Study on demolding temperature in thermal imprint lithography via finite element analysis

Demolding in thermal imprint lithography is a process to overcome chemical and mechanical interactions of all levels between stamp and substrate formed by the process history, stamp geometries, and materials used, accounting for most of imprint failure. Undesired deformation in resist may occur depending on the stress generated in the resist with respect to the yield stress. In thermal imprint lithography, temperature is one of the most important parameters not only for the flow of resist during the molding process, but also for the demolding because the thermal stress, the friction and adhesion forces, and the mechanical strength of resist are all dependent on temperature even at below the glass transition temperature of the resist. In this paper, we have developed a method via a numerical simulation of the demolding process to determine an optimal demolding temperature that leads to the least resist deformation. Considering the viscoelasticity of the resist and the temperature dependence of friction coefficients at the resist/stamp interface, an optimal temperature was extracted via normalization of the local Von Mises stress in the resist by its yield stress. For a simple two-dimensional model system with Si stamp and PMMA as a resist layer, the optimal demolding temperature was found to be ∼70°C.