Silver-based mirror coatings in astronomical telescopes have the potential to increase the efficiency of new and existing telescopes by roughly 30% or more. However, silver thin films must be over-coated with transparent protective layers to prevent tarnish and corrosion. Furthermore, we need these mirror coatings to stand up to 5-to-10 years of use at the telescope. To date, a suitable silver-based coating has remained elusive. The traditional method of depositing these coatings, called physical vapor deposition (PVD), can leave pinholes and/or crystal grain boundaries where contaminants can penetrate the protective layer and cause the silver to corrode. There is a relatively new deposition technique, atomic layer deposition (ALD) that produces excellent pinhole-free coatings that lack large crystal structures. ALD is quite common in the silicon industry, but it has not been explored much for use in optics, and particularly not on large surfaces. It works by alternating the introduction of two carefully designed chemical reagents into a vacuum chamber: these chemicals react at the substrate surface to build up the thin film by a molecule on each cycle. Each cycle requires about one-to-two minutes on small substrates like silicon wafers, the majority of which is spent pumping out the leftover reagents and by-products of the reaction. This means that ALD is a very slow process, often taking hours to make a given layer.

This project explored whether ALD could be used on large-diameter optics. We designed and had built a custom ALD coating chamber capable of coating optics up to 37-inches in diameter in reasonable times. We do this by using the optic as one wall of a thin reaction chamber, which enables us keep the volume for reaction quite small. In turn, this keeps the pumping times short. The primary goal was to demonstrate the proof-of-concept. The new chamber was used to make excellent thin films of metal oxides, like aluminum oxide, across a 37-inch diameter surface and keep the film thickness very uniform. We accomplished this with thickness uniformity of 2% or better, which is ideal for the needed protective layers in a silver mirror coating.  We developed processes that operate at relatively low temperature, which is required to reduce risks of breakage or other damage with large glass optics during the coating. We compared our ALD processes to those optimized for a small ALD system and determined how the process parameters would scale with size. We see no impediment to scaling up this design to handle large telescope mirrors potentially a few meters in diameter.  An ALD chamber like this is relatively inexpensive compared to those required for PVD.

We found that protective layers produced by ALD protect the silver layer significantly better than PVD deposited film, as expected. Another goal was to develop new processes using non-traditional reagents, such as ozone or hydrogen peroxide for oxidizing. This work is on-going. The coating chamber will be used for research for many years to come, and it has attracted a number of potential collaborators for use in other projects.

While our research has focused on mirrors for astronomy, a durable silver-based mirror coating could also be used, for example, to increase the light gathered by solar concentrators for energy production.

Last Modified: 07/02/2020
Modified by: Andrew C Phillips