Silicon nitride circuit board

Abstract

To provide a silicon nitride circuit board capable of improving a removal rate of an unnecessary brazing material while securing bonding strength with a metal circuit and a metal heat radiation plate.SOLUTION: A silicon nitride circuit substrate includes: a silicon nitride substrate having a first main surface and a second main surface; a metal circuit bonded to the first main surface of the silicon nitride substrate; and a metal heat dissipation plate bonded to the second main surface of the silicon nitride substrate. The metal heat dissipation plate is 0.1 mm or more thicker than the metal circuit. With the silicon nitride substrate, surface roughness Ra2 of the second main surface is 0.10 μm or more greater than that of Ra1 of the first main surface.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ceramic substrate, a ceramic circuit board, and a method for manufacturing a ceramic substrate, and particularly to a ceramic substrate having different surface roughness Ra values on one main surface and the other main surface, a ceramic circuit board using the ceramic substrate, and a method for manufacturing a ceramic substrate having different surface roughness Ra values on one main surface and the other main surface.

Background Art

[0002] Conventionally, a ceramic circuit board in which a ceramic substrate is joined to a metal circuit and a metal heat sink has been used in semiconductor modules, power modules, and the like.

[0003] The ceramic circuit board used in such a power module or the like is formed by joining a metal circuit and a metal heat sink to a ceramic substrate having high insulation, high mechanical strength, high thermal conductivity, etc., and a semiconductor chip or the like is further joined to the metal circuit. Since this semiconductor chip generates heat during its operation, the ceramic substrate, the metal circuit, and the metal heat sink are required to have good thermal conductivity so that the heat can be dissipated. At the same time, the ceramic substrate is also required to have high insulation (electrical resistivity).

[0004] In recent years, as a ceramic substrate having such characteristics, those mainly composed of silicon nitride (Si3N4) have attracted attention and have been variously studied (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In order to form a ceramic circuit board using such a ceramic substrate, for example, a brazing material is applied to the surface of the ceramic substrate, and a metal circuit or a metal heat sink is joined to the ceramic substrate via the brazing material.

[0007] When joining via a brazing material, generally, the surface of the ceramic substrate to which the brazing material is applied is roughened to sufficiently roughen the surface, thereby increasing the bonding strength between the ceramic substrate and the metal circuit or the metal heat sink.

[0008] By the way, in forming a metal circuit joined to a ceramic substrate, first, a metal plate is joined to the surface of the ceramic substrate via a brazing material, and a circuit pattern is formed by etching the metal plate or the like. Therefore, the brazing material in the portion removed by the etching process or the like is exposed on the surface and is unnecessary for the ceramic circuit board, so a removal process is performed.

[0009] At this time, the surface of the ceramic substrate is roughened to ensure the bonding strength, and there are cases where the brazing material cannot be sufficiently removed. That is, the roughened substrate surface has relatively large irregularities, and the brazing material enters the inside of the irregularities, and it may be difficult to remove the brazing material that has entered the inside of the irregularities.

[0010] Therefore, the present invention has been made for the purpose of providing a silicon nitride circuit board that can easily remove unnecessary brazing material while ensuring the bonding strength with a metal circuit or a metal heat sink.

Means for Solving the Problems

[0011] The ceramic substrate in one embodiment is a silicon nitride substrate having a first main surface and a second main surface, a metal circuit bonded to the first main surface of the silicon nitride substrate, and a metal heat sink bonded to the second main surface of the silicon nitride substrate. The silicon nitride circuit substrate is characterized in that the metal heat sink is 0.1 mm or more thicker than the metal circuit, and the surface roughness Ra2 of the second main surface of the silicon nitride substrate is 0.10 μm or more greater than the surface roughness Ra1 of the first main surface.

Advantages of the Invention

[0012] According to the silicon nitride circuit substrate of one embodiment, it is possible to easily remove unnecessary brazing material while ensuring the bonding strength with the metal circuit and the metal heat sink.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

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Figure 8

Embodiments for Carrying Out the Invention

[0014] In all the drawings for explaining the embodiments, the same members are generally denoted by the same reference numerals, and repeated explanations thereof are omitted. Note that, in order to make the drawings easy to understand, hatching may be added even to a plan view or a side view.

[0015] <Ceramic substrate> The ceramic substrate in the present embodiment is a ceramic substrate having two surfaces as main surfaces. For example, as shown in FIG. 1, there is a ceramic substrate 1 having a main surface 1a on the front side and a main surface 1b on the back side which is the opposite surface. Note that the terms “front surface” and “back surface” are used for convenience to distinguish each surface.

[0016] In the present embodiment, the main surface 1a and the main surface 1b respectively indicate surfaces on which a metal circuit and a metal heat sink are formed, and this ceramic substrate 1 is a substrate used for forming a ceramic circuit board. Hereinafter, this case will be described as an example.

[0017] In the present embodiment, the surface roughness Ra1 of the main surface 1a is 0.5 μm or less, and the surface roughness Ra2 of the main surface 1b is 0.5 μm or more. And the surface roughness Ra2 is 0.10 μm or more larger than the surface roughness Ra1. That is, the difference between these surface roughnesses Ra1 and Ra2 is 0.10 μm or more.

[0018] Here, by setting the surface roughness Ra1 to 0.5 μm or less, it is possible to easily remove unnecessary solder on the surface of the ceramic substrate exposed by the etching process for forming the metal circuit. Further, the surface roughness Ra1 is preferably 0.2 μm or more, and more preferably 0.35 μm or more. By setting it within such a range, the bonding strength between the ceramic substrate and the metal circuit can be ensured. That is, by setting the surface roughness Ra1 to be 0.2 μm or more and 0.5 μm or less, the surface roughness is adjusted to be relatively small so that the bonding strength between the ceramic substrate and the metal circuit is not impaired and the solder is less likely to remain in the unevenness.

[0019] Moreover, by setting the surface roughness Ra2 to 0.5 μm or more, the bonding strength between the ceramic substrate and the metal heat sink can be improved. In addition, by improving the bonding strength between the ceramic substrate and the metal heat sink, it becomes possible to increase the thickness of the metal heat sink. If the metal heat sink is thickened, for example, heat generated by the operation of a semiconductor chip mounted on a metal circuit can be efficiently released to the outside through the metal heat sink. Also, the surface roughness Ra2 is preferably 1.0 μm or less, and more preferably 0.7 μm or less. By setting it within such a range, it is possible to make it difficult for the brazing material to remain on the ceramic portion exposed by etching. That is, by setting the surface roughness Ra2 to 0.5 μm or more and 1.0 μm or less, the bonding strength between the ceramic substrate and the metal heat sink is improved, the heat dissipation efficiency by the metal heat sink is made good, and it is adjusted to a range in which it is easy to remove excess brazing material.

[0020] Furthermore, the surface roughness Ra2 is made 0.10 μm or more greater than the surface roughness Ra1. Thereby, both the actions and effects on the above-described respective main surfaces can be ensured in a preferable state. The difference between the surface roughness Ra1 and the surface roughness Ra2 is preferably 0.80 μm or less, more preferably 0.65 μm or less, still more preferably 0.50 μm or less, and particularly preferably 0.35 μm or less.

[0021] In this specification, the surface roughness Ra is the arithmetic mean roughness calculated in accordance with JIS B 0601:2001.

[0022] <Method for manufacturing a ceramic substrate> The ceramic substrate of the present embodiment can be used, for example, as an insulating substrate used in a power module. A power module is, for example, an electronic device that constitutes an inverter circuit for controlling a motor provided in an electric vehicle, a hybrid electric vehicle, a railway vehicle, or industrial equipment.

[0023] As the ceramic substrate, as described above, a ceramic substrate for providing a metal circuit and a metal heat sink on each main surface is preferable. For example, ceramic substrates made of silicon nitride, aluminum nitride, silicon carbide, alumina, etc. can be mentioned, and a silicon nitride substrate is preferable.

[0024] Next, the manufacturing method of the ceramic substrate of the present embodiment will be described. The ceramic substrate can be manufactured according to a known manufacturing method and can be obtained by processing the surface roughness Ra of each main surface to have a desired relationship. Hereinafter, the manufacturing of a silicon nitride substrate will be described as an example.

[0025] (1-1) Preparation of slurry (slurry preparation process) First, rare earth element oxides and magnesium compounds are added to silicon powder as a raw material for the substrate to obtain raw material powder, which is pulverized by a method such as media dispersion to prepare a slurry. Hereinafter, the raw materials used will be described in detail.

[0026] (a) Silicon As the silicon used here, industrially available grade silicon powder can be used. The silicon before pulverization is preferably a powder having a median diameter D50 of 6 μm or more, a BET specific surface area of 3 m2 / g or less, an oxygen content of 1.0 mass% or less, and an impurity C content in silicon of 0.15 mass% or less, and more preferably a powder having a median diameter D50 of 7 μm or more, a BET specific surface area of 2.5 m2 / g or less, an oxygen content of 0.5 mass% or less, and an impurity C content in silicon of 0.10 mass% or less. The purity of the silicon powder is preferably 99% or more, and more preferably 99.5% or more.

[0027] Impurity oxygen contained in silicon is one of the factors that inhibit the heat conduction of a silicon nitride substrate obtained by reaction sintering, and it is preferably as little as possible. Further, in the present embodiment, as will be described later, it is preferable to adjust the total amount of impurity oxygen contained in the silicon powder and oxygen from the magnesium compound by restricting the amount of oxygen from the magnesium compound. At this time, in the powder raw material, it is preferable that the total amount of oxygen is in the range of 0.1 to 1.1% by mass with respect to silicon converted to silicon nitride.

[0028] In addition, impurity carbon contained in silicon may inhibit the growth of silicon nitride particles in a silicon nitride substrate obtained by reaction sintering. As a result, it becomes one of the factors causing insufficient densification and deterioration of heat conduction and insulation.

[0029] In the present specification, the BET specific surface area (m2 / g) is a value obtained by the BET single-point method (JIS R 1626:1996 "Method for Measuring Specific Surface Area of Fine Ceramics Powder by Gas Adsorption BET Method") using a BET specific surface area meter, and the median diameter D50 (μm) is the particle diameter when the cumulative frequency becomes 50% in the particle size distribution obtained by the laser diffraction / scattering method.

[0030] Although it is not essential in the manufacturing method of the present embodiment, the raw material powder may contain silicon nitride powder. However, since the cost is high when using silicon nitride compared to silicon, it is better to use as little silicon nitride as possible. Further, when silicon nitride is used in the raw material powder, the contribution of the nitriding process described later becomes small, and thus the difference in surface roughness between the main surfaces required in the present embodiment may be difficult to obtain. Therefore, the amount of silicon nitride used is preferably 20 mol% or less, more preferably 10 mol% or less, and still more preferably 5 mol% or less of silicon (converted to silicon nitride).

[0031] (b) Rare earth element oxide As the rare earth element oxide used herein, oxides such as Y, Yb, Gd, Er, Lu, etc., which are easily available and stable as oxides, are preferred. Specific examples of the rare earth element oxide include Y2O3, Yb2O3, Gd2O3, Er2O3, Lu2O3, etc. The content of the rare earth element oxide is 0.5 mol% or more and less than 3 mol% based on the total of silicon (converted to silicon nitride), rare earth element oxide (converted to trivalent oxide), and magnesium compound (converted to MgO).

[0032] When the content of the rare earth element oxide is less than 0.5 mol%, the effect as a sintering aid becomes insufficient and the density does not increase sufficiently, which is not preferable. On the other hand, when the content of the rare earth element oxide is 3 mol% or more, the grain boundary phase with low thermal conductivity increases, reducing the thermal conductivity of the sintered body and increasing the usage amount of the expensive rare earth element oxide, which is not preferable. The content of the rare earth element oxide is preferably 0.6 mol% or more and less than 3 mol%, and more preferably 1 mol% or more and 2 mol% or less.

[0033] In this specification, the total of the number of moles of silicon nitride (Si3N4) obtained when all silicon is nitrided, the number of moles when the rare earth element oxide is converted to trivalent oxide RE2O3 (RE is a rare earth element), and the number of moles when the magnesium compound is converted to MgO may be simply referred to as "the total of silicon (converted to silicon nitride), rare earth element oxide (converted to trivalent oxide), and magnesium compound (converted to MgO)".

[0034] (c) Magnesium compound As the magnesium compound, one or more magnesium compounds containing silicon (Si), nitrogen (N), or oxygen (O) can be used. In particular, it is preferable to use magnesium oxide (MgO), magnesium silicon nitride (MgSiN2), magnesium silicide (Mg2Si), magnesium nitride (Mg3N2), etc.

[0035] Here, it is preferable to select such that 87 mass% or more becomes MgSiN₂ with respect to the total of the magnesium compounds. By using 87 mass% or more of MgSiN₂, the oxygen concentration in the obtained silicon nitride substrate can be reduced. When MgSiN₂ in the magnesium compound is less than 87 mass%, the amount of oxygen in the silicon nitride particles after sintering increases, and there is a possibility that the thermal conductivity of the sintered body becomes low. MgSiN₂ in the magnesium compound is preferably 90 mass% or more.

[0036] The content (in terms of MgO) of the magnesium compound in the silicon nitride substrate is 8 mol% or more and less than 15 mol% with respect to the total of silicon (converted to silicon nitride), rare earth element oxide (converted to trivalent oxide), and magnesium compound (in terms of MgO). When the content of the magnesium compound is less than 8 mol%, the effect as a sintering aid becomes insufficient and the density does not increase sufficiently, which is not preferable. When the content of the magnesium compound is 15 mol% or more, the grain boundary phase with low thermal conductivity increases, reducing the thermal conductivity of the sintered body, which is not preferable. The content of the magnesium compound is preferably 8 mol% or more and less than 14 mol%, more preferably 9 mol% or more and less than 13 mol%.

[0037] (d) Grinding To silicon powder, rare earth element oxide and magnesium compound are added as sintering aids so as to have a predetermined ratio, a dispersion medium (organic solvent) and, if necessary, a dispersant are added, and the mixture is ground with a ball mill to prepare a slurry (dispersion of raw material powder). The media preferably have a diameter of 5 mm or more, the concentration of the raw material powder in the slurry (also referred to as the slurry concentration) is preferably 40 mass% or more, and it is preferable to grind for 6 hours or more. The media are preferably made of a material that does not contain Al or Fe, which are factors that lower the thermal conductivity of silicon nitride, and silicon nitride is particularly preferable. The types of the dispersion medium and the dispersant are not particularly limited and can be arbitrarily selected according to the method of forming a sheet or the like.

[0038] As the dispersion medium, ethanol, n-butanol, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), etc. can be used. As the dispersant, for example, sorbitan ester type dispersants, polyoxyalkylene type dispersants, etc. can be used. The amount of the dispersion medium used is preferably, for example, 40 to 70% by mass based on the total amount of the above powder, and the amount of the dispersant used is preferably, for example, 0.3 to 2% by mass based on the total amount of the above powder. Note that after dispersion, the dispersion medium may be removed or replaced with another dispersion medium as necessary.

[0039] The time for performing pulverization varies depending on the milling device used, the amount and characteristics of the starting materials, etc., and is not particularly limited, but it is preferable to select the time so that the raw material powder can be sufficiently pulverized and mixed. The pulverization time is preferably, for example, 6 hours or more and 48 hours or less, and more preferably 12 hours or more and 24 hours or less. If the pulverization time is too short, sufficient pulverization may not be achieved, and a silicon nitride substrate having the characteristics required in this embodiment may not be obtained. If the pulverization time is too long, the amount of impurity oxygen may gradually increase, and the thermal conductivity of the silicon nitride substrate may decrease.

[0040] The oxygen content of the silicon particles after pulverization is preferably 1.0% by mass or less, and more preferably 0.7% by mass or less. By making the silicon particles have as little oxygen content as possible, the thermal conductivity of silicon nitride can be improved. Note that since it is difficult to measure only the oxygen content of the silicon particles after mixing with the sintering aid, if a slurry of only silicon particles without the sintering aid is prepared under the same pulverization conditions as a sample, this slurry can be used for measuring the oxygen content of the silicon particles. For example, the silicon particles can be extracted from the slurry, and the oxygen content can be measured using an oxygen analyzer of the inert gas fusion-non-dispersive infrared absorption method for the silicon particles.

[0041] The BET specific surface area (m2 / g), median diameter D50 (μm), and oxygen content of the silicon particles in the slurry obtained by pulverization are obtained as values measured using silicon particles pulverized in the same manner except that rare earth element oxides and magnesium compounds are not added. Since only very small amounts of the rare earth element oxide powder and magnesium compound powder are added to the silicon powder, they have little effect on the pulverization efficiency, and the values obtained in this way are considered to be substantially the same as the silicon particles in the slurry.

[0042] (1-2) Production of sheet-shaped molded body (sheet molding process) To the obtained slurry, a dispersion medium, an organic binder, a dispersant, etc. are added as necessary, vacuum degassing is performed as necessary, the viscosity is adjusted within a predetermined range, and a slurry for coating is produced. It is preferable to adjust the viscosity of the above slurry within the range of 1 Pa·s or more and less than 15 Pa·s. The viscosity of the slurry is a value measured using a rotational viscometer at a temperature of 25°C and a rotation speed of 10 rpm. In some cases, as described above, removal or replacement of the dispersion medium may be performed.

[0043] The produced slurry for coating is formed into a sheet using a sheet molding machine, cut to a predetermined size, and then dried to obtain a sheet-shaped molded body. The organic binder used for producing the slurry for coating is not particularly limited, and examples include PVB-based resins (polyvinyl butyral resins), ethyl cellulose-based resins, acrylic resins, etc. The addition amounts of the dispersion medium, organic binder, dispersant, etc. are preferably adjusted appropriately according to the coating conditions.

[0044] The method of forming the slurry for coating into a sheet is not particularly limited, and for example, a sheet molding method such as the doctor blade method can be used. In the method for manufacturing the silicon nitride substrate of this embodiment, in order to make the surface roughness Ra between the main surfaces of the substrate different, it is preferable to make the surface roughness Ra different at the stage of forming into a sheet.

[0045] When forming a sheet-shaped molded body by the doctor blade method, after making it into a sheet shape, one main surface contacts the conveyance film, and the other main surface is dried in a released state. Therefore, it is easy to adjust so that the surface roughness Ra of one main surface (main surface 1a) in contact with the conveyance film is small and the surface roughness Ra of the other main surface (main surface 1b) in the released state is large. Note that at this stage, it is not necessary to satisfy the relationship between the surface roughness Ra of the ceramic substrate required for the surface roughness Ra.

[0046] At this time, the surface roughness Ra of the contact surface of the conveyance film used with the sheet-shaped molded body is preferably 0.03 to 0.20 μm, and more preferably 0.04 to 0.10 μm. By using a conveyance film having such a surface roughness Ra, a sheet-shaped molded body having a main surface with a smaller surface roughness Ra than the surface in the released state can be obtained.

[0047] Also, the material of the conveyance film is not particularly limited as long as it is used for known conveyance films. For example, polyesters such as polyethylene terephthalate (PET) can be mentioned.

[0048] In this doctor blade method, the forming speed of the coating slurry is preferably 600 mm / min or less. Since the coating slurry used in this embodiment has thixotropy, when the coating slurry passes through the doctor blade of the doctor blade method, shear stress is applied to the coating slurry, and the viscosity of the coating slurry decreases. Therefore, when the forming speed exceeds 600 mm / min, the coating slurry easily flows and is likely to entrap bubbles that cause voids, and densification may be inhibited.

[0049] The sheet-shaped slurry formed on the conveying film is then conveyed to a drying chamber set at a predetermined temperature and humidity, and the solvent is evaporated to obtain a dried sheet-shaped molded body. At this time, the drying rate of the coating slurry is preferably 0.8 mass% / min or less. When the drying rate of the coating slurry exceeds 0.8 mass% / min, bubbles may be easily generated in the sheet due to the rapid volatilization of the dispersion medium. The sheet after coating is passed through a drying zone and gradually heated and dried, so the drying rate in the drying process fluctuates and is not constant. Therefore, it is preferable that the maximum drying rate does not exceed 0.8 mass% / min.

[0050] The thickness of the sheet-shaped molded body formed in the molding process can be adjusted so that the thickness of the finally obtained ceramic substrate is a desired thickness, for example, 0.15 mm or more and 0.8 mm or less. Also, the sheet-shaped molded body can be cut into a predetermined size with a punching machine or the like as necessary.

[0051] (1-3) Sintering of the molded body (sintering process) By heating the obtained sheet-shaped molded body, the silicon contained in the molded body is nitrided and then densified. This sintering process includes a debinding process for removing the organic binder in the molded body, a nitriding process for reacting the silicon (Si) and nitrogen (N) contained in the molded body to nitride it, and a densification sintering process for densifying after nitriding. These processes may be sequentially performed in separate furnaces or continuously performed in the same furnace.

[0052] In this embodiment, when performing this sintering process, powdery boron nitride (BN) is applied to the main surface 1a of the sheet-shaped molded body to form a boron nitride (BN powder) layer. This boron nitride (BN) also functions as a separating material that facilitates separation after sintering when a plurality of sheet-shaped molded bodies are laminated. Therefore, when a plurality of sheet-shaped molded bodies are laminated, boron nitride (BN) exists between the sheet-shaped molded bodies, and each sintered body can be easily separated from the sintered body laminate obtained after sintering.

[0053] The sheet-shaped molded body coated with boron nitride powder (BN powder) in this way is placed in an electric furnace, degreased (removing organic binders, etc.), decarburized at 900 - 1300 °C in a nitriding apparatus, heated to a predetermined temperature and nitrided under a nitrogen atmosphere, and then sintered in a sintering apparatus. At this time, it is preferable to heat while applying a load of 10 - 1000 Pa to the molded body. The degreasing is preferably performed at a temperature of 800 °C or lower.

[0054] In addition, it is preferable to use a BN powder layer with a thickness of about 3 - 20 μm as the above-mentioned separation material. A BN powder layer can be formed by applying a slurry of BN powder, for example, by spraying, brush coating, or screen printing on one surface of each sheet-shaped molded body. The BN powder preferably has a purity of 95% or more and an average particle size (D50) of 1 - 20 μm.

[0055] In the nitriding process, the nitrogen partial pressure during nitriding is preferably 0.05 - 0.7 MPa, more preferably 0.07 - 0.2 MPa. The nitriding temperature is preferably 1350 - 1500 °C, more preferably 1400 - 1450 °C. The holding time after heating to the nitriding temperature is preferably 3 - 12 hours, more preferably 5 - 10 hours.

[0056] When the nitriding temperature is less than 1350 °C, or when the holding time is less than 3 hours, unreacted silicon powder may remain in the sheet-shaped molded body, and it may not be possible to obtain a dense body by the densification sintering process performed after the nitriding process. When the nitriding temperature exceeds 1500 °C, the silicon powder may melt before nitriding and remain without nitriding, or the sintering aid component may volatilize and the sintering aid component may be insufficient in the densification sintering process, making it difficult to obtain a dense sintered body. When the holding time exceeds 12 hours, the sintering aid component may volatilize and the sintering aid component may be insufficient in the densification sintering process, making it difficult to obtain a dense sintered body.

[0057] The nitrogen partial pressure during densification sintering is preferably 0.1 to 0.9 MPa, more preferably 0.5 to 0.9 MPa. The sintering temperature is preferably 1800 to 1950 °C, more preferably 1850 to 1900 °C. The holding time (sintering time) after heating to the sintering temperature is preferably 3 to 12 hours, more preferably 5 to 12 hours. When the sintering temperature is less than 1800 °C or the holding time is less than 3 hours, the growth and rearrangement of silicon nitride particles may be insufficient and a dense body may not be obtained. When the sintering temperature exceeds 1950 °C or the holding time exceeds 12 hours, the sintering aid component may volatilize and become insufficient, making it difficult to obtain a dense sintered body.

[0058] By the method as described above, the ceramic substrate (silicon nitride substrate) according to the present embodiment is obtained. In addition, at the stage of obtaining the sheet-shaped compact described above, it is generally the case that the difference in surface roughness between the two main surfaces of the sheet-shaped compact is not more than 0.10 μm. However, in the present embodiment, by the subsequent sintering process (including the nitriding process), the difference in surface roughness Ra between the two main surfaces can be made 0.10 μm or more.

[0059] The reason for this is that although the principle is not clear, boron nitride (BN) present on the surface of the sheet-shaped compact is considered to suppress the growth of sintered particles formed by sintering, and thus the difference in surface roughness Ra between the two main surfaces of the sheet-shaped compact becomes larger due to the sintering operation. In particular, when silicon powder is used as the raw material powder and a nitriding process is performed before sintering, (compared with the case where silicon nitride powder is used as the raw material powder) the content ratio of silicon nitride increases, particle growth becomes easier, and the difference becomes more prominent. That is, it is presumed that the main surface 1b, which is the surface of the sheet-shaped compact where the BN powder layer is not formed, is more likely to have a larger surface roughness Ra than the main surface 1a where the BN powder layer is formed.

[0060] After the sintering process, blast treatment may be appropriately performed. Blast treatment generally roughens the surface roughness Ra of the substrate surface and can be used without particularly limiting known methods. Examples of blast treatment include dry blast treatment in which abrasive is sprayed with compressor air and wet blast treatment in which a mixture of abrasive and solution is sprayed with compressor air.

[0061] This blast treatment may be carried out, for example, to finely adjust the surface roughness Ra of the obtained ceramic substrate, or when the difference in surface roughness Ra between the main surfaces of the ceramic substrate obtained after the sintering process does not satisfy the above-specified relationship, this blast treatment may be carried out to satisfy the relationship.

[0062] The silicon nitride substrate thus obtained has, for example, a shape having two main surfaces and four side surfaces (FIG. 1), the surface roughness Ra1 of one main surface 1a is 0.50 μm or less, the surface roughness Ra2 of the other main surface 1b is 0.50 μm or more, and the surface roughness Ra2 is 0.10 μm or more greater than the surface roughness Ra1.

[0063] This silicon nitride substrate has a rectangular shape, and it is preferably obtained with each side being 100 mm or more. The silicon nitride substrate has good thermal conductivity and is suitable for the above-described applications, for example, applications such as power modules. Its thermal conductivity is preferably 100 W / m·K or more, more preferably 110 W / m·K or more, and even more preferably 130 W / m·K or more. In the method for manufacturing a silicon nitride substrate having a nitriding step described in detail above, since the purity of silicon nitride can be improved, it is easy to manufacture a substrate having a thermal conductivity of 110 W / m·K or more, which is preferable.

[0064] The sintered silicon nitride substrate contains β-phase silicon nitride as a main component and contains rare earth elements and magnesium. The rare earth elements may be in a single state or may form a compound with other substances. The magnesium contained in the silicon nitride substrate may be in a single state or may be a compound with other substances.

[0065] The silicon nitride substrate formed as described above becomes a silicon nitride sintered body having silicon nitride particles and a grain boundary phase that forms the grain boundaries of the silicon nitride particles. The content of rare earth elements (in terms of trivalent oxide RE2O3, where RE is a rare earth element) in the grain boundary phase is preferably 0.5 to 3.0 mol%, and the content of magnesium (in terms of MgO) is preferably 0.5 to 10 mol%. In the silicon nitride substrate of the present embodiment, the content of rare earth elements and the content of magnesium are values obtained by setting the total of the number of moles of silicon nitride (Si3N4), the number of moles when the rare earth element is converted to trivalent oxide RE2O3 (RE is a rare earth element), and the number of moles when the magnesium is converted to MgO to 100 mol%. Hereinafter, the total may be simply referred to as "the total of silicon nitride, rare earth elements (in terms of trivalent oxide conversion), and magnesium (in terms of MgO)".

[0066] Here, the total content of rare earth elements (in terms of trivalent oxide RE2O3, where RE is a rare earth element) and magnesium (in terms of MgO) in the grain boundary phase (the total amount of the grain boundary phase) is preferably 1.0 to 12.3 mol%.

[0067] The contents of silicon nitride, rare earth elements, and magnesium in the silicon nitride substrate depend on the amount of silicon powder added during production and the amounts of rare earth element oxides and magnesium compounds added as sintering aids.

[0068] In the above method, during firing, mainly the magnesium compound decreases due to volatilization. Therefore, the content of magnesium in the silicon nitride substrate after sintering decreases compared to the amount added during production. On the other hand, since the rare earth element oxide hardly volatilizes, the content ratio with respect to the total of silicon nitride, rare earth elements (in terms of trivalent oxide conversion), and magnesium (in terms of MgO) may increase slightly due to the decrease in the magnesium compound. The amount of volatilization of the magnesium compound varies depending on the shape of the compact, firing conditions, etc.

[0069] The amount of oxygen in the silicon nitride particles is preferably 0.05% by mass or less. If the amount of oxygen exceeds 0.05% by mass, it may not be possible to obtain high thermal conductivity. In addition, two silicon nitride substrates under the same conditions can be prepared as samples. One silicon nitride substrate can be used as a substrate, and the other silicon nitride substrate can be used for measuring the amount of oxygen. For example, the other silicon nitride substrate is pulverized, and the silicon nitride particles are extracted by pickling (removing the grain boundary phase by pickling), and the amount of oxygen in the silicon nitride particles is measured using an oxygen analyzer of an inert gas fusion-non-dispersive infrared absorption method.

[0070] (1-4) Others The silicon nitride substrate manufactured as described above preferably has a dense structure with a relative density of 98% or more. If the relative density of the silicon nitride substrate is less than 98%, high thermal conductivity cannot be obtained. Such a dense silicon nitride substrate is less likely to cause inhibition of heat conduction due to voids. In particular, the silicon nitride substrate of the present embodiment preferably has a thermal conductivity in the thickness direction of 110 W / m·K or more.

[0071] Also, the bending strength of the silicon nitride substrate is preferably, for example, 600 MPa or more. As will be described later, when the silicon nitride substrate is used as a silicon nitride circuit board for a power module in which a circuit such as a metal plate is joined via a brazing material, high stress is applied during mounting or driving. Therefore, depending on the method, the bending strength is preferably 600 MPa or more. In addition, since the bending strength is as high as 600 MPa or more, it is also possible to make the silicon nitride substrate thinner.

[0072] The thickness of the silicon nitride substrate is not particularly limited and can be any thickness. For example, when used as an insulating heat dissipation substrate for a semiconductor element or an electronic device, the thickness is preferably 0.05 to 2.5 mm, more preferably 0.1 to 1 mm. In particular, when used as a silicon nitride circuit board for a power module, it is even more preferably 0.2 to 0.7 mm. The thickness of the sintered silicon nitride substrate can be adjusted to a desired thickness by adjusting the thickness of the sheet molded body in the sheet molding step in consideration of the influence on the thickness during sintering.

[0073] <Ceramic circuit board> The ceramic circuit board of this embodiment includes the ceramic substrate described above, a metal circuit joined to one main surface of the ceramic substrate, and a metal heat sink joined to the other main surface of the ceramic substrate. The surface roughness Ra1 of one main surface is 0.50 μm or less, the surface roughness Ra2 of the other main surface is 0.50 μm or more, and the surface roughness Ra2 is 0.10 μm or more greater than the surface roughness Ra1.

[0074] This ceramic circuit board is, for example, as shown in FIG. 2, a ceramic circuit board 10 including a ceramic substrate 1, a metal circuit 11 joined to one main surface 1a of the ceramic substrate 1, and a metal heat sink 12 joined to the other main surface 1b of the ceramic substrate 1. Further, this ceramic circuit board 10 has a brazing material layer 13 for joining the ceramic substrate 1 and the metal circuit 11, and a brazing material layer 14 for joining the ceramic substrate 1 and the metal heat sink 12.

[0075] As the ceramic substrate 1, the ceramic substrate of this embodiment described above can be used.

[0076] The metal circuit 11 can have the same configuration as a known metal circuit formed on this type of ceramic circuit board and is not particularly limited.

[0077] The metal circuit 11 can be formed of, for example, copper, aluminum, or their alloys. The thickness of the metal circuit 11 is not particularly limited, but is preferably 0.2 mm or more and 2.0 mm or less, and more preferably 0.5 mm or more and 1.2 mm or less. For example, it is 0.8 mm.

[0078] The metal heat sink 12 can have the same configuration as a known metal heat sink formed on this type of ceramic circuit board and is not particularly limited.

[0079] The metal heat dissipation plate 12 can be formed of, for example, copper, aluminum, or their alloys. The thickness of the metal heat dissipation plate 12 is not particularly limited, but is preferably 0.2 mm or more and 2.0 mm or less, and more preferably 0.5 mm or more and 1.2 mm. For example, it is 0.8 mm.

[0080] The thickness of this metal heat dissipation plate 12 may be made thicker than that of the metal circuit 11. By doing so, the amount of heat transfer from the metal heat dissipation plate 12 to a heat sink or the like can be increased, and heat dissipation can be performed efficiently. For example, it is preferable that the thickness of the metal heat dissipation plate 12 is 0.1 mm or more thicker than that of the metal circuit 11.

[0081] The brazing material layer 13 is a brazing material for joining the ceramic substrate 1 and the metal circuit 11, and the brazing material layer 14 is a brazing material for joining the ceramic substrate 1 and the metal heat dissipation plate 12.

[0082] These brazing material layers 13 and 14 may be formed using a known brazing material layer used in this type of ceramic circuit board. Generally, they are composed of a paste containing brazing material powder and an organic binder. Examples of the brazing material powder used here include brazing material powder containing silver, copper, etc. in a predetermined composition, and various organic resins can be used as the organic binder. For example, an Ag-Cu based active brazing material with a eutectic composition mainly composed of Ag and Cu and added with active metals such as Ti, Zr, Hf is preferable in terms of obtaining high strength, high sealing performance, etc. Further, from the viewpoint of the bonding strength between the ceramic substrate and the metal plate, a ternary Ag-Cu-In based active brazing material in which In is added to the above Ag-Cu based active brazing material is more preferable.

[0083] The thickness of the brazing material layer 13 is not particularly limited, but is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 30 μm or less. Also, the thickness of the brazing material layer 14 is not particularly limited, but is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 30 μm or less.

[0084] At this time, the brazing material layer 14 may be made thicker than the brazing material layer 13. By doing so, it becomes easier for the brazing material layer to absorb the thermal stress and the like generated during the operation of the semiconductor device, and the influence of deformation of the substrate due to heat can be suppressed.

[0085] <Method for manufacturing a ceramic circuit board> Next, a method for manufacturing the ceramic circuit board according to the present embodiment will be described. The manufacturing of the ceramic circuit board can be carried out according to a known manufacturing method and is not particularly limited. Hereinafter, it will be described in detail with reference to FIGS. 3 to 6.

[0086] (2-1) Formation of brazing material layer (brazing material layer forming step) First, prepare the ceramic substrate 1 described in the above embodiment. In this ceramic substrate 1, as described above, it has main surfaces with different surface roughness Ra, and note that the members to be joined are different depending on the surface roughness Ra.

[0087] At this stage, in order to join a metal circuit or a metal heat sink to each main surface, a brazing material layer is formed on both main surfaces. Specifically, the brazing material layer 13 is formed on the first main surface 1a side, and the brazing material layer 14 is formed on the second main surface 1b side by applying the brazing material by a method such as screen printing (FIG. 3).

[0088] Here, the brazing material used to form the brazing material layers 13 and 14 is the brazing material described for the above ceramic circuit board.

[0089] (2-2) Joining of metal plates (joining step) The metal plate 21 for circuit formation is laminated and fixed to the ceramic substrate 1 via the brazing material layer 13, and the metal plate 22 for heat sink formation is laminated and fixed to the ceramic substrate 1 via the brazing material layer 14.

[0090] By heating the obtained laminated material, the ceramic substrate 1 and the metal plate 21 for circuit formation, and the ceramic substrate 1 and the metal plate 22 for heat sink formation are joined via the brazing material layer respectively, and a laminated board is formed (FIG. 4).

[0091] The heating during this joining is preferably carried out in a vacuum or a reducing atmosphere. Also, in order to remove the organic components in the brazing material paste during the heating-up process, it is preferable to hold it once near the volatilization temperature of the organic binder, and then heat it up to the brazing temperature. This heating is preferably continued for 10 minutes or more. The brazing temperature means a temperature at which a brazing material layer can be appropriately formed, that is, a temperature equal to or higher than the melting point of the brazing material. The brazing temperature is usually the highest temperature in the above heating-up process.

[0092] After the above heating, the temperature can be lowered to room temperature by a known method, and the method is not particularly limited. For example, it is preferable to lower the temperature to 350 °C over a period of 50 minutes or more, and then lower the temperature from 350 °C to room temperature.

[0093] In the holding step during the joining process, if the holding temperature for removing the organic binder is low, the organic binder components may not be able to volatilize, and there is a risk that residues of the organic binder remain. Therefore, the holding temperature for removing the organic binder is preferably 300 °C or higher. For example, in the case of an organic binder containing an acrylic resin, this holding temperature is preferably 360 °C or higher. In order to avoid oxidation of the active metal in the brazing material by oxygen contained in the resin or the like in the organic binder, the holding temperature for removing the organic binder is set lower than the brazing temperature in the heating step (brazing step).

[0094] Using the brazing material described in the above joining process, the joining of the ceramic substrate and the metal plate can be carried out as described above using a brazing material paste containing brazing material powder and an organic binder. When using a brazing material having a predetermined melting point, for example, the brazing temperature is preferably set to a temperature near the melting point. Heating above the melting point ensures sufficient melting of the brazing material and can suppress the formation of voids. Also, it is preferable not to set the temperature too high far from the melting point so that the brazing material does not spread excessively.

[0095] The holding time at the brazing temperature depends on the amount used, but considering normal productivity, it is preferably within 5 hours, more preferably within 2 hours. The holding time at the brazing temperature is appropriately adjusted and set according to the number of samples to be input, and also, for example, in the case of a vacuum atmosphere, according to the volume of the bonding heating furnace and the exhaust volume of the vacuum pump. It is preferable to apply a load and fix it in a fixed state during the placement process so that the metal plate and the ceramics are joined without voids, and then heat them in the joining process.

[0096] (2-3) Formation of Circuit Pattern (Pattern Formation Process) For the laminated board obtained in the above joining process, a circuit pattern is formed. First, resist films are formed on the surfaces of the circuit-forming metal plate 21 and the heat-radiating plate-forming metal plate 22 in desired patterns, respectively, for the formation of the circuit pattern.

[0097] The formation of the resist film can be performed by a known method. Once a resist film is formed on the entire surface of the metal plate, a resist film of a desired pattern can be formed by photolithography or the like. Using the formed resist film, a metal pattern is formed by etching, and then the resist film is removed to form the metal circuit 11 and the metal heat-radiating plate 12 (Fig. 5).

[0098] The thickness of the resist film formed in the pattern formation process is preferably 10 to 80 μm, more preferably 30 to 70 μm. As the resist film, an ultraviolet curable resist material is preferred.

[0099] In Fig. 5, for the sake of explanation, it is shown as a figure in which a pair of metal circuits 11 and metal heat-radiating plates 12 are formed. However, in this pattern formation, usually, a plurality of sets of metal circuits 11 and metal heat-radiating plates 12 are formed on the ceramic substrate 1. After going through the processes described later, the plurality of sets of metal circuits 11 and metal heat-radiating plates 12 formed in this way are finally cut by dicing or the like to form a plurality of ceramic circuit boards.

[0100] (2-4) Brazing Material Layer Removal Process After the above-mentioned (3) pattern formation step, since an unnecessary brazing material layer exists on the ceramic substrate 1 in the etched portion, this is removed (Fig. 6).

[0101] The removal of the brazing material can be carried out by a known method. Examples of the solvent used for removal include a chemical solution containing hydrogen peroxide and ammonium fluoride. More specifically, an aqueous solution containing 10 to 40% by mass (2.9 to 8.8 mol / L) of hydrogen peroxide and 1 to 8% by mass (0.7 to 2.1 mol / L) of ammonium fluoride can be exemplified as a preferred chemical solution.

[0102] When the hydrogen peroxide is less than 10% by mass, the ability to remove the brazing material may be insufficient. When it exceeds 40% by mass, the metal plate may be excessively eroded, and the dimensional accuracy of the metal plate may deteriorate. When the ammonium fluoride is less than 1% by mass, the ability to remove the reaction layer containing the active metal generated at the bonding interface between the brazing material layer and the ceramic substrate may decrease. On the other hand, when it exceeds 8% by mass, the crystal grains constituting the ceramic substrate may be dissolved, and the electrical insulation and strength required for the ceramic substrate may decrease.

[0103] (2-5) Cleaning step After removing the brazing material layer, the patterned laminate is immersed in a chemical solution for cleaning. For example, if it is not sufficient to reduce oxygen from the processing atmosphere and the surface of the metal plate is oxidized in the above heating step, it may cause a decrease in electrical conductivity and solderability, which is not preferable. Therefore, by immersing the patterned laminate in a chemical solution containing at least one selected from hydrogen peroxide, sulfuric acid, hydrochloric acid, and ammonium chloride for cleaning, the oxide on the surface of the metal plate can be removed. It is preferable to use sulfuric acid as the above chemical solution.

[0104] After going through the above pattern formation step to cleaning step, the deposits that reduce the insulation resistance between the metal plates on the substrate are removed or reduced, which is preferable.

[0105] Furthermore, after the cleaning step, a plating layer such as nickel, gold, or silver may be formed on the surface of the metal plate. When forming such a plating layer, for example, in the case of nickel plating, the metal plate is immersed in an electroless plating solution (85°C) with nickel (Ni) as the main component and the phosphorus (P) concentration adjusted to 8 mass% for 20 to 30 minutes, whereby a Ni plating layer with a thickness of about 5 μm can be formed on the surface of the metal plate.

[0106] By forming a plating layer on the brazing material layer forming step to the cleaning step as needed, the ceramic circuit board 10 of the present embodiment can be obtained.

[0107] As shown in FIG. 7, the circuit board 10 thus obtained can be made into a semiconductor device by connecting a semiconductor chip 51 to the metal circuit 11, and connecting a heat sink 52 or the like to the metal heat sink 12 to improve its heat dissipation characteristics.

Example

[0108] The present embodiment will be described in more detail by way of examples, but the present invention is not limited thereto.

[0109] (1) Fabrication of silicon nitride substrate (Slurry preparation step) To silicon powder with a BET specific surface area of 2.1 m2 / g, a median diameter D50 of 8.2 μm, and an oxygen content of 0.3 mass%, 1.2 mol% of Y2O3 powder and 9.8 mol% of MgSiN2 powder were added as sintering aids with respect to the total of silicon (in terms of silicon nitride), rare earth element oxide (in terms of trivalent oxide), and magnesium compound (in terms of MgO) to obtain raw material powder. To this raw material powder, a dispersion medium (toluene) and 0.5 mass% of a dispersant (sorbitan trioleate) with respect to the total of the raw material powder were added to make a slurry with a concentration of 42 mass%. Using a ball mill and using 5φ silicon nitride balls as media, pulverization was performed for 24 hours.

[0110] The addition amount of the magnesium compound was expressed in mol% when all the magnesium compounds were converted to MgO. The BET specific surface area, median diameter D50, and oxygen content of the silicon powder before pulverization were measured using a BET specific surface area meter by the BET single-point method, a particle size distribution meter by the laser diffraction / scattering method, and an oxygen analyzer by the inert gas fusion-non-dispersive infrared absorption method, respectively.

[0111] (Sheet forming process) The obtained slurry was adjusted in concentration by adding a dispersion medium and an organic binder (acrylic resin), and defoaming treatment was performed to obtain a slurry-like coating liquid. This coating slurry was applied to a transport film by the doctor blade method, formed into a sheet shape with a thickness of 0.38 mm at a forming speed of 600 mm / min or less, and cut into a size of 240 mm × 200 mm to obtain a sheet-shaped formed body. Note that the surface roughness Ra of the coating surface of the transport film is 0.1 μm. Here, the surface in contact with the transport film of the sheet-shaped formed body is one main surface (main surface 1a), and the surface not in contact with the transport film and in an open state is the other main surface (main surface 1b).

[0112] (Sintering process) For the obtained sheet-shaped formed body, a boron nitride (BN powder) layer is formed on the main surface 1a. As shown in FIG. 8, a laminate 100A is produced by laminating a plurality of sheet-shaped formed bodies with a boron nitride powder (BN powder) layer (thickness 4.5 μm) not shown in between, and is installed on a BN placement plate 200 with a separator sandwiched therebetween. A BN weight plate 300 is placed on the laminate 100A.

[0113] Such laminates 100A are stacked in multiple stages, and further, a plurality of such multi-stage stacked laminates 100A are prepared.

[0114] Next, these multiple laminates 100A were degreased at 750 °C for 5 hours in a nitrogen atmosphere (nitrogen partial pressure 0.1 MPa) (degreasing process), and nitrided at 1400 °C for 10 hours in a nitrogen atmosphere (nitrogen partial pressure 0.5 MPa) in a nitriding apparatus (nitriding process).

[0115] Next, the laminate 100A taken out of the nitriding apparatus was placed in a BN crucible, then carried into a sintering apparatus, and sintered at 1900 °C for 12 hours in a nitrogen atmosphere (nitrogen partial pressure 0.9 MPa) (densification sintering step). The BN powder layer was removed to obtain a silicon nitride substrate made of a silicon nitride sintered body.

[0116] Finally, liquid honing treatment was performed for the purpose of cleaning the surface of the silicon nitride substrate and giving it an appropriate roughness. In the honing treatment, an appropriate amount of alumina abrasive grains was added to water, and it was sprayed onto the front and back surfaces of the sintered body at a pressure of 0.5 MPa. The size of the obtained silicon nitride substrate was 200 mm × 170 mm, and the thickness was 0.32 mm.

[0117] After cleaning and liquid honing treatment, 10 pieces were randomly selected from the obtained large number of silicon nitride substrates, and for the main surfaces 1a and 1b of those silicon nitride substrates, the surface roughness Ra was measured at 6 locations for one surface, and the average value was taken as the surface roughness Ra of that surface, and the results were summarized and shown in Table 1.

[0118] Also, as another lot, when silicon nitride substrates were produced by the same operation, the surface roughness Ra of the main surfaces 1a and 1b was similarly measured and shown in Table 2.

[0119]

Table 1

[0120]

Table 2

[0121] (Comparative Example) To silicon nitride powder with a BET specific surface area of 2.1 m2 / g, a median diameter D50 of 8.2 μm, and an oxygen content of 0.3 mass%, powders of Y2O3 at 1.2 mol% and MgSiN2 at 9.8 mol% were added as sintering aids with respect to the total of silicon nitride, rare earth element oxides (in terms of trivalent oxides), and magnesium compounds (in terms of MgO), to obtain raw material powder.

[0122] The above comparative examples were made into silicon nitride substrates by the same operations as in the examples, except that the nitriding process in the sintering process was not performed. The surface roughness Ra (μm) of the two main surfaces of these silicon nitride substrates was measured, and the results including the difference (μm) in the surface roughness Ra between the main surfaces are shown below.

[0123] Comparative substrate 1: Main surface 1a 0.581, main surface 1b 0.585, difference 0.004 Comparative substrate 2: Main surface 1a 0.583, main surface 1b 0.572, difference 0.011 For comparative substrates 1 and 2, the difference in the surface roughness Ra between the main surfaces is less than 0.10 μm, and both surface roughnesses Ra1 and Ra2 exceed 0.50 μm. Also, in comparative substrate 2, Ra2 is smaller than Ra1.

[0124] From the above results, it was found that the ceramic substrate of the present embodiment produced by the manufacturing method including the nitriding process in which the BN powder layer is formed on the main surface 1a of the sheet-shaped compact and the sheet-shaped compact is nitrided can have suitable surface roughnesses Ra on the formation surfaces of the metal circuit and the metal heat sink plate, respectively, and their surfaces can be in a desired relationship. Thereby, when joining the ceramic substrate, the metal circuit, and the metal heat sink plate, the surface roughness can be made relatively small so that the brazing material is easily removed on the main surface 1a to which the metal circuit is joined, while the surface roughness can be made relatively large so that the joining strength with the metal heat sink plate is improved on the main surface 1b to which the metal heat sink plate is joined.

[0125] As described above, the invention made by the present inventor has been specifically described based on its embodiments. However, it goes without saying that the present invention is not limited to the above embodiments and can be variously modified without departing from the gist thereof.

Explanation of reference numerals

[0126] 1 Ceramic substrate 1a, 1b Main surfaces 10 Ceramic circuit board 11 Metal circuit 12 Metal heat sink plate 13,14 solder layer 51 semiconductor chip 52 heat sink

Claims

1. A silicon nitride substrate having a first main surface and a second main surface, a metal circuit bonded to the first main surface of the silicon nitride substrate, a metal heat sink bonded to the second main surface of the silicon nitride substrate, a silicon nitride circuit board comprising: the metal heat sink is 0.1 mm or more thicker than the metal circuit, the silicon nitride substrate has a surface roughness Ra2 of the second main surface that is 0.10 μm or more greater than a surface roughness Ra1 of the first main surface, a silicon nitride circuit board.

2. the silicon nitride substrate has a thermal conductivity of 100 W / (m·K) or more, the silicon nitride circuit board according to Claim 1.

3. the silicon nitride substrate has a thickness of 0.2 mm or more and 0.7 mm or less, the silicon nitride circuit board according to Claim 1 or 2.

4. the silicon nitride substrate has a rectangular shape, and each side is 100 mm or more, the silicon nitride circuit board according to any one of Claims 1 to 3.

5. the silicon nitride substrate has a flexural strength of 600 MPa or more, the silicon nitride circuit board according to any one of Claims 1 to 4.

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