Significance With the development of new electronic devices for miniaturization, flexibility and intelligence, the diversity of nanomaterial properties and limitations of traditional electrical connection methods bring new challenges in new electronic device preparation. Researchers are encouraged to continue to explore ways to break the limit of the device size. The manufacturing technology has gradually developed to the nanoscale level. Nanowelding is one of the key technologies for integrating nanomaterials with micro and macro systems.
Metal nanomaterials (e.g., Ag, Au, and Cu) and some carbon-based nanomaterials (e.g., carbon nanotubes, and graphene) exhibit excellent electrical and thermal properties. Besides, some wide bandgap semiconductor nanomaterials (e.g., ZnO) have shown great potential in future electronic devices. Not only for homogenous connections but also for the study of electrical and mechanical properties of heterogeneous connections, evaluating their mechanical and electrical properties is crucial for predicting the failure modes of electronic devices.
Stable device performance depends on reliable nanointerconnected structures. The size effect and high specific surface area of nanomaterials make them exhibit different welding characteristics from bulk materials during the welding process. The study on the electrical performance of nanowelding consists of single joints and interconnection networks. The study of a single nanojoint is essential to deepen the understanding of the welding mechanism. For interconnection networks, especially with the rapid development of industries, such as smart touch interactive terminals and wearable electronic equipment flexible solar devices, their performance has attracted significant attention.
Progress Currently, the electrical and mechanical characterization of nanoconnection quality consists of two methods. The first method is aimed at electrical testing and characterization of nanoconnected single-welded joints, such as direct in-suit measurement of one-dimensional (1D) nanowire and nanotube-welding points. The second method indirectly characterizes macroscopic devices based on nanointerconnections, especially for some flexible film structures. For the study on the performance of 1D nanowires and single-nanometer connection joints of tubes, some researchers have used molecular dynamics-related simulation software to simulate the mechanical and electrical properties of their interface and perform atomic simulation of the entire welding process. The morphology and influencing factors are analyzed to obtain theoretical electrical performance before and after welding. For experimental measurement, if the electrical and mechanical properties are to be directly characterized at such a small scale, with the development of characterization technology, direct mechanical measurement of solder joints can be achieved. However, there are still many challenges in the actual measurement process.
The current nanowelding methods are low-temperature cold welding, pressure welding, ultrasonic welding, electric field and chemical-assisted welding, high temperature and Joule welding, high-energy beam welding (e.g., electron and ion beam), and laser-induced plasma welding at local low temperature. During the preparation of nanointerconnection devices, especially for the new generation of flexible nanoelectronics, it is necessary to prepare interconnect joints with high electrical performance and a low-temperature and low-stress welding environment, which does not cause damage to other surrounding nanodevices and substrates. The nanojoints obtained using high-temperature melting are often accompanied by a relatively large heat-affected area, which will also have a thermal impact or even damage to the structure of the nonconnected parts, and then reduce the electrical performance of the overall interconnection structure.
Conclusion and Prospect This study summarizes and prospects the electrical and mechanical properties of different materials from the atomic scale to single welded joints, and then to macroscopic multinanoscale welded joints by combining the characteristics of current different nanowelding technologies and their welding interfaces. The discussion of welding structure and deformation mechanism, welding strength, fatigue characteristics, and electrical performance showed that laser-induced plasma welding with characteristics of self-limiting and low-temperature has great potential in fabricating nanodevices and flexible electronic devices.
Although the current study on laser-induced plasma self-limiting low-temperature welding technology has achieved a certain progress, it still faces huge challenges for achieving high-efficiency, high-precision, and high-resolution laser-induced nanocontrollable interconnection manufacturing. The realization of the energy precise control of the nanoscale joints and interconnection mechanism of materials at the nanoscale still needs further study. Besides, for interconnection functional structures with nanoscale line widths, effective manipulation techniques are often required to arrange and assemble them before the connection. It is necessary to achieve subsequent high-precision positioning. This process relies on the integration of high-precision laser nanowelding equipment; however, related technologies still need further study and development. It is believed that the continuous development of laser nanowelding technology will play a significant role in the next generation of electronic device interconnection packaging.
