Time-dependent breakdown of ultra-thin SiO2 gate dielectrics under pulsed biased stress

Ultra-thin SiO2 films (t(ox) similar to 2.0 nm) were stressed under de. uni polar, and bipolar pulsed bias conditions up to a pulse repetition frequenc y of 50 kHz. The time-to-breakdown (t(BD)), the number of defects at breakd own (N-BD), and the number of defects generated inside the oxide as a funct ion of stress time were monitored during each stress condition, Oxide Lifet ime under unipolar pulsed bias is similar to that under de conditions; howe ver lifetime under bipolar pulsed bias is significantly improved and exhibi ts a dependence on pulse repetition frequency. The observation of a lifetim e increase under bipolar pulsed bias For the oxide thickness and voltage ra nge 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.