Effect of surface quality on transmission performance for (111) CaF2

With the advent of 193 nm optical lithography, calcium fluoride (CaF2) has become the material of choice for excimer laser components and for chromatic aberration correction of stepper lens systems. Paralleling the need to maximize transmission performance is the challenge of processing CaF2 to surface qualities equal to if not superior than for conventional fused silica optics. In this report, (1 1 1) calcium fluoride crystals were processed to two different surface qualities and tested for transmittance from 185 to 400 nm. Results show that surface quality in terms of mid- and high-spatial frequency roughness and subsurface damage strongly affect transmittance with a greater increase in transmittance with improved surface quality at lower wavelengths. Specifically for 193 nm, transmittance increased from 90.30% for a 1.4 nm RMS surface to 91.98% for a 0.8 nm RMS surface (Delta = 1.68%). (C) 2001 Published by Elsevier Science B.V.