Thermally robust Cu interconnects with Cu-Ag alloy for sub 45nm node

Cu interconnects reliability has been drastically improved by the introduction of Ag doped Cu (Cu-Ag) alloy seed. The usage of Cu-Ag remarkably suppressed stress induced voiding (SiV) in the lower wide metal which appeared by the additional anneal to prevent the degradation of the barrier metal. It has been explained by the Cu extrusion model where the void formation was caused by the poor adhesion between the upper barrier metal and the extruded lower metal under the via. The thermal stress hysteresis curve of Cu-Ag which shows the shift of the softening temperature to higher suggests more thermal resistance to Cu extrusion than pure Cu. Also no degradation of interconnects performance and the minimum increase of the resistance were verified. These indicate that Cu-Ag alloy is the most feasible candidate for the improvement of the reliability issues of Cu interconnects with porous low-k film for 45nm node and beyond.