Cryogenic‐Assisted Hydrogen Fluoride Surface Reactions Enabling Reversibly Ultra‐High Selectivity of Atomic Layer Etching Between SiO ₂ and SiN

Atomic-level precision processes are increasingly essential for advanced semiconductor devices with highly-complicated small features. Plasma-enhanced atomic layer etching (ALE) is considered as a promising technique to meet requirements of material diversity and highly selective processing. Here, the ALE processes of SiO₂ and SiN are demonstrated through manipulating hydrogen-fluoride (HF) reactions for surface modification, followed by argon ion bombardment for material removal. By varying substrate temperature (T_s) and introducing ethanol (C₂H₅OH) gas, which provides hydroxyl groups for cryogenic-assisted synergistic reactions, the surface HF reactions and the properties of the surface modification layer can be significantly influenced. The etch amount per cycle (EPC) of SiN ALE decreases to zero with decreasing T_s, regardless of C₂H₅OH addition, due to increased stability of the (NH₄)₂SiF₆ modification layer. No ALE synergy is observed for SiO₂ when C₂H₅OH is not added during the HF dose step, irrespective of T_s. Conversely, the addition of C₂H₅OH at cryogenic temperatures enables the synergistic interactions between HF molecules and hydroxyl groups, enhancing the co-adsorption of HF/C₂H₅OH and lowering activation energy for the fluorination reaction of the SiO₂ that leads to the increased EPC. Consequently, the reversible etching selectivity between ALE SiO₂ and SiN, reaching up to infinity, is achieved.