A method for bonding surface-nanosized copper pillars in an air environment

Abstract

This invention discloses a method for bonding surface-nanosized copper pillars in an air environment, belonging to the field of electronic packaging. The method includes the following steps: depositing Ti and Cu metals on a silicon substrate; preparing an array pattern using photolithography and development; obtaining a copper pillar array through electrodeposition; preparing a Cu nanowire array through anodic oxidation and electrochemical reduction; and preparing, coating, and hot-pressing an organic reducing agent. This invention can fabricate patterned copper nanowire array bumps and, combined with high-temperature resistant photoresist as a thermally conductive medium, eliminates the need for additional filling steps. It can achieve high-quality copper-copper interconnects in a low-temperature air environment, overcoming the excessively high temperature of copper interconnects and dependence on the bonding environment. The copper nanowires of a certain height achieve compression perpendicular to the substrate during interconnection, reducing the requirements for surface flatness and eliminating the need for chemical mechanical polishing, while also improving interconnect defects caused by alignment deviations. This invention features a simple and low-cost fabrication process, suitable for low-temperature interconnect technology used in 3D packaging and ultra-fine pitch packaging.

Description

Technical Field

[0001] This invention belongs to the field of electronic packaging, and specifically relates to a method for bonding surface-nanosized copper pillars in an air environment. Background Technology

[0002] With the trend of miniaturization in electronic products, the size of chips and other electronic components is getting smaller and smaller, and the requirements for electronic packaging interconnects are also getting higher and higher. Traditional Sn-based solder joints will suffer from major reliability problems such as solder extrusion, bridging, and fracture of brittle Cu-Sn intermetallic compounds when the interconnect spacing is reduced. They are difficult to meet the packaging requirements of extremely high I / O density chips in emerging industries (such as artificial intelligence, autonomous driving, Internet of Things, 5G networks, etc.), and there is an urgent need to explore more suitable interconnect solutions.

[0003] Cu-Cu bump interconnects are considered an interconnect technology with higher density and reliability. Compared to Sn-based solder joints, Cu-Cu joints do not experience bridging between adjacent pads, allowing for higher interconnect densities. Furthermore, Cu exhibits superior resistance to electromigration, electrical conductivity, and thermal conductivity, along with good machinability and low cost. Despite these significant advantages, Cu-Cu joints require high temperature, high pressure, high surface flatness, and high vacuum or reducing environments, limiting their widespread industrial application.

[0004] Reducing the temperature, pressure, atmosphere, and flatness requirements of copper-copper interconnects is crucial for the widespread application of this technology. Researchers have lowered the bonding temperature by fabricating nanocrystalline copper layers, used organic materials to prevent copper surface oxidation, and employed nanoporous or nanowire structures to reduce flatness requirements. Despite these efforts to reduce the stringent conditions required for copper-copper bonding, several challenges remain, including the complex fabrication process of copper nanowires (requiring reduction with H2), the limited function of organic protective layers (only preventing copper oxidation), and gaps between the substrates after bonding leading to poor heat dissipation. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for achieving direct Cu-Cu bonding interconnects using an organic layer to protect copper nanowires in a low-temperature, low-pressure air environment. In this method, the copper nanowires are prepared via electrochemical methods, the organic film prevents oxidation of the copper nanowires, and the organic layer, combined with a suitable hot-pressing process, does not hinder the diffusion of copper atoms at the bonding interface. The copper nanowire interconnect structure requires no chemical mechanical polishing, resulting in joints with high strength and excellent electrical properties.

[0006] This invention enables the fabrication of patterned copper nanowire array bumps, utilizing a high-temperature resistant photoresist as a thermally conductive medium without requiring additional filling steps. It achieves high-quality copper-copper interconnects in low-temperature air environments, overcoming the limitations of excessively high copper interconnect temperatures and dependence on the bonding environment. The copper nanowires of a certain height achieve compression perpendicular to the substrate during interconnection, reducing the requirements for surface flatness and eliminating the need for chemical mechanical polishing. This also improves interconnect defects caused by alignment misalignment. The fabrication process of this invention is simple and low-cost, suitable for low-temperature interconnect technology in 3D packaging and ultra-fine pitch packaging.

[0007] The technical solution adopted in this invention is:

[0008] A method for bonding surface-nanosized copper pillars in an air environment includes the following steps:

[0009] (1) First, deposit a Ti seed layer on the silicon substrate, then deposit a Cu seed layer; then apply photoresist to the silicon substrate by spin coating, and bake, expose and develop in sequence to form a cylinder;

[0010] (2) Preparation of nanowire solution for copper hydroxide (Cu(OH)2): Add sodium hydroxide to deionized water and stir until sodium hydroxide is completely dissolved;

[0011] (3) Electrodeposited copper pillar array: The silicon substrate after photolithography in step (1) is filled with copper by electroplating. After electroplating, it is cleaned with deionized water and dried with nitrogen.

[0012] (4) Preparation of Cu(OH)2 nanowires: The silicon substrate obtained in step (3) is used as the anode, the platinum sheet is used as the cathode, and the Ag / AgCl saturated potassium chloride electrode is used as the reference electrode. The cathode, anode and reference electrode are placed in the electrolytic cell of the nanowire solution obtained in step (2). The three-electrode system of the electrochemical workstation is used for oxidation, and the constant potential method is used for anodic oxidation. After the anodic oxidation is completed, the power is turned off, the silicon substrate of the anode is taken out from the nanowire solution, rinsed with deionized water and dried. Cu(OH)2 nanowires are formed at the top of the copper column.

[0013] (5) Anneal the Cu(OH)2 nanowires obtained in step (4) in air to thermally decompose the Cu(OH)2 nanowires into CuO nanowires;

[0014] (6) Prepare potassium hydroxide electroreduction solution: Add potassium hydroxide to deionized water and stir until completely dissolved;

[0015] (7) Electroreducing CuO nanowires to Cu nanowires: Using the silicon substrate with CuO nanowires grown in step (5) as the cathode, graphite as the anode, and Ag / AgCl saturated potassium chloride electrode as the reference electrode, in the potassium hydroxide electroreduction solution obtained in step (6), constant potential polarization is performed using an electrochemical workstation until the current stabilizes and the cathode surface changes from black to reddish-brown. CuO nanowires are electroreduced to Cu nanowires, and copper pillar array bumps with Cu nanowires on the surface are obtained on the silicon substrate.

[0016] (8) After electroreduction, the silicon substrate with Cu nanowire array bumps is placed in an organic reducing agent. In a low vacuum environment of less than 0.1 MPa, the organic material forms a dense protective film on the Cu nanowires. Then, the silicon substrate with Cu nanowire array bumps is taken out, the bumps of the two silicon substrates are aligned, and they are placed in a bonding machine for hot pressing sintering at 150-300℃ and 0-20 MPa for 10-30 minutes.

[0017] The diameters of the Cu(OH)2 nanowires and CuO nanowires are 50-200 nm.

[0018] Further preferred, in step (1), the thickness of the Ti seed layer is 10-100 nm and the thickness of the Cu seed layer is 100-500 nm.

[0019] Further preferred, in step (2), the concentration of sodium hydroxide is 0.1 to 1 M / L.

[0020] Further preferred, the working voltage applied in step (4) is -200mV to -100mV relative to the reference electrode.

[0021] Further optimization is that the Cu(OH)2 nanowire annealing temperature in step (5) is 180-220℃ and the annealing time is 1-3h.

[0022] Further preferred, in step (7), the concentration of potassium hydroxide is 0.1-1 M / L.

[0023] Further preferably, the working voltage applied in step (7) is -0.8V to -1.2V relative to the reference electrode.

[0024] Further preferred, the organic reducing agent used in step (8) is one or a mixture of two or more of the following: ethylene glycol, polyethylene glycol, polyvinylpyrrolidone, ascorbic acid, citric acid, glycerol, 3-dimethylamino-1,2-propanediol (DMAPD), formic acid, and acetic acid.

[0025] The beneficial effects of this invention are:

[0026] (1) The present invention uses a combination of thermal oxidation decomposition and electrochemical reduction to prepare copper nanowire array bumps with a diameter in the range of 50 to 200 nm on a silicon substrate, which reduces the temperature, pressure and other conditions required for direct copper-copper bonding.

[0027] (2) The coating of organic film on the surface of copper nanowires can prevent copper nanowires from oxidizing in the air environment. At the same time, the organic material can reduce copper oxide when heated, so that CuO is not easily generated when the copper nanowire array is hot-pressed in the air environment, thus avoiding the need for a vacuum or reducing environment.

[0028] (3) Copper nanowire array bumps can achieve compression deformation perpendicular to the substrate direction, eliminating the need for chemical mechanical polishing before bonding, and can also improve interconnection defects caused by alignment deviations.

[0029] (4) The preparation process of this invention is simple, low in cost, and has great application value. It is suitable for low-temperature interconnect technology for three-dimensional packaging and ultra-fine pitch packaging. Attached Figure Description

[0030] Figure 1 This is a flowchart of the experiment in the embodiment.

[0031] Figure 2 The images show the surface SEM images and EDS composition analysis results of the copper hydroxide nanowires prepared by anodic oxidation in Example 1.

[0032] Figure 3 The images show the surface SEM images and EDS composition analysis results of copper hydroxide nanowires decomposed by heating in Example 1.

[0033] Figure 4 The images show SEM images and EDS composition analysis results of copper nanowires prepared by electroreduction of copper oxide nanowires obtained in Example 1. Detailed Implementation

[0034] The embodiments of the present invention will be described in detail below with reference to examples. Unless otherwise specified, all reagents or instruments used are commercially available conventional products. The various embodiments employ... Figure 1 Follow the steps shown.

[0035] Example 1

[0036] (1) A 100nm Ti and 100nm Cu metal seed layer was magnetron sputtered on a silicon substrate, followed by spin coating of photoresist, baking, exposure and development.

[0037] (2) Electroplating of copper pillars: The substrate in step (1) is used as the cathode for electroplating of copper pillars. After filling, it is cleaned with deionized water and dried with cold air.

[0038] (3) Preparation of anodic copper nanowire solution: Add 4g sodium hydroxide to 100mL of deionized water, sonicate the solution for 10min, and then magnetically stir for 10min at a speed of 500r / min.

[0039] (4) Using an electrochemical workstation and a three-electrode system, the silicon wafer filled with copper in step (2) was used as the working electrode, a platinum electrode as the counter electrode, and Ag / AgCl saturated potassium chloride as the reference electrode. The distance between the working electrode and the counter electrode was 3 cm. The working electrode, counter electrode, and reference electrode were placed in an electrolytic cell containing sodium hydroxide solution for constant potential oxidation. The oxidation potential relative to the reference electrode was -200 mV, and the temperature was 25 °C.

[0040] (5) After oxidation for 2 hours, the electrochemical workstation was turned off, the working electrode was removed from the solution, rinsed with deionized water, and then dried to obtain Cu(OH)₂ nanowire array microbumps, such as... Figure 2 As shown in the SEM morphology image of the sample, the nanowire diameter is in the range of 100 nm.

[0041] (6) Cu(OH)₂ nanowires were placed in an annealing furnace and heated to 220℃ at a rate of 10℃ / min, held for 1 hour, and then air-cooled to obtain a CuO nanowire array, as shown below. Figure 3 As shown, the SEM image illustrates that the CuO obtained by annealing Cu(OH)2 nanowires still retains the nanowire morphology, and EDS composition analysis confirms that the annealing product is CuO.

[0042] (7) To prepare an electrochemical reduction CuO solution, add 0.56 g of potassium hydroxide to 100 mL of deionized water, sonicate for 10 min, and then magnetically stir for 10 min at a speed of 500 r / min.

[0043] (8) Using an electrochemical workstation with a three-electrode system, the sample from step (6) was used as the working electrode, a platinum sheet electrode as the counter electrode, and Ag / AgCl saturated potassium chloride as the reference electrode. The distance between the working electrode and the counter electrode was 3 cm. The working electrode, counter electrode, and reference electrode were placed in an electrolytic cell containing potassium hydroxide solution for constant potential polarization. The polarization potential relative to the reference electrode was -0.8 V, and the temperature was 25 °C. The polarization continued until the current stopped changing and the surface of the working electrode changed from black to reddish-brown. Figure 4 As shown, the SEM images demonstrate that the Cu obtained after electroreduction still retains the nanowire morphology, and the EDS composition analysis shows that the oxygen content of the electroreduction product is greatly reduced, proving that CuO is reduced to Cu.

[0044] (9) Remove the copper nanowire array substrate of the working electrode, clean the residual solution on the surface with ethylene glycol, and then place it in an ethylene glycol solution containing 0.5g pvp, 0.1g ascorbic acid and 0.5g glycerol. Let it stand in a low vacuum environment of 0.08Mpa for 20min to allow the organic reducing agent to be uniformly covered on the copper nanowire.

[0045] (10) Align the bumps of the copper nanowire array substrate coated with an organic reducing agent film, place it in a hot press bonding machine, apply a pressure of 2 MPa in air, heat to 250°C at 10°C / min, hold for 1 min, remove the bonded sample, and air cool to room temperature. Perform a shear strength test on the bonded joint, and the strength is higher than 10 MPa.

[0046] Example 2

[0047] (1) A 10nm Ti and 500nm Cu metal seed layer was magnetron sputtered on a silicon substrate, followed by spin coating of photoresist, baking, exposure and development.

[0048] (2) Electroplating of copper pillars: The substrate in step (1) is used as the cathode for electroplating of copper pillars. After filling, it is cleaned with deionized water and dried with cold air.

[0049] (3) Preparation of anodic copper nanowire solution: Add 0.4g sodium hydroxide to 100mL of deionized water, sonicate the solution for 10min, and then magnetically stir for 10min at a speed of 500r / min.

[0050] (4) Using an electrochemical workstation, a three-electrode system is adopted. The silicon wafer filled with copper in step (2) is used as the working electrode, the platinum electrode is used as the counter electrode, and the Ag / AgCl saturated potassium chloride is used as the reference electrode. The distance between the working electrode and the counter electrode is 3 cm. The working electrode, the counter electrode and the reference electrode are placed in an electrolytic cell containing sodium hydroxide solution for constant potential oxidation. The oxidation potential relative to the reference electrode is -150 mV and the temperature is 25 °C.

[0051] (5) After oxidation for 2 hours, the electrochemical workstation was turned off, the working electrode was removed from the solution, rinsed with deionized water and dried to obtain Cu(OH)2 nanowire array microbumps.

[0052] (6) Place Cu(OH)2 nanowires into an annealing furnace, raise the temperature to 200℃ at a rate of 10℃ / min and hold for 2 hours, and then air cool to obtain CuO nanowire array.

[0053] (7) To prepare an electrochemical reduction CuO solution, add 5.6g of potassium hydroxide to 100mL of deionized water, sonicate for 10min, and then magnetically stir for 10min at a speed of 500r / min.

[0054] (8) Using an electrochemical workstation and a three-electrode system, the sample from step (6) was used as the working electrode, a platinum sheet electrode as the counter electrode, and Ag / AgCl saturated potassium chloride as the reference electrode. The distance between the working electrode and the counter electrode was 3 cm. The working electrode, counter electrode, and reference electrode were placed in an electrolytic cell containing potassium hydroxide solution and subjected to constant potential polarization. The polarization potential relative to the reference electrode was -1.0 V, and the temperature was 25 °C. The polarization continued until the current stopped changing and the surface of the working electrode changed from black to reddish-brown.

[0055] (9) Remove the copper nanowire array substrate of the working electrode, clean the residual solution on the surface with ethylene glycol, and then place it in an ethylene glycol solution containing 0.5g pvp, 0.1g ascorbic acid and 0.5g glycerol. Let it stand in a low vacuum environment of 0.08Mpa for 20min to allow the organic reducing agent to be uniformly covered on the copper nanowire.

[0056] (10) Align the bumps of the copper nanowire array substrate coated with an organic reducing agent film, place it in a hot press bonding machine, apply a pressure of 5 MPa in air, first heat to 200°C at a heating rate of 50°C / min, hold for 10 min, then heat to 250°C at a rate of 10°C / min, hold for 60 min, remove the bonded sample, and air cool to room temperature. Perform a shear strength test on the bonded joint; the strength is higher than 10 MPa.

[0057] Example 3

[0058] (1) A 20nm Ti and 200nm metal seed layer was magnetron sputtered on a silicon substrate, followed by spin coating of photoresist, baking, exposure and development.

[0059] (2) Electroplating of copper pillars: The substrate in step (1) is used as the cathode for electroplating of copper pillars. After filling, it is cleaned with deionized water and dried with cold air.

[0060] (3) Preparation of anodic copper nanowire solution: Add 1.0g sodium hydroxide to 100mL of deionized water, sonicate the solution for 10min, and then magnetically stir for 10min at a speed of 500r / min.

[0061] (4) Using an electrochemical workstation and a three-electrode system, the silicon wafer filled with copper in step (2) was used as the working electrode, a platinum electrode as the counter electrode, and Ag / AgCl saturated potassium chloride as the reference electrode. The distance between the working electrode and the counter electrode was 3 cm. The working electrode, counter electrode, and reference electrode were placed in an electrolytic cell containing sodium hydroxide solution for constant potential oxidation. The oxidation potential relative to the reference electrode was -100 mV, and the temperature was 25 °C.

[0062] (5) After oxidation for 2 hours, the electrochemical workstation was turned off, the working electrode was removed from the solution, rinsed with deionized water and dried to obtain Cu(OH)2 nanowire array microbumps.

[0063] (6) Place Cu(OH)2 nanowires into an annealing furnace, raise the temperature to 200℃ at a rate of 10℃ / min and hold for 2 hours, and then air cool to obtain CuO nanowire array.

[0064] (7) To prepare an electrochemical reduction CuO solution, add 0.56 g of potassium hydroxide to 100 mL of deionized water, sonicate for 10 min, and then magnetically stir for 10 min at a speed of 500 r / min.

[0065] (8) Using an electrochemical workstation and a three-electrode system, the sample from step (6) was used as the working electrode, a platinum sheet electrode as the counter electrode, and Ag / AgCl saturated potassium chloride as the reference electrode. The distance between the working electrode and the counter electrode was 3 cm. The working electrode, counter electrode, and reference electrode were placed in an electrolytic cell containing potassium hydroxide solution and subjected to constant potential polarization. The polarization potential relative to the reference electrode was -1.2 V, and the temperature was 25 °C. The polarization continued until the current stopped changing and the surface of the working electrode changed from black to reddish-brown.

[0066] (9) Remove the copper nanowire array substrate of the working electrode, clean the residual solution on the surface with ethylene glycol, and then place it in glycerol containing 0.1g ascorbic acid. Let it stand in a low vacuum environment of 0.08Mpa for 20min so that the organic reducing agent is uniformly covered on the copper nanowire.

[0067] (10) Align the bumps of the copper nanowire array substrate coated with an organic reducing agent film, place it in a hot press bonding machine, apply a pressure of 5 MPa in air, first heat to 200°C at a heating rate of 10°C / min, hold for 10 min, then heat to 250°C at a rate of 10°C / min, hold for 20 min, remove the bonded sample, and air cool to room temperature. Perform a shear strength test on the bonded joint; the strength is higher than 10 MPa.

[0068] Example 4

[0069] The method described in Example 3 is the same, except that the component in step (9) is a mixture of 0.5g citric acid and 5g glycerol. Finally, the bonded joint was subjected to a shear strength test, and the strength was higher than 10 MPa.

[0070] Example 5

[0071] The method described in Example 3 is the same, except that the component in step (9) is a mixture of 0.5 g of 3-dimethylamino-1,2-propanediol (DMAPD) ​​and 5 g of glycerol. Finally, the bonded joint was subjected to a shear strength test, and the strength was higher than 10 MPa.

[0072] Example 6

[0073] The method described in Example 3 is the same, except that the component in step (9) is a mixture of 0.5g formic acid and 5g glycerol. Finally, the bonded joint was subjected to a shear strength test, and the strength was higher than 10 MPa.

[0074] Example 7

[0075] The method described in Example 3 is the same, except that the component in step (9) is a mixture of 0.5g acetic acid and 5g glycerol. Finally, the bonded joint was subjected to a shear strength test, and the strength was higher than 10 MPa.

[0076] Comparative Example 1

[0077] The copper nanowire interconnects were prepared according to the method described in Example 3, the difference from Example 1 being that the working voltage applied during the electrochemical reduction process was -0.6V relative to the reference electrode.

[0078] Since CuO needs to be electrochemically reduced to Cu at a certain potential, CuO nanowires cannot be converted into Cu nanowires when the applied working voltage is low, resulting in a decrease in the bonding strength of the copper nanowire interconnect joints.

[0079] Comparative Example 2

[0080] The copper nanowire interconnects were prepared according to the method described in Example 1, except that the organic reducing agent consisted of polyethylene glycol and ethylene glycol.

[0081] Organic reducing agents can prevent oxidation and reduce copper oxides. When the content of reducing organic matter is low, the oxidized copper nanowires cannot be reduced to copper, resulting in a decrease in the bonding strength of the copper nanowire interconnect joints.

Claims

1. A method for bonding surface-nanosized copper pillars in an air environment, characterized in that, Includes the following steps: (1) First, deposit a Ti seed layer on the silicon substrate, then deposit a Cu seed layer; then apply photoresist to the silicon substrate by spin coating, and bake, expose and develop in sequence. (2) Preparation of Cu(OH)2 nanowire solution: Add sodium hydroxide to deionized water and stir until sodium hydroxide is completely dissolved; (3) Electrodeposited copper pillar array: The silicon substrate after photolithography in step (1) is filled with copper by electroplating. After electroplating, it is cleaned with deionized water and dried with nitrogen. (4) Preparation of Cu(OH)2 nanowires: The silicon substrate obtained in step (3) is used as the anode, the platinum sheet is used as the cathode, and the Ag / AgCl saturated potassium chloride electrode is used as the reference electrode. The cathode, anode and reference electrode are placed in the electrolytic cell of the nanowire solution obtained in step (2). The three-electrode system of the electrochemical workstation is used for oxidation, and the anodic oxidation is carried out under a certain voltage range by constant potential. After the anodic oxidation is completed, the power is turned off, the silicon substrate of the anode is taken out from the nanowire solution, rinsed with deionized water and dried. Cu(OH)2 nanowires are formed at the top of the copper column. (5) Anneal the Cu(OH)2 nanowires obtained in step (4) in air to thermally decompose the Cu(OH)2 nanowires into CuO nanowires; (6) Prepare potassium hydroxide electroreduction solution: Add potassium hydroxide to deionized water and stir until completely dissolved; (7) Electroreducing CuO nanowires to Cu nanowires: Using the silicon substrate with CuO nanowires grown in step (5) as the cathode, graphite as the anode, and Ag / AgCl saturated potassium chloride electrode as the reference electrode, in the potassium hydroxide electroreduction solution obtained in step (6), constant potential polarization is performed using an electrochemical workstation until the current stabilizes and the cathode surface changes from black to reddish-brown. CuO nanowires are electroreduced to Cu nanowires, and copper pillar array bumps with Cu nanowires on the surface are obtained on the silicon substrate. (8) After electroreduction, the silicon substrate with Cu nanowire array bumps is placed in an organic reducing agent. In a low vacuum environment of less than 0.1 MPa, the organic material forms a dense protective film on the Cu nanowires. Then, the silicon substrate with Cu nanowire array bumps is taken out, the bumps of the two silicon substrates are aligned, and they are placed in a bonding machine for hot pressing sintering at 150-300℃ and 0-20 MPa for 10-30 minutes.

2. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, In step (1), the thickness of the Ti seed layer is 10-100 nm and the thickness of the Cu seed layer is 100-500 nm.

3. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, In step (2), the concentration of sodium hydroxide is 0.1 to 1 M / L.

4. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, The working voltage applied in step (4) is -200mV to -100mV relative to the reference electrode.

5. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, In step (5), the Cu(OH)2 nanowires are annealed at a temperature of 180-220℃ for 1-3 hours.

6. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, In step (7), the concentration of potassium hydroxide is 0.1-1 M / L.

7. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, The working voltage applied in step (7) is -0.8V to -1.2V relative to the reference electrode.

8. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, The organic reducing agent used in step (8) is one or a mixture of two or more of the following: ethylene glycol, polyethylene glycol, polyvinylpyrrolidone, ascorbic acid, citric acid, glycerol, 3-dimethylamino-1,2-propanediol, formic acid, and acetic acid.

9. The method for bonding surface-nanosized copper pillars in an air environment according to claim 1, characterized in that, The diameters of the Cu(OH)2 nanowires and CuO nanowires are 50-200 nm.

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