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

Abstract

To provide a silicon nitride-based sintered compact and a silicon nitride-based heat dissipation substrate that achieve higher strength while maintaining high thermal conductivity.SOLUTION: A silicon nitride-based sintered compact is mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, the silicon nitride-based sintered compact comprising at least 1.0 to 7.5 wt.% in total of rare earth elements, 0.25 to 2.0 wt.% in total of alkali metal elements and alkaline earth metal elements, 0.010 to 0.30 wt.% in total of one or more elements selected from Fe, Ni, Co, and Al, and 0.30 to 3.0 wt.% in total of group 4 elements, wherein the group 4 elements form one or more compounds containing at least one of nitrogen and carbon, and the total content of the compounds is 0.30 to 3.0 wt.%.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Since silicon nitride has high thermal conductivity and strength, it has attracted attention as an insulating heat dissipation substrate for power modules for inverters mounted on EV (Electric Vehicle) and HV (Hybrid Vehicle). Conventionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material. However, in the case of power modules for large currents such as EVs, the temperature rises to about 250°C, and a large thermal stress is generated in the substrate due to the difference in thermal expansion from metals such as copper joined thereto. Low-strength aluminum nitride has developed cracks and fractures. Therefore, although the thermal conductivity is inferior to that of aluminum nitride, among general insulating ceramics, silicon nitride, which has high thermal conductivity and higher strength, is being increasingly adopted.

[0003] Patent Document 1 discloses a silicon nitride sintered body that is substantially composed of silicon nitride, in which both aluminum and oxygen contained as impurities are 3.5% by weight or less, the density is 3.15 g / cm^3 or more, and the thermal conductivity is 40 W / mK or more.

[0004] Patent Document 2 discloses a silicon nitride sintered body characterized by having silicon nitride particles with a total content of oxygen, Al, Ca, and Fe of 1500 ppm or less and a minor axis diameter of 2 μm or more, and a method for producing a silicon nitride sintered body in which a raw material powder obtained by adding one or more oxides of yttrium and / or lanthanoid group elements to silicon nitride powder is molded and then sintered. The method uses silicon nitride powder containing 300 ppm or less of Al, 1% by weight or less of oxygen, and having an α conversion rate of 70% or less.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Laid-Open No. 4-175268 [Patent Document 2] Japanese Patent Laid-Open No. 2001-19557 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] The silicon nitride sintered body described in Patent Document 1 is a silicon nitride sintered body having a thermal conductivity of 50 W / mK, which has high thermal conductivity among conventional insulating ceramics, but the value of strength, which is important as a heat dissipation substrate, was unknown. In addition, it was also a problem that it was manufactured in an industrially very load-intensive process such as a firing temperature of 2000 °C, a holding time of 20 hours, and a pressing force of 200 kg / cm 2 and the fact that it was produced in such a process with a very high industrial load.

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

[0008] For this reason, when using a silicon nitride sintered body as an insulating heat dissipation substrate for a power device, in order to enhance the reliability as a substrate while maintaining a high thermal conductivity, further strengthening has been required.

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

[0010] (1) To achieve the above object, the silicon nitride-based sintered body of the present invention has taken the following measures. That is, the silicon nitride-based sintered body of the application example of the present invention is a silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, containing at least rare earth elements in a total amount of 1.0 to 7.5 wt%, alkali metal elements and alkaline earth metal elements in a total amount of 0.25 to 2.0 wt%, one or more elements selected from Fe, Ni, Co, and Al in a total amount of 0.010 to 0.30 wt%, and group 4 elements in a total amount of 0.30 to 3.0 wt%. The group 4 elements form one or more compounds containing at least one of nitrogen and carbon, and the total content of the compounds is 0.30 to 3.0 wt%.

[0011] (2) Further, in the silicon nitride-based sintered body of the application example of (1) above, the average particle diameter of the particles of the compound is 8.0 μm or less.

[0012] (3) Further, in the silicon nitride-based sintered body of the application example of (1) or (2) above, the connection of the particles of the compound is less than 3.

[0013] (4) Further, in the silicon nitride-based sintered body of any of the application examples of (1) to (3) above, the group 4 elements contain one or more elements selected from Zr and Hf.

[0014] (5) Further, the silicon nitride-based heat dissipation substrate of the application example of the present invention is composed of the silicon nitride-based sintered body described in any of (1) to (4) above.

[0015] (6) Further, in the silicon nitride-based heat dissipation substrate of the application example of (5) above, the flexural strength is 650 MPa or more.

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

[0017] (8) Further, in the silicon nitride-based heat dissipation substrate of any one of the application examples (5) to (7) above, the thickness in the direction perpendicular to one main surface of the silicon nitride-based heat dissipation substrate is 220 μm or more and 690 μm or less.

Effect of the Invention

[0018] According to the silicon nitride sintered body or the silicon nitride-based heat dissipation substrate of the present invention, it is possible to obtain a silicon nitride sintered body or a silicon nitride-based heat dissipation substrate with higher strength while maintaining high thermal conductivity.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0020] Next, embodiments of the present invention will be described with reference to the drawings. For ease of understanding of the description, the same reference numerals are assigned to the same components in each drawing, and duplicate descriptions are omitted. In the configuration diagrams, the sizes of the respective components are conceptually represented and do not necessarily represent actual dimensional ratios.

[0021] [Embodiment] [Configuration of Silicon Nitride Sintered Body] First, a silicon nitride-based sintered body according to an embodiment of the present invention will be described. The silicon nitride-based sintered body according to the embodiment of the present invention is a silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles. The silicon nitride-based particles refer to silicon nitride particles or sialons. However, since the thermal conductivity of sialon is lower than that of silicon nitride, it is preferable to set an upper limit on the amount generated depending on the content range of Al described later. That the silicon nitride-based sintered body is mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles means that impurities other than the elements described later may be contained in an amount of 0.5 wt% or less.

[0022] The silicon nitride-based sintered body contains a total of 1.0 to 7.5 wt% of rare earth elements. If the amount of rare earth elements is less than this range, the sinterability may decrease and pores may remain, resulting in a decrease in the strength of the silicon nitride-based sintered body. If the amount of rare earth elements is more than this range, the amount of the grain boundary phase may increase, resulting in a decrease in the thermal conductivity of the silicon nitride-based sintered body. It is considered that the same effect can be obtained regardless of which rare earth element is contained. The rare earth elements can be, for example, yttrium (Y), lanthanum (La), erbium (Er), and ytterbium (Yb).

[0023] The silicon nitride-based sintered body contains a total of 0.25 to 2.0 wt% of alkali metal elements and alkaline earth metal elements. If the amount of alkali metal elements and alkaline earth metal elements is less than this range, the strength of the silicon nitride-based sintered body may decrease. If the amount of alkali metal elements and alkaline earth metal elements is more than this range, the thermal conductivity of the silicon nitride-based sintered body may decrease.

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

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

[0026] When the Group 4 elements are within the above range and the Group 4 element compounds are also within the above range, high thermal conductivity and high strength can be achieved simultaneously. Since the Group 4 elements do not dissolve in silicon nitride, they do not cause a decrease in thermal conductivity due to phonon scattering. Also, they do not dissolve in the grain boundary phase formed by the reaction of rare earth elements and silicon nitride. Therefore, they are likely to form compounds alone, and since these compounds fill the residual pores, the strength is improved. In addition, since the Group 4 element compound particles suppress crack propagation, they also contribute to high toughness. The Group 4 element compound may be not only a compound containing only a Group 4 element and nitrogen or carbon, but also a compound containing a Group 4 element, nitrogen, and carbon, and further a compound in which other Group 4 elements or anions (such as oxygen) are dissolved within the range that does not break their crystal structure.

[0027] Due to these characteristics, it is possible to obtain a silicon nitride sintered body with higher strength while maintaining high thermal conductivity.

[0028] The average particle size of the Group 4 element compound particles is preferably 8.0 μm or less. When the average particle size of the Group 4 element compound particles is within the above particle size range, a high-strength silicon nitride sintered body can be obtained. When the particle size is larger than the range, it becomes a fracture initiation point and at the same time the compound localizes, so the residual pores increase and the strength may decrease.

[0029] The average particle size of the Group 4 element compound particles can be determined by SEM (Scanning Electron Microscope) observation. Specifically, for the polished surface of the silicon nitride-based sintered body, 5 locations are randomly selected, and a field of view of 120 μm × 90 μm is observed at a magnification of 2000 times. Then, the equivalent circle diameter is obtained from the area of each particle recognized as a Group 4 element compound, and the average particle size can be obtained by averaging these. The calculation of the average particle size may use image analysis software such as Winroof. In addition, the Group 4 element compound can be identified from the results of XRF (X-ray Fluorescence) and XRD (X-ray Diffraction).

[0030] The connection between the Group 4 element compound particles is preferably less than 3. If the number of connections is less than 3, it is difficult to become a fracture initiation point, and a decrease in strength can be prevented. Also, when the Group 4 element compound is conductive, the connection between the particles is reduced, so the insulation property is easily maintained. The connection between the Group 4 element compound particles is the maximum value of the number of connected particles among all the above 5 fields of view.

[0031] The Group 4 element preferably contains one or more elements selected from Zr and Hf. Thereby, the Group 4 element compound can be specifically configured. When the silicon nitride-based sintered body contains Zr, zirconium nitride (ZrN), zirconium carbide (ZrC), and zirconium carbonitride (ZrCN) are likely to be generated as the Group 4 element compound. When the silicon nitride-based sintered body contains Hf, hafnium nitride (HfN), hafnium carbide (HfC), and hafnium carbonitride (HfCN) are likely to be generated as the Group 4 element compound.

[0032] [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 above-mentioned silicon nitride sintered body. Thereby, while maintaining high thermal conductivity, higher strength can be achieved, and the possibility of problems occurring in the circuit board using this can be reduced. The silicon nitride heat dissipation substrate 10 of the present invention can be suitably used as a heat dissipation substrate for a circuit board for power devices. The silicon nitride heat dissipation substrate 10 is formed in a flat plate shape, for example.

[0033] The silicon nitride heat dissipation substrate 10 preferably has a flexural strength of 650 MPa or more. Thereby, the possibility of the silicon nitride heat dissipation substrate 10 being damaged can be reduced.

[0034] The flexural strength can be measured as follows. In accordance with ISO23242, the silicon nitride heat dissipation substrate is processed into a certain thickness × 12 × 25 mm. Then, the flexural strength can be measured by three-point bending with a span of 15 mm. ISO23242 can be applied to ceramic thin plates with a thickness of 0.2 mm to 1.0 mm.

[0035] The silicon nitride heat dissipation substrate 10 preferably has a thermal conductivity of 85 W / mK or more. Thereby, the performance as a heat dissipation substrate can be sufficiently exhibited.

[0036] The thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed into 0.32 mm × □17 mm, and the thermal diffusivity is measured by the two-dimensional method using a laser flash. Also, the density of the silicon nitride heat dissipation substrate 10 is measured by a method compliant with JIS R1634. The specific heat value is constant at 0.68 cm 2 / sec. Then, from the measured values of the thermal diffusivity and density, the thermal conductivity can be calculated by (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0037] The thickness in the direction perpendicular to one main surface of the silicon nitride heat dissipation substrate 10 is preferably 220 μm or more and 690 μm or less. Thereby, the balance between the strength and heat dissipation of the silicon nitride heat dissipation substrate 10 can be improved. If the thickness is smaller than this range, the strength of the substrate may be low. Also, if the thickness is larger than this range, the heat dissipation performance may decrease.

[0038] [Configuration of Power Device] FIG. 2 is a schematic cross-sectional view showing an example of a power device using the silicon nitride heat dissipation substrate according to an embodiment of the present invention. The power device 100 includes a circuit board 20, a power semiconductor 30, a heat sink 40, and a heat dissipation member 50.

[0039] On one main surface of the circuit board 20, a circuit layer 12 is formed, and on the other main surface facing the one main surface, a conductor layer 14 is formed. The circuit layer 12 and the conductor layer 14 are preferably made of metal, and more preferably made of a metal having copper as a main component. The circuit layer 12 and the conductor layer 14 are joined to the silicon nitride heat dissipation substrate 10 directly or using a joining member such as a brazing material.

[0040] A power semiconductor 30 is mounted on the upper side of the circuit layer 12 of the circuit board 20. The power semiconductor 30 and the circuit layer 12 may be joined using solder 22 or the like. The power semiconductor 30 may be, for example, a semiconductor through which a large current for an EV flows and which easily gets hot. Since the silicon nitride heat dissipation substrate 10 of the present invention has high strength while maintaining a high thermal conductivity, even when a large thermal stress is generated in the silicon nitride heat dissipation substrate 10 due to the difference in thermal expansion between the metal joined to the silicon nitride heat dissipation substrate 10 due to temperature rise, cracks and fractures are less likely to occur.

[0041] A heat sink 40 is joined to the lower side of the conductor layer 14 of the circuit board 20. The heat sink 40 and the conductor layer 14 may be joined using solder 22 or the like. The surface of the heat sink 40 facing the surface joined to the conductor layer 14 is in contact with the heat radiating member 50 via the grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal having copper as a main component. The heat radiating member 50 has heat radiating fins formed thereon. The heat radiating member 50 is preferably made of metal, and more preferably made of a metal having copper or aluminum as a main component.

[0042] [Method for manufacturing silicon nitride sintered body and silicon nitride heat radiating substrate] An example of the above-described method for manufacturing a silicon nitride sintered body and a silicon nitride heat radiating substrate is shown below. First, necessary materials are selected from the raw material powders of the silicon nitride sintered body and weighed so as to have a target composition. The raw material powders of the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of the respective elements contained in the silicon nitride sintered body. Examples of the raw material powders of the silicon nitride sintered body include, in addition to silicon nitride, magnesium carbonate, calcium carbonate, yttrium oxide, ytterbium oxide, erbium oxide, lanthanum oxide, zirconium nitride, and the like.

[0043] Ethanol is added to these raw material powders and wet-mixed and pulverized in a ball mill for, for example, 6 to 60 hours to obtain a slurry. The slurry is dried by hot water evaporation, a spray dryer, or the like to obtain a mixed powder.

[0044] Next, the mixed powder is filled into a mold and uniaxially pressed, for example, at a pressure of 30 MPa to form a desired shape. Thereafter, a CIP treatment (cold isostatic pressing treatment) is performed, for example, at a pressure of 150 MPa to obtain a formed body. The obtained formed body (CIP pressed body) is placed, for example, in a silicon carbide mold with its interior coated with BN and fired by holding it at a maximum temperature of 1800°C to 1900°C for 5 to 20 hours in a nitrogen atmosphere of 9 atmospheres to obtain a silicon nitride sintered body.

[0045] When using a silicon nitride sintered body as a silicon nitride heat dissipation substrate, the outer shape is processed to have a predetermined shape and thickness. The processing can be performed by, for example, grinding, polishing, blasting, or the like.

[0046] By such a manufacturing method, it is possible to manufacture a silicon nitride sintered body or a silicon nitride heat dissipation substrate with higher strength while maintaining high thermal conductivity.

[0047] [Examples, Comparative Examples] (Sample 1) For 100 wt% of silicon nitride powder (average particle size 1.4 μm), 3.2 wt% of magnesium carbonate powder (average particle size 2.5 μm) and 3.0 wt% of yttrium oxide powder (average particle size 1.0 μm) were weighed by external division. Next, the weighed raw material powders were used for ball milling to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were put into a resin pot, and ball stones made of YSZ (Y 2 O 3 (partially stabilized zirconia) were used, and pulverized and mixed at 60 rpm for 24 hours. The obtained mixed slurry was dried by simmering to obtain a mixed powder.

[0048] The obtained mixed powder was subjected to powder press molding by uniaxial pressing and CIP to produce a molded body. First, after filling the mixed powder into a dedicated mold, pre-molding was performed by uniaxial pressing at a pressure of 30 MPa. Next, the pre-molded body was evacuated and put into a dedicated bag, and CIP molding was performed at a pressure of 150 MPa.

[0049] The obtained molded body was fired. The sintering method was atmospheric firing with a gas pressure of 9 atmospheres of nitrogen, and it was held at a maximum temperature of 1900 °C for 5 hours. As the mold, a silicon carbide mold with an internal BN coating was used. In this way, a silicon nitride sintered body of Sample 1 was produced.

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

[0051] (Sample 3) The silicon nitride sintered body of Sample 3 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the addition amount of magnesium carbonate powder was 5.0 wt%.

[0052] (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 addition amount of magnesium carbonate powder was 8.0 wt%.

[0053] (Sample 5) The silicon nitride sintered body of Sample 5 was produced under the same conditions as the silicon nitride sintered body of Sample 3, except that the addition amount of yttrium oxide powder was 6.0 wt%.

[0054] (Sample 6) The silicon nitride sintered body of Sample 6 was produced under the same conditions as the silicon nitride sintered body of Sample 3, except that the addition amount of yttrium oxide powder was 8.5 wt%.

[0055] (Sample 7) The silicon nitride sintered body of Sample 7 was produced under the same conditions as the silicon nitride sintered body of Sample 6, except that yttrium oxide powder was changed to ytterbium oxide powder (average particle size 1.2 μm).

[0056] (Sample 8) The silicon nitride sintered body of Sample 8 was produced under the same conditions as the silicon nitride sintered body of Sample 6, except that yttrium oxide powder was changed to erbium oxide powder (average particle size 1.4 μm).

[0057] (Sample 9) The silicon nitride sintered body of Sample 9 was produced under the same conditions as the silicon nitride sintered body of Sample 6, except that yttrium oxide powder was changed to lanthanum oxide powder (average particle size 1.0 μm).

[0058] (Sample 10) The silicon nitride sintered body of Sample 10 was produced under the same conditions as the silicon nitride sintered body of Sample 5, except that 5.0 wt% of calcium carbonate powder (average particle size 2.5 μm) was further added.

[0059] (Sample 11) The silicon nitride sintered body of Sample 11 was produced under the same conditions as the silicon nitride sintered body of Sample 5, except that the maximum temperature during firing was 1850°C.

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

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

[0062] (Sample 14) The silicon nitride sintered body of Sample 14 was produced under the same conditions as the silicon nitride sintered body of Sample 3, except that the addition amount of yttrium oxide powder was 9.5 wt%.

[0063] (Sample 15) The silicon nitride sintered body of Sample 15 was produced under the same conditions as the silicon nitride sintered body of Sample 5, except that the addition amount of magnesium carbonate powder was 1.5 wt%.

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

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

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

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

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

[0069] (Sample 21) The silicon nitride sintered body of Sample 21 was produced under the same conditions as the silicon nitride sintered body of Sample 5, except that the ball mill media when preparing the mixed slurry was changed from YSZ to silicon nitride.

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

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

[0072] (Sample 24) The silicon nitride sintered body of Sample 24 was produced under the same conditions as the silicon nitride sintered body of Sample 3, except that after putting carbon powder into a silicon carbide mold coated with BN and then putting the green body in for firing.

[0073] [Various Measurements] The sintered bodies of each obtained sample were evaluated by the following measurements and the like.

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

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

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

[0077] (Measurement of Particle Size) The average particle size of the Group 4 element compound particles was determined by SEM observation. Specifically, for the polished surface of the silicon nitride-based sintered body, five locations were randomly selected and observed at a magnification of 2000 times with a field of view of 120 μm × 90 μm. Then, the equivalent circle diameter was determined from the area of each particle recognized as the Group 4 element compound, and the average particle size was determined by averaging them.

[0078] (Calculation of Thermal Conductivity) The sintered body of each sample was processed into 0.32 mm × □17 mm, and the thermal diffusivity was measured by the two-dimensional method using a laser flash. Also, the specific heat value was assumed to be constant at 0.68 cm 2 / sec. Then, the thermal conductivity was calculated from the thermal diffusivity measured by the two-dimensional method and the above density value using (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0079] (Measurement of Bending Strength) In accordance with ISO23242, the sintered body of each sample was processed to a thickness of 0.32×12×25 mm, and the flexural strength was measured by three-point bending with a span of 15 mm.

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

[0081] (Results) Figures 3 and 4 are tables showing the elemental content and the characteristics of the grain boundary phase of each sample. Note that Fe, Ni, Co, and Al in each sample are derived from the raw materials or the equipment used in the manufacturing process. Also, the Group 4 elements in the Group 4 element compounds of each sample except Samples 21 and 23 are derived from the YSZ balls used in the manufacturing process.

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

[0083] Sample 13 had a low flexural strength. This is presumably because the content of rare earth elements was too low. Sample 14 had a low thermal conductivity. This is presumably because the content of rare earth elements was too high.

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

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

[0086] Sample 19 had a low flexural strength. This is presumably because the content of Group 4 elements or Group 4 element compounds was too low. Sample 20 had a low thermal conductivity. This is presumably because the content of Group 4 elements or Group 4 element compounds was too high.

[0087] Sample 21 had a low flexural strength. This is presumably because silicon nitride was used for the ball mill and no external addition of Group 4 element compounds was made, so it contained no Group 4 elements or Group 4 element compounds.

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

[0089] Sample 23 had a flexural strength within the allowable range but was slightly low. This is presumably because more than 3 crystal grains of the Group 4 element compound (ZrN) were connected.

[0090] Although the confirmed Group 4 element compound in Sample 24 was ZrCN containing Group 4 elements, nitrogen, and carbon, both the thermal conductivity and the flexural strength were high. Thus, it was found that the Group 4 element compound does not have to contain only Group 4 elements and nitrogen. It is presumed that the same applies even if the Group 4 element compound is a compound containing Group 4 elements and carbon.

[0091] From the above results, it was confirmed that the silicon nitride sintered body and the silicon nitride heat dissipation substrate of the present invention can be made to have higher strength while maintaining a high thermal conductivity.

[0092] The present invention is not limited to the above embodiments, and it goes without saying that the present invention extends to various modifications and equivalents included in the spirit and scope of the present invention. In addition, the structure, shape, number, position, size, etc. of the components shown in each drawing are for convenience of explanation and can be changed as appropriate.

Explanation of Reference Numerals

[0093] 10 Silicon nitride heat dissipation substrate 12 Circuit layer 14 Conductor layer 20 Circuit board 22 Solder 30 Power semiconductor 40 Heat sink 42 Grease 50 Heat dissipation member 100 Power device

Claims

1. A silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, containing at least rare earth elements in a total amount of 1.0 to 7.5 wt%, alkali metal elements and alkaline earth metal elements in a total amount of 0.25 to 2.0 wt%, one or more elements selected from Fe, Ni, Co, and Al in a total amount of 0.010 to 0.30 wt%, and Group 4 elements in a total amount of 0.30 to 3.0 wt%, wherein at least a part of the Group 4 elements forms one or more compounds containing at least one of nitrogen and carbon, and the total content of the compounds is 0.30 to 3.0 wt%. A silicon nitride-based sintered body characterized by this.

2. The silicon nitride-based sintered body according to Claim 1, wherein the average particle diameter of the particles of the compound is 8.0 μm or less.

3. The silicon nitride-based sintered body according to Claim 1 or Claim 2, wherein the connection of the particles of the compound is less than 3.

4. The silicon nitride-based sintered body according to Claim 1 or Claim 2, wherein the Group 4 elements contain one or more elements selected from Zr and Hf.

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

6. The silicon nitride-based heat dissipation substrate according to Claim 5, wherein the bending strength is 650 MPa or more.

7. The silicon nitride-based heat dissipation substrate according to Claim 5, wherein the thermal conductivity is 85 W / mK or more.

8. The silicon nitride-based heat dissipation substrate according to Claim 5, wherein the thickness in the direction perpendicular to one main surface of the silicon nitride-based heat dissipation substrate is 220 μm or more and 690 μm or less.

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