Power-switch gate-oxide breakdown tolerance techniques for power-gated SRAM (original) (raw)
Related papers
Impacts of gate-oxide breakdown on power-gated SRAM
Microelectronics Journal, 2011
This paper presents a detailed analysis on the impacts of various gate-oxide breakdown (BD) paths in column-based header-and footer-gated SRAMs. It is shown that with gate-oxide BD, the read static noise margin (RSNM) and write margin (WM) degrade in general. Pass-transistor gate-oxide BD between WL and BL is shown to degrade read/write margin and performance, and to affect other healthy cells along the same column as well. The effects of gate-to-source BD of cell transistors are shown to confine to the individual cells, while multiple cells suffering cell transistor drain-to-drain BD in a column could cumulatively affect VVDD (header structure) or VVSS (footer structure), thus influencing other cells in the same column. In particular, we show that the gate-oxide BD of the powerswitches has severe and even detrimental effects on the margin, stability, and performance of the SRAM array. Several techniques to mitigate the power-switch gate-oxide BD have been evaluated, including adding a gate series resistance to the power-switch, dual threshold voltage power-switch, thick gate-oxide power-switch, and dual gate-oxide thickness (dual-T OX) power-switch. It is shown that the dual-T OX power-switch improves the time-to-dielectric-breakdown (T BD) of the power-switch while maintaining the performance without side effect.
Analysis and On-Chip Monitoring of Gate Oxide Breakdown in SRAM Cells
IEEE Transactions on Very Large Scale Integration (VLSI) Systems, 2012
Scaling of device sizes has reduced gate oxide thickness to a few atomic layers, increasing the vulnerability of the gate oxide to breakdown. During breakdown, devices go through a gradual wearout process characterized by increased leakage. Using experimentally verified gate oxide breakdown models, a detailed analysis of the effect of progressive gate oxide breakdown on the performance of a conventional 6T SRAM cell is presented for 45-nm predictive technology. The DC margins (read, write, and retention) and access times (read and write) during wearout are analyzed, and a cell breakdown point due to degradation in each of these parameters is defined. A combination of these results is used to formulate a definition for the hard-breakdown point of a cell. An on-chip process, voltage, and temperature tolerant monitoring scheme is proposed to detect the gradual wearout of SRAM cells. The monitoring scheme enables the detection of impending cell failure, which in turn can trigger reconfiguration of the SRAM with redundant rows and/or columns prior to failure. Index Terms-Gate oxide breakdown (GOBD), on-chip monitor, reliability, testing.
The effect of gate oxide breakdown has long been studied in the context of device functional failure in the past. As technology node scales down to Very Deep SubMicron (VDSM) era, such an effect starts to influence the performance and power consumption of digital circuits within their lifetime. Meanwhile, process variability like threshold voltage shift due to e.g., device dopant fluctuation and/or line edge roughness effects also leads to significant shift of the parametric figures for performance and energy of these circuits at sub 100nm era. Further scaling will definitely lead to the co-existence of both effects in a single circuit. In this paper, we present the experimental analysis on the impact combining gate oxide breakdown and process variability on the energy and delay figures of SRAM cell and Sense Amplifier. Hspice simulations at 65nm technology node indicate a significantly larger shift in both energy and delay of these components than in the cases with either single effect when using the thinner oxide found in 45/32 nm technologies. The actual behavior of the circuits under such a situation becomes more difficult to predict and control, thus bringing a huge challenge to a successful design in the VDSM era.
IEEE Transactions on Very Large Scale Integration (VLSI) Systems, 2011
The threshold voltage (TH) drifts induced by negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) weaken PFETs and high-k metal-gate NFETs, respectively. These long-term TH drifts degrade SRAM cell stability, margin, and performance, and may lead to functional failure over the life of usage. Meanwhile, the contact resistance of CMOS device increases sharply with technology scaling, especially in SRAM cells with minimum size and/or sub-ground rule devices. The contact resistance, together with NBTI/PBTI, cumulatively worsens the SRAM stability, and leads to severe SRAM performance degradation. Furthermore, most state-of-the-art SRAMs are designed with power-gating structures to reduce leakage currents in Standby or Sleep mode. The power switches could suffer NBTI or PBTI degradation and have large contact resistances. This paper presents a comprehensive analysis on the impacts of NBTI and PBTI on power-gated SRAM arrays with high-k metal-gate devices and the combined effects with the contact resistance on SRAM cell stability, margin, and performance. NBTI/PBTI tolerant sense amplifier structures are also discussed.
Prediction and Modeling of Thin Gate Oxide Breakdown Subject to Arbitrary Transient Stresses
IEEE Transactions on Electron Devices, 2010
A reliable dielectric breakdown model under transient stresses via an extension of the power law is demonstrated. The model, which is based on the percolation model and the assumption of no significant detrapping, is successfully used in ramped voltage stress breakdown analysis. A demonstration of the model's validity consists of applying repetitive time-variant voltage waveforms-pulses, sine waves, ramps, and noise-until breakdown and, consequently, comparing prediction to reality. The breakdown distribution is initially derived from DC measurements, with the model predicting both the center and the shape of the distribution. Index Terms-Charged device model (CDM), gate oxide breakdown (GOB), power law (PL), time-dependent dielectric breakdown (TDDB), very fast transmission line pulse (VFTLP).
Encountering gate oxide breakdown with shadow transistors to increase reliability
2008
Device scaling has enabled continuous performance increase of integrated circuits. However, severe reliability and yield concerns are arising against the background of nanotechnology. Traditionally, most causes and countermeasures were solely considered manufacturing issues, but lately, we have seen a shift towards operational reliability issues. Though, besides intense research on soft-errors and system-level approaches very little effort is put into low-level design solutions in order to enhance lifetime reliability. Hence, we demonstrate that redundant transistor insertion does improve system reliability significantly as regards Time-Dependent Dielectric Breakdown (TDDB). Furthermore, we introduce an algorithm which identifies the transistors being most vulnerable to TDDB. Subsequently, redundant transistors (called shadow transistors) are inserted at the previously identified instances. Lastly, we argue for applying high threshold voltage devices for the redundant transistors. Finally, we present results for a set of benchmark circuits and prove the combined approach successful. The enhanced designs were on average 41.8 % more reliable compared to the initial designs in respect of TDDB at the price of moderately increased power consumption and delay.
Dielectric breakdown mechanisms in gate oxides
Journal of Applied Physics, 2005
In this paper we review the subject of oxide breakdown ͑BD͒, focusing our attention on the case of the gate dielectrics of interest for current Si microelectronics, i.e., Si oxides or oxynitrides of thickness ranging from some tens of nanometers down to about 1 nm. The first part of the paper is devoted to a concise description of the subject concerning the kinetics of oxide degradation under high-voltage stress and the statistics of the time to BD. It is shown that, according to the present understanding, the BD event is due to a buildup in the oxide bulk of defects produced by the stress at high voltage. Defect concentration increases up to a critical value corresponding to the onset of one percolation path joining the gate and substrate across the oxide. This triggers the BD, which is therefore believed to be an intrinsic effect, not due to preexisting, extrinsic defects or processing errors. We next focus our attention on experimental studies concerning the kinetics of the final event of BD, during which the gate leakage increases above acceptable levels. In conditions of intrinsic BD, the leakage increase is due to the growth of damage within the oxide in localized regions. Observations concerning this damage are reviewed and discussed. The measurement of the current, voltage, and power dissipated during the BD transient are also reported and discussed in comparison with the data of structural damage. We then describe the current understanding concerning the dependence of the BD current transient on the conditions of electric field and voltage. In particular, as the oxide thickness and, as a consequence, the voltage levels used for accelerated reliability tests have decreased, the BD transient exhibits a marked change in behavior. As the stress voltage is decreased below a threshold value, the BD transient becomes slower. This recently discovered phenomenon has been termed progressive BD, i.e., a gradual growth of the BD spot and of the gate leakage, with a time scale that under operation conditions can be a large fraction of the total time to BD. We review the literature on this phenomenon, describing the current understanding concerning the dependence of the effect on voltage, temperature, oxide thickness, sample geometry, and its physical structure. We also discuss the possible relation to the so-called soft oxide BD mode and propose a simpler, more consistent terminology to describe different BD regimes. The last part of the paper is dedicated to exploratory studies, still at the early stages given the very recent subject, concerning the impact on the BD of materials for the metal-oxide-semiconductor gate stack and, in particular, metal gates.
Gate Oxide Wear-Out and Breakdown Effects on the Performance of Analog and Digital Circuits
IEEE Transactions on Electron Devices, 2008
To investigate the impact of gate oxide degradation and breakdown (BD) on complimentary metal-oxidesemiconductor circuit functionality, an accurate description of the electrical characteristics of the stressed devices, which can be included in circuit simulators, is needed. In this paper, a description of the stressed device performance that considers, on the one hand, the variation of the channel current and, on the other, the increase in the gate current due to the oxide degradation and BD is presented, which is able to account for different levels of oxide damage. The parameters extracted from device experimental data have been introduced in a circuit simulator to evaluate the effect of the oxide degradation and BD on simple analog (current mirror) and digital [reset set (RS) latches] circuits. The impact of the increase in the gate leakage current and the variation of the conduction along the metal-oxide-semiconductor field-effect transistor channel due to the oxide degradation on the circuit performances has been separately analyzed.
Gate oxide breakdown in FET devices and circuits: From nanoscale physics to system-level reliability
Microelectronics Reliability, 2007
Gate oxide breakdown has been historically considered a catastrophic failure mechanism for CMOS technology. With CMOS downscaling the mid 1990's have seen the emergence of soft breakdown as a possible failure mode. At the same time the notion started appearing that the first breakdown event does not necessarily spell the immediate failure of the entire CMOS application. Relaxation of the CMOS circuit reliability criteria, however, requires a thorough understanding of the impact of the breakdown path on FET behavior. This cannot be consistently achieved without the microscopic perspective of the physical effects occurring in the affected device. Future CMOS applications will be able to sustain many soft breakdown events, which will be treated as additional parametric variation. Tools ranging from simulation to circuit monitoring will assure reliability at the functional level.
Reliable cache design with detection of gate oxide breakdown using BIST
2009
Scaling of device sizes has reduced gate oxide thickness to a few atomic layers, increasing the vulnerability of the gate oxide to breakdown. During breakdown, devices go through a gradual wearout process after an initial gate leakage increase leading to device failure. It is proposed that if wearout can be monitored, cache arrays with failing cells can be reliably operated after reconfiguration given available memory redundancy. Using experimentally verified gate oxide breakdown models, a detailed analysis of the effect of progressive gate oxide breakdown on the performance of a conventional 6T SRAM cell is presented for 45 nm predictive technology. The DC margin trends (Read, Write and Retention) and access times (Read and Write) during wearout are analyzed, and a cell breakdown point due to degradation in each of these parameters is defined. A combination of these results is used to formulate a practical definition for the hard-breakdown point of a cell. Using an on-chip PVT (process, voltage, and temperature) tolerant monitoring scheme, it has been shown that gradual wearout in SRAM cells, due to gate oxide breakdown, is detectible, and cell failure can be predicted before its occurrence.