Chemical Mechanism of AlF3 Etching during AlMe3 Exposure: A Thermodynamic and DFT Study

Thermal atomic layer etching (ALE) is a novel approach for the isotropic etching of materials with atomic-level precision. Previous studies have demonstrated the thermal ALE of Al2O3using sequential, self-limiting reactions with HF and AlMe3(Me = CH3) as the reactants. It was found that the HF exposure leads to the conversion of Al2O3to AlF3and the formation of H2O molecules. The present work aims to investigate the subsequent AlF3etching during the AlMe3exposure. On the basis of thermodynamic modeling and density functional theory (DFT) calculations, a two-step mechanism is suggested for the etching of AlF3by AlMe3. In the first step, an AlF3-xMex(0 < x < 1.5) layer is formed on the surface through the ligand-exchange reaction between AlMe3and AlF3, with Al2F2Me4as the main byproduct. The AlF3-xMexsurface layer serves as a key intermediate during the etching process. As compared to the original AlF3, AlF3-xMexhas a lower stability and can be removed more readily. In the second step, the AlF3-xMexsurface layer can be removed via two possible pathways. Pathway I relies on the formation of volatile dimers between AlMe3and the etched species, while Pathway II is accomplished by the direct decomposition of AlF3-xMex. Due to the higher volatility of the reaction products, Pathway I is found to be more favorable than Pathway II. The feasibility of thermal Al2O3ALE using HF with alternative precursors SiCl4, TiCl4, and AlClMe2is evaluated with the above two-step etching mechanism. To realize the AlF3etching during the precursor exposure, both the formation and removal of the AlF3-xMexsurface layer are required to be feasible. Consistent with the experimental results, it is found that only the AlClMe2precursor meets this requirement and can be considered as a candidate precursor. The presented methodology can be used to guide the development of new precursors for the thermal ALE of Al2O3and other materials.