Silicon nitride sintered body and silicon nitride heat dissipation substrate

Abstract

The objective is to provide a silicon nitride sintered body and a silicon nitride heat dissipation substrate that maintain high thermal conductivity while further increasing strength. The solution is a silicon nitride sintered body mainly composed of silicon nitride particles and grain boundary phases surrounding the silicon nitride particles, wherein the sintered body contains at least 1.0 to 6.5 wt% of rare earth elements and 0.3 to 3.0 wt% of alkali / alkaline earth metal elements. When the amount of rare earth elements is set as Re and the amount of alkali / alkaline earth metal elements is set as A in the two grain boundaries of the silicon nitride particles, the average element ratio Re / (A+Re) is in the range of 0.1 to 0.5.

Description

Technical Field

[0001] This invention relates to silicon nitride sintered bodies and silicon nitride heat dissipation substrates. Background Technology

[0002] Silicon nitride (SiN) possesses high thermal conductivity and strength, making it a promising insulating heat dissipation substrate for power modules in inverters of electric vehicles (EVs) and hybrid vehicles (HVs). Previously, aluminum nitride was primarily used as an insulating heat dissipation substrate material. However, in high-current power modules used in EVs and similar applications, temperatures reach around 250°C. Due to the difference in thermal expansion between the silicon nitride and bonded metals such as copper, significant thermal stress is generated on the substrate, causing the low-strength aluminum nitride to crack and break. Therefore, although its thermal conductivity is lower than aluminum nitride, the use of silicon nitride, which offers high thermal conductivity and higher strength, is being promoted in conventional insulating ceramics.

[0003] Patent Document 1 discloses a silicon nitride sintered body comprising a plurality of silicon nitride particles and a grain boundary triple point located between three or more of the plurality of silicon nitride particles. The grain boundary triple point contains Mg and at least one element RE selected from rare earth elements. The ratio of Mg to RE at the grain boundary triple point, Mg / RE, satisfies Mg / RE≤1. The O content in the silicon nitride particles is less than 0.05% by mass.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2022-153934 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] In recent years, the current flowing through inverters has tended to increase, placing very high thermal stress on the silicon nitride substrate and causing problems such as substrate cracking. In addition, the flexural strength of the silicon nitride sintered body described in Patent Document 1 is about 500 MPa. In such silicon nitride substrates, when used in applications with higher than previous temperatures, the possibility of insufficient strength to withstand thermal stress and cracking increases.

[0009] Therefore, when using silicon nitride sintered bodies as insulating heat dissipation substrates for power devices, further increases in strength are required to improve the reliability of the substrate while maintaining high thermal conductivity.

[0010] The present invention was made in view of this situation, and its object is to provide a silicon nitride sintered body and a silicon nitride heat dissipation substrate that further enhance strength while maintaining high thermal conductivity.

[0011] Solution for solving the problem

[0012] (1) In order to achieve the above objective, the silicon nitride sintered body of the present invention employs the following means. That is, the silicon nitride sintered body of the application example of the present invention is mainly composed of silicon nitride particles and grain boundary phase surrounding the silicon nitride particles, and contains at least a total of 1.0 to 6.5 wt% rare earth elements and a total of 0.3 to 3.0 wt% alkali / alkaline earth metal elements. When the amount of rare earth elements is set as Re and the amount of alkali / alkaline earth metal elements is set as A in the two grain boundaries of the silicon nitride particles, the average element ratio Re / (A+Re) is in the range of 0.1 to 0.5.

[0013] (2) In addition, in the silicon nitride sintered body of the application example of (1) above, the alkaline / 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.

[0014] (3) In addition, the silicon nitride sintered body in the application example of (1) or (2) above also contains a group 4 element, and at least a portion of the group 4 element forms one or more compounds containing nitrogen or carbon.

[0015] (4) In addition, the silicon nitride heat dissipation substrate of the application example of the present invention is composed of the silicon nitride sintered body described in any one of (1) to (3) above.

[0016] (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.

[0017] (6) In addition, in the silicon nitride heat dissipation substrate of the application example described in (4) or (5) above, the thermal conductivity is 85 W / mK or higher.

[0018] (7) In addition, in any of the application examples of (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.

[0019] The effects of the invention

[0020] The silicon nitride sintered body or silicon nitride heat dissipation substrate according to the present invention can further increase strength while maintaining high thermal conductivity. Attached Figure Description

[0021] Figure 1This is a schematic perspective view illustrating an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention.

[0022] Figure 2 This is a schematic cross-sectional view illustrating an example of a power device using a silicon nitride heat dissipation substrate according to an embodiment of the present invention.

[0023] Figure 3 It is a table that shows the elemental content, characteristics and properties of grain boundary phases of each sample.

[0024] Figure 4 (a) and (b) are SEM images of silicon nitride sintered bodies of sample 1 and sample 10, respectively. Detailed Implementation

[0025] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. For ease of understanding, the same reference numerals are used to denote the same constituent elements in each drawing, and repeated descriptions are omitted. It should be noted that the sizes of the constituent elements in the structural drawings are conceptual representations and do not necessarily represent actual size ratios.

[0026] [Implementation Method]

[0027] [Composition of silicon nitride sintered bodies]

[0028] First, the 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. 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 use the amount generated according to the range of Al content described later as an upper limit. "Mainly composed of silicon nitride particles and a grain boundary phase surrounding the silicon nitride particles" means that it may contain impurities other than the elements described later, totaling less than 0.5 wt%.

[0029] The silicon nitride sintered body contains a total of 1.0 to 6.5 wt% rare earth elements. If the rare earth element content is less than this range, the sinterability may decrease and residual porosity may occur, resulting in a reduction in the strength of the silicon nitride sintered body. If the rare earth element content is greater than this range, the amount of grain boundary phase increases, 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 type of rare earth element contained. Preferably, the rare earth element includes one or more elements selected from yttrium (Y), lanthanum (La), and ytterbium (Yb).

[0030] The silicon nitride sintered body contains a total of 0.3 to 3.0 wt% alkali / alkaline earth metal elements. If the alkali / alkaline earth metal element content is less than this range, the strength of the silicon nitride sintered body may sometimes be lower. If the alkali and alkaline earth metal elements are more than this range, the thermal conductivity of the silicon nitride sintered body may sometimes be lower. Alkali / alkaline earth metal elements refer to either alkali metal elements or alkaline earth metal elements. Preferably, the alkali / alkaline earth metal elements include one or more elements selected from Mg and Ca.

[0031] For silicon nitride sintered bodies, when the elemental content of rare earth elements is set as Re and the elemental content of alkali / alkaline earth metal elements is set as A at the two-grain boundaries of silicon nitride particles, the average elemental ratio Re / (A+Re) ranges from 0.1 to 0.5. The two-grain boundaries of silicon nitride particles refer to the grain boundaries between adjacent silicon nitride particles. Furthermore, the elemental content of rare earth elements (Re) and the elemental content of alkali / alkaline earth metal elements (A) represent the values ​​of the elemental content obtained at a specific point on the two-grain boundaries of silicon nitride particles through point analysis using STEM-EDS (Scanning Transmission Electron Microscope-Energy-Dispersive-Spectroscopy), as described later.

[0032] By keeping the Re / (A+Re) value within the above range, both the columnar growth of silicon nitride particles and the filling of grain boundary voids by the grain boundary phase due to reduced viscosity can be achieved. If the Re / (A+Re) value is greater than the above range, that is, when the proportion of Re in the two-particle grain boundary increases, the viscosity of the liquid phase grain boundary component increases, making it unable to fully fill the voids with the grain boundary phase, resulting in more voids and sometimes lower strength and lower thermal conductivity. If the Re / (A+Re) value is less than the above range, that is, when the proportion of Re in the two-particle grain boundary decreases, the columnar growth of silicon nitride particles cannot be promoted, and sometimes the strength and toughness decrease.

[0033] The elemental ratio Re / (A+Re) of the two grain boundaries of silicon nitride particles can be determined by STEM-EDS analysis. Specifically, for the machined surface obtained by further ion milling of the polished surface of the silicon nitride sintered body, five points were randomly selected and observed at a magnification of 50,000x within a 5μm × 5μm field of view. Then, one two grain boundary of silicon nitride particles was randomly selected from each of the five fields of view, and the elemental content was determined by point analysis. The elemental content of rare earth elements was then set as Re, and the elemental content of alkali / alkaline earth metals was set as A. The value of Re / (A+Re) was calculated for each point, and its average (arithmetic mean) was obtained.

[0034] Since the grain boundary phase is randomly generated in the sintered body, the Re / (A+Re) value calculated from the STEM-EDS analysis of the ion-milled machined surface of the silicon nitride sintered body is approximately the same in any part of the sintered body.

[0035] The grain boundary phase preferably comprises at least one of the following crystal phases: M phase, J phase, monosilicate phase, and disilicate phase, more preferably including the M phase. The crystal phase of the grain boundary phase can be identified by XRD (X-ray Diffraction) results. The M phase refers to the crystal identified by PDF (Powder Diffraction File) card 00-045-0249. The M phase is also known as the Melilite phase and is a Y2Si3O3N4 type crystal. The J phase refers to the crystal identified by PDF card 01-086-1106. The J phase is a Y4Si2O7N2 type crystal. The monosilicate phase refers to the crystal identified by PDF card 00-052-181. The monosilicate phase is also known as the m phase and is a Y2SiO5 type crystal. The disilicate phase refers to the crystal identified by PDF card 01-072-359. The disilicate phase, also known as the d phase, is a Y₂Si₂O₇ type crystal. These terms represent representative crystal compositions, but the elements can differ as long as the crystal system, space group, and atomic configuration are the same. For example, the M phase can be Yb₂Si₃O₃N₄.

[0036] The silicon nitride sintered body preferably contains a Group 4 element. Furthermore, the silicon nitride sintered body preferably contains a total of 0.30 to 3.0 wt% of a Group 4 element. Additionally, the silicon nitride sintered body preferably contains at least a portion of the Group 4 element forming one or more compounds comprising at least one of nitrogen or carbon.

[0037] If Group 4 elements fall within the aforementioned range, both high thermal conductivity and high strength are achieved. Since Group 4 elements are insoluble in silicon nitride, thermal conductivity is not reduced due to phonon scattering. Furthermore, they are also insoluble in the grain boundary phase formed by the reaction of rare earth elements with silicon nitride. Therefore, compounds can easily form independently, filling residual pores and thus improving strength. Additionally, the Group 4 element compound particles inhibit crack propagation, contributing to high toughness. It should be noted that Group 4 element compounds do not only refer to compounds containing only Group 4 elements and nitrogen or carbon, but can also be compounds containing Group 4 elements, nitrogen, and carbon, or compounds containing other Group 4 elements and anions (such as oxygen) dissolved within a range that does not disrupt their crystal structure. The types of Group 4 element compounds can be identified using XRD (X-ray Diffraction) results.

[0038] The Group 4 elements preferably contain one or more elements selected from zirconium (Zr) and hafnium (Hf). This allows for the specific formation of Group 4 element compounds. When the silicon nitride sintered body contains Zr, zirconium nitride (ZrN), zirconium carbide (ZrC), and zirconium carbonitride (ZrCN) readily form as Group 4 element compounds. When the silicon nitride sintered body contains Hf, hafnium nitride (HfN), hafnium carbide (HfC), and hafnium carbonitride (HfCN) readily form as Group 4 element compounds.

[0039] Silicon nitride sintered bodies can contain more than 0 wt% and less than 0.1 wt% Al. Al is an impurity derived from the raw materials. The presence of Al improves the sinterability of silicon nitride sintered bodies and increases their strength. Furthermore, if the content is within this range, the decrease in thermal conductivity due to silane formation is negligible. If the content exceeds 0.1 wt%, the amount of silane in the silicon nitride sintered body increases, and sometimes the thermal conductivity of the silicon nitride sintered body decreases.

[0040] These characteristics enable the fabrication of silicon nitride sintered bodies that maintain high thermal conductivity while achieving even higher strength.

[0041] [Composition of silicon nitride heat dissipation substrate]

[0042] Figure 1 This is a schematic perspective view illustrating 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 constructed from the aforementioned silicon nitride sintered body. This allows for further high strength while maintaining high thermal conductivity, thereby reducing the possibility of defects occurring in circuit boards using it. The silicon nitride heat dissipation substrate 10 of the present invention is suitable for use as a heat dissipation substrate for circuit boards used in power devices. The silicon nitride heat dissipation substrate 10 is, for example, formed in a flat plate shape.

[0043] The bending strength of the silicon nitride heat dissipation substrate 10 is preferably 700 MPa or higher. This reduces the possibility of breakage of the silicon nitride heat dissipation substrate 10.

[0044] Bending strength can be determined as follows. According to ISO 23242, a silicon nitride heat sink substrate is processed to a certain thickness × 12 × 25 mm. Then, the bending strength can be determined by bending at three points with a span of 15 mm. ISO 23242 is applicable to ceramic sheets with a thickness from 0.2 mm to 1.0 mm.

[0045] The thermal conductivity of the silicon nitride heat dissipation substrate 10 is preferably 85 W / mK or higher. This allows it to fully utilize its performance as a heat dissipation substrate.

[0046] Thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed into a size of 0.32 mm × □10 mm, and the thermal diffusivity is measured using the xenon flash method. Additionally, the density of the silicon nitride heat dissipation substrate 10 is measured according to the method in JIS R1634. The specific heat value is kept constant at 0.68 cm⁻¹. 2 / sec. Then, based on the measured values ​​of thermal diffusivity and density, thermal conductivity can be calculated using (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0047] 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 less than this range, the strength of the substrate may decrease. Conversely, if the thickness is greater than this range, the heat dissipation performance may decrease.

[0048] [Composition of power devices]

[0049] Figure 2 This is a schematic cross-sectional view illustrating 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.

[0050] The circuit board 20 has a circuit layer 12 formed on one main surface of the silicon nitride heat dissipation substrate 10, and a conductor layer 14 formed on the other main surface opposite to the first main surface. The circuit layer 12 and the conductor layer 14 are preferably made of metal, and more preferably of a metal with copper as the main component. The circuit layer 12 and the conductor layer 14 are directly bonded to the silicon nitride heat dissipation substrate 10 or bonded using bonding members such as soldering materials.

[0051] 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 can be bonded using solder 22 or the like. The power semiconductor 30 can be, for example, a semiconductor that is prone to high temperature due to the high current required for EVs. This is because the silicon nitride heat sink 10 of the present invention maintains high thermal conductivity while being high-strength, so even if the difference in thermal expansion between the silicon nitride heat sink 10 and the metal bonded to it causes large thermal stress in the silicon nitride heat sink 10 due to high temperature, cracks and fractures are not easily generated.

[0052] 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 can also be bonded using solder 22 or the like. The surface of the heat sink 40 opposite to the surface bonded to the conductor layer 14 is in contact with the heat dissipation member 50 via grease 42. The heat sink 40 is preferably made of metal, more preferably of a metal primarily composed of copper. The heat dissipation member 50 has heat sink fins. The heat dissipation member 50 is preferably made of metal, more preferably of a metal primarily composed of copper or aluminum.

[0053] [Manufacturing Method of Silicon Nitride Sintered Body and Silicon Nitride Heat Dissipation Substrate]

[0054] The following is an example of the manufacturing method of the aforementioned silicon nitride sintered body and silicon nitride heat dissipation substrate. First, necessary raw material powders for the silicon nitride sintered body are selected and weighed in a manner that achieves the desired composition. The raw material powders for the silicon nitride sintered body can be oxides, carbonates, hydroxides, nitrides, etc., of the elements contained in the silicon nitride sintered body. Besides silicon nitride (Si3N4), examples of raw material powders for the silicon nitride sintered body include magnesium carbonate (MgCO3), calcium carbonate (CaCO3), yttrium oxide (Y2O3), ytterbium oxide (Yb2O3), lanthanum oxide (La2O3), and zirconium nitride (ZrN).

[0055] Ethanol is added to these raw material powders, and the mixture is wet-milled using a ball mill for 6-60 hours to obtain a slurry. The slurry is then dried using a water-cooling method, a spray dryer, or similar technique to obtain a mixed powder. A binder (acrylic-based, etc.) can be added to the obtained mixed slurry, and after mixing for about 30 minutes, it is dried.

[0056] Next, the mixed powder is filled into a mold and uniaxially pressed, for example, at a pressure of 100 MPa, to form the desired shape, thus obtaining a molded body. The molded body can be formed by CIP (cold isostatic pressing). Then, the obtained molded body is heat-treated (degreased) by holding it at a temperature of 600°C for 5 hours, for example, in an atmospheric airflow, to obtain a degreased body.

[0057] The resulting degreased body is then placed in a carbon mold, for example, internally coated with BN, and sintered, for example, at a nitrogen atmosphere of 9 atmospheres and a maximum temperature of 1900°C for 8 hours. By sintering the degreased body in a carbon mold, the reducing atmosphere is strengthened, and the alkali / alkaline earth metal elements added as oxides readily volatilize. If the alkali / alkaline earth metal elements volatilize, the proportion of rare earth elements in the grain boundary phase increases relatively, promoting the columnar growth of silicon nitride particles. Furthermore, if the alkali / alkaline earth metal elements volatilize to a certain extent, the viscosity of the liquid grain boundary phase decreases, filling the voids, and the volatilization of the alkali / alkaline earth metal elements becomes difficult. As a result, the strength of the silicon nitride sintered body is considered to be higher. A silicon nitride sintered body is obtained through this process.

[0058] When using a silicon nitride sintered body as a silicon nitride heat sink substrate, the shape is processed to achieve a specified shape and thickness. Processing methods include, for example, grinding, lapping, and sandblasting.

[0059] This manufacturing method enables the production of silicon nitride sintered bodies or silicon nitride heat dissipation substrates that can achieve higher strength while maintaining high thermal conductivity.

[0060] [Example, Comparative Example]

[0061] (Sample 1)

[0062] Weigh out 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). Then, ball mill the weighed raw material powders to obtain a mixed slurry. Ball milling was performed by adding the raw material powders and ethanol to a resin-made container, using YSZ (Y₂O₃ partially stabilized zirconia) balls, and mixing at 60 rpm for 20 hours. A binder (acrylic-based) was added to the resulting mixed slurry, and after further mixing for 30 minutes, the slurry was dried by spray drying to obtain a mixed powder.

[0063] Next, the obtained mixed powder was powder pressed into a molded body using uniaxial pressing. After filling the mixed powder into a dedicated mold, it was shaped by uniaxial pressing at a pressure of 100 MPa. Then, the resulting molded body was degreased. Degreasing was performed in an atmospheric flow at a maximum temperature of 600°C for 5 hours. Next, the degreased body was sintered. The sintering method was to sinter in an atmosphere using nitrogen at a gas pressure of 9 atmospheres, held at a maximum temperature of 1900°C for 8 hours. A carbon mold with BN coated on the inside was used. Thus, a silicon nitride sintered body of Sample 1 was produced.

[0064] (Sample 2)

[0065] The silicon nitride sintered body of sample 2 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 set to 3.0 wt%.

[0066] (Sample 3)

[0067] The silicon nitride sintered body of sample 3 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 set to 8.0 wt%.

[0068] (Sample 4)

[0069] 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 addition amount of yttrium oxide powder was set to 3.0 wt%, the addition amount of magnesium carbonate powder was set to 1.0 wt%, and the addition amount of calcium carbonate powder (average particle size 2.5 μm) was further 0.5 wt%.

[0070] (Sample 5)

[0071] The silicon nitride sintered body of sample 5 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 set to 3.0 wt% and the amount of magnesium carbonate powder added was set to 5.0 wt%.

[0072] (Sample 6)

[0073] The silicon nitride sintered body of sample 6 was prepared under the same conditions as the silicon nitride sintered body of sample 2, except that the yttrium oxide powder was replaced with ytterbium oxide powder (average particle size 1.2 μm).

[0074] (Sample 7)

[0075] The silicon nitride sintered body of sample 7 was prepared under the same conditions as the silicon nitride sintered body of sample 2, except that the yttrium oxide powder was replaced with lanthanum oxide powder (average particle size 1.0 μm).

[0076] (Sample 8)

[0077] The silicon nitride sintered body of sample 8 was sintered by adding carbon powder into a carbon mold coated with BN and then adding degreased material. Otherwise, it was made under the same conditions as the silicon nitride sintered body of sample 2.

[0078] (Sample 9)

[0079] The silicon nitride sintered body of sample 9 was prepared under the same conditions as the silicon nitride sintered body of sample 2, except that the spheroids used in the preparation of the mixed slurry were changed from YSZ to silicon nitride.

[0080] (Sample 10)

[0081] The silicon nitride sintered body of sample 10 was made using a SiC mold in which BN was coated inside the mold used during firing, except that it was made under the same conditions as the silicon nitride sintered body of sample 2.

[0082] (Sample 11)

[0083] The silicon nitride sintered body of sample 11 was made using a mold made of hBN (hexagonal boron nitride) with BN coated inside the mold during firing, except that it was made under the same conditions as the silicon nitride sintered body of sample 2.

[0084] [Various measurements]

[0085] The sintered bodies of each sample were evaluated by grinding to remove more than 0.25 mm from the sintered surface, and then by the following measurements.

[0086] (Density measurement)

[0087] The density of the sintered body of each sample was determined using the method according to JIS R1634.

[0088] (Determination of elemental amounts)

[0089] The obtained polished surfaces were subjected to X-ray fluorescence (XRF) analysis to determine the types and amounts of constituent elements of the sintered body of each sample.

[0090] (Identification of constituent phases)

[0091] XRD analysis was used to identify the crystal phase of the sintered body of the grain boundary phase in each sample. However, Figure 3 The table only records the components identified as crystalline, and does not record the presence or absence of amorphous components.

[0092] (2. Determination of the elemental ratio at grain boundaries)

[0093] Five randomly selected locations on the surface that underwent further ion milling were subjected to STEM-EDS analysis. The values ​​of Re / (A+Re) and their average values ​​were determined from the results.

[0094] (Calculation of thermal conductivity)

[0095] The sintered bodies of each sample were machined to a size of 0.32 mm × □10 mm, and the thermal diffusivity was determined using the xenon flash method. The specific heat value was kept constant at 0.68 cm⁻¹. 2 / sec. Then, using the thermal diffusivity determined by the xenon flash method and the density value mentioned above, the thermal conductivity is calculated by (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0096] (Determination of bending strength)

[0097] According to ISO 23242, the sintered body of each sample was processed to a thickness of 0.32×12×25mm, and the bending strength was determined by bending at 3 points with a span of 15mm.

[0098] (result)

[0099] Figure 3 This table shows the elemental content, characteristics, and properties of the grain boundary phases for each sample. It should be noted that, except for sample 9, the Group 4 elements in all samples are derived from YSZ-based spheroids used in the manufacturing process. Samples 1-9, with an average Re / (A+Re) ratio at the grain boundaries ranging from 0.1 to 0.5, all exhibit thermal conductivity above 85 W / mK and flexural strength above 700 MPa, demonstrating high values ​​for both thermal conductivity and flexural strength.

[0100] The flexural strength of samples 10 and 11 is low. This is presumably because the average Re / (A+Re) ratio at the grain boundaries of the two grains is less than 0.1. It is believed that because samples 10 and 11 were fired using molds made of SiC and hBN respectively, the reducing atmosphere during firing was weaker, making it difficult for alkali / alkaline earth metal elements to volatilize, thus resulting in a lower average Re / (A+Re) ratio. If the volatilization of alkali / alkaline earth metal elements is reduced, it will not promote the columnar growth of silicon nitride particles but rather their isotropic growth, thus leading to a decrease in strength.

[0101] Figure 4 (a) and (b) are SEM images of silicon nitride sintered bodies of sample 1 and sample 10, respectively. Figure 4 (a) and (b) are SEM images at 2000x magnification of the surfaces of the polished surfaces of each sample after plasma etching. Figure 4 In (a), the silicon nitride particles grow into columnar shapes in all directions. In contrast, Figure 4 As can be seen from (b), the silicon nitride particles did not grow into columnar shapes.

[0102] A comparison of samples 2 and 8 shows that, among the Group 4 element compounds contained in the silicon nitride sintered body, compounds containing Group 4 elements and nitrogen play an equal role as compounds containing Group 4 elements, nitrogen, and carbon. Furthermore, it is presumed that the Group 4 element compounds contained in the silicon nitride sintered body can be compounds containing Group 4 elements and carbon.

[0103] The thermal conductivity and flexural strength of sample 9 are both slightly lower. This is presumably because the silicon nitride sintered body does not contain Group 4 element compounds. Therefore, it is confirmed that the silicon nitride sintered body preferably contains Group 4 element compounds. On the other hand, the flexural strength of sample 9 is higher than that of samples 10 and 11. That is, it can be seen that even in silicon nitride sintered bodies that do not contain Group 4 element compounds, when the average elemental ratio Re / (A+Re) at the two grain boundaries is in the range of 0.1 to 0.5, the flexural strength is higher than that in cases where it is lower.

[0104] The results above confirm that the silicon nitride sintered body and silicon nitride heat dissipation substrate of the present invention, in the silicon nitride sintered body with the contents of rare earth elements and alkali / alkaline earth metal elements within a specified range, can further increase strength while maintaining high thermal conductivity by controlling the sintering conditions so that the average Re / (A+Re) of the two grain boundaries falls within the range of 0.1 to 0.5.

[0105] This invention is not limited to the embodiments described above, and naturally includes various modifications and equivalents encompassed by the spirit and scope of this invention. Furthermore, the structure, shape, quantity, position, and size of the constituent elements shown in the accompanying drawings are for ease of explanation and may be appropriately modified.

[0106] Explanation of reference numerals in the attached figures

[0107] 10 Silicon nitride heat dissipation substrate

[0108] 12 Circuit Layers

[0109] 14 Conductor Layer

[0110] 20 Circuit board

[0111] 22 Solder

[0112] 30 Power Semiconductors

[0113] 40 Heatsink

[0114] 42 Lubricating Grease

[0115] 50 Heat dissipation components

[0116] 100 power devices.

Claims

1. A silicon nitride sintered body, characterized in that, It is mainly composed of silicon nitride particles and grain boundary phases surrounding the silicon nitride particles. It contains at least 1.0 to 6.5 wt% rare earth elements and 0.3 to 3.0 wt% alkali / alkaline earth metal elements. In the two grain boundaries of the silicon nitride particles, when the elemental amount of the rare earth element is set as Re and the elemental amount of the alkali / alkaline earth metal element is set as A, the average elemental ratio Re / (A+Re) is in the range of 0.1 to 0.

5.

2. The silicon nitride sintered body according to claim 1, characterized in that, The alkaline / alkaline earth metal element includes one or more elements selected from Mg and Ca. The rare earth elements include one or more elements selected from Y, La and Yb.

3. The silicon nitride sintered body according to claim 1 or 2, characterized in that, It also contains Group 4 elements. At least a portion of the Group 4 elements form one or more compounds containing nitrogen or carbon.

4. A silicon nitride heat dissipation substrate, characterized in that, It is composed of the silicon nitride sintered body as described in claim 1 or 2.

5. The silicon nitride heat dissipation substrate according to claim 4, characterized in that, Its bending strength is above 700 MPa.

6. The silicon nitride heat dissipation substrate according to claim 4, characterized in that, Its thermal conductivity is above 85 W / mK.

7. The silicon nitride heat dissipation substrate according to claim 4, characterized in that, The thickness of the silicon nitride heat dissipation substrate in the direction perpendicular to one of the main surfaces is more than 220 μm and less than 690 μm.

Images