Franz Schanovsky

The Paradigm Shift in Understanding the Bias Temperature Instability: From Reaction–Diffusion to Switching Oxide Traps

One of the most important degradation modes in CMOS technologies, the bias temperature instability (BTI) has been known since the 1960s. Already in early interpretations, charge trapping in the oxide was considered an important aspect of the degradation. In their 1977 paper, Jeppson and Svensson suggested a hydrogen-diffusion controlled mechanism for the creation of interface states. Their reaction-diffusion model subsequently became the dominant explanation of the phenomenon. While Jeppson and Svensson gave a preliminary study of the recovery of the degradation, this issue received only limited attention for many years. In the last decade, however, a large number of detailed recovery studies have been published, showing clearly that the reaction-diffusion mechanism is inconsistent with the data. As a consequence, the research focus shifted back toward charge trapping. Currently available advanced charge-trapping theories based on switching oxide traps are now able to explain the bulk of the experimental data. We give a review of our perspective on some selected developments in this area.

The time dependent defect spectroscopy (TDDS) for the characterization of the bias temperature instability

We introduce a new method to analyze the statistical properties of the defects responsible for the ubiquitous recovery behavior following negative bias temperature stress, which we term time dependent defect spectroscopy (TDDS). The TDDS relies on small-area metal-oxide-semiconductor field effect transistors (MOSFETs) where recovery proceeds in discrete steps. Contrary to techniques for the analysis of random telegraph noise (RTN), which only allow to monitor the defect behavior in a rather narrow window, the TDDS can be used to study the capture and emission times of the defects over an extremely wide range. We demonstrate that the recoverable component of NBTI is due to thermally activated hole capture and emission in individual defects with a very wide distribution of time constants, consistent with nonradiative multiphonon theory previously applied to the analysis of RTN. The defects responsible for this process show a number of peculiar features similar to anomalous RTN previously observed in nMOS transistors. A quantitative model is suggested which can explain the bias as well as the temperature dependence of the characteristic time constants. Furthermore, it is shown how the new model naturally explains the various abnormalities observed.

Recent advances in understanding the bias temperature instability

Our understanding of the bias temperature instability (BTI) has been plagued by disagreements related to measurement issues. Although even in the early papers on BTI the existence of recovery was acknowledged and discussed [1, 2], for unknown reasons this had little impact on the way we used to think about the phenomenon until recently [3–7]. Even after the re-discovery of recovery [3], it took a few years until it was fully appreciated that any measurement scheme conceived so far considerably interferes with the degradation it is supposed to measure, often accelerating its recovery. Nonetheless, this experimental nuisance has led researchers to think in more detail about the problem and has thus opened the floodgates for fresh ideas [6–11]. Some of these ideas together with the experimental data supporting them are reviewed in the following.

Advanced characterization of oxide traps: The dynamic time-dependent defect spectroscopy

An accurate understanding of oxide traps is essential for a number of reliability issues, including the bias temperature instability, hot carrier degradation, time-dependent dielectric breakdown, random telegraph and 1/f noise. Recent results have demonstrated that hole capture and emission into oxide traps in pMOS transistors are more complicated than the usually assumed Shockley-Read-Hall-like process. In particular, both charging and discharging proceed via a non-radiative multiphonon (NMP) mechanism involving metastable defect states. The existence of these metastable states can be demonstrated by extending the previously introduced time-dependent defect spectroscopy (TDDS) to a more general dynamic case by employing AC stresses and precisely timed discharge pulses during recovery. Application of AC stresses clearly reveals a frequency-dependence of the effective capture time, which confirms the existence of an intermediate metastable state. Application of pulses during recovery, on the other hand, allows extraction of the effective emission time also in depletion as well as in accumulation, thereby clearly revealing a metastable switching state. While all investigated traps show a frequency-dependent capture time constant, suggesting them to be of the same microscopic origin, we find two different kinds of emission behavior, namely fixed positive and switching traps. We finally demonstrate that our multi-state NMP model perfectly captures both cases.

NBTI in Nanoscale MOSFETs—The Ultimate Modeling Benchmark

After nearly half a century of research into the bias temperature instability, two classes of models have emerged as the strongest contenders. One class of models, the reaction-diffusion models, is built around the idea that hydrogen is released from the interface and that it is the diffusion of some form of hydrogen that controls both degradation and recovery. Although various variants of the reaction-diffusion idea have been published over the years, the most commonly used recent models are based on nondispersive reaction rates and nondispersive diffusion. The other class of models is based on the idea that degradation is controlled by first-order reactions with widely distributed (dispersive) reaction rates. We demonstrate that these two classes give fundamentally different predictions for the stochastic degradation and recovery of nanoscale devices, therefore providing the ultimate modeling benchmark. Using detailed experimental time-dependent defect spectroscopy data obtained on such nanoscale devices, we investigate the compatibility of these models with experiment. Our results show that the diffusion of hydrogen (or any other species) is unlikely to be the limiting aspect that determines degradation. On the other hand, the data are fully consistent with reaction-limited models. We finally argue that only the correct understanding of the physical mechanisms leading to the significant device-to-device variation observed in the degradation in nanoscale devices will enable accurate reliability projections and device optimization.

Hydrogen-related volatile defects as the possible cause for the recoverable component of NBTI

The recently suggested time-dependent defect spectroscopy (TDDS) has allowed us to study the recoverable component of NBTI at the single-defect level. To go beyond our previous efforts, we have performed a long-term TDDS study covering also the kilo-second time window. We found that even in this extended window NBTI recovery is due to a collection of first-order reactions. In particular, there is no trace of a diffusion-limited process as assumed in the reaction-diffusion model. Most intriguingly, the responsible traps show various degrees of volatility, that is, they can disappear and reappear. Our observations lend strong support to the idea that the recoverable component of NBTI is due to hydrogen-related defects which are active when a hydrogen atom is at the defect site and inactive when not.

Multiphonon hole trapping from first principles

Nonradiative multiphonon capture of carriers into the gate dielectrics of metal-oxide-semiconductor systems and its involvement with the negative bias temperature instability is discussed. A simple method for the extraction of the line-shape function from an atomistic bulk defect model is suggested and applied to defect models in alpha quartz. Electronic structures are described using density functional theory.

Advanced Modeling of Oxide Defects

During the last couple of years, there is growing experimental evidence which confirms charge trapping as the recoverable component of BTI. The trapping process is believed to be a non-radiative multiphonon (NMP) process, which is also encountered in numerous physically related problems. Therefore, the underlying NMP theory is frequently found as an important ingredient in the youngest BTI reliability models. While several different descriptions of the NMP transitions are available in literature, most of them are not suitable for the application to device simulation. In this chapter, we will present a rigorous derivation that starts out from the microscopic Franck–Condon theory and yields generalized trapping rates accounting for all possible NMP transitions with the conduction and the valence band in the substrate as well as in the poly-gate. Most importantly, this derivation considers the more general quadratic electron–phonon coupling contrary to several previous charge trapping models. However, the pure NMP transitions do not suffice to describe the charge trapping behavior seen in time-dependent defect spectroscopy (TDDS). Inspired by these measurements, we introduced metastable states, which have a strong impact on the trapping dynamics of the investigated defect. It is found that these states provide an explanation for plenty of experimental features observed in TDDS measurements. In particular, they can explain the behavior of fixed as well as switching oxide hole traps, both regularly observed in TDDS measurements.

Understanding correlated drain and gate current fluctuations

Recently, some experimental groups have observed the occurrence of correlated drain and gate current fluctuations, which indicate that both currents are influenced by the charge state of the same defect. Since the physical reason behind this phenomenon is unclear at the moment, we evaluated two different explanations: The first model assumes that direct tunneling of carriers is affected by the electrostatic field of the charged defect. Interestingly, this model inherently predicts the gate bias and temperature dependences observed in the experiments and is therefore quite promising at a first glance. In the second model, our multi-state defect model is employed to describe trap-assisted tunneling as a combination of two consecutive nonradiative multi-phonon transitions - namely hole capture from the substrate followed by hole emission into the poly-gate. The latter transition is found to be in the weak electron-phonon coupling regime, which requires the consideration of all band states instead of only the band edges. Our investigation shows that the electrostatic model must be discarded since it predicts only small changes in the gate current while the extended variant of the multi-state defect model delivers quite promising results.

A detailed evaluation of model defects as candidates for the bias temperature instability

Despite its long research history, the bias temperature instability (BTI) is still not fully understood. Recent advances on both the experimental and theoretical side have deepened our understanding of the phenomenon, but the microscopic origin is still unknown. We report on a detailed evaluation of atomistic models of the oxygen vacancy and the hydrogen bridge defects in SiO2 as candidates for the defect responsible for the BTI. For this purpose, time constants are calculated using a combination of atomistic and semiconductor device modeling. These time constants are then compared to electrical measurement data obtained from BTI experiments on individual defects in small-area MOS transistors. The inherent uncertainty in the energetic position of the energy levels in the density functional calculation with respect to the device simulation is accounted for using an empirical energy shift. Very good agreement with the experimental data is found for the hydrogen bridge defect, while for the oxygen vacancy severe discrepancies between the predicted behavior and the experimental observation arise.