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 (Al 2 O 3 ) using density functional theory (DFT) calculations. Using amorphous aluminum fluoride (a-AlF 3 ) surface models, we simulated reactions with various metal precursors: TiCl 4, SiCl 4, AlCl 3, Al(CH 3 ) 3 (TMA), and AlCl(CH 3 ) 2 (DMAC). Our results indicate that the first two ligand-exchange steps involving TiCl 4 and SiCl 4 are favorable at 250 °C, releasing TiFCl 3 and SiFCl 3, respectively. However, the subsequent release of any Al-containing product is hindered by high activation barriers, which prevent net etching of the Al 2 O 3 film. In contrast, we found that an alternative reaction pathway involving dimer formation is critical for successful etching processes. Although AlCl 3 is ineffective via the simple ligand-exchange mechanism, it can etch the a-AlF 3 layer effectively by forming a stable Al 2 F 2 Cl 4 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.