Green sheets for silicon nitride and silicon nitride substrates

Abstract

To provide a green sheet for silicon nitride and a silicon nitride substrate, which can obtain a silicon nitride sintered body with little poor appearance and poor molding accuracy and exhibits high heat conductivity.SOLUTION: A green sheet for silicon nitride is used as a precursor of a silicon nitride sintered body and contains silicon, a sintering assistant and binder, in which a filling factor of the silicon is 59 volume% or more and 80 volume% or less, and a thickness is 0.1 mm or more and 1 mm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a green sheet for silicon nitride, which is used as a precursor of a sintered body of silicon nitride and contains silicon and a sintering aid, and a silicon nitride substrate.

Background Art

[0002] Silicon nitride (Si3N4) is excellent not only in mechanical properties such as high-temperature strength and wear resistance, but also in heat resistance, low thermal expansion, thermal shock resistance, and corrosion resistance against metals such as Al and Ag. Therefore, sintered bodies of silicon nitride are used as structural materials for gas turbines, engines, steelmaking machines, etc., and materials resistant to molten metal erosion. In addition, due to its excellent electrical insulation, it is used as an electronic component material or electrical insulation material for circuit boards and the like.

[0003] In recent years, the spread of semiconductor devices with a large amount of heat generation, such as high-frequency transistors and power semiconductors, has been progressing. Since the materials of semiconductor devices with a large amount of heat generation are required to have not only excellent electrical insulation but also excellent heat dissipation characteristics, the demand for ceramic substrates showing high resistivity and high thermal conductivity is increasing. Conventionally, aluminum nitride (AlN) has been widely used as a material for ceramic substrates.

[0004] Aluminum nitride is excellent in electrical insulation and thermal conductivity, but has the characteristics of low mechanical strength and fracture toughness. When tightening an aluminum nitride substrate during the assembly of a circuit board unit, there is a problem that it is likely to crack. In addition, since the thermal expansion coefficient of aluminum nitride is significantly different from that of silicon, when a silicon semiconductor device is mounted on an aluminum nitride substrate, cracks and fractures are easily generated due to thermal expansion and contraction.

[0005] Currently, silicon nitride substrates are being increasingly used as a replacement for aluminum nitride substrates, which have issues with implementation reliability. Silicon nitride substrates are sintered silicon nitride substrates, formed from sintered silicon nitride and grain boundary phases derived from sintering aids. Although silicon nitride has a lower thermal conductivity than aluminum nitride, its coefficient of thermal expansion is close to that of silicon, and it forms a sintered body with excellent mechanical strength, fracture toughness, and thermal fatigue resistance.

[0006] Silicon nitride has a stable crystal structure with strong covalent bonds, resulting in excellent heat resistance and high hardness. There are two main crystal structures of silicon nitride: α-type and β-type. The α-type is a trigonal low-temperature phase that exists below approximately 1400°C. The β-type is a hexagonal high-temperature phase that is formed by a phase transition from the α-type between 1400°C and 1600°C.

[0007] A known method for manufacturing sintered silicon nitride is to sinter silicon nitride powder in a nitrogen gas atmosphere with the addition of a sintering aid. Silicon nitride is a non-oxide with strong covalent bonding properties, making dense sintering difficult. Therefore, sintering aids such as aluminum oxide, yttrium oxide, and magnesium oxide are added during heat treatment.

[0008] The raw material used for sintered bodies produced by self-sintering is mainly α-type silicon nitride powder. α-type silicon nitride undergoes a phase transition to the β-type columnar crystal, improving the mechanical properties of the sintered body. Self-sintered bodies are generally known to have excellent mechanical strength, such as bending strength. Self-sintered sintered bodies have been obtained with a thermal conductivity close to 100 W / (m·K).

[0009] Another known method for manufacturing silicon nitride sintered bodies involves reaction sintering and post-sintering of silicon powder under a nitrogen gas atmosphere. While reaction sintered bodies are expected to have higher thermal conductivity, they undergo volume changes due to the silicon nitriding reaction. During reaction sintering, volume expansion occurs due to the nitriding reaction and volume contraction occurs due to sintering, making it difficult to form near-net shape bodies. Furthermore, measures are needed to prevent deformation such as warping, as well as cracks and fractures. Near-net shape refers to a state where the dimensions and shape are close to the finished product, to the point where secondary processing is unnecessary.

[0010] Patent Document 1 describes a silicon nitride sintered body produced by reaction sintering. This sintered body is produced by heating a raw material containing mixed metallic silicon, which includes 80% to 95% by weight of coarse metallic silicon powder with an average particle size of 10 μm to 30 μm and 5% to 20% by weight of fine metallic silicon powder with an average particle size of 1 μm to 10 μm, in a nitrogen atmosphere at a temperature of 1390°C to 1500°C.

[0011] Patent Document 2 describes a silicon nitride sintered body produced by self-sintering using atmospheric pressure sintering or gas pressure sintering. The raw material used is silicon nitride powder having a specific surface area of ​​5 m² / g or more and 20 m² / g or less, a β-type silicon nitride content of 70% by mass or more, a D50 of 0.5 μm or more and 3 μm or less, a D90 of 3 μm or more and 6 μm or less, an iron content of 200 ppm or less, an aluminum content of 200 ppm or less, a total content of metal impurities other than iron and aluminum of 200 ppm or less, a crystallite diameter DC of β-type silicon nitride of 60 nm or more, a ratio DBET / DC (nm / nm) of specific surface area equivalent diameter DBET to crystallite diameter DC of 3 or less, and a crystal strain of β-type silicon nitride of 1.5 × 10⁻⁴ or less.

[0012] Patent Document 3 describes a silicon nitride sintered body produced by self-sintering. This sintered body has silicon nitride particles and a grain boundary phase containing Mg and at least one rare earth element, the ratio of Mg and the rare earth element (RE) when converted to oxide (RExOy / MgO) is in the range of 0.05 to 5, the average value of the long axis length L of the silicon nitride particles in an arbitrarily set 20 × 20 μm region on the processed surface is 5.0 μm or less, the average value of the ratio of the long axis length L to the short axis length S (L / S) is 5 or less, in an arbitrarily set 300 × 300 μm region on the processed surface, the area ratio of pores with an individual area of ​​0.01 μm2 or more is 0.01% to 5%, the average value of the distance between the centroids of the closest adjacent pores is 5 μm or more, and the coefficient of variation of the distance between the centroids is 1.5 or less. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 7-309669 [Patent Document 2] International Publication No. 2018 / 110564 [Patent Document 3] Japanese Patent Publication No. 2014-073945 [Overview of the project] [Problems that the invention aims to solve]

[0014] From the perspective of obtaining high thermal conductivity and fracture toughness, reaction sintering is a promising method for manufacturing silicon nitride sintered bodies. In a reaction sintering manufacturing process, it is important to reduce the voids in the sintered body and increase the packing density of silicon nitride in order to obtain higher thermal conductivity. However, there is room for improvement in conventional techniques regarding increasing the packing density.

[0015] The raw material used in Patent Document 1 is a two-particle distribution obtained by mixing two types of metallic silicon powder. Therefore, the difference in particle size between the coarse powder and the fine powder may be small, or an extremely bimodal particle size frequency distribution may occur. In such cases, the silicon packing ability is poor, making it easy for voids of a certain size or larger to form, or the size and distribution of the voids to vary considerably. As a result, the effect of volume expansion associated with the nitriding reaction may become large.

[0016] Furthermore, in self-sintering methods such as those described in Patent Documents 2 and 3, glass phases, grain boundaries, and voids are easily formed, posing many obstacles to improving thermal conductivity. Generally, one way to increase the packing density of a sintered body is to increase the amount of sintering aid added. However, increasing the amount of sintering aid increases the amount of low-thermal-conductivity glass phases, etc., in the sintered body, which may actually lower the thermal conductivity of the sintered body.

[0017] Furthermore, in the case of manufacturing processes using reaction sintering, a high packing density before the nitriding reaction can also be a problem. The silicon that makes up the precursor in reaction sintering undergoes volume expansion during the nitriding reaction. Therefore, if the packing density of silicon in the precursor is too high, the volume expansion due to the nitriding reaction can cause large deformations and excessive stress, resulting in defects in appearance such as warping, as well as cracks, fractures, and poor molding accuracy.

[0018] In particular, if the silicon packing ratio in the precursor is approximately 82% by volume or higher, volume expansion will exceed the volume of the voids, resulting in significant defects. Patent Document 1 describes reaction sintering, but the packing ratio and void ratio of the precursor are not clear, and this problem of volume expansion is not adequately considered.

[0019] Therefore, the present invention aims to provide a silicon nitride green sheet and a silicon nitride substrate that can produce a silicon nitride sintered body with fewer appearance defects and molding accuracy defects and high thermal conductivity. [Means for solving the problem]

[0020] The green sheet for silicon nitride according to the present invention is used as a precursor of a sintered body of silicon nitride, and is a green sheet for silicon nitride containing silicon, a sintering aid, and a binder, wherein the filling rate occupied by the silicon is 59% by volume or more and 80% by volume or less, and the thickness is 0.1 mm or more and 1 mm or less.

Advantages of the Invention

[0021] According to the present invention, it is possible to provide a green sheet for silicon nitride that can obtain a sintered body of silicon nitride with few appearance defects and poor molding accuracy and high thermal conductivity, and a silicon nitride substrate.

Brief Description of the Drawings

[0022] [Figure 1] It is a flowchart showing a method for manufacturing a green sheet for silicon nitride according to an embodiment of the present invention. [Figure 2] It is a diagram showing the relationship between the particle diameter and the filling rate of silicon in the green sheet for silicon nitride.

Embodiments for Carrying Out the Invention

[0023] Hereinafter, a green sheet for silicon nitride and a method for manufacturing the same according to an embodiment of the present invention will be described with reference to the drawings.

[0024] The green sheet for silicon nitride according to the present embodiment is used as a precursor for manufacturing a sintered body of silicon nitride by reaction sintering. The green sheet for silicon nitride contains powdery silicon (Si) and a sintering aid, and may contain a binder as required.

[0025] A silicon nitride sintered body is obtained by reacting and sintering molded powdered silicon with a sintering aid in a nitrogen gas atmosphere. When silicon (Si) is heat-treated at a high temperature of approximately 1300°C or higher in a nitrogen gas atmosphere, it undergoes a nitriding reaction to become silicon nitride (Si3N4). A silicon nitride sintered body is obtained by the sintering of silicon nitride particles.

[0026] In this specification, a green sheet for silicon nitride refers to a sheet-like molded body composed of powdered silicon and a sintering aid, and means a precursor of silicon before it undergoes a nitriding reaction. Although a green sheet for silicon nitride is a sheet with a thickness that is small compared to its width and length, the width, length, and thickness of the sheet are not particularly limited.

[0027] In this embodiment, the silicon nitride green sheet has a silicon filling rate of 59% to 80% by volume. The remainder of the sheet mainly consists of a small amount of sintering aid, a small amount of binder, and voids.

[0028] When silicon particles undergo a nitriding reaction to become silicon nitride, the volume calculated from the unit cell expands by approximately 1.22 times. By setting the silicon packing ratio in the sheet to between 59% and 80% by volume, the packing ratio of silicon nitride after the nitriding reaction can be increased, resulting in the desirable effects described later.

[0029] In silicon nitride sintered bodies, it is preferable to have a high silicon nitride packing density from the viewpoint of obtaining high thermal conductivity. Therefore, it is also preferable for the silicon nitride green sheet, which is a precursor to the sintered body, to have a high silicon packing density. According to calculations using a packing model, if the silicon particle packing density is 59 volume% or more, the silicon nitride particles will bond more easily during the nitriding and sintering reactions. As a result, it becomes possible to obtain high thermal conductivity.

[0030] If the silicon filling density in the silicon nitride green sheet is too high (more than 80 volume%), the silicon nitride filling density after the nitriding reaction will approach 100 volume%, resulting in insufficient space for volume expansion during the nitriding reaction. In such cases, significant deformation and stress occur in the sintered body due to volume expansion exceeding the volume contraction caused by sintering, as well as constraints on particle rearrangement. In contrast, if voids of approximately 2 volume% or more are secured, even if volume expansion occurs during the nitriding reaction, particle volume expansion and rearrangement are permitted, thus mitigating deformation and stress.

[0031] The filling percentage of silicon in the green sheet is at least 59 volume%, but from the viewpoint of increasing the thermal conductivity of the sintered body, it is preferably 65 volume%, more preferably 70 volume%, and even more preferably 75 volume%. Furthermore, it is at least 80 volume%, but depending on the application and required performance of the sintered body, it can also be 75 volume% or less, 70 volume% or less, 65 volume% or less, etc.

[0032] Defects and defects that occur in sintered bodies include volume expansion due to the nitriding reaction, thermal expansion due to heating during heat treatment, and thermal contraction due to cooling after heat treatment. Specific examples of defects and defects include cracks, fractures, and cosmetic defects such as warping, which is problematic in applications such as printed circuit boards, as well as poor molding accuracy in achieving near-net-shape or target dimensions. These defects and defects can be reduced by lowering the silicon filling rate to create voids within the sintered body.

[0033] In this embodiment, the green sheet for silicon nitride has a particle size ratio of D10 to D90 (D10 / D90) that is greater than 0 and 0.15 or less, and a ratio of D50 to D90 (D50 / D90) that is 0.4 or less, when the particle sizes corresponding to the volume fractions of 10%, 50%, and 90% under cumulative sieving in the volume-based cumulative particle size distribution of silicon are defined as D10, D50, and D90, respectively.

[0034] When D50 / D90 is 0.4 or less, the small particles (particles with D10 or less) and medium particles (particles with D10 or more and D50 or less) that account for 50% of the total volume of silicon powder in the sheet are sufficiently smaller than the large particles (particles with D90 or more) that account for 10% of the total volume. Therefore, a large amount of small and medium particles can be used to fill the gaps between the large particles.

[0035] Furthermore, when D10 / D90 is 0.15 or less, the small particles (particles with a D of 10 or less) that account for 10% of the total volume of silicon powder in the sheet are sufficiently smaller than the large particles (particles with a D of 90 or more) that account for 10% of the total volume, and the medium particles (particles with a D of 10 or more and D of 50 or less) that account for 40% of the total volume. As a result, a large amount of small particles can be used to fill the gaps between the large and medium particles.

[0036] Therefore, by using silicon that satisfies the aforementioned D10 / D90 and D50 / D90 conditions, a sufficient number of medium particles can be tightly arranged around large particles, and a sufficient number of small particles can be tightly arranged around medium particles. Furthermore, the D50 / D90 condition suppresses the size of voids, increases the uniformity of size and distribution, and ensures dispersed space for volume expansion, thus making it less susceptible to the effects of volume expansion associated with the nitriding reaction. Thus, by using silicon that satisfies these conditions, it is possible to obtain a silicon nitride green sheet in which the bonding between particles is easily formed uniformly by the nitriding reaction and sintering, and the filling rate of silicon is in the range of 59 volume% to 80 volume%, making it less prone to deformation and stress during reaction sintering.

[0037] D10 / D90 is preferably 0.01 or higher, more preferably 0.03 or higher. When D10 / D90 is 0.03 or higher, the proportion of silicon with extremely small particle sizes is small, and the overall surface area of ​​the powder becomes small, thus reducing the amount of binder used. D50 / D90 is greater than the D10 / D90 of the powder and is 0.4 or lower.

[0038] The D50 of the silicon powder in the silicon nitride green sheet is preferably between 0.5 μm and 20 μm. If D50 is less than 0.5 μm, the amount of binder is small relative to the total specific surface area of ​​the powder, resulting in lower mechanical strength of the sheet. If D50 exceeds 20 μm, the surface irregularities of the silicon nitride green sheet become larger, and the nitriding reaction rate slows down. In contrast, if D50 is between 0.5 μm and 20 μm, the surface of the silicon nitride green sheet can be made smooth while increasing the nitriding reaction rate.

[0039] The D50 of the silicon powder in the silicon nitride green sheet is preferably 0.8 μm or larger from the viewpoint of obtaining high strength while suppressing the amount of binder. Furthermore, from the viewpoint of making the surface of the silicon nitride green sheet smoother and increasing the nitriding reaction rate, it is preferably 10 μm or less, more preferably 4 μm or less, and even more preferably 3 μm or less. If it is 4 μm or less, sufficient smoothness can be obtained and the nitriding reaction time can be shortened. If it is between 0.8 μm and 3 μm, high strength can be obtained with a small amount of binder, and a surface with fewer irregularities can also be obtained. If it is between 0.8 μm and 2 μm, in addition to high strength and smoothness, a higher packing density can be obtained.

[0040] The particle size and particle size distribution of silicon can be measured using a laser diffraction / scattering particle size distribution analyzer. The particle size and particle size distribution of silicon in a silicon nitride green sheet can be determined by dissolving the silicon nitride green sheet in a solvent such as alcohol, drying it to remove the solvent, and then separating the silicon in the resulting powder by specific gravity, or by observing an arbitrary cross-section of the silicon nitride green sheet with an electron microscope to collect data such as the equivalent circular diameter of the silicon particles.

[0041] The porosity of a silicon nitride green sheet can be determined by observation using a scanning electron microscope (SEM). An arbitrary cross-section of the silicon nitride green sheet is cut out, and the cross-section is photographed with an SEM to obtain an SEM image. The SEM image is then processed using binarization or trinarization, and the low-density regions in the image are assumed to be voids without silicon or sintering aids. The area ratio of voids per unit area is then calculated. SEM images are taken at a magnification of approximately 500x to 1000x for multiple cross-sections or microscope fields.

[0042] The number of cross-sections for which SEM images are taken is preferably two or more, and more preferably five or more, in order to minimize variations due to measurement errors. Based on a large number of SEM images, the average value of the void area ratio is calculated, and when the variation of the average value becomes sufficiently small with increasing numbers of images, the average value of the area ratio is taken as the void ratio of the silicon nitride green sheet. If the amount of sintering aid or binder in the sheet is small, the silicon filling rate [%] can be considered as 100 - void ratio [%].

[0043] As a sintering aid, it is preferable to use a compound containing one or more elements selected from the group consisting of magnesium (Mg), yttrium (Y), and rare earth elements (RE). Examples of compounds containing these elements include oxides, nitrides, and silicides. The sintering aid may contain one or more of these elements.

[0044] Rare earth elements include the lanthanides: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0045] Mg, Y, and rare earth elements have low solid solubility in silicon nitride and do not easily alter its crystal structure or chemical composition, making them effective in increasing the thermal conductivity of silicon nitride itself. In particular, Mg and Y are effective in densifying the sintered body to obtain high thermal conductivity. Preferred compounds containing Mg and Y include magnesium oxide (MgO) and yttrium oxide (Y2O3).

[0046] As rare earth elements, one or more elements selected from the group consisting of La, Ce, Gd, Dy, and Yb are particularly preferred. Using a sintering aid containing these elements allows for sintering to be promoted while suppressing temperature and pressure, thereby reducing the cost of heat treatment.

[0047] The composition of the sintering aid preferably includes La, Y, Gd or Yb, and when Mg is included, the ratio of RExOy / MgO > 1, where RExOy is the mixed weight of the rare earth elements in terms of oxides and MgO is the mixed weight of magnesium in terms of oxides. Increasing the weight ratio of these rare earth elements to Mg makes it easier for fine particles derived from the sintering aid to form during heat treatment. This is because these rare earth elements do not dissolve as easily in silicon nitride compared to Mg.

[0048] The amount of sintering aid is preferably 1% by mass or more, and more preferably 2% by mass or more, based on 100% by mass of the total of silicon and sintering aid, in terms of oxide. Furthermore, 15% by mass or less is preferred, and 10% by mass or less is more preferred. With such an amount, it is possible to promote the sintering of silicon nitride and the purification of the particles while suppressing the formation of glass phases and other structures with low thermal conductivity.

[0049] The thickness of the silicon nitride green sheet is preferably 0.1 mm to 1 mm, and more preferably 0.1 mm to 0.5 mm. Such a thickness allows for the production of thin silicon nitride sintered bodies. A silicon nitride green sheet of this thickness may be obtained by pressurizing after drying during the manufacturing process, or by drying without pressurizing. Reducing the thickness to 0.5 mm or less by pressurizing or other means allows for a higher thermal conductivity.

[0050] According to the above-described green sheet for silicon nitride, since silicon satisfying the D10 / D90 and D50 / D90 conditions is used, the packing density of silicon is in the range of 59% to 80% by volume, and a precursor can be obtained in which space for volume expansion due to the nitriding reaction is dispersed. It is also possible to secure a high packing density close to 80% by volume at the precursor stage of the silicon nitride sintered body, and since the bonding between particles is easily formed uniformly by the nitriding reaction and sintering, a dense silicon nitride sintered body can be obtained. Therefore, when used in a manufacturing process by reaction sintering, a silicon nitride sintered body can be obtained with fewer surface defects such as warping, cracks, fractures, and molding accuracy defects, and exhibiting high thermal conductivity.

[0051] Next, a method for manufacturing the aforementioned silicon nitride green sheet, in which the silicon content of the filling is within a predetermined range, will be described. In the following description, a manufacturing method in which the raw material powders are wet-mixed and finally dried will be illustrated along with a heat treatment method.

[0052] Figure 1 is a flowchart showing a method for manufacturing a silicon nitride green sheet according to an embodiment of the present invention. As shown in Figure 1, the method for manufacturing a silicon nitride green sheet according to this embodiment includes a particle size adjustment step S10, a mixing step S20, a coating step S30, and a drying step S40. After these steps, a silicon nitride green sheet can be obtained in which the silicon content is 59% to 80% by volume, and which serves as a precursor for a silicon nitride sintered body.

[0053] A silicon nitride sintered body is obtained as a silicon nitride substrate, which is a plate-shaped sintered body, by using a silicon nitride green sheet as a precursor and undergoing a heat treatment process S50. The silicon nitride substrate can be used in applications such as printed circuit boards.

[0054] In particle size adjustment step S10, the particle size distribution of powdered silicon used as a raw material for the silicon nitride green sheet is adjusted so that the filling ratio of silicon in the silicon nitride green sheet falls within a predetermined range. The particle size distribution can be adjusted by mixing the powders together so that D10 / D90 is greater than 0 and 0.15 or less, and D50 / D90 is 0.4 or less.

[0055] It is preferable to use multiple classified types of silicon powders with different median diameters (D50) and mode diameters as the silicon powders to be mixed together. It is preferable that the powdered silicon used as a raw material for the silicon nitride green sheet has a D50 of 0.5 μm to 20 μm.

[0056] For example, one method can be used in which 10% to 50% by weight of small particle powder with a median diameter of approximately 0.15 to 0.4 times that of large particle powder with a median diameter greater than 0.5 μm to 20 μm is added to 100% by weight of large particle powder with a median diameter greater than 0.5 μm to 20 μm, or in which 40% or less by weight of medium particle powder with a mode diameter of approximately 0.4 times or less and 10% or less by weight of small particle powder with a mode diameter of approximately 0.15 times or less is added to 100% by weight of large particle powder with a median diameter greater than 0.5 μm to 20 μm.

[0057] In mixing step S20, powdered silicon and a sintering aid are mixed in a dispersion medium to prepare a slurry. The silicon with a controlled particle size distribution and the sintering aid are weighed to a predetermined mixing ratio, and a binder and dispersant are added as needed, and wet mixing is performed in the dispersion medium. Wet mixing allows for the suppression of gaseous component contamination without significantly affecting the particle size distribution, and enables the dispersion of powder and the dissociation of aggregated particles.

[0058] As a sintering aid, it is preferable to use a powder with a particle size smaller than the D50 of silicon, from the viewpoint of dispersibility with silicon.

[0059] As a binder, any suitable type can be used, as long as it decomposes at low temperatures and does not leave behind large amounts of ash, carbon, etc., such as acrylic resins like polyacrylic acid and polyacrylic acid esters, cellulose resins like methylcellulose and ethylcellulose, polyvinyl alcohol, polyvinyl butyral, etc. Using a binder prevents particles from falling off the silicon nitride green sheet after molding, making the sheet easier to handle.

[0060] As a dispersion medium, any suitable type can be used, as long as it volatilizes at low temperatures and dissolves the binder added as needed. When using a water-soluble binder such as polyacrylic acid, lower alcohols such as methanol, ethanol, and butanol can be used. In addition, organic solvents such as hexane, trichloroethylene, and dichloromethane, or water can be used, as long as they do not react excessively with the silicon or binder.

[0061] The raw materials can be mixed using appropriate mixing equipment such as a V-type mixer, W-type mixer, Henschel mixer, ribbon mixer, screw mixer, ball mill, bead mill, planetary mixer, high-pressure homogenizer, ultrasonic homogenizer, or roll mill.

[0062] In coating step S30, the slurry is applied to the substrate to form a coating film containing powdered silicon, a sintering aid, a binder added as needed, and a dispersion medium. The thickness of the coating film can be set to an appropriate thickness depending on the application and required performance of the sintered body. However, from the viewpoint of reducing surface irregularities and variations in thickness within the plane of the sintered body, a thickness of 0.1 mm to 1 mm after drying is preferred.

[0063] As the base material, any suitable type can be used, such as resin films, resin plates, metal plates, ceramic plates, or glass plates, as long as it exhibits a sufficiently high heat resistance temperature relative to the vaporization temperature of the dispersion medium. Resin films are particularly preferred as the base material because they can be wound into a roll as a carrier tape. Specific examples of resin films include polyethylene terephthalate, polyethylene naphthalate, and polycarbonate.

[0064] The slurry can be coated using an appropriate coating device such as a sheet forming applicator equipped with a doctor blade, a slot die coater, a knife coater, a bar coater, a gravure coater, or a spray coater. The coating device can be a roll-type coater, a sheet-type coater, or equipment for casting or extrusion molding.

[0065] In drying step S40, the slurry coated on the substrate is dried to form a green sheet for silicon nitride. The substrate with the slurry coating is placed in a heating and drying oven or the like to dry the coating and remove the dispersion medium. As a result, powdered silicon and sintering aid remain on the substrate along with a binder added as needed, and a green sheet for silicon nitride is obtained in which the powders are bound together.

[0066] The drying temperature and drying time can be set to appropriate conditions within a range that does not cause deterioration of the material, depending on the volatilization temperature of the dispersion medium. The silicon nitride green sheet obtained after drying may or may not be peeled from the substrate before heat treatment. Alternatively, processing such as cutting or punching may be performed before heat treatment.

[0067] After drying, the silicon nitride green sheet can be subjected to pressure treatment to adjust the silicon filling rate, porosity, or sheet thickness. By controlling the silicon particle size distribution, the silicon filling rate in the sheet can be increased to 59% by volume or more. However, to achieve a higher silicon filling rate or to adjust the sheet thickness, the silicon may be compacted by compression during pressure treatment. Pressure treatment can be performed using appropriate pressurizing equipment such as an axial press or a roll press.

[0068] In the heat treatment process S50, the green sheet for silicon nitride is heat-treated in a nitrogen gas atmosphere. When heat treatment is performed at a high temperature, silicon and nitrogen gas gradually react to produce silicon nitride, and a silicon nitride substrate is obtained in which silicon nitride particles are sintered together. The sintering aid melts and volatilizes during the heat treatment, forming an interface such as a liquid phase that serves as a reaction field on the surface of the silicon, and after the heat treatment, a grain boundary phase containing components derived from the sintering aid is formed in the structure.

[0069] The heat treatment temperature and time can be set to appropriate conditions depending on the average particle size of silicon, the type of sintering aid, the mixing ratio of silicon to sintering aid, etc. If a binder is included, a low-temperature heat treatment for degreasing can be performed before the heat treatment for reaction sintering by nitriding.

[0070] A preferred heat treatment method is a multi-stage heat treatment including a heat treatment for reaction sintering by nitriding and a heat treatment to promote sintering between silicon nitride particles. Reaction sintering can be carried out in a nitrogen gas atmosphere, for example, at 1300°C to 1600°C. Sintering between silicon nitride particles can be carried out in a nitrogen gas atmosphere, for example, at 1720°C to 2000°C.

[0071] These processes allow for an increased silicon nitride packing density in the silicon nitride sintered body. The remainder of the sintered body consists mainly of small amounts of grain boundary phases containing components derived from the sintering aid, and voids. It is preferable that the silicon nitride sintered body has a high β fraction of silicon nitride.

[0072] Silicon nitride substrates can be used as final products for substrate applications without further processing. One application of silicon nitride substrates is printed circuit boards. They are particularly suitable as a material for circuit boards in power modules such as high-frequency transistors and power semiconductors, and for multi-chip modules. They can also be used as heat transfer plates for heat sinks, Peltier elements, Seebeck elements, and other applications.

[0073] Figure 2 shows the relationship between silicon particle size and packing density in a green sheet for silicon nitride. In Figure 2, the horizontal axis represents the ratio of D10 to D90 silicon (D10 / D90) and the ratio of D50 to D90 silicon (D50 / D90), while the vertical axis represents the silicon packing density in the silicon nitride green sheet. The silicon particle size and packing density are calculated values. It was assumed that some of the voids were filled with a binder.

[0074] The triangle plots represent the D10 / D90 of silicon obtained without pressure treatment after drying. The circles plots represent the D50 / D90 of silicon obtained without pressure treatment after drying. A1 and A2, B1 and B2, and C1 and C2 are values ​​from the same silicon nitride green sheet. A, B, and C have different particle size distributions.

[0075] The plots marked with ▲ represent the D10 / D90 of silicon obtained after pressure treatment following drying. The plots marked with ● represent the D50 / D90 of silicon obtained after pressure treatment following drying. D1 and D2, E1 and E2, and F1 and F2 are values ​​from the same silicon nitride green sheet, respectively. DEF values ​​have different particle size distributions.

[0076] As shown in Figure 2, the smaller the D10 / D90 or D50 / D90 of silicon, the higher the silicon packing density. One method for reducing the D10 and D50 of silicon is to prepare multiple powders with different median diameters (D50) and mode diameters, and then add the powder with the smallest particles. In particular, the higher the mixing ratio of powders with a small D50, the greater the change towards the small particle size.

[0077] As the ▲ and ● plots show, when silicon is compacted by pressurizing after drying, binders and small particles can be moved into the voids between silicon particles, resulting in a higher packing density compared to when no pressurizing treatment is performed. Pressurizing treatment allows the packing density of silicon in the sheet to be easily adjusted to approximately 80% by volume.

[0078] Next, we will describe specific examples of green sheets for silicon nitride.

[0079] (Examples 1-3) Examples 1-3 were prepared using a raw material mixture of silicon and sintering aid in a weight ratio of 95:5. The following table shows the reconnaissance options. 2 A powder with a controlled particle size distribution as shown, and Y2O as a sintering aid. 3. Polyacrylic acid was used as the binder and butanol as the dispersion medium. The thickness of the green sheet used was set to 0.5 mm. Examples 1 to 3 are shown in Figure 2. It corresponds to C to A.

[0080] (Examples 4-6) Examples 4-6 were prepared using a raw material mixture of silicon and sintering aid in a weight ratio of 95:5. The following table shows the reconnaissance options. 2 A powder with a controlled particle size distribution as shown, and Y2O as a sintering aid. A mixture of 3 and MgO in a weight ratio of 2:3, with polyacrylic acid as the binder and dispersion medium. Butanol was used as the additive. The thickness of the silicon nitride green sheet was 0.5 mm. Examples 4-6 correspond to C-A shown in Figure 2, respectively.

[0081] (Examples 7-8) Examples 7-8 used a raw material in which silicon and a sintering aid were mixed in a weight ratio of 95:5, and It was manufactured by forming a T-shaped coating film and then applying pressure using a roll press machine. The table below is for the contest. 2 A powder with a controlled particle size distribution as shown, and Y2O3 as a sintering aid. Only, or a mixture of Y2O3 and MgO in a weight ratio of 2:3, as a binder. Acrylic acid was used, and butanol was used as the dispersion medium. Thickness of the green sheet for silicon nitride. The thickness was set to 0.5 mm. Examples 7 and 8 correspond to D shown in Figure 2, respectively.

[0082] (Comparative Example 1) Comparative Example 1 was prepared using a raw material in which the silicon particle size distribution of Example 1 was shifted towards the larger particle size side. All conditions other than the silicon particle size were the same as in Example 1.

[0083] (Comparative Example 2) Comparative Example 2 was prepared using a raw material in which the silicon particle size distribution of Example 4 was shifted towards the larger particle size side. All conditions other than the silicon particle size were the same as in Example 4.

[0084] Table 1 shows the composition of the raw materials and manufacturing conditions for the silicon nitride green sheet. Table 2 shows the particle size of the silicon powder used as a raw material, the calculated results for D10 / D90 and D10 / D50, and the calculated results for the silicon packing density.

[0085] [Table 1]

[0086] [Table 2]

[0087] As shown in Tables 1 and 2, in all systems of Examples 1-3 and 4-6, which have different sintering aid compositions, the smaller the D10 / D90 and D50 / D90 values, the higher the silicon packing density. Since this result is obtained by converting silicon particles to spherical size, the relative size relationship of the particles is important, and it can be said that it is preferable to satisfy the above-mentioned D10 / D90 and D50 / D90 conditions depending on the appropriate silicon particle size range for the raw material.

[0088] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, the present invention is not necessarily limited to having all the configurations of the embodiments described above. Some configurations of one embodiment may be replaced with other configurations, some configurations of one embodiment may be added to other forms, or some configurations of one embodiment may be omitted. [Explanation of symbols]

[0089] S10 Particle size adjustment process S20 Mixing process S30 Coating Process S40 Drying process S50 Heat treatment process

Claims

1. A green sheet for silicon nitride, used as a precursor for silicon nitride sintered bodies, comprising silicon, a sintering aid, and a binder, The amount of the sintering aid is 1% by mass or more and 15% by mass or less, based on the total mass of silicon and sintering aid (100% by mass), in terms of oxides. The filling percentage of the aforementioned silicon is 59% by volume or more and 80% by volume or less. A green sheet for silicon nitride with a thickness of 0.1 mm to 1 mm.

2. A green sheet for silicon nitride according to claim 1, A green sheet for silicon nitride, wherein when the particle sizes corresponding to the 10% and 90% volume fractions in the volume-based cumulative particle size distribution of silicon are defined as D10 and D90, respectively, the ratio of D10 to D90 (D10 / D90) is greater than 0 and 0.15 or less.

3. A green sheet for silicon nitride according to claim 1 or 2, A green sheet for silicon nitride, wherein when D50 is defined as the particle size corresponding to 50% of the volume fraction in the volume-based cumulative particle size distribution of the silicon, the D50 of the silicon is 0.5 μm or more and 20 μm or less.

4. A green sheet for silicon nitride according to any one of claims 1 to 3, When D50 is defined as the particle size corresponding to 50% of the volume fraction in the volume-based cumulative particle size distribution of the silicon, the sintering aid is a powder with a particle size smaller than D50 of the silicon, and is a green sheet for silicon nitride.

5. A silicon nitride substrate obtained by using a silicon nitride green sheet according to any one of claims 1 to 4 as a precursor and undergoing a heat treatment process.

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