Silicon nitride substrate and method for manufacturing silicon nitride substrate

Abstract

The present invention relates to a silicon nitride substrate manufactured by well laminating green sheets, and a method for manufacturing the silicon nitride substrate. A silicon nitride substrate according to an embodiment includes a main layer and an intermediate layer darker than the main layer when a cross-section is observed in a dark field of view of an optical microscope. The color difference between the main layer and the intermediate layer is 0.1 or more. And the m + 1 main layers and the m middle layers are alternately arranged in the thickness direction. The color difference between the nth (n is an integer) main layer and the nth intermediate layer from the one surface is set as CDna. The color difference between the (n + 1) th main layer and the nth intermediate layer is set as CDnb. In this case, a first condition and a second condition are satisfied when n is an arbitrary value from 1 to m (inclusive). The first condition is that in a first direction parallel to the cross-section and perpendicular to the thickness direction, a region of 0.1 < = CDna < = 1.4 is present at 100 [mu] m or more contiguously. The second condition is that a region having 0.1 < = CDnb < = 1.4 in the first direction is present at 100 [mu] m or more contiguously.

Description

TECHNICAL FIELD

[0001] Embodiments relate to a silicon nitride substrate and a manufacturing method of a silicon nitride substrate. BACKGROUND

[0002] In recent years, semiconductor elements that require large current, such as power electronic devices and next-generation power semiconductors, have been developed. With the development of these semiconductor elements, the demand for ceramic substrates having high heat dissipation and high electrical insulation has been increasing year by year. In particular, with the miniaturization and high performance of semiconductor elements, the heat generation of semiconductor elements has increased. In order to efficiently dissipate heat, the thickness of the ceramic substrate tends to be thinner.

[0003] On the other hand, larger ceramic substrates have been manufactured in order to reduce the manufacturing cost of ceramic substrates. Among ceramic substrates, silicon nitride substrates have high strength, high toughness, and high heat dissipation. A silicon nitride substrate having a size of 220 mm x 220 mm x 0.32 mm is disclosed in Patent Literature 1.

[0004] In addition, with respect to a heat dissipation member of silicon nitride, a method of obtaining a thicker heat dissipation member by laminating materials is disclosed (Patent Literature 2). According to Patent Literature 2, a sintered body having a thickness of 0.03 to 1.0 mm is laminated. Thereby, a member having a total thickness of 0.1 to 10 mm is obtained.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent No. 6399252 Patent Literature 2: Japanese Patent Application Laid-Open No. 2000-72552 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION In the manufacturing of a silicon nitride substrate, a green sheet is produced by sheet molding. A thinner silicon nitride substrate is manufactured by sintering a single green sheet. As one of the methods of manufacturing a thicker silicon nitride substrate, there is a method of laminating thinner green sheets. A thicker laminate composed of a plurality of green sheets is produced by lamination. A thicker silicon nitride substrate is manufactured by performing debinding sintering on the laminate. According to this method, a silicon nitride substrate that is cost-effective, larger, and thicker can be manufactured.

[0007] In the above method, in the case where a plurality of larger green sheets are laminated, it is preferable that the green sheets are sufficiently tightly bonded to each other. In the case where the tightness of the green sheets to each other is low, there is a problem that the thermal conductivity or the bending strength of the silicon nitride substrate after debinding and sintering is reduced.

[0008] Embodiments of the present application are completed based on this problem, and provide a silicon nitride substrate manufactured by good lamination of green sheets and a manufacturing method thereof.

[0009] Means for solving the problem The silicon nitride substrate of the embodiment includes a main layer and an intermediate layer darker than the main layer when a cross section of the silicon nitride substrate along a thickness direction is observed in a dark field of an optical microscope. A color 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. A color difference between an n-th (n is an integer) main layer and an n-th intermediate layer from one face of the silicon nitride substrate is set as CDna. A color difference between an (n+1)-th main layer and the n-th intermediate layer is set as CDnb. In this case, a first condition and a second condition are satisfied when n is any value of 1 or more and m or less. The first condition is that a region where 0.1≤CDna≤1.4 is continuously present for 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 is continuously present for 100 μm or more in the first direction.

[0010] The manufacturing method of the silicon nitride substrate of the embodiment includes production of a slurry, production of a green sheet, production of a laminate, production of a debound body, and sintering. In the production of the slurry, a binder and an organic solvent are mixed in a silicon nitride powder and a sintering aid powder. In the production of the green sheet, the slurry is molded. In the production of the laminate, a plurality of the green sheets are laminated at a temperature of 60°C or more and a pressure of 0.1 MPa or more. In the production of the debound body, the laminate is heat-treated at a temperature of 1000°C or less. In the sintering, the debound body is heated at a temperature of 1600°C or more and 2000°C or less. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 is a perspective view showing an example of the silicon nitride substrate of the embodiment.

[0012] Figure 2 is a cross-sectional view showing an example of the silicon nitride substrate of the embodiment.

[0013] Figure 3 is an enlarged view of a cross section showing an example of the silicon nitride substrate of the embodiment.

[0014] Figure 4 is an enlarged view of a cross section showing an example of the silicon nitride substrate of the embodiment.

[0015] Figure 5 is a flowchart showing a manufacturing method of the silicon nitride substrate 1 of the embodiment.

[0016] Figure 6 is a cross-sectional view showing an example of lamination of green sheets of the silicon nitride substrate of the embodiment.

[0017] Figure 7 is a cross-sectional view showing an example of a green sheet layer stack of a silicon nitride substrate of an embodiment. DETAILED DESCRIPTION

[0018] The silicon nitride substrate of the embodiment is one in which, when a cross section of the silicon nitride substrate along a thickness direction is observed in a dark field of an optical microscope, the silicon nitride substrate includes a main layer and an intermediate layer that is darker than the main layer. A color 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 arranged alternately. A color difference between an nth (n is an integer) main layer and an nth intermediate layer from one face of the silicon nitride substrate is denoted as CDna. A color difference between an (n+1)th main layer and the nth intermediate layer is denoted as CDnb. In this case, when n is any value of 1 or more and m or less, both a first condition and a second condition are satisfied. The first condition is that, in a first direction parallel to the cross section and perpendicular to the thickness direction, a region in which 0.1≤CDna≤1.4 exists continuously for 100 μm or more. The second condition is that, in the first direction, a region in which 0.1≤CDnb≤1.4 exists continuously for 100 μm or more.

[0019] Figure 1 is a perspective view showing an example of a silicon nitride substrate of an embodiment. Figure 2 is a cross-sectional view showing an example of a silicon nitride substrate of an embodiment.

[0020] In Figure 1 and Figure 2 , 1 is a silicon nitride substrate, and 2 is a first face (front face, upper face). In Figure 2 , 3 is a second face (back face, lower face) on the opposite side of the first face 2. In the silicon nitride substrate of the embodiment, the first face 2 and the second face 3 are substantially parallel. In the illustrated example, the shape in plan view of the first face 2 is a square. The shape in plan view of the first face 2 can also be a polygon such as a rectangle or a circle or the like. Furthermore, a hole can be formed that penetrates the silicon nitride substrate 1 from the first face 2 to the second face 3.

[0021] Figure 3 is an enlarged view showing an example of a portion A of Figure 2 .

[0022] Figure 3 is schematically shown. When a cross section of the silicon nitride substrate 1 along a thickness direction D is observed in a dark field of an optical microscope, the color 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 arranged alternately. A color difference between an nth (n is an integer) main layer and an nth intermediate layer from one face of the silicon nitride substrate is denoted as CDna. A color difference between an (n+1)th main layer and the nth intermediate layer is denoted as CDnb. In this case, when n is any value of 1 or more and m or less, both a first condition and a second condition are satisfied. The first condition is that, in a first direction parallel to the cross section and perpendicular to the thickness direction, a region in which 0.1≤CDna≤1.4 exists continuously for 100 μm or more. The second condition is that, in the first direction, a region in which 0.1≤CDnb≤1.4 exists continuously for 100 μm or more. Figure 3As shown, 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 dark-viewing conditions, the color difference between the main layer 4 and the intermediate layer 5 is greater than 0.1. In the silicon nitride substrate 1, m+1 main layers 4 and m intermediate layers 5 are arranged alternately in the thickness direction D. "m" is an integer greater than or equal to 1. In the example shown, m is "1". That is, the silicon nitride substrate 1 includes two main layers 4 and one intermediate layer 5 located between these main layers 4.

[0023] Here, the color difference between the nth (n is an integer) main layer 4 (starting from the first surface 2) and the nth intermediate layer 5 is defined as CDna. The color difference between the (n+1)th main layer 4 and the nth intermediate layer 5 is defined as CDnb. In this case, the first and second conditions are satisfied for any value of n greater than 1 and less than m. The first condition is that a region of 0.1 ≤ CDna ≤ 1.4 exists in a continuous manner for more than 100 μm in the first direction D1, which is parallel to the cross-section and perpendicular to the thickness direction D. The second condition is that a region of 0.1 ≤ CDnb ≤ 1.4 exists in a continuous manner for more than 100 μm in the first direction D1.

[0024] For example, the color difference between the first main layer 4a and the first intermediate layer 5 starting 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, in the first main layer 4a and the first intermediate layer 5, a region with a density of 0.1 ≤ CD1a ≤ 1.4 exists in connection along the first direction D1 for more than 100 μm. Furthermore, in the second main layer 4b and the first intermediate layer 5, a region with a density of 0.1 ≤ CDnb ≤ 1.4 exists in connection along the first direction D1 for more than 100 μm.

[0025] The measurement range for color difference is set to "thickness of silicon nitride substrate 1" × "1 mm". Five measurement ranges are randomly set in the cross-section of the silicon nitride substrate 1. The measurement ranges are preferably set at positions that are sufficiently separated from each other. According to the embodiment, at any one of the five locations, the color difference CDna satisfies a first condition, and the color difference CDnb satisfies a second condition.

[0026] like Figure 3 As shown, 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 determined as intermediate layers 5 based on their color difference relationship with the main layers 4. In the silicon nitride substrate 1 of the embodiment, region r1 exists for 100 μm or more in connection in the first direction D1.

[0027] "Color difference" is an index defined using the respective colors of two objects. The larger the color difference between the two objects, the easier it is to distinguish the objects. The smaller the color difference, the more difficult it is to distinguish the objects. The method of calculating the color difference is specified by JIS Z 8730 (2002). JIS Z 8730 (2002) corresponds to ISO 7724-3. In the calculation method specified by ISO 7724-3, the color difference is calculated based on Color difference in color system The color difference is calculated by the following equation.

[0028] CIE brightness of two object colors in color system Difference Δa Difference in color coordinates of two objects in color system Difference Δb Difference in color coordinates of two objects in color system Difference With respect to the color difference, in addition to using the calculation method specified by ISO 7724-3, it is also possible to calculate using the components of brightness, chroma, and hue. The difference in color difference resulting from the calculation methods is at most about 0.1%. Therefore, the color differences calculated in either method are substantially the same.

[0029] The color difference in the cross section of the silicon nitride substrate 1 is difficult to directly measure using a measuring device. This is because the size of the measuring portion of the device is larger than the measurement range. In the case where the size of the measuring portion is larger than the measurement range, the color around the measurement range is also measured. As a result, it is difficult to measure an accurate color difference. Therefore, in the case of 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. The photographing device of the cross section can use an industrial microscope STM7 manufactured by Olympus Corporation. The photographing can use automatically set conditions. By enlarging the image of the dark field, the range in which the color difference is measured is made larger than the size of the measuring portion. The color difference of each portion in the cross section is measured using the enlarged image. The measuring device can be appropriately selected as long as it can measure the color difference of a small portion. For example, as the measuring device, a spectrophotometer CM-2600d manufactured by Konica Minolta Optics, Inc. or CM-26d which is a subsequent model thereof can be used. By these spectrophotometers, it is possible to measure Δa and Δb ​​​​The measurements are calculated by rounding to two decimal places. The color difference conforming to ISO 7724-3 is measured using a spectrophotometer CM-2600d or CM-26d.

[0030] 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 the direction perpendicular to the thickness direction D. For example, if the silicon nitride substrate 1 is rectangular, it is cut along the center line of the long side or the center line of the short side. If the silicon nitride substrate 1 is circular, it is cut along a line passing through the center of the circle.

[0031] In the cross-section, the main layer 4 and the intermediate layer 5 are determined by the following steps. First, the main layer 4 is located near the first surface 2 or the second surface 3 of the silicon nitride substrate 1. In the cross-section, the relatively white area near the first surface 2 is identified as the main layer 4. Next, the color difference between the main layer 4 and the area below it is measured. The cross-sectional photograph is preferably magnified so that the measurement range of the color difference is less than 1.0 mm. Depending on the thickness of the main layer 4, the photograph can also be magnified further. Based on the color difference measurement, the area that is darker than the main layer 4 under visual observation 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 area below it is measured. Based on the color difference measurement, the area that is brighter than the intermediate layer 5 under visual observation 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, the same steps are repeated to determine each main layer 4 and each intermediate layer 5.

[0032] Figure 4 It means Figure 2 An enlarged view of another example of part A.

[0033] Figure 3 An example is shown where the silicon nitride substrate 1 includes two main layers 4. For example... Figure 4 As shown, the silicon nitride substrate 1 of the embodiment may include three or more main layers 4. Figure 4 In the example shown, the silicon nitride substrate 1 includes five main layers 4a~4e and four intermediate layers 5a~5d in cross-section. That is, m is "4". In this case, the color differences CD1a~CD4a satisfy the first condition, and the color differences CD1b~CD4b satisfy the second condition.

[0034] When the color difference CDna or the color difference CDnb exceeds 1.4, the adhesion of the main layers 4 to each other can be insufficient. When the adhesion is insufficient, a void (pore) is generated between the main layers 4. For example, even in a case where no void is observed in a cross section, a void (pore) can be generated in a region other than the cross section. In a case where a large void is generated, peeling between the green sheets, which is called delamination, occurs locally or entirely. When peeling occurs, the peeling portion becomes a starting point of a crack, and a crack easily develops. As a result, the bending strength (mechanical property) of the silicon nitride substrate 1 decreases. In addition, in the peeling portion, the transfer of heat is hindered. Therefore, the thermal conductivity (heat dissipation property) of the silicon nitride substrate 1 decreases.

[0035] The silicon nitride substrate 1 is manufactured by laminating and sintering a plurality of green sheets made of silicon nitride. For example, the green sheets are manufactured by sheet molding. In sheet molding, a liquid silicon nitride raw material is continuously applied to a sheet-shaped film and dried. In this case, a thickness deviation and a gentle unevenness are generated on the surface of the green sheet. In addition, in a case where sheet molding is performed in the atmosphere, the surface of the green sheet becomes a state in which the solvent is less and dried, compared to the inside of the green sheet. The unevenness and the drying decrease the adhesion of the green sheets to each other. Therefore, when a plurality of green sheets are laminated, it is preferable to apply sufficient pressure to the green sheets.

[0036] A minute space is present between the laminated green sheets. The space is present between the green sheets, and air is present in the space. The laminated green sheets are sintered in a nitrogen atmosphere. If the green sheets are sintered in a state in which air is dispersed, the green sheets react with oxygen contained in the air. A portion becomes a state in which sintering is performed in an oxidizing environment. The portion sintered in the oxidizing environment is colored. The color of the portion sintered in the oxidizing environment is slightly darker than the color of a portion sintered in a nitrogen atmosphere. As a result, in a cross section, the portion sintered in the nitrogen atmosphere is observed as a relatively bright main layer 4. The portion colored by the reaction with oxygen is observed as a relatively dark intermediate layer 5. In other words, a region away from the surface of the green sheet in a state before sintering corresponds to the main layer 4 after sintering. A region near the surface of the green sheet in a state before sintering corresponds to the intermediate layer 5 after sintering.

[0037] As described above, if a defect caused by a void or delamination occurs due to a large color difference, heat conduction at the defective site decreases. As a result, the heat dissipation property of the silicon nitride substrate decreases. The silicon nitride substrate is used for a silicon nitride circuit substrate provided with a metal circuit, or a semiconductor module in which a semiconductor element is mounted on the surface of the metal circuit, and the like. If the heat dissipation property of the silicon nitride substrate decreases, the heat dissipation property of the silicon nitride circuit substrate or the semiconductor module decreases. If the semiconductor element operates in a state in which the heat dissipation property decreases, the semiconductor element can be damaged by heat generated. In addition, if a defect caused by a void or delamination occurs due to a large color difference, the strength of the silicon nitride substrate can decrease. If the strength of the silicon nitride substrate decreases, peeling can occur in the inside of the silicon nitride substrate due to thermal cycles and the like. For example, the upper surface portion of the silicon nitride substrate and the metal circuit bonded thereto can peel from the other portions of the silicon nitride substrate.

[0038] The silicon nitride substrate 1 according to the embodiment satisfies the first condition and the second condition. The first condition is that a region in which 0.1 ≤ CDna ≤ 1.4 exists continuously in the first direction D1 for 100 μm or more. The second condition is that a region in which 0.1 ≤ CDnb ≤ 1.4 exists continuously in the first direction D1 for 100 μm or more. The existence of a color difference between the main layer 4 and the intermediate layer 5 means that a site in which coloration due to oxidation exists in the intermediate layer 5. The greater the degree of oxidation, the stronger the coloration of the region, and the greater the color difference. The color difference is 0.1 or more and 1.4 or less, which means that the coloration in the intermediate layer 5 is suppressed, and the oxidation of the intermediate layer 5 is suppressed. Oxidation of the intermediate layer 5 can cause a void or delamination to occur. The state in which the first condition and the second condition are satisfied is a state in which the occurrence of a void or delamination is suppressed.

[0039] For example, in a case in which the first condition and the second condition are not satisfied, the length of a region in which the color difference is 1.4 or less is less than 100 μm. In other words, in a case in which the region in which the color difference is large expands, it means that the region in which oxidation occurs expands. This means that the adhesion of the green sheets to each other is insufficient, and it is a state in which a defect caused by a void or delamination is likely to occur.

[0040] In addition, in the embodiment, in a case in which the silicon nitride substrate 1 has m+1 main layers 4 and m intermediate layers 5, the first condition and the second condition are satisfied in any range of 1 ≤ n ≤ m. That is, in any intermediate layer 5 present in the silicon nitride substrate 1, oxidation is suppressed. Thus, the occurrence of a void or delamination can be more reliably suppressed. The decrease in mechanical strength and the decrease in heat dissipation property caused by the occurrence of a void or delamination can be suppressed.

[0041] Further, the color difference is most preferably zero. However, in order to achieve a state where the color difference is zero, it is necessary to press the green sheets against each other for a long time with a large pressure. The productivity of the silicon nitride substrate 1 is reduced, and the manufacturing cost is increased. Therefore, according to the manufacturing process of the product, a color difference can occur between the main layer 4 and the intermediate layer 5.

[0042] The smaller the color difference between the main layer 4 and the intermediate layer 5, the smaller the degree of oxidation in the intermediate layer 5. Therefore, it is more preferable that a region where 0.1 ≤ CDna ≤ 1.2 exists continuously in the first direction D1 for 100 μm or more, and a region where 0.1 ≤ CDnb ≤ 1.2 exists continuously in the first direction D1 for 100 μm or more. It is most preferable that a region where 0.1 ≤ CDna ≤ 1.0 exists continuously in the first direction D1 for 100 μm or more, and a region where 0.1 ≤ CDnb ≤ 1.0 exists continuously in the first direction D1 for 100 μm or more.

[0043] Further, the larger the region where the color difference is 0.1 or more and 1.4 or less, the better the adhesion of the green sheets to each other. Therefore, it is more preferable that a region where 0.1 ≤ CDna ≤ 1.4 exists continuously in the first direction D1 for 200 μm or more, and a region where 0.1 ≤ CDnb ≤ 1.4 exists continuously in the first direction D1 for 200 μm or more. It is most preferable that a region where 0.1 ≤ CDna ≤ 1.4 exists continuously in the first direction D1 for 400 μm or more, and a region where 0.1 ≤ CDnb ≤ 1.4 exists continuously in the first direction D1 for 400 μm or more.

[0044] The number of the main layers 4 is not particularly limited. However, the more the number of the main layers 4 increases, the more the number of the intermediate layers 5 increases. That is, the proportion of the region to be oxidized in the silicon nitride substrate 1 increases. As a result, it can lead to a decrease in thermal conductivity or a decrease in mechanical strength of the silicon nitride substrate 1. Therefore, the number of the main layers 4 is preferably 2 or more and 5 or less.

[0045] The thermal conductivity of the silicon nitride substrate 1 is preferably 50 W / (m·K) or more. According to the embodiment, the defect of the void or the delamination is suppressed, and thus the thermal conductivity of the silicon nitride substrate 1 can be further 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.

[0046] The three-point bending strength of the silicon nitride substrate 1 is preferably 600 MPa or more. According to the embodiment, the defect of the void or the delamination is suppressed, and thus 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 ISO23242 (2020).

[0047] Figure 5is a flowchart showing a manufacturing method of the silicon nitride substrate 1 of the embodiment.

[0048] A manufacturing method of the silicon nitride substrate 1 of the embodiment will be described. As shown in Figure 5 the manufacturing method of the silicon nitride substrate of the embodiment mainly includes preparation of slurry (step S1), preparation of green sheet (step S2), preparation of laminate (step S3), preparation of debinding body (step S4), and sintering (step S5).

[0049] In the preparation of slurry, an organic solvent and a binder are mixed in the silicon nitride powder and the sintering aid powder. In the preparation of green sheet, the slurry is molded. In the preparation of laminate, a plurality of green sheets are overlapped by a pressure of 0.1 MPa or more at a temperature of 60°C or more. In the preparation of debinding body, the laminate is heat-treated at a temperature of 1000°C or less. In the sintering, the debinding body is heat-treated at a temperature of 1600°C or more and 2000°C or less. Hereinafter, each process will be described in detail.

[0050] First, the slurry is prepared by mixing an organic solvent and a binder in the silicon nitride powder and the sintering aid powder. The α conversion rate of the silicon nitride powder is preferably 80% by mass or more. The average particle diameter 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 manufactured silicon nitride substrate 1 can be decreased.

[0051] The sintering aid powder is preferably a metal oxide powder having an average particle diameter of 0.5 μm or more and 3.0 μm or less. As the metal oxide powder, an oxide of an element selected from the group consisting of rare earth elements, magnesium, titanium, and hafnium is used. By adding the sintering aid of the metal oxide, liquid components are easily formed in the sintering process. For example, as the sintering aid, one or more kinds selected from the group consisting of rare earth elements, magnesium, titanium, and hafnium are added as oxides in a total amount of 1% by mass or more and 20% by mass or less. More preferably, the sintering aid is added as oxides in a total amount of 1% by mass or more and 20% by mass or less. It is particularly preferable to add two or more kinds of oxides. By adding two or more kinds, two or more kinds of glassy compound phases having different compositions are easily formed. More preferably, an oxide of a rare earth element is added as a necessary component, and an oxide of one or more kinds of elements selected from the group consisting of magnesium, hafnium, and titanium is added.

[0052] The silicon nitride powder and the sintering aid powder are mixed by crushing in an organic solvent. The organic solvent is toluene, ethanol, or butanol, or the like. Next, a binder is added to prepare the slurry. The binder uses butyl methacrylate, polyvinyl butyral, or polymethyl methacrylate, or the like.

[0053] Next, the raw material slurry is molded to produce a green sheet. The sheet molding can use a known method such as a doctor blade method or an extrusion molding method. In particular, the sheet molding preferably uses the doctor blade method. If the doctor blade method is used, a thin and large green sheet can be produced, and the mass productivity is improved.

[0054] Next, the plurality of green sheets are laminated and sandwiched between the dust-free papers. The plurality of green sheets are pressed at a temperature of 60°C or higher and a pressure of 0.1 MPa or higher through the dust-free papers. Thus, the plurality of green sheets are tightly attached to each other, and a laminate is produced.

[0055] Figure 6 is a cross-sectional view showing an example of a method of producing a laminate. Figure 6 A flat press lamination method 10 is a method of producing a laminate by flat pressing. In the flat press lamination method 10, a pair of resin sheets 12, a pair of dust-free papers 13, and a plurality of green sheets 14 of silicon nitride are arranged between a pair of flat press machines 11. The flat press machines 11 are provided with heating portions. The pair of dust-free 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 dust-free papers 13.

[0056] In the case of using the flat press machines 11, the green sheets 14 are cut to a predetermined size before the laminate is produced. After the cutting, a predetermined number of green sheets 14 are laminated. The resin sheets 12 are arranged to improve the releasability of the flat press machines 11 from the dust-free papers 13 after the lamination. The resin sheets 12 are made of a resin material such as polyethylene (PE), polyethylene terephthalate (PET), or polyvinyl chloride.

[0057] The dust-free papers 13 are papers that are generally used in a dust-free room and have less dust emission. The dust-free papers 13 are made of long fibers, not short fibers. The long fibers are combined with each other to prevent dust emission. By using the dust-free papers 13, the fibers can be prevented from becoming green sheets. Dust-free papers 13 that use conductive fibers can also be used. By using the dust-free papers 13 having conductivity, the generation of static electricity can be suppressed. Thus, dust can be prevented from being adsorbed to the dust-free papers 13 due to static electricity.

[0058] The dust-free papers 13 have strength and softness that are generally possessed by paper. In the case of directly pressing the laminated green sheets 14 from both surfaces using the flat press machines 11, the pressure can be deviated. The dust-free papers 13 are deformed along the unevenness of the surfaces of the green sheets 14. The laminated green sheets 14 are arranged between the pair of dust-free papers 13. By pressing the green sheets 14 through the dust-free papers 13, the pressure is dispersed by the dust-free papers 13. Thus, the pressure can be more uniformly applied to the entire surface of the green sheets 14. For example, depending on the state of the flat press machines 11, the central portion and the end portions of the green sheets 14 are not uniformly pressed and can cause delamination to occur. By using the dust-free papers 13, the pressure can be uniformly applied to the green sheets 14, and the occurrence of delamination can be suppressed.

[0059] The thickness of the dust-free paper 13 is approximately 0.08 to 0.15 mm. In the case of a thinner dust-free paper 13, the deformation at the time of pressurization is smaller. It is easier to directly apply pressure to the green sheet 14. Therefore, a thinner dust-free paper 13 is suitable for a green sheet 14 having less unevenness on the surface. In the case of a thicker dust-free paper 13, the deformation at the time of pressurization is larger. It is easy to more diffusely apply pressure to the green sheet 14. Therefore, a thicker dust-free paper 13 is suitable for a green sheet 14 having more unevenness on the surface.

[0060] Figure 7 is a cross-sectional view showing another example of a method of manufacturing a laminate. Figure 7 A two-side roll lamination method 15 of manufacturing a roll laminate is shown. In the two-side roll lamination method 15, a pair of rolls 16 having a heating portion is used. A plurality of green sheets 14 made of silicon nitride are arranged between the rolls 16 with the dust-free paper 13 interposed therebetween. As needed, a resin sheet 12 is arranged between the rolls 16 and the dust-free paper 13. By arranging the resin sheet 12, the rolls 16 and the dust-free paper 13 are easily peeled.

[0061] The resin sheet 12, the dust-free paper 13, and the plurality of green sheets 14 are continuously supplied between the pair of rolls 16. Therefore, the resin sheet 12 and the dust-free paper 13 corresponding to the length of the green sheet 14 are prepared in advance. The laminate 17 in which the plurality of green sheets 14 are laminated is discharged from between the pair of rolls 16. In the two-side roll, there is no limitation on the length in the moving direction of the green sheet 14. Therefore, the method using the rolls 16 is excellent in mass productivity. In the case where the width of the roll 16 is smaller than the green sheet width, the green sheet is cut to a predetermined width.

[0062] The plurality of green sheets 14 are partially pressurized while being extruded by the rolls 16. Therefore, compared with the method using the flat press 11, the method using the rolls 16 is excellent in uniformity of pressurization. In addition, in the method using the rolls 16, depending on the state of the rolls 16, it is possible that a pressure difference is generated between the central portion and the end portion of the roll. Therefore, as with the method using the flat press 11, it is preferable to use the dust-free paper 13. By interposing the dust-free paper 13 between the green sheet 14 and the roll 16, the dust-free paper 13 is deformed depending on the unevenness of the surface of the green sheet 14 or the state of the roll 16. By using the dust-free paper 13, it is possible to uniformly apply pressure to the green sheet 14, and to suppress the occurrence of delamination.

[0063] The temperature at the time of making the laminate is preferably set to 60°C or higher. The "temperature" is the temperature of the upper and lower flat press 11 or the temperature of the upper and lower rollers 16. By heating, the green sheets 14 are bonded to each other. By further applying pressure, the green sheets 14 are well tightly bonded to each other. If the temperature is less than 60°C, the bonding based on heating cannot be sufficiently performed. In the made laminate, air is easily left between the green sheets 14. As a result, the green sheets 14 are oxidized due to the air, and the color difference between the main layer 4 and the intermediate layer 5 becomes large. That is, defects such as voids or delamination are easily generated. The higher the temperature, the more the green sheets 14 can be bonded to each other in a short time. In the case where the green sheets 14 are thick, or in the case where the number of the laminated green sheets 14 is large, since the heat is difficult to transfer to the inside, it is preferable to perform heating at a higher temperature. On the other hand, if the temperature is too high, the surface of the green sheets 14 is easily dried. Therefore, the temperature is preferably 140°C or lower.

[0064] The pressure at the time of making the laminate is preferably 0.1 MPa or higher. The "pressure" is the pressure applied by the upper and lower flat press 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 cannot be sufficiently bonded to each other, and defects such as voids or delamination are easily generated. In the case where the pressure is high, the bonding can be performed in a short time. On the other hand, if the pressure is too high, the green sheets 14 can be damaged in the thickness direction. Therefore, the pressure is preferably 0.5 MPa or lower.

[0065] Next, the laminate is subjected to debinding at a temperature of 1000°C or lower, thereby making a debound body. The environment of the debinding step is preferably a non-active environment such as nitrogen or argon. Oxygen can be added to the non-active environment, and the environment can be set to an oxygen-containing environment. The debinding temperature is preferably 1000°C or lower, and more preferably in the range of 500°C or higher and 800°C or lower. By performing the debinding step in this temperature range, the speed of thermal decomposition of the binder can be controlled. By the release of the thermal decomposition gas accompanying debinding, the debound body can be prevented from being damaged. If the debinding temperature exceeds 1000°C, the high-molecular binder is rapidly burned off, and the debound body is easily damaged. On the other hand, when the debinding temperature is lower than 500°C, the thermal decomposition of the binder becomes insufficient, and it can be impossible to densify the sintered body. Therefore, it is preferable to thermally decompose the organic matter such as the binder at a temperature of 1000°C or lower, and more preferably 500°C or higher and 800°C or lower.

[0066] Next, the debound 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 environment. When the sintering temperature is lower than 1600°C, the densification of the sintered body becomes insufficient. When the sintering temperature exceeds 2000°C and the ambient pressure in the furnace is low, the silicon nitride contained in the sintered body can be decomposed into silicon (Si) and nitrogen (N). Therefore, the sintering temperature is preferably in the range of 1700°C or higher and 1900°C or lower. When in this temperature range, a plurality of glass compounds having different compositions are generated by the sintering aid and the impurity oxygen. As a result, liquid sintering of the silicon nitride is easily performed.

[0067] (Examples 1 to 8, Comparative Examples 1 to 8) A silicon nitride powder having an a conversion rate of 95% and an average particle diameter of 0.8 μm was prepared. As a sintering aid, an oxide of a rare earth element, an oxide of titanium, an oxide of hafnium, and an oxide of magnesium were prepared. 10 mass% of the sintering aid was mixed with the silicon nitride powder, converted to the oxide. An organic solvent and a binder were mixed in the silicon nitride powder and the sintering aid, and a slurry was prepared. The prepared slurry was molded by a sheet molding machine, and a large silicon nitride green sheet was prepared. The large green sheet was cut by a cutter so that the size of one green sheet would be 200 mm x 200 mm. The cut green sheets were overlapped. The overlapped green sheets were sandwiched by a sheet of polyethylene terephthalate, and were pressed by a flat press machine or a two-sided roller while being heated. Thus, the overlapped green sheets were bonded, and a laminate was prepared.

[0068] The preparation conditions of the laminates are described in Table 1. In Table 1, "substrate thickness" is the thickness of the silicon nitride substrate after final sintering. "Number of layers" is the number of overlapped green sheets. In Examples 1 to 8, a dust-free paper was disposed on the upper surface and the lower surface of the overlapped green sheets, respectively. The thickness of the dust-free paper was 1 mm. In Comparative Examples 1 to 8, no dust-free paper was disposed. "Lamination method" indicates the method of preparing the laminate. "Flat" means the method using a flat press machine as shown in Figure 6 "Roller" means the method using a roller as shown in Figure 7 "Roller" means the method using a roller as shown in

[0069] Next, the laminate was debound at a temperature of 500°C or higher and 800°C or lower in a nitrogen atmosphere. Then, the debound body was sintered at a temperature of 1600°C or higher and 2000°C or lower in a nitrogen pressurized atmosphere. The obtained sintered body was cut in the thickness direction in a manner passing through the central portion in plan view. The cross section was observed in a dark field using an industrial microscope STM7 manufactured by Olympus Corporation, and a magnified photograph was taken. Next, the main layer and the intermediate layer in the cross section were determined using a spectrophotometer (CM-2600d manufactured by Konica Minolta, Inc.). The color difference between the main layer and the intermediate layer was measured in order along the first direction D1. The measurement range in the first direction D1 was set to “the thickness of the ceramic substrate” x “1 mm”. Five measurement ranges separated by 1 mm from each other were selected. From the measurement results of the color difference, the length of the region existing continuously in the first direction D1 was measured for each color difference. In each example, the smallest color difference among the color differences in which a region of 100 μm or more existed continuously was extracted. The measurement results are described in Table 2. For example, in Example 1, a region in which the color difference CD1a was 1.1 existed continuously in the first direction D1 by 100 μm or more in any measurement range. This indicates that the length of the region in which the color difference CD1a was 1.0 was less than 100 μm in at least one measurement range.

[0070] As can be seen from Table 2, in the silicon nitride substrates of the examples, it was confirmed that the color difference CDna and the color difference CDnb satisfied the first condition and the second condition. This is because the lamination conditions of the green sheet were appropriate. In contrast, in the silicon nitride substrates of the comparative examples, the region in which the color difference was large expanded, and the first condition and the second condition were not satisfied. This is because the lamination conditions of the green sheet were not appropriate, and the close contact of the green sheet was not sufficient. As a result, the intermediate layer was discolored, and a larger color difference was measured compared to the examples.

[0071] Next, the silicon nitride substrates of the examples and the comparative examples were processed into 40 mm x 20 mm to produce test pieces. The test pieces were used to measure the bending strength. The processing of the test pieces was in accordance with JIS R1607 (2015). The measurement of the bending strength was performed by three-point bending with a span of 30 mm in accordance with ISO 23242 (2020). Next, the silicon nitride substrates of the examples and the comparative examples were processed into a diameter of 10 mm to measure the thermal conductivity. The measurement results of the bending strength and the thermal conductivity are shown in Table 3.

[0072] As can be known from Table 3, the bending strength and the thermal conductivity of the silicon nitride substrates of the examples are in the preferable ranges. This is because the lamination conditions of the green sheets are appropriate, and thus the occurrence of voids and delamination between the layers is suppressed. In contrast, the bending strength and the thermal conductivity of the silicon nitride substrates of the comparative examples are outside the preferable ranges. This is because the lamination of the green sheets is insufficient, voids and delamination are generated in the intermediate layers, and thus the bending strength and the thermal conductivity are reduced.

[0073] Embodiments of the present application include the following features.

[0074] (FEATURE 1) A silicon nitride substrate, wherein, when a cross section of the silicon nitride substrate along a thickness direction is observed in a dark field of an optical microscope, the silicon nitride substrate includes a main layer and an intermediate layer darker than the main layer, a 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, a color difference between an nth (n is an integer) main layer from one face of the silicon nitride substrate and an nth intermediate layer is set as CDna, a color difference between an (n+1)th main layer and the nth intermediate layer is set as CDnb, the first condition that a region where 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, and the second condition that a region where 0.1≤CDnb≤1.4 exists continuously for 100 μm or more in the first direction are satisfied for any value of n that is 1 or more and m or less.

[0075] (FEATURE 2) The silicon nitride substrate according to FEATURE 1, wherein the number of the main layers is 2 or more and 5 or less.

[0076] (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.

[0077] (FEATURE 4) The silicon nitride substrate according to any one of FEATURES 1 to 3, wherein the bending strength is 600 MPa or more.

[0078] (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.

[0079] (FEATURE 6) A method for manufacturing a silicon nitride substrate, wherein a slurry is prepared by mixing an organic solvent and a binder in silicon nitride powder and sintering aid powder, a green sheet is prepared by molding the slurry, a plurality of the green sheets are laminated at a temperature of 60°C or higher and a pressure of 0.1 MPa or higher to prepare a laminate, and the laminate is heat-treated at a temperature of 1000°C or lower to prepare a debindered body, and the debindered body is sintered at a temperature of 1600°C or higher and 2000°C or lower.

[0080] (Feature 7) The method for manufacturing a silicon nitride substrate according to Feature 6, wherein, in the preparation of the laminate, the plurality of green sheets are laminated by a pair of rollers.

[0081] (Feature 8) The method for manufacturing a silicon nitride substrate according to Feature 6 or 7, wherein, in the preparation of the laminate, the plurality of green sheets are arranged between a pair of dustless papers, and the plurality of green sheets are heated and pressed through the pair of dustless papers.

[0082] The above illustrates several embodiments of the present application, but these embodiments are suggested as examples and are not intended to limit the scope of the application. These new embodiments can be implemented in other various ways, and various omissions, substitutions, changes, etc. can be made within the scope of the gist of the application. These embodiments, their modified examples, are included in the scope and gist of the application, and are included in the scope of the application and its equivalents described in the patent claim. In addition, the above-described embodiments can be implemented in combination with each other.

[0083] Explanation of Reference Signs 1: Silicon nitride substrate 2: First surface 3: Second surface 4, 4a to 4e: Main layer 5, 5a to 5d: Intermediate layer 10: Flat pressing laminate 11: Flat pressing machine 12: Resin sheet 13: Dustless paper 14: Green sheet 15: Two-sided roller lamination 16: Two-sided roller 17: Green sheet laminate

Claims

1. A silicon nitride substrate, wherein, When observing a cross-section of the silicon nitride substrate along its thickness direction in a dark-field optical microscope, the silicon nitride substrate comprises a main layer and an intermediate layer that is darker than the main layer. The color difference between the main layer and the intermediate layer is greater than 0.

1. In the thickness direction, m+1 main layers and m intermediate layers are arranged alternately. When the color difference between the nth main layer and the nth intermediate layer, starting from one side of the silicon nitride substrate, is set to CDna, and the color difference between the (n+1)th main layer and the nth intermediate layer is set to CDnb, the first and second conditions are satisfied for any value of n greater than or equal to 1 and less than or equal to m, where n is an integer. The first condition is that, in a first direction parallel to the cross-section and perpendicular to the thickness direction, regions with a thickness of 0.1 ≤ CDna ≤ 1.4 exist in a continuous manner for more than 100 μm. The second condition is that, in the first direction, regions with a density of 0.1 ≤ CDnb ≤ 1.4 exist in a continuous manner for a depth of more than 100 μm.

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

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

4. The silicon nitride substrate according to claim 1 or 2, wherein, The bending strength is above 600 MPa.

5. The silicon nitride substrate according to claim 1 or 2, wherein, The thermal conductivity is above 50 W / (m·K).

6. A method for manufacturing a silicon nitride substrate, wherein, A slurry is prepared by mixing organic solvents and binders with silicon nitride powder and sintering aid powder. The slurry is shaped to produce raw sheets. A laminate is produced by overlapping multiple green sheets at a temperature above 60°C and a pressure above 0.1 MPa. The laminate is heat-treated at a temperature below 1000°C to produce a degreased body. The degreased body is sintered at a temperature above 1600°C and below 2000°C.

7. The method for manufacturing a silicon nitride substrate according to claim 6, wherein, In the fabrication of the laminate, a pair of rollers are used to overlap multiple green sheets.

8. The method for manufacturing a silicon nitride substrate according to claim 6 or 7, wherein, In the fabrication of the laminate, multiple green sheets are arranged between a pair of cleanroom sheets, and the green sheets are heated and pressurized through the pair of cleanroom sheets.

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