Comparison of edge detection methods using a prototype overlay calibration artifact

Accurate overlay measurements rely on robust, repeatable, and accurate feature position determination. In our effort to develop traceable overlay standards we have examined a number of edge detection methods and the parameters which affect those measurements. The samples used in this study are a comprehensive set of prototype overlay wafer standards. The methods for determining the position of a feature generally rely on some determination of edges, and the resulting feature centerline can be significantly affected by the method of choice. We have compared cross-correlation, centroid, and edge-threshold methods as well as an integrated least squares method. This paper is focused on empirical results obtained through the measurement of relevant overlay targets and pitch specimens. The data presented in this paper was acquired using charge coupled device (CCD)-based arrays.