Sputtering and the formation of nanometre voids and holes in aluminium in a scanning transmission electron microscope

摘要

Nanometre voids and holes have been produced in aluminium films up to 220 nm thick by the stationary focused 100 keV high-current-density electron probe in a dedicated scanning transmission electron microscope. The electron energy is below the threshold for bulk displacements in aluminium but is sufficient to cause sputtering of aluminium atoms from the electron-exit surface. The sputtering leads to the formation of a pit with a diameter determined by the electron probe size at the electron-exit surface. As the pit aspect ratio increases, atoms are sputtered from the pit base onto the pit side walls where they experience a much reduced electron intensity, rather than being sputtered directly out of the pit. Eventually the pit seals to leave a void, separated from the end of the small pit that remains at the electron-exit surface. By repeatedly interrupting the irradiation so as to image a projection of the irradiated volume, it is shown that the void then moves from near the electron-exit surface to the electron-entrance surface along the irradiated volume, presumably by electron-stimulated surface diffusion and sputtering of atoms around the void faces in the general direction of the flow of electrons. The process of pit growth and void formation at the electron-exit surface repeats itself, producing a row of voids extending away from the electron-exit surface along the irradiated volume. Under continuous irradiation, the voids formed in the irradiated volume have lengths of 25-30 nm, independent of sample thickness, and diameters comparable with the electron-beam diameter at the electron-exit surface, which increases with thickness owing to electron-lattice atom elastic scattering. Voids reaching the electron-entrance surface cause the growth of a pit at the surface, which eventually forms a continuous hole through the aluminium. Monte Carlo simulations have been used to follow the electron trajectories and estimate electron-exit surface sputtering rates for scanning transmission electron microscopy (STEM) electron probes with near-Gaussian radial intensity distributions used in this study. The simulations are consistent with the void formation rates observed and the total time for hole formation of typically tens of minutes for the 0.1-1 nA STEM electron probes of 2 nm diameter used.