Mechanism of date retention improvement by high temperature annealing of Al₂O₃ blocking layer in flash memory device

The mechanism underlying improved data retention via high-temperature oxygen annealing of the Al₂O₃ blocking layer in a charge-trap type flash memory device is investigated in comparison with the case of high-temperature nitrogen annealing. The results show that significant improvement of the retention property can be achieved by oxygen annealing at 1100 °C, compared to nitrogen annealing. Experimental evidence indicated that the underlying mechanism does not arise from suppression of the trap-assisted tunneling current through the blocking oxide; instead it is caused by a reduction of the thermionic emission component of the charge loss factor. This is possibly due to changes of the conduction band offset of the crystallized Al₂O₃.