Novel Program Method of String Select Transistors for Layer Selection in Channel-Stacked NAND Flash Memory

摘要

In this paper, we propose new string select transistors (SSTs)/dummy SSTs (DSSTs) threshold voltage (Vth) setting methods in simplified channel-stacked array with layer selection by multilevel operation (SLSM). In these methods, SSTs/DSSTs on each layer are set to their targeted Vth values by incremental step pulse program/one erase with various erase voltages, respectively. In the fabricated pseudo-SLSM, the validity of the new methods is verified. As a result, it is confirmed that the Vth values of SSTs/DSSTs are set to the targeted Vth values by the new methods and SSTs with extremely narrow Vth distribution can be obtained in the consequence. Moreover, memory operations such as erase, program, and read are performed in the fabricated structure after setting the Vth values of all the SSTs/DSSTs by the new methods. Despite unique LSM operations, stable memory operations are obtained successfully without the interference between stacked layers.