Electromigration threshold has been measured for unpassivated damascene Al(Cu) interconnects by drift experiments on Blech-type test structures, and results are compared to equivalent plasma-etched lines. When using a damascene implementation, the critical product of current density and stripe length increases significantly. As a result, the incubation period, needed for Cu depletion beyond a critical length, is increased over a wide temperature range (155–230 °C). This is shown to be an intrinsic effect, directly controlling the relative electromigration performance of both metallization schemes. © 1998 American Institute of Physics.