Preferential sputtering on multicomponent optical surfaces

In this study, we report the effect of preferential sputtering during ion beam machining multicomponent optics. A binary continuum model is developed to investigate the surface preferential sputtering mechanism. Diverse mechanisms, such as sputtering, Fick diffusion, Mullins diffusion, ion-enhanced viscous flow, and ballistics drift, are integrated to characterize the surface evolution. Model simulations and experiments on ultra-smooth SiO2 substrates with the roughness of several nanometers are performed and compared. Both results demonstrate a consistent trend that surface composition is preferentially re-organized during ion beam sputtering. As ions impinge, surface Si concentration increases from initial 32.5% of un-sputtered surface to about 50% and then falls and eventually stabilizes at 41 ± 1.5%, both in experiments and model simulations. Furthermore, preferential sputtering enhances surface reflectivity (>45%) while scarcely changing surface roughness (<2%).