Silicon nitride substrate and production method for same

Abstract

Provided is a silicon nitride substrate that has greatly improved strength and still maintains various characteristics such as thermal conductivity and dielectric breakdown strength, even when a large amount of iron or another inevitable substance is included, and a production method for the silicon nitride substrate. This silicon nitride substrate 1 includes iron (Fe). The silicon nitride substrate 1 is characterized in that the average major axis diameter, the average minor axis diameter, and the average aspect ratio (average major axis diameter / average minor axis diameter) of silicon nitride particles 11 at a 64 μm×48 μm mirror-polished surface as measured by observation with a scanning electron microscope are 5.5–9.0 μm, 2.2–3.1 μm, and 2.4–3.8, respectively.

Description

Silicon nitride substrate and method for manufacturing the same

[0001] The present invention relates to a silicon nitride substrate and a method for manufacturing the same.

[0002] In recent years, silicon nitride (Si ３ N ４ Attempts are being made to apply alumina (Al) substrates to semiconductor circuit boards such as power semiconductors. ２ O ３ Alumina substrates and aluminum nitride (AlN) substrates are used. The thermal conductivity of alumina substrates is about 30 W / m·K, but alumina substrates can be manufactured at a low cost. In addition, the thermal conductivity of aluminum nitride substrates is 160 W / m·K or higher, making it possible to increase the thermal conductivity of the substrate. On the other hand, silicon nitride substrates with a thermal conductivity of 50 W / m·K or higher have been developed.

[0003] Although silicon nitride substrates have a lower thermal conductivity than aluminum nitride substrates, their three-point bending strength is over 500 MPa, which is superior to that of aluminum nitride substrates. Aluminum nitride substrates typically have a three-point bending strength of around 300-400 MPa, and this strength tends to decrease as thermal conductivity increases. By leveraging the advantage of high strength, silicon nitride substrates can be made thinner. Thinning the substrate reduces thermal resistance, thus improving heat dissipation.

[0004] Taking advantage of these properties, silicon nitride substrates are widely used as circuit boards by providing circuit sections such as metal plates. Furthermore, they can also be used as circuit boards for pressure-welded structures, as shown in international publication number WO2011 / 010597 (Patent Document 1).

[0005] Furthermore, the present inventors provide a silicon nitride substrate having excellent thermal conductivity in the thickness direction and a method for manufacturing the same (Patent Document 2).

[0006] International Publication Number WO2011 / 010597, Brochure JP 2022-027444

[0007] “Effects of Impurity Iron Content on Characteristics of Sintered Reaction-Bonded Silicon Nitride”, Dai Kusano et. al., Int. J. Appl. Ceram. Technol., 10 [4] 690-700 (2013)

[0008]

[0009] However, as mentioned above, the thermal conductivity of conventional silicon nitride substrates (for example, Patent Document 1) is lower than that of aluminum nitride and the like. Therefore, when the substrate is used in a semiconductor circuit board, the heat generated by the semiconductor chip cannot be efficiently dissipated to the heat sink, and the power that can be supplied to the semiconductor circuit board is also limited. Consequently, there is a need for a silicon nitride substrate with higher thermal conductivity, especially in the thickness direction.

[0010] Furthermore, even silicon nitride substrates that have excellent thermal conductivity in the thickness direction, as described in Patent Document 2, may contain trace amounts of unavoidable substances such as iron during manufacturing. It is known that the presence of iron in silicon nitride substrates reduces various properties such as bending strength, thermal conductivity, and dielectric breakdown strength (Non-Patent Document 1).

[0011] Therefore, even with the silicon nitride substrate disclosed in Patent Document 2, various properties deteriorate due to the inclusion of iron. However, using raw material powders with a low amount of iron or using equipment to prevent contamination during manufacturing presents not only economic constraints (for example, a change of an order of magnitude in silicon purity quadruples the unit price) but also the challenge of requiring complicated processing and creating time constraints during manufacturing. Furthermore, even with the silicon nitride substrate disclosed in Patent Document 1, there is still the problem of insufficient strength.

[0012] This invention was made to solve the above-mentioned problems. Specifically, the present invention aims to provide a silicon nitride substrate and a method for manufacturing the same, which significantly improves strength while maintaining various properties such as thermal conductivity and dielectric breakdown strength, even though it contains a large amount of iron or other unavoidable substances.

[0013] To achieve the above objective, the silicon nitride substrate is a silicon nitride substrate containing iron (Fe), characterized in that, on a 64 μm × 48 μm surface that has been mirror-polished, the average major axis diameter of the particles measured by scanning electron microscopy is 5.5 μm or more and 9.0 μm or less, the average minor axis diameter is 2.2 μm or more and 3.1 μm or less, and the average aspect ratio (average major axis diameter / average minor axis diameter) is 2.4 or more and 3.8 or less.

[0014] Furthermore, in the silicon nitride substrate described above, it is preferable that the mass of iron is 0.12% by mass or more and 0.2% by mass or less with respect to the total mass of the silicon nitride substrate.

[0015] Furthermore, in any of the silicon nitride substrates described above, it is preferable that the substrate contains an impurity consisting of at least one element selected from calcium (Ca), nickel (Ni), zirconium (Zr), and copper (Cu), and that the total mass of the impurity and the iron is 0.12% by mass or more and 0.4% by mass or less of the total mass of the silicon nitride substrate.

[0016] Furthermore, in any of the silicon nitride substrates described above, it is preferable that the bending strength measured by the three-point method is 700 MPa or more, and the thickness is 0.2 mm or more and 0.7 mm or less.

[0017] Furthermore, in any of the silicon nitride substrates described above, the density is 3.22 g / cm³. ３ 3.25g / cm or more ３ The following is preferable:

[0018] Furthermore, a silicon nitride substrate as described in any of the above, characterized in that the dielectric strength is 30 kV / mm or more and 45 kV / mm or less.

[0019] Furthermore, in any of the silicon nitride substrates described above, it is preferable that the thermal conductivity in the thickness direction of the substrate is 83 W / m·K or more and 95 W / m·K or less.

[0020] To achieve the above objective, a method for manufacturing a silicon nitride substrate is characterized by comprising: a step of preparing a slurry by mixing silicon powder, a sintering aid, and a dispersion medium containing 0.2% to 0.4% by mass of iron with respect to the total mass of the powder; a molding step of preparing a sheet molded body from the slurry; a nitriding treatment step of heat-treating the sheet molded body in a nitrogen-containing atmosphere to nitride the silicon constituting the sheet molded body; and a firing step of sintering the sheet molded body that has undergone the nitriding treatment step at a temperature of 1750°C to 2000°C for a period of 6 to 20 hours to prepare a silicon nitride substrate.

[0021] When there are many fine particles in a silicon nitride substrate, the bending strength improves because cracks generated when the substrate breaks are less likely to propagate due to the presence of numerous grain boundaries. However, the thermal conductivity decreases because heat transfer is also inhibited by these grain boundaries. The opposite is true when there are too many large particles: the proportion of heat transfer within the particles increases, reducing inhibition by grain boundaries and improving thermal conductivity. However, the bending strength decreases because cracks propagate rapidly along the grain boundaries of the large particles.

[0022] The silicon nitride substrate described in any of the above descriptions and the silicon nitride substrate manufactured by the above manufacturing method have the average major axis diameter and average minor axis diameter of the particles controlled within a predetermined numerical range. Therefore, by keeping the major axis diameter or minor axis diameter of the particles within a predetermined particle size without excessive growth, it is possible to construct a crystalline structure that exhibits high strength while maintaining sufficient thermal conductivity.

[0023] Furthermore, according to the manufacturing method described in any of the above, a predetermined amount of iron contained in the silicon nitride substrate is dispersed in the liquid phase, thereby suppressing the excessive growth of silicon nitride particles, and thus enabling the formation of a structure with controlled average major axis diameter and minor axis diameter.

[0024] Therefore, with the silicon nitride substrate described in any of the above descriptions and the silicon nitride substrate manufactured by the manufacturing method described above, it is possible to significantly improve strength while maintaining various properties such as thermal conductivity and dielectric breakdown strength, even though they contain a large amount of iron and other unavoidable substances.

[0025] This is a schematic cross-sectional view of a silicon nitride substrate according to one embodiment of the present invention. This is a diagram showing the Kα1 map of the iron intrinsic X-rays of the silicon nitride substrate according to one embodiment of the present invention.

[0026] (Configuration of Silicon Nitride Substrate) Figure 1 is a schematic cross-sectional view of a silicon nitride substrate 1 in an embodiment of the present invention (Example 1 described later). The silicon nitride substrate 1 of the present invention is a silicon nitride substrate containing iron (Fe). The silicon nitride substrate 1 is composed of silicon nitride particles 11, a first grain boundary phase 12, and a second grain boundary phase 13. In Figure 1, the brighter areas (white areas) compared to other areas indicate the second grain boundary phase 13, which contains iron, and the first grain boundary phase 12, which does not contain iron. The first grain boundary phase 12 and the second grain boundary phase 13 are distinguished by a diagram (Figure 2) showing the Kα1 map of the iron's characteristic X-rays. For example, in the Kα1 map of the iron's characteristic X-rays in the cross-sectional view shown in Figure 1, the brighter areas (white areas) compared to other areas are areas containing iron, and the grain boundary phase in Figure 1 corresponding to this area is the first grain boundary phase 12. In Figure 1, the darker regions (gray areas) represent silicon nitride particles 11.

[0027] The silicon nitride content of the silicon nitride substrate 1 is preferably 85% to 95% by mass, and more preferably 87% to 93% by mass, relative to the total mass of the silicon nitride substrate 1. This improves the thermal conductivity in the thickness direction of the silicon nitride substrate 1, due to the crystal system (crystal structure) of the silicon nitride, as will be explained below. If the silicon nitride content is less than 85% by mass, the improvement in the thermal conductivity in the thickness direction of the silicon nitride substrate will be insufficient because the proportion of silicon nitride will be low. Also, if the silicon nitride content exceeds 95% by mass, the particle size will not reach the predetermined length, as will be explained later, resulting in insufficient improvement in strength.

[0028] The iron content of the silicon nitride substrate 1 is preferably 0.1% by mass or more and 0.3% by mass or less, and more preferably 0.12% by mass or more and 0.2% by mass or less, relative to the total mass of the silicon nitride substrate 1. If the iron content is less than 0.1% by mass, the particle diameter will not reach the predetermined length, as will be described later, resulting in insufficient improvement in strength. On the other hand, if the iron content exceeds 0.3% by mass, the silicon nitride particles will grow excessively, and as will be described later, the particle diameter will exceed the predetermined length, leading to a decrease in thermal conductivity and dielectric breakdown strength.

[0029] The content of impurities other than iron in the silicon nitride substrate 1 is preferably 0.07% by mass or less, and more preferably 0.06% by mass or less, relative to the total mass of the silicon nitride substrate 1. If the content of impurities other than iron exceeds 0.07% by mass, the particle size will not reach the predetermined length, resulting in insufficient improvement in strength and thermal conductivity.

[0030] The total content of iron and impurities in the silicon nitride substrate 1 is preferably 0.1% by mass or more, and more preferably 0.12% by mass or more, relative to the total mass of the silicon nitride substrate 1. The total content of iron and impurities in the silicon nitride substrate 1 is preferably 0.4% by mass or less, and more preferably 0.3% by mass or less, relative to the total mass of the silicon nitride substrate 1. If the total content of iron and impurities falls outside the above numerical range, the particle size will not reach the predetermined length, resulting in insufficient improvement in strength and thermal conductivity.

[0031] The sheets obtained in the process of manufacturing silicon nitride substrates contain silicon powder, sintering aids, organic solvents as dispersion media, binders, plasticizers and other additives, and also contain iron and other impurities. In addition, the impurities other than iron include at least one element selected from calcium (Ca), nickel (Ni), zirconium (Zr), and copper (Cu).

[0032] The average major axis diameter of the silicon nitride particles 11 constituting the silicon nitride substrate 1 is preferably 5.0 μm or more and 10.0 μm or less, and more preferably 5.5 μm or more and 9.0 μm or less. If the average major axis diameter of the silicon nitride particles 11 exceeds 10.0 μm, the proportion of long particles in the silicon nitride substrate 1 increases. Cracks that occur when the silicon nitride substrate 1 breaks are assumed to propagate mainly along the particle interface, and the larger the silicon nitride particles 11, the more rapidly the cracks propagate at that interface, leading to a decrease in strength. If the average major axis diameter becomes too large, the proportion of long silicon nitride particles 11 increases, and the rate at which cracks propagate rapidly increases, resulting in insufficient improvement in strength. On the other hand, if the average major axis diameter of the silicon nitride particles 11 is less than 5.0 μm, the proportion of short particles in the silicon nitride substrate 1 increases. Heat is mainly transferred through the silicon nitride particles 11, but if each silicon nitride particle 11 is short, the heat conduction path itself becomes short, leading to a decrease in thermal conductivity. In addition, the particle interfaces or liquid phase portions where auxiliary components are concentrated between the silicon nitride particles 11 have lower thermal conductivity than the silicon nitride particles 11, thus hindering heat conduction, and the more of these there are between the silicon nitride particles 11, the greater the decrease in thermal conductivity. If the average major axis diameter becomes too small, the proportion of short silicon nitride particles 11 increases, the heat conduction path itself becomes shorter, and the particle interfaces or liquid phase portions that hinder heat conduction increase, resulting in insufficient improvement in the thermal conductivity in the thickness direction of the silicon nitride substrate 1.

[0033] The average short-axis diameter of the silicon nitride particles 11 constituting the silicon nitride substrate 1 is preferably 1.8 μm or more and 4.0 μm or less, and more preferably 2.2 μm or more and 3.1 μm or less. If the average short-axis diameter of the silicon nitride particles 11 exceeds 4.0 μm, the proportion of thick particles in the substrate increases. Cracks that occur when the silicon nitride substrate 1 breaks are assumed to propagate mainly along the particle interface, and the larger the silicon nitride particles 11, the more rapidly the cracks propagate at that interface, leading to a decrease in strength. If the average short-axis diameter becomes too large, the proportion of thick silicon nitride particles 11 increases, and the rate at which cracks propagate rapidly increases, resulting in insufficient improvement in strength. On the other hand, if the average short-axis diameter of the silicon nitride particles 11 is less than 1.8 μm, the proportion of thin silicon nitride particles 11 in the silicon nitride substrate 1 increases. Heat is mainly transferred through the silicon nitride particles 11, but if each silicon nitride particle 11 is thin, the heat conduction path itself becomes thin, leading to a decrease in thermal conductivity. In addition, the particle interfaces or liquid phase portions where auxiliary components are concentrated between the silicon nitride particles 11 have lower thermal conductivity than the silicon nitride particles 11, thus hindering heat conduction, and the more of these there are between the silicon nitride particles 11, the greater the decrease in thermal conductivity. If the average short axis diameter becomes too small, the proportion of thin silicon nitride particles 11 increases, the heat conduction path itself becomes thin, and the particle interfaces or liquid phase portions that hinder heat conduction increase, resulting in insufficient improvement in the thermal conductivity in the thickness direction of the silicon nitride substrate 1.

[0034] The average aspect ratio (average major axis diameter / average minor axis diameter) of the silicon nitride particles 11 constituting the silicon nitride substrate 1 is preferably 1.4 or more and 4.5 or less, and more preferably 1.7 or more and 4.1 or less. If the average aspect ratio of the silicon nitride particles 11 exceeds 4.5 (i.e., if the number of elongated silicon nitride particles 11 increases), the rate at which cracks propagate rapidly along the elongated silicon nitride particles 11 increases, resulting in insufficient improvement in strength. On the other hand, if the average aspect ratio of the silicon nitride particles 11 is less than 1.4 (i.e., if the number of silicon nitride particles 11 that are close to cubes increases), the heat conduction paths within the silicon nitride particles 11 become shorter, and heat conduction inhibition by the particle interface or liquid phase increases, resulting in insufficient improvement in the thermal conductivity in the thickness direction of the silicon nitride substrate 1.

[0035] Here, the average major axis diameter, average minor axis diameter, and average aspect ratio of the silicon nitride particles 11 are calculated by measuring particles measured by scanning electron microscope observation on at least a 128 μm × 96 μm surface, preferably a 64 μm × 48 μm surface, that has been mirror-polished. The major axis of the silicon nitride particles 11 is the axis in the direction of the maximum length of the particles observed on the surface that has been mirror-polished, and the minor axis of the silicon nitride particles 11 is the axis in the direction orthogonal to the major axis.

[0036] In the silicon nitride substrate 1 of the present invention, the thermal conductivity in the thickness direction is 80 W / m·K or more and 100 W / m·K or less, preferably 85 W / m·K or more and 95 W / m·K or less. Thereby, for example, even when the silicon nitride substrate 1 is used as a semiconductor circuit substrate, heat generated in the semiconductor chip can be efficiently released to the heat sink, and the power that can be input to the semiconductor circuit substrate can be improved. That is, in combination with the excellent strength of the silicon nitride substrate 1 described later, it can be applied to various semiconductor circuit substrates including power semiconductors.

[0037] Note that the above thermal conductivity can be obtained by satisfying the requirements such as the content of the above-described silicon nitride, iron, and impurities other than iron, and the particle size of the silicon nitride particles 11 in the silicon nitride substrate 1 of the present invention.

[0038] Here, in the present invention, the thermal conductivity is calculated by the product of the density, thermal diffusivity, and specific heat of a test piece cut out from the same substrate. The specific heat is preferably 0.665 J / g·K, and the density and thermal diffusivity are preferably measured by the measurement methods described later.

[0039] Further, in the silicon nitride substrate 1, the flexural strength by the three-point method is preferably 650 MPa or more, more preferably 700 MPa or more. At this time, the thickness of the silicon nitride substrate 1 is preferably 0.1 mm or more and 1.0 mm or less, more preferably 0.2 mm or more and 0.7 mm or less. Note that the flexural strength by the three-point method is preferably as high as possible, but from the viewpoint of ensuring the above-described thermal conductivity, it is preferably 900 MPa or less, more preferably 850 MPa or less.

[0040] Here, the flexural strength by the three-point method in the present invention is preferably measured in accordance with JIS R1601:2008 for a test piece of 24 mm × 40 mm. The temperature at this time is room temperature (25°C), the interval between the two supports is preferably 30 mm, and it is preferably measured as the three-point flexural strength when bent from the midpoint between the two supports. Also, it is preferably measured as the average value of the three-point flexural strengths of six test pieces.

[0041] Further, the size of the main surface of the silicon nitride substrate 1 is preferably 20000 mm 2 or more and 40000 mm 2 or less, more preferably 25000 mm 2 or more and 35000 mm 2 or less. The density of the silicon nitride substrate 1 is preferably 3.0 g / cm 3 or more and 3.5 g / cm 3 or less, more preferably 3.1 g / cm 3 or more and 3.3 g / cm 3 or less. The dielectric breakdown voltage of the silicon nitride substrate 1 is preferably 25 kV / mm or more and 50 kV / mm or less, more preferably 30 kV / mm or more and 45 kV / mm.

[0042] (Method for manufacturing a silicon nitride substrate) Next, the method for manufacturing the silicon nitride substrate of the present invention will be described.

[0043] First, as raw materials, silicon powder and sintering aid powder are prepared. The amount of the sintering aid is preferably 12 parts by mass with respect to 100 parts by mass of the silicon powder. The amounts of the silicon powder and the sintering aid powder are adjusted so that the iron content of the silicon nitride substrate 1 after sintering becomes the aforementioned amount. The iron content of the silicon powder used as a raw material is preferably 0.1% by mass or more and 0.4% by mass or less, more preferably 0.13% by mass or more and 0.39% by mass or less, with respect to the total mass of the silicon powder, so that the iron content of the silicon nitride substrate 1 after sintering becomes the aforementioned amount.

[0044] The sintering aid is preferably, for example, a metal compound powder. Examples of metal compound powders include oxides of rare earth elements, magnesium, titanium, and hafnium, but more preferably rare earth element oxides and magnesium compounds (magnesia, etc.).

[0045] Next, a dispersion medium is added to the silicon powder and sintering aid, and the mixture is dispersed in a ball mill, for example, and then ground and mixed to produce a slurry. Organic solvents such as toluene, ethanol, and butanol can be used as the dispersion medium.

[0046] Next, a binder, plasticizer, etc., are added to the slurry as needed, and the slurry is further degassed under vacuum to adjust its viscosity. Organic binders such as butyl methacrylate, polyvinyl butyral, and polymethyl methacrylate can be used as binders.

[0047] Next, the viscosity-adjusted slurry is formed into a sheet using a sheet forming method such as the doctor blade method or the roll method, to form a sheet molded body with a thickness of, for example, 0.38 mm. This sheet molded body can be obtained, for example, by applying the slurry onto a film, and then removing the film after drying.

[0048] Next, if necessary, a slurry consisting of ceramic powder and a dispersion medium is applied to the main surface of the sheet molded body to form a separation agent layer. As for the dispersion medium, organic solvents such as toluene, ethanol, and butanol can be used, as described above. As for the application method, spraying, bar coating, screen printing, etc., can be used.

[0049] Next, if necessary, the sheet molded body is degreased, for example, in a non-oxidizing atmosphere at a temperature of 600°C or less for several hours. After that, the sheet molded body is held in a nitrogen-containing atmosphere at a temperature of 1200 to 1500°C for 2 to 8 hours, during which the silicon constituting the sheet molded body undergoes nitridation, forming silicon nitride. The partial pressure of nitrogen in the nitrogen-containing atmosphere is, for example, 0.05 to 0.5 MPa.

[0050] Next, silicon nitride is sintered by holding it in a nitrogen-containing atmosphere, preferably at a temperature of 1750°C to 2000°C, more preferably at 1800°C to 1900°C, for preferably 6 hours to 20 hours, and more preferably 8 hours to 15 hours.

[0051] By controlling the temperature and sintering time, a silicon nitride substrate 1 is manufactured that contains iron within the above numerical range and comprises silicon nitride particles 11 controlled to the above numerical range.

[0052] In addition, the present invention also employs a method of using a weight plate in the nitriding sintering process, but may also be used, such as (1) a method in which the upper surface is left free without using a weight plate during silicon nitriding and the weight plate is used only during sintering, (2) a method in which a porous plate is used as the weight plate and a load is applied to the molded body continuously during silicon nitriding and silicon nitride sintering, or (3) a method in which a dense plate is used as the weight plate and a separating agent layer is provided between the molded body and the dense plate.

[0053] While boron nitride is preferred as the separating agent layer, the material is not limited to boron nitride. Any ceramic powder that is thermally stable during nitriding and sintering and can separate the dense plate after sintering is complete is acceptable.

[0054] Metallic silicon powder, a sintering aid, a dispersant, and a dispersion medium were mixed using a ball mill. The metallic silicon powder contained iron, and the mixing ratio was adjusted so that the iron content of the silicon nitride substrate 1 after sintering would be as shown in Table 1 below. Subsequently, a dispersion medium, an organic binder, and a plasticizer were added to the mixture and remixed to produce a slurry. Next, the prepared slurry was removed from the ball mill and transferred to a degasser, where its viscosity was adjusted by vacuum degassing, and it was formed into a sheet to produce a sheet molded body. The doctor blade method was used as the sheet molding method.

[0055] Subsequently, a ceramic slurry consisting of boron nitride was applied to the sheet molded body to form a separating agent layer on the surface of the sheet molded body, after which the sheet molded body was subjected to a degreasing treatment in a non-oxidizing atmosphere.

[0056] Next, a sheet molded body with a boron nitride separation agent layer formed on its main surface was subjected to nitriding in a nitrogen-containing atmosphere. Furthermore, sintering was carried out in a nitrogen-containing atmosphere under predetermined heat treatment conditions to produce silicon nitride substrates for Examples 1 to 6 and Comparative Examples 1 to 6.

[0057]

[0058] Quantitative analysis of the mass percentage of iron and the mass percentage of other metal impurities was performed by X-ray fluorescence analysis using a Rigaku ZSX Primus II.

[0059] Figure 2 shows the Kα1 map of the iron intrinsic X-rays of the silicon nitride substrate 1 of Example 1 (Fe-K SEM-EDX elemental map image). Here, regions that are brighter than other regions (white regions) indicate regions containing iron. Referring to Figure 2, the average number of iron particles measured on a 256 μm × 192 μm surface of the silicon nitride substrate 1 was 75. The average value was measured on 256 μm × 192 μm surfaces at seven arbitrary locations on the silicon nitride substrate 1. Similarly, Table 1 shows the average number of iron particles per 256 μm × 192 μm surface of the silicon nitride substrate 1 that were measured.

[0060] As shown in Table 1, it was clear that the number of iron particles in the 256 μm × 192 μm surface of Examples 1 to 6 was greater than that of Comparative Examples 1 to 6. This makes it possible to improve the strength of the silicon nitride substrate 1, as will be described later. The number of iron particles in the 256 μm × 192 μm surface is preferably 30 to 90, and more preferably 35 to 85.

[0061] The particle shapes of Examples 1-6 and Comparative Examples 1-6 were measured by scanning electron microscopy observation of a mirror-polished silicon nitride substrate 1. Statistical information such as averages was calculated based on multiple particles measured on a 64 μm × 48 μm surface of the mirror-polished silicon nitride substrate 1. Table 1 shows the average major axis diameter, average minor axis diameter, and average aspect ratio of Examples 1-6 and Comparative Examples 1-6, measured by this method.

[0062] Thermal diffusivity was measured using the flash method with a NETZSCH LFA 467 Hyper Flash instrument. Using this instrument, xenon flash light was irradiated, and the AC temperature response was measured with an IR detector. Thermal diffusivity was calculated from the amplitude of the temperature response and its attenuation rate with respect to position. Measurements were performed on 10 mm x 10 mm test specimens after blackening treatment of the surface.

[0063] Density measurements were performed using the Archimedes method.

[0064] The three-point bending strength was measured for 24 mm x 40 mm test specimens according to JIS R1601:2008 at room temperature (25°C), with a distance of 30 mm between the two supports, and the bending strength was measured from the midpoint between the two supports. The average of the three-point bending strengths of 10 test specimens was then calculated.

[0065] The dielectric breakdown strength was measured for a 40 mm x 50 mm test specimen according to the short-time test of JIS C2110-1. The test specimen was placed between cylindrical stainless steel electrodes with a diameter of φ25 mm, and the voltage was increased at a rate of 1000 V / s. The voltage at which dielectric breakdown occurred was then divided by the thickness of the test specimen.

[0066] Table 1 summarizes the evaluation results of the silicon nitride substrate 1 for Examples 1 to 6 and Comparative Examples 1 to 6.

[0067] As shown in Table 1, a silicon nitride substrate 1 in which the amount of iron is predetermined and the particle size is adjusted to a predetermined shape has a three-point bending strength of 740 MPa or more and a thermal conductivity of 85.0 W / m·K or more. Therefore, it has become clear that such a silicon nitride substrate 1 can significantly improve strength while maintaining various properties such as thermal conductivity and dielectric breakdown strength, even though it contains a large amount of iron and other unavoidable substances.

[0068] It should be noted that the present invention is not limited to the embodiments and examples described above, and can be implemented by making appropriate changes and modifications without departing from the spirit of the invention.

[0069] 1...Silicon nitride substrate, 11...Silicon nitride particles, 12...First grain boundary phase, 13...Second grain boundary phase

Claims

1. A silicon nitride substrate containing iron (Fe), characterized in that, on a 64 μm × 48 μm surface that has been mirror-polished, the average major axis diameter of the particles measured by scanning electron microscopy is 5.5 μm or more and 9.0 μm or less, the average minor axis diameter is 2.2 μm or more and 3.1 μm or less, and the average aspect ratio (average major axis diameter / average minor axis diameter) is 2.4 or more and 3.8 or less.

2. A silicon nitride substrate according to claim 1, characterized in that the mass of iron is 0.12% by mass or more and 0.2% by mass or less with respect to the total mass of the silicon nitride substrate.

3. A silicon nitride substrate according to claim 1, wherein the silicon nitride substrate contains an impurity consisting of at least one element selected from calcium (Ca), nickel (Ni), zirconium (Zr), and copper (Cu), and the total mass of the impurity and the iron is 0.12% by mass or more and 0.4% by mass or less with respect to the total mass of the silicon nitride substrate.

4. A silicon nitride substrate according to claim 1, characterized in that the bending strength measured by the three-point method is 700 MPa or more, and the thickness is 0.2 mm or more and 0.7 mm or less.

5. The silicon nitride substrate according to claim 1, wherein the density is 3.2 g / cm³. 3 3.3g / cm or more 3 A silicon nitride substrate characterized by the following:

6. A silicon nitride substrate according to claim 1, characterized in that the dielectric strength is 33 kV / mm or more and 42 kV / mm or less.

7. A silicon nitride substrate according to claim 1, characterized in that the thermal conductivity in the thickness direction of the substrate is 80 or more.

8. A method for producing a silicon nitride substrate, comprising: a step of preparing a slurry by mixing silicon powder, a sintering aid, and a dispersion medium, which contain 0.13% to 0.39% by mass of iron relative to the total mass of the powder; a molding step of producing a sheet molded body from the slurry; a nitriding treatment step of heat-treating the sheet molded body in a nitrogen-containing atmosphere to nitride the silicon constituting the sheet molded body; and a firing step of sintering the sheet molded body that has undergone the nitriding treatment step at a temperature of 1800°C to 2000°C for a period of 7 to 15 hours to produce a silicon nitride substrate.

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