Refined NBTI characterization of arbitrarily stressed PMOS devices at ultra-low and unique temperatures

Abstract

We reexamine degradation and recovery dynamics in the negative bias temperature instability (NBTI) of p-channel metal oxide semiconductor field effect transistors (PMOSFETs) by making use of the recently developed in situ polyheater technique. The capability of switching the device temperature extremely fast and almost arbitrarily allows for measuring differently stressed devices directly after the termination of stress at a unique and much lower characterization temperature (e.g. −60°C). This procedure (‘degradation quenching’) is a powerful extension of the conventional measure–stress–measure (MSM) technique and provides a cleaner way for comparing threshold voltage shifts and charge pumping (CP) currents of arbitrarily stressed devices. We find that increasing the stress bias predominantly activates a larger number of defects with similar (short) recovery time constants causing steeper threshold voltage recovery transients after the termination of stress. Increasing the stress temperature has a very similar effect on the threshold voltage shift as increasing the stress time. In both cases, defects with larger recovery time constants are activated while the number of defects with short recovery time constants remains essentially the same. A comparison of VTH shift and CP data suggests that the total threshold voltage shift is due to at least two fundamentally different types of defects, one being readily recoverable and uncorrelated to the CP current while the other is ‘quasi-permanent’ and proportional to the CP current. By converting CP currents into corresponding threshold voltage shifts, we find that only about 50% of the ‘quasi-permanent’ VTH damage is due to slowly-recoverable interface states. The remaining fraction is due to another, yet undefined, positively charged defect generated at virtually the same rate.