Effect of indenter shapes on bond pad hardness studies of low-k devices

The idea of using low-k materials compared to conventional silica (SiO2) poses a large challenge to the back end processes such as wire bonding. In this study, numerical methods such as finite element method is used to characterize the permissible compressive bonding load at the bond pad. Since micro and nano hardness test conventionally use a conical indenter while the wirebond process experiences a spherical indentation, a comparison is made in terms of the force-indentation depth graphs. Results shows that the conical indenter induces early damage and their prediction of the force can be taken as a conservative measure during the wirebond process design. Further a numerical model can be made to verify for its sufficiency to squash the gold ball for a required dimension and to check for any damage within the device. In addition, the strain contours within the device at different metal layers provides better insight to modify the device’s active circuit layout below bond pad to distribute the forces evenly.