Radiation-resistant circuits from mechanical parts

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.”

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.”