Scaling Atomic Layer Deposition to Astronomical Optic Sizes: Low-Temperature Aluminum Oxide in a Meter-Sized Chamber

Atomic Layer Deposition (ALD) is very attractive for producing optical quality thin films, including transparent barrier films on metal-coated astronomical mirrors. To date, ALD of mirror coatings has been limited to relatively small-sized substrates. A new ALD tool has been designed, constructed, and tested to apply uniform protective coatings over a 0.9 m diameter substrate in a 1 m diameter scale deposition plane. The new tool, which we have named the meter scale ALD system (MSAS), employs a unique chamber design that isolates a large substrate surface to be coated by utilizing the substrate as a wall of the reaction chamber. The MSAS is mechanically designed to be rapidly reconfigurable for selective area coating of custom substrates with arbitrary shape, size, and permanent backside hardware attachments. The design, implementation, results, and future applications of this new tool are discussed for coating large-area optical substrates, specifically protective coatings for silver mirrors, and other future large astronomical optics. To demonstrate the potential of this new design, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60 degrees C. Growth rate and uniformity, which are dependent on precursor pulse times and chamber purge times, show that the two half-reactions occur in a saturated regime, matching typical characteristics of ideal ALD behavior. Aluminum oxide deposition process parameters of the MSAS are compared with those of a conventional 100 mm wafer-scale ALD tool, and saturated ALD growth over the 0.9 m substrate is realized with a simple scaling factor applied to precursor pulse and purge times. This initial test shows that lateral thickness uniformity across a 0.9 m substrate is within 2.5% of the average film thickness, and simple steps to realize 1% uniformity have been identified for next growths. Results show promising application of transparent robust dielectric films as uniform coatings across large optical components scaled to meter-sized substrates.