Ultraprecise mirror devices show considerable potential with view to applications in the visible and the ultraviolet spectral ranges. Aluminum alloys gather good mechanical and excellent optical properties and thus they emerge as important mirror construction materials. However, ultraprecision machining and polishing of optical aluminum surfaces are challenging, which originates from the high chemical reactivity and the heterogeneous matrix structure. Recently, several ion beam-based techniques have been developed to qualify aluminum mirrors for short-wavelength applications. We give an overview of the state-of-the-art ion beam-processing techniques for figure error correction and planarization, either by direct aluminum machining or with the aid of polymer or inorganic, amorphous surface films.
The surface error topography of optical aluminium surfaces after common manufacturing by single-point diamond turning meets the requirements for applications in the infrared spectral range. However, for short-wavelength applications in the (E)UV spectral range the requirements in the optical surface quality increase immensely. Reactive ion beam etching (RIBE) is a promising process route, which allows direct surface machining rather than the use of a NiP coating. Lowenergy ion beams driven by a reactive process control permit a roughness preservation up to 1 μm etching depth. The effect of RIBE machining on roughness features is evaluated suggesting a model scheme for smoothing of high-frequency features.
Aluminum mirrors offer great potential for satisfying the increasing demand in high-performance optical components for visible and ultraviolet applications. Ion beam figuring is an established finishing technology and in particular a promising technique for direct aluminum figure error correction. For the machining of strongly curved or arbitrarily shaped surfaces as well as the correction of low-to-mid spatial frequency figure errors, the usage of a high-performance ion beam source with low tool width is mandatory. For that reason, two different concepts of ion beam generation with high ion current density and narrow beam width are discussed. (1) A concave ion beam extraction grid system is used for apertureless constriction of ion beams in the low millimeter range. An oxygen ion beam with a full-width at half-maximum (FWHM) of 4.0 mm with an ion current density of 29.8 mA / cm2 was achieved. (2) For even smaller ion beams, a conic aperture design with a submillimeter-sized exit opening was tested. A nitrogen ion beam with an FWHM down to 0.62 mm with an ion current density of 4.6 mA / cm2 was obtained. In situ ion current density mapping is performed by scanning Faraday probe measurements. Special interest is set on the data evaluation for submillimeter ion beam analysis.
Ion beam figuring with low-energy ion beam tools is a widely used finishing technique for most precise optical devices 1-3. For the machining of strongly curved surfaces as well as the correction of low-to-mid spatial frequency figure errors the usage of a high performance ion beam source with low tool width is mandatory. For that reason two different concepts of ion beam generation with high ion current density and narrow beam width are discussed: 1) A concave ion beam extraction grid system is used for aperture-less constriction of ion beams in the low millimeter range. An oxygen ion beam with a full-width at half maximum (FWHM) of 4.0mm with an ion current density of 29.8mA=cm<sup>2</sup> was achieved. 2) For even smaller ion beams a conic aperture design with a sub-millimeter sized exit opening has been tested. A nitrogen ion beam with a FWHM down to 0.62mm with an ion current density of 4.6mA=cm<sup>2</sup> was obtained.