We report the observation of self-limited layer-by-layer etching of Si by alternated chlorine adsorption and low energy Ar+ ion irradiation using an ultraclean electron-cyclotron-resonance plasma apparatus. The etch rate per cycle increased with the chlorine supplying time and saturated to a constant value of about 1/2 atomic layer per cycle for Si(100) and 1/3 for Si(111), which was independent of the chlorine partial pressure in the range of 1.3-6.7 mPa. These results indicate that etching was determined by self-limited adsorption of chlorine. Moreover, the chlorine adsorption rate was found to be described by a Langmuir-type equation with an adsorption rate constant k=83 and 110 (Pa s)-1 for Si(100) and Si(111), respectively.
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