Growth behavior of Cu/Al intermetallic compounds and cracks in copper ball bonds during isothermal aging

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. There are many potential benefits for use of copper in these applications, including better electrical and mechanical properties, and lower cost. Usually, wires are bonded to aluminum contact pads. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood, and if excessive would increase the contact resistance and degrade the bond reliability.To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250°C for up to 196h has been used to accelerate the aging process of the bonds. The Cu/Al IMCs growth behavior was then recorded and the IMC formation rate of 6.2±1.7×10−14cm2/s was obtained. In addition to the conventional yz-plane cross-section perpendicular to the bonding interface, a xy-plane cross-section parallel through the interfacial layers is reported. Three IMC layers were distinguished at the Cu/Al interfaces by their different colors under optical microscopy on the xy-plane cross-sections of ball bonds. The results of micro-XRD analysis confirmed that Cu9Al4, and CuAl2 were the main IMC products, while a third phase is found which possibly is CuAl. During the aging process, IMC film growth starts from the periphery of the bond and propagates inward towards the centre area. Subsequently, with increased aging time, cavities are observed to develop between the IMC layer and the Cu ball surface, also starting at the bond periphery. The cavitation eventually links up and progresses toward the centre area leading to a nearly complete fracture between the ball and the intermetallic layer, as observed after 81h.