Creating Self-Organized Submicrometer Contact Instability Patterns in Soft Elastic Bilayers with a Topographically Patterned Stamp

Abstract

The surface of a thin elastic bilayer becomes spontaneously unstable when it is brought in proximity to another rigid contactor. The instability patterns, which are random and isotropic, exhibit a dominant lateral length scale of instability λ, which linearly scales with the bilayer thickness (h) as: λ = <i>R</i>_F<i>h</i>. It is known that for an elastic bilayer, <i>R</i>_F exhibits a nonlinear dependence on the ratios of individual film thicknesses (<i>H</i>) and shear moduli (<i>M</i>) of the two constituent layers, and can have values as low as 0.5 under specific conditions. This is in contrast to a near constant value of <i>R</i>_F ≈ 3 for a single layer elastic film. These isotropic contact instability patterns in a bilayer can be ordered, aligned and modulated using a topographically patterned stamp. The precise morphology of the aligned structures depends on commensuration between λ and the stamp periodicity (λ_P), and on the intersurface separation distance. A variety of patterns, like an array of circular holes, double periodic channels, etc., in addition to a positive and a negative replica of the stamp pattern, can be engineered with a simple stamp having 1D grating structure. A lower value of <i>R</i>_F in a bilayer allows generating patterns with sub 500 nm lateral resolution, which is impossible to create by elastic contact lithography (ECL) of a single layer film due to strong surface tension effects in ultrathin films. Thus, control of elastic instability in a bilayer with a patterned stamp represents a flexible soft lithography tool allowing modulation of length scales, morphology, and order