The optical properties of very-thin chromium films and their suitability for use in precision, scanning, reflection interferometry have been studied. The thickness range was from 0–36 nm. The reflectances and transmittances were measured with an absolute reflectometer. The phase change F = 2β1-Φ1-Φ1′ of the film was measured by scanning interferometry, where ø1 and Φ1′ are the phase changes at reflection from the air/film and silica substrate/film interfaces, respectively, and β1 is the phase change on transmission through the film. F influences the form and symmetry of the reflection interference fringes. The phase changes ø1 and ø1′ have also been measured directly. For films 6–10 nm thick (air/film reflectance 20–40%), and with a highly reflecting second surface, the reflection fringes are transmission-like fringes (F→2n π) of high contrast and good symmetry. Fringes of optimum contrast (visibility nearly unity) and symmetry are obtained for films with air/film reflectances of 20–30%, which was obtained with thicknesses from 6–8 nm. Accurate settings on these fringes can be made to the order of 0.001 fringe in a scanning-interferometer system. The results indicate that the films have continuous structures, with optical properties insensitive to wavelength, and with the phase properties very different from metal films such as silver.
C. F. Bruce and W. K. Clothier, "Optical properties of thin chromium films," J. Opt. Soc. Am. 64, 823-829 (1974)