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Degradation behavior of oxide–nitride–oxide (ONO) stacked gate dielectrics used in metal–oxide–nitride–oxide–silicon (MONOS)-based flash memories has been studied. Occurrence of the soft breakdown (SBD), which is well known in single-layer ultrathin silicon dioxides, has been revealed even in these thick and stacked dielectrics. The SBD of ONO dielectrics shows similar characteristics with that of silicon dioxides in locality, current/voltage fluctuation, and the dependencies of the ratio of SBD on the gate area and stress condition. These findings require the countermeasure for SBD even in MONOS-based flash memories and also give useful information on the reliability of high-k stacked gate dielectrics. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]