Low-temperature atomic layer deposition of aluminum oxide scaled up to a 36" chamber for observatory optics

A new atomic layer deposition (ALD) tool has been designed, constructed, and tested to apply uniform protective coatings over a substrate 36" in diameter. The new tool, named the meter-scale ALD system (MSAS), employs a novel chamber design which utilizes a large substrate to be coated as a wall of the chamber. Conceptual design and implementation of this new tool are discussed with potential applications to large astronomical telescope optics, specifically protective coatings for silver mirrors, and other future large structures. In this work, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60°C. Thickness uniformity across a 36" substrate is within 2.5% of the average film thickness. MSAS aluminum oxide deposition process parameters are compared with those of a conventional 4" wafer-scale ALD tool. The results show promising application of transparent robust dielectric films as uniform barriers across large optical components as well as multiple wafer assemblies encountered in semiconductor processing.