Voltage variant source side injection for multilevel charge storage in flash EEPROM

The growing demand for high-density Flash memories in portable computing, smart cards, and telecommunications applications has boosted the efforts on Flash memory cell size scaling and cost reduction. In order to further increase the storage capability and, consequently, reduce the cost per bit of Flash memories, multilevel charge storage (MLCS) techniques have recently gained a lot of interest. Furthermore, MLCS is considered a viable route for increasing embedded Flash density as well. The devices investigated so far, rely either on conventional channel hot electron (CHE) injection or on Fowler-Nordheim tunneling (FNT) for programming. For the first time, this paper shows that source side injection (SSI) is also an excellent candidate for MLCS. The main advantages of SSI for MLCS are the very narrow threshold-voltage distributions after SSI programming, the symmetrical threshold-voltage window and the overerase immunity, which allows an overall wider threshold-voltage window, and hence more separated distributions.