Preparation by glancing incidence ion irradiation of CaF2 surfaces with angstrom-scale RMS roughness

We have subjected both an air-cleaved, nominally (111)-oriented and a 4 degrees-miscut, mechanically polished (111)-oriented CaF2 surface to irradiation by 4.5 keV Ar+ ions incident at a glancing angle of 10 degrees to the surface. The evolution of the surface morphology was followed using atomic force microscopy with a lateral resolution of approximately 10 nm. The initial root mean square roughness of the mechanically polished surface was 0.64 +/- 0.05 nm, this value being the mean of 30 separate measurements on randomly selected 0.5 x 0.5 mu m(2) areas of the surface. Following a fluence of 3 x 10(17) ions mm(-2), the RMS roughness had decreased to 0.12 +/- 0.007 nm. This value is only 75% of the Ca-F interlayer spacing of the (111)-oriented crystal. The data suggest that surface irregularities are removed by sputtering at the edges of atomic steps. The planarization mechanism on vicinal surfaces appears to require diffusion along step edges or repulsive step-step interactions to limit statistical roughening of the step edges during ion irradiation.