Surface electromigration in copper interconnects

RF-magnetron sputter deposited copper films have been characterised in terms of microstructure, stress and resistivity. The electromigration behaviour of this copper has been compared to that of sputtered aluminium. Atomic force microscopy (AFM) has been used to study the void sites. The morphology of the damaged regions, together with the microstructural information, has been used to gain an insight into the mechanism by which electromigration voiding proceeds. The observed void sites in copper are entirely different from those in aluminium; hence, a different failure mechanism must be responsible. The voiding process for copper can be explained by a grain-boundary grooving model, and it was observed that surface impurities controlled the damage mechanism and the resulting void morphology.