Silicon nitride plate and method for producing silicon nitride plate

Abstract

To provide a silicon nitride plate in which variation in insulation is reduced.SOLUTION: A silicon nitride plate having a rectangular main surface, wherein when a portion defined by a first virtual rectangle R1 of length * width = 40 mm * 10 mm at a center C of the main surface is defined as a central portion X, and portions defined by four second virtual rectangles R2 of length * width = 40 mm * 10 mm at four corners of the main surface are defined as outer edge portions Y, a difference between an oxygen content at the central portion X and an oxygen content at each of the outer edge portions Y is 0.030 mass% or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to silicon nitride plates and methods for manufacturing silicon nitride plates. [Background technology]

[0002] In recent years, power modules for controlling large amounts of power have been used in industrial equipment such as motors and products such as electric vehicles. These power modules use circuit boards with ceramic plates to efficiently diffuse heat generated from semiconductor elements and suppress leakage current. These circuit boards usually use plate-shaped ceramic sintered bodies.

[0003] Known ceramic sintered bodies are those made of nitrides, carbides, borides, silicides, etc. For example, Patent Document 1 describes that a silicon nitride sintered body with improved breakdown voltage is useful for circuit boards of electronic components such as power modules. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-166445 Summary of the Invention [Problem to be solved by the invention]

[0005] Electronic components such as power modules require reliable electrical characteristics. Therefore, silicon nitride plates used in circuit boards are required to have reduced variations in insulation properties. The present disclosure provides silicon nitride plates with reduced variations in insulation properties. It also provides a method for smoothly producing such silicon nitride plates. [Means for solving the problem]

[0006] One aspect of the present disclosure provides the following silicon nitride plate:

[0007] [1] A silicon nitride plate having a rectangular main surface, A portion defined by a first imaginary rectangle having a length x width of 40 mm x 10 mm at a center C of the main surface is defined as a central portion, When the portions defined by four second imaginary rectangles of length x width = 40 mm x 10 mm at the four corners of the main surface are defined as outer edges, A silicon nitride plate, wherein the difference between the oxygen content in the central portion and the oxygen content in each of the outer edge portions is 0.030 mass % or less.

[0008] The oxygen contained in silicon nitride plates comes from the oxide-based sintering aid used during firing, and from oxides produced by reaction with oxygen in the firing atmosphere. It is believed that some of the oxygen derived from the sintering aid volatilizes during firing. This results in uneven distribution of the oxide-based sintering aid in the silicon nitride plate. Such uneven distribution of oxides can cause variations in insulation properties. The silicon nitride plate described in [1] above has a sufficiently small difference in oxygen content between the center and the outer edge, which allows for a high degree of uniformity in oxide distribution. Therefore, variations in the insulation properties of the silicon nitride plate can be reduced.

[0009] The silicon nitride plate of the above [1] may be any one of the following [2] to [5].

[0010] [2] The silicon nitride plate according to [1], wherein the oxygen content of the central portion is higher than the oxygen content of the outer edge portion. [3] The silicon nitride plate according to [1] or [2], wherein both the oxygen content of the central portion and the oxygen content of the outer edge portion are 2.00 mass % or more. [4] The area of ​​the main surface is 5000 to 50,000 mm 2 The silicon nitride plate according to any one of [1] to [3], wherein [5] Volume resistivity is 3.0 × 10 14The silicon nitride plate according to any one of [1] to [4], having a resistivity of Ω·cm or more.

[0011] The silicon nitride plate [2] above is sintered in a well-balanced and uniform manner, and therefore has high insulating properties.

[0012] The silicon nitride plate described in [3] above has a sufficiently high oxygen content in the center and outer edge, which gives the plate high insulating properties.

[0013] The silicon nitride plate of [4] above is not too large, so the difference in oxygen content between the center and the outer edge of the silicon nitride plate can be further reduced. Therefore, such a silicon nitride plate can further reduce the variation in insulation properties. Furthermore, because it has a certain size, it can be suitably used not only as a component for circuit boards but also for various other applications.

[0014] The silicon nitride plate [5] above has high insulating properties, and therefore when used as a component of a circuit board, it can improve the reliability of the circuit board.

[0015] One aspect of the present disclosure provides the following method for producing a silicon nitride plate.

[0016] [6] A preparation step of preparing a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder; a green sheet preparation step of preparing a green sheet containing the mixed raw material; a lamination step of laminating 20 to 50 of the green sheets to prepare a laminate; and a firing step of firing the laminate.

[0017] In the silicon nitride plate manufacturing method [6] above, the number of stacked green sheets in the laminate is 20 to 50. Although it is believed that the amount of volatilization of sintering aid near the surface of the sintered body can be reduced when the number of stacked green sheets is greater than 50, the green sheets cannot be fired with high uniformity, resulting in uneven distribution of the sintering aid. In contrast, when the number of stacked green sheets is less than 20, the amount of volatilization of sintering aid near the surface of the sintered body increases, resulting in excessive volatilization of the sintering aid near the surface of the sintered body, resulting in uneven distribution of the sintering aid in the fired sintered body. Therefore, if the number of stacked green sheets during firing is too large or too small, the distribution of the oxide-based sintering aid in the sintered body tends to be uneven. In a laminate with 20 to 50 stacked green sheets, uneven distribution of the oxide-based sintering aid after firing can be suppressed in the green sheets stacked near the center. This sufficiently reduces the difference in oxygen content between the center and the outer edge, allowing the production of a silicon nitride plate with highly uniform oxide distribution. The silicon nitride plate produced in this manner has a sufficiently small difference in oxygen content between the center and the outer edge, making it possible to reduce variations in the insulating properties of the silicon nitride plate.

[0018] The method for producing a silicon nitride plate according to [6] above may be the following method [7].

[0019] [7] At least one of the plurality of silicon nitride plates obtained in the firing step has a rectangular main surface, A portion defined by a first imaginary rectangle having a length x width of 40 mm x 10 mm at a center C of the main surface is defined as a central portion, When the portions defined by four second imaginary rectangles of length x width = 40 mm x 10 mm at the four corners of the main surface are defined as outer edges, The method for producing a silicon nitride plate according to [6], wherein the difference between the oxygen content in the central portion and the oxygen content in each of the outer edge portions is 0.030 mass % or less.

[0020] The silicon nitride plate obtained by the manufacturing method [7] above has a small difference in oxygen content between the center and the outer edge, resulting in a highly uniform distribution of oxides. Therefore, the silicon nitride plate obtained by this manufacturing method has high insulating properties. [Effects of the Invention]

[0021] The present disclosure can provide a silicon nitride plate with reduced variations in insulation properties, and also a method for smoothly producing such a silicon nitride plate. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a perspective view showing an example of a silicon nitride plate. [Figure 2] FIG. 2 is a plan view showing an example of the center and outer edge of a silicon nitride plate. [Figure 3] FIG. 2 is a perspective view showing an example of a circuit board. [Figure 4] FIG. 2 is a plan view showing the center and outer edge of a silicon nitride plate in an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] Embodiments of the present disclosure will be described below. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content. In the description, the same reference numerals will be used for identical elements or elements having the same functions, and redundant explanations will be omitted where appropriate. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right will be based on the positional relationships based on the orientation of the reference numerals shown in the drawings. The dimensional ratios of each element are not limited to those shown. The numerical ranges exemplified as "a to b" are numerical ranges inclusive of a and b, with a lower limit being a and an upper limit being b. The present disclosure also includes those in which the upper or lower limit of each numerical range is replaced with the numerical value of any of the examples. When multiple materials are exemplified, one of the materials may be used alone, or multiple materials may be used in combination.

[0024] [Silicon nitride plate] FIG. 1 is a perspective view showing an example of a silicon nitride plate. The silicon nitride plate 50 has rectangular main surfaces (first main surface 50A, second main surface 50B). In this disclosure, a main surface is a surface whose area is larger than the other surfaces. Typically, the two surfaces that are perpendicular to the thickness direction of the silicon nitride plate are the main surfaces. A rectangle refers to a quadrilateral with all four corners being right angles.

[0025] The thickness of the silicon nitride plate 50 may be, for example, 0.20 to 0.50 mm, 0.25 to 0.40 mm, or 0.25 to 0.35 mm. When the thickness is within the above range, it can be suitably used as a member for a circuit board.

[0026] The area of ​​the first main surface 50A of the silicon nitride plate 50 is set to 5000 mm 2 in order to ensure a sufficient size for use as a circuit board member. 2 Over 10,000 mm 2 or more, or 15,000 mm 2 In order to further reduce the difference between the inside and outside of the oxygen content, the area of ​​the first main surface 50A is set to 50,000 mm 2 Below, 45000mm 2 Below, 40000mm 2 or less than 35,000 mm 2 The silicon nitride plate 50 has further reduced variations in insulation properties and can be more suitably used as a component of a circuit board. The area of ​​the first main surface 50A may be, for example, 5,000 to 50,000 mm 2 , 10000~45000mm 2 , or 15,000 to 40,000 mm 2 may be.

[0027] The vertical length of the first main surface 50A may be 100 to 250 mm, 110 to 240 mm, or 120 to 230 mm. The horizontal length of the first main surface 50A may be 50 to 200 mm, 70 to 190 mm, or 90 to 180 mm. The above-described shape further improves the uniformity of the silicon nitride plate 50, further reducing variations in insulation properties. Furthermore, the silicon nitride plate 50 having the above-described shape has excellent processability.

[0028] FIG. 2 is a plan view showing the center and outer edge of the silicon nitride plate 50 of FIG. 1. The silicon nitride plate 50 has a center X, which is a portion defined by a first imaginary rectangle R1 measuring 40 mm x 10 mm at the center C of the first main surface 50A, and an outer edge Y, which is a portion defined by a second imaginary rectangle R2 measuring 40 mm x 10 mm at the first main surface 50A, with corner K being the point on the outer edge of the first main surface 50A that is farthest from the center C of the first main surface 50A. In other words, the outer edge Y is defined by four second imaginary rectangles R2 at the four corners of the first main surface 50A. The outer edge Y is a collective term for the four outer edges Y1, Y2, Y3, and Y4. The center X is defined so that the center of the center X coincides with the center C of the first main surface 50A.

[0029] The difference in oxygen content between the center X and the outer edge portion Y (inner / outer difference) is 0.030% by mass or less. When the oxygen contents of the four outer edge portions Y1, Y2, Y3, and Y4 are different from one another, the difference in oxygen content between the center X and each of the four outer edge portions Y1, Y2, Y3, and Y4 (inner / outer difference) is 0.030% by mass or less. Such a silicon nitride plate 50 has a small inner / outer difference in oxygen content and high uniformity, thereby reducing variations in insulation properties. Furthermore, such a silicon nitride plate 50 can be suitably used as a component of a circuit board. The oxygen contents of the four outer edge portions Y1, Y2, Y3, and Y4 may be the same or different from one another. The difference between the maximum and minimum oxygen contents of the four outer edge portions Y1, Y2, Y3, and Y4 may be 0.01% by mass or less.

[0030] The oxygen content in the silicon nitride plate 50 is derived from, for example, oxide-based sintering aids and oxides (e.g., silica) produced by firing. Here, oxygen derived from the sintering aids may volatilize and decrease during firing. Because volatilization occurs near the surface of the silicon nitride plate 50, it is thought that when green sheets are stacked and fired, the amount of volatilization differs between the center X and the outer edge Y. This can be a factor in the difference in oxygen content between the center X and the outer edge Y. Therefore, by adjusting the amount of volatilization of the sintering aid during firing and suppressing uneven distribution of the sintering aid, the difference in oxygen content between the center X and the outer edge Y can be reduced. Since the uneven distribution of the insulating oxide-based sintering aid is suppressed in such a silicon nitride plate 50, variations in insulation properties can be reduced.

[0031] The oxygen content can be measured, for example, by subjecting the pulverized products obtained by pulverizing the center portion X and the outer peripheral portion Y (outer peripheral portions Y1, Y2, Y3, Y4) to an oxygen / nitrogen analyzer (product name: EMGA-920, manufactured by Horiba, Ltd.). X ) and the oxygen content of the outer edge Y (O Y ) and calculate the difference between the measured values ​​to determine the difference in the oxygen content between the inside and outside of the sample. X ) and the oxygen content of the outer edge Y (O Y ) absolute difference (|O X -O Y |).

[0032] The difference in oxygen content between the inside and outside of the silicon nitride plate 50 may be 0.028% by mass or less, or 0.025% by mass or less. Such a silicon nitride plate 50 has higher uniformity and further reduced variation in insulation properties. From the viewpoint of ease of manufacture, the difference in oxygen content between the inside and outside may be 0.010% by mass or more. The difference in oxygen content between the inside and outside may be, for example, 0.010 to 0.030% by mass.

[0033] The silicon nitride plate 50 has an oxygen content (O X ) is the oxygen content of the outer edge Y (OY ) The silicon nitride plate 50 having such a thickness can have a sufficiently high insulating property.

[0034] Oxygen content of the central part X (O X ) may be 2.00 to 4.00 mass %, 2.50 to 3.50 mass %, or 2.80 to 3.20 mass %. Such a silicon nitride plate 50 has higher insulation properties because the central portion X contains an insulating oxide-based sintering aid.

[0035] Oxygen content of outer edge Y (O Y ) may be 2.00 to 4.00 mass %, 2.50 to 3.50 mass %, or 2.80 to 3.20 mass %. Since the silicon nitride plate 50 contains an insulating oxide-based sintering aid in the outer edge portion Y, it has even higher insulation properties.

[0036] Oxygen content of the central part X (O X ) and the oxygen content of the outer edge Y (O Y ) may be 2.00 mass % or more, 2.50 mass % or more, or 2.80 mass % or more. In such a silicon nitride plate 50, the oxygen contents of both the center portion X and the outer edge portion Y are sufficiently high. Therefore, the silicon nitride plate 50 has high insulation properties while suppressing variations in insulation properties. The oxygen content (O X ) and the oxygen content of the outer edge Y (O Y ) may be 4.00 mass % or less, 3.50 mass % or less, or 3.20 mass % or less. X ) and the oxygen content of the outer edge Y (O Y ) may be in the range of, for example, 2.00 to 4.00 mass %, 2.50 to 3.50 mass %, or 2.80 to 3.20 mass %.

[0037] In the present disclosure, the variation in the insulating properties of the silicon nitride plate 50 refers to the variation in the insulating properties at different positions on a single silicon nitride plate 50. Such variation in the insulating properties is caused by the volume resistivity (E X ) to the volume resistivity (E Y) can be expressed as the ratio of E X E for Y The ratio (E Y / E X The closer the value of E is to 1, the more uniform the oxygen is near the center X and near the outer edge Y, and the less unevenly the sintering aid is distributed. Therefore, the silicon nitride plate 50 has reduced variations in insulation properties. X E for Y The ratio may be 2.10 or less, 2.00 or less, or 1.90 or less.

[0038] The volume resistivity of the silicon nitride plate 50 is 3.0 x 10 14 Ω cm or more, 5.0×10 14 Ω·cm or more, or 7.0×10 14 The volume resistivity of the silicon nitride plate 50 means the volume resistivity of the entire silicon nitride plate 50, including the central portion X and the outer edge portion Y. A silicon nitride plate 50 having a volume resistivity within the above range has excellent insulating properties, and therefore can improve the reliability of a circuit board when used as a component of the circuit board. The volume resistivity of the silicon nitride plate 50 is 20.0×10 14 Ω cm or less, or 15.0×10 14 The volume resistivity of the silicon nitride plate 50 may be, for example, 3.0×10 14 ~20.0×10 14 It may be Ω·cm.

[0039] The volume resistivity of the silicon nitride plate 50 can be calculated, for example, by measuring the insulation resistance value in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance" and converting it into volume resistivity using the volume of the test piece.

[0040] Volume resistivity of the center X (E X ) is 3.0 × 10 14 Ω cm or more, 4.0×10 14 Ω·cm or more, or 5.0×10 14 It may be Ω·cm or more. X The silicon nitride plate 50 having E in the above range has better insulating properties.X is 20.0 × 10 14 Ω cm or less, or 15.0×10 14 The volume resistivity of the center portion X may be, for example, 3.0×10 14 ~20.0×10 14 It may be Ω·cm.

[0041] E X The insulation resistance of the test piece including the center X is measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance" and can be calculated by converting it into volume resistivity using the volume of the test piece. X As a test piece including a center point X for measuring , a center point X having a length x width of 40 mm x 10 mm may be used as is. The test piece including the center point X may be a test piece larger in size than the center point X, as long as it includes the center point X. The length of the test piece including the center point X may be, for example, 40 to 50 mm. The width of the test piece including the center point X may be, for example, 10 to 40 mm. The size of the test piece including the center point X may be, for example, length x width = 40 mm x 30 mm. In the case of a test piece larger in size than the center point X, the center C of the center point X may coincide with the center of the test piece.

[0042] The volume resistivity (E Y ) is 7.0 × 10 14 Ω cm or more, 10.0×10 14 Ω·cm or more, or 12.0×10 14 It may be Ω·cm or more. Y The silicon nitride plate 50 having E in the above range has better insulating properties. Y is 20.0 × 10 14 Ω cm or less, or 16.0×10 14 It may be Ω·cm or less. E Y For example, 7.0 x 10 14 ~20.0×10 14 When the volume resistivities of the four outer edge portions Y1, Y2, Y3, and Y4 are different from each other, the EY is within the above range.

[0043] E Y The insulation resistance of the test piece including the outer edge Y is measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance" and can be calculated by converting it into volume resistivity using the volume of the test piece. Y As a test piece including outer edge portion Y for measuring the thickness, an outer edge portion Y having a length x width of 40 mm x 10 mm may be used as is. The test piece including outer edge portion Y may be a test piece larger in size than outer edge portion Y, as long as it includes outer edge portion Y. The length of the test piece including outer edge portion Y may be, for example, 40 to 50 mm. The width of the test piece including outer edge portion Y may be, for example, 10 to 40 mm. The size of the test piece including outer edge portion Y may be, for example, length x width = 40 mm x 30 mm.

[0044] 3 is a perspective view showing an example of a circuit board. The circuit board 150 includes a silicon nitride plate 50 and a metal plate 70 bonded to the silicon nitride plate 50. Such a circuit board 150 has excellent reliability because it includes a silicon nitride plate 50 with reduced variation in insulation properties. The metal plates 70 are disposed on the first main surface 50A and the second main surface 50B of the silicon nitride plate 50 so as to face each other. The pair of metal plates 70 are bonded to the silicon nitride plate 50 so as to cover a portion of the first main surface 50A and a portion of the second main surface 50B of the silicon nitride plate 50. The metal plate 70 may be a copper plate or the like.

[0045] The silicon nitride plate 50 and the metal plate 70 may have the same shape and size or may differ from each other. The metal plate 70 may have a circuit pattern. The circuit pattern may be formed by etching the metal plate 70 using a resist. This makes it possible to form a circuit board or a heat sink that can sufficiently suppress leakage current, etc. By mounting such a circuit board 150 on a power module, the reliability of the power module can be improved.

[0046] [Method for manufacturing silicon nitride plates] The method for manufacturing a silicon nitride plate includes a preparation step of preparing a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder; a green sheet preparation step of producing a green sheet containing the mixed raw material; a lamination step of laminating 20 to 50 of the green sheets to produce a laminate; and a firing step of firing the laminate.

[0047] In the preparation step, a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder is prepared. The content of the silicon nitride powder in the mixed raw material may be, for example, 85.00 mass% or more, or 87.00 mass% or more. When the content of the silicon nitride powder is within the above range, the silicon nitride can be sufficiently sintered. Furthermore, the content of the silicon nitride powder may be 95.00 mass% or less, or 93.00 mass% or less. The content of the silicon nitride powder may be, for example, 85.00 to 95.00 mass%.

[0048] The content of magnesium oxide powder in the mixed raw material may be, for example, 0.50% by mass or more, or 0.70% by mass or more. By having the content of magnesium oxide powder in the above range, sintering can be promoted and the silicon nitride substrate can be obtained more smoothly. Furthermore, the content of magnesium oxide powder may be 3.00% by mass or less, or 2.00% by mass or less. The content of magnesium oxide powder may be, for example, 0.50 to 3.00% by mass.

[0049] The content of yttrium oxide powder in the mixed raw material may be, for example, 3.00 mass% or more, or 4.00 mass% or more. When the content of yttrium oxide powder is within the above range, the compatibility between magnesium oxide powder and yttrium oxide powder is improved, making it easier to form a sintering aid phase and more smoothly obtaining a silicon nitride substrate. Furthermore, the content of yttrium oxide powder may be 9.00 mass% or less, or 8.00 mass% or less. The content of yttrium oxide powder may be, for example, 3.00 to 9.00 mass%.

[0050] The mixed raw material may contain sintering aids other than magnesium oxide powder and yttrium oxide powder. Examples of other sintering aids include silicon dioxide powder. The content of silicon dioxide powder in the mixed raw material may be, for example, 0.10 to 2.00 mass %, or 0.50 to 1.50 mass %. The mixed raw material may also contain other components such as a binder, a dispersant, and a dispersion medium. Examples of the binder include those containing an organic component. Examples of the binder include an acrylic copolymer. Examples of the dispersant include an unsaturated fatty acid.

[0051] In the green sheet preparation process, a green sheet containing the prepared mixed raw material is prepared. The green sheet is prepared, for example, by the following procedure. First, a mixed raw material (raw material slurry) containing silicon nitride powder, sintering aid powder, binder, and dispersant is prepared. The raw material slurry is applied to a release film to a predetermined thickness by, for example, a doctor blade method, a calendar method, or an extrusion method. The applied raw material slurry is then dried and peeled off from the release film to obtain a green sheet. The green sheet may be processed into a desired shape by, for example, cutting. The materials and shapes of multiple green sheets may be the same or different from each other.

[0052] The green sheet may be flat. The size of the green sheet may be, for example, 120 to 300 mm, 130 to 280 mm, or 140 to 260 mm in longitudinal length. The width of the green sheet may be 80 to 250 mm, 100 to 240 mm, or 120 to 230 mm. Having the green sheet size within the above ranges facilitates the production of a laminate and allows sufficient firing to produce a silicon nitride plate with even more excellent insulation properties. The thickness of the green sheet may be, for example, 0.2 to 2.0 mm, 0.2 to 1.0 mm, 0.2 to 0.6 mm, or 0.2 to 0.5 mm.

[0053] In the lamination process, 20 to 50 of the prepared green sheets are stacked to produce a laminate. By stacking 20 to 50 sheets, the green sheets can be fired more uniformly than when the number of sheets is greater than 50, and uneven distribution of the sintering aid can be suppressed. Furthermore, excessive volatilization of the sintering aid from near the surface of the sintered body can be suppressed compared to when the number of sheets is less than 20. Therefore, by stacking 20 to 50 sheets, uneven distribution of the oxide-based sintering aid can be suppressed in the green sheets stacked near the center after firing. The silicon nitride plate obtained in this manner has a sufficiently small difference in oxygen content between the center and the outer edge, allowing for high uniformity of oxide distribution. This reduces variation in the insulating properties of the silicon nitride plate.

[0054] The number of stacked green sheets may be 23 or more, or 26 or more, from the viewpoint of further suppressing volatilization of the sintering aid. Furthermore, from the viewpoint of firing the green sheets with higher uniformity and obtaining a silicon nitride board with further reduced variation in insulation properties, the number may be 45 or less, 40 or less, or 35 or less. The number of stacked green sheets may be, for example, 23 to 45, 23 to 40, 23 to 35, 26 to 45, 26 to 40, or 26 to 35.

[0055] The laminate may be formed by stacking multiple green sheets so that their main surfaces are in contact with each other, and a release agent may be applied to the main surface of each green sheet to prevent the green sheets from adhering to each other. The release agent may be, for example, a ceramic powder such as boron nitride, or graphite powder.

[0056] Before the firing step, the prepared laminate may be heated to remove carbon in a degreasing step. In the degreasing step, the laminate is heated to reduce the binder content in each green sheet. In the degreasing step, the laminate is placed in a degreasing furnace and heated, for example, at 300°C to 700°C for 15 to 25 hours. This volatilizes the binder and dispersant contained in the green sheets, resulting in a degreased green sheet with a reduced organic component content.

[0057] In the firing step, the laminate (degreased body) is fired. The mixed raw materials in the green sheet are fired in the firing step to obtain a silicon nitride plate. When a firing furnace is used in the firing step, the degreasing furnace and the firing furnace may be the same furnace or different furnaces.

[0058] The firing temperature in the firing step may be, for example, 1700 to 2000°C, or 1750 to 1900°C. The heating time may be, for example, 4 to 20 hours, or 5 to 10 hours. The heating time refers to the time during which the predetermined firing temperature is maintained after it is reached. The predetermined firing temperature can be selected arbitrarily from within the above-mentioned firing temperature range.

[0059] The firing step can be carried out, for example, in a nitrogen gas atmosphere pressurized to 0.70 to 1.00 MPa. In such an atmosphere, the generation of insulating oxides caused by firing can be suppressed, and a silicon nitride substrate with even less variation in insulation properties can be obtained.

[0060] After the firing step, the sintered body can be cooled to room temperature to obtain a silicon nitride plate. The temperature drop rate when cooling to room temperature is not particularly limited. The temperature drop rate may be, for example, 1.0°C / min or more, or 1.2°C / min or more. The temperature drop rate may also be 3.0°C / min or less. The temperature drop rate may be in the range of, for example, 1.0 to 3.0°C / min. In this manner, multiple silicon nitride plates can be obtained.

[0061] The cooling may be performed by changing the temperature drop rate. For example, the cooling may be performed by cooling from the firing temperature to a first temperature T1 at a first temperature drop rate V1, and then cooling from the first temperature T1 to a second temperature T2 at a second temperature drop rate V2. The cooling from the second temperature T2 to room temperature may be performed by furnace cooling.

[0062] The first temperature T1 may be 1550 to 1650°C, or 1570 to 1620°C. The second temperature T2 may be 1450 to 1540°C, or 1470 to 1520°C. The first temperature decrease rate V1 may be 1.0 to 2.0°C / min, or 1.2 to 1.5°C / min. The second temperature decrease rate V2 may be a value different from the first temperature decrease rate V1 and may be 1.0 to 2.0°C / min, or 1.2 to 1.5°C / min. Under these temperature decrease conditions, cooling proceeds more uniformly, and a silicon nitride substrate with further reduced variation in insulation properties can be obtained.

[0063] The silicon nitride plate derived from the green sheet located near the center of the laminate has a more uniform distribution of the oxide-based sintering aid than the silicon nitride plate derived from the green sheet located at the top or bottom of the laminate. Therefore, it is not necessary for all of the silicon nitride plates obtained in the firing process to have the same properties as the silicon nitride plate 50. That is, of the multiple silicon nitride plates obtained in the firing process, only the silicon nitride plate obtained from the green sheet located near the center of the laminate may have the same properties as the silicon nitride plate 50. The above description can be applied directly to the silicon nitride plate 50 obtained by this manufacturing method. Because at least one of the multiple silicon nitride plates obtained in the firing process has the same properties as the silicon nitride plate 50, the manufacturing method can smoothly produce silicon nitride plates with excellent insulating properties.

[0064] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. [Example]

[0065] The present disclosure will be described in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0066] [Example 1] (Fabrication of silicon nitride plates) Silicon nitride powder and, as sintering aids, magnesium oxide powder, yttrium oxide powder, and silicon dioxide powder were prepared. The silicon nitride powder, magnesium oxide powder, yttrium oxide powder, and silicon dioxide powder were mixed in a mass ratio of Si3N4:MgO:Y2O3:SiO2 = 91.35:1.58:6.00:1.07 to obtain a raw material powder. A binder, dispersant, and dispersion medium were added to this raw material powder to prepare a mixed raw material (raw material slurry). Next, the raw material slurry was applied to a release film using a doctor blade method, adjusting the coating thickness to 0.440 mm, to produce a green sheet.

[0067] The prepared green sheets were cut into lengths of 190 to 260 mm and 160 to 200 mm, and 30 sheets were stacked to obtain a laminate. This laminate was placed in an electric furnace equipped with a carbon heater and heated in air at 500°C for 20 hours to degrease, obtaining a degreased body.

[0068] Next, the pressure inside the sintering furnace was reduced to below 100 Pa, the temperature was raised to 900°C, and the degreased body was heat-treated under vacuum. Nitrogen gas was then introduced into the sintering furnace, and the temperature was increased under a pressure of approximately 0.9 MPa. The sintering temperature was maintained at 1785°C under a pressure of approximately 0.9 MPa for 6 hours. The sintered body was then cooled to 1600°C (first temperature T1) at a first temperature decrease rate of 1.45°C / min (second temperature decrease rate V2). It was then cooled from 1600°C to 1500°C (second temperature T2) at a second temperature decrease rate of 1.33°C / min (second temperature decrease rate V2), and after reaching 1500°C, it was cooled to room temperature by furnace cooling. In this way, a silicon nitride plate having a rectangular parallelepiped shape was obtained.

[0069] (Volume resistivity measurement) The 15th silicon nitride plate from the top of the sintered silicon nitride plate stack was removed. The length and width of the removed silicon nitride plate were measured and found to be within the range of 150-210 mm and 120-160 mm. The removed silicon nitride plate was laser scribed, and the first main surface 50A was divided into multiple sections by scribe lines SL1 and SL2, as shown in Figure 4. The divisions yielded a section XA defined by a rectangle measuring 40 mm x 30 mm and including a center X at its center, and a section YA defined by a rectangle measuring 40 mm x 30 mm and including four corners with corner K as a vertex. The center of section XA coincided with the center C of the first main surface 50A. That is, section XA included the center X defined by the first imaginary rectangle R1, and section YA included the outer edge Y defined by the second imaginary rectangle R2. The center of section XA coincided with the center of section XA.

[0070] The silicon nitride plate was cut along the scribe lines SL1 and SL2, and the portion XA and the portion YA were used as test pieces. The insulation resistance values ​​of the obtained test pieces were measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance." The measured insulation resistance values ​​were calculated as the volume resistivity (×10 14 The measurement results for the test piece of section XA were taken as the volume resistivity of the center, and the measurement results for the test piece of section YA were taken as the volume resistivity of the outer edge. The measured volume resistivity values ​​for the four sections YA were identical. The ratio of the volume resistivity of section YA (outer edge) to the volume resistivity of section XA (center) was calculated as the variation in insulation (insulation ratio). The results are shown in Table 1.

[0071] (Oxygen content measurement) A laser scribing process was performed on the portion XA where the insulation resistance value of the center was measured, forming a scribe line along a first imaginary rectangle R1. A center portion X measuring 40 mm long x 10 mm wide was cut out along this scribe line. The cut-out center portion X was pulverized and introduced into an oxygen / nitrogen analyzer (product name: EMGA-920, manufactured by Horiba, Ltd.) to measure the oxygen content (mass%) of the center portion X. The results are shown in Table 1.

[0072] Next, a laser scribing process was performed on the portion YA where the insulation resistance value of the outer edge was measured, forming a scribe line along the second imaginary rectangle R2. An outer edge portion Y measuring 40 mm x 10 mm was cut out along this scribe line. The cut-out outer edge portion Y was pulverized and introduced into an oxygen / nitrogen analyzer (product name: EMGA-920, manufactured by Horiba, Ltd.) to measure the oxygen content (mass%) of the outer edge portion Y. The measurement results are shown in Table 1. The measured oxygen content values ​​of the four outer edge portions Y were identical. In addition, the difference between the oxygen content of the center portion and the oxygen content of the outer edge portion (inside / outside difference in oxygen content) was calculated. The results are shown in Table 1.

[0073] [Comparative Example 1] Silicon nitride plates were produced in the same manner as in Example 1, except that 70 green sheets were stacked. The 35th silicon nitride plate from the top was removed, and the volume resistivity and oxygen content were measured in the same manner as in Example 1. The difference between the inner and outer oxygen contents and the insulation ratio were also calculated. The results are shown in Table 1.

[0074] [Table 1]

[0075] As shown in Table 1, when a stack of 30 sheets was fired, the silicon nitride plate obtained from the center of the stack had a smaller difference in oxygen content between the inside and outside and a smaller insulation ratio than when the number of sheets was 70. This is thought to be because the smaller difference in oxygen content between the inside and outside reduced the uneven distribution of the oxide-based sintering aid in the silicon nitride plate. [Industrial Applicability]

[0076] According to the present disclosure, it is possible to provide a silicon nitride plate with reduced variation in insulation properties, and a method for manufacturing such a silicon nitride plate that can be smoothly produced. [Explanation of symbols]

[0077] 50...silicon nitride plate, 50A...first main surface, 50B...second main surface, C...center, K...corner, R1...first imaginary rectangle, R2...second imaginary rectangle, X...center, Y, Y1, Y2, Y3, Y4...outer edge, SL1, SL2...scribe line, XA, YA...portion, 70...metal plate, 150...circuit board.

Claims

1. A silicon nitride plate having a rectangular main surface, a portion defined by a first imaginary rectangle having a length x width of 40 mm x 10 mm at a center C of the main surface is defined as a central portion; When the portions defined by four second imaginary rectangles of length x width = 40 mm x 10 mm at the four corners of the main surface are defined as outer edges, A silicon nitride plate, wherein the difference between the oxygen content in the central portion and the oxygen content in each of the outer edge portions is 0.030 mass% or less.

2. The silicon nitride plate according to claim 1 , wherein the oxygen content of the central portion is greater than the oxygen content of the outer edge portion.

3. 3. The silicon nitride plate according to claim 1, wherein both the oxygen content of the central portion and the oxygen content of the outer edge portion are 2.00 mass% or more.

4. The area of ​​the main surface is 5,000 to 50,000 mm ２ 3. The silicon nitride plate according to claim 1 or 2, wherein:

5. Volume resistivity 3.0 x 10 １４ The silicon nitride plate according to claim 1 or 2, having a resistivity of Ω·cm or more.

6. A preparation step of preparing a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder; a green sheet preparation step of preparing a green sheet containing the mixed raw material; a lamination step of laminating 20 to 50 of the green sheets to prepare a laminate; and a firing step of firing the laminate.

7. At least one of the plurality of silicon nitride plates obtained in the firing step has a rectangular main surface, a portion defined by a first imaginary rectangle having a length x width of 40 mm x 10 mm at a center C of the main surface is defined as a central portion; When the portions defined by four second imaginary rectangles of length x width = 40 mm x 10 mm at the four corners of the main surface are defined as outer edges, 7. The method for producing a silicon nitride plate according to claim 6, wherein the difference between the oxygen content in the central portion and the oxygen content in each of the outer edge portions is 0.030 mass % or less.

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