(378c) 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.