Method for producing silicon nitride substrate

Abstract

Provided is a method for producing a silicon nitride substrate having high heat dissipation and excellent mechanical characteristics by preventing a rapid nitriding reaction and performing a uniform nitriding reaction of the silicon. The method comprises a first step for forming a composite powder by mixing a metal silicon powder and a silicon nitride powder and subjecting the mixture to a fine pulverization treatment, a second step for 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, a third step for molding the slurry for forming a sheet body to form a sheet body, a fourth step for heating the sheet body at 250-600°C and degreasing the resin component to form a degreased sheet body, a fifth step for heating the degreased sheet body at 1200-1500°C to nitride the silicon contained in the sheet body to form a nitrided sheet body, and a sixth step for sintering the nitrided sheet body to form a silicon nitride substrate. The composite powder is characterized by being formed by the adsorption of metal silicon particles and silicon nitride particles.

Description

Method for manufacturing silicon nitride substrate

[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.

[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 required to have high heat dissipation properties and excellent mechanical properties. In other words, for insulating substrates used in power devices, heat dissipation technology for 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 general 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 viewpoint, a method for producing a dense silicon nitride substrate has been proposed, which can be produced from a raw material powder containing silicon powder and 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, which 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 after 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 silicon nitride-forming step 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 that is a sintered body composed of a main phase consisting of silicon nitride particles and a grain boundary phase consisting of a sintering aid, and that has a surface waviness of 1.0 μm or less and 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.

[0009] Japanese Patent No. 5836522 Japanese Patent Application Laid-Open No. 2022-027444 International Publication No. 2013 / 054852

[0010] However, each of the above-mentioned conventional technologies has the following problem to 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, if 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 the heat generation of silicon contained in the silicon powder as the 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 as the 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 does not allow the use of 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 body, 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 nitriding reaction of silicon powder.

[0013] Thus, silicon nitride substrates produced by conventional methods for producing silicon nitride substrates have not satisfied all of these requirements from the two perspectives of production cost and symbiosis of thermal conductivity and mechanical properties. Therefore, from the perspective of reducing the cost required for raw material powder, even when using a so-called reaction sintering method in which inexpensive silicon powder is used as the raw material powder and a compact thereof is nitrided in nitrogen and then sintered at high temperature, there is a need for a method for producing silicon nitride substrates that includes a nitriding step that suppresses heat generation due to a rapid nitriding reaction of the silicon powder and 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.

[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. The method comprises 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 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 sintered body, 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 is characterized in that: (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 is 1 / 2 or 1 / 4 of the silicon nitride particles; ＳＮ５０ the average particle diameter D of the metal silicon particles Ｓ５０(c) in the first step, the metal silicon powder and the silicon nitride powder are contained in a molar ratio of 70:30 to 95:5 in terms of silicon nitride; (d) the metal silicon particles have fine particles of the sintering aid uniformly dispersed on their surfaces; (e) the sintering aid is a magnesium compound, and the magnesium compound contains one or more magnesium compounds selected from magnesium oxide, magnesium silicide, and magnesium silicon nitride; (f) the sintering aid is a 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 a xenon flash method.

[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.

[0019] 1 is a flow chart showing each step of a method for producing a silicon nitride plate according to the present invention. It is an SEM image of the 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 they are pulverized. It is an SEM image of the 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 they have been pulverized. It is an EDS image of the metal silicon particles and silicon nitride particles that constitute the composite powder after they have been pulverized. It is an enlarged image of the EDS image of the metal silicon particles and silicon nitride particles that constitute the composite powder after they have been pulverized. It is an SEM image of composite silicon particles in which fine particles made of a sintering aid are dispersed on the surface of the metal silicon particles that constitute the composite powder. It is an EDS image of composite silicon particles in which fine particles made of a sintering aid are dispersed on the surface of the metal silicon particles that constitute the composite powder.

[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 be described below. Fig. 1 is a flow chart showing each step of the method for manufacturing a silicon nitride plate according to this embodiment. As shown in Figure 1, the method for producing 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 them 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 nitride the metal silicon contained in the degreased sheet to form a nitriding sheet; and a sixth step of sintering the nitriding sheet to form a silicon nitride substrate. The composite powder is formed by adsorption of metal silicon particles and silicon nitride particles. Each step of the method for producing a silicon nitride plate according to this embodiment will be described below.

[0022] <First Step: Forming Composite Powder> The method for manufacturing a silicon nitride plate according to this embodiment includes a composite powder formation step in which metal silicon powder and silicon nitride powder are mixed together to form a composite powder through a mechanochemical effect caused by a fine pulverization process. The composite powder formation step is a step of forming a composite powder, which is the raw material for the sintered silicon nitride sheet required to manufacture a silicon nitride substrate. That is, the first step, the composite powder formation step, is a step of mixing metal silicon powder and silicon nitride powder together to form a composite powder through a mechanochemical effect caused by a fine pulverization process, as the raw material for the silicon nitride sintered body required to manufacture a silicon nitride substrate. Here, the mechanochemical effect is the effect in which the application of mechanical energy to a substance changes the bonding state of the substance, activating it. The mechanochemical effect occurs when particles are subjected to continuous mechanical energy such as impact, compression, shear, shear stress, and friction during the process of being crushed into fine particles. This causes the crystalline structure of the particles to change, activating the particle surface and causing a chemical reaction with the substances surrounding 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, the method for producing a silicon nitride substrate according to this embodiment uses silicon nitride powder in addition to metal silicon powder as the silicon source for the silicon nitride sintered body required for producing the silicon nitride substrate. That is, the method for producing 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 metallic silicon powder and silicon nitride powder is used as the silicon source for the silicon nitride sintered body required to produce 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, as this makes it possible to reduce 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 the silicon nitride substrate is preferably 80 mol% or more, more preferably 90 mol% or more, and 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 the raw material powder contains 3 mol of silicon powder and 1 mol of silicon nitride powder as the silicon source, the silicon nitride powder will account for 50 mol% of the silicon source 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. 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 imparting the mechanochemical effect 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. Note that the average particle size refers to the particle size at 50% of the integrated value in the particle size distribution determined by laser diffraction / scattering. Generally, commercially available silicon nitride powders and silicon powders 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 reaction sintering and uses metal silicon powder as the starting material, which has a significant advantage in terms of reducing the amount of oxygen compared to using only silicon nitride as the starting material. This is because, when silicon powder is used as the starting material, a sheet-forming process is performed in which a composite powder consisting of the starting materials, metal silicon powder and silicon nitride powder, is formed into a sheet, followed by a nitriding process in which a sheet containing the composite powder is nitrided. In the nitriding process, the nitriding reaction shown in the following reaction formula (1) proceeds.

[0030] 3Si+2N ２ = Si ３ N ４　　　　　　　　　 (1)

[0031] The weight of the sheet containing the composite powder increases by about 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 composed 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] That is, the method for producing 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 composed of metal silicon powder and silicon nitride powder as a silicon supply source. As a result, the influence of the impurity oxygen content in the metal silicon powder and silicon nitride powder constituting the composite powder is negligible. Therefore, in the method for producing a silicon nitride substrate according to this embodiment, a variety of metal silicon powders can be used, ranging from low-quality metal silicon powders with high impurity oxygen concentrations to high-quality 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 powders with low impurity oxygen concentrations. Here, the metal silicon particles constituting the metal silicon powder may be silicon particles having fine particles composed 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, 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 slurry formation step of the second step described below. The particle diameter of the fine particles made of the sintering aid is preferably 0.5 μm or less. By uniformly dispersing the fine particles made 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 Pulverization) In the composite powder formation 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. The fine pulverization is performed using any one or combination of a jet mill, ball mill, bead mill, planetary mill, attritor, and mechanochemical mill. For example, a jet mill ejects compressed air, high-pressure steam, or high-pressure gas at several atmospheres or more from a jet nozzle, accelerating the raw material particles, metal silicon particles and silicon nitride particles, with the jet stream, and pulverizing these particles by collisions between the accelerated particles or by impact or friction with the accelerated particles. Jet milling is performed using a jet mill fine pulverizer that can impart such effects to the metal silicon powder and silicon nitride powder to further pulverize them.

[0034] When metal silicon particles and silicon nitride particles are finely pulverized by dry milling using a jet mill, a fine powder having a particle size of 0.1 to 10.0 μm, preferably 1.0 to 5.0 μm, can be obtained. Finely pulverizing using a jet mill results in little temperature rise and is suitable for pulverizing heat-sensitive materials. However, finely pulverizing using a jet mill 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 material. 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] Jet mill pulverizers include micronizers that use jets that create horizontal swirling flows, jet-o-mizers that use vertical swirling flows, Blaw-Knox or Trost jet mills that use counter-impingement of solid-gas mixed flows, methods in which a solid-gas mixed jet impinges on an impingement plate, and methods in which a solid-gas mixed flow is mixed in an ultrasonic nozzle. These methods can be appropriately adopted and used in the composite powder formation process included in the silicon nitride plate manufacturing method of this embodiment. Furthermore, many jet mills used in the composite powder formation process not only perform dry pulverization, but also effectively utilize airflow to perform classification, or may be directly connected to 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 contain metal silicon particles coated with silicon nitride powder. That is, the composite particles may have a metal silicon particle serving as a core particle located at the center of the composite particle as a base material. The composite particles have silicon nitride particles that coat or adsorb onto the entire surface or a portion of the metal silicon particle serving as a core particle. The number of silicon nitride particles used to coat or adsorb onto the entire surface or a portion of the metal silicon particle is greater than the number of metal silicon particles serving as core particles. The average particle size of the silicon nitride particles used to coat or adsorb onto the entire surface or a portion of the metal silicon particle may be smaller than the average particle size of the metal silicon particle serving as a core particle. 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 adsorb 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 particles 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 the entire surface or a portion of the silicon nitride particles may be smaller than the average particle size of the silicon nitride particles that are the core particles.

[0038] For this reason, when a mixed powder formed from a metal silicon powder and a silicon nitride powder is used as a base material for large particle diameter metal silicon particles, small particle diameter silicon nitride particles may be adsorbed to the surface by the mechanochemical effect of the fine grinding process. As a result, composite particles are formed as particles constituting the composite powder, in which the entire surface or a portion of the metal silicon particles located at the center of the composite particle are coated or adsorbed by silicon nitride particles. Furthermore, when a mixed powder formed from a silicon nitride powder and a metal silicon powder is used as a base material for large particle diameter silicon nitride, small particle diameter metal silicon particles may be adsorbed to the surface by the mechanochemical effect of the fine grinding process. As a result, composite particles are formed as particles constituting the composite powder, in which the entire surface or a portion of the silicon nitride particles located at the center of the composite particle are coated or adsorbed by metal silicon particles. Furthermore, composite particles are formed as particles constituting the composite powder, in which the entire surface or a portion of the silicon nitride particles located at the center of the composite particle are adsorbed by 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 composed 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 silicon nitride particles in a nitriding step of a sheet body containing the composite powder, which will be described later. 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. Furthermore, silicon nitride particles may 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 to silicon nitride particles in a nitriding step of a sheet body containing the composite powder, which will be described later. 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 manufacturing a silicon nitride plate 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 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 described below, the method for producing a silicon nitride substrate according to this embodiment involves forming a sheet-forming slurry containing metallic silicon powder, removing the binder contained in the sheet formed from the sheet-forming slurry, and then nitriding the silicon contained in the degreased sheet molded body, followed by a sintering process to produce a silicon nitride substrate. However, since silicon nitride is difficult to sinter, it 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] To reduce the amount of dissolved oxygen in the silicon nitride particles in the 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. To reduce the low-thermal-conductivity grain boundary glass phase in the 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 evaporates during sintering at high temperatures. Thus, in the slurry formation process, 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, when the silicon in the raw material powder is converted into silicon nitride, the raw material powder contains the rare earth compound in an amount of 1.0 mol % to 7.0 mol % in terms of oxide. The amount (molar concentration) of the rare earth compound added means the molar concentration of the rare earth compound in the three components of the composite powder as a silicon source, the rare earth compound, and the magnesium compound, and the same applies hereinafter.

[0047] Rare earth element 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 element compound added is less than 1.0 mol%, oxygen cannot be trapped in the grain boundary glass phase, the amount of dissolved oxygen contained 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 element compound added is more than 7.0 mol%, the amount of low thermal conductive grain boundary phase containing the rare earth element 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, in terms of oxide, and it is even more preferable to add the rare earth element compound so that the raw material slurry 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 the rare earth element compound include rare earth element oxides. Specific examples of the rare earth element compound 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 % or more and 15.0 mol % or less 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 specialized sintering furnace and increases process costs. Furthermore, sintering silicon nitride sintered bodies at ultra-high temperatures results in significant grain growth of the particles constituting the silicon nitride sintered body, resulting in a deterioration of the mechanical properties of the silicon nitride sintered body. From this technical perspective, in order to promote densification of the silicon nitride sintered body during post-sintering and thereby enable the silicon nitride sintered body to exhibit high strength and toughness, it is preferable to add a magnesium compound as a sintering aid simultaneously with the addition of the rare earth element compound. The addition of a magnesium compound allows Mg ions to function as modifying ions in the oxynitride glass generated during heating, reducing the viscosity of the oxynitride glass and promoting densification of the silicon nitride sintered body structure, 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 inhibit 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] As the magnesium compound, for example, magnesium silicide, fluoride, boride, nitride, and ternary compounds thereof can be used. In particular, magnesium oxide (MgO), magnesium silicide (Mg ２ Si) or magnesium silicon nitride (MgSiN ２) is preferably added to the raw material powder. The magnesium compound added to the raw material powder is preferably one or more selected from magnesium oxide (MgO), magnesium silicide (Mg ２ Si) or magnesium silicon nitride (MgSiN ２ ) is more preferably one or more magnesium compounds selected from the group consisting of magnesium compounds having hydroxypropyl methyl group and magnesium compounds having hydroxypropyl methyl group.

[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 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. Below, specific examples of conditions for forming the raw material slurry by adding the sintering aid, dispersion medium, and organic binder to the composite powder in the slurry formation step will be described.

[0056] The composite powder and sintering aid are weighed out. A milling vessel such as a ball mill is prepared, charged with 0.5 to 2 wt % of a dispersant and 30 to 70 wt % of an organic solvent, etc., based on the total weight of the composite powder and sintering aid. The composite powder and sintering aid are added to the milling vessel equipped with a milling device that also contains the dispersant. 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. Note that it is also possible to use only the dispersion medium without adding a dispersant. The raw material powders can be mixed and milled using a ball mill.

[0057] The time for which mixing and grinding is carried out is not particularly limited, as it varies depending on the function of the milling device 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 uniformly mixed into the raw material slurry, and unevenness will occur in the nitriding sheet after sintering the nitriding sheet, which is not preferable. On the other hand, if the mixing and grinding time is longer than 48 hours, the mixing and grinding of the composite powder and sintering aid will not significantly change the mixed state, and there is a risk of impurities being mixed in from the balls or pot. In addition, after mixing and grinding the composite powder and sintering aid in the slurry formation process, 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 % of an organic binder (organic binding agent) can be added and then mixed 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 can be preferably used.

[0060] The mixing time after adding the organic binder is not particularly limited, as it varies depending on the performance of the equipment 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 during sheet molding, which is undesirable. Furthermore, since there is usually no significant change in the mixed state of the organic binder and raw material powder even if mixing is performed for a time longer than 24 hours, a mixing time of 24 hours or less is preferred from the standpoint of productivity.

[0061] After adding an organic binder and mixing, the prepared slurry is vacuum degassed to adjust the viscosity of the slurry, thereby forming a sheet body-forming slurry. The sheet body-forming slurry thus 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 body-forming slurry is 100 to 200 cps before vacuum degassing, and is 6,000 to 7,000 cps after vacuum degassing. As a result, the sheet body-forming slurry after vacuum degassing becomes a slurry suitable for forming a sheet body.

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

[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 body obtained in the sheet body forming step is preferably 45% or more, more preferably 50% or more. When the sheet body formed in the sheet body forming step is cut to a predetermined size, the relative density of the cut sheet body is preferably 45% or more. The relative density of the sheet body formed in the sheet body forming step can be adjusted by the amount of raw material powder (solid content concentration) contained in the sheet body forming slurry formed in the sheet body forming slurry forming step and the amount of binder added to the slurry. Here, by making the relative density of the sheet body formed in the sheet body forming step 45% or more, the pores in the sheet body can be sufficiently reduced, and the sintered density of the silicon nitride substrate obtained after the sintering step described below can be increased, which is preferable.

[0066] The upper limit of the relative density of the sheet body formed in the sheet body forming step is not particularly limited. In order to increase the relative density of the sheet body, it is necessary to increase the solid content of the sheet body forming slurry by reducing the amount of binder or the like added in the sheet body forming slurry forming step. If the solid content of the sheet body forming slurry becomes high, cracks or the like will occur in the sheet body formed from the sheet body forming slurry, making it difficult to handle. Therefore, the relative density of the sheet body obtained in the sheet body forming step is preferably 65% ​​or less, more preferably 60% or less.

[0067] <Fourth Step: Degreasing Step for Forming a Degreased Sheet Body> The method for manufacturing a silicon nitride plate according to this embodiment further 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 molding step is heated in an atmosphere of air or 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 at 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 (resin) contained in the sheet body.

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

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

[0070] In the nitriding step, the metal silicon particles present on the surface of the composite particles are nitrided to become silicon nitride particles. All of the metal silicon particles present on the surface of the composite particles become silicon nitride. In addition, in the nitriding step, the metal silicon particles exposed from the surface of the composite particles are nitrided to become silicon nitride particles. All of the metal silicon particles exposed from the surface of the composite particles 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 step is formed from the silicon nitride powder prepared initially and the silicon nitride powder produced by the nitriding reaction of the metal silicon powder.

[0071] Furthermore, before starting the nitriding step (step 5), 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 evacuate to 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, for example, 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 step is 1000°C or higher, or the treatment time of the nitriding step 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 step 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 less, or if the treatment time in the nitriding step is 15 hours or less, the sintering aid, which is a composition component contained in 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 there is no case where it becomes 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 composed 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 in 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 type electric furnace, which is one 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. Thus, by using a vacuum / pressurized atmosphere furnace in the nitriding step included in the method for producing a silicon nitride plate according to this embodiment, productivity of silicon nitride substrates can be increased.

[0077] Furthermore, when a degreasing treatment sheet containing a composite powder is nitrided, the weight of the degreasing treatment sheet increases without any dimensional change. Therefore, the relative density of the nitrided degreasing treatment sheet is about 15% higher than that of 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 treatment sheet has the advantages of shortening the sintering time of the silicon nitride sheet in the subsequent sintering process and preventing excessive grain growth, which adversely affects mechanical properties. Thus, the method for manufacturing a silicon nitride substrate according to this embodiment utilizes reactive sintering, which provides excellent features in terms of increasing the thermal conductivity of the silicon nitride substrate.

[0078] <Sixth Step: Sintering of Nitriding Sheet Body> The method for manufacturing a silicon nitride plate according to this embodiment includes a sintering step in which the silicon nitride sintered body formed by sintering the nitriding sheet body is converted into a silicon nitride substrate. That is, the final step, the sintering step, is a step in which the nitriding sheet body formed after the nitriding step is sintered in a nitrogen atmosphere. Then, in the sintering step, the sintered nitriding sheet body becomes the final product, the silicon nitride substrate.

[0079] The heating temperature of the nitrided sheet body in the sintering step is not particularly limited as long as it is a temperature that can densify the structure of the nitrided sheet body, 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 body 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 under 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 more, more preferably 0.9 MPa or more. However, if the pressure of the nitrogen atmosphere becomes too high, it becomes necessary to use a special furnace with high pressure resistance, so for example, the nitrogen atmosphere pressure is preferably 1 MPa or less, more preferably 0.92 MPa or less.

[0082] By carrying out the sintering step, 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 step can be a dense silicon nitride substrate free of an altered layer. Therefore, the silicon nitride substrate obtained after the sintering step 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 in terms of oxide. Furthermore, the amount of magnesium present in the silicon nitride substrate after the sintering step is preferably 2.0 mol% or less in terms of oxide. 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 the 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, 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] Thus, the method for producing a silicon nitride plate 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 plate. 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 producing a silicon nitride plate according to this embodiment uses metal silicon as the silicon source contained in the sheet that constitutes the silicon nitride substrate, and can produce a silicon nitride substrate containing silicon nitride obtained by nitriding the metal silicon.

[0086] As explained above, the method for producing a silicon nitride plate according to the first embodiment prevents heat generation due to a rapid nitridation reaction of the silicon contained in the metal silicon powder and allows a uniform nitridation reaction of the 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 a 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 above embodiment except that the average particle diameter D ＳＮ５０ the average particle diameter D of the metal silicon particles Ｓ５０ The particle size ratio X is 0.2 to 2.0. The characteristic features of the method for producing a silicon nitride plate according to this embodiment will be described below.

[0088] The first step of the method for producing a silicon nitride plate according to this embodiment, which is the step of forming a composite powder, is to form a silicon nitride powder having an average particle diameter D ＳＮ５０ Average particle diameter D of metal silicon particles Ｓ５０The method for producing a silicon nitride plate according to this embodiment is characterized in that the particle diameter ratio X, which is the ratio of the above, is 0.2 to 2.0. That is, 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, using silicon nitride particles as a base material to form metal silicon-silicon nitride particles coated or adsorbed with metal silicon particles. 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, using metal silicon particles as a base material to form metal silicon-silicon nitride particles coated with silicon nitride particles. In other words, the silicon nitride particles are adsorbed around the metal silicon particles, and the structure of the above composite particles 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. The fine pulverization process is performed using one or a combination of a jet mill, a ball mill, a planetary mill, and an attritor. From this technical viewpoint, the method for producing a silicon nitride plate according to this embodiment includes, in the first step, determining the particle diameter D of the metal silicon particles constituting the metal silicon powder. Ｓ５０ and the particle diameter D of silicon nitride constituting the silicon nitride powder ＳＮ５０ By controlling the above, the entire surface of the silicon nitride particles, which are the base material, is uniformly coated with the metal silicon particles, or the metal silicon particles are adsorbed onto the entire surface or part of the silicon nitride particles to form composite fine particles. Ｓ５０and the particle diameter D of silicon nitride constituting 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 manufacturing method of the silicon nitride plate according to this embodiment, the average particle diameter D of the silicon nitride particles ＳＮ５０ Average particle diameter D of metal silicon particles 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 D of metal silicon particles Ｓ５０ / average particle diameter D of silicon nitride particles ＳＮ５０　　 (1)

[0092] The particle size ratio X can be set taking into account the particle size of the metal silicon particles and the particle size of the silicon nitride particles that make up the composite microparticles contained in the composite powder. A particle size ratio X of 0.2 or more is preferred because the silicon nitride particles can be adsorbed onto the surface of the metal silicon particles that are the core particles that make up the composite powder microparticles. A particle size ratio X of 2.0 or less is preferred because the silicon nitride particles do not adsorb onto the surface of the metal silicon particles that are the core particles that make up the composite powder microparticles, and the silicon nitride particles do not adhere to each other. The particle size ratio X may be appropriately set depending on the particle size of the metal silicon particles that make up the composite powder, the particle size of the silicon nitride particles, and the pulverization method used, 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 Average particle diameter D of silicon nitride particles ＳＮ５０ By setting the particle size ratio X, which is the ratio of the average particle size D of the metal silicon particles to the silicon nitride particles, it is possible to adsorb the metal silicon particles and the silicon nitride particles. Ｓ５０ Average particle diameter D of silicon nitride particlesＳＮ５０ By setting the particle size ratio X, which is the ratio of the above, it is possible to form a composite powder containing silicon nitride particles coated with metal silicon particles. Ｓ５０ Average particle diameter D of silicon nitride particles 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 producing 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 exposed from the silicon nitride particles adsorbed on the surface of the metal silicon particles that 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. In the method for producing 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 that 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 producing a silicon nitride board according to this embodiment uses metal silicon as the silicon source contained in the sheet body that makes up the silicon nitride substrate, and can produce a silicon nitride substrate containing silicon nitride obtained by the nitriding reaction of the metal silicon particles.

[0095] As described above, the method for manufacturing 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 making it possible to manufacture a silicon nitride substrate with high heat dissipation properties and excellent mechanical properties.

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

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

[0100] (Composite Formation Step: Production of Powder Seed) 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 the metal silicon powder and β-silicon nitride powder were mixed using the jet mill to form a metal silicon-silicon nitride composite. The composite powder produced in this way in Invention Example 1 was designated Powder Seed 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 composite powder consisting of metal silicon and silicon nitride. The composite powder formation conditions were adjusted so that the average particle size of the particles constituting the metal silicon-silicon nitride composite powder was 2.0 to 3.0 μm. The formation conditions for Powder Type No. 1, a composite powder consisting of metal silicon and silicon nitride, are shown in Table 1.

[0102]

[0103] (Preparation of Slurry for Sheet Formation) Next, a slurry for sheet formation was prepared containing Powder Type No. 1, the composite powder used in Example 1, a sintering aid, a dispersion, and a binder. Table 2 shows the amounts of Powder Type No. 1, the composite powder used in the silicon nitride substrate produced in Example 1, the rare earth compound, the magnesium compound, the dispersion, and the binder. As shown in Table 2, the sintering aid contained in the slurry for sheet formation containing the raw composite powder, the sintering aid, the dispersion medium, and the binder was 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. Furthermore, 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. Table 2 shows only the proportions of the magnesium compound and the rare earth element compound, with the remainder being Powder Type No. 1, which is silicon powder. The molar ratios shown in Table 2 are those where silicon (Si) is silicon nitride (Si ３ N ４ ), the silicon is converted to silicon nitride, and the magnesium compound is converted to magnesium oxide (MgO).

[0104]

[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 pre-weighed to achieve a slurry concentration of 50 wt% and added to 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 mixed for another 24 hours. The viscosity was then adjusted using a vacuum degasser (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 becomes 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.). After forming the sheet-forming slurry into a sheet, the formed sheet was cut into a size of 50 x 50 x 0.4 mm, and the relative density of the sheet was evaluated. The relative density of the sheet was evaluated by length measurement. As a result, the relative density of the obtained sheet was 60% to 70%.

[0107] (Production of Degreased Sheet Body) The sheet body produced in the sheet forming process was molded 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 set in a boron nitride case (hereinafter also referred to as "BN crucible"). Then, the BN crucible containing the 12 degreased sheet bodies coated with BN powder was set 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] (Production of nitrided sheet body) The BN crucible in which the degreased sheet body was set was set in a vacuum / pressurized atmosphere furnace (manufactured by Shimadzu Industrial Systems Co., Ltd., model: PVLgr10 VESTA), and the furnace was temporarily heated to 1000 K. －１ After the pressure was reduced to 0.1 Pa, nitrogen was introduced into the furnace and nitriding was carried out at 1480°C for 8 hours in a nitrogen atmosphere of 0.1 MPa. 99.9 vol% nitrogen gas was used as the nitrogen gas. The degreased sheet body obtained after nitriding was used as the nitrided sheet body of Production Example 1. In Invention Example 1, the temperature rise rate used 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 that had been nitrided in the nitriding step for the sample of Production Example 1 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 observed in the silicon nitride substrate. Note that the nitride sheet body was sintered using the same vacuum / pressurized atmosphere furnace as the nitriding step, with the nitride sheet body stacked in the same manner placed in a BN crucible, and the BN crucible then placed in a vacuum / heated atmosphere furnace.

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

[0111] (Structural Observation) The silicon nitride substrate produced in Invention 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 Invention Example 1. The structural observation was evaluated as follows: ◯: Sintered body whose structure could be observed -: Sintered body whose density was low due to melting and therefore structural observation was not performed

[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 plate. Figure 2A is an SEM image of the metal silicon particles and silicon nitride particles that make up the composite powder before they are pulverized, 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 pulverized. As is clear from Figure 2, it can be confirmed that the particle size of the composite particles that make up the composite powder obtained by the mechanochemical effect of the fine pulverization process is smaller than that before the composite powder is synthesized (before the mechanochemical effect).

[0113] Next, the cross section of the silicon nitride substrate produced in Example 1 was observed under 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 in Example 1 by color-coding the element distribution based on the element peak information detected using EDS. 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, nitrogen was found to be distributed on part of the surface of the composite particles that make up the composite powder. This confirms that the silicon nitride particles are bonded to the surface of the metal silicon particles to form a composite. Furthermore, Figure 5 shows an SEM image of the composite silicon particles produced in Example 1, in which fine particles of a sintering aid (a magnesium compound) are dispersed on the surface of the metal silicon particles. Figure 6 shows an EDS image of composite silicon particles in which fine particles of sintering aid are dispersed on the surface of the metal silicon particles that make up the composite powder. As is clear from Figures 5 and 6, it was found that the fine particles of sintering aid are uniformly dispersed on the surface of the metal silicon particles. By using such metal silicon particles as a component of the composite powder, in combination with the addition of the 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] (Measurement of relative density) 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 Invention 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), and the thermal conductivity was measured by 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 Invention Example 1.

[0116] (Measurement of Mechanical Properties) The mechanical properties of the silicon nitride substrate produced in Invention 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 specifically as follows. Specifically, the three-point bending strength (ISO 23242:2020) and fracture toughness (ISO 21113:2018) of the produced 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 produced in Invention Example 1.

[0117]

[0118] First, as is clear from Figure 2, a 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 substrate had a bimodal structure.

[0119] Furthermore, SEM observation of the cross section of the silicon nitride substrate produced in Invention Example 1 confirmed the absence of an altered layer. 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 Invention 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 2-24, Comparative Examples 1-16> Silicon nitride substrates for Invention Examples 2-24 and Comparative Examples 1-16 were manufactured in the same manner as Invention Example 1, except that the raw materials (powder type, magnesium compound), raw material composition, composite formation conditions for the powder type, the heating rate during nitriding of the degreasing treatment sheet, and sintering conditions were changed. Specifically, Powder Types Nos. 2-10, which were manufactured under modified conditions, were used as the powder type constituting the silicon nitride substrates manufactured in Invention Examples 2-24. Magnesium silicon nitride powder was used as the magnesium compound contained in the sheet molding slurry. In Invention Examples 2-24, the heating rate during nitriding of the degreasing treatment sheet was 0.2-1.0°C / min. Tables 1-5 show the raw materials, composite formation conditions, sheet formation conditions, nitriding conditions, and sintering conditions for the silicon nitride substrates manufactured in Invention Examples 2-24 and Comparative Examples 1-16. It was confirmed that the silicon nitride substrate produced in Inventive Example 17 was a dense body with a relative density of 99% or more and a thermal conductivity of 121 W / mK.

[0121]

[0122]

[0123] As is clear from Tables 1 to 5, increasing the proportion of metal silicon powder contained 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 generation caused by the rapid nitridation 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 generation. On the other hand, increasing the proportion of silicon nitride powder contained in the composite powder that constitutes the silicon nitride sintered body (Table 1, Powder Type No. 10, Comparative Examples 13 to 16) suppresses the heat generation caused by the rapid nitridation reaction of silicon because the proportion of metal silicon powder added is small. 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 can be increased. As a result, the silicon nitride sintered body obtained by the sintering step contains a large amount of oxygen inside the silicon nitride sintered body, resulting in a decrease in its thermal conductivity.

[0124] Examples 25-48 and Comparative Examples 17-24 Silicon nitride substrates for Examples 25-48 and Comparative Examples 17-24 were manufactured in the same manner as Example 1, except that the raw materials (powder type, magnesium compound), raw material composition, composite formation conditions for the powder type, the heating rate during nitriding of the degreasing treatment sheet, and sintering conditions were changed. Specifically, powder types 11-18, which were manufactured under modified conditions, were used as the powder type constituting the silicon nitride substrates manufactured in Examples 25-48. Magnesium oxide powder was used as the magnesium compound contained in the sheet molding slurry. In Examples 25-48, the heating rate during nitriding of the degreasing treatment sheet was 0.2-1.0°C / min. Tables 1 and 7-8 show the raw materials, composite formation conditions, sheet formation conditions, nitriding conditions, and sintering conditions for the silicon nitride substrates manufactured in Examples 25-48 and Comparative Examples 17-24.

[0125]

[0126]

[0127]

[0128] These experimental data show that silicon nitride substrates manufactured using a composite powder of metal silicon-silicon nitride formed by pulverizing a mixed powder 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 of metal silicon-silicon nitride formed by pulverizing a mixed powder of metal silicon powder and silicon nitride powder have high heat dissipation properties. Furthermore, the silicon nitride substrates manufactured using such composite powder of metal silicon-silicon nitride gave good results in the measurement and evaluation of three-point bending strength and fracture toughness, which indicate mechanical properties, and were therefore found to have excellent mechanical properties.

[0129] Thus, the method for producing a silicon nitride substrate according to the present invention uses a composite powder made 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 become silicon nitride particles through a nitriding reaction. As a result, after the nitriding reaction is complete, the composite powder becomes a silicon source powder composed solely 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.

[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 method for producing a silicon nitride substrate, comprising: 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 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.

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 a jet mill, a ball mill, a bead mill, a planetary mill, an attritor, and a mechanochemical mill.

3. 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. A method for producing a silicon nitride substrate as described in claim 1 or 2, characterized in that 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.

5. A method for producing a silicon nitride substrate according to claim 1 or 2, characterized in that the metal silicon particles have fine particles of the sintering aid uniformly dispersed on the surface of the metal silicon particles.

6. A method for producing a silicon nitride substrate according to claim 1 or 2, characterized in that 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.

7. A method for producing a silicon nitride substrate according to claim 1 or 2, characterized in that the sintering aid is a rare earth element compound, and the rare earth element compound contains one or more elements selected from Y, Sc, La, Ce, Nd, Sm, Gd, Dy, Ho, Er and Yb.

8. The method for producing a silicon nitride substrate according to claim 1 or 2, wherein the sheet has a relative density of 45% or more.

9. The method for producing a silicon nitride substrate according to claim 1 or 2, 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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