Jupiter, Juno, and Why Space Qualification Still Doesn’t Cut It
I have always been fascinated with space exploration. The endless possibilities of discovery captured my imagination as a kid. It wasn’t until I began working with space development that I could fully appreciate what goes into the development of a new space craft. Watching the Juno probe perform its Jupiter Orbit Insertion maneuver brought tears to my eyes and inspired me to learn more about the probe and Earth’s biggest neighbor.
There are many things which engineers at NASA, Space X, and at a semiconductor company like ADI must consider when developing products for use in space that anyone else wouldn’t have to worry about for earth-bound applications, most notably of which is radiation (of course unless you’re working on a nuclear power plant). Radiation can wreak havoc on any CMOS device, causing glitches and even damage beyond repair in many cases. We’ll cover these failure modes in a little bit.
For those with limited experience working in the space market, any device which is targeted to companies like NASA must adhere to a set of standards set by the Defense Logistics Agency (DLA). Among these specifications are different radiation tolerance levels which a “rad-hard” space qualified device must pass. This typically entails a standard level of radiation testing up to 100Krads.
While 100Krads may sound like a lot of radiation, even a standard “rad-hard” device as tested to DLA standards wouldn’t stand a chance around Jupiter. Jupiter has an enormous magnetic field, and it’s extremely strong. If we could see Jupiter’s magnetosphere from earth, it might look something like this. See Figure 1.
Jupiter’s Magnetosphere if it could be seen from earth; Photo Credit: NASA
Just like Earth’s, Jupiter’s magnetic field is distorted by the solar winds. The tail shown here extends all the way past Saturn’s orbit! This strong magnetic field traps solar wind particles, as well as those ejected by the volcanic moon Io, and flings them around the planet at ridiculous speeds. It’s these energetic particles that Juno’s electronics will need to worry about. The radiation levels seen around Jupiter can reach upwards of 20 million rads. To put that into perspective, that’s the equivalent of being blasted by 100 million dental x-rays at once! Juno’s orbit will thankfully keep it outside of the worst areas of radiation, but it still must punch through these zones in order to take the data it needs. In its closest approaches, Juno will be just about 3,100 miles (5,000 km) from Jupiter’s cloud tops.
With all of that radiation, even a space qualified device wouldn’t last long without extra shielding. Juno’s sensitive and most important electronics are housed inside a heavy titanium box to block as much radiation from reaching the inside.
Let’s take a look at a few aspects of radiation testing that any DLA space device must go through.
There is an inherent amount of radiation present in our solar system. Some of it comes from our own sun in the form of solar flares and solar wind, and some comes from other cosmic rays from other distant stars.
Imagine for a minute that we are testing a waterproof fabric. It would clearly have to be able to withstand a light mist that lasts all day as well as a sudden storm that rains cats and dogs and then goes away. This kind of basic resistance testing would be the same as the Total Ionizing Dose, or TID test.
An IC is exposed to gamma rays from a cobalt-60 source over a certain period of time and at a certain rate depending on the testing being performed. There are two categories of TID testing: High Dose Rate (HDR) and Low Dose Rate (LDR) and each simulates a different situation. HDR simulates the “raining cats and dogs” example, such as a blast from a solar flare. LDR ensures that the device can survive drifting through cosmic space radiation, or a “light mist of rain all day”. Devices are then tested to ensure they still pass production testing post-irradiation.
Typically, the radiation dose rate is somewhere between 50 to 300 rad(Si)/s, though smaller rates are sometimes used depending on the intended application environment. Since these rays are charged particles, any that get stuck inside the crystal structure can alter device performance. This can cause any number of hard and soft failures, though this is rare and these kinds of failures are more common with Heavy Ion Radiation than it is with stray electrons darting through the cosmos.
Let’s return to our waterproof fabric analogy. Any waterproof fabric should be able to withstand a rainstorm or a quick dunk in a pool of water without damage or degradation, otherwise why call it waterproof! DD, or Displacement Damage (NOT Designated Driver), helps simulate this kind of scenario.
Neutrons and protons have a significant amount of mass compared to electrons, so they are more likely to cause damage to the actual atomic structure of the IC than to cause other performance degradation. Also, radiation in the form of these particles tends to cause more destructive damage than gamma rays, which just excite otherwise immobile charges into an excited state before they settle back down. A frequent failure mode seen from Neutron Irradiation is actual shifting of aluminum traces, creating voids and migration of atoms. It’s also possible for neutron radiation to actually damage the silicon crystalline structure, though this is rare.
It is possible to combine TID and Displacement Damage testing into one test by using protons, which have significant mass and a charge associated with them. Though on some occasions it’s more favorable to do these separately so it’s possible to determine which effect caused the failure in initial studies.
That’s not to say it’s impossible to determine which effect caused the failure. One method is to use different biasing conditions to separate TID effects from DD effects.
We’ve covered the basics of radiation testing. Next time, we’ll dive into detail on Heavy Ion radiation and different transient effects it can cause. What do you think Juno will find out about Jupiter?
Radiation Testing Documentation
Analog Devices offers radiation reports for many devices in our space portfolio. A good example is the ADA4077-2S, a dual low offset and drift, high precision amplifier that was recently released. For examples of the HDR and LDR reports, check out the product page here.
And be sure to take a look at the full space products portfolio