Liquid electromigration in gallium-based biphasic thin films

Abstract

Liquid metals have recently gained interest as a material of choice for soft and stretchable electronic circuits, thanks to their virtually infinite mechanical failure strain and high electrical conductivity. Gallium-based thin films are obtained by depositing gallium in the vapor phase to form a class of liquid metal conductors. The films, with an average thickness below 1 µm, withstand mechanical strain in excess of 400%. However, modes of failure other than mechanical ones have not yet been thoroughly investigated. In particular, electromigration, a well-known cause of failure in solid thin film traces for integrated circuits, also occurs in bulk liquid metals. In this work, microscopic observation of the thin conductive traces reveals that gallium is displaced from the anode terminal toward the cathode terminal after direct current stressing. This results in a catastrophic increase in the trace resistance and electrical failure. The mean time to failure decreases with increasing current density, following Black’s equation, an empirical mathematical model originally developed to describe failure in solid metal thin-film tracks due to electromigration. We show that using alternating current, e.g., symmetric square wave, rather than direct current can extend the lifetime of the thin liquid metal film conductor by several orders of magnitude. These results may help stretchable circuit designers who select liquid metal thin-film conductors as the stretchable interconnect technology to predict devices’ lifetime and implement mitigation strategies at the system level or at the material level.