Pupil wavefront manipulation for optical nanolithography

As semiconductor lithography is pushed to smaller dimensions, process yields tend to suffer due to subwavelength topographical imaging effects. Three dimensional or “thick mask” effects result in such things as a pitch dependent best focus and, for alternating phase shift masks (AltPSMs), an intensity imbalance between etched and un-etched features. Corrective mask structures such as the dual trench AltPSM have been introduced to compensate for such intensity imbalances. In this work, the compensation of thick mask effects is explored using the manipulation of the pupil wavefront through the addition of spherical aberration. The wavefront has been experimentally varied through the manipulation of the lens aberration in a state of the art full field scanner. Results reveal that the influence of spherical aberration on best focus is predictable, allowing focus deviation through pitch to be tuned. Simulations further predict that aberration manipulation can provide compensation for thick mask effects by increasing the useable depth of focus for a particular set of features on both AltPSM and thicker film attenuated PSM masks. Such pupil wavefront correction has the potential to compensate for mask topography by matching thick mask effects to those of thin masks.