Illumination spot profile correction in digital holographic microscopy for overlay metrology

Background: Precise, accurate, and fast overlay (OV) metrology is an important step in semiconductor device manufacturing. With the increasing demand for better OV over a larger range of different process layers, the optics used in OV metrology tools become more complex, bulky, and expensive. OV, which is to be measured with sub-nanometer precision, is susceptible to many small imperfections in the measurement system. Aim: We present a dark-field digital holographic microscope (DHM) that measures the complex field of the OV targets using simple optics, followed by computational algorithms to correct for hardware imperfections. With the setup, we aim to correct the effects of the absolute intensity of the illumination beam as well as the spatial profile. Approach: The spatial profiles of two oblique illumination beams for diffraction based OV metrology are calibrated using large gratings as calibration targets using DHM, and thereafter OV target images are corrected by the calibrated illumination spot profiles. Results: OVs are calculated for test targets with known OV values, and illumination spot correction removes errors originating from intensity imbalance and intensity variation. Conclusion: We present an optical OV measurement method that is more robust against non-uniform illumination beams using simple calibration steps.