Influence of macro- And nanotopography, thin film thermomechanical behavior and process parameters on the stability of thermocompression bonding

The quality of wafer-level, gold thermocompression bonds is critically dependent on the interaction between the wafer topography, the thin film properties, the process parameters and tooling used to achieve the bonds. This study presents mechanics modeling of the effect of wafer topography. An analytical expression for the strain energy release rate associated with the elastic deformation required to overcome wafer bow is developed. Furthermore, a simple contact yielding criterion is used to examine the pressure and temperature conditions required to flatten nano-scale asperities in order to achieve bonding over the full apparent area. The analytical results combined with experimental data for the interface bond toughness obtained from four-point bend testing indicate that the overall wafer shape is a negligible contributor to bond quality. A micro-scale bond characterization based on microscopic observations and AFM measurements show that the bond yield is increased with increasing bond pressure.