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Subsurface Scanning Probe Microscopy (SSPM) complements present optical and electron-beam based techniques (e.g. CD-SEM, x-ray scatterometry, etc.) for measuring through optically opaque layers (for e.g. metals like Ta, W, Ti) by being non-destructive in nature with greater depth sensitivity. With the potential CMOS scaling road-map extension by IMEC presenting Gate All Around (GAA) and Complementary Field Effect Transistors (CFET) as the future of most CMOS devices, there is a rise in stacked 3D architectures with relatively less stiffness difference between adjacent nanostructures. The processing of gate all-around Si transistors requires isolating vertically stacked nanometers-thick Si sheets or wires. For this purpose, the SiGe layers of a SiGe/Si superlattice are etched selectively and laterally. Controlling the quantity of etched SiGe material, i.e., the cavity depth, is critical for optimal device performance. The critical dimension (CD) of the underetch can only be measured by cross-sectional electron microscopy providing limited statistics and hence control of the underetch across wafers. This process also requires more time reducing the overall efficiency. With SSPM, one can sensitively distinguish such cavity structures based on the relative stiffness difference. This process is comparatively faster providing sub nanometers resolution with statistically significant direct local 3D information. This work showcases the enhanced achievable sensitivity in mixed-frequency excitation scheme in comparison to single-frequency excitation scheme in SSPM for distinguishing the critical dimensions of the given samples based on the increasing etch recess. The achieved performance is for distinguishing non-destructively the difference in sub 10nm etch difference lying below a capping layer of approximately 100nm of hard and opaque layer. The top layer can be treated like a gate material due to relatively similar material and physical characteristics. Thus, this work can be treated as an example of subsurface measurements of buried nanostructures through gate like material.
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