Optimized overlay metrology based on polarization eigenstate of Mueller matrix.

Overlay metrology is critical for the management and control of the lithography quality in semiconductor manufacturing. In industrial applications, the empirical DBO (diffraction-based overlay) method based on differential signals is one of the main techniques for this task. However, how to select and identify an overlay indicator with good linearity, sensitivity, and robustness for a wide overlay range is the key challenge for constructing the empirical DBO formulations. In this work, we propose what we believe to be a new overlay indicator based on the polarization eigenstate of the retardance Mueller matrix for ellipsometry-based overlay metrology. The general property requirements and optimization mechanism for the overlay indicator selection are also theoretically analyzed and discussed. Then, the effectiveness of the proposed method is validated by both simulations and experiments. Compared to the existing Mueller matrix-based indicators, the results prove that our method shows better accuracy and robustness for the typical double-layer-grating overlay targets. This work can provide a theoretical basis for the DBO indicator design as well as a potentially new tool for the overlay metrology practice.