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

A significant increase in the ultraviolet laser-damage threshold of CaF2 (1 1 1) single-crystal surfaces after surface finishing by chemomechanical po lishing (CMP) with colloidal silica has been demonstrated as compared to co nventional mechanical-abrasive polishing (MAP). It was shown that CMP yield s 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 irra diation. Even after 5-on-1 irradiations, the damage threshold remains as hi gh 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 mean s of scanning electron microscopy (SEM) using electron beam-induced charge- up phenomena. These were mainly detected by the variation of emitted second ary electron (SE) yield delta(SE) depending on the primary electron (PE) en ergy. Two kinds of charge-up phenomena were employed: (i) the onset or vani shing of statistically fluctuating SE yield bursts during slow-scan imaging ("stripe pattern" method), and (ii) the temporal decay of the electron bea m-induced charge-up inside an electrically conducting mask (charge decay me thod). Both these phenomena disappeared after CMP. It is concluded that th is disappearance results from removing the subsurface damage layer wh ich i s typical of MAP. (C) 1998 Kluwer Academic Publishers.