Optimized wavelength selection for diffraction-based overlay measurement by minimum asymmetry factor variation with finite-difference time-domain simulation

A robust and wafer-less wavelength selection methodology was proposed. An overlay calculation model considering the asymmetric bottom grating structure showed that additional intensities diffracted from the asymmetric structure caused the overlay error. An asymmetry factor was introduced to describe the intensity ratio of the original overlay mark and a mark with bottom grating only. Based on the simulation results, the optimized wavelength was selected by analyzing the wavelength at which there is the minimum variation of the asymmetry factor. Four cases were tested by the simulation, and the maximum overlay error of the optimized wavelength selected by this method was 0.21 nm.