Reliability of Flash Nonvolatile Memories

This paper reviews the reliability of Flash nonvolatile memories, with particular attention to dielectric degradation. These memories are manufactured with the same materials and processing steps as are other types of IC's, but unique features of Flash memories create very different reliability challenges. These features stem from the three unique functions of the Flash cell: to program, to erase, and to retain charge for years. Each of these functions has its own characteristic degradation mechanisms. A fascinating aspect of Flash reliability is the insight that can be gained into dielectric degradation mechanisms. In normal use, Flash dielectrics experience levels of electric field, hot-carrier bombardment, and charge fluence that other IC processes experience only in capacitor breakdown studies and other accelerated stresses. Catastrophic breakdown does not occur, however, because the stress from charging and discharging the floating-gate capacitor is fundamentally self-limiting. At the same time, dielectric leakage of a few electrons a day can be measurable in a Flash cell. The combination of high stress, self-limiting degradation, and high leakage sensitivity allows prebreakdown dielectric degradation to be studied in great detail. Although Flash memory technology poses unique development challenges, engineering optimization can make these memories highly reliable.