Diffusion Studies on Stressed Tantalum-Tantalum Oxide Capacitors

Tantalum films, 3000 angstroms thick, evaporated on fused-quartz substrates and activated by neutron irradiation, were anodized at 82 volts to form an oxide layer of approximately 1600 angstroms. Counterelectrodes of gold were evaporated on the oxide and the completed thin-film capacitors were placed under thermal and electrical stress. The radioactive profiles of unstressed tantalum units show a pronounced variation of the tantalum-to-oxygen ratio from that of pure tantalum at the metal-oxide interface to a limiting value 600 angstroms away from the interface. Application of thermal and electrical stresses alter the shape of the gradient and extend it several hundred angstroms further into the oxide layer. By normalizing the data with respect to the gradient formed by anodization in the unstressed unit, it was shown that thermal diffusion creates regions in the oxide near the tantalum interface where the tantalum concentration was depleted below that of the original unstressed unit and regions farther away from the interface where tantalum was in excess. Direct-current electrical bias accelerated the tantalum diffusion, and in combination with short-term thermal stress created regions where the tantalum concentration was in excess of that of the unstressed unit. Longer term thermal and electrical stress gave rise to an over-all region of tantalum depletion in the oxide. Diffusion constants calculated as a function of oxide thickness were of the order of 10¿13 cm2/day with the larger constants associated with electrically stressed units.