Copper paste

Abstract

A copper paste according to the present invention contains copper powder and an alcohol solvent. The alcohol solvent contains at least one first alcohol selected from the group consisting of monovalent alcohols and divalent alcohols having a viscosity of 3-70 mPa·s at 25°C, and at least one second alcohol selected from the group consisting of divalent alcohols and trivalent alcohols having a viscosity of 300-1000 mPa·s at 25°C. The copper paste exhibits a viscosity η10 of 1-50 Pa·s at 25°C at a shear rate of 10 s-1. The square root √σ0 of the Casson yield stress σ0 of the copper paste is 10 Pa1 / 2 or less.

Description

[Technical Field]

[0001] The present invention relates to a copper paste. [Background technology]

[0002] A power module is composed of a semiconductor element that controls power, an insulating heat dissipation substrate, and cooling fins, all of which are bonded together. Materials such as Si, SiC, GaN, and Ga2O3 are used for the semiconductor element. Ceramic materials with excellent thermal conductivity, such as Al2O3, AlN, and Si3N4, are used for the insulating heat dissipation substrate. Furthermore, aluminum is used for the heat dissipation fins. To bond these components, a thin metal film made of Cu, Ni, Ag, or other metal is formed on the surface where the semiconductor element is bonded, and a thin copper plate called direct bonding of copper (DBC) or active metal brazing (AMB) is formed on the surface of the insulating heat dissipation substrate. An additional thin Ag film may also be formed on the surface of this thin copper plate.

[0003] The process of joining a semiconductor device to an insulating heat dissipation substrate is generally called "die bonding." This die bond is formed by the following steps: applying a conductive paste to the surface of a thin Cu plate formed on an insulating heat dissipation substrate using a screen printing or dispensing method (application step); drying the paste (drying step); placing a semiconductor device on the applied conductive paste (die mounting step); and heating the conductive paste while applying a stress of approximately 20 MPa in the stacking direction of the resulting laminate (pressure sintering step). This results in a thin metal film formed as a sintered body of the conductive paste on the surface of the semiconductor device, which is then bonded to the substrate. Typical sintering conditions are a temperature of 250–300°C for 3–10 minutes, and the atmosphere can be air, nitrogen, or hydrogen, depending on the paste used. The die bond formed in this manner requires a die shear strength of at least 30 MPa.

[0004] Here, because the sintered body formed from the conductive paste has excellent thermal conductivity, the heat generated when the semiconductor element operates under high voltage and high current can be efficiently transferred to the insulating heat dissipation substrate and dissipated through the cooling fins. As such, solder paste containing high concentrations of lead has traditionally been used as the paste used for die bonding, but in order to cope with the increasing operating temperatures of elements, silver paste, which has better thermal conductivity, has replaced it, and in recent years, copper paste, which can be used at low cost, has been attracting attention.

[0005] However, when copper paste is pressure-sintered under the above-mentioned sintering conditions, the sintering time is shorter and the temperature is lower than usual, so it is not sintered sufficiently, and it is difficult to obtain the die shear strength required for the product. Therefore, in order to promote sintering, it has been proposed to prepare a paste using fine copper particles with a large surface area.

[0006] For example, Non-Patent Document 1 proposes the use of copper nanoparticles with an average particle size of 50 to 60 nm obtained by reducing a mixed solution of copper hydroxide and disodium nitrilotriacetate with hydrazine in a copper paste. It has been reported that such copper paste forms a sintered body by heating at a low temperature of 200°C for 30 minutes in a nitrogen atmosphere, and that the die shear strength of the die bond is up to 39 MPa. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Y. Kamikoriyama, H. Imamura, A. Muramatsu, K. Kanie, Sci. Rep., 9, 899 (2019). Summary of the Invention [Problem to be solved by the invention]

[0008] However, when the particles are miniaturized as in Non-Patent Document 1, the particles tend to aggregate and form holes, protrusions, or cracks after the coating and drying processes. These defects remain as defects in the bonded body even after the pressure sintering process and can cause a decrease in die shear strength. Furthermore, when the particles are miniaturized, the specific surface area increases, and the particle surface is oxidized during storage of the copper particles or in the drying process after the copper paste is applied, resulting in a decrease in sinterability, which in turn decreases die shear strength and the thermal conductivity required for heat dissipation.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a copper paste that has good oxidation resistance and produces a sintered body having excellent die shear strength. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have discovered a method for producing a coating composition comprising copper powder and an alcohol solvent, the alcohol solvent including one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s or more and 1000 mPa·s or less, the coating composition being capable of being sintered at 25°C at a shear rate of 10 s -1 Viscosity η at 10 is between 1 Pa·s and 50 Pa·s, and the square root of the Casson yield stress σ0√σ0 is 10 Pa 1 / 2 The inventors have found that the copper paste described below has good oxidation resistance and produces a sintered body having excellent die shear strength, and have thus completed the present invention. That is, the present invention provides the following.

[0011] (1) A method for producing a copper powder solution containing copper powder and an alcohol solvent, the alcohol solvent including one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s or more and 1000 mPa·s or less, the method comprising: -1 Viscosity η at 10 is between 1 Pa·s and 50 Pa·s, and the square root of the Casson yield stress σ0√σ0 is 10 Pa 1 / 2 Below is the copper paste.

[0012] (2)Casson viscosity η ∞ Square root of √η ∞ is 1 (Pa·s) 1 / 2 The copper paste according to (1) below.

[0013] (3) The copper paste according to (1) or (2), wherein the copper powder comprises first copper particles having an average particle diameter of 50 nm or more and 900 nm or less, second copper particles having an average particle diameter of 150 nm or more and 1 μm or less and being 100 nm or more larger than the first copper particles, and third copper particles having a plate-like, scale-like, flat or flake-like shape and an average particle diameter of 1.5 μm or more and 20 μm or less.

[0014] (4) The copper paste according to (3), wherein the copper powder contains 1% by mass or more and 30% by mass or less of the second copper particles and 5% by mass or more and 60% by mass or less of the third copper particles relative to 100% by mass of the copper powder.

[0015] (5) The copper paste according to (1) or (2), wherein the total amount of the first alcohol and the second alcohol is 5% by mass or more and 50% by mass or less, relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol.

[0016] (6) The copper paste according to (1) or (2), which does not contain a resin or contains the resin in an amount of more than 0% by mass and not more than 10% by mass relative to 100% by mass of the copper powder.

[0017] (7) A copper paste according to (1) or (2), which does not contain a silane coupling agent having an epoxy group and a silane coupling agent having an amino group, or which contains the silane coupling agent having an epoxy group and the silane coupling agent having an amino group in a total amount of more than 0% by mass and not more than 0.05% by mass relative to 100% by mass of the copper powder.

[0018] (8) The copper paste according to (1) or (2), wherein the first alcohol includes one or more selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, and 2,3-butanediol.

[0019] (9) The copper paste according to (3), wherein at least one of the first copper particles and the second copper particles has at least a portion of its surface coated with a polysaccharide.

[0020] (10) The copper paste according to (3), wherein at least one of the first copper particles and the second copper particles contains, on at least a portion of its surface, one or more acids selected from the group consisting of octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a copper paste that has good oxidation resistance and produces a sintered body that has excellent die shear strength. [Brief explanation of the drawings]

[0022] [Figure 1] 10 is a Casson plot of the paste of Comparative Example 2 (η10=185 Pa·s). [Figure 2] 1 is a Casson plot of the paste of Comparative Example 1 (η10=11 Pa·s). [Figure 3]1 is a Casson plot of the paste of Example 4 (η=8 Pa·s). [Figure 4] FIG. 2 is an optical microscope photograph of the dried paste of Comparative Example 1. [Figure 5] FIG. 1 is an optical microscope photograph of the dried paste of Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the description of the embodiments and can be practiced with appropriate modifications.

[0024] <Copper paste> The copper paste according to this embodiment contains copper powder and an alcohol solvent. The alcohol solvent contains one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s or more and 1000 mPa·s or less, and the copper paste is sintered at 25°C at a shear rate of 10 s -1 Viscosity η at 10 is between 1 Pa·s and 50 Pa·s, and the square root of the Casson yield stress σ0√σ0 is 10 Pa 1 / 2 The following is the result.

[0025] The viscosity of the first alcohol and the second alcohol refers to the viscosity at any shear rate, since both of these two alcohols are Newtonian viscous bodies and the viscosity does not depend on the shear rate.

[0026] In the case of copper paste, since it is a non-Newtonian viscous material, measurements are made using a cone-plate type dynamic viscoelasticity measuring device (e.g., Brookfield RST Cone-Plate Rheometer) and the shear rate is 10 s -1 This means the viscosity of the copper paste at this temperature.

[0027] In the copper paste according to this embodiment, at least two types of alcohol are used as the organic solvent of the dispersion medium. This prevents oxidation of the copper powder during storage and sintering. The sintered body obtained in this manner has a reduced amount of oxides present inside, resulting in excellent thermal conductivity and bonding strength.

[0028] In the copper paste according to this embodiment, the shear rate was 10 s -1 Viscosity η at 10 The shear rate is 1 Pa·s or more and 50 Pa·s or less. -1 Viscosity η at 10 By making the viscosity η equal to or greater than the required value, it is possible to prevent the paste from getting under the printing plate and becoming smeared during application by the printing method. In addition, it is possible to prevent the paste from flowing after application by the dispensing method, and to maintain the desired shape. 10 By keeping the required value or less, the paste in the gap between the semiconductor element and the insulating heat dissipation substrate can be uniformly distributed in the die mounting process, which reduces variations in sintering of each part of the pressure-sintered body and reduces part-dependent variations in die shear strength.

[0029] Shear rate 10s -1 Viscosity η at 10 is not particularly limited as long as it is 1 Pa·s or more and 50 Pa·s or less, but is preferably, for example, 1.5 Pa·s or more, 2 Pa·s or more, 2.5 Pa·s or more, 3 Pa·s or more, 3.5 Pa·s or more, 4 Pa·s or more, 4.5 Pa·s or more, 5 Pa·s or more, 5.5 Pa·s or more, 6 Pa·s or more, 6.5 Pa·s or more, 7 Pa·s or more, 7.5 Pa·s or more, 8 Pa·s or more, 8.5 Pa·s or more, 9 Pa·s or more, 9.5 Pa·s or more, or 10 Pa·s or more. -1 Viscosity η at 10 The viscosity is preferably 49 Pa·s or less, 47 Pa·s or less, 45 Pa·s or less, 42 Pa·s or less, 40 Pa·s or less, 37 Pa·s or less, 35 Pa·s or less, 32 Pa·s or less, or 30 Pa·s or less.

[0030] In the copper paste according to this embodiment, the square root √σ0 of the Casson yield stress σ0 is 10 Pa 1 / 2 The square root √σ0 of the Casson yield stress σ0 is an index of the size and cohesive strength of the aggregates, and by keeping √σ0 at or below the required value, the aggregates are prevented from becoming large or strongly cohesive, and therefore it is possible to prevent holes or protrusions from forming on the paste surface after drying, which would reduce the die shear strength after pressure sintering.

[0031] The square root of the Casson yield stress σ0 is √σ0, which is 10Pa. 1 / 2 There is no particular limitation as long as it is equal to or less than 9.7 Pa, for example. 1 / 2 Below, 9.5Pa 1 / 2 Below, 9.2Pa 1 / 2 Below, 9Pa 1 / 2 Below, 8.7Pa 1 / 2 Below, 8.5Pa 1 / 2 Below, 8.2Pa 1 / 2 Below, 8Pa 1 / 2 Below, 7.7Pa 1 / 2 Below, 7.5Pa 1 / 2 Below, 7.2Pa 1 / 2 Below, 7Pa 1 / 2 Below, 6.7Pa 1 / 2 Below, 6.5Pa 1 / 2 Below, 6.2Pa 1 / 2 Below, 6Pa 1 / 2 Below, 5.7Pa 1 / 2 Below, 5.5Pa 1 / 2 Below, 5.2Pa 1 / 2 Below 5Pa 1 / 2 It is preferable that:

[0032] In the copper paste according to this embodiment, Casson viscosity η ∞ is not particularly limited, but its square root √η ∞ is 1 (Pa·s) 1 / 2 Below, 0.97(Pa s) 1 / 2 Below, 0.95(Pa s) 1 / 2 Below, 0.92 (Pa s) 1 / 2 Below, 0.9 (Pa s) 1 / 2 Below, 0.87(Pa s)1 / 2 Below, 0.85(Pa s) 1 / 2 Below, 0.82(Pa s) 1 / 2 Below, 0.8(Pa s) 1 / 2 Below, 0.77(Pa s) 1 / 2 Below, 0.75(Pa s) 1 / 2 Below, 0.72(Pa s) 1 / 2 Below, 0.7(Pa s) 1 / 2 Below, 0.67(Pa s) 1 / 2 Below, 0.65(Pa s) 1 / 2 Below, 0.62 (Pa s) 1 / 2 Below, 0.6(Pa s) 1 / 2 Below, 0.57(Pa s) 1 / 2 Below, 0.55(Pa s) 1 / 2 Below, 0.52(Pa s) 1 / 2 Below, 0.5(Pa s) 1 / 2 It is preferable that the Casson viscosity η is equal to or less than η. ∞ Square root of √η ∞ can be used as an index of the ease of paste movement during the paste application process, and √η ∞ When the thickness is equal to or less than the required value, the paste can easily follow the movement of the squeegee during screen printing, and defects in the paste application area and unevenness in the paste thickness can be prevented.

[0033] (Measurement of dynamic viscoelastic behavior of copper paste) Measurements were performed using a dynamic viscoelasticity measuring device equipped with a cone-plate type spindle (e.g., Brookfield RST Cone-Plate Rheometer) at a shear rate of 10 s -1 The viscosity of the copper paste at this time is η 10 In addition, a Casson plot was obtained with the square root (√σ) of shear stress (σ) on the vertical axis and the square root (√γ) of shear rate (γ) on the horizontal axis. When an approximate straight line is obtained on this plot, the shear rate is 10 s -1 The intercept of the approximation line in the above region with the vertical axis is the square root of the Casson yield stress (√σ0), and the slope is the square root of the Casson viscosity (√η ∞ )

[0034] (alcohol solvent) The organic solvent used in the copper paste according to this embodiment is an alcohol solvent that combines a monohydric or dihydric alcohol with a dihydric or trihydric alcohol of different viscosities. When a polyhydric alcohol with a valence of 4 or more is used as the solvent, the alcohol may remain in the sintered body, reducing electrical conductivity and adhesion strength, especially when sintering is performed at low temperatures below approximately 300°C in a reducing or nitrogen atmosphere. On the other hand, when monohydric alcohol alone is used as the solvent, the copper paste is prone to volatilization during storage and printing, which can change the viscosity of the copper paste and worsen workability. The copper paste according to this embodiment avoids these problems by combining a monohydric or dihydric alcohol with a dihydric or trihydric alcohol of different viscosities, providing a copper paste with excellent physical properties and workability, with uniformly dispersed copper powder. In particular, the inclusion of a second alcohol with a high viscosity suppresses deformation from the desired shape due to sagging of the paste after application, and as described below, the viscosity of the copper paste can be adjusted to an appropriate value without the need for binder components such as resins. If the copper paste does not contain a resin component, there is no need to consider the generation of carbon residue derived from the resin component, and sintering can be carried out in a non-oxidizing atmosphere at a relatively low temperature.

[0035] In this specification, the term "alcohol solvent" refers to a mixed solvent primarily composed of alcohol, and may include a mixed solvent containing a small amount of water or an organic solvent other than alcohol, for example, 1 to 20 mass%, 2 to 17 mass%, 3 to 15 mass%, 4 to 12 mass%, or 5 to 10 mass% of one or more selected from ethers, ketones, esters, etc. In addition, hydrocarbon solvents, halogenated hydrocarbon solvents, etc. may be contained, but nitrogen-containing solvents such as amines and amides tend to remain in the dried product, so it is preferable that they are not contained, or if they are contained, they are contained in an amount of 5 mass% or less, 4 mass% or less, 3 mass% or less, 2 mass% or less, 1 mass% or less, 0.7 mass% or less, 0.5 mass% or less, 0.2 mass% or less, or 0.1 mass% or less.

[0036] The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 5% by mass or more, 5.5% by mass or more, 6% by mass or more, 6.5% by mass or more, 7% by mass or more, 7.5% by mass or more, or 8% by mass or more relative to 100% by mass of the total copper paste. On the other hand, the total amount of the first alcohol and the second alcohol is preferably 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, or 20% by mass or less relative to 100% by mass of the total copper paste. By ensuring that the total amount of the first alcohol and the second alcohol is greater than the required amount, the copper paste can be applied to the entire interface with a uniform layer thickness, resulting in excellent bonding strength. Furthermore, by ensuring that the total amount of the first alcohol and the second alcohol is less than the required amount, no solvent remains during firing, thereby preventing a decrease in electrical conductivity and bonding strength.

[0037] The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 5% by mass or more, 7% by mass or more, 10% by mass or more, 12% by mass or more, 15% by mass or more, 17% by mass or more, 20% by mass or more, 22% by mass or more, 25% by mass or more, 27% by mass or more, or 30% by mass or more relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol. On the other hand, the total amount of the first alcohol and the second alcohol is preferably 50% by mass or less, 47% by mass or less, 45% by mass or less, 42% by mass or less, 40% by mass or less, 37% by mass or less, 35% by mass or less, 32% by mass or less, 30% by mass or less, 27% by mass or less, or 25% by mass or less relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol.

[0038] The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 95% by mass or more, 97% by mass or more, 98% by mass or more, 99% by mass or more, 99.9% by mass or more, or 99.99% by mass or more, relative to 100% by mass of the total solvent in the copper paste. Because alcohols, particularly trihydric alcohols, have a reducing effect, increasing their content in the solvent of the copper paste can more effectively suppress oxidation of the copper powder.

[0039] The ratio (X / Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) in the copper paste is not particularly limited, but is preferably 0.2 or more, 0.3 or more, 0.4 or more, or 0.5 or more. On the other hand, the ratio (X / Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) is preferably 8 or less, 7 or less, 6 or less, or 5 or less. To ensure sufficient bonding strength of the copper paste, such as the die shear strength between the chip and the substrate, a copper paste layer must be printed with a substantially uniform thickness at the interface between the chip and the substrate. By ensuring that the ratio (X / Y) is equal to or greater than the required value, the viscosity is suitable for application, resulting in excellent bonding strength. By ensuring that the ratio (X / Y) is equal to or less than the required value, the reduction effect derived from the alcohol is fully expressed, resulting in particularly good sinterability of the resulting sintered body, resulting in high electrical conductivity and bonding strength.

[0040] (First Alcohol) The first alcohol is one or more alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity of 3 mPa·s or more and 70 mPa·s or less at 25°C. If the viscosity of the first alcohol is within this range, the application of the copper paste becomes easy and good workability is ensured.

[0041] The boiling point of the first alcohol is not particularly limited, but is preferably 150°C or higher, 155°C or higher, 160°C or higher, 165°C or higher, 170°C or higher, 175°C or higher, 180°C or higher, 185°C or higher, or 190°C or higher. On the other hand, the boiling point of the first alcohol is preferably 250°C or lower, 245°C or lower, 240°C or lower, 235°C or lower, 230°C or lower, 225°C or lower, 220°C or lower, 215°C or lower, 210°C or lower, 205°C or lower, or 200°C or lower. Having a boiling point of 150°C or higher can prevent bumping during heating, which can cause voids in the paste and reduce sinterability, thereby improving the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the first alcohol is 150°C or higher, the solvent will not volatilize and cause a change in viscosity within a short period of time, even when the copper paste is stored at room temperature. Therefore, there is no need to store the product in a refrigerator or freezer, which reduces storage costs.In this specification, the "boiling point" refers to the boiling point at atmospheric pressure unless otherwise specified.

[0042] The boiling point of the first alcohol is not particularly limited, but is preferably more than 50° C. lower than the firing temperature of the copper paste.

[0043] The vapor pressure of the first alcohol is not particularly limited, but is preferably 0.1 Pa or more, 0.2 Pa or more, 0.3 Pa or more, 0.4 Pa or more, 0.5 Pa or more, 0.6 Pa or more, 0.7 Pa or more, 0.8 Pa or more, 0.9 Pa or more, 1 Pa or more, 1.2 Pa or more, 1.5 Pa or more, 1.7 Pa or more, 2 Pa or more, 2.2 Pa or more, 2.5 Pa or more, 2.7 Pa or more, or 3 Pa or more at about room temperature, for example, 25°C. On the other hand, the vapor pressure of the first alcohol is not particularly limited, but is preferably 100 Pa or less, 90 Pa or less, 80 Pa or less, 70 Pa or less, 60 Pa or less, 50 Pa or less, 40 Pa or less, or 30 Pa or less. Having the vapor pressure of the first alcohol within the required range results in excellent storage stability and printability.

[0044] Specific examples of the first alcohol include 1-hexanol (viscosity 4.58 mPa·s, boiling point 158°C, vapor pressure 80 Pa), 1-heptanol (viscosity 5.81 mPa·s, boiling point 176°C, vapor pressure 44 Pa), 2-heptanol (viscosity 3.96 mPa·s, boiling point 159°C, vapor pressure 78 Pa), 1-octanol (viscosity 7.29 mPa·s, boiling point 195°C, vapor pressure 24 Pa), 2-octanol (viscosity 6.49 mPa·s, boiling point 180°C, vapor pressure 42 Pa), and 2-ethyl-1-hexanol (viscosity 6.27 mPa·s, boiling point 1 Examples of suitable first alcohols include monohydric alcohols such as benzyl alcohol (viscosity 5.47 mPa·s, boiling point 205°C, vapor pressure 18 Pa) and ethylene glycol (viscosity 16.1 mPa·s, boiling point 197°C, vapor pressure 20 Pa), 1,2-propanediol (viscosity 40.4 mPa·s, boiling point 188°C, vapor pressure 28 Pa), 1,3-propanediol (viscosity 47 mPa·s, boiling point 214°C, vapor pressure 5 Pa), and 2,3-butanediol (viscosity 45 mPa·s, boiling point 182°C, vapor pressure <100 Pa). Among these, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, ethylene glycol, and 1,2-propanediol are preferred. The first alcohol may be used alone or in combination of two or more types as long as it satisfies the above-mentioned requirements for the first alcohol. The viscosity and vapor pressure are both measured at 25°C. As described above, the first alcohol has a low viscosity, so the viscosity of the copper paste can be adjusted to an appropriate value by adding a smaller amount. This reduces the total amount of organic solvent in the copper paste, making it possible to suppress the residual organic solvent components during firing.

[0045] (Second Alcohol) The second alcohol is one or more alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity of 300 mPa·s or more and 1000 mPa·s or less at 25° C. If the viscosity of the second alcohol is within this range, it is possible to prevent the copper paste before sintering from sagging, which would make it impossible to form the desired shape, and the workability of the copper paste is not impaired.

[0046] The boiling point of the second alcohol is not particularly limited, but is preferably 150°C or higher, 160°C or higher, 170°C or higher, 180°C or higher, 190°C or higher, 195°C or higher, 200°C or higher, 205°C or higher, 210°C or higher, 215°C or higher, 220°C or higher, 225°C or higher, 230°C or higher, 235°C or higher, 240°C or higher, 245°C or higher, 250°C or higher, 255°C or higher, 260°C or higher, 265°C or higher, 270°C or higher, 275°C or higher, 280°C or higher, or 285°C or higher. On the other hand, the boiling point of the second alcohol is preferably 320°C or lower, 315°C or lower, 310°C or lower, 305°C or lower, 300°C or lower, or 295°C or lower. By having the boiling point of the second alcohol within this range, it does not remain in the gaps between the copper particles in the sintered body even after low-temperature firing, thereby not reducing thermal conductivity. By ensuring that the boiling point of the second alcohol is equal to or higher than the required value, it is possible to prevent bumping during heating, which would cause voids in the paste and reduce sinterability, thereby improving the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the second alcohol is equal to or higher than the required value, the solvent will not volatilize and the viscosity will not change in a short period of time even if the copper paste is stored at room temperature. This eliminates the need for refrigeration or freezing, thereby reducing storage costs.

[0047] The boiling point of the second alcohol is not particularly limited, but is preferably more than 50° C. lower than the firing temperature of the copper paste.

[0048] It is preferable to use a second alcohol with a boiling point higher than that of the first alcohol. The inclusion of a low-viscosity first alcohol provides the copper paste of this embodiment with an appropriate viscosity and excellent workability. However, there is no need to adjust the viscosity after paste application, and from the viewpoint of preventing the copper paste from sagging, it is preferable that the first alcohol has disappeared. On the other hand, among alcohols, dihydric alcohols and trihydric alcohols, especially trihydric alcohols, have a high reducing effect, so it is preferable that the second alcohol containing at least one of these be present at a high concentration during firing. Therefore, by using a second alcohol with a boiling point higher than that of the first alcohol and that evaporates near the firing temperature of the copper paste, good workability can be maintained and oxidation of the copper powder can be more effectively suppressed.

[0049] Furthermore, it is preferable that the vapor pressure at around room temperature, for example, 25°C, is 1 mPa or more and 5 Pa or less, further 1.5 Pa or less, and particularly 1 Pa or less, because this improves storage stability and further enhances the effect of inhibiting oxidation during baking. This effect is particularly remarkable when the vapor pressure of the second alcohol is lower than that of the first alcohol.

[0050] The vapor pressure of the second alcohol is not particularly limited, but is preferably 1 mPa or more, 2 mPa or more, 3 mPa or more, 4 mPa or more, 5 mPa or more, 6 mPa or more, 7 mPa or more, 8 mPa or more, 9 mPa or more, or 10 mPa or more at about room temperature, for example, 25°C. On the other hand, the vapor pressure of the second alcohol is not particularly limited, but is preferably 100 Pa or less, 90 Pa or less, 80 Pa or less, 70 Pa or less, 60 Pa or less, 50 Pa or less, 40 Pa or less, or 30 Pa or less. Having the vapor pressure of the first alcohol within the required range results in excellent storage stability and printability.

[0051] Specifically, the second alcohol may be a dihydric alcohol such as 2-ethyl-1,3-hexanediol (viscosity 323 mPa·s, boiling point 244°C, vapor pressure <1.4 Pa) or a trihydric alcohol such as glycerol (viscosity 934 mPa·s, boiling point 290°C, vapor pressure 0.01 Pa). As the second alcohol, one type may be used alone, or two or more types may be mixed together, as long as they satisfy the above-mentioned requirements for the second alcohol.

[0052] (copper powder) The copper powder is contained in a copper paste, and the paste is sintered to form a sintered body.

[0053] The copper powder is not particularly limited and may be any of various commercially available products. Copper powder can be produced using methods such as the high-pressure water atomization method described in International Publication No. 99 / 11407 and the wet reduction precipitation method described in International Publication No. 2014 / 80662. The high-pressure water atomization method is a method for producing metal powder (e.g., copper) from molten metal. The molten metal is split by gas near the nozzle outlet through the center of a nozzle through which gas flows downstream of the molten metal. The split molten metal is then further split by liquid ejected in an inverted cone shape. This method allows for the continuous splitting of the molten metal by gas and liquid, resulting in the industrial, large-scale, and low-cost production of metal powder (e.g., copper) with fine particle size, spherical or granular shape, and low oxygen content. The wet reduction precipitation method involves wet reduction of copper ions using a reducing agent such as hydrazine, using an organic solvent that is compatible with water and reduces the surface tension of water as a solvent. Specifically, this method uses water and an organic solvent as the liquid medium, mixes a reaction solution containing monovalent or divalent copper ions with a reducing agent, and reduces the copper ions to produce copper particles. Generally, high-pressure water atomization can produce particles of 0.7 μm or larger. For producing finer particles smaller than this, the wet reduction precipitation method is suitable.

[0054] In one embodiment, the average particle size of the copper powder (copper particles) is not particularly limited, but is preferably, for example, 0.05 μm or more, 0.06 μm or more, 0.07 μm or more, 0.08 μm or more, 0.09 μm or more, 0.1 μm or more, 0.12 μm or more, 0.15 μm or more, 0.17 μm or more, 0.2 μm or more, 0.22 μm or more, 0.25 μm or more, 0.27 μm or more, or 0.3 μm or more. On the other hand, the average particle size of the copper powder is preferably 2.0 μm or less, 1.9 μm or less, 1.8 μm or less, 1.7 μm or less, 1.6 μm or less, 1.5 μm or less, 1.4 μm or less, 1.3 μm or less, 1.2 μm or less, 1.1 μm or less, 1 μm or less, 0.9 μm or less, 0.8 μm or less, or 0.7 μm or less. When the average particle size of the copper powder is equal to or less than the required value, the surface area of ​​the copper powder increases relatively, making it easier to sinter at low temperatures. On the other hand, when the average particle size of the copper powder is equal to or greater than the required value, it is possible to suppress an increase in the price of the copper powder. Furthermore, when the average particle size is equal to or greater than the required value, it is possible to prevent a large number of particles from agglomerating and forming defects in the sintered compact. In this specification, the "average particle size" refers to the 50% particle size (D50), and more specifically, it is the median value in the particle diameter distribution measured using a laser particle size distribution analyzer or the like.

[0055] In one embodiment, the copper powder preferably includes first copper particles having an average particle diameter of 50 nm to 900 nm, second copper particles having an average particle diameter of 150 nm to 1 μm, which are at least 100 nm larger than the first copper particles, and third copper particles having a plate-like, scale-like, flat, or flake-like shape and an average particle diameter of 1.5 μm to 20 μm. When the first copper particles and the second copper particles have an average particle diameter difference of 100 nm or more, the first copper particles having a smaller average particle diameter fill the gaps between the larger second copper particles, resulting in a dense sintered body. Furthermore, the inclusion of third copper particles in a flake-like shape or the like can suppress the occurrence of cracks after the paste is applied and dried.

[0056] The shape of the first copper particles is not particularly limited, but is preferably, for example, spherical, ellipsoidal, polyhedral, irregular, wire-like, dendritic, or other shapes.

[0057] The average particle size of the first copper particles is preferably, for example, 60 nm or more, 70 nm or more, 80 nm or more, 90 nm or more, 100 nm or more, 120 nm or more, 150 nm or more, 170 nm or more, 200 nm or more, 220 nm or more, or 250 nm or more. On the other hand, the average particle size of the first copper particles is preferably 850 nm or less, 800 nm or less, 750 nm or less, 700 nm or less, 650 nm or less, 600 nm or less, 550 nm or less, 500 nm or less, 450 nm or less, 400 nm or less, 350 nm or less, or 300 nm or less.

[0058] The content of the first copper particles is not particularly limited, and is preferably, for example, 20% by mass or more, 25% by mass or more, 30% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, 60% by mass or more, 65% by mass or more, or 70% by mass or more relative to 100% by mass of the copper powder. On the other hand, the content of the first copper particles in the copper powder is preferably 90% by mass or less, 85% by mass or less, or 80% by mass or less relative to 100% by mass of the copper powder.

[0059] The shape of the second copper particles is not particularly limited, but is preferably, for example, spherical, ellipsoidal, polyhedral, irregular, wire-like, dendritic, or other shapes.

[0060] The average particle size of the second copper particles is preferably, for example, 160 nm or more, 170 nm or more, 180 nm or more, 190 nm or more, 200 nm or more, 220 nm or more, 250 nm or more, 270 nm or more, 300 nm or more, 320 nm or more, or 350 nm or more. On the other hand, the average particle size of the first copper particles is preferably 950 nm or less, 900 nm or less, 850 nm or less, 800 nm or less, 750 nm or less, 700 nm or less, 650 nm or less, 600 nm or less, 550 nm or less, 500 nm or less, 450 nm or less, or 400 nm or less.

[0061] The average particle size of the second copper particles is preferably at least 100 nm larger than that of the first copper particles, and is preferably at most 900 nm, 800 nm, 700 nm, 600 nm, 500 nm, 400 nm, 300 nm, or 200 nm larger than that of the first copper particles.

[0062] The content of the second copper particles is not particularly limited, and is preferably, for example, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 11% by mass or more, or 12% by mass or more relative to 100% by mass of the copper powder. On the other hand, the content of the second copper particles in the copper powder is preferably 30% by mass or less, 27% by mass or less, 25% by mass or less, 22% by mass or less, 20% by mass or less, or 18% by mass or less relative to 100% by mass of the copper powder.

[0063] The average particle size of the third copper particles is, for example, preferably 1.5 μm or more, 2 μm or more, 2.5 μm or more, 3 μm or more, 3.5 μm or more, 4 μm or more, 4.5 μm or more, 5 μm or more, 5.5 μm or more, or 6 μm or more. On the other hand, the average particle size of the third copper particles is preferably 20 μm or less, 19 μm or less, 18 μm or less, 17 μm or less, 16 μm or less, 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, 10 μm or less, 9 μm or less, or 8 μm or less.

[0064] The content of the third copper particles in the copper powder is not particularly limited, and is preferably, for example, 5% by mass or more, 5.5% by mass or more, 6% by mass or more, 6.5% by mass or more, 7% by mass or more, 7.5% by mass or more, 8% by mass or more, 8.5% by mass or more, 9% by mass or more, or 9.5% by mass or more, relative to 100% by mass of the copper powder. On the other hand, the content of the third copper particles in the copper powder is preferably 60% by mass or less, 55% by mass or less, 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 12% by mass or less, relative to 100% by mass of the copper powder.

[0065] The copper powder (copper particles) may have a portion of its surface coated with an organic substance. The organic substance is preferably a polysaccharide or fatty acid compound. When polysaccharide molecules coat the copper powder, the outer side (the side in contact with the solvent) becomes hydrophilic, interacting with the hydroxyl groups of the organic solvent in the copper paste to provide appropriate viscosity. Meanwhile, the carboxyl groups of fatty acids bond to the copper particle surface, making the opposite end of the fatty acid hydrophobic, thereby improving the dispersibility of the copper particles and suppressing particle aggregation. These effects of polysaccharides and fatty acids allow the copper paste to spread uniformly across the entire interface, resulting in excellent bonding strength.

[0066] Specific examples of polysaccharides that can be used include, but are not limited to, one or more selected from gum arabic, carboxymethyl cellulose, hydroxyethyl cellulose, cellulose nanofiber, starch, glycogen, agarose (agar), pectin, alginic acid, and salts thereof. Among these, it is preferable to use one or more selected from gum arabic and sodium alginate.

[0067] Specific examples of the fatty acid that can be used include medium-chain fatty acids such as pentanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid. Of these, it is preferable to use one or more selected from octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid.

[0068] The surface coverage of the organic material is not particularly limited, but is preferably such that the carbon content is 0.05% to 0.8% by mass, preferably 0.1% to 0.5% by mass, and the oxygen content is 0.05% to 1.5% by mass, preferably 0.1% to 1% by mass, based on 100% by mass of the copper powder containing the coating layer. By ensuring that the carbon and oxygen contents are above the required levels, the hydrophilic properties of the polysaccharide molecules on the copper powder surface are fully realized, reducing the viscosity of the copper paste and forming a uniform paste layer, resulting in excellent bond strength for the resulting sintered body. On the other hand, by ensuring that the carbon and oxygen contents are below the required levels, it is possible to prevent carbon- and oxygen-containing components from remaining inside the sintered body, for example, during firing in a nitrogen atmosphere, thereby improving thermal conductivity and bond strength.

[0069] When the copper powder is made of the three types of copper particles, i.e., the first copper particles, the second copper particles, and the third copper particles, it is preferable that at least one of the first copper particles and the second copper particles has at least a portion of its surface coated with an organic substance. Polysaccharides and fatty acids can be used as the organic substance, with polysaccharides being preferred. The third copper particles may also have at least a portion of their surface coated with an organic substance (e.g., polysaccharides, fatty acids, etc.).

[0070] In the copper paste according to this embodiment, the total content of elements other than copper in the copper powder is not particularly limited, but is preferably, for example, 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, relative to 100% by mass of the copper powder. Among the components other than copper, metal elements, in particular, may segregate on the surface of the copper powder or form oxides, thereby impairing sinterability and potentially reducing the electrical conductivity of the sintered body by dissolving in the copper powder. When the total content of metal elements such as As, Co, Cr, Fe, Ir, P, S, Sb, Se, Te, Ti, V, and Zr is below the required level, the electrical resistivity of the copper paste sintered body can be reduced and the sintered body can exhibit superior thermal conductivity. Such thermal conductivity allows, for example, heat generated by a power module to be efficiently dissipated to the outside.

[0071] (Other ingredients) In addition to the components described above, the copper paste according to this embodiment may contain a dispersant made of amines, a surfactant, an antioxidant, a reducing agent such as hydrazine, glass frit, a binder including a resin component, and the like.

[0072] The resin component is not particularly limited, but may be one or more selected from, for example, cellulose-based resins such as methyl cellulose, ethyl cellulose, and carboxymethyl cellulose, acrylic resins, butyral resins, alkyd resins, epoxy resins, and phenolic resins.

[0073] The resin component content is not particularly limited, but is preferably greater than 0% by mass, 0.01% by mass or more, 0.05% by mass or more, or 0.1% by mass or more relative to 100% by mass of copper powder. The resin component content may be 10% by mass or less, 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.1% by mass or less, 0.05% by mass or less, or 0.01% by mass or less relative to 100% by mass of copper particles, but is preferably copper powder-free. If the copper paste contains a resin component, sinterability (especially at temperatures below 350°C) may be impaired. For example, thermosetting resins such as epoxy resins remain in the copper paste sintered body even after sintering. Even when cellulose resins are used, thermal decomposition begins at around 300°C, but complete thermal decomposition requires heating to temperatures above 400°C. Furthermore, sintering in an oxygen atmosphere is required to remove the carbon residue produced by thermal decomposition, which may oxidize the copper powder. Resin-free copper pastes can be sintered at relatively low temperatures in a non-oxidizing atmosphere, resulting in the formation of high-density sintered copper bodies without the risk of copper powder oxidation causing a decrease in conductivity. The copper paste according to this embodiment contains a second alcohol with high viscosity, making it possible to adjust the viscosity to an appropriate value without the need for a resin component.

[0074] Furthermore, it is preferable that the copper paste does not contain either a silane coupling agent having an epoxy group or a silane coupling agent having an amino group, or contains a silane coupling agent having an epoxy group and a silane coupling agent having an amino group in a total amount of more than 0 mass% and 0.05 mass% or less relative to 100 mass% of the copper powder.

[0075] (Copper paste applications) As described above, the copper paste according to this embodiment has high electrical and thermal conductivity, and is excellent in storage stability and workability. Furthermore, the copper paste according to this embodiment can be fired at low temperatures and in short periods of time, exhibiting high die shear strength. Therefore, it can be used for wiring formation in electronic components such as power modules, chip resistors, chip capacitors, and solar cells, as well as in electronic packaged products such as printed wiring boards and substrates with through-holes. For example, a copper sintered body can be formed by applying the copper paste according to this embodiment to a power module, a solar cell substrate, a substrate for mounting electronic packaged products, a printed wiring board, or a substrate with through-holes, and then sintering it. Examples of substrate materials that can be used include silicon substrates, oxide substrates such as silicate glass, alumina, and quartz, nitride substrates such as silicon nitride and aluminum nitride, carbide substrates such as silicon carbide and titanium carbide, resin substrates such as polyimide, polyethylene terephthalate, and polyethylene naphthalate, and substrates having a transparent conductive film (TCO) or a metal film on their surfaces.

[0076] (manufacturing copper paste) The copper paste according to this embodiment can be produced by mixing the copper powder and the solvent described above and kneading the mixture using a device such as a planetary mixer, if necessary. Furthermore, the dispersibility of the copper powder can be improved using a three-roll mill, if necessary. Furthermore, the paste may be filtered or degassed.

[0077] (Firing of copper paste) When firing the copper paste according to this embodiment, the method and conditions are not particularly limited, and any method can be used depending on the target product and the mating material to which the paste is applied. However, prior to firing the copper paste according to this embodiment, it is preferable to dry and remove the first alcohol. This increases the proportion of the second alcohol around the copper powder during firing, thereby more effectively preventing oxidation of the copper powder during firing. The drying conditions are not particularly limited and can be set as desired depending on the boiling point of the first alcohol and the target product. For example, heating in an air atmosphere at 50 to 200°C, particularly 60 to 150°C, for 1 to 60 minutes is preferred. Drying can also be performed under reduced pressure to further reduce the heating temperature. Heat drying can also be performed in an inert gas atmosphere or a reducing atmosphere.

[0078] The copper paste according to this embodiment can be fired at low temperatures and in a short time, and the firing conditions are not particularly limited. For example, a sintered body having excellent electrical conductivity, thermal conductivity, and die shear strength can be obtained by sintering in an inert gas atmosphere such as nitrogen or argon gas, or in a reducing atmosphere containing approximately 0.1% by volume to 30% by volume of hydrogen, ammonia, carbon monoxide, alcohol vapor, or the like, at 150 to 400°C or 200 to 350°C, particularly 250 to 300°C, for 10 seconds to 60 minutes, particularly 2 minutes to 30 minutes. [Example]

[0079] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0080] [Evaluation of sample properties and performance] The pastes prepared under the respective compositions and conditions described below were evaluated for their physical properties and performance according to the evaluation methods described below.

[0081] (Measurement of viscosity, and measurement of Casson yield stress and Casson viscosity) The viscosity of the prepared paste was measured using a dynamic viscoelasticity measuring device (Brookfield, RST Cone-Plate Rheometer) equipped with a cone-plate type spindle, and the viscosity was measured at a shear rate of 10 s -1 The viscosity of the copper paste at this time is η 10 In addition, a Casson plot was obtained with the square root (√σ) of shear stress (σ) on the vertical axis and the square root (√γ) of shear rate (γ) on the horizontal axis. When an approximate straight line is obtained in this plot, the Casson plot is possible, and the shear rate is 10 s -1 The intercept of the approximation line in the above region with the vertical axis is the square root of the Casson yield stress (√σ0), and the slope of the approximation line is the square root of the Casson viscosity (√η ∞ ) Here, the fact that an approximate straight line can be obtained means that R 2 This means that the value (coefficient of determination) is 0.9 or higher.

[0082] Hereinafter, a description will be given with reference to specific examples. As a representative example, Casson plots were created for three types of copper paste (Comparative Example 2, Comparative Example 1, and Example 4, details of which will be described later). Figure 1 shows the results of Comparative Example 2 (η 10 = 185 Pa s) paste. -1 R in the above areas 2 The value was 0.0615, and as is clear from FIG. 1, it is not linear and therefore cannot be fitted using the Casson equation.

[0083] On the other hand, FIG. 2 shows Comparative Example 1 (η 10 3 is a Casson plot of the paste of Example 4 (η 10 The Casson plots shown in Figures 2 and 3 are for pastes with a shear rate of 10 s -1 R in the above areas 2The values ​​are 0.9994 and 0.9996, respectively, and the data can be approximated linearly. In both cases, a rapid increase in √σ in the slow shear rate region and a slow increase in √σ in the fast shear rate region are observed. The rapid increase corresponds to the phenomenon of breaking down the aggregates of fine powder contained in the paste, so the fast shear rate (10 s) reflects the characteristics of a homogeneous paste. -1 (Above) From the approximate line of the area, intercept: √σ0, slope: √η ∞ obtained.

[0084] (Observation of tissue defects) The prepared paste was applied to a glass substrate by stencil printing using a metal mask to form a square with sides of 20 mm, and then dried in air at 100°C for 5 minutes. The structure of the dried paste surface was observed with a stereoscopic optical microscope at 10x magnification to check for the presence of structural defects such as holes, protrusions, and cracks.

[0085] The following describes specific examples. The pastes of Comparative Example 1 and Example 4, which were capable of linear approximation in the Casson plot, were printed on a glass substrate, dried, and the surface of the resulting dried product was observed. The results are shown below. FIG. 4 is an optical microscope photograph of the dried product of the paste of Comparative Example 1. FIG. 5 is an optical microscope photograph of the dried product of the paste of Example 4. Numerous holes were observed in the dried product of the paste of Comparative Example 1 (positions indicated by arrows in FIG. 4). On the other hand, no holes or other structural defects were observed in the dried product of the paste of Example 4. Similar tests were performed on pastes prepared under various process conditions to check for the presence or absence of surface defects and printing defects.

[0086] (Measurement of electrical resistivity) The paste thus prepared was applied to a glass substrate by stencil printing using a metal mask to form a square with sides of 20 mm, and then dried in air at 100°C for 5 minutes. Subsequently, pressure sintering was carried out in a nitrogen atmosphere using a high-temperature press under a load of 20 MPa at a heating temperature of 280°C for a heating time of 2 minutes, yielding a copper paste sintered body with a thickness of approximately 20 μm. The electrical resistivity of this sintered body was measured using a DC four-probe electrical resistance measuring device with a probe spacing set to 1 mm. The electrical resistivity was converted to thermal conductivity according to the Wiedemann-Franz law, yielding a value of 134 Wm -1 K -1 This corresponds to the above.

[0087] (Measurement of die shear strength) A 1 mm thick copper plate was used as a substrate, and copper paste was applied to a thickness of 100 μm. A silicon carbide (SiC) semiconductor chip measuring 2 mm × 2 mm × 0.4 mm was then placed on top of the copper paste. A 500 nm Ti layer and a 500 nm Cu layer were deposited on the surface of the SiC chip that contacted the copper paste using a sputtering method. The resulting laminate was subjected to a load of 20 MPa using a high-temperature press in a nitrogen atmosphere, and pressure sintered at a firing temperature of 280°C for 3 minutes. After cooling to room temperature, the adhesion strength between the SiC chip and the copper substrate was measured as die shear strength using a die shear tester (Nordson DAGE4000).

[0088] (Evaluation criteria) Based on these measurement results, if all four conditions were met, (1) there were no surface defects on the dried paste, (2) there were no printing defects on the dried paste, (3) the electrical resistivity of the sintered body was 5 μΩ cm or less, and (4) the die shear strength of the sintered body was 30 MPa or more, the product was rated as AA. If three conditions were met, the product was rated as A, if two conditions were met, the product was rated as B, and if one condition or none of the conditions were met, the product was rated as C.

[0089] [Test 1: Effect of paste viscosity and Casson yield stress] Example 1 The copper particles used were first copper particles with a D50 of 270 nm and a roughly spherical shape, second copper particles with a D50 of 380 nm and a roughly spherical shape, and third copper particles with a D50 of 7 μm and a flake shape. The surfaces of the two types of copper particles other than the flake-shaped particles were coated with gum arabic, a polysaccharide.

[0090] First, the first copper particles, second copper particles, and third copper particles were mixed in a mass ratio of 65:30:5. The amounts of impurities contained in all of the first to third copper particles were 0.3% carbon, 0.7% oxygen, and 0.2% metal elements other than copper. Ethylene glycol as the first alcohol and glycerol as the second alcohol were prepared, and weighed out to a mass ratio of copper particles:ethylene glycol:glycerol of 60:20:20, and kneaded in a planetary mixer to produce a copper paste. The evaluation results are shown in Table 1.

[0091] (Examples 2 to 7 and Comparative Examples 1 to 4) Copper pastes were prepared under the same conditions as in Example 1, except that the mass ratio of the copper powder to the solvent and the mass ratio of the first to third copper particles were changed. The evaluation results are shown in Table 1. In Table 1, "N / A" indicates that a good approximate straight line (fitting) was not obtained in the Casson plot.

[0092] [Table 1]

[0093] The first alcohol (ethylene glycol) with a viscosity in the range of 3 mPa·s or more and 70 mPa·s or less and the second alcohol (glycerol) with a viscosity in the range of 300 mPa·s or more and 1000 mPa·s or less were used as solvents, and the paste composition was adjusted to achieve η 10 , √σ0, √η ∞ It was found that the value of η 10 It was found that by adjusting and √σ0, a paste with good evaluation could be obtained.

[0094] Test 2: Effect of the first alcohol species (Examples 8 to 13 and Comparative Examples 5 to 7) Pastes were prepared and evaluated in the same manner as in Example 3, except that the ethylene glycol used as the first alcohol was changed to the alcohols shown in Table 2. The evaluation results are shown in Table 2.

[0095] [Table 2]

[0096] From Table 2, it was found that the first alcohol is not limited to ethylene glycol, and that a sintered body having the desired performance can be obtained by using a copper paste that uses a monohydric or dihydric alcohol having a viscosity within the required viscosity range.

[0097] Test 3: Effect of a second alcohol species (Example 14, Comparative Examples 8 to 9) Pastes were prepared and evaluated in the same manner as in Example 3, except that the glycerol used as the second alcohol was changed to the alcohol shown in Table 3. The evaluation results are shown in Table 3.

[0098] [Table 3]

[0099] From Table 3, it was found that the second alcohol is not limited to ethylene glycol, and that a sintered body having the desired performance can be obtained by using a copper paste that uses a dihydric or trihydric alcohol having a viscosity within the required viscosity range.

Claims

1. Contains copper powder and alcohol solvent, The alcohol solvent is a first alcohol, which is one or more selected from the group consisting of monohydric alcohols and dihydric alcohols, having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less; and a second alcohol, which is one or more selected from the group consisting of dihydric alcohols and trihydric alcohols, having a viscosity at 25°C of 300 mPa s or more and 1000 mPa s or less, 25°C, shear rate 10 s －１ Viscosity η at １０ is 1 Pa s or more and 50 Pa s or less, Casson yield stress σ ０ Square root of √σ ０ is 10 Pa １／２ is Copper paste.

2. Casson viscosity η ∞ Square root of √η ∞ is 1 (Pa s) １／２ is The copper paste according to claim 1 .

3. The copper powder is First copper particles having an average particle diameter of 50 nm or more and 900 nm or less; second copper particles having an average particle diameter of 150 nm or more and 1 μm or less, and an average particle diameter 100 nm or more larger than that of the first copper particles; and third copper particles that are plate-like, scale-like, flat, or flake-like and have an average particle size of 1.5 μm or more and 20 μm or less. The copper paste according to claim 1 or 2.

4. The copper powder contains the second copper particles in an amount of 1% by mass or more and 30% by mass or less and the third copper particles in an amount of 5% by mass or more and 60% by mass or less, relative to 100% by mass of the copper powder. The copper paste according to claim 3.

5. The total amount of the first alcohol and the second alcohol is 5% by mass or more and 50% by mass or less with respect to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol. The copper paste according to claim 1 or 2.

6. The resin is not contained, or the resin is contained in an amount of more than 0 mass % and not more than 10 mass % relative to 100 mass % of the copper powder. The copper paste according to claim 1 or 2.

7. The composition does not contain a silane coupling agent having an epoxy group and a silane coupling agent having an amino group, or contains the silane coupling agent having an epoxy group and the silane coupling agent having an amino group in a total amount of more than 0% by mass and 0.05% by mass or less relative to 100% by mass of the copper powder. The copper paste according to claim 1 or 2.

8. The first alcohol includes one or more selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, and 2,3-butanediol. The copper paste according to claim 1 or 2.

9. At least one of the first copper particles and the second copper particles has a surface at least partially coated with a polysaccharide. The copper paste according to claim 3.

10. At least one of the first copper particles and the second copper particles contains, on at least a portion of the surface thereof, one or more selected from the group consisting of octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid. The copper paste according to claim 3.

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