Aluminum nitride sintered substrate, method for producing same, and insulating substrate for electronic circuit

Abstract

In order to improve the heat cycle resistance of an aluminum nitride sintered substrate via a technique which is favorable for improving partial discharge resistance, the present invention provides an aluminum nitride sintered substrate comprising a plate-like aluminum nitride sintered body, at least one surface layer section of which is an unevenly-distributed-zirconium surface layer section which contains more zirconium atoms than does a plate-thickness center section thereof, if a region extending from the surface to a depth of 1 / 3 the plate thickness is called a surface layer section, and the region besides the surface layer sections is called the plate-thickness center section. The aluminum nitride sintered substrate can be obtained by a manufacturing method including: a step for obtaining a substrate sheet by applying a paste A containing aluminum nitride powder so as to form a plate-like shape; a step for obtaining a multilayer sheet by applying a paste B containing aluminum nitride powder and powder of a zirconium-containing substance to at least one surface of the substrate sheet; and a step for heating and sintering the multilayer sheet.

Description

Aluminum nitride sintered substrate, method for producing the same, and insulating substrate for electronic circuits

[0001] The present invention relates to an aluminum nitride sintered substrate useful as an insulating substrate for an electronic circuit on which a semiconductor element is mounted, and a method for producing the same. It also relates to an insulating substrate for an electronic circuit using the sintered substrate.

[0002] Aluminum nitride sintered substrates have good thermal conductivity and are widely used as insulating substrates for electronic circuits. In the manufacturing process of aluminum nitride sintered substrates, additive substances may be mixed for the purpose of improving manufacturability and the properties of the insulating substrate.

[0003] For example, Patent Documents 1 to 3 describe aluminum nitride sintered bodies containing zirconium for the purpose of improving mechanical strength.

[0004] JP-A-2016-98159, JP-A-2017-100937, JP-A-2018-184316

[0005] In high-voltage equipment, partial discharge is likely to occur at locations where the electric field is concentrated, such as the boundary between the electrode and the insulator or defective portions inside the insulator. If the occurrence of partial discharge is left unattended for a long time, the intensity and frequency of partial discharge may gradually increase, leading to damage to the member and insulation breakdown. Generally, partial discharge tends to be a problem in high-voltage components such as generators, power transformers, transmission lines, and power cables, but high-voltage-resistant power modules also require countermeasures. In power modules, it is desirable to apply an insulating substrate for an electronic circuit on which a semiconductor element is mounted that has excellent characteristics (hereinafter referred to as "partial discharge resistance") capable of suppressing the occurrence frequency of partial discharge accompanied by a charge movement higher than a predetermined level.

[0006] Figure 1 schematically illustrates the cross-sectional structure near the edge of an insulating substrate for an electronic circuit on which semiconductor elements are mounted. A circuit metal member 2 made of a copper-based or aluminum-based material is bonded to one side of a ceramic substrate 1 made of an aluminum nitride sintered body or the like. A heat dissipation metal member 3 made of a copper-based or aluminum-based material is bonded to the other side of the ceramic substrate 1 as needed. In this example, the insulating substrate 10 for the electronic circuit is composed of the ceramic substrate 1, the circuit metal member 2, and the heat dissipation metal member 3. In such a laminated structure, electric fields tend to concentrate at the bonding interface edge 20 between the ceramic substrate 1 and the circuit metal member 2, and at the bonding interface edge 30 between the ceramic substrate 1 and the heat dissipation metal member 3. Partial discharge due to corona discharge mainly occurs starting from these electric field concentration areas.

[0007] Aluminum nitride sintered substrates are useful as insulating substrates for power modules and other applications requiring excellent insulation and thermal conductivity. In the future, to meet the demands of improving the performance of electronic devices, enhanced partial discharge resistance will also become crucial for insulating substrates used in electronic circuits.

[0008] On the other hand, in the manufacturing process of power modules, aluminum nitride sintered substrates are subjected to several processes involving heating and cooling, such as the process of mounting semiconductor elements and the process of joining them to base members, after circuit metal components and heat dissipation components are bonded to the surface. Furthermore, even when power modules are in use, aluminum nitride sintered substrates are exposed to thermal cycles of repeated heating and cooling. Therefore, aluminum nitride sintered substrates used as insulating substrates for electronic circuits are required to have excellent "heat cycle resistance," that is, the characteristic of being less prone to the occurrence of "problematic cracks" near the bonding interface with circuit metal components when subjected to thermal cycles. Note that initial cracks that occur at the bonding interface are often not visible from the outside, but as these cracks propagate, they appear on the exposed parts of the aluminum nitride sintered substrate. Here, cracks that have progressed to the point of appearing on the exposed parts of the aluminum nitride sintered substrate are considered "problematic cracks."

[0009] Ceramic plates containing zirconium throughout the aluminum nitride sintered structure, as shown in Patent Documents 1 to 3, can be expected to show some improvement in heat cycle resistance compared to aluminum nitride sintered substrates that do not contain zirconium, due to improved mechanical strength. However, resistance to partial discharge is not considered.

[0010] The present invention aims to improve the heat cycle resistance of an aluminum nitride sintered substrate by using a method that is advantageous for improving partial discharge resistance.

[0011] As a result of their research, the inventors found that distributing zirconium unevenly, particularly in the surface layer near at least one surface, rather than throughout the entire aluminum nitride sintered structure, is advantageous for improving partial discharge resistance and is also extremely effective in improving heat cycle resistance.

[0012] Based on the above findings, the following inventions are disclosed in this specification: [1] An aluminum nitride sintered substrate comprising a plate-shaped aluminum nitride sintered body, wherein the region from the surface to a depth of 1 / 3 of the plate thickness is called the surface layer, and the region other than the surface layer is called the central part of the plate thickness, and at least one of the surface layers is a zirconium-uneven surface layer containing a larger amount of zirconium atoms than the amount of zirconium atoms in the central part of the plate thickness. [2] The aluminum nitride sintered substrate according to [1], wherein in the zirconium-uneven surface layer, zirconium exists in the form of zirconium-containing particles. [3] The aluminum nitride sintered substrate according to [1] or [2], wherein in at least one depth region of the zirconium-uneven surface layer, the average zirconium concentration measured by an EPMA (electron probe microanalyzer) is 0.30 mass% or more, comprising a depth region 1 of which is 10 to 40 μm from the surface, a depth region 2 of which is 40 to 70 μm from the surface, and a depth region 3 of which is 70 to 100 μm from the surface. [4] The aluminum nitride sintered substrate according to [3], wherein the average zirconium concentration measured by EPMA (electron probe microanalyzer) in at least one of the depth regions 1, 2, and 3 is 0.90% by mass or more. [5] The aluminum nitride sintered substrate according to any one of [1] to [4], wherein the zirconium-uneven surface layer contains zirconium nitride. [6] A method for manufacturing an aluminum nitride sintered substrate according to any one of [1] to [5] above, comprising: a base sheet manufacturing step of preparing a paste A containing aluminum nitride powder and a sintering aid powder, and coating the paste A in a plate shape to obtain a base sheet; a multilayer sheet manufacturing step of preparing a paste B containing aluminum nitride powder, a zirconium-containing substance powder, and a sintering aid powder, and applying the paste B to at least one surface of the base sheet to obtain a multilayer sheet in which a zirconium-containing coating layer is formed on at least one surface of the base sheet; and a sintering step of heating the multilayer sheet to obtain an aluminum nitride sintered body in which the layers of the base sheet and the zirconium-containing coating layer are integrated.[7] In the step of manufacturing the multilayer sheet, a paste B containing 0.5 parts by mass or more and 15.0 parts by mass or less of zirconium-containing powder in terms of zirconium oxide (ZrO

[0013] )) is prepared with respect to 100 parts by mass of aluminum nitride powder. The manufacturing method according to [6] above. [8] The thickness of the base sheet is t Ａ (μm), and the thickness of the zirconium-containing coating layer is t Ｂ (μm). In the step of manufacturing the multilayer sheet, a multilayer sheet where t Ａ > t Ｂ is obtained. The manufacturing method according to [6] or [7] above. [9] When the thickness of the zirconium-containing coating layer is t Ｂ (μm), in the step of manufacturing the multilayer sheet, a multilayer sheet where t Ｂ is 8 μm or more and 120 μm or less is obtained. The manufacturing method according to any one of [6] to [8] above.

[10] When the thickness of the zirconium-containing coating layer is t Ｂ (μm), in the step of manufacturing the multilayer sheet, a multilayer sheet where t Ｂ is 20 μm or more and 120 μm or less is obtained. The manufacturing method according to any one of [6] to [8] above.

[11] An insulating substrate for an electronic circuit using the aluminum nitride sintered substrate according to any one of [1] to [5] above, having a structure in which a conductive layer of copper or a copper alloy is formed on at least one surface of the aluminum nitride sintered substrate.

[12] The insulating substrate for an electronic circuit according to

[11] above, wherein the thickness of the aluminum nitride sintered substrate is 0.1 to 3.0 mm.

[13] The insulating substrate for an electronic circuit according to

[11] above, wherein the thickness of the aluminum nitride sintered substrate is 0.25 to 2.0 mm.

[0013] According to the present invention, in an insulating substrate for an electronic circuit using an aluminum nitride sintered substrate, it has become possible to provide one excellent in heat cycle resistance by a method advantageous for improving partial discharge resistance.

[0014] A schematic cross-sectional view illustrating the cross-sectional structure near the edge of an insulating substrate for electronic circuits. A schematic diagram showing the cross-sectional structure of the aluminum nitride sintered substrate of the present invention parallel to the thickness direction. A diagram illustrating a backscattered electron image obtained by SEM of a cross-section parallel to the thickness direction of the aluminum nitride sintered substrate according to the present invention. A diagram showing the EDS analysis area in a backscattered electron image of a magnified portion of region B in Figure 3. A diagram illustrating the EDS spectrum for particle a in Figure 4. A diagram illustrating the EDS spectrum for particle b in Figure 4. A diagram illustrating the EDS spectrum for particle c in Figure 4. A diagram showing the dimensions and shape of a copper-coated aluminum nitride sintered substrate used in a heat cycle resistance test. A diagram showing the dimensions and shape of a copper-coated aluminum nitride sintered substrate used for partial discharge measurement.

[0015] [Aluminum Nitride Sintered Substrate] The aluminum nitride (AlN) sintered substrate targeted by this invention has at least one surface portion composed of a sintered structure in which zirconium (Zr) is scattered within the aluminum nitride sintered body. Here, the aluminum nitride sintered body is formed when adjacent aluminum nitride particles are sintered together to form a single unit.

[0016] In this specification, in a plate-shaped aluminum nitride sintered body constituting an aluminum nitride sintered substrate, the region from the surface to a depth of 1 / 3 of the plate thickness is referred to as the surface layer, and the region other than the surface layer is referred to as the center of the plate thickness. For example, in an aluminum nitride sintered substrate with a plate thickness of 0.3 mm, the thickness of each surface layer and the center of the plate thickness as referred to in this specification is 0.1 mm. In the aluminum nitride sintered substrate of the present invention, in the distribution of zirconium in the plate thickness direction, zirconium is unevenly distributed in at least one of the surface layers. That is, at least one of the surface layers contains a larger amount of zirconium atoms than the amount of zirconium atoms in the center of the plate thickness. The surface layer (the region up to a depth of 1 / 3 of the plate thickness) that contains a larger amount of zirconium than the center of the plate thickness is referred to as the zirconium unevenly distributed surface layer. When the amount of zirconium atoms in the center of the plate thickness is p (moles), and the amount of zirconium atoms in the surface layer on the side with the greater amount of zirconium atoms is q (moles), it is preferable, for example, that p ≤ 0.5q from the viewpoint of sufficiently improving partial discharge resistance. However, p may be 0.

[0017] The zirconium distribution in the zirconium-uneven surface layer does not necessarily have to be uniform in the direction of the plate thickness, and variations in zirconium concentration within the zirconium-uneven surface layer are acceptable. For example, even if the zirconium distribution in the zirconium-uneven surface layer is such that zirconium is unevenly distributed near the surface (for example, in a very shallow region with a depth of 10 μm or less from the surface, or in a shallow region with a depth of 30 μm or less from the surface), the presence of this zirconium-uneven surface layer provides a significant improvement in heat cycle resistance. Furthermore, for example, in the case of an aluminum nitride sintered substrate, which is the subject of this invention, with a plate thickness of 0.3 mm or more, the aluminum nitride sintered substrate has a zirconium-uneven surface layer with a thickness of 0.1 mm or more (100 μm or more) on at least one side. In this case, a more preferable target is a zirconium plate thickness distribution such that, in at least one of the following depth regions of the zirconium-unevenly distributed surface, the average zirconium concentration measured by EPMA (Electron Probe Microanalyzer) is 0.30 mass% or more. This is achieved in depth region 1, depth region 2, depth region 3, depth region 3, where the average zirconium concentration measured by EPMA is 0.30 mass% or more. Furthermore, it is more preferable that the average zirconium concentration in at least one of the depth regions 1, 2, and 3 is 0.90 mass% or more, and even more preferable that it is 1.10 mass% or more. In particular, in depth region 1, it is even more preferable that the average zirconium concentration is 0.30 mass% or more, even more preferable that it is 0.90 mass% or more, and even more preferable that it is 1.10 mass% or more. The average zirconium concentration in each depth region 1, 2, and 3 is usually 5.00 mass% or less. Preferably, the concentration is 3.00% by mass or less. Typically, the average zirconium concentration in depth regions 1 to 3 is highest in depth region 1, followed by the next highest in depth region 2, and the lowest in depth region 3. The method for determining the average zirconium concentration in each depth region based on EPMA measurements will be explained in detail in the examples described later.

[0018] In the surface layer where zirconium is unevenly distributed, some or all of the zirconium often exists in the form of zirconium-containing particles. Typical forms of zirconium-containing particles include those mainly composed of zirconium nitride or zirconium oxynitride.

[0019] Figure 2 schematically shows the cross-sectional structure of the aluminum nitride sintered substrate of the present invention parallel to the thickness direction. The vertical direction in the figure corresponds to the thickness direction. Near the thickness-direction edge surface 41 of the aluminum nitride sintered substrate 4, there is a zirconium-unevenly distributed surface layer 5 where zirconium is unevenly distributed. That is, in its thickness-direction distribution, zirconium atoms are concentrated in the surface layer (the region from the surface to a depth of 1 / 3 of the thickness). Figure 2 illustrates an example in which the zirconium-unevenly distributed surface layer 5 is formed on both surface sides of the aluminum nitride sintered substrate 4. As mentioned above, the thickness-direction distribution of zirconium in the zirconium-unevenly distributed surface layer 5 does not necessarily have to be uniform, and there may be fluctuations in zirconium concentration within the zirconium-unevenly distributed surface layer.

[0020] Figure 3 illustrates a backscattered electron image obtained by a scanning electron microscope (SEM) of a cross-section parallel to the thickness direction of an aluminum nitride sintered substrate according to this specification. The vertical direction of the image corresponds to the thickness direction, and one surface of the aluminum nitride sintered substrate is located at the top of the image. The region indicated by symbol A in the figure corresponds to the portion derived from the base sheet obtained in the base sheet manufacturing process described later, and the region indicated by symbol B corresponds to the portion derived from the zirconium-containing coating layer formed in the multilayer sheet manufacturing process described later. These regions are integrated by the sintering process described later, and the whole becomes an aluminum nitride sintered body. In the region indicated by symbol B, tiny particles with high brightness (appearing whiter) are scattered, which are not seen in the region indicated by symbol A. These high-brightness particles are zirconium-containing particles, and the region indicated by symbol B, where these high-brightness particles are unevenly distributed in the thickness direction, corresponds to the zirconium-uneven surface layer 5 described above. In this example, it is considered that most of the zirconium atoms exist in the form of zirconium-containing particles. The relatively large particles with low brightness observed in the regions marked A and B are yttrium-containing particles.

[0021] Figure 4 shows a magnified backscattered electron image of a portion of region B in Figure 3. The following are examples of elemental analysis (point analysis) performed on particles a, b, and c shown in Figure 4 using EDS (Energy Dispersive X-ray Spectroscopy), with the electron beam aimed at the center of each particle.

[0022] Figure 5 shows the EDS spectrum for particle a, which has high brightness. A zirconium (Zr) peak is observed, indicating that particle a is a zirconium-containing particle. Figure 6 shows the EDS spectrum for particle b, which has high brightness. A zirconium (Zr) peak is observed, indicating that particle b is a zirconium-containing particle. The higher amount of aluminum (Al) detected compared to particle a (Figure 5) is thought to be because particle b has a smaller diameter than particle a, and therefore picked up more fluorescent X-rays from the aluminum substrate. These zirconium-containing particles are thought to be mainly composed of zirconium nitride (ZrN). In addition, in the aluminum nitride sintered substrate of the present invention, zirconium oxide particles or zirconium oxynitride particles may also be included as zirconium-containing particles.

[0023] Figure 7 shows the EDS spectrum for particle c, which has a slightly lower brightness (appears grayer) than particles a and b. Particle c can be distinguished from zirconium-containing particles by the observation of a strong peak of yttrium (Y), which originates from the yttrium compound used as a sintering aid. Small amounts of zirconium may also be detected in particles containing large amounts of metallic elements, which are components of this type of sintering aid.

[0024] The size of the zirconium-containing particles present in the zirconium-unevenly distributed surface layer 5 is preferably such that the average particle diameter on the cross-sectional backscattered electron image is in the range of, for example, 0.2 to 2.0 μm. Here, "particle diameter" refers to the "major axis," which is the maximum possible length of the line segment (limited to those that do not extend outside the particle) connecting two points on the contour line of the particle on the image plane.

[0025] In this specification, zirconium-containing particles refer to particles in which, excluding Al, the most abundant element detected in the elemental analysis (point analysis) by EDS described above is Zr. The typical form is considered to be particles mainly composed of the ZrN phase. Since ZrN is conductive, zirconium-containing particles dispersed in the surface layer of the aluminum nitride sintered substrate are thought to have the effect of mitigating electric field concentration. On the other hand, regions other than the surface layer maintain higher insulation properties compared to the surface layer. It is presumed that the synergistic effect of "mitigation of electric field concentration" and "ensuring insulation properties," resulting from this unique dispersion form of zirconium-containing particles, contributes to the improvement of partial discharge resistance.

[0026] The unevenly distributed zirconium surface layer 5 increases the strength and toughness of the aluminum nitride sintered substrate 4, thereby improving the heat cycle resistance of the aluminum nitride sintered substrate 4. The unevenly distributed zirconium surface layer 5 may be provided on one side or on both sides of the aluminum nitride sintered substrate 4. From the viewpoint of enhancing the effect of improving heat cycle resistance, it is preferable that it be provided on both sides.

[0027] Furthermore, zirconium, in any form, has inferior thermal conductivity compared to aluminum nitride. In ceramic plates in which zirconium is incorporated throughout the aluminum nitride sintered structure, as shown in Patent Documents 1 to 3, the deterioration of thermal conductivity due to zirconium affects the entire structure. Therefore, the aluminum nitride sintered substrate 4 of the present invention is superior to the conventional technology in terms of thermal conductivity. In addition, since zirconium is expensive, the present invention, which involves uneven distribution of zirconium, is superior in terms of raw material costs compared to the conventional technology of incorporating it throughout the aluminum nitride sintered structure. In particular, forming an unevenly distributed zirconium surface layer 5 is extremely effective in improving partial discharge resistance.

[0028] The thickness of the aluminum nitride sintered substrate 4 described above can be, for example, 0.1 to 3.0 mm. From the viewpoint of manufacturing costs and miniaturization of equipment using the aluminum nitride sintered substrate 4, the thickness is preferably 0.25 to 2.0 mm, and more preferably 0.3 to 1.5 mm. It may also be possible to control the thickness to ensure a thickness of 0.5 mm or more.

[0029] [Method for manufacturing aluminum nitride sintered substrate] The above aluminum nitride sintered substrate can be manufactured, for example, by the method disclosed below.

[0030] (Substrate Sheet Manufacturing Process) A paste A is prepared containing aluminum nitride powder and sintering aid powder as raw material powders. Preferably, the aluminum nitride powder used has a cumulative 50% particle size D50 in the volume-based particle size distribution measured by laser diffraction / scattering after being dispersed in water containing a dispersant, which is, for example, 0.5 μm or more and 20.0 μm or less, and more preferably 0.8 μm or more and 10.0 μm or less.

[0031] The sintering aid can be any known substance that has been conventionally used in the production of aluminum nitride sintered bodies. For example, a compound containing one or more alkali metals, alkaline earth metals, or rare earth elements can be used. Lithium is an example of the alkali metal. Beryllium, magnesium, calcium, strontium, barium, etc. are examples of the alkaline earth metal elements. Yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, etc. Of these, oxides of calcium, strontium, barium, yttrium, lanthanum, cerium, and neodymium are more preferred. These compounds may be used individually or in combination of two or more. The cumulative 50% particle size D50 in the volume-based particle size distribution of the sintering aid powder, measured by laser diffraction / scattering after dispersion in water containing a dispersant, is not particularly limited, but for example, a particle size of 1.0 μm or more and 3.2 μm or less can be used.

[0032] The mixing ratio of the sintering aid powder should be set within the conventional range. Yttrium oxide (Y) can be used as the sintering aid powder. ２ O ３For example, when using powder, the amount of yttrium oxide powder can be in the range of 1.0 to 10.0 parts by mass per 100 parts by mass of aluminum nitride powder, and it is more preferable to use an amount of 2.5 to 6.0 parts by mass of yttrium oxide powder.

[0033] In addition to the above raw material powders, a binder component and a solvent component consisting of a substance that dissolves the binder component are added and mixed using a ball mill or the like to obtain paste A. Conventional binder components can be used, and examples include resin components such as polyvinyl butyral resin, acrylic resin, epoxy resin, and ethylcellulose resin, phthalate-based plasticizers such as dibutyl phthalate, bis(2-ethylhexyl) phthalate, and diisononyl phthalate, phosphoric acid-based plasticizers such as tricresyl phosphate, tris(2-butoxyethyl) phosphate, and tris(2-ethylhexyl) phosphate, and adipic acid-based plasticizers such as bis(2-ethylhexyl) adipate, diisononyl adipate, and diisodecyl adipate. Examples of solvent components include methanol, ethanol, isopropyl alcohol, 1-butanol, acetone, methyl ethyl ketone, ethylene glycol, propylene glycol, 1,3-butylene glycol, methyl acetate, ethyl acetate, toluene, xylene, and tetrahydrofuran.

[0034] The obtained paste A (viscosity of, for example, 1,000 to 20,000 mPa·s) is formed into a sheet using coating means such as a doctor blade, knife coater, roll coater, or die coater. The sheet thickness can be set by lamination as needed, taking into account the thickness of the zirconium-containing coating layer described later, so that the target plate thickness (for example, 0.1 mm to 3.0 mm) can be obtained in the aluminum nitride sintered substrate product. The base sheet can then be obtained by drying the sheet to evaporate the solvent. The drying method may be natural drying or heating drying at 150°C or below.

[0035] (Multilayer sheet manufacturing process) A paste B is prepared containing aluminum nitride powder, zirconium-containing material powder, and sintering aid powder as raw material powders.

[0036] For the aluminum nitride powder, one with the same particle size distribution as that used for the base sheet described above should be used. For the sintering aid, for example, one or more of the compounds described above can be used. Usually, the same type as that used for the base sheet described above should be used. The mixing ratio of the sintering aid powder should be set within the same range as when conventionally obtaining a general aluminum nitride sintered body. Yttrium oxide (Y ２ O ３ The mixing ratio when using powder is the same as in the case of paste A described above.

[0037] Paste B is mixed with a zirconium-containing powder (hereinafter sometimes referred to as zirconium-containing powder). Suitable zirconium-containing substances include zirconium oxide, zirconium hydroxide, zirconium nitrate, zirconium carbonate, zirconium sulfate, zirconium oxychloride, zirconium tetrachloride, zirconium acetate, and ammonium zirconium carbonate. The aforementioned zirconium oxide (ZrO) ２ ) as pure ZrO ２ In addition, stabilized zirconia, partially stabilized zirconia, etc. can be used. The mixing ratio of zirconium-containing powder in paste B (i.e., in the zirconium-containing coating layer described later) is 100 parts by mass of aluminum nitride powder to zirconium oxide (ZrO ２ It is preferable that the amount be between 0.5 parts by mass and 18.0 parts by mass in terms of ) conversion. Within this range, excellent improvements in heat cycle resistance and partial discharge resistance can be obtained. Zirconium oxide (ZrO ２ It is more effective to use an amount of 0.8 parts by mass or more and 15.0 parts by mass or less in terms of ) conversion. ２ ) Conversion means that the total amount of Zr supplied from the zirconium-containing powder is converted to ZrO ２ The ZrO that is required when supplying it ２ This means expressing it in terms of its mass.

[0038] For example, the zirconium-containing powder should be one in which the cumulative 50% particle size D50 in the volume-based particle size distribution, measured by laser diffraction / scattering after dispersion in water containing a dispersant, is between 0.4 μm and 4.9 μm. The D90 of the zirconium-containing powder is preferably smaller than the D90 of the mixed powder after mixing with aluminum nitride powder or the like for the preparation of paste B. For example, zirconium-containing powders with a D90 of 10.0 μm or less, 5.0 μm or less, 2.5 μm or less, or 1.2 μm or less can be used. The lower limit of the D90 of the zirconium-containing powder is not particularly limited, but for example, D90 can be 0.40 μm or more.

[0039] In addition to the raw material powders mentioned above, a binder component and a solvent component consisting of a substance that dissolves the binder component are added and mixed using a ball mill or the like to obtain paste B. The binder component and solvent component used to prepare paste B are those listed in the description of paste A above.

[0040] Next, paste B is applied to at least one surface of the base sheet to form a zirconium-containing coating layer, thereby creating a "multilayer sheet" consisting of the base sheet and the zirconium-containing coating layer. From the viewpoint of heat cycle resistance, it is preferable to form the zirconium-containing coating layer on both surfaces of the base sheet. From the viewpoint of significantly improving partial discharge resistance, it is preferable that the thickness of the zirconium-containing coating layer be thinner than the thickness of the base sheet. That is, the thickness of the base sheet is t Ａ (μm), the thickness of the zirconium-containing coating layer is t Ｂ When (μm), t Ａ >t Ｂ It is preferable to adjust the application thickness of paste B so that a multi-layer sheet is obtained. Ａ > 5t Ｂ It is more effective to use a multi-layer sheet that satisfies the following conditions: Ａ >7t Ｂ A multi-layer sheet that satisfies the following conditions may also be used. ＢIt is preferable to set it in the range of 8 μm to 120 μm, for example, but it may also be set in the range of 20 μm to 120 μm, or 25 μm to 110 μm. It is even more preferable to set it in the range of 30 μm to 90 μm. Here, t Ｂ t is the thickness of the zirconium-containing coating layer formed on one of the surfaces of the base sheet. If zirconium-containing coating layers are formed on both surfaces of the base sheet, then t is the thickness of each zirconium-containing coating layer. Ｂ It is preferable that all of them satisfy the above conditions.

[0041] The resulting multilayer sheet is heated and dried to remove solvent components derived from paste B before being subjected to the degreasing process. Here, heating can be done at a relatively low temperature, for example, between 80°C and 150°C.

[0042] (Degreasing process) Next, a degreasing process is carried out to remove the binder components contained in the dried multilayer sheet before subjecting it to the sintering process. As a method of removal, it is preferable to apply a method of heating and holding the sheet in a temperature range (for example, 300°C to 700°C) in which the volatilization of the organic substances that are binder components is promoted and sintering does not occur. In this case, the heating atmosphere can be air.

[0043] (Sintering Process) The multilayer sheet that has undergone the degreasing process is heated and sintered to obtain an aluminum nitride sintered body in which the base sheet layer and the zirconium-containing coating layer are integrated. Sintering can be carried out by firing the degreasing multilayer sheet in a nitrogen atmosphere at a temperature of 1700°C to 1900°C, more preferably 1730°C to 1800°C. The firing time (holding time in a predetermined temperature range) can be set, for example, in the range of 1 hour to 10 hours. This sintering process can be carried out using conventional aluminum nitride sintered substrate manufacturing equipment. In this sintering process, zirconium diffusion occurs, so the zirconium distribution in the thickness direction of the sheet changes somewhat, and the zirconium concentration tends to be higher in the region relatively close to the surface, even in the portion of the obtained aluminum nitride sintered substrate that originates from the zirconium-containing coating layer.

[0044] As described above, an aluminum nitride sintered substrate can be obtained, which consists of a plate-shaped aluminum nitride sintered body and has a surface layer (zirconium-unevenly distributed surface layer) on at least one surface side in which zirconium is unevenly distributed.

[0045] [Insulating Substrate for Electronic Circuits] When an insulating substrate for electronic circuits is constructed using the aluminum nitride sintered substrate described above, by forming a conductive layer on at least one surface of the aluminum nitride sintered substrate, the partial discharge resistance of the insulating substrate for electronic circuits can be significantly improved. Furthermore, the presence of a surface layer in which zirconium is unevenly distributed improves strength and also improves heat cycle resistance. The thickness of the aluminum nitride sintered substrate can be set in the range of 0.1 to 3.0 mm, for example, depending on the application. It may also be 0.25 to 2.0 mm thick. It may also be controlled to ensure a thickness of 0.5 mm or more. As the conductive layer, for example, a plate-shaped member made of copper or a copper alloy can be suitably applied. The method of joining the aluminum nitride sintered substrate and the conductive layer is not limited, but it is preferable that they be joined via a brazing material containing an active metal, for example.

[0046] [Example 1] (Preparation of base sheet) The following materials were prepared. The mixing ratio of each is expressed in parts by mass per 100 parts by mass of aluminum nitride powder. (A1) Aluminum nitride (AlN) powder (D50 = 1.08 μm): 100 parts by mass (A2) Yttrium oxide (Y ２ O ３ (A3) Powder (D50 = 1.21 μm): 5 parts by mass (A4) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 parts by mass (A5) Dibutyl phthalate (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 parts by mass (A6) Ethanol + Toluene: 200 parts by mass The above (A3) and (A4) correspond to binder components, and (A5) corresponds to solvent components.

[0047] The materials (A1) to (A5) listed above were mixed in a mill pot to obtain paste A. Paste A was degassed under vacuum, its viscosity was adjusted, and then it was formed into a sheet approximately 0.77 mm thick (770 μm) using a sheet molding machine (manufactured by Tester Industries Co., Ltd.) with the doctor blade method. This sheet was dried in room temperature air for 18 hours to obtain a base sheet.

[0048] (Preparation of multilayer sheet) The following materials were prepared to form a zirconium-containing coating layer: (B1) Aluminum nitride (AlN) powder (D50 = 1.38 μm): 100 parts by mass (B2) Yttrium oxide (Y ２ O ３ ) Powder (D50 = 1.23 μm): 5 parts by mass (B3) Yttria-stabilized zirconia (YSZ; manufactured by Tosoh Corporation) powder (D50 = 0.56 μm): ZrO ２ Converted to 1.0 part by mass (B4) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 parts by mass (B5) Dibutyl phthalate (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 parts by mass (B6) Ethanol + Toluene: 200 parts by mass The above (B4) and (B5) correspond to binder components, and (B6) corresponds to solvent components.

[0049] The materials (B1) to (B6) above were mixed in a mill pot to obtain paste B. A 200 μm gap was created on the upper surface of the base sheet, paste B was poured in, and paste B was applied with a bar coater to form a zirconium-containing coating layer. After that, it was dried in air at 100°C to obtain a multilayer sheet in which the zirconium-containing coating layer was formed on one side of the base sheet. The thickness of the zirconium-containing coating layer is approximately 90 μm. In this case, the thickness of the base sheet is t Ａ = 770 μm, thickness t of the zirconium-containing coating layer Ｂ = It can be considered to be 90 μm, t Ａ >t Ｂ It fully satisfies the requirements.

[0050] In this example, the mixing ratio of zirconium-containing powder in paste B (i.e., zirconium-containing coating layer) is 100 parts by mass of aluminum nitride powder to zirconium oxide (ZrO ２ This is equivalent to 1.0 parts by mass.

[0051] (Fabrication of aluminum nitride sintered substrate) The above multilayer sheet was degreased by holding it in a degreasing furnace at 500°C for 2 hours in air, and then fired in a firing furnace at 1750°C for 8 hours in a nitrogen gas atmosphere to obtain an aluminum nitride sintered substrate (i.e., a plate-shaped aluminum nitride sintered body) with a thickness of approximately 0.75 mm, in which the base sheet layer and the zirconium-containing coating layer were integrated.

[0052] (SEM-EDS analysis of the cross-section) The cross-section of the obtained aluminum nitride sintered substrate, parallel to the thickness direction, was observed using an SEM (Scanning Electron Microscope, Hitachi High-Tech Corporation, SU3800), and elemental analysis of the particles observed in the cross-section was performed by EDS (Energy Dispersive X-ray Spectroscopy). The observation surface was prepared by polishing with diamond abrasive grains (in three stages: particle size 9 μm, 3 μm, and 1 μm). It was confirmed that the aluminum nitride sintered substrate in this example exhibits a unique aluminum nitride sintered structure in which zirconium is unevenly distributed in the surface layer, including the thickness direction region derived from the zirconium-containing coating layer, in the form of zirconium-containing particles. It was found that the average particle size of the zirconium-containing particles, based on their major axis, was in the range of 0.2 to 2.0 μm (the same applies to each of the following examples).

[0053] (Fabrication of Copper-Coated Aluminum Nitride Sintered Substrate) A 68 mm square plate was cut from the above-mentioned aluminum nitride sintered substrate. Using this plate, a copper (Cu) coated aluminum nitride sintered substrate (modeled after an insulating substrate for electronic circuits) with the dimensions and shape shown in Figure 8 (34 mm square) was fabricated as follows. In Figure 8, the upper diagram shows the surface on which copper material is formed as a metal component for circuits (hereinafter referred to as the "circuit surface"), and the lower diagram shows the surface on the back side on which copper material is formed as a metal component for heat dissipation (hereinafter referred to as the "back surface"). The surface layer in which zirconium is unevenly distributed in the aluminum nitride sintered substrate is formed on the circuit surface side.

[0054] Using a screen printing plate (200 mesh, stainless steel mesh, wire diameter 40 μm, emulsion 10 μm), a paste-like brazing material containing Ag, Cu, and Ti as metal components in a mass ratio of Ag:Cu:Ti = 87.8:10:2.2 was screen printed on both sides of a 68 mm square aluminum nitride sintered substrate to form a brazing layer with a thickness of 40 μm per side. Copper plates were placed on each side of the aluminum nitride sintered substrate via this brazing layer to form a copper plate-aluminum nitride sintered substrate-copper plate laminate. This laminate was heated to 850°C in a vacuum to bond the aluminum nitride sintered substrate to the copper plates on both sides. In this case, copper plates with a thickness of 0.25 mm were used on both sides.

[0055] A UV-curable alkaline peel-off resist was screen-printed onto copper plates bonded to both sides of an aluminum nitride sintered substrate. The coating pattern corresponds to four copper patterns shown in Figure 8. After curing the resist with UV light, unwanted copper was removed using an etching solution consisting of copper chloride, hydrochloric acid, and water, and then the resist was removed with an aqueous sodium hydroxide solution. Subsequently, the plates were pickled by immersion in dilute sulfuric acid for 20 seconds, and then immersed in a chelate aqueous solution containing EDTA, ammonia water, and hydrogen peroxide to remove unwanted brazing material remaining on the surface of the aluminum nitride sintered substrate around the copper plates (the brazing material located beneath the copper removed by the etching solution). After that, a medium-phosphorus Ni-P plating (3-4 μm thick) was applied to the surface of the copper material by electroless plating. The obtained 68 mm square material was divided into four sections to obtain 34 mm square copper-coated aluminum nitride sintered substrates with the shape shown in Figure 8. These were used as test materials for the following investigations.

[0056] (Heat cycle resistance test) Using a batch furnace equipped with a carbon hot plate capable of heating and cooling, the above 34 mm square copper-clad aluminum nitride sintered substrate was subjected to repeated heat loads as described below, and the number of cycles at which cracks occurred in the ceramic substrate was investigated.

[0057] The copper-coated aluminum nitride sintered substrate sample, which was the test material, was placed on the hot plate with its reverse side (the side opposite the circuit side) in contact with the hot plate. The batch furnace was maintained in a reducing atmosphere of hydrogen:nitrogen = 20:80 (vol %). The hot plate functions as a heating element when the rod heater inside it is energized, and as a cooling element when the power to the heater is turned off and a cooling plate is pressed against the underside of the hot plate.

[0058] Using the heating and cooling functions of the hot plate described above, a copper-coated aluminum nitride sintered substrate sample placed on the hot plate was subjected to a heat cycle consisting of the following heat pattern for up to 25 cycles: The hot plate temperature was raised to 380°C at an average heating rate of 1.0°C / s → held at 380°C for 10 minutes → cooled from 380°C to 100°C at an average cooling rate of 2.5°C / s, and then cooled from 100°C to 40°C at an average cooling rate of 0.5°C / s.

[0059] At the end of cycles 1, 2, 3, 5, 7, 8, 10, 13, 15, 18, 20, 23, and 25, the samples were removed from the furnace, allowed to cool to room temperature, and the exposed ceramic portions on the circuit and back surfaces of the samples were observed with a microscope to check for crack formation in the ceramics. Samples in which no crack formation was observed, except for the sample that completed 25 cycles, were returned to the batch furnace and the heat cycling was continued. The test was conducted on five samples (number of tests n=5) (the same applies to Examples 2, 3, and Comparative Example 1 below). As a result, crack formation was first observed in one sample at the end of cycle 18 and in one sample at the end of cycle 23. The results are shown in Table 1.

[0060] (Ultrasonic testing of bonding interfaces) For the copper-coated aluminum nitride sintered substrate, which was the test material, ultrasonic testing images were acquired of the ceramic-copper material bonding area on the circuit side and the ceramic-copper material bonding area on the back side using an ultrasonic testing device (SAT) (FS100II, manufactured by Hitachi Construction Machinery Fine Tech Co., Ltd.) to check for the presence or absence of bonding defects at each bonding area. As a result, no bonding defects that could cause problems in actual use were observed, and it was confirmed that the aluminum nitride sintered substrate in this example has no problems with bonding to copper material.

[0061] [Example 2] Except that the operation of applying paste B to form a zirconium-containing coating layer was performed on both sides of the base sheet during the preparation of the multilayer sheet, an aluminum nitride sintered substrate and a copper-coated aluminum nitride sintered substrate were prepared in the same manner as in Example 1 and subjected to the same investigation as in Example 1. The thickness of the zirconium-containing coating layer in the multilayer sheet prepared in this example is approximately 90 μm on both sides. In this case, the thickness of the base sheet t Ａ = 770 μm, thickness t of the zirconium-containing coating layer Ｂ = It can be considered to be 90 μm, t Ａ >t Ｂ The requirements are fully met. The thickness of the resulting aluminum nitride sintered substrate is approximately 0.75 mm.

[0062] SEM-EDS analysis confirmed that the aluminum nitride sintered substrate in this example exhibits a unique aluminum nitride sintered structure, similar to Example 1, in which zirconium is unevenly distributed in the surface layer, including the thickness-direction region derived from the zirconium-containing coating layer, in the form of zirconium-containing particles. Heat cycle resistance testing revealed that in this example, crack formation was first observed in one sample of the copper-coated aluminum nitride sintered substrate after 25 cycles. No crack formation was observed in the remaining four samples after 25 cycles. The results are shown in Table 1. Ultrasonic testing revealed no bonding defects that could cause problems in actual use, confirming that the aluminum nitride sintered substrate in this example has no problems with bonding to copper material.

[0063] [Example 3] Except that the material composition of paste B for forming the zirconium-containing coating layer in the preparation of the multilayer sheet was as follows, and the operation of applying paste B to form the zirconium-containing coating layer was performed on both sides of the base sheet, an aluminum nitride sintered substrate and a copper-coated aluminum nitride sintered substrate were prepared in the same manner as in Example 1 and subjected to the same investigation as in Example 1. (B1) Aluminum nitride (AlN) powder (D50 = 1.30 μm): 100 parts by mass (B2) Yttrium oxide (Y ２ O ３ ) Powder (D50 = 1.30 μm): 5 parts by mass (B3) Yttria-stabilized zirconia (YSZ; manufactured by Tosoh Corporation) powder (D50 = 0.49 μm): ZrO ２ Converted to 3.0 parts by mass (B4) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 parts by mass (B5) Dibutyl phthalate (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 parts by mass (B6) Ethanol + Toluene: 200 parts by mass In this example, the mixing ratio of zirconium-containing powder in paste B (i.e., zirconium-containing coating layer) is 100 parts by mass of aluminum nitride powder to zirconium oxide (ZrO ２ This is equivalent to 3.0 parts by mass in terms of t. Furthermore, the thickness of the zirconium-containing coating layer in the multilayer sheet is approximately 90 μm on both sides. In this case, the thickness of the base sheet is t. Ａ = 770 μm, thickness t of the zirconium-containing coating layer Ｂ = It can be considered to be 90 μm, t Ａ >t Ｂ The requirements are fully met. The thickness of the resulting aluminum nitride sintered substrate is approximately 0.73 mm.

[0064] SEM-EDS analysis confirmed that the aluminum nitride sintered substrate in this example exhibits a unique aluminum nitride sintered structure, similar to Example 1, in which zirconium is unevenly distributed in the surface layer, including the thickness-direction region derived from the zirconium-containing coating layer, in the form of zirconium-containing particles. Heat cycle resistance testing showed no cracks in the copper-coated aluminum nitride sintered substrate in this example even after 25 cycles. The results are shown in Table 1. Ultrasonic testing revealed no bonding defects that could cause problems in actual use, confirming that the aluminum nitride sintered substrate in this example has no problems with bonding to copper material.

[0065] [Comparative Example 1] In this example, an aluminum nitride sintered substrate (approximately 0.64 mm thick) made of a conventional aluminum nitride sintered body that does not contain zirconium was prepared according to the procedure for preparing the base sheet shown in Example 1. Using this, a copper-coated aluminum nitride sintered substrate having the same dimensions and shape as in Example 1 was prepared, and the same investigation as in Example 1 was conducted.

[0066] In the heat cycle resistance test, 18 samples (test number n=18) were subjected to the same heat cycles as in Example 1 for up to 20 cycles. As a result, cracks were first observed in 4 samples of the copper-coated aluminum nitride sintered substrate in this example after 10 cycles, 7 samples after 15 cycles, and 3 samples after 20 cycles. The results are shown in Table 1. Ultrasonic testing revealed no bonding defects that could be problematic in actual use, confirming that conventional aluminum nitride sintered substrates without zirconium have no problems with bonding to copper material.

[0067]

[0068] As can be seen from Table 1, a copper-clad aluminum nitride sintered substrate (insulating substrate for electronic circuits) using an aluminum nitride sintered substrate in which zirconium is unevenly distributed in the surface layer showed improved heat cycle resistance compared to a conventional aluminum nitride sintered substrate without added zirconium (Comparative Example 1). In other words, even without zirconium being present throughout the entire substrate, forming a surface layer in which zirconium is unevenly distributed improves heat cycle resistance, thereby improving quality stability during manufacturing and reliability during product use. In particular, using an aluminum nitride sintered substrate with a surface layer in which zirconium is unevenly distributed on both sides yields an even better improvement in heat cycle resistance.

[0069] Next, using the aluminum nitride sintered substrates obtained in Example 3, Comparative Example 1, and Comparative Example 2 described below, copper-coated aluminum nitride sintered substrates were fabricated and subjected to evaluation of their partial discharge resistance.

[0070] [Comparative Example 2] A paste was prepared using the material formulations B1 to B6 described in Example 3, and a sheet material was prepared using the paste in accordance with the manufacturing method for the base sheet described in Example 1. The sheet material was degreased by holding it in a degreasing furnace at 500°C in air for 2 hours, and then fired in a firing furnace at 1750°C in a nitrogen gas atmosphere for 8 hours to obtain an aluminum nitride sintered substrate with a thickness of approximately 0.64 mm in which zirconium was present throughout the entire thickness of the plate.

[0071] [Evaluation of Partial Discharge Resistance] (Fabrication of Copper-Coated Aluminum Nitride Sintered Substrate) A 50 mm x 44.2 mm plate was cut from the aluminum nitride sintered substrate prepared by the methods described in Example 3, Comparative Example 1, and Comparative Example 2 above. Copper plates were bonded to both sides of this plate via a brazing layer to obtain a copper-coated aluminum nitride sintered substrate with the dimensions and shape shown in Figure 9. In Figure 9, hatching is shown on the copper plate portion. The left diagram of Figure 9 shows the arrangement of the copper plate bonded to one side of the plate, and the right diagram shows the arrangement of the copper plate bonded to the back side. The thickness of the copper plate in the left diagram (which will be used as the circuit metal component) is 0.3 mm, and the thickness of the copper plate in the right diagram (which will be used as the heat dissipation metal component) is 0.2 mm. Brazing and molding using resist were performed in accordance with the fabrication method of the copper-coated aluminum nitride sintered substrate shown in Figure 8 above. Here, no plating was applied to the surface of the copper plate.

[0072] Ten copper-coated aluminum nitride sintered substrates for partial discharge measurement, fabricated in this manner, were prepared for each example and used as test materials in the following tests with n=10 test numbers.

[0073] (Measurement of partial discharge) For the measurement of partial discharge, the partial discharge measurement system of Soken Electric Co., Ltd. (partial discharge measuring instrument: DAC-PD-9, partial discharge detector: DAC-PDE-2, power control unit: DAC-WTC-1, partial discharge calibrator: DAC-CP-2) was used, and the measurement of partial discharge was performed according to the following procedure.

[0074] For the setting procedure, the copper-coated aluminum nitride sintered substrate, which was the test material, was immersed in insulating oil (Fluorinert FC-3283), a heat dissipation metal component was connected to GND, and a contact probe for applying the test voltage was brought into contact with the circuit metal component.

[0075] Calibration Procedure: A partial discharge calibrator was connected in parallel to the test material in this state to generate a calibration pulse equivalent to a partial discharge of 100 pC. After calibrating the partial discharge measuring instrument, the partial discharge calibrator was removed.

[0076] The holding voltage setting value V-HI of the measurement operation power control unit was set to the initial test voltage of 6kV. The power control unit's Auto mode was used to automatically increase the voltage to the above test voltage, and then it was held at that voltage. Twenty seconds after reaching the above test voltage, partial discharge data was collected for 5 seconds using the φ-Q mode of the partial discharge meter, and then the voltage was lowered. The measurement frequency settings of the partial discharge meter were a measurement center frequency f0 of 180kHz and a measurement bandwidth BW of 300kHz. The voltage increase and decrease speed by the power control unit was both 600V / s, and the frequency of the test voltage was 60Hz.

[0077] For each test material, the number of partial discharge pulses with a charge of 10 pC or more was counted from the 5-second partial discharge data collected. A lower count indicates better partial discharge resistance. Partial discharge resistance was evaluated using the average of the above counts over 10 tests (n=10). The results for each example are shown in Table 2.

[0078]

[0079] As can be seen from Table 2, it was confirmed that a copper-clad aluminum nitride sintered substrate (insulating substrate for electronic circuits) using an aluminum nitride sintered substrate with a surface layer in which zirconium is unevenly distributed exhibits excellent partial discharge resistance.

[0080] Examples 4 and 5 below show the elemental distribution in the thickness direction of the plate in the zirconium-unevenly distributed surface layer, as investigated by EPMA.

[0081] [Example 4] An aluminum nitride sintered substrate and a copper-coated aluminum nitride sintered substrate were prepared in the same manner as in Example 1, except that the material composition for forming the zirconium-containing coating layer in the preparation of the multilayer sheet was as follows: (B1) Aluminum nitride (AlN) powder (D50 = 1.30 μm): 100 parts by mass (B2) Yttrium oxide (Y ２ O ３ ) Powder (D50 = 1.30 μm): 5 parts by mass (B3) Yttria-stabilized zirconia (YSZ; manufactured by Tosoh Corporation) powder (D50 = 0.49 μm): ZrO ２Converted to 5.0 parts by mass (B4) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 parts by mass (B5) Dibutyl phthalate (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 parts by mass (B6) Ethanol + Toluene: 200 parts by mass In this example, the mixing ratio of zirconium-containing powder in paste B (i.e., zirconium-containing coating layer) is zirconium oxide (ZrO) per 100 parts by mass of aluminum nitride powder ２ This is equivalent to 5.0 parts by mass in terms of the thickness of the base sheet t. Ａ = 770 μm, thickness t of the zirconium-containing coating layer Ｂ = It can be considered to be 90 μm, t Ａ >t Ｂ The requirements are fully met. The thickness of the resulting aluminum nitride sintered substrate is approximately 0.76 mm.

[0082] Ultrasonic testing revealed no bonding defects that could cause problems in actual use, confirming that the aluminum nitride sintered substrate in this example has no issues with bonding to copper material.

[0083] (Elemental analysis by EPMA) For cross-sections parallel to the thickness direction near the surface of the aluminum nitride sintered substrate, semi-quantitative analysis was performed by point analysis using an EPMA (electron probe microanalyzer, JXA-8200 manufactured by JEOL Ltd.). The average zirconium concentration was determined in each of three depth regions: depth region 1 (10-40 μm), depth region 2 (40-70 μm), and depth region 3 (70-100 μm). The sample surface was prepared by polishing with diamond abrasive grains (in three stages: particle size 9 μm, 3 μm, and 1 μm).

[0084] The measurement was performed using an electron beam with a diameter of 30 μm irradiated onto the sample surface, as follows: A wide-area backscattered electron image of the sample surface was obtained. A position on the sample surface at a depth of 25 μm from the surface of the aluminum nitride sintered substrate was randomly selected, the beam's central axis was aligned with this position, and a 30 μm diameter electron beam was irradiated to perform point analysis. This operation was performed at four positions where the beam irradiation ranges did not overlap, and the arithmetic mean of the zirconium analysis values ​​(mass%) at the four positions was calculated. Since the beam diameter was 30 μm, this arithmetic mean can be considered to reflect the concentration of the element in the region with a depth of 10 to 40 μm. Therefore, the above arithmetic mean is adopted here as the average zirconium concentration (mass%) in depth region 1, which is 10 to 40 μm from the surface. Using a similar method, point analyses are performed at four locations on the sample surface, with the beam center axis aligned to positions at a depth of 55 μm from the surface of the aluminum nitride sintered substrate and at positions at a depth of 85 μm from the surface of the aluminum nitride sintered substrate. The arithmetic mean of the zirconium analysis values ​​(mass%) at each of the four locations is calculated, and these arithmetic mean values ​​are adopted as the average zirconium concentration (mass%) in depth region 2, where the depth from the surface is 40 to 70 μm, and as the average zirconium concentration (mass%) in depth region 3, where the depth from the surface is 70 to 100 μm.

[0085] The more detailed EPMA measurement conditions were as follows: Irradiation current: 3.0 × 10⁻⁶ －７ A. Integration time: 200 msec, step size: 50 μm, observation magnification: 800x, acceleration voltage: 15 kV, beam diameter during analysis: φ30 μm, spectroscopic crystals: LDE2H, LDE1, TAP, LIF, PETH. Qualitative analysis confirmed the elements present at the observation site. Quantitative analysis was performed using the ZAF method, quantifying the detected peak intensity (≒height). For elements with multiple detected peaks, only the strongest peak (≒main peak) was quantified. Note that nitrogen (N) was quantitatively analyzed using the LDE1 peak instead of LDE2H.

[0086] The measurement results are shown in Table 3.

[0087] In the table, a Zr content of 0% by mass means that no zirconium was detected (the same applies to Table 4 below).

[0088] In the aluminum nitride sintered substrate of this example, the average zirconium concentration in depth region 1, where the depth from the surface is 10 to 40 μm, was 1.393 mass%, in depth region 2, where the depth from the surface is 40 to 70 μm, was 0.397 mass%, and in depth region 3, where the depth from the surface is 70 to 100 μm, was 0.137 mass%. Furthermore, when elemental analysis by EPMA was performed in regions 30 μm deeper increments from depth region 3 up to a depth of 2 / 3 of the thickness of the aluminum nitride sintered substrate, the Zr concentration was less than 0.137 mass% in all regions.

[0089] [Example 5] Except for the following material composition for forming the zirconium-containing coating layer in the preparation of the multilayer sheet, an aluminum nitride sintered substrate and a copper-coated aluminum nitride sintered substrate were prepared in the same manner as in Example 1 and subjected to the same investigation as in Example 4. (B1) Aluminum nitride (AlN) powder (D50 = 1.30 μm): 100 parts by mass (B2) Yttrium oxide (Y ２ O ３ ) Powder (D50 = 1.30 μm): 5 parts by mass (B3) Yttria-stabilized zirconia (YSZ; manufactured by Tosoh Corporation) powder (D50 = 0.49 μm): ZrO ２ Converted to 15.0 parts by mass (B4) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 parts by mass (B5) Dibutyl phthalate (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 parts by mass (B6) Ethanol + Toluene: 200 parts by mass In this example, the mixing ratio of zirconium-containing powder in paste B (i.e., zirconium-containing coating layer) is zirconium oxide (ZrO) per 100 parts by mass of aluminum nitride powder ２ This is equivalent to 15.0 parts by mass in terms of the thickness of the base sheet t. Ａ = 770 μm, thickness t of the zirconium-containing coating layer Ｂ = It can be considered to be 90 μm, t Ａ >t ＢThe requirements are fully met. The thickness of the resulting aluminum nitride sintered substrate is approximately 0.75 mm.

[0090] Ultrasonic testing revealed no bonding defects that could cause problems in actual use, confirming that the aluminum nitride sintered substrate in this example has no issues with bonding to copper material.

[0091] Table 4 shows the results of elemental analysis performed by EPMA using the same method as in Example 4.

[0092]

[0093] In the aluminum nitride sintered substrate of this example, the average zirconium concentration in depth region 1, where the depth from the surface is 10 to 40 μm, was 2.433 mass%, in depth region 2, where the depth from the surface is 40 to 70 μm, was 0.269 mass%, and in depth region 3, where the depth from the surface is 70 to 100 μm, was 0.087 mass%. Furthermore, when elemental analysis by EPMA was performed in regions 30 μm deeper increments from depth region 3 up to a depth of 2 / 3 of the thickness of the aluminum nitride sintered substrate, the Zr concentration was less than 0.087 mass% in all regions.

[0094] Furthermore, the aluminum nitride sintered substrates obtained in Examples 4 and 5 are considered to exhibit better heat cycle resistance than those of Example 1.

[0095] [Example 6-16] In the preparation of a multilayer sheet, the amount of yttria-stabilized zirconia powder shown in Example 1 (B3) was changed to ZrO ２ Paste B was prepared in quantities of 5.0 parts by mass (Examples 6-9), 10.0 parts by mass (Examples 10-12), or 15.0 parts by mass (Examples 13-16) by conversion, and paste B was applied to a thickness t. Ｂ An aluminum nitride sintered substrate and a copper-coated aluminum nitride sintered substrate were prepared in the same manner as in Example 1, except that a zirconium-containing coating layer, adjusted as described below, was formed on both sides of the base sheet. The thickness of the base sheet was t. Ａ = 770 μm, and the thickness of the obtained aluminum nitride sintered substrate is as follows: (ZrO ２Example of equivalent to 5.0 parts by mass) Example 6: t Ｂ = 10 μm, thickness of aluminum nitride sintered substrate = approximately 0.67 mm Example 7: t Ｂ = 30 μm, thickness of aluminum nitride sintered substrate = approximately 0.68 mm Example 8: t Ｂ = 60 μm, thickness of aluminum nitride sintered substrate = approximately 0.72 mm Example 9: t Ｂ = 90 μm, thickness of aluminum nitride sintered substrate = approximately 0.76 mm (ZrO ２ Example of equivalent to 10.0 parts by mass) Example 10: t Ｂ = 10 μm, thickness of aluminum nitride sintered substrate = approximately 0.65 mm Example 11: t Ｂ = 30 μm, thickness of aluminum nitride sintered substrate = approximately 0.67 mm Example 12: t Ｂ = 60 μm, thickness of aluminum nitride sintered substrate = approximately 0.73 mm (ZrO ２ Example of equivalent to 15.0 parts by mass) Example 13: t Ｂ = 10 μm, thickness of aluminum nitride sintered substrate = approximately 0.65 mm Example 14: t Ｂ = 30 μm, thickness of aluminum nitride sintered substrate = approximately 0.68 mm Example 15: t Ｂ = 60 μm, thickness of aluminum nitride sintered substrate = approximately 0.71 mm Example 16: t Ｂ = 90 μm, thickness of aluminum nitride sintered substrate = approximately 0.75 mm

[0096] A 34 mm square copper-coated aluminum nitride sintered substrate, prepared using the same method as in Example 1, was used as the test material, and the same heat cycle resistance test as in the example was performed. In this example, 10 samples were examined in each example (test number n = 10). The test was performed for a maximum of 50 cycles, and the test for that example was stopped when the total number of samples in which cracks occurred reached 4 or more (except in Example 13, when it was 3 or more). The results are shown in Table 5. Comparative Example 1, shown in Table 1, is reproduced in Table 5. Note that no cracks were observed in any example with fewer than 10 cycles, so the results for fewer than 10 cycles are omitted from Table 5.

[0097]

[0098] As can be seen from Table 5, the copper-clad aluminum nitride sintered substrate (insulating substrate for electronic circuits) of Example 6-16, which used an aluminum nitride sintered substrate in which zirconium is unevenly distributed in the surface layer, showed an improvement in heat cycle resistance compared to a conventional general aluminum nitride sintered substrate without added zirconium (Comparative Example 1).

[0099] For the test material of Example 6-16 (copper-coated aluminum nitride sintered substrate), ultrasonic flaw detection testing of the bonding interface was performed using the same method as in Example 1. As a result, no bonding defects that could cause problems in actual use were observed in any of the test materials of Example 6-16, confirming that there were no problems with bonding with copper material.

[0100] 1. Ceramic substrate 2. Metal component for circuitry 3. Metal component for heat dissipation 4. Aluminum nitride sintered substrate 5. Zirconium unevenly distributed surface 10. Insulating substrate for electronic circuits 20, 30. Bonding interface end 41. End surface in the thickness direction

Claims

1. An aluminum nitride sintered substrate comprising a plate-shaped aluminum nitride sintered body, wherein the region from the surface to a depth of 1 / 3 of the plate thickness is called the surface layer, and the region other than the surface layer is called the central part of the plate thickness, and at least one of the surface layers is a zirconium-unevenly distributed surface layer containing a larger amount of zirconium atoms than the amount of zirconium atoms in the central part of the plate thickness.

2. The aluminum nitride sintered substrate according to claim 1, wherein in the zirconium-unevenly distributed surface layer, zirconium exists in the form of zirconium-containing particles.

3. The aluminum nitride sintered substrate according to claim 1, wherein in at least one of the following depth regions of the zirconium-uneven surface layer, the average zirconium concentration measured by an EPMA (electron probe microanalyzer) is 0.30% by mass or more: depth region 1 having a depth of 10 to 40 μm from the surface, depth region 2 having a depth of 40 to 70 μm from the surface, and depth region 3 having a depth of 70 to 100 μm from the surface.

4. The aluminum nitride sintered substrate according to claim 3, wherein the average zirconium concentration measured by an EPMA (electron probe microanalyzer) is 0.90% by mass or more in at least one of the depth regions 1, 2, and 3.

5. The aluminum nitride sintered substrate according to claim 1, wherein the zirconium-unevenly distributed surface layer contains zirconium nitride.

6. A method for manufacturing an aluminum nitride sintered substrate according to claim 1, comprising: a base sheet manufacturing step of preparing a paste A containing aluminum nitride powder and a sintering aid powder, and coating the paste A in a plate shape to obtain a base sheet; a multilayer sheet manufacturing step of preparing a paste B containing aluminum nitride powder, a zirconium-containing substance powder, and a sintering aid powder, and applying the paste B to at least one surface of the base sheet to obtain a multilayer sheet in which a zirconium-containing coating layer is formed on at least one surface of the base sheet; and a sintering step of heating the multilayer sheet to obtain an aluminum nitride sintered body in which the layers of the base sheet and the zirconium-containing coating layer are integrated.

7. In the multilayer sheet manufacturing process, 100 parts by mass of aluminum nitride powder is mixed with zirconium oxide (ZrO ２ The manufacturing method according to claim 6, comprising preparing a paste B containing 0.5 parts by mass or more and 15.0 parts by mass or less in terms of conversion.

8. The thickness of the base sheet is t Ａ (μm), the thickness of the zirconium-containing coating layer is t Ｂ When (μm), in the multilayer sheet manufacturing process, t Ａ >t Ｂ A manufacturing method according to claim 7, which yields a multilayer sheet.

9. The thickness of the zirconium-containing coating layer is t Ｂ When (μm), in the multilayer sheet manufacturing process, t Ｂ The manufacturing method according to claim 7, which provides a multilayer sheet having a thickness of 8 μm or more and 120 μm or less.

10. When the thickness of the zirconium-containing coating film layer is t Ｂ (μm), in the step of producing the multilayer sheet, a multilayer sheet in which t Ｂ is 20 μm or more and 120 μm or less is obtained. The manufacturing method according to claim 7.

11. An insulating substrate for an electronic circuit using an aluminum nitride sintered substrate as described in claim 1, wherein the insulating substrate for an electronic circuit has a structure in which a conductive layer of copper or a copper alloy is formed on at least one surface of the aluminum nitride sintered substrate.

12. The insulating substrate for electronic circuits according to claim 11, wherein the thickness of the aluminum nitride sintered substrate is 0.1 to 3.0 mm.

13. The insulating substrate for electronic circuits according to claim 11, wherein the thickness of the aluminum nitride sintered substrate is 0.25 to 2.0 mm.

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