A degradation mechanism of EEPROM cell operational margins which remains undetected by conventional quality assurance

We report an electron-discharge mechanism from the floating gate of charged EEPROM cells during the first charging operation after baking (250/spl deg/C, 24 h). For an ensemble of measured EEPROM tells the discharge occurs statistically with threshold-voltage reductions up to over 1 V. Responsible is Fowler-Nordheim (FN) tunneling through the interpolyoxide at the edge where the control gate wraps over the floating gate. This FN tunneling is normally suppressed by a localized highly stable electrical passivation, which is automatically generated by programming operations. Baking partly destroys this passivation so that subsequent cell charging removes more electrons from the floating gate by FN tunneling via the interpolyoxide than it adds via the tunneling oxide.