Continuous atomic layer deposition: Explanation for anomalous growth rate effects

Spatial atomic layer deposition (ALD) on moving substrates has recently been the subject of increasing interest and development. Recent results of deposition on flexible substrates in a cylindrical rotating continuous ALD system showed that in certain regions of operation, deviations from ideal ALD behavior occurred showing excess deposition during the trimethylaluminium (TMA)/water process for aluminum oxide. It was speculated that this was due to boundary layer gas entrainment at the surface of the moving substrate and consequent drag-through of precursors between the different precursor vapor zones. In this paper a study has been made of these gas entrainment effects by using helium as a tracer gas to determine how the transport between zones takes place. A simple model of the process based on physical principles has been constructed which replicates the observed helium transport behavior in the boundary layer. Based on this, it has been shown that gas entrainment is not the reason for the anomalous excess growth in this system. As an alternative explanation, adsorption of excess water molecules on the substrate surface and their carry over to the TMA zone has been proposed as the cause of the anomalous growth. A physical model for this process has been constructed and it has been shown that simulations based on this model reproduce the observed behavior over a range of substrate temperatures. (C) 2012 American Vacuum Society. [DOI: 10.1116/1.3662861]