20 April 2011 Accuracy of diffraction-based and image-based overlay
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There is no overlay standard in the world. For critical dimension (CD), we may use the VLSI standard or programmed pitch offsets to determine the CD accuracy or CD sensitivity. Programmed overlay offsets can provide relatively accurate sub-nanometer level overlay splits but it is only on a single layer and does not contain layer-to-layer process variations. The splits of scanner magnification can check the trend of overlay sensitivity but it cannot provide the exact value of overlay offsets. Transmission electron microscopes (TEM) can be used as a final overly error verification tool. However, TEM sample preparation for after-development-inspection (ADI) will introduce even more sample distortion errors. Therefore, unlike CD metrology, there is no clean and systematic way to verify the accuracy of overlay metrology. These technical barriers necessitate matching diffraction-based overlay and image-based overlay, especially for sub-nanometer point-to-point matching requirement. In this paper, we compare the correlation of ADI to after-etch-inspection (AEI) by using image-based box-in-box overlay measurement and diffraction-based overlay measurement on the same wafer. The ADI-to-AEI overlay data consistency plays a key role for lithography overlay APC success and AEI overlay should be treated as the final standard for overlay accuracy. We found that process-induced asymmetric profiles of overlay marks will lead to ADI-to-AEI overlay bias. This bias is proportional to the degree of profile asymmetry and different color/wavelength have different sensitivity to this ADI-to-AEI bias. Our experimental results show that the ADI-to-AEI overlay data bias can indeed be significantly improved by selecting the color/wavelength with minimum sensitivity to the asymmetry profile. These results make us believe that overlay metrology recipe setup is quite critical no matter for image-based overlay or diffraction-based overlay. Otherwise, problematic overlay data will be taken into APC feedback loop and lead to wrong overlay correction.
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Chih-Ming Ke, Chih-Ming Ke, Guo-Tsai Huang, Guo-Tsai Huang, Jacky Huang, Jacky Huang, Rita Lee, Rita Lee, } "Accuracy of diffraction-based and image-based overlay", Proc. SPIE 7971, Metrology, Inspection, and Process Control for Microlithography XXV, 79711E (20 April 2011); doi: 10.1117/12.880037; https://doi.org/10.1117/12.880037

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