Coma Aberration Measurement by Relative Shift of Displacement with Pattern Dependence

Abstract

In lithography for LSI pattern exposure, pattern displacement was caused by aberrations of the projection lens. This displacement depended on the pattern position as well as the pattern size. Relative pattern shifts of the pattern to a 2.0 µm isolated pattern were measured using submicron bars-in-bars marks, which are similar to the overlay measurement marks. The measurement marks lie on the same mask and are exposed at the same time by a KrF excimer stepper with a numerical aperture (NA) of 0.6 and wavelength of 248 nm. The relative pattern shift showed that the lens had magnification errors which depended on the pattern size. Coma aberration was presumed from the measurement results of aerial image simulation. The coma was expressed in terms of Seidel aberrations. Wave aberration from the simulation corresponded with the measurement results for the two types of illuminations, i.e., such as a standard circuler source and an annular source.