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29 August 2008 Optimal measurement method for diffraction-based overlay metrology
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We propose a method to measure the overlay by choosing an optimal measurement design with a modified scatterometer system. This method is capable of measuring zero and non-zero diffraction orders at theta (zenith) and phi (azimuth) angles of incidence by carefully modulating the optical system. Thus a large quantity of angular scatterometry data can be measured in a short period of time with no mechanical or vibrational movement. We used a rigorous diffraction theory to model the measurement sensitivity using an overlay with two layer gratings at a fixed wavelength in the range of the theta (zenith, 0° to 90°) and phi (azimuth, 0° to 180°) incident angles. We compared the measurement sensitivities at theta and phi dependence. In addition, we compared the optical responses of zero order and first order diffractive overlays. We propose a methodology to measure the overlay using overlay targets with two gratings, designed with an intentional offset difference between the top and bottom gratings to maximize the measurement sensitivity and minimize the response to the process noise.
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Wei-Te Hsu and Yi-Sha Ku "Optimal measurement method for diffraction-based overlay metrology", Proc. SPIE 7066, Two- and Three-Dimensional Methods for Inspection and Metrology VI, 70660L (29 August 2008);

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