Enhancing re-detection efficacy of defects on blank wafers using stealth fiducial markers

Abstract

To qualify tools of semiconductor manufacturing, particles unintentionally deposited in these tools are characterized using blank wafers. With fast optical inspection tools one can quickly localize these particle defects. An example is TNO's Rapid Nano, which operates in optical dark field. The next step is defect review for further defect characterization. When the blank wafers are transferred to another tool, e.g. a SEM or an AFM the absolute defect position information is lost. Therefore, the re-detection of the defects in the review tool is time consuming. To enhance the re-detection speed, a fiducial marker system can be used that couples the coordinates of the fast inspection tool to the coordinates of the characterization (review) tool.In this work such a fiducial marker system was designed and validated. The influences of the height and the composition of the fiducial markers on the performance of the marker system were investigated using finite element analysis (by COMSOL) and experiments. The optimized fiducial markers are very visible in optical bright field and in SEM, while almost invisible (“stealth”) in optical dark field. These properties make the markers both easily visible and accurately localizable in the characterization tools. The stealth fiducial marker system was fabricated and validated by re-detecting programmed test defects on a blank wafer. The experimental results are compared to a Monte Carlo simulation that takes into account the uncertainties in the coordinate transformation and localization of the test defects.Our results show that a fiducial marker system greatly enhances the re-detection efficacy of defects on blank wafers. Using the fiducial marker system, 100% of the test defects were re-detected in SEM and AFM. A single 7×7μm2 SEM image suffices to meet the ITRS requirement for particles as small as 70nm in diameter.