Surface wrinkling by chemical modification of poly(dimethylsiloxane)-based networks during sputtering

Abstract

Wrinkling is an important mechanical phenomenon that generates periodic topographical patterns across a surface. This paper presents experimental evidence that surface wrinkles, which form consequent to thin film magnetron sputtering of either indium tin oxide (ITO) or aluminum on poly(dimethylsiloxane) networks (PDMS-N) made from a commercial Sylgard-184 kit, result from chemical modification of the PDMS-N surface as opposed to extrinsic thermomechanical stresses originating from differential thermal expansion. X-ray photoelectron spectroscopy results reveal that the PDMS-N surface becomes depleted in carbon and concurrently enriched in oxygen relative to silicon due to sputtering. This silica-like surface layer possesses intrinsic compressive stress that leads to wrinkle formation during the first ≈5 seconds of sputtering. The wrinkles maintain their periodicity irrespective of the thickness of the ITO film formed during subsequent deposition. Furthermore, upon removal of the ITO layer, the wrinkles persist with their periodicity unchanged. A narrow sputtering pressure window between 2 and 12 mTorr generates wrinkles. Pressures below this range cannot sustain a radio frequency plasma, while pressures above this range provide sufficient thermalization of kinetic energy as to eliminate the energetic bombardment that modifies the PDMS-N. This study provides a new understanding of the origins of wrinkling in sputtered films on polymeric substrates and creates opportunities to manipulate the topography produced by spontaneous surface wrinkling.