Zero-G Radiation Assurance
Commercial-off-the-shelf (COTS) electronic components have been utilized in space systems for decades. They offer advantages in performance, size, weight, and cost, which cannot be matched with traditional radiation-hardened (radhard) components. However, COTS have unpredictable reliability & radiation performance, which necessitate proper vetting to an acceptable level of failure probability and ensure mission success.
EEE parts traditionally used for space applications are chosen with relatively strict standards of quality and reliability. For example, microcircuits from the qualified manufactures list (QML) go through various levels of screening & qualification to guarantee a level of acceptable risk. QML-V level parts require additional radiation qualification.
However, COTS are not bound to these standards. Therefore, COTS can be more susceptible to a higher level and wider distribution of infant mortal failures, random failures, and long term reliability concerns. "Space-grade" radhard components often have its own isolated wafer fabrication lines with proprietary processes that are unique from a comparable COTS product line. For example, when enhanced-low-dose-rate-sensitivity (ELDRS) became a widely known concern in the space community, a number of manufacturers spent significant resources to design and install dedicated fabrication processes that significantly reduced the level of ELDRS in their products. These radhard processes consisted of completely redesigned transistors that minimized the radiation-induced charge trapping in the oxide and interface regions. This illustrates an example of radiation-hardened by process (RHBP), where specific changes to the device process improved the radiation tolerance. Similarly, manufacturer can implement radiation-hardened by design (RHBD) with modifications to the circuit design - e.g. triple mode redundancy to mitigate single-event upsets (SEU). These powerful techniques produce EEE components that are robust against the effects of harsh radiation environments. The dedicated fabrication flows also allow the manufacturer to control and sustain overall reliability.
Many COTS have relatively robust radiation tolerance, either due to the intrinsic characteristics of its transistor process or just by luck of draw. However, variability of the radiation reliability can be dangerous for radiation assurance when it's not fully understood. Variability of the radiation susceptibility of COTS is a widely known problem that can be difficult to manage. The figure below shows the cumulative probability of the failure dose for the SFT2907A bipolar transistor irradiated with gamma rays. We observe a very wide range for the failure dose, from approximately 20 to 80 krad(Si). This range of TID encompasses a variety of mission types in either LEO or GEO environments. The high levels of variability highlights the risks of using non-lot-specific radiation data.
The above example shows gain degradation failures, which are parametric drifts. Similarly, the variability can occur for functional failures. For example, commercial NAND flash memories can exhibit a wide range of functional failures from 20 to 50 krad(Si) for devices from the same lot code. These failures are caused by radiation-induced charge buildup in the charge pumps. After charge buildup to a sufficient level, devices typically fail to carry out erase or program functions. In other more rare cases, parts can exhibit bimodal characteristics, such that two groups of the same device will show different degradation trends . The variability of radiation susceptibility can impact the single-event effect (SEE) response as well. The variability can contribute to uncertainty of the on-orbit SEU rate prediction. Or worse, the variability can lead to a false sense of safety from destructive effects such as single-event latchup (SEL), single-event gate-rupture (SEGR), or single-event burnout (SEB).
When faced with a parts list that consists entirely of COTS, an impossibly tight launch schedule, and very limited resources, how can we proceed to meet the mission performance and reliability targets for radiation? This issue has recently become the focal point of discussions, due to the increasing number of missions built with mostly COTS components. There are different approaches to the problem. Regardless of the approach, it is vital to understand the underlying failure or degradation mechanism, when evaluating the suitability of COTS for space missions. Only with that understanding and a knowledge of the mission radiation environment, can one move towards parts testing and radiation data analysis.
Our recently developed radiation database consists of ground test data for over 4000 entries, searchable by part number, function, SEL LET, TID, DDD and more. This tool should provide a much needed boost for the industry by bringing the largest collection of data just a mouse-click away.
M.A. Xapsos, C. Stauffer, A. Phan, S.S. McClure, R.L. Ladbury, J.A. Pellish, M.J. Campola, and K.A. LaBel, "Inclusion of radiation environment variability in total dose hardness assurance methodology," IEEE Tran. Nucl. Sci., vol. 64, no. 1, Jan 2017, pp 325 - 331.
R.L Ladbury and M.J. Campola, "Statistical modeling for radiation hardness assurance: towards bigger data," IEEE Tran. Nucl. Sci., vol. 62, no. 5, Oct 2015, p. 2141 - 2154.
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Figure 1. TID failure distribution for the SFT2907A bipolar transistor. The line is a lognormal fit to the data .