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Proceedings Paper

Simulation of linearity in optical microscopes
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Paper Abstract

Different optical metrology systems such as broadband confocal microscopes, broadband coherence probe microscopes, and broadband brightfield microscopes show different linearity characteristics for different layers and linewidths. Linearity of response is dependent not only on the layer specifics but on the optical system parameters as well. These include the type of microscope, the bandwidth of illumination, the numerical aperture, the partial coherence, etc. Algorithm parameters such as focus- offset, threshold, and phase filter strength and placement (coherence probe) also make a difference. Since computer simulation is now able to predict (with good accuracy) the linewidths measured by these technologies and parameters, it is natural to begin a systematic study of the theoretical predictions. For example, the effects of wavelength on linearity for four types of microscopes are shown. All show improved linearity in the shorter wavelength region, extending the linear range of the instruments. The simulations suggest that variation of wavelength is a key to optimizing linearity around a given feature size. Linearity optimization analysis is performed for several microscope types and measurement algorithms for a simple layer geometry. The optimization program varies threshold and focus offset to achieve the best linearity. The combination of simulation with linearity optimization provides a testbench for metrology system design and evaluation. A new complex-difference algorithm is presented which clearly shows better linearity in at least one simulated case (when resist sidewall angles are changing) than the algorithms commonly used in optical metrology.

Paper Details

Date Published: 1 June 1992
PDF: 16 pages
Proc. SPIE 1673, Integrated Circuit Metrology, Inspection, and Process Control VI, (1 June 1992); doi: 10.1117/12.59789
Show Author Affiliations
Mark P. Davidson, Spectel Co. (United States)
Kevin M. Monahan, Metrologix, Inc. (United States)

Published in SPIE Proceedings Vol. 1673:
Integrated Circuit Metrology, Inspection, and Process Control VI
Michael T. Postek, Editor(s)

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