Initial structure design of coaxial six-ten mirror central-obscured extreme ultraviolet lithographic objective

The high numerical aperture projection objective for industrial extreme ultraviolet lithography (EUVL) usually includes several mirrors with central obscuration in the light path. We apply the grouping design method on the spherical initial structure of central-obscured six-ten mirrors EUVL objective. A new condition called obscuration condition is introduced for the parameter calculation of the central-obscured mirror groups. The non-obscured mirror groups in the projection objective were calculated by the non-obstruction condition and conjugation condition, which is based on the object-image relationship and the pupil-stop relationship. By analyzing the expression of obscuration condition and the non-obstruction condition, the combined available ranges of parameters can be obtained. The available mirror group is picked up from that available range and connected to form a whole initial spherical structure. The design process of a six-mirror objective with central obscuration and initial structures with eight-ten mirrors designed by the same process is shown in this article.