On 24 January 1994, the “Clementine” spacecraft was launched as part of the Deep Space Program Science Experiment (DSPSE), which aimed to make scientific observations of the Moon and some asteroids, in particular 1620 Geographos. Almost four months later, on 7 May 1994, the central computer sent an unintentional command that caused an altitude controller to burn up all its fuel. This prevented the mission from continuing and was almost certainly caused by a Single Event Upset (SEU), one of the most common effects of cosmic radiation on electronics.
An environment not only hostile to life but also to electronics
The solar wind, cosmic rays and van Allen belts put a strain on electronic circuits. This radiation consists mainly of electrons, protons, neutrinos, positrons, photons and other particles with ionising power, capable of breaking the covalent bonds between atoms.
Perturbations from space. Credits: www.etantonio.it
Most problematic are high-energy particles (greater than or equal to 10^9 eV/cm^3) which, although rare, can affect devices at any time and compromise the operation of an entire satellite, probe or any other spacecraft.
The effects of radiation on electronics can be classified into two major groups:
- Cumulative Effects
- Single Event Effects (SEE)
Electronic Circuitry. Credits: www.vnbamboo.com.vn
The first are the damages that accumulate irreparably over the years, until the electronics inside the space devices become unusable. This damage is predictable in the laboratory and allows us to establish an average service life for each aircraft. We can differentiate them into Total Ionising Dose (TID) and Displacement Damage (DD).
The latter, on the other hand, are unpredictable and can appear at any time since the electronic equipment was put into space. SEE in turn, are grouped into two categories: transient effects (or soft error) such as Single Event Transient (SET) and Single Event Upset (SEU); catastrophic effects such as Single Event Burnout (SEB), Single Event Gate Rupture (SEGR) and Single Event Latch-up (SEL).
Ionisation damage occurs in transistors, both field-effect transistors (such as MOSFETs) and bipolar junction transistors (BJTs). In the case of MOSFETs, ionisation is caused by a highly charged particle impinging on the silicon dioxide (insulating layer); it causes the electron-lacuna pairs to break up, directing them towards the oxide-silicon interface where it creates “trap” states that decrease the number of effective carriers and thus the transistor’s threshold voltage. In the BJT, on the other hand, these particles increase the base current and consequently the overall gain.
How Displacement Damage occurs. Credits:wpo-altertechnology.com
Displacement Damage is associated with the structure of the crystal lattice of materials. When a neutron interacts with an atom in the lattice, it imparts enough energy to displace it, and it travels a certain distance creating other displacements which disrupt the periodicity of the crystal and produce energy levels in the forbidden band. These energy levels alter the electrical properties of materials and therefore of devices.
Single Event Effects
Single Event Effects are phenomena caused by highly ionising particles (such as heavy ions). These events cause immediate malfunctions of one or more transistors that can affect the entire circuit.
A particle passing through a CMOS. Credits: www.esa.int
The mechanism underlying any SEE is the accumulation of charge in a sensitive area of a device as a result of the passage of the particle. In a semiconductor device, a column of electron-lunar pairs, varying in diameter from a few hundred nanometres to a few microns, is released along the path by coulombic interaction.
Depending on various factors, the particle may cause unobservable effects (SET), transient perturbations of the microprocessor circuit operations, changes in logical states (SEU, SEL), or permanent damage to the device or integrated circuit (SEGR, SEBO).
The main countermeasures
Once one of these effects has occurred, one can use the “Annealing” technique, which consists of applying high temperatures (or injecting electrons) to the transistors, to give the charge carriers sufficient energy to reactivate and re-establish the conduction or crystal lattice structure of the materials.
Example of a hard-rad component. Credits: www.militaryaerospace.com
For prevention, satellites are avoided in the van Allen belt areas; they are switched off during the periods of greatest solar wind flow; shielding against radiation is done (even if sometimes it can be heavy); but above all, when possible, we try to use “Hard-rad” components which, even if expensive, are made and tested in the laboratory, much more resistant than normal COTS (commercial components).