Threshold Voltage Fluctuation by Random Telegraph Noise in Floating Gate NAND Flash Memory String

Read current fluctuation (Delta I(read)) due to random telegraph noise was measured from a cell in a NAND flash memory cell string, and its effect on threshold voltage fluctuation (Delta V(th)) was analyzed. Sixteen-level fluctuation (four traps) was observed in a 60-nm cell of a cell string (Delta I(read)/I(read) of similar to 0.4). Delta I(read) increased with decreasing L(g), and Delta I(read)/I(read) up to 0.75 was observed at 48 nm. Delta I(read), Delta V(th), and their relation were clearly analyzed with program/erase mode of a cell and pass cells in a string. Although Delta I(read) is largest when a read cell and pass cells are erased, Delta V(th) is largest when a read cell is erased and pass cells are programmed in a cell string. We also observed the specific noise amplitude under various conditions, such as the bit-line bias, the pass bias of unselected cells in the NAND strings, and the temperature.