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Proceedings Paper

Frequency-domain subpixel position estimation algorithm for overlay measurement
Author(s): Scott C. Douglas
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Paper Abstract

In lithographic metrology tasks such as overlay measurement, the pixel size of the imaging device is often much larger than the desired accuracy of the measurement tool. Interpolating the imaged data prior to the application of a measurement algorithm gives an accurate measurement but increases the amount of data to be processed, placing demands upon computational resources. In this paper, we propose a computationally-simple and highly- accurate algorithm for determining the subpixel shift between two waveforms. The algorithm utilizes the fast Fourier transform of each waveform to estimate the relative subpixel shift between the two waveforms using the complex phases of the frequency-domain-transformed data. The algorithm uses a simple iterative search for the position parameter that typically converges in less than ten iterations with optical data. We show that this algorithm estimates subpixel position more accurately than cross-correlation followed by interpolated-peak-fitting using both synthetic and real image data. Moreover, in high signal-to-noise ratio situations, the algorithm's precision can be shown to approach the fundamental precision limit determined by the statistics of the image data. Results of the algorithm's application to overlay data from a commercial scanning-confocal optical microscope are presented.

Paper Details

Date Published: 4 August 1993
PDF: 10 pages
Proc. SPIE 1926, Integrated Circuit Metrology, Inspection, and Process Control VII, (4 August 1993); doi: 10.1117/12.148962
Show Author Affiliations
Scott C. Douglas, Univ. of Utah (United States)


Published in SPIE Proceedings Vol. 1926:
Integrated Circuit Metrology, Inspection, and Process Control VII
Michael T. Postek, Editor(s)

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