New developments in the design of ring-field projection cameras for EUV lithography: Passive pupil correction

In this paper we have discussed how the concept of passive pupil correction can be applied to improve the field performance of a ring field system. This improvement involves an optical surface coincident with the stop, or with a pupil of the system. To implement passive pupil correction, such a surface carries a non-axially symmetric asphericity that cancels aberrations along one meridian of the ring field. Because this surface is non-axially symmetric, the axial symmetry of original camera is broken and therefore the aberration compensation is not perfect for other meridians. However, an improvement of a factor of 4 can be expected if the original aberration residual consists of simple terms. The distortion characteristics of the original axially symmetric system are unchanged because the non-axially symmetric asphericity does not affect propagation of chief rays.