Copper powder and copper paste

Abstract

To provide a copper paste capable of yielding a copper sintered body having low electric resistivity and excellent bonding strength by low-temperature sintering, and a copper powder usable for the same.　A copper paste according to the present disclosure contains a polyether having a hydroxyl group at one end of the chain, and has, in a volume-based particle size frequency distribution, a first peak that indicates the highest frequency value among peaks having a peak top in the range of 50-500 nm, and a second peak that indicates the highest frequency value among peaks having a peak top in the range of more than 500 nm to 10 μm and at least 100 nm larger than the peak top of the first peak. The ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak is 5-60.

Description

Copper powder and copper paste

[0001] This disclosure relates to copper powder and copper paste.

[0002] Submicron-sized copper powder with a particle size of 1 μm or less may be used, when incorporated into a conductive paste, as a material for the internal and external electrodes of multilayer ceramic capacitors or inductors and other electronic components, as well as in the manufacture of inkjet wiring.

[0003] Conductive pastes containing copper powder (hereinafter referred to as "copper paste") are used for purposes such as forming circuits and bonding semiconductor elements to substrates. They are printed onto substrates, heated, and the copper powder is sintered. The copper powder used in copper paste may require low-temperature sintering, i.e., low-temperature sinterability. Copper powder that sinters at low temperatures has the advantage of lower heating costs compared to copper powder that sinters at high temperatures, and can also be used on substrates with low heat resistance.

[0004] Generally, in the sinterability of copper paste, low-temperature sintering can result in insufficient sintering of the copper powder, leaving a large amount of voids within the copper sintered body. On the other hand, if the copper powder is sufficiently melted, the molten copper attempts to fill these voids, leading to a greater thermal shrinkage rate compared to the pre-sintered state, which can cause cracks. When voids or cracks occur in the copper sintered body in this way, electrical resistance may increase, or the bonding strength with substrates may decrease.

[0005] Thus, in order to suppress the occurrence of voids and cracks in the metal sintered body, reduce electrical resistance, and increase bonding strength with substrates, etc., multiple metal particles of different particle sizes are combined and used in copper paste. In such copper paste, even in the dry state before sintering, the gaps between the large particles are filled by the small particles, reducing voids and resulting in a copper sintered body with low electrical resistance and high bonding strength.

[0006] For example, Patent Document 1 discloses a conductive paste comprising large-particle powder with a median diameter D50 greater than 5.0 μm and 20 μm or less, medium-particle powder with a median diameter D50 greater than 1.5 μm and 5.0 μm or less, and small-particle powder with a median diameter D50 of 0.1 μm or more and 1.5 μm or less. The blending ratio of the three types of conductive powders is such that, when the total is 100 parts by mass, the medium-particle powder is 4 parts by mass or more and less than 50 parts by mass, and the small-particle powder is 0.1 parts by mass or more and less than 20 parts by mass, and the shrinkage initiation temperature of the large-particle and medium-particle powders is higher than the debindering temperature. Copper is given as an example of the conductive powder. Furthermore, Patent Document 1 also discloses that the resistance value of the sintered body can be suppressed with such a conductive paste.

[0007] Japanese Patent Publication No. 2019-102240

[0008] In recent years, copper paste has attracted attention as a die bond material for joining semiconductor elements to insulating heat dissipation substrates, even in power semiconductors operating under high voltage and high current. Such copper pastes require lower electrical resistivity, as well as higher bonding strength from the perspective of ensuring reliability, and there is still room for improvement in terms of both electrical resistivity and bonding strength.

[0009] Here, according to the inventors' findings, particularly with small-diameter copper particles, aggregation can occur, resulting in coarse particles that may not adequately fill the gaps between larger-diameter copper particles. Patent Document 1 uses a binder resin such as ethyl cellulose, which surrounds the copper particles in the paste and suppresses aggregation to a certain extent, but does not strongly suppress aggregation. Furthermore, in order to avoid such binder resin remaining in the copper sintered body, heating to around 600°C is required for decomposition, making it difficult to take advantage of the benefits of low-temperature sintering.

[0010] This disclosure has been made in view of the above circumstances, and aims to provide a copper paste and copper powder that can be used therein, which can be used to obtain a copper sintered body with low electrical resistivity and excellent bonding strength by low-temperature sintering.

[0011] The inventors diligently conducted research to solve the above-mentioned problems. As a result, they found that a copper paste using copper powder containing a polyether having a hydroxyl group at one end of the chain, having a first peak with the highest frequency value among peaks having a peak top in the range of 50 nm to 500 nm in the volume-based particle size frequency distribution, and a second peak with the highest frequency value among peaks having a peak top in the range of greater than 500 nm and less than or equal to 10 μm, and 100 nm or more larger than the peak top of the first peak, and having a ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak to the frequency value at the peak top of the second peak to a copper paste using copper powder with a lower electrical resistivity and superior bonding strength can be obtained by sintering at a lower temperature, and have completed the technology relating to this disclosure. That is, this disclosure provides the following.

[0012] [1] A copper powder comprising a polyether having a hydroxyl group at one end of its chain, wherein, in a volume-based particle size frequency distribution, it has a first peak showing the highest frequency value among peaks having a peak top in the range of 50 nm to 500 nm, and a second peak showing the highest frequency value among peaks having a peak top in the range of greater than 500 nm and 10 μm or less, and 100 nm or more greater than the peak top of the first peak, and the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak is 5 to 60.

[0013] [2] The polyether is the copper powder described in [1], which is represented by the following formula (1) and / or formula (2). (1) RO(C ２ H ４ O) ｎ H (In formula (1), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.) (2) RO(C ３ H ６ O) ｎ H (In formula (2), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.)

[0014] [3] A copper paste comprising the copper powder described in [1] or [2] and a dispersion medium.

[0015] According to the present disclosure, it is possible to provide a copper paste capable of obtaining a copper sintered body with low electrical resistivity and excellent bonding strength by sintering at a lower temperature, and a copper powder that can be used therefor.

[0016] Hereinafter, the copper powder according to the embodiment of the present disclosure will be described with specific embodiments, but the copper powder of the present disclosure is not limited to the following embodiments at all.

[0017] In the present specification, the "copper particles" mainly refer to fine particles containing copper (copper is 60 atomic% or more). The "copper powder" refers to a state of an aggregate formed by gathering a plurality of copper particles.

[0018] Also, in the present specification, the notation "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

[0019] ≪Copper Powder≫ The copper powder according to the embodiment of the present disclosure contains a polyether having a hydroxyl group at one chain end, and in the particle size frequency distribution based on volume, among the peaks having a peak top in the range of 50 nm or more and 500 nm or less, the first peak showing the value of the maximum frequency, and a second peak having a peak top in the range of more than 500 nm and 10 μm or less and 100 nm or more larger than the peak top of the first peak, and the ratio of the value of the frequency at the peak top of the first peak to the value of the frequency at the peak top of the second peak is 5 or more and 60 or less.

[0020] Such copper powder can be coated with a polyether having a hydroxyl group at one end of the chain. Since the hydroxyl group of this polyether is attracted to the surface of the copper particles, the polyether is modified with the other end facing outward from the copper particles. This modification with polyether can strongly suppress the aggregation of copper particles. Here, the copper powder has a volume-based particle size frequency distribution, with a first peak showing the highest frequency value among peaks having a peak top in the range of 50 nm to 500 nm, and a second peak showing the highest frequency value among peaks having a peak top in the range of greater than 500 nm and less than or equal to 10 μm, and at least 100 nm larger than the peak top of the first peak. This means that there are at least two types of particle sizes in the copper powder. Thus, because there are at least two types of particle sizes in the copper powder, when these are used in a copper paste, in the dry state of the copper paste, the gaps between the copper particles showing the second peak, which have a larger particle size, are filled with copper particles showing the first peak, which have a smaller particle size. In this process, the modification of copper particles with polyether ensures good dispersion and suppresses aggregation in the copper paste. When the copper paste is dried in this state to remove the dispersion medium, smaller copper particles more densely fill the gaps between larger particles in the dried paste, reducing the amount of resulting gaps. This allows for the formation of a dense copper sintered body. Furthermore, it was found that, surprisingly, when using a mixture of copper particles exhibiting a first peak and copper particles exhibiting a second peak, such polyether can be removed without leaving any residue such as carbon after combustion, even during low-temperature sintering.

[0021] Furthermore, the waveform may have peaks other than the first peak in the range of 50 nm to 500 nm, peaks other than the second peak in the range of over 500 nm and up to 10 μm, and peaks other than the first and second peaks in other ranges.

[0022] In the volume-based particle size frequency distribution of the copper powder, the range indicating the peak top of the first peak is not particularly limited, but it is preferably 50 nm or more, more preferably 100 nm or more, still more preferably 150 nm or more, and particularly preferably 200 nm or more. On the other hand, the range indicating the peak top of the first peak is preferably 700 nm or less, more preferably 650 nm or less, still more preferably 600 nm or less, and particularly preferably 550 nm or less. The volume-based particle size frequency distribution of the copper powder is measured by a laser diffraction scattering type particle size distribution analysis method. When there is an overlap of peaks such as a shoulder peak in the particle size frequency distribution, peak separation is performed by using ORIGIN9 of ORIGIN LAB Corporation, assuming a Gaussian function, and performing peak fitting by the least squares method. The same applies hereinafter.

[0023] In the volume-based particle size frequency distribution of the copper powder, the range indicating the peak top of the second peak is not particularly limited as long as it is more than 500 nm and 10 μm or less, and 100 nm or more larger than the peak top of the first peak, but it is preferably 500 nm or more, more preferably 600 nm or more, still more preferably 700 nm or more, further preferably 800 nm or more, particularly preferably 900 nm or more, and most preferably 1 μm or more. On the other hand, the range indicating the peak top of the second peak is preferably 1 μm or less, more preferably 9 μm or less, still more preferably 7 μm or less, particularly preferably 5 μm or less, and most preferably 3 μm or less.

[0024] In the volume-based particle size frequency distribution of the copper powder, the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak (frequency at the peak top of the first peak / frequency at the peak top of the second peak) is not particularly limited as long as it is 5 or more and 60 or less, but it is preferably 5 or more, more preferably 6 or more, and even more preferably 7 or more. On the other hand, the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak is preferably 60 or less, more preferably 55 or less, and even more preferably 50 or less. By having the ratio as described above, copper particles showing the first peak with a smaller particle size are more densely filled in the gaps between copper particles showing the second peak with a larger particle size.

[0025] The oxygen content of the copper powder is not particularly limited as long as it is 0.50% by mass or more and 10% by mass or less, but it is preferably 0.55% by mass or more, more preferably 0.60% by mass or more, even more preferably 0.65% by mass or more, and particularly preferably 0.70% by mass or more. On the other hand, the oxygen content of the copper powder may be 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, or 5% by mass or less. Oxygen is mainly assumed to be derived from the surface of copper particles and from polyether, but it may be derived from the surface of copper particles, from polyether, or from other sources. Also, the oxygen content is measured by the inert gas fusion-infrared absorption method. Specifically, a TC600 type manufactured by LECO is used as the oxygen-nitrogen analyzer, a steel pin is used for the calibration curve, and the copper powder is put into a nickel capsule for measurement.

[0026] The BET specific surface area of the copper powder is not particularly limited, but for example, it is preferably 0.1 m ２ / g or more, 0.2 m ２ / g or more, 0.5 m ２ / g. On the other hand, the BET specific surface area of the copper powder is ２It can be set to less than / g. The BET specific surface area of ​​copper nanoparticles can be measured and calculated using Macsorb HM model-1201 (Mountec Co., Ltd.).

[0027] [Polyether] Polyethers are a general term for polymer compounds having ether links as the main chain, and in this disclosure, they are particularly those having a hydroxyl group at one end of the chain.

[0028] The other end of the chain (opposite the end with the hydroxyl group) preferably has a hydrophobic group. Examples of hydrophobic groups include alkyl groups, hydrocarbon groups having 1 to 3 double bonds, phenyl groups having alkyl groups as side chains, alkyl-substituted or unsubstituted phenyl groups, alkyl-substituted or unsubstituted phenylene groups, naphthyl groups, bisphenol A skeleton, bisphenol F skeleton, castor oil skeleton, hydrogenated castor oil skeleton, lanolin alcohol ether chain, rosin acid ester chain, fatty acid ester chain, alkylamide chain, etc. Examples of hydrocarbon groups having 1 to 3 double bonds include -C ｎ H ２ｎ－１ Base (where n is an integer greater than or equal to 2, for example, n is between 8 and 20), -C ｎ H ２ｎ－３ Base (where n is an integer greater than or equal to 2, for example, n is between 8 and 20), -C ｎ H ２ｎ－５ It can be a group (where n is an integer greater than or equal to 3, for example n is between 8 and 20), and can be an alkenyl group, for example.

[0029] Specifically, it is preferable to use a polyether represented by the following formula (1) and / or formula (2): (1) RO(C) ２ H ４ O) ｎ H (In formula (1), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.) (2) RO(C ３ H ６ O) ｎ H (In formula (2), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.)

[0030] More specifically, as a compound of general formula (1), H ２ｍ＋１ C ｍ O(C) ２ H ４ O) ｎ Examples include polyoxyethylene alkyl ethers of H (m: integer from 4 to 25). Also, as a compound of general formula (2), H ２ｍ＋１ C ｍ O(C) ３ H ６ O) ｎ Examples include polyoxypropylene alkyl ethers of H (m: integer from 4 to 25). Also, H is a compound in which the polyoxyethylene moiety and the polyoxypropylene moiety are combined. ２ｍ＋１ C ｍ O(C) ２ H ４ O) ｎ (C ３ H ６ O) ｌ Examples include H polyoxyethylene polyoxypropylene alkyl ethers.

[0031] The polyether contained in the copper powder preferably has an alkyl chain (i.e., a linear or branched alkyl group) at the end opposite the hydroxyl group. Having an alkyl chain improves the dispersibility of the copper powder in the paste. This is because the alkyl chain is a hydrophobic functional group and therefore has good affinity with the dispersion medium, which is also a hydrophobic organic solvent. For this reason, when copper powder having an alkyl chain is mixed with an organic solvent, it is thought that the dispersion state of the copper powder can be maintained for a long time.

[0032] Copper powder may contain elements other than copper as additive elements to exhibit some function, or as impurity elements. In such cases, the total content of elements other than copper is not particularly limited, but it is preferable that it be, for example, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.2% by mass or less, or 0.1% by mass or less, relative to 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 copper particles 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, Zr is below the required amount, the electrical resistivity of the copper paste sintered body can be made lower, and it will also exhibit better thermal conductivity.

[0033] Furthermore, copper powder may contain elements other than copper as additives or impurities to exhibit some function. In such cases, the total content of elements other than copper is not particularly limited, but it is preferably, for example, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.2% by mass or less, or 0.1% by mass or less, based on 100% by mass of copper powder.

[0034] ≪Copper Paste≫ The copper paste according to the embodiment of this disclosure comprises the copper powder described above and a dispersion medium.

[0035] [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.

[0036] 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.

[0037] 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.

[0038] Furthermore, in one embodiment, an alcohol dispersion medium can be used that combines monovalent or divalent alcohols of different viscosities with a divalent or trivalent alcohol. If a polyhydric alcohol with a valency of 4 or higher is used as the dispersion medium, the alcohol may remain in the sintered body, especially when sintering is performed at low temperatures of around 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 dispersion medium, 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. By combining monovalent or divalent alcohols of different viscosities with a divalent or trivalent alcohol, 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 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.

[0039] In this specification, "alcohol dispersion medium" means a mixed dispersion medium mainly composed of alcohol, and may also include a small amount of water or an organic dispersion medium other than alcohol, such as a mixed dispersion medium 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, other dispersion media such as hydrocarbon dispersion media and halogenated hydrocarbon dispersion media may be included, but since nitrogen-containing dispersion media such as amines and amides tend to remain in the dry matter, it is preferable not to include other dispersion media, or if they are included, the content should be 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.

[0040] 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 dispersion medium remains during firing, suppressing a decrease in thermal conductivity and bonding strength.

[0041] 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.

[0042] 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, and 99.99% by mass or more, based on 100% by mass of the total dispersion medium in the copper paste. On the other hand, the total amount of the first and second alcohols is preferably 100% by mass or less, 99.99% by mass or less, 99.9% by mass or less, 99% by mass or less, 98% by mass or less, 97% by mass or less, and 95% by mass or less. Since alcohols, especially trivalent alcohols, have a reducing effect, increasing their content in the dispersion medium of the copper paste can more effectively suppress the oxidation of copper particles.

[0043] The ratio of the mass of the first alcohol to the mass of the second alcohol (mass of the first alcohol / mass of the second alcohol) is not particularly limited, but is preferably 0.2 or more, 0.3 or more, 0.4 or more, or 0.5 or more. On the other hand, the ratio of the mass of the first alcohol to the mass of the second alcohol is preferably 8 or less, 7 or less, 6 or less, or 5 or less. In order to achieve a sufficient bonding strength of the copper paste, it is desirable that the copper paste layer be printed at the interface between the chip and the substrate with a substantially uniform thickness. When the ratio of the mass of the first alcohol to the mass of the second alcohol is below the required value, the viscosity becomes suitable for application, and as a result, the bonding strength is excellent. Furthermore, when the ratio of the mass of the first alcohol to the mass of the second alcohol is below the required value, the reducing effect derived from the alcohol is fully expressed, resulting in particularly good sinterability of the sintered body, and high thermal conductivity and bonding strength are obtained.

[0044] [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 this range, the copper paste will be easy to apply and good workability will be ensured.

[0045] The boiling point of the first alcohol is not particularly limited, but is preferably 150°C or higher, 155°C or higher, 160°C or higher, 165°C or higher, 170°C or higher, 175°C or higher, 180°C or higher, 185°C or higher, or 190°C or higher. On the other hand, the boiling point of the first alcohol is preferably 250°C or lower, 245°C or lower, 240°C or lower, 235°C or lower, 230°C or lower, 225°C or lower, 220°C or lower, 215°C or lower, 210°C or lower, 205°C or lower, or 200°C or lower. By having a boiling point of 150°C or higher for the first alcohol, it is possible to prevent bumping during heating, which would create voids in the paste and reduce sinterability, thereby increasing the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the first alcohol is 150°C or higher, even if the copper paste is stored at room temperature, the dispersion medium will not volatilize and cause a change in viscosity in a short period of time. Therefore, there is no need to store the product in a refrigerator or freezer, which can reduce storage costs. In this specification, "boiling point" refers to the boiling point at atmospheric pressure.

[0046] The boiling point of the first alcohol is not particularly limited, but it is preferably more than 50°C lower than, for example, the firing temperature of the copper paste.

[0047] The vapor pressure of the first alcohol is not particularly limited, but it is preferable that the vapor pressure at room temperature, for example at 25°C, is 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, or 3 Pa or higher. On the other hand, it is preferable that the vapor pressure of the first alcohol is 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, or 30 Pa or lower. By having the vapor pressure of the first alcohol within the required range, storage stability and printability are improved.

[0048] 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 dispersion medium in the copper paste and suppress the residue of organic dispersion medium components during firing.

[0049] [Second Alcohol] The second alcohol 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 at 25°C. 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 it does not impair the workability of the copper paste.

[0050] 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 dispersion medium will not volatilize and cause a rapid change in viscosity even when the copper paste is stored at room temperature. Therefore, there is no need to store it under refrigeration or freezing, which can reduce storage costs.

[0051] 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.

[0052] 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 disclosure. 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] [Other Components] In addition to the components described above, the copper paste relating to this disclosure may also contain dispersants and surfactants made of amines, antioxidants, reducing agents such as hydrazine, glass frit, resin components and other binders.

[0057] The resin component is not particularly limited, but for example, one or more can be selected from cellulosic resins such as methylcellulose, ethylcellulose, and carboxymethylcellulose, acrylic resins, butyral resins, alkyd resins, epoxy resins, phenolic resins, etc.

[0058] When the copper paste contains a resin component, the resin component content is not particularly limited, but for example, it may be more than 0% by mass, 0.01% or more by mass, 0.02% or more by mass, 0.05% or more by mass, 0.07% 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, 0.7% or less by mass, 0.5% or less by mass, 0.2% or less by mass, 0.1% or less by mass, 0.07% or less by mass, 0.05% or less by mass, 0.02% or less by mass, or 0.01% or less by mass, relative to 100% by mass of copper particles, but it is preferable that the copper paste according to this disclosure does not contain a resin component. 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 cellulose resins are used, although thermal decomposition begins at around 300°C, complete thermal decomposition requires heating at temperatures above 400°C, and furthermore, sintering in an oxygen atmosphere is required 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 carried out at a relatively low temperature in a non-oxidizing atmosphere, so a high-density copper sintered body can be formed, and a decrease in conductivity due to oxidation of copper powder is not caused. In the copper paste according to this disclosure, since a second alcohol with high viscosity is contained, it is possible to adjust the viscosity to an appropriate value even without containing resin components.

[0059] Furthermore, the copper paste may contain epoxy group-containing silane coupling agents and amino group-containing silane coupling agents in total amounts of more than 0% by mass and 0.05% by mass or less, more than 0% by mass and 0.04% by mass or less, more than 0% by mass and 0.03% by mass or less, more than 0% by mass and 0.02% by mass or less, or more than 0% by mass and 0.01% by mass or less, based on 100% by mass of copper powder, but it is preferable that it does not contain any of these.

[0060] [Applications] As described above, the copper paste according to the embodiments of this disclosure has low electrical resistivity. Furthermore, the copper paste according to the embodiments of this disclosure can be fired at low temperatures and in a short time, and can exhibit high bonding strength. For this reason, 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 disclosure 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 having a transparent conductive film (TCO) or a metal film on the surface can be used.

[0061] [Manufacturing of Copper Paste] The copper paste according to this disclosure can be manufactured by mixing the above-mentioned copper powder with a dispersion medium and kneading the mixture using a planetary mixer or other device as needed. The dispersibility of the copper powder can also be improved by using a three-roll mill as needed. Furthermore, the paste may be subjected to filtering or degassing treatment.

[0062] [Casturing of Copper Paste] When calcining the copper paste according to this disclosure, 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, when the first alcohol and the second alcohol described above are used as the dispersion medium, it is preferable to dry and remove the first alcohol prior to calcining the copper paste. This increases the proportion of the second alcohol present 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 according to 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.

[0063] 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 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.

[0064] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0065] [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.

[0066] (Particle Size Frequency Distribution) The volume-based particle size frequency distribution of the copper powder samples was measured by laser diffraction scattering particle size distribution analysis. For the copper powder samples in the examples and comparative examples, peak separation was not performed because no overlapping peaks, such as shoulder peaks, were observed in the particle size frequency distribution.

[0067] (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.

[0068] (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).

[0069] [Preparation of Samples] Copper powder samples and copper paste samples for Examples 1 to 6 and Comparative Examples 1 to 7 were prepared according to the following method.

[0070] (Example 1) As copper powder, an irregularly shaped copper powder with a peak top position of 305 nm in the volume-based particle size frequency distribution, an irregularly shaped copper powder with a peak top position of 1.2 μm, and polyether RO(C) ２ H ４ O) ｎ H (where R is a cetyl group or an oleyl group, and n is 2 to 30) was mixed and prepared so that the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak was 20, thereby obtaining a copper powder sample.

[0071] Ethylene glycol as the first alcohol and glycerol as the second alcohol were prepared. The copper powder samples were weighed and mixed in a planetary mixer in a mass ratio of 85:7.5:7.5 to produce the copper paste sample of Example 1. The evaluation results are shown in Table 1.

[0072] (Example 2) The copper powder sample and copper paste sample of Example 2 were prepared in the same manner as in Example 1, except that instead of using irregularly shaped copper powder with a peak top position of 305 nm, irregularly shaped copper powder with a peak top position of 124 nm was used.

[0073] (Example 3) The copper powder sample and copper paste sample of Example 3 were prepared in the same manner as in Example 1, except that instead of using irregularly shaped copper powder with a peak top position of 1.2 μm, irregularly shaped copper powder with a peak top position of 5.0 μm was used as the copper powder.

[0074] (Example 4) The copper powder sample and copper paste sample of Example 4 were prepared in the same manner as in Example 1, except that the ratio of irregularly shaped copper powder with a peak top position of 305 nm to irregularly shaped copper powder with a peak top position of 1.2 μm was changed so that the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak was 10.

[0075] (Example 5) The copper powder sample and copper paste sample of Example 5 were prepared in the same manner as in Example 1, except that the ratio of irregularly shaped copper powder with a peak top position of 305 nm to irregularly shaped copper powder with a peak top position of 1.2 μm was changed to adjust the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak to 50.

[0076] (Example 6) RO(C) as polyether ３ H ６ O) ｎ Except for using H (where R is a cetyl group or an oleyl group, and n is 2 to 30), the copper powder sample and copper paste sample of Example 6 were prepared in the same manner as in Example 1.

[0077] (Comparative Example 1) A copper powder sample and a copper paste sample of Comparative Example 1 were prepared in the same manner as in Example 1, except that only irregularly shaped copper powder with a peak top position of 1.2 μm was used as the copper powder.

[0078] (Comparative Example 2) Except for using only irregularly shaped copper powder with a peak top position of 305 nm as the copper powder, the copper powder sample and copper paste sample of Comparative Example 2 were prepared in the same manner as in Example 1.

[0079] (Comparative Example 3) Except for using irregularly shaped copper powder with a peak top position of 455 nm and irregularly shaped copper powder with a peak top position of 519 nm as the copper powder, the copper powder sample and copper paste sample of Comparative Example 3 were prepared in the same manner as in Example 1.

[0080] (Comparative Example 4) The copper powder sample and copper paste sample of Comparative Example 4 were prepared in the same manner as in Example 1, except that the ratio of irregularly shaped copper powder with a peak top position of 305 nm to irregularly shaped copper powder with a peak top position of 1.2 μm was changed so that the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak was 2.

[0081] (Comparative Example 5) The copper powder sample and copper paste sample of Comparative Example 5 were prepared in the same manner as in Example 1, except that the ratio of irregularly shaped copper powder with a peak top position of 305 nm to irregularly shaped copper powder with a peak top position of 1.2 μm was changed so that the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak was 80.

[0082] (Comparative Example 6) RO(C) as a polyether ２ H ４ O) ｎ The copper powder sample and copper paste sample of Comparative Example 6 were prepared in the same manner as in Example 1, except that H (where R is a cetyl group or an oleyl group, and n is 2 to 30) was not used.

[0083] (Comparative Example 7) RO(C) as a polyether ２ H ４ O) ｎ The copper powder sample and copper paste sample of Comparative Example 7 were prepared in the same manner as in Example 1, except that glycerin was used instead of H (where R is a cetyl group or an oleyl group, and n is 2 to 30).

[0084] Table 1 shows the peak position indicating the peak top of the first peak, the peak position indicating the peak top of the second peak, the difference between the peak position indicating the peak top of the first peak and the peak position indicating the peak top of the second peak, the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak (indicated as "first peak / second peak" in the table), and the type of additive, die shear strength (MPa), and electrical resistivity (μΩcm) of the copper paste sample using the copper powder sample.

[0085]

Claims

1. Copper powder comprising a polyether having a hydroxyl group at one end of its chain, wherein, in a volume-based particle size frequency distribution, it has a first peak showing the highest frequency value among peaks with peak tops in the range of 50 nm to 500 nm, and a second peak showing the highest frequency value among peaks with peak tops in the range of greater than 500 nm and 10 μm or less, and 100 nm or more greater than the peak top of the first peak, and the ratio of the frequency value at the peak top of the first peak to the frequency value at the peak top of the second peak is 5 to 60.

2. The polyether is the copper powder according to claim 1, represented by the following formula (1) and / or formula (2). (1) RO(C) ２ H ４ O) ｎ H (In formula (1), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.) (2) RO(C ３ H ６ O) ｎ H (In formula (2), R is a saturated or unsaturated hydrocarbon of C4 to C25, and n is an integer from 2 to 30.) 3. A copper paste comprising the copper powder according to claim 1 or 2 and a dispersion medium.

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