From finite-amplitude equilibrium structures to dewetting in thin polymer films on chemically patterned substrates

Abstract

We study templating and patterning strategies of an ultrathin (<100 nm) polystyrene (PS) film on substrates with varying lateral confinement produced by linear chemical patterns. The patterned surfaces are fabricated by coating self-assembled monolayers (SAMs) of alternating less (–CH₃) and more wettable (–COOH) micro-stripes on gold-coated silicon surfaces. The width of the less wettable stripe was changed from 1 to 15 μm to uncover the influence of the lateral confinement on the dewetted patterns while keeping constant the film thickness (65 nm) and the width of the more wettable stripes (3 μm). When the less wettable stripes are highly confined (<6 μm), both experiments and simulations show a finite amplitude periodic deformation of the PS free surface that replicates the substrate pattern without film rupture. For moderately confined less wettable stripes, ideal templating of the substrate patterns on the polymer surface was observed by film rupture. Combined ridge and drop morphologies were observed when the less wettable stripes have higher widths (low confinement). Shallow ridges are found to form over the more wettable stripes at the initial stages of dewetting whereas larger droplets are found to form over the less wettable stripes at the later stages of dewetting. Interestingly, the arrangement of the droplets does not randomize even when the lateral confinement of the less wettable patterns is rather low, reflecting a long-term memory of boundary effects. A parametric kinetic study uncovered the following morphologies: (i) small aspect ratio pattern replication when the less wettable stripes are highly confined; (ii) larger aspect ratio for the ridges on a substrate with moderately confined less wettable stripes; (iii) near equilibrium morphology when the ridges and droplets coexist on a less wettable stripe with very low confinement. In the latter case, although the droplets are much larger than the ridges, they have similar aspect ratios.