离子束溅射制备低应力深紫外光学薄膜

Low stress DUV optical coatings deposited by ion beam sputtering

采用离子束溅射制备了Al F3、Gd F3单层膜及193 nm减反和高反膜系,分别使用分光光度计、原子力显微镜和应力仪研究了薄膜的光学特性、微观结构以及残余应力。在优选的沉积参数下制备出消光系数分别为1.1×10^(-4)和3.0×10^(-4)的低损耗AlF_3和GdF_3薄膜,对应的折射率分别为1.43和1.67,193 nm减反膜系的透过率为99.6%,剩余反射几乎为零,而高反膜系的反射率为99.2%,透过率为0.1%。应力测量结果表明,AlF_3薄膜表现为张应力而GdF_3薄膜具有压应力,与沉积条件相关的低生长应力是AlF_3和GdF_3薄膜残余应力较小的主要原因,采用这两种材料制备的减反及高反膜系应力均低于50 MPa。针对平面和曲率半径为240 mm的凸面元件,通过设计修正挡板,250 mm口径膜厚均匀性均优于97%。为亚纳米精度的平面元件镀制193 nm减反膜系,镀膜后RMS由0.177 nm变为0.219 nm。

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- 4and 3. 0 × 10- 4can 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 com-pressive 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.