Atomic Layer Etching of Nickel Using N₂/H₂ Plasma Exposure and Hexafluoroacetylacetone

Nickel (Ni) and its aluminides are key materials in extreme ultraviolet lithography masks and nanoscale interconnects, where precise patterning is essential. However, the engineering of Ni-based intermetallics poses significant challenges due to their high physical stability and chemical inertness. This study introduces a plasma-enhanced atomic layer etching (ALE) method for Ni, relying on a surface modification by a N₂/H₂ plasma mixture followed by selective removal of the modified layer with hexafluoroacetylacetone vapor. Optimizing plasma chemistry, power, and exposure time promotes a controlled surface modification, which minimizes surface roughness and enhances process control. Half-reactions are shown to be self-limited, leading to an etch per cycle of 0.21 ± 0.03 nm at 350 °C. Periodic O₂ plasma steps are incorporated to eliminate carbon residues from the surface. X-ray photoelectron spectroscopy reveals a mechanism involving surface nitridation and subsequent removal of the Ni _x N layer. The ALE process is demonstrated on blanket substrates and assessed on prepatterned 3D nanostructures to examine the etching directionality. Transmission electron microscopy studies conducted on the blanket and 3D-structured Ni demonstrate the damage-free characteristics and anisotropic nature of the ALE process. The proposed method represents a significant advance in ALE technology and paves the way for anisotropic Ni patterning, which is essential for the fabrication of future nanoscale devices.