Optical Characterization of Low-Scatter, Plasma-Deposited Thin Films

Plasma-deposited thin films, sometimes called plasma CVD or plasma-enhanced CVD, are deposited in a low pressure gas phase reaction that is controlled by a glow discharge plasma. Substrate temperatures of 200 – 300°C, low compared to conventional CVD processes, are typical for plasma-coating processes. This new coating method has wide possible applications in the optics area. In this paper we will describe studies that have been made to determine if the films are suitable for critical optical applications requiring highly uniform, low-scatter films that, for example, could be used as mirror coatings or as graded index antireflection coatings. We have evaluated the surface quality of SiO₂ and Si₃N₄ films approximately 1000 Å thick, plasma-deposited onto silicon and silicon carbide substrates. Evaluation techniques have included surface profiling using a Talystep instrument, Nomarski microscopy, and total integrated scattering. In all cases, comparisons have been made between the coated and uncoated substrates. The best Si₃N₄ films have been found to contour the very smooth surfaces onto which they were deposited, while the SiO₂ films added only a minimal additional roughness, of the order of 2.8 Å rms. Some problems have been encountered, however, with particulates contained in the films; techniques for minimizing these effects will be discussed.