Constraint effect in deformation of copper interconnect lines subjected to cyclic Joule heating

Abstract

Temperature distributions and von Mises strains produced by cyclic Joule heating of damascene copper interconnect structures of varying geometries were calculated by finite element analysis. Constraining the top surface of the lines with rigid dielectric increased the temperature range required to produce a given strain range, relative to unconstrained lines. Much narrower lines required much higher temperatures to reach this same range of strain. Electrical tests to failure of actual structures were conducted. Scanning electron microscope images of unconstrained damascene copper lines after testing showed significant topographic features that have been associated in previous reports with mechanical fatigue. None of the constrained lines – including those cycled at ranges of total strain similar to those of the unconstrained lines – exhibited these features, but they did have large voids. Present and previously reported observations suggest that both cyclic plastic deformation and void formation can contribute to damage and failure of small-scale structures subjected to cyclic Joule heating, and that mechanical constraint has a strong effect on the operation of these mechanisms.