Investigation of Electron and Hole Lateral Migration in Silicon Nitride and Data Pattern Effects on V_t Retention Loss in a Multilevel Charge Trap Flash Memory

We investigate electron and hole lateral migration in V_t retention loss in a multilevel charge trap flash memory. We use hot electron program and band-to-band tunneling hot hole erase to inject various amounts of electrons and holes at the two ends of a SONOS cell. A random telegraph signal (RTS) method is used to distinguish electron and hole lateral movements in silicon nitride. In V_t retention measurement, we apply a voltage to the gate or the source/drain to enhance or retard trapped charge vertical loss and lateral migration. From the evolution characteristics of RTS and V_t traces in retention, we are able to identify the separate roles of electron vertical loss, electron lateral migration, and hole lateral migration in different data patterns. Due to the interaction of stored electrons and holes, we find that V_t retention loss in a program state exhibits a turnaround characteristic as program V_t level increases. V_t loss at low program levels is attributed to hole lateral migration from a neighboring bit. At higher program levels, the influence of hole lateral migration is reduced and V_t loss is dominated by electron vertical loss and lateral migration.