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 [25]. Alternatively,
can be directly obtained by enforcing the initial
threshold current
[27] or by the shift of complete
curves recorded
using ultra-short pulses [28, 29].
However, the MSM method suffers from an unavoidable measurement delay [30], 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 [31], 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 [27].
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
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 [55].
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 [51]. 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 [51], 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 [56]. 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.