Parallax method for diffraction-based single-cell overlay and film thickness measurement

Abstract

A diffraction-based measurement of overlay requires a target composed of two cells per direction of measurement, with induced shifts of opposite signs designed into each of the cells. We present a method for a measurement which only requires a single cell per direction. This is achieved by resolving the image in the pupil plane and using the angle of incidence inlieu of the induced shift. The use of single-celled targets reduces the target size by half and enables the placement of the target in-die, as well as reducing the measurement time. This single-cell measurement requires the calibration of the target’s optical stack height, which is done on a small number of two-cell targets. This calibration also produces a stable map of the aligned layers’ height profile across the wafer.