Characterization of intermetallic compound formation and copper diffusion of copper wire bonding

The trend to replace Au and Al wires with Cu in wire bonding has become an emerging trend for IC packaging nowadays. Although some research works have been carried out for the applications of Cu wire bonding, they are mainly focused on the processing and material issues of Cu wire bonds. However, the Cu in the wire bonds may diffuse into the Si chip and impose reliability threats to the silicon devices. There is no study yet on the Cu-to-Si diffusion in Cu wire bonding. In the present study, the intermetallic compound (IMC) growth in the wire bond and the Cu-to-Si diffusion behavior are investigated. The correlation between IMC growth and Cu diffusion is established. From the experimental characterization, it is found that the IMC growth of Cu-Al is less severe than that of Au-Al. On the other hand, it is also found that the depth of Cu-to-Si diffusion is much larger than that of Au-to-Si. Furthermore, the barrier layer appears to be effective for reducing the Cu and Au diffusion depth in silicon. Nevertheless, it seems that the barrier layer also promotes the growth of IMC in the wire bond. Detailed comparison and discussion of results will be given.