10 September 2005 Through-focus algorithm to improve overlay tool performance
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Current optical overlay measurement tools utilize visible light and operate with optical resolution of approximately 0.5-1.0 μm. Such tools cannot resolve the targets based on the design rule features. Hence, reliable theoretical model-based measurements and enhanced algorithms are required to address this problem. The test targets, with features similar to those specified by the design rule, were fabricated by a high precision E-beam writer. An optical bright field overlay metrology tool was applied to acquire the optical images of the test targets. The best focus position of test target is selected using an auto focus algorithm. The focus offset is specified relative to the best focus position and the optical image data is measured with a full field-of-view CCD array. The through focus image data are analyzed to obtain the relationship between the intensity profile and the structural parameters of the test targets. These structural parameters are also verified with the CD-SEM. This work experimentally analyzes the through-focus behavior of the test targets. These targets are based on grating patterns, and while they provide more information than traditional targets, they are more sensitive to the focus position. The through focus image formed at the image plane depends on the relative focus position between the target and the optical system. An appropriate design for the optical configuration and target geometry produces a unique image at each focus position, for a specific physical feature.
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An-Shun Liu, An-Shun Liu, Yi-Sha Ku, Yi-Sha Ku, Nigel Smith, Nigel Smith, } "Through-focus algorithm to improve overlay tool performance", Proc. SPIE 5908, Optical Information Systems III, 59081E (10 September 2005); doi: 10.1117/12.616422; https://doi.org/10.1117/12.616422

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