(488ag) Surface Reaction Mechanism During Atomic Layer Deposition of Titanium Dioxide: A Comparative Study Using Ozone, Water and Oxygen Plasma as Oxidizers

In this presentation, the authors will discuss the surface reaction mechanisms during the atomic layer deposition (ALD) of TiO₂ using titanium tetraisopropoxide (TTIP) and various oxidizers such as O₃, H₂O, and O radicals. In situ attenuated total reflection Fourier-transform infrared (IR) spectroscopy was used to detect surface species generated/consumed during each half-reaction cycle with a sensitivity down to a fraction of a monolayer. The surface coverage and the growth per cycle (GPC) were determined by recording the mass uptake during each half-cycle using the quartz crystal microbalance (QCM). Figure 1 shows IR difference spectra recorded during a complete ALD cycle involving TTIP and O₂ plasma half-reaction cycles. Sub-monolayer surface absorbates were detected in real time and were identified in these spectra. We had recently reported metal carbonates as the reactive sites using O₃ as the oxidizer¹ and ?OH groups have previously been reported with H₂O as the oxidant. In case of O₂-plasma-assisted ALD of TiO₂, both metal carbonates and surface ?OH groups were observed in 1400-1700 cm^-1 and 3200-3800 cm^-1 regions, respectively indicating a combination of reaction pathways of O₃- and H₂O-based ALD chemistry. We hypothesize the following reaction mechanism. A fraction of CO₂, generated due to combustion of isopropoxy ligands, readsorbs on the surface to produce metal carbonates whereas plasma activated dissociation of generated gas-phase H₂O leads to ?OH groups on the surface simultaneously. The latter step does not occur when O₃ is used as the oxidant, and hence, no ?OH groups were observed on the surface. 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. GPC 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. The ALD window was observed over the temperature range of 50-150 °C. 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.

¹Rai, V. R.; Agarwal, S. J. Phys. Chem. C 2008, 112, 9552.