Modeling the charge decay mechanism in nitrogen-rich silicon nitride films

The stability of negative charge in nitrogen-rich silicon nitride films deposited by plasma-enhanced chemical vapor deposition is investigated by analyzing the influence of storage temperature, postdeposition thermal annealing, and the presence of a tunnel oxide. The results are compared to a charge decay model. Comparison of experimental and modeled results indicates that (i) the tunnel oxide is almost entirely responsible for charge retention in samples with an oxide-nitride-oxide (ONO) structure, with the trap properties playing an insignificant role; (ii) thermionic emission over the tunnel oxide barrier is the limiting charge decay mechanism; and (iii) thermal annealing of the films at 800 °C leads to an increase in the oxide-nitride barrier height by ∼0.22 eV, which results in a significant increase in the charge stability. Annealed ONO samples are predicted to maintain a negative charge density of >5× 1012 cm-2 for well in excess of 100 years at a storage temperature of 100°C.