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Proceedings Paper

Through-focus algorithm to improve overlay tool performance
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Paper Abstract

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.

Paper Details

Date Published: 10 September 2005
PDF: 9 pages
Proc. SPIE 5908, Optical Information Systems III, 59081E (10 September 2005); doi: 10.1117/12.616422
Show Author Affiliations
An-Shun Liu, Industrial Technology Research Institute (Taiwan)
Yi-Sha Ku, Industrial Technology Research Institute (Taiwan)
Nigel Smith, Accent Optical Technologies (Taiwan)


Published in SPIE Proceedings Vol. 5908:
Optical Information Systems III
Bahram Javidi; Demetri Psaltis, Editor(s)

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