Radiation Effects and Soft Errors in Integrated Circuits and Electronic Devices

The following sections are included:

• Preface

• CONTENTS

Single event effects in electronics caused by the atmospheric neutrons have been an issue for systems using large blocks of random access memory (RAM) in avionics applications as well as those on the ground. At ground level there are two main sources of single event effects, alpha particles from the packaging materials as well as the neutrons, but at aircraft altitudes, where the neutron flux is about 300 times higher than the ground, the alpha particles make a negligible contribution. We review the trends over the last 5-10 years in the response of COTS computer systems to single event effects, taking into the response of devices as well as fault tolerant measures incorporated into the systems.

The once-ephemeral soft error has recently caused considerable concern for manufacturers of advanced silicon technology as this phenomenon now has the potential for inducing the highest failure rate of all other reliability mechanisms combined. We briefly review the three radiation mechanisms responsible for causing soft errors in commercial electronics and the basic physical mechanism by which ionizing radiation can produce a soft error. We then focus on the soft error sensitivity trends in commercial DRAM, SRAM, and peripheral logic devices as a function of technology scaling and discuss some of the solutions used for mitigating the impact of soft errors in high reliability systems.

Single-event effects are a serious concern for high-speed III-V semiconductor devices operating in radiation-intense environments. GaAs integrated circuits (ICs) based on field effect transistor technology exhibit single-event upset sensitivity to protons and very low linear energy transfer (LET) particles. The current understanding of single-event effects in III-V circuits and devices, and approaches for mitigating their impact, are discussed.

This paper describes the use of a pulsed laser for studying radiation-induced single-event transients in integrated circuits. The basic failure mechanisms and the fundamentals of the laser testing method are presented. Sample results are presented to illustrate the benefits of using a pulsed laser for studying single-event transients.

Use of a systems engineering process and the application of techniques and methods of fault tolerant systems are applicable to the development of a mitigation strategy for Single Event Upsets (SEU). Specific methods of fault avoidance, fault masking, detection, containment, and recovery techniques are important elements in the mitigation of single event upsets. Fault avoidance through the use of SEU hardened technology, fault masking using coding and redundancy provisions, and solutions applied at the subsystem and system level are available to the system developer. Validation and verification of SEU mitigation and performance of fault tolerance provisions are essential elements of systems design for operation in energetic particle environments.

Modern semiconductor processes can provide significant intrinsic hardness against radiation effects in digital and analog circuits. Current design techniques using commercial processes for radiation-tolerant integrated circuits are summarized, with an emphasis on their application in high performance mixedsignal circuits and systems. Examples of “radiation hardened by design” (RHBD) methodologies are illustrated for reducing the vulnerability of circuits and components to total dose, single-event, and doserate effects.

This paper describes design choices and tradeoffs made when designing total-dose hardness into an advanced CMOS integrated circuit. Closed geometry transistors are described and compared, emphasizing their radiation tolerant performance. Speed and area tradeoffs incurred in circuit design when using such closed geometry transistors are illustrated in the design of an advanced IEEE 1394 cable physical layer mixed-signal interface chip.

With the construction of the Large Hadron Collider at the European Center for Nuclear Research (CERN), the radiation levels at large High Energy Physics (HEP) experiments are significantly increased with respect to past experience. The approach the HEP community is using to ensure radiation tolerance of the electronics installed in these new generation experiments is described. Particular attention is devoted to developments that led to original work: the estimate of the SEU rate in the complex LHC radiation environment and the use of hardness by design techniques to achieve radiation hardness of ASICs in a commercial CMOS technology.

Assessing the risk of using optocouplers in satellite applications offers challenges that incorporate those of commercial off-the-shelf devices compounded by hybrid module construction techniques. In this paper, we discuss these challenges with regard to radiation-induced effects, e.g., single event transients in the photodiodes and displacement damage effects in light emitting diodes. Radiation-induced effects can depend on several important variables including circuit application, radiation environment, annealing, temperature, and others. Despite the ominous list of test variables above, it is possible to make a conservative estimate of optocoupler performance in the space radiation environment. However, these predictions could result in an overestimation of on-orbit performance by an order of magnitude or more.

This review concerns radiation effects in silicon Charge-Coupled Devices (CCDs) and CMOS active pixel sensors (APSs), both of which are used as imagers in the visible region. Permanent effects, due to total ionizing dose and displacement damage, are discussed in detail, with a particular emphasis on the space environment. In addition, transient effects are briefly summarized. Implications for ground testing, effects mitigation and device hardening are also considered. The review is illustrated with results from recent ground testing.

Power MOSFETs are a commonly used device for many switching and power control applications. Their upper frequency limit spans a fairly broad range, from 1 MHz to 10 MHz. These devices are frequently used in spaceborne electronic systems where they encounter radiation exposure during operation. This paper reviews the current technology, its high frequency capability, the future trends for power MOSFET technology, and the degradation that the power VDMOS technology experiences in the space radiation environment.

The context of SOI technology is briefly presented in terms of wafer fabrication, configuration/performance of SOI devices, and operation mechanisms in partially and fully depleted MOSFETs. Typical radiation effects, induced by single particles and cumulated dose, are evoked: BOX degradation, parasitic bipolar action, coupling effects, transistor latch, and back-channel conduction. The future of SOI is tentatively explored, by discussing the further scalability of SOI-MOSFETs as well as the innovating architectures proposed for the ultimate generations of SOI transistors.

We present an overview of radiation effects in silicon-germanium heterojunction bipolar transistors (SiGe HBT). We begin by reviewing SiGe HBTs, and then examine the impact of ionizing radiation on both the dc and ac performance of SiGe HBTs, the circuit-level impact of radiation-induced changes in the transistors, followed by single-event phenomena in SiGe HBT circuits. While ionizing radiation degrades both the dc and ac properties of SiGe HBTs, this degradation is remarkably minor, and is far better than that observed in even radiation-hardened conventional Si BJT technologies. This fact is particularly significant given that no intentional radiation hardening is needed to ensure this level of both device-level and circuit-level tolerance (typically multi-Mrad TID). SEU effects are pronounced in SiGe HBT circuits, as expected, but circuit-level mitigation schemes will likely be suitable to ensure adequate tolerance for many orbital missions. SiGe HBT technology thus offers many interesting possibilities for space-borne electronic systems.

The current gain of irradiated bipolar junction transistors decreases due to increased recombination current in the emitter-base depletion region and the neutral base. This recombination current depends on the interaction of two factors: (1) decreased minority-carrier lifetime at the Si/SiO₂ interface or in the bulk Si and (2) changes in surface potential caused by charge in the oxide. In npn transistors, these two factors both result in increased base current, while in pnp devices, positive charge in the oxide moderates the increase in base current due to surface recombination. In some technologies, the amount of degradation that occurs at a given total dose increases as the dose rate decreases. This enhanced low-dose-rate sensitivity results from space-charge effects produced by slowly transporting holes and protons in the oxide that covers the emitter-base junction.

Electronics systems that operate in space or strategic environments can be severely damaged by exposure to ionizing radiation. Space-based systems that utilize linear bipolar integrated circuits are particularly susceptible to radiation-induced damage because of the enhanced sensitivity of these circuits to the low rate of radiation exposure. The phenomenon of enhanced low-dose-rate sensitivity (ELDRS) demonstrates the need for a comprehensive understanding of the mechanisms of total dose effects in linear bipolar circuits. The majority of detailed bipolar total dose studies to date have focused on radiation effects mechanisms at either the process or transistor level. The goal of this text is to provide an overview of total dose mechanisms from the circuit perspective; in particular, the effects of transistor gain degradation on specific linear bipolar circuit parameters and the effects of circuit parameter degradation on select linear bipolar circuit applications.

An approach to hardness assurance for commercial microelectronics is presented based on hardness assurance guidelines developed by the US Department of Defense in the late 1970s and early 1980s. Modifications are made to accommodate commercial variability in radiation response and frequent changes in design and process.

We have briefly reviewed the most important degradation mechanisms affecting ultra-thin gate oxides after exposure to ionizing irradiation. The increase of the gate leakage current seems the most crucial issue for device lifetime, especially for non-volatile memory and dynamic logic. The build-up of positive charge in the oxide and the subsequent threshold voltage shift, which was the major concern for thicker oxide, are no longer appreciable in today's devices due to the reduced oxide thickness permitting a fast recombination of trapped holes with electrons from interfaces. Among the leakage currents affecting thin oxides we have considered here the Radiation Induced Leakage Current (RILC) and the Radiation Soft Breakdown (RSB). RILC is observed after irradiation with a low Linear Energy Transfer (LET) radiation source and comes from a trap-assisted tunneling of electrons mediated by the neutral traps produced by irradiation. RILC depends on the applied bias during irradiation and the maximum is measured when devices are biased in flat band. Contrarily to RILC, RSB is observed after irradiation with high LET ions and derives from the formation of several conductive paths across the oxide corresponding to the ion hits. RSB conduction is explained by the theory of the Quantum Point Contact as also proposed for the electrically induced Soft breakdown. Finally, we present some preliminary results, which indicate that although the direct effects of irradiation (in terms of gate leakage current increase) are small for oxide thinner than 3nm, it is possible that these devices may experience an accelerated wear-out and/or breakdown after subsequent electrical stress relative to a fresh (not irradiated) device.

Two contrasting behaviors have been observed for H in Si/SiO₂ structures: a) Radiation experiments established that protons released in SiO₂ migrate to the Si/SiO₂ interface where they induce new defects; b) For oxides exposed first to high-temperature annealing and then to molecular hydrogen, mobile positive charge believed to be protons can be cycled to and from the interface by reversing the oxide electric field. First-principles density functional calculations identify the atomic-scale mechanisms for the two types of behavior and conditions that are necessary for each. Using the results of the atomic-scale calculations we develop a model for enhanced interface-trap formation at low dose rates due to space charge effects in the base oxides of bipolar devices. We find that the hole trapping in the oxide cannot be responsible for all the Enhanced Low-Dose-Rate Sensitivity (ELDRS) effects in SiO₂, and the contribution of protons is also essential. The dynamics of interface-trap formation are defined by the relation between the proton mobility (transport time of the protons across the oxide) and the time required for positive-charge buildup near the interface due to trapped holes. The analytically estimated and numerically calculated interface-trap densities are found to be in very good agreement with available experimental data.

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.

A new generation of Optically stimulated materials has been synthesized at the University Montpellier II. The very high sensitivity of these phosphors, the short time constant of the luminescence and the perfectly separated spectra enable many applications in real time and online dosimetry. Dosimetry can be considered as real-time when the dose change between two measurements is considered as low enough. For satellite applications, we have developed an integrated sensor to measure the dose received orbit by orbit. In radiotherapy, OSL has been used to control the dose deposited during intra-operative in-vivo irradiation. At CERN, we have proposed an online system to monitor the dose simultaneously in about one hundred locations inside the Compact Muon Solenoid (CMS) experiment.