Thermal oxidation mechanism and stress evolution in Ta thin films

Oxidation-induced stress evolutions in Ta thin films were investigated using ex situ microstructure analyses and in situ wafer curvature measurements. It was revealed that Ta thin films are oxidized to a crystalline TaO₂ layer, which is subsequently oxidized to an amorphous tantalum pentoxide (a-Ta₂O₅) layer. Initial layered oxidation from Ta to TaO₂ phases abruptly induces high compressive stress up to about 3.5 GPa with fast diffusion of oxygen through the Ta layer. Subsequently, it is followed by stress relaxation with the oxidation time, which is related to the slow oxidation from TaO₂ to Ta₂O₅ phases. The initial compressive stress originates from the molar volume expansion during the layered formation of TaO₂ from the Ta layer, while the relaxation of the compressive stresses is ascribed to the amorphous character of the a-Ta₂O₅ layer. According to Kissinger's analysis of the stress evolution during an isochronic heating process, the oxygen diffusion process through the a-Ta₂O₅ layer is the rate-controlling stage in the layered oxidation process of forming a a-Ta₂O₅/TaO₂/Ta multilayer and has an activation energy of about 190.8 kJ/mol.