Data retention reliability model of NROM nonvolatile memory products

Post cycling data retention reliability model of NROM devices is presented. The degradation rate of the threshold voltage of cycled cells is shown to be a multiplication of three functions: 1) bit density; 2) endurance; and 3) storage time and temperature. The functions are fitted to experimental results of products of three technology nodes. The retention loss is interpreted in terms of thermally activated lateral migration of trapped holes in the ONO layer. The holes' migration quenches the electrons' field over the channel of the device, degrading its threshold voltage. The migration process is presented as a dispersive transport process. Saturation of the retention loss is demonstrated at threshold voltage levels well above the neutral state of the device. From the retention loss function we derive a time-to-failure formula and an expression for the thermal acceleration factor of NROM products useful for determining stress conditions for accelerated reliability tests.