Charge Spreading Effect of Stored Charge on Retention Characteristics in SONOS NAND Flash Memory Devices

Abstract

This research investigates the impact of charge spreading on the data retention of three-dimensional (3D) siliconoxide-nitride-oxide-silicon (SONOS) flash memory where the charge trapping layer is shared along the cell string. In order to do so, this study conducts an electrical analysis of the planar SONOS test pattern where the silicon nitride charge storage layer is not isolated but extends beyond the gate electrode. Experimental results from the test pattern show larger retention loss in the devices with extended storage layers compared to isolated devices. This retention degradation is thought to be the result of an additional charge spreading through the extended silicon nitride layer along the width of the memory cell, which should be improved for the successful 3-D application of SONOS flash devices.

Reliability of NAND Flash Memories: Planar Cells and Emerging Issues in 3D Devices

“…In 3D devices, the importance of such effects is shown in Figure 37, where lateral diffusion along the nitride is modulated by the state of adjacent cells [305]: having all cells programmed decreases the concentration gradient, slowing down diffusion and reducing the V T shift. Similar results obtained via simulations were also presented in [306][307][308] and, changing the device size, in [309]. Note also the thermal activation, accelerating both leakage mechanisms and making pattern-dependent charge spreading effects more relevant at high temperatures.…”