Grouping design of eight-mirror projection objective for high-numerical aperture EUV lithography

A grouping design method for all-sphere initial design of an eight-mirror projection objective is proposed for extreme ultraviolet lithography (EUVL). By separating the eight-mirror objective into three mirror groups (the object side group, the image side group, the middle group), this method allows designers to calculate the parameters of an eight-mirror objective by the nonobstruction constraints and the conjugation relationships of object image and pupils. Exhaustive paraxial search for the middle group is implemented while a designer-chosen combination of object side group and image side group is considered. The grouping design process is visualized and steerable. The load of calculation is well controlled in a practical acceptable span. The final eight-mirror design optimized with aspheric parameters achieves an 0.4 numerical aperture on the image side, and the image resolution achieves a diffraction limit with almost no distortion.