Contributing Editor, MEMS Investor Journal
Many MEMS sensors have a moving part that responds to environmental stimuli with motion. An accelerometer, for instance, capacitively senses a moving "proof mass" that responds to displacements of just a few microns. However, a MEMS device's sensitivity could be vastly increased if motions as small as a few nanometers could be sensed.
Now Tel Aviv University claims that standard MEMS devices can sense nanometer-scale motions by switching from capacitive sensing to sensing the change in resistance in a tiny carbon nanotube. Resistance changes in nanotubes can be detected when they are stretched by just a few nanometers, allowing them to multiply the sensitivity of a MEMS sensor by as much as 100 times.
"Our overall approach to MEMS sensors is unchanged, just instead of measuring capacitance, we are measuring the change in resistance of a carbon nanotube," said professor Yael Hanein at Tel Aviv University (Israel). She performed the work with fellow professor Slava Krylov and doctoral candidate Assaf Ya'akobovitz.
The Tel-Aviv process enables standard silicon fabrication steps to be used for a conventional MEMS sensor, but with the addition of carbon nanotubes to increase sensitivity.
"We are using a completely conventional MEMS silicon-on-insulator (SOI) process -- the only difference is that we introduce a titanium nitride electrode on top of the silicon and add some islands of silicon dioxide -- and these two things allow us to control the growth of, and electrical connections to, the carbon nanotubes. The islands control where the nanotubes grow from, and the titanium nitride makes the electrical contacts," said Hanein.
For instance, a conventional MEMS accelerometer suspends a silicon proof mass between electrodes so that its movement can be sensed capacitively. The Tel-Aviv process goes through this same fabrication process, but also adds the steps necessary to grow a few nanotubes between titanium nitride electrodes and over the proof mass. As a consequence, when the proof mass moves just a few nanometers, its motion can be sensed by a resistance change in the nanotubes that are stretched by its movement.
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