E. Youm, S. Sunil-Shim, M. Park, Y. Shin, E. Youm, B.D. Choi
Samsung Electronics / Sungkyunkwan University,
Korea
Keywords: 3D NAND, channel hole, channel poly, sub-threshold swing, cell reliability, grain boundary, grain size, interface trap site
Summary:
3D-NAND is a successful alternative to overcome the scaling limits of 2D-NAND. While 2D-NAND has been scaled down laterally, 3D-NAND stacks more gate WLs vertically to collect more cells per unit area. However, as the number of WLs increases, the total height of cell increases as well. The increase in cell height makes the process highly difficult. Therefore, reducing the Lgate / Lspace in the vertical direction is a major attainment target of 3D-NAND. However, reducing gate length will degrade gate control ability, so cell stacks (ONOP) should be also scaled down to compensate the gate control ability degradation. However, from a cell reliability perspective, the cell stacks cannot be continuously reduced. Therefore, it is very important to ensure cell performance and reliability through improving the physical properties of cell stacks. In this study, we have researched the 3D-NAND cell characteristics according to different physical properties of channel poly at the same thickness. In particular, we have studied the effects of channel poly grain size on initial cell characteristics and reliability of 3D-NAND. We fabricated 3D-NAND strings consisting of 32 layers of WL and three groups of samples with different channel poly grain sizes. Initial Id-Vg measurement showed that when the channel poly grain size increased by 37.8%, the sub-threshold swing decreased by 7.9% and the on-current increased by 38.6%. The erase Vth shift after 100K Program/Erase cycle also decreased by 10.3%. In addition, we measured charge pumping current to compare the interfacial trap. We found out that the charge pumping current decreased as the channel poly grain size increased. We interpret the results that the decrease of trap sites in grain boundaries improves the cell switching characteristic and reliability. We believe that elemental technologies for deposition of macaroni channel poly with large grain size within channel hole are crucial to improve cell performance and reliability degradation resulting from scaling 3D-NAND.