Selective Surface Smoothening of Polymer Microlenses by Depth Confined Softening

Polymeric refractive micro‐optical devices simultaneously demand striking smooth 3D topographies and precise shape accuracy for high performance and low stray light. Here, a surface selective smoothening of thermoplastic, polymeric material has been established while maintaining the high curvature corners required for a 50 µm tall, refractive, optical diffuser device. The 3D master structures are fabricated using direct write laser‐lithography with two‐photon absorption. Master structures are replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate copying step and subsequently smoothed out. Here, various high‐energy radiations have been considered to have a surface selective exposure and have been narrowed down to 172 nm ultraviolet exposure to be the ideal fit for this application. The 172 nm exposure provides selective modification of an up to 400 nm thick surface skin layer and negligible etching, which allows smoothening out up to 420 nm surface steps and reducing the RMS roughness from 22 nm down to below 10 nm by thermally driven material displacement, the so–called reflow, on a global pattern scale. Extreme ultraviolet exposure, as an alternative method, shows a higher modification efficiency than the 172 nm but has severe practical limitations.