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A surface selective smoothening of thermoplastic polymer is established while maintaining the high curvature corners required for a 50 µm tall, refractive, optical diffuser device. 172 nm exposure provides selective modification of a 400 nm thick surface skin layer in 3D master structures, which allows smoothening RMS roughness from 22 nm down to below 10 nm by thermal reflow.

A two-step replication process chain is developed for a microlens array structure with deep three dimensional (3D) reliefs and sharp features enabling the transfer of a photocured acrylic resist patterns into thermoplastic poly-methyl methacrylate (PMMA) with the same structural polarity via an intermediate stamp. By using ultraviolet (UV)-curable polydimethyl siloxane (PDMS), high fidelity negatives were cast from the original microstructures made by two-photon-polymerization and subsequently replicated into PMMA using thermal imprint. The mechanical properties of the new UV-PDMS (X-34-4184, Shin-Etsu Chemical Company, Ltd.), along with its nearly zero process shrinkage, proved to be highly suitable to replicate both 50 lm high concave features and sharp tips with an apex diameter of 500 nm. The results prove that silicone rubber, despite its elasticity, has specific advantages in thermal imprint in structures where both tall microstructures and submicron surface structures have to be replicated. This way, high fidelity PMMA structures with low defects could be prepared by the optimized processing found in this work to have a replication of 3D masters for further upscaling. V

Most polymeric refractive micro-optics simultaneously demand ultra-smooth 3D topographies and precise geometry for high optical performance and low stray light. We have established a surface selective smoothening for thermoplastic polymers that does not affect the designed optical geometry. For example, high curvature corners required for a 50 µm tall optical diffuser device were maintained while the surface roughness was reduced to about 10 nm RMS. 3D master structures were fabricated using direct write laser-lithography with two-photon absorption. Master structures were then replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate copying step and subsequently smoothed-out by high-energy photon exposure and thermal post-processing. The smoothening results in a lower roughness compared to a direct writing strategy using even about 50 nm vertical discretization steps still enables 10 times faster writing times.

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