New Cu “Bulge-Out” Mechanism Supporting SubMicron Scaling of Hybrid Wafer-to-Wafer Bonding

In order to scale wafer-to-wafer hybrid bonding to sub-micrometer pitches, a tight control of surface topography after CMP is required. The post CMP Cu pad recess with respect to the bonding dielectric surface, must be limited to only a few nanometers. This is required to obtain a full dielectric-to-dielectric bonding at room temperature, and to obtain a good Cu-to-Cu contact after post-bond wafer anneal. During this post-bond anneal the Cu pads expand to make contact and bond. For pads with equal size, the expansion is small and largely elastic, and drops with reducing Cu pad volume and anneal temperature. When using unequal sized pads, a much more significant inelastic Cu “bulge-out” is observed. This phenomenon relaxes CMP requirement and enables sub-micron scaling at strongly reduced post-bond anneal times and temperatures. A dedicated test methodology was developed to study this Cu bulge-out phenomenon in detail.