Overerase phenomena: an insight into flash memory reliability

The most important reliability issues related to the erasing operation in flash memories are, still today, caused by single bit failures. In particular, the overerase of tail and fast bits affects the threshold voltage distribution width, causing bit-line leakage that produces read/verify circuitry malfunctions, affects the programming efficiency due to voltage drop, and causes chargepump circuitry failure. This brief overview explores the most important characteristics of these anomalous bits, their relation with the erratic erase phenomena and their impact on flash memory reliability. Identification techniques, experimental results, and physical models are also discussed.