Degradation of the tunnel gate oxide in a flash EEPROM device under high-field stress

The degradation of thin tunnel gate oxide under constant Fowler-Nordheim current stress was studied using flash EEPROM structures. The degradation is a strong function of the amount of injected charge density (Q{dollar}sb{lcub}rm inj{rcub}{dollar}), oxide thickness, and the direction of stress. Positive charge trapping is usually dominant at low Q{dollar}sb{lcub}rm inj{rcub}{dollar} followed by negative charge trapping at high Q{dollar}sb{lcub}rm inj{rcub}{dollar}, causing a turn-around of gate voltage, flatband voltage, and threshold voltage. Interface trap generation continues to increase with increasing stress, as observed from subthreshold slope and transconductance. Gate injection stress creates more positive charge traps and interface traps than does substrate injection stress. Oxide degradation gets more severe for thicker oxide, due to more oxide charge trapping and interface trap generation by impact ionization. A simple model of oxide degradation and breakdown was established based on the experimental results. It indicates that the damage in the oxide is more serious near the anode interface by impact ionization and oxide breakdown is also closely related to surface roughness at the cathode interface. When all the damage sites in the oxide connect and a conductive path between cathode and anode is formed, oxide breakdown occurs. If the cathode interface is rough or weak, this makes it easier for a conductive leakage path between cathode and anode to form because the anode is heavily damaged by the stress. This is the reason why charge-to-breakdown of gate injection stress is lower than that of substrate injection stress. The damage is more serious for thicker oxide because a thicker oxide is more susceptible to impact ionization. It has also been observed that bipolar stressing of thin oxides leads to higher time-to-breakdown than does dc or unipolar stress. The characteristics of transistors under unipolar stress and low frequency bipolar stress are very similar to those under dc stress, suggesting that just turning the signal off or using low frequency stress is not as effective as bipolar stress for extending oxide lifetime.