NASA has begun qualifying MEMS oscillators for space exploration and has found that MEMS oscillators also operated over a wide range of temperatures. In an internal paper co-authored with Ahmad Hammoud at ASRC Aerospace Inc. (Cleveland), titled "MEMS Silicon Oscillators at Extreme Temperatures," NASA program manager in extreme temperature electronics, Richard Patterson, at its Glenn Research Center (Cleveland), said, "The small size of the MEMS oscillators along with their reliability and thermal stability make them ideal candidates for space exploration missions."
As a first step to qualifying MEMS oscillators for space missions, Patterson and Hammoud tested an off-the-shelf MEMS oscillator from SiTime at extremely low and high temperatures.
"NASA got an absolutely typical part, and we frankly do not have the equipment to test for temperatures as cold as spacecraft can get," said SiTime's Partridge.
Patterson and Hammoud discovered that SiTime's MEMS oscillator functioned from +100°Cdown to –110°C, a range that exceeded their specified –40°C to +85°C. Also, the MEMS oscillator was able to cold re-start at –110°C and exhibited no change in performance during thermal cycling up and down between the extremes. NASA proposes to evaluate SiTime's MEMS oscillators for extended space exploration missions under extreme temperatures.
MEMS oscillators are just the latest MEMS devices used by NASA's exploration of space. MEMS accelerometers, gyroscopes and other specialized inertial devices have been keeping astronauts safe and spacecraft navigational systems on track for a decade.
Like MEMS oscillators, the big advantage of MEMS inertial devices is their resistance to vibration and shock, as well as their small SWAP (NASA-speak for size, weight and power).
"MEMS gyroscopes are orders of magnitude smaller and use much less power, but they are also much more tolerant to vibration and electromagnetic pulses," said Adam Champy, an applications engineer at ADI. "Their immunity to vibration and shock comes from their being so small that their resonant frequency is very high--around 14 KHz--compared to normal gyros, whose resonant frequency is only about 100 Hz, which makes conventional gyros much more sensitive to the vibrations of launching."
MEMS devices in space also share many of the requirements for other earthly applications, in particular the same need for ultrahigh reliability as ADI's accelerometers in cars, which are used to trigger airbags.
"MEMS is ultrareliable compared to other types of inertial sensing technologies, which have many moving parts that are put together by watch makers," said Weinberg. "Those big, bulky inertial sensors also need circuit boards for their controllers, which can develop broken solder joints and all sorts of other failures. Our accelerometers and gyros, on the other hand, use a single piece of silicon to replace all that complexity. There is just not very much that can go wrong."
The failure mechanisms of electronic devices are most often their interconnects, according to NASA. So by getting the interconnects off the board and integrated into on-chip circuitry, a MEMS chip all but eliminates the sources of fatigue in a mechanical device.
"Silicon is an almost ideal mechanical element when it is used over a small range," said Weinberg. "There are simply no wear-out mechanisms. As proof, ADI has 300 million MEMS devices in the field, and we have never had a single case of one wearing out."
In addition, even when MEMS devices do fail, by virtue of mechanisms unrelated to fatigue, they often have built-in self-testing capabilities that enable ground-based NASA engineers to diagnose the problem remotely.
"There is nothing that humans have, can or will build that has 100 percent reliability, so we build our MEMS devices to tell you when something is wrong," said Weinberg. "All of our MEMS devices are built with internal self-test systems with very high coverage--meaning that if our part says it passes self-testing, then you can have a very high confidence that our part is OK."