Effects of partial coherence on contact hole projection lithography

Calculated image intensity profiles (IIP’s) are presented for contact holes printed with partial coherence levels in the S=0.5 to S=0.7 range and are compared to IIP’s of the correspondingly dimensioned line–space gratings. Improvements in the depth of focus (DOF) latitude and resolution at higher coherence levels (lower pupil fill S) are quantified for contact hole projection lithography with high-numerical aperture (NA), g-line lenses. The relationship between contact and grating DOF is quite complex and changes with the coherence level. Higher pupil fills produce greater differences between the contact and grating DOF. Workable contact hole resolution levels for 0.7, 0.6, and 0.5 pupil fills are estimated to occur at k factors of 0.89, 0.83, and 0.79, respectively, where the k factor normalizes feature size to the resolution factor λ/NA of the lens.