Comparison of copper, silver and gold wire bonding on interconnect metallization

Wire bonding technology has been extensively used to interconnect IC chips and substrates. Gold (Au) and aluminium (AI) has been used for wire bonding interconnect for decades. Recently, copper (Cu) wire bonding is used in high temperature applications and general cost down approaches. Despite its many benefits, copper wire has not yet been widely used like gold wire, as copper wire bonding also introduces many new challenges. One challenge is that copper wire bonding process needs more ultrasonic energy and a higher bonding force, which can damage the Si substrate, form die cratering and induce cracking and peeling of the bonding pad. Compared with copper, Ag is similar in conductivity, but softer in terms of mechanical properties. The lower Young's modulus and Yield stress of Ag could help to reduce the bond force and ultrasonic power during wire bonding process which leads to less damage on interconnect metallization under bond pad. In this paper, transient mechanical responses of the interconnect metallization beneath bump pad are investigated. Under the assumption of elastic-plastic behavior of the bonding pad and elastic behavior of the oxide, parametric studies are carried out to examine the effect of wire bonding material, interconnect metallization structure and thickness of bump pad on the stress distribution and elastic deformation of interconnect metallization layers.