A NEW PUPIL FILTER FOR ANNULAR ILLUMINATION IN OPTICAL LITHOGRAPHY

Several super-resolution photolithography techniques are investigated. In annular illumination, both the resolution and depth of focus (DOF) are enhanced most effectively when 0.6 < sigma < 0.7, whether or not contrast enhancement techniques, such as pupil filters or edge-enhancing phase-shift masks (PSMs), are used. Various contrast enhancement techniques are investigated from the viewpoint of spatial frequency characteristics, since flat characteristics are required for good mask-to-image fidelity. We propose a new high-spatial-frequency enhancing filter for annular illumination system which meets this requirement. We also optimize edge-enhancing PSMs for use with annular illumination. It is shown that both methods can improve the resolution/DOF characteristics while maintaining good mask-to-image fidelity. A practical resolution analysis shows that 0.2 (0.3) mum patterns can be delineated with a DOF of +/- 0.6 (+/- 0.75) mum, if a KrF (i-line) stepper with an NA0.5 lens and high-contrast resist materials are used, with few restrictions on the pattern layout.