Low stress DUV optical coatings deposited by ion beam sputtering

AlF₃, GdF₃ single layers, AR and HR coatings at 193 nm were deposited by ion beam sputtering. The optical properties, microstructural properties and residual stress were investigated by spectral photometer, atomic force microscope and film stress measurement system, respectively. With proper deposition conditions, low loss AlF₃, GdF₃ films with the extinction coefficients of 1.1×10^-4 and 3.0×10^-4 can be fabricated. The corresponding refractive indexes are 1.43 and 1.67. At 193 nm, the transmittance of AR coating is 99.6% and the reflectance is almost zero. While the reflectance is 99.2% and the transmittance is 0.1% for HR coating. From the results of stress measurement, AlF₃ film presents tensile stress, while GdF₃ film has compressive stress. The low growing stress is the major reason for the small residual stress of AlF₃ and GdF₃ films. The stress of multilayer coatings consisting of these two materials is less than 50 MPa. By designing shadowing masks for a flat and a convex substrate with the radius of curvature of 240 mm, thickness uniformity of both substrates in diameter of 250 mm is better than 97%. AR coating at 193 nm was deposited on the flat substrate with sub-nanometer accuracy surface and the roughness varied from 0.177 nm RMS to 0.219 nm RMS after coating.