Investigation on copper diffusion depth in copper wire bonding

Due to the dramatic price increase of precious metals, the replacement of Au with Cu in wire bonding has become an emerging trend for IC packaging nowadays. Similar to the Pb-free soldering transition, such a replacement is not just a simple drop-in material change. Comprehensive processing and reliability investigations are required before a mass production of electronic devices with Cu wire bonding can be implemented. However, among the existing studies on Cu wire bonding, it appears that most researchers just focused on issues above the wire bond pads. In fact, the Cu in the wire bonds may diffuse into the Si chip and impose reliability threats to the electronic devices. So far there was no research on the Cu-to-Si diffusion issue in Cu wire bonding. In this paper, an experimental study on the Cu-to-Si diffusion in Cu wire bond is reported. The Cu diffusion depth was characterized with the secondary ion mass spectrometry (SIMS) technique. Specimens with various configurations were designed and fabricated to investigate the effects of several parameters on the Cu-to-Si diffusion depth. The issues of concern include the amount of Cu supply, the bond pad deformation, and the barrier layer under the bond pad. In addition, some samples with conventional Au wire bonding were fabricated and tested in parallel for comparison.