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Koehler Illumination for High-Resolution Optical Metrology



Martin Y. Sohn, Bryan M. Barnes, Lowell P. Howard, Richard M. Silver, Ravikiran Attota, Michael T. Stocker


Accurate preparation of illumination is critical for high-resolution optical metrology applications such as line width and overlay measurements. To improve the detailed evaluation and alignment of the illumination optics, we have separated Koehler illumination into three components.  The three Koehler illumination components are defined as full field spatial intensity variation (Koehler factor 1), angular intensity homogeneity (Koehler factor 2), and wavefront phase/intensity homogeneity (Koehler factor 3).  We have also proposed a field aperture pattern transfer method to analyze the illumination properties with respect to systematic variations, such as the shape of the source, the intensity distribution at the back focal plane, and the displacements of elements along and off the optical axis. These factors were investigated in both ideal and practical illumination systems. In particular, any angular asymmetry in the illumination proves to have a detrimental effect upon the distribution of light that illuminates the target. Wavefront asymmetry is also studied in the context of an optical system with a coherent or partially coherent light source.
Proceedings Title
Proceedings of SPIE
Conference Dates
February 19-24, 2006
Conference Location
Santa Clara, CA
Conference Title
Metrology, Inspection, and Process Control for Microlithography XX


angular homogeneity, back focal plane, field pattern transfer method, high-resolution metrology, Koehler factor, Koehler illumination


Sohn, M. , Barnes, B. , Howard, L. , Silver, R. , Attota, R. and Stocker, M. (2006), Koehler Illumination for High-Resolution Optical Metrology, Proceedings of SPIE, Santa Clara, CA (Accessed February 28, 2024)
Created March 1, 2006, Updated February 19, 2017