Atomic Force Microscopy Observation of Water-Induced Morphological Changes in Yz03 Monolayer Coatings

Environmental stability studies for thin-film coating materials employed on the upgraded OMEGA laser system are essential for defining acceptable air humidity levels and their control, as well as for facilitating proper thin-film coating design of long-term-use OMEGA laser optics. Moisture penetration into porous, dielectric thin-film coatings causes generally undesirable changes in the film's optical perforniance parameters-a fact that has been well known for some time now.'.2 The major effects previously observed and studied in this regard are shifts in the transmission characteristics of narrow-band filt e r ~ , ~ , ~ caused by the replacement of air with a 1.3-index medium inside the pores; enhanced absorption in the IR water band;5 and changing mechanical stress (either compressive or t e n ~ i l e ) ~ that renders difficult maintaining stringent wavefront-error requirements on reflective components. Thermal annealing7 ion bombardment, and. more generally, preparing coatings with dense-packing structure are aniong the methods devised in combating water effects in dielectric films.