Role of physisorption in atomic layer etching of silicon nitride

Atomic layer etching (ALE) techniques are growing in popularity due to their various benefits such as low damage, high selectivity, and precise and controlled etching. In ALE, surface modification is typically achieved by chemisorption of radicals generated in a plasma followed by ion assisted removal of the modified surface. A surface modification process assisted by a plasma may lead to unwanted side effects such as excessive polymerization or spontaneous etching. In this work, the authors demonstrate the feasibility of atomic layer etching of silicon nitride by physisorption of hydrofluorocarbon gas precursors [CH_xF_(4−x), x = 1−4] in the absence of a plasma. Self-limited SiN etching was observed for all hydrofluorocarbon gases, with the largest etch depth per cycle observed for CF₄ and CHF₃ adsorption (∼15 Å/cycle), and smallest etch depth per cycle observed using CH₃F adsorption (∼6 Å/cycle). Etch precursor availability on the surface was manipulated by varying absolute pressure, partial pressure of the adsorbate, and adsorption time. The results of these experiments indicated that of physisorption of precursors is enough to modify the surface, leading to its removal in the subsequent plasma assisted desorption step.