One of the biggest threats to the deployed aerospace and defense electronics systems is radiation. Outside the protective cover of the Earth’s atmosphere, the solar system is filled with radiation. The natural space environment consists of electrons and protons trapped by Earth’s magnetic field, protons and small amount of heavy nuclei produced by Solar events, and heavy nuclei, i.e. cosmic rays produced outside the Solar system.
The radiation environment close to Earth is divided into two categories: particles trapped in the Van Allen belts and transient radiation. The particles trapped in the Van Allen belts are composed of energetic protons, electrons, and heavy ions. The transient radiation consists of galactic cosmic ray particles and particles from solar events (coronal mass ejections and solar flares). This force can occur naturally in space and at high altitudes on Earth or can come in massive doses from the detonation of nuclear weapons.
Deep-space and long-duration missions, where both crew members and spacecraft no longer benefit from the protection of Earth’s magnetic fields, are considered high risk for adverse radiation impacts. Aircraft flying at altitude, at about 30,000 feet and above, also are starting to experience radiation-induced effects. “There are 500 times more neutrons at 30,000 feet than there are on the ground,” points out Aitech’s Romaniuk.
Some of these charged particles have sufficient energy to penetrate into space vehicles and can damage their electronics. These damages from charged particles significantly cause ionization. The ionizing radiation of space accelerates the aging of electronic parts and materials, leading to degraded electrical performance or even permanent failures. In addition to radiation damage, electronics that operate in spacecraft applications can be exposed to extreme temperatures – ranging from -55°C to 125°C – over mission lifetimes that can exceed 15 years.
The mission parameters including orbit parameters such as inclination, period, perigee height, apogee height, and eccentricity, additionally launch time, and mission duration are related to the definition of the radiation environment. As far as missions in Earth’s orbit are concerned, three different environment profiles are distinguished: Low Earth orbit (LEO): altitude below 2 000 km, with polar inclination. LEO is the standard orbit for Earth observation satellites. Medium Earth orbit (MEO): altitude between 2 000 km and ~36 000 km at which most navigation systems satellites are located. Geostationary Earth orbit (GEO): altitude of ~36 000 km, this orbit is mainly used for telecommunication satellites.
There are large variations in the levels of and types of radiation a spacecraft may encounter. Missions flying at low Earth orbits, highly elliptical orbits, geostationary orbits, and interplanetary missions have vastly different environments. In addition, those environments are changing. Radiation sources are affected by the activity of the Sun. The solar cycle is divided into two activity phases: the solar minimum and the solar maximum. Will your spacecraft mission occur during a solar minimum, a solar maximum period, or both? The key point here is that there are vastly different environments in space. The requirements for a launch vehicle are much different from that of a geostationary satellite or a Mars rover. Each space program has to be evaluated in terms of reliability, radiation tolerance, environmental stresses, the launch date, and the expected life cycle of the mission.
In low altitudes, the main dose source is caused by electrons and protons, while in high altitudes in other words geostationary earth orbit, the main dose source is from electrons and solar protons. An LEO space system receives 0.1 kRads/year (1 Gy/year) dose rates for a mission. For a typical 3-5 year mission, the total dose is 0.5 kRads. A space system in GEO is exposed to outer radiation belts, solar flares, and cosmic rays. In GEO, a space system receives approximately 10 kRads/year (0.1 kGy/year) dose rate. For a typical 10 year mission, the total dose is 100 kRad. Since a space system is mostly within the Van Allen belts in MEO, it is harshly exposed to radiation. The environment in MEO is highly affected by the solar cycle effects and the dose rate is in the order of 100 kRads/year (1 kGy/year).
The total ionizing dose (TID) effect, caused by trapped protons and electrons contained in radiation belts and protons emitted by solar flares, can result in device failure or biological damage to astronauts in space systems.
The evaluation of the effects of space radiation on electronic systems or components. The worst-case definition of the environment provides sufficient information to consider the level of the damage on the electronic systems or components. Ionizing radiation takes a few forms: Alpha, beta, and neutron particles, and gamma and X-rays. The ability of Radiation to affect electronic devices depends on its ability to penetrate the electronic equipment and then to penetrate the packages with semiconductor devices in them. Usually, it will be beta and gamma radiation that will have this ability; alpha particles will usually be stopped by outer packaging very easily.
The common quality that is measured in Radiation is its ability to ionise materials. In semiconductors this ionising radiation can have two major effects: one is to produce electron-hole pairs which can create “soft” errors (errors in operation but not permanent damage) and, if the radiation is sufficient, permanent damage by creating large numbers of charges with sufficient energy to be injected into Silicon dioxide regions (where they stick) and change the characteristics of a transistor. Such high levels of radiation can also disrupt the crystal lattice and damage the transistors in that way.
The natural space radiation environment can damage electronic devices and the effects range from degradation in parametric performance to a complete functional failure. These effects can result in reduced mission lifetimes and major satellite system failures.
For, weather, communications, and surveillance satellites to be cost-effective, they need to be operational for at least 10 to 15 years. Onboard electronic equipment failures are a common reason for early satellite failure. The computers in space must meet the radiation-hardness requirements of the severest space environment. However, the biggest market driver for radiation-hardened electronics is low-Earth-orbit space applications in communications and Earth observation, where missions might last only for a few years — or even just months — and where threats from space radiation are relatively low.
Ensuring the reliable operation of microcircuits and software in outer space is an important scientific and economic objective. There are various measures used to protect electronic circuits from radiation, One is the shielding of electronics which is not only expensive, but it also can be heavy enough to adversely influence launch costs. There are other ways of dealing with space radiation, ranging from redundant subsystems, selective shielding, and upscreening commercial off-the-shelf (COTS) electronics for enhanced reliability. There are several ways that electronic parts designers can radiation-harden their devices. One of the most common is to harden for total-ionizing-dose radiation – or the amount of radiation the device is expected to withstand for its entire life before problems occur. A typical requirement is for 100 kilorads of total-dose radiation hardness.
Several experiments aboard the International Space Station are testing whether the space agency can move beyond traditional rad-hard components. NASA has long specified traditional and expensive radiation hardening of electronic components either by adding redundant circuits or using insulating substrates on semiconductor wafers. That brute force approach is expensive. HPE engineers came up with what they say is a cheaper approach for hardening electronics: “…simply slowing down a system in adverse conditions can avoid glitches and keep the computer running,” according to an HPE blog post.
Radiation hazards associated with a trip to Mars — at least 35 million miles at its closest approach to Earth — would likely be far greater than those in Earth’s orbit. Nevertheless, HPE engineers note that current commercial electronic components far exceed current radiation hardening requirements for the space station. Meanwhile, key components for future space computing platforms are expected to be available in the next few years. For example, the space agency expects its next general-purpose processor, a variant of the ARM Cortex-A53, to be cleared for launch in 2020.
Radiation effects on Electronics
There are two primary ways that radiation can effect satellite electronics: total ionizing dose (TID) and single event effects (SEEs). TID is a long-term failure mechanism vs. SEE, which is an instantaneous failure mechanism. SEE is expressed in terms of a random failure rate, whereas TID is a failure rate that can be described by a mean time to failure.
Radiation has the potential to interfere with electronic devices and systems, creating so-called radiation-induced effects. Radiation effects on electronics are normally divided into 3 different categories according to their effect on the electronic components:
Total ionizing dose (TID):
TID is a time dependent, accumulated charge in a device over the lifetime of a mission. A particle passing through a transistor generates electron hole pairs in the thermal oxide. The accumulated charges can create leakage currents, degrade the gain of a device, affect timing characteristics, and, in some cases, result in complete functional failure. The total accumulated dose depends on the orbit and time. In LEO, the main source of radiation is from electrons and protons (inner belt) and in GEO, the primary source is from electrons (outer belt) and solar protons. It is worth noting that device shielding can be used to effectively reduce the accumulation of TID radiation.
Total Ionizing Dose effects on modern integrated circuits cause the threshold voltage of MOS transistors to change because of trapped charges in the silicon dioxide gate insulator. For sub-micron devices these trapped charges can potentially “escape” by tunneling effects. Leakage currents are also generated at the edge of (N)MOS transistors and potentially between neighbor N-type diffusions. Commercial digital CMOS processes can normally stand a few Krad without a significant increase in power consumption. Modern sub-micron technologies tend to be more resistant to total dose effects than older technologies (in some cases up to several hundred Krad). High performance analog devices ( e.g. amplifiers, ADC, DAC) may though potentially be affected at quite low doses. Total dose is measured in Rad or Gray ( 1 Gray = 100 Rad.)
Hadrons may displace atoms (therefore called displacement effect) in the silicon lattice of active devices and thereby affect their function. Bipolar devices and especially optical devices ( e.g. Lasers, LEDs, Optical receivers, Opto-couplers) may be very sensitive to this effect. CMOS integrated circuits are normally not considered to suffer degradation by displacement damage. The total effect of different types of hadrons at different energies are normalized to 1 Mev Neutrons using the NIEL ( Non Ionizing Energy Loss) equivalent.
Single event effects (SEE):
Single Event Effects refer to the fact that it is not a cumulative effect but an effect related to single individual interactions in the silicon. Highly ionizing particles can directly deposit enough charge locally in the silicon to disturb the function of electronic circuits. Energetic Hadrons ( > ~20Mev) can by nuclear interactions within the component itself generate recoils that also deposits sufficient charge locally to disturb the correct function. The different SEE effects are normally characterized by an energy threshold and a sensitivity cross-section at energies well above the threshold.
Typically, SEEs are divided into soft errors and hard errors. The Joint Electron Device Engineering Council (JEDEC) defines soft errors as nondestructive, functional errors induced by energetic ion strikes. Soft errors are a subset of SEEs and includes single event upsets (SEUs), multiple-bit upsets (MBUs), single event functional interrupts (SEFIs), single event transients (SETs), and single event latch-up (SEL). A SEL is where the formation of parasitic bipolar action in CMOS wells induces a low impedance path between power and ground, producing a high current condition. Therefore, a SEL can cause latent and hard errors.
Examples of soft errors would be bit flips or changes in the state of memory cells or registers. A SET is a transient voltage pulse generated by a charge injected into the device from a high energy particle. These transient pulses can cause SEFIs. SEFIs are soft errors that cause the component to reset, lock-up, or otherwise malfunction in a detectable way, but does not require power cycling of the device to restore operability. A SEFI is often associated with an upset in a control bit or register.
JEDEC defines a hard error as an irreversible change in operation that is typically associated with permanent damage to one or more elements of a device or circuit (for example, gate oxide rupture, or destructive latch-up events). The error is hard because the data is lost and the component or device no longer functions properly, even with a power reset. SEE hard errors are potentially destructive. Examples of hard errors are single event latch-up (SEL), single event gate rupture (SEGR), and single event burnout (SEB). SEEs hard errors can destroy the device, drag down the bus voltage, or even damage the system power supply.
Single event upset (SEU):
The deposited charge is sufficient to flip the value of a digital signal. Single Event Upsets normally refer to bit flips in memory circuits ( RAM, Latch, flip-flop) but may also in some rare cases directly affect digital signals in logic circuits.
Single event latchup (SEL):
Bulk CMOS technologies (not Silicon On Insulat