Unveiling the effects of the surface and in-depth nanostructure on the far-UV optical reflectance of thin fluoride multilayer coatings

Abstract

Developing high-quality optical components for far ultraviolet (FUV) spectrum is gaining significance in space observations. Optical elements designed for a narrowband FUV response rely on coatings made of thin multilayers (MLs), alternating between two materials with low optical absorption and significantly contrasting refractive indices. Recent research shows that (AlF3/LaF3) systems outperform (MgF2/LaF3) coatings in FUV optical reflectance due to two critical factors: 1) AlF3′s lower refractive index compared to MgF2, and 2) structural properties resulting from a trade-off between the number of bilayers, contributing to higher reflectance, and the tendency for roughness to increase with the total coating thickness, limiting FUV optical performance. This study conducts a comprehensive examination of the nano-microstructure of these MLs, using three techniques: AFM for surface characterization, cross-sectional SEM micrographs with backscattered electrons, and Rutherford Backscattering Spectrometry (RBS) for in-depth structural and compositional analysis. A strong correlation is observed between roughness, grain size, composition, and intermixing, impacting the optical response in the FUV range. Nanometer-scale intermixing between layers is estimated using RBS. These findings provide a foundation for modeling FUV optical reflectance in MLs, enabling the optimization of their design through materials engineering to achieve more efficient optical devices for FUV applications in space observations.