Nonerratic behavior of overerased bits in flash EEPROM

We report the effects of flash "channel" programming, or severe gate distur b, on the threshold voltage of fast or overerased bits. Experiments have be en performed to establish that this class of fast bits are nonerratic and r emain fast after 250 degrees C bake. These fast bits exhibit identical subt hreshold characteristics similar to that of a normal bit after ultraviolet erase, thus establishing that the initial charge stored on the floating gat e is the same for both normal and fast bits. Polysilicon grain boundary enh anced electric fields which result in impact ionization by tunneling electr ons, thus generating trapped positive charges in the grain boundary oxide r idges are believed to play an important role in the generation of fast bits . (C) 1998 American Vacuum Society. [S0734-211X(98)11906-6].