Surface morphology of HCl etched silicon wafers

When silicon is etched by HCl, either a smooth or a pitted surface structure results, depending on temperature and HCl input pressure. The transition from a smooth to a pitted surface structure appears to be independent of the type of crystallographic orientation of the surface planes. The type of carrier gas does have an influence; using argon instead of hydrogen, the transition line shifts to lower values of the HCl-input concentration. It is shown that the etching process of the smooth surface is related to the equilibrium concentration of SiCl2 in the gas phase, whereas surface reactions are responsible for the pit formation. In this formalism etchpit formation is caused by local undersaturation of the main reaction product, SiCl2.