Silicon nitride sintered body

Abstract

[Problem] To reduce volume shrinkage during cooling after sintering, to reduce voids in a silicon nitride sintered body, and to reduce warpage of the silicon nitride sintered body. [Solution] Provided is a silicon nitride sintered body which contains silicon nitride and a grain boundary phase comprising a sintering aid, and which is obtained by sintering a silicon nitride powder using 2-3 mass% of MgO as the sintering aid and 2.7-4 mass% of a rare earth oxide having an oxidation state of 3 (however, the amount of the rare earth oxide is greater than that of the MgO). The silicon nitride (101) has a grain boundary phase having an amorphous structure, and in an X-ray diffraction pattern obtained using an X-ray diffraction device provided with a semiconductor detector, the maximum integrated intensity of the peaks of the crystal compound present in the grain boundary phase having a diffraction angle 2 [theta] in the range of 28-32 DEG is 2.4% or less with respect to the integrated intensity of the silicon nitride (101) plane.

Description

Technical Field

[0001] The present invention relates to a silicon nitride sintered body useful as a circuit board, a heat dissipation member, and the like. Background Art

[0002] Examples of materials for insulating ceramics (sintered bodies) used as circuit boards, heat dissipating members, and the like include aluminum nitride (AlN) and silicon nitride (Si 3 N 4 ).

[0003] The thermal conductivity of aluminum nitride is as high as 150 W / m·K or more, but due to its low mechanical strength, it is easy to crack and is inconvenient to use.

[0004] Although the thermal conductivity of silicon nitride is not as good as that of aluminum nitride, it is over 50W / m·K, and it has advantages such as high mechanical strength, so it is not easy to crack and can be made thinner. Therefore, the development and adoption of silicon nitride sintered bodies are being carried out in recent years.

[0005] Patent document 1 describes a silicon nitride sintered body and a method for manufacturing the same, wherein the intensity ratio of the crystal phase existing in the grain boundary phase of the silicon nitride grains in the X-ray diffraction peak is 0.05 to 0.40 (when silicon nitride is 1), and the manufacturing method is to sinter a raw material sheet of the silicon nitride sintered body at 1600 to 1900°C in a nitrogen atmosphere, and then remove the residual glass phase at 1100 to 1700°C.

[0006] Patent document 2 describes a silicon nitride substrate and a method for manufacturing the same, wherein the silicon nitride substrate improves the bonding properties of circuit components, etc., and the dielectric breakdown voltage measured using a 3 mm thick substrate is 36 to 47 kV / mm (Table 6 of the document). The manufacturing method comprises adding a raw sheet of the silicon nitride substrate into a sintering furnace containing a common material of magnesium oxide and erbium oxide to suppress volatilization of the components, and sintering the raw sheet at 1750°C for 3 to 5 hours.

[0007] Patent document 3 describes a silicon nitride substrate and a method for manufacturing the same, wherein the silicon nitride substrate has a porosity of 0 to 1.0%, a maximum pore diameter of 0.2 to 3 μm, a thickness of 0.15 to 0.635 mm, and an insulation strength of 17 to 29 kV / mm (Tables 7 and 8 of the document). The manufacturing method is to sinter the material raw sheet of the silicon nitride substrate at 1800 to 1900°C in a non-oxidizing atmosphere.

[0008] Patent document 4 describes a silicon nitride substrate and a method for manufacturing the same, wherein the warping of the silicon nitride substrate is less than 2.0 μm / mm. The manufacturing method comprises sintering the raw material sheet of the silicon nitride substrate at 1800 to 2000° C. in a nitrogen pressurized atmosphere for 8 to 18 hours, and then heat treating the raw material sheet at 1550 to 1700° C. while applying a load to suppress the warping.

[0009] Patent document 5 describes a silicon nitride substrate and a method for manufacturing the same, wherein the silicon nitride substrate has little warping and high strength. The manufacturing method comprises adding a raw material sheet of the silicon nitride substrate to a sintering container that is provided with filling powder such as magnesium oxide to suppress the volatilization of silicon nitride and magnesium oxide, and sintering the container at 1860°C for 5 hours.

[0010] Patent document 6 describes a silicon nitride substrate and a method for manufacturing the same. The silicon nitride substrate has a pore ratio of 0.1 to 4%, a thickness of 0.15 to 0.25 mm, and a dielectric breakdown strength of 32 to 36 kV / mm (Table 3 of the document). The manufacturing method is to sinter the material raw sheet of the silicon nitride substrate at 1850 to 1900°C for 3 to 5 hours in a nitrogen atmosphere.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 5-279124

[0014] Patent Document 2: International Publication No. 2011-087055

[0015] Patent Document 3: Japanese Patent Application Publication No. 2017-178776

[0016] Patent Document 4: Japanese Patent Application Publication No. 2009-218322

[0017] Patent Document 5: Japanese Patent Application Publication No. 2020-93978

[0018] Patent Document 6: Japanese Patent Application Publication No. 2014-73937 Summary of the invention

[0019] Problem that the invention aims to solve

[0020] However, none of Patent Documents 1 to 6 describes a silicon nitride sintered body in which the grain boundary phase formed by the sintering aid is an amorphous structure. On the contrary, Patent Document 1 describes the following: When the sintering aid remains in the grain boundary phase in the form of a glass phase, high temperature properties such as high temperature strength and creep resistance are reduced, so the residual glass phase is crystallized and removed at 1100 to 1700° C. after sintering as described above.

[0021] In this regard, the inventors have found that when the grain boundary phase formed by the sintering aid is a crystalline phase, the volume shrinkage during cooling after sintering becomes larger, the pores in the silicon nitride sintered body become more, and the warpage of the silicon nitride sintered body becomes larger. In addition, it is also found that by making the grain boundary phase formed by the sintering aid an amorphous structure, the volume shrinkage during cooling after sintering becomes smaller, the pores in the silicon nitride sintered body become fewer, and the warpage of the silicon nitride sintered body becomes smaller. The present invention is completed by further advancing this research.

[0022] Solutions for solving problems

[0023] [1] A silicon nitride sintered body comprising silicon nitride and a grain boundary phase formed by a sintering aid, which is sintered using the following materials: 2 to 3 mass % of MgO as a sintering aid is added to silicon nitride powder, and 2.7 to 4 mass % of a rare earth oxide with an oxidation state of 3 is added (however, the amount used is greater than the aforementioned MgO). The aforementioned grain boundary phase is an amorphous structure, and in an X-ray diffraction pattern obtained using an X-ray diffraction device equipped with a semiconductor detector, the maximum integrated intensity of the peaks of crystalline compounds present in the grain boundary phase with a diffraction angle 2θ in the range of 28° to 32° is less than 2.4% relative to the integrated intensity of the silicon nitride (101) plane.

[0024] [2] The grain boundary phase preferably contains at least MgO or MgSiN 2 and does not contain SrO.

[0025] [3] The thermal conductivity is preferably 72 W / mK or more.

[0026] [4] The silicon nitride sintered body was processed into a test piece with a size of 40 mm×20 mm×0.32 mm and the three-point bending strength measured at a crosshead speed of 0.5 mm / min, a support distance of 30 mm and room temperature (23±2°C) was 625 MPa or more.

[0027] [5] Preferably, the plane projected area ratio of pores in any at least one 64 μm×48 μm region of the polished surface obtained by polishing the surface of the silicon nitride sintered body by 50 μm or more is 1.0% or less.

[0028] [6] The thermal conductivity is preferably 80 W / m·K or more.

[0029] [7] A circuit board using the silicon nitride sintered compact according to any one of 1 to 6 above.

[0030] [8] A heat dissipation member using the silicon nitride sintered body according to any one of 1 to 6 above.

[0031] [9] An insulating member using the silicon nitride sintered body according to any one of 1 to 6 above.

[0032] [effect]

[0033] By making the grain boundary phase an amorphous structure, and in the X-ray diffraction pattern obtained using an X-ray diffraction device with a semiconductor detector, the maximum integrated intensity of the peak of the crystalline compound in the grain boundary phase with a diffraction angle 2θ in the range of 28° to 32° is 2.4% or less relative to the integrated intensity of the silicon nitride (101) plane, so that during cooling after sintering, the volume shrinkage becomes smaller, and the sintering aid exists in the form of a liquid phase at a lower temperature and spreads to the narrow part between the silicon nitride grains, so that the pores in the silicon nitride sintered body are reduced, the internal stress of the silicon nitride sintered body is reduced, and the warpage is reduced. In addition, the unevenness of the pore shape becomes smaller.

[0034] In addition, by adding alkaline earth metal as a sintering aid, it has the effect of lowering the liquid phase melting point. However, even if it is an alkaline earth metal, SrO will remain after firing because it is more difficult to volatilize than MgO or MgSiN2, becoming a factor that hinders heat conduction. Therefore, by containing at least MgO or MgSiN2 and not containing SrO, a silicon nitride sintered body with high thermal conductivity can be obtained.

[0035] Effects of the Invention

[0036] According to the present invention, the number of pores in the silicon nitride sintered body is reduced, and the warpage of the silicon nitride sintered body is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows X-ray diffraction patterns of silicon nitride sintered bodies, where (a) is the pattern of Example 1 and (b) is the pattern of Comparative Example 1. FIG.

[0038] FIG. 2 shows SEM photographs of a silicon nitride sintered body, where (a) is the photograph of Example 1 and (b) is the photograph of Comparative Example 1. FIG.

[0039] Figure 3 This is a diagram illustrating the unevenness of pores.

[0040] Figure 4 It is a figure explaining the measuring method of the warpage of the silicon nitride sintered body.

[0041] FIG. 5 is a diagram showing an example of application of a silicon nitride sintered body. DETAILED DESCRIPTION

[0042] The silicon nitride sintered body of the present invention is a silicon nitride sintered body comprising silicon nitride and a grain boundary phase formed by a sintering aid, wherein the grain boundary phase has an amorphous structure. In addition to the preferred embodiments exemplified in the above embodiment, the following preferred embodiments are also exemplified.

[0043] 1. Manufacturing method

[0044] In the method for producing a silicon nitride sintered body, the sintering step of sintering a mixture of silicon nitride powder and a sintering aid is preferably set to: 1930≤sintering temperature (°C)+sintering time (hr)×50≤2200, and the grain boundary phase formed by the sintering aid is an amorphous structure.

[0045] Preferably, no peak derived from the grain boundary phase is detected in an X-ray diffraction pattern obtained using an X-ray diffraction apparatus equipped with a semiconductor detector.

[0046] Preferably, in the sintering step, a previously sintered plate-shaped silicon nitride sintered body separate from the silicon nitride sintered body to be produced is arranged in a sealed housing for sintering.

[0047] As the sintering aid, it is preferred that at least MgO or MgSiN 2 is contained and SrO is not contained.

[0048] By setting 1930≤sintering temperature (℃) + sintering time (hr) × 50≤2200, sintering is achieved, and the volatilization of SiO2 during sintering is suppressed, and the crystallization of the grain boundary phase is suppressed. By setting the grain boundary phase formed by the sintering aid to an amorphous structure, the volume shrinkage is reduced during cooling after sintering, and the sintering aid also exists in the form of a liquid phase at a lower temperature, spreading to the narrow part between the silicon nitride grains, so the pores in the silicon nitride sintered body are reduced, the internal stress of the silicon nitride sintered body is reduced, and the warping is reduced. In addition, the unevenness of the pore shape is reduced.

[0049] In addition, in the sintering process, when a pre-sintered plate-shaped silicon nitride sintered body (hereinafter referred to as a "pseudo silicon nitride sintered body") separate from the silicon nitride sintered body to be manufactured is arranged in a closed shell for sintering, the volatilization of SiO2 of the silicon nitride sintered body to be manufactured is suppressed by volatilization of SiO2 during sintering, so that crystallization is also suppressed and the reduction of sintering density can be prevented. The pseudo silicon nitride sintered body does not need to have the same composition as the silicon nitride sintered body to be manufactured, but preferably has the same auxiliary agent system.

[0050] In addition, by adding alkaline earth metal as a sintering aid, it has the effect of lowering the liquid phase melting point. However, even if it is an alkaline earth metal, SrO will remain after firing because it is more difficult to volatilize than MgO or MgSiN2, becoming a factor that hinders heat conduction. Therefore, by containing at least MgO or MgSiN2 and not containing SrO, a silicon nitride sintered body with high thermal conductivity can be obtained.

[0051] In addition, by densifying the silicon nitride sintered body to a relative density of 98% or more, the bending strength is increased and the dielectric breakdown voltage is also increased.

[0052] 2. Porosity

[0053] In any at least one 64μm×48μm area of ​​the polished surface obtained by polishing the surface of the silicon nitride sintered body by more than 50μm, the number of pores having a roughness of 0.9 or more calculated by dividing the area within the contour line of the pore by the area within the envelope line of the pore preferably accounts for more than 10% of the total number of pores.

[0054] In any at least one 64μm×48μm area of ​​the polished surface obtained by polishing the surface of the silicon nitride sintered body by more than 50μm, the number of pores having a roughness of 0.8 or more calculated by dividing the area within the contour line of the pore by the area within the envelope line of the pore preferably accounts for more than 30% of the total number of pores.

[0055] In the aforementioned region, the plane projected area ratio of pores is preferably 1.0% or less.

[0056] By making the pores with a roughness of 0.9 or more account for 10% or more, or making the pores with a roughness of 0.8 or more account for 30% or more, local discharge that would occur in the roughness of the pore shape when voltage is applied is less likely to occur and is reduced due to the small roughness, thereby increasing the dielectric breakdown voltage.

[0057] Furthermore, by setting the plane projected area ratio of the pores to 1.0% or less, the warpage of the silicon nitride sintered body is reduced.

[0058] 3. Warping

[0059] The plate-shaped silicon nitride sintered body is kept at 120°C for more than 1 hour and then placed on a flat sample table at 25°C. The warpage measured before 1 minute is preferably 0.2% or less, and the warpage is calculated as the ratio of the difference between the height of the highest point on the upper surface of the silicon nitride sintered body from the sample table and the height of the lowest point from the sample table to the maximum cross-sectional length of the silicon nitride sintered body.

[0060] Here, the maximum cross-sectional length of the silicon nitride sintered body refers to the maximum length of a line segment that extends from one point on the edge of the plate surface of the silicon nitride sintered body to another point, for example, the diagonal length when the plate surface is rectangular and the diameter length when the plate surface is circular.

[0061] By limiting the warpage measured as described above to less than 0.2%, even if the silicon nitride sintered body used as a circuit board, heat dissipation component, etc. is exposed to a high temperature environment exceeding 100°C, the warpage of the silicon nitride sintered body is small, so a sufficient heat dissipation effect can be obtained and breakage is not likely to occur.

[0062] 4. Dielectric breakdown voltage

[0063] The dielectric breakdown voltage when an AC voltage is applied to a plate-like silicon nitride sintered body having a thickness of 100 μm is preferably 5 kV or more.

[0064] By setting the dielectric breakdown voltage to 5 kV or more when an AC voltage is applied to a plate-shaped silicon nitride sintered body having a thickness of 100 μm, it is possible to cope with applications requiring a high dielectric breakdown voltage when a silicon nitride sintered body having a thickness of about 100 μm is actually formed.

[0065] It should be noted that "thickness 100 μm" only specifies the measurement condition of the dielectric breakdown voltage, and does not specify the thickness of the silicon nitride sintered product. That is, the silicon nitride sintered product can be of any thickness, and it is preferred that the dielectric breakdown voltage measured after processing it to a thickness of 100 μm is 5 kV or more.

[0066] 5. Purpose

[0067] The use of the silicon nitride sintered body is not particularly limited, but the following uses can be exemplified.

[0068] As shown in FIG. 5( a ), a circuit board used in semiconductor modules, LED packages, Peltier modules, printers, multi-function devices, semiconductor lasers, optical communications, high frequencies, etc.

[0069] A general heat dissipation component as shown in Figure 5(b).

[0070] A heat dissipation member (heat sink) for a power semiconductor module as shown in FIG. 5( c ).

[0071] Insulating plate as shown in Figure 5(d).

[0072] An insulating plate for bonding wafers as shown in FIG. 5( e ).

[0073] As shown in FIG. 5( f ), the heat dissipation member is embedded in a flexible resin or the like.

[0074] Not shown but used for high frequency windows of vibrating gyroscopes, klystrons, etc.

[0075] Example

[0076] Next, examples of the present invention will be described in comparison with comparative examples with reference to the accompanying drawings. It should be noted that the materials, quantities, and conditions of each part of the examples are illustrative only and can be appropriately changed without departing from the spirit of the invention.

[0077] Silicon nitride sintered bodies of Examples 1 to 21 shown in Tables 1 and 2 and silicon nitride sintered bodies of Comparative Examples 1 to 8 shown in Table 3 were prepared. Hereinafter, "each example" refers to each of Examples 1 to 21 and Comparative Examples 1 to 8. It should be noted that Example 21 is a reference example.

[0078]

Table 1

[0079]

[0080]

Table 2

[0081]

[0082]

Table 3

[0083]

[0084] [1] Materials

[0085] As for silicon nitride (Si 3 N 4 ) as a main raw material, silicon nitride powder having an average particle size (D50) of about 1.0 μm produced by an imide thermal decomposition method or a direct nitridation method was used in each example.

[0086] As sintering aids, two selected from MgO, MgSiN2, Y2O3, La2O3, Nd2O3, Sm2O3, and Dy2O3 powders were used in each example as shown in Tables 1 to 3. In Examples 1 to 21, at least MgO or MgSiN2 was used, and SrO was not used.

[0087] [2] Manufacturing method

[0088] (i) Material mixing process

[0089] In each example, silicon nitride powder was mixed with sintering aid powder in the mass % shown in Tables 1 to 3 (the total of silicon nitride powder and sintering aid powder was 100 mass %). 0.3 weight part of a surfactant dispersant and about 50 weight parts of a mixed solvent of toluene and ethanol were added to 100 weight parts of the mixed powder, and the mixture was pulverized and mixed in a ball mill using a resin container and silicon nitride jade.

[0090] A binder solution comprising 10 parts by weight of polyvinyl butyral as a binder, 4 parts by weight of dioctyl adipate as a plasticizer, and about 20 parts by weight of a mixed solvent of toluene and ethanol was further added to the pulverized mixture, and the mixture was stirred and mixed using a ball mill until the binder solution and the pulverized mixture were completely mixed to prepare a slurry. The slurry was then heated and placed in a vacuum to degas and volatilize the solvent, thereby adjusting the viscosity at 25° C. to 15,000 cps.

[0091] (ii) Green Sheet Production Process

[0092] Next, a plate-shaped green sheet was obtained from the prepared slurry of each example by a doctor blade method. The final drying temperature in the doctor blade forming apparatus was set at 90° C. The obtained green sheet was punched into a rectangular shape of 180 mm×250 mm by die punching.

[0093] Boron nitride (BN) powder slurry, which serves as a demolding material, is sprayed onto the surface of the raw sheet after demolding using a sprayer. A raw sheet stack formed by stacking multiple raw sheets is placed in a BN shell, heated at 500°C for about 4 hours in a dry air flow, and a degreasing process is performed to remove organic components such as adhesives.

[0094] (iii) Green Sheet Sintering Process

[0095] Regarding Examples 1 to 21, a green sheet stack is arranged on a BN base plate, a BN setter is placed thereon, a tungsten block as a carrier is placed on the setter, and the above-mentioned plate-shaped pseudo silicon nitride sintered body is arranged on the carrier.

[0096] Next, a BN side plate and a top plate are placed on the bottom plate to assemble a closed housing. The housing containing the green sheets and the like is placed in a sintering furnace, and the sintering furnace is set to a nitrogen atmosphere of 0.9 MPa. Since the housing is a closed state that allows nitrogen to flow in, but is not completely sealed, the housing also becomes a nitrogen atmosphere of 0.9 MPa.

[0097] In this state, for each example, the green sheet stack was sintered by heating at the sintering temperature and for the time shown in Tables 1 to 2, and the sintered stack was separated into silicon nitride sintered bodies. The separated silicon nitride sintered bodies were honed to remove the BN demolding material. The outer four sides of the honed silicon nitride sintered bodies were fractured by a diamond dicing machine, and the shape and size of the silicon nitride sintered bodies finally obtained were rectangular plates of 139.6 mm × 190.5 mm × 0.32 mm.

[0098] In Examples 1 to 21, the sintering temperature was set in the range of 1830 to 1920° C., and the sintering time was short so as to satisfy the following formula 1.

[0099] 1930≤Firing temperature (℃)+Firing time (hr)×50≤2200···(Formula 1)

[0100] In Comparative Examples 1 to 7, the sintering temperature was set in the range of 1860 to 1880° C., but the sintering time was long enough to exceed the upper limit of the above formula 1.

[0101] In Comparative Example 8, the sintering temperature was set to 1800° C. and the sintering time was short so as to be lower than the lower limit of Formula 1.

[0102] [3] Features

[0103] As the properties of the silicon nitride sintered body of each example, relative density, three-point bending strength, thermal conductivity, grain boundary phase identification by X-ray diffraction, porosity, warpage, and dielectric breakdown voltage were measured (as shown in Tables 1 to 3).

[0104] (i) Relative density and three-point bending strength

[0105] The relative density of the silicon nitride sintered body is the measured density / theoretical density. The measured density is measured by the Archimedean method of depositing the silicon nitride sintered body in pure water. For the theoretical density, Si3N4=3.18 g / cm is used as the density of the raw material powder. 3 MgO = 3.60 g / cm 3 MgSiN2=3.07g / cm 3 、Y2O3=5.01g / cm 3 、La2O3=6.51g / cm 3 、Nd2O3=7.24g / cm 3 、Sm2O3=7.60g / cm 3 、Dy2O3=7.81g / cm 3 The values ​​are calculated from the mixing ratio of the raw material powders.

[0106] Regarding the three-point bending strength, the silicon nitride sintered body was processed into a test piece with a size of 40 mm×20 mm×0.32 mm, and measured using a universal testing machine manufactured by Shimadzu Corporation: Model "AG-IS" at a crosshead speed of 0.5 mm / min, a distance between fulcrums of 30 mm, and room temperature (23±2°C).

[0107] Examples 1 to 21 and Comparative Examples 1 to 4, 6, and 7 had relative densities of 98% or more and were able to be sufficiently densified, and therefore had three-point bending strengths of 600 MPa or more.

[0108] The relative density of Comparative Examples 5 and 8 was less than 98%, and the densification was not sufficient. Therefore, the three-point bending strength was less than 600 MPa, and a high-strength silicon nitride sintered body could not be obtained.

[0109] (ii) Thermal conductivity

[0110] The thermal conductivity was measured by processing the silicon nitride sintered body into a test piece with a size of 10 mm×10 mm×0.32 mm, performing surface treatment (Ag film deposition + carbon black treatment) using a thermal conductivity measuring instrument manufactured by NETZSCH: Model “LFA467HyperFlash”.

[0111] (iii) Grain boundary phase identification using X-ray diffraction

[0112] The silicon nitride sintered body was processed into a test piece with a size of 10 mm × 10 mm × 0.32 mm, and an X-ray diffraction pattern of the plane of the test piece was obtained by a powder X-ray diffraction method using Cu-Kα rays using an X-ray diffraction device manufactured by Rigaku Corporation: Model "Ultima IV" (Cu and Ni filters were used as targets in the enclosed arena, and a one-dimensional semiconductor type was used as the detector).

[0113] 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 maximum peak among the peaks of the Si-YNO compound in the grain boundary phase with a diffraction angle 2θ in the range of 28° to 32° (hereinafter referred to as "I grain boundary phase") are calculated in the following order, and the integrated intensity ratio (I grain boundary phase / I silicon nitride) is obtained.

[0114] (1) Perform background removal, Kα2 removal and smoothing preprocessing, and perform peak search.

[0115] (2) The peak curve is subtracted from the measurement data to calculate the background curve, and the calculated data is fitted with a B-spline function.

[0116] (3) The peak shape is represented by quasi-split Voigt functions, and the integrated intensity is calculated.

[0117] Fig. 1(a) shows the X-ray diffraction pattern of Example 1. No peak derived from the grain boundary phase formed by the sintering aid was detected, and as shown in Table 1, the integrated intensity ratio was 0. This means that there was no grain boundary phase and the grain boundary phase was substantially an amorphous structure.

[0118] Examples 2 to 19 are also the same as Example 1.

[0119] In Example 20, a peak derived from a grain boundary phase formed by the sintering aid was detected, but as shown in Table 1, the integrated intensity ratio was only 2.4%, which also indicated an amorphous structure.

[0120] Fig. 1(b) shows the X-ray diffraction pattern of Comparative Example 1. Peaks derived from the grain boundary phase formed by the sintering aid were detected, and the integrated intensity ratio was 24.6% as shown in Table 3. This indicates that not only the grain boundary crystalline phase exists but also the grain boundary phase is substantially composed of a crystalline phase.

[0121] Comparative Examples 2 to 7 are also substantially the same as Comparative Example 1 (although the integrated intensity ratios are different).

[0122] In Comparative Example 8, no peak derived from the grain boundary phase formed by the sintering aid was detected, and as shown in Table 1, the integrated intensity ratio was 0. This means that there was no grain boundary crystalline phase and the grain boundary phase was substantially an amorphous structure. However, in Comparative Example 8, as described later, the relative density was low and there were few pores with a roughness of 0.8 or more.

[0123] (iv) Porosity

[0124] The silicon nitride sintered body was subjected to the following surface treatment.

[0125] The silicon nitride sintered body was processed into a test piece of 8 mm × 8 mm × 0.32 mm, and the test piece was fixed to the Aluminum specimen stand.

[0126] The sample table was set on a sample rotating machine manufactured by AMT Co., Ltd.: Model "SP-L1", and a table grinder manufactured by AMT Co., Ltd.: Model "IM-P2" was used. The surface of the silicon nitride sintered body was ground using diamond grinding pads (manufactured by AMT Co., Ltd.) in the order of #80, #600, and #1200 (grinding load: 15N, grinding disc rotation speed: 150rpm, sample rotation speed: 150rpm), and the flatness was adjusted. The final grinding amount of the diamond grinding pad was adjusted to about 50μm. Then, diamond slurries (manufactured by AMT Co., Ltd.) with particle sizes of 15μm, 6μm, and 1μm were used, and surface grinding was performed for 5 minutes with each diamond slurry (grinding load: 15N, grinding disc rotation speed: 150rpm, sample rotation speed: 150rpm).

[0127] Furthermore, the surface was mirror-finished by polishing for 20 minutes using alumina slurry (manufactured by Buehler) having a particle size of 0.05 μm as a finishing abrasive.

[0128] After mirror finishing, plasma etching was performed in CF4 gas for 4 minutes using a plasma etching device (model "SEDE-PHL") manufactured by Meiwafosis Co., Ltd. to adjust the microstructure observation surface.

[0129] Then, in order to conduct electricity on the surface of the observation sample, an Au film was formed using an ion sputtering machine model "E-1010" manufactured by Hitachi High-Technologies Co., Ltd. The sputtering time was set to 120 seconds, and according to the manual, the thickness of the formed Au film was about 15 to 20 nm.

[0130] The surface treated silicon nitride sintered body was observed at an accelerating voltage of 10 kV using a scanning electron microscope (SEM) model "S-3400N" manufactured by Hitachi High-Technologies Corporation, and SEM photographs were taken. FIG2(a) shows the SEM photograph of Example 1, and FIG2(b) shows the SEM photograph of Comparative Example 1.

[0131] The SEM photos taken were analyzed using the software "A-ZO-KUN Ver.2.58" manufactured by Asahi Kasei Engineering Corporation, and the roughness of the pores in any 64μm×48μm area on the polished surface was measured. The roughness was divided into 6 areas (above 0.9, above 0.8 and less than 0.9, above 0.7 and less than 0.8, above 0.6 and less than 0.7, above 0.5 and less than 0.6, and less than 0.5), and the proportion of the number of pores in each area to the total number of pores was calculated.

[0132] Here, regarding the concavity, Figure 3 As shown, based on the contour line and envelope of the pores, calculation is performed according to the following formula 2. The closer the concavity is to 1, the less concavity there is, and the less the concavity is than 1, the more concavity there is.

[0133] Concavity = Area within the pore contour / Area within the pore envelope (Formula 2)

[0134] In Examples 1 to 21, pores having an asperity of 0.9 or more accounted for more than 10%, and pores having an asperity of 0.8 or more accounted for more than 30%.

[0135] In Comparative Examples 1 to 8, the number of pores having a roughness of 0.9 or more was less than 10%, and the number of pores having a roughness of 0.8 or more was less than 30%.

[0136] Next, the above-mentioned software was used to perform image analysis on a 64 μm×48 μm region of the SEM photograph of the silicon nitride sintered body, and the plane projected area ratio (%) of pores was calculated according to the following formula 3.

[0137] Plane projection area ratio = (total plane projection area of ​​pores / area of ​​region) × 100 ... (Formula 3)

[0138] In Examples 1 to 21 and Comparative Example 3, the plane projected area ratio was 1.0% or less.

[0139] In Comparative Examples 1, 2, and 4 to 8, the plane projection area ratio exceeded 1.0%.

[0140] (v) Warping

[0141] like Figure 4As shown, for each example, 3 pieces of silicon nitride sintered bodies (139.6mm×190.5mm×0.32mm, diagonal length 236mm) were placed in a heating furnace adjusted to 120°C and relative humidity 1% rh. After being kept at this temperature for 1 hour, they were taken out of the heating furnace and placed on a flat natural stone sample table (25°C) equipped with an optical three-dimensional measuring instrument: model "MikroCAD" manufactured by GFMesstechnik. Before 1 minute had passed, the difference (μm) between the height of the highest point of the silicon nitride sintered body from the sample table and the height of the lowest point from the sample table was measured using the measuring instrument, and the average value of the difference for the three pieces was calculated. The ratio (%) of the average value to the maximum cross-sectional length of the plate surface of the silicon nitride sintered body (diagonal length in this example) was set as the warp value.

[0142] The warpage (average value) of Examples 1 to 21 and Comparative Example 8 was 0.2% or less.

[0143] The warpage (average value) of Comparative Examples 1 to 7 exceeded 0.2%.

[0144] In addition, regarding Example 1, the warpage was measured in the same manner as above except that the holding time at 120°C was extended to 2 hours, 4 hours, and 8 hours. However, the measurement results were all within ±1% relative to the holding time for 1 hour, so no significant difference due to the holding time was found.

[0145] In addition, the warpage was measured in the same manner as above except that the time from taking out of the heating furnace to placing on a flat sample stand at 25°C was changed to 20 seconds and 40 seconds later, but the measurement results were all within ±3% relative to the measurement results after 1 minute, so as long as it was within 1 minute, no significant difference was found due to the time from taking out of the heating furnace. It should be noted that within ±3% means a change between 0.194% and 0.206% relative to a silicon nitride sintered body with a warpage of 0.2%, which can be said to be no significant difference.

[0146] In addition, as shown in Table 4 below, with respect to Example 14 (the composition and firing conditions are averaged in all examples), the warpage after being maintained at 120°C was measured in the same manner as described above for the case where the second piece of silicon nitride sintered body for which the warping was measured was divided into 4 parts and the size was reduced (69.8 mm × 95.3 mm × 0.32 mm, diagonal length 118 mm), the case where the size was further divided into 2 parts and the size was reduced (69.8 mm × 47.6 mm × 0.32 mm, diagonal length 85 mm), and the case where the size was further divided into 2 parts and the size was reduced (34.9 mm × 47.6 mm × 0.32 mm, diagonal length 59 mm).

[0147]

Table 4

[0148]

[0149] The warpage before splitting (diagonal length 236 mm) was 0.14%, while the warpage after splitting (diagonal length 118 mm, 85 mm, 59 mm) was 0.12%, 0.13%, and 0.15%, respectively. Therefore, even if the size is split into smaller pieces, the warpage is almost unchanged from that before splitting.

[0150] (vi) Dielectric breakdown voltage

[0151] The silicon nitride sintered body was cut into single pieces of 20 mm × 20 mm, and the thickness was made to 100 μm by double-sided grinding to prepare the test sample. It should be noted that the surface roughness (Sa) within the range of 200 μm × 200 μm (the magnification of the objective lens was 50 times) of the ground sample surface was measured using a laser microscope model "VKX-150" manufactured by Keyence Co., Ltd. The result was Sa = 0.48 to 0.52 μm. As the measuring electrode, Conductive copper foil adhesive tape was attached to both sides of the sample, and an AC voltage (sine wave) was applied in a fluorine-based inert liquid (3M Co., Ltd., Fluorinert FC-43) using a withstand voltage tester model "TOS5101" of Kikusui Electronics Co., Ltd. The AC voltage step-up rate was set to 500 V / s, and the average dielectric breakdown voltage of the three samples was measured.

[0152] In Examples 1 to 21, the dielectric breakdown voltage was 5 kV or more.

[0153] In Comparative Examples 1 to 7, the dielectric breakdown voltage was less than 5 kV.

[0154] The dielectric breakdown voltage is often measured using a sintered body with a thickness greater than 100 μm (e.g., 300 μm), but the value obtained by converting the measurement result of such a sintered body per 100 μm is always a theoretical value. Therefore, it cannot be guaranteed that the dielectric breakdown voltage of the sintered body will also be the converted value when it is actually formed to about 100 μm. The present invention can make this guarantee possible.

[0155] It should be noted that the present invention is not limited to the above-described embodiments, and can be embodied with appropriate changes within the scope of the invention.

Claims

1. A silicon nitride sintered body, comprising silicon nitride and a grain boundary phase formed by a sintering aid, which is sintered using the following materials: 2 to 3 mass % of MgO as a sintering aid is added to silicon nitride powder, and 2.7 to 4 mass % of a rare earth oxide with an oxidation state of 3 is added (however, the amount used is greater than the MgO.), wherein: The grain boundary phase has an amorphous structure. In the X-ray diffraction pattern obtained using an X-ray diffraction device equipped with a semiconductor detector, the maximum integrated intensity of the peaks of the crystalline compounds in the grain boundary phase with a diffraction angle 2θ in the range of 28° to 32° is less than 2.4% relative to the integrated intensity of the silicon nitride (101) surface.

2. The silicon nitride sintered body according to claim 1, wherein The grain boundary phase contains at least MgO or MgSiN2 and does not contain SrO.

3. The silicon nitride sintered body according to claim 2, wherein: Thermal conductivity is above 72W / mK.

4. The silicon nitride sintered body according to any one of claims 1 to 3, wherein The silicon nitride sintered body was processed into a test piece with a size of 40 mm×20 mm×0.32 mm and the three-point bending strength measured at a crosshead speed of 0.5 mm / min, a support distance of 30 mm and room temperature (23±2° C.) was 625 MPa or more.

5. The silicon nitride sintered body according to any one of claims 1 to 4, wherein In any at least one 64 μm×48 μm region of a polished surface obtained by polishing the surface of the silicon nitride sintered body by 50 μm or more, the plane projected area ratio of pores is 1.0% or less. 6 . The silicon nitride sintered body according to claim 1 , which has a thermal conductivity of 80 W / m·K or more. 7 . A circuit board using the silicon nitride sintered body according to claim 1 . 8 . A heat dissipation member using the silicon nitride sintered body according to claim 1 . 9 . An insulating member using the silicon nitride sintered body according to claim 1 .

Images