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Driven by the increasing demand for a better on-product overlay control, the metrology specifications continue to tighten when nodes advance. Metrology accuracy budgets are reaching levels close to 1 angstrom. To obey these stringent requirements on metrology accuracy, the understanding of the interplay between the metrology target and metrology tool is of paramount importance to advance diffraction based overlay (DBO) to the next level of accuracy. In this contribution we will first present a model for the signal formation of DBO. We will show how overlay is translated into a measurable intensity asymmetry using biased gratings. When the metrology target is symmetric, the accuracy of the measurement is limited by photon shot noise only. We will show how an asymmetric target deformation leads to an overlay ambiguity that deteriorates the accuracy of the overlay metrology well beyond the fundamental photon shot noise limit. To recover from the effects of asymmetric target deformation, we will show that additional information is needed on top of the traditional single wavelength overlay reading on one target. When the target deformation is small, that additional information can, for example, come from multiple wavelengths. For large asymmetric target deformations causing a significant center-of-gravity shift, a plurality of targets or a description of the stack is needed. These specific solution directions will be discussed in detail. In conclusion, in this paper we will dive into the fundamentals of diffraction based overlay, where we explore the physics of the signal formation in diffraction based overlay in the presence of asymmetric target deformation. We will show that a better understanding of the signal formation helps to develop methods that advance diffraction based overlay metrology to the next level of accuracy, which is needed to fulfill the increasing demand for a better on product overlay control.
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