Uncertainty estimation and design optimization of 2D diffraction-based overlay metrology targets
Scatterometry is an optical metrology technique, in which light scattered from a specifically designed grating stack (overlay target) is measured in the far-field. Using 1D periodic overlay target designs the technique has been shown to have nanometer-scale sensitivity to spatial misalignments of subsequent patterned layers, which are also known as overlay errors. However, while scatterometry is highly sensitive to overlay errors, multiple sources of systematic errors hinder its absolute accuracy. Here, we investigate how an extended version of scatterometry called Fourier scatterometry in combination with more complex overlay target designs can help addressing those challenges. To this end, we developed a statistical method, which can determine the influence of 2D overlay targets on the overlay measurement uncertainty. We study periodic and deterministic aperiodic designs as well as designs that emerged from simulated annealing optimizations. Our results suggest that current overlay target designs could be augmented by more complex 2D designs to fulfill specific purposes, such as fabrication robustness and high sensitivity over a large overlay range.