Method of producing nitride ceramic substrate

Abstract

To provide a method of producing a nitride ceramic substrate, capable of more effectively preventing color unevenness from occurring on the surface thereof, relative to a conventional method.SOLUTION: There is provided a method of producing a nitride ceramic substrate using a sheet-shaped molded article as a precursor, comprising an assembling step of assembling an assembly 1 in which heating containers 10 each containing a laminate 2 of the plurality of molded articles are stacked in a plurality of stages, and a nitriding step of nitriding the molded article by a heat treatment in a nitrogen atmosphere subsequent to the assembling step. The heating container 10 includes a plate material 101 whereon the laminate 2 is to be placed, the assembly 1 includes the heating containers 10 stacked in a plurality of stages via a heat conductive material 11 at least in a part, and the heat conductive material 11 has higher heat conductivity relative to the plate material 101.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a nitride ceramic substrate. [Background technology]

[0002] Conventionally, there is known a method for manufacturing a silicon nitride substrate in which a plurality of stacked sheet-like molded bodies are nitrided to obtain a silicon nitride substrate, and the method reduces color unevenness that occurs on the surface of the silicon nitride substrate (see Patent Document 1).

[0003] According to the method described in Patent Document 1, color unevenness occurring on the surface of a silicon nitride substrate is reduced by controlling the heating conditions for the nitriding treatment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-123942 Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the method described in Patent Document 1, if the number of stacked sheet-like bodies is increased or the temperature rise rate during nitriding treatment is increased in order to improve the productivity of silicon nitride substrates, it becomes impossible to suppress the occurrence of color unevenness on the surface of the silicon nitride substrate. Therefore, a method that can more effectively suppress the occurrence of color unevenness on the surface is desired.

[0006] An object of the present invention is to provide a method for producing a nitride ceramic substrate that can more effectively prevent the occurrence of color unevenness on the surface than conventional methods. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a nitride ceramic substrate, which uses a sheet-shaped molded body as a precursor to manufacture a nitride ceramic substrate, comprising: an assembly process for assembling an assembly in which heating containers containing stacks of a plurality of the molded bodies are stacked in multiple tiers; and, after the assembly process, a nitriding process for nitriding the molded bodies by heat treatment in a nitrogen atmosphere, wherein the heating container includes a plate material on which the stacks are placed, and the assembly is formed by stacking the heating containers in multiple tiers at least in part via a thermally conductive material, and the thermally conductive material has a higher thermal conductivity than the plate material. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for manufacturing a nitride ceramic substrate that can more effectively prevent the occurrence of color unevenness on the surface than conventional methods. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a vertical cross-sectional view of an assembly used in a method for manufacturing a nitride ceramic substrate according to an embodiment of the present invention. [Figure 2] FIG. 2 is an exploded perspective view of a portion of the assembly including one each of the heating enclosure and the thermally conductive material. [Figure 3] Figure 3(a) is a photograph of the appearance of a molded body after the nitriding process, which is manufactured using a manufacturing method for a nitride ceramic substrate according to an embodiment of the present invention, in which a thermally conductive material is included in the assembly, and which has reduced color unevenness on the surface. Figure 3(b) is a photograph of the appearance of a molded body after the nitriding process, which is manufactured using a conventional manufacturing method in which a thermally conductive material is not included in the assembly, and which has developed color unevenness on the surface. [Figure 4] FIG. 4 is a table showing the evaluation results of the effect of using a thermally conductive material on suppressing discoloration of a nitride substrate. DETAILED DESCRIPTION OF THE INVENTION

[0010] The method for manufacturing a nitride ceramic substrate according to an embodiment of the present invention is a method for manufacturing a nitride ceramic substrate using a sheet-shaped molded body as a precursor, and includes an assembly process for assembling an assembly 1 in which heating containers 10 containing stacks 2 of multiple sheet-shaped molded bodies are stacked in multiple stages, and a nitriding process for nitriding the molded body by heat treatment in a nitrogen atmosphere after the assembly process.

[0011] (Assembly structure) 1 is a vertical cross-sectional view of an assembly 1 used in a method for manufacturing a nitride ceramic substrate according to an embodiment of the present invention. The assembly 1 has a multi-stage structure in which a plurality of heating containers 10, each containing a laminate 2, are stacked with a thermally conductive material 11 interposed therebetween.

[0012] FIG. 2 is an exploded perspective view of a portion of the assembly 1, including one heating receptacle 10 and one heat conducting material 11. As shown in FIG.

[0013] As shown in Figures 1 and 2, the heating container 10 typically includes a plate 101, an annular frame 102 placed on the plate 101 and surrounding the periphery of the laminate 2, and a plate 103 placed on the frame 102 and covering the top of the laminate 2.

[0014] In the laminate 2, a plurality of sheet-like molded bodies (for example, 10 or more and 25 or less) are stacked, with intermediate layers provided between the molded bodies to prevent them from adhering to each other. That is, the laminate 2 is composed of an alternating laminate of sheet-like molded bodies and intermediate layers. The number of sheet-like molded bodies included in the laminate 2 is preferably 13 or more, and more preferably 15 or more. Since the thermally conductive material 11 is used in the assembly 1, discoloration of the nitride ceramic substrate can be suppressed even if the number of sheet-like molded bodies included in the laminate 2 is greater than when the thermally conductive material 11 is not used.

[0015] The sheet-like molded body constituting the laminate 2 is a precursor of the nitrided ceramic substrate and contains an element that bonds with nitrogen in the nitriding step. For example, if the nitrided ceramic substrate is a silicon nitride substrate, the molded body contains silicon. The intermediate layer is made of a material that can withstand the heat treatment temperatures in the nitriding step and the sintering step after the nitriding step and that has excellent releasability from the molded body, such as BN powder.

[0016] 1, it is preferable to place a weight plate 12 on the laminate 2 to prevent warping of the laminate 2. The weight plate 12 is made of a material that can withstand the temperatures of the heat treatments in the nitriding and sintering steps and that does not react with the compact, such as BN.

[0017] In the above-described nitriding step, a gap is preferably provided between the weight plate 13 and the plate material 103 above it to prevent a temperature rise in the laminate 2. The size of this gap (the distance between the weight plate 13 and the plate material 103 above it) is preferably 1 mm or more and 10 mm or less. By making the size of the gap 1 mm or more, it is possible to provide a space for supplying sufficient nitrogen for reaction with the compact, and to prevent contact between the weight plate 13 and the plate material 103 above it. Furthermore, by making the size of the gap 10 mm or less, it is possible to reduce the area occupied by a gas with low thermal conductivity (an atmosphere containing nitrogen), which makes it easier for heat to be conducted uniformly to the laminate 2.

[0018] Plate material 101 is a plate-like member that supports laminate 2, frame material 102, plate material 103, etc., which are placed thereon. Since laminate 2 is placed on plate material 101, plate material 101 is made of a material that can withstand the temperatures of the heat treatments in the nitriding step and sintering step and that has excellent releasability from the molded body, such as BN.

[0019] The frame material 102 is made of a material that can withstand the heat treatment temperatures in the nitriding and sintering processes, such as BN. In the example shown in Fig. 2, the outer shape of the frame material 102 is rectangular, but this is not limited thereto, and a shape can be appropriately selected according to, for example, the planar shape of the laminate 2.

[0020] The plate material 103 is a plate-shaped member that supports the heat conductive material 11 and other components placed thereon. The plate material 103 is made of a material that can withstand the temperatures of the heat treatments in the nitriding and sintering steps, such as BN.

[0021] The thermally conductive material 11 has a higher thermal conductivity than the plate material 101 and the plate material 103, and is used to efficiently release heat from the laminate 2 to the outside. The thermally conductive material 11 has, for example, a plate shape. It is preferable that the size of the thermally conductive material 11 in the planar direction is larger than that of the frame material 102 in order to prevent damage to the plate material 103 due to its weight.

[0022] The thermally conductive material 11 is made of a material that can withstand the heat treatment temperature in the nitriding step (for example, a heat-resistant temperature of 1400°C or higher (a melting point of 1400°C or higher)), has a low linear expansion coefficient, and has high thermal conductivity (for example, 5 W / m·K or higher), such as carbon (graphite), ceramics such as Si3N4, or a metal such as tungsten. Carbon is particularly preferred as a material for the thermally conductive material 11 because of its high heat-resistant temperature, low linear expansion coefficient, and high thermal conductivity. Carbon produced by extrusion molding is even more preferred because of its particularly low linear expansion coefficient. Note that the thermally conductive material 11 is used to prevent overheating of the molded body in the nitriding step, and is therefore usually removed from the assembly 1 in the subsequent sintering step. However, if the heat-resistant temperature of the thermally conductive material 11 is higher than the temperature in the sintering step, it does not need to be removed.

[0023] In the assembly process for assembling the assembly 1, the heating containers 10 are typically stacked one by one from the bottom up, but the assembly procedure is not limited to this.

[0024] The laminate 2 housed in each heating container 10 is sandwiched between thermally conductive materials 11 on the top and bottom, allowing heat to be effectively released to the outside, thereby preventing overheating (thermal runaway) due to reaction heat. By preventing overheating of the laminate 2, color unevenness on the surface of the nitride ceramic substrate obtained using the multiple sheet-like molded bodies contained in the laminate 2 as precursors can be reduced.

[0025] The color unevenness on the surface of the nitrided ceramic substrate is thought to occur because part of the sheet-like molded body melts due to overheating during the nitriding process, and remains on the surface of the nitrided ceramic substrate as a discolored area without being nitrided.

[0026] Fig. 3(a) is a photograph of the appearance of a molded body after the nitriding step, which has reduced color unevenness on the surface, manufactured using a manufacturing method for a nitride ceramic substrate according to an embodiment of the present invention, in which the thermally conductive material 11 is included in the assembly 1. Fig. 3(b) is a photograph of the appearance of a molded body after the nitriding step, which has developed color unevenness on the surface, manufactured using a conventional manufacturing method in which the thermally conductive material 11 is not included in the assembly 1. Note that the discoloration that occurs in the molded body after the nitriding step remains even after the molded body is sintered to become a silicon nitride substrate.

[0027] In addition, since the stack 2 of the heating container 10 placed at the top is sandwiched between thermally conductive materials 11 on the top and bottom, it is preferable to also place thermally conductive material 11 on the top of the uppermost heating container 10, as shown in Figure 1.

[0028] 1, thermally conductive materials 11 are typically placed at all positions between the stacked heating receptacles 10 in the assembly 1, but thermally conductive materials 11 may also be placed at some positions between the stacked heating receptacles 10. That is, in the assembly 1, the heating receptacles 10 are stacked in multiple stages with thermally conductive materials 11 having a higher thermal conductivity than the plate materials 101 at least in some places interposed therebetween.

[0029] For example, since color unevenness on the surface of the nitride ceramic substrate due to overheating of the laminate 2 is likely to occur near the center of the assembly 1, a thermal conductive material 11 may be placed between some of the heating containers 10 located near the center of the height of the assembly 1.

[0030] To obtain a sufficient heat dissipation effect, the thickness of the thermally conductive material 11 is preferably 2 mm or more, and more preferably 5 mm or more. Furthermore, to prevent the assembly 1 from becoming too large, the thickness of the thermally conductive material 11 is preferably 20 mm or less, and more preferably 10 mm. Furthermore, the thickness of the thermally conductive material 11 may vary depending on the position where it is placed.

[0031] For example, since color unevenness on the surface of the nitride ceramic substrate due to overheating of the laminate 2 is likely to occur near the center of the assembly 1, the thickness of the thermally conductive material 11 arranged near the center in the height direction of the assembly 1 may be made thicker than the thickness of the thermally conductive material 11 arranged in other positions. More specifically, for example, the thickness of the thermally conductive material 11 arranged closest to the center in the height direction of the assembly 1 is made the thickest.

[0032] When carbon is used as the material for the thermally conductive material 11, it has been confirmed that if the atmosphere contains a large amount of oxygen during the sintering process, part of the sheet-like compact contained in the laminate 2 will be carbonized. This is thought to be because oxygen in the atmosphere combines with carbon released from the thermally conductive material 11 to generate CO, which then reacts with the compact. For example, when a silicon nitride substrate is manufactured as the nitride ceramic substrate, part of the compact containing silicon is carbonized to produce silicon carbide.

[0033] To prevent carbonization of the compact, for example, the air in the furnace that heats the assembly 1 is removed with a pump, and then nitrogen gas for nitriding the compact is introduced into the furnace to reduce the oxygen content in the atmosphere. This method of removing the air from the furnace can be used when the sintering process is carried out in a batch furnace.

[0034] Furthermore, when the sintering process is performed in a continuous furnace, and it is not possible to evacuate the furnace, the oxygen in the atmosphere can be reduced by placing an oxygen-absorbing substance, such as Mg powder, near the assembly 1. In this case, the Mg powder is placed near the assembly 1 in a crucible, for example.

[0035] When carbon is used as the material for the thermally conductive material 11, by surrounding the top, bottom, and periphery of the laminate 2 with the plate material 101, the frame material 102, and the plate material 103, it is possible to prevent carbon released from the thermally conductive material 11 from adhering to the compact, thereby effectively preventing carbonization of the compact. However, if the space surrounded by the plate material 101, the frame material 102, and the plate material 103 is completely sealed, nitrogen for nitriding the compact cannot be introduced into the heating container 10. Therefore, it is preferable that a gap of 0.1 mm or less is formed, for example, between the plate material 101 and the frame material 102, or between the plate material 103 and the frame material 102. The gap between the plate material 101 (plate material 103) and the frame material 102 is formed, for example, by deflection of the plate material 101 and the plate material 103. If the gap is too large, carbon detached from the thermal conductive material 11 may adhere to the compact, or the magnesium added as a sintering aid may volatilize excessively, which may make it difficult for the sintering reaction to proceed during the sintering process.

[0036] (Method of manufacturing a molded body) Hereinafter, as an example of a molded body, a method for producing a silicon-containing sheet-like molded body, which is a precursor of a silicon nitride substrate as a nitride ceramic substrate, will be described in detail.

[0037] First, a raw material powder obtained by adding a rare earth element oxide and a magnesium compound as sintering aids to silicon powder is pulverized by a method such as media dispersion to prepare a slurry.

[0038] As the silicon powder, industrially available grade silicon powder can be used. The silicon powder before pulverization has a median diameter D50 of 6 μm or more and a BET specific surface area of ​​3 m 2 / g or less, oxygen content is 1.0 mass% or less, and the amount of C impurity in silicon is 0.15 mass% or less. The powder preferably has a median diameter D50 of 7 μm or more and a BET specific surface area of ​​2.5 m 2 More preferably, the powder has a SiO2 content of 0.1 / g or less, an oxygen content of 0.5 mass% or less, and a carbon impurity content of 0.10 mass% or less in silicon. The purity of the silicon powder is preferably 99% or more, and more preferably 99.5% or more. The impurity oxygen contained in silicon is one of the factors that inhibit the thermal conduction of the silicon nitride substrate obtained by reaction sintering, so it is preferable to keep it as low as possible. Furthermore, as described below, it is preferable to adjust the raw material powder by limiting the amount of oxygen from the magnesium compound so that the total amount of impurity oxygen contained in the silicon powder and the oxygen from the magnesium compound is in the range of 0.1 to 1.1 mass% relative to the silicon converted to silicon nitride. Furthermore, the impurity carbon contained in silicon inhibits the growth of silicon nitride particles in the silicon nitride substrate obtained by reaction sintering. As a result, insufficient densification occurs, which is one of the factors that reduces thermal conduction and insulation.

[0039] The above BET specific surface area (m 2 / g) is a value determined by the BET single-point method using a BET specific surface area meter (JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method"), and the median diameter D50 (μm) is the particle size at which the cumulative frequency is 50% in the particle size distribution determined by laser diffraction / scattering method.

[0040] The raw material powder may contain silicon nitride powder. However, since using silicon nitride increases costs, it is preferable to use as little silicon nitride as possible. The amount of silicon nitride used is preferably 20 mol % or less of silicon (silicon nitride equivalent), more preferably 10 mol % or less, and even more preferably 5 mol % or less.

[0041] The rare earth element oxide is preferably an oxide of Y, Yb, Gd, Er, Lu, or the like, which is readily available and stable as an oxide. Specific examples include Y2O3, Yb2O3, Gd2O3, Er2O3, and Lu2O3. The content of the rare earth element oxide is 0.5 mol% or more but less than 2 mol% based on the total of silicon (calculated as silicon nitride), rare earth element oxide (calculated as trivalent oxide), and magnesium compound (calculated as MgO). A rare earth element oxide content of less than 0.5 mol% is undesirable because its effect as a sintering aid is insufficient and the density does not increase sufficiently. A rare earth element oxide content of 2 mol% or more is undesirable because the increase in the low-thermal-conductivity grain boundary phase reduces the thermal conductivity of the sintered compact and increases the amount of expensive rare earth element oxide used. The content of the rare earth element oxide is preferably 0.6 mol% or more but less than 2 mol%, and more preferably 1 mol% or more but less than 1.8 mol%.

[0042] As the magnesium compound, one or more magnesium compounds containing Si, N, or O can be used. In particular, magnesium oxide (MgO), magnesium silicon nitride (MgSiN2), magnesium silicide (Mg2Si), magnesium nitride (Mg3N2), etc. are preferably used. Here, the magnesium compounds are selected so that 87 mass% or more of the total amount of MgSiN2 is MgSiN2. By using 87 mass% or more of MgSiN2, the oxygen concentration in the resulting silicon nitride substrate can be reduced. If the MgSiN2 content in the magnesium compound is less than 87 mass%, the amount of oxygen in the silicon nitride particles after sintering will be high, resulting in a low thermal conductivity of the sintered body, which is undesirable. The MgSiN2 content in the magnesium compound is preferably 90 mass% or more.

[0043] The content of magnesium compound in the silicon nitride substrate (moles converted to MgO) is 8 mol% or more and less than 15 mol% of the total of the moles of silicon nitride (Si3N4) obtained when all silicon is nitrided, the moles of rare earth element oxides converted to trivalent oxide RE2O3 (RE is a rare earth element), and the moles of magnesium compound converted to MgO. If the content of magnesium compound is less than 8 mol%, the effect as a sintering aid will be insufficient, resulting in an insufficient increase in density, which is undesirable. If the content of magnesium compound is 15 mol% or more, the increase in low thermal conductivity grain boundary phase will undesirably reduce the thermal conductivity of the sintered body. The content of magnesium compound is preferably 8 mol% or more and less than 14 mol%, and more preferably 9 mol% or more and less than 13 mol%.

[0044] A rare earth oxide and a magnesium compound are added to silicon powder as sintering aids in a predetermined ratio, followed by the addition of a dispersion medium (organic solvent) and, if necessary, a dispersant. The mixture is then pulverized in a ball mill to produce a slurry (a dispersion of the raw material powder). The media preferably has a diameter of 5 mm or more, and the concentration of the raw material powder in the slurry (also referred to as the slurry concentration) is 40% by mass or more. The pulverization is preferably carried out for 6 hours or more. It is preferable to use media made of a material that does not primarily contain Al or Fe, which can reduce the thermal conductivity of silicon nitride, and silicon nitride is particularly preferred. The types of dispersion medium and dispersant are not particularly limited and can be selected as desired depending on the sheet molding method, etc.

[0045] Examples of the dispersion medium that can be used include ethanol, n-butanol, toluene, MEK, and MIBK. Examples of the dispersant that can be used include sorbitan ester dispersants and polyoxyalkylene dispersants. The amount of the dispersion medium used is preferably 40 to 70% by mass, based on the total amount of the powder, and the amount of the dispersant used is preferably 0.3 to 2% by mass, based on the total amount of the powder. After dispersion, the dispersion medium may be removed or replaced with another dispersion medium, as necessary.

[0046] The milling time is not particularly limited, as it varies depending on the milling equipment used, the amount and characteristics of the starting materials, etc., but it is preferable to select a time that allows the raw material powder to be sufficiently milled and mixed. For example, the milling time is preferably 6 to 48 hours, and more preferably 12 to 24 hours. If the milling time is too short, sufficient milling may not be achieved, and milled silicon powder that meets the requirements of the present invention may not be obtained. If the milling time is too long, the amount of impurity oxygen may gradually increase, which may reduce the thermal conductivity of the silicon nitride substrate.

[0047] The silicon particles after milling preferably have an oxygen content of 1.0% by mass or less, more preferably 0.7% by mass or less. By minimizing the oxygen content of the silicon particles, the thermal conductivity of silicon nitride can be improved. Since it is difficult to measure the oxygen content of silicon particles alone after mixing with a sintering aid, a sample slurry containing only silicon particles without sintering aids can be prepared under the same milling conditions. This slurry can then be used to measure the oxygen content of the silicon particles. For example, silicon particles can be extracted from the slurry and the oxygen content of the silicon particles measured using an inert gas fusion-non-dispersive infrared absorption oxygen analyzer.

[0048] The BET specific surface area (m 2 The values ​​of the sieve density (D50 / g), median diameter D50 (μm), and oxygen content were measured using silicon particles pulverized in the same manner except that no rare earth element oxide or magnesium compound was added. Since only a small amount of rare earth element oxide powder and magnesium compound powder was added to the silicon powder, they had almost no effect on the pulverization efficiency, and the values ​​obtained in this manner are considered to be substantially the same as those of the silicon particles in the slurry.

[0049] To the resulting slurry, a dispersion medium, an organic binder, a dispersant, etc. are added as needed, and vacuum degassing is performed as needed to adjust the viscosity within a predetermined range, thereby preparing a coating slurry. The slurry viscosity is preferably adjusted to a range of 1 Pa·s or more and less than 15 Pa·s. The viscosity of the slurry is measured using a rotational viscometer at a temperature of 25°C and a rotation speed of 10 rpm. In some cases, the dispersion medium may be removed or replaced, as described above. The prepared coating slurry is formed into a sheet using a sheet forming machine, cut to a predetermined size, and dried to obtain a sheet-like molded product. The organic binder used to prepare the coating slurry is not particularly limited, but examples include PVB resin (polyvinyl butyral resin), ethyl cellulose resin, and acrylic resin. The amounts of the dispersion medium, organic binder, dispersant, etc. added are preferably adjusted appropriately depending on the coating conditions.

[0050] The method for forming the coating slurry into a sheet is not particularly limited, and sheet forming methods such as the doctor blade method and extrusion molding method can be used. In the method for producing a silicon nitride substrate of this embodiment, an altered layer is not formed and does not need to be removed, so the thickness of the altered layer does not need to be considered. For this reason, sheet forming, which can be easily formed into a thin sheet, is preferably used.

[0051] When forming the coating slurry into a sheet by the doctor blade method or extrusion molding method, the forming speed of the coating slurry is preferably 600 mm / min or less. Because the coating slurry used in the present invention is thixotropic, when the coating slurry passes through a doctor blade in the doctor blade method or a mold in the extrusion method, shear stress is applied to the coating slurry, causing a decrease in viscosity of the coating slurry. Therefore, if the forming speed exceeds 600 mm / min, the coating slurry will easily flow and will easily entrap bubbles that cause voids, which may hinder densification.

[0052] The drying rate of the coating slurry is preferably 0.8% by mass / min or less. If the drying rate of the coating slurry exceeds 0.8% by mass / min, the dispersion medium may volatilize rapidly, which may lead to the formation of bubbles in the sheet. The coated sheet is passed through a drying zone where the temperature is gradually increased and the sheet is dried, so the drying rate during the drying process is not constant but fluctuates. Therefore, it is preferable that the maximum drying rate does not exceed 0.8% by mass / min.

[0053] The thickness of the sheet-like molded body formed in the molding step is, for example, 0.15 to 0.8 mm. The obtained sheet-like molded body can be cut to a predetermined size, for example, by a punching machine, if necessary.

[0054] (Assembly process details) An example of a specific flow of the assembly process in the method for manufacturing a nitride ceramic substrate according to an embodiment of the present invention will be shown below.

[0055] First, a stack 2 of multiple sheet-like molded bodies and a weight plate 12 are placed on a plate 101 placed on a thermally conductive material 11. Next, a frame 102 is placed on the outer periphery of the plate 101 so as to surround the stack 2 and the weight plate 12. Next, a plate 103 is placed on the frame 102 so as to cover the stack 2 and the weight plate 12. Thereafter, the placement of the thermally conductive materials 11 to the plate 103 is repeated depending on the number of nitride ceramic substrates to be manufactured, thereby stacking the heating containers 10 in multiple layers with the thermally conductive materials 11 interposed between them. Finally, the thermally conductive material 11 is placed on the topmost plate 103, completing the assembly of the assembly 1.

[0056] (Details of the heat treatment process) After the assembly step, a heat treatment step is carried out in which the assembly 1 is heat treated in a furnace to produce a nitride ceramic substrate from the sheet-like compact.

[0057] The heat treatment process includes a degreasing process for removing an organic binder if present in the green body, a nitriding process for nitriding the green body by bonding nitrogen to the metal or semiconductor elements contained in the green body after the degreasing process, and a sintering process for sintering the green body to obtain a nitride ceramic substrate after the nitriding process. These processes may be performed sequentially in separate furnaces or may be performed continuously in the same furnace. Note that the components (e.g., plate material 101, frame material 102, and plate material 103) provided around the laminate 2, in which sheet-like green bodies are stacked, have different heat resistance temperature and other required conditions in the nitriding process and the sintering process, so components made of different materials can be used for the nitriding process and the sintering process.

[0058] A specific example of the heat treatment step when producing a silicon nitride substrate as a nitride ceramic substrate from a silicon-containing sheet-like molded body will be described below.

[0059] For example, the laminate 2, in which the produced sheet-like compacts are stacked with an intermediate layer sandwiched between them, is placed on a BN plate 101 and placed in an electric furnace. After removing organic binders and the like from the compacts (debinding step), the compacts are decarbonized at a temperature of 900 to 1300°C in a nitriding device, heated to a predetermined temperature in a nitrogen atmosphere to be nitrided (nitriding step), and then sintered in a sintering device (sintering step). At this time, it is preferable to heat the compacts while applying a load of 10 to 1000 Pa using a weight plate 12. The debinding step is preferably carried out at a temperature of 800°C or less.

[0060] It is preferable to use a boron nitride (BN) powder layer having a thickness of approximately 3 to 20 μm as the intermediate layer used in the laminate 2. The BN powder layer is intended to facilitate separation of the silicon nitride sintered body substrate after the sintering step, and can be formed by, for example, spraying, brushing, or screen printing a BN powder slurry onto one surface of each sheet-like compact. The BN powder preferably has a purity of 95% or higher and an average particle size (D50) of 1 to 20 μm.

[0061] In the nitriding step, the nitrogen partial pressure during nitriding is preferably 0.05 to 0.7 MPa, more preferably 0.07 to 0.2 MPa. The nitriding temperature is preferably 1350 to 1500°C, more preferably 1400 to 1450°C. The holding time after heating to the nitriding temperature is preferably 3 to 12 hours, more preferably 5 to 10 hours. If the nitriding temperature is less than 1350°C or the holding time is less than 3 hours, unreacted silicon powder may remain in the sheet-shaped compact, making it impossible to obtain a dense body by the sintering step performed after the nitriding step. If the nitriding temperature is higher than 1500°C, the silicon powder may melt before nitriding and remain unnitrided, or the sintering aid components may volatilize, resulting in a shortage of sintering aid components in the sintering step, making it difficult to obtain a dense sintered body. If the holding time is longer than 12 hours, the sintering aid components may volatilize, resulting in a shortage of sintering aid components in the sintering step, making it difficult to obtain a dense sintered body.

[0062] The nitrogen partial pressure in the sintering step is preferably 0.1 to 0.9 MPa, and more preferably 0.5 to 0.9 MPa. The sintering temperature is preferably 1800 to 1950°C, and more preferably 1850 to 1900°C. The holding time (sintering time) after heating to the sintering temperature is preferably 3 to 12 hours, and more preferably 5 to 12 hours. If the sintering temperature is less than 1800°C or the holding time is less than 3 hours, the growth and rearrangement of silicon nitride particles may be insufficient, making it impossible to obtain a dense body. If the sintering temperature is higher than 1950°C or the holding time is longer than 12 hours, the sintering aid components may volatilize and become insufficient, making it difficult to obtain a dense sintered body.

[0063] The sintered silicon nitride substrate contains β-phase silicon nitride as a main component, a rare earth element, and magnesium. The rare earth element may be in a simple state or may form a compound with other substances. The magnesium contained in the silicon nitride substrate may be in a simple state or may form a compound with other substances.

[0064] The silicon nitride substrate is a silicon nitride sintered body having silicon nitride particles and a grain boundary phase that forms the grain boundaries of the silicon nitride particles. The grain boundary phase preferably contains 0.5 to 2.3 mol% of rare earth elements (equivalent to trivalent oxides RE2O3 (RE is a rare earth element)) and 0.5 to 10 mol% of magnesium (equivalent to MgO). The rare earth element and magnesium contents of the silicon nitride substrate are calculated by taking the sum of the number of moles of silicon nitride (Si3N4), the number of moles of rare earth elements converted to trivalent oxides RE2O3 (RE is a rare earth element), and the number of moles of magnesium converted to MgO as 100 mol%. Hereinafter, the above total may be simply referred to as "the total of silicon nitride, rare earth elements (equivalent to trivalent oxides), and magnesium (equivalent to MgO)."

[0065] Here, the total content of rare earth elements (converted to trivalent oxides RE2O3 (RE is a rare earth element)) and magnesium (converted to MgO) in the grain boundary phase (total amount of grain boundary phase) is preferably 1.0 to 12.3 mol%.

[0066] The contents of silicon nitride, rare earth elements, and magnesium in the silicon nitride substrate depend on the amounts of silicon powder added during production and rare earth oxides and magnesium compounds added as sintering aids. In the method according to this specific example, the magnesium compound is mainly reduced by volatilization during firing, so the magnesium content in the silicon nitride substrate after sintering is reduced relative to the amount added during production. On the other hand, since rare earth oxides are hardly volatilized, the reduction in magnesium compound may result in a slight increase in the total content of silicon nitride, rare earth elements (in terms of trivalent oxides), and magnesium (in terms of MgO). The amount of volatilized magnesium compound varies depending on the shape of the compact, firing conditions, etc.

[0067] The oxygen content in silicon nitride particles is 0.05% by mass or less. If the oxygen content exceeds 0.05% by mass, high thermal conductivity cannot be obtained. Two silicon nitride substrates are prepared under the same conditions as samples, and one silicon nitride substrate is used as the substrate, while the other silicon nitride substrate is used to measure the oxygen content. For example, the other silicon nitride substrate can be crushed and pickled to extract silicon nitride particles (the grain boundary phase is removed by pickling), and the oxygen content of the silicon nitride particles can be measured using an oxygen analyzer that uses inert gas fusion and non-dispersive infrared absorption.

[0068] The silicon nitride substrate preferably has a dense structure with a relative density of 98% or more. If the relative density of the silicon nitride substrate is less than 98%, high thermal conductivity cannot be obtained. Such a dense silicon nitride substrate is less susceptible to voids that hinder thermal conduction, and the silicon nitride substrate of this specific example in particular has a thermal conductivity of 115 W / m K or more in the thickness direction.

[0069] The bending strength of a silicon nitride substrate is, for example, 600 MPa or more. When a silicon nitride circuit substrate is used for a power module in which a circuit such as a copper plate is joined to the silicon nitride substrate via a brazing material, high stress is applied during mounting and operation, so the bending strength is preferably 600 MPa or more, although this depends on the mounting method. Furthermore, a high bending strength of 600 MPa or more makes it possible to make the silicon nitride substrate thinner.

[0070] The silicon nitride substrate has a rectangular shape with each side being 100 mm or more. The thickness of the silicon nitride substrate is not particularly limited and can be any thickness, but when used as an insulating heat dissipation substrate for semiconductor elements or electronic devices, it is preferably 0.05 to 2.5 mm, more preferably 0.1 to 1 mm, and particularly when used as a silicon nitride circuit substrate for power modules, it is even more preferably 0.2 to 0.6 mm. The thickness of the silicon nitride substrate after the sintering step can be adjusted by adjusting the thickness of the sheet-like molded body formed in the sheet molding step.

[0071] (Evaluation of discoloration prevention effect) The method and results of evaluating the effect of suppressing discoloration of a silicon nitride substrate by a method for manufacturing a nitride ceramic substrate according to an embodiment of the present invention will be described below.

[0072] In this evaluation, a silicon nitride substrate was produced by the method for producing a nitride ceramic substrate according to the embodiment of the present invention using a sheet-like molded body containing silicon as a precursor.

[0073] First, the BET specific surface area is 2.1m 2 To silicon powder with a sintering aid of 1.2 mol% Y2O3 powder and 9.8 mol% MgSiN2 powder (based on the total of silicon (silicon nitride), rare earth oxides (trivalent oxides), and magnesium compound (MgO)) was added. A 42% slurry was prepared by adding a dispersion medium (toluene) and a dispersant (sorbitan trioleate) to the silicon powder. The slurry was milled for 24 hours using 5φ silicon nitride balls. The amount of magnesium compound added was expressed as mol% when all magnesium compounds were converted to MgO. The BET specific surface area, median diameter D50, and oxygen content of the silicon powder before grinding were measured using a BET specific surface area analyzer using the BET single-point method, a particle size distribution analyzer using the laser diffraction / scattering method, and an oxygen analyzer using the inert gas fusion / non-dispersive infrared absorption method, respectively.

[0074] The resulting slurry was mixed with a dispersion medium and an organic binder (acrylic resin) to adjust the concentration, and then degassed to form a coating solution. This coating slurry was applied to a carrier film by a doctor blade method, formed into a 0.38 mm thick sheet, and cut into a 240 mm x 200 mm sheet. The speed at which the carrier film was fed during coating corresponded to the forming speed, which was set to 600 mm / min or less.

[0075] Next, an assembly process was carried out, and the obtained sheet-like molded body was used to assemble the assembly 1 under a plurality of different conditions. For the thermal conductive material 11, carbon (PSG-12 manufactured by Toyo Carbon Co., Ltd.) having a thickness of 5 mm or silicon nitride (47 silicon nitride substrates having a thickness of 0.32 mm stacked) having a thickness of 15 mm was used.

[0076] After that, the degreasing process, nitriding process, and sintering process were performed to obtain a rectangular silicon nitride substrate with a long side of 200 mm and a short side of 140 mm. In the nitriding process, the temperature was raised from room temperature (about 25°C) to 1400°C in about 10 hours.

[0077] 4 is a table showing the evaluation results of the effect of using thermally conductive material 11 on inhibiting discoloration of silicon nitride substrates. In this evaluation, discoloration of a sheet-shaped molded body after the nitriding process and before the sintering process was examined. Since the discoloration that occurred in the molded body after the nitriding process remains even after the molded body has been sintered to become a silicon nitride substrate, the effect of inhibiting discoloration in silicon nitride substrates can be evaluated by examining the discoloration of the molded body after the nitriding process.

[0078] The "number of processed sheets" in the table of FIG. 4 refers to the number of sheet-like molded bodies included in the assembly 1 that underwent the heat treatment processes (degreasing process, nitriding process, and sintering process) in this evaluation. In the test where the number of processed sheets was 60, four stacked bodies 2, each containing 15 molded bodies, were accommodated in the assembly 1 in which four heating containers 10 were stacked. In the test where the number of processed sheets was 78, six stacked bodies 2, each containing 13 molded bodies, were accommodated in the assembly 1 in which six heating containers 10 were stacked. In the test where the number of processed sheets was 90, six stacked bodies 2, each containing 15 molded bodies, were accommodated in the assembly 1 in which six heating containers 10 were stacked.

[0079] As shown in Figure 4, it was found that the more substrates were processed, the more likely discoloration occurred in the silicon nitride substrate obtained through the heat treatment process. This is thought to be because the more substrates were processed, the more heat was generated in the molded body contained in the laminate 2 during the nitriding process, making thermal runaway more likely to occur. Furthermore, discoloration of the silicon nitride substrate was more common in the laminate 2 arranged near the center of the assembly 1.

[0080] Furthermore, as shown in Figure 4, it was found that discoloration of the silicon nitride substrate was suppressed by using the thermally conductive material 11 in the assembly 1. This is thought to be because the thermally conductive material 11 suppresses heat buildup in the laminate 2, thereby suppressing thermal runaway during the nitriding process. Furthermore, it was found that when carbon is used as the material for the thermally conductive material 11, discoloration of the silicon nitride substrate is suppressed more effectively than when silicon nitride is used as the material for the thermally conductive material 11. Note that in tests in which the thermally conductive material 11 was not used in the assembly 1 and 78 or 90 substrates were processed, the molded bodies melted, making it impossible to evaluate discoloration.

[0081] (Effects of the embodiment) According to the embodiment of the present invention, by using the thermally conductive material 11 in the assembly 1, it is possible to manufacture a nitride ceramic substrate such as a silicon nitride substrate in which color unevenness on the surface is effectively suppressed.

[0082] (Summary of the embodiment) Next, the technical ideas grasped from the above-described embodiments will be described by using the reference numerals and the like in the embodiments. However, the reference numerals and the like in the following description do not limit the components in the claims to the members and the like specifically shown in the embodiments.

[0083] [1] A method for producing a nitride ceramic substrate using a sheet-like molded body as a precursor, the method comprising: an assembly step of assembling an assembly (1) in which heating containers (10) containing a stack (2) of a plurality of the molded bodies are stacked in multiple stages; and a nitriding step of nitriding the molded bodies by heat treatment in a nitrogen atmosphere after the assembly step, wherein the heating container (10) includes a plate material (101) on which the stack (2) is placed, and the assembly (1) is formed by stacking the heating containers (10) in multiple stages at least in part via a heat conductive material (11), and the heat conductive material (11) has a higher thermal conductivity than the plate material (101).

[0084] [2] The method for producing a nitride ceramic substrate according to the above [1], wherein the heat conductive material (11) contains carbon (graphite).

[0085] [3] The method for producing a nitride ceramic substrate according to the above [1], wherein the thermally conductive material (11) is made of silicon nitride.

[0086] [4] The method for producing a nitride ceramic substrate according to [1] above, wherein the thermally conductive material has a thermal conductivity of 5 W / m·K or more.

[0087] [5] The method for producing a nitride ceramic substrate according to [1] above, wherein the heat-conductive material has a heat-resistant temperature of 1400°C or higher.

[0088] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and various modifications are possible within the scope of the gist of the invention. Furthermore, the components of the above embodiments can be combined as desired within the scope of the gist of the invention. Furthermore, the above-described embodiments do not limit the invention according to the claims. Furthermore, it should be noted that not all of the combinations of features described in the embodiments are necessarily essential to the means for solving the problems of the invention. [Explanation of symbols]

[0089] 1 assembly 2. Laminate 10 Heating container 101 Board material 102 Frame material 103 Board material 11 Thermal Conductive Materials 12 Weight board

Claims

1. A method for producing a nitride ceramic substrate, which produces a nitride ceramic substrate using a sheet-shaped molded body as a precursor, comprising: an assembling process for assembling an assembly in which heating containers each containing a stack of a plurality of the molded bodies are stacked in multiple stages; a nitriding step of nitriding the compact by heat treatment in a nitrogen atmosphere after the assembling step; Including, the heating container includes a plate on which the laminate is placed, The assembly is at least partially formed by stacking the heating containers in multiple stages via a thermally conductive material, The thermally conductive material has a higher thermal conductivity than the plate material. A method for manufacturing a nitride ceramic substrate.

2. The thermally conductive material includes carbon. The method for producing the nitride ceramic substrate according to claim 1.

3. The thermally conductive material is made of silicon nitride. The method for producing the nitride ceramic substrate according to claim 1.

4. The thermal conductivity of the thermally conductive material is 5 W / m·K. That's all. The method for producing the nitride ceramic substrate according to claim 1.

5. The heat-conductive material has a heat-resistant temperature of 1400°C or higher. The method for producing the nitride ceramic substrate according to claim 1.

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