Analysis of Mechanical Stress on Fowler-Nordheim Tunneling for Program Operation in 3D NAND Flash Memory

This study investigated the relationship between mechanical stress and program efficiency in three-dimensional (3D) NAND flash memory devices. A stacked memory array transistor (SMArT) 3D NAND flash structure was modeled using a technology computer-aided design (TCAD) simulation. The mechanical stress distribution in the device depended on the deposition temperature (T_D) of the constituent material. In particular, the T_D of tungsten (T_D,W) dominated the mechanical stress. The tensile stress on the polycrystalline silicon (poly-Si) channel increased as the T_D,W decreased, and the compressive stress on the tunneling oxide (T_ox) decreased. Consequently, the barrier height between T_ox and poly-Si, and the effective electron mass decreased as the electric field in the T_ox increased. These changes significantly increased the Fowler-Nordheim (FN) tunneling process and program efficiency, indicating the crucial performance of 3D NAND flash. Moreover, the mechanical stress caused by the differences in T_D,W improved the program efficiency at a lower program voltage (V_PGM). Therefore, a change in the mechanical stress based on decreasing T_D,W improved the program efficiency through a higher FN tunneling process.