Emerging Radiation Hardness Assurance (RHA) Issues: A NASA Approach for Space Flight Programs

Abstract Spacecraft performance requirements drive the utilization of commercial-off-the-shelf (COTS) components and emerging technologies in systems. The response of these technologies to radiation is often complex. This engenders a set of emerging radiation hardness assurance (RHA) issues which include displacement damage in optocouplers, highprecision and hybrid devices, enhanced low dose rate (ELDR) and proton damage enhancement (PDE) in linear circuits, linear transients, and catastrophic single event effects (SEEs) phenomena. NASA has developed an approach to designing reliable space systems which addresses these emerging RHA issues. This programmatic methodology includes hazard definition, hazard evaluation, requirements definition, evaluation of device usage, and application of radiation engineering techniques with the active involvement of designers. Risk assessment is an integral constituent in the approach as is an established program to assess future technology needs for programs. I. INTRODUCTION Spacecraft and instrument design needs have changed greatly in the recent past. Increased performance requirements such as data handling capabilities are coupled with decreased spacecraft parameters such as power consumption, weight, volume, and cost to drive spacecraft design toward the use of COTS and newer emerging technologies. Projects are no longer able to meet their design and performance requirements by using radiation hardened (RH) components. In many cases, RH devices are no longer available, and the RH integrated circuits (ICs) that are available do not come close to being "state of the art" (SOTA). A relevant example is in the semiconductor memory arena where RH devices are at least two generations behind COTS. The current SOTA RH devices are 1 to 4 Mbit static random access memories (SRAMs) compared to the 64 Mbit (or even 256 Mbit) COTS dynamice random access memories (DRAMs) that are available at this time. This performance gap is widening rapidly. Three other issues that influence methodology in RHA programs involve the use of SOTA commercial and emerging technologies. These are: -COTS and emerging technology devices are more susceptible to radiation effects (and in some cases have new effects) than their predecessors. -There is much greater uncertainty surrounding radiation hardness because of the limited control and frequent processing changes associated with COTS device. -With a trend toward minimization of spacecraft size as well as the use of composite structures, the amount of effective shielding against the external radiation environment has been greatly reduced, increasing the internal radiation environment. The consequence is that we are now using more radiation sensitive devices with less protection. II. REPRESENTATIVE RHA ISSUES Several new phenomena have been observed in the last ten years that can heavily influence the use of COTS and emerging technology components in space Examples of the more significant new effects are highlighted below, divided into various categories. The list is not meant to be inclusive nor are the discussions comprehensive. The reader is strongly encouraged to supplement this information with the references provided. A. Displacement Damage As recently as two years ago, displacement damage in optocouplers was considered a "non-issue". For most NASA missions the radiation levels were low enough so that proton displacement damage on system electronics was a secondorder problem. Also, devices that had been tested for displa cem ent dam a g e had show n rela tive insensitivity to this effect Recent work has shown that some device technologies are far more affected by displacement damage than previously thought. Proton exposure of a different type of light-emitting diode (LED) used in certain optocouplers led to device degradation at fairly low fluence