WO2024111484A1 - Bonded body and power module - Google Patents

Abstract

Provided is a bonded body comprising a ceramic plate, a metal plate, and a brazing material layer that bonds the main surface of the ceramic plate and the main surface of the metal plate. In a cross-section obtained by cutting the ceramic plate along the thickness direction thereof, the average value of the area ratio of pores is 2.0% or less in a region from the main surface of the ceramic plate to a depth of 200 μm and including directly below the outer edge at the side of the metal plate.

Description

Joint and power module

This disclosure relates to a joint and a power module.

In recent years, power modules for controlling large amounts of power have been used in industrial equipment such as motors, and in products such as electric vehicles. In these power modules, a circuit board having a ceramic plate and a circuit pattern is used to efficiently diffuse heat generated by the semiconductor device and suppress leakage current. For example, Patent Document 1 proposes reducing the bonding void rate in the brazing material layer that bonds the ceramic plate and the circuit pattern to a predetermined value or less.

The use of wide-gap semiconductors in semiconductor devices has been proposed (for example, Patent Document 2). Semiconductor devices using wide-gap semiconductors have better pressure resistance and heat resistance than semiconductor devices equipped with Si semiconductors. Such wide-gap semiconductors have excellent pressure resistance and are capable of high-temperature operation.

International Publication No. 2019/022133 JP 2018-200918 A

Power modules and other electronic components are becoming more compact and more sophisticated. As a result, the reliability of the various products used in these electronic components is becoming increasingly higher. For example, assemblies such as circuit boards on which semiconductor elements using wide-gap semiconductors are mounted are required to have higher insulation performance than ever before at high temperatures.

The present disclosure therefore provides a joint that has excellent insulation reliability at high temperatures. The present disclosure also provides a power module that has excellent reliability by including such a joint.

One aspect of the present disclosure provides the following ceramic sintered body:

[1] A joint comprising a ceramic plate, a metal plate, and a brazing material layer joining the main surface of the ceramic plate and the main surface of the metal plate, in which the average area ratio of pores in a region from the main surface of the ceramic plate to a depth of 200 μm, including directly below the outer edge of the side of the metal plate, on a cut surface obtained by cutting the ceramic plate along the thickness direction of the ceramic plate, is 2.0% or less.

When a semiconductor element or the like is mounted on the joint, the temperature fluctuates due to heat generation from the semiconductor element. Here, since the ceramic plate and the metal plate have different thermal expansion coefficients, thermal stress occurs in the ceramic plate as the temperature of the joint changes. This thermal stress is greatest in the ceramic plate near the end of the metal plate. Therefore, when a high voltage is applied to the metal plate of such a joint, insulation breakdown is likely to occur in the ceramic plate near the end of the metal plate as the temperature of the joint increases. In the joint of [1] above, the average area ratio of pores in the region including directly below the outer edge of the metal plate and up to a depth of 200 μm from the main surface of the ceramic plate, which corresponds to the portion where large thermal stress occurs, is sufficiently low. Therefore, insulation breakdown of the joint at high temperatures can be sufficiently suppressed. Therefore, the joint has excellent insulation reliability at high temperatures.

The bonded body of the above [1] may be any one of the following [2] to [5].

[2] The bonded body according to the above-mentioned [1], wherein the maximum value of the equivalent circle diameter of the pores included in the region of the cut surface is 10 μm or less.
[3] The joined body according to [1] or [2], wherein the ceramic plate has a thickness of 0.2 mm or more.
[4] The joined body according to any one of [1] to [3], wherein the metal plate has a thickness of 0.5 mm or less.
[5] The bonded structure according to any one of [1] to [4], which is a circuit board on which a semiconductor element made of a semiconductor material having a band gap exceeding 1.12 eV is mounted.

In the joint of [2] above, the maximum value of the circular equivalent diameter of the pores in the region of the ceramic plate is 10 μm or less. If there are large pores, insulation breakdown is likely to occur from those pores. By reducing the maximum value of the circular equivalent diameter of the pores, the variation in insulation reliability can be sufficiently reduced. Therefore, the joint of [2] above has even better insulation reliability at high temperatures. Since the ceramic plate in the joint of [3] above has a sufficient thickness, the durability of the insulation at high temperatures can be improved. In the joint of [4] above, the thickness of the metal plate is 0.5 mm or less, so that the thermal stress generated in the ceramic plate can be reduced. Therefore, the insulation reliability at high temperatures can be further improved. In [5] above, a power module capable of operating at high frequencies can be obtained by using the joint with excellent insulation performance at high temperatures as a circuit board on which a semiconductor element made of a semiconductor material with a band gap exceeding 1.12 eV is mounted.

One aspect of the present disclosure provides the following power module:

[6] A power module comprising the assembly described in any one of [1] to [5] above and a semiconductor element electrically connected to the metal plate of the assembly.

The power module includes the semiconductor element and the bonded body, which has excellent insulation performance at high temperatures. Therefore, the power module has excellent reliability.

The power module [6] above may be any one of the following [7] to [9].

[7] The power module according to [6], wherein the band gap of the semiconductor material constituting the semiconductor element exceeds 1.12 eV.
[8] The power module according to [6] or [7], wherein the ceramic plate has a thickness of 0.35 mm or less.
[9] The power module according to any one of [6] to [8], wherein the thickness of the metal plate in the joined body is 0.3 mm or more.

The power module of [7] above has semiconductor elements made of semiconductor materials with a large band gap, and can therefore be driven at high frequencies to improve control performance. When such semiconductors with a large band gap are used, the amount of heat generated by the semiconductor elements increases. Therefore, by reducing the thickness of the ceramic plates as in [8] above, the joint can be made smaller and heat dissipation can be improved. Also, by increasing the thickness of the metal plates as in [9] above, heat dissipation can be improved, further increasing reliability.

The present disclosure can provide a joint that has excellent insulation reliability at high temperatures. Furthermore, the present disclosure can provide a power module that has excellent reliability by including such a joint.

FIG. 1 is a plan view of the joint body. FIG. 2 is a cross-sectional view of the bonded body taken along the thickness direction. FIG. 3 is an enlarged cross-sectional view showing a cross section obtained by cutting the ceramic plate along the thickness direction, the cross section including the area just below the outer edge of the side of the metal plate and extending to a depth of 200 μm from the main surface of the ceramic plate. FIG. 4 is a cross-sectional view of the power module. FIG. 5 is a diagram showing an example of a volume-based particle size distribution of a sintering additive powder obtained by a laser diffraction/scattering method. FIG. 6 is a diagram showing an example of an image of grain growth as sintering progresses in one example of a manufacturing method. FIG. 7 is a schematic diagram showing an example of an inspection device for performing a Vt test. FIG. 8A is a photograph (500x) showing an image of the cut surface of the ceramic plate of Example 1 taken by a scanning electron microscope, and FIG. 8B is a diagram showing the image after being binarized. FIG. 9 is a diagram showing an image of grain growth as sintering progresses in a conventional manufacturing method. FIG. 10 is a photograph taken by a scanning electron microscope (200x) of a cut surface of a conventional molded body (ceramic green sheet) and of aggregates of sintering additive powder contained in the cut surface. FIG. 11A is a photograph (500x) showing an image of the cut surface of the ceramic plate of Comparative Example 1 taken with a scanning electron microscope, and FIG. 11B is a diagram showing the photograph after being binarized.

Below, embodiments of the present disclosure will be described, with reference to the drawings where necessary. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following content. Note that the numerical ranges indicated with the symbol "to" include the lower limit and the upper limit. In other words, the numerical range indicated by "A to B" means A or more and B or less. This disclosure also includes numerical ranges that combine a numerical range having only an upper limit with a numerical range having only a lower limit. This disclosure also includes numerical ranges in which the upper or lower limit of each numerical range is replaced with the numerical value of any of the examples. When multiple materials are exemplified, one of the materials may be used alone, or multiple materials may be used in combination.

The bonded body according to one embodiment includes a ceramic plate, a metal plate, and a brazing material layer that bonds a main surface of the ceramic plate to a main surface of the metal plate. The ceramic plate is composed of a ceramic sintered body that contains ceramic particles.

The ceramic particles constituting the ceramic sintered body may contain at least one selected from the group consisting of silicon nitride particles, aluminum nitride particles, and alumina particles. The ceramic plate may contain pores. Examples of the ceramic plate include a silicon nitride plate containing silicon nitride particles as a main component, an aluminum nitride plate containing aluminum nitride particles as a main component, and an alumina plate containing alumina particles as a main component. The ceramic plate may also be composed of a composite sintered body containing multiple types of ceramic particles.

In the bonded body of this embodiment, the average area ratio of pores in the region of the cut surface obtained by cutting the ceramic plate along the thickness direction of the ceramic plate, including directly below the outer edge of the side of the metal plate and extending to a depth of 200 μm from the main surface of the ceramic plate, is 2.0% or less. The average area ratio in this region may be 1.9% or less, 1.8% or less, 1.7% or less, 1.6% or less, 1.5% or less, 1.4% or less, or 1.3% or less. A bonded body including ceramic plates having such a small average area ratio in this region has sufficiently excellent insulation reliability at high temperatures. The average area ratio in this region may be 0.1% or more, 0.3% or more, 0.5% or more, or 0.8% or more. An example of the average range of the area ratio in this region is 0.1 to 1.9%.

The side of the metal plate may be inclined so as to widen toward the ceramic plate. In this case, the "outer edge of the side of the metal plate" is the outer edge of the main surface of the metal plate facing the ceramic plate. The brazing filler metal may creep up onto the side of the metal plate and be covered with the brazing filler metal. If the side of the metal plate is covered with a coating layer containing a brazing filler metal component, the "outer edge of the side of the metal plate" is the outer edge of the coating layer that covers the side of the metal plate.

In this specification, insulation reliability at high temperatures refers to insulation reliability in an environment of, for example, 80 to 100°C. If the assembly is used as a part of a power module, it will be exposed to such a temperature environment for a long time. In particular, a power module using a wide-gap semiconductor generates a large amount of heat because it is driven at high frequency, so the assembly (circuit board) built into the power module is frequently exposed to such high temperatures. In the assembly of this embodiment, the average value of the area ratio of pores in the region including just below the outer edge of the metal plate and up to a depth of 200 μm from the main surface of the ceramic plate, which corresponds to the portion where thermal stress increases at high temperatures, is sufficiently low. Therefore, insulation breakdown at high temperatures of the assembly of this embodiment can be sufficiently suppressed. Therefore, the assembly of this embodiment has excellent insulation reliability at high temperatures.

The junction may be a circuit board on which a semiconductor element is mounted, the semiconductor element being made of a semiconductor material having a band gap of more than 1.12 eV, 2.0 eV or more, 2.5 eV or more, or more than 3.0 eV. The semiconductor element may be a wide-gap semiconductor element. That is, examples of wide-gap semiconductor materials constituting the semiconductor element include silicon carbide, diamond, gallium oxide, and gallium nitride. The semiconductor may also be a semiconductor doped with impurities. In this specification, a wide-gap semiconductor (wide-gap semiconductor material) refers to one having a band gap of 2.2 eV or more.

The average value of the area ratio in the region is the arithmetic average value of the area ratio in at least five regions. The at least five regions are different from each other and are selected so that the regions do not overlap. The area of each region is, for example, ^0.05 mm2. The maximum value of the circle-equivalent diameter of the pores in the at least five regions is 10 μm or less, 9 μm or less, 8 μm or less, or 7 μm or less. By reducing the maximum value of the circle-equivalent diameter of the pores in this way, the variation in insulation reliability can be sufficiently reduced. The area of the pores in the region and the maximum value of the circle-equivalent diameter can be determined from an image magnified 500 times by a scanning electron microscope.

The thickness of the ceramic plate may be 0.2 mm or more, 0.25 mm or more, or 0.3 mm or more. Increasing the thickness of the ceramic plate in this manner can improve the durability of the insulation at high temperatures. The thickness of the ceramic plate may be 0.35 mm or less, or 0.31 mm or less. This allows the joint to be made smaller, and can improve heat dissipation when mounted on a device that generates a large amount of heat, such as a power module having a wide handgap semiconductor element. This can further increase the reliability of devices such as power modules. One example of the thickness range of the ceramic plate may be 0.2 to 0.35 mm, from the viewpoint of achieving both of the above-mentioned characteristics.

The thickness of the metal plate may be 0.5 mm or less, 0.45 mm or less, or 0.4 mm or less. By reducing the thickness of the metal plate in this way, the thermal stress generated in the ceramic plate can be reduced. Therefore, the insulation reliability at high temperatures can be further improved. The thickness of the metal plate may be 0.3 mm or more, or 0.35 mm or more. This improves heat dissipation and further increases the reliability of the device when the metal plate is mounted on a device that generates a large amount of heat, such as a power module having a wide handgap semiconductor element. An example of the thickness range of the metal plate may be 0.3 to 0.5 mm from the viewpoint of achieving both the above-mentioned characteristics. The metal plate may be, for example, a copper plate from the viewpoint of increasing thermal conductivity and electrical conductivity.

FIG. 1 is a plan view showing an example of a joint. FIG. 2 is a cross-sectional view obtained by cutting along line II-II in FIG. 1. The joint 100 shown in FIGS. 1 and 2 comprises a ceramic plate 10, a metal plate 41 joined to one main surface 10A of the ceramic plate 10 via a brazing material layer 51, and a metal plate 42 joined to the other main surface 10B of the ceramic plate 10 via the brazing material layer 51. The metal plate 41 is patterned and functions, for example, as a circuit. In this specification, such a metallic circuit pattern is also referred to as a metal plate. The metal plate 42 is not patterned and functions, for example, as a heat sink.

2 shows a cut surface along the thickness direction of the ceramic plate 10. In the cut surface thus obtained, the average area ratio of pores in the region RE including directly below the outer edge 44 on the side of the metal plate 41 and extending to a depth of 200 μm from the main surface 10A of the ceramic plate 10 is 2.0% or less. Directly below the outer edge 44 on the side of the metal plate 41 is directly below the outer edge of the side (side surface) of the metal plate 41 when the joint body 100 is viewed in plan as shown in FIG. 1. As shown in FIG. 1, regions RE1, RE2, RE3, RE4, and RE5 are selected directly below at least five outer edge portions 44a, 44b, 44c, 44d, and 44e of the outer edge 44 on the side of the metal plate 41. Regions RE1, RE2, RE3, RE4, and RE5 all include the area directly below the outer edge 44 ( outer edge portions 44a, 44b, 44c, 44d, and 44e) and are areas up to a depth of 200 μm from the main surface 10A of the ceramic plate 10. Note that region RE5, which includes the outer edge portion 44e and the area directly below it, is not shown in FIG. 2. Region RE5, like regions RE1, RE2, RE3, and RE4, can also be defined by obtaining a cut surface obtained by cutting the ceramic plate 10 along the thickness direction so as to pass directly below the outer edge 44 of the metal plate 41.

Five regions RE are shown in FIG. 1 (four in FIG. 2), but six or more regions RE may be selected to determine the average value of the pore area ratio in the regions RE. Furthermore, the positions at which the ceramic plate 10 is cut in the thickness direction are not limited to those shown in FIG. 1 and FIG. 2. For example, five cut surfaces may be obtained, the regions RE may be determined in each cut surface, and the pore area ratio in each region RE may be determined. The average value of the pore area ratio in the bonded body 100 may be calculated by arithmetically averaging the pore area ratios determined in each region RE. The thickness ranges of the ceramic plate 10 and the metal plates 41 and 42 are as described above. The thicknesses of the metal plates 41 and 42 may be the same or different from each other.

FIG. 3 is a diagram showing an example of an image obtained when a part of the cut surface 10C of the ceramic plate 10 is observed with a scanning electron microscope (SEM, magnification: 500 times). As shown in FIG. 3, the region RE in the cut surface 10C of the ceramic plate 10 includes ceramic particles and pores 20. The region RE includes a plurality of pores 20. FIG. 3 shows a schematic enlargement of one pore 20. In this way, the two-dimensional image of the pores 20 includes pores that are not perfect circles. Note that the region RE usually includes a large number of ceramic particles, but in FIG. 3, the ceramic particles are omitted for convenience. The area of each pore 20 can be obtained using commercially available image processing software (e.g., Image J). The area of each pore 20 may be obtained by binarizing the SEM image.

The total area of the pores 20 is calculated from the area of each pore 20 included in the region RE shown in FIG. 3. The total area of the pores 20 can be calculated by dividing the total area by the area of the region RE. The area of the region RE is, for example, 0.05 ^mm2 . Using the area ratio of the pores 20 in each region RE, the average area ratio of the pores 20 can be calculated as described above. The average area ratio of the pores 20 is 2.0% or less as described above. Other examples of numerical ranges are as described above.

In a joint 100 including such a ceramic plate 10, large thermal stress occurs in region RE as the temperature changes. In region RE, the area ratio of pores 20 is low, so the occurrence and progression of cracks can be sufficiently suppressed. Therefore, the joint 100 has excellent insulation reliability at high temperatures.

The circular equivalent diameter of the pores 20 contained in the region RE in Figure 3 can be obtained using commercially available image processing software (e.g., Image J). When five regions RE1, RE2, RE3, RE4, and RE5 are selected as the region RE, the maximum value of the circular equivalent diameter of the pores 20 in the five regions is obtained. In other words, the maximum value is obtained from among the five maximum values. The maximum value of the circular equivalent diameter obtained in this way is within the above-mentioned range.

In this example, only multiple regions from the main surface 10A to a depth of 200 μm were selected, but in another example, the area ratio of pores in the region RE from the main surface of the ceramic plate to a depth of 200 μm, including directly below the outer edge of the side of the metal plate 42 on the main surface 10B, may be obtained. For example, three regions RE from the main surface 10A to a depth of 200 μm may be selected, and two regions RE from the main surface 10B to a depth of 200 μm may be selected.

The region RE may contain a sintering aid phase in addition to the pores 20 and ceramic particles. The average area ratio of the ceramic particles (ceramic phase) in the region RE may be 70 to 90%, or 75 to 85%. The average area ratio of the sintering aid phase in the region RE may be 10 to 25%, or 14 to 22%. A ceramic plate 10 having a region RE (cut surface 10C) containing ceramic particles and a sintering aid phase at such average area ratios has a sufficiently high thermal conductivity. The average area ratio of the ceramic particles and the sintering aid phase is calculated as the arithmetic average of the area ratios measured in at least five regions RE, similar to the average area ratio of the pores 20.

1, 2, and 3 are an example of a bonded body. The shapes of the ceramic plate 10 and the metal plates 41, 42 are not limited to those shown in FIG. 1 and FIG. 2. In a modified example of the bonded body 100, the metal plate 42 may also have a circuit pattern. In another modified example, both the metal plate 41 and the metal plate 42 do not need to be patterned. For example, when a metal plate is etched to form a circuit pattern, both before and after the circuit pattern is formed correspond to the bonded body. In yet another modified example, there may be multiple metal plates (circuit patterns) bonded to one main surface of the ceramic plate. A metal plate may be bonded to only one main surface of the ceramic plate.

The brazing material layers 51, 52 are layers that join the ceramic plate 10 and the metal plates 41, 42, and contain brazing material components. The brazing material layers 51, 52 may contain, for example, silver derived from the brazing material, or silver and copper. The brazing material layers 51, 52 may further contain one or more metals selected from the group consisting of tin and active metals derived from the brazing material. In the brazing material layers 51, 52, the two or more metals may be an alloy. The active metal may contain one or more metals selected from the group consisting of titanium, hafnium, zirconium, and niobium. The silver and copper contained in the brazing material layers 51, 52 may be contained as an alloy, such as an Ag-Cu eutectic alloy.

The silver content in the brazing material layers 51, 52 may be 45-95 mass% or 50-95 mass% in Ag equivalent. The total silver and copper content in the brazing material layers 51, 52 may be 65-100 mass%, 70-99 mass%, or 90-98 mass% in Ag and Cu equivalent, respectively. The thickness and composition of the brazing material layers 51, 52 may be the same as or different from each other.

The power module according to one embodiment includes a joint (circuit board) and a semiconductor element electrically connected to the metal plate of the joint. The joint may be the joint 100 described above or a modified version thereof. The description of the joint and the modified versions applies to the power module of this embodiment. Such a power module includes a joint (circuit board) that has excellent insulation reliability at high temperatures. Therefore, even when used in a high-temperature environment, high performance can be maintained. In this way, the power module has excellent reliability.

FIG. 4 is a cross-sectional view showing an example of a power module. The description of this example is not limited to this example, and is also applicable to modified examples of the power module. The power module 200 in FIG. 4 includes a base plate 90 and a joint body 100 that is joined to one side of the base plate 90 via solder 82. A metal plate 42 (heat sink) on one side of the joint body 100 is joined to the base plate 90 via solder 82.

A semiconductor element 80 is attached to the metal plate 41 (circuit pattern) on the other side of the joint body 100 via solder 81. The semiconductor element 80 is connected to a predetermined location of the metal plate 41 with a metal wire 84 such as an aluminum wire. In this way, the semiconductor element 80 and the metal plate 41 are electrically connected. To electrically connect the outside of the housing 86 to the metal plate 41, one of the metal plates 41, metal plate 41a, is connected to an electrode 83 that penetrates the housing 86 via solder 85.

A housing 86 is disposed on one of the main surfaces of the base plate 90, and is integrated with the main surface to house the joint body 100. The housing space formed by the one of the main surfaces of the base plate 90 and the housing 86 is filled with resin 95. The resin 95 seals the joint body 100 and the semiconductor element 80. The resin may be, for example, a thermosetting resin or a photocurable resin.

A cooling fin 92, which serves as a heat dissipation member, is joined to the other main surface of the base plate 90 via grease 94. Screws 93 are attached to the ends of the base plate 90 to secure the cooling fin 92 to the base plate 90. The base plate 90 and the cooling fin 92 may be made of aluminum. The base plate 90 and the cooling fin 92 function well as heat dissipation parts due to their high thermal conductivity.

The metal plate 41 and the metal plate 42 are electrically insulated by the ceramic plate 10. The metal plate 41 (41a) may form an electric circuit. The metal plate 41 and the metal plate 42 are respectively joined to the main surface 10A and the main surface 10B of the ceramic plate 10 by a brazing material layer (not shown) containing a brazing material component. The power module 200 has excellent reliability because it includes the joint 100.

The semiconductor element 80 in the power module 200 may be made of a semiconductor material having a band gap of more than 1.12 eV, 2.0 eV or more, 2.5 eV or more, or more than 3.0 eV. The semiconductor element may be made of a wide-gap semiconductor material. Examples of wide-gap semiconductor materials include silicon carbide, diamond, and gallium nitride. The junction 100 and its variations have excellent insulation reliability at high temperatures, so that the power module 200 including the wide-gap semiconductor element can operate at a high power level. The band gap of the semiconductor material constituting the semiconductor element 80 may be 6.0 eV or less, 5.5 eV or less, or 5.0 eV or less. An example of the range of the band gap of the semiconductor material is more than 1.12 eV and 6.0 eV or less.

An example of a manufacturing method for the ceramic plate 10 provided in the joint body 100 will be described. This manufacturing method includes a crushing process in which the sintering aid raw material is crushed in a crusher to obtain sintering aid powder with a D50 (median diameter) of 0.5 to 1.1 μm, a mixing process in which a mixed raw material containing ceramic powder and sintering aid powder is prepared, and a firing process in which a compact of the mixed raw material is fired.

The sintering aid powder may contain at least one selected from the group consisting of alkaline earth metal oxides, rare earth oxides, transition metal oxides different from the rare earth oxides, silica, and alumina, and may contain two or more, or three or more.

The alkaline earth metal oxide has an alkaline earth metal and oxygen as constituent elements. The alkaline earth metal oxide may include at least one selected from the group consisting of magnesium oxide, calcium oxide, and strontium oxide. The rare earth oxide has a rare earth element and oxygen as constituent elements. The rare earth oxide may include, for example, at least one selected from the group consisting of yttrium oxide and cerium oxide. The transition metal oxide different from the rare earth oxide has a transition metal different from the rare earth and oxygen as constituent elements. Such a transition metal oxide may include, for example, iron oxide.

An example of a sintering aid powder includes magnesium oxide, rare earth oxide, and silica. In this case, when the total amount of the sintering aid powder is 100 parts by mass, the content of the rare earth oxide may be 30 to 80 parts by mass, or may be 40 to 70 parts by mass. In this case, the content of the magnesium oxide may be 5 to 40 parts by mass, or may be 10 to 30 parts by mass. In this case, the content of the silica may be 5 to 40 parts by mass, or may be 10 to 30 parts by mass.

The D50 (median diameter) of the sintering aid powder in the grinding process may be prepared, for example, by grinding the sintering aid raw material with a grinder. A bead mill type grinder may be used as the grinder. The particle size distribution of the sintering aid powder may be adjusted by changing at least one condition selected from the group consisting of the diameter of the beads of the bead mill type grinder, the peripheral speed, and the grinding time. The diameter of the beads may be 0.1 to 0.3 mm. The peripheral speed of the rotor may be 8 to 12 m/sec. The grinding time may be 5 to 20 minutes. Examples of grinders other than the bead mill type grinder include a ball mill, a vibration mill, and a pot mill.

When the D50 of the sintering aid powder exceeds 1.1 μm, the number and size of the pores 20 increase, and the average area ratio of the pores 20 in the region RE tends to increase. Also, when the D50 of the sintering aid powder is less than 0.5 μm, the crushed particles tend to aggregate due to the relationship between the input energy applied to the sintering aid raw material from the crusher and the crushing ratio. This is thought to be due to the fact that as the crushing progresses, the frequency of contact between the crushed particles increases, and the potential energy becomes attractive. In this case too, the average area ratio of the pores 20 in the region RE tends to increase.

The D50 of the sintering aid powder is determined based on the volumetric particle size distribution measured by a particle size distribution measuring device using a laser diffraction/scattering method. By having the D50 of the sintering aid powder within the above range, the particles contained in the sintering aid powder are sufficiently small and the particles can be prevented from agglomerating together. This makes it possible to prevent the generation of pores due to the particles and agglomerates of the sintering aid powder when producing a ceramic plate (ceramic sintered body). Therefore, the average value of the area ratio of the pores 20 in the region RE can be reduced, and the maximum value of the circle equivalent diameter of the pores 20 contained in the region RE can be reduced. In addition, the grain growth of the ceramic particles can be promoted with high uniformity.

Figure 5 shows an example of a volumetric particle size distribution of sintering aid powder by laser diffraction and scattering. The horizontal axis is particle size [μm] in logarithmic scale, and the vertical axis is frequency [volume %]. The particle size distribution in this disclosure is measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method". An LS-13 320 (product name) manufactured by Beckman Coulter is used to measure the particle size distribution. The measurement conditions are a particle refractive index of 2.2 and a solvent refractive index of 1.33.

The sintering aid powder may have only one peak in the particle size distribution (frequency %) as shown in Figure 5. Such sintering aid powder is sufficiently inhibited from agglomerating, so that the size and number of pores in the ceramic sintered body can be sufficiently reduced. The peak in the particle size distribution may be sharp. For example, the D100 of the sintering aid powder may be 5.5 μm or less, or may be less than 5.0 μm. For example, the ratio of D100 to D50 may be 5 or less. An example of the lower limit of D100 is 2 μm. An example of the lower limit of the ratio of D100 to D50 is 2.

In the mixing process, the sintering aid powder obtained by pulverization is mixed with the ceramic powder and, if necessary, additives, and mixed using, for example, a ball mill. In this way, a mixed raw material containing the sintering aid powder and the ceramic powder is prepared. Examples of additives include binders, plasticizers, dispersion media, and release agents. Examples of binders include methylcellulose-based binders that have plasticity or surface activity, and acrylic ester-based binders that have excellent thermal decomposition properties. Examples of plasticizers include glycerin. Examples of dispersion media include ion-exchanged water and ethanol.

As the ceramic powder, for example, silicon nitride powder, aluminum nitride powder, or aluminum oxide powder can be used. The D50 (median diameter) of the ceramic powder may be 0.1 to 6 μm, or may be 0.5 to 4 μm. This allows a sufficiently densified ceramic sintered body to be obtained. The D50 of the ceramic powder is determined in the same manner as the D50 of the sintering aid powder. The number of peaks in the particle size distribution (frequency %) of the ceramic powder may also be one.

The mass ratio of the sintering aid powder to the ceramic powder may be 0.03 to 0.12, or may be 0.05 to 0.1. This makes it easier to densify the ceramic sintered body, and allows the flexural strength to be sufficiently high.

The mixed raw material obtained in the mixing step is applied to a release film at a specified thickness by a doctor blade method, a calendar method, an extrusion method, or the like, and then dried and molded to obtain a molded body. The molding pressure may be 3 to 30 MPa. The molded body may be produced by uniaxial pressing or by CIP. It may also be fired while being molded by hot pressing. For example, after producing a ceramic green sheet substrate by the above-mentioned methods such as the doctor blade method, the ceramic green sheet substrate may be punched out using a mold equipped with a die and a punch to obtain a molded body.

The solid content of the ceramic green sheet substrate when punched with a die may be 65 to 85% by mass, or 75 to 85% by mass. The solid content may be adjusted by carrying out a drying process to dry the ceramic green sheet substrate before punching with a die.

The molded body may be degreased before being sintered in the sintering step. The degreasing method is not particularly limited, and may be performed, for example, by heating the molded body to 300 to 700°C in air or a non-oxidizing atmosphere such as nitrogen. The heating time may be, for example, 1 to 10 hours.

Ceramic plates (plate-shaped ceramic sintered bodies) can be obtained by firing the molded body. The atmosphere, temperature, and time during firing can be set appropriately depending on the type of ceramic sintered body. When manufacturing a silicon nitride plate as the ceramic plate, it may be performed in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure during firing may be 0.7 to 1 MPa. The firing temperature may be 1800 to 2100°C, 1800 to 2000°C, or 1800 to 1900°C. The firing time at the firing temperature may be 3 to 20 hours, or 4 to 16 hours.

When manufacturing an aluminum nitride plate as the ceramic plate, the sintering temperature may be, for example, 1760 to 1840°C. The holding time in the temperature range of 1760 to 1840°C may be, for example, 1 to 10 hours. Firing may be carried out under atmospheric pressure. When manufacturing ceramic plates other than silicon nitride plates and aluminum nitride plates (for example, and aluminum oxide plates), the sintering conditions may be appropriately set so that the sintered body is sufficiently densified.

Figure 6 is a diagram showing an image of grain growth as sintering progresses in the manufacturing method of this example. In this example, as shown in (a) of Figure 6, fine sintering aid powder 32 is dispersed with high uniformity in ceramic particles 12 in the molded body. When such a molded body is fired, the liquefied sintering aid phase 32a diffuses to the grain boundaries by capillary action as shown in (b) of Figure 6. When the sintering aid phase 32a diffuses, the shrinkage of the molded body (ceramic sintered body) progresses, and the pores 22 disappear as shown in (c) of Figure 6. When heating is continued, the ceramic particles 12 melt into the sintering aid phase 32a, and columnar ceramic particles 14 are generated as shown in (d) of Figure 6. In this way, as liquid phase sintering progresses, the pores 22 disappear sufficiently with the smooth grain growth of the ceramic particles, so that the pores 20 contained in the ceramic plate 10 can be sufficiently reduced. A part of the sintering aid phase 32a may remain in the ceramic plate.

Figure 9 is a diagram showing an image of grain growth as sintering progresses in a conventional manufacturing method. In the conventional manufacturing method, as shown in (a) of Figure 9, agglomerates 132 of sintering aid powder are contained in ceramic particles 112 in the molded body. A cross-sectional photograph of such a conventional molded body is shown in Figure 10. When such a molded body is fired, as shown in (b) of Figure 9, the liquefied sintering aid phase 132a diffuses from the agglomerates 132 to the grain boundaries by capillary action starting from the agglomerates 132. As diffusion by capillary action progresses, pores 122 are formed in the agglomerates 132 because of the large size of the agglomerates 132. Because the pores 122 are large in size, they do not disappear even when the molded body shrinks, and the pores 122 remain in the ceramic plate as shown in (c) of Figure 9. In this way, the number of large pores contained in the ceramic plate increases.

Compared to the conventional manufacturing method, in the manufacturing method of the ceramic plate 10 of this example, the particles of the sintering aid powder are sufficiently fine, and the aggregation of the particles is suppressed. Therefore, it is possible to reduce the pores remaining as traces of the sintering aid powder. This reduces the number of pores generated during the sintering process and makes the size of the pores smaller. The average area ratio of the pores 20 in the region RE of the ceramic plate 10 obtained in this way becomes sufficiently small. In addition, the maximum value of the circle equivalent diameter of the pores 20 in the region RE also becomes sufficiently small. Such a ceramic plate can sufficiently suppress the occurrence and progression of cracks when thermal stress occurs due to temperature change. Therefore, it has excellent insulation reliability at high temperatures.

FIG. 7 is a schematic diagram showing an example of an inspection device for performing a V-t test on a joint (circuit board). The inspection device 400 includes an AC power source 60 and a voltage resistance tester 50 connected to the AC power source 60. One terminal of the voltage resistance tester 50 is electrically connected to a conductive support part 72a that contacts the metal plate 41 joined to the ceramic plate 10. The other terminal of the voltage resistance tester 50 is electrically connected to a conductive support part 72b that contacts the metal plate 42 via an electrode 70 that is placed in a storage tank 77 that stores insulating oil 76.

The electrode 70 is arranged along the bottom surface and one side surface of the storage tank 77. As shown in FIG. 7, the electrode 70 has an L-shape when viewed in vertical cross section. Two insulating support parts 74 are provided on the electrode 70 adjacent to the conductive support part 72b. The two insulating support parts 74 each contact the metal plate 42, and support the assembly 100 in the insulating oil 76.

The electrode 70 and the conductive support parts 72a, 72b may be made of, for example, oxygen-free copper. The insulating oil 76 may be, for example, a fluorine-based inert liquid. The voltage resistance tester 50 may be a commercially available product. In this inspection device 400, a voltage of, for example, about 10 to 15 kV is applied between the metal plates 41, 42 that sandwich the ceramic plate 10, and the voltage resistance tester 50 measures the presence or absence of leakage current. By heating the insulating oil 76, the insulating performance of the ceramic plate 10 in the bonded body 100 at high temperatures, for example, above 100°C, may be evaluated.

The inspection device is not limited to the configuration shown in FIG. 7, and can be used without any particular restrictions as long as it is an inspection device capable of performing a V-t test when a voltage is applied between the metal plates 41 and 42 at a temperature of, for example, 100°C or higher.

Although the embodiments of the present disclosure have been described above, the present disclosure is in no way limited to the above-described embodiments. The contents of the description regarding the embodiments of the ceramic sintered body also apply to the bonded body, the power module, and the manufacturing method of the ceramic sintered body. The contents of the description regarding the embodiments of the manufacturing method of the ceramic sintered body also apply to the ceramic sintered body.

　Numerical ranges that combine the upper and lower limit values of the numerical ranges specifically described in each of the above embodiments are also included in the present disclosure. In addition, numerical ranges in which the upper and/or lower limit values are replaced with the values of the examples described below are also included in the present disclosure.

The contents of this disclosure will be explained in more detail with reference to examples, reference examples, and comparative examples, but this disclosure is not limited to the specific examples below.

(Preparation of sintering aid powder)
As raw materials for the sintering aid powder, commercially available _yttrium oxide powder, magnesium oxide powder, and silica powder were prepared. These were mixed to obtain a mixed powder with a mass ratio of _Y2O3 :MgO: _SiO2 = 5:2:2. The mixed powder was pulverized using a bead mill type pulverizer (manufactured by Ashizawa Finetech Co., Ltd., device name: Star Mill LMZ) to obtain a sintering aid powder. The pulverization conditions (bead diameter, rotor circumferential speed, and pulverization time) of the bead mill type pulverizer were changed as shown in Tables 1 and 2 to prepare eight types of sintering aid powders with different pulverization conditions.

The volumetric particle size distribution of each sintering aid powder was measured using a particle size distribution measuring device using the laser diffraction/scattering method (Nikkiso Co., Ltd., device name: particle size distribution measuring device MT3000II). From the particle size distribution measurement results, D50 (median size) and D100 (maximum particle size) were calculated. The results are shown in Tables 1 and 2. Tables 1 and 2 also show the ratio of D100 to D50. The particle size distributions (frequency %) of numbers 5 to 8 all had only one peak. In other words, these particle size distributions had the shape shown in Figure 5.

The results for numbers 1 to 3 in Table 1 confirm that the D50 and D100 of the sintering aid can be reduced by reducing the diameter of the beads. Additionally, the results for numbers 3, 4, and 5 in Tables 1 and 2 confirm that the D50 and/or D100 can be reduced by increasing the circumferential speed of the rotor. The results for numbers 5 and 6 confirm that the D50 and D100 can be reduced by extending the grinding time. On the other hand, the results for numbers 6, 7, and 8 confirm that the D50 and D100 increase when the grinding time is too long. This is thought to be due to the aggregation of the ground powder.

Example 1
[Preparation of silicon nitride plate]
Commercially available silicon nitride powder (D50: 0.7 μm), sintering aid powder number 6 in Table 2, and additives (solvent-based binder) were mixed in a bead mill to prepare a raw material slurry. The compounding ratio of silicon nitride powder and sintering aid powder was silicon nitride powder: sintering aid powder = 91:9. Next, the above-mentioned raw material slurry was applied to a release film by a doctor blade method to prepare a green sheet. The prepared ceramic green sheet was cut to a length x width = 250 mm x 180 mm, and 70 sheets were stacked to obtain a laminate. The above laminate was placed in an electric furnace equipped with a carbon heater and heated in air at 500 ° C for 20 hours to degrease.

The degreased molded body was placed in a sintering furnace, the pressure in the furnace was reduced to less than 100 Pa, and the temperature was raised to 900°C. Nitrogen gas was then introduced into the furnace, and the temperature was raised to 1500°C under a pressure of approximately 0.9 MPa and held at that temperature for 4 hours. After that, the temperature was raised to 1830°C and held at 1830°C for 5 hours. In this way, a silicon nitride plate with a thickness of 3 mm was obtained.

[Preparation of Joint]
A brazing filler metal was prepared containing 89.5 parts by mass of Ag powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name: Ag-HWQ, average particle size D50: 2.5 μm, specific surface area: 0.4 ^{m2 /} g), 9.5 parts by mass of Cu powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name: Cu-HWQ, average particle size D50: 3.0 μm, specific surface area: 0.4 m2/g), and 1.0 part by mass of Sn powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd.: Sn-HPN, average particle size D50: 3 μm, specific surface area: 0.1 m2/ ^g ) for a total of 100 parts by mass, and 3.5 parts by mass of titanium hydride powder (manufactured by Toho Tech Co., Ltd., product name: TCH-100). This brazing filler metal was applied by screen printing to both main surfaces of a silicon nitride plate so that the application amount was 8 mg/ ^cm2 .

A copper plate for forming a circuit was placed on the brazing material layer on one of the main surfaces of the silicon nitride plate, and a copper plate for forming a heat sink (both C1020 oxygen-free copper plates with a thickness of 0.3 mm and a purity of 99.60% by mass) was placed on the brazing material layer on the other main surface of the silicon nitride plate to obtain a laminate. This laminate was heated at 830° C. for 30 minutes in a vacuum of 1.0×10 ⁻³ Pa or less to obtain a bonded body. An etching resist was printed on the bonded copper plate for forming a circuit, and the copper plate for forming a circuit was etched with a ferric chloride solution to form a circuit pattern as shown in FIG. 1. Furthermore, the brazing material layer protruding beyond the side surface of the metal plate was removed with an ammonium fluoride/hydrogen peroxide solution. In this way, a circuit board was produced. A plurality of circuit boards were produced for use in the following cross-sectional observation and V-t test.

[Cutting surface observation]
The silicon nitride plate was cut along the thickness direction of the silicon nitride plate to obtain a cut surface. Cutting was performed at multiple locations to select five regions including the area immediately below the outer edge of the side of the circuit pattern and extending to a depth of 200 μm from the main surface of the silicon nitride plate. The identified regions RE were observed at a magnification of 500 times using a scanning electron microscope (SEM). The total area of pores and the maximum value of the circle-equivalent diameter of the pores in the five regions RE (each area: 0.05 mm ² ) were obtained. Each region included the area immediately below the outer edge of the side of the circuit pattern. The size and number of pores were measured using image processing software (ImageJ). The total area of pores was divided by the area of the field of view (0.05 mm ² ) to obtain the area ratio of the pores. The maximum circle-equivalent diameter of the pores and the area ratio of the pores were obtained in each of the five regions RE. The arithmetic mean value of the pore area ratio and the maximum value of the pore equivalent circle diameter (maximum value in the five regions RE) are shown in Table 3.

[Insulation test]
Using an inspection device as shown in FIG. 7, a V-t test of the circuit board was performed in accordance with JIS C2110-1:2010. For this inspection, an AC 20 kV withstand voltage tester (model: 7473) manufactured by Keisoku Gijutsu Kenkyusho Co., Ltd. was used. Silicon oil was used as the insulating oil. An inspection jig manufactured by Onishi Electronics Co., Ltd. was used as the reservoir 77, the electrode 70, the conductive support parts 72a, 72b, and the insulating support part 74. The electrode 70 was made of oxygen-free copper, and the conductive support parts 72a, 72b were made of carbon tool steel material (SK material) plated with rhodium.

The insulating oil in the testing device was heated to 100°C, the circuit board 100 was fixed in the insulating oil, and a voltage of 10 kV was applied between the circuit pattern 41 and the copper plate 42, and the time until dielectric breakdown occurred (maximum: 746 hours) was measured. The results are shown in Table 3.

Example 2
A silicon nitride plate and a bonded body were produced and each evaluation was performed in the same manner as in Example 1, except that the sintering aid powder No. 5 in Table 2 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

Example 3
A silicon nitride plate and a bonded body were produced and each evaluation was performed in the same manner as in Example 1, except that the sintering aid powder No. 7 in Table 2 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

Example 4
A silicon nitride plate and a bonded body were produced and each evaluation was carried out in the same manner as in Example 1, except that the sintering aid powder No. 8 in Table 2 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

(Comparative Example 1)
A silicon nitride plate and a bonded body were produced and each evaluation was performed in the same manner as in Example 1, except that the sintering aid powder No. 3 in Table 1 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

(Comparative Example 2)
A silicon nitride plate and a bonded body were produced and each evaluation was performed in the same manner as in Example 1, except that the sintering aid powder No. 1 in Table 1 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

(Comparative Example 3)
A silicon nitride plate and a bonded body were produced and each evaluation was performed in the same manner as in Example 1, except that the sintering aid powder No. 2 in Table 1 was used instead of the sintering aid powder No. 6 in Table 2. The results are shown in Table 3.

As shown in Table 3, it was confirmed that the area ratio of pores is reduced by using a sintering aid powder with a small D50. In Example 1, which used sintering aid powder No. 6 with the smallest D50 and D100/D50, the area ratio of pores was the smallest. This Example 1 had the best insulation reliability without any insulation breakdown. Figure 8 (A) is a photograph showing an SEM image (500x) of one of the above-mentioned regions selected from the cut surface of the silicon nitride plate of Example 1, and Figure 8 (B) is a diagram showing the SEM image after binarization. Figure 11 (A) is a photograph showing an SEM image (500x) of one of the above-mentioned regions selected from the cut surface of the silicon nitride plate of Comparative Example 1, and Figure 11 (B) is a diagram showing the SEM image after binarization. Comparing Figures 8 and 11, the number of pores and the size of the pores were clearly smaller in Example 1 than in Comparative Example 1.

From the results shown in Table 3, the average pore area ratio and maximum pore circle equivalent diameter in the above-mentioned region of the silicon nitride plate were smaller in Examples 1 to 4 than in Comparative Examples 1 to 3. In the V-t test of the circuit board of Example 1, no insulation breakdown occurred until the end of the test (746 hours). Furthermore, it was confirmed that Examples 2 to 4 also took longer to break down than Comparative Examples 1 to 3, and therefore had superior insulation reliability.

According to the present disclosure, a bonded body having excellent insulation reliability at high temperatures is provided. In addition, according to the present disclosure, a power module having excellent reliability is provided by including the bonded body.

10...ceramic plate, 10A, 10B...main surface, 10C...cut surface, 12, 14...ceramic particles, 20, 22, 122...pores, 32...sintering aid powder, 32a, 132a...sintering aid phase, 41...metal plate (circuit pattern), 42...metal plate (copper plate), 44...outer edge, 44a, 44b, 44c, 44d, 44e...outer edge portion, 50...voltage tester, 51, 52...brazing material layer, 60...AC power source, 70, 83...electrodes, 72a , 72b...conductive support part, 74...insulating support part, 76...insulating oil, 77...reservoir, 80...semiconductor element, 81, 82, 85...solder, 84...metal wire, 86...housing, 90...base plate, 92...cooling fin, 93...screw, 94...grease, 95...resin, 100...joint (circuit board), 132...aggregate, 200...power module, 400...inspection device, RE, RE1, RE2, RE3, RE4, RE5...area.

Claims (9)

1. A ceramic plate, a metal plate, and a brazing material layer that joins a main surface of the ceramic plate and a main surface of the metal plate,
   A bonded body, wherein, in a cut surface obtained by cutting the ceramic plate along the thickness direction of the ceramic plate, the average area ratio of pores in a region including directly below the outer edge of the side of the metal plate and extending to a depth of 200 μm from the main surface of the ceramic plate is 2.0% or less.
2. The bonded body according to claim 1, wherein the maximum circle equivalent diameter of the pores contained in the region of the cut surface is 10 μm or less.
3. The bonded body according to claim 1, wherein the ceramic plate has a thickness of 0.2 mm or more.
4. The joint body according to claim 1, wherein the thickness of the metal plate is 0.5 mm or less.
5. The junction according to claim 1, which is a circuit board on which a semiconductor element made of a semiconductor material having a band gap exceeding 1.12 eV is mounted.
6. A power module comprising a joint body according to any one of claims 1 to 5 and a semiconductor element electrically connected to the metal plate of the joint body.
7. The power module according to claim 6, wherein the band gap of the semiconductor material constituting the semiconductor element exceeds 1.12 eV.
8. The power module according to claim 6, wherein the thickness of the ceramic plate is 0.35 mm or less.
9. The power module according to claim 6, wherein the thickness of the metal plate in the joint is 0.3 mm or more.

Images