Trenches are fundamental structures used to build advanced optical planar waveguide components. In this letter, the fabrication of trenches across silica-on-silicon waveguides using inductively coupled plasma etching is presented. These trenches were etched deep into the silicon substrate and their widths were varied between 24 and 100 m. The insertion loss for waveguides with 24-and 28-m-wide trenches etched across them were measured for trenches filled with air and oil. The measured results followed those expected from simulations.