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Based on the well-known fact that thermoplastic polymers feature a decrease of viscosity at increased frequency we propose a novel tool for frequency assisted thermal nanoimprint. The system is equipped with a stepper motor to drive into contact and to apply a static loading. In addition a piezo-unit is available that allows the superposition of the static load with a dynamic excitation. Detailed analysis of the frequency response of the overall system makes obvious that the frequency range available is limited not only by the frequency cut-off of the piezo amplifier, but also by its power or rather by the limited output current available. As a consequence the maximum frequency at full displacement is only 10 Hz. Nonetheless this should be enough to reduce the viscosity of typical imprint polymers at a low imprint temperature. The measurement system is sensitive enough to detect the small changes induced by the polymeric layer in the imprint stack, when the temperature is raised to typical imprint temperatures. Decay times for the residual force during imprint with a conventional imprint stack could be obtained from a relaxation experiment, where the piezos are used as step-displacement sources. The data are in excellent agreement with values calculated from dynamic rheological characterization experiments.

In order to utilise both techniques in the best and easiest way, thermal nanoimprint (T-NIL) and optical/UV lithography (UVL) were combined in a hybrid process to pattern the same resist layer. Typically, T-NIL as the first step defines the small, submicron patterns, UVL as the second step the larger patterns, well above 1 μm. The hybrid process is performed with a conventional Si stamp for imprint and a conventional photomask for UVL and does not require any special infrastructure. Conventional, commercially available positive and negative tone resists were used, SU-8 and AR-P 3510. In the case of SU-8, a chemically amplified negative resist with low glass temperature, the imprint is uncritical and does not result in any optical degradation. However, the post exposure bake required to finalise the crosslink reaction has to be adapted and performed as a temperature ramp. In the case of AR-P 3510 the imprint step is critical, as the photoactive component is degenerated at suitable imprint temperatures. We found that at temperatures up to 130°C this degradation can be compensated by an increase of the exposure dose. Though more critical in processing, the positive tone resist features a better definition of the transition region between imprinted patterns and UV patterns.