Robust Semiconductor Overlay Metrology with Non-Uniform Illumination Beams Using Digital Holographic Microscopy

Precise, accurate, and fast characterization of overlay between two layers of semiconductors during chip fabrication is an important monitoring and feedback step. Overlay, which is to be measured within the accuracy of a nanometer, is susceptible to many small imperfections in the measurement system. A digital holographic microscope measures the complex field of overlay targets using simple optics followed by computational algorithms for overlay metrology. We improved the precision of overlay measurement by correcting the inhomogeneities and asymmetries caused by the illumination spots, presenting a robust method resulting from simple calibration steps.