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

Abstract

[Problem] To provide a silicon nitride-based sintered compact and a silicon nitride-based heat dissipation substrate that achieve higher strength while maintaining high thermal conductivity. [Solution] This silicon nitride-based sintered compact mainly comprises silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, and contains at least a total of 1.0-7.5 wt% of rare-earth elements, a total of 0.25-2.0 wt% of alkali metal elements and alkaline earth metal elements, a total of 0.010-0.30 wt% of at least one element selected from Fe, Ni, Co, and Al, and a total of 0.30-3.0 wt% of group-4 elements. At least a portion of the group-4 elements forms at least one compound including nitrogen and / or carbon. The compound is contained in an amount of 0.30-3.0 wt% in total.

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] Patent Document 1 discloses a silicon nitride sintered body that is essentially made of silicon nitride, contains aluminum and oxygen as impurities each at 3.5% by weight or less, has a density of 3.15 g / cm^3 or more, and has a thermal conductivity of 40 W / mK or more.

[0004] Patent Document 2 discloses a silicon nitride sintered body characterized by having silicon nitride particles with a total content of oxygen, Al, Ca, and Fe of 1500 ppm or less and a minor axis diameter of 2 μm or more, and a method for producing the silicon nitride sintered body by molding and sintering a raw material powder obtained by adding an oxide of yttrium and / or one or more lanthanoid elements to silicon nitride powder, wherein the method uses silicon nitride powder containing 300 ppm or less of Al and 1 wt % or less of oxygen and having an alpha conversion rate of 70% or less.

[0005] Japanese Patent Laid-Open No. 4-175268 Japanese Patent Laid-Open No. 2001-19557

[0006] The silicon nitride sintered body described in Patent Document 1 has a thermal conductivity of 50 W / mK, which is high compared to conventional insulating ceramics, but its strength, which is important as a heat dissipation substrate, was unknown. The firing temperature was 2000°C, the holding time was 20 hours, and the pressing force was 200 kg / cm. ２ Another problem was that it was produced through a process that was extremely industrially demanding.

[0007] The manufacturing method described in Patent Document 2 aims to increase the thermal conductivity of the substrate by reducing the amount of impurities in the material, but because sintering requires a relatively long period of time, at 1900°C for 8 to 48 hours, grain growth is likely to occur and residual pores are generated, resulting in relatively low strength.

[0008] For these reasons, when using silicon nitride sintered bodies as insulating heat dissipation substrates for power devices, further strengthening is required to improve the reliability of the substrate while maintaining high thermal conductivity.

[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 have higher strength while maintaining high thermal conductivity.

[0010] (1) In order 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, and contains at least 1.0 to 7.5 wt% in total of rare earth elements, 0.25 to 2.0 wt% in total of alkali metal elements and alkaline earth metal elements, 0.010 to 0.30 wt% in total of one or more elements selected from Fe, Ni, Co, and Al, and 0.30 to 3.0 wt% in total of Group 4 elements, and the Group 4 elements form one or more compounds at least a portion of which contains at least one of nitrogen and carbon, and the content of such compounds is 0.30 to 3.0 wt% in total.

[0011] (2) In the silicon nitride sintered body according to the application example of (1) above, the average particle size of the compound particles is 8.0 μm or less.

[0012] (3) In the silicon nitride sintered body according to the application example of (1) or (2) above, the number of connections between the particles of the compound is less than three.

[0013] (4) In the silicon nitride sintered body according to any one of the application examples (1) to (3) above, the Group 4 element contains one or more elements selected from Zr and Hf.

[0014] (5) A silicon nitride heat dissipation substrate according to an application example of the present invention comprises the silicon nitride sintered body according to any one of (1) to (4) above.

[0015] (6) In addition, in the silicon nitride heat dissipation substrate of the application example of (5) above, the bending strength is 650 MPa or more.

[0016] (7) In the silicon nitride heat dissipation substrate according to the application example of (5) or (6) above, the thermal conductivity is 85 W / mK or more.

[0017] (8) In the silicon nitride heat dissipation substrate according to any one of the application examples (5) to (7) 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.

[0018] 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 having higher strength while maintaining high thermal conductivity.

[0019] Fig. 1 is a schematic perspective view showing an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention; 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; Fig. 3 is a table showing the element contents and grain boundary phase characteristics of each sample; Fig. 4 is a table showing the element contents and grain boundary phase characteristics of each sample; Fig. 5 is a table showing the characteristics of each sample.

[0020] 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.

[0021] [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.

[0022] The silicon nitride sintered body contains a total of 1.0 to 7.5 wt% of rare earth elements. If the rare earth element content is less than this range, sinterability may decrease, pores may remain, and the strength of the silicon nitride sintered body may be reduced. If the rare earth element content is more than this range, the amount of grain boundary phase may increase, and the thermal conductivity of the silicon nitride sintered body may be reduced. It is believed that the same effect 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).

[0023] The silicon nitride sintered body contains 0.25 to 2.0 wt % of alkali metal elements and alkaline earth metal elements in total. If the alkali metal elements and alkaline earth metal elements are less than this range, the strength of the silicon nitride sintered body may be reduced. If the alkali metal elements and alkaline earth metal elements are more than this range, the thermal conductivity of the silicon nitride sintered body may be reduced.

[0024] The silicon nitride sintered body contains one or more elements selected from Fe, Ni, Co, and Al in a total content of 0.010 to 0.30 wt%. If the total content of one or more elements selected from Fe, Ni, Co, and Al is less than this range, sinterability may decrease, pores may remain, and the strength of the silicon nitride sintered body may be reduced. If the total content of one or more elements selected from Fe, Ni, Co, and Al is greater than this range, these elements may dissolve in the silicon nitride in large amounts, resulting in phonon scattering and a decrease in the thermal conductivity of the silicon nitride sintered body. Of these, the content of Al is preferably 0.1 wt% or less.

[0025] The silicon nitride sintered body contains 0.30 to 3.0 wt% of Group 4 elements in total. Furthermore, the silicon nitride sintered body contains 0.30 to 3.0 wt% of compounds in total, in which at least a portion of the Group 4 elements form one or more compounds containing at least one of nitrogen and carbon. In the following description, a compound containing a Group 4 element and at least one of nitrogen and carbon is referred to as a Group 4 element compound.

[0026] When the Group 4 element content is within the above range and the Group 4 element compound content is also within the above range, both high thermal conductivity and high strength are achieved. Group 4 elements do not form a solid solution in silicon nitride, so they do not cause a decrease in thermal conductivity due to phonon scattering. They also do not form a solid solution in the grain boundary phase formed by the reaction of rare earth elements with silicon nitride. Therefore, they easily form compounds by themselves, and these compounds fill residual pores, improving strength. Furthermore, the Group 4 element compound particles suppress crack propagation, contributing to increased toughness. The Group 4 element compound 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.

[0027] These features make it possible to obtain a silicon nitride sintered body having higher strength while maintaining high thermal conductivity.

[0028] The average particle size of the Group 4 element compound particles is preferably 8.0 μm or less. If the average particle size of the Group 4 element compound particles is within the above particle size range, a high-strength silicon nitride sintered body can be obtained. If the particle size is larger than this range, fracture origins will occur and the compound will be localized, resulting in an increase in residual pores and possibly reduced strength.

[0029] The average particle size of the Group 4 element compound particles can be determined by SEM (Scanning Electron Microscope) observation. Specifically, five randomly selected locations on the polished surface of the silicon nitride sintered body are observed at 2000x magnification in a 120 μm × 90 μm field of view. The average particle size can then be determined by calculating the equivalent circle diameter from the area of ​​each particle identified as a Group 4 element compound and averaging these. Image analysis software such as Winroof can be used to calculate the average particle size. The Group 4 element compound can be identified from the results of XRF (X-ray Fluorescence) and XRD (X-ray Diffraction).

[0030] The number of connections between particles of the Group 4 element compound is preferably less than three. If the number of connections is less than three, it is less likely to become a fracture origin, and a decrease in strength can be prevented. Furthermore, if the Group 4 element compound is conductive, the number of connections between particles will be reduced, making it easier to maintain insulation. The number of connections between particles of the Group 4 element compound is the maximum number of connected particles among all of the above five fields of view.

[0031] The Group 4 element preferably contains one or more elements selected from Zr and Hf. This allows for the specific formation of a Group 4 element compound. When the silicon nitride sintered body contains Zr, zirconium nitride (ZrN), zirconium carbide (ZrC), and zirconium carbonitride (ZrCN) are likely to be produced as Group 4 element compounds. When the silicon nitride sintered body contains Hf, hafnium nitride (HfN), hafnium carbide (HfC), and hafnium carbonitride (HfCN) are likely to be produced as Group 4 element compounds.

[0032] [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.

[0033] The silicon nitride heat dissipation substrate 10 preferably has a bending strength of 650 MPa or more, which reduces the risk of the silicon nitride heat dissipation substrate 10 being damaged.

[0034] 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.

[0035] 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.

[0036] 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).

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] [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 nitride.

[0043] 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.

[0044] The mixed powder is then filled into a mold and pressed uniaxially 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 compact. The resulting compact (CIP-pressed compact) 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 to obtain a silicon nitride sintered body.

[0045] 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.

[0046] 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.

[0047] [Examples and Comparative Examples] (Sample 1) 3.2 wt% magnesium carbonate powder (average particle size 2.5 μm) and 3.0 wt% yttrium oxide powder (average particle size 1.0 μm) were weighed out relative to 100 wt% silicon nitride powder (average particle size 1.4 μm). Next, the weighed raw material powder was ball milled to obtain a mixed slurry. For ball milling, the raw material powder and ethanol were placed in a resin pot, and milled and mixed for 24 hours at 60 rpm using YSZ (Y2O3 partially stabilized zirconia) balls. The obtained mixed slurry was dried in a hot water bath to obtain a mixed powder.

[0048] 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.

[0049] The obtained molded body was sintered. The sintering method was atmospheric sintering under a nitrogen gas pressure of 9 atmospheres, and the maximum temperature was 1900°C, held for 5 hours. 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.

[0050] (Sample 2) The silicon nitride sintered body of Sample 2 was produced under the same conditions as those for the silicon nitride sintered body of Sample 1, except that the amount of magnesium carbonate powder added was 4.0 wt%.

[0051] (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 1, except that the amount of magnesium carbonate powder added was 5.0 wt %.

[0052] (Sample 4) The silicon nitride sintered body of Sample 4 was produced under the same conditions as those for the silicon nitride sintered body of Sample 1, except that the amount of magnesium carbonate powder added was 8.0 wt %.

[0053] (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 3, except that the amount of yttrium oxide powder added was 6.0 wt %.

[0054] (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 3, except that the amount of yttrium oxide powder added was 8.5 wt %.

[0055] (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 6, except that the yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 µm).

[0056] (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 6, except that the yttrium oxide powder was changed to erbium oxide powder (average particle size 1.4 μm).

[0057] (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 6, except that the yttrium oxide powder was changed to lanthanum oxide powder (average particle size 1.0 μm).

[0058] (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 5, except that 5.0 wt % of calcium carbonate powder (average particle size 2.5 μm) was further added.

[0059] (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 5, except that the maximum temperature during firing was 1850°C.

[0060] (Sample 12) The silicon nitride sintered body of Sample 12 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the maximum temperature during firing was set to 1800°C.

[0061] (Sample 13) The silicon nitride sintered body of Sample 13 was produced under the same conditions as those for the silicon nitride sintered body of Sample 3, except that the amount of yttrium oxide powder added was 1.0 wt %.

[0062] (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 3, except that the amount of yttrium oxide powder added was 9.5 wt %.

[0063] (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 5, except that the amount of magnesium carbonate powder added was 1.5 wt %.

[0064] (Sample 16) The silicon nitride sintered body of Sample 16 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the amount of magnesium carbonate powder added was 11.5 wt %.

[0065] (Sample 17) The silicon nitride sintered body of Sample 17 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the silicon nitride powder was changed to high-purity silicon nitride powder (average particle size 0.9 μm).

[0066] (Sample 18) The silicon nitride sintered body of Sample 18 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the silicon nitride powder was changed to a low-purity silicon nitride powder (average particle size 1.2 μm).

[0067] (Sample 19) The silicon nitride sintered body of Sample 19 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the mixing time for producing the mixed slurry was 6 hours.

[0068] (Sample 20) The silicon nitride sintered body of Sample 20 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the mixing time for producing the mixed slurry was 60 hours.

[0069] (Sample 21) The silicon nitride sintered body of Sample 21 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the balls used to prepare the mixed slurry were changed from YSZ to silicon nitride.

[0070] (Sample 22) The silicon nitride sintered body of Sample 22 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that the maximum temperature was maintained for 20 hours during firing.

[0071] (Sample 23) The silicon nitride sintered body of Sample 23 was produced under the same conditions as those for the silicon nitride sintered body of Sample 5, except that 1.0 wt % of zirconium nitride powder (average particle size 1.6 μm) was further added.

[0072] (Sample 24) The silicon nitride sintered body of Sample 24 was produced under the same conditions as the silicon nitride sintered body of Sample 3, except that carbon powder was placed in a BN-coated silicon carbide mold, and then the molded body was placed in the mold and sintered.

[0073] [Various Measurements] The sintered bodies of the obtained samples were evaluated by the following measurements.

[0074] (Measurement of Density) The density of the sintered body of each sample was measured by a method in accordance with JIS R1634.

[0075] (Measurement of Element Amount) The types and amounts of constituent elements of the sintered body of each sample were measured by X-ray fluorescence analysis (XRF).

[0076] (Identification of Constituent Phases) The constituent phases of the sintered body of each sample were identified by XRD analysis. In addition, the amount of a compound containing a Group 4 element and at least one of nitrogen and carbon (Group 4 element compound) was calculated from the results of XRF and XRD. However, the tables in Figures 3 and 4 only list components identified as crystalline, and do not list the presence or absence of amorphous phases. Note that the J phase listed in the grain boundary crystalline phase column in Figures 3 and 4 is a Y phase. ４ Si ２ O ７ N ２ The M phase is Y ２ Si ３ O ３ N ４ This shows:

[0077] (Measurement of particle size) The average particle size of the Group 4 element compound particles was determined by SEM observation. Specifically, five randomly selected locations on the polished surface of the silicon nitride sintered body were observed at 2000x magnification in a field of view of 120 μm × 90 μm. The circle-equivalent diameter was then determined from the area of ​​each particle recognized as a Group 4 element compound, and the average diameter was calculated.

[0078] (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).

[0079] (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.

[0080] (Measurement of fracture toughness) The measurement surface was mirror-polished and the fracture toughness value was calculated by the IF method in accordance with JIS R1607, using the Miyoshi formula.

[0081] (Results) Figures 3 and 4 are tables showing the element contents and grain boundary phase characteristics of each sample. Note that the Fe, Ni, Co, and Al in each sample originate from the raw materials or the equipment used in the manufacturing process. Furthermore, the Group 4 elements and Group 4 element compounds in each sample, except for samples 21 and 23, originate from the YSZ spheres used in the manufacturing process.

[0082] Figure 5 is a table showing the properties of each sample. Samples 1 to 12 and 22 to 24 all had high thermal conductivity of 85 W / mK or more and bending strength of 650 MPa or more.

[0083] Sample 13 had a low bending strength, presumably because the rare earth element content was too low. Sample 14 had a low thermal conductivity, presumably because the rare earth element content was too high.

[0084] Sample 15 had a low bending strength, presumably because the content of alkali metals and alkaline earth metal elements was too low. Sample 16 had a low thermal conductivity, presumably because the content of alkali metals and alkaline earth metal elements was too high.

[0085] Sample 17 had a low bending strength. This is presumably because the total content of Fe, Ni, Co, and Al was too low. Sample 18 had a low thermal conductivity. This is presumably because the total content of Fe, Ni, Co, and Al was too high. Sample 17 was produced using high-purity silicon nitride powder, so the total content of Fe, Ni, Co, and Al was low. This shows that although raw materials contain unavoidable impurities, it is difficult to control them as in the composition of the present invention by simply increasing the purity of the raw materials.

[0086] Sample 19 had a low bending strength, presumably because the content of the Group 4 element or Group 4 element compound was too low. Sample 20 had a low thermal conductivity, presumably because the content of the Group 4 element or Group 4 element compound was too high.

[0087] Sample 21 had a low bending strength, presumably because silicon nitride was used for the balls and no Group 4 element compounds were added externally, so the sample did not contain any Group 4 elements or Group 4 element compounds.

[0088] Although the bending strength of Sample 22 was within the allowable range, it was a slightly low value. This is presumably because the particle size of the Group 4 element compound (ZrN) was larger than 8 μm.

[0089] Although the bending strength of Sample 23 was within the allowable range, it was a little low. This is presumably because more than three crystal grains of the Group 4 element compound (ZrN) were connected together.

[0090] Although the Group 4 element compound identified in Sample 24 was ZrCN, which contains a Group 4 element, nitrogen, and carbon, both the thermal conductivity and bending strength were high. This indicates that the Group 4 element compound does not have to contain only a Group 4 element and nitrogen. It is estimated that the same applies to a Group 4 element compound containing a Group 4 element and carbon.

[0091] 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.

[0092] 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.

[0093] 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 mainly composed of silicon nitride particles and a grain boundary phase surrounding said silicon nitride particles, characterized in that it contains at least 1.0 to 7.5 wt% rare earth elements in total, 0.25 to 2.0 wt% alkali metal elements and alkaline earth metal elements in total, 0.010 to 0.30 wt% of one or more elements selected from Fe, Ni, Co and Al in total, and 0.30 to 3.0 wt% Group 4 elements in total, wherein the Group 4 elements form one or more compounds at least a portion of which contains at least one of nitrogen and carbon, and the silicon nitride sintered body contains 0.30 to 3.0 wt% of said compounds in total.

2. The silicon nitride sintered body according to claim 1, wherein the average particle size of the compound particles is 8.0 μm or less.

3. A silicon nitride sintered body according to claim 1 or 2, characterized in that the number of connections between particles of said compound is less than three.

4. The silicon nitride sintered body according to claim 1 or 2, characterized in that the Group 4 elements contain one or more elements selected from Zr and Hf.

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

6. The silicon nitride heat dissipation substrate according to claim 5, characterized in that the bending strength is 650 MPa or more.

7. The silicon nitride heat dissipation substrate according to claim 5, characterized in that the thermal conductivity is 85 W / mK or more.

8. The silicon nitride heat dissipation substrate according to claim 5, 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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