Impacts of Poly-Si Channel on Cell Variations in Vertical Scaled Charge-trap (CT) 3D NAND Flash Memory

In this work, the impacts of poly-Si grain distributions on cell variations are investigated in charge-trap 3D NAND flash memory. Statistical distributions of On current (Ion), threshold voltage (Vth), and subthreshold swing (S.S.) are extracted from devices with random grain distributions. 3D TCAD simulations show that larger grain size with fewer GBs is important for achieving better cell performance. Moreover, the grain boundaries in the cell region can enlarge the variations, which is even more serious with vertical scaling. More importantly, it is shown that Vth represents a good indicator of the On-Off current ratios and S.S. values, which can be utilized in redundancy designs to improve the reliabilities of 3D NAND chip-based memory systems.