Plasma-Assisted Atomic Layer Deposition of Titanium Dioxide: Reaction Mechanism Studies Using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

Surface reaction mechanism studies during gas-solid interactions lead to improved insight in the thin film deposition process to efficiently control the film composition, nanostructure, and its electrical properties. Here, authors will present the surface reaction mechanisms during the atomic layer deposition (ALD) of TiO₂ from titanium tetraisopropoxide (TTIP) and an O₂-Ar plasma using in situ real-time attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy having sub-monolayer sensitivity for adsorbed surface species. We have also recently reported the surface reaction mechanism during O₃-based ALD of TiO₂. The IR spectra recorded during each TTIP and O₂ plasma half-reaction cycles show that reactive sites for TTIP chemisorption are both surface carbonates and –OH groups, identified in the 1450-1700 and 3400-3800 cm^-1 regions, respectively. Based on this observation, we conclude the presence of a combination of both O₃- and H₂O-based ALD reaction chemistry during plasma-assisted ALD of TiO₂. A fraction of CO₂, generated due to combustion of isopropoxy ligands, adsorbs on the surface to produce metal carbonates. Unlike O₃, O₂ plasma activates the generated gas-phase H₂O, further dissociating it into –OH groups which adsorb on the surface, and hence, are present along with metal carbonates. The ratio of the carbonates and the surface –OH groups could be varied by controlling the residence time of the reaction products in the plasma. A growth per cycle of ~0.8 Å was obtained at 150 °C, which was significantly higher compared to H₂O- and O₃-based ALD of TiO₂ at similar temperatures. In situ and ex situ IR measurements showed no significant carbon contamination in the films. Ex situ IR data showed the Ti-O-Ti transverse optic mode at 440 cm^-1, a characteristic of anatase. The ex situ x-ray diffraction measurements further confirmed anatase as the dominant crystal phase. The crystallinity of the films may be the reason for the higher growth per cycle compared to that observed for amorphous films deposited from the same metal precursor.