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

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


Rai, V. R. - Presenter, Colorado School of Mines
Agarwal, S. - Presenter, Colorado School of Mines

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.

Figure 1 IR difference spectra showing the different surface species due to the ligand-exchange reactions during (a) TTIP and (b) O2-plasma half-reaction cycles at a substrate temperature of 150 °C. The almost identical change in absorbance in each spectrum shows a one-to-one exchange of ligands, which is typical of ALD.



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