Coated copper or copper alloy foil and method for producing coated copper or copper alloy foil

Abstract

The present invention provides a coated metal foil for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, or between a solid substrate and a metal foil. The coated metal foil comprises or consists of a copper, first copper alloy, or copper-plated nickel-iron alloy metal foil having a first surface and a second surface, and a composition coated on the first surface and / or the second surface of the metal foil, the composition comprising or consisting of an organic binder and flakes of copper, the first copper alloy, or the second copper alloy, the flakes containing internal pores.

Description

[Technical Field]

[0001] The present invention relates to a copper or copper alloy coated metal foil, a method for producing a copper or copper alloy coated metal foil, and the use of a copper or copper alloy coated metal foil for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate. In the sense of the present invention, the intermediate layer is understood as an interconnection layer, i.e., a solid metal interconnection between the first solid substrate and the second solid substrate. [Background technology]

[0002] Die attach bonding is a key process for achieving high-temperature operation of power semiconductor devices. High-lead solder has been the preferred and established choice over the past few decades. However, strict regulations regarding hazardous substances have restricted the use of high-lead solder, and the few exceptions that currently exist are expected to be banned in the near future. As the use of wide-bandgap (WBG) semiconductor devices increases, it is essential to find sustainable and reliable alternatives, both economically and technically. Interconnect materials and technologies must be able to meet the stringent requirements of WBG semiconductor devices while simultaneously being economical to enable mass production.

[0003] Currently, there are two established methods for producing high-temperature bonds while taking advantage of relatively low bonding temperatures: transient liquid phase (TLP) bonding and particle sintering. Although various studies have reported successful creation of in-situ phases with higher remelting points, the thermomechanical fracture behavior of TLP joints appears to have significant drawbacks. This is due to the microstructure of TLP joints, which are composed of brittle intermetallic compounds (IMCs). However, in the case of particle sintering, some reports suggest that Ag sintered joints are promising candidates for high-temperature WBG applications. Upon sintering, an all-metal connection can be achieved. In addition to the material's high thermal conductivity, the interconnects have the advantage of being established at relatively low temperatures (250°C) yet stable at high temperatures (above 300°C). However, sintered Ag particles have drawbacks, such as high cost and poor electromigration resistance.

[0004] Copper is approximately 100 times cheaper than silver, more abundant, easily recycled, and readily available. Copper has an overall lower carbon footprint than silver. It has a lower coefficient of thermal expansion than silver and roughly the same electrical and thermal conductivity. However, copper's higher melting point means that sintering temperatures are also slightly higher than for silver. Additionally, copper's tendency to oxidize rapidly in air is a major drawback, adversely affecting the mechanical and thermal integrity of the joint.

[0005] According to Non-Patent Document 1, paste formulations were obtained by mixing etched copper or brass flakes with PEG600 as a binder. Prior to preparing the paste formulations, the flakes were treated with 12 M HCl to etch zinc from the brass flakes and copper oxide from the copper flakes. After etching, the flakes were thoroughly washed, first with distilled water and then with isopropanol. This treatment removed zinc, chlorine, and all organic components from the flakes. The use of PEG600 in the paste formulations was reported to enable in situ reduction of Cu oxide during sintering.

[0006] Non-Patent Document 2 discloses a sintering paste composed of etched microscale brass metal pigment flakes or pure copper flakes and a binder mixture of α-terpineol and polyethylene glycol 600 (PEG600). Before compounding the paste, the flakes were treated with HCl for etching, washed with distilled water, washed with isopropanol, and dried in air. This treatment removed zinc, chlorine, and all organic components from the flakes.

[0007] Patent Document 1 describes a metal paste containing 65 to 85% by weight of metal particles and 10 to 35% by weight of an organic solvent, in which 70 to 100% by weight of the metal particles have a specific surface area of ​​1.9 to 3.7 m 2 A metal paste is disclosed that comprises organically coated copper flakes having a carbon content in the range of 0.15 to 0.9 / g, a total oxygen content in the range of 2 to 4 wt%, and a total carbon to total oxygen weight ratio in the range of 0.25 to 0.9. The proportion of the organic coating can be in the range of 2 to 5 wt% based on the weight of the organically coated copper flakes.

[0008] Patent Document 2 discloses a method for fabricating conductive features on a substrate, the method including the steps of: (a) providing a precursor composition containing a copper metal precursor compound, wherein the precursor composition has a viscosity of 1000 centipoise or less; (b) depositing the precursor composition on the substrate using a direct-write tool; and (c) heating the precursor composition to a conversion temperature of about 350°C or less to form conductive features having a resistivity of about 40 times or less that of bulk copper. The substrate can be selected from the group consisting of polyfluorine compounds, polyimides, epoxies (including glass-filled epoxies), polycarbonates, cellulosic materials (i.e., wood or paper), acetate, polyesters, polyethylene, polypropylene, polyvinyl chloride, acrylonitrile butadiene (ABS), flexible fiberboard, nonwoven polymer fabrics, cloth, metal foils, semiconductors, ceramics, glass, and combinations thereof. Also disclosed is a method for fabricating copper conductive features on a substrate surface, the method comprising the steps of: (a) providing a precursor composition comprising a copper metal precursor compound having a viscosity of 100 centipoise or less; (b) depositing the precursor composition on the substrate using an aerosol jet device to form traces having a minimum size of about 100 microns or less; and (c) heating the precursor composition to a temperature of about 250°C or less to form conductive features having a minimum feature size of about 100 microns or less and a resistivity of about 100 times or less that of bulk copper metal.

[0009] Non-Patent Document 3 discloses the introduction of Triton X-100 detergent into a metal-organic solution and the application of the detergent / metal-organic solution to a substrate to form a uniform spin-on thin film. The precursor copper formate film is spin-deposited using a metal-organic solution prepared with a solvent mixture of methanol and Triton X-100 (volume ratio 50:1). Direct writing of copper patterns on glass and polyimide substrates has been achieved using a KrF laser (248 nm).

[0010] Patent Document 3 discloses a nanometer conductive ink containing an organic copper salt, a solvent, a surfactant, and nanometer metal conductive particles, and a method for preparing the same. The method includes a first step of uniformly mixing the organic copper salt, the solvent, the surfactant, and the nanometer metal conductive particles in a certain 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] Patent Document 4 discloses a method for forming metal traces on a substrate. The method includes inkjet printing a chemical ink containing a metal-containing compound onto a substrate surface to form ink droplets thereon, and heating the substrate to a suitable elevated temperature. The ink droplets undergo at least one of decomposition and reduction due to the substrate temperature, forming metal on the substrate in a controlled atmosphere.

[0012] The Die Top System (DTS®) from Heraeus Electronics consists of copper foil that is plated on one side with NiPdAu, NiAu, or NiAg, and a silver-based sinter paste that is pre-applied to the plating. The die and DTS® can be sintered together in one step using standard sintering equipment. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] European Patent No. 3626785 [Patent Document 2] International Patent Application Publication No. 2006 / 041657 [Patent Document 3] Chinese Patent Application Publication No. 104341860 [Patent Document 4] US Patent Application Publication No. 2006 / 0258136 [Non-patent literature]

[0014] [Non-Patent Document 1] Bhogaraju, S.K. et al., "Novel approach to copper sintering using surface enhanced brass micro flakes for microelectronics packaging," Journal of Alloys and Compounds, Vol. 844, 2020, pp. 156043 [Non-patent document 2] Bhogaraju, SK et al., "Die-attach bonding with etched micro brass metal pigment flakes for high-power electronics packaging," ACS Appl. Electron. Mater. 2021, Vol. 3, No. 10, pp. 4587-4603 [Non-patent document 3] Lin CT et al., "Effect of surfactant on casting metalorganic films and writing copper metal patterns," J. Mater. Res., Vol. 6, No. 4, April 1991, pp. 760-765 Summary of the Invention [Problem to be solved by the invention]

[0015] The problem solved by the present invention is to provide an alternative interconnect material, a method for manufacturing the alternative interconnect material, and uses of the alternative interconnect material. The alternative interconnect material should enable, in particular, die topside connections, first-level interconnects (i.e., die to substrate), and second-level interconnects (i.e., substrate to baseplate), the formation of conductive and / or thermally conductive paths on solid substrates, and the bonding of substrate connections. The alternative interconnect material should enable sintering with short sintering times, low sintering temperatures, and / or low sintering pressures. Sintering the alternative interconnect material results in a copper-containing interlayer with excellent thermomechanical fatigue resistance, high thermal conductivity, and high electrical conductivity. Furthermore, sintering the alternative interconnect material results in a homogeneous bonding interface. [Means for solving the problem]

[0016] This problem is solved by the subject matter of claims 1, 12 and 16. Embodiments of the invention are the subject matter of claims 2-11 and 13-15.

[0017] The present invention provides a coated metal foil for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, or between a solid substrate and a metal foil. The coated metal foil comprises or consists of a copper, first copper alloy, or copper-plated nickel-iron alloy metal foil having a first surface and a second surface, and a composition coated on the first surface and / or the second surface of the metal foil. The composition comprises or consists of an organic binder and flakes of copper, the first copper alloy, or the second copper alloy. The flakes contain internal pores.

[0018] In this application, the term "metal foil" refers to the metal foil itself, i.e., not including any composition coated on the first and / or second surfaces of the metal foil. However, organic residues may be present on the foil that remain on the foil as a result of standard manufacturing processes. The organic residues may be oils.

[0019] Metal foils coated with the composition are referred to as "coated metal foils." Metal foils and flakes are formed from elemental metals, i.e., metals in elemental, rather than ionic, form. The metal is copper or a copper alloy. In the case of metal foils, the metal can also be a copper-plated nickel-iron alloy. The nickel-iron alloy can be FeNi36, commonly known as Invar®. Copper-plated nickel-iron alloy foils are usually coated with copper on both sides. Such foils based on FeNi36 are particularly interesting for applications with low thermal expansion coefficients. It can be a commercially available foil, such as a CIC (copper / Invar® / copper) foil, in which the core layer is made of an iron-nickel alloy with a nickel content of 36% and accounts for 60% of the composite volume. The copper on both sides accounts for 20% of the composite volume. Such foils are commercially available as CIC 20 / 60 / 20 from Schlenk Metallfolien GmbH & Co. KG, Germany.

[0020] The inventors of the present invention have discovered that the coated metal foils of the present invention can be manufactured using standard metal foils with various material properties (e.g., various tensile strengths or thicknesses). This ability to fine-tune the material properties of the coated metal foils of the present invention, thereby enabling them to meet the requirements of a desired application, is highly advantageous. The material properties of the metal foil can be complemented by providing a porous microstructure by coating the metal foil with a composition containing flakes with internal pores. The thickness of the coating can be independently varied on either side of the metal foil using suitable standard application methods, such as screen printing, stencil printing, spray coating, or spin coating. All of this allows for fine tuning of the interconnects, their thickness, and thermomechanical fatigue resistance. This allows for application-specific and / or package-specific requirements to be met.

[0021] Furthermore, the coated metal foil of the present invention allows for sintering with short sintering times, low sintering temperatures, and / or low sintering pressures. Because organic binders typically evaporate or decompose at temperatures applied before or at the beginning of the sintering process, the coated copper-containing metal foil of the present invention allows for sintering with minimal organic residue. Therefore, the coated metal foil can be used in certain applications where relatively low sintering pressures (especially pressures of 250 kPa or less) and relatively low sintering temperatures (especially temperatures of 260°C or less) are applied. For miniaturization in certain applications, such as batteries or high-power light-emitting diodes, the metal foil may be an ultrathin metal foil with a thickness of less than 10 μm. On this foil, a thin layer of the composition can be applied to the first and / or second surfaces to create a thin bond line thickness, such as a bond line thickness of less than 5 μm. The bond line thickness is the thickness of the intermediate layer between the first solid substrate and the second solid substrate.

[0022] The composition can be coated on a first surface and an additional composition can be coated on a second surface, the additional composition comprising or consisting of an organic binder or additional organic binder and copper, a first copper alloy, or a second copper alloy flake or additional flakes, the additional flakes comprising internal pores.

[0023] By coating a first surface with a composition and a second surface with an additional composition, it is possible to tailor each of these compositions to the specific requirements of each surface. For example, if the first surface is in a more oxidized state than the second surface, the composition can be configured to be more suitable than the additional composition for in situ reduction of copper oxide during the manufacturing process of the coated metal foil. In such cases, the additional composition need not contain the same amount of a reducible organic binder, such as a diol or polymeric glycol, as the composition.

[0024] The average grain size of the metal foil on the first and / or second surfaces of the metal foil may be 2 μm or more and 100 μm or less. The average grain size of the crystal grains in the mutually independent flakes and / or further flakes may be 5 nm or more and 200 nm or less, particularly 20 nm or more and 200 nm or less. The crystal grains are any crystals that make up a solid metal. In the case of the first and / or second surfaces, this metal is copper or a first copper alloy or a second copper alloy.

[0025] "Average grain size" refers to the average grain size given as the average diameter measured from the average area measured by electron backscatter diffraction (EBSD) according to ISO / DIS 13067(en). It is a value determined from two-dimensional measurements related to the average three-dimensional size of the grains or clusters of crystals that form the polycrystalline material by their steric relationships. The grains or clusters of crystals may contain 30 to 1000 particles or crystals, in particular 40 to 400 particles or crystals, in particular 50 to 200 particles or crystals.

[0026] Grain boundaries act as pathways for the diffusion of atoms or molecules during sintering. During sintering, the application of sintering temperatures causes atoms or molecules to move across these boundaries, improving diffusion and therefore the sintering process. It has been found that a given average grain size results in relatively fast sintering and a relatively high densification rate.

[0027] In one embodiment, the ratio of the average grain size in the flakes to the average grain size on the first surface and / or the second surface of the metal foil, and / or the ratio of the average grain size in the further flakes to the average grain size on the second surface of the metal foil, each independently of the other, does not exceed 0.002.

[0028] The effect of this ratio is to create an energy change between the surface of the flake or additional flakes and the first and / or second surfaces of the metal foil. This is because the flake or additional flakes have more grains in the same volume, and therefore more grain boundaries than the foil on its first and / or second surfaces. This difference in the number of grain boundaries promotes surface diffusion and sintering between the flakes. The smaller grains of the flakes tend to densify more quickly than the larger grains on the first and / or second surfaces of the metal foil. Therefore, there is a difference between the sintering of the flakes and the sintering of the first and / or second surfaces of the metal foil. In this way, the mechanical properties of the copper foil can be maintained and contributed to by the mechanical properties of the sintered flakes.

[0029] The first copper alloy can be bronze or brass. When the metal foil is made of the first copper alloy, the coated metal foil according to the present invention can have good thermal conductivity. Higher thermal conductivity of the coated metal foil according to the present invention can be obtained when the metal foil is made of copper. The brass can be a brass containing 0.5% or more by weight of zinc, particularly 1% or more by weight of zinc, and 36% or less by weight of zinc, particularly 30% or less by weight of zinc, particularly 25% or less by weight of zinc, particularly 20% or less by weight of zinc, and particularly 15% or less by weight of zinc. In particular, the brass can be alpha brass containing 36% or less by weight of zinc. The bronze can be a bronze containing 0.5% or more by weight of tin, particularly 1% or more by weight of tin, and 20% or less by weight of tin, particularly 15% or less by weight of tin, particularly 10% or less by weight of tin, and particularly 5% or less by weight of tin. The purity of the copper or first copper alloy may be 93% or more, in particular 95% or more, in particular 99% or more, in particular 99.95% or more, and up to 99.99%.

[0030] The thickness of the metal foil can be 5 μm or more, particularly 10 μm or more, particularly 25 μm or more, particularly 50 μm or more, and 200 μm or less, particularly 150 μm or less, particularly 100 μm or less, particularly 75 μm or less. 2For bonding areas of 100 μm or greater and for applications that may be exposed to high thermomechanical stresses during their operational life, bond line thicknesses greater than 100 μm are typically desired. This is because such thicknesses can compensate for the relatively high warpage and non-uniformity present on the surfaces of the first and second solid substrates being connected. For example, the first solid substrate can be an electronic component and / or the second solid substrate can be a base plate. Furthermore, bond line thicknesses greater than 100 μm provide superior ability to handle high thermomechanical stresses between the first and second substrates.

[0031] The present inventors have discovered that bond line thicknesses of greater than 100 μm can be provided when the metal foil is relatively thick, particularly 80, 90, or 100 μm or thicker, and the composition is coated on the first surface and / or second surface at a relatively thin thickness, particularly less than 10 μm.

[0032] Alternatively, the thermomechanical fatigue resistance can be adjusted by varying the porosity of the sintered composition and / or the additional composition coated on the first surface and / or the second surface of the metal foil.

[0033] The porosity of the sintered composition and / or additional compositions can be varied by varying the sintering parameters, individually or in combination. For example, sintering for 3 minutes instead of 5 minutes generally results in higher porosity. Similarly, applying a pressure of 1 MPa instead of 20 MPa during sintering will increase porosity. Also, sintering at 250°C instead of 300°C will increase porosity.

[0034] In addition to, or independently of, varying the porosity of the sintering composition and / or additional compositions, the metal foil and foil thickness can be selected to achieve the desired strength. Therefore, depending on the design and application requirements, both strategies can be used in combination or alone. The current state-of-the-art technology for large-area interconnects is the use of printed solder or sinter pastes with thicknesses of 150–400 μm, which requires relatively long pre-drying times and can result in voids after soldering / sintering.

[0035] Compared to the relatively thick solder pastes or sintering pastes used in the art, these coated metal foils have good sinterability, allowing for a faster sintering process with a relatively small amount of organic binder compared to a bond line of the same thickness achieved using sintering paste alone. Pre-drying the coated metal foil before the sintering process can further reduce the amount of organic binder. This low amount of organic binder results in a uniform interlayer with few or no unwanted irregular voids within the interlayer. This is particularly advantageous for large-area bonds, where bond thicknesses of at least 100 μm or more are desired and where a relatively large amount of organic binder would normally result in the formation of a relatively large amount of undesirable irregular voids and cavities. These cavities and voids reduce the reliability, thermal conductivity, and electrical conductivity of the interlayer and can cause localized heat spots during operation.

[0036] The metal foil, particularly copper foil, may be a single crystal foil or a polycrystalline foil. The first and / or second surfaces of the metal foil may be fully or partially oriented in a single crystallographic orientation, particularly the 111 crystallographic orientation, or may have a fully or partially disordered crystal structure.

[0037] Metal foils can be purchased, for example, as high strength alloy copper foils (e.g., HTA-520, HTA-600, HTA-750), ultra-smooth copper foils, or roll-clad / plain copper foils, all of which are offered by Schlenk Metallfolien GmbH & Co. KG.

[0038] The flakes and / or further flakes are formed from copper or a first or second copper alloy. The first or second copper alloy can be bronze or brass. If the flakes and / or further flakes are formed from a first or second copper alloy instead of copper, particularly if the first or second copper alloy is bronze or brass, good thermal conductivity of the coated metal foil according to the present invention is still possible. Higher thermal conductivity of the coated metal foil according to the present invention can be obtained when the flakes are formed from copper. The brass can be a brass containing 0.5% or more by weight of zinc, particularly 1% or more by weight of zinc, and 36% or less by weight of zinc, particularly 30% or less by weight of zinc, particularly 25% or less by weight of zinc, particularly 20% or less by weight of zinc, particularly 15% or less by weight of zinc. In particular, the brass can be alpha brass containing 36% or less by weight of zinc. In particular, the bronze may be a bronze containing 0.5% by weight or more of tin, in particular 1% by weight or more of tin, and not more than 20% by weight of tin, in particular not more than 15% by weight of tin, in particular not more than 10% by weight of tin, in particular not more than 5% by weight of tin. The purity of the copper or the first copper alloy or the second copper alloy may be 93% or more, in particular 95% or more, in particular 99% or more, in particular 99.95% or more and not more than 99.99%.

[0039] The flakes and / or further flakes may have an average particle size D50 measured by laser granulometry of 1 μm or more, particularly 2 μm or more, particularly 2.5 μm or more, particularly 3 μm or more, and 10 μm or less, particularly 7 μm or less, particularly 5 μm or less, particularly 4 μm or less, particularly 3.4 μm or less. Laser granulometry may be performed using laser diffraction measurements in accordance with ISO 13320 "Particle size analysis - laser diffraction method." Laser diffraction measurements may be performed using a Helos (trademark) laser diffractometer manufactured by Sympatec GmbH (38678 Clausthal-Zellerfeld, Germany). The average particle size D50 is the particle size corresponding to a cumulative percentage of 50%. For example, a powder sample with D50 = 50 μm means that 50% of the particles are larger than 50 μm and 50% of the particles are smaller than 50 μm.

[0040] The flakes, particularly copper flakes, may be monocrystalline or polycrystalline flakes. The copper flakes may be fully or partially oriented in a single crystallographic orientation (particularly the 111 crystallographic orientation), or may have a fully or partially disordered crystal structure.

[0041] The flakes can be obtained by ball milling and can have a layered, irregular, or cornflake-like shape. The inventors of the present invention have discovered that the layered, irregular, or cornflake-like shape of the flakes (especially flakes having an average particle size D50 of 5 μm or less, particularly an average particle size D50 of 4 μm or less, particularly an average particle size D50 of 3.4 μm or less, and particularly an average particle size D50 of 3 μm or less) allows the flakes to pack tightly together even when a low bonding force (e.g., a bonding force of less than 1 MPa) is applied, resulting in a better contact surface compared to spherical particles. The layered, irregular, or cornflake-like shape of the flakes, combined with their relatively thin thickness, also results in a relatively large surface area of ​​the flakes, which improves densification and coarsening of the flakes during sintering. This further promotes the formation of sintering necks and allows material movement during sintering. Any flake may, independently of one another, have a thickness in the range of 5 nm to 400 nm, in particular in the range of 20 nm to 350 nm, in particular in the range of 50 nm to 300 nm, in particular in the range of 100 nm to 275 nm, in particular in the range of 150 nm to 250 nm, in particular in the range of 175 nm to 200 nm.

[0042] The flakes and / or further flakes contain internal pores. Internal pores are defined as void spaces within the flakes or further flakes that are not filled with the copper, first copper alloy, or second copper alloy forming the flakes. Any internal pores are present within the grains of the copper, first copper alloy, or second copper alloy forming the flakes and / or further flakes. The diameter of any internal pore can be in the range of 2 nm to 30 nm, particularly in the range of 3 nm to 25 nm, particularly in the range of 5 nm to 20 nm, and particularly in the range of 10 nm to 15 nm. The diameter of the internal pores can be measured, for example, by image-based analysis of transmission electron microscope (TEM) or scanning electron microscope (SEM) images. The inventors of the present invention have discovered that, unlike interparticle pores, internal pores do not shrink during sintering, thereby enabling relatively high stress absorption (particularly relatively high thermomechanical and / or mechanical stress absorption) when temperature and / or pressure are applied to the intermediate layer formed from the coated metal foil. This stress absorption by the internal pores of the flakes or additional flakes provides a relatively high resistance to thermomechanical fatigue, thus improving the thermomechanical performance of the interlayer formed from the coated metal foil. The internal pores also limit crack propagation during the operational life of the interlayer. In the context of the present invention, thermomechanical stress refers to stresses generated in the interlayer due to mismatches in thermal expansion coefficients (i.e., due to differential expansion and contraction of the first and second solid substrates and the interlayer caused by thermal effects during and after sintering and / or during operation of a device including the interlayer between the first and second solid substrates). The inventors have discovered that the combination of a metal foil with a coating including flakes and / or additional flakes with internal pores is particularly advantageous in terms of high thermomechanical stress absorption of the interlayer formed between the first and second solid substrates, especially when the foil is coated on the first and second surfaces. This is because the metal foil, in addition to the flakes or additional flakes with internal pores, provides exceptional thermomechanical fatigue resistance as an interconnect / interlayer.This is further strengthened by the fact that the composition of the first surface of the foil and the further composition of the second surface can be different, thereby allowing for precise adaptation according to the design specifications, requirements and coefficients of thermal expansion (CTE) of the first and second solid substrates.

[0043] Furthermore, the inventors of the present invention have found that mechanical stresses applied to the intermediate layer are also more efficiently absorbed by the intermediate layer obtained from the coated metal foil of the present invention, which includes flakes and / or additional flakes containing internal pores, compared to intermediate layers obtained from copper or copper alloy flakes without internal pores. The intermediate layer obtained from the coated metal foil of the present invention after sintering exhibits improved resistance to thermomechanical fatigue compared to intermediate layers obtained from copper or copper alloy coated metal foils including copper or copper alloy flakes without internal pores. In the context of the present invention, thermomechanical fatigue refers to the superposition of cyclic mechanical loads and cyclic thermal loads, which leads to material fatigue. Therefore, the intermediate layer obtained from the coated metal foil of the present invention exhibits good thermomechanical performance, thereby ensuring good reliability, especially when relatively high thermomechanical stresses and / or relatively high mechanical stresses are applied to the intermediate layer.

[0044] The copper, first copper alloy, or second copper alloy flakes and / or further flakes have internal pores of 1 μm or less at any cross section, particularly at all cross sections, through any flake or further flake. 2 2 to 40 per pore, especially internal pores 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2 3 to 25 per pore, especially for internal pores of 1 μm 2 5 to 20 per pore, especially the internal pores are 1 μm 2 7 to 18 per pore, especially the internal pores are 1 μm 2 10-15 per pore, especially the internal pores are 1 μm 2In one embodiment, the copper, first copper alloy, or second copper alloy flakes and / or further flakes, particularly only the flakes, may have an average intra-particle pore density in the range of 12 to 13 per 1000 microns. 2 7 to 40 per pore, especially the internal pores are 1 μm 2 The flakes and / or further flakes have an average intra-particle pore density in the range of 7 to 30 per flake. In the context of the present invention, the average intra-particle pore density of the flakes or further flakes is defined as the average number of internal pores per area of ​​any given cross-section through the flake or further flake. The inventors of the present invention have discovered that the average intra-particle pore density of the flakes and / or further flakes in the composition and / or further composition defined in accordance with the present invention is not affected by sintering of the coated metal foil according to the present invention. Thus, the sintered flakes and / or further flakes in the copper-containing intermediate / interconnect layer between two solid substrates have internal pores of 1 μm or less at any given cross-section, and in particular at all given cross-sections, through the intermediate / interconnect layer. 2 2 to 40 per pore, especially internal pores 1 μm 2 7 to 40 per pore, especially the internal pores are 1 μm 2 7 to 30 per pore, especially the internal pores are 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2 3 to 25 per pore, especially for internal pores of 1 μm 2 5 to 20 per pore, especially the internal pores are 1 μm 2 7 to 18 per pore, especially the internal pores are 1 μm 2 10-15 per pore, especially the internal pores are 1 μm 2The average intra-particle pore density may range from 12 to 13 per particle. Even when single particles or flakes and / or additional flakes are sintered together within the intermediate / interconnecting layer, and there is no single particle / flake / additional flake present within the intermediate / interconnecting layer, the pores still originate from the particle / flake / additional flake, and the term "intrace pore density" is retained. The average intra-particle pore density of the flakes / additional flakes can be measured, for example, by gravimetric analysis, computed tomography analysis, or image-based analysis. The average intra-particle pore density of the flakes / additional flakes can be determined, for example, using the following method: 1. Embedding the flakes / further flakes in resin (especially epoxy resin), 2. Preparing Ultrathin Sections of Embedded Flakes / Further Flakes 3. Taking transmission electron microscope (TEM) and / or scanning electron microscope (SEM) images of ultrathin sections of embedded / further flakes; 4. Determining multiple intra-particle pore densities by calculating the number of internal pores within the ultra-thin sections of the flakes / further flakes relative to the total area of ​​the ultra-thin sections of the flakes / further flakes in multiple transmission electron microscope (TEM) images and / or multiple scanning electron microscope (SEM) images; 5. Calculate the average of the multiple intra-particle pore densities thus determined.

[0045] In this regard, step 4 can be performed by computer-assisted image analysis of the TEM and / or SEM images.

[0046] The inventors of the present invention have found that the internal pores are 1 μm 2 2 to 40 per pore, especially internal pores 1 μm 2 7 to 40 per pore, especially the internal pores are 1 μm 2 7-30 per pore, or internal pores 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2It has been discovered that metal foils coated with compositions comprising copper, first copper alloy, or second copper alloy flakes and / or additional flakes and / or additional compositions having an average intra-particle pore density in the range of 5 to 20 flakes per micron have improved resistance to thermo-mechanical fatigue of the interlayer, as opposed to interlayers obtained from copper or copper alloy metal foils coated with compositions comprising copper or copper alloy flakes without internal pores. Furthermore, it has been discovered that the interlayers have improved resistance to thermo-mechanical fatigue of the interlayer, as compared to interlayers obtained from copper or copper alloy metal foils coated with compositions comprising copper or copper alloy flakes without internal pores. 2 2 to 40 per pore, especially internal pores 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2 7-30 per pore, or internal pores 1 μm 2 The thermal and electrical conductivity of an intermediate layer obtained from a metal foil coated with a composition containing flakes and / or additional flakes and / or additional compositions having an average intra-particle pore density in the range of 5 to 20 per micron is not affected by the internal pores. This is the same or nearly the same as that of an intermediate layer obtained from a metal foil coated with a composition containing copper or copper alloy flakes without internal pores. Therefore, the thermal and electrical conductivity of an intermediate layer obtained from a metal foil coated with a composition containing copper or copper alloy flakes without internal pores is not affected by the internal pores. 2 2 to 40 per pore, especially internal pores 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2 7-30 per pore, or internal pores 1 μm 2 Flakes and / or additional flakes having an average intra-particle pore density in the range of 5-20 per flake enable relatively high thermo-mechanical performance, relatively high thermal conductivity, and relatively high electrical conductivity of the resulting interlayer. 2 2 to 40 per pore, especially internal pores 1 μm 2 2 to 30 per pore, especially internal pores 1 μm 2 7-30 per pore, or internal pores 1 μm 2The average intra-particle pore density of the flakes / further flakes, in the range of 5-20 per flake, is not affected by sintering of the composition and / or further composition defined in accordance with the present invention. The resulting relatively high thermomechanical performance allows the intermediate layer / interconnect layer to be rapidly cooled from the sintering temperature to the cooling temperature in a short period of time without excessive stress on the intermediate layer (e.g., without crack formation in the intermediate layer).

[0047] In contrast to the average intragranular pore density, the average intergranular void density in the context of the present invention is defined as the average percentage of the area between sintered flakes / additional flakes from the total area in the cross section of an intermediate layer formed from the composition defined in accordance with the present invention or the additional composition. The area between the sintered flakes and the additional flakes is not filled with metal. The average intergranular void density of the flakes / additional flakes in this intermediate layer, especially at the start of sintering of the flakes / additional flakes, can be in the range of 5% to 50%, particularly 10% to 45%, particularly 15% to 40%, particularly 20% to 35%, and particularly 25% to 30%. The average intergranular void density of the flakes / additional flakes in the intermediate layer can be measured, for example, by scanning electron microscopy (SEM), computed tomography, or image-based analysis of the cross section of the intermediate layer. In this regard, the cross section of the intermediate layer can be prepared by ion beam milling to avoid contamination or clogging of the internal pores and / or intergranular voids. The average interparticle void density of the flakes / further flakes in the intermediate layer can be determined, for example, using the following method. 1. Preparation of ultrathin sections of the middle layer 2. Take SEM images of these ultrathin sections. 3. Determine the interparticle void density by calculating the area between the flakes / further flakes not filled with metal in the ultrathin sections relative to the total area of ​​these ultrathin sections of the interlayer in the multiple SEM images; 4. Calculate the average of the interparticle void densities thus determined.

[0048] In this regard, step 3 can be performed by computer-assisted image analysis of the SEM images.

[0049] The inventors of the present invention have discovered that while the average intraparticle pore density of the flakes / further flakes is not affected during sintering of the coated metal foil of the present invention, the average interparticle void density of the flakes / further flakes in the coated composition or additional composition defined according to the present invention decreases during sintering of the coated metal foil of the present invention. In this regard, the decrease in the average interparticle void density depends on the sintering time, the pressure applied during sintering, and the sintering temperature. The pressure applied when sintering the coated metal foil between the first solid substrate and the second solid substrate is usually the bonding pressure applied to press the first substrate and the second substrate together. A relatively long sintering time, a relatively high pressure applied during sintering, and / or a relatively high sintering temperature results in a relatively large decrease in the average interparticle void density. After sintering of the coated metal foil according to the present invention, the average interparticle void density of the copper-containing intermediate layer / interconnect layer (especially between the sintered flakes / further flakes within the copper-containing intermediate layer / interconnect layer) may be in the range of 0.5% to 45%, particularly in the range of 1% to 45%, particularly in the range of 5% to 40%, particularly in the range of 10% to 35%, particularly in the range of 15% to 30%, particularly in the range of 20% to 25%. The reduction in the average interparticle void density during sintering of the coated metal foil according to the present invention may result in the complete or nearly complete disappearance of the interparticle pores of the copper-containing intermediate layer / interconnect layer (especially between the sintered flakes / further flakes within the copper-containing intermediate layer / interconnect layer).

[0050] The flakes / further flakes are stacked in a uniform stacking pattern, particularly by stacking horizontally on top of each other. Thus, a relatively large surface contact between these flakes / further flakes is provided. The relatively large surface contact between the flakes / further flakes allows for sintering in a relatively short sintering time, a relatively low sintering pressure, and / or a relatively low sintering temperature. Furthermore, the flakes / further flakes exhibit a relatively high surface energy. Surface energy can be defined as the excess energy of a material surface compared to the bulk of the material.

[0051] The flakes / further flakes can be purchased, for example, as Cubrotec 8000 copper flakes or Cubrotec 8001 copper flakes, both of which are supplied by Carl Schlenk SE, Germany.

[0052] The flakes and / or further flakes may have a multi-lamellar structure (i.e., a structure in which multiple smaller flakes forming lamellae are loosely stacked on top of each other but held together by van der Waals forces). The total thickness of this structure preferably does not exceed 200 nm. The inventors of the present invention have discovered that copper or copper alloy flakes and / or further flakes having a multi-lamellar structure can be sintered at a relatively short sintering time (particularly, a sintering time of 5 minutes or less), a relatively low sintering temperature (particularly, a sintering temperature of 250°C or less), and / or a relatively low sintering pressure (particularly, a sintering pressure of 15 MPa or less) compared to copper or copper alloy flakes not having a lamellar structure (particularly, not having a multi-lamellar structure). This is made possible by the relatively large surface area of ​​the flakes and / or further flakes having a multi-lamellar structure, which results in an increased density of the flakes and / or further flakes in the composition and / or further composition during sintering and rapid grain growth. The multilamellar structure of the flakes or further flakes can be determined by surface analysis, in particular by metal surface analysis (including the methods described above), and / or by analysis of a cross section of the flakes or further flakes. Cross-sectional analysis can be carried out, for example, by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), glow discharge spectroscopy, and image-based metal surface inspection. These methods can also be applied to the analysis of a cross section of the copper-containing intermediate layer, a cross section of the coated metal foil according to the present invention, a cross section of the metal foil, and / or a cross section of the composition or further composition defined according to the present invention, respectively, before and / or after sintering.

[0053] The multilamellar structure may comprise or consist of 2 or more, in particular 3 or more, in particular 4 or more, in particular 5 or more, in particular 6 or more, in particular 7 or more, in particular 8 or more, and in particular 20 or less, in particular 18 or less, in particular 16 or less, in particular 14 or less, in particular 12 or less, in particular 11 or less, in particular 10 or less, in particular 9 or less lamellae. Any one of the lamellae, independently of the other, may have a thickness in the range of 5 nm to 20 nm, in particular in the range of 6 nm to 18 nm, in particular in the range of 8 nm to 16 nm, in particular in the range of 10 nm to 14 nm, in particular in the range of 11 nm to 13 nm. The number of lamellae in the multilamellar structure and / or the thickness of any of the lamellae in the multilamellar structure can be determined by analyzing a cross-section of the flake / further flake. Analysis of the cross-section can be carried out, for example, by SEM or TEM of the cross-section of the flake / further flake. The number of lamellae in the multilamellar structure of the flakes / further flakes can be determined, for example, using the following method. 1. Embedding the flakes / further flakes in resin, especially epoxy resin, 2. Preparing Ultrathin Sections of Embedded Flakes / Further Flakes 3. Taking transmission electron microscope (TEM) and / or scanning electron microscope (SEM) images of ultrathin sections of embedded / further flakes; 4. Count the number of lamellae of the multilayered lamellar structure in the flakes / ultrathin sections of the flakes in the TEM images and / or SEM images.

[0054] In this regard, step 4 can be performed by computer-assisted image analysis of the TEM and / or SEM images.

[0055] The multilamellar structure may extend over the entire flake or over a portion of the flake. The multilamellar structure may have a total thickness of at least 20 nm, particularly at least 50 nm, particularly at least 100 nm, and at most 400 nm, particularly at most 350 nm, particularly at most 300 nm, particularly at most 250 nm, particularly at most 200 nm.

[0056] The copper or copper alloy forming the lamellae may have a nanocrystalline structure containing nanoscale grains or consisting of nanoscale grains. The copper or copper alloy forming the lamellae may be the same as the copper or copper alloy forming the flakes / further flakes. The nanoscale grain size may be in the range of 5 nm to 200 nm, particularly in the range of 5 nm to 100 nm, particularly in the range of 10 nm to 90 nm, particularly in the range of 15 nm to 80 nm, particularly in the range of 20 nm to 70 nm, particularly in the range of 25 nm to 60 nm, particularly in the range of 30 nm to 50 nm, particularly in the range of 35 nm to 40 nm. If the metal forming the lamellae (i.e., copper or the first copper alloy or the second copper alloy) has a nanocrystalline structure containing nanoscale grains or consisting of nanoscale grains, rapid grain growth of the flakes / further flakes is possible during sintering. In this regard, rapid grain growth is particularly evident in the later stages of sintering, when the interparticle void density decreases and the interparticle pores of the flakes / further flakes in the copper-containing intermediate / interconnecting layer disappear completely or almost completely, while intraparticle pores remain. In the later stages of sintering, grain boundary migration becomes the primary process mechanism for grain growth, which is particularly evident when the lamella-forming metal has a nanocrystalline structure, including or consisting of nanoscale grains. Therefore, when the lamella-forming metal has a nanocrystalline structure, including or consisting of nanoscale grains, sintering can be facilitated with a relatively short sintering time, a relatively low sintering pressure, and / or a relatively low sintering temperature. The grain structure can be detected and analyzed, for example, by using electron backscatter diffraction (EBSD) analysis of the flakes / further flakes.

[0057] Within the multilamellar structure, each lamella may contact other lamella(s) by at least one lamellar boundary. Furthermore, a lamella may be part of one layer bent at the edge of the flake. This means that the lamella forms a unit in the form of a flake with a multilamellar structure.

[0058] The flakes and / or further flakes can be coated with stearic acid. The total weight of stearic acid relative to the total weight of the stearic acid-coated flakes and / or the stearic acid-coated further flakes can be, independently of one another, 0.001% by weight or more, particularly 0.005% by weight or more, particularly 0.01% by weight or more, particularly 0.05% by weight or more, particularly 0.075% by weight or more, and 1.5% by weight or less, particularly 1.25% by weight or less, particularly 1% by weight or less, particularly 0.75% by weight or less, particularly 0.5% by weight or less, particularly 0.25% by weight or less, particularly 0.1% by weight or less. In particular, the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes and / or the further flakes can be, independently of one another, 1.5% by weight or less. The present inventors have discovered that stearic acid coated on the surface of the flakes or additional flakes prevents the flakes or additional flakes from agglomerating and cold welding. However, due to its boiling point of 361°C, the stearic acid on the surface of the flakes or additional flakes does not completely evaporate during sintering at relatively low temperatures and relatively short sintering times. Copper-containing particles coated with an organic coating agent (especially stearic acid) typically exhibit relatively low sintering efficiency. The present inventors have discovered that relatively low sintering efficiency occurs only when the coated metal foil contains a relatively high total weight of stearic acid-coated flakes and / or additional flakes relative to the total weight of the stearic acid-coated flakes and / or additional flakes (especially when the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes and / or additional flakes exceeds 1.5 wt.%).

[0059] In one embodiment, the total weight of stearic acid relative to the total weight of the stearic acid-coated copper or copper alloy flakes is 0.7 wt.% or more and 1.5 wt.% or less. A total weight of 0.7 wt.% stearic acid relative to the total weight of the flakes has been found to be high enough to prevent agglomeration and cold welding of the flakes under all relevant ball milling conditions. A total weight of 1.5 wt.% stearic acid relative to the total weight of the flakes has been found to be low enough not to interfere with subsequent sintering. The inventors have discovered that total weights above this value interfere with sintering because the flakes contain a high organic content that must first be removed from compositions containing these flakes before surface diffusion can occur to enable sintering. 1.5 wt.% stearic acid relative to the total weight of the flakes has been found to be the limit at which sintering results become unsatisfactory.

[0060] A total weight of stearic acid relative to the total weight of the flakes of 0.001% by weight or more, particularly 0.005% by weight or more, particularly 0.01% by weight or more, particularly 0.05% by weight or more, especially 0.075% by weight or more has been found to be sufficiently high to prevent agglomeration and cold welding of the flakes under many, but not all, relevant conditions.

[0061] The present inventors have discovered that a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less stearic acid), relative to the total weight of the stearic acid-coated flakes and / or further flakes in the composition or further composition, is sufficient to prevent agglomeration and cold welding of the flakes and / or further flakes, and also improves or at least does not prevent sintering of these flakes. The inventors have further discovered that compositions and / or additional compositions comprising flakes and / or additional flakes coated with a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less stearic acid) relative to the total weight of the stearic acid-coated flakes and / or additional flakes exhibit improved particle contact during sintering of the coated metal foil, in contrast to copper or copper alloy coated metal foils comprising copper or copper alloy flakes and / or additional flakes coated with a higher total weight of stearic acid (particularly 1.6 wt. % or more stearic acid, particularly 2 wt. % or more stearic acid, particularly 4 wt. % or more stearic acid) relative to the total weight of the stearic acid-coated flakes and / or additional flakes. Therefore, coated metal foils comprising flakes and / or additional flakes coated with a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less stearic acid) relative to the total weight of the stearic acid-coated flakes and / or additional flakes allow for improved sintering neck formation, and therefore allow for sintering at shorter sintering times, lower sintering temperatures, and / or lower sintering pressures.

[0062] Furthermore, in contrast to flakes without an organic coating (particularly flakes without a stearic acid coating), flakes and / or additional flakes coated with a relatively low total weight of stearic acid (i.e., 1.5 wt.% or less stearic acid), relative to the total weight of the stearic acid-coated flakes and / or additional flakes in the composition and / or additional compositions, do not agglomerate, particularly in the process of forming the composition and / or additional compositions defined in accordance with the present invention, and / or during storage, and when the composition is coated on the first surface and / or second surface of the metal foil and / or when the additional composition is coated on the second surface of the metal foil. Agglomeration prevents the composition and / or additional compositions from being formulated as a paste by forming substantially large clumps that prevent homogeneous mixing of the flakes or additional flakes with the organic binder or additional organic binder. Agglomeration of the flakes or additional flakes can result in uneven and inconsistent coating of the first and / or second surfaces of the metal foil with the composition or additional composition, as well as poor adhesion of the coated metal foil to the first and / or second substrate. This can occur, in particular, when the composition coated on the first and / or second surfaces or the additional composition coated on the second surface of the metal foil, or the first and / or second surfaces of the metal foil together with the composition and / or additional composition, are heated to a pre-drying temperature before sintering. Pre-drying results in evaporation (especially partial evaporation) of the organic binder or additional organic binder in the composition or additional composition defined in accordance with the present invention, and evaporation (especially partial evaporation) of the optional polar organic solvent or optional turpentine in the composition or additional composition. This can result in significant agglomeration of the flakes or additional flakes in the case of flakes or additional flakes of copper, first copper alloy, or second copper alloy without an organic coating. Significant agglomeration of the flakes or further flakes results in a non-homogeneous distribution of the composition or further composition on the first surface and / or the second surface of the metal foil.Furthermore, due to the inhomogeneity caused by the agglomerated or further flakes, the contact area between the coated composition and the first surface and / or the second surface, or the contact area between the coated further composition and the second surface of the metal foil, is relatively small, which leads to an uneven pressure distribution during sintering of the coated metal foil and therefore to uneven sintering of the coated metal foil.

[0063] Thus, preventing agglomeration of the flakes and / or additional flakes in the composition allows for homogeneous and consistent coating and / or application, particularly printability, and sufficient adhesion of the composition and / or additional composition to the first and / or second surface of the metal foil, particularly when the composition on the first and / or second surface and / or the additional composition on the second surface of the metal foil, or the first and / or second surface together with the composition and / or additional composition, is heated to a pre-drying temperature before sintering. This is particularly important when the composition is applied to the first and / or second surface and / or the additional composition is applied to the second surface of the metal foil at a relatively thin thickness (e.g., less than 20 μm). Thus, after sintering, the coated metal foil allows for sufficient copper bonding between the first and second substrates with relatively high arithmetic mean shear strength values, particularly when the coated metal foil comprises flakes and / or additional flakes coated with a relatively low total weight of stearic acid (i.e., 1.5 wt.% or less stearic acid, based on the total weight of the stearic acid-coated flakes and / or additional flakes). Furthermore, coated metal foils comprising copper or copper alloy flakes coated with a higher total weight of stearic acid, particularly 1.6 wt.% or more stearic acid, particularly 2 wt.% or more stearic acid, and particularly 4 wt.% or more stearic acid, will leave stearic acid residues after sintering. As the total weight of stearic acid increases, the adhesion of the organic binder to the first surface and / or the second surface of the metal foil and / or the adhesion of the further organic binder to the second surface of the metal foil becomes stronger than the cohesion of the organic binder and / or the further organic binder within the composition and / or the further composition, resulting in an effect commonly referred to as bleed-out.Bleeding of the organic binder or additional organic binder may result in uneven distribution of the organic binder or additional organic binder on the first and / or second surfaces of the metal foil when the composition and / or additional composition are coated on the second surface of the metal foil, resulting in entrapment of the flakes and / or additional flakes due to the uneven distribution of the organic binder and / or additional organic binder on the first and / or second surfaces of the metal foil. Thus, the composition or additional composition defined in accordance with the present invention, comprising a relatively low total weight of stearic acid-coated flakes or additional flakes relative to the total weight of the stearic acid-coated flakes or additional flakes, exhibits improved processability and sintering efficiency compared to compositions comprising copper or copper alloy flakes without an organic coating and compared to compositions comprising copper or copper alloy flakes coated with a relatively high total weight of stearic acid (particularly 2 wt. % or more stearic acid) relative to the total weight of the stearic acid-coated flakes.

[0064] The inventors of the present invention have discovered that after the sintering process, the sintered flakes and / or additional flakes are uniformly dispersed in the copper-containing intermediate layer, particularly between the first surface of the metal foil and the first solid substrate, and between the second surface of the metal foil and the second solid substrate. This allows for efficient blending of the composition and / or additional composition defined in accordance with the present invention, and allows for improved coating and / or applicability (particularly printability) of the composition and / or additional composition defined in accordance with the present invention onto the first surface and / or second surface of the metal foil. Furthermore, the coated metal foil of the present invention allows for relatively good surface control of the sintered copper-containing intermediate layer, particularly by ensuring a relatively low surface roughness and a uniform bond thickness of the copper-containing intermediate layer during and / or after sintering. Furthermore, when the flakes and / or further flakes are coated with a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less of stearic acid based on the total weight of the stearic acid-coated flakes and / or further flakes), agglomeration of the flakes and / or further flakes is prevented, and thus the uniform distribution of the flakes and / or further flakes within the copper-containing intermediate layer (particularly between the first surface of the metal foil and the first solid substrate, and between the second surface of the metal foil and the second solid substrate) after the sintering process is further improved, which further improves the coating and / or applicability (particularly printability) of the composition on the first surface and / or second surface of the metal foil and / or the further composition on the second surface. Furthermore, when the flakes and / or further flakes are coated with a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less stearic acid) relative to the total weight of the stearic acid-coated flakes and / or further flakes, the surface control of the sintered copper-containing intermediate layer is also further improved by ensuring a relatively low surface roughness and uniform bond thickness of the copper-containing intermediate layer, particularly during and / or after sintering.

[0065] The inventors of the present invention have discovered that in addition to the multi-lamellar structure of the flakes and / or further flakes, the following properties of the flakes and / or further flakes in the composition defined according to the present invention and / or the further composition: a relatively large surface contact on the first surface and / or the second surface of the metal foil due to a uniform stacking pattern by the flakes in the composition defined according to the present invention and / or the further flakes in the further composition defined according to the present invention, the metal forming the lamellae of the flakes and / or further flakes having a nanocrystalline structure comprising nanoscale particles or consisting of nanoscale crystal grains, and the coating of the flakes and / or further flakes with a relatively low total weight of stearic acid (i.e., 1.5 wt. % or less of stearic acid, based on the total weight of the stearic acid-coated flakes and / or further flakes), also contribute to enabling relatively short sintering times, relatively low sintering pressures, and / or sintering at the relatively low sintering temperatures indicated above, respectively. The inventors of the present invention have further discovered that when the flakes and / or further flakes not only have a multilayer lamellar structure but also satisfy two or even three of the above properties, compared to copper or copper alloy flakes that not only have a multilayer lamellar structure but also do not have a lamellar structure (in particular, do not have a multilayer lamellar structure), or compared to copper or copper alloy flakes that only have a multilayer lamellar structure but do not satisfy any one of the above properties, or compared to copper or copper alloy flakes that have a multilayer lamellar structure but do not satisfy all of the above properties, the multilayer lamellar structure of the flakes and / or further flakes and each of the above properties reinforce each other, resulting in a synergistic effect, thus allowing for shorter sintering times, lower sintering pressures, and / or lower sintering temperatures.

[0066] The flakes and / or further flakes may have a total oxygen content in the range of 3% to 8% by weight, particularly in the range of 4% to 7.9% by weight, particularly in the range of 4.1% to 7.5% by weight, particularly in the range of 4.2% to 7% by weight, particularly in the range of 4.3% to 6.5% by weight, particularly in the range of 4.4% to 6% by weight, particularly in the range of 4.5% to 5.5% by weight, particularly in the range of 4.6% to 5% by weight. In the context of the present invention, the total oxygen content of the flakes and / or further flakes refers to the oxygen content of the flakes and / or further flakes themselves, resulting from oxygen atoms in any form (e.g., in the form of copper oxide) on the surface of the flakes and / or further flakes, as well as oxygen trapped, for example, within internal pores or lamellar structures. If the flakes and / or further flakes are coated with stearic acid, the total oxygen content also refers to the oxygen content of the stearic acid coating of the flakes / further flakes.

[0067] The organic binder and / or the additional organic binder may be a liquid organic binder. The organic binder and / or the additional organic binder may each, independently of one another, be terpineol, a primary alcohol, a diol, a triol, a polymeric glycol, or a mixture of at least two of terpineol, a primary alcohol, a diol, a triol, and a polymeric glycol. In particular, the terpineol forming the organic binder or the additional organic binder may be α-terpineol. The α-terpineol may be (R)-(+)-α-terpineol, (S)-(-)-α-terpineol, or a mixture of (R)-(+)-α-terpineol and (S)-(-)-α-terpineol. The (R)-(+)-α-terpineol and / or (S)-(-)-α-terpineol may each, independently of one another, have a purity of 90% or more, particularly 93% or more. The primary alcohol forming the organic binder can be 1-butanol or 1-octanol. The diol forming the organic binder or further organic binder can be a diol, particularly ethylene glycol, diethylene glycol, propylene glycol, or butylene glycol, having an average molar mass of 60 g / mol to 110 g / mol. The triol forming the organic binder or further organic binder can be a triol, particularly glycerol, having an average molar mass of 75 g / mol to 110 g / mol. The polymeric glycol forming the organic binder or further organic binder can be a polymeric ethyl glycol having an average molar mass of 200 g / mol or more, particularly 300 g / mol or more, particularly 400 g / mol or more, particularly 500 g / mol or more, particularly 550 g / mol or more, and 800 g / mol or less, particularly 750 g / mol or less, particularly 700 g / mol or less, particularly 650 g / mol or less.

[0068] During the sintering process, the organic binder and / or additional organic binder, particularly the residual organic binder and / or the residual additional organic binder, typically evaporates or decomposes. During the evaporation or decomposition of the diol and / or polymer glycol, a reducing atmosphere is generated, which prevents the copper of the coated metal foil from oxidizing to copper oxide without the need for an additional protective gas atmosphere from the outside. Therefore, the diol and / or polymer glycol in the composition or additional composition defined in accordance with the present invention allows for the in situ reduction of Cu oxide during the sintering process with little effort. The in situ reduction of copper oxide (particularly CuO) during the sintering process also forms copper nanoparticles, whose high surface area and reactivity further promote sintering. The organic binder or additional organic binder, particularly the polymer glycol, particularly the polymer glycol with an average molar mass of 550 g / mol to 650 g / mol, has a reducing effect on the coated metal foil.

[0069] In contrast to polymeric glycols, particularly polymeric ethyl glycols, especially those with an average molar mass greater than 200 g / mol, evaporation of diols (especially diols with an average molar mass greater than or equal to 60 g / mol and less than or equal to 110 g / mol) and primary alcohols (especially 1-butanol and 1-octanol) is achieved at relatively low temperatures (especially temperatures below 200°C). For example, evaporation of ethylene glycol is achieved at a temperature of 197°C. For example, evaporation of 1-butanol is achieved at a temperature of 117.7°C. For example, evaporation of 1-octanol is achieved at a temperature of 195°C. To evaporate polymeric ethyl glycol, a relatively high temperature of at least 200°C is required. However, evaporation of the organic binder or further organic binder at relatively high temperatures requires a relatively high sintering temperature or a relatively long sintering time at a relatively high sintering temperature to allow complete evaporation of the organic binder or further organic binder. By using a mixture of ethylene glycol and α-terpineol, it is also possible to achieve the combined effect of relatively low sintering temperatures, such as sintering temperatures below 250°C, and relatively fast sintering, such as sintering times of less than 5 minutes, while simultaneously utilizing a reducing atmosphere.

[0070] In one embodiment of the present invention, the organic binder or further organic binder can be a mixture of ethylene glycol and α-terpineol. In another embodiment of the present invention, the organic binder or further organic binder can be a mixture of α-terpineol, ethylene glycol, and polyethylene glycol. In this mixture, the total weight of α-terpineol relative to the total weight of the organic binder or further organic binder can be 80% by weight, the total weight of ethylene glycol relative to the total weight of the organic binder or further organic binder can be 18% by weight, and the total weight of polyethylene glycol relative to the total weight of the organic binder or further organic binder can be 2% by weight. In another embodiment of the present invention, the organic binder or further organic binder can be a mixture of α-terpineol, polyethylene glycol, and glycerol. In this mixture, the total weight of α-terpineol relative to the total weight of the organic binder or further organic binders may be 80% by weight, the total weight of polyethylene glycol relative to the total weight of the organic binder or further organic binders may be 19.5% by weight, and the total weight of glycerol relative to the total weight of the organic binder or further organic binders may be 0.5% by weight.

[0071] In a further embodiment of the present invention, the organic binder or further organic binder can be a mixture of ethylene glycol, α-terpineol, polyethylene glycol, and glycerol. The inventors of the present invention have discovered that a mixture of α-terpineol, ethylene glycol, and polyethylene glycol, a mixture of α-terpineol, polyethylene glycol, and glycerol, and a mixture of ethylene glycol, α-terpineol, polyethylene glycol, and glycerol, can provide a composition defined in accordance with the present invention that allows for relatively fast pre-drying without drying out. This results in relatively high stability of the composition, particularly after pre-drying, and a relatively long usable life of the composition defined in accordance with the present invention (especially the composition defined in accordance with the present invention after pre-drying), up to one week. Furthermore, the composition defined in accordance with the present invention can sufficiently adhere to the first and / or second surface of the coated metal foil even after pre-drying, eliminating the need for an additional adhesive.

[0072] The composition and / or further compositions defined according to the present invention may further comprise, independently of each other, a polar organic solvent or turpentine. The polar organic solvent may be a liquid polar organic solvent. The turpentine may be liquid turpentine. The polar organic solvent and turpentine may each function as a diluent. In the present invention, a polar solvent is defined as a solvent having a dielectric constant of 7 or greater measured at 20°C. A nonpolar solvent is defined as a solvent having a dielectric constant of less than 7 at 20°C. Here, the dielectric constant of a given solvent is the ratio of the electric permeability of the solvent to the electric permeability of free space (i.e., vacuum). The dielectric constant can be measured by an LCR meter or an impedance analyzer using methods known in the art. The polar solvent may be, in particular, alcohol, acetone, or a mixture of alcohol and acetone. The alcohol may be methanol, ethanol, propanol, isopropanol, or a mixture of at least two of methanol, ethanol, propanol, and isopropanol. The inventors of the present invention have discovered that the composition or further composition further comprising a polar organic solvent or turpentine oil can be coated onto the first surface and / or the second surface of a metal foil, respectively and independently of one another, at a relatively thin thickness (particularly a thickness of 10 μm or less, particularly 5 μm or less, particularly 1 μm or less).

[0073] The total solids content of the composition or further composition defined in accordance with the present invention may be 80% by weight or less, particularly 75% by weight or less, particularly 70% by weight or less, particularly 65% ​​by weight or less, particularly 63% by weight or less, particularly 60% by weight or less, particularly 55% by weight or less, particularly 50% by weight or less, particularly 40% by weight or less, particularly 30% by weight or less, particularly 20% by weight or less, particularly 10% by weight or less. Therefore, the total solids content of the composition or further composition defined in accordance with the present invention is significantly lower than that of a silver sintering paste having a total solids content of about 90% by weight. The relatively low total solids content results in a relatively low viscosity of the composition defined in accordance with the present invention. The viscosity of the composition defined in accordance with the present invention may be in the range of 50 mPas to 400,000 mPa·s, particularly in the range of 100 mPas to 350,000 mPas, particularly in the range of 200 mPas to 200,000 mPa·s. The viscosity of the composition defined according to the invention or the further composition can be measured at room temperature (particularly 20°C) using a standard viscometer (particularly a standard classical rotational viscometer, in particular a Thermo Scientific™ HAAKE™ Viscotester™ C). These rotational viscometers measure the resistance of the composition defined according to the invention or the further composition against a preset speed. The resulting torque or resistance is a measure of the viscosity of the composition defined according to the invention or the further composition. The higher the torque, the higher the viscosity. The low viscosity of the composition defined according to the invention or the further composition allows for its easy processability and good and uniform application onto the first and / or second surface of the metal foil.

[0074] The total weight of the flakes or further flakes, optionally coated with stearic acid, relative to the total weight of the composition or further composition defined according to the invention can be at most 80% by weight, in particular at most 75% by weight, in particular at most 70% by weight, in particular at most 65% by weight, in particular at most 63% by weight, in particular at most 60% by weight, in particular at most 55% by weight, in particular at most 50% by weight, in particular at most 45% by weight, in particular at most 40% by weight, in particular at most 30% by weight, in particular at most 20% by weight, in particular at most 10% by weight. The remainder of the composition or further composition can be formed by the organic binder or further organic binder. This means that the total weight of the organic binder or further organic binder relative to the total weight of the composition or further composition defined in accordance with the present invention can be 20% by weight or more, in particular 25% by weight or more, in particular 30% by weight or more, in particular 35% by weight or more, in particular 37% by weight or more, in particular 40% by weight or more, in particular 45% by weight or more, in particular 50% by weight or more, in particular 55% by weight or more, in particular 60% by weight or more, in particular 70% by weight or more, in particular 80% by weight or more, in particular 90% by weight or more. When the composition or further composition defined in accordance with the present invention further comprises a polar organic solvent or turpentine, the remainder of the composition or further composition can be formed by the polar organic solvent or turpentine. The ratio of the total weight of the polar organic solvent to the total weight of the flakes plus the organic binder, or the total weight of the further flakes plus the organic binder or further organic binder, can be 10:1 or less, particularly 5:1 or less, particularly 4:1 or less, particularly 3:1 or less, particularly 2:1 or less, particularly 1:1 or less, particularly 1:2 or less, particularly 1:3 or less, and particularly 1:4 or less. The ratio of the total weight of the turpentine oil to the total weight of the flakes plus the organic binder, or the total weight of the further flakes plus the organic binder or further organic binder, can be 10:1 or less, particularly 5:1 or less, particularly 4:1 or less, particularly 3:1 or less, particularly 2:1 or less, particularly 1:1 or less, particularly 1:2 or less, particularly 1:3 or less, and particularly 1:4 or less.The inventors have discovered that a relatively high ratio of the total weight of the polar organic solvent or turpentine to the total weight of the flakes plus the organic binder in the composition defined according to the present invention, or the total weight of the organic binder or further flakes plus the further organic binder in the further composition defined according to the present invention, makes it possible to coat the composition or further composition, respectively, independently of one another, onto the first surface and / or the second surface of the metal foil at a relatively thin thickness, in particular 10 μm or less, in particular 5 μm or less, in particular 1 μm or less. The total weight of the flakes, further flakes, organic binder, further organic binder, optional stearic acid coated on the flakes or further flakes, and / or optional polar organic solvent or optional turpentine, respectively, can be measured by nuclear magnetic resonance spectroscopy, thermogravimetry, mass spectroscopy, and infrared spectroscopy. The total weight of the flakes, further flakes, organic binder, further organic binder, optional stearic acid coated on the flakes or further flakes, and / or optional polar organic solvent or optional turpentine can be determined by weighing the flakes, further flakes, organic binder, further organic binder, optional stearic acid, and / or optional polar organic solvent or optional turpentine prior to forming the composition or further composition defined in accordance with the present invention.

[0075] 10% to 100%, particularly 50% to 100%, particularly 55% to 95%, particularly 60% to 90%, particularly 65% ​​to 85%, particularly 70% to 80% of the total area of ​​the first surface and / or the total area of ​​the second surface of the metal foil may be coated independently with a composition as defined according to the present invention. Alternatively, 10% to 100%, particularly 50% to 100%, particularly 100% of the total area of ​​the first surface of the metal foil may be coated with a composition, and a further 10% to 100%, particularly 50% to 100%, particularly 100% of the total area of ​​the second surface of the metal foil may be coated with a further composition. This area may be 100% of the total area of ​​the first surface and / or the total area of ​​the second surface. The composition can be coated onto the first surface, particularly a region of the first surface, and / or the second surface, particularly a region of the second surface, of the metal foil, each independently of the other, to a thickness of 1 μm to 500 μm, particularly 1 μm to 50 μm, or 5 μm to 400 μm, particularly 10 μm to 300 μm, particularly 20 μm to 250 μm, particularly 50 μm to 200 μm, particularly 75 μm to 150 μm, particularly 100 μm to 125 μm. Alternatively, the composition can be coated onto the first surface of the metal foil, and the additional composition can be coated onto the second surface, each independently of the other, to a thickness of 1 μm to 500 μm, particularly 1 μm to 50 μm. When the composition or the additional composition is coated to a thickness of 1 μm to 50 μm, the sintered layer obtained from the composition or the additional composition can each have a thickness of 200 nm to 25 μm.

[0076] The coated metal foil may be disposed between a first solid substrate and a second solid substrate as a copper-containing intermediate layer connecting the first solid substrate and the second solid substrate, or may be disposed on the solid substrate to connect the solid substrate and the metal foil. The composition coated on the metal foil after the sintering process may provide electrical and / or thermal conductivity between the solid substrate and the metal foil or between the first solid substrate and the metal foil. When the solid substrate is connected only to the metal foil, the metal foil can dissipate heat from the solid substrate and function as a heat sink, or it can dissipate heat to another heat sink. A solid substrate connected only to the metal foil can also be a precursor to a conductor plate.

[0077] The method for forming flakes or further flakes includes grinding and / or milling copper particles or copper alloy particles. When obtaining stearic acid-coated flakes or further flakes, grinding and / or milling the copper particles or copper alloy particles is performed in the presence of stearic acid. The grinding and / or milling can be performed, for example, in a ball mill. The flakes or further flakes are obtained during grinding and / or milling the copper particles or copper alloy particles. The stearic acid-coated flakes or further flakes are obtained during grinding and / or milling the copper particles or copper alloy particles in the presence of stearic acid. During grinding and / or milling the copper particles or copper alloy particles in the presence of stearic acid, the total weight of stearic acid relative to the total weight of the copper particles or copper alloy particles can be relatively low. Furthermore, the total number of individual grinding and / or milling steps of the copper particles or copper alloy particles in the presence of stearic acid can be relatively small. In particular, the total number of individual grinding and / or milling steps of copper particles or copper alloy particles in the presence of stearic acid can be 3 or less, particularly 2 or less, and particularly 1 or less. This makes it possible to obtain flakes or further flakes coated with stearic acid, and the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes or further flakes is 1.5 wt.% or less. The total weight of stearic acid coated on the flakes or further flakes relative to the total weight of the stearic acid-coated flakes or further flakes can be determined by nuclear magnetic resonance spectroscopy, thermogravimetric analysis, Rutherford backscattering spectroscopy, mass spectrometry, infrared spectroscopy, Raman spectroscopy, and X-ray spectroscopy combined with nuclear magnetic resonance spectroscopy.

[0078] A method for forming a composition defined in accordance with the present invention comprises providing an organic binder, optionally stearic acid-coated flakes, and optionally a polar organic solvent or turpentine, and mixing the organic binder, optionally stearic acid-coated flakes, and optionally a polar organic solvent or turpentine to obtain a composition defined in accordance with the present invention.A method for forming a further composition defined in accordance with the present invention comprises providing an organic binder or further organic binder, optionally stearic acid-coated flakes or further flakes, and optionally a polar organic solvent or turpentine, and mixing the organic binder or further organic binder, optionally stearic acid-coated flakes or further flakes, and optionally a polar organic solvent or turpentine to obtain a further composition defined in accordance with the present invention.The further composition may differ from the composition not only by the individual components, but also by different weight ratios of the components, if they are the same.

[0079] The present invention also provides a method for producing a coated metal foil according to the present invention, comprising the steps of: a) a copper or first copper alloy or copper-plated nickel-iron alloy metal foil having a first surface and a second surface as defined above; Flakes of copper or a first copper alloy or a second copper alloy as defined above, and optionally further flakes, comprising internal pores; an organic binder as defined above and optionally a further organic binder, and optionally a polar organic solvent as defined above or optionally turpentine as defined above; providing a b) mixing flakes, an organic binder, and optionally a polar organic solvent or optionally turpentine to obtain a composition, and optionally mixing flakes or further flakes, an organic binder or further organic binder, and optionally a polar organic solvent or optionally turpentine to obtain a further composition; c) applying the composition obtained in step b) to a first surface and / or a second surface of the metal foil, or applying the composition obtained in step b) to a first surface of the metal foil and applying a further composition obtained in step b) to a second surface of the metal foil, to obtain a coated metal foil; The present invention relates to a method comprising:

[0080] Thus, the coated metal foil of the present invention can be produced by the above-described method. The composition obtained in step b) can be a composition defined according to the present invention, and the further composition obtained in step b) can be a further composition defined according to the present invention. Applying the composition obtained in step b) to the first surface and / or the second surface of the metal foil, or applying the composition obtained in step b) to the first surface of the metal foil and the further composition obtained in step b) to the second surface of the metal foil, can both be carried out by coating (particularly doctor blade coating or spray coating), dip coating, printing (particularly stencil printing, inkjet printing, aerosol jet printing, or screen printing), direct writing, or material dispensing.

[0081] The composition and / or further composition may be applied independently to 10% to 100%, in particular 50% to 100%, in particular 55% to 95%, in particular 60% to 90%, in particular 65% to 85%, in particular 70% to 80% of the area of ​​the first surface and / or the area of ​​the second surface of the metal foil, which may be the entire area of ​​the first surface and / or the entire area of ​​the second surface. The composition is applied to a first surface (particularly a region of the first surface) of the metal foil, and the composition or further composition is applied to a second surface (particularly a region of the second surface) of the metal foil, each independently of the other, in a thickness of 1 μm to 500 μm, in particular 5 μm to 400 μm, in particular 10 μm to 300 μm, in particular at least 20 μm to 250 μm, in particular at least 50 μm to 200 μm, in particular at least 75 μm to 150 μm, in particular at least 100 μm to 125 μm.

[0082] Prior to step c), the metal foil may be heated to a first pretreatment temperature in the range of 170°C to 220°C for at least 1 minute, particularly at least 2 minutes, particularly at least 3 minutes, and for at most 2 hours, particularly at most 1 hour, particularly at most 30 minutes, particularly at most 20 minutes, particularly at most 10 minutes, particularly at most 8 minutes, particularly at most 5 minutes. The inventors of the present invention have discovered that heating the metal foil to the first pretreatment temperature removes organic residues present on the metal foil as a result of standard manufacturing processes, such as rolling. Removing organic residues from the first and / or second surfaces allows for efficient sintering and relatively high wettability of the first and / or second surfaces. After heating the metal foil to the first pretreatment temperature, the metal foil may be cooled to a second pretreatment temperature in the range of 15°C to 40°C (particularly, 20°C to 30°C). Cooling can be achieved by active cooling, but is typically passive cooling (i.e., by allowing the coated metal foil to cool to the second pretreatment temperature, which is typically ambient temperature). Optionally, heating to the first pretreatment temperature and / or cooling to the second pretreatment temperature may be carried out in a reducing atmosphere, an inert atmosphere, and / or a vacuum to prevent oxidation of the first surface and / or the second surface of the metal foil during heating to the first pretreatment temperature and / or cooling to the second pretreatment temperature. The vacuum may be a relatively high vacuum of 10 mbar or more, particularly 1 mbar or more, and particularly 0.3 mbar or more.

[0083] After the composition defined according to the present invention and / or the further composition has been applied to the first surface and / or the second surface of the metal foil according to step c), an optional step d) can be carried out in which the composition on the first surface and / or the second surface of the metal foil, or the composition and the further composition plus the composition on the first surface and / or the second surface, is heated to a pre-drying temperature in the range from 100°C to 160°C, in particular in the range from 110°C to 150°C, in particular in the range from 120°C to 140°C, in particular in the range from 125°C to 130°C.

[0084] Alternatively, after applying the composition obtained in step b) to a first surface, the metal foil can be heated to a pre-drying temperature in the range of 100°C to 160°C, in particular in the range of 110°C to 150°C, in particular in the range of 120°C to 140°C, in particular in the range of 125°C to 130°C, for 1 minute to 30 minutes, followed by a cooling step, applying the composition obtained in step b) or a further composition to a second surface and heating the metal foil to a pre-drying temperature in the range of 100°C to 160°C, in particular in the range of 110°C to 150°C, in particular in the range of 120°C to 140°C, in particular in the range of 125°C to 130°C, for 1 minute to 30 minutes.

[0085] The pre-drying temperature may be maintained for at least 30 seconds, in particular 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 10 minutes, in particular at most 8 minutes, in particular at most 6 minutes, in particular at most 5 minutes, in particular at most 4 minutes. Heating to the pre-drying temperature and maintaining the pre-drying temperature by optional step d) may be carried out in a reducing atmosphere, in an inert atmosphere, and / or in vacuum.

[0086] During the optional step d), 70% by weight or more, particularly 75% by weight or more, particularly 80% by weight or more, particularly 85% by weight or more, particularly 90% by weight or more, particularly 95% by weight or more, particularly 99% by weight or more of the initial weight of the organic binder and / or the further organic binder and / or the optional polar organic solvent or the optional turpentine can be evaporated. Evaporation refers to the conversion of the liquid organic binder and / or the further organic binder and / or the optional liquid polar organic solvent or the optional turpentine into vapor below the boiling point of the organic binder and / or the further organic binder and / or the optional polar organic solvent or the optional turpentine, respectively. If step d) is carried out in a vacuum, the evaporation of the organic binder and / or the further organic binder and / or the optional polar organic solvent or the optional turpentine is further increased. The initial weight of the organic binder and / or further organic binder can be determined by weighing the organic binder and / or further organic binder and / or optional polar organic solvent or optional turpentine before providing / preparing the composition and / or further composition defined according to the present invention, and / or by mass spectrometry or thermogravimetric analysis before and after heating in step d). The optional polar organic solvent or optional turpentine may evaporate completely during step d), but residual organic binder and / or further organic binder (especially 30% by weight or less, particularly 25% by weight or less, particularly 20% by weight or less, particularly 15% by weight or less, particularly 10% by weight or less, particularly 5% by weight or less, particularly 1% by weight or less of the initial weight of the organic binder and / or further organic binder) may remain on the coated metal foil (especially the composition and / or further composition coated independently of each other on the first surface and / or second surface of the metal foil). The residual organic binder and / or further organic binder in the coated metal foil allows for good adhesion of the flakes and / or further flakes to the first surface and / or second surface of the metal foil.Furthermore, the residual organic binder and / or further organic binder may have a reducing effect on the coated metal foil during sintering of the coated metal foil, providing adhesion for placing and firmly bonding the coated foil to the surface(s) of a solid substrate. The weight of the residual organic binder and / or further organic binder in the coated metal foil may be determined by weighing the coated metal foil before and after heating according to optional step d) and / or by mass spectrometry or thermogravimetric analysis.

[0087] After the method according to the invention, in particular after step c) or optionally after step d), the coated metal foil produced by the method according to the invention can be applied to a method for forming a copper-containing interconnect layer between two solid substrates or between a solid substrate and a metal foil, which method comprises the steps of: e) providing a coated metal foil of the present invention produced by the method of the present invention, providing a solid substrate, or providing a first solid substrate and providing a second solid substrate; f) applying the coated metal foil according to the invention to a solid substrate or a first solid substrate; g) if the copper-containing interconnect layer is to be formed between two solid substrates, placing a second solid substrate on the coated metal foil of the present invention; h) applying a bonding pressure to press the solid substrate and the coated metal foil, or the first solid substrate and the second solid substrate, together, wherein the bonding pressure may be at least 100 kPa, in particular 1 MPa, in particular 2.5 MPa, in particular 5 MPa, in particular 7.5 MPa, in particular 10 MPa, and at most 40 MPa, in particular 35 MPa, in particular 30 MPa, in particular 25 MPa, in particular 20 MPa, in particular 15 MPa; i) heating the coated metal foil or solid substrate or the first solid substrate and / or the second solid substrate together with the coated metal foil to a sintering temperature in the range of 200°C to 300°C, particularly in the range of 205°C to 275°C, particularly in the range of 210°C to 270°C, particularly in the range of 215°C to 265°C, particularly in the range of 220°C to 260°C, particularly in the range of 225°C to 255°C, particularly in the range of 230°C to 250°C, particularly in the range of 235°C to 245°C, and maintaining the sintering temperature and bonding pressure until a copper-containing intermediate layer is formed between the solid substrate and the metal foil or between the first solid substrate and the second solid substrate; Includes:

[0088] Applying the coated metal foil according to the invention to the solid substrate or to the first solid substrate and / or placing the second solid substrate on the coated metal foil can be performed manually, by transfer (in particular by vacuum-mediated foil transfer or vacuum-mediated substrate transfer) or by a pick-and-place machine.

[0089] The sintering temperature and bonding pressure may be maintained for 1 second or more, particularly 5 seconds or more, particularly 30 seconds or more, particularly 1 minute or more, particularly 3 minutes or more, particularly 4 minutes or more, and for 30 minutes or less, particularly 15 minutes or less, particularly 10 minutes or less, particularly 8 minutes or less, particularly 5 minutes or less.

[0090] The method for forming a copper-containing intermediate layer between two solid substrates or between a solid substrate and a metal foil may further include step j) of cooling the copper-containing intermediate layer obtained in step i) between the first solid substrate and the second solid substrate, or between the solid substrate and the metal foil or the solid substrate or the first solid substrate and / or the second solid substrate, together with the copper-containing intermediate layer obtained in step i), to a cooling temperature in the range of 15°C to 40°C (particularly, in the range of 20°C to 30°C). The cooling temperature may be ambient temperature. The cooling in step j) can be performed by active cooling, but is typically passive cooling (i.e., by cooling the copper-containing intermediate layer or the solid substrate or the first solid substrate and / or the second solid substrate together with the copper-containing intermediate layer to a cooling temperature (typically ambient temperature)).

[0091] At least step c) of the method according to the invention, in particular at least steps b) and c), in particular at least steps a) to c), can be carried out in a reducing atmosphere, in an inert atmosphere, and / or in a vacuum. Optionally, step d) of the method according to the invention can be carried out in a reducing atmosphere, in an inert atmosphere, and / or in a vacuum. At least step i) of the method for forming a copper-containing intermediate layer between two solid substrates, in particular at least steps i) and h), in particular at least steps g) to i), in particular at least steps f) to i), in particular steps e) to i), can be carried out in a reducing atmosphere, inert atmosphere, and / or in a vacuum. Optionally, step j) can be carried out in a reducing atmosphere, inert atmosphere, and / or in a vacuum.

[0092] The reducing atmosphere can be provided by a mixture of evaporated formic acid and nitrogen gas or a gas mixture of hydrogen and an inert gas, independently of each other. The gas mixture of hydrogen and an inert gas can be a gas mixture of nitrogen and hydrogen or a gas mixture of argon and hydrogen. The inert atmosphere can be provided by an inert gas, independently of each other. The inert gas can be included in the inert gas mixture. The inert gas can be nitrogen gas, carbon dioxide gas, helium gas, neon gas, or argon gas. The inert gas mixture can be a mixture of at least two gases selected from nitrogen gas, carbon dioxide gas, helium gas, neon gas, and argon gas.

[0093] The inventors of the present invention have discovered that the coated metal foils of the present invention provide relatively good electrical conductivity of the copper joints after sintering because they have relatively low oxide impurities, and because the oxide impurities are so low, there is no need to add a reducing atmosphere during sintering.

[0094] The vacuum pressures can be, independently of one another, in the range of 0.1 mbar to 10 mbar, particularly in the range of 0.5 mbar to 5 mbar, and particularly in the range of 1 mbar to 3 mbar. The inventors of the present invention have discovered that sintering in a vacuum allows the organic binder and / or further organic binder and / or optional polar organic solvent or optional turpentine to further evaporate at a relatively low temperature because the vacuum lowers the boiling point of the organic binder and / or further organic binder and / or optional polar organic solvent or optional turpentine. The further evaporation of the organic binder and / or further organic binder and / or optional polar organic solvent or optional turpentine further improves the sintering efficiency of the coated metal foil of the present invention and contributes to sintering at a relatively low sintering temperature.

[0095] During step e), 70% by weight or more, particularly 75% by weight or more, particularly 80% by weight or more, particularly 85% by weight or more, particularly 90% by weight or more, particularly 95% by weight or more, particularly 99% by weight or more, particularly 100% by weight of the initial weight of the organic binder and / or further organic binder, particularly the residual organic binder and / or the residual further organic binder, and / or the optional polar organic solvent or optional turpentine, can be evaporated. If optional step d) is performed before step e), the residual organic binder and / or the residual further organic binder can be completely evaporated during step e). The initial weight of the organic binder and / or the further organic binder can be determined by weighing the organic binder and / or the further organic binder before providing / preparing the composition and / or the further composition defined according to the present invention. The evaporation of the organic binder and / or further organic binder (in particular the residual organic binder and / or the residual further organic binder) can be determined by weighing the first solid substrate and / or the second solid substrate together with the coated metal foil according to the present invention before, during or after the heating in step e) and / or by mass spectrometry or thermogravimetric analysis.

[0096] The first solid substrate can be made of or consist of a metal or metal oxide, and the second solid substrate can be made of or consist of a metal, an additional metal, a metal oxide, an additional metal oxide, or a surface-mounted device component. The metal and / or additional metal can be in the form of an additional foil. The metal or additional metal can be gold, silver, nickel, palladium, copper, pretreated copper, or tin. The metal oxide or additional metal oxide can be aluminum oxide. The surface-mounted device component can 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, a transistor, a silicon carbide (SiC) device, or a gallium nitride (GaN) device. The diode can be a silicon diode. The transistor can be a metal-oxide-semiconductor field-effect transistor (MOSFET). The inventors of the present invention have discovered that the organic binder and / or further organic binder, in particular the residual organic binder and / or the residual further organic binder, can also reduce oxides on the first solid substrate and / or the second solid substrate. Thus, the coated metal foil according to the present invention allows sintering on copper, silver, and / or nickel substrates, even if these substrates are covered with an oxide layer (in particular an oxide layer of CuO).

[0097] Pretreated copper can be prepared, for example, by coating the copper substrate with an organic surface protection (OSP) layer, which protects the copper from oxidation and dissolves during the soldering process. Alternatively, other methods (e.g., sol-gel or CVD) can be used to coat the copper substrate with a protective layer to protect the copper from oxidation.

[0098] The present invention further relates to the use of the coated metal foil of the present invention to form a copper-containing intermediate layer / interconnect layer between two surfaces of a solid substrate (i.e., between a surface of a first solid substrate and a further surface of a second solid substrate, or between a surface of a solid substrate and a metal foil), particularly for die attach bonding, microelectronic packaging, electric vehicle technology, hybrid electric vehicle technology, high-power electronics packaging, thick film technology, large-area bonding, and / or battery applications. In particular, the present invention relates to the use of the coated metal foil of the present invention for die attach bonding. The present invention also relates to the use of the coated metal foil of the present invention to form conductive paths on a solid substrate. The conductive paths on the solid substrate may be electrical and / or thermal conductive paths on the solid substrate. The present invention also relates to the use of the coated metal foil of the present invention for substrate attachment. The microelectronic package may be a WBG semiconductor package. The high-power electronics package may be a high-power light-emitting diode package. The present inventors have discovered that the use of the coated metal foil of the present invention to form a copper-containing intermediate / interconnect layer between two solid substrates or to form a conductive path on a solid substrate can result in a relatively low thermal load on the solid substrate(s) and any electronic components (particularly surface mount device components as defined above) on the solid substrate(s) when forming the interconnect layer or conductive path, due to the relatively low sintering temperatures required to sinter the coated metal foil of the present invention.

[0099] When the coated metal foil of the present invention is used to form an electrical and / or thermally conductive path on a solid substrate, the solid substrate can be any of the substrates defined above as a first or second substrate or solid substrate, as long as it is not an electrically conductive first or second substrate. When the coated metal foil of the present invention is used for substrate attachment, the substrate can be any of the substrates defined above as a first or second substrate, an aluminum substrate, particularly a metallized aluminum substrate, particularly a copper-metallized aluminum substrate, a ceramic substrate, particularly a metallized ceramic substrate, particularly a copper-metallized ceramic substrate, a direct-bonded copper substrate, or a polymer substrate, particularly a polyimide substrate.

[0100] All features shown in this specification should be understood as features applicable to all embodiments of the present invention. This means, for example, that features shown for the coated metal foil of the present invention can also be applied to the composition defined in accordance with the present invention, the method for producing the coated metal foil of the present invention, the method for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate of the present invention, and / or the use of the present invention, and vice versa. Furthermore, unless otherwise specified, the term "average" always means "arithmetic average," and the term "mean" always means "arithmetic mean." The abbreviation "wt.%" means "weight percent." The abbreviation "v / v" means "volume ratio."

[0101] The present invention will be explained in more detail with reference to the following figures and embodiments. [Brief explanation of the drawings]

[0102] [Figure 1] 1 shows an SEM image of a cross section of a copper flake containing internal pores. [Figure 2] 1 shows an SEM image of a cross section of a copper flake with a multilayer lamellar structure. [Figure 3a]1 shows a scanning transmission electron microscope image of the multilayer lamellar structure. [Figure 3b] 1 shows a scanning transmission electron microscope image of the multilayer lamellar structure. [Figure 4] 1 shows an SEM image of a foil according to the present invention. [Figure 5] Figure 4 shows SEM images of the foil at different magnifications. [Figure 6] Figure 4 shows SEM images of the foil at different magnifications. [Figure 7] 1 shows an SEM image of another foil according to the present invention coated with a thin layer of flakes. [Figure 8] 8 shows SEM images of the foil of FIG. 7 at different magnifications. [Figure 9] 1 shows a focused ion beam scanning electron microscope (FIB-SEM) image of another foil according to the present invention coated with a thin layer of flakes. [Figure 10] 10 shows a crop of the image of FIG. 9 at a different magnification. [Figure 11] 1 shows a FIB-SEM image of another foil according to the present invention coated with a thick layer of flakes. [Figure 12] 1 shows an SEM image of a cross section of an interconnection between a chip and a copper substrate resulting from sintering a coated copper foil according to the present invention. [Figure 13] 1 shows an SEM image of a cross section of an interconnection between a copper foil and a copper substrate resulting from sintering of a coated copper foil according to the present invention. [Figure 14] Figure 1 shows an SEM image of a cross section of an interconnect between two ceramic substrates obtained from sintering of a coated metal foil. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0103] Average intraparticle pore density and average interparticle cavity density of copper flakes The compositions defined in accordance with the present invention include flakes containing internal pores. For example, these flakes containing internal pores can be purchased commercially as Cubrotec 8001 copper flakes from Carl Schlenk SE, Germany. A cross section of Cubrotec 8001 copper flakes is shown in FIG. 1. In FIG. 1, the numerous internal pores of the copper flakes are exemplarily indicated by solid arrows. The spaces between the flakes, which become interparticle voids after sintering, are exemplarily indicated by open arrows.

[0104] As can be seen from Figure 1, Cubrotec 8001 copper flakes have a relatively large number of internal pores and a relatively high average density of intra-particle pores. 2 The Cubrotec 8001 copper flakes have an average intra-particle pore density of 30 per flake. The inter-flake space is 40%. [Example]

[0105] Copper or copper alloy flakes with multilayer lamellar structure If the flake is a Cubrotec 8001 copper flake, the flake has a multilamellar structure. Cubrotec 8000 flakes have a simple lamellar structure, but not a multilamellar structure. Figure 2 shows a cross section of a Cubrotec 8001 copper flake, illustrating the multilamellar structure of the copper flake. In Figure 2, a stack of three copper flakes is shown within the white dotted box as an example. Each multilamellar structure consists of a variable number of lamellae, ranging from 2 to 20. Furthermore, the overall thickness of the multilamellar structure varies from 20 nm to 400 nm.

[0106] Figures 3a and 3b show scanning transmission electron microscope (STEM) images of an exemplary multilamellar structure consisting of 10 lamellae. In Figure 3a, the lamellar boundaries between lamellae (lamellae 1-10) in the stacked multilamellar structure are shown by black dotted lines. In this regard, for example, lamella 1 contacts lamella 2 by one lamellar boundary. However, lamella 2 contacts both lamella 1 and lamella 3 by one lamellar boundary each, and so on. This results in a stacked multilamellar structure consisting of lamellae in contact with each other. Furthermore, Figure 3b shows the nanocrystalline structure of copper forming the lamellae within the stacked multilamellar structure. [Example]

[0107] composition The following compositions were tested:

[0108] [Table 1]

[0109] Table 1 shows different compositions that can be coated on the first and / or second surfaces of copper or copper alloy foil. Cubrotec 8001 copper particles were used as an example in each of Compositions 1 through 12. As can be seen from Table 1, Compositions 1 and 2 do not contain copper flakes coated with stearic acid. Compositions 3 through 12 contain copper flakes coated with stearic acid. Composition 2 illustratively used isopropanol as the optional polar organic solvent. Composition 4 illustratively used acetone as the optional polar organic solvent. Composition 6 illustratively used turpentine.

[0110] In each of Compositions 3, 5, 7, and 12, the total weight of the stearic acid-coated copper flakes relative to the total weight of the composition ranges from 60% to 76% by weight. The total weight of the stearic acid-coated copper flakes includes the total weight of the stearic acid coated on the copper flakes and the total weight of the copper flakes. For example, in Composition 3, 60% by weight of the stearic acid-coated copper flakes contains 58.5% by weight of the copper flakes and 1.5% by weight of the stearic acid coated on the copper flakes. Therefore, the total solids content in Composition 3 is 58.5% by weight. Furthermore, the organic binder in Composition 3 is a mixture of ethylene glycol and α-terpineol. In this regard, the total weight of the ethylene glycol relative to the total weight of the composition is 16% by weight, and the total weight of the α-terpineol relative to the total weight of the composition is 24% by weight. Thus, composition No. 3 contains 40% by weight of the total weight of organic binders relative to the total weight of the composition. [Example]

[0111] Preparation of the composition The following are exemplary formulations of Composition No. 1 and Composition No. 5. The formulation of Composition No. 1 includes the following steps:

[0112] 63 g of copper flakes are added to a standard paste mixing jar commercially available for high-speed mixers and planetary mixers. 37 g of ethylene glycol, an exemplary organic binder, is then added to the copper flakes. The mixture is mixed in a planetary rotary mixer at 1000 rpm for 4 minutes, followed by mixing at 500 rpm for 5 minutes. Alternatively, the mixture can be mixed in a high-speed mixer at 1500 rpm for 30 seconds.

[0113] The formulation of Composition No. 5 involves the following steps:

[0114] 15.5g of ethylene glycol, 24g of α-terpineol, and 0.5g of PEG600 were placed in a beaker and mixed using a magnetic stirrer at 500 rpm for 3 minutes. This resulted in the preparation of an organic binder. 60g of stearic acid-coated copper flakes (flakes with a maximum 1.5% stearic acid coating) were added to a planetary mixer. The organic binder was then added to the coated copper flakes and mixed in the planetary mixer at 1000 rpm for 4 minutes and 500 rpm for 5 minutes. [Example]

[0115] Copper foil pretreatment Copper foil with a thickness of 10 μm was purchased from Schlenk Metallfolien GmbH & Co. KG. The foil was produced by a traditional rolling process, which resulted in the presence of oily residues on the surface. To remove the oily residues, the foil was pretreated as follows:

[0116] The foil was cut into 50 mm x 50 mm pieces. Each piece was placed on the heat plate of a reflow oven RSS-160-S manufactured by UniTemp GmbH (Luitpoldstr. 6, D-85276 Pfaffenhofen, Germany) at 25 °C. The oven was then closed, a vacuum of 1 mbar was applied, and the foil was subsequently heated from 25 °C to 180 °C at 60 K / s, maintaining the vacuum and temperature for 30 minutes. Finally, the oven containing the foil was cooled to 25 °C at 30 K / min under vacuum. Finally, nitrogen was introduced into the oven and the vacuum was released before the foil was removed. After these steps, both sides of the foil pieces were free of organic oily residues and were ready for spray coating, as outlined below. [Example]

[0117] Spray coating of pretreated copper foil Each of Compositions Nos. 2-7 was independently applied by spray coating to the first and / or second surfaces of separate pretreated copper foils as shown in Examples 3 and 4. Each of Compositions Nos. 1 and 8-12 was independently applied by hand coating to the first and / or second surfaces of separate pretreated copper foils as follows.

[0118] Spray coating was performed using a SHOTMASTER 300 OMEGAX, SM300OMEGAX-3A-SS, manufactured by MUSASHI ENGINEERING EUROPE GMBH (Marcel-Breuer-Str. 15, 80807 München, Germany). The following spray coating parameters were applied: Nozzle opening 100~120μm (screw valve mechanism) Cartridge inlet air pressure: -0.3 to 0.4 MPa Chamber air pressure -0.3MPa Nozzle air pressure -0.4~0.5MPa

[0119] The spray coating was carried out by printing 30 mm lines with a 0.5 mm pitch at a coating speed of 200 mm / s.

[0120] Although spray coating is illustratively shown as an application method, other application methods such as aerosol jet printing, screen printing, or material dispensing have also been implemented in other examples to apply the composition to the first and / or second surfaces of the copper foil.

[0121] For compositions 1 and 8-12, manual coating was performed using a metal squeeze tool. This is because the absence of polar organic solvents makes these formulations highly viscous, making a spray coating process impossible. In this case, the formulation was applied to the edge of the foil and manually spread across the entire foil surface with a single stroke of the handheld metal squeeze tool to obtain a uniform layer on the foil. [Example]

[0122] Preparation of coated copper foil by sintering After the spray coating and manual coating were completed, a pre-drying step was first performed, and then a sintering step was performed to sinter the foil as follows:

[0123] Pre-drying was performed in a hot-air sterilizer SN30 manufactured by Memmert GmbH+Co.KG (Aeussere Rittersbacher Strasse 38, 91126 Schwabach, Germany). For this purpose, the sterilizer was preset to 120 °C. Once the temperature was reached, the coated foil was placed in the oven, with the fan speed at 100% for 5 minutes. The sterilizer door was then opened, and the foil was removed from the 120 °C oven and allowed to cool to room temperature outdoors. Therefore, a hot-in / hot-out process was employed to pre-dry the foil. This pre-drying evaporated the organic binder. However, residual organic binder remained on each coated copper foil, allowing the flakes to adhere to the first and / or second surfaces of the foil, respectively. Following this step, sintering was performed, as detailed below.

[0124] To obtain the foils shown in Figures 4 to 11, sintering was carried out in a reflow oven RSS-160-S manufactured by UniTemp GmbH under nitrogen atmosphere at 250°C for 30 minutes under pressureless conditions as follows.

[0125] The pre-dried foil was placed on the heat plate of a reflow oven at 25°C and the oven was closed. A vacuum of 1 mbar was applied at 25°C. Nitrogen was then flowed into the chamber for 1 minute at 25°C. The oven was then ramped up to 250°C at 30 K / s under nitrogen, then held at 250°C under nitrogen for 30 minutes, before being cooled to 25°C at a rate of 30 K / s under nitrogen, and the sintered foil was removed.

[0126] The foils were examined by SEM and FIB-SEM, and the results are shown in Figures 4-11.

[0127] Figure 4 shows the flakes' ability to uniformly cover the copper foil area, and the arrows in Figure 5 indicate sintered connections in the form of sintered necks between the flakes, demonstrating that the flakes exhibit excellent sintering ability even in an unpressurized state at 250°C for 30 minutes in an inert atmosphere.

[0128] Figure 6 shows that the surface of the flakes has a very fine grain structure. Analysis of the grain structure by electron backscatter diffraction (EBSD) showed a preference for a 0101 grain orientation relative to the surface normal.

[0129] Figures 7 and 8 show copper foil coated with a thin flake layer, leaving a portion of the foil uncoated. The arrows added to Figure 7 indicate the uncoated portion of the copper foil.

[0130] Figure 9 shows a single flake sintered to copper foil. The grain size of the flake is significantly smaller than that of the foil. As is evident from Figure 9, the grains on the foil are significantly larger (combined with Figure 3, where the grains on the flake are in the sub-50 nm range), thus creating a large energy gradient between the flake and the foil. This accelerates material transport via surface diffusion, allowing the formation of sintering necks even under pressureless conditions, as clearly observed in Figure 10. As seen in Figures 12, 13, and 14, applying uniform bonding pressure increases the contact area between the flake and the foil, improving the bond between the flake and the copper foil. The arrows added to Figure 10 indicate sintered bridges formed between the flake and the foil.

[0131] Figure 11 shows that the flakes can stack on top of each other and connect to each other by sintering, thus offering the possibility of applying thin, homogeneous coatings on foils.

[0132] To prepare the sample shown in Figure 12, the top surface of the copper foil (i.e., the interface with the chip) was coated by spray coating as outlined above, because a thin layer was desired. However, on the bottom surface (i.e., the interface with the substrate), a thick sintered layer was desired between the foil and the substrate. To achieve this, after pre-drying the spray-coated layer, a thick layer of Composition No. 1 was printed directly onto the bottom surface of the copper foil using the manual coating process described above. Following this process, the hand-coated layer was also pre-dried, and then the foil with the thin spray-coated, pre-dried layer on one side and the thick hand-coated, pre-dried layer on the other side was used as a sandwich to bond the chip to the bare copper substrate.

[0133] Figure 12 shows an SEM image of the cross section of a sintered interconnect, demonstrating the possibility of applying different sinter paste thicknesses on both sides of the foil and sintering a test chip onto a bare copper substrate. Sintering was carried out (according to sample number 5 in Table 1 below) in an SP300 sintering press from budatec GmbH (Melli-Beese-Strasse 28, 12487 Berlin, Germany) at 275°C for 5 minutes in an inert atmosphere under a bonding pressure of 1 MPa.

[0134] Figure 13 shows an SEM image of the cross section of a sintered interconnect, demonstrating the feasibility of bonding only foil onto a bare copper substrate. Sintering was carried out at 275 °C for 5 min under a bonding pressure of 1 MPa.

[0135] To prepare the sample shown in Figure 14, a coated copper foil was prepared by applying the composition to one surface of the copper foil at a thickness of 20 μm and to the second surface of the copper foil at a thickness of 75 μm, followed by pre-drying. Composition No. 1 was manually coated as described above. First, the paste was applied to one side and pre-dried in air at 120°C for 5 minutes. Next, the composition was applied to the other side and pre-dried again in air at 120°C for 5 minutes. Finally, the double-sided coated metal foil was precisely placed on the first ceramic substrate using a pick-and-place machine. Next, a second ceramic substrate with copper edge metallization was placed on top of the coated metal foil using the pick-and-place machine for precise placement. Sintering was then carried out in an open bond chamber at 260 °C for 3 min with a nitrogen flow rate of 3 L / min and a ramp rate of 1 K / s, using a Fineplacer Sigma flip-chip bonder from Finetech GmbH & Co. KG (Box-berger Str. 14, 12681 Berlin) to bond the second substrate at a bonding pressure of 2 MPa. A cross section of the resulting interconnect is shown in Figure 14.

[0136] As shown in Figure 14, the thicknesses of the layers between the first ceramic substrate and the copper foil and the second ceramic substrate and the copper foil are reduced to 10-15 μm and 30-40 μm, respectively, by sintering. The coated metal foil sintered between the ceramic substrates exhibits uniform and regular interconnections. [Example]

[0137] Running a shear test In accordance with the present invention, a 1 mm 2Dummy test chips with a footprint of 10 μm were sintered onto bare copper substrates via foil. Ten samples were prepared per test. The foil was prepared by applying a thin sintered layer of less than 5 μm to both sides of a 10 μm-thick copper foil manufactured by Schlenk Metallfolien GmbH & Co. KG. Sintering was performed in an SP300 sintering press manufactured by budatec GmbH in an inert nitrogen atmosphere. After sintering, shear tests were performed according to the MLT-STD-883 test method (Mechanical Test Method 2019.5 Die Shear Strength). The arithmetic mean shear strength values ​​of the sintered interconnects were measured using an XYZ Condor Sigma Lite shear tester at a shear rate of 100 μm / s. The results are shown in Table 3 below.

[0138] [Table 2]

[0139] Although the above only shows experiments using copper foil and copper flakes, experiments using bronze foil, brass foil, bronze flakes, and brass flakes were also carried out, and all of these experiments yielded results similar to those of the above experiments.

Claims

1. A coated metal foil for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, or between a solid substrate and a metal foil, the coated metal foil comprising: a copper or first copper alloy or copper-plated nickel-iron alloy metal foil having a first surface and a second surface; a composition coated on the first surface and / or the second surface of the metal foil; wherein the composition comprises or consists of an organic binder; flakes of copper, a first copper alloy, or a second copper alloy; comprising or consisting of The flakes are coated metal foils containing internal pores.

2. the composition is coated on the first surface and a further composition is coated on the second surface; The further composition comprises: the organic binder or a further organic binder; the flakes or further flakes of copper, the first copper alloy, or the second copper alloy; comprising or consisting of The coated metal foil of claim 1 , wherein the additional flakes include internal pores.

3. The flakes and / or further flakes of copper, the first copper alloy, or the second copper alloy have internal pores of 1 μm or less. ２ 3. The coated metal foil of claim 1, having an average intra-particle pore density in the range of 7 to 30 pores per particle.

4. the average grain size of the crystal grains in the metal foil on the first surface and / or the second surface of the metal foil is 2 μm or more and 100 μm or less; and / or the average grain size of the flakes and / or the further flakes, which are independent of one another, is between 20 nm and 200 nm; and / or 4. The coated metal foil according to claim 1, wherein the ratio of the average grain size of the crystal grains in the flakes to the average grain size of the crystal grains at the first surface and / or the second surface of the metal foil and / or the ratio of the average grain size of the crystal grains in the further flakes to the average grain size of the crystal grains at the second surface of the metal foil each does not exceed 0.

002.

5. the first copper alloy is bronze or brass; and / or The coated metal foil according to any one of claims 1 to 4, wherein the thickness of the metal foil is 5 μm to 150 μm, in particular 100 μm or less.

6. the flakes and / or the further flakes are made of the second copper alloy, the second copper alloy being bronze or brass; and / or 6. The coated metal foil according to claim 1, wherein the flakes and / or the further flakes have an average particle size D50 of 5 μm or less, measured by laser granulometry.

7. The coated metal foil according to any one of claims 1 to 6, wherein the flakes and / or further flakes have a multi-lamellar structure.

8. 8. The coated metal foil according to claim 1, wherein the flakes and / or further flakes are coated with stearic acid, and the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes and / or the stearic acid-coated further flakes is, independently of one another, 1.5% by weight or less.

9. 9. The coated metal foil according to claim 1, wherein the organic binder and / or further organic binder are each independently of one another terpineol, a primary alcohol, a diol, a triol, a polymeric glycol, or a mixture of at least two of terpineol, a primary alcohol, a diol, a triol, and a polymeric glycol.

10. 10. The coated metal foil according to claim 1, wherein the composition and / or the further composition, independently of one another, further comprise turpentine or a polar organic solvent, in particular an alcohol, acetone, or a mixture of an alcohol and acetone.

11. 10% to 100%, in particular 50% to 100%, of the area of ​​the first surface and / or the area of ​​the second surface of the metal foil are coated, independently of one another, with the composition; and / or the composition is coated onto the first surface and / or the second surface of the metal foil, each independently of the other, to a thickness of from 1 μm to 500 μm, in particular from 1 μm to 50 μm, or 10% to 100%, in particular 50% to 100% of the area of ​​the first surface of the metal foil is coated with the composition and 10% to 100%, in particular 50% to 100% of the area of ​​the second surface of the metal foil is coated with a further composition; and / or 11. The coated metal foil according to claim 1, wherein the composition is coated independently on the first surface of the metal foil and the further composition is coated independently on the second surface of the metal foil, to a thickness of from 1 μm to 500 μm, in particular from 1 μm to 50 μm.

12. A method for producing a coated metal foil according to any one of claims 1 to 11, comprising the steps of: a) the metal foil of copper or a first copper alloy or a copper-plated nickel-iron alloy having the first surface and the second surface; the flakes and optionally further flakes of copper or the first copper alloy or the second copper alloy, the flakes comprising internal pores; said organic binder and optionally a further organic binder; providing a b) mixing said flakes with said organic binder to obtain a composition, and optionally mixing said flakes or said further flakes with said organic binder or said further organic binder to obtain a further composition; c) applying the composition obtained in step b) to the first surface and / or the second surface of the metal foil, or applying the composition obtained in step b) to the first surface of the metal foil and the further composition obtained in step b) to the second surface of the metal foil, to obtain the coated metal foil. A method comprising:

13. the metal foil is made of bronze or brass and / or has a thickness of 5 μm to 150 μm, in particular 100 μm or less; and / or the flakes and / or the further flakes are made of a second copper alloy, the second copper alloy being bronze or brass; and / or the flakes and / or the further flakes have an average particle size D50 of 5 μm or less, as measured by laser granulometry; and / or the flakes and / or the further flakes have a multi-lamellar structure, and / or the flakes and / or the further flakes are coated with stearic acid, and the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes and / or the stearic acid-coated further flakes is, independently of one another, not more than 1.5% by weight; and / or 13. The method of claim 12, wherein the organic binder and / or the further organic binder are each independently of one another terpineol, a primary alcohol, a diol, a triol, a polymeric glycol, or a mixture of at least two of terpineol, a primary alcohol, a diol, a triol, and a polymeric glycol.

14. 14. The method of claim 12 or 13, wherein prior to step c), the metal foil is heated to a first pre-treatment temperature in the range of 170°C to 220°C for at least 1 minute and at most 2 hours, and cooled to a second pre-treatment temperature in the range of 15°C to 40°C.

15. 15. The method according to any one of claims 12 to 14, wherein after step c) the metal foil is heated to a pre-drying temperature in the range of 100°C to 160°C for at most 1 minute and at most 30 minutes, or during step c) and after applying the composition obtained in step b) to the first surface the metal foil is heated to a pre-drying temperature in the range of 100°C to 160°C for at most 1 minute and at most 30 minutes, followed by a cooling step, applying the composition obtained in step b) or the further composition to the second surface and heating the metal foil to a pre-drying temperature in the range of 100°C to 160°C for at most 1 minute and at most 30 minutes.

16. Use of the coated metal foil of any one of claims 1 to 11 for forming a copper-containing intermediate layer between two surfaces of a solid substrate or between a surface of a solid substrate and said metal foil, for forming conductive paths on a solid substrate, or for attaching a substrate.

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