Random Telegraph Signal in Flash Memory: Its Impact on Scaling of Multilevel Flash Memory Beyond the 90-nm Node

Abstract

Threshold-voltage /40 /41fluctuation due to random telegraph signal (RTS) in flash memory was observed for the first time. A large amount of data of fluctuation was acquired by using a 90-nm-node memory array, and it was confirmed that a few memory cells have large RTS fluctuation exceeding 0.2 V. It was found that program-and-erase cycles increase amplitude in a flash memory. It was also found by simulation and measurement that tail-bits are generated due to RTS in multilevel flash opera- tion. The amount of broadening due to the tail-bits was es- timated to become larger as the scaling of memory cells advances and reaches more than 0.3 V in the 45-nm node. These results thus demonstrate that RTS will become a prominent issue in designing multilevel flash memory in the 45-nm node and beyond.