K. Shubhakar

Single vacancy defect spectroscopy on HfO2 using random telegraph noise signals from scanning tunneling microscopy

Random telegraph noise (RTN) measurements are typically carried out at the device level using standard probe station based electrical characterization setup, where the measured current represents a cumulative effect of the simultaneous response of electron capture/emission events at multiple oxygen vacancy defect (trap) sites. To better characterize the individual defects in the high-κ dielectric thin film, we propose and demonstrate here the measurement and analysis of RTN at the nanoscale using a room temperature scanning tunneling microscope setup, with an effective area of interaction of the probe tip that is as small as 10 nm in diameter. Two-level and multi-level RTN signals due to single and multiple defect locations (possibly dispersed in space and energy) are observed on 4 nm HfO2 thin films deposited on n-Si (100) substrate. The RTN signals are statistically analyzed using the Factorial Hidden Markov Model technique to decode the noise contribution of more than one defect (if any) and estimate t...

The “buffering” role of high-к in post breakdown degradation immunity of advanced dual layer dielectric gate stacks

Post breakdown (BD) reliability is an important area of study in ultra-thin gate dielectrics as it has significant implications on the performance degradation, lifetime, reliability margin and power dissipation of advanced sub-22 nm transistors and circuits. A prolonged phase of post-BD can ensure we can live with the circuit with moderate performance and error-free operation, even if the soft breakdown (SBD) events occur early. While analysis of post-BD is simple and straightforward for single layer SiO2/SiON stacks, the number of possible scenarios of post-BD increases when analyzing high-κ-interfacial layer (HK-IL) based technology. This is because the sequence of BD (whether HK or IL fails first followed by the other one) and the competition between multiple SBD in one of these layers, dilative wear-out of a single SBD spot and the possibility of a successive localized BD above/below the HK/IL BD percolation spot (with or without metal filamentation) are all possible phenomena that can be classified as post-BD. The likelihood of occurrence of these various possibilities will determine the immunity of the stack to post-BD degradation. We will investigate each of these scenarios in detail in this work in order to provide a comprehensive assessment of post-BD reliability of state-of-the-art technology. Our analysis on a HK:IL = 25:12Å stack supported by electrical, physical and modeling results provides clear evidence that circuit failure at operating conditions can only be due to multiple SBD events within the IL layer and that the HK is very robust and resilient to breakdown.

Localized characterization of charge transport and random telegraph noise at the nanoscale in HfO2 films combining scanning tunneling microscopy and multi-scale simulations

Charge transport and Random Telegraph Noise (RTN) are measured successfully at the nanoscale on a thin polycrystalline HfO2 film using room temperature Scanning Tunneling Microscopy (STM). STM is used to scan the surface of the sample with the aim of identifying grains and grain boundaries, which show different charge transport characteristics. The defects responsible for charge transport in grains and grain boundaries are identified as positively charged oxygen vacancies by matching the localized I-V curves measured at the nanoscale with the predictions of physics-based multi-scale simulations. The estimated defect densities at grains and grain boundaries agree with earlier reports in the literature. Furthermore, the current-time traces acquired by STM at fixed bias voltages on grains show characteristic RTN fluctuations. The high spatial resolution of the STM-based RTN measurement allows us to detect fluctuations related to individual defects that typically cannot be resolved by the conventional device-...

Leakage current and structural analysis of annealed HfO2/La2O3 and CeO2/La2O3 dielectric stacks: A nanoscopic study

Grain boundaries in the polycrystalline microstructure of post-annealed high-κ (HK) dielectrics are a major limitation in the reliability of HK dielectrics used for advanced CMOS technologies. Another challenge in the field of HK dielectrics is to ensure higher drain drive current in CMOS, while maintaining low leakage current. In this work, the authors demonstrate enhanced performance of HfO2 and CeO2 dielectrics by incorporating lanthanum. The resulting stacks show promising dielectric characteristics with reduced leakage current and uniform (amorphous) crystal structure. The improved HK characteristics were shown to occur even over nanometer-length scales using scanning probe microscopy and transmission electron microscopy, in agreement with previous studies based on micron-scale device-level measurement.

Monte Carlo evidence for need of improved percolation model for non-weibullian degradation in high-κ dielectrics

Dielectric breakdown is one of the critical failure mechanisms and showstopper for ultra-large scale integrated (ULSI) circuits as it impacts the performance and functioning of the transistor, which is the fundamental unit governing the operation of all the advanced microprocessors that we have today. As a reliability engineer, it is essential that the failure mode and mechanism be best described using statistical distributions that correlate with the physical mechanism and driving forces causing failure. In many cases, the distributions used to represent the time to failure data are empirically assumed, without carefully considering its implications on the extrapolated predictions of field lifetime. Application of a wrong distribution can give lifetime estimates that vary by many orders of magnitude, which nullify the very purpose of the reliability study in itself. The Weibull distribution is commonly used to describe random defect generation induced percolation failure of the oxide (dielectric) by means of the “weakest link” phenomenology [1, 2]. While the assumption of a Weibull distribution is well justified for silicon oxide (SiO2) and silicon oxynitride (SiON) materials [3, 4], the application of the same stochastics for high permittivity (high-κ) dielectrics is questionable [5] - [7]. This is fundamentally attributable to the different microstructure of the grown / deposited dielectrics, which we will discuss in detail, along with strong physical analysis evidence. We will present further evidence using Kinetic Monte Carlo (KMC) simulations to explain the origin of the non-Weibullian trends observed. The key motivation of this study is to caution microelectronics reliability scientists against the use of standard statistical distributions for all scenarios. We may have to resort to the need for non-standard distributions or selectively use the standard distributions only over confined percentile ranges, as material and device failure mechanisms become increasingly complex and interdependent in nanoscale integrated circuits.

Real-time analysis of ultra-thin gate dielectric breakdown and recovery - A reality

Switching behaviours have been observed after gate dielectric breakdown under certain favourable conditions. In our recent report in IEDM 2009, the conductive breakdown path in gate dielectric can be “switched-off” if a reverse bias, as opposed to the stressing voltage, is applied, a condition required for observing SET and RESET conduction in bipolar switching material systems. Similar phenomenon has also been observed for unipolar switching. This means that breakdown transistor can be “repaired” electrically by a reverse threshold voltage to expand its lifetime. More recently, detailed insights of the dielectric breakdown and recovery were reported by real-time transmission electron microscopy analysis. In this invited talk, the real-time TEM analysis of the physical structure and morphology of breakdown paths in high-k/metal gate system while under electrical stress is discussed. The results are further correlated with the chemical composition of the breakdown path dynamically during breakdown and recovery. Oxygen vacancies and metal atoms from the anode constitute the chemistry of the nanoscale breakdown path.

Impact of local variations in high-k dielectric on breakdown and recovery characteristics of advanced gate stacks

Nanometer scale variations in property of polycrystalline high-κ (HK) dielectrics significantly affect the reliability and performance of HK-based metal-oxide-semiconductor (MOS) devices. Electrical and physical insight into the kinetics and variability in degradation and breakdown trends of dielectrics is essential to understand the physics and stochastics of failure in transistors. This study will in turn help to understand charge trapping mechanisms in Flash memory and filamentary resistive switching in RRAM. In this work, we present a study on impact of local variations of HK dielectric properties on its degradation, breakdown and recovery process using a combination of Monte Carlo simulations and experimental results with nanometer scale resolution.

An SEM/STM based nanoprobing and TEM study of breakdown locations in HfO 2 /SiO x dielectric stacks for failure analysis

Abstract The formation of conductive percolation path in high-κ (HK)/interfacial layer (IL) dielectric stack is accompanied by dynamic changes in the electrical and chemical properties at the nanometer length scale. It is therefore essential to study these breakdown (BD) events using high-precision nanoscale characterization tools to investigate the physical mechanisms of failure for advanced HK dielectric based devices. In this work, we carry out a new method for electrical nanoprobing of HfO2/SiOx (x

Random telegraph noise in 2D hexagonal boron nitride dielectric films

This study reports the observation of low frequency random telegraph noise (RTN) in a 2D layered hexagonal boron nitride dielectric film in the pre- and post-soft breakdown phases using conductive atomic force microscopy as a nanoscale spectroscopy tool. The RTN traces of the virgin and electrically stressed dielectric (after percolation breakdown) were compared, and the signal features were statistically analyzed using the Factorial Hidden Markov Model technique. We observe a combination of both two-level and multi-level RTN signals in h-BN, akin to the trends commonly observed for bulk oxides such as SiO2 and HfO2. Experimental evidence suggests frequent occurrence of unstable and anomalous RTN traces in 2D dielectrics which makes extraction of defect energetics challenging.

Nanoscale investigations of soft breakdown events in few layered fluorinated graphene

In this study, we perform scanning tunneling spectroscopy (STS) on bi/tri-layered fluorinated graphene (FG) dielectrics, enabling investigation of the degradation and the breakdown phenomenon at the sub-nanometer scale. Our characterization results show that the energy gap can be tailored by surface functionalization of graphene with fluorine ions. Experimental evidence of electrical stress induced degradation and breakdown trends at localized spots across bi/tri-layered FG films is presented. Statistical analysis on bi-layered FG film breakdown voltage data reveals a tri-modal Weibull distribution trend possibly due to variations in the effective FG thickness due to imperfect fluorine incorporation at all C-sites during the fluorine diffusion process. Although preliminary, the results presented provide insight into the kinetics of degradation in graphene based 2-D dielectric materials.