The reliability issues of Flash electrically erasable programmable read-only memory (Flash EEPROM) are reviewed in this paper. The reduction of the memory cell size and improvement in the reliability have been realized by several breakthroughs in the device technology; in particular, the reliability of the ETOX and NAND structure EEPROM will be discussed in detail. Flash EEPROM is expected to be a very promising device for a large nonvolatile memory market. One of the most promising applications is the replacement of the conventional magnetic hard disk by nonvolatile memories.

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For the replacement of conventional harddisks by NAND EEPROM, a very high density and a high programming speed are required. An increased density can be achieved by using multi-level memory cells. With the new method, using staircase programming pulses combined with a bit-by-bit verify, a very narrow threshold voltage distribution of 0.7V, necessary for 4-level or 2-bit operation, and a high programming speed of 300*/page or 59Ons/byte can be obtained.