Progress in sidewall morphology smoothing of Si wire waveguides by thermal oxidization was observed using a focused-ion-beam (FIB) transmission-electron-microscopy technique. The roughness of Si/SiO2 interface was drastically reduced with increasing oxidation time and temperature. Deformation of the cross-sectional profile of the waveguides was observed with secondary electron images of scanning FIB irradiation. The initial rectangular profile is transformed due to stress concentration at the rectangle corners at oxidation temperatures of 900 and 1000 degreesC. In contrast, at 1100 degreesC, the profile maintains the original rectangular profile due to the stress release by the viscous flowing of SiO2. These results indicate that the optimum oxidation condition for the Si wire waveguide has been found, which provides an extremely smooth sidewall without deformation of the cross-sectional profile. (C) 2004 American Vacuum Society.