Fatigue effect in MNOS memory devices

In an MNOS memory device, after a certain number of positive and negative erase/write pulses, the achievable threshold voltage window representing the difference between "1" and "0" states decreases with increased number of erase/write cycles. This decrease has been reported as a serious concern for the device and high-field stress has been suspected as the cause. The purpose of this paper is to report the extensive investigations made into this phenomenon, the findings, and a model for this failure mechanism. Instantaneous observation of the MNOS switching phenomenon has revealed that there is a large amount of "transient charge" involved in the switching process. Transient charge is the charge that tunnels back through the thin oxide after the termination of the excitation pulse before the flat-band voltage can be measured by conventional C-V methods. This transient charge has been neglected by all previous theories and analyses. During the rapid reversal of the erase and write pulse polarity, an exceedingly high field is exerted on the thin oxide due to the combined effect of the applied pulse and the transient charge. The high field causes what appears to be fast states and these states cause the stored charge to decay so rapidly that the observable threshold window becomes small. These fast states can be annealed out and the device can be recovered. Various experimental evidences are presented.