Silicon nitride sintered body and circuit board

Abstract

To provide a silicon nitride sintered body having high thermal conductivity.SOLUTION: A silicon nitride sintered body has a surface including a first main surface and a second main surface. In an internal region located at 10 μm or more away from the surface among cut surfaces obtained by cutting in a direction orthogonal to the first main surface and the second main surface, an average value of an area of voids contained in a plurality of measurement regions each having 100 μm square and different from each other is 1.400 μm2 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a silicon nitride sintered body and a circuit board. [Background technology]

[0002] In recent years, power modules for controlling large amounts of power have been used in industrial equipment such as motors and in products such as electric vehicles. Such power modules use circuit boards or the like equipped with ceramic plates in order to efficiently diffuse heat generated from semiconductor elements and suppress leakage current (see, for example, Patent Document 1). The ceramic sintered body used in such ceramic plates is usually produced by forming a ceramic raw material powder into a predetermined shape to form a ceramic compact, and then firing the ceramic compact.

[0003] Known ceramic sintered bodies are composed of nitrides, carbides, borides, silicides, etc. When producing such ceramic sintered bodies, sintering aids are used to promote sintering. For example, Patent Document 2 proposes using Si powder, MgO powder, and YO powder when producing a silicon nitride sintered substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 022133 [Patent Document 2] International Publication No. 2017 / 170247 Summary of the Invention [Problem to be solved by the invention]

[0005] In electronic components such as power modules, silicon nitride sintered bodies are required to have high thermal conductivity to improve heat dissipation efficiency. Sintering aids used in sintering silicon nitride melt when heated to form a sintering aid phase. The sintering aid phase dissolves the surface of silicon nitride particles and contracts due to surface tension, covering the surface, causing phase transition and grain growth of silicon nitride. Therefore, controlling the internal structure of the silicon nitride sintered body by adjusting the blend of sintering aids used and firing conditions such as firing temperature may potentially improve the thermal conductivity of the silicon nitride sintered body. Therefore, the present disclosure provides a silicon nitride sintered body with high thermal conductivity. It also provides a circuit board including a silicon nitride sintered body with high thermal conductivity. [Means for solving the problem]

[0006] One aspect of the present disclosure provides the following silicon nitride sintered body.

[0007] [1] A silicon nitride sintered body having a surface including a first main surface and a second main surface, In an internal region of a cut surface obtained by cutting along a direction perpendicular to the first principal surface and the second principal surface, the average area of ​​voids contained in a plurality of measurement regions of 100 μm square that are different from each other is 1.400 μm. 2 The silicon nitride sintered body is as follows:

[0008] The silicon nitride sintered body described above [1] has an average void area of ​​1.400 μm in the internal region of the cut surface. 2 The following is true: Such a silicon nitride sintered body has little variation in the area of ​​voids in the internal region. This improves the internal uniformity of the silicon nitride sintered body. Such a silicon nitride sintered body has high thermal conductivity.

[0009] The silicon nitride sintered body of the above [1] may be any one of the following [2] to [4].

[0010] [2] The variance of the void area in the plurality of measurement areas is 3.50 μm 2 The silicon nitride sintered body according to [1], which is as follows: [3] The silicon nitride sintered body according to [1] or [2], wherein when the number of measurement regions is nine, the total number of voids is 3,000 or less. [4] The silicon nitride sintered body according to any one of [1] to [3], wherein the average value of the ratio of the area of ​​the voids determined in each of the plurality of measurement regions is 3.00% or less.

[0011] The silicon nitride sintered body of [2] above has smaller variations in void area, which results in improved uniformity of the silicon nitride sintered body and higher thermal conductivity.

[0012] The silicon nitride sintered body described in [3] above has a small total number of voids, which results in a small amount of pores in the silicon nitride sintered body, and therefore has a higher thermal conductivity.

[0013] The silicon nitride sintered body described in [4] above has a small void area ratio in multiple measurement regions. This results in a small amount of pores in the silicon nitride sintered body. Therefore, such a silicon nitride sintered body has a higher thermal conductivity.

[0014] The silicon nitride sintered body of the above [4] may be the following [5].

[0015] [5] The silicon nitride sintered body according to [4], wherein the standard deviation of the void area ratio is 0.200% or less.

[0016] The silicon nitride sintered body described in [5] above has a small variation in the ratio of void area, which reduces internal porosity while further improving the uniformity of the silicon nitride sintered body and resulting in higher thermal conductivity.

[0017] One aspect of the present disclosure provides the following circuit board.

[0018] [6] The silicon nitride sintered body according to any one of [1] to [5] above, a metal plate bonded to the silicon nitride sintered body.

[0019] The circuit board of the above item [6] is made of sintered silicon nitride, which has high thermal conductivity. Such a circuit board has even greater reliability. [Effects of the Invention]

[0020] The present disclosure can provide a silicon nitride sintered body having high thermal conductivity, and can also provide a circuit board including a silicon nitride sintered body having high thermal conductivity. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a perspective view showing a silicon nitride sintered body according to one embodiment. [Figure 2] 1 is a cross-sectional view showing an enlarged portion of a cross section of a silicon nitride sintered body cut along a direction perpendicular to the surface. FIG. [Figure 3] FIG. 1 is a perspective view showing a circuit board according to an embodiment. [Figure 4] FIG. 2 is a diagram showing an SEM image (magnification: 200 times) of a cross section in Example 1. [Figure 5] FIG. 1 is a diagram showing an SEM image (magnification: 200 times) of a cross section in Example 2. [Figure 6] FIG. 1 is a diagram showing an SEM image (magnification: 200 times) of a cross section in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present disclosure will be described. 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. The upper or lower limit of a numerical range specified in this disclosure may be replaced with any value shown in the examples. Furthermore, the upper and lower limits individually stated may be arbitrarily combined. The symbol "~" used in a numerical range indicates a numerical range that includes the upper and lower limit. For example, "X~Y" indicates a numerical range of "greater than or equal to X and less than or equal to Y." Unless otherwise specified, the materials or components exemplified in this disclosure can be used alone or in combination of two or more.

[0023] [Silicon nitride sintered body] 1 is a perspective view showing a silicon nitride sintered body according to one embodiment. The silicon nitride sintered body 50 has a surface including a first main surface 50A and a second main surface 50B, and in a cross section obtained by cutting the silicon nitride sintered body 50 along a direction X perpendicular to the first main surface 50A and the second main surface 50B, in an internal region 10 μm or more away from the surface (50A, 50B), the average area of ​​voids included in multiple measurement regions of 100 μm square that are different from each other is 1.400 μm. 2 By measuring the area of ​​voids in a plurality of different regions, it is possible to reduce variations in the measurement regions and obtain more reliable values.

[0024] The silicon nitride sintered body 50 has, for example, a plate shape. The silicon nitride sintered body 50 may have a thickness of, for example, 0.20 mm or more, or 0.25 mm or more. The silicon nitride sintered body 50 may have a thickness of, for example, 0.50 mm or less, 0.40 mm or less, or 0.35 mm or less. The silicon nitride sintered body 50 may have a thickness of, for example, 0.20 to 0.50 mm, 0.25 to 0.40 mm, or 0.25 to 0.35 mm.

[0025] The silicon nitride sintered body 50 may contain silicon nitride as the main component and a compound having at least one of Si, Mg, and Y as a secondary component, as a sintering aid phase. The sintering aid phase is derived from the sintering aid. The compound may be an oxide (complex oxide), nitride, or oxynitride. Specific examples of the secondary component include YMgSiON and YSiNO. 14 , and Y2Si3N4O3. These components may be amorphous or crystalline.

[0026] FIG. 2 is an enlarged cross-sectional view of a portion of a cross section of a silicon nitride sintered body 50 cut along a direction X perpendicular to the first and second main surfaces 50A and 50B. The silicon nitride sintered body 50 has an internal region C sandwiched between imaginary lines 51A and 51B that are 10 μm apart from the first and second main surfaces 50A and 50B. The internal region C also has multiple measurement regions D1, D2, and D3, each 100 μm square, that do not overlap with each other. The number of measurement regions may be 3 or more, 4 or more, or 5 or more. Alternatively, the number may be 10 or less. The number of measurement regions may be, for example, 3 to 10. Such a cross section can be obtained using a scanning electron microscope (SEM). In this case, the nine measurement regions may be obtained by dividing an SEM image magnified at 200 times into nine sections.

[0027] The average value y of the area of ​​the voids 30 contained in the measurement areas D1, D2, and D3 is the arithmetic mean of the area of ​​each void 30 contained in D1, the area of ​​each void 30 contained in D2, and the area of ​​each void 30 contained in D3. In other words, it is the value obtained by dividing the total number of voids 30 contained in the entire measurement area by the total number of voids 30. When the average value y of the area of ​​the voids 30 is 1.400 μm 2 By keeping the average area y of the voids 30 at 1.300 μm or less, the variation in the area of ​​the voids in each measurement region is reduced. Therefore, the variation in the area of ​​the voids in the entire silicon nitride sintered body is also reduced, improving the internal uniformity. This improves the thermal conductivity of the silicon nitride sintered body. 2 Less than or equal to 1.100 μm 2The average area of ​​the voids 30 may be 0.400 μm or less. Such a silicon nitride sintered body has improved internal uniformity and thermal conductivity. 2 It may be more than that.

[0028] In the present disclosure, a void refers to a void where the distance between the two most distant points on the outline of the void observed in a cross section such as that shown in Figure 2 is 0.5 μm or more. The area of ​​the void 30 can be determined as follows. In each of the measurement areas D1, D2, and D3, the void 30 can be detected using image analysis software, and the area of ​​the void 30 can be determined. In the present disclosure, a void is an open space in the cross section of the silicon nitride sintered body, and can be detected by image analysis because it appears dark in the image.

[0029] The variance of the area of ​​the voids 30 included in the measurement areas D1, D2, and D3 is 3.50 μm 2 The variance of the area of ​​the voids 30 within the above range further improves the uniformity of the silicon nitride sintered body, resulting in a higher thermal conductivity. From the viewpoint of further improving the uniformity of the silicon nitride sintered body, the variance of the area of ​​the voids 30 included in the measurement regions D1, D2, and D3 may be 3.00 μm or less. 2 Below, 2.500μm 2 Less than or equal to 2,000 μm 2 The variance of the area of ​​the voids 30 included in the measurement regions D1, D2, and D3 may be 0.200 μm or less. 2 The total number of voids is n, and the area of ​​the void is x i , when the average value of the void area is y, the variance σ 2 can be calculated using the following formula (1), where n is a natural number.

[0030]

number

[0031] When there are nine measurement regions, the total number of voids 30 may be 3,000 or less. Such a silicon nitride sintered body has a low pore content and therefore has a higher thermal conductivity. From the viewpoint of further reducing the pore content and further improving the thermal conductivity, the total number of voids 30 contained in the nine measurement regions may be 2,500 or less, 2,000 or less, or 1,500 or less. The total number of voids 30 contained in the nine measurement regions may be 10 or more, or 50 or more.

[0032] The average value w of the area ratio of voids 30 determined in each of the measurement regions D1, D2, and D3 may be 3.00% or less. The area ratio of voids 30 is determined as the ratio of the area of ​​voids 30 to the area of ​​one measurement region. The average value w of the area ratio of voids 30 can be determined by determining the area ratio of voids for each of the measurement regions D1, D2, and D3 and dividing the total value of the ratios by the number of measurement regions (three in this embodiment). When the average value w of the area ratio of voids 30 determined in this manner is 3.00% or less, the pore content in the silicon nitride sintered body is reduced. Therefore, such a silicon nitride sintered body has a higher thermal conductivity. From the viewpoint of further reducing the pore content and further improving the thermal conductivity, the average value w of the area ratio of voids 30 determined in each of the measurement regions D1, D2, and D3 may be 2.00% or less, 1.50% or less, or 1.200% or less. The average value w of the area ratio of the voids 30 determined in each of the measurement regions D1, D2, and D3 may be 0.500% or more.

[0033] The standard deviation of the area ratio of the voids 30 may be 0.200% or less. Such a silicon nitride sintered body has small variations in the voids 30. Therefore, the uniformity of the silicon nitride sintered body is further improved, and the thermal conductivity is further increased. From the viewpoint of further improving the uniformity of the silicon nitride sintered body, the standard deviation of the area ratio of the voids 30 may be 0.060% or less, or 0.050% or less. The standard deviation of the area ratio of the voids 30 may be 0.010% or more. When the number of measurement regions is m and the area ratio of the voids in one measurement region is v, iWhen the average value of the void area is w, the standard deviation σ can be calculated using the following formula (2), where m is a natural number.

[0034]

number

[0035] The magnesium content in the silicon nitride sintered body 50, calculated as MgO, may be 1.40% by mass or more. In such a silicon nitride sintered body 50, the magnesium oxide used as a sintering aid remains sufficiently. Magnesium oxide used as a sintering aid melts at a lower temperature than other components of the sintering aid. Therefore, a silicon nitride sintered body containing a certain amount of magnesium oxide is accelerated in sintering at a low temperature, and is densified with high uniformity. Such a silicon nitride sintered body has a higher thermal conductivity. From the viewpoint of obtaining a higher thermal conductivity of the silicon nitride sintered body 50, the magnesium oxide content may be 1.50% by mass or more, or 1.60% by mass or more.

[0036] From the viewpoint of reducing the amount of magnesium oxide powder used and reducing production costs, the magnesium content in terms of MgO may be 3.00 mass% or less, or 2.00 mass% or less, and an example of the magnesium content in terms of MgO may be 1.40 to 3.00 mass%.

[0037] The magnesium content in terms of MgO can be determined by converting the elemental magnesium content obtained by X-ray fluorescence (XRF) measurement of any surface of the silicon nitride sintered body 50 into the magnesium oxide content. That is, when the elemental magnesium content obtained by XRF measurement is A mass %, the magnesium content in terms of MgO can be calculated by A × (molecular weight of magnesium oxide / atomic weight of magnesium). The XRF measurement device can be, for example, a "ZSX Primus II" (trade name, manufactured by Rigaku Corporation).

[0038] The thermal conductivity of the silicon nitride sintered body 50 may be 93 W / (m·K) or more. Because such a silicon nitride sintered body 50 has sufficiently high thermal conductivity, it can be suitably used as a material for circuit boards. The thermal conductivity may be 95 W / (m·K) or more, 97 W / (m·K) or more, or 100 W / (m·K) or more. Having the thermal conductivity of the silicon nitride sintered body 50 within this range makes it even more suitable for use as a material for circuit boards. The thermal conductivity of the silicon nitride sintered body 50 may be 110 W / (m·K) or less. The thermal conductivity of the silicon nitride sintered body 50 can be measured by the laser flash method in accordance with JIS R 1601:2010, "Method for measuring thermal diffusivity, specific heat capacity, and thermal conductivity of fine ceramics by the flash method."

[0039] The flexural strength of the silicon nitride sintered body 50 may be 720 MPa or more, 750 MPa or more, or 800 MPa or more. The flexural strength of the silicon nitride sintered body 50 can be measured by a three-point bending test in accordance with JIS R 1601:2008 "Room temperature bending strength test method for fine ceramics." The flexural strength may be 900 MPa or less.

[0040] An example of a method for producing a silicon nitride sintered body is described below. The method for producing a silicon nitride sintered body 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, and a firing step of firing the mixed raw material at a temperature of 1730°C or higher but lower than 1800°C.

[0041] In the mixed raw material, the content of magnesium oxide powder relative to the total of silicon nitride powder and sintering aid powder may be 2.0% by mass or more. When the content of magnesium oxide powder is 2.0% by mass or more, the sintering aid powder melts at a low temperature and shrinks due to surface tension, allowing the surfaces of the silicon nitride particles to be covered with a sintering aid phase. By covering the surfaces of the silicon nitride particles with the sintering aid phase at a low temperature in this manner, the silicon nitride can be sintered at a low temperature during the temperature rise, thereby obtaining a densified silicon nitride sintered body 50. Furthermore, the silicon nitride sintered body 50 obtained by densification can have reduced variation in void area.

[0042] In the mixed raw material, the content of magnesium oxide powder relative to the total of silicon nitride powder and sintering aid powder may be 2.3 mass% or more, 2.5 mass% or more, or 2.8 mass% or more. When the content of magnesium oxide powder is within the above range, the sintering-promoting effect of magnesium oxide in the sintering aid is further enhanced, and a silicon nitride sintered body 50 with a uniform interior and higher thermal conductivity can be obtained.

[0043] The content of magnesium oxide powder may be 3.5% by mass or less, 3.2% by mass or less, or 3.1% by mass or less. By keeping the content of magnesium oxide within the above range, the amount of magnesium oxide powder used can be reduced, thereby reducing production costs. In the mixed raw material, the content of magnesium oxide powder relative to the total of silicon nitride powder and sintering aid powder may be, for example, 2.0 to 3.5% by mass.

[0044] In the mixed raw material, the content of the yttrium oxide powder relative to the total of the silicon nitride powder and the sintering aid powder may be 5.0 mass% or less, 4.5 mass% or less, or 4.0 mass% or less. When the content of the yttrium oxide powder is within the above range, the compatibility between the magnesium oxide powder and the yttrium oxide powder is improved, the sintering aid phase is easily formed, and the silicon nitride sintered body 50 can be smoothly produced. Furthermore, the content of the yttrium oxide powder may be 2.0 mass% or more, or 2.5 mass% or more. In the mixed raw material, the content of the yttrium oxide powder relative to the total of the silicon nitride powder and the sintering aid powder may be, for example, 2.0 to 5.0 mass%.

[0045] The mass ratio of the magnesium oxide powder to the yttrium oxide powder in the mixed raw material may be 0.5 to 1.5, 0.5 to 1.2, 0.6 to 1.2, 0.7 to 1.1, 0.8 to 1.1, or 0.9 to 1.1. When the mass ratio of the magnesium oxide powder to the yttrium oxide powder in the mixed raw material is within the above range, the compatibility between the magnesium oxide powder and the yttrium oxide powder is further improved, the sintering aid phase is more easily formed, and the silicon nitride sintered body 50 can be smoothly produced. From the viewpoint of further improving the compatibility between the magnesium oxide powder and the yttrium oxide powder and more smoothly producing the silicon nitride sintered body 50, the mass ratio of the magnesium oxide powder to the yttrium oxide powder in the mixed raw material may be, for example, 1:1.

[0046] In the mixed raw material, the content of silicon dioxide powder relative to the total of silicon nitride powder and sintering aid powder may be less than 1.0 mass% or 0.5 mass% or less. By ensuring that the content of silicon dioxide powder as a sintering aid is within the above range, the content of silicon dioxide contained in the silicon nitride sintered body 50 can be reduced. This can suppress the precipitation of silver contained in the brazing filler metal when manufacturing a circuit board, thereby improving the insulation of the circuit board. Therefore, such silicon nitride sintered body 50 can be more suitably used as a material for circuit boards. Note that the content of silicon dioxide powder relative to the total of silicon nitride powder and sintering aid powder in the mixed raw material may be 0 mass%.

[0047] The mixed raw material may contain a binder and a dispersant. A green sheet may be prepared using the mixed raw material prepared in this manner. The green sheet is produced, for example, by the following procedure. First, a mixed raw material (raw material slurry) containing silicon nitride powder, sintering aid powder, a binder, and a dispersant is prepared. The binder may include one containing an organic component. The binder may be, for example, an acrylic copolymer. The dispersant may be, for example, an unsaturated fatty acid.

[0048] 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 the multiple green sheets may be the same or different from each other.

[0049] The green sheet may have a flat plate shape. The size of the green sheet is not particularly limited, but may be, for example, 170 to 300 mm or 170 to 200 mm in diagonal length. The thickness of the green sheet may be, for example, 0.2 to 2 mm, 0.2 to 1 mm, 0.2 to 0.6 mm, or 0.2 to 0.5 mm.

[0050] Before the firing step, a degreasing step may be performed in which the prepared green sheet is heated to remove carbon. In the degreasing step, a laminate formed by stacking a plurality of green sheets prepared as described above is heated to reduce the content of the binder component in each green sheet. The number of green sheets constituting the laminate may be, for example, 20 to 150 sheets, or 50 to 100 sheets. By keeping the number of green sheets constituting the laminate within the above range, deformation of the green sheets themselves due to their weight can be further suppressed, and productivity can be improved.

[0051] 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.

[0052] In the degreasing step, the laminate is placed in a degreasing furnace and heated to, for example, 300° C. to 700° C. This causes the binder and dispersant contained in the green sheet to volatilize, reducing the content of organic components in the green sheet.

[0053] In the firing step, the mixed raw material is fired at a firing temperature of 1730°C or higher and lower than 1800°C. The mixed raw material is fired in the firing step to obtain a flat-plate sintered silicon nitride body. The mixed raw material may be fired in the form of a green sheet, or may be fired directly using a hot press or the like without being formed into a green sheet. When a firing furnace is used in the firing step, the degreasing furnace used for degreasing and the firing furnace used for firing may be the same furnace or different furnaces.

[0054] The heating time in the calcination step may be, for example, 8 to 20 hours or 8 to 12 hours. The heating time refers to the time during which the predetermined calcination temperature is maintained after the temperature reaches the predetermined calcination temperature. The predetermined calcination temperature can be selected arbitrarily within the above-mentioned calcination temperature range. A portion of the magnesium oxide may fly off during the calcination step.

[0055] After the firing step, the fired product can be cooled to room temperature to obtain the silicon nitride sintered body 50. The temperature drop rate when cooling to room temperature is not particularly limited. The temperature drop rate may be, for example, 10°C / min or more, or 20°C / min or more.

[0056] [Circuit board] 3 is a perspective view showing a circuit board according to one embodiment. The circuit board 150 includes a silicon nitride sintered body 50 and a metal plate 70 bonded to the silicon nitride sintered body 50. Such a circuit board 150 has excellent reliability because it includes a silicon nitride sintered body 50 with high thermal conductivity. The metal plates 70 are arranged on the first main surface 50A and the second main surface 50B of the silicon nitride sintered body 50 so as to face each other. The pair of metal plates 70 are bonded to the silicon nitride sintered body 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 sintered body 50. The metal plate 70 may be a copper plate or the like.

[0057] The silicon nitride sintered body 50 and the metal plate 70 may have the same shape and size or may be different 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.

[0058] An example of a method for manufacturing a circuit board is described below. The method for manufacturing a circuit board includes a bonding step of bonding a metal plate 70 to a silicon nitride sintered body 50. A brazing filler metal can be used to bond the silicon nitride sintered body 50 and the metal plate 70.

[0059] The brazing filler metal may contain Ag in the form of a metal element or a metal compound (alloy), and may contain, in addition to Ag, one or more metals selected from the group consisting of Cu, Sn, and active metals. Each metal may be contained as an alloy or compound. The active metal may include one or more metals selected from the group consisting of Ti, Hf, Zr, and Nb. The ratio of Ag to the total of Ag and Cu in the brazing filler metal may be 80% by mass or more, 91% by mass or more, 95% by mass or more, 98% by mass or more, or 100% by mass.

[0060] In the bonding step, a brazing filler metal is applied to the first main surface 50A and the second main surface 50B of the silicon nitride sintered body 50, and a metal plate 70 is laminated on the laminate. The resulting laminate is then heated to obtain a circuit board 150 in which the silicon nitride sintered body 50 and the metal plate 70 are bonded together via a bonding layer. In the bonding step, the laminate may be heated using a heating furnace. Heating may be performed while pressing the laminate in the stacking direction. The heating bonds the silicon nitride sintered body 50 and the multiple metal plates 70 together via the bonding layer. The heating temperature may be, for example, 700 to 900°C. The atmosphere in the heating furnace may be an inert gas such as nitrogen, and the heating may be performed under reduced pressure below atmospheric pressure or in a vacuum. The circuit board 150 thus obtained is highly reliable because it includes the silicon nitride sintered body 50 with high thermal conductivity.

[0061] A power module may be manufactured using the circuit board 150. The power module can be manufactured by mounting a semiconductor element electrically connected by soldering, wire bonding, or the like on a copper plate of the circuit board, housing the circuit board and the semiconductor element in the housing space of a housing, and then sealing with resin.

[0062] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. [Example]

[0063] The present disclosure will be described in more detail with reference to examples, comparative examples, and reference examples, but the present disclosure is not limited to the following examples.

[0064] (Reference example 1-1) <Preparation of sintered silicon nitride> Silicon nitride powder and, as sintering aids, magnesium oxide powder and yttrium oxide powder were prepared. These were mixed in the compounding ratio (mass ratio) shown in Table 1 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 onto a release film by the doctor blade method, adjusting the coating thickness to 0.440 mm, and a green sheet was produced.

[0065] The green sheets were cut into a size of 250 mm x 180 mm, and 70 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 the laminate, obtaining a degreased body.

[0066] Next, the pressure inside the firing furnace was reduced to 100 Pa or less, the temperature was raised to 900°C, and the degreased body was heat-treated under vacuum. Nitrogen gas was then introduced into the firing furnace, and the temperature was raised to 1500°C at a heating rate of 2.1°C / min under a pressure of approximately 0.9 MPa to initiate sintering. After the firing temperature reached 1500°C, the body was allowed to cool to room temperature to obtain a silicon nitride sintered body. The sintered body density (g / cm) of the silicon nitride sintered bodies fired at each temperature was 3 ) was measured according to JIS R 1634:1998 "Method of measuring sintered density and open porosity of fine ceramics." The results are shown in Table 1.

[0067] (Reference example 1-2) A silicon nitride sintered body was produced in the same manner as in Reference Example 1-1, except that the firing temperature was set to 1600° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0068] (Reference example 1-3) A silicon nitride sintered body was produced in the same manner as in Reference Example 1-1, except that the firing temperature was set to 1700° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0069] (Reference example 2-1) A silicon nitride sintered body was produced and the density of the sintered body was determined in the same manner as in Reference Example 1-1, except that the compounding ratio of silicon nitride powder, magnesium oxide powder, and yttrium oxide powder was set as shown in Table 1. The results are shown in Table 1.

[0070] (Reference example 2-2) A silicon nitride sintered body was produced in the same manner as in Reference Example 2-1, except that the firing temperature was set to 1600° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0071] (Reference example 2-3) A silicon nitride sintered body was produced in the same manner as in Reference Example 2-1, except that the firing temperature was set to 1700° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0072] (Reference example 3-1) A silicon nitride sintered body was produced and the density of the sintered body was determined in the same manner as in Reference Example 1-1, except that the compounding ratio of silicon nitride powder, magnesium oxide powder, and yttrium oxide powder was set as shown in Table 1. The results are shown in Table 1.

[0073] (Reference example 3-2) A silicon nitride sintered body was produced in the same manner as in Reference Example 3-1, except that the firing temperature was set to 1600° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0074] (Reference example 3-3) A silicon nitride sintered body was produced in the same manner as in Reference Example 3-1, except that the firing temperature was set to 1700° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0075] (Reference example 4-1) A silicon nitride sintered body was produced and the density of the sintered body was determined in the same manner as in Reference Example 1-1, except that the compounding ratio of silicon nitride powder, magnesium oxide powder, and yttrium oxide powder was set as shown in Table 1. The results are shown in Table 1.

[0076] (Reference example 4-2) A silicon nitride sintered body was produced in the same manner as in Reference Example 4-1, except that the firing temperature was set to 1600° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0077] (Reference example 4-3) A silicon nitride sintered body was produced in the same manner as in Reference Example 4-1, except that the firing temperature was set to 1700° C., and the density of the sintered body was calculated. The results are shown in Table 1.

[0078] [Table 1]

[0079] As shown in Table 1, Reference Examples 1-1 to 1-3, which contain a high magnesium oxide content, had higher sintered body densities at 1500°C, 1600°C, and 1700°C than Reference Examples 2-1 to 2-3, which contain a low magnesium oxide content, demonstrating promoted densification at low temperatures. Furthermore, as shown in Reference Examples 3-1 to 3-3 and 4-1 to 4-3, the sintered body density did not change significantly even when the yttrium oxide content was increased from Reference Examples 1-1 to 1-3 and 2-1 to 2-3. Therefore, it was confirmed that magnesium oxide promotes densification of silicon nitride sintered bodies at 1500 to 1700°C.

[0080] Example 1 <Preparation of sintered silicon nitride> Silicon nitride powder and, as sintering aids, magnesium oxide powder, yttrium oxide powder, and silicon dioxide powder were prepared. These were mixed in the compounding ratio (mass ratio) shown in Table 2 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 by the doctor blade method, adjusting the coating thickness to 0.440 mm, and a green sheet was produced.

[0081] The green sheets were cut into a size of 250 mm x 180 mm, and 70 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 the laminate, obtaining a degreased body.

[0082] Next, the pressure inside the firing furnace was reduced to 100 Pa or less, the temperature was raised to 900°C, and the degreased body was heat-treated under vacuum. Nitrogen gas was then introduced into the firing furnace, and the temperature was raised under a pressure of approximately 0.9 MPa at a rate of 0.5°C / min to 1600°C and then 1.4°C / min to 1775°C. The firing temperature of 1775°C was maintained for 5.5 hours, and then cooled to obtain a silicon nitride sintered body.

[0083] <Measurement of magnesium content converted to MgO> The magnesium element content was measured for an arbitrary cross section of the silicon nitride sintered body using an X-ray fluorescence (XRF) analyzer "ZSX Primus II" (trade name, manufactured by Rigaku Corporation). The magnesium element content obtained was multiplied by (molecular weight of MgO / atomic weight of Mg) to calculate the magnesium content (mass%) converted to MgO. The results are shown in the MgO column in Table 3.

[0084] <Analysis of voids in the measurement area> A cross section of the silicon nitride sintered body cut along a direction perpendicular to the main surface was observed at 200x magnification using a scanning electron microscope, and a photograph of the cross section was obtained. The photograph of the cross section is shown in Figure 4. Nine measurement areas, each 100 µm square, were selected without overlapping in the internal region of the cross section, located at least 10 µm inward from the first main surface 50A and the second main surface 50B. Voids 30 with a distance between the two most distant points on the contour of 0.5 µm or more were detected by image analysis in the nine measurement areas. The average void area, area variance, and total number of voids in the nine measurement areas were calculated. Furthermore, the void area ratio was calculated for each of the nine measurement areas, and the average void area ratio and standard deviation of the void area ratio among the nine measurement areas were calculated. The results are shown in Table 3.

[0085] <Measurement of thermal conductivity> After surface treatment (carbon blackening) of the silicon nitride sintered body, the thermal conductivity was measured in accordance with JIS R 1601:2010, "Method for measuring thermal diffusivity, specific heat capacity, and thermal conductivity of fine ceramics by the flash method." A laser flash method thermal property measuring device (product name: TC-7SB RT, manufactured by ULVAC) was used to measure the thermal conductivity. The results are shown in Table 3.

[0086] <Measurement of bending strength> The flexural strength of the silicon nitride sintered body was measured by a three-point bending test in accordance with JIS R 1601:2008 "Room temperature bending strength test method for fine ceramics." The results are shown in Table 3.

[0087] Example 2 A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the compounding ratio (mass ratio) of the raw material powders was set to the compounding ratio shown in Table 2. The magnesium content in terms of MgO, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. The results are shown in Table 3.

[0088] In addition, a cross section obtained by cutting the silicon nitride sintered body in a direction perpendicular to the main surface using the same procedure as in Example 1 was observed with a scanning electron microscope at 200x magnification, and a photograph of the cross section was obtained. The photograph of the cross section is shown in Figure 5. In the cross section, voids in the measurement area were analyzed using the same procedure as in Example 1, and the average void area, area variance, and total number of voids in the nine measurement areas were calculated. Furthermore, the void area ratio was calculated for each of the nine measurement areas, and the average void area ratio among the nine measurement areas and the standard deviation of the void area ratio were calculated. The results are shown in Table 3.

[0089] Example 3 A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the firing temperature was set to 1738°C. The magnesium content in terms of MgO, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. The results are shown in Table 3.

[0090] In addition, a cross section obtained by cutting the silicon nitride sintered body along a direction perpendicular to the main surface using the same procedure as in Example 1 was observed using a scanning electron microscope at 200x magnification, and a photograph of the cross section was taken. In the cross section, voids in the measurement area were analyzed using the same procedure as in Example 1, and the average void area, area variance, and total number of voids in the nine measurement areas were calculated. Furthermore, the void area ratio was calculated for each of the nine measurement areas, and the average void area ratio between the nine measurement areas was calculated. The results are shown in Table 3.

[0091] Example 4 A silicon nitride sintered body was obtained in the same manner as in Example 3, except that the compounding ratio (mass ratio) of the raw material powders was set to the compounding ratio shown in Table 2. The magnesium content (MgO equivalent), thermal conductivity, and flexural strength were measured in the same manner as in Example 1. The results are shown in Table 3.

[0092] In addition, a cross section obtained by cutting the silicon nitride sintered body in a direction perpendicular to the main surface using the same procedure as in Example 1 was observed using a scanning electron microscope at 200x magnification, and a photograph of the cross section was taken. In the cross section, voids in the measurement area were analyzed using the same procedure as in Example 1, and the average void area, area variance, and total number of voids in the nine measurement areas were calculated. Furthermore, the void area ratio was calculated for each of the nine measurement areas, and the average void area ratio and standard deviation of the void area ratio among the nine measurement areas were calculated. The results are shown in Table 3.

[0093] (Comparative Example 1) A silicon nitride sintered body was obtained in the same manner as in Example 3, except that the compounding ratio (mass ratio) of the raw material powders was set to the compounding ratio shown in Table 2. The magnesium content in terms of MgO and thermal conductivity were measured in the same manner as in Example 1. The results are shown in Table 3.

[0094] In addition, a cross section obtained by cutting the silicon nitride sintered body in a direction perpendicular to the main surface using the same procedure as in Example 1 was observed with a scanning electron microscope at 200x magnification, and a photograph of the cross section was obtained. The photograph of the cross section is shown in Figure 6. In the cross section, voids in the measurement area were analyzed using the same procedure as in Example 1, and the average void area, area variance, and total number of voids in the nine measurement areas were calculated. Furthermore, the void area ratio was calculated for each of the nine measurement areas, and the average void area ratio among the nine measurement areas and the standard deviation of the void area ratio were calculated. The results are shown in Table 3.

[0095] [Table 2]

[0096] [Table 3]

[0097] As shown in Table 3, Examples 1 to 4, which had a small average area, had higher thermal conductivities than Comparative Example 1, which had a large average area. Therefore, it is believed that the lack of unevenness in voids across multiple measurement regions and the uniformity of the interior result in a silicon nitride sintered body with high thermal conductivity. [Industrial Applicability]

[0098] According to the present disclosure, a silicon nitride sintered body having high thermal conductivity is provided, and a circuit board including the silicon nitride sintered body having high thermal conductivity is also provided. [Explanation of symbols]

[0099] 50...silicon nitride sintered body, 50A...first main surface, 50B...second main surface, 51A, 51B...imaginary lines, C...internal area, D1, D2, D3...measurement area, X...direction, 70...metal plate, 150...circuit board, 30...void.

Claims

1. A silicon nitride sintered body having a surface including a first main surface and a second main surface, In an internal region of a cut surface obtained by cutting along a direction perpendicular to the first main surface and the second main surface, the average area of ​​voids contained in a plurality of measurement regions of 100 μm square that are different from each other is 1.400 μm. ２ The silicon nitride sintered body is as follows:

2. The variance of the void area in the plurality of measurement regions is 3.50 μm ２ 2. The silicon nitride sintered body according to claim 1, wherein:

3. 3. The silicon nitride sintered body according to claim 1, wherein the number of voids is 3,000 or less when the number of measurement regions is nine.

4. 3. The silicon nitride sintered body according to claim 1, wherein the average value of the ratio of the area of ​​the voids determined in each of the plurality of measurement regions is 3.00% or less.

5. 5. The silicon nitride sintered body according to claim 4, wherein the standard deviation of the void area ratio is 0.200% or less.

6. The silicon nitride sintered body according to claim 1 or 2, a metal plate bonded to the silicon nitride sintered body.

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