Anhydrous Atomic Layer Deposition of HfO ₂ : Mechanistic Analysis of the Tetrakis(dimethylamido)hafnium (TDMAH)–O ₂ Process

Atomic layer deposition (ALD) is widely used to produce uniform hafnium oxide (HfO 2 ) thin films with sub-nanometer thickness control. However, most studies on HfO 2 ALD have focused on ligand exchange reactions between surface hydroxyl (−OH) groups and Hf precursors. Here, we investigate the ALD reaction of HfO 2 on an OH-free HfO 2 surface using tetrakis(dimethylamido)hafnium (TDMAH) and molecular O 2 . Theoretical approaches including machine learning interatomic potential (MLIP) and density functional theory (DFT) calculations reveal that TDMAH can adsorb onto the pristine monoclinic HfO 2 (−111) surface without OH moieties through a ligand-mediated mechanism. At the initial stage, TDMAH reacts with the HfO 2 surface via a Lewis acid–base interaction, producing surface-bound −Hf[N(CH 3 ) 2 ] x and −N(CH 3 ) 2 species. Subsequent C–H cleavage of these adsorbed ligands leads to the formation of dimethylamine (DMA) and methylmethylenimine (MMI). Upon exposure to molecular O 2, the adsorbed ligands can subsequently undergo oxidation, yielding energetically stable nitrogen-containing intermediates such as −NCO and −NO 2 . In situ FTIR spectroscopy corroborates such reaction pathways by identifying vibrational features related to amido ligands following TDMAH adsorption. Upon subsequent O 2 exposure, these peaks diminish and distinct peaks consistent with the formation of −NCO and −NO 2 species emerge. Regardless of repeated exposures to TDMAH and O 2, 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.