实现纳米材料焊接的纳米连接技术不仅是研制高性能纳米器件的关键, 还是 "自下而上" 进行纳米结构制造的重要手段, 决定着新一代纳米器件的性能及可靠性。其中, 纳米焊焊技术对焊接母材损伤小, 可以连接同种或不同种类的纳米材料, 并获得优异的力学及电学性能, 是纳米连接技术的重要发展方向。基于分子动力学, 对纳米颗粒焊料在 SiO₂ 基底上的激光熔融过程进行了仿真分析, 分析了激光辐照导致的不同温度下银纳米颗粒的原子构型变化, 并探究了焊料熔融过程。为了探讨基底对熔化过程的影响, 进一步分析了基底与纳米颗粒之间的接触角、接触长度以及吸附能变化等。研究结果表明 : 为了获得可靠的纳米互连结点, 激光辐照下温度对纳米颗粒与衬底之间吸附能的调控是影响纳米互连结点稳定性的主要因素。该仿真结果为实现激光熔融纳米颗粒的连接提供了参考。

Objective  As a technology of welding nanomaterials, nano-welding is not only an important "bottom-up" means for manufacturing nanostructures, but also a key technology for the development of high-performance integrated circuits with reliable interconnection points. Among all nano-welding methods, the nanometer brazing technology of melting nanomaterials under laser irradiation, as one of most reliable methods, is utilized to realize nano-device-level interconnection. This technology reduces the damage to the welding base materials, achieves the interconnection points with high mechanical strength, and even maintains the excellent electrical performance of the devices. However, the previous theoretical models of nano-welding have only considered the atomic configuration evolution process of nanoparticles at different temperatures, ignoring the effect of substrate materials on the energy exchange process for achieving the best welding quality. Moreover, the simulation of nanoparticle melting under laser irradiation without substrates cannot completely represent the evolution of actual atomic configuration of nanoparticles as a reliable interconnection node during the brazing process. Therefore, in view of the actual brazing process of nanometer brazing, the change of atomic configuration of Ag nanoparticles on a SiO₂ substrate under laser irradiation is simulated and analyzed. More importantly, the adsorption energy between the substrate and nanoparticles during the melting process is discussed in detail. These results provide a theoretical basis for the realization of actual nanometer brazing. 

Methods  To obtain the melting evolution process of nanoparticles at the atomic level under laser irradiation, molecular dynamics (MD) simulation based on classical mechanics is used for establishing the simulation model. In the simulation model, single and multiple Ag nanoparticles are considered. Also, amorphous silica is obtained by the energy minimization process for supporting an energy-exchanging substrate in the melting evolution process of nanoparticles. This paper simulates the melting process of silver nanoparticles induced by a laser. In the melting simulation, geometric structure optimization is first executed as an initial system state. The laser irradiation energy is applied by controlling the corresponding evolution temperature of an atomic structure. The melting process utilizes a canonical ensemble NVT to carry out the relaxation of an atomic configuration. The Nose-Hoover thermostat method is used to set the temperature and bath time for matching the requirement of energy exchange. The boundary condition is an aperiodic boundary. Three bottom atoms of the amorphous SiO₂ substrate are selected to exert fixed constraints in three directions in the simulation. After simulation, the atomic configuration and energy change are extracted and analyzed for the subsequent discussion of contact length, contact angle, and adhesion energy. 

Results and Discussions  When the applied temperature is low, the shape of silver nanoparticles is spherical. With the increase of applied temperature, the shape of silver nanoparticles gradually changes to a hemispherical shape (Fig. 2). The hemispherical shape is attributed to the restriction of the substrate at the interface between nanoparticles and SiO₂ substrate during the evolution of atomic configuration. The changes of contact length and contact angle between silver nanoparticles and substrate at different temperatures are analyzed [Fig. 3(a)]. The contact length and contact angle increase first and then reach a flat state with the increase of temperature. The adsorption energy between a single silver nanoparticle and an amorphous SiO₂ substrate versus temperature is discussed [Fig. 3(b)]. When the applied temperature is 4001000 K, the adsorption energy increases linearly with temperature. When the temperature is higher than 1000 K, the adsorption decreases rapidly. The changes of the atomic configuration of two silver nanoparticles on the amorphous SiO₂ substrate at different time are conducted (Fig. 4). The original two nanoparticles fuse into one after high-temperature relaxation. The adsorption energy of two silver nanoparticles melted on the substrate is significantly higher than that of a single nanoparticle, which was attributed to the increase of contact area (Fig. 7).

Conclusions  In summary, based on the molecular dynamics method, the evolution process of the atomic configuration of 4 nm-diameter nanoparticles on SiO₂ substrate at different temperatures is discussed, and the melting process of nanoparticles caused by laser irradiation in the actual brazing process is simulated. When the temperature reaches 800 K, the atomic configuration of a single Ag nanoparticle forms a hemispherical shape, and the adsorption capacity of a single Ag nanoparticle reaches the maximum at 1000 K. At a temperature of 1200 K, the atomic lattice change, sintering neck formation, and melting of two nanoparticles into a single particle occur. The atomic configuration can completely form a single nano-interconnection node. The adsorption capacity with a SiO₂ substrate can reach the maximum, higher than the adsorption capacity of a single particle as an interconnection node. In addition, the adsorption energy increases first and then decreases with temperature based on different numbers of Ag nanoparticles and the SiO₂ substrate. Therefore, there is an optimal critical temperature to maximize the adsorption energy and to ensure a stable nano-interconnect structure after welding. The above simulation results lay a theoretical foundation for the subsequent realization of laser melting of nanoparticles and nanomaterial brazing.