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

Read current fluctuation (ΔI
_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 (ΔV
_th
) was analyzed. Sixteen-level fluctuation (four traps) was observed in a 60-nm cell of a cell string (ΔI
_read
/I
_read
of ~0.4). ΔI
_read
increased with decreasing L
_g
, and ΔI
_read
/I
_read
up to 0.75 was observed at 48 nm. ΔI
_read
, ΔV
_th
, and their relation were clearly analyzed with program/erase mode of a cell and pass cells in a string. Although ΔI
_read
is largest when a read cell and pass cells are erased, ΔV
_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.