Dry-etch of As₂S₃ thin films for optical waveguide fabrication

Abstract

Plasma etching to As₂S₃ thin films for optical waveguide fabrication has been studied using a helicon plasmaetcher. The etching effects using the processing gases or gas mixtures of O₂, Ar, and CF₄ were compared. It was found that the O₂plasma 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 CF₄plasma exhibited a too strong chemical etch to the As₂S₃, leading to serious undercutting and very rough sidewalls of the waveguides. Ar and O₂ gases were compared as the additives to dilute the CF₄ processing gas. The etch rate of the As₂S₃ was reduced dramatically from over 2000nm∕min to a few hundred nm/min when the pure CF₄ gas was heavily diluted with 70% Ar or O₂ 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 O₂ showed a better dilution effect than the Ar in reducing the etch rate of the As₂S₃; and the O₂∕CF₄plasma also enabled a much lower erosion rate to Al mask layers than the Ar∕CF₄plasma at similar plasma conditions. The As₂S₃ waveguides with near vertical and very smooth sidewalls were obtained using an optimized O₂∕CF4plasma. 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.