Radiation-induced Charge Trapping Research Articles

Effects of Fast Neutrons on the Electromechanical Properties of Materials Used in Microsystems

Microelectromechanical systems (MEMS) technology is expected to play a significant role in avionics and in future space missions provided that the effect of radiation on the reliability of MEMS devices is well understood. Most tests, to date, have been performed on MEMS that operate based on electrostatic principles, and radiation-induced dielectric charging has been identified as a mechanism which can potentially limit the reliability of such devices. Experiments with piezoresistive and piezoelectrically actuated MEMS have also revealed changes in the operation of devices after exposure to different types of ionizing radiation. However, not many reports on the damage effects in MEMS due to fast neutrons exist. In this paper, we present results from mechanical and electrical tests on different materials which are typically used in microsystems, before and after exposure to fast neutrons. Changes were detected in the residual stress and in the resonant frequencies of silicon nitride microbeams. There was also evidence of radiation-induced charge trapping in metal-insulator-semiconductor capacitors fabricated from thermal oxide, silicon nitride, and aluminum oxide deposited by atomic layer deposition. Finally, we observed that the electrical properties of ferroelectric capacitors fabricated from lead zirconate titanate and lead strontium titanate thin films do not degrade after exposure to fast neutrons.

Comprehensive study on the TID effects of 0.13 μm partially depleted SOI NMOSFETs

Total ionizing dose (TID) effects of 0.13μm partially depleted (PD) SOI NMOSFETs are overviewed in this paper. For core devices with floating body, the 2nd peak of the front-gate transconductance (gm) degrades with incremental TID. Moreover, the 1st and 2nd peak of the drain current (Ids) hysteresis also declines due to the increased recombination current at the back-gate interface after irradiation. The front-gate subthreshold hump effect is more obvious under the OFF (off-state) and TG (transmission gate) bias conditions. By contrast, the back-gate subthreshold hump exhibits a maximum negative shift under the ON (on-state) bias condition and causes the most significant hump effect owing to the largest charge accumulation at the bottom of the shallow trench isolation (STI) oxide. The maximum negative shift of the back-gate threshold voltage is observed under the TG bias condition, leading to the coupling effect. Finally, for narrow-channel devices, it is found that the radiation-induced positive trapped charge in the STI oxide will degrade the electron mobility in the front-gate channel region, leading to a gm decrease because of the increased surface roughness scattering.

Bias dependence of TID radiation responses of 0.13 μm partially depleted SOI NMOSFETs

The bias dependence of total ionizing dose (TID) radiation responses of 0.13μm Partially Depleted (PD) SOI NMOSFETs is presented. Experiments and three dimensional (3D) simulations are used to analyze the buildup of trapped charge in the STI oxide as well as in the buried oxide (BOX) and its impact on the negative shift of back-gate threshold voltage and subthreshold hump. It is demonstrated that the worst-case bias conditions for the largest back-gate threshold voltage shift and increase of the off-state leakage current (or the negative shift of back-gate subthreshold hump) are different. The radiation-induced positive trapped charge in the BOX can fully deplete the body silicon for input/output (I/O) NMOSFETs as a result of the lower body doping concentration, giving rise to the coupling effect between the front-gate and back-gate threshold voltage shift.

Enhanced Radiation-Induced Narrow Channel Effects in Commercial <formula formulatype="inline"><tex Notation="TeX">${\hbox {0.18}}~\mu$</tex> </formula>m Bulk Technology

Total ionizing dose effects are investigated in input/output transistors that are fabricated by using a commercial 0.18 μm bulk process. An enhanced radiation-induced narrow channel effect is demonstrated in N-type metal-oxide semiconductor (NMOS) and P-type metal-oxide semiconductor (PMOS) transistors, leading to a significant threshold voltage shift which may compromise circuit operations. Calculations using a code dedicated to radiation-induced charge trapping in oxides show that the radiation-induced positive charge trapping in trench oxides leads to the modifications of the electrical characteristics experimentally evidenced. Radiation hardening issues are finally discussed as a function of the device geometry and design.

Proton Irradiation Effects on Resistive Random Access Memory With ZrO$_{\rm x}$/HfO$_{\rm x}$ Stacks

In this study, we investigated proton irradiation effects on resistive random access memory (ReRAM) comprising ZrO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$_{\rm x}$</tex></formula> /HfO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$_{\rm x}$</tex></formula> stacks. After irradiation, changes of current were observed in the initial state (IS). From the electrical conduction mechanism in the IS, we have concluded that the different initial conditions of the active layer lead to different radiation effects. The radiation-induced leakage paths have been concluded as main origin of the increased leakage current, whereas radiation-induced charge trapping is dominant fact of the decreased leakage current in the IS. From the results of noise analysis in the low resistance state (LRS) and high resistance state (HRS), we observed that the radiation effects became negligible because of the formed local conducting path during forming process.

Effect of Ionizing Radiation on Defects and $1/f$ Noise in Ge pMOSFETs

The irradiation and annealing responses of Ge pMOSFETs have been investigated under transmission gate bias. Both the radiation-induced charge trapping and the low frequency (1/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> ) noise increase with total ionizing dose and decrease with annealing time. The smallest increases in noise after irradiation are observed for Ge pMOSFETs with the lowest halo implantation doses. The smallest increases in oxide and interface trap charge densities are obtained for devices with eight monolayers of Si at the interface, as compared to devices with five Si monolayers.

Total ionizing dose effect of 0.18 m nMOSFETs

A 0.18 m MOSFET with shallow trench isolation is exposed to a -ray radiation. The parameters such as off-state leakage current, threshold voltage, transconductance, gate leakage current, and subthreshold slope are analyzed for pre- and post-irradiation. By introducing constant sheet charges at the shallow trench isolation oxide sidewall, good agreement between 3D simulation and experiment result is demonstrated. We believe that the thin gate oxide is insensitive to radiation, and the radiation induced charge trapping in the shallow trench isolation still leads to macroscopic effects such as drain-to-source leakage current, ultimately limiting the tolerance of CMOS circuits.

Radiation sensitivity of microelectromechanical system devices

The sensitivity of microelectromechanical system (MEMS) devices to radiation is reviewed, with an emphasis on radiation levels representative of space missions rather than of operation in nuclear reactors. As a purely structural material, silicon has shown no mechanical degradation after radiation doses in excess of 100 Mrad. MEMS devices, even when excluding control/readout electronics, have, however, failed at doses of only 20 krad, though some devices have been shown to operate correctly for doses greater than 10 Mrad. Radiation sensitivity depends strongly on the sensing or actuation principle, device design, and materials, and is linked primarily to the impact on device operation of radiation-induced trapped charge in dielectrics. MEMS devices operating on electrostatic principles can be highly sensitive to charge accumulation in dielectric layers, especially for designs with dielectrics located between moving parts. In contrast, thermally and electromagnetically actuated MEMS are much more radiation tolerant. MEMS operating on piezoresitive principles start to slowly degrade at low doses, but do not fail catastrophically until doses of several Mrad. A survey of all published reports of radiation effects on MEMS is presented, as well as a summary of techniques that can improve their radiation tolerance.

Effect of thermal annealing on charge exchange between oxygen interstitial defects within HfO2 and oxygen-deficient silicon centers within the SiO2/Si interface

We compare the charging response of rapid thermally annealed (800 and 1000 °C) 4 nm thick HfO2 to as-deposited HfO2 on Si by measuring the surface potential of the HfO2 layers after vacuum ultraviolet (VUV) irradiation with 11.6 eV photons. From VUV spectroscopy, we determined all HfO2 layers show the presence of oxygen-interstitial defects (OIDs). The electronic states of OID in HfO2 line up in energy with oxygen-deficient Si centers within the SiO2 interfacial layer. This implies charge exchange between OIDs within HfO2 and the O-deficient silicon centers within the SiO2 interfacial layer are very important for controlling the radiation-induced trapped charge in HfO2 dielectric stacks.

Total Ionizing Dose Effects on Strained ${\rm HfO}_{2}$-Based nMOSFETs

Radiation-induced charge trapping and mobility degradation are measured on uniaxially stressed HfO2-based nMOSFETs. Controlled external mechanical stress is applied via a four-point bending jig while the samples are irradiated using 10-keV X-rays. Positive charge trapping is observed for unstressed devices, and for devices irradiated under both compressive and tensile stress. Reduced trapped charge is measured as the uniaxial stress level increases. These results suggest that the increased stress leads to a reconfiguration of defect microstructure, as compared to the unstressed devices. The reconfiguration consists of changes in bond lengths and angles and a corresponding change in trap energy levels, which can (1) reduce the probability that a defect can capture holes, (2) increase the probability for electron and trapped-hole recombination, and/or (3) increase the mobility of the transporting holes in the oxides of the devices that are irradiated under stress. These results show that increased mechanical stress in high-k dielectrics does not degrade their radiation hardness. We also observe that the post-irradiation channel mobility degrades less in uniaxially stressed devices than unstressed devices.

Channel Hot Carrier Stress on Irradiated 130-nm NMOSFETs

We investigate how X-ray exposure impact the long term reliability of 130-nm NMOSFETs as a function of device geometry and irradiation bias conditions. This work focuses on electrical stresses on n-channel MOSFETs performed after irradiation with X-ray up to 136 Mrad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) in different bias conditions. Irradiation is shown to negatively affect the degradation during subsequent hot carrier injection. Increasing the bias during irradiation slightly reduces the impact on following electrical stress in core MOSFETs. Through device simulations, we attribute these effects to an enhanced impact ionization at the bulk-STI interfaces due to radiation-induced trapped charge and defects.

Radiation Induced Charge Trapping in Ultrathin ${\rm HfO}_{2}$-Based MOSFETs

Radiation induced charge trapping in ultrathin HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based n-channel MOSFETs is characterized as a function of dielectric thickness and irradiation bias following exposure to 10 keV X-rays and/or constant voltage stress. Positive and negative oxide-trap charges are observed, depending on irradiation and bias stress conditions. No significant interface-trap buildup is found in these devices under these irradiation and stress conditions. Enhanced oxide-charge trapping occurs in some cases for simultaneous application of constant voltage stress and irradiation, relative to either type of stress applied separately. Room temperature annealing at positive bias after irradiation of transistors with thicker gate dielectric films leads to positive oxide-trapped charge annihilation and/or neutralization in these devices, and net electron trapping. The oxide thickness dependence of the radiation response confirms the extreme radiation tolerance of thin HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectric layers of relevance to device applications, and suggests that hole traps in the thicker layers are located in the bulk of the dielectric. A revised methodology is developed to estimate the net effective charge trapping efficiency, f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ot</sub> , for high-kappa dielectric films. As a result, estimates of f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ot</sub> for Hf silicate capacitors and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> transistors in previous work are reduced by up to 18%.

Total Dose and Bias Temperature Stress Effects for HfSiON on Si MOS Capacitors

We have performed an experimental study of the effects of ionizing radiation and bias-temperature stress on Si MOS devices with HfSiON gate dielectrics. We compare the responses of homogeneous high-SiN films and low-SiN films that contain crystalline HfO. We observe that the low-SiN films are more sensitive to ionizing radiation than the high-SiN films. In particular, the low-SiN film that includes crystalline HfO is especially vulnerable to electron trapping due to substrate injection under positive irradiation bias. Both film types exhibit reduced radiation-induced charge trapping relative to previous Hf silicates. The high-SiN film exhibits less radiation-induced net oxide-trap charge density than earlier Hf silicate films processed without nitride. We also find that these devices are relatively robust against bias-temperature stress instabilities. Consistent with the radiation response, the low-SiN devices also display elevated levels of charge trapping relative to the high-SiN devices during bias-temperature stress.

Electrical stress on irradiated thin gate oxide partially depleted SOI nMOSFETs

The effect of hot-carrier stress on 60 MeV proton irradiated thin gate oxide partially depleted SOI nMOSFETs has been studied. The results are compared with those from the electrical stress of non-irradiated devices. Whereas no significant differences are observed for the front channel degradation, hot-electron trapping in the buried oxide is found to be enhanced in the irradiated devices. This hot-electron trapping leads to a compensation or neutralization of the effects caused by the radiation-induced positive trapped charges. It is shown that a similar hot-electron trapping enhancement can be achieved in non-irradiated devices stressed under certain back gate bias conditions.

Electrical bias stressing and radiation induced charge trapping in HfO2/SiO2 dielectric stacks

Electrical bias stressing and X irradiation experiments have been carried out on HfO2/SiO2 dielectric stacks. Evidence is found for a negative electric field induced positive charge injection and trapping and radiation induced positive charge trapping. Positive electric fields associated with radiation result in negative charge trapping. Potential-induced cyclic injection/removal of positive charge is also observed. It is suggested that these structures are radiation soft.

Modelling the Back-Gate Coupling effect in Triple-, Π- and Ω-Gate FETs

In multiple-gate devices, coupling effects due to the 3-D architecture modify the electrical characteristics of the transistor. In this paper, electrical measurements have been carried out in order to examine the role of the back-gate voltage. In agreement with numerical simulations, it is shown that the back-gate influence is reduced for narrow devices due to a strong coupling between the different faces of the main gate. In narrow Fin devices, the Pi- and Omega-shapes of the gate provide an excellent electrostatic control of the field lines and potential, both in the buried oxide and in the silicon channel. The radiation-induced charge trapping in the BOX underneath the channel can be then much limited: narrow Fin devices exhibit a higher tolerance to ionizing radiations than its single-gate counterparts. Additionally, it is shown that the 2D dimensional coupling in these structures can be analytical calculated for Triple- and Pi-gate FETs. These models are compared with numerical simulations and show a very good agreement.

Differences Between Charge Trapping States in Irradiated Nano-Crystalline HfO$_{2}$ and Non-Crystalline Hf Silicates

This paper provides an explanation for qualitative differences between radiation-induced charge trapping states in nano-crystalline HfO 2 and non-crystalline Hf silicate alloys in high-kappa gate stacks by combining electrical measurements with spectroscopic studies and theory. Differences in the observed electrical response to X-ray and gamma-ray irradiation are consistent with fundamental differences in electronic structures between high-kappa dielectrics that are nano-crystalline and have a film thickness in excess of 4 nm, and high-kappa dielectrics that are non-crystalline and devoid of grain boundaries. Oxygen vacancy and interstitial defects are shown to be natural candidates for the electron and hole traps in these high-kappa dielectrics

Optically stimulated luminescence in forensics

Tracking the former location of a radioactive source represents an attractive attribute in today's society, where the threat of a radiological dispersal device (RDD) is considered high. Development of a novel, retrospective detection technique based on optically stimulated luminescence is underway. This will allow confirmation of the former location of a radioactive source. Materials near the source's former location may be probed for information on accumulated dose by taking advantage of radiation-induced charge trapping that occurs in many common materials. The prototype detector design and analysis techniques are described.

Impact of Radiation-Induced Back-Channel Leakage and Back-Gate Bias on Drain Current Transients of Thin-Gate-Oxide Partially Depleted Silicon-On-Insulator n-channel Metal–Oxide–Semiconductor Field-Effect Transistors

In this study, we analyze the impact of back-channel radiation-induced leakage and back-gate bias on switch-off drain current transients of thin-gate-oxide partially depleted (PD) silicon-on-insulator (SOI) n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs). The presence of radiation-induced positive trapped charges in the buried oxide after 60 MeV proton irradiation is found to reduce the “switch-off” transient times for gate voltages above and below the front-gate threshold voltage for body-to-gate electron valence band tunnelling. An increase in steady-state drain current and an increase in the amplitude of weak inversion drain current transients are observed. A similar effect is observed when applying a positive bias to the back-gate, which is found to generate an “irradiation-like” subthreshold leakage. The observed switch-off drain current transient behavior is explained by taking into account an edge parasitic back-channel transient component, which is added to the conventional front-gate drain current transient.

SWITCHING OXIDE TRAPS

Positive oxide trapped charge is one of the main factors determining the radiation response of a CMOS device. The most widely accepted model for oxide-trapped charge is the dipole model, originally proposed by Lelis et al. The annealing of radiation-induced positive trapped charge proceeds (usually) via the tunneling of electrons, which form metastable dipoles, compensating the trapped positive charge without removing it. Under appropriate bias, these compensating electrons can tunnel back to the Si substrate, restoring the trapped positive charge. The experimental work leading to the development of this model is summarized. By now there is a large body of experimental and theoretical work by others, confirming and extending the original model. In particular, the relevance of the model to some electron trapping studies has been shown, and its application to the larger topic of oxide reliability is discussed.