1.2 Two Main Contributions to NBTI

The change in the threshold voltage Vth  , the signature of NBTI, originates from trapped charges which are immobile and therefore cannot carry any drain current. Hence, the central question arises whether NBTI must be ascribed to charges trapped at the Si∕SiO2  interface (interface traps), in the dielectric (oxide traps), or even a combination thereof.

Interface traps stem from the lattice mismatch caused by the abrupt transition from crystalline bulk silicon to amorphous SiO2  . Even the amorphous nature of modern high-quality oxides is not fully capable of compensating the lattice mismatch through the flexibility of its bonding network. As a result, a certain fraction of Si  atoms cannot establish four bonds to their neighbor atoms and thus leave behind unsaturated bonds, the so-called dangling bonds [89]. The corresponding orbitals can carry up to two electrons and feature two trap levels found to lie within the substrate bandgap. Three different types of interface defects are observed experimentally: Pb  centers have been found at (111)Si∕SiO2  interfaces, while Pb0  and Pb1  centers are present at the technologically more relevant (100)Si∕SiO2  interfaces [81011]. The creation or annealing of charged interface states induces a non-negligible threshold voltage shift ΔVth  , which is considered as an undesired degradation by engineers. However, these states can be eliminated by exposure to a hydrogen ambient, where the interfacial dangling bonds are passivated and their corresponding energy levels are shifted out of the substrate bandgap [1210].

There exists a series of traps, such as cycling positive charges [13], anti-neutralization positive charges [13], border traps [14], switching traps [1516], oxide traps, E ′ centers [17], and Kn  centers [18]. However, no precise distinction has been made between them. Experimentally, these traps are characterized by either their trapping time constants or their defect structures. Furthermore, their properties have been found to strongly depend on the local environment of their host material, such as E ′ centers in amorphous SiO2   [1920]. Nevertheless, all these types of oxide traps have been linked to the NBTI phenomenon [2122] since they are capable of exchanging charge carriers with the substrate. Electron or hole injection is assumed to proceed by some kind of elastic [23] or inelastic trapping mechanism [24] into spatially and/or energetically distributed traps. Ongoing research is now dealing with the exact physical description of this process, including all dependences on the oxide field and the temperature.