for more information, contact Kenneth A. LaBel
Radiation damage to on-board electronics may be separated into two categories: total ionizing dose and single event effects. Total ionizing dose (TID) is a cumulative long-term degradation of the device when exposed to ionizing radiation. Single event effects (SEEs) are individual events which occur when a single incident ionizing particle deposits enough energy to cause an effect in a device.
There are many device conditions and failure modes due to SEE, depending on the incident particle and the specific device. It may be convenient to think of two types of SEEs: soft errors and hard errors. Soft errors are nondestructive to the device and may appear as a bit flip in a memory cell or latch, or as transients occurring on the output of an I/O, logic, or other support circuit. Also included are conditions that cause a device to interrupt normal operations and either perform incorrectly or halt. Hard errors may be (but are not necessarily) physically destructive to the device, but are permanent functional effects. Different device effects, hard or soft, may or may not be acceptable for a given design application.
Unlike TID degradation, SEE rates are not evaluated in terms of a time or dose until failure, where the stopwatch begins at launch, but a probability that an SEE will occur within a known span of time. Devices are tested in ground test facilities to characterize the device in a radiation environment. Calculations are also performed to predict the radiation environment for a particular mission orbit. Environment predictions are used with the experimental device data to calculate the probability of occurrence of SEEs in the device for the mission.
Device failure is, of course, of great concern. The effects of propagation of SEEs through a circuit, subsystem, and system are also often of particular importance. The level of impact on the affected circuit, box, subsystem, etc. depends on the type and location of the SEE, as well as on the design. For example, a device error or failure may have effects propagating to critical mission elements, such as a command error affecting thruster firing. There are also cases where SEEs may have little or no observable effect on a system level. In fact, in most designs, there are specific areas which have less system impact from certain radiation effects. The data storage memory in a solid state recorder, for example, may have error detection and correction coding (EDAC) which makes bit errors in the devices transparent to the system. Evaluating the severity of the single event effect hazard involves knowledge from several technical fields including radiation physics, parts engineering, solid state physics, electrical engineering, reliability analysis, and systems engineering.
Both the functional impact of an SEE to the system or spacecraft and the probability of its occurrence provide the foundation for setting a design requirement. System-level SEE requirements may be fulfilled through a variety of mitigation techniques, including hardware, software, and device tolerance requirements. The most cost efficient approach may be an appropriate combination of SEE-hard devices and other mitigation. However, the availability, power, volume, performance, and cost of radiation-hardened devices prohibits their use. Hardware or software design also serve as effective mitigation, but design complexity may present a problem. A combination of the two may be the selected option.
Terms and Definitions:
Single Event Upset (SEU) is a change of state or transient induced by an ionizing particle such as a cosmic ray or proton in a device. This may occur in digital, analog, and optical components or may have effects in surrounding circuitry. These are "soft" bit errors in that a reset or rewriting of the device causes normal behavior thereafter.
Single Hard Error (SHE) is an SEU which causes a permanent change to the operation of a device. An example is a permanent stuck bit in a memory device.
Single Event Functional Interrupt (SEFI) is a condition where the device stops normal functions, and usually requires a power reset to resume normal operations. It is a special case of SEU changing an internal control signal.
Single Event Latchup (SEL) is a potentially destructive condition involving parasitic circuit elements. In traditional SEL, the device current may exceed device maximum specification and destroy the device if not current limited. A "microlatch" is a subset of SEL where the device current remains below the maximum specified for the device. A removal of power to the device is required in all non-catastrophic SEL conditions in order to recover device operations.
Single Event Burnout (SEB) is a highly localized burnout of the drain-source in power MOSFETs. SEB is a destructive condition.
Single Event Gate Rupture (SEGR) is the burnout of a gate insulator in a power MOSFET. SEGR is a destructive condition.
Linear Energy Transfer (LET) is a measure of the energy transferred to the device per unit length as an ionizing particle travels through a material. The common unit is MeV*cm2/mg of material (Si for MOS devices).
LET threshold (LETth) is the minimum LET to cause an effect. The JEDEC recommended definition is the first effect when the particle fluence = 1x107 ions/cm2.
Cross section (sigma) is the device SEE response to ionizing radiation. For an experimental test for a specific LET, sigma = #errors/ion fluence. The units for cross section are cm2 per device or per bit.
Asymptotic or saturation cross section (sigmasat) is the value that the cross section approaches as LET gets very large.
Sensitive volume refers to the device volume affected by SEE-inducing radiation. The geometry of the sensitive volume is not easily known, but some information is gained from test cross section data.
1. The SEE Problem
2. Functional Analysis and Criticality
3. Ionizing Radiation Environment Concerns
4. Effects in Electronic Devices and SEE Rates
5. SEU Propagation Analysis: System Level Effects
6. SEE Mitigation: Methods of Reducing SEE Impacts
7. Managing SEEs: System Level Planning
8. SEE Criticality Assessment Case Studies