Embedded single and multimode waveguides on fused quartz substrates have been fabricated by high‐speed silica deposition and accurate reactive sputter etching by a C2F6gas plasma. These waveguides have rectangular cross sections with an aspect ratio close to unity. Total waveguide loss was approximately 5 dB/cm, and absorption loss measured by laser calorimetry at 637‐nm wavelength was less than 0.1 dB/cm.