Silicon nitride substrate

Abstract

This silicon nitride substrate is made such that the ratio Oe / Oc of the solute oxygen content Oe of an end part and the solute oxygen content Oc of a center part is 0.95–1.05 and the ratio Te / Tc of the thermal conductivity Te of the end part and the thermal conductivity Tc of the center part is 0.95–1.05.　The present invention makes it possible to provide a highly uniform silicon nitride substrate that makes it possible to obtain individual pieces that have high stability of quality with respect to thermal cycle resistance.

Description

Silicon nitride substrate

[0001] The present invention relates to silicon nitride substrates.

[0002] Because silicon nitride sintered bodies have excellent mechanical strength, thermal conductivity, and electrical insulation, they are used as circuit substrates by bonding a metal circuit plate to the surface. For example, Patent Document 1 discloses a silicon nitride substrate containing silicon nitride and magnesium, in which the ratio of the amount of magnesium at the intersection of diagonal lines on the surface to the average amount of magnesium at four points located 3 mm inward from a corner of the surface toward the intersection of the diagonal lines is 0.80 to 1.00.

[0003] WO2020 / 203787

[0004] From the viewpoint of manufacturing efficiency, a method of manufacturing silicon nitride substrates is sometimes used in which a large silicon nitride substrate is manufactured and then cut into multiple pieces. In this case, if there is variation in the quality of the silicon nitride substrate before cutting, there will be variation in the quality among the pieces obtained by cutting. Therefore, in order to obtain pieces with high quality stability, high uniformity of the silicon nitride substrate is required.

[0005] In particular, silicon nitride substrates, taking advantage of their high thermal conductivity, are used in locations where heat dissipation performance is important, so it is important that they do not peel off from metal circuit plates even when subjected to repeated thermal cycles. While studies have been conducted to date to improve the uniformity of the magnesium content of silicon nitride substrates from the perspective of warping, as in Patent Document 1, sufficient studies have not been conducted from the perspective of thermal cycle resistance. The present invention has been made in light of these circumstances, and aims to provide a highly uniform silicon nitride substrate from which individual pieces with high quality stability in thermal cycle resistance can be obtained.

[0006] The present inventors have conducted extensive research to solve the above problems, and as a result have discovered that in order to obtain individual pieces with high quality stability from the viewpoint of thermal cycle resistance, it is important for a silicon nitride substrate to have uniformity in both the amount of dissolved oxygen, which is a factor that affects the state of silicon nitride particles, and the thermal conductivity, which reflects the characteristics of the sintered body structure, and this has led to the completion of the present invention.

[0007] That is, the present invention provides a silicon nitride substrate having a ratio Oe / Oc of the amount of dissolved oxygen in the central portion Oc to the amount of dissolved oxygen in the edge portion Oe of 0.95 to 1.05, and a ratio Te / Tc of the thermal conductivity Tc of the central portion Te to the thermal conductivity Te of the edge portion of 0.95 to 1.05. It is preferable that the amount of dissolved oxygen in the central portion Oc is 0.05% or less, and that the thermal conductivity Tc of the central portion is 90 W / m K or more. Furthermore, it is preferable that the silicon nitride substrate of the present invention has a main surface having a size of 140 mm x 140 mm or more.

[0008] The silicon nitride substrate of the present invention makes it possible to obtain individual pieces with high quality stability from the viewpoint of thermal cycling, which facilitates efficient production of individual pieces with excellent quality stability.

[0009] The silicon nitride substrate of the present invention has a ratio Oe / Oc of the amount of dissolved oxygen Oe at the center to the amount of dissolved oxygen Oe at the edges in the range of 0.95 to 1.05, and a ratio Te / Tc of the thermal conductivity Tc at the center to the thermal conductivity Te at the edges in the range of 0.95 to 1.05, which means that the amount of dissolved oxygen and the thermal conductivity are uniform between the center and the edges of the silicon nitride substrate.

[0010] In the present invention, the "central region" refers to the vicinity of the intersection of two diagonal lines drawn from the four corners of the main surface of the silicon nitride substrate. Specifically, to measure the thermal conductivity Tc of the central region, a test piece cut to a size of 25 mm x 25 mm with the intersection of the diagonal lines as the center is used. The amount of dissolved oxygen Oc in the central region is measured by first cutting the test piece for measuring the thermal conductivity of the central region, and then taking the required amount of sample from around the test piece.

[0011] In the present invention, the term "edge" refers to the vicinity of the four corners of the main surface. Specifically, to measure the thermal conductivity Te of the edge, a test piece cut to a size of 25 mm x 25 mm is used, centered at a point 50 mm diagonally from the corner of the main surface. The dissolved oxygen content Oe of the edge is measured by first obtaining the test piece for measuring the thermal conductivity of the edge, and then collecting the required amount of sample from around the test piece. The dissolved oxygen content Oe of the edge is measured at all four points and determined as the value with the greatest difference from Oc, i.e., the point at which Oe / Oc is the value farthest from 1.00 among the four points. Similarly, the thermal conductivity Te of the edge is measured at all four points and determined as the value with the greatest difference from Tc and the value with Te / Tc the farthest from 1.00.

[0012] The dissolved oxygen content in this invention can be measured by inert gas fusion-infrared absorption spectroscopy using an oxygen / nitrogen analyzer. It refers to the amount of oxygen dissolved in the silicon nitride particle crystals of the substrate. Specifically, the measurement involves crushing a silicon nitride substrate, then acid-treating it to remove the auxiliary phase, sealing 25 mg of the sample in a tin capsule, placing it in a graphite crucible, and heating it at 5.5 kW for 20 seconds. After degassing the adsorbed gas, the sample is heated at 0.8 kW for 10 seconds, and then heated from 0.8 kW to 4 kW over 350 seconds. The amount of carbon dioxide generated during this period is measured and converted into oxygen content to determine the total oxygen content. During the 350-second heating period, the oxygen generated initially corresponds to oxygen derived from oxides present on the surface of the silicon nitride particles (external oxygen), while the oxygen generated later corresponds to dissolved oxygen (internal oxygen) in the silicon nitride crystals. Therefore, a perpendicular line is drawn from the valley of these two measurement peaks, after subtracting a previously measured background, to separate the two peaks. The amount of dissolved oxygen (internal oxygen) and the amount of external oxygen are calculated by proportionally allocating the areas of the respective peaks. Small peaks due to impurities or the measurement environment may be detected in addition to the two peaks, but these small peaks are ignored and the two largest peaks are used to calculate the amount of dissolved oxygen. The valley between the two measurement peaks is the point where the amount of oxygen is lowest between the two measurement peaks.

[0013] The silicon nitride substrate of the present invention has a ratio Te / Tc of the thermal conductivity Tc of the central portion to the thermal conductivity Te of the edge portion in the range of 0.95 to 1.05, which means that the thermal conductivity is uniform between the central portion and the edge portion of the silicon nitride substrate.

[0014] In the present invention, thermal conductivity is measured by the laser flash method. Specifically, the surface of a test piece is blasted, then coated with gold and carbon, and the measurement is performed using a laser flash method thermal constant measurement device. Thermal conductivity is calculated by multiplying the thermal diffusivity, density, and specific heat. The specific heat of a silicon nitride substrate is set to 0.68 (J / g K), and the density is measured using an automatic hydrometer.

[0015] Although it is unclear why a silicon nitride substrate having an Oe / Oc ratio in the range of 0.95 to 1.05 and a Te / Tc ratio in the range of 0.95 to 1.05 results in a silicon nitride substrate with high quality stability and thermal cycle resistance, the inventors believe as follows: The amount of dissolved oxygen affects the state of the silicon nitride particles, and if the amount of dissolved oxygen differs between the individual pieces obtained by cutting the silicon nitride substrate, the state of the silicon nitride particles will differ between the individual pieces, which is presumably to affect the behavior of expansion and contraction during thermal cycles. Since one of the main causes of delamination between metal circuit boards during thermal cycles is stress generated due to differences in expansion and contraction between silicon nitride and metal during thermal cycles, differences in the amount of dissolved oxygen between the individual pieces lead to differences in thermal cycle resistance. Furthermore, if the thermal conductivity differs between the individual pieces obtained by cutting the silicon nitride substrate, the way heat is transferred during a thermal cycle will differ between the individual pieces. As a result, differences will arise in the expansion and contraction behavior of the individual pieces during a thermal cycle, and therefore differences in thermal conductivity between the individual pieces will lead to differences in resistance to thermal cycles.

[0016] Generally, silicon nitride substrates are most likely to exhibit differences in properties between the center and edges due to differences in the sintering environment. Therefore, uniformity between the center and edges can be considered to be uniform throughout. Therefore, individual pieces cut from a silicon nitride substrate having an Oe / Oc within the range of 0.95 to 1.05 and a Te / Tc within the range of 0.95 to 1.05 all have the same amount of dissolved oxygen and thermal conductivity, resulting in equivalent thermal cycle resistance. This is presumably what results in a silicon nitride substrate with high quality stability from the perspective of thermal cycle resistance. The silicon nitride substrate of the present invention preferably has an Oe / Oc within the range of 0.97 to 1.03, more preferably within the range of 0.99 to 1.01. Furthermore, the silicon nitride substrate of the present invention preferably has a Te / Tc within the range of 0.97 to 1.03, more preferably within the range of 0.99 to 1.01.

[0017] The dissolved oxygen content in the silicon nitride substrate of the present invention is preferably 0.055% or less, and more preferably 0.050% or less, in terms of Oc. By ensuring that Oc is within this range, the silicon nitride substrate can have high thermal conductivity, improve heat dissipation at high temperatures, and extend its lifespan during thermal cycling. Similarly, Oe is preferably 0.055% or less, and more preferably 0.050% or less.

[0018] The silicon nitride substrate of the present invention preferably has a Tc of 90 W / m K or more, more preferably 100 W / m K or more. Having a Tc within this range improves heat dissipation at high temperatures, enabling an extended lifespan in thermal cycling. Similarly, Te is preferably 90 W / m K or more, more preferably 100 W / m K or more. There are no particular upper limits for Tc and Te, but they are typically 150 W / m K or less.

[0019] The silicon nitride substrate of the present invention is preferably plate-shaped, and the shape of the main surface is preferably rectangular. The size of the main surface is preferably 140 mm x 140 mm or more. As the size of the main surface increases, the number of individual pieces obtained by cutting it increases, thereby improving manufacturing efficiency. However, the environmental difference between the center and edges during firing generally becomes greater, making it difficult to improve quality stability, and therefore the effects of the present invention are significant. The upper limit of the size of the main surface is not particularly limited, but is generally 200 mm x 200 mm or less. The ratio of the long side to the short side of the main surface is not particularly limited. The thickness of the silicon nitride substrate of the present invention is not particularly limited, but is generally 0.1 mm to 2.0 mm.

[0020] The silicon nitride substrate of the present invention can be obtained, for example, by mixing silicon nitride powder, a sintering aid, and water for 20 hours or more, adding a binder and mixing for 20 hours or more to obtain a raw material slurry, and then molding the slurry to obtain a green body, which can be obtained by firing the green body.

[0021] While the reason why the silicon nitride substrate of the present invention can be obtained by the above-described manufacturing method is unclear, the inventors believe it is as follows. In a typical silicon nitride substrate manufacturing process, the surface of silicon nitride powder gradually oxidizes during processes such as raw material mixing, molding, and firing. Oxidation of the silicon nitride powder surface to form an oxide film can increase dissolved oxygen and disrupt the crystalline structure of the silicon nitride after sintering, resulting in a decrease in thermal conductivity. However, because it is difficult to control the oxidation reaction during the substrate manufacturing process, areas with relatively advanced oxidation and areas with relatively less oxidation are generated, making it difficult to obtain a silicon nitride substrate with extremely high dissolved oxygen content and thermal conductivity uniformity. Here, as described above, by mixing silicon nitride powder, a sintering aid, and water in a ball mill for 20 hours or more, and then adding a binder and mixing in a water-containing state for 20 hours or more, the oxidation of the silicon nitride powder surface is accelerated by the action of water, resulting in the formation of a sufficient oxide film over almost the entire surface of the silicon nitride powder in the green body. In particular, a binder is essential for molding silicon nitride powder, and in the present invention, it is particularly important to add the binder and continue mixing for an additional 20 hours or more. Mixing after adding the binder reduces impact, preventing the silicon nitride powder from being pulverized. If the silicon nitride powder is pulverized, new surfaces will be generated, resulting in variations in the oxidation state of the silicon nitride powder surface. However, by adding a binder and mixing for 20 hours or more while preventing pulverization, the surface of the silicon nitride powder can be sufficiently oxidized, thereby achieving uniform oxidation. Once a sufficient oxide film is formed on the surface of the silicon nitride powder, no further oxidation reaction occurs. Therefore, it is presumed that the progress of the oxidation reaction in subsequent steps will be uniform within the silicon nitride substrate, resulting in a silicon nitride substrate with extremely high uniformity in dissolved oxygen content and thermal conductivity.

[0022] The time for mixing the silicon nitride powder, sintering aid, and water in the ball mill before adding the binder is not particularly limited as long as it is 20 hours or more, but a longer mixing time will reduce production efficiency, so it is preferably 50 hours or less, and particularly 30 hours or less. The mixing time after adding the binder is also not particularly limited as long as it is 20 hours or more, but a longer mixing time will reduce production efficiency, so it is preferably 50 hours or less, and particularly 30 hours or less.

[0023] The rotation speed of the ball mill during ball mill mixing is set to 10 rpm to 50 rpm, preferably 20 rpm to 30 rpm, which allows for sufficient oxidation while preventing the silicon nitride powder from being excessively pulverized, which would reduce the uniformity of the surface oxidation state.

[0024] The silicon nitride powder may be made of α-type silicon nitride, β-type silicon nitride, or a mixture thereof. However, β-type silicon nitride is preferred because β-type powder is less susceptible to grain growth during sintering than α-type powder and is easier to control uniformity. The β-phase ratio of the silicon nitride powder is preferably 80% or more, more preferably 90% or more, and even more preferably 99% or more. The β-phase ratio of the silicon nitride powder refers to the peak intensity ratio of the β-phase to the total of the α-phase and β-phase in the silicon nitride powder [100 × (peak intensity of β-phase) / (peak intensity of α-phase + peak intensity of β-phase)], and is determined by powder X-ray diffraction (XRD) measurement using CuKα radiation. More specifically, see C. P. Gazzara and D. R. Messier: Ceram. Bull. , 56 (1977), 777-780, by calculating the weight ratio of the α phase and the β phase of the silicon nitride powder.

[0025] The particle size of the silicon nitride powder is not particularly limited, but the average particle size D50 is preferably 0.4 to 1.2 μm, and more preferably 0.6 to 1.0 μm. The average particle size D50 can be measured using a laser diffraction / scattering particle size distribution analyzer. The specific surface area of ​​the silicon nitride powder is preferably 12 to 30 m. ２ / g, and 18 to 24m ２In the present invention, the specific surface area refers to a BET specific surface area measured by a BET single-point method using nitrogen gas adsorption.

[0026] The sintering aid may be any known sintering aid without any particular limitation, and examples thereof include oxides such as yttria, magnesia, ceria, and calcia, and compounds not containing oxygen such as carbonitride compounds and nitride compounds. ２ Si ４ N ６ C, Yb ２ Si ４ N ６ C, Ce ２ Si ４ N ６ C, MgSi ４ N ６ C. Examples of nitride compounds include MgSiN ２ These sintering aids may be used alone or in combination of two or more. The amount of the sintering aid to be added is not particularly limited, but is preferably 5 to 20 parts by mass, and more preferably 7 to 10 parts by mass, per 100 parts by mass of the silicon nitride powder.

[0027] The amount of water is not particularly limited, but from the viewpoint of thoroughly mixing and forming a sufficient oxide film on the surface of the silicon nitride powder, it is preferable to use an amount that results in a solids concentration in the slurry (slurry concentration) of 50 to 70 mass %.

[0028] The binder may be any known binder without particular limitation, and examples thereof include polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, acrylic resins, polyacrylamide, urethane resins, polyesters, polyethers, melamine, epoxy resins, cellulose resins, and starch. These binders may be used alone or in combination of two or more. The content of the binder is preferably 1 to 30 parts by mass per 100 parts by mass of the silicon nitride powder.

[0029] The raw material slurry may contain other components in addition to the silicon nitride powder, sintering aid, binder, and water. Examples of other components include dispersants, plasticizers, and antifoaming agents. The other components may be added before adding the binder, when adding the binder, or at any time after adding the binder. For example, a dispersant is used to improve the dispersibility of the silicon nitride powder and sintering aid powder in the slurry, and generally, a surfactant can be suitably used. Any known surfactant can be used without any restrictions. Specific examples of surfactants that can be suitably used in the present invention include carboxylated trioxyethylene tridecyl ether, diglycerol monooleate, diglycerol monostearate, carboxylated heptaoxyethylene tridecyl ether, tetraglycerol monooleate, hexaglycerol monooleate, sorbitan laurate, sorbitan oleate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate. These surfactants may be used alone or in combination. The amount of the dispersant can be selected appropriately, but is typically within the range of 0.1 to 5 parts by weight per 100 parts by weight of the silicon nitride powder and the sintering aid powder. Within this range, the upper limit of the amount of dispersant is preferably 3 parts by weight or less, more preferably 2 parts by weight or less, and even more preferably 1 part by weight or less.

[0030] The raw material slurry can be molded into a desired shape to obtain a green body. A doctor blade method can be used as a molding method for easily obtaining a plate-shaped green body from the raw material slurry.

[0031] Since the green body contains water, it is preferable to dry it to remove the water before firing. The drying conditions are not particularly limited, but the green body can be dried by heating it to about 30°C to 150°C.

[0032] Furthermore, since the green body contains organic components such as binders, dispersants, plasticizers, and antifoaming agents, degreasing to remove the organic components may be performed after drying and before firing to facilitate firing. The degreasing conditions are not particularly limited, but may be performed, for example, by heating the green body to 450 to 650°C in air or in an inert atmosphere such as nitrogen or argon.

[0033] The green body can be fired to obtain a silicon nitride substrate. The firing conditions are not particularly limited, but the firing temperature is preferably 1700°C to 2000°C, and more preferably 1800°C to 1950°C, and the firing time is preferably 1 to 30 hours, and more preferably 3 to 20 hours. The firing pressure may be normal pressure or may be increased pressure.

[0034] The silicon nitride substrate of the present invention can be cut into a plurality of pieces. Because the silicon nitride substrate of the present invention has a high uniformity in dissolved oxygen content and thermal conductivity, when cut into a plurality of pieces, it is easy to obtain pieces with equivalent thermal cycle characteristics, and it can be said that the substrate has excellent quality stability. In other words, by using the silicon nitride substrate of the present invention, pieces with excellent quality stability can be efficiently produced.

[0035] The method for cutting the silicon nitride substrate is not particularly limited, and can be performed by a slicer, a laser scribe, etc. The number of pieces obtained by cutting one silicon nitride substrate is not particularly limited, but it is preferable to cut it into six or more pieces, and more preferably nine or more pieces, in which the pieces are different in size in the vicinity of the center and the vicinity of the edge.

[0036] Examples will be described below to specifically explain the present invention, but the present invention is not limited to these examples. The measurements of the various items in the examples and comparative examples were performed by the following methods.

[0037] (1) Amount of dissolved oxygen in silicon nitride substrate The amount of dissolved oxygen in silicon nitride powder was measured by inert gas fusion-infrared absorption spectrometry. Measurements were performed using an oxygen / nitrogen analyzer (EMGA-920, manufactured by HORIBA). 25 mg of measurement sample was obtained from the silicon nitride substrate obtained in each Example and Comparative Example. The measurement sample was encapsulated in a tin capsule (a tin capsule manufactured by LECO was used), introduced into a graphite crucible, heated at 5.5 kW for 20 seconds, and after degassing of adsorbed gases, heated at 0.8 kW for 10 seconds, and then heated from 0.8 kW to 4 kW over 350 seconds. The amount of carbon dioxide generated during this period was measured and converted into oxygen content. During the 350-second temperature rise, the oxygen generated initially corresponds to oxygen derived from oxides present on the surface of the silicon nitride particles (external oxygen), while the oxygen generated later corresponds to dissolved oxygen (internal oxygen) that is solid-dissolved in the silicon nitride crystals. Therefore, a perpendicular line was drawn from the valley of these two measurement peaks, after subtracting a previously measured background, to separate the two peaks. The amount of dissolved oxygen (internal oxygen) and the amount of external oxygen were calculated by proportionally allocating the areas of the respective peaks.

[0038] (2) Thermal Conductivity of Silicon Nitride Substrates The silicon nitride substrates obtained in each Example and Comparative Example were cut out to obtain test pieces with a main surface of 25 mm x 25 mm. The surfaces of the test pieces were blasted, then coated with gold and carbon, and measurements were performed using a laser flash thermal constant measurement device (TC-9000, manufactured by Advance Riko Co., Ltd.). Thermal conductivity was calculated by multiplying the thermal diffusivity, density, and specific heat. The specific heat of the silicon nitride substrate was set to 0.68 (J / g K), and density was measured using an automatic hydrometer (DMA-220H, manufactured by Shinko Denshi Co., Ltd.).

[0039] (3) Thermal Cycle Resistance The silicon nitride substrate was cut into 40 mm x 10 mm pieces to obtain 20 silicon nitride samples for evaluation. A metallized substrate was prepared by bonding a copper plate to the evaluation sample using the AMB (Active Metal Brazing) method. In the AMB method, a brazing material containing an active metal (Ag-Cu-Ti 75:21:4 (mass ratio)) was screen-printed on both sides of the silicon nitride sample, a 0.3 mm thick copper plate was placed on the sample, and the sample was heated in a vacuum at 850°C for 20 minutes. The sample was then immersed in an aqueous ferric chloride solution to form a pattern. To remove any remaining composite material, the copper plate was eroded in an aqueous acidic ammonium fluoride solution and then washed with water to prepare a metallized sample. The metallized sample was then subjected to a heat cycle test (3000 cycles at -40 to 200°C) and visually evaluated according to the following criteria. ○: No cracks or peeling were found in any of the metallized samples ×: Cracks or peeling were found in some of the metallized samples If the evaluation results in ○, it can be said that all of the individual pieces obtained by cutting the large silicon nitride substrate have stable and high thermal cycle resistance, and that the silicon nitride substrate obtained has high quality stability in thermal cycle resistance and is highly uniform.

[0040] The silicon nitride substrate was manufactured using raw materials containing the following silicon nitride powder and sintering aid. <Silicon nitride powder> - β-phase ratio: 99% - Average particle size D50: 0.9 μm <Sintering aid> - Yttria (manufactured by Shin-Etsu Chemical Co., Ltd.) - Magnesia (manufactured by Ube Materials Co., Ltd.) <Binder and dispersant> - Binder resin: acrylic resin (manufactured by Fujikura Kasei Co., Ltd.) - Dispersant: Cerna D735 (manufactured by Chukyo Yushi Co., Ltd.)

[0041] Example 1: 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, and 0.5 parts by mass of dispersant were weighed out, and mixed with water as a solvent using a resin pot and silicon nitride balls in a ball mill at 25 rpm for 24 hours. Water was pre-weighed so that the slurry concentration was 60% by mass and added to the resin pot. Then, 22 parts by mass of binder was added, and mixing was continued for another 24 hours at 25 rpm. Next, degassing and viscosity adjustment were performed using a vacuum degassing machine (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then formed into a sheet using the doctor blade method, dried in air at 100°C to evaporate the solvent, and a green body (green sheet) with a width of 750 mm and a thickness of 420 μm was obtained. The green sheet obtained as described above was cut into a size of 200 mm x 268 mm for the main surface, and then degreased in dry air at 550°C. The degreased green sheet was then placed in a firing vessel and fired at 1880°C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPa G, yielding a silicon nitride substrate with a main surface of 150 mm x 200 mm. The evaluation results are shown in Table 1.

[0042] Example 2: 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, and 0.5 parts by mass of dispersant were weighed out, and mixed with water as a solvent using a resin pot and silicon nitride balls in a ball mill at 25 rpm for 20 hours. Water was pre-weighed so that the slurry concentration was 60% by mass and added to the resin pot. Then, 22 parts by mass of binder was added, and mixing was continued for another 24 hours at 25 rpm. Next, degassing and viscosity adjustment were performed using a vacuum degassing machine (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then formed into a sheet using the doctor blade method, dried in air at 100°C to evaporate the solvent, and a green body (green sheet) with a width of 750 mm and a thickness of 420 μm was obtained. The green sheet obtained as described above was cut into a size of 200 mm x 268 mm for the main surface, and then degreased in dry air at 550°C. The degreased green sheet was then placed in a firing vessel and fired at 1880°C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPa G, yielding a silicon nitride substrate with a main surface of 150 x 200 mm. The evaluation results are shown in Table 1.

[0043] Comparative Example 1: 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, and 0.5 parts by mass of dispersant were weighed out, and mixed with water as a solvent using a resin pot and silicon nitride balls in a ball mill at 25 rpm for 24 hours. Water was pre-weighed so that the slurry concentration was 60% by mass and added to the resin pot. Then, 22 parts by mass of binder was added, and mixing was continued for another 12 hours at 25 rpm. Next, degassing and viscosity adjustment were performed using a vacuum degasser (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then formed into a sheet using a doctor blade method, dried in air at 100°C to evaporate the solvent, and a green body (green sheet) with a width of 750 mm and a thickness of 420 μm was obtained. The green sheet obtained as described above was cut into a size of 200 mm x 268 mm for the main surface, and then degreased in dry air at 550°C. The degreased green sheet was then placed in a firing vessel and fired at 1880°C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPa G, yielding a silicon nitride substrate with a main surface of 150 mm x 200 mm. The evaluation results are shown in Table 1.

[0044]

Claims

1. A silicon nitride substrate having a ratio Oe / Oc of the amount of dissolved oxygen Oc in the center to the amount of dissolved oxygen Oe in the edges, which is 0.95 to 1.05, and a ratio Te / Tc of the thermal conductivity Tc of the center to the thermal conductivity Te of the edges, which is 0.95 to 1.

05.

2. The silicon nitride substrate according to claim 1, wherein the amount of dissolved oxygen Oc in said central portion is 0.05% or less, and the thermal conductivity Tc of said central portion is 90 W / m·K or more.

3. The silicon nitride substrate according to claim 1, wherein the size of the main surface is 140 mm x 140 mm or more.

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