Ambient effects on the laser durability of 157 nm optical coatings

We present results of the durability of antireflectance (AR) coatings under laser irradiation with emphasis on the interplay between coating materials and ambient. We find that introducing ppm-levels of water has a dramatic impact on the performance of certain coatings. In particular, no significant degradation of a coating was observed for up to 1MJ/cm(2) dose in the presence of similar to20 ppm H2O, Whereas linear transmission drop of several percent was observed when irradiating a coating of similar design in <0.1 pprn H2O but under 1.5 ppm O-2. Cycling water concentration on and off leads to a corresponding cycling of transmission of the coatings. Adding water vapor to the ambient has a much greater benefit to coating durability than adding corresponding amounts of gas phase oxygen. In a series of experiments involving the same coating stack with different degrees of porosity of the outer layer, moisture was found to have the greatest impact on the most porous coating.