Mechanical stress in 355 nm LaF3/MgF2 high reflectors with various layer-pair number and methods for reduction

Layer-pair number ranged from 9 to 22 of 355 nm LaF3/MgF2 high reflectors were deposited by boat resistant heating onto substrates of fused silica and K9 glasses, and the total stress was obtained by curvature measurements. The stress increased with incremental layers until crazing happed because of huge tensile stress at higher layer-pair number. Two methods called post-heat treatment in vacuum and double stack design with combination of LaF3/MgF2 and HfO2/SiO2 coatings were found to be effective to reduce the stress and avoid crazing. Especially in the double stack designed coatings, both low stress and high optical properties with transmittance smaller than 0.4% at 355 nm were obtained. Substrates and age effects were also discussed.