Recent progress in optical coating technology: low-voltage ion plating deposition

Abstract

After fairly extensive discussions of the advantages and disadvantages of low energy and high energy ion beam bombardment of a growing film, we review briefly a number of experimental results obtained with various samples made with low voltage reactive ion plating deposition. The availability of a state-of-the-art high vacuum coating machine specifically equipped for this process is the foundation for a major leap toward achieving near-perfect optical coatings. The high density of ion plated thin films makes them impermeable to water vapor and corrosive solutions. This has been demonstrated with protected aluminum mirrors, polarizers, and infrared anti-reflection coatings. An indication of the high packing density is the substantially higher refractive index than that of comparable layers deposited with either conventional electron beam evaporation or ion assisted deposition. The spectral transmittance of multilayer stacks of oxide thin films is lower than expected from theoretical predictions which assume absorption-free dielectrics. The observed absorption is primarily of an interface nature rather than a volume effect and occurs predominantly in combinations of Ti02 and Si02 thin films.