Enhancing diffraction-based overlay metrology capabilities in digital holographic microscopy using model-based signal separation

BackgroundThe continuous shrink of device dimensions in the semiconductor industry drives the need to improve optical microscopy techniques that are often used in overlay metrology. Achieving sub-nanometer overlay metrology precision requires near-perfect imaging conditions and an almost complete suppression of imaging artifacts.AimDigital holographic microscopy (DHM) has been presented as a promising new overlay tool measuring the full complex-valued field, giving one computational access to the pupil plane. The unique characteristics of a holographic imaging system in combination with prior knowledge of the object under study show the capability to advance semiconductor metrology. This technique enables a further reduction in metrology target size while also offering opportunities to improve precision.ApproachWe present experimental results on a model-based signal separation technique using digital pupil filtering for two different metrology challenges and demonstrate strong suppression of nuisance signals without resolution loss.ResultsWe will present two experimental examples of different types of pupil filtering in DHM allowing for larger region-of-interest selection to improve the diffraction-based overlay metrology precision and accuracy.ConclusionWe experimentally demonstrate that model-based signal separation in the pupil plane can significantly enhance the overlay metrology capabilities in cases where prior knowledge of the sample is present.