Characterization and Modeling of Temperature Effects in 3-D NAND Flash Arrays—Part II: Random Telegraph Noise

Abstract

This paper investigates the temperature dependence of random telegraph noise (RTN) in 3-D NAND Flash technologies. Experimental results on memory arrays reveal an increase of RTN when temperature is reduced, which is mainly attributed to the growth of the amplitude of the fluctuations arising from RTN traps. A direct proof of this growth is provided by directly monitoring some RTN waveforms arising from single traps in cells out of either memory arrays or test elements. In order to explain this evidence, the TCAD model for polysilicon NAND strings presented in Part I of this paper is adopted and extended to address the amplitude of the threshold-voltage shift coming from the trapping of a single electron at different positions in cell channel. Numerical results successfully reproduce the experimental trends with temperature and allow to explain them in terms of stronger nonuniformities in polysilicon inversion when temperature decreases. In particular, the reduction of temperature makes the string current more controlled by the polysilicon grain boundaries, increasing, in turn, the amplitude of the RTN fluctuations arising from traps placed on or close to them.