Lateral shearing interferometry for high-resolution EUV optical testing

Abstract

Next generation EUV optical systems are moving to higher resolution optics to accommodate the smaller length scales targeted by the semiconductor industry. As the numerical apertures (NA) of the optics become larger, it becomes increasingly difficult to characterize aberrations, which broaden the point-spread function and thus limit the ultimate resolution of an optical system. Lateral shearing interferometry (LSI) provides an attractive alternative to conventional interferometric techniques such as point diffraction interferometry due to its experimental simplicity, stability, relaxed coherence requirements, and its ability to scale to high numerical apertures. In this paper we present an analytical solution to the LSI interferogram in various NA regimes. We demonstrate that systematic aberrations present in high NA interferograms due to grating distortion of the diffracted order angular spectrum are measurable and must be compensated for in the reconstruction algorithm.