A DEGRADATION MECHANISM OF EEPROM CELL OPERATIONAL MARGINS WHICH REMAINS UNDETECTED BY CONVENTIONAL QUALITY ASSURANCE

We report an electron-discharge mechanism front the floating gate of c harged EEPROM cells during the first charging operation after baking ( 250 degrees C, 24 h), For an ensemble of measured EEPROM cells the dis charge occurs statistically with threshold-voltage reductions up to ov er 1 V, Responsible is Fowler-Nordheim (FN) tunneling through the inte rpolyoxide at the edge where the control gate wraps over the floating gate. This FN tunneling is normally suppressed by a localized highly s table electrical passivation, which is automatically generated by prog ramming operations. Baking partly destroys this passivation so that su bsequent cell charging removes more electrons from the floating gate b y FN tunneling via the interpolyoxide than it adds via the tunneling o xide.