Plasma etching to As2S3 thin films for optical waveguide fabrication has been studied using a helicon plasma etcher. The etching effects using the processing gases or gas mixtures of O-2, Ar, and CF4 were compared. It was found that the O-2 plasma had no chemical etching effect to the As2S3, but it could oxidize the surface of the As2S3. The Ar plasma provided a strong ion sputtering effect to the films. The CF4 plasma exhibited a too strong chemical etch to the As2S3, leading to serious undercutting and very rough sidewalls of the waveguides. Ar and O-2 gases were compared as the additives to dilute the CF4 processing gas. The etch rate of the As2S3 was reduced dramatically from over 2000 nm/min to a few hundred nm/min when the pure CF4 gas was heavily diluted with 70% Ar or O-2 gas. The undercutting and sidewall roughness of the etched waveguides were also decreased greatly when above dilution was made, which was associated with an enormous weakening of the isotropic chemical etch induced by neutral reactants in the plasma. In addition, the O-2 showed a better dilution effect than the Ar in reducing the etch rate of the As2S3; and the O-2/CF4 plasma also enabled a much lower erosion rate to Al mask layers than the Ar/CF4 plasma at similar plasma conditions. The As2S3 waveguides with near vertical and very smooth sidewalls were obtained using an optimized O-2/CF4 plasma. Moreover, the etching behaviors and mechanisms were explained base on the etching results, and on the characteristics of the applied plasma diagnosed using Langmuir probe and optical spectroscopy techniques. (c) 2005 American Vacuum Society.