Investigation of novel inspection capability for 3D NAND device wordline inspection

Migration to a 3D implementation for NAND flash devices is seen as the leading contender to replace traditional planar architectures. The new 3D NAND architectures involve the stacking of multiple cells formed inside a high aspect ratio channel hole. However, the strategy of replacing shrinking design rules for greater aspect ratios is not without its own set of challenges, especially related to defectivity. Process integration of the wordline is a particular challenge, both for integrators as well as for defect inspection and reduction. Key defects of interest are typically sub-surface (especially voids), buried in the stack, or are residues in high aspect ratio structures. In our collaboration we investigated the performance of both current optical inspection systems as well as the value of new inspection techniques identified using simulations for buried defect detection. This simulation investigation showed the potential value of near infrared wavelengths. A prototype inspection tool was investigated which lead to the development of near infrared optimized optics for a production capable inspection tool. This study validates this capability in R&D and is now being implemented as a production monitoring solution.