EUV coherent diffraction imaging (CDI) is a powerful technique enabling non-destructive characterization of buried nano-structures. Ptychography is a type of CDI technique where the sample is scanned with a focused light beam to reach the necessary amount of over-sampled data. The technique allows reconstruction of both amplitude and phase distributions in the sample space, much like holography, using special iterative phase retrieval algorithms. Utilizing information on both absorption/reflection and phase shift allows to decouple different subsurface layers of buried structures.
So far, EUV ptychography was used for imaging of mostly thin-film samples. Successful imaging of more complex buried 3D structures had been demonstrated before, but to enable 3D imaging, multi-angle tomographic approach was utilized. This drastically increases the length and complexity of both the measurements and data processing.
Here we present the results on the first-time quantitative characterization of the geometry of complex, 3D periodic buried nano-structures on a bulk substrate measured at near-grazing geometry with 13.5nm HHG source performed without angle-scanning.
New 3D architectures are especially challenging structures as the structures of interest are often hidden in the stack. Currently, relatively slow and destructive techniques such as TEM are being used which require extensive sample preparation.
The structures used in this experiment are architectures commonly used for 3D memory applications like 3D-NAND and are composed of three pairs of alternating PolySi and SiO₂ layers (thickness 70nm and 30nm, respectively). These multi-layer stacks are patterned to achieve memory holes with varying CDs (50nm - 200nm). Horizontal cavities were created by sacrificial PolySi etching and were partially filled with a conformal deposition of a dielectric material, a workfunction metal and a metal ¹.               
We were able to resolve the general geometry of all three levels with resolution down to 30 nm while the exercised resolved features had the size of ~70nm. This technique might become highly relevant for metrology of new-generation semiconductor devices since it enables a relatively fast, non-destructive measurement of the critical dimensions of buried nanoscale structures.
1.    Rachidi, S. et al. At the Extreme of 3D-NAND Scaling: 25 nm Z-Pitch with 10 nm Word Line Cells. 2022 IEEE International Memory Workshop, IMW 2022 - Proceedings (2022).