Continuous Patterns with Height Gradients by Nanoimprint Lithography and Thermal Gradient Annealing
Yifu Ding, HyunWook Ro, Jirun Sun, Jing Zhou, Nancy J. Lin,
Sheng Lin-Gibson, Christopher L. Soles
Besides its promise in the lithographic arena, nanoimprint lithography (NIL) is a versatile technique to fabricate nanostructures for both basic research and technological developments. Whether it is through thermal embossing or UV-induced crosslinking reaction, the features on the rigid mold can be replicated onto the polymer resist layer. For such a replication process, the as-imprinted structures are entirely determined by the design of the mold.
In this work, we utilize the viscoelastic properties of polymer materials to create patterns that have a gradient in the height (or depth), from the as-imprinted value to a flat film. This is achieved by annealing nanoimprinted polymer patterns under a temperature gradient. At elevated temperature, the patterned polymer surface will tend to smooth out due to the surface tension, and the rate of the pattern decay is determined by the polymer viscosity. Since the latter is a function of the temperature, a gradient in annealing temperature creates a gradient of viscosity, which in turn results in a gradient of height across the pattern after annealing for a fixed amount of time. Quantitative control of the pattern height gradient is achieved based on the understanding of temperature dependence of polymer viscosity. We further demonstrate that these gradient substrates have applicability as a general combinatorial measurement platform for studying surface topology effects on cellular responses, wettability, and adhesion.
Author name: Yifu Ding
Mentor name: Christopher L. Soles
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