Optimization of the deposition parameters of MgF 2 /LaF 3 narrowband reflective FUV multilayers

The development of efficient dielectric coatings in the far UV (FUV) is demanded for upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph) in LUVOIR mission, among other applications. Multilayers (MLs) based on MgF 2 and LaF 3 have been developed in the last decades for the 157-nm and 193-nm lithography, demonstrating excellent optical properties. Yet, the deposition procedure to obtain coatings with optimal performance has not been fully detailed in the open literature, such as the dependence of ML performance with deposition and post-deposition temperature. This research investigates the effect of the substrate deposition temperature of MgF 2 /LaF 3 ML coatings prepared by thermal evaporation on FUV reflectance, stress, roughness, as well as the performance of the coatings and their evolution over time. The relatively higher expansion coefficient of these two fluorides in comparison with fused silica substrates results in a large tensile stress for coatings deposited at high temperature and later cooled down to room temperature. Such stress may result in coating cracking and delamination. A compromise deposition temperature of ∼240°C and 13 bilayers was found for optimal ML reflectance peaked at ∼160 nm before cracks are generated. Above this deposition temperature, stress increased, which resulted in an extension of the cracked area and in a slight roughness increase and FUV reflectance decrease. MLs that were deposited at room temperature and later annealed resulted in a similar reflectance and stress to those of hot-deposited coatings for a given temperature.