Silicon nitride-based sintered body and silicon nitride-based heat dissipation substrate

Abstract

[Problem] To provide a silicon nitride-based sintered body and a silicon nitride-based heat dissipation substrate that exhibit both high thermal conductivity and high strength. [Solution] The silicon nitride-based sintered body is formed mainly from silicon nitride-based particles and a grain boundary phase that surrounds the silicon nitride-based particles. In a cross section of the silicon nitride-based sintered body, the area of the grain boundary phase is in the range from at least 1% to not more than 10% with respect to the overall area. The grain boundary phase comprises an amorphous region A and a crystalline region C, and the area ratio C / (A + C) × 100 of the crystalline region C to the area of the grain boundary phase is in the range from at least 20% to not more than 70%. A crystal phase forming the crystalline region C contains at least one or more of an M phase, a J phase, a monosilicate phase, and a disilicate phase.

Description

Silicon nitride sintered body and silicon nitride heat dissipation substrate

[0001] The present invention relates to a silicon nitride sintered body and a silicon nitride heat dissipation substrate.

[0002] Silicon nitride, with its high thermal conductivity and strength, has attracted attention as an insulating heat dissipation substrate for inverter power modules installed in electric vehicles (EVs) and hybrid vehicles (HVs). Traditionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material. However, in the case of high-current power modules such as those used in EVs, temperatures reach approximately 250°C, and the difference in thermal expansion between the substrate and the copper or other metals to which it is bonded generates significant thermal stress, causing the aluminum nitride, which has low strength, to crack or break. Therefore, silicon nitride, which has higher thermal conductivity than common insulating ceramics and even higher strength, is increasingly being adopted, although its thermal conductivity is inferior to that of aluminum nitride.

[0003] In Patent Document 1, oxides of Mg and Ce are expressed as MgO and CeO ２ The silicon nitride ceramic material is characterized in that it contains a total of 5 to 15% by weight of both in a weight ratio of 5:4 to 4:1, with the remainder being silicon nitride, and the silicon nitride crystal grains are acicular grains with a minor axis diameter of 1 μm or less, the grain boundaries are amorphous, and the porosity is 0.5% or less.

[0004] Patent Document 2 discloses a silicon nitride sintered body comprising silicon nitride and a grain boundary phase formed by a sintering aid, the grain boundary phase having an amorphous structure.

[0005] JP 7-267735 A JP 2022-166444 A

[0006] The silicon nitride sintered body described in Patent Document 1 or Patent Document 2 relates to a sintered body composed of silicon nitride particles and a grain boundary phase, where the grain boundary phase is mainly composed of amorphous material. The aim is to improve strength, and it is described that the strength can be at least 600 MPa or more through mechanisms such as strengthening of the grain boundaries and filling of voids with amorphous material.

[0007] Silicon nitride is a highly thermally conductive material, with theoretical thermal conductivity values ​​reported to exceed 200 W / mK. The higher the thermal conductivity of a silicon nitride material, the better its heat dissipation properties. On the other hand, the thermal conductivity of the amorphous phase is generally low, said to be at most 1 W / mK. When the grain boundary phase is dominated by amorphous matter, as in the techniques described in Patent Document 1 or Patent Document 2, the thermal conductivity of the silicon nitride material decreases, and it is thought that it will not be able to exhibit satisfactory heat dissipation characteristics as a heat dissipation substrate.

[0008] As a result of extensive research, the present inventors have discovered that even in the case of a silicon nitride sintered body containing a grain boundary phase, by adjusting the area ratio of the grain boundary phase and the area ratio of the crystalline region in the grain boundary phase to fall within a predetermined range while adjusting the crystals constituting the crystalline region to include specific crystals, a silicon nitride sintered body having both high thermal conductivity and high strength can be obtained, and have completed the present invention.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon nitride sintered body and a silicon nitride heat dissipation substrate that combine high thermal conductivity and high strength.

[0010] (1) To achieve the above object, the silicon nitride sintered body of the present invention employs the following measures: Specifically, the silicon nitride sintered body of an application example of the present invention is a silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles, in which, in a cross section of the silicon nitride sintered body, the area of ​​the grain boundary phase is in the range of 1% to 10% of the total area, the grain boundary phase is composed of amorphous regions A and crystalline regions C, the area ratio of the crystalline regions C to the area of ​​the grain boundary phase (C / (A+C)×100) is in the range of 20% to 70%, and the crystalline phase forming the crystalline regions C includes at least one of an M phase, a J phase, a monosilicate phase, and a disilicate phase.

[0011] (2) In the silicon nitride sintered body of the application example of (1) above, the Al content is greater than 0 ppm and not more than 650 ppm.

[0012] (3) A silicon nitride heat dissipation substrate according to an application example of the present invention is made of the silicon nitride sintered body described in (1) or (2) above.

[0013] (4) In addition, in the silicon nitride heat dissipation substrate of the application example of (3) above, the thermal conductivity is 85 W / mK or more.

[0014] (5) In the silicon nitride heat dissipation substrate according to the application example of (3) or (4) above, the bending strength is 500 MPa or more.

[0015] (6) In the silicon nitride heat dissipation substrate according to any one of the application examples (3) to (5) above, the thickness of the silicon nitride heat dissipation substrate in a direction perpendicular to one of the main surfaces is 220 μm or more and 690 μm or less.

[0016] According to the silicon nitride sintered body or silicon nitride heat dissipation substrate of the present invention, it is possible to obtain a silicon nitride sintered body or silicon nitride heat dissipation substrate that has both high thermal conductivity and high strength.

[0017] The present invention relates to a silicon nitride heat dissipation substrate, ...

[0018] Next, an embodiment of the present invention will be described with reference to the drawings. To facilitate understanding of the description, the same reference numerals are used to designate the same components in the drawings, and duplicated descriptions will be omitted. Note that in the configuration diagrams, the size of each component is shown conceptually and does not necessarily represent the actual dimensional ratio.

[0019] [Embodiments] [Configuration of Silicon Nitride Sintered Body] First, a silicon nitride sintered body according to an embodiment of the present invention will be described. The silicon nitride sintered body according to an embodiment of the present invention is a silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles. The silicon nitride particles refer to silicon nitride particles or sialon. However, since sialon has a lower thermal conductivity than silicon nitride, it is preferable to set the upper limit of the amount of sialon produced according to the range of the Al content described below. "Mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles" means that the body may contain 0.5 wt% or less of impurities other than the elements described below.

[0020] In the cross section of the silicon nitride sintered body, the area of ​​the grain boundary phase is in the range of 1% to 10% of the total area. The grain boundary phase is composed of amorphous regions A and crystalline regions C, and the area ratio of the crystalline regions C to the area of ​​the grain boundary phase, C / (A+C)×100, is in the range of 20% to 70%. The crystalline phases forming the crystalline regions C include at least one of the M phase, J phase, monosilicate phase, and disilicate phase.

[0021] The area ratio of the grain boundary phase can be determined by observation with a scanning electron microscope (SEM). Specifically, five locations are randomly selected on the ion-milled surface of a silicon nitride sintered body, and a 50 μm × 50 μm field of view is observed at 2000x magnification. The areas of the regions or particles recognized as grain boundary phase in the five fields of view are then added together, and the ratio to the area of ​​the entire field of view is calculated to determine the area ratio of the grain boundary phase. Image analysis software such as Winroof may be used to calculate the area ratio.

[0022] The area ratio of the crystalline region C in the grain boundary phase can be determined by STEM (Scanning Transmission Electron Microscope) observation. Specifically, STEM observation is performed on the same processed surface as that observed by SEM, and the crystalline / amorphous nature of each grain boundary phase region can be determined from electron diffraction analysis of the grain boundary phase. This makes it possible to determine that the grain boundary phase consists of amorphous region A and crystalline region C, and to calculate the area ratio of the crystalline region C to the area of ​​the grain boundary phase, C / (A+C) × 100.

[0023] The crystalline phase that forms the crystalline region C of the grain boundary phase can be identified from the results of XRD (X-ray Diffraction). The M phase is a crystal identified by 00-045-0249 on the PDF (Powder Diffraction File) card. The M phase is also known as the melilite phase, and is a Y ２ Si ３ O ３ N ４ The J phase is a crystal identified as 01-086-1106 on the PDF card. ４ Si２ O ７ N ２ The monosilicate phase is a crystal identified as 00-052-181 on the PDF card. The monosilicate phase is also known as the m phase, and is ２ SiO ５ The disilicate phase is a crystal identified as 01-072-359 on the PDF card. The disilicate phase is also known as the d phase, and is a Y ２ Si ２ O ７ Although each of these crystals shows a typical crystal composition, the elements may be different as long as the crystal system, space group, and arrangement of constituent atoms are the same. For example, the M phase is Yb ２ Si ３ O ３ N ４ That's fine too.

[0024] The grain boundary phase or the amorphous regions A or crystalline regions C that make up the grain boundary phase are generated randomly within the sintered body, and therefore the area ratio of the grain boundary phase or the area ratio of the crystalline regions in an SEM image of the ion-milled surface of the silicon nitride sintered body can be regarded as the volume fraction within the silicon nitride sintered body.

[0025] The silicon nitride sintered body preferably contains more than 0 ppm but not more than 650 ppm of Al. Al is an impurity derived from the raw materials, and its inclusion improves the sinterability of the silicon nitride sintered body, thereby increasing its strength. Furthermore, as long as the content is within this range, the effect of the decrease in thermal conductivity due to the formation of sialon is virtually negligible. If the content is greater than 650 ppm, the amount of sialon in the silicon nitride sintered body increases, which may result in a decrease in the thermal conductivity of the silicon nitride sintered body.

[0026] The silicon nitride sintered body preferably contains a rare earth element. Because rare earth elements primarily form grain boundary phases, the content of the rare earth element is preferably the amount necessary to form the grain boundary phase. The specific amount varies depending on the composition of the compounds forming the amorphous region A and the crystalline region C, but, for example, a total of 1.0 to 7.5 wt% of rare earth elements is preferred. If the rare earth element content is less than this range, the sinterability may decrease, pores may remain, and the strength of the silicon nitride sintered body may decrease. If the rare earth element content is greater than this range, the amount of grain boundary phase may increase, resulting in a decrease in the thermal conductivity of the silicon nitride sintered body. It is believed that similar effects can be obtained regardless of the rare earth element contained. The rare earth element can be, for example, yttrium (Y), lanthanum (La), erbium (Er), or ytterbium (Yb).

[0027] The silicon nitride sintered body preferably contains an alkali metal element or an alkaline earth metal element. By containing an alkali metal element or an alkaline earth metal element, the sinterability of the silicon nitride sintered body is improved, and the strength of the silicon nitride sintered body can be increased. The silicon nitride sintered body preferably contains, for example, 0.25 to 2.0 wt% of alkali metal element and alkaline earth metal element in total. If the alkali metal element and alkaline earth metal element are contained below this range, the strength of the silicon nitride sintered body may be reduced. If the alkali metal element and alkaline earth metal element are contained above this range, the thermal conductivity of the silicon nitride sintered body may be reduced.

[0028] The silicon nitride sintered body preferably contains a Group 4 element. The silicon nitride sintered body preferably contains, for example, a total of 0.30 to 3.0 wt % of the Group 4 element. In this case, the Group 4 element may form a compound containing at least one of nitrogen and carbon. Because the Group 4 element does not form a solid solution in silicon nitride, it does not cause a decrease in thermal conductivity due to phonon scattering. Furthermore, it does not form a solid solution in the grain boundary phase formed by the reaction of rare earth elements with silicon nitride. Therefore, it is easy to form a compound by itself, and this compound fills residual pores, thereby improving strength. Furthermore, compound particles containing a Group 4 element and at least one of nitrogen and carbon suppress crack propagation, thereby contributing to increased toughness. The compound containing at least one of a Group 4 element and nitrogen or carbon may be not only a compound containing only a Group 4 element and nitrogen or carbon, but also a compound containing a Group 4 element, nitrogen, and carbon. It may also be a compound in which other Group 4 elements or anions (such as oxygen) are solid-solved to the extent that the crystal structure is not disrupted.

[0029] These features make it possible to obtain a silicon nitride sintered body having higher strength while maintaining high thermal conductivity.

[0030] [Configuration of Silicon Nitride Heat Dissipating Substrate] Figure 1 is a schematic perspective view showing an example of a silicon nitride heat dissipating substrate according to an embodiment of the present invention. The silicon nitride heat dissipating substrate 10 of the present invention is made of the silicon nitride sintered body described above. This allows for increased strength while maintaining high thermal conductivity, reducing the risk of defects in circuit boards using the same. The silicon nitride heat dissipating substrate 10 of the present invention can be suitably used as a heat dissipating substrate for circuit boards for power devices. The silicon nitride heat dissipating substrate 10 is formed, for example, in a flat plate shape.

[0031] The silicon nitride heat dissipation substrate 10 preferably has a bending strength of 500 MPa or more, which reduces the risk of the silicon nitride heat dissipation substrate 10 being damaged.

[0032] Bending strength can be measured as follows. In accordance with ISO 23242, a silicon nitride heat dissipation substrate is processed to a specified thickness x 12 x 25 mm. Then, bending strength can be measured using a three-point bending test with a span of 15 mm. ISO 23242 is applicable to ceramic thin plates with a thickness of 0.2 mm to 1.0 mm.

[0033] The silicon nitride heat dissipation substrate 10 preferably has a thermal conductivity of 85 W / mK or more, so that it can fully exhibit its performance as a heat dissipation substrate.

[0034] The thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed to a size of 0.32 mm x 17 mm, and the thermal diffusivity is measured by a two-dimensional method using a laser flash. The density of the silicon nitride heat dissipation substrate 10 is also measured by a method in accordance with JIS R1634. The specific heat value is 0.68 cm ２ The thermal conductivity can be calculated from the measured thermal diffusivity and density values ​​by the formula (thermal conductivity) = (density) x (specific heat) x (thermal diffusivity).

[0035] The thickness of the silicon nitride heat dissipation substrate 10 in the direction perpendicular to one of its main surfaces is preferably 220 μm or more and 690 μm or less. This allows for a good balance between the strength and heat dissipation of the silicon nitride heat dissipation substrate 10. If the thickness is smaller than this range, the strength of the substrate may be reduced. If the thickness is larger than this range, the heat dissipation performance may be reduced.

[0036] 2 is a schematic cross-sectional view showing an example of a power device using a silicon nitride heat dissipation substrate according to an embodiment of the present invention. The power device 100 includes a circuit board 20, a power semiconductor 30, a heat sink 40, and a heat dissipation member 50.

[0037] The circuit board 20 comprises a silicon nitride heat dissipation substrate 10 having a circuit layer 12 formed on one main surface thereof and a conductor layer 14 formed on the other main surface thereof opposite the one main surface. The circuit layer 12 and the conductor layer 14 are preferably made of metal, and more preferably made of a metal containing copper as a main component. The circuit layer 12 and the conductor layer 14 are joined to the silicon nitride heat dissipation substrate 10 directly or by using a joining material such as brazing material.

[0038] A power semiconductor 30 is mounted on the upper side of the circuit layer 12 of the circuit board 20. The power semiconductor 30 and the circuit layer 12 may be joined using solder 22 or the like. The power semiconductor 30 may be, for example, a semiconductor used in an EV that carries a large current and is prone to high temperatures. The silicon nitride heat dissipation substrate 10 of the present invention has high strength while maintaining high thermal conductivity, and is therefore less likely to crack or break even if high temperatures cause large thermal stress in the silicon nitride heat dissipation substrate 10 due to the difference in thermal expansion between the silicon nitride heat dissipation substrate 10 and the metal to which it is joined.

[0039] A heat sink 40 is bonded to the underside of the conductor layer 14 of the circuit board 20. The heat sink 40 and the conductor layer 14 may be bonded using solder 22 or the like. The surface of the heat sink 40 opposite the surface bonded to the conductor layer 14 is in contact with a heat sink member 50 via grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal primarily containing copper. The heat sink member 50 has heat dissipation fins formed thereon. The heat sink 50 is preferably made of metal, and more preferably made of a metal primarily containing copper or aluminum.

[0040] [Method for manufacturing silicon nitride sintered body and silicon nitride heat dissipation substrate] An example of a method for manufacturing the silicon nitride sintered body and silicon nitride heat dissipation substrate is shown below. First, the necessary raw material powders for the silicon nitride sintered body are selected and weighed to obtain the desired composition. The raw material powder for the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of the elements contained in the silicon nitride sintered body. In addition to silicon nitride, examples of raw material powders for the silicon nitride sintered body include magnesium carbonate, calcium carbonate, yttrium oxide, ytterbium oxide, erbium oxide, lanthanum oxide, and zirconium oxide.

[0041] Ethanol is added to these raw material powders, and they are mixed and pulverized in a wet manner in a ball mill for, for example, 6 to 60 hours to obtain a slurry. The slurry is dried in a hot water bath or a spray dryer, etc., to obtain a mixed powder.

[0042] Next, the mixed powder is filled into a mold and uniaxially pressed at a pressure of, for example, 30 MPa to form the desired shape. This is followed by cold isostatic pressing (CIP) at a pressure of, for example, 150 MPa to obtain a green body. The resulting green body (CIP-pressed body) is placed in a silicon carbide mold, the interior of which is coated with BN, and sintered at a maximum temperature of 1800°C to 1900°C for 5 to 20 hours in a nitrogen atmosphere at 9 atmospheres. A silicon nitride sintered body is then obtained by cooling from the maximum temperature to 1400°C at a cooling rate of, for example, 50°C / hr to 500°C / hr.

[0043] When the silicon nitride sintered body is used as a silicon nitride heat dissipation substrate, the outer shape is processed to a predetermined shape and thickness by, for example, grinding, polishing, blasting, etc.

[0044] By using such a manufacturing method, it is possible to manufacture a silicon nitride sintered body or a silicon nitride heat dissipation substrate that has increased strength while maintaining high thermal conductivity.

[0045] [Examples and Comparative Examples] (Sample 1) 3.0 wt% of yttrium oxide powder (average particle size 1.0 μm), 3.0 wt% of magnesium carbonate powder (average particle size 2.5 μm), and 1.0 wt% of zirconium oxide powder (average particle size 1.6 μm) were weighed relative to silicon nitride powder (average particle size 1.4 μm). Next, the weighed raw material powders were ball milled to obtain a mixed slurry. For the ball milling, the raw material powders and ethanol were placed in a resin pot, and milled and mixed for 16 hours at 60 rpm using silicon nitride balls. The obtained mixed slurry was dried in a hot water bath to obtain a mixed powder.

[0046] The resulting mixed powder was subjected to powder press molding using uniaxial pressing and CIP to produce a compact. First, the mixed powder was filled into a dedicated mold and then pre-molded using uniaxial pressing at a pressure of 30 MPa. Next, the pre-molded body was placed in a dedicated bag under vacuum and then subjected to CIP molding at a pressure of 150 MPa.

[0047] The obtained molded body was sintered. The sintering method was atmospheric sintering under a nitrogen gas pressure of 9 atmospheres, with the maximum temperature being 1900°C, held for 12 hours. Thereafter, the mold was cooled from the maximum temperature to 1400°C at a cooling rate of 200°C / hr. A mold made of silicon carbide with a BN coating on the inside was used. In this way, a silicon nitride sintered body of Sample 1 was produced.

[0048] (Sample 2) The silicon nitride sintered body of Sample 2 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 5.0 wt % and the amount of magnesium carbonate powder added was 4.0 wt %.

[0049] (Sample 3) The silicon nitride sintered body of Sample 3 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the amount of magnesium carbonate powder added was 6.0 wt %.

[0050] (Sample 4) The silicon nitride sintered body of Sample 4 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the amount of magnesium carbonate powder added was 3.0 wt % and the holding time at the maximum temperature during firing was 6 hours.

[0051] (Sample 5) The silicon nitride sintered body of Sample 5 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the maximum temperature was maintained for 9 hours during firing.

[0052] (Sample 6) The silicon nitride sintered body of Sample 6 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the maximum temperature was maintained for 18 hours during firing.

[0053] (Sample 7) The silicon nitride sintered body of Sample 7 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the mixing time for producing the mixed slurry was 24 hours.

[0054] (Sample 8) The silicon nitride sintered body of Sample 8 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the mixing time for producing the mixed slurry was 36 hours.

[0055] (Sample 9) The silicon nitride sintered body of Sample 9 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that zirconium oxide was not added.

[0056] (Sample 10) The silicon nitride sintered body of Sample 10 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 μm).

[0057] (Sample 11) The silicon nitride sintered body of Sample 11 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the magnesium carbonate powder was changed to calcium carbonate powder (average particle size 2.5 μm).

[0058] (Sample 12) The silicon nitride sintered body of Sample 12 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 1.0 wt % and the amount of magnesium carbonate powder added was 2.0 wt %.

[0059] (Sample 13) The silicon nitride sintered body of Sample 13 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the amount of yttrium oxide powder added was 7.0 wt % and the amount of magnesium carbonate powder added was 10.0 wt %.

[0060] (Sample 14) The silicon nitride sintered body of Sample 14 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the cooling rate was set to 500°C / hr.

[0061] (Sample 15) The silicon nitride sintered body of Sample 15 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the cooling rate was set to 50°C / hr.

[0062] (Sample 16) The silicon nitride sintered body of Sample 16 was produced under the same conditions as the silicon nitride sintered body of Sample 8, except that the amount of yttrium oxide powder added was 3.0 wt %, the amount of magnesium carbonate powder added was 10.0 wt %, and instead of using a carbon case, a silicon carbide mold was placed in a silicon carbide case and sintering was carried out.

[0063] [Various Measurements] The sintered bodies of the obtained samples were evaluated by the following measurements.

[0064] (Measurement of Density) The density of the sintered body of each sample was measured by a method in accordance with JIS R1634.

[0065] (Measurement of Area Ratio of Grain Boundary Phase) The area ratio of the grain boundary phase was determined by SEM observation. Specifically, five locations were randomly selected on the ion-milled surface of the silicon nitride sintered body, and a 50 μm × 50 μm field of view was observed at 2000x magnification. The areas of the regions or particles recognized as grain boundary phase in the five fields of view were then added together, and the ratio to the area of ​​the entire field of view was calculated to determine the area ratio of the grain boundary phase.

[0066] (Measurement of Area Ratio of Crystalline Region) By STEM observation, it was determined that the grain boundary phase consisted of amorphous region A and crystalline region C, and the area ratio of crystalline region C to the area of ​​the grain boundary phase was determined as C / (A+C)×100(%). Specifically, STEM observation was performed on the same processed surface as that observed by SEM, and the crystalline / amorphous nature of each grain boundary phase region was determined from electron diffraction analysis of the grain boundary phase, and the area percentage and area ratio of each were calculated.

[0067] (Identification of the constituent phases of the crystalline region) The constituent phases of the crystalline region C of the grain boundary phase of each sample were identified by XRD analysis. However, the constituent phases of the amorphous region were not confirmed.

[0068] (Calculation of thermal conductivity) The sintered body of each sample was processed into a size of 0.32 mm x 17 mm, and the thermal diffusivity was measured by a two-dimensional method using a laser flash. The specific heat value was 0.68 cm ２ The thermal conductivity was calculated from the thermal diffusivity measured by the two-dimensional method and the density value described above by the formula (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0069] (Measurement of bending strength) In accordance with ISO23242, the sintered body of each sample was processed to a thickness of 0.32×12×25 mm, and the bending strength was measured by three-point bending with a span of 15 mm.

[0070] (Results) Figure 3 is a table showing the characteristics of the grain boundary phase, the material of the crystalline region, and the properties of each sample. Samples 1 to 11 all had high thermal conductivity of 85 W / mK or more and bending strength of 500 MPa or more.

[0071] Sample 12 had a low bending strength, presumably because the amount of additive was small and the area ratio of the grain boundary phase was small. Sample 13 had a low thermal conductivity, presumably because the amount of additive was large and the area ratio of the grain boundary phase was large.

[0072] Sample 14 had a low thermal conductivity. This is presumably because the cooling rate was relatively fast, resulting in a small area ratio of the crystalline region to the grain boundary phase. Sample 15 had a low bending strength. This is presumably because the cooling rate was relatively slow, resulting in a large area ratio of the crystalline region to the grain boundary phase.

[0073] Sample 16 had a very low thermal conductivity. This is presumably because crystallization of the grain boundaries did not occur as a result of firing the sample without using a carbon case and placing a silicon carbide mold in a silicon carbide case.

[0074] From the above results, it was confirmed that the silicon nitride sintered body and silicon nitride heat dissipation substrate of the present invention can be made stronger while maintaining high thermal conductivity.

[0075] The present invention is not limited to the above-described embodiments, and various modifications and equivalents are included within the spirit and scope of the present invention. Furthermore, the structure, shape, number, position, size, etc. of the components shown in each drawing are for the convenience of explanation and may be changed as appropriate.

[0076] REFERENCE SIGNS LIST 10 silicon nitride heat dissipation substrate 12 circuit layer 14 conductor layer 20 circuit board 22 solder 30 power semiconductor 40 heat dissipation plate 42 grease 50 heat dissipation member 100 power device

Claims

1. A silicon nitride sintered body consisting essentially of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles, wherein in a cross section of the silicon nitride sintered body, the area of ​​the grain boundary phase is in the range of 1% or more and 10% or less of the total area, the grain boundary phase is composed of amorphous regions A and crystalline regions C, the area ratio C / (A+C)×100 of the crystalline regions C to the area of ​​the grain boundary phase is in the range of 20% or more and 70% or less, and the crystalline phase forming the crystalline regions C includes at least one of an M phase, a J phase, a monosilicate phase and a disilicate phase.

2. The silicon nitride sintered body according to claim 1, characterized in that the Al content is greater than 0 ppm and not more than 650 ppm.

3. A silicon nitride heat dissipation substrate comprising the silicon nitride sintered body according to claim 1 or 2.

4. The silicon nitride heat dissipation substrate according to claim 3, characterized in that the thermal conductivity is 85 W / mK or more.

5. The silicon nitride heat dissipation substrate according to claim 3, characterized in that the bending strength is 500 MPa or more.

6. The silicon nitride heat dissipation substrate according to claim 3, wherein the thickness of the silicon nitride heat dissipation substrate in a direction perpendicular to one of its main surfaces is 220 μm or more and 690 μm or less.

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