Progress and new techniques for protected-silver coatings

Abstract

We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed, and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition (ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely promising for protecting silver from corrosion.