Radiation-resistant circuits from mechanical parts can survive in space, damaged nuclear plants

Jun 12, 2012
These are microscopic images of two "logic gates" made of microscopic mechanical parts and thus designed to resist ionizing radiation that fries conventional silicon electronics. The top gate performs the logic function named "exclusive or" and the gate in the bottom image performs the function "and." These devices, designed at the University of Utah, are so small that four of them would fit in the cross section of a single human hair. Credit: Massood Tabib-Azar, University of Utah

University of Utah engineers designed microscopic mechanical devices that withstand intense radiation and heat, so they can be used in circuits for robots and computers exposed to radiation in space, damaged nuclear power plants or nuclear attack.

The researchers showed the devices kept working despite intense ionizing radiation and heat by dipping them for two hours into the core of the University of Utah's research reactor. They also built simple circuits with the devices.

Ionizing radiation can quickly fry , so heavy shielding must be used on robots such as those sent to help contain the meltdowns at the Daiichi after Japan's catastrophic 2011 earthquake and tsunami.

"Robots were sent to control the troubled reactors, and they ceased to operate after a few hours because their electronics failed," says Massood Tabib-Azar, a professor of electrical and computer engineering at the University of Utah and the Utah Science Technology and Research initiative.

"We have developed a unique technology that keeps on working in the presence of ionizing radiation to provide computation power for critical defense infrastructures," he says. "Our devices also can be used in deep space applications in the presence of cosmic ionizing radiation, and can help robotics to control troubled nuclear reactors without degradation."

The new devices are "" that perform logical operations such as "and" or "not" and are a type of device known as MEMS or micro-electro-mechanical systems. Each gate takes the place of six to 14 switches made of conventional silicon electronics.

Development of the new logic gates and their use to build circuits such as adders and multiplexers is reported in a study set for online publication this month in the journal Sensors and Actuators. The research was conducted by Tabib-Azar, University of Utah electrical engineering doctoral student Faisal Chowdhury and Daniel Saab at Case Western Reserve University in Cleveland.

Tabib-Azar says that if he can obtain more research funding, "then the next stage would be to build a little computer" using the logic gates and circuits.

The study was funded by the Defense Advanced Research Projects Agency.

Shown here are three kinds of micro-electro-mechanical systems (MEMS) circuits built from microscopic mechanical devices, known as "logic gates," designed to resist ionizing radiation. The circuits are a 2-bit multiplexer (top), a 1-bit full adder (center) and a 2-bit full adder (bottom). Credit: Massood Tabib-Azar, University of Utah

"Its premier goal is to keep us ready," says Tabib-Azar. "If there is a nuclear event, we need to be able to have control systems, say for radars, to be working to protect the nation. There are lots of defense applications both in peacetime and wartime that require computers that can operate in the presence of ionizing radiation."

In April, the Defense Advanced Research Projects Agency 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 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.

But by having one MEMS device 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 in the center square. Each bridge is made of a glass-like silicon nitride insulator with polysilicon under it to give rigidity. The insulator is etched and covered by metallic strips of tungsten that serve as electrodes.

"When you charge them, they attract each other and they move and contact each other. Then current flows," says Tabib-Azar.

He and his colleagues put the logic gates and conventional silicon switches to the test, showing the logic gates kept working as they were repeatedly turned on and off under extreme heat and radiation, while the silicon switches "shorted out in minutes."

University of Utah electrical engineer Massood Tabib-Azar has developed micro-electro-mechanical "logic gates" that resist intense heat and ionizing radiation and can be used to build circuits for robots and computers exposed to radiation from nuclear attack, in space and in damaged nuclear power plants. A microscope image of one such circuit is shown on the computer screen. Credit: Lee J. Siegel, University of Utah

The devices were placed on a hot plate in a vacuum chamber and heated to 277 degrees Fahrenheit for an hour.

Three times, the researchers lowered the devices for two hours into the core of the university's 90-kilowatt TRIGA research reactor, with wires extending to the control room so the researchers could monitor their operation. The logic gates did not fail.

The researchers also tested the logic gates outside the reactor and oven, running them for some two months and more than a billion cycles without failure. But to be useful, Tabib-Azar wants to improve that reliability a millionfold.

Two Kinds of Logic Gates

For the study, Tabib-Azar and colleagues built two kinds of logic gate, each with two inputs (0 or 1) and thus four possible combinations of inputs (0-0, 0-1, 1-0, 1-1). The input and output are electrical voltages:

  • An AND gate, which means "and." If both inputs – A and B – are true (or worth 1 each), then the output is true (or equal to 1). If input A or B or both are false (worth 0), then the output is false (or equal to 0).
  • An XOR gate, which means "exclusive or." If input A doesn't equal B (so A is 0 and B is 1 or A is 1 and B is 0), the output is true (equal 1). If both A and B are either true (1) or false (0), the output is false (0).
"In a sense, you can say these are switches with multiple outcomes," rather than just off-on (0-1), says Tabib-Azar. "But instead of using six [silicon] switches separately, you have one structure that gives you the same logic functionality."

"Let's say you want to decide whether to go to dinner tonight, and that depends on if the weather is nice, if you feel like it," he says. "In order to make that decision, you have a bunch of 'or' statements and a bunch of 'and' statements: 'I'll go to dinner if the weather is nice and I feel like it.' 'I like to eat Italian or French.' You put these statements together and then you can make a decision."

"To analyze this using silicon computers," Tabib-Azar says, "you need a bunch of on-off switches that have to turn on or off in a particular sequence to give you the output, whether you go to dinner or not. But just a single one of these [MEMS logic gate] devices can be designed to perform this computation for you."

Explore further: Researchers increase the switching contrast of an all-optical flip-flop

Related Stories

'Chaogates' hold promise for the semiconductor industry

Nov 16, 2010

In a move that holds great significance for the semiconductor industry, a team of researchers has created an alternative to conventional logic gates, demonstrated them in silicon, and dubbed them "chaogates." The researchers ...

Recommended for you

Intelligent materials that work in space

Oct 23, 2014

ARQUIMEA, a company that began in the Business Incubator in the Science Park of the Universidad Carlos III de Madrid, will be testing technology it has developed in the International Space Station. The technology ...

Using sound to picture the world in a new way

Oct 22, 2014

Have you ever thought about using acoustics to collect data? The EAR-IT project has explored this possibility with various pioneering applications that impact on our daily lives. Monitoring traffic density ...

User comments : 5

Adjust slider to filter visible comments by rank

Display comments: newest first

Eikka
not rated yet Jun 12, 2012
"To analyze this using silicon computers," Tabib-Azar says, "you need a bunch of on-off switches that have to turn on or off in a particular sequence to give you the output, whether you go to dinner or not. But just a single one of these [MEMS logic gate] devices can be designed to perform this computation for you."


That's actually the difference between ASICs and general purpose processors. Nothing says you can't make a complex logic block out of a silicon switch - it's just that if you do design such a part, you can't change its function once it's built. It's made for that one specific task.
hb_
not rated yet Jun 12, 2012
Again a temperature in Fahrenheit..!?
antialias_physorg
5 / 5 (1) Jun 12, 2012
It's an interesting concept. But they really need to test this for more than two hours. Long term exposure could lead to embrittlement of MEMS which would also lead to failure.

Though, depending on the timeframe of the task at hand, that can be an acceptable risk.

Nothing says you can't make a complex logic block out of a silicon switch - it's just that if you do design such a part, you can't change its function once it's built.

Why not? If you already have switches then you can also use them to control the connection of the MEMS.

If you think back: That's what mechanical looms did (which were the forerunners of today's computers). They used punchcards to reconfigure so that different patterns could be woven on one loom.
Eikka
not rated yet Jun 12, 2012
Why not? If you already have switches then you can also use them to control the connection of the MEMS.


Because the circuit itself is not programmable. The ASIC, once built, does nothing else than what it is designed to do, which is why it can substitute the programmable bits with simpler and more efficient shortcuts. It's an Application Specific Integrated Circuit.

What you are thinking about is an FPGA, which is kinda in the middle between a programmable processor and an ASIC. Overall, they may be even more complex than the processor, as in, you can implement a processor inside an FPGA.

What the article talks about is similiar to an ASIC, because it specifically mentions the sort of efficiency where you make a single gate perform a complex, specific function. Trying to re-purpose that same gate then is extremely difficult, just like trying to make the loom knit instead of weave.
Gino
not rated yet Jun 13, 2012
back in the 1890,s the computers in telephone exchanges ran in a similar manner.