Development of Overlay Target's Centre Positioning Algorithms Using Customizable Shape Fitting for High-Precision Wafer Bonding

Wafer bonding is a critical process in 3D integration, and overlay (OVL) metrology is essential for its success. Accurately positioning the centre of OVL targets is fundamental for effective metrology. However, the identification and localization of target centres become challenging due to complex shapes and unexpected features, such as rounded corners, that can arise during manufacturing. An algorithm is proposed to tackle this challenge by employing customizable shape fitting. This method begins with the extraction of sub-pixel edge points, followed by applying a Hough transform to group and smooth these points, thereby enhancing contour quality. By parameterizing the target shape based on specific points, the algorithm integrates sub-pixel traversal techniques with an optimization objective, achieving sub-pixel accuracy in centre positioning. Simulation results indicate that the algorithm can achieve a positioning accuracy of +/- 0.03 pixels and demonstrates robustness against noise and blur. Finally, the proposed algorithm was used to test the OVL target pair arrays fabricated by electron beam etching, confirming an accuracy of +/- 0.04 pixels (+/- 6.9 nm). These results validate the algorithm's capability to meet high precision requirements for OVL target centre positioning in wafer applications.