Capillary instability in nanoimprinted polymer films

Capillary forces play an active role in defining the equilibrium structure of nanoscale patterns. This effect can be especially pronounced in soft materials such as polymers near or above their glass transition temperature (T-g) 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 upon thermal annealing of nanoimprinted polystyrene line-space gratings with an underlying residual layer. This novel instability is characterized by the development of lateral undulations of the lines, culminating in the localized coalescence of adjacent imprinted lines. An exact analytic model of this undulatory instability is not tractable, but we introduce a simple physical model for this lateral instability based on the driving force to reduce the surface energy, as in the well-known Rayleigh Plateau instability, which is likewise surface-energy driven. Good agreement is obtained between this simplified model and our observations. Our insights into the nature of this instability have implications for controlling the thermal stability of nanoscale patterns fabricated by nanoimprint lithography or other lithography techniques.