Radiation-resistant circuits from mechanical parts

can operate in the presence of ionizing radiation.”

In April DARPA issued a call for the development of robots to deal with stricken nuclear reactors to reduce human exposure to deadly radiation. In May NASA said it was seeking proposals for new shields or materials able to resist radiation in space.

Circuits built with the new devices also could resist intense heat in engines to monitor performance, Tabib-Azar says.

MEMS: Ability to withstand radiation overcomes drawbacks
Current radiation-resistant technologies fall into two categories: conventional complementary silicon-oxide semiconductor electronics shielded with lead or other metals, and the use of different materials that inherently resist radiation.

“Electronic materials and devices by their nature require a semiconducting channel to carry current, and the channel is controlled by charges,” Tabib-Azar says. Radiation creates current inside the semiconductor channel, and “that disrupts the ability of the normal circuitry to control the current, so the signal gets lost.”

He says the MEMS logic gates are not degraded by ionizing radiation because they lack semiconducting channels. Instead, electrical charges make electrodes move to touch each other, thus acting like a switch.

MEMS have their drawbacks, which Tabib-Azar believes is why no one until now has thought to use them for radiation-resistant circuits. Silicon electronics are 1,000 times faster, much smaller, and more reliable because they have no moving parts.

By having one MEMS device, however, act as a logic gate, instead of using separate MEMS switches, the number of devices needed for a computer is reduced by a factor of 10 and the reliability and speed increases, Tabib-Azar says.

Also, “mechanical switches usually require large voltages for them to turn on,” Tabib-Azar says.

“What we have done is come up with a technique to form very narrow gaps between the bridges in the logic gates, and that allows us to activate these devices with very small voltages, namely 1.5 volts” versus 10 or 20 volts. Unlike conventional electronics, which get hot during use, the logic gates leak much less current and run cooler, so they would last longer if battery-operated.

Design and reactor testing of the logic gates
Each logic gate measures about 25-by-25 microns, or millionths of a meter, “so you could put four of these on the cross section of a human hair,” says Tabib-Azar. Each gate is only a half-micron thick.

The logic gates each have two “bridges,” which look somewhat like two tiny microscope slides crossing each other to form a tic-tac-toe pattern, with tungsten electrodes