Influences of off-axis illumination on optical lens aberration

Through modification of the illumination system of optical lithography tool s, resolution and focal depth improvements can allow for near diffraction l imited imaging. Most often, lens aberrations are evaluated assuming full us e of a lens pupil. With off-axis illumination (OAI), diffraction informatio n is distributed selectively over the lens pupil, influencing the impact of aberrations on imaging. This article describes these effects through a mod ified set of aberration coefficients and their variance over a pupil illumi nated using OAI. Results in general show that astigmatic effects can worsen while spherical aberration and defocus effects can be improved. Coma induc ed image placement can be further aggravated with OAI unless rebalanced wit h tilt. The analysis presented here can also be extended to phase-shift mas king approaches. (C) 1998 American Vacuum Society.