Through-focus technique for overlay metrology

An-Shun Liu; Yi-Sha Ku; Nigel Smith

doi:10.1117/12.712030

5 April 2007 Through-focus technique for overlay metrology

An-Shun Liu, Yi-Sha Ku, Nigel Smith

Proceedings Volume 6518, Metrology, Inspection, and Process Control for Microlithography XXI; 65182X (2007) https://doi.org/10.1117/12.712030
Event: SPIE Advanced Lithography, 2007, San Jose, California, United States

Abstract

Optical overlay metrology is thought to face the challenges in precision improvement with edge-determination based algorithm because the design rule is gradually decreasing in advanced semiconductor process. We develop a novel algorithm for determining the overlay error of grating structures with an optical bright-field imaging tool. By evaluating the intensity variation of the different acquired images through the analysis of the optical images obtained at different defocus positions, the analysis curves of the focus measure versus the overlay error experimentally demonstrate the nanometer sensitivity with the overlay of the grating structure. When plenty of images of different target pattern captured with image sensor at different amounts of defocus position, the image intensity variation in each image can be calculated by focus criteria. In our application, the gradient energy is the best metric of the image within the grating area because it discriminates the main and side lobe local maximum more sharply that is a key phenomenon in our algorithm application. Empirical models were developed to fit the experiment results of image intensity variation versus overlay error. The experimental data show that the variations of the focus measure has nano-scale sensitivity to the overlay of the grating structure, so that it can be used to determine the overlay error by implementing the algorithm. Thus, the through-focus method has potential application in overlay metrology for the process control in future semiconductor manufacturing.

Citation Download Citation

An-Shun Liu, Yi-Sha Ku, and Nigel Smith "Through-focus technique for overlay metrology", Proc. SPIE 6518, Metrology, Inspection, and Process Control for Microlithography XXI, 65182X (5 April 2007); https://doi.org/10.1117/12.712030

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