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.