Image displacement sensing (NDSE) for achieving overlay alignment
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In this paper we discuss the application of NDSE  (Hewlett Packard’s nanoscale displacement sensing and estimation technology) as an overlay metrology tool. We describe a method where nanoscale displacement sensing forms the basis of a precision alignment measurement. We will then provide a review of experiments performed to assess the accuracy of one particular NDSE algorithm, tracking silicon targets as they translate on a piezoelectric stage under an optical microscope. We conclude by describing upcoming experiments which will incorporate NDSE as an alignment sensor in a nanoimprint lithography application.
Current methods of overlay metrology and many methods of displacement metrology require precise alignment targets, such as symmetric geometric figures or extremely high-Q diffraction gratings. Such patterns are expensive to produce and/difficult to fabricate consistently. On the other hand, NDSE provides displacement sensing by tracking totally arbitrary patterns. As long as the patterns remain fixed, NDSE can provide extraordinary precision. We extend this advantage into a method for alignment sensing, which retains displacement sensing as the key underlying measurement. Hence, as with displacement sensing, the alignment targets need not be held to any absolute standard, pattern asymmetries caused by process variations are not an issue, and precision gratings are not required.
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