A TRANSMISSION ELECTRON-MICROSCOPY STUDY OF HILLOCKS IN THIN ALUMINUM FILMS

Hillocks, small outgrowths on a film surface, form when compressional stresses in an aluminum film are relaxed at elevated temperature (greater-than-or-equal-to 90-degrees-C), for instance during the phase of rising temperature in an annealing cycle. This paper reports a study of hillock formation in A1 films of thickness in the interval 0.25-2.2-mu-m and which have been deposited by electron beam evaporation. Hillock sizes, shapes, number and formation temperatures were determined, the latter on a heating stage in situ in a scanning electron microscope. The internal structure of the hillock was studied by cross-sectional transmission electron microscopy technique. These studies provided strong support for the idea that hillocks are formed by migration of material along grain boundaries, presumably at triple junctions, up to the surface where it is deposited in a growing hillock. Initially, the hillocks are separated from the original film surface by a grain boundary-like interface, but prolonged annealing will cause underlying grains to grow into the hillocks, until they become integrated in the film.