Increased demand for connectivity to high-speed data networks is leading to increased deployment of space platforms for services ranging from GPS to military intelligence to commercial high-speed data for home internet applications.
Outside the Earth’s atmosphere, though, the solar system is filled with radiation that can damage electronic devices. The effects range from degradation in performance to leakage currents, lowering the gain of a device, upsetting timing characteristics, and, in some cases, resulting in complete functional failure.
Radiation in space comes in many flavors, including electromagnetic waves and various energetic particles (Fig. 1). The Van Allen radiation belts around the Earth contain charged particles—protons, electrons, and heavy ions—trapped in the Earth’s magnetic field. They’re typically found is very high orbits such as the geosynchronous orbits (GEO) that are approximately 36,000 km above the planet. The inner Van Allen belt consists largely of highly energetic protons, with energy exceeding 30,000,000 electron-volts. Protons trapped in the magnetic field also exist in high concentrations in low Earth orbit (LEO), which is defined as 1400 to 2000 km from the Earth’s surface.
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1. A spacecraft can encounter a range of different types of radiation, including the inner and outer trapped radiation belts, solar flares, and galactic cosmic radiation. (Source: NASA)
The sun also emits solar radiation continuously in all directions. This stream of charged particles emitted into space—the so-called “solar wind”—consists of lower-energy photons, plasma, and magnetic flux. Solar flares and coronal mass ejections can generate intense particle bursts with much higher energies and fluxes than the steady-state solar wind.
Not to be forgotten are cosmic rays, which come from all directions and consist of approximately 85% protons, 14% alpha particles, and 1% heavy ions, together with x-ray and gamma-ray radiation. Heavy ions are massive, highly charged particles that can cause severe damage to electronic devices.
In space, heavy-ion particles generally have so much energy that they easily ionize atoms, freeing negatively charged electrons. In electronic devices, this ionization process creates excess charge, which can produce both transient and lasting effects. Most effects are caused by particles with energies between 0.1 and 20 giga-electron-volts (GeV).
Impact on Electronics
Radiation can affect satellite electronics in two primary ways: total ionizing dose (TID) and single event effects (SEEs). SEEs are random, instantaneous disruptions triggered by the passage of a single particle or photon. A TID, with exposure quantified in units of radiation-absorbed dose, or rads, is a long-term failure mechanism, while SEE 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.
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. An example of soft errors would be bit changes in the state of memory cells or registers.
Soft errors can include temporary errors due to single-event upsets (SEUs, a change of state caused by a single ionizing particle striking a sensitive node in a device) (Figs. 2 and 3), multiple-bit upsets (MBUs, occurring in memory), single-event functional interrupts (SEFIs, where control regions of a device are hit by a particle, possibly resulting in system lockup or reset), single-event transients (SETs, which can cause voltage dropouts in logic devices) and single-event latch-up (SEL).
2. A particle event in sequential logic can become a persistent SEU in digital systems. The erroneous bit has some chance of being transmitted downstream and may affect a machine state or be written into memory. (Source: TI Radiation Handbook for Electronics)
3. In this diagram of a 1T-1C DRAM bit cell, the red arrows indicate where ion strikes are likely to inject charge that will cause an upset. (Source TI Radiation Handbook for Electronics)
An SEL is a potentially catastrophic short-circuit mechanism that can occur due to the inadvertent creation of a low-impedance path that disrupts proper functioning of the part. It could even lead to its destruction due to overcurrent if power isn’t current-limited and removed in time.
JEDEC defines a hard error as an irreversible change in operation that’s typically associated with permanent damage to one or more elements of a device or circuit (for example, gate oxide rupture). 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 can destroy the device, reduce the bus voltage, or even damage the system power supply.
Electronic devices in space environments may contain numerous types of oxides and insulators. Ionizing radiation can induce significant charge buildup in these oxides and insulators, leading to device degradation and failure. The key mechanism driving TID is the generation, transport, and trapping of holes in the insulation used as gate and isolation oxides in metal-oxide semiconductor (MOS) and in bipolar devices at or near the silicon-oxide interface.
In CMOS processes, the reduction in feature sizes over the years has generally resulted in an improvement in TID survivability. The limiting factor became the field oxide; charged field oxide created leakage paths underneath the oxide.
Generally speaking, structures 90 nm and below are good to 300 krad or even into the Mrad levels. The exception is fully depleted CMOS structures on SOI substrates. Charging of the buried oxide can impact these structures. As noted in TI’s Radiation Handbook for Electronics, unlike CMOS processing, gradual evolutionary changes in bipolar process technology have had little impact on TID survivability. In bipolar transistors, TID reduces the current gain of the device.
Heavy ions in space can also degrade the oxides in electronic devices through several different mechanisms. These include single-event gate rupture, reduction in device lifetime, and large voltage shifts in power MOSFETs.
Mitigating the Effects of Radiation
Three methods are typically used to minimize dose exposure for people and equipment: limiting time near the source of radiation, maximizing the distance between the user and the source (the flux will decrease with the square of the distance), and shielding the source of radiation.
In a space environment, the mission dictates the time and distance in the radiation zone. Therefore, the only recourse to reduce the exposure levels is to shield the electronics. Shielding impacts both the total dose and SEEs.
Radiation shielding usually consists of single or multiple barriers of metal, ceramic plates, or enclosures (Fig. 4). On Earth, lead is often used as a shielding material for gamma rays and X-rays due to its high density and low cost. But a severe constraint in spacecraft is the mass and size of the final payload. Large, heavy shielding usually isn’t a viable option due to these constraints.
4. Shown are effective shielding materials for different specific particle radiation that can be encountered (Source: TI Radiation in Electronics Handbook)
Two fundamental methods harden microelectronics against radiation effects and can be used individually or in combination. The first is called radiation hardening by process (RHBP). It focuses on modifying the baseline semiconductor process. RHBP alone will seldom result in a complete elimination of radiation effects, but it can reduce them so that a component will pass the metric with the modified process.
Texas Instruments’ space-grade products go through a single process flow. Because it has its own wafer foundries, TI is able to control process changes that could impact radiation performance. As found in TI Space Products, the space-grade ADC08D1520QML-SP and ADC14155QML-SP analog-to digital converters (ADCs) are on the same CMOS 180-nm process. The ADC08D1520QML-SP digitizes signals to 8 bits of resolution at sample rates up to 1.7 Gsamples/s. It uses 1.9-V cells and is rated to 300 krad. The ADC14155QML-SP is an ADC capable of converting analog input signals into 14-bit digital words at rates up to 155 Msamples/s. It uses the 3.3-V modules available on this process and is rated to 100 krad.
Design solutions ranging from layout-based changes to circuit-design alterations is another method of attenuating the effects of radiation. Known as radiation hardening by design (RHBD) it has a few drawbacks: only new from-the-ground-up designs can benefit from RHBD methods; it will add to layout area; it likely will increase design complexity; and RHBD parts are more costly to manufacture (NASA has found that the cost to harden a satellite to the natural space radiation environment by using rad-hard electronics is typically about 1% of the total system cost).
Ionization from space-based radiation events, which continually increases over the lifetime of a spacecraft, can cause errors leading to catastrophic failure in electronic devices. Proper selection of components and Improvements in circuit design can be easily incorporated to mitigate certain radiation effects. Early involvement of radiation specialists in system design and design review (part-by-part verification) is essential.
Attempting to use a commercial product in place of the space-grade version of a device can be risky or even disastrous. For instance, the space-grade versions of TI’s ADC128S102 and DAC121S101 are radiation-hardened by design, while the commercial versions are not and will experience both SEL and SEFI at low ion energies.
Texas Instruments has supplied space-grade products for six decades. The company’s space-grade products are manufactured, tested, and qualified per military specification MILPRF-38535 QML Class V. Most are listed on the Defense Logistics Agency’s Qualified Manufacturers List (QML) and they’re radiation hardness assured (RHA). To aid in device selection and design-in, Texas Instruments provides radiation test data with TID and SEE reports.
There’s a wealth of radiation test reports and publications about Texas Instruments and other suppliers‘ products online. TI has an extensive list of space products and reference designs located here.