A comparative study of diluted HF etch rates was performed on thermally grown oxide (1120 °C; O2pressure ≊1.1 atm) and buried oxide layers [formed by implantingn‐andp‐type (100)Si maintained at ≊600 °C with 150–200 keV O+ions to a dose of ≊1.8×1018cm−2, and subsequent annealing at 1250–1325 °C]. From accurate mechanical thickness measurements, a difference in etch rate between thermally grown and buried oxide is observed. This is direct evidence for a structural and/or stoichiometrical difference between both oxides. Additionally, plots of the etch rate as a function of oxide thickness reveal detailed information on the local structure of the oxide layers.