Antioxidant copper nanoparticles, slurries, their preparation methods and applications

Abstract

This invention provides an antioxidant copper nanoparticle, a slurry, a preparation method thereof, and an application thereof. The antioxidant copper nanoparticle is prepared by a method comprising the following steps: contacting the copper nanoparticle with a composite etching solution containing halide ions to perform anisotropic chemical etching treatment, thereby increasing the exposure ratio of the (111) crystal plane and decreasing the exposure ratio of the (110) and (100) crystal planes on the surface of the copper nanoparticle.

Description

Technical Field

[0001] This invention relates to the field of semiconductor power device packaging technology, and in particular to an antioxidant copper nanoparticle, a paste, its preparation method, and its application. Background Technology

[0002] With the rapid development of third-generation semiconductor power devices (such as silicon carbide (SiC) and gallium nitride (GaN), electronic devices are evolving towards higher power density, higher operating temperature, and higher reliability. The junction temperature of these devices can exceed 200°C, and even reach above 300°C. Traditional tin-based solders (with melting points typically below 250°C) are no longer sufficient to meet the requirements for high-temperature operation. Therefore, developing novel packaging and connection materials that can achieve "low-temperature interconnection and high-temperature operation" has become a research hotspot in the field of power electronics packaging.

[0003] Nanomaterials, due to their unique size effect, can be sintered at temperatures far below their bulk melting points, enabling low-temperature bonding and high-temperature operation. Among them, nanosilver, with its excellent electrical and thermal conductivity and good sintering activity, has been widely studied and applied. However, nanosilver suffers from two prominent problems: firstly, its high price significantly increases packaging costs; secondly, in humid and hot environments, silver is prone to electrochemical migration, leading to decreased device insulation performance or even short-circuit failure. In contrast, nanocopper possesses electrical and thermal conductivity close to that of silver, and its price is only about 1% of silver's, with far superior resistance to electrochemical migration. Therefore, nanocopper is considered one of the most promising materials to replace nanosilver.

[0004] However, nano-copper slurries face severe oxidation problems in practical applications. Nano-copper particles have a large specific surface area and high surface activity, making them highly reactive with oxygen in the air during storage or use at room temperature, forming a dense layer of copper oxide or cuprous oxide on the particle surface. This oxide layer severely hinders the diffusion and necking growth between copper atoms during sintering, resulting in a loose and porous sintered structure with low mechanical strength. Especially under pressureless sintering conditions, the presence of the oxide layer makes atomic diffusion even more difficult due to the lack of external pressure, making it almost impossible to form a dense bonding layer. Existing technologies for addressing nano-copper oxidation mainly include coating the copper particle surface with organic protective agents (such as long-chain fatty acids, amines, thiols, etc.) or adding reducing agents (such as formic acid, ethylene glycol, etc.) to the slurry. However, research shows that with prolonged storage time, the organic coating layer may detach or become ineffective, and the activity of the reducing agents will gradually decrease, failing to fundamentally solve the long-term storage problem. More importantly, these methods only focus on "isolating" or "reducing" the generated oxides, without improving the intrinsic antioxidant capacity of the copper nanoparticles themselves based on their crystallographic properties.

[0005] Therefore, how to improve the intrinsic antioxidant properties of copper nanoparticles by starting with their microstructure, while ensuring the formation of a dense and high-strength interconnected structure under low-temperature pressureless sintering conditions, is a technical problem that urgently needs to be solved in this field.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide an antioxidant copper nanoparticle, slurry, preparation method and application thereof, which solves at least one of the problems mentioned in the background art.

[0008] In a first aspect, the present invention provides an antioxidant copper nanoparticle, the copper nanoparticle being prepared by a method comprising the following steps: contacting the copper nanoparticle with a composite etching solution containing halide ions to perform anisotropic chemical etching treatment, thereby increasing the exposure ratio of the (111) crystal plane and decreasing the exposure ratio of the (110) and (100) crystal planes on the surface of the copper nanoparticle.

[0009] A second aspect of the present invention provides a method for preparing the above-mentioned nano-copper particles, comprising the following steps: S1: Add the nano-copper particles to the organic dispersion medium to prepare suspension A; S2: Add a composite corrosive solution B containing halide ions to suspension A and carry out anisotropic chemical corrosion reaction treatment. S3: The mixed solution obtained after S2 treatment is centrifuged, washed, and dried to obtain antioxidant copper nanoparticles.

[0010] In a third aspect, the present invention provides an antioxidant nano-copper paste, comprising the above-mentioned nano-copper particles or nano-copper particles prepared by the above-mentioned preparation method, and an organic carrier; after the nano-copper paste is stored in a room temperature air environment for 6 months, the surface atomic percentage of oxygen on the surface of the nano-copper particles is ≤5%, and it can be sintered at 200°C under pressureless conditions to form a dense bonding layer with a shear strength of 30 MPa or more.

[0011] In a fourth aspect, the present invention provides a method for preparing the above-mentioned nano-copper slurry, wherein the above-mentioned nano-copper particles or nano-copper particles obtained by the above preparation method are mixed with an organic carrier, and the mixture is degassed, rolled, and degassed again to obtain the nano-copper slurry.

[0012] A fifth aspect of the present invention provides an application of the above-mentioned nano-copper paste in the packaging of semiconductor power devices.

[0013] A sixth aspect of the present invention provides a method for fabricating an encapsulated interconnect structure, comprising the following steps: A) Pre-treat the lower substrate by coating the above-mentioned nano-copper paste onto the surface of the lower substrate; B) The upper substrate connection surface is attached to the lower substrate nano-copper paste to form a stacked structure; C) The stacked structure is sintered in an inert or reducing atmosphere to obtain an encapsulated interconnect structure.

[0014] The present invention has at least the following beneficial effects: This invention performs corrosion treatment on the surface of nano-copper particles, utilizing the thermodynamic stability principle driven by the surface energy difference of different crystal planes and the anisotropic selective dissolution principle. During the corrosion process, the atoms of the (110) and (100) crystal planes, with high surface energy, are in an unsaturated coordination state, possessing higher chemical potentials and more easily breaking free from their bonds to enter the solution, thus preferentially dissolving; while the (111) crystal plane, due to its lowest surface energy and the most thermodynamic stability, is selectively retained in the corrosion environment. Furthermore, halide ions (especially Cl-) are also removed. - The adsorption energies on different crystal planes vary significantly, and the adsorption strength order is: (110) plane > (100) plane > (111) plane. - After preferential adsorption onto the (110) facet, a soluble Cu-Cl complex is formed, significantly reducing the dissolution potential of that facet and thus accelerating the dissolution of the (110) facet. Through this selective etching of the crystal facet, the exposure of the (111) crystal facet on the surface of the nano-copper particles is ultimately increased, especially the (111) crystal facet. Due to the atomically dense arrangement and extremely high oxygen diffusion barrier of the (111) crystal facet, it exhibits excellent room temperature oxidation resistance and can be stored for a long time without oxidation, thus effectively solving the technical problem of short room temperature storage time of nano-copper paste. At the same time, since there is no oxide layer on the surface, the nano-copper particles diffuse more fully during sintering. Therefore, the copper paste of the present invention can be sintered without pressure at 200°C and maintain a high bonding strength. Attached Figure Description

[0015] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the surface corrosion process of nano-copper particles in an embodiment of the present invention; Figure 2 This is a schematic diagram illustrating the preparation of copper paste for pressureless sintering in an embodiment of the present invention; Figure 3 This is a schematic diagram of pressureless copper paste printing, bonding, and sintering in an embodiment of the present invention; Figure 4 The image shows a scanning electron microscope (SEM) image of the sintered structure using copper paste from Embodiment 1 of the present invention. Figure 5 Scanning electron microscope image of the sintered structure of copper paste prepared from uncorroded nano-copper particles in Comparative Example 1; Figure 6 The copper paste used in Embodiment 1 of the present invention is used as a bonding layer after sintering; Figure 7 Scanning electron microscope image of the copper paste prepared from uncorroded nano-copper particles in Comparative Example 1 as the bonding layer after sintering; Figure 8 Images showing the changes in oxygen content on the surface of the copper paste prepared in this embodiment of the invention and the copper paste prepared from uncorroded copper nanoparticles after long-term storage; Figure 9 Images showing the strength changes of copper paste prepared in this embodiment of the invention and copper paste prepared with uncorroded nano-copper particles after long-term storage for sintering. Detailed Implementation

[0017] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0018] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form includes the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0019] It should be noted that if the text uses terms such as "first" or "second", these terms are only used to distinguish similar objects and should not be interpreted as indicating or implying their relative importance, order of precedence, or implicitly indicating the number of technical features indicated. It should be understood that the data in the descriptions of "first" and "second" can be interchanged where appropriate.

[0020] Throughout the accompanying drawings, identical elements are represented by the same or similar reference numerals. Conventional structures or configurations may be omitted where they might cause confusion in understanding the invention. Furthermore, the shapes, dimensions, and positional relationships of the components in the drawings do not reflect actual size, scale, or actual positional relationships. Additionally, any reference symbols placed within parentheses in this invention should not be construed as limiting the scope of the invention.

[0021] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] One aspect of the present invention provides antioxidant copper nanoparticles, which are prepared by a method comprising the following steps: contacting the copper nanoparticles with a composite etching solution containing halide ions to perform anisotropic chemical etching treatment, thereby increasing the exposure ratio of the (111) crystal plane and decreasing the exposure ratio of the (110) and (100) crystal planes on the surface of the copper nanoparticles. The surface of the copper nanoparticles may have halide ion adsorption residues.

[0023] This invention provides an antioxidant copper nanoparticle, a copper nanoparticle slurry containing the copper nanoparticle, a method for preparing the same, and a pressureless sintering application. This invention utilizes the principle of thermodynamic stability driven by the difference in surface energy of different crystal planes and the principle of anisotropic selective dissolution, combined with the selective adsorption of halide ions on crystal planes, to increase the exposure ratio of the (111) crystal plane and decrease the exposure ratio of the (110) and (100) crystal planes. Simultaneously, halide ion adsorption residues are left on the particle surface, thereby endowing the copper nanoparticle with excellent room-temperature antioxidant properties.

[0024] The (110) and (100) crystal planes have high surface areas and are prone to oxidation, while the (111) crystal plane has the lowest surface energy, the densest atomic packing, and the most thermodynamic stability. Increasing the exposure ratio of the (111) crystal plane can effectively prevent the oxidation of the copper nanoparticles, effectively extend the storage time of the copper nanoparticles, and ensure the long-term storage of the copper nanoparticle paste. Furthermore, reducing the obstruction of the surface oxide layer is more conducive to the diffusion between copper nanoparticles during sintering. Therefore, it is possible to form a dense sintered structure under no-pressure conditions, improving the strength and density of pressureless sintering.

[0025] In some embodiments, the surface of the nano-copper particles is dominated by the (111) crystal plane, for example, reaching more than 55% or more, or more than 70%, and the exposure ratio of the (110) crystal plane and the (100) crystal plane is reduced relative to the uncorroded nano-copper particles, for example, by more than 10%.

[0026] To ensure sufficient surface activity for low-temperature sintering while maintaining particle integrity during the etching process, in some embodiments, the average particle size of the copper nanoparticles is 30 nm to 1 μm, such as 40 nm, 100 nm, 250 nm, 400 nm, 600 nm, 750 nm, 900 nm, etc., preferably 50 nm to 500 nm, to achieve better results. Experiments show that when the particle size is less than 30 nm, the particles are easily over-dissolved in the etching solution; when the particle size is greater than 1 μm, the sintering activity decreases significantly, and it is difficult to form dense bonds during pressureless sintering at 200°C.

[0027] In some embodiments, the morphology is spherical or near-spherical, which is beneficial for forming a uniform etched layer and subsequent printing / dispensing processes.

[0028] A second aspect of the present invention provides a method for preparing the above-mentioned copper nanoparticles, comprising the following steps: S1: Add the nano-copper particles to the organic dispersion medium to prepare suspension A; S2: Add a composite corrosive solution B containing halide ions to suspension A and carry out anisotropic chemical corrosion reaction treatment. S3: The mixed solution obtained after S2 treatment is centrifuged, washed, and dried to obtain antioxidant copper nanoparticles.

[0029] This invention uses the above method to etch the surface of nano-copper particles, utilizing the thermodynamic stability principle driven by the surface energy difference of different crystal planes and the anisotropic selective dissolution principle. During the etching process, the atoms of the (110) and (100) crystal planes, with high surface energy, are in an unsaturated coordination state, possessing higher chemical potentials and more easily breaking free from their bonds to enter the solution, thus dissolving preferentially; while the (111) crystal plane, due to its lowest surface energy and the most thermodynamic stability, is selectively retained in the etching environment. Furthermore, halide ions (especially Cl-) are also etched away. - The adsorption energies on different crystal planes vary significantly, with the adsorption strength in the order: (110) plane > (100) plane > (111) plane. Halogen ions (such as Cl...) - After being preferentially adsorbed on the (110) surface, it will form a soluble Cu-halogen complex, which will significantly reduce the dissolution potential of the surface and thus accelerate the dissolution of the (110) surface.

[0030] Through this selective etching of crystal planes, the exposure ratio of the (111) crystal planes on the surface of the copper nanoparticles is ultimately increased. Due to the atomically dense arrangement and extremely high oxygen diffusion barrier of the (111) crystal planes, excellent room-temperature oxidation resistance is exhibited, allowing for long-term storage without oxidation, thus effectively solving the technical problem of short room-temperature storage time for copper nanoparticle paste. Simultaneously, because there is no oxide layer hindering the surface, atomic diffusion of the copper nanoparticles is more complete during sintering. Therefore, the copper nanoparticle paste (also called copper paste) prepared from the aforementioned copper nanoparticles of this invention can be pressurelessly sintered at 200°C while maintaining high bonding strength.

[0031] In some embodiments, in step S1, the solid content of suspension A is 5 wt% to 20 wt%, for example, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, etc. Too low a solid content will lead to low processing efficiency, while too high a content will result in uneven particle dispersion, affecting corrosion consistency.

[0032] In some embodiments, the organic dispersion medium consists of a main solvent and a dispersant. The main solvent is selected from one or more of anhydrous ethanol, isopropanol, ethylene glycol, diethylene glycol, and glycerol, accounting for 85 wt% to 99.9 wt% of the organic dispersion medium, for example, 87 wt%, 90 wt%, 94 wt%, 98 wt%, etc., preferably 85 wt% to 97 wt%. The dispersant is selected from one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), oleic acid, and cetyltrimethylammonium bromide (CTAB), accounting for 0.1 wt% to 15 wt% of the organic dispersion medium, for example, 0.5 wt%, 2 wt%, 5 wt%, 8 wt%, 12 wt%, 14 wt%, etc., preferably 3 wt% to 15 wt%. The function of the dispersant is to prevent the agglomeration of nano-copper particles and ensure the uniformity of the corrosion treatment. Experiments show that adding 3% to 10% PVP K30 can effectively increase the particle dispersion to over 95%.

[0033] In some embodiments, after the nano-copper particles are added to the organic dispersion medium, they are first mechanically stirred at a speed of 200-500 rpm for at least 5 minutes, preferably 10-30 minutes, and then ultrasonically dispersed for at least 10 minutes, preferably 20-50 minutes. Mechanical stirring achieves preliminary mixing, and ultrasonic dispersion further breaks down agglomerates. After this treatment, the absolute value of the Zeta potential of the particles in suspension A can reach more than 30 mV, and the dispersion stability is good.

[0034] In some embodiments, in step S2, the composite etching solution B includes a crystal plane selector. The crystal plane selector (halogen ions) preferentially adsorbs on the high surface energy (110) and (100) crystal planes to accelerate the dissolution of these crystal planes, thereby achieving crystal plane selective etching.

[0035] In some embodiments, the composite etching solution B further includes one or more of an oxidant, a complexing agent, an acid reagent, a pH adjuster, and a diluent. The role of adding an oxidant to the composite etching solution B is to oxidize the copper surface to form Cu. 2+ Ions; addition of complexing agents and Cu 2+ Soluble complexes are formed, promoting dissolution. Furthermore, by rationally adding acidic reagents, pH adjusters, and diluents to the composite etching solution B, the anisotropic chemical etching process on the copper nanoparticles can be precisely controlled, thereby stably and efficiently preparing copper nanoparticles with controllable surface structure and excellent antioxidant properties.

[0036] In some embodiments, the oxidant is selected from one or more of hydrogen peroxide (H2O2, such as 30% by mass), ammonium persulfate, and cerium ammonium nitrate, and the amount used is 0.2 to 1.5 times the volume of suspension A, for example, 0.5 times, 0.8 times, 1.0 times, 1.2 times, etc. If the oxidant concentration is too low, the corrosion rate is slow; if it is too high, the particles dissolve excessively, and the yield decreases. When the oxidant amount is 0.2 to 1.5 times the volume of suspension A, a good balance is achieved between corrosion effect and yield.

[0037] In some embodiments, the complexing agent is selected from one or more of ammonia (NH3·H2O, such as 25%~28% by mass), ethylenediamine, and triethanolamine, and the amount used is 0.1~1.0 times the volume of suspension A, for example, 0.3 times, 0.5 times, 0.7 times, 0.9 times, etc. Ammonia can be used as a complexing agent to react with Cu²⁺. + [Cu(NH3)4]² is formed + Complex ions can also act as pH adjusters to regulate the acidity or alkalinity of a solution. The chelation effect is optimal when the amount of ammonia used is within the above-mentioned range.

[0038] In some embodiments, the crystal facet selector is selected from one or more of NaCl, KCl, NaBr, and KI, and its concentration in the composite etching solution B is 1~10 mmol / L, for example, 2 mmol / L, 5 mmol / L, 8 mmol / L, etc. Halogen ions (especially Cl-) - The adsorption energy of the (110) facet is much higher than that of the (111) facet, thus preferentially adsorbing and accelerating the dissolution of the (110) facet. When the concentration of the crystal facet selector is within the above range, the exposure ratio of the (111) crystal facet increases significantly, thereby enhancing crystal facet selectivity.

[0039] In some embodiments, the acid reagent is selected from one or more of nitric acid (e.g., 5-20% by mass), sulfuric acid (e.g., 5-10% by mass), citric acid, and ascorbic acid. The amount of acid reagent can be 0.05-0.5 times the volume of suspension A, for example, 0.08 times, 0.15 times, 0.25 times, 0.4 times, 0.45 times, etc. The acid reagent can assist in removing the natural oxide layer on the surface of the copper nanoparticles, and can also directly participate in the chemical corrosion process of copper. By adjusting the acid concentration, the initiation rate and uniformity of the corrosion reaction can be effectively controlled.

[0040] In some embodiments, the pH adjuster is selected from one or more of ammonia, ethylenediamine, and triethanolamine, and can be used to adjust the pH of the composite corrosion solution B to 3-5.

[0041] In some embodiments, the diluent is deionized water, and the amount used is 1 to 3 times the volume of suspension A, such as 1.5 times, 2 times, 2.5 times, etc.

[0042] Composite etching solution B can be divided into acidic and alkaline systems. The acidic system may include, for example, acidic reagents and crystal facet selectors. The alkaline system may include, for example, complexing agents, oxidizing agents, and crystal facet selectors. These two different systems, one relying on acidic reagents for etching and the other on ammonia and strong oxidizing agents, represent two different routes. The crystal facet selector is used to accelerate the etching of the other two crystal facets, thereby increasing the proportion of the 111 crystal facet.

[0043] In some embodiments, the chemical etching reaction is carried out at a temperature of 35–60°C, preferably 35–45°C. Too low a temperature results in a slow reaction rate; too high a temperature can lead to excessive dissolution or agglomeration of particles. The stirring speed is 200–800 rpm, preferably 300–500 rpm, to ensure sufficient contact between the etching solution and the particles. The reaction time is 30–120 min, preferably 60–90 min, to obtain the best crystal plane selective etching effect. The chemical etching reaction can be carried out with stirring in an oil bath.

[0044] In step S3, centrifugation can be used to collect the reaction products.

[0045] In some embodiments, the separated copper nanoparticles are washed 2-3 times with anhydrous ethanol, each time with ultrasonic or agitation for 15-25 minutes, to remove residual corrosion liquid and byproducts. Finally, the washed copper nanoparticles are dried under a protective atmosphere (such as nitrogen or argon). Drying can be carried out at 30-60°C for 1-3 hours.

[0046] A third aspect of the present invention provides an antioxidant nano-copper paste comprising the aforementioned nano-copper particles and an organic carrier. After being stored in air at room temperature for 6 months, the nano-copper paste exhibits a surface oxygen atom percentage of ≤5% on the surface of the nano-copper particles and can be sintered at 200°C under pressureless conditions to form a dense bonding layer with a shear strength exceeding 30 MPa.

[0047] The antioxidant copper nanoparticles of the present invention achieve an increased exposure ratio of the (111) crystal plane through selective etching of the crystal planes. Due to the atomically dense arrangement and extremely high oxygen diffusion barrier of the (111) crystal plane, they exhibit excellent room temperature antioxidant properties and can be stored for a long time without oxidation. Therefore, the slurry made using the antioxidant copper nanoparticles of the present invention effectively solves the technical problem of short room temperature storage time of nano slurries.

[0048] In some embodiments, the organic carrier comprises volatile alcoholic organic compounds and a binder. The volatile alcoholic organic compounds may include one or more of ethylene glycol, diethylene glycol, terpineol, propylene glycol, glycerol, butanediol, and hexanediol. Preferably, the mixture is formulated in the following weight ratios: 20-25 parts diethylene glycol, 20-35 parts terpineol, 20-30 parts propylene glycol, and 20-25 parts glycerol. This mixed alcohol system exhibits a moderate evaporation rate and good dispersibility, ensuring stable rheological properties of the paste during printing / dispensing.

[0049] In some embodiments, the binder includes one or more of methylcellulose, hydroxypropyl methylcellulose, hydroxyethylcellulose, and ethylcellulose to provide thixotropy and moldability of the slurry.

[0050] If the mass ratio of volatile alcoholic organic compounds to binder is too low, sedimentation and stratification will occur during the storage of copper paste. If the ratio is too high, the viscosity of the copper paste will be too high, which is not conducive to printing. Therefore, in some embodiments, the mass ratio of volatile alcoholic organic compounds to binder in the organic carrier can be 100:1 to 15, such as 100:3, 100:5, 100:9, 100:13, etc., preferably 100:2 to 8.

[0051] Too low a solid content will result in a loose sintered layer; too high a solid content will result in excessively high slurry viscosity and printing difficulties. Therefore, in some embodiments, the mass percentage of nano-copper particles in the nano-copper slurry is 80% to 93%, for example, 83%, 86%, 89%, 92%, etc., preferably 88% to 91%; the mass percentage of the organic carrier is 7% to 20%, for example, 8%, 10%, 13%, 16%, 19%, etc., preferably 9% to 12%.

[0052] The fourth aspect of the present invention provides a method for preparing a nano-copper paste, wherein the above-mentioned nano-copper particles are mixed with an organic carrier, and then degassed, rolled, and degassed again to obtain an antioxidant nano-copper paste.

[0053] To effectively remove air bubbles introduced during the mixing process and prevent voids from forming after sintering, in some embodiments, degassing is performed at a speed of 1000-2500 rpm for 3-15 minutes, preferably 5-8 minutes. Re-degassing can also be performed at a speed of 1000-2500 rpm for 3-15 minutes, preferably 5-8 minutes. The speed and degassing time can be the same as or different from those used during degassing.

[0054] In order to uniformly disperse the copper nanoparticles in the organic carrier during the rolling process and to break up any possible soft agglomerates, in some embodiments, the rolling is performed using a three-roll mill for 3 to 8 passes, with the roll gap gradually decreasing from 50 μm to 5 μm.

[0055] The fifth aspect of the present invention provides the application of the above-mentioned nano-copper paste in the packaging of semiconductor power devices, which can significantly reduce thermal stress and improve the thermal cycle life and reliability of power devices.

[0056] A sixth aspect of the present invention provides a method for fabricating an encapsulated interconnect structure, comprising the following steps: A) Pre-treat the lower substrate by coating the above-mentioned nano-copper paste onto the surface of the lower substrate; B) The upper substrate connection surface is attached to the lower substrate nano-copper paste to form a stacked structure; C) The stacked structure is sintered in an inert or reducing atmosphere to obtain an encapsulated interconnect structure.

[0057] In some embodiments, in step A), the lower substrate is selected from a pure copper substrate, a silver-plated substrate, a copper-clad ceramic substrate (DBC), or an active metal brazing ceramic substrate (AMB).

[0058] In some embodiments, the pretreatment includes: ultrasonically washing the substrate in anhydrous ethanol for 3-6 minutes, and then vacuum drying it at 40-60°C for 4-6 minutes to remove surface oil and moisture and ensure good wetting of the slurry with the substrate.

[0059] In some embodiments, in step B), the upper substrate is selected from a pure copper block, a silver-plated copper block, a silver-plated chip, a gold-plated chip, a silver-plated silicon wafer, or a gold-plated silicon wafer. A bonding pressure of 0.1~0.3 MPa can be applied during bonding to ensure uniform contact between the chip and the copper paste, avoiding internal defects caused by the bonding process.

[0060] In some embodiments, step C) of sintering includes: first, heating to 100-140°C at a heating rate of 8-12°C / min and holding for 5-15 min to allow the low-boiling-point solvent in the organic carrier to evaporate; then heating to 180-210°C at a heating rate of 18-22°C / min and holding for 20-30 min to sinter the nano-copper particles. No pressure is applied throughout the sintering process. An inert atmosphere (such as nitrogen and / or argon) or a reducing atmosphere (such as a formic acid / nitrogen mixture) can prevent copper oxidation. Experiments show that after sintering at 200°C for 30 min, the relative density of the bonding layer can reach over 85%, and the shear strength is ≥30 MPa.

[0061] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0062] Example 1 1. Etching the surface of nano-copper particles Combination Figure 1 As shown, the surface corrosion of the nano-copper particles is as follows: 1) At room temperature, 10 g of copper nanoparticles with a particle size of 300-500 nm and 0.3 g of PVP K30 were added to 90 ml of anhydrous ethanol. The mixture was first mechanically stirred at 300 rpm for 10 min, and then placed in an ultrasonic cleaner for ultrasonic dispersion for 20 min to obtain suspension A.

[0063] 2) At room temperature, measure 30 ml of 28% ammonia water, 50 ml of 30% hydrogen peroxide, and 100 ml of deionized water, mix them evenly (volume ratio 3:5:10), and then add 20 ml of 10 mmol / L KCl solution. The final KCl concentration is about 1 mmol / L, and the corrosion solution B is obtained.

[0064] 3) Place suspension A in an oil bath, maintain the temperature at 40℃, and stir at 300 rpm. Add corrosion solution B to suspension A and react for 60 min.

[0065] 4) At room temperature, centrifuge the resulting mixture after the reaction is complete to remove the supernatant. Add anhydrous ethanol to the obtained copper nanoparticles, sonicate for 20 min, and centrifuge to remove the supernatant. Repeat the operation 2-3 times. Dry the obtained copper nanoparticles under nitrogen as a protective gas to obtain dry copper nanoparticles.

[0066] 2. Preparation of pressureless sintering copper paste (nano copper paste) Combination Figure 2 The preparation process of pressureless sintering copper paste is as follows: 1) At room temperature, add 30% diethylene glycol, 30% terpineol, 20% propylene glycol and 20% glycerol to the same container by mass fraction, mix, and then add 3% ethyl cellulose by mass ratio of organic solution to obtain organic solution C.

[0067] 2) At room temperature, 90% of the dried and surface-corroded nano-copper particles were mixed with 10% organic solution C, and vacuum degassed at 2000 rpm for 8 min.

[0068] 3) The obtained paste (slurry) is rolled 6 times by a three-roll mill, and the roller gap is reduced from 50 μm to 5 μm; then vacuum degassing is performed at 1500 rpm for 5 min to obtain pressureless sintering copper paste.

[0069] 3. Packaging and interconnection Combination Figure 3 The encapsulation and interconnection process is as follows: The upper substrate of the interconnect structure is made of pure copper, and the lower substrate is made of pure copper block. The area of ​​the upper substrate is 3×3mm. 2 The area of ​​the lower substrate is 10×10 mm. 2 The interconnection process is as follows: 1) Process the substrate by immersing the pure copper substrate in anhydrous ethanol and ultrasonically washing it for 5 minutes to remove impurities from its surface, and then drying it in a vacuum drying oven at 50°C for 5 minutes to remove the anhydrous ethanol from the surface of the copper substrate.

[0070] 2) The sintered copper paste is uniformly coated onto the surface of the lower substrate treated in step 1) using a semi-automatic printing machine with stencil printing. The coating amount of the sintered copper paste is 5×5 mm. 2 The thickness is 100 μm, forming a sintered copper paste-substrate stacked structure.

[0071] 3) The upper substrate connection surface is attached to the above-mentioned sintered copper paste-lower substrate stack structure. The stack structure is placed in a sintering machine for pressureless sintering. Nitrogen gas is introduced, and the temperature is first raised to 130°C at a heating rate of 10°C / min under an inert atmosphere, and held at 130°C for 10 min; then the temperature is raised to 200°C at a heating rate of 20°C / min, and held at 200°C for 30 min. The sintered copper paste coating is sintered to form a connection layer, resulting in an encapsulated interconnect structure.

[0072] Scanning electron microscope images of the sintered microstructure as follows Figure 4 As shown. Figure 4 The results show that the copper paste prepared in this embodiment exhibits better sintering performance during the sintering process, and a dense sintering neck is formed between the nano-copper particles.

[0073] Ultrasonic non-destructive scanning image of the sintered bonding layer, as shown Figure 6 As shown. Figure 6 The results show that the bonding layer formed by sintering the copper paste prepared in this embodiment has a uniform structure and no obvious defects.

[0074] The sintering strength of the packaged interconnect structure was tested using a shearing machine, and the results are shown in Table 1 below.

[0075] Example 2 Except for the difference in etching the surface of the nano-copper particles (2), the rest is basically the same as in Example 1. Step 2) of this example is as follows: at room temperature, 10 g of citric acid (0.05 mol / L), 3 g of ascorbic acid (0.017 mol / L), 0.015 g of KCl (2 mmol / L), and 100 ml of deionized water are measured, mixed evenly, and the pH is adjusted to 4.0 to obtain etching solution B.

[0076] The sintering strength of the packaged interconnect structure was tested using the method in Example 1, and the results are shown in Table 1 below.

[0077] Compare with Example 1 Except for the use of uncorroded nano-copper particles, the preparation of pressureless sintering copper paste and the encapsulation interconnection process are the same as in Example 1. Scanning electron microscope images of the sintered microstructure as follows Figure 5 As shown. Figure 5 The results showed that the copper paste prepared in this control example had poor sintering performance, and no obvious sintering necks were formed between the nano-copper particles.

[0078] Ultrasonic non-destructive scanning image of the sintered bonding layer, as shown Figure 7 As shown. Figure 7 The results show that the bonding layer formed by sintering the copper paste prepared in this control example has an uneven microstructure and obvious defects.

[0079] The sintering strength of the packaged interconnect structure was tested using the method in Example 1, and the results are shown in Table 1 below.

[0080] Compare with Example 2 Except that the nano-copper particles were not etched with a halogen-containing solution (i.e., no KCl solution was added to the etchant B), the rest was basically the same as in Example 1.

[0081] Table 1 Performance test results of the packaged interconnect structure

[0082] Performance testing The copper pastes from Example 1 and Comparative Example 1 were stored at room temperature for one year. The surface oxygen content of the copper nanoparticles within the paste was measured monthly. Sintering was performed according to the encapsulation interconnection steps, and the sintering strength of the encapsulated interconnection structure was tested using the method of Example 1. The measured surface oxygen content of the copper nanoparticles is as follows: Figure 8 As shown, the joint shear strength is as follows Figure 9 As shown.

[0083] from Figure 8 It can be seen that the surface oxygen content of the nano-copper particles in the copper paste prepared in Example 1 is less than 2%. Even after six months of storage, the surface oxygen content of the nano-copper particles remains below 4%, and the surface oxygen content of the nano-copper particles increases slowly with the increase of storage time. In contrast, the surface oxygen content of the nano-copper particles in the copper paste prepared in Comparative Example 1 initially reached more than 5%, and the surface oxygen content of the nano-copper particles increased rapidly with the increase of storage time.

[0084] from Figure 9 It can be seen that the copper paste prepared in Example 1 has a shear strength of nearly 40 MPa. After six months of storage, the shear strength remains basically unchanged, and even after twelve months of storage, the shear strength remains above 35 MPa. In contrast, the copper paste prepared in Comparative Example 1 has a shear strength only slightly higher than 20 MPa. As the storage time increases, the shear strength decreases rapidly. After six months of storage, the shear strength drops to about 10 MPa, and after twelve months of storage, the shear strength drops to about 5 MPa.

[0085] The results of the above examples and comparative examples show that the present invention significantly improves the exposure ratio of the (111) crystal plane by anisotropically chemically etching the nano-copper particles and utilizing the selective adsorption and dissolution of halide ions on the crystal plane. The prepared nano-copper particles have excellent room temperature oxidation resistance. The nano-copper slurry prepared from it can still maintain a surface oxygen content below 5 at% after 6 months of storage in room temperature air, and even after 12 months it is still below 5 at% and can be sintered at 200°C under pressureless conditions to form a dense bonding layer with a shear strength of over 30 MPa. In contrast, the nano-copper particles without etching treatment (Comparative Example 1) or without halide ion etching (Comparative Example 2) have a low (111) crystal plane exposure ratio, poor oxidation resistance, rapid increase in oxygen content after storage, and a significant decrease in sintering strength, which cannot meet the application requirements of long-term storage and low-temperature pressureless sintering.

[0086] Therefore, the method of the present invention fundamentally solves the problems of easy oxidation and difficulty in long-term storage of nano-copper particles through crystal plane engineering, and achieves high-strength sintering at 200℃ without pressure, which has broad application prospects in the field of power semiconductor packaging.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An antioxidant copper nanoparticle, characterized in that, The nano-copper particles are prepared by a method comprising the following steps: contacting the nano-copper particles with a composite etching solution containing halide ions to perform anisotropic chemical etching treatment, thereby increasing the exposure ratio of the (111) crystal plane and decreasing the exposure ratio of the (110) and (100) crystal planes on the surface of the nano-copper particles.

2. The antioxidant copper nanoparticles according to claim 1, characterized in that, The surface of the nano-copper particles is dominated by (111) crystal planes, and the exposure ratio of (110) crystal planes and (100) crystal planes is reduced compared with uncorroded nano-copper particles. Preferably, the average particle size of the copper nanoparticles is 30 nm to 1 μm, and more preferably 50 nm to 500 nm. Preferably, the morphology of the copper nanoparticles is spherical or near-spherical.

3. A method for preparing the nano-copper particles according to claim 1 or 2, characterized in that, Includes the following steps: S1: Add the nano-copper particles to the organic dispersion medium to prepare suspension A; S2: Add a composite corrosive solution B containing halide ions to suspension A and carry out anisotropic chemical corrosion reaction treatment. S3: The mixed solution obtained after S2 treatment is centrifuged, washed, and dried to obtain antioxidant copper nanoparticles.

4. The preparation method according to claim 3, characterized in that, In step S1, the solid content of suspension A is 5wt%~20wt%; Preferably, the organic dispersion medium is composed of a main solvent and a dispersant. The main solvent is selected from one or more of anhydrous ethanol, isopropanol, ethylene glycol, diethylene glycol, and glycerol, accounting for 85 wt% to 99.9 wt% of the organic dispersion medium, preferably 85 wt% to 97 wt%. The dispersant is selected from one or more of polyvinylpyrrolidone, polyethylene glycol, oleic acid, and cetyltrimethylammonium bromide, accounting for 0.1 wt% to 15 wt% of the organic dispersion medium, preferably 3 wt% to 15 wt%. Preferably, after the nano-copper particles are added to the organic dispersion medium, they are first mechanically stirred at a speed of 200~500 rpm for more than 5 minutes, preferably 10~30 minutes, and then ultrasonically dispersed for more than 10 minutes, preferably 20~50 minutes.

5. The preparation method according to claim 3 or 4, characterized in that, In step S2, the composite etching solution B includes a crystal plane selector; Preferably, the crystal plane selector is selected from one or more of NaCl, KCl, NaBr, and KI, and its concentration in the composite etching solution B is 1~10 mmol / L; Preferably, the composite corrosive solution B further includes one or more of the following: oxidant, complexing agent, acid reagent, pH adjuster, and diluent; Preferably, the oxidant is selected from one or more of hydrogen peroxide, ammonium persulfate, and cerium ammonium nitrate, and the amount used is 0.2 to 1.5 times the volume of suspension A; Preferably, the complexing agent is selected from one or more of ammonia, ethylenediamine, and triethanolamine, and the amount used is 0.1 to 1.0 times the volume of suspension A; Preferably, the acid reagent is selected from one or more of nitric acid, sulfuric acid, citric acid, and ascorbic acid, and the amount used is 0.05 to 0.5 times the volume of suspension A; Preferably, the pH adjuster is selected from one or more of ammonia, ethylenediamine, and triethanolamine; Preferably, the diluent is deionized water, and the amount used is 1 to 3 times the volume of suspension A; Preferably, the pH of the composite corrosion solution B is adjusted to 3-5; Preferably, the chemical corrosion reaction is carried out at a temperature of 35-60°C and a stirring speed of 200-800 rpm for 30-120 min, and more preferably at a temperature of 35-45°C and a stirring speed of 300-500 rpm for 60-90 min. Preferably, in step S3, the separated copper nanoparticles are washed 2-3 times with anhydrous ethanol, and then dried under a protective atmosphere.

6. An antioxidant nano-copper paste, characterized in that, The nano-copper particles described in claim 1 or 2, or nano-copper particles prepared by any one of claims 3 to 5, and an organic carrier are included; after the nano-copper slurry is stored in a room temperature air environment for 6 months, the surface atomic percentage of oxygen on the surface of the nano-copper particles is ≤5%, and it can be sintered at 200°C under pressureless conditions to form a dense bonding layer with a shear strength of 30 MPa or more.

7. The nano-copper paste according to claim 6, characterized in that, The organic carrier includes volatile alcoholic organic compounds and binders; Preferably, the volatile alcoholic organic compounds include one or more of ethylene glycol, diethylene glycol, terpineol, propylene glycol, glycerol, butanediol, and hexanediol; Preferably, the volatile alcoholic organic compounds include, by weight, 20-25 parts diethylene glycol, 20-35 parts terpineol, 20-30 parts propylene glycol, and 20-25 parts glycerol; Preferably, the binder comprises one or more of methylcellulose, hydroxypropyl methylcellulose, hydroxyethylcellulose, and ethylcellulose; Preferably, the mass ratio of the volatile alcoholic organic compound and the binder in the organic carrier is 100:1~15, more preferably 100:2~8; Preferably, the mass percentage of the nano-copper particles in the nano-copper paste is 80%~93%, more preferably 88%~91%, and the mass percentage of the organic carrier is 7%~20%, more preferably 9%~12%.

8. A method for preparing the nano-copper paste according to claim 6 or 7, characterized in that, The nano-copper particles described in claim 1 or 2, or the nano-copper particles prepared by any one of the preparation methods described in claims 3 to 5, are mixed with an organic carrier, and then degassed, rolled, and degassed again to obtain the nano-copper slurry. Preferably, degassing is performed at a speed of 1000~2500 rpm for 3~15 min; Preferably, the rolling process is performed using a three-roll mill for 3 to 8 passes, with the roll gap gradually decreasing from 50 μm to 5 μm.

9. The application of the nano-copper paste according to claim 6 or 7 in semiconductor power device packaging.

10. A method for fabricating an encapsulated interconnect structure, characterized in that, Includes the following steps: A) Pre-treat the lower substrate by coating the nano-copper paste as described in claim 6 or 7 onto the surface of the lower substrate; B) The upper substrate connection surface is attached to the lower substrate nano-copper paste to form a stacked structure; C) The stacked structure is sintered in an inert or reducing atmosphere to obtain an encapsulated interconnect structure; Preferably, the lower substrate in step A) is selected from a pure copper substrate, a silver-plated substrate, a copper-clad ceramic substrate, or an active metal brazing ceramic substrate. Preferably, the pretreatment includes: ultrasonically washing the substrate in anhydrous ethanol for 3-6 min, and then vacuum drying it at 40-60°C for 4-6 min; Preferably, the upper substrate in step B) is selected from pure copper block, silver-plated copper block, silver-plated chip, gold-plated chip, silver-plated silicon wafer or gold-plated silicon wafer; Preferably, the sintering in step C) includes: first heating to 100-140°C at a heating rate of 8-12°C / min and holding for 5-15 min; then heating to 180-210°C at a heating rate of 18-22°C / min and holding for 20-30 min. Preferably, no pressure is applied during the entire sintering process.

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