5 April 2012 Evaluating diffraction-based overlay
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Abstract
We evaluate diffraction-based overlay (DBO) metrology using two test wafers. The test wafers have different film stacks designed to test the quality of DBO data under a range of film conditions. We present DBO results using traditional empirical approach (eDBO). eDBO relies on linear response of the reflectance with respect to the overlay displacement within a small range. It requires specially designed targets that consist of multiple pads with programmed shifts. It offers convenience of quick recipe setup since there is no need to establish a model. We measure five DBO targets designed with different pitches and programmed shifts. The correlations of five eDBO targets and the correlation of eDBO to image-based overlay are excellent. The targets of 800nm and 600nm pitches have better dynamic precision than targets of 400nm pitch, which agrees with simulated results on signal/noise ratio. 3σ of less than 0.1nm is achieved for both wafers using the best configured targets. We further investigate the linearity assumption of eDBO algorithm. Simulation results indicate that as the pitch of DBO targets gets smaller, the nonlinearity error, i.e., the error in the overlay measurement results caused by deviation from ideal linear response, becomes bigger. We propose a nonlinearity correction (NLC) by including higher order terms in the optical response. The new algorithm with NLC improves measurement consistency for DBO targets of same pitch but different programmed shift, due to improved accuracy. The results from targets with different pitches, however, are improved marginally, indicating the presence of other error sources.
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Jie Li, Asher Tan, JinWoo Jung, Gary Goelzer, Nigel Smith, Jiangtao Hu, Boo-Hyun Ham, Min-Cheol Kwak, Cheol-Hong Kim, Suk-Woo Nam, "Evaluating diffraction-based overlay", Proc. SPIE 8324, Metrology, Inspection, and Process Control for Microlithography XXVI, 83243A (5 April 2012); doi: 10.1117/12.918706; https://doi.org/10.1117/12.918706
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