Fabrication of ultralow-loss Si/SiO_2 waveguides by roughness reduction

We demonstrate $0.8‐\mathrm{dB}/\mathrm{cm}$ transmission loss for a single-mode, strip $\mathrm{Si}/{\mathrm{SiO}}_2$ waveguide with submicrometer cross-sectional dimensions. We compare the conventional waveguide-fabrication method with two smoothing technologies that we have developed, oxidation smoothing and anisotropic etching. We observe significant reduction of sidewall roughness with our smoothing technologies, which directly results in reduced scattering losses. The rapid increase in the scattering losses as the waveguide dimension is miniaturized, as seen in conventionally fabricated waveguides, is effectively suppressed in the waveguides made with our smoothing technologies. In the oxidation smoothing case, the loss is reduced from $32 \mathrm{dB}/\mathrm{cm}$ for the conventional fabrication method to $0.8 \mathrm{dB}/\mathrm{cm}$ for the single-mode waveguide width of $0.5 \mu \mathrm{m}$. This is to our knowledge the smallest reported loss for a high-index-difference system such as a $\mathrm{Si}/{\mathrm{SiO}}_2$ strip waveguide.