Direct aerial image measurement as a method of testing high numerical aperture microlithographic lenses

A method for testing high numerical aperture (NA) microlithographic lenses using direct aerial image measurements has been developed. By measuring contrast versus focus for dense line/space patterns, this system can quantify the amount of astigmatism, distortion, field curvature, and spherical aberration produced by a lens under test. Experimental results show high correlation with more traditional lens testing methods such as transmissive interferometry and evaluating exposed resist patterns. The lens used in these experiments is an 0.53 NA, 248 nm objective designed for imaging 0.35 μm features over a 31 mm diam field.