Employing an Aluminum Fluoride Thin Film Interference layer to find Optical Constants in the Extreme Ultraviolet*

 Interference fringes in reflectance and transmission are invaluable in obtaining thicknesses and optical constants of thin-films.  However, interference fringes may not be produced for absorbing materials.  A technique to deal with this, in the visible is to place a transparent layer beneath the absorbing thin-film (Hilfiker, et al.) A portion of the light passing through that semitransparent film, reflecting off the substrate and after transmitting again through the film, interferes with the front-surface, reflected light producing interference fringes whose position depends on the layers’ thicknesses and indices, and the light’s wavelength and angle of incidence. The damping of the fringes highly constrains the optical constants of this overlayer. We have extended this approach into the extreme ultraviolet to obtain the optical constants of aluminum fluoride between 17.1 and 49.5 nm using evaporated aluminum as a “semitransparent” interference layer. The AlF₃ is evaporated within minutes after the aluminum and without breaking vacuum so as to minimize the presence of oxygen on the Al film. The AlF₃ also acts as a barrier layer, drastically retarding the oxidation of the aluminum film after it is removed from the deposition system.