Characterization of homogeneous and inhomogeneous Si‐based optical coatings deposited in dual‐frequency plasma

Silicon-compound optical coatings (silicon nitride, dioxide, oxynitrides) were prepared by plasma-enhanced chemical vapor deposition (PECVD) at near-ambient substrate temperature from SiH4/N2O/NH3 mixtures, using a dual-mode microwave/radiofrequency plasma system. The refractive index profile of the films was adjusted with depth in two ways: (1) between 1.45 (SiO2) and 1.90 (SiN1.3) by varying the N2O/NH3 feed gas ratio, and (2) between 1.65 and 1.90 for SiN1.3 films, by increasing the energy of bombarding ions from 5 eV to 400 eV. The film composition was changed either abruptly (multilayers) or continuously (graded-index, inhomogeneous layers). The resulting optical properties are correlated with the depth profile analysis of the chemical composition, and with mechanical characteristics such as adhesion, stress, and scratch resistance.