Low-voltage MEMS switch developed for high-speed electronics

Microelectronics researchers at the University of Illinois have developed a low-loss, wide-bandwidth microelectromechanical systems (MEMS) switch that can be integrated with existing technologies for high-speed electronics.

The new low-voltage switch could be used in switching networks for phased-array radars, multibeam satellite communications systems and wireless applications.

"The switch has a tiny metal pad that can move up or down in less than 25 microseconds," said Milton Feng, the Holonyak Professor of Electrical and Computer Engineering at the UI. "This simple configuration provides a very low insertion loss of less than 0.1 dB, and the metal-to-metal contact has the inherently wide band response of a larger, more typical mechanical switch."

The switches are fabricated in the UI's Micro and Nanotechnology Laboratory using standard MEMS processing techniques. To create the unique metal pull-down pad, Feng and graduate students David Becher, Richard Chan and Shyh-Chiang Shen first deposit a thin layer of gold on a sacrificial layer of photosensitive material. Then they dissolve the substrate, pick up the pad and place it in position on the switch.

The metal pad – about 150 microns wide and 200 microns long – is supported at the four corners by serpentine cantilevers, which allow mechanical movement up and down.

"When in the 'up' position, the metal pad forms a bridge that spans a segment of the coplanar waveguide and allows the signal to pass through," Feng said. "But an applied voltage will pull the pad down into contact with the signal line, creating a short circuit that blocks the signal transmission."

The gap between the metal pull-down pad and the bottom electrode is about 3 microns wide, which provides an isolation of greater than 22 dB for signal frequencies up to 40 GHz. Currently, an activation voltage of 15 volts is required to operate the switch.

One major problem Feng and his students had to overcome was stiction – a tendency for the metal pad to stick to a dielectric layer beneath the bottom electrode as a result of accumulated electrostatic charge. To prevent the charge from building up, the researchers added a tiny post that limits the downward motion of the pad. "This hard stop prevents the pad from moving past the bottom electrode and contacting the dielectric," Feng said.

In reliability tests, the switches have demonstrated lifetimes in excess of 780 million switching cycles. To further enhance the reliability, the researchers are attempting to lower the actuation voltage to less than 10 volts.

"For any device to be used in a practical application it must be reliable," Feng said. "Our results show that good reliability is possible with low voltage operation."

Becher will describe the MEMS switch at the 2002 GaAs MANTECH International Conference on Compound Semiconductor Manufacturing Technology, to be held April 8-11 in San Diego.

The Defense Advanced Research Projects Agency funded the work.