Paper TF2-TuA8
Plasma-Assisted Atomic Layer Deposition of Titanium Dioxide: Reaction Mechanism Studies using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

Tuesday, November 10, 2009, 4:20 pm, Room B4

In this presentation, the authors will discuss the surface reaction mechanism during the plasma-assisted atomic layer deposition (ALD) of TiO₂ using titanium tetraisopropoxide and an O₂-Ar plasma at substrate temperatures ≤150 °C. In situ attenuated total reflection Fourier-transform infrared (IR) spectroscopy was used to detect surface species generated or consumed during each half-reaction cycle with a sensitivity down to a fraction of a monolayer. Our IR data showed that the reactive species on the TiO₂ surface for TTIP chemisorption were both surface carbonates and –OH groups, identified in the 1450-1700 and 3400-3800 cm^-1 regions, respectively. Based on this observation, we conclude that plasma-assisted ALD of TiO₂ involves a combination of the mechanisms previously reported for O₃- and H₂O-based ALD using TTIP as the metal precursor: we had recently reported surface carbonates as the reactive sites using O₃ as the oxidizing agent and –OH groups have previously been reported with H₂O as the oxidant. We hypothesize the following reaction mechanism. Combustion products such as CO₂, CO, and H₂O are formed during the O₂-plasma cycle due to the plasma-assisted combustion of the isopropoxy ligands on the surface. A fraction of this CO₂ reacts with the surface to generate the carbonates, similar to the O₃-based ALD mechanism. We explain the simultaneous presence of the surface –OH groups in addition to the carbonates due to the plasma activation of H₂O, also generated during the O₂ plasma cycle, and the subsequent reaction of these activated species with the surface. The latter step does not occur when O₃ is used as the oxidant. In fact, 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.