Optical Design for Soft X-Ray Projection Lithography

We investigated the effect of soft X-ray multilayer mirrors on the characteristics of image formation and determined that the phase shift of the X-ray on the mirror plays an important role in image formation as well as reflectivity. Under such a condition that the effect on the characteristics of image formation can be neglected, we designed an optical system that has a resolution of 0.18 µm and 0.25 µm at the wavelength of 5 nm and 13 nm, respectively, and distortion of less than 0.01 µm in a whole exposure field of 20×40 mm2. The necessary surface accuracy of mirrors and tolerance of setting mirrors are discussed.