Manufacturing method of high thermal conductive silicon nitride sintered body

Abstract

To provide a high thermal conductive silicon nitride sintered body with a controlled solid solution oxygen content.SOLUTION: A manufacturing method of a high thermal conductive silicon nitride sintered body of an embodiment includes: a wet type cracking and mixing step of mixing a silicon nitride powder having an average grain size of 2.5 μm or under and an impurity oxygen content of 2 mass% or under with a sintering aid having an average grain size of 3 μm or under; a molding step of performing molding by using a raw material powder slurry; a degreasing step of degreasing a molded body at 400 to 800°C; and a sintering step of sintering a degreased body. The sintering step includes: a first holding step of holding the degreased body in a non-oxidation atmosphere at 1,500 to 1,650°C for 2 to 10 hours; and a second holding step of holding degreased body in a non-oxidation atmosphere at 1,750 to 2,000°C for 5 to 30 hours. A thermal conductivity of the obtained silicon nitride sintered body is 80 W / (m K) or over, a mean value of a solid solution oxygen content is 0.2 wt% or under, a mean value of a major axis of a silicon nitride crystal grain is 1 to 10 μm, an average of an aspect ratio of the silicon nitride crystal grain is 2 to 10, and a content of grain boundary phases is 1 to 20 mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments described below relate to a method for producing a highly thermally conductive silicon nitride sintered body. [Background technology]

[0002] In recent years, attempts have been made to use silicon nitride (Si3N4) substrates as semiconductor circuit substrates. Alumina (Al2O3) substrates and aluminum nitride (AlN) substrates are used for semiconductor circuit substrates. Alumina substrates have a thermal conductivity of about 30 W / (m·K), but can be manufactured at low cost. Aluminum nitride substrates can be made to have a high thermal conductivity of 160 W / (m·K) or more. Meanwhile, silicon nitride substrates with a thermal conductivity of 50 W / (m·K) or more have been developed. The thermal conductivity of silicon nitride substrates is lower than that of aluminum nitride substrates. However, the three-point bending strength of silicon nitride substrates is excellent, at over 500 MPa. The three-point bending strength of aluminum nitride substrates is usually around 300 to 400 MPa, and the strength tends to decrease as the thermal conductivity increases. By taking advantage of the high strength, silicon nitride substrates can be made thinner. Thinning the substrate makes it possible to reduce thermal resistance, improving heat dissipation. For example, Japanese Patent No. 6293772 (Patent Document 1) discloses a silicon nitride substrate having a thermal conductivity of 50 W / (m·K) or more and a three-point bending strength of 600 MPa or more. In Patent Document 1, the distribution ratio of the grain boundary phase in the thickness direction of the substrate is controlled. As a result, Patent Document 1 suppresses variations in dielectric strength and improves the temperature dependence of volume resistivity. Furthermore, Patent Document 1 also controls the relative permittivity at 50 Hz and 1 kHz. In recent years, the guaranteed operating temperature of semiconductor elements has become higher as their performance improves. The guaranteed operating temperature of SiC and GaN elements is expected to reach approximately 250°C. Furthermore, the operating frequency of semiconductor elements is expected to increase to approximately 1 MHz. Therefore, silicon nitride substrates are also required to maintain their insulation properties even in high-temperature environments of approximately 250°C or high-frequency environments of approximately 1 MHz. Although the silicon nitride substrate described in Patent Document 1 has good insulating properties, further improvements in performance have been demanded in recent years. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6293772 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-24548 [Patent Document 3] Japanese Patent Publication No. 2022-71426 Summary of the Invention [Problem to be solved by the invention]

[0004] When the cause of this was investigated, it was found that it was influenced by the amount of dissolved oxygen within the silicon nitride crystal grains. Dissolved oxygen occurs when part of the crystal lattice of the silicon nitride crystal is replaced by oxygen element, or when oxygen penetrates between the crystal lattices. In other words, dissolved oxygen is oxygen that is taken up within the silicon nitride crystal grains. Dissolved oxygen is distinct from oxygen present in the grain boundary phase of silicon nitride sintered bodies. Furthermore, because dissolved oxygen causes defects in the crystal lattice, it was necessary to control the amount of dissolved oxygen. For example, Japanese Patent Laid-Open Publication No. 2018-24548 (Patent Document 2) discloses a silicon nitride sintered body in which the dissolved oxygen concentration of silicon nitride crystal grains is 1 to 2500 ppm. Patent Document 2 uses secondary ion mass spectrometry (SIMS) to measure the amount of dissolved oxygen. Patent Document 2 uses a raster region of 3 μm. The method of Patent Document 2 can measure the amount of dissolved oxygen in silicon nitride crystal grains of 3 μm or larger. However, it cannot measure the amount of dissolved oxygen in silicon nitride crystal grains of 3 μm or smaller in a silicon nitride sintered body. Furthermore, because SIMS is a surface analysis method, it is susceptible to oxidation of the sample surface. For this reason, it cannot be said that the amount of dissolved oxygen in the silicon nitride sintered body was necessarily controlled sufficiently. The present invention is intended to address these problems and to provide a highly thermally conductive silicon nitride sintered body in which the amount of dissolved oxygen is controlled. [Means for solving the problem]

[0005] The method for producing a high thermal conductive silicon nitride sintered body according to the embodiment comprises: a wet-crushing and mixing step of wet-crushing and mixing, in an organic solvent, a raw material powder containing silicon nitride powder having an average particle size of 2.5 μm or less and an impurity oxygen content of 2 mass % or less, and a sintering aid having an average particle size of 3 μm or less; a molding step of forming a green body using the raw material powder slurry obtained by the wet-crushing and mixing step; a degreasing step of degreasing the obtained green body at a temperature of 400°C to 800°C to obtain a degreased body; and a sintering step of sintering the obtained degreased body to obtain a silicon nitride sintered body, the sintering step comprising: a first holding step of holding the green body at a temperature of 1500°C to 1650°C in a non-oxidizing atmosphere for 2 hours to 10 hours; and a second holding step of holding the green body at a temperature of 1750°C to 2000°C in a non-oxidizing atmosphere for 5 hours to 30 hours. The obtained silicon nitride sintered body is a high thermal conductivity silicon nitride sintered body containing silicon nitride crystal grains and a grain boundary phase, and the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average amount of dissolved oxygen of the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in any cross section of the silicon nitride sintered body is 0.2 wt% or less, the average value of the major axis of the silicon nitride crystal grains present in a unit area of ​​50 μm × 50 μm in any cross section of the silicon nitride sintered body is 1 μm or more and 10 μm or less, the average aspect ratio of the silicon nitride crystal grains present in the unit area of ​​50 μm × 50 μm is 2 or more and 10 or less, and the content of the grain boundary phase is 1 mass% or more and 20 mass% or less. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a schematic diagram showing an example of a cross-sectional structure of a silicon nitride sintered body according to an embodiment. FIG. [Figure 2] FIG. 10 is a diagram showing an example of a first plot diagram. [Figure 3] FIG. 10 is a diagram showing an example of a second plot diagram. [Figure 4] FIG. 10 is a diagram showing an example of a third plot diagram. [Figure 5] 1 is a schematic view showing an example of a silicon nitride circuit substrate according to an embodiment. [Figure 6] 1 is a schematic diagram showing an example of a semiconductor device according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0007] The method for producing a high thermal conductive silicon nitride sintered body according to the embodiment comprises: a wet-crushing and mixing step of wet-crushing and mixing, in an organic solvent, a raw material powder containing silicon nitride powder having an average particle size of 2.5 μm or less and an impurity oxygen content of 2 mass % or less, and a sintering aid having an average particle size of 3 μm or less; a molding step of forming a green body using the raw material powder slurry obtained by the wet-crushing and mixing step; a degreasing step of degreasing the obtained green body at a temperature of 400°C to 800°C to obtain a degreased body; and a sintering step of sintering the obtained degreased body to obtain a silicon nitride sintered body, the sintering step comprising: a first holding step of holding the green body at a temperature of 1500°C to 1650°C in a non-oxidizing atmosphere for 2 hours to 10 hours; and a second holding step of holding the green body at a temperature of 1750°C to 2000°C in a non-oxidizing atmosphere for 5 hours to 30 hours. The obtained silicon nitride sintered body is a high thermal conductivity silicon nitride sintered body containing silicon nitride crystal grains and a grain boundary phase, and the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average amount of dissolved oxygen of the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in any cross section of the silicon nitride sintered body is 0.2 wt% or less, the average value of the major axis of the silicon nitride crystal grains present in a unit area of ​​50 μm × 50 μm in any cross section of the silicon nitride sintered body is 1 μm or more and 10 μm or less, the average aspect ratio of the silicon nitride crystal grains present in the unit area of ​​50 μm × 50 μm is 2 or more and 10 or less, and the content of the grain boundary phase is 1 mass% or more and 20 mass% or less. 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 Fig. 1, reference numeral 1 denotes a high thermal conductive silicon nitride sintered body, reference numeral 2 denotes silicon nitride crystal grains, and reference numeral 3 denotes a grain boundary phase. The high thermal conductive silicon nitride sintered body 1 may also be simply referred to as silicon nitride sintered body 1. The silicon nitride sintered body 1 comprises silicon nitride crystal grains 2 and a grain boundary phase 3. The grain boundary phase 3 is distributed in the gaps between the silicon nitride crystal grains 2. The grain boundary phase 3 is formed by the reaction of a sintering aid, which will be described later. The presence of the grain boundary phase 3 firmly bonds the silicon nitride crystal grains 2 together, making it possible to form a silicon nitride sintered body 1 with high thermal conductivity. The silicon nitride sintered body 1 may also have pores (not shown). The thermal conductivity of the silicon nitride sintered body 1 according to the embodiment is 80 W / (m·K) or more. High thermal conductivity improves heat dissipation. For this reason, the thermal conductivity is preferably 80 W / (m·K) or more, and more preferably 100 W / (m·K) or more.

[0008] The silicon nitride crystal particles 2 have an average major axis length of 1 μm or more and 10 μm or less, and an average aspect ratio of 2 or more and 10 or less. Scanning electron microscope (SEM) photographs are used to measure the average length of the major axis and the average aspect ratio. SEM photographs are taken at 2000x magnification. The maximum diameter of each silicon nitride crystal particle 2 in the SEM photograph is measured. The maximum diameter of each silicon nitride crystal particle 2 in the SEM photograph is taken as the major axis. The average value of the maximum diameters of each silicon nitride crystal particle 2 in a unit area of ​​50 μm × 50 μm is taken as the average length of the major axis.

[0009] The aspect ratio is calculated by dividing the major axis by the minor axis. The major axis is the maximum diameter mentioned above. The minor axis is the length of the silicon nitride crystal particle along a line extending perpendicularly from the center point of the major axis. The average aspect ratio is the average of the aspect ratios of the individual silicon nitride crystal particles photographed in a unit area of ​​50 μm × 50 μm. The major and minor axes are measured using the portion of the silicon nitride crystal particle visible in the SEM photograph. For example, for silicon nitride crystal particles whose outlines are not fully visible because they overlap with other silicon nitride crystal particles, the major and minor axes are measured using only the visible portion (the portion visible in the SEM photograph). Furthermore, for silicon nitride crystal particles whose outlines are discontinued at the edges of a unit area of ​​50 μm × 50 μm, the major and minor axes are measured using only the visible portion (the portion visible in the SEM photograph). If the outlines of the silicon nitride crystal particles are difficult to see, the grain boundary phase may be removed by etching.

[0010] The silicon nitride crystal particles 2 have an average major axis length of 1 μm or more and 10 μm or less. The silicon nitride crystal particles 2 also have an average aspect ratio of 2 or more and 10 or less. Within these ranges, the thermal conductivity can be improved. Strength can also be improved. If the average length of the major axis is less than 1 μm, the silicon nitride crystal particles 2 may be too small, resulting in a decrease in thermal conductivity. If the average length exceeds 10 μm, the thermal conductivity may improve, but the strength may decrease. If the average aspect ratio is less than 2, the strength may decrease due to the small number of elongated silicon nitride crystal particles. If the average aspect ratio exceeds 10, the gaps between the silicon nitride crystal particles may become large. The larger the gaps between the silicon nitride crystal particles, the larger the grain boundary phase. A large grain boundary phase may cause a decrease in thermal conductivity. As shown in Figure 1, it is preferable that silicon nitride crystal particles 2a (first silicon nitride crystal particles) with a major axis of less than 5 µm and silicon nitride crystal particles 2b (second silicon nitride crystal particles) with a major axis of 5 µm or more are present. The presence of small and large crystal particles allows the small crystal particles to exist in the gaps between the large crystal particles. This improves thermal conductivity and strength. From this perspective, it is more preferable that silicon nitride crystal particles 2b with a major axis of less than 3 µm are present.

[0011] In the silicon nitride sintered body 1, the amount of dissolved oxygen in the silicon nitride crystal grains 2 present in a unit area of ​​20 μm×20 μm in any cross section is 0.2 wt % or less. The amount of dissolved oxygen in silicon nitride crystal particles 2 is measured using TEM-EDS. TEM is an abbreviation for transmission electron microscope. EDS is an abbreviation for energy dispersive X-ray spectrometer. A measurement method using TEM-EDS is sometimes simply called EDS analysis. A method for measuring the amount of dissolved oxygen in silicon nitride crystal particles 2 using EDS analysis is disclosed in Japanese Patent Application No. 2020-180386. An arbitrary cross section of the silicon nitride sintered body 1 is used as a sample for EDS analysis. A sample is taken from the arbitrary cross section by focused ion beam (FIB) processing or ion milling processing. The thickness of the sample is preferably in the range of 0.05 μm to 0.5 μm. To prevent surface oxidation of the sample, it is desirable to prepare and store the sample in a vacuum or in an inert gas atmosphere. The EDS device used is a JEOL JED-2300T or a device with equivalent or better performance. The TEM used is a JEOL JEM-200CX (accelerating voltage 200 kV) or a device with equivalent or better performance. The recommended conditions for EDS analysis are an accelerating voltage of 200 kV, a probe current of 1.00 nA, and a spot diameter of 1 nm during analysis. The recommended analysis time is 30 seconds, with a sample tilt angle of X = 10°, Y = 0°. While the measurement conditions may be changed, the first plot diagram described below is measured under the recommended conditions.

[0012] By using TEM-EDS, silicon nitride crystal grains can be selected as measurement points. Measurements using the SIMS method, such as those in Patent Document 2, can only measure large silicon nitride crystal grains. Even with the nano-SIMS method, which reduces the irradiation diameter, it is difficult to image silicon nitride crystal grains. For this reason, the amount of dissolved oxygen in small silicon nitride crystal grains cannot be measured. There is also a total dissolution method for measuring the amount of dissolved oxygen. This method dissolves the grain boundary phase of a silicon nitride sintered body and extracts the silicon nitride crystal grains. The oxygen content of the extracted silicon nitride crystal grains is then measured. However, dissolving and removing the grain boundary phase is difficult, and the residual grain boundary phase can result in reduced measurement accuracy and poor reproducibility. By setting the measurement spot diameter at 1 nm, TEM-EDS can measure only silicon nitride crystal grains, and the amount of dissolved oxygen can be measured regardless of the size of the silicon nitride crystal grains.

[0013] First, in EDS analysis, at least 10 measurement points are set for silicon nitride crystal particles within a unit area of ​​20 μm × 20 μm. These 10 measurement points are selected from silicon nitride crystal particles 2 that are as different as possible. For example, one measurement point is set for each of 10 silicon nitride crystal particles. The atomic ratios of silicon (Si), oxygen (O), and nitrogen (N) are measured using EDS analysis. Ten or more measurement points are set so that there are at least three measurement points where the Si count is 300,000 cps or higher. If there are fewer than three measurement points where the Si count is 300,000 cps or higher, the number of measurement points is increased until there are at least three measurement points where the Si count is 300,000 cps or higher. A Si count of 300,000 cps or higher indicates that the oxygen content can be measured without being affected by surface oxygen. Therefore, the amount of dissolved oxygen in silicon nitride crystal particles 2 can be measured even if the sample surface is naturally oxidized. A first plot diagram is created in which the atomic ratio of oxygen element / silicon element is plotted against the Si count number. In the first plot diagram, the horizontal axis shows the Si count number (cps) and the vertical axis shows the O / Si atomic ratio. FIG. 2 shows an example of the first plot diagram. FIG. 3 shows an example of the second plot diagram. FIG. 4 shows an example of the third plot diagram. FIGS. 2 to 4 are plot diagrams based on the measurement results of Example 3, which will be described later.

[0014] Next, a second plot is created in which the atomic ratio of nitrogen to silicon is plotted against the Si count number, with the Si count number (cps) on the horizontal axis and the N / Si atomic ratio on the vertical axis. Next, the oxygen / silicon atomic ratio in the first plot is corrected using the second plot. This is because oxygen (O), a light element, has a higher X-ray absorption rate than silicon (Si) in silicon nitride sintered bodies. The absorption characteristics of oxygen (O) are similar to those of nitrogen (N). Furthermore, since the main phase of silicon nitride sintered body 1 is Si3N4, the theoretical N / Si atomic ratio is 4 / 3. Therefore, the O / Si atomic ratio is corrected using approximate data for the Si and N atomic ratios. In this correction method, the O / Si atomic ratio in the first plot is corrected using the N / Si atomic ratio at each measurement point in the second plot. The correction is performed using the difference between the N / Si atomic ratio at each measurement point and the theoretical value of 4 / 3 (= 1.33). For example, if the N / Si atomic ratio is 0.70, the correction coefficient is 1.9 (= 1.33 / 0.70). The correction value is calculated by multiplying the O / Si atomic ratio by the correction coefficient. By this method, the O / Si atomic ratio in the first plot is corrected, and the plot obtained by correcting the first plot is designated as the third plot.

[0015] In the third plot, the horizontal axis shows the Si count number (cps) and the vertical axis shows the corrected O / Si atomic ratio. From the third plot, multiple combinations of three or more measurement points are extracted. For each combination of measurement points, an approximate line y = aX + b is calculated. In the approximate line y = aX + b, X is the horizontal axis, y is the vertical axis, a is the slope, and b is the point of contact with the vertical axis (y axis). From the multiple approximate lines obtained, -4 × 10 -8 ≦a≦4×10-8 The approximate line is extracted so that the measurement points used for the extracted approximate line are in a convergence region where the O / Si atomic ratio has little variation. The approximate line is created using the approximation function of spreadsheet software. An example of such spreadsheet software is Microsoft Excel.

[0016] The convergence region obtained from the third plot is the region where the influence of natural oxidation on the sample surface and grain boundary phases is minimized. When the sample surface is influenced by natural oxidation or grain boundary phases, the variation in the O / Si atomic ratio also increases. Therefore, the slope a does not fall within the aforementioned range. When the slope a of the approximation line is -4×10 -8 Over 4×10 -8 The fact that the O / Si atomic ratio is within the range below indicates that the variation in the O / Si atomic ratio is reduced. Because the variation in the O / Si atomic ratio is reduced, it can be seen that the effects of natural oxidation and grain boundary phases are sufficiently small. Therefore, the O / Si atomic ratio at a measurement point included in the convergence region indicates the amount of dissolved oxygen. In the convergence region, three measurement points are extracted in descending order of Si count number. The amount of dissolved oxygen is calculated using the average O / Si atomic ratio. The average O / Si atomic ratio of the three extracted measurement points is a value in which the effects of natural oxidation and grain boundary phase 3 are further reduced. Since the silicon nitride crystal grains 2 are Si3N4, the amount of dissolved oxygen (wt%) can be calculated by (3 / 7) × (average value of O / Si atomic ratio). This is a method of calculation based on the amount of oxygen corresponding to the amount of Si in the Si3N4 crystal grains. Also, the slope a of the approximation line of three or more points is -4 × 10 -8 Over 4×10 -8 The convergence region within the range below is determined from three or more measurement points where the Si count is 300,000 cps or higher. EDS analysis cannot selectively measure only points where the count is 300,000 cps or higher. For this reason, it is effective to measure 10 or more points using EDS analysis. The oxygen content obtained by measuring 10 or more points using EDS analysis and extracting measurement points where the Si count is 300,000 cps or higher and falls within the convergence region is the average amount of dissolved oxygen in silicon nitride crystal grains.

[0017] In the high thermal conductive silicon nitride sintered body according to the embodiment, the amount of dissolved oxygen measured by the above method is 0.2 wt% or less. The amount of dissolved oxygen measured by TEM-EDS is an average value corresponding to the number of measurement points. In other words, the average amount of dissolved oxygen in silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm is 0.2 wt% or less. Furthermore, the fact that the cross-sectional structure is arbitrary means that the average amount of dissolved oxygen in silicon nitride crystal grains present in any unit area of ​​20 μm × 20 μm is 0.2 wt% or less.

[0018] As described above, silicon nitride sintered bodies in which the amount of dissolved oxygen in the silicon nitride crystal grains is controlled can improve thermal conductivity. Furthermore, they can improve the dielectric constant characteristics. As will be described later, the frequency dependence and temperature dependence of the dielectric constant can be improved. Solute oxygen reduces the thermal conductivity of silicon nitride crystal grains. It also causes lattice defects. By controlling the amount of dissolved oxygen, the causes of lattice defects can be reduced. If the amount of dissolved oxygen exceeds 0.2 wt%, the thermal conductivity, dielectric constant, etc. decrease.

[0019] Furthermore, it is preferable that the dissolved oxygen content of each silicon nitride crystal grain within the unit area of ​​20 μm × 20 μm is within the range of 0.01 wt% to 0.2 wt%. As described above, the method using TEM-EDS can measure only silicon nitride crystal grains. Controlling the dissolved oxygen content of all silicon nitride crystal grains can further improve performance. To measure the dissolved oxygen content of each silicon nitride crystal grain, one or more measurement points are set for each silicon nitride crystal grain within the unit area of ​​20 μm × 20 μm. The analysis method is as described above. The analysis method is repeated until the measurement points set for each silicon nitride crystal grain are included in the convergence region and the dissolved oxygen content can be measured. Setting measurement points for each silicon nitride crystal grain and finding that the dissolved oxygen content is 0.01 wt% to 0.20 wt% indicates that the dissolved oxygen content of each silicon nitride crystal grain is controlled.

[0020] For the first silicon nitride crystal particles 2a with major axes less than 5 μm and the second silicon nitride crystal particles 2b with major axes of 5 μm or more present within the unit area of ​​20 μm × 20 μm, the difference between the dissolved oxygen content of the first silicon nitride crystal particles 2a and the dissolved oxygen content of the second silicon nitride crystal particles 2b is preferably 0.03 wt% or less. The dissolved oxygen content measured by setting measurement points only on the first silicon nitride crystal particles 2a within a unit area of ​​20 μm × 20 μm is defined as "dissolved oxygen content A." The dissolved oxygen content measured by setting measurement points only on the second silicon nitride crystal particles 2b is defined as "dissolved oxygen content B." It is preferable that |dissolved oxygen content A - dissolved oxygen content B| ≦ 0.03 wt%. As mentioned above, the presence of both small and large silicon nitride crystal grains improves thermal conductivity and strength. Regardless of grain size, by suppressing variations in the amount of dissolved oxygen, thermal conductivity can be increased to 100 W / (m K) or higher, or even 120 W / (m K) or higher.

[0021] Furthermore, by reducing the difference in the amount of dissolved oxygen between small and large particles, the dielectric constant can be further stabilized, leading to improved electrical properties. The dielectric constant is the capacitance of a capacitor when the medium between the electrodes is filled, divided by the capacitance when the capacitor is in a vacuum. In this embodiment, the medium is a silicon nitride sintered body. Silicon nitride sintered bodies are both insulators and dielectrics, and polarization occurs in silicon nitride sintered bodies under an electric field. The greater the polarization, the greater the dielectric constant. It has been found that the amount of dissolved oxygen affects the frequency and temperature dependence of the dielectric constant. An increase in the dielectric constant increases the likelihood of polarization, leading to a decrease in insulation. Dissolved oxygen causes lattice defects. Controlling the amount of dissolved oxygen and improving the frequency dependence of the dielectric constant leads to improved insulation. Previously, only the amount of dissolved oxygen in limited silicon nitride crystal particles could be measured. This made it impossible to control the amount of dissolved oxygen.

[0022] The content of the grain boundary phase 3 in the silicon nitride sintered body 1 is preferably 1% by mass or more and 20% by mass or less. The grain boundary phase 3 is formed by reactions between sintering aids and between sintering aids and impurity oxygen on the surface of the silicon nitride powder. The grain boundary phase 3 also has the effect of firmly bonding the silicon nitride crystal grains 2 together and suppressing the generation of pores. Controlling the amount of the grain boundary phase 3 can improve properties dependent on thermal conductivity, strength, and dielectric constant. When the grain boundary phase is less than 1% by mass, the proportion of the grain boundary phase is low. When the grain boundary phase 3 is low, pores are likely to occur. When the grain boundary phase 3 exceeds 20% by mass, pore generation can be suppressed, but thermal conductivity is likely to decrease. Therefore, the content of the grain boundary phase 3 is preferably 1% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 15% by mass or less. Furthermore, by setting the porosity to 2% or less and the pore size to 20 μm or less, the strength can be increased to 500 MPa or more, and even 600 MPa or more. The porosity and pore size can be measured using the aforementioned SEM photographs with a unit area of ​​50 μm × 50 μm. The amount (mass%) of the grain boundary phase in the high thermal conductive silicon nitride sintered body 1 can be determined by qualitative and quantitative analysis of components other than silicon nitride. Furthermore, when the amount of sintering aid added can be determined, the amount of sintering aid added can be considered as the mass% of the grain boundary phase. The grain boundary phase 3 preferably contains one or more elements selected from the group 2A elements, the group 3A elements, and the group 4A elements. The grain boundary phase 3 preferably further contains one or more elements selected from rare earth elements, magnesium, titanium, and hafnium. The rare earth elements include yttrium (Y) and lanthanoid elements. The rare earth elements include one or more elements selected from yttrium (Y), erbium (Er), ytterbium (Yb), and cerium (Ce). Furthermore, the grain boundary phase 3 contains both a rare earth element and magnesium, thereby improving sinterability. More preferably, the grain boundary phase 3 contains one or both of titanium and hafnium in addition to a rare earth element and magnesium. Titanium or hafnium has the effect of strengthening the grain boundary phase 3. Strengthening the grain boundary phase 3 leads to improved strength. The notations for Groups 2A, 3A, and 4A are based on the Japanese Periodic Table. Group 2A elements are Be, Mg, Ca, Sr, Ba, and Ra. Group 3A elements are Sc, Y, the lanthanides, and the actinides. Group 4A elements are Ti, Zr, and Hf.

[0023] With the silicon nitride sintered body 1 described above, the dielectric constant at 50 Hz can be set to 10 or less at room temperature. Room temperature refers to 25°C. The dielectric constant is determined by measuring capacitance. Capacitance is the charge induced and accumulated at an electrode in an AC electric field. If the insulation is poor, the charge increases and the capacitance also increases. As the capacitance increases, the dielectric constant also increases. In other words, if the insulation is good, the dielectric constant decreases. The relative permittivity at 50 Hz is ε 50-25 The relative dielectric constant at 50 Hz at 300°C is ε 50-300 Then, ε 50-300 / ε 50-25 is preferably in the range of 0.9 or more and 1.2 or less. The relative permittivity at 1 MHz at room temperature is ε 1M-25 The relative dielectric constant at 300°C and 1 MHz is ε 1M-300 Then, ε 1M-300 / ε 1M-25 is preferably in the range of 0.9 or more and 1.2 or less. Also, ε1M-300 / ε 50-300 is preferably in the range of 0.8 or more and 1.2 or less. The dielectric constant is measured in accordance with JIS-C-2141 (1992). It is measured using the complex dielectric constant measurement method (three-terminal method) of JIS-C-2141. JIS-C-2141 corresponds to ISO672-2.

[0024] The silicon nitride sintered body 1 according to the embodiment has a relative dielectric constant ε 50-25 can be set to 10 or less. 50-25 The fact that the value of ρ is 10 or less indicates that polarization of the silicon nitride sintered body 1 can be suppressed under these conditions. This indicates that the insulating properties of the silicon nitride sintered body 1 are high. This makes it possible to improve the reliability of a semiconductor device in which a semiconductor element is mounted on the silicon nitride sintered body 1. Also, ε 50-300 / ε 50-25 The fact that the dielectric constant is in the range of 0.9 to 1.2 indicates that the relative dielectric constant at 50 Hz changes little even when the temperature changes. In other words, this indicates that the relative dielectric constant at 50 Hz has little temperature dependency. Therefore, even if the guaranteed operating temperature of the semiconductor element increases, the characteristics that depend on the relative dielectric constant are unlikely to deteriorate. Also, ε 1M-300 / ε 1M-25 The fact that the dielectric constant is in the range of 0.9 to 1.2 indicates that the relative dielectric constant at 1 MHz changes little even when the temperature changes. In other words, this indicates that the relative dielectric constant at 1 MHz has little temperature dependency. Therefore, even if the guaranteed operating temperature of the semiconductor element increases, the characteristics that depend on the relative dielectric constant are unlikely to deteriorate. Also, ε 1M-300 / ε 50-300 The fact that the dielectric constant is between 0.8 and 1.2 indicates that the change in the dielectric constant is small even when the frequency changes. In other words, even if the operating frequency of the semiconductor device changes, the characteristics that depend on the dielectric constant do not deteriorate. Also, ε 50-300 / ε 50-25 , ε 1M-300 / ε 1M-25 , and ε1M-300 / ε 50-300 However, it is preferable that both are within the above range.

[0025] The operating frequencies of semiconductor elements vary widely, from several tens of Hz to 1 MHz. Semiconductor elements include bipolar transistors, MOSFETs, IGBTs, and GTOs. These semiconductor elements are called power semiconductors. Power semiconductors can switch on and off depending on the operating frequency. Each semiconductor element has a different operating frequency range. By improving the temperature and frequency dependence of the dielectric constant of sintered silicon nitride, insulation can be easily maintained regardless of the type of semiconductor element mounted. This makes it possible to provide highly reliable semiconductor devices.

[0026] Therefore, the silicon nitride sintered body according to the embodiment is suitable for use as a silicon nitride substrate. The thickness of the substrate is preferably 0.1 mm or more and 3 mm or less. Even if the thickness of the silicon nitride substrate is as thin as 0.1 mm or more and 3 mm or less, the dielectric constant is improved, resulting in high reliability. The substrate thickness is more preferably within the range of 0.1 mm or more and 0.4 mm or less. Thinning the substrate has the effect of reducing thermal resistance, thereby improving heat dissipation.

[0027] The invention according to the embodiment is suitable for a silicon nitride circuit board in which a circuit portion is provided on a silicon nitride substrate. Fig. 5 is a schematic diagram showing an example of a silicon nitride circuit board according to the embodiment. In Fig. 5, reference numeral 4 denotes a bonding layer, reference numeral 5 denotes a metal plate (front metal plate), reference numeral 6 denotes a metal plate (rear metal plate), reference numeral 10 denotes a silicon nitride substrate, and reference numeral 20 denotes a silicon nitride circuit board. The metal plate 5 has a circuit shape. The metal plate 5 is used as a circuit portion for mounting a semiconductor element. The metal plate 5 used as a circuit portion is sometimes called a front metal plate. The metal plate 6 is used as a heat sink. The metal plate 6 used as a heat sink is sometimes called a back metal plate. In the example shown in FIG. 5, two front metal plates 5 are provided. The number of front metal plates 5 is not limited to the example shown, and is arbitrary. The back metal plate 6 may be used as a circuit portion instead of a heat sink. Examples of the metal plate 5 and the metal plate 6 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. Oxygen-free copper has a copper purity of 99.96 wt% or higher, as specified in JIS-H-3100 (e.g., ISO 1337). The thermal conductivity of copper is approximately 400 W / (m·K). The thermal conductivity of aluminum is approximately 240 W / (m·K). Copper plates have a higher thermal conductivity than aluminum plates. Therefore, using copper plates can further improve heat dissipation. The thickness of the metal plates 5 and 6 is preferably within a range of 0.2 mm to 5 mm. Increasing the thickness of the metal plates can improve heat dissipation and current-carrying capacity. The bonding layer 4 may be an active metal bonding layer. When the metal plate is a copper plate, the active metal bonding layer may be a material containing Ti and primarily Ag or Cu. When the metal plate is an aluminum plate, the active metal bonding layer may be a material containing Si and primarily Al. The active metal bonding layer is a layer containing Ti or Si as an active metal. Although the circuit section is an example of bonding metal plates, the invention according to the embodiment is not limited to this configuration. The circuit section may be formed of a thin metal film or a thick metal film. The thin metal film is a metal film formed by a film formation method such as sputtering or vapor deposition. Examples of thin metal films include films of Ti, Pt, Au, Ni, Cu, Al, or Ag. The thick metal film is a film formed by firing a metal paste. The thick metal film is sometimes called a metallized film. Examples of thick metal films include films of Ag, Cu, Ti, W, or Mo.

[0028] FIG. 6 is a schematic diagram showing an example of a semiconductor device according to an embodiment. In FIG. 6, reference numeral 30 denotes a semiconductor device, reference numeral 31 denotes a semiconductor element, and reference numeral 32 denotes a lead frame. In the semiconductor device 30 shown in FIG. 6, the semiconductor element 31 is mounted on one of two metal plates 5. The lead frame 32 is connected to the other of the two metal plates 5. The structure of the semiconductor device 30 according to the embodiment is not limited to the example shown in the figure. The number and size of the metal plates 5, the number and size of the semiconductor elements 31, etc. can be changed as needed. Furthermore, the metal plate 6 may be used as a circuit unit, and the semiconductor elements 31 may be mounted on the metal plate 6.

[0029] Next, a method for manufacturing the high thermal conductive silicon nitride sintered body 1 according to the embodiment will be described. The high thermal conductive silicon nitride sintered body 1 according to the embodiment can be manufactured by any method as long as it has the above-described configuration. Here, a method for obtaining the silicon nitride sintered body 1 with a high yield will be described.

[0030] First, silicon nitride powder is prepared. The silicon nitride powder preferably has an average particle size of 2.5 μm or less and an impurity oxygen content of 2 mass% or less. The impurity oxygen in silicon nitride powder includes components dissolved in the powder and components attached to the surface of the powder. The less impurity oxygen there is, the more the amount of oxygen dissolved in the silicon nitride particles of the silicon nitride sintered body can be reduced. For this reason, the impurity oxygen content of the silicon nitride powder is preferably 2 mass% or less, and more preferably 1 mass% or less.

[0031] Silicon nitride powders are primarily produced by the imide decomposition method or the direct nitridation method. The imide decomposition method is preferred because it minimizes the amount of dissolved oxygen impurities in the powder. Silicon nitride powders are available in α-type and β-type. β-type silicon nitride powder has a crystal structure that minimizes dissolved oxygen. On the other hand, α-type silicon nitride powder is easier to densify due to its higher sinterability than β-type silicon nitride. Using α-type silicon nitride as the raw powder can produce high-strength sintered silicon nitride. A mixture of α-type silicon nitride powder and β-type silicon nitride powder can also be used. Furthermore, the amount of β-type silicon nitride is preferably between 1 and 30 parts by mass, based on a total of 100 parts by mass of α-type silicon nitride powder and β-type silicon nitride powder. More than 30 parts by mass of β-type silicon nitride may result in reduced sinterability. Less than 1 part by mass of β-type silicon nitride may result in insufficient benefits. Commercially available α-type silicon nitride powder has an α-type content of 90% or more by mass. In other words, commercially available α-type silicon nitride powder inevitably contains a small amount of β-type silicon nitride powder. A material made by mixing α-type silicon nitride powder and β-type silicon nitride powder refers to a material in which β-type silicon nitride powder has been intentionally added to commercially available α-type silicon nitride powder. In other words, a material made only of α-type silicon nitride powder that inevitably contains β-type silicon nitride powder is not considered to be a material made by mixing α-type silicon nitride powder and β-type silicon nitride powder.

[0032] It is also effective to subject the silicon nitride powder to a treatment to reduce the impurity oxygen. Examples of treatments to reduce impurity oxygen include reduction treatment and chemical treatment. Examples of reduction treatment include heat treatment in a hydrogen atmosphere and heat treatment in an atmosphere containing carbon. Chemical treatment is a method of treating the silicon nitride powder with an acidic or alkaline solution.

[0033] Next, a sintering aid is prepared. The sintering aid is a component that promotes sintering and becomes a grain boundary phase. The sintering aid is preferably one or more elements selected from Group 2A elements, Group 3A elements, and Group 4A elements. The notation of Group 2A, Group 3A, and Group 4A is based on the Japanese Periodic Table. The sintering aid preferably further contains one or more elements selected from rare earth elements, magnesium, titanium, and hafnium. Titanium exists in the grain boundary phase as titanium nitride (TiN) particles. The titanium nitride particles have a pinning effect, which strengthens the grain boundary phase. The sintering aid is preferably added as an oxide powder. The oxides react with each other to form a stable grain boundary phase. More preferably, the sintering aid is a metal oxide powder with an average particle size of 3 μm or less.

[0034] Next, a process of mixing silicon nitride powder and sintering aid powder is carried out. To control the long diameter and aspect ratio of silicon nitride crystal grains in a silicon nitride sintered body, uniform sinterability is required. To achieve this, it is necessary to uniformly mix the silicon nitride powder and sintering aid powder. During the sintering process, the sintering aid reacts to form a grain boundary phase. The growth reaction of silicon nitride particles progresses through this grain boundary phase. During this process, oxygen escapes from the silicon nitride particles. By uniformly mixing the silicon nitride powder and sintering aid powder, the reaction mediated by the grain boundary phase can be homogenized. In the mixing step, a ball mill or a bead mill is used. Silicon nitride powder and sintering aid powder often exist as agglomerated secondary particles. Secondary particles hinder uniform sintering. By uniformly mixing the secondary particles while crushing them into unagglomerated primary particles, uniform sintering can be improved. During this mixing process, which involves crushing, it is preferable not to apply strong stress that would further destroy the primary particles. When the primary particles are destroyed, a fracture surface is formed in the silicon nitride powder. Because the fracture surface is an active surface, an oxide film forms on it for stabilization, increasing the amount of oxygen attached. The amount of oxygen increases beyond that attached to the primary particles. Therefore, it is effective to prevent the formation of fracture surfaces on the primary particles. Wet crushing using a solvent is suitable for crushing such secondary particles. A solvent with high wettability and low reactivity with the particle surface is used. This reduces the stress required for crushing and prevents the destruction of primary particles. Organic solvents are suitable for mixing silicon nitride powder and sintering aid powder with crushing. Alcohols and ketones are suitable organic solvents. A mixture of alcohols and ketones can also be used. Alcohols are a general term for hydrocarbons in which some of the hydrogen atoms have been replaced with hydroxyl groups (OH groups). Ketones are represented by the formula RC(=O)-R', where R and R' are alkyl groups, for example. These organic solvents have high wettability and low reactivity with silicon nitride powder and sintering aid powder. Sintering aid powder is often added as an oxide powder. For raw material compositions containing a mixture of silicon nitride powder and oxide powder, a mixture of alcohols and ketones is preferable. A dispersant may also be added if necessary. Dispersants have the effect of stabilizing primary particles in a solvent and suppressing re-aggregation. Examples of dispersants include surfactants.

[0035] When the crushing process is performed using a ball mill, the diameter of the media is preferably 20 mm or less, and even 12 mm or less. The media is ceramic balls. Ball milling is a method in which powder and media are placed in a cylindrical container and the cylindrical container is rotated to crush the powder. As mentioned above, the ball milling process using an organic solvent is a wet crushing and mixing process. Selecting an organic solvent suitable for the powder has the effect of reducing the stress required for crushing. In other words, crushing is possible using ceramic balls with a small media diameter, which has the effect of reducing the energy with which the media hits the powder. By reducing the energy with which the media hits the powder, the formation of fracture surfaces on the primary particles can be suppressed. The minimum diameter of the media is preferably 3 mm or more. If the media is too small, work efficiency may decrease. The time for wet disintegration and mixing using a ball mill is preferably within the range of 5 hours to 40 hours. If it is less than 5 hours, the disintegration effect may be insufficient, and a large amount of secondary particles may remain. If it is longer than 40 hours, there is a high possibility that fracture surfaces will form on the primary particles. For this reason, the time for wet disintegration and mixing using a ball mill is preferably within the range of 5 hours to 40 hours, and even more preferably within the range of 10 hours to 30 hours. Furthermore, the rotation speed of the cylindrical container in the ball mill process is preferably within the range of 50 rpm to 500 rpm. The fact that secondary particles have been crushed can be determined by examining the particle size distribution before and after crushing. As the secondary particles are crushed and the number of primary particles increases, the peak position of the particle size distribution (frequency distribution) shifts to the smaller particle size side. The peak of the particle size distribution also becomes sharper. The suppression of fracture surface formation of primary particles can be understood by measuring the amount of oxygen before and after crushing. The amount of oxygen before crushing is the amount of oxygen in the raw material powder. There is no problem as long as the amount of oxygen in the raw material powder after crushing does not increase significantly compared to before crushing.

[0036] A raw material powder slurry is obtained by carrying out the wet disintegration and mixing process. A molding process is carried out to form a compact using the raw material powder slurry. Examples of molding processes include sheet molding and mold molding. Sheet molding is performed using a doctor blade method, for example. When sheet molding is performed, a cutting process is carried out to cut a long sheet into the required size. The slurry may be processed into granulated powder suitable for the molding method before use. The compact is subjected to a degreasing process. The degreasing process is preferably carried out within a temperature range of 400°C to 800°C. By carrying out the degreasing process, organic matter in the compact can be removed. The compact that has undergone the degreasing process is called a degreased body.

[0037] Next, the degreased body is sintered in a sintering step, which preferably includes a first holding step in the range of 1500°C to 1650°C and a second holding step in the range of 1750°C to 2000°C. The first holding step is a step of holding the degreased body at a temperature ranging from 1500°C to 1650°C. The holding time is preferably 2 hours or longer. The temperature range of 1500°C to 1650°C corresponds to the temperature range at which silicon nitride grain growth begins. Furthermore, this temperature range also corresponds to the temperature at which impurity oxygen begins to desorb from the silicon nitride powder. The impurity oxygen desorbs as SiO (silicon monoxide). By performing the first holding step, the amount of dissolved oxygen in the silicon nitride powder can be efficiently reduced. Furthermore, the grain growth of the silicon nitride crystal grains can be homogenized. The holding time for the first holding step is preferably 10 hours or shorter. If the holding time exceeds 10 hours, there may be a shortage of oxygen necessary to form a liquid phase together with the sintering aid, resulting in a decrease in sinterability. Therefore, the holding time for the first holding step is preferably 2 hours to 10 hours, and more preferably 3 hours to 6 hours. Furthermore, this holding step is preferably performed under a reduced pressure atmosphere of atmospheric pressure (0.1 MPa) or lower. In a so-called pressurized atmosphere exceeding 0.1 MPa, the desorption of SiO tends to be suppressed. The second holding step is a step of holding the degreased body at a temperature between 1750°C and 2000°C. The holding temperature in the second holding step is the so-called sintering temperature. This temperature range allows the sintered body to be densified. Furthermore, the major axis and aspect ratio of the silicon nitride crystal particles can be controlled. Holding temperatures below 1750°C may result in insufficient densification. Temperatures above 2000°C may result in excessive grain growth of the silicon nitride crystal particles. The holding time for the second holding step is preferably between 5 hours and 30 hours. The second holding step is preferably performed in a pressurized atmosphere of 0.5 MPa or more. Under a pressure atmosphere of 0.1 MPa (atmospheric pressure) or less, silicon nitride is likely to self-decompose at temperatures above 1700°C, resulting in insufficient densification. The sintering step is preferably carried out in a non-oxidizing atmosphere, such as a nitrogen atmosphere, an argon atmosphere, or a vacuum atmosphere, which can prevent oxygen from being dissolved in the silicon nitride crystal grains.

[0038] A sintered body can be obtained through the sintering process. The sintered body may also be subjected to a reheat treatment. By including a step of holding the sintered body at a temperature between 1400°C and 1600°C in the reheat treatment, SiO desorption can be more effectively promoted. Because the sintered body has a stable crystal structure, the adverse effect of SiO desorption on the crystal structure due to reheat treatment is minimal. The holding time in the reheat treatment range between 1400°C and 1600°C is preferably 2 hours or longer. This holding step is also preferably performed under a reduced pressure atmosphere at atmospheric pressure (=0.1 MPa) or less. A pressurized atmosphere exceeding 0.1 MPa tends to suppress SiO desorption. The reheat treatment is preferably performed after returning the body to room temperature following the second holding step. Alternatively, after the second holding step, the temperature may be lowered to 1300°C or less and held there, and then the sintered body may be treated by heating it again. Below 1300°C, the liquid phase consisting of sintering additives and the like solidifies and no longer contributes to the sintering reaction. That is, the sintered body can be considered to be in a state in which it maintains a stable crystal structure, similar to the state when it is returned to room temperature, and therefore the adverse effect on the crystal structure caused by SiO elimination due to reheat treatment is small. Furthermore, the maximum holding temperature in the reheat treatment is lower than the holding temperature in the second holding step of sintering. The difference between the maximum holding temperature in the reheat treatment and the holding temperature in the second holding step is preferably 50°C or higher and 300°C or lower. For example, if the holding temperature in the second holding step is 1800°C, the reheat treatment temperature is preferably within the range of 1500°C or higher and 1750°C or lower. The reheat treatment not only eliminates oxygen from the sintered body as SiO, but also has the effect of suppressing variations in the amount of dissolved oxygen among the silicon nitride crystal grains. Furthermore, by lowering the maximum holding temperature in the reheat treatment below the holding temperature in the second holding step of sintering, grain growth of the silicon nitride crystal grains can be suppressed. The reheat treatment is preferably performed in a non-oxidizing atmosphere. By performing the reheat treatment in a non-oxidizing atmosphere, oxygen incorporation into the silicon nitride crystal grains can be suppressed.

[0039] The silicon nitride sintered body 1 according to the embodiment can be manufactured by the above steps. In addition, a silicon nitride substrate can be manufactured by producing a silicon nitride sintered body 1 in the shape of a substrate. A silicon nitride circuit board can be manufactured by providing a circuit part on the silicon nitride substrate. A semiconductor device can be manufactured by mounting a semiconductor element on the circuit part of the silicon nitride circuit board.

[0040] (Example) (Examples 1 to 7, Comparative Examples 1 to 3) The silicon nitride powders shown in Table 1 were prepared. The mixing ratio is the mass ratio when the total of α-type and β-type is 100 parts by mass. Furthermore, the α-type silicon nitride powder used had an α-phase ratio of 90 wt% or more.

[0041] [Table 1]

[0042] Next, the sintering aid powders shown in Table 2 were mixed in. The average particle size of the sintering aid powders was 3 μm or less. The mixing ratio of the sintering aids is the ratio when the total of the silicon nitride powder and the sintering aid powder is 100 mass %.

[0043] [Table 2]

[0044] Next, the silicon nitride powder and the sintering aid powder were mixed with an organic solvent to carry out a wet crushing and mixing process. The wet crushing and mixing process was carried out using a ball mill under the conditions shown in Table 3. The rotation speed of the cylindrical container of the ball mill was set within the range of 50 rpm to 500 rpm. In Comparative Example 2, wet mixing was carried out using water. In Comparative Example 3, dry mixing was carried out.

[0045] [Table 3]

[0046] A raw material powder slurry was prepared by a wet crushing and mixing process. The raw material powder slurry was then molded into sheets using the doctor blade method. The long sheets were cut into predetermined sizes. A degreasing process was then carried out to prepare degreased bodies. Each degreased body was subjected to a sintering process. The sintering process included a first holding process and a second holding process. After the sintering process, a third holding process (reheat treatment) was performed. The conditions for the sintering process are as shown in Tables 4 and 5. The first holding process, second holding process, and third holding process were performed in a non-oxidizing atmosphere.

[0047] [Table 4]

[0048] [Table 5]

[0049] The silicon nitride sintered bodies according to the examples and comparative examples were manufactured by the above sintering process. The silicon nitride sintered bodies were processed into silicon nitride substrates measuring 100 mm long x 80 mm wide. In examples 1 to 3, examples 6 to 7, and comparative examples 1 to 3, silicon nitride substrates with a thickness of 0.32 mm were manufactured. In examples 4 and 5, silicon nitride substrates with a thickness of 0.25 mm were manufactured. In the silicon nitride substrates according to the examples and comparative examples, the grain boundary phase content was within the range of 1% by mass or more and 20% by mass or less. The amount of dissolved oxygen in the silicon nitride crystal grains contained in the silicon nitride substrate, the average length of the major axis of the silicon nitride crystal grains, and the average aspect ratio were measured. The dissolved oxygen content of silicon nitride crystal particles was measured using TEM-EDS, with a unit area of ​​20 μm × 20 μm of any cross section set as a measurement point. The TEM-EDS measurement conditions were as described above. The dissolved oxygen content of each silicon nitride crystal particle within a unit area of ​​20 μm × 20 μm was measured, and the average value was calculated. The method for measuring the dissolved oxygen content was as described above. Figures 2 to 4 are first, second, and third plot diagrams, respectively, obtained when measuring Example 3. The range of dissolved oxygen content of each silicon nitride crystal particle, the average dissolved oxygen content of the first silicon nitride crystal particles, and the average dissolved oxygen content of the second silicon nitride crystal particles were also calculated. As described above, the first silicon nitride crystal particles are silicon nitride crystal particles with a major axis of less than 5 μm. The second silicon nitride crystal particles are silicon nitride crystal particles with a major axis of 5 μm or more. The average length of the major axis and the average aspect ratio of the silicon nitride crystal particles 2 were measured by using SEM photographs, with a unit area of ​​50 μm × 50 μm of any cross section set as the measurement point. The method using SEM photographs was as described above. The results of these measurements are shown in Tables 6 and 7.

[0050] [Table 6]

[0051] [Table 7]

[0052] As can be seen from Table 6, the silicon nitride substrates according to the examples had a dissolved oxygen content of 0.2 wt% or less. In contrast, the silicon nitride substrates according to the comparative examples had a dissolved oxygen content of more than 0.2 wt%. There were no significant differences between the examples and the comparative examples in the average major axis length and average aspect ratio of the silicon nitride crystal particles. In examples 1 to 7, silicon nitride crystal particles with major axes of less than 3 μm were present. Next, the thermal conductivity and fracture toughness of each silicon nitride substrate were measured. Thermal conductivity was measured using the laser flash method. Fracture toughness was measured in accordance with JIS-R-1607 (IF method) and calculated using Niihara's formula. The results are shown in Table 8. JIS-R-1607 corresponds to ISO15732.

[0053] [Table 8]

[0054] As can be seen from Table 8, the silicon nitride substrates according to the examples had thermal conductivities of 80 W / (m K) or more. Furthermore, the thermal conductivities of Examples 1, 2, and 4 were 100 W / (m K) or more. There was no significant difference in strength between the examples and the comparative examples. Next, the dielectric constants of the silicon nitride substrates according to the examples and comparative examples were measured. The dielectric constants were measured in accordance with the complex dielectric constant measurement method (three-terminal method) of JIS-C-2141. The measurement frequencies were changed to 50 Hz and 1 MHz, and the measurement temperatures were changed to room temperature (25°C) and 300°C. The results are shown in Tables 9 and 10.

[0055] [Table 9]

[0056] [Table 10]

[0057] As can be seen from Tables 9 and 10, the frequency dependence and temperature dependence of the dielectric constant are improved for the silicon nitride substrates according to the examples. In contrast, the frequency dependence and temperature dependence of the dielectric constant are reduced for the comparative examples. From the above, it can be seen that the silicon nitride substrate according to the embodiment is suitable as a substrate for mounting semiconductor elements with high guaranteed operating temperatures or semiconductor elements with high operating frequencies.

[0058] Embodiments of the present invention may include the following features. (Appendix 1) A highly thermally conductive silicon nitride sintered body comprising silicon nitride crystal grains and a grain boundary phase, The thermal conductivity of the silicon nitride sintered body is 80 W / (m K) or more, the average amount of dissolved oxygen in the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in any cross section is 0.2 wt % or less, the average value of the major axis of the silicon nitride crystal grains present in a unit area of ​​50 μm × 50 μm in any cross section is 1 μm or more and 10 μm or less; The highly thermally conductive silicon nitride sintered body has an average aspect ratio of the silicon nitride crystal grains present in the unit area of ​​50 μm×50 μm of 2 or more and 10 or less. (Appendix 2) 2. The highly thermally conductive silicon nitride sintered body according to claim 1, wherein the amount of dissolved oxygen in each silicon nitride crystal grain present within the unit area of ​​20 μm×20 μm is within the range of 0.01 wt % to 0.2 wt %. (Appendix 3) Within the unit area of ​​20 μm×20 μm, there are present first silicon nitride crystal grains with major axes of less than 5 μm and second silicon nitride crystal grains with major axes of 5 μm or more, 3. The high thermal conductive silicon nitride sintered body according to claim 1, wherein the difference between the amount of dissolved oxygen in the first silicon nitride crystal grains and the amount of dissolved oxygen in the second silicon nitride crystal grains is 0.03 wt % or less. (Appendix 4) 4. The highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 3, wherein the content of the grain boundary phase is 1% by mass or more and 20% by mass or less. (Appendix 5) 5. The highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 4, having a relative dielectric constant of 10 or less at 50 Hz and room temperature. (Appendix 6) 50Hz, relative permittivity at room temperature is ε 50-25 The relative dielectric constant at 50 Hz and 300°C is ε 50-300 Then, ε 50-300 / ε 50-25 6. The highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 5, wherein the value of σ is in the range of 0.9 or more and 1.2 or less. (Appendix 7) The relative permittivity at 1MHz and room temperature is ε 1M-25 Let the relative permittivity at 1MHz and 300℃ be ε 1M-300 Then, ε 1M-300 / ε 1M-25 7. The highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 6, wherein the value of σ is in the range of 0.9 or more and 1.2 or less. (Appendix 8) 8. The highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 7, wherein the thermal conductivity is 100 W / (m·K) or more. (Appendix 9) A silicon nitride substrate using the highly thermally conductive silicon nitride sintered body according to any one of claims 1 to 8. (Appendix 10) 10. The silicon nitride substrate according to claim 9, having a thickness of 0.1 mm or more and 3 mm or less. (Appendix 11) A silicon nitride substrate according to claim 9 or 10; a circuit portion provided on the silicon nitride substrate; A silicon nitride circuit board comprising: (Appendix 12) A silicon nitride circuit substrate according to claim 11; a semiconductor element mounted on the circuit section; A semiconductor device comprising:

[0059] 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. [Explanation of symbols]

[0060] REFERENCE SIGNS LIST 1...High thermal conductive silicon nitride sintered body, 2...Silicon nitride crystal grains, 3...Grain boundary phase, 4...Joining layer, 5...Metal plate (front metal plate), 6...Metal plate (back metal plate), 10...Silicon nitride substrate, 20...Silicon nitride circuit board, 30...Semiconductor device, 31...Semiconductor element, 32...Lead frame

Claims

1. a wet crushing and mixing step of wet crushing and mixing a raw material powder containing silicon nitride powder having an average particle size of 2.5 μm or less and an impurity oxygen content of 2 mass % or less and a sintering aid having an average particle size of 3 μm or less in an organic solvent; a molding step of forming a compact using the raw material powder slurry obtained by the wet crushing and mixing step; a degreasing step of degreasing the obtained compact at a temperature in the range of 400°C to 800°C to obtain a degreased body; A sintering step of sintering the obtained degreased body to obtain a silicon nitride sintered body, a first holding step of holding the material in a non-oxidizing atmosphere at a temperature in the range of 1500°C to 1650°C for 2 hours to 10 hours; a second holding step of holding the material in a non-oxidizing atmosphere at a temperature in the range of 1750°C to 2000°C for 5 hours to 30 hours; The sintering step includes: The obtained silicon nitride sintered body is a high thermal conductive silicon nitride sintered body containing silicon nitride crystal grains and a grain boundary phase, The thermal conductivity of the silicon nitride sintered body is 80 W / (m K) or more, the average amount of dissolved oxygen in the silicon nitride crystal grains present in a unit area of ​​20 μm × 20 μm in any cross section of the silicon nitride sintered body is 0.2 wt % or less; the average value of the major axis of the silicon nitride crystal grains present in a unit area of ​​50 μm × 50 μm in any cross section of the silicon nitride sintered body is 1 μm or more and 10 μm or less; the average aspect ratio of the silicon nitride crystal grains present in the unit area of ​​50 μm × 50 μm is 2 or more and 10 or less, A method for producing a highly thermally conductive silicon nitride sintered body, wherein the content of the grain boundary phase is 1% by mass or more and 20% by mass or less.

2. 2. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 1, wherein the peak position of the particle size distribution of the raw material powder is shifted to smaller particle sizes by carrying out the wet disintegration and mixing step.

3. 3. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 1, wherein the first holding step is carried out in an atmosphere of atmospheric pressure or a reduced pressure atmosphere lower than atmospheric pressure.

4. 3. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 1, wherein the second holding step is carried out in a pressurized atmosphere of 0.5 MPa or more.

5. 4. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 3, wherein the second holding step is carried out in a pressurized atmosphere of 0.5 MPa or more.

6. 3. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 1, further comprising the step of subjecting the obtained silicon nitride sintered body to a reheat treatment step in which the temperature is maintained within a range of 1400°C or higher and 1600°C or lower.

7. 6. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 5, further comprising the step of subjecting the obtained silicon nitride sintered body to a reheat treatment step of maintaining the temperature within a range of 1400°C to 1600°C.

8. 7. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 6, wherein the reheat treatment step is carried out under a reduced pressure atmosphere of atmospheric pressure or lower.

9. 8. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 7, wherein the reheat treatment step is carried out under a reduced pressure atmosphere of atmospheric pressure or lower.

10. 7. The method for producing a high thermal conductive silicon nitride sintered body according to claim 6, wherein the difference between the holding temperature in said re-heat treatment step and the holding temperature in said second holding step is within a range of 50°C or more and 300°C or less.

11. 10. The method for producing a high thermal conductive silicon nitride sintered body according to claim 9, wherein the difference between the holding temperature in the re-heat treatment step and the holding temperature in the second holding step is within a range of 50°C or more and 300°C or less.

12. 3. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 1, wherein the obtained silicon nitride sintered body is a silicon nitride substrate having a thickness of 0.1 mm to 3 mm.

13. 12. The method for producing a highly thermally conductive silicon nitride sintered body according to claim 11, wherein the obtained silicon nitride sintered body is a silicon nitride substrate having a thickness of 0.1 mm or more and 3 mm or less.

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