Fatigue Behavior of Sub-Micron Silver and Copper Films

Fatigue compromises the reliability of macroscopic metallic components utilized in a variety of technological applications. However, the fatigue behavior of thin metal films and smallscale components used in microelectronics and mechanical microdevices has yet to be explored in detail. The fatigue behavior in (sub)micron thin films is likely to differ from that in bulk material, since the volume necessary for the formation of dislocation structures typical of cyclic deformation is larger than that available in thin films. The microscopic processes responsible for fatigue are, therefore, affected by the thin film dimensions and microstructure. This work focused on the characterization of such mechanisms and the resulting fatigue damage. In particular, the effect of grain size and/or film thickness were investigated. The fatigue behavior of 0.2-1.5 μm thick, Ag films on SiO2 and 0.4-3.0 μm thick,Cu films on polyimide substrates was investigated. The films were tested using cantilever microbeam deflection and cyclic tensile testing. Extrusions similar to those observed in bulk material were found at the Ag and Cu film surfaces after cyclic loading. Voids observed beneath the extrusions, at the film-substrate interface, contributed significantly to thin film failure. The occurrence of these extrusions and voids was qualitatively explained by a model that combines mechanisms of bulk fatigue with the constraints exerted on dislocations by thin film dimensions. Fatigue lifetime decreased with increasing cyclic amplitude. Moreover, thin films were more fatigue resistant and contained fewer, smaller extrusions than thicker films. We found that a small thickness and/or grain size inhibits void nucleation. This observation is explained in terms of vacancy diffusion and annihilation at free surfaces or grain boundaries. These investigations shine a new light on a well-known phenomenon and provide a basis for designing thin film devices against cyclic loading. Max-Planck-Institut für Metallforschung und Institut für Metallkunde der Universität Stuttgart, 2001 Alle Rechte, auch das des auszugsweisen Nachdrucks, der auszugsweisen oder vollständigen Wiedergabe (Photokopie, Mikroskopie), der Speicherung in Datenverarbeitungsanlagen und das der Übersetzung, vorbehalten. Als Manuskript gedruckt. Printed in Germany.