Halogen-Free Anisotropic Atomic-Layer Etching of HfO₂ at Room Temperature

Hafnium(IV) oxide (HfO₂)-based materials have attracted substantial interest owing to their outstanding performance in advanced ultrathin semiconductor devices. However, achieving atomic-level precision and smoothness in HfO₂ etching remains a major challenge, primarily due to the nonvolatility of reaction products formed with halogen-based chemicals at room temperature. Herein, a facile cyclic atomic-layer etching (ALE) process capable of etching HfO₂ films at room temperature without the use of halogen-based chemicals is reported. The ALE process consists of a surface nitrogenation step via N⁺-ion bombardment during N₂ plasma exposure, followed by O₂ plasma treatment to remove the surface-modified layer through the formation of volatile etching byproducts-most likely hafnium nitrates. This process enables precise, subatomic-level etching of HfO₂, achieving an etch depth per cycle ranging from 0.23 to 1.07 Å/cycle, depending on the N⁺ ion energy. Additionally, this cyclic ALE method effectively smooths the HfO₂ surface, yielding a 60% reduction in surface roughness after 20 cycles. Based on the proposed mechanism, this facile ALE process can be extended to other transition metal oxides and offers a sustainable route for fabricating advanced functional oxide-based devices, without generating corrosive or toxic wastes.