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 (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 −HfN(CH3)2x 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.