Impact of String Pattern on the Threshold-Voltage Spread of Program-Inhibited Cell in NAND Flash

An anomalous threshold-voltage (
$V_{t} $
) spread of the program-inhibited cell is investigated for the first time in NAND flash memory. The program disturb characteristics are studied by applying the program-inhibited stress on the
${N}$
th cell of the unselected bitline with various string patterns for the 0th to (
$N-1$
)th cell, using the global self-boosting method. Distinguishing features of the variance of the number of injected electrons (
$\sigma _{n}^{2} $
) into the floating gate are observed. The variance is proportional to the mean value of injected electrons (
$\bar {n}$
) times 10. The other is proportional to
$\bar {n}$
times 20 and occurs only when the (
$N-1$
)th cell is programmed in a high
$V_{t} $
level and the other cells are in the erased state. A 3-D TCAD simulation reveals that the former case is attributed to Fowler–Nordheim tunneling from the insufficiently boosting channel, and the latter is explained by hot-electron injection owing to the strong lateral electric field between the
${N}$
th and (
$N-1$
)th cells.