Ion beam machining technology has been extensively adopted to obtain an ultraprecision surface in ultraviolet lithography optics. However, there exist complex mechanisms leading the surface to evolve complicated topographies and increasing roughness. We build a kinetic model integrating with the typical sputter theory and a bond-counting Monte Carlo algorithm based on the compound materials to investigate the surface roughness evolution during ion beam sputtering. The influences of primary sputter, reflection, secondary sputter, geometrical shadowing, redeposition, and thermal diffusion were all taken into consideration to compose a dynamic evolution process. In calculation, using this model the surface first possesses a period of smoothing and then goes into a roughening stage, where the roughness follows the regular power law. Quantitative analyses of surface roughness derived from calculations are also examined and compared with experiments.
Ion beam machining has become an important means adopted to high-precision large optical mirror processing. This study has conducted a bitmap-style model, calculation and analysis on low-energy Ar+ ion beam sputtering optical surface, based on Sigmund Theory and the CCOS principle. We have obtained the relationship of the removal function and the removal rate with major technological parameters of ion beam machining (e.g. dwell time, work distance, ion energy, etc.) also via orthogonal experiments of single point removal. Results indicated that the removal rate of amorphous SiO2 (fused silica) by Ar+ ions with 600~1200 electron volts increases with the increase of ion energy and dwell time at different extents, decreases exponentially with the increase in work distance. On the basis of computational analysis and experimental investigations, we optimized process conditions and further figured the plane mirror with the clear aperture of 130 millimeters, utilizing technologically optimized low-energy Ar+ ion beam machining. Eventually we obtained the high-precision figure shape with the post-machined surface roughness of 0.43~0.44 nm rms and the post-machined figure error of 1 nm rms.
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