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 Nth 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(2)(n)) into the floating gate are observed. The variance is proportional to the mean value of injected electrons ((n) over bar) times 10. The other is proportional to (n) over bar times 20 and occurs only when the (N - 1) th cell is programmed in a high Vt 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 Nth and (N - 1) th cells.