Atomic scale etching (ASE) of poly-Si, which can give etching with atomic scale accuracy, was investigated in inductively coupled Ar and He plasmas. ASE used a cyclic operation of gas adsorption and ion beam irradiation, which is the same concept as atomic layer etching of single crystal substrates. The etch rate gradually increased, reached the saturated region, and then rapidly increased with increasing bias voltage (or ion energy) in both Ar and He plasmas. This saturation region offered a process window for the realization of ASE. At the bias voltage within the process window for ASE, the etch rate was self-limited with respect to the duration of ion beam irradiation for both Ar and He plasmas, confirming that ASE of poly-Si was successfully achieved in both Ar and He plasmas. The range of the process window for ASE using He plasmas was about 10 times wider than that using Ar plasmas. This is because heavier Ar ions impart a greater fraction of ion energy to the surface atoms compared to lighter He ions.