Copper-containing metal foil and method for producing a copper-containing metal foil

Abstract

The invention concerns a metal foil produced by the following steps: a) Providing a copper formate, a solvent and a foil of copper or of a copper alloy or of a nickel-iron alloy plated on both sides with copper which foil has a first surface and a second surface, b) mixing the copper formate and the solvent for obtaining a mixture in which the copper formate is completely dissolved, c) applying the mixture obtained in step b) to the first surface and the second surface of the foil provided at step a), d) heating to a temperature in the range of 175 °C to 300 °C and maintaining the temperature until a layer comprising copper nanoparticles is formed on the first surface and the second surface of the foil provided at step a) for obtaining the coated metal foil, wherein at least steps d) is performed in a reducing atmosphere or in an inert atmosphere, wherein the copper nanoparticles have an arithmetic average particle diameter in the range of 20 nm to 200 nm.

Description

[0001] Copper-containing Metal Foil and Method for Producing a Copper-containing

[0002] Metal Foil

[0003] The present invention relates to a copper-containing metal foil, a method for producing a copper-containing metal foil and a use of the copper-containing metal foil.

[0004] Die-attach bonding is a key process to realize high-temperature operation of power semiconductor devices. High Pb solders have been a preferred and well established choice over the past decades. However strict regulations concerning hazardous substances restrict the usage of high Pb solders and the few exceptions that exist today are also foreseen to be banned in the near future. With the increase in the usage of wide-bandgap (WBG) semiconductor devices, it is imperative to find sustainable and reliable alternatives both on the economical and the technical fronts. Interconnect materials and technologies need to be able to fulfill the challenging requirements of the WBG semiconductor devices and at the same time need to be economical for enabling mass production.

[0005] Currently, there are two established methods to produce high-temperature bonds while benefiting from the comparatively low-temperature bonding temperature: Transient liquid phase (TLP) bonding and particle sintering. Although various studies reported successful production of an in-situ phase with a higher re-melting point, thermo-mechanical fracture behavior of TLP bonded joint appeared to have a major drawback. That is due to microstructure of the TLP bonded joint as it is made of brittle intermetallic compounds (IMCs). However, for the case of particle sintering, there are promising reports that suggest Ag-sintered joint as a candidate for high-temperature WBG applications. During sintering, it is possible to realize a connection, which consists of one metal throughout. Beside the high thermal conductivity of the material, the advantage is that the interconnect is established at relative low temperature (250 °C) but is stable at higher temperature (over 300 °C). However, sintered Ag particles have its own drawbacks: High cost and low-electromigration resistance. Copper is approximately 100 times cheaper than silver and also more abundant, easily recyclable and easily available. Copper has an overall lower carbon footprint than silver. It has a lower co-efficient of thermal expansion than silver and has nearly the same electrical and thermal conductivities. However, the higher melting point of copper means that the sintering temperatures are also slightly higher than in case of silver. Furthermore, the susceptibility to oxidize quickly under atmosphere is a major drawback when using copper as this is detrimental to the mechanical and thermal integrity of the joint.

[0006] Bhogaraju, S. K. et al. "Copper die bonding using copper formate based pastes with alpha-terpineol, amino-2-propanol and hexylamine as binders", 2020 IEEE 8thElectronics System-Integration Technology Conference (ESTC), 2020, pp. 1-7, discloses copper pastes based on Cu(ll) formate tetrahydrate mixed with either PEG600, alpha-terpineol, amino-2-propanol or hexylamine as binder. After completion of the thermal decomposition process, the formation of pure metal copper nanoparticles is observed. The average shear strength values of the sinter pastes were 30 MPa, 9 MPa, 2 MPa for the binders PEG600, alpha-terpineol and amino- 2-propanol, respectively, whereas hexylamine as binder established no sufficient bond strength to perform shear tests. However, in all cases, a high degree of surface contamination was observed which is undesired and renders the process and the realized product incompatible for use.

[0007] Bhogaraju, S. K. et al. "Die-attach bonding for high temperature applications using thermal decomposition of copper(ll) formate with polyethylene glycol", Ser. Mater. 2020, 182, 74-80, discloses copper pastes based on Cu(ll) formate mixed with either PEG600, alpha-terpineol or a mixture of alpha-terpineol and polyvinyl butyral. From the tested sintering pastes, the Cu(ll) formate / PEG600 paste at a weight ratio of 63 / 37 showed the highest shear strength value of 60 MPa. In addition, thermal decomposition of Cu(ll) formate results in the in situ formation of copper nanoparticles. According to Bhogaraju, S. K. et al. "Novel approach to copper sintering using surface enhanced brass micro flakes for microelectronics packaging", Journal of Alloys and Compounds, Volume 844, 2020, 156043, etched copper or brass flakes were mixed with PEG600 as binder to obtain a paste formulation. The use of PEG600 in the paste formulation enables the in situ reduction of Cu oxides during sintering. The reduction products are copper nanoparticles which act as nano joiners between the micro-sized flakes thereby realizing a bulk like microstructure with a shear strength value of 62 MPa in case of the etched copper flakes.

[0008] WO 2006 / 041657 A2 discloses a method for the fabrication of a conductive feature on a substrate, said method comprising the steps of: (a) providing a precursor composition comprising a copper metal precursor compound, wherein said precursor composition has a viscosity not greater than 1000 centipoise; (b) depositing said precursor composition on said substrate using a direct-write tool; and (c) heating said precursor composition to a conversion temperature of not greater than about 350 °C to form a conductive feature having a resistivity of not greater than about 40 times the resistivity of bulk copper. Said substrate may be selected from the group consisting of polyfluorinated compounds, polyimides, epoxies (including glass-filled epoxy), polycarbonate, cellulose-based materials (i.e. wood or paper), acetate, polyester, polyethylene, polypropylene, polyvinyl chloride, acrylonitrile, butadiene (ABS), flexible fiber board, non-woven polymeric fabric, cloth, and metallic foil, semiconductors, ceramics, glass and combinations thereof. Furthermore, a method for the fabrication of a copper conductive feature on a substrate surface is disclosed, comprising the steps of: (a) providing a precursor composition comprising a copper metal precursor compound, wherein said precursor composition has a viscosity not greater than 100 centipoise; (b) depositing said precursor composition on said substrate using an aerosol jet device to form a trace having a minimum size of not greater than about 100 microns; and (c) heating said precursor composition to a temperature of not greater than about 250 °C to form a conductive feature having a minimum feature size of not greater than about 100 microns and a resistivity of not greater than about 100 times the resistivity of bulk copper metal. Jo K.H. and Lee J. "Die attachment method on a Cu finish by pressure-assisted sinter bonding in air using Cu formate paste", Arch. Metal. Mater. 65, 2020, 3, 1057-1061 , discloses a method of high-speed die attachment for wide-bandgap power chips on Cu finish in air by sinter bonding of a paste containing Cu(ll) formate rods at 250 °C under a pressure of 10 MPa. Synthesis of the plate-type Cu formate particles from CuO is accomplished through a wet reaction for 180 min. Copper, formed in situ in the bondline by pyrolysis of the formate during heating for the attachment, was sufficiently active to lead high-speed sintering within a carbon dioxide-hydrogen atmosphere derived from the pyrolysis, and the oxide layer on the Cu finish was reduced by the hydrogen. As a result, sinter bonding for 10 min formed a bonding with a shear strength approaching 27 MPa.

[0009] Lin C.T. et al. "Effect of surfactant on casting metalorganic films and writing copper metal patterns", J. Mater. Res., 6(4), Apr 1991 , 760-765, discloses the introduction of Triton X-100 detergent to metalorganic solutions and application of the deter- gent / metalorganic solution to a substrate to form a uniform spin-on thin film. The precursor, copper formate film, is spin-deposited with a metalorganic solution prepared by a solvent mixture of methanol and Triton X-100 (50:1 by volume). Using a KrF laser (248 nm) direct writing of copper patterns on glass and polyimide substrates has been achieved.

[0010] CN 104341860 A discloses a nanometer conductive ink comprising an organic copper salt, a solvent, a surfactant and nanometer metal conductive particles and a preparing method thereof. The preparing method includes: a first step of uniformly mixing the organic copper salt, the solvent, the surfactant and the nanometer metal conductive particles according to a ratio to form a solution; and a second step of filtering the solution obtained in the first step through a screen to obtain the nanometer conductive ink.

[0011] US 2006 / 0258136 A1 discloses a method of forming a metal trace on a substrate.

[0012] The method includes inkjet printing a chemical ink comprising a metal containing compound on the substrate surface to form an ink drop thereon, and heating the substrate to a suitable elevated temperature. The ink drop undergoes at least one of decomposition and reduction due to the substrate temperature to form a metal on the substrate in a controlled atmosphere.

[0013] CN104812944 A discloses a copper foil which is equipped with a carrier, a release layer, an ultra-thin copper layer, and an optional resin layer. The average value of the roughness (Rz) of the surface of the ultra-thin copper layer as measured in accordance with JIS B0601-1982 using a contact roughness meter is at most 1.5 pm, and the standard deviation of the roughness (Rz) is at most 0.1 pm. Roughness may be achieved by an electrochemical treatment.

[0014] The Die Top System (DTS®) distributed by Heraeus Electronics consists of a copper foil with a NiPdAu-, NiAu- or NiAg plating on one side of the foil and a silver based sinter paste pre-applied to the plating. A die and the DTS® can be sintered together in one step using standard sinter equipment.

[0015] US 2022 / 0371089 A1 discloses a sintering composition, consisting essentially of a solvent; and a metal complex dissolved in the solvent, wherein the sintering composition contains at least 60 wt. % of the metal complex, based on the total weight of the sintering composition and the sintering composition contains at least 20 wt. % of the metal of the metal complex, based on the total weight of the sintering composition. Typically, the metal complex is at a high concentration in the solvent. Preferably, the solvent is supersaturated with the metal complex. The viscosity of the sintering composition may be from 60,000 to 120,000 cP, preferably from 70,000 to 80,000 cP. Such a viscosity may result from the large amount of metal complex contained in the sintering composition. The sintering composition may be metallic gel. According to an example, the copper metallic gel was synthesized as follows: 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethyla- mino)-1 ,2-propanediol was added to it. This mixture was heated at 130° C with occasional stirring and then cooled to room temperature. 1 g 1-ethyl-2-pyrrolidinone was added to this mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and then homogenized in an orbital mixture. The resulting copper gel was printed on direct bond copper (DBC) as a rigid substrate and then dried at 160° C for 30 minutes in nitrogen atmosphere. The pre-dried material was then allowed to cool at room temperature. Die elements such as gold coated Si dies were placed on the pre-dried printed area. The assembly was then wrapped in an aluminum foil with a graphite sheet for cushioning effect. The vehicle was then subjected to 260° C at 10 MPa pressure for 2 minutes in a press for sintering. The resulting bond-line thickness for a print using a 200 micron stencil is given as around 11 microns.

[0016] The problem to be solved by the present invention is to provide an alternative interconnecting material, a method for producing the alternative interconnecting material and a use of the alternative interconnecting material. The alternative interconnecting material shall allow die-attach bonding, the formation of an electrically and / or thermally conductive path on a solid substrate and substrate attachment. Furthermore, the alternative interconnecting material shall be free or at least essentially free of volatile organic compounds (VOCs) and the method shall enable a sintering free or at least essentially free of VOCs with relative short sintering times, with relative low sintering temperature and / or with no or relative low sintering pressure. Sintering of the alternative interconnecting material shall result in a homogenous bonding interface or in a bonding interface which is homogenous in at least 95 % of its area.

[0017] The problem is solved by the subject-matter of claims 1 , 13 and 15. Embodiments of the invention are subject-matter of claims 2 to 12 and claim 14.

[0018] According to the invention a coated metal foil for forming a copper-containing interlayer between a first solid substrate and a second solid substrate is provided. The coated metal foil is produced by the following steps: a) Providing a copper formate, a solvent and a foil of a nickel-iron alloy plated on both sides with copper or of copper or of a copper alloy which foil has a first surface and a second surface, b) mixing the copper formate and the solvent for obtaining a mixture in which the copper formate is completely dissolved, in particular directly dissolved, i. e. without being complexed by a complexing agent, and either c1 ) applying the mixture obtained in step b) to the first surface and the second surface of the foil and d1 ) heating the mixture on the first surface and the second surface of the foil or the foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface and the second surface of the foil for obtaining the coated metal foil, or c2) applying the mixture obtained in step b) to the first surface of the foil, heating the mixture on the first surface of the foil or the foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface of the foil for obtaining a first coating on the first surface and d2) applying the mixture obtained in step b) to the second surface of the foil, heating the mixture on the second surface of the foil or the foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the second surface of the foil for obtaining a second coating on the second surface and thus the coated metal foil, wherein at least the heating and the maintaining is performed in a reducing atmosphere or in an inert atmosphere. The copper nanoparticles have an arithmetic average particle diameter in the range of 20 nm to 200 nm, wherein each particle diameter is determined by image analysis of electron microscope or scanning electron microscope images.

[0019] The feature that the copper formate is completely dissolved means that usually no solids are contained in the mixture obtained in step b). For obtaining a mixture in which the copper formate is completely dissolved, the amount of copper formate can be chosen such that it completely dissolves in the solvent, i. e. such that no undissolved copper formate remains in the mixture. For example, if the solvent is water it is possible to dissolve up to 12,5 wt.% copper formate in the solvent.

[0020] Another possibility to obtain a mixture in which the copper formate is completely dissolved is to add such an amount of copper formate to the solvent that a part of the copper formate remains undissolved upon mixing. Then the undissolved copper formate is allowed to settle and the supernatant is taken as the mixture in which the copper formate is completely dissolved.

[0021] Heating the mixture on the first surface and the second surface of the foil or the foil together with the mixture results in the in situ formation of copper nanoparticles. The formation of the layer comprising copper nanoparticles can be observed as a change in the appearance of the first surface and the second surface on which surfaces the mixture has been applied from the color of the mixture, which is usually blue or light blue or almost colorless, to a shiny copper red color. The in situ formation of copper nanoparticles is achieved by heating of the copper formate to the temperature in the range from 175 °C to 300 °C, in particular in the range from 175 °C to 274°C, in particular in the range from 175 °C to 250°C, in particular in the range from 180 °C to 245 °C, in particular in the range from 185 °C to 240 °C, in particular in the range from 190 °C to 235 °C, in particular in the range from 195 °C to 230 °C, in particular in the range from 200 °C to 225 °C, in particular in the range from 205 °C to 220 °C, in particular in the range from 210 °C to 215 °C, wherein the copper formate decomposes and the copper nanoparticles are formed. Since the in situ formation of copper nanoparticles is achieved by heating in a reducing atmosphere or in an inert atmosphere, oxidation of the formed copper nanoparticles is prevented.

[0022] Depending on the duration of the maintaining at step d1 ) or steps c2) and d2) an arithmetic average particle diameter of the copper nanoparticles of at least 20 nm, in particular at least 30 nm, in particular at least 40 nm, in particular at least 50 nm, in particular at least 60 nm, in particular at least 80 nm, and at most 200 nm, in particular at most 180 nm, in particular at most 160 nm, in particular at most 140 nm, in particular at most 120 nm, in particular at most 100 nm, is obtained. In particular, the arithmetic average particle diameter of the copper nanoparticles may be at least 50 nm and at most 200 nm. The inventors of the present invention further found that the temperature during the heating and maintaining affects the arithmetic average particle diameter of the copper nanoparticles. A relatively high temperature during the heating and maintaining results in a relatively high arithmetic average particle diameter. A relatively low temperature during the heating and maintaining results in a relatively low arithmetic average particle diameter.

[0023] The technical effect of the relatively small copper nanoparticles having an arithmetic average particle diameter in the range of 20 nm to 200 nm is a strong activation of the first and the second surface which strong activation results from the relatively high surface energy of the relatively small particles. When forming the copper-containing interlayer between the first and the second solid substrate this activation enables sintering at a relatively low temperature in a relative short time with no or only low pressure. This enables a relatively low thermal and / or mechanical load on the first and second substrates during sintering. This is important in case the first and / or second substrate(s) is / are (a) sensitive electronic compound^).

[0024] The purpose of the coating of the metal foil is not to provide a large amount of material to be sintered, as is the case with sintering pastes, but the strong activation of the metal foil such that a foil results that is ready to by used for forming a copper-containing interlayer between the first and second solid substrate which foil enables a quick sintering at low temperature with no or only low pressure. The reason for the strong activation is that the small nanoparticles lead to a very high surface area and many contact points between particles. The smaller the copper particles are the stronger is the activation of the coated copper foil. The greatly increased contact area facilitates fast diffusion of atoms or molecules across the boundaries of the particles and thus facilitates the whole sintering process. The specific steps of production of the coated metal foil according to the invention have an influence on the resulting features of this foil.

[0025] For example, the complete solving of the copper formate in the solvent promotes the formation of the small nanoparticles and a very even distribution of these nanoparticles on the first and the second surfaces. The resulting surface of the coated metal foil is macroscopically much more even and has a much higher surface area than a surface obtained by electrochemical roughening such as it is known from CN104812944 A. The relatively even surface enables a high amount and a high density of contact points between the coated metal foil and the first and the second solid substrate and thus further facilitates fast sintering at relatively low temperature. Furthermore, the complete solving of the copper formate in the solvent enables its application to the first surface and the second surface by spraying of the mixture obtained in step b) on the first surface and the second surface. Application by spraying facilitates an even distribution of the mixture on the first surface and the second surface and thus an even distribution of the resulting nanoparticles on these surfaces. Furthermore, the nanoparticles formed from completely dissolved copper formate are much smaller than particles that are formed from dissolved complexes of copper as known, e. g. from US 2022 / 0371089 A1 .

[0026] The in situ formed copper nanoparticles, in particular the copper nanoparticles having an arithmetic average particle diameter of at least 50 nm and at most 200 nm, are efficiently sintered to each other and onto the first surface and the second surface of the foil during the heating and maintaining and show a relatively strong adhesion to the foil. This means that the nanoparticles on the first surface and on the second surface are not to be understood as isolated nanoparticles but as nanoparticles that stick directly or indirectly by sticking to other nanoparticles to the first surface and to the second surface.

[0027] The sintering of the nanoparticles onto the first surface and the second surface of the foil also results in a relatively high arithmetic average shear strength value obtainable with the coated copper-containing metal foil according to the invention after sintering of the coated copper-containing metal foil to the first solid substrate and the second solid substrate, in particular in an arithmetic average shear strength value in the range of 20 MPa to 40 MPa. The arithmetic average shear strength value can be measured by performing a mandrel band test or a MLT- STD-883E shear test standard method. In addition, relatively small nanoparticles, in particular nanoparticles having an arithmetic average particle diameter of at least 20 nm and at most 200 nm, in particular of at least 50 nm and at most 100 nm, have a relatively high surface energy and form independently from each other on the first surface and the second surface of the foil a relatively dense and a relatively thin layer comprising copper nanoparticles. In particular, the relatively thin layer comprising copper nanoparticles formed by relatively small nanoparticles may have in each case independently from each other a thickness of at least 80 nm and at most 500 nm. The relatively high surface energy of the relatively small nanoparticles enables sintering of the nanoparticles within a relatively short sintering time at a relatively low sintering temperature to the first solid substrate and the second solid substrate, in particular if a copper-containing interlayer between the first solid substrate and the second solid substrate is formed. Furthermore, the relatively dense and relatively thin layer comprising copper nanoparticles on the first surface and the second surface provides a relatively high number of activation sites and / or bonding sites during the sintering of the copper nanoparticles to the first solid substrate and the second solid substrate. This further reduces the sintering time and the sintering temperature and improves the adhesion to the first solid substrate and the second solid substrate, in particular if a copper-containing interlayer between the first solid substrate and the second solid substrate is formed.

[0028] The composition can be applied to the first surface and / or the second surface by inkjet printing, by aerosoljet printing, in particular nanojet™ printing, by piezojet printing or by spray coating, in particular with a screw nozzle. Inkjet printing usually allows application of a fixed volume of the mixture in the range of 1 -10 pL.

[0029] During the maintaining and forming of the layer comprising copper nanoparticles according to step d1 ) or step(s) c2) and / or d2) at least 85 wt.%, in particular at least 90 wt.%, in particular at least 95 wt.%, and at most 99.5 wt.%, of the initial weight of the mixture applied to the foil may be evaporated. In case the mixture contains 12,5 wt.% copper formate and the solvent is water, at most 96.5 wt.%, of the initial weight of the mixture applied to the foil may be evaporated and 3.5 wt% remain in the form of copper particles. The initial weight of the mixture can be determined by weighing of the foil before and after application of the mixture as difference between the two weights determined in this way. The evaporation of the mixture can be determined either by weighing of the foil together with the mixture during or before and after the maintaining according to step d1 ) or step(s) c2) and / or d2) and / or by mass spectrometry or thermal gravimetric analysis.

[0030] The inventors of the present invention further found that the foil and the copper nanoparticles formed on the first surface and the second surface of the foil do not oxidize during the maintaining according to step d1 ) or step(s) c2) and / or d2) due to the reducing atmosphere or due to the inert atmosphere. The inventors found that the inert atmosphere prevents oxidation of the foil and the formed copper nanoparticles. Furthermore, the reducing atmosphere provides a reducing effect on the foil and the copper nanoparticles formed, thus preventing oxidation of the foil and the formed copper nanoparticles. Thus, a further agent for application on the first surface and the second surface of the foil for preventing oxidation of the foil and / or a further agent in the mixture obtained in step b) for preventing oxidation of the formed copper nanoparticles does not have to be provided.

[0031] Furthermore, the copper nanoparticles formed do not agglomerate during the maintaining according to step d1 ) or step(s) c2) and / or d2). The inventors of the present invention found that the copper nanoparticles formed are dispersed uniformly in the layer comprising copper nanoparticles on the first surface and the second surface of the coated copper-containing metal foil. Thus, a further agent for preventing agglomeration of the formed copper nanoparticles does not have to be provided in the mixture obtained in step b).

[0032] The inventors of the present invention further found that the layer comprising copper nanoparticles on the first surface and the second surface of the foil shows a relatively high film adhesion to the copper-containing foil. Film adhesion in the context of the present invention is determined according to the international standard test methods for rating adhesion by tape test (Designation: D3359-17; https: / / www.astm.org / d3359-17.html). The D3359-17 standard methods cover procedures for assessing the adhesion of relatively ductile coating films to metallic substrates by applying and removing pressure-sensitive tape over cuts made in the film. Test Method A (X-cut tape test) is primarily intended for use in the field while Test Method B (cross-cut tape test) is more suitable for use in laboratory or shop environments. The film adhesion of the layer comprising copper nanoparticles on the first surface and the second surface of the foil is determined according to Test Method B (cross-cut tape test) of the international standard test method D3359-17. The procedure of Test Method B is described in detail in the international standard test method D3359-17 (https: / / www.astm.org / d3359-17.html). After performing Test Method B, the adhesion is rated in accordance with the following scale: 5B The edges of the cuts are completely smooth; none of the squares of the lattice is detached.

[0033] 4B Small flakes of the coating are detached at intersections; less than 5 % of the area is affected.

[0034] 3B Small flakes of the coating are detached along edges and at intersections of the cuts. The area affected is 5 to 15 % of the lattice.

[0035] 2B The coating has flaked along the edges and on parts of the squares. The area affected is 15 to 35 % of the lattice.

[0036] 1 B The coating has flaked along the edges of cuts in large ribbons and whole squares have detached. The area affected 35 to 65 % of the lattice.

[0037] OB Flaking and detachment worse than Classification 1 B.

[0038] The film adhesion of the layer comprising in situ formed copper nanoparticles as specified according to the invention on the first surface and the second surface of the foil is rated 4B according to the above scale. In contrast, application of a mixture already comprising copper nanoparticles and a solvent or application of a mixture comprising a copper formate, a complexing agent complexing the copper formate and a solvent, to the first surface and the second surface of the foil and heating those mixtures as specified in steps d1 ), c2) and d2) results in a relatively low film adhesion according to the above scale of the resulting layer. For example, application as specified in steps c1 ), c2) and d2) and heating as specified in steps d1 ), c2) and d2) of a mixture consisting of copper nanoparticles coated with an organic polymer, in particular coated with polyvinylpyrrolidone, and a solvent, in particular an alcohol or a glycol-based solvent, results in a layer on the first surface and the second surface of the foil having a film adhesion of 1 B according to the above scale. Furthermore, application as specified in steps c1 ), c2) and d2) and heating as specified in steps d1 ), c2) and d2) of a mixture consisting of copper formate complexed with an alkanolamine and a solvent, in particular an alcohol-gly- col based solvent, results in a layer on the first surface and the second surface of the foil having a film adhesion of OB. Thus, the layer comprising copper nanoparticles formed on the first surface and the second surface of the foil for obtaining the coated copper-containing metal foil according to the invention shows a relatively high film adhesion.

[0039] A possibility of measuring the properties of an adhesive bond is the performance of a peel test which is a basic form of mechanical testing. Peel tests involve applying a tensile force to normally a flexible substrate that is bound by an adhesive to either another flexible substrate - such as tape, thin film, or rubber - or a rigid substrate - such as metal, rigid plastic, or composite. Common results from a peel test are initial peak force, average force over the seal, and peel strength (average force per width of the specimen). Peel tests are performed on universal testing machines that consists of a test frame that is equipped with a load cell, testing software, and application-specific grips and accessories. Many peel tests require the use of a specialized test fixture that maintains the specified peel angle throughout the test.

[0040] To determine peel strength of Cu containing Cu foils attached to the substrate after pressure sintering, a 90° angle peel test is performed. A fixture consists of a bearing-mounted sled that is linked to the testing machine’s crosshead by a cable and pulley system. This fixture has a T-slotted sled that allows the substrate clamps, which secure the substrate to the sled, to adjust to a variety of substrate widths. The tests are performed as per the standards ASTM B571 and ASTM D2861 .

[0041] The peel test of the layer comprising in situ formed copper nanoparticles as specified according to the invention on the first surface and the second surface of the foil resulted in a peak force of 0.8N across 10 samples, translating to 2.5 to 5 N / cm based on test specimen dimensions.

[0042] The inventors of the present invention further found that the thermal ramp rate of heating of the mixture on the first surface and the second surface of the foil or of the foil together with the mixture to the temperature in the range of 175 °C to 300 °C, affects the arithmetic average particle diameter of the copper nanoparticles. In the context of the present invention, the thermal ramp rate is defined as a change in the temperature over an interval of time (Delta T / Delta t). The thermal ramp rate is dependent on the means of heating, in particular the heat source. The heat source may be a sintering furnace, in particular a reflow oven, a laser or a flash lamp. The laser may be a CO2 laser, a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, a fiber laser or any other laser that can be applied in sintering, in particular in selective laser sintering. The flash lamp may be a xenon flash lamp or any other flash lamp that can be applied in sintering, in particular in intense pulsed light sintering.

[0043] If the heat source is a sintering furnace, the thermal ramp rate may be a relatively low thermal ramp rate, in particular a thermal ramp rate in the range of 5 °C / min to 45 °C / min, in particular in the range of 10 °C / min to 25 °C / min. Thus, the duration of heating to the temperature in the range of 175 °C to 300 °C by using a sintering furnace as a heat source is relatively high. In particular, the duration of heating to the temperature may be at least 5 min, in particular at least 10 min, and at most 30 min, in particular at most 20 min, in particular at most 15 min. If the heat source is a laser or a flash lamp, the thermal ramp rate may be a relatively high thermal ramp rate, in particular a thermal ramp rate in the range of 300 °C / ms to 1000 °C / ms, in particular in the range of 400 °C / ms to 900 °C / ms, in particular in the range of 500 °C / ms to 800 °C / ms, in particular in the range of 600 °C / ms to 700 °C / ms.

[0044] If the heat source is a laser, the thermal ramp rate is dependent on the power and the scanning speed of the laser. Thus, the duration of heating to the temperature in the range of 175 °C to 300 °C by using a laser as a heat source is relatively low. In particular, the duration of heating to the temperature may be at least 0.1 ms, in particular at least 0.2 ms, in particular at least 0.25 ms, and at most 0.5 ms, in particular at most 0.4 ms, in particular at most 0.3 ms. The inventors of the present invention found that a relatively high thermal ramp rate and thus a relatively low duration of heating result in a relatively fast evaporation of the solvent. The evaporation of the solvent can be measured by determination of a loss of total weight of the foil and the mixture on the foil. The initial weight of the solvent can be determined by weighing of the solvent before providing / preparing the mixture. The evaporation of the solvent can be determined either by weighing of the foil together with the mixture during or before and after the heating and maintaining according to step d1 ) or steps c2) and d2) and / or by mass spectrometry or thermal gravimetric analysis. A relatively fast evaporation of the solvent results in a relatively weak bubble formation during evaporation of the solvent which enables a relatively low surface irregularity and a relatively high homogeneity of the resulting layer comprising copper nanoparticles. Thus, a relatively high thermal ramp rate enables a relatively low surface roughness of the layer comprising copper nanoparticles. Surface roughness may be measured by atomic force microscopy, by optical microscopy, in particular by three-dimensional optical microscopy, or by profilometry, in particular by three-dimensional profilometry, in particular on basis of analysis of confocal laser scanning microscopy data.

[0045] Furthermore, a relatively high thermal ramp rate results in the formation of relatively small copper nanoparticles, in particular copper nanoparticles having an arithmetic average particle diameter in the range of 10 nm to 30 nm.

[0046] The inventors of the present invention further found that the duration of maintaining according to step d1 ) or steps c2) and d2) affects the sinterability of the copper nanoparticles. The maintaining according to step d1 ) or steps c2) and d2) may be for at least 1 ps, in particular for at least 100 ps, in particular for at least 0.5 ms, in particular for at least 1 ms, in particular for at least 2 ms, in particular for at least 5 ms, in particular for at least 100 ms, in particular for at least 1 second, in particular for at least 5 seconds, in particular for at least 1 minute, in particular for at least 2 minutes, in particular for at least 3 minutes, in particular for at least 5 minutes, in particular for at least 7 minutes, and for at most 30 minutes, in particular for at most 25 minutes, in particular for at most 20 minutes, in particular for at most 15 minutes, in particular for at most 10 minutes, in particular for at most 8 minutes. The duration of maintaining according to step d1 ) or steps c2) and d2) is dependent on the means of heating, in particular the heat source. If the heat source is a sintering furnace, the duration of maintaining may be relatively long, in particular at least 1 minute and at most 30 minutes, until the layer comprising copper nanoparticles is formed. A relatively long duration of the maintaining according to step d1 ) or steps c2) and d2), in particular maintaining for at least 5 minutes, results in a relatively high sinterability of the copper nanoparticles. If the heat source is a laser or a flash lamp, the duration of maintaining may be relatively short, in particular at least 1 ps and at most 100 ms, until the layer comprising copper nanoparticles is formed.

[0047] In an embodiment, the mixture has a viscosity in the range of 2.5 mPas to 10 mPas, measured at 20 °C. Said low viscosity promotes the formation of a very thin layer of the mixture on the first and on the second surface and thus the formation of relatively small nanoparticles. In the same or a further embodiment the total weight of the copper formate in the mixture obtained in step b) in relation to the total weight of this mixture is at most 5 wt.%. This concentration of the copper formate in the mixture also promotes the formation of relatively small nanoparticles.

[0048] The layer comprising copper nanoparticles on the first surface and on the second surface may have in each case independently from each other a thickness of at least 80 nm and at most 500 nm. Such a thin layer is sufficient for the desired activation of the foil but has only little influence on the features of the coated metal foil. Thus, the features of the copper-containing interlayer to be formed are essentially only influenced by the features of the foil of copper or of the copper alloy or of the nickel-iron alloy plated on both sides with copper. Desired features of the copper- containing interlayer can therefore be easily determined by the choice of this foil.

[0049] A thickness of the layer of 500 nm can, e. g., be obtained by application of a mixture containing water as the solvent and 5 wt.% of copper format on the first and / or second surface in a thickness of 1 .5 pm. For that purpose application can, e. g., be performed by inkjet printing. The copper alloy may be bronze or brass. Though copper usually has a higher thermal conductivity, copper alloy still enables a good thermal conductivity during the heating and the maintaining according to step d1 ) or steps c2) and d2). The purity of the copper, the copper alloy or the nickel-iron alloy plated on both sides with copper may be at least 95 %, in particular at least 99 %, in particular at least 99.95 %, and at most 99.99 %. The purity refers to the mentioned metals. This means, e. g., in case of 95 % purity of the nickel-iron alloy plated on both sides with copper that 95 % of the nickel-iron alloy is nickel and iron and 95 % of the copper plated on the foil of nickel-iron alloy is copper. The foil provided at step a) may have a thickness of at least 5 pm, in particular of at least 10 pm, in particular of at least 25 pm, in particular of at least 50 pm, and of at most 200 pm, in particular of at most 100 pm, in particular of at most 75 pm.

[0050] The foil, in particular the foil of copper, may be a single crystal foil or a polycrystalline foil, i. e. the metal forming the foil may have a single crystal orientation or a polycrystalline orientation. The orientation of the metal depends on the manufacturing process of the foil and single crystal foils as well as polycrystalline foils are commercially available. The first surface and the second surface of the foil may be fully or partially oriented in a single crystallographic orientation, in particular in a 111 crystallographic orientation, or may have a fully or partially disordered crystal structure.

[0051] In case the metal foil is of the copper plated nickel-iron alloy, the nickel-iron alloy may be FeNi36 which is generally known as Invar®. In such a foil, the nickel-iron alloy is usually plated on both sides with copper. Such a foil based on FeNi36 is of specific interest for low coefficient of thermal expansion applications. It can be a commercially available foil, e. g. a CIC (copper / lnvar® / copper) foil where the core layer is made of iron-nickel alloy with 36% nickel content and represents 60% of the volume of the composite material. The copper on either side represents 20% of the volume of the composite. Such a foil can be commercially procured as CIC 20 / 60 / 20 from Schlenk Metallfolien GmbH & Co. KG, Germany. The foil provided at step a) can be purchased for example as SE-Cu58 R360 blank copper foil or phosphorus deoxidized copper foil, which are both offered by the company Carl Schlenk AG, Germany. Phosphorus deoxidized copper can be purchased as Cu-HCP or SE-Cu58 from the Carl Schlenk AG.

[0052] The first surface and / or the second surface of the foil may have a number of superficial recesses. The superficial recesses on the first surface and / or the second surface of the foil may be formed by means of etching with an acid before step c1 ) or c2) or by means of plasma treatment before step c1 ) or c2). In particular, the superficial recesses of the foil may be formed prior to step a). This means that the foil may be provided in step a) as a foil with superficial recesses on the first surface and / or the second surface. Any one of the superficial recesses may extend either from the first surface in the direction from the first surface to the second surface of the foil or from the second surface in the direction from the second surface to the first surface of the foil.

[0053] A maximal superficial extension of any of the superficial recesses of the foil may be 2 pm, in particular 1 pm. A minimal superficial extension of any of the superficial recesses of the foil may be 5 nm, in particular 20 nm. In particular, the superficial extension of any of the superficial recesses of the foil may be in the range of 10 nm to 100 nm, in particular in the range of 15 nm to 80 nm, in particular in the range of 20 nm to 60 nm, in particular in the range of 30 nm to 40 nm. The recesses ensure a relatively large surface area of the foil and enable a relatively high wettability of the first surface and the second surface of the foil when applying the mixture obtained in step b) to the first surface and the second surface of the foil. A relatively large surface area of the foil results in a relatively low total surface energy. A relatively low total surface energy of the foil enables a relatively high rate of sinter neck formation between the copper nanoparticles formed in situ and the foil during the maintaining according to step d1 ) or steps c2) and d2). The maximal superficial extension of any of the superficial recesses of the foil can be determined by surface analysis, in particular by metal surface analysis, of the first surface and the second surface of the foil. The surface analysis, in particular the metal surface analysis, of the first surface and the second surface of the foil can comprise microscopic morphology analysis, surface structure analysis, surface elemental composition analysis and depth analysis. The surface analysis, in particular the metal surface analysis, can be performed for example by scanning electron microscopy (SEM), x-ray diffraction (XRD), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), glow discharge spectrometry and imagebased metal surface inspection. These methods can also be applied to the surface analysis of the coated copper-containing metal foil according to the invention, in particular to the surface analysis of the layer comprising copper nanoparticles formed on the first surface and the second surface of the foil.

[0054] The absence of solids results in a relatively low viscosity of the mixture obtained in step b). The viscosity of the mixture obtained in step b) may be in the range of 2.5 mPas to 10 mPas. The viscosity of the mixture obtained in step b) may be measured at room temperature, in particular at 20 °C, using a standard viscosimeter, in particular a standard classical rotational viscosimeter, in particular the Thermo Scientific™ HAAKE™ Viscotester™ C. These rotational viscometers measure the resistance of the mixture obtained in step b) against a preset speed. The resulting torque or resistance is a measure for the viscosity of the mixture obtained in step b). The higher the torque, the higher the viscosity. A relatively low viscosity of the mixture obtained in step b) enables its easy processability and its good and even spreadability on the first surface and the second surface of the foil.

[0055] A surfactant may be optionally provided in step a). The optional surfactant may be mixed with the copper formate and the solvent in step b). The surfactant may be chosen such that its boiling point allows its evaporation during the maintaining according to step d1 ) or steps c2) and d2). The evaporation of the surfactant can be determined by mass spectrometry or thermal gravimetric analysis. The surfactant may be a water-soluble polymer, a silicon-based organic polymer or a mixture of ethyl cellulose and an organic solvent dissolving the ethyl cellulose. In the context of the present invention, the water-soluble polymer is defined as a polymer having a solubility in water at 20 °C of at least 0.1 g / L, in particular of at least 0.5 g / L, in particular of at least 1 g / L. In the context of the present invention, a silicon-based organic polymer is defined as an organic polymer having a repeating unit comprising silicone, carbon and hydrogen. The water-soluble polymer may be polyvinyl pyrrolidone. The silicon-based organic polymer may be polydimethylsiloxane. The organic solvent in the mixture of ethyl cellulose and the organic solvent may be an alcohol, a monocyclic aromatic hydrocarbon or an ester. The alcohol may be methanol or ethanol. The monocyclic aromatic hydrocarbon may be toluene. The ester may be ethyl acetate. The total weight of ethyl cellulose in relation to the total weight of the mixture of ethyl cellulose and the organic solvent may be at most 10 wt.%, in particular at most 5 wt.%, in particular at most 1 wt.%. The remaining part of the mixture of ethyl cellulose and the organic solvent may be formed by the organic solvent. The polydimethylsiloxane may be a polyether-modified polydimethylsiloxane, in particular Byk®-333. The inventors of the present invention further found that the surfactant enables a relatively high and even wettability of the first surface and the second surface of the foil when applying the mixture obtained in step b) to the first surface and the second surface of the foil. The inventors of the present invention further found that the surfactant affects the arithmetic average particle diameter of the copper nanoparticles formed in situ during the maintaining as specified in step d1 ) or step c2) and d2). The presence of the surfactant results in the formation of relatively small copper nanoparticles, in particular copper nanoparticles having an arithmetic average particle diameter the range of 20 nm to 80 nm.

[0056] The copper formate may be Cu(ll) formate or a hydrate of copper formate. The Cu(ll) formate may be Cu(HCOO)2. The hydrate of copper formate may be copper formate tetrahydrate (Cu(HCOO)2*4H2O).

[0057] The total weight of the copper formate in relation to the total weight of the mixture obtained in step b) may be at most 12 wt.%, in particular at most 8 wt.%, in particular at most 6 wt.%, in particular at most 5 wt.%, in particular at most 4 wt.%, in particular at most 3.5 wt.%, in particular at most 3 wt.% and at least 0.5 wt.%. The total weight of the optional surfactant in relation to the total weight of the mixture obtained in step b) may be at most 2 wt.%, in particular at most 1 .5 wt.%, in particular at most 1 wt.%, in particular at most 0.5 wt.%, in particular at most 0.1 wt.%, in particular at most 0.01 wt.% and at least 0.001 wt.%. The remaining part of the mixture obtained in step b) may be formed by the solvent. The total weight of the copper formate, the optional surfactant and / or the solvent can each be determined by nuclear magnetic resonance spectroscopy, thermogravimetric analysis, mass spectrometry and infrared spectroscopy. The total weight of the copper formate, the optional surfactant and / or the solvent can be determined by weighing the copper formate, the optional surfactant and / or the solvent before forming the mixture obtained in step b).

[0058] The solvent is generally a liquid solvent. The solvent may be water or an acidic aqueous solution or a mixture of water and ethanol. The acidic aqueous solution may be an aqueous solution of an organic acid. The organic acid may be formic acid. The volume concentration of the organic acid in the acidic aqueous solution may be at least 0.1 % v / v, in particular at least 0.5 % v / v, in particular at least 1 % v / v, in particular 2 % v / v, and at most 5 % v / v, in particular at most 4 % v / v, in particular at most 3 % v / v. The inventors of the present invention found that an acidic aqueous solution as solvent, in particular an aqueous solution of an organic acid, in particular formic acid, further provides a reducing effect on the copper nanoparticles formed, thus further preventing oxidation of the formed copper nanoparticles.

[0059] In the context of the present invention, a solvent is considered neutral when the pH of the solvent is in a range of 7.0 to 7.5. In the context of the present invention, a solvent is considered acidic when the pH of the solvent is in a range below 7.0. In the context of the present invention, a solvent is considered basic when the pH of the solvent is in a range above 7.5.

[0060] The pH of the solvent is determined at 20 °C. The pH can be adjusted as desired by adding acids or bases, e.g. 1 M HCI or 1 M NaOH. The coated copper-containing metal foil according to the invention may have at least one, in particular at least two, in particular at least five, and at most 20, in particular at most 15, in particular at most ten, layer(s) comprising copper nanoparticles on each of the first surface and the second surface of the foil. The number of layers comprising copper nanoparticles on the first surface and the second surface of the foil may be selected independently from each other. The first layer comprising copper nanoparticles on each of the first surface and the second surface of the foil is formed by the method according to the invention. Any further layer comprising copper nanoparticles can be produced by the following steps: a2) Providing the copper formate as specified according to the invention, a solvent as specified according to the invention, and the coated metal foil according to the invention having a coated first surface and a coated second surface, b2) mixing the copper formate and the solvent for obtaining the mixture as specified according to step b) of the invention, and either c3) applying the mixture obtained in step b2) to the coated first surface and / or the coated second surface of the coated metal foil provided at step a2) and d3) heating the mixture on the coated first surface and / or the coated second surface of the coated metal foil or the coated metal foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the coated metal foil together with the mixture at the temperature until a further layer comprising copper nanoparticles is formed on the coated first surface and / or the coated second surface of the coated metal foil, or c4) applying the mixture obtained in step b2) to the coated first surface of the coated metal foil, heating the mixture on the coated first surface of the coated metal foil or the coated metal foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the coated metal foil together with the mixture at the temperature until a further layer comprising copper nanoparticles is formed on the coated first surface of the coated metal foil for obtaining a further coating on the coated first surface and d4) applying the mixture obtained in step b2) to the coated second surface of the coated metal foil, heating the mixture on the coated second surface of the coated metal foil or the coated metal foil together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the coated metal foil together with the mixture at the temperature until a further layer comprising copper nanoparticles is formed on the coated second surface of the coated metal foil for obtaining a further coating on the coated second surface.

[0061] At least the heating and the maintaining, in particular at least the application of the mixture, the heating and the maintaining, in particular at least the mixing of the copper formate and the solvent, the application of the mixture, the heating and the maintaining, in particular all of the above steps a2) to d3) or d4), may be performed in a reducing atmosphere, in an inert atmosphere and / or in a vacuum. For obtaining more than one further layer, at least the application of the mixture, the heating and the maintaining), in particular at least the mixing of the copper formate and the solvent, the application of the mixture, the heating and the maintaining, in particular all of the above steps, can be repeated, wherein for each repetition, the formerly obtained coated metal foil is provided in step a2) and / or c3) and d3) or c4) and d4) as the coated metal foil. The inventors of the present invention found that the layer(s) comprising copper nanoparticles on the first surface and the second surface of the foil is / are (a) relatively homogenous and regular layer(s) comprising copper nanoparticles. Each of the layer(s) comprising copper nanoparticles on the first surface and the second surface of the foil may have in each case independently from each other a thickness of at least 80 nm, in particular of at least 100 nm, in particular of at least 150 nm, in particular of at least 200 nm, and of at most 500 nm, in particular of at most 400 nm, in particular of at most 300 nm, in particular of at most 250 nm.

[0062] If the coated metal foil according to the invention has more than one layer comprising copper nanoparticles on the first surface and the second surface of the foil, a total thickness of the layer(s) comprising copper nanoparticles on the first surface and the second surface of the foil may be in each case independently from each other in the range of 160 nm to 10 pm, in particular in the range of 200 nm to 5 pm, in particular in the range of 500 nm to 1 pm.

[0063] The mixture obtained in step b) may be applied in each case independently from each other to 90 % to 100 %, in particular to 95 % to 100 %, in particular to 99 % to 100 %, in particular to 100 %, of an area of the first surface and of an area of the second surface of the foil. The area may be the total area of the first surface and / or the total area of the second surface.

[0064] The coated metal foil may be arranged between the first solid substrate and the second solid substrate as a copper-containing interlayer connecting the first substrate with the second substrate. An electrical and / or thermal conduct may be formed between the first solid substrate and the coated metal foil by the layer comprising copper nanoparticles on the first surface of the coated metal foil and between the second solid substrate and the coated metal foil by the layer comprising copper nanoparticles on the second surface of the coated metal foil. Optionally, a sinter paste may be arranged between the coated metal foil and the first solid substrate and / or between the coated metal foil and the second solid substrate when the interlayer is formed. Arranging of the sinter paste between the coated metal foil and the first solid substrate and / or arranging of the sinter paste between the coated metal foil and the second solid substrate further improves the arithmetic average shear strength value obtainable with the coated metal foil according to the invention after sintering of the coated metal foil to the first solid substrate and / or after sintering of the coated metal foil to the second solid substrate. The shear strength value obtainable by use of the sinter paste may be in the range of 60 MPa to 80 MPa.

[0065] The invention also concerns a method for producing the coated metal foil according to the invention comprising the steps of a) providing copper formate, a solvent and a foil of copper or of a copper alloy or of a nickel-iron alloy plated on both sides with copper which foil has a first surface and a second surface as specified above, b) mixing the copper formate and the solvent for obtaining a mixture in which the copper formate is completely dissolved, in particular directly dissolved, i. e. without being complexed by a complexing agent, and either c1 ) applying the mixture obtained in step b) to the first surface and the second surface of the foil provided at step a) and d1 ) heating the mixture on the first surface and the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface and the second surface of the foil for obtaining the coated metal foil, or c2) applying the mixture obtained in step b) to the first surface of the foil provided at step a), heating the mixture on the first surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface of the foil provided at step a) for obtaining a first coating on the first surface and d2) applying the mixture obtained in step b) to the second surface of the foil provided at step a), heating the mixture on the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the second surface of the foil provided at step a) for obtaining a second coating on the second surface and thus the coated metal foil, wherein at least the heating and the maintaining is performed in a reducing atmosphere or in an inert atmosphere. The copper nanoparticles have an arithmetic average particle diameter in the range of 20 nm to 200 nm, wherein each particle diameter is determined by image analysis of electron microscope or scanning electron microscope images.

[0066] The temperature in the range of 175 °C to 300 °C may be a temperature, in the range of 175 °C to 274 °C, in particular in the range of 175 °C to 250 °C, in particular in the range of 180 °C to 245 °C, in particular in the range of 185 °C to 240 °C, in particular in the range of 190 °C to 235 °C, in particular in the range of 195 °C to 230 °C, in particular in the range of 200 °C to 225 °C, in particular in the range of 205 °C to 220 °C, in particular in the range of 210 °C to 215 °C.

[0067] The temperature may be maintained for at least 1 ps, in particular for at least 100 ps, in particular for at least 0.5 ms, in particular for at least 1 ms, in particular for at least 2 ms, in particular for at least 5 ms, in particular for at least 100 ms, in particular for at least 1 second, in particular for at least 5 seconds, in particular for at least 1 minute, in particular at least 2 minutes, in particular at least 3 minutes, in particular for at least 5 minutes, in particular for at least 7 minutes, and for at most 30 minutes, in particular for at most 25 minutes, in particular for at most 20 minutes, in particular for at most 15 minutes, in particular for at most 10 minutes, in particular for at most 8 minutes.

[0068] The method may further comprise a step e) of cooling of the coated metal foil obtained in step d1 ) or d2) to a further temperature in the range from 15 °C to 40 °C, in particular 20 °C to 30 °C. Cooling may be performed by active cooling but usually is passive cooling, i. e. by letting the coated metal foil cool down to the further temperature which usually is ambient temperature. Optionally, step e) may be performed in a reducing atmosphere, in an inert atmosphere and / or in a vacuum.

[0069] As indicated above, the coated metal foil according to the invention may have at least one and at most 20 layer(s) comprising copper nanoparticles on each the first surface and the second surface of the foil, respectively. The method according to the invention may further comprise above steps a2) to d3) or d4) as well as repetitions of steps a2) to d3) or d4) in order to obtain the further coated metal foil.

[0070] Before providing the foil according to step a), a pre-treatment of the first surface and the second surface of the foil may be performed. The pre-treatment of the first surface and the second surface of the foil may comprise heating of the foil to a pre-treatment temperature in the range from 150 °C to 200 °C, in particular in the range from 160 °C to 190 °C, in particular in the range from 170 °C to 180 °C. The pre-treatment temperature may be maintained for at least 10 minutes, in particular at least 20 minutes, in particular at least 30 minutes, and at most 3 hours, in particular at most 2 hours, in particular at most 1 hour. The pre-treatment may be performed in a reducing atmosphere, in an inert atmosphere and / or in a vacuum. The inventors of the present invention found that the pre-treatment of the foil results in the removal of organic residues and / or oxide compounds, in particular removal of copper oxide, from the first surface and the second surface of the foil. Removal of organic residues and / or oxide compounds from the first surface and the second surface, in particular removal of copper oxide, enables an efficient sintering and a relatively high wettability of the first surface and the second surface.

[0071] The pre-treatment of the first surface and the second surface of the foil may further comprise plasma treatment of the first surface and / or the second surface of the foil and / or treatment or etching of the first surface and / or the second surface of the foil with an acid or an aqueous solution of an acid. The plasma treatment is a treatment of the first surface and / or the second surface of the foil by an ionized gas.

[0072] The ionized gas may be ionized oxygen gas and / or ionized argon gas. The plasma treatment results in a surface modification of the first surface and / or the second surface of the foil to a rough surface texture. Furthermore, the plasma treatment as well as the acid treatment result in the removal or at least partial removal of oxides and / or organic residues from the first surface and / or the second surface of the foil.

[0073] The acid may be hydrochloric acid, citric acid or formic acid. The aqueous solution of the acid may be an aqueous solution of hydrochloric acid, citric acid or formic acid. In particular, the aqueous solution of the acid may be an aqueous solution of hydrochloric acid. The volume concentration of the acid in the aqueous solution of the acid may be at least 0.1 % v / v, in particular at least 0.5 % v / v, in particular at least 1 % v / v, in particular 2 % v / v, and at most 5 % v / v, in particular at most 4 % v / v, in particular at most 3 % v / v. Etching refers to the wet chemical activation of the first surface and / or the second surface of the foil resulting in surface modification to a rough surface texture. A relatively rough surface texture enables a relatively high wettability of the first surface and the second surface of the foil when applying the mixture obtained in step b) to the first surface and the second surface of the foil even without the optional surfactant. Thus, a surfactant does not have to be provided and mixed with copper formate and the solvent in step b), if the pre-treatment of the foil comprises etching or plasma treatment.

[0074] Pre-treatment of the foil by etching, in particular by selective etching, or by plasma treatment may result in the formation of the number of superficial recesses. The number of superficial recesses result in the foil having a first and / or second surface having high free surface energy. The increased surface area leads to a higher specific surface energy y (J / m2) distributed over the total surface area A (m2) and therefore to a reduction of the total surface energy which is the driving force for sintering. The inventors of the present invention found that a surface modification to a rough surface texture of the first surface and / or the second surface of the foil results in a relatively high surface interaction between the in situ formed copper nanoparticles and the foil. In the case of the foil consisting of a copper alloy, etching can also lead to selective dealloying of the alloying element. For instance, in a brass foil, the alloying element zinc is selectively etched. Nevertheless, a residual amount of the alloying element always remains in the foil after etching, regardless of the original content of the alloying element. In particular, hydrochloric acid enables selective etching of zinc out of the brass foil without forming any unwanted residues, in particular without forming residues containing a metal chloride and / or a metal oxide. With hydrochloric acid, the etching byproducts may be ZnCl2 and CuCI2, which are both highly soluble in water. Another etching byproduct may be CuCI which is highly soluble in concentrated hydrochloric acid. Further byproducts may be water and H2. Etching with formic acid may lead to the formation of zinc formate and copper formate. Zinc formate and copper formate are both soluble in water.

[0075] Applying the mixture obtained in step b) to the first surface and the second surface of the foil may be performed by spraying, in particular spray atomization, by coating, in particular dip coating, by printing, in particular stencil printing, inkjet printing, aerosoljet printing, piezojet printing or screen-printing, by direct writing or by material dispensing.

[0076] At least step d1 ) or steps c2) and d2) may be performed in a vacuum. At least steps c1 ) and d1 ) or c2) and d2), in particular at least steps b) to d1 ) or to d2), in particular a) to d1 ) or to d2) or steps a) to e), of the method according to the invention may be performed in a reducing atmosphere, in an inert atmosphere and / or in a vacuum. The pressure of the vacuum may be a pressure in the range of 0.1 mbar to 10 mbar, in particular in the range of 0.5 mbar to 5 mbar, in particular in the range of 1 mbar to 3 mbar. If step d1 ) or c2) and d2) is / are performed in a vacuum, the evaporation of the solvent and / or the optional surfactant is further increased. The reducing atmosphere may be provided by means of a mixture of evaporated formic acid and nitrogen gas or by means of a gas mixture of hydrogen and an inert gas. The mixture of evaporated formic acid and nitrogen gas may be produced by passing nitrogen gas through formic acid. The gas mixture of hydrogen and the inert gas may be a gas mixture of nitrogen and hydrogen or a gas mixture of argon and hydrogen. The inert atmosphere may be provided by means of an inert gas or a mixture of at least two inert gases. The inert gas may be nitrogen gas, carbon dioxide gas, helium gas, neon gas or argon gas. The inert gas mixture may be a mixture of at least two of nitrogen gas, carbon dioxide gas, helium gas, neon gas and argon gas.

[0077] After the method according to the invention, in particular after step d1 ) or d2), or, optionally, after step e), the coated metal foil produced by the method according to the invention may be applied in a method for forming a copper-containing interconnection layer between two solid substrates comprising the steps of g) providing the coated metal foil produced by the method according to the invention, providing a first substrate and providing a second substrate, h) applying the coated metal foil according to the invention to the first substrate, i) placing the second substrate on the coated metal foil, j) applying a bonding pressure to press the first substrate and the second substrate against one another, wherein the bonding pressure may be at least 100 kPa, in particular at least 1 MPa, in particular at least 2.5 MPa, in particular at least 5 MPa, in particular at least 7.5 MPa, in particular at least 10 MPa, and at most 40 MPa, in particular at most 35 MPa, in particular at most 30 MPa, in particular at most 25 MPa, in particular at most 20 MPa, in particular at most 15 MPa, k) heating the coated metal foil to a sintering temperature and maintaining the sintering temperature and the bonding pressure, wherein the sintering temperature is a temperature in the range from 200 °C to 295 °C, in particular a temperature in the range from 205 °C to

[0078] 275 °C, in particular a temperature in the range from 210 °C to

[0079] 270 °C, in particular a temperature in the range from 215 °C to

[0080] 265 °C, in particular a temperature in the range from 220 °C to

[0081] 260 °C, in particular a temperature in the range from 225 °C to

[0082] 255 °C, in particular a temperature in the range from 230 °C to

[0083] 250 °C, in particular a temperature in the range from 235 °C to

[0084] 245 °C, wherein the sintering temperature is optionally maintained for at least 1 ps, in particular for at least 100 ps, in particular for at least 0.5 ms, in particular for at least 1 ms, in particular for at least 2 ms, in particular for at least 5 ms, in particular for at least 100 ms, in particular for at least 1 second, in particular for at least 5 seconds, in particular for at least 1 minute, in particular at least 2 minutes, in particular at least 3 minutes and for at most 30 minutes, in particular at most 20 minutes, in particular at most 15 minutes, in particular at most 10 minutes, 6 minutes, in particular at most 5 minutes, in particular at most 4 minutes, wherein the bonding pressure is optionally maintained for at least 1 ps, in particular for at least 100 ps, in particular for at least 0.5 ms, in particular for at least 1 ms, in particular for at least 2 ms, in particular for at least 5 ms, in particular for at least 100 ms, in particular for at least 1 second, in particular for at least 5 seconds, in particular for at least 3 minutes, in particular at least

[0085] 4 minutes, in particular at least 5 minutes, and for at most 8 minutes, in particular at most 7 minutes, in particular at most 6 minutes, until an interconnection between the two solid substrates is formed,

[0086] I) cooling the first substrate, the sintered foil and the second substrate at least to a cooling temperature, wherein the cooling temperature is a temperature in the range from 15 °C to 40 °C, in particular 20 °C to 30 °C.

[0087] Applying the coated metal foil according to the invention to the first substrate, optionally coated by a sinter paste, and / or placing the second substrate on the coated copper-containing metal foil, optionally coated by a sinter paste, may be performed manually, by foil transfer, in particular by vacuum-mediated foil transfer, or by means of a pick-and-place machine.

[0088] Cooling according to step I) may be performed by active cooling but usually is passive cooling, i. e. by letting the first substrate, the sintered copper-containing foil and the second substrate cool down to the cooling temperature which usually is ambient temperature.

[0089] At least step k), in particular at least steps k) and I), in particular at least steps j) to I), in particular at least steps i) to I), in particular at least steps h) to I), in particular steps g) to I), of the method for forming the copper-containing interconnection layer between two solid substrates may be performed in a reducing atmosphere or in an inert atmosphere. The reducing atmosphere may be provided by means of a mixture of evaporated formic acid and nitrogen gas or by means of a gas mixture of hydrogen and an inert gas. The gas mixture of hydrogen and the inert gas may be a gas mixture of nitrogen and hydrogen or a gas mixture of argon and hydrogen. The inert atmosphere may be provided by means of an inert gas. The inert gas may be contained in an inert gas mixture. The inert gas may be nitrogen gas, carbon dioxide gas, helium gas, neon gas or argon gas. The inert gas mixture may be a mixture of at least two of nitrogen gas, carbon dioxide gas, helium gas, neon gas and argon gas. The inventors of the present invention found that sintering in an inert atmosphere or in a reducing atmosphere, in particular sintering by means of a mixture of evaporated formic acid and nitrogen gas, results in relatively high arithmetic average shear strength value of the resulting copper bond, in particular in an arithmetic average shear strength value in the range of 15 to 20 MPa.

[0090] At least step k), in particular at least steps k) and I), in particular at least steps j) to I), in particular at least steps i) to I), in particular at least steps h) to I), in particular steps g) to I), of the method for forming the copper-containing interconnection layer between two solid substrates may be performed in a vacuum. The inventors of the present invention found that sintering in a vacuum, in particular sintering in a vacuum, results in the removal of residual organic residues, in particular removal of residues of the optional surfactant, from the first surface and the second surface of the coated metal foil according to the invention. Removal of residual organic residues, in particular removal of residues of the optional surfactant, enables an efficient sintering.

[0091] Sintering of the coated metal foil produced by the method according to the invention ensures relatively high shear strength values of the resulting bonds. In addition, the coated metal foil according to the invention enables a relatively energyefficient production of a copper bond, since the in situ formation of nanoparticles was already realized during the production of the coated metal foil.

[0092] In contrast to sintering of a sintering paste, in particular sintering of a sintering paste with a relatively high solvent content, in particular a relatively high water content, and / or a relatively high content of organic compounds, sintering of the coated metal foil according to the invention does not result in a relatively high volume shrinkage and degassing of organic components. This can result in relatively high surface irregularities and a relatively high inhomogeneity of the resulting copper-containing bond and thus result in a relatively high surface roughness of the copper-containing bond. Surface roughness may be measured by atomic force microscopy, by optical microscopy, in particular by three-dimensional optical microscopy, or by profilometry, in particular by three-dimensional profilometry. Thus, sintering of the coated metal foil according to the invention enables relatively low surface irregularities and a relatively high homogeneity of the resulting copper-containing bond and thus enables a relatively low surface roughness of the copper- containing bond.

[0093] By forming nanoparticles in situ before sintering, the coated metal foil according to the invention avoids a relatively high surface irregularity and a relatively high inhomogeneity of a copper containing bond. Therefore, the coated metal foil according to the invention enables a relatively good surface control by ensuring a relatively low surface roughness. The relatively low surface roughness and relatively good surface control of the coated metal foil according to the invention is enabled by a relatively low total weight of solvent, in particular water, and / or by a relatively low total weight of the optional surfactant in relation to the total weight of the coated metal foil according to the invention. The total weight of solvent, in particular water, and the optional surfactant in relation to the total weight of the coated metal foil according to the invention may be in particular at most 0.5 wt.%, in particular at most 0.1 wt.%, in particular at most 0.05 wt.%, in particular at most 0.01 wt.%, in particular at most 0.005 wt.%, in particular at most 0.001 wt.%. Furthermore, sintering of the coated metal foil according to the invention results in a relatively thin copper- containing interlayer / interconnect layer between the two solid substrates. Since the coated metal foil comprises a relatively low total weight of residual solvent and / or the optional surfactant after formation of the copper nanoparticles on the first surface and the second surface of the foil, the properties of the resulting copper-containing interlayer / interconnect layer formed by the coated metal foil according to the invention are similar to the properties of bulk metal, in particular of bulk copper. Thus, the copper-containing interlayer / interconnect layer shows a relatively high electrical conductivity and a relatively high thermal conductivity. If the foil is of a copper alloy, a good thermal conductivity during the sintering and a good electrical conductivity after sintering is enabled, which can be both be further increased if the foil is of copper. In particular, the electrical conductivity and the thermal conductivity of the copper-containing interlayer / interconnect layer of the coated metal foil according to the invention are respectively higher than the electrical conductivity and the thermal conductivity of a copper-containing interlayer / interconnect layer obtained by a conventional copper-containing sinter paste having a relatively high total weight of solvents and / or other organic components.

[0094] The first substrate may be made of or consist of a metal or a metal oxide and the second substrate may be made of or consist of the metal, a further metal, the metal oxide, a further metal oxide or a surface mount device component. The metal or the further metal may be gold, silver, nickel, copper, pre-treated copper or tin. The metal oxide or the further metal oxide may be aluminum oxide. The surface mount device component may be a capacitor, a chip resistor, a crystal oscillator, a diode, a fuse, an inductor, an integrated circuit, an LED, a network resistor, a transformer or a transistor.

[0095] The pre-treated copper may be pre-treated for example by coating a copper substrate with an organic surface protection (OSP) layer, wherein the OSP layer protects the copper against oxidation and is dissolved during the soldering process. Alternatively, other methods, e.g. sol-gel application or CVD, can be used to coat the copper substrate with a protective layer in order to protect the copper against oxidation.

[0096] The invention further concerns the use of the coated metal foil according to the invention for forming a copper-containing interlayer / interconnect layer between two surfaces of solid substrates, in particular for die-attach bonding, in microelectronics packaging, in electric vehicle technologies, in hybrid electric vehicle technologies, in high power electronics packaging, and / or in thick film technology. In particular, the invention concerns the use of the coated metal foil according to the invention for die-attach bonding. The invention further concerns the use of the coated metal foil according to the invention for forming a conductive path on a solid substrate. The invention further concerns the use of the coated metal foil according to the invention for substrate attach. The microelectronics packaging may be WBG semiconductor packaging. The high power electronics packaging may be high power light emitting diode packaging. The inventors of the present invention found that the use of the coated metal foil according to the invention for forming a copper-containing interlayer / interconnect layer between two solid substrates or for forming a conductive path on a solid substrate enables a relatively low thermal load of the solid substrate(s) and any electronic component, in particular the surface mount device component as specified above, on the substrate(s) when forming the interconnection layer or conductive path. This is due to the relatively low sintering temperature required to sinter the coated metal foil according to the invention.

[0097] If the coated metal foil according to the invention is used for forming an electrically and / or thermally conductive path on a solid substrate, the solid substrate may be any substrate defined above as first or second substrate as far as it is not an electrically conductive first or second substrate. If the coated metal foil according to the invention is used for substrate attach, the substrate may be any substrate defined above as first or second substrate, an aluminum substrate, in particular a metallized aluminum substrate, in particular a copper-metallized aluminum substrate, a ceramic substrate, in particular a metallized ceramic substrate, in particular a copper-metallized ceramic substrate, a directed bonded copper substrate or a polymer substrate, in particular a polyimide substrate.

[0098] All features indicated in the specification are to be understood as features applicable to all embodiments of the invention. This means, for example, that a feature indicated for the coated metal foil according to the invention can also be applied to the method for producing the coated metal foil according to the invention, the method for forming a copper-containing interlayer / interconnect layer between two solid substrates, and / or the use according to the invention, and vice versa. Furthermore, as far as not specified the term “average” always means “arithmetic average”. As far as not specified the term “particle diameter” always means a diameter determined by image analysis of electron microscope or scanning electron microscope images. This means that the diameter is always a diameter determined from a plan view, i. e. a top view, of the particle and that material from the foil to which the particle is sintered is not considered when the diameter is determined. In case of an irregularly shaped particle, the diameter of this particle can be determined for the purpose of the present invention by measuring the largest and the smallest diameter of this particle and calculating the arithmetic mean of the two values thus obtained. The arithmetic average particle diameter can be determined by determining the diameters of all or of a representative number of the particles and calculating the arithmetic average of these diameters. The representative number can be a value between 30 and 1000, in particular between 40 and 400, in particular between 50 and 200. The abbreviation "wt.%" means "percent by weight".

[0099] The invention will be explained in more detail with reference to the following embodiments.

[0100] Figs. 1 a) and 1 b) show SEM images of the layer comprising copper nanoparticles on the first surface,

[0101] Fig. 2 shows a profilometer analysis of a coated metal foil having five copper nanoparticle layers or ten copper nanoparticle layers,

[0102] Fig. 3 shows SEM images of the layer comprising copper nanoparticles after heating to different temperatures,

[0103] Fig. 4 shows SEM images of the layer comprising copper nanoparticles after heating with different thermal ramp rates and Fig. 5 shows SEM images of the layer comprising copper nanoparticles on different foil materials.

[0104] Example 1 : Mixtures

[0105] The following mixtures were tested:

[0106] Table 1: Mixtures Table 1 shows different mixtures I inks and their respective compositions. As can be seen in Table 1 , the mixtures No. 1 to 4 do not contain the optional surfactant.

[0107] Example 2: Mixture production

[0108] Copper formate tetrahydrate is mixed with distilled water. The solubility of copper formate tetrahydrate in water is 125 g / L. Based on this, copper formate tetrahydrate is added to water in a ratio of 1 :8. The solution is stirred in a magnetic stirrer for 1 hour at room temperature at 500 rpm. After stirring, the suspension is left for 2 to 3 hours in order for the undissolved salt particles to settle. Subsequently, the solution is filtered to obtain the mixture as specified according to step b) of the invention. The solution is then stored at room temperature until further use.

[0109] After the mixture I ink is prepared, it is characterized using viscosity and contact angle measurements. To improve the wettability of the substrate the mixture is applied to, optionally surfactants are added in different weight ratios to the mixture. For example, in order to improve the wetting of mixture on the foil, optionally BYK®-333 can be used. BYK®-333 is a polyether-modified polydimethylsiloxane. Exemplarily, BYK®-333 is added in a weight ratio of 0.5 wt.% to the mixture. The mixture is filled into an atomizer for further use.

[0110] Example 3: Application of the mixture to the foil and in situ copper nanoparticle formation

[0111] After preparation of the mixtures No. 1 to 9 as indicated in examples 1 and 2, the mixtures were each applied independently from each other to a first surface of nine pre-treated copper foils, respectively, by spray coating using an atomizer. Pre-treatment of the copper foils occurred before application of the mixtures by heating to 180 °C for 30 minutes in high vacuum followed by plasma treatment. Even though spray coating as application method is exemplarily shown, other application methods, for example aerosol jet printing, screen printing or inkjet printing were also performed in other examples in order to apply the mixtures to the first surface of the copper foil. Once the mixtures were applied, each of the copper foils together with each of the mixtures were heated in a reflow oven to a temperature of 250 °C and maintained for 5 minutes under a formic acid enriched nitrogen atmosphere. This results in the in situ formation of a layer comprising copper nanoparticles on the first surface by decomposition of the mixture on the first surface.

[0112] After the respective formation of the layer comprising copper nanoparticles on the first surface, each of the mixtures were applied on the respective second surface of the same copper foil and heated as indicated above. This results in the in situ formation of a layer comprising copper nanoparticles on the second surface by decomposition of the mixtures on the second surfaces. Thus, the coated metal foil according to the invention is obtained.

[0113] After in situ copper nanoparticle formation, exemplarily the layer comprising copper nanoparticles on the first surface by application and heating of mixture 3 as indicated in example 1 is analyzed using a Zeiss Leo 1430 scanning electron microscope (SEM). The results are given in Fig. 1 a and 1 b. Fig. 1 a and Fig. 1 b show SEM images of the layer comprising copper nanoparticles of the coated metal foil after oven sintering.

[0114] As can be seen in Fig. 1 a and Fig. 1 b, the layer comprising copper nanoparticles shows a homogenous and regular surface structure. Furthermore, sinter neck formation between the copper nanoparticles can be seen in Fig. 1 b.

[0115] Example 4: Formation of a copper-containing interlayer connecting a first substrate with a second substrate via the coated metal foil

[0116] A coated metal foil is placed on a copper substrate by means of a pick-and-place machine for accurate placing. A test chip is then placed on the coated metal foil by means of a pick-and-place machine for accurate placing. Subsequently, sintering is performed under application of 10 MPa bonding pressure at 275 °C for 30 min under nitrogen atmosphere with a ramp rate of 1 K / s. After sintering, the arithmetic average shear strength value of the sintered interconnect is measured by a XYZ Condor Sigma Lite shear tester under a shear speed of 25 pm / s. The arithmetic average shear strength value of the sintered interconnect is 30 MPa.

[0117] Example 5: Modulating the thickness of the layer comprising copper nanoparticles by number of layers comprising copper nanoparticles

[0118] Exemplarily, mixture No. 5 as indicated in example 1 is applied to each the first surface and the second surface of two copper foils and heated as indicated in example 3. This results in the formation of a first layer comprising copper nanoparticles on each the first surface and the second surface of the two copper foils. Subsequently, mixture No. 5 as indicated in example 1 is applied to each first layer comprising copper nanoparticles on each the first surface and the second surface of the coated metal foils and heated as indicated in example 3. This is repeated until either a total of five cycles for or a total of ten cycles is achieved. Thus, a total of five layers comprising copper nanoparticles is formed on one copper foil and a total of ten layers comprising copper nanoparticles is formed on the other copper foil.

[0119] Both copper foils are analyzed by profilometer measurement. The results are given in Fig. 2. The left part of Fig. 2 shows the profilometer measurement of the coated metal foil having a total of five layers comprising copper nanoparticles. The right part of Fig. 2 shows the profilometer measurement of the coated metal foil having a total of ten layers comprising copper nanoparticles.

[0120] As can be seen in Fig. 2, the average thickness of the coated metal foil having a total of five layers comprising copper nanoparticles is approximately 450 nm, whereas the average thickness of the coated metal foil having a total of ten layers comprising copper nanoparticles is approximately 900 nm. Thus, by repeating the application and heating of the mixture on the foil, a modulation of the thickness of the layer comprising copper nanoparticles is enabled. Example 6: Effect of the temperature on arithmetic average particle diameter of copper nanoparticles

[0121] Exemplarily, mixture No. 7 as indicated in example 1 is applied to each the first surface and the second surface of two copper foils as indicated in example 3. The heating as indicated in example 3 was performed by oven sintering at either 200 °C for 15 min or at 250 °C for 15 min.

[0122] After in situ copper nanoparticle formation, each of the layers comprising copper nanoparticles on the first surface is analyzed using a Zeiss Leo 1430 scanning electron microscope (SEM). The results are given in Fig. 3. The left part of Fig. 3 shows an SEM image of a layer comprising copper nanoparticles of the coated metal foil formed after oven sintering at 200 °C for 15 min. The right part of Fig. 3 shows an SEM image of a layer comprising copper nanoparticles of the coated metal foil formed after oven sintering at 250 °C for 15 min.

[0123] As can be seen on the left part of Fig. 3, oven sintering at a lower temperature (200 °C, 15 min) results in the formation of copper nanoparticles having an arithmetic average particle diameter of 180 nm. In contrast, as can be seen on the right part of Fig. 3, oven sintering at a higher temperature (250 °C, 15 min) results in the formation of copper nanoparticles having an arithmetic average particle diameter of 500 nm. Thus, a higher temperature of heating results in a bigger arithmetic average particle diameter of the in situ formed copper nanoparticles on the copper foil.

[0124] Example 7: Effect of the thermal ramp rate on arithmetic average particle diameter of copper nanoparticles

[0125] Exemplarily, mixture No. 7 as indicated in example 1 is applied to each the first surface and the second surface of two copper foils as indicated in example 3. Once the mixtures were applied, one copper foil together with the mixture was heated in a reflow oven to a temperature of 250 °C with a thermal ramp rate of 50 °C / min for 5 minutes under a formic acid enriched nitrogen atmosphere. The other copper foil together with the mixture was heated by laser sintering using a 100 W laser with a scanning speed of 6 mm / s at 500 °C / ms for 0.5 ms under a formic acid enriched nitrogen atmosphere.

[0126] After in situ copper nanoparticle formation, each of the layers comprising copper nanoparticles on the first surface is analyzed using a Zeiss Leo 1430 scanning electron microscope (SEM). The results are given in Fig. 4. The left part of Fig. 4 shows an SEM image of a layer comprising copper nanoparticles of the coated metal foil formed after oven sintering. The right part of Fig. 4 shows an SEM image of a layer comprising copper nanoparticles of the coated metal foil formed after laser sintering.

[0127] As can be seen on the left part of Fig. 4, oven sintering at a lower thermal ramp rate results in the formation of copper nanoparticles having an arithmetic average particle diameter of 200 nm. In contrast, as can be seen on the right part of Fig. 4, laser sintering at a higher thermal ramp rate results in the formation of copper nanoparticles having an arithmetic average particle diameter of 50 nm. Thus, a higher thermal ramp rate results in a smaller arithmetic average particle diameter of the in situ formed copper nanoparticles on the copper foil.

[0128] Example 8: Effect of the foil material on arithmetic average particle diameter of copper nanoparticles

[0129] Exemplarily, mixture No. 1 as indicated in example 1 is applied as indicated in example 3 to each the first surface and the second surface of an aluminum foil, a copper foil or a polyimide foil. Subsequently, heating as indicated in example 3 was performed.

[0130] After in situ copper nanoparticle formation, each of the layers comprising copper nanoparticles on the first surface of each surface material is analyzed using a Zeiss Leo 1430 scanning electron microscope (SEM). The results are given in Fig. 5. The top part of Fig. 5 shows an SEM image of a layer comprising copper nanoparticles of a coated aluminum foil formed after oven sintering. The middle part of Fig. 5 shows an SEM image of a layer comprising copper nanoparticles of a coated copper foil formed after oven sintering. The bottom part of Fig. 5 shows an

[0131] SEM image of a layer comprising copper nanoparticles of a coated polyimide foil formed after oven sintering.

[0132] As can be seen in Fig. 5, only the layer comprising copper nanoparticles on the copper foil showed the formation of a homogenous and regular copper nanoparticle layer showing effective sinter neck formation between the copper nanoparticles. Thus, the in situ formed copper nanoparticle layer is dependent on the material on which the mixture as specified according to step b) of the invention is applied.

Claims

Claims1. A coated metal foil for forming a copper-containing interlayer between a first solid substrate and a second solid substrate, wherein the coated metal foil is produced by the following steps: a) Providing a copper formate, a solvent and a foil of copper or of a copper alloy or of a nickel-iron alloy plated on both sides with copper which foil has a first surface and a second surface, b) mixing the copper formate and the solvent for obtaining a mixture in which the copper formate is completely dissolved, and either c1) applying the mixture obtained in step b) to the first surface and the second surface of the foil provided at step a) and d1 ) heating the mixture on the first surface and the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface and on the second surface of the foil provided at step a) for obtaining the coated metal foil, or c2) applying the mixture obtained in step b) to the first surface of the foil provided at step a), heating the mixture on the first surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C andmaintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface of the foil provided at step a) for obtaining a first coating on the first surface and d2) applying the mixture obtained in step b) to the second surface of the foil provided at step a), heating the mixture on the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the second surface of the foil provided at step a) for obtaining a second coating on the second surface and thus the coated metal foil, wherein at least the heating and the maintaining is performed in a reducing atmosphere or in an inert atmosphere, wherein the copper nanoparticles have an arithmetic average particle diameter in the range of 20 nm to 200 nm, wherein a particle diameter is determined by image analysis of electron microscope or scanning electron microscope image(s).

2. Coated metal foil according to claim 1 , wherein the mixture has a viscosity in the range of 2.5 mPas to 10 mPas, measured at 20 °C, and / or wherein the total weight of the copper formate in the mixture obtained in step b) in relation to the total weight of the mixture is at most 5 wt.%.

3. Coated metal foil according to claim 1 or 2, wherein the layer comprising copper nanoparticles on the first surface and on the second surface has in each case independently from each other a thickness of at least 80 nm and at most 500 nm.

4. Coated metal foil according to any of the preceding claims, wherein the copper alloy is bronze or brass, and / or wherein the foil provided at step a) has a thickness of at least 5 pm and at most 200 pm.

5. Coated metal foil according to any of the preceding claims, wherein the first surface and / or the second surface of the foil provided at step a) has / have a number of superficial recesses formed by means of etching with an acid before step c1 ) or c2) or formed by means of plasma treatment before step c1 ) or c2), wherein any one of the superficial recesses extends either from the first surface in the direction from the first surface to the second surface of the foil provided at step a) or from the second surface in the direction from the second surface to the first surface of the foil provided at step a).

6. Coated metal foil according to any of the preceding claims, wherein a surfactant is further provided in step a) and wherein the surfactant is mixed with the copper formate and the solvent in step b).

7. Coated metal foil according to claim 6, wherein the surfactant is a water-soluble polymer, a silicon-based organic polymer or a mixture of ethyl cellulose and an organic solvent.

8. Coated metal foil according to any of the preceding claims, wherein the copper formate is Cu(ll) formate or a hydrate of copper formate, in particular Cu(ll) formate tetrahydrate.

9. Coated metal foil according to any of the preceding claims, wherein the solvent is water or an acidic aqueous solution or a mixture of water and ethanol.

10. Coated metal foil according to any of the preceding claims, wherein the coated metal foil has on each of the first surface and the second surface of the foil provided at step a) independently from each other at least one andat most 20 layer(s) comprising copper nanoparticles and wherein each of the layers has in each case independently from each other a thickness of at least 80 nm and at most 500 nm.11 . Coated metal foil according to any of the preceding claims, wherein the mixture obtained in step b) is applied in each case independently from each other to 90 % to 100 %, in particular to 100 %, of an area of the first surface and of an area of the second surface of the foil provided at step a).

12. A method for producing the coated metal foil according to any of the preceding claims comprising the steps of a) providing a copper formate, a solvent and a foil of copper or of a copper alloy or of a nickel-iron alloy plated on both sides with copper which foil has a first surface and the second surface, b) mixing the copper formate and the solvent for obtaining a mixture in which the copper formate is completely dissolved, and either c1) applying the mixture obtained in step b) to the first surface and the second surface of the foil provided at step a) and d1 ) heating the mixture on the first surface and the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface and on the second surface of the foil provided at step a) for obtaining the coated metal foil,or c2) applying the mixture obtained in step b) to the first surface of the foil provided at step a), heating the mixture on the first surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the first surface of the foil provided at step a) for obtaining a first coating on the first surface and d2) applying the mixture obtained in step b) to the second surface of the foil provided at step a), heating the mixture on the second surface of the foil provided at step a) or the foil provided at step a) together with the mixture to a temperature in the range of 175 °C to 300 °C and maintaining the mixture or the foil provided at step a) together with the mixture at the temperature until a layer comprising copper nanoparticles is formed on the second surface of the foil provided at step a) for obtaining a second coating on the second surface and thus the coated metal foil, wherein at least the heating and the maintaining) is performed in a reducing atmosphere or in an inert atmosphere, wherein the copper nanoparticles have an arithmetic average particle diameter in the range of 20 nm to 200 nm, wherein a particle diameter is determined by image analysis of electron microscope or scanning electron microscope images.

13. Method according to claim 12, wherein the copper formate is Cu(ll) formate or a hydrate of copper formate, in particular Cu(ll) formate tetrahydrate, and / or wherein the solvent is water or an acidic aqueous solution or a mixture of water and ethanol and / or wherein the first surface and / or the second surface of the foil provided at step a) has a number of superficial recessesformed by means of etching with an acid before step c1 ) or c2) or formed by means of plasma treatment before step c1 ) or c2), wherein any one of the superficial recesses extends either from the first surface in the direction from the first surface to the second surface of the foil provided at step a) or from the second surface in the direction from the second surface to the first surface of the foil provided at step a) and / or wherein the maintaining is at least 1 minute and at most 30 minutes.

14. Method according to claim 13 or 14, wherein the reducing atmosphere is provided by means of a mixture of evaporated formic-acid and nitrogen gas or by means of a gas mixture of hydrogen and an inert gas, in particular a gas mixture of nitrogen and hydrogen or a gas mixture of argon and hydrogen, and wherein the inert atmosphere is provided by means of an inert gas, in particular nitrogen gas, carbon dioxide gas, helium gas, neon gas or argon gas, or a mixture of at least two inert gases.

15. Use of the coated metal foil according to any of claims 1 to 11 for forming a copper-containing interlayer between two surfaces of solid substrates for forming a conductive path on a solid substrate or for substrate attach.

Images