Investigation of Vth Distribution Tails of Ground-Select-Line Cells and Edge Dummy Cells in a 3-D NAND Flash Memory

The threshold voltage ( Vth) distributions of ground-select-line (GSL) cells and edge dummy (DMY0) cells in a 3-D NAND flash memory are investigated. We characterize the Vth distributions in 3-D NAND flash samples with different fabrication processes and bitline voltages. Large DMY0 and GSL Vth distribution tails are observed in certain fabrication process and operation voltage conditions. The DMY0 Vth tail is attributed to a random distribution of grain boundary (GB) traps in a poly-silicon channel between GSL and DMY0. The GSL Vth tail is affected by ion implant energy in an epitaxial silicon layer underneath a GSL. A 3-D TCAD simulation is performed to study the effects of a GB trap position and a bitline voltage on the Vth distribution of DMY0. The influence of an implant dose profile in the epitaxial silicon on the GSL Vth distribution is also analyzed by 3-D simulation. The GSL and the DMY0 Vth distributions can be significantly improved by optimizing a fabrication process and choosing an appropriate bitline voltage.