Impact of lens aberrations on optical lithography

All optical projection systems for microlithography depart from perfection because of various lens aberrations, especially when large image field size is combined with high numerical aperture (NA). Such aberrations have a variety of effects on lithographic imaging: shifts in the image position, image asymmetry, reduction of the process window, and the appearance of undesirable imaging artifacts. These undesirable effects are sometimes exacerbated through use of resolution enhancement techniques such as phase-shift masks or nonstandard illumination. This paper examines the impact of different types of aberrations on lithographic imagery through simulation. New techniques for measuring aberrations by examining lithographically printed resist patterns are considered.