Dynamic positive bias temperature instability characteristics of ultra-thin HfO2 NMOSFET

We present the threshold voltage instability characteristics of high-k HfO₂ NMOSFET dielectric with SiON interface layer under dynamic stress. Compared to DC stress, reduced threshold voltage shift was observed at higher frequency and lower duty cycle in AC unipolar stress. Similarly, the degradation of maximum transconductance was also reduced with AC stress conditions. However, the degradation in subthreshold swing was found to be negligible and fairly independent of stress frequencies and duty cycles in AC unipolar stress. The traps in bulk of HfO₂ dielectric, which is proportional to its physical thickness, is believed as the primary factor for larger threshold voltage shift as the thickness of HfO₂ increases. Compared to the result under DC constant voltage stress, AC unipolar stress allows higher 10-year lifetime operating voltage.