Atomic layer deposition of titanium oxide thin films using a titanium precursor with a linked amido-cyclopentadienyl ligand

Abstract

We studied the atomic layer deposition (ALD) of titanium oxide (TiO₂) thin films using a newly developed heteroleptic titanium precursor with a linked ligand. The titanium precursor, [2-(N-methylamino) 1-methyl ethyl cyclopentadienyl] bis(dimethylamino) titanium (CMENT), was designed to enhance the maximum ALD process temperature by linking the Cp ligand of tris(dimethylamino)cyclopentadienyl titanium [CpTi(NMe₂)₃] to an amine ligand. The maximum temperature for the self-limiting ALD growth using CMENT and O₃ was 300 °C, 75 °C higher than tetrakis(dimethylamino)titanium (TDMAT) or CpTi(NMe₂)₃. At 300 °C, high-density (4.0–4.1 g cm⁻³) high-purity TiO₂ films were prepared with a growth rate of ∼0.7 Å per cycle, good step coverage, and wide bandgap (3.0–3.1 eV). CMENT can grow TiO₂ films using H₂O, unlike heteroleptic precursors with a cyclopentadienyl ligand. The density functional theory (DFT) simulation on the thermolysis of titanium precursors showed that TDMAT has the best thermal stability as a molecule, which could not explain the highest ALD temperature of CMENT. The simulation of the surface reactions of the precursors expected that the linked ligand is the only surface species for CMENT, whereas N(CH₃)₂ (NMe₂) ligands remain on the surface for TDMAT or CpTi(NMe₂)₃. The absence of thermally unstable NMe₂ in the CMENT case explains the highest ALD temperature of CMENT.