Electromigration drift and threshold in Cu thin-film interconnects

The electromigration-induced ionic drift velocity and critical length-current density product, (jl/sub c/) of Cu thin film conductors, were measured using the Blech-Kinsbron edge-displacement technique. Unencapsulated Cu edge-displacement segments on TiN conductors were stressed in vacuum at a modest current density of 6/spl times/10/sup 5/ A/cm/sup 2/ in the temperature range of 175-275/spl deg/C. Drift velocity was observed to be between 1-1/2 to 3 orders-of-magnitude lower than that previously measured for unencapsulated Al in this temperature range. We measured an activation energy for EM-induced drift of 1.25/spl plusmn/0.08 eV which corresponds to grain boundary diffusion in Cu. Critical lengths were measured and the jl/sub c/ threshold was estimated to range between 900-1600 A/cm. We calculated a Cu grain boundary Z* value of -0.7, to our knowledge, this study is the first to measure Z* for electromigration in Cu thin film conductors.