Abstract

Dielectric multilayer coatings of SiO₂ and Si₃N₄ were prepared by reactive rf diode sputtering and by reactive dc magnetron sputtering on glass substrates and, in the latter case, also on plastic substrates. Both types of coating show very good environmental stability which surpasses that of conventional vapor-deposited coatings. It is generally assumed that sputtered films have a higher packing density which makes them less susceptible to water penetration. Transmission electron micrographs of cross-sectional replicas of these multilayer structures indeed show a denser columnar microstructure of the individual Si₃N₄ films compared with many conventional vapor-deposited thin films. But since the microstructure is still columnar, voids must also exist in these films. Hence, other mechanisms than just densification may contribute to the improved resistance of the sputtered coatings to water penetration. One possible explanation is that the plasma discharge which is inherent to the sputtering process has made the thin-film material hydrophobic, so that the voids between the columns in the film are not able to adsorb water. The actual process by which this occurs may be the plasma polymerization of residual hydrocarbons in the vacuum chamber during the sputter deposition of the dielectric films.

© 1986 Optical Society of America