Optical properties of obliquely evaporated aluminium films

Vacuum-evaporated aluminium films can show substantially different optical properties depending on the angle at which the vapour stream impinges on the substrates. Films deposited at 85° oblique incidence developed diffuse reflecting surfaces and showed greater absorption than those condensed at normal incidence. Electron micrographs of such films reveal chains of crystallites whose long axes are inclined to the substrate. The orientation of these crystallites gives rise to optical anisotropy and the optical transmission of the films strongly depends on the direction of the vapour beam. The surface density at which the films start to show surface roughness can be increased if the films are condensed in an argon atmosphere. These films were shown to have superior reflectivity and to exhibit very low absorption or scattering. Optical anisotropy in these films could hardly be observed.