Stray-light sources from pupil mask edges and mitigation techniques for the TPF Coronagraph

Stray-light sources from pupil plane masks that may limit Terrestrial Planet Finder Coronagraph (TPF-C) performance are characterized1,2 and mitigation strategies are discussed to provide a guide for future development. Rigorous vector simulation with the Finite-Difference Time-Domain (FDTD) method is used to characterize waveguiding effects in narrow openings, sidewall interactions, manufacturing tool-marks, manufacturing roughness, mask tilt, and cross-wavelength performance of thick Silicon mask structures. These effects cause stray-light that is not accounted for in scalar thin-mask diffraction theory, the most important of which are sidewall interactions, waveguiding effects in narrow openings, and tilt. These results have been used to improve the scalar thin-mask theory used to simulate the TPF-C with the Integrated Telescope Model.3 Of particular interest are simulations of 100m thick vertical sidewall openings that model features typically found on Ripple masks4 fabricated by Reactive Ion Etching (RIE) processes.5 This paper contributes fundamental data for systematically modeling these effects in end-to-end system simulation. Leakage straight through the mask material varies greatly with wavelength, especially in Silicon (an attractive mask material due to the precision manufacturing techniques developed by the IC industry). Coating Silicon with 200nm of Chrome effectively mitigates the leakage without causing additional scattering. Thick-mask diffraction differs from the predictions of scalar thin-mask theory because diffraction spreading is confined by the mask's sidewalls. This confinement can make a mask opening look electro-magnetically larger or smaller than designed, by up to 3λ per vertical sidewall on a 50μm thick mask yet this can be reduced an order of magnitude by undercutting the sidewalls 20°. These confinement effects are sensitive to mask tilt (if light reaches the sidewalls) which can lead to an imbalance in stray-light sources and an extra wavelength of effective opening change on the illuminated sidewall.