Low-loss silicon waveguides with sidewall roughness reduction using a SiO₂ hard mask and fluorine-based dry etching

High optical field confinement-possible silicon waveguides are used in various fields. The performance of a silicon waveguide depends on its sidewall roughness, which is responsible for scattering loss. The fabrication process for a silicon waveguide with a smooth sidewall for low loss was studied. The propagation loss of an optical silicon waveguide was reduced by using a SiO₂ hard mask and reactive ion etching (RIE) with fluorine gases. The sidewall angle was controlled by Si etching with a SF₆ and CF₄ gas mixture. The thickness of the SiO₂ hard mask affects the sidewall smoothing treatment. The roughness of the sidewall was reduced in the SiO₂ etching process with a CF₄ and O₂ gas mixture as the thickness of the SiO₂ hard mask was increased. The minimum propagation loss (0.89 dB cm^-1) of the fabricated silicon waveguide was achieved by using the fabrication method introduced in this paper.