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–Nordheim (FN) current stress was studied using flash EEPROM structures. The degradation is a strong function of the amount of injected charge density (Qinj), oxide thickness, and the direction of stress. Positive charge trapping is usually dominant at low Qinj followed by negative charge trapping at high Qinj, causing a turnaround of gate voltage and threshold voltage. Interface trap generation continues to increase with increasing stress, as evidenced by 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. The damage is more serious for thicker oxide because a thicker oxide is more susceptible to impact ionization.