JP2022094464A - Green sheet of silicon nitride and production method thereof - Google Patents

Abstract

To provide a green sheet of silicon nitride for obtaining a sintered body of silicon nitride which exhibits a high thermal conductivity with little appearance defect and poor molding accuracy, and to provide a production method for the same.SOLUTION: A green sheet of silicon nitride is used as a precursor of a sintered body of silicon nitride, and is a sheet containing silicon and a sintering aid. When particle diameters corresponding to the volume fractions of 10%, 50% and 90% in the volume-based integrated particle diameter distribution of silicon are D10, D50 and D90, D10/D90 is greater than 0 and 0.15 or less, D50/D90 is 0.4 or less, and a filling ratio occupied by silicon is 59 volume% or more and 80 volume% or less.SELECTED DRAWING: Figure 1

Description

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

Silicon nitride (Si _{3N 4 )} is excellent in heat resistance, low thermal expansion, heat impact resistance, and corrosion resistance to metals such as Al and Ag, in addition to mechanical properties such as high temperature strength and wear resistance. Therefore, the sintered body of silicon nitride is used as a structural material for gas turbines, engines, steelmaking machines, etc., and as a material for resistance to melting and damage to molten metal. Further, since it has excellent electrical insulation properties, it is used as an electronic component material such as a circuit board and an electrical insulation material.

In recent years, semiconductor devices with a large calorific value, such as high-frequency transistors and power semiconductors, have become widespread. Since the material of a semiconductor element having a large calorific value is required to have not only electrical insulation but also excellent heat dissipation characteristics, there is an increasing demand for a ceramic substrate exhibiting high resistivity and high thermal conductivity. Conventionally, aluminum nitride (AlN) is often used as a material for a ceramic substrate.

Aluminum nitride has excellent electrical insulation and thermal conductivity, but has low mechanical strength and fracture toughness. If the aluminum nitride substrate is tightened when the circuit board unit is assembled, there is a problem that cracks are likely to occur. Further, since aluminum nitride has a coefficient of thermal expansion significantly different from that of silicon, when a silicon semiconductor element is mounted on an aluminum nitride substrate, cracks and cracks are easily generated due to thermal expansion and thermal shrinkage.

Currently, the use of a silicon nitride substrate is being promoted in place of the aluminum nitride substrate, which has a problem in mounting reliability. The silicon nitride substrate is a sintered body of silicon nitride, and is formed of sintered silicon nitride or a grain boundary phase derived from a sintering aid. Silicon nitride has a lower thermal conductivity than aluminum nitride, but has a coefficient of thermal expansion close to that of silicon, and forms a sintered body having excellent mechanical strength, fracture toughness, heat fatigue characteristics, and the like.

Silicon nitride has a stable crystal structure that is firmly bonded by a covalent bond, so that it has excellent heat resistance and high hardness. There are two main types of crystal structure of silicon nitride, α-type and β-type. The α type is a trigonal low temperature phase and exists on the lower temperature side than around 1400 ° C. The β-type is a hexagonal high-temperature phase, and is produced by a phase transition from the α-type at 1400 ° C. or higher and 1600 ° C. or lower.

As a method for producing a sintered body of silicon nitride, a method of sintering silicon nitride powder in a nitrogen gas atmosphere by adding a sintering aid is known. Silicon nitride is a non-oxide, has a strong covalent bond, and is difficult to be densely sintered. Therefore, during the heat treatment, a sintering aid such as aluminum oxide, yttrium oxide, and magnesium oxide is added.

As a raw material for a self-sintered sintered body, α-type silicon nitride powder is mainly used. The α-type silicon nitride undergoes a phase transition to the β-type of columnar crystals, improving the mechanical properties of the sintered body. It is generally known that a self-sintered sintered body is excellent in mechanical strength such as bending strength. As the self-sintered sintered body, a sintered body having a thermal conductivity close to 100 W / (m · K) has been obtained.

Further, as a method for producing a sintered body of silicon nitride, a method of react-sintering / post-sintering silicon powder in a nitrogen gas atmosphere is also known. The sintered body by reaction sintering is expected to obtain higher thermal conductivity, but causes a volume change due to the nitriding reaction of silicon. During reaction sintering, volume expansion due to nitriding reaction and volume contraction due to sintering occur, so that there is a problem that it is difficult to form a near-net shape. In addition, measures to prevent deformation such as warpage, cracks, cracks, etc. are also required. The near-net shape means that the dimensions and shape are close to those of a finished product to the extent that secondary processing is not required.

Patent Document 1 describes a silicon nitride sintered body obtained by reaction sintering. In this sintered body, coarse metal silicon powder having an average particle size of 10 μm or more and 30 μm or less is 80% by weight or more and 95% by weight or less, and fine metal silicon powder having an average particle size of 1 μm or more and 10 μm or less is 5% by weight or more and 20% by weight. The raw material containing the following mixed metallic silicon is produced by a method of heating to a temperature of 1390 ° C. or higher and 1500 ° C. or lower in a nitrogen atmosphere.

Patent Document 2 describes a silicon nitride sintered body obtained by self-sintering using normal pressure sintering or gas pressure sintering. As raw materials, the specific surface area is 5 m ² / g or more and 20 m ² / g or less, the ratio of β-type silicon nitride is 70% by mass or more, D50 is 0.5 μm or more and 3 μm or less, and D90 is 3 μm or more and 6 μm. The iron content is 200 ppm or less, the aluminum content is 200 ppm or less, the total content of metal impurities other than iron and aluminum is 200 ppm or less, and the crystallite diameter of β-type silicon nitride is 200 ppm or less. The DC is 60 nm or more, the ratio _DBET / _DC (nm / nm ₎ between the specific surface area equivalent diameter _DBET and the crystallite diameter _DC is 3 or less, and the crystal strain of β-type silicon nitride is 1.5. Silicon nitride powder having a size of × 10 ^-4 or less is used.

Patent Document 3 describes a silicon nitride sintered body by self-sintering. This sintered body has a grain boundary phase containing silicon nitride particles, Mg and at least one rare earth element, and the ratio (RExOy / MgO) when each of Mg and the rare earth element (RE) is converted into an oxide. However, the average value of the major axis length L of the silicon nitride particles existing in the region of 20 × 20 μm arbitrarily set on the processed surface is 5.0 μm or less and the minor axis length is in the range of 0.05 or more and 5 or less. The average value of the ratio (L / S) of the major axis length L to S is 5 or less, and in a region of 300 × 300 μm arbitrarily set on the processed surface, pores having an individual area of 0.01 μm ² or more are formed. The area ratio includes 0.01% or more and 5% or less, the average value of the distance between the centers of gravity of the adjacent pores is 5 μm or more, and the coefficient of variation of the distance between the centers of gravity is 1.5 or less. There is.

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

As a method for producing a sintered body of silicon nitride, a method by reaction sintering is expected from the viewpoint of obtaining high thermal conductivity and fracture toughness. In order to obtain higher thermal conductivity in the manufacturing process by reaction sintering, it is important to reduce the voids in the sintered body and increase the filling rate of silicon nitride. However, there is room for improvement in the conventional technique in terms of increasing the filling rate.

Since the raw material used in Patent Document 1 has a two-particle distribution in which two types of metallic silicon powder are mixed, the particle size difference between the coarse powder and the fine powder is small, or the particle size frequency distribution is extremely bifurcated. In some cases. In such a case, the filling property of silicon is poor, and voids of a certain size or larger are likely to be formed, and the size and distribution of the voids vary, so that the effect of volume expansion due to the nitriding reaction May be large.

Further, in self-sintering as in Patent Documents 2 and 3, glass phases, grain boundaries and voids are likely to be formed, and there are many obstacles to the improvement of thermal conductivity. Generally, as a method of increasing the filling rate of the sintered body, a method of increasing the addition amount of the sintering aid can be considered. However, if the amount of the sintering aid is increased, the glass phase having a low thermal conductivity increases in the sintered body, so that the thermal conductivity of the sintered body may be rather low.

Further, in the case of the manufacturing process by reaction sintering, it is also a problem that the filling rate before the nitriding reaction is too high. Silicon, which constitutes a precursor of reaction sintering, undergoes volume expansion with the nitriding reaction. Therefore, if the filling rate of silicon in the precursor is too high, volume expansion due to the nitriding reaction causes large deformation and excessive stress, resulting in poor appearance such as warpage, cracks, cracks, and poor molding accuracy. And so on.

In particular, when the filling rate of silicon in the precursor is about 82% by volume or more, the volume expansion exceeds the volume of the voids, so that defects / defects are remarkably generated. In Patent Document 1, reaction sintering is performed, but the filling rate and porosity of the precursor are not clear, and such a problem of volume expansion is not fully considered.

Therefore, an object of the present invention is to provide a green sheet for silicon nitride capable of obtaining a sintered body of silicon nitride showing high thermal conductivity with few defects in appearance and molding accuracy, and a method for manufacturing the same. ..

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

Further, the method for producing a green sheet for silicon nitride according to the present invention is a method for producing 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. The ratio of D10 to D90 (D10 / D90) when the particle diameters corresponding to the volume fractions of 10%, 50%, and 90% in the volume-based integrated particle size distribution of silicon are D10, D50, and D90, respectively. ) Is greater than 0 and 0.15 or less, and the ratio of D50 to D90 (D50 / D90) is 0.4 or less. It includes a step of preparing a slurry by mixing in a dispersion medium, a step of applying the slurry on a substrate, and a step of drying the coated slurry, and the filling ratio occupied by the silicon after drying. A green sheet for silicon nitride having a value of 59% by volume or more and 80% by volume or less is obtained.

According to the present invention, it is possible to provide a green sheet for silicon nitride capable of obtaining a sintered body of silicon nitride exhibiting high thermal conductivity with few defects in appearance and molding accuracy, and a method for producing the same.

It is a flow figure which shows the manufacturing method of the green sheet for silicon nitride which concerns on embodiment of this invention. It is a figure which shows the relationship between the particle size of silicon, and the filling rate in a green sheet for silicon nitride.

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

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

The sintered body of silicon nitride is obtained by a method of reaction-sintering 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 ) by a nitriding reaction. A sintered body of silicon nitride is obtained by sintering the particles of silicon nitride with each other.

In the present specification, the green sheet for silicon nitride is a sheet-shaped molded body composed of powdered silicon and a sintering aid, and means a precursor before silicon undergoes a nitriding reaction. The green sheet for silicon nitride is in the form of a sheet having a thickness smaller than the width and length, but the width, length and thickness of the sheet are not particularly limited.

In the green sheet for silicon nitride according to the present embodiment, the filling rate of silicon in the sheet is 59% by volume or more and 80% by volume or less. The rest in 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 by the nitriding reaction, the volume calculated from the unit cell expands by about 1.22 times. When the filling rate of silicon in the sheet is 59% by volume or more and 80% by volume or less, the filling rate of silicon nitride after the nitride reaction can be increased, and the preferable effects described later can be obtained.

The silicon nitride sintered body preferably has a high filling rate of silicon nitride from the viewpoint of obtaining high thermal conductivity. Therefore, it is preferable that the green sheet for silicon nitride, which is a precursor of the sintered body, also has a high silicon filling rate. According to the calculation using the filling model, if the filling rate of the silicon particles is 59% by volume or more, the silicon nitride particles are likely to be bonded in the process of the nitride reaction and the sintering reaction. As a result, it becomes possible to obtain high thermal conductivity.

If the filling rate of silicon in the green sheet for silicon nitride is higher than 80% by volume, the filling rate of silicon nitride after the nitriding reaction becomes close to 100% by volume, so that there is space for volume expansion due to the nitriding reaction. Is insufficient. In such a case, large deformation and stress occur in the sintered body due to volume expansion exceeding volume shrinkage due to sintering and restrictions on particle rearrangement. On the other hand, if a void of about 2% by volume or more is secured, even if the volume expansion due to the nitriding reaction occurs, the volume expansion and rearrangement of the particles are allowed, so that the deformation and stress can be alleviated.

The filling rate of silicon in the green sheet is at least 59% by volume or more, 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 further. It is preferably 75% by volume or more. Further, although it is at least 80% by volume or less, it may be 75% by volume or less, 70% by volume or less, 65% by volume or less, or the like, depending on the use of the sintered body, the required performance, and the like.

Defects / defects that occur in the sintered body include volume expansion due to the nitriding reaction, thermal expansion due to heating during heat treatment, and thermal shrinkage due to cooling after heat treatment. Specific examples of defects / defects include appearance defects such as cracks, cracks, and warpage that is a problem in applications such as printed circuit boards, and improper molding accuracy for molding into near-net shapes and molding to target dimensions. .. These defects / defects can be reduced by lowering the filling rate of silicon and securing voids in the sintered body.

The green sheet for silicon nitride according to the present embodiment has particle diameters corresponding to 10%, 50% and 90% volume fractions under a cumulative sieve in the volume-based integrated particle size distribution of silicon, respectively, at D10 and D50. And D90, the ratio of D10 to D90 (D10 / D90) is larger than 0 and 0.15 or less, and the ratio of D50 to D90 (D50 / D90) is 0.4 or less.

When D50 / D90 is 0.4 or less, small particles (particles of D10 or less) and medium particles (particles of D10 or more and D50 or less) that occupy 50% by volume of the whole silicon powder in the sheet are the whole. Since it is sufficiently smaller than large particles (particles of D90 or more) occupying 10% by volume of the above, a large amount of small particles or medium particles can be filled in the voids between the large particles.

When D10 / D90 is 0.15 or less, small particles (particles of D10 or less) occupying 10% by volume of the whole silicon powder in the sheet are large particles (particles of D10 or less) occupying 10% by volume of the whole. Since it is sufficiently smaller than the particles (D90 or more) and the medium particles (D10 or more and D50 or less) that occupy 40% by volume of the whole, a large amount of small particles are added to the voids between the large particles and the medium particles. Can be filled.

Therefore, when silicon satisfying the above conditions of D10 / D90 and D50 / D90 is used, a sufficient number of medium particles are closely arranged around the large particles, and a sufficient number of small particles are placed around the medium particles. The particles can be arranged in close contact with each other. Further, depending on the conditions of D50 / D90, the size of the void is suppressed, the uniformity of the size and distribution is increased, and the space for volume expansion is dispersedly secured, so that the volume expansion accompanying the nitriding reaction is performed. It becomes difficult to be affected by. Therefore, when silicon satisfying such conditions is used, the bonds between the particles are likely to be uniformly formed by the nitriding reaction or sintering, and the filling rate occupied by the silicon is in the range of 59% by volume or more and 80% by volume or less. It is possible to obtain a green sheet for silicon nitride that is less likely to be deformed or stressed during reaction sintering.

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

The D50 of the silicon powder in the green sheet for silicon nitride is preferably 0.5 μm or more and 20 μm or less. When D50 is less than 0.5 μm, the amount of binder is small with respect to the total specific surface area of the powder, so that the mechanical strength of the sheet is low. Further, when D50 exceeds 20 μm, the unevenness of the surface of the green sheet for silicon nitride becomes large and the nitriding reaction rate becomes slow. On the other hand, when D50 is 0.5 μm or more and 20 μm or less, the nitriding reaction rate can be increased while smoothing the surface of the green sheet for silicon nitride.

The D50 of the silicon powder in the green sheet for silicon nitride is preferably 0.8 μm or more from the viewpoint of obtaining high strength while suppressing the amount of binder. Further, from the viewpoint of smoothing the surface of the green sheet for silicon nitride and increasing the nitriding reaction rate, it is preferably 10 μm or less, more preferably 4 μm or less, still more preferably 3 μm or less. When it is 4 μm or less, sufficient smoothness can be obtained and the nitriding reaction time can be shortened. When 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. When it is 0.8 μm or more and 2 μm or less, a higher filling rate can be obtained in addition to high strength and smoothness.

The particle size and particle size distribution of silicon can be measured by using a laser diffraction / scattering type particle size distribution measuring device. Regarding the particle size and particle size distribution of silicon in the green sheet for silicon nitride, the green sheet for silicon nitride is dissolved in a solvent such as alcohol and then dried to remove the solvent, and the silicon in the obtained powder has a specific gravity. It can be obtained by a method of separating and measuring with a method such as, or a method of collecting data such as the equivalent circle diameter of silicon particles by observing an arbitrary cross section of a green sheet for silicon nitride with an electron microscope.

The porosity of the green sheet for silicon nitride can be determined by observation with a scanning electron microscope (SEM). An arbitrary cross section of the green sheet for silicon nitride is cut out, and the cross section is photographed by SEM to acquire an SEM image. Then, the SEM image is image-processed by binarization processing or binarization processing, and the area ratio per unit area of the void is calculated assuming that the low-density region in the image is a void without silicon or a sintering aid. do. The SEM image is taken at a magnification of 500 times or more and 1000 times or less for a plurality of cross sections and a microscope field of view.

The number of cross sections for which SEM images are taken is preferably 2 or more, and more preferably 5 or more, in that variation due to measurement error is small. The average value of the area ratio of the voids is calculated based on a large number of SEM images, and when the variation of the average value becomes sufficiently small with respect to the increase in the number of images, the average value of the area ratio is calculated for the green sheet for silicon nitride. The porosity. When the amount of the sintering aid or binder in the sheet is small, it can be considered that the filling rate of silicon [%] = 100-porosity [%].

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 element (RE). Examples of the compound containing these elements include oxides, nitrides, silicides and the like. The sintering aid may contain one of these elements, or may contain a plurality of these elements.

Rare earth elements include lanthanoids such as lanthanum (La), cerium (Ce), placeodim (Pr), neodym (Nd), promethium (Pm), samarium (Sm), uropyum (Eu), gadrinium (Gd), and terbium. (Tb), dysprosium (Dy), formium (Ho), elbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) can be mentioned.

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 that 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 the compound containing Mg and Y, magnesium oxide ₍ MgO) and yttrium oxide ₍ Y2O3) 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 preferable. According to the sintering aid containing these elements, sintering can be promoted while suppressing the temperature and pressure, so that the cost of heat treatment can be reduced.

When La, _Y , Gd or Yb and Mg are contained in the composition of the sintering aid, the mixed weight of rare earth elements in terms of oxide is RE _x Oy, and the mixed weight of magnesium in terms of oxide is MgO. , RE _{x Oy} / MgO> 1. Increasing the weight ratio of these rare earth elements to Mg facilitates the formation of fine particles derived from the sintering aid during heat treatment. This is because these rare earth elements are less likely to dissolve in silicon nitride than Mg.

The amount of the sintering aid 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 the sintering aid in terms of oxide. Further, 15% by mass or less is preferable, and 10% by mass or less is more preferable. With such an amount, it is possible to suppress the formation of a glass phase or the like having a low thermal conductivity while promoting the sintering of silicon nitride and the purification of particles.

The thickness of the green sheet for silicon nitride is preferably 0.1 mm or more and 1 mm or less, and more preferably 0.1 mm or more and 0.5 mm or less. With such a thickness, a thin silicon nitride sintered body can be produced. The green sheet for silicon nitride having such a thickness may be obtained by applying pressure after drying during the manufacturing process, or may be obtained by applying no pressure after drying. When the thickness is reduced to 0.5 mm or less by pressurization or the like, higher thermal conductivity can be obtained.

According to the above green sheet for nitriding silicon, since silicon satisfying the above conditions of D10 / D90 and D50 / D90 is used, the filling factor occupied by the silicon is in the range of 59% by volume or more and 80% by volume or less, and the nitriding reaction is carried out. It is possible to obtain a precursor in which the space for the accompanying volume expansion is dispersedly secured. It is possible to secure a high filling rate close to 80% by volume at the stage of the precursor of the sintered body of silicon nitride, and it is easy to form a uniform bond between particles by the nitriding reaction or sintering. , A dense sintered body of silicon nitride can be obtained. Therefore, when used in a manufacturing process by reaction sintering, it is possible to obtain a sintered silicon nitride that exhibits high thermal conductivity with few appearance defects such as warpage, cracks, cracks, and molding accuracy defects.

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

FIG. 1 is a flow chart showing a method for manufacturing a green sheet for silicon nitride according to an embodiment of the present invention.
As shown in FIG. 1, the method for manufacturing a green sheet for silicon nitride according to the present embodiment includes a particle size adjusting step S10, a mixing step S20, a coating step S30, and a drying step S40. Through these steps, the filling rate of silicon is 59% by volume or more and 80% by volume or less, and a green sheet for silicon nitride that is a precursor of a sintered body of silicon nitride can be obtained.

The silicon nitride sintered body can be obtained as a plate-shaped sintered body as a silicon nitride substrate by passing through the heat treatment step S50 using a green sheet for silicon nitride as a precursor. The silicon nitride substrate can be used for applications such as a printed circuit board.

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

As the silicon powder to be mixed with each other, it is preferable to use a plurality of classified powders having different median diameters (D50) and mode diameters. The powdered silicon used as a raw material for the green sheet for silicon nitride preferably has a D50 of 0.5 μm or more and 20 μm or less.

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

In the mixing step S20, powdered silicon and a sintering aid are mixed in a dispersion medium to prepare a slurry. Silicon and the sintering aid whose particle size distribution is controlled are weighed so as to have a predetermined mixing ratio, a binder and a dispersant are added as necessary, and wet mixing is performed in a dispersion medium. According to the wet mixing, it is possible to suppress the mixing of gas components without significantly affecting the particle size distribution, and to obtain the dispersion action of the powder and the dissociation action for the aggregated particles.

As the sintering aid, it is preferable to use a powder having a particle size smaller than that of D50 of silicon from the viewpoint of dispersibility in silicon.

As a binder, acrylic resin such as polyacrylic acid and polyacrylic acid ester, cellulosic resin such as methyl cellulose and ethyl cellulose, polyvinyl alcohol, etc. Appropriate types such as polyvinyl butyral can be used. By using a binder, it is possible to prevent particles from falling off from the green sheet for silicon nitride after molding, and to facilitate the handling of the sheet.

As the dispersion medium, an appropriate type can be used as long as it volatilizes at a low temperature and dissolves a binder added as needed. When a water-soluble binder such as polyacrylic acid is used, lower alcohols such as methanol, ethanol and butanol can be used. Further, as long as it does not excessively react with silicon or a binder, an organic solvent such as hexane, trichlorethylene or dichloromethane, water or the like can be used.

The raw materials should be mixed using an appropriate mixing device such as a V-type mixer, a W-type mixer, a Henchel mixer, a ribbon mixer, a screw mixer, a ball mill, a bead mill, a planetary mixer, a high-pressure homogenizer, an ultrasonic homogenizer, and a roll mill. Can be done.

In the coating step S30, the slurry is applied onto 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 an appropriate thickness depending on the application of the sintered body, the required performance, and the like. However, from the viewpoint of reducing the unevenness of the surface of the sintered body and the variation in the thickness in the surface, the thickness of 0.1 mm or more and 1 mm or less after drying is preferable.

As the base material, an appropriate type such as a resin film, a resin plate, a metal plate, a ceramic plate, and a glass plate can be used as long as the heat resistant temperature is sufficiently high with respect to the vaporization temperature of the dispersion medium. As the base material, a resin film is particularly preferable because it can be wound into a roll as a carrier tape. Specific examples of the resin film include polyethylene terephthalate, polyethylene naphthalate, polycarbonate and the like.

The slurry coating can be performed by 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, and a spray coater. As the coating apparatus, any of a roll type coater, a single-wafer type coater and the like may be used, and an apparatus of a casting method or an extrusion molding method may be used.

In the drying step S40, the slurry coated on the substrate is dried to form a green sheet for silicon nitride. When the base material on which the coating film formed by the slurry is formed is put into a heating / drying furnace or the like and the coating film is dried to remove the dispersion medium, powdered silicon and a sintering aid are added as necessary. A green sheet for silicon nitride that remains on the substrate together with the binder and the powders are bonded to each other can be obtained.

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

After drying, the green sheet for silicon nitride can also be subjected to a pressure treatment in order to adjust the filling rate, porosity or sheet thickness of silicon. By controlling the particle size distribution of silicon, the filling rate of silicon in the sheet can be 59% by volume or more. However, in the case of increasing the filling rate of silicon or adjusting the sheet thickness, the silicon may be consolidated by compression by a pressure treatment. The pressurizing process can be performed by an appropriate pressurizing device such as a shaft press machine or a roll press machine.

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

The heat treatment temperature and the heat treatment time can be set to appropriate conditions according to the average particle size of silicon, the type of sintering aid, the mixing ratio of silicon and the sintering aid, and the like. When a binder is included, a low temperature heat treatment for degreasing can be performed before the heat treatment for reaction sintering by the nitriding reaction.

A preferred heat treatment method is a multi-stage heat treatment including a heat treatment for reaction sintering by a nitriding reaction and a heat treatment for advancing the sintering of silicon nitride particles. The reaction sintering can be performed in a nitrogen gas atmosphere, for example, at 1300 ° C. or higher and 1600 ° C. or lower. Sintering of silicon nitride particles can be performed in a nitrogen gas atmosphere, for example, at 1720 ° C. or higher and 2000 ° C. or lower.

The silicon nitride sintered body can increase the filling rate of silicon nitride by these steps. The remainder in the sintered body is mainly composed of a small amount of grain boundary phase containing components derived from the sintering aid and voids. It is preferable that the sintered body of silicon nitride has a high β fraction of silicon nitride.

The silicon nitride substrate can be a final product used in the application of the substrate without performing secondary processing. Applications of silicon nitride substrates include printed circuit boards. In particular, it is suitable as a material for a circuit board of a power module such as a high-frequency transistor or a power semiconductor, or a multi-chip module. It can also be used as a heat transfer plate for a heat sink, a Pelche element, a Zeebeck element, or the like.

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

The plot of Δ is D10 / D90 of silicon obtained when the pressure treatment is not performed after drying. The plot of ◯ is D50 / D90 of silicon obtained when the pressure treatment is not performed after drying. A1 and A2, B1 and B2, and C1 and C2 are the same numerical values of the green sheet for silicon nitride, respectively. ABCs have different particle size distributions from each other.

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

As shown in FIG. 2, the smaller the particle size side of the silicon D10 / D90 or D50 / D90, the higher the silicon filling factor. As a method of reducing the D10 and D50 of silicon, there is a method of preparing a plurality of powders having different median diameters (D50) and mode diameters and adding smaller powders. In particular, the higher the mixing ratio of the powder having a small D50, the larger the amount of change toward the small particle size side.

As shown in the plots of ▲ and ●, if the silicon is compacted by pressure treatment after drying, binders and small particles can be moved to the voids between the silicon particles, so compared with the case without pressure treatment. Therefore, a high filling rate can be obtained. According to the pressure treatment, the filling rate of silicon in the sheet can be easily adjusted to about 80% by volume.

Next, specific examples of the green sheet for silicon nitride will be described.

(Examples 1 to 3)
Examples 1 to 3 were prepared from a raw material in which silicon and a sintering aid were mixed at a weight ratio of 95: 5. As silicon, a powder having a controlled particle size distribution shown in Table ₁ below was used, Y2O3 was used as a sintering aid, polyacrylic acid was used as a binder _, and butanol was used as a dispersion medium. The thickness of the green sheet for silicon nitride was 0.5 mm. Examples 1 to 3 correspond to C to A shown in FIG. 2, respectively.

(Examples 4 to 6)
Examples 4 to 6 were made from a raw material in which silicon and a sintering aid were mixed at a weight ratio of 95: 5. The silicon is a powder with a controlled particle size distribution shown in Table 1 below, the sintering aid is a mixture of Y2O3 and MgO at a weight ratio of ₂ : ₃ , the binder is polyacrylic acid, and the dispersion medium. Butanol was used. The thickness of the green sheet for silicon nitride was 0.5 mm. Examples 4 to 6 correspond to C to A shown in FIG. 2, respectively.

(Examples 7 to 8)
In Examples 7 to 8, a method of forming a sheet-shaped coating film using a raw material in which silicon and a sintering aid are mixed at a weight ratio of 95: 5 and then applying a pressure treatment using a roll press machine is performed. Made. As silicon, a powder with a controlled particle size distribution shown in Table 1 below, as a sintering aid, only _Y2O3 , or a mixture of _Y2O3 and MgO in a weight ratio of ₂ : ₃ , a binder. Polyacrylic acid was used as the dispersion medium, and butanol was used as the dispersion medium. The thickness of the green sheet for silicon nitride was 0.5 mm. Examples 7 to 8 correspond to D shown in FIG. 2, respectively.

(Comparative Example 1)
Comparative Example 1 was prepared from a raw material in which the particle size distribution of silicon of Example 1 was shifted to the large particle size side. The conditions other than the particle size of silicon are the same as in Example 1.

(Comparative Example 2)
Comparative Example 2 was prepared from a raw material in which the particle size distribution of silicon of Example 4 was shifted to the large particle size side. The conditions other than the particle size of silicon are the same as in Example 4.

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

As shown in Tables 1 and 2, in any of the systems of Examples 1 to 3 and Examples 4 to 6 in which the compositions of the sintering aids are different, the smaller D10 / D90 or D50 / D90 is, the more silicon is filled. The rate is high. Since this result is the result of converting silicon particles into spheres, the relative magnitude relationship of the particle size is important, and the above-mentioned D10 / D90 and D50 / depending on the silicon particle size range suitable as a raw material. It can be said that it is preferable to satisfy the condition of D90.

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 can be made without departing from the spirit of the present invention. For example, the present invention is not necessarily limited to those having all the configurations included in the above embodiment. Replacing part of the configuration of one embodiment with another, adding part of the configuration of one embodiment to another, or omitting part of the configuration of one embodiment. Can be done.

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

Claims (6)

A green sheet for silicon nitride used as a precursor of a sintered body of silicon nitride and containing silicon and a sintering aid.
The ratio of D10 to D90 (D10 /) when the particle diameters corresponding to the volume fractions of 10%, 50%, and 90% in the volume-based integrated particle size distribution of silicon are D10, D50, and D90, respectively. D90) is greater than 0 and 0.15 or less, and the ratio of D50 to D90 (D50 / D90) is 0.4 or less.
A green sheet for silicon nitride having a filling rate of 59% by volume or more and 80% by volume or less. The green sheet for silicon nitride according to claim 1.
The sintering aid is a green sheet for silicon nitride, which is a compound containing one or more elements selected from the group consisting of magnesium, yttrium and rare earth elements. The green sheet for silicon nitride according to claim 1.
A green sheet for silicon nitride in which the amount of the sintering aid is 1% by mass or more and 15% by mass or less with respect to 100% by mass of the total of the silicon and the sintering aid. The green sheet for silicon nitride according to claim 1.
The silicon D50 is a green sheet for silicon nitride having a D50 of 0.5 μm or more and 20 μm or less. The green sheet for silicon nitride according to claim 1.
The silicon D50 is a green sheet for silicon nitride having a D50 of 0.5 μm or more and 4 μm or less. The method for manufacturing a green sheet for silicon nitride according to any one of claims 1 to 5.
The ratio of D10 to D90 (D10 /) when the particle diameters corresponding to the volume fractions of 10%, 50%, and 90% in the volume-based integrated particle size distribution of silicon are D10, D50, and D90, respectively. A step of preparing powdered silicon so that D90) is greater than 0 and 0.15 or less, and the ratio of D50 to D90 (D50 / D90) is 0.4 or less.
The step of mixing the silicon and the sintering aid in a dispersion medium to prepare a slurry, and
The process of applying the slurry on the base material and
Including the step of drying the coated slurry,
A method for producing a green sheet for silicon nitride, which obtains a green sheet having a filling rate of 59% by volume or more and 80% by volume or less after drying.

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