Excimer Laser Micromachining of Polymers using Half-tone Masks: Mask Design and Process Optimization

Laser micromachining by projection with half-tone masks is a useful technique for fabricating microstructures with stepped multi-level or continuous surface relief. However, design of the mask transmission function to achieve a particular surface profile is a non-trivial problem except when the surface relief is shallow compared to the focal depth of the projection optics. In this paper we describe recent progress in the development of computer simulation tools aimed at assisting with this design process. Simulation is based on pulse-by-pulse propagation of the etched surface, using an empirically derived etch function to represent the laser-material interaction. The fluence distribution over the etched surface is estimated by free-space propagation of the angular spectrum at the focal plane. This approach has been used in the iterative design of half-tone masks for a range of microfluidic and optical components. Results are presented for parts fabricated in polycarbonate by halftone ablation at 248 nm wavelength.