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Overlay accuracy fundamentals Daniel Kandel, Vladimir Levinski, Noam Sapiens, Guy Cohen, Eran Amit, Dana Klein, Irina Vakshtein KLA-Tencor Corporation, 1 Halavyan Street, Migdal Haemek 23100, Israel ABSTRACT Currently, the performance of overlay metrology is evaluated mainly based on random error contributions such as precision and TIS variability. With the expected shrinkage of the overlay metrology budget to < 0.5nm, it becomes crucial to include also systematic error contributions which affect the accuracy of the metrology. Here we discuss fundamental aspects of overlay accuracy and a methodology to improve accuracy significantly. We identify overlay mark imperfections and their interaction with the metrology technology, as the main source of overlay inaccuracy. The most important type of mark imperfection is mark asymmetry. Overlay mark asymmetry leads to a geometrical ambiguity in the definition of overlay, which can be ~1nm or less. It is shown theoretically and in simulations that the metrology may enhance the effect of overlay mark asymmetry significantly and lead to metrology inaccuracy ~10nm, much larger than the geometrical ambiguity. The analysis is carried out for two different overlay metrology technologies: Imaging overlay and DBO (1st order diffraction based overlay). It is demonstrated that the sensitivity of DBO to overlay mark asymmetry is larger than the sensitivity of imaging overlay. Finally, we show that a recently developed measurement quality metric serves as a valuable tool for improving overlay metrology accuracy. Simulation results demonstrate that the accuracy of imaging overlay can be improved significantly by recipe setup optimized using the quality metric. We conclude that imaging overlay metrology, complemented by appropriate use of measurement quality metric, results in optimal overlay accuracy. Keywords: Overlay Accuracy, Measurement Quality Metric.

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OVERLAY MARK IMPERFECTIONS, OVERLAY AMBIGUITY AND ACCURACY

The standard evaluation of the capability of an overlay metrology tool relies mostly on a Total Measurement Uncertainty (TMU), which includes effects of precision, TIS variability and tool matching. Overlay control requirements of future nodes lead to the very tight requirement of TMU < 0.5nm. It is known, however, that small TMU does not guarantee that the overlay metrology budget is met, because some metrology errors are not taken into account in the TMU. The most important additional errors are associated with process (litho, etch, CMP, etc.) induced overlay mark imperfections and their interaction with the metrology technology, which may lead to inaccurate overlay measurement. Such inaccuracy may be reflected, for example, in bias between after develop and after etch measurements, difference between measurements carried out with different wavelengths or different focus positions of the metrology tool. In extreme cases, the resulting inaccuracy can be much larger than 1nm, and can consume the whole overlay control budget. In this work we explain the origin of overlay mark related inaccuracy, and show that a measurement quality metric developed by KLA-Tencor is a valuable tool to address this problem. We show that using this quality metric it is possible to eliminate outlier measurements and to select a recipe setup which enables an accurate overlay metrology. We start by realizing that in overlay metrology we measure the distance between the centers of symmetry of features of the overlay mark, printed in different layers. Such a methodology leads to accurate well defined overlay only if each feature of the overlay mark is symmetric to reflection. Unfortunately, a variety of process effects generate asymmetric features. This asymmetry can have a random nature, where no specific wafer level signature is observed, or a systematic nature where overlay correctibles are affected directly.

Metrology, Inspection, and Process Control for Microlithography XXVI, edited by Alexander Starikov, Proc. of SPIE Vol. 8324, 832417 路 漏 2012 SPIE 路 CCC code: 0277-786X/12/$18 路 doi: 10.1117/12.916369

Proc. of SPIE Vol. 8324 832417-1 Downloaded from SPIE Digital Library on 10 Apr 2012 to 192.146.1.12. Terms of Use: http://spiedl.org/terms


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