Silicon nitride sintered body, and silicon nitride heat dissipation substrate

Abstract

To provide a silicon nitride sintered body and a silicon nitride heat dissipation substrate that have both high thermal conductivity and high strength.SOLUTION: A silicon nitride sintered body is mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles. In a cross section of the silicon nitride sintered body, an area of the grain boundary phase relative to the whole area is in the range of 1% or more and 10% or less, the grain boundary phase consists of an amorphous region A and a crystalline region C, an area ratio C / (A+C)×100 of the crystalline region C to the area of the grain boundary phase is in the range of 20% or more and 70% or less, and a crystalline phase forming the crystalline region C includes at least one of an M phase, a J phase, a monosilicate phase, and a diesilicate phase.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Since silicon nitride has high thermal conductivity and strength, it has been attracting attention as an insulating heat dissipation substrate for power modules for inverters mounted in electric vehicles (EVs) and hybrid vehicles (HVs). Conventionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material. However, in the case of power modules for large currents such as EVs, the temperature rises to about 250°C, and a large thermal stress is generated in the substrate due to the difference in thermal expansion from metals such as copper to which it is joined. As a result, aluminum nitride with low strength has cracks and fractures. Therefore, although its thermal conductivity is inferior to that of aluminum nitride, among general insulating ceramics, silicon nitride with high thermal conductivity and higher strength is being increasingly adopted.

[0003] Patent Document 1 discloses a silicon nitride ceramic containing oxides of Mg and Ce in a total amount of 5 to 15% by weight in a weight ratio of 5:4 to 4:1 in terms of MgO and CeO conversion, with the balance being silicon nitride, wherein the silicon nitride crystal grains are acicular grains with a minor axis diameter of 1 μm or less and the grain boundaries are amorphous, and the porosity is 0.5% or less. 2 Patent Document 2 discloses 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.

[0004]

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[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, and the grain boundary phase is mainly composed of an amorphous substance. The aim is to improve the strength, and it is described that the strength is at least 600 MPa or more by mechanisms such as strengthening the grain boundaries and filling voids with an amorphous substance.

[0007] Silicon nitride is a high thermal conductivity material, such as reports that its theoretical thermal conductivity value exceeds 200 W / mK. The higher the thermal conductivity of the silicon nitride material, the better the heat dissipation performance. On the other hand, generally, the thermal conductivity of the amorphous phase is low, and it is said to be at most about 1 W / mK. When the grain boundary phase is occupied by an amorphous substance as in the technology described in Patent Document 1 or Patent Document 2, the thermal conductivity of the silicon nitride material decreases, so it is considered that satisfactory heat dissipation characteristics cannot be exhibited as a heat dissipation substrate.

[0008] As a result of intensive research, the present inventors have found that even in a silicon nitride-based sintered body containing a grain boundary phase, by setting the area ratio of the grain boundary phase and the area ratio of the crystalline region in the grain boundary phase within a predetermined range and adjusting the crystals constituting the crystalline region so that specific crystals are included, a silicon nitride-based sintered body having both high thermal conductivity and high strength can be obtained, and the present invention has been completed.

[0009] The present invention has been made in view of such circumstances, and an object thereof is to provide a silicon nitride-based sintered body and a silicon nitride-based heat dissipation substrate having both high thermal conductivity and high strength.

Means for Solving the Problems

[0010] (1) To achieve the above object, the silicon nitride-based sintered body of the present invention has taken the following means. That is, the silicon nitride-based sintered body of the application example of the present invention is a silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles. In the cross-section of the silicon nitride-based sintered body, the area of the grain boundary phase is in the range of 1% or more and 10% or less with respect to the total area. The grain boundary phase is composed of an amorphous region A and a crystalline region C. The area ratio C / (A + C)×100 of the crystalline region C to the area of the grain boundary phase is in the range of 20% or more and 70% or less. The crystal phase forming the crystalline region C includes at least one or more of an M phase, a J phase, a monosilicate phase, and a disilicate phase.

[0011] (2) Further, in the silicon nitride-based sintered body of the application example of (1) above, the content of Al is greater than 0 ppm and 650 ppm or less.

[0012] (3) Further, the silicon nitride-based heat dissipation substrate of the application example of the present invention is composed of the silicon nitride-based sintered body described in (1) or (2) above.

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

[0014] (5) Further, in the silicon nitride-based heat dissipation substrate of the application example of (3) or (4) above, the flexural strength is 500 MPa or more.

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

Advantages of the Invention

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

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

[0018] Next, embodiments of the present invention will be described with reference to the drawings. For ease of understanding of the description, the same reference numerals are assigned to the same components in each drawing, and duplicate descriptions are omitted. In the configuration diagrams, the sizes of the respective components are conceptually represented and do not necessarily represent actual dimensional ratios.

[0019] [Embodiment] [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 to the amount generated depending on the content range of Al described later. Mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles means that impurities other than the elements described later may be contained in an amount of 0.5 wt% or less.

[0020] In the 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 with respect to the total area. Further, the grain boundary phase is composed of an amorphous region A and a crystalline region C, and the area ratio C / (A + C)×100 of the area of the crystalline region C to the area of the grain boundary phase is in the range of 20% or more and 70% or less. Further, the 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.

[0021] The area ratio of the grain boundary phase can be determined by SEM (Scanning Electron Microscope) observation. Specifically, for the ion-milled processed surface of the silicon nitride sintered body, five locations are randomly selected, and a field of view of 50 μm × 50 μm is observed at a magnification of 2000 times. Then, the area ratio of the grain boundary phase can be determined by calculating the ratio of the total area of the regions or particles recognized as the grain boundary phase in the five fields of view to the area of the entire field of view. The calculation of the area ratio may use image analysis software such as Winroof.

[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, for the same processed surface observed by SEM, STEM observation is performed, and the crystal / amorphous state of each grain boundary phase region can be determined from the electron diffraction analysis of the grain boundary phase. Thereby, it can be determined that the grain boundary phase is composed of an amorphous region A and a crystalline region C, and the area ratio C / (A + C)×100 of the area of the crystalline region C to the area of the grain boundary phase can be calculated.

[0023] The crystal phase forming 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 PDF (Powder Diffraction File) card 00-045-0249. The M phase is also called the merlite phase and is a crystal of the Y 2 Si 3 O 3 N 4 type crystal. The J phase is a crystal identified by PDF card 01-086-1106. The J phase is a Y4 Si 2 O 7 N 2 is a type of crystal. The monosilicate phase is the crystal identified by PDF card 00-052-181. The monosilicate phase is also known as the m phase, and Y 2 SiO 5 is a type of crystal. The disilicate phase is the crystal identified by PDF card 01-072-359. The disilicate phase is also known as the d phase, and Y 2 Si 2 O 7 is a type of crystal. All of them show typical crystal compositions, but even if the elements are different, as long as the crystal system, space group, and arrangement of constituent atoms are the same. For example, the M phase can be Yb 2 Si 3 O 3 N 4 as well.

[0024] Since the grain boundary phase or the amorphous region A or crystalline region C constituting the grain boundary phase are randomly generated in the sintered body, the area ratio of the grain boundary phase or the area ratio of the crystalline region in the SEM image of the ion-milled processed surface of the silicon nitride sintered body can be regarded as the volume ratio in the silicon nitride sintered body.

[0025] The silicon nitride sintered body preferably contains Al in an amount greater than 0 ppm and not more than 650 ppm. Al is an impurity derived from the raw material, and by containing it, the sinterability of the silicon nitride sintered body can be improved, and the strength of the silicon nitride sintered body can be increased. Also, if the content is within this range, the influence of the decrease in thermal conductivity due to the formation of sialon can be almost ignored. If the content exceeds 650 ppm, the amount of sialon in the silicon nitride sintered body increases, and the thermal conductivity of the silicon nitride sintered body may decrease.

[0026] The silicon nitride sintered body preferably contains a rare earth element. Since the rare earth element mainly forms a grain boundary phase, the content of the rare earth element is preferably the amount necessary for forming the above grain boundary phase. The specific amount varies depending on the composition of the compound forming the amorphous region A and the crystalline region C. For example, the total content of the rare earth element is preferably 1.0 to 7.5 wt%. If the rare earth element is less than this range, the sinterability may decrease and pores may remain, resulting in a decrease in the strength of the silicon nitride sintered body. If the rare earth element is more than this range, the amount of the grain boundary phase may increase, resulting in a decrease in the thermal conductivity of the silicon nitride sintered body. It is considered that the same effect can be obtained with any rare earth element contained. The rare earth element can be, for example, yttrium (Y), lanthanum (La), erbium (Er), 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 can be improved and the strength of the silicon nitride sintered body can be increased. The silicon nitride sintered body preferably contains, for example, a total of 0.25 to 2.0 wt% of the alkali metal element and the alkaline earth metal element. If the alkali metal element and the alkaline earth metal element are less than this range, the strength of the silicon nitride sintered body may decrease. If the alkali metal element and the alkaline earth metal element are more than this range, the thermal conductivity of the silicon nitride sintered body may decrease.

[0028] The silicon nitride-based sintered body preferably contains a Group 4 element. For example, the silicon nitride-based sintered body preferably contains a total of 0.30 to 3.0 wt% of the Group 4 element. At this time, the Group 4 element may form a compound containing at least one of nitrogen or carbon. Since the Group 4 element does not dissolve in silicon nitride, it does not cause a decrease in the thermal conductivity due to phonon scattering. Also, it does not dissolve in the grain boundary phase generated by the reaction of the rare earth element and silicon nitride. Therefore, it is easy to form a compound alone, and since this compound fills the residual pores, the strength is improved. In addition, since the compound particles containing at least one of the Group 4 element and nitrogen or carbon suppress crack propagation, it also contributes to high toughness. Note that the compound containing at least one of the Group 4 element and nitrogen or carbon may be not only a compound containing only the Group 4 element and nitrogen or carbon, but also a compound containing the Group 4 element, nitrogen, and carbon, and further, within a range where their crystal structures are not destroyed, a compound in which other Group 4 elements or anions (such as oxygen) are dissolved may also be used.

[0029] Due to these characteristics, it is possible to obtain a silicon nitride-based sintered body with higher strength while maintaining a high thermal conductivity.

[0030] [Configuration of Silicon Nitride-Based Heat Dissipation Substrate] FIG. 1 is a schematic perspective view showing an example of a silicon nitride-based heat dissipation substrate according to an embodiment of the present invention. The silicon nitride-based heat dissipation substrate 10 of the present invention is made of the above-described silicon nitride-based sintered body. Thereby, it is possible to achieve higher strength while maintaining a high thermal conductivity, and the possibility of problems occurring in the circuit board using this can be reduced. The silicon nitride-based heat dissipation substrate 10 of the present invention can be suitably used as a heat dissipation substrate for a circuit board for a power device. The silicon nitride-based heat dissipation substrate 10 is formed in a flat plate shape, for example.

[0031] The silicon nitride-based heat dissipation substrate 10 preferably has a flexural strength of 500 MPa or more. Thereby, the possibility of the silicon nitride-based heat dissipation substrate 10 being damaged can be reduced.

[0032] The bending strength can be measured as follows. In accordance with ISO23242, a silicon nitride heat dissipation substrate is processed into a certain thickness × 12 × 25 mm. Then, the bending strength can be measured by three-point bending with a span of 15 mm. ISO23242 can be applied 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. Thereby, the performance as a heat dissipation substrate can be sufficiently exhibited.

[0034] The thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed into 0.32 mm × □17 mm, and the thermal diffusivity is measured by a two-dimensional method using a laser flash. Also, the density of the silicon nitride heat dissipation substrate 10 is measured by a method in accordance with JIS R1634. The specific heat value is constant at 0.68 cm 2 / sec. Then, the thermal conductivity can be calculated from the measured values of the thermal diffusivity and density by (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0035] The thickness in the direction perpendicular to one main surface of the silicon nitride heat dissipation substrate 10 is preferably 220 μm or more and 690 μm or less. Thereby, the balance between the strength and heat dissipation of the silicon nitride heat dissipation substrate 10 can be improved. If the thickness is smaller than this range, the strength of the substrate may be low. Also, if the thickness is larger than this range, the heat dissipation performance may decrease.

[0036] [Configuration of Power Device] FIG. 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 has a circuit layer 12 formed on one main surface of the silicon nitride heat dissipation substrate 10, and a conductor layer 14 formed on the other main surface facing 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 having 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 using a joining member such as a 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 through which a large current for an EV flows and which is likely to heat up. Since the silicon nitride heat dissipation substrate 10 of the present invention has high strength while maintaining a high thermal conductivity, even when a large thermal stress is generated in the silicon nitride heat dissipation substrate 10 due to a difference in thermal expansion between the metal joined to the silicon nitride heat dissipation substrate 10 due to temperature rise, cracks and fractures are less likely to occur.

[0039] A heat sink 40 is joined to the lower side of the conductor layer 14 of the circuit board 20. The heat sink 40 and the conductor layer 14 may be joined using solder 22 or the like. The surface of the heat sink 40 facing the surface joined to the conductor layer 14 is in contact with a heat dissipation member 50 via grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal having copper as a main component. The heat dissipation member 50 has heat dissipation fins formed thereon. The heat dissipation member 50 is preferably made of metal, and more preferably made of a metal having copper or aluminum as a main component.

[0040] [Method for Manufacturing Silicon Nitride Sintered Body and Silicon Nitride Heat Dissipation Substrate] An example of the method for manufacturing the above-described silicon nitride sintered body and the silicon nitride heat dissipation substrate is shown below. First, necessary materials are selected from the raw material powders of the silicon nitride sintered body and weighed to obtain the target composition. The raw material powders of the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of each element contained in the silicon nitride sintered body. The raw material powders of the silicon nitride sintered body include, in addition to silicon nitride, for example, magnesium carbonate, calcium carbonate, yttrium oxide, ytterbium oxide, erbium oxide, lanthanum oxide, zirconium oxide, etc.

[0041] Ethanol is added to these raw material powders and wet-mixed and pulverized in a ball mill for, for example, 6 to 60 hours to obtain a slurry. The slurry is dried by, for example, evaporation or a spray dryer to obtain a mixed powder.

[0042] Next, the mixed powder is filled into a mold and uniaxially pressed at, for example, a pressure of 30 MPa to form it into a desired shape. Then, CIP treatment (cold isostatic pressing treatment) is performed at, for example, a pressure of 150 MPa to obtain a formed body. The obtained formed body (CIP pressed body) is placed in, for example, a silicon carbide mold with an inner surface coated with BN and fired by holding it at a maximum temperature of 1800°C to 1900°C for 5 to 20 hours in a nitrogen atmosphere of 9 atmospheres. Then, a silicon nitride sintered body is obtained by cooling, for example, from the maximum temperature to 1400°C at a cooling rate of 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 have a predetermined shape and thickness. The processing can be performed, for example, by grinding, polishing, blasting treatment, etc.

[0044] By such a manufacturing method, a silicon nitride sintered body or a silicon nitride heat dissipation substrate with higher strength while maintaining high thermal conductivity can be manufactured.

[0045] [Examples, Comparative Examples] (Sample 1) For silicon nitride powder (average particle size 1.4 μm), 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 by internal ratio. Next, using the weighed raw material powders, ball milling was carried out to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were put into a resin pot, and silicon nitride balls were used for grinding and mixing at 60 rpm for 16 hours. The obtained mixed slurry was dried by simmering to obtain a mixed powder.

[0046] The obtained mixed powder was subjected to powder press molding by uniaxial pressing and CIP to produce a molded body. First, after filling the mixed powder into a dedicated mold, pre-molding was carried out by uniaxial pressing at a pressure of 30 MPa. Next, the pre-molded body was evacuated and put into a dedicated bag, and CIP molding was carried out at a pressure of 150 MPa.

[0047] The obtained molded body was fired. The sintering method was atmospheric firing under a gas pressure of 9 atmospheres of nitrogen, and it was held at a maximum temperature of 1900 °C for 12 hours. Then, it was cooled from the maximum temperature to 1400 °C at a cooling rate of 200 °C / hr. As the mold, a silicon carbide mold with an internal BN coating 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 addition amount of yttrium oxide powder was 5.0 wt% and the addition amount of magnesium carbonate powder was 4.0 wt%.

[0049] (Sample 3) The silicon nitride sintered body of Sample 3 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of magnesium carbonate powder 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 addition amount of magnesium carbonate powder 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 the silicon nitride sintered body of Sample 2, except that the holding time at the maximum temperature during firing was 9 hours.

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

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

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

[0055] (Sample 9) The silicon nitride sintered body of Sample 9 was produced under the same conditions as 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 the silicon nitride sintered body of Sample 2, except that yttrium oxide powder was changed to yttrium orthovanadate 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 the silicon nitride sintered body of Sample 2, except that 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 addition amount of yttrium oxide powder was 1.0 wt% and the addition amount of magnesium carbonate powder 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 addition amount of yttrium oxide powder was 7.0 wt% and the addition amount of magnesium carbonate powder was 10.0 wt%.

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

[0061] (Sample 15) The silicon nitride sintered body of Sample 15 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the cooling rate was 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 addition amount of yttrium oxide powder was 3.0 wt% and the addition amount of magnesium carbonate powder was 10.0 wt%, and without using a carbon case, a silicon carbide mold was placed in a silicon carbide case for firing.

[0063] [Various Measurements] The sintered bodies of each obtained sample were evaluated by the following measurements and the like.

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

[0065] (Measurement of Grain Boundary Phase Area Ratio) The area ratio of the grain boundary phase was determined by SEM observation. Specifically, for the ion-milled processed surface of the silicon nitride sintered body, five locations were randomly selected, and a field of view of 50 μm × 50 μm was observed at a magnification of 2000 times. Then, the total area of the regions or particles recognized as the grain boundary phase in the five fields of view was summed, and the area ratio of the grain boundary phase was determined by calculating the ratio to the total area of the entire field of view.

[0066] (Measurement of the area ratio of the crystalline region) By STEM observation, it was found that the grain boundary phase consists of 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 was determined. Specifically, for the same processed surface observed by SEM, STEM observation was performed, and the crystal / amorphous state of each grain boundary phase region was determined from the electron diffraction analysis of the grain boundary phase, and the respective area ratios and area ratios 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) Each sintered sample was processed into 0.32 mm × □17 mm, and the thermal diffusivity was measured by the two-dimensional method using a laser flash. Also, the specific heat value was kept constant at 0.68 cm 2 / sec. Then, the thermal conductivity was calculated from the thermal diffusivity measured by the two-dimensional method and the above density value using (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0069] (Measurement of flexural strength) In accordance with ISO23242, each sintered sample was processed into a thickness of 0.32 × 12 × 25 mm, and the flexural 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 of each sample, the material of the crystalline region, and the characteristics of each sample. Samples 1 to 11 all had a thermal conductivity of 85 W / mK or more and a flexural strength of 500 MPa or more, resulting in high values for both thermal conductivity and flexural strength.

[0071] Sample 12 had a low flexural strength. This is presumably because the amount of the auxiliary agent was small and the area ratio of the grain boundary phase became small. Sample 13 had a low thermal conductivity. This is presumably because the amount of the auxiliary agent was large and the area ratio of the grain boundary phase became large.

[0072] Sample 14 had a low thermal conductivity. This is presumably because the cooling rate was relatively fast and the area ratio of the crystalline region to the area of the grain boundary phase became small. Sample 15 had a low flexural strength. It is presumed that this was because the cooling rate was relatively slow and the area ratio of the crystalline region to the area of the grain boundary phase became large.

[0073] Sample 16 had a very low thermal conductivity. This is presumably because, without using a carbon case, a silicon carbide mold was placed in a silicon carbide case and fired, resulting in no crystallization at the grain boundaries.

[0074] From the above results, it was confirmed that the silicon nitride sintered body and the silicon nitride heat radiating substrate of the present invention can achieve higher strength while maintaining a high thermal conductivity.

[0075] The present invention is not limited to the above-described embodiments, and it goes without saying that the present invention extends to various modifications and equivalents included in the spirit and scope of the present invention. In addition, the structure, shape, number, position, size, etc. of the components shown in each drawing are for convenience of explanation and can be changed as appropriate.

Explanation of Reference Numerals

[0076] 10 Silicon nitride heat radiating substrate 12 Circuit layer 14 Conductor layer 20 Circuit board 22 Solder 30 Power semiconductor 40 Heat sink 42 Grease 50 Heat dissipation member 100 Power device

Claims

1. A silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, in the cross-section of the silicon nitride-based sintered body, the area of the grain boundary phase is in the range of 1% or more and 10% or less with respect to the total area, the grain boundary phase is composed of 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 of 20% or more and 70% or less, the crystal phase forming the crystalline region C includes at least one or more of an M phase, a J phase, a monosilicate phase, and a disilicate phase. A silicon nitride-based sintered body characterized by this.

2. The silicon nitride-based sintered body according to Claim 1, characterized in that the Al content is greater than 0 ppm and 650 ppm or less.

3. A silicon nitride-based heat dissipation substrate, characterized by being made of the silicon nitride-based sintered body according to Claim 1 or Claim 2.

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

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

6. The silicon nitride-based heat dissipation substrate according to Claim 3, characterized in that the thickness in the direction perpendicular to one main surface of the silicon nitride-based heat dissipation substrate is 220 μm or more and 690 μm or less.

Images