Paper TF-MoA4
Atomic Layer Deposition of Titanium Dioxide: Reaction Mechanism Studies using In Situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

Monday, October 20, 2008, 3:00 pm, Room 302

The authors have investigated the surface reaction mechanism during the atomic layer deposition (ALD) of TiO₂ using titanium tetraisopropoxide (TTIP) as the metal precursor and O₃, atomic O, and H₂O as the oxidizers. The surface species during each half-reaction cycle were detected using in situ, real-time attenuated total reflection Fourier-transform infrared spectroscopy. Sub-monolayer sensitivity was obtained by multiple internal reflections through the ZnSe internal reflection crystal, which has a refractive index that is closely matched to that of amorphous TiO₂. We find that the surface reaction mechanism is very different for O₃- and H₂O-based ALD. Surface hydroxyl groups are generally the reactive sites for the metal precursors in a H₂O-based ALD process. In contrast, we do not detect any hydroxyl groups on the surface after the O₃ cycle. Our data shows that the reactive sites for the adsorption of TTIP after O₃ exposure of the surface are carbonate groups with symmetric and anti-symmetric stretching absorptions in the 1400 – 1700 cm^-1 region. These surface carbonates can be present as mono-, bi-, or poly-dentates. Due to their thermal stability at the growth temperature, all three types of carbonated are present simultaneously resulting in several overlapping absorption bands in the 1400 – 1700 cm^-1 region, which could not be deconvoluted. We hypothesize that during O₃ exposure, CO₂ and H₂O are formed due to the combustion of isopropoxy ligands and a small fraction of these molecules react with the TiO₂ surface producing these surface carbonates. CO₂ was in fact detected as one of the reaction products in the gas-phase by the IR beam passing through the chamber. When the isopropoxy ligands react with the surface, CO₂ from the carbonates was released into the gas phase and was again detected by the IR beam. Using O₃ as the oxidizer, relatively contaminant-free TiO₂ films were deposited at 150 °C: this is nearly 100 °C lower than the widely-accepted minimum temperature for the TTIP-H₂O ALD window. Ex situ spectroscopic ellipsometry data shows a refractive index of ~2.2 and a nearly constant growth rate of ~0.52 Å/cycle over the temperature range of 150-250 °C.