Electrical/thermal properties of nonplanar polyoxides and the consequent effects for EEPROM cell operation

The electrical/thermal properties of nonplanar polyoxides and the resulting effects for EEPROM operational margins are reported. The polyoxide between floating gate (FG) and control gate (CG) of FLOTOX-type EEPROM cells is nonplanar because it always contains edges, where CG wraps over FG. At such edges a highly stable electrical passivation of Fowler-Nordheim (FN) leakage currents occurs, which can cause a degradation of EEPROM operational margins, due to an electron discharge mechanism from the FG of charged EEPROM cells during the first charging operation after conventional baking. The EEPROM cell study includes the dependence on repeated passivation/depassivation of the polyoxide, on baking temperature and baking time. It is found that the average magnitude of the electron discharge is reduced after each passivation/depassivation cycle, which points to a progressive increase of the number of electrons captured in deep neutral electron traps at the polyoxide edges. Analysis of the temperature dependence leads to an activation energy (thermal detrapping energy of the electrons) of 1.3 eV for the degradation mechanism of EEPROM cell operational margins as well as the nonplanar polyoxide depassivation.