Copper paste

Abstract

The present invention provides a copper paste for manufacturing a copper sintered body having higher thermal conductivity and bonding strength by causing copper particles to be filled in more densely when the copper paste is in a dried state.　The copper paste of the present disclosure contains copper powder and a dispersion medium, and has, in a volume-based particle diameter frequency distribution of a copper powder in the copper paste, a first peak representing the maximum frequency value from among peaks having a peak top in the range of 300 nm to 1 μm, and a second peak representing the maximum frequency value of peaks having a peak top in the range of greater than 3 μm and less than or equal to 10 μm. The ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 to 2.

Description

Copper paste

[0001] The present disclosure relates to a copper paste.

[0002] A power module is formed by joining a semiconductor element for controlling power, an insulating heat dissipation substrate, and each member of cooling fins. Among these, as the semiconductor element, materials such as Si, SiC, GaN, Ga ２ O ３ etc. are used. Also, as the insulating heat dissipation substrate, Al ２ O ３ , AlN, Si ３ N ４ etc., ceramic materials with excellent thermal conductivity are used. Further, as the heat dissipation fins, Al is used. In order to join each member, a metal thin film such as Cu, Ni, Ag, etc. is formed on the surface where the semiconductor element is joined, and a Cu thin plate called Direct Bonding of Copper (DBC) or Active Metal Brazing (AMB) is formed on the surface where the insulating heat dissipation substrate is joined. An Ag thin film may be further formed on the surface of this Cu thin plate.

[0003] The joining of a semiconductor element and an insulating heat dissipation substrate is generally called "die bonding". This die bonding includes a process of applying a conductive paste on the surface of the Cu thin plate formed on the insulating heat dissipation substrate by a screen printing method, a dispensing method, etc. (coating process), a process of drying the paste (drying process), a process of arranging the semiconductor element at the location where the conductive paste is applied (die mounting process), and a process of heating the conductive paste in a state where a stress of about 20 MPa is applied in the stacking direction of the obtained laminate (pressure sintering process) in this order, and a metal thin film as a sintered body of the conductive paste is formed on the surface of the semiconductor element and joined. Typical sintering conditions are 250 to 300 °C for 3 to 10 minutes, and the atmosphere depends on the paste used, and examples include air, nitrogen, hydrogen, etc. The joining strength of the die bonding formed in this way is required to be at least 30 MPa or more as the die shear strength.

[0004] Here, the sintered body formed from the conductive paste has excellent thermal conductivity, allowing it to efficiently transfer the heat generated when the semiconductor device operates under high voltage and high current to the insulating heat dissipation substrate and dissipate the heat through the cooling fins. Conventionally, high-concentration lead-containing solder paste has been used as the paste for die bonding, but in order to cope with the higher operating temperatures of the device, silver paste, which has better thermal conductivity, has taken its place, and in recent years, copper paste, which can be made more cost-effective, has been attracting attention.

[0005] When copper paste is pressure-sintered under the aforementioned sintering conditions, the process is less efficient than under normal sintering conditions due to the lower temperature and shorter time, making it difficult to achieve the required die shear strength for the product. Therefore, to promote sintering, it has been proposed to prepare copper paste using fine copper particles with a large surface area.

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

[0007] In sintering at such low temperatures, copper particles do not melt completely, resulting in a considerable amount of voids between them. These voids can reduce the die shear strength of the die bond. To further increase the die shear strength of the die bond, a technique has been proposed that uses a mixture of copper particles of different sizes. By using such copper particles, while the smaller particles have high low-temperature sinterability, the combination of large and small particles allows the copper particles to densely fill the copper paste after drying, thereby reducing the voids in the copper sintered body, which are reflected to a certain extent by this structure.

[0008] For example, Patent Document 1 proposes a copper paste using a first flake-shaped copper particle with a particle size of 2 μm or more and a second copper particle with a particle size of 0.8 μm or less. It has been reported that with such a copper paste, the die shear strength of the die bond can exceed 40 MPa at most.

[0009] Japanese Patent Publication No. 2020-020015

[0010] As described above, in low-temperature sintering, by densely packing copper particles into the copper paste after application while it is dry, the copper sintered body can obtain higher thermal conductivity and bonding strength. On the other hand, further improvements are desired regarding the thermal conductivity and bonding strength of the copper paste.

[0011] This disclosure has been made in view of the foregoing, and aims to provide a copper paste for manufacturing a copper sintered body having higher thermal conductivity and bonding strength by more densely packing copper particles in the dry state of the copper paste.

[0012] The inventors diligently conducted research to solve the above-mentioned problems. As a result, they found that a copper paste containing copper powder and a dispersion medium, wherein the volume-based particle size frequency distribution of the copper powder in the copper paste has a first peak showing the highest frequency value among peaks having a peak top in the range of 300 nm to 1 μm, and a second peak showing the highest frequency value among peaks having a peak top in the range of more than 3 μm and 10 μm or less, and the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 or more and 2 or less, allows for denser packing of copper particles in the dry state of the copper paste, enabling the production of a copper sintered body with higher thermal conductivity and bonding strength, and thus completed the technology related to this disclosure. That is, this disclosure provides the following.

[0013] (1) A copper paste comprising copper powder and a dispersion medium, wherein the volume-based particle size frequency distribution of the copper powder in the copper paste has a first peak that shows the highest frequency value among peaks having a peak top in the range of 300 nm to 1 μm, and a second peak that shows the highest frequency value among peaks having a peak top in the range of more than 3 μm and 10 μm or less, and the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 or more and 2 or less.

[0014] (2) The copper paste according to (1), wherein the dispersion medium comprises one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols having a viscosity of 3 mPa·s or more and 70 mPa·s or less at 25°C, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols having a viscosity of 300 mPa·s or more and 1000 mPa·s or less at 25°C.

[0015] According to this disclosure, a copper paste can be provided for manufacturing a copper sintered body having higher thermal conductivity and bonding strength by more densely packing copper particles in the dry state of the copper paste.

[0016] The copper powder according to the embodiments of this disclosure will be described below with reference to specific embodiments, but the copper powder according to this disclosure is not limited in any way to the embodiments described below.

[0017] In this specification, "copper particles" refers to fine particles that primarily contain copper (60 atomic percent or more). "Copper powder" refers to an aggregate state formed by the aggregation of multiple copper particles.

[0018] Furthermore, in this specification, the notation "X to Y" (where X and Y are any numerical values) means "X or greater and Y or less".

[0019] ≪Copper Paste≫ The copper paste according to the embodiment of this disclosure comprises copper powder and a dispersion medium. In the volume-based particle size frequency distribution of the copper powder in the copper paste, the copper paste has a first peak that shows the highest frequency value among peaks having a peak top in the range of 300 nm to 1 μm, and a second peak that shows the highest frequency value among peaks having a peak top in the range of more than 3 μm and 10 μm or less, and the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 or more and 2 or less.

[0020] In the aforementioned Patent Document 1, the particle sizes of two types of copper particles used in copper paste are specified, but these particle sizes are those of the copper particles themselves supplied as raw materials. According to the inventor's knowledge, aggregation occurs depending on the dispersion medium of the copper paste, the state of surface modification of the copper particles, and the particle size of the copper particles and their relationships, and the particle size frequency distribution does not exist as that of the copper particles themselves supplied as raw materials. In particular, when multiple types of copper particles are mixed, more complex aggregation occurs in the copper paste. Therefore, the particle size frequency distribution of copper particles in the paste differs greatly from the particle size frequency distribution of the copper particles themselves supplied as raw materials, and when copper paste is manufactured using a mixture of multiple copper particles, the particle size frequency distribution in the copper paste is not simply a superposition of the particle size frequency distributions of the multiple copper particles (see Examples).

[0021] Based on the above findings, the inventors considered that in order to densely pack copper particles in the dry state before sintering and obtain a dense copper sintered body, it is necessary to further increase the density of copper particles in the dry state of the paste. Therefore, they considered it important to precisely control the particle size of the copper powder used, taking into account not only the particle size of the copper powder supplied as raw material, but also the aggregation of particles in the paste. The inventors then investigated the particle size of copper powder considering aggregation in the paste and found that, in the volume-based particle size frequency distribution of copper powder in the copper paste, a copper paste having a first peak with the highest frequency value among peaks with peak tops in the range of 300 nm to 1 μm, and a second peak with the highest frequency value among peaks with peak tops in the range of more than 3 μm and 10 μm or less, and a ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak being 0.5 or more and 2 or less, results in a more densely packed copper particle in the applied and dried paste, and a more dense copper sintered body after sintering. As a result, the copper powder of this disclosure, when sintered, yields a copper sintered body with higher thermal conductivity and bonding strength.

[0022] (Copper powder) Copper powder is contained in the copper paste, and the sintering of this copper paste forms the framework of the copper sintered body which serves as a conductive path.

[0023] The copper powder used is one in which, in a volume-based particle size frequency distribution when copper particles are dispersed in a copper paste, i.e., in the dispersion medium of the copper paste, the volume-based particle size frequency distribution has a first peak that shows the highest frequency value among peaks with peak tops in the range of 300 nm to 1 μm, and a second peak that shows the highest frequency value among peaks with peak tops in the range of more than 3 μm and 10 μm or less, and the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 or more and 2 or less. The copper powder is not particularly limited as long as it is such, but the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak may be, for example, 0.6 or more and 0.7 or more. On the other hand, the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak may be 1.9 or less, 1.8 or less and 1.7 or less.

[0024] The volume-based particle size frequency distribution of copper powder in the copper paste is measured using laser diffraction scattering particle size distribution analysis. Specifically, after manufacturing the copper paste, the measurement is performed on a dispersion obtained by dispersing the copper paste in methanol. Furthermore, if overlapping peaks such as shoulder peaks are observed in the particle size frequency distribution, peak separation is performed by peak fitting using the least squares method with the assumption of a Gaussian function, using ORIGIN9 from ORIGIN Lab.

[0025] As described above, the copper powder used in the copper paste has at least two peaks, a first peak and a second peak, in the volume-based particle size frequency distribution of the copper powder, but the copper powder exhibiting each peak is not limited. More specifically, copper powder exhibiting the first peak and copper powder exhibiting the second peak may be mixed and used, or copper powder exhibiting the first and second peaks may be used without mixing multiple types of copper powder. Furthermore, the copper powder exhibiting the first peak and the copper powder exhibiting the second peak may be independently aggregated or not aggregated. In addition, copper powder in which a portion exhibits the first peak without aggregation and another portion exhibits the second peak may be used.

[0026] Furthermore, the copper powder may be coated with an organic substance. Depending on its compatibility with the dispersion medium, this allows for control of the dispersion and aggregation states. Moreover, even with the same particles, by changing the presence or absence of surface coating or the type of organic substance, it is possible to control the dispersion and aggregation states and obtain copper powder exhibiting both a first and a second peak from a single type of copper powder.

[0027] Furthermore, it is preferable to use polysaccharides or fatty acid compounds as the organic material used for coating the copper powder. When polysaccharide molecules coat the copper powder, the outer side (the side in contact with the solvent) becomes hydrophilic, and interacts with the hydroxyl groups of the organic solvent in the copper paste to provide appropriate viscosity. On the other hand, fatty acids have carboxyl groups that bind to the surface of the copper particles, and the opposite end of the fatty acid becomes hydrophobic, thereby improving the dispersibility of the copper particles and suppressing particle aggregation. As a result of these effects of polysaccharides and fatty acids, the copper paste can spread evenly across the entire interface with a uniform layer thickness, resulting in excellent bonding strength.

[0028] Specifically, polysaccharides that can be used include, but are not limited to, one or more selected from, for example, gum arabic, carboxymethylcellulose, hydroxyethylcellulose, cellulose nanofiber, starch, glycogen, agarose (agar), pectin, and alginic acid, as well as their salts. Among these, it is preferable to use one or more selected from gum arabic and sodium alginate.

[0029] Specifically, medium-chain fatty acids such as pentanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid can be used as fatty acids. Among these, it is preferable to use one or more selected from octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid.

[0030] The surface coverage of the organic material is not particularly limited, but it is preferable that the coating is applied such that, per 100% by mass of copper powder having a coating layer, the carbon content is 0.05% by mass or more and 0.8% by mass or less, preferably 0.1% by mass or more and 0.5% by mass or less, and the oxygen content is 0.05% by mass or more and 1.5% by mass or less, preferably 0.1% by mass or more and 1% by mass or less. By having carbon and oxygen content above the required amount, the hydrophilicity due to polysaccharide molecules on the surface of the copper powder is sufficiently expressed, the viscosity of the copper paste is reduced, a uniform paste layer is formed, and the resulting sintered body has excellent rigidity. On the other hand, by having carbon and oxygen content below the required amount, it is possible to prevent carbon and oxygen-containing components from remaining inside the sintered body during firing in a nitrogen atmosphere, for example, and to have excellent thermal conductivity and bonding strength.

[0031] The total content of elements other than copper in the copper powder is not particularly limited, but it is preferably, for example, 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, based on 100% by mass of copper powder. Among the components other than copper, metallic elements in particular may worsen sinterability by segregating on the surface of the copper powder or forming oxides, and may also reduce the thermal conductivity of the sintered body by solid dissolution inside the copper powder. If the total content of metallic 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 made lower, and it will also exhibit better thermal conductivity. With such thermal conductivity, for example, heat generated from a power module can be efficiently dissipated to the outside.

[0032] (Dispersion medium) The dispersion medium used in the copper paste according to this disclosure is not particularly limited, and is not particularly limited as long as it is a liquid capable of dispersing copper powder.

[0033] The dispersion medium is not particularly limited, but for example, a solvent with a boiling point of 50°C to 250°C can be used. Examples of such solvents include ethers, ketones, aromatic compounds, terpenes, alcohols, and glycols.

[0034] In one embodiment, one or more dispersion media 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, 2,3-butanediol, 2-ethyl-1,3-hexanediol, glycerol, triethylene glycol monobutyl ether, etc., can be used as the dispersion medium.

[0035] Furthermore, in one embodiment, it is preferable to use an alcohol solvent as the dispersion medium, which is a combination of a monovalent or divalent alcohol with a divalent or trivalent alcohol. If a polyhydric alcohol with a valency of 4 or higher is used as the solvent, the alcohol may remain in the sintered body, especially when sintering is performed at a low temperature of about 300°C or lower and in a reducing or nitrogen atmosphere, which can reduce thermal conductivity and bonding strength. On the other hand, if only a monovalent alcohol is used as the solvent, it is prone to volatilization during storage and printing of the copper paste, which can change the viscosity of the copper paste and worsen its workability. Therefore, in the copper paste according to this embodiment, by combining a monovalent or divalent alcohol with a divalent or trivalent alcohol with a different viscosity, these problems can be avoided, and a copper paste with excellent physical properties and workability, in which the copper powder is uniformly dispersed, is provided. In particular, because it contains a second alcohol with high viscosity, changes from the desired shape due to the paste sagging after application are suppressed, and as will be described later, the viscosity of the copper paste can be adjusted to an appropriate value without binder components such as resin. If the copper paste does not contain resin components, there is no need to consider the generation of carbon residue derived from the resin components, and sintering can be carried out at a relatively low temperature in a non-oxidizing atmosphere.

[0036] Thus, by using at least two types of alcohol as the organic solvent in the dispersion medium, oxidation of the copper powder is suppressed during storage and sintering. The sintered body obtained in this way has a reduced amount of oxides present inside, resulting in excellent thermal conductivity and bonding strength.

[0037] In this specification, "alcohol solvent" means a mixed solvent mainly composed of alcohol, and may also include a small amount of water or an organic solvent other than alcohol, such as a mixed solvent containing 1 to 20% by mass, 2 to 17% by mass, 3 to 15% by mass, 4 to 12% by mass, or 5 to 10% by mass of one or more selected from ethers, ketones, esters, etc. In addition, hydrocarbon solvents and halogenated hydrocarbon solvents may be included, but nitrogen-containing solvents such as amines and amides tend to remain in the dry material, so it is preferable that they are not included, or if included, the amount is 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, 0.7% by mass or less, 0.5% by mass or less, 0.2% by mass or less, or 0.1% by mass or less.

[0038] The total amount of the first and second alcohols is not particularly limited, but it 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, based on 100% by mass of the total amount of copper paste. On the other hand, the total amount of the first and second alcohols 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, based on 100% by mass of the total amount of copper paste. When the total amount of the first and second alcohols is above the required amount, the copper paste can be applied to the entire interface with a uniform layer thickness, resulting in excellent bonding strength. Furthermore, when the total amount of the first and second alcohols is below the required amount, no solvent remains during firing, suppressing a decrease in thermal conductivity and bonding strength.

[0039] 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, based on 100% by mass of the total amount of 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, based on 100% by mass of the total amount of copper powder, the first alcohol and the second alcohol.

[0040] The total amount of the first and second alcohols 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% 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. Since alcohols, especially trivalent alcohols, have a reducing effect, increasing their content in the solvent of the copper paste can more effectively suppress the oxidation of the copper powder.

[0041] The ratio (X / Y) of the mass (X) of the first alcohol to the mass (Y) of the second alcohol in the copper paste is not particularly limited, but it is preferably 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more. On the other hand, the ratio (X / Y) of the mass (X) of the first alcohol to the mass (Y) of the second alcohol is preferably 8 or less, 7 or less, 6 or less, 5 or less. In order to make the bonding strength of the copper paste, for example, the bonding strength between the chip and the substrate, a sufficient value, the copper paste layer needs to be printed on the interface between the chip and the substrate with a substantially uniform thickness. When the ratio (X / Y) is at or above the required value, it becomes a viscosity suitable for coating, and as a result, the bonding strength becomes excellent. When the ratio (X / Y) is at or below the required value, the sinterability of the sintered body obtained by fully expressing the reducing action derived from the alcohol becomes particularly good, and high thermal conductivity and bonding strength can be obtained.

[0042] (The first alcohol) The first alcohol is one or more alcohols 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. If the viscosity of the first alcohol is within such a range, the application of the copper paste becomes easy and good workability is ensured.

[0043] Although the boiling point of the first alcohol is not particularly limited, it 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, 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, 200 °C or lower. When the boiling point of the first alcohol is 150 °C or higher, it is possible to prevent bumping during heating and the generation of voids in the paste, and to prevent a decrease in sinterability, thereby enhancing the thermal conductivity and bonding strength of the sintered body. Further, if the boiling point of the first alcohol is 150 °C or higher, even when the copper paste is stored at room temperature, the solvent will not volatilize and cause a viscosity change in a short period of time. Therefore, there is no need to store it in a refrigerator or freezer, and the storage cost can also be reduced. In this specification, "boiling point" refers to the boiling point at atmospheric pressure unless otherwise specified.

[0044] Although the boiling point of the first alcohol is not particularly limited, it is preferably 50 °C lower than the firing temperature of the copper paste.

[0045] Although the vapor pressure of the first alcohol is not particularly limited, the vapor pressure at around room temperature, for example, 25 °C, is preferably 0.1 Pa or higher, 0.2 Pa or higher, 0.3 Pa or higher, 0.4 Pa or higher, 0.5 Pa or higher, 0.6 Pa or higher, 0.7 Pa or higher, 0.8 Pa or higher, 0.9 Pa or higher, 1 Pa or higher, 1.2 Pa or higher, 1.5 Pa or higher, 1.7 Pa or higher, 2 Pa or higher, 2.2 Pa or higher, 2.5 Pa or higher, 2.7 Pa or higher, 3 Pa or higher. On the other hand, the vapor pressure of the first alcohol is preferably 100 Pa or lower, 90 Pa or lower, 80 Pa or lower, 70 Pa or lower, 60 Pa or lower, 50 Pa or lower, 40 Pa or lower, 30 Pa or lower. When the vapor pressure of the first alcohol is within the required range, the storage stability and printability will be excellent.

[0046] For example, the first alcohol could be 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), or 2-ethyl-1-hexanol (viscosity 6.27 mPa·s, boiling point 1 Monohydric alcohols such as benzyl alcohol (viscosity 5.47 mPa·s, boiling point 205°C, vapor pressure 18 Pa), 85°C, vapor pressure 35 Pa; and dihydric alcohols such as 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) can be used. Among these, it is preferable to use 1-octanol, 2-octanol, 2-ethyl-1-hexanol, ethylene glycol, and 1,2-propanediol as the first alcohol. As the first alcohol, one type may be used alone or a mixture of two or more types, as long as they meet the requirements for the first alcohol described above. Note that viscosity and vapor pressure are both values ​​at 25°C. As described above, since the first alcohol has low viscosity, the viscosity of the copper paste can be adjusted to an appropriate value with a smaller amount added. Therefore, it is possible to reduce the total amount of organic solvent in the copper paste and suppress the residue of organic solvent components during firing.

[0047] (Second alcohol) The second alcohol is one or more alcohols 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. If the viscosity of the second alcohol is within this range, it is possible to prevent the copper paste from becoming sloppy before sintering and making it impossible to form the desired shape, and the workability of the copper paste is not impaired.

[0048] 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. Because the boiling point of the second alcohol is within this range, it does not remain in the gaps between copper particles in the sintered body even after low-temperature firing, and this does not reduce the thermal conductivity. By ensuring that the boiling point of the second alcohol is above the required value, it is possible to prevent bumping during heating, which would create voids in the paste and reduce its sinterability, thereby improving the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the second alcohol is above the required value, the copper paste will not undergo viscosity changes in a short period of time even if stored at room temperature due to solvent evaporation. Therefore, there is no need to store it in a refrigerator or freezer, which can reduce storage costs.

[0049] The boiling point of the second alcohol is not particularly limited, but it is preferably more than 50°C lower than the firing temperature of the copper paste.

[0050] It is preferable to use a second alcohol with a higher boiling point than the first alcohol. The inclusion of a low-viscosity first alcohol results in a copper paste with appropriate viscosity and excellent workability according to this embodiment. However, there is no need to adjust the viscosity after applying the paste, and from the viewpoint of preventing the copper paste from sagging, it is preferable that the first alcohol disappears. On the other hand, among alcohols, dihydric 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 higher boiling point than the first alcohol that evaporates near the firing temperature of the copper paste, good workability can be maintained, and the oxidation of copper powder can be suppressed even more effectively.

[0051] Furthermore, it is preferable that the vapor pressure at room temperature, for example at 25°C, is 1 mPa or more and 5 Pa or less, and more preferably 1.5 Pa or less, because this improves storage stability and further enhances the oxidation suppression effect during firing. This effect is particularly pronounced when the vapor pressure of the second alcohol is lower than that of the first alcohol.

[0052] The vapor pressure of the second alcohol is not particularly limited, but it is preferable that the vapor pressure at room temperature, for example at 25°C, be 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. On the other hand, the vapor pressure of the second alcohol is not particularly limited, but it is preferable that it be 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. By having the vapor pressure of the first alcohol within the required range, storage stability and printability are improved.

[0053] Specifically, as the second alcohol, dihydric alcohols such as 2-ethyl-1,3-hexanediol (viscosity 323 mPa·s, boiling point 244°C, vapor pressure < 1.4 Pa) and trihydric alcohols such as glycerol (viscosity 934 mPa·s, boiling point 290°C, vapor pressure 0.01 Pa) can be used. As long as the second alcohol satisfies the requirements for the second alcohol described above, one type may be used alone or two or more types may be used as a mixture.

[0054] (Other components) In addition to the components described above, the copper paste according to this embodiment may also contain dispersants and surfactants made of amines, antioxidants, reducing agents such as hydrazine, glass frit, binders including resin components, etc.

[0055] The resin component is not particularly limited, but for example, one or more selected from cellulosic resins such as methylcellulose, ethylcellulose, and carboxymethylcellulose, acrylic resins, butyral resins, alkyd resins, epoxy resins, and phenolic resins can be used.

[0056] The resin component content is not particularly limited, but may be greater than 0% by mass, 0.01% or more by mass, 0.05% or more by mass, or 0.1% or more by mass, relative to 100% by mass of copper powder. The resin component content may be 10% or less by mass, 9% or less by mass, 8% or less by mass, 7% or less by mass, 6% or less by mass, 5% or less by mass, 4% or less by mass, 3% or less by mass, 2% or less by mass, 1% or less by mass, 0.5% or less by mass, 0.1% or less by mass, 0.05% or less by mass, or 0.01% or less by mass, relative to 100% by mass of copper particles, but it is preferable that it does not contain copper powder. If the copper paste contains a resin component, the sinterability (especially below 350°C) may deteriorate. For example, thermosetting resins such as epoxy resins remain in the copper paste sintered body even after sintering. Even when using cellulose resins, thermal decomposition begins at around 300°C, but complete decomposition requires heating to 400°C or higher. Furthermore, sintering in an oxygen atmosphere is necessary to remove the carbon residue generated by thermal decomposition, which may cause oxidation of the copper powder. With a copper paste that does not contain resin components, sintering can be performed at a relatively low temperature in a non-oxidizing atmosphere, thus forming a high-density copper sintered body and preventing a decrease in conductivity due to oxidation of the copper powder. In the copper paste according to this embodiment, since it contains a second alcohol with high viscosity, it is possible to adjust the viscosity to an appropriate value even without containing resin components.

[0057] Furthermore, it is preferable that the copper paste does not contain either an epoxy group-containing silane coupling agent or an amino group-containing silane coupling agent, or contains both an epoxy group-containing silane coupling agent and an amino group-containing silane coupling agent in a total amount of more than 0% by mass and 0.05% by mass or less based on 100% by mass of copper powder.

[0058] (Applications of Copper Paste) As described above, the copper paste according to this embodiment has high thermal conductivity, and as a result, high electrical conductivity. Furthermore, the copper paste according to this embodiment can be fired at low temperatures and in a short time, and can exhibit high bonding 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 printed circuit 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 substrate for solar cells or a substrate for mounting electronic components, a printed circuit board, or a substrate with through-holes, and then sintering it. As substrate materials, for example, 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 with a transparent conductive film (TCO) or a metal film on the surface can be used.

[0059] (Manufacturing of copper paste) The copper paste according to this embodiment can be manufactured by mixing the above-mentioned copper powder with a solvent and kneading it using a planetary mixer or other device as needed. Furthermore, the dispersibility of the copper powder can be improved by using a three-roll mill as needed. In addition, the paste may be subjected to filtering or degassing treatment.

[0060] (Casturing of copper paste) When calcining the copper paste according to this embodiment, the method and conditions are not particularly limited and can be carried out by any method depending on the target product and the material to which the paste is applied. However, it is preferable to dry and remove the first alcohol prior to calcining the copper paste according to this embodiment. This increases the proportion of the second alcohol around the copper powder during calcination, thereby more effectively preventing oxidation of the copper powder during calcination. The drying conditions are not particularly limited and can be set arbitrarily depending on the boiling point of the first alcohol and the target product, but it is preferable to heat at 50 to 200°C, particularly 60 to 150°C, for 1 to 60 minutes in an atmospheric environment. Drying can also be carried out under reduced pressure to further lower the heating temperature. Heat drying can also be carried out in an inert gas atmosphere or a reducing atmosphere.

[0061] Since such copper pastes can be fired at low temperatures and for short periods of time, the firing conditions are not particularly limited. For example, by sintering at 150 to 400°C or 200 to 350°C, particularly 250 to 300°C, for 10 seconds to 60 minutes, especially 2 minutes to 30 minutes, under an inert gas atmosphere such as nitrogen or argon gas, or under a reducing atmosphere containing approximately 0.1 to 30 volume percent of hydrogen, ammonia, carbon monoxide, alcohol vapor, etc., a sintered body with excellent thermal conductivity and bonding strength can be obtained.

[0062] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0063] [Evaluation of Sample Properties and Performance] For pastes prepared under the compositions and conditions described below, the particle size frequency distribution of copper powder samples and the physical properties and performance of copper paste samples were evaluated according to the evaluation methods in the following sections.

[0064] (Particle Size Frequency Distribution) The volume-based particle size frequency distribution of copper powder in copper paste was measured by laser diffraction scattering particle size distribution analysis. Specifically, after manufacturing the copper paste, the dispersion obtained by dispersing the copper paste in methanol was measured. In addition, for each copper powder sample in the examples and comparative examples, no overlap of peaks such as shoulder peaks was observed in the particle size frequency distribution, so peak separation was not performed.

[0065] (Measurement of Electrical Resistivity) The prepared paste was stencil printed onto a glass substrate using a metal mask to create a 20 mm square, and then dried in air at 100°C for 5 minutes. Subsequently, pressure sintering was performed in a nitrogen atmosphere using a high-temperature press with a load of 20 MPa, at a heating temperature of 280°C for 2 minutes, to obtain 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 of 1 mm. According to the Wiedemann-Franz law, electrical resistivity and thermal conductivity are inversely proportional; the lower the electrical resistivity, the higher the thermal conductivity. Therefore, electrical resistivity can serve as an indicator of thermal conductivity.

[0066] (Measurement of die shear strength) A copper plate with a thickness of 1 mm was used as a substrate, and copper paste was applied to it to a thickness of 100 μm. A semiconductor chip made of silicon carbide (SiC) with dimensions of 2 mm × 2 mm × 0.4 mm was then placed on top of the paste. On the surface of the SiC chip that was in contact with the copper paste, a Ti layer of 500 nm and a Cu layer of 500 nm were deposited using the sputtering method. The resulting laminate was subjected to a load of 20 MPa using a high-temperature press in a nitrogen atmosphere, and pressure sintering was performed at a firing temperature of 280°C for 3 minutes. In the sample that had cooled to room temperature, the bonding strength between the SiC chip and the copper substrate was measured as die shear strength using a die shear tester (Nordson DAGE4000).

[0067] [Preparation of Samples] Copper paste samples for Examples 1-2 and Comparative Examples 1-3 were prepared according to the following method.

[0068] (Example 1) As copper powder, a mixture was used consisting of copper powder 1, which had an irregular shape with a D50 of 312 nm and was coated with gum arabic; copper powder 2, which had an irregular shape with a D50 of 431 nm; and copper powder 4, which was in flake form with a D50 of 7 μm. Note that copper powders 2 and 4 were not coated with organic matter.

[0069] Copper powder was mixed in a mass ratio of 1:2:4, which was 75:16:9. Ethylene glycol was prepared as the first alcohol and glycerol as the second alcohol. These were weighed and mixed in a planetary mixer in a mass ratio of the sum of all copper powders:ethylene glycol:glycerol of 85:7.5:7.5 to produce a copper paste. The evaluation results are shown in Table 1.

[0070] (Example 2) A copper paste was prepared in the same manner as in Example 1, except that copper powder 3, which has an irregular shape with a D50 of 530 nm, and copper powder 4, which has a flake shape with a D50 of 7.0 μm, were mixed as copper powder in a mass ratio of copper powder 3:copper powder 4 = 91:9. Note that copper powder 3 and copper powder 4 were not coated with organic matter. The evaluation results are shown in Table 1.

[0071] (Comparative Example 1) A copper paste was prepared in the same manner as in Example 1, except that a third copper powder with an irregular shape and a D50 of 530 nm, and a flake-shaped copper powder 5 with a D50 of 11.9 μm were used as copper powders, mixed in a mass ratio of copper powder 3:copper powder 5 = 91:9. Note that copper powders 3 and 5 were not coated with organic matter.

[0072] (Comparative Example 2) A copper paste was prepared in the same manner as in Example 2, except that the mass ratio of copper powder 3 to copper powder 4 was changed to copper powder 3:copper powder 4 = 80:20. Note that copper particles 4 and copper powder 3 were not coated with organic matter. The evaluation results are shown in Table 1.

[0073] (Comparative Example 3) A copper paste was prepared in the same manner as in Example 2, except that the mass ratio of copper powder 3 to copper powder 4 was changed to copper powder 3:copper powder 4 = 95:5. Note that copper powder 3 and copper powder 4 were not coated with organic matter. The evaluation results are shown in Table 1.

[0074]

Claims

1. A copper paste comprising copper powder and a dispersion medium, wherein the volume-based particle size frequency distribution of the copper powder in the copper paste has a first peak showing the highest frequency value among peaks having a peak top in the range of 300 nm to 1 μm, and a second peak showing the highest frequency value among peaks having a peak top in the range of more than 3 μm and 10 μm or less, and the ratio of the frequency value at the peak top of the second peak to the frequency value at the peak top of the first peak is 0.5 or more and 2 or less.

2. The copper paste according to claim 1, wherein the dispersion medium comprises one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols having a viscosity of 3 mPa·s or more and 70 mPa·s or less at 25°C, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols having a viscosity of 300 mPa·s or more and 1000 mPa·s or less at 25°C.

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