Tabular base substance of ceramic substrate and production method therefor

Abstract

To provide a ceramic substrate with properties not existing conventionally, which can realize thermal conductivity equal to or higher than a conventional ceramic substrate including a granular aluminum nitride polycrystalline substance in a base substance, and realize fracture toughness superior to a conventional ceramic substrate including a granular silicon nitride polycrystalline substance in a base substance.SOLUTION: A ceramic substrate according to an embodiment includes a fiber-like AlN single crystal substance in a base substance.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present embodiment relates to a ceramic substrate that constitutes a control module of, for example, industrial equipment, etc. Furthermore, the present embodiment relates to an AlN single crystal, AlN whiskers, and AlN whisker composites contained in the ceramic substrate. [Background technology]

[0002] For example, control modules that perform power control or motor control for electric vehicles, self-driving cars, railways, machine tools, data centers, high-brightness LEDs, etc. are modules to which high voltages are applied, and ceramic substrates are used as the substrates for these modules. Also, various improvements have been attempted for this type of ceramic substrate (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-63042 Summary of the Invention [Problem to be solved by the invention]

[0004] Ceramic substrates used in the control modules described above require high heat dissipation performance, and therefore, the thermal conductivity of the ceramic substrate itself needs to be improved. Furthermore, control modules using ceramic substrates undergo repeated high-temperature and low-temperature cycles. Therefore, to prevent cracking due to thermal stress during such cycles, ceramic substrates used in control modules also need high mechanical strength. Thus, ceramic substrates used in control modules need to combine high thermal conductivity and high mechanical strength. Currently, the majority of ceramic substrates on the market contain granular SiN polycrystalline substrates or granular AlN polycrystalline substrates. Ceramic substrates using granular SiN polycrystalline substrates have excellent fracture toughness but low thermal conductivity. On the other hand, ceramic substrates using granular AlN polycrystalline substrates have excellent thermal conductivity but low fracture toughness. Thus, conventional ceramic substrates have both advantages and disadvantages, and there is a need for the development of ceramic substrates that combine high thermal conductivity and high mechanical strength.

[0005] Therefore, we provide a ceramic substrate with previously unavailable properties that can achieve thermal conductivity equal to or greater than that of conventional ceramic substrates containing granular aluminum nitride polycrystalline bodies in the base, and fracture toughness superior to that of conventional ceramic substrates containing granular silicon nitride polycrystalline bodies in the base, as well as AlN crystals, AlN whiskers, and AlN whisker composites contained in the ceramic substrate. [Means for solving the problem]

[0006] The ceramic substrate according to this embodiment is characterized in that the base contains a fibrous AlN single crystal.

[0007] The ceramic substrate according to this embodiment can achieve a higher thermal conductivity than conventional ceramic substrates made of granular AlN polycrystalline bodies, and can also achieve a higher fracture toughness than conventional ceramic substrates made of granular SiN polycrystalline bodies. In other words, it is possible to obtain a ceramic substrate that achieves both high thermal conductivity and high mechanical strength. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram schematically showing an example of the configuration of a power module according to an embodiment of the present invention. [Figure 2] 5A and 5B are diagrams showing characteristics of the ceramic substrate according to the embodiment. [Figure 3] FIG. 1 is a perspective view schematically showing the structure of a fiber-shaped AlN single crystal mass according to an embodiment of the present invention. [Figure 4] 1A and 1B are diagrams schematically illustrating an example of the configuration of an apparatus for performing X-ray diffraction on a ceramic substrate according to an embodiment of the present invention, and an example of the structure of the ceramic substrate. [Figure 5] FIG. 2 is a diagram showing an X-ray diffraction pattern for the ceramic substrate according to the present embodiment in comparison with a conventional ceramic substrate. [Figure 6] FIG. 2 is a graph showing the correlation between the fracture toughness of the ceramic substrate according to the present embodiment and the “a / c” value. [Figure 7] FIG. 4 is a diagram showing the correlation between fracture toughness and oxygen content of the ceramic substrate according to the embodiment. [Figure 8] FIG. 4 is a diagram showing the correlation between the thermal conductivity and the amount of oxygen of the ceramic substrate according to the embodiment. [Figure 9] FIG. 2 is a diagram showing the microstructure of the ceramic substrate according to the embodiment in comparison with a conventional ceramic substrate. [Figure 10] FIG. 2 is a diagram showing data relating to the length and thickness of fibrous AlN single crystals contained in the ceramic substrate according to the present embodiment. [Figure 11] FIG. 2 is a diagram showing the presence of fibrous AlN single crystals having a size of 100 μm or more in the structure of the ceramic substrate according to the present embodiment. [Figure 12] 3A and 3B are views showing a fracture surface of the ceramic substrate according to the embodiment in comparison with a conventional ceramic substrate. [Figure 13] FIG. 2 is a graph showing the correlation between the fracture toughness of the ceramic substrate according to the embodiment and the arithmetic mean roughness of the fracture surface. [Figure 14] FIG. 10 is a diagram showing data on the diameter and oxygen concentration of AlN whiskers contained in the ceramic substrate according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of a ceramic substrate will be described below with reference to the drawings. The power module 1 illustrated in FIG. 1 is an example of a control module for power control and motor control in, for example, electric vehicles, self-driving cars, railways, machine tools, data centers, and high-brightness LEDs, and includes a ceramic substrate 10 according to this embodiment. The ceramic substrate 10 is formed in a plate shape and has a metal layer 11 provided on both sides of the plate in the thickness direction. A so-called power semiconductor 12 is provided on one end surface of the ceramic substrate 10 in the thickness direction, in this case, the upper surface in FIG. 1. A heat sink 13 with a heat dissipation function is provided on the other end surface of the ceramic substrate 10 in the thickness direction, in this case, the lower surface in FIG. 1.

[0010] As shown by arrow H in FIG. 1 , heat generated by the power semiconductor 12 is transferred to the heat sink 13 via the ceramic substrate 10, thereby dissipating heat from the power module 1. Therefore, the ceramic substrate 10 is required to have a higher thermal conductivity than conventional ceramic substrates containing granular AlN polycrystalline bodies in their base material. Furthermore, from the standpoint of improving reliability, for example, the ceramic substrate 10 is required to have a higher fracture toughness, i.e., a higher mechanical strength, than conventional ceramic substrates containing only granular AlN polycrystalline bodies. The ceramic substrate 10 according to this embodiment is designed to achieve both improved thermal conductivity and improved mechanical strength. This point will be described in detail below.

[0011] First, a description will be given of an example of a method for manufacturing the ceramic substrate 10. The manufacturing process for the ceramic substrate 10 includes a kneading step, a drying step, a granulating step, a molding step, a degreasing step, and a sintering step.

[0012] In the kneading process, fibrous AlN single crystals (i.e., fibrous aluminum nitride single crystals) are dispersed in a mixture of a well-known dispersant containing oil and fat components and an organic solvent. Then, yttria, a sintering aid, and granular AlN polycrystalline (i.e., aluminum nitride powder) are added and kneaded. This forms a slurry, which is the raw material for the ceramic substrate 10.

[0013] In the drying step, the slurry obtained in the kneading step is dried, for example, at a temperature of 130° C. and a pressure of −0.1 MPa for a predetermined time, for example, about one hour.

[0014] In the granulation step, the lumps of the slurry obtained in the drying step are broken down and rolled in, for example, a pod mill to granulate, or pelletize, the raw material.

[0015] In the molding step, the granular raw material obtained in the granulation step is put into a mold and pressed, for example, by a press, to form the raw material into a plate shape.

[0016] In the debinding process, the plate-shaped raw material obtained in the molding process is debound. In other words, the dispersed material is mainly removed from the raw material. The debinding process is carried out, for example, in a nitrogen atmosphere or an air atmosphere. The debinding process is also carried out at a temperature ranging from about 500 to 650 degrees Celsius for a predetermined time, for example, about 4 to 6 hours.

[0017] In the sintering step, the plate-shaped raw material that has been subjected to the degreasing step is sintered under conditions of a temperature of 1900° C. and a pressure of 40 MPa for a predetermined time, for example, about one hour.

[0018] The above steps allow for the manufacture of a ceramic substrate 10 containing a fibrous AlN single crystal and granular AlN polycrystalline body in the base. The temperature, pressure, time, and other conditions in each of the above steps can be modified as appropriate. The term "fibrous" refers to the AlN single crystal extending in a fibrous, elongated shape. As long as the AlN single crystal is fibrous overall, it may extend linearly or may be partially curved or bent.

[0019] 2 shows the characteristics of samples A and B of the ceramic substrate 10 obtained by the above-mentioned manufacturing method. The content of the fibrous AlN single crystal in both samples A and B is "10 weight percent," and the amount of yttria added is "5 weight percent." The difference between samples A and B is the amount of oxygen contained in the plate-shaped raw material after the above-mentioned degreasing process.

[0020] 2 shows the characteristic values ​​of samples A and B of the ceramic substrate 10 obtained by the above-mentioned manufacturing method, and, as a comparative example, the characteristic values ​​of sample C, a conventional ceramic substrate that does not contain fibrous AlN single crystals. The content of fibrous AlN single crystals in sample C is "0 weight percent," and the amount of yttria added is "3 weight percent."

[0021] As is clear from the characteristic values ​​shown in FIG. 2, the thermal conductivity of sample B is 150 W / mK or more, in this case 160 W / mK. In other words, the thermal conductivity of sample B is higher than the thermal conductivity of conventional sample C, which is 149 W / mK. Furthermore, the thermal conductivity of the ceramic substrate 10 according to this embodiment can be further improved by, for example, annealing after firing. Furthermore, the fracture toughness of sample B is 4.0 MPa. 1 / 2 In this case, 6.4MPam 1 / 2 In other words, the fracture toughness of sample B is 3.6 MPa, which is the fracture toughness of conventional sample C. 1 / 2The breakdown voltage of sample B is 20 kV / mm. As described above, the ceramic substrate according to this embodiment is characterized by achieving both improved thermal conductivity and improved fracture toughness by introducing fibrous AlN single crystals.

[0022] As described above, it was confirmed that the above-described manufacturing method can provide a ceramic substrate 10 having improved thermal conductivity and fracture toughness compared to conventional ceramic substrates that do not contain fibrous AlN single crystals. It was also confirmed that the above-described manufacturing method can provide a ceramic substrate 10 exhibiting a high breakdown voltage of 20 kV / mm.

[0023] In this embodiment, the thermal conductivity is calculated based on the values ​​of thermal diffusivity, specific heat, and density. Thermal diffusivity was measured using, for example, a Kyoto Electronics Manufacturing Co., Ltd. "LFA501" instrument in accordance with "JIS R1603 Method for Measuring Thermal Diffusivity, Specific Heat Capacity, and Thermal Conductivity of Fine Ceramics by the Flash Method" using the laser flash method. Specific heat was measured using, for example, a Shimadzu Corporation "DSC-60A" instrument in accordance with "JIS R1603 Method for Measuring Thermal Diffusivity, Specific Heat Capacity, and Thermal Conductivity of Fine Ceramics by the Flash Method" using differential scanning calorimetry. Density was measured using, for example, an A&D Corporation "AD-1653" instrument in accordance with "JIS Z8807 Method for Measuring Density and Specific Gravity of Solids" using the liquid weighing method.

[0024] The fracture toughness was measured using a micrometer manufactured by Mituyo Corporation, a Vickers hardness tester HV-115 manufactured by Mituyo Corporation, a universal testing machine Model 5582 manufactured by Instron Corporation, a MEASURESCOPE 10 manufactured by Nikon Corporation, and the like, in accordance with "JIS R1607 Room-temperature fracture toughness (toughness) test method for fine ceramics" and using the SEPB method.

[0025] In addition, sample A has a lower oxygen content after the degreasing process than sample B. As is clear from the characteristic values ​​shown in Figure 2, the fracture toughness of sample A is 4.0 MPa. 1 / 2 In this case, 9.8MPam 1 / 2 As described above, it was confirmed that the above-described manufacturing method can provide a ceramic substrate 10 with improved fracture toughness compared to conventional ceramic substrates that do not contain fibrous AlN single crystals. It was also confirmed that sample A, which contains a lower amount of oxygen in the base than sample B, has higher fracture toughness; in other words, it was confirmed that the smaller the amount of oxygen contained in the base 20 of the ceramic substrate 10, the higher the fracture toughness. Furthermore, as shown in FIG. 7, which will be described later, it was confirmed that the lower the oxygen content, the higher the fracture toughness tends to be.

[0026] The ceramic substrate 10 according to this embodiment is not limited to the above-mentioned samples A and B, but includes those having a thermal conductivity of 150 W / mK or more. 1 / 2 The ceramic substrate 10 according to this embodiment also includes those having a breakdown voltage of 20 kV / mm or more.

[0027] Next, the characteristics of the ceramic substrate 10 according to this embodiment will be described in relation to the structural features of the fibrous AlN single crystal. As shown in FIG. 3, the fibrous AlN single crystal has a hexagonal wurtzite crystal structure. The fibrous AlN single crystal has a (10-10) plane, a (0002) plane, and a (11-20) plane. The (10-10) plane and the (11-20) plane are examples of "planes along the longitudinal direction of the AlN single crystal." The (0002) plane is an example of "planes perpendicular to the longitudinal direction of the AlN single crystal." Hereinafter, the (10-10) plane will be referred to as the "a-plane," and the (0002) plane will be referred to as the "c-plane."

[0028] As shown in the lower part of Figure 4, in the ceramic substrate 10, the numerous AlN single crystals are oriented in a direction along the surface of the base 20 of the ceramic substrate 10, in this case, along the end face in the plate thickness direction. That is, as described above, the ceramic substrate 10 is formed into a plate by pressing the raw material. At this time, the long fibrous AlN single crystals are subjected to the pressing force of the press and are oriented in a direction perpendicular to the pressing direction, i.e., the plate thickness direction of the base 20. Therefore, within the base 20 of the ceramic substrate 10, the fibrous AlN single crystals are oriented in a direction perpendicular to the pressing direction, i.e., along the end face in the plate thickness direction of the base 20.

[0029] When X-ray diffraction is performed on the ceramic substrate 10 in which the fibrous AlN single crystals are oriented in a direction along the end face in the plate thickness direction of the base 20, the following results can be obtained. As illustrated in the upper part of FIG. 4, the X-ray diffraction apparatus 100 includes an X-ray source 101 that generates X-rays, an incident-side collimator 102, a receiving-side collimator 103, and a detector 104. The X-rays generated by the X-ray source 101 are irradiated via the incident-side collimator 102 onto the object to be measured, in this case, the end face in the plate thickness direction of the ceramic substrate 10. The X-rays diffracted by the object to be measured are incident on the detector 104 via the receiving-side collimator 103. The diffraction pattern is then measured by the detector 104.

[0030] In X-ray diffraction using such an X-ray diffractometer 100, the angle 2θ of detector 104 relative to the direction of X-ray irradiation on the measurement object can be changed within a predetermined range, for example, from 20 to 80 degrees, to obtain diffraction peaks representing each face of the AlN single crystal, i.e., each face such as the "a-face" or "c-face." The peak intensity obtained by X-ray diffraction is also the maximum count number for each face of the AlN crystal in the ceramic substrate, i.e., the number of faces present.

[0031] Figure 5 shows the X-ray diffraction pattern for the ceramic substrate 10 according to this embodiment, i.e., a ceramic substrate containing fibrous AlN single crystals in the base 20, compared with the X-ray diffraction pattern for a conventional ceramic substrate, i.e., a ceramic substrate not containing fibrous AlN single crystals in the base.

[0032] That is, the peak intensity ratio representing the "a-plane" (i.e., the (10-10) plane) in the X-ray diffraction pattern obtained when X-rays are irradiated onto the end face in the thickness direction of the base 20 of the ceramic substrate 10 is greater than the peak intensity ratio representing the "a-plane" (i.e., the (10-10) plane) in the X-ray diffraction pattern obtained when X-rays are irradiated onto the end face in the thickness direction of the base of a conventional ceramic substrate that does not contain a fibrous AlN single crystal. The peak in the detected value representing the "a-plane" (i.e., the (10-10) plane) is detected when the angle of the detector 104 is approximately 33.21 degrees. However, the peak in the detected value representing the "a-plane" (i.e., the (10-10) plane) may also be detected when the angle of the detector 104 is slightly different from approximately 33.21 degrees, depending on, for example, the shape of the sample or the positional relationship of the device.

[0033] Furthermore, the peak intensity ratio indicating the "c-plane" or (0002) plane in the X-ray diffraction pattern obtained when X-rays are irradiated onto the end face in the thickness direction of the base 20 of the ceramic substrate 10 is smaller than the peak intensity ratio indicating the "c-plane" or (0002) plane obtained when X-rays are irradiated onto the end face in the thickness direction of the base of a conventional ceramic substrate that does not contain a fibrous AlN single crystal. The peak of the detected value indicating the "c-plane" or (0002) plane is detected when the angle of the detector 104 is approximately 36.04 degrees. However, the peak of the detected value indicating the "c-plane" or (0002) plane may also be detected when the angle of the detector 104 is slightly different from approximately 36.04 degrees, for example, depending on the shape of the sample and the positional relationship of the device.

[0034] Based on these X-ray diffraction results, it can be confirmed that in the ceramic substrate 10 according to this embodiment, the "a-planes" of the numerous fibrous AlN single crystal bodies are aligned along the end face of the substrate 20 in the thickness direction. In other words, it can be confirmed that the numerous fibrous AlN single crystal bodies are aligned and oriented in a direction along the end face of the substrate 20 in the thickness direction.

[0035] Next, we will explain the relationship between the ratio of the peak intensity of the "a-plane" to the peak intensity of the "c-plane" in the X-ray diffraction pattern obtained by X-ray diffraction and the fracture toughness of the ceramic substrate 10. Hereinafter, the ratio of the peak intensity of the detected value of the "a-plane" to the peak intensity of the detected value of the "c-plane" is referred to as the "a / c" value. A higher "a / c" value indicates a stronger orientation of the fibrous AlN single crystals contained within the substrate 20 along the end face in the plate thickness direction of the substrate 20, or indicates a greater amount of fibrous AlN single crystals present within the substrate 20. Specifically, the "a / c" value and fracture toughness were measured for multiple samples of the ceramic substrate 10 obtained by the above-mentioned manufacturing method. As shown in Figure 6, it was confirmed that the larger the "a / c" value, the higher the fracture toughness of the ceramic substrate 10.

[0036] In particular, when the "a / c" value is 2.00 or more, higher fracture toughness can be achieved than in the comparative example not including the fibrous AlN single crystal, as indicated by points P6a, P6b, P6c, and P6d. Furthermore, when the "a / c" value is 20.00 or more, even higher fracture toughness can be achieved than in the comparative example, as indicated by points P6e, P6f, P6g, P6h, P6i, P6j, P6k, and P6l. Note that the "a / c" value of the ceramic substrate according to this embodiment may be any value greater than the "a / c" value of a conventional commercially available product, for example, approximately 1.1. Even if the "a / c" value is 1.5 or less than 2.00, high fracture toughness can be achieved.

[0037] X-ray diffraction was performed using the well-known θ-2θ method using an X-ray diffraction instrument 100, such as the Rigaku "Ultima IV." The X-ray diffraction conditions were: voltage: 40 kV, current: 30 mA, divergence slit: 1 / 2 degree, scattering slit: 1 / 2 degree, receiving slit: 0.3 mm, scan step: 0.02 degree, and 2θ range: 20 to 80 degrees. The peak positions in the X-ray diffraction pattern were determined based on the X-ray spectrum of AlN in the "AtomWork" inorganic materials database of the National Institute for Materials Science (NIMS). The peak intensities in the X-ray diffraction pattern were determined by the maximum counts of the peaks.

[0038] Furthermore, the amount of oxygen contained in the plate-shaped raw material and the fracture toughness of multiple samples of ceramic substrate 10 obtained by the above-described manufacturing method were measured after the degreasing process. As a result, as illustrated in Fig. 7, it was confirmed that the fracture toughness of ceramic substrate 10 tends to increase as the amount of oxygen contained in base 20 decreases. In particular, when the amount of oxygen contained in base 20 of ceramic substrate 10 is 0.07 wt% or less, high fracture toughness can be achieved, as shown by points P7a and P7b.

[0039] Furthermore, the amount of oxygen contained in the plate-shaped raw material and the thermal conductivity after the degreasing process were measured for multiple samples of ceramic substrate 10 obtained by the above-mentioned manufacturing method. As a result, as shown by points P8a, P8b, P8c, and P8d in Fig. 8, it was confirmed that the thermal conductivity of ceramic substrate 10 tends to increase as the amount of oxygen contained in base 20 decreases.

[0040] The oxygen content was measured using, for example, a PerkinElmer "2400II Fully Automatic Elemental Analyzer." The oxygen content was measured as follows: Approximately 5 mg of sample was filled into a tin holder and placed in the analyzer. The sample was then decomposed by pyrolysis, and the oxygen was reacted with a carbon catalyst to produce carbon monoxide, which was then analyzed.

[0041] Furthermore, in the ceramic substrate 10 according to this embodiment, the lengths of the fibrous AlN single crystals added in the kneading process are different. That is, as illustrated in the upper part of Fig. 10, the base 20 of the ceramic substrate 10 contains a plurality of AlN single crystals each having a different major axis, i.e., length. For example, 50% by volume of the plurality of AlN single crystals contained in the base 20 are longer than 20 µm. Furthermore, 10% by volume of the plurality of AlN single crystals contained in the base 20 are longer than 134 µm.

[0042] Furthermore, in the ceramic substrate 10 according to this embodiment, the thickness of the fibrous AlN single crystals added in the kneading process varies from approximately 1 μm to 10 μm. That is, as illustrated in the lower part of Fig. 10, the base 20 of the ceramic substrate 10 contains a plurality of AlN single crystals each having a different minor axis, i.e., thickness; for example, 50% by volume of the plurality of AlN single crystals contained in the base 20 are thicker than 1.6 μm. Furthermore, 10% by volume of the plurality of AlN single crystals contained in the base 20 are thicker than 2.9 μm.

[0043] The fibrous AlN single crystals introduced in the kneading step are preferably longer than 10 μm, and more preferably longer than 15 μm. The ceramic substrate 10 according to this embodiment preferably contains fibrous AlN single crystals without breaking them as much as possible. Furthermore, when fibrous AlN single crystals longer than 100 μm are used in the kneading step, the fibrous AlN single crystals longer than 100 μm actually remain within the base 20 even after the firing step, as shown in FIG. 11 . The greatest feature of the method for producing the ceramic substrate 10 according to this embodiment is that the fibrous AlN single crystals introduced in the kneading step remain within the base 20 after the firing step without losing their length.

[0044] In addition, when multiple samples of the ceramic substrate 10 according to this embodiment were examined, it was confirmed that, as long as the AlN single crystals contained in the base 20 of the ceramic substrate 10 had a length of 10 μm or more, and regardless of the length ratio, as long as fibrous AlN single crystals were contained, higher fracture toughness than conventional ceramic substrates could be obtained regardless of the content of the fibrous AlN single crystals. Furthermore, it was confirmed that, as long as the AlN single crystals contained in the base 20 of the ceramic substrate 10 had a thickness in the range of 1 μm to 10 μm, as long as fibrous AlN single crystals were contained, higher fracture toughness than conventional ceramic substrates could be obtained regardless of the content of the fibrous AlN single crystals.

[0045] Here, from the viewpoint of improving water resistance, the surface of the AlN single crystal contained in the ceramic substrate is preferably coated with an oxygen-containing layer. The oxygen-containing layer is formed when the AlN single crystal incorporates at least oxygen atoms during the manufacturing process of the AlN single crystal. When AlN reacts with oxygen molecules or water molecules, an oxygen-containing layer containing at least one of Al2O3, AlON, and Al(OH)3 can be formed so as to cover the surface of the AlN single crystal. However, from the viewpoint of improving water resistance, it is preferable that the oxygen-containing layer contains AlON.

[0046] If an "AlN whisker" refers to an entity consisting of an AlN single crystal and an oxygen-containing layer covering the surface of the AlN single crystal, the oxygen concentration in the AlN whisker (corresponding to the concentration in the oxygen-containing layer) is preferably 7.0 mass% or less, more preferably 4.0 mass% or less, and most preferably 2.0 mass% or less. This is based on the fact that, as shown in Figure 14, when multiple AlN whiskers used in this embodiment were examined, a correlation was found between the oxygen concentration in the AlN whisker and the diameter (thickness) of the AlN whisker for oxygen-containing layers of various thicknesses (6 nm, 10 nm, 20 nm, 30 nm). In other words, as mentioned above, in order to improve the fracture toughness of the ceramic substrate 10, it is preferable that the diameter (thickness) of the AlN single crystal be 1 μm or more. In this regard, referring to Figure 14, it can be seen that in order to make the diameter of the AlN whiskers (which in this explanation, for convenience, is assumed to be substantially the same as the diameter of the AlN single crystal) 1.0 μm or more, it is preferable that the oxygen concentration be 7.0 mass% or less, more preferably 4.0 mass% or less, and most preferably 2.0 mass% or less.

[0047] The ceramic substrate 10 also contains a plurality (a large number) of AlN whiskers (AlN single crystals). That is, in the kneading step described above, a plurality (a large number) of AlN whiskers (AlN single crystals) are dispersed in a mixture of a dispersion material and an organic solvent. Here, if the plurality of AlN whiskers dispersed in the mixture are collectively referred to as an "AlN whisker composite" for convenience, the AlN whisker composite contains a plurality of AlN whiskers (a plurality of AlN single crystals) each having a different diameter (thickness). As described above, the diameter of the AlN whiskers is preferably 1.0 μm or more. However, referring to FIG. 10, the content of AlN whiskers with a diameter of less than 1.0 μm in the AlN whisker composite is preferably 20% by volume or less (the content of AlN whiskers with a diameter of 1.0 μm or more is preferably 80% by volume or more).

[0048] The AlN single crystal contained in the ceramic substrate 10 according to this embodiment was subjected to a particle shape image analyzer "PITA-04" manufactured by Seishin Enterprise Co., Ltd., to obtain analytical data relating to the length (major axis in FIG. 10) and thickness (minor axis in FIG. 10) of the AlN single crystal shown in Fig. 10. As an example, the analytical data shown in Fig. 10 was obtained by analyzing approximately 5,000 prepared AlN single crystals.

[0049] 9, the structure of a conventional ceramic substrate is composed of particles of about 2 μm to 3 μm. On the other hand, the ceramic substrate 10 according to this embodiment is composed of a characteristic structure in which three types of particles are mixed: fibrous AlN single crystals, particles of about 2 μm to 3 μm, and particles of about 10 μm.

[0050] 12, when the ceramic substrate 10 according to this embodiment, which includes a substrate 20 containing fibrous AlN single crystals, is fractured in a fracture toughness test, the fracture surface is non-smooth and has numerous irregularities. That is, the substrate 20 of the ceramic substrate 10 according to this embodiment contains not only "granular" AlN polycrystals but also numerous "fibrous" AlN single crystals. Therefore, the fracture surface of the substrate 20 after the fracture toughness test is bent, particularly in the areas where the fibrous AlN single crystals are present. As a result, the fracture surface of the substrate 20 is non-smooth and has numerous irregularities compared to conventional ceramic substrates.

[0051] As shown by points P11a, P11b, P11c, P11d, and P11e in FIG. 13 , when multiple samples of the ceramic substrate 10 according to this embodiment were examined, it was confirmed that the lower the arithmetic mean roughness of the fracture surface of the substrate 20, the lower the fracture toughness of the ceramic substrate 10, and the higher the arithmetic mean roughness of the fracture surface of the substrate 20, the higher the fracture toughness of the ceramic substrate 10. In particular, when the arithmetic mean surface roughness of the fracture surface of the substrate 20 is 3 μm or more, higher fracture toughness can be achieved compared to a comparative example not including a fibrous AlN single crystal. That is, a high arithmetic mean roughness of the fracture surface means that the cracks formed in the structure of the substrate 20 upon fracture proceed in a non-uniform direction. Therefore, it can be said that the higher the fracture toughness of a substrate 20, the more irregularities there are on the fracture surface, and the higher the arithmetic mean roughness of the fracture surface.

[0052] The surface height of the fractured surface was measured by a well-known height measurement method, for example, using an "OPTELICSH1200" device manufactured by Lasertec. The measurement conditions for the surface height of the fractured surface were a lens magnification of 50x and a resolution of 0.01 μm. The arithmetic mean roughness was determined in an arbitrary 300 μm square area on the ceramic substrate in accordance with "JIS B0601 Geometrical Product Specifications (GPS) - Surface Texture: Profile Method - Terms, Definitions, and Surface Texture Parameters."

[0053] According to the ceramic substrate 10 according to the present embodiment exemplified above, the base 20 constituting the main body thereof has a structure in which a large number of fibrous AlN single crystals are oriented in a direction along the end face in the plate thickness direction of the base 20. Due to the presence of fibrous AlN single crystals, the ceramic substrate 10 having such a structure can achieve a further improvement in thermal conductivity compared to conventional ceramic substrates, and can also achieve a further improvement in fracture toughness, i.e., a further improvement in mechanical strength, compared to conventional ceramic substrates.

[0054] Furthermore, since the fracture toughness, i.e., mechanical strength, can be improved, the thickness of the ceramic substrate 10 can be reduced, which makes it easier to transfer heat generated from the power semiconductors 12 to the heat sink 13, thereby further improving the heat dissipation performance.

[0055] As described above, according to this embodiment, it is possible to obtain a ceramic substrate 10 that has both high thermal conductivity and high mechanical strength.

[0056] The characteristics of the ceramic substrate 10 exemplified in this embodiment show similar trends even when detection values ​​indicating a "plane along the longitudinal direction of the AlN single crystal," such as the (11-20) plane, are used instead of the detection values ​​indicating the (10-10) plane obtained by X-ray diffraction. The peak of the detection values ​​indicating the (11-20) plane is detected when the angle of the detector 104 is approximately 59.34 degrees. However, the peak of the detection values ​​indicating the (11-20) plane may also be detected when the angle of the detector 104 is slightly different from approximately 59.34 degrees, depending on, for example, the shape of the sample or the positional relationship of the device.

[0057] The present embodiment illustrated above shows one embodiment of a ceramic substrate, and AlN crystals, AlN whiskers, and AlN whisker composites contained in the ceramic substrate, and various modifications and extensions can be made without departing from the spirit of the present invention. For example, a ceramic substrate containing a fibrous AlN single crystal in a substrate may be identified by parameters other than thermal conductivity, fracture toughness, X-ray diffraction pattern, and the amount of oxygen contained in the substrate.

[0058] In the present disclosure, a numerical range indicated using "from" indicates a range that includes the numerical values ​​stated before and after "from" as the minimum and maximum values, respectively.

[0059] This disclosure is based on and benefits from the priority rights of the following Japanese patent applications, the entire contents of which are incorporated herein by reference: (1) Japanese Patent Application No. 2020-134777, entitled "Ceramic Substrate," filed on August 7, 2020

Claims

1. A ceramic substrate containing a fibrous AlN single crystal body as a base.

2. Thermal conductivity is 150 W / mK or more, and fracture toughness is 4.0 MPa １／２ 2. The ceramic substrate according to claim 1, wherein the ceramic substrate is a ceramic substrate having the above structure.

3. The fibrous AlN single crystal has a hexagonal wurtzite structure, The ratio of the peak intensity of the (10-10) plane of the AlN single crystal obtained when the end surface in the plate thickness direction of the substrate is irradiated with X-rays to the peak intensity of the (0002) plane is 2.00 or more. The ceramic substrate according to claim 1 or 2.

4. 4. The ceramic substrate according to claim 1, wherein the amount of oxygen contained in the base is 0.07% by weight or less.

5. 5. The ceramic substrate according to claim 1, wherein the fracture surface of the base containing the AlN single crystal has an arithmetic mean roughness of 3 [mu]m or more.

6. 6. The ceramic substrate according to claim 1, wherein the base further contains granular AlN single crystals.

7. A ceramic substrate comprising a fibrous AlN single crystal having a hexagonal wurtzite structure and an oxygen-containing layer covering the surface of the AlN single crystal, the substrate containing AlN whiskers having an oxygen concentration of 7.0 mass% or less.

8. A ceramic substrate having an AlN whisker composite contained in a base, the AlN whisker composite having a plurality of AlN whiskers each consisting of a fibrous AlN single crystal having a hexagonal wurtzite structure and an oxygen-containing layer covering the surface of the AlN single crystal, and the content of the AlN whiskers having a diameter of less than 1.0 μm being 20 volume % or less.

9. A fibrous AlN single crystal having a hexagonal wurtzite structure, in which the ratio of the peak intensity of the (10-10) plane to the peak intensity of the (0002) plane obtained when an end face in the thickness direction is irradiated with X-rays is 2.00 or more.

10. An AlN whisker is comprised of a fibrous AlN single crystal having a hexagonal wurtzite structure and an oxygen-containing layer covering the surface of the AlN single crystal, the oxygen concentration of which is 7.0 mass % or less.

11. An AlN whisker composite having a plurality of AlN whiskers each consisting of a fibrous AlN single crystal having a hexagonal wurtzite structure and an oxygen-containing layer covering the surface of the AlN single crystal, wherein the content of the AlN whiskers having a diameter of less than 1.0 μm is 20 volume % or less.

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