Atomic layer etching of Si₃N₄ with high selectivity to SiO₂ and poly-Si

Atomic layer etching (ALE) is usually classified into ion-driven anisotropic etching or thermally driven isotropic etching. In this work, we present a thermal ALE process for Si₃N₄ with high selectivity to SiO₂ and poly-Si. This ALE process consists of exposure to a CH₂F₂/O₂/Ar downstream plasma to form an (NH₄)₂SiF₆-based surface-modified layer, followed by infrared (IR) annealing to remove the modified layer. CH₂F₂-based chemistry was adopted to achieve high selectivity to SiO₂ and poly-Si. This chemistry was expected to reduce the number density of F atoms (radicals), which contributes to decreasing the etching rate of SiO₂ and poly-Si films. X-ray photoelectron spectroscopy analysis confirmed the formation of an (NH₄)₂SiF₆-based modified layer on the surface of the Si₃N₄ after exposure to the plasma and subsequent removal of the modified layer using IR annealing. An in situ ellipsometry measurement revealed that the etch per cycle of the ALE process saturated with respect to the radical exposure time at 0.9 nm/cycle, demonstrating the self-limiting nature of this etching process. In addition, no etching was observed on SiO₂ and poly-Si films, successfully demonstrating the high selectivity of this ALE process. This high selectivity to SiO₂ and poly-Si is attributed to the fact that the spontaneous etching rates of these films are negligibly small and that there is no surface reaction to etch these films during the IR annealing step.