Leakage current and reliability evaluation of ultra-thin reoxidized nitride and comparison with silicon dioxides

In this work, we have conducted a systematic investigation of leakage current and reliability for re-oxidized nitride, both in planar films and deposited in the deep trenches for DRAM storage-capacitor applications. It was found that for the same equivalent thickness (Teq), the leakage current of re-oxidized nitride is anomalously higher than that of SiO₂. We demonstrate that this increase in leakage current is caused by a reduction of oxide barrier height from -3 eV to -2.2 eV. In addition, the species release and injection process at the anode, by the energetic electrons, is greatly enhanced by the barrier-height reduction. Within the framework of the current understanding of oxide breakdown, this reduction in oxide barrier-height can self-consistently explain the breakdown data in reoxidized nitride films in many aspects: 1) T_BD polarity and thickness dependence; 2) the disappearance of T_BD polarity dependence for thinner films; 3) a much stronger T_BD (Q_BD) thickness dependence causing a crossover effect in comparison with SiO₂. This result suggests that the defect generation rate in reoxidized nitride is thickness dependent. Using a cell-based analytical model, we found that the critical defect density at breakdown extracted from the thickness dependence of Weibull slopes is higher than SiO₂. The similarities and differences in T_BD (Q_BD) voltage- and temperature dependences between reoxidized nitrides and silicon dioxides are discussed. As compared to reoxidized nitride, it is shown that high quality SiO₂ can offer a thickness scaling option for storage capacitors assuming silicon dioxide can be successfully fabricated in deep trenches with sufficiently low defect density as required by DRAM applications.