MAINTAINING RELIABILITY UNDER EXTREME CONDITIONS IN SPACE
The electronics on-board space exploration vehicles should be able to withstand the extreme temperature of space. Operating it beyond the normal limits is an option worth considering.
Space exploration vehicles like satellites, probes, shuttles and spacecrafts all rely on electronics. These electronic devices have to withstand for their working not only the extreme (very low to very high) temperatures but also the radiation hazards prevailing in the space. In fact, very little of a space system falls within the conventional electronics ; temperature range. Its temperature in space depends on its proximity and orientation to other bodies, absorption and emission of energy, and internal heat generation. Therefore extreme-temperature electronics is a key technology for space exploration.
USE OF CONVENTIONAL ELECTRONICS
Conventional-temperature-range electronics can be used in space (an extreme-temperature environment) by means of insulation and heating (for low-temperature environments) or refrigeration (for high-temperature environments). This can be combined with thermal sinks or thermal sources. For example, the well-logging electronic devices may be placed in a dewar flask (vacuum-insulated vessel) to protect these from the hot environment.
In addition, a thermal sink thermally connected to the electronic devices will absorb a large amount of heat without a substantial increase in temperature. This is usually done by employing the material's phase change from solid to liquid, which absorbs a large amount of heat (the latent heat of fusion). For low temperatures, the opposite effect may be used to provide a thermal source. Thus the same material may serve both as a thermal sink for high temperatures and a thermal source for low temperatures. It might be as basic as ice/water or less familiar such as a bismuth alloy.
However, in many situations, the techniques described above would be undesirable or impractical due to various reasons. The passive techniques might have a limited lifetime that is insufficient for the application, while the active techniques require additional power and subsystems. Also, for some applications, active techniques might be too disturbing to the environment because of the additional heat that needs to be dumped. All such techniques add weight, bulk and some degree of complexity.
Operating electronics beyond the normal limits is thus an option worth considering. This special electronics will be able to withstand the extreme temperatures of space.
EXTREME TEMPERATURE ELECTRONICS
The term 'extreme-temperature electronics'(ETE) is used for electronics operating outside the traditional temperature range of -55/-65C to +125C. It covers both the very low temperatures-down to essentially absolute zero (0K or -273C)-and the high temperatures (+125C and above).
In low-temperature electronics (LTE), operation of semiconductor-based devices and circuits has often been reported down to temperatures as low as a few degrees above absolute zero. These devices are based on silicon (Si), germanium (Ge), gallium arsenide (GaAs) and other semiconductor materials. Moreover, there is no reason to believe that operation might not extend all the way down to absolute zero.
In high-temperature electronics (HTE), laboratory operation of discrete semiconductor devices has been reported at temperatures as high as +700C (for a diamond Schottky diode) and 650C (for a silicon carbide (SiC) MOSFET). Integrated circuits (ICs) based on Si and GaAs have operated at 400500C. Si ICs have been reported to operate at +300C for a thousand hours or longer. Covering both extremes, there are reports of the same transistor working at about -270C to +400C temperature range. Also, many passive components are usable to the lowest temperatures or up to several hundred degrees Celsius.
However, operation at extreme temperatures is not true for every semiconductor device or passive component; it depends on a number of material and design factors. Practical operation of devices and circuits is reasonably achievable to as low temperature as desired, provided materials and designs appropriate to the temperature are used. However, the various characteristics of the device might improve or degrade. In particular, below about 40K (-230C), Si devices often exhibit significant changes in characteristics.
High-temperature electronics presents more difficulty. The practical upper temperature limit is determined by many factors and the inherent temperature limit is often not reflected for semiconductor devices. The limit is frequently determined by the interconnections and packaging-both for active devices and passive components. As an indication of the practical upper limit, circuits have been offered commercially for operation at up to +300C.
Parts availability is a major obstacle to practical ETE. There are few components specified for either low- or high-temperature use. To construct ETE hardware, often the conventional-temperature- range components are selected and adapted. Custom fabrication is done if resources and time permit.
PROBLEMS WITH SPACE ELECTRONICS
Reliability of components: Space agencies place reliability at the top of their priorities since the failure of just one component can lead to the loss of a multi-million dollar mission. A clear counter trend is the use of commercial off-the-shelf (COTS) components. While these parts are generally more advanced in terms of processor performance and logic or memory density than those designed specifically for use in military or space borne systems, COTS devices do not have the background of design and extensive testing that ensures reliability.
Radiation effect: When a spacebound electronic component passes its test, there remains one big problem- radiation. Radiation is one of the main characterstics of space weather. Radiations of galactic and solar origin determine radiation hazards for people and technology, computer and memory upsets and failures, solar cell damage, radio wave propagation disturbances, and failures in communication and navigation systems.
The effect of ionising radiation on hard-wired logic circuits is less pronounced. These errors are typically transient and often non-destructive. A review conducted by NASA in 1996 of a hundred failures and problems on its spacecraft found that one third of the failures were caused by ionising radiation leading to single event upsets (state changes in logic or memory) or permanent degradation in the performance of on-board electronic devices. Sometimes these single event upsets are even capable of destroying computer memories on the earth. But obviously with a much larger probability in spacecraft systems during periods of large energetic particle fluxes, it is advisable to switch off some part of the electronics to protect computer memories.
High-energy particles ionise the medium through which these pass, leaving behind a wake of electron-hole pairs. These pairs can change the state of a memory cell or a logic flip-flop. As a result, a radiation strike might change not just the state of a memory cell but also the design of the circuit it controls, potentially leading to catastrophic failure.
ADVANCES BEING MADE
A five-year project led by the Georgia Institute of Technology has developed a novel approach to space electronics that could change the way space vehicles and instruments are designed. The new capabilities are based on silicon-germanium (SiGe) technology, which can produce electronics that is highly resistant to both wide temperature variations and space radiation.
The team's overall task was to develop a tested infrastructure that included everything needed to design and build extreme-environment electronics for space missions. The result is a robust material that offers important gains in toughness, speed and flexibility. The robustness is crucial for SiGe's ability to function in space without bulky radiation shields or large power-hungry temperature control devices. Compared to conventional approaches, SiGe electronics can provide major reductions in weight, size, complexity, power and cost, as well as increased reliability and adaptability.
The Radiation Hardened Electronics for Space Environments (RHESE) project endeavours to expand the radiation-hardened electronics by developing high-performance devices robust enough to withstand the extreme radiation and temperature levels of the space environment. The project is a part of the Exploration Technology Development Program (ETDP), which funds an entire suite of technologies needed for accomplishing the goals of the vision for space exploration.
|Silicon Germanium chip|
NASA's Marshall Space Flight Center (MSFC) manages the RHESE project. RHESE's investment areas include novel materials, design processes to implement radiation hardening, reconfigurable hardware techniques, software development tools, and radiation environment modeling tools.
Near-term emphasis within the multiple RHESE tasks is on hardening field-programmable gate arrays (FPGAs) for use in reconfigurable architectures and developing electronic components using semiconductor processes and materials (such as SiGe) to enhance the tolerance of a device to radiation events and low-temperature environments.
As these technologies mature, the project will shift its focus to developing low-power, high-efficiency total- processor hardening techniques and hardening of volatile and non-volatile memories. This phased approach to distributing emphasis between technology developments allows RHESE to provide hardened FPGA devices and environmentally-hardened electronic units for mission infusion into early constellation projects.
Once these technologies begin the infusion process, the RHESE project will shift its technology development focus to hardened high-speed processors with associated memory elements and high-density storage for longer-duration missions, such as the Lunar Lander, Lunar Outpost, and eventual mars exploration missions occurring later in the Constellation schedule.The individual tasks of RHESE are diverse, yet united in the common endeavour to develop electronics capable of operating within the harsh environment of space. Specifically, the RHESE tasks include SiGe integrated electronics for extreme environments, modeling of radiation effects on electronics, single-event-effects- immune reconfigurable FPGA, radiation-hardened high-performance processors and reconfigurable computing.Though the tasks are diverse in their specific key performance parameters, these target to accomplish specific goals-improved total ionisation dose tolerance, reduced single event upset rates, increased threshold for single-event latch-up, increased sustained processor performance, increased processor efficiency, increased speed of dynamic reconfigurability, reduced lower bound of the operating temperature range, increased available levels of redundancy and reconfigurability, and increased reliability and accuracy of radiation effects modeling.