Impact of String Pattern on the Threshold-Voltage Spread of Program-Inhibited Cell in NAND Flash
Yung-Yueh Chiu, Minoru Aoki, Masaru Yano, R. Shirota · IEEE Journal of the Electron Devices Society · 2016
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.