Reactive ion etching of PECVD a‐Si:H and silicon nitride films was investigated using CF4, C2F6, CHF3, CF3Cl, and CF2Cl2 gases. Under the same power the a‐Si:H etch rate decreases in the order of: CF4∼CF3Cl≫CF2Cl2≫C2F6≫CHF3; and the nitride etch rate decreases in the order of: CF4≫CF3Cl∼C2F6≫CF2Cl2≫CHF3. The etch selectivity is controlled by two competitive mechanisms: plasma chemistry and surface reactions. When no surface mass‐transfer‐limit layer is formed, the plasma phase chemistry dominates the etch rate of the film. For example, the silicon etch rate increases with the chlorine concentration in the plasma phase. When the surface is covered with a thin film, the etch rate is controlled by the ion bombardment energy. For example, the film etch rate in the C2F6 plasma depends on the self‐bias voltage only. ESCA and emission spectra results support the above theory.