Objective Copper (Cu) nanoparticle exhibits high potential as an interconnecting material in electronic devices due to its relatively lower cost and similar conductivity compared with other noble metals. The interconnection between Cu nanoparticles can optimize the electrical conductivity and optical and mechanical properties of fabricated Cu microstructures. Compared with other traditional joining technologies, laser-induced nanojoining has the advantages of high precision, low damage, and high efficiency. In particular, femtosecond lasers with high peak power and ultrashort pulse duration would limit the heat-affected zone and result in less damage of joint than other lasers with longer pulse duration or continuous wave. When femtosecond laser pulse interacts with metallic nanomaterials, electrons absorb photons and quickly reach a higher temperature, while the lattice remains unchanged, resulting in less thermal effect and local melting during processing. It is expected to have potential in joining materials at nanoscale. At present, some reports focus on the reduction of Cu nanoparticles by femtosecond laser irradiation, whereas the effect of femtosecond laser on the joining process of Cu nanoparticles is yet to be understood. The joining mechanism and laser thermal effect on the joining of Cu nanoparticles need to be given more effort to optimize the femtosecond laser processing. In this work, femtosecond laser direct-writing is used to in-situ reduce Cu nanoparticles and join them to form a conductive copper microstructure. The effect of laser power on the composition, microstructure, and conductivity of Cu microstructures are investigated. Furthermore, the effect of single-shot laser pulses on the electron and lattice temperature in the “hotspot” between a Cu nanoparticle dimer is calculated. Simulation experimental results are compared to understand the joining process and mechanism of Cu nanoparticles under femtosecond laser irradiation.
Methods In a typical experimental procedure, the aqueous solution of polymethacrylic acid sodium salt (PMAA-Na, 30%, 1 μL), and polyvinyl pyrrolidone (PVP, 0.25 g/mL, 1200 μL) are added to the aqueous solution of copper nitrate hydrate (Cu(NO3)2·3H2O, 1.208 g/mL, 1000 μL) to form a Cu ion precursor. The as-prepared Cu ion precursor (200 μL) is coated on a polycarbonate flexible substrate (PC, 2.5 cm × 5 cm) and then dried at 50 ℃ in an oven. The femtosecond laser is used to scan the dry precursor film to reduce Cu ion to Cu nanoparticles and join the nanoparticles to form a conductive Cu microstructure. After laser writing, deionized water is used to clean the Cu microstructure to leave the as-written structures on a substrate. The electrical properties of the Cu microstructure are measured with a source meter using the four-point probe method. Then, the morphology of the Cu microstructure is characterized by field emission scanning electron microscopy and high-resolution transmission electron microscopy. X-ray diffraction is used to verify the chemical composition of the Cu microstructure. The effect of laser power on the chemical composition, microstructure, and conductivity of Cu are studied. Finally, the electric field and temperature field distribution characteristics of the Cu nanoparticle dimer under femtosecond laser irradiation are simulated using COMSOL Multiphysics, and the effect of single-shot laser pulses on the electron and lattice temperature of Cu nanoparticles is calculated.
Results and Discussions The sheet resistance of as-fabricated Cu microstructure presents a tendency to decrease first and then slowly increase with the increase of laser power (Fig.1). The Cu microstructure obtained at 960 mW laser power exhibits the lower sheet resistance of 11.2 Ω·sq -1. When the laser power is 322 mW, insufficient laser energy input results in the reduction of only a few dispersed Cu nanoparticles from the precursor, leading to high sheet resistance. As the laser power increases to 960 mW, more Cu nanoparticles are reduced and joined to form a dense network structure because of more hot-spots induced by the plasmonic effect, which greatly enhances its conductivity (Fig.3). Further increasing the laser power to 1690 mW or above, the high local temperature can melt Cu nanoparticles to form large micron-sized Cu, resulting in the increased sheet resistance. The simulation results show that the lattice temperature at the contact area of the Cu nanoparticle dimer increases as the incident laser power increases (Fig.5). When the laser power is 960 mW, the lattice temperature of the “hotspot” between the Cu nanoparticle dimer is up to 698 K. It induces surface melting of Cu nanoparticles and facilitates their interconnection (Fig.6). As the laser power increases to 1690 mW, the lattice temperature increases to 1175 K, resulting in intensive melting and interconnection of nanoparticles. These also have been observed in the experiment.
Conclusions In this work, femtosecond laser direct-writing was used to reduce Cu nanoparticles and in-situ joins them to fabricate the Cu microstructure with high conductivity. The Cu microstructure obtained at 960 mW laser power and 3 mm/s scan rate exhibited the lowest sheet resistance of 11.2 Ω sq -1. A two-temperature model during single-pulse femtosecond laser irradiation was employed to calculate the electron and lattice temperature of Cu nanoparticles using COMSOL Multiphysics. Laser-induced localized surface plasmon effect on the Cu nanoparticle dimer enhanced the local temperature greatly at the contact area of Cu nanoparticles, contributing to the interconnection of nanoparticles. As the incident laser power increased, the lattice temperature at the contact area of the Cu nanoparticle dimer increased, leading to intensive joining. When the laser power was 960 mW, the lattice temperature of the “hotspot” between the Cu nanoparticle dimer was up to 698 K, which can cause surface melting to facilitate joining. The consistent experimental and simulation results provide a further understanding of the joining process and mechanism of Cu nanoparticles under femtosecond laser irradiation.
