Super-stretchable metallic interconnects on polymer with a linear strain of up to 100%

Abstract

Metal interconnects in flexible and wearable devices are heterogeneous metal-polymer systems that are expected to sustain large deformation without failure. The principal strategy to make strain tolerant interconnect lines on flexible substrates has comprised of creating serpentine structures of metal films with either in-plane or out-of-plane waves, using porous substrates, or using highly ductile materials such as gold. The wavy and helical serpentine patterns preclude high-density packing of interconnect lines on devices, while ductile materials such as Au are cost prohibitive for real world applications. Ductile copper films can be stretched if bonded to the substrate, but show high level of cracking beyond few tens of % strain. In this paper, we demonstrate a material system consisting of Indium metal film over an elastomer (PDMS) with a discontinuous Cr layer such that the metal interconnect can be stretched to extremely high linear strain (up to 100%) without any visible cracks. Such linear strain in metal interconnects exceeds that reported in literature and is obtained without the use of any geometrical manipulations or porous substrates. Systematic experimentation is carried out to explain the mechanisms that allow the Indium film to sustain the high strain level without failure. The islands forming the discontinuous Cr layer are shown to move apart from each other during stretching without delamination, providing strong adhesion to the Indium film while accommodating the large strain in the system. The Indium film is shown to form surface wrinkles upon release from the large strain, confirming its strong adhesion to PDMS. A model is proposed based upon the observations that can explain the high level of stretch-ability of the Indium metal film over the PDMS substrate.