Charge Localization during Program and Retention in Nitrided Read Only Memory-Like Nonvolatile Memory Devices

An alternative solution to standard Flash memories is represented by nitride-trap memories as silicon–oxide–nitride–oxide–silicon (SONOS) or nitrided read only memory (NROM) memories. However these structures are facing retention issues at high temperature and a quantitative analysis of the location of the charge during program and retention is required. This work investigates the location of charge of NROM Nonvolatile memory devices in order to evaluate the trapped charge distribution in program and retention conditions for Si3N4, Al2O3, and HfO2 trapping layers. During programming, the charge is initially injected on a 40–60-nm-length region in the trapping layer, then after reaching a trapped charge saturation level, it broadens. The charge saturation level is explained through electrostatic considerations and not by a limit of available number of traps. During retention, the lateral migration (inside the trapping layer) and vertical migration (charge loss) of the trapped charge are quantitatively evaluated. Thanks to a one-dimensional (1D) drift model, the characterization of lateral migration at room temperature is put in relation to the trap properties of Si3N4, Al2O3, and HfO2 layers.