Relaxation Behavior of Polymer Structures Fabricated by Nanoimprint Lithography

We study the decay of imprinted polystyrene (PS) patterns under thermal annealing using light diffraction. The first-order diffraction intensity from the imprinted gratings was measured as a function of annealing time. A local intensity maximum is observed as a function of annealing time. This "abnormal" intensity variation can be qualitatively understood, using rigorous coupled wave approximation calculations, as a characteristic diffraction from patterns with specific shape/height. We demonstrate that this diffraction anomaly can be used to characterize the temperature dependence of the pattern decay rate. The activation energies of the pattern decay are found to be similar to those of the segmental and chain relaxations. Comparisons between PS samples of different molecular mass reveal that the patterns decay through different mechanisms. For unentangled PS, the decay of the imprinted pattern follows the surface-tension-driven viscous flow, with a viscosity similar to the steady-state viscosity. However, for highly entangled PS, large residual stresses introduced from the imprinting process cause the pattern to decay much faster than expected from surface-tension-driven viscous flow.