EVIDENCE FOR NONUNIFORM TRAP DISTRIBUTIONS IN THIN OXIDES AFTER HIGH-VOLTAGE STRESSING

During high voltage stressing of thin silicon oxides, traps are genera ted inside of the oxides and at the oxide interfaces. In the trap gene ration/breakdown model, it has been assumed that the high-voltage indu ced traps are generated randomly throughout the oxide during wearout. The oxide breaks down when the local density of traps exceeds a critic al value. The results of two experiments have indicated that the traps are not generated uniformly inside of the oxide during wearout. These two experiments were 1) the measurement of the time-to-breakdown (TTB ) during bipolar stressing and 2) the measurement of the thickness and field dependence of the TTB. The bipolar stressing experiment showed that, in addition to random generation of traps, asperities at the cat hodes introduced high local trap densities. The increased TTB during b ipolar stressing was correlated with the higher trap densities, implyi ng that the traps generated near asperities at one interface did not a lign themselves with traps generated near asperities at the other inte rface. When analyzing the thickness and field dependences of the TTB d ata, it was found that the TTB was independent of oxide thickness and only depended on the bride field. Fitting of the TTB data to TDDB dist ributions showed that the trap densities needed to fit the data had to increase as the oxide thickness decreased in order to maintain consta nt times-to-breakdown for different oxide thicknesses. This result imp lied a nonuniform distribution of traps throughout the thickness of th e oxide, with thinner oxides requiring more traps per unit volume to t rigger breakdown than did thicker oxides and more traps being generate d near the cathode than near the anode.