Demolding temperature in thermal nanoimprint lithography

In thermal nanoimprint lithography, temperature is one of the most important process parameters. Temperature is not only important for the flow of resist during molding but also for demolding, the process by which the imprint stamp is removed from the molded resist/substrate. This is because thermal stress and friction and adhesion forces generated at the stamp/resist interface and the mechanical strength of the resist are all dependent on temperature. In this paper, we demonstrate via both experimentation and numerical simulation that an optimal temperature (T _d) leading to minimal deformation of molded resist exists for demolding. The ease of demolding was directly accessed by measuring demolding force at different T _d for a Si stamp/PMMA/Si substrate system of 4-in.-diameter using a mechanical tester. Numerically, the demolding process for a simple two-dimensional model of a Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate system was simulated using a finite-element method for different T _d, assuming viscoelasticity of the PMMA resist and temperature dependence of friction coefficients at the stamp/PMMA interface. We found that a temperature leading to the minimum in both the demolding force and the normalized stress vs. T _d curves exists below the glass transition temperature of the PMMA resist, from which the optimal T _d was derived.