Proton Testing

Proton irradiation basics:

High energy proton irradiation has been commonly utilized to evaluate displacement damage (DD), single-event effect (SEE), and total-ionizing dose (TID). Proton testing has been primarily used by the aerospace/defense industry to test devices sensitive to both DD and TID (e.g. charge coupled devices, optocouplers, etc.). In some cases, the test article exhibits temperature sensitive degradation and annealing characteristics, from room temperature to cryogenic regime. To accurately characterize the degradation in space, specially manufactured cryogenic dewars are used for in-situ irradiation and measurements. Figure 1 shows an example of such a dewar [1]. Notice the window near the bottom of the dewar that allows for beam exposure of the test sample inside. A thorough review of proton displacement damage effects can be found in the 1999 IEEE NSREC short course.

Proton SEE testing:

High energy proton irradiation can also be a useful source to assess SEE, depending on the radiation environment and system requirements for the mission. The table below shows a generic categorization of proton SEE severity by the mission profile. A spacecraft in a medium earth orbit (MEO) or high LEO environment is exposed to the trapped protons from the inner Van Allen belt during solar minimum and additional contributions from solar protons during solar max, in turn producing a very harsh proton SEE environment. A spacecraft in geosynchronous orbit (GEO) is free from the trapped protons, and the only major source of protons comes from the sun.

The proton energy is an important consideration for testing. One needs to take into account the mission environment to determine the range of relevant proton energies. Higher energy protons are more likely to produce the recoil ions that cause SEE via indirect ionization. Typically, 200 MeV or greater is recommended to evaluate proton-induced SEE, but always consider what's relevant for your mission environment.

Proton-induced SEE from indirect ionization is generally a wider concern. However, protons can also cause SEE through direct ionization for some highly scaled technologies. As transistor dimensions continue to shrink, the critical charge required to cause upset will also diminish, and proton direct ionization could become increasingly problematic.

Advantages & what to watch out for:

One benefit of proton testing is that high energy protons are also highly penetrating, so that part decapsulation is typically not necessary, unlike heavy ion testing. This eliminates a layer of test preparation, along with the likelihood of destroying the test samples, and simplifies test setups. Proton testing can also be relatively cheaper compared to heavy ion testing.

These characteristics make proton irradiation suitable for board-level testing, because it offers a cost effective method to expose multiple on-chip components simultaneously. Statistical confidence is a concern for board-level testing. A system-level upset can be caused by a variety of single-event upsets (SEU) at the part-level. So, one needs an even larger number of events for statistical confidence, compared to what's typically needed when testing a single part type.

A potentially fatal shortfall of relying solely on proton testing is its limited ability to gauge destructive SEE. The linear energy transfer (LET) from protons is limited to the LET of the secondary recoil ions in the target device. Typically, that's silicon, which has a maximum LET of ~14 MeV-cm2/mg. Additionally, the secondary recoil ions have much shorter range than galactic cosmic ray ions or solar particles. Destructive SEE, including single-event latchup (SEL), and particularly gate-rupture (SEGR) and burnout (SEB) have deep collection volumes and require highly penetrating ions to trigger the effects. Therefore, proton irradiation may underestimate the sensitivity for destructive SEE, if it's used as a proxy for the heavy ion susceptibility. These are just some characteristics of proton irradiation that should be considered.

Test facilities:

High energy proton facilities in the US include: Texas A&M University, Lawrence Berkeley National Laboratory (LBNL), Crocker Nuclear Laboratory at University of California Davis, Proton Therapy Center at Massachusetts General Hospital, Proton Therapy Treatment Center at Loma Linda, Northwestern Chicago Proton Center [2]-[4], [5]-[7]. The facilities at Massachusetts General Hospital, Loma Linda, and Northwestern offer proton energies up to 200 MeV or higher. The other facilities have energies up to ~60 MeV.

Test logistics & methodologies:

JEDEC JESD234 describes general test methodologies for proton SEE testing [8]. There are also several test guidelines and lessons learned released by NASA and other organizations. Please see the links under resources for further reading.

Protons are highly penetrating, and produce recoil ions. Therefore, proton irradiation chambers are often more isolated from the control room area for the experimenters, which require even longer cable feeds. So, the test equipment will almost always be positioned inside the irradiation cave, where they can be susceptible to the proton beam and recoil ions. Therefore, it is necessary to properly shield the test equipment during testing. Equipment failure can, and have happened, due to unintended hits from recoil particles. Additionally, test articles, especially metallic materials, can become activated during irradiation. So, it's not uncommon for the facility to withhold the test articles for days, weeks, or months after testing until the radioactivity of the test article diminishes to a safe level.

For SEE testing, it is advisable to determine the upset sensitivity for a range of proton energies. Ideally, one would like to find the threshold energy and saturating cross section. The cross section data can be inputted into CREME96 to calculate on-orbit event rates [8]. Our SEE analysis app assists the user to plot cross section data, find appropriate Weibull fits, and estimate proton upset rates under various environments and shielding levels.

Additional resources:

NASA's presentation on proton SEE guideline
Proton Test Guideline Development - Lessons Learned
Cheryl Marshall and Paul Marshall, "Proton effects and test issues for satellite designers: displacement effects," NSERC Short Course, 1999.