Elucidating the capability of electron backscattering for 3D nano-structure determination

Reflection electron energy loss spectroscopy (REELS) is well established for the study of homogeneous materials with flat surfaces. Here we extend the use of this technique to nano-structures consisting of silicon and silica and show that the experimentally-observed peculiar dependence of the REELS spectra on the sample orientation can be reproduced by Monte Carlo simulations using the known sample morphology. A sample with a 3D structure, resembling those found in FinFET transistors, was analyzed through electron Rutherford backscattering (ERBS, revealing the mass of the atoms near the surface) and REELS (revealing the electronic structure). ERBS/REELS spectra were taken at two incoming electron energies (5 and 40 keV) and in two experimental geometries with the component of the outgoing propagation direction along the surface being either parallel or perpendicular to the fins. The measured spectra were different for the two geometries due to attenuation effects within the fins, especially at 5 keV where the inelastic mean free path is of the order of the fin dimensions. This means that the 3D structure induces shadowing effects which suppress the elastic peaks and enhanced the inelastic signal. A Monte Carlo code was used to simulate multiple elastic and inelastic interactions of the electrons with these 3D structures and was indeed able to reproduce these experimental results, including the shadowing effects. A sub-angstrom layer of Au was evaporated on the sample and the changes induced by the Au layer were dependent on the orientation of the fins and were again reproduced by the simulation.