SPUTTERING AND THE FORMATION OF NANOMETER VOIDS AND HOLES IN ALUMINUMIN A SCANNING-TRANSMISSION ELECTRON-MICROSCOPE

Nanometre voids and holes have been produced in aluminium films up to 220 nm thick by the stationary focused 100 keV high-current-density el ectron 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 fro m the electron-exit surface. The sputtering leads to the formation of a pit with a diameter determined by the electron probe size at the ele ctron-exit surface. As the pit aspect ratio increases, atoms are sputt ered 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 f rom the end of the small pit that remains at the electron-exit surface . By repeatedly interrupting the irradiation so as to image a projecti on 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 diff usion and sputtering of atoms around the void faces in the general dir ection of the flow of electrons. The process of pit growth and void fo rmation at the electron-exit surface repeats itself, producing a row o f voids extending away from the electron-exit surface along the irradi ated volume. Under continuous irradiation, the voids formed in the irr adiated volume have lengths of 25-30 nm, independent of sample thickne ss, and diameters comparable with the electron-beam diameter at the el ectron-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 for ms a continuous hole through the aluminium. Monte Carlo simulations ha ve been used to follow the electron trajectories and estimate electron -exit surface sputtering rates for scanning transmission electron micr oscopy (STEM) electron probes with near-Gaussian radial intensity dist ributions 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.