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Bottom grating asymmetry-induced inaccuracy in diffraction-based overlay measurement

Abstract. Background: Overlay (OV) is a very important indicator for characterizing the pattern position accuracy between two layers in an integrated circuit in the manufacturing industry. The self-aligned double-patterning (SADP) process, which is limited to the resolution of deep ultraviolet lithography, is being extensively used for the volume manufacturing of integrated circuits. However, compared to the planar process, the SADP process poses new challenges for OV control. Aim: We investigate the bottom grating asymmetry of a microdiffraction-based overlay (μDBO) target in the SADP process and its effect on OV measurement. Approach: The bottom grating of the OV metrology target is analyzed through scanning electron microscopy and transmission electron microscopy. The imbalance of the −1st- and +1st-order diffraction light intensities of the asymmetrical bottom OV target is characterized. Finally, the effects of light polarization and wavelength on the OV accuracy are simulated. Results: Asymmetric behavior of bottom OV grating is identified. OV dependency on the measurement-light polarization is determined. The simulation results indicate that wavelength and polarization affect the OV accuracy simultaneously. Conclusions: We studied the bottom grating asymmetry-induced inaccuracy in DBO measurement and can assist in understanding the mechanism of measurement light-interaction with the OV grating to enhance the DBO accuracy to a new level.