder 1 of Al powder and Al2O3 powder was placed in the above-mentioned heating furnace, and argon gas (oxygen partial pressure 1 × 10 -6 Atm) was supplied into the heating furnace.

[0068] Then, argon gas was continuously supplied and discharged at a flow rate of 2 L / min into a heating furnace under atmospheric pressure, and heat treatment (reduction treatment) was carried out at 580° C. for 20 hours.

[0069] Furthermore, the same heat-resistant container 10 and the same dummy substrate 40 were reused, and the heat treatment (reduction treatment) of the LT crystal 2 was repeated 20 times under the same conditions described above.

[0070] In this embodiment, the LT crystal with the outer periphery beveled is heat-treated (reduced) to differentiate it from the dummy substrate 40 and prevent chipping of the end face, but it goes without saying that the LT crystal without the outer periphery beveled may also be heat-treated (reduced).

[0071] [confirmation] (1) Color unevenness in the center of the LT board A visual inspection was conducted on a total of 500 LT substrates, consisting of 25 substrates (one reduction treatment) x 20 times, and no color unevenness was found in the center of the substrate surfaces 21 and 22 of any of the LT crystals 2, including the LT crystal 2a closest to the inner wall surface 10a on the bottom side of the heat-resistant container 10.

[0072] (2) Volume resistivity of LT substrate The volume resistivity of a total of 500 LT substrates was measured by the three-terminal method in accordance with JIS K-6911, with 25 substrates (one reduction treatment per substrate) being performed 20 times.

[0073] As a result, the volume resistivity of all LT substrates is within a given range [10 10 (Ω·cm)~10 11 (Ω·cm)]. [Comparative Example 1] The same procedure as in Example 1 was carried out 20 times (25 substrates per reduction treatment), except that no dummy substrate 40 was placed on the inner wall surface 10a on the bottom side of the heat-resistant container (cylindrical container) 10 made of stainless steel.

[0074] That is, a mixed powder 1 of 10 wt % Al powder and 90 wt % Al2O3 powder and LT crystals 2 with a diameter of φ100 mm and beveled outer periphery were alternately placed into a heat-resistant container (cylindrical container) 10 made of stainless steel and having an inner diameter of φ110 mm as shown in Figure 6, with 5 mm intervals between the LT crystals 2, and after 25 LT crystals 2 were embedded in the mixed powder 1, the open part of the heat-resistant container 10 was closed with a lid.

[0075] Next, the heat-resistant container 10 in which 25 LT crystals 2 were embedded in the mixed powder 1 of Al powder and Al2O3 powder was placed in the above-mentioned heating furnace, and argon gas (oxygen partial pressure 1 × 10 -6 Atm) was supplied into the heating furnace.

[0076] Then, argon gas was continuously supplied and discharged at a flow rate of 2 L / min into a heating furnace under atmospheric pressure, and heat treatment (reduction treatment) was carried out at 580° C. for 20 hours.

[0077] Furthermore, the same heat-resistant container 10 was reused, and the heat treatment (reduction treatment) of the LT crystal 2 was repeated 20 times under the same conditions as above.

[0078] [confirmation] (1) Color unevenness in the center of the LT board A visual inspection was conducted on a total of 500 LT substrates, consisting of 25 substrates (one reduction treatment) x 20 times, and color unevenness was confirmed in the center of the substrate surface of the five LT crystals 2a closest to the inner wall surface 10a on the bottom side of the heat-resistant container 10.

[0079] (2) Volume resistivity of LT substrate The volume resistivity was measured for a total of 500 LT substrates, 25 substrates (one reduction treatment) x 20 times. As in Example 1, the volume resistivity of all LT substrates 2 was within the predetermined range [10 10 (Ω·cm)~10 11 (Ω·cm)]. [Explanation of symbols]

[0080] OF Oriental Flat 1. Mixture of aluminum powder (Al powder) and aluminum oxide powder (Al2O3 powder) 2 LT crystal 21 Board surface 22 Board surface 2a LT crystal (LT substrate) 2b LT crystal (LT substrate) 10 Heat-resistant container 10a Inner wall surface 40 Dummy board 50 Dummy board 30 Outer edge 31 Crystal holding stand 32 Chamfering stone

Claims

1. A method for producing an oxide single crystal substrate, comprising embedding a plurality of flat oxide single crystals having substrate surfaces in a powder containing a powdered reducing agent filled in a heat-resistant container with the powder interposed therebetween, and heat-treating the resulting oxide single crystals at a temperature lower than the Curie temperature of the oxide single crystals, A method for producing an oxide single crystal substrate, characterized in that a dummy substrate is placed between the inner wall surface and the substrate surface of the oxide single crystal, the substrate surface facing the inner wall surface of the heat-resistant container via the powder.

2. 2. The method for producing an oxide single crystal substrate according to claim 1, wherein the powder containing the powdery reducing agent is a mixture of aluminum powder and aluminum oxide powder.

3. 3. The method for producing an oxide single crystal substrate according to claim 1, wherein the heat-resistant container is made of stainless steel.

4. 2. The method for producing an oxide single crystal substrate according to claim 1, wherein the dummy substrate is made of the same material as the oxide single crystal.

5. 2. The method for producing an oxide single crystal substrate according to claim 1, wherein the dummy substrate and the oxide single crystal have a circular shape, and the dummy substrate has a larger diameter than the oxide single crystal.

6. The method for manufacturing an oxide single crystal substrate according to claim 1, wherein the dummy substrate is differentiated so as to be distinguishable from the oxide single crystal.

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