Ultrathin protective coatings by atomic layer engineering for far ultraviolet aluminum mirrors

Conventional aluminum-coated mirrors operating at far ultraviolet wavelengths (90–200 nm) utilize protective overcoats of metal fluoride thin films deposited by physical vapor deposition. The use of atomic layer deposition (ALD) holds promise in improving spatial reflectance uniformity and reducing the required thickness of the protective layers. Achieving a stable, pinhole-free, ultrathin (<3 nm) overcoat would allow protected Al mirrors to approach the ideal Al intrinsic reflectivity in the challenging, but spectrally-rich, 90–115 nm range. However, combining ALD methods with high performance evaporated Al layers has technical challenges associated with the formation of undesirable interfacial oxide. To overcome this issue, we demonstrate the use of thermal atomic layer etching (ALE) methods to remove this oxide prior to ALD encapsulation. This paper describes our continuing work to optimize new ALD processes for the metal fluoride materials of MgF₂, AlF₃ and LiF. We also describe new work on low temperature (<200 °C) ALE methods utilizing a fluorination-volatilization approach that has been incorporated into our mirror development efforts. The scalability of this overall approach and the environmental stability of ALD/ALE Al mirrors is discussed in the context of possible future astrophysics applications such as the NASA LUVOIR and HabEx mission concepts. The use of this combined ALE/ALD method may also enable a fabrication platform in space that can renew or reconfigure protective overcoats on Al mirrors on-orbit, as an alternative to other space-based metal coating methods considered previously.