Article pubs.acs.org/Langmuir
Atmospheric Pressure Atomic Layer Deposition of Al2O3 Using Trimethyl Aluminum and Ozone Moataz Bellah M. Mousa,* Christopher J. Oldham, and Gregory N. Parsons
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Department of Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, United States ABSTRACT: High throughput spatial atomic layer deposition (ALD) often uses higher reactor pressure than typical batch processes, but the specific effects of pressure on species transport and reaction rates are not fully understood. For aluminum oxide (Al2O3) ALD, water or ozone can be used as oxygen sources, but how reaction pressure influences deposition using ozone has not previously been reported. This work describes the effect of deposition pressure, between ∼2 and 760 Torr, on ALD Al2O3 using TMA and ozone. Similar to reports for pressure dependence during TMA/water ALD, surface reaction saturation studies show self-limiting growth at low and high pressure across a reasonable temperature range. Higher pressure tends to increase the growth per cycle, especially at lower gas velocities and temperatures. However, growth saturation at high pressure requires longer O3 dose times per cycle. Results are consistent with a model of ozone decomposition kinetics versus pressure and temperature. Quartz crystal microbalance (QCM) results confirm the trends in growth rate and indicate that the surface reaction mechanisms for Al2O3 growth using ozone are similar under low and high total pressure, including expected trends in the reaction mechanism at different temperatures.
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INTRODUCTION High-speed continuous atomic layer deposition (ALD) processes under ambient conditions are offering new advanced capabilities to many different industries for their surface depositions ranging from nanoscale to microscale.1 ALD is being used to create gas diffusion barriers on flexible polymers,2,3 for example, for advanced organic light emitting displays and solar energy conversion cells.4−6 ALD coatings on fibrous materials are also of interest for applications including filtration and protective fabrics.7 A key hurdle for process commercialization is transitioning ALD from a low deposition rate batch process to a continuous process with higher throughput while maintaining the same material properties and functionality, especially under low temperature and high pressure conditions. ALD is a sequential, self-limiting, vapor phase, nanocoating process compatible with many oxide, nitride, and elemental materials. The process can produce very uniform and conformal coatings even on high aspect ratio and 3D structures. In a typical ALD batch process, the reactants pass over the substrate in distinct steps separated by a purging gas flow. The purge step eliminates gas-phase mixing of the coreactants, thereby promoting ultra-uniform coatings. However, the gas cycling extends the time needed for deposition, especially when long purge times are necessary. Water is a common oxygen source for ALD of metal oxides. The reaction between water and trimethylaluminum (TMA) to form aluminum oxide (Al2O3) is the most widely studied and best understood ALD process.8−10 Although water is readily available, it has a high sticking coefficient to oxides and tends to © 2014 American Chemical Society
form hydrogen bonds with surfaces. Water desorption and removal out of the reactor during the purge step becomes challenging under low temperature and high pressure conditions. Groner et al.11 showed that under low pressure ALD conditions, Al2O3 grown with TMA and water at ∼1 Torr and ∼60 °C required a water purge time of 30 s compared to 5 s to achieve the same growth rate at 177 °C. Decreasing the temperature to 33 °C required even longer purge times (∼180 s). They also observed higher hydrogen concentrations for films deposited at lower temperatures. Also, Jur et al.12 showed that atmospheric pressure ALD of Al2O3 and ZnO using TMA/ water and diethylzinc/water, respectively, produced film growth rates up to 2× larger than that observed in the same flow tube reactor operating under low pressure conditions (∼2 Torr). Further work showed that the higher growth rates extended over a relatively wide temperature range, from ∼50 to 200 °C, though the growth rate could be decreased using higher gas flow rates.13 The results suggested that excess physisorbed water present at high pressure and low gas flow produced the excess growth per cycle (GPC). Studies by Al-Abadleh et al.14 showed that low relative humidity (
