Some TEM Observations on Electromigrated Al and Al Alloy Interconnects

Transmission electron microscopy has been performed on the stressed (typical current density 2 × 106 amp cm¿2) as well as failed regions of Al, Al-Si, Al-Ti, Al-Ti-Si, Al-Cu-Si and Al-Cr-Si test conductors. Metal thickness was 0.5 ¿m and linewidth was 4-5 ¿m. The metal films were sputter deposited orn SiO2 substrdtes (300-3000A) that were thermally grown by oxidizing single crystal Si wafers. Irregular shaped voids were observed in the pure Al at triple points of grair bourndaries and at the conductor edges, suggesting enharnced electromigration at these locations. In Al-alloys, the shdpe and location of voids was dependent on the type of impurity added. It appears that prectpitation of added impurities at the grdin bounddries in Al-alloy systems does rnot always ensure ernhanced electromigration resistance. Pheromenological models explaining the observed open-circuit failures in Al and Al-alloys are proposed.