Impact of Source/Drain Junction and Cell Shape on Random Telegraph Noise in NAND Flash Memory

Abstract

A comprehensive numerical study of threshold voltage fluctuation (Delta V-T) in scaled NAND flash memory caused by random telegraph noise (RTN) and discrete dopant fluctuation (RDF) in both the channel and the cell-to-cell space [source/drain (S/D)] region was carried out. Following a three-dimensional (3D) Monte Carlo (MC) procedure, the statistical distribution of Delta V-T is estimated, considering the effects of both the random placement of discrete doping atoms and a discrete single trap at the tunnel oxide/substrate interface. The result demonstrates the significant influence of the doping in the S/D regions. For the cells with and without an S/D junction, the electron concentration in the S/D region is determined by the pass voltage of the unselected cell (V-pass) and the neighboring cell V-T (V-T(n)), owing to the fringing fields of neighboring floating gates (FGs). As a result, Delta V-T increases in the S/D region as V-pass - V-T(n) decreases. The fluctuation amplitude strongly depends on the [single-trap RTN] position along the cell length (L) and width (W) directions. For the cell shape with rounding of the active area (AA) at the shallow trench isolation (STI) edge, the results indicate that the high Delta V-T area moves from the AA edge towards the center area along the W-direction. (C) 2013 The Japan Society of Applied Physics