Surface quality and laser-damage behaviour of chemo-mechanically polished CaF2 single crystals characterized by scanning electron microscopy

Abstract

A significant increase in the ultraviolet laser-damage threshold of CaF₂ (1 1 1) single-crystal surfaces after surface finishing by chemomechanical polishing (CMP) with colloidal silica has been demonstrated as compared to conventional mechanical-abrasive polishing (MAP). It was shown that CMP yields an up to 12-fold increase of the damage threshold fluence up to F_th = 30 J cm⁻² for 1-on-1 nanosecond pulses of 248 and 193 nm excimer laser irradiation. Even after 5-on-1 irradiations, the damage threshold remains as high as F_th = 15 J cm⁻² in the case of CMP. For both polishing procedures, the change in dielectric surface properties has been characterized by means of scanning electron microscopy (SEM) using electron beam-induced charge-up phenomena. These were mainly detected by the variation of emitted secondary electron (SE) yield δ_SE depending on the primary electron (PE) energy. Two kinds f charge-up phenomena were employed: (i) the onset or vanishing of statistically fluctuating SE yield bursts during slow-scan imaging (“stripe pattern” method), and (ii) the temporal decay of the electron beam-induced charge-up inside an electrically conducting mask (charge decay method). Both these phenomena disappeared after CMP. It is concluded that this disappearance results from removing the subsurface damage layer which is typical of MAP.