(333j) Atomic Layer Etching of Ni3Al for Advanced Photolithography Masks

Semiconductor device scaling to dimensions previously thought impossible has been accomplished using extreme ultraviolet (EUV) lithography. To continue improving EUV capabilities, new materials with precisely tuned optical constants are required for the absorber layer of EUV masks. Ni₃Al is emerging as an important absorber layer material due to its high extinction coefficient and index of refraction close to unity. However, Ni₃Al is difficult to pattern while maintaining etch selectively, anisotropy, and film composition. This work presents an atomic layer etch (ALE) approach to patterning Ni₃Al that involves repeating cycles consisting of a remote nitrogen plasma to form Ni_xAl_yN at the film surface, formic acid vapor to remove the Ni_xAl_yN selective to the metallic Ni₃Al, and an Ar⁺ ion beam to remove residues left by the formic acid. Compared to an earlier, oxidation-based Ni₃Al ALE process, these new nitridation-based results indicate a 10x improvement in selectivity to Ru. Ni is preferentially etched over the Al, changing the film’s optical properties, but basic etchants are explored as a method to counteract this.