Dimer Formation as the Key Removal Pathway in the Isotropic Atomic Layer Etching of Al ₂ O ₃ : A First-Principles Study

We elucidate the removal reaction mechanisms during isotropic atomic layer etching (ALE) of aluminum oxide (Al2O3) using density functional theory (DFT) calculations. Using amorphous aluminum fluoride (a-AlF3) surface models, we simulated reactions with various metal precursors: TiCl4, SiCl4, AlCl3, Al(CH3)3 (TMA), and AlCl(CH3)2 (DMAC). Our results indicate that the first two ligand-exchange steps involving TiCl4 and SiCl4 are favorable at 250 °C, releasing TiFCl3 and SiFCl3, respectively. However, the subsequent release of any Al-containing product is hindered by high activation barriers, which prevent net etching of the Al2O3 film. In contrast, we found that an alternative reaction pathway involving dimer formation is critical for successful etching processes. Although AlCl3 is ineffective via the simple ligand-exchange mechanism, it can etch the a-AlF3 layer effectively by forming a stable Al2F2Cl4 dimer product with low activation energies. The heteroleptic precursor DMAC is the most effective, offering multiple favorable reaction pathways that release various Al-containing dimers. DMAC features the lowest energy barriers, which explains its superior performance in experiments. Our calculations show that breaking the surface metal-fluoride bond is the most difficult part of the etching process, and dimer formation can offset this high-energy penalty by forming additional bonds. These theoretical results align well with experimental observations from in situ analyses, confirming that DFT is a powerful predictive tool for screening precursors and explaining reaction mechanisms in ALE processes.