Negative Bias Temperature Instability (NBTI) is one of the most pressing
reliability issues in modern CMOS semiconductors. It is caused by application
of a negative gate voltage to a MOSFET and manifests itself as a shift in the
device parameters, first and foremost the threshold voltage. The effect is
considerably enhanced at elevated temperatures, and p-channel FETs are more
affected than n-channel FETs. Although a great deal of effort is going into the
research on this topic, there is still no universally accepted microscopic
model.
As with modeling in general, accurate and reliable measurements are the key
to successful model development. Especially with sophisticated devices in the
sub-micron range, second-order effects cause the measured device
characteristics to deviate considerably from the idealized closed-form
analytical solutions derived from the semiconductor equations. It is therefore important that
measurement data be assessed by means of simulation of the device within
the chosen measurement setup. This is true to an even greater extent in the case of NBTI, since
measurement data from different groups often do not match and sometimes even
contradict each other. This discrepancy is largely due to the fact that the relaxation of
NBTI sets in very quickly, but lasts for several decades in time.
One particular measurement method called On-The-Fly (OTF) avoids relaxation
by continuously stressing the device while measuring the drain current
degradation. Since no degradation is 'lost' during some measurement cycles,
this method was considered superior in the past. Recently, the concern was raised
regarding the extent to which errors in the initial measurement of the drain current, to
which all subsequent drain current readouts are compared to, may impair the calculated
degradation of the threshold voltage. Our
simulations clearly showed that uncertainties in the initial drain current
measurement seriously impact the extracted threshold voltage shift. In
addition, mobility variations induced by NBTI may add spurious threshold
voltage shifts.
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