Silicon nitride substrate

Abstract

To provide a silicon nitride substrate with extremely high uniformity, from which individual pieces with high quality stability in dielectric strength are obtained.SOLUTION: The ratio Be / Bc of the dielectric strength Bc at the center to the dielectric strength Be at the edges of the silicon nitride substrate is 0.90 to 1.10.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Silicon nitride sintered bodies have excellent mechanical strength, thermal conductivity, and electrical insulation properties, and are therefore used as substrates, particularly in applications requiring high voltages and currents, such as power modules (see, for example, Patent Documents 1 and 2).

[0003] From the viewpoint of manufacturing efficiency, silicon nitride substrates are sometimes manufactured by a method in which a large silicon nitride substrate is manufactured and then cut into a plurality of individual 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 individual pieces obtained by cutting. Therefore, in order to obtain individual pieces with high quality stability, high uniformity of the silicon nitride substrate is required.

[0004] When manufacturing large silicon nitride substrates, quality variations are likely to occur depending on the location on the substrate. In response to this, Patent Document 1 investigates ways to improve the uniformity of the magnesium content of silicon nitride substrates from the perspective of warping, but does not focus on the uniformity of dielectric strength.

[0005] Patent Document 2 focuses on the void fraction in the center and the void fraction in the edge. It describes that the breakdown voltage (dielectric strength) can be evaluated by the partial discharge voltage, and that the void fraction and the partial discharge voltage are correlated. However, looking at the data disclosed in the examples, no correlation is observed between the ratio of the void fractions in the center and the edge and the Weibull coefficient of the breakdown voltage. The Weibull coefficient is an index that indicates variation, and the lack of correlation between the void fraction ratio and the Weibull coefficient suggests that the variation in dielectric strength is mainly caused by factors other than voids. Therefore, there is room for further improvement in terms of the variation in dielectric strength. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] WO2020 / 203787 issue [Patent Document 2] Japanese Patent Application Publication No. 2019-059639 Summary of the Invention [Problem to be solved by the invention]

[0007] According to the research of the present inventors, with the recent trend toward higher voltages and currents, it has been found that even variations in dielectric strength to a degree that did not previously cause problems in quality stability can cause dielectric breakdown in some individual pieces. The present invention has been made in consideration of these circumstances, and an object of the present invention is to obtain a silicon nitride substrate with extremely high uniformity, from which individual pieces with high quality stability in dielectric strength can be obtained. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors have conducted extensive research and have found that by controlling the state of the glass phase, it is possible to obtain a silicon nitride substrate having extremely high quality stability in terms of dielectric strength.

[0009] That is, the present invention provides a silicon nitride substrate having a ratio Be / Bc of the dielectric strength Bc of the central portion to the dielectric strength Be of the edge portion of the silicon nitride substrate, of 0.90 to 1.10. The dielectric strength Bc of the central portion is preferably 42 kV / mm or more. The silicon nitride substrate of the present invention preferably has a main surface having a size of 140 mm × 140 mm or more. [Effects of the Invention]

[0010] The silicon nitride substrate of the present invention makes it possible to obtain pieces with high quality stability from the viewpoint of dielectric strength, which facilitates efficient production of pieces with excellent quality stability. DETAILED DESCRIPTION OF THE INVENTION

[0011] The silicon nitride substrate of the present invention has a ratio Be / Bc of the dielectric strength Bc of the center to the dielectric strength Be of the edge, of 0.90 to 1.10. This means that the dielectric strength is uniform between the center and edge of the silicon nitride substrate. The dielectric strength in the present invention is measured by a withstand voltage tester method using a withstand voltage insulation resistance tester.

[0012] In the present invention, the "central portion" 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 dielectric strength Bc of the central portion, a test piece cut to a size of 50 mm x 50 mm and centered on the intersection of the diagonal lines is used.

[0013] In the present invention, the term "edges" refers to the vicinity of the four corners of the main surface. Specifically, to measure the dielectric strength Be at the edges, a test piece cut to a size of 50 mm x 50 mm is used, centered at a point 40 mm diagonally from the corner of the main surface. The dielectric strength Be at the edges is measured at all four points, and the point with the largest difference from Bc is used, i.e., the point at which Be / Bc is the value farthest from 1.00.

[0014] Generally, silicon nitride substrates are most susceptible to differences in properties between the center and edges due to differences in the sintering environment, and therefore uniformity between the center and edges can be said to be uniform throughout. Therefore, individual pieces cut from a silicon nitride substrate having a Be / Bc ratio in the range of 0.90 to 1.10 all have the same dielectric strength, and can be said to be silicon nitride substrates with high quality stability. The silicon nitride substrate of the present invention preferably has a Be / Bc ratio in the range of 0.95 to 1.05, and more preferably in the range of 0.97 to 1.03.

[0015] The dielectric strength Bc of the silicon nitride substrate of the present invention is preferably 42 kV / mm or more, more preferably 45 kV / mm or more. Having Bc in the above range makes it easy to use the silicon nitride substrate in high-voltage applications. Similarly, Be is preferably 42 kV / mm or more, more preferably 45 kV / mm or more.

[0016] In the silicon nitride substrate of the present invention, the ratio Oe / Oc of the amount of dissolved oxygen in the center Oc to the amount of dissolved oxygen in the edge Oe is preferably in the range of 0.95 to 1.05, more preferably in the range of 0.97 to 1.03, and even more preferably in the range of 0.99 to 1.01, which means that the amount of dissolved oxygen is uniform between the center and the edge of the silicon nitride substrate.

[0017] The uniformity of the dissolved oxygen content between the center and edges is believed to indicate that the sintering aid is uniformly distributed and the sintering reaction proceeds uniformly. During sintering of silicon nitride substrates, oxygen is important for promoting the reaction between silicon nitride particles. Therefore, if the Oe / Oc ratio is within the above range, the sintering reaction proceeds uniformly. In silicon nitride substrates, dielectric breakdown primarily originates from impurities or the glass phase. Therefore, a uniformly formed glass phase facilitates the improvement of dielectric strength uniformity. Although it is difficult to analyze the uniformity of the glass phase state, since the glass phase originates from the sintering aid and is formed in conjunction with the sintering reaction of silicon nitride particles, if the silicon nitride particles are uniform, i.e., if the Oe / Oc ratio is within the above range, it means that the sintering reaction proceeds uniformly and the glass phase is formed uniformly. This is believed to facilitate achieving the Be / Bc ratio within the above range.

[0018] The dissolved oxygen content in this invention can be measured by inert gas fusion-infrared absorption spectroscopy using an oxygen / nitrogen analyzer, and refers to the amount of oxygen dissolved in the crystals of silicon nitride particles in the substrate. Specifically, the silicon nitride substrate was crushed and then acid-treated to remove the additive phase. 25 mg of the sample was sealed in a tin capsule and placed in a graphite crucible. The crucible was heated at 5.5 kW for 20 seconds to degas the adsorbed gases, then 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 monoxide generated during this process, converted to carbon dioxide in the oxidizer, was measured, and the total oxygen content was calculated by converting the amount of carbon monoxide into carbon dioxide. During the 350-second heating period, the oxygen generated early 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) dissolved in the silicon nitride crystals. Therefore, a perpendicular line is drawn from the valley of these two measurement peaks, after subtracting the previously measured background, to separate the two peaks. The amounts of dissolved oxygen (internal oxygen) and external oxygen are calculated by proportionally allocating the respective peak areas. Note that, in addition to the two peaks, small peaks derived from impurities or the measurement environment may be detected. However, these small peaks are ignored and the largest two peaks are used to calculate the amount of dissolved oxygen. The valley of the two measurement peaks is the point between the two measurement peaks where the amount of oxygen is lowest.

[0019] The amount of dissolved oxygen in the center, Oc, is measured by first preparing a test piece for measuring the dielectric strength of the center and a test piece for XRF measurement (described later), and then collecting a required amount of sample from around the test piece. The amount of dissolved oxygen in the edge, Oe, is measured by first preparing a test piece for measuring the dielectric strength of the edge and a test piece for XRF measurement (described later), and then collecting a required amount of sample from around the test piece. The amount of dissolved oxygen in the edge, Oe, is measured at all four points, and the value with the largest difference from Oc is used, i.e., the point at which Oe / Oc is the furthest from 1.00 among the four points.

[0020] The amount of dissolved oxygen in the silicon nitride substrate of the present invention, Oc, is preferably 0.055% by mass or less, more preferably 0.050% by mass or less. By keeping Oc within this range, it becomes easier to achieve high thermal conductivity for the silicon nitride substrate. Similarly, Oe is preferably 0.055% by mass or less, more preferably 0.050% by mass or less.

[0021] In the silicon nitride substrate of the present invention, the ratio Ie / Ic of the amount of impurities at the center Ic to the amount of impurities at the edge Ie is preferably in the range of 0.90 to 1.10, more preferably 0.93 to 1.07, which means that the amount of impurities is uniform between the center and the edge of the silicon nitride substrate.

[0022] As mentioned above, impurities present in the glass phase of a silicon nitride substrate can be the starting point of dielectric breakdown, so uniform control of the impurity amount improves the uniformity of dielectric strength and makes it easier to keep Be / Bc within the above range. Note that, in the present invention, impurities refer to metal elements unintentionally contained during the production of a silicon nitride substrate, and metal elements contained in the sintering aid used are not included in the metal impurities. For example, when yttria is used as a sintering aid, the amount of metal elements other than yttrium is the content of the metal impurities in the present invention; when yttria and magnesia are used, the amount of metal elements other than yttrium and magnesium is the content of the metal impurities in the present invention.

[0023] The impurities particularly include iron, and the ratio of the amount of iron in the center to the amount of iron in the edge is preferably within a range of 0.90 to 1.10, and more preferably within a range of 0.93 to 1.07.

[0024] The impurity amount in the present invention can be measured by X-ray fluorescence analysis (XRF). The impurity amount Ic in the center portion is measured by taking a test piece for measuring the dielectric strength of the center portion, and then taking 25 mm x 25 mm test pieces from around the center portion. The impurity amount Ie in the edge portion is measured by taking a test piece for measuring the dielectric strength of the edge portion, and then taking 25 mm x 25 mm test pieces from around the center portion. The impurity amount Ie in the edge portion is measured at all four points, and the value with the largest difference from Ic is used, that is, the point at which Ie / Ic is the value farthest from 1.00 among the four points.

[0025] The amount of impurities in the silicon nitride substrate of the present invention is preferably Ic 0.10% by mass or less, more preferably 0.07% by mass or less. By having Ic in this range, it becomes easy to increase Bc. Similarly, Ie is preferably 0.10% by mass or less, more preferably 0.07% by mass or less.

[0026] The silicon nitride substrate of the present invention preferably has a three-point bending strength of 600 MPa or more and a fracture toughness of 6.0 MPa m 1 / 2 It is preferable that the thermal conductivity is 80 (W / (m·K)) or more.

[0027] The silicon nitride substrate of the present invention is preferably plate-shaped. The size of the main surface is preferably 140 mm × 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 × 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.

[0028] As described above, the silicon nitride substrate of the present invention comprises silicon nitride particles and a glass phase derived from a sintering aid. The glass phase preferably contains yttrium and magnesium.

[0029] The silicon nitride substrate of the present invention can be obtained, for example, by mixing silicon nitride powder having an impurity content of 0.1% by mass or less, a sintering aid having an impurity content of 0.1% by mass or less, 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.

[0030] While the reason why the silicon nitride substrate of the present invention can be obtained by the above-described manufacturing method is unclear, the present inventors believe it to be 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. Since a small amount of oxygen is required for sintering silicon nitride particles, if the oxide film on the surface of the silicon nitride powder is not uniform during the sintering reaction, the sintering reaction may proceed unevenly, with the sintering reaction proceeding relatively quickly in areas with sufficient oxygen and relatively slowly in areas with insufficient oxygen. If the sintering reaction of silicon nitride proceeds unevenly, the reaction of the surrounding sintering aids will also be uneven, resulting in an uneven glass phase state. In silicon nitride substrates, dielectric breakdown generally originates in the glass phase, and therefore an uneven glass phase state can also cause uneven dielectric strength values. On the other hand, 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 for 20 hours or more in a water-containing state, 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 another 20 hours or more. Mixing after adding the binder reduces impact, preventing the silicon nitride powder from being pulverized. Pulverization of silicon nitride powder creates new surfaces, 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 silicon nitride powder surface can be sufficiently oxidized, resulting in uniform oxidation. Once a sufficient oxide film has formed on the surface of the silicon nitride powder, no further oxidation reaction occurs. Therefore, the progress of the oxidation reaction in subsequent steps becomes uniform within the silicon nitride substrate, resulting in a uniform glass phase state, and it is presumed that a silicon nitride substrate with extremely uniform dielectric strength can be obtained.

[0031] In addition, because impurities present in the glass phase can become the starting point for dielectric breakdown, it is important to maintain a uniform impurity content throughout the substrate. However, it is difficult to control the distribution of impurities to prevent localized high concentrations. Therefore, by using silicon nitride powder with an impurity content of 0.1% by mass or less and a sintering aid with an impurity content of 0.1% by mass or less, the absolute amount of impurities can be reduced. Even if localized high impurity concentrations occur, the small absolute amount prevents significant adverse effects on dielectric strength. This prevents localized decreases in dielectric strength, and thus is believed to result in silicon nitride substrates with extremely high uniformity in dielectric strength.

[0032] 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 reduces 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 reduces production efficiency, so it is preferably 50 hours or less, and particularly 30 hours or less.

[0033] The rotation speed of the ball mill during ball mill mixing is 10 rpm to 50 rpm, preferably 20 rpm to 30 rpm, which allows for sufficient oxidation while preventing excessive pulverization of the silicon nitride powder, which would reduce the uniformity of the surface oxidation state.

[0034] The silicon nitride powder may be composed 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 ratio of the peak intensity 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, it can be determined by calculating the weight ratio of the α-phase and β-phase in the silicon nitride powder using the method described in C.P. Gazzara and D.R. Messier: Ceram. Bull., 56 (1977), 777-780.

[0035] The silicon nitride powder has an impurity content of 0.1% by mass or less, and more preferably 0.05% by mass or less. The impurity content can be measured by X-ray fluorescence analysis (XRF). Iron is particularly an example of such impurities. The silicon nitride powder has an iron content of preferably 0.1% by mass or less, and particularly preferably 0.05% by mass or less.

[0036] Silicon nitride powder with an impurity content of 0.1% by mass or less can be obtained by reducing the impurities of known silicon nitride powder through acid washing or magnetic separation. Acid washing can be performed, for example, by treating the powder with a mixed aqueous solution of hydrochloric acid and hydrofluoric acid at 10°C to 80°C for 0.5 to 5 hours. Commercially available 35% hydrochloric acid may be used, or it may be diluted as needed. Commercially available 55% hydrofluoric acid may be used, or it may be diluted as needed. The mixing ratio of hydrochloric acid to hydrofluoric acid can be, for example, 1:5 to 5:1. Magnetic separation can be performed by passing the powder through a strong magnetic field, for example, of 1.0 to 10.0 T, particularly 2.0 to 3.0 T. For example, a method using a permanent magnet to remove magnetic impurities can be used, and this can be performed using a cylindrical container or an apparatus combining a lattice frame and a permanent magnet. It can also be done using an electromagnetic sieving device with a magnetized screen stacked on top of it, as described in WO2021 / 124961.

[0037] 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 2 In the present invention, the specific surface area refers to the BET specific surface area measured by the BET single-point method using nitrogen gas adsorption.

[0038] 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, as well as oxygen-free compounds such as carbonitride compounds and nitride compounds. Examples of carbonitride compounds include Y2Si4N6C, Yb2Si4N6C, Ce2Si4N6C, and MgSi4N6C. Examples of nitride compounds include MgSiN2. These sintering aids may be used alone or in combination of two or more. The amount of the sintering aid 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.

[0039] The sintering aid preferably contains impurities in an amount of 0.1% by mass or less, more preferably 0.05% by mass or less. The amount of impurities in the sintering aid can be measured by XRF.

[0040] The sintering aid may be a commercially available, high-purity product containing 0.1% or less impurities. Alternatively, a sintering aid powder containing more than 0.1% impurities may be acid washed or magnetically separated to reduce the impurities.

[0041] 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 %.

[0042] The binder can be any known binder without any particular limitation, and examples thereof include polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, acrylic resins, polyacrylamide, urethane resins, polyester, polyether, 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.

[0043] 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, at the same time as adding the binder, or at any time after adding the binder.

[0044] For example, the 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. Known surfactants 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 of two or more. The amount of the dispersant can be appropriately selected, and can typically be selected from the range of 0.1 to 5 parts by mass per 100 parts by mass of the silicon nitride powder and the sintering aid powder combined. Within this range, the upper limit of the amount of dispersant is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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 particularly preferably 1800°C to 1950°C, and the firing time is preferably 1 to 30 hours, and particularly preferably 3 to 20 hours. The firing pressure may be normal pressure or may be increased.

[0049] 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 of dielectric strength, when cut into a plurality of pieces, it is easy to obtain pieces with equivalent insulating properties, and it can be said that the quality is stable. In other words, by using the silicon nitride substrate of the present invention, pieces with excellent quality stability can be efficiently produced.

[0050] There are no particular limitations on the method for cutting the silicon nitride substrate, and it can be done by a slicer, laser scribing, 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 central region and the edge regions. [Example]

[0051] 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.

[0052] (1) Dielectric strength of silicon nitride substrate The dielectric strength of the silicon nitride substrate was determined by a voltage resistance tester method using a voltage resistance tester manufactured by Tama Densoku Co., Ltd.

[0053] (2) Amount of dissolved oxygen in silicon nitride substrate The amount of dissolved oxygen in the silicon nitride substrate was measured by inert gas fusion-infrared absorption spectroscopy using an oxygen / nitrogen analyzer (HORIBA: EMGA-920). A 25 mg measurement sample was obtained from the silicon nitride substrate obtained in each Example and Comparative Example. The measurement sample was enclosed in a tin capsule (LECO Tin Cupsule was used for the tin capsule) and introduced into a graphite crucible. It was heated at 5.5 kW for 20 seconds to degas the adsorbed gas, then heated at 0.8 kW for 10 seconds. The temperature was then increased from 0.8 kW to 4 kW over 350 seconds. The amount of carbon monoxide generated during this period was measured and converted to carbon dioxide in the oxidizer. This was then converted to oxygen content. During the 350-second temperature increase, the oxygen generated initially corresponded to oxygen derived from oxides present on the surface of the silicon nitride particles (external oxygen), while the oxygen generated later corresponded to dissolved oxygen (internal oxygen) solid-dissolved in the silicon nitride crystals. Therefore, a perpendicular line was drawn from the valley of these two measurement peaks, after subtracting the 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 respective peak areas.

[0054] (3) Amount of impurities in silicon nitride substrate Test pieces (25 mm x 25 mm) cut from the center or edge of the silicon nitride substrate were subjected to fluorescent X-ray analysis, and the amount of impurities was determined from the results obtained.

[0055] (4) Amount of impurities in silicon nitride powder The silicon nitride powder was subjected to X-ray fluorescence analysis, and the amount of impurities was determined from the results obtained.

[0056] (5) Amount of impurities in sintering aids The sintering aid was subjected to X-ray fluorescence analysis, and the amount of impurities was determined from the results obtained.

[0057] The silicon nitride substrate was manufactured using the following raw materials containing silicon nitride powder and a sintering aid. <Silicon nitride powder> ·Beta conversion rate: 99% ·Average particle size D50: 0.9μm <Sintering aid> Yttria (manufactured by Japan Yttrium, purity 99.99% or higher) Magnesia (manufactured by Tateho Chemical Industries, purity 99.99% or higher) <Binders and dispersants> Binder resin: Acrylic resin (manufactured by Fujikura Kasei Co., Ltd.) Dispersant: Cerna D735 (manufactured by Chukyo Yushi Co., Ltd.)

[0058] Example 1 The silicon nitride powder was subjected to magnetic separation and acid washing until the impurity content was reduced to 0.05% by mass or less. Specifically, the powder was magnetically separated by passing it through a strong magnetic field of 2.0 to 3.0 T, and then treated with a 1:1 aqueous solution of 35% hydrochloric acid and 55% hydrofluoric acid at 60°C for 2 hours. The powder was then filtered, washed with water, vacuum dried at 200°C, and pulverized in a vibrating ball mill for 12 hours. Commercially available yttria and magnesia with a purity of 99.99% or more (impurity content of 0.01% by mass or less) were used. Next, 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 by 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 × 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 chamber and fired at 1880 °C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPaG to obtain a silicon nitride substrate with a main surface of 150 mm × 200 mm. The evaluation results are shown in Table 1.

[0059] <Example 2> The silicon nitride powder was subjected to an impurity reduction treatment in the same manner as in Example 1. Next, 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 in a ball mill using water as a solvent and a resin pot and silicon nitride balls at 25 rpm for 20 hours. The water was pre-weighed so that the slurry concentration was 60% by mass and added to the resin pot. 22 parts by mass of binder was then added, and mixing was continued for another 24 hours at 25 rpm. Next, defoaming and viscosity adjustment were performed using a vacuum defoamer (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then formed into a sheet by 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 × 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 chamber and fired at 1880 °C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPaG to obtain a silicon nitride substrate with a main surface of 150 × 200 mm. The evaluation results are shown in Table 1.

[0060] <Comparative Example 1> The silicon nitride powder was subjected to an impurity reduction treatment in the same manner as in Example 1. Next, 100 parts by mass of silicon 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 in a ball mill using water as a solvent and a resin pot and silicon nitride balls at 25 rpm for 24 hours. The 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, defoaming and viscosity adjustment were performed using a vacuum defoamer (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then formed into a sheet by 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 × 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 chamber and fired at 1880 °C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPaG to obtain a silicon nitride substrate with a main surface of 150 mm × 200 mm. The evaluation results are shown in Table 1.

[0061] <Comparative Example 2> Except for not carrying out the treatment to reduce impurities in the silicon nitride powder, a silicon nitride substrate having a main surface of 150 mm×200 mm was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0062] [Table 1]

Claims

1. A silicon nitride substrate, in which the ratio Be / Bc of the dielectric strength Bc of the center portion to the dielectric strength Be of the edge portion is 0.90 to 1.

10.

2. 2. The silicon nitride substrate in accordance with claim 1, wherein the dielectric strength Bc of said central portion is 42 kV / mm or more.

3. 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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