Method for manufacturing silicon nitride substrate and silicon nitride substrate

Abstract

The present invention provides a silicon nitride substrate that has high heat dissipation and excellent mechanical properties due to a uniform nitriding reaction, by suppressing the generation of decomposition products caused by plasticizers from within the green sheet and suppressing the generation of pores within the green sheet, thereby preventing shrinkage and warping of the silicon nitride substrate. The method includes the following steps: a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a binder, a plasticizer, and a dispersant to form a green sheet-forming slurry; and a third step of molding the green sheet-forming slurry to form a green sheet, wherein the binder has a molecular weight of 5.0 × 10 3 ~5.0×10 5 Binder A has a molecular weight of 7.0 × 10 5 ~7.0×10 6 and binder B, wherein the plasticizer is a polyoxyethylene glycol derivative.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silicon nitride substrate and a silicon nitride substrate. More specifically, the present invention relates to a method for producing a silicon nitride substrate using a green sheet for a silicon nitride substrate, which uses a silicon source, two types of binders with different molecular weights, and a polyalkylene glycol derivative plasticizer, and which suppresses the generation of decomposition products caused by the plasticizer from within the green sheet, thereby suppressing the generation of pores, and the silicon nitride substrate. [Background technology]

[0002] In recent years, the demand for power devices has been increasing year by year. That is, with the spread of hybrid vehicles, electric vehicles, etc., the trend in power devices is toward higher output, higher density, and higher temperature operation. To meet these trends in power devices, insulating substrates used in power devices are required to not only have high heat dissipation properties and excellent mechanical properties, but also to be able to prevent shrinkage, warping, etc. of silicon nitride substrates. In other words, in insulating substrates used in power devices, technology that can prevent shrinkage, warping, etc. of silicon nitride substrates has become extremely important.

[0003] Aluminum nitride is used as a material for heat-dissipating insulating substrates for power modules. Aluminum nitride combines excellent insulating properties with high thermal conductivity, making it suitable for use as a heat-dissipating insulating substrate for power modules. However, aluminum nitride has poor mechanical properties such as strength and fracture toughness, making it unreliable, and can cause shrinkage and warpage in silicon nitride substrates, limiting its applications.

[0004] On the other hand, silicon nitride sintered bodies are widely known as an excellent structural ceramic material that combines 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 a material for heat-dissipating insulating substrates.

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

[0006] From this technical viewpoint, methods for producing highly thermally conductive silicon nitride sintered bodies with excellent mechanical strength and enhanced thermoelectric conductivity have been proposed (for example, Patent Document 1). That is, Patent Document 1 describes a method for producing a silicon nitride sintered body, which comprises blending 1 to 50 parts by weight of silicon nitride powder having a β fraction of 30 to 100%, an oxygen content of 0.5 wt% or less, an average particle size of 0.2 to 10 μm, and an aspect ratio of 10 or less, and 99 to 50 parts by weight of α-type silicon nitride powder having an average particle size of 0.2 to 4 μm, and firing the blend.

[0007] Also, a method for producing a silicon nitride sintered body with a thickness suitable for use in heat dissipation circuit boards in power devices using a simplified manufacturing process has been proposed (for example, Patent Document 2). That is, Patent Document 2 describes a method for producing a silicon nitride sintered body, including the steps of preparing a slurry composition for tape casting containing silicon nitride powder, forming the slurry composition for tape casting to produce a green sheet, subjecting the green sheet to a first heat treatment to degreasing, and subjecting the degreased green sheet to a second heat treatment to sintering.

[0008] Furthermore, a method for producing a silicon nitride sintered body substrate with high fracture toughness in both the vertical and planar directions and improved isotropy of fracture toughness has been proposed (for example, Patent Document 3). That is, Patent Document 3 describes a method for producing a silicon nitride sintered body, comprising the steps of mixing silicon nitride powder with a β-Si3N4 ratio of 7% or less and a sintering aid powder in a predetermined compounding ratio, adding a solvent to the mixed powder to form a slurry, adjusting the viscosity of the slurry to 13,000 cps or more, forming the viscosity-adjusted slurry into a sheet compact of a predetermined thickness, and sintering the sheet compact in a non-oxidizing atmosphere to obtain a silicon nitride sintered body substrate. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-97005 [Patent Document 2] Special Publication No. 2020-528861 [Patent Document 3] Japanese Patent Application Publication No. 2019-52072 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the above-mentioned conventional techniques have the following problems that must be solved. Specifically, the method for producing a high-thermal-conductivity silicon nitride sintered body described in Patent Document 1 includes, for example, blending predetermined amounts of powdered silicon nitride powder and α-type silicon nitride powder, further blending a plasticizer, a sintering agent, and a dispersant (Leoguard GP), and then adding the plasticizer to a ball mill container filled with ethanol and mixing. The plasticizer is an ester compound such as dimethyl phthalate, which is hydrolyzed in the presence of an alkaline solution. Therefore, the silicon nitride sintered body produced by the method for producing a high-thermal-conductivity silicon nitride sintered body described in Patent Document 1 has problems such as unevenness in the sheet due to decomposition products caused by the plasticizer, and shrinkage and warpage of the silicon nitride substrate formed by sintering the sheet.

[0011] Furthermore, in the method for producing sintered silicon nitride described in Patent Document 2, a plasticizer is added to suppress aggregation of the raw material powder and improve the plasticity of the slurry composition. The plasticizer can be one or more selected from the group consisting of di-2-ethylhexyl phthalate, di-normal butyl butalate, butyl phthalyl butyl glycolate, and dioctyl phthalate. Therefore, depending on the type of plasticizer used in the method for producing sintered silicon nitride described in Patent Document 2, decomposition products generated by hydrolysis of the plasticizer may be present in the slurry composition. As a result, the silicon nitride sintered body obtained from such a slurry composition suffers from problems such as shrinkage and warpage of the silicon nitride substrate due to the decomposition products.

[0012] In the method for producing sintered silicon nitride described in Patent Document 3, the slurry used for the sintered silicon nitride is prepared by adding a dispersant and an organic solvent to the raw material powder and then grinding it in a conventional manner. The plasticizer used in the preparation of the slurry is an ester compound such as dioctyl adipate. For this reason, in the method for producing a silicon nitride sintered body described in Patent Document 3, decomposition products formed by hydrolysis of the plasticizer, which is an ester compound, are present in the slurry composition. As a result, the silicon nitride sintered body obtained from the slurry composition produced in the production process of the silicon nitride sintered body described in Patent Document 3 has problems such as shrinkage and warpage.

[0013] Thus, silicon nitride sintered bodies produced from conventional green sheets have not been able to satisfy all of the requirements in terms of suppression of shrinkage, warpage, etc., and thermal conductivity and mechanical properties of the silicon nitride substrate obtained by sintering the green sheet. Therefore, there is a need for a method for producing a silicon nitride substrate that can produce a silicon nitride substrate in which shrinkage, warpage, etc. are suppressed by using a green sheet with reduced porosity as the raw material.

[0014] The present invention has been made in consideration of the above-mentioned problems of the conventional technology, and aims to provide a silicon nitride substrate that prevents shrinkage and warping of the silicon nitride substrate by suppressing the generation of decomposition products caused by plasticizers from within the green sheet during the manufacturing process of the silicon nitride substrate and suppressing the generation of pores within the green sheet, and that has high heat dissipation properties and excellent mechanical properties due to a uniform nitriding reaction. [Means for solving the problem]

[0015] In view of these problems, the inventors have conducted extensive research and have found that by forming a green sheet, which is a precursor to a silicon nitride substrate, using a silicon source, two types of binders with different molecular weights, and a polyalkylene glycol derivative as a plasticizer, and by suppressing the generation of decomposition products caused by the plasticizer from within the green sheet and suppressing the generation of pores within the green sheet, it is possible to prevent shrinkage, warping, etc. of the silicon nitride substrate, thereby arriving at the present invention.

[0016] That is, the present invention provides a method for producing a silicon nitride substrate that advantageously solves the above-mentioned problems, 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 binder, a plasticizer, and a dispersant to form a green sheet-forming slurry; and a third step of molding the green sheet-forming slurry to form a green sheet, wherein the binder has a molecular weight of 5.0 × 10 3 ~5.0×10 5 Binder A has a molecular weight of 7.0 × 10 5 ~7.0×10 6 and binder B, wherein the plasticizer is a polyoxyethylene glycol derivative.

[0017] The method for producing a silicon nitride substrate according to the present invention includes the steps of: (a) the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (1), [ka] (b) the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (2): [ka] (c) A more preferable means for solving the problems is to contain the binder A and the binder B in a weight ratio of 70:30 to 90:10. (d) A silicon nitride substrate according to the present invention is characterized in that it is manufactured by the above-mentioned method for manufacturing a silicon nitride substrate. [Effects of the Invention]

[0018] According to the present invention, in the process for producing a silicon nitride substrate, a green sheet, which is a precursor to the silicon nitride substrate, is formed using two types of binders with different molecular weights and a polyalkylene glycol derivative as a plasticizer, and by suppressing the generation of decomposition products caused by the plasticizer from within the green sheet and suppressing the generation of pores within the green sheet, shrinkage and warping of the silicon nitride substrate can be prevented, and a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be produced through a uniform nitriding reaction. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a flow chart showing each step of a method for manufacturing a silicon nitride substrate according to the present invention. [Figure 2] 1 is an SEM image of a green sheet used in the method for producing a silicon nitride plate according to the present invention. [Figure 3] 1 is an SEM image of a green sheet used in the method for producing a silicon nitride plate according to the present invention. [Figure 4] 1 is an SEM image of a green sheet used in the method for producing a silicon nitride plate according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0021] [First embodiment] A method for manufacturing a silicon nitride substrate according to a first embodiment will now be described. Fig. 1 is a flow chart showing the steps of the method for manufacturing a silicon nitride substrate according to this embodiment. As shown in Fig. 1, the method for manufacturing a silicon nitride substrate according to this embodiment includes 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, a binder, a plasticizer, and a dispersant to form a green sheet-forming slurry, and a third step of molding the green sheet-forming slurry to form a green sheet, wherein the binder has a molecular weight of 5.0 x 10 3 ~5.0×10 5 Binder A has a molecular weight of 7.0 × 10 5 ~7.0×10 6 and binder B, wherein the plasticizer is a polyoxyethylene glycol derivative. Hereinafter, each step included in the method for manufacturing a silicon nitride substrate according to this embodiment will be described.

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

[0023] In the conventional method for producing a silicon nitride sintered body, 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 of the present 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 metal silicon powder and silicon nitride powder is used as the silicon source for the silicon nitride sintered body required for producing the silicon nitride substrate. Of the silicon sources for the silicon nitride substrate contained in the mixed powder, the silicon nitride powder preferably accounts for 20 mol% or less, more preferably 10 mol% or less. More preferably, it is 5 mol% or less, which is preferable because it allows for reducing the production costs for producing the silicon nitride substrate. That is, the metallic silicon contained in the mixed powder containing metallic silicon powder and silicon nitride powder as the silicon source for the silicon nitride sintered body required to produce a silicon nitride substrate is preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 95 mol % or less.

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

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

[0028] The amount of oxygen contained as an impurity in silicon nitride powder and metal silicon powder varies depending on the properties of these powders, but for example, in the case of silicon nitride powder, it is about 1.2 mass%, and in the case of metal silicon powder, it is about 0.2 mass% to several mass%. The purity of the silicon nitride powder used in the method for producing a silicon nitride substrate according to this embodiment is preferably in the range of 98.00% to 99.00%, and more preferably in the range of 99.00% 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 99.00% or more, and more preferably in the range of 99.50% 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 of this embodiment uses reaction sintering and uses 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. 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+2N2=Si3N4(1)

[0031] The weight of the green sheet containing the composite powder increases by about 70% due to the nitriding reaction. Therefore, the amount of oxygen contained as an impurity in the green 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 supply source, thereby reducing the amount of oxygen in the crystals of the silicon nitride sintered body compared to when only silicon nitride is used as the silicon supply source starting material.

[0032] In other words, the method for manufacturing a silicon nitride substrate of this embodiment can relatively reduce the amount of oxygen, which is an impurity in the composite powder, by undergoing a nitriding reaction of a green sheet containing a composite powder made of metal silicon powder and silicon nitride powder as a silicon source. As a result, the influence of the amount of oxygen, which is an impurity in the metal silicon powder and silicon nitride powder that constitute the composite powder, is negligible. Therefore, in the method for manufacturing a silicon nitride substrate of this embodiment, a variety of metal silicon powders can be used, ranging from low-grade metal silicon powders with a high impurity oxygen concentration to high-grade metal silicon powders with a low impurity oxygen content.

[0033] In particular, when it is necessary to reduce the amount of impurity oxygen in the silicon nitride substrate, it is preferable to use a high-quality metal silicon powder with a low impurity oxygen concentration. Here, the metal silicon particles constituting the metal silicon powder may be silicon particles with fine particles of a sintering aid uniformly dispersed on the surface of the metal silicon particles. That is, the metal silicon powder contains a sintering aid. The sintering aid may be any sintering aid with a lower melting point than the silicon nitride contained in the green sheet, which is the precursor of the silicon nitride substrate. It may be a rare earth element compound or magnesium compound, which is the sintering aid used in the second slurry formation step described below, or it may be boron, carbon, a phenolic resin, or the like. Uniformly dispersing fine particles of a sintering aid on the surface of the metal silicon particles reduces the grain boundary energy of the metal silicon particles, thereby improving the sinterability of the silicon nitride particles.

[0034] (Fine grinding process) In the step of forming a composite powder included in the method for manufacturing a silicon nitride substrate according to this embodiment, the composite powder is formed by pulverizing a mixed powder of metal silicon powder and silicon nitride powder. The pulverization is carried out using any one or a 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 carried out using a jet mill pulverizer that can apply this action to the metal silicon powder and silicon nitride powder to pulverize them.

[0035] When metal silicon particles and silicon nitride particles are finely pulverized by dry pulverization using a jet mill pulverizer, a fine powder having a particle size of 0.1 to 10.0 μm, preferably 1.0 to 5.0 μm, can be obtained. Jet mill pulverization produces little temperature rise and is suitable for pulverizing heat-sensitive materials. However, jet mill pulverization has low energy efficiency, and while it can produce fine powders of metal silicon particles and silicon nitride particles, it consumes a lot of power and generally only produces small amounts. 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.

[0036] There are various types of jet mill fine grinders, including the Micronizer type, which uses a jet jet that creates a horizontal swirling flow, the Jet Omizer type, which uses a vertical swirling flow, the Blaw-Knox or Trost jet mill type, which uses counter-collision of solid-gas mixed flows, a method in which a solid-gas mixed jet collides against an impact plate, and a method in which a solid-gas mixed flow is mixed in an ultrasonic nozzle. These methods can be appropriately selected and used in the composite powder forming step included in the method for manufacturing a silicon nitride substrate according to this embodiment. Furthermore, many jet mills used in the composite powder forming step 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.

[0037] In this way, in the composite powder forming 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 forming 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.

[0038] 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 particle may have a metal silicon particle as a base material, which is a core particle located at the center of the composite particle. The composite particle has silicon nitride particles coating the entire surface or a portion of the metal silicon particle as the core particle, or adsorbed to the entire surface or a portion of the metal silicon particle as the core particle.

[0039] The composite powder may also contain silicon nitride powder coated with metal silicon particles. That is, the composite particles constituting the composite powder may have, as a base material, silicon nitride particles that are core particles located at the center of the composite particles. The composite particles have metal silicon particles that coat the entire surface or a portion of the silicon nitride core particles, or that are adsorbed onto the entire surface or a portion of the silicon nitride core particles.

[0040] 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 in the nitriding step of the green sheet containing the composite powder, which will be described later, to become silicon nitride particles. Then, all of the metal silicon particles present on the surface of the composite particles become silicon nitride. Ultimately, the composite powder is formed only from silicon nitride particles.

[0041] <Second step: Step of forming slurry for forming green sheets> The method for producing a silicon nitride substrate according to this embodiment includes a slurry formation step of forming a green sheet slurry by mixing the composite powder with a binder and a plasticizer. The slurry formation step is a step of forming a raw material slurry for producing a green sheet, which is a precursor of the silicon nitride substrate.

[0042] In the slurry formation step, a raw material slurry is formed by mixing a binder, a plasticizer, etc. with the composite powder. A dispersant and a dispersion medium may be used to form the raw material slurry. Water or an organic solvent can be used as the dispersion medium. The raw material slurry can be formed by adding and mixing the composite powder, binder, plasticizer, dispersant, etc., and then mixing them using a ball mill, bead mill, or planetary mill in a conventional manner.

[0043] 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 the binder and the plasticizer. Alternatively, in the slurry formation step, the composite powder and the sintering aid may be simply weighed, and then the powders of the composite powder and the sintering aid may be introduced into a ball mill or the like containing the binder and the plasticizer, and the mixed powder of the composite powder and the sintering aid may be simultaneously mixed with the binder, the plasticizer, etc.

[0044] The binder used in the second step of the method for producing a silicon nitride substrate according to this embodiment is composed of binder A and binder B. Binder A has a molecular weight of 5.0 × 10 3 ~5.0×10 5 Binder B has a molecular weight of 7.0 × 10 5 ~7.0×10 6 That is, the molecular weights of binder A and binder B that make up the binder used in the second step are different. Binder A is a low molecular weight binder, and binder B is a high molecular weight binder. The type of binder A and the type of binder B may be the same or different, as long as they have different molecular weights.

[0045] Therefore, in the second step, when the green sheet-forming slurry is formed, the plurality of binder particles constituting binder B can enter the voids formed between the plurality of binder particles constituting binder A. As a result, the binder used when forming the green sheet-forming slurry does not have voids formed between the binder particles constituting the binder. In this way, by using a polymer binder and a low molecular weight binder with different molecular weights in the second step included in the method for producing a silicon nitride substrate according to this embodiment, the formation of voids in the binder can be suppressed.

[0046] The mixing ratio of binder A and binder B that constitute the binder is preferably 70:30 to 90:10 by weight. If the mixing ratio of binder A and binder B is within the above range, it is possible to avoid the binder not filling in the spaces between the particles that constitute the composite powder, compared to when only binder B, a polymer binder, is used as the binder. Furthermore, if the mixing ratio of binder A and binder B is within the above range, it is possible to avoid the binder not filling in the spaces between the particles that constitute the composite powder, compared to when only binder A, a low molecular weight binder, is used as the binder.

[0047] In this way, by setting the mixing ratio of binder A to binder B to 70:30 to 90:10 by weight, it is possible to promote bonding of the particles that make up the composite powder and suppress separation of the particles.Furthermore, by setting the mixing ratio of binder A to binder B to 80:20 to 95:5 by weight, it is not necessary to use a large amount of low molecular weight binder, and it is possible to prevent the green sheet from being degreased for a long time, and carbon does not remain inside the green sheet, and it is possible to prevent cracks from occurring in the green sheet.

[0048] The binder is not particularly limited as long as it has dispersibility, film-forming properties, flexibility, and thermal decomposition properties, and may be a butyral resin, an acrylic resin, or the like. Acrylic resins are classified into two types: polyacrylic acid esters and polymethacrylic acid esters. Binders are classified into thermoplastic resins and linear resins. For example, butyral resins are produced by saponifying vinyl acetate resin with potassium hydroxide to form polyvinyl alcohol resin, which is then reacted with butylaldehyde. Since saponification and butyralization do not occur completely, functional groups consisting of acetal groups, acetyl groups, and hydroxyl groups are present in the side chains of the butyral resin.

[0049] Furthermore, the plasticizer used in the second step of the method for producing a silicon nitride substrate according to this embodiment is a polyoxyethylene glycol derivative. Here, the polyoxyethylene glycol derivative is a polymer having oxyethylene glycol as a repeating unit, and has specific atoms or atomic groups at both ends. The atoms and atomic groups at both ends may be the same or different.

[0050] That is, the polyoxyethylene glycol derivative has predetermined atoms or functional groups at both ends thereof, and the predetermined atoms or functional groups may be the same or different. Examples of the predetermined atoms include hydrogen, alkali metals such as lithium and potassium, and halogen elements such as fluorine, chlorine, and bromine. The predetermined atomic group may be a functional group, and examples of the functional group include a hydroxyl group, a hydroxyl group, a phenolic hydroxyl group, an aldehyde group, a carboxyl group, an amino group, a nitro group, a sulfo group, and an ether group. The molecular weight of the polyoxyethylene glycol derivative is 2.0 x 10 2 ~2.0×10 4 As described above, the dispersant used in the method for manufacturing a silicon nitride substrate according to this embodiment has oxyethylene glycol as a base unit, and has a predetermined atom or a predetermined atomic group at both ends of the base unit.

[0051] In this way, the polyoxyethylene glycol derivative has an ether bond but not an ester bond, and therefore the polyoxyethylene glycol derivative, which is a plasticizer, is not hydrolyzed in the step of forming a green sheet slurry, which is the second step in the method for producing a silicon nitride substrate according to this embodiment. In other words, the method for manufacturing a silicon nitride substrate according to this embodiment is not affected by decomposition products generated from the polyoxyethylene glycol derivative, which is a plasticizer, in the second step, the slurry formation step, in which a slurry for forming a green sheet is formed.

[0052] As a result, in the method for producing a silicon nitride substrate according to this embodiment, the green sheet formed from the sheet-forming slurry is not affected by bubbles generated by decomposition products, etc. As a result, the method for producing a silicon nitride substrate according to this embodiment can produce a silicon nitride substrate that is prevented from shrinking or warping by degreasing, nitriding, and sintering a green sheet with such an extremely reduced porosity.

[0053] As described above, the method for manufacturing a silicon nitride substrate according to this embodiment includes a second step of forming a slurry for forming a green sheet, and the slurry for forming a green sheet formed in the second step contains a composite powder, a binder, a plasticizer, etc. Hereinafter, specific examples of conditions for forming a slurry for forming a green sheet, which is a raw material slurry, by adding a plasticizer and a binder to the composite powder in the slurry forming step will be described.

[0054] The composite powder and sintering aid are weighed out. A milling container such as a ball mill is prepared, which is charged with 0.5 to 2 wt% of a dispersant and 30 to 70 wt% of an organic solvent, etc., based on the total amount of the composite powder and sintering aid. The composite powder and sintering aid are added to the milling container equipped with a milling device that has the dispersant charged. Examples of milling devices include jet mills, ball mills, planetary mills, and attritors. Dispersants that can be used include sorbitan esters and polyoxyalkylenes. Organic solvents that can be used as the dispersion medium include ethanol and toluene. It is also possible to use only the dispersion medium without adding a dispersant. The raw material powders can be mixed and pulverized using a ball mill.

[0055] The time for mixing and grinding is not particularly limited, as it varies depending on the capabilities of the milling machine used, the amounts of the composite powder and sintering aid as starting materials, etc., but it is preferable to select a time that allows the composite powder and sintering aid to be sufficiently ground and mixed. The time for mixing and grinding is preferably 6 hours to 48 hours, and more preferably 12 hours to 24 hours.

[0056] If the mixing and grinding time is less than 6 hours, the sintering aid will not be mixed uniformly into the raw material slurry, and unevenness will occur in the nitrided sheet after sintering, which is not preferable. On the other hand, if the mixing and grinding time is longer than 48 hours, there will be no significant change in the mixed state even after mixing and grinding the composite powder and sintering aid, and there is a risk of impurities being mixed in from the balls or pot. In the slurry formation step, after the composite powder, sintering aid, binder, plasticizer, dispersant, etc. are mixed and pulverized, the dispersion medium contained in the raw material slurry can be removed, if necessary.

[0057] Alternatively, after the grinding and mixing, a binder of 5 to 30 wt % or less may be added, followed by mixing to prepare a slurry for forming a green sheet. The mixing time after adding the binder is not particularly limited, as it varies depending on the performance of the mixing device, such as a milling device, but is preferably 1 hour to 24 hours, and more preferably 6 hours to 12 hours. If the mixing time is less than 1 hour, the binder and raw material powder will not be mixed evenly, which may result in the formation of a green sheet and the generation of many pores and cracks in the produced green sheet, which is undesirable. Normally, mixing for a time longer than 24 hours does not significantly change the mixed state of the binder and raw material powder, so from the perspective of productivity, a mixing time of 24 hours or less is preferred.

[0058] After adding the binder and mixing, the prepared slurry is vacuum degassed to adjust the viscosity of the slurry, thereby forming a green sheet slurry. The green sheet 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 green sheet forming step. The viscosity of the prepared green sheet-forming slurry is 100 to 200 cps before vacuum degassing, and becomes 6000 to 7000 cps after vacuum degassing. As a result, the green sheet-forming slurry after vacuum degassing becomes a slurry suitable for forming a green sheet.

[0059] <Third step: green sheet forming step for forming green sheets> The method for manufacturing a silicon nitride substrate according to this embodiment includes a green sheet forming step of forming the green sheet slurry into a green sheet. The green sheet forming step is a step of forming a green sheet by forming the green sheet slurry having a predetermined composition and a predetermined viscosity into a sheet. Here, the sheet formed into a sheet is a so-called "green sheet." Specifically, in the third step, the composite powder, sintering aid, and plasticizer are mixed using a ball mill or a planetary mill as described above, and then the dispersion medium is removed as necessary, and a binder consisting of two types of binders with different molecular weights is added, and the green sheet-forming slurry is then formed into a sheet to form a green sheet.

[0060] Here, the plasticizer used in the method for manufacturing a silicon nitride substrate according to this embodiment is not an ester compound such as dimethyl phthalate, but a polyoxyethylene glycol derivative. Because the polyoxyethylene glycol derivative does not have an ester bond, it is not hydrolyzed even in the presence of an alkaline solution. In other words, there is no room for decomposition products to be generated by hydrolysis of the polyoxyethylene glycol derivative, which is the plasticizer. As a result, the green sheet formed in the third step included in the method for manufacturing a silicon nitride substrate according to this embodiment does not contain any decomposition products generated by hydrolysis of the plasticizer, and therefore the components are uniformly dispersed, resulting in a green sheet with almost no pores inside and free of wrinkles and unevenness.

[0061] The method for forming the green sheet-forming slurry containing the composite powder, binder, plasticizer, and dispersant into a sheet is not particularly limited as long as it is a method that can form a green sheet from the green sheet-forming slurry, and examples that can be used include mold forming, sheet forming, extrusion forming, and isostatic pressing (CIP) forming.

[0062] The shape and size of the green sheet formed in the green sheet forming step are not particularly limited, and can be any shape and size depending on the shape and size required for the silicon nitride substrate. For example, the green sheet formed in the green sheet forming step preferably has a thickness of 0.05 to 2.5 mm, more preferably 0.25 to 1.0 mm. In the green sheet forming step, the obtained green sheet may be cut to a predetermined size using a punching machine or the like, as needed.

[0063] The relative density of the green sheet obtained in the green sheet forming step is preferably 45% or more, more preferably 50% or more. When the green sheet formed in the green sheet forming step is cut to a predetermined size, the relative density of the green sheet after cutting is preferably 45% or more. The relative density of the green sheet formed in the green sheet forming step can be adjusted by the amount of raw material powder (solid content) contained in the green sheet forming slurry formed in the green sheet forming slurry forming step and the amount of binder added to the green sheet forming slurry. Here, by making the relative density of the green sheet formed in the green sheet molding step 45% or more, the pores in the green sheet can be sufficiently reduced, and the relative density of the silicon nitride substrate obtained after the sintering step described below can be made higher, which is preferable.

[0064] Furthermore, the green sheet formed in the green sheet forming process does not contain decomposition products generated by hydrolysis of the plasticizer made of polyoxyethylene glycol derivative. Therefore, the green sheet formed in the green sheet forming process is formed with composition components, such as two types of binders with different molecular weights, uniformly dispersed, which prevents the formation of pores inside the green sheet and results in a molded product without wrinkles or unevenness, so that the green sheet does not develop cracks or the like.

[0065] The upper limit of the relative density of the green sheet formed in the green sheet forming step is not particularly limited. In order to increase the relative density of the green sheet, it is necessary to increase the solid content of the green sheet forming slurry by reducing the amount of two types of binders with different molecular weights used in the green sheet forming slurry forming step. If the solid content of the green sheet-forming slurry becomes high, many pores will be generated in the green sheet formed from the green sheet-forming slurry, and cracks and the like will occur, making the green sheet difficult to handle. Therefore, the relative density of the green sheet obtained in the green sheet forming step is preferably 65% ​​or less, and more preferably 60% or less.

[0066] <Fourth step: Degreasing step of forming a degreasing treatment sheet body> Furthermore, the method for manufacturing a silicon nitride substrate according to this embodiment may include a degreasing step in which a green sheet in which the generation of pores has been suppressed is manufactured by the above-mentioned steps 1 to 3, and the green sheet is heated at 250 to 600°C to remove the binder contained in the green sheet, thereby forming a degreased green sheet.

[0067] In the degreasing process, the green sheet formed in the green sheet forming process is heated in an atmosphere of air, inert gas such as nitrogen, or a mixture of these gases, thereby removing all of the resin (binder, etc.) contained in the green sheet. The residual carbon content in the green sheet is 0.01% or less. The green sheet formed in the third step of the method for producing a silicon nitride substrate according to this embodiment does not contain decomposition products generated by hydrolysis of the plasticizer, and the components are uniformly dispersed. Therefore, by heating the green sheet, the generation of bubbles can be suppressed and the resin (binder) contained in the sheet can be effectively removed. In the fourth step, the temperature to which the green sheet is heated can be appropriately set depending on the planar shape of the green sheet, the thickness of the green sheet, and the amount of the resin (binder) contained in the green sheet.

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

[0069] In the nitriding step, the metal silicon particles present on the surface of the composite particle are nitrided to become silicon nitride particles. Then, all of the metal silicon particles present on the surface of the composite particle become silicon nitride. Also, in the nitriding step, the metal silicon particles exposed from the surface of the composite particle are nitrided to become silicon nitride particles. Then, all of the metal silicon particles exposed from the surface of the composite particle become silicon nitride. Ultimately, the silicon source that serves as the raw material for the silicon nitride substrate contained in the green sheet formed in the green sheet forming process is formed from the silicon nitride powder prepared at the beginning and silicon nitride powder produced by the nitriding reaction of the metal silicon powder. Furthermore, the green sheet formed in the third step included in the method for producing a silicon nitride substrate according to this embodiment does not contain decomposition products generated by hydrolysis of the plasticizer made of a polyoxyethylene glycol derivative, and the components that make up the green sheet are formed so that pores are suppressed from occurring inside the green sheet, and the components that make up the green sheet are uniformly dispersed. As a result, the nitriding-treated green sheet formed after degreasing such a pore-suppressed green sheet does not suffer from shrinkage, warping, or the like that accompanies the nitriding treatment.

[0070] The method for heating the degreased green sheet in the nitriding step is not particularly limited, but can be, for example, by stacking the degreased sheets between hexagonal boron nitride (BN) powder or hexagonal boron nitride (BN) plates for release, and placing them in a vacuum / pressurized atmosphere furnace consisting of graphite insulation and a graphite heater. Here, hexagonal boron nitride powder (h-BN) is a white powder with a flaky crystal structure similar to graphite, and is a material with excellent thermal conductivity, heat resistance, corrosion resistance, electrical insulation, lubrication, and release properties.

[0071] The vacuum / pressurized atmosphere furnace used for the nitriding of the degreased green sheet can be, for example, a tight-box type electric furnace. By using such a vacuum / pressurized atmosphere furnace, gas generated inside the furnace can be discharged to the outside of the furnace. Furthermore, by using a tight-box electric furnace, which is a type of vacuum / pressurized atmosphere furnace, for example, when a binder removal step is performed to remove the organic binder used in forming the green sheet, the binder removal step, nitriding step, and sintering step can be performed using a single electric furnace. In this way, by using a vacuum / pressurized atmosphere furnace in the nitriding step included in the method for producing a silicon nitride substrate according to this embodiment, productivity of silicon nitride substrates can be increased.

[0072] <Sixth step: Sintering the nitrided green sheet> The method for manufacturing a silicon nitride substrate according to this embodiment includes a sintering step in which the nitriding green sheet is sintered to form a silicon nitride sintered body, which then becomes a silicon nitride substrate. That is, the final sintering step is a step in which the nitriding green sheet formed after the nitriding step is sintered in a nitrogen atmosphere. Then, in the firing step, the sintered nitriding green sheet becomes the final product, a silicon nitride substrate.

[0073] The heating temperature of the nitrided green sheet in the sintering step is not particularly limited as long as it is a temperature that can densify the structure of the nitrided green sheet, but for example, heating is preferably at 1700 to 1950° C. or less, and more preferably at 1750 to 1900° C. In the sintering step, the time for sintering the nitrided green sheet in a nitrogen atmosphere is preferably 1 to 48 hours, and more preferably 5 to 24 hours.

[0074] By carrying out the sintering process, a silicon nitride substrate with a relative density of 95% or more can be obtained. As a result, the silicon nitride substrate obtained after the sintering process can be a dense silicon nitride substrate without an altered layer. Therefore, the silicon nitride substrate obtained after the sintering process can have a thermal conductivity of 85 W / mK or more, preferably 120 W / mK or more, in the unprocessed state, as measured by the laser flash method.

[0075] 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, for example, it is preferably 0.05 mm or more and 2.5 mm or less. The thickness of the silicon nitride substrate obtained after the sintering step can be selected by adjusting the thickness of the green sheet formed in the green sheet forming step.

[0076] Thus, in the method for producing a silicon nitride substrate according to this embodiment, the green sheet, which is the precursor of the silicon nitride substrate, contains a polyoxyethylene glycol derivative as a plasticizer. Therefore, unlike ester-based plasticizers, the green sheet, which is the precursor of the silicon nitride substrate, does not contain decomposition products generated by hydrolysis of the plasticizer, and the components are uniformly dispersed throughout the green sheet. Moreover, the green sheet, which is the precursor of the silicon nitride substrate used in the method for producing a silicon nitride substrate according to this embodiment, is less likely to develop pores caused by decomposition products. Therefore, the green sheet is degreased and then further nitrided, and the resulting nitrided green sheet is sintered to produce a silicon nitride substrate that does not suffer from shrinkage, warping, or the like that accompanies the combustion process.

[0077] In other words, the method for manufacturing a silicon nitride substrate according to this embodiment employs a polyoxyethylene glycol derivative as the plasticizer used in the green sheet, which is the precursor of the silicon nitride substrate, so that the green sheet does not contain any decomposition products generated by hydrolysis of the plasticizer, and employs a green sheet in which the generation of pores inside the green sheet is extremely suppressed. As a result, the method for manufacturing a silicon nitride substrate according to this embodiment uses a green sheet formed by uniformly dispersing composition components such as composite powder, plasticizer, and binder, and by carrying out degreasing, nitriding, and sintering processes, it is possible to manufacture a silicon nitride substrate that does not suffer from shrinkage, warping, etc., which are associated with each of these processes.

[0078] As explained above, according to the method for producing a silicon nitride substrate in accordance with the first embodiment, by carrying out the necessary processing on a green sheet that does not contain decomposition products generated by hydrolysis of a plasticizer and that is formed by uniformly dispersing compositional components such as two types of binders with different molecular weights, it is possible to produce a silicon nitride substrate that does not suffer from shrinkage, warping, etc., and that has high heat dissipation properties and excellent mechanical properties.

[0079] That is, the method for manufacturing a silicon nitride substrate according to the first embodiment makes it possible to manufacture a silicon nitride substrate having a thermal conductivity of 80 W / mK or more as measured by the laser flash method, without shrinkage, warpage, etc. occurring during the manufacturing process. Furthermore, the method for manufacturing a silicon nitride substrate according to the first embodiment makes it possible to provide a highly thermally conductive silicon nitride substrate and its application products, without shrinkage or warpage occurring.

[0080] [Second embodiment] A method for manufacturing a silicon nitride substrate according to a second embodiment will now be described. The method for manufacturing a silicon nitride substrate according to this embodiment is characterized in that, in the above-described embodiment, the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (1). The following describes the characteristic features of the method for manufacturing a silicon nitride substrate according to this embodiment. [ka]

[0081] The method for producing a silicon nitride substrate according to this embodiment is characterized in that the plasticizer contained in the green sheet-forming slurry formed in the second step is a polyoxyethylene glycol derivative represented by the above general formula (1). That is, in the method for producing a silicon nitride substrate according to this embodiment, the plasticizer contained in the green sheet-forming slurry formed in the second step is limited to a specific polyoxyethylene glycol derivative.

[0082] Specifically, the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, has a repeating unit with polyethylene glycol as the basic structure in the above general formula (1). In the polyoxyethylene glycol derivative, which is the dispersant used in the method for producing a silicon nitride substrate according to this embodiment, x, which represents the number of repeating units, is preferably 1 or more and 21 or less. If x, which represents the number of repeating units, is 1 or more, the plasticity of the slurry for forming a green sheet can be ensured, which is preferable. If x, which represents the number of repeating units, is 21 or less, the viscosity of the slurry for forming a green sheet can be ensured, which is preferable, and thus the formation of a green sheet can be facilitated.

[0083] Furthermore, in the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, in the above general formula (1), y, which represents the number of repeating units, is preferably 1 or more and 5 or less. If y, which represents the number of repeating units, is 1 or more, the structure of the polyoxyethylene glycol derivative is chemically stable, which is preferable. If y, which represents the number of repeating units, is 5 or less, the plasticity of the slurry for forming a green sheet can be ensured, which is preferable. Thus, the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, has polyethylene glycol and polypropylene glycol as repeating units in the above general formula (1), and has a basic structure consisting of a set number of repeating units as the skeleton of the polyoxyethylene glycol derivative.

[0084] The polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, has the general formula (1) above, where either R1 or R2 is H (hydrogen). That is, in the general formula (1) above, H (hydrogen) may be bonded to the terminal oxygen of the polyethylene glycol where R1 is located, or to the terminal oxygen of the polypropylene glycol where R2 is located.

[0085] Furthermore, the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, is a compound represented by the general formula (1) above, in which the other of R1 and R2 is C 10 H 21 (decyl group). That is, in the above general formula (1), C 10 H 21 (decyl group) may be bonded to the terminal oxygen of the polyethylene glycol on which R1 is located, or may be bonded to the terminal oxygen of the polypropylene glycol on which R2 is located.

[0086] C 10 H 21 (decyl group) may be a straight-chain hydrocarbon group or a branched hydrocarbon group. 10 H 21 By changing either R1 or R2 to C (decyl group), it is possible to impart an appropriate degree of hydrophobicity to the polyoxyethylene glycol derivative, which is a plasticizer, and ensure the plasticity of the slurry for forming the green sheet. 10 H 21 (decyl group), the molecular weight of the polyoxyethylene glycol derivative is increased to 2.0 × 10 2 ~2.0×10 4 By setting the temperature at 1000° C. or more, the plasticity of the green sheet forming slurry can be ensured.

[0087] As described above, according to the method for producing a silicon nitride substrate in the second embodiment, the green sheet does not contain any decomposition products generated by hydrolysis of the plasticizer, and the components are uniformly dispersed. The green sheet is used in which the generation of pores inside the green sheet is suppressed. By carrying out degreasing, nitriding, and sintering processes, it is possible to produce a silicon nitride substrate that has high heat dissipation properties and excellent mechanical properties without shrinkage, warping, etc.

[0088] [Third embodiment] A method for manufacturing a silicon nitride substrate according to a third embodiment will now be described. The method for manufacturing a silicon nitride substrate according to this embodiment is characterized in that, in the above-described embodiment, the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (2). The following describes the characteristic features of the method for manufacturing a silicon nitride substrate according to this embodiment. [ka]

[0089] The method for producing a silicon nitride substrate according to this embodiment is characterized in that the plasticizer contained in the slurry for forming a green sheet formed in the second step is a polyoxyethylene glycol derivative represented by the above general formula (2). That is, in the method for producing a silicon nitride substrate according to this embodiment, the plasticizer contained in the slurry for forming a green sheet formed in the second step is limited to the specific polyoxyethylene glycol derivative represented by the above general formula (2). Specifically, the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, has a repeating unit in the above general formula (2) whose basic structure is solely polyethylene glycol.

[0090] Furthermore, in the polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, z, which represents the number of repeating units, is preferably 1 or more and 19 or less. If z, which represents the number of repeating units, is 1 or more, the plasticity of the green sheet-forming slurry can be ensured, which is preferable. If z, which represents the number of repeating units, is 19 or less, the viscosity of the green sheet-forming slurry can be ensured, which makes it easier to form the green sheet, which is preferable. Thus, the polyoxyethylene glycol derivative, which is the plasticizer used in the method for manufacturing a silicon nitride substrate according to this embodiment, has polyethylene glycol and polypropylene glycol as repeating units in the above general formula (2), and has a basic structure consisting of a set number of repeating units as the skeleton of the polyoxyethylene glycol derivative.

[0091] The polyoxyethylene glycol derivative, which is the plasticizer used in the method for manufacturing a silicon nitride substrate according to this embodiment, has one of its two terminals represented by the general formula (2) as OH (hydroxyl group). That is, in the general formula (2), the OH (hydroxyl group) is located at the terminal of the polyethylene glycol. The polyoxyethylene glycol derivative, which is the plasticizer used in the method for producing a silicon nitride substrate according to this embodiment, is a compound represented by the general formula (2) above, in which either one of the two terminals of the polyoxyethylene glycol derivative is C 10 H 21 (decyl group). That is, in the above general formula (2), C 10 H 21 (decyl group) is located at the end of the polyethylene glycol.

[0092] In the polyoxyethylene glycol derivative represented by the above general formula (2), C 10 H 21 The decyl group may be a straight-chain hydrocarbon group or a branched hydrocarbon group. Either one of the two terminals of the polyoxyethylene glycol derivative may be C 10 H 21 By using a decyl group, it is possible to impart an appropriate degree of hydrophobicity to the polyoxyethylene glycol derivative, which is a plasticizer, and ensure the plasticity of the slurry for forming the green sheet. Furthermore, either one of the two terminals of the polyoxyethylene glycol derivative represented by the general formula (2) is C 10 H 21(decyl group), the molecular weight of the polyoxyethylene glycol derivative is increased to 2.0 × 10 2 ~2.0×10 4 By setting the temperature at 1000° C. or more, the plasticity of the green sheet forming slurry can be ensured.

[0093] As described above, according to the method for producing a silicon nitride substrate in accordance with the third embodiment, the green sheet used for the silicon nitride substrate does not contain decomposition products resulting from the hydrolysis of the plasticizer, which is a polyoxyethylene glycol derivative having a specific structure, and is formed with a uniform dispersion of two types of binders and other components with different molecular weights. Therefore, the green sheet has reduced pores within it. In other words, according to the method for producing a silicon nitride substrate in accordance with the third embodiment, by subjecting the resulting green sheet to degreasing, nitriding, and sintering, a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be produced without shrinkage, warping, or the like.

[0094] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the configuration and details of the present invention that are understandable to those skilled in the art within the technical scope of the present invention. Furthermore, systems or devices that combine the separate features included in each embodiment in any manner are also included in the technical scope of the present invention. [Example]

[0095] The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

[0096] <Invention Example 1 (Example 1)> Using the method for producing a silicon nitride substrate according to the present invention, green sheets for silicon nitride substrates and silicon nitride substrates were produced as follows. Furthermore, the properties of the green sheets for silicon nitride substrates and silicon nitride substrates obtained using the method for producing a silicon nitride substrate according to the present invention were evaluated.

[0097] The metallic silicon powder was prepared with a purity of 99.0%, an average particle size of 25.0 μm, and an impurity oxygen content of 0.40 mass%. The silicon nitride powder was prepared with a purity of 99.0%, an average particle size of 40.0 μm (manufactured by Shinano Electric Smelting Co., Ltd.). The impurity oxygen content of the metallic silicon powder was measured using a nitrogen and oxygen simultaneous analyzer (manufactured by Horiba, Ltd., model: EMGA-20E).

[0098] (Complex formation process: production of powder seeds) A jet mill (Kurimoto, Model KJ-25) was prepared as the equipment required for mixing metal silicon powder and β-silicon nitride powder to form a metal silicon-silicon nitride composite. Metal silicon powder and silicon nitride powder were weighed and mixed in a predetermined ratio, and then mixed using the jet mill to form a metal silicon-silicon nitride composite. The composite powder produced in Example 1 was designated Powder Type No. 1.

[0099] The production conditions for powder type No. 1 were a rotor rotation speed of 12,000 rpm and a nozzle pressure of 0.60 MPa, and involved pulverizing metal silicon powder and β-silicon nitride powder and synthesizing a metal silicon-silicon nitride composite powder. The composite powder formation conditions were adjusted so that the average particle size of the particles making up the metal silicon-silicon nitride composite powder was 2.0 to 3.0 μm. Table 1 shows the conditions for forming powder type No. 1, which is a composite powder made of metal silicon and silicon nitride.

[0100] [Table 1]

[0101] (Production of slurry for forming green sheets) Next, a slurry for forming a green sheet was produced containing Powder Type 1, which is the composite powder used in Invention Example 1, as well as a binder, a plasticizer, a dispersant, etc. Table 1 shows the production conditions for Powder Type No. 1, the composite powder used in the silicon nitride substrate produced in Invention Example 1.

[0102] In addition, three types of methyl methacrylate / methyl acrylate copolymers were prepared as binders contained in the slurry for forming the green sheet, and two types of binders were selected from the three types of binders and used. The two types of binders were mixed at a predetermined ratio (parts by weight). The three types of binders were named binder A and binder B according to the molecular weight of the methyl methacrylate / methyl acrylate copolymer. Here, binder A had a molecular weight of 5.0 x 10 4 and binder B has a molecular weight of 5.0 × 10 6 is.

[0103] Furthermore, an A-type polyoxyethylene glycol derivative or a B-type polyoxyethylene glycol derivative was used as a plasticizer contained in the green sheet-forming slurry. Here, the A-type polyoxyethylene glycol derivative is a polymer represented by the general formula (1) and has an average molecular weight of 868 (x=10, y=3).

[0104] The sintering aid contained in the green sheet slurry 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, and the rare earth element compound was yttrium oxide powder with an average particle size of 1.5 μm (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0105] Table 2 shows the powder species, plasticizer, binder, and the composition of each of these components contained in the slurry for forming the green sheet. As shown in Table 2, two types of binders were used: a high molecular weight binder and a low molecular weight binder. The mass ratio of the plasticizer to the powder species was approximately 1.0 (0.89), and the mass ratio of the binder consisting of the two types of binder to the powder species was also approximately 1.0 (0.88).

[0106] Furthermore, the prepared powder No. 1 was milled and mixed for 24 hours using a silicon nitride ball mill with ethanol as the dispersion medium. Ethanol was pre-weighed to a slurry concentration of 50 wt% and added to the resin pot. After milling and mixing, 0.88 parts by weight of alkyl methacrylate / alkyl acrylate copolymer as a binder and 0.89 parts by weight of A-type polyoxyethylene glycol derivative as a plasticizer were added to 1.0 parts by weight of the composite powder, and the mixture was mixed for 24 hours. The viscosity was then adjusted using a vacuum defoamer (manufactured by Eiko Co., Ltd.) to prepare a coating slurry for use in forming green sheets. The viscosity of the coating slurry after viscosity adjustment was adjusted to 300 mPa·s.

[0107] [Table 2]

[0108] (Green sheet formation and evaluation) The slurry for forming green sheets obtained after viscosity adjustment was formed into sheets of 0.4 mm thick using a doctor blade (manufactured by Sansho Industry Co., Ltd.). After forming the slurry for forming green sheets into sheets, the formed green sheets were cut into 50 x 50 x 0.4 mm, and the relative density of the green sheets was evaluated. The relative density of the green sheets was evaluated by length measurement. As a result, the relative density of the obtained green sheets was 60% to 70%.

[0109] (structural observation) Furthermore, the structure of the green sheet obtained in Example 1 was observed. After the green sheet was formed, the green sheet was observed using a scanning electron microscope (SEM) (Model: JSM-IT210, manufactured by JEOL Ltd.). Based on the structure observation of the green sheet obtained in Example 1, the void ratio of the green sheet was measured.

[0110] (Measurement of green sheet tensile strength, evaluation of peelability) Furthermore, the tensile strength of the obtained green sheets was measured and their peelability was evaluated as follows: Finally, the green sheets were comprehensively evaluated, taking into consideration the tensile strength, the presence or absence of breaks / cracks, the peelability, and the unevenness of the structure of the obtained green sheets.

[0111] (Measurement of tensile strength) The tensile strength of the green sheet was measured as follows. Test specimens used to measure the tensile strength of the green sheet were prepared in accordance with JIS K7139:2009, a JIS standard that specifies test specimens machined from sheets directly produced by molding plastic materials. The tensile strength of the green sheet was measured using a test specimen, which was gripped and fixed using a gripping jig connected to a load cell (load meter) to measure the force applied to the test specimen, in accordance with JIS K7161. An extensometer to measure strain was then attached to the test specimen's evaluation section, and after applying a slight preload, the test specimen was pulled at a constant speed to measure the tensile strength of the green sheet. The tensile strength of the green sheet was measured at a pulling speed of 50 mm / min.

[0112] (Evaluation of peelability) The peelability of the green sheet was evaluated as follows. Specifically, after forming the green sheet, it was evaluated whether the green sheet to be evaluated could be peeled without resistance from the film constituting the sheet forming apparatus used to form the green sheet. The green sheet was peeled from the film manually or mechanically with a predetermined tensile strength.

[0113] (Green Sheet Overall Rating) The overall evaluation of the green sheets was carried out as follows: Overall evaluation ○: (a) tensile strength of 10 MPa, (b) no breaks or cracks, (c) good peelability, (d) no uneven structure, all of (a) to (d) are satisfied. Overall rating ×: Does not meet any one of (a) to (d). Based on the above, the evaluation results of the green sheets are shown in Table 3 according to the above evaluation criteria.

[0114] [Table 3]

[0115] (Production of degreased green sheets) Boron nitride powder (hereinafter also referred to as "BN powder") was applied to the surface of the green sheet produced in the green sheet forming process. Twelve degreased green sheets coated with BN powder were stacked as a set and then placed in a boron nitride case (hereinafter also referred to as "BN crucible"). The BN crucible containing the 12 degreased green sheets coated with BN powder was then placed in a box furnace (manufactured by Motoyama Corporation, model: DC-6060) and heated at 600°C for 60 hours in an air atmosphere to perform a debinding process. That is, the binder, which is a resin component contained in the green sheet formed from the green sheet forming slurry, was removed to obtain a degreased green sheet.

[0116] (Production of nitride green sheets) The BN crucible containing the degreased green sheet was placed in a vacuum / pressurized atmosphere furnace (Shimadzu Industrial Systems Co., Ltd., Model: PVLgr10 VESTA), and the furnace was temporarily heated to 1.0 × 10 -1 After reducing the pressure 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. The nitrogen gas used was 99.9 vol% nitrogen gas. The degreased green sheet obtained after nitriding was used as the nitrided green sheet of Production Example 1. In Invention Example 1, the temperature rise rate used to obtain the nitrided green sheet was set to 0.2°C / min.

[0117] (Sintering of nitride green sheets) Next, as post-sintering, the nitrided green sheet obtained by nitriding the sample of Production Example 1 in the nitriding step was fired under specified conditions. The fired nitrided green sheet of Production Example 1 was used as the silicon nitride substrate of Invention Example 1. X-ray diffraction measurement (Model: Mini Flex 600, manufactured by Rigaku Corporation) was performed on the silicon nitride substrate obtained in Invention Example 1. The sintering of the nitrided green sheets was carried out using the same vacuum and pressurized atmosphere furnace as used in the nitriding process, with the nitrided green sheets stacked in the same manner being placed in a BN crucible, and the BN crucible then being placed in a vacuum and heated atmosphere furnace.

[0118] (Measuring the mechanical properties of silicon nitride substrates) After sintering the nitride green sheet, the sintered silicon nitride green sheet was removed from the BN crucible, and the BN powder and other materials adhering to the surface were removed using a sandblasting machine (manufactured by Fuji Manufacturing Co., Ltd.). The final product was a silicon nitride substrate. The silicon nitride substrate manufactured in this way was evaluated. Furthermore, various properties of the silicon nitride substrate were measured. Specifically, the mechanical properties of the silicon nitride substrate, such as three-point bending strength and Weibull modulus, were measured.

[0119] (Measuring the mechanical properties of silicon nitride substrates) The mechanical properties of the silicon nitride substrate manufactured in Example 1 were measured. Three-point bending strength and fracture toughness were adopted as the mechanical properties of the silicon nitride substrate. The mechanical properties of the silicon nitride substrate were measured as follows. Specifically, the three-point bending strength of the manufactured silicon nitride substrate was measured and evaluated in accordance with ISO 23242:2020. Furthermore, the Weibull modulus of the produced silicon nitride substrate was measured and evaluated in accordance with JIS R1625: 2010. Table 3 shows the measurement results of the mechanical properties of the silicon nitride substrate produced in Invention Example 1.

[0120] <Invention Examples 2 to 3 (Examples 2 to 3), Comparative Examples 1 to 3> Except for changing the raw materials (plasticizer, dispersant) and raw material composition of the silicon nitride sintered body, green sheets and silicon nitride substrates of Inventive Examples 2-3 and Comparative Examples 1-3 were produced in the same manner as Inventive Example 1. Specifically, Powder Type No. 1 was used as the powder type constituting the silicon nitride substrates produced in Inventive Examples 2-3. Furthermore, as the plasticizer contained in the sheet molding slurry, an A-type polyoxyethylene glycol derivative or a B-type polyoxyethylene glycol derivative was used. Here, the A-type polyoxyethylene glycol derivative is a polymer represented by general formula (1) and has an average molecular weight of 868 (x=10, y=3), and the B-type polyoxyethylene glycol derivative is a polymer represented by general formula (2) and has an average molecular weight of 598 (Z=10).

[0121] In addition, three types of methyl methacrylate / methyl acrylate copolymers were prepared as binders contained in the green sheet forming slurry, and two types of binders were selected from the three types of binders and used. The two types of binders were mixed at a predetermined ratio (parts by weight). The three types of binders were named Binder A, Binder C, and Binder B according to the molecular weight of the methyl methacrylate / methyl acrylate copolymer. Here, Binder A had a molecular weight of 5.0 x 10 4 and binder C has a molecular weight of 8.0 × 105 and binder B has a molecular weight of 5.0 × 10 6 is.

[0122] In Inventive Examples 2-3 and Comparative Examples 1-3, the temperature rise rate used when nitriding the degreased green sheets was 0.2°C / min. Tables 1-2 show the powder species, plasticizer, binder, and composition of each of these components contained in the green sheets produced in Inventive Examples 2-3 and Comparative Examples 1-3. Table 3 also shows the tensile strength measurement results, peelability evaluation results, and overall evaluation of the green sheets produced in Inventive Examples 2-3 and Comparative Example 1. Table 3 also shows the measurement results of the mechanical properties of the silicon nitride substrates produced in Inventive Examples 2-3 and Comparative Examples 1-3.

[0123] (Measurement of void ratio of green sheet) 2 to 4 show scanning electron microscope (SEM) images of the green sheets produced in Example 2 and Comparative Example 1. FIG. 2 shows the results of three-dimensional observation of the green sheets in Example 2 and Comparative Example 1. That is, the air surface, the surface located in the center, and the film surface of the green sheets produced in Example 2 and Comparative Example 1 were observed. From the SEM images shown in FIGS. 2 to 4, it can be seen that pores are formed inside the green sheets produced in Example 2 and Comparative Example 1.

[0124] Therefore, the porosity of the produced green sheet was calculated by calculating the total area of ​​the pores formed within the green sheet relative to the total area of ​​the green sheet. For example, in Figures 3 and 4, it can be seen that the porosity of the air side, the side located in the center, and the film side of the green sheet produced in Example 2 was approximately 14.0%, while the porosity of the air side, the side located in the center, and the film side of the green sheet produced in Comparative Example 1 was approximately 27.0%. As shown in Figure 2, the green sheet of Example 2, which is the inventive product, has pores that are uniformly dispersed not only on the top and bottom (air side and film) but also in the center forming the middle layer.

[0125] That is, it was found that the green sheet produced in Example 2 had a structure with fewer pores than the green sheet produced in Comparative Example 1. In other words, the green sheets produced in Examples 1 to 3 are understood to be precursors of silicon nitride substrates in which the components of the green sheet, such as the composite powder, binder, and plasticizer, are uniformly mixed by using two types of binders with different molecular weights and a polyalkylene glycol derivative as a plasticizer, thereby reducing the number of pores formed therein.

[0126] Furthermore, the silicon nitride substrates manufactured using the green sheets produced in Examples 1 to 3 with reduced porosity were found to have excellent mechanical properties, as evidenced by good results in the measurement and evaluation of three-point bending strength and fracture toughness, which indicate mechanical properties.

[0127] Thus, the method for producing a silicon nitride substrate according to the present invention uses a plasticizer that is a polyoxyethylene glycol derivative having a specific structure, and therefore does not contain decomposition products generated by hydrolysis, and the compositional components of the composite powder, such as two types of binders with different molecular weights, are uniformly dispersed. Moreover, a green sheet using a plasticizer that is a polyoxyethylene glycol derivative having a specific structure is a green sheet in which the generation of pores within the green sheet is suppressed. In other words, according to the method for producing a silicon nitride substrate according to this embodiment, by subjecting the obtained green sheet to a degreasing treatment, nitriding treatment, and sintering treatment, a silicon nitride substrate with high heat dissipation properties and excellent mechanical properties can be produced without shrinkage, warping, or the like.

[0128] In other words, the method for producing a silicon nitride substrate according to the present invention uses a plasticizer that is a polyoxyethylene glycol derivative having a specific structure, and two types of binders with different molecular weights, so that the molecules of the binder with a smaller molecular weight penetrate into the voids formed by the particles of the binder with a larger molecular weight, thereby reducing the number of pores that occur in the green sheet. Thus, it has become clear that the method for producing a silicon nitride substrate according to the present invention can provide a silicon nitride substrate that can be endowed with excellent thermal conductivity and mechanical properties by using a green sheet in which the generation of pores due to decomposition products generated by hydrolysis of the plasticizer is suppressed. [Industrial Applicability]

[0129] According to the present invention, by using two types of binders with different molecular weights and a polyalkylene glycol derivative as a plasticizer, it is possible to uniformly mix the composite powder, binder, plasticizer, etc., which are the components of the green sheet, and to produce a green sheet with reduced pores formed therein.Furthermore, it is possible to produce a silicon nitride substrate with high heat dissipation properties and excellent mechanical properties, which will contribute to the development of the semiconductor-related industry and be extremely useful industrially.

Claims

1. a first step of mixing metal silicon powder and silicon nitride powder and pulverizing the mixture to form a composite powder; a second step of mixing the composite powder with a binder, a plasticizer, and a dispersant to form a slurry for forming a green sheet; a third step of forming a green sheet by molding the green sheet-forming slurry, The binder has a molecular weight of 5.0×10 ３ ~5.0 x 10 ５ Binder A having a molecular weight of 7.0 × 10 ５ ~7.0 x 10 ６ and a binder B, A method for producing a silicon nitride substrate, wherein the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (1): 【Chemistry 1】

2. A method for producing a composite powder by mixing and pulverizing a silicon metal powder and a silicon nitride powder; a second step of mixing the composite powder with a binder, a plasticizer, and a dispersant to form a slurry for forming a green sheet; a third step of forming a green sheet by molding the green sheet-forming slurry, The binder comprises a binder A having a molecular weight of 5.0×10 3 to 5.0×10 5 and a binder B having a molecular weight of 7.0×10 5 to 7.0×10 6 ; A method for producing a silicon nitride substrate, wherein the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (2): 【Chemistry 2】

3. 3. The method for producing a silicon nitride substrate according to claim 1, wherein the binder A and the binder B are contained in a weight ratio of 70:30 to 90:10.

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