Silicon nitride sintered body

Abstract

To provide a silicon nitride sintered compact for which a high breakdown voltage is required when it is formed to a thickness of about 100 μm, and to reduce warpage of the silicon nitride sintered compact.SOLUTION: A silicon nitride sintered compact has a planar projected area ratio of voids of 1.0% or less and a breakdown voltage of 5 kV or more when an AC voltage is applied to a plate-shaped silicon nitride sintered compact having a thickness of 100 μm in at least one area of 64 μm×48 μm of a polished surface of a silicon nitride sintered compact whose surface has been polished 50 μm or over.SELECTED DRAWING: Figure 2

Description

【Technical Field】 【0001】 The present invention relates to a silicon nitride sintered body used as a circuit board, a heat radiating member, etc. 【Background Art】 【0002】 As materials for insulating ceramics (sintered bodies) used as circuit boards, heat radiating members, etc., aluminum nitride (AlN) and silicon nitride (Si3N4) can be mentioned. Although aluminum nitride has a high thermal conductivity of 150 W / m·K or more, its mechanical strength is low, so cracks are likely to occur and it is difficult to use. Although the thermal conductivity of silicon nitride is not as high as that of aluminum nitride but is 50 W / m·K or more, and it has high mechanical strength, so it has advantages such as being less likely to crack and being able to be thinned. Therefore, in recent years, the development and adoption of silicon nitride sintered bodies have been progressing. 【0003】 Patent Document 1 describes a silicon nitride sintered body in which the crystal phase existing in the grain boundary phase of silicon nitride crystal grains has an intensity ratio of X-ray diffraction peaks of 0.05 to 0.40 (taking silicon nitride as 1), and a manufacturing method in which a green sheet of the material is sintered at 1600 to 1900 °C in a nitrogen atmosphere and then the residual glass phase is removed at 1100 to 1700 °C. 【0004】 Patent Document 2 describes a silicon nitride substrate that improves the bonding property of circuit members, etc., and has an insulation breakdown voltage of 36 to 47 kV / mm (Table 6 of the same document) measured with a substrate having a thickness of 3 mm, and a manufacturing method in which a green sheet of the material is placed in a firing furnace in which a co-material of magnesium oxide and erbium oxide is arranged to suppress the volatilization of components and sintered at 1750 °C for 3 to 5 hours. 【0005】 Patent Document 3 describes a silicon nitride substrate having a porosity of 0-1.0%, a maximum pore diameter of 0.2-3 μm, and a thickness of 0.15-0.635 mm, with an dielectric strength of 17-29 kV / mm (Tables 7 and 8 in the same document), and a manufacturing method for sintering a green sheet of the same material at 1800-1900°C in a non-oxidizing atmosphere. 【0006】 Patent Document 4 describes a silicon nitride substrate having a warp of 2.0 μm / mm or less, and a manufacturing method for suppressing warp by sintering a green sheet of the same material at 1800 to 2000°C for 8 to 18 hours in a nitrogen pressurized atmosphere, and then heat-treating it at 1550 to 1700°C while applying a load. 【0007】 Patent Document 5 describes a silicon nitride substrate having low warping and high strength, and a manufacturing method in which a green sheet of the material is placed in a firing container containing packing powder such as magnesium oxide to suppress the volatilization of silicon nitride and magnesia, and sintered at 1860°C for 5 hours. 【0008】 Patent document 6 describes a silicon nitride substrate having a void ratio of 0.1 to 4% and a thickness of 0.15 to 0.25 mm, with a dielectric breakdown strength of 32 to 36 kV / mm (Table 3 in the same document), and a manufacturing method for a green sheet of the same material, which is sintered in a nitrogen atmosphere at 1850 to 1900°C for 3 to 5 hours. [Prior art documents] [Patent Documents] 【0009】 [Patent Document 1] Japanese Patent Application Publication No. 5-279124 [Patent Document 2] International Publication No. 2011-087055 [Patent Document 3] Japanese Patent Publication No. 2017-178776 [Patent Document 4] Japanese Patent Publication No. 2009-218322 [Patent Document 5] Japanese Patent Publication No. 2020-93978 [Patent Document 6] Japanese Patent Publication No. 2014-73937 [Overview of the Initiative] [Problems that the invention aims to solve] 【0010】 Patent Document 3 states that "it is possible to reduce the substrate thickness to 0.1 mm," but the examples described only go up to a substrate with a thickness of 0.15 mm in which the dielectric strength measured is 23 kV / mm. Patent Document 6 states that "even thin silicon nitride substrates with a thickness of about 0.1 to 0.4 mm have a dielectric breakdown strength of 31 kV / mm or more, and even 35 kV / mm or more," but the examples described only go up to a substrate with a thickness of 0.15 mm in which the dielectric breakdown strength measured is 32 kV / mm. 【0011】 Dielectric breakdown voltage is often measured for sintered bodies with a thickness greater than 100 μm (e.g., 300 μm), but the value obtained by converting the measurement result for such sintered bodies to a value per 100 μm is merely a theoretical value. Therefore, it cannot be guaranteed that the dielectric breakdown voltage of the sintered body when actually formed to a thickness of approximately 100 μm will be the same as the converted value. This invention makes that guarantee possible. [Means for solving the problem] 【0012】 [1] In any one 64 μm × 48 μm area of ​​the polished surface of a silicon nitride sintered body polished to a thickness of 50 μm or more, the planar projected area ratio of voids is 1.0% or less. A silicon nitride sintered body having a thickness of 100 μm and exhibiting a dielectric breakdown voltage of 5 kV or more when an AC voltage is applied to the plate-shaped silicon nitride sintered body. [2] A circuit board using the silicon nitride sintered body described in item 1 above. [3] A heat dissipation member using the silicon nitride sintered body described in item 1 above. [4] An insulating member using the silicon nitride sintered body described in item 1 above. 【0013】 [Function] When an alternating voltage is applied to a plate-shaped silicon nitride sintered body with a thickness of 100 μm, the dielectric breakdown voltage is 5 kV or more, so it can meet the applications of silicon nitride sintered bodies that require a high dielectric breakdown voltage when actually formed to a thickness of about 100 μm. Also, since the planar projection area ratio of voids is 1.0% or less, the warpage of the silicon nitride sintered body is reduced. [Advantages of the Invention] 【0014】 According to the present invention, it can meet the applications of silicon nitride sintered bodies that require a high dielectric breakdown voltage when actually formed to a thickness of about 100 μm, and also reduces the warpage of the silicon nitride sintered body. [Brief Description of the Drawings] 【0015】 [Figure 1] FIG. 1 shows an X-ray diffraction pattern diagram of a silicon nitride sintered body, where (a) is the same diagram for Example 1 and (b) is the same diagram for Comparative Example 1. [Figure 2] FIG. 2 shows SEM photographs of a silicon nitride sintered body, where (a) is the same photograph for Example 1 and (b) is the same photograph for Comparative Example 1. [Figure 3] FIG. 3 is a diagram for explaining the unevenness of voids. [Figure 4] FIG. 4 is a diagram for explaining the method of measuring the warpage of a silicon nitride sintered body. [Figure 5] FIG. 5 is a diagram showing an application example of a silicon nitride sintered body. [Embodiments for Carrying Out the Invention] 【0016】 The silicon nitride sintered body of the present invention is characterized in that in any at least one 64 μm × 48 μm area of the polished surface obtained by polishing the surface of the silicon nitride sintered body by 50 μm, the planar projection area ratio of voids is 1.0% or less, and the dielectric breakdown voltage when an alternating voltage is applied to a plate-shaped silicon nitride sintered body with a thickness of 100 μm is 5 kV or more. In addition to the preferred embodiments exemplified above, the following preferred forms are exemplified. 【0017】 1. Manufacturing method In a method for producing a silicon nitride sintered body, the sintering step in which a mixture of silicon nitride powder and a sintering aid is sintered is preferably performed with a firing temperature (°C) + firing time (hr) × 50 ≤ 2200, and the grain boundary phase formed by the sintering aid is preferably amorphous. It is preferable that no peaks originating from the grain boundary phase are detected in the X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector. In the aforementioned sintering process, it is preferable to place a pre-sintered, plate-shaped silicon nitride sintered body, separate from the silicon nitride sintered body to be manufactured, inside a closed enclosure for firing. Preferably, the sintering aid contains at least MgO or MgSiN2 and does not contain SrO. 【0018】 By setting the firing temperature (°C) + firing time (hr) × 50 ≤ 2200, sintering is achieved, and the volatilization of SiO2 during sintering is suppressed, thereby inhibiting the crystallization of the grain boundary phase. By making the grain boundary phase formed by the sintering aid an amorphous structure, volume shrinkage during cooling after sintering is reduced, and the sintering aid remains in the liquid phase until lower temperatures, reaching even the narrow spaces between silicon nitride crystal grains. As a result, the number of voids in the silicon nitride sintered body decreases, reducing internal stress and minimizing warping. In addition, the irregularities of the void shape are reduced. Furthermore, in the sintering process, if a pre-sintered, plate-shaped silicon nitride sintered body (hereinafter referred to as "dummy silicon nitride sintered body") separate from the silicon nitride sintered body to be manufactured is placed inside a closed enclosure for firing, the SiO2 in the dummy silicon nitride sintered body volatilizes during firing, thereby suppressing the volatilization of SiO2 in the silicon nitride sintered body to be manufactured. This also suppresses crystallization and prevents a decrease in sinter density. The dummy silicon nitride sintered body does not need to have the same composition as the silicon nitride sintered body to be manufactured, but it is preferable that it has the same additive system. Furthermore, adding alkaline earth metals as sintering aids has the effect of lowering the melting point of the liquid phase. However, even among alkaline earth metals, SrO is less volatile than MgO or MgSiN2, so it remains after firing and becomes a factor that inhibits heat conduction. Therefore, by containing at least MgO or MgSiN2 and not SrO, a silicon nitride sintered body with high thermal conductivity can be obtained. Furthermore, as the silicon nitride sintered body is densified to a relative density of 98% or higher, its bending strength and dielectric breakdown voltage increase. 【0019】 2. Grain boundary phase A silicon nitride sintered body consists of silicon nitride and a grain boundary phase formed by a sintering aid, and it is preferable that the grain boundary phase has an amorphous structure. The grain boundary phase preferably contains at least MgO or MgSiN2 and does not contain SrO. The grain boundary phase is preferably an amorphous structure containing at least Mg, rare earth elements (RE), and Si. In an X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector, an amorphous structure is defined as a crystalline compound in the grain boundary phase where the diffraction angle 2θ is in the range of 28° to 32°, where the largest integrated intensity is 5% or less of the integrated intensity of the silicon nitride (101) plane. It is preferable that the planar projected area ratio of voids is 1.0% or less in at least one 64 μm × 48 μm area of ​​the polished surface of the silicon nitride sintered body, which has been polished to a thickness of 50 μm or more. It is preferable that the thermal conductivity of the silicon nitride sintered body is 80 W / m·K or higher. 【0020】 Because the grain boundary phase has an amorphous structure, volume shrinkage during cooling after sintering is reduced, and the sintering aid remains in the liquid phase until lower temperatures, reaching even the narrow spaces between silicon nitride crystal grains. As a result, there are fewer voids in the silicon nitride sintered body, the internal stress of the silicon nitride sintered body decreases, and warping is reduced. In addition, the irregularities of the void shape are reduced. 【0021】 3. Curvature It is preferable that the warpage, calculated as the ratio of the difference between the height of the highest point and the lowest point on the upper surface of the silicon nitride sintered body relative to the maximum transverse length of the silicon nitride sintered body, measured before 1 minute has elapsed after the plate-shaped silicon nitride sintered body has been held at 120°C for 1 hour or more and placed on a flat sample stage at 25°C, is 0.2% or less. Here, the maximum transverse length of the silicon nitride sintered body refers to the longest line segment length that crosses the surface of the silicon nitride sintered body from one point on its edge to another. For example, if the surface is rectangular, it is the diagonal length; if the surface is circular, it is the diameter length. 【0022】 As described above, the measured warpage is 0.2% or less. Therefore, even when products using silicon nitride sintered bodies as circuit boards, heat dissipation components, etc., are exposed to high-temperature environments exceeding 100°C, the warpage of the silicon nitride sintered body is small, resulting in sufficient heat dissipation and reduced risk of damage. 【0023】 4. Dielectric Breakdown Voltage It is preferable that the dielectric breakdown voltage of a 100 μm thick, plate-shaped silicon nitride sintered body is 5 kV or higher when an AC voltage is applied to it. 【0024】 The dielectric breakdown voltage of a 100 μm thick silicon nitride sintered body when an AC voltage is applied is 5 kV or higher, which allows it to meet the requirements for applications of silicon nitride sintered bodies that require a high dielectric breakdown voltage when actually formed to a thickness of approximately 100 μm. It should be noted that the "thickness of 100 μm" is merely a specification for measuring the dielectric breakdown voltage and does not specify the thickness of the silicon nitride sintered body product. In other words, the silicon nitride sintered body product can be of any thickness, but it is preferable that the dielectric breakdown voltage measured after processing it to a thickness of 100 μm is 5 kV or higher. 【0025】 5.Applications The applications of silicon nitride sintered bodies are not particularly limited, but the following are some examples of applications. Circuit boards used in semiconductor modules, LED packages, Peltier modules, printers, multifunction devices, semiconductor lasers, optical communications, and high-frequency applications, as shown in Figure 5(a). A general-purpose heat dissipation component as shown in Figure 5(b). A heat dissipation component (heat sink) for a power semiconductor module, as shown in Figure 5(c). An insulating board as shown in Figure 5(d). An insulating plate for bonding wafers, as shown in Figure 5(e). A heat dissipation component embedded in a flexible resin or the like, as shown in Figure 5(f). Although not shown in the diagram, this is a high-frequency window used in devices such as gyrotrons and klystrons. [Examples] 【0026】 Next, embodiments of the present invention will be described with reference to the drawings, in comparison with comparative examples. Note that the materials, quantities, and conditions of each part of the embodiments are illustrative and can be modified as appropriate without departing from the spirit of the invention. 【0027】 Silicon nitride sintered bodies were prepared as shown in Examples 1 to 21 in Tables 1 and 2, and as well as as comparative examples 1 to 8 in Table 3. Hereinafter, "each example" refers to each of Examples 1 to 21 and Comparative Examples 1 to 8. 【0028】 [Table 1] 【0029】 [Table 2] 【0030】 [Table 3] 【0031】 [1] Material For each example, silicon nitride powder with an average particle size (D50) of approximately 1.0 μm, produced by either imide thermal decomposition or direct nitriding, was used as the main raw material, silicon nitride (Si3N4). 【0032】 As sintering aids, two types selected from the following powders were used in each example, as shown in Tables 1-3: MgO, MgSiN2, Y2O3, La2O3, Nd2O3, Sm2O3, and Dy2O3. In Examples 1-21, at least MgO or MgSiN2 was used, but SrO was not. 【0033】 [2] Manufacturing method (i) Mixing process of materials For each example, silicon nitride powder was mixed with sintering aid powder in the mass % shown in Tables 1-3 (total of silicon nitride powder and sintering aid powder was 100% by mass). To 100 parts by weight of this mixed powder, 0.3 parts by weight of a surfactant dispersant and approximately 50 parts by weight of a mixed solvent of toluene and ethanol were added, and the mixture was crushed and mixed using a ball mill with a resin container and silicon nitride pebbles. 【0034】 To this pulverized mixture, a binder solution consisting of 10 parts by weight of polyvinyl butyral as a binder, 4 parts by weight of dioctyl adipicate as a plasticizer, and approximately 20 parts by weight of a mixed solvent of toluene and ethanol was added. The mixture was then stirred and mixed using a ball mill until the binder solution and the pulverized mixture were completely mixed to produce a slurry. The slurry was then heated and left to stand in a vacuum to remove air bubbles and evaporate the solvent, adjusting the viscosity at 25°C to 15,000 cps. 【0035】 (ii) Process for manufacturing green sheets Next, plate-shaped green sheets were obtained from the slurries of each prepared example using the doctor blade method. The final drying temperature in the doctor blade molding apparatus was 90°C. The obtained green sheets were die-cut into rectangular shapes of 180 mm x 250 mm using die press processing. 【0036】 A boron nitride (BN) powder slurry, used as a release agent, was sprayed onto the surface of a die-cut green sheet. Multiple sheets of this laminated green sheet were then placed in a BN enclosure, and the assembly was heated to 500°C for approximately 4 hours in a flow of dry air to perform a degreasing process that removed organic components such as binders. 【0037】 (iii) Sintering process of the green sheet In Examples 1 to 21, a green sheet laminate was placed on a BN base plate, a BN setter was placed on top of it, a tungsten block was placed on top of the setter as a loader, and the aforementioned plate-shaped dummy silicon nitride sintered body was placed on top of the loader. Next, the BN side and top plates were placed on the bottom plate to assemble a closed enclosure. The enclosure, now containing the green sheet and other materials, was then placed in a firing furnace, and the inside of the furnace was filled with a nitrogen atmosphere of 0.9 MPa. The enclosure was not completely sealed, but was closed to the extent that nitrogen could flow in, so the inside of the enclosure also became a nitrogen atmosphere of 0.9 MPa. 【0038】 In this state, for each example, the green sheet laminate was sintered by heating at the firing temperature shown in Tables 1 and 2 for the firing time, and the sintered laminate was separated into individual silicon nitride sintered bodies. The BN release agent was removed from the separated silicon nitride sintered bodies by honing. The outer edges of the four sides of the honed silicon nitride sintered bodies were broken with a diamond scribing tool, and the final shape and dimensions of the obtained silicon nitride sintered bodies were rectangular plates measuring 139.6 mm × 190.5 mm × 0.32 mm. In Examples 1 to 21, the firing temperature was set in the range of 1830 to 1920°C, and sintering was carried out for a relatively short time so as to satisfy the following equation 1. 1930 ≤ firing temperature (°C) + firing time (hr) × 50 ≤ 2200 ... (Equation 1) 【0039】 In Comparative Examples 1 to 7, the firing temperature was set in the range of 1860 to 1880°C, but the sintering was carried out for a relatively long time, exceeding the upper limit of Equation 1 above. In Comparative Example 8, the firing temperature was set to 1800°C, and sintering was performed for a short time so as to be below the lower limit of Equation 1. 【0040】 [3] Characteristics For each example of silicon nitride sintered body, the relative density, three-point bending strength, thermal conductivity, identification of grain boundary phases by X-ray diffraction, voids, warpage, and dielectric breakdown voltage were measured (shown in Tables 1-3). 【0041】 (i) Relative density and three-point bending strength The relative density of a silicon nitride sintered body is calculated as measured density / theoretical density. The measured density was determined by the Archimedes method, which involves immersing the silicon nitride sintered body in pure water. The theoretical density was calculated using the density of the raw material powder, Si3N4 = 3.18 g / cm³. 3 MgO = 3.60 g / cm³ 3 MgSiN2 = 3.07 g / cm³ 3 Y2O3 = 5.01 g / cm³ 3 La2O3 = 6.51 g / cm³ 3 Nd2O3 = 7.24 g / cm³ 3 Sm2O3 = 7.60 g / cm³ 3 Dy2O3 = 7.81 g / cm³ 3 These values ​​were used and calculated from the mixing ratio of the raw material powders. The three-point bending strength was measured using a silicon nitride sintered body, processed into a test specimen measuring 40 mm x 20 mm x 0.32 mm, and a universal testing machine (model "AG-IS") manufactured by Shimadzu Corporation, with a crosshead speed of 0.5 mm / min, a support distance of 30 mm, and at room temperature (23 ± 2 °C). 【0042】 Examples 1-21 and Comparative Examples 1-4, 6, and 7 had a relative density of 98% or higher, indicating sufficient densification, resulting in a three-point bending strength of 600 MPa or higher. Comparative Examples 5 and 8 had a relative density of less than 98% and were not sufficiently densified, resulting in a three-point bending strength of less than 600 MPa, making it impossible to obtain a high-strength silicon nitride sintered body. 【0043】 (ii) Thermal conductivity Thermal conductivity was measured using a NETZSCH LFA 467 HyperFlash thermal conductivity meter after processing a silicon nitride sintered body into a 10mm x 10mm x 0.32mm test specimen, surface treatment (Ag film deposition + carbon blackening treatment), and then processing. 【0044】 (iii) Identification of grain boundary phases by X-ray diffraction A silicon nitride sintered body was processed into a test specimen measuring 10 mm × 10 mm × 0.32 mm. An X-ray diffraction pattern of the specimen plane was obtained using powder X-ray diffraction with Cu-Kα rays, employing a Rigaku Corporation X-ray diffractometer: model "Ultima IV" (enclosed tube target using Cu and Ni filters, detector using a one-dimensional semiconductor method). In the obtained X-ray diffraction pattern, the integrated intensity of the (101) plane of α-Si3N4 (hereinafter referred to as "I silicon nitride") and the integrated intensity of the largest peak among the Si-YNO compounds in the grain boundary phase with diffraction angles 2θ in the range of 28° to 32° (hereinafter referred to as "I grain boundary phase") were calculated using the following procedure, and the integrated intensity ratio (I grain boundary phase / I silicon nitride) was determined. (1) Preprocessing is performed by background removal, Kα2 removal, and smoothing, and then a peak search is performed. (2) The background profile is calculated by subtracting the peak profile from the measured data, and the calculated data is fitted using a B-spline function. (3) The peak shape is represented by a divided pseudovoit function, and the integral intensity is calculated. 【0045】 Figure 1(a) shows the X-ray diffraction pattern of Example 1. No peaks originating from the grain boundary phase formed by the sintering aid were detected, and the integrated intensity ratio was 0, as shown in Table 1. This indicates that there is no grain boundary crystalline phase and that the grain boundary phase is substantially amorphous. Examples 2 to 19 were similar to Example 1. In Examples 20 and 21, peaks originating from the grain boundary phase formed by the sintering aid were detected, but as shown in Table 1, the integrated intensity ratios were 2.4% and 3.8%, respectively. Thus, a structure in which a crystalline phase exists in the grain boundary phase but its integrated intensity ratio is 5% or less is defined as an amorphous structure. 【0046】 Figure 1(b) shows the X-ray diffraction pattern of Comparative Example 1. Peaks originating from the grain boundary phase formed by the sintering aid were detected, and as shown in Table 3, the integrated intensity ratio was 24.6%. This indicates that not only is a grain boundary crystalline phase present, but the grain boundary phase is substantially composed of a crystalline phase. Comparative Examples 2-7 were basically the same as Comparative Example 1 (although the integrated intensity ratios were different). In Comparative Example 8, no peaks originating from the grain boundary phase formed by the sintering aid were detected, and the integrated intensity ratio was 0, as shown in Table 1. This indicates that there is no grain boundary crystalline phase and the grain boundary phase is substantially amorphous. However, as will be discussed later, Comparative Example 8 has a low relative density and few voids with a roughness of 0.8 or higher. 【0047】 (iv) Void The silicon nitride sintered body was surface-treated as follows. A silicon nitride sintered body was processed into a test specimen measuring 8 mm x 8 mm x 0.32 mm, and fixed to a φ40 aluminum sample stage using Alcowax "5402SL" manufactured by Nichika Seiko Co., Ltd. The sample stage was set on a sample rotating machine (model "SP-L1") manufactured by IMT Co., Ltd., and the silicon nitride sintered body was surface polished using diamond polishing pads (manufactured by IMT Co., Ltd.) in the order of #80, #600, and #1200 using the company's benchtop polishing machine (model "IM-P2") to adjust the flatness. The final polishing amount with the diamond polishing pad was adjusted to approximately 50 μm. Subsequently, surface polishing was performed for 5 minutes with diamond slurries (manufactured by IMT Co., Ltd.) with particle sizes of 15 μm, 6 μm, and 1 μm, respectively (polishing load: 15 N, polishing plate rotation speed: 150 rpm, sample rotation speed: 150 rpm). Furthermore, a mirror finish was achieved by polishing for 20 minutes using an alumina slurry (manufactured by Buehler) with a particle size of 0.05 μm as a finishing abrasive. After mirror finishing, plasma etching was performed in CF4 gas for 4 minutes using a plasma etching device (model "SEDE-PHL") manufactured by Meiwa Forsis Co., Ltd. to prepare the surface for microstructure observation. Subsequently, an Au film was formed on the surface of the observation sample using an ion sputter (model "E-1010") manufactured by Hitachi High-Tech Corporation, with the aim of applying a conductive treatment. The sputtering time was 120 seconds, and according to the operation manual, the thickness of the formed Au film was approximately 15-20 nm. 【0048】 The silicon nitride sintered body after the above surface treatment was observed using a scanning electron microscope (SEM): model "S-3400N" manufactured by Hitachi High-Tech Corporation, at an acceleration voltage of 10kV, and SEM images were taken. Figure 2(a) shows the SEM image of Example 1, and Figure 2(b) shows the SEM image of Comparative Example 1. 【0049】 The captured SEM images were analyzed using Asahi Kasei Engineering Corporation's software "A-zo-kun Ver.2.58" to measure the roughness of voids in any 64 μm × 48 μm area of ​​the polished surface. The roughness was then divided into six categories (0.9 or more, 0.8 or more but less than 0.9, 0.7 or more but less than 0.8, 0.6 or more but less than 0.7, 0.5 or more but less than 0.6, and less than 0.5). The number of voids in each category and the percentage of the total number of voids in each category were calculated. Here, the degree of unevenness is calculated using Equation 2 below, based on the void contour and envelope as shown in Figure 3. The closer the degree of unevenness is to 1, the less unevenness there is, and the less than 1, the more unevenness there is. Degree of unevenness = Area within the void's contour line / Area within the void's envelope ... (Equation 2) 【0050】 In Examples 1 to 21, voids with a roughness degree of 0.9 or higher accounted for more than 10%, and voids with a roughness degree of 0.8 or higher accounted for more than 30%. In comparative examples 1 to 8, less than 10% of the voids had a roughness of 0.9 or higher, and less than 30% of the voids had a roughness of 0.8 or higher. 【0051】 Next, an area of ​​64 μm × 48 μm in the SEM image of the silicon nitride sintered body was image-analyzed using the software described above, and the planar projected area ratio (%) of the voids was calculated using the following equation 3. Planar projection area ratio = (Total planar projection area of ​​voids / Area of ​​the area) × 100 …(Equation 3) Examples 1-21 and Comparative Example 3 had a planar projected area ratio of 1.0% or less. Comparative Examples 1, 2, 4-8 had a planar projection area ratio exceeding 1.0%. 【0052】 (v) curve As shown in Figure 4, for each example, three silicon nitride sintered bodies (139.6 mm × 190.5 mm × 0.32 mm, diagonal length 236 mm) were placed in a heating furnace adjusted to 120°C and 1% rh relative humidity and held at the same temperature for 1 hour. After removing them from the heating furnace, they were placed on a flat natural stone sample stage (25°C) equipped with a GFMesstechnik optical 3D measuring instrument: model "MikroCAD". Before 1 minute had elapsed, the difference (μm) between the highest point and the lowest point of the silicon nitride sintered body from the sample stage was measured using the instrument. The average of these differences for the three bodies was calculated, and the ratio (%) of this average value to the maximum transverse length of the silicon nitride sintered body surface (diagonal length in this example) was taken as the warp value. 【0053】 Examples 1-21 and Comparative Example 8 all exhibited a warp (average value) of 0.2% or less. Comparative Examples 1-7 all showed a warp (average value) exceeding 0.2%. 【0054】 Furthermore, for Example 1, the holding time at 120°C was increased to 2 hours, 4 hours, and 8 hours, and the warpage was measured in the same manner as above. However, the results were within ±1% of the measurement results when held for 1 hour, so no significant difference was observed due to the holding time. Furthermore, the time elapsed from removal from the heating furnace to measurement on a flat sample stage at 25°C was varied to 20 seconds and 40 seconds, and the warpage was measured in the same manner as above. However, the results were within ±3% of the measurement results after 1 minute, indicating that no significant difference was observed due to the time elapsed since removal from the heating furnace if the measurement time was within 1 minute. Note that ±3% means a variation between 0.194% and 0.206% for a silicon nitride sintered body with a warpage of 0.2%, which can be considered statistically insignificant. 【0055】 Furthermore, as shown in Table 4 below, for Example 14 (whose composition and firing conditions are average among all examples), the warpage of the second silicon nitride sintered body, which had its warpage measured, was measured in the same manner as above for the following: one piece was divided into four to reduce its size (69.8 mm × 95.3 mm × 0.32 mm, diagonal length 118 mm), another piece was further divided into two to reduce its size (69.8 mm × 47.6 mm × 0.32 mm, diagonal length 85 mm), and yet another piece was further divided into two to reduce its size (34.9 mm × 47.6 mm × 0.32 mm, diagonal length 59 mm). 【0056】 [Table 4] 【0057】 The warp before division (diagonal length 236 mm) was 0.14%, while the warp after division (diagonal lengths 118 mm, 85 mm, and 59 mm) was 0.12%, 0.13%, and 0.15%, respectively. This indicates that even when the size is reduced, the warp remains almost unchanged from the original state. 【0058】 (vi) Dielectric breakdown voltage Silicon nitride sintered bodies were cut into 20mm x 20mm pieces and polished on both sides to a thickness of 100μm to prepare the measurement samples. The surface roughness (Sa) of the polished sample surface in a 200μm x 200μm area (objective lens magnification 50x) was measured using a laser microscope (model VKX-150) manufactured by Keyence Corporation. The result showed Sa to be in the range of 0.48 to 0.52μm. Conductive copper foil adhesive tape with a diameter of φ10.4mm was attached to both sides of the sample as measurement electrodes, and an AC voltage (sine wave) was applied in a fluorine-based inert liquid (Fluorinert FC-43, manufactured by 3M Japan Ltd.) using a voltage withstand tester (model TOS5101) manufactured by Kikusui Electronics Co., Ltd. The AC voltage boosting rate was set to 500V / s, and the average dielectric breakdown voltage of the three samples was measured. 【0059】 Examples 1-21 showed dielectric breakdown voltages of 5kV or higher. Comparative Examples 1-7 had dielectric breakdown voltages of less than 5kV. 【0060】 Dielectric breakdown voltage is often measured for sintered bodies with a thickness greater than 100 μm (e.g., 300 μm), but the value obtained by converting the measurement result for such sintered bodies to a value per 100 μm is merely a theoretical value. Therefore, it cannot be guaranteed that the dielectric breakdown voltage of the sintered body when actually formed to a thickness of approximately 100 μm will be the same as the converted value. This invention makes that guarantee possible. 【0061】 It should be noted that the present invention is not limited to the embodiments described above, and can be appropriately modified and implemented without departing from the spirit of the invention.

Claims

[Claim 1] In any one 64 μm × 48 μm area of ​​the polished surface of a silicon nitride sintered body polished to a thickness of 50 μm or more, the planar projected area ratio of voids is 0.6% or less. A silicon nitride sintered body having a thickness of 100 μm and exhibiting a dielectric breakdown voltage of 5 kV or more when an AC voltage is applied to the plate-shaped silicon nitride sintered body. [Claim 2] A circuit board using the silicon nitride sintered body described in claim 1. [Claim 3] A heat dissipation member using a silicon nitride sintered body as described in claim 1. [Claim 4] An insulating member using a silicon nitride sintered body as described in claim 1.

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