Electrical characterisation of Ta2O5 thin films for application in microelectronics

Two methods of preparing Ta2O5 thin films on Si are used here: RF sputtering of Ta in Ar+O2 mixture and thermal oxidation of sputtered tantalum layer on Si. Their dielectric and electric properties have been investigated for high density 256 M bit and 1 G bit DRAM application. Dielectric constant and capacitance density are obtained from quasistatic C-V measurements (8-10 mF/m2 for d > 40 nm and 4 - 5,5 mF/m2 for d ∼ 27 nm ). Leakage current densities for both gate polarities defined from I-V measurements show decreasing with decreasing of the substrate temperature and with thermal annealing at higher temperature. Breakdown field is ∼ 3 MV/cm and is decreasing with increasing of oxide thickness. Conduction mechanism obtained for these films is Pool-Frenkel with or without compensation for almost all the investigated structures with possibility for Schottky emission. The change of leakage current density and conduction mechanism with short time (6 s) and long time (2720 s) constant current stress is investigated and the change of barrier height is observed with the amount of injected charge. The conduction mechanism is not changing. The UG - t characteristics for both polarities of injected charge are different. Instantaneous degradation of films is observed for injection from the substrate with positive charge trapping. For injection from the gate we have trapping of both charges, positive and negative. It has been found that the formation of ultra thin SiO2 film at the interface with Si affects the characteristics of these MOS structures. (Author)