Investigation of pupil-fill factors as process window indicators for dry optical lithography

This work presents two pupil-fill factors as process window indicators for optical projection lithography when resolution enhancement techniques are employed. The formulations of these two pupil-fill factors are based on two different concepts of the correlation between the process window and the diffraction orders captured at the pupil of the imaging lens. One pupil-fill factor considers the amplitude of the diffracted orders. The other pupil-fill factor considers the extent of overlap between the 0th and the 1st diffraction orders at the pupil. This work investigates how accurately the variation of the two pupil-fill factors indicates the influences of source configuration and mask features on the process window. The Chromeless Phase Shift Lithography (CPL™) technology, in the context of dry optical lithography, is used as a case study. The results suggest that the pupil-fill factor, which considers the amplitude of the diffraction spectrum, is a better process window indicator than the other.