Anhydrous Atomic Layer Deposition of HfO₂: Mechanistic Analysis of the Tetrakis(dimethylamido)hafnium (TDMAH)-O₂...

Atomic layer deposition (ALD) is widely used to produce uniform hafnium oxide (HfO₂) thin films with sub-nanometer thickness control. However, most studies on HfO₂ ALD have focused on ligand exchange reactions between surface hydroxyl (-OH) groups and Hf precursors. Here, we investigate the ALD reaction of HfO₂ on an OH-free HfO₂ surface using tetrakis(dimethylamido)hafnium (TDMAH) and molecular O₂. Theoretical approaches including machine learning interatomic potential (MLIP) and density functional theory (DFT) calculations reveal that TDMAH can adsorb onto the pristine monoclinic HfO₂ (-111) surface without OH moieties through a ligand-mediated mechanism. At the initial stage, TDMAH reacts with the HfO₂ surface via a Lewis acid-base interaction, producing surface-bound -Hf[N(CH₃)₂]_x and -N(CH₃)₂ species. Subsequent C-H cleavage of these adsorbed ligands leads to the formation of dimethylamine (DMA) and methylmethylenimine (MMI). Upon exposure to molecular O₂, the adsorbed ligands can subsequently undergo oxidation, yielding energetically stable nitrogen-containing intermediates such as -NCO and -NO₂. In situ FTIR spectroscopy corroborates such reaction pathways by identifying vibrational features related to amido ligands following TDMAH adsorption. Upon subsequent O₂ exposure, these peaks diminish and distinct peaks consistent with the formation of -NCO and -NO₂ species emerge. Regardless of repeated exposures to TDMAH and O₂, features related to the OH groups on the surface are not detected, consistent with the theoretical analysis. These insights provide a foundation for the development of fully anhydrous ALD processes for water-sensitive substrates.