Dong-Yean Oh

3-dimensional analysis on the cell string current of NAND flash memory

The cell string current of NAND flash memory is very small due to large resistance from the cells connected in series. In this paper, scaling effects on the cell current are analyzed for 70/60/50 nm NAND flash technologies using 3-dimensional TCAD simulation. The geometrical and process parameters are varied and their effects are quantified. It is identified that the coupling ratio has the most significant impact on the cell current and the LDD engineering is more relevant for higher cell current

New Cell Structure with Edge-thick Tunnel Oxide for Highly Reliable NAND Flash Memory Devices

One of the most important performances of NAND flash memory is reliability characteristics, such as program/erase cycling and data retention. Tunnel oxide quality is essential to the reliability and it is well known that tunnel oxide degradation during FN (Fowler-Nordheim) stress is due to the oxide trap and interface trap generation. It is believed that trapping mainly occurs where tunnel oxide is locally thin. For example, conventional SAP (self-aligned poly) process with shallow trench isolation, tunnel oxide at active edge is necessarily thinner than active channel. In this paper, we proposed a new process scheme to fabricate optimized tunnel oxide thickness varying from active center to edge, and we confirmed the improvement of reliability characteristics such as Vth shift and interface trap density during endurance cycling