Characterization of an expanded-field Schwarzschild objective for extreme ultraviolet lithography

Abstract

The performance of a new 10×-reduction Schwarzschild system for projection imaging at 13.4 nm wavelength is reported. The optical design is optimized to achieve 0.1 μm resolution over a 0.4 mm image field of view, an increase in area of a factor of 100 over previous designs. An offset aperture, located on the convex primary, defines an unobscured 0.08 numerical aperture. The system is illuminated using extreme ultraviolet (EUV) radiation emitted from a laser plasma source and collected by an ellipsoidal condenser. A 45° turning mirror is used to relay the collected EUV radiation onto a near-normal reflecting mask. Multiple sets of primary and secondary elements were fabricated, matched, and clocked to minimize the effects of small figure errors on imaging performance. Optical metrology indicates that the wave-front error within the subaperture used is within a factor of 2 of the design value. Images recorded in poly(methyl methacrylate) and ZEP 520 (Nippon Zeon) resists reveal good imaging fidelity over much of the 0.4 mm field with equal line/space gratings being resolved to 0.1 μm.