Metallized SU-8 thin film patterns on stretchable PDMS

Abstract

We show how a rigid patterned SU-8 thin film in tandem with the mechanical Poisson effect can be used beneficially to enable robust metallization on stretchable polydimethylsiloxane (PDMS). Two generic planar processes used to form a thin, mechanically-robust strain-shielding photoresist on a soft substrate are developed to demonstrate these ideas. The first process is a self-aligned metallization of photolithographically patterned SU-8 features on PDMS. The second process is a fully photolithographic lift-off metallization of pre-patterned SU-8 features on PDMS. In both cases, the SU-8 has a sub-micrometre thickness (~800 nm)—supporting a thermally evaporated Ti/Au (5 nm/50 nm) thin film metallization. The resulting samples were characterized electromechanically—the results demonstrate that the electrical continuity of metal lines (width = 150 µm and length up to 1 cm) is maintained up to ~70% strain between the lines. The electrical resistance of such lines remains relatively stable: 35 Ω at zero-strain and 63 Ω at 69% average interline strain. The electrical resistivity of the evaporated gold is near to that of pure gold, and remains so even at high strains (49 nΩ m at 10% and 70 nΩ m at 41%)—this proves the absence strain-induced micro-cracking of the metal lines. Although a specific evaporated metal combination is used here to demonstrate a working prototype system, in principle, any thin-film material, which can be deposited by microfabrication, e.g. insulators, semiconductors…, could be integrated into the generic processes. The basic lithographic processes expounded here are also potentially extendible to encompass the integration of other microfabrication techniques, e.g. soft lithography, contact printing…, enabling more complex flexible systems to be envisaged and realized.