Optical critical dimension measurement and illumination analysis using the through-focus focus metric

In this paper we present a unique method of evaluating the angular illumination homogeneity in an optical microscope using the through-focus focus metric. A plot of the sum of the mean square slope throughout an optical image as the target moves through the focus is defined as the through-focus focus metric. Using optical simulations we show that the angular illumination inhomogeneity causes the through-focus focus metric value to proportionately increase at specific focus positions. Based on this observation, we present an experimental method to measure angular illumination homogeneity by evaluating the through-focus focus metric values on a grid across the field of view. Using the same through-focus focus metric, we present a detailed study to measure critical dimensions with nanometer sensitivity with the aid of simulations.