Local smoothing of optical aluminum surfaces by reactive ion beam etching

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.