Voids in the aluminum metallization of integrated circuits can be created under certain device fabrication conditions. The formation and characteristics of these defects have been studied in detail. It is found that the density and size of the metal voids have a strong functional dependence on the compressive stress of the passivation film. The study shows that metal voids occur in a wide class of Al systems, including E-beam evaporated pure Al, and those sputtered with Si and Si-Cu. Physical characterization, quantification of stress relationships, grain size dependence, temperature stability, and the effect of the metal voids on electromigration are discussed.