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 simultaneously achieving high thermal conductivity and high intensity.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 grain includes a β type silicon nitride. The grain boundary phase includes an Re2Si3O3N4 type crystal phase (Re denotes a rare-earth element). A Lotgering factor f(hk0) of the β type silicon nitride is 0.01 to 0.35, and a Lotgering factor f(hk0) of the Re2Si3O3N4 type crystal phase is 0.01 to 0.35, within a range of 2θ=20 to 80° in an X-ray diffraction pattern by a θ-2θ method.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 method for utilizing superplasticity to form silicon nitride and sialon sintered bodies as single materials without forming them into composite materials, and a sintered body formed by this method. The method aims to provide such a ceramic, which is essentially composed of silicon nitride or sialon and contains only a sintering aid as an added component, and is characterized in that the degree of texture orientation on a two-dimensional cross section of the sintered body after forming, in a direction parallel to the tensile or compressive axis direction during forming, is in the range of 5 to 80%, as defined by the method defined by Saltykov, and the linear density of crystal grains per 50 μm length parallel to the tensile axis direction or perpendicular to the compressive axis direction on the two-dimensional cross section, is 80 to 200. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3624225 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, with the rise in temperature of power modules, silicon nitride substrates are required to have both improved heat dissipation through high thermal conductivity and high strength to withstand thermal stress. When silicon nitride crystals are oriented in a certain direction, as in the silicon nitride-based or sialon-based ceramics described in Patent Document 1, for example, strength can be improved by orienting the c-axis of β-type silicon nitride in a direction parallel to the substrate plane, but in the direction perpendicular to the substrate plane, the a-axis orientation occurs, which has low thermal conductivity relative to the c-axis, resulting in a decrease in thermal conductivity in the perpendicular direction.

[0006] For these reasons, when using silicon nitride sintered bodies as insulating heat dissipation substrates for power devices, new technology was needed to achieve both high thermal conductivity and high strength.

[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 combine high thermal conductivity and high strength. [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, wherein the silicon nitride particles contain β-type silicon nitride and the grain boundary phase contains a Re2Si3O3N4-type crystalline phase (Re is a rare earth element), and in an X-ray diffraction pattern measured by the θ-2θ method, the Lotgering factor f(hk0) of the β-type silicon nitride is 0.01 to 0.35 in the 2θ=20 to 80° range, and the Lotgering factor f(hk0) of the Re2Si3O3N4-type crystalline phase is 0.01 to 0.35.

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

[0010] (3) Furthermore, 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) In addition, in the silicon nitride sintered body according to any one of the application examples of (1) to (3) above, the alkali and alkaline earth metal elements are contained in a total amount of 0.3 to 3.0 wt%, and the rare earth elements are contained in a total amount of 1.0 to 6.5 wt%.

[0012] (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.

[0013] (6) In addition, in the silicon nitride heat dissipation substrate of the application example of (5) above, the bending strength is 700 MPa or more.

[0014] (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.

[0015] (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 the direction perpendicular to one of the main surfaces is 220 μm or more and 690 μm or less. [Effects of the Invention]

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

[0017] [Figure 1] 1 is a schematic diagram showing a cross section of a silicon nitride sintered body in a direction perpendicular to a main surface thereof. FIG. [Figure 2] 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 3] 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 4] 1 is a table showing the element contents, grain boundary phase characteristics, and properties of each sample. DETAILED DESCRIPTION OF THE INVENTION

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

[0019] [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. The phrase "mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles" means that impurities other than the elements described below may be contained in an amount of up to 0.5 wt%. The silicon nitride particles include β-type silicon nitride.

[0020] The grain boundary phase contains a Re2Si3O3N4-type crystalline phase (Re is a rare earth element). The crystalline phase of the grain boundary phase can be identified from the results of XRD (X-ray diffraction). The Re2Si3O3N4-type crystal may be, for example, Y2Si3O3N4. Y2Si3O3N4 is also known as the M phase or melilite phase and is a crystal identified by 00-045-0249 in the ICDD (International Centre for Diffraction Data) PDF (Powder Diffraction File) card. The Re2Si3O3N4-type crystal may contain different elements as long as the crystal system, space group, and arrangement of constituent atoms are the same. For example, the Re2Si3O3N4-type crystal may be Yb2Si3O3N4.

[0021] In the X-ray diffraction pattern of the silicon nitride sintered body by the θ-2θ method, the Lotgering factor f(hk0) of β-type silicon nitride in the 2θ range of 20 to 80° is 0.01 to 0.35. The Lotgering factor f(hk0) of the Re2Si3O3N4 crystalline phase is 0.01 to 0.35. In the present invention, the Lotgering factor is a value calculated by measuring the X-ray diffraction pattern of the main surface (the surface with the largest area) of the silicon nitride sintered body. The Lotgering factor can be calculated using software accompanying the X-ray diffractometer.

[0022] The Lotgering factor f(hk0) is a numerical value calculated by the following formula (1). f(hk0)=(ρ-ρ0) / (1-ρ0) … (1) however, ρ0=ΣI0(hk0) / ΣI0(hkl) … (2) ρ=ΣI(hk0) / ΣI(hkl) … (3) Let's say. ρ0 indicates a value calculated from the X-ray diffraction intensity of a non-oriented comparative sample. ρ0 may also be calculated using the ICDD PDF card. ρ indicates a value calculated from the X-ray diffraction intensity of the sample for which the Lotgering factor is to be calculated. Furthermore, I0(hk0), I0(hkl), I(hk0), and I(hkl) indicate the peak values in the X-ray diffraction pattern of the non-oriented comparative sample or silicon nitride sintered compact corresponding to the following crystal planes. In other words, the Lotgering factor is a value indicating the degree of orientation relative to a non-oriented sample.

[0023] Beta silicon nitride: ·(hkl)=(110), (200), (101), (120), (111), (300), (201), (220), (121), (130), (301), (400), (221), (131) , (230), (002), (140), (401), (102), (112), (321), (202), (500), (141), (330), (122), (240), (302), (501) ·(hk0)=(110), (200), (120), (300), (220), (130), (400), (230), (140), (500), (330), (240)

[0024] Re2Si3O3N4 type crystal: ·(hkl)=(101), (200), (111), (210), (201), (211), (220), (002), (310), (221), (1 02), (301), (112), (311), (202), (212), (321), (400), (410), (222), (330), (401) , (411), (312), (420), (331), (103), (322), (113), (402), (412), (332), (511), (422), (303), (521), (440), (530), (502), (600), (403), (601), (413), (611), (333) ·(hk0)=(200), (210), (220), (310), (400), (410), (330), (420), (440), (530), (600)

[0025] FIG. 1 is a schematic diagram showing a cross section perpendicular to the main surface 2 of a silicon nitride sintered body 1. By setting the Lotgering factor f(hk0) of β-silicon nitride within the above range, the c-axis of the β-silicon nitride 3 can be oriented parallel to the main surface 2 of the silicon nitride sintered body 1, as shown in FIG. 1, thereby improving strength. Furthermore, by setting the Lotgering factor f(hk0) of the Re2Si3O3N4 crystalline phase within the above range, the a-axis of the Re2Si3O3N4 crystalline phase 4 can be oriented perpendicular to the main surface 2 of the silicon nitride sintered body 1, as shown in FIG. 1, thereby improving thermal conductivity.

[0026] The a-axis orientation refers to the fact that the crystal orientation of specific crystals on a certain surface is statistically aligned along the a-axis relative to the random state, while the c-axis orientation refers to the fact that the crystal orientation of specific crystals on a certain surface is statistically aligned along the c-axis relative to the random state.

[0027] The rare earth element preferably includes one or more elements selected from yttrium (Y), lanthanum (La), and ytterbium (Yb). The silicon nitride sintered body preferably contains 1.0 to 6.5 wt% of rare earth elements in total. If the rare earth element content is less than this range, the sinterability may decrease, pores may remain, and the strength of the silicon nitride sintered body may decrease. If the rare earth element content is more than this range, the amount of grain boundary phase may increase, and the thermal conductivity of the silicon nitride sintered body may decrease. It is believed that similar effects can be obtained even if the rare earth element contained is a rare earth element other than those listed above.

[0028] The silicon nitride sintered body preferably contains alkali / alkaline earth metal elements. In this case, the alkali / alkaline earth metal elements preferably include one or more elements selected from Mg and Ca. The silicon nitride sintered body also preferably contains 0.3 to 3.0 wt% of alkali / alkaline earth metal elements in total. If the alkali / 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. The alkali / alkaline earth metal elements refer to alkali metal elements or alkaline earth metal elements.

[0029] 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.

[0030] 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.

[0031] 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).

[0032] 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.

[0033] These characteristics make it possible to produce a silicon nitride sintered body that combines high thermal conductivity with high strength.

[0034] [Configuration of silicon nitride heat dissipation substrate] FIG. 2 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 composed of the silicon nitride sintered body 1 described above, and the main surface 11 of the silicon nitride heat dissipation substrate 10 is a surface generally parallel to the main surface 2 of the silicon nitride sintered body 1. That is, in the silicon nitride heat dissipation substrate 10 of the present invention, the β-type silicon nitride has a c-axis orientation in a plane parallel to the main surface 11, and the Re2Si3O3N4 crystal has an a-axis orientation in a plane perpendicular to the main surface 11. This allows for both high thermal conductivity and high strength, reducing the risk of defects 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 may be formed, for example, in a flat plate shape.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] [Power device configuration] 3 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] [Methods for producing 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 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).

[0045] 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.

[0046] 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. A cold isostatic pressing (CIP) process is then performed at a pressure of, for example, 100 MPa to obtain a compact. The resulting compact (CIP-pressed compact) is then heat-treated (debound) by, for example, holding it in an air flow at 600°C for 2 hours to obtain a debound compact.

[0047] The resulting degreased body is then placed in, for example, a silicon carbide mold with the inside coated with BN, and sintered for 8 hours at a maximum temperature of 1900°C in a nitrogen atmosphere at 9 atmospheres while applying a load in a predetermined direction. A crystalline phase adjustment step is then added in which the temperature is maintained at a crystalline phase adjustment temperature lower than the maximum temperature for a predetermined period of time during both the temperature rise and fall. The crystalline phase adjustment temperature can be, for example, 1800°C. The holding time in the crystalline phase adjustment step can be 25% to 100% of the holding time at the maximum temperature. A silicon nitride sintered body is obtained through these steps.

[0048] 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.

[0049] By using this manufacturing method, it is possible to manufacture a silicon nitride sintered body or a silicon nitride heat dissipation substrate that has both high thermal conductivity and high strength.

[0050] [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 24 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.

[0051] Next, 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 special mold and then pre-molded using uniaxial pressing at a pressure of 30 MPa. Next, the pre-molded body was placed in a special bag under vacuum and then subjected to CIP molding at a pressure of 100 MPa.

[0052] The resulting green body was then degreased. This was done by holding it in an air flow at a maximum temperature of 600°C for two hours. The resulting degreased body was then sintered. The sintering method was atmospheric sintering under a nitrogen gas pressure of 9 atmospheres, with the maximum temperature being 1900°C and held for eight hours. In addition, a crystal phase adjustment step was added in which the temperature was held at 1800°C for four hours each during the temperature rise and fall periods. A silicon carbide mold with a BN coating on the inside was used. In this way, the silicon nitride sintered body of Sample 1 was produced.

[0053] (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 %.

[0054] (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 %.

[0055] (Sample 4) The silicon nitride sintered body of Sample 4 was prepared 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.

[0056] (Sample 5) The silicon nitride sintered body of Sample 5 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 μm).

[0057] (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 lanthanum oxide powder (average particle size 1.0 μm).

[0058] (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 holding times in the crystal phase adjustment steps during temperature increase and temperature decrease were changed to 2 hours each.

[0059] (Sample 8) The silicon nitride sintered body of Sample 8 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the holding times in the crystal phase adjustment steps during temperature increase and temperature decrease were changed to 8 hours each.

[0060] (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 carbon powder was placed in a BN-coated silicon carbide mold, and then the degreased body was placed in the mold and fired.

[0061] (Sample 10) The silicon nitride sintered body of Sample 10 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the balls used to produce the mixed slurry were changed from YSZ to silicon nitride.

[0062] (Sample 11) The silicon nitride sintered body of Sample 11 was produced under the same conditions as those for the silicon nitride sintered body of Sample 2, except that the crystal phase adjustment step during cooling was not carried out.

[0063] (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 2, except that the crystal phase adjustment step during heating was not carried out.

[0064] (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 2, except that no crystal phase adjustment steps were performed during either the temperature increase or decrease.

[0065] [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.

[0066] (density measurement) The density of the sintered body of each sample was measured by a method in accordance with JIS R1634.

[0067] (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.

[0068] (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 4 lists only the components identified as crystalline, and does not include information on whether or not there is amorphous matter.

[0069] (Calculation of Lotgaering Factor (LF)) The Lotgering factor was calculated using the above definition formula and the card data of the ICDD PDF as a non-oriented comparative sample.

[0070] (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 by 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).

[0071] (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.

[0072] (result) Figure 4 is a table showing the element content, grain boundary phase characteristics, and properties of each sample. The Group 4 elements in each sample except for sample 10 originate from the YSZ balls used in the manufacturing process. Samples 1 to 10 all had high thermal conductivity of 85 W / mK or more and bending strength of 700 MPa or more.

[0073] Sample 11 had a low thermal conductivity. This is presumably because, although the Lotgering factor of the β-type silicon nitride met the standard, the Lotgering factor of the Re2Si3O3N4-type crystal did not. The reason why the Lotgering factor of the Re2Si3O3N4-type crystal did not meet the standard is thought to be because a crystal phase adjustment process was not performed when the temperature was lowered during sintering.

[0074] Sample 12 had a low bending strength. This is presumably because, although the Lotgering factor of the Re2Si3O3N4 crystal met the standard, the Lotgering factor of the β-type silicon nitride did not. The reason why the Lotgering factor of the β-type silicon nitride did not meet the standard is thought to be because a crystalline phase adjustment process was not performed during the sintering temperature rise.

[0075] Sample 13 had low thermal conductivity and bending strength. This is presumably because the Lotgering factors of the β-type silicon nitride and the Re2Si3O3N4-type crystal did not meet the standards. This is thought to be because no crystal phase adjustment process was performed during either the temperature rise or temperature fall of the sintering process.

[0076] A comparison of Sample 2 and Sample 9 revealed that the Group 4 element compounds contained in the silicon nitride sintered body play an equivalent role to compounds containing a Group 4 element and nitrogen and compounds containing a Group 4 element, nitrogen, and carbon. It is also presumed that compounds containing a Group 4 element and carbon may also be used.

[0077] Sample 10 had slightly lower thermal conductivity and bending strength. This is presumably because the silicon nitride sintered body did not contain a Group 4 element compound. This confirmed that it is preferable for silicon nitride sintered bodies to contain a Group 4 element compound. On the other hand, Sample 10 had higher thermal conductivity and bending strength than Sample 13. In other words, even for silicon nitride sintered bodies that do not contain a Group 4 element compound, if the Lotgering factors of both the β-type silicon nitride and the Re2Si3O3N4-type crystal satisfy the criteria, both the thermal conductivity and bending strength will be higher than when they do not satisfy the criteria.

[0078] From the above results, it was confirmed that the silicon nitride sintered body and silicon nitride heat dissipation substrate of the present invention can achieve both high thermal conductivity and high strength.

[0079] 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]

[0080] 1. Silicon nitride sintered body 2 Main Surfaces 3 β-type silicon nitride 4. Re2Si3O3N4 type crystal 10 Silicon nitride heat dissipation substrate 11 Main surface 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, the silicon nitride particles contain β-type silicon nitride, The grain boundary phase is Re ２ Si ３ O ３ N ４ type crystalline phase (Re is a rare earth element), In an X-ray diffraction pattern by the θ-2θ method, the Lotgering factor f(hk0) of the β-type silicon nitride in the range of 2θ=20 to 80° is 0.01 to 0.35, and the Re ２ Si ３ O ３ N ４ The silicon nitride sintered body is characterized in that the Lotgering factor f(hk0) of the crystalline phase is 0.01 to 0.

35.

2. Further containing alkali and alkaline earth metal elements, 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. The alkali / alkaline earth metal elements are contained in a total amount of 0.3 to 3.0 wt %, 3. The silicon nitride sintered body according to claim 2, wherein the rare earth elements are contained in a total amount of 1.0 to 6.5 wt %.

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

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

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

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