Silicon nitride substrate

Abstract

To provide a silicon nitride substrate that is isotropic while possessing high fracture toughness. [Solution] A silicon nitride substrate having silicon nitride particles, wherein the average angle between the main surface of the silicon nitride substrate and the major 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 equivalent circular diameter of the silicon nitride particles is greater than 3 μm.

Description

[Technical Field]

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

[0002] Silicon nitride exhibits excellent mechanical strength, thermal conductivity, and electrical insulation properties, and is used in semiconductor module substrates and other applications. Conventional silicon nitride substrates generally suffer from an isotropy problem, where the fracture toughness in the planar direction is lower than the fracture toughness in the perpendicular direction of the substrate. To address this problem, for example, Patent Document 1 discloses a silicon nitride sintered substrate in which the isotropy is improved by arranging columnar silicon nitride crystal particles randomly or uniformly in both the longitudinal and transverse directions. Specifically, the fracture toughness is 6-7 MPa·m in both the first direction parallel to the substrate plane and the second direction perpendicular to the substrate plane. 1 / 2 A silicon nitride substrate has been obtained. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-052072 [Overview of the project] [Problems that the invention aims to solve]

[0004] As described above, Patent Document 1 describes a fracture toughness of 6-7 MPa·m with improved isotropy. 1 / 2 While silicon nitride substrates of a certain degree have been obtained, there is a desire to further improve their fracture toughness. Therefore, the object of the present invention is to provide a silicon nitride substrate that is isotropic while possessing even higher fracture toughness. [Means for solving the problem]

[0005] To solve the aforementioned problems, the inventors conducted intensive research. As a result, they discovered that the aforementioned problems could be solved by controlling the isotropic orientation of relatively large silicon nitride particles with a low aspect ratio, and thus completed the present invention.

[0006] In other words, the present invention relates to a silicon nitride substrate having silicon nitride particles, wherein the average angle between the main surface of the silicon nitride substrate and the major 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 equivalent circular diameter of the silicon nitride particles is greater than 3 μm. The fracture toughness of the silicon nitride substrate is 8 MPa·m 1 / 2 Preferably, the thermal conductivity of the silicon nitride substrate is 80 W / (m·K) or higher. [Effects of the Invention]

[0007] According to the present invention, a silicon nitride substrate having isotropy and high fracture toughness can be obtained. [Modes for carrying out the invention]

[0008] The silicon nitride substrate of the present invention has silicon nitride particles, wherein the average angle between the main surface of the silicon nitride substrate and the major 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 equivalent circular 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 major axis of the silicon nitride particles being 35° to 55° indicates that the orientation of the major axes of the silicon nitride particles is random and unoriented. Since the properties of the silicon nitride substrate depend largely on the properties of its main component, the silicon nitride particles, having this property allows the silicon nitride substrate to exhibit isotropic properties. The average angle is preferably 37° to 53°, and more preferably 40° to 50°.

[0010] The measurement of the average angle can be performed using SEM images obtained by observing a cross-section of a silicon nitride substrate cut along the thickness direction of the substrate using a scanning electron microscope (SEM). Specifically, first, 1000 silicon nitride particles are randomly selected from the SEM image. If 1000 silicon nitride particles cannot be selected from a single image, multiple images obtained from the same substrate may be used. Next, for each selected silicon nitride particle, the longest distance between the edges of the particle is taken as the major axis, and the angle between this major axis and the main surface of the substrate is measured. The smallest value of the angle between the major axis and the main surface of the substrate is adopted, and the value takes the range from 0° to 90°. After measuring the angle between the major axis and the main surface of the substrate for all silicon nitride particles, the average angle between the major axis and the silicon nitride particle can be determined by calculating a weighted average value that takes into account the area of ​​each particle in the SEM image.

[0011] The reason the average angle is calculated as a weighted average considering the area of ​​each particle is that larger silicon nitride particles contribute more to the physical properties. The weighted average considering the area of ​​each particle can be calculated using the following procedure. First, for all selected silicon nitride particles, calculate the value (value A) obtained by multiplying the area on the SEM image by the angle between the major axis and the main surface of the substrate. Next, calculate the sum of values ​​A for all selected silicon nitride particles (value B). Separately, calculate the sum of the areas on the SEM image of all selected silicon nitride particles (value C). Then, by dividing value B by value C, the average angle between the major axis of the silicon nitride particle and the particle can be obtained.

[0012] Furthermore, depending on the shape of the silicon nitride particles, there may be multiple straight lines connecting the edges of the particles that have the longest distance between them. In such cases, all of these lines are taken as the major axes, and the angle between the major axes and the main surface of the substrate is measured and the arithmetic mean is taken to obtain the angle between the major axis and the main surface of the substrate for that particle. If the particle is circular and there are infinitely many major axes, the angle between the major axis and the main surface of the substrate will be 45°.

[0013] The fact that the average aspect ratio of the silicon nitride particles is less than 2 means that there are many silicon nitride particles close to circular shape and few acicular silicon nitride particles. And, in combination with the fact that the average equivalent circle diameter of the silicon nitride particles is more than 3 μm, it indicates that in the silicon nitride substrate of the present invention, the crystals of the silicon nitride particles have a shape close to a relatively large circle. It is presumed that by being composed of such large silicon nitride particles, it has become possible to prevent the propagation of cracks and improve the fracture toughness. In addition, it is considered that such large silicon nitride particles also make it easy to reduce the number of particle interfaces that cause thermal resistance and increase the thermal conductivity.

[0014] The average aspect ratio of the silicon nitride particles and the average equivalent circle diameter of the silicon nitride particles can be obtained from the results of measuring the aspect ratio and the equivalent circle diameter for all the silicon nitride particles selected in the measurement of the average angle.

[0015] The aspect ratio of the silicon nitride particles is obtained by dividing the long axis by the short axis, where in each selected silicon nitride particle, the longest straight line connecting edge to edge of the particle is taken as the long axis, and the longest straight line connecting edge to edge of the particle while being orthogonal to this long axis is taken as the short axis. Note that depending on the shape of the silicon nitride particles, there may be a plurality of the longest straight lines connecting edge to edge of the particle, and in that case, the one with the largest aspect ratio is adopted.

[0016] The average aspect ratio of the silicon nitride particles in the present invention is a weighted average value considering the area of each particle, similar to the average angle, and can be calculated by a calculation method in which the "angle formed by the long axis and the main surface of the substrate" in the calculation of the average angle is replaced with the "aspect ratio".

[0017] The equivalent circular diameter of the silicon nitride particles in the present invention means the diameter of a circle having the same area as the particle, which is calculated from the area of the particle confirmed from the SEM image for each selected silicon nitride particle. The average equivalent circular diameter in the present invention means the equivalent circular diameter of the particle that becomes the cumulative 50% value based on the area when the particle areas are arranged in ascending order.

[0018] The lower limit of the average aspect ratio is not particularly limited, and it may be 1 or more, which is the theoretical lower limit value. The average equivalent circular diameter is preferably 4 μm or more, and more preferably 5 μm or more. The upper limit of the average equivalent circular diameter is not particularly limited. If it is large, it becomes easier to prevent the propagation of cracks and the number of particle interfaces decreases, making it easier to increase the fracture toughness and thermal conductivity. Generally, it is 30 μm or less, particularly 20 μm or less.

[0019] The silicon nitride substrate of the present invention can have high fracture toughness. This makes it easy to prevent breakage during use. The fracture toughness value of the silicon nitride substrate is preferably 8 MPa·m 1 / 2 or more, and more preferably 9 MPa·m 1 / 2 or more. Although the higher the fracture toughness value of the silicon nitride substrate, the more preferable it is, generally it can be 15 MPa·m 1 / 2 or less, particularly 12 MPa·m<​​​​​​​​​​​​In the present invention, the thermal conductivity of the silicon nitride substrate is preferably 80 W / (m·K) or higher, and more preferably 90 W / (m·K) or higher. A higher thermal conductivity of the silicon nitride substrate is preferable, but is generally 150 W / (m·K) or lower, and particularly 120 W / (m·K) or lower. In the present invention, the thermal conductivity of the silicon nitride substrate is 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 in the examples. Both λ1 and λ2 are preferably 80 W / (m·K) or higher, more preferably 90 W / (m·K) or higher, and generally 150 W / (m·K) or lower, and particularly 120 W / (m·K) or lower. The ratio of λ1 to λ2 (λ1 / λ2) is preferably 0.90 to 1.10, and more preferably 0.95 to 1.05, from the viewpoint of isotropy.

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

[0022] The silicon nitride substrate of the present invention is preferably in the form of a plate. The size of the main surface is not particularly limited, but for example, the area of ​​the main surface is 100 mm². 2 Above 80,000 mm 2 The following, in particular, 900mm 2 The above is 40,000 yen. 2 The following is possible. The shape of the main surface is not particularly limited and can be circular or rectangular. In the case of a rectangle, the ratio of the length of the long side to the short side of the main surface is not particularly limited. The main surface is the surface with the largest area among the surfaces of the silicon nitride substrate, and if there are surfaces with the same area on the top and bottom, such as a cylindrical or rectangular parallelepiped, either can be designated as the main surface. The thickness of the silicon nitride substrate of the present 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 method for manufacturing a silicon nitride substrate, which includes at least a firing step of firing a green body 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 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. Although silicon nitride powder exists in α-type and β-type forms, α-type is prone to grain growth during firing, resulting in the formation of needle-shaped crystals with a high aspect ratio, whereas β-type is less prone to grain growth during firing. Therefore, it is presumed that the average aspect ratio of silicon nitride particles in the silicon nitride substrate obtained by preventing the formation of needle-shaped crystal particles can be kept below 2. It should be noted that even if a small amount of α-type silicon nitride powder is included, the occurrence of grain growth during firing is limited and does not lead to an increase in the aspect ratio. Therefore, in this invention, β-type silicon nitride powder refers to silicon nitride powder with a β-conversion rate of 98% or more. A β-conversion rate of 99% or more is more preferable. The β-conversion rate of silicon nitride powder refers to the ratio of the peak intensity of the β phase to the sum of the α and β phases in silicon nitride powder [100 × (peak intensity of β phase) / (peak intensity of α phase + peak intensity of β phase)], and is determined by powder X-ray diffraction (XRD) measurement using CuKα rays. More specifically, it can be determined by calculating the weight ratio of the α and β phases of silicon nitride powder using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780.

[0025] By setting the average particle size of the β-type silicon nitride powder to 1.5 μm or more, it is presumed that even when using β-type silicon nitride powder, which is less prone to grain growth, as a raw material, the average equivalent circular diameter of the silicon nitride particles in the resulting silicon nitride substrate can be set to more than 3 μm. Preferably, the average particle size of the β-type silicon nitride powder is 2 μm or more. The upper limit of the average particle size of the β-type silicon nitride powder is not particularly limited and can be appropriately determined according to the size of the silicon nitride particles in the target silicon nitride substrate; for example, it may be 10 μm or less. The average particle size of the β-type silicon nitride powder is the median diameter D50 in the volume-based particle size distribution and can be measured by laser diffraction scattering.

[0026] By setting the average aspect ratio of the β-type silicon nitride powder to less than 1.5, it becomes easy to set the average aspect ratio of the silicon nitride particles in the resulting silicon nitride substrate to less than 2. The lower limit of the average aspect ratio of the β-type silicon nitride powder is not particularly limited; it is sufficient if it is 1.0 or higher, which is the theoretical lower limit. The average aspect ratio of the β-type silicon nitride powder can be calculated from the SEM image of the β-type silicon nitride powder. Specifically, 1000 silicon nitride particles are randomly selected from the SEM image of the β-type silicon nitride powder, the aspect ratio of all selected silicon nitride particles is measured, and the average aspect ratio can be calculated by calculating a weighted average value that takes into account the area of ​​each particle, similar to the measurement of the average aspect ratio of silicon nitride particles in the silicon nitride substrate described above.

[0027] The aforementioned green material may contain, in addition to β-type silicon nitride powder, sintering aids, binder resins, and the like.

[0028] The sintering aid can be any agent commonly used for sintering silicon nitride powder, without any particular limitations, such as yttria, magnesia, ceria, silica, and calcia. Carbonitride compounds such as Y2Si4N6C, Yb2Si4N6C, Ce2Si4N6C, and MgSi4N6C, and nitride compounds such as MgSiN2 may also be used as sintering aids. The amount of sintering aid in the green body is preferably 1 to 20 parts by mass, and more preferably 3 to 10 parts by mass, per 100 parts by mass of β-type silicon nitride powder.

[0029] The binder resin is not particularly limited, but examples include polyvinyl alcohol, polyvinyl butyral, methylcellulose, alginic acid, polyethylene glycol, carboxymethylcellulose, ethylcellulose, and acrylic resin. The binder resin content in the green is preferably 1 to 40 parts by mass, and more preferably 5 to 30 parts by mass, per 100 parts by mass of β-type silicon nitride powder.

[0030] The method for producing the green body is not particularly limited. The green body may be produced by using a powder obtained by dry mixing the components of the green body, or by using granules obtained by drying a slurry for granule production, which is made by wet mixing the components of the green body, using a spray dryer or the like, and then performing press molding.

[0031] Alternatively, the components of the green body may be mixed wet to prepare a slurry for green body molding, and the slurry for green body molding may be formed into a sheet. The slurry for green body molding can be prepared, for example, by weighing out each component in a predetermined amount and stirring and mixing so that the powder is dispersed in the dispersion medium. The dispersion medium is not particularly limited and can be, 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; halogenated hydrocarbons such as trichloroethylene, tetrachloroethylene, and bromochloromethane, or one or a mixture of two or more of these. Examples of dispersion devices for stirring and mixing include ultrasonic dispersion devices, bead mills, ball mills, roll mills, homomixers, ultramixers, disper mixers, homomixers, through-type high-pressure dispersion devices, impact-type high-pressure dispersion devices, porous-type high-pressure dispersion devices, clump-catching type high-pressure dispersion devices, (impact + through-type) high-pressure dispersion devices, ultra-high-pressure homogenizers, and the like. Furthermore, after stirring and mixing, if necessary, operations such as filter filtration may be performed to remove clumps from the slurry for forming the green body.

[0032] The method for producing the green body from the green body molding slurry is not particularly limited, and known molding methods can be used. However, from the viewpoint of good uniformity of sheet thickness, etc., it is preferable to mold by the doctor blade method. The resulting green body can be processed to an appropriate size and desired shape. For example, it is common to process it into a roughly rectangular parallelepiped shape with sides of 100 mm to 2000 mm and a thickness of 0.3 mm to 1.2 mm. Alternatively, the green body may be molded to a size larger than that to be used for firing, and then cut to the desired shape. Cutting may be performed before or after drying, as described later.

[0033] When obtaining a green body from a slurry for green body molding, it is preferable to perform a drying step as needed. The drying step is a process of further removing the dispersion medium from the molded body, which facilitates the subsequent firing of the green body. In the case of water as the dispersion medium, the drying step can be performed by letting the molded body stand at a temperature of approximately 30°C to 150°C, and it is preferable to dry the green body so that its moisture content is 10% or less.

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

[0035] By firing the aforementioned green material at a maximum temperature of 1700-1900°C for 10-50 hours, the silicon nitride substrate of the present invention can be obtained. As a result, even when using β-type silicon nitride powder, which is less prone to grain growth, it is presumed that appropriate growth of silicon nitride particles will occur, and the average equivalent circular diameter of the silicon nitride particles in the resulting silicon nitride substrate can be made to more than 3 μm.

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

[0037] The applications of the silicon nitride substrate of the present invention are not particularly limited, but examples include various heat dissipation substrates, power module substrates (for automotive, railway, and high-power semiconductor applications), high-frequency circuit boards, LED packages, and optical pickup submounts (for DVDs and CDs). [Examples]

[0038] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Each test method is as follows. Each test method is as follows.

[0039] <Measurement of the average angle, average aspect ratio, and average equivalent circle 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 with a 2000x magnification SEM to obtain an SEM image. From the obtained SEM image, 1000 silicon nitride particles were randomly selected, and the angle between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particle, the aspect ratio, and the area of ​​each particle were measured using image analysis software (ImageJ). Furthermore, the equivalent circular diameter of each particle was calculated from the measured area of ​​each particle. From the angle between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particle, the aspect ratio, and the equivalent circular diameter of each particle, the average angle between the main surface of the silicon nitride substrate and the major axis of the silicon nitride particle were determined.

[0040] <Measurement of fracture toughness value> The fracture toughness values ​​were measured in directions parallel to and perpendicular to the substrate surface according to the method conforming to JIS R1607:2015. An AVK-CO Vickers hardness tester manufactured by Akashi Co., Ltd. was used. Specifically, a silicon nitride substrate was cut perpendicular to the main surface, the cut surface was polished to a mirror finish, and then an indentation was created using the Vickers hardness tester so that the two diagonals of the indentation were perpendicular to and parallel to the substrate surface. For the resulting indentation, the diagonal length a1 of the indentation parallel to the substrate surface and the length c1 of the crack parallel to the substrate surface were measured. In addition, for the resulting indentation, the diagonal length a2 of the indentation perpendicular to the substrate surface and the length c2 of the crack perpendicular to the substrate surface were measured. Next, in the following equation (Equation 1), the fracture toughness value Kc1 in the direction parallel to the main surface of the substrate was obtained by setting a to (a1 / 2) and c to (c1 / 2), and the fracture toughness value Kc2 in the direction perpendicular to the main surface of the substrate was obtained by setting a to (a2 / 2) and c to (c2 / 2). Furthermore, the average value of Kc1 and Kc2 ((Kc1+Kc2) / 2) was 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 (Formula 1) In Equation 1, E is the modulus of elasticity, and is obtained from the ratio of bending stress σ to bending strain ε (E = σ / ε) obtained in a three-point bending test performed by applying a compressive load perpendicular to the main surface. P is the compressive load.

[0041] <Measurement of thermal conductivity> The thermal conductivity λ1 in the direction parallel to the main surface of the silicon nitride substrate and the thermal conductivity λ2 in the direction perpendicular to the main surface were measured using spot periodic heating radiation thermometry. Furthermore, the average value of λ1 and λ2 ((λ1+λ2) / 2) was calculated as the average thermal conductivity of the silicon nitride substrate.

[0042] The raw materials and abbreviations used in the test are as follows: <Silicon Nitride Powder> ·C1: Beta conversion rate 100%, average particle size 2.13μm, average aspect ratio 1.27 ·C2: Beta conversion rate 100%, average particle size 1.69μm, average aspect ratio 1.32 ·C3: Beta conversion rate 100%, average particle size 0.72μm, average aspect ratio 1.86 <Sintering aid> • S1: Yttrium oxide (specific surface area 20 m²) 2 / g) • S2: Magnesium silicon nitride (specific surface area 12 m²) 2 / g) <Binder> • B1: Acrylic resin emulsion (manufactured by Fujikura Chemical Co., Ltd.: Acrybase EMK-02)

[0043] <Example 1> A silicon nitride aqueous slurry was obtained by stirring and mixing 100 parts by mass of silicon nitride powder C1, 3 parts by mass of sintering aid S1, 3 parts by mass of sintering aid S2, 20 parts by mass of binder B1, and 50 parts by mass of water for 48 hours. The obtained silicon nitride aqueous slurry was placed in a container, and the container was subjected to a reduced pressure treatment process by reducing the pressure to -0.1 MPa·G and letting it stand at 30°C for 2.5 hours to obtain a slurry for green body molding. After removing coarse particles from the obtained slurry for green body molding by passing it through a polyethylene mesh (mesh opening 229 μm), a sheet molded body was prepared using the doctor blade method. The obtained sheet molded body was placed on a plate-shaped heater set to 50°C and left to stand for 3 hours to dry and obtain a sheet-like green body. The obtained green body was degreased at 500°C for 60 hours, and then fired at a maximum temperature of 1850°C for 48 hours under conditions of 1 MPa·G to obtain a silicon nitride substrate. Table 1 shows the silicon nitride powder used and the firing conditions, and Table 2 shows the evaluation results of the obtained silicon nitride substrate.

[0044] <Examples 2-3, Comparative Examples 1-2> A silicon nitride substrate was obtained and evaluated in the same manner as in Example 1, except that the silicon nitride powder and firing conditions were changed as shown in Table 1. 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 having silicon nitride particles, wherein the average angle between the main surface of the silicon nitride substrate and the major 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 equivalent circular diameter of the silicon nitride particles is greater than 3 μm.

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

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

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