Projection lithography below lambda/7 through deep-ultraviolet evanescent optical imaging

Abstract

An approach is presented to allow for imaging down to 26 nm using 193 nm projection reduction lithography together with evanescent wave optical effects at the imaging plane. The physical limitations of imaging imposed by the refractive indices of the materials were surpassed through the interference of evanescent waves at near fields. This has allowed for the imaging into photoresist of half-pitch resolution below lambda/7. A special situation where imaging at a system numerical aperture above the resist refractive index is discussed. Lithography results are shown using image media including air, water, and a high refractive index fluid together with a systematic gauge control to determine gap requirements and tolerances to achieve adequate image contrast into the resist. Gauging methods include both optical and mechanical approaches to achieve nanometer level gap control. It is found that gap depths as large as 50 nm can be tolerated using this approach, well beyond the levels associated with alternative high resolution approaches such as those using surface plasmons.