Optical design of illumination systems for lithography applications

Abstract

Proper illumination of the mask and the entrance pupil of the reduction system is crucial in projection lithography. The typical subsystems involved in the illumination system needed in the transport to the mask and entrance pupil of the reduction system include focusing optical system into a beam uniformer, beam uniformer, optical relay from the exit plane of the uniformer to the mask plane, and the entrance pupil of the reduction system. The aberration levels needed for these various subsystems, considerations of Kohler versus critical illumination, coherence requirements (if the source is a laser), and partial coherence requirements for the ideal imaging properties all play crucial roles. We examine these for a practical projection lithography system working with a 248-nm laser (with ±1% wavelength spread) and a 20× reduction system. We find that the illumination system plays the role of both a typical condenser-type system (as used in microscopy) and an imaging system. This is because typical uniformers require a focused input beam with good aberrational properties and the imaging of the uniform output plane of the uniformer to the mask plane again requires good imaging. We examine these aspects in detail with optical design analyses using CODE V and OPTICAD.