Co films with various thicknesses were selectively deposited as Cu capping layers by chemical-vapor-deposition technique. Selectivity of the Co deposition between Cu and dielectric surfaces was improved by raising the deposition pressure. Degree of electromigration (EM) resistance enhancement was observed to be dependent on the deposited Co thickness. Compared to the no-Co control, significant EM lifetime enhancement was observed when the Co cap is thicker than 6 nm. © 2011 IEEE.