Silicon nitride substrate and silicon nitriide substrate production method

Abstract

Provided are: a silicon nitride substrate produced by excellent lamination of green sheets; and a method for producing the same. The silicon nitride substrate according to one embodiment, when observed in cross section in a dark field of an optical microscope, includes a main layer and an intermediate layer that is darker than the main layer. The colour difference between the main layer and the intermediate layer is 0.1 or more. In the thickness direction, m+1 main layers and m intermediate layers are alternately arranged. The colour difference between the n-th (n is an integer) main layer and the n-th intermediate layer from one surface is defined as CDna. The colour difference between the (n+1)-th main layer and the n-th intermediate layer is defined as CDnb. A first condition and a second condition are satisfied with any value for n of 1 to m. The first condition is that, in a first direction parallel to the cross section and perpendicular to the thickness direction, a region where 0.1 ≤ CDna ≤ 1.4 is present over a range of 100 μm or more. The second condition is that, in the first direction, a region where 0.1 ≤ CDnb ≤ 1.4 is present over a range of 100 μm or more.

Description

Silicon nitride substrate and method for manufacturing silicon nitride substrate

[0001] The embodiments relate to a silicon nitride substrate and a method for manufacturing the silicon nitride substrate.

[0002] In recent years, semiconductor elements requiring large currents, such as power electronics and next-generation power semiconductors, have been developed. With the development of these semiconductor elements, the demand for ceramic substrates with high heat dissipation and electrical insulation properties is increasing year by year. In particular, the heat generated by semiconductor elements is increasing as they become smaller and more powerful. To ensure efficient heat dissipation, ceramic substrates tend to be thinner.

[0003] On the other hand, in order to reduce the manufacturing cost of ceramic substrates, larger ceramic substrates are being manufactured. Among ceramic substrates, silicon nitride substrates have high strength, high toughness, and high heat dissipation. Patent Document 1 discloses a silicon nitride substrate measuring 220 mm x 220 mm x 0.32 mm.

[0004] Furthermore, a method for obtaining a thick silicon nitride heat dissipating member by laminating materials has been disclosed (Patent Document 2). According to Patent Document 2, sintered bodies with a thickness of 0.03 to 1.0 mm per layer are laminated, resulting in a member with an overall thickness of 0.1 to 10 mm.

[0005] Japanese Patent No. 6399252 Japanese Patent Laid-Open No. 2000-72552

[0006] In the manufacture of silicon nitride substrates, green sheets are produced by sheet molding. Thin silicon nitride substrates are produced by sintering a single green sheet. One method for producing thick silicon nitride substrates is to stack thin green sheets. By stacking, a thick laminate consisting of multiple green sheets is produced. By degreasing and sintering this laminate, a thick silicon nitride substrate is produced. This method allows for the cost-effective production of large, thick silicon nitride substrates.

[0007] In the above method, when multiple large green sheets are laminated, it is preferable that the green sheets are sufficiently adhered to each other. If the adhesion between the green sheets is poor, the thermal conductivity or bending strength of the degreased and sintered silicon nitride substrate will decrease.

[0008] The embodiments of the present invention have been made based on this problem, and provide a silicon nitride substrate manufactured by properly stacking green sheets, and a method for manufacturing the same.

[0009] A silicon nitride substrate according to an embodiment includes a main layer and an intermediate layer that is darker than the main layer when a cross section of the silicon nitride substrate along the thickness direction is observed under a dark field of an optical microscope. The color difference between the main layer and the intermediate layer is 0.1 or more. m+1 main layers and m intermediate layers are alternately arranged in the thickness direction. The color difference between the nth main layer (n is an integer) from one surface of the silicon nitride substrate and the nth intermediate layer is defined as CDna. The color difference between the n+1th main layer and the nth intermediate layer is defined as CDnb. In this case, the first and second conditions are satisfied for any value of n between 1 and m. The first condition is that a region satisfying 0.1≦CDna≦1.4 exists in a continuous pattern of 100 μm or more in a first direction parallel to the cross section and perpendicular to the thickness direction. The second condition is that a region where 0.1≦CDnb≦1.4 exists in the first direction and extends for 100 μm or more.

[0010] A method for manufacturing a silicon nitride substrate according to an embodiment includes preparing a slurry, preparing a green sheet, preparing a laminate, preparing a degreased body, and sintering. In preparing the slurry, silicon nitride powder and sintering aid powder are mixed with an organic solvent and a binder. In preparing the green sheet, the slurry is molded. In preparing the laminate, multiple green sheets are stacked at a temperature of 60°C or higher and a pressure of 0.1 MPa or higher. In preparing the degreased body, the laminate is heat-treated at a temperature of 1000°C or lower. In sintering, the degreased body is heated at a temperature of 1600°C or higher and 2000°C or lower.

[0011] Fig. 1 is a perspective view showing an example of a silicon nitride substrate according to an embodiment. Fig. 2 is a cross-sectional view showing an example of a silicon nitride substrate according to an embodiment. Fig. 3 is an enlarged cross-sectional view showing an example of a silicon nitride substrate according to an embodiment. Fig. 4 is an enlarged cross-sectional view showing an example of a silicon nitride substrate according to an embodiment. Fig. 5 is a flowchart showing a method for manufacturing a silicon nitride substrate 1 according to an embodiment. Fig. 6 is a cross-sectional view showing an example of a green sheet lamination for a silicon nitride substrate according to an embodiment. Fig. 7 is a cross-sectional view showing an example of a green sheet lamination for a silicon nitride substrate according to an embodiment.

[0012] A silicon nitride substrate according to an embodiment includes a main layer and an intermediate layer that is darker than the main layer when a cross section of the silicon nitride substrate along the thickness direction is observed under a dark field of an optical microscope. The color difference between the main layer and the intermediate layer is 0.1 or more. m+1 main layers and m intermediate layers are alternately arranged in the thickness direction. The color difference between the nth main layer (n is an integer) from one surface of the silicon nitride substrate and the nth intermediate layer is defined as CDna. The color difference between the n+1th main layer and the nth intermediate layer is defined as CDnb. In this case, the first and second conditions are satisfied for any value of n between 1 and m. The first condition is that a region satisfying 0.1≦CDna≦1.4 exists in a continuous pattern of 100 μm or more in a first direction parallel to the cross section and perpendicular to the thickness direction. The second condition is that a region where 0.1≦CDnb≦1.4 exists in the first direction and extends for 100 μm or more.

[0013] FIG. 1 is a perspective view showing an example of a silicon nitride substrate according to an embodiment. FIG. 2 is a cross-sectional view showing an example of a silicon nitride substrate according to an embodiment. In FIGS. 1 and 2, 1 denotes a silicon nitride substrate, and 2 denotes a first surface (front surface, upper surface). In FIG. 2, 3 denotes a second surface (rear surface, lower surface) opposite the first surface 2. In the silicon nitride substrate according to an embodiment, the first surface 2 and the second surface 3 are substantially parallel. In the illustrated example, the shape of the first surface 2 in a plan view is a square. The shape of the first surface 2 in a plan view may be a polygon such as a rectangle, or a circle. Furthermore, a hole may be formed penetrating the silicon nitride substrate 1 from the first surface 2 to the second surface 3.

[0014] FIG. 3 is an enlarged view showing an example of portion A in FIG. 2 . FIG. 3 schematically shows a cross section of a silicon nitride substrate 1 along the thickness direction D, as observed under a dark field of an optical microscope. As shown in FIG. 3 , the silicon nitride substrate 1 includes a main layer 4 and an intermediate layer 5. The intermediate layer 5 is darker than the main layer 4. Specifically, in the dark field, the color difference between the main layer 4 and the intermediate layer 5 is 0.1 or more. In the silicon nitride substrate 1, m+1 main layers 4 and m intermediate layers 5 are alternately arranged in the thickness direction D. "m" is an integer greater than or equal to 1. In the illustrated example, m is "1." That is, the silicon nitride substrate 1 includes two main layers 4, with one intermediate layer 5 located between the main layers 4.

[0015] Here, CDna denotes the color difference between the nth (n is an integer) main layer 4 from the first surface 2 and the nth intermediate layer 5. CDnb denotes the color difference between the (n+1)th main layer 4 and the nth intermediate layer 5. In this case, the first and second conditions are satisfied regardless of whether n is 1 or greater or m or less. The first condition is that, in a first direction D1 parallel to the cross section and perpendicular to the thickness direction D, there exists a continuous region satisfying 0.1≦CDna≦1.4 for a length of 100 μm or more. The second condition is that, in the first direction D1, there exists a continuous region satisfying 0.1≦CDnb≦1.4 for a length of 100 μm or more.

[0016] For example, the color difference between the first main layer 4a and the first intermediate layer 5 from the first surface 2 is CD1a. The color difference between the second main layer 4b and the first intermediate layer 5 is CD1b. In the silicon nitride substrate 1, a region where 0.1≦CD1a≦1.4 exists between the first main layer 4a and the first intermediate layer 5, and extends for 100 μm or more in the first direction D1. Furthermore, a region where 0.1≦CDnb≦1.4 exists between the second main layer 4b and the first intermediate layer 5, and extends for 100 μm or more in the first direction D1.

[0017] The color difference measurement range is set to "thickness of silicon nitride substrate 1" x "1 mm." Five measurement ranges are set at random locations on the cross section of silicon nitride substrate 1. The measurement ranges are preferably set at positions sufficiently separated from one another. According to the embodiment, at all five locations, the color difference CDna satisfies the first condition, and the color difference CDnb satisfies the second condition.

[0018] As shown in FIG. 3 , regions r1, r2, and r3 may exist between adjacent main layers 4 in the thickness direction D. The color difference between region r1 and each adjacent main layer 4 is 0.1 or more and 1.4 or less. The color difference between region r2 and each adjacent main layer 4 is greater than 1.4. The color difference between region r3 and each adjacent main layer 4 is less than 0.1. Regions r1 and r2 are identified as intermediate layers 5 based on the relationship of the color difference between them and the main layers 4. In the silicon nitride substrate 1 according to the embodiment, region r1 exists in a continuous manner for 100 μm or more in the first direction D1.

[0019] "Color difference" is an index defined using the respective colors of two objects. The larger the color difference between two objects, the easier it is to distinguish between them. The smaller the color difference, the harder it is to distinguish between them. The calculation method for color difference is specified in JIS Z8730 (2002). JIS Z8730 (2002) corresponds to ISO 7724-3. In the calculation method specified in ISO 7724-3, L ＊ a ＊ Color difference ΔE according to the b color system ＊ ａｂ is calculated by the following formula: ＊ ａｂ = [(ΔL ＊ ) ２ + (Δa ＊ ) ２ + (Δb ＊ ) ２ ] １／２ ΔL ＊ : L ＊ a ＊ CIE lightness L of two object colors in the b color system ＊ Difference Δa ＊ : L ＊ a ＊ Color coordinates of two objects in the b color system＊ Difference Δb ＊ : L ＊ a ＊ Color coordinates of two objects in the b color system ＊ The difference

[0020] In addition to the calculation method specified in ISO 7724-3, color difference can also be calculated using the components of lightness, chroma, and hue. The difference in color difference between calculation methods is at most about 0.1%. Therefore, the calculated color difference is substantially the same regardless of the method.

[0021] It is difficult to directly measure the color difference in the cross section of the silicon nitride substrate 1 using a measuring device. This is because the size of the measuring section of the device is larger than the range to be measured. If the size of the measuring section is larger than the range to be measured, the color around the range to be measured is also measured. As a result, it is difficult to measure the color difference accurately. Therefore, when measuring the color difference in the cross section of the silicon nitride substrate 1, a photograph of the cross section is used. The cross section is photographed in a dark field. An industrial microscope STM7 manufactured by Olympus Corporation can be used as the device to photograph the cross section. Automatically set conditions may be used for the photograph. By enlarging the dark field image, the range in which the color difference is measured is made larger than the size of the measuring section. The enlarged image is used to measure the color difference at each location on the cross section. Any measuring device can be selected as long as it can measure the color difference at small locations. For example, a spectrophotometer CM-2600d by Konica Minolta or its successor, the CM-26d, can be used as the measuring device. These spectrophotometers can measure the ΔL between two objects. ＊ , Δa ＊ , and Δb ＊ The color difference ΔE can be measured. ＊ ａｂ The values ​​are calculated by rounding off the first decimal place. The spectrophotometer CM-2600d or CM-26d measures color difference in accordance with ISO 7724-3.

[0022] The cut surface of the silicon nitride substrate 1 preferably passes through the center in the in-plane direction of the silicon nitride substrate 1. The in-plane direction is a direction perpendicular to the thickness direction D. For example, if the silicon nitride substrate 1 is rectangular, the silicon nitride substrate 1 is cut along the center line in the long side direction or the center line in the short side direction. If the silicon nitride substrate 1 is circular, the silicon nitride substrate 1 is cut along a line passing through the center of the circle.

[0023] In the cross section, the main layer 4 and the intermediate layer 5 are identified by the following procedure. First, the main layer 4 is present near the first surface 2 or the second surface 3 of the silicon nitride substrate 1. In the cross section, a relatively white region that is present near the first surface 2 is identified as the main layer 4. Next, the color difference between the main layer 4 and the region below it is measured. The photograph of the cross section is preferably enlarged so that the color difference measurement range is 1.0 mm or less. The photograph may be enlarged further depending on the thickness of the main layer 4. As a result of the color difference measurement, a region that is visually darker than the main layer 4 and has a color difference of 0.1 or more relative to the main layer 4 is identified as the intermediate layer 5. Then, the color difference between the intermediate layer 5 and the region below it is measured. As a result of the color difference measurement, a region that is visually lighter than the intermediate layer 5 and has a color difference of 0.1 or more relative to the intermediate layer 5 is identified as the main layer 4. When the silicon nitride substrate 1 includes three or more main layers 4 and two or more intermediate layers 5, each main layer 4 and each intermediate layer 5 can be identified by repeating the same procedure.

[0024] Fig. 4 is an enlarged view showing another example of portion A in Fig. 2. Fig. 3 shows an example in which the silicon nitride substrate 1 includes two main layers 4. As shown in Fig. 4, the silicon nitride substrate 1 according to the embodiment may include three or more main layers 4. In the example shown in Fig. 4, the silicon nitride substrate 1 includes, in cross section, five main layers 4a to 4e and four intermediate layers 5a to 5d. That is, m is "4." In this case, the first condition is satisfied for each of the color differences CD1a to CD4a, and the second condition is satisfied for each of the color differences CD1b to CD4b.

[0025] If the color difference CDna or CDnb exceeds 1.4, the adhesion between the main layers 4 may be insufficient. Insufficient adhesion can cause pores to form between the main layers 4. For example, even if no pores are observed in the cross section, pores may have formed in areas other than the cross section. If large pores are present, delamination, a type of peeling between the green sheets, occurs partially or entirely. When delamination occurs, the peeled portion becomes the starting point and fractures tend to progress. As a result, the bending strength (mechanical properties) of the silicon nitride substrate 1 decreases. Furthermore, heat transfer is hindered in the peeled portion. This reduces the thermal conductivity (heat dissipation properties) of the silicon nitride substrate 1.

[0026] The silicon nitride substrate 1 is manufactured by stacking and sintering multiple green sheets made of silicon nitride. The green sheets are manufactured, for example, by sheet molding. In sheet molding, liquid silicon nitride raw material is continuously applied to a sheet film and then dried. In this case, thickness variations and gentle irregularities occur on the surface of the green sheet. Furthermore, when sheet molding is performed in the atmosphere, the surface of the green sheet contains less solvent and is drier than the interior of the green sheet. The irregularities and drying reduce the adhesion between the green sheets. For this reason, it is preferable to apply sufficient pressure to the green sheets when stacking multiple green sheets.

[0027] There are small spaces between the stacked green sheets. Spaces are scattered between the green sheets, and air exists in these spaces. The stacked green sheets are sintered in a nitrogen atmosphere. When the green sheets are sintered with scattered air, the green sheets react with the oxygen contained in the air. The green sheets are locally sintered in an oxidizing atmosphere. The areas sintered in the oxidizing atmosphere are colored. The color of the areas sintered in the oxidizing atmosphere is slightly darker than the color of the areas sintered in the nitrogen atmosphere. As a result, in a cross section, the areas sintered in the nitrogen atmosphere are observed as a relatively light main layer 4. The areas colored by the reaction with oxygen are observed as a relatively dark intermediate layer 5. In other words, the area away from the surface of the green sheets before sintering corresponds to the main layer 4 after sintering. The area located near the surface of the green sheets before sintering corresponds to the intermediate layer 5 after sintering.

[0028] As described above, when color difference is large and defects due to voids or delamination occur, thermal conduction at the defective area decreases. As a result, the heat dissipation performance of the silicon nitride substrate decreases. Silicon nitride substrates are used in silicon nitride circuit boards with metal circuits or semiconductor modules with semiconductor elements mounted on the surface of the metal circuits. If the heat dissipation performance of the silicon nitride substrate decreases, the heat dissipation performance of the silicon nitride circuit board or semiconductor module also decreases. If the semiconductor element operates in a state where heat dissipation performance is reduced, the semiconductor element may be destroyed by the generated heat. Furthermore, when color difference is large and defects due to voids or delamination occur, the strength of the silicon nitride substrate may decrease. If the strength of the silicon nitride substrate decreases, peeling may occur inside the silicon nitride substrate due to thermal cycling, etc. For example, the top surface of the silicon nitride substrate and the metal circuit bonded thereto may peel off from the rest of the silicon nitride substrate.

[0029] According to the silicon nitride substrate 1 of the embodiment, the first and second conditions are satisfied. The first condition is that a region where 0.1≦CDna≦1.4 exists in a continuous pattern extending over 100 μm or more in the first direction D1. The second condition is that a region where 0.1≦CDnb≦1.4 exists in a continuous pattern extending over 100 μm or more in the first direction D1. The presence of a color difference between the main layer 4 and the intermediate layer 5 indicates that there is a region in the intermediate layer 5 that has been discolored due to oxidation. The greater the degree of oxidation, the more severe the discoloration of that region and the greater the color difference. A color difference of 0.1 or more and 1.4 or less indicates that discoloration in the intermediate layer 5 is suppressed and oxidation of the intermediate layer 5 is suppressed. Oxidation of the intermediate layer 5 leads to the occurrence of voids or delamination. A state in which the first and second conditions are satisfied indicates a state in which the occurrence of voids or delamination is suppressed.

[0030] For example, if the first and second conditions are not met, the length of the region where the color difference is 1.4 or less is less than 100 μm. In other words, if the region with a large color difference is expanding, it means that the oxidized region is expanding. This means that the green sheets are not sufficiently adhered to each other, and voids or delamination defects are likely to occur.

[0031] Furthermore, in the embodiment, when the silicon nitride substrate 1 includes m+1 main layers 4 and m intermediate layers 5, the first and second conditions are satisfied in any range of 1≦n≦m. That is, oxidation is suppressed in all intermediate layers 5 present in the silicon nitride substrate 1. This more reliably suppresses the occurrence of voids or delamination. This can suppress the reduction in mechanical strength and heat dissipation due to the occurrence of voids or delamination.

[0032] It is most desirable that the color difference be zero. However, to achieve zero color difference, the green sheets must be tightly attached to each other with a large pressure for a long time. This reduces the mass productivity of the silicon nitride substrate 1 and increases manufacturing costs. Therefore, color differences may occur between the main layer 4 and the intermediate layer 5 according to the manufacturing process for mass-produced products.

[0033] A smaller color difference between the main layer 4 and the intermediate layer 5 indicates a smaller degree of oxidation in the intermediate layer 5. Therefore, more preferably, regions satisfying 0.1≦CDna≦1.2 are continuous for 100 μm or more in the first direction D1, and regions satisfying 0.1≦CDnb≦1.2 are continuous for 100 μm or more in the first direction D1. Most preferably, regions satisfying 0.1≦CDna≦1.0 are continuous for 100 μm or more in the first direction D1, and regions satisfying 0.1≦CDnb≦1.0 are continuous for 100 μm or more in the first direction D1.

[0034] Furthermore, the wider the region where the color difference is 0.1 or more and 1.4 or less, the better the adhesion between the green sheets. Therefore, more preferably, the region where 0.1≦CDna≦1.4 exists over a distance of 200 μm or more in the first direction D1, and the region where 0.1≦CDnb≦1.4 exists over a distance of 200 μm or more in the first direction D1. Most preferably, the region where 0.1≦CDna≦1.4 exists over a distance of 400 μm or more in the first direction D1, and the region where 0.1≦CDnb≦1.4 exists over a distance of 400 μm or more in the first direction D1.

[0035] The number of main layers 4 is not particularly limited. However, as the number of main layers 4 increases, the number of intermediate layers 5 also increases. In other words, the proportion of oxidized regions in the silicon nitride substrate 1 increases. As a result, there is a possibility that the thermal conductivity or mechanical strength of the silicon nitride substrate 1 will decrease. Therefore, the number of main layers 4 is preferably 2 or more and 5 or less.

[0036] The thermal conductivity of the silicon nitride substrate 1 is preferably 50 W / (m·K) or more. According to the embodiment, defects such as voids or delamination are suppressed, so the thermal conductivity of the silicon nitride substrate 1 can be increased. The thermal conductivity of the silicon nitride substrate 1 is more preferably 60 W / (m·K) or more. The thermal conductivity is measured by a laser flash method.

[0037] The three-point bending strength of the silicon nitride substrate 1 is preferably 600 MPa or more. According to the embodiment, defects such as voids or delamination are suppressed, so that the three-point bending strength of the silicon nitride substrate 1 can be further increased. The three-point bending strength of the silicon nitride substrate 1 is preferably 650 MPa or more. The three-point bending strength is measured in accordance with ISO 23242 (2020).

[0038] 5 is a flowchart showing a method for manufacturing a silicon nitride substrate 1 according to an embodiment. The method for manufacturing a silicon nitride substrate 1 according to an embodiment will be described. As shown in FIG. 5, the method for manufacturing a silicon nitride substrate according to an embodiment mainly includes preparing a slurry (step S1), preparing a green sheet (step S2), preparing a laminate (step S3), preparing a degreased body (step S4), and sintering (step S5).

[0039] In preparing the slurry, silicon nitride powder and sintering aid powder are mixed with an organic solvent and a binder. In preparing the green sheet, the slurry is molded. In preparing the laminate, multiple green sheets are stacked at a temperature of 60°C or higher and a pressure of 0.1 MPa or higher. In preparing the degreased body, the laminate is heat-treated at a temperature of 1000°C or lower. In sintering, the degreased body is heat-treated at a temperature of 1600°C or higher and 2000°C or lower. Each step will be explained in detail below.

[0040] First, a slurry is prepared by mixing silicon nitride powder and sintering aid powder with an organic solvent and a binder. The alpha conversion rate of the silicon nitride powder is preferably 80% by mass or more. The average particle size is preferably 0.4 μm or more and 2.5 μm or less. The impurity oxygen content is preferably 2% by mass or less. The impurity oxygen content is preferably 1.0% by mass or less, and more preferably 0.1% by mass or more and 0.8% by mass or less. If the impurity oxygen content exceeds 2% by mass, the thermal conductivity of the silicon nitride substrate 1 produced may decrease.

[0041] The sintering aid powder is preferably a metal oxide powder having an average particle size of 0.5 μm to 3.0 μm. The metal oxide powder is an oxide of an element selected from rare earth elements, magnesium, titanium, and hafnium. Adding a metal oxide sintering aid facilitates the formation of a liquid phase component during the sintering process. For example, one or more elements selected from rare earth elements, magnesium, titanium, and hafnium are added as the sintering aid in a total amount of 1% by mass to 20% by mass in terms of oxide. More preferably, the sintering aid is added in a total amount of 1% by mass to 20% by mass in terms of oxide. Adding two or more oxides is particularly preferred. Adding two or more oxides facilitates the formation of two or more glass compound phases with different compositions. More preferably, an oxide of a rare earth element is used as an essential component, and an oxide of one or more elements selected from magnesium, hafnium, and titanium is added.

[0042] Silicon nitride powder and sintering aid powder are crushed and mixed in an organic solvent such as toluene, ethanol, or butanol. A binder is then added to create a slurry. Examples of binders include butyl methacrylate, polyvinyl butyral, and polymethyl methacrylate.

[0043] Next, the raw material slurry is molded to produce a green sheet. For sheet molding, known methods such as the doctor blade method and extrusion molding can be used. In particular, for sheet molding, the doctor blade method is preferably used. The doctor blade method allows for the production of thin and large green sheets, improving mass productivity.

[0044] Next, multiple green sheets are stacked and sandwiched between clean paper. A pressure of 0.1 MPa or more is applied to the multiple green sheets through the clean paper at a temperature of 60°C or more. This causes the multiple green sheets to adhere to each other, producing a laminate.

[0045] FIG. 6 is a cross-sectional view showing an example of a method for producing a laminate. FIG. 6 shows a flat press lamination method 10 in which a laminate is produced by flat press. In the flat press lamination method 10, a pair of resin sheets 12, a pair of clean papers 13, and a plurality of silicon nitride green sheets 14 are arranged between a pair of flat presses 11. The flat presses 11 are equipped with a heating section. The pair of clean papers 13 and the plurality of green sheets 14 are arranged between the resin sheets 12. The plurality of green sheets 14 are arranged between the clean papers 13.

[0046] When using the flat press 11, the green sheet 14 is cut to a predetermined size before the laminate is produced. After cutting, a predetermined number of green sheets 14 are stacked. The resin sheet 12 is placed to improve the releasability of the flat press 11 and the clean paper 13 after lamination. The resin sheet 12 is made of a resin material such as polyethylene (PE), polyethylene terephthalate (PET), or polyvinyl chloride.

[0047] The clean paper 13 is a type of paper that is generally used in clean rooms and generates little dust. The clean paper 13 is made using long fibers instead of short fibers. The long fibers are bonded together to prevent dust generation. The use of the clean paper 13 can prevent the fibers from becoming green sheets. Clean paper 13 using conductive fibers may also be used. The use of conductive clean paper 13 can suppress the generation of static electricity. This can prevent dust from being attracted to the clean paper 13 due to static electricity.

[0048] The clean paper 13 has the strength and flexibility of ordinary paper. When the stacked green sheets 14 are directly pressed from both sides by the flat press 11, uneven pressure can occur. The clean paper 13 deforms along the unevenness of the surface of the green sheets 14. The stacked green sheets 14 are placed between a pair of clean papers 13. By pressing the green sheets 14 through the clean paper 13, the pressure is dispersed across the clean paper 13. This allows pressure to be applied more uniformly across the entire surface of the green sheets 14. For example, depending on the state of the flat press 11, the center and edges of the green sheets 14 may not be pressed uniformly, which could lead to delamination. By using the clean paper 13, pressure can be applied uniformly to the green sheets 14, preventing delamination.

[0049] The thickness of the clean paper 13 is generally 0.08 to 0.15 mm. When the clean paper 13 is thin, deformation when pressure is applied is small. Pressure is more likely to be applied more directly to the green sheet 14. For this reason, thin clean paper 13 is suitable for green sheets 14 with few irregularities on the surface. When the clean paper 13 is thick, deformation when pressure is applied is large. Pressure is more likely to be applied to the green sheet 14 in a more dispersed manner. For this reason, thick clean paper 13 is suitable for green sheets 14 with many irregularities on the surface.

[0050] Figure 7 is a cross-sectional view showing another example of a method for producing a laminate. Figure 7 shows a double-sided roll lamination method 15 for producing a roller laminate. In the double-sided roll lamination method 15, a pair of rollers 16 having heating units is used. A plurality of green sheets 14 made of silicon nitride are arranged between the rollers 16 with clean paper 13 interposed therebetween. If necessary, a resin sheet 12 is arranged between the roller 16 and the clean paper 13. By arranging the resin sheet 12, the roller 16 and the clean paper 13 can be easily peeled off.

[0051] Resin sheets 12, clean paper 13, and multiple green sheets 14 are continuously fed between the pair of rollers 16. For this reason, resin sheets 12 and clean paper 13 corresponding to the length of the green sheets 14 are prepared in advance. A laminate 17 in which multiple green sheets 14 are stacked is discharged from between the pair of rollers 16. With the double-sided roll, there is no length restriction in the direction of movement of the green sheets 14. For this reason, the method using rollers 16 is excellent for mass production. If the width of rollers 16 is smaller than the width of the green sheets, the green sheets are cut to the specified width.

[0052] The plurality of green sheets 14 are partially pressed as they are pushed out by the rollers 16. Therefore, the method using the rollers 16 provides more uniform pressure than the method using the flat press 11. Furthermore, even in the method using the rollers 16, there is a possibility that a difference in pressure may occur between the center and the ends of the rollers depending on the state of the rollers 16. For this reason, it is preferable to use clean paper 13, as in the method using the flat press 11. By sandwiching the clean paper 13 between the green sheets 14 and the rollers 16, the clean paper 13 deforms according to the unevenness of the surface of the green sheets 14 or the state of the rollers 16. By using the clean paper 13, pressure can be applied uniformly to the green sheets 14, thereby suppressing the occurrence of delamination.

[0053] The temperature used to fabricate the laminate is preferably set to 60°C or higher. This "temperature" refers to the temperature of the upper and lower flat presses 11 or the temperature of the upper and lower rollers 16. Heating bonds the green sheets 14 together. Further pressure ensures good adhesion between the green sheets 14. If the temperature is below 60°C, the green sheets 14 are likely to remain between the green sheets 14 in the resulting laminate. As a result, air tends to oxidize the green sheets 14, increasing the color difference between the main layer 4 and the intermediate layer 5. This increases the likelihood of defects such as voids or delamination. The higher the temperature, the shorter the bonding time. When the green sheets 14 are thick or when a large number of green sheets 14 are stacked, the heat is less likely to be transferred to the interior, so heating at a higher temperature is preferred. On the other hand, if the temperature is too high, the surfaces of the green sheets 14 tend to dry out. Therefore, a temperature of 140°C or lower is preferred.

[0054] The pressure applied when producing the laminate is preferably 0.1 MPa or more. This "pressure" refers to the pressure applied by the upper and lower flat presses 11 or the pressure applied by the upper and lower rollers 16. If the pressure is less than 0.1 MPa, the green sheets 14 will not be sufficiently bonded together, and defects such as voids or delamination will likely occur. If the pressure is high, bonding will be possible in a short time. On the other hand, if the pressure is too high, there is a possibility that the green sheets 14 will break in the thickness direction. For this reason, the pressure is preferably 0.5 MPa or less.

[0055] Next, the laminate is degreased at a temperature of 1000°C or less to produce a degreased body. The atmosphere used in the degreasing process is preferably an inert atmosphere such as nitrogen gas or argon gas. Oxygen may be added to the inert atmosphere to create an oxygen-containing atmosphere. The degreasing temperature is preferably 1000°C or less, and more preferably in the range of 500°C to 800°C. Performing the degreasing process within this temperature range allows for control of the thermal decomposition rate of the binder. This prevents the degreased body from being destroyed by the release of pyrolysis gases associated with degreasing. If the degreasing temperature exceeds 1000°C, the polymer binder is rapidly burned away, making the degreased body prone to damage. On the other hand, if the degreasing temperature is less than 500°C, the thermal decomposition of the binder may be insufficient, potentially making it impossible to densify the sintered body. For this reason, it is preferable to thermally decompose organic materials such as binders at a temperature of 1000°C or less, more preferably 500°C to 800°C.

[0056] Next, the degreased body is sintered at a temperature of 1600°C or higher and 2000°C or lower. The pressure in the sintering furnace is preferably a pressurized atmosphere. If the sintering temperature is lower than 1600°C, the sintered body will not be sufficiently densified. If the sintering temperature exceeds 2000°C and the atmospheric pressure in the furnace is low, the silicon nitride contained in the sintered body may decompose into silicon (Si) and nitrogen (N). For this reason, the sintering temperature is preferably within the range of 1700°C or higher and 1900°C or lower. Within this temperature range, the sintering aid and impurity oxygen produce multiple glass compounds with different compositions. As a result, liquid phase sintering of silicon nitride is more likely to proceed.

[0057] (Examples 1-8, Comparative Examples 1-8) Silicon nitride powder with an alpha content of 95% and an average particle size of 0.8 μm was prepared. Sintering aids were prepared: oxides of rare earth elements, titanium oxide, hafnium oxide, and magnesium oxide. 10% by mass of sintering aid, calculated as oxide, was mixed with the silicon nitride powder. The silicon nitride powder and sintering aid were mixed with an organic solvent and a binder to prepare a slurry. The prepared slurry was molded using a sheet molding machine to produce large silicon nitride green sheets. Large green sheets were cut with a cutter to measure 200 mm x 200 mm. Multiple cut green sheets were stacked. Multiple green sheets were sandwiched between polyethylene terephthalate sheets and heated and pressed using a flat press or double-sided roll. This bonded the multiple green sheets to produce a laminate.

[0058] Table 1 shows the conditions for producing the laminate. In Table 1, "substrate thickness" refers to the thickness of the silicon nitride substrate after final sintering. "Number of layers" refers to the number of green sheets stacked together. In Examples 1 to 8, clean paper was placed on the top and bottom surfaces of the stacked green sheets. The thickness of the clean paper was 1 mm. In Comparative Examples 1 to 8, no clean paper was placed. "Lamination method" refers to the method for producing the laminate. "Plane" refers to the method using a planar press as shown in Figure 6. "Roll" refers to the method using a roller as shown in Figure 7. "Temperature" refers to the temperature of the planar press or roller when producing the laminate. "Pressure" refers to the pressure applied to the green sheets from the planar press or roller.

[0059]

[0060] Next, the laminate was degreased in a nitrogen atmosphere at a temperature of 500°C to 800°C. The degreased body was then sintered in a pressurized nitrogen atmosphere at a temperature of 1600°C to 2000°C. The resulting sintered body was cut in the thickness direction, passing through the center in a plan view. The cross section was observed in a dark field using an Olympus STM7 industrial microscope, and enlarged photographs were taken. Next, the main layer and intermediate layer in the cross section were identified using a spectrophotometer (Konica Minolta CM-2600d). The color difference between the main layer and intermediate layer was measured sequentially along the first direction D1. The measurement range in the first direction D1 was set to "thickness of the ceramic substrate" x "1 mm." Five measurement ranges, each 1 mm apart, were selected. From the color difference measurement results, the length of the continuous region in the first direction D1 was measured for each color difference. In each example, the smallest color difference was extracted from among the color differences that had a continuous region of 100 μm or more. The measurement results are shown in Table 2. For example, in Example 1, in all measurement ranges, regions in which the color difference CD1a was 1.1 were continuous for 100 μm or more in the first direction D1. This indicates that in at least one measurement range, the length of the region in which the color difference CD1a was 1.0 was less than 100 μm.

[0061]

[0062] As can be seen from Table 2, it was confirmed that the color difference CDna and color difference CDnb of the silicon nitride substrate according to the example satisfied the first and second conditions. This is because the lamination conditions for the green sheets were appropriate. In contrast, the silicon nitride substrate of the comparative example had a wide area of ​​large color difference, and did not satisfy the first and second conditions. This is because the lamination conditions for the green sheets were not appropriate and the green sheets were not sufficiently adhered to each other. As a result, the intermediate layer discolored, and the color difference was measured to be larger than that of the example.

[0063] Next, the silicon nitride substrates according to the examples and comparative examples were processed to 40 mm x 20 mm to prepare test specimens. The bending strength was measured using the test specimens. The test specimens were processed in accordance with JIS R1607 (2015). The bending strength was measured in accordance with ISO 23242 (2020) by three-point bending with a span of 30 mm. Next, the silicon nitride substrates according to the examples and comparative examples were processed to a diameter of 10 mm to measure the thermal conductivity. The measurement results of bending strength and thermal conductivity are shown in Table 3.

[0064]

[0065] As can be seen from Table 3, the bending strength and thermal conductivity of the silicon nitride substrates according to the examples were within the preferred ranges. This is because the lamination conditions for the green sheets were appropriate, which prevented the occurrence of voids and delamination between layers. In contrast, the bending strength and thermal conductivity of the silicon nitride substrates of the comparative examples were outside the preferred ranges. This is because the lamination of the green sheets was insufficient, causing voids and delamination in the intermediate layers, which resulted in a decrease in bending strength and a decrease in thermal conductivity.

[0066] The present invention includes the following features: (Feature 1) A silicon nitride substrate, wherein, when a cross section of the silicon nitride substrate along a thickness direction is observed under a dark field of an optical microscope, the silicon nitride substrate includes main layers and intermediate layers that are darker than the main layers, the color difference between the main layers and the intermediate layers is 0.1 or more, m+1 main layers and m intermediate layers are arranged alternately in the thickness direction, and when the color difference between the nth main layer (n is an integer) from one surface of the silicon nitride substrate and the nth intermediate layer is CDna and the color difference between the n+1th main layer and the nth intermediate layer is CDnb, the silicon nitride substrate satisfies a first condition and a second condition for any value of n between 1 and m, and the first condition is that a region with 0.1≦CDna≦1.4 exists continuously for 100 μm or more in a first direction parallel to the cross section and perpendicular to the thickness direction. A silicon nitride substrate, wherein the second condition is that a region satisfying 0.1≦CDnb≦1.4 exists in the first direction, the region being continuous for 100 μm or more. (Feature 2) The silicon nitride substrate according to Feature 1, wherein the number of main layers is 2 or more and 5 or less. (Feature 3) The silicon nitride substrate according to Feature 1 or 2, wherein the thickness is 0.3 mm or more and 2.0 mm or less. (Feature 4) The silicon nitride substrate according to any one of Features 1 to 3, wherein the bending strength is 600 MPa or more. (Feature 5) The silicon nitride substrate according to any one of Features 1 to 4, wherein the thermal conductivity is 50 W / (m·K) or more. (Feature 6) A method for manufacturing a silicon nitride substrate, comprising mixing silicon nitride powder and sintering aid powder with an organic solvent and a binder to prepare a slurry, molding the slurry to prepare green sheets, stacking a plurality of the green sheets at a temperature of 60° C. or higher under a pressure of 0.1 MPa or higher to prepare a laminate, heat-treating the laminate at a temperature of 1000° C. or lower to prepare a degreased body, and sintering the degreased body at a temperature of 1600° C. or higher and 2000° C. (Feature 7) The method for manufacturing a silicon nitride substrate according to Feature 6, wherein the plurality of green sheets are stacked with a pair of rollers in preparing the laminate.(Feature 8) The method for producing a silicon nitride substrate according to Feature 6 or 7, wherein in producing the laminate, the plurality of green sheets are placed between a pair of clean papers, and the plurality of green sheets are heated and pressed via the pair of clean papers.

[0067] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

[0068] DESCRIPTION OF SYMBOLS 1...Silicon nitride substrate 2...First surface 3...Second surface 4, 4a to 4e...Main layer 5, 5a to 5d...Intermediate layer 10...Flat press lamination 11...Flat press 12...Resin sheet 13...Clean paper 14...Green sheet 15...Double-sided roll lamination 16...Double-sided roll 17...Green sheet laminate

Claims

1. A silicon nitride substrate, comprising: a silicon nitride substrate, when a cross section of the silicon nitride substrate along a thickness direction is observed under a dark field of an optical microscope, the silicon nitride substrate includes a main layer and an intermediate layer darker than the main layer, the color difference between the main layer and the intermediate layer is 0.1 or more, m+1 main layers and m intermediate layers are alternately arranged in the thickness direction, the color difference between the nth main layer (n is an integer) from one side of the silicon nitride substrate and the nth intermediate layer is CDna, and the color difference between the n+1th main layer and the nth intermediate layer is CDnb, the silicon nitride substrate satisfies a first condition and a second condition for any value of n between 1 and m, the first condition being that a region of 0.1≦CDna≦1.4 exists continuously for 100 μm or more in a first direction parallel to the cross section and perpendicular to the thickness direction, The silicon nitride substrate, wherein the second condition is that a region satisfying 0.1≦CDnb≦1.4 exists in the first direction and extends for 100 μm or more.

2. The silicon nitride substrate according to claim 1, wherein the number of said main layers is 2 or more and 5 or less.

3. The silicon nitride substrate according to claim 1 or 2, having a thickness of 0.3 mm or more and 2.0 mm or less.

4. The silicon nitride substrate according to claim 1 or 2, having a bending strength of 600 MPa or more.

5. The silicon nitride substrate according to claim 1 or 2, having a thermal conductivity of 50 W / (m·K) or more.

6. A method for manufacturing a silicon nitride substrate, comprising mixing silicon nitride powder and sintering aid powder with an organic solvent and a binder to prepare a slurry, molding the slurry to prepare a green sheet, stacking a plurality of the green sheets at a temperature of 60°C or higher and a pressure of 0.1 MPa or more to prepare a laminate, heat-treating the laminate at a temperature of 1000°C or less to prepare a degreased body, and sintering the degreased body at a temperature of 1600°C or more and 2000°C or less.

7. The method for producing a silicon nitride substrate according to claim 6, wherein in producing said laminate, said plurality of green sheets are superimposed by a pair of rollers.

8. A method for producing a silicon nitride substrate as described in claim 6 or 7, wherein in producing the laminate, the plurality of green sheets are placed between a pair of clean papers, and the plurality of green sheets are heated and pressed via the pair of clean papers.

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