Far-ultraviolet optical properties and performance of physical vapor deposited aluminum mirrors protected with XeLiF and XeMgF2

We describe efforts to develop broadband mirror coatings with high performance that will extend from the far-ultraviolet (FUV) to infrared wavelengths. Our team at the Goddard Space Flight Center has developed a reactive physical vapor deposition (rPVD) process that combines a fluorination with a XeF2 gas (which grants a thin AlF3 layer) in between the Al and the metal-fluoride protection layer (either LiF or MgF2) that are done with the conventional PVD process. This recently developed rPVD process produces protected Al mirrors coatings with an improved average FUV reflectance between 10% and 15% higher (when compared with conventionally prepared samples). We have termed these coatings as XeLiF when the dielectric overcoat is LiF or XeMgF2 when the dielectric overcoat is MgF2. The XeLiF-coated Al mirrors meet current goals for advanced broadband mirrors in the FUV (R>70% at 103 nm and R>80 above 110 nm), whereas the XeMgF2 provides R>80% above 115 nm. The IR/Vis/UV reflectance for either XeLiF or XeMgF2 mirrors is similar to the theoretical reflectance of bare aluminum at wavelengths >200 nm. In addition, long-term lifetime testings of XeLiF mirrors indicate the rPVD process produces more environmentally stable coatings, where a XeLiF sample showed a degradation in the average FUV reflectance of around 1% to 2% when stored in a relative humidity of 40% over a period of 3.5 years. These results are a remarkable improvement when compared with conventionally prepared Al+LiF samples that would degrade their FUV reflectance in a matter of weeks or months when exposed to those kinds of relative humidity levels. Surface topographies on several XeLiF samples with varying Al and LiF thicknesses have been measured with an atomic force microscope (AFM). The root mean square (RMS) roughness (σ) values derived from these AFM results have ranged between 0.6 and 0.9 nm. For comparison, samples without the Xe process start off by having an RMS roughness that is 30% larger than samples treated with the XeF2 gas. We have also determined that these roughness values are showing a slight increase, ranging between 0.9 and 1.0 nm, when samples are exposed to room temperature and relative humidity as high as 50% over one week. Both of these key performance parameters (environmental stability in reflectance and smoothness of ≤1 nm) are key considerations for using the XeLiF coating in the primary and secondary mirrors of the Habitable Worlds Observatory (HWO). We also show evidence that the rPVD coating process is compatible with deposition on Si-based gratings. It is known that XeF2 vapor is a strong Si etchant, thus the demonstration that the native SiO2 layer on Si test samples is sufficient to protect the groove profile of E-beam-ruled Si gratings from degradation is an important and significant finding.