Ben Kaczer

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.

Impact of MOSFET gate oxide breakdown on digital circuit operation and reliability

The influence of FET gate oxide breakdown on the performance of a ring oscillator circuit is studied using statistical tools, emission microscopy, and circuit analysis. It is demonstrated that many hard breakdowns can occur in this circuit without affecting its overall function. Time-to-breakdown data measured on individual FETs are shown to scale correctly to circuit level. SPICE simulations of the ring oscillator with the affected FET represented by an equivalent circuit confirm the measured influence of the breakdown on the circuits frequency, the stand-by and the operating currents. It is concluded that if maintaining a digital circuits logical functionality is the sufficient reliability criterion, a nonzero probability exists that the circuit will remain functional beyond the first gate oxide breakdown. Consequently, relaxation of the present reliability criterion in certain cases might be possible.

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.

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 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.

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.

Disorder-controlled-kinetics model for negative bias temperature instability and its experimental verification

A model for NBTI is proposed based on disorder-controlled diffusion and drift in amorphous dielectrics. Experimental data on finFETs confirm all major predictions of the model: temperature dependence of the NBTI exponent, non-Arrhenius behavior of NBTI, log(t) and electric field dependencies of recovery. Experimental challenges with determining NBTI parameters are also highlighted.

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.

AC NBTI studied in the 1 Hz -- 2 GHz range on dedicated on-chip CMOS circuits

We describe on-chip circuits specially designed and fabricated for the purpose of measuring the effect of AC NBTI on an individual, well-defined device in the wide frequency range on a single wafer. The circuits are designed to allow measurements in multiple modes, specifically, DC and AC NBTI (both interrupted and on-the-fly), on a single pFET and on a CMOS inverter, as well as charge-pumping characterization of the stressed pFET. The results indicate that AC NBTI is independent of the frequency in the 1 Hz-2 GHz range. The voltage and stress time acceleration is observed to be identical for both AC and DC NBTI stress

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.