Highly Stretchable Metal Films on Polymer Substrates: Mechanics and Mechanisms

Stretchable interconnects are required for flexible electronic devices where a typical system comprising metallic interconnect and elastomer substrate needs to undergo large deformation while maintaining its mechanical and electrical integrity. This paper reports on a highly stretchable interconnect system consisting of indium metal periodically bonded to a compliant polydimethylsiloxane (PDMS) substrate via a thin, cracked chromium interlayer. It is shown that the system can sustain large tensile strain of ~100% without mechanical breakage and about 30% strain without significant electrical and mechanical damage. It is shown that the In-layer overlying the narrow gaps between adjacent Cr islands are strained much more than that possible in bulk In. The increased failure strain is demonstrated to be associated with microscale effective gauge lengths between Cr islands using digital image correlation (DIC). It is further demonstrated via experiments on freestanding thin In foils that the ductility of the foil increases with decreasing sample length, particularly when the sample length is less than 100 μm. This suggests that the root cause for obtaining such large strain-to-failure in periodically bonded In films on PDMS is the delay in the film plastic instability as a result of the reduction in the sample gauge length.