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13 Permanent Component of Negative Bias Temperature Instabilities

As discussed in the previous parts of this thesis, BTI is often considered to be the sum of a recoverable and a permanent threshold voltage shift. To understand both partial contributions to this puzzling phenomenon, tremendous efforts have been put towards an explanation of the observed threshold voltage shift. In contrast to the recoverable component, the study of the permanent threshold voltage shift is even more involved s it is mostly overshadowed by the former. According to the prevalent opinion, the permanent threshold voltage shift is due to single defects with large time constants, commonly referred to as interface states [139, 185, 145, 144, 186, 156, 187]. In the following a measurement sequence deliberately designed to study the recoverable and permanent component simultaneously is introduced. By performing ultra-long time experiments over several month, the evolution of the permanent threshold voltage shift is recorded and analyzed. To explain the newly collected data the hydrogen release model, see Section 7.4.2, is used.

13.1 Experimental Characterization of the Permanent Component

To monitor threshold voltage shift due to NBTI in pMOSFETs, simple stress/measure sequences are commonly used. We extended this procedure by a set of (math image) sweeps, recorded prior and after the stress/measure cycle is applied, see Figure 13.1.

(-pstool- diagram)

Figure 13.1:  The measurement sequence used to determine the permanent component of NBTI is shown. (top) During stress the defects get charged, resulting in a threshold voltage shift (math image). At the end of the recovery a significant (math image) remains which can be assessed by our modified measurement sequence. Using subsequent (math image) sweeps, a significant amount of the charge trapped during stress is removed and thus a basically flat recovery is observed at the end of the proposed measurement sequence. (bottom) As can be seen, prior and after the stress/measure cycle ten additional (math image) sweeps are performed [MWC7].

As can be seen, the (math image) accumulated during stress is not completely reversed after recovery. Obviously, a significant number of defects with very large time constants is present which determine the permanent component of (math image). To access the remaining (math image) typically ten (math image) sweeps are measured after recovery thereby removing a significant amount of the trapped charge. Next, the permanent (math image) can be extracted at different gate biases using the (math image) sweeps, see Figure 13.2.


Figure 13.2:  (top) The (math image) sweeps are used to extract the permanent contribution (math image) to the total (math image). (bottom) The voltage range of the (math image) sweeps allows to evaluate (math image) at different gate biases. As can be seen, during the first up-sweep (math image) is significantly larger, a consequence of the large number of trapped charges available immediately after the recovery cycle. In the subsequent sweeps only a weak change of (math image) is visible. In addition, the location of the interface states (math image) is shown [137]. At the corresponding gate biases a strong impact of interface states would be expected. These interface states are typically aligned \( \SI {0.25}{\electronvolt } \) above the valence band, but no particular shift is observed in this region [MWC7].

We define the permanent component as the difference in the gate voltage between the down-sweep (math image) and corresponding up-sweep (math image) of the measured (math image) sweeps

(13.1) \begin{equation} \Perm (\VGdown ) = \VGdown - \VGup (@\IDS (\VGdown )), \end{equation}

extracted at a certain gate bias by considering the same drain-source current. Remarkably, (math image) extracted from the first (math image) sweep performed from inversion to accumulation is strongly reduced towards decreasing absolute gate bias. This is a consequence of a larger number of traps remaining charged at the end of the recovery phase. In contrast, the analysis of the subsequent (math image) sweeps show a nearly flat (math image) dependence on (math image). As the interface states are typically supposed to be the main culprit for (math image), a peak arranged slightly above the (math image) of the pMOSFET is expected. Figure 13.2 b shows the area where contributions of interface states are expected, calculated considering (math image) at \( \SI {0.25}{\electronvolt } \) above the valence band [137]. However, no contribution is this region is measured. Finally,

(13.2) \begin{equation} \label {equ:extractP} \PermMin = \mathrm {min}(\Perm ) \end{equation}

is taken as a measure for the permanent component remaining after our proposed measurement sequence.

In order to elucidate the evolution of (math image), the TMI together with a computer controlled furnace is used. The latter is necessary to achieve defined device temperatures and controlled temperature gradients when the temperature is changed. Using our sophisticated setup, (math image) is studied over several month on a single transistor mounted into a conventional ceramic package. As the time constants of the defects responsible for (math image) are very large, temperature accelerated experiments are performed, see Figure 13.3.

(-pstool- diagram)

Figure 13.3:  The measurement sequence used to monitor (math image) over several month employs the temperature acceleration of charge trapping. At a reference temperature of \( \SI {200}{\celsius } \) 20 (math image) sweeps are performed before the device is stress at higher temperature. Again, after cooling back to the reference temperature, 20 (math image) sweeps are recorded. Note that the stress bias is applied during heating and cooling phases. To obtain an accurate description by simulations these phases have to be considered [MWC6].

To minimize the measurement noise, the permanent component of (math image) is again extracted using equation (13.2).

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