Method for manufacturing silicon nitride substrate

Abstract

A method for producing a silicon nitride substrate that has high heat dissipation and excellent mechanical properties by preventing a rapid nitridation reaction of silicon and achieving a uniform nitridation reaction includes the following steps: a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a sintering aid and a dispersing medium to form a sheet-forming slurry in which the sintering aid is dispersed in the composite powder; a third step of molding the sheet-forming slurry to form a sheet; a fourth step of heating the sheet at 250 to 600°C to degrease the resin component to form a degreased sheet; a fifth step of heating the degreased sheet at 1200 to 1500°C to nitride the silicon contained in the sheet to form a nitrided sheet; and a sixth step of sintering the nitrided sheet to form a silicon nitride substrate, wherein the composite powder is formed by adsorption of metal silicon particles and silicon nitride particles.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silicon nitride substrate, and more particularly to a method for producing a silicon nitride substrate having high heat dissipation properties and excellent mechanical properties by using a composite powder formed by mixing metal silicon powder and silicon nitride powder as a silicon source and finely pulverizing the mixture to prevent a rapid nitriding reaction of silicon. [Background technology]

[0002] In recent years, the demand for power devices has been increasing year by year. That is, with the spread of hybrid vehicles, electric vehicles, etc., the trend in power devices is toward higher output, higher density, and higher temperature operation. To meet these trends in power devices, insulating substrates used in power devices are now required to have high heat dissipation properties and excellent mechanical properties. In other words, in insulating substrates used in power devices, technology for dissipating heat generated from semiconductor elements has become extremely important.

[0003] Aluminum nitride is used as a material for insulating substrates with heat dissipation properties. Because aluminum nitride combines excellent insulating properties with high thermal conductivity, it is used as a material for heat dissipation insulating substrates for power modules. However, aluminum nitride has poor mechanical properties such as strength and fracture toughness, and is unreliable, so its applications have been very limited.

[0004] On the other hand, silicon nitride sintered bodies are widely known as excellent structural ceramic materials that combine high strength and high toughness. Furthermore, silicon nitride sintered bodies are predicted to exhibit extremely high thermal conductivity of 200-320 W / mK in single crystal form. Therefore, silicon nitride sintered bodies are expected to be used as materials for heat-dissipating insulating substrates.

[0005] However, in typical silicon nitride sintered bodies, impurities such as oxygen are dissolved inside the silicon nitride particles that make up the silicon nitride sintered body, which causes the phonons responsible for thermal conduction to be scattered, resulting in a thermal conductivity of 20 to 80 W / mK, which is far lower than the theoretical value predicted for a single crystal.

[0006] From this technical perspective, a method for producing a dense silicon nitride substrate has been proposed that can be produced from a raw material powder containing silicon powder and that does not require the removal of an affected layer after forming a sintered body (for example, Patent Document 1). That is, Patent Document 1 describes a method for producing a silicon nitride substrate that includes a raw material powder preparation step of preparing a predetermined raw material powder containing silicon powder, a rare earth element compound, and a magnesium compound, a sheet forming step of forming the raw material powder into a sheet body, a nitriding step of heating the sheet body in a nitrogen atmosphere to nitridize the silicon contained in the sheet body, and a sintering step of sintering the sheet body that has undergone the nitriding step in a nitrogen atmosphere.

[0007] Furthermore, a method for producing a silicon nitride substrate having excellent thermal conductivity in the thickness direction has been proposed (for example, Patent Document 2). Patent Document 2 describes a method for producing a silicon nitride substrate, including the steps of mixing silicon powder, a sintering aid, and a dispersion medium to prepare a slurry, forming a sheet from the slurry, heat-treating the sheet in a nitrogen-containing atmosphere to nitride the silicon in the sheet to form silicon nitride, and sintering the silicon nitride-containing sheet to produce a silicon nitride substrate. The step of forming silicon nitride, included in the method for producing a silicon nitride substrate described in Patent Document 2, is characterized by controlling the volatilization of the sintering aid and orienting the silicon nitride particles in the thickness direction, which is the direction of movement of the sintering aid.

[0008] Furthermore, a manufacturing method has been proposed that enables the formation of silicon nitride substrates with high separability without damaging the ceramic substrate (for example, Patent Document 3). That is, Patent Document 3 describes a silicon nitride substrate, which is a sintered body composed of a main phase consisting of silicon nitride particles and a grain boundary phase consisting of a sintering aid, with a surface waviness of 1.0 μm or less and which has desired bending strength, thermal conductivity, and bondability to a metal plate, and a manufacturing method that enables the formation of silicon nitride substrates with high separability without damaging the ceramic substrates when separating individual ceramic substrates from a stack of multiple ceramic substrates after sintering. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 5836522 [Patent Document 2] Japanese Patent Publication No. 2022-027444 [Patent Document 3] International Publication No. 2013 / 054852 Summary of the Invention [Problem to be solved by the invention]

[0010] However, each of the above-mentioned conventional technologies has the following problems that must be solved. Specifically, in the method of producing a silicon nitride substrate from a raw material powder containing silicon powder, as described in Patent Document 1, when a large amount of silicon powder is used, heat is generated due to a rapid nitriding reaction of the silicon contained in the silicon powder. This method of producing a silicon nitride substrate requires a very long nitriding reaction time to control the rapid heat generation of silicon contained in the silicon powder, which is the raw material powder. Furthermore, this rapid reaction causes problems such as melting of the metallic silicon or leaving it unreacted.

[0011] In order to suppress heat generation from the silicon contained in the silicon powder (raw material powder), it has been considered to use a mixed powder obtained by mixing silicon powder with silicon nitride powder as the raw material for the silicon nitride substrate. However, simply mixing silicon powder and silicon nitride powder does not result in a mixed powder in which the silicon powder and silicon nitride powder are uniformly mixed. Therefore, the heat generated by the nitriding reaction of the silicon contained in the silicon powder (raw material powder) makes it impossible to control the nitriding reaction. Thus, the method for producing a silicon nitride substrate from a raw material powder containing silicon powder described in Patent Document 1 cannot use a large amount of metallic silicon powder as the silicon source for the silicon nitride substrate.

[0012] Furthermore, the methods for manufacturing silicon nitride substrates described in Patent Documents 2 and 3 involve nitriding silicon in a sheet, promoting the volatilization of sintering aids in the process of forming silicon nitride, and orienting silicon nitride particles in the thickness direction, but do not take into consideration the heat generated by the rapid nitridation reaction of silicon powder.

[0013] Thus, silicon nitride substrates manufactured by conventional methods for manufacturing silicon nitride substrates have not been able to satisfy all of these requirements from the two standpoints of manufacturing cost and the coexistence of thermal conductivity and mechanical properties. Therefore, from the viewpoint of reducing the cost required for the raw material powder, even when using a so-called reactive sintering method in which inexpensive silicon powder is used as the raw material powder, and the resulting compact is nitrided in nitrogen and then sintered at high temperature, there is a need for a method for producing a silicon nitride substrate that suppresses heat generation due to the rapid nitriding reaction of the silicon powder and includes a nitriding step that involves a uniform nitriding reaction of the silicon powder.

[0014] The present invention has been made in view of the above-mentioned problems of the conventional technology, and has as its object to provide a silicon nitride substrate which prevents a rapid nitridation reaction of silicon during the manufacturing process of the silicon nitride substrate, and which has high heat dissipation properties and excellent mechanical properties due to a uniform nitridation reaction. [Means for solving the problem]

[0015] In view of these problems, the inventors have conducted extensive research and have discovered that by using a composite powder formed by mixing metal silicon powder and silicon nitride powder and then pulverizing the mixture as the silicon source for silicon nitride substrates, it is possible to prevent the rapid nitridation reaction of silicon and non-uniformity of the reaction, and have arrived at the present invention.

[0016] The present invention provides a method for producing a silicon nitride substrate that advantageously solves the above-mentioned problems, comprising the steps of: a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a sintering aid and a dispersant to form a sheet-forming slurry in which the sintering aid is dispersed in the composite powder; a third step of molding the sheet-forming slurry to form a sheet; a fourth step of heating the sheet at 250 to 600°C to degrease the resin components contained in the sheet to form a degreased sheet; a fifth step of heating the degreased sheet at 1200 to 1500°C to nitriding the silicon contained in the degreased sheet to form a nitrided sheet; and a sixth step of sintering the nitrided sheet to form a silicon nitride substrate, wherein the composite powder is formed by adsorption of metal silicon particles and silicon nitride particles.

[0017] The method for producing a silicon nitride substrate according to the present invention includes the steps of: (a) The fine pulverization treatment is carried out by any one or combination of a jet mill, a ball mill, a bead mill, a planetary mill, an attritor, and a mechanochemical mill; (b) the average particle diameter D of the silicon nitride particles SN50 The average particle diameter D of the metal silicon particles S50 The particle size ratio X is 0.2 to 2.0. (c) in the first step, the metal silicon powder and the silicon nitride powder are contained in a molar ratio, calculated as silicon nitride, of 70:30 to 95:5; (d) the metal silicon particles have fine particles of the sintering aid uniformly dispersed on the surface of the metal silicon particles; (e) the sintering aid is a magnesium compound, and the magnesium compound includes one or more magnesium compounds selected from magnesium oxide, magnesium silicide, and magnesium silicon nitride; (f) the sintering aid is the rare earth element compound, and the rare earth element contains one or more elements selected from Y, Sc, La, Ce, Nd, Sm, Gd, Dy, Ho, Er, and Yb; (g) the relative density of the sheet body is 45% or more; (h) A more preferable means for solving the problems is that the thermal conductivity of the silicon nitride substrate is 80 W / mK or more as measured by the xenon flash method. [Effects of the Invention]

[0018] According to the present invention, a silicon nitride substrate having high heat dissipation properties and excellent mechanical properties can be produced by preventing a rapid nitridation reaction of silicon. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a flow chart showing each step of a method for manufacturing a silicon nitride plate according to the present invention. [Figure 2A] 1 is an SEM image of metal silicon particles and silicon nitride particles that constitute the composite powder used in the method for producing a silicon nitride plate according to the present invention before being pulverized. [Figure 2B] 1 is an SEM image of metal silicon particles and silicon nitride particles that constitute the composite powder used in the method for producing a silicon nitride plate according to the present invention after being crushed. [Figure 3] This is an EDS image of the silicon metal particles and silicon nitride particles that make up the composite powder after they have been crushed. [Figure 4] This is an enlarged EDS image of the silicon metal particles and silicon nitride particles that make up the composite powder after they have been crushed. [Figure 5]This is an SEM image of composite silicon particles in which fine particles of a sintering aid are dispersed on the surface of metal silicon particles that make up the composite powder. [Figure 6] This is an EDS image of composite silicon particles in which fine particles of a sintering aid are dispersed on the surface of metal silicon particles that make up the composite powder. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0021] [First embodiment] A method for manufacturing a silicon nitride plate according to a first embodiment will now be described. FIG. 1 is a flow chart showing the steps of the method for manufacturing a silicon nitride plate according to this embodiment. As shown in FIG. 1, the method for manufacturing a silicon nitride plate according to this embodiment includes the following steps: a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a sintering aid and a dispersion medium to form a sheet-forming slurry in which the sintering aid is dispersed in the composite powder; a third step of molding the sheet-forming slurry to form a sheet; a fourth step of heating the sheet at 250 to 600°C to degrease the resin components contained in the sheet to form a degreased sheet; a fifth step of heating the degreased sheet at 1200 to 1500°C to nitride the metal silicon contained in the degreased sheet to form a nitrided sheet; and a sixth step of sintering the nitrided sheet to form a silicon nitride substrate. The composite powder is characterized by being formed by adsorption of metal silicon particles and silicon nitride particles. Hereinafter, each step included in the method for manufacturing a silicon nitride plate according to this embodiment will be described.

[0022] <Step 1: Step of forming composite powder> The method for manufacturing a silicon nitride plate according to this embodiment includes a composite powder forming step in which metal silicon powder and silicon nitride powder are mixed together and a composite powder is formed by a mechanochemical effect caused by a fine pulverization process. The composite powder forming step is a step of forming composite powder, which is the raw material for the sintered silicon nitride sheet body required for producing a silicon nitride substrate. That is, the first step, the composite powder formation step, is a step in which metal silicon powder and silicon nitride powder are mixed as raw materials for the silicon nitride sintered body required to manufacture a silicon nitride substrate, and composite powder is formed by the mechanochemical effect of a fine grinding process. The mechanochemical effect is an effect in which the bonding state of a substance changes and becomes activated when mechanical energy is applied to the substance. The mechanochemical effect occurs when, during the process of particles being reduced to fine particles by a crushing operation, mechanical energy such as impact, compression, shear, shear stress, and friction is continuously applied to the particles, causing a change in the crystal structure of the particles, activating the particle surface and causing a chemical reaction with the substances present around the particles.

[0023] Here, in conventional methods for producing silicon nitride sintered bodies, a method has been proposed in which only silicon nitride powder is used as the silicon source for the silicon nitride sintered body. However, because silicon nitride powder is expensive, using only silicon nitride powder as the silicon source for silicon nitride raises the problem of increased production costs. In contrast, in the method for producing a silicon nitride substrate according to this embodiment, metal silicon particles are used as the raw material for the silicon nitride sintered body, and a silicon nitride substrate can be produced by a sintering reaction.

[0024] Furthermore, in the method for producing a silicon nitride substrate according to this embodiment, silicon nitride powder is used in addition to metal silicon powder as a silicon supply source for the silicon nitride sintered body required for producing a silicon nitride substrate. That is, the method for manufacturing a silicon nitride substrate according to this embodiment is characterized by using a mixed powder of metal silicon powder and silicon nitride powder as the silicon source for the silicon nitride sintered body, and using a composite powder of metal silicon-silicon nitride formed by finely pulverizing the mixed powder of metal silicon powder and silicon nitride powder.

[0025] (metallic silicon powder and silicon nitride powder) In the method for producing a silicon nitride substrate according to this embodiment, a mixed powder containing metal silicon powder and silicon nitride powder is used as the silicon source for the silicon nitride sintered body required for producing the silicon nitride substrate. Of the silicon sources for the silicon nitride substrate contained in the mixed powder, the silicon nitride powder is preferably 20 mol% or less, more preferably 10 mol% or less. More preferably, it is 5 mol% or less, which is preferable because it allows for reducing the production costs for producing the silicon nitride substrate. That is, the metallic silicon contained in the mixed powder containing metallic silicon powder and silicon nitride powder as the silicon source for the silicon nitride sintered body required to produce a silicon nitride substrate is preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or less.

[0026] The molar concentration of silicon nitride in the mixed powder is a value calculated by converting the silicon contained in the mixed powder into silicon nitride, i.e., a value calculated assuming that 1 mol of silicon in the silicon powder is 1 / 3 mol. Specifically, for example, if a raw material powder contains 3 mol of silicon powder and 1 mol of silicon nitride powder as silicon sources, the silicon nitride powder will account for 50 mol% of the silicon sources contained in the raw material powder.

[0027] The average particle size of the metal silicon particles constituting the metal silicon powder is 0.1 to 10.0 μm, preferably 2.0 to 5.0 μm, and the average particle size of the silicon nitride particles constituting the silicon nitride powder is 1.0 to 25.0 μm, preferably 3.0 to 10.0 μm. Here, the average particle size of the metal silicon particles before the mechanochemical effect is imparted by the pulverization treatment employed in the method for producing a silicon nitride substrate according to this embodiment is 20.0 to 30.0 μm, and the average particle size of the silicon nitride particles is 40.0 to 50.0 μm. The average particle size refers to the particle size at 50% of the cumulative value in the particle size distribution determined by laser diffraction / scattering. Commercially available silicon nitride powders and silicon powders generally contain unavoidable impurities.

[0028] The amount of oxygen contained as an impurity in silicon nitride powder and metal silicon powder varies depending on the properties of these powders, but for example, in the case of silicon nitride powder, it is about 1.2 mass%, and in the case of metal silicon powder, it is about 0.2 mass% to several mass%. The purity of the silicon nitride powder used in the method for producing a silicon nitride substrate according to this embodiment is preferably in the range of 98.0% to 99.99%, and more preferably in the range of 99.0% to 99.90%. The purity of the silicon powder used in the method for producing a silicon nitride substrate according to this embodiment is preferably in the range of 99.0% or more, and more preferably in the range of 99.5% or more.

[0029] When manufacturing a silicon nitride substrate, it is preferable to reduce the amount of dissolved oxygen contained in the crystals of the silicon nitride sintered body in order to improve the thermal conductivity of the silicon nitride substrate. The method for manufacturing a silicon nitride substrate according to this embodiment uses reactive sintering and uses metal silicon powder as the starting material, which offers a significant advantage in terms of reducing the amount of oxygen compared to using only silicon nitride as the silicon source. When silicon powder is used as the starting material, a composite powder consisting of the starting materials, metal silicon powder and silicon nitride powder, is formed into a sheet, followed by a nitriding step in which the sheet containing the composite powder is nitrided. In the nitriding step, the nitriding reaction shown in the following reaction formula (1) proceeds.

[0030] 3Si+2N2=Si3N4(1)

[0031] The weight of the sheet containing the composite powder increases by approximately 70% due to the nitriding reaction. Therefore, the impurity oxygen content of the sheet containing the composite powder is relatively reduced. Thus, the method for producing a silicon nitride substrate according to this embodiment uses a composite powder consisting of metal silicon powder and silicon nitride powder as the silicon source, thereby reducing the oxygen content in the crystals of the silicon nitride sintered body compared to when silicon nitride alone is used as the silicon source starting material.

[0032] In other words, the method for manufacturing a silicon nitride substrate according to this embodiment can relatively reduce the amount of impurity oxygen in the composite powder by undergoing a nitriding reaction of a sheet body containing a composite powder made of metal silicon powder and silicon nitride powder as a silicon supply source. As a result, the influence of the amount of impurity oxygen in the metal silicon powder and silicon nitride powder that make up the composite powder is negligible. Therefore, in the method for manufacturing a silicon nitride substrate according to this embodiment, a variety of metal silicon powders can be used, from low-grade metal silicon powders with high impurity oxygen concentrations to high-grade metal silicon powders with low impurity oxygen concentrations. In particular, when it is necessary to reduce the amount of impurity oxygen in the silicon nitride substrate, it is preferable to use high-quality metal silicon powder with a low impurity oxygen concentration. Here, the metal silicon particles constituting the metal silicon powder may be silicon particles having fine particles of a sintering aid uniformly dispersed on the surface of the metal silicon particles. That is, the metal silicon powder contains a sintering aid. The sintering aid may have a melting point lower than that of the silicon nitride contained in the sheet body, which is the precursor of the silicon nitride substrate, and may be a rare earth element compound or magnesium compound, which is the sintering aid used in the second slurry formation step described below. The particle diameter of the fine particles of the sintering aid is preferably 0.5 μm or less. By uniformly dispersing the fine particles of the sintering aid on the surface of the metal silicon particles, the grain boundary energy of the metal silicon particles can be reduced, thereby improving the sinterability of the silicon nitride particles.

[0033] (Fine grinding process) In the composite powder forming step included in the method for producing a silicon nitride plate according to this embodiment, the composite powder is formed by finely pulverizing a mixed powder of metal silicon powder and silicon nitride powder using any one or a combination of mills selected from the group consisting of a jet mill, a ball mill, a bead mill, a planetary mill, an attritor, and a mechanochemical mill. For example, jet milling involves ejecting compressed air, high-pressure steam, or high-pressure gas at several atmospheres or more from a jet nozzle, accelerating the raw material particles, i.e., metal silicon particles and silicon nitride particles, with the jet stream, and pulverizing these particles by collisions between the accelerated particles or by impact action or friction with the accelerated particles. Jet milling is carried out using a jet mill fine pulverizer that can impart such action to the metal silicon powder and silicon nitride powder to advance pulverization.

[0034] When metal silicon particles and silicon nitride particles are finely pulverized by dry pulverization using a jet mill pulverizer, a fine powder having a particle size of 0.1 to 10.0 μm, preferably 1.0 to 5.0 μm, can be obtained. Jet mill pulverization produces little temperature rise and is suitable for pulverizing heat-sensitive materials. However, jet mill pulverization has low energy efficiency, and while it can produce a fine powder composed of metal silicon particles and silicon nitride particles, it consumes a lot of power and generally produces a small amount of powder. For this reason, in addition to jet mills, any one or combination of other fine powder processing means such as ball mills, bead mills, planetary mills, and attritors may also be used.

[0035] There are various types of jet mill fine grinders, including the Micronizer type, which uses a jet jet that creates a horizontal swirling flow, the Jet Omizer type, which uses a vertical swirling flow, the Blaw-Knox or Trost jet mill type, which uses counter-collision of solid-gas mixed flows, a method in which a solid-gas mixed jet collides against an impact plate, and a method in which a solid-gas mixed flow is mixed in an ultrasonic nozzle. These methods can be appropriately selected and used in the composite powder formation step included in the method for producing a silicon nitride plate according to this embodiment. Furthermore, many jet mills used in the composite powder formation step not only perform dry pulverization, but also effectively utilize airflow to perform classification, or directly connect a high-performance airflow classifier to perform as sharp a classification as possible.

[0036] In this way, in the composite powder formation step, a mixed powder obtained by mixing metal silicon powder and silicon nitride powder is finely pulverized to produce a composite powder that serves as the silicon source for the silicon nitride substrate, due to the mechanochemical effect of the pulverization treatment. In other words, the composite powder formation step is a step in which a mixed powder obtained by mixing metal silicon powder and silicon nitride powder is finely pulverized to produce a composite powder from the metal silicon powder and silicon nitride powder by utilizing a mechanochemical reaction or mechanical alloying reaction.

[0037] The composite powder is characterized by being formed by adsorption of metal silicon particles and silicon nitride particles. The composite particles constituting the composite powder formed from metal silicon powder and silicon nitride powder may include metal silicon particles coated with silicon nitride powder. That is, the composite particle may have a metal silicon particle as a base material, which is a core particle located at the center of the composite particle, and the composite particle has silicon nitride particles that coat or adsorb to the entire surface or a portion of the metal silicon particle that is the core particle. The number of silicon nitride particles used to coat or adsorb onto the entire surface or a portion of the metal silicon particles is greater than the number of metal silicon particles that are the core particles. The average particle size of the silicon nitride particles used to coat all or part of the metal silicon particles may be smaller than the average particle size of the metal silicon particles that are the core particles. On the other hand, the composite particles constituting the composite powder formed from metal silicon powder and silicon nitride powder may contain silicon nitride particles coated with metal silicon particles. That is, the composite particles may have a silicon nitride particle that is a core particle located at the center of the composite particle. The composite particles may have metal silicon particles that coat the entire surface or a portion of the silicon nitride particle that is the core particle, or that are adsorbed to the entire surface or a portion of the silicon nitride particle that is the core particle. The number of metal silicon particles used to coat the entire surface or a portion of the silicon nitride particle is greater than the number of silicon nitride particles that are the core particles. The average particle size of the metal silicon particles used to coat all or part of the silicon nitride particles may be smaller than the average particle size of the silicon nitride particles that are the core particles.

[0038] Therefore, when a mixed powder formed from a metal silicon powder and a silicon nitride powder is used as a base material with large particle diameters, the surface of the large metal silicon particles may be adsorbed with small silicon nitride particles by the mechanochemical effect of the fine grinding treatment, resulting in the formation of composite particles in which the metal silicon particles located at the center of the composite particles are entirely or partially coated with or adsorbed by silicon nitride particles. Furthermore, when a mixed powder formed from silicon nitride powder and metal silicon powder is used as a base material with a large particle size, metal silicon particles with a small particle size may be adsorbed onto the surface by the mechanochemical effect of a fine grinding process. As a result, composite particles are formed in which the silicon nitride particles located at the center of the composite particles are coated or adsorbed entirely or partially with metal silicon particles. Furthermore, composite particles are formed in which the silicon nitride particles located at the center of the composite particles are adsorbed entirely or partially with metal silicon particles.

[0039] Metal silicon particles may be present on the surface of the composite particles that make up the composite powder. Furthermore, a metal silicon particle layer consisting of metal silicon particles formed by agglomeration of metal silicon particles may be formed on the surface of the composite particles. The metal silicon particles present on the surface of the composite particles are nitrided to become silicon nitride particles in the nitriding step of the sheet body containing the composite powder described below. Then, all of the metal silicon particles present on the surface of the composite particles become silicon nitride. Ultimately, the composite powder is formed only from silicon nitride particles. Silicon nitride particles may also be present on the surface of the composite particles that make up the composite powder. Furthermore, metal silicon particles may be exposed from the surface of the composite particles that make up the composite powder. The metal silicon particles exposed from the surface of the composite particles are nitrided in the nitriding step of the sheet body containing the composite powder, which will be described later, to become silicon nitride particles. Then, all of the metal silicon particles present on the surface of the composite particles become silicon nitride. Ultimately, the composite powder is formed only from silicon nitride particles.

[0040] <Second step: Step of forming slurry for forming sheet body> The method for producing a silicon nitride substrate according to this embodiment includes a slurry formation step of mixing the composite powder with a sintering aid and a dispersion medium to form a slurry for forming a sheet body in which the sintering aid is dispersed in the composite powder. The slurry formation step is a step of forming a raw material slurry for producing a sheet body, which is a precursor of the silicon nitride substrate.

[0041] In the slurry formation process, a rare earth element compound and a magnesium compound, which are sintering aids, are added to the composite powder, and a dispersion medium is added and mixed to form a raw material slurry. The dispersion medium used to form the raw material slurry can be water or an organic solvent, and an organic binder (organic binding agent) can be used as needed. The raw material slurry can be formed by adding and mixing the composite powder, sintering aid, dispersion medium, etc., and mixing them using a ball mill, bead mill, or planetary mill in the usual way.

[0042] In the slurry formation step, the composite powder and the sintering aid may be mixed in advance to form a mixed powder, and the mixed powder may then be mixed with a dispersion medium, etc. Alternatively, in the slurry formation step, the composite powder and the sintering aid may be weighed out separately, and the composite powder and the sintering aid powder may be introduced into a ball mill or the like containing a dispersion medium, etc., and the mixed powder of the composite powder and the sintering aid may be mixed with the dispersion medium, etc. at the same time.

[0043] As will be described later, the method for producing a silicon nitride substrate according to this embodiment involves forming a sheet-forming slurry containing metal silicon powder, removing the binder contained in a sheet formed from the sheet-forming slurry, and then carrying out a nitriding step to nitride the silicon contained in the resulting degreased sheet molding, followed by a sintering step to produce a silicon nitride substrate. However, silicon nitride is difficult to sinter, and therefore cannot be sintered by itself, making it impossible to obtain a silicon nitride sintered body with a dense structure. Therefore, a sintering aid is added to sinter silicon nitride to obtain a silicon nitride sintered body with a dense structure.

[0044] In order to improve the thermal conductivity of a silicon nitride sintered body, it is necessary to reduce the amount of dissolved oxygen contained in the silicon nitride particles contained in the silicon nitride sintered body, reduce the low thermal conductive grain boundary glass phase that constitutes the silicon nitride sintered body, etc. Therefore, the method for producing a silicon nitride substrate according to this embodiment is able to achieve high thermal conductivity of the silicon nitride substrate by selecting a sintering aid in the slurry formation step.

[0045] In order to reduce the amount of dissolved oxygen contained in silicon nitride particles in a silicon nitride sintered body, it is preferable to use a rare earth element compound as a sintering aid, which has a high affinity for oxygen and is excellent at trapping oxygen in the grain boundary glass phase. Furthermore, in order to reduce the grain boundary glass phase with low thermal conductivity in a silicon nitride sintered body, it is preferable to use a magnesium compound as a sintering aid, which lowers the melting point of the melt generated during heating, contributes to densification in the early stages of sintering, and is also evaporated and evaporated during sintering at high temperatures. In this way, in the slurry formation step, a rare earth element compound and a magnesium compound can be preferably used as sintering aids. The rare earth element compound and magnesium compound used as sintering aids can be prepared as raw materials for the raw material slurry together with the composite powder, which is the silicon source.

[0046] The amount of rare earth compound added is preferably such that the raw material powder contains 1.0 mol % to 7.0 mol % of the rare earth compound in terms of oxide when the silicon in the raw material powder is converted into silicon nitride. The amount (molar concentration) of the rare earth compound added refers to the molar concentration of the rare earth compound in the three components: the composite powder as a silicon source, the rare earth compound, and the magnesium compound, and the same applies hereinafter.

[0047] Rare earth compounds have a high affinity for oxygen and are excellent at trapping oxygen in the grain boundary glass phase that constitutes the silicon nitride substrate. Therefore, if the amount of rare earth compound added is less than 1.0 mol%, oxygen cannot be trapped in the grain boundary glass phase, the amount of dissolved oxygen in the silicon nitride particles increases, and the thermal conductivity of the silicon nitride substrate decreases, which is undesirable. On the other hand, if the amount of rare earth compound added exceeds 7.0 mol%, the amount of low-thermal-conductivity grain boundary phase containing the rare earth compound increases, which may decrease the thermal conductivity of the silicon nitride substrate.

[0048] In particular, when the silicon in the metal silicon powder is converted into silicon nitride, it is more preferable to add the rare earth element compound so that the raw material slurry contains 1.5 mol% or more and 5.0 mol% or less of the rare earth element compound in terms of oxide, and it is even more preferable to add it so that it contains 2.0 mol% or more and 4.0 mol% or less.

[0049] The rare earth element compound preferably contains one or more rare earth elements selected from Y, Sc, La, Ce, Nd, Sm, Gd, Dy, Ho, Er, and Yb. Examples of rare earth element compounds include rare earth element oxides. Specific examples of rare earth element compounds that can be preferably used include yttrium oxide, cerium oxide, ytterbium oxide, and scandium oxide. The rare earth element compound is not limited to one type, and two or more types of rare earth element compounds can be used simultaneously.

[0050] The amount of magnesium compound added is preferably 8.0 mol % to 15.0 mol % in terms of oxide when the silicon in the composite powder is converted into silicon nitride. The content (molar concentration) of the magnesium compound means the molar concentration of the magnesium compound in the three components of the silicon source, rare earth element compound, and magnesium compound contained in the composite powder.

[0051] When silicon nitride sintered bodies are produced using only rare earth element compounds as sintering aids, densification requires sintering at ultra-high temperatures of up to 2000°C under high nitrogen pressures of approximately 10 MPa, which requires a special sintering furnace, resulting in high process costs. Furthermore, sintering silicon nitride sintered bodies at ultra-high temperatures causes significant grain growth of the particles that make up the silicon nitride sintered bodies. This results in a deterioration of the mechanical properties of the silicon nitride sintered bodies. From this technical viewpoint, it is preferable to add a magnesium compound as a sintering aid simultaneously with the addition of a rare earth element compound in order to promote densification of the silicon nitride sintered body during post-sintering, thereby enabling the silicon nitride sintered body to exhibit high strength and high toughness. By adding a magnesium compound, Mg ions become modifying ions in the oxynitride glass generated during heating, reducing the viscosity of the oxynitride glass and promoting densification of the structure of the silicon nitride sintered body, while also reducing the amount of grain boundary phase that evaporates and remains during sintering.

[0052] If the amount of magnesium compound added is less than 8.0 mol%, the magnesium volatilizes before sintering shrinkage occurs, making it impossible to obtain a densified silicon nitride sintered body. Furthermore, if the amount of magnesium compound added is more than 15.0 mol%, a large amount of magnesium remains even after the sintering process, which may impair the thermal conductivity of the sintered body. Therefore, it is preferable to add the magnesium compound so that the content, calculated as oxide, of the silicon in the composite powder is 8.0 mol% to 15.0 mol%, calculated as silicon nitride.

[0053] Examples of magnesium compounds that can be used include magnesium silicide, fluoride, boride, nitride, and ternary compounds thereof. In particular, from the viewpoints of ease of handling, stability during processing, and the absence of harmful substances, the magnesium compound added to the raw material powder preferably includes one or more magnesium compounds selected from magnesium oxide (MgO), magnesium silicide (MgSi), and magnesium silicon nitride (MgSiN). Furthermore, the magnesium compound added to the raw material powder is more preferably one or more magnesium compounds selected from magnesium oxide (MgO), magnesium silicide (MgSi), and magnesium silicon nitride (MgSiN).

[0054] The slurry raw material formed in the slurry formation step described above is a mixed powder obtained by mixing a composite powder as a silicon supply source, a rare earth element compound, and a magnesium compound. Furthermore, the powders contained in the raw material powder can be mixed and pulverized together with other components contained in the raw material slurry when forming the raw material slurry formed in the slurry formation step, without being mixed after weighing each powder contained in the raw material powder.

[0055] In the slurry formation step, the types, amounts, and methods of addition of the dispersion medium, organic binder, and dispersant added when preparing the raw material slurry for forming the sheet body are not particularly limited, and can be selected as desired depending on the method for forming the sheet body, etc. Hereinafter, specific examples of conditions for forming a raw material slurry by adding a sintering aid, a dispersion medium, and an organic binder to the composite powder in the slurry forming step will be described.

[0056] The composite powder and sintering aid are weighed out. A milling container such as a ball mill is prepared, which is charged with 0.5 to 2 wt% of a dispersant and 30 to 70 wt% of an organic solvent, etc., based on the total amount of the composite powder and sintering aid. The composite powder and sintering aid are added to the milling container equipped with a milling device that has the dispersant charged. Examples of milling devices include jet mills, ball mills, planetary mills, and attritors. Dispersants that can be used include sorbitan esters and polyoxyalkylenes. Organic solvents that can be used as the dispersion medium include ethanol and toluene. It is also possible to use only the dispersion medium without adding a dispersant. The raw material powders can be mixed and pulverized using a ball mill.

[0057] The time for mixing and grinding is not particularly limited, as it varies depending on the capabilities of the milling machine used, the amounts of the composite powder and sintering aid as starting materials, etc., but it is preferable to select a time that allows the composite powder and sintering aid to be sufficiently ground and mixed. The time for mixing and grinding is preferably 6 hours to 48 hours, and more preferably 12 hours to 24 hours.

[0058] If the mixing and grinding time is less than 6 hours, the sintering aid will not be mixed uniformly into the raw material slurry, and unevenness will occur in the nitrided sheet after sintering, which is not preferable. On the other hand, if the mixing and grinding time is longer than 48 hours, there will be no significant change in the mixed state even after mixing and grinding the composite powder and sintering aid, and there is a risk of impurities being mixed in from the balls or pot. In the slurry formation step, after the composite powder and the sintering aid are mixed and pulverized, the dispersion medium contained in the raw material slurry can be removed, if necessary.

[0059] Furthermore, after the pulverization and mixing, 5 to 30 wt % or less of an organic binder (organic binding agent) may be added, followed by mixing to prepare a slurry for forming a sheet. The organic binder is not particularly limited, but for example, a PVB (polyvinyl butyral resin), an ethyl cellulose resin, or an acrylic resin may be preferably used.

[0060] The mixing time after adding the organic binder is also not particularly limited because it differs depending on the performance of the device used for mixing, such as a milling device, but is preferably, for example, from 1 hour to 24 hours, and more preferably from 6 hours to 12 hours. If the mixing time is less than 1 hour, the organic binder and raw material powder will not be mixed evenly, which may cause cracks in the produced sheet when the sheet is formed, which is undesirable. Also, since there is usually no significant change in the mixed state of the organic binder and raw material powder even if the mixing time is longer than 24 hours, it is preferable from the viewpoint of productivity to keep the mixing time to 24 hours or less.

[0061] After adding an organic binder and mixing, the prepared slurry is vacuum degassed to adjust the viscosity of the slurry, thereby forming a slurry for forming a sheet body. The slurry for forming a sheet body formed in the second step is applied to a sheet forming substrate using a sheet forming machine, and a sheet body can be formed in the next step, the sheet body forming step. The viscosity of the prepared sheet-forming slurry is 100 to 200 cps before vacuum degassing, and becomes 6000 to 7000 cps after vacuum degassing. As a result, the sheet-forming slurry after vacuum degassing becomes a slurry suitable for forming a sheet.

[0062] <Third step: Sheet body forming step for forming a sheet body> The method for producing a silicon nitride plate according to this embodiment includes a sheet forming step of forming the sheet-forming slurry into a sheet. The sheet forming step is a step of forming a sheet-forming slurry having a predetermined composition and a predetermined viscosity into a sheet. The sheet-shaped molded body is a green sheet. Specifically, as described above, the composite powder, the sintering aid, and the dispersion medium are mixed using a ball mill or a planetary mill, and then the dispersion medium is removed as necessary, an organic binder is added, and the sheet-forming slurry is then molded into a sheet to form the sheet.

[0063] The method for forming the sheet-forming slurry containing the raw material composite powder, sintering aid, dispersion medium, and binder into a sheet is not particularly limited as long as it is a method that can form a sheet from the sheet-forming slurry, and examples that can be used include mold forming, sheet forming, extrusion forming, and isostatic pressing (CIP) forming.

[0064] The shape and size of the sheet formed in the sheet forming step are not particularly limited, and can be any shape and thickness depending on the shape and thickness required for the silicon nitride substrate. For example, the sheet formed in the sheet forming step preferably has a thickness of 0.05 to 2.5 mm, more preferably 0.25 to 1.0 mm. In the sheet forming step, the obtained sheet may be cut to a predetermined size using a punching machine or the like, as needed.

[0065] The relative density of the sheet obtained in the sheet molding step is preferably 45% or more, more preferably 50% or more. When the sheet formed in the sheet molding step is cut to a predetermined size, the relative density of the cut sheet is preferably 45% or more. In the sheet forming step, the relative density of the sheet formed can be adjusted by adjusting the amount of raw material powder (solid content) contained in the sheet-forming slurry formed in the sheet-forming slurry forming step and the amount of binder added to the slurry. Here, by making the relative density of the sheet formed in the sheet forming step 45% or more, the voids in the sheet can be sufficiently reduced, which is preferable because it allows for a higher sintered density of the silicon nitride substrate obtained after the sintering step described below.

[0066] The upper limit of the relative density of the sheet formed in the sheet forming step is not particularly limited. To increase the relative density of the sheet, it is necessary to increase the solids concentration of the sheet-forming slurry by reducing the amount of binder or other additives used in the sheet-forming slurry forming step. If the solids concentration of the sheet-forming slurry becomes too high, cracks or the like may occur in the sheet formed from the sheet-forming slurry, making it difficult to handle. Therefore, the relative density of the sheet obtained in the sheet forming step is preferably 65% ​​or less, more preferably 60% or less.

[0067] <Fourth step: Degreasing step of forming a degreasing treatment sheet body> Furthermore, the method for manufacturing a silicon nitride plate according to this embodiment includes a degreasing step in which the sheet body is heated at 250 to 600°C to remove the resin (binder) contained in the sheet body, thereby forming a degreased sheet body. In the degreasing step, the sheet body formed in the sheet forming step is heated in an atmosphere of air, an inert gas such as nitrogen, or a mixed gas thereof, thereby making it possible to remove all of the resin (binder, etc.) contained in the sheet body. The residual carbon content in the sheet body is 0.01% or less. In the fourth step, the temperature to which the sheet body is heated can be appropriately set depending on the planar shape of the sheet body, the thickness of the sheet body, and the amount of binder, which is the resin content, contained in the sheet body.

[0068] <Fifth step: Nitriding step for forming nitrided sheet body> The method for producing a silicon nitride plate according to this embodiment includes a nitriding step in which the degreasing treatment sheet is heated at 1200 to 1500°C to nitride the silicon contained in the degreasing treatment sheet, thereby forming a nitrided treatment sheet. In the nitriding step, the sheet formed in the sheet forming step is heated in an inert gas atmosphere such as nitrogen to obtain a degreasing treatment sheet, and then all of the metallic silicon powder, which is a compositional component contained in the degreasing treatment sheet, can be nitrided.

[0069] The sheet formed in the sheet forming step contains a composite powder formed from a metal silicon powder and a silicon nitride powder. The composite particles constituting the composite powder include metal silicon particles coated with silicon nitride particles. Furthermore, the composite particles have silicon nitride particles adsorbed to the entire surface or a portion of the metal silicon particle, which is a core particle located at the center of the composite particle. On the other hand, the composite particles constituting the composite powder may include composite particles in which metal silicon particles are adsorbed to silicon nitride particles, and the metal silicon particles may be exposed from the composite particles. They may also include silicon nitride particles coated with metal silicon particles. Furthermore, the composite particles have metal silicon particles adsorbed to the entire surface or part of the silicon nitride particle, which is the core particle located at the center of the composite particle. The composite particles constituting the composite powder may include composite particles in which metal silicon particles are adsorbed to silicon nitride particles, and the metal silicon particles may be exposed from the composite particles.

[0070] In the nitriding step, the metal silicon particles present on the surface of the composite particle are nitrided to become silicon nitride particles. Then, all of the metal silicon particles present on the surface of the composite particle become silicon nitride. Also, in the nitriding step, the metal silicon particles exposed from the surface of the composite particle are nitrided to become silicon nitride particles. Then, all of the metal silicon particles exposed from the surface of the composite particle become silicon nitride. Ultimately, the silicon source that serves as the raw material for the silicon nitride substrate contained in the sheet formed in the sheet forming process is formed from the silicon nitride powder prepared from the beginning and silicon nitride powder produced by the nitriding reaction of the metal silicon powder.

[0071] Furthermore, before starting the fifth nitriding step, it is necessary to remove gas present in the furnace used to carry out the nitriding reaction of silicon, a composition component contained in the degreasing treatment sheet. For this reason, the furnace may be first depressurized and evacuated, and then nitrogen gas may be supplied into the furnace before starting the nitriding step. The pressure when depressurizing and evacuating the furnace before supplying nitrogen gas is not particularly limited, but it is preferable to depressurize the furnace to, for example, 1.0 Pa or less, and more preferably 0.1 Pa or less.

[0072] The heating temperature of the degreasing treatment sheet in the nitriding step is not particularly limited, but is preferably from 600° C. to 1500° C., and more preferably from 1000° C. to 1480° C. The treatment time of the degreasing treatment sheet in the nitriding step is preferably in the range of from 1 hour to 20 hours, and more preferably from 5 hours to 15 hours.

[0073] It is preferable that the heating temperature of the degreasing treatment sheet in the nitriding process is 1000°C or higher, or the treatment time of the nitriding process is 1 hour or longer, because unreacted silicon powder remains in the degreasing treatment sheet, and a silicon nitride sintered body with a dense structure cannot be obtained after the sintering process of the degreasing treatment sheet.

[0074] On the other hand, if the heating temperature of the sheet body in the nitriding step is 1480°C or lower, or if the treatment time in the nitriding step is 15 hours or shorter, the sintering aid, which is a compositional component of the degreasing treatment sheet body, will volatilize, resulting in a shortage of the sintering aid component in the sintering step. As a result, even if the degreasing treatment sheet body is sintered, it is preferable because it is not difficult to obtain a silicon nitride sintered body with a dense structure.

[0075] The method for heating the degreasing treatment sheet in the nitriding step is not particularly limited, but for example, the degreasing treatment sheet can be stacked between hexagonal boron nitride (BN) powder or hexagonal boron nitride (BN) plates for mold release, and then set in a vacuum / pressurized atmosphere furnace consisting of graphite insulation and a graphite heater. Here, hexagonal boron nitride powder (h-BN) is a white powder with a flaky crystal structure similar to graphite, and is a material with excellent thermal conductivity, heat resistance, corrosion resistance, electrical insulation, lubrication, and mold release properties.

[0076] The vacuum / pressurized atmosphere furnace used for the nitriding of the degreasing treatment sheet body can be, for example, a tight-box type electric furnace. By using such a vacuum / pressurized atmosphere furnace, gas generated inside the furnace can be discharged to the outside of the furnace. Furthermore, by using a tight-box electric furnace, which is a type of vacuum / pressurized atmosphere furnace, for example, when a binder removal step is performed to remove the organic binder used in forming the sheet body, the binder removal step, nitriding step, and sintering step can be performed using a single electric furnace. In this way, by using a vacuum / pressurized atmosphere furnace in the nitriding step included in the method for producing a silicon nitride substrate according to this embodiment, productivity of silicon nitride substrates can be increased.

[0077] Furthermore, when a degreasing treatment sheet containing composite powder is nitrided, the weight of the degreasing treatment sheet increases without any dimensional change. As a result, the relative density of the nitrided degreasing treatment sheet increases by about 15% compared to the unnitrided degreasing treatment sheet, facilitating densification of the silicon nitride sheet in the subsequent sintering process. Furthermore, the increased relative density of the nitrided degreasing sheet body has the advantage of enabling the sintering time of the silicon nitride sheet body to be shortened in the subsequent sintering step, and preventing excessive grain growth that adversely affects mechanical properties, etc. As such, the method for manufacturing a silicon nitride substrate according to this embodiment utilizes reactive sintering, which has excellent characteristics from the viewpoint of increasing the thermal conductivity of the silicon nitride substrate.

[0078] <Sixth step: step of sintering the nitrided sheet body> The method for producing a silicon nitride plate according to this embodiment includes a sintering step in which the nitriding treated sheet body is sintered to form a silicon nitride sintered body into a silicon nitride substrate. That is, the final sintering step is a step in which the nitrided sheet formed after the nitriding step is sintered in a nitrogen atmosphere, and the sintered nitrided sheet becomes the final product, a silicon nitride substrate.

[0079] The heating temperature of the nitrided sheet in the sintering step is not particularly limited as long as it is a temperature that can densify the structure of the nitrided sheet, but for example, heating at 1700 to 1950° C. is preferable, and heating at 1750 to 1900° C. is more preferable. In the sintering step, the time for sintering the nitrided sheet in a nitrogen atmosphere is preferably 1 to 48 hours, and more preferably 5 to 24 hours.

[0080] If the sintering temperature of the nitriding sheet in the sintering step is less than 1700°C or the sintering time is less than 1 hour, the microstructure of the nitriding sheet cannot be sufficiently densified, which is not preferred. On the other hand, if the heating temperature in the sintering step is higher than 1950°C or the sintering time exceeds 48 hours, excessive grain growth occurs in the particles constituting the nitriding sheet, which may reduce the strength of the silicon nitride substrate obtained after sintering the nitriding sheet, which is not preferred.

[0081] The sintering step is preferably carried out by heating in a nitrogen atmosphere. The pressure of the nitrogen atmosphere employed in the sintering step is not particularly limited, and it is preferable to carry out heating at a pressure such that the silicon nitride produced in the nitriding step does not decompose due to the heating temperature of the sintering step. Specifically, the pressure of the nitrogen atmosphere is preferably set to 0.1 MPa or higher, and more preferably 0.9 MPa or higher. However, if the pressure of the nitrogen atmosphere becomes too high, it becomes necessary to use a special furnace with high pressure resistance. Therefore, for example, the nitrogen atmosphere pressure is preferably set to 1 MPa or lower, and more preferably 0.92 MPa or lower.

[0082] By carrying out the sintering process, a silicon nitride substrate with a relative density of 95% or more can be obtained. As a result, the silicon nitride substrate obtained after the sintering process can be a dense silicon nitride substrate without an altered layer. Therefore, the silicon nitride substrate obtained after the sintering process can have a thermal conductivity of 85 W / mK or more, preferably 120 W / mK or more, in the unprocessed state, as measured by the laser flash method.

[0083] Furthermore, the sintered silicon nitride sheet obtained after the sintering step is the silicon nitride substrate manufactured by the silicon nitride plate manufacturing method according to this embodiment. Such a silicon nitride substrate can be a silicon nitride substrate containing β-phase silicon nitride as a main component and a rare earth element. The rare earth element contained in the silicon nitride substrate may be in a simple state or may form a compound with other substances. In this case, the silicon nitride substrate preferably contains 1.0 mol % to 4.0 mol % of rare earth elements calculated as oxides. Furthermore, the amount of magnesium present in the silicon nitride substrate after the sintering step is preferably 2.0 mol % or less calculated as oxides. When the silicon nitride substrate after the sintering step contains magnesium, the magnesium may be in a simple state or may form a compound with other substances.

[0084] The thickness of the silicon nitride substrate, which is a sintered silicon nitride sheet, is not particularly limited and can be any thickness, but when used as a heat-dissipating insulating substrate for semiconductor elements or electronic devices, for example, it is preferably 0.05 mm or more and 2.5 mm or less. The thickness of the silicon nitride substrate obtained after the sintering step can be selected by adjusting the thickness of the sheet formed in the sheet forming step.

[0085] As described above, the method for producing a silicon nitride substrate according to this embodiment uses a composite powder containing silicon nitride particles coated with metal silicon particles as the silicon source for the silicon nitride substrate. Therefore, the method for producing a silicon nitride substrate according to this embodiment does not generate heat due to the rapid progress of the nitriding reaction of the metal silicon particles contained in the metal silicon powder, and the nitriding reaction proceeds uniformly. In other words, the method for manufacturing a silicon nitride plate according to this embodiment uses metallic silicon as the silicon source contained in the sheet body constituting the silicon nitride substrate, and can manufacture a silicon nitride substrate containing silicon nitride obtained by nitriding the metallic silicon.

[0086] As described above, the method for producing a silicon nitride plate according to the first embodiment prevents heat generation due to a rapid nitridation reaction of silicon contained in metal silicon powder and allows a uniform nitridation reaction of silicon to proceed, thereby producing a silicon nitride substrate with high heat dissipation and excellent mechanical properties. In other words, the method for producing a silicon nitride plate according to the first embodiment can produce a silicon nitride substrate with a thermal conductivity of 80 W / mK or more as measured by the laser flash method. Furthermore, it is possible to provide a highly thermally conductive silicon nitride substrate produced by the method for producing a silicon nitride plate according to the first embodiment, and products using the silicon nitride substrate.

[0087] [Second embodiment] A method for producing a silicon nitride plate according to the second embodiment will be described. The method for producing a silicon nitride plate according to this embodiment is the same as the method according to the first embodiment, except that the average particle diameter D SN50The average particle diameter D of the metal silicon particles S50 The particle diameter ratio X is 0.2 to 2.0. The following describes the characteristic features of the method for producing a silicon nitride board according to this embodiment.

[0088] The first step in the method for producing a silicon nitride plate according to this embodiment, which is the step of forming composite powder, is to form a silicon nitride powder having an average particle diameter D SN50 The average particle size of metal silicon particles D S50 The particle diameter ratio X is set to 0.2 to 2.0. In other words, the method for manufacturing a silicon nitride plate according to this embodiment specifies the conditions for forming the composite particles that make up the composite powder, which is the silicon source contained in the sintered silicon nitride sheet, into metal silicon-silicon nitride particles that are coated or adsorbed by metal silicon particles, with silicon nitride particles as the base material. Furthermore, the method for producing a silicon nitride plate according to this embodiment specifies the conditions for forming the composite particles that make up the composite powder, which is the silicon source contained in the sintered silicon nitride sheet, into metal silicon-silicon nitride particles that are coated with silicon nitride particles and have metal silicon particles as the base material. In other words, the silicon nitride particles are adsorbed around the metal silicon particles, and the structure of the composite particles described above can also be reversed. In other words, in the method for manufacturing a silicon nitride plate according to this embodiment, the structure of the composite particles that make up the composite powder, which is the silicon source contained in the sintered silicon nitride sheet, can be appropriately selected depending on the average particle size of the silicon nitride particles that serve as the raw material for the composite powder before the mechanochemical effect is imparted by the fine grinding process, the average particle size of the metal silicon particles, the fine grinding process conditions, and the nitriding reaction conditions.

[0089] The composite powder is formed by mixing metal silicon powder and silicon nitride powder and applying a mechanochemical effect through a fine pulverization process using any one or a combination of a jet mill, a ball mill, a planetary mill, and an attritor. From such a technical viewpoint, in the manufacturing method of the silicon nitride plate according to the present embodiment, in the first step, the particle diameter D of the metal silicon particles constituting the metal silicon powder S50 and the particle diameter D of silicon nitride that constitutes the silicon nitride powder SN50 By controlling the temperature, the entire surface of the silicon nitride particles, which are the base material, is uniformly coated with metal silicon particles, or composite fine particles are formed in which metal silicon particles are adsorbed onto the entire surface or part of the silicon nitride particles. In addition, in the first step of the method for producing a silicon nitride plate according to this embodiment, the particle diameter D of the metal silicon particles constituting the metal silicon powder is S50 and the particle diameter D of silicon nitride that constitutes the silicon nitride powder SN50 By controlling the above, the entire surface of the metal silicon particle, which is the core particle or base material, is uniformly coated with silicon nitride particles, thereby forming composite microparticles in which the silicon nitride particles are adsorbed onto the entire surface or part of the metal silicon particle.

[0090] In the method for producing a silicon nitride plate according to this embodiment, the average particle diameter D of the silicon nitride particles SN50 The average particle size of metal silicon particles D S50 The particle size ratio X, which is the ratio of the above, can be expressed by the following relational expression (1).

[0091] Particle size ratio X= Average particle size of metal silicon particles D S50 / Average particle size of silicon nitride particles D SN50 (1)

[0092] The particle size ratio X can be set taking into consideration the particle sizes of the metal silicon particles and silicon nitride particles that make up the composite fine particles contained in the composite powder. A particle size ratio X of 0.2 or more is preferable because it allows the silicon nitride particles to be adsorbed onto the surfaces of the metal silicon particles that are the core particles that make up the composite powder fine particles. A particle size ratio X of 2.0 or less is preferable because the silicon nitride particles are not adsorbed to the surfaces of the metal silicon particles, which are the core particles that make up the composite powder fine particles, and the silicon nitride particles do not stick to each other. The particle size ratio X may be set appropriately depending on the particle size of the metal silicon particles and silicon nitride particles constituting the composite powder, and the pulverization method used for the pulverization, which may be any one or a combination of a jet mill, a ball mill, a planetary mill, and an attritor.

[0093] As described above, the method for producing a silicon nitride plate according to this embodiment uses, as the silicon source for forming the silicon nitride plate, metal silicon particles having an average particle diameter D S50 The average particle size of silicon nitride particles D SN50 By setting the particle size ratio X, which is the ratio of the average particle size D of the metal silicon particles, it is possible to adsorb the metal silicon particles and the silicon nitride particles. S50 The average particle size of silicon nitride particles D SN50 By setting the particle size ratio X, which is the ratio of: ##EQU1## it is possible to form a composite powder containing silicon nitride particles coated with metal silicon particles. In addition, the average particle diameter D of the metal silicon particles S50 The average particle size of silicon nitride particles D SN50 By setting the particle size ratio X, which is the ratio of: ##EQU1## it is possible to form a composite powder containing metal silicon particles coated or adsorbed by silicon nitride particles.

[0094] Therefore, in the method for manufacturing a silicon nitride substrate according to this embodiment, there is no heat generation due to the rapid progress of the nitridation reaction of the metal silicon particles exposed from the silicon nitride particles adsorbed on the surface of the metal silicon particles, which are the core particles that make up the composite powder microparticles, and the nitridation reaction proceeds uniformly over the entire surface of the metal silicon particles. In the method for manufacturing a silicon nitride substrate according to this embodiment, there is no heat generation due to the rapid progress of the nitriding reaction of the metal silicon particles adsorbed on the surface of the silicon nitride particles, which are the core particles that make up the composite powder microparticles, and the nitriding reaction proceeds uniformly over the entire surface of the metal silicon particles adsorbed on the surface of the silicon nitride particles. In other words, the method for manufacturing a silicon nitride plate according to this embodiment uses metallic silicon as the silicon source contained in the sheet body constituting the silicon nitride substrate, and can manufacture a silicon nitride substrate containing silicon nitride obtained by the nitriding reaction of metallic silicon particles.

[0095] As described above, the method for producing a silicon nitride substrate according to the second embodiment prevents heat generation due to a rapid nitriding reaction of the metal silicon particles contained in the metal silicon powder and allows the uniform nitriding reaction of silicon to proceed, thereby producing a silicon nitride substrate with high heat dissipation properties and excellent mechanical properties.

[0096] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the configuration and details of the present invention that are understandable to those skilled in the art within the technical scope of the present invention. Furthermore, systems or devices that combine the separate features included in each embodiment in any manner are also included in the technical scope of the present invention. [Example]

[0097] The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

[0098] <Example 1> A silicon nitride substrate was produced using the method for producing a silicon nitride substrate according to the present invention as follows. Furthermore, the properties of the silicon nitride substrate obtained using the method for producing a silicon nitride substrate according to the present invention were evaluated.

[0099] The metal silicon powder was prepared with a purity of 99.0%, an average particle size of 25.0 μm, and an impurity oxygen content of 0.40 mass %. The silicon nitride powder was prepared with a purity of 99.0%, an average particle size of 40.0 μm (manufactured by Shinano Electric Smelting Co., Ltd.). The amount of oxygen impurity in the metal silicon powder was measured using a nitrogen and oxygen simultaneous analyzer (Horiba, Ltd., Model: EMGA-20E).

[0100] (Complex formation process: production of powder seeds) A jet mill (Kurimoto, Model KJ-25) was prepared as the equipment required for mixing metal silicon powder and β-silicon nitride powder to form a metal silicon-silicon nitride composite. Metal silicon powder and silicon nitride powder were weighed and mixed in a predetermined ratio, and then mixed using the jet mill to form a metal silicon-silicon nitride composite. The composite powder produced in Example 1 was designated Powder Type No. 1.

[0101] The production conditions for powder type No. 1 were a rotor rotation speed of 12,000 rpm and a nozzle pressure of 0.60 MPa, and involved pulverizing metal silicon powder and β-silicon nitride powder and synthesizing a metal silicon-silicon nitride composite powder. The composite powder formation conditions were adjusted so that the average particle size of the particles making up the metal silicon-silicon nitride composite powder was 2.0 to 3.0 μm. Table 1 shows the conditions for forming powder type No. 1, which is a composite powder made of metal silicon and silicon nitride.

[0102] [Table 1]

[0103] (Production of slurry for forming sheet body) Next, a sheet-forming slurry was produced containing Powder Type No. 1, the composite powder used in Invention Example 1, a sintering aid, a dispersion liquid, and a binder. Table 2 shows the amounts of Powder Type No. 1, the composite powder used in the silicon nitride substrate produced in Invention Example 1, the rare earth compound, the magnesium compound, the dispersion liquid, and the binder. As shown in Table 2, magnesium oxide powder with an average particle size of 0.5 μm (manufactured by Kyowa Chemical Industry Co., Ltd.) or magnesium silicon nitride powder with an average particle size of 1.0 μm was used as the sintering aid contained in the sheet-forming slurry containing the raw material composite powder, sintering aid, dispersion medium, and binder. Also, yttrium oxide powder with an average particle size of 1.5 μm (manufactured by Shin-Etsu Chemical Co., Ltd.) was used as the rare earth element compound. In Table 2, only the proportions of the magnesium compound and the rare earth element compound are shown, but the remainder is powder type No. 1, which is silicon powder. The molar ratios shown in Table 2 are calculated based on the assumption that silicon (Si) is completely nitrided to silicon nitride (Si3N4), and the magnesium compound is calculated based on the molar ratio when converted to silicon nitride and magnesium oxide (MgO).

[0104] [Table 2]

[0105] Using ethanol as the dispersion medium, the prepared powder type No. 1 was milled and mixed for 24 hours using a silicon nitride ball mill with a resin pot and silicon nitride balls. Ethanol was weighed out in advance so that the slurry concentration would be 50 wt% and poured into the resin pot. After milling and mixing the composite powder, 14.3 wt% of an organic resin binder (Sekisui Chemical Co., Ltd., product name "S-LEC") was added, and the mixture was mixed for another 24 hours. The viscosity was then adjusted using a vacuum defoamer (manufactured by Eiko Co., Ltd.) to prepare a coating slurry. The viscosity of the coating slurry after viscosity adjustment was adjusted to 300 mPa·s.

[0106] (Forming of sheet body) The sheet-forming slurry, which was to be used as the coating slurry after viscosity adjustment, was formed into a sheet with a thickness of 0.4 mm using a doctor blade (manufactured by Sansho Industry Co., Ltd.) for the sample. After forming the sheet-forming slurry into a sheet, the formed sheet was cut into a size of 50 × 50 × 0.4 mm, and the relative density of the sheet was evaluated. The relative density of the sheet was evaluated by length measurement, and the result was that the relative density of the obtained sheet was 60% to 70%.

[0107] (Manufacturing of degreasing treatment sheet body) The sheet bodies produced in the sheet forming process were formed and evaluated. Then, boron nitride powder (hereinafter also referred to as "BN powder") was applied to the surface of the sheet body. 12 degreased sheet bodies coated with BN powder were stacked as a set and then placed in a boron nitride case (hereinafter also referred to as "BN crucible"). The BN crucible containing the 12 degreased sheet bodies coated with BN powder was then placed in a box furnace (manufactured by Motoyama Corporation, model: DC-6060) and heated at 600°C for 60 hours in an air atmosphere to perform a debinding process. That is, a degreased sheet body was obtained by degreasing the binder, which is a resin component contained in the sheet body formed from the sheet body-forming slurry.

[0108] (Manufacturing of nitrided sheet bodies) The BN crucible containing the degreased sheet was placed in a vacuum / pressurized atmosphere furnace (Shimadzu Industrial Systems Co., Ltd., Model: PVLgr10 VESTA), and the furnace was temporarily heated to 10 -1 After the pressure was reduced to 0.1 Pa, nitrogen was introduced into the furnace, and nitriding treatment was carried out in a nitrogen atmosphere of 0.1 MPa at 1480° C. for 8 hours. Nitrogen gas used was 99.9 vol % nitrogen gas. The degreased sheet body obtained after nitriding in this manner was designated as the nitrided sheet body of Production Example 1. In Invention Example 1, the temperature rise rate employed to obtain the nitrided sheet body was set to 0.2°C / min.

[0109] (Sintering of nitride sheet body) Next, as post-sintering, the nitride sheet body obtained by nitriding the sample of Production Example 1 in the nitriding step was fired under specified conditions. The fired nitride sheet body of Production Example 1 was used as the silicon nitride substrate of Invention Example 1. X-ray diffraction measurement (manufactured by Rigaku Corporation, model: Mini Flex 600) was performed on the silicon nitride substrate obtained in Invention Example 1. The results of the X-ray diffraction measurement are shown in Table 3. As shown in Table 3, it was found that no residual Si was found in the silicon nitride substrate. The nitrided sheet body was sintered using the same vacuum and pressurized atmosphere furnace as in the nitriding process, with the nitrided sheet body stacked in the same manner being placed in a BN crucible, and the BN crucible then being placed in a vacuum and heated atmosphere furnace.

[0110] <Measurement and evaluation of various properties of silicon nitride substrates> After sintering the nitride sheet, the sintered silicon nitride sheet was removed from the BN crucible, and the BN powder adhering to the surface was removed using a sandblasting device (manufactured by Fuji Seisakusho), resulting in a silicon nitride substrate as the final product. The silicon nitride substrates thus produced were subjected to structural observation and evaluation. Furthermore, the properties of the silicon nitride substrates were measured. Specifically, the silicon nitride substrates were subjected to structural observation, and the relative density, thermal conductivity, and mechanical properties (three-point bending strength, fracture toughness) of the silicon nitride substrates were measured. The measurement results and evaluation of the properties of the silicon nitride substrates are shown in Table 3.

[0111] (structural observation) The silicon nitride substrate produced in Example 1 was subjected to structural observation. The silicon nitride substrate was polished in cross section and observed using a scanning electron microscope (SEM) (Model JSM-IT210, manufactured by JEOL Ltd.). Table 3 shows the results of SEM observation of the cross section of the silicon nitride substrate produced in Example 1. The structural observation was evaluated as follows: ○: Sintered body whose structure could be observed -: Sintered body that was melted and had low density, so the structure was not observed.

[0112] Figure 2 shows an SEM photograph of a cross section of the silicon nitride substrate produced in Example 1. Figure 2 is an SEM image of the fine particles that make up the composite powder used in the method for producing a silicon nitride substrate. Figure 2A is an SEM image of the metal silicon particles and silicon nitride particles that make up the composite powder before they are crushed, and Figure 2B is an SEM image of the metal silicon particles and silicon nitride particles that make up the composite powder after they have been crushed. As is clear from Figure 2, the composite powder obtained by the mechanochemical effect of the fine grinding process has smaller particle sizes of the composite particles that make up the composite powder compared to before the composite powder was synthesized (before the mechanochemical effect).

[0113] Next, a cross section of the silicon nitride substrate produced in Example 1 was observed using an SEM, and then the elements contained in the silicon nitride substrate were identified using energy dispersive X-ray spectroscopy (EDS). Figures 3 and 4 show two-dimensional images obtained by color-coding the element distribution based on the element peak information detected using EDS in Example 1. Figure 3 is an EDS image of the metal silicon particles and silicon nitride particles that make up the composite powder after being crushed. Figure 4 is an enlarged image of the EDS image of the metal silicon particles and silicon nitride particles that make up the composite powder after being crushed. As is clear from Figures 3 and 4, it was found that nitrogen was distributed on part of the surface of the composite particles that make up the composite powder, which confirms that silicon nitride particles are bonded to the surfaces of metal silicon particles to form composites. Furthermore, Figure 5 shows an SEM image of composite silicon particles produced by dispersing fine particles of a sintering aid (magnesium compound) on the surface of metal silicon particles that constitute the composite powder used in Example 1. Figure 6 shows an EDS image of composite silicon particles by dispersing fine particles of a sintering aid on the surface of metal silicon particles that constitute the composite powder. As is clear from FIGS. 5 and 6, it was found that fine particles of sintering aid were uniformly dispersed on the surfaces of the metal silicon particles. By using such metal silicon particles as a component of the composite powder, in combination with the addition of a sintering aid used in producing the slurry for forming the sheet body, the sinterability of the silicon nitride particles contained in the sheet body can be further improved.

[0114] (Relative density measurement) The relative density of the silicon nitride substrate produced in Invention Example 1 was measured. The relative density of the silicon nitride substrate was measured by phase identification using X-ray diffraction and according to the Archimedes method. Table 3 shows the measurement results of the relative density of the silicon nitride substrate produced in Invention Example 1.

[0115] (Measurement of thermal conductivity) The thermal conductivity of the silicon nitride substrate produced in Example 1 was measured. The thermal conductivity of the silicon nitride substrate was measured specifically as follows. The produced silicon nitride substrate was cut into 10 mm squares, and the surface was coated with Au (manufactured by Sanyu Electronics Co., Ltd., model: SC-701AT). The thermal conductivity was measured using the xenon flash method (manufactured by NETZSCH Japan Co., Ltd., model: LFA467). Table 3 shows the measurement results of the thermal conductivity of the silicon nitride substrate produced in Example 1.

[0116] (Mechanical property measurements) The mechanical properties of the silicon nitride substrate manufactured in Example 1 were measured. Three-point bending strength and fracture toughness were adopted as the mechanical properties of the silicon nitride substrate. The mechanical properties of the silicon nitride substrate were measured as follows. Specifically, the three-point bending strength (ISO 23242:2020) and fracture toughness (ISO 21113:2018) of the manufactured silicon nitride substrate were measured and evaluated in accordance with ISO. Table 3 shows the measurement results of the mechanical properties of the silicon nitride substrate manufactured in Example 1.

[0117] [Table 3]

[0118] First, as is clear from Figure 2, the cross section of the silicon nitride substrate was polished, and the phase of the silicon nitride substrate was identified by X-ray diffraction based on a scanning electron microscope (SEM) photograph, confirming that silicon nitride was obtained from the metal silicon powder used in Invention Example 1. The SEM observation also confirmed that there were few voids and that the silicon nitride had a bimodal structure.

[0119] Furthermore, when the cross section of the silicon nitride substrate produced in Invention Example 1 was observed using an SEM, it was confirmed that no altered layer was present. As is clear from the measurement results of the silicon nitride substrate shown in Table 3, it was confirmed that the silicon nitride substrate produced in Example 1 was a dense body with a relative density of 99% or more and a thermal conductivity of approximately 80 W / mK.

[0120] <Invention Examples 2 to 24, Comparative Examples 1 to 16> The silicon nitride substrates of Invention Examples 2 to 24 and Comparative Examples 1 to 16 were manufactured in the same manner as Invention Example 1, except that the raw materials for the silicon nitride sintered body (powder species, magnesium compound), raw material composition, conditions for forming the composite which is the powder species, the heating rate during nitriding of the degreasing treatment sheet body, and sintering conditions were changed. Specifically, powder types Nos. 2 to 10 were used as the powder types constituting the silicon nitride substrates manufactured in Examples 2 to 24, with the manufacturing conditions changed. Magnesium silicon nitride powder was used as the magnesium compound contained in the sheet molding slurry. In Examples 2 to 24, the temperature rise rate used when nitriding the degreased sheet was 0.2 to 1.0°C / min. Tables 1 to 5 show the raw materials, composite formation conditions, sheet formation conditions, nitriding treatment conditions, and sintering conditions for the silicon nitride substrates produced in invention examples 2 to 24 and comparative examples 1 to 16. It was confirmed that the silicon nitride substrate produced in Example 17 was a dense body with a relative density of 99% or more and a thermal conductivity of 121 W / mK.

[0121] [Table 4]

[0122] [Table 5]

[0123] As is clear from Tables 1 to 5, increasing the proportion of metal silicon powder in the composite powder that constitutes the silicon nitride sintered body (Table 1, Powder Type No. 9, Comparative Examples 9 to 12) makes it impossible to suppress the heat generated by the rapid nitriding reaction of silicon. Therefore, in the sintering step included in the manufacturing process of silicon nitride sintered body, the rate of increase in the sintering temperature for the nitride sheet body when sintering the nitride sheet body cannot be increased. As a result, the silicon nitride sintered body obtained by the sintering step melts due to the heat generated. On the other hand, when the proportion of silicon nitride powder added to the composite powder that constitutes the silicon nitride sintered body is increased (Table 1, Powder Type No. 10, Comparative Examples 13 to 16), the proportion of metal silicon powder added is small, so heat generation due to the rapid nitriding reaction of silicon can be suppressed. Therefore, in the sintering step included in the manufacturing process of silicon nitride sintered body, the rate of temperature rise for the nitride sheet when sintering the nitride sheet can be increased. As a result, the silicon nitride sintered body obtained by the sintering step will contain a lot of oxygen inside the silicon nitride sintered body, and its thermal conductivity will be reduced.

[0124] <Invention Examples 25 to 48, Comparative Examples 17 to 24> Silicon nitride substrates of invention examples 25 to 48 and comparison examples 17 to 24 were manufactured in the same manner as invention example 1, except that the raw materials for the silicon nitride sintered body (powder species, magnesium compound), raw material composition, conditions for forming the composite which is the powder species, the heating rate during nitriding of the degreasing treatment sheet body, and sintering conditions were changed. Specifically, powder types Nos. 11 to 18, which were produced under modified manufacturing conditions, were used as the powder types constituting the silicon nitride substrates produced in Examples 25 to 48. Magnesium oxide powder was used as the magnesium compound contained in the sheet molding slurry. In Examples 25 to 48, the temperature rise rate used when nitriding the degreasing treatment sheet was 0.2 to 1.0°C / min. Tables 1 and 7 to 8 show the raw materials, composite formation conditions, sheet formation conditions, nitriding treatment conditions, and sintering conditions for the silicon nitride substrates produced in Examples 25 to 48 and Comparative Examples 17 to 24.

[0125] [Table 6]

[0126] [Table 7]

[0127] [Table 8]

[0128] From these experimental data, it was found that silicon nitride substrates manufactured using composite powder consisting of metal silicon and silicon nitride, which is formed by finely pulverizing a mixture of metal silicon powder and silicon nitride powder, have a thermal conductivity of approximately 80 W / mK or more, with the highest thermal conductivity being 120 W / mK or more. As a result, it was found that silicon nitride substrates manufactured using a composite powder consisting of metal silicon and silicon nitride, formed by finely pulverizing a mixed powder of metal silicon powder and silicon nitride powder, have high heat dissipation properties. Furthermore, silicon nitride substrates manufactured using such composite powders of metal silicon and silicon nitride were found to have excellent mechanical properties, as evidenced by good results in the measurement and evaluation of three-point bending strength and fracture toughness, which indicate mechanical properties.

[0129] Thus, the method for producing a silicon nitride substrate according to the present invention uses a composite powder of metal silicon-silicon nitride, which is a mixture of metal silicon powder and silicon nitride powder. Therefore, the silicon nitride particles adsorbed to the metal silicon particles are uniformly present in the composite powder without agglomerating, preventing the metal silicon particles exposed from the composite particles from undergoing a rapid nitriding reaction. Furthermore, the metal silicon particles coating the silicon nitride particles undergo a uniform nitriding reaction. That is, the composite powder contains metal silicon particles as core particles, and the metal silicon particles exposed from the silicon nitride particles coating the surfaces of the metal silicon particles are converted into silicon nitride particles by a nitriding reaction. As a result, after the nitriding reaction is completed, the composite powder becomes a silicon source powder composed only of silicon nitride particles.

[0130] In other words, the method for producing a silicon nitride substrate according to the present invention allows for the use of a large amount of metallic silicon powder as the silicon source for the sintered silicon nitride sheet, thereby reducing the production cost of the silicon nitride substrate. Thus, it has become clear that the method for producing a silicon nitride substrate according to the present invention can satisfy both the requirements for production cost and the symbiosis of thermal conductivity and mechanical properties of the produced silicon nitride substrate. [Industrial Applicability]

[0131] According to the present invention, it is possible to produce a silicon nitride substrate having high heat dissipation properties and excellent mechanical properties by preventing a rapid nitridation reaction of silicon, and therefore the invention contributes to the development of the semiconductor-related industry and is extremely useful industrially.

Claims

1. a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a sintering aid and a dispersion medium to form a sheet-forming slurry in which the sintering aid is dispersed in the composite powder; a third step of forming the sheet body by molding the sheet body-forming slurry; a fourth step of heating the sheet body at 250 to 600°C to degrease the resin component contained in the sheet body, thereby forming a degreased sheet body; a fifth step of heating the degreasing treatment sheet at 1200 to 1500°C to nitride the silicon contained in the degreasing treatment sheet, thereby forming a nitriding treatment sheet; and a sixth step of sintering the nitriding sheet body to form a silicon nitride sintered body into a silicon nitride substrate, The composite powder and the sintering aid are used as raw material powders, the composite powder contains the metal silicon powder and the silicon nitride powder in a molar ratio of 70:30 to 80:20 calculated as silicon nitride, It is formed by the adsorption of metal silicon particles and silicon nitride particles, the sintering aid comprises a rare earth element compound and a magnesium compound; the rare earth element compound contains one or more elements selected from Y, Sc, La, Ce, Nd, Sm, Gd, Dy, Ho, Er, and Yb; The rare earth element compound is such that, when silicon in the composite powder is converted into silicon nitride, the raw material powder contains the rare earth element compound in an amount of 1.0 mol % or more and 7.0 mol % or less in terms of oxide, and the magnesium compound includes one or more magnesium compounds selected from magnesium oxide, magnesium silicide, and magnesium silicon nitride; A method for producing a silicon nitride substrate, characterized in that the raw material powder contains the magnesium compound in an amount of 8.0 mol % or more and 15.0 mol % or less, calculated as oxide, when the silicon in the composite powder is converted into silicon nitride.

2. 2. The method for producing a silicon nitride substrate according to claim 1, wherein the fine pulverization is carried out by any one or combination of mills selected from the group consisting of a jet mill, a ball mill, a bead mill, a planetary mill, an attritor, and a mechanochemical mill.

3. The average particle diameter D of the silicon nitride particles ＳＮ５０ The average particle diameter D of the metal silicon particles Ｓ５０ 3. The method for producing a silicon nitride substrate according to claim 1, wherein the particle size ratio X is 0.2 to 2.

0.

4. 3. The method for producing a silicon nitride substrate according to claim 1, wherein the metal silicon particles have fine particles of the sintering aid uniformly dispersed on the surfaces of the metal silicon particles.

5. 3. The method for manufacturing a silicon nitride substrate according to claim 1, wherein the sheet has a relative density of 45% or more.

6. 3. The method for producing a silicon nitride substrate according to claim 1, wherein the thermal conductivity of the silicon nitride substrate is 80 W / mK or more as measured by a xenon flash method.

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