Chlorine reactive ion etching of silicon requires the initial removal of the native oxide prior to rapid etching of silicon. The threshold energy for sputter removal of the native oxide on silicon was measured from the apparent oxide thickness on the silicon surface as determined by x‐ray photoemission spectroscopy of the Si 2pcore level. Using model computations, the threshold energy to sputter the modified native oxide was determined to be 72±5 eV. The surface film chemistry during etching is different above and below the sputter threshold energy. Above the threshold, the silicon surface is contaminated with 1–2 monolayers of SiClx(x=1,2,3) and residual SiClOy. Below the threshold, the oxide is not etched and SiClOyforms on the native oxide surface. The film thickness is observed to increase with ion energy to the threshold energy indicating that ion induced chemisorption phenomena control the film thickness.