A SIMS study of ion-assisted etching mechanisms; adsorbed fluorine on Si removed by ion bombardment

Abstract

Secondary ion mass spectrometry (SIMS), previously calibrated using quartz crystal microbalance (QCM) methods, has been used to measure the cross-section for ion-bombardment-induced removal of fluorine adsorbed on silicon. Large (100 Å2) cross-sections have been found at high fluorine coverages. It is believed that ion bombardment of the adsorbed fluorine layer forms relatively large quantities of volatile silicon fluorides which spontaneously desorb from the surface. At low fluorine coverages, where the cross-sections are much smaller (5-10 Å2), there is insufficient fluorine to form volatile silicon fluorides and the adsorbed fluorine must be removed primarily by physical sputtering. This phenomenon of ion-bombardment-induced gasification or ion-induced clearing of adsorbed layers is believed to be responsible for the ion-enhanced gas-surface chemistry observed in directed beam studies involving fluorine and silicon. Furthermore, it is suspected that many of the observations of anisotropic etching in reactive gas glow discharges may be caused by this process. © 1982.