Effect of storage conditions on the performance of Al/LiF/MgF2 mirrors

Mirror stability is crucial for their applications, especially in the far ultraviolet (FUV) where Al mirrors protected with fluoride are the preferred choice. This study evaluates the effect of storage conditions on the performance of Al/LiF/MgF2 mirrors and compares them with previous investigations of Al/LiF/eMgF2 mirrors. This study focuses on FUV reflectivity, structural characteristics, and the long-term stability of mirrors stored in various environments. Coatings were deposited on the super-polished silicon wafers using thermal evaporation. Three sets of mirrors were stored in the environment with 20% relative humidity (RH), 40% RH, and 80% RH, respectively. Additionally, three other sets were stored in vacuum, nitrogen (N2), and oxygen (O2), respectively, in sealed bags kept in a 40% RH environment. Mirrors stored in a 20% RH environment demonstrated the best performance, exhibiting the smallest decline in reflectivity and a relatively stable surface morphology. Conversely, mirrors stored in 80% RH experienced a significant reduction in reflectivity, accompanied by the most pronounced deterioration in surface morphology. It was concluded that the chemical changes and high roughness of the films contributed to the decrease in the reflectivity.