Origin of Post-Irradiation Vₜₕ-Shift Variability in 3-D NAND Memory Array

In this article, we report total-ionizing-dose (TID)-induced threshold-voltage (
${V}_{\text {th}}$
) shift characteristics of commercial-off-the-shelf (COTS) 64-layer 3-D NAND flash memory chips. Our measurements of cell
${V}_{\text {th}}$
distributions after irradiation up to 50 krad(Si) indicate that significant
${V}_{\text {th}}$
-shift variability exists among the memory cells of the chip. We find that a certain fraction of cells is very responsive to ionizing irradiation while a significant amount of the cells stays surprisingly resilient. We model the TID-tolerance behavior of these cells using pre-existing trap states in the tunnel oxide and provide three independent supporting evidence.