Capillary forces play an active role in defining the equilibrium structure of nanoscale structures. This effect can be especially pronounced in soft materials such as polymers near or above their glass transition temperatures where material flow is possible. In these situations, the effect of surface tension can produce varied and complex capillary instabilities, even in relatively simple geometries such as parallel line-space grating patterns. Here we investigate a novel capillary instability that arises in polystyrene line-space gratings with a residual layer (created by nanoimprint lithography) upon thermal annealing these patterns. This instability is characterized by a developing lateral undulation of the lines with a micron scale periodicity that culminates in the local coalescence of neighboring imprinted lines. We further provide a simple physical model for this new lateral instability based on energy considerations, analogous to the well-known Rayleigh-Plateau instability, and we show good agreement between this model and the data. These findings also have implications for controlling the thermal stability of nanoscale patterns made by nanoimprint lithography or other lithography techniques.
nano imprint lighography, relaxation, stability, surface tension