Silicon nitride sintered body and method for producing same, and circuit board and method for producing same

Abstract

The silicon nitride sintered body satisfies formula (1), where I1 is the maximum peak intensity showing the YMgSi2O5N crystal phase detected when x-ray diffraction measurement is performed, I2 is the maximum peak intensity showing the Y8SiN4O14 crystal phase, I3 is the maximum peak intensity showing the Y2Si3N4O3 crystal phase, and I0 is the maximum peak intensity showing the silicon nitride crystal phase, and the magnesium content in terms of MgO is 1.25 mass% or more.　I1 / (I1+I2+I3+I0)×100≤5 (1)

Description

Silicon nitride sintered body and its manufacturing method, and circuit board and its manufacturing method

[0001] The present disclosure relates to a silicon nitride sintered body and a method for producing the same, and a circuit board and a method for producing the same.

[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. Circuit boards and the like equipped with ceramic plates are used in these power modules to efficiently diffuse heat generated from semiconductor elements and suppress leakage current (see, for example, Patent Document 1). The ceramic sintered compacts used in these ceramic plates are typically produced by forming 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, in Patent Document 2, when producing a silicon nitride sintered substrate, Si powder, MgO powder, and Y powder are used. ２ O ３ It has been proposed to use powder.

[0004] International Publication No. 2019 / 022133 International Publication No. 2017 / 170247

[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 and a method for manufacturing the same. It also provides a circuit board including a silicon nitride sintered body with high thermal conductivity and a method for manufacturing the same.

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

[0007] [1] YMgSi detected during X-ray diffraction measurement ２ O ５ The maximum value of the peak intensity showing the N crystalline phase is I １ , Y ８ SiN ４ O １４ The maximum value of the peak intensity indicating the crystalline phase of ２ , Y ２ Si ３ N ４ O ３ The maximum value of the peak intensity indicating the crystalline phase of ３ , and the maximum value of the peak intensity indicating the silicon nitride crystalline phase is I ０ A silicon nitride sintered body, which satisfies the following formula (1) when expressed as I, and has a magnesium content calculated as MgO of 1.25 mass% or more. １ / (I １ +I ２ +I ３ +I ０ ) × 100≦5 (1)

[0008] The silicon nitride sintered body [1] is YMgSi detected by X-ray diffraction measurement. ２ O ５ The ratio of the maximum value of the peak intensity showing the N crystalline phase to the YMgSi ２ O ５ N.Y. ８ SiN ４ O １４ , Y ２ Si ３ N ４ O ３ and the maximum peak intensity of silicon nitride. The magnesium content, calculated as MgO, is 1.25 mass % or more. Such a silicon nitride sintered body contains YMgSi derived from a sintering aid. ２ O ５The N crystalline phase is sufficiently reduced, forming an amorphous structure. On the other hand, magnesium oxide, which is used as a sintering aid, melts at a lower temperature than other components of the sintering aid. Therefore, silicon nitride sintered bodies containing a certain amount of magnesium oxide are promoted to be sintered at low temperatures. This allows for the production of silicon nitride sintered bodies with a dense interior. Such silicon nitride sintered bodies have 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 silicon nitride sintered body according to [1], wherein the magnesium content, calculated as MgO, is 3.0% by mass or less. [3] The silicon nitride sintered body according to [1] or [2], wherein the void fraction in the cut surface is 1.20% or less. [4] The silicon nitride sintered body according to any one of [1] to [3], wherein the thermal conductivity is 93 W / (m K) or more. [5] The silicon nitride sintered body according to any one of [1] to [4], wherein the flexural strength is 720 MPa or more.

[0011] The silicon nitride sintered body described in [2] above has a magnesium content of 3.0 mass % or less, calculated as MgO, and has a higher thermal conductivity. In addition, the amount of magnesium oxide used can be reduced, thereby reducing production costs.

[0012] The silicon nitride sintered body of [3] has a void ratio of 1.20% or less on the cut surface, which reduces the air content in the silicon nitride sintered body, and therefore has a higher thermal conductivity.

[0013] The silicon nitride sintered body [4] above has improved heat dissipation properties and can be suitably used as a material for circuit boards.

[0014] The silicon nitride sintered body of [5] above has improved mechanical properties and can be suitably used as a material for circuit boards.

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

[0016] [6] A method for producing a silicon nitride sintered body, comprising: 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 firing temperature range Ts of 1730°C or higher and lower than 1800°C, wherein the content of the magnesium oxide powder in the mixed raw material relative to the total of the silicon nitride powder and the sintering aid powder is 2.0 mass% or higher.

[0017] In the method for producing a silicon nitride sintered body described in [6] above, the sintering aid magnesium oxide particles melt at low temperatures and coat the surfaces of the silicon nitride particles, allowing the silicon nitride sintered body to be densified at low temperatures. Therefore, a sufficiently densified silicon nitride sintered body can be obtained in the sintering step at a firing temperature Ts of 1730°C or higher but lower than 1800°C. Furthermore, the silicon nitride sintered body obtained in this manner is amorphous and has high thermal conductivity.

[0018] The method for producing a silicon nitride sintered body according to the above item [6] may be any one of the following items [7] to

[10] .

[0019] [7] The method for producing a silicon nitride sintered body according to [6], wherein the mixed raw material contains 3.5 mass% or less of the magnesium oxide powder relative to the total of the silicon nitride powder and the sintering aid powder. [8] The method for producing a silicon nitride sintered body according to [6] or [7], wherein the mixed raw material contains 5.0 mass% or less of the yttrium oxide powder relative to the total of the silicon nitride powder and the sintering aid powder. [9] The method for producing a silicon nitride sintered body according to any one of [6] to [8], wherein the mixed raw material contains less than 1.0 mass% of the silicon dioxide powder relative to the total of the silicon nitride powder and the sintering aid powder.

[10] The method for producing a silicon nitride sintered body according to any one of [6] to [9], further comprising, after the firing step, a cooling step of cooling the fired product from 1730°C to 1500°C at an average cooling rate of 2.0°C / min or more.

[0020] The silicon nitride sintered body obtained by the method for producing a silicon nitride sintered body described in [7] above has a higher thermal conductivity. In addition, the amount of magnesium oxide powder used can be reduced, thereby reducing production costs.

[0021] In the method for producing a silicon nitride sintered body described in [8] above, the content of yttrium oxide powder relative to the total of silicon nitride powder and sintering aid powder in the mixed raw material is 5.0 mass% or less, which improves the compatibility between the yttrium oxide powder and magnesium oxide powder and makes it easier to form a sintering aid phase. Therefore, sintering can be further promoted. The silicon nitride sintered body obtained by this production method has even higher thermal conductivity.

[0022] In the method for producing a silicon nitride sintered body described in [9] above, the content of silicon dioxide powder relative to the total of silicon nitride powder and sintering aid powder in the mixed raw material is less than 1.0 mass%, thereby suppressing the formation of crystalline phases during sintering. The silicon nitride sintered body obtained by this production method has high thermal conductivity. Furthermore, since the content of silicon dioxide powder contained in the silicon nitride sintered body is low, precipitation of silver contained in the brazing material can be suppressed during the production of circuit boards. Therefore, if the silicon nitride sintered body obtained by this production method is used in circuit boards, the insulation properties of the circuit boards can be improved.

[0023] The method for producing a silicon nitride sintered body described in

[10] above includes a cooling step after the firing step in which the fired body is cooled from 1730°C to 1500°C at an average cooling rate of 2.0°C / min or more, thereby reducing the location-dependent bias in the phase transition and grain growth of silicon nitride and achieving highly uniform phase transition and grain growth. The silicon nitride sintered body obtained by this production method has high flexural strength regardless of the stacking position during firing.

[0024] One aspect of the present disclosure provides the following circuit board and method for manufacturing the circuit board.

[0025]

[11] A circuit board comprising the silicon nitride sintered body according to any one of [1] to [5] above, and a metal plate bonded to the silicon nitride sintered body.

[12] A method for manufacturing a circuit board, comprising a bonding step of bonding a metal plate to the silicon nitride sintered body obtained by the manufacturing method according to any one of [6] to

[10] above.

[0026] The circuit board of the above

[11] includes a silicon nitride sintered body having high thermal conductivity, and such a circuit board has even greater reliability.

[0027] The circuit board obtained by the method for producing a circuit board according to the above item

[12] includes a silicon nitride sintered body having high thermal conductivity, and the circuit board obtained by such a production method has even greater reliability.

[0028] The present disclosure can provide a silicon nitride sintered body having high thermal conductivity and a method for manufacturing the same, and can also provide a circuit board including a silicon nitride sintered body having high thermal conductivity and a method for manufacturing the same.

[0029] Fig. 1 is a perspective view showing a silicon nitride sintered body according to one embodiment. Fig. 2 is a perspective view showing a circuit board according to one embodiment. Fig. 3 is a view showing an SEM image (magnification: 200 times) of a cross section in Example 1. Fig. 4 is a view showing an SEM image (magnification: 200 times) of a cross section in Comparative Example 4.

[0030] 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 "to" used in a numerical range indicates a numerical range that includes the upper and lower limit values. For example, "X to Y" indicates a numerical range "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 types.

[0031] 1 is a perspective view showing a silicon nitride sintered body according to one embodiment. The silicon nitride sintered body 50 contains silicon nitride and is detected as YMgSi when measured by X-ray diffraction (XRD). ２ O ５ The maximum value of the peak intensity showing the N crystalline phase is I １ , Y ８ SiN ４ O １４ The maximum value of the peak intensity indicating the crystalline phase of ２ , Y ２ Si ３ N ４ O ３ The maximum value of the peak intensity indicating the crystalline phase of ３ , and the maximum value of the peak intensity indicating the silicon nitride crystalline phase is I ０ When the silicon nitride sintered body 50 is formed as follows, the following formula (1) is satisfied, and the magnesium content in terms of MgO is 1.25 mass % or more. The silicon nitride sintered body 50 has a first main surface 50A and a second main surface 50B. １ / (I １ +I ２ +I ３ +I ０ ) × 100≦5 (1)

[0032] The silicon nitride sintered body 50 has, for example, a plate shape. The thickness of the silicon nitride sintered body 50 may be, for example, 0.20 mm or more, or 0.25 mm or more. The thickness of the silicon nitride sintered body 50 may be, for example, 0.50 mm or less, 0.40 mm or less, or 0.35 mm or less. The thickness of the silicon nitride sintered body 50 may be, for example, 0.20 to 0.50 mm, 0.25 to 0.40 mm, or 0.25 to 0.35 mm.

[0033] The silicon nitride sintered body 50 may contain silicon nitride as a 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), a nitride, or an oxynitride. Specific examples of the secondary component include YMgSi ２ O ５ N.Y. ８ SiN ４ O １４ , and Y ２Si ３ N ４ O ３ These components may be amorphous or crystalline.

[0034] The peaks detected by XRD measurement indicate crystalline phases. Therefore, if no peaks indicating these crystalline phases are detected, the sintering aid phase in the silicon nitride sintered body 50 does not have a crystalline structure and is amorphous. In the present disclosure, if the above formula (1) is satisfied, the sintering aid phase in the silicon nitride sintered body 50 has a sufficiently high amorphous ratio.

[0035] XRD measurement can be performed using any surface of the silicon nitride sintered body as the measurement surface, for example, using an X-ray diffractometer (trade name: D8 ADVANCE, manufactured by Bruker) with a Cu-Ka X-ray source, a diffraction angle of 10°<2θ<70°, a voltage of 40 kV, and a current of 40 mA. The intensity of the measured peak is expressed as the area of ​​the peak (integral intensity) relative to the baseline.

[0036] YMgSi ２ O ５ Among the peaks showing the N crystalline phase, the peak with the greatest intensity is detected at 2θ=29.5 to 30.5°. ８ SiN ４ O １４ Among the peaks showing the crystalline phase of Y, the peak with the greatest intensity is detected at 2θ=28.5 to 29.5°. ２ Si ３ N ４ O ３ Among the peaks representing the crystalline phase of silicon nitride, the one with the greatest peak intensity is detected at 2θ = 31.5 to 32.5°. Furthermore, among the peaks representing the crystalline phase of silicon nitride, the one with the greatest peak intensity is detected at 2θ = 26.5 to 27.5°.

[0037] I １ , I ２ , I ３ , and I ０ satisfies the above formula (1), the YMgSi derived from the sintering aid ２ O ５The N crystalline phase is sufficiently reduced, and the amorphous fraction can be sufficiently increased. Furthermore, by having a magnesium content of 1.25 mass% or more in terms of MgO, the magnesium oxide particles used as a sintering aid melt at low temperatures and coat the surfaces of the silicon nitride particles, accelerating sintering and sufficiently densifying the silicon nitride sintered body. Such silicon nitride sintered body 50 has high thermal conductivity.

[0038] I in the above formula (1) １ / (I １ +I ２ +I ３ +I ０ ) × 100 may be 4 or less, or 3 or less. Such a silicon nitride sintered body 50 is a YMgSi ２ O ５ The N crystalline phase is further reduced. Such a silicon nitride sintered body 50 has a higher thermal conductivity. １ may be 0. That is, YMgSi ２ O ５ It is not necessary to detect a peak indicating a crystalline phase of N.

[0039] I ２ / (I １ +I ２ +I ３ +I ０ ) × 100 may be 3 or less, or 2 or less. ３ / (I １ +I ２ +I ３ +I ０ ) × 100 may be 1 or less, or 0.5 or less. Such a silicon nitride sintered body 50 has a sufficiently high amorphous ratio and is sufficiently densified by the action of magnesium oxide powder, which is a sintering aid. Such a silicon nitride sintered body 50 has an even higher thermal conductivity. ２ and I ３ may be 0. That is, Y ８ SiN ４ O １４ and Y ２ Si ３ N ４ O ３For example, the entire sintering aid phase contained in the silicon nitride sintered body 50 may be amorphous.

[0040] The magnesium content in the silicon nitride sintered body 50, calculated as MgO, is 1.25% by mass or more. In such a silicon nitride sintered body 50, the magnesium oxide used as a sintering aid remains in a sufficient amount. Therefore, densification at low temperatures is promoted by the magnesium oxide, and the silicon nitride sintered body 50 has high thermal conductivity. From the viewpoint of achieving even higher thermal conductivity in the silicon nitride sintered body 50, the magnesium oxide content may be 1.30% by mass or more, 1.40% by mass or more, 1.50% by mass or more, or 1.60% by mass or more.

[0041] 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 3.0 mass% or less, or 2.50 mass% or less, and an example of the magnesium content in terms of MgO may be 1.25 to 3.0 mass%.

[0042] The magnesium content in terms of MgO can be determined by measuring any surface of the silicon nitride sintered body 50 by X-ray fluorescence (XRF) and converting the obtained magnesium content into magnesium oxide content. That is, when the magnesium content obtained by XRF measurement is A mass%, the magnesium content in terms of MgO can be calculated by A × (molecular weight of MgO / atomic weight of Mg). For example, a "ZSX Primus II" (trade name, manufactured by Rigaku Corporation) can be used as the XRF measurement device.

[0043] The silicon nitride sintered body 50 may have a void fraction of 1.20% or less, 1.10% or less, or 1.00% or less at the cut surface. The silicon nitride sintered body 50 contains a sufficient amount of magnesium oxide used as a sintering aid. During sintering, magnesium oxide particles melt at a low temperature, allowing the sintering aid phase to cover the surfaces of the silicon nitride particles without gaps, thereby reducing the number of voids that occur during sintering. With a void fraction within the above range, highly insulating air is reduced inside the silicon nitride sintered body 50. Therefore, such a silicon nitride sintered body 50 has even higher thermal conductivity. The void fraction may be 0.3% or more, or 0.5% or more.

[0044] The void fraction can be determined by the following procedure. In a cross section of the silicon nitride sintered body 50 observed with a scanning electron microscope (SEM) or the like, voids with a major axis of 10 μm or more are detected in an internal region 10 μm or more away from the first main surface 50A and the second main surface 50B using image analysis software, and the area ratio of the voids to the entire internal region is calculated. In the present disclosure, voids are gaps in the cross section of the silicon nitride sintered body, and can be detected by image analysis because they appear dark in the image.

[0045] 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 a 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 a 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."

[0046] The flexural strength of the silicon nitride sintered body 50 may be 720 MPa or more, 750 MPa or more, 800 MPa or more, or 850 MPa or more. Such silicon nitride sintered body 50 has sufficiently high mechanical properties and can be suitably used as a material for circuit boards. The flexural strength of the silicon nitride sintered body 50 can be measured by a three-point bending test in accordance with the description of JIS R 1601:2008 "Room temperature bending strength test method for fine ceramics." The flexural strength may be 1000 MPa or less, or 900 MPa or less. The flexural strength may be in the range of, for example, 720 to 1000 MPa, or 800 to 1000 MPa.

[0047] The insulation resistance of the silicon nitride sintered body 50 is 13.0 × 10 １２ Ω or more, 15.0×10 １２ Ω or more, 17.0×10 １２ Ω or more, or 18.0 x 10 １２ Such a silicon nitride sintered body 50 is highly densified and has sufficiently high insulating properties, so it can be suitably used as a material for circuit boards. The insulation resistance of the silicon nitride sintered body 50 can be 100×10 １２ Ω or less, 80.0×10 １２ Ω or less, 70.0×10 １２ Ω or less, 50.0×10 １２ Ω or less, 30.0×10 １２ Ω or less, or 25.0 x 10 １２ An example of the range of the insulation resistance is 13.0×10 １２ ~100 x 10 １２ The insulation resistance may be Ω. The insulation resistance can be measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance."

[0048] [Method for producing silicon nitride sintered body] A method for producing a silicon nitride sintered body according to one embodiment 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 firing temperature Ts of 1730°C or higher but lower than 1800°C.

[0049] In the mixed raw material, the content of magnesium oxide powder relative to the total of silicon nitride powder and sintering aid powder is 2.0% by mass or more. By having a magnesium oxide powder content of 2.0% by mass or more, the sintering aid powder melts at low temperatures 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 low temperatures in this manner, the silicon nitride can be sintered at a low temperature during the temperature rise, resulting in a densified silicon nitride sintered body 50. Furthermore, the sintering aid phase in the densified silicon nitride sintered body 50 has a sufficiently high amorphous proportion and high thermal conductivity.

[0050] 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 promoted, making it possible to obtain a silicon nitride sintered body 50 with a higher amorphous proportion and higher thermal conductivity.

[0051] 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 having the magnesium oxide content 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.

[0052] 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, making it easier to form a sintering aid phase, and enabling the smooth production of the silicon nitride sintered body 50. 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%.

[0053] The mass ratio of the magnesium oxide powder to the yttrium oxide powder in the mixed raw material may be 0.3 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-mentioned 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.

[0054] 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 having the content of silicon dioxide powder as a sintering aid 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 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 0 mass%.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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 multiple green sheets prepared as described above is heated to reduce the binder component content 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.

[0059] 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.

[0060] 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.

[0061] In the firing step, the mixed raw material is fired at a firing temperature range Ts of 1730°C or higher but lower than 1800°C. The firing temperature range Ts may be 1730 to 1780°C, or 1730 to 1770°C. In the firing step, the mixed raw material is fired to obtain a flat plate-shaped silicon nitride sintered 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.

[0062] In the firing step, the rate of temperature rise to the firing temperature may be 2.0°C / min or less, 1.5°C / min or less, 1.2°C / min or less, or 1.0°C / min or less. By keeping the temperature rise rate within the above range, it is possible to obtain a silicon nitride sintered body having even higher flexural strength regardless of the stacking position of the silicon nitride sintered body. The rate of temperature rise to the firing temperature may be 0.1°C / min or more, or 0.3°C / min or more. An example of the range of the rate of temperature rise to the firing temperature may be 0.1 to 2.0°C / min.

[0063] The heating time in the firing step may be, for example, 4 to 20 hours or 4 to 12 hours. The heating time refers to the time during which the temperature is maintained within the firing temperature range Ts. The firing temperature range Ts is 1730°C or higher and lower than 1800°C. Some of the magnesium oxide may fly off during the firing step.

[0064] After the firing step, a temperature-reducing step may be performed in which the fired product is cooled from 1730°C to 1500°C at an average temperature-reducing rate of 2.0°C / min or more. After the temperature-reducing step, the fired product is cooled to room temperature to obtain the silicon nitride sintered body 50. The average temperature-reducing rate can be calculated from the time (min) required to reduce the temperature of the fired product from 1730°C to 1500°C by 230°C. In other words, the average temperature-reducing rate can be determined by calculating 230°C / (time required to cool to 230°C).

[0065] By setting the average temperature decreasing rate to 2.0°C / min or more, the location-dependent bias in the phase transition and grain growth of silicon nitride is reduced, and the phase transition and grain growth can be performed with high uniformity. Therefore, the flexural strength of the silicon nitride sintered body can be increased regardless of the stacking position during firing. The temperature decreasing rate when cooling to room temperature is not particularly limited. From the viewpoint of further increasing the flexural strength of the silicon nitride sintered body, the average temperature decreasing rate may be, for example, 5.0°C / min or more, 10°C / min or more, or 20°C / min or more. Furthermore, the average temperature decreasing rate may be 50°C / min or less. An example of the range of the average temperature decreasing rate may be 5.0 to 50°C / min.

[0066] The temperature decreasing rate in the temperature decreasing step may be the same as or different from the average temperature decreasing rate. When the temperature decreasing rate and the average temperature decreasing rate are the same, the temperature decreasing rate is constant. For example, the temperature decreasing rate may be adjusted arbitrarily so that the average temperature decreasing rate in the temperature decreasing step satisfies the above range. The temperature decreasing rate in the temperature decreasing step may be, for example, 1.0°C / min or more, 5.0°C / min or more, 10°C / min or more, or 20°C / min or more. Furthermore, the temperature decreasing rate may be 100°C / min or less, 50°C / min or less. An example of the temperature decreasing rate range may be 1.0 to 100°C / min.

[0067] [Circuit Board] Figure 2 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 due to the silicon nitride sintered body 50 having high thermal conductivity. The metal plates 70 are disposed 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. Examples of the metal plate 70 include a copper plate.

[0068] 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.

[0069] [Method for manufacturing circuit board] A method for manufacturing a circuit board according to one embodiment includes a joining step of joining a metal plate 70 to a silicon nitride sintered body 50. A brazing filler metal can be used to join the silicon nitride sintered body 50 and the metal plate 70.

[0070] 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 brazing filler metal may have a ratio of Ag to the total of Ag and Cu of 80% by mass or more, 91% by mass or more, 95% by mass or more, 98% by mass or more, or 100% by mass.

[0071] In the bonding process, 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 the 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 process, 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 process 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 a silicon nitride sintered body 50 with high thermal conductivity.

[0072] A power module may be manufactured using the circuit board 150. The power module can be manufactured by mounting a semiconductor element electrically connected to a copper plate of the circuit board using solder, wire bonding, or the like, and housing the circuit board and the semiconductor element in a housing space of a housing, followed by resin sealing.

[0073] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments.

[0074] 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.

[0075] (Reference Example 1-1) <Preparation of silicon nitride sintered body> 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, a dispersant, and a 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, and the applied thickness was adjusted to 0.440 mm, to produce a green sheet.

[0076] 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, thereby obtaining a degreased body.

[0077] 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 sintering was initiated by raising the temperature at a rate of 2.1°C / min under a pressure of approximately 0.9 MPa. 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 The density and open porosity of sintered fine ceramics were measured in accordance with JIS R 1634:1998. The results are shown in Table 1.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] (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.

[0084] 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.

[0085] 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.

[0086] (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.

[0087] 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.

[0088] 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.

[0089]

[0090] As shown in Table 1, Reference Examples 1-1 to 1-3, which had 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 had a low magnesium oxide content, and densification at low temperatures was promoted. 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 the silicon nitride sintered body at 1500 to 1700°C.

[0091] Example 1 <Preparation of silicon nitride sintered body> 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 ratios (mass ratios) 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, and the applied thickness was adjusted to 0.440 mm, to produce a green sheet.

[0092] 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, thereby obtaining a degreased body.

[0093] 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 1600°C at a rate of 0.5°C / min and to 1775°C at a rate of 1.4°C / min under a pressure of approximately 0.9 MPa. The firing temperature of 1775°C was maintained for 5.5 hours, and then cooled to 1500°C at a rate of 0.9°C / min. The body was then allowed to cool naturally to room temperature (20°C), yielding a silicon nitride sintered body.

[0094] <X-ray diffraction (XRD) measurement of silicon nitride sintered body> The silicon nitride sintered body was processed into a test piece measuring length x width x thickness = 10 mm x 10 mm x 0.32 mm, and XRD measurement was performed using an X-ray diffractometer (product name: Ultima IV, manufactured by Rigaku Corporation). X-ray diffraction patterns of the main surfaces of the test pieces were obtained by powder X-ray diffraction using Cu-Kα radiation in the diffraction angle range of 10° < 2θ < 70°. The target of the sealed tube was Cu, a Ni filter was used, and a one-dimensional semiconductor detector was used.

[0095] YMgSi detected in the range of 2θ = 29.5 to 30.5° ２ O ５ The maximum value of the peak intensity indicating the crystalline phase of N, I １ was calculated as the peak height relative to the baseline. ８ SiN ４ O １４ The maximum peak intensity I indicating the crystalline phase ２ , Y detected in the range of 2θ = 31.5 to 32.5° ２ Si ３ N ４ O ３ The maximum peak intensity I indicating the crystalline phase ３ , the maximum value I of the peak intensity indicating the silicon nitride crystalline phase detected in the range of 2θ = 26.5 to 27.5° ０ was calculated from the following formula (1). １ , I ２ , I ３ , and I ０ I for the sum of １ The ratio R1 was calculated from the following formulas (2) and (3). １ , I ２ , I ３ , and I ０ I for the sum of ２ , I ３ The ratios R2 and R3 were calculated. The results are shown in Table 3. In Table 3, the calculation results of Equation (1), Equation (2), and Equation (3) are shown. When no peak was detected in the above range and the calculation result was 0, it was indicated by "-". R1 = I １ / (I １ +I ２ +I３ +I ０ )×100 (1) R2=I ２ / (I １ +I ２ +I ３ +I ０ )×100 (2) R3=I ３ / (I １ +I ２ +I ３ +I ０ ) x 100 (3)

[0096] <Measurement of magnesium content in terms of MgO> The magnesium content of an arbitrary cross section of the silicon nitride sintered body was measured using an X-ray fluorescence (XRF) measurement device "ZSX Primus II" (trade name, manufactured by Rigaku Corporation). The obtained magnesium content was multiplied by (molecular weight of MgO / atomic weight of Mg) to calculate the magnesium content (mass %) in terms of MgO. The results are shown in the MgO column in Table 3.

[0097] <Measurement of Void Fraction> An arbitrary cross section of the silicon nitride sintered body 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 3. In the internal region of the cross section, which is 10 µm or more inward from the first main surface 50A and the second main surface 50B, voids 30 with a major axis of 10 µm or more were detected by image analysis. The area ratio of the voids 30 to the entire observed internal region was calculated as the void fraction. The results are shown in Table 3.

[0098] <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.

[0099] <Measurement of flexural 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 "Test method for room temperature flexural strength of fine ceramics." The results are shown in Table 3.

[0100] <Measurement of insulation resistance> The insulation resistance of the silicon nitride sintered body was measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials -- Measurement method of insulation resistance." The results are shown in Table 3.

[0101] Example 2 A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the blending ratio (mass ratio) of the raw material powders was set to the blending ratio shown in Table 2. XRD, magnesium oxide content, void fraction, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0102] Comparative Example 1 A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the blending ratio (mass ratio) of the raw material powders was set to the blending ratio shown in Table 2. XRD, magnesium oxide content, void fraction, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0103] Examples 3 and 4 Silicon nitride sintered bodies were obtained in the same manner as in Example 1, except that the blending ratios (mass ratios) of the raw material powders were set to those shown in Table 2. XRD, magnesium oxide content, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0104] (Comparative Example 2, Example 5) A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the compounding ratios (mass ratios) of the raw material powders were set to those shown in Table 2. XRD, magnesium oxide content, void fraction, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0105] Example 6 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. XRD, magnesium oxide content, void fraction, thermal conductivity, flexural strength, and insulation resistance were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0106] (Examples 7 to 10, Comparative Example 3) Silicon nitride sintered bodies were obtained in the same manner as in Example 6, except that the compounding ratios (mass ratios) of the raw material powders were set to those shown in Table 2. XRD, magnesium oxide content, void fraction, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0107] (Example 11) A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the firing temperature was set to 1795°C. XRD, magnesium oxide content, void fraction, thermal conductivity, and flexural strength were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3.

[0108] Comparative Example 4 A silicon nitride sintered body was obtained in the same manner as in Example 11, except that the blending ratio (mass ratio) of the raw material powders was set to the blending ratio shown in Table 2. XRD, magnesium oxide content, void fraction, thermal conductivity, flexural strength, and insulation resistance were measured in the same manner as in Example 1. R1, R2, and R3 were calculated from the XRD measurement results. The results are shown in Table 3. In addition, an SEM image (magnification 200x) of the cross section of Comparative Example 4 is shown in FIG.

[0109]

[0110]

[0111] As shown in Table 3, Examples 1 to 11, which had a high amorphous proportion in the sintering aid phase, had higher thermal conductivities than Comparative Examples 1, 2, 3, and 4, which had a high crystalline proportion. Furthermore, all of the Examples with a high amorphous proportion had a magnesium oxide content of more than 1.25 mass%. From these results, it is believed that magnesium oxide as a sintering aid promotes sintering at low temperatures and densifies the material, thereby allowing for the production of a silicon nitride sintered body with a high amorphous content and high thermal conductivity.

[0112] Example 12 Silicon nitride powder and magnesium oxide powder and yttrium oxide powder as sintering aids were mixed in the same blending ratio (mass ratio) as in Example 1 to obtain a raw material powder. Seventy green sheets were produced from the raw material powder using the same procedure as in Example 1. A laminate of the green sheets was degreased using the same procedure as in Example 1 to obtain a degreased body. The degreased body was fired using the same procedure as in Example 1, except that the firing temperature was 1795°C. After firing, the degreased body was cooled from 1795°C to 1500°C at a temperature decrease rate of 3.9°C / min. The body was then allowed to cool naturally to room temperature (20°C) to obtain a silicon nitride sintered body. Note that the temperature decrease rate was constant, so the average temperature decrease rate was the same as the temperature decrease rate. One of the obtained silicon nitride sintered bodies was randomly selected and subjected to XRD measurement and measurement of the magnesium content (MgO equivalent) using the same procedure as in Example 1. The results are shown in Table 4. In Table 4, calculation results of 0 are indicated by "-".

[0113] Of the obtained silicon nitride sintered bodies, the third, 36th, and 68th silicon nitride sintered bodies from the top were each taken out. The thermal conductivity, flexural strength, void fraction, and insulation resistance of each silicon nitride sintered body were measured in the same manner as in Example 1. The results are shown in Table 5. In Table 5, the void fraction of the silicon nitride sintered body at the 68th stacking position was not measured, and is therefore indicated by "-".

[0114] Example 13 Silicon nitride powder and magnesium oxide powder and yttrium oxide powder as sintering aids were mixed in the same blending ratio (mass ratio) as in Example 1 to obtain a raw material powder. 70 green sheets were produced from the raw material powder using the same procedure as in Example 1. A laminate of the green sheets was degreased using the same procedure as in Example 1 to obtain a degreased body. The degreased body was fired using the same procedure as in Example 1, except that the firing temperature was 1755°C and the heating rate from 1600°C to 1755°C was 1.0°C / min. After firing, the degreased body was cooled from 1755°C to 1500°C at a temperature decrease rate of 10°C / min. The body was then allowed to cool naturally to room temperature (20°C) to obtain a silicon nitride sintered body. Since the temperature decrease rate was constant, the average temperature decrease rate was the same as the temperature decrease rate. One of the silicon nitride sintered bodies obtained was randomly selected and subjected to XRD measurement and measurement of the magnesium content in terms of MgO in the same manner as in Example 1. The results are shown in Table 4. In Table 4, calculation results of 0 are indicated by "-".

[0115] Of the obtained silicon nitride sintered bodies, the third, 36th, and 68th silicon nitride sintered bodies from the top were each taken out. The thermal conductivity, flexural strength, void fraction, and insulation resistance of each silicon nitride sintered body were measured using the same procedures as in Example 1. The results are shown in Table 5. In Table 5, the void fraction of the silicon nitride sintered body at the 68th stacking position was not measured, and is therefore indicated by "-".

[0116] Example 14 A silicon nitride sintered body was obtained in the same manner as in Example 13, except that the firing temperature was 1775°C. One of the obtained silicon nitride sintered bodies was taken out at random, and XRD measurement and measurement of the magnesium content in terms of MgO were carried out in the same manner as in Example 1. The results are shown in Table 4. In Table 4, calculation results of 0 are indicated by "-".

[0117] Of the obtained silicon nitride sintered bodies, the third, 36th, and 68th silicon nitride sintered bodies from the top were each taken out. The thermal conductivity, flexural strength, void fraction, and insulation resistance of each silicon nitride sintered body were measured using the same procedures as in Example 1. The results are shown in Table 5. In Table 5, the void fraction of the silicon nitride sintered body at the 68th stacking position was not measured, and is therefore indicated by "-".

[0118]

[0119]

[0120] Examples 12 to 14 in Table 4 had different heating rates and cooling rates from Examples 1 to 11 in Table 2. Even when the heating rate and cooling rate during firing were changed, the MgO content was 1.25 mass% or more, and the sintering aid phase was amorphous. Furthermore, as shown in Table 5, the silicon nitride sintered bodies of Examples 12 to 14 exhibited not only high thermal conductivity but also high flexural strength.

[0121] Comparing Example 12 in Table 3 with Example 11 in Table 5, which differ only in the cooling rate, Example 12 exhibited higher flexural strength than Example 11. The cooling rate for Example 12 was 3.9°C / min, which was higher than the 0.9°C / min cooling rate for Example 11. This indicates that increasing the cooling rate significantly increases the flexural strength of the silicon nitride sintered body. Furthermore, comparing Example 14 in Table 5 with Example 1 in Table 3, Example 14 exhibited higher flexural strength than Example 1. The cooling rate for Example 14 (10°C / min) was significantly higher than the cooling rate for Example 1 (0.9°C / min), which is thought to have significantly improved the flexural strength.

[0122] Comparing Example 13 and Example 14 in Tables 4 and 5, Example 13, in which the firing temperature was 1755° C., had a higher flexural strength than Example 14, in which the firing temperature was 1775° C. On the other hand, Example 14 had a higher thermal conductivity than Example 13. In other words, it was confirmed that the flexural strength and thermal conductivity could be adjusted by changing the firing temperature.

[0123] According to the present disclosure, there are provided a silicon nitride sintered body having high thermal conductivity and a method for manufacturing the same, and a circuit board including a silicon nitride sintered body having high thermal conductivity and a method for manufacturing the same.

[0124] 50...silicon nitride sintered body, 50A...first main surface, 50B...second main surface, 70...metal plate, 150...circuit board, 30...void.

Claims

1. YMgSi detected during X-ray diffraction measurement ２ O ５ The maximum value of the peak intensity showing the N crystalline phase is I １ , Y ８ SiN ４ O １４ The maximum value of the peak intensity indicating the crystalline phase of ２ , Y ２ Si ３ N ４ O ３ The maximum value of the peak intensity indicating the crystalline phase of ３ , and the maximum value of the peak intensity indicating the silicon nitride crystalline phase is I ０ A silicon nitride sintered body, which satisfies the following formula (1) when expressed as I, and has a magnesium content calculated as MgO of 1.25 mass% or more. １ / (I １ +I ２ +I ３ +I ０ ) × 100≦5 (1) 2. The silicon nitride sintered body according to claim 1, wherein the magnesium content calculated as MgO is 3.0 mass % or less.

3. The silicon nitride sintered body according to claim 1 or 2, having a void ratio of 1.20% or less in the cut surface.

4. The silicon nitride sintered body according to claim 1 or 2, having a thermal conductivity of 93 W / (m·K) or more.

5. The silicon nitride sintered body according to claim 1 or 2, having a bending strength of 720 MPa or more.

6. A method for producing a silicon nitride sintered body, comprising: 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 firing temperature range Ts of 1730°C or higher and lower than 1800°C, wherein the content of the magnesium oxide powder in the mixed raw material relative to the total of the silicon nitride powder and the sintering aid powder is 2.0 mass% or higher.

7. A method for producing a silicon nitride sintered body according to claim 6, wherein the content of the magnesium oxide powder in the mixed raw material relative to the total of the silicon nitride powder and the sintering aid powder is 3.5 mass % or less.

8. A method for producing a silicon nitride sintered body according to claim 6 or 7, wherein the content of the yttrium oxide powder in the mixed raw material relative to the total of the silicon nitride powder and the sintering aid powder is 5.0 mass% or less.

9. A method for producing a silicon nitride sintered body according to claim 6 or 7, wherein the content of silicon dioxide powder in the mixed raw material relative to the total of the silicon nitride powder and the sintering aid powder is less than 1.0 mass%.

10. A method for producing a silicon nitride sintered body according to claim 6 or 7, which includes, after the firing step, a temperature-lowering step of cooling the fired product from 1730°C to 1500°C at an average temperature-lowering rate of 2.0°C / min or more.

11. A circuit board comprising: the silicon nitride sintered body according to claim 1 or 2; and a metal plate bonded to the silicon nitride sintered body.

12. A method for producing a circuit board, comprising a bonding step of bonding a metal plate to the silicon nitride sintered body obtained by the method of claim 6 or 7.

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