Ion beam finishing techniques are commonly used for improvement of surface error topography of optical devices. Optical aluminum surfaces after manufacturing by single-point diamond turning meet the requirements for applications in the infrared spectral range. However, optics used for applications in the short-wavelength visible and ultraviolet spectral range demand improved surface qualities. To overcome the limitations mainly caused by structural and compositional inhomogeneities of aluminum alloys, a reactive ion beam machining process using oxygen and nitrogen operating gas is applied. This technology enables direct surface machining while preserving the initial roughness up to a 1-μm etching depth using low-energy ion beams. Moreover, the use of oxygen allows us to smooth the surface in the microroughness regime. Based on Monte-Carlo simulations and roughness evolution measured by atomic force microscopy, a more detailed discussion of the ion beam process is presented. Hence, a model scheme for direct smoothing of high-frequency surface features is suggested.
For fabrication of high-performance mirror devices, technical aluminum alloys Al6061 or Al905 are widely used. The surface error topography after manufacturing by single-point 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.
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=cm2 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=cm2 was obtained.