21-09-2020 | | By Robin Mitchell
Since its discovery by Marie Curie, radiation has provided engineers with countless applications as well as design issues. What is radiation, how does it affect circuits, and how can engineers protect against it?
Radioactivity is the natural phenomena where unstable atomic nuclei release energy in the form of radiation to become more stable, and this release of energy comes in one of three types; Alpha, Beta, and Gamma. Alpha decay is when an atomic nucleus releases an alpha particle (a helium nucleus, or two protons and two neutrons), while Beta decay is when an atomic nucleus releases a Beta particle (an electron) as a neutron in the nucleus decays into a proton. Gamma radiation is a high-energy photon whose frequency is typically greater than 1018 Hz, but Gamma radiation is always emitted during both Alpha and Beta decays.
Radiation is a particular problem for electronic circuits as radiation can interfere in several different ways. The first method in which radiation affects circuits is through its destructive properties. Alpha particles are highly ionising and as a result, can denature materials by ripping electrons away, causing physical damage. However, Alpha particles have the least penetrative capabilities, thus are only able to damage the most outer layer of material. Beta particles are also able to cause physical damage by either making materials ionising, or by knocking electrons off atoms. While they are not as destructive as Alpha, they can penetrate deeper into materials and thus damage internal components. Gamma rays are the least ionising of all radiation types, and their destructive physical power comes from excitation of electrons or nuclei. However, they have the strongest penetrative power and can damage materials behind large amounts of shielding.
The second method in which radiation can cause havoc in electronic circuits comes from the ability to induce current. While radiation itself is not able to induce large currents, the ability to ionise materials combined with the increasingly small sizes of transistors results in the ability for radiation to trigger transistor gates. This could be in the form of briefly allowing a transistor to conduct enough current to flip a bit stored in memory, or even interfere with the floating gate layer in a FLASH chip. The result is that information can be interfered with, thus removing the ability for a computer to operate reliably.
Radiation hardened parts are components that have measures built into them to resist the effects of radiation. The first method used by components is in the form of metallic shielding, and a glance of such parts show gold plated packages (gold nuclei are extremely heavy and ideal for absorbing radiation). Radiation protection is also done using semiconductors that sit on insulating materials; this can prevent effects such as latch-up caused when a particle interferes with the bulk semiconductor material. Another method for protecting against radiation is to use different circuit technology such as the use of BJTs over CMOS. Bipolar Junction Transistors (BJT), are current based and use larger silicon space making them far more resilient to damage from radiation and ESD. In contrast, CMOS technology uses MOS transistors that utilise very thin insulating gates which are susceptible to damage. Radiation hardness can be further improved with the use of high band-gap materials such as silicon carbide and gallium nitride, which have strong dielectric properties and ability to resist spikes in voltages. In scenarios where neutron radiation is an issue, boron can be utilised in the IC packaging to absorb the neutrons. Still, the result is the emission of alpha particles (however, these are easier to defend against).
All of the methods of radiation hardening above involve physical shields and mechanisms to prevent damage; however, the effects of radiation can be mitigated against with the use of software. If the effects of radiation are not mechanical, but instead corrupted bits in memory, then error correction can be utilised instead. For example, Error-Correcting Memory, or ECC, is often used in applications that may experience such a fault. Upon a bit in memory being flipped, additional error bits (such as parity), can quickly flag up errors in memory, and then be used to restore the memory to its original contents.
Radiation hardened parts will always be found in applications that are susceptible to radiation but can also be found in applications that require high degrees of reliability. To start, electronics found in nuclear reactors and test sites will incorporate such electronics for the apparent reason that those applications directly involve radiation. Aerospace applications will also utilise radiation parts as cosmic rays can wreak havoc in circuits either in space or in high altitude (such as commercial airliners). Radiation hardened parts can also be found in select military products such as tanks and communications equipment in the event of radioactive weaponry. While server farms may not utilise radiation-hardened components, they will often use ECC protection in memory as stray cosmic rays can cause bit flips in memory.