Large area smoothing of optical surfaces by low-energy ion beams

This contribution focuses on ion beam-assisted processes targeted at the preparation of ultra-smooth surfaces with root mean square (rms) roughness values Rq<0.2 nm. Especially, ion beam direct smoothing in combination with smoothing using planarization or sacrificial layers are investigated. The roughness reduction of the surfaces is monitored by atomic force microscopy (AFM) covering a broad range of spatial frequencies (>0.1 μm−1). These smoothing techniques were discussed for common optical substrate materials like silicon, quartz glass and a special glass ceramic. By optimization and combination of these different techniques a sub-nanometer scale roughness reduction down to 0.1 nm was demonstrated. Due to the application of broad beam ion sources the results are also applicable to large-scale optical components.