Circuit substrate and semiconductor package

Abstract

This circuit substrate comprises: a ceramic substrate which contains silicon nitride as the main component; a conductive part which is formed of copper and connected to a semiconductor element; and a bonding part which bonds the ceramic substrate and the conductive part. The bonding part: includes a first bonding portion which is disposed on the ceramic substrate side and contains titanium nitride as the main component, and a second bonding portion which is disposed between the first bonding portion and the conductive part and contains copper and titanium; and has a titanium concentration of 0.1-10 atm% in the range of 500 nm or less from the interface with the conductive part.

Description

Circuit board and semiconductor package

[0001] The present invention relates to a circuit board and a semiconductor package.

[0002] Conventionally, a circuit board including a conductive portion connected to a semiconductor element and a ceramic substrate has been known (for example, Patent Document 1).

[0003] Japanese Unexamined Patent Application Publication No. 2016-184606

[0004] However, even with the prior art such as that disclosed in Patent Document 1, there is still room for improvement in the technology capable of improving the durability of the circuit board in thermal cycles.

[0005] An object of the present invention is to provide a technology capable of improving the durability of a circuit board in thermal cycles.

[0006] The present invention has been made to solve at least part of the above-described problems, and can be implemented in the following embodiments.

[0007] (1) According to one aspect of the present invention, a circuit board is provided. This circuit board includes a ceramic substrate containing silicon nitride as a main component, a conductive portion formed of copper and connected to a semiconductor element, and a joining portion that joins the ceramic substrate and the conductive portion. The joining portion has a first joining portion disposed on the ceramic substrate side and containing titanium nitride as a main component, and a second joining portion disposed between the first joining portion and the conductive portion and containing copper and titanium. The concentration of titanium in a range of 500 nm or less from the interface with the conductive portion is 0.1 atm% or more and 10 atm% or less.

[0008] In this configuration, the joint connecting the ceramic substrate and the conductive part formed of copper has a second joint on the conductive part side containing copper and titanium, and the titanium concentration in the range of 500 nm or less from the interface with the conductive part is between 0.1 atm% and 10 atm%. In this way, because a certain amount of titanium is present on the conductive part side of the joint, the difference in thermal expansion coefficients between the ceramic substrate and the conductive part can be mitigated. Therefore, durability in thermal cycling can be improved.

[0009] (2) In the circuit board of the above form, the joint portion contains titanium particles formed of titanium, and the average particle diameter of the titanium particles contained in the range of 500 nm or less from the interface with the conductive portion may be 10 nm or more and 500 nm or less. With this configuration, the titanium concentration in the conductive portion side of the joint portion is within a certain range. This makes it possible to mitigate the difference in thermal expansion coefficients between the ceramic substrate and the conductive portion, thereby improving durability in thermal cycling.

[0010] (3) In the circuit board of the above form, the copper concentration in the joint portion within a range of 500 nm or less from the interface with the conductive portion may be 80 atm% or more and 99 atm% or less. With this configuration, the copper concentration of the material forming the conductive portion is within a certain range in the conductive portion side of the joint portion. This makes it possible to improve the bonding strength between the conductive portion and the joint portion, as well as improve thermal conductivity.

[0011] (4) In the circuit board of the above form, the thickness of the joint may be 0.1 μm or more and 5 μm or less. With this configuration, the thickness of the joint is smaller than, for example, the thickness of a joint formed by brazing. This makes it easier for heat to be transferred between the ceramic substrate and the conductive part, thereby improving the thermal conductivity between the ceramic substrate and the conductive part.

[0012] (5) In the above-described circuit board, the joint portion does not need to contain silver. With this configuration, the joint portion is not formed with a silver-containing brazing material. Therefore, the thickness of the joint portion can be made relatively small, and thus the thermal conductivity between the ceramic substrate and the conductive portion can be improved.

[0013] (6) In the circuit board of the above form, the ceramic substrate has a pair of main surfaces, the conductive portion has a first conductive portion disposed on one of the pair of main surfaces of the ceramic substrate and a second conductive portion disposed on the other of the pair of main surfaces of the ceramic substrate, the joint portion has a first joint portion that joins the ceramic substrate and the first conductive portion and a second joint portion that joins the ceramic substrate and the second conductive portion, and the thermal conductivity between the first conductive portion and the second conductive portion may be 170 W / (m·K) or more. With this configuration, a circuit board having two conductive portions sandwiched between a ceramic substrate can efficiently transfer heat from, for example, a semiconductor element or electronic component connected to the first conductive portion to the second conductive portion.

[0014] (7) According to another embodiment of the present invention, a semiconductor package is provided. This semiconductor package comprises the circuit board described above and a power semiconductor element connected to the conductive part. With this configuration, a semiconductor package equipped with a power semiconductor element that generates a relatively large amount of heat can improve the durability of the semiconductor package in thermal cycling because it is equipped with a circuit board in which the difference in thermal expansion coefficients between the ceramic substrate and the conductive part is mitigated.

[0015] Furthermore, the present invention can be realized in various forms, for example, in the form of a method for manufacturing a circuit board and a semiconductor package, an apparatus and system comprising a circuit board and a semiconductor package, a method for controlling such apparatus and system, a computer program that causes such apparatus and system to supply power to electronic equipment and to perform mutual conversion between AC and DC, a server apparatus for distributing the computer program, and a non-temporary storage medium that stores the computer program.

[0016] This is a cross-sectional view of the circuit board of the first embodiment. This is a cross-sectional view of the semiconductor package of the first embodiment. This is an SEM image of part A in Figure 1. This is a schematic diagram of part A in Figure 1. This is a diagram illustrating the results of the evaluation test of the circuit board.

[0017] <First Embodiment> Figure 1 is a cross-sectional view of the circuit board 1 of this embodiment. Figure 2 is a cross-sectional view of the semiconductor package P1 of this embodiment. The circuit board 1 of this embodiment is a substrate on which a power semiconductor element 5 can be mounted, and as shown in Figure 1, comprises a ceramic substrate 10, conductive parts 21, 22, and bonding parts 31, 32. The semiconductor package P1 shown in Figure 2 comprises the circuit board 1 and a power semiconductor element 5 connected to the conductive part 21. The semiconductor package P1 performs various power conversions by switching operation, specifically supplying power to electronic devices and performing mutual conversion between AC and DC. In Figures 1 and 2, the direction along the stacking direction of the ceramic substrate 10, conductive parts 21, 22, and bonding parts 31, 32 is defined as the z-axis direction, the direction perpendicular to the z-axis is defined as the x-axis direction, and the direction perpendicular to the z-axis and x-axis is defined as the y-axis direction. Note that the thickness relationships between the ceramic substrate 10, the conductive parts 21 and 22, and the bonding parts 31 and 32 in Figure 1, and the thickness relationship between the circuit board 1 and the power semiconductor element 5 in Figure 2, are illustrated in a way that differs from the actual thickness relationships for the sake of explanation.

[0018] The ceramic substrate 10 is made of silicon nitride (Si3N4, thermal expansion coefficient: 3 × 10⁻⁶). -6 The ceramic substrate 10 is a flat plate-shaped member mainly composed of silicon nitride (K). Here, "mainly composed of silicon nitride" means that it contains 86 wt% or more silicon nitride. The ceramic substrate 10 has a pair of main surfaces 10a and 10b. The arithmetic mean roughness Ra of each of the pair of main surfaces 10a and 10b of the ceramic substrate 10 is preferably 0.6 μm or less in order to avoid extremely uneven shapes. The thickness of the ceramic substrate 10 is, for example, 220 μm or more and 690 μm or less. However, the thickness of the ceramic substrate 10 is not limited to this range.

[0019] Each of the conductive parts 21 and 22 is made of copper (Cu, thermal expansion coefficient: 17 × 10⁻⁶).-6 It is formed by (K). The conductive part 21 is connected to the power semiconductor element 5. The conductive part 21 becomes part of the electrical circuit in the semiconductor package P1. The thickness of each of the conductive parts 21 and 22 is, for example, 200 μm or more and 1500 μm or less. The conductive part 21 is arranged on one of the pair of main surfaces 10a and 10b of the ceramic substrate 10. The conductive part 22 is arranged on the other of the pair of main surfaces 10a and 10b of the ceramic substrate 10. The conductive part 21 corresponds to the "first conductive part" in the claims, and the conductive part 22 corresponds to the "second conductive part" in the claims.

[0020] The joints 31 and 32 contain titanium nitride and copper. The copper in the joints 31 and 32 is due to the diffusion of copper forming the conductive parts 21 and 22 into the titanium film that forms the joints 31 and 32 during the manufacturing of the circuit board 1. Neither of the joints 31 and 32 in this embodiment contains silver. Here, "does not contain silver" means that no silver element is detected in an analysis of the silver element using an energy-dispersive X-ray spectroscope, specifically meaning that the concentration of the silver element is less than 1 atm%. Joint 31 joins the ceramic substrate 10 and the conductive part 21, and joint 32 joins the ceramic substrate 10 and the conductive part 22. Joint 31 corresponds to the "first joint" in the claims, and joint 32 corresponds to the "second joint" in the claims.

[0021] Figure 3 is an SEM image of part A in Figure 1. Figure 3 is an image taken with a scanning electron microscope (SEM) of a cross-section of the circuit board 1 including the joint 31, the ceramic substrate 10 and the conductive part 21 joined to the joint 31. The joint 31 is a first joint 311 located on the ceramic substrate 10 side, which has titanium nitride as its main component, and a second joint 312 located between the first joint 311 and the conductive part 21, which has copper and titanium as its main component. Here, in the first joint 311, "having titanium nitride as its main component" means that it contains 50 wt% or more of titanium nitride. Figure 3 includes a dashed line B1 indicating the interface between the joint 31 and the conductive part 21, making it easier to identify the interface between the joint 31 and the conductive part 21. The method for defining the interface between the joint and the conductive part in this embodiment will be described later. Although Figure 3 describes the joint 31, the joint 32 also has, similar to the joint 31, a first joint located on the ceramic substrate 10 side and mainly composed of titanium nitride, and a second joint located between the first joint and the conductive part 22 and containing copper and titanium.

[0022] Figure 4 is a schematic diagram of part A in Figure 1. The joint portion 31 of the circuit board 1 in this embodiment has a titanium concentration of 1.6 atm%, where the titanium concentration in the range R1 of 500 nm or less from the interface with the conductive portion 21 is between 0.1 atm% and 10 atm%. Here, we will explain how to define the "interface of the joint with the conductive portion," which serves as the reference for the range R1 in which the titanium concentration of the joint portion is measured in this embodiment. First, information on the distribution of titanium in the cross-section of the circuit board including the joint portion and the conductive portion is obtained by energy-dispersive X-ray spectroscopy (EDS) using a scanning electron microscope. Next, using the obtained information on the distribution of titanium elements, the position of the titanium element located furthest towards the conductive portion in the cross-section of the circuit board including the joint portion and the conductive portion is identified. In the schematic diagram of the cross-section of the circuit board 1 shown in Figure 4, the position of the titanium element located furthest towards the conductive portion 21 in the joint portion 31 is position Pt. Next, a virtual straight line is created that passes through the position of the titanium element located closest to the conductive portion 21, and is parallel to the virtual straight line based on the main surface 10a of the ceramic substrate 10. Here, the "virtual straight line based on the main surface of the ceramic substrate" is a straight line created using the least squares method with respect to the outline line representing the main surface 10a of the ceramic substrate 10 in a cross-section including both the ceramic substrate 10 and the joint portion 31. Specifically, in a cross-section as shown in Figure 4, the straight line created using the least squares method with respect to the solid line L10a, which is the outline line representing the main surface 10a of the ceramic substrate 10, becomes the "virtual straight line based on the main surface 10a of the ceramic substrate 10". In this embodiment, when creating the "virtual straight line based on the main surface of the ceramic substrate", the size of the cross-section including the outline line representing the main surface 10a of the ceramic substrate 10 is, for example, 6000 nm in the direction along the main surface 10a of the ceramic substrate 10, which is the range for measuring the titanium concentration, as shown in Figure 4. However, the size of the cross-section when creating a "virtual straight line based on the main surface of the ceramic substrate" is not limited to this.

[0023] In this embodiment, the virtual line created using the virtual line based on the main surface of the ceramic substrate as described above, passing through the position of the titanium element located closest to the conductive part, is defined as the outline of the "interface between the joint and the conductive part" in the cross-section of the circuit board. In Figure 4, the dashed line F1 is the outline of the "interface between the joint and the conductive part". Hereafter, for convenience, the outline of the "interface between the joint and the conductive part" created in the cross-section of the circuit board will simply be referred to as the "interface between the joint and the conductive part". In this embodiment, similar to the joint 31, the titanium concentration in the range R1 of 500 nm or less from the interface with the conductive part 22 is 0.1 atm% to 10 atm%.

[0024] In this embodiment, the calculation of the "titanium concentration in the range R1 of 500 nm or less from the interface with the conductive part 21" is performed using the titanium concentration obtained by energy-dispersive X-ray spectroscopy for the range R1 (size in the z-axis direction: 500 nm, size in the x-axis direction: 6000 nm) enclosed by the dashed line shown in Figure 4. The boundary B1 shown in Figures 3 and 4 is a line that continuously connects the positions of the titanium closest to the conductive part 21 at multiple positions on the x-axis when the cross-section of the circuit board 1 is viewed along the x-axis.

[0025] In the circuit board 1 of this embodiment, the junction 31 contains titanium particles Tp formed from titanium, and the average particle diameter of the titanium particles Tp contained in the range R1 of 500 nm or less from the interface with the conductive part 21 is 28 nm, which is between 10 nm and 500 nm. As shown in Figure 3, the junction 31 contains multiple titanium particles Tp. In the junction 31, the average particle diameter of the titanium particles Tp contained in the above-mentioned range R1 (see Figure 4) is between 10 nm and 500 nm, and the titanium concentration in the portion of the junction 31 on the conductive part 21 side is within a constant range. In calculating the average particle diameter of the titanium particles Tp in this embodiment, first, the portion containing titanium elements contained in the range R1 shown in Figure 4 is identified using the analysis results by energy-dispersive X-ray spectroscopy and the analysis results of backscattered electron images captured using a scanning electron microscope. Next, the size of the identified portion containing titanium elements is measured and taken as the particle size of one titanium particle Tp. The particle size of the titanium particles Tp is measured for each of the five ranges R1 at different locations. Finally, the average value of the particle sizes of multiple titanium particles Tp obtained in each of the five ranges R1 is defined as the "average particle diameter of titanium particles Tp". Note that, similar to the average particle diameter of titanium particles Tp contained in the joint 31, the average particle diameter of titanium particles Tp contained within the range of 500 nm or less from the interface with the conductive part 22 is between 10 nm and 500 nm.

[0026] In the circuit board 1 of this embodiment, the copper concentration in the joint 31 in the range R1 from 500 nm or less from the interface with the conductive part 21 is 80 atm% to 99 atm%, specifically 95 atm%. Thus, in the joint 31, the copper concentration of the material forming the conductive part 21 is within a constant range in the portion of the joint 31 that is on the conductive part 21 side. The copper concentration in this embodiment is calculated using the copper concentration obtained by energy-dispersive X-ray spectroscopy for the range R1 shown in Figure 4. The copper concentration in the joint 32 is also the same as that in the joint 31, with the copper concentration in the range from 500 nm or less from the interface with the conductive part 22 being 80 atm% to 99 atm%.

[0027] The thickness of the joint portion 31 is 0.1 μm or more and 5 μm or less. The thickness of the joint portion 31 can be determined using a scanning electron microscope. In this embodiment, the thickness of the joint portion 31 is measured by first cutting the circuit board 1 perpendicular to the main surface 10a of the ceramic substrate 10 and polishing the cut surface. Next, 10 locations are randomly selected from the polished cut surface, including the ceramic substrate 10, the joint portion 31, and the conductive portion 21, and a 4 μm × 6 μm area is imaged at a magnification of 20,000 times. Next, the lengths of 10 line segments drawn perpendicular to a 6 μm line segment drawn on the main surface 10a of the ceramic substrate 10 and at equal intervals are determined. In this embodiment, the average of these determined values ​​is taken as the thickness of the joint portion. In addition, the "main surface of the ceramic substrate" used in measuring the thickness of the joint 31 is the "virtual straight line based on the main surface of the ceramic substrate" created in the method for defining the "interface of the joint with the conductive part" described above. In this embodiment, the thickness of the joint 32 is 0.1 μm or more and 5 μm or less, similar to the thickness of the joint 31.

[0028] In the circuit board 1 of this embodiment, the thickness of the joints 31 and 32 tends to be greater than the thickness of the titanium film produced in the manufacturing method of the circuit board 1 described later. This is because, during the bonding of the ceramic substrate 10 and the conductive parts 21 and 22 via the joints 31 and 32, nitrogen atoms and silicon atoms are supplied from the ceramic substrate 10, and copper atoms are supplied from the conductive parts 21 and 22.

[0029] In the circuit board 1 of this embodiment, the joint 31 contains a titanium compound. The titanium compound contained in the joint 31 is a compound bonded with nitrogen (Ti-N), a compound bonded with silicon (Ti-Si), a compound bonded with copper (Ti-Cu), a compound bonded with oxygen (Ti-O), etc. The titanium compound in this embodiment is identified using the concentrations of titanium, nitrogen, silicon, copper, and oxygen obtained by energy-dispersive X-ray spectroscopy on a cross-section including the joint 31, as shown in Figure 3. The titanium compound contained in the joint 32 is the same as the titanium compound contained in the joint 31.

[0030] In the circuit board 1 of this embodiment, the thermal conductivity between the conductive part 21 and the conductive part 22 is 190 W / (m·K), which is 170 W / (m·K) or higher. As a result, even if the power semiconductor elements 5 arranged on both sides of the circuit board 1 generate a large amount of heat, the heat can be released to the outside through the ceramic substrate 10.

[0031] Next, the manufacturing method of the circuit board 1 will be described. In the manufacturing method of the circuit board 1, a ceramic substrate 10 is first manufactured. In the manufacturing of the ceramic substrate 10, the raw material powder of the silicon nitride sintered body is weighed. The raw material powder of the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of each element contained in the silicon nitride sintered body. In addition to silicon nitride, the raw material powder of the silicon nitride sintered body may include, for example, magnesium carbonate, calcium carbonate, yttrium oxide, etc. In this embodiment, the powders are weighed to be 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). These weighed powders and ethanol are added to a resin pot, and a mixed slurry is produced by wet mixing and grinding using a ball mill, for example at 60 rpm for 24 hours. The prepared slurry is dried by a water bath to produce a mixed powder.

[0032] In the fabrication of the ceramic substrate 10, the mixed powder is then filled into a mold and molded by uniaxial pressing at a pressure of, for example, 30 MPa. After that, a molded body is produced by performing CIP (cold isohydrostatic pressing) at a pressure of, for example, 150 MPa. The fabricated molded body is then placed in a silicon carbide mold coated with BN on the inside and fired in a nitrogen atmosphere of 0.9 MPa at a maximum temperature of 1900°C for 10 hours to produce a silicon nitride sintered body. The fabricated silicon nitride sintered body is then processed to achieve a predetermined shape, for example, a flat plate shape and thickness, thereby manufacturing the ceramic substrate 10. The processing can be done by cutting, grinding, polishing, etc. At this time, each of the pair of main surfaces 10a and 10b on which the conductive parts 21 and 22 of the ceramic substrate 10 are arranged is polished so that the arithmetic mean roughness Ra is 0.6 μm or less.

[0033] In the manufacturing method of the circuit board 1, separate from the manufacturing of the ceramic substrate 10, a plate material of a predetermined thickness made of oxygen-free copper, which will become the conductive parts 21 and 22, is prepared. The plate material that will become the conductive parts 21 and 22 may be made of a metal containing copper or a metal whose main component is copper, in addition to oxygen-free copper. Next, a titanium film, which will become the bonding parts 31 and 32, is formed on each of the pair of main surfaces 10a and 10b of the ceramic substrate 10 by vapor deposition or sputtering. This makes it possible to form a relatively thin titanium film, with a thickness of 0.5 μm. In the manufacturing method of the circuit board 1 of this embodiment, the thickness of the titanium film is preferably, for example, 0.3 μm or more and 1.0 μm or less. It is desirable to remove organic matter such as oil adhering to the surface of the ceramic substrate 10 and the plate material before forming the titanium film. It is thought that by sufficiently removing organic matter from the surface, the gas generated during heat treatment is suppressed, which contributes to the diffusion of copper that forms the conductive parts 21 and 22. The process of removing organic matter can be carried out, for example, by degreasing, washing with water, or washing with acetone. These can also be combined, or a step for drying the surface may be included.

[0034] Next, the plate material that will become the conductive parts 21 and 22 and the ceramic substrate 10 are laminated so that a titanium film is sandwiched between the plate material that will become the conductive parts 21 and 22 and the ceramic substrate 10. The laminate in which the plate material that will become the conductive parts 21 and 22 and the ceramic substrate 10 are laminated is subjected to HP treatment (hot pressing treatment) or HIP treatment (hot isostatic pressing treatment) to bond the plate material that will become the conductive parts 21 and 22 and the ceramic substrate 10. The conditions for HP treatment are, for example, a maximum pressure of 12 MPa, a maximum temperature of 1000°C, and a maximum temperature holding time of 30 minutes. Note that the manufacturing method of the circuit board 1 is not limited to these methods.

[0035] Next, we will explain the results of the circuit board evaluation test. In this evaluation test, for each of the five types of circuit boards (hereinafter simply referred to as "samples") with different methods of creating the joints, the relationship between the titanium concentration, the average particle size of titanium particles, and the copper concentration at the joints, and the mechanical strength and thermal properties was measured or calculated and evaluated.

[0036] Figure 5 illustrates the results of the circuit board evaluation test. Each of the samples 1 to 5 shown in Figure 5 was manufactured by a method corresponding to the manufacturing method of the circuit board 1 of this embodiment. In the manufacture of sample 1, titanium, which will form the bonding area, was sputtered to both sides of the ceramic substrate to a thickness of 0.5 μm. In the manufacture of sample 2, titanium, which will form the bonding area, was sputtered to both sides of the ceramic substrate to a thickness of 0.3 μm. In the manufacture of sample 3, titanium, which will form the bonding area, was sputtered to both sides of the ceramic substrate to a thickness of 0.8 μm. In the manufacture of sample 4, titanium, which will form the bonding area, was sputtered to both sides of the ceramic substrate to a thickness of 1.0 μm. In the manufacture of sample 5, a brazing material containing titanium, copper, and silver was applied to the ceramic substrate to a thickness of 15 μm, and the ceramic substrate and the conductive part were bonded by heating to 800°C. The dimensions of each sample are as follows: the ceramic substrate is 100 mm x 100 mm x 0.32 mm, and the conductive part is 100 mm x 100 mm x 0.3 mm.

[0037] The "Titanium Concentration (atm%)" shown in Figure 5 indicates the concentration of titanium at the joint of the sample. The "Titanium Concentration (atm%)" was measured and calculated for each of the four types of samples using the same method as the titanium concentration at the joints 31 and 32 of the circuit board 1 of this embodiment. Furthermore, the interface between the joint and the conductive part in sample 5, where the ceramic substrate and the conductive part are joined by brazing, was defined in the same way as the method for setting the range R1 in the joints 31 and 32 of the circuit board 1 of this embodiment, by replacing the titanium concentration with the silver concentration. Note that if the ceramic substrate and the conductive part are joined with a brazing material that does not contain silver, the interface between the joint and the conductive part may be defined in the same way as the method for setting the range R1 in the joints 31 and 32 of the circuit board 1 of this embodiment, by replacing the titanium concentration with the concentration of a component contained in the brazing material.

[0038] The "average particle diameter (μm)" shown in Figure 5 represents the average particle diameter of titanium particles formed by titanium contained in the joints of the sample. The "average particle diameter (μm)" was calculated for each of the five types of samples using the same method as the method for calculating the average particle diameter of titanium particles Tp contained in the joints 31 and 32 of the circuit board 1 of this embodiment.

[0039] The "copper concentration (atm%)" shown in Figure 5 indicates the copper concentration at the junction of the sample. The "copper concentration (atm%)" was measured and calculated for each of the five types of samples using the same method as the copper concentration at the junctions 31 and 32 of the circuit board 1 of this embodiment.

[0040] The "bonding strength (kN / m)" shown in Figure 5 represents the peel strength of the sample. For each of the five types of samples, the "bonding strength (kN / m)" was calculated by first making a 10 mm wide slit in a copper plate bonded to a ceramic substrate, then peeling off a portion of the edge of the copper plate, clamping it in a tensile testing machine, and measuring the load value when the copper plate was peeled off by pulling it vertically. Next, the "bonding strength (kN / m)" was calculated by dividing the measured load value by the measured peel width and averaging it.

[0041] The "Thermal Conductivity (W / (m·K))" shown in Figure 5 represents the thermal conductivity of the sample. For each of the five types of samples, a measurement sample with sides of 10 mm was first prepared, coated with blackbody spray, and then the thermal diffusivity and specific heat were measured using the flash method. Next, the thermal conductivity was calculated as the product of the thermal diffusivity, specific heat, and density, using the density measured separately. Note that the measurement sample may also be prepared using a semiconductor package from which the semiconductor element has been removed.

[0042] The "thermal cycle test results" shown in FIG. 5 indicate the resistance to thermal shock. In the thermal cycle test, for each of the 5 types of samples, etching was performed such that the size of the conductive part bonded to the ceramic substrate was 50 mm × 50 mm so that the ceramic substrate was exposed, then a cycle of raising the temperature from -25°C to 150°C and then lowering the temperature to -25°C was performed 3000 times, and ultrasonic flaw inspection was used to confirm whether damage such as cracks or peeling had occurred. When no damage was confirmed, the "thermal cycle test result" was set to "Yes", and when damage was confirmed, the "thermal cycle test result" was set to "No".

[0043] As shown in FIG. 5, in each of Sample 1 to Sample 4, the titanium concentration at the joint portion was 0.1 atm% or more and 10 atm% or less, while in Sample 5, the titanium concentration at the joint portion was 0 atm%. In each of Sample 1 to Sample 4, the average particle diameter of the titanium particles contained in the joint portion was 10 nm or more and 500 nm or less, while in Sample 5, the presence of titanium particles was not confirmed. It was confirmed that each of Sample 1 to Sample 4 was superior to Sample 5 in both "bonding strength (kN / m)" and "thermal conductivity (W / (m·K))".

[0044] According to the circuit board 1 of the present embodiment described above, the joint portion 31 has the second joint portion 312 containing copper and titanium on the conductive part 21 side, and the titanium concentration in the range R1 of 500 nm or less from the interface with the conductive part 21 is 0.1 atm% or more and 10 atm% or less. Further, the joint portion 32 has a second joint portion containing copper and titanium on the conductive part 22 side, and the titanium concentration in the range of 500 nm or less from the interface with the conductive part 22 is 0.1 atm% or more and 10 atm% or less. As described above, since a certain amount of titanium is present in the portions on the conductive parts 21, 22 side formed of copper having a relatively large thermal expansion coefficient in each of the joint portions 31, 32, the difference in thermal expansion coefficient between the ceramic substrate 10 and the conductive parts 21, 22 can be reduced. Therefore, the durability in thermal cycles can be improved.

[0045] Further, according to the circuit board 1 of the present embodiment, in the joint portion 31, the average particle diameter of titanium particles contained in a range R1 of 500 nm or less from the interface with the conductive portion 21 is 10 nm or more and 500 nm or less, and in the joint portion 32, the average particle diameter of titanium particles contained in a range of 500 nm or less from the interface with the conductive portion 22 is 10 nm or more and 500 nm or less. As described above, since the titanium concentration in the portions of the joint portions 31 and 32 on the conductive portions 21 and 22 side is within a certain range, the difference in thermal expansion coefficient between the ceramic substrate 10 and the conductive portions 21 and 22 can be further reduced. Thereby, the durability in thermal cycles can be further improved.

[0046] Further, according to the circuit board 1 of the present embodiment, in the joint portion 31, the copper concentration in a range R1 of 500 nm or less from the interface with the conductive portion 21 is 80 atm% or more and 99 atm% or less, and in the joint portion 32, the copper concentration in a range of 500 nm or less from the interface with the conductive portion 22 is 80 atm% or more and 99 atm% or less. As described above, in the portions of the joint portions 31 and 32 on the conductive portions 21 and 22 side, the concentration of copper, which is the material forming the conductive portions 21 and 22, is within a certain range. Thereby, the bonding strength between the conductive portions 21, 22 and the joint portions 31, 32 can be improved, and the thermal conductivity can also be improved.

[0047] Further, according to the circuit board 1 of the present embodiment, the thickness of each of the joint portions 31 and 32 is 0.1 μm or more and 5 μm or less, which is smaller than, for example, a joint portion formed by brazing. This facilitates heat transfer between the ceramic substrate 10 and the conductive portions 21 and 22, so that the thermal conductivity between the ceramic substrate 10 and the conductive portions 21 and 22 can be improved.

[0048] Further, according to the circuit board 1 of the present embodiment, the joint portions 31 and 32 are not formed of a silver-containing brazing material. Accordingly, the thickness of each of the joint portions 31 and 32 can be made relatively small, so that the thermal conductivity between the ceramic substrate 10 and the conductive portions 21 and 22 can be improved.

[0049] Furthermore, according to the circuit board 1 of this embodiment, the thermal conductivity between the conductive part 21 and the conductive part 22 is 170 W / (m·K) or higher. As a result, the circuit board 1, which has two conductive parts 21 and 22 sandwiching the ceramic substrate 10, can efficiently transfer the heat of the power semiconductor element 5 connected to the first conductive part 21 to the second conductive part 22.

[0050] Furthermore, according to the semiconductor package P1 of this embodiment, the semiconductor package P1 equipped with a power semiconductor element 5 that generates a relatively large amount of heat includes a circuit board 1 in which the difference in thermal expansion coefficients between the ceramic substrate 10 and the conductive parts 21 and 22 is mitigated. This improves the durability of the semiconductor package P1 in thermal cycling.

[0051] <Modifications of this Embodiment> The present invention is not limited to the above-described embodiments, and can be implemented in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0052] [Modification 1] According to the above embodiment, the joint contains titanium particles formed of titanium, and the average particle diameter of the titanium particles contained in the range of 500 nm or less from the interface with the conductive part is 10 nm or more and 500 nm or less. The joint does not have to contain titanium particles.

[0053] [Modification 2] According to the above embodiment, the copper concentration in the joint portion in the range of 500 nm or less from the interface with the conductive portion is 80 atm% or more and 99 atm% or less. The copper concentration in the joint portion in the range of 500 nm or less from the interface with the conductive portion is not limited to this.

[0054] [Modification 3] According to the above embodiment, the thickness of the joint is 0.1 μm or more and 5 μm or less. The thickness of the joint is not limited to this.

[0055] [Modification 4] According to the above embodiment, the circuit board 1 is provided with two conductive parts 21 and 22 on a single ceramic substrate 10, and each of the two conductive parts 21 and 22 is provided with a joint 31 and 32 in which the titanium concentration in the range of 500 nm or less from the interface with the conductive parts 21 and 22 is 0.1 atm% or more and 10 atm% or less. Either the joint 31 or 32 is sufficient if the titanium concentration in the range of 500 nm or less from the interface with the conductive parts 21 and 22 is 0.1 atm% or more and 10 atm% or less. The titanium particles contained in the joint, the average particle size of the titanium particles, the copper concentration, the thickness of the joint, and whether or not silver is included are also acceptable as long as either the joint 31 or 32 has any of these characteristics.

[0056] [Modification 5] According to the above embodiment, the semiconductor package P1 comprises a circuit board 1 and a power semiconductor element 5 connected to the conductive part 21. The electronic components included in the semiconductor package P1 are not limited to the power semiconductor element 5.

[0057] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0058] <Application Example 1> A circuit board comprising: a ceramic substrate mainly composed of silicon nitride; a conductive part formed of copper and connected to a semiconductor element; and a joint that joins the ceramic substrate and the conductive part, wherein the joint comprises: a first joint disposed on the ceramic substrate side and mainly composed of titanium nitride; and a second joint disposed between the first joint and the conductive part and containing copper and titanium, wherein the concentration of titanium in the range of 500 nm or less from the interface with the conductive part is 0.1 atm% or more and 10 atm% or less. <Application Example 2> The circuit board according to Application Example 1, wherein the joint contains titanium particles formed of titanium, and the average particle size of the titanium particles included in the range of 500 nm or less from the interface with the conductive part is 10 nm or more and 500 nm or less. <Application Example 3> A circuit board according to Application Example 1 or Application Example 2, characterized in that the copper concentration in the joint portion in a range of 500 nm or less from the interface with the conductive portion is 80 atm% or more and 99 atm% or less. <Application Example 4> A circuit board according to any one of Application Examples 1 to 3, characterized in that the thickness of the joint portion is 0.1 μm or more and 5 μm or less. <Application Example 5> A circuit board according to any one of Application Examples 1 to 4, characterized in that the joint portion does not contain silver. <Application Example 6> A circuit board according to any one of Application Examples 1 to 5, wherein the ceramic substrate has a pair of main surfaces, the conductive portion has a first conductive portion disposed on one of the pair of main surfaces of the ceramic substrate and a second conductive portion disposed on the other of the pair of main surfaces of the ceramic substrate, the joint portion has a first joint portion that joins the ceramic substrate and the first conductive portion and a second joint portion that joins the ceramic substrate and the second conductive portion, and the thermal conductivity between the first conductive portion and the second conductive portion is 170 W / (m·K) or more.<Application Example 7> A semiconductor package comprising a circuit board described in any one of Application Examples 1 to 6, and a power semiconductor element connected to the conductive portion.

[0059] 1...Circuit board 5...Power semiconductor element 10...Ceramic substrate 21, 22...Conductive part 31, 32...Bond P1...Semiconductor package

Claims

1. A circuit board comprising: a ceramic substrate mainly composed of silicon nitride; a conductive portion formed of copper and connected to a semiconductor element; and a joint portion that joins the ceramic substrate and the conductive portion, wherein the joint portion comprises: a first joint portion disposed on the ceramic substrate side and mainly composed of titanium nitride; and a second joint portion disposed between the first joint portion and the conductive portion and containing copper and titanium, wherein the concentration of titanium in a range of 500 nm or less from the interface with the conductive portion is 0.1 atm% or more and 10 atm% or less.

2. A circuit board according to claim 1, wherein the bonding portion contains titanium particles formed of titanium, and the average particle size of the titanium particles contained in a range of 500 nm or less from the interface with the conductive portion is 10 nm or more and 500 nm or less.

3. A circuit board according to claim 1 or claim 2, characterized in that the copper concentration in the joint portion is 80 atm% or more and 99 atm% or less in a range of 500 nm or less from the interface with the conductive portion.

4. A circuit board according to claim 1 or claim 2, characterized in that the thickness of the joint portion is 0.1 μm or more and 5 μm or less.

5. A circuit board according to claim 1 or claim 2, characterized in that the joint portion does not contain silver.

6. A circuit board according to claim 1 or claim 2, wherein the ceramic substrate has a pair of main surfaces, the conductive portion has a first conductive portion disposed on one of the pair of main surfaces of the ceramic substrate and a second conductive portion disposed on the other of the pair of main surfaces of the ceramic substrate, the joint portion has a first joint portion that joins the ceramic substrate and the first conductive portion and a second joint portion that joins the ceramic substrate and the second conductive portion, and the thermal conductivity between the first conductive portion and the second conductive portion is 170 W / (m·K) or more.

7. A semiconductor package comprising a circuit board according to claim 1 or claim 2, and a power semiconductor element connected to the conductive portion.

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