Reconfigurable Antenna Elements Using RF MEMS Switches

These days it seems that every research institution has a program to develop MEMS components; sensors, switches, optics, or fluidics. Some of these components are already finding their way into the marketplace, and MEMS/microelectronic systems-on-a-chip are already being designed. HRL Laboratories, LLC, as been building RF MEMS switches for the last three years. [1,2] The design and fabrication of metal-metal contact cantilever switches on GaAs substrates, and the measured performance of the switches will be presented, including switching speed, insertion loss, and isolation. Some examples of how these switches can be integrated into simple filters and antenna elements will be exhibited to show the potential for MEMS applications in electronic systems Microwave and millimeter wave phased-array antennas typically operate over a narrow bandwidth, but for many applications multiple frequency operation would be highly desirable. This would require either a tunable antenna element with a multiband feed network or a multiband antenna array and the tunable feed network. PIN diodes and ferrites have been used for decades as tuning devices for antennas [3,4]. Recently, optoelectronically tunable antennas have been reported [5-7] where photoconductive switches are used to connect dipole segments for frequency tuning or beam scanning. With the advent of MEMS processing, new RF switch structures are possible, and have the advantages over the other tuning techniques in simplicity, ultralow insertion loss, high isolation, power efficiency, and compactness. Two canonical RF MEMS switch structures are being actively developed, the capacitive switch and the contacting switch. The capacitive switch uses a metal membrane that is physically in contact with the ground electrode of a planar transmission line. A DC voltage causes opposite charges to be induced between the membrane and the ground electrode, which in turn results in an attractive force which pulls the membrane toward the oppositely charged electrode. A thin dielectric insulator (on the order of a few microns) prevents the switch from shorting. The effective RF capacitance of this switch increases from the switch "off" state to the switch "on: state, with an on-off capacitance ratio up to the order of 100 [8]. Reliable contacts are essential for the metal-contact RF MEMS switch, which depends on the making and breaking of an actual metal-to-metal joint, a feat complicated by the low forces available from microactuation. In the force regime of tens to hundreds of micronewtons, surface states, adsorbed contaminants, and nanometerdepth material properties dominate the properties of metallic contact, and limited information has been available to the MEMS switch designer. Recent studies into this phenomenon by [9] have determined useful design criteria for pure gold contacts electroplated from sodium sulfite solution. Based on this work and other recent low-force contact studies [10,11], switch designs were developed to provide 200 µN of contact force per contact and maximal-heat sinking to the substrate. During the past three years, HRL Laboratories has developed a metal-contact RF MEMS switch for operation up through 40 GHz, and beyond. The design and fabrication of the switches will be presented in this paper. In addition, the performance of this switch, including some limited lifetime data and power handling capability, will be provided. Finally, examples of the integration of these switches into frequency tuned filters and antennas will demonstrate the potential application of these switches to subsystem components. 2. CANTILEVER RF MEMS SWITCH CONFIGURATION AND PERFORMANCE The RF MEMS switch, shown in figure 1, has a suspended cantilever trilayer structure of metal sandwiched between insulating layers to improve the thermal stability of the devices and reduce the effect of as-fabricated stress mismatch between the layers. A microphotograph of the switch is shown in figure 2. The cantilever beam includes the top actuation electrode and the RF switch. The actuation electrodes are isolated from the RF transmission line, thus no bias circuit is required for this switch. In addition, the switch actuation, being electrostatic, does not draw any DC current (there is a very small transient capacitance charging current), thus the required power for applications where arrays of switches are needed can remain reasonable.