Kirkendall void formation during room-temperature air-oxidation of thin aluminium and aluminium-lithium alloy films

The effect of room-temperature (~20°C) air-oxidation on void formation in sputter-deposited thin films of aluminum and its alloys was investigated using a transmission electron microscope. It was found that after air-oxidation, only lithium-bearing aluminum alloy films exhibited a high (~4×10 ¹⁶ cm^-3) density of small (~2 nm) voids, whereas pure aluminum or lithium-free aluminum alloy films did not contain any voids. In lithium-bearing aluminum alloy films, both aluminum and lithium atoms migrate to the surfaces to form their surface oxide during room-temperature ageing after film deposition. In the course of the atom migration, excess vacancies are generated as a result of the large diffusivity difference existing between aluminum and lithium atoms (_{DLi in Al}≫D_Al) in the alloy matrix. The agglomeration of these excess vacancies led to the formation of so-called Kirkendall voids inside the alloy. Thus the presence of both aluminum and lithium in the alloys was a key factor for generating these Kirkendall voids in the films.