ELECTROMIGRATION IN VACUUM-EVAPORATED CU FILMS

Copper is a potential substitute for Al in integrated circuit metallizations because of its lower electrical resistivity and its greater resistance to electromigration damage. Changes in the properties of Al and Cu films when they undergo electromigration stressing are compared. Measurements, carried out in vacuum, were made of the electrical resistance, times to failure, and activation energies. Scanning electron microscopy was used to characterize topographic changes. It is shown that a careful electrical resistance change method for measuring activation energies for electromigration damage is more reproducible than the time-to-failure method. Cu also shows less film thinning prior to failure than Al. For the same fractional change in electrical resistance, the hillocks on Cu are more crystallographic and the voids are larger and more numerous than in Al.