Exploring the mechanisms of image formation in nanoscale subsurface imaging with Atomic Force Microscopy

Multi-frequency Atomic Force Microscopy (AFM) is emerging as a powerful platform for non-destructive subsurface imaging with nanoscale spatial resolution -- lateral and depth resolution. A combination of AFM and acoustic imaging has been explored to probe the volume of heterogeneous samples but the approach lacks quantitative interpretation of the depth at which structures are buried. Here we designed a set of samples composed of a 80 nm layer of Ni embedded between layers of Au of different thicknesses. These layers are in the range from 50 nm to 200 nm. These samples were prepared to determine the effect of depth on acoustic wave propagation in the sample, and on the resonance frequencies of the AFM cantilever used for imaging. The frequency and amplitude spectra acquired indicate that the position of the Ni layer in the volume (or depth) of the sample greatly impacts the signals measured by acoustic AFM. Significant frequency shifts could be observed, with great impact on the image formation previously reported in the literature. Our results pave the way to a deeper understanding of the processes of image formation for nanoscale subsurface imaging.