Method for producing active metal paste, method for producing ceramic circuit board, method for producing semiconductor device, active metal paste, ceramic circuit board, and semiconductor device

Abstract

A method for producing an active metal paste according to an embodiment comprises a premixing step and a kneading step. In the premixing step, a binder obtained by dissolving a resin in an organic solvent is mixed with a metal powder containing copper and an active metal to produce a mixture containing the metal powder at 60 to 95 mass%. In the kneading step, rotation and revolution are applied to this mixture to produce an active metal paste.

Description

Method for manufacturing active metal paste, method for manufacturing ceramic circuit board, method for manufacturing semiconductor device, active metal paste, ceramic circuit board, and semiconductor device

[0001] The embodiments generally relate to a method for manufacturing an active metal paste, a method for manufacturing a ceramic circuit board, a method for manufacturing a semiconductor device, an active metal paste, a ceramic circuit board, and a semiconductor device.

[0002] Ceramic circuit boards are used in semiconductor devices equipped with semiconductor elements such as power elements. The ceramic substrate and metal circuit section are bonded to each other via a bonding layer using an active metal brazing material or the like. This improves bonding strength and heat cycle characteristics. As reliability improves, ceramic circuit boards are used in automobiles (including electric vehicles), electric railway vehicles, solar power generation facilities, inverters for industrial machinery, and the like. In semiconductor devices such as power modules, semiconductor elements are mounted on the circuit section. Wire bonding or metal terminals may also be bonded to the semiconductor elements for electrical continuity. In the manufacture of semiconductor devices, semiconductor elements, wire bonding, metal terminals, and the like are bonded to the circuit section.

[0003] A method for forming a bonding layer on a ceramic circuit board is disclosed in Patent Document 1, which involves forming an active metal brazing material into a paste and printing it on a ceramic substrate. According to Patent Document 1, a mixing process for mixing brazing material component powders is carried out for 10 hours or more, and then a binder and a solvent are added to the mixed powder to prepare a brazing material paste. The mixing process of the mixed powder and binder, etc. is carried out for 10 hours or more. In the bonding layer using the paste disclosed in Patent Document 1, the distribution of Ag (silver) is controlled, thereby suppressing differences in etching effect. It is disclosed that controlling the size of the protrusion of the bonding portion by etching in this way leads to improved temperature cycle test (TCT) characteristics.

[0004] As a method for producing a paste, a method of subjecting a kneaded material to mechanical dispersion treatment is disclosed (Patent Document 2). According to Patent Document 2, examples of mechanical dispersion treatment include ultrasonic dispersion, a disperser, a three-roll mill, a ball mill, a bead mill, a two-axis kneader, and a planetary mixer. Patent Document 2 lists ultrasonic dispersion and a three-roll mill as modes for carrying out the invention.

[0005] Furthermore, a method for producing a paste using a planetary stirrer has been disclosed (Patent Document 3). Patent Document 3 describes a silver paste produced by kneading silver powder, terpineol, and a resin using a planetary stirrer at a centrifugal force of 420 G.

[0006] International Publication No. 2022 / 085654 International Publication No. 2012 / 070262 Patent No. 5790433

[0007] As the reliability of semiconductor substrates that use ceramic circuit boards increases, the challenges of improving the bonding strength and heat cycle characteristics of ceramic circuit boards become apparent. To improve these characteristics, a brazing filler metal tailored to the purpose is used, and it is necessary to uniformly and efficiently mix these brazing filler metal components to form a paste. The powder shape and density of the metal materials used in brazing filler metals vary depending on the type. Furthermore, mixing of the metal materials with solvents and resin components to form a paste must also be considered. While the paste components can be uniformly mixed by extending the mixing time, this increases the work time and costs proportionally.

[0008] The present invention aims to solve these problems by providing a method for producing an active metal paste that can more uniformly knead the paste while suppressing increases in cost, and a method for producing a ceramic circuit board and a semiconductor device using the active metal paste. The present invention also provides a more uniformly kneaded active metal paste, and a ceramic circuit board and a semiconductor device using the active metal paste.

[0009] The method for producing an active metal paste according to the embodiment includes a pre-mixing step and a kneading step. In the pre-mixing step, a metal powder containing copper and an active metal is mixed with a binder in which a resin is dissolved in an organic solvent to produce a mixture containing 60 to 95 mass% of the metal powder. In the kneading step, the mixture is rotated and revolved to produce the active metal paste.

[0010] Fig. 1 is a side view showing an example of a ceramic circuit board according to an embodiment; Fig. 2 is a process flow diagram showing an example of a method for manufacturing an active metal paste according to an embodiment; Fig. 3 is a schematic diagram showing an example of a kneading process of an active metal paste according to an embodiment; Fig. 4 is a cross-sectional view showing an example of a manufacturing process of a ceramic circuit board according to an embodiment; Fig. 5 is a side view showing an example of a semiconductor device according to an embodiment.

[0011] A method for producing an active metal paste according to an embodiment is a method for producing an active metal paste containing 60 to 95 mass % of a metal powder containing copper and at least one active metal. This production method includes a premixing step of mixing the metal powder with a binder comprising a resin dissolved in an organic solvent, and a kneading step of kneading the premixed paste. In the kneading step, a kneading treatment is performed in which the premixed paste is subjected to rotation and revolution. This active metal paste is used in the production of ceramic circuit boards.

[0012] FIG. 1 is a cross-sectional view showing an example of a ceramic circuit board according to an embodiment. In FIG. 1, 1 is a ceramic circuit board. 2 is a ceramic substrate. 3 is a metal circuit. 4 is a bonding layer. The metal circuit 3 is bonded to a ceramic substrate 2 via a bonding layer 4. In the example of FIG. 1, three metal circuits 3 are bonded to the upper surface of the ceramic substrate 2 via three bonding layers 4. One metal circuit 3 is bonded to the lower surface of the ceramic substrate 2. The metal circuit 3 on the lower surface can function as a heat sink rather than as a circuit. The embodiment is not limited to the example shown. For example, one, two, or four or more metal circuits 3 may be bonded to the upper surface of the ceramic substrate 2.

[0013] The ceramic substrate 2 is preferably one of a silicon nitride substrate, an aluminum nitride substrate, and an aluminum oxide substrate. An Argil substrate is one type of aluminum oxide substrate. Argil is a sintered body containing 20 to 80 mass% aluminum oxide and the remainder zirconium oxide. The three-point bending strength of both the aluminum nitride substrate and the aluminum oxide substrate is approximately 300 to 450 MPa. The strength of the Argil substrate is approximately 550 MPa. The three-point bending strength of the silicon nitride substrate is 600 MPa or more, and can be further increased to 700 MPa or more. The thermal conductivity of the silicon nitride substrate is 50 W / (m·K) or more, and can even be increased to 80 W / (m·K) or more. In particular, in recent years, silicon nitride substrates have been developed that combine both high strength and high thermal conductivity. Due to their high strength, silicon nitride substrates can be made thin, enabling improved heat dissipation. To improve heat dissipation, the thickness of the silicon nitride substrate is preferably 0.635 mm or less, more preferably 0.3 mm or less. There is no particular limitation on the lower limit of the thickness, but it is preferably 0.1 mm or more. This is to ensure the electrical insulation of the silicon nitride substrate. The thickness here refers to the dimension of the ceramic substrate 2 in the direction connecting the upper and lower surfaces of the ceramic substrate 2. These ceramic substrates 2 may be single plates or may have a three-dimensional structure such as a multilayer structure. There is no particular limitation on the thickness of the ceramic substrate 2. Heat dissipation is improved by making the ceramic circuit board thinner and the metal circuit thicker.

[0014] The metal used for the metal circuit 3 is copper, a copper alloy, or the like. Copper and copper alloys have high electrical conductivity and are suitable for electrical circuits. Copper and copper alloys also have high thermal conductivity, so they are excellent in dissipating heat from semiconductor elements mounted on the metal circuit 3. For example, the metal circuit 3 is made of copper or a copper alloy, excluding impurities and the like.

[0015] The ceramic substrate 2 and the metal circuit 3 are preferably bonded via a bonding layer 4. The bonding layer 4 preferably contains one or more active metals selected from the group consisting of copper (Cu) and silver (Ag) and one or more active metals selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), and niobium (Nb). The bonding layer 4 may further contain one or more active metals selected from the group consisting of tin (Sn), indium (In), zinc (Zn), magnesium (Mg), and aluminum (Al). The bonding layer 4 may also contain inorganic substances other than metals, such as silicon (Si) or carbon (C). When the metal circuit 3 is copper or a copper alloy, the bonding layer 4 preferably contains Cu and Ti.

[0016] 2 is a process flow diagram showing an example of a method for producing an active metal paste according to an embodiment. The production method according to the embodiment includes a binder preparation step, a pre-mixing step, and a kneading step.

[0017] First, an organic solvent, a resin powder, and a metal powder are prepared. The organic solvent is not particularly limited as long as it is burned away during the drying process or the bonding process. Preferred examples of the organic solvent include terpineol and butyl carbitol. The resin is not particularly limited as long as it is burned away during the drying process or the bonding process. Preferred examples of the resin include ethyl cellulose, polyvinyl butyral, and acrylic resin.

[0018] The metal powder contains one or more selected from Cu and Ag and one or more active metals selected from the group consisting of Ti, Zr, Hf, and Nb. The metal powder may further contain metal components such as Sn, In, Zn, Al, and Mg. The metal powder may further contain inorganic components such as C and Si. For example, when the metal powder contains Cu and Ti, the Cu content in the metal powder is preferably 50% by mass or more, the Ti content is 4% by mass to 30% by mass, the content of one or both selected from Sn and In is 5% by mass to 45% by mass, and the C content is 0% by mass to 2% by mass. When the metal powder contains Ag, Cu, and Ti, the Ag content in the metal powder is preferably 20% by mass to 60% by mass, the Cu content is 15% by mass to 40% by mass, the Ti content is 1% by mass to 15% by mass, and the content of one or two elements selected from Sn and In is preferably 5% by mass to 25% by mass. If necessary, one or more elements selected from the group consisting of Al, Si, and Mg may be added in an amount within the range of 1 to 15% by mass.

[0019] In the binder preparation step, a resin is dissolved in an organic solvent to prepare the binder. In addition to the resin and organic solvent, a dispersant or surfactant may be added as needed to aid dispersion and prevent aggregation. Note that the binder preparation step may be omitted by preparing a commercially available binder in which a resin is dissolved in an organic solvent.

[0020] Next, a pre-mixing step is carried out in which the prepared binder is mixed with the metal powder. In the pre-mixing step, the binder and the metal powder are mixed and blended together using a stirrer or a crusher. Since some metal powder may remain unmixed, the mixing is carried out so that no agglomerates of the metal powder are visible. A mixture of the binder and the metal powder is obtained by the pre-mixing step.

[0021] Next, a kneading step is carried out to make the mixture into a paste, which disperses the metal powder more uniformly in the mixture, resulting in a paste suitable for printing.

[0022] FIG. 3 is a schematic diagram illustrating an example of a kneading process for an active metal paste according to an embodiment. In FIG. 3, 5 denotes a kneading vessel containing a mixture of a binder and metal powder. The kneading vessel 5 rotates about an axis AX1. The kneading vessel 5 also revolves about an axis AX2. The axis AX1 is inclined relative to the axis AX2. For example, the axis AX1 is inclined relative to the vertical direction. The axis AX2 is parallel to the vertical direction. The angle θ between the axes AX1 and AX2 is preferably 25 degrees or greater and 50 degrees or less, more preferably 30 degrees or greater and 45 degrees or less. The rotation direction RD1 of the kneading vessel 5 when viewed from a direction D1 parallel to the axis AX1 is opposite to the rotation direction RD2 of the kneading vessel 5 when viewed from a direction D2 parallel to the axis AX2. In the illustrated example, the kneading vessel 5 rotates counterclockwise when viewed from the direction D1. Furthermore, the kneading vessel 5 revolves clockwise when viewed from direction D2. The revolution of the kneading vessel 5 acts to apply a centrifugal force to the mixture of binder and metal powder, degassing the mixture. Furthermore, the rotation of the kneading vessel 5 causes convection and agitation of the mixture.

[0023] In order to efficiently perform kneading in the kneading process, it is desirable to change the kneading conditions and divide the steps. The kneading process includes primary kneading, secondary kneading, and tertiary kneading. In the primary kneading, kneading is performed substantially by revolution only. In the secondary kneading, kneading is performed by revolution and rotation. The revolution speed in the secondary kneading is lower than the revolution speed in the primary kneading. The rotation speed of the rotation speed in the rotation speed in the rotation speed in the secondary kneading is lower than the revolution speed in the secondary kneading. In the tertiary kneading, kneading is performed by revolution and rotation. The revolution speed in the tertiary kneading is higher than the revolution speed in the secondary kneading. The rotation speed of the rotation speed in the rotation speed in the rotation speed in the rotation speed in the tertiary kneading is lower than the revolution speed in the tertiary kneading. In the primary kneading, the kneading container 5 may rotate on its axis, but it is preferable that the rotation speed be sufficiently lower than the rotation speed in the secondary kneading or the tertiary kneading. By carrying out the steps from the first kneading to the third kneading, the mixture can be kneaded efficiently.

[0024] In the primary kneading, the mixture is kneaded by revolution for a preset time while maintaining the rotation speed. In the secondary kneading and tertiary kneading, the mixture is kneaded by revolution and rotation for a preset time while maintaining the respective rotation speeds.

[0025] In the primary kneading, the mixture is essentially kneaded by revolution alone, thereby promoting degassing. Air entrained during the manufacturing process exists in the mixture as bubbles. Printing a paste containing residual bubbles may result in defects such as voids. Therefore, it is desirable to remove as many bubbles as possible. If stirring is continued without sufficient degassing, the bubbles in the mixture will be broken down into smaller pieces. Although the mixture appears degassed and bubbles are difficult to detect, this is undesirable because it merely disperses small defects. To achieve sufficient degassing, the centrifugal acceleration in the primary kneading is preferably 80% or more of the centrifugal acceleration in the tertiary kneading. If the centrifugal acceleration is less than 80%, the mixture may not be sufficiently degassed. The centrifugal acceleration in the primary kneading is more preferably 85% or more of the centrifugal acceleration in the tertiary kneading, and most preferably 90% or more of the centrifugal acceleration in the tertiary kneading. On the other hand, if the centrifugal acceleration in the primary kneading is too high, small defects may remain dispersed, as described above. For this reason, the centrifugal acceleration in the primary kneading is preferably 100% or less of the centrifugal acceleration in the tertiary kneading, and more preferably 95% or less of the centrifugal acceleration in the tertiary kneading.

[0026] The time for the primary kneading is preferably 10% or more of the total kneading time. If the kneading time is less than 10%, the mixture may not be sufficiently degassed. The longer the primary kneading time, the better the mixture will be degassed. However, if the primary kneading time is longer than necessary, the total time for the kneading process will be longer and the temperature of the mixture will increase. As a result, the kneading process may affect physical properties such as paste viscosity. For this reason, the time for the primary kneading is preferably 20% or less of the total kneading time, and more preferably 15% or less.

[0027] In the secondary mixing, the degassed mixture is agitated by rotation in addition to revolution. The large rotation of the mixture promotes mixing of the metal powder particles with each other and with the binder, resulting in a more uniform dispersion of the metal powder. The purpose of the secondary mixing is to agitate the entire mixture. Therefore, in the secondary mixing, mixing is performed by revolution at a lower rotation speed than in the primary mixing and rotation at a lower rotation speed than the revolution. By reducing the revolution speed compared to the primary mixing, the force that moves the mixture in the centrifugal direction is suppressed. By suppressing the centrifugal force, the degassing effect of the mixture is reduced, but material flow is more likely to occur. Furthermore, rotation at a lower rotation speed than the revolution promotes agitation of the mixture. The mixture contains metal powder and a binder. If the metal powder particles are not sufficiently agitated with each other and with the binder, a paste in which the metal powder is uniformly dispersed within the binder cannot be obtained. For this reason, it is desirable to agitate the metal powder in the binder as much as possible. If the paste is formed without sufficient stirring, the paste may contain areas of only binder without metal powder, or areas where the metal components are unevenly concentrated, which is undesirable.

[0028] In order to perform sufficient mixing, the centrifugal acceleration of the secondary kneading is preferably 50% or more of the centrifugal acceleration of the tertiary kneading. If the centrifugal acceleration is less than 50%, the metal powder may not be sufficiently mixed. The centrifugal acceleration of the secondary kneading is more preferably 60% or more of the centrifugal acceleration of the tertiary kneading, and most preferably 70% or more of the centrifugal acceleration of the tertiary kneading. On the other hand, if the centrifugal acceleration of the secondary kneading is too high, there is a possibility that pasting will proceed without mixing, as described above. For this reason, the centrifugal acceleration of the secondary kneading is preferably 90% or less of the centrifugal acceleration of the tertiary kneading, and more preferably 80% or less of the centrifugal acceleration of the tertiary kneading.

[0029] The secondary mixing time is preferably 10% or more of the total mixing time. If the mixing time is less than 10%, the metal powder may not be sufficiently mixed. The longer the secondary mixing time, the more the metal powder can be mixed, but if the secondary mixing time is longer than necessary, the total mixing time will be longer and the temperature of the mixture will rise. As a result, the mixing process may affect physical properties such as paste viscosity. For this reason, the secondary mixing time is preferably 20% or less of the total mixing time, and more preferably 15% or less.

[0030] In the tertiary kneading, the kneaded material that has been further mixed by the secondary kneading is further mixed by applying even greater revolution and rotation. Mixing is performed by applying shear force to the mixture through revolution with a higher rotation speed than in the secondary kneading and rotation with a lower rotation speed than the revolution. Mixing with shear force promotes the mixture to become a paste. The force applied to the mixture by the tertiary kneading is preferably greater than the force applied to the mixture by the primary kneading and greater than the force applied to the mixture by the secondary kneading. In other words, the centrifugal acceleration in the tertiary kneading is preferably greater than the centrifugal acceleration in the primary kneading and greater than the centrifugal acceleration in the secondary kneading. In order to make the centrifugal acceleration of the tertiary kneading higher than that of the primary kneading and secondary kneading, it is preferable to make the revolution speed in the tertiary kneading higher than that of the primary kneading and secondary kneading, and to make the rotation speed in the tertiary kneading higher than that of the primary kneading and secondary kneading. It is also possible to increase the centrifugal acceleration by making the revolution speed or the rotation speed in the tertiary kneading higher than that of the primary kneading and secondary kneading, but this may result in the centrifugal force of the revolution or the rotation speed becoming too large, placing an excessive load on the device or the mixture.

[0031] It is possible to skip the second mixing step and proceed directly from the first mixing step to the third mixing step, but if the preliminary mixing is insufficient or if the stirring in the third mixing step is insufficient, a paste in which the metal powder is not sufficiently dispersed may be produced. For this reason, the mixing process preferably includes three steps: the first mixing step, the second mixing step, and the third mixing step. If the mixing process includes the first, second, and third mixing steps, a separate mixing step may be added between the first and second mixing steps, between the second and third mixing steps, or after the third mixing step. However, as described above, an increase in the overall mixing time results in an increase in the temperature of the mixture. Therefore, if a separate step is added, it is preferable that the time for that step be short.

[0032] The above process yields an active metal paste. The kneaded paste is then printed onto a ceramic substrate. The printing thickness of the paste is preferably 15 to 40 μm. If the printing thickness is less than 15 μm, the thickness of the active metal brazing material layer may vary, potentially reducing the bonding strength. On the other hand, if the printing thickness exceeds 40 μm, the bonding strength effect is not improved, resulting in increased costs. The paste is printed to a uniform thickness on the end surface of the ceramic component using a screen printing method or the like. If the printing thickness is uneven, the active metal brazing material will be excessive in thicker areas, causing brazing material pools and cracks due to thermal stress. In thinner areas, a lack of active metal brazing material will result in areas with reduced strength. For this reason, the difference in thickness between the thickest and thinnest parts of the printed paste is preferably 10 μm or less, and more preferably 5 μm or less.

[0033] 4(A) to 4(E) are cross-sectional views illustrating an example of a manufacturing process for a ceramic circuit board according to an embodiment. FIG. 4(A) shows a ceramic substrate 2 before printing an active metal paste 6. As shown in FIG. 4(B), the active metal paste 6 is printed on the upper surface of the ceramic substrate 2 and dried. In the illustrated example, the active metal paste 6 is selectively printed only in the area where a metal circuit will be formed. Next, as shown in FIG. 4(C), the active metal paste 6 is printed on the lower surface of the ceramic substrate 2 and dried. As shown in FIG. 4(D), a metal plate 7 is placed on the surface of the dried active metal paste 6 and bonded. The active metal paste 6 melts and solidifies, forming a bonding layer 4. Then, as shown in FIG. 4(E), the metal plate 7 is processed into the shape of the metal circuit by etching or the like to form the metal circuit 3. Through the above steps, a ceramic circuit substrate 1 including a ceramic substrate 2 and a metal circuit 3 is manufactured.

[0034] In the step shown in Fig. 4(B), the active metal paste 6 may be printed on the upper surface of the ceramic substrate 2 over an area wider than the metal circuit 3. In this case, after a portion of the metal plate 7 is etched, a portion of the bonding layer 4 is etched. It is also possible to prepare pre-processed metal circuits 3 and place these metal circuits 3 on the active metal paste 6. In this case, the etching step shown in Fig. 4(D) is omitted.

[0035] A metal thin film may be provided on the surface of the metal circuit 3 of the ceramic circuit board 1. The metal thin film has one selected from the group consisting of Ni (nickel), Ag (silver), and Au (gold) as its main component. Here, "main component" refers to a component that is contained in an amount of 50% or more. The metal thin film may be a plated film, a sputtered film, or the like. By providing a metal thin film, corrosion resistance, solder wettability, and the like can be improved.

[0036] Figures 5(A) and 5(B) are side views showing an example of a semiconductor device according to an embodiment. The ceramic circuit board 1 according to the embodiment is suitable for a semiconductor device. In the semiconductor device, a semiconductor element is mounted on a metal circuit 3 via a bonding layer. In Figure 5(A), 1 is a ceramic circuit board. 8 is a bonding layer. 9 is a semiconductor element. 10 is a semiconductor device. 11 is wire bonding. 12 is a lead frame. In Figure 5(B), 13 is a mold resin. Figure 5(B) shows the semiconductor device 10 shown in Figure 5(A) covered with mold resin 13.

[0037] As shown in FIG. 5A , a semiconductor element 9 is mounted on a metal circuit 3 of a ceramic circuit board 1 via a bonding layer 8. Adjacent metal circuits 3 are electrically connected to each other via wire bonding 11. The semiconductor device 10 according to the embodiment is not limited to this structure. For example, multiple semiconductor elements 9 and multiple wire bonding 11 may be provided on the upper metal circuit 3. Furthermore, semiconductor elements, wire bonding, etc. may be bonded to the lower metal circuit 3 as needed. Metal terminals such as lead frames may also be bonded to the ceramic circuit board 1. The bonding layer 8 that bonds the semiconductor element 9 includes solder, brazing filler metal, etc. Lead-free solder is preferred. "Solder" refers to a bonding material with a melting point of 450°C or less. "Brazing filler metal" refers to a bonding material with a melting point exceeding 450°C. A bonding material with a melting point of 500°C or higher is called a high-temperature brazing filler metal. Examples of high-temperature brazing filler metals include materials primarily composed of Ag.

[0038] While semiconductor elements 9 are becoming smaller, the amount of heat generated by the semiconductor elements 9 is increasing. Therefore, improved heat dissipation is required for the ceramic circuit substrate 1 on which the semiconductor elements 9 are mounted. To improve the performance of the semiconductor device 10 (semiconductor module), multiple semiconductor elements 9 can be mounted on the ceramic circuit substrate 1. When any of the semiconductor elements 9 exceeds its intrinsic temperature, its resistance changes to a negative temperature coefficient. This can lead to thermal runaway, in which power flows intensively, resulting in the instantaneous destruction of the semiconductor device 10. Therefore, improving the reliability of the bond between the semiconductor elements 9 and the metal circuit 3 is highly effective. Furthermore, the semiconductor device 10 according to the embodiment can be used in PCUs, IGBTs, and IPM modules. PCUs, IGBTs, and IPM modules are used in inverters for automobiles (including electric vehicles), electric railcars, industrial machinery, air conditioners, and the like. Regarding automobiles, electric vehicles are becoming increasingly popular. The more reliable the semiconductor device, the greater the safety of the automobile. The same is true for electric railcars, industrial equipment, and the like.

[0039] A specific example of a method for manufacturing the ceramic circuit board 1 according to the embodiment will be described. As long as the ceramic circuit board 1 has the above-described configuration, the manufacturing method is not particularly limited. Here, an example of a method for obtaining the ceramic circuit board 1 with a high yield will be described.

[0040] First, a ceramic substrate 2 and a metal plate 7 are prepared. The ceramic substrate 2 is preferably one selected from a silicon nitride substrate, an aluminum nitride substrate, and an aluminum oxide substrate. In particular, considering the overall heat dissipation properties of the ceramic circuit substrate 1, the ceramic substrate 2 is preferably a silicon nitride substrate having a thermal conductivity of 50 W / (m·K) or more and a three-point bending strength of 600 MPa or more. The metal plate 7 is preferably made of one selected from copper or a copper alloy. When the metal circuit 3 provided on the upper surface of the ceramic substrate 2 is electrically connected to the metal circuit 3 provided on the lower surface through through holes, a ceramic substrate 2 having through holes is prepared. When providing through holes in the ceramic substrate 2, the through holes may be provided in advance at the molded body stage. Alternatively, the through holes may be provided in the ceramic substrate 2 (ceramic sintered body). The through holes may be provided by laser processing, cutting, or the like. Cutting processing includes drilling using a drill or the like.

[0041] The ceramic substrate 2 is preferably joined to a copper plate or a copper alloy plate by an active metal bonding method. The active metal bonding method uses an active metal brazing material composed of a mixture of an active metal such as Ti and Cu. Examples of active metal brazing materials include a mixture of Ti and Cu, and a mixture of Ti, Ag, and Cu. For example, in the case of a mixture of Ti and Cu, the inorganic components contained in the mixture contain 50% or more by mass of Cu, 4% to 30% by mass of Ti, 5% to 45% by mass of one or two elements selected from Sn and In, and 0% to 2% by mass of C. The term "inorganic components" refers to components other than organic components contained in organic solvents, resins, etc., and includes metal elements such as Ag, Cu, and active metals contained in metal powders, as well as inorganic components other than metals such as Si and C. In the mixture of Ti, Ag, and Cu, the inorganic components contained in the mixture have an Ag content of 20% by mass to 60% by mass, a Cu content of 15% by mass to 40% by mass, a Ti content of 1% by mass to 15% by mass, and a content of one or two elements selected from Sn and In of 5% by mass to 25% by mass. If necessary, one or more elements selected from the group consisting of Al, Si, and Mg may be added in an amount within the range of 1 to 15% by mass.

[0042] Next, the active metal brazing material is made into a paste. A metal powder with the metal mass ratio used for the active metal brazing material paste is prepared. If the metal powder is too fine, it tends to aggregate and is difficult to disperse uniformly. Furthermore, if the metal powder is too coarse, the metal components are difficult to mix when melted during joining. For this reason, with the above-mentioned composition, the average particle size of the metal powder is preferably 1 to 5 μm. If the proportion of metal powder in the paste is too small, the brazing material metal components required for joining will not be obtained when the organic components evaporate due to drying and heating during joining. Furthermore, if the proportion of metal powder is too high, the paste will not have the viscosity required for printing. For this reason, although the overall specific gravity can change depending on the compounding ratio of the active metal brazing material, it is preferable that the metal powder be contained in the paste at 60 to 95 mass%.

[0043] Next, a binder is prepared for mixing with the metal powder. The binder is prepared by dissolving a resin component in an organic solvent. A preferred example of the resin used in the binder preparation is ethyl cellulose. A preferred example of the organic solvent is terpineol. If the binder contains a large amount of resin and a small amount of organic solvent, the binder's viscosity increases, making it difficult to mix with the metal powder. If the binder contains a small amount of resin and a large amount of organic solvent, the binder's viscosity decreases, making it difficult to obtain a paste suitable for printing. For this reason, the amount of resin contained in the binder is preferably 5 to 15% by mass. In addition to the resin and organic solvent, dispersants or surfactants such as sodium lauroyl sarcosinate or oleic acid may be added as needed to aid dispersion and prevent aggregation. The amount of dispersant or surfactant added is 0.1 to 5% by mass. If the resin is difficult to dissolve in the organic solvent, stirring with a stirrer or mixer, or stirring by rotating the mixture in a sealed container using a pot roller or the like, will ensure that the resin component does not remain dissolved in the organic solvent.

[0044] In premixing, the binder prepared in the binder preparation step is mixed with the metal components. In premixing, the binder and metal powder are mixed and blended using a stirrer or crusher. Since some metal powder may remain unmixed, mixing is carried out so that no clumps of metal powder are visible. Since premixing does not involve sufficient mixing or stirring, there is a possibility that the binder and metal components will separate if left unattended. For this reason, kneading is carried out promptly after premixing.

[0045] In the kneading process, a premixed mixture of binder and metal powder is made into a paste. The premixed mixture is placed in a kneading container with a sealed lid made of polyethylene or polypropylene. The mixture in the kneading container is kneaded by rotating it using a revolutionary device. Examples of revolutionary devices include the SK series from Photo Chemical Co., Ltd. and the ARE series from Thinky Corporation.

[0046] In the kneading step, the mixture is degassed and stirred to form a paste. It is preferable to set rotation and revolution conditions suitable for degassing, stirring, and forming a paste, and to divide the steps into primary kneading, secondary kneading, and tertiary kneading to achieve efficient kneading.

[0047] In the primary kneading, the mixture is kneaded essentially by revolution alone, thereby promoting degassing. In order to sufficiently degas the centrifugal acceleration in the primary kneading is preferably 80% or more of the centrifugal acceleration in the tertiary kneading required for forming a paste. The time for the primary kneading is preferably 10% or more of the total kneading time. In the primary kneading of a mixture of the aforementioned metal powder and binder, the centrifugal acceleration is preferably 80 G or more and the kneading time is preferably 45 seconds or more.

[0048] The secondary kneading is carried out immediately after the primary kneading. In the secondary kneading, rotation is carried out in addition to revolution. This promotes the stirring of the degassed mixture by the rotation. Since the purpose of the secondary kneading is to stir the entire mixture, the kneading is carried out by revolution at a lower rotation speed than the primary kneading and rotation at a lower rotation speed than the revolution. In order to perform sufficient stirring, the centrifugal acceleration of the secondary kneading is preferably 50% or more of the centrifugal acceleration of the tertiary kneading. The time for the secondary kneading is preferably 10% or more of the total kneading time. In the secondary kneading of a mixture composed of the above-mentioned metal powder and binder, the centrifugal acceleration is preferably 65 G or more and the kneading time is preferably 45 seconds or more.

[0049] The tertiary kneading is performed immediately after the secondary kneading. In the tertiary kneading, the kneaded material that has been further stirred by the secondary kneading is subjected to even greater revolution and rotation, thereby stirring the mixture vigorously. By performing revolution at a higher rotation speed than in the secondary kneading and rotation at a lower rotation speed than in the orbital kneading, shear force is applied to the mixture to stir it. This stirring with shear force promotes the mixture to become a paste. The force applied to the mixture by the tertiary kneading is preferably greater than that of the primary kneading and secondary kneading. In other words, the centrifugal acceleration in the tertiary kneading is preferably greater than that of the primary kneading and secondary kneading. In the case of the tertiary kneading of a mixture composed of the metal powder and binder described above, the centrifugal acceleration is preferably 100 G or more, and the kneading time is preferably 250 seconds or more.

[0050] The active metal paste completed by kneading is printed onto a ceramic substrate. The active metal paste is printed in a uniform thickness by screen printing or the like. The printing thickness of the paste is preferably 15 to 40 μm. In the case of a ceramic circuit board having metal circuits on both sides, the active metal paste is printed on one side and then dried by heating in the air. Thereafter, the active metal paste is printed on the other side, and the paste is similarly dried.

[0051] Next, a metal plate is placed on the ceramic substrate on which the active metal paste has been printed and dried. When the metal plate is a copper plate or a copper alloy plate, the ceramic substrate on which the metal plate has been placed is heated at 700 to 900°C to bond the ceramic substrate and the metal plate. The heating step is carried out in a vacuum or a non-oxidizing atmosphere as required. When carried out in a vacuum, the pressure is 1 x 10 －２ The pressure is preferably 0.05 Pa or less. Examples of non-oxidizing atmospheres include a nitrogen atmosphere and an argon atmosphere. By performing the heat bonding, the active metal paste melts and solidifies. The active metal paste functions as a bonding layer that bonds the ceramic substrate and the metal plate.

[0052] When a metal plate is pre-processed into a circuit shape, a ceramic circuit board can be obtained by bonding the processed metal circuit to a ceramic substrate. When a metal plate that has not been processed into a circuit shape is bonded to a ceramic substrate, the metal circuit is formed by etching or other processes. Not only the metal plate, but also the bonding layer can be processed into a circuit shape by etching.

[0053] A ceramic circuit board can be manufactured by the above-described steps. Next, a step of bonding a semiconductor element or the like to the ceramic circuit board is performed. A bonding layer is provided at the location where the semiconductor element is to be bonded. The bonding layer preferably contains solder or brazing material. The semiconductor element is provided on the bonding layer via the bonding layer. Wire bonding may be provided as needed. Multiple semiconductor elements and multiple wire bonding may be provided as needed.

[0054] According to an embodiment of the present invention, an active metal paste can be obtained in which the components are dispersed more uniformly and with fewer bubbles. In particular, active metals are chemically active and highly reactive. Therefore, it is desirable to disperse them uniformly in the paste efficiently and in a shorter time. According to an embodiment of the present invention, the kneading process can disperse each component of the metal powder, including the active metal, more uniformly in the paste in a shorter time.

[0055] According to an embodiment of the present invention, it is possible to produce an active metal paste in which the dispersibility of the copper element and the active metal element is each less than 1.0. A dispersibility of the copper element and the active metal element less than 1.0 means that the copper element and the active metal element are sufficiently dispersed in the active metal paste. A dispersibility of the copper element and the active metal element less than 1.0 suppresses aggregation and improves the fluidity of the paste. This makes it easier to print the active metal paste. Furthermore, since each particle easily enters between other particles, the reaction proceeds efficiently and the bonding properties are improved.

[0056] (Examples 1 to 12, Comparative Examples 1 to 10) Ceramic substrates shown in Table 1 were prepared. The ceramic substrates were silicon nitride substrates or aluminum nitride substrates. The thermal conductivity of the silicon nitride substrate was 90 W / (m·K) and the three-point bending strength was 650 MPa. The thermal conductivity of the aluminum nitride substrate was 170 W / (m·K) and the three-point bending strength was 300 MPa. The size of the ceramic substrate was 30 mm length x 50 mm width. The thickness of the silicon nitride substrate was 0.32 mm, and the thickness of the aluminum nitride substrate was 0.635 mm. In Table 1, the silicon nitride substrate was Si ３ N ４ The aluminum nitride substrate is indicated as AlN.

[0057] Next, metal powders were prepared, which were the metal components of the pastes shown in Table 1. The metal powders were silver-based powder (Ag-Cu-Sn-Ti) or copper-based powder (Cu-Sn-TiH). The ratios of each metal component in the silver-based powder were 58 mass% Ag, 30 mass% Cu, 10 mass% Sn, and 2 mass% Ti. The ratios of each metal component in the copper-based powder were 68 mass% Cu, 20 mass% Sn, and 12 mass% TiH. The average particle size of the Ag powder was 2 μm. The average particle size of the Cu powder was 1 μm. The average particle size of the Sn powder was 5 μm. The average particle size of the Ti powder was 5 μm. The average particle size of the TiH powder was 6 μm.

[0058] Next, a binder was prepared by placing 7% by mass of ethyl cellulose, 3% by mass of sarcosinate, and 90% by mass of terpineol in a wide-mouth polyethylene bottle with a lid, and rotating the bottle on a pot roller for 24 hours.

[0059] Next, premixing was performed by mixing the metal powder and binder. 20% by mass of binder was placed in a polyethylene cylindrical container with a total of 100% by mass of metal powder. A stirring blade was fixed perpendicular to the opening of the cylindrical container, and the cylindrical container was rotated at 50 rpm for 3 minutes to perform premixing, producing a mixture of metal powder and binder.

[0060] Next, the premixed mixture of binder and metal powder was kneaded to form a paste. A sealing lid was attached to the polyethylene cylindrical container containing the premixed mixture, and primary, secondary, and tertiary kneading were carried out using a kneading device. The kneading device used was a SK-3000 manufactured by Photo Chemical Co., Ltd. The kneading conditions for each step, including the rotation speed (rpm), revolution speed (rpm), centrifugal acceleration (G), and kneading time (seconds), are shown in Tables 1 and 2.

[0061]

[0062]

[0063] The ratio (G1 / G3) of the centrifugal acceleration of the primary kneading to the centrifugal acceleration of the tertiary kneading (G3) and the ratio (T1 / T1-3) of the primary kneading time (T1) to the total kneading time (T1-3 = T1 + T2 + T3) are shown in Table 1. Table 2 shows the ratio (G2 / G3) of the centrifugal acceleration of the secondary kneading to the centrifugal acceleration of the tertiary kneading (G3) and the ratio (T2 / T1-3) of the secondary kneading time (T2) to the total kneading time (T1-3).

[0064] Regarding the kneading conditions for the pastes in Examples 1 to 12, the ratio of the centrifugal acceleration of the primary kneading to the centrifugal acceleration of the tertiary kneading (G1 / G3) and the ratio of the primary kneading time to the total kneading time (T1 / T1-T3) were within the preferred ranges. The ratio of the centrifugal acceleration of the secondary kneading to the centrifugal acceleration of the tertiary kneading (G2 / G3) and the ratio of the secondary kneading time to the total kneading time (T2 / T1-T3) were also within the preferred ranges. On the other hand, in Comparative Examples 1 to 10, these values ​​were outside the preferred ranges.

[0065] Next, to confirm the cohesion of the metal powder in the paste, the maximum agglomerated particle size was measured using a grind gauge. The grind gauge is a metal block gauge with a groove with a slanted bottom carved into its surface. The depth of one end of the groove is zero. The maximum agglomerated particle size can be easily measured by placing paste in the deep part of the groove and pulling the paste from the deep side to the shallow side with a metal squeegee. Pastes with a maximum agglomerated particle size measured with the grind gauge of less than 10 μm were considered to be pastes without large agglomerates. In Table 3, examples in which the measured maximum agglomerated particle size was less than 10 μm were marked "OK," and examples in which the measured maximum agglomerated particle size was 10 μm or more were marked "NG."

[0066] The active metal paste completed by kneading was printed onto a ceramic substrate. The active metal paste was printed using a screen plate measuring 320 mm x 320 mm and having a mesh size of 200 μm. First, three rectangular circuit shapes were printed on one side (top side) of the ceramic substrate. The active metal paste printed on one side was dried by heating at 100°C in air for 1 minute. Then, one rectangular circuit shape was printed on the other side (bottom side). The active metal paste printed on the other side was dried by heating at 100°C in air for 1 minute.

[0067] Next, the dispersibility of Cu particles and active metal particles was evaluated to confirm the dispersion of the metal powder in the paste after printing and drying. The paste was printed and dried on a polyethylene terephthalate (PET) film under the same conditions as for the ceramic substrate. The flat surface of the paste on the film side was observed using an energy dispersive X-ray analyzer (SEM-EDX). A Hitachi High-Technologies S-5500 or equivalent device was used for the SEM-EDX. The size of the observation area was set to 100 μm x 80 μm. Mapping data of Cu elements and active metal elements present in the observation area were acquired. From the mapping data of Cu elements, particle size analysis software was used to calculate the particle size distribution. Specifically, the mapping data of Cu elements was subjected to Voronoi division. The divided area distribution was measured, and the average area and standard deviation were calculated. The standard deviation was divided by the average value to calculate the value as dispersibility (standard deviation / average value). Similarly, for active metal elements, the particle size distribution was calculated from the mapping data of active metal elements using particle size analysis software. The divided area distribution is measured, and the average area and standard deviation are calculated. The standard deviation is divided by the average area to calculate the dispersibility (standard deviation / average area). ImageJ can be used as particle size analysis software.

[0068] Next, oxygen-free copper plates measuring 30 mm long x 50 mm wide x 0.8 mm thick were placed on both sides of the ceramic substrate on which the paste had been printed and dried, and thermal bonding was performed. When silver-based paste (Ag-Cu-Sn-Ti) was used, the bonding temperature was set to 820°C. When copper-based paste (Cu-Sn-TiH) was used, the bonding temperature was set to 600°C. The bonding time for each was set to 10 minutes, and the bonding was performed in a vacuum (1 x 10 －２ The bonding was carried out at a pressure of 1000 kJ / cm2 or less.

[0069] Next, the oxygen-free copper plate to which the ceramic substrate was bonded was etched. By etching, metal circuits were formed in three locations on the front surface of the ceramic substrate, and a metal circuit was formed in one location on the back surface. For the three circuit shapes on the front surface, pullbacks of 5 mm were formed from the periphery of the ceramic substrate, and non-circuit areas of 5 mm were also formed between the circuit shapes. For the circuit shape on the back surface, pullbacks of 5 mm were formed from the periphery of the ceramic substrate.

[0070] Next, to confirm the micro-bonding between the metal circuit and the ceramic substrate using the paste, voids in the bonding layer of the surface circuit were measured using a scanning acoustic tomograph (SAT). Defects with a diameter of 1 mm or more were determined to be void defects. In Table 3, those that were not determined to be void defects are marked as "OK," and those that were determined to be void defects are marked as "NG."

[0071] Next, to confirm the macroscopic bonding strength between the metal circuit and the ceramic substrate by the paste, the peel strength was measured as the bonding strength between the metal circuit and the ceramic substrate. The peel strength was measured by fixing the ceramic circuit substrate to a jig and peeling off a part of the upper surface metal circuit in the vertical direction at 50 mm / min. The measurement results are shown in Table 3.

[0072]

[0073] The pastes of Examples 1 to 12 did not exhibit poor cohesion, and the dispersibility values ​​were within the preferred range. This is thought to be because the kneading conditions for the pastes were set within the preferred range, thereby suppressing the occurrence of bubbles and aggregation within the paste. In contrast, in Comparative Examples 1 to 10, poor cohesion was observed in some cases, and the dispersibility values ​​were outside the preferred range. This is thought to be because bubbles and aggregation remained in the paste without being eliminated.

[0074] Furthermore, in the ceramic circuit boards of Examples 1 to 12, there were no void defects, and the bonding strength values ​​were within the preferred range. This is thought to be because the ceramic and metal circuit were joined using a paste with few bubbles and agglomerations. In contrast, in Comparative Examples 1 to 10, there were cases where void defects were observed, and the bonding strength values ​​were outside the preferred range. This is thought to be because the ceramic and metal circuit were joined using a paste with many bubbles and agglomerations, which resulted in an increase in bonding defects.

[0075] As described above, the ceramic circuit board according to the embodiment has few bonding defects and high bonding strength, and is therefore ideal for semiconductor devices equipped with semiconductor elements.

[0076] The embodiments of the present invention include the following features: (Feature 1) A method for producing an active metal paste, comprising: a pre-mixing step of mixing a metal powder containing copper and an active metal with a binder in which a resin is dissolved in an organic solvent to produce a mixture containing 60 to 95 mass % of the metal powder; and a kneading step of applying rotation and revolution to the mixture to produce an active metal paste. (Feature 2) The method for producing an active metal paste according to Feature 1, wherein the kneading step includes primary kneading in which kneading is performed by revolution, secondary kneading in which kneading is performed by revolution and rotation after the primary kneading, and tertiary kneading in which kneading is performed by revolution and rotation after the secondary kneading, wherein the rotation speed of the revolution in the secondary kneading is smaller than the rotation speed of the revolution in the primary kneading, the rotation speed of the rotation in the secondary kneading is smaller than the rotation speed of the revolution in the secondary kneading, the rotation speed of the revolution in the tertiary kneading is larger than the rotation speed of the revolution in the secondary kneading, and the rotation speed of the rotation in the tertiary kneading is smaller than the rotation speed of the revolution in the tertiary kneading. (Feature 3) The method for producing an active metal paste according to Feature 2, wherein, in the kneading step, the centrifugal acceleration of the primary kneading is 80% or more of the centrifugal acceleration of the tertiary kneading, and the centrifugal acceleration of the secondary kneading is 50% or more of the centrifugal acceleration of the tertiary kneading. (Feature 4) The method for producing an active metal paste according to Feature 2 or Feature 3, wherein, in the kneading step, the time for the primary kneading and the time for the secondary kneading are each 10% or more of the total kneading time. (Feature 5) A method for producing a ceramic circuit board, comprising: a step of carrying out the method for producing an active metal paste according to any one of Features 1 to 4; and a bonding step of bonding a metal member to a ceramic substrate via the active metal paste. (Feature 6) A method for producing a semiconductor device, comprising: a step of carrying out the method for producing an active metal paste according to any one of Features 1 to 4; a bonding step of bonding a metal member to a ceramic substrate via the active metal paste; and a mounting step of mounting a semiconductor element on the metal member on which a circuit has been formed. (Feature 7) An active metal paste containing copper and at least one active metal, characterized in that the dispersibility of the copper element and the active metal element is each less than 1.0.(Feature 8) The active metal paste according to Feature 7, wherein, among the inorganic components, the copper content is 50% by mass or more, the active metal content is 4% by mass or more and 30% by mass or less, the content of one or two types selected from tin and indium is 5% by mass or more and 45% by mass or less, and the carbon content is 0% by mass or more and 2% by mass or less. (Feature 9) The active metal paste according to Feature 7, wherein, among the inorganic components, the silver content is 20% by mass or more and 60% by mass or less, the copper content is 15% by mass or more and 40% by mass or less, the active metal content is 1% by mass or more and 15% by mass or less, and the content of one or two types selected from tin and indium is 5% by mass or more and 25% by mass or less. (Feature 10) A ceramic circuit board comprising: a ceramic substrate; and a metal circuit joined to the ceramic substrate via a joining layer made of the active metal paste according to any one of Features 7 to 9. (Feature 11) The ceramic circuit board according to Feature 10, wherein the ceramic substrate is one of an aluminum oxide substrate, an aluminum nitride substrate, and a silicon nitride substrate. (Feature 12) The ceramic circuit board according to Feature 11, wherein the metal circuit is made of copper or a copper alloy. (Feature 13) A semiconductor device comprising: the ceramic circuit board according to any one of Features 10 to 12; and a semiconductor element mounted on the metal circuit via a bonding layer.

[0077] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

[0078] REFERENCE SIGNS LIST 1... ceramic circuit board 2... ceramic substrate 3... metal circuit 4... bonding layer 5... kneading container 6... active metal paste 7... metal plate 8... bonding layer 9... semiconductor element 10... semiconductor device 11... wire bonding 12... lead frame 13... molding resin

Claims

1. a pre-mixing step of mixing a metal powder containing copper and an active metal with a binder having a resin dissolved in an organic solvent to produce a mixture containing 60 to 95 mass % of the metal powder; and a kneading step of revolving a container containing the mixture to knead the mixture, and then rotating and revolving the container to knead the mixture, thereby producing an active metal paste.

2. The kneading step includes a primary kneading step in which kneading is performed by revolving the container, a secondary kneading step in which kneading is performed by revolving and rotating the container after the primary kneading, and a tertiary kneading step in which kneading is performed by revolving and rotating the container after the secondary kneading, The rotation speed of the revolution in the secondary kneading is smaller than the rotation speed of the revolution in the primary kneading, The rotation speed of the rotation in the secondary kneading is smaller than the rotation speed of the revolution in the secondary kneading, The rotation speed of the revolution in the tertiary kneading is greater than the rotation speed of the revolution in the secondary kneading, The method for producing an active metal paste according to claim 1 , wherein the rotation speed in the tertiary kneading is smaller than the revolution speed in the tertiary kneading.

3. 3. The method for producing an active metal paste according to claim 2, wherein in the kneading step, a centrifugal acceleration of the first kneading is 80% or more of a centrifugal acceleration of the third kneading, and a centrifugal acceleration of the second kneading is 50% or more of a centrifugal acceleration of the third kneading.

4. The method for producing an active metal paste according to claim 2 , wherein in the kneading step, a time for the first kneading and a time for the second kneading are each 10% or more of a total kneading time.

5. A step of carrying out the method for producing an active metal paste according to any one of claims 1 to 4; a bonding step of bonding a metal member to a ceramic substrate via the active metal paste; A method for manufacturing a ceramic circuit board comprising the steps of:

6. A step of carrying out the method for producing an active metal paste according to any one of claims 1 to 4; a bonding step of bonding a metal member to a ceramic substrate via the active metal paste; a mounting step of mounting a semiconductor element on the metal member on which a circuit is formed; A method for manufacturing a semiconductor device comprising the steps of:

7. An active metal paste comprising copper and at least one active metal, the active metal paste being characterized in that the dispersibility of the copper element and the active metal element are each less than 1.

0.

8. 8. The active metal paste according to claim 7, wherein the inorganic components include a copper content of 50% by mass or more, an active metal content of 4% by mass or more and 30% by mass or less, a content of one or two selected from tin and indium of 5% by mass or more and 45% by mass or less, and a carbon content of 0% by mass or more and 2% by mass or less.

9. 8. The active metal paste according to claim 7, wherein the inorganic components include a silver content of 20% by mass or more and 60% by mass or less, a copper content of 15% by mass or more and 40% by mass or less, an active metal content of 1% by mass or more and 15% by mass or less, and a content of one or two selected from tin and indium of 5% by mass or more and 25% by mass or less.

10. A ceramic substrate; A metal circuit joined to the ceramic substrate via a joining layer made of the active metal paste according to any one of claims 8 to 9; A ceramic circuit board comprising:

11. 11. The ceramic circuit board according to claim 10, wherein the ceramic substrate is one of an aluminum oxide substrate, an aluminum nitride substrate, and a silicon nitride substrate.

12. The ceramic circuit board according to claim 11, wherein the metal circuit is made of copper or a copper alloy.

13. The ceramic circuit board according to claim 12; a semiconductor element mounted on the metal circuit via a bonding layer; A semiconductor device comprising: