Multilayer structures of active semiconductor devices (1), novel memories (2) and semiconductor interconnects are becoming increasingly three-dimensional (3D) with simultaneous decrease of dimensions down to the few nanometres length scale (3). Ability to test and explore these 3D nanostructures with nanoscale resolution is vital for the optimization of their operation and improving manufacturing processes of new semiconductor devices. While electron and scanning probe microscopes (SPMs) can

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... vide necessary lateral resolution, their ability to probe underneath the immediate surface is severely limited. Cross-sectioning of the structures via focused ion beam (FIB) to expose the subsurface areas often introduces multiple artefacts that mask the true features of the hidden structures, negating benefits of such approach. In addition, the few tens of micrometre dimension of FIB cut, make it unusable for the SPM investigation. Here, we present two complementary advanced characterization methodologies enabling mapping of 3D nanostructure of semiconductor devices, memories and interconnects with nanometre scale resolution. First one is based on the wellknown ability of elastic stress to propagate in the bulk of the material. In order to create such nanoscale stress we use the combination of SPM and ultrasound -Ultrasonic Force Microscopy (UFM) (4) known for its ability to detect subsurface features in the solid state materials (5). UFM is using the standard contact atomic force microscope (AFM) setup where the high frequency (HF) several MHz range, but very small sub-nanometre amplitude ultrasonic vibration is applied to the studied sample. As vibration frequency is much higher than the resonance frequency of AFM tip and cantilever, nanoscale sized apex of the tip dynamically presses the sample, creating the HF oscillating strain field in the subsurface of the sample (6) as shown in Fig.1a . The subsurface features in the material in the volume reached by this strain field result in the modification of the dynamic reaction force acting on the probe tip. The change in the oscillating reaction force is detected as the average "ultrasonic" force due to the force "rectification" at the nonlinear tip-surface contact (4) resulting in a mapping of surface and near-surface properties of the material. While observing subsurface defects with significant mismatch of mechanical properties have been already reported reported by the authors who pioneered this work (5) and several other groups -including observing voids and delaminations (7-9), vacuoles (10) or inorganic inclusions in the cells (11), the question remains whether nanomechanical mapping can reveal compositional differences in the solid state inorganic materials, a task essential for the semiconductor Figure 1 . a) FEA simulation of the dynamic strain created by the 10 nm size ultrasonic force microscopy (UFM) tip with 10 nN average force probing the subsurface of the GaAs sample. The absolute strain is below 10 -5 suggesting fully non-destructive testing with no plastic deformation or sample modification. b) Example of the AFM contact mode imaging of topography of a surface of 50 nm thick graphite flake on the patterned DVD polymeric substrate. c) The same area imaged via UFM clearly shows both the areas where the flake contacts the subsurface matrix as well as internal stress induced subsurface defects (ripples, marked by arrows). Topo b) c) a) 100 nm UFM