Correction of thickness uniformity of multilayer fluoride coatings on steep spherical substrates using shadowing mask...

In this study, the deposition angle parameter was introduced into the traditional deposition model to eliminate the coating thickness errors caused by large deposition angles and differences in the microstructures of coatings from the expected values. Based on the different thickness distributions of MgF₂ and LaF₃ coatings, a method that involves switching shadowing masks and a stepwise numerical optimization algorithm were proposed to control each coating thickness distribution accurately. When designing shadowing masks for a multilayer antireflective (AR) coating with a wide incident angle range, the correlation between the thickness distributions of the MgF₂ and LaF₃ layers was added to the merit function to ensure consistency in the two distributions. Two-layer deep-ultraviolet (DUV) AR fluoride coatings and a six-layer DUV low-polarization antireflection fluoride coating at 193 nm were fabricated on spherical substrate holders using thermal evaporation. In the experiment, when the ratio of the clear aperture to the radius of curvature was increased from -1.68 to 1.63, the thickness uniformities of the MgF₂ and LaF₃ layers of the two-layer DUV AR coatings on the fused silica substrates increased to a minimum of 98.49%. Owing to the transmission spectra and the incidence angle-resolved transmission, uniform optical performance and low-variation polarization transmittance were also achieved for the 193 nm AR coating deposited on the fused silica substrates.