Surface microroughness of ion-beam etched optical surfaces

Ion-beam etching (IBE) and ion-beam figuring techniques using low-energy ion-beam sources have been applied for more than ten years in the fabrication and finishing of extremely smooth high-performance optics. We used optical interferometric techniques and atomic force microscopy to study the evolution of the surface root-mean-square (rms) microroughness, Rq, as a function of depth of a material removed (0-3000 nm) by a broad ion-beam source (Ar+ ions of energy 600 eV and ion current density of 1 mA cm-2). Highly polished samples of fused silica and Zerodur (Rq∼3.5 A) showed a small decrease in microroughness (to 2.5 A) after 3000-nm IBE removal while an ultrapolished single-crystal sapphire sample (Rq∼1 A rms) retained its very low microroughness during IBE. Power spectral density functions over the spatial frequency interval of measurement (f=5x10-3-25 μm-1) indicate that the IBE surfaces have minimal subsurface damage and low optical scatter