Ion beam planarization of optical aluminum surfaces

Abstract

For fabrication of high-performance mirror devices, technical aluminum alloys Al6061 or Al905 are widely used. The surface error topography after manufacturing by singlepoint diamond turning is applicable in the infrared spectral range. For increasing demands on the optical surface quality in the shortwave visible and ultraviolet spectral range, further improvement of the surface roughness is required. Hence, a promising alternative process to attain the required surface quality is evaluated. Within the ion beam planarization technique, a photoresist layer is deposited by conventional spin coating or spray coating technologies exhibiting an ultrasmooth surface. When removing the resist by reactive ion beam etch (RIBE) processing using nitrogen process gas, the ultrasmooth surface topography of the resist is transferred into the substrate. We optimized the photoresist thermal pretreatment to realize roughness preservation and a steady-state material removal rate during RIBE machining. The optimum preparation steps are explored based on roughness evaluation, chemical modification, and etch resistance of the negative photoresist. Reactive ion beam etching-based planarization is conducted on single-point diamond turned RSA Al905 and RSA Al6061 samples made of rapidly solidified aluminum (RSA) in a two-step process. The optimum process and the roughness evaluation are explored by topographic analysis applying a combination of white light interferometry and atomic force microscopy measurements. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Improvement of the optical properties after surface error correction of aluminium mirror surfaces

“…Reactive ion beam planarization with the aid of a sacrificial layer combined with RIBE-based finishing using oxygen Hereafter, ion beam planarization of Al905 with the aid of a sacrificial layer and subsequent O 2 finishing are elucidated. A detailed discussion of RIBE planarization process of optical aluminium surfaces with the aid of a sacrificial layer is described elsewhere [21]. As brief summary, the ion beam process parameter and application steps of the sacrificial layer were optimized to ensure an optimum transfer of the smooth sacrificial layer surface into the underlying aluminium substrate.…”

“…To ensure an optimum transfer of the smooth sacrificial layer surface into the underlying aluminium substrate, RIBE planarization investigations were performed at 1.2 keV with nitrogen. During investigations on ion beam planarization technique with the aid of a sacrificial layer using nitrogen, these surface error features were successfully reduced by overall 82% on RSA Al905 [21].…”

“…However, during ion beam processing with nitrogen an aluminium nitride layer is formed, which is accompanied by an increased microroughness and thus reduced reflectivity properties of the optical surface [21]. Since high reflection coefficient over a broad wavelength range is required for high-performance mirror devices, this paper is intended to face the drawbacks of the surface nitride after ion beam finishing with the nitrogen process for figure error correction or planarization by means of an optimized ion beam based post-processing step.…”