Space Radiation Effects For Future Technologies and Missions

by:

Professor Clive S Dyer, Chief Scientist (Space), QinetiQ Space Department, UK Dr Gordon R Hopkinson, Project Manager, Sira Electro-Optics Ltd, UK

(Report reference QINETIQ/KI/SPACE/TR010690/1.1)

Abstract

A brief review is presented of radiation effects in microelectronics and electrical systems, sensors and detectors (including optoelectronic sensors), materials and astronaut crews, as well as current methods for quantifying the effects. Trends in technologies and future mission requirements are examined in order to identify the highest priority areas for the development of a modelling capability.

1 Introduction

1.1 Contractual

This report has been produced by QinetiQ Space Department and Sira Electro-Optics Ltd for the European Space Agency; the research described was conducted under Contract Number 14968/00/NL/EC (ESA Technology Research Programme, Space Environment and Effects Major Axis), and this document is issued in fulfilment of milestone 1.

1.2 Purpose

This document provides a review of the potential space radiation effects to microelectronics, optoelectronics, electrical systems, sensors/detectors, materials and astronaut crews in order to identify the highest priority areas for the development of tools in later work packages. In particular, the trends in technology and space mission requirements are examined and the gaps in our current understanding of radiation effects to address future systems identified. Priority is assigned to identify those components and systems where there is least understanding of and ability to predict potential effects, and high relevance to future space missions.

1.3 Content

The remaining chapters of this report cover the following topics:

• Chapter 2 provides a summary of the space radiation environment; • Chapter 3 describes in overview of basic mechanisms for radiation effects on components,

systems, materials and crews; • Chapter 4 identifies the general trends in spacecraft mission; • Chapter 5 summarises the trends in technology that may affect future space system susceptibility.

1.4 Scope

The emphasis of this report is towards radiation effects on spacecraft equipment for future missions, with the exception of specialist scientific equipment. It is intended that the latter be covered in greater detail under a separate ESA-sponsored study.

2 The space radiation environment

2.1 Introduction

Recent reviews of space radiation environment models and observations have been given by Barth [1] and Dyer [2] respectively. Essential features are summarised below.

2.2 Cosmic rays

The Earth's magnetosphere is bombarded by a nearly isotropic flux of energetic charged particles, primarily the nuclei of atoms stripped of all electrons. These comprise 85% protons (hydrogen nuclei),

14% α -particles or helium nuclei, and 1% heavier ions covering the full range of elements, some of the more abundant being, for example, carbon and iron nuclei. They are partly kept out by the Earth's magnetic field and have easier access at the poles compared with the equator. From the point of view of space systems it is particles in the energy range 1-20 GeV/nucleon which have most influence. An important quantity is the rigidity of a cosmic ray which measures its resistance to bending in a magnetic field and is defined as the momentum-to-charge ratio for which typical units are GV. The radius of curvature of the particle is then the ratio between its rigidity and the magnetic field. At each point on the Earth it is possible to d efine a threshold rigidity or cut-off as a function of direction which a particle must exceed to be able to arrive from that direction. Values vary from 0GV at the poles to about 17GV at the equator. The influence of solar cycle is to provide a modulation in antiphase with the sunspot cycle and with a phase lag which is dependent on particle energy. In addition, short term "Forbush decreases" lasting several days can be produced by fast solar wind streams and/or coronal mass ejections.

2.3 Radiation belts

The space-flight of a radiation monitor in 1958 showed unusual regions of high counts and detector saturation which Van Allen identified as regions of radiation trapped in the Earth's magnetic field. Subsequent research showed that these divide into two belts, an inner belt extending to 2.5 Earth radii and comprising energetic protons up to 600MeV together with electrons up to several MeV, and an outer belt comprising mainly electrons extending out to 10 Earth radii. In addition the outer belt has a component of soft protons (0.1 to 5 MeV). The slot region between the belts has lower intensities but may be greatly enhanced for up to a year following one or two solar events in each solar cycle. The outer belt is naturally highly time-variable and is driven by solar wind conditions.

The Earth's atmosphere removes particles from the radiation belts and low Earth orbits (LEO) can be largely free of trapped particles. However because of the displacement of the dipole term in the geomagnetic field away from the Earth's centre, there is a region in the South Atlantic where the trapped radiation mirrors lower altitudes. This is called the South Atlantic Anomaly (SAA) or Brazilian Anomaly and dominates the radiation received in LEO. In addition, highly inclined LEOs intersect the outer belt electrons at high latitudes in the so-called horn regions.

Other planets in the Solar System also possess trapped radiation belts. These are become important for interplanetary missions since, for example, the Jovian particle belts are considerably more intense and extensive compared to the van Allen belts.

2.4 Solar particles

In the years around solar maximum the sun is an additional sporadic source of lower energy particles accelerated during certain solar flares and/or in the subsequent coronal mass ejections. These solar particle events last for up to several days at a time and comprise both protons and heavier ions with variable composition from event to event. Energies typically range up to several hundred MeV and have most influence on high inclination or high altitude systems. Occasional events produce particles of several GeV in energy and these can reach equatorial latitudes.

2.5 Atmospheric secondaries

On the Earth's surface we are shielded by the atmosphere. The primary cosmic rays interact with air nuclei to generate a cascade of secondary particles comprising protons, neutrons, mesons and nuclear fragments. The intensity of radiation builds up to a maximum at 18km (this is known as the Pfotzer maximum after its discoverer who flew a detector on a very high altitude balloon in 1936) and then slowly drops off to sea level. At normal aircraft cruising altitudes the radiation is several hundred times the ground level intensity and at 18km a factor three higher again. Solar particles are less penetrating and only a few events in each cycle can reach aircraft altitudes or ground level. Some of the neutrons are emitted by the atmosphere to give a significant albedo neutron flux at LEO spacecraft. It is the decay of these albedo neutrons into protons that is believed to be the source of the inner radiation belt.

2.6 Spacecraft secondaries

Spacecraft shielding is complicated by the production of secondary products. For example, electrons produce penetrating X-radiation, or bremsstrahlung, as they scatter and slow on atomic nuclei. Cascades of secondary particles, similar to those produced in the atmosphere, are also produced in spacecraft and can become very significant for heavy structures, such as Space Shuttle, the International Space Station and the large observatories, where path lengths can reach values equivalent to the atmospheric Pfotzer maximum (product of density and thickness is around 100g/cm2).

3 Basic mechanisms and their trends

3.1 Total dose effects

Total dose effects in semiconductor devices depend on the creation of electron-hole pairs within dielectric layers (oxides, nitrides etc.) and subsequent generation of traps at or near the interface with the semiconductor or of trapped charge in the dielectric. This can produce a variety of device effects such as flatband and threshold voltage shifts and surface leakage currents.

There is an extensive literature on total dose effects. One of the most recent and detailed reviews has been given by Dressendorfer in his 1998 NSREC Short Course Notes [3].

Although the concept of total ionising dose is a useful first order approximation for quantifying effects there are dependencies on a number of other parameters. Notably, the linear energy transfer (energy deposited per unit pathlength) of the radiation and the applied electric field both influence the rate of recombination of electron-hole pairs, while the dose-rate influences the relative importance of hole traps and interface states. These are extensively discussed in [3]. Strictly dose should be defined in relationship to the material affected. This is commonly silicon dioxide rather than silicon. Giving average doses at a device can lead to significant errors due to dose enhancement effects at boundaries.

Basic mechanisms of total dose effects continue to be a matter of research but it is now well established that hydrogen plays an important role [4]. In previous years considerable effort has been devoted to process optimisations to 'harden' the dielectric (usually silicon dioxide). With the trend to thinner oxides (discussed in more detail in section 5.1) commercial 'soft' gate oxides are tolerant to typical spacecraft exposures and attention has shifted to the removal of 'parasitic' leakage paths in the thicker field oxides. This can often be achieved 'by design' so that, provided a circuit is designed using cells from a radiation tolerant library, performance can be guaranteed regardless of the particular foundry used [5]. However the device scaling which results in thinner oxides also leads to microdosimetry effects where the total dose deposited by single energetic ions can cause permanent effects such as stuck bits in memories [6] or permanent damage to CMOS readout circuits for imag ers [7].

In ultra-thin oxides new phenomena such as radiation induced leakage current (RILC) are becoming apparent - due to mechanisms such as trap assisted tunnelling. Also the trend to nitrided oxides (to suppress hot carrier effects) has implications for radiation tolerance [8].

It will also be seen in section 5 that, although local oxidation of silicon (LOCOS) isolations are common

down to 0.5µm, other technologies are needed for smaller scales; for example, polysilicon-buffered and

polysilicon-encapsulated LOCOS and shallow trench isolation (for 0.18µm or smaller). These bring new issues for total dose effects and the design of the isolation becomes important.

Another area of new technology is the use of high-K dielectrics (i.e. materials with a high dielectric constant) and copper interconnections.

Total ionising dose can also influence materials properties. For example transparent materials, such as cover glasses, can become opaque via the population of colour centres, while plastic materials can become embrittled. Radiation-induced conductivity is an important phenomenon in mitigating charging of dielectric materials.

In general, issues which are important for total dose effects are:

• Dependence on bias during irradiation (irradiation whilst the device is biased is usually worst case).

• Annealing effects (trapped charge reduces after irradiation, while interface traps tend to build-up). • Dependence on dose rate (mainly because of annealing effects). • Dependence on package and burn-in (especially for some types of plastic package). • Variability from batch to batch and device to device (especially for commercial-off-the-shelf

devices). • In linear devices with junction isolated bipolar transistors there is a pronounced "enhanced low

dose rate sensitivity" (ELDRS) effect where the damage is greater at low dose rates. • Of relevance to potential work under this project is the problem of dose-enhancement under

electron or bremsstrahlung irradiation where there are boundaries between materials of widely differing atomic number. These can occur in packaging and shielding (see 5.3) as well as on the die (e.g. copper interconnects, high-Z bump bonds, metallized layers, Au-Si die attachments). Enhancement factors can approach a factor two [9].

3.2 Displacement damage

Energetic particles such as neutrons, protons, electrons, α-particles and heavy ions can create damage in semiconductor materials by displacing atoms in the crystal lattice. Secondary electrons produced by high-energy photons will also produce displacement effects. The result is that stable defect states are created within the bandgap that can give rise to any of the five effects illustrated in figure 1, depending on the temperature, carrier concentration and the location at which the defect resides [10]:

• generation of electron-hole pairs (leading to thermal dark current in detectors); • recombination of electron-hole pairs (leading to reduction of minority carrier lifetime and effects in

LEDs and laser diodes); • trapping of carriers, leading to loss in charge transfer efficiency in CCDs (minority carrier trapping)

or carrier removal (majority carrier trapping); • compensation of donors or acceptors, also leading to carrier removal in some devices (for

example the resistance in a lightly doped collector in a bipolar transistor can increase); • tunnelling of carriers, leading to increased current in reverse biased junctions - particularly for

small bandgap materials and high electric fields;

Figure 1: Illustration of the five basic effects of a defect energy level (Et) on the electrical performance of a device (after [11]).

The usually accepted view is that:

• Displacement damage is proportional to the non-ionising energy loss, NIEL (usually defined in units of keVcm2/g, though in high energy physics the displacement damage cross section (D) in MeVmb is usually used). The NIEL hypothesis can be used to relate damage due to different particles and energies - this greatly reduces the amount of testing needed (usually only one particle and energy, e.g. 10MeV proton, is used). The NIEL scaling hypothesis leads to the concept of displacement damage equivalent dose (=NIEL x particle fluence) [12]. This can be measured in keV/g or in (non-ionising) rads.

• Displacement damage does not depend significantly on irradiation bias or temperature, hence irradiations can be performed unbiased at room temperature - this simplifies experimental procedures.

These assumptions appear to be adequate for many cases. However when the exact nature of the defect is important then damage may not always scale with NIEL. Such an exception has been discussed by Dressendorfer [3] (in that case the difference between diffusion length damage in n- and p-type silicon solar cells). Another case has recently emerged in the field of high energy physics where neutron and proton effects in oxygen doped silicon microstrip detectors have been found to differ [13,14]. The reasons for this probably lie in the differences in damage clustering (and subsequent defect kinetics) between different particle types and energies.

It is now well established that the amount of formation of defect clusters depends on the particle type. Electron irradiation gives primary knock-on atoms (PKAs) with low recoil energies and hence leads to almost exclusive production of point defects; whereas neutrons give a flat PKA spectrum and a much greater proportion of cluster formation. For protons the situation is in between. In some cases the amount of clustering may not matter, only the total number of defects. However the clustering can be expected to affect the defect kinetics. Recent work by Watts [15] suggests that when impurity related defects (e.g. the E-centre) are involved then NIEL scaling may not always be strictly valid. The implication for space instrumentation is that tests at a single proton energy may not allow an accurate prediction in all cases. Fortunately there are still many cases where the NIEL hypothesis is valid, in particular where non-impurity related defects, such as divacancies are involved. R ecently Srour [16] has suggested a universal damage factor for displacement-damage-induced dark current in silicon devices. This suggests a common defect such as the divacancy is involved and it is seen that NIEL scaling is effective.

Other factors which can affect the generation of defects are the irradiation temperature and post-irradiation annealing. Usually it is assumed that neither irradiation temperature nor bias has an important effect and that annealing at room temperature takes place in only the first few weeks after irradiation. However this has not been studied in detail for situations (such as CCDs at low temperature) where the details of the defect kinetics may be important.

Although the largest body of displacement damage results applies to silicon devices, effects in other semiconductors are becoming increasingly important, particularly for photonics devices involving materials such as GaAs, InP and SiGe. Prediction of the NIEL for advanced materials is therefore an important topic, especially as some devices (such as optocouplers, amphoterically doped and single-heterojunction LEDs) are especially vulnerable in orbits subjected to intense proton fluxes. A simple technique for predicting NIEL using the SRIM code has been discussed recently [17]. However this does not include nuclear reactions which are important at high energies. Note that discrepancies between measured and predicted high energy NIEL for GaAs devices have been actively discussed over recent years and the issue is not yet fully resolved. This is discussed further in section 5.2.

As for silicon devices, it is well known that imaging devices, such as CCDs and CMOS active pixel sensors, show dark current spikes due to displacement damage. These are individual pixels with higher than average dark current. The dark current non-uniformity then depends not only on the average NIEL but also on its variance (as recently reviewed by Robbins [18]). In fact there are cases where a detailed Monte Carlo simulation of the damage cascades is necessary - e.g. when interactions of high energy protons within small depletion volumes have to be considered.

Displacement damage in linear devices (such as operational amplifiers) has recently been discovered to be a potentially important phenomenon [19], making prediction of displacement effects important for almost all spacecraft systems.

For systems inside spacecraft (as opposed to external systems such as solar panels) it is important to be able to predict effects from secondary particles. For shielding thicknesses of greater than ~10mm Al the flux of secondary neutrons can be significant. However, at present there are no readily available engineering codes for calculating the production of secondary particles and their contribution to NIEL. SHIELDOSE2 [20] makes a crude attempt to allow for the contribution of secondaries to dose while BRYNTRN [21] provides a fast numerical method for predicting secondaries. However, the latter is not available in Europe.

3.3 Single event effects

Single event effects arise from the interaction of single particles (e.g. protons, neutrons or heavy ions) with the semiconductor causing either transient or permanent effects:

• Single dark current generation centres (spikes) and single electron traps in imagers (permanent) due to individual lattice defects such as vacancy-phosphorous complexes and divacancies.

• Single event upset in memories (i.e. bit-flips leading to change of stored information. • Single event transients in imagers or linear circuits (i.e. a current transient which can be

interpreted as a false signal or be propagated to cause an output error in combinational logic). • Single event latch-up in CMOS circuits (a potentially destructive triggering of a parasitic pnpn

thyristor structure in the device). • Single event burnout in power transistors (a destructive triggering of a vertical n-channel

transistor accompanied by regenerative feedback). • Single event snapback in NMOS devices, particularly in SOI devices (a destructive triggering of a

lateral npn transistor accompanied by regenerative feedback). • Single event functional interrupt in control circuitry, e.g. in processors or ADCs, (transient

corruption of a control path). • Single event gate rupture (destructive rupture of gate dielectric due to high field generated by high

current).

These effects have been observed in many devices over the years but recently there has been a growing emphasis on the following areas:

• Observation of single event transients in linear circuits such as operational amplifiers, comparators and voltage regulators [22,23].

• Single event effects in optocouplers [24]. • Single event functional interrupts in ADCs and processors - which can lead to complex effects

such as lingering errors and 'mini-latch' events [25]. Bit-line errors can also be caused in high density memories [26].

• Effects of device scaling on threshold LET for single event upsets and on the shape of the cross-section versus LET curve [27].

• Observation of multiple-bit upsets now that cell sizes are becoming comparable with particle track dimensions. This is particularly true for high density DRAM's [28,29]. The directionality of secondary products has been shown to be of importance here [30].

• Observation of single event gate rupture in operational amplifiers. This occurs for susceptible devices only at near normal incidence and high power supply voltages [31] and calculation of events rates follows a different methodology to SEU prediction.

• Observation of single event gate rupture in DRAMs and FPGAs and also SRAMS and capacitors with thin gate oxides (<6nm), sometimes termed 'soft failure' [32].

• Neutron-induced single event effects in ground-based memories [33], avionics [34], and heavy spacecraft.

• Observation of microdose effects in DRAMs and 4-T SRAMs, which can cause sub-threshold leakage ('soft failure').

• Use of silicon-on-insulator (SOI) substrates to reduce the charge-collection volume and hence the sensitivity to SEU. However this technology can be susceptible to parasitic bipolar amplification of charge and triggering of high current states (snapback) [35].

• Possible susceptibility to thermal neutrons due to the presence of boron, in which the 10B isotope has a large cross-section for generation of nuclear fragments. This may be present as an implant or in borophosphosilicate glass in devices.

3.4 Biological effects

Ionisation produces free hydroxyl radicals which may lead to base damage in DNA. In addition individual particles can break chemical bonds leading to loss of bases or rupturing of the sugar phosphate backbone of DNA. The latter is analogous to single event effects in electronics and can result in single or double-strand breaks that may not repair successfully resulting in the possibility of cancer. As with SEE, the charge deposition can be by direct ionisation or by nuclear interactions. The microdosimetric volumes of interest are, however, smaller than in microelectronics (10nm and even 1nm compared to 100nm for microelectronics feature sizes). The effects increase with the linear energy transfer (LET) of the radiation so that heavy ions and neutrons are important. To account for this, doses are weighted by "Quality Factors", which are functions of LET, to arrive at dose equivalent. However it is doubtful whether such a concept applies to damage from individual high-Z particles or nuclear interactions.

Effects are divided into stochastic (e.g. cancer induction) where probability is a function of dose and non-stochastic (e.g. eye cataracts) which definitely occur beyond a threshold dose. Individual highly ionising particles can give light flashes in the retina.

Currently human exposure is limited to Space Shuttle and International Space Station orbits for which trapped protons in the SAA are a major concern. Heavy ions in cosmic rays and occasional solar particle events are also of concern, while electrons can be important for extravehicular activity. In large space structures, secondary neutrons become very significant and can provide a third of the dose equivalent for certain missions [2,36]. For interplanetary travel, cosmic-ray ions and solar particle events are most significant and very large solar events could provide debilitating doses if inadequate shielding is provided.

Important issues include:

• Determination of quality factors; • Microdosimetric calculations at the cell nucleus level, both for heavy ions (track structure

important) and for nuclear interactions by neutrons and protons; • Accurate shielding calculations to account for fragmentation of heavy ions and production of

secondary neutrons.

A review of this area has been given by Reitz et al [37].

3.4 Spacecraft charging

This can arise from energetic plasmas (10s of keV), leading to surface charging, or from energetic electrons (MeV), which can penetrate the spacecraft skin and collect in insulators leading to deep dielectric charging. The subsequent discharges can couple into spacecraft systems leading to anomalies and damage. Tools for predicting charge deposition and build-up already exist [38,39] and improvements should probably involve better definition of material properties (such as conductivity and its dependence on temperature and radiation) rather than radiation transport.

4 Trends in space systems

4.1 Introduction

A good review of trends in spacecraft systems has been given recently by LaBel and Cohn [40], albeit from a US rather than a European perspective. The well known trend towards use of COTS parts is discussed as well as the cost, size and performance pressures on system designers to use state-of-the-art components.

4.2 Commercial satellites

Commercial satellites are predominantly used in the fields of telecommunications and commercial imaging. Whilst conventional TV broadcast and telephone satellites tend to use traditional designs and components, not least because of the need for high reliability, there is a growing need to use state-of-the-art components. Use of ASICs in radiation tolerant technologies is common.

In all areas there is a growing use of on-board processing and large solid-state memories.

There is a growing need for precise pointing of telecommunication satellites (both for greater efficiency and for specific targeting of broadcasts). This leads to increasing use of electro-optic systems such as star trackers. Add-on systems such as laser communications terminals are also being actively considered as the pressure for increased bandwidth increases. This leads to needs for advanced photonics devices (laser diodes, detectors, optical signal processors and multiplexers).

Although fibre-optic data buses have been slower to develop than in the US, it is likely that the pronounced advantages of simplicity and lower mass will see fibre replacing copper in spacecraft buses in future. This again points to a growing need for use of advanced photonics in space (which mirrors the widespread trend in terrestrial communications systems).

Increasing communications bandwidths also leads to the use of advanced materials for transistors (e.g. InP HEMTs).

There is continuing cost pressure to use state-of-the-art solar cells (e.g. GaAs/Ge and InP) again requiring knowledge of effects in advanced materials.

Despite the current problems in the financing of telecommunications constellations (e.g. Globalstar, Teledesic and Iridium), there is still an interest to develop such systems, with a push to systems which will perform in high altitude LEO (e.g. at 1400km). Higher orbits lead to higher proton fluxes and a requirement to consider displacement damage effects in all components as well as proton-induced SEE. Generation of secondaries from proton interactions with spacecraft structures becomes increasingly important. Imaging systems (e.g. star trackers and laser terminals) have particular problems in high fluence proton orbits.

Satellite constellations are particularly demanding on cost and push the requirements for COTS devices - but still need performance in environments giving in excess of 100krad(Si) total dose at end of life. Since this dose comes primarily from trapped protons it is difficult to shield against.

There is a growing interest in payloads to be added on to telecommunications satellites so as to tap into the growing market for Earth observation. Low-cost imagers and imaging spectrometers are of interest here - but again the problems of maintaining performance in a high fluence proton environment can be severe. Most commercial Earth imaging is performed from LEO where radiation requirements are not so demanding (examples being the SPOT and ICONOS satellites). However use of high resolution imagers requires a continuing trend to use and understand state-of-the-art imagers and associated electronics (high performance amplifiers, ADCs and processors). Many hardened or known radiation tolerant components are simply too slow for use in advanced imager systems and there is a continuing need to characterise state-of-the-art components and to understand new effects such as single event functional interrupts and transients in linear and mixed signal devices. Use of gate arrays for imager cloc king and signal processing is widespread. The use of ASICs is not usually feasible in the low volumes of many systems and so there is a growing need for understanding of effects in such devices. Use of programmable FPGAs is of great interest but this requires a greater understanding and mitigation of single event effects.

4.3 Military satellites

Many of the trends stated above apply but greater demands are made on reliability and survivability. Additional specifications may be applied in terms of transient radiation effects and enhanced dose levels resulting from nuclear bursts. There is a trend to raise orbit altitudes for surveillance missions making natural radiation effects more significant, particularly those resulting from trapped protons. In addition sensors may have to operate in high radiation portions of orbits.

4.4 Science missions

The topic of science missions includes both space science (e.g. astronomy and planetary science) and Earth remote sensing. The size of spacecraft can range from the large facility with multiple instruments (such as Envisat) to the small demonstrator (such as PROBA). The orbit is usually low (e.g. HST, CGRO) or elliptical (eg. XMM, INTEGRAL). These orbits spend most of their time outside the belts and so the environment is not usually harsh from the total dose or damage standpoint; however lifetimes are often 10 years. There is a trend towards use of Lagrangian points for observatories. The L1 point is typically used for solar observations (e.g. SOHO) while use of the L2 point is planned for many astronomy missions in order to minimise contamination from earth and solar emissions (e.g. Herschel-Planck, NGST, GAIA, Eddington). For all missions in interplanetary space there is full exposure to cosmic rays and solar particle events and so single event effects and noise in sensors are important. Missions to Jupiter and Saturn experience intense trapped radiation in the planetary magnetospheres.

By their nature, science missions demand high performance and there is a push to use advanced electronic, optical and structural components. Examples are shown in Table 1.

Mission Concern

Space Science

X-ray astronomy (e.g. XMM-

Newton, XEUS)

Low energy proton scattering and loss of CTE

in the cooled CCD detectors

Infrared & Optical

Astronomy (e.g. ISO,

Herschel-Planck, Eddington,

NGST)

Transient effects in detectors (background

rate)

Gamma ray astronomy (e.g.

INTEGRAL)

Background rate from activated materials

Astrometry (e.g. GAIA) CTE effects in large CCD focal planes

Planetary Science Long duration and low temperatures during

cruise phase, harsh environment in Jupiter

belts

Earth observation

Laser sounders Radiation effects in photonics (e.g. laser

diodes)

Infrared imagers Radiation effects in infrared detectors (and

associated readout circuits)

Optical instruments Radiation effects in precision optics and

lightweight structures (e.g. graphite epoxy

composites or SiC)

Synthetic aperture radar Radiation effects in light-weight active

antenna arrays

Table 1: Summary of radiation effects on future space science / Earth observation missions.

Space science missions often require giga-bit mass memories for the storage of data. This leads to use of high density devices and increased concerns for single hard errors and multiple bit upsets.

4.5 Spacecraft size issues

Spacecraft sizes now range from nanosatellites (<10kg) to major observatories (16 tonnes) and the International Space Station. Radiation transport and effects tools have to cover situations ranging from those where primaries dominate to those where secondaries dominate (e.g. by a factor 20 on the Compton Gamma Ray Observatory [41]).

5 Trends in technology

5.1 Microelectronics

5.1.1 CMOS

Radiation effects on advanced microelectronics technologies have recently been discussed by Johnston [24] and Claeys and Simoen [8].

Over the past few years there has been a strong tendency to replace hardened electronics for space by commercial-off-the-shelf (COTS) components, fabricated in a mainstream commercial CMOS

technology. Scaling is projected to reduce feature sizes from the current value of 0.18µm to 0.07µm by 2010, together with a reduction in device voltages to 1 volt or less. A strong motivation is the expectation that scaled technologies should inherently be more radiation tolerant. This relies on the fact that as the channel length of MOS transistors reduces, so does the thickness of the gate dielectric. Hence the amount of positive trapped-hole charge also decreases. This follows from the expression for the flatband voltage(the voltage at which a constant Fermi level is achieved across the device), in this case for positive irradiation bias [8]:

Equation 1

where q is the charge on an electron, ε0x and ε0 the dielectric constants of the gate oxide and vacuum, and b the faction of holes which are trapped. The parameter h1 is the distance over which trapped holes can recombine with electrons tunnelling from the substrate or from the gate. h1 depends on the time t between irradiation and measurement (as lnt) but for typical times is ~3nm. For thin and ultra thin oxides with less than 6nm, essentially no net hole trapping will occur - and this is indeed found in practice.

In fact, the conclusion could be made that all (deep) submicron CMOS technologies should be radiation hard - particularly for low dose-rate space applications - and no extra measures need be taken. Unfortunately other effects come into play:

• Interface traps are also important and in scaled devices the damage can also be non-uniform. • Aggressive process steps (e.g. plasma etching, e-beam and x-ray lithography) are often used and

lead to direct or latent radiation damage. • Reliability concerns have lead to the development of nitrided and re-oxidised oxides which have a

particular radiation response (showing both electron and hole trapping and a different bias response).

• Ultra thin oxides can show enhanced gate leakage current (RILC) at low gate biases, believed to be due to trap-assisted tunnelling of carriers through the oxide barrier.

• Device isolation remains a difficult issue. For many devices field oxide inversion is still important,

even for feature sizes in the 0.8 to 2µm range. Although the thickness of LOCOS isolations is reduced to 300-6000 nm in newer technologies, the geometry of the 'birds beak' region plays an

important part. Below 0.6µm, modifications such as polysilicon buffered and polysilicon encapsulated LOCOS are needed [24]. The radiation tolerance is likely to be dependent on the design and the process. Although simulations have shown that good radiation tolerance can be achieved these technologies have not been experimentally studied in detail. For deep submicron

technologies (below 0.25µm) LOCOS is being replaced by shallow trench isolation.

Particular concerns for memories are that the trend to small cell size leads to microdose issues such as single hard errors and a decrease in the critical charge for upset. However manufacturers of SRAMs and DRAMs anyway need to ensure low upset rate from alpha particles arising from metallizations and other parts of the devices. Thus there tends to be a lower bound to SEU sensitivity. Smaller feature sizes also lead to increasing numbers of multiple-bit upsets from proton and neutron reaction products as well as heavy ion tracks.

Upset thresholds for COTS processors have remained essentially unchanged at 2-3 MeVcm2/mg over the past decade. Registers tend to dominate the response but the net cross section is low because the registers occupy a small area. There is a trend towards cu interconnects and high-K dielectrics. The implications for total dose response have not yet been fully explored. The capacity for novel forms of single event functional interrupt is expanded with the increase in the complexity of control circuitry. Successful use in space will depend on identification and recovery. It can happen that the packaging of high speed processors interferes with the needs for on-ground radiation testing.

Silicon-on-insulator (SOI) technology has many potential advantages. The charge collection depth in

epitaxial structures is typically 2µm but this can be reduced by an order of magnitude in SOI, which also shows lower power consumption. There are some disadvantages, such as memory effects, snapback and reduced thermal conductivity. However technologies are not yet fully established.

Some technologies, such as flash memories, require selective use of higher internal voltages, which in turn require thicker gate oxides. Also charge pump circuits are often used in complex circuits, such as FPGAs, to provide slightly boosted internal voltages for start-up or switching purposes. These circuits tend to operate at low currents and can be affected by small threshold shifts.

As well as a susceptibility to total dose damage in charge pump circuits, FPGAs suffer from single event upsets. In reprogrammable devices those upsets can produce a reconfiguration of the device, potentially resulting in high current (and therefore damaging) configurations. Current monitoring and periodic scrubbing of the device is therefore needed to make sure that the failure probability is low.

5.1.2 Bipolar devices

As mentioned in section 3.1, junction isolated bipolar devices are subject to enhanced low dose rate sensitivity (ELDRS) effects. These depend on the design of the device. Lateral PNP transistors show the largest normalised current gain degradation compared with substrate or vertical p-n-p transitions and the low dose rate effect is higher. Other important issues for linear devices are:

• Transient effects, especially if the transient state can trigger external circuitry. Effects depend on the device, its operating configuration and external circuitry. Hence testing or simulation of the complete circuit is needed for sensitive devices or critical applications.

• Displacement damage can be important in some cases [19]. • Single event gate rupture has been observed [31].

5.1.3 GaAs devices

The high electron mobility of GaAs and related III-V compounds makes these materials very suitable for high-speed digital and microwave/millimetre wave applications. The high operating frequency, low high-frequency noise and low power dissipation make GaAs devices ideal for telecommunications systems. Since no dielectric layers are used the devices show extreme radiation tolerance - up to several hundred Mrad (GaAs), which is two orders of magnitude higher than for equivalent Si-based technologies.

III-V materials are also extensively used in photonics applications (e.g. LEDs and laser diodes) and also in multi-quantum-well infrared detectors, but these will be discussed later (in section 3.1).

Radiation effects in GaAs and related materials has been reviewed in detail by Claeys and Simoen [8]. MESFETs and HEMTs are fabricated as SI (semi-insulating) GaAs substrates. The semi-insulating character arises from the nature of the native donor trap (EL2) which is attributed to an AsGa antisite defect (or an associated complex). The partial compensation of the deep EL2 donor by acceptor levels from trace impurities introduced during crystal growth pins the Fermi level near mid gap, yielding a semi-insulating substrate. The radiation hardness arises from the immovability of the antisite (since there are no dielectric layers only displacement damage effects are important).

Prediction of displacement damage effects is straightforward based on the NIEL hypothesis, however the current discrepancies between theoretical and measured NIEL at high proton energies (mentioned in section 3.2 and in greater detail in 5.2) suggest that caution is needed until the matter is resolved.

In general GaAs technology is very susceptible to SEU but hardening with low temperature buffer layers offers orders of magnitude improvement.

5.1.4 SiGe devices

The properties and applications of SiGe have recently been reviewed in [8]. SiGe alloys can be formed with band gaps covering the range 1.12eV (Si) to 0.74eV (Ge). There is also a band gap shift introduced by strain due to lattice mismatch. SiGe devices are compatible with silicon processing and hence offer

high yield and low cost. In SiGe HBTs a high current gain can be achieved and the hole mobility is increased. Such devices are therefore competitors for GaAs devices for use in satellite communications systems. Implementing SiGe in BiCMOS [42] opens the way for mixed mode analogue-digital applications. In addition, the SiGe HBT can be optimised for cryogenic applications.

As for GaAs devices, displacement damage is the only radiation concern. The nature of the predominant lattice defects is becoming clear, though identification is difficult due to the Ge-fraction dependence of the trap parameters. Recent results [43] indicate good tolerance against proton and gamma irradiation.

5.1.5 InP

This material shows promise for high speed applications but is in the early stages of development. Displacement damage is probably the key issue.

5.1.6 Wide band gap semiconductors

Materials such as SiC and GaN have advantages in terms of high temperature operation, power handling and speed and may replace current technology in areas such as power conditioning and HF power amplification.

5.1.7 Solar cells and cover glasses

An excellent review of effects in solar cells has recently been given by Marshall and Marshall [10].

Solar cells are basically large area photodiodes, sometimes with complex multi-junction designs to optimize their conversion efficiency. As expected for a minority carrier device, the degradation in power output is a result of a radiation-induced reduction in the minority carrier lifetime. There is a growing tendency for modern solar cells to be made from direct bandgap materials such as GaAs, where the short optical absorption length (a few µm) leads to much shallower active regions than for Si devices. Device radiation sensitivity is also reduced for materials with short initial lifetimes. For example, minority carrier lifetimes in GaAs are typically much shorter (tens of nanoseconds) as compared to the much longer Si lifetimes of tens to even hundreds of microseconds. Key electrical parameters include not only the power output, but also the open circuit voltage, and short circuit current.

Although crystalline Si and GaAs/Ge solar cells are common, multi-junction GaAs cells, InP and amorphous Si cells are being investigated for future use in space. The drive to operate spacecraft in ever more harsh environments (including the more intense part of the proton belts) has spurred interest in more radiation hardened cells. The presence of multiple junctions provides additional design flexibility to achieve increased hardness, as well as higher power-conversion efficiency. Note that InP has the potential for increased survivability as a result of injection annealing. At very high fluences, minority carrier devices begin to show the effects of carrier removal, and solar cells are no exception. Failure, from carrier removal effects, of a Si solar cell flown in an elliptical orbit through the Van Allen belts has already been observed.

Since solar cells are flown in space with very thin shielding (e.g. cover glasses as thin as 100 µm), the lower energy portion of the proton spectrum contributes a large part of the displacement damage. This means that deviations of the damage factors from the GaAs NIEL curve are less significant. The low shielding also implies that damage from electrons must be considered in addition to the proton contribution. It has been recognized that a linear relationship between electron damage factors and NIEL

is not always observed in Si. In the case of GaAs solar cells, a recent detailed review recommends separate fits to the NIEL in order to describe proton and electron degradation. Proton data correlated well with NIEL over the energy range from 0.2 to 9.5 MeV. For electrons the best agreement was obtained when the calculated NIEL was raised to the power of 1.7.

Cover glasses are designed for protection against the soft protons which give a very steep dose and damage profile. Questions have been raised as to whether population of colour centres by ionisation can lead to significant opacity. Improved knowledge of low energy proton environments is probably required.

5.1.7 Power devices

Power MOSFETs are susceptible to single event burn-out in the "off"-state through turn on of a parasitic bipolar junction transistor by intense ionisation. This can result from proton and neutron interactions [44] as well as from heavy ions.

In addition single event gate rupture can result from localised dielectric breakdown of the gate oxide caused by dense ionisation at the Si-SiO2 interface in the gate-drain overlap region.

5.2 Photonics

5.2.1 Introduction

Effects in photonics have been exhaustively reviewed by Marshall and Marshall [10] and Johnston [45]. A brief discussion is given below.

5.2.2 LEDs and laser diodes

Displacement damage is the primary effect of radiation on LEDs and laser diodes. The main effect is to reduce the light output of LEDs (particularly for amphoterically doped devices) and to increase the threshold current of laser diodes.

Since GaAs technology is normally used, the validity of NIEL scaling is important in making on-orbit predictions. The latest results available in the literature [46] indicate that the discrepancy between experimental data and theoretical NIEL values is significant at high proton energies (see figure 2). It is possible that effects depend on whether cluster or point defects are involved - which may depend on the device and the damage involved (e.g. minority carrier lifetime or junction dark current).

Figure 2: Lifetime-damage factor measurements for GaAs LEDs (scaled to the right abscissa), normalised to agree with calculated NIEL values (solid line - scaled to left abscissa) for 10MeV protons.

Particular points to note are [47]:

• LEDs: o Damage depends on current, generally being less at high values. o I-V characteristics can change with radiation as well as light output. o Loss in output often recovers by way of injection-enhanced annealing. o Some devices can degrade abnormally, with large increases in non-radiative current at

moderate injection levels. o Damage is not linear with proton fluence.

• Laser diodes: o As with LEDs, devices which are biased during or after irradiation show recombination-

enhanced annealing.

5.2.3 Optocouplers

Optocouplers are hybrid modules comprised of an LED, a coupling medium and a detector (often a phototransistor). Being hybrid modules they can suffer from a large part-to-part variability. A given commercial device may have internal components (such as LEDs) that cannot be traced and may come from several sources. They are susceptible to direct ionisation-induced transients in the detector (because of its large volume) [24]. Permanent damage can be caused by displacement damage, for example loss in LED power (though devices with a hardened LED will ultimately suffer from photo-response degradation) [48] and from ionising damage in the photodetector. The main performance metric is the current transfer ratio (CTR) which is the ratio of the photodetector collector current to the LED forward current. Optocouplers can be of linear or digital type and many different designs exist, each with a different radiation response. As with LEDs, CTR degradation is not always linear with fluence. Op tocoupler failures occurred on the Topex-Poseidon mission [48] at TID levels of 20-30 krad (Si). For

systems which use a lens coupling, radiation darkening of the lens, due to TID, can sometimes be a problem (particularly for graded index, GRIN, lenses).

5.2.4 Fibre-optics

Total dose radiation damage is an active area of research for the nuclear industry, however for the short lengths (<50m) used on spacecraft there should be no problems posed by total dose darkening, though it is sensible to check on new component types. The expected TID tolerance follows in part from the annealing which takes place in the low dose rate environment [36]. Certainly some fibres will be more resistant to darkening than others (e.g. pure silicon versus phosphorous doped) and these will be the best candidates for use in the space environment [49]. Typically, attenuation of less than 1dB might be expected over the course of ~100-200 krad(Si) delivered over a 7-10 year mission.

TID effects in couplings also need to be considered.

Erbium doped fibre amplifiers can be used for space communications systems, for example those using phased array antennas. There appears to be some TID sensitivity but further research is needed. Polarisation-maintaining fibres are another area where TID sensitivity may be encountered.

5.2.5 Fibre-optic data links

Fibre optic data links have been the subject of development in the US for several years and are now becoming of interest in Europe. Among the advantages are ease of integration, low mass and immunity from electromagnetic interference. A fibre optic bus consists of a light source (e.g. laser diode), a fibre, and a photodetector (usually a photodiode). Design trade-offs, radiation effects and future trends are discussed in reference [50]. Current designs tend to use InGaAs lasers and detectors for 1300nm detection. Use of a direct bandgap semiconductor allows for a thinner depletion region in the photodetector (and hence better immunity from transient effects) and a lower capacitance (allowing higher speed). For avionics systems, which sometimes require stability at temperature over 80 C, 830nm technology is often chosen.

For typical space missions it is expected that displacement damage effects on III-IV sources will be minimal and dark current increases should only be a problem in photodiode circuits with extreme noise sensitivity.

The receiver photodiode will, however, be sensitive to transient effects, the parameter of interest being the bit error rate (BER). At speeds below ~100-200 Mbps it is possible to discriminate against short-lived particle events, otherwise encoding schemes can be used. Recent advances in GaAs, SiGe, InP and deep submicron silicon-on-insulator (SOI) technology (see section 5.1) can provide Gbps to tens of Gbps rates and further radiation evaluation of these technologies is required. Conventional alternative high speed technologies such as GaAs MESFET or silicon ECL have a high power dissipation and high SEU rates [50]. GaAs ICs have shown good immunity to TID and latch-up but are susceptible to single event upset, but work is underway in the US on promising GaAs HIGFET technologies. InP and SiGe are also extremely hard to TID effects. Metal-semiconductor-metal (MSM) technology photodiode is also being developed to allow monolithic low-noise, high-bandwidth receivers. The MSM is a va riation on the Schottky barrier diode in which the active region is a 2-D depletion region near the metal contact which results in a high electron mobility.

Vertical cavity surface emitting lasers (VCSELs) will be increasingly used as sources. They can be used for parallel links as monolithic, one- and two-dimensional arrays are possible. Preliminary indications are

that VCSELs show a similar good radiation tolerance to laser diodes. There is also an interest in the US to integrate VCSEL technology with FETs and detectors to make a monolithic smart pixel array (SPA).

Although scintillation effects (due to single particle events) can be seen in special fibres, they are not usually considered in fibre data links.

5.2.6 Sensors

5.2.6.1 Silicon imagers

Silicon imagers include charge coupled devices (CCDs), charge injection devices (CIDs) and CMOS active pixel sensors (APSs). These are all susceptible, to varying degrees, to radiation effects.

All are sensitive to transient signals. One electron-hole pair is created, on average, for each 3.6eV of energy deposited. Hence transient signals can be large, typically a significant faction of full well capacity. Contamination of images by transients has to be taken into account for all orbits. Transient generation cannot be avoided but can be mitigated by software rejection algorithms. Tailoring of pixel size and epitaxial thickness/resistivity can be used to give a favourable ratio of signal-to-transient amplitude.

All silicon imagers are vulnerable to displacement damage induced dark current spikes. Modelling of dark current non-uniformity has recently been discussed by Robbins [18]. The largest spikes are enhanced by local electric fields (e.g. at the edge of a pixel) and their size is dependent on the pixel architecture. In critical applications the device often has to be cooled to temperatures at or below 0C in order to reduce the amplitude of the spikes, which tend to show random telegraph switching with a wide range of amplitudes and time constants. Apart from the field-enhanced spikes, the dark current histograms can be simulated using the NIEL hypothesis and a universal damage constant [18].

CCDs are particularly sensitive to displacement damage as packets of signal charge have to be moved large distances through the silicon from the generation sites to the readout amplifier. Charge will be lost from a packet each time a trap is encountered. A variety of trapping energy levels can be created by particle irradiation; the most common has an energy level ~0.44eV below the conduction band and is usually identified as the E Centre. However other traps have been reported [51,52]. The nature of the traps and the variation (if any) in trap concentration ratios with particle energy is likely to be important for space science missions such as GAIA. It is possible that NIEL scaling will not hold.

The loss in CTE caused by displacement damage depends on the signal and background level and can vary by over three orders of magnitude [53]. This has to be taken into account when making performance predictions.

Since CCDs are especially sensitive to displacement damage it is important to be able to reliably estimate the fluence of both primary and secondary particles. The CCD itself is normally well shielded and secondaries often contribute a significant proportion of the damage. However, no convenient tools are available at present to estimate the NIEL from secondary particles.

CTE damage tends to preclude the use of CCDs in high altitude, proton-rich orbits, though use of p- rather than n-channel devices has been investigated [54,55] and is undergoing development in the US. An alternative is to use the charge injection device (CID) or the active pixel sensor, both of which use column/row addressing (or even direct pixel addressing in some cases).

Active pixel sensors also have advantages in ease of clocking (5V levels) and ease of incorporation of additional on-chip functions (e.g. digitisation).

Total dose effects in silicon imagers should not be ignored as the dielectric layers are soft and moderately thick (~100nm). Surface dark current damage can be suppressed in CCDs using inverted mode operation, IMO, sometimes called MPP operation. CMOS APS devices can show TID-induced dark current and also leakage currents due to field-oxide-inversion [56] (unless a radiation tolerant CMOS design is used). APS devices are potentially vulnerable to latch-up unless hardened by design. If an on-chip ADC is used then this may show TID and transient effects.

CCD detectors show flatband voltage shifts of order 50mV/krad(Si) when biased but can usually accommodate changes of order ~1V in effective operating voltage.

5.2.6.2 Infrared sensors

Radiation effects in IR imaging arrays were reviewed in [57]. Both the readout circuit (usually a CMOS Si X-Y addressed IC) and the infrared photodetector array have to be considered. The CMOS readout circuit will be susceptible to TID and single event effects as usual. For the photodetector array, effects will depend on the materials technology employed, e.g. HgCdTe, InSb, InGaAs, extrinsic silicon etc. Calculation of energy deposition in these materials is therefore of importance. Information is available on particular devices but effects will be very much device dependent.

5.3 Packaging and shielding issues

Advanced techniques and packaging schemes are being used to shrink volume and weight while optimising system performance. For example, by removing packages and attaching the die directly to a substrate, such as diamond, a high surface utilisation is obtained. Such designs can reduce shielding to sensitive regions.

The problem of dose-enhancement due to high-Z materials within chips has been discussed above. The converse of this is that appropriate use of high-Z materials in packaging, coatings or spot shields can significantly reduce the dose (or the weight of shielding for a given dose) in environments where electron and bremsstrahlung dose are dominant. Rad-pakTM and Rad-coatTM are commercially available packages and coatings. However users do not have control over the exact shielding advantage and may wish to apply spot shielding of their own design. Appropriate ordering of high-Z and low-Z materials can give factor-of-three improvements, while inappropriate ordering can negate any advantage.

All of the above require accurate application of electron-gamma transport codes, including adjoint applications and the ability to calculated dose in thin layers.

5.4 Micro electromechanical structures (MEMS)

These integrate the mechanical world with the electrical and are a rapid growth area with applications to sensors, propulsion, mass data storage and smart structures. Accumulation of trapped charge in insulators can provide a total dose problem but this is very design dependent. It has recently been shown that the mechanical properties of polysilicon, itself, are not affected by even high (10Mrad) doses of cobalt-60 [58].

5.5 Insulator materials

Bulk trapping of electrons in cables and other insulators can lead to build up of high electric fields with subsequent discharge producing upsets and damage. This "deep dielectric charging" mechanism has been a major cause of anomalies and some losses in geostationary orbit where energetic electron fluxes are extremely variable. Key properties of the materials are their conductivities and the temperature dependence thereof. Accurate assessment of charge deposition profiles are also required but are very dependent on the hardness of the electron environment and so it is usual to design in terms of worst case environments using, for example, the FLUMIC model of Wrenn [59].

Insulators, such as polymer-matrix composites, are becoming used for spacecraft structures because of their light weight, high specific strength and low coefficient of thermal expansion. Examples are precision, planar, phased-array antennas and reflectors. Total doses can be as high as 5000Mrad for a geosynchronous 30 year orbit and so radiation effects on mechanical properties are a significant concern [60]. Other applications use insulators for their dielectric properties. Important parameters which can be affected by radiation include dielectric strength, dielectric constant and dielectric loss, conductivity and, again, mechanical properties [61].

5.6 Optical materials

Radiation causes the formation of colour centres in optical materials as well as changes in refractive index and mechanical properties. Because there is considerable annealing in the low dose rate space environment and doses are limited for shielded components the parameter changes (loss in transmission, change in refractive index and dimensions) are generally small and vary linearly with dose(D) so that a dose coefficient approach can be used (as suggested by Gusarov and co-workers [62]).

Xm = αD

where α is a dose coefficient. The changes in refractive index will produce changes in the focus of optical systems which can be analysed using optical design programs such as CodeV (as suggested by Gusarov et al [62]). Alternatively, many systems can be modelled as a single lens of appropriate focal length so that focus shifts are easily calculated - Fruit et al [63] take an example of an refractive optical

system with a focal length F=350mm with a focus tolerance of ±5µm. Assuming a measured dose coefficient for refractive index Vp =10-10/rad (typical of LaK9G15 components) the maximum allowable total dose is 150krad.

5.7 Detector materials for astrophysics

Charge-deposition spectra of ionising radiation in detector materials is a major source of background in astronomy and astrophysics missions ranging from the infrared [64] to the gamma ray. In the latter case the contribution of radioactivity limits sensitivity even outside the regimes of intense particle radiation [41]. Extensive work has been performed on NaI, CsI, bismuth germanate and germanium but further calculations are required for newer materials, such as gadolinium oxyothosilicate, cadmium zinc telluride, barium fluoride. The use of heavy spacecraft and detector systems necessitates the accurate computation of secondary radiation and its spectrum. Thermal neutron capture cross-sections are very large for certain materials commonly employed (e.g. iodine).

Superconducting materials and cryogenics are likely to be employed in future detectors. It is known that high temperature superconductors suffer a reduction in critical temperature under irradiation [65]. Intense radiation regimes can lead to macroscopic temperature changes in cryogenically cooled materials [66]. Future missions such as LISA intend to detect gravity waves using very-long baseline

laser interferometry between free-falling masses within the spacecraft. These could be sensitive to thermal, electrostatic and potentially kinetic (i.e. momentum transfer) effects resulting from the incident cosmic radiation.

5.8 Summary

Table 2 summarises this chapter by identifying the key radiation effects parameters associated for each of the technologies.

Technology Key parameter to be

calculated

TID on scales down to nm

allowing for local materials

(dose build-up) etc.

CMOS

Single Event charge depositions

in volumes down to nm cube,

including mapping of

simultaneous depositions in

adjacent volumes

TID allowing for dose rate

Single event charge depositions

Bipolar

Displacement damage using

NIEL

NIEL GaAs

Single Event charge depositions

SiGe/InP NIEL

Solar cells NIEL

Cover glass TID

Power Devices Single event charge depositions

LEDs and laser diodes NIEL

NIEL

TID

Optocouplers

Single Event charge depositions

TID Fibre-optic data links

Single Event charge deposition

in receivers

Single Event charge deposition

NIEL

Silicon sensors

TID

TID Infra-red sensors

Single Event charge deposition

MEMS TID

Bulk trapped charge Insulator materials

TID

Optical materials TID

X-and gamma-ray detectors Energy-loss spectra, both prompt

& due to induced radioactivity

Cryogenic systems Heating by TID

Charge from incident cosmic

rays and secondaries

Gravity-wave detectors

Momentum imparted

TID is Total Ionising Dose in J/kg NIEL is Non-Ionising energy loss in J/kg Single Event charge depositions in Coulombs (usually of order fC to pC) from direct ionisation (as determined by linear energy transfer of particle at position of interest) or indirect ionisation via nuclear reactions (including nuclear recoil and secondary products).

Table 2: Summary of important radiation effects parameter(s) as a function of technology

Abbreviations

ADC analogue-to-digital converter

APS active pixel sensor

ASIC application-specific integrated circuit

BER bit error rate

CCD charge coupled device

CID charge injection device

CMOS complementary metal oxide semiconductor

COTS commercial off-the-shelf

CTE charge-transfer efficiency

CTR charge transfer ratio

DNA deoxyribonucleic acid

DRAM dynamic random access memory

ECL emitter coupled logic

ELDRS enhanced low dose rate sensitivity

ESA European Space Agency

ESTEC European Space Technology Centre

FET field effect transistor

FPGA field-programmable gate array

GaAs galluim arsenide

GRIN graded index

HBT heterojunction bipolar transistor

HEMT high electron-mobility transistor

HF high-frequency

HgCd Temercury cadmium telluride

HIGFET heterojunction insulated gate field effect transistor

IC integrated circuit

IMO inverted mode operation

InGaAs indium gallium arsenide

InP indium phosphide

InSb indium antimonide

IR infrared

LED light emitting diode

LEO low Earth orbit

LET linear energy transfer

LOCOS local oxidation of silicon

MEMS micro-electromechanical structure

MESFET metal-semiconductor field effect transistor

MOS metal oxide semiconductor

MOSFET metal oxide semiconductor field effect transistor

MSM metal - semi-conductor - metal

NASA National Aeronautics and Space Administration

NIEL non-ionising energy loss

NMOS N-type metal oxide semiconductor

NSREC Nuclear and Space Radiation Effects Conference

PKA primary knock-on atoms

RILC radiation induced leakage current

SAA South Atlantic Anomaly

SEE single event effects

SEU single event upset

SI semi-insulating

SOI silicon-on-insulator

SPA small pixel array

SRAM static random access memory

SRIM Stopping powers and Ranges of Ions in Materials

TID total ionising dose

VCSEL vertical cavity surface emitting laser

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