Effect of ultrasonic energy on interfacial structure and bond strength in copper wire bonding

The effect of ultrasonic energy on the interfacial microstructure in Cu-Al bonds is investigated to establish a relationship between ultrasonic energy, interfacial structure and bonding strength. It is shown that formation of intermetallic compounds (IMCs) at a Cu/Al interface is strongly dependent on the character of ultrasonic vibration; higher levels of ultrasonic energy enhance fragmentation of an oxide layer, resulting in continuous alloy interfaces. In addition, a stronger ultrasonic vibration heats up the interface to a higher temperature, and simultaneously creates more grain boundaries and dislocations, promoting migration of metal and leading to thicker IMCs. Formation of continuous thick IMCs at the Cu/Al interface significantly improves the bonding strength.