Traditionally, the total measurement uncertainty (TMU) of overlay metrology focuses on dynamic precision, toolinduced-shift, and matching, while rarely examining inaccuracy. However, some researchers have recently shown that measurement inaccuracy can still be large despite optimized small TMU. Moreover, this inaccuracy can consume a significant portion of the overlay budget in the advanced nodes. In addition to qualifying the overlay error of inline wafers, overlay metrology is also used for improving on-product overlay as it provides corrective feedback to the lithography scanner. The accuracy of the correction terms as a result depends directly upon the measurement accuracy. As such, enhanced overlay accuracy will improve the overlay performance of reworked wafers, or subsequently exposed wafers. We have previously shown that a segmented Blossom target is more prone to asymmetry-induced inaccuracy than a nonsegmented target is . Since target segmentation is inevitable for SADP and SAQP patterning processes, their resulting overlay performance leaves a lot to be desired. In our quest to reach for the true overlay, this paper reports our investigations on accuracy enhancement techniques for image-based targets, such as redundancy and self-calibration, and on the use of simulation-optimized scatterometry-based targets.
Chiew-seng Koay, Bassem Hamieh, Nelson Felix, and John Gaudiello, "Reaching for the true overlay in advanced nodes," Proc. SPIE 10145, Metrology, Inspection, and Process Control for Microlithography XXXI, 101450B (Presented at SPIE Advanced Lithography: February 28, 2017; Published: 28 March 2017); https://doi.org/10.1117/12.2260007.
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