In-situ electromigration studies using resistometry

Abstract

The effect of electromigration on unpassivated aluminum thin films has been studied using the resistometry method. Interim information during current-stressing can be obtained by this method. Ultimate open-circuit failure has also been monitored by the presence of a very large increase in resistance. The relations between the mean-time-to failure (MTTF) with the current density and with the ambient temperature have been determined. From the resistometry measurements, it was found that there are three stages of temporal evolution for sputtered films. An abrupt resistance change of about 0.02–0.04% was observed in the films under low or moderate current density. Subsequently, a randomly fluctuating period followed, where the fluctuating amplitude was about 0.01%. Finally, a fatal stage with an abrupt and tremendous resistance increase was reached until an open-circuit failure occurred. For sputtered films under high stressing-current density (J ≥ 2.35 MA cm^-2), and for all evaporated films, the initial stage of abrupt resistance change essentially disappeared. From the scanning electron microscopy (SEM) micrograph, film thinning, surface coarsening, voids in irregular shape, and continuous cracks were found near the cathode. “Fan-shape” erosion appeared in the cathode under high current density. In the anode, hillocks parallel to the conductor direction were observed. Perpendicular extrusions extending out of the conductor were also present.