Thermal reflow of polymers for innovative and smart 3D structures: A review

This is a topical review on advanced lithographic methods used to create 3D topographies. We start the discussion with the principal capabilities of grayscale electron beam lithography and multi-photon lithography for initial patterning. The innovative structures in this work become only possible by combining the initial patterns with methods to reshape them. By this, new functionalities such as aspheric, freeform and ultra-smooth surfaces can be provided. All the covered methods have in common that they exploit a specific material contrast that enables a post-processing and thus a transformation of the initial pattern by polymer reflow into a new surface topography or shape. To enhance the understanding of current reflow methods, we review the history of polymer reflow and discuss its simulation. In-depth examples cover optical and biomimetic applications. Furthermore, we provide new results and new insights into dynamic material changes during thermoplastic reflow. An outlook on emerging 3D MEMS fabrication using thermoplastic polymer reflow for actuation will illuminate the way towards smart, i.e, stimuli responsive 3D structures. This review is mainly meant for applied scientists and engineers.