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1 April 2010 Proximity-associated errors in contour metrology
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In contour metrology the CD-SEM (critical dimension scanning electron microscope) assigns a continuous boundary to extended features in an image. The boundary is typically assigned as a simple function of the signal intensity, for example by a brightness threshold or gradient. However, the neighborhood of different points along the feature boundary may vary considerably. Some parts of the boundary may have close neighboring features while others are relatively isolated. Neighboring features can obstruct the escape of secondary electrons. Varying proximity of neighbors therefore represents an influence on detected intensity. An intensity difference caused by a neighborhood difference can be incorrectly interpreted as a contour shift, for example when the contour passes from an isolated neighborhood to a dense one. The magnitude of this offset variation is estimated using images produced by JMONSEL, a Monte Carlo simulator of SEM secondary electron imaging, from simple model test patterns with varying neighborhoods. Similar structures were subsequently measured by both SEM and atomic force microscopy (AFM). Apparent shifts (i.e., errors) on the order of 0.5 nm to 1.0 nm for each edge were observed in both modeled and measured SEM images as compared to AFM when edge positions were assigned by using a fixed image brightness contour. Assignment of edges by brightness relative to the local background and local maximum brightness resulted in measurements that were less sensitive to neighborhood differences.
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John S. Villarrubia, Ronald G. Dixson, and András E. Vladár "Proximity-associated errors in contour metrology", Proc. SPIE 7638, Metrology, Inspection, and Process Control for Microlithography XXIV, 76380S (1 April 2010);

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