Damage of alumina films by medium energy hydrogen and helium ions

Following previous observations of detachment of amorphous, anodic alumina films from aluminium by 100 keV H+ ions, further experiments have been carried out using H+, D+, 3He+ and 4He+ ions, at energies in the range of 0.5–270 keV, to irradiate anodized aluminium with oxides of thickness from 30 to 500 nm. Surface damage was investigated by field-emission-gun scanning electron microscopy and transmission electron microscopy. Detachment of the oxide, which takes place close to, or at, the metal/oxide interface, occurred only following irradiation by H+ and D+ ions, with the ions being stopped in the metal rather than the oxide. The threshold fluence for initiation of detachment is approximately 3×1015 ions cm−2. No detachment was detected following irradiations by 3He+ and 4He+ ions with fluences up to 5×1016 ions cm−2 and ranges similar to those of H+ and D+ ions, although vacancy production is much greater for helium ions. These findings indicate that detachment is associated with accumulation of hydrogen species in the vicinity of the metal/oxide interface, rather than condensation of vacancies. Such behaviour is consistent with the expected mobility of hydrogen, but not of helium, in aluminium near room temperature