JP7615652B2 - Silicon nitride green sheet and its manufacturing method - Google Patents

Description

The present invention relates to a silicon nitride green sheet containing silicon and a sintering aid, which is used as a precursor for a silicon nitride sintered body, and a method for producing the same.

Silicon nitride (Si ₃ N ₄ ) has excellent mechanical properties such as high-temperature strength and abrasion resistance, as well as heat resistance, low thermal expansion, thermal shock resistance, and corrosion resistance against metals such as Al and Ag. Therefore, silicon nitride sintered bodies are used as structural materials for gas turbines, engines, steelmaking machines, etc., and as materials resistant to melting damage by molten metals. In addition, because of its excellent electrical insulation, it is used as an electronic component material for circuit boards, etc., and as an electrical insulating material.

In recent years, semiconductor elements that generate a large amount of heat, such as high-frequency transistors and power semiconductors, have become increasingly popular. Materials for semiconductor elements that generate a large amount of heat must have not only electrical insulation but also excellent heat dissipation properties, so there is a growing demand for ceramic substrates that exhibit high resistivity and high thermal conductivity. Traditionally, aluminum nitride (AlN) has been widely used as a material for ceramic substrates.

Aluminum nitride has excellent electrical insulation and thermal conductivity, but low mechanical strength and fracture toughness. When tightening the aluminum nitride substrate during assembly of a circuit board unit, there is a problem that it is prone to cracking. In addition, because the thermal expansion coefficient of aluminum nitride is significantly different from that of silicon, when silicon semiconductor elements are mounted on an aluminum nitride substrate, cracks and breakage easily occur due to thermal expansion and contraction.

Currently, silicon nitride substrates are being used in place of aluminum nitride substrates, which have problems with mounting reliability. Silicon nitride substrates are sintered bodies of silicon nitride, and are formed from sintered silicon nitride and grain boundary phases derived from sintering aids. Silicon nitride has a lower thermal conductivity than aluminum nitride, but its thermal expansion coefficient is close to that of silicon, and it forms sintered bodies with excellent mechanical strength, fracture toughness, and thermal fatigue resistance.

Silicon nitride has a stable crystal structure that is strongly bonded by covalent bonds, and is therefore characterized by its excellent heat resistance and hardness. There are two main crystal structures of silicon nitride: α type and β type. α type is a low-temperature phase of the trigonal crystal system that exists at temperatures lower than around 1400°C. β type is a high-temperature phase of the hexagonal crystal system that is formed by a phase transition from α type at temperatures between 1400°C and 1600°C.

A known method for producing 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, has strong covalent bonds, and is difficult to sinter densely. For this reason, sintering aids such as aluminum oxide, yttrium oxide, and magnesium oxide are added during heat treatment.

Alpha-type silicon nitride powder is mainly used as the raw material for sintered bodies produced by self-sintering. Alpha-type silicon nitride undergoes a phase transition to beta-type columnar crystals, improving the mechanical properties of the sintered body. Sintered bodies produced by self-sintering are generally known to have excellent mechanical strength, including bending strength. Sintered bodies produced by self-sintering have been shown to have a thermal conductivity close to 100 W/(m·K).

Another known method for producing silicon nitride sintered bodies is to subject silicon powder to reactive sintering and post-sintering in a nitrogen gas atmosphere. Sintered bodies produced by reactive sintering are expected to have higher thermal conductivity, but undergo volumetric changes due to the nitriding reaction of silicon. During reactive sintering, volumetric expansion due to the nitriding reaction and volumetric contraction due to sintering occur, making it difficult to mold the body into a near-net shape. Measures are also required to prevent deformations such as warping, cracks, and breakage. Note that near-net shape refers to a finished product that is close in size and shape to the finished product, to the extent that secondary processing is not required.

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

Patent Document 2 describes a silicon nitride sintered body produced by self-sintering using normal pressure sintering or gas pressure sintering. The raw material used is a silicon nitride powder having a specific surface area of 5 m ² /g or more and 20 m ² /g or less, a ratio of β-type silicon nitride of 70 mass% or more, D50 of 0.5 μm or more and 3 μm or less, 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 D _C of β-type silicon nitride of 60 nm or more, a ratio D _BET /D _C (nm/nm) of the specific surface area equivalent diameter D _BET to the crystallite diameter D _C of 3 or less, and a crystal strain of β-type silicon nitride of 1.5×10 ⁻⁴ or less.

Patent Document 3 describes a silicon nitride sintered body by self-sintering. The sintered body has silicon nitride particles and a grain boundary phase containing Mg and at least one rare earth element, the ratio (RExOy/MgO) of Mg and rare earth element (RE) converted into oxide is in the range of 0.05 to 5, the average long axis length L of silicon nitride particles present in an arbitrarily set 20×20 μm area on the processed surface is 5.0 μm or less, the average ratio (L/S) of the long axis length L to the short axis length S is 5 or less, the area ratio of pores each having an area of 0.01 ^μm2 or more is 0.01% to 5% in an arbitrarily set 300×300 μm area on the processed surface, the average value of the distance between the centers of gravity of the most adjacent pores among the pores is 5 μm or more, and the variation coefficient of the distance between the centers of gravity is 1.5 or less.

Japanese Unexamined Patent Publication No. 7-309669 International Publication No. 2018/110564 JP 2014-073945 A

Reaction sintering is a promising method for producing sintered silicon nitride from the standpoint of obtaining high thermal conductivity and fracture toughness. In order to obtain higher thermal conductivity in the reaction sintering manufacturing process, it is important to reduce the voids in the sintered body and increase the packing rate of silicon nitride. However, there is room for improvement in conventional technology when it comes to increasing the packing rate.

The raw material used in Patent Document 1 is a two-particle distribution mixture of two types of metal silicon powder, so the difference in particle size between the coarse powder and the fine powder may be small, or the particle size frequency distribution may have an extremely two-peak shape. In such cases, the filling properties of the silicon are poor, voids of a certain size or larger may be easily formed, and the size and distribution of the voids may vary widely, which may increase the impact of volume expansion associated with the nitriding reaction.

Furthermore, in the case of self-sintering as described in Patent Documents 2 and 3, glass phases, grain boundaries, and voids are likely to form, which poses many obstacles to improving thermal conductivity. In general, one method of increasing the packing rate of a sintered body is to increase the amount of sintering aid added. However, increasing the amount of sintering aid increases the amount of glass phases with low thermal conductivity in the sintered body, which may actually lower the thermal conductivity of the sintered body.

In addition, in the case of a manufacturing process using reactive sintering, a filling rate that is too high before the nitriding reaction can also be a problem. The silicon that constitutes the precursor for reactive sintering expands in volume as the nitriding reaction occurs. Therefore, if the filling rate of silicon in the precursor is too high, the volume expansion that accompanies the nitriding reaction can cause large deformations and excessive stress, resulting in poor appearance such as warping, cracks, breakage, poor molding accuracy, etc.

In particular, when the filling rate of silicon in the precursor is approximately 82 volume percent or more, the volume expansion exceeds the volume of the voids, resulting in significant defects and failures. In Patent Document 1, reactive sintering is performed, but the filling rate and porosity of the precursor are not clear, and the problem of such volume expansion is not fully taken into consideration.

Therefore, the present invention aims to provide a green sheet for silicon nitride that can produce a silicon nitride sintered body that has high thermal conductivity and little appearance or molding accuracy defects, and a method for manufacturing the same.

In order to solve the above problems, the silicon nitride green sheet according to the present invention is used as a precursor for a silicon nitride sintered body, and contains silicon and a sintering aid. When the particle sizes corresponding to 10%, 50% and 90% volume fractions in the volume-based cumulative particle size distribution of the silicon are D10, D50 and D90, respectively, the ratio of D10 to D90 (D10/D90) is greater than 0 and less than or equal to 0.15, and the ratio of D50 to D90 (D50/D90) is less than or equal to 0.4, and the filling rate of the silicon is 59% by volume or more and 80% by volume or less.

The method for producing a silicon nitride green sheet according to the present invention is used as a precursor for a silicon nitride sintered body, and contains silicon and a sintering aid. The method includes the steps of: preparing powdered silicon such that the ratio of D10 to D90 (D10/D90) is greater than 0 and less than 0.15 and the ratio of D50 to D90 (D50/D90) is less than 0.4, where D10, D50, and D90 are particle sizes corresponding to 10%, 50%, and 90% volume fractions in the volume-based cumulative particle size distribution of the silicon, respectively; mixing the silicon and the sintering aid in a dispersion medium to prepare a slurry; applying the slurry to a substrate; and drying the applied slurry. The method obtains a silicon nitride green sheet having a packing ratio of 59% to 80% by volume of the silicon after drying.

The present invention provides a silicon nitride green sheet that can produce a silicon nitride sintered body that has high thermal conductivity and little defects in appearance or molding precision, and a method for producing the same.

FIG. 2 is a flow diagram showing a method for producing a silicon nitride green sheet according to an embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the particle size and the filling rate of silicon in a silicon nitride green sheet.

The following describes a silicon nitride green sheet according to one embodiment of the present invention and its manufacturing method, with reference to the drawings.

The silicon nitride green sheet according to this embodiment is used as a precursor for producing a silicon nitride sintered body by reactive sintering. The silicon nitride green sheet contains powdered silicon (Si) and a sintering aid, and may contain a binder as necessary.

A silicon nitride sintered body is obtained by reactive sintering of molded powdered silicon together with a sintering aid in a nitrogen gas atmosphere. When silicon (Si) is heat-treated at a high temperature of about 1300°C or higher in a nitrogen gas atmosphere, it becomes silicon nitride (Si ₃ N ₄ ) through a nitriding reaction. The silicon nitride particles are sintered together to obtain a silicon nitride sintered body.

In this specification, a green sheet for silicon nitride is a sheet-like molded body composed of powdered silicon and a sintering aid, and refers to the precursor before the silicon undergoes a nitriding reaction. A green sheet for silicon nitride is in the form of a sheet that is thin compared to its width and length, but the width, length and thickness of the sheet are not particularly limited.

The silicon nitride green sheet according to this embodiment has a silicon filling rate of 59% to 80% by volume. The remainder of the sheet is mainly composed of a small amount of sintering aid, a small amount of binder, and voids.

When silicon particles become silicon nitride through a nitriding reaction, the volume calculated from the unit cell expands by approximately 1.22 times. By making the filling rate of silicon in the sheet between 59 volume % and 80 volume %, the filling rate of silicon nitride after the nitriding reaction can be increased, resulting in the desirable effects described below.

In order to obtain high thermal conductivity, it is preferable for the silicon nitride sintered body to have a high silicon nitride filling rate. Therefore, it is also preferable for the silicon nitride green sheet, which is the precursor of the sintered body, to have a high silicon filling rate. According to calculations using a filling model, if the filling rate of silicon particles is 59 volume percent or more, the silicon nitride particles will be more likely to bond during the nitriding and sintering reactions. As a result, it is possible to obtain high thermal conductivity.

If the silicon filling rate in the silicon nitride green sheet is higher than 80% by volume, the silicon nitride filling rate after the nitriding reaction will be close to 100% by volume, leaving insufficient space for the volume expansion that accompanies the nitriding reaction. In such cases, the sintered body will experience significant deformation and stress due to volume expansion exceeding the volume contraction caused by sintering and restrictions on particle rearrangement. In contrast, if a void of approximately 2% by volume or more is secured, even if volume expansion occurs due to the nitriding reaction, the volume expansion and rearrangement of particles is permitted, and deformation and stress can be alleviated.

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

Defects and defects that occur in sintered bodies are due to volume expansion associated with 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, breaks, and appearance defects such as warping that can be problematic in applications such as printed circuit boards, as well as poor molding accuracy when molding to a near net shape or to target dimensions. These defects and defects can be reduced by lowering the silicon filling rate to ensure voids in the sintered body.

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

When D50/D90 is 0.4 or less, small particles (particles of D10 or less) and medium particles (particles of D10 to D50) that account for 50% by volume of the silicon powder in the sheet are sufficiently smaller than the large particles (particles of D90 or more) that account for 10% by volume of the sheet, allowing a large amount of small and medium particles to be filled into the gaps between the large particles.

Furthermore, when D10/D90 is 0.15 or less, small particles (particles of D10 or less), which account for 10% by volume of the total silicon powder in the sheet, are sufficiently smaller than large particles (particles of D90 or more), which account for 10% by volume of the total, and medium particles (particles of D10 to D50), which account for 40% by volume of the total, etc., so that a large amount of small particles can be filled into the gaps between the large and medium particles.

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

D10/D90 is preferably 0.01 or more, more preferably 0.03 or more. If D10/D90 is 0.03 or more, the ratio of silicon with an extremely small particle size is low, and the surface area of the powder as a whole is small, so the amount of binder used can be reduced. D50/D90 is greater than the D10/D90 of the powder and is 0.4 or less.

The D50 of the silicon powder in the silicon nitride green sheet is preferably 0.5 μm or more and 20 μm or less. If D50 is less than 0.5 μm, the amount of binder is small relative to the total specific surface area of the powder, and the mechanical strength of the sheet is low. If D50 exceeds 20 μm, the surface of the silicon nitride green sheet becomes uneven and the nitriding reaction rate slows. In contrast, if D50 is 0.5 μm or more and 20 μm or less, the surface of the silicon nitride green sheet can be made smooth while nitriding reaction rate can be increased.

The D50 of the silicon powder in the silicon nitride green sheet is preferably 0.8 μm or more from the viewpoint of obtaining high strength while suppressing the amount of binder. Also, from the viewpoint of smoothing the surface of the silicon nitride green sheet and accelerating the nitridation 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 nitridation reaction time can be shortened. If it is 0.8 μm or more and 3 μm or less, high strength can be obtained with a small amount of binder and a surface with few irregularities can be obtained. If it is 0.8 μm or more and 2 μm or less, in addition to high strength and smoothness, a higher filling rate can be obtained.

The particle size and particle size distribution of silicon can be measured using a laser diffraction/scattering type particle size distribution measuring device. 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 to remove the solvent, and separating and measuring the silicon in the resulting powder based on specific gravity, or by observing any cross section of a silicon nitride green sheet with an electron microscope to collect data such as the circle equivalent diameter of silicon particles.

The porosity of silicon nitride green sheets can be determined by observation with 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 ternarization, and the area ratio of voids per unit area is calculated, assuming that low-density areas in the image are voids that are free of silicon or sintering aids. SEM images are taken at a magnification of 500x to 1000x for multiple cross sections and microscope fields.

The number of cross sections taken for SEM images is preferably 2 or more, and more preferably 5 or more, in order to reduce variation due to measurement error. The average void area ratio is calculated based on a large number of SEM images, and when the variation in the average value becomes sufficiently small with an increase in the number of images, the average area ratio is taken as the void ratio of the silicon nitride green sheet. When the sheet contains only a small amount of sintering aid and binder, the silicon filling rate [%] can be considered to be 100 - void ratio [%].

As the 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.

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).

Mg, Y and rare earth elements have low solid solubility in silicon nitride and are difficult to change the crystal structure and chemical composition, so they are 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. As a compound containing Mg or Y, magnesium oxide (MgO) and yttrium oxide (Y ₂ O ₃ ) are preferable.

As the rare earth element, one or more elements selected from the group consisting of La, Ce, Gd, Dy and Yb are particularly preferred. Sintering aids containing these elements can promote sintering while suppressing temperature and pressure, thereby reducing the cost of heat treatment.

When the composition of the sintering aid contains La, Y, Gd or Yb and Mg, when the mixed weight of the rare earth elements converted into oxides is _{RExOy and} the mixed weight of magnesium converted into oxides is MgO, it is preferable that _RExOy /MgO>1. When the weight ratio of these rare earth elements to Mg is increased, fine particles derived from the sintering aid are easily formed during heat treatment. _This is because these rare earth elements are less likely to form a solid solution in silicon nitride than Mg.

The amount of sintering aid, calculated as oxide, is preferably 1% by mass or more, more preferably 2% by mass or more, based on 100% by mass of the total of silicon and sintering aid. Also, it is preferably 15% by mass or less, more preferably 10% by mass or less. With such an amount, it is possible to promote sintering of silicon nitride and high purification within the particles, while suppressing the formation of a glass phase with low thermal conductivity.

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. With such a thickness, a thin silicon nitride sintered body can be produced. A silicon nitride green sheet of such a thickness may be obtained by applying pressure after drying during the manufacturing process, or may be obtained without applying pressure after drying. By applying pressure or the like to reduce the thickness to 0.5 mm or less, a higher thermal conductivity can be obtained.

The silicon nitride green sheet described above uses silicon that satisfies the above-mentioned D10/D90 and D50/D90 conditions, so that the silicon filling rate is in the range of 59% to 80% by volume, and a precursor can be obtained in which the space for volume expansion accompanying the nitriding reaction is dispersedly secured. It is also possible to secure a high filling rate close to 80% by volume at the stage of the silicon nitride sintered body precursor, and since the nitriding reaction and sintering make it easier for the particles to be bonded uniformly, a dense silicon nitride sintered body can be obtained. Therefore, when used in a manufacturing process by reactive sintering, a silicon nitride sintered body that has high thermal conductivity and has little appearance defects such as warping, cracks, breaks, or molding precision defects can be obtained.

Next, a method for manufacturing the silicon nitride green sheet in which the silicon filling rate is within a predetermined range will be described. In the following explanation, a manufacturing method in which the raw material powders are wet mixed and finally dried will be exemplified together with a heat treatment method.

FIG. 1 is a flow diagram showing a method for producing a silicon nitride green sheet according to an embodiment of the present invention.
1, the method for producing a silicon nitride green sheet according to this embodiment includes a particle size adjusting step S10, a mixing step S20, a coating step S30, and a drying step S40. Through these steps, a silicon nitride green sheet having a silicon filling rate of 59 volume % or more and 80 volume % or less, which serves as a precursor of a silicon nitride sintered body, can be obtained.

The silicon nitride sintered body is obtained as a silicon nitride substrate, which is a plate-shaped sintered body, by subjecting the silicon nitride green sheet as a precursor to a heat treatment process S50. The silicon nitride substrate can be used for applications such as printed circuit boards.

In the particle size adjustment step S10, the particle size distribution of the powdered silicon used as the raw material for the silicon nitride green sheet is adjusted so that the filling rate of silicon in the silicon nitride green sheet is 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 less than or equal to 0.15, and D50/D90 is less than or equal to 0.4.

As the silicon powder to be mixed, it is preferable to use multiple types of classified powder with different median diameters (D50) and mode diameters. It is preferable that the powdered silicon used as the raw material for the silicon nitride green sheet is adjusted to have a D50 of 0.5 μm or more and 20 μm or less.

For example, a method of adding 10% to 50% by weight of small particle powder with a median diameter of about 0.15 to 0.4 times that of the large particles to 100% by weight of large particle powder with a median diameter of 0.5 to 20 μm or a method of adding 40% or less by weight of medium particle powder with a mode diameter of about 0.4 times or less and 10% or less by weight of small particle powder with a mode diameter of about 0.15 times or less to 100% by weight of large particle powder with a median diameter of 0.5 to 20 μm can be used.

In the mixing step S20, powdered silicon and sintering aid are mixed in a dispersion medium to prepare a slurry. Silicon with controlled particle size distribution and sintering aid are weighed out to a predetermined mixing ratio, and binders and dispersants are added as necessary, followed by wet mixing in the dispersion medium. Wet mixing can suppress the inclusion of gas components without significantly affecting the particle size distribution, and can provide a dispersion effect for the powder and a dissociation effect for agglomerated particles.

From the standpoint of dispersibility in silicon, it is preferable to use a powder with a particle size smaller than the D50 of silicon as a sintering aid.

As the binder, any suitable type can be used, such as acrylic resins such as polyacrylic acid and polyacrylic acid esters, cellulose resins such as methyl cellulose and ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, etc., as long as they are pyrolyzed at low temperatures and do not leave behind a large amount of ash, carbon, etc. Use of a binder can prevent particles from falling off the silicon nitride green sheet after molding, making the sheet easier to handle.

Any suitable type of dispersion medium can be used as long as it volatilizes at low temperatures and dissolves the binder that is 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, as well as water, can be used as long as they do not react excessively with the silicon or binder.

The raw materials can be mixed using an appropriate mixing device 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.

In the coating step S30, the slurry is applied onto the substrate to form a coating film containing powdered silicon, sintering aids, a binder added as necessary, and a dispersion medium. The thickness of the coating film can be set appropriately depending on the application and required performance of the sintered body. However, from the viewpoint of reducing the unevenness of the surface of the sintered body and the variation in thickness within the surface, a thickness of 0.1 mm to 1 mm after drying is preferable.

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

The slurry can be applied using a suitable 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 may be a roll coater or a sheet-fed coater, or may be a device using a casting method or an extrusion molding method.

In the drying step S40, the slurry applied to the substrate is dried to form a silicon nitride green sheet. The substrate on which the slurry coating is formed is placed in a heating and drying furnace or the like, and the coating is dried to remove the dispersion medium, leaving powdered silicon and sintering aid on the substrate together with a binder added as necessary, resulting in a silicon nitride green sheet in which the powder particles are bonded together.

The drying temperature and drying time can be set as appropriate according to the volatilization temperature of the dispersion medium, as long as the material is not altered. The silicon nitride green sheet obtained after drying may or may not be peeled off from the substrate before heat treatment. In addition, cutting, punching, or other processing may be performed before heat treatment.

After drying, the silicon nitride green sheet can be subjected to a pressure treatment to adjust the silicon filling rate, porosity, or sheet thickness. By controlling the silicon particle size distribution, the filling rate of silicon in the sheet can be increased to 59 volume % or more. However, to increase the silicon filling rate or adjust the sheet thickness, the silicon can be compressed and densified by pressure treatment. The pressure treatment can be performed using an appropriate pressure device such as an axial press or a roll press.

In the heat treatment step S50, the silicon nitride green sheet is heat treated in a nitrogen gas atmosphere. When heat treatment is performed at high temperatures, 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 of a liquid phase, etc., which serves as a reaction site, on the silicon surface, etc., and after heat treatment, a grain boundary phase containing components derived from the sintering aid is formed in the structure.

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

A preferred heat treatment method is a multi-stage heat treatment that includes a heat treatment for reactive sintering by a nitriding reaction and a heat treatment to promote sintering of silicon nitride particles together. Reactive sintering can be carried out in a nitrogen gas atmosphere, for example, at 1300°C to 1600°C. Sintering of silicon nitride particles together can be carried out in a nitrogen gas atmosphere, for example, at 1720°C to 2000°C.

These processes make it possible to increase the packing rate of silicon nitride in the sintered silicon nitride body. The remainder of the sintered body is mainly composed of small amounts of grain boundary phases containing components derived from the sintering aid, and voids. It is preferable that the sintered silicon nitride body have a high β fraction of silicon nitride.

Silicon nitride substrates can be used as final products for substrate applications without secondary processing. Examples of applications for silicon nitride substrates include printed circuit boards. They are particularly suitable as materials for high-frequency transistors, power modules such as power semiconductors, and circuit boards for multi-chip modules. They can also be used as heat transfer plates for heat sinks, Peltier elements, Seebeck elements, etc.

FIG. 2 is a diagram showing the relationship between the particle size and the packing rate of silicon in a silicon nitride green sheet.
In Fig. 2, the horizontal axis indicates the ratio of D10 to D90 of silicon (D10/D90) and the ratio of D50 to D90 of silicon (D50/D90), and the vertical axis indicates the filling rate of silicon in the silicon nitride green sheet. The silicon particle size and filling rate are calculated values. It is assumed that some of the voids are filled with binder.

The △ plots show the D10/D90 of silicon obtained when pressure treatment is not applied after drying. The ◯ plots show the D50/D90 of silicon obtained when pressure treatment is not applied after drying. A1 and A2, B1 and B2, C1 and C2 are each the values for the same silicon nitride green sheet. A, B and C have different particle size distributions.

The ▲ plot shows the D10/D90 of silicon obtained when pressure treatment is applied after drying. The ● plot shows the D50/D90 of silicon obtained when pressure treatment is applied after drying. D1 and D2, E1 and E2, F1 and F2 are each the values for the same silicon nitride green sheet. The DEFs have different particle size distributions.

As shown in Figure 2, the smaller the particle diameter of silicon's D10/D90 or D50/D90, the higher the silicon filling rate. One method for reducing silicon's D10 or D50 is to prepare multiple powders with different median diameters (D50) or mode diameters, and add smaller particle powder. In particular, the higher the mixing ratio of powder with a small D50, the greater the change to the smaller particle diameter side.

As the ▲ and ● plots show, when the silicon is compressed by pressure treatment after drying, the binder and small particles can be moved into the gaps between the silicon particles, resulting in a higher packing rate compared to when pressure treatment is not applied. By applying pressure treatment, the packing rate of silicon in the sheet can be easily adjusted to approximately 80% by volume.

Next, we will explain some specific examples of silicon nitride green sheets.

(Examples 1 to 3)
Examples 1 to 3 were made from raw materials in which silicon and sintering aid were mixed in a weight ratio of 95:5. Silicon powder with a controlled particle size distribution shown in Table 1 below was used as the silicon, Y ₂ O ₃ as the sintering aid, polyacrylic acid as the binder, and butanol as the dispersion medium. The thickness of the silicon nitride green sheet was 0.5 mm. Examples 1 to 3 correspond to C to A in FIG. 2, respectively.

(Examples 4 to 6)
Examples 4 to 6 were made from raw materials in which silicon and sintering aid were mixed in a weight ratio of 95:5. The silicon used was a powder with a controlled particle size distribution as shown in Table ₁ below, the sintering aid was a mixture of _Y2O3 and MgO in a weight ratio of 2:3, the binder was polyacrylic acid, and the dispersion medium was butanol. The thickness of the silicon nitride green sheet was 0.5 mm. Examples 4 to 6 correspond to C to A in Figure 2, respectively.

(Examples 7 to 8)
Examples 7 to 8 were produced by using a raw material in which silicon and a sintering aid were mixed in a weight ratio of 95:5, forming a sheet-like coating film, and then applying pressure using a roll press. Silicon was a powder with a controlled particle size distribution as shown in Table 1 below, sintering aid was either Y ₂ O ₃ alone or a mixture of Y ₂ O ₃ and MgO in a weight ratio of 2:3, binder was polyacrylic acid, and dispersion medium was butanol. The thickness of the silicon nitride green sheet was 0.5 mm. Examples 7 to 8 correspond to D in FIG. 2, respectively.

(Comparative Example 1)
Comparative Example 1 was produced using a raw material in which the silicon particle size distribution of Example 1 was shifted to the larger particle size side. The conditions other than the silicon particle size were the same as those of Example 1.

(Comparative Example 2)
Comparative Example 2 was produced using a raw material in which the silicon particle size distribution of Example 4 was shifted to the larger particle size side. The conditions other than the silicon particle size were the same as those of Example 4.

Table 1 shows the composition of the raw materials and manufacturing conditions for silicon nitride green sheets. Table 2 shows the particle size of the silicon powder used as the raw material, the calculation results of D10/D90 and D10/D50, and the calculation results of the silicon filling rate.

As shown in Tables 1 and 2, in both the systems of Examples 1 to 3 and Examples 4 to 6, which have different compositions of sintering aids, the smaller the D10/D90 and D50/D90, the higher the silicon filling rate. This result is based on converting silicon particles into spherical particles, so the relative size relationship of the particle diameter is important, and it can be said that it is preferable to satisfy the above D10/D90 and D50/D90 conditions depending on the particle diameter range of silicon suitable as a raw material.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention. For example, the present invention is not necessarily limited to having all of the configurations of the above-described embodiments. It is possible to replace part of the configuration of an embodiment with another configuration, add part of the configuration of an embodiment to another form, or omit part of the configuration of an embodiment.

S10 Particle size adjustment step S20 Mixing step S30 Coating step S40 Drying step S50 Heat treatment step

Claims (6)

A silicon nitride green sheet used as a precursor of a silicon nitride sintered body, the green sheet containing silicon , a sintering aid , and a binder ,
When particle sizes corresponding to 10%, 50% and 90% volume fractions in the volume-based cumulative particle size distribution of silicon are designated as D10, D50 and D90, respectively, the ratio of D10 to D90 (D10/D90) is greater than 0 and is not greater than 0.15, and the ratio of D50 to D90 (D50/D90) is not greater than 0.4,
The filling rate of the silicon is 59% by volume or more and 80% by volume or less,
A green sheet for silicon nitride having a thickness of 0.1 mm or more and 1 mm or less . 2. The silicon nitride green sheet according to claim 1,
The sintering aid is a compound containing one or more elements selected from the group consisting of magnesium, yttrium and rare earth elements. 2. The silicon nitride green sheet according to claim 1,
A green sheet for silicon nitride, wherein the amount of the sintering aid is 1 mass % or more and 15 mass % or less with respect to 100 mass % in total of the silicon and the sintering aid. 2. The silicon nitride green sheet according to claim 1,
The silicon nitride green sheet has a D50 of 0.5 μm or more and 20 μm or less. 2. The silicon nitride green sheet according to claim 1,
The silicon nitride green sheet has a D50 of 0.5 μm or more and 4 μm or less. A method for producing a silicon nitride green sheet according to any one of claims 1 to 5, comprising the steps of:
a step of preparing powdered silicon such that the ratio of D10 to D90 (D10/D90) is greater than 0 and not greater than 0.15 and the ratio of D50 to D90 (D50/D90) is not greater than 0.4, where D10, D50 and D90 are particle sizes corresponding to 10%, 50% and 90% volume fractions in a volume-based cumulative particle size distribution of the silicon, respectively;
mixing the silicon and a sintering aid in a dispersion medium to prepare a slurry;
applying the slurry onto a substrate;
and drying the applied slurry.
A method for producing a green sheet for silicon nitride, which produces a green sheet having a filling rate of silicon of 59 volume % or more and 80 volume % or less after drying.

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