Ceramic circuit board and method for manufacturing a ceramic circuit board

Abstract

To provide a ceramic substrate capable of improving the removal rate of an unnecessary brazing material while ensuring the bonding strength with a metal circuit or a metal heat sink.SOLUTION: A ceramic substrate 1 has a main surface 1a and a main surface 1b, and the surface roughness Ra1 of the main surface 1a is 0.50 μm or less, the surface roughness Ra2 of the main surface 1b is 0.50 μm or more, and the surface roughness Ra2 is greater than the surface roughness Ra1 by 0.10 μm or more.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 in which the surface roughness Ra of one main surface is different from that of the other main surface, a ceramic circuit board using the ceramic substrate, and a method for manufacturing a ceramic substrate in which the surface roughness Ra of one main surface is different from that of 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 a semiconductor module, a power module, etc.

[0003] The ceramic circuit board used in this 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) at the same time.

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] 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 metal circuits or metal heat sinks are joined to the ceramic substrate via the brazing material.

[0007] When joining materials using brazing material, it is common practice to roughen the surface of the ceramic substrate to which the brazing material is applied, thereby increasing the bonding strength between the ceramic substrate and the metal circuit or metal heat sink.

[0008] Incidentally, when forming a metal circuit bonded to a ceramic substrate, a metal plate is first bonded to the surface of the ceramic substrate via a brazing material, and then the circuit pattern is formed by etching the metal plate. As a result, the brazing material removed by etching is exposed on the surface and is unnecessary for the ceramic circuit board, so it is removed.

[0009] In this case, the surface of the ceramic substrate was roughened to ensure bonding strength, and there were instances where the brazing material could not be completely removed. In other words, the roughened substrate surface had relatively large irregularities, and the brazing material could get trapped inside these irregularities, making it difficult to remove the brazing material that had entered these irregularities.

[0010] Therefore, the present invention aims to provide a ceramic substrate that can easily remove unnecessary brazing material while ensuring bonding strength with metal circuits and metal heat sinks, and a ceramic circuit board using the same ceramic substrate. [Means for solving the problem]

[0011] In one embodiment, the ceramic substrate is a ceramic substrate having a first main surface and a second main surface, wherein the surface roughness Ra1 of the first main surface is 0.50 μm or less, and the surface roughness Ra2 of the second 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.

[0012] A ceramic circuit board in one embodiment comprises a ceramic substrate having a first main surface and a second main surface, a metal circuit bonded to the first main surface of the ceramic substrate, and a metal heat sink bonded to the second main surface of the ceramic substrate, wherein the surface roughness Ra1 of the first main surface is 0.50 μm or less, the surface roughness Ra2 of the second 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.

[0013] A method for manufacturing a ceramic substrate in one embodiment comprises the steps of (a) preparing a slurry containing silicon powder, (b) obtaining a sheet-like molded body from the slurry, and (c) sintering the sheet-like molded body, wherein step (c) includes a nitriding step, and the ceramic substrate has a first main surface and a second main surface, the surface roughness Ra1 of the first main surface is 0.50 μm or less, the surface roughness Ra2 of the second 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. [Effects of the Invention]

[0014] According to one embodiment of the ceramic substrate, it is possible to easily remove unnecessary brazing material while ensuring bonding strength with metal circuits and metal heat sinks. A ceramic circuit board using this ceramic substrate can easily remove unnecessary brazing material while ensuring bonding strength with metal circuits and metal heat sinks, thereby enabling the formation of a highly reliable semiconductor device. [Brief explanation of the drawing]

[0015] [Figure 1] This figure shows the schematic configuration of the ceramic substrate in the embodiment. [Figure 2] This figure shows the schematic configuration of a ceramic circuit board in an embodiment. [Figure 3] This is a diagram illustrating the manufacturing process of a ceramic circuit board in an embodiment. [Figure 4] It is a diagram for explaining the manufacturing process of a ceramic circuit board following FIG. 3. [Figure 5] It is a diagram for explaining the manufacturing process of a ceramic circuit board following FIG. 4. [Figure 6] It is a diagram for explaining the manufacturing process of a ceramic circuit board following FIG. 5. [Figure 7] It is a diagram in which a semiconductor chip and a heat sink are connected to the ceramic circuit board shown in FIG. 2. [Figure 8] It is a diagram showing a schematic configuration of a laminate of a ceramic substrate during sintering, which was used in the example.

Embodiments for Carrying Out the Invention

[0016] In all the diagrams for explaining the embodiments, the same members are generally denoted by the same reference numerals, and repeated explanations thereof are omitted. Note that, for the sake of clarity of the drawings, hatching may be added even in a plan view or a side view.

[0017] <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 surface and a main surface 1b on the back surface, which is the opposite surface. Note that the terms “surface” and “back surface” are used for convenience to distinguish each surface.

[0018] 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 to form a ceramic circuit board. Hereinafter, this case will be described as an example.

[0019] In this 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. Furthermore, the surface roughness Ra2 is 0.10 μm or more greater than the surface roughness Ra1. In other words, the difference between the surface roughness Ra1 and the surface roughness Ra2 is 0.10 μm or more.

[0020] Here, by setting the surface roughness Ra1 to 0.5 μm or less, it is possible to easily remove unwanted brazing material from the surface of the ceramic substrate exposed by the etching process for forming the metal circuit. Furthermore, it is preferable that the surface roughness Ra1 be 0.2 μm or more, and more preferably 0.35 μm or more. By setting it within this range, the bonding strength between the ceramic substrate and the metal circuit can be ensured. In other words, by setting the surface roughness Ra1 to 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, while preventing brazing material from remaining in the irregularities.

[0021] Furthermore, 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, the improved bonding strength between the ceramic substrate and the metal heat sink makes it possible to make the metal heat sink thicker. By making the metal heat sink thicker, heat generated by the operation of a semiconductor chip mounted on a metal circuit can be efficiently dissipated to the outside through the metal heat sink. The surface roughness Ra2 is preferably 1.0 μm or less, and more preferably 0.7 μm or less. By setting it within this range, it is possible to make it less likely for brazing material to remain on the ceramic portion exposed by etching. In other words, 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 improved, and it is adjusted to a range that makes it easier to remove excess brazing material.

[0022] Furthermore, the surface roughness Ra2 is set to be at least 0.10 μm greater than the surface roughness Ra1. This ensures that the effects and properties of each main surface described above are in a favorable state. The difference between the surface roughness Ra1 and Ra2 is preferably 0.80 μm or less, more preferably 0.65 μm or less, even more preferably 0.50 μm or less, and particularly preferably 0.35 μm or less.

[0023] In this specification, surface roughness Ra is the arithmetic mean roughness calculated in accordance with JIS B 0601:2001.

[0024] <Method for manufacturing ceramic substrates> The ceramic substrate of this embodiment can be used, for example, as an insulating substrate used in power modules. A power module is an electronic device that constitutes an inverter circuit for controlling motors in electric vehicles, hybrid electric vehicles, railway vehicles, or industrial equipment.

[0025] As described above, the ceramic substrate is preferably a ceramic substrate on which metal circuits and metal heat sinks are provided on each main surface. Examples include ceramic substrates made of silicon nitride, aluminum nitride, silicon carbide, alumina, etc., with silicon nitride substrates being preferred.

[0026] Next, the method for manufacturing the ceramic substrate of this embodiment will be described. The ceramic substrate can be manufactured according to known manufacturing methods and can be obtained by treating the surface roughness Ra of each main surface to have a desired relationship. The manufacturing of a silicon nitride substrate will be described below as an example.

[0027] (1-1) Slurry preparation (slurry preparation process) First, silicon powder, which is the raw material for the substrate, is mixed with rare earth element oxides and magnesium compounds as sintering aids to create a raw material powder. This powder is then pulverized using methods such as media dispersion to produce a slurry. The raw materials used will be described in detail below.

[0028] (a) Silicon For the silicon used here, industrially available grade silicon powder can be used. The silicon before grinding should have a median diameter D50 of 6 μm or more and a BET specific surface area of ​​3 m². 2 Preferably, the powder has a concentration of 1.0% by mass or less per gram, an oxygen content of 1.0% by mass or less, and a silicon impurity C content of 0.15% by mass or less, a median diameter D50 of 7 μm or more, and a BET specific surface area of ​​2.5 m². 2 Powders with a purity of 0.5% by mass or less, an oxygen content of 0.5% by mass or less, and an impurity C content of 0.10% by mass or less in silicon are more preferable. The purity of the silicon powder is preferably 99% or higher, and more preferably 99.5% or higher.

[0029] The impurity oxygen contained in silicon is one of the factors that inhibits the heat conduction of the silicon nitride substrate obtained by reaction sintering, and it is preferable to keep it as low as possible. Furthermore, in this 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 limiting the amount of oxygen from the magnesium compound. In this case, it is preferable that the total amount of oxygen in the powder raw material is in the range of 0.1 to 1.1 mass% relative to the silicon converted to silicon nitride.

[0030] Furthermore, the carbon impurities contained in silicon may inhibit the growth of silicon nitride particles in silicon nitride substrates obtained by reaction sintering. As a result, insufficient densification can occur, which is one of the factors that reduces thermal conductivity and insulation.

[0031] Furthermore, in this specification, the BET specific surface area (m²) 2 The value ( / g) is determined using a BET specific surface area meter by the BET single-point method (JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by gas adsorption BET method"), and the median diameter D50 (μm) is the particle size at which the cumulative frequency reaches 50% in the particle size distribution determined by laser diffraction and scattering method.

[0032] Although not essential in the manufacturing method of this embodiment, silicon nitride powder may be included in the raw material powder. However, since using silicon nitride is more expensive than using silicon, it is preferable to use as little silicon nitride as possible. Also, when silicon nitride is used in the raw material powder, the contribution of the nitriding process described later becomes smaller, which may make it difficult to achieve the difference in surface roughness between the main surfaces required in this embodiment. For this reason, the amount of silicon nitride used is preferably 20 moles or less of silicon (silicon nitride equivalent), more preferably 10 moles or less, and even more preferably 5 moles or less.

[0033] (b) Rare earth element oxide The rare earth element oxides used here are preferably those of Y, Yb, Gd, Er, and Lu, which are readily available and stable as oxides. Specific examples of rare earth element oxides include Y2O3, Yb2O3, Gd2O3, Er2O3, and Lu2O3. The content of rare earth element oxides is 0.5 mol% or more and less than 3 mol% relative to the total of silicon (converted to silicon nitride), rare earth element oxides (converted to trivalent oxides), and magnesium compounds (converted to MgO).

[0034] If the content of rare earth element oxides is less than 0.5 mol%, the effect as a sintering aid is insufficient and the density does not increase sufficiently, which is undesirable. On the other hand, if the content of rare earth element oxides is 3 mol% or more, the amount of grain boundary phases with low thermal conductivity increases, which lowers the thermal conductivity of the sintered body and also increases the amount of expensive rare earth element oxides used, which is undesirable. The content of rare earth element oxides is preferably 0.6 mol% or more and less than 3 mol%, and more preferably 1 mol% or more and 2 mol% or less.

[0035] In this specification, the sum of the number of moles of silicon nitride (Si3N4) obtained when all silicon is nitrided, the number of moles of rare earth element oxides converted to trivalent oxides RE2O3 (RE is a rare earth element), and the number of moles of magnesium compounds converted to MgO may simply be referred to as "the sum of silicon (converted to silicon nitride), rare earth element oxides (converted to trivalent oxides), and magnesium compounds (converted to MgO)."

[0036] (c) Magnesium compounds As the magnesium compound, one or more magnesium compounds containing silicon (Si), nitrogen (N), or oxygen (O) can be used. In particular, magnesium oxide (MgO), magnesium silicon nitride (MgSiN2), magnesium silicide (Mg2Si), magnesium nitride (Mg3N2), etc. are preferred.

[0037] Here, it is preferable to select the magnesium compound such that 87% by mass or more is MgSiN2. By using 87% by mass or more of MgSiN2, the oxygen concentration in the resulting silicon nitride substrate can be reduced. If the amount of MgSiN2 in the magnesium compound is less than 87% by mass, the amount of oxygen in the silicon nitride particles after sintering will be high, which may result in a low thermal conductivity of the sintered body. Preferably, the amount of MgSiN2 in the magnesium compound is 90% by mass or more.

[0038] The magnesium compound content (in MgO equivalent) in the silicon nitride substrate is 8 mol% or more and less than 15 mol% relative to the total of silicon (in silicon nitride equivalent), rare earth element oxides (in trivalent oxide equivalent), and magnesium compounds (in MgO equivalent). If the magnesium compound content is less than 8 mol%, the effect as a sintering aid is insufficient and the density does not increase sufficiently, which is undesirable. If the magnesium compound content is 15 mol% or more, the amount of grain boundary phases with low thermal conductivity increases, which lowers the thermal conductivity of the sintered body, which is also undesirable. The magnesium compound content is preferably 8 mol% or more and less than 14 mol%, and more preferably 9 mol% or more and less than 13 mol%.

[0039] (d) Crushing To prepare a slurry (dispersion of raw material powder), silicon powder is mixed with rare earth element oxides and magnesium compounds in predetermined ratios as sintering aids, a dispersion medium (organic solvent) and, if necessary, a dispersant, and the mixture is ground in a ball mill. The media should have a diameter of 5 mm or more, the concentration of raw material powder in the slurry (also called slurry concentration) should be 40% by mass or more, and the grinding should be carried out for 6 hours or more. The media should preferably be made of a material that does not mainly contain Al or Fe, which can reduce the thermal conductivity of silicon nitride, and silicon nitride is particularly preferred. The types of dispersion medium and dispersant are not particularly limited and can be arbitrarily selected depending on the sheet molding method, etc.

[0040] As the dispersion medium, ethanol, n-butanol, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), etc., can be used, and as the dispersant, for example, sorbitan ester type dispersants, polyoxyalkylene type dispersants, etc., can be used. The amount of dispersion medium used is preferably 40 to 70% by mass of the total amount of the powder, and the amount of dispersant used is preferably 0.3 to 2% by mass of the total amount of the powder. After dispersion, the dispersion medium may be removed or replaced with another dispersion medium as needed.

[0041] The grinding time is not particularly limited as it varies depending on the milling equipment used, the amount and characteristics of the starting material, etc., but it is preferable to select a time that allows the raw material powder to be sufficiently ground and mixed. For example, the grinding time is preferably 6 hours or more and 48 hours or less, and more preferably 12 hours or more and 24 hours or less. If the grinding time is too short, sufficient grinding may not be achieved, and a silicon nitride substrate with the characteristics required in this embodiment may not be obtained. If the grinding time is too long, the amount of impurity oxygen may gradually increase, and the thermal conductivity of the silicon nitride substrate may decrease.

[0042] The silicon particles after grinding preferably have an oxygen content of 1.0% by mass or less, and more preferably 0.7% by mass or less. By using silicon particles with as little oxygen as possible, the thermal conductivity of silicon nitride can be improved. Since it is difficult to measure the oxygen content of silicon particles alone after mixing with a sintering aid, if a slurry of only silicon particles without the sintering aid is prepared as a sample under the same grinding conditions, this slurry can be used to measure the oxygen content of the silicon particles. For example, silicon particles can be extracted from the slurry, and the oxygen content of the silicon particles can be measured using an oxygen analyzer that employs the inert gas fusion-non-dispersive infrared absorption method.

[0043] The BET specific surface area (m²) of silicon particles in the slurry obtained by grinding. 2 The particle size ( / g), median diameter D50 (μm), and oxygen content were obtained using silicon particles ground in the same manner, except that rare earth element oxides and magnesium compounds were not added. Since only very small amounts of rare earth element oxide powder and magnesium compound powder were added to the silicon powder, they had little effect on the grinding efficiency, and the values ​​obtained in this way are considered to be substantially the same as those of silicon particles in the slurry.

[0044] (1-2) Fabrication of sheet-like molded products (sheet molding process) To the obtained slurry, a dispersion medium, organic binder, dispersant, etc., are added as needed, and vacuum degassing is performed as necessary to adjust the viscosity to a predetermined range and prepare a slurry for coating. Preferably, the slurry viscosity is adjusted to a range of 1 Pa·s or more and less than 15 Pa·s. The viscosity of the slurry was measured using a rotational viscometer at a temperature of 25°C and a rotational speed of 10 rpm. In some cases, the dispersion medium may be removed or replaced as described above.

[0045] The prepared coating slurry is formed into a sheet using a sheet molding machine, cut to a predetermined size, and then dried to obtain a sheet-like molded body. The organic binder used in preparing the coating slurry is not particularly limited, but examples include PVB resin (polyvinyl butyral resin), ethyl cellulose resin, and acrylic resin. The amounts of the dispersion medium, organic binder, dispersant, etc. added are preferably adjusted as appropriate according to the coating conditions.

[0046] The method for forming the coating slurry into a sheet is not particularly limited, but for example, a sheet forming method such as the doctor blade method can be used. In the silicon nitride substrate manufacturing method of this embodiment, it is preferable to make the surface roughness Ra different between the main surfaces of the substrate at the stage of forming it into a sheet.

[0047] When forming a sheet-like molded body using the doctor blade method, after forming the sheet, one main surface contacts the transport film while the other main surface is left open during drying. This makes it easy to adjust the surface roughness Ra of the main surface in contact with the transport film (main surface 1a) to be small, and the surface roughness Ra of the other main surface (main surface 1b) that is left open to be large. At this stage, the surface roughness Ra does not necessarily have to satisfy the required relationship with the surface roughness Ra of the ceramic substrate.

[0048] The transport film used in this process preferably has a surface roughness Ra of 0.03 to 0.20 μm at the contact surface with the sheet-like molded body, and more preferably 0.04 to 0.10 μm. By using a transport film with such a surface roughness Ra, a sheet-like molded body can be obtained that has a main surface with a smaller surface roughness Ra than the surface in the open state.

[0049] Furthermore, the material of the transport film is not particularly limited as long as it is one of the materials used in known transport films, and examples include polyester such as polyethylene terephthalate (PET).

[0050] In this doctor blade method, the molding speed of the coating slurry is preferably 600 mm / min or less. The coating slurry used in this embodiment is thixotropic, so when the coating slurry passes through the doctor blade in the doctor blade method, shear stress is applied to the coating slurry, causing a decrease in the viscosity of the coating slurry. Therefore, if the molding speed exceeds 600 mm / min, the coating slurry flows easily and tends to entrain bubbles that cause voids, which may hinder densification.

[0051] The sheet-like slurry formed on the transport film is then transported to a drying chamber set to a predetermined temperature and humidity, where the solvent is evaporated to dry the sheet-like molded body. At this time, it is preferable that the drying rate of the coating slurry be 0.8 mass% / min or less. If the drying rate of the coating slurry exceeds 0.8 mass% / min, the dispersion medium may rapidly volatilize, making it easier for air bubbles to form within the sheet. After coating, the sheet is passed through a drying zone, where the temperature is gradually raised and it is dried, so the drying rate during the drying process is not constant but fluctuates. Therefore, it is preferable that the maximum drying rate does not exceed 0.8 mass% / min.

[0052] The thickness of the sheet-like molded body formed during the molding process can be adjusted so that the final thickness of the resulting ceramic substrate is a desired thickness, for example, 0.15 mm to 0.8 mm. Furthermore, the sheet-like molded body can be cut to a predetermined size using a punching machine or the like, as needed.

[0053] (1-3) Sintering of the molded body (sintering process) The resulting sheet-like molded body is heated to nitride the silicon contained in the molded body, and then densified. This sintering process includes a degreasing step to remove the organic binder from the molded body, a nitriding step to react the silicon (Si) and nitrogen (N) contained in the molded body to nitride it, and a densification sintering step to densify the material after nitriding. These steps may be carried out sequentially in separate furnaces or continuously in the same furnace.

[0054] In this embodiment, when performing the sintering process, powdered boron nitride (BN) is applied to the main surface 1a of the sheet-like molded body to form a boron nitride (BN powder) layer. When multiple sheet-like molded bodies are stacked, this boron nitride (BN) also functions as a separating agent that facilitates separation after sintering. Therefore, when multiple sheet-like molded bodies are stacked, boron nitride (BN) is present between the sheet-like molded bodies, allowing each sintered body to be easily separated from the sintered body stack obtained after sintering.

[0055] The sheet-like molded body coated with boron nitride powder (BN powder) is placed in an electric furnace, degreased (removed organic binders, etc.), decarbonized in a nitriding apparatus at 900-1300°C, nitrided under a nitrogen atmosphere by raising the temperature to a predetermined level, and then sintered in a sintering apparatus. At this time, it is preferable to heat the molded body while applying a load of 10-1000 Pa. Degreasing is preferably performed at a temperature of 800°C or lower.

[0056] It is preferable to use a BN powder layer with a thickness of approximately 3 to 20 μm as the separating agent. The BN powder layer can be formed on one surface of each sheet-like molded body by, for example, spraying, brushing, or screen printing a slurry of BN powder. The BN powder preferably has a purity of 95% or more and an average particle size (D50) of 1 to 20 μm.

[0057] In the nitriding process, the nitrogen partial pressure during nitriding is preferably 0.05 to 0.7 MPa, and more preferably 0.07 to 0.2 MPa. The nitriding temperature is preferably 1350 to 1500°C, and more preferably 1400 to 1450°C. The holding time after heating to the nitriding temperature is preferably 3 to 12 hours, and more preferably 5 to 10 hours.

[0058] If the nitriding temperature is less than 1350°C, or if the holding time is less than 3 hours, unreacted silicon powder may remain in the sheet-like molded body, making it impossible to obtain a dense body through the densification sintering process performed after the nitriding process. If the nitriding temperature exceeds 1500°C, the silicon powder may melt before nitriding, leaving it unnitrided, or the sintering aid components may volatilize, resulting in a deficiency of sintering aid components in the densification sintering process, making it difficult to obtain a dense sintered body. If the holding time exceeds 12 hours, the sintering aid components may volatilize, resulting in a deficiency of sintering aid components in the densification sintering process, making it difficult to obtain a dense sintered body.

[0059] The nitrogen partial pressure during densification sintering is preferably 0.1 to 0.9 MPa, and more preferably 0.5 to 0.9 MPa. The sintering temperature is preferably 1800 to 1950°C, and more preferably 1850 to 1900°C. The holding time (sintering time) after heating to the sintering temperature is preferably 3 to 12 hours, and more preferably 5 to 12 hours. If 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. If the sintering temperature is greater than 1950°C, or the holding time is greater than 12 hours, the sintering aid components may volatilize and become insufficient, making it difficult to obtain a dense sintered body.

[0060] By the method described above, the ceramic substrate (silicon nitride substrate) of this embodiment can be obtained. Generally, at the stage where the sheet-like molded body is obtained, the difference in surface roughness between the two main surfaces of the sheet-like molded body is not 0.10 μm or more. However, in this embodiment, the difference in surface roughness Ra between the two main surfaces can be made 0.10 μm or more through the subsequent sintering process (including the nitriding process).

[0061] The reason for this is that, although the principle is not entirely clear, boron nitride (BN) present on the surface of the sheet-like molded body suppresses the growth of sintered particles formed by sintering, and the difference in surface roughness Ra between the two main surfaces of the sheet-like molded body increases with the sintering operation. In particular, it is presumed that using silicon powder as the raw material and performing a nitriding process before sintering increases the silicon nitride content (compared to using silicon nitride powder as the raw material), making particle growth easier and thus making the difference more pronounced. In other words, it is presumed that the main surface 1b of the sheet-like molded body, which does not have a BN powder layer formed on it, tends to have a larger surface roughness Ra than the main surface 1a, which has a BN powder layer formed on it.

[0062] Furthermore, blast treatment may be performed as appropriate after the sintering process. Blasting treatment generally increases the surface roughness Ra of the substrate surface, and known methods can be used without particular limitation. Examples of blast treatments include dry blast treatment, in which abrasive material is sprayed with compressed air, and wet blast treatment, in which a mixture of abrasive material and solution is sprayed with compressed air.

[0063] This blasting treatment may be performed, for example, to fine-tune the surface roughness Ra of the ceramic substrate obtained above, or if the difference in surface roughness Ra between the main surfaces of the ceramic substrate obtained after the sintering process does not satisfy the relationship specified above, this blasting treatment may be performed to satisfy that relationship.

[0064] The silicon nitride substrate obtained in this manner has a shape having, for example, two main surfaces and four side surfaces (Figure 1), where the surface roughness Ra1 of one main surface 1a is 0.50 μm or less, and the surface roughness Ra2 of the other main surface 1b is 0.50 μm or more, with surface roughness Ra2 being 0.10 μm or more greater than surface roughness Ra1.

[0065] The silicon nitride substrate is preferably rectangular in shape, with each side measuring 100 mm or more. The silicon nitride substrate has good thermal conductivity and is suitable for the above-mentioned applications, such as power modules. Its thermal conductivity is preferably 100 W / m·K or higher, more preferably 110 W / m·K or higher, and even more preferably 130 W / m·K or higher. In the silicon nitride substrate manufacturing method having a nitriding process as described in detail above, the purity of the silicon nitride can be improved, so it is preferable that a substrate with a thermal conductivity of 110 W / m·K or higher can be easily manufactured.

[0066] The silicon nitride substrate after sintering mainly consists of β-phase silicon nitride and contains rare earth elements and magnesium. The rare earth elements may be in elemental form or may form compounds with other substances. The magnesium contained in the silicon nitride substrate may be in elemental form or may be in compounds with other substances.

[0067] The silicon nitride substrate formed as described above is 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 (converted to trivalent oxide RE2O3 (RE is a rare earth element)) in the grain boundary phase is preferably 0.5 to 3.0 mol%, and the content of magnesium (converted to MgO) is preferably 0.5 to 10 mol%. In this embodiment, the content of rare earth elements and magnesium in the silicon nitride substrate is calculated by taking the total of the number of moles of silicon nitride (Si3N4), the number of moles of the rare earth elements converted to trivalent oxide RE2O3 (RE is a rare earth element), and the number of moles of the magnesium converted to MgO as 100 mol%. Hereinafter, the above total may simply be referred to as "the total of silicon nitride, rare earth elements (converted to trivalent oxide), and magnesium (converted to MgO)."

[0068] Here, the total content of rare earth elements (equivalent to trivalent oxide RE2O3 (RE is a rare earth element)) and magnesium (equivalent to MgO) in the grain boundary phase (total amount of grain boundary phase) is preferably 1.0 to 12.3 mol%.

[0069] The content of silicon nitride, rare earth elements, and magnesium in the silicon nitride substrate depends on the amount of silicon powder added during manufacturing, as well as the amount of rare earth element oxides and magnesium compounds added as sintering aids.

[0070] In the above method, since magnesium compounds are mainly reduced by volatilization during firing, the magnesium content in the silicon nitride substrate after sintering decreases compared to the amount added during manufacturing. On the other hand, since rare earth element oxides hardly volatilize, the decrease in magnesium compounds may result in a slight increase in the total content of silicon nitride, rare earth elements (in terms of trivalent oxides), and magnesium (in terms of MgO). The amount of magnesium compounds that volatilize varies depending on the shape of the molded body, firing conditions, etc.

[0071] The oxygen content within the silicon nitride particles is preferably 0.05% by mass or less. If the oxygen content exceeds 0.05% by mass, high thermal conductivity may not be obtained. Two silicon nitride substrates can be prepared under the same conditions as samples. One silicon nitride substrate can be used as the substrate, and the other can be used to measure the oxygen content. For example, the silicon nitride particles can be extracted by crushing the other silicon nitride substrate and pickling it (removing the grain boundary phase by pickling), and the oxygen content of the silicon nitride particles can be measured using an oxygen analyzer that employs the inert gas fusion-non-dispersive infrared absorption method.

[0072] (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%, a high thermal conductivity cannot be obtained. Such a dense silicon nitride substrate is less susceptible to inhibition of thermal conduction due to voids, and in particular, the silicon nitride substrate of this embodiment preferably has a thermal conductivity of 110 W / m·K or more in the thickness direction.

[0073] Furthermore, the bending strength of the silicon nitride substrate is preferably 600 MPa or higher. As will be described later, when a silicon nitride circuit board for a power module is used in which circuits such as metal plates are bonded to the silicon nitride substrate via brazing material, high stress is applied during mounting and operation, so depending on the method, a bending strength of 600 MPa or higher is preferable. In addition, a high bending strength of 600 MPa or higher makes it possible to make the silicon nitride substrate thinner.

[0074] 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 semiconductor elements or electronic equipment, the thickness is preferably 0.05 to 2.5 mm, more preferably 0.1 to 1 mm, and especially when used as a silicon nitride circuit board for power modules, a thickness of 0.2 to 0.7 mm is even more preferable. The thickness of the silicon nitride substrate after sintering can be adjusted to the desired thickness by adjusting the thickness of the sheet molded body in the sheet molding process, taking into account the influence of the thickness during sintering.

[0075] <Ceramic circuit board> The ceramic circuit board of this embodiment comprises the ceramic substrate described above, a metal circuit bonded to one main surface of the ceramic substrate, and a metal heat sink bonded to the other main surface of the ceramic substrate, wherein 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.

[0076] This ceramic circuit board is, for example, a ceramic circuit board 10, which comprises 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, as shown in Figure 2. Furthermore, this ceramic circuit board 10 has a brazing layer 13 that joins the ceramic substrate 1 and the metal circuit 11, and a brazing layer 14 that joins the ceramic substrate 1 and the metal heat sink 12.

[0077] The ceramic substrate 1 can be the ceramic substrate of this embodiment as described above.

[0078] The metal circuit 11 can have the same configuration as known metal circuits formed on this type of ceramic circuit substrate, and is not particularly limited.

[0079] The metal circuit 11 can be formed from, for example, copper, aluminum, or alloys thereof. The thickness of the metal circuit 11 is not particularly limited, but is preferably 0.2 mm to 2.0 mm, and more preferably 0.5 mm to 1.2 mm. For example, it is 0.8 mm.

[0080] The metal heat sink 12 can have the same configuration as known metal heat sinks formed on this type of ceramic circuit board, and is not particularly limited.

[0081] The metal heat sink 12 can be made of, for example, copper, aluminum, or an alloy thereof. The thickness of the metal heat sink 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.

[0082] The thickness of the metal heat sink 12 may be greater than that of the metal circuit 11. By doing so, the amount of heat transferred from the metal heat sink 12 to the heat sink etc. can be increased, and heat dissipation can be performed efficiently. For example, it is preferable that the thickness of the metal heat sink 12 be 0.1 mm or more greater than that of the metal circuit 11.

[0083] The brazing layer 13 is a bonding material for joining the ceramic substrate 1 and the metal circuit 11, and the brazing layer 14 is a bonding material for joining the ceramic substrate 1 and the metal heat sink 12.

[0084] These brazing layers 13 and 14 can be formed using known brazing layers used in this type of ceramic circuit board, and are generally composed of a paste containing brazing powder and an organic binder. Examples of brazing powders used here include brazing powders containing silver, copper, etc., in a predetermined composition, and various organic resins can be used as the organic binder. For example, an Ag-Cu activated brazing material, mainly composed of Ag and Cu in a eutectic composition with active metals such as Ti, Zr, and Hf added, is preferred in terms of obtaining high strength and high sealing properties. Furthermore, from the viewpoint of bonding strength between the ceramic substrate and the metal plate, a ternary Ag-Cu-In activated brazing material, in which In is added to the above Ag-Cu activated brazing material, is even more preferred.

[0085] The thickness of the brazing material layer 13 is not particularly limited, but is preferably 5 μm to 50 μm, and more preferably 10 μm to 30 μm. Similarly, the thickness of the brazing material layer 14 is not particularly limited, but is preferably 5 μm to 50 μm, and more preferably 10 μm to 30 μm.

[0086] In this case, the brazing layer 14 may be thicker than the brazing layer 13. By doing so, thermal stress and other stresses generated during the operation of the semiconductor device can be more easily absorbed by the brazing layer, and the effects of thermal deformation of the substrate can be suppressed.

[0087] <Method for manufacturing ceramic circuit boards> Next, the manufacturing method for the ceramic circuit board of this embodiment will be described. The ceramic circuit board can be manufactured according to known manufacturing methods and is not particularly limited. The following will be described in detail with reference to Figures 3 to 6.

[0088] (2-1) Formation of the brazing layer (brazing layer formation process) First, prepare the ceramic substrate 1 described in the above embodiment. Note that this ceramic substrate 1 has main surfaces with different surface roughness Ra values, and the members to be joined will differ depending on the surface roughness Ra value.

[0089] At this stage, a brazing layer is formed on both main surfaces in order to join a metal circuit or metal heat sink to either of them. Specifically, a brazing layer 13 is formed on the first main surface 1a, and a brazing layer 14 is formed on the second main surface 1b, by applying brazing material using a method such as screen printing (Figure 3).

[0090] The brazing material used to form the brazing layers 13 and 14 here is the same brazing material described above for the ceramic circuit board.

[0091] (2-2) Joining of metal plates (joining process) A metal plate 21 for circuit formation is laminated and fixed onto the ceramic substrate 1 via a brazing layer 13, and a metal plate 22 for heat sink formation is laminated and fixed onto the ceramic substrate 1 via a brazing layer 14.

[0092] By heating the resulting 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 brazing layers, forming a laminate (Figure 4).

[0093] The heating during this joining process is preferably carried out in a vacuum or a reducing atmosphere. Furthermore, in order to remove organic components from the brazing paste during the heating process, it is preferable to temporarily hold the temperature near the volatilization temperature of the organic binder, and then heat it to the brazing temperature. This heating is preferably continued for 10 minutes or more. The brazing temperature refers to the temperature at which a brazing layer can be properly formed, that is, a temperature above the melting point of the brazing material. The brazing temperature is usually the highest temperature reached during the heating process described above.

[0094] After the heating described above, the temperature can be lowered to room temperature by a known method, and this 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 it from 350°C to room temperature.

[0095] During the holding step in the joining process, if the holding temperature for removing the organic binder is too low, the organic binder components may not be able to volatilize, potentially leaving organic binder residue. Therefore, it is preferable to set the holding temperature for removing the organic binder to 300°C or higher. For example, in the case of an organic binder containing acrylic resin, it is preferable to set this holding temperature to 360°C or higher. To avoid oxidation of the active metal in the brazing material by oxygen contained in the resin in the organic binder, the holding temperature for removing the organic binder should be lower than the brazing temperature in the heating step (brazing step).

[0096] As described above, the bonding of the ceramic substrate and the metal plate can be performed using the brazing material described in the bonding process, using a brazing paste containing brazing material powder and an organic binder. When using a brazing material with a predetermined melting point, it is preferable to set the brazing temperature to a temperature near that melting point. Heating above the melting point ensures sufficient melting of the brazing material and suppresses the formation of voids. Furthermore, it is preferable not to set the temperature far above the melting point to prevent the brazing material from wetting and spreading excessively.

[0097] The holding time at the brazing temperature depends on the amount used, but considering normal productivity, it is preferably within 5 hours, and more preferably within 2 hours. The holding time at the brazing temperature is set appropriately according to the number of samples to be placed, and, for example, if a vacuum atmosphere is used, according to the volume of the joining heating furnace and the exhaust volume of the vacuum pump. It is preferable to apply a load to fix the metal plate and ceramics in place during the placement process so that they are joined without voids, and then heat them during the joining process.

[0098] (2-3) Formation of circuit patterns (pattern formation process) Circuit patterns are formed on the laminate obtained in the above bonding process. To form the circuit patterns, first, a resist film is formed on the surface of the circuit-forming metal plate 21 and the heat sink-forming metal plate 22 in a desired pattern.

[0099] The resist film can be formed by known methods. After forming a resist film over the entire surface of the metal plate, a resist film with a desired pattern can be created using photolithography or the like. Using the formed resist film, a metal pattern can be formed by etching, and then the resist film can be removed to form the metal circuit 11 and the metal heat sink 12 (Figure 5).

[0100] The resist film formed in the pattern formation process preferably has a thickness of 10 to 80 μm, and more preferably 30 to 70 μm. A UV-curable resist material is preferred as the resist film.

[0101] In Figure 5, for illustrative purposes, a pair of metal circuits 11 and metal heat sinks 12 are shown as being formed. However, in this pattern formation, typically multiple sets of metal circuits 11 and metal heat sinks 12 are formed on the ceramic substrate 1. These multiple sets of metal circuits 11 and metal heat sinks 12 are then cut by dicing or the like, after going through a process described later, to create multiple ceramic circuit boards.

[0102] (2-4) Brazing layer removal process After the pattern formation process described in (3) above, an unnecessary brazing layer is present on the ceramic substrate 1 in the etched area, so this is removed (Figure 6).

[0103] The wax material can be removed by known methods, and examples of solvents used for removal include chemical solutions containing hydrogen peroxide and acidic ammonium fluoride. More specifically, an aqueous solution containing 10-40% by mass (2.9-8.8 mol / L) of hydrogen peroxide and 1-8% by mass (0.7-2.1 mol / L) of acidic ammonium fluoride can be exemplified as a preferred chemical solution.

[0104] If the hydrogen peroxide content is less than 10% by mass, the ability to remove the brazing material may be insufficient, and if it exceeds 40% by mass, the metal plate may be excessively eroded, potentially degrading the dimensional accuracy of the metal plate. If the acidic ammonium fluoride content is less than 1% by mass, the ability to remove the reaction layer containing active metal that forms at the bonding interface between the brazing material layer and the ceramic substrate may decrease, while if it exceeds 8% by mass, it may dissolve the crystalline particles constituting the ceramic substrate, potentially reducing the electrical insulation and strength required for the ceramic substrate.

[0105] (2-5) Washing process After removing the brazing layer, the patterned laminate is immersed in a chemical solution for cleaning. For example, if the oxygen in the processing atmosphere is not sufficiently reduced and the surface of the metal plate oxidizes during the heating process, this can lead to a decrease in electrical conductivity and solderability, which is undesirable. Therefore, the oxides on the surface of the metal plate can be removed by immersing the patterned laminate in a chemical solution containing at least one selected from hydrogen peroxide, sulfuric acid, hydrochloric acid, and ammonium chloride. Sulfuric acid is preferred as the chemical solution.

[0106] After the pattern formation process and cleaning process described above, any deposits that reduce the insulation resistance between metal plates on the substrate are removed or reduced, which is preferable.

[0107] Furthermore, after the cleaning process, a plating layer of nickel, gold, silver, or the like may be formed on the surface of the metal plate. When forming such a plating layer, for example, when applying nickel plating, a nickel plating layer with a thickness of about 5 μm can be formed on the surface of the metal plate by immersing it for 20 to 30 minutes in an electroless plating solution (85°C) mainly composed of nickel (Ni) with a phosphorus (P) concentration adjusted to 8% by mass.

[0108] By forming a plating layer as needed during the above brazing layer formation process to cleaning process, the ceramic circuit board 10 of this embodiment can be obtained.

[0109] As shown in Figure 7, the circuit board 10 obtained in this way can be used as a semiconductor device by connecting a semiconductor chip 51 to the metal circuit 11, and its heat dissipation characteristics can be improved by connecting a heat sink 52 or the like to the metal heat sink 12. [Examples]

[0110] This embodiment will be described in more detail by reference to examples, but the present invention is not limited to these examples.

[0111] (1) Fabrication of silicon nitride substrate (Slurry preparation process) BET specific surface area is 2.1m² 2 To silicon powder with 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 relative to the total amount of silicon (calculated as silicon nitride), rare earth element oxides (calculated as trivalent oxides), and magnesium compounds (calculated as MgO) to obtain a raw material powder. To this raw material powder, a dispersion medium (toluene) and 0.5 mass% of a dispersant (sorbitanic acid triolate) relative to the total amount of raw material powder were added to obtain a slurry with a concentration of 42 mass%, and grinding was performed for 24 hours using a ball mill with 5φ silicon nitride balls as the media.

[0112] The amount of magnesium compound added is expressed as mol% when all magnesium compounds are converted to MgO. The BET specific surface area, median diameter D50, and oxygen content of the silicon powder before grinding were measured using a BET single-point BET specific surface area meter, a laser diffraction / scattering particle size analyzer, and an inert gas fusion-non-dispersive infrared absorption oxygen analyzer, respectively.

[0113] (Sheet forming process) The obtained slurry was adjusted in concentration by adding a dispersion medium and an organic binder (acrylic resin), and then degassed to obtain a slurry-like coating liquid. This coating slurry was coated onto the transport film using the doctor blade method, and formed into a sheet with a thickness of 0.38 mm at a molding speed of 600 mm / min or less. The sheet was then cut to a size of 240 mm x 200 mm to obtain a sheet-like molded body. The surface roughness Ra of the coated surface of the transport film was 0.1 μm. Here, the surface of the sheet-like molded body that was in contact with the transport film is referred to as one main surface (main surface 1a), and the surface that was not in contact with the transport film and was in an open state is referred to as the other main surface (main surface 1b).

[0114] (Sintering process) A boron nitride (BN powder) layer is formed on the main surface 1a of the obtained sheet-like molded body. As shown in Figure 8, a laminate 100A is prepared by stacking multiple sheet-like molded bodies with a boron nitride powder (BN powder) layer (thickness 4.5 μm) (not shown) in between, and is placed on a BN mounting plate 200 with a separating material in between. A BN weight plate 300 is placed on top of the laminate 100A.

[0115] Multiple layers of these laminated structures 100A were stacked and arranged, and furthermore, several such multi-layered laminated structures 100A were prepared.

[0116] Next, these multiple laminates 100A were degreased at 750°C for 5 hours under a nitrogen atmosphere (nitrogen partial pressure 0.1 MPa) (degreasing process), and then nitrided in a nitriding apparatus at 1400°C for 10 hours under a nitrogen atmosphere (nitrogen partial pressure 0.5 MPa) (nitriding process).

[0117] Next, the laminate 100A, which had been removed from the nitriding apparatus, was placed in a BN crucible and then transferred to a sintering apparatus. There, it was sintered at 1900°C for 12 hours under a nitrogen atmosphere (nitrogen partial pressure 0.9 MPa) (densification sintering process). The BN powder layer was then removed to obtain a silicon nitride substrate made of a silicon nitride sintered body.

[0118] Finally, the silicon nitride substrate surface was subjected to liquid honing treatment with the aim of cleaning and achieving a suitable roughness. The honing treatment was carried out by adding an appropriate amount of alumina abrasive grains to water and spraying it onto both sides of the sintered body at a pressure of 0.5 MPa. The resulting silicon nitride substrate measured 200 mm × 170 mm and had a thickness of 0.32 mm.

[0119] After cleaning and liquid honing, ten silicon nitride substrates were randomly selected from the numerous substrates obtained. The surface roughness Ra of the main surfaces 1a and 1b of these silicon nitride substrates was measured at six locations on each surface, and the average value was taken as the surface roughness Ra of that surface. The results are summarized in Table 1.

[0120] Furthermore, the surface roughness Ra of the main surfaces 1a and 1b of silicon nitride substrates produced by the same procedure in a separate lot was measured in the same manner and is shown in Table 2.

[0121] [Table 1]

[0122] [Table 2]

[0123] (Comparative example) BET specific surface area is 2.1m² 2 To silicon nitride powder with 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 relative to the total of silicon nitride, rare earth element oxides (in terms of trivalent oxides), and magnesium compounds (in terms of MgO) to obtain the raw material powder.

[0124] The above-mentioned examples were prepared using the same procedure as the examples, except that the nitriding process in the sintering process was omitted. The surface roughness Ra (μm) of the two main surfaces of these silicon nitride substrates was measured, and the results, including the difference in surface roughness Ra (μm) between the main surfaces, are shown below.

[0125] Comparison substrate 1: Main surface 1a 0.581, Main surface 1b 0.585, Difference 0.004 Comparison substrate 2: Main surface 1a 0.583, Main surface 1b 0.572, Difference 0.011 Comparison substrates 1 and 2 have a surface roughness Ra difference of less than 0.10 μm between their main surfaces, and both surface roughness Ra1 and Ra2 exceed 0.50 μm. Furthermore, in comparison substrate 2, Ra2 is smaller than Ra1.

[0126] From the above results, it was found that the ceramic substrate of this embodiment, manufactured by a manufacturing method that includes a nitriding step in which a BN powder layer is formed on the main surface 1a of a sheet-like molded body, can have a suitable surface roughness Ra on the formation surfaces of the metal circuit and the metal heat sink, and that these surfaces can be made to have a desired relationship. As a result, when joining the ceramic substrate to the metal circuit and the metal heat sink, the surface roughness on the main surface 1a to which the metal circuit is joined can be made relatively small so that the brazing material can be easily removed, while the surface roughness on the main surface 1b to which the metal heat sink is joined can be made relatively large so that the bonding strength with the metal heat sink is improved.

[0127] The present inventors have described the invention in detail based on its embodiments, but it goes without saying that the present invention is not limited to the embodiments described above, and can be modified in various ways without departing from its essence. [Explanation of Symbols]

[0128] 1. Ceramic substrate 1a,1b Main surface 10 Ceramic circuit boards 11 Metal circuit 12 Metal heat sink 13,14 Brazing layer 51 Semiconductor chips 52 Heatsink

Claims

1. A ceramic substrate having a first main surface and a second main surface, A metal circuit bonded to the first main surface of the ceramic substrate, A ceramic circuit board comprising a metal heat sink bonded to the second main surface of the ceramic substrate, The aforementioned ceramic substrate is a silicon nitride substrate, The ceramic substrate and the metal circuit are joined via a first brazing layer, and the ceramic substrate and the metal heat sink are joined via a second brazing layer. The thickness of the second brazing layer is greater than the thickness of the first brazing layer. The surface roughness Ra1 of the first main surface is 0.35 μm or more and 0.50 μm or less. The surface roughness Ra2 of the second main surface is 0.50 μm or more and 1.0 μm or less. A ceramic circuit board in which the surface roughness Ra2 is 0.10 μm or greater than the surface roughness Ra1.

2. In the ceramic circuit board according to claim 1, A ceramic circuit board having a thermal conductivity of 110 W / (m·K) or higher.

3. In the ceramic circuit board according to claim 1 or 2, A ceramic circuit board in which the thickness of the metal heat sink is greater than the thickness of the metal circuit.

4. (a) A step of preparing a slurry containing silicon powder, (b) A step of obtaining a sheet-like molded body from the slurry, (c) A step of obtaining a ceramic substrate by sintering the sheet-like molded body, (d) A step of joining a metal circuit to the first main surface of the ceramic substrate via a first brazing layer, and joining a metal heat sink to the second main surface of the ceramic substrate via a second brazing layer. (e) A step of forming a circuit pattern on the metal circuit by etching, (f) A step of removing the first brazing material layer from the etched portion, A method for manufacturing a ceramic circuit board, comprising: The (c) step includes a nitriding step, The aforementioned ceramic substrate is a silicon nitride substrate, The thickness of the second brazing layer is greater than the thickness of the first brazing layer. The surface roughness Ra1 of the first main surface is 0.35 μm or more and 0.50 μm or less. The surface roughness Ra2 of the second main surface is 0.50 μm or more and 1.0 μm or less. A method for manufacturing a ceramic circuit board, wherein the surface roughness Ra2 is 0.10 μm or larger than the surface roughness Ra1.

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