The etching of SiO2films on a Si substrate in an rf plasma in CF4has been studied withinsituellipsometry. The etch rate was measured as a function of flow, rf power and pressure. An accurate analysis of the experimental data using numerical simulations based on multilayer models has yielded information both on the refractive index of the etched SiO2film and on the existence of a top layer. It could be established that a layer is present on top of the SiO2during etching, which is probably caused by roughening of the SiO2layer. Furthermore at high pressures (>8 Pa) it was demonstrated that after the complete removal of the SiO2film a polymer layer starts growing on the Si substrate.