Effect of Ti doping on Ta 2O 5 stacks with Ru and Al gates

Abstract

The Ti-doped Ta 2O 5 thin films (<10 nm) obtained by rf sputtering are studied with respect to their composition, dielectric and electrical properties. The incorporation of Ti is performed by two methods – a surface doping, where a thin Ti layer is deposited on the top of Ta 2O 5 and a bulk doping where the Ti layer is sandwiched between two layers of Ta 2O 5. The effect of the process parameters (the method and level of doping) on the elemental distribution in-depth of the films is investigated by the time of flight secondary ion mass spectroscopy (ToF-SIMS). The Ti and Ta 2O 5 are intermixed throughout the whole thickness but the layers are very inhomogeneous. Two sub-layers exist in all the samples — a near interfacial region which is a mixture of Ta-, Ti-, Si-oxides as well as TaSiO, and an upper Ti-doped Ta 2O 5 sub-layer. For both methods of doping, Ti tends to pile-up at the Si interface. The electrical characterisation is performed on capacitors with Al- and Ru-gate electrodes. The two types of MIS structures exhibit distinctly different electrical behavior: the Ru gate provides higher dielectric permittivity while the stacks with Al electrode are better in terms of leakage currents. The specific metal-dielectric reactions and metal-induced electrically active defects for each metal electrode/high- k dielectric stack define its particular electrical behavior. It is demonstrated that the Ti doping of Ta 2O 5 is a way of remarkable improvement of leakage characteristics (the current reduction with more than four orders of magnitude as compared with undoped Ta 2O 5) of Ru-gated capacitors which originates from Ti induced suppression of the oxygen vacancy related defects.