Radiation hardening is a process that makes electronic parts and circuits less likely to be damaged or stop working when exposed to high levels of ionizing radiation, such as particle radiation and high-energy electromagnetic radiation. This process is especially important in environments like outer space, particularly beyond low Earth orbit, around nuclear reactors and particle accelerators, or during nuclear accidents or nuclear warfare.

Most electronic parts made from semiconductors are vulnerable to radiation damage. Radiation-hardened (rad-hard) components are similar to regular ones but have design and manufacturing changes that make them less likely to be damaged by radiation. Because there is not much demand and because creating radiation-tolerant chips requires a lot of time and testing, the technology for these chips often lags behind newer developments. They usually cost more than regular commercial chips.

Radiation-hardened products are usually tested using one or more tests that check for effects like total ionizing dose (TID), enhanced low dose rate effects (ELDRS), neutron and proton displacement damage, and single event effects (SEEs).

Problems caused by radiation

Environments with high levels of ionizing radiation make it difficult to design reliable electronic systems. A single charged particle can dislodge thousands of electrons, creating electrical interference and sudden spikes in signals. In digital circuits, this can lead to incorrect or unclear results. This issue is especially important in the design of satellites, spacecraft, future quantum computers, military aircraft, nuclear power stations, and nuclear weapons. To ensure these systems work properly, companies that make integrated circuits and sensors for military or aerospace use apply special techniques to protect against radiation. These systems are called rad-hardened, rad-hard, or simply hardened.

Major radiation damage sources

Typical sources of exposure to ionizing radiation for electronics include the Van Allen radiation belts for satellites, nuclear reactors in power plants for sensors and control circuits, particle accelerators for control electronics (especially particle detector devices), residual radiation from isotopes in chip packaging materials, cosmic radiation for spacecraft and high-altitude aircraft, and nuclear explosions for potentially all military and civilian electronics.

Secondary particles are created when other types of radiation interact with structures near electronic devices.

• The Van Allen radiation belts contain high-energy electrons (up to about 10 MeV) and protons (up to hundreds of MeV) trapped by Earth's magnetic field. The number of particles in regions farther from Earth can change greatly depending on conditions in the Sun and magnetosphere. These belts are a concern for satellites because of their location.
• Nuclear reactors produce gamma radiation and neutron radiation, which can affect sensors and control circuits in nuclear power plants.
• Particle accelerators generate high-energy protons and electrons. The secondary particles created by their interactions can cause significant radiation damage to sensitive control and particle detector components, such as in the Large Hadron Collider, with damage levels reaching about 10 MRad[Si]/year.
• Chip packaging materials were a hidden source of radiation discovered in the 1970s. Small amounts of radioactive elements in the packaging produced alpha particles that occasionally caused errors in DRAM chips by disrupting capacitors storing data. These issues are now reduced by using purer materials and error-correcting codes to detect and correct errors.
• Cosmic rays come from all directions and are mostly protons (about 85%), with smaller amounts of alpha particles (about 14%) and heavy ions (about 1%), along with X-ray and gamma-ray radiation. Most effects are caused by particles with energies between 0.1 and 20 GeV. The atmosphere blocks most of these, so they mainly affect spacecraft and high-altitude aircraft, though they can also impact ordinary computers on Earth.
• Solar particle events release large numbers of high-energy protons and heavy ions from the Sun, along with X-ray radiation.
• Nuclear explosions create a short but extremely intense burst of electromagnetic radiation (an electromagnetic pulse or EMP), neutron radiation, and a flow of primary and secondary charged particles. In the event of a nuclear war, these could threaten all civilian and military electronics.

Radiation effects on electronics

Two main types of damage occur in semiconductor materials when exposed to radiation:

Lattice displacement happens when neutrons, protons, alpha particles, heavy ions, or very high-energy gamma photons strike the material. These particles move atoms out of their normal positions in the crystal lattice, causing permanent damage. This damage increases the number of recombination centers, which reduce the number of minority carriers and worsen the performance of semiconductor junctions. Interestingly, higher radiation doses delivered quickly can partially repair (anneal) the damage, resulting in less harm than the same doses given slowly over time (LDR or low dose rate). This issue is especially important for bipolar transistors, which rely on minority carriers in their base regions. Increased recombination reduces transistor gain (see neutron effects). Components labeled as ELDRS (enhanced low dose rate sensitive)-free do not show damage when exposed to radiation fluxes below 0.01 rad(Si)/s = 36 rad(Si)/h.

Ionization effects occur when charged particles, even those with too little energy to cause lattice displacement, interact with the material. These effects are usually temporary, causing brief errors or glitches, but can lead to device failure if they trigger other damage mechanisms, such as latchup. Photocurrent from ultraviolet or X-ray radiation may also fall into this category. Over time, the buildup of trapped charges in the oxide layer of MOSFET transistors can degrade their performance, eventually causing device failure at high radiation doses (see total ionizing dose effects).

The impact of radiation depends on many factors, including the type of radiation, total dose, radiation intensity, combinations of radiation types, and the device's operating conditions (such as voltage, frequency, and transistor state during exposure). These variables make thorough testing difficult, time-consuming, and require many test samples.

End-user effects can be grouped into several categories:

Neutrons interacting with a semiconductor lattice displace atoms, increasing recombination centers and deep-level defects. This shortens the lifetime of minority carriers, affecting bipolar devices more than CMOS ones. Bipolar devices on silicon show electrical changes at neutron levels of 10 to 10 neutrons/cm², while CMOS devices remain unaffected until 10 neutrons/cm². Device sensitivity may increase with higher integration and smaller component sizes. Neutron activation can also cause induced radioactivity, a major noise source in high-energy astrophysics instruments. Induced radiation and residual radiation from material impurities can lead to single-event problems over time. GaAs LEDs in optocouplers are highly sensitive to neutrons. Lattice damage can alter the frequency of crystal oscillators. Charged particle kinetic energy effects (lattice displacement) also fall into this category.

Total ionizing dose effects describe the gradual damage to semiconductor lattices caused by long-term exposure to ionizing radiation. Measured in rads, this damage slowly degrades device performance. Silicon-based devices exposed to over 5000 rads within seconds to minutes experience long-term degradation. In CMOS devices, radiation creates electron-hole pairs in gate insulation layers, causing photocurrents during recombination. Trapped holes in insulator defects create persistent gate biasing, altering transistor threshold voltages. This makes N-type MOSFETs easier to switch on and P-type ones harder. Accumulated charge can permanently lock transistors open or closed, leading to failure. Some self-healing occurs over time, but it is minimal. This effect is similar to hot carrier degradation in high-speed electronics. Crystal oscillators are somewhat sensitive to radiation, with frequency changes possible. Sensitivity can be reduced by using swept quartz instead of natural quartz. Radiation performance curves for total ionizing dose (TID) testing show device behavior trends and are included in radiation test reports.

Transient dose effects occur from brief, high-intensity radiation pulses, such as those from nuclear explosions. High radiation flux creates photocurrents in semiconductors, causing transistors to randomly open, altering logical states in flip-flops and memory cells. Permanent damage may occur if the pulse lasts too long or causes junction damage or latchup. Latchups are often triggered by X-rays or gamma radiation from nuclear explosions. Crystal oscillators may stop during the flash due to photoconductivity in quartz.

SGEMP effects happen when radiation flashes cause localized ionization and electric currents in chips, circuit boards, cables, and cases.

Single-event effects (SEEs) have been studied since the 1970s. High-energy particles passing through semiconductors leave ionized tracks, causing localized effects similar to transient dose effects. These may include harmless glitches, memory or register bit flips, or destructive latchups and burnouts in high-power transistors. SEEs are critical for electronics in satellites, aircraft, and aerospace applications. In circuits without latches, adding RC time constant circuits can help reduce the impact of SEEs by slowing reaction times.

An SET (single-event transient) occurs when charge from an ionization event creates a spurious signal in a circuit. This is similar to an electrostatic discharge and is considered a soft error, which can be reversed.

Single-event upsets (SEUs) or transient radiation effects happen when a single ion interaction changes memory or register bits. These do not cause lasting device damage but may cause system issues if errors cannot be recovered. In highly sensitive devices, a single ion may flip multiple adjacent memory bits (MBU). SEUs can become single-event functional interrupts (SEFI) when they disrupt control circuits, placing devices in undefined states that require resets or power cycles to fix.

An SEL (single-event latchup) occurs in chips with parasitic PNPN structures. A heavy ion or high-energy proton passing through inner-transistor junctions activates a thyristor-like structure, causing a short circuit (latchup) until power is cycled. This can lead to destructive currents and device failure. This is a hard error and cannot be reversed. Bulk CMOS devices are most vulnerable.

A single-event snapback is similar to an SEL but does not require a PNPN structure. It can occur in N-channel MOS transistors switching large currents when an ion hits near the drain junction.

Radiation-hardening techniques

Hardened chips are sometimes made on insulating materials instead of regular semiconductor wafers. Silicon on insulator (SOI) and silicon on sapphire (SOS) are common choices. Regular commercial chips can survive radiation doses between 50 and 100 gray (5 to 10 krad). Space-grade SOI and SOS chips can handle doses between 1000 and 3000 gray (100 to 300 krad). In the past, many 4000 series chips were available in radiation-hardened versions (RadHard). While SOI prevents latchup events, improvements in tolerance to total ionizing dose (TID) and single-event effects (SEE) are not guaranteed.

Choosing substrates with wide band gaps, such as silicon carbide or gallium nitride, increases tolerance to deep-level defects.

Using a special process node improves radiation resistance. Due to high development costs, the smallest "true" rad-hard (RHBP) process available in 2016 was 150 nm. However, rad-hard 65 nm FPGAs were available that used some techniques from "true" rad-hard processes (RHBD). By 2019, 110 nm rad-hard processes were available.

Bipolar integrated circuits generally have higher radiation tolerance than CMOS circuits. The low-power Schottky (LS) 5400 series can survive 1000 krad, and many ECL devices can withstand 10,000 krad. Edgeless CMOS transistors, which have an unusual design and layout, can also improve radiation resistance.

Magnetoresistive RAM (MRAM) is considered a promising option for radiation-hard, rewritable, non-volatile memory. Early tests suggest MRAM is not affected by ionization-induced data loss.

Capacitor-based DRAM is often replaced by more durable (but larger and more expensive) SRAM. SRAM cells use more transistors per cell (4T or 6T), making them more resistant to single-event upsets (SEUs) but increasing power use and size.

Shielding the package from radioactivity is a simple way to protect the device.

To protect against neutron radiation and neutron-induced material activation, chips can be shielded using depleted boron (which contains only boron-11) in the borophosphosilicate glass layer. Naturally occurring boron-10 captures neutrons and causes alpha decay, which can lead to errors.

Error-correcting code memory (ECC memory) uses extra bits to detect and correct corrupted data. Since radiation can damage memory even when the system is idle, a "scrubber" circuit continuously checks and corrects data in RAM.

Redundant parts can be used at the system level. Three separate microprocessor boards may independently perform calculations and compare results. If a board produces an unusual result, it will recalculate. Additional logic can shut down a board if repeated errors occur from the same system.

Redundant elements can also be used at the circuit level. A single bit may be replaced with three bits, with "voting logic" to determine the correct value (triple modular redundancy). This increases chip size by about five times and is best used for smaller designs. It also provides real-time fail-safety, as voting logic can correct single-bit failures without relying on a watchdog timer. System-level voting between three processors usually requires circuit-level voting logic to compare results.

Hardened latches may be used to improve reliability.

A watchdog timer forces a system reset if no activity is detected, such as a write operation from an onboard processor. During normal operation, software regularly updates the watchdog timer to prevent a reset. If radiation causes the processor to malfunction, the software may fail to update the timer. The watchdog will then reset the system, acting as a last-resort measure for radiation hardening.

Military and space industry applications

Radiation-hardened and radiation-tolerant components are frequently used in military and aerospace systems, such as power management systems, satellite power supplies, voltage regulators that reduce power, microprocessors, FPGAs, power sources for FPGAs, and high-efficiency, low-voltage power systems.

However, not all military-grade components are radiation-hardened. For example, the US MIL-STD-883 standard includes many tests related to radiation effects but does not include a requirement for single event latchup frequency. The Fobos-Grunt space probe may have failed due to a related assumption.

The market size for radiation-hardened electronics used in space applications was estimated to be $2.35 billion in 2021. A new study predicts this market will grow to about $4.76 billion by 2032.

Nuclear hardness for telecommunication

In telecommunication, the term nuclear hardness has these meanings: 1) a way to describe how much a system, facility, or device is expected to perform worse in a nuclear environment, and 2) the physical features of a system or electronic part that help it survive in an environment with nuclear radiation and electromagnetic pulses (EMP).

• Nuclear hardness can be described by how easily a system is affected or how likely it is to be harmed.
• How much a system's performance gets worse (like time without working, data lost, or equipment damage) must be clearly stated. The environment's details (like radiation levels, pressure, speed, energy, and electrical stress) also need to be clearly stated.
• The physical features of a system or part that help it survive in a nuclear environment caused by a weapon.
• Nuclear hardness is checked under specific environmental conditions (like radiation levels, pressure, speed, energy, and electrical stress). It is designed into systems and tested using analysis and experiments.

Examples of rad-hard computers

• The System/4 Pi, created by IBM and used on the Space Shuttle (AP-101 variant), uses the System/360 design.
• The RCA1802 8-bit CPU, introduced in 1976, was the first microprocessor made in large numbers and resistant to radiation.
• The PIC 1886VE, a Russian 50 MHz microcontroller designed by Milandr and made by Sitronics-Mikron using 180 nm bulk-silicon technology.
• m68k based: The Coldfire M5208 used by General Dynamics is a low power (1.5 W) and radiation hardened option.
• MIL-STD-1750A based: The RH1750 is produced by GEC-Plessey.
• The Proton 100k SBC by Space Micro Inc., introduced in 2003, uses TTMR technology to reduce single event upsets (SEU) in one processor. The processor is Equator BSP-15.
• The Proton200k SBC by Space Micro Inc., introduced in 2004, reduces SEU with patented TTMR technology and uses H-Core technology to handle single event function interrupts (SEFI). The processor is the high-speed Texas Instruments 320C6Xx series digital signal processor. The Proton200k operates at 4000 MIPS while reducing SEU.
• MIPS based: The RH32 is made by Honeywell Aerospace. The Mongoose-V used by NASA is a 32-bit microprocessor for spacecraft computers, such as on the New Horizons mission. The KOMDIV-32 is a 32-bit microprocessor, compatible with MIPS R3000, developed by NIISI and made by Kurchatov Institute, Russia.
• PowerPC/POWER based: The RAD6000 SBC, made by BAE Systems, includes a rad-hard POWER1 CPU. The RHPPC is made by Honeywell Aerospace, based on hardened PowerPC 603e. The SP0 and SP0-S, made by Aitech Defense Systems, are 3U cPCI SBCs using SOI PowerQUICC-III MPC8548E, PowerPC e500 based, capable of processing speeds from 833 MHz to 1.18 GHz. The RAD750 SBC, made by BAE Systems, is based on the PowerPC 750 processor and is the successor to the RAD6000. The SCS750, built by Maxwell Technologies, uses three PowerPC 750 cores to reduce radiation effects. Seven of these are used by the Gaia spacecraft. The Boeing Company produces a radiation hardened space computer variant based on the PowerPC 750. The BRE440 by Moog Inc. is an IBM PPC440 core based system-on-a-chip with 266 MIPS, PCI, 2x Ethernet, 2x UARTs, a DMA controller, and L1/L2 cache. The RAD5500 processor is the successor to the RAD750, based on the PowerPC e5500.
• SPARC based: The ERC32 and LEON 2, 3, 4, and 5 are radiation hardened processors designed by Gaisler Research and the European Space Agency. They are described in synthesizable VHDL code available under specific open-source licenses. The Gen 6 SBC, made by Cobham Semiconductor Solutions, is enabled for the LEON microprocessor.
• ARM based: The Vorago VA10820 is a 32-bit ARMv6-M Cortex-M0 microprocessor. The Vorago Cortex-M4 is also used. The ESA DAHLIA is an ARM Cortex-R52 based processor.
• RISC-V based: Cobham Gaisler NOEL is a 32/64-bit RISC-V processor. NASA's Jet Propulsion Laboratory chose Microchip Technology to create a new HPSC processor using SiFive Intelligence X280 technology.

Books and Reports

• Calligaro, Christiano; Gatti, Umberto (2018). Rad-hard Semiconductor Memories. River Publishers Series in Electronic Materials and Devices. River Publishers. ISBN 978-8770220200.
• Holmes-Siedle, Andrew; Adams, Len (2002). Handbook of Radiation Effects (Second ed.). Oxford University Press. ISBN 0-19-850733-X.
• León-Florian, E.; Schönbacher, H.; Tavlet, M. (1993). Data compilation of dosimetry methods and radiation sources for material testing (Report). CERN Technical Inspection and Safety Commission. CERN-TIS-CFM-IR-93-03.
• Ma, Tso-Ping; Dressendorfer, Paul V. (1989). Ionizing Radiation Effects in MOS Devices and Circuits. New York: John Wiley & Sons. ISBN 0-471-84893-X.
• Messenger, George C.; Ash, Milton S. (1992). The Effects of Radiation on Electronic Systems (Second ed.). New York: Van Nostrand Reinhold. ISBN 0-442-23952-1.
• Oldham, Timothy R. (2000). Ionizing Radiation Effects in MOS Oxides. International Series on Advances in Solid State Electronics and Technology. World Scientific. doi: 10.1142/3655. ISBN 978-981-02-3326-6.
• Platteter, Dale G. (2006). Archive of Radiation Effects Short Course Notebooks (1980–2006). IEEE. ISBN 1-4244-0304-9.
• Schrimpf, Ronald D.; Fleetwood, Daniel M. (July 2004). Radiation Effects and Soft Errors in Integrated Circuits and Electronic Devices. Selected Topics in Electronics and Systems. Vol. 34. World Scientific. doi: 10.1142/5607. ISBN 978-981-238-940-4.
• Schroder, Dieter K. (1990). Semiconductor Material and Device Characterization. New York: John Wiley & Sons. ISBN 0-471-51104-8.
• Schulman, James Herbert; Compton, Walter Dale (1962). Color Centers in Solids. International Series of Monographs on Solid State Physics. Vol. 2. Pergamon Press.
• Holmes-Siedle, Andrew; van Lint, Victor A. J. (2000). "Radiation Effects in Electronic Materials and Devices". In Meyers, Robert A. (ed.). Encyclopedia of Physical Science and Technology. Vol. 13 (Third ed.). New York: Academic Press. ISBN 0-12-227423-7.
• van Lint, Victor A. J.; Flanagan, Terry M.; Leadon, Roland Eugene; Naber, James Allen; Rogers, Vern C. (1980). Mechanisms of Radiation Effects in Electronic Materials. Vol. 1. New York: John Wiley & Sons. p. 13073. Bibcode: 1980STIA…8113073V. ISBN 0-471-04106-8. {{cite book}}: |journal= ignored (help).
• Watkins, George D. (1986). "The Lattice Vacancy in Silicon". In Pantelides, Sokrates T. (ed.). Deep Centers in Semiconductors: A State-of-the-Art Approach (Second ed.). New York: Gordon and Breach. ISBN 2-88124-109-3.
• Watts, Stephen J. (1997). "Overview of radiation damage in silicon detectors — Models and defect engineering". Nuclear Instruments and Methods in Physics Research Section A. 386 (1): 149–155. Bibcode: 1997NIMPA.386..149W. doi: 10.1016/S0168-9002(96)01110-2.
• Ziegler, James F.; Biersack, Jochen