Search

Vertical, gigapascal-level mechanical stress (MS) is induced at different locations along the channel of 40-nm effective gate length planar CMOS field-effect transistor (FET) devices and electrical parameter variations are investigated. In both p-and n-channel devices, the threshold voltage, mobility, and ON-current are seen to change proportionally with the additional MS, while gate-induced drain-leakage current increases exponentially. A clear effect of the location of the applied force along the source–drain direction is observed on the transistor parameters. Simulations show that a mechanical load located closer to the FET source induces a stronger asymmetry between the source and drain stresses. This leads to asymmetric subband splitting/warping, which reduces the backscattering rate at the source, in line with theoretical predictions on the importance of the channel barrier near the source for current in quasi-ballistic transistors. [ABSTRACT FROM AUTHOR]

Silicon bandgap limits the reduction of operation voltage when downscaling device sizes. This increases the electrical field within-a-device and hot carrier aging (HCA) is becoming an important reliability issue again for some CMOS technologies. For nanodevices, there are a number of challenges for characterizing their HCA: the random charge–discharge of traps in gate dielectric causes “within-a-device-fluctuation (WDF),” making the parameter shift uncertain after a given HCA. This can introduce errors when extracting HCA time exponents and it will be shown that the lower envelope of the WDF must be used. Nanodevices also have substantial device-to-device variation (DDV) and multiple tests are needed for evaluating their standard deviation ( $\sigma )$ and mean value ( $\mu $ ). Repeating the time-consuming HCA tests is costly and a voltage-step-stress method is applied to reduce the number of tests by 80%. For a given number of devices under tests (DUTs), there is a little information on the accuracy of the extracted $\sigma $ and $\mu $ . We will develop a method to provide this information, based on the defect-centric model. For 40 DUTs with an average of ten traps per device, the extracted $\mu $ and $\sigma $ has an accuracy of ±14% and ±24%, respectively, with a 95% confidence. [ABSTRACT FROM PUBLISHER]

The access transistor of SRAM can suffer both positive bias temperature instability (PBTI) and hot carrier aging (HCA) during operation. The understanding of electron traps (ETs) is still incomplete and there is little information on their similarity and differences under these two stress modes. The key objective of this paper is to investigate ETs in terms of energy distribution, charging and discharging properties, and generation. We found that both PBTI and HCA can charge ETs which center at 1.4 eV below conduction band ( Ec ) of high-k dielectric, agreeing with theoretical calculation. For the first time, clear evidences are presented that HCA generates new ETs, which do not exist when stressed by PBTI. When charged, the generated ETs’ peak is 0.2 eV deeper than that of preexisting ETs. In contrast with the power law kinetics for charging the preexisting ETs, filling the generated ETs saturates in seconds, even under an operation bias of 0.9 V. ET generation shortens device lifetime and must be included in modeling HCA. A cyclic and antineutralization ETs model is proposed to explain PBTI and HCA degradation, which consists of preexisting cyclic ETs, generated cyclic ETs, and antineutralization ETs. [ABSTRACT FROM AUTHOR]

Ge is a candidate for replacing Si, especially for pMOSFETs, because of its high hole mobility. For Si-pMOSFETs, negative-bias temperature instabilities (NBTI) limit their lifetime. There is little information available for the NBTI of Ge-pMOSFETs with Ge/GeO2/Al2O3 stack. The objective of this paper is to provide this information and compare the NBTI of Ge- and Si-pMOSFETs. New findings include: 1) the time exponent varies with stress biases/field when measured by either the conventional slow dc or pulse I-V technique, making the conventional Vg -accelerated method for predicting the lifetime of Si-pMOSFETs inapplicable to Ge-pMOSFETs used in this paper; 2) the NBTI is dominated by positive charges (PCs) in dielectric, rather than generated interface states; 3) the PC in Ge/GeO2/Al2O3 can be fully annealed at 150 ^\circC ; and 4) the defect losses reported for Si sample were not observed. For the first time, we report that the PCs in oxides on Ge and Si behave differently, and to explain the difference, an energy-switching model is proposed for hole traps in Ge-MOSEFTs: their energy levels have a spread below the edge of valence band, i.e., Ev , when neutral, lift well above Ev after charging, and return below Ev following neutralization. [ABSTRACT FROM PUBLISHER]

Due to a significantly reduced negative-bias temperature instability (NBTI), (Si)Ge channel pMOSFETs are shown to offer sufficient reliability at ultrathin equivalent oxide thickness. The intrinsically superior NBTI robustness of the MOS system consisting of a Ge-based channel and a \SiO2/ \HfO2 dielectric stack is ascribed to a reduced availability of interface precursor defects and to a significantly reduced interaction of channel carriers with gate dielectric defects due to a favorable energy decoupling. Owing to this effect, a significantly reduced time-dependent variability of nanoscale devices is also observed. The superior reliability is shown to be process and architecture independent by comparing both our results on a variety of Ge-based device families and published data of other groups. [ABSTRACT FROM PUBLISHER]

The role of trap-assisted tunneling (TAT) in the degradation of the subthreshold swing (SS) in n-type line tunnel field-effect transistors (TFETs) is investigated through the experiments and simulations. A two to fourfold increase in the interface state density is achieved by applying a positive or a negative stress between the gate and the source. The negative stress shows no impact on the SS in spite of nearly fourfold increase in the interface state density. A nearly twofold increase in interface state density and improvement in SS are observed under the application of positive stress. The improvement in SS is attributed to H^+ species released from the Si/SiO2 interface during stress, which moves toward the bulk Si, passivating boron and bulk Si traps, thereby improving the SS. Under negative stress bias, the released H^+ species drifts toward the gate electrode, and hence no change in SS was observed. These experiments suggest that the SS degradation is mainly caused by TAT through bulk Si traps and insensitive to interface traps. A good control of bulk semiconductor trap density will be required to achieve sub-60-mV/decade SS in line TFETs. [ABSTRACT FROM PUBLISHER]

We report extensive experimental results of the negative bias temperature instability (NBTI) reliability of SiGe channel pMOSFETs as a function of the main gate-stack parameters. The results clearly show that this high-mobility channel technology offers significantly improved NBTI robustness compared with Si-channel devices, which can solve the reliability issue for sub-1-nm equivalent-oxide-thickness devices. A physical model is proposed to explain the intrinsically superior NBTI robustness. [ABSTRACT FROM AUTHOR]

The time-dependent variability of nanoscaled \Si0.45 \Ge0.55 pFETs with varying thicknesses of the Si passivation layer is studied. Single charge/discharge events of gate oxide defects are detected by measuring negative bias-temperature instability (NBTI)-like threshold voltage (Vth) shift relaxation transients. The impact of such individually charged defect on device Vth is observed to be exponentially distributed. SiGe channel devices with a reduced thickness of their Si passivation layer show a reduced average number of active defects and a reduced average impact per charged defect on device Vth. Our model for the superior reliability of the SiGe channel technology previously proposed in Part I, which is based on the energy decoupling between channel holes and dielectric defects, is shown to also explain these experimental observations. Other reliability mechanisms, such as \1/f noise, body biasing during NBTI, channel hot carriers, and time-dependent dielectric breakdown, are also investigated. None of these mechanisms are observed to constitute a showstopper for the reliability of this promising novel technology. [ABSTRACT FROM AUTHOR]

New insights into the negative/positive bias temperature instability (N/PBTI) degradation mechanisms in the sub-1-nm equivalent oxide thickness (EOT) regime are presented in this paper. The electric field requirements suggested by the International Roadmap for Semiconductors demand an even higher value in the sub-1-nm-EOT regime, which is practically difficult to meet with the increased hole trapping mechanism involved. Thus, a fixed electric field target of 5 MV/cm is considered as well here, which might be a reasonable target to achieve. The sub-1-nm-EOT devices in this paper are obtained by adopting a thinner TiN metal gate inducing Si in-diffusion and reducing the interfacial oxide layer thickness. NBTI degradation follows an isoelectric field model in over an EOT of 1 nm due to the degradation mechanism of \Si/SiO2 interface state generation combined with a hole trapping mechanism. However, in the sub-1-nm-EOT regime, the probability of hole trapping into the gate dielectric increases, and it is strongly dependent on the thickness of the interfacial oxide layer. Several experimental proofs of this increased bulk defect effect are shown in this paper. In addition, the bulk defect affecting NBTI is shown to be mostly a preexisting defect, although the permanently generated defects are relatively higher in sub-1-nm-EOT devices. Therefore, NBTI in the sub-1-nm-EOT regime faces the lifetime limit by both electric field dependence and increased degradation by increased hole trapping into bulk defects. Further, we found a minimum interfacial layer thickness of 0.4 nm that is required to prevent the accelerated NBTI degradation by increased direct tunneling. The main degradation mechanism of PBTI in sub-1-nm EOT is the electron trapping into bulk defects, which is the same as in over 1-nm-EOT devices. This enables us to modulate the bulk defect energetic locations in the oxide and to improve PBTI. [ABSTRACT FROM AUTHOR]

In this paper, the potential of Silicon-Germanium (SiGe) technology for VLSI logic applications is investigated from a circuit perspective for the first time. The study is based on experimental measurements on 45-nm SiGe pMOSFETs with a high-\kappa/metal gate stack, as well as on 45-nm Si pMOSFETs with identical gate stack for comparison. In the reference SiGe technology, an innovative technological solution is adopted that limits the SiGe material only to the channel region. The resulting SiGe device merges the higher speed of the Ge technology with the lower leakage of the Si technology. Appropriate circuit- and system-level metrics are introduced to identify the advantages offered by SiGe technology in VLSI circuits. Analysis is performed in the context of next-generation VLSI circuits that fully exploit circuit- and system-level techniques to improve the energy efficiency through aggressive voltage scaling, other than low-leakage techniques. Analysis shows that the SiGe technology has more efficient leakage-delay and dynamic energy-delay trade-offs at nominal supply, compared to Si technology. Moreover, it is shown that the traditional analysis performed at nominal supply actually underestimates the benefits of SiGe pMOSFETs, since the speed advantage of SiGe VLSI circuits is further emphasized at low voltages. This demonstrates that SiGe VLSI circuits benefit from aggressive voltage scaling significantly more than Si circuits, thereby making SiGe devices a very promising alternative to Si transistors in next-generation VLSI systems. [ABSTRACT FROM PUBLISHER]

Extraction of interfacial trap density Nit in extremely reduced gate oxides with equivalent oxide thickness (EOT) below 1 nm by conventional charge pumping is virtually impossible due to the high gate leakage current through the very thin oxide. However, interface quality assessment in subnano EOT devices is essential for the reliability and performance improvement of future logic devices. In this paper, an accurate approach to determine the interfacial trap density in a 5.8- \\rm\AA EOT device is performed by an advanced charge pumping technique employing ring-oscillator-connected devices. A consistency comparison of this technique to the conventional charge pumping is done by a frequency sweep on the 10.1-\\rm \AA EOT device. Clear charge pumping currents are obtained on the 5.8- \\rm\AA EOT oxide, and further analysis by varying the applied frequency and amplitude is performed. The interface trap density in the 5.8-\\rm\AA EOT device is found to be higher than that in the 10.1- \\rm\AA EOT device due to the physically reduced interfacial layer in the thinner EOT device. Moreover, direct tunneling-based calculation gives the charge injection distance as about 2 \\rm\AA inside the oxide. Stress-induced defect generation is investigated by applying dc stress between charge pumping and Idrain - Vgate measurements. The 5.8- \\rm\AA EOT device shows higher initial Nit but lower stress-induced Nit as compared with the 10.1- \\rm\AA EOT device. The bulk trap Not generated after stress is higher in the 5.8- \\rm\AA EOT device due to the higher initial bulk trap density. [ABSTRACT FROM AUTHOR]

The off-state degradation of n-channel laterally diffused metal–oxide–semiconductor (MOS) silicon-controlled-rectifier electrostatic-discharge (ESD) devices for high-voltage applications in standard low-voltage complementary MOS technology is studied. Based on experimental data and technology computer-aided design simulations, impact ionization induced by conduction-band electrons tunneling from an \n^+ poly-Si gate to an n-well is identified to be the driving force of device degradation. Device optimization is proposed, which improves both off-state and ESD reliability. [ABSTRACT FROM AUTHOR]

Negative bias temperature instability (NBTI) is a serious reliability concern for pMOS transistors. Although discovered more than 40 years ago, the phenomenon remains highly controversial in both experimental and theoretical terms. A considerable number of recent publications suggest that NBTI is caused by the following two independent mechanisms: 1) generation of defects close to the silicon/silicon dioxide interface and 2) hole trapping in the oxide. We have performed stress and recovery experiments at different temperatures and voltages that, quite surprisingly, reveal that all data can be scaled onto a single universal curve during stress, recovery, and restress. This suggests that in our samples, NBTI is caused by either a single dominant mechanism, which is more complicated than previously anticipated, or that the previously suggested mechanisms, i.e., hole trapping and defect creation, are actually tightly coupled. [ABSTRACT FROM AUTHOR]

A comparative study of the reliability issues of triple-gate and planar FETs processed on the same silicon-on-insulator wafer is presented. It is shown that the triple-gate architecture does not alter the behavior of the time-dependent dielectric breakdown (BD) for different gate voltages and temperatures. The apparent higher Weibull slope observed in planar devices with respect to the triple-gate devices is ascribed to the area dependence of the time-to-BD detection. In spite of the different surface orientations, low-frequency noise measurements indicate similar values of the interface trap density for triple-gate and planar FETs. [ABSTRACT FROM AUTHOR]

The high hole mobility of Ge makes it a strong candidate for end of roadmap pMOSFETs and low interface states have been achieved for the Al2O3/GeO2/Ge gate-stack. This structure, however, suffers from significant negative bias temperature instability (NBTI), dominated by positive charge (PC) in Al2O3GeO2. An in-depth understanding of the PCs will assist in the minimization of NBTI and the defect energy distribution will provide valuable information. The energy distribution also provides the effective charge density at a given surface potential, a key parameter required for simulating the impact of NBTI on device and circuit performance. For the first time, this letter reports the energy distribution of the PC in Al2O3GeO2 on Ge. It is found that the energy density of the PC has a clear peak near Ge Ec at the interface and a relatively low level between Ec and Ev. Below Ev at the interface, it increases rapidly and screens 20% of the Vg rise. [ABSTRACT FROM PUBLISHER]

Negative bias temperature instability (NBTI) reliability in p-FinFET devices is studied with respect to the silicon substrate orientation. Interface trap density Nit is lower in the 45^\circ rotated devices compared with the 0^\circ rotated devices because of lower density of Si dangling bond at the (100) side walls than the (110) side walls. This improves NBTI reliability in the 45^\circ rotated FinFET devices. Furthermore, we demonstrate that the lower inversion charge density Ninv—exhibited when transitioning from planar to FinFET architecture at 45^\circ rotation—plays an important role in the whole NBTI degradation. NBTI clearly improves in the 45^\circ rotated FinFET devices compared with the planarlike device because of the lower Ninv. Leakage current density analysis is shown as an experimental proof, in addition to simulation results of Cho [ABSTRACT FROM AUTHOR]

The reliability of ultra-thin silicon oxynitride gate dielectrics was investigated in terms of the Weibull slope of the tbd distributions, and the voltage acceleration factor. High field tBD statistics were extrapolated using a worst-case linear fit, to the low field region. The samples showed low voltage acceleration, which degrades reliability, but this is overcome by high Weibull slopes (β), which improve reliability. The high β values were investigated by monitoring the change in β as gate current increased during CVS. We saw the Weibull slope change from ≈1 at low current to 2 at high current, indicating that breakdown results from 2-trap conduction. Charge pumping measurements showed that the trap generation rate at the SiON/Si interface, where most of the Nitrogen resides was very low. Thus it was concluded that there is a much higher trap generation rate in the bulk than at the interface and this is responsible for the values of β. [Copyright &y& Elsevier]