Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/16—Oxides
  • C30B29/22—Complex oxides
  • C30B29/30—Niobates; Vanadates; Tantalates
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
  • C30B15/02—Single-crystal growth by pulling from a melt, e.g. Czochralski method adding crystallising materials or reactants forming it in situ to the melt
  • C30B15/04—Single-crystal growth by pulling from a melt, e.g. Czochralski method adding crystallising materials or reactants forming it in situ to the melt adding doping materials, e.g. for n-p-junction
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
  • C30B15/20—Controlling or regulating
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/16—Oxides
  • C30B29/20—Aluminium oxides
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
  • C30B33/02—Heat treatment
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02535—Details of surface acoustic wave devices
  • H03H9/02614—Treatment of substrates, e.g. curved, spherical, cylindrical substrates ensuring closed round-about circuits for the acoustical waves
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3451—Structure
  • H10P14/3452—Microstructure
  • H10P14/3458—Monocrystalline
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P95/00—Generic processes or apparatus for manufacture or treatments not covered by the other groups of this subclass
  • H10P95/90—Thermal treatments, e.g. annealing or sintering

Definitions

• the present invention relates to a lithium niobate single crystal substrate used for a surface acoustic wave device or the like, and more particularly to an improvement in a lithium niobate single crystal substrate in which a yield reduction in an element manufacturing process hardly occurs and a method for manufacturing the same.
• a lithium niobate (LiNbO 3 ; hereinafter abbreviated as LN) single crystal is an artificial ferroelectric crystal having a melting point of about 1250 ° C. and a Curie temperature of about 1140 ° C.
• LN substrate a lithium niobate single crystal substrate obtained from an LN single crystal is mainly used for a surface acoustic wave element (SAW filter) for removing electric signal noise used in mobile communication equipment.
• SAW filter surface acoustic wave element
• the SAW filter has a structure in which a comb-shaped electrode is formed of a metal thin film such as an AlCu alloy on a substrate made of a piezoelectric material such as an LN single crystal, and this comb-shaped electrode has an important effect on the characteristics of the device. Have a role to play.
• the comb-shaped electrode is formed by forming a metal thin film on a piezoelectric material by sputtering or the like, and then removing unnecessary portions by etching using a photolithographic technique while leaving a comb-shaped pattern.
• LN single crystals as materials for SAW filters are industrially mainly produced by the Choral Ski method, usually using a platinum crucible, and in an electric furnace in a nitrogen-oxygen mixed gas atmosphere with an oxygen concentration of about 20%. After being grown and cooled at a predetermined cooling rate in the electric furnace, it is taken out from the electric furnace.
• the grown LN single crystal is colorless and transparent or has a light yellow color with high transparency.
• a heat treatment is performed under a soaking temperature close to the melting point, and further a poling treatment for making a single polarization, that is, the LN single crystal from room temperature to the Curie temperature.
• the temperature is raised to a temperature, a voltage is applied to the single crystal, the temperature is lowered to a predetermined temperature lower than the Curie temperature while the voltage is applied, and then a series of processes of stopping the voltage application and cooling to room temperature is performed.
• the LN single crystal (ingot) ground to grind the outer shape of the single crystal becomes an LN substrate through machining such as slicing, lapping and polishing.
• the finally obtained substrate is almost colorless and transparent, and the volume resistivity is about 1 ⁇ 10 15 ⁇ ⁇ cm or more.
• pyroelectric breakdown becomes a problem in the SAW filter manufacturing process.
• Pyroelectric breakdown is the characteristic of LN single crystal, which is pyroelectric, so that the charge is charged up on the surface of the LN substrate due to the temperature change received in the process, and the spark generated thereby forms on the surface of the LN substrate. This is a phenomenon in which the comb-shaped electrode is broken and the crack of the LN substrate occurs. Pyroelectric breakdown is a major factor in yield reduction in the element manufacturing process.
• the high light transmittance of the substrate causes a problem that the light transmitted through the substrate in the photolithography process, which is one of the element manufacturing processes, is reflected on the back surface of the substrate and returns to the front surface, thereby degrading the resolution of the formation pattern.
• the LN substrate is selected from argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen, and combinations thereof within the range of 500 to 1140 ° C.
• a method of reducing the resistance value of the LN substrate and reducing the pyroelectricity by exposing it to a chemically reducing atmosphere such as the generated gas and making it black.
• the heat treatment By applying the heat treatment, the LN crystals that are colorless and transparent become colored and opaque. Since the observed color tone of the color opacification appears to be brown to black in the transmitted light, this color opacification phenomenon is referred to as “blackening” here.
• the blackening phenomenon is considered to be due to the formation of a color center by introducing oxygen defects (vacancies) into the LN substrate by reduction treatment.
• the change in the resistance value compensates for the charge balance deviation due to the generation of oxygen defects, because the number of free electrons released from the Nb ions in the substrate increases as the valence of the Nb ions changes from +5 to +4. it is conceivable that. Therefore, the degree of blackening and the resistance value are in a proportional relationship.
• Patent Document 1 heats the LN substrate (crystal) to a high temperature of 500 ° C. or higher, so that the processing time is short, but the blackening tends to vary between the processing batches.
• color unevenness due to blackening that is, in-plane distribution of resistivity is likely to occur in the substrate, and the yield reduction in the element manufacturing process cannot be sufficiently prevented.
• the LN substrate (crystal) is composed of at least one element selected from the group consisting of Al, Ti, Si, Ca, Mg, and C.
• a method of heat treatment at a low temperature of 300 ° C. or higher and lower than 500 ° C. in a state of being embedded in a powder has been proposed.
• An object of the present invention is to provide an LN substrate having a very small in-plane value distribution and a method for manufacturing the same.
• the present inventor has grown an LN single crystal containing a small amount of Fe and attempted a reduction experiment thereof.
• the LN crystal (substrate) containing Fe does not contain Fe. It has been confirmed that the properties are less likely to be reduced compared to the LN crystal (substrate), and further, the reduction of the LN crystal (substrate) is less likely to be substantially proportional as the Fe content in the crystal increases. I came to find out.
• the present invention has been completed based on such findings.
• the first invention according to the present invention is: A lithium niobate single crystal substrate whose volume resistivity is controlled in the range of 1 ⁇ 10 8 ⁇ ⁇ cm to 1 ⁇ 10 10 ⁇ ⁇ cm, wherein the Fe concentration in the lithium niobate single crystal is 50 mass ppm Above, it is characterized by being 1000 ppm by mass or less,
• the second invention according to the present invention is: In a method for producing a lithium niobate single crystal substrate using a lithium niobate single crystal grown by the chocolate ski method, A lithium niobate single crystal whose Fe concentration in the single crystal is 50 mass ppm or more and 1000 mass ppm or less and processed into a substrate state is embedded in Al powder or a mixed powder of Al and Al 2 O 3 and 450 ° C.
• the third invention according to the present invention is: In the method for producing a lithium niobate single crystal substrate according to the second invention, The arithmetic average roughness Ra of the surface of the lithium niobate single crystal processed into the state of the substrate is 0.2 ⁇ m or more and 0.4 ⁇ m or less,
• the fourth invention is: In the method for producing a lithium niobate single crystal substrate according to the second invention or the third invention, The heat treatment is performed in a vacuum atmosphere or an inert gas decompression atmosphere, In addition, the fifth invention, In the method for producing a lithium niobate single crystal substrate according to the second invention, the third invention or the fourth invention, The heat treatment is performed for 1 hour or more.
• the volume resistivity after the reduction treatment is in the range of 1 ⁇ 10 8 ⁇ ⁇ cm to 1 ⁇ 10 10 ⁇ ⁇ cm. Is controlled stably.
• a lithium niobate single crystal whose Fe concentration in the single crystal is 50 mass ppm or more and 1000 mass ppm or less and processed into a substrate state is embedded in Al powder or a mixed powder of Al and Al 2 O 3 , and 450 ° C. or more.
• Stable lithium niobate single crystal substrate whose volume resistivity after reduction treatment is controlled in the range of 1 ⁇ 10 8 ⁇ ⁇ cm or more and 1 ⁇ 10 10 ⁇ ⁇ cm or less and in-plane distribution of volume resistivity is small. It can be manufactured.
• volume resistivity and pyroelectric effect (pyroelectricity) of LN single crystal The LN single crystal changes in volume resistivity and color (light transmittance spectrum) depending on the concentration of oxygen defects present in the crystal.
• the valence of some Nb ions changes from 5+ to 4+ due to the need to compensate for the charge balance due to deficiency of -2 oxygen ions, and the volume resistivity is increased. Make a change.
• light absorption is caused by the generation of color centers due to oxygen defects.
• the change in volume resistivity is considered to occur because electrons as carriers move between Nb 5+ ions and Nb 4+ ions.
• the volume resistivity of a crystal is determined by the product of the number of carriers per unit volume and the mobility of carriers. If the mobility is the same, the volume resistivity is proportional to the number of oxygen vacancies.
• the color change due to light absorption is considered to be caused by a color center formed due to metastable electrons trapped in oxygen defects.
• the control of the number of oxygen vacancies can be performed by a so-called atmosphere heat treatment.
• the concentration of oxygen vacancies in a crystal at a specific temperature changes so as to balance with the oxygen potential (oxygen concentration) of the atmosphere in which the crystal is placed. If the oxygen concentration in the atmosphere is lower than the equilibrium concentration, the oxygen vacancy concentration in the crystal increases. Also, by increasing the temperature while keeping the oxygen concentration in the atmosphere constant, the oxygen vacancy concentration increases even if the oxygen concentration in the atmosphere is lower than the equilibrium concentration. Therefore, in order to increase the oxygen vacancy concentration and increase the opacity, the oxygen concentration in the atmosphere should be lowered at a high temperature.
• the pyroelectric effect is caused by lattice deformation caused by a change in crystal temperature. In crystals with electric dipoles, it can be understood that this occurs because the distance between the dipoles varies with temperature.
• the pyroelectric effect occurs only with materials with high electrical resistance. Due to the displacement of ions, a charge is generated in the dipole direction on the surface of the crystal. However, in a material having a low electrical resistance, this charge is neutralized due to the electrical conductivity of the crystal itself.
• the normal transparent LN single crystal has a volume resistivity of 1 ⁇ 10 15 ⁇ ⁇ cm, so that the pyroelectric effect appears remarkably. However, in the blackened opaque LN single crystal, its volume resistivity is significantly reduced by several orders of magnitude, so that the charge is neutralized in a very short time and pyroelectricity is not observed.
• Patent Document 2 that solves the problem of Patent Document 1 has a low processing temperature of less than 500 ° C., so that there is little color unevenness due to blackening, that is, the in-plane distribution of volume resistivity. This is a method that makes it possible to provide an LN substrate with reduced electrical properties.
• the LN single crystal (melting point: 1250 ° C.) is more easily reduced than the lithium tantalate single crystal (LT single crystal) having a melting point of 1650 ° C.
• the method described in Patent Document 2 requires strict management of processing conditions, and there is room for improvement accordingly.
• the reducibility (easy to be reduced) of the LN single crystal is lowered (becomes difficult to reduce) by containing a small amount of Fe in the crystal, and the Fe content is reduced. As the amount increases, the reducibility of the LN single crystal further decreases.
• the method for producing an LN substrate according to the present invention is made by paying attention to the reducibility of the LN single crystal, and the Fe concentration in the single crystal is 50 mass ppm or more and 1000 mass ppm or less, and the reducing atmosphere.
• an LN substrate whose volume resistivity is controlled to 1 ⁇ 10 8 ⁇ ⁇ cm or more and 1 ⁇ 10 10 ⁇ ⁇ cm or less is obtained by heat-treating the LN single crystal (LN substrate) processed into the substrate state. It is characterized by manufacturing.
• the LN substrate to be heat-treated is preferably a substrate obtained by processing the LN single crystal after the poling process.
• the heat treatment of the LN substrate containing a small amount of Fe is performed using a metal Al powder having a low free energy for oxide formation in order to promote the reduction reaction at a low temperature.
• a heat treatment is performed by embedding an LN substrate containing a small amount of Fe in a mixed powder of metal Al powder or metal Al powder and Al 2 O 3 powder.
• the heating temperature of the LN substrate is 450 ° C. or higher and lower than 500 ° C.
• preferable temperature is the range of 480 to less than 500 degreeC.
• the atmosphere for the heat treatment is preferably a vacuum or an inert gas (nitrogen gas, argon gas, etc.), and the treatment time is preferably 1 hour or longer. Furthermore, in order to increase the surface area of the LN substrate and promote the reduction reaction, the arithmetic average roughness Ra of the LN substrate surface is preferably set to 0.2 ⁇ m or more and 0.4 ⁇ m or less.
• an LN substrate cut out from the LN crystal ingot after the poling process is used. It is effective to embed the LN substrate in a mixed powder of Al and Al 2 O 3 and perform heat treatment in an inert gas atmosphere such as nitrogen gas or argon gas or in an atmosphere such as vacuum. Note that the vacuum atmosphere is preferable to the black atmosphere of the LN substrate in a relatively short time than the inert gas atmosphere.
• the surface potential of the LN substrate is at most several hundreds V, and no sparking phenomenon is observed on the LN substrate surface. Therefore, the determination of the presence or absence of blackening is useful as a practical determination method for pyroelectricity.
• Example 1 Using a raw material having a congruent composition, an Fe-added LN single crystal having a diameter of 4 inches was grown by the chocolate ski method.
• the growing atmosphere is a nitrogen-oxygen mixed gas having an oxygen concentration of about 20%.
• the Fe addition concentration in the crystal was 50 ppm.
• the obtained crystal was red.
• the crystal was subjected to a heat treatment for removing residual thermal strain and a poling treatment for single polarization under soaking, and then peripherally ground and sliced to prepare an LN substrate for adjusting the crystal shape.
• the obtained LN substrate was embedded in Al powder and heat-treated at 495 ° C. for 20 hours in a vacuum atmosphere.
• the LN substrate after the heat treatment was dark green brown, the volume resistivity was about 1 ⁇ 10 8 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• Ave is the average value of volume resistivity measured in 5 in-plane measurements at 1 point at the center of the substrate and 4 points at the outer periphery, and ⁇ is their standard deviation.
• the volume resistivity was measured by a three-terminal method in accordance with JIS K-6911.
• the Curie temperature of the obtained LN substrate was 1140 ° C., and the physical property values affecting the characteristics of the SAW filter were not different from those of the conventional product not subjected to blackening treatment.
• Example 2 The heat treatment was performed under substantially the same conditions as in Example 1 except that the heat treatment temperature was 450 ° C.
• the obtained LN substrate was dark green brown, the volume resistivity was about 1 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 60 V or less, and no sparking phenomenon was observed on the LN substrate surface.
• Example 3 Heat treatment was performed under substantially the same conditions as in Example 1 except that the atmosphere was a nitrogen gas atmosphere.
• the obtained LN substrate was dark green brown, the volume resistivity was about 1.5 ⁇ 10 8 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 20 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 4 The heat treatment was performed under substantially the same conditions as in Example 1 except that the heat treatment time of the LN substrate was 1 hour.
• the obtained LN substrate was brown, the volume resistivity was about 1 ⁇ 10 10 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) within the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 100 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 5 Heat treatment was performed under substantially the same conditions as in Example 1 except that the Fe addition concentration in the LN crystal was 1000 ppm.
• the obtained LN substrate was dark green brown, the volume resistivity was about 5 ⁇ 10 8 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 40 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 6 Heat treatment was performed under substantially the same conditions as in Example 2 except that the Fe addition concentration in the LN crystal was 1000 ppm.
• the obtained LN substrate was dark green brown, the volume resistivity was about 5 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 90 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 7 Heat treatment was performed under substantially the same conditions as in Example 3 except that the Fe concentration in the LN single crystal was 1000 ppm.
• the obtained LN substrate was dark green brown, the volume resistivity was about 7.7 ⁇ 10 8 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) within the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 50 V or less, and no sparking phenomenon was observed on the LN substrate surface.
• Example 8 The heat treatment was performed under substantially the same conditions as in Example 7 except that the treatment temperature of the LN substrate was set to 450 ° C.
• the obtained LN substrate was dark green brown, the volume resistivity was about 7.7 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 100 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 9 Heat treatment was performed under substantially the same conditions as in Example 1 except that the LN substrate was embedded in a mixed powder of 10 mass% Al and 90 mass% Al 2 O 3 .
• the obtained LN substrate was dark green brown, the volume resistivity was about 1 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 60 V or less, and no sparking phenomenon was observed on the LN substrate surface.
• Example 10 Heat treatment was performed under substantially the same conditions as in Example 2 except that the LN substrate was embedded in a mixed powder of 10% by mass of Al and 90% by mass of Al 2 O 3 .
• the obtained LN substrate was dark green brown, the volume resistivity was about 1 ⁇ 10 10 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 100 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 11 Heat treatment was performed under substantially the same conditions as in Example 5 except that the LN substrate was embedded in a mixed powder of 10% by mass of Al and 90% by mass of Al 2 O 3 .
• the obtained LN substrate was dark green brown, the volume resistivity was about 5 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 90 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• Example 12 Heat treatment was performed under substantially the same conditions as in Example 3 except that the LN substrate was embedded in a mixed powder of 10% by mass of Al and 90% by mass of Al 2 O 3 .
• the obtained LN substrate was dark green brown, the volume resistivity was about 1.5 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 60 V or less, and no sparking phenomenon was observed on the LN substrate surface.
• Example 13 Heat treatment was performed under substantially the same conditions as in Example 7 except that the LN substrate was embedded in a mixed powder of 10 mass% Al and 90 mass% Al 2 O 3 .
• the obtained LN substrate was dark green brown, the volume resistivity was about 7.7 ⁇ 10 9 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface was less than 3%. Even in visual observation, color unevenness did not occur.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 100 V or less, and no phenomenon of sparking on the surface of the LN substrate was observed.
• This crystal was subjected to a heat treatment for removing residual strain and a poling process for single polarization under uniform temperature, and then peripherally ground and sliced to prepare a crystal shape.
• the obtained LN substrate was heat-treated at 800 ° C. for 1 minute in nitrogen.
• the LN substrate after the heat treatment was black, but color unevenness was observed by visual observation.
• the average volume resistivity measured value in the LN substrate surface was about 1 ⁇ 10 9 ⁇ ⁇ cm, but it varies depending on the measurement location ( ⁇ / Ave) About 30%.
• the surface potential generated at the moment when the LN substrate was placed on the hot plate was 60 V or less, and no sparking phenomenon was observed on the LN substrate surface.
• Comparative Example 2 A transparent light yellow LN single crystal having a diameter of 4 inches was grown in the same manner as in Comparative Example 1, and an LN substrate was manufactured in the same manner as in Comparative Example 1.
• the obtained LN substrate was embedded in aluminum (Al) powder and heat-treated at 480 ° C. for 20 hours in a nitrogen gas atmosphere.
• the LN substrate after the heat treatment is black, the volume resistivity is about 1 ⁇ 10 8 ⁇ ⁇ cm, and the variation in volume resistivity ( ⁇ / Ave) in the substrate surface is less than 3% as in this example. It was. Even in visual observation, color unevenness did not occur.
• the volume resistivity after the reduction treatment is controlled in the range of 1 ⁇ 10 8 ⁇ ⁇ cm or more and 1 ⁇ 10 10 ⁇ ⁇ cm or less, and an LN substrate having a small volume resistivity in-plane distribution is stabilized. Can be obtained. For this reason, it has the industrial applicability applied to the material for a surface acoustic wave element (SAW filter).
• SAW filter surface acoustic wave element

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