Atomic Layer Controlled Digital Etching of Silicon

Layer-by-layer etching of silicon on atomic scale has been achieved by repeating the reaction cycles of fluorine (F) atom adsorption on a cooled Si surface and subsequent Ar⁺ ion (≃20 eV) irradiation which induces fluorine/Si surface reactions. The digital etch rate first increases and reaches a plateau region with an increase of Ar⁺ ion irradiation time. For the case of CF₄/O₂ downstream plasma as a fluorine source, CF_x radical accumulation appears to be a self-limiting stop of the F/Si reaction to promote atomic layer etching, while F atoms produced by a remote NF₃ plasma or an F₂/95%He discharge also cause similar atomic layer etching in which the amount of physiosorbed fluorine molecules on Si surfaces controls the etch rate. The etching in the plateau region exhibits no microloading effect because the fluorine coverage is independent of pattern size. Anisotropic etching of Si with a 20 nm PMMA mask pattern and an aspect ratio of 5 is attained.