Sintered silicon nitride substrate

Abstract

The present invention pertains to a sintered silicon nitride substrate in which, in a cross-section of the sintered silicon nitride substrate having a silicon nitride phase and a grain boundary phase, the unit equivalent length based on the distance between the interfaces of the silicon nitride phase and the grain boundary phase is 0.1-2 [mu] m. According to the present invention, it is possible to provide a sintered silicon nitride substrate having more excellent insulating properties than in the prior art.

Description

Technical Field

[0001] This invention relates to a silicon nitride sintered substrate. Background Technology

[0002] Silicon nitride sintered substrates have attracted much attention as ceramic materials in various industrial fields due to their excellent properties such as high thermal conductivity, high insulation and high strength. For example, they are used as circuit boards (silicon nitride circuit boards) for power modules in automotive and semiconductor applications.

[0003] For example, Patent Document 1 discloses an invention of a silicon nitride substrate, which is used as a circuit board for mounting high-voltage and high-current power modules and the like, and the silicon nitride substrate used therein, characterized in that it has a surface characteristic with a centerline average roughness (Ra) of 0.2 to 20 μm.

[0004] Patent Document 2 discloses an invention of a silicon nitride sintered body, which serves as a ceramic substrate suitable for forming high-power semiconductor modules. The sintered body has silicon nitride grains, grain boundary phases, and pores, wherein the porosity of the pores with a circular equivalent diameter of 0.5 μm or more is 0.1 to 4%, and it contains a specific pore aggregate.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2002-201076

[0008] Patent Document 2: Japanese Patent Application Publication No. 2014-073937 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] Silicon nitride sintered substrates, which have excellent properties such as high thermal conductivity, high insulation and high strength, have also been used as automotive heat dissipation substrates. However, in recent years, with the increase in the output power of power modules, silicon nitride sintered substrates with higher insulation than before are required.

[0011] Therefore, the objective of this invention is to provide a silicon nitride sintered substrate with superior insulation properties compared to previous methods.

[0012] Methods for solving problems

[0013] To achieve the above objectives, the inventors conducted in-depth research. As a result, the inventors focused on the unit equivalent length of the interfacial distance between the silicon nitride phase and the grain boundary phase in silicon nitride sintered substrates, which had not been previously explored. By limiting this length to a specific range, they were able to provide silicon nitride sintered substrates with excellent insulation properties, thus completing the present invention.

[0014] The main points of this invention are as follows [1] to [3]:

[0015] [1] A silicon nitride sintered substrate, wherein, in the cross section of the silicon nitride sintered substrate having a silicon nitride phase and a grain boundary phase, the unit equivalent length based on the interfacial distance between the silicon nitride phase and the grain boundary phase is 0.1 to 2 μm.

[0016] [2] The silicon nitride sintered substrate described in [1] above has a cross-sectional porosity of less than 1%.

[0017] [3] The silicon nitride sintered substrate described in [1] or [2] above is an insulating substrate.

[0018] Invention Effects

[0019] According to the present invention, a silicon nitride sintered substrate with excellent insulation properties can be provided. Attached Figure Description

[0021] [ Figure 1 This is an example of a reflected electron image obtained by observing the cross-section of a silicon nitride sintered substrate using a scanning electron microscope.

[0022] [ Figure 2 [Illustrated diagram] is a schematic illustration of the method for measuring the unit equivalent length of the present invention. Detailed Implementation

[0023] [Silicon nitride sintered body]

[0024] The silicon nitride sintered substrate of the present invention will now be described in detail.

[0025] <Silicon nitride sintered substrate>

[0026] The silicon nitride sintered substrate of the present invention has a silicon nitride phase and a grain boundary phase, and the unit equivalent length based on the interfacial distance between the silicon nitride phase and the grain boundary phase in the cross section of the silicon nitride sintered substrate is 0.1 to 2 μm.

[0027] If the equivalent length per unit exceeds 2 μm, the insulation strength (insulation withstand capability) of the silicon nitride sintered substrate decreases, resulting in reduced insulation performance. Therefore, from the perspective of improving insulation, the equivalent length per unit should be below 2 μm. Furthermore, the smaller the equivalent length per unit is compared to 2 μm, the higher the insulation strength, indicating a trend towards improved insulation performance. However, to prevent loss of insulation due to the electron tunneling effect, the equivalent length per unit should be above 0.1 μm.

[0028] From the viewpoint of improving the insulation of silicon nitride sintered substrates, the above-mentioned unit equivalent length is preferably 0.1 to 1.5 μm, more preferably 0.1 to 1.2 μm, even more preferably 0.1 to 1.0 μm, and even more preferably 0.1 to 0.5 μm.

[0029] Typically, silicon nitride sintered bodies are obtained by sintering raw silicon nitride powder at high temperatures in the presence of sintering aids. During the sintering process, the individual silicon nitride grains constituting the silicon nitride powder grow and form silicon nitride phases, thereby densifying the material and ultimately forming a silicon nitride sintered body. During this process, grain boundary phases are formed at the interfaces between adjacent silicon nitride phases within the sintered body.

[0030] Figure 1 This paper presents a reflected electron image of a cross-section of a silicon nitride sintered substrate as observed by scanning electron microscopy (SEM).

[0031] exist Figure 1 In the image, the bright white areas observed are grain boundary phases, while the other grayish-gray areas are silicon nitride phases.

[0032] When investigating the relationship between the morphology of the silicon nitride phase and grain boundary phase in a silicon nitride sintered substrate and its physical properties such as insulation, the common practice is to measure the grain size of the silicon nitride phase and the grain boundary phase and study their relationship with physical properties. In this invention, as... Figure 1 As shown, scanning electron microscopy (SEM) revealed that the silicon nitride phase and grain boundary phase appeared to be interconnected and distributed. More specifically, as... Figure 1 As shown, the cross-section of the silicon nitride sintered substrate was observed using SEM reflectance electron microscopy. Continuous silicon nitride phases and discontinuous grain boundary phases were observed, exhibiting an image similar to an island structure. This image is likely observed because the grain boundary phases are not easily observed, making it difficult to accurately determine their grain size.

[0033] Therefore, it is deemed inappropriate to assess the relationship between particle size and insulation solely based on particle size. Consequently, the relationship was explored using the unit equivalent length of the interfacial distance between the silicon nitride phase and the grain boundary phase. The results show that, as described above, insulation can be improved by adjusting the aforementioned unit equivalent length to a relatively small range of 0.1–2 μm.

[0034] In this invention, the aforementioned unit equivalent length is based on the interfacial distance between the silicon nitride phase and the grain boundary phase. A smaller unit equivalent length means a smaller interfacial distance between the silicon nitride phase and the grain boundary phase.

[0035] That is, by reducing the unit equivalent length of the interfacial distance between the silicon nitride phase and the grain boundary phase to a certain extent, the insulation strength can be improved, thereby enhancing the insulation performance.

[0036] Typically, when a voltage is applied to a mixture of materials with different dielectric constants (ε), the electric field strength of the phase with the lower dielectric constant increases, causing insulation breakdown to propagate in a chain from the lower dielectric constant phase to the higher dielectric constant phase, ultimately leading to overall insulation breakdown. In silicon nitride sintered substrates, the dielectric constants of the silicon nitride phase and the grain boundary phase are rarely identical; in most cases, their dielectric constants differ, resulting in a state where high and low dielectric constant phases coexist. Furthermore, as described below, it is known that the insulating strength of solids generally increases with decreasing thickness.

[0037] Typically, the insulation breakdown voltage (V) of a solid is known. B The relationship between the solid thickness d and the solid thickness d is shown in equation (1) (see Kunihiko Hidaka, "New Electrical Systems Engineering: High Voltage Optics", JURSU, p. 103).

[0038]

[0039] Here, in order to measure the insulation breakdown voltage (V) B ) converted to insulation strength (V) B / d), divide both sides of equation (1) by d to obtain the following equation (2).

[0040]

[0041] Equation (2) can be rewritten as Equation (3) as shown below.

[0042]

[0043] Equation (3) shows that the smaller the solid thickness d, the lower the insulation strength (V). B / d) is higher.

[0044] If the thickness of the silicon nitride phase and the grain boundary phase mentioned above is regarded as the solid thickness d in equation (3), then theoretically it can be understood that by making the silicon nitride phase and the grain boundary phase thinner, the insulation strength will be improved.

[0045] It should be noted that the insights discovered in this invention are that in silicon nitride sintered substrates in which the silicon nitride phase and grain boundary phase are dispersed at the micrometer scale, a relationship similar to that of the commonly known relationship between insulation strength and solid thickness shown by equations (1) to (3) can also be observed.

[0046] Next, the method for measuring the unit equivalent length in this invention will be described.

[0047] The unit equivalent length of the silicon nitride phase and grain boundary phase is equivalent to the alternation period of the silicon nitride phase and grain boundary phase, that is, the distance between the interfaces between the silicon nitride phase and the grain boundary phase.

[0048] The equivalent length of the above unit is obtained as follows: using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), at any n (n≥9) measurement sites inside the cross section of the silicon nitride sintered substrate at a distance of more than 1 μm from the surface, the concentration data of the grain boundary phase-specific elements in the analysis area are obtained, and the standard deviation of the obtained n concentration data is calculated.

[0049] The aforementioned grain boundary phase-specific elements refer to elements detected in the grain boundary phase but not in the silicon nitride phase during SEM-EDX analysis, and are typically derived from sintering aids. For example, when using yttrium oxide (Y₂O₃) as a sintering aid, yttrium (Y) can be used as a grain boundary phase-specific element to obtain its concentration data.

[0050] Next, the magnification of each measurement site is gradually changed, and the concentration data within the analysis area is obtained at each magnification using the method described above. While gradually changing the magnification, the analysis area is fixed using the device's analysis area designation function (it should be noted that although the analysis area is fixed for different devices, the actual size of the analysis area will naturally decrease as the magnification increases).

[0051] Then, the standard deviation of the obtained concentration data (n) is calculated at each magnification, and the square root of the actual size area of ​​the analytical region at the magnification with the largest calculated standard deviation is taken as the "unit equivalent length".

[0052] It should be noted that there is no particular limitation on the range of magnification changes; for example, it can be set to 5,000 to 1.5 million times. Moreover, the more magnifications used, the higher the measurement accuracy, and the more ideal it is. It is best to use at least 15, preferably more than 18, different magnifications to determine the above-mentioned standard deviation.

[0053] A standard deviation closer to zero indicates a more uniform distribution of the grain boundary phase (or silicon nitride phase) within the analytical region. Conversely, a larger standard deviation indicates a more uneven distribution of the grain boundary phase (or silicon nitride phase) within the analytical region. Specifically, at the maximum standard deviation, we find analytical regions with extremely high concentrations of elements characteristic of grain boundary phases (i.e., the analytical region is almost entirely composed of grain boundary phase) and analytical regions with extremely low concentrations of elements characteristic of grain boundary phases (i.e., the analytical region is almost entirely composed of silicon nitride phase). In other words, the actual area of ​​the analytical region at this point is similar to the size of the grain boundary phase (or silicon nitride phase).

[0054] That is, at the magnification with the largest standard deviation, the actual area of ​​the analytical region is equivalent to the size of the grain boundary phase (or silicon nitride phase), and the square root of the actual area is equivalent to the "unit equivalent length" based on the interfacial distance between the silicon nitride phase and the grain boundary phase.

[0055] In this invention, the unit equivalent length can be adjusted by modifying the average particle size of the silicon nitride particles used in manufacturing the silicon nitride sintered substrate, the amount of sintering aid, and the sintering time. The manufacturing method of the silicon nitride sintered substrate will be described in detail later.

[0056] <Porosity>

[0057] The porosity of the cross-section of the silicon nitride sintered substrate of the present invention is not particularly limited, but is preferably 1% or less. When the porosity is 1% or less, the insulation of the silicon nitride sintered substrate is easily improved.

[0058] The aforementioned pore area ratio is preferably 0.8% or less, more preferably 0.6% or less, and even more preferably 0.5% or less. The smaller the pore area ratio, the better; ideally, it should be 0%.

[0059] Pore ​​area ratio can be determined by observing the cross-section of a silicon nitride sintered substrate using a scanning electron microscope (SEM) and performing image analysis. Specifically, pore area ratio refers to the ratio of the total pore area to the total area of ​​the region observed by SEM.

[0060] The porosity can be adjusted by modifying the average particle size of silicon nitride particles, the amount of sintering aid, and the sintering time used in manufacturing silicon nitride sintered substrates.

[0061] Insulation strength (insulation endurance)

[0062] As described above, the silicon nitride sintered substrate of the present invention has a specific range of unit equivalent lengths. Furthermore, the silicon nitride sintered substrate of the present invention preferably has the aforementioned porosity. Such a silicon nitride sintered substrate exhibits high insulation strength and excellent insulation properties.

[0063] The insulating strength of the silicon nitride sintered substrate of the present invention is preferably 35 kV / mm or higher, more preferably 40 kV / mm or higher, and even more preferably 45 kV / mm or higher. Higher insulating strength is better, but practically it is 100 kV / mm or lower.

[0064] It should be noted that the insulation strength of the silicon nitride sintered substrate can be measured using the method described in the examples.

[0065] <Thickness>

[0066] The thickness of the silicon nitride sintered substrate of the present invention is not particularly limited and can be appropriately set according to the application, for example, 0.1 to 2.0 mm, preferably 0.1 to 1.0 mm.

[0067] [Manufacturing method of silicon nitride sintered body]

[0068] The silicon nitride sintered substrate of the present invention can be obtained by molding silicon nitride powder and then sintering it. Specifically, it can be obtained by molding a mixture of raw materials containing silicon nitride powder and sintering aids and then sintering it.

[0069] (Silicon nitride powder)

[0070] The average particle size of the silicon nitride powder contained in the mixed raw materials is not particularly limited, but from the viewpoint of keeping the unit equivalent length of the silicon nitride sintered substrate within the above-mentioned desired range, it is preferably 0.1 to 1.5 μm, more preferably 0.1 to 1 μm, and even more preferably 0.2 to 0.6 μm.

[0071] In this way, if silicon nitride powder with a small average particle size is used as raw material, the size of the silicon nitride phase in the silicon nitride sintered body can be easily reduced, thereby making it easier to adjust the unit equivalent length to the above-mentioned desired range.

[0072] The average particle size of silicon nitride powder can be measured by a laser diffraction and scattering particle size distribution measuring device to obtain a volume frequency distribution curve with particle size (μm) as the horizontal axis and volume frequency as the vertical axis. The particle size (D50) corresponding to the cumulative percentage of the measured volume reference particle size distribution cumulative curve reaching 50% is defined as the average particle size.

[0073] Silicon nitride powder can be manufactured by known methods. Examples of methods for manufacturing silicon nitride powder include: reduction nitriding, which uses silicon dioxide powder as a raw material and introduces nitrogen gas in the presence of carbon powder to generate silicon nitride; direct nitriding, which involves reacting silicon powder and nitrogen at high temperatures; and imide decomposition, which involves reacting silicon halide with ammonia. Furthermore, silicon nitride powder can also be manufactured using combustion synthesis (a direct nitriding method utilizing auto-ignition). Combustion synthesis allows for the stable production of silicon nitride powder without incurring energy costs.

[0074] Combustion synthesis is a method of synthesizing silicon nitride by forcibly igniting a portion of the silicon powder under a nitrogen atmosphere, thereby achieving a self-heating reaction of the raw material compound. Combustion synthesis is a known method; for example, see Japanese Patent Application Publication No. 2000-264608 and International Publication No. 2019 / 167879.

[0075] (Sintering aid)

[0076] Metal oxides are preferred as sintering aids. Using metal oxides as sintering aids facilitates the sintering of silicon nitride powder, resulting in a denser, higher-strength sintered body. Examples of metal oxides include yttrium oxide (Y₂O₃), cerium oxide (CeO), and magnesium oxide (MgO). Yttrium oxide is preferred. One type of metal oxide can be used alone, or two or more can be used in combination.

[0077] As a sintering aid, it is preferable to use an oxygen-free compound in conjunction with the aforementioned metal oxides. By using an oxygen-free compound as a sintering aid, dissolved oxygen originating from the sintering aid can be reduced. As a result, a silicon nitride sintered body with high thermal conductivity can be obtained.

[0078] Examples of oxygen-free compounds include carbonitride compounds containing rare earth elements and carbonitride compounds containing magnesium. Among oxygen-free compounds, carbonitride compounds containing magnesium are preferred, and compounds containing both rare earth elements and magnesium are more preferred.

[0079] Examples of rare earth element-containing carbonitride compounds include Y₂Si₄N₆C, Yb₂Si₄N₆C, and Ce₂Si₄N₆C. Among these, Y₂Si₄N₆C and Yb₂Si₄N₆C are preferred, and Y₂Si₄N₆C is more preferred, from the viewpoint of easily obtaining silicon nitride sintered bodies with high thermal conductivity.

[0080] Examples of magnesium-containing carbonitride compounds include MgSi4N6C.

[0081] When using an oxygen-free compound as a sintering aid, the content of the oxygen-free compound in the mixed raw materials is preferably 1 to 10 parts by mass, more preferably 1 to 5 parts by mass, relative to 100 parts by mass of silicon nitride powder.

[0082] By adjusting the content of the oxygen-free compound used as a sintering aid to the above range, the concentration of dissolved oxygen can be easily reduced while promoting sintering.

[0083] In the mixed raw materials, the content of sintering aids (the total amount of all sintering aids used) is preferably 1 to 17 parts by mass relative to 100 parts by mass of silicon nitride powder, more preferably 2 to 10 parts by mass, and even more preferably 2 to 7 parts by mass.

[0084] By adjusting the content of sintering aid to the above range, i.e., to a relatively small amount, the size of the grain boundary phase in the silicon nitride sintered body can be reduced, and the grain growth of the silicon nitride phase can be suppressed, thereby reducing the size of the silicon nitride phase. As a result, it is easy to adjust the unit equivalent length to the above-mentioned desired range.

[0085] (Sintering conditions)

[0086] The silicon nitride sintered substrate of the present invention can be obtained by sintering a molded body containing a mixed powder comprising the aforementioned silicon nitride powder and sintering aid. The mixed powder may contain other components besides the aforementioned silicon nitride powder and sintering aid. Examples of other components include: binders, dispersants, plasticizers, defoamers, etc. A molded body of a desired shape is obtained from the aforementioned mixed powder. The molding method for obtaining the molded body is not particularly limited, but a doctor blade coating method for sheet forming of a slurry containing the aforementioned mixed powder comprising a dispersing medium is preferred. When the molded body contains organic components such as a dispersing medium and a binder, drying to remove the dispersing medium and / or degreasing to remove the binder, etc., can be performed before firing to facilitate firing.

[0087] The firing temperature can be, for example, between 1500 and 1900°C. The maximum firing temperature is preferably between 1700 and 1800°C, more preferably between 1750 and 1800°C. Furthermore, from the viewpoint of easily adjusting the unit equivalent length in this invention to the above range, the holding time at the maximum temperature is, for example, 0.5 hours or more, less than 3 hours, and more preferably 0.5 to 2 hours. Thus, by setting a shorter holding time at the maximum temperature, grain growth can be suppressed, the size of the silicon nitride phase can be reduced, and the unit equivalent length can be easily adjusted to the above range.

[0088] Then, as described above, the temperature is maintained at the highest temperature for a certain period of time before cooling. The cooling rate from the highest temperature to 1000°C is preferably 15–100°C / min, more preferably 20–80°C / min. By setting a faster cooling rate, grain growth can be suppressed, the size of the silicon nitride phase can be reduced, and the unit equivalent length can be easily adjusted to the aforementioned range.

[0089] It should be noted that when cooling the temperature from 1000°C to near room temperature (e.g., 25°C), the effect on the unit equivalent length is not significant, so the cooling conditions can be set appropriately.

[0090] The firing process is preferably carried out in a non-reactive gas atmosphere. Examples of non-reactive gas atmospheres include nitrogen and argon, with nitrogen being the preferred atmosphere.

[0091] As described above, because the unit equivalent length of the silicon nitride sintered substrate of the present invention is within a specific range, it has high insulation strength and excellent insulation properties. Therefore, it is very suitable for use as a variety of insulating substrates.

[0092] Example

[0093] Hereinafter, embodiments will be shown to illustrate the present invention in more detail, but the present invention is not limited to these embodiments.

[0094] [Measurement Method]

[0095] The various physical properties in the examples and comparative examples were measured by the following methods.

[0096] <Equivalent Length per Unit>

[0097] The unit equivalent length is measured in the following ways (1) to (3).

[0098] (1) Measurement 1

[0099] Using Hitachi High-Tech SEM-EDX equipment "SU9000", the concentration data of yttrium, a grain boundary phase-specific element, in the analytical region (9.5 μm long × 12.7 μm wide) A at the magnification of 15 kV and 10,000x was acquired at nine randomly selected measurement locations on a cross-section (more than 1 μm from the surface) of a silicon nitride sintered substrate. The standard deviation A of the concentration data at the above nine measurement locations was then calculated. Note that the sample used was one whose cross-section was mechanically polished to a mirror finish.

[0100] Specifically, such as Figure 2 As shown, yttrium concentration data at nine adjacent measurement locations are obtained, and the standard deviation A of the yttrium concentration is calculated. It is noted that in the embodiment, as... Figure 2 The figure shows nine adjacent measurement locations, but the measurement locations can also be non-adjacent, or more than nine measurement locations can be randomly selected.

[0101] (2) Measurement 2

[0102] Next, a portion of the region observed in 1 will be measured (e.g., Figure 1 The central part of the observation area of ​​measurement 1 (represented by the dashed line) was observed at a magnification of 20,000. As in (1) above, the concentration data of yttrium, an intrinsic element of the grain boundary phase, was obtained in the same analytical area (4.8 μm long and 6.4 μm wide) B at 9 measurement positions. The standard deviation B of the concentration data at the above 9 measurement positions was calculated. Specifically, as Figure 2 As shown, yttrium concentration data were acquired at nine adjacent measurement locations, and the standard deviation B of the yttrium concentration was calculated. It is noted that in the embodiment, as... Figure 2 The figure shows nine adjacent measurement locations, but the measurement locations can also be non-adjacent, or more than nine measurement locations can be randomly selected.

[0103] (3) Measure 3-19

[0104] Similar to measurements 1-2, the observation magnification was gradually increased. At each magnification, the concentration data of yttrium, a grain boundary phase-specific element, was obtained in the same set analysis area C-S (actual size values ​​are shown in Table 1). The standard deviation of the concentration data at the above 9 measurement locations was calculated.

[0105] Among the obtained standard deviations A to S, the square root of the actual size area of ​​the analysis region at the maximum observation magnification is taken as the unit equivalent length.

[0106] Perform the steps (1) to (3) above on 5 samples, and take the average value of the unit equivalent length of the 5 samples as the unit equivalent length of the present invention.

[0107] [Table 1]

[0108]

[0109] <Porosity>

[0110] Using the JCM-7000 SEM-EDX apparatus manufactured by Nippon Electronics Corporation, silicon nitride sintered substrates manufactured for each embodiment and comparative example were prepared at an accelerating voltage of 15 kV and a magnification of 1000x. Five samples of each type were prepared, and three fields of view were observed in the cross-section of each sample, resulting in a total of 15 SEM images. The image analysis software "ImageJ" was used to analyze the SEM images of the 15 fields of view, determine the porosity of each, and calculate the average value of the 15 fields of view.

[0111] Insulation strength (insulation endurance)

[0112] After measuring the thickness of the silicon nitride sintered substrates manufactured in each embodiment and comparative example, the insulation breakdown voltage was measured using an AC endurance testing device "7473" manufactured by KYC TechKU. The insulation strength was then calculated by dividing the insulation breakdown voltage by the thickness of the silicon nitride sintered substrate.

[0113] The following materials are used as raw materials for manufacturing silicon nitride sintered substrates.

[0114] <Silicon Nitride Powder>

[0115] • Silicon nitride powder 1: Silicon nitride powder with an average particle size of 0.48 μm obtained by combustion synthesis.

[0116] • Silicon nitride powder 2: Silicon nitride powder with an average particle size of 1.54 μm obtained by combustion synthesis.

[0117] Sintering aids

[0118] • Metal oxide: Yttrium oxide (Y₂O₃)

[0119] • Carbonitride compounds containing rare earth elements: Y2Si4N6C

[0120] The following reaction is used for heating synthesis to produce it.

[0121]

[0122] Carbonitride compounds containing magnesium: MgSi4N6C

[0123] The following reaction is used for heating synthesis to produce it.

[0124]

[0125] <Example 1>

[0126] To 100 parts by weight of silicon nitride powder with an average particle size of 0.48 μm, 2 parts by weight of Y₂O₃, 1 part by weight of Y₂Si₄N₆C, and 2 parts by weight of MgSi₄N₆C as sintering aids were added, and the mixture was then mixed using a planetary ball mill to obtain a mixed raw material. Next, using this mixed raw material, sheet forming was performed by a scraper method, and the resulting molded body was introduced into a sintering furnace and sintered under a nitrogen atmosphere at atmospheric pressure. Firing was carried out at a maximum temperature of 1760°C for 1 hour. Afterward, the temperature was cooled from the maximum temperature to 1000°C at a cooling rate of 50°C / min. Then, it was naturally cooled from 1000°C to room temperature to obtain a silicon nitride sintered substrate.

[0127] The evaluation results of the fabricated silicon nitride sintered substrate are shown in Table 2.

[0128] <Examples 2-3, Comparative Examples 1-2>

[0129] The composition of the mixed raw materials and sintering conditions were changed as shown in Table 2. Otherwise, the same procedure as in Example 1 was followed to fabricate a silicon nitride sintered substrate. The evaluation results of the fabricated silicon nitride sintered substrate are shown in Table 2.

[0130] [Table 2]

[0131]

[0132] As shown in Table 2, the unit equivalent length of the silicon nitride sintered substrate in each embodiment, based on the interfacial distance between the silicon nitride phase and the grain boundary phase, is in the range of 0.1 to 2 μm, exhibiting high insulation strength and excellent insulation properties.

[0133] In comparison, it can be seen that the unit equivalent length of the silicon nitride sintered substrates in each comparative example is not in the range of 0.1 to 2 μm, and the insulation strength is lower and the insulation performance is worse compared with the silicon nitride sintered substrates of the examples.

Claims

1. A silicon nitride sintered substrate, wherein, In the cross-section of a silicon nitride sintered substrate having a silicon nitride phase and a grain boundary phase, the unit equivalent length based on the interfacial distance between the silicon nitride phase and the grain boundary phase is 0.1 to 2 μm.

2. The silicon nitride sintered substrate according to claim 1, wherein the porosity of its cross-section is less than 1%.

3. The silicon nitride sintered substrate according to claim 1 or 2 is an insulating substrate.

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