Layer-by-layer etching of Si(I 0 0) has been investigated by alternating chlorine adsorption and Ar+ ion irradiation. As the Ar+ ion source. a helicon plasma has been used to invoke the surface reaction of Si with the adsorbed chlorine. At a chlorine partial pressure of 10 mPa, the Si surface was found to be fully saturated with adsorbed chlorine in 20 s. In order to achieve the layer-by-layer etching of Si, the Ar+ ion acceleration energy should be above 20 eV at the time of chlorine adsorption. At energies above 40 eV, however. sputtering of Si was found to occur. The self-limited etch rate was measured to be half monolayer per cycle, which was 0.68 Angstrom per cycle. The roughness of Si surface was found to increase during the etching by about 22% after 900 cycles. (C) 2002 Elsevier Science B.V. All rights reserved.