Optimization of multilayer reflectors for extreme ultraviolet lithography

Multilayer interference coatings on reflective elements in extreme ultraviolet (EUV) projection lithography systems introduce phase and amplitude variations at reflection, which have important implications for imaging properties, e.g., resolution, depth of focus, and tolerances. We discuss the numerical results of the optical effects of multilayers (MLs) and the inclusion of these effects in optical design. This numerical study demonstrates the advantages of spatially varying (graded) MLs compared to multilayers with constant layer thicknesses. We present a new method to calculate the optimum grading of multilayers. Using this new method, we are able to fully optimize the wavefronts emerging from the projection system toward the image plane.