Effect of Noncircular Channel on Distribution of Threshold Voltage in 3D NAND Flash Memory

The instability in threshold voltage (VTH) and charge distributions in noncircular cells of three-dimensional (3D) NAND flash memory are investigated. Using TCAD simulation, we aim to identify the main factors influencing the VTH of noncircular cells. The key focus is on the nonuniform trapped electron density in the charge trapping layer (CTL) caused by the change in electric field between the circular region and the spike region. There are less-trapped electron (LT) regions within the CTL of programmed noncircular cells, which significantly enhances current flow. Remarkably, more than 50% of the total current flows through these LT regions when the spike size reaches 15 nm. We also performed a comprehensive analysis of the relationship between charge distribution and VTH in two-spike cells with different heights (HSpike) and angles between spikes (theta). The results of this study demonstrate the potential to improve the reliability of next-generation 3D NAND flash memory.