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

Abstract

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. Given 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.

Molecular Mechanism of Thermal Dry Etching of Iron in a Two-Step Atomic Layer Etching Process: Chlorination Followed by Exposure to Acetylacetone

“…This observation leads us to think that there may be a disconnect between the numerous current computational investigations of the ALE process and the realistic processing conditions. The majority of the computational studies of the diketonate ligands participating in the metal ALE start with the most thermodynamically stable bidentate structure ,, and find it thermodynamically impossible at reasonable processing conditions to form a metal-containing product that could be removed from the terrace site of the surface. However, in realistic processing conditions, at reasonably high pressures, the kinetic factors and surface crowding by the ligands could easily lead to the formation of stable monodentate diketonate ligands as dominant surface species (even if there is a presence of stable bidentate ligands) that could fundamentally change the energy landscape for analysis of ALE schemes.…”

Thermodynamics of Atomic Layer Etching Chemistry on Copper and Nickel Surfaces from First Principles

“…7 It is likely, however, that the magnitude of such barriers will depend to a significant extent on the surface morphology. 11 The postplasma surface morphology is far from clear at this point, and we do not claim that the surface structure used here corresponds to the postplasma surface. In other words, the effect of kinetic energy of the ion is a missing link, deferred to a future study.…”

“…Existing data suggests that the desorption of the metal-organic complex is the limiting step, and often high temperature is required to overcome the barrier 7 . It is likely, however, the magnitude of such barriers will dependent to a significant extent on the surface morphology 11 .…”

“…The existing thermal ALE processes on metal substrates require the oxidation of pristine metals by either oxygen (O 2 and/or O 3 ) or halogens, followed by the introduction of diketone molecules to form volatile metal complexes. Existing studies center on a single process, using combinations of experimental and computational tools to propose a mechanism. − These allow for the classification of existing ALE processes, and the rationalization can be generalized to other substrates, modifiers, and etchants to discover new processes. However, difficulties of in situ characterization of the surface and the low pressures of the products formed prevent direct confirmation of these mechanisms.…”

Recent Progress of Atomic Layer Technology in Spintronics: Mechanism, Materials and Prospects

“…The entire process begins with the oxidation of Ni by oxygen to generate nickel oxide, which is subsequently reacts with Hhfac to achieve etching [157]. Furthermore, in 2020, Basher et al investigated the formation and desorption of nickel hexafluoroacetylacetonate Ni(hfac) 2 on a nickel oxide surface using atomic layer etching operations [155].…”