Traditionally, ALD has been a process based on the sequential pulsing and purging of precursors into a common chamber containing the substrate material. Recently, several approaches have emerged that provide the sequential precursor exposure by movement of the substrate through physically separated regions containing the precursors, which enables direct compatibility with continuous roll to roll processing. This mode of ALD eliminates the time required for precursor introduction, saturation, and purge with each cycle, providing the promise of substantially reducing processing cost by increasing deposition speed and efficiency. Broadly, there are two general approaches to processing using this technique, which use different methods to keep the precursors separated from each other, to prevent gas phase reaction. One method involves operating in a pressure regime near to or slightly higher than atmospheric pressure, and uses tightly confined geometries and viscous flow to control the exhaust path of the precursors and prevent migration into the purge areas. Another method involves operating in the pressure regime of conventional pulse-based ALD, and utilizes differential flow and pumping to keep the precursors confined to their individual zones. This presentation will provide a survey of the various approaches to continuous ALD based on substrate translation, including recent developments and prospects for scaling to large area roll to roll processing. Additional detail will be presented on one particular approach under development for the deposition of gas diffusion barriers on polymer substrates, which employs a serpentine geometry to weave the flexible substrate back and forth between the two separated precursor zones.
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