Electromigration Reliability in Nanoscale Cu Interconnects

Electromigration behavior in Cu damascene wires was studied for various Cu grain struc-tures. The grain size was modulated by Cu linewidth and thickness, and by adjusting the wafer annealing process step after Cu electroplating and before Cu chemical mechanical polishing. A larger variation of Cu grain size between the samples was achieved on CMOS 65 nm node tech-nology than previous nodes which was due to the finer line width and thinner metal thickness. The Cu lifetime and mass flow in samples with bamboo, near bamboo, bamboo-polycrystalline mixture, and polycrystalline grain structures were measured. The effects of a Cu(2.5 wt.% Ti) alloy seed, Cu surface pre-clean, and selective electroless CoWP deposition techniques on Cu electromigration were also observed and a significantly improved Cu lifetime was found. The electromigration activation energies for Cu in Cu(Ti) alloy, along Cu/amorphous a-SiCxNyHz in-terface and grain boundary were found to be 1.3, 0.95 and 0.79 ± 0.05 eV, respectively. In addition the Cu line size effect on the Cu conductivity for Cu area less than 4×104 nm2 was found to be a linear function of the Cu line area.