The negative bias temperature instability (NBTI) falls into the category of reliability
issues which considerably affect the device behavior of metal oxide semiconductor
field effect transistors (MOSFETs). After its discovery several decades ago, NBTI
has risen to one of the most serious reliability concerns for modern CMOS
technologies and has therefore increasingly attracted industrial as well as
scientific interest. In its idealized form it occurs when the device is subjected to
elevated temperatures and high gate voltages while the remaining terminals are
grounded. Temperatures typically encountered in practice range from to
, while the field across the dielectric reaches values up to approximately
. These conditions, usually referred to as stress, have detrimental
effects on the device characteristics, for instance a shift in the threshold
voltage
and a change in the subthreshold slope. However, as soon as
the stress conditions are removed, the device characteristics are found to
recover, meaning that the device parameters slowly revert towards their initial
values. For operating stress conditions outside these parameter ranges, NBTI
occurs in a combination with other reliability phenomena, such as hot carrier
degradation [1, 2, 3] or time-dependent dielectric breakdown [4, 5] among
others.
Interestingly, NBTI becomes increasingly pronounced for modern devices: First, the aggressive down-scaling of device geometry goes hand in hand with higher electric fields across the dielectric. Second, compact device integration gives rise to high power dissipation and thus to high operating temperatures. Both strong fields as well as increased temperatures enhance the NBTI device degradation. Also, the introduction of new technologies, especially nitrided oxides, has turned out to reduce other reliability issues but enhances NBTI [6, 7]. On the other hand, advances in device processing improved the oxide reliability, which is accompanied by a reduction of defects at the interface as well as in the dielectric. Even to date, NBTI has eluded our detailed understanding regarding the phenomenological behavior as well as the underlying microscopic origins.