Low-loss silicon waveguides with sidewall roughness reduction using a SiO 2 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 SiO 2 hard mask and reactive ion etching (RIE) with fluorine gases. The sidewall angle was controlled by Si etching with a SF 6 and CF 4 gas mixture. The thickness of the SiO 2 hard mask affects the sidewall smoothing treatment. The roughness of the sidewall was reduced in the SiO 2 etching process with a CF 4 and O 2 gas mixture as the thickness of the SiO 2 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.