Enhanced optical performance of aluminum films by copper inclusion

Abstract

The aluminum film is a standard coating material for optics. It yields a high reflectance over a broad wavelength region, and films can be made sufficiently smooth for high quality optical applications. However, it suffers from a high sensitivity to hillock formation and severe degradation of its low light scattering properties, already at temperatures of 70–80°C. In order to raise the temperature for onset of hillock formation we have co-evaporated a slight amount (3 at.%) of Cu into the Al film. The anti-hillocking performance was indeed confirmed by raising this temperature to ∼130°C. But, we also greatly reduced the surface roughness of the as-deposited 3600 Å films to the outstanding values of 1-3 Å rms as measured by the ASTM F 1048-87 total integrated scattering (TIS) method. This yields very low light scattering films despite their thickness. Also, the specular reflectance was coming out just as high as for a pure Al film over a wide wavelength range. The enhanced optical performance is attributed to less light scattering of the co-evaporated film and to the strengthening effect in the film.