Evaluation of NBTI in HfO₂ Gate-Dielectric Stacks With Tungsten Gates

Negative-bias temperature instability (NBTI) of the threshold voltage in ultrathin HfO2 p-type field-effect transistors (pFET) with tungsten gates is reported. The dependence of threshold voltage, transconductance peak, and interface trap density on stress time is investigated for various negative stress voltages and temperatures. The measurements show that the threshold voltage shifts with a concomitant decrease in transconductance peak and increase in interface trap density as assessed by subthreshold slope and dc current-voltage (DCIV) method. The threshold voltage shift data are fitted with a stretched exponential equation and the fits are used for estimating lifetime. The measurements show that NBTI-related degradation in HfO2 stacks is comparable to that observed in SiO2/poly Si pFETs.