Team hardens SiGe circuits
By Ron Wilson, Executive Editor -- EDN, November 12, 2009
Manufacturers once aimed certain process technologies at radiation-hardened designs. CMOS, for example, started out that way, when RCA developed its silicon-on-sapphire process. However, the overwhelming success of bulk-silicon CMOS processes has driven most of the alternatives into niches, forcing up their cost to the point that only the best-funded programs can use an alternative process technology to harden their circuitry. That situation leaves everyone else with plain-vanilla CMOS.
As a result, a lot of work over the years has taken place in hardening bulk CMOS. Several foundries offer radiation-resistant processes that, by manipulating the device geometry, reduce the amount of charge that a radiated particle releases as it passes through an IC. Alternatively, these processes could increase the rate of recombination to mop up the charge before it can upset a logic node. In another method, designers use more resistant circuit-design techniques to help reduce either the probability of an upset or its consequences. These measures can take you only so far, however. Circuits that must operate in deep-space conditions, in which they will encounter not only ordinary ionizing radiation but also cosmic rays, also must rely on shielding and redundancy. Both techniques are costly in total mission weight.
Now, John Cressler, a professor at the Georgia Institute of Technology’s School of Electrical and Computer Engineering, is leading a team exploring the traditional approach: hardening the process so that the circuits can survive radiation exposure without extensive shielding or redundancy. Cressler’s team chose SiGe (silicon germanium) because of its ability to endure exposure to heavy, high-energy particles in cosmic rays. The team first built computer models of the response of SiGe devices to the passage of an ionizing particle through them. The team now intends to use an extremely high-speed oscilloscope to observe the picosecond-level electrical traces of particle impacts and to calibrate the computer models from the hard data. Cressler hopes to then refine device designs to produce a library of SiGe devices that can operate in space conditions without additional process hardening or shielding.
