Formation and desorption of nickel hexafluoroacetylacetonate Ni(hfac)$_{2}$ on a nickel oxide surface in atomic layer etching processes

Thermal atomic layer etching (ALE) of nickel (Ni) may be performed with a step of thin-layer oxidation of its surface and another step of its removal by gas-phase hexafluoroacetylacetone (hfacH) as an etchant. In this study, adsorption of hfacH and possible formation of volatile nickel hexafluoroacetylacetonate Ni(hfac)$_{2}$ on a NiO surface were investigated based on the density functional theory (DFT) with more realistic surface material models than those used in the previous study [A. H. Basher et al., J. Vac. Sci. Technol. A 38, 022610 (2020)]. It has been confirmed that an hfacH molecule approaching a NiO surface deprotonates without a potential barrier and adsorbs on the surface exothermically. In addition, stable adsorption of two deprotonated hfacH molecules on a NiO (100) surface was found to occur not on a single Ni atom but over a few Ni atoms instead, which makes the formation of a Ni(hfac)$_{2}$ complex on the flat surface very unlikely even at elevated temperature. However, if the surface is rough and a Ni atom protrudes from the surrounding atoms, two hexafluoroacetylacetonate anions (hfac$^{-}$) can bond to the Ni atom stably, which suggests a possibility of desorption of a Ni(hfac)$_{2}$ complex from the surface at elevated temperature. ... mehrGiven the experimentally observed fact that desorption of Ni(hfac)$_{2}$ complexes typically takes place on a NiO surface at a temperature of ∼300–400 °C, our DFT calculations indicate that the surface roughness of an oxidized Ni surface facilitates the formation and desorption of organometallic complexes Ni(hfac)$_{2}$, and therefore, the resulting Ni surface after ALE can be smoother than the initial surface.