Temperature control

Temperature control

The extreme conditions of space exploration call for the development of electronic devices that can withstand huge variations in temperature.

For a probe to go to Neptune, it needs to be able to operate in temperatures as low as -230°C (43K). A lander that can operate on Venus would have to tolerate temperatures of close to +500°C (773K). Such low temperatures are not just seen near the outer planets. Simply being in shadow in orbit around the Earth or at one of the lunar poles is enough to have temperatures plunge to 40K, below the temperature at which nitrogen condenses.

Low temperatures can be problematic, as electronic behaviour can change substantially as temperatures head towards absolute zero. However it is not a straightforward situation. Cooling electronics a long way below 0°C can help improve performance. Several years ago, KryoTech built PCs based on the AMD Athlon processor that cooled the CPU to -40°C so that the processors could be overclocked.

Organisations such as the European Space Agency and NASA have busily been experimenting with a variety of techniques to deal with extreme temperatures. Traditionally, the option the agencies' subcontractors have selected has been the simplest: use heaters and coolers to keep the temperature within the range of conventional electronics. A large interplanetary probe such as Cassini had close to 150 individual radioisotope heaters on it, each one the size of a roll of coins. Ultimately, NASA wants to dispense with the heaters as they increase cost, weight and complexity.

Battling conditions

The James Webb infrared telescope, which is meant to go into Earth orbit early next decade, will use a sunshield that will keep one side of the probe in a permanent freeze while the other side will be hot. Infrared sensors tend to depend on very low temperatures to operate. Putting a heater next door will cause problems where, on one side of the probe, the electronics will be operating at temperatures as low as 40K.

Finding electronic devices that will work at 77K or below is a painstaking process. NASA typically buys in off-the-shelf parts although some components will have to be custom-made. Tests have revealed that silicon bipolar devices fail below 77K although there are indications that silicon CMOS will work down to 40K. Working out which will operate demands many tests.

In 2005, NASA handed BAE Systems' Manassas, Virginia operation a $12.5m grant to perform research on ultralow-temperature silicon technology. This is only one part of a larger programme to build entire electronic systems that need no protection from the intense cold of deep space. BAE hopes to create the basis for a structured ASIC based on a 150nm process that will work at temperatures as low as -230°C (43K), the temperature around the orbit of Pluto, and up to +120°C (393K), the highest temperature at which military-grade CMOS devices work today.

For bipolar electronics, NASA and other researchers are investigating silicon germanium, which improves in key factors such as current gain as it gets colder. Non-silicon devices based on gallium arsenide or germanium are other contenders.

Research on CMOS has suggested that increasing the operating voltage helps it function below 77K, although that might cause problems in systems that see big swings in temperature because higher voltages damage the transistors above freezing. One way to avoid that is to dynamically alter the operating voltage but this brings problems with design.

The heat is on

Operating in cold or hot conditions is not the only problem. Sudden changes in temperature will affect Moon and Mars landers as well as satellites as they move out of night-time shadow and into direct sunlight. The temperature cycling could cause bond wires to break suddenly and damage packages. It is another area where the researchers need to perform many tests.

High temperatures are far more problematic. Although silicon has a very high melting point, most electronic devices fail long before the temperature reaches it. It is not so much that the circuits just don't work. It is that the aging effects that plague semiconductors speed up dramatically as the temperature rises.

On top of that, bipolar devices suffer from thermal runaway above +200°C because charge carriers are produced spontaneously as the temperature rises. The interconnect also has problems. Solder melts at a much lower temperature than the semiconductors themselves, which demands new materials with far less information on their long-term reliability.

For higher temperatures, silicon-on-insulator (SOI) shows promise as a high-temperature semiconductor. The thin silicon sandwich limits the effects of unwanted thermal charge carriers. SOI is already used commercially by suppliers such as Belgium-based Cissoid. The company recently introduced a family of logic devices that will function above +200°C and research indicates that SOI can get to at least +250°C.

To get up to the temperatures found on the Venusian surface, NASA is looking at the material silicon carbide. It's not only an extremely hard material - Infineon Technologies has to use high-powered lasers to drill holes through its power transistors to make vias it can function at temperatures up to +600°C. The material glows red hot but it still functions.

A couple of years ago, researchers from NASA's Glenn Research Centre demonstrated an amplifier based on silicon carbide that could function at +500°C. Placed on a ceramic carrier and coupled with aluminium oxide capacitors, the amplifier ran stably during tests lasting more than 400 hours, indicating that the team were not just getting close to semiconductors that could survive the horrific conditions on planets such as Venus, but that the packaging technology could as well.

Although silicon carbide is not easy to deal with, it may provide the key to getting probes to last longer than a couple of minutes on the hottest planets of the Solar System.

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