Bi-modal erase method for eliminating cycling-induced flash EEPROM cell write/erase threshold closure

摘要

A method to erase data from a flash EEPROM while electrical charges trapped in the tunneling oxide of a flash EEPROM are eliminated to maintain proper separation of the programmed threshold voltage and the erased threshold voltage after extended programming and erasing cycles. The method to erase a flash EEPROM cell begins by channel erasing to remove charge from the floating gate of the flash EEPROM cell. The channel erasing consists of applying a first relatively large negative voltage pulse to the control gate of said EEPROM cell and concurrently applying a first moderately large positive voltage pulse to a first diffusion well. At the same time a ground reference potential is applied to the semiconductor substrate, while the drain and a second diffusion well is allowed to float. The method to erase then proceeds with the source erasing to detrap the tunneling oxide of the flash EEPROM cell. The source erasing consists continued floating the drain and the second diffusion well and concurrently applying the ground reference potential to the semiconductor substrate and the first diffusion well. Concurrently a second relatively large negative voltage pulse is applied to the control gate, as a second moderately large positive voltage pulse is applied to said source.