An Investigation of Active Optics for Extreme-Ultraviolet Projection Lithography

Abstract

Projection lithography systems are being developed to print 0.1 μm features over a large field, using extreme ultraviolet (EUV) light of a wavelength of around 13 nm. At this wavelength the optics must be all-reflective with multilayer coatings. Current designs of the projection optics consist of off-axis ringfield systems with three or four mirrors (see Fig. I). The demands of microlithography are that the image must be of high quality across the entire field and within a large depth of focus, and that image distortion must be less than 10 nm. These requirements put severe constraints on the quality of the mirrors, which, in general, must have figure errors better than 0.2 nm rms. Although such tolerances can be met by the present state of the art there are various sources of error which can degrade the optical performance of the system. Of those sources, many may cause only low-frequency, quasi-static errors. These error sources include deformation due to stress of the multilayer coatings, thermal and gravitational loading, optical manufacture, and maintenance and alignment of the system. We are investigating the possibility of actively controlling the figure of the mirrors to correct for such errors and achieve the desired imaging performance. Control of the surface figure may also allow a greater throughput of the system, by being able to accommodate a higher heat load from a more intense EUV source.