NASA looks to SiGe for radiation-immune circuits

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.
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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 highspeed
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.

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