Comparative near infrared through-focus scanning optical microscopy for 3D memory subsurface defect detection and classification

Through-focus scanning optical microscopy (TSOM) is a model-based optical metrology method that involves the scanning of a target through the focus of an optical microscope. Nanometer scale sensitive information is then extracted by matching the target TSOM data/image to reference TSOM data/images that are either experimentally or computationally collected. The nanometer sensitivity was previously confirmed by several theoretical and optical implementations. However, these studies all involved application to wafer patterns on the top surface. The present study extends the TSOM method to subsurface defect detection and classification without destruction, which becomes extremely important due to increasingly widely employed 3D semiconductor technologies. First, we apply a near-infrared (NIR) beam as illumination light in order to allow defect identification over the entire device depth. In addition, we adopt a model-less TSOM approach since the construction of a TSOM reference database for 3D pattern structures such as 3D NAND flash memory is hardly practical. We therefore employ a comparative TSOM method in which a TSOM data cube/image is compared with an image of an adjacent die or that of a “golden” die known to be defect free. We report the results of the first application of this method to an Intel 3D NAND flash and show that substantial subsurface defects are detected and classified.