The role of chemisorption in plasma etching

Abstract

It is shown that the sticking probability (i.e., the probability for dissociative chemisorption plus the probability for nondissociative chemisorption where E≳20 kcal/mole) for CF4, CF3H, and CF3Cl on polycrystalline films of SiO2, Si 3N4, and silicon is less than 10-7. The sticking probability for CF2Cl2 and CCl4 is detectable on some of these surfaces but still less than 10-6. Electron collisions with nondissociatively adsorbed molecules (E≳20 kcal/mole) are shown to result in the rapid buildup of adsorbed carbon and chlorine which will then remain on the surface indefinitely. It is further shown that CFx radicals have sticking probabilities between 0.08 and 0.75 on clean silicon. Based on these results, it is concluded that the dissociative chemisorption of stable gases is probably not important in plasma etching while radiation-induced-dissociative chemisorption and the chemisorption of CF x radicals are expected to play an important role.