Update on UCO's advanced coating lab development of silver-based mirror coatings

Scaling Atomic Layer Deposition to Astronomical Optic Sizes: Low-Temperature Aluminum Oxide in a Meter-Sized Chamber

“…With an established scalable deposition process of low-temperature AlO x , future work will utilize the scaling process parameters identified in Table to study scalability of different thin-film chemistry processes in the MSAS. While AlO x is generally a proficient corrosion barrier for many applications, extensive studies have shown that it is not an optimized stand-alone coating material to protect against environmental corrosion because of its eventual dissolution from atmospheric moisture. − Therefore, future work will explore low-temperature deposition processes with the MSAS of thin-film materials that have shown promising corrosion barrier properties on Ag in previous work when deposited by PVD techniques, such as TiO 2 and Si 3 N 4 . − The optimized process conditions and lessons in this work will be used to further optimize MSAS process performance for metal oxide thin films, specifically with the use of alternative oxidizing reactants that can be purged quicker than H 2 O, such as ozone, oxygen plasma, and wet Ar plasma. Detecting and eliminating vacuum leaks, minimizing gas flow-dependent growth patterns with improved flow diverting hardware, and precisely calibrating and controlling chamber and substrate temperatures are unique challenges of this new large-area ALD technique that will be addressed in future work exploring protected Ag mirror barrier coatings.…”

“…Even the Gemini coating comes with the cost of sacrificing deep blue and UV portions of the spectrum because of unwanted absorption primarily from the nickel–chromium nitride adhesion layer, , which is an unacceptable compromise for many astronomical programs. The Advanced Coatings Lab of the University of California Observatories (UCO) has been identifying and developing high-performance coatings useful for astronomical optics. − The Thirty-Meter Telescope (TMT) project has stringent requirements of broadband high reflectivity (0.34 < λ < 28 μm) and “lifetimes” of 5–10 years between recoating, which has supplied the motivation for this research and development of durable Ag mirror coatings.…”

“…Aluminum oxide was chosen as a demonstration thin-film barrier material for this initial experiment because of the previous success observed with AlO x deposited by ALD, its acceptable transmission properties, its chemical and mechanical durability, and its standardized acceptance of an ideal and well-documented ALD chemistry process. − This work introduces a new implementation of low-temperature thermal ALD capable of coating meter-sized substrates with the ability to be rapidly reconfigured to coat custom optics with arbitrary shape, size, and backside hardware attachments. The key concept of this new chamber design was originally introduced by Phillips et al, describing an ALD chamber where the reaction volume was kept small by using the substrate as part of the reaction chamber, and initial results of the chamber deposition testing have been presented . Film deposition which utilizes a substrate to be coated as a chamber wall has been previously demonstrated using atmospheric pressure ALD onto a ∼7.5 cm diameter area of a car windshield, but the coated surface perimeter was isolated by a purge gas curtain instead of an O-ring .…”

Enhanced Durability and Antireflective Performance of Ag-Based Transparent Conductors Achieved via Controlled N-Doping

“…70 Improving the conformality of Ag encapsulation layers is a well accepted means to improve stability, but the lower interface is rarely considered. 6,16,36,37 This lower interface should not be neglected: Ag films directly encapsulated with Al 2 O 3 by atomic layer deposition were shown to be much more durable than Ag films which had an evaporated dielectric coating between the Al 2 O 3 and the Ag layer. 36 This supports the idea that conformal contact at the interfaces is required to optimize durability and that simply generating a low permeability encapsulating layer is insufficient.…”

“…While Ag has attracted considerable attention due to its potential in TC applications, the chemical stability of these films (and that of most Ag-based TCs) has gone largely unaddressed. Whereas ITO is fairly stable when exposed to moisture, Ag-based coatings require particular encapsulation to achieve similar durability. ,− In addition to encapsulation in dielectrics, Ag layers are frequently stabilized using NiCrN x or other transition metal barrier layers: while these are well-known for their ability to protect Ag coatings from chemical degradation, as their thickness increases, transmission rapidly decreases, reducing the optical performance. − Although not discussed at length in this work, appropriate metallic dopants can enable improved durability with less severe impact on optical performance . The specificity of encapsulation used to protect Ag films can be disadvantageous in that they may require specific techniques or equipment, such as solution processed layers or atomic layer deposition, and may limit the selection of materials that can be used to improve Ag durability.…”