Molecular dynamics study of pattern transfer in nanoimprint lithography

A molecular dynamics simulations model of nanoimprint lithography (NIL) is proposed in order to study the pattern transfer and its related phenomena. The proposed model is similar to a real NIL process imprinting an alpha-quartz stamp with a rectangular line pattern into an amorphous poly-(methylmethacrylate) (PMMA) film. The polymer deformation behavior and the adhesion and friction effects between the stamp and the polymer film are investigated and their dependency on the pattern aspect ratio is discussed. Force fields including bond, angle, torsion, van der Waals, and electrostatic potentials are used to describe intermolecular and intramolecular interacting forces. Nose-Hoover thermostat is used to control the temperature of the polymer film and cell multipole method is adopted to treat long range interactions. The deformation of the polymer film is observed for two stamps having different aspect ratio patterns. The distributions of density and stress in the polymer film are calculated for the detail analysis of deformation behavior. For a high aspect ratio pattern (aspect ratio = 2.5, imprint depth = 8.0 nm), large amount of springback of the residual polymer film is observed, which is mainly due to the residual compressive stress left in the polymer film. However, for a low aspect ratio pattern (aspect ratio = 1.0, imprint depth = 3.0 nm), the springback is not observed. In addition, adhesion and friction forces are obtained by dividing the polymer film into subregions and calculating the interacting force between each subregion and the stamp. While the adhesion force is nearly constant regardless of the pattern aspect ratio, the friction force increases as the pattern aspect ratio grows, so the friction force becomes larger than the adhesion force when the pattern aspect ratio increases.