Abstract

Gold diffraction gratings overcoated with Al + MgF2 to increase their efficiency in the vacuum ultraviolet suffered a severe loss in efficiency within six months to a year after coating; for example, from 50% to 2% at λ1216 Å. The cause of this loss was assumed to be interdiffusion of Au and Al; therefore, a more complete study of Au–Al film combinations was performed. The coatings were aged at room and elevated temperatures. Reflectance measurements were made in the visible and vacuum ultraviolet spectral regions. For wavelengths longer than λ900 Å, the measurements show very little change until the diffusion boundary reaches the penetration depth of the radiation. If Al is the first surface layer, however, reflectance measurements at λ584 Å permit measuring the progress of the diffusion boundary toward the Al surface because of the low absorptance of Al at this wavelength. Interdiffusion can be effectively eliminated by the use of thin dielectric layers such as SiO and the natural oxide of Al. Such protected coatings have been exposed for one week at a temperature of 170°C with no visible sign of diffusion, whereas a similar coating without the barrier layer would become useless in less than 1 h. Some preliminary studies have been made with Pt–Al film combinations.

© 1972 Optical Society of America

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