Changes in the phase structure and formation of chemical compounds by ion implantation of tantalum thin films

Evaporated films of tantalum+tantalum oxide, 500 Å thick, were implanted with argon, oxygen and nitrogen ions. The phase structure of the films was determined by transmission electron microscopy and the dose dependence of resistivity and thermal coefficient of resistivity was satisfactorily explained in terms of the precipitation of compounds formed in the film. Argon bombardment had little effect on the structure of a film until sputter etching had reduced the film thickness considerably, when precipitates of TaO₂ in the form of rounded islands were created. Oxygen bombardment resulted in the precipitation of the b.c.c. phase of tantalum, and at higher doses precipitates of TaO₂ were formed. Nitrogen bombardment resulted in a sudden phase change and the precipitation of Ta₄N₅ in the form of hexagonal platelets; the conduction process in these films was highly activated, probably as a result of the presence of an amorphous oxide matrix. The dependence of changes in electrical properties on oxygen content of the film is explained by this phase structure.