The lithographic lens: its history and evolution

The history of Nikon's projection lens development for optical microlithography started with the first "Ultra Micro-Nikkor" in 1962, which was used for making photo-masks. Nikon's first wafer stepper "NSR-1010G" was developed with a g-line projection lens in 1980. Since then, many kinds of projection lenses have been developed for each generation of stepper or scanner. In addition to increasing numerical aperture (NA) and field size, there have been many technical transitions for the projection lens, such as shortening the wavelength, controlling Zernike aberrations with phase measurement interferometry (PMI) for low k1 lithography, using aspherical lenses, applying kinematic optomechanical mounts, and utilizing free asphere re-polishing steps in the lens manufacturing process. The most recent advancement in projection lens technology is liquid immersion and polarization control for high NA imaging. NA now exceeds 1.0, which is the theoretical limit for dry (in air) imaging. At each transition, the amount of information that goes through the projection lens has been increased. In this paper, the history of the microlithographic lens is reviewed from several different points of view, such as specification, optical design, lens manufacturing, etc. In addition, future options of the projection lens are discussed briefly.