Atomic Layer Deposition Barrier Layers on Large Silver-coated Mirrors

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

Even with all the dazzling technology that has been brought to bear on astronomical observations, and the furthering of that technology in turn that has been inspired by astronomical applications, it is still difficult to make a metal mirror coating that is highly reflective at all optical wavelengths yet durable enough to last in field use. Traditional aluminum coatings are durable, but not highly reflective at all wavelengths. Silver has better all-wavelength reflectance, but will develop pin-holes and flake away in a short time unless clever methods are used to deposit it on a glass mirror blank. The development of such methods is the goal of this investigation, and a goal with far-reaching applications to many different kinds of future instruments. The essence of the technical approach to improved durability, high-reflectance silver coatings, is Atomic Layer Deposition (ALD). ALD is a relatively new, sequential chemical vapor process taking advantage of self-limiting surface reactions. ALD coatings have several desirable properties, notably excellent film structure, conformal coverage, and excellent thickness uniformity and control. ALD is now a mature process used widely in the silicon wafer industry. To date, however, ALD has been limited to relatively small substrate sizes and is rarely used for optics, owing to its relatively slow speed. This project will place ALD barrier layers on top of silver deposited by conventional physical vapor deposition (PVD) to get films more durable than PVD alone. The process proposed is a thus hybrid process, with most of the coating done with PVD processes and the barrier layers added with the ALD system. Improvements in ALD deposition speed are also planned to allow coating of larger substrates in reasonable times. This project has immense broader impacts in that it enables dramatically improved practical performance of a vast range of astronomical instrumentation. In addition, students will be involved, and there is a strong commitment to plain-language explication of the research and its results. Funding for this project is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.