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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 ΔVt in NOR-type SONOS multilevel cells (MLCs). Our measurement shows the following: 1) A single-charge-loss-induced ΔVt 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.