Silicon nitride substrate and method of producing the same

Abstract

To improve the performance of a silicon nitride substrate, namely, to realize high thermal conductivity and high stress resistance in a silicon nitride substrate for the purpose of, for example, efficiently dissipating heat generated in a silicon nitride substrate which is an insulation substrate whereon a semiconductor device is mounted.SOLUTION: The silicon nitride substrate comprising a plurality of silicon nitride particles and grain boundary phases present between the plurality of silicon nitride particles, in which not less than 90% of the silicon nitride particles having a major axis size of less than 2 μm each have an oxygen content of less than 2.5 at% among the plurality of silicon nitride particles, is used.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride substrate manufactured using silicon powder and a technique that is effective when applied to a manufacturing method thereof. [Background technology]

[0002] A power module is an electronic device that constitutes an inverter circuit that controls a motor installed in an electric vehicle, a hybrid electric vehicle, a railcar, or industrial equipment. The power module includes a semiconductor device having a switching element used in the inverter circuit and an insulating substrate (wiring board) on which the semiconductor device is mounted. Examples of switching elements include a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an IGBT (Insulated Gate Bipolar Transistor).

[0003] Japanese Patent Application Laid-Open No. 2018-184333 (Patent Document 1) describes a technology related to a silicon nitride substrate manufactured using silicon powder. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-184333 Summary of the Invention [Problem to be solved by the invention]

[0005] Heat is generated when the switching elements are operated, that is, in the power module, and therefore it is necessary to prevent malfunctions and breakdowns of the power module caused by heat.

[0006] Therefore, in order to efficiently dissipate the heat generated in the power module, the insulating substrate on which the semiconductor device is mounted is required to have high heat dissipation characteristics. In other words, the insulating substrate is required to have high thermal conductivity. The insulating substrate is also required to be resistant to stress caused by temperature changes. For this reason, there is a need for innovations to realize an insulating substrate that has high thermal conductivity and high stress resistance. [Means for solving the problem]

[0007] In one embodiment, the silicon nitride substrate has a plurality of silicon nitride particles and a grain boundary phase present between the plurality of silicon nitride particles, and of the plurality of silicon nitride particles, 90% or more of the silicon nitride particles having a major axis diameter of less than 2 μm each have an oxygen content of less than 2.5 at%. [Effects of the Invention]

[0008] According to one embodiment, the performance of the silicon nitride substrate can be improved. [Brief explanation of the drawings]

[0009] [Figure 1] 1A and 1B are a perspective view and a schematic diagram of a cross-sectional structure of a silicon nitride substrate according to an embodiment; [Figure 2] 1 is a graph of Weibull distribution probability. [Figure 3] 10 is a graph showing the relationship between cumulative failure probability and strength σ. [Figure 4] 10 is a formula explaining a method for calculating bending strength σ0. [Figure 5] 1 is a graph showing the relationship between the major axis diameter and the amount of oxygen in Example 1. [Figure 6] 10 is a graph showing the relationship between the major axis diameter and the amount of oxygen in Example 2. [Figure 7] 10 is a graph showing the relationship between the major axis diameter and the amount of oxygen in Example 3. [Figure 8] 1 is a graph showing the relationship between the major axis diameter and the oxygen content in Comparative Example 1. [Figure 9]10 is a graph showing the relationship between the major axis diameter and the oxygen content in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0011] Below, we will explain how specifying the amount of oxygen in small-diameter particles among the particles that make up the silicon nitride substrate allows the silicon nitride substrate to have both high thermal conductivity and high mechanical strength, while also reducing manufacturing costs.

[0012] (Embodiment) <Room for improvement> For example, silicon nitride substrates used as insulating substrates for power modules require high thermal conductivity. Since silicon nitride substrates need to dissipate heat from semiconductor elements mounted on one side to the other, thermal conductivity in the thickness direction is particularly important. Defects within silicon nitride particles are cited as a factor that reduces the thermal conductivity of silicon nitride substrates. Defects within silicon nitride particles can be reduced primarily by reducing the amount of oxygen within the particles.

[0013] As described in Patent Document 1, one possible method for increasing the thermal conductivity of silicon nitride substrates is to reduce the oxygen content of the silicon and sintering aids that are the raw materials for the silicon nitride substrate, and to increase the average major axis diameter of the silicon nitride particles, thereby reducing the oxygen content within the particles to 0.05% or less.

[0014] However, attempts to reduce the oxygen content of the silicon and sintering aid raw materials lead to an increase in raw material costs, and attempts to increase the average major axis diameter of the silicon nitride particles result in a decrease in the mechanical strength (e.g., bending strength) of the silicon nitride substrate.

[0015] Furthermore, Patent Document 1 describes that the amount of oxygen in silicon nitride particles in a silicon nitride substrate is reduced by setting the temperature in the degreasing step to a relatively high temperature and using a nitrogen atmosphere. However, setting the temperature in the degreasing step to a relatively high temperature requires costs for a furnace used in the degreasing that can withstand high temperatures, and running costs for introducing nitrogen gas during the degreasing process.

[0016] Thus, there is room for improvement in silicon nitride substrates, in order to suppress increases in manufacturing costs and to achieve both high thermal conductivity and high mechanical strength of silicon nitride substrates.

[0017] In this embodiment, a device is implemented to solve the above-mentioned room for improvement. The technical concept of this embodiment that implements this device will be described below.

[0018] <Structure of silicon nitride substrate> The silicon nitride substrate according to this embodiment is primarily composed of silicon nitride (Si3N4). FIG. 1 shows a perspective view of the silicon nitride substrate according to this embodiment and a schematic diagram of its cross-sectional structure. The silicon nitride substrate 1 shown in FIG. 1 has a first main surface and a second main surface opposite the first main surface, and has a side surface connecting the first and second main surfaces. The schematic diagram of the cross-sectional structure shown in FIG. 1 shows a cross section along the thickness direction of the silicon nitride substrate 1. The thickness direction here is a direction perpendicular to each of the first and second main surfaces of the silicon nitride substrate 1. The vertical direction of the schematic diagram is the thickness direction, and the horizontal direction is a plane direction along each of the first and second main surfaces.

[0019] As shown in FIG. 1, the silicon nitride substrate 1 is composed of a plurality of silicon nitride particles 2 and a grain boundary phase 3 present between the silicon nitride particles 2. In other words, the black areas including the outlines of the silicon nitride particles 2 in the schematic diagram are the grain boundary phase 3. The silicon nitride particles 2 are composed of, for example, silicon nitride (Si3N4). The grain boundary phase 3 contains, for example, a rare earth element oxide and a magnesium compound. Examples of rare earth element oxides include Y2O3, Yb2O3, Gd2O3, Er2O3, and Lu2O3. Examples of magnesium compounds include magnesium oxide (MgO), magnesium silicon nitride (MgSiN2), magnesium silicide (Mg2Si), and magnesium nitride (Mg3N2).

[0020] The schematic diagram is a simulation of an image (field of view) obtained by the inventors by imaging the cross section of a silicon nitride substrate 1 using a scanning electron microscope (SEM). When the inventors observed the entire cross section of the silicon nitride substrate 1, they found that there were no coarse particles with a major axis diameter exceeding 100 μm, and the structure was uniform. Although the structure is uniform, some particles with a major axis diameter of 10 μm or more and 20 μm or less are present. The majority of the particles constituting the structure have a minor axis diameter of about 1 μm and a major axis diameter of 10 μm or less. In this application, the major axis diameter refers to the longest length of each of the multiple particles observed in the image showing the cross section of the silicon nitride substrate. The minor axis diameter refers to the length of each of the multiple particles observed in the image, for example, in a direction perpendicular to the center of the major axis diameter.

[0021] <Method for manufacturing silicon nitride substrate> Silicon nitride substrates are manufactured through the following steps in order: (1) mixing silicon nitride powder with sintering aids (such as MgO), binders, plasticizers, etc. to form a slurry, (2) degassing and thickening the slurry to form a sheet, which is then dried and degreased to obtain a green body, (3) heating the green body (sheet) in a nitrogen atmosphere to nitride the silicon contained in the green body, and (4) sintering the nitrided green body to obtain a sintered body. In other words, silicon nitride substrates are manufactured by performing the mixing, degreasing, nitriding, and sintering steps in that order.

[0022] <Examples and Comparative Examples> The main feature of the silicon nitride substrate of this embodiment is that, among the multiple silicon nitride particles constituting the silicon nitride substrate, 90% or more of the silicon nitride particles having a major axis diameter of less than 2 μm all have an oxygen content of less than 2.5 at%. Examples 1 to 3 that satisfy this condition and Comparative Examples 1 and 2 that do not satisfy this condition are shown in Tables 1 to 5 below.

[0023] [Table 1]

[0024] [Table 2]

[0025] [Table 3]

[0026] The silicon nitride substrates of Examples 1 to 3 and Comparative Examples 1 and 2 were manufactured under the conditions shown in Tables 1 and 2, respectively. Specifically, in the manufacturing process for the silicon nitride substrate, a slurry was formed by mixing silicon (Si) powder, sintering aid powder, binder, plasticizer, and the like. The sintering aid powder consisted of a mixture of an Mg compound and Y2O3. In Examples 2 and 3 and Comparative Example 2, MgSiN2 was used as the Mg compound. In the subsequent degreasing step, a sheet made of the slurry was degreased by heating it in an air atmosphere at 750°C or 550°C. In the subsequent nitriding step, the sheet compact was nitrided in a nitrogen atmosphere at 1400°C. Here, the heating rate in Comparative Examples 1 and 2 was 0.5°C / min, while the heating rate in Examples 1 to 3 was 1.0°C / min. Thus, Examples 1 to 3 have a faster heating rate than Comparative Examples 1 and 2. In Examples 1 to 3, the amount of oxygen taken into the silicon nitride particles is reduced by increasing the temperature rise rate in the nitriding treatment. Next, the compact is sintered by heating it to 1860°C or 1850°C in a nitrogen atmosphere to produce a silicon nitride substrate.

[0027] Among the sintering aids in Table 1, the amounts of Y2O3 and Mg compounds added are shown in mol% relative to the total of silicon (converted to silicon nitride), Y2O3, and Mg compounds (converted to MgO). Among the sintering aids in Table 1, the MgSiN2 ratio is the ratio of MgSiN2 in the Mg compounds (MgO + MgSiN2).

[0028] Table 3 shows the thermal conductivity, bending strength, and bending strength σ0, which are the characteristics of the silicon nitride substrates of each example and comparative example. The silicon nitride substrates of Examples 1 to 3 achieved a high thermal conductivity of 110 W / m K or more and a high bending strength σ0 of 700 MPa or more.

[0029] Here, the silicon nitride substrate (sintered body) obtained by the above sintering was subjected to liquid honing treatment in order to clean it and give it a suitable roughness. Honing treatment is carried out by adding an appropriate amount of alumina abrasive grains to water and spraying it onto the front and back of the sintered body at a pressure of 0.5 MPa.

[0030] Table 3 shows the measured or calculated results of the relative density, bending strength, and thermal conductivity of silicon nitride substrates after cleaning and liquid honing. The relative density was calculated as sintered density / true density, using the sintered density measured by the underwater displacement method (Archimedes method) and the true density of silicon nitride. The true density of each sample was calculated from the true densities and composition ratios of Si3N4, Y2O3, MgO, and MgSiN2. Bending strength was measured using 4 mm wide and 10 mm long test pieces prepared from the silicon nitride substrates and a three-point bending test (span 7 mm, crosshead speed 0.5 mm / min). Thermal conductivity (thickness direction) was measured using the flash method using similarly cut 10 mm square test pieces. The calculation method for bending strength σ0 is described below.

[0031] [Table 4]

[0032] [Table 5]

[0033] Table 4 shows the results of the inventors' observation of the silicon nitride substrates of Examples 1 to 3 and Comparative Examples 1 and 2 by SEM images. The long-axis diameters of the particles visible in the images were defined as r, and the results of analysis by dividing them into three levels were shown. These three levels are: (1) r < 2 μm, (2) 2 μm ≤ r ≤ 4 μm, and (3) 4 μm < r. Here, for each of the silicon nitride substrates of Examples 1 to 3 and Comparative Examples 1 and 2, particles of sizes corresponding to each of these three levels were arbitrarily extracted from three SEM images (fields of view) in the cross section. The number of particles extracted for each of these three levels is in the range of 10 or more and 30 or less.

[0034] Table 5 shows the number of particles with an oxygen content of less than 2.5 at%, the number of particles with a long-axis diameter of less than 2 μm, and the ratio of particles with a long-axis diameter of less than 2 μm and an oxygen content of less than 2.5 at% in each of the silicon nitride substrates of Examples 1 to 3 and Comparative Examples 1 and 2. For example, in Example 1, there were 17 particles with a long-axis diameter of less than 2 μm, and among them, 17 particles had an oxygen content of less than 2.5 at%, so the ratio was 100%. In Example 2 as well, out of 15 particles with a long-axis diameter of less than 2 μm, 15 particles had an oxygen content of less than 2.5 at%, so the ratio was 100%. Also, in Example 3, out of 22 particles with a long-axis diameter of less than 2 μm, 20 particles had an oxygen content of less than 2.5 at%, so the ratio was 91%. On the other hand, the ratios of Comparative Examples 1 and 2 were 87% respectively, which were relatively low. Thus, in Examples 1 to 3, the ratio of particles with a low oxygen content among the fine particles with a long-axis diameter of less than 2 μm is larger than that of Comparative Examples 1 and 2. Here, the oxygen content of each particle was measured by point analysis using SEM-EDX at 3000 times magnification.

[0035] The thermal conductivities shown in Table 3 are average values ​​calculated from the thermal conductivities of multiple samples obtained from a single silicon nitride substrate manufactured under the conditions of each example and comparative example shown in Tables 1 and 2. These multiple samples are multiple individual pieces cut from a single silicon nitride substrate, and the number of samples is, for example, two. Here, the thermal conductivities of a sample at the center of a single silicon nitride substrate in a planar view and a sample at an edge in a planar view were measured, and the thermal conductivity of each example and comparative example was calculated as the average of these thermal conductivities. The "planar view" here refers to the field of view when looking down on the silicon nitride substrate in its thickness direction. The silicon nitride substrate has a rectangular planar shape, and the center of the silicon nitride substrate in a planar view refers to the region including the intersection of the diagonals of the rectangular silicon nitride substrate.

[0036] The bending strength and bending strength σ0 shown in Table 3 are the average values ​​of multiple samples obtained from one silicon nitride substrate manufactured under the conditions of each example and comparative example shown in Tables 1 and 2. These multiple samples are multiple slices cut out from one silicon nitride substrate and separated into individual pieces, and the number of samples is, for example, 13.

[0037] As an example, the following describes a method for calculating the bending strength σ from the bending strength value measured for the sample of Example 2. The bending strength σ is the value at which the cumulative fracture probability F is 63.2% in the Weibull distribution, and is a value defined as a scale parameter in JIS R 1625. The following procedure is described in more detail in JIS R 1625.

[0038] To explain how to calculate the bending strength σ0, the data in Table 6 below will be used as an example.

[0039] [Table 6]

[0040] As shown in Table 6, the bending strength (strength σ) is measured for test pieces 1 to 13. Next, as shown in Table 7, the n pieces of strength data (here, 13 pieces) to be subjected to statistical analysis are sorted in ascending order of strength.

[0041] [Table 7]

[0042] Next, for each rank i (i = 1 to n) data, the cumulative failure probability F is calculated using the median rank method (1). i Calculate i, σ i , F i Here, n is the number of strength data (total number of test pieces), i is the rank of the strength data sorted in ascending order of strength, and F i is the i-th cumulative failure probability. F i =(i-0.3) / (n+0.4) (1)

[0043] [Table 8]

[0044] Next, i, σ according to the strength ranking in Table 8 i and F i The set of vertical axis lnln(1-F) -1 The horizontal axis is lnσ and the plot is as shown in Figure 2. Figure 2 is a graph of the Weibull distribution probability that explains how to calculate the bending strength σ0. Table 9 shows the lnln(1-F) -1 and the value of lnσ.

[0045] [Table 9]

[0046] The slope m (Weibull coefficient) and intercept were calculated from the regression line of the Weibull plot shown in Figure 2. From the slope and intercept, the cumulative fracture probability was calculated as F = 63.2% (i.e., lnln(1-F)-1 = 0), the bending strength σ0 is calculated. In other words, when the data ranked with the smallest cumulative failure probability F is 0% and the data ranked with the largest cumulative failure probability F is 100%, the strength at the position of 63.2% is the bending strength σ0. In the following formula (2), F is the cumulative failure probability, m is the shape parameter (Weibull coefficient), σ is the bending strength, and β is the scale parameter. lnln(1-F) -1 = mlnσ-mlnβ (2)

[0047] The left side of equation (2) is the vertical axis of the Weibull plot in Figure 2, and lnσ on the right side of equation (2) is the horizontal axis of Figure 2. Therefore, lnln(1-F) -1 Let Y be the slope, m be the slope a, lnσ be X, and mlnβ be b, then it can be expressed as a linear function Y=aX+b. Add lnln(1-F) to the above equation. -1 = 0 and find the slope and intercept of the plotted graph shown in Figure 3, the bending strength σ0 can be calculated using equation (3) shown in Figure 4. Figure 3 is a graph showing the relationship between the cumulative fracture probability (vertical axis) and strength σ (horizontal axis).

[0048] 3 has a slope of 26.1 and an intercept of -180, so that σ0 = 976 MPa is obtained. By the above procedure, the bending strength σ0 of each example and each comparative example in Table 3 can be calculated.

[0049] The graphs in Figures 5, 6, 7, 8, and 9 show the relationship between the major axis diameter and the oxygen content for each of Example 1, Example 2, Example 3, Comparative Example 1, and Comparative Example 2. These graphs show the major axis diameter (horizontal axis) and oxygen content (vertical axis) of silicon nitride particles extracted from the three SEM images (fields of view) mentioned above. The number of plots in each of the graphs in Figures 5 to 9 corresponds to the total number of particles in Table 4.

[0050] As shown in Figures 8 and 9 and Table 5, in Comparative Examples 1 and 2, the proportion of silicon nitride particles with a major axis diameter of less than 2 µm and an oxygen content of less than 2.5 at% was 87%. Thus, if the proportion of silicon nitride particles with an oxygen content of less than 2.5 at% among the silicon nitride particles with a major axis diameter of less than 2 µm is less than 90%, it is difficult to achieve both high thermal conductivity and high strength (bending strength). That is, as shown in Table 3, the silicon nitride substrates of Comparative Examples 1 and 2 cannot simultaneously achieve the two properties of a thermal conductivity of 110 W / m K or more and a bending strength σ0 of 700 MPa or more.

[0051] If the thermal conductivity of a silicon nitride substrate is less than 110 W / m K, a power module using such a silicon nitride substrate will be unable to efficiently remove heat during operation. Also, if the bending strength σ0 of the silicon nitride substrate is less than 700 MPa, a power module using such a silicon nitride substrate will be susceptible to breakage due to stress caused by temperature changes during operation, resulting in low reliability.

[0052] In contrast, as shown in Figures 5, 6, and Table 5, in Examples 1 and 2, all silicon nitride particles with a major axis diameter of less than 2 μm had an oxygen content of less than 2.5 at%. That is, the proportion of silicon nitride particles with a major axis diameter of less than 2 μm and an oxygen content of less than 2.5 at% was 100%. Furthermore, as shown in Figure 7 and Table 5, in Example 3, the proportion of silicon nitride particles with a major axis diameter of less than 2 μm and an oxygen content of less than 2.5 at% was 91%. That is, in Examples 1 to 3, 90% or more of the silicon nitride particles with a major axis diameter of less than 2 μm had an oxygen content of less than 2.5 at%. Therefore, as shown in Table 3, the silicon nitride substrates of Examples 1 to 3 simultaneously achieved the two properties of a thermal conductivity of 110 W / m·K or more and a bending strength σ0 of 700 MPa or more.

[0053] <Effects of the embodiment> Patent Document 1 specifies the oxygen content of the entire particle, as described in the following: "The grain boundary phase was removed from the pulverized silicon nitride sintered body by acid washing, and the silicon nitride particles were extracted and then measured with an oxygen analyzer." In contrast, in the present embodiment, the oxygen content of particles with a major axis diameter of less than 2 μm is specified.

[0054] The inventors have found through experiments and calculations that the assumption that a silicon nitride substrate with a lower oxygen content overall has higher thermal conductivity and bending strength does not necessarily hold true, and that the thermal conductivity and bending strength of the silicon nitride substrate depend on the oxygen content of silicon nitride particles with a relatively small major axis diameter. As shown in Tables 3 and 5, when comparing the proportion of particles with a low oxygen content limited to silicon nitride particles with a major axis diameter of less than 2 μm, Example 3 has a higher proportion than Comparative Examples 1 and 2, and Example 3 has a higher thermal conductivity and bending strength σ0 than Comparative Example 1.

[0055] In this embodiment, particles that contribute to improving thermal conductivity are arranged to a degree that does not reduce bending strength, thereby achieving a good balance in the microstructure. The relatively small particles (approximately 1 to 2 μm) that make up the matrix have a relatively high oxygen content because they have not grown as much as the coarse particles. However, because such small particles account for a relatively large proportion of the overall structure, the thermal conductivity (≒ oxygen content) of the small particles becomes important. Therefore, in this embodiment, we focus on small particles and fine-tune their properties to achieve both high thermal conductivity and high bending strength in the silicon nitride substrate. Specifically, among the multiple silicon nitride particles that make up the silicon nitride substrate, the oxygen content of 90% or more of the silicon nitride particles with a major axis diameter of less than 2 μm is specified to be less than 2.5 at%. This allows the silicon nitride substrate to achieve both high thermal conductivity and high bending strength. In other words, the performance of the silicon nitride substrate can be improved.

[0056] In the silicon nitride substrate of this embodiment, of the silicon nitride particles having a major axis diameter of less than 2 μm, 90% or more of the silicon nitride particles have an oxygen content of less than 2.5 at%. However, it is more preferable that there are no particles having an oxygen content of 2.5 at% or more. In other words, it is more preferable that of the silicon nitride particles having a major axis diameter of less than 2 μm, 100% of the silicon nitride particles have an oxygen content of less than 2.5 at%.

[0057] Furthermore, in Examples 1 to 3, the temperature rise rate during nitriding was 1.0°C / min, while the temperature rise rate was 0.5°C / min in Comparative Examples 1 and 2. Therefore, when the temperature rise rate is slow (the nitriding process takes a long time) as in Comparative Examples 1 and 2, the amount of oxygen taken in increases, and it is thought that the amount of oxygen in particles with major axis diameters of less than 2 µm in the final silicon nitride substrate increases.

[0058] As described above, the amount of oxygen incorporated into silicon nitride particles can be reduced by controlling the atmosphere and temperature rise rate in the nitriding step. This manufacturing method can reduce the amount of oxygen in silicon nitride particles with a major axis diameter of less than 2 μm in the final silicon nitride substrate.

[0059] In addition, the nitriding process (nitriding treatment) may be performed using a vacuum furnace or an atmospheric furnace. A vacuum furnace is a furnace capable of nitriding a molded body in a sealed, evacuated container. An atmospheric furnace is a furnace in which the interior of the furnace is open to the atmospheric atmosphere, and nitrogen is supplied to the molded body passing through the interior using a nitrogen supply mechanism and heating is performed to perform the nitriding process. In a vacuum furnace, inexpensive materials such as oxides cannot be used for the insulation and heating mechanism in order to make the furnace vacuum-compatible. Furthermore, a furnace body structure and vacuum pump are required to maintain vacuum-compatible sealing, resulting in high equipment costs. Therefore, when a vacuum furnace is used for the nitriding process, the manufacturing cost of the silicon nitride substrate tends to be high. Using an atmospheric furnace for the nitriding process eliminates the need for vacuum-compatible materials and auxiliary equipment, thereby reducing the manufacturing cost of the silicon nitride substrate. Furthermore, the manufacturing cost of the silicon nitride substrate is reduced by using an atmospheric atmosphere for the degreasing process.

[0060] The invention made by the present inventors has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention. [Explanation of symbols]

[0061] 1. Silicon nitride substrate 2 Silicon nitride particles 3 Grain boundary phase 4 Long axis diameter

Claims

1. A composite material having a plurality of silicon nitride particles and a grain boundary phase present between the plurality of silicon nitride particles, A silicon nitride substrate, wherein 90% or more of the silicon nitride particles having a major axis diameter of less than 2 μm among the plurality of silicon nitride particles have an oxygen content of less than 2.5 at %.

2. (a) mixing silicon nitride powder, a sintering aid, and a binder to form a slurry; (b) forming the slurry into a sheet and degreasing the sheet to obtain a formed body made of the slurry; (c) heating the compact in a nitrogen atmosphere to nitride the silicon contained in the compact; (d) after the step (c), sintering the molded body to form a silicon nitride substrate; and In the step (c), the temperature rising rate during nitriding is 1.0°C / min or more, the silicon nitride substrate has a plurality of silicon nitride grains and a grain boundary phase present between the plurality of silicon nitride grains, A method for producing a silicon nitride substrate, wherein, of the plurality of silicon nitride particles, 90% or more of the silicon nitride particles having a major axis diameter of less than 2 μm each have an oxygen content of less than 2.5 at %.

3. 3. The method for manufacturing a silicon nitride substrate according to claim 2, In the step (c), the molded body is heated in a nitrogen atmosphere using an atmospheric furnace to nitride the silicon contained in the molded body.

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