Nano-structure of GdF3 thin film evaluated by variable angle spectroscopic ellipsometry

As excimer lasers extend to deep-ultraviolet and vacuum-ultraviolet wavelengths at 193nm and 157nm, optical coatings experience the challenge of eliminating possible environmental contamination, reducing scattering loss, and increasing laser irradiation durability. Wide band-gap metal fluorides become the materials of choice for the laser optics applications. In order to understand the optical properties of nanostructured fluoride films, thin GdF3 films grown on CaF2 (111) substrates were evaluated by variable angle spectroscopic ellipsometry. An effective medium approximation model was used to determine both the film porosity and the surface roughness. Structural evolution of the GdF3 film was revealed with improved ellipsometric modeling, suggesting the existence of 3-layer structure, a densified bottom layer, a porous middle layer and a rough top surface. The nanostructure of the film and the surface roughness were confirmed by atomic force microscopy. The attraction of the nano-structure to environmental contamination was experimentally demonstrated.