Electromigration and stress-induced-voiding in dual damascene Cu/low-k interconnects: a complex balance between vacancy and stress gradients

The influence of residual vacancy concentrations and stress gradients on electromigration both in the metal layer below and above copper via's with a diameter of 90 nm integrated in low-k materials has been investigated. Variations in stress gradients and vacancy concentrations were created by applying different post-plating anneal conditions. The impact of these variations was quantified based on high temperature storage tests both at the optimum stress-induced-voiding temperature and around the copper stress free temperature. By linking the results from these high temperature storage tests to electromigration data, we observe residual vacancy concentrations contribute more to upstream electromigration, while downstream electromigration is more vulnerable to residual stress gradients.