The limits of image-based optical metrology

An overview of the challenges encountered in imaging device-sized features using optical techniques recently developed in our laboratories is presented in this paper. We have developed a set of techniques we refer to as scatterfield microscopy which allows us to engineer the illumination in combination with appropriately designed metrology targets. The techniques have previously been applied to samples with sub-50 nm sized features having pitches larger than the conventional Rayleigh resolution criterion which results in images having edge contrast and elements of conventional imaging. In this paper we extend these methods to targets composed of features much denser than the conventional Rayleigh resolution criterion. For these applications, a new approach is presented which uses a combination of zero order optical response and edge-based imaging. The approach is, however, more general and a series of analyses based on theoretical methods is presented. This analysis gives a direct measure of the ultimate size and density of features which can be measured with these techniques and addresses what measurement resolution can be obtained. We present several experimental results, optical simulations using different electromagnetic scattering packages, and statistical analyses to evaluate the ultimate sensitivity and extensibility of these techniques.