High thermal conductivity silicon nitride sintered body, silicon nitride substrate, silicon nitride circuit board, and semiconductor device

Abstract

The highly thermally conductive silicon nitride sintered compact according to an embodiment comprises silicon nitride crystal grains and a grain boundary phase. The silicon nitride sintered compact has a heat conductivity of 80 W / (m·K) or more. The average value of solute oxygen levels in the silicon nitride crystal grains present in a unit surface area of 20 μm × 20 μm in an arbitrary cross-sectional surface is 0.2 wt% or less. The average value of major axis diameters of the silicon nitride crystal grains present in a unit surface area of 50 μm × 50 μm in an arbitrary cross-sectional surface is 1 to 10 μm inclusive. The average value of aspect ratios of the silicon nitride crystal grains present in the unit surface area of 50 μm × 50 μm is 2 to 10 inclusive.

Description

Technical Field

[0001] The embodiments described below relate to a high thermal conductivity silicon nitride sintered body, a silicon nitride substrate, a silicon nitride circuit board, and a semiconductor device.

Background Art

[0002] In recent years, attempts have been made to apply a silicon nitride (Si3N4) substrate to a semiconductor circuit board. As the semiconductor circuit board, an alumina (Al2O3) substrate and an aluminum nitride (AlN) substrate are used. The alumina substrate has a thermal conductivity of about 30 W / (m·K) and can be manufactured at low cost. Further, the aluminum nitride substrate can achieve high thermal conductivity with a thermal conductivity of 160 W / (m·K) or more. On the other hand, as the silicon nitride substrate, a substrate having a thermal conductivity of 50 W / (m·K) or more has been developed. The thermal conductivity of the silicon nitride substrate is lower than that of the aluminum nitride substrate. However, the three-point bending strength of the silicon nitride substrate is 500 MPa or more, which is excellent. The three-point bending strength of the aluminum nitride substrate is usually about 300 to 400 MPa, and the strength tends to decrease as the thermal conductivity increases. By taking advantage of the high strength, the silicon nitride substrate can be thinned. Since the thermal resistance can be reduced by thinning the substrate, the heat dissipation performance is improved. 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. Thereby, in Patent Document 1, the variation in the insulation breakdown voltage is suppressed, and the temperature dependence of the volume resistivity value is improved. Further, in Patent Document 1, the relative dielectric constant at 50 Hz and 1 kHz is also controlled. In recent years, with the improvement of the performance of semiconductor devices, the guaranteed operating temperature has been increasing. In the case of SiC devices and GaN devices, it is expected that the guaranteed operating temperature will be as high as about 250°C. Also, it is expected that the operating frequency of semiconductor devices will increase up to about 1 MHz. Therefore, for silicon nitride substrates, it is required to maintain insulation even in a high-temperature environment of about 250°C or a high-frequency environment of about 1 MHz. Although the silicon nitride substrate described in Patent Document 1 has good insulation, further performance improvement has been required in recent years.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] When investigating the cause, it was found that it is affected by the amount of dissolved oxygen in silicon nitride crystal particles. Dissolved oxygen is generated when a part of the crystal lattice of silicon nitride crystals is replaced by oxygen elements or when oxygen penetrates between the crystal lattices. That is, dissolved oxygen is the oxygen taken into silicon nitride crystal particles. Dissolved oxygen is distinguished from the oxygen present in the grain boundary phase of the silicon nitride sintered body. Also, since dissolved oxygen causes defects in the crystal lattice, it was necessary to control the amount of dissolved oxygen. For example, Japanese Patent Application Laid-Open No. 2018-24548 (Patent Document 2) discloses a silicon nitride sintered body in which the solid solution oxygen concentration of silicon nitride crystal particles is 1 to 2500 ppm. In Patent Document 2, secondary ion mass spectrometry (SIMS) is used to measure the amount of solid solution oxygen. In Patent Document 2, the raster region is 3 μm. According to the method of Patent Document 2, the amount of solid solution oxygen in silicon nitride crystal particles of 3 μm or more can be measured. The amount of solid solution oxygen in silicon nitride crystal particles of 3 μm or less in the silicon nitride sintered body has not been measured. Further, since SIMS is a surface analysis method, it is a method that is easily oxidized on the sample surface. Therefore, it cannot be said that the control of the amount of solid solution oxygen in the silicon nitride sintered body was always sufficient. The present invention is for solving such problems, and for providing a high thermal conductivity silicon nitride sintered body with controlled solid solution oxygen amount.

Means for Solving the Problems

[0005] The high thermal conductivity silicon nitride sintered body according to the embodiment includes silicon nitride crystal particles and a grain boundary phase, the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, and the average value of the amount of solid solution oxygen of the silicon nitride crystal particles present in a unit area of 20 μm×20 μm in an arbitrary cross section is 0.2 wt% or less, the average value of the major axis of the silicon nitride crystal particles present in a unit area of 50 μm×50 μm in an arbitrary cross section is 1 μm or more and 10 μm or less, and the average aspect ratio of the silicon nitride crystal particles present in the unit area of 50 μm×50 μm is 2 or more and 10 or less.

Brief Description of the Drawings

[0006]

Figure 1

Figure 2

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Figure 6

Mode for Carrying Out the Invention

[0007] The high thermal conductivity silicon nitride sintered body according to the embodiment includes silicon nitride crystal particles and a grain boundary phase. The thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more. The average value of the amount of dissolved oxygen in the silicon nitride crystal particles 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 length of the silicon nitride crystal particles 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 average aspect ratio of the silicon nitride crystal particles present in the unit area of 50 μm × 50 μm is 2 or more and 10 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 is a high thermal conductivity silicon nitride sintered body, reference numeral 2 is a silicon nitride crystal particle, and reference numeral 3 is a grain boundary phase. The high thermal conductivity silicon nitride sintered body 1 may also be simply referred to as the silicon nitride sintered body 1. The silicon nitride sintered body 1 includes silicon nitride crystal particles 2 and a grain boundary phase 3. The grain boundary phase 3 is distributed in the gaps between the silicon nitride crystal particles 2. The grain boundary phase 3 is formed by the reaction of a sintering aid described later. The presence of the grain boundary phase 3 enables the silicon nitride crystal particles 2 to be firmly bonded to each other, forming a silicon nitride sintered body 1 with high thermal conductivity. Also, the silicon nitride sintered body 1 may 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. The high thermal conductivity improves the heat dissipation performance. Therefore, the thermal conductivity is preferably 80 W / (m·K) or more, and more preferably 100 W / (m·K) or more.

[0008] The average major axis length of the silicon nitride crystal particles 2 is 1 μm or more and 10 μm or less. The average aspect ratio of the silicon nitride crystal particles 2 is 2 or more and 10 or less. For the measurement of the average length of the major axis and the average aspect ratio, scanning electron microscope (SEM) photographs are used. The SEM photographs are taken at a magnification of 2000 times. Also, in the SEM photographs, the individual maximum diameters of the silicon nitride crystal particles 2 are measured. The maximum diameter of each individual silicon nitride crystal particle 2 shown in the SEM photograph is taken as the major axis. The average value of the maximum diameters of the individual silicon nitride crystal particles 2 shown 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 major axis / minor axis. The major axis is the aforementioned maximum diameter. The length of the silicon nitride crystal particle along the line extending vertically from the center point of the major axis is taken as the minor axis. The average value of the individual aspect ratios of the silicon nitride crystal particles shown in a unit area of 50 μm × 50 μm is taken as the average aspect ratio. Note that the major axis and the minor axis are measured using the portion of the silicon nitride crystal particle shown in the SEM photograph. For example, for a silicon nitride crystal particle that overlaps with other silicon nitride crystal particles and whose entire contour cannot be seen, only the visible portion (the portion shown in the SEM photograph) is used to measure the major axis and the minor axis. Also, for a silicon nitride crystal particle whose contour is interrupted at the edge of the SEM photograph of a unit area of 50 μm × 50 μm, only the visible portion (the portion shown in the SEM photograph) is used to measure the major axis and the minor axis. When the contour of the silicon nitride crystal particle is difficult to confirm, the grain boundary phase may be removed by etching.

[0010] The average length of the major axis of the silicon nitride crystal particles 2 is in the range of 1 μm or more and 10 μm or less. Also, the average aspect ratio of the silicon nitride crystal particles 2 is 2 or more and 10 or less. Being within this range can improve the thermal conductivity. Also, the strength can be improved. If the average length of the major axis is less than 1 μm, the silicon nitride crystal particles 2 are too small, which may reduce the thermal conductivity. On the other hand, if it exceeds 10 μm, although the thermal conductivity will improve, the strength may decrease. If the average aspect ratio is less than 2, there are few elongated silicon nitride crystal particles, which may reduce the strength. If the average aspect ratio exceeds 10, the gaps between the silicon nitride crystal particles may become larger. When the gaps between the silicon nitride crystal particles become larger, the grain boundary phase becomes larger. A large grain boundary phase may cause a decrease in the thermal conductivity. Also, as shown in FIG. 1, it is preferable that there are silicon nitride crystal particles 2a (first silicon nitride crystal particles) with a major axis length of less than 5 μm and silicon nitride crystal particles 2b (second silicon nitride crystal particles) with a major axis length of 5 μm or more. The presence of small crystal particles and large crystal particles allows small crystal particles to be present in the gaps between the large crystal particles. Thereby, improvement in thermal conductivity and strength can be achieved. From this viewpoint, it is more preferable that silicon nitride crystal particles 2b with a major axis length 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 particles 2 present per unit area of 20 μm × 20 μm of an arbitrary cross-section is 0.2 wt% or less. The measurement of the amount of dissolved oxygen in the silicon nitride crystal particles 2 shall be performed using TEM-EDS. TEM is an abbreviation for a transmission electron microscope. EDS is an abbreviation for an energy dispersive X-ray spectrometer. The measurement method using TEM-EDS is sometimes simply called EDS analysis. The method for measuring the amount of dissolved oxygen in the silicon nitride crystal particles 2 using EDS analysis is described in Japanese Patent Application No. 2020-180386 to No. as shown. As a sample for performing EDS analysis, an arbitrary cross-section of the silicon nitride sintered body 1 is used. The sample is collected from an 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 or more and 0.5 μm or less. In order to prevent surface oxidation of the sample, it is desirable that the sample is produced and stored in a vacuum or an inert gas atmosphere. As the EDS device, a device such as JED-2300T manufactured by JEOL Ltd. or a device having performance equivalent to or better than that is used. As the TEM, a device such as JEM-200CX (acceleration voltage 200 kV) manufactured by JEOL Ltd. or a device having performance equivalent to or better than that is used. The recommended conditions in EDS analysis are an acceleration voltage of 200 kV, an irradiation current of 1.00 nA, and a spot diameter of 1 nm during analysis. An analysis time of 30 seconds, a sample tilt angle of X = 10° and Y = 0° are recommended. The measurement conditions may be changed, but the first plot diagram described later is measured under the recommended conditions described above.

[0012] By using TEM-EDS, silicon nitride crystal particles can be selected as the measurement points. In the measurement using the SIMS method as in Patent Document 2, only large silicon nitride crystal particles can be measured. Also, even in the nano-SIMS method with a reduced irradiation diameter, it was difficult to recognize the image of silicon nitride crystal particles. For this reason, the amount of dissolved oxygen in small silicon nitride crystal particles could not be measured. Also, for the measurement of the amount of dissolved oxygen, there is the total dissolution method. The total dissolution method is a method of dissolving the grain boundary phase of a silicon nitride sintered body and taking out the silicon nitride crystal particles. The amount of oxygen in the taken-out silicon nitride crystal particles is measured. However, it is difficult to dissolve and remove the grain boundary phase, and problems such as a decrease in measurement accuracy and reproducibility due to the remaining grain boundary phase have occurred. By setting the spot diameter of the measurement point to 1 nm in TEM-EDS, only silicon nitride crystal particles can be included in the measurement point. Also, regardless of the size of the silicon nitride crystal particles, it is possible to measure the amount of dissolved oxygen.

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

[0014] Next, create a second plot showing the atomic ratio of nitrogen element / silicon element against the Si count. In the second plot, the Si count (cps) is shown on the horizontal axis and the N / Si atomic ratio is shown on the vertical axis. Next, correct the oxygen element / silicon element atomic ratio of the first plot diagram from the second plot diagram. This is because in the silicon nitride sintered body, the light element oxygen (O) has a greater X-ray absorption than silicon (Si). The absorption characteristics of oxygen (O) are similar to those of nitrogen (N). Also, since the main phase of the silicon nitride sintered body 1 is Si3N4, the N / Si atomic ratio has a theoretical value of 4 / 3. Therefore, correct the O / Si atomic ratio from the approximate data of the Si and N atomic ratios. In this correction method, correct the O / Si atomic ratio of the first plot diagram using the N / Si atomic ratio of each measurement point in the second plot diagram. Correct using the difference between the N / Si atomic ratio of each measurement point and the theoretical value of 4 / 3 (= 1.33). For example, when the N / Si atomic ratio is 0.70, the correction coefficient is 1.9 (= 1.33 / 0.70). Calculate the corrected value by multiplying the O / Si atomic ratio by the correction coefficient. By this method, correct the O / Si atomic ratio in the first plot diagram. The diagram obtained by correcting the first plot diagram is defined as the third plot diagram.

[0015] In the third plot diagram, the Si count number (cps) is shown on the horizontal axis, and the corrected O / Si atomic ratio is shown on the vertical axis. Extract a plurality of combinations of three or more measurement points from the third plot diagram. For each combination of measurement points, calculate the approximate straight line y = aX + b. In the approximate straight line y = aX + b, X is the horizontal axis, y is the vertical axis, a is the slope, and b is the intersection point with the vertical axis (y-axis). From the plurality of obtained approximate straight lines, -4 × 10 -8 ≦ a ≦ 4 × 10 -8 Extract the approximate straight lines that satisfy this condition. The region where the measurement points used for the extracted approximate straight lines exist is a convergence region with a small variation in the O / Si atomic ratio. The approximate straight line is created using the approximation function of spreadsheet software. Examples of spreadsheet software include Excel from Microsoft.

[0016] The convergence region obtained from the third plot diagram is a region that minimizes the influence of natural oxidation on the sample surface and the grain boundary phase. When affected by natural oxidation on the sample surface and the grain boundary phase, the variation in the O / Si atomic ratio also increases. Therefore, the slope a of the approximate straight line does not fall within the aforementioned range. When the slope a of the approximate straight line is -4 × 10 -8 or more and 4 × 10-8 Being within the following range indicates that the variation in the O / Si atomic ratio is reduced. Since the variation in the O / Si atomic ratio is reduced, it can be seen that the effects of natural oxidation and the grain boundary phase are sufficiently small. Therefore, the O / Si atomic ratio of the measurement points included in the convergence region indicates the amount of dissolved oxygen. In the convergence region, extract three measurement points in descending order of the Si count number. Calculate the amount of dissolved oxygen using the average value of the O / Si atomic ratio. The average value of the O / Si atomic ratio of the three extracted measurement points is a value with even lower influence of natural oxidation and the grain boundary phase 3. Since the silicon nitride crystal particles 2 are Si3N4, the amount of dissolved oxygen (wt%) can be calculated by (3 / 7)×(average value of the O / Si atomic ratio). This is a method of calculation from the amount of oxygen corresponding to the amount of Si in the Si3N4 crystal particles. Also, the slope a of the approximate straight line of three or more points is -4×10 -8 or more and 4×10 -8 The convergence region within the following range is determined from three or more measurement points where the Si count number is 300000 cps or more. In EDS analysis, only points of 300000 cps or more cannot be selectively measured. Therefore, a method of measuring ten or more points by EDS analysis is effective. The amount of oxygen obtained by measuring ten or more points by EDS analysis and extracting the measurement points where the Si count number is 300000 cps or more and included in the convergence region is the average value of the amount of dissolved oxygen in the silicon nitride crystal particles.

[0017] In the high thermal conductivity 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 the average value according to the number of measurement points. That is, the average value of the amount of dissolved oxygen in the silicon nitride crystal particles existing in a unit area of 20 μm × 20 μm is 0.2 wt% or less. Also, the fact that it is an arbitrary cross-sectional structure indicates that no matter which unit area of 20 μm × 20 μm the amount of dissolved oxygen in the silicon nitride crystal particles is measured, the average value is 0.2 wt% or less.

[0018] As described above, the silicon nitride sintered body with the amount of dissolved oxygen in silicon nitride crystal particles controlled can improve the thermal conductivity. Furthermore, the relative permittivity characteristics can be improved. As will be described later, the frequency dependence and temperature dependence of the relative permittivity can be improved. Dissolved oxygen is a factor that reduces the thermal conductivity of silicon nitride crystal particles. Also, it causes lattice defects. By controlling the amount of dissolved oxygen, the cause of lattice defects can be reduced. When the amount of dissolved oxygen exceeds 0.2 wt%, the thermal conductivity, relative permittivity, etc. decrease.

[0019] Also, it is preferable that the amount of dissolved oxygen in each silicon nitride crystal particle present in the unit area of 20 μm × 20 μm is in the range of 0.01 wt% or more and 0.2 wt% or less. As described above, the method using TEM-EDS can use only silicon nitride crystal particles as the measurement region. By controlling the amount of dissolved oxygen in all silicon nitride crystal particles, the performance can be further improved. In the measurement of the amount of dissolved oxygen in each silicon nitride crystal particle, one or more measurement points are set for each individual silicon nitride crystal particle in the unit area of 20 μm × 20 μm. The analysis method is as described above. The analysis method is repeatedly executed until the measurement points set for each individual silicon nitride crystal particle are included in the convergence region and the amount of dissolved oxygen can be measured. After setting measurement points for each individual silicon nitride crystal particle, the fact that the amount of dissolved oxygen is 0.01 wt% or more and 0.20 wt% or less indicates that the amount of dissolved oxygen in each individual silicon nitride crystal particle is controlled.

[0020] Regarding the first silicon nitride crystal particles 2a with a major axis length of less than 5 μm and the second silicon nitride crystal particles 2b with a major axis length of 5 μm or more, which are present in the unit area of 20 μm × 20 μm, the difference between the amount of dissolved oxygen in the first silicon nitride crystal particles 2a and the amount of dissolved oxygen in the second silicon nitride crystal particles 2b is preferably 0.03 wt% or less. In the unit area of 20 μm × 20 μm, the amount of dissolved oxygen obtained by setting measurement points only for the first silicon nitride crystal particles 2a is defined as "amount of dissolved oxygen A". The amount of dissolved oxygen obtained by setting measurement points only for the second silicon nitride crystal particles 2b is defined as "amount of dissolved oxygen B". It is preferable that |amount of dissolved oxygen A - amount of dissolved oxygen B| ≦ 0.03 wt%. As described above, the presence of small silicon nitride crystal particles and large silicon nitride crystal particles can improve the thermal conductivity and strength. Regardless of the particle size, by suppressing the variation in the amount of dissolved oxygen, the thermal conductivity can be increased to 100 W / (m·K) or more, and further to 120 W / (m·K) or more.

[0021] Furthermore, by reducing the difference in the amount of dissolved oxygen between small particles and large particles, the relative permittivity can be stabilized and the electrical properties can be improved. The relative permittivity is the value obtained by dividing the capacitance of a capacitor when the medium between the electrodes is filled by the capacitance when it is in a vacuum. In this embodiment, the medium is a silicon nitride sintered body. The silicon nitride sintered body is an insulator and a dielectric, and polarization occurs in the silicon nitride sintered body due to an electric field. The larger the polarization, the larger the relative permittivity. It has been found that the amount of dissolved oxygen affects the frequency dependence and temperature dependence of this relative permittivity. An increase in the relative permittivity leads to easy occurrence of polarization and a decrease in insulation. Dissolved oxygen causes lattice defects. Controlling the amount of dissolved oxygen and improving the frequency dependence of the relative permittivity lead to an improvement in insulation. Conventionally, only the amount of dissolved oxygen in limited silicon nitride crystal particles could be measured. Therefore, the amount of dissolved oxygen could not be controlled.

[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 the reaction of sintering aids with each other, and the impurities oxygen on the surface of the silicon nitride powder and the sintering aids. Further, the grain boundary phase 3 has the effect of firmly bonding the silicon nitride crystal particles 2 to each other and suppressing the generation of pores. By controlling the amount of the grain boundary phase 3, the properties depending on the thermal conductivity, strength, and relative permittivity can be improved. When the grain boundary phase is less than 1% by mass, the proportion of the grain boundary phase is small. When the grain boundary phase 3 is small, pores are likely to occur. Further, when the grain boundary phase 3 exceeds 20% by mass, although the generation of pores can be suppressed, the 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, more preferably 3% by mass or more and 15% by mass or less. Further, by setting the porosity to 2% or less and the pore size to 20 μm or less, the strength can be 500 MPa or more, more preferably 600 MPa or more. For the measurement of the porosity and the pore size, the SEM photograph with a unit area of 50 μm × 50 μm described above can be used. The amount (mass%) of the grain boundary phase in the high thermal conductivity silicon nitride sintered body 1 can be determined by qualitative analysis and quantitative analysis of components other than silicon nitride. Further, when the addition amount as a sintering aid can be grasped, the addition amount of the sintering aid may be regarded as the mass% of the grain boundary phase. The grain boundary phase 3 preferably contains one or more selected from Group 2A elements, Group 3A elements, and Group 4A elements. The grain boundary phase 3 further preferably contains one or more selected from rare earth elements, magnesium, titanium, and hafnium. The rare earth elements are yttrium (Y), lanthanoid elements, etc. Examples of the rare earth element include one or more selected from yttrium (Y), erbium (Er), ytterbium (Yb), and cerium (Ce). Further, by the grain boundary phase 3 containing both a rare earth element and magnesium, the sinterability can be improved. More preferably, the grain boundary phase 3 contains one or two of titanium or hafnium in addition to the rare earth element and magnesium. Titanium or hafnium has the effect of strengthening the grain boundary phase 3. The strengthening of the grain boundary phase 3 leads to an improvement in strength. Note that the notations of Group 2A, Group 3A, and Group 4A are based on the periodic table of Japan. The Group 2A elements are Be, Mg, Ca, Sr, Ba, and Ra. The Group 3A elements are Sc, Y, lanthanoid elements, and actinoid elements. The Group 4A elements are Ti, Zr, and Hf.

[0023] According to the silicon nitride sintered body 1 as described above, the relative permittivity at 50 Hz at room temperature can be made 10 or less. Room temperature means 25°C. The relative permittivity is obtained by measuring the capacitance. The capacitance is the charge induced and accumulated on the electrodes in an alternating electric field. When the insulation property is poor, the charge becomes large and the capacitance also becomes large. When the capacitance becomes large, the relative permittivity also becomes large. That is, when the insulation property is good, the relative permittivity becomes small. Let the relative permittivity at 50 Hz at room temperature be ε 50-25 and the relative permittivity at 50 Hz at 300°C be ε 50-300 When this is the case, it is preferable that ε 50-300 / ε 50-25 is in the range of 0.9 or more and 1.2 or less. Let the relative permittivity at 1 MHz at room temperature be ε 1M-25 and the relative permittivity at 1 MHz at 300°C be ε 1M-300 When this is the case, it is preferable that ε 1M-300 / ε 1M-25 is 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 relative permittivity is measured in accordance with JIS-C-2141 (1992). It is measured by the complex relative permittivity 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 permittivity ε of 50 Hz at room temperature 50-25 which can be 10 or less. The relative permittivity ε 50-25 being 10 or less indicates that the polarization of the silicon nitride sintered body 1 can be suppressed under this condition. Therefore, it shows that the silicon nitride sintered body 1 has high insulation. For this reason, the reliability of a semiconductor device in which a semiconductor element is mounted on the silicon nitride sintered body 1 can be improved. Also, the fact that ε 50-300 / ε 50-25 is in the range of 0.9 or more and 1.2 or less indicates that the relative permittivity at 50 Hz changes little even when the temperature changes. That is, it shows that the relative permittivity at 50 Hz has little temperature dependence. Therefore, even if the operating guarantee temperature of the semiconductor element increases, the characteristics depending on the relative permittivity are less likely to deteriorate. Also, the fact that ε 1M-300 / ε 1M-25 is in the range of 0.9 or more and 1.2 or less indicates that the relative permittivity at 1 MHz changes little even when the temperature changes. That is, it shows that the relative permittivity at 1 MHz has little temperature dependence. Therefore, even if the operating guarantee temperature of the semiconductor element increases, the characteristics depending on the relative permittivity are less likely to deteriorate. Also, the fact that ε 1M-300 / ε 50-300 is 0.8 or more and 1.2 or less indicates that the change amount of the relative permittivity is small even when the frequency changes. That is, even if the operating frequency of the semiconductor element changes, the characteristics depending on the relative permittivity do not deteriorate. Also, ε 50-300 / ε 50-25 ε 1M-300 / ε 1M-25 and ε1M-300 / ε 50-300 are preferably all within the above ranges.

[0025] The operating frequencies of semiconductor devices vary widely from several 10 Hz to 1 MHz. Semiconductor devices include bipolar transistors, MOSFETs, IGBTs, GTOs, etc. These semiconductor devices are called power semiconductors. Power semiconductors can switch the on / off of the switch according to the operating frequency. The operating frequency ranges are different for each semiconductor device. By improving the temperature dependence and frequency dependence of the relative permittivity of the silicon nitride sintered body, insulation is likely to be maintained regardless of what semiconductor device is mounted. Therefore, a highly reliable semiconductor device can be provided.

[0026] Therefore, the silicon nitride sintered body according to the embodiment is suitable for a silicon nitride substrate. Also, 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 becomes as thin as 0.1 mm or more and 0.4 mm or less, the relative permittivity is improved, so the reliability is high. It is more preferable that the substrate is within the range of 0.1 mm or more and 0.4 mm or less. By thinning the substrate, there is an effect of reducing the thermal resistance. For this reason, the heat dissipation performance is improved.

[0027] The invention according to the embodiment is suitable for a silicon nitride circuit board in which a circuit section 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 is a bonding layer, reference numeral 5 is a metal plate (front metal plate), reference numeral 6 is a metal plate (back metal plate), reference numeral 10 is a silicon nitride substrate, and reference numeral 20 is a silicon nitride circuit board. The metal plate 5 has a circuit shape. The metal plate 5 is used as a circuit section for mounting semiconductor devices. The metal plate 5 used as a circuit section may also be called a front metal plate. Also, the metal plate 6 is used as a heat sink. The metal plate 6 used as a heat sink may also be 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 illustrated example and is arbitrary. The back metal plate 6 may be used as a circuit section instead of a heat sink. Examples of the metal plates 5 and 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 more, as shown in JIS-H-3100 (ISO1337, etc.). The thermal conductivity of the copper plate is approximately 400 W / (m·K). The thermal conductivity of aluminum is approximately 240 W / (m·K). The copper plate has a higher thermal conductivity than the aluminum plate. Therefore, using a copper plate can improve the heat dissipation performance more effectively. Also, the thickness of the metal plates 5 and 6 is preferably in the range of 0.2 mm or more and 5 mm or less. Increasing the thickness of the metal plate can improve the heat dissipation performance and the current-carrying capacity. Examples of the bonding layer 4 include an active metal bonding layer. When the metal plate is a copper plate, examples of the active metal bonding layer include members mainly composed of Ag or Cu and containing Ti. When the metal plate is an aluminum plate, examples of the active metal bonding layer include members mainly composed of Al and containing Si. The active metal bonding layer is a layer containing Ti or Si as an active metal. Also, as an example of the circuit portion, a case where metal plates are joined is shown, but the invention according to the embodiment is not limited to this form. The circuit portion may be formed of a metal thin film or a metal thick film. A metal thin film is a metal film formed by a film-forming method such as sputtering or evaporation. Examples of the metal thin film include films of Ti, Pt, Au, Ni, Cu, Al, or Ag. A metal thick film is a film formed by firing a metal paste. A metal thick film is sometimes called a metallized film. Examples of the metal thick film 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, a semiconductor element 31 is mounted on one of two metal plates 5. A 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 illustrated example. The number and size of the metal plates 5, the number and size of the semiconductor elements 31, etc. can be appropriately changed as needed. Further, a metal plate 6 may be used as a circuit portion, and a semiconductor element 31 may be mounted on the metal plate 6.

[0029] Next, a method for manufacturing the high thermal conductivity silicon nitride sintered body 1 according to the embodiment will be described. The manufacturing method of the high thermal conductivity silicon nitride sintered body 1 according to the embodiment is not limited as long as it has the above configuration. Here, a method for obtaining the silicon nitride sintered body 1 with good yield is given.

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

[0031] As silicon nitride powder, there are mainly those produced by the imide decomposition method or the direct nitridation method. The imide decomposition method is preferable because the amount of impurity oxygen dissolved in the powder is small. Also, silicon nitride powder has an α-type and a β-type. β-type silicon nitride powder can reduce the amount of dissolved oxygen in terms of crystal structure. On the other hand, α-type silicon nitride powder has higher sinterability than the β-type and is thus easily densified. By using the α-type as the raw material powder, a silicon nitride sintered body with high strength can be obtained. As the silicon nitride powder, a mixture of α-type silicon nitride powder and β-type silicon nitride powder may be used. Also, when the total of the α-type silicon nitride powder and the β-type silicon nitride powder is 100 parts by mass, the β-type is preferably in the range of 1 part by mass or more and 30 parts by mass or less. If the β-type exceeds 30 parts by mass, the sinterability may decrease. Also, if it is less than 1 part by mass, the effect of using the β-type may be insufficient. Note that commercially available α-type silicon nitride powder has an α-conversion rate of 90 wt% or more. That is, commercially available α-type silicon nitride powder unavoidably contains a small amount of β-type silicon nitride powder. The material obtained by mixing α-type silicon nitride powder and β-type silicon nitride powder refers to a material in which β-type silicon nitride powder is actively added to commercially available α-type silicon nitride powder. In other words, a material consisting only of α-type silicon nitride powder that unavoidably contains β-type silicon nitride powder is not treated as a material obtained by mixing α-type silicon nitride powder and β-type silicon nitride powder.

[0032] Also, it is also effective to perform a treatment for reducing the impurity oxygen in the silicon nitride powder. Examples of the treatment for reducing impurity oxygen include reduction treatment and chemical solution treatment. Examples of the reduction treatment include heat treatment in a hydrogen atmosphere and heat treatment in an atmosphere in which carbon is present. Also, the chemical solution treatment is a method of treating silicon nitride powder with an acidic or alkaline solution.

[0033] Next, a sintering aid is prepared. The sintering aid promotes sintering and is a component that forms a grain boundary phase. Preferably, the sintering aid is one or more selected from Group 2A elements, Group 3A elements, and Group 4A elements. The notations of Group 2A, Group 3A, and Group 4A are based on the periodic table of Japan. Preferably, the sintering aid further contains one or more selected from rare earth elements, magnesium, titanium, and hafnium. Titanium exists as titanium nitride (TiN) particles in the grain boundary phase. The titanium nitride particles have an effect of strengthening the grain boundary phase due to the pinning effect. Preferably, the sintering aid is added as an oxide powder. Oxides can 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 step of mixing silicon nitride powder and sintering aid powder is performed. To control the major axis length and aspect ratio of the silicon nitride crystal particles in the silicon nitride sintered body, sintering homogeneity is required. For this purpose, it is necessary that the silicon nitride powder and the sintering aid powder are uniformly mixed. In the sintering process, the sintering aid reacts to form a grain boundary phase. The growth reaction of the silicon nitride particles proceeds through this grain boundary phase. In the process, oxygen escapes from the silicon nitride particles. By uniformly mixing the silicon nitride powder and the sintering aid powder, the reaction through the grain boundary phase can be homogenized. Also, in the mixing step, a ball mill or a bead mill is used. The silicon nitride powder and the sintering aid powder often exist as aggregated secondary particles. The secondary particles are an inhibiting factor for homogeneous sinterability. By uniformly mixing while crushing the secondary particles into primary particles without aggregation, the homogeneity of sinterability can be improved. In this mixing step accompanied by crushing, it is preferable not to apply a strong stress that further breaks the primary particles. When the primary particles are broken, a fracture surface is formed on the silicon nitride powder. Since the fracture surface is an active surface, an oxide film is formed for stabilization, and the amount of oxygen attachment increases. The amount of oxygen increases compared to the amount attached to the primary particles. Therefore, it is effective to suppress the formation of a fracture surface on the primary particles. For the comminution of such secondary particles, wet comminution using a solvent is suitable. A solvent with high wettability on the particle surface and low reactivity with the particles is used. This way, the stress required for comminution can be reduced, and the destruction of primary particles can be suppressed. For the mixing accompanied by the comminution of silicon nitride powder and sintering aid powder, an organic solvent is suitable. As the organic solvent, alcohols and ketones are suitable. An organic solvent obtained by mixing alcohols and ketones may also be used. Alcohols are a general term for substances obtained by replacing a part of the hydrogen contained in hydrocarbons with Ki hydroxy groups (OH groups). Also, ketones are represented by R-C(=O)-R’. R and R’ are alkyl groups and the like. These organic solvents have high wettability with silicon nitride powder and sintering aid powder and low reactivity with the powder. The sintering aid powder is often added as an oxide powder. In a raw material composition in which silicon nitride powder and oxide powder are mixed, it is desirable to use a mixed liquid of alcohols and ketones. Also, a dispersant may be added as necessary. The dispersant has the effect of stabilizing primary particles in the solvent and suppressing re-aggregation. Examples of the dispersant include surfactants.

[0035] When the comminution process is carried out in a ball mill, the diameter of the media is preferably 20 mm or less, and more preferably 12 mm or less. The media are ceramic balls. The ball mill is a method of putting powder and media into a cylindrical container and comminuting the powder while rotating the cylindrical container. As described above, the ball mill process using an organic solvent is wet comminution mixing. By selecting an organic solvent suitable for the powder, there is an effect of reducing the stress required for comminution. That is, it becomes possible to perform comminution with ceramic balls having a small media diameter, and there is an effect of weakening the energy with which the media collide with the powder. By weakening the energy with which the media collide with the powder, it is possible to suppress the formation of fracture surfaces on the primary particles. In addition, the minimum value of the diameter of the media is preferably 3 mm or more. If the media are too small, the working efficiency may decrease. The wet crushing and mixing time by the ball mill is preferably in the range of 5 hours or more and 40 hours or less. If it is less than 5 hours, the crushing effect may be insufficient and a large number of secondary particles may remain. If it is longer than 40 hours, the possibility of forming fracture surfaces on the primary particles increases. Therefore, the wet crushing and mixing time by the ball mill is preferably in the range of 5 hours or more and 40 hours or less, more preferably 10 hours or more and 30 hours or less. Also, the rotational speed of the cylindrical container in the ball mill process is preferably in the range of 50 rpm or more and 500 rpm or less. Whether the secondary particles are crushed can be grasped by examining the particle size distribution before and after crushing. When the secondary particles are crushed and the number of primary particles increases, the peak position of the particle size distribution (frequency distribution) shifts toward smaller particle sizes. Also, the peak of the particle size distribution becomes sharp. Suppression of the formation of fracture surfaces on the primary particles can be grasped by measuring the oxygen amount before and after crushing. The oxygen amount before crushing is the oxygen amount of the raw material powder. There is no problem if the oxygen amount of the raw material powder after crushing does not increase significantly compared to before crushing.

[0036] When the wet crushing and mixing process is performed, a raw material powder slurry is obtained. A forming process for forming a compact is performed using the raw material powder slurry. Examples of the forming process include sheet forming and die forming. Sheet forming is, for example, the doctor blade method. When sheet forming is performed, a cutting process for cutting the long sheet into the required size is performed. The slurry may be used after being processed into granulated powder suitable for the forming method. A debinding process of the compact is performed. The debinding process is preferably performed in the range of 400°C or more and 800°C or less. By performing the debinding process, the organic substances in the compact can be removed. Also, the product after the debinding process is called a debound body.

[0037] Next, a sintering process for sintering the debound body is performed. The sintering process preferably includes a first holding process in the range of 1500°C or more and 1650°C or less and a second holding process in the range of 1750°C or more and 2000°C or less. The first holding step is a step of holding the degreased body within a range of 1500 °C or higher and 1650 °C or lower. The holding time is preferably 2 hours or longer. The temperature range of 1500 °C or higher and 1650 °C or lower corresponds to the temperature range at which silicon nitride starts grain growth. Furthermore, this temperature range also corresponds to the temperature at which impurity oxygen starts 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. Moreover, the grain growth of the silicon nitride crystal particles can be homogenized. The holding time of the first holding step is preferably 10 hours or shorter. If the holding time exceeds 10 hours, there may be insufficient oxygen required for the formation of a liquid phase composed of a sintering aid, and the sinterability may decrease. For this reason, the holding time of the first holding step is preferably 2 hours or longer and 10 hours or shorter, and more preferably 3 hours or longer and 6 hours or shorter. Also, this holding step is preferably performed under normal pressure (0.1 MPa) or a reduced pressure atmosphere below that. Under 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 within a range of 1750 °C or higher and 2000 °C or lower. The holding temperature of the second holding step is a so-called sintering temperature. Within this temperature range, the sintered body can be densified. Also, the major axis and aspect ratio of the silicon nitride crystal particles can be controlled. If the holding temperature is less than 1750 °C, there may be insufficient densification. If it exceeds 2000 °C, the silicon nitride crystal particles may grow too much. Also, the holding time of the second holding step is preferably within a range of 5 hours or longer and 30 hours or shorter. The second holding step is preferably performed under a pressurized atmosphere of 0.5 MPa or higher. Under a pressure atmosphere of 0.1 MPa (normal pressure) or lower, self-decomposition of silicon nitride is likely to occur at 1700 °C or higher, so there may be insufficient densification. The sintering step is preferably performed in a non-oxidizing atmosphere. Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon atmosphere, or a vacuum atmosphere. In a non-oxidizing atmosphere, the dissolution of oxygen into the silicon nitride crystal particles can be suppressed.

[0038] A sintered body can be obtained through a sintering process. Additionally, the sintered body may be subjected to a heat treatment. By providing a process of holding the sintered body within the range of 1400 °C or higher and 1600 °C or lower in the heat treatment, the desorption of SiO can be further effectively promoted. Since the sintered body has a stable crystal structure, the adverse effect on the crystal structure due to the desorption of SiO by the heat treatment is small. The holding time within the range of 1400 °C or higher and 1600 °C or lower in the heat treatment is preferably 2 hours or longer. This holding process is also preferably carried out under normal pressure (= 0.1 MPa) or a reduced pressure atmosphere below that. In a so-called pressurized atmosphere exceeding 0.1 MPa, the desorption of SiO tends to be suppressed. The heat treatment is preferably carried out after returning to room temperature after the second holding process. Also, after the second holding process, after cooling to 1300 °C or lower and holding, and then heating up again to process the sintered body may be acceptable. 1300 °C or lower is the temperature at which the liquid phase composed of a sintering aid and the like solidifies and no longer contributes to the sintering reaction. That is, similar to the state of returning to room temperature, since the sintered body can be regarded as being in a form maintaining a stable crystal structure, the adverse effect on the crystal structure due to the desorption of SiO by the heat treatment is reduced. Also, the maximum holding temperature in the heat treatment is lower than the holding temperature in the second holding process in sintering. The difference between the maximum holding temperature in the heat treatment and the holding temperature in the second holding process is preferably 50 °C or higher and 300 °C or lower. For example, when the holding temperature of the second holding process is 1800 °C, the heat treatment temperature is preferably within the range of 1500 °C or higher and 1750 °C or lower. The heat treatment not only desorbs oxygen in the sintered body as SiO outside the sintered body but also has the effect of suppressing the variation in the amount of solid solution oxygen between silicon nitride crystal particles. Also, by setting the maximum holding temperature of the heat treatment lower than the holding temperature of the second holding process in sintering, the grain growth of silicon nitride crystal particles can be suppressed. The heat treatment is preferably carried out in a non-oxidizing atmosphere. By carrying out the heat treatment in a non-oxidizing atmosphere, the incorporation of oxygen into the silicon nitride crystal particles can be suppressed.

[0039] Through the above steps, the silicon nitride sintered body 1 according to the embodiment can be manufactured. Further, by fabricating the silicon nitride sintered body 1 in the shape of a substrate, a silicon nitride substrate can be manufactured. By performing the step of providing a circuit portion on the silicon nitride substrate, a silicon nitride circuit board can be manufactured. By mounting a semiconductor element on the circuit portion of the silicon nitride circuit board, a semiconductor device can be manufactured.

[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 the α-type and β-type is 100 parts by mass. Further, as the α-type silicon nitride powder, a powder with an α conversion rate of 90 wt% or more was used.

[0041]

Table 1

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

[0043]

Table 2

[0044] Next, an organic solvent was mixed with the silicon nitride powder and the sintering aid powder, and a wet pulverization and mixing step was performed. The wet pulverization and mixing step 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 or more and 500 rpm or less. In Comparative Example 2, wet mixing was performed using water. In Comparative Example 3, dry mixing was performed.

[0045]

Table 3

[0046] A raw material powder slurry was prepared by a wet disintegration and mixing process. The raw material powder slurry was molded into sheets. The sheet molding was performed using a doctor blade method. The long sheet was cut into a predetermined size. Then, a degreasing process was performed to prepare a degreased body. A sintering process was carried out on each degreased body. The sintering process included a first holding process and a second holding process. In addition, a third holding process (reheat treatment) was carried out after the sintering process. The conditions for the sintering process are as shown in Tables 4 and 5. In addition, the first holding process, the second holding process, and the third holding process were carried out 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 in length and 80 mm in width. In examples 1 to 3, examples 6 to 7, and comparative examples 1 to 3, silicon nitride substrates having a thickness of 0.32 mm were manufactured. In examples 4 to 5, silicon nitride substrates having 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 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. In the measurement of the amount of dissolved oxygen in silicon nitride crystal particles, a unit area of 20 μm × 20 μm of an arbitrary cross-section was set as the measurement point, and TEM-EDS was used. The measurement conditions of TEM-EDS were as described above. The amount of dissolved oxygen in each individual silicon nitride crystal particle contained in the unit area of 20 μm × 20 μm was measured, and their average value was calculated. The method for measuring the amount of dissolved oxygen was as described above. FIGS. 2 to 4 are, respectively, the first plot diagram, the second plot diagram, and the third plot diagram when Example 3 was measured. Also, the range of the amount of dissolved oxygen in each individual silicon nitride crystal particle, the average value of the amount of dissolved oxygen in the first silicon nitride crystal particles, and the average value of the amount of dissolved oxygen in the second silicon nitride crystal particles were calculated, respectively. As described above, the first silicon nitride crystal particles are silicon nitride crystal particles having a major axis of less than 5 μm. The second silicon nitride crystal particles are silicon nitride crystal particles having a major axis of 5 μm or more. In the measurement of the average length of the major axis and the average aspect ratio of the second silicon nitride crystal particles, a unit area of 50 μm × 50 μm of an arbitrary cross-section was set as the measurement point, and SEM photographs were used. 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, for the silicon nitride substrates according to the examples, the amount of dissolved oxygen was 0.2 wt% or less. On the other hand, for the silicon nitride substrates according to the comparative examples, the amount of dissolved oxygen exceeded 0.2 wt%. Regarding the average value of the major axis length and the average aspect ratio of the silicon nitride crystal particles, there was no significant difference between the examples and the comparative examples. In Examples 1 to 7, silicon nitride crystal particles having a major axis of less than 3 μm were present. Next, the thermal conductivity and fracture toughness values of each silicon nitride substrate were measured. The thermal conductivity was measured by the laser flash method. The fracture toughness value was measured according to JIS-R-1607 (IF method) and determined by Shinohara'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 substrate according to the examples had a thermal conductivity of 80 W / (m·K) or more. Further, for Examples 1, 2, and 4, the thermal conductivity was 100 W / (m·K) or more. Regarding the strength, there was no significant difference between the examples and the comparative examples. Next, the relative permittivity of the silicon nitride substrates according to the examples and the comparative examples was measured. The relative permittivity was measured in accordance with the complex relative permittivity measurement method (3-terminal method) of JIS-C-2141. The measurement was carried out by changing the measurement frequency to 50 Hz and 1 MHz, and the measurement temperature to room temperature (25°C) and 300°C. The results are shown in Table 9 and Table 10.

[0055]

Table 9

[0056]

Table 10

[0057] As can be seen from Table 9 and Table 10, it can be seen that for the silicon nitride substrates according to the examples, the frequency dependence and temperature dependence of the relative permittivity have been improved. On the other hand, in the comparative examples, the frequency dependence and temperature dependence of the relative permittivity have decreased. From the above, it can be seen that the silicon nitride substrate according to the examples is suitable as a substrate for mounting a semiconductor element with a high operating guarantee temperature or a semiconductor element with a high operating frequency.

[0058] Embodiments of the present invention may include the following configurations. (Appendix 1) A high thermal conductivity silicon nitride sintered body comprising silicon nitride crystal particles and a grain boundary phase, wherein the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average value of the amount of dissolved oxygen in the silicon nitride crystal particles present per unit area of 20 μm × 20 μm in any cross section is 0.2 wt% or less, the average value of the major axis length of the silicon nitride crystal particles present per unit area of 50 μm × 50 μm in any cross section is 1 μm or more and 10 μm or less, A high thermal conductivity silicon nitride sintered body, wherein the average aspect ratio of the silicon nitride crystal particles present in the unit area of 50 μm × 50 μm is 2 or more and 10 or less. (Appendix 2) The high thermal conductivity silicon nitride sintered body according to Appendix 1, wherein the amount of dissolved oxygen in each of the silicon nitride crystal particles present in the unit area of 20 μm × 20 μm is in the range of 0.01 wt% or more and 0.2 wt% or less. (Appendix 3) In the unit area of 20 μm × 20 μm, there are present first silicon nitride crystal particles having a major axis length of less than 5 μm and second silicon nitride crystal particles having a major axis length of 5 μm or more, The high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 2, wherein the difference between the amount of dissolved oxygen in the first silicon nitride crystal particles and the amount of dissolved oxygen in the second silicon nitride crystal particles is 0.03 wt% or less. (Appendix 4) The high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 3, wherein the content of the grain boundary phase is 1 mass% or more and 20 mass% or less. (Appendix 5) The high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 4, having a relative permittivity of 10 or less at 50 Hz and room temperature. (Appendix 6) Taking the relative permittivity at 50 Hz and room temperature as ε 50-25 and the relative permittivity at 50 Hz and 300 °C as ε 50-300 when, ε 50-300 / ε 50-25 is in the range of 0.9 or more and 1.2 or less. The high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 5. (Appendix 7) The relative dielectric constant at 1 MHz and room temperature is ε 1M-25 and the relative dielectric constant at 1 MHz and 300 °C is ε 1M-300 When it is, ε 1M-300 / ε 1M-25 is in the range of 0.9 or more and 1.2 or less, and the high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 6. (Appendix 8) The high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 7, wherein the thermal conductivity is 100 W / (m·K) or more. (Appendix 9) A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to any one of Appendices 1 to 8. (Appendix 10) The silicon nitride substrate according to Appendix 9, having a thickness of 0.1 mm or more and 3 mm or less. (Appendix 11) The silicon nitride substrate according to Appendix 9 or 10, and a circuit portion provided on the silicon nitride substrate, and A silicon nitride circuit board comprising. (Appendix 12) The silicon nitride circuit board according to Appendix 11, and a semiconductor element mounted on the circuit portion, and A semiconductor device comprising.

[0059] As mentioned above, several embodiments of the present invention have been illustrated. However, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, changes, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope. Also, the above-described embodiments can be implemented in combination with each other.

Explanation of Reference Numerals

[0060] 1…High thermal conductivity silicon nitride sintered body, 2…Silicon nitride crystal particles, 3…Grain boundary phase, 4…Bonding 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 high thermal conductivity silicon nitride sintered body comprising silicon nitride crystal particles and a grain boundary phase, wherein the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average value of the amount of dissolved oxygen in the silicon nitride crystal particles 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 length of the silicon nitride crystal particles 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 average aspect ratio of the silicon nitride crystal particles 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, a high thermal conductivity silicon nitride sintered body.

2. The high thermal conductivity silicon nitride sintered body according to claim 1, wherein the amount of dissolved oxygen in each of the silicon nitride crystal particles present in the unit area of 20 μm × 20 μm is in the range of 0.01 wt% or more and 0.2 wt% or less.

3. In the unit area of 20 μm × 20 μm, there are present first silicon nitride crystal particles having a major axis length of less than 5 μm and second silicon nitride crystal particles having a major axis length of 5 μm or more, and the difference between the amount of dissolved oxygen in the first silicon nitride crystal particles and the amount of dissolved oxygen in the second silicon nitride crystal particles is 0.03 wt% or less, the high thermal conductivity silicon nitride sintered body according to claim 1 or claim 2.

4. The high thermal conductivity silicon nitride sintered body according to claim 1 or claim 2, having a relative permittivity of 10 or less at 50 Hz and room temperature.

5. A high thermal conductivity silicon nitride sintered body comprising silicon nitride crystal particles and a grain boundary phase, wherein the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average value of the amount of dissolved oxygen in the silicon nitride crystal particles 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 length of the silicon nitride crystal particles 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 average aspect ratio of the silicon nitride crystal particles present in the unit area of 50 μm × 50 μm is 2 or more and 10 or less, in the unit area of 20 μm × 20 μm, there are present first silicon nitride crystal particles having a major axis length of less than 5 μm and second silicon nitride crystal particles having a major axis length of 5 μm or more, and the difference between the amount of dissolved oxygen in the first silicon nitride crystal particles and the amount of dissolved oxygen in the second silicon nitride crystal particles is 0.03 wt% or less, a high thermal conductivity silicon nitride sintered body.

6. A high thermal conductivity silicon nitride sintered body comprising silicon nitride crystal particles and a grain boundary phase, wherein the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average value of the amount of dissolved oxygen in the silicon nitride crystal particles 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 lengths of the silicon nitride crystal particles 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 average of the aspect ratios of the silicon nitride crystal particles present in the unit area of 50 μm × 50 μm is 2 or more and 10 or less, A high thermal conductivity silicon nitride sintered body having a relative dielectric constant of 10 or less at 50 Hz and room temperature.

7. Let the relative permittivity at 50 Hz and room temperature be ε ５０－２５ and the relative permittivity at 50 Hz and 300 °C be ε ５０－３００ When this is the case, the high thermal conductivity silicon nitride sintered body according to claim 1, claim 5, or claim 6, wherein ε ５０－３００ / ε ５０－２５ is in the range of 0.9 or more and 1.2 or less.

8. Let the relative permittivity at 1 MHz and room temperature be ε １Ｍ－２５ and the relative permittivity at 1 MHz and 300 °C be ε １Ｍ－３００ When, ε １Ｍ－３００ / ε １Ｍ－２５ is in the range of 0.9 or more and 1.2 or less, the high thermal conductivity silicon nitride sintered body according to claim 1, claim 5, or claim 6.

9. The high thermal conductivity silicon nitride sintered body according to claim 1, claim 5, or claim 6, wherein the thermal conductivity is 100 W / (m·K) or more.

10. A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to claim 1, claim 5, or claim 6.

11. The silicon nitride substrate according to claim 10, having a thickness of 0.1 mm or more and 3 mm or less.

12. The silicon nitride substrate according to claim 10, and a circuit portion provided on the silicon nitride substrate, A silicon nitride circuit board comprising the same.

13. The silicon nitride circuit board according to claim 12, and a semiconductor element mounted on the circuit portion, A semiconductor device comprising the same.

14. Let the relative permittivity at 1 MHz and room temperature be ε １Ｍ－２５ and the relative permittivity at 1 MHz and 300 °C be ε １Ｍ－３００ . When ε １Ｍ－３００ / ε １Ｍ－２５ is in the range of 0.9 or more and 1.2 or less, the high thermal conductivity silicon nitride sintered body according to claim 7.

15. In the unit area of 20 μm × 20 μm, there are present first silicon nitride crystal particles having a major axis length of less than 5 μm and second silicon nitride crystal particles having a major axis length of 5 μm or more, The high thermal conductivity silicon nitride sintered body according to claim 14, wherein the difference between the amount of dissolved oxygen in the first silicon nitride crystal particles and the amount of dissolved oxygen in the second silicon nitride crystal particles is 0.03 wt% or less.

16. A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to claim 14.

17. The silicon nitride substrate according to claim 16, and a circuit portion provided on the silicon nitride substrate, A silicon nitride circuit board comprising the same.

18. The silicon nitride circuit board according to claim 17, and a semiconductor element mounted on the circuit portion, A semiconductor device comprising the same.

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