Influences of off-axis illumination on optical lens aberration

Through modification of the illumination system of optical lithography tools, resolution and focal depth improvements can allow for near diffraction limited imaging. Most often, lens aberrations are evaluated assuming full use of a lens pupil. With off-axis illumination (OAI), diffraction information is distributed selectively over the lens pupil, influencing the impact of aberrations on imaging. This article describes these effects through a modified set of aberration coefficients and their variance over a pupil illuminated using OAI. Results in general show that astigmatic effects can worsen while spherical aberration and defocus effects can be improved. Coma induced image placement can be further aggravated with OAI unless rebalanced with tilt. The analysis presented here can also be extended to phase-shift masking approaches.