$V_{t}$ Retention Distribution Tail in a Multitime-Program MLC SONOS Memory Due to a Random-Program-Charge-Induced Current-Path Percolation Effect

A
Vt
retention distribution tail in a multitime-program (MTP) silicon-oxide-nitride-oxide-silicon (SONOS) memory is investigated. We characterize a single-program-charge-loss-induced Δ
Vt
in NOR-type SONOS multilevel cells (MLCs). Our measurement shows the following: 1) A single-charge-loss-induced Δ
Vt
exhibits an exponential distribution in magnitudes, which is attributed to a random-program-charge-induced current-path percolation effect, and 2) the standard deviation of the exponential distribution depends on the program-charge density and increases with a program
Vt
level in an MLC SONOS. In addition, we measure a
Vt
retention distribution in a 512-Mb MTP SONOS memory and observe a significant
Vt
retention tail. A numerical
Vt
retention distribution model including the percolation effect and a Poisson-distribution-based multiple-charge-loss model is developed. Our model agrees with the measured
Vt
retention distribution in a 512-Mb SONOS well. The observed
Vt
tail is realized mainly due to the percolation effect.