al 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 sub- strate, pick up the pad, and place it in position on the switch. The metal pad- about 150 um wide and 200 um 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 segment of the coplanar waveguide and allows the signal to pass through," Feng says. "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 um 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 says. 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 says. "Our results show that good reliability is possible with low-voltage operation." 2.13.2 StressedMetal MEMS The StressedMetal MEMS process is a proprietary technology for building three-dimensional MEMS devices. It is one of several MEMS technologies under development at Xerox's Palo Alto Research Center (PARC). Trad