Modelling of the threshold voltage distributions of sub-20nm NAND flash memory

Abstract

The proliferation of NAND flash memory in consumer devices has driven their aggressive cost reduction by continuous scaling to smaller technology nodes. However, this relentless cost per capacity improvement has diminished the reliability of flash memory to a degree that advanced signal processing and error correction are needed to enhance signal integrity in current flash-based systems. Accurate models of flash readback signals are necessary to properly design such advanced signal enhancement schemes. We propose a new parametric model of the flash readback signal based on fitting threshold voltage distributions from NAND flash devices. We show accurate fitting results for flash devices cycled up to 10 times longer than their nominal endurance specification, and provide simple expressions of the model parameters as a function of program/erase cycles. Finally, we also demonstrate that the proposed model can be used to capture effects such as programming errors, that occur in over-stressed flash devices.