Last time (Jupiter: The IC Danger Zone, Part 1) we looked at Total Ionizing Dose and Displacement Damage and how they relate to testing a waterproof fabric and the environment that the Juno probe will encounter in orbit around Jupiter. This month we’ll dive into the details of Heavy Ion Radiation, which is a different kind of radiation testing. Some of these failure modes are deal-breakers, some of them are not.
Heavy Ion Radiation
Sometimes, you may want to bring your high-performance waterproof fabric into a water gun fight. Let’s face it, being soaked can be a cold experience, so keeping yourself dry is important! And how terrible would it be if that fabric didn’t stop the intense pressure of being pelted with a Super Soaker!? Or, assuming you’re wearing a full body suit, what if you get shot in the eye or get the wind blown out of you? If a high-pressure water stream was enough to cause the fabric to fail and leak water to your skin, you’d be in for a cold battle. Heavy Ion testing puts ICs through a similar test.
In any, particularly CMOS, there are parasitic bipolar transistors which in normal operating conditions cannot ever be excited into latch-up on earth unless there is some sort of other catastrophic failure. However, in space there are free energetic particles floating around which could trigger a latch-up if they happen to hit in just the right place. As the particle punches through the device, it can ricochet off of the crystalline structure. Often latch-up occurs through a combination of the lateral and vertical parasitic transistors as the particles bounce around. Let’s look at a CMOS inverter as an example.
Latch-up current flow in a CMOS inverter
The parasitic BJTs are shown in red, and the latch-up current flow is shown by the blue arrows. It’s just like latch-up in any other situation, except now you have to worry about ions which can hit at random. If any high energy particle were to hit in just the right spot, it could trigger these BJTs to turn on in such a way that normally wouldn’t be possible. Since they connect VDD to VSS, the only way to stop this failure is to turn the inverter completely off. This is of course assuming that these kinds of huge currents don’t cause any fatal damage to the device! Since these high energy particles will hit at random times and on random parts of the die, any space application device must be checked that these latch-ups do not occur. And of course, this effect can be exacerbated by any lingering charges lodged in the IC from other ionizing radiation.
This is another important parameter to check for in any of Juno’s electronics. Much of the radiation which exists around the Red Giant is very high energy particles moving close to the speed of light. If any one of these particles were to barrel into a sensitive device, it could be catastrophic. That titanium box protects these sensitive devices from what their design modifications cannot.
Moving slightly away from our waterproof fabric example, let’s say you’re, again, in a water-gun fight. Someone nails you right in the face. Agh! That hurt! Your face probably got contorted for a few seconds, but then you were fine and back at it and ready to go. Interruptions to device function like this caused by radiation are referred to as Single Event Effects, or SEE. They go by different names depending on the device and interruption in question, but they are all caused by similar radiation events.
SEFI, or Single Event Functional Interrupt, is a term used for disruptions in functionality of a device, usually a data converter, where a register bit gets flipped by an injected ion. In this kind of interruption, an ion is injected into a register and gets trapped there, changing the register state, and thus the device operation. It could be as simple as putting a data converter into a different mode of operation, or it could affect whether the device is enabled or disabled. It is considered a soft failure if a device reboot or a register re-write clears the error. There are many methods used to prevent these kinds of soft failures in the field, including redundant memory and checksums, which are used to trigger a device reset or register re-write so that the IC can return to normal functionality. The equivalent term used for memory devices is referred to as a Single Event Upset, or SEU.