Mechanism of silicon etching by a CF4 plasma

The mechanisms for the reactive ion etching of silicon by CF4 plasma are investigated. A model is proposed whereby silicon is etched by chemical reaction with free fluorine to produce a volatile species, and also by physical sputtering. The chemical etching is shown to be enhanced by ion bombardment of the reacting surface. This etching process, together with a model for cracking CF4 in the plasma, is evaluated by comparison to actual etch rates. Experimentally, the silicon etch rate is observed to decrease with increasing silicon area, by what is called the loading effect. The functional form of the loading effect, as predicted by the model, is fitted to experimental loading curves. The contributions of the various etching components are separated, to yield empirical values for the enhancement of the chemical reaction by physical sputtering.