Time-dependent breakdown of Ultra-thin SiO ₂ gate dielectrics under pulsed biased stress

Ultra-thin SiO ₂ films (t _ox ∼ 2.0 nm) were stressed under dc, unipolar, and bipolar pulsed bias conditions up to a pulse repetition frequency of 50 kHz. The time-to-breakdown (t _BD), the number of defects at breakdown (N _BD), and the number of defects generated inside the oxide as a function of stress time were monitored during each stress condition. Oxide lifetime under unipolar pulsed bias is similar to that under dc conditions; however lifetime under bipolar pulsed bias is significantly improved and exhibits a dependence on pulse repetition frequency. The observation of a lifetime increase under bipolar pulsed bias for the oxide thickness and voltage range used in this study suggests that a different physical mechanism may be responsible for the lifetime increase from that assumed in earlier studies for thicker films.