A TRANSMISSION ELECTRON-MICROSCOPY STUDY OF ULTRASONIC WIRE BONDING

Ultrasonic bonding of aluminum wire is a technique widely used for joining wire to substrates and integrated circuit chips. Despite numerous metallurgical studies of wire bonding, the basic mechanism of bonding is still not well understood due to the limited resolution of the experimental techniques used to study bond interfaces. In this study, transmission electron microscopy (TEM) is used to examine ultrasonic bonding of aluminum wire to various metal substrates. A special technique for preparing cross section TEM specimens is described which involves embedding the sample in epoxy, dimpling, and electropolishing. The materials systems examined are Al wire bonded to Al foil, and Al wire bonded to Al foil onto which copper or gold was evaporated. Examination of the bond interface in the TEM reveals that in some areas, the surface oxides and contaminants were dispersed, allowing the bare metals to contact, whereas in other nearby areas the bond interface were filled with debris. The debris could be oxides, surface contaminants, or sheared metal particles. It also appears that the debris is dispersed to low-lying regions between asperities. In studies of the Al/Au/Al and Al/Cu/Al interfaces, microchemical analysis showed no evidence for diffusion or melting along the interface. Therefore, it is concluded that ultrasonic wire bonding does not involve significant heating along the bonding interface. However, it is shown that dynamic annealing of aluminum wire can occur during bonding. The experimental results are compared with the previously proposed theories on the mechanism of bonding in ultrasonic welding. © 1990 IEEE.