Charge trapping characteristics of SONOS capacitors with control gates of different work functions during programferase operations

The control gate materials are important in adjusting charge trapping characteristics of oxide- nitride-oxide capacitors in SONGS flash memory. In this study, we have investigated program/erase characteristics of SONGS capacitors with the control gates of different work functions. Gate work functions make little difference during program operation for both aluminum gate and n poly-silicon gate, but they become prominently different during erase operation. At the erase voltages of -12 V and -13 V, flat band voltages did not come back to initial values. When aluminum gate is used, the flat band voltage differences were 0.10 V and 0.58 V, respectively. When n poly-silicon gate is used, the flat band voltage differences after erase operation were 1.33 V and 2.15 V, respectively. These results can be explained by the fact that energy barrier height of poly-silicon gate (3.12 eV) is lower than that of aluminum gate (3.2 eV), which gives higher probability of the electron tunneling from control gate to the silicon nitride layer through the top oxide.