Ceramic substrate, power module, and aluminum nitride sintered body

Abstract

The present invention relates to a ceramic substrate having improved bending strength in addition to thermal conductivity and fracture toughness, and more particularly, to a ceramic substrate comprising an aluminum nitride sintered body obtained by sintering a fibrous aluminum nitride single crystal and a particulate aluminum nitride, the maximum minor diameter of the aluminum nitride particles constituting the sintered body is 15.0 [mu] m or less in a structure image of the sintered body.

Description

TECHNICAL FIELD

[0001] The present application relates to a ceramic substrate, a power module, and an aluminum nitride sintered body, and particularly relates to a ceramic substrate excellent in thermal conductivity, fracture toughness, and bending strength. BACKGROUND

[0002] For example, a control module that performs power control or motor control of an electric automobile, an autonomous vehicle, a railway, a working machine, a data center, a high-brightness LED, or the like is a module to which a high voltage is applied, and a ceramic substrate is used as a substrate thereof.

[0003] It is widely known that a ceramic substrate of such a control module uses a silicon nitride sintered body and an aluminum nitride sintered body. On the other hand, when a silicon nitride sintered body is used for a ceramic substrate, the ceramic substrate has a problem that the thermal conductivity is low. In addition, when an aluminum nitride sintered body is used for a ceramic substrate, the ceramic substrate, although excellent in thermal conductivity, has a problem that the fracture toughness is reduced.

[0004] In International Publication No. 2022 / 030637 (Patent Literature 1), it is described that, for a ceramic substrate composed of an aluminum nitride sintered body, by using a fibrous aluminum nitride single crystal, a ceramic substrate that combines thermal conductivity and mechanical properties (fracture toughness) can be provided.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: International Publication No. 2022 / 030637 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The present inventors have found, when studying a ceramic substrate using a fibrous aluminum nitride single crystal, that a ceramic substrate using a fibrous aluminum nitride single crystal, although excellent in fracture toughness, has a tendency that the bending strength is reduced, and there is room for improvement in the mechanical strength of the ceramic substrate.

[0010] Therefore, an object of the present application is to provide a ceramic substrate in which the bending strength is improved in addition to the thermal conductivity and the fracture toughness. In addition, another object of the present application is to provide an aluminum nitride sintered body that is suitable for a power module using such a ceramic substrate and such a ceramic substrate.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The present inventors, first, found, upon observing the microstructure of an aluminum nitride sintered body not using a fibrous aluminum nitride single crystal and an aluminum nitride sintered body using a fibrous aluminum nitride single crystal, that, although sintered under the same conditions, the aluminum nitride sintered body using a fibrous aluminum nitride single crystal had a large number of large particles in the microstructure image thereof, as compared with the aluminum nitride sintered body not using a fibrous aluminum nitride single crystal.

[0013] In the growth process of aluminum nitride via a liquid phase in sintering, the particles of aluminum nitride grow by repeated dissolution and precipitation. At this time, the smaller the radius of curvature of the particles, the more easily they dissolve, whereas the larger the radius of curvature, the more easily they grow (this is a phenomenon known as Ostwald ripening). Therefore, if a material in which a fibrous aluminum nitride single crystal is added to a particulate aluminum nitride serving as a base material is sintered, the fibrous aluminum nitride single crystal selectively grows over time, and the small particles of the particulate aluminum nitride dissolve in the aid phase, and a large number of large particles (i.e., particles based on the fibrous aluminum nitride single crystal) appear in the microstructure of the aluminum nitride sintered body.

[0014] Therefore, the present inventors further found, upon further research, that, when an aluminum nitride sintered body is manufactured using a fibrous aluminum nitride single crystal and a particulate aluminum nitride, by adjusting the maximum minor axis of the aluminum nitride particles constituting the obtained sintered body to be within a range of 15.0 μm or less, the bending strength can be improved. In addition, the present inventors found that, even if the maximum minor axis of the aluminum nitride particles constituting the aluminum nitride sintered body is adjusted to be within the above specific range, the thermal conductivity and the fracture toughness are not affected, or even if they are affected, the degree is small, and high thermal conductivity and high fracture toughness can be maintained, and thus an aluminum nitride sintered body and a ceramic substrate excellent in thermal conductivity, fracture toughness, and bending strength can be provided, thereby completing the present invention.

[0015] Therefore, the ceramic substrate of the present invention is characterized by being a ceramic substrate constituted by an aluminum nitride sintered body sintered from a fibrous aluminum nitride single crystal and a particulate aluminum nitride, in which the maximum minor axis of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a microstructure image of the sintered body.

[0016] In a preferred example of the ceramic substrate of the present invention, the longest distance of a straight line connecting two points on the outer periphery of a void present in the sintered body is 2.5 μm or less in a microstructure image of the sintered body with respect to the size of each void.

[0017] In another preferred example of the ceramic substrate of the present invention, the thermal conductivity of the ceramic substrate is 175 to 183 W / mK.

[0018] In another preferred example of the ceramic substrate of the present invention, the fracture toughness of the ceramic substrate satisfies 4.2 to 5.8 MPa m1 / 2 in at least one fracture direction in the SEPB method.1 / 2 .

[0019] In another preferred example of the ceramic substrate of the present application, the ceramic substrate has a bending strength of 306 to 344 MPa.

[0020] In another preferred example of the ceramic substrate of the present application, the ceramic substrate has a thermal conductivity of 175 to 183 W / mK, a fracture toughness of 4.2 to 5.8 MPa.m1 / 2in at least one fracture direction in the SEPB method, and a bending strength of 306 to 344 MPa. 1 / 2

[0021] In another preferred example of the ceramic substrate of the present application, the ceramic substrate contains oxygen in an amount of 2.8 mass% or less.

[0022] Further, the power module of the present application is a power module having the above-described ceramic substrate of the present application.

[0023] Further, the aluminum nitride sintered body of the present application is characterized by being an aluminum nitride sintered body sintered from fibrous aluminum nitride single crystals and particulate aluminum nitride, wherein the maximum minor axis of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a microstructure image of the sintered body.

[0024] Effects of the Invention

[0025] The ceramic substrate of the present application can provide a ceramic substrate excellent in thermal conductivity, fracture toughness, and bending strength. Further, the power module of the present application can provide a power module using such a ceramic substrate. Further, the aluminum nitride sintered body of the present application can provide an aluminum nitride sintered body suitable for such a ceramic substrate. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a perspective view schematically showing the crystal structure of the fibrous aluminum nitride single crystals.

[0027] Figure 2 is a view schematically showing a structure example of an apparatus for performing X-ray diffraction on a ceramic substrate.

[0028] Figure 3 is a view schematically showing a structure example of a power module.

[0029] Figure 4 is a microstructure image of the aluminum nitride sintered body of Sample No. 2.

[0030] Figure 5 is a view showing the values of the "bending strength (MPa)" shown in Table 1 as the vertical axis and the values of the "maximum minor axis (μm)" as the horizontal axis, and is a view plotting the measurement results of Sample No. 1 to Sample No. 8. ​

[0031] Figure 6 A graph showing the value of "void maximum diameter (μm)" shown in Table 1 as the vertical axis and the value of "bending strength (MPa)" as the horizontal axis is a graph in which the measurement results of Test Material No. 1 to Test Material No. 8 are plotted.

[0032] Figure 7 A schematic diagram and dimensions of a test sample for a cold-heat cycle test are shown.

[0033] Figure 8 Results of the cold-heat cycle test are shown. DETAILED DESCRIPTION

[0034] Hereinafter, the present application will be described in detail. The present application relates to a ceramic substrate, a power module having the ceramic substrate, and an aluminum nitride sintered body suitable for the ceramic substrate.

[0035] In the present specification, the ceramic substrate refers to a substrate composed of a sintered body of an inorganic compound such as aluminum nitride, silicon nitride, alumina, alumina zirconia, beryllia, or the like. In a practical ceramic substrate, good thermal conductivity, insulating properties, heat resistance, mechanical strength, and the like are required depending on its use. The ceramic substrate is mainly used as a power module.

[0036] In the present specification, the power module refers to a module for performing power control or motor control of a light emitting diode, a laser diode, an electric automobile, an autonomous driving vehicle, a railway, a working machine, a data center, an industrial robot, solar power generation, wind power generation, an uninterruptible power supply (UPS), a semiconductor manufacturing device, or the like.

[0037] In the present specification, the aluminum nitride sintered body refers to a dense substance obtained by adding a sintering aid to a powder-like aluminum nitride and sintering to bind it. In the present application, in addition to the above-mentioned raw material, a fibrous aluminum nitride single crystal is also sintered. There are cases where the aluminum nitride is expressed as AlN.

[0038] The ceramic substrate of the present application is a substrate composed of an aluminum nitride sintered body, and the aluminum nitride sintered body is an aluminum nitride sintered body obtained by sintering a fibrous aluminum nitride single crystal and a particulate aluminum nitride. By adding the fibrous aluminum nitride single crystal to the particulate aluminum nitride, high thermal conductivity and high fracture toughness of the obtained aluminum nitride sintered body and the ceramic substrate composed of the aluminum nitride sintered body can be realized. In the present specification, the aluminum nitride sintered body is also referred to as "the aluminum nitride sintered body of the present application". Also, the aluminum nitride sintered body of the present application is suitable for a ceramic substrate, but can also be used for, for example, a member for a semiconductor manufacturing device, a ceramic heater, an electrostatic chuck, a Peltier element, or the like.

[0039] The fibrous aluminum nitride single crystal can also be expressed as a fibrous aluminum nitride single crystal. In other words, the so-called fibrous means that the AlN single crystal extends in a fiber-like manner. Also, it is sufficient if the entire body is fibrous, and for example, it can be linearly elongated, or it can have a portion that is bent or curved.

[0040] The fibrous aluminum nitride single crystal has a high aspect ratio, and the aspect ratio of the fibrous aluminum nitride single crystal is preferably 2 to 100, and more preferably 5 to 50. The aspect ratio is obtained from the ratio of the median particle diameter of the length to the median particle diameter of the short diameter (length median particle diameter / short diameter median particle diameter) of the fibrous aluminum nitride single crystal described later.

[0041] The median particle diameter of the length of the fibrous aluminum nitride single crystal is preferably 5 to 100 μm, and more preferably 10 to 50 μm. In addition, the median particle diameter of the short diameter of the fibrous aluminum nitride single crystal is preferably 1 to 3 μm, and more preferably 1 to 2 μm.

[0042] In the present specification, the median particle diameter of the length and the median particle diameter of the short diameter of the fibrous aluminum nitride single crystal can be determined from a microscope image obtained using an optical microscope. Specifically, at least 5000 fibrous aluminum nitride single crystals are selected from the microscope image, the maximum length of each fibrous aluminum nitride single crystal is obtained as the length, the maximum length in the perpendicular direction with respect to the length is obtained as the short diameter, a cumulative distribution based on volume conversion is generated from the obtained values, and for the length and the short diameter, the diameter (median particle diameter) at which each cumulative reaches 50% is taken as the median particle diameter of the length and the median particle diameter of the short diameter of the fibrous aluminum nitride single crystal. In the measurement of the median particle diameter of the length and the median particle diameter of the short diameter of the fibrous aluminum nitride single crystal, for example, a particle shape image analyzer "PITA-04" manufactured by Seishin Enterprise Co., Ltd. can be used.

[0043] The fibrous aluminum nitride single crystal, as Figure 1 The fibrous aluminum nitride single crystal has a crystal structure that is a so-called hexagonal wurtzite-type structure. In addition, the fibrous aluminum nitride 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 fibrous aluminum nitride single crystal". The (0002) plane is an example of "a plane orthogonal to the longitudinal direction of the fibrous aluminum nitride 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".

[0044] The orientation of the particle based on the fibrous aluminum nitride single crystal constituting the aluminum nitride sintered body (i.e., the particle having a long-short diameter ratio in which the grain growth is performed with the fibrous aluminum nitride single crystal as a nucleus) in the ceramic substrate is not particularly limited, and the particle based on the fibrous aluminum nitride single crystal constituting the aluminum nitride sintered body may, for example, be oriented in the plate thickness direction of the ceramic substrate, may be oriented in a direction perpendicular to the plate thickness direction, or may be a mixture of the particle oriented in the plate thickness direction of the ceramic substrate and the particle oriented in a direction perpendicular to the plate thickness direction.

[0045] The fibrous aluminum nitride single crystal maintains the wurtzite-type structure of the hexagonal crystal even after sintering (i.e., the particle based on the fibrous aluminum nitride single crystal constituting the aluminum nitride sintered body also has the wurtzite-type structure of the hexagonal crystal), and thus the orientation of the particle based on the fibrous aluminum nitride single crystal in the ceramic substrate can be confirmed by performing X-ray diffraction on the ceramic substrate.

[0046] Figure 2 is a diagram schematically showing an example of the structure of an apparatus for performing X-ray diffraction on a ceramic substrate. The X-ray diffraction apparatus 100 has an X-ray source 101 that generates X-rays, an incident-side collimator 102, a light-receiving-side collimator 103, and a detector 104. The X-rays generated by the X-ray source 101 are irradiated to the end surface in the plate thickness direction of the ceramic substrate 10 (i.e., the surface in a direction perpendicular to the plate thickness direction) via the incident-side collimator 102. Then, the X-rays diffracted by the measurement object are incident on the detector 104 via the light-receiving-side collimator 103. Then, the diffraction pattern is measured in the detector 104.

[0047] In the X-ray diffraction performed by such an X-ray diffraction apparatus 100, the value of the angle 2θ of the detector 104 with respect to the irradiation direction of the X-rays on the measurement object is changed in a predetermined range, for example, from 20 degrees to 80 degrees, and thus the diffraction peaks of each of the surfaces of the wurtzite-type structure of the hexagonal crystal, i.e., the "a surface" and the "c surface", and the like can be obtained. In addition, the peak intensity obtained by the X-ray diffraction is also the maximum statistical number of each of the surfaces of the wurtzite-type structure of the hexagonal crystal in the ceramic substrate, in other words, the number of each of the surfaces present.

[0048] As described in International Publication No. 2022 / 030637, the peak indicating the detection value of the "a surface", i.e., the (10-10) surface, of the X-ray diffraction pattern obtained when the X-rays are irradiated to the end surface in the plate thickness direction of the ceramic substrate 10 is detected at an angle of about 33.21 degrees of the detector 104. However, the peak indicating the detection value of the "a surface", i.e., the (10-10) surface, can also be detected at an angle somewhat deviated from 33.21 degrees of the detector 104, for example, due to the shape of the sample or the positional relationship of the apparatus, and the like.

[0049] In addition, the peak indicating the detection value of the "c plane" (0002) of the X-ray diffraction pattern obtained when X-rays are irradiated to the end surface in the direction of the thickness of the ceramic substrate 10 is detected at an angle of about 36.04 degrees of the detector 104. However, the peak indicating the detection value of the "c plane" (0002) can also be detected at an angle somewhat deviated from 36.04 degrees of the detector 104, for example, due to the shape of the sample or the positional relationship of the device, and the like.

[0050] Thus, as compared with the X-ray diffraction pattern of the ceramic substrate in which the fibrous aluminum nitride single crystal is not used, the peak intensity ratio of the "a plane" (10-10) of the X-ray diffraction pattern obtained when X-rays are irradiated to the end surface in the direction of the thickness of the ceramic substrate is large, and the peak intensity ratio of the "c plane" (0002) is small, and it can be said that the particles based on the fibrous aluminum nitride single crystal in the ceramic substrate are oriented in the direction perpendicular to the direction of the thickness.

[0051] In addition, the higher the ratio of the peak intensity of the "a plane" to the peak intensity of the "c plane" (a / c value) of the X-ray diffraction pattern obtained by the X-ray diffraction of the ceramic substrate, the stronger the orientation of the particles based on the fibrous aluminum nitride single crystal in the ceramic substrate in the direction of the end surface in the direction of the thickness, or the larger the amount of the particles based on the fibrous aluminum nitride single crystal in the ceramic substrate.

[0052] Further, the X-ray diffraction is performed by the well-known θ-2θ method, and for example, the device "Ultima IV" manufactured by Rigaku Corporation can be used. In addition, the implementation conditions of the X-ray diffraction are voltage: 40 kV, current: 30 mA, divergence slit: 1 / 2 degree, scattering slit: 1 / 2 degree, light receiving slit: 0.3 mm, scanning step: 0.02 degree, and the range of 2θ: from 20 degrees to 80 degrees. In addition, the peak position of the X-ray diffraction pattern can be determined based on the X-ray spectrum of AlN of the inorganic material database "AtomWork" of the National Institute for Materials Science (NIMS). In addition, the peak intensity of the X-ray diffraction pattern is the maximum number of statistics of the peak.

[0053] The fibrous aluminum nitride single crystal is preferably at least a part of the surface thereof is coated with an oxygen-containing layer. By coating the surface of the fibrous AlN single crystal with an oxygen-containing layer, water resistance can be improved. The oxygen-containing layer is formed by at least incorporating oxygen atoms into the AlN single crystal during the production of the AlN single crystal. If AlN reacts with oxygen molecules or water molecules, an oxygen-containing layer containing at least one of Al2O3, AlON, Al(OH)3 is formed in a manner covering the surface of the AlN single crystal. From the viewpoint of improving water resistance, it is preferable that the oxygen-containing layer contain AlON.

[0054] In the present specification, the fibrous aluminum nitride single crystal whose surface is coated with an oxygen-containing layer is referred to as "aluminum nitride whisker" or "AlN whisker".

[0055] The amount of oxygen contained in the aluminum nitride whisker is preferably 3.0 mass% or less, more preferably 2.0 mass% or less, and most preferably 1.0 mass% or less. The lower the amount of oxygen contained in the aluminum nitride whisker, the better. On the other hand, due to reasons such as manufacturing processes, it is sometimes inevitable to contain oxygen. As a lower limit value for such a case, from the viewpoint of manufacturing costs and the like, it is preferable that the amount of oxygen contained in the aluminum nitride whisker be 0.01 mass% or more.

[0056] The measurement of the amount of oxygen contained in the aluminum nitride whisker can be performed using, for example, an apparatus "EMGA-920" manufactured by HORIBA Ltd. Specifically, the measurement of the amount of oxygen can be performed in the following manner. About 10 mg of a sample is filled into a nickel capsule and is put into the apparatus. Then, the sample is decomposed by thermal decomposition, and oxygen is analyzed by reacting with carbon monoxide using a carbon catalyst.

[0057] The method for producing the fibrous aluminum nitride single crystal and the aluminum nitride whisker is already known, and is described in detail in, for example, Japanese Patent Application Publication No. 2018-154534 and the like.

[0058] The amount of the fibrous aluminum nitride single crystal used for the production of the aluminum nitride sintered body is preferably 1 to 30 parts by mass, and more preferably 3 to 5 parts by mass, relative to 100 parts by mass of the particulate aluminum nitride as a base material.

[0059] The particulate aluminum nitride is an aluminum nitride used as a base material for an aluminum nitride sintered body, and has an aspect ratio of 1.1 or less. The aspect ratio is obtained from the ratio of the median particle diameter of the long diameter to the median particle diameter of the short diameter (median particle diameter of long diameter / median particle diameter of short diameter) of the particulate aluminum nitride.

[0060] The median particle diameters of the long diameter and the short diameter of the particulate aluminum nitride are each preferably in the range of 0.3 to 5 μm, and more preferably in the range of 0.5 to 3 μm.

[0061] In the present specification, the median particle diameter of the aspect ratio and the median particle diameter of the short diameter of the particulate aluminum nitride can be determined from a microscope image obtained by an optical microscope. Specifically, at least 5000 particulate aluminum nitrides are selected from the microscope image, in the case of a circular particulate aluminum nitride, the diameter thereof is determined as the aspect ratio and the short diameter, in the case of a particulate aluminum nitride having a shape other than a circle, the maximum length thereof is determined as the aspect ratio, and the maximum length in the direction perpendicular to the aspect ratio is determined as the short diameter, a cumulative distribution based on volume conversion is generated from the obtained values, and the particle diameter at which the cumulative amount of each of the aspect ratio and the short diameter reaches 50% (the median particle diameter) is determined as the median particle diameter of the aspect ratio and the median particle diameter of the short diameter. In the measurement of the median particle diameter of the aspect ratio and the median particle diameter of the short diameter of the particulate aluminum nitride, for example, a particle shape image analyzer "PITA-04" manufactured by Seishin Enterprise Co., Ltd. can be used.

[0062] The amount of oxygen contained in the particulate aluminum nitride is preferably 2.0% by mass or less, more preferably 1.0% by mass or less, and most preferably 0.5% by mass or less. The amount of oxygen contained in the particulate aluminum nitride is also preferably as low as possible. On the other hand, it is sometimes difficult to avoid containing oxygen due to manufacturing processes and the like, and as a lower limit value in such a case, the amount of oxygen contained in the particulate aluminum nitride is preferably 0.01% by mass or more. The amount of oxygen contained in the particulate aluminum nitride can be measured in the same manner as the "measurement of the amount of oxygen contained in the aluminum nitride whisker" described above.

[0063] In the material used for the production of the aluminum nitride sintered body, in addition to the aluminum nitride, as other components, there are sintering aids, dispersing materials, defoaming materials, organic solvents, water, and the like, and these components can be appropriately used according to the purpose.

[0064] A sintering aid is often used in the production of the aluminum nitride sintered body. As the sintering aid, for example, oxides of rare earth elements, oxides of alkaline earth elements, carbonates, oxalates, nitrates, and the like can be exemplified. Among them, oxides of rare earth elements are preferred, and yttrium oxide (Y2O3) is particularly preferred.

[0065] When a sintering aid is used, the amount of the sintering aid is preferably 1 to 10 parts by mass, and more preferably 3 to 7 parts by mass, with respect to 100 parts by mass of the particulate aluminum nitride as the base material.

[0066] In the aluminum nitride sintered body, the maximum short diameter of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in the microstructure image of the sintered body, preferably 13.0 μm or less, and more preferably 12.0 μm or less. According to the present application, by making the maximum short diameter of the aluminum nitride particles constituting the sintered body within the above specific range, the bending strength of the aluminum nitride sintered body and the ceramic substrate composed of the aluminum nitride sintered body can be improved. The smaller the maximum short diameter of the aluminum nitride particles, the better. On the other hand, from the viewpoint of manufacturing and handling, the maximum short diameter of the aluminum nitride particles constituting the sintered body is preferably 1.0 μm or more as a lower limit value. In addition, the thermal conductivity and the fracture toughness of the aluminum nitride sintered body in which the maximum short diameter of the aluminum nitride particles is within the above specific range show the same degree of values as those of the aluminum nitride sintered body in which the maximum short diameter of the aluminum nitride particles is more than 15.0 μm. Therefore, the aluminum nitride sintered body according to the present application and the ceramic substrate of the present application can provide an aluminum nitride sintered body and a ceramic substrate excellent in thermal conductivity, fracture toughness, and bending strength.

[0067] The reason why the aluminum nitride sintered body of the present application can obtain the same degree of thermal conductivity as that of the existing aluminum nitride sintered body in the state excellent in bending strength is considered to be that the contribution of the amount of oxygen in the crystal to the thermal conductivity is high. If sintering is performed, the amount of oxygen in the crystal is simultaneously discharged to the system along with the growth of the crystal grains. As a result, generally, the larger the maximum short diameter, the longer the sintering time, and the amount of oxygen in the crystal is sufficiently discharged, so the thermal conductivity becomes high. In other words, it is considered that the maximum short diameter and the thermal conductivity show a pseudo correlation. The aluminum nitride sintered body of the present application, because the fibrous aluminum nitride single crystal having a large curvature radius of the particle is added to the particulate aluminum nitride as a base material, the fibrous aluminum nitride single crystal selectively grows in a short time as time passes. As a result, the discharge rate of the amount of oxygen in the crystal is accelerated, the growth of the crystal grains is suppressed, and the thermal conductivity is improved.

[0068] The reason why the aluminum nitride sintered body of the present application can obtain the same degree of fracture toughness as that of the existing aluminum nitride sintered body in which the maximum short diameter of the aluminum nitride particles is large is that, generally, in a ceramic body composed of a single structure, the larger the particle diameter, the higher the fracture toughness. The so-called fracture toughness is a parameter indicating the degree to which a crack is difficult to progress. The crack of the aluminum nitride sintered body develops at the grain boundary. At this time, the larger the particle, the more the crack will expand around the particle, and the fracture toughness increases. Because the aluminum nitride sintered body of the present application adds the fibrous aluminum nitride single crystal, the detour of the crack frequently occurs (bridge effect). Thereby, high fracture toughness can be obtained regardless of the particle diameter.

[0069] Here, the so-called aluminum nitride particles constituting the aluminum nitride sintered body refer to the aluminum nitride particles formed through the sintering process, the particles having a long-short diameter ratio in which the fibrous aluminum nitride single crystal grows as a nucleus, and mixed in the particulate aluminum nitride as a base material which grows by repeated dissolution and deposition.

[0070] In the present specification, in order to obtain the maximum minor diameter of the aluminum nitride particles constituting the aluminum nitride sintered body, and the void size described later existing in the aluminum nitride sintered body, an "aluminum nitride sintered body microstructure image" is used, which is an electron microscope image taken using a scanning electron microscope (SEM). In order to distinguish between the particles, a backscattered electron image (BSE image) is taken, which shows a difference in contrast due to the orientation direction of the crystal grains. At least 200 or more aluminum nitride particles are photographed in the aluminum nitride sintered body microstructure image. Since the aluminum nitride sintered body is a homogeneous microstructure to some extent, a microstructure image of a specific region in which at least 200 or more aluminum nitride particles are photographed can represent the microstructure of the aluminum nitride sintered body. Therefore, by observing with such a microstructure image as one field of view, it can be determined whether the constituent elements of the present application are satisfied. Also, in the aluminum nitride sintered body, depending on the manufacturing method and the like, a small amount of coarse aluminum nitride particles and coarse voids will inevitably occur, but even if such coarse particles and coarse voids exist in a small amount, they will not affect the characteristics of the sintered body. In the present specification, the photographing is performed under the conditions of an acceleration voltage of 3 to 5 KeV, a current of 20 μA, and a working distance of 4 mm. In order to avoid the formation of a contrast due to the unevenness of the sample surface and to prevent detection, the sample surface is polished, and then processed to Ra < 0.2 μm or so using ion milling or the like. Also, Ra is the arithmetic average roughness, which can be measured in accordance with JIS B0601 (2013).

[0071] In the present specification, the "short diameter" of the aluminum nitride particles constituting the aluminum nitride sintered body is the maximum length in the direction perpendicular to the longest length among the straight lines connecting two points on the outer periphery of the aluminum nitride particles, and, if the aluminum nitride particles are circular, the diameter thereof is the short diameter. Then, the maximum minor diameter of the aluminum nitride particles constituting the aluminum nitride sintered body refers to the maximum value among the short diameters of all the aluminum nitride particles in the aluminum nitride sintered body microstructure image.

[0072] In the present application, as a means for adjusting the maximum minor diameter of the aluminum nitride particles constituting the aluminum nitride sintered body to be within the range of 15.0 μm or less, a method of adjusting the firing temperature and the firing time in the sintering process can be cited. Specifically, by setting the firing temperature to be lower and the firing time to be shorter, the fibrous aluminum nitride single crystals and the particulate aluminum nitride are fired, whereby an aluminum nitride sintered body constituted by aluminum nitride particles having a maximum minor diameter within the range of 15.0 μm or less can be obtained. In addition, by using a fine metal as a raw material, the maximum minor diameter can also be adjusted by the pinning effect of the grain growth.

[0073] A gap can be generated between the aluminum nitride particles that constitute the aluminum nitride sintered body. In the present application, the gap present in the aluminum nitride sintered body, with respect to the size thereof, is preferably such that the longest distance of a straight line connecting two points on the outer circumference of the gap in a microstructure image of the sintered body is 2.5 μm or less, and more preferably 1 μm or less. Since the gap present in the aluminum nitride sintered body can become a starting point of destruction, it is preferable that there be no large gaps. Also, even if there is a gap having a large size (for example, a linearly extending gap, etc.) only in a certain specific direction, since this can become a starting point of destruction of the aluminum nitride sintered body, it is preferable to control the longest distance of a straight line connecting two points on the outer circumference of the gap in a microstructure image of the sintered body. According to the present application, since the particle diameter of the aluminum nitride particles that constitute the aluminum nitride sintered body is small, the gap that can be generated between the aluminum nitride particles can be reduced. In the case where it is difficult to make the gap extremely small, the lower limit value of the longest distance of a straight line connecting two points on the outer circumference of the gap in a microstructure image of the sintered body is preferably 0.1 μm or more.

[0074] The thermal conductivity of the ceramic substrate is preferably 170 to 250 W / mK, more preferably 170 to 230 W / mK, further preferably 170 to 220 W / mK, and most preferably 175 to 183 W / mK. The preferable range of the thermal conductivity of the ceramic substrate is also applicable to the thermal conductivity of the aluminum nitride sintered body of the present application. Although the higher the thermal conductivity is, the better, from the viewpoint of the increase in manufacturing cost due to the change in the conditions required for high thermal conductivity, the thermal conductivity is preferably 220 W / mK or less.

[0075] In the present specification, the thermal conductivity is measured by the hot disc method in accordance with "ISO 22007-2 "Plastics - Determination of thermal conductivity and thermal diffusivity - Part 2: Transient plane heat source (hot disc) method" ", and for example, an apparatus "TPS-2500" manufactured by Kyoto Electronics Manufacturing Co., Ltd. can be used.

[0076] In the present specification, the fracture toughness is measured by the SEPB method in accordance with "JIS R1607 Test method of room temperature fracture toughness of fine ceramics", and for example, a micrometer manufactured by Mituyo Co., Ltd., a Vickers hardness tester HV-115 manufactured by Mituyo Co., Ltd., a universal testing machine Model 5582 manufactured by Instron Corporation, a MEASURESCOPE 10 manufactured by Nikon Corporation, etc. can be used.

[0077] The fracture toughness of the ceramic substrate of the present application differs depending on the orientation direction when a particle having a long-short diameter ratio, which is grown in a grain form with a fibrous AlN single crystal as a nucleus (hereinafter, simply referred to as "fibrous particle"), is oriented in-plane. For example, when two kinds of test pieces having different orientation directions of the fibrous particle are measured by the SEPB method, the fracture toughness of the test piece in which the fibrous particle is oriented in parallel with respect to the fracture direction in the SEPB method exhibits 4.1 MPa · m 1 / 2 , and the fracture toughness of the test piece in which the fibrous particle is oriented in a direction intersecting with respect to the fracture direction in the SEPB method exhibits 6.1 MPa · m 1 / 2 . In general, the test piece in which the fibrous particle is oriented in a direction intersecting with respect to the fracture direction tends to have higher fracture toughness. The "fracture direction in the SEPB method" refers to the direction in which a crack progresses from the thickness direction of the test piece when contacting the crosshead.

[0078] The fracture toughness of the ceramic substrate preferably satisfies 2.5 to 10.0 MPa · m 1 / 2 , more preferably 3.5 to 7.0 MPa · m 1 / 2 , and further preferably 4.2 to 5.8 MPa · m 1 / 2 in at least one fracture direction in the SEPB method. The preferable range of the fracture toughness of the ceramic substrate is also applicable to the fracture toughness of the aluminum nitride sintered body of the present application.

[0079] The bending strength of the ceramic substrate is preferably 250 to 450 MPa, more preferably 300 to 450 MPa, and particularly preferably 306 to 344 MPa. The preferable range of the bending strength of the ceramic substrate is also applicable to the bending strength of the aluminum nitride sintered body of the present application.

[0080] In the present specification, the bending strength is measured by the SEPB method according to "JIS R1601 Test Method for Room Temperature Bending Strength of Fine Ceramics", and for example, Autograph AGX-10KNVV manufactured by Shimadzu Corporation or the like can be used.

[0081] In the preferable embodiment of the present application, the ceramic substrate and the aluminum nitride sintered body have a thermal conductivity of 170 to 220 W / mK, a fracture toughness of 4 to 8 MPa · m 1 / 2 in at least one fracture direction in the SEPB method, and a bending strength of 300 to 450 MPa. In addition, in the more preferable embodiment of the present application, the ceramic substrate and the aluminum nitride sintered body have a thermal conductivity of 175 to 183 W / mK, a fracture toughness of 4.2 to 5.8 MPa · m 1 / 2 in at least one fracture direction in the SEPB method, and a bending strength of 306 to 344 MPa.

[0082] The ceramic substrate preferably contains 3 mass% or less of oxygen, more preferably 2.8 mass% or less. The preferable range of the amount of oxygen contained in the ceramic substrate is also applicable to the amount of oxygen contained in the aluminum nitride sintered body of the present application. The amount of oxygen contained in the ceramic substrate and the amount of oxygen contained in the aluminum nitride sintered body are also preferably as low as possible. On the other hand, it is sometimes inevitable to contain oxygen due to manufacturing processes and the like. As a lower limit value in such a case, the ceramic substrate preferably contains 0.01 mass% or more of oxygen.

[0083] In the present specification, the amount of oxygen in the ceramic substrate is measured, for example, using a device "EMGA-920" manufactured by HORIBA Ltd. Specifically, the measurement of the amount of oxygen can be performed in the following manner. About 10 mg of a sample is filled into a nickel capsule and is put into the device. Then, the sample is decomposed by thermal decomposition, and oxygen is analyzed by reacting with carbon monoxide using a carbon catalyst. As a pretreatment, the ceramic substrate is pulverized using an alumina mortar or the like. After pulverization, a mesh screen with a mesh size of 77 μm is used to remove coarse particles, and then the measurement is performed.

[0084] The ceramic substrate preferably has a density of 3.26 to 3.31 g / cm 3 , more preferably 3.28 to 3.31 g / cm 3 . The preferable range of the density of the ceramic substrate is also applicable to the density of the aluminum nitride sintered body of the present application. In the present specification, the density is a value measured by a liquid displacement method in accordance with "JIS Z8807 Measurement Methods for Density and Specific Gravity of Solids".

[0085] The ceramic substrate preferably has a thickness of 0.1 to 1.5 mm. The preferable range of the thickness of the ceramic substrate is also applicable to the thickness of the aluminum nitride sintered body of the present application.

[0086] Next, a manufacturing method of the ceramic substrate will be described. The manufacturing method of the ceramic substrate generally includes a mixing step, a molding step, a debinding step, and a sintering step. In addition, in the manufacturing method of the ceramic substrate, a drying step can be performed after the mixing step, and a granulation step can be performed before the molding step. Also, the manufacturing method of the ceramic substrate described herein can also be said to be a manufacturing method of the aluminum nitride sintered body.

[0087] The mixing step is a step of mixing materials used for the manufacturing of the ceramic substrate to prepare a slurry. For example, fibrous aluminum nitride single crystals are put into and dispersed in a mixed solution of a dispersing material and an organic solvent. Thereafter, a sintering aid and particulate aluminum nitride as a base material are added and mixed. Thus, a slurry can be prepared.

[0088] The drying process, sometimes performed after the mixing process when the solvent content in the slurry is high, is a step to dry the solvent and adjust the aluminum nitride concentration in the slurry. There are no particular restrictions on the drying conditions of the slurry; for example, it can be carried out at a temperature of 130°C and a pressure of -0.1 MPa for a specified time, such as approximately one hour. When performing subsequent processes (e.g., granulation and forming processes), it is preferable that the amount of aluminum nitride contained in the slurry is 20–70% by mass.

[0089] Granulation is a process that is sometimes performed from the perspective of easy shaping. It is the process of granulating the slurry after the mixing process. Granulation can be carried out after the mixing process by methods such as spray drying, or after the drying process using a milling machine.

[0090] The forming process is the process of shaping the material used in the manufacture of ceramic substrates. In the forming process, slurries obtained through mixing or drying processes can be formed, as can granules obtained through granulation processes. Forming can employ forming methods suitable for the required shape, depending on the application and size, such as compression molding, cold isostatic pressing, slip casting, and tape casting. For example, slurries obtained through mixing are fed into a doctor blade tape casting machine for tape casting, thereby obtaining sheet-like molded bodies. A predetermined number of sheet-like molded bodies are stacked and pressed using a hot isostatic press, thereby obtaining plate-like molded bodies. At this time, fibrous aluminum nitride single crystals are oriented along the coating direction of the tape casting. When stacking sheet-like molded bodies, they can be stacked in the same direction to obtain plate-like molded bodies oriented in one direction. Alternatively, by stacking in any direction, plate-like molded bodies with disordered orientation in the horizontal direction can be obtained. Particles with a length-to-diameter ratio that grow from fibrous aluminum nitride single crystals are oriented in a direction perpendicular to the thickness direction of the ceramic substrate.

[0091] The degreasing process is a process of removing organic matter (such as dispersion materials) from the materials used in the manufacture of ceramic substrates. When dispersion materials are used in the manufacture of ceramic substrates, this process is typically performed after the molding process. The degreasing process is performed, for example, in a nitrogen atmosphere or an atmospheric atmosphere. The degreasing temperature is, for example, 400–650°C. The degreasing time is, for example, 4–24 hours.

[0092] The sintering step is a step of densifying the aluminum nitride (particulate aluminum nitride and fibrous aluminum nitride single crystal) by sintering to obtain an aluminum nitride sintered body. The sintering is preferably performed in a nitrogen atmosphere. In addition, as for the pressure at the time of sintering, although both normal pressure sintering and pressure sintering are possible, normal pressure sintering is preferable from the viewpoint of economy and the like. In the present application, in order to adjust the maximum short diameter of the aluminum nitride particles constituting the aluminum nitride sintered body to a range of 15.0 μm or less, it is important to set the sintering temperature to be relatively low and the sintering time to be relatively short. The sintering temperature is preferably 1800 to 1900°C, and more preferably 1800 to 1850°C. The sintering time is preferably 2 to 24 hours, and more preferably 3 to 15 hours.

[0093] Next, the power module of the present application will be described. The power module of the present application is a power module having the ceramic substrate of the present application described above, and one embodiment thereof is shown in Figure 3 .

[0094] Figure 3 is an example of a power module 1, for example, a module for performing power control or motor control of a light emitting diode, a laser diode, an electric automobile, an autonomous driving vehicle, a railway, a working machine, a data center, an industrial robot, solar power generation, wind power generation, an uninterruptible power supply (UPS), a semiconductor manufacturing device, and the like, having the ceramic substrate 10 of the present application. The ceramic substrate 10 is formed in a plate shape, and a metal layer 11 is provided on both surfaces in the plate thickness direction thereof. In addition, on one end surface of the ceramic substrate 10 (the upper surface in Figure 3 ), a semiconductor 12 of a so-called power system is provided. In addition, on the other end surface of the ceramic substrate 10 (the lower surface in Figure 3 ), a heat sink 13 having a heat dissipation function is provided.

[0095] As illustrated by an arrow H in Figure 3 , heat generated from the semiconductor 12 of the power system is transmitted to the heat sink 13 via the ceramic substrate 10, and thus heat dissipation of the power module 1 is performed.

[0096] Example

[0097] Hereinafter, the present application will be described in more detail by citing examples, but the present application is not limited to the following examples at all.

[0098] 1. Mixing step

[0099] In a granular aluminum nitride (Tokuyama H-grade), an appropriate amount of an organic solvent, a dispersing material, and an antifoaming material were mixed. To the granular aluminum nitride, a fibrous aluminum nitride single crystal (U-MAP short fiber broken type) was added. The median particle diameter of the short diameter of the granular aluminum nitride single crystal used was 1.0 μm, the median particle diameter of the long diameter was 1.0 μm, the ratio of the long diameter to the short diameter was 1, and the oxygen content was 1.1 mass%. The median particle diameter of the short diameter of the fibrous aluminum nitride single crystal used was 2.5 μm, the median particle diameter of the long diameter was 20 μm, the ratio of the long diameter to the short diameter was 8, and the oxygen content was 2.8 mass%. As a sintering aid, yttrium oxide (Japan Yttrium Co., Ltd. fine particle product 3N) was added. The kneading was performed for 24 hours. Thus, a slurry was obtained.

[0100] The ratio (mass%) of the fibrous aluminum nitride single crystal in the raw material and the ratio (mass%) of the sintering aid in the raw material for each test piece are shown in Table 1.

[0101] 2. Forming Process

[0102] The slurry obtained in the kneading process was formed into a sheet having a thickness of 100 μm by a flow casting. At this time, in order to orient the fibrous AlN single crystal in the formed sheet in the direction of forming of the sheet, a plurality of sheets cut to 50 mm square were alternately overlapped by turning the sheets by 90 degrees, so that the fibrous AlN single crystal was cross-oriented with respect to the planar direction of the sheet and stacked. By performing hot pressing on the obtained stack, a formed body having a thickness of 4.5 mm was produced.

[0103] 3. Debinding Process

[0104] The formed body obtained in the forming process was subjected to a debinding process. The formed body was arranged on a ceramic susceptor plate, and left in a furnace. After being heated at a temperature increasing rate of 40°C / h to 470°C in an atmosphere, it was held for 4 hours, and then cooled by furnace cooling. Thus, a debound body was obtained.

[0105] 4. Sintering Process

[0106] The debound body obtained in the debinding process was sintered by a reducing atmosphere furnace. The debound body was left on a jig of boron nitride provided in a furnace covered with carbon-based heat insulating material. On the debound body, a desired number of jigs of boron nitride and debound bodies were further provided. For the purpose of preventing warping of the debound body, a jig of boron nitride and an aluminum nitride substrate were provided as dead weights on the uppermost layer. After nitrogen replacement, the temperature was increased at 20°C / min, and after reaching a prescribed temperature (holding temperature), it was held for a prescribed time, and then cooled. Thus, a ceramic substrate was produced.

[0107] The holding temperature and the holding time for each test piece are shown in Table 1. Also, the holding temperature and the holding time are referred to as the sintering temperature and the sintering time.

[0108] 5. Properties of ceramic substrate

[0109] The ceramic substrate obtained via the sintering process exhibited a sintering texture on its surface, and thus was removed by cutting, grinding, polishing, or the like. The test piece (ceramic substrate) from which the sintering texture was removed was cut to the size required for the measurement method described above, and various measurements were performed. Regarding the fracture toughness, the measurement was performed in the direction in which the fibers orthogonal to the fracture direction were present, when measured by the SEPB method. The results are shown in Table 1.

[0110] [Table 1]

[0111]

[0112] In the table, "sintering aid addition amount (mass %)" indicates the proportion (mass %) of the sintering aid in the raw material, "fiber addition amount (mass %)" indicates the proportion (mass %) of the fibrous aluminum nitride single crystal in the raw material, "holding temperature (°C)" indicates the holding temperature (firing temperature) in the sintering process, and "holding time (time)" indicates the time (firing time) of holding at the holding temperature (firing temperature) in the sintering process.

[0113] In addition, "density (g / cm 3 )" in the table indicates the density of the ceramic substrate, and the measurement method is as described above. "Thermal conductivity (W / mK)" indicates the thermal conductivity of the ceramic substrate, and the measurement method is as described above. "Fracture toughness (MPa m 1 / 2 )" indicates the fracture toughness of the ceramic substrate, and the measurement method is as described above. Regarding the measurement direction of the fracture toughness, the measurement was performed from above with respect to one direction of the cross, and in parallel with the other direction, with respect to the fibers oriented in a cross in the forming process. "Bending strength (MPa)" indicates the bending strength of the ceramic substrate, and the measurement method is as described above. "Oxygen content (mass %)" indicates the oxygen content contained in the ceramic substrate, and the measurement method is as described above. "Maximum diameter of void (pm)" indicates the largest value among the longest distances of the respective voids present in the aluminum nitride sintered body constituting the ceramic substrate. "Maximum minor axis (pm)" indicates the maximum minor axis of the aluminum nitride particles constituting the sintered body constituting the ceramic substrate. The photographing method of the "microstructure image of the aluminum nitride sintered body" used to obtain the longest distances of the voids present in the aluminum nitride sintered body constituting the ceramic substrate and the maximum minor axis of the aluminum nitride particles constituting the sintered body is as described above. Figure 4 is a "microstructure image of the aluminum nitride sintered body" of Test Piece No. 2.

[0114] Sample No. 1 is a ceramic substrate manufactured without using fibrous aluminum nitride single crystals, and although the result is high in bending strength, the thermal conductivity and fracture toughness are low. Sample Nos. 2 to 8 are ceramic substrates composed of aluminum nitride sintered bodies formed by sintering fibrous aluminum nitride single crystals and particulate aluminum nitride, and exhibit excellent results in thermal conductivity and fracture toughness. Among these, Sample Nos. 2, 7, and 8, by adjusting the maximum short diameter of the aluminum nitride particles constituting the aluminum nitride sintered body to 15.0 μm, are also superior in bending strength compared to Sample Nos. 3 to 6.

[0115] Figure 5 A graph showing the value of "bending strength (MPa)" shown in Table 1 as the vertical axis and the value of "maximum short diameter (μm)" as the horizontal axis is a graph plotted from the measurement results of Sample Nos. 1 to 8. In each graph, "fiber addition amount (mass %)" and "holding time (time)" (i.e., sintering time) are depicted.

[0116] Figure 6 A graph showing the value of "void maximum diameter (μm)" shown in Table 1 as the vertical axis and the value of "bending strength (MPa)" as the horizontal axis is a graph plotted from the measurement results of Sample Nos. 1 to 8.

[0117] Next, a cold-heat cycle test (thermal shock test) was performed using a small cold-heat shock device TSE-12-A manufactured by ESPEC Corporation. The set values of the cold-heat cycle test (thermal shock test) were 150°C for the maximum temperature and -55°C for the minimum temperature, and the cold-heat cycle test (thermal shock test) was performed for 1200 cycles.

[0118] A sample in which copper was joined to both surfaces of the ceramic substrate in the plate thickness direction was prepared for the cold-heat cycle test. Figure 7 A schematic diagram and dimensions of the sample for the cold-heat cycle test are shown in the center. The corners of the copper foil were subjected to R processing with a radius of 0.5 mm. Also, the dimensions of the ceramic substrate and the copper, except for the thickness, are expressed in mm.

[0119] The ceramic substrate used was as follows.

[0120] 1. Ceramic substrate (thickness 0.635 mm) to which fibrous AlN single crystals were added

[0121] 2. AlN ceramic substrate (thickness 0.635 mm) manufactured by MARUWA Corporation

[0122] 3. SiN ceramic substrate (thickness 0.32 mm) manufactured by MARUWA Corporation

[0123] This paper illustrates a method for fabricating a ceramic substrate with added fibrous AlN single crystals. A slurry, obtained by mixing with particulate aluminum nitride, is granulated by spray drying. The resulting granulated powder is fed into a mold and pressed using a press to obtain a plate-shaped molded body. The obtained molded body is then subjected to cold isostatic pressing (CIP) at 200 MPa for 1 minute to obtain a shaped body. The obtained molded body is then subjected to a debinding and firing process using the same methods as described in "4. Debinding Process" and "5. Firing Process" above to obtain a ceramic substrate. In the obtained ceramic substrate, the maximum minor diameter of the aluminum nitride particles is 15.0 μm or less.

[0124] The results of the hot and cold cycling test showed that Figure 8 middle.

[0125] Figure 8 The "Invention / AlN / 0.635mm thickness" refers to the results of thermal cycling tests on a ceramic substrate with added fibrous AlN single crystals. Figure 8 The "existing product / AlN / 0.635mm thickness" refers to the results of thermal cycling tests using a sample (existing product A) made of AlN ceramic substrate manufactured by MARUWA Corporation. Figure 8 The "existing product / SiN / 0.32mm thickness" refers to the results of thermal cycling tests using a sample (existing product B) made of SiN ceramic substrate manufactured by MARUWA Corporation.

[0126] Figure 8 The document "Invention / AlN / 0.635mm thickness" shows ultrasonic flaw detection images (SAT images) of the specimen before testing, after 300 cycles, after 600 cycles, after 900 cycles, after 1200 cycles, and a photograph of the specimen after 1200 cycles. Figure 8 The section titled "Existing Product / AlN / 0.635mm Thickness" shows SAT images of the sample before testing, after 300 cycles, and a photograph of the sample after 600 cycles. Figure 8 The “Existing / SiN / 0.32mm thickness” section shows SAT images of the sample before testing, after 300 cycles, after 600 cycles, after 900 cycles, and after 1200 cycles.

[0127] exist Figure 8 In the image, the white areas in the SAT image, which is an image taken by an ultrasonic flaw detection device, indicate that the copper foil has peeled off from the ceramic substrate.

[0128] The ceramic substrate with added fibrous AlN single crystals (the present invention) did not break even after 1200 cycles of thermal cycling. On the other hand, the existing product A, which uses the same AlN ceramic substrate, broke after 600 cycles.

Claims

1. A ceramic substrate, characterized by, A ceramic substrate composed of an aluminum nitride sintered body made of fibrous aluminum nitride single crystals and particulate aluminum nitride, wherein the maximum short diameter of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a microstructure image of the sintered body.

2. The ceramic substrate of claim 1, wherein, The longest distance of a straight line connecting two points on the outer circumference of the voids present in the sintered body with respect to the size of each of the voids is 2.5 μm or less in a microstructure image of the sintered body.

3. The ceramic substrate of claim 1, wherein, The thermal conductivity of the ceramic substrate is 175 to 183 W / mK.

4. The ceramic substrate of claim 1, wherein, The ceramic substrate has a fracture toughness of 4.2 to 5.8 MPa·m in at least one fracture direction in the SEPB method 1 / 2 .

5. The ceramic substrate of claim 1, wherein, The bending strength of the ceramic substrate is 306 to 344 MPa.

6. The ceramic substrate of claim 1, wherein, The ceramic substrate has a thermal conductivity of 175 to 183 W / mK, a fracture toughness of 4.2 to 5.8 MPa·m in at least one fracture direction in the SEPB method, and a bending strength of 306 to 344 MPa. 1 / 2 , a bending strength of 306 to 344 MPa.

7. The ceramic substrate of claim 1, wherein, The oxygen content included in the ceramic substrate is 2.8 mass% or less.

8. A power module having the ceramic substrate according to any one of claims 1 to 7.

9. An aluminum nitride sintered body, characterized by comprising: An aluminum nitride sintered body made of fibrous aluminum nitride single crystals and particulate aluminum nitride, wherein the maximum short diameter of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a microstructure image of the sintered body.

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