Atomic Layer Deposited (ALD) coatings for future astronomical telescopes: recent developments

Atomic Layer Deposition (ALD) can create conformal, near stoichiometric and pinhole free transmissive metal fluoride coatings to protect reflective aluminum films. Spectral performance of astronomical mirror coatings strongly affect the science capabilities of astronomical satellite missions. We are utilizing ALD to create a transmissive overcoat to protect aluminum film mirrors from oxidation with the goal of achieving high reflectance (> 80%) from the UV ( 100 nm) to the IR ( 2,000 nm). This paper summarizes the recent developments of ALD aluminum fluoride (AlF₃) coatings on Al. Reflectance measurements of aluminum mirrors protected by ALD AlF₃ and future applications are discussed. These measurements demonstrate that Al + ALD AlF₃, even with an interfacial oxide layer of a few nanometers, can provide higher reflectance than Al protected by traditional physical vapor deposited MgF₂ without an oxide layer, below 115 nm.