Radiation hardening

[2] Radiation-hardened products are typically tested to one or more resultant-effects tests, including total ionizing dose (TID), enhanced low dose rate effects (ELDRS), neutron and proton displacement damage, and single event effects (SEEs).

In order to ensure the proper operation of such systems, manufacturers of integrated circuits and sensors intended for the military or aerospace markets employ various methods of radiation hardening.

Two fundamental damage mechanisms take place: Lattice displacement is caused by neutrons, protons, alpha particles, heavy ions, and very high energy gamma photons.

They change the arrangement of the atoms in the crystal lattice, creating lasting damage, and increasing the number of recombination centers, depleting the minority carriers and worsening the analog properties of the affected semiconductor junctions.

The ionization effects are usually transient, creating glitches and soft errors, but can lead to destruction of the device if they trigger other damage mechanisms (e.g., a latchup).

This leads to an increase in the count of recombination centers and deep-level defects, reducing the lifetime of minority carriers, thus affecting bipolar devices more than CMOS ones.

There is also a risk of induced radioactivity caused by neutron activation, which is a major source of noise in high energy astrophysics instruments.

A total dose greater than 5000 rads delivered to silicon-based devices in a timespan on the order of seconds to minutes will cause long-term degradation.

In CMOS devices, the radiation creates electron–hole pairs in the gate insulation layers, which cause photocurrents during their recombination, and the holes trapped in the lattice defects in the insulator create a persistent gate biasing and influence the transistors' threshold voltage, making the N-type MOSFET transistors easier and the P-type ones more difficult to switch on.

Transient dose effects result from a brief high-intensity pulse of radiation, typically occurring during a nuclear explosion.

The high radiation flux creates photocurrents in the entire body of the semiconductor, causing transistors to randomly open, changing logical states of flip-flops and memory cells.

Single event effects have importance for electronics in satellites, aircraft, and other civilian and military aerospace applications.

An SET happens when the charge collected from an ionization event discharges in the form of a spurious signal traveling through the circuit.

Single-event upsets (SEU) or transient radiation effects in electronics are state changes of memory or register bits caused by a single ion interacting with the chip.

In very sensitive devices, a single ion can cause a multiple-bit upset (MBU) in several adjacent memory cells.

A heavy ion or a high-energy proton passing through one of the two inner-transistor junctions can turn on the thyristor-like structure, which then stays "shorted" (an effect known as latch-up) until the device is power-cycled.

As the effect can happen between the power source and substrate, destructively high current can be involved and the part may fail.

A single-event snapback is similar to an SEL but not requiring the PNPN structure, and can be induced in N-channel MOS transistors switching large currents, when an ion hits near the drain junction and causes avalanche multiplication of the charge carriers.

White neutron beams—ostensibly the most representative SEE test method—are usually derived from solid target-based sources, resulting in flux non-uniformity and small beam areas.

However, recent studies have indicated that, to the contrary, mono-energetic neutrons—particularly 14 MeV neutrons—can be used to quite accurately understand SEE cross-sections in modern microelectronics.

[14] Choosing a substrate with wide band gap gives it higher tolerance to deep-level defects; e.g. silicon carbide or gallium nitride.

[18] Magnetoresistive RAM, or MRAM, is considered a likely candidate to provide radiation hardened, rewritable, non-volatile conductor memory.

Physical principles and early tests suggest that MRAM is not susceptible to ionization-induced data loss.

In the event of a single-bit failure (which may be unrelated to radiation), the voting logic will continue to produce the correct result without resorting to a watchdog timer.

If radiation causes the processor to operate incorrectly, it is unlikely the software will work correctly enough to clear the watchdog timer.