Life tests have demonstrated the longevity of an electrostatically actuated capacitive switch of a microelectromechanical systems (MEMS) type suitable for handling radio signals having frequencies of multiple gigahertz. The tests were performed to contribute to understanding of factors that affect the reliability of MEMS switches in general and of how improvements in designs and materials can increase operational lifetimes of MEMS capacitive switches. The tests were based partly on the concept that data obtained in monitoring both high-speed and low-speed switching characteristics provide valuable insight into quantifying the lifetime properties of the switches and enable estimation of switching lifetimes under a variety of operating conditions.
In the absence of applied electrostatic force, the membrane was normally suspended in air at a distance of 2.2 μm above the switch insulator, which was a dielectric layer on the lower switch electrode. Application of a control potential between 25 and 35 V to the bottom electrode produced electrostatic attraction that pulled the membrane into contact with the switch insulator, thus forming a 120-by-80-μm capacitor to shunt the RF signal to ground; this condition was the higher-capacitance or "on" switch state, characterized by a capacitance between 280 and 340 ff. When the control potential was removed, the membrane sprang back to its fully suspended position (the lower-capacitance or "off" switch state) wherein the capacitance was between 15 and 20 fF.
In the tests, the switch was actuated by a trapezoidal waveform at a repetition rate of 60 kHz for a total time of 476 hours, amounting to slightly more than 100 billion switch cycles. Quantities measured in these tests included (1) detector output potentials indicative of on- and off-state capacitances and (2) pull-in and release potentials, both which showed small drifts in switch characteristics (see Figure 2). The drifts in detector output potentials were not considered to represent significant deterioration of performance. The drifts in pull-in and release potentials were interpreted as being partly attributable to dielectric charging.
This work was done by C. L. Goldsmith and D. I. Forehand of MEMtronics Corp., and Z. Peng and J. C. M. Hwang of Lehigh University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. AFRL-0031
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Life Tests of a Microwave MEMS Capacitive Switch
(reference AFRL-0031) is currently available for download from the TSP library.
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