Photochemically assisted patterning: An interfacial hydrodynamic model perspective

Abstract

Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.