We show that an overlay (OVL) metrology system based on a scanning electron microscope can achieve accurate registration of buried and resist (top) structures. The positions were determined by both Back Scattered Electrons (BSE) and Secondary Electrons (SE). The accuracy was quantified for After-Development Inspection (ADI) of an advanced EUVL process. Results by linear tracking showed accuracy below 0.4nm, robust across process variation and target designs. The influence of various measurement conditions, e.g. Field of View, on position and OVL tracking was negligible. The measurement methodology presented is applicable for both standalone High Voltage SEM (HV-SEM) registration targets and optical targets, such as the Advanced Imaging Metrology (AIM®) target used by Imaging Based Overlay (IBO) metrology systems. Using SEM ADI OVL results as a calibration for optical overlay metrology tools we can demonstrate significant improvements in the optical ADI OVL accuracy on small targets like AIM in-die (AIMid).
On-product overlay (OPO) challenges are quickly becoming yield limiters for the latest IC technology nodes, requiring new and innovative solutions to meet the technology demands. One of the primary means for reducing OPO error is the measurement of the grid (on target) at after-develop inspection (ADI) correctly and accurately. To reduce the optical error in the measurement, signals from both high voltage scanning electron microscope (HV-SEM) technology and imaging based overlay (IBO) measurements at ADI can be leveraged. Using key performance indicators (KPIs) and information produced by multiple optical measurement conditions, it is possible to optimize SEM sampling across the wafer and to capture all relevant target deformations. The objective is to improve the accuracy of optical measurements by efficiently combining information from HV-SEM and optical metrology systems. This paper will demonstrate that the information extracted from electron-based metrology and IBO measurements can be used for direct measurement of target deformations, which feeds into advanced optical target diagnostics and utilized for de-correlation between asymmetries and overlay (OVL).
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