Silicon nitride sintered body and silicon nitride circuit board

Abstract

A silicon nitride sintered body according to an embodiment of the present invention comprises silicon nitride crystal particles. Observation by SEM of an area of 256 μm × 172 μm in an arbitrary first cross-section of the silicon nitride sintered body reveals that the ratio of a second number of silicon nitride crystal particles having an aspect ratio of 2 or less and a short diameter of 1.8 μm to 2.2 μm to a first number of silicon nitride crystal particles having an aspect ratio of 2 or less is in the range from 5% to 20%.

Description

Silicon nitride sintered body and silicon nitride circuit board

[0001] The embodiments described below generally relate to silicon nitride sintered bodies and silicon nitride circuit boards.

[0002] Silicon nitride sintered bodies are used in a variety of fields, including substrates for semiconductor devices and bearing balls. For example, Japanese Patent No. 6293772 (Patent Document 1) discloses a silicon nitride substrate with a thermal conductivity of 50 W / m·K or higher. In the technology described in Patent Document 1, the average diameter of the long axis of silicon nitride crystal particles is controlled to 1.5 to 10 μm. In the technology described in Japanese Patent No. 5380277 (Patent Document 2), the silicon nitride particles include acicular crystal particles. The long axis L of the acicular crystal particles is controlled to 10 μm or less, and the ratio of the long axis L to the short axis S (L / S) is controlled to 5 or more.

[0003] The techniques described in Patent Documents 1 and 2 control the major axis of silicon nitride crystal grains. The silicon nitride sintered body has high strength due to the intricate entanglement of columnar silicon nitride crystal grains with large aspect ratios.

[0004] Patent No. 6293772 Patent No. 5380277

[0005] On the other hand, the silicon nitride sintered bodies described in Patent Document 1 and Patent Document 2 sometimes suffered from a phenomenon in which mechanical properties deteriorated depending on the direction of stress application. Investigation into the cause of this finding revealed that the distribution of minor diameters of silicon nitride crystal grains was related. Patent Documents 1 and 2 focused only on the major diameter, and did not control the minor diameters of the silicon nitride crystal grains.

[0006] The embodiments are intended to address such problems and to provide a silicon nitride sintered body and a silicon nitride circuit board in which the distribution of minor diameters of silicon nitride crystal grains is controlled.

[0007] A silicon nitride sintered body according to the embodiment contains silicon nitride crystal grains, and when a 256 μm × 172 μm area in any first cross section of the silicon nitride sintered body is observed with an SEM, the ratio of a second number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm to a first number of silicon nitride crystal grains having an aspect ratio of 2 or less is in the range of 5% to 20%.

[0008] 1A and 1B are diagrams showing an example of a cross-sectional structure of a silicon nitride sintered body according to an embodiment, a silicon nitride substrate, and a silicon nitride circuit substrate.

[0009] A silicon nitride sintered body according to the embodiment contains silicon nitride crystal grains, and when a 256 μm × 172 μm area in any first cross section of the silicon nitride sintered body is observed with an SEM, the ratio of a second number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm to a first number of silicon nitride crystal grains having an aspect ratio of 2 or less is in the range of 5% to 20%.

[0010] Fig. 1 is a schematic diagram showing an example of the cross-sectional structure of a silicon nitride sintered body according to an embodiment, in which reference numeral 1 denotes the cross-sectional structure, reference numeral 2 denotes silicon nitride crystal grains with an aspect ratio of 2 or less, reference numeral 3 denotes silicon nitride crystal grains with an aspect ratio of more than 2, and reference numeral 4 denotes the grain boundary phase.

[0011] The cross-sectional structure 1 is the structure of an arbitrary cross section of a silicon nitride sintered body. The cross-sectional structure 1 includes silicon nitride crystal grains 2 having an aspect ratio of 2 or less, silicon nitride crystal grains 3 having an aspect ratio of more than 2, and a grain boundary phase 4. Pores (not shown) may be present in the cross-sectional structure 1. Here, the silicon nitride crystal grains 2 having an aspect ratio of 2 or less are sometimes referred to as spherical silicon nitride crystal grains 2. The silicon nitride crystal grains 3 having an aspect ratio of more than 2 are sometimes referred to as columnar silicon nitride crystal grains 3. The spherical silicon nitride crystal grains 2 may include spherical sialon crystal grains. The columnar silicon nitride crystal grains 3 may include columnar sialon crystal grains. The sialon grains refer to compounds containing Si, Al, O, and N as constituent elements.

[0012] "Aspect ratio" refers to the ratio of the minor axis to the major axis. Two points with the greatest distance between them are set on the outer edge of a single silicon nitride crystal particle. The greatest distance between those two points is the "major axis." A straight line is set that passes through the midpoint of the line segment connecting those two points and is perpendicular to that line segment. The distance between the two intersections of the perpendicular line and the outer edge of the silicon nitride crystal particle is the "minor axis."

[0013] The cross-sectional structure 1 has a complex intertwining structure of spherical silicon nitride crystal grains 2 and columnar silicon nitride crystal grains 3. A grain boundary phase 4 exists in the gaps between the spherical silicon nitride crystal grains 2 and columnar silicon nitride crystal grains 3.

[0014] The cross-sectional structure 1 can be obtained by observing a measurement area of ​​256 μm × 172 μm in an arbitrary cross section with a SEM. In the observation with a scanning electron microscope (SEM), the measurement area is photographed at a magnification of 500 times. If the size of the measurement area is larger than 256 μm × 172 μm, the resolution may decrease.

[0015] As described below, at least five measurement areas of 256 μm × 172 μm are observed, and the number ratio is measured based on the observation results. As long as the area of ​​one measurement area corresponds to 256 μm × 172 μm, the vertical and horizontal lengths of the measurement area may be changed. The vertical and horizontal orientations relative to the silicon nitride sintered body may also be changed as appropriate. Multiple measurement areas smaller than 256 μm × 172 μm and adjacent to each other may also be observed. In this case, the observation results of multiple measurement areas equivalent to 256 μm × 172 μm are used as the observation results of a single measurement area. Here, a measurement area of ​​256 μm × 172 μm was used as an area that is easy to measure. Furthermore, a measurement area size of 256 μm × 172 μm allows for the observation of a sufficient number of silicon nitride crystal particles for quantitative evaluation.

[0016] When five measurement areas of 256 μm×172 μm are observed, the number of silicon nitride crystal grains with an aspect ratio of 2 or less is generally 5,000 or more. The proportion of the number can be calculated while reducing the influence of coarse particles, etc.

[0017] The SEM used is a JOEL JCM-7000 or JSM-7200F, or a device with equivalent or better performance. Cross-sectional SEM photographs are sometimes simply called SEM photographs. SEM photographs use secondary electron images.

[0018] A method for analyzing the cross-sectional structure 1 will now be described. The cross-sectional structure 1 has a complex structure in which spherical silicon nitride crystal particles 2 and columnar silicon nitride crystal particles 3 are intertwined. The silicon nitride crystal particles are randomly oriented. Even silicon nitride crystal particles with a large aspect ratio will become spherical silicon nitride crystal particles 2 or columnar silicon nitride crystal particles 3 in the cross-sectional structure 1 depending on the orientation direction. The spherical silicon nitride crystal particles 2 may originally be spherical crystal particles, such as α-sialon crystal particles.

[0019] It is possible to determine the size of relatively large particles from SEM photographs. However, it is difficult to directly determine the size and distribution of small particles, such as spherical particles, from SEM photographs. In this embodiment, the SEM photographs are subjected to the following analytical processing.

[0020] For the analysis, a 1280 x 1024 pixel image is prepared. Image analysis software is used for the analysis process. The executable file for the image analysis software is, for example, SemAnalyzer.exe. The following steps S1 to S4 are performed on the prepared image.

[0021] (Step 1) Slightly smooth (blur) the image to the extent that the outlines of silicon nitride crystal particles (including sialon crystal particles) do not disappear. Smoothing can remove noise caused by minute irregularities on the sample surface. The smoothed image is called the smoothed image. To operate the executable file, modify the value on the line below "# smooth..." in the settings file. The value represents the density (coarseness) of the smoothing filter, and sets the density of the filter to remove noise to the extent that the outlines of silicon nitride crystal particles do not disappear.

[0022] (Step 2) Due to the influence of large irregularities on the sample surface, the brightness distribution in the SEM photograph can vary greatly. A process is performed to reduce this variation in brightness distribution. First, the smoothed image is divided into grids (lattice-like). The number of grids to be divided is set to 9 or more.

[0023] Each grid contains multiple pixels. For each grid, the median of the luminance values ​​of those pixels is calculated. Luminance is an index of brightness, and pixels have a luminance value as their pixel value. After calculating the median for each grid, the average of those medians is calculated. Next, the difference between the average and the median is calculated for each grid. The luminance values ​​of the pixels included in each grid are corrected so that this difference is small. Preferably, the correction is performed so that the difference between the median and the average for each grid is close to zero (including zero). This evens out the luminance differences between each grid, creating a bias image that smoothly connects the entire image. Then, by subtracting the bias image from the smoothed image, a normalized image is generated in which the luminance differences within the image have been subtracted.

[0024] The normalized image is called a normalized image. One normalized image is obtained for one measurement area of ​​256 μm×172 μm. Five normalized images are obtained by observing five measurement areas of 256 μm×172 μm.

[0025] When operating the executable file, edit the numerical values ​​representing the number of grids in the horizontal and vertical directions in the configuration file. The numerical values ​​are set to 9 or more. If the brightness distribution in the SEM image is small and normalization of the brightness distribution within the image is unnecessary, this process can be omitted by setting the number of grids in either the horizontal or vertical direction to 0. For example, if the observation surface is polished, the brightness distribution variation may be sufficiently small.

[0026] (Step 3) Further correction of the luminance distribution is performed to reduce variations in the luminance distribution among at least five images. The overall signal value histogram of the normalized or smoothed image is considered to be a normal distribution, and the luminance values ​​of each image are linearly transformed so that the full width at half maximum is the same as the standard normal distribution. An image that has undergone linear transformation is called a linearly transformed image. In other words, after correcting at least five smoothed or normalized images, a frequency histogram is created for each and averaged to perform linear transformation. In step 3, there are no adjustment parameters for the executable file.

[0027] (Step 4) Extract the particle shapes of silicon nitride crystal particles that are clearly depicted in the linearly transformed image. In this process, grain boundary phases are excluded from extraction. The minimum size of silicon nitride crystal particle diameter is set to 2.0 pixels by manipulating the executable file. In this case, particles whose circumscribing circle radius is less than 2.0 pixels are excluded from extraction. The minimum size of silicon nitride particle diameter can be adjusted by changing the value of "th_dis" in the configuration file. The aspect ratio threshold can be adjusted by changing the value of "th_shape." By adjusting the aspect ratio threshold, SiN particles with a large aspect ratio in the image can be excluded from extraction. For example, by setting the minimum size to 2.0 pixels, the aspect ratio threshold can be set to 2.0. In other words, silicon nitride crystal particles with an aspect ratio greater than 2.0 in the image can be excluded from extraction. This process allows for the quantification of silicon nitride crystal particles with an aspect ratio of 2 or less.

[0028] The above analysis method can also be performed using the image processing software ImageJ. When using ImageJ, the image is analyzed in four steps, just like SemAnalyzer.exe.

[0029] (Step 1) Create a smoothed image from the SEM image. Apply a smoothing filter using the executable file. Use "Process" -> "smooth" as the filter. Gaussian Blur can also be used as the filter.

[0030] (Step 2) Create a normalized image from the smoothed image. First, divide the smoothed image created in step 1 into nine parts. From "Analyze" on the menu bar, select "Grid" under "Tools," set the number of rows and columns to "3," and create a grid divided into nine parts. It is preferable to divide the grid into nine or more parts. Therefore, it is preferable to set the number of rows and columns to "3" or more.

[0031] Next, set the ROI area. From the menu bar, select "Analyze," then "Tools," then "ROI Manager," and add each grid cell to the "ROI Manager." Calculate the median brightness value. From the menu bar, select "Analyze," then "Set Measurements," then check "Median." Select each ROI in the "ROI Manager" and click "Measure." This calculates the median brightness value for each grid. Calculate the average of multiple medians. Calculate the difference between the average and median for each grid. Correct the brightness values ​​of the pixels included in each grid to minimize this difference. To perform this correction, select each ROI in the "ROI Manager" and select "File" under "Edit" on the menu bar. Select "Median" from the "File" options and repeat this process for each grid cell to create a bias image. Then, subtract the bias image from the smoothed image to generate an image with normalized brightness for the entire image.

[0032] (Step 3) To create a linearly transformed image, the brightness values ​​are linearly transformed. The linear transformation method is the same as when SemAnalyzer.exe is used.

[0033] (Step 4) Extract the particle shapes of silicon nitride crystal particles from the linearly transformed image obtained in Step 3. First, display the "Bright & Contrast" window, adjust "Minimum" and "Maximum," and place the cursor on the left side of the histogram. To remove noise, select "Process" → "Noise" → "Despeckle." Binarize the image ("Process" → "Binary" → "Make Binary"), then select "Analyze Particles" under "Analyze" to analyze the particles. Check the data displayed in the "Results" window and filter out silicon nitride crystal particles with an aspect ratio of 2 or less based on the aspect ratio (Major / Minor).

[0034] Any cross section of a silicon nitride sintered body is analyzed using the above method. The above analysis method allows the number of silicon nitride crystal grains with an aspect ratio of 2 or less to be measured. In the silicon nitride sintered body according to the embodiment, when a measurement area of ​​256 μm × 172 μm is observed using an SEM, the ratio of the second number to the first number is within the range of 5% to 20%. The first number is the total number of silicon nitride crystal grains with an aspect ratio of 2 or less. The second number is the number of silicon nitride crystal grains with an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm. Furthermore, according to the above analysis method, when five measurement areas of 256 μm × 172 μm are analyzed, for example, 5,000 or more silicon nitride crystal grains with an aspect ratio of 2 or less are observed.

[0035] This embodiment is characterized in that the ratio of the number of silicon nitride crystal grains (second number) having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm to the number of silicon nitride crystal grains (first number) having an aspect ratio of 2 or less is within the range of 5% to 20%. In other words, [(number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm / number of silicon nitride crystal grains having an aspect ratio of 2 or less)] × 100 = 5 to 20%.

[0036] By having the ratio of the second number to the first number be within the range of 5% to 20%, it is possible to suppress a decrease in three-point bending strength against stress in the direction perpendicular to the observation surface. Here, the ratio of the second number to the first number is referred to as the "number ratio A." If the number ratio A is less than 5%, the amount of silicon nitride crystal particles with an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm is low. Silicon nitride crystal particles with an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm contribute to improving three-point bending strength but may result in a decrease in thermal conductivity. If the number ratio A is less than 5%, the three-point bending strength of the silicon nitride sintered body may decrease. Furthermore, if the number ratio A exceeds 20%, the thermal conductivity of the silicon nitride sintered body may decrease. For this reason, the number ratio A is preferably within the range of 5% to 20%, and more preferably within the range of 8% to 18%.

[0037] In this embodiment, no matter how the five measurement areas of 256 μm × 172 μm are set on the cross section, the number ratio A in the measurement results is within the range of 5% to 20%. Preferably, the number ratio A is within the range of 5% to 20% in any of the measurement areas of 256 μm × 172 μm.

[0038] Silicon nitride sintered bodies have a structure in which silicon nitride crystal particles of various sizes are dispersed in a complex manner. Even in structures that appear to have no orientation at first glance, a decrease in three-point bending strength occurs depending on the direction of stress application. By controlling the number fraction A, it is possible to suppress a decrease in three-point bending strength against stress in a direction perpendicular to the observation surface. The cross section used for the observation surface can be arbitrary. In other words, no matter how the silicon nitride sintered body is cut, the number fraction A in the cross section is within the range of 5% to 20%. Therefore, a decrease in three-point bending strength against stress can be suppressed in any direction.

[0039] For example, a silicon nitride sintered body is cut in an arbitrary direction to obtain a first cross section. Next, the silicon nitride sintered body is cut in a direction perpendicular to the first cross section to obtain a second cross section. Furthermore, the silicon nitride sintered body is cut in a direction perpendicular to the first and second cross sections to obtain a third cross section. When five measurement areas of 256 μm × 172 μm are observed with an SEM in all of the first, second, and third cross sections, the number proportion A is within the range of 5% to 20%.

[0040] The ratio of the number of silicon nitride crystal grains (third number) having an aspect ratio of 2 or less and a minor axis of 4.0 μm or more to the first number is preferably within the range of 0% to 10%. Here, the ratio of the third number to the first number is referred to as "number ratio B." Silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 4.0 μm or more are relatively large crystal grains. If number ratio B exceeds 10%, and the number of large crystal grains increases, although this contributes to improving thermal conductivity, it may result in a decrease in three-point bending strength in the stress acting direction. For this reason, number ratio B is preferably 10% or less.

[0041] Large crystal grains have the effect of improving thermal conductivity. When the thermal conductivity of the silicon nitride sintered body is to be increased to 90 W / m K or more, the number ratio B is preferably in the range of 0.01% to 10%, and more preferably in the range of 0.02% to 5%.

[0042] There is no particular upper limit to the minor axis of silicon nitride crystal grains having an aspect ratio of 2 or less. Preferably, the maximum minor axis is less than 15 μm. If large crystal grains with minor axes exceeding 15 μm exist, it may be difficult to control the number ratio A and other parameters.

[0043] The ratio of the number of silicon nitride crystal grains (fourth number) having an aspect ratio of 2 or less and a minor axis of less than 1.8 μm to the first number is preferably within the range of 45% to 75%. Here, the ratio of the fourth number to the first number is referred to as "number ratio C." Silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of less than 1.8 μm are relatively small crystal grains. For example, many small crystal grains may be arranged between silicon nitride crystal grains having a minor axis of more than 2.2 μm. Therefore, number ratio C is larger than number ratios A and B.

[0044] The orientation direction of small crystal grains is unlikely to cause a decrease in three-point bending strength in the stress acting direction. Furthermore, small crystal grains exist in the gaps between crystal grains of different sizes and play a role in forming a dense cross-sectional structure 1. For this reason, the number ratio C is preferably in the range of 45% to 75%, and more preferably in the range of 50% to 75%.

[0045] Although there is no particular limitation on the lower limit of the minor axis of silicon nitride crystal particles having an aspect ratio of 2 or less, it is preferably 1 μm or more. If the minimum minor axis value is small, the proportion of the fourth number increases, which may make it difficult to control the number proportions A and B.

[0046] The ratio of the number of silicon nitride crystal particles (fifth number) having an aspect ratio of 2 or less and a minor axis greater than 2.2 μm and less than 4.0 μm to the first number is preferably within the range of 10% to 25%. The number ratio of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis greater than 2.2 μm and less than 4.0 μm is referred to as "number ratio D." Silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis greater than 2.2 μm and less than 4.0 μm are medium-sized. Silicon nitride crystal particles of this size contribute to improving three-point bending strength and thermal conductivity. A predetermined amount of number ratio D can suppress a decrease in three-point bending strength in the stress acting direction. Furthermore, medium-sized crystal particles also contribute to improving thermal conductivity.

[0047] It is preferable that at least one of number ratio B, number ratio C, and number ratio D satisfy the above-mentioned ranges. It is most preferable that number ratio A is satisfied and then all of number ratio B, number ratio C, and number ratio D satisfy the above-mentioned ranges.

[0048] The difference between number proportion A and number proportion D is preferably 10% or less. In other words, it is preferable that |number proportion A - number proportion D|≦10%. Number proportion A relates to silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of 1.8 μm or more and 2.2 μm or less, and number proportion D relates to silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of more than 2.2 μm and 4.0 μm or less. A small difference in the number proportion between slightly small particles and medium-sized particles can reduce the gaps between silicon nitride crystal particles, thereby improving fracture toughness.

[0049] The minor axis (D ５０ The minor axis (D ) is preferably 1.30 μm or more and less than 1.80 μm. ５０ ) refers to the particle size corresponding to a cumulative frequency of 50% in the distribution of minor diameters of silicon nitride crystal grains with an aspect ratio of 2 or less. 50% of silicon nitride crystal grains with an aspect ratio of 2 or less have minor diameters D ５０ In other words, the minor axis (D ５０ ) is also the average diameter (median diameter) of the minor axes of silicon nitride crystal particles having an aspect ratio of 2 or less. As a specific example, in five measurement areas of 256 μm × 172 μm, there are 6,000 silicon nitride crystal particles having an aspect ratio of 2 or less. In this case, counting these silicon nitride crystal particles in order of smallest minor axis, the minor axis of the 3,000th silicon nitride crystal particle is the minor axis (D ５０ )

[0050] The minor axis (D １００ The minor axis (D) is preferably 3.00 μm or more and less than 15.00 μm. １００ ) refers to the particle size corresponding to a cumulative frequency of 100% in the distribution of minor diameters of silicon nitride crystal particles with an aspect ratio of 2 or less. １００) corresponds to the maximum minor axis of silicon nitride crystal grains having an aspect ratio of 2 or less.

[0051] As described above, silicon nitride crystal grains with an aspect ratio of 2 or less contribute to improving three-point bending strength or thermal conductivity depending on their minor diameter. Furthermore, the entanglement of these silicon nitride crystal grains contributes to improving fracture toughness. By controlling the average minor diameter and the maximum minor diameter, and by controlling the proportion of each grain, it is possible to improve three-point bending strength, thermal conductivity, and fracture toughness.

[0052] The total area ratio of silicon nitride crystal grains with an aspect ratio of 2 or less is preferably within the range of 30% to 70%. This indicates that (total area of ​​silicon nitride crystal grains with an aspect ratio of 2 or less / total area of ​​all silicon nitride crystal grains) × 100 (%) = 30 to 70%. The total area of ​​all silicon nitride crystal grains refers to the total area of ​​silicon nitride crystal grains with an aspect ratio of 2 or less and silicon nitride crystal grains with an aspect ratio exceeding 2. Note that when the relative density of the silicon nitride sintered body is 95% to 100%, the area of ​​the measurement area of ​​256 μm × 172 μm may be used as the total area of ​​all silicon nitride crystal grains. This is because the influence of grain boundary phases and pores is sufficiently small at a relative density of 95% or more. However, even in this case, the above analysis process is performed to calculate the total area of ​​silicon nitride crystal grains with an aspect ratio of 2 or less.

[0053] According to the silicon nitride sintered body of the above-mentioned embodiment, the three-point bending strength can be made 400 MPa or more. The thermal conductivity can be made 90 W / m·K or more. The fracture toughness value can be made 7 MPa·m １／２ It can be more than that.

[0054] Three-point bending strength is measured in accordance with JIS-R-1601 (2008). JIS-R-1601 corresponds to ISO 14704. Thermal conductivity is measured using the flash method. Fracture toughness can be measured using the Niihara formula in accordance with the IF method of JIS-R-1607 (2015). JIS-R-1607 corresponds to ISO 15732.

[0055] The thickness of the silicon nitride sintered body may be 0.2 mm or more and 3 mm or less. A silicon nitride sintered body having a thickness of 0.2 mm or more and 3 mm or less is called a silicon nitride substrate. FIG. 2 is a perspective view showing an example of a silicon nitride substrate. In FIG. 2, reference numeral 5 denotes a silicon nitride substrate. Although a rectangular substrate is illustrated in FIG. 2, the shape of the substrate is not limited to this. The silicon nitride substrate may have various shapes, such as a circle (including an oval), an L-shape, or a U-shape. The vertical and horizontal dimensions of the substrate can also be freely designed.

[0056] A silicon nitride circuit board can be obtained by providing a circuit section on a silicon nitride substrate. Circuit sections may be provided on both sides of the silicon nitride substrate. Alternatively, a circuit section may be provided on the front surface of the silicon nitride substrate and a heat dissipation section may be provided on the back surface.

[0057] Various materials such as metal plates, metallized layers, and thin films can be used for the circuit section and heat sink. Examples of metal plates include copper plates, copper alloy plates, aluminum plates, and aluminum alloy plates. The metal plate is preferably a copper plate made of oxygen-free copper. As specified in JIS-H-3100 (ISO 1337, etc.), oxygen-free copper has a copper purity of 99.96 wt% or more.

[0058] The thermal conductivity of a copper plate is approximately 400 W / m·K, which is higher than the thermal conductivity of aluminum, which is approximately 240 W / m·K. Therefore, by using a copper plate as the metal plate, the heat dissipation properties of the silicon nitride circuit board can be improved compared to when an aluminum plate is used.

[0059] The thickness of the metal plate is preferably within a range of 0.2 mm to 5 mm. By increasing the thickness of the metal plate, heat dissipation and current carrying capacity can be improved. The bonding layer 6 is, for example, an active metal bonding layer. When the metal plate is a copper plate, for example, the active metal bonding layer contains Ti and is mainly composed of Ag or Cu. When the metal plate is an aluminum plate, for example, the active metal bonding layer contains Si and is mainly composed of Al. The active metal bonding layer is a bonding layer containing Ti or Si as the active metal. Here, an example in which a metal plate is bonded to a silicon nitride substrate as a circuit portion has been described, but the silicon nitride circuit board according to the embodiment is not limited to this form.

[0060] The thin film is a metal film formed by a method such as sputtering or vapor deposition. The metal thin film includes Ti, Pt, Au, Ni, Cu, Al, Ag, etc. The metallized layer is a film formed by sintering a metal paste. The metallized layer is sometimes called a metal thick film. The metallized layer includes Ag, Cu, Ti, W, Mo, etc.

[0061] Fig. 3 is a side view showing an example of a silicon nitride circuit substrate. In Fig. 3, reference numeral 5 denotes a silicon nitride substrate, reference numeral 6 denotes a bonding layer, reference numeral 7 denotes a metal plate, reference numeral 8 denotes a stress, and reference numeral 10 denotes a silicon nitride circuit substrate. The metal plate 7 is bonded to the silicon nitride substrate 5 via the bonding layer 6. In the example shown in Fig. 3, the metal plates 7 are bonded to both sides of the silicon nitride substrate.

[0062] For example, the metal plate 7 on one side is used as a circuit portion, and the metal plate 7 on the other side is used as a heat sink. The silicon nitride circuit board according to the embodiment is not limited to such a structure. For example, the metal plates on both sides may each be used as a circuit portion. Multiple metal plates may be bonded to one side, and each metal plate may be used as a circuit portion.

[0063] In recent years, there has been a trend toward using thicker metal plates. As the metal plate becomes thicker, the stress applied to the silicon nitride substrate also increases. For example, when a thermal cycle test (TCT) is performed, the joint edge of the metal plate is subjected to not only tensile stress and compressive stress in the in-plane direction, but also an upward stretching stress. In Figure 3, the left direction indicates tensile stress, the right direction indicates compressive stress, and the upward direction indicates upward stretching stress.

[0064] In conventional circuit boards, cracks can occur in silicon nitride substrates when tensile stress, compressive stress, or upward stretching stress is applied to the joining edge of the metal plate. In other words, a decrease in strength occurs in the direction in which the stress acts. This problem is particularly pronounced when the metal plate is thick. For this reason, cracks and the like are likely to occur in silicon nitride substrates during TCT, making it difficult to obtain good TCT characteristics. In the silicon nitride substrate according to the embodiment, the grain size is controlled, so that the occurrence of cracks in the silicon nitride substrate can be suppressed regardless of where the joining edge of the metal plate is located on the surface of the silicon nitride substrate. This increases the degree of freedom in the position at which the metal plate is joined.

[0065] For example, a silicon nitride circuit substrate is cut in the thickness direction of the silicon nitride substrate. When the cross section is observed with an SEM, a portion of the silicon nitride substrate located directly below the lateral edge of the metal plate is observed. In the silicon nitride circuit substrate according to the embodiment, when a 256 μm × 172 μm area located directly below the lateral edge of the metal plate is observed with an SEM, the number ratio A is within the range of 5% to 20%. Preferably, the number ratios B to D are also within the above-mentioned ranges. According to the embodiment, the number ratio of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm is controlled directly below the lateral edge of the metal plate, where stress is likely to concentrate. This prevents a decrease in strength and suppresses the occurrence of cracks, even when stress is applied to the silicon nitride substrate.

[0066] The silicon nitride substrate according to the embodiment suppresses a decrease in strength in the direction of stress action. For example, when the metal plate is thicker than 0.6 mm, tensile stress, compressive stress, and upward stretching stress all increase. The silicon nitride substrate according to the embodiment suppresses a decrease in strength due to stress acting in a direction perpendicular to any observation surface. Therefore, good TCT characteristics can be obtained regardless of the direction of stress increase.

[0067] Furthermore, by controlling the number ratios A, B, C, and D, a cross-sectional structure in which small silicon nitride crystal particles are distributed in a complex manner can be obtained. When joining a metal plate to a silicon nitride substrate, an active metal bonding method is used. In the active metal bonding method, when Ti (titanium) is used as the active metal, a titanium nitride (TiN) layer is formed between the silicon nitride substrate and the brazing material. The titanium nitride layer is formed by the reaction between Ti and silicon nitride crystal particles. The structure in which small silicon nitride crystal particles are distributed in a complex manner increases the surface area on which the titanium nitride layer is formed, thereby improving the bonding strength. Furthermore, local variations in the bonding strength are suppressed, which also leads to improved TCT characteristics. In other words, according to the embodiment, a silicon nitride substrate suitable for a silicon nitride circuit substrate to which a metal plate is joined by the active metal bonding method can be provided.

[0068] A method for producing a silicon nitride sintered body according to an embodiment will be described. The method for producing the silicon nitride sintered body according to the embodiment is not limited as long as it has the above-mentioned configuration. Here, a method for producing a silicon nitride sintered body with a high yield will be described.

[0069] First, silicon nitride powder and sintering aid powder are prepared as raw materials. The silicon nitride powder is produced, for example, by the imide decomposition method or the direct nitriding method. The average particle size of the silicon nitride powder is preferably in the range of 0.1 μm to 4 μm. The particle size D of the silicon nitride powder ｐ１００ It is preferable that the particle diameter D is 10 μm or less. ｐ１００ indicates the particle size at which the cumulative frequency is 100% in the particle size distribution of the silicon nitride powder. ｐ１００ is the maximum particle size in the silicon nitride powder. If there are many coarse grains in the raw material powder, coarse grains with a minor axis exceeding 4.0 μm are likely to be formed.

[0070] The sintering aid may be, for example, one or more selected from rare earth elements, titanium (Ti), hafnium (Hf), zirconium (Zr), magnesium (Mg), calcium (Ca), tungsten (W), molybdenum (Mo), and compounds thereof. The compound may be an oxide, nitride, or oxynitride. The rare earth element may be yttrium (Y) or a lanthanoid element. Examples of lanthanoid elements include ytterbium (Yb), erbium (Er), europium (Eu), lutetium (Lu), lanthanum (La), cerium (Ce), and dysprosium (Dy). The average particle size of the sintering aid powder is preferably in the range of 0.1 μm to 4 μm.

[0071] When the total of the silicon nitride powder and the sintering aid powder is taken as 100% by mass, the amount of the sintering aid added is preferably within the range of 1% by mass to 20% by mass. The amount of the rare earth element compound in the sintering aid is preferably within the range of 1% by mass to 10% by mass. The rare earth element compound is one or more selected from oxides of rare earth elements, nitrides of rare earth elements, and oxynitrides of rare earth elements. The sintering aid other than the rare earth element compound is preferably one or more selected from titanium (Ti), hafnium (Hf), zirconium (Zr), magnesium (Mg), calcium (Ca), tungsten (W), molybdenum (Mo), and compounds thereof.

[0072] It is preferable to control the amounts of rare earth compound and other sintering aids added. Rare earth compound improves sinterability and promotes grain growth of silicon nitride crystal particles in the c-axis direction. Sintering aids other than rare earth compound have the effect of reacting with the rare earth compound to form or strengthen a grain boundary phase. The amount of rare earth compound added is A (mol), and the amount of other sintering aid added is B (mol). When the molar ratio A / B is low, the aspect ratio of the silicon nitride crystal particles tends to be small and the minor axis size tends to be large. On the other hand, when the molar ratio A / B is high, the aspect ratio of the silicon nitride crystal particles tends to be large and the minor axis size tends to be small. By controlling the ratio of the rare earth compound to other components in the sintering aid, it is possible to control the aspect ratio and minor axis size of the silicon nitride crystal particles. The molar ratio A / B is preferably 0.5 or more, more preferably 1.0 or more and 1.2 or less.

[0073] Silicon nitride powder and sintering aid powder are mixed, followed by a crushing step. In the crushing step, the powders are mixed and crushed using a pulverizer. The pulverizer is, for example, a ball mill or a bead mill. In the crushing step, it is preferable to suppress the particle size variation of the crushed particles. Before being processed in the pulverizer, the silicon nitride powder and sintering aid powder are often agglomerated. It is preferable to reduce the number of agglomerated particles by the crushing step using a pulverizer.

[0074] Particle size D of mixed powder after crushing process ｍｐ１０ , D ｍｐ５０ , D ｍｐ９０ The difference between the particle diameter D and the particle size D is preferably small. ｍｐ１０ , D ｍｐ５０ , and D ｍｐ９０ and D respectively refer to particle sizes corresponding to cumulative frequencies of 10%, 50%, and 90% in the particle size distribution of the mixed powder. ｍｐ１０ 50% of the particles contained in the mixed powder have a particle size of less than particle size D ｍｐ５０ Particle size D ｍｐ５０ is also the median diameter. 90% of the particles contained in the mixed powder have a particle size D ｍｐ９０ is less than.

[0075] D ｍｐ５０ -D ｍｐ１０ ≦1.0 μm or D ｍｐ９０ -D ｍｐ５０ It is preferable that at least one of the following conditions is satisfied: ≦2.0 μm. The particle size is measured by weighing 40 mg of the mixed powder after the crushing step and measuring the particle size distribution. The particle size distribution is measured using a laser diffraction method.

[0076] A binder, a solvent, etc. are added to the mixed powder after the crushing step to prepare a raw material powder slurry. If necessary, the raw material powder slurry may be subjected to a kneading step.

[0077] Next, a molding step is carried out using the raw material powder slurry. For the molding step, sheet molding, injection molding, mold molding, or the like can be used. For sheet molding, for example, a doctor blade method is used. When producing a silicon nitride substrate, the doctor blade method is preferred. By using the doctor blade method, a long molded body can be produced, improving mass productivity. The molded body obtained by the molding step may be cut as necessary. When producing a molded body in a shape other than a substrate, mold molding may be used.

[0078] Next, the compact is subjected to a degreasing process. By performing the degreasing process, a degreased body can be obtained. The degreasing process is preferably performed within a range of 400°C or higher and 800°C or lower. By the degreasing process, organic substances such as binders can be removed. Furthermore, the solvent remaining in the compact can also be removed.

[0079] In the sintering step, the degreased body is heated in a non-oxidizing atmosphere. The sintering step is carried out for a period of 4 hours to 24 hours, and the sintering temperature is set to a range of 1650°C to 1950°C. The non-oxidizing atmosphere is preferably a nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas. The pressure inside the sintering furnace is preferably a pressurized atmosphere.

[0080] If the degreased body is sintered at a sintering temperature below 1650°C, it is difficult to obtain a dense sintered body. On the other hand, if the degreased body is sintered at a sintering temperature higher than 1950°C, self-decomposition of silicon nitride is likely to occur, making it difficult to obtain a dense sintered body. Therefore, it is preferable to control the sintering temperature within the above range.

[0081] The pressure in the sintering furnace is preferably a pressurized atmosphere of 0.6 MPa or more. By setting the pressurized atmosphere at 0.6 MPa or more, it is possible to suppress the self-decomposition of silicon nitride and the grain growth of silicon nitride crystal particles. With regard to the pressurized atmosphere, it is preferable that the pressurization is started between 1400°C and the start of sintering.

[0082] In the sintering step, it is preferable to use a setter. The setter is a plate material on which the degreased body is placed, a wall material surrounding the degreased body, a container for storing the degreased body, etc. The material of the setter is, for example, carbon, silicon carbide, silicon nitride, etc. The setter has the role of preventing the degreased body from adhering to the inside of the sintering furnace. The setter also has the role of controlling the atmosphere inside the sintering furnace.

[0083] The degreased body is preferably surrounded by a setter. Surrounded by a setter means that the degreased body and the setter are arranged so that the degreased body placed on the setter is surrounded on all four sides by other setters. When surrounding the degreased body on all four sides, there may be gaps between the setters. Alternatively, the degreased body may be covered with a setter after being surrounded on all four sides. When a lid is provided, the setter may be in the shape of a container. Surrounding the degreased body with a setter improves the effect of controlling the atmosphere in the sintering furnace. The thickness of the setter on which the degreased body is placed is preferably 1 mm or more and 5 mm or less, and the thickness of the setter surrounding the degreased body is preferably 6 mm or more.

[0084] The volume of the degreased body placed in the space surrounded by the setter is preferably 40 vol% or more. When multiple degreased bodies are placed, the "volume of the degreased body" is the total volume of those multiple degreased bodies. By setting the volume of the degreased body to 40 vol% or more, a sufficient amount of gas can be generated from the degreased body to control the partial pressure in the processing atmosphere. This makes it easy to maintain a pressurized atmosphere with a pressure of 0.6 MPa or more inside the sintering furnace. There is no particular upper limit for the volume of the degreased body, but 90 vol% or less is preferable. If the volume of the degreased body exceeds 90 vol%, the gap between the setter and the degreased body is small, which may make it difficult to remove the sintered body from the setter after sintering.

[0085] When multiple degreased bodies are arranged in a space surrounded by a setter, it is preferable that the distance between the degreased bodies does not exceed 10 mm. For example, when the degreased bodies are arranged in a stacked manner, it is preferable that there are no gaps between the degreased bodies that exceed 10 mm in the thickness direction. When the degreased bodies are arranged side by side, it is preferable that there are no gaps between the degreased bodies that exceed 10 mm in the arranging direction. For example, when multiple sheet-shaped degreased bodies are stacked in close contact with each other, the distance between the degreased bodies is 0 mm. When the degreased bodies are arranged in a stacked manner, a bedding powder may be used. It is preferable that one degreased body is located within 10 mm of another degreased body. This makes it possible to more uniformly distribute the paths of the atmospheric gas during the sintering process and the gas generated from the degreased bodies. This also allows for control of the atmospheric partial pressure.

[0086] The use of a setter in sintering, the surrounding of the degreased bodies with the setter, and the control of the distance between the degreased bodies may be carried out by any one method alone or in combination of two or more methods. A combination of all the methods is most preferred.

[0087] While the degreased body is being heated to the sintering temperature, a heat treatment step is preferably carried out in which the temperature is maintained at 1400°C to 1650°C for 1 hour to 8 hours. By carrying out the heat treatment step, it is possible to suppress the variation in grain growth of silicon nitride crystal grains. By controlling the particle size of the mixed powder after the crushing step, the effect of suppressing the variation in grain growth can be further enhanced.

[0088] If necessary, a reheat treatment step may be performed after the sintering step. When performing the reheat treatment step, the sintered body is cooled to room temperature once and then heated again. The reheat treatment temperature is preferably in the range of 1500°C to 1850°C. The reheat treatment step is preferably performed in a pressurized atmosphere of 0.6 MPa or more. The temperature at which pressurization begins in the reheat treatment step is preferably lower than the temperature at which pressurization begins in the sintering step. For example, if pressurization begins at 1000°C during the temperature rise toward the sintering step, pressurization begins at less than 1000°C during the temperature rise toward the reheat treatment step. By performing the reheat treatment step, it is possible to suppress variation in particle size of silicon nitride crystal particles. By setting the pressurization start temperature in the reheat treatment step to a temperature lower than the pressurization start temperature in the sintering step, it is possible to suppress variation in particle size.

[0089] The above steps can produce a silicon nitride substrate (sintered silicon nitride). If necessary, the obtained silicon nitride substrate may be subjected to a surface treatment such as honing. When the silicon nitride substrate is to be used as a silicon nitride circuit substrate, a step of providing a circuit portion on the silicon nitride substrate is carried out.

[0090] (Examples 1 to 5, Comparative Examples 1 and 2) First, silicon nitride powder and sintering aid powder were prepared. The mixing ratio of silicon nitride powder and sintering aid powder, the average particle size of silicon nitride powder, and the particle size D of silicon nitride powder were determined. ｐ１００ The raw materials of the sintering aid powder and the average particle size of the sintering aid powder are shown in Table 1.

[0091]

[0092] The silicon nitride powder and the sintering aid powder were mixed and crushed using a ball mill. ｍｐ１０ , D ｍｐ５０, D ｍｐ９０ The particle size distribution was measured using a laser diffraction method. ｍｐ５０ and D ｍｐ１０ and the difference between ｍｐ９０ and D ｍｐ５０ The difference between the values ​​was calculated. The results are shown in Table 2.

[0093]

[0094] The mixed powder after the crushing process was mixed with an organic binder and a solvent to prepare a raw material powder slurry. The raw material powder slurry was then formed into a sheet by the doctor blade method. The long sheet was cut into a sheet-like compact.

[0095] The sheet-shaped compact was subjected to a debinding process. In some examples, a heat treatment process was carried out while the temperature was rising to the sintering temperature. In the heat treatment process, the debinding body was held at 1400°C or higher and 1650°C or lower for 1 hour or higher and 8 hours or lower. Table 3 shows whether or not the heat treatment process was carried out and the time of the heat treatment process.

[0096] The sheet-shaped degreased body was subjected to a sintering process. In the sintering process, multiple sheet-shaped degreased bodies were stacked with powder interposed between them. In other words, the multiple sheet-shaped degreased bodies were arranged so that there were locations where the distance between the degreased bodies was 0 mm. Table 3 shows the sintering temperature and the presence or absence of a pressurized atmosphere in the sintering process. In the examples, the stacked sheet-shaped degreased bodies were surrounded by a setter. In examples 1 to 5, the total volume of the stacked sheet-shaped degreased bodies was in the range of 40 vol% to 90 vol% of the volume of the space surrounded by the setter. In example 6, the total volume was 95 vol% of the volume of the space. In comparative example 1, only the stacked sheet-shaped degreased bodies were placed on a plate-shaped setter. In other words, in comparative example 1, the sheet-shaped degreased bodies were not surrounded by a setter.

[0097] In some examples and comparative examples, the sintered body was returned to room temperature after the sintering step and then subjected to a reheat treatment step. In the reheat treatment step, the sintered body was heat-treated in a pressurized atmosphere at a temperature in the range of 1500°C to 1850°C. The temperature and pressure in the reheat treatment step are shown in Table 3. The pressurization start temperature during the temperature increase to the reheat treatment step was set lower than the pressurization start temperature during the temperature increase to the sintering step.

[0098]

[0099] Silicon nitride substrates were fabricated through the above steps. The particle size distribution of the minor axes of silicon nitride crystal particles with an aspect ratio of 2 or less was measured in any cross section of the silicon nitride substrate. Five areas of 256 μm × 172 μm were measured. The particle size distribution of the minor axes was measured using the method described above.

[0100] From the results of the particle size distribution, the number of silicon nitride crystal particles having an aspect ratio of 2 or less (first number) was measured. The number of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of 1.8 to 2.2 μm (second number), the number of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of 4.0 μm or more (third number), the number of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of less than 1.8 (fourth number), and the number of silicon nitride crystal particles having an aspect ratio of 2 or less and a minor axis of 2.2 to 4.0 μm (fifth number) were measured. Based on these measurement results, the ratio of the number of the second to fifth numbers to the first number was calculated. The minor axis D of silicon nitride crystal particles having an aspect ratio of 2 or less was calculated. ５０ and D １００ The ratio of the total area of ​​silicon nitride crystal grains with an aspect ratio of 2 or less to the area of ​​the measurement area was also calculated. The results are shown in Table 4.

[0101]

[0102] In all Examples, the ratio of the second number to the first number was 5% or more and 20% or less, which was within the preferred range. In contrast, in the Comparative Examples, the ratio of the second number to the first number was outside the preferred range. In Comparative Examples 1 and 2, although the raw materials and ratios of the sintering aid were almost the same as in Example 1, the particle size distribution of the minor axis was different from that of Example 1. This is because in Comparative Examples 1 and 2, the average particle size of the mixed powder after the crushing process was outside the preferred range, and the conditions of the sintering process were outside the preferred range.

[0103] Furthermore, in Examples 1 to 6, the silicon nitride substrate was measured in a direction perpendicular to the cross section where the particle size distribution was measured, and a new cross section was obtained. In the new cross section, five areas of 256 μm × 172 μm were measured to obtain a particle size distribution. From the results, the ratio of the number of particles in the second to fifth numbers to the number of particles in the first number was calculated. As a result, in the new cross section, the ratio of the number of particles in the second to fifth numbers to the number of particles in the first number was also 5% or more and 20% or less, which was within the preferred range.

[0104] Next, the three-point bending strength, fracture toughness, and thermal conductivity of the silicon nitride substrate were measured, and the results are shown in Table 5. The methods for measuring the three-point bending strength, fracture toughness, and thermal conductivity were as described above.

[0105]

[0106] In each example and comparative example, a silicon nitride substrate measuring 60 mm in length and 50 mm in width was fabricated under the conditions shown in Tables 1 to 3. In examples 1 to 3, example 6, and comparative example 1, the silicon nitride substrate had a thickness of 0.32 mm. In examples 4, 5, and comparative example 2, the silicon nitride substrate had a thickness of 0.25 mm.

[0107] Using an active metal process, copper plates with a thickness of 0.8 mm were bonded to both sides of a silicon nitride substrate. The copper plates on the surfaces were subjected to an etching process to give them a circuit pattern. In this way, a silicon nitride circuit substrate was prepared.

[0108] The bonding strength and TCT characteristics of the copper plate in the silicon nitride circuit substrate were measured. Bonding strength was measured by a peel test. The TCT conditions were set as -40°C x 30 minutes → room temperature x 10 minutes → 170°C x 30 minutes → room temperature x 10 minutes, and 3,000 cycles were performed. The presence or absence of cracks in the silicon nitride substrate after TCT was measured. In each example and comparative example, the bonding strength and TCT characteristics of 10 silicon nitride circuit substrates were measured. The results are shown in Table 6. The bonding strength in Table 6 is the average bonding strength of the 10 silicon nitride circuit substrates. Regarding the TCT characteristics, examples in which no cracks occurred in any of the silicon nitride substrates were recorded as "absent," and examples in which cracks were observed in any of the silicon nitride substrates were recorded as "present."

[0109]

[0110] The results in Table 6 show that higher bonding strength was obtained in Examples 1 to 6 compared to Comparative Examples 1 and 2. Furthermore, improved TCT characteristics were confirmed in Examples 1 to 6 compared to Comparative Examples 1 and 2. For example, small cracks were observed in the silicon nitride substrate in Comparative Examples 1 and 2. These results show that controlling the minor axis of silicon nitride crystal particles can improve strength against stresses generated in various directions. It is also clear that this is effective in improving bonding strength.

[0111] Embodiments of the present invention include the following features: (Feature 1) A silicon nitride sintered body containing silicon nitride crystal grains, wherein, when an area of ​​256 μm × 172 μm in any first cross section is observed with an SEM, a ratio of a second number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 1.8 μm or more and 2.2 μm or less to a first number of silicon nitride crystal grains having an aspect ratio of 2 or less is within a range of 5% to 20%. (Feature 2) The silicon nitride sintered body according to Feature 1, wherein, in the area, a ratio of a third number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 4.0 μm or more to the first number is within a range of 0% to 10%. (Feature 3) The silicon nitride sintered body according to Feature 1 or Feature 2, wherein in the area, a fourth number of the silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of less than 1.8 μm relative to the first number is in the range of 45% to 75%. (Feature 4) The silicon nitride sintered body according to any one of Feature 1 to Feature 3, wherein in the area, a fifth number of the silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of more than 2.2 μm and less than 4.0 μm relative to the first number is in the range of 10% to 25%. (Feature 5) In the area, the minor axis (D ５０ ) is 1.30 μm or more and less than 1.80 μm, and the minor diameter of 50% of the silicon nitride crystal grains having an aspect ratio of 2 or less is ５０ ) or less. (Feature 6) The silicon nitride sintered body according to any one of Features 1 to 4, wherein the silicon nitride crystal grains having an aspect ratio of 2 or less have a maximum minor axis value of 3.00 μm or more and less than 15.00 μm. (Feature 7) The silicon nitride sintered body according to any one of Features 1 to 6, wherein the silicon nitride crystal grains having an aspect ratio of 2 or less occupy an area of ​​256 μm × 172 μm in a range of 30% or more and 70% or less. (Feature 8) The silicon nitride sintered body according to any one of Features 1 to 6, wherein the silicon nitride crystal grains having an aspect ratio of 2 or less occupy an area of ​​256 μm × 172 μm in a range of 30% or more and 70% or less. １／２(Feature 9) The silicon nitride sintered body according to any one of Features 1 to 7, having a thermal conductivity of 90 W / m K or more. (Feature 10) The silicon nitride sintered body according to any one of Features 1 to 9, having a thickness of 0.2 mm or more and 3 mm or less. (Feature 11) The silicon nitride sintered body according to any one of Features 1 to 10, having a three-point bending strength of 400 MPa or more. (Feature 12) The silicon nitride sintered body according to any one of Features 1 to 11, wherein, when an area of ​​256 μm × 172 μm in a second cross section perpendicular to the first cross section is observed by SEM, the ratio of the number of silicon nitride crystal grains having an aspect ratio of 2 or less and a minor axis of 1.8 μm to 2.2 μm to the number of silicon nitride crystal grains having an aspect ratio of 2 or less is within the range of 5% to 20%. (Feature 13) A silicon nitride circuit board comprising: a silicon nitride substrate using the silicon nitride sintered body according to any one of Features 1 to 12; a metal plate joined to at least one surface of the silicon nitride substrate; and a brazing layer joining the silicon nitride sintered body and the metal plate. (Feature 14) The silicon nitride circuit board according to Feature 13, wherein the metal plate is a copper plate.

[0112] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

[0113] REFERENCE SIGNS LIST 1... Cross-sectional structure 2... Silicon nitride crystal grains with an aspect ratio of 2 or less 3... Silicon nitride crystal grains with an aspect ratio of more than 2 4... Grain boundary phase 5... Silicon nitride substrate 6... Bonding layer 7... Metal plate 8... Stress 10... Silicon nitride circuit substrate

Claims

1. A silicon nitride sintered body containing silicon nitride crystal grains, wherein, when an area of ​​256 μm x 172 μm in any first cross section is observed with an SEM, the ratio of a second number of silicon nitride crystal grains having an aspect ratio of 2 or less and a short axis of 1.8 μm or more and 2.2 μm or less to a first number of silicon nitride crystal grains having an aspect ratio of 2 or less is within the range of 5% or more and 20% or less.

2. The silicon nitride sintered body according to claim 1, wherein in the area, the ratio of a third number of the silicon nitride crystal grains having an aspect ratio of 2 or less and a short axis of 4.0 μm or more to the first number is within the range of 0% or more and 10% or less.

3. A silicon nitride sintered body as described in claim 1 or 2, wherein in the area, the ratio of a fourth number of silicon nitride crystal grains having an aspect ratio of 2 or less and a short axis of less than 1.8 μm to the first number is within the range of 45% or more and 75% or less.

4. A silicon nitride sintered body according to any one of claims 1 to 3, wherein in the area, the ratio of a fifth number of silicon nitride crystal grains having an aspect ratio of 2 or less and a short axis exceeding 2.2 μm and less than 4.0 μm to the first number is within the range of 10% or more and 25% or less.

5. In the area, the minor axis (D ５０ ) is 1.30 μm or more and less than 1.80 μm, and the minor axis of 50% of the silicon nitride crystal grains having an aspect ratio of 2 or less is ５０ 5. The silicon nitride sintered body according to claim 1, wherein the surface roughness of the silicon nitride sintered body is 0.1 or less.

6. A silicon nitride sintered body according to any one of claims 1 to 5, wherein the maximum minor axis value of said silicon nitride crystal grains having an aspect ratio of 2 or less is 3.00 µm or more and less than 15.00 µm.

7. A silicon nitride sintered body according to any one of claims 1 to 6, wherein the area of ​​256 μm x 172 μm occupied by silicon nitride crystal grains having an aspect ratio of 2 or less is within the range of 30% or more and 70% or less.

8. Fracture toughness value is 7MPa・m １／２ The silicon nitride sintered body according to any one of claims 1 to 7.

9. A silicon nitride sintered body according to any one of claims 1 to 8, having a thermal conductivity of 90 W / m·K or more.

10. A silicon nitride sintered body according to any one of claims 1 to 9, having a thickness of 0.2 mm or more and 3 mm or less.

11. A silicon nitride sintered body according to any one of claims 1 to 10, having a three-point bending strength of 400 MPa or more.

12. A silicon nitride sintered body according to any one of claims 1 to 11, wherein, when an area of ​​256 μm x 172 μm in a second cross section perpendicular to the first cross section is observed with an SEM, the ratio of the number of silicon nitride crystal grains having an aspect ratio of 2 or less and a short axis of 1.8 μm or more and 2.2 μm or less to the number of silicon nitride crystal grains having an aspect ratio of 2 or less is within the range of 5% or more and 20% or less.

13. A silicon nitride circuit board comprising: a silicon nitride substrate using the silicon nitride sintered body according to any one of claims 1 to 12; a metal plate joined to at least one surface of the silicon nitride sintered body; and a brazing layer joining the silicon nitride sintered body and the metal plate.

14. The silicon nitride circuit board according to claim 13, wherein said metal plate is a copper plate.

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