Circuit board and power device

Abstract

This circuit board 50 comprises: a ceramic board 10 composed of a material containing silicon nitride as a main component; a bonding layer 20 containing Ti and formed on one principal surface 12 of the ceramic board 10; and a conductor layer 30 containing Cu and bonded to the ceramic board 10 with the bonding layer 20 therebetween, wherein the bonding layer 20 includes a Ti compound layer 22 formed on the ceramic board 10 side and a Ti-Cu mixed layer 24 formed on the conductor layer 30 side, and in a cross section in a direction perpendicular to the one principal surface, the area ratio of a Ti compound in the Ti-Cu mixed layer 24 is 10-75%.

Description

Circuit boards and power devices

[0001] The present invention relates to a circuit board and a power device.

[0002] Silicon nitride, with its high thermal conductivity and strength, has attracted attention as an insulating heat dissipation substrate for inverter power modules installed in electric vehicles (EVs) and hybrid vehicles (HVs). Traditionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material. However, in the case of high-current power modules such as those used in EVs, temperatures reach approximately 250°C, and the difference in thermal expansion between the substrate and the copper or other metals to which it is bonded generates significant thermal stress, resulting in cracks and fractures in the aluminum nitride, which has low strength. Therefore, silicon nitride, which has a higher thermal conductivity than common insulating ceramics and is even stronger, is increasingly being adopted, although its thermal conductivity is inferior to that of aluminum nitride. The bonding of silicon nitride heat dissipation substrates to conductor layers is typically performed using a brazing method involving the use of a brazing filler metal.

[0003] Patent Document 1 discloses a metal ceramic bonded substrate formed by laminating a conductor layer made of copper or a copper alloy on at least one surface side of a silicon nitride substrate, wherein a bonding layer containing at least one compound selected from the group consisting of titanium nitride, zirconium nitride, vanadium nitride, and aluminum nitride and not containing silver is interposed between the silicon nitride substrate and the conductor layer, the silicon nitride substrate and the conductor layer being bonded together with the bonding layer interposed therebetween, and wherein the oxygen concentration in the bonding layer is 3×10 atoms / cm or more.

[0004] Patent No. 6499545

[0005] Patent Document 1 describes that by bonding a silicon nitride substrate and a conductor layer with a relatively thin bonding layer, it is possible to form a fine circuit pattern and also to exhibit the required heat dissipation performance. However, the bonding strength described in Patent Document 1 was not sufficient in some cases to withstand thermal cycles.

[0006] For these reasons, when using a circuit board in which a silicon nitride heat dissipation substrate and a conductor layer are bonded as a circuit board for a power device, there has been a demand for a circuit board that maintains heat dissipation properties while improving bonding strength.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a circuit board and a power device that can increase the bonding strength between the ceramic substrate and the conductor layer while maintaining the heat transfer between the ceramic substrate and the conductor layer, thereby improving reliability.

[0008] (1) In order to achieve the above object, the circuit board of the present invention employs the following measures: That is, a circuit board according to an application example of the present invention includes a ceramic substrate made of a material primarily composed of silicon nitride, a bonding layer containing titanium formed on one main surface of the ceramic substrate, and a conductor layer containing copper bonded to the ceramic substrate via the bonding layer, wherein the bonding layer includes a Ti compound layer formed on the ceramic substrate side and a Ti—Cu mixed layer formed on the conductor layer side, and the area ratio of the Ti compound in the Ti—Cu mixed layer is 10% or more and 75% or less in a cross section perpendicular to the one main surface.

[0009] (2) In the circuit board of the application example of (1), the area ratio is 10% or more and 75% or less.

[0010] (3) In the circuit board of the application example of (1), the area ratio is 1% or more and less than 10%.

[0011] (4) In the circuit board according to any one of the application examples (1) to (3) above, the thickness of the Ti compound layer is thinner than the thickness of the Ti—Cu mixture layer in the cross section.

[0012] (5) In the circuit board according to any one of the application examples (1) to (4) above, the thickness of the Ti--Cu mixed layer in the cross section is 0.2 μm or more and 2.0 μm or less.

[0013] (6) In addition, in the circuit board of any of the application examples (1) to (5) above, a second bonding layer containing titanium formed on the other main surface opposite the one main surface of the ceramic substrate, and a second conductor layer containing copper bonded to the ceramic substrate via the second bonding layer, wherein the thermal conductivity in a direction perpendicular to the one main surface of the circuit board is 118 W / mK or more.

[0014] (7) A power device according to an application example of the present invention includes the circuit board according to any one of (1) to (6) above, and a power semiconductor mounted on the conductor layer.

[0015] (8) Furthermore, a method for manufacturing a circuit board according to an application example of the present invention includes the steps of preparing a ceramic substrate made of a material primarily composed of silicon nitride, preparing a plate made of a metal containing copper, cleaning the surfaces of the ceramic substrate and the plate, respectively, forming a metal film primarily composed of titanium on one main surface of the ceramic substrate or the plate by sputtering, vapor deposition, or plating, and stacking the ceramic substrate and the plate so that the metal film is sandwiched between them, and joining them by hot pressing.

[0016] According to the circuit board or power device of the present invention, the bonding strength between the ceramic substrate and the conductor layer can be increased while maintaining the heat transfer between the ceramic substrate and the conductor layer, resulting in a circuit board or power device with improved reliability.

[0017] FIG. 1 is a schematic cross-sectional view showing an example of a circuit board according to an embodiment. FIG. 2 is a schematic plan view showing an example of a circuit board according to an embodiment. FIG. 3 is a schematic partially enlarged cross-sectional view showing an example of a circuit board according to an embodiment. FIG. 4 is a schematic cross-sectional view showing a modified example of a circuit board according to an embodiment. FIG. 5 is a schematic bottom view showing a modified example of a circuit board according to an embodiment. FIG. 6 is a schematic partially enlarged cross-sectional view showing a modified example of a circuit board according to an embodiment. FIG. 7 is a schematic cross-sectional view showing an example of a power device according to an embodiment. FIG. 8 is a table showing the thickness, bonding strength, and thermal conductivity of each layer of circuit boards of examples and comparative examples. FIG. 9A is an SEM image of Example 1. FIG. 9B is an SEM image of Comparative Example 1. FIG. 9C is an SEM image of Comparative Example 2.

[0018] Next, an embodiment of the present invention will be described with reference to the drawings. To facilitate understanding of the description, the same reference numerals are used to designate the same components in the drawings, and duplicated descriptions will be omitted. Note that in the configuration diagrams, the size of each component is shown conceptually and does not necessarily represent the actual dimensional ratio.

[0019] [Configuration of Circuit Board] (Embodiment) First, a circuit board according to an embodiment of the present invention will be described. Fig. 1 is a schematic cross-sectional view showing an example of a circuit board 50 according to an embodiment of the present invention. Fig. 2 is a schematic plan view showing an example of a circuit board 50 according to an embodiment of the present invention. Fig. 3 is a schematic partially enlarged cross-sectional view showing an example of a circuit board 50 according to an embodiment of the present invention. The circuit board 50 according to an embodiment of the present invention includes a ceramic substrate 10, a bonding layer 20, and a conductor layer 30.

[0020] The ceramic substrate 10 is made of a material containing silicon nitride as a main component. "Containing silicon nitride as a main component" means that the material contains 86 wt % or more of silicon nitride. The ceramic substrate 10 may also contain sialon. The ceramic substrate 10 is formed, for example, in a flat plate shape.

[0021] The thickness of the ceramic substrate 10 in the direction perpendicular to one main surface 12 is preferably 220 μm or more and 690 μm or less. This allows a good balance between the strength and heat dissipation of the ceramic substrate 10. If the thickness is smaller than this range, the strength of the ceramic substrate 10 may be reduced. On the other hand, if the thickness is larger than this range, the heat dissipation properties may be reduced.

[0022] The bonding layer 20 is formed on one main surface 12 of the ceramic substrate 10. The bonding layer 20 contains titanium (Ti). The bonding layer 20 also includes a Ti compound layer 22 formed on the ceramic substrate 10 side and a Ti—Cu mixed layer 24 formed on the conductor layer 30 side. In addition, in the cross section of the bonding layer 20 perpendicular to the one main surface 12, the area ratio of Ti compounds in the Ti—Cu mixed layer 24 is 10% or more and 75% or less. This can increase the bonding strength between the ceramic substrate 10 and the conductor layer 30 while maintaining heat transfer between the ceramic substrate 10 and the conductor layer 30, thereby improving the reliability of the circuit board 50. The bonding layer 20 may also include layers other than the Ti compound layer 22 and the Ti—Cu mixed layer 24 (layers that can be distinguished from the Ti compound layer 22 and the Ti—Cu mixed layer 24 by SEM image or EPMA).

[0023] In a cross section perpendicular to one main surface 12, the Ti compound layer is preferably thinner than the Ti-Cu mixed layer. This increases the bonding strength between the ceramic substrate 10 and the conductor layer 30. In addition, in a cross section perpendicular to one main surface 12, the thickness of the Ti-Cu mixed layer 24 is preferably 0.2 μm or more and 2.0 μm or less. The thickness of the Ti compound layer 22 is preferably 0.05 μm or more and 0.50 μm or less. The thickness of the bonding layer 20 is preferably 0.25 μm or more and 2.50 μm or less.

[0024] The thicknesses of the Ti—Cu mixed layer 24, the Ti compound layer 22, and the bonding layer 20 can be determined by SEM (Scanning Electron Microscope) observation. Specifically, five randomly selected locations on the polished surface of a cross section perpendicular to one of the main surfaces 12 of the ceramic substrate 10 are observed at 20,000x magnification, with a field of view of 10.0 μm × 7.0 μm. Next, the lengths of ten lines drawn at equal intervals perpendicular to a 9 μm line drawn at the interface between the ceramic substrate 10 and the bonding layer 20 are determined. The average of these values ​​is then used as the thickness of the Ti—Cu mixed layer 24, the Ti compound layer 22, or the bonding layer 20. Note that if the interface between the ceramic substrate 10 and the bonding layer 20 is not a straight line in the SEM image, a line drawn at the interface in a lower magnification SEM image where the interface between the ceramic substrate 10 and the bonding layer 20 can be considered a straight line is used as the reference line. Furthermore, each layer has a different color tone in the SEM image, so the interfaces between the layers can be distinguished.

[0025] The types and proportions of elements contained in the ceramic substrate 10, the bonding layer 20, and the conductor layer 30 can be measured by qualitative and quantitative analysis using an EPMA (Electron Probe Micro Analyzer) and a TEM (Transmission Electron Microscopy) on a polished surface of a cut surface perpendicular to one main surface 12 of the ceramic substrate 10 of the circuit board 50. The Ti compound layer 22 is a layer containing Ti and Ti compounds as main components in an area ratio of 90% or more. The Ti compound layer 22 may contain fine-particle Ti compounds and N, Si, Cu, O, etc. in a total area ratio of less than 10% relative to the fine-particle Ti compounds, as determined by EPMA and TEM. The Ti-Cu mixed layer 24 is a layer formed by interdiffusion of Ti and Cu, and is a layer containing Ti, Ti compounds, Cu, and Cu compounds as main components in an area ratio of 90% or more. EPMA and TEM have shown that the Ti-Cu mixed layer 24 may contain a mixture of fine particles of Ti, Ti, N, Si, Cu, O, etc. The area ratio of Ti compounds per unit area of ​​the Ti-Cu mixed layer 24 can be calculated from an SEM analysis image using image analysis software such as ImageJ. The Ti compounds in the Ti-Cu mixed layer 24 are those compounds whose main component is Ti, as determined by EPMA and TEM.

[0026] The conductor layer 30 is bonded to the ceramic substrate 10 via the bonding layer 20. The conductor layer 30 is preferably made of a metal containing copper, more preferably made of a metal containing copper as a main component, and even more preferably made of oxygen-free copper. A metal containing copper as a main component refers to a metal containing 99 wt% or more of copper. The thickness of the conductor layer 30 is preferably 0.2 mm or more and 1.5 mm or less.

[0027] (Modification) Fig. 4 is a schematic cross-sectional view showing a modification of the circuit board 50 according to the embodiment of the present invention. Fig. 5 is a schematic bottom view showing a modification of the circuit board 50 according to the embodiment of the present invention. Fig. 6 is a schematic partially enlarged cross-sectional view showing a modification of the circuit board 50 according to the embodiment of the present invention. The circuit board 50 according to the embodiment of the present invention includes a ceramic substrate 10, a bonding layer 20, a conductor layer 30, a second bonding layer 26, and a second conductor layer 32. The configurations of the ceramic substrate 10, the bonding layer 20, and the conductor layer 30 are similar to those of the circuit board 50 described above.

[0028] The second bonding layer 26 is formed on the other main surface 16 opposite the one main surface 12 of the ceramic substrate 10. The second bonding layer 26 preferably contains titanium. The second bonding layer 26 also preferably contains a Ti compound layer 22 formed on the ceramic substrate 10 side and a Ti—Cu mixed layer 24 formed on the conductor layer 30 side. This increases the bonding strength between the ceramic substrate 10 and the second conductor layer 32 while maintaining heat transfer between the ceramic substrate 10 and the second conductor layer 32, thereby further improving the reliability of the circuit board 50. When the second bonding layer 26 contains the Ti compound layer 22 or the Ti—Cu mixed layer 24, the second bonding layer 26 may also contain a layer other than the Ti compound layer 22 and the Ti—Cu mixed layer 24 (a layer that can be distinguished from the Ti compound layer 22 and the Ti—Cu mixed layer 24 by SEM imaging or EPMA).

[0029] When second bonding layer 26 includes Ti compound layer 22 and Ti-Cu mixed layer 24, Ti compound layer 22 included in second bonding layer 26 is preferably thinner than Ti-Cu mixed layer 24 included in second bonding layer 26. This increases the bonding strength between ceramic substrate 10 and second conductor layer 32. Furthermore, Ti-Cu mixed layer 24 included in second bonding layer 26 preferably has a thickness of 0.2 μm or more and 2.0 μm or less. Ti compound layer 22 included in second bonding layer 26 preferably has a thickness of 0.05 μm or more and 0.5 μm or less. Second bonding layer 26 preferably has a thickness of 0.25 μm or more and 2.5 μm or less.

[0030] The thickness of second bonding layer 26 may be the same as or different from the thickness of bonding layer 20. When second bonding layer 26 includes Ti compound layer 22, the thickness of Ti compound layer 22 of second bonding layer 26 may be the same as or different from the thickness of Ti compound layer 22 of bonding layer 20. When second bonding layer 26 includes Ti—Cu mixed layer 24, the thickness of Ti—Cu mixed layer 24 of second bonding layer 26 may be the same as or different from the thickness of Ti—Cu mixed layer 24 of bonding layer 20.

[0031] The second conductor layer 32 is bonded to the ceramic substrate 10 via a second bonding layer 26. The second conductor layer 32 is preferably made of a metal containing copper, more preferably made of a metal containing copper as a main component, and even more preferably made of oxygen-free copper. The material of the second conductor layer 32 may be different from the material of the conductor layer 30, but is preferably the same. The thickness of the second conductor layer 32 is preferably 0.2 mm or more and 1.5 mm or less. The thickness of the second conductor layer 32 may be the same as or different from the thickness of the conductor layer 30.

[0032] 5, the second conductor layer 32 is preferably bonded to an area that occupies 75% or more of the area of ​​the other main surface 16 of the ceramic substrate 10. This allows heat to be dissipated efficiently from the second conductor layer 32, improving the heat dissipation properties of the circuit board 50.

[0033] The thermal conductivity of the circuit board 50 in a direction perpendicular to the one main surface 12 is preferably 118 W / mK or more. This sufficiently improves the heat dissipation properties of the circuit board 50. Note that the thermal conductivity in a direction perpendicular to the one main surface 12 of the circuit board 50 is measured for a circuit board 50 that includes a conductor layer 30 and a second conductor layer 32, such as the circuit board 50 of the modified example.

[0034] The thermal conductivity in the direction perpendicular to one main surface 12 of the circuit board 50 can be measured and calculated by a laser flash method.

[0035] These features enable the bonding strength between the ceramic substrate 10 and the conductor layer 30 to be increased while maintaining the heat transfer between the ceramic substrate 10 and the conductor layer 30, thereby improving the reliability of the circuit board 50.

[0036] [Configuration of Power Device] Fig. 7 is a schematic cross-sectional view showing an example of a power device according to an embodiment of the present invention. The power device 100 includes a circuit board 50 and a power semiconductor 60. In Fig. 7, the bonding layer 20, the second bonding layer 26, etc. of the circuit board 50 are omitted.

[0037] The circuit board 50 is the above-described circuit board 50. The circuit board 50 has a conductor layer 30 formed on at least one main surface 12 of the ceramic substrate 10. The circuit board 50 may have a second conductor layer 32 formed on the other main surface 16 opposite the one main surface 12.

[0038] A power semiconductor 60 is mounted on the upper side of the conductor layer 30 of the circuit board 50. The power semiconductor 60 and the conductor layer 30 may be joined using solder 52 or the like. The power semiconductor 60 may be, for example, a semiconductor for an EV that flows a large current and is prone to high temperatures. The circuit board 50 of the present invention has enhanced bonding strength while maintaining heat dissipation properties, and therefore has high resistance to thermal cycles. Therefore, even if high temperatures cause large thermal stress in the ceramic substrate 10 due to the difference in thermal expansion between the ceramic substrate 10 and the joined metal, cracks and breakage are unlikely to occur.

[0039] When the circuit board 50 includes the second conductor layer 32, the heat sink 70 may be joined to the underside of the second conductor layer 32. The heat sink 70 and the second conductor layer 32 may be joined using solder 52 or the like.

[0040] The surface of the heat sink 70 opposite to the surface bonded to the second conductor layer 32 may be in contact with the heat dissipation member 80 via grease 72 or the like. The heat sink 70 is preferably made of metal, more preferably made of a metal containing copper as a main component, and even more preferably made of oxygen-free copper. The heat dissipation member 80 preferably has heat dissipation fins formed thereon. The heat dissipation member 80 is preferably made of metal, more preferably made of a metal containing copper or aluminum as a main component.

[0041] [Method for manufacturing circuit board] An example of a method for manufacturing the circuit board is shown below. A general silicon nitride sintered body can be used as the ceramic substrate. The silicon nitride sintered body can be manufactured, for example, by the following method. First, raw material powder for the silicon nitride sintered body is weighed. The raw material powder for the silicon nitride sintered body may be an oxide, carbonate, hydroxide, nitride, etc. of each element contained in the silicon nitride sintered body. In addition to silicon nitride, examples of raw material powder for the silicon nitride sintered body include magnesium carbonate, calcium carbonate, yttrium oxide, etc.

[0042] Ethanol is added to these raw material powders, and the mixture is wet-mixed and pulverized in a ball mill at, for example, 40 to 100 rpm for 6 to 60 hours to obtain a slurry. The slurry is dried in a hot water bath or with a spray dryer, etc., to obtain a mixed powder.

[0043] The mixed powder is then filled into a mold and pressed uniaxially at a pressure of 30 MPa, for example, to form the desired shape. A cold isostatic pressing (CIP) process is then performed at a pressure of 150 MPa, for example, to obtain a compact. The resulting compact (CIP-pressed body) is placed in a silicon carbide mold, for example, with the interior coated with BN, and sintered at a maximum temperature of 1800°C to 1900°C for 5 to 30 hours in a nitrogen atmosphere at 9 atmospheres, to obtain a silicon nitride sintered body.

[0044] The resulting silicon nitride sintered body is processed to a predetermined shape and thickness to produce a ceramic substrate. Processing can be performed by, for example, cutting, grinding, polishing, etc. The main surface of the ceramic substrate on which the conductor layer or the second conductor layer is bonded is preferably polished to a surface roughness Ra of 0.5 μm or less.

[0045] Separately from the manufacture of the ceramic substrate, a plate material of a predetermined thickness is prepared to serve as the conductor layer or second conductor layer. The plate material is preferably made of a metal containing copper, more preferably made of a metal primarily composed of copper, and even more preferably made of oxygen-free copper. Next, a metal film primarily composed of Ti to serve as a bonding layer is formed on one main surface of the plate material or ceramic substrate. The metal film can be formed by sputtering, vapor deposition, plating, or the like. The thickness of the metal film is preferably 0.01 μm or more and 10.0 μm or less. In this way, by forming a thin metal film primarily composed of Ti in advance and directly bonding it by hot pressing, a Ti compound layer and a Ti-Cu mixed layer are formed. Furthermore, the area ratio of Ti compounds in the Ti-Cu mixed layer is 10% or more and 75% or less. A metal film to serve as a second bonding layer may be formed on the other main surface. Furthermore, prior to forming the metal film, a process of removing organic matter such as oil from the surfaces of the ceramic substrate and plate material is preferably carried out. It is believed that sufficient removal of organic matter from the surface suppresses the generation of gas during heat treatment, contributing to the diffusion of titanium or copper. Various methods can be used to remove the organic matter, such as degreasing, washing with water, washing with acetone, etc. These methods may be combined, or a drying step may be added.

[0046] Next, the ceramic substrate and the plate material are stacked so that the metal film is sandwiched between them. Then, HP treatment (hot pressing) or HIP treatment (hot isostatic pressing) can be performed to bond the plate material and the ceramic substrate. The HP treatment conditions can be, for example, a pressure of 5 MPa to 30 MPa, a maximum temperature of 700°C to 980°C, and a maximum temperature holding time of 10 minutes to 2 hours.

[0047] This manufacturing method makes it possible to increase the bonding strength between the ceramic substrate and the conductor layer while maintaining the heat transfer between the ceramic substrate and the conductor layer, thereby manufacturing a circuit board with improved reliability.

[0048] [Examples and Comparative Examples] (Example 1) 94 wt% silicon nitride powder (average particle size 1.4 μm), 3 wt% magnesium carbonate powder (average particle size 2.5 μm), and 3 wt% yttrium oxide powder (average particle size 1.0 μm) were weighed out. Next, the weighed raw material powders were ball milled to obtain a mixed slurry. For the ball milling, the raw material powders and ethanol were placed in a resin pot, and milled and mixed for 24 hours at 60 rpm using silicon nitride balls. The obtained mixed slurry was dried in a hot water bath to obtain a mixed powder.

[0049] The obtained mixed powder was subjected to powder press molding using uniaxial pressing and CIP to produce a molded body. First, the mixed powder was filled into a dedicated mold and then pre-molded using uniaxial pressing at a pressure of 30 MPa. Next, the pre-molded body was vacuumed and placed in a dedicated bag, and CIP molding was performed at a pressure of 150 MPa. The obtained molded body was fired. The sintering method involved atmospheric firing under a nitrogen gas pressure of 9 atmospheres, and the maximum temperature was maintained at 1900°C for 10 hours. A silicon carbide mold with a BN-coated interior was used. The fired silicon nitride sintered body was cut into a size of 100 mm x 100 mm x 0.32 mm and polished so that the surface roughness Ra of the bonding surface of the conductor layer was 0.5 μm or less to prepare a ceramic substrate.

[0050] Separately, two oxygen-free copper plates (conductor layer, second conductor layer) measuring 100 mm x 100 mm x 0.3 mm were prepared. Next, the surfaces of the ceramic substrate and the plates were each cleaned. Next, Ti, which would serve as a bonding layer, was sputtered to a thickness of 1.0 μm on one main surface and the other main surface (the bonding surface with the plate) of the ceramic substrate. Then, the conductor layer, ceramic substrate, and conductor layer (second conductor layer) were stacked in this order and bonded by HP treatment (hot pressing). The pressure was 10 MPa, the maximum temperature was 900°C, and the maximum temperature holding time was 30 minutes. In this way, the circuit board of Example 1 was produced.

[0051] Example 2 The circuit board of Example 2 was produced under the same conditions as the circuit board of Example 1, except that the thickness of Ti sputtered onto the ceramic substrate was 0.8 μm.

[0052] Example 3 The circuit board of Example 3 was produced under the same conditions as the circuit board of Example 1, except that the thickness of Ti sputtered onto the ceramic substrate was 0.5 μm.

[0053] Example 4 The circuit board of Example 4 was produced under the same conditions as the circuit board of Example 1, except that the thickness of Ti sputtered onto the ceramic substrate was set to 0.2 μm.

[0054] Example 5 The circuit board of Example 5 used oxygen-free copper plates measuring 100 mm × 100 mm × 0.5 mm as the conductor layer and the second conductor layer. The circuit board was fabricated under the same conditions as the circuit board of Example 1, except that the thickness of the Ti sputtered onto the ceramic substrate was 1.0 μm and the maximum temperature of the HP treatment was 1000° C.

[0055] Example 6 The circuit board of Example 6 uses oxygen-free copper plates measuring 100 mm × 100 mm × 0.5 mm as the conductor layer and the second conductor layer. The circuit board was fabricated under the same conditions as the circuit board of Example 1, except that the thickness of the Ti sputtered onto the ceramic substrate was 0.5 μm and the maximum temperature of the HP treatment was 1000° C.

[0056] Example 7 The circuit board of Example 6 uses oxygen-free copper plates measuring 100 mm × 100 mm × 0.5 mm as the conductor layer and the second conductor layer, and was fabricated under the same conditions as the circuit board of Example 1, except that the thickness of the Ti sputtered onto the ceramic substrate was 0.2 μm and the maximum temperature of the HP treatment was 1000° C.

[0057] Comparative Example 1 The circuit board of Comparative Example 1 was produced under the same conditions as the circuit board of Example 1, except that 1.0 μm Ti foil was hot-pressed onto both sides of the ceramic substrate.

[0058] (Comparative Example 2) The circuit board of Comparative Example 2 was produced under the same conditions as the circuit board of Example 1, except that instead of sputtering Ti onto the plate material, a brazing material containing Ti, Cu, and Ag was applied to a thickness of 15 μm and heated to 800° C. to bond the plate material.

[0059] [Various Measurements] The obtained circuit boards of the examples and comparative examples were evaluated by the following measurements.

[0060] (Calculation of Thermal Conductivity) The thermal conductivity of the circuit boards of the examples and comparative examples was determined by a laser flash method at room temperature.

[0061] (Cold-Heat Cycle Test) The circuit boards of the examples and comparative examples were placed in a cold-heat cycle tester and subjected to the following cold-heat cycle test. One cycle of the cold-heat cycle test consisted of -40°C for 30 minutes, 25°C for 10 minutes, 150°C for 30 minutes, 25°C for 10 minutes, and cooling to -40°C. The circuit boards were observed after 500 cycles. Those that did not have any bulging of the conductor layer or bonding layer or cracks in the ceramic substrate were judged as passing (◯), and those that had any bulging of the conductor layer or bonding layer or cracks in the ceramic substrate were judged as failing (×).

[0062] (Confirmation of Ti compound layer and Ti-Cu mixed layer, measurement of thickness) The circuit board was cut perpendicular to one main surface of the ceramic substrate and polished. Then, the presence of the Ti compound layer and the Ti-Cu mixed layer was confirmed by observing SEM images of the cross section at 20,000 times magnification. Next, SEM images of the cross section at 20,000 times magnification were taken at five locations. Next, for each SEM image, a straight line was drawn at the interface between the ceramic substrate and the bonding layer, and 10 line segments were drawn at equal intervals perpendicular to the line. The distance between each interface on the 10 line segments was measured and the average value was calculated to determine the thickness of each layer.

[0063] (Measurement of Elements Contained in Each Layer) The type and relative amount of elements contained in each layer were measured using EPMA and TEM for the same cross section as above.

[0064] (Calculation of Area Ratio of Ti Compounds in Ti—Cu Mixed Layer) Using ImageJ, the area ratio of Ti compounds per unit area of ​​the Ti—Cu mixed layer was calculated from the SEM analysis image.

[0065] (Bonding Strength Test) A 5 mm wide slit was made in the copper plate (conductor layer) bonded to the circuit board of the Example or Comparative Example, and a portion of the edge of the copper plate was peeled off. Next, the edge of the peeled copper plate was clamped in a tensile tester (Imada Seisakusho, SL-6001) and pulled in a direction perpendicular to the bonding surface to measure the load value when the copper plate peeled off. The load value was then divided by 5 mm to obtain the bonding strength value of each circuit board.

[0066] (Results) Figure 8 is a table showing the results of the thickness, bonding strength, and thermal conductivity of each layer of the circuit boards of the Examples and Comparative Examples. Examples 1 to 7, which had a Ti compound layer and a Ti-Cu mixed layer and in which the area ratio of the Ti compound in the Ti-Cu mixed layer was 1% or more and 75% or less, had high bonding strength and maintained high thermal conductivity. Furthermore, Examples 5 to 7, in which the area ratio of the Ti compound in the Ti-Cu mixed layer was 1% or more and less than 10%, showed higher thermal conductivity and bonding strength than Examples 1 to 4, in which the area ratio of the Ti compound in the Ti-Cu mixed layer was 10% or more and 75% or less. In contrast, Comparative Example 1, in which a Ti-Cu mixed layer was not formed, had a fairly high thermal conductivity, but the bonding strength was lower than that of the Examples. Comparative Example 2, in which bonding was performed using a conventional brazing material, showed both low bonding strength and low thermal conductivity. This revealed that it is preferable to have a Ti compound layer and a Ti--Cu mixed layer, and that the area ratio of the Ti compound in the Ti--Cu mixed layer is 10% or more and 75% or less.

[0067] Figure 9A is an SEM image of Example 1. Figure 9B is an SEM image of Comparative Example 1. Figure 9C is an SEM image of Comparative Example 2. Example 1 and Comparative Example 1 are SEM images at 20,000x magnification, and Comparative Example 2 is at 5,000x magnification. Comparative Example 2 was taken at a lower magnification than the others because the bonding layer was thicker. Observing the SEM image of Example 1, the ceramic substrate 10, the Ti compound layer 22 in the bonding layer 20, the Ti-Cu mixed layer 24, and the conductor layer 30 all had different colors and were clearly distinguishable. On the other hand, observing the SEM images of Comparative Examples 1 and 2, no clearly distinguishable layers were observed within the bonding layer. Furthermore, quantitative EPMA and TEM analysis of Example 1 confirmed that the types and amounts of elements contained in the Ti compound layer and the Ti-Cu mixed layer of the bonding layer were different. On the other hand, quantitative EPMA and TEM analysis of Comparative Example 1 confirmed the presence of a Ti-Cu mixed layer. Furthermore, it was confirmed that no Ti compound layer was formed by quantitative EPMA and TEM analysis of Comparative Example 2. Therefore, the thicknesses of the Ti compound layer, Ti—Cu mixed layer, and bonding layer in the examples and comparative examples were calculated from SEM images.

[0068] As described above, in the circuit board of the present invention, the bonding layer includes a Ti compound layer and a Ti-Cu mixed layer, and the area ratio of the Ti compound in the Ti-Cu mixed layer is 10% or more and 75% or less, thereby increasing the bonding strength between the ceramic substrate and the conductor layer while maintaining the heat transfer between the ceramic substrate and the conductor layer. As a result, the circuit board or power device of the present invention can improve the reliability against thermal cycles.

[0069] The present invention is not limited to the above-described embodiments, and various modifications and equivalents are included within the spirit and scope of the present invention. Furthermore, the structure, shape, number, position, size, etc. of the components shown in each drawing are for the convenience of explanation and may be changed as appropriate.

[0070] REFERENCE SIGNS LIST 10 ceramic substrate 12 one principal surface 16 other principal surface 20 bonding layer 22 Ti compound layer 24 Ti-Cu mixed layer 26 second bonding layer 30 conductor layer 32 second conductor layer 50 circuit board 52 solder 60 power semiconductor 70 heat sink 72 grease 80 heat dissipation member 100 power device

Claims

1. A circuit board comprising: a ceramic substrate made of a material primarily composed of silicon nitride; a bonding layer containing titanium formed on one main surface of the ceramic substrate; and a conductor layer containing copper bonded to the ceramic substrate via the bonding layer, wherein the bonding layer includes a Ti compound layer formed on the ceramic substrate side and a Ti-Cu mixed layer formed on the conductor layer side, and wherein the area ratio of Ti compounds in the Ti-Cu mixed layer is 1% or more and 75% or less in a cross section perpendicular to the one main surface.

2. The circuit board according to claim 1, wherein the area ratio is 10% or more and 75% or less.

3. The circuit board according to claim 1, wherein the area ratio is 1% or more and less than 10%.

4. The circuit board according to claim 1, wherein the thickness of the Ti compound layer is thinner than the thickness of the Ti-Cu mixed layer in the cross section.

5. The circuit board according to any one of claims 1 to 4, wherein the thickness of the Ti-Cu mixed layer in the cross section is 0.2 μm or more and 2.0 μm or less.

6. A circuit board according to any one of claims 1 to 4, further comprising: a second bonding layer containing titanium formed on the other main surface opposite the one main surface of the ceramic substrate; and a second conductor layer containing copper bonded to the ceramic substrate via the second bonding layer, wherein the thermal conductivity of the circuit board in a direction perpendicular to the one main surface is 118 W / mK or more.

7. A power device comprising: a circuit board according to any one of claims 1 to 4; and a power semiconductor mounted on the conductor layer.

8. A method for manufacturing a circuit board, comprising the steps of: preparing a ceramic substrate made of a material primarily composed of silicon nitride; preparing a plate made of a metal containing copper; cleaning the surfaces of the ceramic substrate and the plate; forming a metal film primarily composed of titanium on one main surface of the ceramic substrate or the plate by sputtering, vapor deposition, or plating; and stacking the ceramic substrate and the plate so that the metal film is sandwiched between them, and joining them by hot pressing.