Patent ID: 7714307

Claim:
A method of designing a projection system for a lithographic projection apparatus, comprising: obtaining a projection system starting configuration comprising a plurality of surfaces and having a merit function value corresponding to a local minimum in a merit function space; inserting two additional surfaces at a reference surface in the starting configuration, with a separation between the additional surfaces and between the additional surfaces and the reference surface, with the optical material between the additional surfaces being substantially the same as that at the reference surface and with the curvatures of the two additional surfaces being substantially the same as the curvature of the reference surface to generate a saddle point configuration of a projection system corresponding to a saddle point in merit function space, wherein the saddle point has a Morse Index value of 1; perturbing the saddle point configuration and performing optimization on at least one side of the saddle point to obtain a new projection system configuration having a merit function value corresponding to a local minimum in the merit function space; increasing the separation between the two inserted surfaces and between the inserted surfaces and the reference surface at which they were inserted; and outputting the resulting projection system configuration.