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Proceedings Paper

Overlay accuracy fundamentals
Author(s): Daniel Kandel; Vladimir Levinski; Noam Sapiens; Guy Cohen; Eran Amit; Dana Klein; Irina Vakshtein
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Paper 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.

Paper Details

Date Published: 5 April 2012
PDF: 10 pages
Proc. SPIE 8324, Metrology, Inspection, and Process Control for Microlithography XXVI, 832417 (5 April 2012); doi: 10.1117/12.916369
Show Author Affiliations
Daniel Kandel, KLA-Tencor Corp. (Israel)
Vladimir Levinski, KLA-Tencor Corp. (Israel)
Noam Sapiens, KLA-Tencor Corp. (Israel)
Guy Cohen, KLA-Tencor Corp. (Israel)
Eran Amit, KLA-Tencor Corp. (Israel)
Dana Klein, KLA-Tencor Corp. (Israel)
Irina Vakshtein, KLA-Tencor Corp. (Israel)


Published in SPIE Proceedings Vol. 8324:
Metrology, Inspection, and Process Control for Microlithography XXVI
Alexander Starikov, Editor(s)

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