|MANNARA AURELIO||Cycle: XXXIII |
Tutor: SOTTOCORNOLA SPINELLI ALESSANDRO
Advisor: MONZIO COMPAGNONI CHRISTIAN Major Research topic
:TCAD modeling of current transport and main reliability issues of polysilicon-channel 3-D NAND
NAND Flash technology represents today one of the leading solutions for highly performing non-volatile memories. The continuous development of its manufacturing processes brought the characteristic cell feature size (F) to current values around 15 mm. Although the scaling of the arrays allowed to drastically increase storage density and the reduction of costs over the past few decades, it also brought many reliability issues which were overcame by developing 3-D NAND Flash structures, employing the third dimension, thus relaxing the scaling of the cell and stacking multiple layers in the vertical direction. One of the drawbacks coming with these new structures is given by the channel being polycrystalline, due to the involved manufacturing processes. This is one of the major source of issues in 3-D NAND Flash strings, due to the presence of grain boundaries with a high concentration of defect states. More specifically, this causes both an increase of the resistivity of the string channel and also threshold voltage fluctuations caused by random trapping/detrapping events at these trap states. Moreover, the random configuration of grain profiles causes variability issues that have to be taken into account to properly design the device. Due to the complexity of such devices and the involved reliability issues, care must be taken in order to avoid issues during operation. In this framework, this thesis aims to describe current transport and main reliability issues in vertical-channel 3-D NAND Flash strings by means of TCAD modeling, accounting for the polycrystalline nature of the channel. In particular, simulations were performed to compare the results coming from different models to describe current transport in the polysilicon channel. Then, the TCAD model was calibrated on experimental data and it was used to statistically analyze the temperature activation of threshold voltage variability and Random Telegraph Noise (RTN) fluctuations coming from the haphazardness of grain configuration.