Mechanism of Threshold Voltage Widening in Sub-30 nm MLC NAND FLASH Cells after Erase/Write Cycling

Abstract

From the investigation of endurance failures in sub-30 nm multi-level NAND flash cells, we observe that the threshold voltage (VTH) distribution of the lowest state cells has the broadest dispersion, leading to failures after erase/write (E/W) cycling. This is due to the combined effect of trapped charges in the tunnel oxide on the cell junction region and to cross coupling between a victim cell and neighbor cells. In the gate edge, the trapped charges that were generated during E/W cycling induce a cell VTH shift by causing a depletion of junction, and neighbor cells have a larger VTH due to victim-cell coupling.