Tibor Grasser

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.

Origin of NBTI variability in deeply scaled pFETs

The similarity between Random Telegraph Noise and Negative Bias Temperature Instability (NBTI) relaxation is further demonstrated by the observation of exponentially-distributed threshold voltage shifts corresponding to single-carrier discharges in NBTI transients in deeply scaled pFETs. A SPICE-based simplified channel percolation model is devised to confirm this behavior. The overall device-to-device ΔVth distribution following NBTI stress is argued to be a convolution of exponential distributions of uncorrelated individual charged defects Poisson-distributed in number. An analytical description of the total NBTI threshold voltage shift distribution is derived, allowing, among other things, linking its first two moments with the average number of defects per device.

Stochastic charge trapping in oxides: From random telegraph noise to bias temperature instabilities

Charge trapping at oxide defects fundamentally affects the reliability of MOS transistors. In particular, charge trapping has long been made responsible for random telegraph and 1/f noise. Recently, it has been identified as a significant contributor to bias temperature instabilities. Conventional defect models assume that the defect has two states, one of them neutral and the other charged. The transition rates between the two states are calculated using some extended Shockley–Read–Hall theory, which neglects the configurational changes occurring at the defect site following a charge trapping or emission event. In order to capture these changes, multiphonon models have been in use for many decades but have not found their way into the mainstream of reliability modeling yet. Furthermore, recent experimental results demonstrate that defects have more states than the two assumed in the conventional model. These additional states together with multiphonon charge transfer mechanisms are essential for the understanding of the complex defect dynamics. The present review summarizes the basic principles of how to model stochastic defect transitions with a particular focus on multi-state defects. After discussing the limitations of Shockley–Read–Hall theory, the relatively simple semiclassical approximation of multiphonon theory is introduced which already provides a much better description. Finally, the transition rates for multi-state defects are estimated using multiphonon theory, which gives a very accurate description of the latest experimental data.

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.

A review of hydrodynamic and energy-transport models for semiconductor device simulation

Since Stratton published his famous paper four decades ago, various transport models have been proposed which account for the average carrier energy or temperature in one way or another. The need for such transport models arose because the traditionally used drift-diffusion model cannot capture nonlocal effects which gained increasing importance in modern miniaturized semiconductor devices. In the derivation of these models from Boltzmanns transport equation, several assumptions have to be made in order to obtain a tractable equation set. Although these assumptions may differ significantly, the resulting final models show various similarities, which has frequently led to confusion. We give a detailed review on this subject, highlighting the differences and similarities between the models, and we shed some light on the critical issues associated with higher order transport models.

A two-stage model for negative bias temperature instability

Based on the established properties of the most commonly observed defect in amorphous oxides, the E′ center, we suggest a coupled two-stage model to explain the negative bias temperature instability. We show that a full model that includes the creation of E′ centers from their neutral oxygen vacancy precursors and their ability to be repeatedly charged and discharged prior to total annealing is required to describe the first stage of degradation. In the second stage a positively charged E′ center can trigger the depassivation of Pb centers at the Si/SiO2 interface or KN centers in oxynitrides to create an unpassivated silicon dangling bond. We evaluate the new model to experimental data obtained from three vastly different technologies (thick SiO2, SiON, and HK) and obtain very promising results.

The statistical analysis of individual defects constituting NBTI and its implications for modeling DC- and AC-stress

The physical origin of the Negative Bias Temperature Instability (NBTI) is still under debate. In this work we analyze the single defects constituting NBTI. We introduce a new measurement technique stimulating a charging of these defects. By employing a statistical analysis of many stochastic stimulation processes of the same defect we are able to determine the electric field and the temperature dependence of these defects with great precision. Based on our experiments we present and verify a new, physics-based, quantitative model allowing a precise prediction of NBTI degradation and recovery. This model takes the stress history into account and also provides a prediction for degradation due to AC-NBTI and an understanding of the special features seen in conjunction with AC-NBTI.

Ubiquitous relaxation in BTI stressing—New evaluation and insights

The ubiquity of threshold voltage relaxation is demonstrated in samples with both conventional and high-k dielectrics following various stress conditions. A technique based on recording short traces of relaxation during each measurement phase of a standard measure-stress-measure sequence allows monitoring and correcting for the otherwise-unknown relaxation component. The properties of relaxation are discussed in detail for pFET with SiON dielectric subjected to NBTI stress. Based on similarities with dielectric relaxation, a physical picture and an equivalent circuit are proposed.

The Universality of NBTI Relaxation and its Implications for Modeling and Characterization

As of date many NBTI models have been published which aim to successfully capture the essential physics. As such, these models have mostly focused on the stress phase. The relaxation phase, on the other hand, has not received as much attention, possibly because of the contradictory results published so far. Particularly noteworthy are the very long relaxation tails of almost logarithmic nature, which cannot be successfully described by the reaction-diffusion model. The authors argue that understanding the nature of the relaxation phase could hold the key to unraveling the underlying NBTI mechanism. In particular, the authors stipulate that the relaxation phase follows a universal relaxation law, demonstrate the valuable consequences resulting therefrom, and use this universality to classify presently available NBTI models.

Simultaneous Extraction of Recoverable and Permanent Components Contributing to Bias-Temperature Instability

Measuring the degradation of modern devices subjected to bias temperature stress has turned out to be a formidable challenge. Interestingly, measurement techniques such as fast- Vth, on-the-fly ID,lin, and charge-pumping give quite different results. This has often been explained by the inherent recovery in non-on-the-fly techniques. Still, all these techniques deliver important information on the degradation and recovery behavior and a rigorous understanding linking these results is still missing. Based on our detailed studies of the recovery, we propose a new measurement technique which allows the simultaneous extraction of two distinctly different components, a fast universally recovering component and a slow, nearly permanent component.