Silicon nitride substrate manufacturing method and silicon nitride substrate

Abstract

Provided is a silicon nitride substrate that has a high heat dissipation property and superior mechanical characteristics due to a uniform nitridation reaction, and in which shrinkage and warpage of the silicon nitride substrate are prevented by suppressing the production, from the interior of a green sheet, of decomposition products that are due to a plasticizer, and by suppressing air holes that are produced in the interior of the green sheet. The present invention is characterized by including: a first step, in which a composite powder is formed by mixing a metal silicon powder and a silicon nitride powder and subjecting the result to fine pulverization processing; a second step, in which a slurry for forming a green sheet is formed by mixing a binder, a plasticizer, and a dispersant into the composite powder; and a third step, in which the slurry for forming a green sheet is molded to form a green sheet, wherein the binder comprises a binder A that has a molecular weight of 5.0×103 to 5.0×105, and a binder B that has a molecular weight of 7.0×105 to 7.0×106, and the plasticizer is a polyoxyethylene glycol derivative.

Description

Method for manufacturing silicon nitride substrate and silicon nitride substrate

[0001] The present invention relates to a method for manufacturing a silicon nitride substrate and to a silicon nitride substrate. More specifically, the present invention relates to a method for manufacturing a silicon nitride substrate and to a silicon nitride substrate using a green sheet for silicon nitride substrates that suppresses the generation of decomposition products caused by the plasticizer from inside the green sheet by using two types of binders with different silicon sources and molecular weights and a polyalkylene glycol derivative as a plasticizer, thereby suppressing the generation of pores.

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

[0003] Aluminum nitride is used as a material for heat-dissipating insulating substrates for power modules. In other words, aluminum nitride is used as a material for heat-dissipating insulating substrates for power modules because it possesses both excellent insulation properties and high thermal conductivity. However, aluminum nitride has low mechanical properties such as strength and fracture toughness, lacking reliability, and can cause shrinkage and warping of silicon nitride substrates, 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 to 320 W / mK in single crystal form. For this reason, 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 within the silicon nitride particles that make up the sintered body. As a result, phonons, which are responsible for heat conduction, are scattered, and the thermal conductivity of the silicon nitride sintered body is 20 to 80 W / mK, which is far lower than the theoretical value predicted for single crystals.

[0006] From this technical standpoint, a method for manufacturing a high thermal conductivity silicon nitride sintered body with excellent mechanical strength and enhanced thermoelectric conductivity has been proposed (for example, Patent Document 1). Specifically, Patent Document 1 describes a method for manufacturing a silicon nitride sintered body characterized by 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 diameter of 0.2 to 10 μm, and an aspect ratio of 10 or less, with 99 to 50 parts by weight of α-type silicon nitride powder having an average particle diameter of 0.2 to 4 μm, and then firing the mixture.

[0007] Furthermore, a method for manufacturing silicon nitride sintered bodies of a thickness suitable for use in heat dissipation circuit boards within power devices has been proposed using a simplified manufacturing process (for example, Patent Document 2). Specifically, Patent Document 2 describes a method for manufacturing silicon nitride sintered bodies, which includes the steps of: preparing a tape casting slurry composition containing silicon nitride powder; molding the tape casting slurry composition to produce a green sheet; degreasing the green sheet by primary heat treatment; and sintering the degreased green sheet by secondary heat treatment.

[0008] Furthermore, a method for manufacturing a silicon nitride sintered substrate that has high fracture toughness in both the vertical and planar directions and improves the isotropy of fracture toughness has been proposed (for example, Patent Document 3). Specifically, Patent Document 3 describes β-Si ３ N ４A method for producing a silicon nitride layer is described, comprising the steps of: mixing silicon nitride powder having a ratio of 7% or less with a sintering aid powder in a predetermined blending 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 slurry with the adjusted viscosity into a sheet molded body of a predetermined thickness; and sintering the sheet molded body in a non-oxidizing atmosphere to obtain a silicon nitride sintered body substrate.

[0009] Japanese Patent Publication No. 2002-97005, Japanese Patent Publication No. 2020-528861, Japanese Patent Publication No. 2019-52072

[0010] However, each of the above-mentioned conventional technologies has the following problems that need to be solved. Specifically, the method for manufacturing a high thermal conductivity silicon nitride sintered body described in Patent Document 1 includes, for example, a step of mixing a predetermined amount of powdered silicon nitride powder and α-type silicon nitride powder, further mixing a plasticizer, a sintering agent and a dispersant (Leogard GP), and then adding the plasticizer to a ball mill container filled with ethanol and mixing. Here, since the plasticizer is an ester compound such as dimethyl phthalate, it is hydrolyzed in the presence of an alkaline solution. For this reason, the silicon nitride sintered body manufactured by the method for manufacturing a high thermal conductivity silicon nitride sintered body described in Patent Document 1 has the problem that unevenness occurs in the sheet body due to decomposition products caused by the plasticizer, and shrinkage, warping, etc. occur in the silicon nitride substrate sintered from the sheet body.

[0011] Furthermore, in the method for producing a silicon nitride sintered body described in Patent Document 2, a plasticizer is added to suppress the aggregation of the raw material powder and improve the plasticity of the slurry composition. One or more types of plasticizers can be selected from the group consisting of di-2-ethylhexyl phthalate, di-n-butyl butate, butylphthalyl butyl glycolate, and dioctyl phthalate. Therefore, in the method for producing a silicon nitride sintered body described in Patent Document 2, depending on the type of plasticizer adopted, decomposition products generated by the 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 defects such as shrinkage and warping of the silicon nitride substrate due to the decomposition products.

[0012] Furthermore, in the method for producing a silicon nitride sintered body described in Patent Document 3, the slurry used for the silicon nitride sintered body is prepared by adding a dispersant and an organic solvent to the raw material powder and using a commonly performed mixing and grinding method. The plasticizer used in the preparation of the slurry is an ester compound such as dioctyl adipate. Therefore, in the method for producing a silicon nitride sintered body described in Patent Document 3, decomposition products generated by the hydrolysis of the ester compound plasticizer are present in the slurry composition. As a result, in the manufacturing process for the method for producing a silicon nitride sintered body described in Patent Document 3, the silicon nitride sintered body obtained from the manufactured slurry composition has the problem of shrinkage, warping, etc.

[0013] Thus, conventional silicon nitride sintered bodies manufactured from green sheets have not been able to fully satisfy the requirements in terms of suppressing shrinkage and warping of the silicon nitride substrate obtained by sintering the green sheet, as well as thermal conductivity and mechanical properties. Therefore, there is a need for a method of manufacturing silicon nitride substrates that can produce silicon nitride substrates with suppressed shrinkage and warping by using green sheets with reduced porosity as raw materials.

[0014] The present invention has been made in view of the problems of the above prior art, and suppresses the generation of decomposition products due to a plasticizer from the inside of a green sheet in the manufacturing process of a silicon nitride substrate, and suppresses the generation of pores inside the green sheet, thereby preventing shrinkage and warping of the silicon nitride substrate, and providing a silicon nitride substrate having high heat dissipation and excellent mechanical properties by a uniform nitridation reaction.

[0015] As a result of intensive studies in view of these problems, the inventors formed a green sheet, which is a precursor of a silicon nitride substrate, using a silicon source, two types of binders having different molecular weights, and a polyalkylene glycol derivative as a plasticizer, and suppressed the generation of decomposition products due to the plasticizer from the inside of the green sheet, and suppressed the generation of pores inside the green sheet, thereby finding that the generation of shrinkage, warping, etc. of the silicon nitride substrate can be prevented, and reaching the present invention.

[0016] That is, a method for manufacturing a silicon nitride substrate according to the present invention that advantageously solves the above problems includes a first step of forming a composite powder by mixing metal silicon powder and silicon nitride powder and subjecting the mixture to fine pulverization treatment, a second step of forming a slurry for forming a green sheet by mixing a binder, a plasticizer, and a dispersant with the composite powder, and a third step of molding the slurry for forming a green sheet to form a green sheet, wherein the binder consists of binder A having a molecular weight of 5.0×10 ３ ~5.0×10 ５ and binder B having a molecular weight of 7.0×10 ５ ~7.0×10 ６ and the plasticizer is a polyoxyethylene glycol derivative.

[0017] In addition, the method for manufacturing a silicon nitride substrate according to the present invention is: (a) the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (1), (b) the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (2). (c) It is more preferable as a problem-solving means to contain the binder A and the binder B at a weight ratio of 70:30 to 90:10. (d) The silicon nitride substrate according to the present invention is characterized in that it is a silicon nitride substrate manufactured by the method for manufacturing a silicon nitride substrate described above.

[0018] According to the present invention, in the manufacturing process of a silicon nitride substrate, two types of binders having different molecular weights are used, and a polyalkylene glycol derivative is used as a plasticizer to form a green sheet which is a precursor of the silicon nitride substrate, suppressing the generation of decomposition products caused by the plasticizer from inside the green sheet, and suppressing the generation of pores inside the green sheet, thereby preventing shrinkage and warpage of the silicon nitride substrate, and manufacturing a silicon nitride substrate having high heat dissipation and excellent mechanical properties by a uniform nitridation reaction.

[0019] It is a flow chart showing each process of the method for manufacturing a silicon nitride substrate according to the present invention. It is a SEM image of a green sheet used in the method for manufacturing a silicon nitride plate according to the present invention. It is a SEM image of a green sheet used in the method for manufacturing a silicon nitride plate according to the present invention. It is a SEM image of a green sheet used in the method for manufacturing a silicon nitride plate according to the present invention.

[0020] Hereinafter, embodiments according to the present invention will be specifically described. Note that each drawing is schematic and may be different from the actual one. Also, the following embodiments illustrate devices and methods for embodying the technical idea of the present invention, and do not specify the configuration to be the following. That is, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0021] [First Embodiment] A method for manufacturing a silicon nitride substrate according to the first embodiment will be described. FIG. 1 is a flow chart showing each step 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 forming a composite powder by mixing metal silicon powder and silicon nitride powder and subjecting the mixture to fine pulverization treatment, a second step of forming a slurry for forming a green sheet by mixing a sintering aid, a binder, a plasticizer, and a dispersant with the composite powder, and a third step of forming a green sheet by molding the slurry for forming a green sheet. The binder is 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 and is characterized in that 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] <First Step: Step of Forming Composite Powder> The method for manufacturing a silicon nitride substrate according to this embodiment includes a step of forming a composite powder by mixing metal silicon powder and silicon nitride powder and subjecting the mixture to a mechanochemical effect by fine pulverization treatment. Such a step of forming a composite powder is a step of forming a composite powder which is a raw material of a sintered green sheet required for manufacturing a silicon nitride substrate. That is, the step of forming a composite powder which is the first step is a step of forming a composite powder by mixing metal silicon powder and silicon nitride powder and subjecting the mixture to a mechanochemical effect by fine pulverization treatment as a raw material of a silicon nitride sintered body required for manufacturing a silicon nitride substrate. Here, the mechanochemical effect is an effect in which when mechanical energy is applied to a substance, the bonding state of the substance changes and is activated. The mechanochemical effect is obtained when mechanical energy such as impact, compression, shear, shear stress, and friction is continuously applied to particles during the process of particle size reduction by a pulverization operation, the crystal structure of the particles changes, the particle surface is activated, and a chemical reaction occurs with substances existing around the particles.

[0023] In conventional methods for manufacturing 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 leads to increased manufacturing costs. In contrast, in the method for manufacturing silicon nitride substrates of this embodiment, metallic silicon particles are used as the raw material for the silicon nitride sintered body, and a silicon nitride substrate can be manufactured by a sintering reaction.

[0024] Furthermore, the silicon nitride substrate manufacturing method according to this embodiment uses silicon nitride powder in addition to metallic silicon powder as a silicon supply source for the silicon nitride sintered body necessary for manufacturing the silicon nitride substrate. That is, the silicon nitride substrate manufacturing method according to this embodiment is characterized by using a mixed powder of metallic silicon powder and silicon nitride powder as a silicon supply source for the silicon nitride sintered body, and using a composite powder consisting of metallic silicon and silicon nitride formed by fine grinding of the mixed powder of metallic silicon powder and silicon nitride powder.

[0025] (Metallic Silicon Powder and Silicon Nitride Powder) In the method for manufacturing a silicon nitride substrate according to this embodiment, a mixed powder containing metallic silicon powder and silicon nitride powder is used as a silicon source for the silicon nitride sintered body necessary for manufacturing the silicon nitride substrate. Of the silicon source for the silicon nitride substrate contained in the mixed powder, the silicon nitride powder is preferably 20 mol% or less, and more preferably 10 mol% or less. More preferably 5 mol% or less is preferable because it is possible to suppress the manufacturing cost for manufacturing the silicon nitride substrate. That is, the metallic silicon contained in the mixed powder containing metallic silicon powder and silicon nitride powder as a silicon source for the silicon nitride sintered body necessary for manufacturing 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 calculated by converting the silicon contained in the mixed powder to silicon nitride, that is, the value calculated by assuming that 1 mol of silicon in 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 silicon sources, then silicon nitride powder will account for 50 mol% of the silicon sources contained in the raw material powder.

[0027] The average particle size of the metallic silicon particles constituting the metallic 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 metallic silicon particles before the mechanochemical effect is imparted by the fine grinding treatment employed in the silicon nitride substrate manufacturing method according to this embodiment is 20.0 to 30.0 μm, and the average particle radius 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 cumulative value in the particle size distribution determined by laser diffraction / scattering. Generally, commercially available silicon nitride powder and silicon powder contain unavoidable impurities.

[0028] The amount of oxygen contained as an impurity in silicon nitride powder and metallic 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 metallic silicon powder, it is about 0.2 mass% to several mass%. The purity of the silicon nitride powder used in the method for manufacturing the 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 manufacturing the silicon nitride substrate according to this embodiment is preferably in the range of 99.00% or higher, and more preferably in the range of 99.50% or higher.

[0029] When manufacturing silicon nitride substrates, it is preferable to reduce the amount of dissolved oxygen contained in the crystal of the silicon nitride sintered body in order to improve the thermal conductivity of the silicon nitride substrate. The silicon nitride substrate manufacturing method of this embodiment uses reaction sintering and uses silicon powder as a starting material, and therefore has a significant advantage in terms of reducing oxygen content compared to the case in which only silicon nitride is used as the starting material. This is because when silicon powder is used as the starting material, after a sheet forming step in which a composite powder consisting of metallic silicon powder and silicon nitride powder, which are the starting materials, is formed into a sheet, a nitriding step is carried out in which the sheet containing the composite powder is nitrided. In the nitriding step, the nitriding reaction shown in the following reaction equation (1) proceeds.

[0030] 3Si + 2N ２ = Si ３ N4 (1)

[0031] The weight of the green sheet containing the composite powder increases by approximately 70% due to the nitriding reaction. As a result, the amount of oxygen contained as an impurity in the green sheet containing the composite powder decreases relatively. Thus, the silicon nitride substrate manufacturing method according to this embodiment, by using a composite powder consisting of metallic silicon powder and silicon nitride powder as a silicon source, can reduce the amount of oxygen in the crystal of the silicon nitride sintered body compared to the case where only silicon nitride is used as the starting material.

[0032] In other words, the silicon nitride substrate manufacturing method of this embodiment can relatively reduce the amount of oxygen, an impurity in the composite powder, by undergoing a nitridation reaction of a green sheet containing a composite powder consisting of metallic silicon powder and silicon nitride powder as a silicon supply source. As a result, the influence of the amount of oxygen, an impurity in the metallic silicon powder and silicon nitride powder constituting the composite powder becomes negligible. Therefore, in the silicon nitride substrate manufacturing method of this embodiment, a wide range of metallic silicon powders can be used, from low-grade metallic silicon powders with high impurity oxygen concentrations to high-grade metallic silicon powders with low impurity oxygen levels.

[0033] In particular, when it is necessary to reduce the amount of impurity oxygen in the silicon nitride substrate, it is preferable to use high-grade metallic silicon powder with a low impurity oxygen concentration. Here, the metallic silicon particles constituting the metallic silicon powder may be silicon particles in which fine particles made of a sintering aid are uniformly dispersed on the surface of the metallic silicon particles. That is, the metallic silicon powder contains a sintering aid. The sintering aid can be any substance with a lower melting point than the silicon nitride contained in the green sheet, which is a precursor of the silicon nitride substrate. It may be a rare earth element compound or a magnesium compound, which are sintering aids used in the slurry formation step of the second step described later, or it may be boron, carbon, phenolic resin, etc. By uniformly dispersing fine particles made of a sintering aid on the surface of the metallic silicon particles, the grain boundary energy of the metallic silicon particles can be reduced, and the sinterability of the silicon nitride particles can be improved.

[0034] (Fine Grinding Process) In the process of forming the composite powder included in the method for manufacturing a silicon nitride substrate according to this embodiment, the composite powder is formed by fine grinding a mixed powder obtained by mixing metallic silicon powder and silicon nitride powder. The fine grinding process is carried out by one type selected from jet mills, ball mills, bead mills, planetary mills, attritors, and mechanochemicals, or a combination thereof. For example, in a jet mill, compressed air or high-pressure steam or high-pressure gas of several atmospheres or more is ejected from an injection nozzle, and the raw material particles, metallic silicon particles and silicon nitride particles, are accelerated by this jet stream. The grinding of these particles is carried out by collisions between the accelerated particles, or by impact or grinding with the accelerated particles. The jet mill is carried out by a jet mill fine grinding machine that can apply such action to metallic silicon powder and silicon nitride powder to advance the grinding.

[0035] When metallic silicon particles and silicon nitride particles are finely ground by dry grinding using a jet mill, fine powder having a particle size of 0.1 to 10.0 μm, preferably 1.0 to 5.0 μm, can be obtained. Fine grinding with a jet mill is suitable for grinding heat-sensitive materials because it causes little temperature rise. However, fine grinding with a jet mill has low energy efficiency; although it can produce fine powder of metallic silicon particles and silicon nitride particles, it consumes a lot of power and generally results in a small processing volume. For this reason, one type or a combination thereof may be used, selected from other fine powder processing methods such as ball mills, bead mills, planetary mills, and attritors, in addition to jet mills.

[0036] Jet mill pulverizers come in various types, including micronizers that use a jet stream to create a horizontal swirling flow, jet omizers that use a vertical swirling flow, Blaw-Knox or Trost jet mills that use opposing collisions of solid-gas mixed flows, methods that involve impacting a solid-gas mixed jet onto an impact plate, and methods that involve mixing solid-gas mixed flows in an ultrasonic nozzle. These methods can be appropriately selected and used in the process of forming the composite powder included in the manufacturing method of silicon nitride substrate according to this embodiment. Furthermore, many jet mills used in the process of forming the composite powder may not only perform dry pulverization but also utilize airflow effectively for classification, or connect high-performance airflow classification directly to achieve the sharpest possible classification.

[0037] Thus, in the process of forming the composite powder, a mixed powder of metallic silicon powder and silicon nitride powder is subjected to a mechanochemical effect through fine grinding, thereby forming a composite powder that serves as the silicon source for the silicon nitride substrate. In other words, the composite powder formation process is a process in which a mixed powder of metallic silicon powder and silicon nitride powder is subjected to a mechanochemical reaction or mechanical alloying reaction through fine grinding to form a composite powder from metallic silicon powder and silicon nitride powder.

[0038] The composite powder is characterized by being formed by the adsorption of metallic silicon particles and silicon nitride particles. The composite particles constituting the composite powder formed from metallic silicon powder and silicon nitride powder may include metallic silicon particles coated with silicon nitride powder. That is, the composite particles may have a metallic silicon particle, which is a core particle located at the center of the composite particles, as the base material. The composite particles have silicon nitride particles that coat all or part of the core metallic silicon particle, or that are adsorbed all or part of the core metallic silicon particle.

[0039] Furthermore, the composite powder may also contain silicon nitride powder coated with metallic silicon particles. That is, the composite particles constituting the composite powder may have a silicon nitride particle, which is a core particle located at the center of the composite particle, as the base material. The composite particles have metallic silicon particles that coat all or part of the silicon nitride particle, which is the core particle, or that are adsorbed all or part of the silicon nitride particle, which is the core particle.

[0040] Furthermore, silicon nitride particles may be present on the surface of the composite particles constituting the composite powder. In addition, metallic silicon particles may be exposed from the surface of the composite particles constituting the composite powder. The metallic silicon particles exposed from the surface of the composite particles are nitrided in the nitriding process of the green sheet containing the composite powder, as described later, and become silicon nitride particles. As a result, all metallic silicon particles present on the surface of the composite particles become silicon nitride. Ultimately, the composite powder is formed solely from silicon nitride particles.

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

[0042] The slurry formation process involves mixing a binder and plasticizer with the composite powder to form the raw material slurry. A dispersant and dispersion medium may also 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 process, the composite powder and sintering aid may be pre-mixed to form a mixed powder, and this mixed powder may be mixed with the binder and plasticizer. Alternatively, in the slurry formation process, the composite powder and sintering aid may be weighed separately, and each powder may be added to a ball mill or the like containing the binder and plasticizer, and the mixed powder of the composite powder and sintering aid may be mixed with the binder and plasticizer at the same time.

[0044] The binder used in the second step of the silicon nitride substrate manufacturing method according to this embodiment consists of binder A and binder B. Binder A has a molecular weight of 5.0 × 10 ３ ~5.0 x 10 ５ Binder B has a molecular weight of 7.0 × 10 ５ ~7.0 x 10 ６ Therefore, the molecular weights of binder A and binder B, which constitute the binder used in the second step, are different. Binder A becomes a low-molecular-weight binder, and binder B becomes a high-molecular-weight binder. Note that the types of binder A and binder B may be the same or different, as long as their molecular weights are different.

[0045] Therefore, in the second step, when forming the slurry for forming the green sheet, multiple binder particles constituting binder B can enter into the voids formed by multiple binder particles constituting binder A. As a result, no voids are formed between the binder particles constituting the binder in the binder used when forming the slurry for forming the green sheet. In this way, by using a high molecular weight binder and a low molecular weight binder with different molecular weights in the second step included in the method for manufacturing a silicon nitride substrate according to this embodiment, the generation of voids formed 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. It is preferable that the mixing ratio of binder A and binder B be within the above range, compared to using only binder B, which is a high-molecular-weight binder, because it can avoid the binder not filling the spaces between the particles constituting the composite powder. Furthermore, it is preferable that the mixing ratio of binder A and binder B be within the above range, compared to using only binder A, which is a low-molecular-weight binder, because it can avoid the binder not sufficiently filling the spaces around the particles constituting the composite powder.

[0047] Thus, by setting the mixing ratio of binder A and binder B that constitute the binder to 70:30 to 90:10 by weight, the bonding of the particles constituting the composite powder can be promoted while suppressing the separation of those particles. Furthermore, by setting the mixing ratio of binder A and binder B that constitute the inder to 80:20 to 95:5 by weight, it is not necessary to use a large amount of low molecular weight binder, the resulting green sheet can be formed without prolonged degreasing, no carbon remains inside the green sheet, and cracks do not occur in the green sheet.

[0048] The binder is not particularly limited as long as it has properties such as dispersibility, film-forming ability, flexibility, and easy thermal decomposition, and may include butyral resin, acrylic resin, etc. 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 resin is produced by saponifying vinyl acetate resin with potassium hydroxide to obtain polyvinyl alcohol resin, and then reacting it with butyraldehyde. Since saponification and butyralization are not carried out completely, the side chains of butyral resin contain functional groups consisting of acetal groups, acetyl groups, and hydroxyl groups.

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

[0050] In other words, polyoxyethylene glycol derivatives have predetermined atoms or functional groups at both ends, and these predetermined atoms or functional groups may be the same or different. Examples of 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 also be a functional group, and examples of functional groups include hydroxyl groups, phenolic hydroxyl groups, aldehyde groups, carboxyl groups, amino groups, nitro groups, sulfo groups, and ether groups. The molecular weight of the polyoxyethylene glycol derivative is 2.0 × 10⁻⁶. ２ ~2.0 x 10 ４ It is preferable that this is the case. Thus, the dispersant used in the method for manufacturing a silicon nitride substrate according to this embodiment has oxyethylene glycol as its basic unit and has a predetermined atom or a predetermined group of atoms at both ends thereof.

[0051] Thus, the polyoxyethylene glycol derivative has an ether bond but no ester bond. For this reason, in the second step of the silicon nitride substrate manufacturing method according to this embodiment, which is the step of forming a slurry for green sheet formation, the polyoxyethylene glycol derivative, which is the plasticizer, does not undergo hydrolysis. In other words, the silicon nitride substrate manufacturing method according to this embodiment is not affected by decomposition products generated from the polyoxyethylene glycol derivative, which is the plasticizer, in the slurry formation step of the second step, which is the step of forming a slurry for green sheet formation.

[0052] As a result, in the silicon nitride substrate manufacturing method according to this embodiment, the green sheet formed from the sheet-forming slurry is not affected by bubbles or other factors generated due to decomposition products. Consequently, the silicon nitride substrate manufacturing method according to this embodiment can produce a silicon nitride substrate in which shrinkage and warping are prevented by degreasing, nitriding, and sintering the green sheet, which has such extremely low 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 green sheet formation, and the slurry for green sheet formation formed in the second step is formed by including a composite powder, a binder, a plasticizer, etc. Hereinafter, specific examples of conditions for forming the raw material slurry, the green sheet formation slurry, by adding a plasticizer and a binder to the composite powder in the slurry formation step will be described.

[0054] The composite powder and sintering aid are weighed. A milling container, such as a ball mill, is prepared with 0.5 to 2 wt% of a dispersant and 30 to 70 wt% of an organic solvent, etc., relative to the total amount of the composite powder and sintering aid. The composite powder and sintering aid are added to the milling container equipped with the milling device containing the dispersant. Examples of milling devices include jet mills, ball mills, planetary mills, and attritors. As dispersants, sorbitan ester type or polyoxyalkylene type can be used. As organic solvents for the dispersion medium, ethanol or toluene can be used. Alternatively, the dispersion medium may be used alone without adding a dispersant. The raw material powders can be mixed and ground using a ball mill.

[0055] The time required for mixing and grinding is not particularly limited, as it varies depending on the function of the milling equipment used, the amount of composite powder and sintering aid used as starting materials, etc. However, it is preferable to select a time that allows for sufficient grinding and mixing of the composite powder and sintering aid. The mixing and grinding time is preferably 6 hours or more and 48 hours or less, and more preferably 12 hours or more and 24 hours or less.

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

[0057] Furthermore, a slurry for forming green sheets can also be prepared by adding a binder of 5 to 30 wt% or less after grinding and mixing, and then mixing again. The mixing time after adding the binder is not particularly limited, as it varies depending on the performance of the mixing equipment such as a milling machine, but it 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 uniformly, and when forming the green sheet, many pores may be generated in the green sheet, which may cause cracks, so this is undesirable. Generally, even if mixing is performed for a longer time than 24 hours, there is no significant change in the mixing state of the binder and raw material powder, so from the viewpoint of productivity, it is preferable to keep the mixing time to 24 hours or less.

[0058] After adding a binder and mixing, the prepared slurry is vacuum-degassed to adjust its viscosity, thereby forming a slurry for green sheet formation. In this way, the slurry for green sheet formation formed in the second step is coated onto a sheet-forming substrate using a sheet forming machine, and the sheet body can be formed in the next step, the green sheet forming step. Here, the viscosity of the prepared slurry for green sheet formation is 100 to 200 cps before vacuum degassing, and becomes 6000 to 7000 cps after vacuum degassing. As a result, the slurry for green sheet formation after vacuum degassing is suitable for forming green sheets.

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

[0060] Here, the plasticizer used in the silicon nitride substrate manufacturing method according to this embodiment is not an ester compound such as dimethyl phthalate, but a polyoxyethylene glycol derivative. Since 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 possibility of decomposition products being generated by the hydrolysis of the polyoxyethylene glycol derivative, which is the plasticizer. As a result, the green sheet formed in the third step included in the silicon nitride substrate manufacturing method according to this embodiment does not contain decomposition products generated by the hydrolysis of the plasticizer, so the compositional components are uniformly dispersed, and the green sheet has almost no pores inside and is free from wrinkles and unevenness.

[0061] The method for forming a green sheet from a slurry containing a composite powder, a binder, a plasticizer, and a dispersant is not particularly limited as long as it can form a green sheet from the green sheet forming slurry. For example, mold forming, sheet forming, extrusion forming, hydrostatic pressure forming (CIP forming), etc., can be used.

[0062] The shape and size of the green sheet formed in the green sheet molding process are not particularly limited and can be any shape and size depending on the required shape and size when used as a silicon nitride substrate. For example, the green sheet formed in the green sheet molding process is preferably 0.05 to 2.5 mm thick, and more preferably 0.25 to 1.0 mm thick. In the green sheet molding process, the formed green sheet may be cut to a predetermined size by using a punching machine or the like as appropriate.

[0063] The relative density of the green sheet obtained in the green sheet molding process is preferably 45% or more, and more preferably 50% or more. When the green sheet formed in the green sheet molding process is cut to a predetermined size, it is preferable that the relative density of the green sheet after cutting is 45% or more. In the green sheet molding process, the relative density of the green sheet formed can be adjusted by the amount of raw material powder (solid content concentration) contained in the green sheet forming slurry formed in the green sheet forming slurry formation process and the amount of binder added to the green sheet forming slurry. Here, it is preferable to set the relative density of the green sheet formed in the green sheet molding process to 45% or more, as this can sufficiently reduce the pores in the green sheet and allow for a higher relative density of the silicon nitride substrate obtained after the sintering process described later.

[0064] Furthermore, the green sheets formed in the green sheet molding process do not contain decomposition products generated by the hydrolysis of the plasticizer, which is made of polyoxyethylene glycol derivatives. As a result, the green sheets formed in the green sheet molding process are formed with two types of binders and other components with different molecular weights uniformly dispersed, suppressing the formation of pores inside the green sheet and resulting in a molded product free from wrinkles and unevenness, thus preventing cracks from forming in the green sheet.

[0065] There is no particular upper limit to the relative density of the green sheet formed in the green sheet molding process. In order to increase the relative density of the green sheet, it is necessary to increase the solid content concentration of the slurry for green sheet formation by reducing the amount of two types of binders with different molecular weights used in the slurry formation process for green sheet formation. When the solid content concentration of the slurry for green sheet formation is high, many pores and cracks occur in the green sheet formed from the slurry, making it difficult to handle. For this reason, the relative density of the green sheet obtained in the green sheet molding process is preferably 65% ​​or less, and more preferably 60% or less.

[0066] <Fourth step: Degreasing step to form a degreasing treated sheet> Furthermore, the method for manufacturing a silicon nitride substrate according to this embodiment may include a degreasing step in which a green sheet is manufactured by the first to third steps above to suppress the generation of pores, and the green sheet is heated at 250 to 600°C to remove the binder contained in the green sheet in order to form a degreasing treated green sheet.

[0067] In the degreasing process, the green sheet formed in the green sheet molding process is heated in an atmosphere of air, an inert gas such as nitrogen, or a mixture thereof, thereby completely removing the resin component (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 silicon nitride substrate manufacturing method according to this embodiment does not contain decomposition products generated by the hydrolysis of plasticizers, and its composition is uniformly dispersed. Therefore, by heating the green sheet, the generation of bubbles can be suppressed, and at the same time, the resin component (binder) contained in the sheet can be effectively removed. In the fourth step, the temperature at 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 binder, which is the resin component, contained in the green sheet.

[0068] <Step 5: Nitriding step to form a nitrided green sheet> The method for manufacturing a silicon nitride substrate according to this embodiment may include a nitriding step to form a nitrided green sheet by heating the degreased green sheet at 1200 to 1500°C to nitride the silicon contained in the degreased green sheet. In the nitriding step, after obtaining a degreased green sheet by heating the green sheet formed in the green sheet molding step in an inert gas atmosphere such as nitrogen, all of the metallic silicon powder, which is a component of the degreased green sheet, can be nitrided.

[0069] In the nitriding process, metallic silicon particles present on the surface of the composite particles are nitrided to become silicon nitride particles. All metallic silicon particles present on the surface of the composite particles become silicon nitride. Furthermore, in the nitriding process, metallic silicon particles exposed from the surface of the composite particles are nitrided to become silicon nitride particles. All metallic silicon particles exposed from the surface of the composite particles become silicon nitride. Finally, the silicon source that serves as the raw material for the silicon nitride substrate contained in the green sheet formed in the green sheet molding process is formed from silicon nitride powder prepared from the outset and silicon nitride powder produced by the nitriding reaction of metallic silicon powder. Moreover, the green sheet molded in the third step of the silicon nitride substrate manufacturing method according to this embodiment does not contain decomposition products generated by the hydrolysis of the plasticizer made of polyoxyethylene glycol derivatives, and the formation of the green sheet is uniformly dispersed, suppressing the generation of pores within the green sheet. As a result, the nitrided green sheet formed after degreasing the green sheet body, in which the generation of such pores is suppressed, does not experience shrinkage, warping, etc. associated with the nitriding process.

[0070] In the nitriding process, there are no particular limitations on the method of heating the degreased green sheet. For example, the degreased sheet can be stacked between hexagonal boron nitride (h-BN) powder or hexagonal boron nitride (h-BN) plates for mold release, and then set in a vacuum / pressurized atmosphere furnace composed of graphite insulation and graphite heaters. Here, hexagonal boron nitride powder (h-BN) is a white powder having a flaky crystal structure similar to graphite, and is a material with excellent thermal conductivity, heat resistance, corrosion resistance, electrical insulation, and lubrication / mold release properties.

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

[0072] <Step 6: Sintering the Nitrided Green Sheet> The method for manufacturing a silicon nitride substrate according to this embodiment includes a sintering step in which a silicon nitride sintered body formed by sintering the nitrided green sheet becomes the silicon nitride substrate. That is, the final step, the sintering step, is a step in which the nitrided green sheet formed after the nitriding step is sintered in a nitrogen atmosphere. Then, in the firing step, the sintered nitrided green sheet becomes the silicon nitride substrate, which is the final product.

[0073] The heating temperature of the nitrided green sheet in the sintering process is not particularly limited as long as it is a temperature that can make the structure of the nitrided green sheet dense. However, it is preferable to heat it at 1700 to 1950°C or lower, and more preferably at 1750 to 1900°C or lower. In the sintering process, 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 performing a 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 that does not contain 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, as measured by the laser flash method in its unprocessed state.

[0075] The thickness of the silicon nitride substrate, which is a sintered silicon nitride sheet, is not particularly limited and can be any thickness. However, when used as a heat-dissipating insulating substrate in semiconductor elements or electronic devices, it is preferable to have a thickness of 0.05 mm to 2.5 mm. The thickness of the silicon nitride substrate obtained after the sintering process can be selected by adjusting the thickness of the green sheet formed in the green sheet molding process.

[0076] As described above, in the method for manufacturing a silicon nitride substrate according to this embodiment, the green sheet, which is a precursor of the silicon nitride substrate, contains a polyoxyethylene glycol derivative as a plasticizer. Therefore, unlike ester-based plasticizers, the green sheet, which is a precursor of the silicon nitride substrate, does not contain decomposition products generated by the hydrolysis of the plasticizer, and its composition is uniformly dispersed within the green sheet. Moreover, the green sheet, which is a precursor of the silicon nitride substrate used in the method for manufacturing a silicon nitride substrate according to this embodiment, rarely generates pores caused by decomposition products. Therefore, the silicon nitride substrate obtained by sintering a nitrided green sheet, which is obtained by degreasing the green sheet and then undergoing a nitriding treatment, does not experience shrinkage, warping, etc., associated with the combustion treatment.

[0077] In other words, the silicon nitride substrate manufacturing method according to this embodiment employs a green sheet that does not contain decomposition products generated by the hydrolysis of the plasticizer, and in which the generation of pores inside the green sheet is extremely suppressed, by using a polyoxyethylene glycol derivative as the plasticizer used in the green sheet, which is the precursor of the silicon nitride substrate. As a result, the silicon nitride substrate manufacturing method according to this embodiment can produce a silicon nitride substrate that does not shrink or warp associated with any of the processes by using a green sheet formed in which the compositional components such as composite powder, plasticizer, and binder are uniformly dispersed and then performing degreasing, nitriding, and sintering treatments.

[0078] As described above, according to the method for manufacturing a silicon nitride substrate as embodied in the first embodiment, by performing the necessary processing on a green sheet that does not contain decomposition products generated by the hydrolysis of plasticizers and is formed in which two types of binders and other compositional components with different molecular weights are uniformly dispersed, it is possible to manufacture a silicon nitride substrate that does not shrink, warp, etc., and has high heat dissipation and excellent mechanical properties.

[0079] In other words, according to the silicon nitride substrate manufacturing method of the first embodiment, a silicon nitride substrate with a thermal conductivity of 80 W / mK or higher, as measured by the laser flash method, can be manufactured without shrinkage, warping, etc., occurring during the manufacturing process. Furthermore, the silicon nitride substrate manufacturing method of the first embodiment can provide a highly thermally conductive silicon nitride substrate and its application products without shrinkage or warping.

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

[0081] The method for manufacturing 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). In other words, the method for manufacturing 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 limited to a specific polyoxyethylene glycol derivative.

[0082] Specifically, the polyoxyethylene glycol derivative, which is a plasticizer used in the silicon nitride substrate manufacturing method according to this embodiment, has repeating units with polyethylene glycol as the basic structure in the above general formula (1). In the polyoxyethylene glycol derivative, which is a dispersant used in the silicon nitride substrate manufacturing method 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, it is preferable because it can ensure the plasticity of the slurry for forming the green sheet. If x, which represents the number of repeating units, is 21 or less, it is preferable because it can ensure the viscosity of the slurry for forming the green sheet, thereby facilitating the formation of the green sheet.

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

[0084] The polyoxyethylene glycol derivative used as a plasticizer in the silicon nitride substrate manufacturing method according to this embodiment is R in the general formula (1) above. １ or R ２ Either one of them is H (hydrogen). That is, in the above general formula (1), H (hydrogen) is R １ It may be located bonded to the terminal oxygen of polyethylene glycol, and R ２ It may be located bonded to the terminal oxygen of the polypropylene glycol in which it is located.

[0085] Furthermore, the polyoxyethylene glycol derivative used as a plasticizer in the silicon nitride substrate manufacturing method according to this embodiment is R in the above general formula (1). １ or R ２ Either the other is C １０ H ２１ (decyl group). That is, in the above general formula (1), C １０ H ２１ (decyl group) is R １ It may be located bonded to the terminal oxygen of polyethylene glycol, and R ２ It may be located bonded to the terminal oxygen of the polypropylene glycol in which it is located.

[0086] C １０ H ２１ The (decyl group) may be a straight-chain hydrocarbon group or a branched hydrocarbon group. １ or R ２ C １０ H ２１ By using a (decyl group), the polyoxyethylene glycol derivative, which is a plasticizer, can be given appropriate hydrophobicity, thereby ensuring the plasticity of the slurry for forming green sheets. Furthermore, R １ or R ２ C １０ H ２１ By using a decyl group, the molecular weight of the polyoxyethylene glycol derivative is increased to 2.0 × 10⁻⁶. ２ ~2.0 x 10 ４ By setting it to this value, the plasticity of the slurry for forming the green sheet can be ensured.

[0087] As described above, according to the method for manufacturing a silicon nitride substrate as embodied in the second embodiment, the green sheet does not contain decomposition products generated by the hydrolysis of the plasticizer, is formed with uniformly dispersed components, and has suppressed pore formation inside the green sheet. By using this green sheet and performing degreasing, nitriding, and sintering treatments, a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be manufactured without shrinkage, warping, etc.

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

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

[0090] Furthermore, in the polyoxyethylene glycol derivative used as a plasticizer in the silicon nitride substrate manufacturing method according to this embodiment, it is preferable that z, which represents the number of repeating units, is 1 or more and 19 or less. If z, which represents the number of repeating units, is 1 or more, it is preferable because it can ensure the plasticity of the slurry for forming the green sheet. If z, which represents the number of repeating units, is 19 or less, it is preferable because it can ensure the viscosity of the slurry for forming the green sheet, thereby facilitating the formation of the green sheet. Thus, the polyoxyethylene glycol derivative used as a plasticizer in the silicon nitride substrate manufacturing method 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 backbone of the polyoxyethylene glycol derivative.

[0091] In the method for manufacturing a silicon nitride substrate according to this embodiment, the polyoxyethylene glycol derivative used as a plasticizer is such that, in the general formula (2) above, one of the two ends of the polyoxyethylene glycol derivative is an OH (hydroxyl group). That is, in the general formula (2) above, the OH (hydroxyl group) is located at the end of polyethylene glycol. In the method for manufacturing a silicon nitride substrate according to this embodiment, the polyoxyethylene glycol derivative used as a plasticizer is such that, in the general formula (2) above, the other end of the polyoxyethylene glycol derivative is C １０ H ２１ (decyl group). That is, in the above general formula (2), C １０ H ２１ The decyl group is located at the end of polyethylene glycol.

[0092] In the polyoxyethylene glycol derivative represented by the general formula (2) above, C １０ H ２１ The (decyl group) may be a straight-chain hydrocarbon group or a branched hydrocarbon group. One of the ends of the polyoxyethylene glycol derivative is C １０ H ２１ By using a (decyl group), the polyoxyethylene glycol derivative, which is a plasticizer, can be given appropriate hydrophobicity, thereby ensuring the plasticity of the slurry for forming green sheets. Furthermore, if either end of the polyoxyethylene glycol derivative shown in the above general formula (2) is C １０ H ２１ By using a decyl group, the molecular weight of the polyoxyethylene glycol derivative is increased to 2.0 × 10⁻⁶. ２ ~2.0 x 10 ４ By setting it to this value, the plasticity of the slurry for forming the green sheet can be ensured.

[0093] As described above, according to the silicon nitride substrate manufacturing method of the third embodiment, the green sheet used in the silicon nitride substrate does not contain decomposition products generated by the hydrolysis of the plasticizer, which is a polyoxyethylene glycol derivative having a specific structure, and is formed by the uniform dispersion of two types of binders and other compositional components with different molecular weights. Therefore, the green sheet is a green sheet in which the generation of pores inside the green sheet is suppressed. In other words, according to the silicon nitride substrate manufacturing method of the third embodiment, by degreasing, nitriding, and sintering the obtained green sheet, a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be manufactured without shrinkage, warping, etc.

[0094] [Other Embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the embodiments described above. Various modifications to the structure and details of the present invention can be made, which can be understood by those skilled in the art within the technical scope of the present invention. Furthermore, any system or apparatus that combines the different features included in each embodiment is also within the technical scope of the present invention.

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

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

[0097] For the metallic 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. For the silicon nitride powder, 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 metallic silicon powder was measured using a nitrogen / oxygen simultaneous analyzer (model: EMGA-20E, manufactured by Horiba, Ltd.).

[0098] (Composite Formation Process: Powder Species Production) A jet mill (Kurimoto Iron Works Co., Ltd., Model: KJ-25) was prepared as the necessary equipment for forming a metallic silicon-silicon nitride composite by mixing metallic silicon powder and β-silicon nitride powder. After weighing and mixing metallic silicon powder and silicon nitride powder in predetermined proportions, the metallic silicon powder and β-silicon nitride powder were mixed using the jet mill to form a metallic silicon-silicon nitride composite. The composite powder produced in this manner in Invention Example 1 was designated as Powder Species No. 1.

[0099] The manufacturing conditions for powder type No. 1 were a rotor speed of 12,000 rpm and a nozzle pressure of 0.60 MPa. This involved grinding metallic silicon powder and β-silicon nitride powder, and synthesizing a composite powder consisting of metallic silicon and silicon nitride. The formation conditions for the composite powder were adjusted so that the average particle size of the particles constituting the metallic silicon-silicon nitride composite powder was 2.0 to 3.0 μm. Table 1 shows the formation conditions for powder type No. 1, which is a composite powder consisting of metallic silicon and silicon nitride.

[0100]

[0101] (Manufacturing of Slurry for Green Sheet Formation) Next, a slurry for green sheet formation was manufactured containing powder type 1, a composite powder used in Invention Example 1, a binder, a plasticizer, a dispersant, etc. Table 1 shows the manufacturing conditions for powder type No. 1, a composite powder used in the silicon nitride substrate manufactured in Invention Example 1.

[0102] Furthermore, three types of methyl methacrylate / methyl acrylate copolymers were prepared as binders to be included in the slurry for forming the green sheet, and two types of binders were selected and used from the three types. The two types of binders were mixed in a predetermined ratio (parts by weight) and used. The three types of binders were designated as Binder A and Binder B according to the molecular weight of the methyl methacrylate / methyl acrylate copolymer. Here, Binder A has a molecular weight of 5.0 × 10⁻⁶. ４ Binder B has a molecular weight of 5.0 × 10 ６ That is the case.

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

[0104] Furthermore, the sintering aid included in the slurry for forming the green sheet was either 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. In addition, yttrium oxide powder with an average particle size of 1.5 μm (manufactured by Shin-Etsu Chemical Co., Ltd.) was used as a rare earth element compound.

[0105] Table 2 shows the powder species, plasticizer, and binder contained in the green sheet forming slurry, as well as the formulation of each component. 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 binders, to the powder species was also approximately 1.0 (0.88).

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

[0107]

[0108] (Forming and Evaluation of Green Sheets) After viscosity adjustment, the obtained slurry for forming green sheets was formed using a doctor blade (manufactured by Sansho Industry Co., Ltd.) to a sheet thickness of 0.4 mm for each sample. After forming the green sheet slurry into sheets, the formed green sheets were cut to 50 × 50 × 0.4 mm, and the relative density of the green sheets was evaluated. The relative density of the green sheets was evaluated by measuring the length. As a result, the relative density of the obtained green sheets was 60% to 70%.

[0109] (Tissue Observation) Furthermore, the tissue of the green sheet obtained in Invention Example 1 was observed. The green sheet was observed using a scanning electron microscope (SEM) (JEOL Ltd., model: JSM-IT210) after its formation. Based on the tissue observation of the green sheet obtained in Invention Example 1, the void ratio of the green sheet was measured.

[0110] (Measurement of tensile strength and evaluation of peelability of green sheets) Furthermore, the tensile strength and peelability of the obtained green sheets were measured and evaluated as follows. Finally, an overall evaluation of the green sheets was made considering the tensile strength, presence or absence of cracks, peelability, and structural irregularities 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. JIS K7139:2009 is a JIS standard that specifies test specimens made by machining from sheets directly produced by molding plastic materials. The tensile strength of the green sheet was measured using a test specimen, and the specimen was gripped and fixed via a gripping jig connected to a load cell (load cell) for measuring the force applied to the specimen, in accordance with JIS K7161. After that, an extensometer for measuring strain was attached to the evaluation part of the test specimen, and after applying a small preload, the test specimen was pulled at a constant speed to measure the tensile strength of the green sheet. The tensile speed used when measuring the tensile strength of the green sheet was set to 50 mm / min.

[0112] (Evaluation of Peelability) The peelability of the green sheets was evaluated as follows. Specifically, after the green sheet was formed, it was evaluated by whether or not the green sheet to be evaluated could be peeled off without resistance from the film constituting the sheet forming apparatus used to form the green sheet. The peeling of the green sheet from the film was performed manually or mechanically with a predetermined tensile strength.

[0113] (Overall evaluation of green sheets) The overall evaluation of green sheets was conducted as follows: Overall evaluation ○: (a) Tensile strength of 10 MPa, (b) No cracks or fissures, (c) Good peelability, (d) No structural irregularities, all of (a) to (d) are met. Overall evaluation ×: Not meeting any one of (a) to (d). Based on the above, the evaluation results of the green sheets according to the evaluation criteria are shown in Table 3.

[0114]

[0115] (Manufacturing of degreased green sheets) Boron nitride powder (hereinafter also referred to as "BN powder") was applied to the surface of the green sheet manufactured in the green sheet molding process. Twelve degreased green sheets coated with BN powder were stacked as one set and then placed in a boron nitride case (hereinafter also referred to as "BN crucible"). Subsequently, the BN crucible containing the twelve degreased green sheets coated with BN powder was placed in a BOX furnace (model: DC-6060, manufactured by Motoyama Co., Ltd.) and heated at 600°C for 60 hours in an atmospheric environment to perform a binder removal process. In other words, a degreased green sheet was obtained by degreasing the binder, which is a resin component contained in the green sheet formed from the green sheet forming slurry.

[0116] (Manufacturing of Nitrided Green Sheets) A BN crucible containing degreased green sheets is placed in a vacuum / pressure atmosphere furnace (Shimadzu Industrial Systems Co., Ltd., Model: PVLgr10 VESTA), and the inside of the furnace is temporarily conditioned to 1.0 x 10 －１ After reducing the pressure to Pa, nitrogen was introduced into the furnace, and nitriding treatment was performed at 1480°C for 8 hours in a nitrogen atmosphere of 0.1 MPa. 99.9 vol% nitrogen gas was used. The degreased green sheet obtained after nitriding treatment in this manner was used as the nitrided green sheet of Production Example 1. In Invention Example 1, the heating rate used to obtain the nitrided green sheet was set to 0.2°C / min.

[0117] (Sintering of Nitrided Green Sheets) Next, as a post-sintering step, the Nitrided Green Sheets, which had been subjected to nitriding treatment in the nitriding process using the samples from Manufacturing Example 1, were fired according to predetermined conditions. The fired Nitrided Green Sheets from Manufacturing Example 1 were used as the silicon nitride substrate for Invention Example 1. X-ray diffraction measurements (Rigaku Corporation, Model: Mini Flex 600) were 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 in the nitriding process. The stacked Nitrided Green Sheets were set in a BN crucible, and the BN crucible was then set in a vacuum and heated atmosphere furnace.

[0118] (Measurement of mechanical properties of silicon nitride substrates) After sintering the silicon nitride green sheet, the sintered silicon nitride green sheet was removed from the BN crucible, and BN powder and other materials adhering to the surface were removed using a sandblasting device (manufactured by Fuji Seisakusho). The final product was then a silicon nitride substrate. The silicon nitride substrate manufactured in this manner was evaluated. Furthermore, various properties of the silicon nitride substrate were measured. Specifically, the three-point bending strength and Weibull coefficient, which are mechanical properties of the silicon nitride substrate, were measured.

[0119] (Measurement of Mechanical Properties of Silicon Nitride Substrates) The mechanical properties of the silicon nitride substrate manufactured in Invention Example 1 were measured. Three-point bending strength and fracture toughness were selected as the mechanical properties of the silicon nitride substrate. Specifically, the measurement of the mechanical properties of the silicon nitride substrate was carried out 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 coefficient of the manufactured 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 manufactured in Invention Example 1.

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

[0121] Furthermore, three types of methyl methacrylate / methyl acrylate copolymers were prepared as binders to be included in the green sheet forming slurry, and two types of binders were selected and used from the three types. The two types of binders were mixed in a predetermined ratio (parts by weight) and used. The three types of binders were designated as Binder A, Binder C, and Binder B according to the molecular weight of the methyl methacrylate / methyl acrylate copolymer. Here, Binder A has a molecular weight of 5.0 × 10⁻⁶. ４ Binder C has a molecular weight of 8.0 × 10 ５ Binder B has a molecular weight of 5.0 × 10 ６ That is the case.

[0122] In Invention Examples 2-3 and Comparative Examples 1-3, the heating rate used when nitriding the degreased green sheets was 0.2°C / min. Tables 1-2 show the powder species, plasticizers, binders, and their respective component formulations contained in the green sheets produced in Invention Examples 2-3 and Comparative Examples 1-3. Furthermore, Table 3 shows the tensile strength measurement results, peelability evaluation results, and overall evaluation of the green sheets produced in Invention Examples 2-3 and Comparative Example 1. In addition, Table 3 shows the measurement results of the mechanical properties of the silicon nitride substrates produced in Invention Examples 2-3 and Comparative Examples 1-3.

[0123] (Measurement of the void ratio of the green sheet) Figures 2 to 4 show SEM images obtained by scanning electron microscope (SEM) observation of the green sheets manufactured in Invention Example 2 and Comparative Example 1. Figure 2 shows the three-dimensional observation results of the green sheets in Invention Example 2 and Comparative Example 1. Specifically, the air surface, the central surface, and the film surface of the green sheets manufactured in Invention Example 2 and Comparative Example 1 were observed. From the SEM images shown in Figures 2 to 4, it can be seen that pores are formed inside the green sheets manufactured in Invention Example 2 and Comparative Example 1.

[0124] Therefore, the porosity of the manufactured green sheet was calculated by determining the total area of ​​pores formed inside the green sheet relative to the total area of ​​the green sheet. For example, in Figures 3-4, it can be seen that the porosity of the air surface, the central surface, and the film surface of the green sheet manufactured in Invention Example 2 is approximately 14.0%, while the porosity of the air surface, the central surface, and the film surface of the green sheet manufactured in Comparative Example 1 is approximately 27.0%. Furthermore, as shown in Figure 2, the green sheet of Invention Example 2, which is the inventive product, has pores uniformly distributed not only on the top and bottom (air surface and film) but also in the center that forms the middle layer.

[0125] In other words, it was found that the green sheet produced in Invention Example 2 had a structure in which no pores were formed, compared to the green sheet produced in Comparative Example 1. That is, the green sheets produced in Invention Examples 1 to 3 are understood to be precursors of silicon nitride substrates in which the composite powder, binder and plasticizer components are uniformly mixed by using two types of binders with different molecular weights and a polyalkylene glycol derivative as a plasticizer, thereby reducing the pores formed inside.

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

[0127] Thus, the silicon nitride substrate manufacturing method 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 components of the composite powder, such as two types of binders with different molecular weights, are uniformly dispersed and formed. Moreover, the green sheet produced using a plasticizer that is a polyoxyethylene glycol derivative having a specific structure is a green sheet in which the generation of pores inside the green sheet is suppressed. In other words, according to the silicon nitride substrate manufacturing method of this embodiment, by degreasing, nitriding, and sintering the obtained green sheet, a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be manufactured without shrinkage, warping, etc.

[0128] In other words, the silicon nitride substrate manufacturing method according to the present invention uses a plasticizer which is a polyoxyethylene glycol derivative having a specific structure, and uses two types of binders with different molecular weights. This allows molecules constituting the binder with a smaller molecular weight to enter the voids formed by the particles constituting the binder with a larger molecular weight, thereby reducing the pores generated in the green sheet. Thus, it has become clear that the silicon nitride substrate manufacturing method according to the present invention can provide a silicon nitride substrate that can be given thermal conductivity and mechanical properties by using a green sheet in which the generation of pores caused by decomposition products generated by the hydrolysis of the plasticizer has been suppressed.

[0129] According to the present invention, by using two binders with different molecular weights and a polyalkylene glycol derivative as a plasticizer, the composite powder, binder, and plasticizer components of the green sheet can be uniformly mixed, thereby producing a green sheet with reduced pore formation. Furthermore, a silicon nitride substrate with high heat dissipation and excellent mechanical properties can be produced, thus contributing to the development of the semiconductor-related industry and being extremely useful industrially.

Claims

A first step involves mixing metallic silicon powder and silicon nitride powder and then performing a fine grinding process to form a composite powder, A second step involves mixing a binder, a plasticizer, and a dispersant with the aforementioned composite powder to form a slurry for green sheet formation. The third step includes forming the aforementioned green sheet slurry to form a green sheet, The aforementioned binder has a molecular weight of 5.0 × 10 ３ ~5.0 x 10 ５ Binder A, which has a molecular weight of 7.0 × 10 ５ ~7.0 x 10 ６ It consists of binder B, A method for producing a silicon nitride substrate, characterized in that the plasticizer is a polyoxyethylene glycol derivative. 　 The method for producing a silicon nitride substrate according to claim 1, characterized in that the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (1). The method for producing a silicon nitride substrate according to claim 1, characterized in that the plasticizer is a polyoxyethylene glycol derivative represented by the following general formula (2). A method for manufacturing a silicon nitride substrate according to claims 1 to 3, characterized in that the binder A and the binder B are contained in a weight ratio of 70:30 to 90:

10. A silicon nitride substrate manufactured by the silicon nitride substrate manufacturing method described in claims 1 to 4.

Images