Life Tests of a Microwave MEMS Capacitive Switch

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

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.

Figure 1. This MEMS Capacitive Switch was subjected to life tests in which the potentials shown in Figure 2 were measured.

The tests were performed on a stateof- the-art metal-dielectric-metal radiofrequency (RF) MEMS capacitive switch fabricated on a glass substrate (see Figure 1). The top switch electrode was a 0.3-μm thick flexible aluminum-alloy membrane that was suspended over an air gap and was electrically tied to DC and RF ground. The bottom switch electrode was composed of chromium/gold and served as the center conductor of a 50-Ω-impedance coplanar waveguide for the RF signal. Thick copper posts, approximately 3 μm tall, served as anchor points for the suspended membrane as well as RF-transmission-line conductors.

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.

Figure 2. Characteristic Potentials measured at intervals during life tests showed small drifts but no significant deterioration of performance.

The switch insulator was made from a 0.28-μm-thick SiO₂ layer sputtered onto the lower electrode, but this layer was not continuous: instead, it was patterned into a series of hexagonal posts about 4 μm wide at an 8-μm pitch. As a result, the proportion of switch area occupied by air as the dielectric exceeded that occupied by SiO₂ as the dielectric. This patterning of the switch insulator undesirably reduced the on-state capacitance but desirably reduced the contact area accessible to dielectric charging. Trading away some of the on-state capacitance to reduce dielectric charging could, potentially, be a way of increasing operational lifetimes of switches like this one.

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