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Tiny MEMS devices to filter, amplify electronic signals

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August 13, 2009

Prof Jeffrey Rhoads and graduate student Venkata Bharadwaj Chivukula have created a new ME...

Prof Jeffrey Rhoads and graduate student Venkata Bharadwaj Chivukula have created a new MEMS device that could improve cell phone reception (Photo: Andrew Hancock)

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Researchers are developing a new class of tiny mechanical devices, made up of vibrating structures the thickness of a human hair, that could be used to filter electronic signals in cell phones and other applications. Only the size of a grain of sand, these microelectromechanical systems (MEMS) will, nonetheless, improve performance and reduce power usage.

The devices, or resonators, vibrate in specific patterns, and can be used as ‘band-pass’ filters. That is, they are able to cancel out some signals with certain frequencies while allowing others to pass. Band-pass filters are common in a cell phone’s circuit, said Jeffrey Rhoads, an assistant professor of mechanical engineering at Purdue University.

He says the filters are critical for cell phones and other portable electronics because they allow devices to process signals with minimal interference and maximum transmission efficiency.

The filter is an example of a microelectromechanical system, or a MEMS, which contain tiny moving parts. Incoming signals generate voltage that produces an electrostatic force, causing the MEMS filters to vibrate.

While researchers in the past have proposed linking tiny beams in straight chains, Rhoads has arranged the structures in rings and other shapes. One prototype, which resembles spokes attached to a wheel's hub, is about 160 microns in diameter, or the size of a grain of sand.

Rhoads’ findings are detailed in a research paper written with mechanical engineering graduate student Venkata Bharadwaj Chivukula.

Other uses

Apart from their future cell phone use, it is envisaged that the resonators also could be used for advanced chemical and biological sensors in medical and homeland-defense applications, possibly even a new type of ‘mechanical memory element’ that harnesses vibration patterns to store information.

"The potential computer-memory application is the most long term and challenging," Rhoads said. "We are talking about the possibility of creating complex behaviors out of relatively simple substructures, similar to how in cellular biology you can have a relatively complex behavior by combining hundreds or thousands of simple cells."

This new level of band-pass filter design promises higher performance than previous MEMS technology because it is more stringent in determining which frequencies can pass. The new design also might be more robust than the traditional linear form, meaning devices that have manufacturing flaws can still perform well.

A tenth the width of human hair

The devices are made of silicon and are manufactured using the common ‘silicon-on-insulator’ procedure the electronics industry employs for computer chips and electronic circuits. The small, vibrating mechanical structures contain beams about 10 microns in diameter, which is roughly one-tenth the width of a human hair. The beams can be connected mechanically, like tiny springs, or they can be linked using electric fields and magnetic attractions.

After testing the new resonators in experiments to ensure the concept works, Rhoads is moving to the next stage. "We are in the process of making a second prototype," he said.

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