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

Abstract

To provide a silicon nitride sintered body having high thermal conductivity.SOLUTION: When X-ray diffraction measurement is conducted and a maximum value of a peak intensity indicating a crystal phase of YMgSi2O5N is represented by I1, a maximum value of a peak intensity indicating a crystal phase of Y8SiN4O14 is represented by I2, a maximum value of a peak intensity indicating a crystal phase of Y2Si3N4O3 is represented by I3, and a maximum value of a peak intensity indicating a crystal phase of silicon nitride is represented by I0, the following formula (1) is satisfied, and a content of magnesium in terms of MgO is 1.25 mass% or more in a silicon nitride sintered body. I1 / (I1+I2+I3+I0)×100≤5 (1)SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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. [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 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. [Means for solving the problem]

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

[0007] [1] The maximum peak intensity indicating the crystalline phase of YMgSi2O5N detected in X-ray diffraction measurement is I1, Y8SiN4O 14 When the maximum value of the peak intensity indicating the crystalline phase of Y2Si3N4O3 is I2, the maximum value of the peak intensity indicating the crystalline phase of Y2Si3N4O3 is I3, and the maximum value of the peak intensity indicating the crystalline phase of silicon nitride is I0, the following formula (1) is satisfied: A silicon nitride sintered body having a magnesium content, calculated as MgO, of 1.25 mass% or more. I1 / (I1+I2+I3+I0)×100≦5 (1)

[0008] The silicon nitride sintered body described above in [1] has a maximum ratio of the peak intensity of the YMgSi2O5N crystalline phase detected in X-ray diffraction measurement to the peak intensity of YMgSi2O5N, Y8SiN4O 14The content of magnesium, calculated as MgO, is 5% or less of the total maximum peak intensity of Y2Si3N4O3 and silicon nitride. The magnesium content, calculated as MgO, is 1.25% by mass or more. In such silicon nitride sintered bodies, the crystalline phase of YMgSi2O5N derived from the sintering aid is sufficiently reduced, forming an amorphous structure. Meanwhile, magnesium oxide, 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 facilitated to sinter 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 mass % or less. [3] The silicon nitride sintered body according to [1] or [2], wherein the void ratio in the cut surface is 1.20% or less. [4] The silicon nitride sintered body according to any one of [1] to [3], which has a thermal conductivity of 93 W / (m·K) or more.

[0011] The silicon nitride sintered body [2] has a magnesium content of 3.0 mass % or less calculated as MgO, which gives it a higher thermal conductivity. It also allows for a reduction in the amount of magnesium oxide used, which reduces production costs.

[0012] The silicon nitride sintered body described in [3] above has a void fraction 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] One aspect of the present disclosure provides the following method for producing a silicon nitride sintered body.

[0015] [5] A preparation step of preparing a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder; A firing step of firing the mixed raw material at 1730°C or higher and lower than 1800°C, A method for producing a silicon nitride sintered body, wherein the content of the magnesium oxide powder in the mixed raw material is 2.0 mass % or more relative to the total of the silicon nitride powder and the sintering aid powder.

[0016] In the method for producing sintered silicon nitride described in [5] 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 at a temperature of 1730°C or higher but lower than 1800°C during the sintering process. Furthermore, the silicon nitride sintered body obtained in this manner is amorphous and has high thermal conductivity.

[0017] The method for producing a silicon nitride sintered body according to the above [5] may be any one of the following [6] to [8].

[0018] [6] The method for producing a silicon nitride sintered body according to [5], 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. [7] The method for producing a silicon nitride sintered body according to [5] or [6], 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. [8] The method for producing a silicon nitride sintered body according to any one of [5] to [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%.

[0019] The silicon nitride sintered body obtained by the method for producing a silicon nitride sintered body described in [6] above has a higher thermal conductivity. In addition, the amount of magnesium oxide powder used can be reduced, thereby reducing production costs.

[0020] In the method for producing a silicon nitride sintered body described in [7] 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 accelerated. The silicon nitride sintered body obtained by this production method has even higher thermal conductivity.

[0021] In the method for producing a silicon nitride sintered body described in [8] above, the silicon dioxide powder content of the mixed raw material relative to the total of the silicon nitride powder and sintering aid powder 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, because the silicon dioxide powder content in the silicon nitride sintered body is low, precipitation of silver contained in the brazing filler metal can be suppressed during the production of circuit boards. Therefore, using the silicon nitride sintered body obtained by this production method in circuit boards can improve the insulation properties of the circuit boards.

[0022] One aspect of the present disclosure provides the following circuit board and method for manufacturing the circuit board.

[0023] [9] The silicon nitride sintered body according to any one of [1] to [4] above, a metal plate bonded to the silicon nitride sintered body.

[10] 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 according to any one of [5] to [8] above.

[0024] The circuit board of the above item [9] is made of a silicon nitride sintered body having high thermal conductivity, and has excellent reliability.

[0025] The circuit board obtained by the method for producing a circuit board according to

[10] above comprises a silicon nitride sintered body having high thermal conductivity, and the circuit board obtained by such a production method has even greater reliability. [Effects of the Invention]

[0026] 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. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a perspective view showing a silicon nitride sintered body according to one embodiment. [Figure 2] FIG. 1 is a perspective view showing a circuit board according to an embodiment. [Figure 3] FIG. 2 is a diagram showing an SEM image (magnification: 200 times) of a cross section in Example 1. [Figure 4] FIG. 10 is a diagram showing an SEM image (magnification: 200 times) of a cross section in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0028] 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.

[0029] [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 contains silicon nitride, and when measured by X-ray diffraction (XRD), the maximum values ​​of the peak intensities showing the crystalline phase of YMgSi2O5N are I1, Y8SiNO 14 When the maximum value of the peak intensity showing the crystalline phase of Y2Si3N4O3 is I2, the maximum value of the peak intensity showing the crystalline phase of Y2Si3N4O3 is I3, and the maximum value of the peak intensity showing the crystalline phase of silicon nitride is I0, the following formula (1) is satisfied, and the magnesium content, calculated as 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. I1 / (I1+I2+I3+I0)×100≦5 (1)

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 (product 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 relative to the baseline (integrated intensity).

[0034] Among the peaks indicating the crystalline phase of YMgSi2O5N, the peak with the greatest intensity is detected at 2θ = 29.5 to 30.5°. 14 Of the peaks representing the crystalline phase of Y2Si3N4O3, the one with the greatest peak intensity is detected at 2θ = 28.5 to 29.5°. Of the peaks representing the crystalline phase of Y2Si3N4O3, the one with the greatest peak intensity is detected at 2θ = 31.5 to 32.5°. Furthermore, of 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°.

[0035] When I1, I2, I3, and I0 satisfy the above formula (1), the crystalline phase of YMgSiON derived from the sintering aid is sufficiently reduced, resulting in a sufficiently high amorphous fraction. Furthermore, when the magnesium content (calculated as MgO) is 1.25 mass% or more, the magnesium oxide particles used as the 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.

[0036] In the above formula (1), I1 / (I1+I2+I3+I0)×100 may be 4 or less, or 3 or less. In such a silicon nitride sintered body 50, the YMgSiON crystalline phase derived from the sintering aid is further reduced. Such a silicon nitride sintered body 50 has a higher thermal conductivity. I1 may be 0. In other words, a peak indicating the YMgSiON crystalline phase may not be detected.

[0037] I2 / (I1+I2+I3+I0)×100 may be 3 or less, or 2 or less. Furthermore, I3 / (I1+I2+I3+I0)×100 may be 1 or less, or 0.5 or less. Such a silicon nitride sintered body 50 has a sufficiently high amorphous fraction 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. I2 and I3 may be 0. That is, Y8SiN4O 14 For example, the silicon nitride sintered body 50 may contain only amorphous sintering aid phases.

[0038] 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 providing the silicon nitride sintered body 50 with even higher thermal conductivity, 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.

[0039] 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%.

[0040] The magnesium content in terms of MgO can be determined by measuring any surface of the silicon nitride sintered body 50 with 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.

[0041] 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 on the cut surface. The silicon nitride sintered body 50 contains a sufficient amount of magnesium oxide used as a sintering aid. During sintering, the 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. As a result, 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.

[0042] The void fraction can be determined by the following procedure. In a cross section of a 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 open spaces in the cross section of the silicon nitride sintered body, and can be detected by image analysis because they appear dark in color on the image.

[0043] 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."

[0044] 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.

[0045] The insulation resistance of sintered silicon nitride 50 is 13.0 x 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 The insulation resistance of the silicon nitride sintered body 50 may be 30.0×10 Ω or more. Since the silicon nitride sintered body 50 is densified and has sufficiently high insulation properties, it can be suitably used as a material for circuit boards. The insulation resistance of the silicon nitride sintered body 50 is 30.0×10 12 Ω or less, or 25.0 x 10 12 The insulation resistance may be Ω or less. The insulation resistance can be measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measurement method for insulation resistance."

[0046] [Method for manufacturing sintered silicon nitride] 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 temperature of 1730°C or higher but lower than 1800°C.

[0047] 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 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, 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.

[0048] 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, resulting in a silicon nitride sintered body 50 with a higher amorphous fraction and higher thermal conductivity.

[0049] 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.

[0050] 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%.

[0051] 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.

[0052] 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%.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] [Circuit board] 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 because it includes a silicon nitride sintered body 50 with 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. The metal plate 70 may be a copper plate or the like.

[0063] 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.

[0064] [Circuit board manufacturing method] The method for manufacturing a circuit board according to one embodiment 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 together.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. [Example]

[0069] 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.

[0070] (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.

[0071] 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.

[0072] 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 ) 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.

[0073] (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.

[0074] (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.

[0075] (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.

[0076] (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.

[0077] (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.

[0078] (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.

[0079] (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.

[0080] (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.

[0081] (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.

[0082] (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.

[0083] (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.

[0084] [Table 1]

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] <X-ray diffraction (XRD) measurement of silicon nitride sintered body> The silicon nitride sintered body was processed into a test piece measuring 10 mm x 10 mm x 0.32 mm in length x width x thickness, 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 using powder X-ray diffraction with 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.

[0090] The maximum peak intensity I1, which indicates the YMgSiON crystalline phase detected in the 2θ range of 29.5 to 30.5°, was calculated as the peak height relative to the baseline. 14The maximum peak intensity I2 indicating the Y2Si3N4O3 crystalline phase, the maximum peak intensity I3 detected in the 2θ range of 31.5 to 32.5° indicating the Y2Si3N4O3 crystalline phase, and the maximum peak intensity I0 detected in the 2θ range of 26.5 to 27.5° indicating the silicon nitride crystalline phase were determined. The ratio R1 of I1 to the sum of I1, I2, I3, and I0 was calculated using the following formula (1). Furthermore, the ratios R2 and R3 of I2 and I3 to the sum of I1, I2, I3, and I0 were calculated using the following formulas (2) and (3). The results are shown in Table 3. Table 3 shows the calculation results for formulas (1), (2), and (3), respectively. When no peaks were detected in the above ranges and the calculation result was 0, this is indicated by "-". R1=I1 / (I1+I2+I3+I0)×100 (1) R2=I2 / (I1+I2+I3+I0)×100 (2) R3=I3 / (I1+I2+I3+I0)×100 (3)

[0091] <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.

[0092] <Void fraction measurement> An arbitrary cross section of the silicon nitride sintered body 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 3. Voids 30 with a major axis of 10µm or more were detected by image analysis in an internal region of the cross section that was 10µm or more inward from the first main surface 50A and the second main surface 50B. The area ratio of the voids 30 to the entire internal region observed was calculated as the void fraction. The results are shown in Table 3.

[0093] <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.

[0094] <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.

[0095] <Insulation resistance measurement> The insulation resistance of the silicon nitride sintered body was measured in accordance with JIS C 2140:2009 "Solid electrical insulating materials - Measuring method of insulation resistance." The results are shown in Table 3.

[0096] 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. 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.

[0097] (Comparative Example 1) 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. 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.

[0098] (Examples 3 and 4) 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. 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.

[0099] (Comparative Example 2, Example 5) 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. 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.

[0100] 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.

[0101] (Examples 7 to 10, Comparative Example 3) A silicon nitride sintered body was obtained in the same manner as in Example 6, 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] 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.

[0103] Comparative Example 4 A silicon nitride sintered body was obtained in the same manner as in Example 11, except that the compounding ratio (mass ratio) of the raw material powders was set to the compounding 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.

[0104] [Table 2]

[0105] [Table 3]

[0106] As shown in Table 3, Examples 1 to 14, 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%. These results suggest that magnesium oxide as a sintering aid promotes sintering at low temperatures and densifies the material, resulting in a silicon nitride sintered body with a high amorphous content and high thermal conductivity. [Industrial Applicability]

[0107] 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. [Explanation of symbols]

[0108] 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 When this is the case, the following formula (1) is satisfied: A silicon nitride sintered body having a magnesium content, calculated as MgO, of 1.25 mass% or more. I １ / (I １ +I ２ +I ３ +I ０ )×100≦5 (1)

2. 2. The silicon nitride sintered body according to claim 1, wherein the magnesium content calculated as MgO is 3.0 mass % or less.

3. 3. The silicon nitride sintered body according to claim 1, wherein the void ratio in the cut surface is 1.20% or less.

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

5. A preparation step of preparing a mixed raw material containing silicon nitride powder and a sintering aid powder containing magnesium oxide powder and yttrium oxide powder; A firing step of firing the mixed raw material at 1730°C or higher and lower than 1800°C, a 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 of 2.0 mass % or more;

6. 6. The method for producing a silicon nitride sintered body according to claim 5, wherein the content of said magnesium oxide powder relative to the total of said silicon nitride powder and said sintering aid powder in said mixed raw material is 3.5 mass % or less.

7. 7. The method for producing a silicon nitride sintered body according to claim 5, 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.

8. 7. The method for producing a silicon nitride sintered body according to claim 5, 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%.

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

10. 7. 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 according to claim 5 or 6.

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