Abstract
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.
Highlights
Optical mirrors are designed to reflect light for a variety of applications, including beam steering, interferometry, imaging, or illumination.[1,2,3] An alternative to conventional glass as a substrate material for a metallization layer as a mirror device is aluminum
This paper focuses on ion beam planarization (IBP) using a nitrogen-containing process gas at normal ion incidence angle of commercially available rapidly solidified aluminum (RSA) alloys RSA Al905 and RSA Al6061 with the aid of a photoresist layer
3.1 Influence of Preparation Parameters on ma-N 2405 Negative Resist The DUV sensitive negative tone photoresist ma-N 2405 is composed of a novolak as polymeric matrix, the biazide as photoactive compound (PAC), and an organic solvent consisting of anisole
Summary
Optical mirrors are designed to reflect light for a variety of applications, including beam steering, interferometry, imaging, or illumination.[1,2,3] An alternative to conventional glass as a substrate material for a metallization layer as a mirror device is aluminum. The machined surface after SPDT still has the periodic groove pattern microstructures left by the cutting tool Those so-called turning marks have a pitch of some microns and an amplitude ranging between 10 and 30 nm. Ulitschka et al.: Ion beam planarization of optical aluminum surfaces removing these structures, one technological solution is the electroless plating with an amorphous nickel phosphorous (NiP) layer.[3,8,9]. Diamond-turned NiP reveals even smoother surfaces than aluminum; it can be further improved by the ion beam planarization (IBP) process, and turning marks are successfully reduced. For applications in the short-wavelength VIS or UV spectral range, it is necessary to improve the high-spatial frequency properties of the surface after SPDT.
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