Silicon nitride substrate

Abstract

[Problem] To provide a silicon nitride substrate having isotropy and high fracture toughness. [Solution] A silicon nitride substrate having silicon nitride particles, the silicon nitride substrate being characterized in that the average angle between the main surface of the silicon nitride substrate and the long axis of the silicon nitride particles is 35-55 DEG, the average aspect ratio of the silicon nitride particles is less than 2, and the average circle equivalent diameter of the silicon nitride particles is greater than 3 [mu] m.

Description

Technical Field

[0001] This invention relates to a novel silicon nitride substrate. Background Technology

[0002] Silicon nitride (SiN) exhibits excellent mechanical strength, thermal conductivity, and electrical insulation, making it suitable for use as substrates in semiconductor modules. However, conventional SiN substrates often suffer from isotropy, exhibiting lower fracture toughness in the planar direction compared to the perpendicular direction. To address this issue, Patent Document 1 discloses a method to improve isotropy by arranging the columnar grains of SiN randomly or uniformly in both the longitudinal and transverse directions. Specifically, this results in a SiN sintered substrate with fracture toughness (fracture toughness) of 6–7 MPa·m in both a first direction parallel to the substrate plane and a second direction perpendicular to the substrate plane. 1 / 2 Silicon nitride substrate. Existing technical documents

[0003] Patent documents Patent Document 1: Japanese Patent Application Publication No. 2019-052072 Summary of the Invention The problem that the invention aims to solve

[0004] As described above, in Patent Document 1, an improved isotropic fracture toughness of 6–7 MPa·m was obtained. 1 / 2 While silicon nitride substrates are generally available, there is a desire to further improve their fracture toughness. Therefore, the objective of this invention is to provide a silicon nitride substrate that is isotropic and exhibits higher fracture toughness. Methods for solving problems

[0005] To address the aforementioned issues, the inventors conducted in-depth research. They discovered that by controlling silicon nitride particles with low aspect ratios and large sizes to be oriented isotropically, the problems could be solved, thus completing this invention.

[0006] That is, the present invention is a silicon nitride substrate having silicon nitride particles, wherein the average angle between the main surface of the silicon nitride substrate and the long axis of the silicon nitride particles is 35° to 55°, the average aspect ratio of the silicon nitride particles is less than 2, and the average circular equivalent diameter of the silicon nitride particles is greater than 3 μm. The fracture toughness of the silicon nitride substrate is preferably 8 MPa·m. 1 / 2 The thermal conductivity of the silicon nitride substrate is preferably 80 W / (m·K) or higher. Invention Effects

[0007] According to the present invention, a silicon nitride substrate having isotropy and high fracture toughness can be obtained. Detailed Implementation

[0008] The silicon nitride substrate of the present invention has silicon nitride particles, the average angle between the main surface of the silicon nitride substrate and the long axis of the silicon nitride particles is 35° to 55°, the average aspect ratio of the silicon nitride particles is less than 2, and the average circular equivalent diameter of the silicon nitride particles is greater than 3 μm.

[0009] The average angle (hereinafter sometimes simply referred to as the "average angle") between the main surface of the silicon nitride substrate and the long axis of the silicon nitride particles is 35° to 55°, indicating that the orientation of the long axis of the silicon nitride particles is random and unoriented. Since the properties of the silicon nitride substrate largely depend on the properties of the silicon nitride particles, which are its main component, this characteristic enables the silicon nitride substrate to exhibit isotropic properties. The average angle is preferably 37° to 53°, and more preferably 40° to 50°.

[0010] The aforementioned average angle can be measured using a scanning electron microscope (SEM) to observe SEM images obtained by examining a cross-section of a silicon nitride substrate cut along its thickness direction. Specifically, firstly, 1000 silicon nitride particles are randomly selected from the SEM images. If 1000 silicon nitride particles cannot be selected from a single image, multiple images obtained from the same substrate can be used. Next, for each selected silicon nitride particle, the longest straight line connecting the edges of that particle is taken as its major axis, and the angle between this major axis and the main surface of the substrate is measured. Note that the angle between the major axis and the main surface of the substrate is taken as the minimum value, any value between 0° and 90°. After measuring the angle between the major axis and the main surface of the substrate for all silicon nitride particles, the average angle with the major axis of the silicon nitride particles can be calculated by taking into account the area of ​​each particle in the SEM image.

[0011] To clarify, the average angle mentioned above is calculated using a weighted average that considers the area of ​​each particle because larger silicon nitride particles contribute more to the physical properties. The weighted average that considers the area of ​​each particle can be calculated in the following order: First, for all selected silicon nitride particles, calculate the product of the area of ​​each particle on the SEM image and the angle between its major axis and the main surface of the substrate (value A). Then, calculate the sum of values ​​A for all selected silicon nitride particles (value B). Additionally, calculate the sum of the areas of all selected silicon nitride particles on the SEM image (value C). Finally, by dividing value B by value C, the average angle with the major axis of the silicon nitride particle can be obtained.

[0012] Furthermore, depending on the shape of the silicon nitride particles, there may sometimes be the longest straight line among multiple straight lines connecting the edges of the particles. In this case, by taking all the straight lines as the major axis, measuring the angle between the major axis and the main surface of the substrate, and calculating the arithmetic mean, the angle formed by the major axis of the particle and the main surface of the substrate can be obtained. When the particle is circular and the major axis exists infinitely, the angle between the major axis and the main surface of the substrate is 45°.

[0013] The fact that the average aspect ratio of the aforementioned silicon nitride particles is less than 2 indicates that there are more near-circular silicon nitride particles and fewer needle-like silicon nitride particles. Combined with the fact that the average spherical equivalent diameter of the aforementioned silicon nitride particles is greater than 3 μm, this indicates that the silicon nitride particles in the silicon nitride substrate of the present invention have a relatively large near-circular crystal shape. It is speculated that the composition of such large silicon nitride particles can prevent crack propagation and improve fracture toughness. Furthermore, it is believed that such large-sized silicon nitride particles can also reduce the number of particle interfaces, which are a cause of thermal resistance, thereby easily improving thermal conductivity.

[0014] Regarding the average aspect ratio and average circular equivalent diameter of the aforementioned silicon nitride particles, the average aspect ratio and average circular equivalent diameter of the silicon nitride particles can be calculated based on the measurement results of the aspect ratio and circular equivalent diameter of all silicon nitride particles selected in the above average angle measurement.

[0015] The aspect ratio of silicon nitride particles is determined as follows: Among the selected silicon nitride particles, the longest straight line connecting the edges of the particle is taken as the major axis, and the longest straight line connecting the edges of the particle that is orthogonal to the major axis is taken as the minor axis. The major axis is then divided by the minor axis. It should be noted that, depending on the shape of the silicon nitride particles, sometimes there are multiple longest straight lines connecting the edges of the particles; in this case, the line with the largest aspect ratio is used.

[0016] The average aspect ratio of the silicon nitride particles of the present invention is a weighted average of the areas of each particle, similar to the average angle mentioned above. It can be calculated by replacing the "angle between the major axis and the main surface of the substrate" in the calculation of the average angle mentioned above with the calculation method of "aspect ratio".

[0017] The circular equivalent diameter of the silicon nitride particles in this invention refers to the diameter of a circle with the same area as the particle, calculated from the particle area observed in the SEM image of each selected silicon nitride particle. The average circular equivalent diameter in this invention refers to the circular equivalent diameter of the particles whose cumulative area value reaches 50% when the particle areas are sorted from smallest to largest.

[0018] The lower limit of the aforementioned average aspect ratio is not particularly limited, as long as it is at least 1 (the theoretical lower limit). The aforementioned average equivalent circular diameter is preferably 4 μm or more, more preferably 5 μm or more. The upper limit of the aforementioned average equivalent circular diameter is not particularly limited, but a larger upper limit makes it easier to prevent crack propagation, reduce the number of particle interfaces, and thus easily improve fracture toughness and thermal conductivity. Generally, it is below 30 μm, especially below 20 μm.

[0019] The silicon nitride substrate of the present invention can possess high fracture toughness. Therefore, it is easy to prevent breakage during use. The fracture toughness (fracture strength) value of the silicon nitride substrate is preferably 8 MPa·m. 1 / 2 The above is preferred, and more preferably is 9 MPa·m 1 / 2 The above applies. A higher fracture toughness value for silicon nitride substrates is better, but it is generally acceptable to have a value of 15 MPa·m. 1 / 2 The following, especially 12 MPa·m 1 / 2 Hereinafter, it will be explained that the fracture toughness value of the silicon nitride substrate of the present invention is the average value ((Kc1+Kc2) / 2) of the fracture toughness value Kc1 in the direction parallel to the main surface of the substrate and the fracture toughness value Kc2 in the direction perpendicular to the main surface of the substrate, as described later in the embodiments. Both Kc1 and Kc2 are preferably 8 MPa·m. 1 / 2 The above is preferred, and more preferably is 9 MPa·m 1 / 2 The above is generally 15 MPa·m 1 / 2 The following, especially 12 MPa·m 1 / 2 From the point of view of isotropy, the ratio of Kc1 to Kc2 (Kc1 / Kc2) is preferably 0.90 to 1.10, and more preferably 0.95 to 1.05.

[0020] In this invention, the thermal conductivity of the silicon nitride substrate is preferably 80 W / (m·K) or higher, more preferably 90 W / (m·K) or higher. Higher thermal conductivity of the silicon nitride substrate is better, but it is generally 150 W / (m·K) or lower, particularly 120 W / (m·K) or lower. It should be noted that the thermal conductivity of the silicon nitride substrate of this invention refers to the average value ((λ1+λ2) / 2) of the thermal conductivity λ1 in the direction parallel to the main surface of the substrate and the thermal conductivity λ2 in the direction perpendicular to the main surface of the substrate, as described later in the embodiments. Both λ1 and λ2 are preferably 80 W / (m·K) or higher, more preferably 90 W / (m·K) or higher, generally 150 W / (m·K) or lower, particularly 120 W / (m·K) or lower. From the viewpoint of isotropy, the ratio of λ1 to λ2 (λ1 / λ2) is preferably 0.90 to 1.10, more preferably 0.95 to 1.05.

[0021] The three-point bending strength of the silicon nitride substrate of the present invention is preferably 600 MPa or higher.

[0022] The silicon nitride substrate of the present invention is preferably plate-shaped. The size of the main surface (main face) is not particularly limited; for example, the area of ​​the main surface can be 100 mm². 2 Above 80000mm 2 Below, especially 900mm 2 Above 40000mm 2 The shape of the main surface is not particularly limited; it can be circular or rectangular. In the case of a rectangular main surface, the ratio of the length of the longer side to the shorter side is not particularly limited. It should be noted that the main surface is the surface with the largest area on the silicon nitride substrate. In the case of a cylindrical or cuboid shape with surfaces of equal area on both sides, either surface can be used as the main surface. The thickness of the silicon nitride substrate of this invention is not particularly limited, but is generally 0.1 mm to 2.0 mm.

[0023] The silicon nitride substrate of the present invention can be easily obtained by a silicon nitride substrate manufacturing method including at least the following firing process: a green compact containing β-type silicon nitride powder with an average particle size of 1.5 μm or more and an average aspect ratio of less than 1.5 is fired at a maximum temperature of 1700 to 1900°C for 10 to 50 hours.

[0024] In the above manufacturing method, the silicon nitride powder constituting the green body includes β-type silicon nitride powder. Silicon nitride powder exists in both α-type and β-type forms, but α-type powder tends to generate high aspect ratio needle-like crystals during firing due to grain growth. In contrast, β-type powder is less prone to grain growth during firing. Therefore, it is speculated that the average aspect ratio of silicon nitride particles in the silicon nitride substrate obtained by preventing needle-like grain formation can be less than 2. It should be noted that when a small amount of α-type silicon nitride powder is present, grain growth during firing is limited and does not lead to an increase in aspect ratio. Therefore, the β-type silicon nitride powder of the present invention refers to silicon nitride powder with a β-saturation rate of 98% or higher. More preferably, the β-saturation rate is 99% or higher. It should be noted that the β-saturation rate of the silicon nitride powder refers to the ratio of the peak intensity of the β phase relative to the sum of the α and β phases of the silicon nitride powder [100 × (peak intensity of β phase) / (peak intensity of α phase + peak intensity of β phase)], obtained by measuring using CuKα-ray powder X-ray diffraction (XRD). More specifically, it is determined by calculating the weight ratio of the α and β phases of the silicon nitride powder, according to the method described in CPGazzara and DR Messier: Ceram. Bull., 56 (1977), 777-780.

[0025] It is speculated that by making the average particle size of the aforementioned β-type silicon nitride powder 1.5 μm or more, even when using β-type silicon nitride powder, which is difficult to grow grains, as raw material, the average spherical equivalent diameter of the silicon nitride particles in the obtained silicon nitride substrate can be greater than 3 μm. The average particle size of the aforementioned β-type silicon nitride powder is preferably 2 μm or more. There is no particular upper limit to the average particle size of the aforementioned β-type silicon nitride powder; it can be appropriately determined according to the size of the silicon nitride particles in the desired silicon nitride substrate, for example, it can be 10 μm or less. The average particle size of the β-type silicon nitride powder is the median particle size D50 in the volume-based particle size distribution, which can be determined by laser diffraction scattering.

[0026] By ensuring the average aspect ratio of the aforementioned β-type silicon nitride powder is less than 1.5, it is easy to obtain an average aspect ratio of less than 2 for the silicon nitride particles in the resulting silicon nitride substrate. There is no particular limitation on the lower limit of the average aspect ratio of the β-type silicon nitride powder, as long as it is at least theoretically lower than 1.0. The average aspect ratio of the β-type silicon nitride powder can be calculated from SEM images of the β-type silicon nitride powder. Specifically, 1000 silicon nitride particles are randomly selected from the SEM images of the β-type silicon nitride powder, and the aspect ratio of all selected silicon nitride particles is measured. Similar to the determination of the average aspect ratio of the silicon nitride particles in the silicon nitride substrate, the aspect ratio can be calculated by taking into account the area of ​​each particle.

[0027] In addition to β-type silicon nitride powder, the above-mentioned green blank may also contain sintering aids, binder resins, etc.

[0028] As sintering aids, substances commonly used in the sintering of silicon nitride powder can be used without particular limitation, such as yttrium oxide, magnesium oxide, cerium oxide, silicon dioxide, and calcium oxide. Additionally, carbonitride compounds such as Y₂Si₄N₆C, Yb₂Si₄N₆C, Ce₂Si₄N₆C, and MgSi₄N₆C, and nitride compounds such as MgSiN₂ can also be used as sintering aids. The amount of sintering aid in the green body is preferably 1 to 20 parts by weight, more preferably 3 to 10 parts by weight, relative to 100 parts by weight of β-type silicon nitride powder.

[0029] The binder resin mentioned above is not particularly limited, and examples include: polyvinyl alcohol, polyvinyl butyral, methylcellulose, alginate, polyethylene glycol, carboxymethyl cellulose, ethyl cellulose, acrylic resins, etc. The content of the binder resin in the green body relative to 100 parts by weight of β-type silicon nitride powder is preferably 1 to 40 parts by weight, more preferably 5 to 30 parts by weight.

[0030] There are no particular limitations on the manufacturing method of the green body. Powder obtained by dry mixing of the components of the green body, or granules obtained by drying the granules made by wet mixing of the components of the green body using a spray dryer or similar means, can be used to mold the green body.

[0031] Alternatively, the components of the green body can be wet-mixed to prepare a green body forming slurry, and the green body forming slurry can be sheet-formed to produce a green body. The aforementioned green body forming slurry can be prepared by measuring each component in a prescribed proportion and stirring to disperse the powder in a dispersion medium. The dispersion medium is not particularly limited; for example, water; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; alcohols such as ethanol, propanol, and butanol; aromatic hydrocarbons such as benzene, toluene, and xylene; and halogenated hydrocarbons such as trichloroethylene, tetrachloroethylene, and bromochloromethane can be used, or a mixture of one or more of these can be used. Examples of dispersing devices used for mixing and stirring include: ultrasonic dispersers, bead mills, ball mills, roller mills, high-speed mixers (Homo mixers), ultra-fine mixers, dispersing mixers, homogenizers, through-type high-pressure dispersers, impact-type high-pressure dispersers, porous high-pressure dispersers, collision-type high-pressure dispersers, (collision + through-type) high-pressure dispersers, and ultra-high-pressure homogenizers. Additionally, after mixing, filtration or other operations can be performed as needed to remove agglomerates from the slurry used for green body forming.

[0032] There are no particular limitations on the method for preparing the green body (blank) from the green body forming slurry; any known forming method can be used. However, from the viewpoint of ensuring good uniformity of sheet thickness, the scraper forming method is preferred. The resulting green body can be processed into an appropriate size and desired shape. For example, it is generally processed into a roughly rectangular prism shape with one side of 100mm to 2000mm and a thickness of 0.3mm to 1.2mm. It can also be noted that after forming to a size larger than that used during firing, it can be cut to obtain the desired shape. The cutting process can be performed before or after drying, as described later.

[0033] When obtaining a green body from the green body forming slurry, a drying process is preferred, depending on the requirements. The drying process is a process of further removing the dispersion medium from the formed body, thereby facilitating the subsequent firing of the green body. When the dispersion medium is water, the drying process can be carried out, for example, by letting the formed body stand at around 30°C to 150°C, and preferably the moisture content of the dried green body is less than 10%.

[0034] When the green body contains organic components such as binders, it is preferable to degrease the green body before firing. The degreasing temperature for degreasing the green body is preferably 300°C to 1200°C, more preferably 400°C to 1000°C. The degreasing of the green body is usually carried out in an atmosphere of oxidizing gas such as oxygen or air, reducing gas such as hydrogen, inactive gas such as argon or nitrogen, carbon dioxide or a mixture thereof, or a humidifying atmosphere of a mixture of these gases and water vapor. In addition, the degreasing time at the above-mentioned degreasing temperature can be appropriately selected according to the type, amount and degreasing atmosphere of the organic components such as binders used in the green body, and is usually 30 minutes to 12 hours, preferably 2 hours to 10 hours.

[0035] By firing the above-mentioned green blank at a maximum temperature of 1700–1900°C for 10–50 hours, the silicon nitride substrate of the present invention can be obtained. It is therefore speculated that even when using β-type silicon nitride powder, which is not prone to grain growth, as raw material, moderate growth of silicon nitride particles will occur, and the average spherical equivalent diameter of the silicon nitride particles in the obtained silicon nitride substrate will be greater than 3 μm.

[0036] The above-described firing is preferably carried out under an inert gas atmosphere. An inert gas atmosphere refers to, for example, a nitrogen atmosphere or an argon atmosphere. The firing pressure is not particularly limited, but high pressures incur equipment costs, so it is preferable to carry out the firing at a pressure of 10 MPa·G or less, more preferably 3 MPa·G or less, and even more preferably 1 MPa·G or less. Furthermore, silicon nitride may decompose during firing at low pressures, so a pressure of 0.5 MPa·G or more is preferred.

[0037] The application of the silicon nitride substrate of the present invention is not particularly limited. Examples include various heat dissipation substrates, power module substrates (for automotive, electrified railway, and high-power semiconductors), high-frequency circuit substrates, LED packaging, and optical pickup sub-mounts (for DVDs and CDs). Example

[0038] The present invention will be specifically described below through examples, but the present invention is not limited to these examples. The various test methods are described below.

[0039] Determination of the average angle, average aspect ratio, and average equivalent circular diameter between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particles. A silicon nitride substrate was cut perpendicular to its main surface, and the cut surface was observed using SEM at 2000x magnification to obtain SEM images. From the obtained SEM images, 1000 silicon nitride particles were randomly selected, and image analysis software (ImageJ) was used to measure the angle, aspect ratio, and area between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particle for each particle. Then, the equivalent circular diameter of each particle was calculated from the measured area. Based on the angle, aspect ratio, and equivalent circular diameter between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particle for each particle, the average angle, average aspect ratio, and average equivalent circular diameter were determined.

[0040] <Determination of Fracture Toughness Value> The fracture toughness values ​​in the direction parallel to and perpendicular to the main surface of the substrate were determined according to the method of JIS R1607:2015. An AVK-CO Vickers hardness tester manufactured by AKASHI Corporation of Japan was used. Specifically, a silicon nitride substrate was cut perpendicular to the main surface (main surface), and the cut surface was mirror-finished. An indentation was then created using a Vickers hardness tester, such that the two diagonals of the indentation were parallel to the direction perpendicular to the main surface of the substrate. The diagonal length a1 of the indentation parallel to the main surface of the substrate and the length c1 of the crack parallel to the main surface of the substrate were measured. Additionally, the diagonal length a2 of the indentation perpendicular to the main surface of the substrate and the length c2 of the crack perpendicular to the main surface of the substrate were measured. Next, in Equation 1 below, by setting a to (a1 / 2) and c to (c1 / 2), the fracture toughness value Kc1 in the direction parallel to the main surface of the substrate is calculated; by setting a to (a2 / 2) and c to (c2 / 2), the fracture toughness value Kc2 in the direction perpendicular to the main surface of the substrate is calculated. Furthermore, the average value of Kc1 and Kc2 ((Kc1+Kc2) / 2) is calculated as the fracture toughness (average value) of the silicon nitride substrate. Kc = 0.026 × E 1 / 2 ×P 1 / 2 ×a / c 3 / 2 (Equation 1) To clarify, E in Equation 1 is the elastic modulus, which is obtained by the ratio of bending stress σ to bending strain ε (E=σ / ε) in a three-point bending test performed by applying an indentation load in a direction perpendicular to the principal surface. P is the indentation load.

[0041] <Determination of Thermal Conductivity> The thermal conductivity λ1 in the direction parallel to the main surface and the thermal conductivity λ2 in the direction perpendicular to the main surface of the silicon nitride substrate were measured by point cyclic heating radiation thermometry. Then, the average value of λ1 and λ2 (λ1+λ2) / 2 was calculated as the thermal conductivity (average value) of the silicon nitride substrate.

[0042] The raw materials used in the experiment and their abbreviations are as follows. <Silicon Nitride Powder> • C1: 100% β-concentration, average particle size 2.13 μm, average aspect ratio 1.27 • C2: 100% β-concentration, average particle size 1.69 μm, average aspect ratio 1.32 • C3: 100% β-concentration, average particle size 0.72 μm, average aspect ratio 1.86 <Sintering aids> • S1: Yttrium oxide (specific surface area 20m²) 2 / g) •S2: Silicon magnesium nitride (specific surface area 12m²) 2 / g) <Adhesive> • B1: Acrylic resin emulsion (manufactured by Fujikura Chemicals Co., Ltd.: Acrybase EMK-02)

[0043] <Example 1> 100 parts by weight of silicon nitride powder C1, 3 parts by weight of sintering aid S1, 3 parts by weight of sintering aid S2, 20 parts by weight of binder B1, and 50 parts by weight of water were stirred and mixed for 48 hours to obtain a silicon nitride aqueous slurry. The obtained silicon nitride aqueous slurry was placed in a container, and the container was placed under reduced pressure at -0.1 MPa·G. It was allowed to stand at 30°C for 2.5 hours to perform a reduced pressure treatment process, resulting in a green body forming slurry. The obtained green body forming slurry was passed through a polyethylene mesh (229 μm mesh) to remove coarse particles, and then a sheet was formed using a doctor blade method. The obtained sheet was placed on a plate heater set to 50°C and allowed to stand for 3 hours to dry, resulting in a sheet-like green body. The obtained green body was degreased at 500°C for 60 hours, and then fired at 1 MPa·G at a maximum temperature of 1850°C for 48 hours to obtain a silicon nitride substrate. The silicon nitride powder used and the firing conditions are shown in Table 1, and the evaluation results of the obtained silicon nitride substrates are shown in Table 2.

[0044] <Examples 2-3, Comparative Examples 1-2> The silicon nitride powder and sintering conditions were modified as shown in Table 1. Otherwise, the process was the same as in Example 1 to obtain a silicon nitride substrate, which was then evaluated. The evaluation results of the obtained silicon nitride substrate are shown in Table 2.

[0045] [Table 1]

[0046] [Table 2]

Claims

1. A silicon nitride substrate, wherein, is a silicon nitride substrate having silicon nitride particles, wherein, The average angle between the main surface of the silicon nitride substrate and the long axis of the silicon nitride particles is 35° to 55°, the average aspect ratio of the silicon nitride particles is less than 2, and the average circular equivalent diameter of the silicon nitride particles is greater than 3 μm.

2. The silicon nitride substrate according to claim 1, wherein, The fracture toughness of the silicon nitride substrate is 8 MPa·m. 1 / 2 above.

3. The silicon nitride substrate according to claim 1 or 2, wherein, The thermal conductivity of the silicon nitride substrate is above 80 W / (m·K).

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