String select transistor leakage suppression by threshold voltage modulation in 3D NAND Flash Memory

Single drain select transistor (DSL) in 3D Flash memory may exhibit higher leakage as compared with DSL in 2D NAND, because of worse subthreshold swing characteristics due to poly-Si channel. Select transistor leakage suppression is essential in 3D NAND Flash Memory, in consideration of boosting potential and program disturbance. Compared to single Si drain select transistor in 2D planner Flash Memory, a novel dual cylindrical thin film transistor is proposed as drain select device to suppress leakage for good boosting performance in 3D NAND flash. And a novel measurement approach is also proposed to quantify leakage of drain select transistor in program inhibit case. The effect of Vth modulation on leakage is evaluated with this new DSL approach in this work. Single poly-Si cylindrical transistors demonstrated to be not applicable in 3D NAND because of high leakage. And the optimized Vth modulation pattern of dual transistors, with lower T1 Vth and higher T2 Vth has been found for good leakage suppression, which is beneficial for self-boosting performance of 3D NAND cell string.