Focus and edge detection algorithms and their relevance to the development of an optical overlay calibration standard

We present results of investigations into optical focus and edge detection algorithms relevant to overlay metrology. We compare gradient energy, standard deviation, contrast and summed intensity of acquired images as focus metrics for bright-field, scanning confocal (Nipkow disc), and confocal (single aperture) microscopy. For our purposes, gradient energy calculated via Sobel filtering was found to be the best criterion (of those tested) for an autofocus algorithm. We predict, based on theoretical results, that all of the focus algorithms we considered will focus in different heights relative to the object depending on the material properties of the object. Edge detection is accomplished via a window and spline technique for whole image data (e.g., charge-coupled device (CCD) array), and by application of a multiple line regression algorithm for single scan data (e.g., photometer). Measurements accomplished through these techniques are compared to state of the art scattering and analysis models.