Circuit board and power device

Abstract

To provide a circuit board and a power device that can increase the bonding strength between a ceramic substrate and a conductor layer while maintaining heat transfer between the ceramic substrate and the conductor layer, thereby improving reliability.SOLUTION: A circuit board 50 includes a ceramic substrate 10 made of a material primarily composed of silicon nitride, a bonding layer 20 containing titanium or a compound containing titanium formed on one main surface 12 of the ceramic substrate 10, and a conductor layer 30 containing copper bonded to the ceramic substrate 10 via the bonding layer 20, wherein the bonding layer 20 includes a bonding-reinforcing layer 22 that is primarily composed of titanium or a compound containing titanium and contains microparticles 24 made of copper or a compound containing copper.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Silicon nitride, with its high thermal conductivity and strength, is attracting 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, causing the aluminum nitride, which has low strength, to crack and break. 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 generally performed using a brazing method involving the use of a brazing filler metal containing metal.

[0003] Patent Document 1 describes 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, and the silicon nitride substrate and the conductor layer are bonded together with the bonding layer interposed therebetween, and the oxygen concentration in the bonding layer is 3×10 19 atoms / cm 3 The metal-ceramic bonded substrate is disclosed above. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6499545 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[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 formed on one main surface of the ceramic substrate and containing titanium or a compound containing titanium, and a conductor layer containing copper bonded to the ceramic substrate via the bonding layer, wherein the bonding layer includes a bonding-reinforcing layer that is primarily composed of titanium or a compound containing titanium and that contains fine particles made of copper or a compound containing copper.

[0009] (2) In the circuit board of the application example of (1) above, the particles are present within 0.3 μm from the interface between the ceramic substrate and the bonding layer, with an area ratio of 5% or more.

[0010] (3) In addition, in the circuit board of the application example (1) or (2) above, the fine particles are present within 0.2 μm of the end of the void formed at the interface between the ceramic substrate and the bonding layer on the bonding layer side.

[0011] (4) Furthermore, in the circuit board of any of the application examples (1) to (3) above, a second bonding layer containing titanium or a compound containing titanium is formed on the other main surface opposite the one main surface of the ceramic substrate, and a second conductor layer containing copper is bonded to the ceramic substrate via the second bonding layer, and the thermal conductivity in a direction perpendicular to the one main surface of the circuit board is 118 W / mK or more.

[0012] (5) A power device according to an application example of the present invention includes the circuit board according to any one of (1) to (4) above, and a power semiconductor mounted on the conductor layer. [Effects of the Invention]

[0013] 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. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating an example of a circuit board according to an embodiment. [Figure 2] FIG. 2 is a schematic plan view illustrating an example of a circuit board according to an embodiment. [Figure 3] FIG. 10 is a schematic cross-sectional view showing a modified example of the circuit board according to the embodiment. [Figure 4] FIG. 10 is a schematic bottom view showing a modified example of the circuit board according to the embodiment. [Figure 5] 1 is a schematic cross-sectional view showing an example of a power device according to an embodiment. [Figure 6]The image on the left shows each layer color-coded based on the element mapping results of Example 2, and the image on the right is an SEM image. [Figure 7] 1 is a table showing the results of various tests on the circuit boards of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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.

[0016] [Circuit board configuration] (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. 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.

[0017] 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 the shape of a flat plate.

[0018] 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.

[0019] The bonding layer 20 is formed on one main surface 12 of the ceramic substrate 10. The bonding layer 20 contains titanium or a titanium-containing compound. The bonding layer 20 also includes a bonding-strengthening layer 22 that is primarily composed of titanium or a titanium-containing compound and contains fine particles 24 made of copper or a copper-containing compound. This inhibits crack propagation in the bonding layer 20 and increases the bonding strength between the ceramic substrate 10 and the conductor layer 30. As a result, the reliability of the circuit board 50 can be improved while maintaining heat transfer between the ceramic substrate 10 and the conductor layer 30. The titanium-containing compound of the bonding layer 20 refers to a compound that contains titanium but does not contain copper. Examples of compounds that contain titanium but do not contain copper include TiO2, TiN, and TiSi3. "Containing titanium or a titanium-containing compound as a primary component" refers to a material that contains titanium or a titanium-containing compound in an area ratio of 60% or more in a cross section, as described below. The bonding layer 20 may include a layer other than the bonding-strengthening layer 22 (a layer that can be distinguished from the bonding-strengthening layer 22 by SEM image or EPMA) that has titanium or a compound containing titanium as its main component and contains microparticles 24 made of copper or a compound containing copper.

[0020] The thickness of the bonding-strengthening layer 22 is preferably 3 μm or less. Furthermore, the thickness of the bonding-strengthening layer 22 is preferably 0.1 μm or more. The thickness of the bonding-strengthening layer 22 can be determined by SEM (Scanning Electron Microscope) observation. Specifically, three randomly selected locations on a polished cross section perpendicular to one of the main surfaces of the ceramic substrate 10 are observed at 20,000x magnification in a 5 μm × 5 μm field of view. Next, ten lines perpendicular to a line drawn at equal intervals at the interface between the ceramic substrate 10 and the bonding layer 20 are drawn, and the lengths between the interfaces of each layer are determined. The average of these values ​​is then used as the thickness of the bonding-strengthening layer 22. 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 is considered to be a straight line is used as the reference line. Furthermore, the interfaces of each layer can be distinguished because each layer has a different color tone in the SEM image.

[0021] 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 performing qualitative and quantitative analysis using an EPMA (Electron Probe Micro Analyzer) on a polished surface of a cut surface perpendicular to one of the main surfaces 12 of the ceramic substrate 10.

[0022] The bond-strengthening layer 22 is a layer primarily composed of titanium or a titanium-containing compound. EPMA analysis reveals that the layer is composed of titanium or a titanium-containing compound, such as TiO2, TiN, or TiSi3. The copper or copper-containing compound microparticles 24 are particles formed by the diffusion of copper contained in the conductor layer 30. EPMA results reveal that the microparticles contain at least copper, and may contain both copper and titanium. In addition to copper and titanium, the microparticles 24 may also contain copper oxide, Cu4Ti, Cu3Ti2, CuTi, or CuTi2. The maximum major axis diameter of the microparticles 24 contained in the bond-strengthening layer 22 is preferably 100 nm or less. If the microparticles 24 contained in the bond-strengthening layer 22 are too large, the stress between the bond-strengthening layer 22 and the microparticles 24 increases, potentially reducing their effectiveness in inhibiting crack propagation. The minimum major axis diameter of the microparticles 24 contained in the bond-strengthening layer 22 may be, for example, 1 nm or greater.

[0023] The particles 24 are preferably present in an area ratio of 5% or more, and more preferably 10% or more, within 0.3 μm from the interface between the ceramic substrate 10 and the bonding layer 20. This increases the strength of the bonding layer 20 near the interface, and more effectively inhibits crack propagation. As a result, the bonding strength between the ceramic substrate 10 and the conductor layer 30 is increased. The region within 0.3 μm from the interface between the ceramic substrate 10 and the bonding layer 20 is referred to as the vicinity of the substrate.

[0024] For voids 26 formed at the interface between the ceramic substrate 10 and the bonding layer 20, it is preferable that the particles 24 be present within 0.2 μm of the edge of the void 26 on the bonding layer 20 side. Because the formation of voids 26 at the interface between the ceramic substrate 10 and the bonding layer 20 reduces the bonding strength, it is preferable to have as few voids 26 as possible, but it is difficult to eliminate the voids 26 entirely. The circuit board 50 of the present invention is manufactured by adjusting the manufacturing process so that the particles 24 are present within 0.2 μm of the edge of the void 26 on the bonding layer 20 side of the void 26 formed at the interface between the ceramic substrate 10 and the bonding layer 20. This increases the strength of the bonding layer 20 near the void 26 and prevents cracks extending to the void 26 from propagating into the bonding layer 20. As a result, the bonding strength between the ceramic substrate 10 and the conductor layer 30 is stronger. The region within 0.2 μm of the edge of the void 26 on the bonding layer 20 side is referred to as the void vicinity. The maximum distance between two points of one hole 26 is preferably 5 μm or less.

[0025] The size and area ratio of the particles 24, or the presence of particles near the voids, can be determined by EPMA and SEM observation. Specifically, three randomly selected locations are observed on the polished surface of a cross section perpendicular to one of the main surfaces 12 of the ceramic substrate 10 at 20,000x magnification, with a field of view of 5 μm x 5 μm. Next, the EPMA results are used to confirm all particles recognized as particles 24 present in the bonding strengthening layer 22. The size of the particles 24 formed in the bonding layer 20, their distance from the interface, and their distance from the end of the voids 26 do not vary significantly depending on the cut location, so the cut surface may be random.

[0026] When determining the maximum major axis of the particles 24, the major axis is determined for each particle 24, and the maximum value in all images is determined. When determining the area ratio, first, the vicinity of the substrate within 0.3 μm from the interface between the ceramic substrate 10 and the bonding layer 20 in the image is determined. Then, the ratio of the area of ​​the particles 24 present in the region that is both the bonding strengthening layer 22 and the vicinity of the substrate is calculated to the area that is both the bonding strengthening layer 22 and the vicinity of the substrate. When confirming the presence of particles near voids, first, the vicinity of the void within 0.2 μm from the edge of the void 26 in the image is determined. Then, it is confirmed whether or not a particle 24 is present in the vicinity of the void.

[0027] The vicinity of the substrate is taken in a direction perpendicular to the interface or its tangent. If there are voids 26 at the interface between the ceramic substrate 10 and the bonding layer 20, the interface between the voids 26 and the bonding layer 20 is used to determine the vicinity of the substrate. The vicinity of the voids is taken in a direction perpendicular to the interface between the voids 26 and the bonding strengthening layer 22 or its tangent. Image analysis software such as Winroof may be used to determine the maximum diameter of the major axis of the microparticles 24, the area ratio, the interface, the vicinity of the substrate, or the vicinity of the voids.

[0028] 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 its main component, and even more preferably made of oxygen-free copper. A metal containing copper as its 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.

[0029] (Variation) Fig. 3 is a schematic cross-sectional view showing a modified example of a circuit board 50 according to an embodiment of the present invention. Fig. 4 is a schematic bottom view showing a modified example of a circuit board 50 according to an embodiment of the present invention. The circuit board 50 according to the modified example of the embodiment of the present invention includes a ceramic substrate 10, a bonding layer 20, a conductor layer 30, a second bonding layer 28, and a second conductor layer 32. The configurations of the ceramic substrate 10, the bonding layer 20, and the conductor layer 30 are the same as those of the circuit board 50 described above.

[0030] The second bonding layer 28 is formed on the other principal surface 16 of the ceramic substrate 10, opposite the one principal surface 12. The second bonding layer 28 preferably contains titanium. The second bonding layer 28 preferably includes a bond-strengthening layer 22 that is primarily composed of titanium or a titanium-containing compound and contains microparticles 24 made of copper or a copper-containing compound. This inhibits crack propagation in the second bonding layer 28 and increases the bonding strength between the ceramic substrate 10 and the second conductor layer 32. As a result, the reliability of the circuit board 50 can be improved while maintaining heat transfer between the ceramic substrate 10 and the second conductor layer 32. When the second bonding layer 28 includes the bond-strengthening layer 22, it preferably has the same characteristics as described above. When the second bonding layer 28 includes the bond-strengthening layer 22, the second bonding layer 28 may include a layer other than the bond-strengthening layer 22 (a layer that can be distinguished from the bond-strengthening layer 22 by SEM imaging or EPMA).

[0031] When the second bonding layer 28 includes the bonding-strengthening layer 22, the thickness of the bonding-strengthening layer 22 is preferably 3 μm or less. When the second bonding layer 28 includes the bonding-strengthening layer 22, the thickness of the bonding-strengthening layer 22 included in the second bonding layer 28 may be the same as or different from the thickness of the bonding-strengthening layer 22 included in the bonding layer 20.

[0032] The second conductor layer 32 is bonded to the ceramic substrate 10 via a second bonding layer 28. The second conductor layer 32 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. 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.

[0033] 4, 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 performance of the circuit board 50.

[0034] The thermal conductivity of the circuit board 50 in the direction perpendicular to the one main surface 12 is preferably 118 W / mK or more. This sufficiently improves the heat dissipation of the circuit board 50. Note that the thermal conductivity in the 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.

[0035] 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.

[0036] These features make it possible to increase the bonding strength between the ceramic substrate 10 and the conductor layer 30 while maintaining the heat transfer between the ceramic substrate 10 and the conductor layer 30, thereby improving the reliability of the circuit board 50.

[0037] [Power device configuration] Fig. 5 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 28, etc. of the circuit board 50 are omitted.

[0038] 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.

[0039] 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 increased bonding strength while maintaining heat dissipation properties, and therefore has high resistance to thermal cycles. Therefore, even if a large thermal stress is generated in the ceramic substrate 10 due to the difference in thermal expansion between the ceramic substrate 10 and the joined metal due to high temperatures, cracks or breakage are unlikely to occur.

[0040] When the circuit board 50 includes the second conductor layer 32, the heat sink 70 may be bonded to the underside of the second conductor layer 32. The heat sink 70 and the second conductor layer 32 may be bonded using solder 52 or the like.

[0041] 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 its 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 its main component.

[0042] [Circuit board manufacturing method] An example of a method for manufacturing the above-mentioned 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, and yttrium oxide.

[0043] 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.

[0044] Next, the mixed powder is filled into a mold and pressed uniaxially at a pressure of, for example, 30 MPa to form it into the desired shape. After that, a CIP (cold isostatic pressing) process is performed at a pressure of, for example, 150 MPa to obtain a green body. The obtained green body (CIP pressed body) is placed in a silicon carbide mold with the inside 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.

[0045] 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.

[0046] Separately from the production 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 a metal primarily composed of copper, and even more preferably oxygen-free copper. Next, a metal film primarily composed of Ti is formed on one main surface of the plate material or ceramic substrate to serve as a bonding layer. The metal film can be formed by sputtering, vapor deposition, plating, or the like. The thickness of the metal film is preferably 3 μm or less. A metal film serving as a second bonding layer may be formed on the other main surface. If a metal foil of a similar thickness can be prepared, forming the metal film is not necessary. Furthermore, prior to forming the metal film, it is preferable to include a step of removing organic matter, such as oil, from the surface of the ceramic substrate or plate material. Sufficient removal of organic matter from the surface is believed to suppress gas generation during heat treatment and contribute to copper diffusion. Various methods can be used for the organic matter removal step, such as degreasing, washing with water, or washing with acetone. These methods may be combined, or a drying step may be included.

[0047] Next, the plate material with the metal film formed thereon and the ceramic substrate are laminated so that the metal film is sandwiched between them. If no metal film is formed, the plate material, metal foil, and ceramic substrate are laminated in this order. Then, HP (hot pressing) or HIP (hot isostatic pressing) can be used 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 850°C to 1050°C, and a maximum temperature holding time of 10 minutes to 2 hours. In this way, by directly bonding the plates by hot pressing at a relatively high temperature with a titanium-based metal film sandwiched between them, fine particles of copper or copper-based compounds are diffused into the layer primarily composed of titanium or titanium-based compounds, forming a bonding-strengthening layer.

[0048] 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.

[0049] [Examples and Comparative Examples] Example 1 The raw material powders were weighed out to a total of 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). Next, the weighed raw material powders were ball milled to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were placed in a resin pot and milled and mixed at 60 rpm for 24 hours using silicon nitride balls. The resulting mixed slurry was dried in a hot water bath to obtain a mixed powder.

[0050] The resulting mixed powder was subjected to powder press molding using uniaxial pressing and CIP to produce a compact. First, the mixed powder was filled into a dedicated mold and then preformed using uniaxial pressing at a pressure of 30 MPa. Next, the preform was placed in a dedicated bag through vacuum suction and subjected to CIP molding at a pressure of 150 MPa. The resulting compact was then fired. The sintering method involved atmospheric firing under a nitrogen gas pressure of 9 atmospheres, with the maximum temperature held at 1900°C for 10 hours. A silicon carbide mold with a BN coating on the inside was used. The fired silicon nitride sintered compact was cut into a size of 100 mm x 100 mm x 0.32 mm and polished to a surface roughness Ra of 0.5 μm or less on the bonding surface of the conductor layer to prepare a ceramic substrate.

[0051] Separately, two oxygen-free copper plates (conductor layer, second conductor layer) measuring 100 mm × 100 mm × 0.3 mm were prepared. Next, Ti, which would become 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. The conductor layer, ceramic substrate, and conductor layer (second conductor layer) were then stacked in this order and bonded by HP treatment (hot pressing). The pressure was 10 MPa, the maximum temperature was 850°C, and the maximum temperature holding time was 30 minutes. In this way, the circuit board of Example 1 was produced.

[0052] Example 2 The circuit board of Example 2 was produced under the same conditions as those for the circuit board of Example 1, except that the maximum temperature of the HP treatment was changed to 900°C.

[0053] Example 3 The circuit board of Example 3 was produced under the same conditions as those for the circuit board of Example 1, except that the maximum temperature of the HP treatment was changed to 950°C.

[0054] (Comparative Example 1) The circuit board of Comparative Example 1 was produced under the same conditions as those for the circuit board of Example 1, except that the maximum temperature of the HP treatment was changed to 800°C.

[0055] [Various measurements] The obtained circuit boards of the examples and comparative examples were evaluated by the following measurements.

[0056] (Measurement of contained elements, confirmation of fine particles in the bonding layer, etc.) The circuit boards of the examples and comparative examples were cut perpendicular to one main surface of the ceramic substrate, and the cross sections were polished. SEM images of the cross sections were then taken at 20,000x magnification in three locations. Next, the same cross sections as the SEM images were measured using an EPMA to measure the type and relative amount of elements contained in each part, and elemental mapping was performed. Based on the elemental mapping, the presence of fine particles made of copper or copper-containing compounds in the bonding layer was confirmed, the area ratio of fine particles near the substrate was calculated, the presence of fine particles near voids was confirmed, and the maximum diameter of the fine particles was calculated.

[0057] (Bonding strength test) A 5 mm wide slit was made in the copper plate (conductor layer) bonded to the circuit board of each Example or Comparative Example, and a portion of the edge of the copper plate was peeled off. The edge of the peeled copper plate was then clamped in a tensile tester (Imada Seisakusho, SL-6001) and pulled perpendicular to the bonding surface to measure the load value at which the copper plate peeled off. The load value was then divided by 5 mm to obtain the bonding strength value for each circuit board.

[0058] In Figure 6, the left image shows each layer color-coded based on the element mapping results of Example 2, and the right image shows an SEM image. The magnification is 20,000 times. Observation of the element mapping image of Example 2 confirmed that a bonding-strengthening layer containing titanium or a titanium-containing compound as a main component and containing fine particles made of copper or a copper-containing compound was formed between the copper (Cu) and silicon nitride.

[0059] (result) 7 is a table showing the results of various tests on the circuit boards of the examples and comparative examples. Examples 1 to 3, in which a bonding-reinforcing layer containing titanium or a titanium-containing compound as a main component and containing fine particles made of copper or a copper-containing compound was formed on the bonding layer, had high bonding strength. In contrast, Comparative Example 1, in which no bonding-reinforcing layer was formed, had lower bonding strength than the examples. This confirmed that the formation of a bonding-reinforcing layer on the bonding layer increased the bonding strength.

[0060] The bonding strength of Example 2 was higher than that of Example 1. This is thought to be because the area ratio of the fine particles near the substrate was higher than that of Example 1, and the bonding-reinforcing layer near the substrate was compounded to increase its strength and suppress cracks from progressing in the bonding-reinforcing layer. This revealed that the area ratio of the fine particles near the substrate is preferably 5% or more, and more preferably 10% or more.

[0061] The bonding strength of Example 3 was higher than that of Examples 1 and 2. This is thought to be because not only was the area ratio of the fine particles near the substrate greater than that of Examples 1 and 2, but the presence of fine particles near the voids also suppressed cracks from propagating from the voids to the bonding-strengthening layer. This shows that the presence of fine particles near the voids can further increase the bonding strength.

[0062] As described above, in the circuit board of the present invention, the bonding layer includes a bonding-strengthening layer, which increases 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 the reliability of the circuit board or power device of the present invention against thermal cycles.

[0063] 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. [Explanation of symbols]

[0064] 10 Ceramic substrate 12 One main surface 16 Other main surface 20 Bonding layer 22 Bonding reinforcement layer 24 Fine particles 26 Vacancies 28 Second bonding layer 30 Conductor Layer 32 Second conductor layer 50 Circuit Board 52 Solder 60 Power Semiconductors 70 Heat sink 72 Grease 80 Heat dissipation material 100 Power Devices

Claims

1. a ceramic substrate made of a material containing silicon nitride as a main component; a bonding layer including titanium or a compound including titanium formed on one main surface of the ceramic substrate; a conductor layer containing copper bonded to the ceramic substrate via the bonding layer, The circuit board, wherein the bonding layer includes a bonding-reinforcing layer containing titanium or a compound containing titanium as a main component and containing fine particles made of copper or a compound containing copper.

2. 2. The circuit board according to claim 1, wherein the fine particles are present within 0.3 [mu]m from the interface between the ceramic substrate and the bonding layer in an area ratio of 5% or more.

3. 3. The circuit board according to claim 1, wherein the fine particles are present within 0.2 μm of the end of a void formed at the interface between the ceramic substrate and the bonding layer on the bonding layer side.

4. a second bonding layer including titanium or a compound containing titanium formed on the other main surface opposite to the one main surface of the ceramic substrate; a second conductor layer including copper bonded to the ceramic substrate via the second bonding layer, 3. The circuit board according to claim 1, wherein the thermal conductivity of the circuit board in a direction perpendicular to the one main surface is 118 W / mK or more.

5. The circuit board according to claim 1 or 2; a power semiconductor mounted on the conductor layer.

Images