Graded Multilayer Film Design for Anamorphic Magnification EUV Lithographic Objective

High numerical aperture (NA) projection objectives with anamorphic magnification are demanded for extreme ultraviolet (EUV) lithography down to 10 nm resolution, which results in extreme increase of incident angle and incident angle range of objective lens system. Traditional normalized multilayer film and laterally graded multilayer film cannot satisfy the requirement of reflectivity and image quality in the projection objectives. A method combining laterally graded multilayer film with depth graded multilayer film is presented. The laterally graded multilayer film is used to increase the reflectivity and the depth graded multilayer film is used to enhance the reflectivity uniformity and compensate the wavefront aberration introduced by the laterally graded multilayer film. The method is used to design the multilayer film of an anamorphic magnification EUV lithographic objective with NA of 0.50. The results show that the average reflectivity of each mirror is higher than 60% and the reflectivity peak-to-valley value of each mirror is less than 3.5% with imaging performance unchanged. Which satisfies the lithographic requirement, and verifies the feasibility of this method.