Objective In recent years, nanowires have achieved great progress in the fields of nanoelectronics, nanophotonics, nanomedicine, and nanoelectromechanical systems. Nanowires play an important role in the miniaturization of micro/nanoelectronic devices. However, the effective interconnection of nanowires is an extremely critical concern in nanomanufacturing. Thus, a good welding method is required to attain effective interconnection of the nanowires. Recently, among the nanowire interconnection methods, the braze welding method presents less damage to the base material than other methods and has the advantages of homogenous and heterogeneous connections of the nanowires. Because of that, it has the greatest potential among all methods. Nevertheless, it is very difficult to locate and transport the nanosolders, which limit the development of nano brazing technology. Therefore, it is important to study the positioning and assembling of flexible nanosolders and nanowires to solve this problem. As a “bottom-up” manufacturing, assembling, and testing method, nanomanipulation technology is an important method to obtain a new generation of electronic components, optical device manufacturing, and nanomaterial manipulation and testing. Among the nanomanipulation technologies, the nanomanipulation technology based on scanning electron microscope (SEM) is suitable for the assembly of nanosolders and nanowires because it has several benefits such as real-time imaging and large operating space. Simultaneously, it has a focused electron beam with high energy, appropriating for performing the work of nanosolders melting.
Methods Based on the above-mentioned limitations of the nanowire braze welding, a new method that can facilitate in situ assemble and braze the interconnection structure of nanowires and nanosolders under SEM is proposed. This method is basically divided into three steps. At first, a self-built SEM nanomanipulation platform was used to finish all steps (Fig.1). This platform consists of an SEM observation system, a double probe nanomanipulation system, the electrical test module, the nanomanipulation sample stage, and the tungsten probe with a tip diameter of 300 nm. With the ZnO nanowires as the target nanowires and Ag nanoclusters as the solder, the nanomanipulation uses the nanoprecision manipulator that clamps the tungsten probe to assemble the interconnection structure of ZnO nanowires and push Ag nanoclusters to the interconnection joint of the interconnection, which complete the assembly of brazing interconnection structure for nanowires and nanosolder on the same base. Secondly, this study uses the SEM in situ focused electron beam to accurately irradiate the nanosolders, making them absorb energy and produce melting, thus achieving the brazing of nanowire interconnections. Finally, double probes of the nanomanipulation system were used to finish the I-V tests on the soldered nanowires and to verify whether the soldering was successfully achieved.
Results and Discussions The melting experiment of a single Ag nanocluster irradiated by the electron beam was completed. Thus, through focused electron beam irradiation for approximately 5 min, the nanocluster achieved local melting, which confirmed the feasibility of melting the Ag nanosolders with the electron beam (Fig.5). Using the nanomanipulation platform, the assembly experiment that includes the interconnection structure of Ag nanoclusters with diameters of ~188, 200, 243, and 290 nm and nanowires with a diameter of ~100 nm was completed using tungsten probes (Fig.7). The electron beam focusing was then used to obtain joints with different sizes. The nanoclusters composed of multiple nanoparticles achieved edge melting and bonding after approximately 5 min of irradiation (Fig.8). As the size of the solder increases, the irradiation time is longer, and the nanowires are also slightly damaged by the irradiation. But if the size is too small, the difficulty of nanomanipulation increases. Therefore, for 100-nm nanowires, the transportation of nanosolders of 200--250 nm can ensure high efficiency of pushing and can also that of the joint connection is not too large at the same time. Finally, the I-V electrical test was accomplished by double-probe contacting the two ends of the interconnected nanowires. The curve is basically linear and symmetrical, indicating that the ZnO nanowires and Ag solder have formed a satisfactory ohmic contact (Fig.9). The resistance of the loop calculated using the I-V curve is ~0.3 MΩ and the resistivity ρ=1.69 Ω·cm; this value is very similar to that previously reported, confirming the reliability of this method.
Conclusions In this study, based on the SEM nanomanipulation platform, the assembly and interconnection of nanowires and nanosolders were achieved successfully. The assembly and interconnection processes were integrated flexibly in situ. Moreover, the electrical testing of the structure after welding was completed in situ. The nanomanipulator manipulated the nanoscale probe to complete the two-dimensional operation of the ZnO nanowires with a diameter of ~100 nm and Ag nanosolders with a diameter of 180--300 nm, and the ZnO-Ag-ZnO nanowire interconnection structure was assembled successfully. The experiment of the electron beam focusing melting was processed and a high beam brazed joint of the nanowire interconnection structure achieved a good interconnection of the ZnO nanowires. The in situ electrical performance test was performed using the double-probe as the electrode. The I-V curve of the ZnO nanowires after brazing was obtained and the calculated resistivity is ρ=1.69 Ω·cm, confirming the reliability of the electron beam irradiation brazing method. The proposed research method is flexible and has a high success rate, providing an important reference for future homogenous and heterogeneous nanowire soldering interconnections and their applications.
