Chemical sputtering of carbon films by simultaneous irradiation with argon ions and molecular oxygen

The erosion of hard amorphous hydrocarbon films by bombardment with argon ions and simultaneous exposure to thermal molecular oxygen is studied as a function of oxygen flux density ( 0 - 11 400 times the ion flux density), ion energy ( 20 - 800 eV), and surface temperature ( 110 - 875 K). While erosion due to Ar+ ions only is dominated by physical sputtering, the additional presence of molecular oxygen leads to a marked increase of erosion, indicating chemical sputtering. The erosion yield increases with both ion energy and oxygen flux density. Starting from about 700K thermal chemical erosion ( combustion) by O-2 is observed even without ion bombardment. Additional ion bombardment in this temperature range causes an increase of the erosion rate over the sum of thermal chemical erosion and the rate observed at room temperature. Below approximate to 300 K, the rate increases with decreasing temperature. We explain the latter behavior by the ion- induced reaction of adsorbed oxygen which constitutes a significant surface coverage only at low temperatures. A rate equation model is presented, which incorporates the mechanisms of physical sputtering, chemical reaction of O2 at reactive sites created by ion bombardment, the ion- induced reaction of adsorbed oxygen and ion- enhanced thermal chemical erosion. The model's nine free parameters are optimized by fitting 68 experimental data points. The model yields good agreement in all investigated dependences.