Reactive ion beam smoothing of rapidly solidified aluminum (RSA) 501 surfaces for potential visible and ultraviolet light applications

Abstract

Optical elements made of aluminum with surface roughness in the sub-nanometer range are required for applications in the visible (VIS) and ultraviolet (UV) spectral range. Rapidly solidified aluminum (RSA) 501 is an interesting candidate to produce sufficiently smooth surfaces for these applications in an ultra-precision (UP) fly-cutting process. However, the polycrystalline grain structure and precipitates contained in the material limit the achievable surface roughness. In this research, a reactive ion beam finishing process of RSA 501 is investigated employing an electron cyclotron resonance (ECR)-driven ion source using CF4 and N2 gasses. Emphasis is placed on understanding the etching mechanisms that influence the topographic evolution to provide the basis for future production processes using the material for UV optics. Material composition characterization is applied to analyze the interaction between the reactive ion species and the RSA material. Surface modification transferring the native oxide top layer to an in situ forming and developing aluminum fluoride or -nitride etch front layer is found to prevent grain orientation-dependent etching. The effect of the ion incidence angle on the etch rates of precipitates and bulk material can be used to reduce the height of precipitates protruding from the surface after the UP-machining process. A surface roughness value of Sq = 0.9 nm ± 0.1 nm (areal root mean square height) is achieved in the micro-roughness range when etching at 40° and 80° angle of incidence using collimated ion beams with CF4 process gas.