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 Al₄Cu₉ can 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 Al₂ Cu with lattice parameters a = 6.067 Å and c = 4.864 Å, and the Cu side is copper-rich Al₄Cu₉ with lattice parameter a = 8.706 Å. Bonding strength and bondability increase significantly after forming the Cu/Al intermetallic phases.