Significance Since the end of the 18th century, mankind has experienced three industrial revolutions, represented by the applications of the steam engine, electric power, and electronic information technology. Each revolution has brought about a huge increase in productivity. Now, the fourth industrial revolution has quietly occurred and industrial production has changed from mechanization and automation to informatization and intelligence. The semiconductor integration industry has become an important carrier of the fourth industrial revolution. Currently, semiconductor integrated circuits (ICs) are developing toward high integration, high density, high performance, and low power consumption, and their manufacturing process has entered the era of the 5-nm node. However, the reduction in feature size has led the bottom-up development model based on photolithography to face huge challenges, such as the restriction of manufacturing processes and applications in ICs. The latest international semiconductor technology roadmap shows that the feature size of ICs will approach its physical limit and the size effect will greatly affect the performance of the device. This will cause electronic devices to fail according to traditional semiconductor physics principles. Solving the size effect caused by the ever-decreasing IC feature size has become the frontier and hot spot of domestic and foreign scholars.
Carbon nanotubes (CNTs) have become ideal next-generation electrical wire materials, owing to their unique electrical, mechanical, and thermal properties, and have attracted the attention of scholars worldwide. The CNT is a typical one-dimensional nanostructured material that only propagates along the axial direction, which greatly reduces the probability of scattering during electron transport. CNTs can withstand 70% higher carrier mobility than silicon materials and their current density is more than 1000 times that of copper wires after interconnection. Therefore, CNTs can not only replace both copper wires and the doped silicon to become the next generation of semiconductor device materials, but also unify semiconductor device materials, which will greatly simplify the manufacturing processes and reduce the cost of ICs. Electrical contact is an indispensable part of ICs. Because of the small contact area between CNTs and metal electrodes, electrical coupling between CNTs and metal electrodes is difficult. Although CNTs have high conductivity, their large interface-contact resistance hinders their practical electronic applications. Therefore, to realize the various applications of CNTs in the field of micro-nanoelectronics in the future, a key prerequisite is to establish reliable and stable mechanical and electrical connections between CNTs and micro-nanoelectrodes. Hence, it is important and necessary to summarize the existing research on CNT-metal connections to guide the future development of this field rationally.
Progress As can be seen from the interface behavior of CNTs and metal, there are two contact modes between them: weak contact in physics and strong contact in chemistry. Experimental and simulation results show that strong chemical contact can not only ensure high mechanical connection strength, but also ensure stable and efficient energy transfer. However, the precise application of an energy source to form a stable chemical connection between CNTs and metal remains an urgent technical problem. Because CNTs are one-dimensional nanomaterials, physical or chemical methods are currently used to achieve closer contact or connections between CNTs and metals in the microscopic fields, such as annealing, deposition, ultrasonic welding, and high-energy beam irradiation. At present, the lowest interface contact resistance and resistivity between the CNT bundle and metal electrode based on interconnection technology are approximately 0.6 Ω and 10-3-10-2 Ω·cm, respectively. In contrast, the resistivity of a copper wire interconnection in 22-nm technology is 5.8×10-6 Ω·cm. Interconnection technologies, such as high-temperature annealing, electron- or ion-beam deposition, and ultrasonic welding, are not suitable for the above applications. For CNT-based micro-nanodevices, achieving high-quality connections between a single CNT or multiple CNTs and metal electrodes is still an urgent problem. So far, the minimum contact resistance between a single CNT and the metal electrode is 116 Ω, and the interface contact resistivity varies from 10-5 Ω·cm to 7.5×10-4 Ω·cm. Therefore, CNT interconnection technology based on IC applications still requires much work.
Conclusions and Prospects Compared with other interconnection methods, such as electron- and ion-beam interconnection technology, laser-beam irradiation technology has the advantages of shape control and versatility. Laser near-net shaping technology, selective laser sintering technology, and other laser irradiation technologies can effectively form strong and effective connections between CNTs and nanoscale metal powder, nanoscale metal particles, micro-nanoscale metal bulk materials, etc. When the laser beam is coupled with the nanoscale operating system, heterogeneous joints of different geometric shapes can also be created across scales, and the performance of each joint can be controlled according to the application requirements. When the laser processing system and high-speed automation system are combined, the focused laser beam can irradiate or process various nanomaterials with high efficiency and high precision in a large work area. This is a good method to prepare high-quality heterogeneous connections between CNTs and metal in large quantities and areas. The interconnection technology for CNTs and metal electrodes is developing toward stability, convenience, large area, and environmental friendliness to effectively reduce the contact resistance of the CNT/metal interface and promote its industrial applications.
