Sensitivity analysis in subsurface scanning probe microscopy measurements through stiff capping layers

Subsurface Scanning Probe Microscopy (SSPM) compliments present optical and electron-beam based techniques (e.g. CD-SEM, x-ray scatterometry, etc.) of measuring through optically opaque layers(for e.g. metals like Ta, W, Ti) by being non-destructive with a greater depth sensitivity for a possible 3-D tomographic reconstruction. With the presence of a certain structure below the surface there is a resultant effective change in local stiffness and, thus, contact resonance. This change can be attributed as a possible reconstruction of what may lie underneath giving directly the local 3-D information. In this work, we investigate imaging of hard copper gratings buried under hard and opaque semiconductor layer of SiCN. The relative difference in stiffness between features is minute and it is hard to sensitively image structures at the scale reported in this work. This work proves that with SSPM, one can sensitively distinguish structures based on the relatively low stiffness difference. Consequently, it is proven as a capable technique that can mitigate the present depth sensitivity and optically opaque subsurface metrological challenges for semiconductor samples. We report a lateral resolution down to 20nm while distinguishing structural variations, edge roughness of the gratings, focusing/ de-focusing effect as a function of ultrasonic actuation frequency and depth sensitivity as a function of applied indentation force. As a result, this work emphasizes that SSPM is capable of revealing buried nanostructures and has a potential industrial application in subsurface precision measurement technologies.