Composition for forming a copper-containing intermediate layer containing internal pores

Abstract

The present invention relates to a composition for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, the composition comprising or consisting of the following components: An organic binder and flakes of copper or a copper alloy, wherein the flakes contain internal pores and have a multilayer lamellar structure.

Description

Technical Field

[0001] The present invention relates to a composition for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, a method for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, and the use of the composition. In the context of the present invention, the intermediate layer should be understood as an interconnecting layer, i.e., a solid metal interconnect between the first solid substrate and the second solid substrate.

Background Art

[0002] Die attach bonding is an important process for realizing the high-temperature operation of power semiconductor devices. High-Pb solder has been a preferred and well-established option over the past few decades. However, strict regulations regarding hazardous substances limit the use of high-Pb solder, and the few exceptions that exist today are also expected to be prohibited in the near future. With the increasing use of wide-bandgap (WBG) semiconductor devices, it is essential to find sustainable and reliable alternatives in both the economic and technological fronts. Interconnect materials and technologies need to be able to meet the challenging requirements of WBG semiconductor devices and at the same time be economical to enable mass production.

[0003] Currently, there are two established methods for creating high-temperature joints while benefiting from relatively low bonding temperatures: transient liquid phase (TLP) bonding and particle sintering. While various studies have reported the successful creation of in situ phases with higher remelting temperatures, the thermomechanical fracture behavior of TLP-bonded joints appears to have significant drawbacks. This is due to the microstructure of TLP-bonded joints, as they are made from brittle intermetallic compounds (IMCs). However, in the case of particle sintering, there are promising reports suggesting Ag-sintered joints as a candidate for high-temperature WBG applications. During sintering, it is possible to achieve a connection consisting of a single metal uniformly. Besides the high thermal conductivity of the material, an advantage is that the interconnection is established at relatively low temperatures (250°C) but stable at higher temperatures (above 300°C). However, sintered Ag particles have their own drawbacks: high cost and low electromigration resistance.

[0004] Copper is approximately 100 times cheaper than silver, is also abundant, easily recyclable, and readily available. Copper has a lower overall carbon footprint than silver. It has a lower coefficient of thermal expansion than silver and possesses nearly the same electrical and thermal conductivity. However, copper's higher melting point means that its sintering temperature is also somewhat higher than that of silver. Furthermore, its tendency to oxidize rapidly in the atmosphere is a major drawback when using copper, as this is detrimental to the mechanical and thermal integrity of the joint.

[0005] German Patent Application Publication No. 102005005876 discloses a thermoplastic molding composition containing A) 4.99 to 39.99 wt.% of a thermoplastic polymer, B) 60 to 95 wt.% of copper, C) 0.01 to 3 wt.% of a transition metal salt or alkali salt, and D) 0 to 35 wt.% of further additives, wherein the total weight percentage of components A) to D) is 100%.

[0006] U.S. Patent Application Publication No. 2018 / 342478 discloses a joint comprising a first member; a second member; and a sintered metal layer joining the first member and the second member. The sintered metal layer comprises a structure derived from flake-shaped copper particles oriented approximately parallel to the interface between the first member or the second member and the sintered metal layer, and the amount of copper contained in the sintered metal layer is 65% by volume or more, based on the volume of the sintered metal layer.

[0007] U.S. Patent Application Publication No. 2011 / 236687 discloses a method for converting spherical or amorphous metal particles into lamellar flakes that promote improved adhesion and aggregation properties when incorporated into a coating composition. The resulting metal flakes exhibit compatibility with binder chemistry such as isocyanates, titanates, and titanate hybrids, and are suitable for use in conjunction with advanced topcoating techniques such as electrodeposition. The particles produced by the method can be used in coatings and, when used in conjunction with an electrodeposited topcoat, may exhibit improved substrate adhesion and improved aggregation characteristics.

[0008] International Publication No. 2022 / 079983 discloses a metal powder containing a large number of metal nanoparticles. The metal nanoparticles include laminated metal nanoparticles. Each laminated metal nanoparticle has a first layer which is in the shape of a flake, and a second layer which is in the shape of a flake, laminated to the first layer and integral with the first layer. There is a space between the first layer and the second layer.

[0009] Japanese Patent Publication No. 6332058 discloses copper powder having an aggregate morphology formed by the aggregation of a plurality of individual copper particles, wherein the copper particles are flat and have an average major axis diameter of 0.5 μm to 5 μm and an average cross-sectional thickness of 0.02 μm to 0.5 μm, as determined by observation with a scanning electron microscope (SEM), and an average particle size (D50) of 1.0 μm to 30 μm, as determined by laser diffraction / scattering particle size distribution measurement.

[0010] International Publication No. 2020 / 035859 discloses a process for preparing copper metal flakes, which includes the mechanical shaping of copper metal particles to obtain copper flakes. The copper metal particles are prepared by reduction of at least one copper metal precursor, such as a copper salt or copper complex. The mechanical shaping of the copper metal particles may include at least one of mechanical milling, mechanical crushing, mechanical bending, and mechanical planarization.

[0011] Bhogaraju, SK et al. "Copper die bonding using copper formate based pastes with alpha-terpineol, amino-2-propanol and hexylamine as binders", 2020 IEEE 8 Electronics System-Integration Technology Conference (ESTC), 2020, pp. 1-7 discloses copper pastes based on Cu(II) formate tetrahydrate mixed with either PEG600, alpha-terpineol, amino-2-propanol, or hexylamine as binders. After completion of the thermal decomposition process, the formation of pure metallic copper nanoparticles is observed. The average shear strength values ​​of the sintered pastes were 30 MPa, 9 MPa, and 2 MPa for PEG600, alpha-terpineol, and amino-2-propanol as binders, respectively, but hexylamine as a binder did not establish sufficient bonding strength for shear testing.

[0012] Bhogaraju, SK et al. "Die-attach bonding for high temperature applications using thermal decomposition of copper(II) formate with polyethylene glycol", Scr. Mater. 2020, 182, 74-80 discloses copper pastes based on Cu(II) formate mixed with either PEG600, alpha-terpineol, or a mixture of alpha-terpineol and polyvinyl butyral. From the sintered pastes tested, the Cu(II) formate / PEG600 paste at a weight ratio of 63 / 37 showed the highest shear strength value of 60 MPa. In addition, the thermal decomposition of Cu(II) formate results in the in situ formation of copper nanoparticles.

[0013] According to Bhogaraju, SK et al. "Novel approach to copper sintering using surface-enhanced brass micro flakes for microelectronics packaging", Journal of Alloys and Compounds, Volume 844, 2020, 156043, etched copper or brass flakes were mixed with PEG600 as a binder to obtain a paste formulation. Before preparing the paste formulation, the flakes were treated with 12M HCl to etch zinc from the brass flakes and copper oxide from the copper flakes. After etching, the flakes were thoroughly rinsed first in distilled water and then in isopropanol. This treatment removed zinc, chlorine, and all organic components from the flakes. The use of PEG600 in the paste formulation has been reported to enable in situ reduction of Cu oxide during sintering.

[0014] Bhogaraju, SK et al. "Die-attach bonding with etched micro brass metal pigment flakes for high-power electronics packaging", ACS Appl. Electron. Mater. 2021, 3, 10, 4587-4603 discloses a sintered paste comprising etched microscale brass metal pigment flakes or pure copper flakes and a binder mixture of alpha-terpineol and polyethylene glycol 600 (PEG600). Prior to the formulation of the paste, the flakes were treated with HCl for etching, washed in distilled water, washed in isopropanol, and dried in air. This treatment removed zinc, chlorine, and all organic components from the flakes.

[0015] European Patent No. 3626785 discloses a metal paste comprising 65–85% by weight of metal particles and 10–35% by weight of an organic solvent, wherein 70–100% by weight of the metal particles consists of organically coated copper flakes having a specific surface area in the range of 1.9–3.7 m / g, a total oxygen content in the range of 2–4% by weight, and a total carbon-to-total oxygen weight ratio in the range of 0.25–0.9. The proportion of the organic coating may be in the range of 2–5% by weight based on the weight of the organically coated copper flakes.

[0016] U.S. Patent Application Publication No. 2014 / 0287158 discloses a conductive paste for screen coating comprising a resin containing a mixture of copper flakes having an average diameter of 1 μm to 8 μm and copper nanoparticles having an average diameter of 10 nm to 100 nm (where the ratio of copper flakes to nanoparticles is between 2:1 and 5:1 by weight), and a polymer having a molecular weight greater than approximately half of 10,000 and one or more solvents. [Overview of the project] [Problems that the invention aims to solve]

[0017] The problem to be solved by the present invention is to provide an alternative composition for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, an alternative method for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate, and the use of the alternative composition. The alternative composition shall allow die-attach bonding, formation of electrical and / or thermal conduction paths on the solid substrate, and substrate adhesion. The alternative composition and alternative method shall enable sintering with a short sintering time, a low sintering temperature, and / or a low sintering pressure. Sintering of the alternative composition shall result in a copper-containing intermediate layer having high resistance to thermomechanical fatigue, high thermal conductivity, and high electrical conductivity. [Means for solving the problem]

[0018] The problem is solved by the subject matter of claims 1, 12, and 15. Aspects of the present invention are the subject matter of claims 2 to 11, and claims 13 and 14.

[0019] According to the present invention, a composition is provided for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate. The composition includes or consists of the following components:

[0020] Organic binder and flakes of copper or copper alloy. The flakes contain internal pores and have a multilayer lamellar structure.

[0021] The inventors of the present invention have found that copper or copper alloy flakes containing internal pores yield an intermediate layer after sintering, and this intermediate layer has increased stress absorption compared to an intermediate layer obtained from copper or copper alloy flakes without internal pores. Thus, due to the internal pores, the intermediate layer obtained from flakes containing internal pores can efficiently absorb the thermomechanical stress applied to the intermediate layer. In the context of the present invention, thermomechanical stress refers to the stress generated in the intermediate layer due to a mismatch in thermal expansion coefficients, i.e., due to the different expansion and contraction of the first and second solid substrates and the intermediate layer caused by thermal effects during and after sintering, and / or during the operation of a device containing an intermediate layer between a first solid substrate and a second solid substrate. Furthermore, the inventors of the present invention have found that, due to the sponge-like microstructure of the intermediate layer, the mechanical stress applied to the intermediate layer is also efficiently absorbed by the intermediate layer obtained from flakes containing internal pores, compared to an intermediate layer obtained from copper or copper alloy flakes without internal pores. Interlayers resulting from compositions according to the present invention after sintering exhibit improved resistance to thermomechanical fatigue compared to interlayers resulting from compositions containing copper or copper alloy flakes without internal pores. In the context of the present invention, thermomechanical fatigue refers to the overlap of periodic thermal loads and periodic mechanical loads leading to material fatigue. Therefore, interlayers resulting from compositions according to the present invention exhibit good thermomechanical performance, and thus ensure good reliability, specifically when relatively high thermomechanical stress and / or relatively high mechanical stress are applied to the interlayer.

[0022] Furthermore, the inventors have found that the thermal and electrical conductivity of the intermediate layer resulting from a composition according to the present invention after sintering is less affected by internal pores compared to an intermediate layer resulting from a composition containing copper or copper alloy flakes without internal pores. Thus, it is also shown that a composition according to the present invention results in a sintered intermediate layer having relatively high thermal and relatively high electrical conductivity.

[0023] The inventors of the present invention have further found that the internal pores of the flakes of copper or copper alloy are maintained during and after the sintering of the composition according to the present invention. Due to the good thermo-mechanical properties, the rapid cooling of the copper-containing intermediate layer from the sintering temperature to the cooling temperature does not cause excessive stress in the copper-containing intermediate layer. Thus, no fractures are formed in the copper-containing intermediate layer during and after rapid cooling, specifically after sintering and / or during the thermal shock test.

[0024] The inventors of the present invention have further found that flakes of copper or copper alloy having a multilayer lamellar structure enable sintering within a relatively short sintering time, specifically within a sintering time of at most 5 minutes, at a relatively low sintering temperature, specifically at a sintering temperature of at most 250 °C, and / or at a relatively low sintering pressure, specifically at a sintering pressure of at most 15 MPa, compared to flakes of copper or copper alloy not having a lamellar structure, specifically not having a multilayer lamellar structure. This is enabled by the relatively large surface area of the flakes having a multilayer lamellar structure, which results in improved densification and rapid grain growth of the flakes of copper or copper alloy in the composition according to the present invention during sintering. The multilayer lamellar structure of the flakes of copper or copper alloy can be determined by surface analysis of the flakes of copper or copper alloy, specifically by metal surface analysis, and / or by cross-sectional analysis. Surface analysis of the flakes of copper or copper alloy, specifically metal surface analysis, can include microscopic geometry analysis, surface structure analysis, surface element composition analysis, and depth direction analysis. Surface analysis and / or cross-sectional analysis can be performed, for example, by scanning electron microscopy (SEM), X-ray diffraction (XRD), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), glow discharge spectroscopy, and image-based metal surface inspection. These methods can also be applied to surface analysis and / or cross-sectional analysis of the copper-containing intermediate layer and / or of the composition according to the present invention before and / or after sintering.

[0025] The crystal grains in a flake of copper or copper alloy may have an average grain size of at least 5 nm and at most 200 nm, specifically at least 20 nm and at most 200 nm, independently of each other. A crystal grain is any of the crystallites that make up a solid metal. The term "average grain size" refers to the average grain size given as the average diameter determined from the average area determined by electron backscatter diffraction (EBSD) according to ISO / DIS 13067(en). It is a value determined from two-dimensional measurements relating to the average three-dimensional size of a crystal grain or aggregate of crystals that forms a polycrystalline material by stereochemical relationships. A crystal grain or aggregate of crystals may contain 30 to 1000, specifically 40 to 400, specifically 50 to 200 crystal grains or crystals.

[0026] Grain boundaries act as pathways for atomic or molecular diffusion during sintering. During sintering, when a sintering temperature is applied, atoms or molecules move across these boundaries, which promotes diffusion and thus improves 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] The copper alloy from which flakes can be formed can be bronze or brass. When the flakes are a copper alloy, good thermal conductivity of the sintered composition according to the present invention is enabled. However, when the flakes are copper, the thermal conductivity of the sintered composition according to the present invention is usually higher. Specifically, the brass can be a brass containing at least 0.5 wt.% zinc, specifically at least 1 wt.% zinc, and at most 36 wt.% zinc, specifically at most 30 wt.% zinc, specifically at most 25 wt.% zinc, specifically at most 20 wt.% zinc, specifically at most 15 wt.% zinc. Specifically, the brass can be an alpha brass containing at most 36 wt.% zinc. Specifically, the bronze can be a bronze containing at least 0.5 wt.% tin, specifically at least 1 wt.% tin, and at most 20 wt.% tin, specifically at most 15 wt.% tin, specifically at most 10 wt.% tin, specifically at most 5 wt.% tin. The purity of the copper or copper alloy can be at least 93%, specifically at least 95%, specifically at least 99%, specifically at least 99.95%, and at most 99.99%.

[0028] The copper or copper alloy, specifically copper, from which flakes are formed can be single crystal or polycrystalline. The flakes of copper or copper alloy can be completely or partially oriented in a single crystal orientation, specifically the 111 crystal orientation, or can have a completely or partially disordered crystal structure.

[0029] Copper or copper alloy flakes may have an average particle size D50 determined by laser particle size measurement, with a particle size of at least 1 μm, specifically at least 2 μm, specifically at least 2.5 μm, specifically at least 3 μm, and at most 10 μm, specifically at most 7 μm, specifically at most 5 μm, specifically at most 4 μm, and specifically at most 3.4 μm. Laser particle size measurement may be performed by using laser diffraction measurement in accordance with the specifications of ISO 13320 "Particle size analysis - Laser diffraction method". Laser diffraction measurement may be performed by means of a Helos® laser diffractometer from Sympatec GmbH, 38678 Clausthal-Zellerfeld, Germany. The average particle size D50 is the corresponding particle size when the cumulative percentage reaches 50%. For example, in a powder sample with D50 = 50 μm, it means that 50% of the particles are larger than 50 μm and 50% of the particles are smaller than 50 μm.

[0030] Copper or copper alloy flakes can be obtained by grinding in a ball mill and may have a lamellar, irregular, or cornflake-like shape. The inventors of the present invention have found that the lamellar, irregular, or cornflake-like shape of copper or copper alloy flakes, specifically those having an average particle size D50 of at most 5 μm, at most 4 μm, at most 3.4 μm, or at most 3 μm, results in a better contact surface compared to spherical microparticles. The lamellar, irregular, or cornflake-like shape also results in a larger surface area of ​​the copper or copper alloy flakes, which leads to improved densification and coarsening of the flakes during sintering. This further promotes the formation of a sintering neck and allows for mass transfer during sintering. Either one of the copper or copper alloy flakes may independently have a thickness in the range of 5 nm to 400 nm, specifically in the range of 20 nm to 350 nm, specifically in the range of 50 nm to 300 nm, specifically in the range of 100 nm to 275 nm, specifically in the range of 150 nm to 250 nm, and specifically in the range of 175 nm to 200 nm.

[0031] Internal pores are defined as void spaces in the flake that are not filled with copper or copper alloy. Any of the internal pores are located within the crystal grains of copper or copper alloy forming the flake. Any one of the internal pores may have a diameter in the range of 2 nm to 30 nm, specifically in the range of 3 nm to 25 nm, specifically in the range of 5 nm to 20 nm, and specifically in the range of 10 nm to 15 nm. The inventors of the present invention have found that the number of internal pores allows for relatively high stress absorption of the temperature and / or pressure applied to the intermediate layer formed from the composition, specifically relatively high thermomechanical and / or mechanical stress absorption. This stress absorption by the internal pores of the copper or copper alloy flakes allows for relatively high resistance to thermomechanical fatigue of the intermediate layer formed from the composition according to the present invention, and thus good thermomechanical performance. The diameter of the internal pores can be measured, for example, by gravimetric analysis, computed tomography analysis, or image-based analysis.

[0032] Flakes containing internal pores in a composition according to the present invention may have an average particle pore density in any cross-section of the flake, specifically in any cross-section, ranging from 2 internal pores / μm to 40 internal pores / μm, specifically from 2 internal pores / μm to 30 internal pores / μm, specifically from 3 internal pores / μm to 25 internal pores / μm, specifically from 5 internal pores / μm to 20 internal pores / μm, specifically from 7 internal pores / μm to 18 internal pores / μm, specifically from 10 internal pores / μm to 15 internal pores / μm, and specifically from 12 internal pores / μm to 13 internal pores / μm. Specifically, flakes containing internal pores in a composition according to the present invention may have an average intraparticle pore density in any cross-section of the flake, specifically in any cross-section, ranging from 10 internal pores / μm to 40 internal pores / μm, specifically from 10 internal pores / μm to 30 internal pores / μm, and specifically from 15 internal pores / μm to 20 internal pores / μm. In the context of the present invention, the average intraparticle pore density of a flake is defined as the average number of internal pores per unit area of ​​any cross-section of the flake. The inventors of the present invention have found that the average intraparticle pore density of flakes in a composition according to the present invention is not affected by sintering of the composition according to the present invention. Therefore, the sintered flakes in the copper-containing intermediate / interconnection layer between the two solid substrates, and thus the intermediate / interconnection layer itself, may have an average particle pore density in any cross-section of the intermediate / interconnection layer, specifically in any cross-section, ranging from 2 to 40 internal pores / μm, specifically from 2 to 30 internal pores / μm, specifically from 3 to 25 internal pores / μm, specifically from 5 to 20 internal pores / μm, specifically from 7 to 18 internal pores / μm, specifically from 10 to 15 internal pores / μm, and specifically from 12 to 13 internal pores / μm.Specifically, the sintered flakes in the copper-containing interlayer / interconnection layer between two solid substrates, and therefore the interlayer / interconnection layer itself, may have an average intraparticle pore density in any cross-section of the interlayer / interconnection layer, specifically in any cross-section, ranging from 10 to 40 intraparticle pores / μm, specifically from 10 to 30 intraparticle pores / μm, and specifically from 15 to 20 intraparticle pores / μm. A single particle or flake is sintered together in the interlayer / interconnection layer in such a way that no single particle / flake exists in the interlayer / interconnection layer, but the term "intraparticle pore density" is maintained because the origin of the pores lies in the particle / flake. The average intraparticle pore density of copper or copper alloy flakes can be measured, for example, by gravimetric analysis, computed tomography analysis, or image-based analysis. The average intraparticle pore density of copper or copper alloy flakes can be determined, for example, by using the following method: 1. Embed the flakes in resin, specifically epoxy resin. 2. Prepare ultrathin sections of the embedded flakes. 3. Obtain transmission electron microscope (TEM) and / or scanning electron microscope (SEM) images of ultrathin sections of the embedded flakes. 4. Determining the intraparticle pore density by calculating the number of internal pores in the ultrathin sections of the flake relative to the total area of ​​these ultrathin sections of the flake in multiple TEM images and / or multiple SEM images, and 5. Calculate the average of the multiple pore densities determined in this manner.

[0033] In this regard, step 4 can be accomplished by means of computer-aided image analysis of TEM images and / or SEM images.

[0034] The inventors of the present invention have found that copper or copper alloy flakes having an average particle pore density in the range of 2 internal pores / μm to 40 internal pores / μm, specifically in the range of 2 internal pores / μm to 30 internal pores / μm, and specifically in the range of 5 internal pores / μm to 20 internal pores / μm, result in increased resistance to thermomechanical fatigue of the intermediate layer, in contrast to that obtained from copper or copper alloy flakes without internal pores. Furthermore, the thermal and electrical conductivity of the intermediate layer obtained from copper or copper alloy flakes having an average particle pore density in the range of 2 internal pores / μm to 40 internal pores / μm, specifically in the range of 2 internal pores / μm to 30 internal pores / μm, and specifically in the range of 5 internal pores / μm to 20 internal pores / μm, is not affected by the internal pores. It is identical or nearly identical to that of the intermediate layer obtained from copper or copper alloy flakes without internal pores. Therefore, relatively high thermomechanical performance, relatively high thermal conductivity, and relatively high electrical conductivity are made possible by sintering an intermediate layer obtained from a composition according to the present invention due to copper or copper alloy flakes having an average particle pore density in the range of 2 internal pores / μm to 40 internal pores / μm, specifically in the range of 2 internal pores / μm to 30 internal pores / μm, and specifically in the range of 5 internal pores / μm to 20 internal pores / μm. The average particle pore density of the copper or copper alloy flakes in the range of 2 internal pores / μm to 40 internal pores / μm, specifically in the range of 2 internal pores / μm to 30 internal pores / μm, and specifically in the range of 5 internal pores / μm to 20 internal pores / μm, is not affected by sintering of the composition according to the present invention. The resulting relatively high thermomechanical performance allows for rapid cooling of the copper-containing intermediate / interconnection layer from the sintering temperature to the cooling temperature in a short period of time, without causing excessive stress in the copper-containing intermediate layer, for example, without fracture formation in the copper-containing intermediate layer.

[0035] Furthermore, in the context of the present invention, internal pores of copper or copper alloy flakes do not refer to surface recesses on any surface of the flakes formed, for example, by etching the flakes. Surface recesses exist only on the surface of the copper or copper alloy flakes and are not independent, closed internal pores within the flakes. Internal pores are completely surrounded by the copper or copper alloy. Therefore, internal pores do not exist on the surface and have no connection to the surface of the copper or copper alloy flakes. Moreover, internal pores are not formed by means of etching the copper or copper alloy flakes, specifically by acid etching.

[0036] In contrast to average intraparticle pore density, in the context of this invention, average interparticle cavity density is defined as the average percentage of the area between sintered copper or copper alloy flakes from the total area in the cross-section of the intermediate layer formed from a composition according to this invention. The area between the sintered copper or copper alloy flakes is not filled, specifically not filled by copper or copper alloy. Specifically, the average interparticle cavity density of copper or copper alloy flakes in this intermediate layer at the beginning of flake sintering may be in the range of 5% to 50%, specifically 10% to 45%, specifically 15% to 40%, specifically 20% to 35%, and specifically 25% to 30%. The average interparticle cavity density of flakes in the intermediate layer may be measured, for example, by scanning electron microscopy (SEM) analysis, computed tomography analysis, or analysis based on images of the cross-section of the intermediate layer. In this regard, the cross-section of the intermediate layer may be prepared by ion beam milling to avoid contamination or clogging of internal pores and / or interparticle cavities. The average interparticle cavity density of the flakes in the intermediate layer may be determined, for example, by the following method: 1. Prepare very thin sections of the intermediate layer. 2. To take SEM images of these ultrathin sections, 3. Determining the density of multiple interparticle cavities by calculating the area between flakes not filled with copper or copper alloy within the ultrathin section relative to the total area of ​​these ultrathin sections in the intermediate layer of multiple SEM images, and 4. Calculate the average of the multiple interparticle cavity densities determined in this manner.

[0037] In this regard, step 3 can be accomplished by means of computer-aided image analysis of SEM images.

[0038] The inventors of the present invention have found that the average intraparticle pore density of copper or copper alloy flakes is not affected during the sintering of the composition according to the present invention, but the average interparticle cavity density of flakes in the composition according to the present invention decreases during the sintering of the composition according to the present invention. In this regard, the decrease in average interparticle cavity density depends on the sintering time, the pressure applied during sintering, and the sintering temperature. The pressure applied during the sintering of the composition between a first solid substrate and a second solid substrate is typically the bonding pressure applied to press the first and second substrates against each other. Relatively long sintering times, relatively high pressures applied during sintering, and / or relatively high sintering temperatures result in a relatively high decrease in average interparticle cavity density. After sintering of a composition according to the present invention, the average interparticle cavity density in the copper-containing intermediate / interconnection layer may be in the range of 0.5% to 45%, specifically 1% to 45%, specifically 5% to 40%, specifically 10% to 35%, specifically 15% to 30%, and specifically 20% to 25%. The decrease in average interparticle cavity density during sintering of a composition according to the present invention may result in the complete or near-complete disappearance of interparticle pores in the copper-containing intermediate / interconnection layer.

[0039] Copper or copper alloy flakes are specifically stacked horizontally on top of each other, resulting in a uniform stacking pattern. This provides relatively large surface contact between these flakes. This relatively large surface contact between flakes allows for sintering within a relatively short sintering time, at a relatively low sintering pressure, and / or at a relatively low sintering temperature. Furthermore, copper or copper alloy flakes exhibit relatively high surface energy. Surface energy can be defined as the excess energy at the surface of the material compared to the bulk energy of the material.

[0040] Copper flakes can be purchased as Cubrotec 8001 copper flakes, supplied by Carl Schlenk SE, Germany. In contrast to these flakes, Cubrotec 8000 flakes are copper flakes that have only a lamellar structure and not a multilayer lamellar structure, and these are also supplied by Carl Schlenk SE.

[0041] A multilayer lamellar structure may contain or consist of at least two, specifically at least three, specifically at least four, specifically at least five, specifically at least six, specifically at least seven, specifically at least eight, and at most 20, specifically at most 18, specifically at most 16, specifically at most 14, specifically at most 12, specifically at most 11, specifically at most 10, and specifically at most nine lamellae. Any one of the lamellae may independently have a thickness in the range of 5 nm to 20 nm, specifically in the range of 6 nm to 18 nm, specifically in the range of 8 nm to 16 nm, specifically in the range of 10 nm to 14 nm, and specifically in the range of 11 nm to 13 nm. The number of lamellae in a multilayer lamellar structure and / or the thickness of any of the lamellae in a multilayer lamellar structure may be determined by cross-sectional analysis of the flake. Cross-sectional analysis may be performed, for example, by SEM or TEM of the cross-section of a copper or copper alloy flake. The number of lamellae in the multilayer lamellar structure of the flake can be determined, for example, by using the following method: 1. Embedding copper or copper alloy flakes in resin, specifically epoxy resin. 2. Prepare ultrathin sections of the embedded flakes. 3. To obtain transmission electron microscope (TEM) images and / or scanning electron microscope (SEM) images of ultrathin sections of the embedded flakes, 4. Count the number of lamellae in the multilayer lamellar structure within ultrathin sections of flakes in TEM and / or SEM images.

[0042] In this regard, step 4 can be accomplished by means of computer-aided image analysis of TEM images and / or SEM images.

[0043] The multilayer lamellar structure may extend throughout the entire copper or copper alloy flake or over a portion of the flake. The multilayer lamellar structure may have a total thickness of at least 20 nm, specifically at least 50 nm, specifically at least 100 nm, and at most 400 nm, specifically at most 350 nm, specifically at most 300 nm, specifically at most 250 nm, and specifically at most 200 nm.

[0044] The copper or copper alloy forming the lamellae may contain or have a nanocrystalline structure consisting of nanoscale grains. The copper or copper alloy forming the lamellae is identical to the copper or copper alloy forming the flakes. The nanoscale grain size may be in the range of 5 nm to 100 nm, specifically 10 nm to 90 nm, specifically 15 nm to 80 nm, specifically 20 nm to 70 nm, specifically 25 nm to 60 nm, specifically 30 nm to 50 nm, and specifically 35 nm to 40 nm. When the copper or copper alloy forming the lamellae contains or has a nanocrystalline structure consisting of nanoscale grains, rapid grain growth of the flakes during sintering is enabled. In this regard, rapid grain growth is particularly pronounced during the later stages of sintering, when the interparticle cavity density decreases and the interparticle pores in the copper-containing intermediate / interconnection layer disappear completely or almost completely. In these later stages of sintering, grain boundary migration becomes the dominant process mechanism for grain growth. This is particularly evident when the copper or copper alloy forming the lamellae contains nanoscale crystal grains or has a nanocrystalline structure composed of them. Therefore, when the copper or copper alloy forming the lamellae contains nanoscale crystal grains or has a nanocrystalline structure composed of them, sintering is facilitated within a relatively short sintering time, at a relatively low sintering pressure, and / or at a relatively low sintering temperature.

[0045] Within a multilayer lamellar structure, each lamellar may be in contact with other lamellae (one or more) by at least one lamellar boundary. Furthermore, a lamellar may be part of a single-layer curved portion at the edge of a flake. This means that the lamellae form units in the form of flakes of copper or copper alloy having a multilayer lamellar structure.

[0046] Copper or copper alloy flakes can be coated with stearic acid. The total weight of stearic acid relative to the total weight of the stearic acid-coated flakes may be at least 0.001 wt.%, specifically at least 0.005 wt.%, specifically at least 0.01 wt.%, specifically at least 0.05 wt.%, specifically at least 0.075 wt.%, and at most 1.5 wt.%, specifically at most 1.25 wt.%, specifically at most 1 wt.%, specifically at most 0.75 wt.%, specifically at most 0.5 wt.%, specifically at most 0.25 wt.%, and specifically at most 0.1 wt.%. Specifically, the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes may be at most 1.5 wt.%. The inventors of the present invention have found that stearic acid coated on the surface of copper or copper alloy flakes prevents flake agglomeration and cold welding. However, due to its boiling point of 361°C, this stearic acid on the surface of the flakes does not evaporate during sintering. Copper or copper alloy particles coated with organic coating agents, specifically stearic acid, typically exhibit relatively low sintering effectiveness. The inventors of the present invention have found that relatively low sintering effectiveness occurs only when the composition contains a relatively high total weight of stearic acid relative to the total weight of the stearic acid-coated flakes, specifically, more than 1.5 wt.% of the total weight of stearic acid-coated copper or copper alloy flakes relative to the total weight of the stearic acid-coated flakes.

[0047] In one embodiment, the total weight of stearic acid relative to the total weight of copper or copper alloy flakes coated with stearic acid is at least 0.7 wt.% and at most 1.5 wt.%. A total weight of 0.7 wt.% stearic acid relative to the total weight of flakes was found to be sufficiently high to prevent flake agglomeration and cold welding under all reasonable ball milling conditions. A total weight of 1.5 wt.% stearic acid relative to the total weight of flakes was found to be sufficiently low so as not to hinder subsequent sintering. The inventors found that a total weight higher than this value hinders sintering because the flakes contain a high organic content, which must first be removed from the composition containing these flakes before surface diffusion can occur and enable sintering. It was found that 1.5 wt.% stearic acid relative to the total weight of flakes is the limit; beyond this, the sintering results are unsatisfactory.

[0048] A total weight of stearic acid of at least 0.001 wt.%, specifically at least 0.005 wt.%, specifically at least 0.01 wt.%, specifically at least 0.05 wt.%, and specifically at least 0.075 wt.%, relative to the total weight of the flakes, was found to be sufficiently high to prevent flake agglomeration and cold welding under many reasonable conditions, though not all.

[0049] The inventors of the present invention have found that a relatively low total weight of stearic acid relative to the total weight of copper or copper alloy flakes coated with stearic acid in the composition, i.e., at most 1.5 wt.% stearic acid, is sufficient to prevent flake agglomeration and cold welding, and also improves the sintering of these flakes. The inventors have further found that, in contrast to compositions containing copper or copper alloy flakes coated with a relatively high total weight of stearic acid relative to the total weight of stearic acid, specifically at least 1.6 wt.% stearic acid, specifically at least 2 wt.% stearic acid, compositions containing copper or copper alloy flakes coated with a relatively low total weight of stearic acid relative to the total weight of stearic acid, i.e., at most 1.5 wt.% stearic acid, exhibit improved particle contact during the sintering of the composition. Therefore, a composition containing copper or copper alloy flakes coated with a relatively low total weight of stearic acid relative to the total weight of the stearic acid-coated flakes, i.e., at most 1.5 wt.% stearic acid, enables improved sintering neck formation, thus allowing sintering at a shorter sintering time, a lower sintering temperature, and / or a lower sintering pressure.

[0050] Furthermore, in contrast to copper or copper alloy flakes without organic coating, specifically copper or copper alloy flakes without stearic acid coating, copper or copper alloy flakes coated with a relatively low total weight of stearic acid relative to the total weight of stearic acid-coated flakes in the composition, i.e., at most 1.5 wt.% stearic acid, do not agglomerate, specifically, do not agglomerate in the method for forming the composition, and / or do not agglomerate during storage and during application of the composition to the first substrate. Specifically, if the composition on the first substrate, or the first substrate and / or second substrate together with the composition, are heated to a pre-drying temperature before sintering, agglomeration(single or multiple) of copper or copper alloy flakes also results in a heterogeneous and inconsistent application of the composition to the first substrate, and insufficient adhesion of the second substrate placed on the composition. Pre-drying leads to evaporation of the organic binder in the composition, specifically partial evaporation, and thus significant flake aggregation in the case of copper or copper alloy flakes without an organic coating. Significant flake aggregation results in a heterogeneous distribution of the composition on the first substrate, which can lead to device cracking during the placement of the second substrate, specifically during the placement of highly delicate and relatively thin devices such as LEDs and diodes. Furthermore, the contact surface between the coated composition and the placed second substrate is relatively small due to the heterogeneity caused by the aggregated copper or copper alloy flakes. This results in an uneven pressure distribution during sintering, and thus uneven sintering.

[0051] Therefore, specifically, when the composition on the first substrate, or the first and / or second substrate together with the composition, is heated to a pre-drying temperature before sintering, the aggregation of copper or copper alloy flakes in the composition is prevented, thereby enabling homogeneous and consistent coating, specifically printability, and sufficient adhesion of the second substrate placed on the composition according to the present invention. Thus, after sintering, sufficient copper bonding between the first and second substrates with a relatively high arithmetic mean shear strength is made possible by a composition containing copper or copper alloy flakes coated with a relatively low total weight of stearic acid relative to the total weight of the stearic acid-coated flakes, i.e., at most 1.5 wt.% stearic acid. Furthermore, a composition containing copper or copper alloy flakes coated with a relatively high total weight of stearic acid, specifically at least 1.6 wt.% stearic acid, specifically at least 2 wt.% stearic acid, results in reduced wettability of the first substrate by the organic binder during the application of the composition. This results in the adhesive force of the organic binder to the first substrate being stronger than the cohesive force of the organic binder within the composition, leading to an effect commonly referred to as bleed-out. Organic binder bleed-out results in the organic binder being unevenly distributed on the first substrate when the composition is applied to it, and the entrainment of copper or copper alloy flakes due to the uneven distribution of the organic binder on the first substrate. Furthermore, the reduced wettability of the first substrate leads to contamination after sintering, specifically contamination by copper sublimation, and hinders surface diffusion during sintering, specifically at relatively low temperatures, specifically at temperatures of no more than 250°C, specifically at temperatures of no more than 225°C.Therefore, compositions according to the present invention, comprising stearic acid-coated copper or copper alloy flakes in a relatively low total weight relative to the total weight of stearic acid-coated flakes, exhibit improved processability and improved sintering effectiveness compared to compositions comprising copper or copper alloy flakes without organic coating, and compositions comprising stearic acid-coated copper or copper alloy flakes in a relatively high total weight relative to the total weight of stearic acid-coated flakes, specifically at least 2 wt.% stearic acid.

[0052] The inventors of the present invention have found that copper or copper alloy flakes are uniformly distributed in the copper-containing intermediate layer between the first and second solid substrates after the sintering process. This allows for efficient formulation of compositions according to the present invention, improved coatability, specifically printability, of the compositions according to the present invention on the first substrate, and improved placement of the second substrate on the compositions according to the present invention. Furthermore, compositions according to the present invention enable relatively good surface control of the sintered copper-containing intermediate layer by ensuring relatively low surface roughness and uniform bonding thickness of the copper-containing intermediate layer during and / or after sintering. Moreover, when the copper or copper alloy flakes are coated with a relatively low total weight of stearic acid relative to the total weight of the flakes coated with stearic acid, i.e., at most 1.5 wt.% stearic acid, agglomeration of the copper or copper alloy flakes is prevented, thus further improving the uniform distribution of flakes in the copper-containing intermediate layer between the first and second solid substrates after the sintering process. This further improves the coatability, specifically the printability, of the composition on the first substrate, and the arrangement of the second substrate on the composition. Furthermore, if the copper or copper alloy flakes are coated with a relatively low total weight of stearic acid relative to the total weight of the stearic acid-coated flakes, i.e., at most 1.5 wt.% stearic acid, the surface control of the sintered copper-containing intermediate layer is also further improved, specifically by ensuring relatively low surface roughness and uniform bonding thickness of the copper-containing intermediate layer during and / or after sintering.

[0053] The inventors of the present invention have found that, in addition to the multilayer lamellar structure of copper or copper alloy flakes, the following properties of the flakes in the composition according to the present invention also contribute to enabling sintering within a relatively short sintering time, at a relatively low sintering pressure, and / or at a relatively low sintering temperature, as indicated above: - Relatively large surface contact due to a uniform layered pattern of copper or copper alloy flakes in the composition according to the present invention. - The copper or copper alloy that forms the lamellae has a nanocrystalline structure that contains or consists of nanoscale crystalline grains, and - Coating of copper or copper alloy flakes with a relatively low total weight of stearic acid relative to the total weight of the stearic acid-coated flakes, i.e., at most 1.5 wt.% stearic acid.

[0054] The inventors of the present invention have further found that, compared to copper or copper alloy flakes that do not have a lamellar structure, specifically, flakes that do not have a multilayer lamellar structure, or flakes that have only a multilayer lamellar structure and do not satisfy any of the above properties, or flakes that have a multilayer lamellar structure but do not satisfy all of the above properties, when a flake not only has a multilayer lamellar structure but also satisfies two or even three of the above properties, the multilayer lamellar structure of the copper or copper alloy flake and each of the above properties reinforce each other, resulting in a synergistic effect, thus allowing sintering in an even shorter sintering time, at an even lower sintering pressure, and / or at an even lower sintering temperature.

[0055] Copper or copper alloy flakes may have a total oxygen content in the range of 3 wt.% to 8 wt.%, specifically in the range of 4 wt.% to 7.9 wt.%, specifically in the range of 4.1 wt.% to 7.5 wt.%, specifically in the range of 4.2 wt.% to 7 wt.%, specifically in the range of 4.3 wt.% to 6.5 wt.%, specifically in the range of 4.4 wt.% to 6 wt.%, specifically in the range of 4.5 wt.% to 5.5 wt.%, and specifically in the range of 4.6 wt.% to 5 wt.%. In the context of the present invention, the total oxygen content of copper or copper alloy flakes refers to oxygen atoms in any form, for example, in the form of copper oxide on the surface of the flakes, and the oxygen content of the flakes themselves. When the flakes are coated with stearic acid, the total oxygen content also refers to the oxygen content of the stearic acid coating of the flakes. However, the oxygen content refers only to the oxygen content of copper or copper alloy flakes that are optionally coated with stearic acid, and therefore does not include oxygen in the spaces between the flakes.

[0056] The organic binder may be terpineol, primary alcohol, diol, triol, polymer glycol, or a mixture of at least two of terpineol, primary alcohol, diol, triol, and polymer glycol. Specifically, the organic binder may be terpineol, primary alcohol, diol, or triol, or a mixture of at least two of terpineol, primary alcohol, diol, and triol. Specifically, the terpineol forming the organic binder may be alpha-terpineol. Alpha-terpineol may be (R)-(+)-alpha-terpineol, (S)-(-)-alpha-terpineol, or a mixture of (R)-(+)-alpha-terpineol and (S)-(-)-alpha-terpineol. (R)-(+)-alpha-terpineol and / or (S)-(-)-alpha-terpineol may each independently have a purity of at least 90%, specifically at least 93%. The primary alcohol forming the organic binder may be 1-butanol or 1-octanol. The diol forming the organic binder may be a diol having an average molar mass of at least 60 g / mol and at most 110 g / mol, specifically ethylene glycol, diethylene glycol, propylene glycol, or butylene glycol. The triol forming the organic binder may be a triol having an average molar mass of at least 75 g / mol and at most 110 g / mol, specifically glycerol. The polymer glycol forming the organic binder may be polymer ethyl glycol having an average molar mass of at least 200 g / mol, specifically at least 300 g / mol, specifically at least 400 g / mol, specifically at least 500 g / mol, specifically at least 550 g / mol, and at most 800 g / mol, specifically at most 750 g / mol, specifically at most 700 g / mol, and specifically at most 650 g / mol.

[0057] In the sintering process, the organic binder typically evaporates or decomposes. During the evaporation or decomposition of the diol and / or polymer glycol, a reducing atmosphere is created, which prevents oxidation from copper to copper oxide without the requirement of providing an additional protective gas atmosphere to be applied from the outside. Thus, the diol and / or polymer glycol in the composition according to the present invention enables the in situ reduction of Cu oxide during the sintering process with little effort. Therefore, the organic binder, specifically the polymer glycol, specifically a polymer glycol having an average molar mass of at least 550 g / mol and at most 650 g / mol, has a reducing effect on copper or copper alloy flakes.

[0058] In contrast to polymer glycols, specifically polymer ethyl glycol, which has an average molar mass greater than 200 g / mol, the evaporation of diols, specifically diols with an average molar mass of at least 60 g / mol and at most 110 g / mol, as well as primary alcohols, specifically 1-butanol and 1-octanol, can be achieved at relatively low temperatures, specifically at a maximum of 200°C. For example, the evaporation of ethylene glycol can be achieved at 197°C. For example, the evaporation of 1-butanol can be achieved at 117.7°C. For example, the evaporation of 1-octanol can be achieved at 195°C. A relatively high temperature of at least 200°C is required to evaporate polymer ethyl glycol. However, evaporation of the binder at relatively high temperatures can lead to the formation of undesirable bubbles during sintering, which can weaken the bond. It is also possible to achieve a combined effect by using a mixture of ethylene glycol and alpha-terpineol, which simultaneously utilizes a reducing atmosphere while producing fewer bubbles during heating. If the formation of bubbles during sintering is to be suppressed or at least reduced, the organic binder should neither contain nor consist of a polymer glycol.

[0059] In one aspect of the present invention, the organic binder may be a mixture of ethylene glycol and alpha-terpineol. In another aspect of the present invention, the organic binder may be a mixture of alpha-terpineol, ethylene glycol, and polyethylene glycol. In this mixture, the total weight of alpha-terpineol relative to the total weight of the organic binder may be 80 wt.%, the total weight of ethylene glycol relative to the total weight of the organic binder may be 18 wt.%, and the total weight of polyethylene glycol relative to the total weight of the organic binder may be 2 wt.%. In another aspect of the present invention, the organic binder may be a mixture of alpha-terpineol, polyethylene glycol, and glycerol. In this mixture, the total weight of alpha-terpineol relative to the total weight of the organic binder may be 80 wt.%, the total weight of polyethylene glycol relative to the total weight of the organic binder may be 19.5 wt.%, and the total weight of glycerol relative to the total weight of the organic binder may be 0.5 wt.%.

[0060] In a further embodiment of the present invention, the organic binder may be a mixture of ethylene glycol, alpha-terpineol, polyethylene glycol, and glycerol. The inventors of the present invention have found that mixtures of alpha-terpineol, ethylene glycol, and polyethylene glycol, mixtures of alpha-terpineol, polyethylene glycol, and glycerol, and mixtures of ethylene glycol, alpha-terpineol, polyethylene glycol, and glycerol result in compositions that allow relatively fast pre-drying without drying out the compositions according to the present invention. This results in relatively high paste stability, specifically relatively high paste stability after pre-drying, and a relatively long pot life of up to one week for the compositions according to the present invention, specifically after pre-drying. Furthermore, sufficient adhesion of the compositions according to the present invention to a first substrate and / or a second substrate is possible even after pre-drying of the compositions, and thus no additional adhesive needs to be provided in the compositions according to the present invention. Furthermore, placement of the second substrate at relatively low temperatures, specifically at room temperature, is possible.

[0061] The total solid content in the composition according to the present invention may be at most 80 wt.%, specifically at most 75 wt.%, specifically at most 70 wt.%, specifically at most 65 wt.%, specifically at most 63 wt.%, specifically at most 60 wt.%, specifically at most 55 wt.%, and specifically at most 50 wt.%. Therefore, the total solid content in the composition according to the present invention is considerably lower compared to silver sintered paste having a total solid content of about 90 wt.%. The relatively low total solid content results in a relatively low viscosity of the composition according to the present invention. The viscosity of the composition according to the present invention may be in the range of 100,000 mPas to 400,000 mPas, specifically in the range of 150,000 mPas to 350,000 mPas, and specifically in the range of 200,000 mPas to 300,000 mPas. The viscosity of the compositions according to the present invention can be measured at room temperature, specifically at 20°C, using a standard viscometer, specifically a standard classical rotational viscometer, specifically Thermo Scientific® HAAKE® Viscotester® C. These rotational viscometers measure the resistance of the compositions according to the present invention to a preset speed. The resulting torque or resistance is a measure of the viscosity of the compositions according to the present invention. Higher torque indicates higher viscosity. The low viscosity of the compositions according to the present invention allows for easy processability, as well as good and uniform spreadability on a first solid substrate and / or a second solid substrate.

[0062] The total weight of optionally stearic acid-coated copper or copper alloy flakes relative to the total weight of the composition according to the present invention may be at most 80 wt.%, specifically at most 75 wt.%, specifically at most 70 wt.%, specifically at most 65 wt.%, specifically at most 63 wt.%, specifically at most 60 wt.%, specifically at most 55 wt.%, specifically at most 50 wt.%, and specifically at most 45 wt.%. The remainder of the composition may be formed by an organic binder. This means that the total weight of the organic binder relative to the total weight of the composition according to the present invention may be at least 20 wt.%, specifically at least 25 wt.%, specifically at least 30 wt.%, specifically at least 35 wt.%, specifically at least 37 wt.%, specifically at least 40 wt.%, specifically at least 45 wt.%, specifically at least 50 wt.%, and specifically at least 55 wt.%. The total weight of copper or copper alloy flakes, organic binder, and / or any stearic acid coated on the flakes can be determined, respectively, by nuclear magnetic resonance spectroscopy, thermogravimetric analysis, mass spectrometry, and infrared spectroscopy. The total weight of copper or copper alloy flakes, organic binder, and / or any stearic acid coated on the flakes can be determined by weighing the flakes, organic binder, and / or any stearic acid before forming the composition according to the present invention.

[0063] A method for forming flakes of copper or copper alloy comprises the step of grinding and / or milling copper particles or copper alloy particles. If the goal is to obtain flakes of copper or copper alloy coated with stearic acid, the grinding and / or milling of the copper particles or copper alloy particles is carried out in the presence of stearic acid. The grinding and / or milling may be carried out, for example, in a ball mill. The flakes of copper or copper alloy are obtained during the grinding and / or milling of the copper particles or copper alloy particles. Flakes of copper or copper alloy coated with stearic acid are obtained during the grinding and / or milling of copper particles or copper alloy particles in the presence of stearic acid. During the grinding and / or milling of copper particles or copper alloy particles in the presence of stearic acid, the total weight of stearic acid relative to the total weight of copper particles or copper alloy particles can be relatively low. Furthermore, 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 relatively low. Specifically, the total number of individual grinding and / or milling steps of copper particles or copper alloy particles in the presence of stearic acid may be at most 3, specifically at most 2, and specifically at most 1. This makes it possible to obtain copper or copper alloy flakes coated with stearic acid, where the total weight of stearic acid relative to the total weight of the stearic acid-coated flakes is at most 1.5 wt.%. The total weight of stearic acid coated on the flakes relative to the total weight of the stearic acid-coated flakes can be determined by nuclear magnetic resonance spectroscopy, thermogravimetric analysis, Rutherford backscatter spectroscopy, mass spectrometry, infrared spectroscopy, Raman spectroscopy, and X-ray spectroscopy in combination with nuclear magnetic resonance spectroscopy.

[0064] A method for forming a composition according to the present invention comprises the steps of providing an organic binder and optionally stearic acid-coated copper or copper alloy flakes to obtain a composition according to the present invention, and mixing the organic binder and optionally stearic acid-coated copper or copper alloy flakes.

[0065] The present invention also relates to a method for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate. The method includes the following steps: a) Providing a composition according to the present invention, providing a first substrate, and providing a second substrate. b) A step of applying a composition according to the present invention to a first substrate, c) The step of placing a second substrate on a composition according to the present invention, d) A bonding pressure is applied to press the first substrate and the second substrate against each other, wherein the bonding pressure is at least 100 kPa, specifically at least 1 MPa, specifically at least 2.5 MPa, specifically at least 5 MPa, specifically at least 7.5 MPa, and at most 40 MPa, specifically at most 35 MPa, specifically at most 30 MPa, specifically at most 25 MPa, specifically at most 20 MPa, specifically at most 15 MPa, specifically at most 10 MPa. e) Heating the composition on the first substrate, or the first and / or second substrate together with the composition, to a sintering temperature in the range of 200°C to 300°C, specifically 205°C to 275°C, specifically 210°C to 270°C, specifically 215°C to 265°C, specifically 220°C to 260°C, specifically 225°C to 255°C, specifically 230°C to 250°C, specifically 235°C to 245°C, and maintaining the sintering temperature and bonding pressure until a copper-containing intermediate layer is formed between the first and second substrates.

[0066] The composition according to the present invention can be applied to a first substrate by coating, specifically by doctor blade coating; by printing, specifically by stencil printing or screen printing; or by cartridge ejection.

[0067] Placing a second substrate on a composition according to the present invention can be done manually, by substrate transport, specifically by vacuum substrate transport, or by a pick-and-place machine.

[0068] After applying a composition according to the present invention to a first substrate and placing a second substrate on the composition according to the present invention, and before applying bonding pressure to press the first and second substrates against each other, the composition on the first substrate, or the first and / or second substrate together with the composition, may be heated to a pre-drying temperature in the range of 80°C to 160°C, specifically 90°C to 150°C, specifically 100°C to 140°C, or specifically 110°C to 130°C. The pre-drying temperature may be maintained for at least 30 seconds, specifically at least 1 minute, specifically at least 2 minutes, specifically at least 3 minutes, and at most 8 minutes, specifically at most 6 minutes, specifically at most 5 minutes, or specifically at most 4 minutes. Heating and maintaining the pre-drying temperature of the composition on the first substrate, or the first and / or second substrate together with the composition, may be done in a reducing atmosphere, an inert atmosphere, an oxygen-containing atmosphere, and / or in a vacuum. Heating the composition on the first substrate, or the first and / or second substrate together with the composition, to a pre-drying temperature may be performed after step c) and before step d).

[0069] The sintering temperature and bonding pressure can be maintained for at least 1 second, specifically at least 5 seconds, specifically at least 30 seconds, specifically at least 1 minute, specifically at least 3 minutes, specifically at least 4 minutes, and at most 30 minutes, specifically at most 15 minutes, specifically at most 10 minutes, specifically at most 8 minutes, and specifically at most 5 minutes.

[0070] A method according to the present invention may further include step f) cooling the copper-containing intermediate layer obtained in step e) between the first substrate and the second substrate, or the first substrate and / or the second substrate together with the copper-containing intermediate layer obtained in step e), to a cooling temperature in the range of 15°C to 40°C, specifically in the range of 20°C to 30°C. The cooling temperature may be room temperature. The cooling according to step f) may be carried out by active cooling, but is usually passive cooling, i.e., by leaving the copper-containing intermediate layer, or the first substrate and / or the second substrate together with the copper-containing intermediate layer, to a cooling temperature which is usually room temperature.

[0071] At least step e), specifically at least steps e) and d), specifically at least steps c) to e), specifically at least steps b) to e), specifically steps a) to e), may be carried out in a reducing atmosphere or an inert atmosphere. Optionally, step f) may be carried out in a reducing atmosphere or an inert atmosphere. The reducing atmosphere may be provided by means of a mixture of evaporated formic acid and nitrogen gas, or by means of a mixture of hydrogen and an inert gas. The mixture of hydrogen and an inert gas may be a mixture of nitrogen and hydrogen gas, or a mixture of argon and hydrogen gas. The inert atmosphere may be provided by means of an inert gas. The inert gas may be contained in an inert gas mixture. The inert gas may be nitrogen gas, carbon dioxide gas, helium gas, neon gas, or argon gas. The inert gas mixture may be a mixture of at least two of nitrogen gas, carbon dioxide gas, helium gas, neon gas, and argon gas. The inventors of the present invention have found that sintering in an inert or reducing atmosphere, specifically by means of a mixture of evaporated formic acid and nitrogen gas, results in a relatively high arithmetic mean shear strength of copper joints, specifically in the range of 20 MPa to 50 MPa, and more specifically in the range of 30 MPa to 40 MPa.

[0072] The inventors of the present invention have found that at least step e), specifically at least steps e) and d), specifically at least steps c) to e), specifically at least steps b) to e), and specifically steps a) to e) of the method according to the present invention can be carried out in an oxygen-containing atmosphere. Optionally, step f) can be carried out in an oxygen-containing atmosphere. In the method according to the present invention, the oxygen-containing atmosphere can be provided by means of a gas mixture containing oxygen and nitrogen. The gas mixture containing oxygen and nitrogen can be air. The inventors of the present invention have found that, due to relatively low oxide impurities, the composition according to the present invention provides relatively good conductivity of the copper joint after sintering. Because the oxide impurities are so low, it is not necessary to apply a reducing atmosphere during sintering.

[0073] During step e), at least 70 wt.%, specifically at least 75 wt.%, specifically at least 80 wt.%, specifically at least 85 wt.%, specifically at least 90 wt.%, specifically at least 95 wt.%, specifically at least 99 wt.%, and specifically at least 100 wt.% of the initial weight of the organic binder may be evaporated. The initial weight of the organic binder may be determined by weighing the organic binder before providing / preparing the composition according to the present invention. The evaporation of the organic binder may be determined by weighing the first and / or second substrate together with the composition during, or before and after heating according to step e), and / or by mass spectrometry or thermogravimetric analysis.

[0074] At least step e), specifically at least steps e) and d), specifically at least steps c) to e), specifically at least steps b) to e), and specifically steps a) to e) of the method according to the present invention, may be carried out in a vacuum. Optionally, step f) may be carried out in a vacuum. The inventors of the present invention have found that sintering in a vacuum results in further evaporation of the organic binder at relatively low temperatures due to the vacuum-induced reduction of the boiling point of the organic binder. Further evaporation of the organic binder further improves the sintering efficiency of the composition according to the present invention and contributes to sintering at relatively low sintering temperatures.

[0075] Furthermore, sintering of compositions according to the present invention by the method according to the present invention avoids the relatively high surface roughness and relatively high heterogeneity of copper-containing bonding. Thus, compositions according to the present invention enable relatively good surface control by ensuring relatively low surface roughness. The relatively low surface roughness and relatively good surface control of compositions according to the present invention are made possible by the relatively low total weight of the organic binder relative to the total weight of the compositions according to the present invention. Furthermore, sintering of compositions according to the present invention results in a relatively thin copper-containing interlayer / interconnection layer between two solid substrates. Since compositions according to the present invention contain a relatively low total weight of organic binder after sintering, the properties of the resulting copper-containing interlayer / interconnection layer formed by the sintered composition are similar to those of bulk copper or bulk copper alloys, specifically bulk copper. Thus, the copper-containing interlayer / interconnection layer exhibits relatively high electrical conductivity and relatively high thermal conductivity. If the flakes consist of a copper alloy, good thermal conductivity during sintering and good electrical conductivity after sintering are possible. However, when the flakes are made of copper, the thermal conductivity during sintering and the electrical conductivity after sintering are usually even higher.

[0076] Sintering of compositions according to the present invention by the method according to the present invention ensures a relatively high shear strength value of the resulting joint. Despite the relatively low solid content, the shear strength value is similar to that achievable with commercially available silver sintering pastes having a solid content of about 90 wt.%. Due to the relatively low solid content and the fact that silver is relatively expensive compared to copper, material costs can be saved by using compositions according to the present invention. In addition, compositions according to the present invention enable the creation of relatively energy-efficient copper joints. Despite these differences from commercially available silver sintering pastes, the sintering process of compositions according to the present invention is compatible with industrialized silver sintering lines. Thus, a simple substitution of commercially available silver sintering pastes with compositions according to the present invention is possible.

[0077] The first substrate may be made from or consist of a metal or metal oxide, and the second substrate may be made from or consist of a metal, a further metal, a metal oxide, a further metal oxide, or a surface mount device component. The metal or further metal may be gold, silver, nickel, copper, pre-treated copper, or tin. The metal oxide or further metal oxide may be aluminum oxide. The surface mount device component may be a capacitor, chip resistor, crystal oscillator, diode, fuse, inductor, integrated circuit, LED, network resistor, transformer, transistor, silicon carbide (SiC) device, or gallium nitride (GaN) device. The diode may be a silicon diode. The transistor may be a metal oxide semiconductor field-effect transistor (MOSFET). The inventors of the present invention have found that the organic binder may also reduce oxides on the first and / or second substrates. Thus, compositions according to the present invention enable sintering on copper, silver, and / or nickel substrates, even when these substrates are covered by an oxide layer.

[0078] Pre-treated copper can be pre-treated by coating the copper substrate with, for example, an organic surface protection (OSP) layer, where the OSP layer protects the copper from oxidation and dissolves during the soldering process. Alternatively, other methods, such as sol-gel coating or CVD, can be used to coat the copper substrate with a protective layer to protect the copper from oxidation.

[0079] The present invention further relates to the use of compositions according to the present invention for forming a copper-containing intermediate layer / interconnection layer between two surfaces of a solid substrate, specifically for die-attach bonding, in microelectronics packaging, electric vehicle technology, hybrid electric vehicle technology, high-power electronics packaging, and / or thick-film technology. Specifically, the present invention relates to the use of compositions according to the present invention for die-attach bonding. The present invention further relates to the use of compositions according to the present invention for forming electrical conduction paths on a solid substrate. Conduction paths on a solid substrate may be electrical and / or thermal conduction paths on the solid substrate. The present invention further relates to the use of compositions according to the present invention for substrate adhesion. Microelectronics packaging may be WBG semiconductor packaging. High-power electronics packaging may be high-power light-emitting diode packaging. The inventors of the present invention have found that the use of compositions according to the present invention for forming a copper-containing intermediate layer / interconnection layer between two solid substrates or for forming conduction paths on solid substrates allows for a relatively low thermal load on the solid substrate(s) and any electronic components on the substrate(s), specifically the surface mount device components as defined above, when forming the interconnection layer or conduction path. This is due to the relatively low sintering temperature required to sinter the compositions according to the present invention.

[0080] When a composition according to the present invention is used to form electrical and / or thermal conduction paths on a solid substrate, the solid substrate may be any of the first or second substrates defined above, unless it is an electrically conductive first or second substrate. When a composition according to the present invention is used for substrate adhesion, the substrate may be any of the first or second substrates defined above, an aluminum substrate, specifically a metallized aluminum substrate, specifically a copper metallized aluminum substrate, a ceramic substrate, specifically a metallized ceramic substrate, specifically a copper metallized ceramic substrate, a directly bonded copper substrate, or a polymer substrate, specifically a polyimide substrate.

[0081] All features indicated herein should be understood as features applicable to all aspects of the present invention. This means, for example, that features indicated for compositions according to the present invention may also be applicable to methods for forming flakes of copper or copper alloy, methods for forming compositions according to the present invention, methods for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate according to the present invention, and / or uses according to the present invention, and vice versa. Furthermore, unless otherwise specified, the term "average" always means "arithmetic mean," and the term "mean" always means "arithmetic mean." The abbreviation "wt.%" means "weight percent."

[0082] The present invention will be described in more detail with reference to the following embodiments. [Brief explanation of the drawing]

[0083] [Figure 1] Figure 1) shows a SEM image of a cross-section of copper flakes containing internal pores. [Figure 2] Figure 2) shows a SEM image of a cross-section of copper flakes having a multilayer lamellar structure. [Figure 3a-b] Figures 3a) and 3b) show scanning transmission electron microscopy images of the multilayer lamellar structure. [Figure 4] Figure 4) shows a cross-sectional SEM image of interconnects resulting from the sintering of a composition containing copper flakes with internal pores. [Modes for carrying out the invention]

[0084] Example 1: Average intraparticle pore density and average interparticle cavity density of copper or copper alloy flakes The compositions according to the present invention comprise flakes of copper or copper alloy containing internal pores. Exemplarily, these internally pore-containing flakes can be purchased as Cubrotec 8001 copper flakes, supplied by Carl Schlenk SE GmbH, Germany. A cross-section of Cubrotec 8001 copper flakes is shown in Figure 1. In Figure 1, several internal pores of the copper flakes are illustratively indicated by filled arrows. Spaces between flakes that result in interparticle cavities after sintering are illustratively indicated by unfilled arrows.

[0085] As can be seen in Figure 1, Cubrotec 8001 copper flakes have a relatively large number of internal pores. For example, Cubrotec 8001 copper flakes have an average intraparticle pore density of 30 internal pores / μm. Therefore, Cubrotec 8001 copper flakes have a relatively high average intraparticle pore density. The space between Cubrotec 8001 copper flakes is 40%. Furthermore, as can be seen in Figure 1, Cubrotec 8001 copper flakes have a multilayer lamellar structure.

[0086] Example 2: Copper or copper alloy flakes having a multilayer lamellar structure Copper or copper alloy flakes have a multilayer lamellar structure. Figure 2 shows a cross-section of Cubrotec 8001 copper flakes illustrating the multilayer lamellar structure of copper flakes. In Figure 2, the layered-like multilayer lamellar structure is illustratively shown by white dotted lines around three copper flakes. Each multilayer lamellar structure consists of a varying number of lamellae, ranging from 1 to 20. Furthermore, the multilayer lamellar structures have varying total thicknesses, ranging from 20 nm to 400 nm.

[0087] Figures 3a and 3b show illustrative scanning transmission electron microscopy (STEM) images of a multilayer lamellar structure consisting of 10 lamellae. In Figure 3a, the lamellar boundaries between lamellae (lamellae 1-10) in the stacked-like multilayer lamellar structure are illustratively shown by black dotted lines. In this regard, for example, lamellar 1 is in contact with lamellar 2 by one lamellar boundary. However, lamellar 2 is in contact with both lamellar 1 and lamellar 3 by one lamellar boundary each, and so on. This results in a stacked-like multilayer lamellar structure consisting of lamellae that are in contact with each other. Furthermore, Figure 3b shows the copper nanocrystalline structure that forms the lamellae in the stacked-like multilayer lamellar structure.

[0088] Example 3: Composition The following compositions were tested.

[0089] [Table 1]

[0090] Table 1 shows the different sintered pastes and their respective compositions. Each of the sintered pastes from composition No. 1 to No. 9 contained Cubrotec 8001 copper flakes with internal pores. As can be seen in Table 1, the sintered paste from composition No. 1 does not contain stearic acid-coated copper flakes. The sintered pastes from composition No. 2 to No. 9 contain stearic acid-coated copper flakes.

[0091] In each of the sintered pastes from compositions No. 1 to No. 9, the total weight of copper flakes optionally coated with stearic acid relative to the total weight of the composition is in the range of 60 wt.% to 76 wt.%. The total weight of copper flakes coated with stearic acid includes the total weight of stearic acid coated on the copper flakes and the total weight of the copper flakes. For example, in composition No. 2, the 60 wt.% copper flakes coated with stearic acid include 58.5 wt.% copper flakes and 1.5 wt.% stearic acid coated on the copper flakes. Therefore, the total solid content in composition No. 2 is 58.5 wt.%. Furthermore, the organic binder in composition No. 2 is a mixture of ethylene glycol and alpha-terpineol. In this regard, the total weight of ethylene glycol relative to the total weight of the composition is 16 wt.%, and the total weight of alpha-terpineol relative to the total weight of the composition is 24 wt.%. Therefore, composition No. 2 contains an organic binder by total weight relative to the total weight of the composition, which is 40 wt.%.

[0092] Example 4: Making sintered paste The following examples illustrate the formulations of sintered pastes from compositions No. 1 and No. 3. The formulation of sintered paste from composition No. 1 includes the following steps: In a planetary rotary mixer, add 63g of copper flakes. Then, add 37g of ethylene glycol as an exemplary organic binder to the copper flakes and mix at 500 rpm for 4 minutes.

[0093] The formulation of the sintered paste from composition No. 3 includes the following steps: Mix 15.5 g of ethylene glycol, 24 g of alpha-terpineol, and 0.5 g of PEG 600 in a beaker using a magnetic stirrer at 500 rpm for 3 min. This yields an organic binder. Add 60 g of stearic acid-coated copper flakes, which contain 58.5 g of copper flakes and 1.5 g of stearic acid coated on the copper flakes, to a planetary rotary mixer. Then, add the resulting organic binder to the copper flakes and mix at 500 rpm for 4 min.

[0094] Example 5: Sintering process and sintered joint strength Each of the sintered pastes from Example 3 is stencil-printed onto a copper substrate (30mm × 30mm × 1.5mm) using a PBT-Uniprint-PMGo3v semi-automatic stencil printer with an electric double-blade squeegee and a 75μm stencil thickness. Exemplarily, the squeegee speed is 13mm / s, the squeegee pressure is 20N, and the stencil spacing is 2.3mm / s. Printing is performed as a double-stroke print.

[0095] Next, a two-step sintering process is carried out, including an optional pre-drying step. First, the stencil-printed composition is pre-dried in a convection oven, either in a nitrogen gas atmosphere or in air. Pre-drying is typically carried out at 100°C for 5 minutes. In this regard, the inventors of the present invention have found that compositions No. 3, 4, 6, and 7, which contain either an organic binder mixture of ethylene glycol, alpha-terpineol, and polyethylene glycol, or an organic binder mixture of ethylene glycol, alpha-terpineol, polyethylene glycol, and glycerol, allow for rapid pre-drying without completely drying the paste, and provide sufficient tackiness and relatively high paste stability with a pot life of up to one week after pre-drying. In contrast, conventional sintered pastes provide a pot life of approximately 8 hours.

[0096] After printing the composition, or after printing the composition and any pre-drying, a silver metallized metal oxide semiconductor field-effect transistor (MOSFET) chip is bonded to the stencil-printed and optionally pre-dried composition using a Fineplacer® Sigma bonder with a bonding chamber under a force of 0.1 N.

[0097] After chip placement, sintering is performed in a Fineplacer® Sigma bonder. Exemplarily, sintering is carried out for 5 minutes at a sintering temperature of 260°C in an open bonding chamber with a constant nitrogen flow and a bonding pressure of 15 MPa. The sintering temperature is reached at a heating rate of 1 K / s. An initial contact force of 5 N is applied by active force control, followed by a ramp to a final bonding force of 500 N at a rate of 1 N / s. The bonding chamber contains residual oxygen. Alternatively, sintering can also be carried out in an industrial sintering press, e.g., a Budatec SP 300 sintering press, in an inert atmosphere with <200 ppm oxygen. Prior to introducing nitrogen into the bonding chamber, the bonding chamber is evacuated twice to obtain a 10 mbar vacuum. In this case, since the top and bottom plates are already heated, the sintering temperature is reached very quickly.

[0098] After sintering, the bonding force is released, and the copper substrate, sintered composition, and chip are cooled to a temperature of 40°C, exemplified by a cooling rate of 1 K / s. The sintered bond strength of the sintered copper joint is measured by shear testing. The shear testing is performed using an XYZTec Condor Sigma Lite shear tester in accordance with the MIL-STD-883E test method standard for microcircuits, with a shear height of 25 μm and a shear speed of 200 μm / s.

[0099] The results are given in the table below.

[0100] [Table 2]

[0101] In Table 2, the sintered joint strength of the sintered copper joints is given as the average of the arithmetic mean shear strength values. As can be seen in Table 2, the sintered joint strength of the sintered copper joints from compositions No. 1 to No. 7 shows arithmetic mean shear strength values ​​in the range of 27 MPa to 50 MPa.

[0102] Furthermore, the arithmetic mean shear strength values ​​of sintered copper joints from compositions No. 2 to No. 9 containing stearic acid-coated copper flakes are higher than those of sintered copper joints from composition No. 1 containing uncoated copper flakes.

[0103] Example 6: Average intraparticle pore density and average interparticle cavity density after sintering The average intraparticle pore density and average interparticle cavity density of the copper-containing intermediate layer / interconnection layer formed after sintering of the composition according to the present invention were determined by analyzing the cross-section of the formed copper-containing intermediate layer / interconnection layer. Figure 4 shows a cross-section of the interconnection obtained by sintering a composition containing Cubrotec 8001 copper flakes. In Figure 4, several internal pores are illustratively shown by filled arrows, and several interparticle cavities are illustratively shown by unfilled arrows.

[0104] As can be seen by comparing Figures 1 and 4, the average intraparticle pore density in the interconnects is not affected by the sintering of the copper particles. The interconnects obtained by sintering a composition containing Cubrotec 8001 copper flakes have an average intraparticle pore density that is the same as or nearly the same as that of unsintered copper flakes. In this respect, the interconnects obtained by sintering a composition containing Cubrotec 8001 copper flakes have a relatively large number of internal pores. However, due to sintering, the spaces between copper flakes are reduced. This results in an average interparticle cavity density in the interconnects that is reduced compared to the average interparticle cavity density of unsintered copper flakes. This can be attributed to sintering neck formation, grain growth, and coarsening of the copper flakes during sintering. Furthermore, measurements of resistance to thermomechanical fatigue showed that interconnects obtained by sintering compositions containing copper flakes with a relatively high average intraparticle pore density exhibited higher resistance to thermomechanical fatigue compared to interconnects obtained by sintering compositions containing copper flakes with a relatively low average intraparticle pore density. Simultaneously, the thermal and electrical conductivity of the interconnects was not negatively affected by the increased number of internal pores.

Claims

1. A composition for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate comprises the following components: Flakes containing internal pores and having a multilayer lamellar structure are organic binders and copper or copper alloy flakes. A composition containing or consisting of the following.

2. The composition according to claim 1, wherein the copper alloy is bronze or brass.

3. The composition according to claim 1 or 2, wherein the flakes have an average particle size D50 determined by laser particle size measurement, which is at most 5 μm.

4. Flakes containing internal pores have 10 internal pores / μm in any cross-section of the flake. 2 ~30 internal pores / μm 2 Within the range of 15 internal pores / μm 2 ~20 internal pores / μm 2 The composition according to any one of claims 1 to 3, having an average particle pore density in the range of [range].

5. The composition according to any one of claims 1 to 4, wherein the multilayer lamellar structure comprises or consists of at least two and at most 20 lamellae, where any one of the lamellae independently has a thickness in the range of 5 nm to 20 nm.

6. The composition according to any one of claims 1 to 5, wherein the flakes are coated with stearic acid, and the total weight of the stearic acid relative to the total weight of the stearic acid-coated flakes is at most 1.5 wt.%.

7. The composition according to any one of claims 1 to 6, wherein the flakes have a total oxygen content in the range of 4.1 wt.% to 7.5 wt.%.

8. The composition according to any one of claims 1 to 7, wherein the organic binder is terpineol, a primary alcohol, a diol or triol, a polymer glycol, or a mixture of at least two of terpineol, a primary alcohol, a diol, and a triol.

9. The composition according to claim 8, wherein the terpineol is alpha-terpineol, the primary alcohol is 1-butanol or 1-octanol, the diol is a diol having an average molar mass of at least 60 g / mol and at most 110 g / mol, specifically ethylene glycol, diethylene glycol, propylene glycol, or butylene glycol, and the triol is a triol having an average molar mass of at least 75 g / mol and at most 110 g / mol, specifically glycerol.

10. The composition according to any one of claims 1 to 9, wherein the total solid content in the composition is at most 80 wt.%, specifically at most 63 wt.%, and specifically at most 55 wt.%.

11. The composition according to any one of claims 1 to 10, wherein the total weight of the flakes is at most 80 wt.% of the total weight of the composition, specifically at most 63 wt.%, specifically at most 60 wt.%, and specifically at most 55 wt.%, where the total weight of the organic binder is at least 20 wt.% of the total weight of the composition, specifically at least 37 wt.%, specifically at least 40 wt.%, and specifically at least 45 wt.%.

12. A method for forming a copper-containing intermediate layer between a first solid substrate and a second solid substrate is as follows: a) Providing a composition according to any one of claims 1 to 11, providing a first substrate, and providing a second substrate, b) A step of coating the composition onto a first substrate, c) The step of placing the second substrate on the composition, d) Apply a bonding pressure to press the first substrate and the second substrate against each other, wherein the bonding pressure is at least 100 kPa and at most 40 MPa. e) Heating the composition on the first substrate, or the first substrate and / or second substrate together with the composition, to a sintering temperature in the range of 200°C to 300°C, and maintaining the sintering temperature and bonding pressure until a copper-containing intermediate layer is formed between the first substrate and the second substrate. Methods that include...

13. The method according to claim 12, wherein the sintering temperature is in the range of 215°C to 265°C, specifically in the range of 225°C to 255°C, the bonding pressure is at least 1 MPa, specifically at least 5 MPa and at most 25 MPa, specifically at most 10 MPa, and / or the sintering temperature and bonding pressure are maintained for at least 1 second, specifically at least 30 seconds and at most 30 minutes, specifically at most 5 minutes.

14. The method according to claim 12 or 13, wherein at least step e) is performed in an inert atmosphere.

15. Use of the composition according to any one of claims 1 to 11 for forming a copper-containing intermediate layer between two surfaces of a solid substrate, for forming a conductive path on a solid substrate, or for substrate adhesion.

Images