Metallic mirror coatings are a key part of intelligence gathering, surveillance, and reconnaissance systems (ISR). As collecting accurate and clear information is critical in these applications, coatings working in these applications must withstand extreme environments, including high humidity and corrosion for extended periods. Gold mirror coatings are a common choice, being chemically inert to be relatively unaffected by environmental conditions and benefiting from high reflectivity from 600nm to into the mid-IR. Aluminum has a lower overall reflectivity than gold but extends its high reflectivity band further into the UV than gold. Silver performs better than gold in the full visible range but suffers from weak environmental survivability performance. As such, silver requires a durable protective coating to withstand exposure to ambient and corrosive environments. Hence, these silver-based coatings must be protected by inert coatings without reduction of their spectral performance in the spectral region of interest. Besides spectral performances, these coatings must also meet other durability requirements according to military specification documents like MIL-F 48616, MIL-810G amongst a few. Amongst the harshest requirements are exposure to Salt Fog spray and extended heat and humidity. Low stress coatings are also desired to meet surface figure requirements for some of these optics. In this article, we provide an overview of performance of our protected silver coating, with reflectance > 95% from 0.45-20 um that can withstand exposure to salt fog spray for over 120 h. These coatings were also exposed for over 24 h of high humidity, temperature, and thermal cycling without any significant deterioration in performance.
As 248nm DUV lithography tools pursue resolution of smaller features, the transition towards higher numerical aperture optics in these tools is pushing the development of high-performance anti-reflection coatings on large-area, highly curved transmitting optics present in these systems. We present the results of one such effort to coat multi-layered Al2O3 and MgF2 antireflection coatings on substrates of planar, spherical, and aspherical geometries. The spectral, surface quality, and pulsed laser damage performance of these coatings are presented.