Ratchet effect in surface electromigration detected with scanning force microscopy in gold micro-stripes

In this work, we present scanning force microscopy studies on the effect of large current densities flowing through a micrometer gold stripe evaporated on a glass substrate. Classical electromigration effects, that is, voids and hillocks growth, can be observed on a relative long-range scale (typically 1 μm) under direct-current stressing. However, on the nanometer scale, direct current stressing induces grain growing due to thermal heating by the Joule effect, and no direct evidence of electromigration is observed. At this scale, observable electromigration effects appear only under alternating current stressing, and it is found that the grain structure, characteristic of a gold thin film, evolves to a faceted structure. We attribute this phenomenon to a ‘ratchet’ effect.