|MANNARA AURELIO||Cycle: XXXIII |
Tutor: SOTTOCORNOLA SPINELLI ALESSANDRO
Advisor: MONZIO COMPAGNONI CHRISTIAN Major Research topic
:Current transport and charge trapping analysis in the polysilicon channel of 3D NAND Flash stringsAbstract:
NAND Flash technology represents currently one of the leading solution for highly performing non-volatile memories. The incessant development of its scaling processes brought the characteristic cell feature size (F) to current values around 15 nm, allowing to obtain outstanding storage density, while keeping a low cost-per-bit ratio, and high read/write speeds, overcoming the performances offered by other technologies, such as magnetic hard-disk drives.
Despite the clear advantages brought from the reduction of F, the scaling of the arrays led to multiple stability issues, becoming harder to control the more F was reduced, limiting the technological growth. In order to overcome these problems, a new approach was taken and the array structure was modified. By employing the third dimension, 3D NAND Flash arrays were developed, allowing greater storage densities while using a feature size big enough to avoid hard to manage issues. The main flaw of the processes involved for the manufacturing of 3D NAND Flash strings is related to the generation of the silicon channel. As a matter of fact, since it employs the deposition of the semiconductor material, the resulting channel is made out of polycrystalline silicon.
The polycrystalline nature of the silicon channel is one of the major source of issues in 3D NAND Flash strings, due to the presence of grain boundaries with an high concentration of defects. Moreover, the random generation of grain profiles causes variability issues that have to be taken into account to properly design the device.
By means of experimental characterizations and TCAD simulations this PhD work aims to study charge transport and trapping mechanisms in cylindrical polysilicon channels of 3D NAND Flash strings, in order to understand the impact on the string current and the variabilities introduced on the threshold voltage of the cells.