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Near-UV to IR optical characterization of YF3 thin films deposited by evaporation and ion beam processes

A comparative study of yttrium fluoride thin films deposited either by conventional e-beam evaporation or ion beam sputtering (IBS) processes was made. Each coating was systematically characterized in terms of chemical composition, density and optical properties in the near UV- visible and IR wavelength ranges. The evaporated films exhibited low densities whereas the IBS coatings were found to be rather dense. As a result, the IR optical properties of the IBS films were more stable without any large losses related to water peak absorption at (lambda) equals 2.9 and 6.1 micrometers. Moreover, the reactive deposition conditions drastically decreased the IR losses of sputtered films in the 10 to 12.5 micrometers wavelength range. The films analyzed by Rutherford backscattering spectroscopy and nuclear reaction analysis were found refractive indices and extinction coefficients were precisely measured using spectrophotometric and ellipsometric methods as well as a photothermal deflection technique. The strong variations of the optical properties from one sample to another were thus correlated to the evolution of the chemical properties of the films. Finally, this work shows that IBS processes used in appropriate operating conditions can lead to low absorbing YF3 thin films usable for applications that require transparency over a very wide wavelength range, from 0.3 to 12.5 micrometers.