Characterization of interfacial intermetallic compounds in gold wire bonding with copper pad

Aluminum (Al) has been used as an interconnection material for the last 40 years. As the ultra-largescale integrated devices become more and more smaller, there are demands for the semiconductor gate performance to be improved through the use of a higher drive current and a lower junction capacitance. One attempt is to replace the conventional Al interconnection with copper (Cu), as Cu has lower electrical resistivity and enhanced electromigration resistance. Thermosonic wire bonding has been extensively developed and applied in bonding technique. It uses both heat and ultrasonic energy to obtain a solid-state diffusion between the bonded ball and pad. For the traditional Al bond pad, it naturally forms a passivation layer, alumina (Al₂O₃), on the surface. The Al₂O₃ prevents the continual oxidation of Al pad, and this brittle Al₂O₃ layer is broken by the ultrasonic energy during the thermosonic bonding. However, for the Cu pad, it is well known that it readily oxidizes at air atmosphere, especially with moisture, as Cu oxide is not a passivation layer and it will continually grow, which block the subsequent solid diffusion during wire bond process. Therefore, it is necessary to study the behavior of Cu pad IMC. In this paper, gold (Au) wire was thermosonic bonded to Cu pad. The IMC (intermetallic compound) coverage was characterized. HTS (high temperature storage) study was conducted and a comprehensive study on the interfacial behavior of thermally aged Au-Cu was conducted using transmission electron microscopy. It was found that Au-Cu IMC grew slower than that of Au-Al and Cu-Al, and the thickness of Au-Cu is 100-250nm after HTS 150°C, 1000 hours.