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 with higher intensity while maintaining high thermal conductivity.SOLUTION: A silicon nitride sintered body primarily includes a silicon nitride grain and a grain boundary phase surrounding the silicon nitride grain. The silicon nitride sintered body at least includes a rare-earth element of 1.0 to 6.5 wt.% in total, and an alkaline / alkaline-earth metal element of 0.3 to 3.0 wt.% in total. If an element content of the rare-earth element is Re and an element content of the alkaline / alkaline-earth metal element is A in a two-grain grain boundary phase of the silicon nitride grain, an average of the element ratio Re / (A+Re) is within a range of 0.1 to 0.5.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 technology]

[0002] Silicon nitride, with its high thermal conductivity and strength, is attracting attention as an insulating heat dissipation substrate for inverter power modules installed in EVs (electric vehicles) and HVs (hybrid vehicles). Traditionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material, but in the case of high-current power modules such as those used in EVs, temperatures reach around 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. For this reason, silicon nitride, which has higher thermal conductivity than common insulating ceramics and even greater strength, is increasingly being adopted, even though its thermal conductivity is inferior to that of aluminum nitride.

[0003] Patent Document 1 discloses a silicon nitride sintered body including a plurality of silicon nitride particles and a grain boundary triple junction located between three or more of the plurality of silicon nitride particles, wherein the grain boundary triple junction contains Mg and at least one element RE selected from rare earth elements, the ratio of Mg to RE at the grain boundary triple junction (Mg / RE) satisfies Mg / RE≦1, and the O content in the silicon nitride particles is 0.05 mass% or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-153934 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the current value flowing through inverters has tended to increase, which places extremely large thermal stresses on silicon nitride substrates, causing problems such as cracking of the substrates. Furthermore, the flexural strength of the silicon nitride sintered body described in Patent Document 1 is approximately 500 MPa, and when such silicon nitride substrates are used in applications where temperatures are higher than conventional ones, they are unlikely to withstand the thermal stress due to their insufficient strength, increasing the risk of cracking.

[0006] 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.

[0007] 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. [Means for solving the problem]

[0008] (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, containing at least 1.0 to 6.5 wt% in total of rare earth elements and 0.3 to 3.0 wt% in total of alkali / alkaline earth metal elements, and at the grain boundaries between two silicon nitride particles, the average element ratio Re / (A+Re) is in the range of 0.1 to 0.5, where Re is the amount of rare earth elements and A is the amount of alkali / alkaline earth metal elements.

[0009] (2) In the silicon nitride sintered body according to the application example of (1) above, the alkali / alkaline earth metal element includes one or more elements selected from Mg and Ca, and the rare earth element includes one or more elements selected from Y, La, and Yb.

[0010] (3) The silicon nitride sintered body according to the application example of (1) or (2) above further contains a Group 4 element, and the Group 4 element forms one or more compounds at least partly containing nitrogen or carbon.

[0011] (4) 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 (3) above.

[0012] (5) In addition, in the silicon nitride heat dissipation substrate of the application example of (4) above, the bending strength is 700 MPa or more.

[0013] (6) In addition, in the silicon nitride heat dissipation substrate according to the application example of (4) or (5) above, the thermal conductivity is 85 W / mK or more.

[0014] (7) In the silicon nitride heat dissipation substrate according to any one of the application examples (4) to (6) above, the thickness of the silicon nitride heat dissipation substrate in the direction perpendicular to one of the main surfaces is 220 μm or more and 690 μm or less. [Effects of the Invention]

[0015] The silicon nitride sintered body or silicon nitride heat dissipation substrate of the present invention can have a higher strength while maintaining a high thermal conductivity. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic perspective view showing an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention. [Figure 2] 1 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. [Figure 3] 1 is a table showing the element contents, grain boundary phase characteristics, and properties of each sample. [Figure 4] (a) and (b) are SEM images of the silicon nitride sintered bodies of Sample 1 and Sample 10, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] [Embodiment] [Structure 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 limit the amount of sialon produced 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 impurities other than the elements described below in a total amount of 0.5 wt% or less.

[0019] The silicon nitride sintered body contains a total of 1.0 to 6.5 wt% of rare earth elements. If the rare earth element content is less than this range, the sinterability will decrease, pores will remain, and the strength of the silicon nitride sintered body may decrease. If the rare earth element content is more than this range, the amount of grain boundary phase will increase, and the thermal conductivity of the silicon nitride sintered body may decrease. It is believed that the same effect can be obtained regardless of the rare earth element contained. The rare earth element preferably includes one or more elements selected from yttrium (Y), lanthanum (La), and ytterbium (Yb).

[0020] The silicon nitride sintered body contains a total of 0.3 to 3.0 wt% of alkali and alkaline earth metal elements. If the alkali and alkaline earth metal elements are present in amounts less than this range, the strength of the silicon nitride sintered body may be reduced. If the alkali and alkaline earth metal elements are present in amounts greater than this range, the thermal conductivity of the silicon nitride sintered body may be reduced. The alkali and alkaline earth metal elements refer to alkali metal elements or alkaline earth metal elements. The alkali and alkaline earth metal elements preferably include one or more elements selected from Mg and Ca.

[0021] In silicon nitride sintered bodies, the average element ratio Re / (A+Re) at the grain boundary between two silicon nitride particles is in the range of 0.1 to 0.5, where Re is the amount of rare earth element and A is the amount of alkali / alkaline earth metal element. The grain boundary between two silicon nitride particles refers to the grain boundary between adjacent silicon nitride particles. The amount of rare earth element Re and the amount of alkali / alkaline earth metal element A are values that indicate the amounts of elements determined at a point on the grain boundary between two silicon nitride particles by point analysis using STEM-EDS (Scanning Transmission Electron Microscope - Energy-Dispersive-Spectroscopy) analysis, which will be described later.

[0022] When the value of Re / (A+Re) is within the above range, columnar growth of silicon nitride particles and filling of grain boundary voids by the grain boundary phase due to reduced viscosity can both be achieved. If the value of Re / (A+Re) is greater than the above range, i.e., if the proportion of Re at the grain boundaries between two particles is high, the viscosity of the grain boundary components that have become liquid increases, and the voids are not sufficiently filled with the grain boundary phase, resulting in an increase in voids and reduced strength and thermal conductivity. If the value of Re / (A+Re) is less than the above range, i.e., if the proportion of Re at the grain boundaries between two particles is low, columnar growth of silicon nitride particles is not promoted, and strength and toughness may be reduced.

[0023] The element ratio Re / (A + Re) at the grain boundary between two silicon nitride particles can be determined by STEM-EDS analysis. Specifically, five randomly selected locations are observed on the surface of a polished silicon nitride sintered compact, which is then ion-milled, at a magnification of 50,000x, over a 5 μm x 5 μm field of view. Next, one randomly selected location on the grain boundary between two silicon nitride particles is analyzed at each of the five locations, and the amount of each element is determined by point analysis. The amount of rare earth element at each point is then defined as Re, and the amount of alkali and alkaline earth metal element at each point is defined as A. The value of Re / (A + Re) is then calculated, and the average (arithmetic mean) is calculated.

[0024] Since the grain boundary phase is generated randomly within the sintered body, the value of Re / (A+Re) calculated from the results of STEM-EDS analysis of the ion-milled surface of a silicon nitride sintered body is roughly the same regardless of the part of the sintered body.

[0025] The grain boundary phase preferably contains at least one of the M phase, J phase, monosilicate phase, and disilicate phase, and more preferably contains the M phase. The grain boundary phase can be identified from the results of X-ray diffraction (XRD). The M phase is a crystal identified by 00-045-0249 on the PDF (Powder Diffraction File) card. The M phase, also known as the melilite phase, is a Y2Si3O3N4-type crystal. The J phase is a crystal identified by 01-086-1106 on the PDF card. The J phase is a Y4Si2O7N2-type crystal. The monosilicate phase is a crystal identified by 00-052-181 on the PDF card. The monosilicate phase, also known as the m phase, is a Y2SiO5-type crystal. The disilicate phase is a crystal identified by 01-072-359 on the PDF card. The disilicate phase, also known as the d-phase, is a Y2Si2O7-type crystal. While each 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 may be Yb2Si3O3N4.

[0026] The silicon nitride sintered body preferably contains a Group 4 element. The silicon nitride sintered body preferably contains a total of 0.30 to 3.0 wt% of the Group 4 element. The silicon nitride sintered body preferably contains at least one compound in which at least a portion of the Group 4 element contains at least one of nitrogen and carbon.

[0027] When the Group 4 element content is within the above range, both high thermal conductivity and high strength are achieved. Group 4 elements do not dissolve in silicon nitride, preventing a decrease in thermal conductivity due to phonon scattering. Furthermore, they do not dissolve in the grain boundary phase formed by the reaction of rare earth elements with silicon nitride. Therefore, they easily form compounds by themselves, which fill residual porosity and improve strength. Furthermore, the Group 4 element compound particles suppress crack propagation, contributing to high toughness. Note that Group 4 element compounds may include not only compounds containing only Group 4 elements and nitrogen or carbon, but also compounds containing Group 4 elements, nitrogen, and carbon. They may also contain other Group 4 elements or anions (such as oxygen) in solid solution to the extent that the crystal structure is not disrupted. The type of Group 4 element compound can be identified using X-ray diffraction (XRD) results.

[0028] The Group 4 element preferably contains one or more elements selected from zirconium (Zr) and hafnium (Hf). This allows for the specific formation of a Group 4 element compound. When the silicon nitride sintered body contains Zr, the Group 4 element compounds that are likely to be formed are zirconium nitride (ZrN), zirconium carbide (ZrC), and zirconium carbonitride (ZrCN). When the silicon nitride sintered body contains Hf, the Group 4 element compounds that are likely to be formed are hafnium nitride (HfN), hafnium carbide (HfC), and hafnium carbonitride (HfCN).

[0029] The silicon nitride sintered body may contain more than 0 wt% but not more than 0.1 wt% 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 0.1 wt%, 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.

[0030] These features make it possible to obtain a silicon nitride sintered body with higher strength while maintaining high thermal conductivity.

[0031] [Configuration of silicon nitride heat dissipation substrate] 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. The silicon nitride heat dissipation 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 occurring in circuit boards using the same. The silicon nitride heat dissipation substrate 10 of the present invention can be suitably used as a heat dissipation substrate for circuit boards for power devices. The silicon nitride heat dissipation substrate 10 is formed, for example, in a flat plate shape.

[0032] The silicon nitride heat dissipation substrate 10 preferably has a bending strength of 700 MPa or more, which reduces the risk of the silicon nitride heat dissipation substrate 10 being damaged.

[0033] Bending strength can be measured as follows. In accordance with ISO 23242, a silicon nitride heat dissipation substrate is processed to a fixed thickness of 12 mm x 25 mm. Then, bending strength can be measured using a 15 mm span and three-point bending test. ISO 23242 is applicable to ceramic thin plates with a thickness of 0.2 mm to 1.0 mm.

[0034] The silicon nitride heat dissipation substrate 10 preferably has a thermal conductivity of 85 W / mK or more, which allows it to fully exhibit its performance as a heat dissipation substrate.

[0035] 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 10 mm, and the thermal diffusivity is measured by the xenon flash method. 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 using the formula (thermal conductivity) = (density) x (specific heat) x (thermal diffusivity).

[0036] 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 decrease. If the thickness is larger than this range, the heat dissipation performance may decrease.

[0037] [Power device configuration] 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.

[0038] 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 bonded to the silicon nitride heat dissipation substrate 10 directly or with a bonding material such as brazing material.

[0039] 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 for use 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.

[0040] 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 dissipation member 50 via grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal containing copper as its 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 containing copper or aluminum as its main component.

[0041] [Methods for producing silicon nitride sintered bodies and silicon nitride heat dissipation substrates] 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 achieve 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 (Si3N4), examples of raw material powders for the silicon nitride sintered body include magnesium carbonate (MgCO3), calcium carbonate (CaCO3), yttrium oxide (YO3), ytterbium oxide (Yb2O3), lanthanum oxide (La2O3), and zirconium nitride (ZrN).

[0042] Ethanol is added to these raw material powders, and the mixture is wet mixed and pulverized 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 spray dryer to obtain a mixed powder. A binder (acrylic or the like) may be added to the obtained mixed slurry, and the mixture may be mixed for about 30 minutes and then dried.

[0043] The mixed powder is then filled into a mold and pressed uniaxially at a pressure of 100 MPa to obtain a green body. The green body may be formed by cold isostatic pressing (CIP). The green body is then heat-treated (debound) by, for example, holding it in an air flow at 600°C for 5 hours to obtain a debound body.

[0044] The resulting debound body is then placed in a carbon mold, for example, with the interior coated with BN, and sintered for 8 hours at a maximum temperature of 1900°C under a nitrogen atmosphere at 9 atmospheres. Sintering the debound body in a carbon mold creates a stronger reducing atmosphere, facilitating the volatilization of the alkali and alkaline earth metal elements added as oxides. As the alkali and alkaline earth metal elements volatilize, the proportion of rare earth elements in the grain boundary phase increases, promoting the columnar growth of silicon nitride particles. Furthermore, once the alkali and alkaline earth metal elements have volatilized to a certain extent, the viscosity of the liquid grain boundary phase decreases, filling the voids and slowing the volatilization of the alkali and alkaline earth metal elements. These factors are believed to result in increased strength of the silicon nitride sintered body. This process allows silicon nitride sintered bodies to be obtained.

[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 this manufacturing method, it is possible to manufacture a silicon nitride sintered body or a silicon nitride heat dissipation substrate that can be made stronger while maintaining high thermal conductivity.

[0047] [Examples and Comparative Examples] (Sample 1) 96.5 wt% silicon nitride powder (average particle size 1.4 μm), 2.0 wt% yttrium oxide powder (average particle size 1.0 μm), and 1.5 wt% magnesium carbonate powder (average particle size 2.5 μm) were weighed out. Next, the weighed raw material powders were ball milled to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were placed in a resin pot and milled and mixed at 60 rpm for 20 hours using YSZ (Y2O3 partially stabilized zirconia) balls. A binder (acrylic) was added to the obtained mixed slurry, and after mixing for an additional 30 minutes, the slurry was dried by spray drying to obtain a mixed powder.

[0048] Next, the resulting mixed powder was subjected to powder press molding using a uniaxial press to produce a compact. The mixed powder was filled into a dedicated mold and then molded using a uniaxial press at a pressure of 100 MPa. The resulting compact was then degreased. Degreasing was performed by holding it at a maximum temperature of 600°C for 5 hours in an air flow. Next, the resulting degreased body was sintered. The sintering method was atmospheric sintering using a nitrogen gas pressure of 9 atmospheres, and the maximum temperature was 1900°C for 8 hours. A carbon mold with a BN coating on the inside was used. In this way, a silicon nitride sintered body of Sample 1 was produced.

[0049] (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 yttrium oxide powder added was 3.0 wt %.

[0050] (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 yttrium oxide powder added was 8.0 wt %.

[0051] (Sample 4) The silicon nitride sintered body of Sample 4 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 3.0 wt%, the amount of magnesium carbonate powder added was 1.0 wt%, and 0.5 wt% of calcium carbonate powder (average particle size 2.5 μm) was also added.

[0052] (Sample 5) The silicon nitride sintered body of Sample 5 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 3.0 wt % and the amount of magnesium carbonate powder added was 5.0 wt %.

[0053] (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 yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 μm).

[0054] (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 yttrium oxide powder was changed to lanthanum oxide powder (average particle size 1.0 μm).

[0055] (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 carbon powder was placed in a BN-coated carbon mold, and then the degreased body was placed in the mold and fired.

[0056] (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 the balls used to produce the mixed slurry were changed from YSZ to silicon nitride.

[0057] (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 a mold made of SiC with a BN coating on the inside was used as the mold during firing.

[0058] (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 a mold made of hBN (hexagonal boron nitride) with a BN coating on the inside was used as the mold during firing.

[0059] [Various measurements] The sintered body of each sample was polished to remove 0.25 mm or more from the sintered surface, and then evaluated by the following measurements.

[0060] (density measurement) The density of the sintered body of each sample was measured by a method in accordance with JIS R1634.

[0061] (Measurement of elemental amounts) The polished surfaces were subjected to X-ray fluorescence analysis (XRF) to measure the types and amounts of constituent elements of the sintered body of each sample.

[0062] (Identification of constituent phases) The crystalline phase of the sintered grain boundary phase of each sample was identified by XRD analysis. However, the table in Figure 3 lists only the components identified as crystalline, and does not list the presence or absence of amorphous phase.

[0063] (2) Measurement of element ratios at grain boundaries The polished surface was further ion-milled, and five randomly selected locations were subjected to STEM-EDS analysis. From the results, the Re / (A+Re) value and its average value were calculated.

[0064] (Calculation of thermal conductivity) The sintered body of each sample was processed into a 0.32mm x 10mm square, and the thermal diffusivity was measured using the xenon flash method. The specific heat value was 0.68cm 2 The thermal conductivity was calculated from the thermal diffusivity measured by the xenon flash method and the density value above using the formula (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0065] (Bending strength measurement) In accordance with ISO23242, the sintered body of each sample was processed into a thickness of 0.32 × 12 × 25 mm, and the bending strength was measured using a 15 mm span, three-point bending test.

[0066] (result) Figure 3 is a table showing the element content, characteristics of the grain boundary phase, and properties of each sample. The Group 4 elements in each sample except for sample 9 originated from the YSZ spheres used in the manufacturing process. Samples 1 to 9, in which the average Re / (A+Re) at the two-particle grain boundaries was in the range of 0.1 to 0.5, all had high thermal conductivities of 85 W / mK or more and bending strengths of 700 MPa or more.

[0067] Samples 10 and 11 exhibited low bending strength. This is presumably due to the average Re / (A+Re) at the two-particle boundary being below 0.1. Samples 10 and 11 were fired using molds made of SiC or hBN, which weakened the reducing atmosphere during firing, making it difficult for alkali and alkaline earth metal elements to volatilize, resulting in a low average Re / (A+Re). It is presumed that reduced volatilization of alkali and alkaline earth metal elements does not promote columnar growth of silicon nitride particles, resulting in isotropic growth, resulting in a decrease in strength.

[0068] Figures 4(a) and (b) are SEM images of the silicon nitride sintered bodies of Sample 1 and Sample 10, respectively. Figures 4(a) and (b) are SEM images at 2000x magnification of the polished surface of each sample that was subjected to plasma etching. In Figure 4(a), the silicon nitride particles have grown in a columnar shape in various directions. In contrast, in Figure 4(b), it can be seen that the silicon nitride particles have not grown in a columnar shape.

[0069] A comparison of Sample 2 and Sample 8 revealed that the Group 4 element compounds contained in the silicon nitride sintered body play equivalent roles when compared with compounds containing a Group 4 element and nitrogen and compounds containing a Group 4 element, nitrogen, and carbon. It is also assumed that the Group 4 element compound contained in the silicon nitride sintered body may also be a compound containing a Group 4 element and carbon.

[0070] Sample 9 had slightly lower thermal conductivity and bending strength. This is presumably because the silicon nitride sintered compact did not contain a Group 4 element compound. This confirmed that it is preferable for silicon nitride sintered compacts to contain a Group 4 element compound. On the other hand, Sample 9 had a higher bending strength than Samples 10 and 11. In other words, even silicon nitride sintered compacts that do not contain a Group 4 element compound have a higher bending strength when the average element ratio Re / (A+Re) at the two grain boundaries is within the range of 0.1 to 0.5 compared to when the average is below this range.

[0071] From the above results, it was confirmed that the silicon nitride sintered body and silicon nitride heat dissipation substrate of the present invention, in which the contents of rare earth elements and alkali / alkaline earth metal elements are within the specified ranges, can be made stronger while maintaining high thermal conductivity by controlling the sintering conditions so that the average Re / (A+Re) at the two grain boundaries is in the range of 0.1 to 0.5.

[0072] 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. [Explanation of symbols]

[0073] 10 Silicon nitride heat dissipation substrate 12 circuit layers 14 Conductor layer 20 Circuit Board 22 Solder 30 Power Semiconductors 40 Heat sink 42 Grease 50 Heat dissipation material 100 Power Devices

Claims

1. A silicon nitride sintered body mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles, Contains at least 1.0 to 6.5 wt% of rare earth elements in total and 0.3 to 3.0 wt% of alkali / alkaline earth metal elements in total, A silicon nitride sintered body characterized in that, at the grain boundary between two of the silicon nitride particles, the average element ratio Re / (A+Re) is in the range of 0.1 to 0.5, where Re is the amount of the rare earth element and A is the amount of the alkali / alkaline earth metal element.

2. The alkali / alkaline earth metal element includes one or more elements selected from Mg and Ca, 2. The silicon nitride sintered body according to claim 1, wherein said rare earth element includes at least one element selected from the group consisting of Y, La, and Yb.

3. Further containing a Group 4 element, 3. The silicon nitride sintered body according to claim 1, wherein said Group 4 element forms one or more compounds at least a portion of which contains nitrogen or carbon.

4. 3. A silicon nitride heat dissipating substrate comprising the silicon nitride sintered body according to claim 1.

5. 5. The silicon nitride heat dissipation substrate according to claim 4, having a bending strength of 700 MPa or more.

6. 5. The silicon nitride heat dissipation substrate according to claim 4, having a thermal conductivity of 85 W / mK or more.

7. 5. The silicon nitride heat dissipation substrate according to claim 4, wherein the thickness of said silicon nitride heat dissipation substrate in a direction perpendicular to one of its main surfaces is 220 [mu]m or more and 690 [mu]m or less.

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