Ceramic substrate, power module, and sintered aluminum nitride body

Abstract

To provide a ceramic substrate with improved bending strength in addition to thermal conductivity and fracture toughness.SOLUTION: A ceramic substrate consists of an aluminum nitride sintered body made by sintering a fiber-shaped single-crystal aluminum nitride body and particle-shaped aluminum nitride, wherein a 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.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a ceramic substrate, a power module, and an aluminum nitride sintered body, and more particularly to a ceramic substrate having excellent thermal conductivity, fracture toughness, and bending strength. [Background technology]

[0002] For example, control modules that perform power control and 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.

[0003] It is widely known that silicon nitride sintered bodies and aluminum nitride sintered bodies are used as ceramic substrates for such control modules. However, when silicon nitride sintered bodies are used for ceramic substrates, there is a problem that the ceramic substrate has low thermal conductivity. On the other hand, when aluminum nitride sintered bodies are used for ceramic substrates, there is a problem that the ceramic substrate has excellent thermal conductivity but low fracture toughness.

[0004] International Publication No. 2022 / 030637 (Patent Document 1) describes that for a ceramic substrate made of an aluminum nitride sintered body, by using a fibrous aluminum nitride single crystal, it is possible to provide a ceramic substrate that combines thermal conductivity and mechanical properties (fracture toughness). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 030637 Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors have conducted research into ceramic substrates using fibrous aluminum nitride single crystals and have found that although ceramic substrates using fibrous aluminum nitride single crystals have excellent fracture toughness, they tend to have low bending strength, and there is room for improvement in the mechanical strength of ceramic substrates.

[0007] Therefore, an object of the present invention is to provide a ceramic substrate having improved bending strength in addition to thermal conductivity and fracture toughness. Another object of the present invention is to provide a power module using such a ceramic substrate and an aluminum nitride sintered body suitable for such a ceramic substrate. [Means for solving the problem]

[0008] The present inventors first observed the structures of an aluminum nitride sintered body that did not use a fibrous aluminum nitride single crystal and an aluminum nitride sintered body that did use a fibrous aluminum nitride single crystal, and found that, despite being sintered under the same conditions, the aluminum nitride sintered body that used the fibrous aluminum nitride single crystal had a greater number of large grains in its structure image than the aluminum nitride sintered body that did not use a fibrous aluminum nitride single crystal.

[0009] In the growth process of aluminum nitride via a liquid phase during sintering, aluminum nitride particles grow by repeated dissolution and deposition. In this process, particles with a smaller radius of curvature are more likely to dissolve, while particles with a larger radius of curvature are more likely to grow (this phenomenon is known as Ostwald ripening). For this reason, when a material in which fibrous aluminum nitride single crystals are added to particulate aluminum nitride as a base material is sintered, the fibrous aluminum nitride single crystals selectively grow over time, and the small particles of particulate aluminum nitride are dissolved in the auxiliary phase, resulting in the appearance of many large particles (i.e. particles based on fibrous aluminum nitride single crystals) in the structure of the aluminum nitride sintered body.

[0010] Therefore, the present inventors have further studied and found that when an aluminum nitride sintered body is manufactured using a fibrous aluminum nitride single crystal and particulate aluminum nitride, the bending strength can be improved by adjusting the maximum short diameter of the aluminum nitride particles constituting the obtained sintered body to be within the range of 15.0 μm or less. Furthermore, the present inventors have found that even when the maximum short diameter of the aluminum nitride particles constituting the aluminum nitride sintered body is adjusted to be within the above-specified range, the thermal conductivity and fracture toughness are not affected, or even if they are affected, the degree of the effect is small, and high thermal conductivity and high fracture toughness can be maintained, and therefore it is possible to provide an aluminum nitride sintered body and a ceramic substrate excellent in thermal conductivity, fracture toughness and bending strength, and have completed the present invention.

[0011] Therefore, the ceramic substrate of the present invention is a ceramic substrate composed of an aluminum nitride sintered body obtained by sintering a fibrous aluminum nitride single crystal and particulate aluminum nitride, and is characterized in that the maximum short diameter of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a structural image of the sintered body.

[0012] In a preferred embodiment of the ceramic substrate of the present invention, for each size of void present in the sintered body, the longest distance of a straight line connecting two points on the periphery of the void in a structure image of the sintered body is 2.5 μm or less.

[0013] In another preferable embodiment of the ceramic substrate of the present invention, the thermal conductivity of the ceramic substrate is 175 to 183 W / mK.

[0014] In another preferable embodiment of the ceramic substrate of the present invention, the fracture toughness of the ceramic substrate is 4.2 to 5.8 MPa m in at least one fracture direction measured by the SEPB method. 1 / 2 Meet the following.

[0015] In another preferable embodiment of the ceramic substrate of the present invention, the bending strength of the ceramic substrate is 306 to 344 MPa.

[0016] In another preferable embodiment of the ceramic substrate of the present invention, the ceramic substrate has a thermal conductivity of 175 to 183 W / mK and a fracture toughness of 4.2 to 5.8 MPa m in at least one fracture direction measured by a SEPB method. 1 / 2 and the bending strength is 306 to 344 MPa.

[0017] In another preferable embodiment of the ceramic substrate of the present invention, the amount of oxygen contained in the ceramic substrate is 2.8 mass % or less.

[0018] A power module of the present invention is a power module including the above-mentioned ceramic substrate of the present invention.

[0019] The aluminum nitride sintered body of the present invention is an aluminum nitride sintered body obtained by sintering a fibrous aluminum nitride single crystal and particulate aluminum nitride, and is characterized in that the maximum short diameter of the aluminum nitride particles constituting the sintered body is 15.0 μm or less in a structural image of the sintered body. Effect of the Invention

[0020] According to the ceramic substrate of the present invention, it is possible to provide a ceramic substrate having excellent thermal conductivity, fracture toughness, and bending strength. Also, according to the power module of the present invention, it is possible to provide a power module using such a ceramic base material. Also, according to the aluminum nitride sintered body of the present invention, it is possible to provide an aluminum nitride sintered body suitable for such a ceramic substrate. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a perspective view showing a schematic crystal structure of a fibrous aluminum nitride single crystal. [Diagram 2]FIG. 1 is a diagram illustrating an example of the configuration of an apparatus for performing X-ray diffraction on a ceramic substrate. [Diagram 3] FIG. 2 is a diagram illustrating an example of the configuration of a power module. [Figure 4] This is a "structural image of aluminum nitride sintered body" of sample No. 2. [Diagram 5] FIG. 1 shows a graph in which the "flexural strength (MPa)" values ​​shown in Table 1 are plotted on the vertical axis and the "maximum minor axis (μm)" values ​​shown on the horizontal axis, and the measurement results of samples No. 1 to No. 8 are plotted. [Figure 6] 1 is a graph showing the "maximum void diameter (μm)" values ​​shown in Table 1 on the vertical axis and the "flexural strength (MPa)" values ​​on the horizontal axis, in which the measurement results of samples No. 1 to No. 8 are plotted. [Figure 7] 1 shows a schematic diagram and dimensions of a sample for thermal cycle testing. [Figure 8] The results of the thermal cycle test are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention relates to a ceramic substrate, a power module including the ceramic substrate, and an aluminum nitride sintered body suitable for the ceramic substrate.

[0023] In this specification, the ceramic substrate is a substrate made of a sintered body of an inorganic compound such as aluminum nitride, silicon nitride, alumina, alumina zirconium, beryllium oxide, etc. Practical ceramic substrates are required to have good thermal conductivity, insulation, heat resistance, mechanical strength, etc. depending on their applications. Ceramic substrates are mainly used as power modules.

[0024] In this specification, a power module refers to a module used for power control or motor control of light-emitting diodes, laser diodes, electric vehicles, self-driving cars, railways, machine tools, data centers, industrial robots, solar power generation, wind power generation, uninterruptible power supplies (UPS), semiconductor manufacturing equipment, and the like.

[0025] In this specification, the aluminum nitride sintered body is a material obtained by adding a sintering aid to powdered aluminum nitride and sintering it to bond and make it dense, and in the present invention, the above-mentioned raw materials are added to the fibrous aluminum nitride single crystal and sintered. Aluminum nitride is sometimes expressed as AlN.

[0026] The ceramic substrate of the present invention 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 body and particulate aluminum nitride. By adding a fibrous aluminum nitride single crystal body to particulate aluminum nitride, it is possible to achieve both high thermal conductivity and high fracture toughness in the obtained aluminum nitride sintered body and the ceramic substrate made of the aluminum nitride sintered body. In this specification, this aluminum nitride sintered body is also referred to as the "aluminum nitride sintered body of the present invention." The aluminum nitride sintered body of the present invention is suitable for a ceramic substrate, but can also be used, for example, as a member for semiconductor manufacturing equipment, a ceramic heater, an electrostatic chuck, a Peltier element, etc.

[0027] The fibrous aluminum nitride single crystal can also be expressed as a fibrous aluminum nitride single crystal. In other words, the term "fibrous" means that the AlN single crystal extends in a fibrous shape. Note that the fibrous shape may be linear as long as it is fibrous, or may be partially curved or bent.

[0028] 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 determined from the ratio of the median major axis to the median minor axis (median major axis / median minor axis) of the fibrous aluminum nitride single crystal described below.

[0029] The median major axis of the fibrous aluminum nitride single crystal is preferably 5 to 100 μm, more preferably 10 to 50 μm, and the median minor axis of the fibrous aluminum nitride single crystal is preferably 1 to 3 μm, more preferably 1 to 2 μm.

[0030] In this specification, the median diameter of the long axis and the median diameter of the short axis of the fibrous aluminum nitride single crystal can be determined from a microscopic image obtained by an optical microscope. Specifically, at least 5,000 fibrous aluminum nitride single crystals are selected from the microscopic image, the maximum length of each fibrous aluminum nitride single crystal is taken as the long axis, and the maximum length in the direction perpendicular to this long axis is taken as the short axis. A cumulative distribution is created by volume conversion from the obtained values, and the diameters (median diameters) at which the cumulative values ​​for each of the long axis and the short axis are 50% are taken as the median diameter of the long axis and the median diameter of the short axis 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 to measure the median diameter of the long axis and the median diameter of the short axis of the fibrous aluminum nitride single crystal.

[0031] As shown in FIG. 1, the crystalline structure of the fibrous aluminum nitride single crystal is a so-called hexagonal wurtzite structure. 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 "planes perpendicular 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 as the "c-plane".

[0032] The orientation of the particles based on the fibrous aluminum nitride single crystal that constitutes the aluminum nitride sintered body in the ceramic substrate (i.e., particles having an aspect ratio that has grown from a fibrous aluminum nitride single crystal as a nucleus) is not particularly limited, and the particles based on the fibrous aluminum nitride single crystal that constitutes the aluminum nitride sintered body may, for example, be oriented along the thickness direction of the ceramic substrate, or along a direction perpendicular to the thickness direction, or a mixture of particles oriented along the thickness direction of the ceramic substrate and particles oriented along a direction perpendicular to the thickness direction.

[0033] Since the fibrous aluminum nitride single crystal maintains a hexagonal wurtzite structure even after sintering (i.e., the particles based on the fibrous aluminum nitride single crystal that make up the aluminum nitride sintered body also have a hexagonal wurtzite structure), the orientation of the particles based on the fibrous aluminum nitride single crystal within the ceramic substrate can be confirmed by performing X-ray diffraction on the ceramic substrate.

[0034] 2 is a schematic diagram showing an example of the configuration of an apparatus for performing X-ray diffraction on a ceramic substrate. 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 an object to be measured, in this case, an end face in the plate thickness direction of the ceramic substrate 10 (i.e., a face along a direction perpendicular to the plate thickness direction). 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 in the detector 104.

[0035] In X-ray diffraction using such an X-ray diffraction device 100, the angle 2θ of the detector 104 relative to the direction of X-ray irradiation on the measurement object is changed within a predetermined range, for example, from 20 degrees to 80 degrees, to obtain diffraction peaks indicating each face of the hexagonal wurtzite structure, i.e., each face such as the "a-face" and the "c-face." The peak intensity obtained by X-ray diffraction is also the maximum count number of each face of the hexagonal wurtzite structure in the ceramic substrate, i.e., the number of faces present.

[0036] As described in International Publication No. 2022 / 030637, the peak of the detection value indicating the "a-plane" of the X-ray diffraction pattern obtained when the end face in the plate thickness direction of the ceramic substrate 10 is irradiated with X-rays, i.e., the (10-10) plane, is detected when the angle of the detector 104 is about 33.21 degrees. However, the peak of the detection value indicating the "a-plane" or (10-10) plane may be detected when the angle of the detector 104 is slightly different from about 33.21 degrees, for example, due to the shape of the sample or the positional relationship of the device.

[0037] Furthermore, the peak of the detection value 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 plate thickness direction of ceramic substrate 10 is detected when the angle of detector 104 is about 36.04 degrees. However, the peak of the detection value indicating the "c-plane" or (0002) plane may also be detected when the angle of detector 104 is slightly different from about 36.04 degrees due to, for example, the shape of the sample or the positional relationship of the apparatus.

[0038] Furthermore, when the X-ray diffraction pattern obtained when an end face in the thickness direction of a ceramic substrate is irradiated with X-rays shows a large peak intensity ratio indicating the "a-plane" or (10-10) plane and a small peak intensity ratio indicating the "c-plane" or (0002) plane, compared to the X-ray diffraction pattern of a ceramic substrate that does not use fibrous aluminum nitride single crystals, it can be said that the particles based on fibrous aluminum nitride single crystals in the ceramic substrate are oriented along a direction perpendicular to the thickness direction of the substrate.

[0039] Furthermore, 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 of a ceramic substrate (a / c value) indicates that the higher the value, the stronger the orientation of the particles based on fibrous aluminum nitride single crystals contained in the ceramic substrate toward the end face in the plate thickness direction, or the greater the amount of particles based on fibrous aluminum nitride single crystals contained in the ceramic substrate.

[0040] The X-ray diffraction is performed by the well-known θ-2θ method, and for example, an apparatus "Ultima IV" manufactured by Rigaku can be used. The conditions for performing the X-ray diffraction are 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 degrees to 80 degrees. The peak positions of the X-ray diffraction pattern can be determined based on the X-ray spectrum of AlN in the inorganic material database "AtomWork" of the National Institute for Materials Science (NIMS). The peak intensity of the X-ray diffraction pattern is the maximum count number of the peak.

[0041] It is preferable that at least a part of the surface of the fibrous aluminum nitride single crystal is covered with an oxygen-containing layer. By covering 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 the AlN single crystal incorporating 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. From the viewpoint of improving water resistance, it is preferable that the oxygen-containing layer contains AlON.

[0042] In this specification, fibrous aluminum nitride single crystal bodies whose surfaces are covered with an oxygen-containing layer are referred to as "aluminum nitride whiskers" or "AlN whiskers".

[0043] The amount of oxygen contained in the aluminum nitride whiskers is preferably 3.0% by mass or less, more preferably 2.0% by mass or less, and most preferably 1.0% by mass or less. The lower the amount of oxygen contained in the aluminum nitride whiskers, the better. On the other hand, oxygen may be unavoidably contained due to the manufacturing process, etc., and in that case, the lower limit of the amount of oxygen contained in the aluminum nitride whiskers is preferably 0.01% by mass or more from the viewpoint of manufacturing costs, etc.

[0044] The amount of oxygen contained in aluminum nitride whiskers can be measured using, for example, an "EMGA-920" device manufactured by HORIBA. Specifically, the amount of oxygen can be measured as follows. Approximately 10 mg of a sample is filled into a nickel capsule and placed in the device. The sample is then decomposed by pyrolysis, and the oxygen is reacted with a carbon catalyst to produce carbon monoxide, which is then analyzed.

[0045] Methods for producing fibrous aluminum nitride single crystals and aluminum nitride whiskers are already known and are described in detail in, for example, JP 2018-154534 A.

[0046] The amount of fibrous aluminum nitride single crystal used in the production of an aluminum nitride sintered body is preferably 1 to 30 parts by mass, and more preferably 3 to 5 parts by mass, per 100 parts by mass of particulate aluminum nitride as the base material.

[0047] Particulate aluminum nitride is aluminum nitride used as a base material for sintered aluminum nitride, and has a small aspect ratio of 1.1 or less. The aspect ratio is calculated from the ratio of the median major axis diameter to the median minor axis diameter of the particulate aluminum nitride (median major axis diameter / median minor axis diameter).

[0048] The median major axis and minor axis of the particulate aluminum nitride are both preferably within a range of 0.3 to 5 μm, and more preferably within a range of 0.5 to 3 μm.

[0049] In this specification, the median diameter of the long axis and the median diameter of the short axis of particulate aluminum nitride can be determined from a microscopic image obtained by an optical microscope. Specifically, at least 5,000 particulate aluminum nitride are selected from the microscopic image, and in the case of circular particulate aluminum nitride in the microscopic image, the diameter is taken as the long axis and the short axis, and in the case of particulate aluminum nitride having a shape other than circular, the maximum length of the particulate aluminum nitride is taken as the long axis, and the maximum length in the direction perpendicular to the long axis is taken as the short axis. A cumulative distribution is created from the obtained values ​​by volume conversion, and the diameter (median diameter) of the value at which the cumulative amount of each of the long axis and the short axis is 50% is taken as the median diameter of the long axis and the median diameter of the short axis. For example, a particle shape image analyzer "PITA-04" manufactured by Seishin Enterprise Co., Ltd. can be used to measure the median diameter of the long axis and the median diameter of the short axis of particulate aluminum nitride.

[0050] The amount of oxygen contained in the particulate aluminum nitride is preferably 2.0 mass% or less, more preferably 1.0 mass% or less, and most preferably 0.5 mass% or less. The lower the amount of oxygen contained in the particulate aluminum nitride, the better. On the other hand, oxygen may be unavoidably contained due to the manufacturing process, etc., and in such a case, the lower limit of the amount of oxygen contained in the particulate aluminum nitride is preferably 0.01 mass% or more. The amount of oxygen contained in the particulate aluminum nitride can be measured in the same manner as in "Measurement of the amount of oxygen contained in aluminum nitride whiskers" described above.

[0051] Materials used in the production of aluminum nitride sintered bodies include, in addition to aluminum nitride, other components such as sintering aids, dispersants, antifoaming agents, organic solvents, and water, which can be used appropriately depending on the purpose.

[0052] Sintering aids are often used in the production of aluminum nitride sintered bodies. Examples of sintering aids include oxides of rare earth elements, oxides of alkaline earth elements, carbonates, oxalates, nitrates, etc. Among these, oxides of rare earth elements are preferred, and yttria (Y2O3) is particularly preferred.

[0053] 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, per 100 parts by mass of particulate aluminum nitride as the base material.

[0054] 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, preferably 13.0 μm or less, and more preferably 12.0 μm or less in the structure image of the sintered body. According to the present invention, by setting the maximum short diameter of the aluminum nitride particles constituting the sintered body within the above-specified range, the bending strength of the aluminum nitride sintered body and the ceramic substrate made 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 production and handling, the lower limit is preferably 1.0 μm or more. In addition, the thermal conductivity and fracture toughness of the aluminum nitride sintered body in which the maximum short diameter of the aluminum nitride particles is within the above-specified range are approximately the same as those of the aluminum nitride sintered body in which the maximum short diameter of the aluminum nitride particles exceeds 15.0 μm. Therefore, according to the aluminum nitride sintered body of the present invention and the ceramic substrate of the present invention, it is possible to provide an aluminum nitride sintered body and a ceramic substrate excellent in thermal conductivity, fracture toughness, and bending strength.

[0055] The reason why the aluminum nitride sintered body of the present invention has excellent bending strength and can obtain the same thermal conductivity as the conventional aluminum nitride sintered body is thought to be that the amount of oxygen in the grains contributes greatly to the thermal conductivity. As the sintering proceeds, the grains grow and the amount of oxygen in the grains is simultaneously discharged to the outside of the system. As a result, the maximum short diameter is generally large, and the sintering time is long and the oxygen in the grains is sufficiently discharged, so that the thermal conductivity is high. In other words, it is thought that the maximum short diameter and the thermal conductivity show a spurious correlation. The aluminum nitride sintered body of the present invention is made by adding a fibrous aluminum nitride single crystal having a large radius of curvature of the grains to the particulate aluminum nitride base material, and therefore the fibrous aluminum nitride single crystal selectively grows in a short time over time. As a result, the rate of discharge of the amount of oxygen in the grains increases, thereby improving the thermal conductivity while suppressing grain growth.

[0056] The aluminum nitride sintered body of the present invention has fracture toughness comparable to that of a conventional aluminum nitride sintered body having a large maximum minor axis of the aluminum nitride grains. Generally, in a ceramic body consisting of a single structure, the larger the grain size, the higher the fracture toughness. Fracture toughness is a parameter that indicates how difficult it is for a crack to propagate. Cracks in aluminum nitride sintered bodies progress along grain boundaries. At this time, the larger the grain, the more the cracks detour around the grains, resulting in higher fracture toughness. Since the aluminum nitride sintered body of the present invention contains fibrous aluminum nitride single crystals, detours of cracks occur frequently (bridging effect). This allows high fracture toughness to be obtained regardless of grain size.

[0057] Here, the aluminum nitride particles that make up the aluminum nitride sintered body refer to aluminum nitride particles formed through a growth process during sintering, and particles having an aspect ratio that results from grain growth using fibrous aluminum nitride single crystals as nuclei are mixed in with the granular aluminum nitride base material that has grown through repeated dissolution and deposition.

[0058] In this specification, the "structure image of the aluminum nitride sintered body" used to determine the maximum short diameter of the aluminum nitride particles constituting the aluminum nitride sintered body and further the size of the voids present in the aluminum nitride sintered body described later is an electron microscope image taken using a scanning electron microscope (SEM). In order to distinguish between particles, a backscattered electron image (BSE image) in which a contrast difference appears depending on the orientation direction of the crystal grains is taken. It is assumed that at least 200 aluminum nitride particles are photographed in the structure image of the aluminum nitride sintered body. Since the aluminum nitride sintered body has a homogeneous structure to a certain extent, the structure of the aluminum nitride sintered body can be represented by a structure image of a specific area in which at least 200 aluminum nitride particles are photographed. Therefore, by observing such a structure image in one field of view, it is possible to determine whether or not it meets the constituent requirements of the present invention. Note that, depending on the manufacturing method, etc., the aluminum nitride sintered body may inevitably have a small amount of coarse aluminum nitride particles and coarse voids, but even if there are a small amount of such coarse particles and coarse voids, it does not affect the properties of the sintered body. In this specification, the images were taken under the conditions of an accelerating voltage of 3 to 5 KeV, a current of 20 μA, and a working distance of 4 mm. In order to prevent the unevenness of the sample surface from being detected as contrast, the sample surface is polished and then processed to approximately Ra < 0.2 μm using ion milling or the like. Note that Ra is the arithmetic mean roughness, and can be measured in accordance with JIS B0601 (2013).

[0059] In this specification, the short diameter of an aluminum nitride particle constituting an aluminum nitride sintered body is the maximum length in a direction perpendicular to the long diameter when the long diameter is the maximum length of a straight line connecting two points on the periphery of the aluminum nitride particle, and in the case of a circular aluminum nitride particle, the short diameter is the diameter of the circular aluminum nitride particle. The maximum short diameter of an aluminum nitride particle constituting an aluminum nitride sintered body refers to the largest value among the short diameters of all aluminum nitride particles in a structure image of the aluminum nitride sintered body.

[0060] In the present invention, the means for adjusting the maximum minor axis of the aluminum nitride particles constituting the aluminum nitride sintered body to within the range of 15.0 μm or less includes a method of adjusting the sintering temperature and sintering time in the sintering process. Specifically, by setting the sintering temperature and sintering time to be relatively low and relatively short for the fibrous aluminum nitride single crystal body and particulate aluminum nitride, an aluminum nitride sintered body composed of aluminum nitride particles having a small maximum minor axis within the range of 15.0 μm or less can be obtained. In addition, by using fine metal as a raw material, it is possible to adjust the maximum minor axis by the pinning effect of grain growth.

[0061] A gap may occur between aluminum nitride particles constituting the aluminum nitride sintered body. In the present invention, for each size of the gap present in the aluminum nitride sintered body, the longest distance of a straight line connecting two points on the periphery of the gap in the structure image of the sintered body is preferably 2.5 μm or less, more preferably 1 μm or less. Since the gap present in the aluminum nitride sintered body can be a starting point of destruction, it is preferable that no large gap exists. In addition, even if a gap having a large size only in a certain direction (for example, a linearly extending gap, etc.) exists, it can be a starting point of destruction of the aluminum nitride sintered body, so it is preferable to control the longest distance of a straight line connecting two points on the periphery of the gap in the structure image of the aluminum nitride sintered body. According to the present invention, since the grain size of the aluminum nitride particles constituting the aluminum nitride sintered body is small, the gap that may occur between the aluminum nitride particles can be made small. When there is a manufacturing difficulty in minimizing the gap, the lower limit of the longest distance of a straight line connecting two points on the periphery of the gap in the structure image of the sintered body is preferably 0.1 μm or more.

[0062] 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 particularly preferably 175 to 183 W / mK. The preferred range of the thermal conductivity of the ceramic substrate also applies to the thermal conductivity of the aluminum nitride sintered body of the present invention. Although the higher the thermal conductivity, the better. From the viewpoint of the large increase in manufacturing costs due to the change in conditions required for high thermal conductivity, the thermal conductivity is desirably 220 W / mK or less.

[0063] In this 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", using, for example, a Kyoto Electronics Manufacturing Co., Ltd. TPS-2500 device.

[0064] In this specification, the fracture toughness is measured by the SEPB method in accordance with "JIS R1607 Test method for room temperature fracture toughness (toughness) of fine ceramics", and for example, 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, MEASURESCOPE10 manufactured by Nikon Corporation, or the like can be used.

[0065] The fracture toughness of the ceramic substrate of the present invention depends on the orientation direction when grains with an aspect ratio that have grown from fibrous AlN single crystals as nuclei (hereinafter simply referred to as "fibrous grains") are oriented in-plane. For example, when two types of samples differing only in the orientation direction of the fibrous grains were measured by the SEPB method, the fracture toughness of the sample in which the fibrous grains were oriented parallel to the fracture direction in the SEPB method was 4.1 MPa m 1 / 2The fracture toughness of the specimen with fibrous particles oriented in a direction perpendicular to the fracture direction in the SEPB method was 6.1 MPa m 1 / 2 Generally, samples with fibrous particles oriented in a direction intersecting the fracture direction tended to have high fracture toughness. The "fracture direction in the SEPB method" refers to the thickness direction of the sample from the surface where the crosshead is placed during measurement, and indicates the direction in which the crack propagates.

[0066] The fracture toughness of the ceramic substrate is 2.5 to 10.0 MPa m at least in one fracture direction in the SEPB method. 1 / 2 It is preferable that the above condition is satisfied, and the range is 3.5 to 7.0 MPa m 1 / 2 More preferably, it satisfies 4.2 to 5.8 MPa m 1 / 2 It is more preferable that the fracture toughness of the ceramic substrate satisfies the following range: The fracture toughness of the aluminum nitride sintered body of the present invention is also preferably within this range.

[0067] 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. This preferred range of the bending strength of the ceramic substrate also applies to the bending strength of the aluminum nitride sintered body of the present invention.

[0068] In this specification, the bending strength is measured by the SEPB method in accordance with "JIS R1601, Room Temperature Bending Strength Test Method for Fine Ceramics", and for example, an Autograph AGX-10KNVV manufactured by Shimadzu Corporation can be used.

[0069] In a preferred embodiment of the present invention, the ceramic substrate and the aluminum nitride sintered body have a thermal conductivity of 170 to 220 W / mK and a fracture toughness of 4 to 8 MPa m in at least one fracture direction measured by the SEPB method. 1 / 2and the bending strength is 300 to 450 MPa. In a further preferred embodiment of the present invention, the ceramic substrate and the aluminum nitride sintered body have a thermal conductivity of 175 to 183 W / mK and a fracture toughness of 4.2 to 5.8 MPa·m in at least one fracture direction measured by the SEPB method. 1 / 2 and the bending strength is 306 to 344 MPa.

[0070] The amount of oxygen contained in the ceramic substrate is preferably 3% by mass or less, and more preferably 2.8% by mass or less. The preferable range of the amount of oxygen contained in the ceramic substrate also applies to the amount of oxygen contained in the aluminum nitride sintered body of the present invention. The lower the amount of oxygen contained in the ceramic substrate and the amount of oxygen contained in the aluminum nitride sintered body, the better. On the other hand, oxygen may be unavoidably contained due to the manufacturing process, etc., and in that case, the lower limit of the amount of oxygen contained in the ceramic substrate is preferably 0.01% by mass or more.

[0071] In this specification, the amount of oxygen in the ceramic substrate can be measured using, for example, an apparatus "EMGA-920" manufactured by HORIBA. Specifically, the amount of oxygen can be measured as follows. Approximately 10 mg of a sample is filled into a nickel capsule and placed in the apparatus. The sample is then decomposed by pyrolysis, and the oxygen is reacted with a carbon catalyst to produce carbon monoxide for analysis. As a pretreatment, the ceramic substrate is pulverized using an alumina mortar or the like. After pulverization, a mesh with a mesh size of 77 μm is used to remove coarse particles before measurement.

[0072] The density of the ceramic substrate is 3.26 to 3.31 g / cm 3 and preferably 3.28 to 3.31 g / cm 3 It is more preferable that the density of the ceramic substrate is in the range of 100 to 2000 nm. 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 invention. In this specification, the density is a value measured by a liquid weighing method in accordance with "JIS Z8807 Measurement method for density and specific gravity of solids".

[0073] The ceramic substrate preferably has a thickness of 0.1 to 1.5 mm. This preferred range of the thickness of the ceramic substrate also applies to the thickness of the aluminum nitride sintered body of the present invention.

[0074] Next, a method for manufacturing a ceramic substrate will be described. The method for manufacturing a ceramic substrate generally includes a kneading step, a molding step, a degreasing step, and a sintering step. In the method for manufacturing a ceramic substrate, a drying step may be performed after the kneading step, or a granulation step may be performed before the molding step. The method for manufacturing a ceramic substrate described here can also be said to be a method for manufacturing an aluminum nitride sintered body.

[0075] The kneading process is a process in which materials used in the manufacture of the ceramic substrate are mixed to prepare a slurry. For example, fibrous aluminum nitride single crystals are put into a mixture of a dispersing agent and an organic solvent and dispersed. Then, a sintering aid and particulate aluminum nitride, which is the base material, are added and kneaded. In this way, the slurry can be prepared.

[0076] The drying step is a step that may be performed after the kneading step when the amount of solvent in the slurry is large, and is a step for adjusting the concentration of aluminum nitride in the slurry by drying the solvent in the slurry. The drying conditions for the slurry are not particularly limited, but for example, the drying is performed under conditions of a temperature of 130°C and a pressure of -0.1 MPa for a predetermined time, for example, about 1 hour. When performing the subsequent steps (for example, a granulation step or a molding step), the amount of aluminum nitride contained in the slurry is preferably 20 to 70 mass%.

[0077] The granulation step is a step that may be performed from the viewpoint of facilitating molding, and is a step of granulating the slurry after the kneading step. The granulation step may be performed by a method such as spray drying after the kneading step, or may be performed using a pod mill or the like after the drying step.

[0078] The forming process is a process for forming the material used in the manufacture of the ceramic substrate. In the forming process, the slurry obtained in the kneading process or the drying process may be formed, or the granulated material obtained in the granulation process may be formed. For forming, a forming means suitable for the shape required depending on the application and size, such as a die press, a rubber press, a casting molding, or a sheet molding, may be adopted. For example, the slurry obtained in the kneading process is put into a doctor blade type sheet forming machine and a sheet molding is performed to obtain a sheet-like molded body. A plate-like molded body can be obtained by stacking a predetermined number of sheet-like molded bodies and pressing them with a hot isostatic press. At this time, the fibrous aluminum nitride single crystal is oriented along the coating direction of the sheet molding. When stacking the sheet-like molded bodies, a plate-like molded body oriented in one direction can be obtained by stacking them in the same direction. In addition, a plate-like molded body oriented randomly in the horizontal direction can be obtained by stacking them in any direction. Particles having an aspect ratio that have grown from the fibrous aluminum nitride single crystal as a nucleus become oriented along a direction perpendicular to the plate thickness direction of the ceramic substrate.

[0079] The degreasing step is a step of removing organic substances (dispersing materials, etc.) from the materials used to manufacture the ceramic substrate, and when a dispersing material, etc. is used in the manufacture of the ceramic substrate, it is usually carried out after the molding step. The degreasing step is carried out, for example, in a nitrogen atmosphere or an air atmosphere. The degreasing temperature is, for example, 400 to 650°C. The degreasing time is, for example, 4 to 24 hours.

[0080] The sintering step is a step of sintering powdered aluminum nitride (particulate aluminum nitride and fibrous aluminum nitride single crystal) to bind and densify the aluminum nitride sintered body. The sintering is preferably performed in a nitrogen atmosphere. The pressure during the sintering may be either normal pressure sintering or pressurized sintering, but normal pressure sintering is preferable from the viewpoint of economy and the like. In the present invention, in order to adjust the maximum minor axis of the aluminum nitride particles constituting the aluminum nitride sintered body to within a range of 15.0 μm or less, it is important to set the sintering temperature relatively low and the sintering time relatively short. The sintering temperature is preferably 1800 to 1900° C., more preferably 1800 to 1850° C. The sintering time is preferably 2 to 24 hours, more preferably 3 to 15 hours.

[0081] Next, the power module of the present invention will be described. The power module of the present invention is a power module including the above-mentioned ceramic substrate of the present invention, and one embodiment thereof is shown in FIG.

[0082] The power module 1 illustrated in Fig. 3 is an example of a control module for controlling power and motors of, for example, light-emitting diodes, laser diodes, electric vehicles, self-driving cars, railways, machine tools, data centers, industrial robots, solar power generation, wind power generation, uninterruptible power supplies (UPS), semiconductor manufacturing equipment, etc., and includes a ceramic substrate 10 of the present invention. The ceramic substrate 10 is formed in a plate shape, and metal layers 11 are provided on both sides in the plate thickness direction. A so-called power semiconductor 12 is provided on one end surface in the plate thickness direction of the ceramic substrate 10 (the upper surface in Fig. 3). A heat sink 13 having a heat dissipation function is provided on the other end surface in the plate thickness direction of the ceramic substrate 10 (the lower surface in Fig. 3).

[0083] As shown by an arrow H in FIG. 3, heat generated from the power semiconductor 12 is transferred to the heat sink 13 via the ceramic substrate 10, thereby dissipating heat from the power module 1. EXAMPLES

[0084] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0085] 1. Mixing process Particulate aluminum nitride (H grade, manufactured by Tokuyama) was mixed with an appropriate amount of organic solvent, dispersant, and defoamer, and fibrous aluminum nitride single crystal (short fiber crushed type, manufactured by U-MAP) was added to the particulate aluminum nitride. The particulate aluminum nitride single crystal used had a median short and long diameter of 1.0 μm, an aspect ratio of 1, and an oxygen content of 1.1 mass%. The fibrous aluminum nitride single crystal used had a median short diameter of 2.5 μm, a median long diameter of 20 μm, an aspect ratio of 8, and an oxygen content of 2.8 mass%. Yttrium oxide (fine grain 3N, manufactured by Nippon Yttrium) was added as a sintering aid. The mixture was kneaded for 24 hours using a ball. This resulted in a slurry. Table 1 shows the proportion (mass %) of fibrous aluminum nitride single crystals in the raw materials and the proportion (mass %) of sintering aid in the raw materials for each sample.

[0086] 2. Molding process The slurry obtained in the kneading process was formed into a sheet with a thickness of 100 μm by sheet forming. At this time, since the fibrous AlN single crystals are oriented in the forming direction of the sheet in the formed sheet, the sheets cut into 50 mm squares were rotated 90 degrees and stacked alternately to laminate the fibrous AlN single crystals so that they were oriented crosswise with respect to the planar direction of the sheet. The obtained laminate was hot pressed to produce a molded body with a thickness of 4.5 mm.

[0087] 3. Degreasing process The green bodies obtained in the molding process were degreased. The green bodies were arranged on a ceramic setter and placed in a furnace. The temperature was raised to 470°C at a heating rate of 40°C / h in an air atmosphere, and then the temperature was held for 4 hours and then cooled by furnace cooling. A degreased body was obtained.

[0088] 4. Sintering process The degreased body obtained in the degreasing process was fired in a reducing atmosphere furnace. A boron nitride jig was placed in a furnace covered with carbon-based insulation material, and the degreased body was placed on top of it. The required number of boron nitride jigs and degreased bodies were placed on top of the degreased body. In order to prevent the degreased body from warping, a boron nitride jig and an aluminum nitride substrate were placed on the top stage as weights. After nitrogen replacement, the temperature was raised at 20°C / min, and after reaching the specified temperature (holding temperature), it was held for the specified time and then cooled. This produced a ceramic substrate. The holding temperature and holding time for each sample are shown in Table 1. The holding temperature and holding time here are referred to as the firing temperature and firing time.

[0089] 5. Characteristics of ceramic substrates The ceramic substrate obtained by the sintering process had a burnt surface, which was removed by cutting, polishing, grinding, etc. After the burnt surface was removed, the sample (ceramic substrate) was cut to the size required for the measurement method described above and various measurements were performed. When measuring fracture toughness using the SEPB method, measurements were performed in the direction where fibers exist perpendicular to the fracture direction. The results are shown in Table 1.

[0090] [Table 1]

[0091] In the table, "amount of sintering aid added (mass %)" indicates the proportion (mass %) of sintering aid in the raw materials, "amount of fiber added (mass %)" indicates the proportion (mass %) of fibrous aluminum nitride single crystals in the raw materials, "holding temperature (°C)" indicates the holding temperature (firing temperature) in the sintering process, and "holding time (hours)" indicates the time (firing time) during which the holding temperature (firing temperature) in the sintering process was held. In addition, the density (g / cm 3"Density (MPa m)" 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 thereof is as described above. The measurement direction of the fracture toughness was measured so that the fibers were oriented in a cross shape in the molding process, facing one direction of the cross from above, and parallel to the remaining direction. "Bending strength (MPa)" indicates the bending strength of the ceramic substrate, and the measurement method thereof is as described above. "Oxygen content (mass%)" indicates the amount of oxygen contained in the ceramic substrate, and the measurement method thereof is as described above. "Maximum void diameter (μm)" indicates the largest value among the longest distances of each void present in the aluminum nitride sintered body constituting the ceramic substrate. "Maximum short diameter (μm)" indicates the maximum short diameter of the aluminum nitride particles constituting the aluminum nitride sintered body constituting the ceramic substrate, for the aluminum nitride sintered body constituting the ceramic substrate. The photographing method of the "structure image of the aluminum nitride sintered body" used to determine the longest distance of the voids present in the aluminum nitride sintered body constituting the ceramic substrate and the maximum short diameter of the aluminum nitride particles constituting the sintered body is as described above. Figure 4 is the "structure image of the aluminum nitride sintered body" of sample No. 2.

[0092] Sample No. 1 is a ceramic substrate manufactured without using a fibrous aluminum nitride single crystal, and has a high bending strength, but low thermal conductivity and fracture toughness. Samples No. 2 to No. 8 are ceramic substrates made of an aluminum nitride sintered body obtained by sintering a fibrous aluminum nitride single crystal and particulate aluminum nitride, and show excellent thermal conductivity and fracture toughness. Among them, Samples No. 2, No. 7, and No. 8 are superior in bending strength compared to Samples No. 3 to No. 6 by adjusting the maximum minor axis of the aluminum nitride particles constituting the aluminum nitride sintered body to 15.0 μm.

[0093] Figure 5 shows a graph with the "flexural strength (MPa)" values ​​shown in Table 1 on the vertical axis and the "maximum minor axis (μm)" values ​​on the horizontal axis, in which the measurement results of samples No. 1 to No. 8 are plotted. Each plot includes the "amount of fiber added (mass%)" and the "retention time (hours)" (i.e., firing time).

[0094] FIG. 6 is a graph showing the "maximum void diameter (μm)" values ​​shown in Table 1 on the vertical axis and the "bending strength (MPa)" values ​​on the horizontal axis, in which the measurement results of samples No. 1 to No. 8 are plotted.

[0095] Next, a thermal cycle test (thermal shock test) was performed using a small thermal shock device TSE-12-A manufactured by Espec Corp. The set values ​​for the thermal cycle test (thermal shock test) were a maximum temperature of 150°C and a minimum temperature of -55°C, and the thermal cycle test (thermal shock test) was performed for 1200 cycles.

[0096] For the thermal cycle test, a sample was prepared by bonding copper to both sides of the ceramic substrate in the thickness direction. Figure 7 shows a schematic diagram and dimensions of the sample for the thermal cycle test. The corners of the copper foil were rounded to a radius of 0.5 mm. The dimensions of the ceramic substrate and copper are shown in mm, except for the thickness.

[0097] The following ceramic substrates were used: 1. Ceramic substrate (thickness 0.635 mm) with fibrous AlN single crystal added 2. MARUWA AlN ceramic substrate (thickness 0.635 mm) 3. MARUWA SiN ceramic substrate (thickness 0.32 mm) The method for producing a ceramic substrate with added fibrous AlN single crystals is described below. The slurry after the kneading process with particulate aluminum nitride was granulated by spray drying. The obtained granulated powder was placed in a mold and pressed with a press to obtain a plate-shaped green body. The obtained green body was held at 200 MPa for 1 minute in a cold isostatic press (CIP) to obtain a green body. The obtained green body was subjected to a degreasing process and a firing process in the same manner as in "4. Degreasing process" and "5. Firing process" described above to obtain a ceramic substrate. In the obtained ceramic substrate, the maximum minor axis of the aluminum nitride particles was 15.0 μm or less.

[0098] The results of the thermal cycle test are shown in Figure 8. "Invention / AlN / 0.635 mm thickness" in Figure 8 shows the result of a thermal cycle test on a ceramic substrate to which a fibrous AlN single crystal was added, "Conventional product / AlN / 0.635 mm thickness" in Figure 8 shows the result of a thermal cycle test on a sample (conventional product A) that used an AlN ceramic substrate manufactured by Maruwa, and "Conventional product / SiN / 0.32 mm thickness" in Figure 8 shows the result of a thermal cycle test on a sample (conventional product B) that used a SiN ceramic substrate manufactured by Maruwa. FIG. 8, "Invention / AlN / 0.635 mm thickness," shows ultrasonic test images (SAT images) of the sample before the test, after 300 cycles, after 600 cycles, after 900 cycles, and after 1200 cycles, as well as a photograph of the sample after 1200 cycles; FIG. 8, "Conventional product / AlN / 0.635 mm thickness," shows SAT images of the sample before the test and after 300 cycles, as well as a photograph of the sample after 600 cycles; and FIG. 8, "Conventional product / SiN / 0.32 mm thickness," shows SAT images of the sample before the test, after 300 cycles, after 600 cycles, and after 900 cycles, and after 1200 cycles. In Figure 8, the white areas in the SAT image and the image taken by the ultrasonic flaw detector indicate peeling between the copper foil and the ceramic substrate.

[0099] The ceramic substrate (invention) to which fibrous AlN single crystals were added did not break even after 1200 cycles of thermal cycling tests, whereas conventional product A, which used the same AlN ceramic substrate, broke after 600 cycles.

Claims

1. A sintered aluminum nitride body obtained by sintering fibrous aluminum nitride single crystals and particulate aluminum nitride, The thermal conductivity is 175–250 W / mK, and the fracture toughness is 4.2–10.0 MPa·m in at least one fracture direction in the SEPB method. １／２ It satisfies the following conditions and has a bending strength of 306 to 450 MPa. Aluminum nitride sintered body.

2. The sintered body is characterized in that 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. The aluminum nitride sintered body according to claim 1.

3. The voids present in the sintered body are characterized in that, for each size, the longest distance of the straight line connecting two points on the outer circumference of the void in the microstructure image of the sintered body is 2.5 μm or less. The aluminum nitride sintered body according to claim 1.

4. The aluminum nitride sintered body is characterized in that its thermal conductivity is 175 to 183 W / mK. The aluminum nitride sintered body according to claim 1.

5. The fracture toughness of the aluminum nitride sintered body is 4.2 to 5.8 MPa·m in at least one fracture direction in the SEPB method. １／２ A feature that satisfies the following conditions: The aluminum nitride sintered body according to claim 1.

6. The aluminum nitride sintered body is characterized in that its bending strength is 306 to 344 MPa. The aluminum nitride sintered body according to claim 1.

7. The aluminum nitride sintered body has a thermal conductivity of 175 to 183 W / mK and a fracture toughness of 4.2 to 5.8 MPa·m in at least one fracture direction according to the SEPB method. １／２ It is characterized by satisfying the following conditions and having a bending strength of 306 to 344 MPa, The aluminum nitride sintered body according to claim 1.

8. The aluminum nitride sintered body is characterized in that the amount of oxygen contained in it is 2.8% by mass or less. The aluminum nitride sintered body according to claim 1.

9. A sintered aluminum nitride body made according to any one of claims 1 to 8, Ceramic substrate.

10. It further comprises a metal layer provided on its surface. The ceramic substrate according to claim 9.

11. The ceramic substrate described in claim 9 is provided. Power module.

12. A method for producing an aluminum nitride sintered body, comprising firing raw materials containing fibrous aluminum nitride single crystals and particulate aluminum nitride, In the raw material, the fibrous aluminum nitride single crystals are oriented, The firing temperature is 1800-1850°C. method.

13. The baking time is 2 to 3 hours. The method according to claim 12.

14. The amount of the fibrous aluminum nitride single crystal is 1 to 5 parts by mass per 100 parts by mass of the particulate aluminum nitride. The method according to claim 12.

15. Before firing, the raw material is formed into a plate shape, and the fibrous aluminum nitride single crystal is oriented along a direction perpendicular to the plate thickness direction. The method according to claim 12.

16. Before firing, The raw material is formed into a sheet, and the fibrous aluminum nitride single crystal is oriented along the coating direction of the sheet formation. The sheets are stacked so that they are oriented in a cross shape. The method according to claim 12.

17. The median diameters of the major and minor axes of the particulate aluminum nitride are both within the range of 0.3 to 5 μm. The method according to claim 12.

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