Portland, Ore. By coupling a nanoscale etched diffraction grating to an optical sensor, Sandia National Laboratories researchers have demonstrated an accelerometer that they say is the most sensitive in the world. The nanoelectromechanical system (NEMS) could measure vibrations as low as 10 nano-G's, compared with the 100 milli-G vibrations measurable with today's accelerometers, by making mechanical motions as small as 10 nanometers visible to the naked eye.
"Because the wavelength of light from our laser is much smaller than the dimensions of our [diffraction] grating, very very small motions have a disproportionate effect," said Sandia researcher Dustin Carr. Team member Bianca Keeler will describe the work later this month at the SPIE Optics East convention in Philadelphia.
Today's best integrated accelerometers, such as those that trigger automotive air bags, can detect hundreds of milli-G's. But many unfulfilled apps need more-sensitive accelerometers, such as those that can anticipate earthquakes or sense a skid where the back end of a car is moving in a different direction than the front end.
"I see Sandia's device as having significant, far-reaching application in areas as diverse as chemical sensing, infrared imaging, accelerometry and displays," said James Walker, an independent consultant and patent agent who was formerly manager of the MEMS Network Element Subsystems Group at Lucent's Bell Laboratories.
By combining optical sensing with a nanoscale diffraction grating, the Sandia researchers claim a thousandfold improvement over today's accelerometers. Next they want to build real sensors around the technology, initially for ultrasensitive seismic applications. Then they plan to develop integrated versions that enable new traction-control algorithms in automobiles.
"We think that in less than a year we can go as low as 10 nano-G's with this technology in a device the size of, say, a can of soda," Carr said. "Geologists could use a device like that to sense earthquakes in advance. At the same time, we want to work toward an integrated version in less than three years for automotive and similar applications." The goal: "a chip that is as cheap as other accelerometer chips, but which senses 1 to 100 micro-G's instead of 100 milli-G's, like today's accelerometers," said Carr.
At Sandia, researchers suspended a movable diffraction grating about 100 to 200 nm above a second, stationary grating. Four tiny NEMS springs held the suspended grating so that even nano-G's of vibration caused it to move tens of nanometers relative to the one below it. By shining a visible laser onto the grating, even motions as small as 10 nm made clear changes in the intensity of the reflected light.
"For a sensor, we plan to shine the laser through the two gratings and onto a photoreceptor beneath it. Then we think that we will be able to sense even the slightest movement of just a few nanometers," said Carr. "No one predicted that light diffracted from such small gratings, which move such very small lateral distances, would undergo such a relatively big, and easily measurable, change."
The entire device was fabricated from polysilicon and measured about 10 microns2. The test device also included actuators along the top and bottom of the top diffraction grating. The comb actuators were used to calibrate the device by moving the top grating a repeatable amount and measuring the intensity of the reflected light.
Carr was recently selected by MIT's science magazine Technology Review as one of the year's top 100 researchers under the age of 35. Previously, as a graduate student at Cornell University, he invented what he called the "nanoguitar."
Sandia National Laboratories has applied for a patent on the NEMS device.