Fundamental mechanisms Two fundamental damage mechanisms take place:
Lattice displacement 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. Counterintuitively, higher doses over a short time cause partial
annealing ("healing") of the damaged lattice, leading to a lower degree of damage than with the same doses delivered in low intensity over a long time (LDR or low dose rate). This type of problem is particularly significant in
bipolar transistors, which are dependent on minority carriers in their base regions; increased losses caused by
recombination cause loss of the transistor
gain (see
neutron effects). Components certified as ELDRS (enhanced low dose rate sensitive)-free do not show damage with fluxes below 0.01 rad(Si)/s = 36 rad(Si)/h.
Ionization effects Ionization effects are caused by charged particles, including ones with energy too low to cause lattice effects. 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).
Photocurrent caused by
ultraviolet and X-ray radiation may belong to this category as well. Gradual accumulation of
holes in the oxide layer in
MOSFET transistors leads to worsening of their performance, up to device failure when the dose is high enough (see
total ionizing dose effects). The effects can vary wildly depending on all the parameters – type of radiation, total dose and radiation flux, combination of types of radiation, and even the kind of device load (operating frequency, operating voltage, actual state of the transistor during the instant it is struck by the particle) – which makes thorough testing difficult, time-consuming, and requiring many test samples.
Resultant effects The "end-user" effects can be characterized in several groups:
Neutron effects A neutron interacting with a semiconductor lattice will displace the atoms in the lattice. 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. Bipolar devices on
silicon tend to show changes in electrical parameters at levels of 1010 to 1011 neutrons/cm2, while CMOS devices aren't affected until 1015 neutrons/cm2. The sensitivity of devices may increase together with increasing level of integration and decreasing size of individual structures. There is also a risk of induced radioactivity caused by
neutron activation, which is a major source of noise in
high energy astrophysics instruments. Induced radiation, together with residual radiation from impurities in component materials, can cause all sorts of single-event problems during the device's lifetime.
GaAs LEDs, common in
optocouplers, are very sensitive to neutrons. The lattice damage influences the frequency of
crystal oscillators. Kinetic energy effects (namely lattice displacement) of charged particles belong here too.
Total ionizing dose effects Total ionizing dose effects represent the cumulative damage of the semiconductor lattice (
lattice displacement damage) caused by exposure to ionizing radiation over time. It is measured in
rads and causes slow gradual degradation of the device's performance. 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. The accumulated charge can be high enough to keep the transistors permanently open (or closed), leading to device failure. Some self-healing takes place over time, but this effect is not too significant. This effect is the same as
hot carrier degradation in high-integration high-speed electronics. Crystal oscillators are somewhat sensitive to radiation doses, which alter their frequency. The sensitivity can be greatly reduced by using
swept quartz. Natural
quartz crystals are especially sensitive. Radiation performance curves for total ionizing dose (TID) testing may be generated for all resultant effects testing procedures. These curves show performance trends throughout the TID test process and are included in the radiation test report.
Transient dose effects 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. Permanent damage may occur if the duration of the pulse is too long, or if the pulse causes junction damage or a latchup. Latchups are commonly caused by the X-rays and gamma radiation flash of a nuclear explosion. Crystal oscillators may stop oscillating for the duration of the flash due to prompt
photoconductivity induced in quartz.
Systems-generated EMP effects SGEMP effects are caused by the radiation flash traveling through the equipment and causing local
ionization and
electric currents in the material of the chips,
circuit boards,
electrical cables and cases.
Digital damage: SEE Single-event effects (SEE) have been studied extensively since the 1970s. When a high-energy particle travels through a semiconductor, it leaves an
ionized track behind. This ionization may cause a highly localized effect similar to the transient dose one - a benign glitch in output, a less benign bit flip in memory or a
register or, especially in
high-power transistors, a destructive latchup and burnout. Single event effects have importance for electronics in satellites, aircraft, and other civilian and military aerospace applications. Sometimes, in circuits not involving latches, it is helpful to introduce
RC time constant circuits that slow down the circuit's reaction time beyond the duration of an SEE.
Single-event transient An SET happens when the charge collected from an ionization event discharges in the form of a spurious signal traveling through the circuit. This is de facto the effect of an
electrostatic discharge. it is considered a soft error, and is reversible.
Single-event upset 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. They do not cause lasting damage to the device, but may cause lasting problems to a system which cannot recover from such an error. It is otherwise a reversible soft error. In very sensitive devices, a single ion can cause a
multiple-bit upset (MBU) in several adjacent memory cells. SEUs can become
single-event functional interrupts (
SEFI) when they upset control circuits, such as
state machines, placing the device into an undefined or incorrect state, a
test mode, or a halt, which would then need a
reset or a
power cycle to recover.
Single-event latchup An SEL can occur in any chip with a
parasitic PNPN structure. 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. This is a hard error, and is irreversible. Bulk CMOS devices are most susceptible.
Single-event snapback 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. The transistor then opens and stays opened, a hard error which is irreversible.
Single-event induced burnout An SEB may occur in power MOSFETs when the substrate right under the source region gets forward-biased and the drain-source voltage is higher than the breakdown voltage of the parasitic structures. The resulting high current and local overheating then may destroy the device. This is a hard error, and is irreversible.
Single-event gate rupture SEGR are observed in power MOSFETs when a heavy ion hits the gate region while a high voltage is applied to the gate. A local breakdown then happens in the insulating layer of
silicon dioxide, causing local overheating and destruction (looking like a microscopic
explosion) of the gate region. It can occur even in
EEPROM cells during write or erase, when the cells are subjected to a comparatively high voltage. This is a hard error, and is irreversible.
SEE testing While proton beams are widely used for SEE testing due to availability, at lower energies proton irradiation can often underestimate SEE susceptibility. Furthermore, proton beams expose devices to risk of total ionizing dose (TID) failure which can cloud proton testing results or result in premature device failure. 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. White neutron beams also have some measure of uncertainty in their energy spectrum, often with high thermal neutron content. The disadvantages of both proton and spallation neutron sources can be avoided by using mono-energetic 14 MeV neutrons for SEE testing. A potential concern is that mono-energetic neutron-induced single event effects will not accurately represent the real-world effects of broad-spectrum atmospheric neutrons. 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. ==Radiation-hardening techniques==