Atomic Layer Deposition of Al2O3 Using Trimethylaluminum and H2O: The Kinetics of the H2O Half-Cycle

Atomic layer deposition (ALD) of Al2O3 using trimethylaluminum and H2O is known to proceed through sequential surface, reactions that leave the surface alternately terminated with AlCH3 and OH groups. Using in situ reflection-absorption infrared spectroscopy in a flow reactor, we monitor the consumption of AlCH3 groups by brief pulses of H2O at temperatures between 105 and 300 degrees C. At low temperatures, the surface reactions occur by what appears to be two stages that can be fit to a biexponential decay. Rate laws based on species of AlCH3 groups that also predict a biexponential decay are found to depend on unrealistic activation energies for their constituent reactions when applied to the data. However, a model in which the effective activation energy changes linearly with AlCH3 coverage does adequately fit the data. This model produces the apparent biexponential decay at low temperatures, and it confirms prior suggestions of a coverage dependence in the rate constant. The decrease in the effective activation energy with increasing coverage can be interpreted in terms of a cooperative effect between adjacent AlCH3 groups. These findings may provide a framework for further studies, and both the kinetic model and parametric fits to the data may be useful in the construction of models of this important ALD process.