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

Abstract

[Problem] To improve the partial discharge resistance of an insulating substrate for an electronic circuit that uses an aluminum nitride sintered substrate. [Solution] Provided is an aluminum nitride sintered substrate containing zirconium-containing particles therein, wherein the aluminum nitride sintered substrate has a composition in which the Zr / Al molar ratio is 0.0017-0.0665, and the region from the end surface in the thickness direction to at least a depth of 10.0 μm is a zirconium-deficient surface layer in which the number density of the zirconium-containing particles in a cross section parallel to the thickness direction is 6.0 / 540μm2 or less.

Description

Aluminum nitride sintered substrate, its manufacturing method and insulating substrate for electronic circuits

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

[0002] Aluminum nitride sintered substrates have good thermal conductivity and are widely used as insulating substrates for electronic circuits. During the manufacturing process of aluminum nitride sintered substrates, additives are sometimes mixed in to improve manufacturability and the properties of the insulating substrate.

[0003] For example, Patent Document 1 shows an example of producing a sintered body by adding zirconium oxide powder and yttrium oxide powder to aluminum nitride powder in order to improve mechanical strength (Table 4). The aluminum nitride powder used is a commercially available product with an average particle size of 2.0 μm (paragraph 0032).

[0004] Japanese Unexamined Patent Publication No. 11-199324

[0005] In high-voltage equipment, partial discharges are likely to occur at locations where electric fields concentrate, such as the boundary between electrodes and insulators and defects inside insulators. If partial discharges are left unchecked for a long period of time, the intensity and frequency of the partial discharges will gradually increase, potentially leading to damage to components and insulation breakdown. Partial discharges are generally a problem in high-voltage components such as generators, power transformers, transmission lines, and power cables, but countermeasures are also necessary for high-voltage power modules. In power modules, it is desirable to use insulating substrates for electronic circuits on which semiconductor elements are mounted that have excellent properties (hereinafter referred to as "partial discharge resistance") that can reduce the frequency of partial discharges that accompany high charge transfer above a certain level.

[0006] FIG. 1 shows a schematic cross-sectional view of an insulating substrate for electronic circuits near an edge on which a semiconductor device is mounted. A circuit metal member 2 made of a copper- or aluminum-based material is bonded to one surface of a ceramic substrate 1 made of an aluminum nitride sintered body or the like. A heat-dissipating metal member 3 made of a copper- or aluminum-based material is bonded to the other surface of the ceramic substrate 1, as needed. In this example, the ceramic substrate 1, the circuit metal member 2, and the heat-dissipating metal member 3 constitute an insulating substrate for electronic circuits 10. In such a laminated structure, electric fields tend to concentrate at the edge 20 of the bonding interface between the ceramic substrate 1 and the circuit metal member 2, and at the edge 30 of the bonding interface between the ceramic substrate 1 and the heat-dissipating metal member 3. Partial discharges due to corona discharges occur primarily at these electric field concentration points.

[0007] Aluminum nitride sintered substrates are useful as insulating substrates for power modules and the like, which require excellent insulation and thermal conductivity. In order to meet the future performance improvements of electronic devices, improving the partial discharge resistance of insulating substrates for electronic circuits will also become important. Patent Document 1 discloses an example in which zirconium oxide powder and yttrium oxide powder are added to aluminum nitride powder as raw material powder for obtaining a sintered body (as mentioned above), but this technology does not use aluminum nitride powder with a special particle size distribution. This conventional technology is unable to improve partial discharge resistance. The object of the present invention is to improve the partial discharge resistance of insulating substrates for electronic circuits that use aluminum nitride sintered substrates.

[0008] As a result of research, the inventors discovered that by providing a surface layer in which the amount of zirconium-containing particles present is extremely small in an aluminum nitride sintered substrate containing zirconium-containing particles, the partial discharge resistance of an insulating substrate for electronic circuits using the aluminum nitride sintered substrate can be significantly improved. Such a sintered structure exhibiting a unique dispersion morphology of zirconium-containing particles can be realized by devising the particle size distribution of the mixed powder used for sintering. The above object can be achieved by the following invention.

[0009] Based on the above findings, the present specification discloses the following invention: [1] An aluminum nitride sintered substrate containing zirconium-containing particles therein, having a composition in which the Zr / Al molar ratio is 0.0017 or more and 0.0665 or less, and in a region from the end surface in the thickness direction to a depth of at least 10.0 μm, the number density of the zirconium-containing particles in a cross section parallel to the thickness direction is 6.0 particles / 540 μm. ２ The aluminum nitride sintered substrate has the following zirconium-deficient surface layer: [2] The aluminum nitride sintered substrate according to the above [1], further comprising yttrium-containing particles therein in addition to the zirconium-containing particles. [3] A method for producing an aluminum nitride sintered substrate, comprising: a molding step of obtaining a plate-shaped body made of a mixed composition containing a mixed powder comprising 100 parts by mass of aluminum nitride powder, 0.5 to 20.0 parts by mass of zirconium oxide powder, and 1.0 to 10.0 parts by mass of a sintering aid, wherein D50 is the cumulative 50% particle diameter (μm) and D90 is the cumulative 90% particle diameter (μm) in a volume-based particle size distribution measured by a laser diffraction / scattering method, and the mixed powder has a D50 of 0.7 μm to 3.0 μm and a D90 / D50 ratio of 2.20 or more, and a binder component; a degreasing step of removing the binder component from the plate-shaped body; and a sintering step of sintering the plate-shaped body that has undergone the degreasing step by heating and maintaining it at a temperature of 1700°C to 1900°C in a nitrogen-containing atmosphere. [4] An insulating substrate for electronic circuits using the aluminum nitride sintered substrate described in [1] or [2] above, wherein the insulating substrate for electronic circuits 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. [5] The insulating substrate for electronic circuits described in [4] above, wherein the thickness of the aluminum nitride sintered substrate is 0.1 to 3.0 mm.

[0010] According to the present invention, it is possible to provide an insulating substrate for electronic circuits using an aluminum nitride sintered substrate that has better partial discharge resistance than conventional insulating substrates.

[0011] FIG. 1 is a cross-sectional view schematically illustrating the cross-sectional structure near an edge of an insulating substrate for electronic circuits. FIG. 2 is a diagram schematically illustrating the cross-sectional structure parallel to the plate thickness direction of the aluminum nitride sintered substrate of the present invention. FIG. 3 is a diagram illustrating the volume-based particle size distribution curve obtained by laser diffraction / scattering for the mixed powder used in Example 1 and Comparative Example 1. FIG. 4 is a diagram showing the dimensions and shapes of the copper-coated aluminum nitride sintered substrates produced in the Examples and Comparative Examples. FIG. 5 is an example of a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in Example 1. FIG. 6 is an example of a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in Example 3. FIG. 7 is an example of a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in Comparative Example 1.

[0012] [Sintered Structure] The aluminum nitride (AlN) sintered substrate of the present invention is a sintered structure in which zirconium (Zr)-containing particles are dispersed within a sintered body of aluminum nitride particles. However, the sintered structure is characterized by having a region in the surface layer where almost no zirconium-containing particles are present. This region is referred to in this specification as a "zirconium-deficient surface layer." Here, the sintered body of aluminum nitride particles is a sintered body in which adjacent aluminum nitride particles are sintered together.

[0013] 2 is a schematic diagram showing the cross-sectional structure of the sintered aluminum nitride substrate of the present invention, taken along a plane parallel to the thickness direction. The vertical direction in the figure corresponds to the thickness direction. A zirconium-deficient surface layer 5 is present near the end surface 41 in the thickness direction of the sintered aluminum nitride substrate 4. The zirconium-deficient surface layer 5 refers to a region where almost no zirconium-containing particles exist. Specifically, the zirconium-deficient surface layer 5 is a region where the number density of zirconium-containing particles is 6.0 particles / 540 μm in a cross-section parallel to the thickness direction. ２ In Fig. 2, the depth of the zirconium-depleted surface layer 5 is indicated as δ. This δ is the number density of zirconium-containing particles measured in the surface layer portion up to the depth at a distance δ from the thickness direction end surface 41 in the cross section parallel to the thickness direction of the plate, and is 6.0 particles / 540 µm ２ The number density of zirconium-containing particles measured in the surface layer portion up to a depth where the distance from the end surface 41 in the thickness direction exceeds δ is 6.0 particles / 540 μm ２This can be considered as the critical depth at which the

[0014] According to the investigations of the inventors, in a zirconium-containing aluminum nitride sintered substrate having a Zr / Al molar ratio of 0.0017 or more, the number density of zirconium-containing particles is 6.0 particles / 540 μm in a region from the end surface in the thickness direction to a depth of at least 10.0 μm. ２ When the zirconium-depleted surface layer is as follows, a significant improvement in partial discharge resistance is observed. That is, it is important that the depth δ of the zirconium-depleted surface layer 5 is 10.0 μm or more. According to the manufacturing method described below, the depth δ of the zirconium-depleted surface layer 5 is generally equal at both thickness-direction end surfaces 41. In the aluminum nitride sintered substrate of the present invention, δ is 10.0 μm or more at both thickness-direction end surfaces 41. FIG. 2 shows the positional relationship between the depth at a distance of 10 μm from the thickness-direction end surface 41 and the δ. Note that in FIG. 2, the distance from the thickness-direction end surface 41 to the position at a depth of 10 μm and the distance to the position at depth δ are exaggerated.

[0015] The number density of zirconium-containing particles in the cross section in the inner region excluding the zirconium-depleted surface layer (i.e., the region deeper than the δ) is usually 10.0 particles / 540 μm when the manufacturing method described below is followed. ２ More than 100.0 pieces / 540μm ２ The number density of the zirconium-containing particles in the cross section of the inner region is 11.0 particles / 540 μm ２ 60.0 pieces / 540μm or more ２ The following ranges may be used:

[0016] The zirconium-containing particles found in the aluminum nitride sintered substrate are believed to be particles primarily composed of ZrN phase. Because ZrN is electrically conductive, the zirconium-containing particles dispersed within the aluminum nitride sintered substrate are believed to mitigate electric field concentration. Meanwhile, the zirconium-depleted surface layer formed around the substrate with a thickness greater than a certain level maintains high insulation in the surface layer of the aluminum nitride sintered substrate. It is speculated that the partial discharge resistance is improved by the synergistic effect of "mitigating electric field concentration" and "ensuring high insulation in the surface layer," which are due to the unique dispersion form of the zirconium-containing particles.

[0017] Among the constituent elements contained in the aluminum nitride sintered substrate, zirconium is important for forming zirconium-containing particles. As a result of various investigations, the molar ratio of zirconium to aluminum contained in the aluminum nitride sintered substrate, i.e., the Zr / Al molar ratio, is specified to be in the range of 0.0017 to 0.0665. Here, the Zr / Al molar ratio of 0.0017 is obtained by combining 100 parts by mass of aluminum nitride (AlN) and 0.5 parts by mass of zirconium oxide (ZrO ２ ), and the Zr / Al molar ratio of 0.0665 corresponds to the ratio of the number of Zr and Al atoms supplied by 100 parts by mass of aluminum nitride (AlN) and 20.0 parts by mass of zirconium oxide (ZrO ２ ) corresponds to the ratio of the number of Zr and Al atoms supplied by the aluminum nitride sintered substrate. By setting the Zr / Al molar ratio to 0.0017 or more in the composition of the entire aluminum nitride sintered substrate, it is possible to ensure a sufficient amount of zirconia-containing particles, which are thought to contribute to the relaxation of electric field concentration. On the other hand, if the Zr / Al molar ratio is too high, partial discharge characteristics may deteriorate and dielectric breakdown may occur, so the upper limit of the Zr / Al molar ratio is set to 0.0665. A preferable range of the Zr / Al molar ratio is 0.0023 or more and 0.0499 or less. A more preferable range of the Zr / Al molar ratio is 0.0033 or more and 0.0332 or less. Here, a Zr / Al molar ratio of 0.0033 is obtained by mixing 100 parts by mass of aluminum nitride (AlN) and 1.0 part by mass of zirconium oxide (ZrO ２The Zr / Al molar ratio of 0.0332 corresponds to the ratio of the number of Zr and Al atoms supplied by 100 parts by mass of aluminum nitride (AlN) and 10.0 parts by mass of zirconium oxide (ZrO ２ The Zr / Al molar ratio may be controlled to be 0.0099 or less. The depth δ of the zirconium-depleted surface layer can be determined as follows.

[0018] (Method of Determining the Depth δ of the Zirconium-Depleted Surface Layer) A cross section parallel to the thickness direction of the aluminum nitride sintered substrate to be measured is observed near the thickness direction end surface using a scanning electron microscope (SEM), and a backscattered electron image is obtained. The cross section parallel to the thickness direction can be revealed, for example, by bending and breaking the aluminum nitride sintered substrate. The observation magnification can be, for example, 3000x. Using the depth position where zirconium-containing particles begin to be scattered in the field of view as a guide, a measurement region is set between the thickness direction end surface and a line passing through the depth position and perpendicular to the thickness direction (hereinafter referred to as the "measurement depth line") so that several zirconium-containing particles are included. Here, the distance from the thickness direction end surface (the average position in the thickness direction of the "thickness direction end surface outline" appearing in the SEM image) to the measurement depth line is defined as the depth of the measurement region. The number of zirconium-containing particles present within this measurement region is counted. Zirconium-containing particles partially present on the boundary line of the measurement region are also counted. In the backscattered electron image, zirconium-containing particles are identified as particles with higher brightness (particles that appear whiter) than the base aluminum nitride. Whether a particle is a zirconium-containing particle can be confirmed by checking whether Zr is detected in the particle using an EDX (energy dispersive X-ray fluorescence spectroscopy) device attached to the SEM. The number density of zirconium-containing particles in the measurement area can be calculated by dividing the above count number by the area of ​​the measurement area. If the number density value is 6.0 particles / 540 μm ２ If it exceeds this, the measurement depth line is shifted toward the surface of the end in the thickness direction, and the measurement depth is set to 6.0 pieces / 540 μm. ２If the density is less than 6.0 particles / 540 μm, the measurement depth line is shifted from the end surface in the thickness direction toward the inside to set a new measurement region, and the number density of the zirconium-containing particles is determined in the same manner as above. By repeating this operation, the number density of the zirconium-containing particles can be determined. ２ The maximum depth (μm) of the measurement area is determined as follows, and this value is taken as the depth δ of the zirconium-depleted surface layer in the field of view. １ The above measurement is carried out for six non-overlapping fields (for example, three fields set at random on each surface), and the depth δ of the zirconium-depleted surface layer in each field is calculated. １ ~δ ６ The arithmetic mean value of the above can be used as the depth δ (μm) of the zirconium-depleted surface layer in the aluminum nitride sintered substrate. ２ The area must be measured to be at least 10 μm in the depth direction and at least 54 μm in the width direction.

[0019] The depth δ of the zirconium-depleted surface layer is not particularly limited as long as it is 10 μm or more, but is preferably 30 μm or less, and more preferably 20 μm or more.

[0020] In addition, when determining whether or not a region extending from the surface at the end in the thickness direction to a depth of at least 10.0 μm is a zirconium-depleted surface layer, it is not necessary to determine the value of δ, and a method can be applied in which the "measurement depth line" is set at a position 10.0 μm away from the surface at the end in the thickness direction, and the number density of zirconium-containing particles is determined in six non-overlapping visual fields. In this case, if the number density of zirconium-containing particles is 6.0 particles / 540 μm in any visual field, the number density of zirconium-containing particles is 6.0 particles / 540 μm. ２ Based on the above, it can be evaluated that the region of the sintered aluminum nitride substrate from the surface at the end in the thickness direction to a depth of at least 10.0 μm is a zirconium-deficient surface layer.

[0021] The aluminum nitride sintered substrate may contain particles containing components derived from a sintering aid. One of the typical sintering aids is an yttrium (Y)-containing substance. When an yttrium-containing substance is used as a sintering aid, the yttrium is present in the sintered body of aluminum nitride particles as yttrium-aluminum-garnet (YAG: 3Y ２ O ３ ・5Al ２ O ３ ), yttrium-aluminum-monoclinic (YAM: 2Y ２ O ３ ・Al ２ O ３ In the present invention, the sintering aid is often a yttrium-containing material (e.g., yttrium oxide Y ２ O ３ When using yttrium and aluminum, the molar ratio of yttrium to aluminum contained in the aluminum nitride sintered substrate, i.e., the Y / Al molar ratio, is usually in the range of 0.0043 to 0.0086.

[0022] [Manufacturing Method] Hereinafter, the cumulative 10% particle diameter (μm) in the volume-based particle size distribution determined by the laser diffraction / scattering method will be referred to as D10, the cumulative 50% particle diameter (μm) as D50, and the cumulative 90% particle diameter (μm) as D90.

[0023] The aluminum nitride sintered substrate having the zirconium-deficient surface layer described above is prepared by using, as raw material powders, aluminum nitride (AlN) powder, zirconium oxide (ZrO ２ ) powder and a powder of a substance that functions as a sintering aid, and a mixed powder having a D50 of 0.7 μm or more and a D90 / D50 ratio of 2.20 or more is used.

[0024] (Particle size distribution of mixed powder) A mixed powder having a particle size distribution in which D50 is 0.7 μm or more and 3.0 μm or less, and D90 / D50 ratio is 2.20 or more, is used. A more preferred range for the D50 is 0.8 μm or more and 2.4 μm or less, and a more preferred range for the D90 / D50 ratio is 2.50 or more. There is no particular need to be particular about D10, but for example, a powder having a D10 / D50 ratio of 0.30 or more and 0.70 or less can be used.

[0025] Figure 3 shows the volumetric particle size distribution curves measured by laser diffraction / scattering for the mixed powders used in Example 1 and Comparative Example 1 described below. Example 1 has a D90 / D50 ratio of 2.798, which is adjusted to the unique particle size distribution specified in the present invention. Comparative Example 1 has a D90 / D50 ratio of 1.734, which is obtained by using a conventional, commercially available aluminum nitride powder. The manufacturing method of the present invention uses a mixed powder in which the proportion of large particle sizes is increased more than usual.

[0026] During the sintering process, zirconium oxide and the sintering aid form a liquid phase in the gaps between aluminum nitride particles. The mixed powder used in the present invention contains many large-diameter particles (e.g., particles with a particle diameter of D90 or more), and there is a time lag between the onset of sintering of small-diameter particles (e.g., particles with a particle diameter of D50 or less) and the large-diameter particles. During the sintering process, voids remain around the large-diameter particles that have not yet been sintered, and the zirconium (Zr)-containing liquid phase is fluidic through these voids. This fluidic state is maintained for a longer period of time than in conventional sintering processes due to the time lag. This increases the amount of zirconium-containing liquid phase that evaporates from the surface of the compact during heating and sintering, presumably resulting in the formation of a "zirconium-depleted surface layer" that is depleted of zirconium to a deeper depth than in conventional processes.

[0027] (Aluminum nitride powder) One effective method for adjusting the particle size distribution of a mixed powder to one with a larger-than-normal proportion of large particles is to prepare a first aluminum nitride powder product with a small average particle size and a second aluminum nitride powder product with a large average particle size and mix them in a ratio that will yield a mixed powder with the desired particle size distribution. This allows a large amount of aluminum nitride particles to be distributed throughout the mixed powder, which is advantageous for forming a zirconium-depleted surface layer by utilizing the local time difference in the onset of sintering. Three or more aluminum nitride powder products with different average particle sizes may also be prepared and mixed.

[0028] When the particle size distribution of the mixed powder is adjusted using a first aluminum nitride powder product having a small average particle size and a second aluminum nitride powder product having a large average particle size, the following particle size distributions can be used for each aluminum nitride powder product. The first aluminum nitride powder may have a D50 of 0.7 μm to 1.5 μm, or 0.8 μm to 1.2 μm. The second aluminum nitride powder may have a D50 of 6.0 μm to 9.0 μm, or 7.2 μm to 8.3 μm. The second aluminum nitride powder may have a D50 of 22.0 μm to 30.0 μm, or 23.5 μm to 27.5 μm. The above is merely an example, and the particle size distribution of the mixed powder can be adjusted by using the first aluminum nitride powder or the second aluminum nitride powder having a particle size distribution other than those described above.

[0029] (Zirconium oxide powder) As a zirconium supply source, various zirconium-containing substances that can form a liquid phase together with a sintering aid and do not impair the effect of the present invention (improvement of partial discharge resistance) can be applied. In the manufacturing method of the present invention, zirconium oxide (ZrO ２) powder is used. The zirconium oxide powder may have a D50 of, for example, 0.4 μm or more and 4.9 μm or less. The D90 of the zirconium oxide powder is preferably smaller than the D90 of the mixed powder, and for example, a powder 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 may be used. The lower limit of the D90 of the zirconium oxide powder is not particularly limited, but it may be, for example, 0.40 μm or more.

[0030] (Sintering Aid) As the sintering aid, known substances conventionally used in producing aluminum nitride sintered bodies can be used. For example, compounds containing one or more of alkali metals, alkaline earth metals, and rare earth elements can be used. Examples of the alkali metal include lithium. Examples of the alkaline earth metal elements include beryllium, magnesium, calcium, strontium, and barium. Examples of the rare earth elements include yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Of these, calcium, strontium, barium, yttrium, lanthanum, cerium, and neodymium are more preferred. Furthermore, oxides are more preferred as the compound. These compounds may be used alone or in combination of two or more.

[0031] The sintering aid is preferably a powder. When a powder sintering aid is used, for example, a sintering aid powder having a D50 of 1.2 μm or more and 3.2 μm or less may be used. Alternatively, a sintering aid powder having a D90 of 10 μm or less, 5.0 μm or less, or 2.5 μm or less may be used. The lower limit of D90 of the sintering aid powder is not particularly limited, but it can be, for example, 1.2 μm or more. One of the sintering aids suitable for forming a liquid phase together with zirconium oxide powder is yttrium oxide (Y ２ O ３ ) powder can be mentioned.

[0032] (Forming Step) A mixed composition containing the mixed powder containing the aluminum nitride powder, zirconium oxide powder, and sintering aid powder, and a binder component is obtained.

[0033] The mixing ratio of zirconium oxide powder is in the range of 0.5 parts by mass to 20.0 parts by mass per 100 parts by mass of aluminum nitride powder. Excellent partial discharge resistance can be achieved within this range. From the viewpoint of achieving high bending strength and fracture toughness, the mixing ratio of zirconium oxide powder is more preferably in the range of 0.7 parts by mass to 12.0 parts by mass per 100 parts by mass of aluminum nitride powder. The mixing ratio of the sintering aid can be in the range of 1.0 parts by mass to 10.0 parts by mass, more preferably in the range of 2.5 parts by mass to 6.0 parts by mass per 100 parts by mass of aluminum nitride powder. As mentioned above, for example, yttrium oxide powder can be used as the sintering aid.

[0034] In addition to the raw material powders, a binder component is added to obtain a mixed powder. Conventional binder components can be used, including, for example, resin components such as polyvinyl butyral resin, acrylic resin, epoxy resin, and ethyl cellulose 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. The binder component may be added together with a solvent component in which the binder component dissolves. Examples of the solvent component 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.

[0035] The above materials are mechanically mixed using a mixing device such as a ball mill to obtain a viscous body. While all materials may be mixed at once in the mixing device to obtain the viscous body, from the viewpoint of efficiency, it is preferable to first mix the aluminum nitride powder, zirconia oxide powder, and sintering aid powder in the mixing device, and then add the other components and further mix them. The particle size distribution of the mixed powder can be determined by taking a sample from the viscous body, removing the resin component, and then measuring it. The viscous body is then formed into a plate using a plate-forming device such as a doctor blade, knife coater, roll coater, or die coater. The thickness of the plate can be set, for example, within the range of 0.1 mm to 3.0 mm, depending on the application, by laminating or the like.

[0036] (Degreasing step) The solvent and binder components contained in the obtained plate-like molded body are removed before the sintering step. As a removal method, it is preferable to evaporate the solvent component as needed, and then heat and hold the body at a temperature range (e.g., 300°C or higher and 700°C or lower) that promotes the volatilization of the organic substance that is the binder component and does not cause sintering. In this case, the heating atmosphere can be air.

[0037] (Sintering Process) After the degreasing process, the plate-shaped compact is sintered by heating and holding it at a temperature of 1700°C or higher and 1900°C or lower in a nitrogen-containing atmosphere. The nitrogen-containing atmosphere serves as a nitrogen supply source for generating zirconium-containing particles mainly composed of ZrN phase. Nitrogen gas can be used as the nitrogen-containing atmosphere. By setting the heating temperature to 1700°C or higher, a liquid phase containing components of the sintering aid can be sufficiently generated, allowing the sintering of aluminum nitride particles to proceed. A heating temperature exceeding 1900°C is uneconomical. A more preferred heating temperature is in the range of 1730°C or higher and 1800°C or lower. The sintering process in the present invention can be carried out using conventional manufacturing equipment for aluminum nitride sintered substrates.

[0038] During the sintering process, the larger particles among the large-diameter aluminum nitride particles sinter more slowly than the smaller particles. Therefore, as mentioned above, the voids between the aluminum nitride particles, through which the Zr-containing liquid phase can flow, are believed to remain for a longer period than usual. Therefore, even Zr present in a relatively deep portion from the end surface in the thickness direction of the plate-shaped compact is easily removed by volatilization into the external space. As a result, a zirconium-depleted surface layer is formed deep, at a depth of 10.0 μm or more. The holding time within the above heating temperature range can be set, for example, within the range of 1 hour to 8 hours.

[0039] [Insulating Substrate for Electronic Circuits] When an insulating substrate for electronic circuits is constructed by forming a conductive layer on at least one surface of the aluminum nitride sintered substrate using the sintered structure described above, the partial discharge resistance of the insulating substrate for electronic circuits can be significantly improved. The thickness of the aluminum nitride sintered substrate can be set, for example, in the range of 0.1 to 3.0 mm depending on the application. A plate-shaped member made of, for example, copper or a copper alloy can be suitably used as the conductive layer. The method for 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.

[0040] [Example 1] The following raw material powders were prepared: (1) First aluminum nitride (AlN) powder (D50 = 1.103 μm, D90 = 2.623 μm): 75 g (2) Second aluminum nitride (AlN) powder (D50 = 7.525 μm, D90 = 24.64 μm): 25 g (3) Zirconium oxide (ZrO ２ ) powder (D50 = 0.644 μm, D90 = 0.819 μm): 1 g (4) Yttrium oxide (Y ２ O ３ ) powder (D50=1.327 μm, D90=2.269 μm): 5 g The following organic substances were also prepared: (5) Polyvinyl butyral resin (manufactured by Sekisui Chemical Co., Ltd.): 8 g (6) Dibutyl phthalate plasticizer (manufactured by Daihachi Chemical Industry Co., Ltd.): 4 g (7) Ethanol + toluene: 200 g The above (5) and (6) correspond to binder components, and (7) corresponds to a solvent component.

[0041] The above (1) to (4) were mixed in a ball mill to obtain a mixed powder, in which 1 part by mass of zirconium oxide powder and 5 parts by mass of yttrium oxide powder were used per 100 parts by mass of aluminum nitride powder.

[0042] The mixed powder samples (1) to (4) and the organic substances (5) to (7) were mixed in a mill pot to obtain a molding composition (viscous body). This molding composition was vacuum degassed, its viscosity adjusted, and then molded into a 1.2 mm thick sheet using a sheet molding machine (manufactured by Tester Sangyo Co., Ltd.) by the doctor blade method to obtain a plate-like molded body (molding process). This plate-like molded body was dried in air at room temperature for 18 hours to evaporate the solvent, and then heated in an air atmosphere at 500°C for 8 hours to remove the binder component (debinding process). After the debinding process, the plate-like molded body was fired in a firing furnace in a nitrogen gas atmosphere at 1750°C for 8 hours to obtain a 1.0 mm thick aluminum nitride sintered substrate (i.e., a plate-like aluminum nitride sintered body).

[0043] (Measurement of Particle Size Distribution of Mixed Powder) A portion of the molding composition was collected, heated at 500°C for 3 hours in air, degreased, and then dispersed in water containing a dispersant (sodium pyrophosphate) using a homogenizer to prepare a measurement sample. Using a laser diffraction / scattering particle size distribution analyzer (Microtrac-Bell, MT3300EXII), wet measurements were performed using water as the solvent to determine the volumetric particle size distribution of the mixed powder. The particle size distribution of this mixed powder sample was as follows: D10 was 0.592 μm, D50 was 1.004 μm, and D90 was 2.809 μm, with a D90 / D50 ratio of 2.798. The particle size distribution curve of this mixed powder sample is shown in Figure 1.

[0044] (Composition Analysis) The aluminum nitride sintered substrate was subjected to Zr and Al composition analysis by adding nitric acid, dilute hydrochloric acid, and hydrogen peroxide to the substrate, decomposing the resulting mixture in a microwave oven, and analyzing the resulting mixture with an ICP (inductively coupled plasma) emission spectrometer (Thermo Fisher, ICAP 7400 Duo). As a result, the Zr / Al molar ratio of the aluminum nitride sintered substrate obtained in this example was 0.0033.

[0045] (Measurement of depth δ of zirconium-depleted surface) The above aluminum nitride sintered body was manually bent and split to form fracture surfaces roughly parallel to the plate thickness direction. The vicinity of the end surface in the thickness direction of the fracture surface was observed using an SEM (Hitachi Corporation, TM3030) to obtain a backscattered electron image at 3000x magnification, and the depth δ (μm) of the zirconium (Zr)-depleted surface layer was determined according to the above-mentioned "Method for determining depth δ of zirconium-depleted surface layer". The depth δ of the zirconium-depleted surface layer of the aluminum nitride sintered substrate obtained in this example was 21 μm.

[0046] Figure 5 shows a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in this example. Small, bright particles (appearing whitish) are scattered throughout the image. These are zirconium-containing particles. Some of these are indicated by arrows. Larger, brighter particles are primarily yttrium-containing particles, most of which would not be counted as zirconium-containing particles in EDX analysis. Figure 5 also shows the positions of the end surfaces in the thickness direction and the depth δ position of the zirconium-depleted surface layer (21 μm deep).

[0047] (Measurement of Volume Resistivity) The volume resistivity (Ω cm) of the above aluminum nitride sintered body was measured at room temperature in accordance with JIS C2141. As a result, the volume resistivity was 5.5×10 １４ The value was Ω·cm.

[0048] (Preparation of copper-coated aluminum nitride sintered substrate) A plate material measuring 50 mm × 44.2 mm × 1.0 mm was cut out from the above-mentioned aluminum nitride sintered substrate. Using this plate material, a copper (Cu)-coated aluminum nitride (AlN) sintered substrate having the dimensions shown in Fig. 4 was prepared as follows.

[0049] A brazing filler metal paste containing Ag, Cu, Sn, and Ti as metal components in a mass ratio of Ag:Cu:Sn:Ti = 83:10:5:2 was screen-printed over almost the entire surface of both sides of the 50 mm × 44.2 mm × 1.0 mm sintered aluminum nitride substrate to form a brazing filler metal layer with a thickness of 20 μm. A 52 mm × 46 mm × 0.3 mm copper plate was placed on one side of the sintered aluminum nitride substrate with the brazing filler metal layer interposed therebetween, and a 52 mm × 46 mm × 0.2 mm copper plate was placed on the other side to form a copper plate-sintered aluminum nitride substrate-copper plate laminate, which was then heated to 850°C in a vacuum to bond the sintered aluminum nitride substrate to the copper plates on both sides.

[0050] An ultraviolet-curable alkaline-peelable resist was applied by screen printing to copper plates bonded to both sides of an aluminum nitride sintered substrate. The coating shape was a 46 mm x 40.2 mm rectangle (with four corners chamfered at a 3.5 mm radius) on the 0.3 mm copper plate side, and a 49 mm x 43.2 mm rectangle (with four corners chamfered at a 2 mm radius) on the 0.2 mm copper plate side. The resist was cured by UV irradiation, and then the unnecessary copper was removed using an etching solution consisting of copper chloride, hydrochloric acid, and water. The resist was then removed using an aqueous sodium hydroxide solution. The substrate was then pickled in dilute sulfuric acid for 20 seconds, and then immersed in a chelating aqueous solution containing EDTA, ammonia water, and hydrogen peroxide to remove the unnecessary brazing material remaining on the surface of the aluminum nitride sintered substrate around the copper plate.

[0051] In this way, a copper-coated aluminum nitride sintered substrate was obtained with the shape shown in Figure 4. This copper-coated aluminum nitride sintered substrate mimics the laminated structure of an insulating substrate for electronic circuits. For convenience, the 0.3 mm thick copper member (with no specific circuit pattern formed) bonded to the aluminum nitride sintered substrate will be referred to as the circuit metal member, and the 0.2 mm thick copper member will be referred to as the heat dissipation metal member. Three of these copper-coated aluminum nitride sintered substrates were prepared and used as test materials for the partial discharge measurements described below.

[0052] (Measurement of Partial Discharge) For the measurement of partial discharge, a partial discharge measurement system (partial discharge meter: DAC-PD-9, partial discharge detector: DAC-PDE-2, power supply control unit: DAC-WTC-1, partial discharge calibrator DAC-CP-2) from Soken Electric Co., Ltd. was used, and the partial discharge was measured according to the following procedure.

[0053] Setting Operation A copper-coated aluminum nitride sintered substrate as a test material was immersed in insulating oil (Fluorinert FC-3283), the heat dissipation metal member was connected to GND, and a contact probe for applying a test voltage was brought into contact with the circuit metal member.

[0054] Calibration Operation A partial discharge calibrator was connected in parallel to the test specimen in this state to generate a calibration pulse equivalent to a partial discharge of 100 pC, and after calibrating the partial discharge measuring instrument, the partial discharge calibrator was removed.

[0055] Measurement operation: The holding voltage setting value V-HI of the power supply control unit was set to 6 kV, which was the initial test voltage. The voltage was automatically increased to the above test voltage in the Auto mode of the power supply control unit, and then maintained at that voltage. 20 seconds after the test voltage was reached, partial discharge data was collected for 5 seconds in the φ-Q mode of the partial discharge meter, and then the voltage was decreased. The measurement frequency settings of the partial discharge meter were a measurement center frequency f0 of 180 kHz, a measurement band BW of 300 kHz, the voltage increase and decrease rates by the power supply control unit were both 600 V / s, and the test voltage frequency was 60 Hz. This test was performed on three test pieces in each example, with the number of tests n = 3.

[0056] For each test material, the number of partial discharge pulses in which the charge of the partial discharge pulse was 10 pC or more was counted from the collected 5-second partial discharge data. The lower this count, the better the partial discharge resistance is judged to be. The partial discharge resistance was evaluated by the average of the counts for the number of tests n = 3. As a result, the average number of partial discharges of 10 pC or more was 2.4 times. The above results are shown in Table 1 (the same applies to each of the following examples).

[0057] [Example 2] An experiment was conducted under the same conditions as in Example 1, except that (1) to (3) in the blending of the raw material powders in Example 1 were changed as follows. (4) to (7) are the same as in Example 1. (1) First aluminum nitride (AlN) powder (D50 = 0.944 μm, D90 = 2.493 μm): 75 g (2) Second aluminum nitride (AlN) powder (D50 = 8.168 μm, D90 = 25.33 μm): 25 g (3) Zirconium oxide (ZrO ２ ) Powder (D50=0.535μm, D90=0.833μm): 3g

[0058] A sample taken from the molding composition obtained in this example was measured for volumetric particle size distribution by laser diffraction / scattering in accordance with "Measurement of particle size distribution of mixed powder" described in Example 1. In the particle size distribution of the mixed powder sample of this example, D10 was 0.573 μm, D50 was 0.980 μm, D90 was 2.653 μm, and the D90 / D50 ratio was 2.707. The amount of zirconium oxide powder used was 3 parts by mass, and the amount of yttrium oxide powder used was 5 parts by mass, relative to 100 parts by mass of aluminum nitride powder.

[0059] The aluminum nitride sintered substrate obtained in this example had a Zr / Al molar ratio of 0.0099, a zirconium-depleted surface layer depth δ of 12 μm, and a volume resistivity of 5.6×10 １４ When the partial discharge resistance of the copper-coated aluminum nitride sintered substrate manufactured using the aluminum nitride sintered substrate obtained in this example was evaluated in the same manner as above, the average number of partial discharges of 10 pC or more was 3.2 times.

[0060] [Example 3] An experiment was conducted under the same conditions as in Example 1, except that (1) to (3) in the blending of the raw material powders in Example 1 were changed as follows. (4) to (7) are the same as in Example 1. (1) First aluminum nitride (AlN) powder (D50 = 1.093 μm, D90 = 2.654 μm): 75 g (2) Second aluminum nitride (AlN) powder (D50 = 8.201 μm, D90 = 27.302 μm): 25 g (3) Zirconium oxide (ZrO ２ ) Powder (D50 = 0.549 μm, D90 = 0.836 μm): 10 g

[0061] A sample taken from the molding composition obtained in this example was measured for volumetric particle size distribution by laser diffraction / scattering in accordance with "Measurement of particle size distribution of mixed powder" described in Example 1. In the particle size distribution of the mixed powder sample of this example, D10 was 0.575 μm, D50 was 0.980 μm, D90 was 2.746 μm, and the D90 / D50 ratio was 2.802. The amount of zirconium oxide powder used was 10 parts by mass, and the amount of yttrium oxide powder used was 5 parts by mass, relative to 100 parts by mass of aluminum nitride powder.

[0062] The aluminum nitride sintered substrate obtained in this example had a Zr / Al molar ratio of 0.0332, a zirconium-depleted surface layer depth δ of 16 μm, and a volume resistivity of 0.84×10 １４ The value was Ω·cm.

[0063] Figure 6 shows a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in this example. Small, bright particles (appearing whitish) are scattered throughout the image. These are zirconium-containing particles. Some of these are indicated by arrows. Larger, brighter particles are primarily yttrium-containing particles, most of which would not be counted as zirconium-containing particles in EDX analysis. Figure 6 also shows the positions of the end surface in the thickness direction and the depth δ position of the zirconium-depleted surface layer (16 μm deep).

[0064] The partial discharge resistance of the copper-coated aluminum nitride sintered substrate manufactured using the aluminum nitride sintered substrate obtained in this example was evaluated in the same manner as above, and the average number of partial discharges of 10 pC or more was 0.3 times.

[0065] Comparative Example 1 An experiment was conducted under the same conditions as in Example 1, except that (1) to (3) in the blending of the raw material powders in Example 1 were changed as follows. (4) to (7) are the same as in Example 1. (1) First aluminum nitride (AlN) powder (D50 = 1.083 μm, D90 = 2.687 μm): 100 g (2) Second aluminum nitride (AlN) powder: Not used (3) Zirconium oxide (ZrO ２ ) Powder (D50=0.556μm, D90=0.836μm): 1g

[0066] A sample taken from the molding composition obtained in this example was subjected to measurement of the volumetric particle size distribution by laser diffraction / scattering according to the "Measurement of particle size distribution of mixed powder" described in Example 1. The particle size distribution of the mixed powder sample in this example was found to be D10 of 0.547 μm, D50 of 0.868 μm, D90 of 1.505 μm, and the D90 / D50 ratio of 1.734. The particle size distribution curve of this aluminum nitride mixed powder sample is shown in Figure 1 above. Relative to 100 parts by mass of aluminum nitride powder, the amount of zirconium oxide powder used was 1 part by mass and the amount of yttrium oxide powder used was 5 parts by mass.

[0067] The aluminum nitride sintered substrate obtained in this example had a Zr / Al molar ratio of 0.0033, a zirconium-depleted surface layer depth δ of 7 μm, and a volume resistivity of 1.0×10 １４ When the partial discharge resistance of the copper-coated aluminum nitride sintered substrate manufactured using the aluminum nitride sintered substrate obtained in this example was evaluated in the same manner as above, the average number of partial discharges of 10 pC or more was 7.4 times.

[0068] Figure 7 shows a backscattered electron image of a cross section parallel to the thickness direction of the aluminum nitride sintered substrate produced in this example. Small, bright particles (appearing whitish) are scattered throughout the image. These are zirconium-containing particles. Some of these are indicated by arrows. Furthermore, the larger, brighter particles are primarily yttrium-containing particles, most of which would not be counted as zirconium-containing particles in EDX analysis. Figure 7 also shows the positions of the end surface in the thickness direction and the depth δ position (7 μm depth) of the zirconium-depleted surface layer.

[0069] Comparative Example 2 An experiment was conducted under the same conditions as in Example 1, except that (1) to (3) in the blending of the raw material powders in Example 1 were changed as follows. (4) to (7) are the same as in Example 1. (1) First aluminum nitride (AlN) powder (D50 = 1.056 μm, D90 = 2.563 μm): 100 g (2) Second aluminum nitride (AlN) powder: Not used (3) Zirconium oxide (ZrO ２ ) Powder (D50 = 1.091 μm, D90 = 2.499 μm): 10 g

[0070] A sample taken from the molding composition obtained in this example was measured for volumetric particle size distribution by laser diffraction / scattering in accordance with "Measurement of particle size distribution of mixed powder" described in Example 1. In the particle size distribution of the mixed powder sample of this example, D10 was 0.551 μm, D50 was 0.893 μm, D90 was 1.613 μm, and the D90 / D50 ratio was 1.806. The amount of zirconium oxide powder used was 10 parts by mass, and the amount of yttrium oxide powder used was 5 parts by mass, relative to 100 parts by mass of aluminum nitride powder.

[0071] The aluminum nitride sintered substrate obtained in this example had a Zr / Al molar ratio of 0.0332, a zirconium-depleted surface layer depth δ of 5 μm, and a volume resistivity of 0.6×10 １４ When the partial discharge resistance of the copper-coated aluminum nitride sintered substrate manufactured using the aluminum nitride sintered substrate obtained in this example was evaluated in the same manner as above, the average number of partial discharges of 10 pC or more was 50.0 times.

[0072]

[0073] As can be seen from Table 1, the zirconium-containing aluminum nitride sintered substrate produced using the aluminum nitride powder having the particle size distribution specified in the present invention had a zirconium-deficient surface layer formed from the surface to a region of 10 μm or more in depth, and had excellent partial discharge resistance when formed into a copper-coated aluminum nitride sintered substrate (insulating substrate for electronic circuits) compared to the zirconium-containing aluminum nitride sintered substrate of the comparative example having a thin zirconium-deficient surface layer.

[0074] Next, the sintered aluminum nitride substrates obtained in Examples 1 to 3 and Comparative Example 3 shown below were subjected to measurements of bending strength and fracture toughness.

[0075] Comparative Example 3 An aluminum nitride sintered substrate was obtained under the same conditions as in Example 1, except that (1) to (3) in the blending of the raw material powders in Example 1 were changed as follows. (4) to (7) were the same as in Example 1. (1) First aluminum nitride (AlN) powder (D50 = 1.066 μm, D90 = 2.293 μm): 100 g (2) Second aluminum nitride (AlN) powder: Not used (3) Zirconium oxide (ZrO ２ ) Powder: Not used

[0076] A sample taken from the molding composition obtained in this example was subjected to measurement of the volumetric particle size distribution by laser diffraction / scattering in accordance with the "Measurement of particle size distribution of mixed powder" described in Example 1. In the particle size distribution of the mixed powder sample of this example, D10 was 0.555 μm, D50 was 0.887 μm, D90 was 1.578 μm, and the D90 / D50 ratio was 1.779. When the partial discharge resistance of a copper-coated aluminum nitride sintered substrate produced using the aluminum nitride sintered substrate obtained in this example was evaluated in the same manner as above, the average number of partial discharges of 10 pC or more was 547.

[0077] [Bending Strength, Fracture Toughness] (Measurement of Bending Strength) A test piece having a width of 4 mm was prepared from the above-mentioned aluminum nitride sintered substrate by surface grinding, and the three-point bending strength was measured in accordance with JIS C2141 at a crosshead speed of 0.5 mm / min and a support distance of 30 mm.

[0078] (Measurement of fracture toughness: IF method) For the above aluminum nitride sintered substrate, the indentation and crack length on the test piece were measured under the conditions of an indentation load of 200 N and a holding time of 15 seconds in accordance with JIS R 1607 to determine the fracture toughness. These results are shown in Table 2.

[0079]

[0080] As can be seen from Table 2, by adding zirconium to the aluminum nitride sintered substrate, the bending strength and fracture toughness can be significantly improved.

[0081] REFERENCE SIGNS LIST 1 ceramic substrate 2 metal member for circuit 3 metal member for heat dissipation 4 aluminum nitride sintered substrate 5 zirconium-deficient surface layer 10 insulating substrate for electronic circuit 20, 30 bonding interface edge 41 thickness direction edge surface

Claims

1. An aluminum nitride sintered substrate containing zirconium-containing particles therein, having a composition in which the Zr / Al molar ratio is 0.0017 or more and 0.0665 or less, and in a region from the end surface in the thickness direction to a depth of at least 10.0 μm, the number density of the zirconium-containing particles in a cross section parallel to the thickness direction is 6.0 particles / 540 μm ２ The following is a zirconium-deficient surface layer of an aluminum nitride sintered substrate.

2. The aluminum nitride sintered substrate according to claim 1, further comprising yttrium-containing particles therein in addition to the zirconium-containing particles.

3. A method for producing an aluminum nitride sintered substrate, comprising: a molding step of obtaining a plate-shaped molded body made from a mixed composition containing a mixed powder containing 100 parts by mass of aluminum nitride powder, 0.5 to 20.0 parts by mass of zirconium oxide powder, and 1.0 to 10.0 parts by mass of a sintering aid, wherein D50 is the cumulative 50% particle diameter (μm) in a volume-based particle size distribution measured by a laser diffraction / scattering method and D90 is the cumulative 90% particle diameter (μm), and wherein D50 is 0.7 μm to 3.0 μm and the D90 / D50 ratio is 2.20 or more, and a binder component; a degreasing step of removing the binder component from the plate-shaped molded body; and a sintering step of sintering the plate-shaped molded body that has undergone the degreasing step by heating and maintaining it at a temperature of 1700°C to 1900°C in a nitrogen-containing atmosphere.

4. An insulating substrate for electronic circuits using the aluminum nitride sintered substrate according to claim 1 or 2, having a structure in which a conductive layer of copper or copper alloy is formed on at least one surface of the aluminum nitride sintered substrate.

5. The insulating substrate for electronic circuits according to claim 4, wherein the thickness of said sintered aluminum nitride substrate is 0.1 to 3.0 mm.

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