NASA looks to SiGe for radiation-immune circuits

Georgia Tech EE professor John Cressler shows a silicon germanium wafer he is designing to take space circuitry from rad-hard to radiation-immune.

The shielded circuitry in current-generation spacecraft meets requirements for radiation hardness. But a joint project among NASA, the U.S. Naval Research Laboratory and Georgia Tech aims to cast the next generation of space circuits in silicon germanium to boost their immunity to bombarding space hazards.

The circuits will need no shielding, the researchers say, because they will continue to operate properly even when cosmic rays cause random localized errors.

“The holy grail in this field is getting sufficient radiation hardness without resorting to any of the high-overhead schemes, such as shielding, process hardening or triple modular redundancy,” said principal investigator John Cressler, an EE professor at the Georgia Institute of Technology. “We are closing in on that goal, using silicon germanium electronics.”

Most of the advanced electronics now used in space were designed for the relatively benign atmosphere of Earth. When used in spacecraft, conventional electronics often require heavy shielding to prevent radiation damage, as well as triple redundancy to compensate for exposure to cosmic rays.

SiGe is naturally resistant to ionizing radiation, which comprises smaller particles, such as electrons and protons, that move at high speeds but do not deeply penetrate circuits. Cosmic rays, however, involve heavy ions moving at speeds so fast that no medium can stop them. When cosmic rays rip through a circuit, they affect charge distribution, causing a local error in the circuit. So Cressler's group is designing its SiGe circuitry to withstand such errors.

Space electronics currently employ triple modular redundancy, wherein three identical but physically separated systems are polled by a master computer. If a random cosmic ray causes an error in one of the three, the computer detects the difference in that system's results and reboots it while the two other systems continue to run. Cressler wants to design circuitry that implements a smarter version of that architecture, to cancel the effects of random errors without requiring triple modular redundancy.

The team has already begun to formulate a model of the effects of particle strikes on SiGe transistors and is using that model to simulate SiGe circuitry. To test the accuracy of their simulations, they fabricate the circuits, then use an ultrafast laser to inject current locally into the test designs, simulating the effect of cosmic rays. Using a high-speed data logger, the team captures the results for different impact points on their circuits, then redesigns them to avoid problems. By characterizing for direct hits on any part of a circuit, they hope to achieve designed-in immunity to all types of space radiation.

Once a circuit is achieved that passes the full laboratory testing regime, it will be shipped to Sandia National Laboratories, where a focused-ion microbeam will bombard it with real cosmic rays. As in the earlier tests, high-speed data loggers will be used to capture the results of each impact. The researchers hope to confirm their computer model's accuracy and fine-tune their predictions for new circuit designs achieved using the model.

See also: Where next in space?

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