Origin of Post-Irradiation <i>V</i>ₜₕ-Shift Variability in 3-D NAND Memory Array

Abstract

In this article, we report total-ionizing-dose (TID)-induced threshold-voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> ) shift characteristics of commercial-off-the-shelf (COTS) 64-layer 3-D NAND flash memory chips. Our measurements of cell <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> distributions after irradiation up to 50 krad(Si) indicate that significant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> -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.