An Analysis of the NBTI-Induced Threshold Voltage Shift Evaluated by Different Techniques

Negative bias temperature instability (NBTI) is limiting the lifetime of pMOSFETs, and it is often monitored by the shift of threshold voltage Delta V-t. Different techniques have been developed to extract Delta V-t, including the conventional extrapolation of the quasi-dc transfer characteristic I-d similar to V-g and the more recent extrapolation of ultrafast pulse I-d similar to V-g and the on-the-fly evaluation at stress bias. After the same stress, these techniques can produce a Delta V-t difference of up to one order of magnitude. The interpretation of this large difference is still controversial. The objective of this paper is to bridge the gap between the Delta V-t values extracted from these techniques. Degradation and recovery during measurement, measurement and truncation errors, and calculation of transconductance are all examined. After taking these factors into account, the gap in Delta V-t still cannot be filled, and hence, the effect of sensing V-g on Delta V-t must now be considered. It is found that vertical bar Delta V-t vertical bar increases with sensing vertical bar V-g vertical bar, and therefore, the popular assumption of Delta V-t being independent of sensing V is invalid. After taking both the effect of sensing Vg and recovery into account, the gap in vertical bar Delta V-t vertical bar is successfully bridged. The difference between the effect of sensing V-g and recovery is explored, and the results show that they are two different phenomena. This paper provides test engineers a method for determining the worst case Delta V-t under a given operation voltage.