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.