In the past, a series of distinct measurement techniques has been established, which includes direct-current current-voltage measurements, measure-stress-measure (MSM) technique, on-the-fly (OTF) measurements, capacitance-voltage measurements, charge-pumping, and electron-spin-resonance (ESR). Each of them is suited and thus employed for the analysis of NBTI. part from equipment issues, these measurement techniques strongly differ in the information they provide. In the following, their basic experimental setup and principle functioning are outlined and their specific shortcomings are discussed.
The measure-stress-measure (MSM) technique has traditionally been employed to probe NBTI experimentally [25, 26]. Before the real measurement of NBTI degradation starts, an curve is taken by scanning the device in a range around the initial threshold voltage . Then the device is subjected to stress bias and only interrupted by short intervals with the gate bias brought back to . During these short measurement intervals, the drain current is monitored and converted to a threshold voltage shift based on the initially scanned curve . Alternatively, can be directly obtained by enforcing the initial threshold current  or by the shift of complete curves recorded using ultra-short pulses [28, 29].
However, the MSM method suffers from an unavoidable measurement delay , which is defined as the time interval between the removal of stress and the first measurement of the drain current. The degradation within this time is not covered by the measurements and so usually leads to an underestimation of the threshold voltage shift. As pointed out in , the different delay times seriously affect the interpretation of the degradation data, for instance, the exponent of a time power-law as discussed in Section 1.4. Thus minimizing the measurement delay has long been the subject of numerous studies .
On-the-fly (OTF) measurements try to circumvent the unintentional measurement delay and are therefore often regarded as the method of choice for experimentally investigating NBTI. In this method, the gate bias is maintained in the linear regime during the entire measurement run while the drain bias is held at a small but constant level. Alternatively, short voltage pulses can be applied to the drain during measurements only. In both cases, the device degradation is monitored based on the drain current . Since the threshold voltage is of central interest for NBTI, the drain current has to be converted to using extrapolation schemes [32, 33]. The simplest is based on the SPICE level 1 compact model[34, 35] and of the undegraded device, respectively. This compact model neglects mobility variations [35, 36, 37] ascribed to the scattering of charge carriers at the trapped charges close at the interface. However, this model benefits from the fact that, in contrast to other extrapolation schemes, only has to be recorded. More complex extrapolations schemes accounting for mobility variations have been proposed but require the determination of the full curve. Note that these curves must be measured before stress, thereby already causing a non-negligible amount of degradation, which is not accounted for in the extrapolation scheme.
Electron spin resonance (ESR) is a powerful tool to identify paramagnetic defects, which are characterized by an unpaired electron in their orbitals. This electron is associated with a spin whose response to an external magnetic field is measured in ESR experiments. For instance, such paramagnetic defects can be -dangling bonds at the interface (the so-called centers) [10, 11, 38, 8] or in the dielectric (the numerous variants of centers) [39, 40, 41, 42, 43, 44]. It thereby gives chemical and structural information about the defect under investigation and provides insight into the chemical processes occurring in the dielectrics. In this measurement technique, the defects in the sample are subjected to a large but slowly varying magnetic field , which splits their energy levels according to the Zeeman effect. An unpaired electron residing in one defect orbital has two possible orientations - namely either parallel or anti-parallel to the magnetic field. The energetical separation of these two orientations equals
Additional structural information of the investigated defect is available via second-order effects: In solids, the spin-orbit interactions vanish for the ground state in solids but affect the excited states. They alter the gyromagnetic factor to an angle-dependent -tensor, which reflects the symmetry of the paramagnetic center. Hence, angle-dependent measurements allow the identification of defects on the basis of this symmetry [8, 45]. In this way, it has been revealed that the central atoms of and centers are tetrahedrally back-bonded to three other atoms. In contrast, centers exhibit a lower symmetry, which is traced back to a surface dimer bond. Another second order effect arises from electron-nuclear hyperfine interactions. Due to different orientations of the nuclei magnetic moments, additional characteristic peaks emerge in the ESR spectrum. For instance, the relative heights of these features — more precisely, a ratio of — is a special signature for the element . Therefore all variants of centers could be identified as dangling bonds. In the context of NBTI, a series of investigations address hydrogen reactions with centers as well as hydrogenated variants of centers, namely doublet and the doublet [40, 46, 41]. Another variant of ESR is spin dependent recombination (SDR) [47, 18], in which the recombination via deep traps in the substrate bandgap is hampered due to a magnetic alignment of electrons in the conduction band and in the trap. With this method, it has been suggested that centers play an important role in the NBTI degradation of silicon oxynitrides.
With the technical advances in the MOSFET technology during the last several years, the device geometries of MOSFETs have been continuously shrunken and reached a point where the device degradation is dominated by the occurrence of single charging or discharging events [48, 49, 50]. As shown in Fig. 1.1, each of these events appears as a step in the recovery traces. Interestingly, one can clearly recognize that those steps differ significantly in their heights. This can be ascribed to the fact that the random distribution of dopants produces a spatially varying electrical potential inside the channel. The resulting inhomogeneous current density from source to drain is frequently referred to as the percolation current path, which is unique for each device. The lateral position of a charged trap within the gate area determines the step height in the drain current and the threshold voltage shift. This height is the signature of each defect and can thus be used for the identification of a single trap. This fact has motivated the use of the so-called spectral maps [51, 52, 53], in which the frequency of emission events is plotted vs. and (see the lower panel of Fig. 1.1). These maps reveal the characteristic emission times for certain stress conditions, which can be varied for the investigation of field and temperature dependence of and (Fig. 1.2).
TDDS has lead to several essential findings [51, 52, 53, 54] outlined in the following:
One should keep in mind that some defects exhibit an exponential oxide field dependence of in normal random telegraph noise (RTN) measurements . This difference to TDDS findings may arise from the fact that these defects are not assessable by the TDDS measurements.
Astonishingly, several TDDS recovery traces display RTN only after stressing . The noise at one recovery trace is physically linked to defects — in this case hole traps — which continuously exchange charge carriers with the substrate. After a while, the RTN signal vanishes and does not reoccur during the remaining measurement time. The termination of the noise signal is ascribed to hole traps which change to their neutral charge state and remain therein. In , this kind of noise has been termed temporary RTN (tRTN) since it occurs only for a limited amount of time.
A similar phenomenon called anomalous RTN (aRTN) has been discovered in the early studies of Kirton and Uren . Therein, electron traps have been observed, which repeatedly produce noise for random time intervals. During the interruptions of the signal, the defects dwell in their negative charge state so that no noise signal is generated. The behavior of these traps has been interpreted by the existence of a metastable defect state.