Fourier analysis determination of best focus in submicron lithography

摘要

In the development and manufacture of integrated circuits, as requirements push closer to the theoretical (Rayleigh) limit of performance, depth of focus decreases as resolution is increased. The advent of easily accessible tools for image processing suggest that a quantitative determination of best focus is possible. Workers in this laboratory have developed a technique for determining `best focus' using 2-D Fourier power spectra of SEM images of exposed patterns. From this a `figure of merit' is extracted by assuming that what is desired is to maximize orthogonal edges (from `as-drawn' features) and minimize intermediate features (edge rounding). This has been shown to provide a quantitative value that is consistent with an `expert' assessment of the same images. The system is consistent with automation of the process, eliminating the need to record hard-copy images for `expert' evaluation.