1.1 Definition of NBTI

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.