Flash Memory Reliability

With reference to the mainstream technology, the most relevant failure mechanisms that affect reliability of Flash memory are reviewed, showing the primary role played by tunnel oxide defects. The degradation of device performance induced by program/erase cycling is discussed, covering the behavior of a typical cell and the single cell failure modes. Oxides traps are demonstrated to be responsible for the most critical failure mechanisms, like the erratic erase. The impact of stress-induced leakage current on data retention is discussed, specifically for what concern the anomalous leakage that affect a very small fraction of memory cells after program/erase cycling. The dependence of the leakage current on tunnel oxide thickness, number of cycles, and temperature are analyzed. The very large distribution of leakage current is explained by trap-assisted tunneling involving one, two or more traps, with decreasing occurrence probability. The current distribution obtained by a simple statistical model is favorably compared with experimental data.