The nanoscale interfacial characteristics of thermosonic copper ball bonding on aluminium metallization were investigated. It was found that ultrasonic vibration swept oxides of aluminium and copper from parts of the contact area, promoting the formation of intermetallic compound Al2Cu (approx.20 nm thick). Where oxides persisted, an amorphous aluminium oxide layer connected with a crystalline copper oxide. It was estimated that ultrasonic vibration caused an effective local temperature increase to 465 °C that accelerated interdiffusion and enhanced the formation of Cu–Al intermetallics.