Interfacial Microstructures and Thermodynamics of Thermosonic Cu-Wire Bonding

The interfacial microstructures of the Cu-wire bonding to an Al pad are investigated first by using an X-ray microdiffractometer and high-resolution transmission electron microscopy. It was found that the intermetallic compounds hardly formed at the Cu/Al interface during the thermosonic Cu-wire bonding process. However, when heating temperature is elevated to 340°C which increases energy levels of Cu/Al, the intermetallic phases Al2Cu and Al4Cu9can form and reach to 130 nm thick within 20 ms due to atomic interdiffusion and reaction activated by ultrasonic energy and heat at the Cu/Al interface. The Al side of the interface is aluminum-rich Al2Cu with lattice parametersa= 6.067 Å andc= 4.864 Å, and the Cu side is copper-rich Al4Cu9with lattice parametera= 8.706 Å. Bonding strength and bondability increase significantly after forming the Cu/Al intermetallic phases.