Abstract
Tantalum oxide thin films were prepared by RF reactive sputtering in various deposition parameters, including oxygen content in gas ambient, self-bias and substrate temperature. XPS analysis was adopted to investigate the chemical bonding of tantalum and oxygen ions, as well as the composition of tantalum oxide thin films. As the oxygen content was in the range 15 ~ 20%, tantalum ions were in the oxidation state of Ta4+, while it became Ta5+ as the oxygen content was increased to 25% due to larger cross section of collisions between oxygen and tantalum species. The composition of the as-sputtered tantalum oxide thin films was not solely controlled by oxygen content in the sputtering gas ambient, instead it was attributed to the flux ratio of oxygen gas to sputtered Ta species in the sputtering reactor.The structure of tantalum oxide remained amorphous after heat-treatment below 700 oC. As the annealing temperature was increased above 700 oC, amorphous tantalum oxide thin films would crystallize into orthorhombic □-Ta2O5 structure and the composition was close to Ta2O5 after annealing in oxygen gas ambient.Electrical properties of tantalum oxide thin films on Pt-metallized Al2O3 substrates were also investigated. The dielectric constant increased after crystallization in consideration of surface roughness of contact area of top electrode and dielectric layer. Leakage current of tantalum oxide films on Pt/Al2O3 was effectively reduced after annealing in oxygen gas ambient by removal of oxygen vacancies and defects. The conduction mechanisms of amorphous and polycrystalline films were also studied for further improvement. In amorphous films, leakage currents were controlled by electron hopping, Poole-Frenkel emission and space-charge-limited conduction with increasing electric fields. In polycrystalline films, conduction behaviors under negative bias showed no difference with those of amorphous films. Under positive bias, leakage currents were contributed from electron hopping, Schottky emission and Fowler-Nordheim tunneling with increasing electric fields due to the different surface state of TiW/Ta2O5 after heat-treatment.