Copper paste taking bimodal distribution copper powder with surface of micron copper sheet tightly wrapped by nano copper particles as filler as well as preparation method and application of copper paste

Abstract

The invention discloses copper paste with bimodal distribution copper powder as filler, and a preparation method and application of the copper paste. The bimodal distribution copper powder is formed by tightly wrapping the surface of a micron copper sheet with nano-copper particles. The preparation method comprises the following steps: preparation of bimodal distribution copper powder: pouring a reducing agent into a prefabricated solution which is subjected to heat preservation and mechanical stirring at the temperature of 80-120 DEG C to obtain a reaction solution, and centrifuging and cleaning after the reaction is finished to obtain the bimodal distribution copper powder; the prefabricated liquid is obtained by mixing a composite coating agent composed of copper salt, organic acid and organic amine with ethylene glycol; and copper paste preparation: mixing the prepared bimodal distribution copper powder with an organic solvent to obtain the copper paste. The copper powder has the advantages of micro-nano cross-scale and high initial stacking density, can be sintered in air to form a compact sintered structure and high-strength interconnection by combining the self-reduction effect of the coating agent and the organic solvent, and is suitable for air low-temperature sintering interconnection packaging of power chips and power devices.

Description

Technical Field

[0001] The present invention relates to a copper paste for interconnection, specifically to a copper paste filled with bimodal distribution copper powder in which micron copper sheets are tightly wrapped by nano copper particles for sintering in air, and its preparation method and application, which are applicable to the technical field of electronic packaging interconnection materials for power devices. Background Art

[0002] In recent years, with the development of electronic devices towards miniaturization, high power density, and high integration, the operating temperature of power devices will exceed 250°C. The third-generation semiconductors represented by GaN and SiC have the advantages of high output power, high operating frequency, high thermal conductivity, and high thermal reliability, ensuring their applications in harsh environments such as electric vehicles, aerospace, high-speed railways, and deep gas / oil extraction. However, traditional materials such as tin-based solders cannot provide reliable packaging interconnection in such harsh working environments. Therefore, new high-temperature interconnection materials need to be developed to meet the requirements of such harsh working environments. Nano silver paste has been favored due to many advantages, including excellent thermal, electrical, and mechanical properties, remarkable oxidation resistance, and long-term reliability. However, due to high material costs and susceptibility to electromigration, its application in the packaging of high-power devices is greatly limited. Copper has similar thermal and electrical conductivities to silver, low cost, and strong electromigration resistance, so nano copper paste is considered a promising alternative material for silver paste.

[0003] As is well known, nano copper powder with a high specific surface energy exhibits excellent sintering performance. However, the sintered structure formed by single-sized nano copper powder often contains a considerable number of pores. To overcome this problem, a method of mixing large-sized micron copper powder with small-sized nano copper powder is adopted to form copper powder with a bimodal particle size distribution. This mixing helps to improve the original packing density of the copper powder and ultimately enhances the densification of the sintered structure and the strength of the interconnect joints. However, most current methods are to prepare copper powder with a bimodal particle size distribution by mechanically mixing existing nano copper powder and micron copper powder. This method cannot ensure the uniform mixing of the two-sized powders and may also lead to the aggregation of nano copper powder.

[0004] Nano copper materials have a high tendency to oxidize, especially during sintering in an air atmosphere, which is prone to oxidation, and the generated oxides seriously hinder the diffusion sintering between copper particles, thus significantly reducing its electrical conductivity and the strength of the interconnect structure. Therefore, usually, harsh sintering conditions such as high temperature (>300°C), high pressure (>10 MPa), and long sintering interconnection time (>1 h), as well as the use of inert, vacuum, or even reducing atmospheres to inhibit copper oxidation, are required to obtain high-strength sintered interconnect joints. Harsh sintering conditions will significantly increase the process difficulty and production cost. Therefore, developing high-performance copper paste for interconnection that can be sintered in air is of great significance for promoting the wide application of copper paste technology in the interconnection field.

[0005] Chinese invention patent CN111408869B discloses a micro-nano copper particle solder paste for low-temperature bonding, its preparation method and application. The preparation method includes: cleaning the micron copper particles to remove impurities, and then drying them for standby; placing the dried micron copper particles in a preset environment for oxidation to form nano copper oxide on their surfaces; placing the oxidized micron copper particles in a reducing agent for reduction to reduce the nano copper oxide on the surface to nano copper particles, obtaining micro-nano copper particles; adding an organic thickener to the micro-nano copper particles, and forming a micro-nano copper particle solder paste after stirring and defoaming. This technology utilizes the pre-oxidation and reduction processes to form a nanostructure on the surface and gaps of the micron copper particles, and effectively reduces the bonding temperature by using its small-size effect, meeting the application requirements of power device packaging. However, this technology adds pre-oxidation and reduction processes to obtain micro-nano copper particles, making the process more complex and greatly increasing the process cost.

[0006] Chinese invention patent CN114799613B discloses a copper solder paste, its preparation method and application. The copper solder paste is made of the following raw material components: copper powder, a complex of a copper precursor and an amine ligand, an organic solvent carrier, and a reducing agent. The hydrogen gas generated by the decomposition of the complex of the copper precursor and the amine ligand added to the solder paste provided by this technology can, to a certain extent, inhibit oxidation in the air. On the other hand, the metal particles in-situ decomposed from the complex of the copper precursor and the amine ligand contribute to sintering and promote interconnection. The copper solder paste provided by this invention can fully achieve the purpose of bonding in the air, obtaining a lower porosity and a higher shear strength. However, before bonding, three preheating steps need to be carried out successively at 60°C - 180°C, adding a self-reduction process and increasing the process cost. And sintering is carried out in the air at a pressure of 10 MPa and a temperature of 250°C, with a pressure holding time of 15 min, and the interconnection joint strength is only 15.04 MPa, and the mechanical properties of the interconnection joint are poor. Summary of the Invention

[0007] Aiming at the problems existing in the prior art, the present invention provides a copper paste filled with bimodal distribution copper powder in which nano copper particles tightly wrap the surface of micron copper sheets, and its preparation method. Through a one-step synthesis method, bimodal distribution copper powder with good sintering performance, antioxidant performance and dense packing is prepared, overcoming the problems that the existing copper paste is easy to oxidize during sintering in an air atmosphere and cannot achieve short-time sintering, and simplifying the sintering process.

[0008] Another object of the present invention is to provide the application of a copper paste filled with bimodal distribution copper powder in which nano copper particles tightly wrap the surface of micron copper sheets in the preparation of interconnection joints in the air, avoiding sintering and interconnecting the copper paste interconnection joints in an inert atmosphere and a reducing atmosphere, greatly reducing the sintering and interconnecting time while obtaining high-quality interconnection joints.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A method for preparing a copper paste using a bimodal distribution copper powder with nano-copper particles tightly coating the surface of micro-copper sheets as a filler, characterized by comprising the following steps:

[0011] 1) Preparation of bimodal distribution copper powder: Pour a reducing agent into a prefabricated solution maintained at a temperature of 80–120°C with heat preservation and mechanical stirring to obtain a reaction solution. After the reaction ends, obtain the bimodal distribution copper powder through centrifugation and washing; the prefabricated solution is obtained by mixing a composite coating agent composed of a copper salt, an organic acid, and an organic amine with ethylene glycol;

[0012] 2) Copper paste preparation: Mix the prepared bimodal distribution copper powder with an organic solvent to obtain a copper paste.

[0013] To further achieve the object of the present invention, preferably, the mass concentration ratio of the copper salt, the reducing agent, and the composite coating agent in the reaction solution is 1:(3–10):(5–10).

[0014] Preferably, the concentration of the copper salt in the prefabricated solution is 30–80 g / L; the copper salt is one or more of copper sulfate pentahydrate, copper nitrate trihydrate, copper acetate, copper hydroxide, basic copper carbonate, and copper chloride.

[0015] Preferably, the concentration of the reducing agent in the reaction solution is 300–800 g / L; the reducing agent is one or more of sodium borohydride, hydrazine hydrate, sodium hypophosphite, and ascorbic acid.

[0016] Preferably, the concentration of the composite coating agent composed of the organic acid and the organic amine is 300–1000 g / L, and the mass concentration ratio of the organic acid to the organic amine is 6:5–4:1.

[0017] Preferably, the organic amine is one or more of methanolamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, and oleylamine; the organic acid is one or more of citric acid, tartaric acid, propionic acid, butyric acid, oxalic acid, lactic acid, glycine, and oleic acid.

[0018] Preferably, the time of the mechanical stirring is 10–60 min, and the rotation speed is 400–800 r / min; the organic solvent is one or more of diethylene glycol, glycerol, ethylene glycol, propylene glycol, polyethylene glycol, and terpineol;

[0019] The mass percentage of the bimodal distribution copper powder in the copper paste is 70–90%, and the mass percentage of the organic solvent is 30–10%.

[0020] The copper paste using the bimodal distribution copper powder with micron copper sheets tightly wrapped by nano copper particles as the filler is prepared by the above preparation method; in the copper paste using the bimodal distribution copper powder with micron copper sheets tightly wrapped by nano copper particles as the filler, the copper powder exhibits bimodal distribution characteristics. The size of the large micron copper sheets is 1–2 μm, and the particle size of the small nano copper particles is 5–15 nm. Among them, the small nano copper particles are tightly coated on the surface and around the large micron copper sheets to form a dense packing structure.

[0021] Application of the copper paste using the bimodal distribution copper powder with micron copper sheets tightly wrapped by nano copper particles as the filler in the preparation of Cu–Cu interconnect joints: The copper paste is printed on a pure copper substrate by screen printing method. The upper pure copper substrate is placed on the surface of the printed copper paste to obtain a sintering joint to be sintered with a pure copper substrate / copper paste / pure copper substrate sandwich structure. It is sintered at 200–280 °C in an air atmosphere for 5–30 min and cooled to room temperature to obtain a Cu–Cu joint.

[0022] Preferably, before sintering, it further includes applying a pressure of 0–10 MPa to the pure copper surfaces at both ends of the sintering joint to be sintered, and then removing the pressure before cooling.

[0023] Compared with the prior art, the advantages of the present invention are as follows:

[0024] 1) The present invention uses a composite coating agent of organic acid and organic amine to regulate the shape and particle size distribution of copper powder. In the initial stage of the reaction, the organic amine can undergo a complexation reaction with copper salt to form a complex, thereby slowing down the rate of redox reaction. At the same time, the selective adsorption effect of the organic amine on copper nanocrystals is conducive to the generation of micron copper sheets. However, the organic acid will increase the activity of copper ions in the copper salt, accelerate the rate of redox reaction, and produce more copper crystal nuclei per unit time. In the later stage of the reaction, the newly formed nanoparticles still maintain a small size. In addition, the organic acid can also react with copper oxide to effectively reduce and inhibit the oxidation of copper powder.

[0025] 2) The present invention uses a one-step chemical reduction method to synthesize copper powder with bimodal distribution characteristics from nano to micron in particle size at one time. In the structure of this copper powder, the surface of 1–2 μm micron copper sheets is tightly wrapped by 5–15 nm nano copper particles. This one-step synthesis method is simple, efficient, and has good repeatability. It can achieve the advantages of uniform distribution and dense packing of micro-nano sized powders, and avoid the problems of uneven powder particle distribution and agglomeration in traditional mechanical mixing methods. Description of the Drawings

[0026] Figure 1 Transmission electron microscope image (TEM) of the micron copper sheets prepared in Example 1 tightly wrapped by small-sized nano copper particles.

[0027] Figure 2Scanning electron microscope image (SEM) of the micron copper flakes prepared in Example 1 tightly wrapped by small-sized nano copper particles.

[0028] Figure 3 Scanning electron microscope image (SEM) of the fracture morphology of the copper paste interconnect joint prepared in Example 1.

[0029] Figure 4 Scanning electron microscope image (SEM) of the surface of the copper paste prepared after sintering at 250 °C in Example 1. Detailed implementation manners

[0030] To better understand the present invention, the present invention will be further described below in conjunction with the drawings and embodiments, but the implementation manners of the present invention are not limited thereto.

[0031] The main measure of the present invention is to use a one-step synthesis method to prepare copper powder with a micro-nano scale distribution, in which small-sized nano copper powder is evenly distributed on the surface of large-sized micron copper powder. In addition to improving the densification of the sintered structure, this method may also reduce the sintering temperature of the copper powder. This one-step synthesis method utilizes the cooperation of a composite coating agent composed of organic acid and organic amine in the prefabricated solution and ethylene glycol; among them, the organic acid in the prefabricated solution is preferably one or more of citric acid, tartaric acid, propionic acid, butyric acid, oxalic acid, lactic acid, glycine and oleic acid, in fact, as long as it is an organic acid; the organic amine is preferably one or more of methanolamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine and oleylamine, and as long as it is an organic amine. In the preparation of the bimodal distribution copper powder, in the initial stage of the reaction, the organic amine can react with the copper salt to form a complex, thereby slowing down the rate of the redox reaction. At the same time, the selective adsorption effect of the organic amine on the copper nanocrystals is conducive to the generation of micron copper flakes. However, the organic acid can enhance the activity of the copper ions in the copper salt, accelerate the rate of the redox reaction, and produce more copper crystal nuclei per unit time. In the later stage of the reaction, the newly formed nano particles still remain small-sized. In particular, the organic acid can also react with copper oxide to effectively reduce and inhibit the oxidation of the copper powder. Therefore, the present invention utilizes organic acid and organic amine to regulate the shape and particle size distribution of the copper powder and inhibit the oxidation of the copper powder.

[0032] Therefore, the preparation method of the copper paste using the bimodal distribution copper powder with nano copper particles tightly wrapping the surface of micron copper flakes as the filler includes the following steps:

[0033] 1) Preparation of bimodal distribution copper powder: Pour the reducing agent into the prefabricated solution at a temperature of 80–120 °C and keep it warm and mechanically stirred to obtain a reaction solution. After the reaction is completed, the bimodal distribution copper powder is obtained through centrifugation and washing; the prefabricated solution is obtained by mixing a composite coating agent composed of copper salt, organic acid and organic amine with ethylene glycol;

[0034] 2) Preparation of copper paste: Mix the prepared bimodal copper powder with an organic solvent to obtain the copper paste.

[0035] Regarding the preparation method and measures of this technology, the mass concentration ratio of copper salt, reducing agent and composite coating agent in the reaction solution can be obtained through experiments on the premise of utilizing the above-mentioned effects of organic acids and organic amines in combination with the purpose of the present invention. The present invention preferably has the mass concentration ratio of copper salt, reducing agent and composite coating agent in the reaction solution as 1:(3–10):(5–10). Control the concentration of the composite coating agent composed of organic acid and organic amine to be 300–1000 g / L, and the mass concentration ratio of organic acid and organic amine to be 6:5–4:1.

[0036] Preferably control the concentration of copper salt in the prefabricated solution to be 30–80 g / L, and the concentration of reducing agent in the reaction solution to be 300–800 g / L

[0037] In the selection of raw materials in the present invention, copper salts are conventionally selected. Preferably, the copper salt is one or more of copper sulfate pentahydrate, copper nitrate trihydrate, copper acetate, copper hydroxide, basic copper carbonate, and copper chloride.

[0038] The reducing agent is also a conventional reducing agent in the art. Preferably, the reducing agent is one or more of sodium borohydride, hydrazine hydrate, sodium hypophosphite, and ascorbic acid. Preferably control the mechanical stirring time in step 1) to be 10–60 min, and the rotation speed to be 400–800 r / min; the organic solvent is one or more of diethylene glycol, glycerol, ethylene glycol, propylene glycol, polyethylene glycol, and terpineol; preferably control the mass percentage of bimodal copper powder in the copper paste to be 70–90%, and the mass percentage of the organic solvent to be 30–10%.

[0039] The one-step synthesis method process of the present invention is not only simple, efficient, and has good repeatability, but also has the advantages of uniform distribution and close packing of micro-nano sized powders. In particular, this one-step synthesis method process avoids the problems of uneven powder particle distribution and agglomeration in the traditional mechanical mixing method.

[0040] The copper paste using the bimodal distribution copper powder with nano copper particles tightly coating the surface of micro copper sheets as the filler is characterized in that the copper powder in the copper paste presents a bimodal distribution. The size of the large micro copper sheets is 1–2 μm, and the particle size of the small nano copper particles is 5–15 nm. The small nano copper particles are tightly coated on the surface and around the large micro copper sheets to form a dense packing structure. The small-sized nano copper particles fill the gaps between the micro copper sheets, increasing the packing density and forming a tightly packed structure. Compared with nano copper particles, the micro copper sheets as the "skeleton" have better oxidation resistance. At the same time, combined with the high specific surface energy and sintering driving force of the small-sized nano copper particles, the small-sized nano copper particles after low-temperature sintering connect the micro copper sheets as the "skeleton" to obtain a dense sintered structure.

[0041] The copper paste obtained by the present invention can avoid the need for sintering under a protective atmosphere in the prior art, which seriously affects the environmental requirements and costs during application. The copper paste can show short-time and high-efficiency sintering under air conditions and obtain excellent interconnect performance. The small-sized nano copper particles help to reduce the sintering temperature and achieve rapid sintering. At the same time, organic acids are adsorbed on the surface of the copper powder. During the sintering process, the organic acids react with copper oxides to form copper organic acids, and then decompose into small-sized nano copper particles at a certain temperature, thereby effectively reducing copper oxides and promoting sintering. The reducing alcohol organic solvents during the sintering process can inhibit the oxidation of the copper powder, thereby realizing air sintering and obtaining copper–copper interconnections with high shear strength.

[0042] To prepare a Cu–Cu interconnect joint using the copper paste with the bimodal distribution copper powder with nano copper particles tightly coating the surface of micro copper sheets as the filler, simply print the copper paste on the pure copper substrate by the stencil printing method. Place the upper and lower pure copper substrates on the surface of the printed copper paste to obtain a sandwich-structured joint to be sintered of pure copper substrate / copper paste / pure copper substrate. Sinter in an air atmosphere at 200–280 °C for 5–30 min, and then cool to room temperature to obtain the Cu–Cu joint. This application method is simple, especially without the need for a protective atmosphere.

[0043] Example 1

[0044] A preparation method of a copper paste using the bimodal distribution copper powder with nano copper particles tightly coating the surface of micro copper sheets as the filler for sintering in air, including two steps: the preparation of the bimodal distribution copper powder and the mixing of the copper paste.

[0045] 1) Preparation of bimodal copper powder with nano - copper particles tightly coating the surface of micron - copper sheets: Mix 2 g of copper hydroxide, 15 g of citric acid, and 5 g of isopropanolamine in 40 ml of ethylene glycol solution. After heating to 90 °C, add 20 g of sodium hypophosphite, and stir at a rate of 600 r / min for reaction for 20 min, then cool to room temperature. Centrifuge the cooled reaction product at a rate of 4000 r / min for 3 minutes, and then wash it repeatedly with ethanol twice to obtain copper powder with a coated structure and a bimodal particle size distribution ranging from nano - scale to micron - scale. As Figure 1 and Figure 2 shown, through particle size statistical analysis, it is confirmed that the particle size of the micron - copper sheets is 1 - 2 μm, and the particle size of the small nano - copper particles is 5 - 15 nm. It can also be seen from the figure that the surface and the surrounding of the micron - copper sheets in the copper powder are tightly coated by small - sized nano - copper particles, and aggregates are formed between these small - sized particles. Therefore, the overall structure shows that small - sized nano - particle aggregates tightly coat the surface and the surrounding of the large - sized micron - sheets. It should be emphasized that the obtained copper powder structure is synthesized by a one - step redox method, and the coating state is very uniform, while the existing multi - scale copper powder is prepared by mechanical mixing, which is prone to uneven particle mixing and coating.

[0046] 2) Copper paste mixing: Mix terpineol and glycerol in a mass ratio of 1:1 and stir evenly. After standing for 30 min to remove bubbles, the organic solvent for the copper paste is obtained. Mix copper powder containing 80% mass percentage of nano - copper particles with the organic solvent containing 20% mass percentage, and uniformly stir and defoam it with a planetary gravity mixer to form a bimodal copper paste.

[0047] Use 1500# sandpaper to polish the pure copper substrate, then soak it in 3 vol.% dilute sulfuric acid solution, and finally wash it with ethanol and dry it for standby. Print the bimodal copper paste prepared in this example with a thickness of 150 μm on the lower pure copper substrate, and attach the upper pure copper substrate to the surface of the bimodal copper paste to ensure full contact between the upper and lower pure copper substrates and the bimodal copper paste. In an air atmosphere, heat the hot - press sintering furnace to 250 °C, then place the pure copper substrate / bimodal copper paste / pure copper substrate interconnection joint on the hot - press sintering furnace and sinter it at a pressure of 10 MPa and a temperature of 250 °C for 10 min, then remove the pressure and cool to room temperature to form an interconnection joint. After shear strength testing, the shear strength of the interconnection joint is measured to be 47.6 MPa, which is significantly higher than the shear strength of 15.04 MPa of the connection joint disclosed in Chinese invention patent CN114799613A. Observe the fracture morphology of the interconnection joint using a scanning electron microscope, and the results are as Figure 3As shown. It can be observed from the figure that there are a large number of dimples and traces of plastic deformation in the large copper structure of the joint fracture surface, which proves that the interconnecting joint has good mechanical properties. The surface morphology of the bimodal copper paste sintered at 250 °C for 10 min was observed using a scanning electron microscope, as Figure 4 shown. It can be clearly observed from the figure that good sintering connection is achieved through obvious grain growth and the formation of sintering necks between the micron copper sheets, forming a dense sintered structure.

[0048] In the bimodal copper powder, small-sized nanoparticle aggregates tightly wrap the surface of large-sized micron sheets, forming a dense initial packing structure. Therefore, it is more likely to form a dense sintered body during the sintering process, thereby improving the strength of the interconnecting layer. At the same time, the composite coating agent used in this embodiment is based on organic acid and can react with copper oxide during sintering heating to generate copper organic acid, and then the organic acid decomposes into small nano-copper particles. In addition, the reducing alcohol organic solvent in the copper paste during the sintering process can also play the role of reducing copper oxide. This process not only eliminates the adverse effects of copper oxide on sintering and interconnecting properties, but also produces small-sized highly active copper particles without coating agent, promoting the further sintering of the copper paste and the diffusion of copper atoms between the copper paste and the upper and lower pure copper substrates, thereby improving the interconnecting performance. In this embodiment, under air conditions, sintering at 250 °C for 10 min under a pressure of 10 MPa can obtain an interconnecting strength of more than 40 MPa. Existing copper paste is extremely easy to oxidize in air and it is very difficult to achieve reliable interconnection. Therefore, compared with existing copper paste, the high-performance bimodal copper paste used in this embodiment can save sintering energy, reduce sintering time and production cost.

[0049] Example 2

[0050] A preparation method of a copper paste filled with bimodal copper powder with nano-copper particles tightly wrapping the surface of micron copper sheets sintered in air, which includes two steps: the preparation of bimodal copper powder and the mixing of copper paste:

[0051] 1) Prepare bimodal copper powder with nano-copper particles tightly wrapping the surface of micron copper sheets: Mix 2 g of copper chloride, 15 g of lactic acid and 5 g of diethanolamine in 40 ml of ethylene glycol solution. After heating to 95 °C, add 18 g of ascorbic acid and stir at a rate of 400 r / min for reaction for 10 min, then cool to room temperature. The cooled reaction product is centrifuged at a rate of 4000 r / min for 3 minutes using a centrifuge, and then washed repeatedly with ethanol 2 times to obtain copper powder with a coating structure and a bimodal particle size distribution characteristic from nano to micron. Through particle size statistical analysis, it is confirmed that the particle size of the micron copper sheets is 1–2 μm, and the particle size of the small nano-copper particles is 5–15 nm.

[0052] 2) Copper paste mixing: Mix polyethylene glycol and diethylene glycol in a mass ratio of 2:1 and stir evenly. After standing for 30 min to remove air bubbles, the organic solvent used for the copper paste is obtained. Mix nano copper particles with 85% mass percentage and the organic solvent with 15% mass percentage, and uniformly stir and defoam with a planetary gravity mixer to form a bimodal distribution copper paste.

[0053] Use 1500# sandpaper to polish the pure copper substrate, then soak it in a 3 vol.% dilute sulfuric acid solution, and finally clean and dry it with ethanol for standby. Use a stencil to print the bimodal distribution copper paste prepared in this example with a thickness of 150 μm on the pure copper substrate, and attach the pure copper substrate to the surface of the bimodal distribution copper paste to ensure full contact between the upper and lower pure copper substrates and the bimodal distribution copper paste. In an air atmosphere, heat the hot press sintering furnace to 280 °C, then place the pure copper substrate / bimodal distribution copper paste / pure copper substrate interconnect joint on the hot press sintering furnace and sinter at 280 °C for 15 min under a pressure of 5 MPa, then remove the pressure and cool to room temperature to form an interconnect joint. After shear strength testing, the shear strength of the interconnect joint was measured to be 24.7 MPa.

[0054] Example 3

[0055] A preparation method of a copper paste filled with bimodal distribution copper powder in which nano copper particles tightly wrap the surface of micron copper sheets and are sintered in air, including two steps: preparation of bimodal distribution copper powder and copper paste mixing:

[0056] 1) Preparation of bimodal distribution copper powder in which nano copper particles tightly wrap the surface of micron copper sheets: Mix 2 g of copper sulfate pentahydrate, 15 g of oleic acid, and 5 g of N,N-dimethylethanolamine in 40 ml of ethylene glycol solution. After heating to 90 °C, add 20 g of sodium borohydride and stir at a rate of 600 r / min for 20 min, then cool to room temperature. Centrifuge the cooled reaction product at a rate of 4000 r / min for 3 minutes, and then wash it repeatedly with ethanol 2 times to obtain copper powder with a coated structure and a bimodal distribution characteristic of particle size from nano to micron. Through particle size statistical analysis, it is confirmed that the particle size of the micron copper sheets is 1–2 μm, and the particle size of the small nano copper particles is 5–15 nm.

[0057] 2) Copper paste mixing: Mix glycerol, propylene glycol, and diethylene glycol in a mass ratio of 1:1:1 and stir evenly. After standing for 30 min to remove air bubbles, the organic solvent used for the copper paste is obtained. Mix nano copper particles with 75% mass percentage and the organic solvent with 25% mass percentage, and uniformly stir and defoam with a planetary gravity mixer to form a bimodal distribution copper paste.

[0058] The pure copper substrate was polished with 1500# sandpaper, then immersed in 5 vol.% dilute sulfuric acid solution, and finally cleaned with ethanol and dried for standby. The bimodal distribution copper paste prepared in this example with a thickness of 150 μm was screen-printed on the lower pure copper substrate, and the upper pure copper substrate was attached to the surface of the bimodal distribution copper paste to ensure full contact between the upper and lower pure copper substrates and the bimodal distribution copper paste. In an air atmosphere, the hot press sintering furnace was heated to 200 °C, and then the pure copper substrate / bimodal distribution copper paste / pure copper substrate interconnect joint was placed on the hot press sintering furnace and sintered at 200 °C for 10 min under a pressure of 10 MPa, then the pressure was removed and cooled to room temperature to form an interconnect joint. After shear strength testing, the shear strength of the interconnect joint was measured to be 27.8 MPa.

[0059] The embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing a copper paste using copper powder with a bimodal distribution in which nano copper particles tightly wrap the surface of a micron copper sheet as a filler, characterized in that It includes the following steps: 1) Preparation of bimodal distribution copper powder: Pour a reducing agent into a prefabricated solution at a temperature of 80–120 °C with heat preservation and mechanical stirring to obtain a reaction solution. After the reaction ends, bimodal distribution copper powder is obtained through centrifugation and washing; the prefabricated solution is obtained by mixing a composite coating agent composed of a copper salt, an organic acid, and an organic amine with ethylene glycol; 2) Preparation of copper paste: Mix the prepared bimodal distribution copper powder with an organic solvent to obtain a copper paste.

2. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 1, characterized in that, The mass concentration ratio of the copper salt, reducing agent, and composite coating agent in the reaction solution is 1:(3–10):(5–10).

3. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 1, characterized in that, The concentration of the copper salt in the prefabricated solution is 30–80 g / L; the copper salt is one or more of copper sulfate pentahydrate, copper nitrate trihydrate, copper acetate, copper hydroxide, basic copper carbonate, and copper chloride.

4. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles according to claim 1, characterized in that, The concentration of the reducing agent in the reaction solution is 300–800 g / L; the reducing agent is one or more of sodium borohydride, hydrazine hydrate, sodium hypophosphite, and ascorbic acid.

5. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles according to claim 1, characterized in that, The concentration of the composite coating agent composed of the organic acid and the organic amine is 300–1000 g / L, and the mass concentration ratio of the organic acid to the organic amine is 6:5–4:

1.

6. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 1 or 5, characterized in that, The organic amine is one or more of methanolamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, and oleylamine; the organic acid is one or more of citric acid, tartaric acid, propionic acid, butyric acid, oxalic acid, lactic acid, glycine, and oleic acid.

7. The preparation method of the copper paste using the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 1 or 5, characterized in that, The time of the mechanical stirring is 10–60 min, and the rotation speed is 400–800 r / min; the organic solvent is one or more of diethylene glycol, glycerol, ethylene glycol, propylene glycol, polyethylene glycol, and terpineol; The mass percentage of the bimodal distribution copper powder in the copper paste is 70–90%, and the mass percentage of the organic solvent is 30–10%.

8. A copper paste with a copper powder having a bimodal distribution, in which micron copper sheets are tightly wrapped by nano copper particles on the surface, is characterized in that: It is prepared by the preparation method according to any one of claims 1-7; in the copper paste filled with bimodal distribution copper powder capable of tightly wrapping the surface of micron copper sheets with nano copper particles, the copper powder presents a bimodal distribution characteristic, the size of the large micron copper sheets is 1–2 μm, and the particle size of the small nano copper particles is 5–15 nm, wherein the small nano copper particles are tightly coated on the surface and around the large micron copper sheets to form a dense stacking structure.

9. Application of the copper paste filled with the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 8 in the preparation of Cu–Cu interconnecting joints, characterized in that: Print the copper paste on a pure copper substrate below by means of screen printing, place the upper pure copper substrate on the surface of the printed copper paste to obtain a sintering joint to be sintered with a pure copper substrate / copper paste / pure copper substrate sandwich structure, sinter in an air atmosphere at 200–280 °C for 5–30 min, and cool to room temperature to obtain a Cu–Cu joint.

10. Application of the copper paste filled with the bimodal distribution copper powder with the surface of the micron copper sheet tightly wrapped by nano copper particles as claimed in claim 9 in the preparation of Cu–Cu interconnect joints, characterized in that: Before sintering, it also includes applying a pressure of 0–10 MPa to the surfaces of the pure copper at both ends of the sintering joint to be sintered, and then removing the pressure before cooling.

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