Srinivasan Chakravarthi

A comprehensive framework for predictive modeling of negative bias temperature instability

A quantitative model is developed for the first time, that comprehends all the unique characteristics of NBTI degradation. Several models are critically examined to develop a reaction/diffusion based modeling framework for predicting interface state generation during NBTI stress. NBTI degradation is found to be dominated by diffusion of neutral atomic and molecular hydrogen related defects. Additionally, the presence of hydrogen gettering sites such as unsaturated grain bound- aries significantly enhance NBTI degradation, whereas hydrogen sources reduce NBTI degradation. The model also suggests the possible mechanisms for saturation. The model is calibrated over a range of stress temperatures and voltages. The model captures recovery, experimental delay and frequency effects successfully.

NBTI impact on transistor and circuit: models, mechanisms and scaling effects [MOSFETs]

We describe a quantitative relationship between I/sub D/ and V/sub T/ driven NBTI specifications. Mobility degradation is shown to be a significant (/spl sim/40%) contributor to I/sub D/ degradation. We report for the first time, degradation in gate-drain capacitance (C/sub GD/) due to NBTI. The impact of this C/sub GD/ degradation on circuit performance is quantified for both digital and analog circuits. We find that C/sub GD/ degradation has a greater impact on the analog circuit studied than the digital circuit. We demonstrate that there is an optimum operating voltage that balances NBTI degradation against transistor voltage headroom. Further, a numerical model based on the reaction-diffusion theory has been developed, which is found to satisfactorily describe degradation, recovery and post-recovery response to stress.

Material dependence of hydrogen diffusion: implications for NBTI degradation

Negative bias temperature instability (NBTI) is known to exhibit significant recovery upon removal of the gate voltage. The process dependence of this recovery behavior is studied by using the time slope (n) as the monitor. We observe a systematic variation of n with oxide thickness, nitrogen concentration, and fluorine implantation. Incorporation of the material dependence of the diffusivity within the reaction-diffusion (R-D) framework captures the observed trends. The consequences of this modification are (a) diffusion limitation is shown to arise from diffusion in poly-Si, rather than oxide, (b) a plausible explanation for low-voltage stress induced leakage current (LV-SILC) naturally appears. Important findings are (a) NBTI degradation remains significant at high frequencies, (b) numerical simulations at moderate frequencies can be used to predict circuit impact in the GHz regime, (c) high frequency operation can be modeled as a lower effective DC stress

Fundamentals of silicon material properties for successful exploitation of strain engineering in modern CMOS manufacturing

Semiconductor industry has increasingly resorted to strain as a means of realizing the required node-to-node transistor performance improvements. Straining silicon fundamentally changes the mechanical, electrical (band structure and mobility), and chemical (diffusion and activation) properties. As silicon is strained and subjected to high-temperature thermal processing, it undergoes mechanical deformations that create defects, which may significantly limit yield. Engineers have to manipulate these properties of silicon to balance the performance gains against defect generation. This paper will elucidate the current understanding and ongoing published efforts on all these critical properties in bulk strained silicon. The manifestation of these properties in CMOS transistor performance and designs that successfully harness strain is reviewed in the last section. Current manufacturable strained-silicon technologies are reviewed with particular emphasis on scalability. A detailed case study on recessed silicon germanium transistors illustrates the application of the fundamentals to optimal transistor design.

Negative bias temperature instability mechanism: The role of molecular hydrogen

The role of dimerization of atomic hydrogen to give molecular hydrogen in determining negative bias temperature instability (NBTI) kinetics is explored analytically. The time dependency of NBTI involving molecular hydrogen was found to obey a power law with a slope of 1∕6, as opposed to the 1∕4 slope derived for a reaction involving atomic hydrogen. The implications of this dimerization reaction for voltage and temperature acceleration are also discussed. Simulation results validating these predictions are also described. The higher slopes typically reported for NBTI are shown to be an artifact of measurement, and experimental data supporting this lower time dependency is shown.

Investigation and Modeling of Fluorine Co-Implantation Effects on Dopant Redistribution

A comprehensive model is developed from ab-initio calculations to understand the effects of co-implanted fluorine (F) on boron (B) and phosphorus (P) under sub-amorphizing and amorphizing conditions. The depth of the amorphous-crystalline interface and the implant depth of F are the key parameters to understand the interactions. Under sub-amorphizing conditions, B and P diffusion are enhanced, in contrast to amorphized regions where the model predicts retarded diffusion. This analysis predicts the F effect on B and P to be entirely due to interactions of F with point-defects.

Effect of nitride sidewall spacer process on boron dose loss in ultrashallow junction formation

A nitride spacer with an underlying deposited tetraethoxysilane oxide, that behaves as a convenient etch stop layer, is a popular choice for sidewall spacer in modern complementary metal–oxide–semiconductor process flows. In this work we have investigated the effect of the silicon nitride spacer process on the boron profile in silicon and the related dose loss of B from the Si into the silicon dioxide. This is reflected as a dramatic decrease in the junction depth. We find that the silicon nitride influences the concentration of hydrogen in the silicon dioxide during the final source/drain anneal. The presence of H enhances the diffusivity of B in the silicon dioxide and thereby results in a significant dose loss from the Si into the silicon dioxide. In this work we have shown this dose loss can be lowered by altering the silicon nitride stoichiometry.

Probing negative bias temperature instability using a continuum numerical framework: Physics to real world operation

Abstract CMOS reliability is facing unprecedented challenges due to the continued scaling of device dimensions. To sustain the current scaling trends, it is imperative to understand the fundamental physics of failure mechanisms. Due to the inherent complexity of these mechanisms, some of the key failure mechanisms can be understood only by a numerical modeling approach. Most failure mechanisms have a characteristic time dependence to failure. Hence in this work, we use a numerical approach to investigate the time dependence of failure mechanism associated with interfacial kinetics at the Si/SiO 2 interface. Several models are critically examined to develop a reaction/diffusion based modeling framework for predicting interface state generation. Our modeling shows reactions at the Si/SiO 2 interface have a direct impact on the time dependence (or time slopes). These time kinetics predictions shed light on the underlying mechanisms behind an technologically important failure mechanism (negative bias temperature instability (NBTI)). In particular, the breaking of an interface SiH bond to release atomic H results in a time slope of 0.25, whereas the release of molecular H 2 results in a time slope of 0.165. Based on this model, we conclude NBTI degradation is dominated by diffusion of neutral molecular hydrogen defects. These models are extended to 2D simulations to study device layout effects. Our simulations suggest differences with device structure (Lgate, Width etc.) and agree with observed experimental results. The developed models are further applied to understand operation under dynamic and static stress.

The Current Understanding of the Trap Generation Mechanisms that Lead to the Power Law Model for Gate Dielectric Breakdown

This paper reviews recent experiments that have shown that the probable mechanism for low voltage trap generation and dielectric breakdown is anode hydrogen release. Vibrational excitation of silicon-hydrogen bonds is the process that provides the most plausible explanation for the existence of a power law model for TDDB.

Modeling the effect of source/drain sidewall spacer process on boron ultra shallow junctions

A novel model is developed to explain the effect of the source/drain sidewall spacer process on boron drain extension formation. A diffusion model for hydrogen in the source/drain sidewall spacer is developed and combined with a model for boron diffusion in oxides. The model is first calibrated to hydrogen out-diffusion data from Nuclear Reaction Analysis (NRA) and then to boron diffusion data from Secondary Ion Mass Spectroscopy (SIMS). Seemingly anomalous changes in boron junction depths with variation in sidewall spacer deposition conditions are explained by this model. The model is applied to TCAD process/device simulations to understand the effect of sidewall spacer on CMOS device performance.