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A small notch or foot existing at the bottom of a polysilicon gate is a common issue for etching processes. The small notch or foot could have a major impact on the length of the polysilicon gate, and the performance of the device would then be impacted significantly, especially for cutting-edge devices. This paper demonstrates the capability of a spectroscopic ellipsometry based profile technology, SpectraCD, as a new metrology tool to monitor polysilicon gate process at 130 nm and 90 nm nodes. Firstly, the capability of SpectraCD as a metrology technology was studied, including dynamic precision and CD correlation. Dynamic precision in the range of 0.1~0.4 nm was demonstrated repeatedly in this study. CD correlation with CDSEM also showed a very linear result. R-squared values of ~0.99 are presented. Secondly, by comparison with images from cross-sectional SEM (XSEM) and TEM (XTEM), it has been proved in this study that SpectraCD can consistently flag different profile excursions of polysilicon gate, e.g., small notching, footing, or undercut. The size of the footing or notch reported by SpectraCD shows a linear correlation with the size extracted from XTEM images, which demonstrates quantitatively SpectraCD capability for detecting profile excursions. Finally, linear correlation between the bottom CD from SpectraCD and the gate lengths determined from electrical test (Lcap) will be presented.

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