RF Microelectromechanical Systems Switches Research Articles

MEMS Electronically Steerable Antennas for Fire Control Radars

Large apertures are of great benefit to applications that are prime powered limited as is found on aerostat and other airborne platforms. Electronically scanned array antennas are often proposed for these applications. However, increasing the aperture area with conventional array technology is met with prohibitive cost, weight, and prime power increases because of the dense spacing of phase shifters and/or active T/R modules. This discusses the recent development of RF MEMS (Microelectromechanical System) switch technology and the use of these switches in a Radanttrade lens configuration for arrays of approximately 10 m2 at X-band. A proof-of-concept 0.4 m2 MEMS Electronically Steerable Antenna (ESA) containing 25,000 MEMS switches has been successfully designed, fabricated, and tested. The 0.4 m2 MEMS ESA was then integrated with an AN/APG-67 radar system to form the MEMS Demonstration Radar System. The MEMS Demonstration Radar System successfully detected both airborne and ground moving targets during a series of extensive radar demonstrations. This is believed to be the first large scale employment of MEMS switches in a scanning antenna and radar system. The low-cost, lightweight, and low power technology demonstrated can enable weight and power constrained platforms with electronic steering.

RF MEMS asymmetric capacitive switch with high‐isolation at selected low‐microwave frequency

AbstractAn asymmetric RF microelectromechanical systems (MEMS) capacitive switch with a novel architecture employing mode switching from a coplanar waveguide (CPW) mode to a coupled slotline (CSL) mode is reported for selected frequency within the 2–10 GHz low microwave frequency range. An additional metal‐insulator‐metal (MIM) capacitor is introduced on one end of the switch membrane as a tuning capacitor. The signal line of CSL is used as a tuning inductor. These two tuning elements together can improve the isolation of a regular capacitive RF MEMS switch by about 20 dB at the induced additional low resonant frequency ranging from 2 to 10 GHz. The measurements of the fabricated asymmetric switch demonstrated excellent isolation improvements. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 702–706, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22235

Multistandard reconfigurable PIFA antenna

In this article, a frequency reconfigurable antenna using the ON/OFF states of a MEMS (micro-electro-mechanical system) switch is presented. This structure is based on a planar inverted-F antenna (PIFA), designed with an L-shaped open slot etched upon its main plate. According to the switch position along the slot, the GSM operating frequency is not detuned while other standards become reconfigurable. To validate our concept, a RF MEMS switch, designed and developed at CEA-LETI, is used. The scattering parameters of the MEMS are measured and compared with simulated results from a simple model using lumped elements and transmission line sections. This active device is then inserted in the slot to validate the switching capabilities of the PIFA. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1975–1977, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21823

ANN based electromagnetic models for the design of RF MEMS switches

Artificial neural network (ANN)-based modeling approach for the design of radio frequency microelectromechanical system (MEMS) switches is presented. In the proposed methodology, a finite element method analysis is performed for the switch characterization and for the creation of training and testing sets for the ANN. Developed full-wave/ANN models allow for circuit level design, simulation, and optimization in a schematic of a circuit simulator. The simulation time is reduced for two orders of magnitude, while the accuracy of the full-wave method is maintained.

A low-loss single-pole six-throw switch based on compact RF MEMS switches

A low-loss single-pole six-throw (SP6T) switch using very compact metal-contact RF microelectromechanical system (MEMS) series switches is presented. The metal-contact MEMS switch has an extremely compact active area of 0.4 mm /spl times/ 0.3 mm, thus permitting the formation of an SP6T MEMS switch into the RF switch with a total area of 1 mm/sup 2/. The MEMS switch shows an effective spring constant of 746 N/m and an actuation time of 8.0 /spl mu/s. It has an isolation loss from -64.4 to -30.6dB and an insertion loss of 0.08-0.19 dB at 0.5-20 GHz. Furthermore, in order to evaluate RF performances of the SP6T MEMS switch, as well as those of the single-pole single-throw RF MEMS series switch, we have performed small-signal modeling based on a parameter-extraction method. Accurate agreement between the measured and modeled RF performances demonstrates the validity of the small-signal model. The SP6T switch performed well with an isolation loss from -62.4 to -39.1dB and an insertion loss of 0.19-0.70 dB from dc to 6 GHz between the input port and each output port.

Effect of nanoscale heating on electrical transport in RF MEMS switch contacts

This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.

Design, fabrication and RF performances of two different types of piezoelectrically actuated Ohmic MEMS switches

In this paper, we have proposed, fabricated and characterized piezoelectrically actuated RF MEMS (radio-frequency micro-electro-mechanical system) switches. They have been designed to operate at a low operation voltage for advanced mobile/wireless handset applications. The proposed switches are largely composed of piezoelectric cantilever actuators with an Au contact electrode and CPW (coplanar wave) transmission lines suspended over the substrate. Two different types of RF MEMS switches have been suggested to find the better geometry. One has the structure of one single piezoelectric cantilever and a contact electrode attached to its edge with three hinges (type-A), and the other contains four piezoelectric cantilevers that are symmetrically combined through each hinge to support a centered contact electrode (type-B). The two different fabricated (type-A and type-B) RF MEMS switches have insertion losses of −0.22 and −0.23 dB at an operation voltage of 2.5 V and a frequency of 2 GHz, respectively. Although the difference in insertion loss is trivial, there exist different dependences of insertion loss on applied voltage between them. The insertion losses of type-A switches are changed with varying operation voltage because the touching area between the contact electrode and the signal transmission lines is variable. Meanwhile, the type-B switches show nearly constant insertion losses regardless of operation voltage. The type-A and type-B switches have isolation values of −40.8 and −42.5 dB at a frequency of 2 GHz, respectively.

A low-voltage and low-power RF MEMS series and shunt switches actuated by combination of electromagnetic and electrostatic forces

This paper reports new RF microelectromechanical systems (MEMS) switches actuated by the combination of electromagnetic and electrostatic forces for low-voltage and low-power operation. The proposed RF MEMS switches have utilized the proper combination of two actuation mechanisms: taking advantage of the large actuation force from electromagnetic actuation for initial movement and the low-power feature from electrostatic actuation for holding the actuator position. Both series- and shunt-type switches have been implemented using the proposed actuation mechanism. From the fabricated switches, feasibility of operation has been successfully demonstrated. The fabricated switches can be operated within several hundred microseconds. In the series-type switch, the isolation has been measured as -34dB and insertion loss as -0.37dB at 20 GHz. In the shunt type switch, the isolation is -20.7dB and insertion loss is -0.85dB at 19.5 GHz. The proposed RF MEMS switches are mechanically robust and the combination of electromagnetic and electrostatic actuations makes it possible to achieve excellent switching characteristics at low power and low voltage below 5 V.

A simple lumped electrical model for an RF MEMS switch considering lossy substrate effects

This work focuses on electrostatically actuated micro-electro-mechanical systems (MEMS) capacitive switches, intended for adaptive output stage of power amplifiers. These devices are particularly attractive because theoretically they are capable of very high quality factor values due to the air gap between plates and the low resistivity of lines and bridge. On the other hand, substrate parasitic effects, overlooked up to date, can seriously impair device performance. In this paper, we introduce a lumped element equivalent circuit taking into account substrate effects. Model validation is based on measurements carried out in the 50 MHz–40 GHz range.

Mechanics-Based Solutions to RF MEMS Switch Stiction Problem

RF micro-electro-mechanical systems (MEMS) switches are an attractive solution to switch antenna bands and transmit/receive switching for future multiband, high bandwidth cell phones. However, Stiction is a major concern for resistive switches with metal-to-metal contact. An iterative-coupled electrostatic-structural analysis is utilized to evaluate the effect of design parameters on restoring force of MEMS switches. Parameters including metal thickness, dielectric thickness, beam-to-ground gap height, metal and dielectric width, and cantilever beam length can be evaluated. The electrostatic force is first calculated based on the electrical field component. A structural analysis is then performed to determine the cantilever beam deflection due to the electrostatic force. A unique integrated empirical-numerical method is used to quantitatively determine the stiction force based on measured actuation voltages for real devices. The analysis can provide quick evaluation and screenings of proposed designs to determine if their actuation voltage falls in the acceptable range. Simulation prediction agrees very well with test measurements. Although increasing cantilever thickness and shortening cantilever length both increase restoring force, the actuation voltage will increase significantly as a result. The most favorable modification is to increase the electrode area. A short and wide structure with a large area can increase restoring force while maintaining low actuation voltage. Compared to similar bi-layer designs, sandwich designs can be actuated at further reduced voltages without changing the beam restoring force. In addition, the sandwich structure, being thermal-stress-balanced, is less sensitive to temperature excursion. With the properly selected design parameters, the new designs will be able to achieve the break away restoring force of the original design at much lower actuation voltages. Switches with good electrical as well as mechanical performances have been successfully fabricated.

Numerical analysis of theoretical model of the RF MEMS switches

An improved electromechanical model of the RF MEMS (radio frequency microelectromechanical systems) switches is introduced, in which the effects of intrinsic residual stress from fabrication processes, axial stress due to stretching of beam, and fringing field are taken into account. Four dimensionless numbers are derived from the governing equation of the developed model. A semi-analytical method is developed to calculate the behavior of the RF MEMS switches. Subsequently the influence of the material and geometry parameters on the behavior of the structure is analyzed and compared, and the corresponding analysis with the dimensionless numbers is conducted too. The quantitative relationship between the presented parameters and the critical pull-in voltage is obtained, and the relative importance of those parameters is given.

MEMS 2-Bit Phase-Shifter Failure Mode and Reliability Considerations for Large>tex<$X$>/tex<-Band Arrays

RF microelectromechanical systems (MEMS) switch technology used in the fabrication of phase-shifter circuits is examined from the perspective of failure mode and reliability implications on the performance of large X-band array antennas. Amplitude and phase-state failure probability density functions (pdfs) conditioned on switch probability of failure are formulated for both the hybrid-T (switched line) and coupled-line phase-shifter circuit topologies. The pdfs are used to assess the phase-shifter failure impact on overall array level performance in terms of gain loss and the increase in RMS sidelobe level. Reliability and lifetime implications are addressed through considering a probability of switch failure that increases with cycling. The phase-shifter lifetime switching considerations are related to radar system lifetime beam switching requirements consistent with plausible radar system applications. The key findings are that RF MEMS switch mean time to failure /spl sim/125000 h or longer, consistent with /spl sim/100-125 phase-shifter state switches per second (/spl sim/10/sup 11/ switch operations), are reasonable expectations for RF MEMS phase-shifter technology to meet in order to be considered viable for a broad range of array antenna applications.

RF MEMS Switches With Enhanced Power-Handling Capabilities

This paper reports on the experimental and theoretical characterization of RF microelectromechanical systems (MEMS) switches for high-power applications. First, we investigate the problem of self-actuation due to high RF power and we demonstrate switches that do not self-actuate or catastrophically fail with a measured RF power of up to 5.5 W. Second, the problem of switch stiction to the down state as a function of the applied RF power is also theoretically and experimentally studied. Finally, a novel switch design with a top electrode is introduced and its advantages related to RF power-handling capabilities are presented. By applying this technology, we demonstrate hot-switching measurements with a maximum power of 0.8 W. Our results, backed by theory and measurements, illustrate that careful design can significantly improve the power-handling capabilities of RF MEMS switches.

Reconfigurable antennas for portable wireless devices

New technologies in communications electronics, such as software-defined radio (SDR) and RF switches implemented using micro-electromechanical systems (MEMS), present new challenges and opportunities for antenna design. In sharp contrast to digital technology where Moore's law reigns, a fundamental law of physics constrains the ability to realize electrically small antennas that are both efficient and broadband. As a result, covering several frequency bands concurrently with a single antenna having enough efficiency and bandwidth is a major challenge. One possible solution to this problem is to use reconfigurable antennas that tune to different frequency bands. Such an antenna would not cover all bands simultaneously, but provides narrower instantaneous bandwidths that are dynamically selectable at higher efficiency than conventional antennas. Such tunable-antenna technology is an enabler for software-definable radios, the RF front ends of which must be reprogrammable on the fly. This paper discusses the practical implementation issues, limitations, and measured results of small, narrowband, tunable antennas within portable handsets. Many of the concepts discussed in this paper will likely become practical and cost effective in the near future because of recent advances in RF MEMS switches.

Monolithic integration of RF MEMS switches with a diversity antenna on PCB substrate

A novel process for monolithic integration of RF microelectromechanical system (MEMS) switches with three-dimensional antenna elements on a microwave laminate printed circuit board (PCB) is presented. This process calls for a low-temperature (90/spl deg/C-170/spl deg/C) high-density inductively coupled plasma chemical vapor deposition technique that allows the choice of any PCB substrate, such as RO4003-FR4 (/spl epsiv//sub r/=3.38, tan/spl delta/=0.002), with the desired electrical properties for antenna applications. A two-element diversity antenna system monolithically integrated with RF MEMS switches is designed and demonstrated.

Electromechanical considerations in developing low-voltage RF MEMS switches

This paper reports on the design, fabrication, and testing of a low-actuation voltage Microelectromechanical systems (MEMS) switch for high-frequency applications. The mechanical design of low spring-constant folded-suspension beams is presented first, and switches using these beams are demonstrated with measured actuation voltages of as low as 6 V. Furthermore, common nonidealities such as residual in-plane and gradient stress, as well as down-state stiction problems are addressed, and possible solutions are discussed. Finally, both experimental and theoretical data for the dynamic behavior of these devices are presented. The results of this paper clearly underline the need of an integrated design approach for the development of ultra low-voltage RF MEMS switches.

MEMS TECHNOLOGY FOR ADVANCED TELECOMMUNICATION APPLICATIONS

MEMS (Micro Electro Mechanical Systems) technology is becoming a crucial enabling technology for optical and RF telecommunication applications due to the performance, cost and integration advantages. All-optical switches and silicon optical benches for optical network systems and RF MEMS components such as RF MEMS switches, tunable capacitors, high Q inductors, and thin film bulk acoustic resonators for miniaturized single chip RF transceivers are discussed. Despite all the promising aspects, practical problems to be addressed for successful commercialization are not trivial. By reviewing optical and RF MEMS research activities and some of the implementation examples, key success factors of MEMS components for the commercialization are discussed.

Film Stress Influence of Bilayer Metallization on the Structure of RF MEMS Switches

AbstractThe performance of microelectromechanical systems (MEMS) switches is highly dependent on the switches' constituent materials. The switch material must be able to provide both structural integrity and high electrical conductivity. Cantilever beams, doubly clamped beams, and membranes are typical MEMS structures used in microwave/millimeter wave applications. In this study, cantilever and doubly clamped beam microswitches were fabricated on GaAs substrates using evaporated bilayers of titanium and gold metallization in which the total thickness was held constant at 1.5 μm while the gold thickness varied from 0.5 μm to 1.5 μm. The lengths of the cantilevers varied from 300 to 500 μm and the doubly clamped beams varied from 600 to 800 μm. An upward deflection of the gold dominated cantilever beams indicated an increasing tensile stress gradient. Results of microwave characterization demonstrated higher switch isolation (off-resistance) for shorter beam switches at the expense of higher insertion loss (on-resistance) and actuation voltage. A discussion of the observed released microswitch structure within the context of the measured film stresses and electrical performance will be presented.