Copper paste

Abstract

A copper paste containing copper powder and an organic solvent, the aforementioned organic solvent being an alcohol-based solvent, the alcohol-based solvent containing: a first alcohol that is one or more selected from the group consisting of monohydric and dihydric alcohols having a viscosity of 3 mPa·s or more and 70 mPa·s or less at 20°C; and a second alcohol that is one or more selected from the group consisting of dihydric and trihydric alcohols having a viscosity of 300 mPa·s or more and 1000 mPa·s or less at 20°C.

Description

TECHNICAL FIELD

[0001] The present application relates to a novel copper paste. BACKGROUND

[0002] Electrically conductive pastes can be used to form wiring in electronic components such as chip resistors, chip capacitors, solar cells, and the like, and electronic mounting products such as printed wiring boards, substrates with through-holes, and the like. In addition, they can also be used as electrodes connected to transistors for controlling pixel switches of displays, wiring, and power modules for operating motors and the like with high power efficiency. In power modules, semiconductor chips such as silicon, silicon carbide, gallium nitride, and the like are bonded to heat-dissipating substrates, but if solder is used as the bonding material, the low thermal conductivity results in poor heat dissipation of the heat generated by the power module to the outside, and high-efficiency operation at high temperatures cannot be performed. Therefore, the use of electrically conductive pastes is particularly important in power modules.

[0003] Current electrically conductive pastes use silver pastes that have excellent oxidation resistance, but silver is not only expensive but also has the problem of easily causing migration defects in fine-pitch wiring. Therefore, in recent years, attempts have been made to use copper pastes that are low in cost and have good thermal conductivity. However, while copper is less expensive than silver, on the other hand, copper has the property of being easily oxidized, and if a reducing gas such as hydrogen, formic acid, an inactive gas such as nitrogen, or the like is not used when firing the copper paste, there is a problem of the copper being oxidized, and the dispersion stability and electrical conductivity being reduced. From the viewpoints of printability of the paste, densification of the wiring, and the like, a copper powder having a fine particle diameter is required as a component, but the oxidation problem of such a copper powder is particularly significant. In addition, the improvement of the bonding strength under conditions of lower temperature and shorter time for the purpose of reducing the number of manufacturing steps and reducing costs is also considered a problem. In order to solve these problems, various technologies for preventing oxidation of the copper powder and improving the dispersion have been studied.

[0004] For example, in Patent Literature 1, collagen peptide-coated copper nanoparticles are disclosed, and it is reported that these copper nanoparticles have excellent oxidation resistance and dispersion stability. In Patent Literature 2, a technology for improving the dispersion of metal fine particles by using a solvent composed of an amide-based organic solvent mixed with an amine-based organic solvent and an alcohol as a solvent component of a paste is described. In Patent Literature 3, a copper paste containing a copper powder having an average particle diameter of 0.1 to 1 μm and an alcohol-based solvent is disclosed. In addition, in Non-Patent Literature 1, a paste is produced by coating the surface of a fine copper particle with a gelatin layer, and oxidation firing in air and reduction firing in N2 and 3% H2 gas are performed to form a sintered body having low resistance.

[0005] PRIOR ART DOCUMENTS

[0006] Patent Literature

[0007] Patent Literature 1: Japanese Patent No. 5450725

[0008] Patent Literature 2: Japanese Patent No. 6097477

[0009] Patent Literature 3: Japanese Patent Application Laid-Open No. 2016-53216

[0010] Non-Patent Literature

[0011] Non-Patent Literature 1: Yonezawa et al., RSC Advances, 2015, 5, 61290-61297 (2015) SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] However, in the copper paste described in Patent Literature 1 and Non-Patent Literature 1, since the collagen peptide and gelatin are coated on the surface of the copper particles, sometimes the sintering of the copper particles is insufficient. As a result, there is a problem that the bonding between the sintered copper particles becomes weak and the mechanical strength as a wiring cannot be maintained. As described above, for the conductive paste, in recent years, sintering in a low temperature and short time and high bonding strength are required, for example, firing under the conditions of 300°C or lower and 10 minutes or less is required, and the bonding strength is required to be increased to 40 MPa or more, but the copper paste described in Patent Literature 1 and Non-Patent Literature 1 cannot satisfy the above requirements. For the copper paste described in Patent Literature 2, sometimes the amide-based organic solvent as the main solvent remains in the wiring after firing, resulting in a decrease in conductivity. In addition, the deterioration of the operation environment caused by the odor generated from the amine-based organic solvent becomes a problem. For the paste described in Patent Literature 3, oxidation of the copper powder at the time of firing cannot be sufficiently prevented, and sometimes problems such as a change in viscosity at the time of storage due to volatilization of the low-boiling-point solvent and the presence of the high-boiling-point solvent remain.

[0014] In view of the above problems, an object of the present application is to provide a copper paste that exhibits good oxidation resistance, has high conductivity and thermal conductivity, and is excellent in storage stability and handleability, and in particular, a copper paste that can be fired under low temperature and short time conditions and exhibits high bonding strength.

[0015] MEANS FOR SOLVING THE PROBLEMS

[0016] The present inventors have conducted intensive studies, and as a result, have found that by using two or more types of alcohol different in type in combination as a dispersion medium in a copper paste, a copper paste that can suppress oxidation of the copper powder at the time of firing, does not cause a change in viscosity at the time of storage, and is excellent in handleability can be obtained, thereby completing the present application. That is, the present application is a copper paste of (1) to (7) below.

[0017] (1) A copper paste containing copper powder and an organic solvent,

[0018] The organic solvent is an alcohol-based solvent, and the alcohol-based solvent contains:

[0019] a first alcohol selected from one or more kinds of monohydric and dihydric alcohol having a viscosity of 3 mPa-s or more and 70 mPa-s or less at 20°C; and

[0020] a second alcohol selected from one or more kinds of dihydric and trihydric alcohol having a viscosity of 300 mPa-s or more and 1000 mPa-s or less at 20°C.

[0021] (2) The copper paste according to the above (1), characterized in that, in the alcohol-based solvent,

[0022] the first alcohol has a boiling point of 150°C or more and 240°C or less at an atmospheric pressure,

[0023] and,

[0024] the second alcohol has a boiling point of 190°C or more and 320°C or less at an atmospheric pressure,

[0025] and,

[0026] the boiling point of the first alcohol is lower than the boiling point of the second alcohol.

[0027] (3) The copper paste according to any one of the above (1) to (2), wherein a ratio (X / Y) of a mass (X) of the first alcohol to a mass (Y) of the second alcohol in the organic solvent is 0.2 or more and 8.0 or less.

[0028] (4) The copper paste according to any one of the above (1) to (3), wherein the first alcohol is one or more kind of alcohol selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, hexanediol, 1,2-propanediol, 1,3-propanediol, and ethylene glycol, and

[0029] the second alcohol is one or more kind of alcohol selected from the group consisting of 2-ethyl-1,3-hexanediol and glycerol.

[0030] (5) The copper paste according to any one of the above (1) to (4), which substantially does not contain a resin component.

[0031] (6) The copper paste according to any one of the above (1) to (5), wherein the copper powder has a coating layer containing at least one compound selected from polysaccharides and fatty acids on at least a part of the surface.

[0032] (7) The copper paste according to (6), wherein, with respect to 100 mass% of the copper powder, the copper powder having the coating layer contains 0.05 mass% or more and 0.8 mass% or less of carbon, and 0.05 mass% or more and 1.5 mass% or less of oxygen.

[0033] Effects of the Invention

[0034] According to the present application, a copper paste which exhibits excellent oxidation resistance, has high electric conductivity and thermal conductivity, is stable in storage, and is excellent in handleability can be provided. The copper paste of the present application can also be fired under low temperature and short time conditions, and can exhibit high joint strength. DETAILED DESCRIPTION

[0035] Hereinafter, an embodiment of the present application will be described. Note that the present application is not limited by the description of the embodiment.

[0036] The copper paste of the present application contains a copper powder and an organic solvent,

[0037] The aforementioned organic solvent is an alcohol-based solvent, and the alcohol-based solvent contains:

[0038] a first alcohol selected from one or more kinds selected from the group consisting of monohydric and dihydric alcohols having a viscosity of 3 mPa-s or more and 70 mPa-s or less at 20°C; and

[0039] a second alcohol selected from one or more kinds selected from the group consisting of dihydric and trihydric alcohols having a viscosity of 300 mPa-s or more and 1000 mPa-s or less at 20°C.

[0040] Here, the use of two or more kinds of alcohols having different types of dispersion medium is an important element of the present application. Thereby, a copper paste which can inhibit oxidation of the copper powder during firing, and which is excellent in handleability and does not change in viscosity during storage can be provided. Note that, in the present specification, the "viscosity" in the case of the aforementioned organic solvent is a Newtonian fluid, and the viscosity is independent of the shear rate, and thus refers to the viscosity at an arbitrary shear rate. On the other hand, in the case of the copper paste, it is a non-Newtonian fluid, and thus refers to the viscosity of the copper paste at a shear rate of 1 sec -1 Hereinafter, each component in the copper paste of the present application will be described in detail.

[0041] (alcohol-based solvent)

[0042] As described above, the organic solvent in the copper paste of the present application is an alcohol-based solvent obtained by combining two or more alcohols having different viscosities from monohydric to trihydric. If a polyhydric alcohol having four or more hydroxyl groups is used as the solvent, it is sometimes left in the sintered body when firing is performed at a low temperature of about 300°C or lower in a reducing atmosphere or a nitrogen atmosphere, resulting in a decrease in the electrical conductivity and the bonding strength. If only a monohydric alcohol is used as the solvent, there is a problem that the alcohol is easily volatilized during storage and printing of the copper paste, the viscosity of the copper paste is changed, and thus the handleability is deteriorated. According to the present application, not only these problems can be avoided, but also by using such alcohols having different viscosities as described above, a copper paste having excellent properties and handleability, in which the copper powder is uniformly dispersed, can be provided. In particular, since the second alcohol having a high viscosity is contained, a change from the desired shape due to dripping of the paste after application can be suppressed, and as described later, even if a binder component such as a resin is not contained, the viscosity of the copper paste can be adjusted to an appropriate value. If the copper paste does not contain a resin component, generation of carbon residues from the resin component does not need to be considered, and firing can be performed at a lower temperature in a non-oxidizing atmosphere.

[0043] In the present application, the "alcohol-based solvent" means a mixed solvent in which alcohol is the main component, and can also include a mixed solvent containing a small amount of water, an organic solvent other than alcohol, for example, an ether, a ketone, an ester, or the like, in an amount of about 1 to 20 mass%, and particularly about 5 to 10 mass%. In addition, although a hydrocarbon solvent, a halogenated hydrocarbon solvent, or the like can be contained, a nitrogen-containing solvent such as an amine or an amide has a tendency to be easily left in the dried solid, and thus it is preferred that it is not contained or is contained in an amount of about 5 mass% or less. The total mass of the first alcohol and the second alcohol is preferably 70 mass% or more, more preferably 80 mass% or more, further preferably 90 mass% or more, and particularly preferably 95 mass% or more, of the total solvent in the copper paste. Since alcohol, and particularly a trihydric alcohol, has a reducing action, by increasing the content of the copper paste in the solvent, oxidation of the copper powder can be more effectively suppressed.

[0044] The first and second alcohols in the present application can be any alcohol as long as they have the number of hydroxyl groups and the viscosity described above, and an alcohol having a boiling point of 150°C or higher is preferred. Note that in this specification, the "boiling point" means the boiling point at atmospheric pressure unless otherwise specified. If the boiling point of the alcohol is lower than 150°C, boiling can occur when heated, and voids can be generated in the paste, resulting in deterioration of the sinterability. On the other hand, if an alcohol having a boiling point of 150°C or higher is used, the copper paste can be fired without these problems, and the electrical conductivity and the thermal conductivity of the sintered body can be improved. In addition, if the boiling point of the alcohol is 150°C or higher, even if the copper paste is stored at room temperature, the solvent will not volatilize and cause a change in the viscosity in a short period of time. Therefore, the copper paste does not need to be stored by refrigeration or freezing, and the storage cost can be reduced.

[0045] In order to make the antioxidation effect of the copper powder in the present application more remarkable, as the second alcohol, an alcohol having a higher boiling point than the first alcohol is preferably selected. By containing the first alcohol having a low viscosity, the viscosity of the copper paste of the present application is appropriate and the handleability is good, and after the paste is applied, there is no need to adjust the viscosity, and from the viewpoint of preventing dripping of the copper paste, the first alcohol is rather preferably absent. On the other hand, among alcohols, di- and tri-, especially tri- alcohols have a high reducing effect, and therefore, for the second alcohol, it is preferable to be present at a high concentration at the time of firing. Therefore, as the second alcohol, by using an alcohol having a higher boiling point than the first alcohol, which evaporates in the vicinity of the firing temperature of the copper paste, it is possible to maintain good handleability while more effectively inhibiting oxidation of the copper powder. In a more preferable mode of the present application, it is preferable that the boiling point of the above-mentioned first alcohol be 150°C or higher and 250°C or lower, especially 240°C or lower, and the boiling point of the second alcohol be 190°C or higher and 320°C or lower.

[0046] In the copper paste of the present application, the content of the first alcohol and the second alcohol is not particularly limited, and the ratio (X / Y) of the mass (X) of the first alcohol to the mass (Y) of the second alcohol in the organic solvent is preferably 0.2 or more and 8.0 or less, and particularly preferably 0.5 or more and 5.0 or less. In order to make the joining strength of the copper paste, for example, the die shear strength of the chip and the substrate, be a sufficient value, it is necessary to print the copper paste layer at a substantially uniform thickness at the interface of the chip and the substrate. If the above-mentioned ratio is 0.2 or more, appropriate tackiness is easily obtained, and a sufficient joining strength can be obtained. If it is 8.0 or less, the reducing effect is sufficiently exhibited, the sinterability becomes particularly good, and high conductivity and joining strength can be obtained. In addition, the content of the organic solvent (alcohol-based solvent) is also not particularly limited, and can be arbitrarily set depending on the viscosity of the copper paste targeted, and is 5 mass% or more and 40 mass% or less, particularly 8 mass% or more and 20 mass% or less, with respect to the total amount of the copper paste 100 mass%, which is preferable in terms of the viscosity of the copper paste formed. If the solvent concentration is 5 mass% or more or so, the copper paste can be spread over the entire interface with a uniform layer thickness, and good joining strength is exhibited. In addition, if the solvent concentration is 40 mass% or less or so, the solvent does not remain at the time of firing, and does not cause a decrease in conductivity or joining strength.

[0047] (first alcohol)

[0048] As described above, the first alcohol in the present application is one or more kinds of alcohol selected from the group consisting of monohydric and dihydric alcohols having a viscosity of 3 mPa-s or more and 70 mPa-s or less. If the viscosity of the first alcohol is within this range, the application of the copper paste becomes easy, and good workability can be ensured. In addition, the boiling point of the first alcohol is preferably 150°C or more. It is further preferable to use an alcohol having a boiling point that is more than 50°C lower than the firing temperature of the copper paste. Note that the firing temperature of the copper paste is not particularly limited, but in the case of general joining applications, it is around 250 to 300°C, and thus the boiling point of the first alcohol in the present application is preferably 150°C or more and 250°C or less, further preferably 150°C or more and 240°C or less, with a range of higher than 150°C and 230°C or less being preferred, and a range of 170°C or more and 200°C or less being particularly preferred. In addition, when the vapor pressure at around room temperature, for example, 20°C, is 0.1 Pa or more and 100 Pa or less, further 1 Pa or more and 50 Pa or less, and particularly 3 Pa or more and 30 Pa or less, the storage stability and workability become even better, and thus this is preferable. As specific examples of such a first alcohol, monohydric alcohols such as 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), 2-ethyl-1-hexanol (viscosity: 6.27 mPa-s, boiling point: 185°C, vapor pressure: 35 Pa), benzyl alcohol (viscosity: 5.47 mPa-s, boiling point: 205°C, vapor pressure: 18 Pa), and the like; 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), 2,3-butanediol (viscosity: 45 mPa-s, boiling point: 182°C, vapor pressure: <100 Pa), hexanediol (viscosity: 34.4 mPa-s, boiling point: 197°C, vapor pressure: 19 Pa), and the like, but are not limited to these. It can also be a mixture of two or more kinds of these alcohols. Note that the above-mentioned viscosity and vapor pressure are values at 20°C or 25°C. In the present application, it is particularly preferable to use 1-octanol, 2-octanol, 2-ethyl-1-hexanol, ethylene glycol, 1,2-propanediol, and hexanediol as the first alcohol. As described above, these first alcohols have low viscosity, and thus the viscosity of the copper paste can be adjusted to an appropriate value by adding a small amount. Thus, the total amount of organic solvent in the copper paste can be reduced, and the residual amount of the organic solvent component at the time of firing can be suppressed.

[0049] (2nd alcohol)

[0050] As described above, the 2nd alcohol in the present application is one or more alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity of 300 mPa-s or more and 1000 mPa-s or less. If the viscosity of the 2nd alcohol is within this range, the dripping of the copper paste before sintering can be prevented so that the desired shape can be formed, and in addition, the workability of the copper paste is not impaired. In addition, the boiling point of the 2nd alcohol is preferably 190°C or more, and more preferably an alcohol having a temperature of the value obtained by subtracting 50°C from the sintering temperature of the copper paste or more is used. In view of the sintering temperature of a general copper paste, 200°C or more is preferable, and 240°C or more is particularly preferable. The upper limit of the boiling point is not particularly limited, but in view of the workability, the heat resistance of the coating object such as a substrate, and the like, 320°C or less is preferable, and 300°C or less is further preferable. If the alcohol has a boiling point within the above range, even after low-temperature sintering, the copper particles remaining in the interstices in the sintered body are not present, and thus the electrical conductivity is not reduced. In addition, when the vapor pressure at around room temperature, for example, 20°C, is 1 mPa or more and 5 Pa or less, further 1.5 Pa or less, and particularly 1 Pa or less, not only the storage stability becomes better, but also the oxidation inhibition effect at the time of sintering is further improved, and thus this is preferable. This effect is particularly significant when the vapor pressure of the 2nd alcohol is lower than the vapor pressure of the 1st alcohol. As specific examples of such a 2nd 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 given, but are not limited to these. A mixture of these alcohols can also be used.

[0051] (Copper powder)

[0052] The copper powder contained in the copper paste of the present application is not particularly limited, and can be any of various commercially available products and the like. However, in the present application, the content of elements other than copper in the copper powder is preferably 1 mass% or less in total with respect to 100 mass% of the copper powder. The components other than copper, particularly metal elements, sometimes deteriorate the sinterability due to segregation on the surface of the copper powder or formation of oxides, and are solid-solved in the inside of the copper powder to reduce the electrical conductivity of the sintered body. If the content of elements other than copper, particularly As, Co, Cr, Fe, Ir, P, S, Sb, Se, Te, Ti, V, Zr, and the like is 1 mass% or less, the electrical resistivity of the copper paste sintered body can be 5 μΩcm or less or so, and a thermal conductivity of 130 W / m-K or more or so is exhibited. If the thermal conductivity is such, heat generated by a power module, for example, can be efficiently dissipated to the outside. If the content of impurities, particularly the above elements, is 0.5 mass% or less, the electrical resistivity is 4 μΩcm or less or so, and the thermal conductivity is 167 W / m-K or more or so, and thus this is more preferable.

[0053] Such copper powder can be produced, for example, using the high-pressure water atomization method described in International Publication No. 99 / 11407, the wet reduction deposition method described in International Publication No. 2014 / 80662, or the like. The high-pressure water atomization method is a method in which, in a method of producing a metal powder (e.g., copper) from molten metal, the falling stream of the molten metal is passed through the center portion of a nozzle through which a gas flows, and the molten metal is broken up by the gas near the outlet of the nozzle, and then the broken-up molten metal is further broken up by liquid that is emitted in an inverted conical shape. According to this method, by continuously applying the breaking up by the gas and the breaking up by the liquid to the molten metal, a metal powder (e.g., copper) having a fine particle diameter and a spherical or granular shape and having a low oxygen content can be produced on an industrial scale and at a low cost. In addition, the wet reduction deposition method is a method in which, in the reduction of copper ions in a wet type using a reducing agent such as hydrazine, an organic solvent that is compatible with water and that can reduce the surface tension of water is used as a solvent. Specifically, it is a method in which a reaction solution containing monovalent or divalent copper ions and a reducing agent are mixed in water and the organic solvent as a liquid medium, and the copper ions are reduced to generate copper particles. Generally, particles of 0.7 μm or more can be produced by the high-pressure water atomization method. The wet reduction deposition method is suitable for producing finer particles than that.

[0054] The average particle diameter of the copper powder (copper particles) is preferably 0.05 μm or more and 2.0 μm or less. By making the average particle diameter of the copper particles 2.0 μm or less, there is a tendency for the surface area of the copper particles to increase relatively and for sintering under low-temperature conditions to become easy. On the other hand, when the average particle diameter of the copper particles is less than 0.05 μm, the price of the copper particles as a raw material is high, and there is a tendency for the copper wiring formed by the method of the present application to be unable to become a low-price alternative to silver wiring. In addition, if the average particle diameter is less than 0.05 μm, there is a tendency for a plurality of particles to agglomerate and to exhibit sintering properties substantially equivalent to those of a paste composed of coarse particles. Here, the average particle diameter refers to the 50% particle diameter (d50) measured using a laser particle size distribution meter or the like. The average particle diameter is the central value in the distribution of particle diameters measured using a laser particle size distribution meter or the like. More preferably, a copper powder having an average particle diameter of 0.08 μm or more and 1.0 μm or less, and particularly preferably 0.3 μm or more and 0.7 μm or less, is used. 50

[0055] ​The copper powder (copper particles) can have a substance coated on the surface within a range that does not affect sinterability. However, the gelatin layer described in Non-Patent Literature 1 has a tendency to be insufficiently sintered, and thus the copper powder in the present application is preferably coated copper powder from which the gelatin layer is removed. In the present application, the copper powder preferably has a coating layer formed of a polysaccharide or a compound of a fatty acid on at least a part of the surface. When the copper powder is coated with a polysaccharide molecule, the outside (the side that contacts a solvent) becomes hydrophilic, and thus interacts with the hydroxyl group of the organic solvent in the copper paste to bring about moderate viscosity. On the other hand, the carboxyl group of the fatty acid binds to the surface of the copper particles, and the terminal on the opposite side of the fatty acid becomes hydrophobic, and thus the dispersibility of the copper particles can be improved to suppress aggregation of the particles. As a result of these effects of the polysaccharide and the fatty acid, the copper paste can spread over the entire interface with a uniform layer thickness, and exhibit good joint strength. As examples of the polysaccharide, arabic gum, carboxymethyl cellulose, hydroxyethyl cellulose, cellulose nanofiber, starch, glycogen, agarose (agar), pectin, and alginic acid, and salts thereof, and the like can be given, but are not limited thereto, and a sulfur-containing polysaccharide such as carrageenan can also be used. Among these, arabic gum and sodium alginate are particularly preferable. As examples of the fatty acid, medium-chain fatty acids such as valeric acid, caprylic acid, nonanoic acid, capric acid, dodecanoic acid, and tetradecanoic acid can be given, and among these, caprylic acid, nonanoic acid, and capric acid are particularly preferable.

[0056] The surface coating area ratio of these coating layers is not particularly limited, and with respect to 100 mass% of the copper powder having the coating layer, the copper powder is preferably coated at a ratio such that the carbon content becomes 0.05 mass% or more and 0.8 mass% or less, and the oxygen content becomes 0.05 mass% or more and 1.5 mass% or less. If the carbon content or the oxygen content is less than 0.05 mass%, the hydrophilicity brought about by the polysaccharide molecule on the surface of the copper powder sometimes fails to be sufficiently exhibited, the viscosity of the copper paste decreases, a uniform paste layer is difficult to form, and the joint strength decreases. If the carbon content exceeds 0.8 mass% or the oxygen content exceeds 1.5 mass%, components containing carbon and oxygen sometimes remain inside the sintered body at the time of firing under a nitrogen atmosphere, for example, and the electrical conductivity or the joint strength decreases. It is more preferable to coat at a ratio such that the carbon content becomes 0.1 to 0.5 mass% and the oxygen content becomes 0.1 to 1.0 mass%. The surface coating area ratio and the carbon content and the oxygen content of the fatty acid are the same as those of the polysaccharide.

[0057] (Production of the Copper Paste)

[0058] The copper paste of the present application can be produced by mixing the above-described copper powder and a solvent, and kneading as necessary using a device such as a planetary mixer. In addition, it is also preferable to use a three-roll mill to improve the dispersibility of the copper powder as necessary. Note that the viscosity of the copper paste is not particularly limited, and can be arbitrarily set according to the target use. For example, by setting the shear rate to 1 sec -1The copper paste of the present application has a viscosity of 30 Pa-s or more and 2000 Pa-s or less, or 100 Pa-s or more and 1000 Pa-s or less, particularly 150 Pa-s or more and 800 Pa-s or less, at around room temperature, for example, at 25°C. The copper paste is easily uniformly applied to a substrate or the like, and the thermal conductivity and the joining strength of the copper sinter obtained can be further improved.

[0059] (Other components)

[0060] The copper paste of the present application can contain, in addition to the above components, a dispersant containing an amine, a surfactant, an antioxidant, a reducing agent such as hydrazine, a glass frit, a binder such as a resin component, and the like. As the resin component, for example, a cellulose-based resin such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, or the like, an acrylic resin, a butyral resin, an alkyd resin, an epoxy resin, a phenol resin, or the like, can be contained at about 0.05 to 5 mass% relative to the mass of the copper particles.

[0061] However, the copper paste of the present application preferably contains substantially no resin component. If the copper paste contains a resin component, the sinterability, particularly the sinterability at 350°C or lower, can sometimes be deteriorated. For example, a thermosetting resin such as an epoxy resin has a tendency to remain in the copper paste sinter even after sintering. In the case of using a cellulose resin, although thermal decomposition starts at around 300°C, complete thermal decomposition requires a temperature of 400°C or higher, and, in order to remove carbon residues generated by thermal decomposition, firing under an oxygen atmosphere is required, thereby causing a problem that the copper powder is oxidized. If the copper paste contains no resin component, firing can be performed under a non-oxidizing atmosphere at a lower temperature, and thus a high-density copper sinter can be formed without causing a decrease in the electrical conductivity due to oxidation of the copper powder. In the copper paste of the present application, since the second alcohol having a high viscosity is contained, the viscosity can be adjusted to an appropriate value even without a resin component.

[0062] (Use of the copper paste)

[0063] As described above, the copper paste of the present application has high electric conductivity and thermal conductivity, and is also excellent in terms of storage stability and workability. The copper paste of the present application can also be fired at low temperature and in a short time, and can exhibit high joining strength. Therefore, it is suitable for use in formation of wiring in electronic components such as power modules, chip resistors, chip capacitors, solar cells, and the like, and electronic mounting products such as printed wiring boards, substrates having through-holes, and the like. For example, the copper paste of the present application can be applied to a power module, a substrate for a solar cell, a substrate on which an electronic mounting product is mounted, a printed wiring board, a substrate having a through-hole, and the like, and fired. Here, as a substrate material, a silicon substrate, a silicate glass, an oxide substrate such as alumina, quartz, a nitride substrate such as silicon nitride, aluminum nitride, a carbide substrate such as silicon carbide, titanium carbide, a resin substrate such as polyimide, polyethylene terephthalate, polyethylene naphthalate, and the like, and a substrate having a transparent conductive film (TCO), a metal film on the surface, and the like can be used.

[0064] (Firing of the copper paste)

[0065] When the copper paste of the present application is fired, the method and conditions are not particularly limited, and any method can be used 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 before firing the copper paste of the present application. By doing so, the presence ratio of the second alcohol around the copper powder at the time of firing is increased, and thus oxidation of the copper powder during firing can be more effectively prevented. The drying conditions are not particularly limited, and can be arbitrarily set depending on the boiling point of the first alcohol used and the target product, and heating is preferably performed at a temperature of 50 to 200°C, particularly 60 to 150°C, for 1 to 60 minutes in an atmospheric atmosphere. Drying can also be performed under reduced pressure, further reducing the heating temperature. Heating and drying can also be performed in a reducing atmosphere.

[0066] The copper paste of the present application can be fired at low temperature and in a short time, and thus the firing conditions can also be set in a wide range. For example, by performing firing in a non-active gas atmosphere such as nitrogen or argon, or in a reducing atmosphere containing hydrogen, ammonia, carbon monoxide, alcohol, or the like at a content of about 0.1% by volume to about 30% by volume, at a temperature of 150°C or higher and 400°C or lower, or 200°C or higher and 350°C or lower, particularly 250°C or higher and 300°C or lower, for a time of 10 seconds or longer and 60 minutes or shorter, particularly 2 minutes or longer and 30 minutes or shorter, a sintered body having high strength and excellent electric conductivity and thermal conductivity can be produced.

[0067] Example

[0068] Hereinafter, the present application will be further described in detail by way of examples, but the present application is not limited by these examples.

[0069] (Example 1)

[0070] A copper powder (50% particle diameter: about 0.4 μm, carbon content: 0.3 mass%, oxygen content: 0.7 mass%, amount of metal elements other than copper: 0.2 mass%) coated with gum arabic as a polysaccharide, ethylene glycol as the first alcohol, and glycerin as the second alcohol were mixed at a mass ratio of 87.0:6.5:6.5 to prepare a copper paste. Here, the mass ratio refers to the mass of each component when the weight of the copper paste is taken as 100.

[0071] For the prepared paste, the viscosity (η0) was measured within 2 hours after preparation using a dynamic viscoelastometer (Rheometer manufactured by Brookfield). Further, the viscosity (η7) after 7 days of storage at 10°C in the atmosphere was measured, and the rate of change in viscosity over time was calculated. The rate of change in viscosity (%) was defined as (η7-η0) / η0x 100.

[0072] The paste was applied to a glass substrate in the form of a square with a side length of 20 mm using a metal mask, dried at 60°C for 5 minutes in the atmosphere, and then pressure-baked at 280°C for 2 minutes in a nitrogen atmosphere using a high-temperature press machine with a load of 20 MPa to produce a copper paste sintered body with a thickness of about 20 μm. For the sintered body, the resistivity was measured using a direct current 4-probe resistance measuring device in which the probe interval had been set to 1 mm.

[0073] In addition, instead of a glass substrate, a copper plate with a thickness of 1 mm was used as a substrate, and the copper paste was applied to the copper plate in such a manner that the thickness became 100 μm. Further, a semiconductor chip formed of silicon carbide (SiC) with a size of 2 mm x 2 mm x 0.4 mm was disposed on the copper paste. On the surface of the SiC chip in contact with the copper paste, a Ti layer was formed in a thickness of 500 nm and a Cu layer was formed in a thickness of 500 nm using a sputtering method. For the thus obtained laminate, pressure-baking was performed at a baking temperature of 280°C for 2 minutes in a nitrogen atmosphere using a high-temperature press machine with a load of 20 MPa. In the sample cooled to room temperature, the adhesion strength between the SiC chip and the copper substrate was measured as the chip shear strength using a chip shear tester (DAGE 4000 manufactured by Nordson).

[0074] For the above measurement results, the viscosity change rate of 10% or less, the resistivity of 5 μΩcm or less, and the chip shear strength of 40 MPa or more were respectively evaluated as acceptable, and the case where all items were acceptable was determined as determination A, the case where two items were acceptable was determined as determination B, and the case where one item was acceptable or all items were not acceptable was determined as determination C. Further, in the results of determination A, the case where the chip shear strength was 70 MPa or more was determined as determination AA.

[0075] The results of the measurement and the determination are shown in the column of Example 1 in Table 1. The determination result is AA.

[0076] (Examples 2 to 7, Comparative Examples 1 to 3)

[0077] The copper paste was produced using the same method as in Example 1 except that the kind of the first alcohol was changed, and the experiment was performed. The results of the measurement and the determination are shown in Table 1 together with the kind of the first alcohol.

[0078] [Table 1]

[0079] [Table 1 Properties of the first alcohol and the copper paste]

[0080]

[0081] In Examples 1 to 7 in which the first alcohol having a viscosity in the range of 3 mPa-s or more and 70 mPa-s or less was used in combination with the second alcohol, excellent results of AA, A or B based on the above-mentioned criteria were obtained according to the present application. Further, in Examples 1, 3 to 5 and 7 in which the alcohol having a boiling point of 150°C or more and 240°C or less and a vapor pressure of 3 Pa or more and 30 Pa or less at 20°C was used as the first alcohol, AA or A was determined, and more preferable results were obtained.

[0082] (Example 8)

[0083] In the experiment of Table 1, the same experiment was performed in which the surface coating layer of the copper powder was replaced from the polysaccharide arabic gum to capric acid which is one of the fatty acids, and as a result, the viscosity of the copper paste produced within 2 hours after the production was reduced by about 8 to 25% compared to the case of the copper powder coated with arabic gum, the coating thickness of the copper paste became about 60 to 85 μm, and the same results as the determination in Table 1 were obtained except for this.

[0084] From this, it was found that the surface coating layer of the copper powder is not limited to polysaccharides, but can also be a medium-chain fatty acid.

[0085] (Example 9, Comparative Examples 4 and 5)

[0086] In the above-mentioned Example 1, the first alcohol was made to be ethylene glycol, the kind of the second alcohol was changed, and the copper paste was produced under the same conditions and the experiment was performed. The results are shown in Table 2.

[0087] [Table 2]

[0088] [Table 2 Properties of the second alcohol and the copper paste]

[0089]

[0090] In Examples 1 and 9 in which a dihydric or trihydric alcohol having a viscosity of 300 mPa-s or more and 1000 mPa-s or less was used as the second alcohol according to the present application, AA or A was determined, and a preferable result was obtained. On the other hand, in Comparative Examples 4 and 5 in which an alcohol having a viscosity outside the above range was used, C was determined. It is considered that in Examples 1 and 9, by making the boiling point of the second alcohol 240°C or more, a further preferable result was determined. In addition, in Example 1 in which glycerin having a vapor pressure of 1 Pa or less at 20°C was used, AA was determined, and a particularly good result was obtained.

[0091] (Examples 10 to 13, Comparative Examples 6 and 7)

[0092] In the above Example 1, the first alcohol was made to be ethylene glycol, the second alcohol was made to be glycerin, the mass ratio (X / Y) of each was changed, and, other than that, a copper paste was produced and an experiment was conducted under the same conditions. The results are shown in Table 3. In Table 3, the standards for passing were the same as in Tables 1 and 2. Since the evaluation items this time were two, a case in which both of the two items passed was determined to be A, and a case in which one or both of the items failed was determined to be C. The standards for determining AA were the same as in Tables 1 and 2.

[0093] [Table 3]

[0094] [Table 3 Mass ratio of first alcohol (X) to second alcohol (Y) and physical properties of copper paste]

[0095]

[0096] If X / Y is 0.2 (1 / 5) or more and 8 (8 / 1) or less, the resistivity and the die shear strength become AA or A, and it is known that a preferable result can be obtained. In particular, in a case in which X / Y is 0.5 (1 / 2) or more and 5 (5 / 1) or less, the die shear strength shows a very high value of 70 MPa or more, AA is determined, and a more preferable result is obtained.

[0097] (Examples 14 to 17, Comparative Examples 8 to 11)

[0098] The amount of carbon and the amount of oxygen in the gum arabic-coated copper powder were changed as in Table 4, and, other than that, the same operation as in Example 1 was performed with the amount of metal impurities in the copper powder being 0.2 mass%. The composition of each copper paste sample and the test results are shown in Table 4.

[0099] [Table 4]

[0100] [Table 4 Amount of carbon and amount of oxygen in gum arabic-coated copper powder and physical properties of copper paste]

[0101]

[0102] *Unit: mass % relative to 100 mass % of copper powder having an arabic gum coating layer

[0103] In any of the copper pastes, the die shear strength was over 30 MPa, but the copper powder having a small amount of carbon and oxygen had a tendency to soften the paste viscosity. On the other hand, as the amount of carbon and oxygen increased, there was a tendency to harden the paste viscosity. In Examples 14 to 17 in which the coated copper powder having an amount of carbon of 0.05 to 0.8 mass % and an amount of oxygen of 0.05 to 1.5 mass % was used, it was shown that a low resistivity of less than 4 μΩcm and a high die shear strength of over 40 MPa were exhibited.

[0104] As described above, the copper paste of the present application has high electric conductivity, excellent storage stability, and exhibits high bonding strength. The copper paste of the present application also shows good oxidation resistance, has high thermal conductivity, and can be fired at low temperature and in a short time.

Claims

1. Copper paste, which contains copper powder and organic solvents. The copper paste does not actually contain any resin components. The organic solvent is an alcohol-based solvent, and the alcohol-based solvent contains: The first alcohol is one or more mono- and di-alcohols selected from the group consisting of alcohols with a viscosity of 3 mPa·s or more and 70 mPa·s or less at 20 °C; and The second alcohol is selected from one or more binary and ternary alcohols with a viscosity at 20°C of 300 mPa·s or more and 1000 mPa·s or less. The copper powder has a coating containing polysaccharides on at least a portion of its surface.

2. The copper paste as described in claim 1, characterized in that, In the alcohol-based solvent, The boiling point of the first alcohol at atmospheric pressure is above 150°C and below 240°C. and, The boiling point of the second alcohol at atmospheric pressure is above 190°C and below 320°C. and, The boiling point of the first alcohol is lower than that of the second alcohol.

3. The copper paste as described in claim 1 or 2, wherein, The ratio (X / Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) in the organic solvent is 0.2 or more and 8.0 or less.

4. The copper paste as described in claim 1 or 2, wherein, The first alcohol is one or more alcohols selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, hexanediol, 1,2-propanediol, 1,3-propanediol, and ethylene glycol. The second alcohol is one or more alcohols selected from the group consisting of 2-ethyl-1,3-hexanediol and glycerol.

5. The copper paste as described in claim 1 or 2, wherein, The coating also contains fatty acids.

6. The copper paste as described in claim 1 or 2, wherein, Relative to 100% by mass of the copper powder, the copper powder having the coating layer contains 0.05% by mass and less than 0.8% by mass of carbon and 0.05% by mass and less than 1.5% by mass of oxygen.

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