Variations of V(t) Retention Loss in a SONOS Flash Memory Due to a Current-Path Percolation Effect

Abstract

Discrete nitride program charge loss in a small-area SONOS Flash memory cell during retention is observed. Our measurement shows that a retention V(t) of a programmed SONOS cell exhibits a stepwise evolution with retention time. Individual single-program charge-loss-induced threshold voltage shifts (Delta V(t)) are characterized. We find the following: 1) The magnitude of Delta V(t) exhibits an exponential distribution, which is believed due to a current-path percolation effect caused by random program charges and substrate dopants, and 2) program-state V(t) retention loss has large variations in different cells and P/E cycles due to the percolation effect. We develop a Monte Carlo analysis to take into account the distribution of Delta V(t) and a tunneling front model to study the spread of a retention V(t) distribution in a SONOS Flash memory.