Feature shape variation using scatterometry

We have studied the ability of ellipsometric spectroscopy to determine slight feature shape variations. In this paper, we show that an optical technique can be a sensitive technique to detect slight profile variations, especially at the top of the features. Simulated results as well as experimental results are presented. A sensitivity metrics, that is better suited to ellipsometric measurements than the conventional mean square error, is proposed. It is used to quantify the sensitivity to measure the feature shape of interest and to examine preferable experimental set-up. Computer simulations and ellipsometry signal libraries have been generated using a common rigorous electromagnetic modal approach (MMFE, Modal Method by Fourier Expansion). Experimental verification has been performed on resist features fabricated using DUV lithography. The ability and sensitivity to measure overhanging profiles such as notched polysilicon gates that are impossible to measure using top-down SEM is also mentioned.