Atomic layer etching of SiO₂ with self-limiting behavior on the surface modification step using sequential exposure of HF and NH₃

Thermal atomic layer etching (ALE) for SiO₂ films with self-limiting behavior on the surface modification step was developed using sequential exposure to HF and NH₃ gases followed by infrared (IR) annealing. X-ray photoelectron spectroscopy analysis showed that an (NH₄)₂SiF₆-based surface-modified layer was formed on the SiO₂ surface after gas exposures and that this layer was removed using IR annealing. The etch per cycle (EPC) of the ALE process saturated at 0.9 nm/cycle as the gas exposure times increased. With this self-limiting behavior, SiO₂ was etched with high selectivity to poly-Si and Si₃N₄. The dependence of the EPC on the partial pressures of HF and NH₃ was found to be in good agreement with the Langmuir adsorption model. This indicated that the HF and NH_­3 molecules were in equilibrium between adsorption and desorption during the exposure, which resulted in the self-limiting formation of the modified layer. In addition to the process with an HF gas flow, it was demonstrated that an H₂/SF₆ plasma can replace the HF gas exposure step to supply the SiO₂ surfaces with HF molecules. The EPC saturated at 2.7 nm/cycle, while no measurable thickness change was observed for poly-Si and Si₃N₄ films.