Investigation of Refractive Index Uniformity in Coatings on a Steep Lens

Objective: The use of large steep lenses is an effective method of increasing the numerical aperture, which has an important impact on the resolution of the lithographic system. Because to the unique geometry of the large steep lens, the distance between the steep lens surface and the evaporation source varies depending on their relative position, and uneven distribution of the film thickness will inevitably occur in the radial direction of the lens. To meet the high demands of a lithographic system with a complex optical system, a steep lens must have a high transmittance over a wide range of incident angles. As a result, controlling the homogeneity of the film on a large steep lens has become a critical issue. As the planetary fixture rotates, the deposition angle will also change during the deposition. In this deposition, the growth state, packing density, and the roughness of film change accordingly, which leads to an uneven refractive index of the film. To investigate the deposition characteristics of the film on a steep convex lens, the mechanism of change of the deposition angle at different positions of the steep convex lens during the rotation of the planetary fixture was studied by computer simulation. The influences of the evolution of the deposition angle and the changes in the substrate temperature on the refractive index of the film were also studied. The results of experimental studies presented in this manuscript provided theoretical and practical guidance on the correction of the refractive index of the film on a steep lens. Methods: Magnesium fluoride (MgF2), a commonly used coating material for deep ultraviolet optical thin films, was selected as the object of study. A single-layer film of magnesium fluoride was prepared by electron beam evaporation technology. The refractive index of the sample was obtained by spectral reflectance/transmittance envelope curve fitting method and the ellipsometer test method. The crystalline properties and morphology of the film were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The evolution of the deposition angle during the rotation of the planetary fixture was obtained using software simulation. Results and Discussions: According to the results of computer simulation, the average deposition angle of the film gradually increases from the center of the lens to the edge, and the range of distribution of the deposition angle gradually increases too. The influence of structural characteristics becomes more obvious. When the substrate temperature is 25 ℃, the deposition angle significantly affects the refractive index of the MgF2 film. As the deposition angle increases, the refractive index of the film gradually decreases. When the deposition angle increases from 0° to 85°, the refractive index of the MgF2 film at 200 nm decreases from 1.42 to 1.28, the change in the refractive index is 0.14, and the non-uniformity of the refractive index is 10.22%. When the substrate temperature is 200 ℃, the refractive index of the MgF2 film at 200 nm decreases from 1.45 to 1.39 with increasing deposition angle from 0° to 85°, the change in refractive index is 0.06 and the non-uniformity of refractive index is 4.51% (Fig.13). For the MgF2 film deposited at a substrate temperature of 25 ℃, the non-uniformity of the refractive index strongly affects the residual reflectivity of the 193 nm AR coating, increasing from 0.04% to 4.72%, whereas for the MgF2 deposited under high substrate temperature, the non-uniformity of the refractive index of the film layer increases the residual reflectivity of the 193 nm AR coating from 0.08% to 0.32% (Fig.13 and Fig.14). Conclusions: The influence of the deposition angle on the uniformity of refractive index of the film was systematically studied. For the convex lens close to a hemispherical shape, the occlusion effect will be more significant with an increase in the deposition angle from 1.41° to 90°, as a result of some parts of the substrate cannot be covered with a film. For the MgF2 film, the non-uniformity of refractive index of the film at different deposition angles is 10.22% at 25 ℃, which significantly affects the optical properties of the 193 nm AR coating. When the substrate temperature increased to 200 ℃, the non-uniformity of refractive index of the film at different deposition angles decreased to 4.51%, while the effect of the deposition angle on the optical properties of the 193 nm AR coating dramatically decreased. It is shown that increasing the substrate temperature in a certain range can effectively increase the uniformity of refractive index distribution. The results of the experimental studies presented in this manuscript provided theoretical and practical guidance on the correction of the refractive index of the film on steep lens. © 2021, Chinese Lasers Press. All right reserved.