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

Abstract

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.