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Optical Critical Dimension Measurement and Illumination Analysis using the Through-focus Focus Metric



Ravikiran Attota, Richard M. Silver, M R. Bishop, Ronald G. Dixson


In this paper we present utility of the out of focus optical microscope images for metrology applications as opposed to the best focus images. Depending on the type of analysis, considerable beneficial information can be deduced from the out of focus optical images. Here we present different types of analyses that can be performed on the out of focus optical images to obtain useful information. One-way of analyzing the out of focus images is to calculate the mean slope of the images as the target moves through-focus. A plot of the sum of the mean square slope is defined as the through-focus focus metric. We present a unique and simple method of evaluating the angular illumination homogeneity in an optical microscope that has K hler illumination scheme, using the through-focus focus metric. Both the theoretical simulations and the experimental results are provided to support the same. We present another application of the through-focus focus metric where it can be used to evaluate critical dimensions (CD) with nanometer sensitivity in combination with the optical simulations. The second way of analyzing out of focus images is to stack the through focus image intensity profiles such that the x-axis represents the position on the target, the y-axis represents the focus (or defocus) position of the target with respect to the lens and the z-axis represents the image intensity. This two-dimensional image is defined as the through focus image map.  Using the simulation results we show application of the through focus image map for CD and overlay analysis with nanometer sensitivity. This technique is especially suitable for CD analysis of a truly isolated line.
Proceedings Title
Proceedings of SPIE
Conference Dates
February 20, 2006
Conference Location
San Diego, CA
Conference Title
Metrology, Inspection, and Process Control for Microlithography XX


angular optical illumination, defect analysis, Kohler factor, optical critical dimension metrology, overlay, through-focus focus metric, through-focus image map


Attota, R. , Silver, R. , Bishop, M. and Dixson, R. (2006), Optical Critical Dimension Measurement and Illumination Analysis using the Through-focus Focus Metric, Proceedings of SPIE, San Diego, CA, [online], (Accessed July 14, 2024)


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Created March 24, 2006, Updated February 19, 2017