DEGRADATION OF THIN TUNNEL GATE OXIDE UNDER CONSTANT FOWLER-NORDHEIM CURRENT STRESS FOR A FLASH EEPROM

The degradation of thin tunnel gate oxide under constant Fowler-Nordhe im (FN) current stress was studied using flash EEPROM structures. The: degradation is a strong function of the amount of injected charge den sity (Q(inj)), oxide thickness, and the direction of stress. Positive charge trapping is usually dominant at low Q(inj) followed by negative charge trapping at high Q(inj), causing a turnaround of gate voltage and threshold voltage. Interface trap generation continues to increase with increasing stress, as evidenced by subthreshold slope and transc onductance. Gate injection stress creates more positive charge traps a nd interface traps than does substrate injection stress. Oxide degrada tion gets more severe for thicker oxide, due to more oxide charge trap ping and interface trap generation by impact ionization. A simple mode l of oxide degradation and breakdown was established based on the expe rimental results. It indicates that the damage in the oxide is more se rious near the anode interface by impact ionization and oxide breakdow n is also closely related to surface roughness at the cathode interfac e. When all the damage sites in the oxide connect and a conductive pat h between cathode and anode is formed, oxide breakdown occurs. The dam age is more serious for thicker oxide because a thicker oxide is more susceptible to impact ionization.