In this study, the etch rate and surface roughness during the Si atomic layer etching using Cl-2 and Ar neutral beam were investigated for Si(100) and Si(111). When the Cl-2 pressure (flux) and the Ar neutral beam irradiation time (dose/cycle) were lower than the critical values, the etch rate (angstrom/cycle) and the surface roughness were varied with Cl2 pressure and Ar neutral beam irradiation time. When both Cl2 pressure and Ar neutral beam irradiation time were higher than the critical values, the saturated etch rates of a monolayer per cycle of 1.36 and 1.57 angstrom/cycle for Si(100) and Si(111), respectively, and the lowest surface roughness of 1.45 angstrom close to the reference sample could be obtained. From these results, it is believed that the silicon etching is controlled by the coverage of silicon chloride during the Cl-2 adsorption by the Langmuir isotherm and the removal of the silicon chlorides by the Ar neutral beam dose/cycle (atom/cm(2) cycle). (c) 2005 The Electrochemical Society.