Mechanisms for hot-carrier-induced degradation in reoxidized-nitrided-oxide n-MOSFET's under combined AC/DC stressing

Abstract

Hot-carrier-induced degradation behaviors of reoxidized-nitrided-oxide (RNO) n-MOSFET's under combined ac/dc stressing have been extensively studied and are compared with conventional-oxide (OX) MOSFET's. As distinct from OX devices, reduced degradation rate can be observed for RNO devices under dynamic stressing versus the corresponding static stressing. A degradation mechanism is proposed in which trapped holes in stressed gate oxide are neutralized by an ensuing hot-electron injection, leaving lots of neutral electron traps in the gate oxide, with no significant generation of interface states. The degradation behaviors of threshold voltage, subthreshold gate-voltage swing, and charge-pumping current during a series of ac/dc stressings support this proposed mechanism. The neutral electron traps introduced by ac stress are believed to be some broken strained bonds caused by both hot-hole injection and recombination of the trapped holes with the injected electrons. RNO device degradation during AC stressing arises mainly from the charge trapping in gate oxide rather than the generation of interface states due to the hardening of Si/SiO2 interface by nitridation/reoxidation steps. Moreover, the ac-stressed RNO devices are significantly inferior to fresh RNO devices in terms of dc stressing due to a lot of neutral electron traps in the gate oxide resulted from the ac stressing.