Sidewall Smoothing for Si/SiO2 Waveguides by Excimer Laser Reformation

Si/SiO₂ waveguides are applicable to modern silicon nano-photonics and are compatible with Si electronics for compact integration. However, for the submicron-scale waveguides, surface roughness sensitively affects the scattering loss and then causes an impediment to use. Many efforts are devoted to reducing the scattering loss by minimizing the roughness of the sidewalls, including hydrogen annealing, dry oxidation, and wet chemical etching. These methods, however, have several drawbacks. The two methods using high temperature have concerns on the thermal budgets and the capability of oxidation method to reduce a large roughness is limited. Here, we demonstrate the fabrication of smooth Si waveguides by laser reformation. This technique has no limitation on thermal budgets in integration of electronics and photonics if protective coatings are used to prevent exposure of the electronics during laser illumination. The process to smooth as-etched Si waveguides is quite simple. The waveguides are illuminated by 248nm homogenized KrF excimer laser with pulse duration of 25ns. During the illumination process, waveguides are placed in a vacuum chamber.