Anisotropic etching of deep trenches in single‐crystal Si has been obtained using reactive ion etching with gas mixtures for the first time. The influence of wafer temperature, total gas pressure, and content on profile control and etch selectivity has been determined. A high anisotropy and selectivity (18:1) have been achieved at 25% content. Scanning electron microscopy and Auger electron spectroscopy techniques have been used to study the trench profile and surface roughness. Methods for post‐reactive ion etching surface treatment have been explored to remove surface roughness.