Atomic Layer Deposition of Al2O3 and ZnO at Atmospheric Pressure in a Flow Tube Reactor

Improving nanoscale thin film deposition techniques such as atomic layer deposition (ALD) to permit operation at ambient pressure is important for high-throughput roll-to-roll processing of emerging flexible substrates, including polymer sheets and textiles. We present and investigate a novel reactor design for inorganic materials growth by ALD at atmospheric pressure. The reactor uses a custom "pressure boost" approach for delivery of low vapor pressure ALD precursors that controls precursor dose independent of reactor pressure. Analysis of continuum gas flow in the reactor shows key relations among reactor pressure, inert gas flow rate, and species diffusion that define conditions needed to efficiently remove product and adsorbed reactive species from the substrate surface during the inert gas purge cycle. Experimental results, including in situ quartz crystal microbalance (QCM) characterization and film thickness measurements for deposition of ZnO and Al2O3 are presented and analyzed as a function of pressure and gas flow rates at 100 degrees C. At atmospheric pressure and high gas flow, ZnO deposition can proceed at the same mass uptake and growth rate as observed during more typical low pressure ALD. However, under the same high pressure and flow conditions the mass uptake and growth rate for Al2O3 is a factor of similar to 1.5-2 larger than at low pressure. Under these conditions, Al2O3 growth at atmospheric pressure in a "flow-through" geometry on complex high surface area textile materials is sufficiently uniform to yield functional uniform coatings.