| AVS 62nd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS+EN-TuM |
| Session: | Mechanistic Insight of Surface Reactions: Catalysis, ALD, etc. - I |
| Presenter: | Vincent Vandalon, Eindhoven University of Technology, Netherlands |
| Authors: | V. Vandalon, Eindhoven University of Technology, Netherlands W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
The nonlinear optical technique of broadband sum-frequency generation (BB-SFG) has been used to study the surface reactions during atomic layer deposition (ALD). Vibrational BB-SFG spectroscopy is excellently suited for in-situ studies of the surface chemistry governing ALD because of its inherent interface selectivity, submonolayer sensitivity, and short acquisition times. In contrast to BB-SFG, conventional absorption spectroscopy, based on the so called “differential” measurements, monitors only changes on the surface. On the other hand, due to its surface selectivity, BB-SFG reveals information about both persistent and changing surface groups. Therefore, with this technique, open questions can be addressed such as the origin of the decrease in growth per cycle (GPC) at low temperatures of the ubiquitous process of thermal ALD of Al2O3 from Al(CH3)3 and H2O. So far, a complete picture of the surface chemistry explaining the reduced GPC is missing and the exact cause of the limited growth at low temperatures remains unclear.
More particularly, the surface chemistry of thermal ALD Al2O3 was followed by monitoring the density of the -CH3 surface groups. In contrast to ALD at high temperatures, below 200oC it was observed that a significant amount of -CH3 could not be removed during the water half-cycle. The observed kinetics could not be explained by a thermally-activated first-order reaction with a constant cross section. We investigated the temperature dependence of the reaction kinetics further by measuring the -CH3 coverage as a function of precursor and co-reactant exposure at different temperatures. It found that the absolute cross section obtained for the TMA half-cycle was independent of temperature, indicating that the chemisorption of TMA is not a thermally activated process. The behavior during the water half-cycle was found to be more complex showing a strong dependence on temperature; it cannot be described as a reaction simply obeying Arrhenius behavior. This is in line with the more complex behavior predicted by recent DFT work carried out by Shirazi and Elliott [Nanoscale 2015] where a so-called “cooperative” effect was observed leading to a coverage dependent reactivity. The observations presented in this work are direct experimental evidence of such a “cooperative” effect and were only possible due to the inherent surface selectivity of BB-SFG.