When Radiation Meets Electronic Systems (Part 2)
In my previous column, we noted that radiation comes in two forms: electromagnetic waves and subatomic particles. We will focus on the latter in this column.
In the case of particle-based radiation, the particles in question are electrons (also known as beta particles), protons, neutrons, and atomic nuclei. In turn, the atomic nuclei range from helium nuclei (also known as alpha particles, symbol) to heavier ions.
Another team one hears is “cosmic rays,” but this is a misnomer based on the fact that — when these “rays” were first detected — they were thought to be electromagnetic radiation in the form of X-rays and gamma rays. It was later determined that cosmic rays are very high-energy particles, predominantly originating outside the Solar System from distant cosmic events such as suns going supernova.
When a primary cosmic ray hits the Earth’s atmosphere, it may produce showers of secondary particles that sometimes reach the surface of the planet.
Why is all this of interest to us here? Well, if a particle is travelling with sufficient speed and/or has sufficient energy, then when it interacts with another atom or molecule it can act as ionizing radiation, which means it can strip one or more electrons away to leave the impacted atom or molecule as a positive ion (this is not considered to be a good thing to happen).
Just how energetic are these little rascals? Well, the highest-energy cosmic rays we’ve encountered thus far pack a kinetic energy punch equivalent to a baseball travelling at 56 mph (90 kph). Think about how you would feel if such a baseball hit you, then consider all this energy being transferred to a microscopic area in a silicon chip in a fraction of a second.
There are all sorts of ways in which radiation can affect silicon chips. For example, it can degrade the crystal matrix and modify the switching thresholds of transistors. Also, it can result in charge being trapped between the various layers forming the chip. Taken together, these effects can degrade the speed of the device while negatively impacting its power consumption. These effects build up over time, which is why you hear terms like the total ionizing dose (TID).
Now, I’m a digital hardware design engineer by trade, so I tend to focus on the ways in which radiation might affect my digital designs. In this context, we tend to talk about single event effects (SEEs), which are caused by a single energetic particle. One type of SEE is a single event upset (SEU), in which a register element or memory cell is flipped from a 0 to a 1, or vice versa.
One way to address this is to apply triple modular redundancy (TMR) at the register level. The idea here is that each register is triplicated and the results from the three registers are passed to a voting circuit. If a radiation event flips one of the register bits, the voting circuit will accept the contents of the other two registers as representing the correct value.
SEUs are regarded as being “soft” errors because they are non-destructive; also, the error will be cleared out on the next active edge of the clock.
Another form of SEE is a single event transient (SET), which is caused when a radiation event impacts a chunk of combinatorial logic triggering a glitch / spike / pulse.
By itself, this type of SET isn’t too worrisome. The problem arises if the SET occurs close to an active edge on the clock, in which case it may be loaded into the register, at which point it is transmogrified into the equivalent of an SEU. One way to address this is to add delay buffers between the inputs to the TMR registers.
The effect of these delay buffers is to ensure that only one of the registers is loaded with the incorrect value. This will be handled by the voting circuit and, once again, the incorrect value will be cleared out on the next active edge of the clock.
It’s not so long ago that radiation effects were of interest only to the designers of electronic systems that were intended to be deployed in high-altitude aircraft, satellites and space probes, and high radiation environments like close to the core of a nuclear power station. Sad to relate, as the sizes of the structures on our chips continue to shrink, these devices become more susceptible to radiation events. On the up-side, engineers who understand these effects and know how to design around them will be in increasingly high demand.
