Efficient cross-sectional evaluation method of three-dimensional NAND flash memory by cooperation of coherence scanning interferometry and scanning electron microscopy

At the 3D-NAND analysis site, the variation in the channel hole shape of the entire laminated structure is evaluated threedimensionally by Scanning Electron Microscopy (SEM) to determine the etching conditions in each stacking process. Then, the feedback is given to the stacking process. In other words, quick observation has been required to evaluate the channel hole shape and the layer number accurately in association with each other. The current observation method identifies all the layers by counting manually. However, there is a concern that the observation time will increase due to the miniaturization and multi-layering of devices. As a solution to this problem, we have devised the method to determine a designated layer by joining together Coherence Scanning Interferometry (CSI) and SEM. CSI has capability of nanometerorder height measurement to identify the target layer. To move SEM field of view (FOV) to the designated layer accurately, CSI height map is converted to the XY coordinates of SEM stage by homography transformation. We introduced a new common specimen holder between CSI and SEM to reduce the position error caused by the difference of instruments. Experimentally, the accuracy of XY directional FOV movement in the SEM after coordinate conversion was verified. It was confirmed that the target FOV movement accuracy of 256-layer was achieved by using the common holder. We have the prospect of a new method that can observe the designated layer with high throughput rate because highly accurate data measured by CSI in Z-direction can be acquired.