5 April 2007 Zero-order imaging of device-sized overlay targets using scatterfield microscopy
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Patterns of lines and trenches with nominal linewidths of 50 nm have been proposed for use as an overlay target appropriate for placement inside the patterned wafer die. The National Institute of Standards and Technology (NIST) Scatterfield Targets feature groupings of eight lines and/or trenches which are not resolvable using visible-wavelength bright-field microscopy. Such repetitive patterns yield zero-order images superimposed by interference effects from these finite gratings. Zero-order imaging is defined as the collection of specular reflection from periodic structures without the collection of any possible diffracted beams. As our lines and trenches are formed in different photolithographic steps, the overlay offset can be derived from the relative displacement of these zero-order responses. Modeling this phenomenon will require a thorough characterization of the transmission of light through all points in the optical path as a function of position, angle, and polarization. Linear polarization parallel and perpendicular to these lines and trenches is investigated as a possible enhancer of overlay offset measurement repeatability. In our particular case, nominally unpolarized light proved most repeatable.
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Bryan M. Barnes, Bryan M. Barnes, Lowell P. Howard, Lowell P. Howard, Jay Jun, Jay Jun, Pete Lipscomb, Pete Lipscomb, Richard M. Silver, Richard M. Silver, } "Zero-order imaging of device-sized overlay targets using scatterfield microscopy", Proc. SPIE 6518, Metrology, Inspection, and Process Control for Microlithography XXI, 65180F (5 April 2007); doi: 10.1117/12.716457; https://doi.org/10.1117/12.716457

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