Substrate orientation dependence of self-limited atomic-layer etching of Si with chlorine adsorption and low-energy Ar+ irradiation

The self-limited atomic-layer etching of Si(100), (111), (110) and (211) has been investigated by alternated chlorine adsorption and low-energy Ar+ ion irradiation using an ultraclean electron cyclotron resonance apparatus. The etch rate per cycle is described by a Langmuir-type equation with the etch rate in saturation and the chlorine adsorption rate for each substrate orientation. There is a possibility that the etch rate per cycle in saturation is expressed by a well-regulated fractional number of the atomic-layer thickness. It has been found that the chlorine radicals are dominant adsorption species in the present experimental conditions, and that the sticking probabilities of chlorine radicals are almost independent of the substrate orientation. In XPS measurements, peaks of Si+ have been observed, but neither of Si2+ nor Si3+ on the chlorine adsorbed surface and the adsorbed chlorine layer has been estimated to be as thin as about one monolayer. It has been proposed that the atomic-layer etch rate per cycle in saturation is explained by a simple equation considering the bond structure of each substrate surface.