Highly heat-conductive silicon nitride sintered body, silicon nitride substrate, silicon nitride circuit substrate, and semiconductor device

Abstract

The high thermal conductive silicon nitride sintered body of the embodiment has silicon nitride 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 value of the solid solution oxygen content of the silicon nitride 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 length diameter of the silicon nitride 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 average value of the length-width ratio of the silicon nitride grains present in the unit area of 50 μm×50 μm is 2 or more and 10 or less.

Description

TECHNICAL FIELD

[0001] Embodiments described later relate to a high thermal conductivity silicon nitride sintered body, a silicon nitride substrate, a silicon nitride circuit substrate, and a semiconductor device. BACKGROUND

[0002] In recent years, a silicon nitride (Si3N4) substrate has been attempted to be applied to a semiconductor circuit substrate. As a semiconductor circuit substrate, an aluminum oxide (Al2O3) substrate, an aluminum nitride (AlN) substrate is used. The thermal conductivity of the aluminum oxide substrate is about 30 W / (m·K), but is capable of being low-cost. In addition, the aluminum nitride substrate is capable of achieving high thermal conduction 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.

[0003] The thermal conductivity of the silicon nitride substrate is lower than that of the aluminum nitride substrate. However, the 3-point bending strength of the silicon nitride substrate is 500 MPa or more, which is excellent. The 3-point bending strength of the aluminum nitride substrate is generally about 300 to 400 MPa, and has a tendency that the higher the thermal conductivity, the lower the strength. By taking advantage of the high strength, the silicon nitride substrate is capable of being thin. The thermal resistance is capable of being reduced by the thinning of the substrate, and thus the heat dissipation property is improved.

[0004] For example, a silicon nitride substrate having a thermal conductivity of 50 W / (m·K) or more and a 3-point bending strength of 600 MPa or more is disclosed in Japanese Patent No. 6293772 (Patent Literature 1). In Patent Literature 1, the distribution ratio of the grain boundary phase in the thickness direction of the substrate is controlled. Thereby, in Patent Literature 1, the variation in insulation resistance is suppressed, and the temperature dependence of the volume specific resistance is improved. In addition, in Patent Literature 1, the relative dielectric constant at 50 Hz and 1 kHz is also controlled.

[0005] In recent years, with the high performance of semiconductor elements, the operation guaranteed temperature has become high. In SiC elements and GaN elements, the operation guaranteed temperature is expected to be raised to about 250°C. In addition, the operation frequency of semiconductor elements is also expected to be raised to about 1 MHz. Therefore, with respect to the silicon nitride substrate, it is required to maintain the insulation even in a high temperature environment of about 250°C or a high frequency environment of about 1 MHz.

[0006] The silicon nitride substrate described in Patent Literature 1 has good insulation, but in recent years, it is required to improve the performance thereof.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent No. 6293772

[0010] Patent Literature 2: Japanese Patent Application Laid-Open No. 2018-24548

[0011] Patent Literature 3: Japanese Patent Application Laid-Open No. 2022-71426 SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] As a result of investigating the cause, it was found that the amount of solid-solution oxygen in the silicon nitride grains had an influence. The solid-solution oxygen is generated by oxygen being substituted for a part of the lattice of the silicon nitride crystal or oxygen invading between the lattices. That is, the solid-solution oxygen refers to oxygen that has entered the silicon nitride grains. The solid-solution oxygen is distinguished from oxygen that is present in the grain boundary phase of the silicon nitride sintered body. In addition, the solid-solution oxygen becomes a cause of lattice defects, and thus the amount of solid-solution oxygen needs to be controlled.

[0014] For example, in Japanese Patent Application Laid-Open No. 2018-24548 (Patent Literature 2), a silicon nitride sintered body in which the solid-solution oxygen concentration of the silicon nitride grains is 1 to 2500 ppm is disclosed. In Patent Literature 2, secondary ion mass spectrometry (SIMS) is used in the measurement of the amount of solid-solution oxygen. In Patent Literature 2, the grating region is 3 μm. According to the method of Patent Literature 2, it is possible to measure the amount of solid-solution oxygen of the silicon nitride grains of 3 μm or more. It is not possible to measure the amount of solid-solution oxygen related to the silicon nitride grains of less than 3 μm in the silicon nitride sintered body. In addition, SIMS is a surface analysis method, and thus is a method that is easily affected by oxidation of the surface of the sample. Therefore, it cannot be said that the control of the amount of solid-solution oxygen in the silicon nitride sintered body is necessarily sufficient.

[0015] The present application has been made to solve such a technical problem, and provides a high thermal conductivity silicon nitride sintered body in which the amount of solid-solution oxygen is controlled.

[0016] MEANS FOR SOLVING THE PROBLEM

[0017] The high thermal conductivity silicon nitride sintered body of the embodiment has silicon nitride grains and a grain boundary phase, the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, the average of the amount of solid-solution oxygen of the silicon nitride grains present in a unit area of 20 μm x 20 μm in any cross section is 0.2 wt% or less, the average of the length diameter of the silicon nitride grains present in a unit area of 50 μm x 50 μm in any cross section is 1 μm or more and 10 μm or less, and the average of the aspect of the silicon nitride grains present in the unit area of 50 μm x 50 μm is 2 or more and 10 or less. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic view showing an example of the cross-sectional structure of the silicon nitride sintered body of the embodiment.

[0019] Figure 2is a view showing an example of a first graph.

[0020] Figure 3 is a view showing an example of a second graph.

[0021] Figure 4 is a view showing an example of a third graph.

[0022] Figure 5 is a view showing an example of a silicon nitride circuit substrate of an embodiment.

[0023] Figure 6 is a view showing an example of a semiconductor device of an embodiment. DETAILED DESCRIPTION

[0024] The high thermal conductivity silicon nitride sintered body of the embodiment has silicon nitride grains 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 solid solution oxygen content of the silicon nitride grains present on 20 μm x 20 μm per unit area in any cross section is 0.2 wt% or less, the average value of the aspect ratio of the silicon nitride grains present on 50 μm x 50 μm per unit area in any cross section is 1 μm or more and 10 μm or less, and the average value of the aspect ratio of the silicon nitride grains present on 50 μm x 50 μm per unit area is 2 or more and 10 or less.

[0025] Figure 1 is a view showing an example of a cross-sectional structure of a silicon nitride sintered body of an embodiment. In Figure 1 , reference numeral 1 is a high thermal conductivity silicon nitride sintered body, reference numeral 2 is a silicon nitride grain, and reference numeral 3 is a grain boundary phase. The high thermal conductivity silicon nitride sintered body 1 is sometimes referred to simply as a silicon nitride sintered body 1.

[0026] The silicon nitride sintered body 1 has silicon nitride grains 2 and a grain boundary phase 3. The grain boundary phase 3 is distributed in the gaps between the silicon nitride grains 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 grains 2 to be firmly bonded to each other, and a silicon nitride sintered body 1 having high thermal conductivity is formed. In addition, pores not shown can also be present in the silicon nitride sintered body 1.

[0027] The thermal conductivity of the silicon nitride sintered body 1 of the embodiment is 80 W / (m·K) or more. The thermal conductivity is high, and thus the heat dissipation property is improved. Therefore, the thermal conductivity is preferably 80 W / (m·K) or more, and more preferably 100 W / (m·K) or more.

[0028] The average length of the aspect ratio of the silicon nitride grains 2 is 1 μm or more and 10 μm or less. The average of the aspect ratio of the silicon nitride grains 2 is 2 or more and 10 or less.

[0029] The average length of the major axis and the average aspect ratio were determined using scanning electron microscopy (SEM) images. SEM images were taken at 2000x magnification. Additionally, the maximum diameter of each silicon nitride grain 2 was measured in the SEM images. The maximum diameter of each silicon nitride grain 2 as reflected in the SEM images was taken as the major axis. The average value of the maximum diameters of each silicon nitride grain 2 reflected in a unit area of ​​50 μm × 50 μm was taken as the average length of the major axis.

[0030] The aspect ratio is calculated as the major axis / minor axis. The major axis is the maximum diameter. The length of the silicon nitride grain along a line extending perpendicularly from the center point of the major axis is taken as the minor axis. The average aspect ratio is the average of the aspect ratios of the individual silicon nitride grains reflected in a unit area of ​​50 μm × 50 μm. Furthermore, the major and minor axes are determined using portions of the silicon nitride grains reflected in SEM images. For example, for silicon nitride grains that overlap with other silicon nitride grains and whose entire outline is not visible, only the visible portion (the portion reflected in the SEM image) is used to determine the major and minor axes. Similarly, for silicon nitride grains whose outline is interrupted at the end of a unit area of ​​50 μm × 50 μm in an SEM image, only the visible portion (the portion reflected in the SEM image) is used to determine the major and minor axes. In cases where the outline of the silicon nitride grain is difficult to confirm, the grain boundary phase can be removed by etching.

[0031] The average length of the major axis of the silicon nitride grain 2 is in the range of 1 μm to 10 μm. Furthermore, the average aspect ratio of the silicon nitride grain 2 is 2 to 10. Within this range, thermal conductivity can be improved. Additionally, increased strength can also be achieved.

[0032] When the average length of the major axis is less than 1 μm, the silicon nitride grains are too small, and the thermal conductivity may decrease. Conversely, if the average length exceeds 10 μm, while thermal conductivity increases, strength may decrease. If the average aspect ratio is less than 2, there are fewer slender silicon nitride grains, thus potentially reducing strength. If the average aspect ratio exceeds 10, there is a possibility that the gaps between silicon nitride grains will increase. Larger gaps between silicon nitride grains result in larger grain boundary phases. Larger grain boundary phases may contribute to a decrease in thermal conductivity.

[0033] In addition, such as Figure 1 As shown, it is preferable to have silicon nitride grains 2a (first silicon nitride grains) with a major diameter less than 5 μm and silicon nitride grains 2b (second silicon nitride grains) with a major diameter of 5 μm or more. By having both small and large grains, small grains can exist in the gaps between large grains. This can improve thermal conductivity and strength. From this point of view, it is more preferable to have silicon nitride grains 2b with a major diameter less than 3 μm.

[0034] In the silicon nitride sintered body 1, the solid solution oxygen amount of the silicon nitride grains 2 present in a unit area of 20 μm x 20 μm of an arbitrary cross section is 0.2 wt% or less.

[0035] The measurement of the solid solution oxygen amount of the silicon nitride grains 2 is performed using TEM-EDS. TEM is an abbreviation of transmission electron microscope. EDS is an abbreviation of energy dispersive X-ray spectrometer. The measurement method using TEM-EDS is sometimes referred to as EDS analysis. The measurement method of the solid solution oxygen amount of the silicon nitride grains 2 using EDS analysis is shown in Japanese Patent Application No. 2020-180386 (Patent Document 3).

[0036] As a sample for performing EDS analysis, an arbitrary cross section of the silicon nitride sintered body 1 is used. The sample is collected by focused ion beam (FIB) processing or ion milling processing from the arbitrary cross section. The thickness of the sample is preferably in a range of 0.05 μm or more and 0.5 μm or less. In order to prevent oxidation of the surface of the sample, it is preferable to produce and store the sample in a vacuum or a non-active gas atmosphere.

[0037] As an EDS device, JED-2300T manufactured by JEOL Ltd. or a device having a performance equal to or higher than that is used. As a TEM, JEM-200CX (acceleration voltage 200 kV) manufactured by JEOL Ltd. or a TEM having a performance equal to or higher than that is used. The recommended conditions in EDS analysis are acceleration voltage 200 kV, irradiation current 1.00 nA, and spot diameter 1 nm at the time of analysis. The recommended analysis time is 30 seconds, the sample inclination angle is X = 10° and Y = 0°. The measurement conditions can be changed, and the first graph described later is measured under the aforementioned recommended conditions.

[0038] By using TEM-EDS, it is possible to select a silicon nitride grain as a measurement point.

[0039] In the measurement using the SIMS method as in Patent Document 2, only large silicon nitride grains can be measured. In addition, even in the nano-SIMS method in which the irradiation diameter is reduced, it is difficult to recognize the image of the silicon nitride grains. Therefore, it is not possible to measure the solid solution oxygen amount of small silicon nitride grains.

[0040] In addition, there is a total dissolution method for measuring the solid solution oxygen amount. The total dissolution method is a method in which the grain boundary phase of the silicon nitride sintered body is melted to extract the silicon nitride grains. The oxygen amount of the extracted silicon nitride grains is measured. However, it is difficult to dissolve and remove the grain boundary phase, and there are problems such as a decrease in measurement accuracy and a decrease in reproducibility caused by the remaining grain boundary phase.

[0041] TEM-EDS can include only silicon nitride grains in the measurement point by setting the spot diameter of the measurement point to 1 nm. In addition, it is possible to measure the solid solution oxygen amount regardless of the size of the silicon nitride grains.

[0042] First, in the EDS analysis, at least 10 measurement points are set from the silicon nitride crystal grains present within 20 μm x 20 μm per unit area. The 10 points are selected as much as possible from different silicon nitride crystal grains 2. For example, one measurement point is set for each of the 10 silicon nitride crystal grains. Through the EDS analysis, the atomic ratios of silicon (Si), oxygen (O), and nitrogen (N) are measured. The measurement points are set so that the number of points at which the Si count is 300,000 cps or more is 3 or more. In the case where the number of points at which the Si count is 300,000 cps or more is less than 3, the measurement points are increased until the number of points at which the Si count is 300,000 cps or more becomes 3 or more. The Si count value of 300,000 cps or more indicates that the oxygen amount can be measured without being affected by surface oxygen. Therefore, even if the sample surface is naturally oxidized, the solid-solution oxygen amount of the silicon nitride crystal grains 2 can be measured.

[0043] A first graph plotting the atomic ratio of the oxygen element / silicon element with respect to the Si count value is prepared. In the first graph, the horizontal axis represents the Si count value (cps), and the vertical axis represents the O / Si atomic ratio. Figure 2 An example of the first graph is shown. Figure 3 An example of the second graph is shown. Figure 4 An example of the third graph is shown. Figures 2 to 4 is a plot based on the measurement results of Example 3 described later.

[0044] Next, a second graph plotting the atomic ratio of the nitrogen element / silicon element with respect to the Si count value is prepared. In the second graph, the horizontal axis represents the Si count value (cps), and the vertical axis represents the N / Si atomic ratio.

[0045] Next, the atomic ratio of the oxygen element / silicon element of the first graph is corrected based on the second graph. This is because, in the silicon nitride sintered body, the X-ray absorption of oxygen (O) as a light element is large compared with that of silicon (Si). The absorption characteristics of oxygen (O) are similar to those of nitrogen (N). In addition, since the main phase of the silicon nitride sintered body 1 is Si3N4, the N / Si atomic ratio of 4 / 3 is a theoretical value. Therefore, the O / Si atomic ratio is corrected based on the approximate data of the atomic ratio of Si and N. In this correction method, the O / Si atomic ratio of the first graph is corrected using the N / Si atomic ratio of each measurement point of the second graph. The correction is performed using the difference between the N / Si atomic ratio of each measurement point and 4 / 3 (=1.33) as a theoretical value. For example, when the N / Si atomic ratio is 0.70, the correction coefficient is 1.9 (=1.33 / 0.70). The correction value is calculated based on the O / Si atomic ratio x the correction coefficient. The O / Si atomic ratio in the first graph is corrected by this method. The graph obtained by correcting the first graph is taken as a third graph.

[0046] In the third graph, the horizontal axis represents the Si count value (cps), and the vertical axis represents the corrected O / Si atomic ratio. From the third graph, a combination of a plurality of 3 or more measurement points is extracted. For each combination of measurement points, an approximate straight line y = aX + b is calculated. 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 with the vertical axis (y axis). From the plurality of approximate straight lines obtained, an approximate straight line that becomes -4 x 10 -8 ≤ a ≤ 4 x 10 -8 is extracted. The approximate straight line extracted uses measurement points present in a convergence region in which the deviation of the O / Si atomic ratio is small. The approximate straight line is created using the approximation function of a table calculation software. As the table calculation software, Microsoft Excel of Microsoft Corporation can be cited.

[0047] The convergence region obtained from the third graph is a region in which the influence of natural oxidation of the sample surface and the grain boundary phase is minimized. In a case where the sample surface is influenced by natural oxidation and the grain boundary phase, the deviation of the O / Si atomic ratio also becomes large. Therefore, the slope a is not in the above range. The slope a of the approximate straight line is in the range of -4 x 10 -8 and 4 x 10 -8 , and indicates that the deviation of the O / Si atomic ratio is reduced. It is known that the deviation of the O / Si atomic ratio is reduced, and thus the influence of natural oxidation and the grain boundary phase is sufficiently small. Therefore, the O / Si atomic ratio of the measurement points included in the convergence region indicates the solid solution oxygen amount.

[0048] In the convergence region, 3 measurement points are extracted in order from large to small in the Si count value. The average value of the O / Si atomic ratio is used to calculate the solid solution oxygen amount. The average value of the O / Si atomic ratio of the 3 measurement points extracted is a value in which the influence of natural oxidation and the grain boundary phase is further reduced.

[0049] Since the silicon nitride crystal grains 2 are Si3N4, the solid solution oxygen amount (wt%) can be calculated by (3 / 7) x (average value of the O / Si atomic ratio). This is a method of calculating from the oxygen amount corresponding to the Si amount in the Si3N4 crystal grains.

[0050] In addition, the convergence region in which the slope a of the approximate straight line of 3 or more is in the range of -4 x 10 -8 or more and 4 x 10 -8 or less is obtained from 3 or more measurement points in which the Si count value is 300,000 cps or more. In the EDS analysis, it is not possible to selectively measure only points of 300,000 cps or more. Therefore, a method of measuring 10 or more places by the EDS analysis is effective. By measuring 10 or more places by the EDS analysis, the oxygen amount obtained by extracting the measurement points included in the convergence region in which the Si count value is 300,000 cps or more is the average value of the solid solution oxygen amount of the silicon nitride crystal grains.

[0051] In the high thermal conductivity silicon nitride sintered body of the embodiment, the solid solution oxygen content is 0.2 wt% or less as measured by the above method. The solid solution oxygen content as measured by TEM-EDS is an average value corresponding to the number of measurement points. That is, the average value of the solid solution oxygen content of the silicon nitride grains present in a unit area of 20 μm x 20 μm is 0.2 wt% or less. Further, the fact that it is an arbitrary cross-sectional structure indicates that the average value of the solid solution oxygen content of the silicon nitride grains present in a unit area of 20 μm x 20 μm is 0.2 wt% or less, regardless of where it is measured.

[0052] As described above, the silicon nitride sintered body in which the solid solution oxygen content of the silicon nitride grains is controlled can improve the thermal conductivity. Further, it can improve the relative dielectric constant characteristics. As described later, it can improve the frequency dependence and the temperature dependence of the relative dielectric constant. The solid solution oxygen is a main cause of reducing the thermal conductivity of the silicon nitride grains. Further, it is a cause of lattice defects. By controlling the solid solution oxygen content, it is possible to reduce the cause of lattice defects. When the solid solution oxygen content exceeds 0.2 wt%, the thermal conductivity, the relative dielectric constant, and the like are reduced.

[0053] Further, it is preferable that the solid solution oxygen content of each of the silicon nitride grains present in a unit area of 20 μm x 20 μm be in a range of 0.01 wt% or more and 0.2 wt% or less. As described above, the method using TEM-EDS can measure only the silicon nitride grains as the measurement region. By controlling the solid solution oxygen content of all the silicon nitride grains, it is possible to further improve the performance. Further, in the measurement of the solid solution oxygen content of each of the silicon nitride grains, one or more measurement points are set for each of the silicon nitride grains present in a unit area of 20 μm x 20 μm. The analysis method is as described above. The analysis method is repeatedly performed until the measurement points set for each of the silicon nitride grains are included in the convergence region and the solid solution oxygen content can be measured. On the basis of the measurement points set for each of the silicon nitride grains, the solid solution oxygen content of 0.01 wt% or more and 0.20 wt% or less indicates that the solid solution oxygen content of each of the silicon nitride grains is controlled.

[0054] With respect to the first silicon nitride grains 2a having a length diameter of less than 5 μm and the second silicon nitride grains 2b having a length diameter of 5 μm or more present in a unit area of 20 μm x 20 μm, the difference between the solid solution oxygen content of the first silicon nitride grains 2a and the solid solution oxygen content of the second silicon nitride grains 2b is preferably 0.03 wt% or less. In a unit area of 20 μm x 20 μm, the solid solution oxygen content obtained by setting measurement points only for the first silicon nitride grains 2a is set as "solid solution oxygen content A". The solid solution oxygen content obtained by setting measurement points only for the second silicon nitride grains 2b is set as "solid solution oxygen content B". | solid solution oxygen content A - solid solution oxygen content B | is preferably 0.03 wt%.

[0055] As described above, there are small silicon nitride grains and large silicon nitride grains, whereby an increase in thermal conductivity and strength can be achieved. By suppressing the variation in solid solution oxygen amount regardless of particle size, the thermal conductivity can be increased to 100 W / (m K) or more, and further to 120 W / (m K) or more.

[0056] Further, by reducing the difference in solid solution oxygen amount between small particles and large particles, the relative dielectric constant can be stabilized, and an increase in electrical properties can be achieved. The relative dielectric constant is a value obtained by dividing the capacitance of an accumulator when a dielectric is filled between electrodes by the capacitance in vacuum. In the present embodiment, the dielectric 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 by an electric field. The greater the polarization, the greater the relative dielectric constant. It is known that the solid solution oxygen amount has an effect on the frequency dependence and temperature dependence of the relative dielectric constant. An increase in the relative dielectric constant easily causes polarization, leading to a decrease in insulation. The solid solution oxygen becomes a cause of lattice defects. Controlling the solid solution oxygen amount and improving the frequency dependence of the relative dielectric constant contribute to improving the insulation. In the past, it was only possible to measure the solid solution oxygen amount of limited silicon nitride grains. Therefore, control of the solid solution oxygen amount was not possible.

[0057] The content of the grain boundary phase 3 in the silicon nitride sintered body 1 is preferably 1 mass% or more and 20 mass% or less. The grain boundary phase 3 is formed by a reaction of the sintering aids with each other, the sintering aids with impurity oxygen on the surface of the silicon nitride powder, and the like. In addition, the grain boundary phase 3 has an effect of firmly binding the silicon nitride grains 2 to each other or suppressing the generation of pores. By controlling the amount of the grain boundary phase 3, the properties depending on the thermal conductivity, the strength, and the relative dielectric constant can be improved. When the grain boundary phase is less than 1 mass%, the proportion of the grain boundary phase is small. When the grain boundary phase 3 is small, pores are easily generated. In addition, when the grain boundary phase 3 exceeds 20 mass%, although the generation of pores can be suppressed, the thermal conductivity is easily decreased. Therefore, the content of the grain boundary phase 3 is preferably 1 mass% or more and 20 mass% or less, and further preferably 3 mass% or more and 15 mass% or less. In addition, by making the porosity 2% or less and the pore diameter 20 μm or less, the strength can be 500 MPa or more, and further 600 MPa or more. In the measurement of the porosity and the pore diameter, the SEM photograph of 50 μm x 50 μm per unit area described above can be used. In addition, the amount (mass%) of the grain boundary phase in the high-thermal-conductivity silicon nitride sintered body 1 can be found by qualitative analysis and quantitative analysis of components other than silicon nitride. In addition, when the amount of addition of the sintering aids can be grasped, the amount of addition of the sintering aids can be regarded as the mass% of the grain boundary phase.

[0058] The grain boundary phase 3 preferably contains one or more selected from the group consisting of a Group 2A element, a Group 3A element, and a Group 4A element. The grain boundary phase 3 preferably further contains one or more selected from the group consisting of a rare earth element, magnesium, titanium, and hafnium. The rare earth element is yttrium (Y), a lanthanoid, or the like. As the rare earth element, one or more selected from the group consisting of yttrium (Y), erbium (Er), ytterbium (Yb), and cerium (Ce) can be mentioned. By the grain boundary phase 3 containing both the rare earth element and magnesium, the sinterability can be improved. More preferably, the grain boundary phase 3 contains one or both of titanium and hafnium in addition to the rare earth element and magnesium. Titanium or hafnium has an effect of strengthening the grain boundary phase 3. The strengthening of the grain boundary phase 3 contributes to the improvement of the strength. The expressions of Group 2A, Group 3A, and Group 4A are based on the periodic table of Japan. The Group 2A element is Be, Mg, Ca, Sr, Ba, and Ra. The Group 3A element is Sc, Y, a lanthanoid, and an actinoid. The Group 4A element is Ti, Zr, and Hf.

[0059] According to the silicon nitride sintered body 1 as described above, the relative dielectric constant at 50 Hz at room temperature is 10 or less. The room temperature means 25°C. The relative dielectric constant is found by measuring the static capacitance. The static capacitance is the electric charge induced and accumulated by an electrode in an alternating electric field. When the insulating property is poor, the electric charge becomes large, and the static capacitance also becomes large. When the static capacitance becomes large, the relative dielectric constant also becomes large. That is, if the insulating property is good, the relative dielectric constant becomes small.

[0060] The relative dielectric constant at 50 Hz at room temperature is denoted by ε 50-25 , and the relative dielectric constant at 50 Hz at 300°C is denoted by ε 50-300 , it is preferable that ε 50-300 / ε 50-25 be in a range of 0.9 or more and 1.2 or less.

[0061] The relative dielectric constant at 1 MHz at room temperature is denoted by ε 1M-25 , and the relative dielectric constant at 1 MHz at 300°C is denoted by ε 1M-300 , it is preferable that ε 1M-300 / ε 1M-25 be in a range of 0.9 or more and 1.2 or less.

[0062] Further, it is preferable that ε 1M-300 / ε 50-300 be in a range of 0.8 or more and 1.2 or less.

[0063] The measurement of the relative dielectric constant is performed in accordance with JIS-C-2141 (1992). The measurement is performed by the complex relative dielectric constant measurement method (3-terminal method) of JIS-C-2141. JIS-C-2141 corresponds to ISO 672-2.

[0064] The silicon nitride sintered body 1 of the embodiment can have a relative dielectric constant εr of 50 Hz at room temperature 50-25 of 10 or less. The relative dielectric constant εr 50-25 of 10 or less indicates that the polarization of the silicon nitride sintered body 1 can be suppressed under the condition. Thus, it indicates that the insulation of the silicon nitride sintered body 1 is high. Therefore, the reliability of a semiconductor device in which the semiconductor element is mounted on the silicon nitride sintered body 1 can be improved.

[0065] In addition, εr 50-300 / εr 50-25 In the range of 0.9 or more and 1.2 or less, it indicates that the amount of change in the relative dielectric constant of 50 Hz is small even if the temperature changes. That is, it indicates that the temperature dependence of the relative dielectric constant of 50 Hz is small. Therefore, even if the operating guaranteed temperature of the semiconductor element becomes high, the characteristics depending on the relative dielectric constant are unlikely to decrease.

[0066] In addition, εr 1M-300 / εr 1M-25 In the range of 0.9 or more and 1.2 or less, it indicates that the amount of change in the relative dielectric constant of 1 MHz is small even if the temperature changes. That is, it indicates that the temperature dependence of the relative dielectric constant of 1 MHz is small. Therefore, even if the operating guaranteed temperature of the semiconductor element becomes high, the characteristics depending on the relative dielectric constant are unlikely to decrease.

[0067] In addition, εr 1M-300 / εr 50-300 of 0.8 or more and 1.2 or less indicates that the amount of change in the relative dielectric constant is small even if the frequency changes. That is, even if the operating frequency of the semiconductor element has changed, the characteristics depending on the relative dielectric constant are unlikely to decrease.

[0068] In addition, it is preferable that εr 50-300 / εr 50-25 , εr 1M-300 / εr 1M-25 , and εr 1M-300 / εr 50-300 all be in the above range.

[0069] The operating frequency of the semiconductor element is diversified to several 10 Hz to 1 MHz. The semiconductor element includes bipolar transistors, MOSFETs, IGBTs, GTOs, and the like. These semiconductor elements are called power semiconductors. The power semiconductors can switch on / off of a switch according to the operating frequency. The range of the operating frequency differs depending on each semiconductor element. By improving the temperature dependence and the frequency dependence of the relative dielectric constant of the silicon nitride sintered body, the insulation is easily maintained regardless of the semiconductor element mounted. Therefore, a semiconductor device with high reliability can be provided.

[0070] Therefore, the silicon nitride sintered body of the embodiment is suitable for a silicon nitride substrate. In addition, 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 thin to 0.1 mm or more and 3 mm or less, the relative dielectric constant is improved, and thus the reliability is high. The substrate is more preferably in a range of 0.1 mm or more and 0.4 mm or less. By thinning the substrate, the effect of reducing the thermal resistance is obtained. Therefore, the heat dissipation property is improved.

[0071] The invention of the embodiment is suitable for a silicon nitride circuit substrate in which a circuit portion is provided in a silicon nitride substrate. Figure 5 is a schematic view showing an example of the silicon nitride circuit substrate of the embodiment. In Figure 5 , reference numeral 4 is a bonding layer, reference numeral 5 is a metal plate (front surface metal plate), reference numeral 6 is a metal plate (back surface metal plate), reference numeral 10 is a silicon nitride substrate, and reference numeral 20 is a silicon nitride circuit substrate.

[0072] 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 the circuit portion is sometimes referred to as a front surface metal plate. In addition, the metal plate 6 is used as a heat dissipation plate. The metal plate 6 used as the heat dissipation plate is sometimes referred to as a back surface metal plate. In Figure 5 , two front surface metal plates 5 are provided. The number of the front surface metal plates 5 is arbitrary, not limited to the example shown in the drawing. The back surface metal plate 6 can be used as a circuit portion instead of a heat dissipation plate.

[0073] As the metal plate 5 and the metal plate 6, a copper plate, a copper alloy plate, an aluminum plate, an aluminum alloy plate, and the like can be given. The metal plate is preferably a copper plate composed of oxygen-free copper. The oxygen-free copper has a copper purity of 99.96 wt% or more as shown in JIS-H-3100 (ISO 1337 or the like). The thermal conductivity of the copper plate is about 400 W / (m·K). The thermal conductivity of aluminum is about 240 W / (m·K). The copper plate has a higher thermal conductivity than the aluminum plate. Therefore, the use of the copper plate can further improve the heat dissipation property. In addition, the thickness of the metal plate 5 and the metal plate 6 is preferably in a range of 0.2 mm or more and 5 mm or less. By thickening the metal plate, the heat dissipation property and the current carrying capacity can be improved.

[0074] As the joining layer 4, an active metal joining layer can be given. In the case where the metal plate is a copper plate, as the active metal joining layer, a member containing Ti in addition to Ag or Cu as a main component can be given. In the case where the metal plate is an aluminum plate, as the active metal joining layer, a member containing Si in addition to Al as a main component can be given. The active metal joining layer is a layer containing Ti or Si as an active metal. In addition, as the circuit portion, an example of joining metal plates is shown, but the application of the embodiment is not limited to this. The circuit portion can also be formed of a metal thin film or a metal thick film. The metal thin film is a metal film formed by a film formation method such as sputtering or evaporation. As the metal thin film, a film of Ti, Pt, Au, Ni, Cu, Al, or Ag, or the like can be given. The metal thick film is a film formed by firing a metal paste. The metal thick film is also sometimes referred to as a metallized film. As the metal thick film, a film of Ag, Cu, Ti, W, or Mo, or the like can be given.

[0075] Figure 6 is a schematic view showing an example of a semiconductor device of the embodiment. In Figure 6 , reference numeral 30 is a semiconductor device, reference numeral 31 is a semiconductor element, and reference numeral 32 is a lead frame. In the semiconductor device 30 shown in Figure 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 configuration of the semiconductor device 30 of the embodiment is not limited to the example shown. The number and size of the metal plates 5, the number and size of the semiconductor elements 31, and the like can be appropriately changed as needed. In addition, the metal plate 6 can be used as a circuit portion, and a semiconductor element 31 can be mounted on the metal plate 6.

[0076] Next, a method for manufacturing the high thermal conductivity silicon nitride sintered body 1 of the embodiment will be described. The method for manufacturing the high thermal conductivity silicon nitride sintered body 1 of the embodiment is not limited as long as the high thermal conductivity silicon nitride sintered body 1 has the above-described configuration. Here, a method for obtaining the silicon nitride sintered body 1 with a good yield will be given.

[0077] First, a silicon nitride powder is prepared. The average particle diameter 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 of the silicon nitride powder has a component dissolved in the powder and a component adhered to the surface of the powder. 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. Therefore, the amount of impurity oxygen of the silicon nitride powder is preferably 2 mass% or less, and more preferably 1 mass% or less.

[0078] As the silicon nitride powder, there are mainly powders produced by an imide decomposition method or a direct nitriding method. The imide decomposition method is preferred because the amount of impurity oxygen solid-solved in the powder is small. In addition, the silicon nitride powder has an α type and a β type. The β type silicon nitride powder reduces solid-solved oxygen in terms of crystal structure. On the other hand, the α type silicon nitride powder has a high sinterability compared to the β type, and thus is easily densified. By using the α type in the raw material powder, a silicon nitride sintered body having a high strength can be obtained. As the silicon nitride powder, a powder in which the α type silicon nitride powder and the β type silicon nitride powder are mixed can also be used. In addition, 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 a range of 1 part by mass or more and 30 parts by mass or less. When the β type exceeds 30 parts by mass, the sinterability can decrease. In addition, when it is less than 1 part by mass, the effect of using the β type can be insufficient. In addition, the α rate of the commercially available α type silicon nitride powder is 90 wt% or more. That is, the commercially available α type silicon nitride powder inevitably contains a small amount of the β type silicon nitride powder. The material in which the α type silicon nitride powder and the β type silicon nitride powder are mixed refers to a material in which the β type silicon nitride powder is positively added to the commercially available α type silicon nitride powder. In other words, a material composed of the α type silicon nitride powder which inevitably contains the β type silicon nitride powder is not treated as a material in which the α type silicon nitride powder and the β type silicon nitride powder are mixed.

[0079] In addition, it is also effective to perform a treatment for reducing the impurity oxygen of the silicon nitride powder. As the treatment for reducing the impurity oxygen, a reduction treatment, a liquid medicine treatment, and the like can be given. The reduction treatment can be given by a heat treatment in a hydrogen atmosphere, a heat treatment in an atmosphere in which carbon is present, and the like. In addition, the liquid medicine treatment is a method of treating the silicon nitride powder with an acidic or basic solution.

[0080] Next, a sintering aid is prepared. The sintering aid is a component which promotes sintering and becomes a grain boundary phase. The sintering aid is preferably one or more selected from the group consisting of a 2A group element, a 3A group element, and a 4A group element. The 2A group, the 3A group, and the 4A group are expressions based on the periodic table of Japan. The sintering aid preferably further contains one or more selected from the group consisting of a rare earth element, 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 by a pinning effect. The sintering aid is preferably added as an oxide powder. The oxides react with each other, and a stable grain boundary phase can be formed. More preferably, the sintering aid is a metal oxide powder having an average particle diameter of 3 μm or less.

[0081] Next, a process of mixing the silicon nitride powder and the sintering aid powder is performed. In order to control the aspect ratio and the length-width ratio of the silicon nitride grains in the silicon nitride sintered body, homogeneity of sinterability is required. For this reason, it is necessary to uniformly mix the silicon nitride powder and the sintering aid powder. In the sintering process, the sintering aid reacts to become a grain boundary phase. The growth reaction of the silicon nitride particles proceeds via the grain boundary phase. In this process, oxygen is detached from the silicon nitride particles. By uniformly mixing the silicon nitride powder and the sintering aid powder, it is possible to homogenize the reaction via the grain boundary phase.

[0082] In addition, in the mixing process, a ball mill or a bead mill is used.

[0083] The silicon nitride powder and the sintering aid powder mostly exist as agglomerated secondary particles. The secondary particles are a hindering factor of homogenization of sinterability. By breaking the secondary particles into primary particles that are not agglomerated while uniformly mixing, it is possible to improve the homogeneity of sinterability. In the mixing process that accompanies this breaking, it is preferable not to apply a strong stress that further damages the primary particles. If the primary particles are damaged, a fracture surface is formed in the silicon nitride powder. Since the fracture surface is an active surface, an oxide film is formed in order to stabilize, and the oxygen adhesion amount increases. The oxygen amount increases more than the amount that adheres to the primary particles. Therefore, it is effective to suppress the formation of a fracture surface on the primary particles.

[0084] Such breaking of the secondary particles is suitable for wet breaking using a solvent. A solvent that has a large wettability to the surface of the particles and a small reactivity with the particles is used. In this way, the stress required for breaking is small, and it is possible to suppress damage to the primary particles. In the mixing that accompanies breaking of the silicon nitride powder and the sintering aid powder, an organic solvent is suitable. As the organic solvent, an alcohol and a ketone are suitable. An organic solvent that is a mixture of an alcohol and a ketone can also be used. The alcohol is a general term for substances obtained by replacing a part of the hydrogen contained in a hydrocarbon with a silicon hydride group (OH group). In addition, the ketone is represented by R-C(=O)-R'. R and R' are alkyl groups or the like. These organic solvents have a large wettability to the silicon nitride powder and the sintering aid powder and a small reactivity with the powder. The sintering aid powder is mostly added as an oxide powder. In the raw material composition in which the silicon nitride powder and the oxide powder are mixed, a mixed liquid of an alcohol and a ketone is preferable. In addition, a dispersant can also be added as needed. The dispersant has an effect of stabilizing the primary particles in the solvent and suppressing re-agglomeration. As the dispersant, a surfactant or the like can be given.

[0085] In the case of performing the breaking process using a ball mill, the diameter of the medium is preferably 20 mm or less, and more preferably 12 mm or less. The medium is a ceramic ball. The ball mill is a method in which a powder and a medium are put in a cylindrical container, and the powder is broken while the cylindrical container is rotated. The ball milling process using an organic solvent as described above is a wet breaking mixing. By selecting an organic solvent suitable for the powder, the stress required for breaking is reduced. That is, the breaking can be performed using a ceramic ball having a small diameter, and the energy of the collision between the medium and the powder is reduced. By reducing the energy of the collision between the medium and the powder, the formation of a fracture surface in the primary particles is suppressed. In addition, the minimum value of the diameter of the medium is preferably 3 mm or more. If the medium is too small, the work efficiency can be reduced.

[0086] The time for the wet breaking mixing performed using the ball mill is preferably in the range of 5 hours or more and 40 hours or less. If less than 5 hours, the effect of breaking is insufficient, and a large amount of secondary particles can remain. If more than 40 hours, the possibility of the formation of a fracture surface in the primary particles increases. Therefore, the time for the wet breaking mixing performed using the ball mill is preferably in the range of 5 hours or more and 40 hours or less, and further preferably in the range of 10 hours or more and 30 hours or less. In addition, the rotation speed of the cylindrical container of the ball milling process is preferably in the range of 50 rpm or more and 500 rpm or less.

[0087] The breaking of the secondary particles can be grasped by investigating the particle size distribution before and after the breaking. By breaking the secondary particles, the primary particles increase, and the peak position of the particle size distribution (frequency distribution) shifts to the side of the small particle size. In addition, the peak of the particle size distribution becomes a sharp shape.

[0088] The suppression of the formation of a fracture surface in the primary particles can be grasped by measuring the oxygen amount before and after the breaking. The oxygen amount before the breaking is the oxygen amount of the raw material powder. If the oxygen amount of the raw material powder after the breaking does not greatly increase compared to before the breaking, there is no problem.

[0089] After the wet breaking mixing process is performed, a raw material powder slurry is obtained. A molding process of forming a molded body using the raw material powder slurry is performed. As the molding process, sheet molding, mold molding, and the like can be given. The sheet molding is a doctor blade method or the like. In the case of performing the sheet molding, cutting processing of cutting a long sheet into a desired size is performed. The slurry can also be used after being treated by a granulation powder or the like suitable for the molding method.

[0090] A debinding process of the molded body 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 matter in the molded body can be removed. In addition, the material after the debinding process is called a debound body.

[0091] Next, a sintering process of the sintered and debound body is performed. The sintering process preferably includes a first holding process in a range of 1500°C or higher and 1650°C or lower and a second holding process in a range of 1750°C or higher and 2000°C or lower.

[0092] The first holding process is a process of holding the debound body in 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 a temperature region in which the silicon nitride starts grain growth. Further, this temperature range also corresponds to a temperature at which impurity oxygen starts to be detached from the silicon nitride powder. The impurity oxygen is detached as SiO (silicon monoxide). By performing the first holding process, the solid-solution oxygen amount in the silicon nitride powder can be effectively reduced. On this basis, the grain growth of the silicon nitride grains can be homogenized. The holding time of the first holding process is preferably 10 hours or shorter. If the holding time exceeds 10 hours, oxygen required for generation of a liquid phase together with a sintering aid is insufficient, and the sinterability can be reduced. Therefore, the holding time of the first holding process is preferably 2 hours or longer and 10 hours or shorter, and further preferably 3 hours or longer and 6 hours or shorter. In addition, this holding process is preferably performed in a reduced pressure atmosphere of normal pressure (0.1 MPa) or lower. In a so-called pressurized atmosphere exceeding 0.1 MPa, there is a tendency that the detachment of SiO is inhibited.

[0093] The second holding process is a process of holding the debound body in a range of 1750°C or higher and 2000°C or lower. The holding temperature of the second holding process is a so-called sintering temperature. If this temperature range, the sintered body can be densified. In addition, the aspect ratio and the length-width ratio of the silicon nitride grains can be controlled. If the holding temperature is lower than 1750°C, there is a possibility that the densification is insufficient. If it exceeds 2000°C, there is a possibility that the silicon nitride grains excessively grain grow. In addition, the holding time of the second holding process is preferably in a range of 5 hours or longer and 30 hours or shorter. The second holding process is preferably performed in a pressurized atmosphere of 0.5 MPa or higher. In a pressure atmosphere of 0.1 MPa (normal pressure) or lower, self-decomposition of the silicon nitride easily occurs at 1700°C or higher, and thus the densification can be insufficient.

[0094] The sintering process is preferably performed in a non-oxidizing atmosphere. As the non-oxidizing atmosphere, a nitrogen atmosphere, an argon atmosphere, or a vacuum atmosphere can be given. In the non-oxidizing atmosphere, oxygen can be inhibited from being solid-solved in the silicon nitride grains.

[0095] The sintered body can be obtained by the sintering process. In addition, the sintered body can be subjected to a reheat treatment. By providing a process of holding the sintered body in a range of 1400°C or higher and 1600°C or lower after the sintering, the SiO can be more effectively removed. The sintered body becomes a state having a stable crystal structure, and thus the adverse effect of the SiO removal by the reheat treatment on the crystal structure is small. The holding time in the range of 1400°C or higher and 1600°C or lower in the reheat treatment is preferably 2 hours or longer. The holding process is also preferably performed in a reduced pressure atmosphere of normal pressure (=0.1 MPa) or lower. In a so-called pressurized atmosphere exceeding 0.1 MPa, there is a tendency that the SiO removal is inhibited. The reheat treatment can be performed after the sintered body is returned to room temperature after the second holding process. In addition, after the second holding process, the sintered body can be processed by being cooled to 1300°C or lower and then being heated again. 1300°C or lower is a temperature at which a liquid phase composed of the sintering aid and the like is solidified without contributing to the sintering reaction. That is, as in the state returned to room temperature, it is considered that the sintered body is in a state of maintaining a stable crystal structure, and thus the adverse effect of the SiO removal by the reheat treatment on the crystal structure is small.

[0096] In addition, the maximum holding temperature in the reheat treatment is lower than the holding temperature in the second holding process in the sintering. The difference between the maximum holding temperature in the reheat treatment and the holding temperature in the second holding process is preferably 50°C or higher and 300°C or lower. For example, in the case where the holding temperature in the second holding process is 1800°C, the reheat treatment temperature is preferably in a range of 1500°C or higher and 1750°C or lower. The reheat treatment not only causes the oxygen in the sintered body to be removed as SiO outside the sintered body, but also has an effect of inhibiting the deviation of the amount of solid-solution oxygen between the silicon nitride grains. In addition, by making the maximum holding temperature in the reheat treatment lower than the holding temperature in the second holding process in the sintering, the grain growth of the silicon nitride grains can be inhibited. The reheat treatment is preferably performed in a non-oxidizing atmosphere. By performing the reheat treatment in a non-oxidizing atmosphere, the entry of oxygen into the silicon nitride grains can be inhibited.

[0097] By the above processes, the silicon nitride sintered body 1 of the embodiment can be manufactured. In addition, by producing the silicon nitride sintered body 1 in the shape of a substrate, a silicon nitride substrate can be manufactured. By performing a process of providing a circuit portion on the silicon nitride substrate, a silicon nitride circuit substrate can be manufactured. By mounting a semiconductor element on the circuit portion of the silicon nitride circuit substrate, a semiconductor device can be manufactured.

[0098] (Example)

[0099] (Examples 1 to 7, Comparative Examples 1 to 3)

[0100] The silicon nitride powder shown in Table 1 was prepared. The mixing ratio is the mass ratio when the total of the α-type and β-type is 100 parts by mass. In addition, as the α-type silicon nitride powder, a powder having an α conversion rate of 90 wt% or more was used.

[0101] [Table 1]

[0102]

[0103] Next, the sintering aid powder shown in Table 2 was mixed. The average particle diameter of the sintering aid powder was 3 μm or less. In addition, the mixing ratio of the sintering aid was the ratio when the total of the silicon nitride powder and the sintering aid powder was 100 mass%.

[0104] [Table 2]

[0105] Silicon nitride powder Sintering aid (mass %) Example 1 Silicon nitride powder 1 Er203(5), MgO (2), Hf02(1) Example 2 Silicon nitride powder 1 [Y2O3(6), MgO(1), HfO2(1)] Example 3 Silicon nitride powder 2 [Y2O3(3), MgO(2), TiO2(0.5)] Example 4 Silicon nitride powder 3 [Y2O3(7), MgO(05), ZrO2(1)] Example 5 Silicon nitride powder 3 CeO2(6), MgO(1), HfO2(3) Example 6 Silicon nitride powder 5 [Y2O3(4), Er2O3(4), MgO(1)] Example 7 Silicon nitride powder 5 [Y2O3(5), MgO(1)] Comparative Example 1 Silicon nitride powder 1 [Y2O3(3), MgO(1), TiO2(0.5)] Comparative Example 2 Silicon nitride powder 4 [Y2O3(7), MgO(1), ZrO2(2)] Comparative Example 3 Silicon nitride powder 1 [Y2O3(3), MgO(1), TiO2(0.5)]

[0106] Next, the organic solvent was mixed in the silicon nitride powder and the sintering aid powder, and a wet-type crushing and mixing process was performed. The wet-type crushing and mixing process was performed using a ball mill under the conditions shown in Table 3. The rotation speed of the cylindrical container of the ball mill was set in the range of 50 rpm or more and 500 rpm or less. In Comparative Example 2, the wet-type mixing was performed using water. In Comparative Example 3, dry-type mixing was performed.

[0107] [Table 3]

[0108]

[0109] The raw material powder slurry was prepared by the wet-type crushing and mixing process. The raw material powder slurry was used for sheet molding. The sheet molding was performed by a doctor blade method. The long sheet was cut to a prescribed size. Then, a debinding process was performed, and a debound body was prepared.

[0110] The sintering process was performed on each debound body. As the sintering process, a first holding process and a second holding process were performed. In addition, a third holding process (reheating treatment) was performed after the sintering process. The conditions of the sintering process are shown in Tables 4 and 5. In addition, the first holding process, the second holding process, and the third holding process were performed in a non-oxidizing atmosphere.

[0111] [Table 4]

[0112]

[0113] [Table 5]

[0114]

[0115] The silicon nitride sintered bodies of the Examples and Comparative Examples were produced by the above sintering process. The silicon nitride sintered bodies were processed into silicon nitride substrates of 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 produced. In Examples 4 to 5, silicon nitride substrates having a thickness of 0.25 mm were produced. In the silicon nitride substrates of the Examples and Comparative Examples, the content of the grain boundary phase was in the range of 1 mass% or more and 20 mass% or less.

[0116] The solid-solution oxygen content of the silicon nitride grains contained in the silicon nitride substrates, the average length of the length-diameter of the silicon nitride grains, and the average aspect ratio were measured.

[0117] In the measurement of the solid-solution oxygen content of the silicon nitride grains, a unit area of 20 μm x 20 μm of an arbitrary cross section was set as a measurement point, and TEM-EDS was used. The measurement conditions of TEM-EDS were as described above. The solid-solution oxygen content of each of the silicon nitride grains contained in the unit area of 20 μm x 20 μm was measured, and the average value thereof was calculated. The method of measuring the solid-solution oxygen content was as described above. Figures 2 to 4 The first graph, the second graph, and the third graph in FIG. 6 are graphs showing the results of measuring the solid-solution oxygen content of the silicon nitride grains in Examples 1 to 3, respectively. In each of the graphs, the horizontal axis represents the solid-solution oxygen content of the silicon nitride grains, and the vertical axis represents the number of the silicon nitride grains. In each of the graphs, the solid-solution oxygen content of each of the silicon nitride grains is shown in the range of 0.1 wt% or more and 0.2 wt% or less. In each of the graphs, the average value of the solid-solution oxygen content of the first silicon nitride grains is shown in the range of 0.1 wt% or more and 0.2 wt% or less. In each of the graphs, the average value of the solid-solution oxygen content of the second silicon nitride grains is shown in the range of 0.1 wt% or more and 0.2 wt% or less. The first silicon nitride grains are silicon nitride grains having a length-diameter of less than 5 μm. The second silicon nitride grains are silicon nitride grains having a length-diameter of 5 μm or more.

[0118] In the measurement of the average length of the length-diameter of the silicon nitride grains 2 and the average aspect ratio, a unit area of 50 μm x 50 μm of an arbitrary cross section was set as a measurement point, and a SEM photograph was used. The method of using the SEM photograph was as described above.

[0119] The results of these measurements are shown in Tables 6 and 7.

[0120] [Table 6]

[0121]

[0122] [Table 7]

[0123]

[0124] As shown in Table 6, in the silicon nitride substrates of the Examples, the solid-solution oxygen content was 0.2 wt% or less. In contrast, in the silicon nitride substrates of the Comparative Examples, the solid-solution oxygen content exceeded 0.2 wt%. In the average value of the length-diameter of the silicon nitride grains and the average aspect ratio, there was no large difference between the Examples and the Comparative Examples. In Examples 1 to 7, there were silicon nitride grains having a length-diameter of less than 3 μm.

[0125] Next, the thermal conductivities and the fracture toughness values of the respective silicon nitride substrates were measured. The thermal conductivities were measured by a laser flash method. The fracture toughness values were measured in accordance with JIS-R-1607 (IF method) and were calculated by a new formula. The results are shown in Table 8. JIS-R-1607 corresponds to ISO 15732.

[0126] [Table 8]

[0127]

[0128] As is apparent from Table 8, the thermal conductivities of the silicon nitride substrates of the examples were 80 W / (m-K) or more. In addition, with respect to Example 1, Example 2, and Example 4, the thermal conductivities were 100 W / (m-K) or more. With respect to the strength, there was no large difference between the examples and the comparative examples.

[0129] Next, the relative dielectric constants of the silicon nitride substrates of the examples and the comparative examples were measured. The relative dielectric constants were measured in accordance with the complex relative dielectric constant measurement method (3-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 Table 9 and Table 10.

[0130] [Table 9]

[0131]

[0132] [Table 10]

[0133]

[0134] As is apparent from Table 9 and Table 10, with respect to the silicon nitride substrates of the examples, the frequency dependence and the temperature dependence of the relative dielectric constant were improved. In contrast, in the comparative examples, the frequency dependence and the temperature dependence of the relative dielectric constant were reduced.

[0135] As is apparent from the above, the silicon nitride substrates of the examples are suitable as substrates for mounting semiconductor elements having a high guaranteed operating temperature or semiconductor elements having a high operating frequency.

[0136] Embodiments of the present application can include the following structures.

[0137] (Postscript 1)

[0138] A high-thermal-conductivity silicon nitride sintered body having silicon nitride grains and a grain boundary phase,

[0139] The thermal conductivity of the silicon nitride sintered body is 80 W / (m-K) or more,

[0140] an average value of the solid-solution oxygen amount of the silicon nitride crystal grains present per unit area of 20 μm x 20 μm in any cross section is 0.2 wt% or less,

[0141] an average value of the aspect ratio of the silicon nitride crystal grains present per unit area of 50 μm x 50 μm in any cross section is 2 or more and 10 or less,

[0142] an average value of the aspect ratio of the silicon nitride crystal grains present per unit area of 50 μm x 50 μm in any cross section is 2 or more and 10 or less.

[0143] (Note 2)

[0144] The high thermal conductivity silicon nitride sintered body according to Note 1, wherein the solid-solution oxygen amount of each of the silicon nitride crystal grains present per unit area of 20 μm x 20 μm is in a range of 0.01 wt% or more and 0.2 wt% or less.

[0145] (Note 3)

[0146] The high thermal conductivity silicon nitride sintered body according to any one of Notes 1 to 2, wherein, in the unit area of 20 μm x 20 μm, there are first silicon nitride crystal grains having an aspect ratio of less than 5 μm and second silicon nitride crystal grains having an aspect ratio of 5 μm or more,

[0147] a difference between the solid-solution oxygen amount of the first silicon nitride crystal grains and the solid-solution oxygen amount of the second silicon nitride crystal grains is 0.03 wt% or less.

[0148] (Note 4)

[0149] The high thermal conductivity silicon nitride sintered body according to any one of Notes 1 to 3, wherein the content of the grain boundary phase is 1 mass% or more and 20 mass% or less.

[0150] (Note 5)

[0151] The high thermal conductivity silicon nitride sintered body according to any one of Notes 1 to 4, wherein a relative dielectric constant at 50 Hz and room temperature is 10 or less.

[0152] (Note 6)

[0153] The high thermal conductivity silicon nitride sintered body according to any one of Notes 1 to 5, wherein, when a relative dielectric constant at 50 Hz and room temperature is set to ε 50-25 , and a relative dielectric constant at 50 Hz and 300°C is set to ε 50-300 , ε 50-300 / ε 50-25 is in a range of 0.9 or more and 1.2 or less.

[0154] (Note 7)

[0155] The high thermal conductivity silicon nitride sintered body according to any one of the supplementary notes 1 to 6, wherein ε 1M-25 / ε 1M-300 is in a range of 0.9 or more and 1.2 or less when a relative dielectric constant at 1 MHz and room temperature is set as ε 1M-300 / ε 1M-25

[0156] (Supplementary Note 8)

[0157] The high thermal conductivity silicon nitride sintered body according to any one of the supplementary notes 1 to 7, wherein the thermal conductivity is 100 W / (m·K) or more.

[0158] (Supplementary Note 9)

[0159] A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to any one of the supplementary notes 1 to 8.

[0160] (Supplementary Note 10)

[0161] The silicon nitride substrate according to the supplementary note 9, wherein the thickness is 0.1 mm or more and 3 mm or less.

[0162] (Supplementary Note 11)

[0163] A silicon nitride circuit substrate comprising:

[0164] the silicon nitride substrate according to the supplementary note 9 or 10; and

[0165] a circuit portion provided on the silicon nitride substrate.

[0166] (Supplementary Note 12)

[0167] A semiconductor device comprising:

[0168] the silicon nitride circuit substrate according to the supplementary note 11; and

[0169] a semiconductor element mounted on the circuit portion.

[0170] The above illustrates several embodiments of the present application, but these embodiments are suggested as examples and are not intended to limit the scope of the application. These new embodiments can be implemented in other various ways, and various omissions, substitutions, changes, and the like can be made within the scope of the gist of the application. These embodiments and their modifications are included in the scope and gist of the application, and are included in the scope of the application and its equivalents recited in the claims. In addition, the above-described embodiments can be implemented in combination with each other.

[0171] Explanation of Reference Signs

[0172] ​1…high-thermal-conductivity silicon nitride sintered body, 2…silicon nitride crystal grains, 3…grain boundary phase, 4…bonding layer, 5…metal plate (front surface metal plate), 6…metal plate (back surface metal plate), 10…silicon nitride substrate, 20…silicon nitride circuit substrate, 30…semiconductor device, 31…semiconductor element, 32…lead frame

Claims

1. A high thermal conductivity silicon nitride sintered body, comprising silicon nitride grains and a grain boundary phase, the thermal conductivity of the silicon nitride sintered body is 80 W / (m·K) or more, an average value of a solid solution oxygen amount of the silicon nitride grains present in a unit area of 20 μm x 20 μm in any cross section is 0.2 wt% or less, an average value of an aspect ratio of the silicon nitride grains present in a unit area of 50 μm x 50 μm in any cross section is 1 μm or more and 10 μm or less, an average value of a length-width ratio of the silicon nitride grains present in the unit area of 50 μm x 50 μm is 2 or more and 10 or less, When the relative dielectric constant at 50 Hz and room temperature is set as ε 50-25 , and the relative dielectric constant at 50 Hz and 300°C is set as ε 50-300 , ε 50-300 / ε 50-25 is in a range of 0.9 or more and 1.2 or less.

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

3. The high thermal conductivity silicon nitride sintered body according to claim 1 or 2, wherein first silicon nitride grains having an aspect ratio of less than 5 μm and second silicon nitride grains having an aspect ratio of 5 μm or more are present in the unit area of 20 μm x 20 μm, a difference between the solid solution oxygen amount of the first silicon nitride grains and the solid solution oxygen amount of the second silicon nitride grains is 0.03 wt% or less.

4. The high thermal conductivity silicon nitride sintered body according to claim 1 or 2, wherein a content of the grain boundary phase is 1 mass% or more and 20 mass% or less.

5. The high thermal conductivity silicon nitride sintered body according to claim 1 or 2, wherein a relative dielectric constant at 50 Hz and room temperature is 10 or less.

6. The high thermal conductivity silicon nitride sintered body according to claim 1 or 2, wherein When the relative dielectric constant at 1 MHz and room temperature is set as ε 1M-25 , and the relative dielectric constant at 1 MHz and 300°C is set as ε 1M-300 , ε 1M-300 / ε 1M-25 is in a range of 0.9 or more and 1.2 or less.

7. The high thermal conductivity silicon nitride sintered body according to claim 1 or 2, wherein the thermal conductivity is 100 W / (m·K) or more.

8. The high thermal conductivity silicon nitride sintered body according to claim 5, wherein When the relative dielectric constant at 1 MHz and room temperature is set as ε 1M-25 , and the relative dielectric constant at 1 MHz and 300°C is set as ε 1M-300 , ε 1M-300 / ε 1M-25 is in a range of 0.9 or more and 1.2 or less.

9. The high thermal conductivity silicon nitride sintered body according to claim 8, wherein first silicon nitride grains having an aspect ratio of less than 5 μm and second silicon nitride grains having an aspect ratio of 5 μm or more are present in the unit area of 20 μm x 20 μm, a difference between the solid solution oxygen amount of the first silicon nitride grains and the solid solution oxygen amount of the second silicon nitride grains is 0.03 wt% or less.

10. A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to claim 8.

11. A silicon nitride circuit substrate, comprising: the silicon nitride substrate according to claim 10; and a circuit portion provided on the silicon nitride substrate.

12. A semiconductor device, comprising: the silicon nitride circuit substrate according to claim 11; and a semiconductor element mounted on the circuit portion.

13. A silicon nitride substrate using the high thermal conductivity silicon nitride sintered body according to claim 1 or 2.

14. The silicon nitride substrate according to claim 13, wherein a thickness is 0.1 mm or more and 3 mm or less.

15. A silicon nitride circuit substrate, comprising: the silicon nitride substrate according to claim 13; and a circuit portion provided on the silicon nitride substrate.

16. A semiconductor device, comprising: the silicon nitride circuit substrate according to claim 15; and A semiconductor element is mounted to the circuit portion. A semiconductor element is mounted to the circuit portion.

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