Thermal imprint lithography or hot embossing is a processing technique using molding to produce surface patterns in polymer resist at micro-and nanoscales. While fast molding is important to improve the yield of the process, the process step that determines the success of imprinting high aspect ratio structures is demolding, a process to separate the mold insert from the patterned resist after conformal molding. In this paper the authors studied the stress and deformation behavior in polymer resist during the cooling and demolding process of thermal imprint lithography via finite element method. A simple model structure of the Si stamp/poly (methyl methacrylate) (PMMA) resist/Si substrate was used for the simulation, assuming that PMMA is viscoelastic. As demolding proceeds, Von Mises stress in the PMMA layer is highly localized in two locations, one at the transition corner zone between the residual layer and the replicated PMMA pattern and the other close to the contact region with the moving stamp edge, creating two maximums. The presence of the second maximum stress indicates that a structural failure may occur not only when demolding starts, but also immediately before demolding ends. The second maximum stress becomes significant as the angular offset from the ideal normal demolding to the substrate surface increases or for the structures located far away from the symmetric centerline. In addition, the authors will discuss the influence of other parameters, including demolding rate and stamp geometries. (C) 2008 American Vacuum Society.