Aluminum nitride sintered body, method for producing same, and circuit board

Abstract

Provided is a method for producing an aluminum nitride sintered body, the method comprising: a molding step for molding a molding raw material that includes an aluminum nitride powder, a sintering aid, and a binder; a degreasing step for heating the molded body in a reduced pressure atmosphere or in an inert gas atmosphere at 500-600°C; and a heating step for firing the degreased molded body for 7 hours or more in a nitrogen gas atmosphere at 1500-1700°C and then firing the same in a nitrogen gas atmosphere at 1750-1880°C for 3-5 hours. The volume resistivity of the aluminum nitride sintered body is 5.0×1012 Ω·cm or more.

Description

Aluminum nitride sintered body, its manufacturing method, and circuit board

[0001] The present disclosure relates to an aluminum nitride sintered body, a method for producing the same, and a circuit board.

[0002] In recent years, power modules for controlling large amounts of power have been used in industrial equipment such as motors and products such as electric vehicles. Circuit boards and the like equipped with ceramic plates are used in these power modules to efficiently diffuse heat generated from semiconductor elements and suppress leakage current. The ceramic sintered compacts used in these ceramic plates are usually produced by sintering a compact formed into a predetermined shape.

[0003] Known ceramic sintered bodies are composed of nitrides, carbides, borides, silicides, etc. Among these, aluminum nitride sintered bodies have excellent thermal conductivity and electrical insulation properties. For this reason, they are used as heat sink materials for electronic components such as power modules. To enhance their suitability for these applications, Patent Document 1 proposes a technology for improving the insulation properties of aluminum nitride sintered bodies by using a specific sintering aid.

[0004] International Publication No. 2021 / 261452

[0005] Electronic components such as power modules are expected to achieve ever higher performance, and as a result, the performance requirements for various products used in the electronic components are expected to become increasingly higher. Therefore, the present disclosure provides an aluminum nitride sintered body having sufficiently high electrical insulation properties and a method for producing the same. It also provides a highly reliable circuit board.

[0006] One aspect of the present disclosure provides an aluminum nitride sintered body according to [1] to [3].

[0007] [1] Volume resistivity is 5.0 × 10 １２ [2] The aluminum nitride sintered body according to [1], having an oxygen content of less than 1.80 mass%. [3] The aluminum nitride sintered body according to [1] or [2], having a bending strength of 430 MPa or more.

[0008] The aluminum nitride sintered bodies of the above [1] to [3] have sufficiently high volume resistivity. Therefore, they have sufficiently high electrical insulation. Such aluminum nitride sintered bodies are suitable for use as components for insulating substrates, power modules, etc.

[0009] One aspect of the present disclosure provides a method for producing an aluminum nitride sintered body according to [4] to [7].

[0010] [4] A method for producing an aluminum nitride sintered body, comprising: a molding step of molding a molding raw material containing aluminum nitride powder, a sintering aid, and a binder to obtain a molded body; a debinding step of heating the molded body in a nitrogen gas atmosphere or a reduced pressure atmosphere at 500 to 600°C; and a heating step of firing the debound molded body in an inert gas atmosphere or a reduced pressure atmosphere at 1500 to 1700°C for 7 hours or more, and then firing it in a nitrogen gas atmosphere at 1750 to 1880°C for 3 to 5 hours. [5] A method for producing an aluminum nitride sintered body according to [4], wherein in the heating step, the molded body is fired in a nitrogen gas atmosphere at 1500 to 1880°C for 11 to 30 hours. [6] A method for producing an aluminum nitride sintered body having a volume resistivity of 5.0 x 10 １２ [7] The method for producing an aluminum nitride sintered body according to [4] or [5], wherein the volume of the compact is 1000 to 9000 mm ３ The method for producing an aluminum nitride sintered body according to any one of [4] to [6],

[0011] In the manufacturing methods of aluminum nitride sintered bodies described above in [4] to [7], the degreasing step is performed in an inert gas atmosphere or a reduced-pressure atmosphere, thereby enabling the oxygen content in the aluminum nitride sintered body to be sufficiently reduced. Here, performing the degreasing step in an inert gas atmosphere or a reduced-pressure atmosphere makes it easier for carbon derived from the binder component to remain in the aluminum nitride sintered body than when performed in air. Therefore, in the manufacturing methods described above, the aluminum nitride sintered body is fired for 7 hours or more in a nitrogen gas atmosphere at 1500 to 1700°C. This allows for sufficient reduction of carbon remaining in the aluminum nitride sintered body. Furthermore, firing for 3 to 5 hours in a nitrogen gas atmosphere at 1750 to 1880°C prevents excessive sintering while sufficiently densifying the aluminum nitride sintered body. These factors enable the production of aluminum nitride sintered bodies with sufficiently high electrical insulation. Such aluminum nitride sintered bodies are suitable for use as components for insulating substrates, power modules, etc.

[0012] One aspect of the present disclosure provides a circuit board according to [8]. [8] The circuit board comprises: an insulating substrate made of the aluminum nitride sintered body according to any one of [1] to [3] above; and a metal circuit bonded to the insulating substrate.

[0013] The circuit board of the above [8] has an insulating substrate made of an aluminum nitride sintered body having sufficiently high electrical insulation, and therefore has excellent reliability. Such a circuit board can be suitably used in semiconductor devices such as power modules.

[0014] According to the present disclosure, it is possible to provide an aluminum nitride sintered body having sufficiently high electrical insulation properties and a method for producing the same, and also to provide a highly reliable circuit board.

[0015] Fig. 1 is a perspective view showing an example of an aluminum nitride sintered body, Fig. 2 is a perspective view showing an example of a circuit board, and Fig. 3 is a perspective view showing an example of a laminate.

[0016] Embodiments of the present disclosure will be described below. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content. In this disclosure, numerical ranges expressed in the format "a to b" are numerical ranges inclusive of a and b, with a being the lower limit and b being the upper limit. This disclosure also includes ranges in which the upper and / or lower limits of each numerical range are replaced with the exemplified numerical values ​​or the numerical values ​​of any of the examples. This disclosure also includes numerical ranges in which any combination of the individually described upper and lower limits is used. Unless otherwise specified, the materials or components exemplified in this disclosure can be used alone or in combination of two or more types. In the description, the same elements or elements having the same functions are denoted by the same symbols, and redundant explanations may be omitted where appropriate.

[0017] The aluminum nitride sintered body contains aluminum nitride as a main component. The aluminum nitride content in the aluminum nitride sintered body may be 90% by mass or more. The aluminum nitride content in the aluminum nitride sintered body may be 93% by mass or more, or may be 95% by mass or more. From the viewpoint of sufficiently increasing the density, the aluminum nitride content in the aluminum nitride sintered body may be 99.5% by mass or less, 99% by mass or less, or 98% by mass or less. An example of the aluminum nitride content in the aluminum nitride sintered body is 90 to 99.5% by mass. The content of each component in the aluminum nitride sintered body can be determined, for example, by X-ray analysis. For X-ray analysis, for example, a D8 ADVANCE (product name) manufactured by Bruker Japan Ltd. can be used.

[0018] The aluminum nitride sintered body in the present disclosure is a solid in which aluminum nitride particles are bonded together. The aluminum nitride sintered body may contain an oxide as a minor component. The oxide may include an oxide (composite oxide) having yttrium and aluminum as constituent elements.

[0019] FIG. 1 is a perspective view showing an aluminum nitride sintered body according to one embodiment. The external shape of the aluminum nitride sintered body 100 is not particularly limited, and may be, for example, a sheet. The volume of the aluminum nitride sintered body 100 is set to 1000 mm from the viewpoint of sufficiently increasing its usefulness as an electronic component. ３ Over 1500mm ３ or more, or 2000 mm ３ The volume of the aluminum nitride sintered body 100 may be 9000 mm or more from the viewpoint of sufficiently reducing the carbon content in the aluminum nitride sintered body 100. ３ Below, 5000mm ３ or less than 3000 mm ３ The volume of the aluminum nitride sintered body 100 may be, for example, 1000 to 9000 mm ３ It may be.

[0020] The volume resistivity of the aluminum nitride sintered body 100 is 5.0×10 １２ Ω cm or more, and 7.0 × 10 １２ Ω・cm or more, 8.0×10 １２ Ω cm or more, or 1.0 x 10 １３ The aluminum nitride sintered body 100 having a sufficiently high volume resistivity has excellent reliability as, for example, an insulating substrate. The volume resistivity of the aluminum nitride sintered body 100 may be, for example, 1.0×10 １４ The volume resistivity of the aluminum nitride sintered body 100 may be, for example, 5.0×10 １２ ~1.0 x 10 １４ It may be Ω·cm.

[0021] The volume resistivity of the aluminum nitride sintered body 100 can be adjusted, for example, by changing the atmosphere in the degreasing step and the temperature profile in the heating step when manufacturing the aluminum nitride sintered body 100. The volume resistivity can be measured in accordance with JIS C2139:2008 "Solid electrical insulating materials - Measurement method for volume resistivity and surface resistivity." The measurement device may be, for example, a Hioki Ultra Megoma Meter (product name: SM-8220). The measurement temperature may be 20°C ± 1°C.

[0022] The oxygen content in the aluminum nitride sintered body 100 may be less than 1.80 mass%, less than 1.70 mass%, or less than 1.65 mass%, from the viewpoint of sufficiently increasing the volume resistivity. The oxygen content in the aluminum nitride sintered body 100 may be 0.50 mass% or more, 1.00 mass% or more, or 1.30 mass% or more, from the viewpoint of sufficiently increasing the density. An example of the oxygen content in the aluminum nitride sintered body 100 may be 0.5 mass% or more and less than 1.80 mass%. Oxygen may be contained in the aluminum nitride sintered body 100, for example, as the above-mentioned oxide.

[0023] The oxygen content of the aluminum nitride sintered body 100 can be adjusted, for example, by changing the synthesis method and oxygen content of the aluminum nitride powder used as a raw material, and the atmosphere in the degreasing step during production of the aluminum nitride sintered body 100. The oxygen content of the aluminum nitride sintered body 100 can be measured using a commercially available oxygen / nitrogen analyzer.

[0024] The thermal conductivity of the aluminum nitride sintered body 100 may be 120 W / m·K or more, 140 W / m·K or more, 150 W / m·K or more, or 160 W / m·K or more. An aluminum nitride sintered body 100 with sufficiently high thermal conductivity is suitable, for example, as a heat sink member for a power module or the like. However, the use of the aluminum nitride sintered body 100 is not limited to this. The thermal conductivity of the aluminum nitride sintered body 100 may be, for example, 200 W / m·K or less. An example of the thermal conductivity of the aluminum nitride sintered body 100 may be 120 to 200 W / m·K.

[0025] The thermal conductivity of the aluminum nitride sintered body 100 can be adjusted, for example, by changing the maximum temperature (temperature in the second temperature range TR2) in the heating step when producing the aluminum nitride sintered body 100. The thermal conductivity can be measured by a laser flash method in accordance with JIS R1611:2010 "Method for measuring thermal diffusivity, specific heat capacity, and thermal conductivity of fine ceramics by the flash method." A measuring device such as TC-7SB RT (product name) manufactured by Advance Riko Co., Ltd. can be used. The measurement temperature may be 20°C ± 1°C.

[0026] The flexural strength of the aluminum nitride sintered body 100 may be 430 MPa or more, 450 MPa or more, 460 MPa or more, or 470 MPa or more. The aluminum nitride sintered body 100 having a high flexural strength has high electrical insulation and can be suitably used as an insulating substrate for a power module or the like. The flexural strength of the aluminum nitride sintered body 100 may be, for example, 800 MPa or less, 700 MPa or less, or 600 MPa or less. An example of the flexural strength of the aluminum nitride sintered body 100 may be 430 to 800 MPa.

[0027] The flexural strength of the aluminum nitride sintered body 100 can be adjusted, for example, by changing the atmosphere in the degreasing step or the temperature profile in the heating step when producing the aluminum nitride sintered body 100. The flexural strength can be measured using an SDT-503NB-50R1 (product name) manufactured by Imada Seisakusho Co., Ltd. The measurement temperature may be 20°C ± 1°C.

[0028] The apparent density of the aluminum nitride sintered body 100 is set to 3.1 g / cm3 from the viewpoint of sufficiently increasing the volume resistivity. ３ or more, 3.2 g / cm ３ The apparent density can be adjusted by changing the blending ratio of the sintering aid used as a raw material, the molding conditions, or the temperature profile in the heating step. The upper limit of the apparent density is, for example, 3.4 g / cm. ３ may be.

[0029] The aluminum nitride sintered body 100 has sufficiently high electrical insulation properties and is therefore suitable for use as an insulating substrate. The insulating substrate made of the aluminum nitride sintered body 100 has excellent insulation reliability and is therefore suitable as a component of a semiconductor device such as a power module.

[0030] 2 is a perspective view showing an example of a circuit board. The circuit board 300 includes an insulating substrate 150 made of an aluminum nitride sintered body 100, a metal circuit 20 bonded to one surface of the insulating substrate 150, and a metal plate 110 bonded to the other surface of the insulating substrate 150. The metal circuit 20 is provided on one main surface 150A of the insulating substrate 150, and the metal plate 110 is provided on the other main surface of the aluminum nitride sintered body 100. The metal circuit 20 may be made of aluminum or copper. The metal circuit 20 is electrically connected to other electronic components to form an electric circuit. The metal plate 110 may be an aluminum plate or a copper plate. When the circuit board 300 is used in a power module, the metal plate 110 may function as a heat dissipation material.

[0031] The circuit board 300 includes an insulating substrate 150 made of an aluminum nitride sintered body 100 having sufficiently high electrical insulation properties. Since the insulating substrate 150 has excellent insulation reliability, the circuit board 300 also has excellent reliability. Therefore, the circuit board 300 can be suitably used as a component of a semiconductor device such as a power module. The aluminum nitride sintered body 100 can be obtained, for example, by the manufacturing method of an aluminum nitride sintered body described below.

[0032] A method for producing an aluminum nitride sintered body according to one embodiment includes a molding step of molding a molding raw material containing aluminum nitride powder, a sintering aid, and a binder to obtain a molded body; a degreasing step of heating the molded body in a nitrogen gas atmosphere or a reduced pressure atmosphere at 500 to 600°C; a heating step of firing the degreasing molded body in a nitrogen gas atmosphere at 1500 to 1700°C (first temperature range TR1) for 7 hours or more, and then firing it in a nitrogen gas atmosphere at 1750 to 1880°C (second temperature range TR2) for 3 to 5 hours; and a temperature-lowering step of stopping the heating and cooling the aluminum nitride sintered body.

[0033] The forming raw material can be prepared by blending aluminum nitride powder, a sintering aid, a binder, and an additive. The aluminum nitride powder may be produced by a direct nitridation method. The direct nitridation method is a process in which aluminum nitride is synthesized by heating aluminum in a nitrogen gas atmosphere. The direct nitridation method can reduce the oxygen content in the aluminum nitride powder compared to a reduction nitridation method using aluminum oxide. Therefore, by using aluminum nitride powder produced by the direct nitridation method, an aluminum nitride sintered body with a sufficiently reduced oxygen content can be obtained.

[0034] The oxygen content of the aluminum nitride powder may be 0.3 to 1.2 mass%, 0.5 to 1.0 mass%, or 0.6 to 0.9 mass% or less. The oxygen content can be measured using a commercially available oxygen / nitrogen analyzer. The oxygen content of the aluminum nitride powder can be adjusted by changing the purity of aluminum or the oxygen concentration in the firing atmosphere when producing the aluminum nitride powder. By using aluminum nitride powder with such an oxygen content, the aluminum nitride sintered body can be sufficiently densified and the residual carbon can be sufficiently reduced.

[0035] In the cumulative volume-based particle size distribution of aluminum nitride powder measured by laser diffraction / scattering, the particle sizes at which the integrated value from the smallest particle size reaches 10%, 50%, and 90% of the total may be defined as d10, d50, and d90, respectively, and may satisfy the following numerical ranges. d10 may be 0.1 to 0.6 μm, or 0.3 to 0.5 μm. d50 may be 1.3 to 5.0 μm, 1.5 to 4.0 μm, or 2.0 to 3.5 μm. d90 may be 3.0 to 10 μm, 4.0 to 8.0 μm, or 4.5 to 7.0 μm. By using aluminum nitride powder having such a particle size distribution, an aluminum nitride sintered body having a sufficiently high volume resistivity can be obtained.

[0036] The specific surface area of ​​aluminum nitride powder is 1.2 to 2.5 m ２ / g, 1.4-2.3m ２ / g, or 1.5 to 2.2 m２ / g. The specific surface area of ​​the aluminum nitride powder is a value measured by the BET single-point method. By using an aluminum nitride powder having such a specific surface area, it is possible to obtain an aluminum nitride sintered body that is sufficiently densified and has a highly uniform microstructure.

[0037] The sintering aid may contain yttrium oxide and aluminum oxide. The sintering aid may be granular. The mass ratio of aluminum oxide to yttrium oxide (aluminum oxide content / yttrium oxide content) may be, for example, 0.1 or more and less than 0.5, or may be 0.1 to 0.25. This can suppress the aggregation of oxides in the aluminum nitride sintered body. The blending ratio of yttrium oxide and aluminum oxide can be adjusted within the above-mentioned range, which can also adjust the oxide composition in the aluminum nitride sintered body. Aluminum oxide and yttrium oxide form a liquid phase of a composite oxide during sintering, promoting sintering. This allows the aluminum nitride sintered body to be sufficiently densified.

[0038] The content of the sintering aid in the forming raw material may be, for example, 1 to 5 parts by mass per 100 parts by mass of aluminum nitride powder. By setting the content of the sintering aid within the above range, the volume resistivity of the aluminum nitride sintered body can be further increased. The content of the sintering aid can be determined by converting the components of the sintering aid into oxides.

[0039] The content of aluminum oxide in the forming raw material may be, for example, 0.1 to 2.0 parts by mass, or 0.3 to 1.0 part by mass, relative to 100 parts by mass of aluminum nitride. This makes it possible to sufficiently densify the aluminum nitride sintered body while increasing the relative content of the aluminum nitride component. This makes it possible to sufficiently increase the volume resistivity of the aluminum nitride sintered body.

[0040] Examples of binders include methylcellulose-based binders with plasticity or surfactant effects, and acrylic ester-based binders with excellent thermal decomposition properties. Examples of additives include plasticizers, dispersion media, and release agents. Examples of plasticizers include glycerin. Examples of dispersion media include ion-exchanged water and ethanol.

[0041] A molding raw material may be prepared by blending and mixing aluminum nitride, a sintering aid, a binder, and additives as needed. The molding raw material may be formed into, for example, a sheet by a known method such as a doctor blade method or extrusion molding. The shape of the molded body obtained in this manner may be the same as the aluminum nitride sintered body 100 shown in FIG. 1.

[0042] The volume of the molded body is 1000 mm from the viewpoint of sufficiently increasing its usefulness as an electronic component. ３ Over 1500mm ３ or more, or 2000m ３ The volume of the compact may be 9000 mm or more from the viewpoint of sufficiently promoting the desorption of carbon in the debinding step and the heating step. ３ Below, 5000mm ３ or less than 3000 mm ３ An example of the volume of the molded body is 1000 to 9000 mm ３ It may be.

[0043] In the debinding step, the compact is heated in an inert gas atmosphere or a reduced pressure atmosphere at 500 to 600°C. By heating the compact in an inert gas atmosphere or a reduced pressure atmosphere, oxidation of the aluminum nitride contained in the compact can be suppressed. This makes it possible to obtain an aluminum nitride sintered body with a sufficiently high volume resistivity.

[0044] The "inert gas atmosphere" in the degreasing step of the present disclosure is an atmosphere containing an inert gas as a main component, and the inert gas content may be 95% by volume or more, 98% by volume or more, or 99% by volume or more. Examples of inert gases include nitrogen gas, argon gas, and carbon dioxide gas. The oxygen content in the "inert gas atmosphere" may be 5% by volume or less, 3% by volume or less, 1% by volume or less, or 0.5% by volume or less. The "reduced pressure atmosphere" in the degreasing step is an atmosphere with an absolute pressure of 5 kPa or less, and may be a vacuum atmosphere. The absolute pressure may be 1 kPa or less, or 0.5 kPa or less.

[0045] In the debinding step, the time for heating the compact in an atmosphere at 500 to 600°C may be 5 to 20 hours, or 6 to 10 hours. When the debinding step is performed in an inert gas atmosphere or a reduced pressure atmosphere, elimination of carbon derived from the binder and the like is more difficult than when the debinding step is performed in air. For this reason, it is necessary to sufficiently eliminate carbon in the subsequent heating step.

[0046] In the heating step, the degreased compact is heated in a nitrogen gas atmosphere. The "nitrogen gas atmosphere" in the heating step of the present disclosure refers to an atmosphere containing nitrogen gas as a primary component, and the nitrogen gas content may be 95% by volume or more, 98% by volume or more, or 99% by volume or more. The heating rate from the heating start temperature (e.g., 20°C) to reach the first temperature range TR1 may be 5 to 30°C / min, or 10 to 20°C / min, on average. The first temperature range TR1 is 1500 to 1700°C. By heating within this temperature range, the carbon content of the compact can be sufficiently reduced. The lower limit of the first temperature range TR1 may be 1550°C. This further reduces the carbon content of the compact, resulting in an aluminum nitride sintered body with excellent appearance. The upper limit of the first temperature range TR1 may be 1680°C. This prevents the aluminum nitride sintered body from being excessively sintered.

[0047] The heating time in the first temperature range TR1 may be 7 hours or more, 8 hours or more, 9 hours or more, or 10 hours or more. This allows the carbon contained in the compact to be sufficiently reduced. The heating time in the first temperature range TR1 may be 20 hours or less, 18 hours or less, or 15 hours or less. This allows the production efficiency of the aluminum nitride sintered body to be improved. An example of the heating time in the first temperature range TR1 is 7 to 20 hours.

[0048] The upper limit of the first temperature range TR1 may be 1650°C or 1630°C. This allows the thermal conductivity of the aluminum nitride sintered body to be sufficiently high. The lower limit of the first temperature range TR1 may be 1580°C. This allows the carbon remaining in the aluminum nitride sintered body to be further reduced, resulting in an aluminum nitride sintered body with even better appearance.

[0049] The second temperature range TR2 is 1750 to 1880°C. Heating within this temperature range allows the aluminum nitride sintered body to be sufficiently densified while suppressing over-sintering. Because the second temperature range TR2 is higher than the first temperature range TR1 (first temperature range TR1 < second temperature range TR2), heating is performed within the first temperature range TR1 for a predetermined time, and then the temperature inside the heating furnace is increased to the second temperature range TR2. The temperature increase may also be performed in a nitrogen gas atmosphere. The time required to reach the second temperature range TR2 from the first temperature range TR1 may be 4 hours or more, 5 hours or more, or 5.5 hours or more. This allows the densification and grain growth to proceed smoothly while sufficiently reducing the carbon remaining in the aluminum nitride sintered body. The time required to reach the second temperature range TR2 from the first temperature range TR1 may be 20 hours or less, or 15 hours or less. This improves the production efficiency of aluminum nitride sintered bodies. An example of the time required for the temperature to reach the second temperature range TR2 from the first temperature range TR1 is 4 to 20 hours.

[0050] The heating time in the second temperature range TR2 is 3 hours or more. This sufficiently promotes densification of the aluminum nitride sintered body, thereby increasing the volume resistivity of the aluminum nitride sintered body. From the same perspective, the heating time in the second temperature range TR2 may be 3.5 hours or more. The heating time in the second temperature range TR2 is 5 hours or less. This prevents over-sintering of the aluminum nitride sintered body, thereby sufficiently increasing the volume resistivity and flexural strength.

[0051] The upper limit of the second temperature range TR2 may be 1850°C, 1830°C, or 1820°C. This sufficiently prevents over-sintering of the aluminum nitride sintered body, and further increases the volume resistivity and flexural strength. The lower limit of the second temperature range TR2 may be 1780°C or 1800°C. This sufficiently promotes densification and grain growth of the aluminum nitride sintered body.

[0052] In the heating step, the heating time in the nitrogen gas atmosphere in the third temperature range TR3 (1500 to 1880°C) may be 11 hours or more, 15 hours or more, or 20 hours or more. This sufficiently reduces the amount of carbon remaining in the aluminum nitride sintered body, improving its appearance. Furthermore, densification and grain growth can be sufficiently promoted to obtain an aluminum nitride sintered body having sufficiently high volume resistivity and flexural strength. In the heating step, the heating time in the nitrogen gas atmosphere in the third temperature range TR3 may be 30 hours or less, or 25 hours or less. This improves the productivity of the aluminum nitride sintered body.

[0053] The lower limit of the third temperature range TR3 may be 1500° C. or 1580° C. from the viewpoint of sufficiently reducing carbon remaining in the aluminum nitride sintered body. The upper limit of the third temperature range TR3 may be 1850° C., 1830° C., or 1820° C. from the viewpoint of suppressing over-sintering.

[0054] After the heating step, a temperature-reducing step may be performed in which the nitrogen gas atmosphere in the heating furnace is cooled from the second temperature range TR2. The temperature-reducing rate is not particularly limited and may be, for example, 5.0 to 50.0°C / min. The temperature-reducing step may also be performed in a nitrogen gas atmosphere. In this manner, an aluminum nitride sintered body having sufficiently high electrical insulation properties can be obtained. The aluminum nitride sintered body may be processed into a desired shape as needed. For example, the aluminum nitride sintered body may be processed into a sheet shape as shown in FIG. 1. The aluminum nitride sintered body may be used as an insulating substrate, to which a metal circuit and a metal plate are attached to form a circuit board as shown in FIG. 2. For example, a laminate may be formed by joining the main surface of a sheet-shaped aluminum nitride sintered body (aluminum nitride plate) to the main surface of a metal plate such as a copper plate or an aluminum plate.

[0055] FIG. 3 is a perspective view showing an example of a laminate. The laminate 200 includes a pair of metal plates 110 arranged facing each other and an insulating substrate composed of an aluminum nitride sintered body 100 between the pair of metal plates 110. Examples of the metal plate 110 include a copper plate and an aluminum plate. The aluminum nitride sintered body 100 and the metal plate 110 may have the same shape and size or may have different shapes and sizes. The metal plate 110 and the insulating substrate 150 may be joined by, for example, a brazing material. The circuit board 300 shown in FIG. 2 may be obtained by using one of the pair of metal plates 110 as a heat dissipation material and processing the other into a circuit pattern. The circuit pattern may be formed by etching the metal plate 110 using a resist. This makes it possible to form a circuit board or a heat sink that can sufficiently suppress leakage current, etc.

[0056] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. For example, the shapes and structures of the circuit board and laminate are not limited to those shown in FIGS. 2 and 3. For example, a circuit pattern may be formed on both main surfaces of an insulating substrate 150 made of an aluminum nitride sintered body 100. Furthermore, instead of forming the metal circuit 20 by etching the metal plate 110, the metal circuit 20 may be formed by spraying metal powder and performing heat treatment.

[0057] The present disclosure will be described in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0058] Example 1 Production and Evaluation of Aluminum Nitride Powder Commercially available aluminum powder (average particle size: 20-30 μm) was nitrided by a direct nitriding method to obtain aluminum nitride powder. The aluminum powder was nitrided by heating to 1600-1700°C in a nitrogen atmosphere. The volumetric particle size distribution of the obtained aluminum nitride powder was measured by a laser diffraction / scattering method. In the cumulative particle size distribution, the particle sizes at which the integrated value from the smallest particle size reached 10%, 50%, and 90% of the total were defined as d10, d50, and d90, respectively. The d10, d50, and d90 values ​​were as shown in Table 1.

[0059] The specific surface area of ​​the aluminum nitride powder was measured by the BET single-point method. The measurement results are shown in Table 1. The oxygen content of the aluminum nitride powder was measured using an oxygen / nitrogen analyzer (product name: EMGA-920) manufactured by Horiba, Ltd., and was found to be 0.72 mass%.

[0060] [Production of Aluminum Nitride Sintered Body] For 100 parts by mass of the aluminum nitride powder obtained by the above procedure, yttrium oxide (Y ２ O ３ 3.87 parts by mass of powder of α-aluminum oxide (Al ２ O ３) powder, and 0.732 parts by mass were blended and mixed using a ball mill to obtain a mixed powder. 6 parts by mass of a cellulose ether binder (manufactured by Shin-Etsu Chemical Co., Ltd., product name: Metrose), 5 parts by mass of glycerin (manufactured by Kao Corporation, product name: Exepar), and 10 parts by mass of ion-exchanged water were added to 100 parts by mass of the mixed powder and mixed for 1 minute using a Henschel mixer to obtain a molding raw material. This molding raw material was molded using a screw extrusion molding machine to produce a sheet-like first molded body (thickness: 1.0 mm). This first molded body was dried at 100 ° C for 1 hour and then cut to obtain a rectangular parallelepiped second molded body having a size of length x width x thickness = 90.0 mm x 90.0 mm x 1.0 mm.

[0061] The second compact was degreased by heating it in a nitrogen gas atmosphere at 550°C for 8 hours using an electric furnace (degreasing step). The obtained degreased body was removed from the electric furnace and cooled to room temperature. The degreased body was then placed in a heating furnace and heated in a nitrogen gas atmosphere from room temperature (T0: approximately 20°C) to temperature T1 shown in Table 2 at an average rate of 15°C / min, and the heating step was initiated. The holding time at temperature T1 was as shown in Table 2. The temperature was then raised to temperature T2. The time required to raise the temperature from T1 to T2 was as shown in the "Heating time required" column in Table 2. The holding time at T2 was as shown in Table 2.

[0062] From the temperature rise profile described above, the heating times for each temperature range in the heating step, namely, the first temperature range TR1 (1500°C to 1700°C), the second temperature range TR2 (1750°C to T2), and the third temperature range TR3 (1500°C to T2), were calculated. The results are shown in Table 3. The heating times in both the first temperature range TR1 and the third temperature range TR3 include the holding time at temperature T1. The heating times in both the second temperature range TR2 and the third temperature range TR3 include the holding time at temperature T2.

[0063] After the holding time at temperature T2 shown in Table 2 had elapsed, the material was cooled from Tmax to 1600°C at an average temperature decrease rate of 10.0°C / min, and then cooled from 1600°C to room temperature at an average temperature decrease rate of 5.0°C / min (cooling down step). In this way, an aluminum nitride sintered body of Example 1 was obtained.

[0064] [Evaluation of Aluminum Nitride Sintered Body] <Evaluation of Insulation Properties of Aluminum Nitride Sintered Body> The volume resistivity of the aluminum nitride sintered body was measured. The measurement was performed using a Hioki E.E. Corporation Super Insulation Meter (product name: SM-8220) in accordance with JIS C 2139:2008 "Solid Electrical Insulating Materials - Measurement Method for Volume Resistivity and Surface Resistivity." Specifically, copper with a diameter of 10 mm was vapor-deposited on both main surfaces of the aluminum nitride sintered body to prepare a measurement sample (substrate). The obtained substrate was sandwiched between measurement electrodes, and a voltage of 1000 V was applied in an environment of 20°C to pass a direct current through the substrate. This state was maintained for 20 seconds. The insulation resistance value was measured after 20 seconds had elapsed, and the volume resistivity was calculated using this value according to the following formula (1). The results are shown in Table 4. Volume resistivity [Ω cm] = Insulation resistance value [Ω] × Copper vapor deposition area [cm ２ ] / substrate thickness [cm] (1)

[0065] <Evaluation of Thermal Conductivity of Aluminum Nitride Sintered Body> Thermal conductivity was measured by the laser flash method in accordance with JIS R1611:2010 "Method for measuring thermal diffusivity, specific heat capacity, and thermal conductivity of fine ceramics by the flash method." The measuring device used was TC-7SB RT (product name) manufactured by Advance Riko Co., Ltd. The measurement temperature was 20°C. The results are shown in Table 4.

[0066] <Evaluation of flexural strength of aluminum nitride sintered body> The three-point bending strength of the aluminum nitride sintered body at 20°C was measured using SDT-503NB-50R1 (product name) manufactured by Imada Seisakusho Co., Ltd. The results obtained are shown in Table 4.

[0067] <Measurement of oxygen content of aluminum nitride sintered body> The oxygen content of the aluminum nitride sintered body was measured using argon gas with an oxygen / nitrogen analyzer (product name: EMGA-920) manufactured by Horiba, Ltd. The measurement results are shown in Table 4.

[0068] (Examples 2 to 7) Aluminum nitride sintered bodies of Examples 2 to 7 were produced and evaluated in the same manner as Example 1, except that at least one selected from the group consisting of T1, the holding time at T1, the time required for temperature rise, and T2 was changed as shown in Table 2. The heating times in the first temperature range TR1, the second temperature range TR2, and the third temperature range TR3 of each Example were as shown in Table 3. The evaluation results of the aluminum nitride sintered bodies of each Example were as shown in Table 4.

[0069] Comparative Example 1 A commercially available aluminum nitride powder produced by a reduction-nitridation method was prepared. The d10, d50, d90, and specific surface area were determined in the same manner as in Example 1. The results are shown in Table 1. An aluminum nitride sintered body was produced and evaluated in the same manner as in Example 1, except that this aluminum nitride powder was used, the degreasing process was performed in an air atmosphere, and T1, the holding time at T1, the required heating time, and T2 were set as shown in Table 2. The heating times in the first temperature range TR1, the second temperature range TR2, and the third temperature range TR3 of Comparative Example 1, as well as the evaluation results of the aluminum nitride sintered body, are shown in Tables 3 and 4, respectively.

[0070] (Comparative Example 2) An aluminum nitride sintered body was produced and evaluated in the same manner as in Comparative Example 1, except that the aluminum nitride powder produced in Example 1 was used. The heating times in the first temperature range TR1, the second temperature range TR2, and the third temperature range TR3 of Comparative Example 2, and the evaluation results of the aluminum nitride sintered body, are shown in Tables 3 and 4, respectively.

[0071] (Comparative Examples 3 to 5) Aluminum nitride sintered bodies of Comparative Examples 3 to 5 were produced and evaluated in the same manner as Example 1, except that at least one selected from the group consisting of T1, the holding time at T1, the time required for temperature rise, and T2 was changed as shown in Table 2. The heating times in the first temperature range TR1, the second temperature range TR2, and the third temperature range TR3 of each Comparative Example were as shown in Table 3. The evaluation results of the aluminum nitride sintered bodies of each Comparative Example were as shown in Table 4.

[0072]

[0073]

[0074]

[0075]

[0076] Comparative Examples 1 and 2, in which the degreasing step was carried out in air, had higher oxygen contents than Examples 1 to 7 and Comparative Examples 3 to 5, in which the degreasing step was carried out in a nitrogen gas atmosphere. It is believed that oxidation of aluminum nitride progressed during the degreasing step in Comparative Examples 1 and 2. The reason why the oxygen content of the aluminum nitride sintered body in Comparative Example 1 was higher than that in Comparative Example 2 is thought to be because the aluminum nitride powder prepared by the reduction nitridation method had a higher oxygen content than the aluminum nitride powder prepared by the direct nitridation method.

[0077] Comparative Examples 3 to 5, in which the heating time in the second temperature range TR2 exceeded 5 hours, had lower volume resistivity and bending strength than Examples 1 to 7. This is thought to be due to excessive sintering. On the other hand, the aluminum nitride sintered bodies of Examples 1 to 7 had sufficiently high volume resistivity and bending strength.

[0078] When the appearances of the aluminum nitride sintered bodies of Examples 1 to 7 were visually inspected, Example 2 had the best appearance. This is thought to be because the heating time in the third temperature range TR3 was the longest, resulting in a sufficient reduction in carbon.

[0079] The present disclosure provides an aluminum nitride sintered body having sufficiently high electrical insulation properties, a method for producing the same, and a highly reliable circuit board.

[0080] 20...metal circuit, 100...aluminum nitride sintered body, 110...metal plate, 150...insulating substrate, 200...laminated body, 300...circuit board.

Claims

1. Volume resistivity is 5.0 x 10 １２ An aluminum nitride sintered body having a resistivity of Ω·cm or more.

2. The aluminum nitride sintered body according to claim 1, having an oxygen content of less than 1.80 mass%.

3. The aluminum nitride sintered body according to claim 1 or 2, having a flexural strength of 430 MPa or more.

4. A method for producing an aluminum nitride sintered body, comprising: a molding step of molding a raw material containing aluminum nitride powder, a sintering aid, and a binder to obtain a molded body; a degreasing step of heating the molded body in an inert gas atmosphere or a reduced pressure atmosphere at 500 to 600°C; and a heating step of firing the degreased molded body in a nitrogen gas atmosphere at 1500 to 1700°C for 7 hours or more, and then firing it in a nitrogen gas atmosphere at 1750 to 1880°C for 3 to 5 hours.

5. The method for producing an aluminum nitride sintered body according to claim 4, wherein in the heating step, the molded body is sintered in a nitrogen gas atmosphere at 1500 to 1880°C for 11 to 30 hours.

6. The volume resistivity of the aluminum nitride sintered body is 5.0×10 １２ The method for producing an aluminum nitride sintered body according to claim 4 or 5, wherein the resistivity is Ω·cm or more.

7. The volume of the molded body is 1000 to 9000 mm ３ The method for producing an aluminum nitride sintered body according to claim 4 or 5, 8. A circuit board comprising: an insulating substrate made of the aluminum nitride sintered body according to claim 1 or 2; and a metal circuit joined to the insulating substrate.

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