Subresolution assist feature and off-axis illumination optimization for 200- and 240-nm contact windows using 248-nm lithography

Sub-resolution assist features, coupled with appropriate off- axis illumination conditions, have been studied with the goal of fabricating 200 and 240 nm contact windows with uniform critical dimensions over a range of pitches and with large depths of focus (DOF). Results show that 240 nm isolated contacts without assist features possessed a useful DOF of less than 0.4 microns. The same features with 140 nm assist slots on each window edge, located 190 nm away, possessed a DOF of over 0.8 microns, using quadrupole illumination. Soft quadrupole illumination, where a mixture of quadrupole and conventional illumination is employed, yielded nearly the same DOF as quadrupole and printed both semi-dense and isolated contact windows near their optimum size as well. Contact holes, 200 nm wide, have been printed with smaller sub- resolution features, soft quadrupole illumination, and higher performance resists with a DOF of over 0.6 microns using a stepper with a numerical aperture of 0.53.