Evaluating operating conditions for continuous atmospheric atomic layer deposition using a multiple slit gas source head

Continuous atmospheric atomic layer deposition (ALD) is based on the separation of ALD reactants in space rather than in time. In this study, operating conditions for continuous atmospheric ALD were evaluated using a multiple slit gas source head. A test apparatus was constructed consisting of a gas source head that sits above a substrate that is driven by a programmable stepper motor. The gas source head was composed of a series of rectangular channels that spatially separate the ALD reactants. With Al2O3 ALD as the model system, the trimethylaluminum (TMA) reactant channel was positioned in the center and the H2O reactant channels were located on both sides of the TMA channel. This design allowed for deposition of two Al2O3 ALD cycles during one complete back-and-forth translation of the substrate. The gap spacing between the gas source head and substrate was fixed and controlled using micrometers. A series of experiments was conducted using He as a tag gas to determine operating conditions that prevent reactant cross diffusion. There was a wider range of suitable exhaust pumping speeds at a 30 μm gap spacing than at a 100 μm gap spacing. However, mechanical tolerances were not sufficient to allow back-and-forth translation of the substrate at a gap spacing of 30 μm. For Al2O3 ALD at a 100 μm gap spacing, the best Al2O3 film uniformity was achieved when there was a slight vacuum of ∼−5 Torr beneath the gas source head relative to ambient. A larger vacuum beneath the gas source head occurred if the pumping speed of the exhaust channel was higher relative to the N2 supplied to the gas source head. These conditions produced a smaller footprint of Al2O3 deposition resulting from an influx of N2 from the higher pressure ambient. The gas source head had a higher pressure than ambient if the exhaust pumping speed was too low relative to the N2 supply. Under these conditions, reactant gas leaked out the sides and formed Al2O3 powder around the perimeter of the gas source head resulting from the chemical vapor deposition reaction of TMA with ambient moisture. A response surface model was generated to predict the dependence of the pressure beneath the gas source head relative to ambient on the various operating conditions.