Influence of Beam Conditions and Energy for SEE Testing

The effects of heavy-ion test conditions and beam energy on device response are investigated. These effects are illustrated with two types of test vehicles: SRAMs and power MOSFETs. In addition, GEANT4 simulations have also been performed to better understand the results. Testing to high fluence levels is required to detect rare events. This increases the probability of nuclear interactions. This is typically the case for power MOSFETs, which are tested at high fluences for single event burnout or gate rupture detection, and for single-event-upset (SEU) measurement in SRAMs below the direct ionization threshold. Differences between various test conditions (e.g., “in air” or vacuum irradiations, with or without degraders) are also explored. Nuclear interactions with any materials in the beam's path can increase the number of high collected charge events potentially impacting the experimental results. A “species” effect has been observed in the power MOSFET devices examined in this work. When the beam energy increases, the single-event-burnout (SEB) voltage is constant, such that the SEB voltage is determined only by the species of the ion beam. The species effect is shown to be due to high collected charge events induced by nuclear interactions, which can lead to premature SEB. If a device is sensitive to the species effect, the worst-case test conditions will be for the heaviest ion species, which can produce the largest linear-energy-transfer (LET) secondaries. SRAMs can also be sensitive to the species effect below the direct ionization threshold LET. For the devices used in this work, the worst-case energy for SEU characterization is
$\sim 10'{\rm s}~{\rm MeV/u}$
where the species dominates the device response. In the 10's MeV/u range the heaviest species result in the largest cross sections. However, at very high energies (100's MeV/u), the species is not the dominant parameter because of differences in the population of secondaries created by nuclear interactions. At very high energies the SEU cross section below the direct ionization threshold LET decreases by several orders of magnitude compared to 10's MeV/u SEU data. The results of this work emphasize that there is no such thing as an “ideal” test facility. Nevertheless, these results can be used by experimenters to optimize the integrity of their results for given test conditions.