978-1-4673-5292-5/12/$31.00 ©2012 IEEE

R FMEMS CIRCUIT FOR SPACE COMMUNICATIONSSYSTEMS

Souad. OUKIL(1)

Abdelmadjid. BOUDJEMAI(2)

Nabil. BOUGHENMI(3)

Faculté de Génie ElectriqueUSTOMB, BP 1505 El M’Naouar

Oran, ALGERIAe-mail: sadoukil@yahoo.fr

Space Mechanics DivisionNational Center of Space

TechniquesP.O. Box 13 Arzew 31200,

ALGERIAe-mail: a_boudjemai@yahoo.fr

Faculté de Génie ElectriqueUSTOMB, BP 1505 El M’Naouar

Oran, ALGERIAe-mail:

boughanminabil@yahoo.com

Abstract— One of the most important considerations in

designing a spacecraft is weight. By reducing the weight of a

spacecraft, it is possible to increase the payload, which

improves agility and also reduces the launch cost [1]. Missions

costs are directly proportional to its total weight, thus, the

trend will be to replace bulky and heavy components of space

carriers, communication and navigation platforms and of

scientific payloads. RF-MEMS technology has the potential of

replacing many of the mechanical and semiconductor switches

used in mobile and satellite communication systems. In many

cases, such RF-MEMS switches would not only reduce

substantially the size and power consumption, but also promise

superior performance. The paper reviews the recent

development of RF MEMS switches and switch matrices.

Several configurations are presented for multi-port RF-MEMS

switches.

Key words—RF, MEMS Switch, Finite element,

Communications systems, Space.

I. INTRODUCTION

Satellite subsystems that are currently being examined inthe MEMS community include attitude determinationsensors and control and actuators, propulsion,communications, power, flight computers, missioninstrumentation, satellite scale thermal management, and thespacecraft chassis. Within this list, the most mature systemsare those that offer the greatest benefit from implementationin MEMS or those with terrestrial market pressure. Thecurrent pressure within the personal communicationindustry is driving a number of technologies that candirectly cross over into the space community. Two of themore dramatic devices that will both soon be ready forsatellite integration and flight testing are MEMS gyroscopesand RF switching components [2, 3, 4, 5].

Wireless satellite communications and space-basedsensors are the major beneficiaries of the MEMStechnology. Minimum size, power consumption, and highreliability are the critical requirements for both theapplications, which can be satisfied by deploying MEMS-based devices and sensors. MEMS technology conserves

energy and minimizes power consumption, weight, and size.RF-MEMS devices such as switches, phase shifters,switched antennas, filters, and amplifiers have becomecritical to increasing communications satellite capabilitywhile realizing significant reduction in launch weight andcost. This is important not only in large satellites employingphased array antennas and switch matrices that are deployedin constellations to downlink data but also in the case ofnano satellites, which are very small, lightweight spacecraftcontaining microelectronic equipment, RF-MEMScomponents, and payloads.

Space communications systems are ‘‘ripe’’ for theinsertion of MEMS-based technologies, in part due to thegrowth in commercial communication developments. Oneof the most exciting applications of MEMS for microwavecommunications in spacecraft concerns the implementationof ‘‘active aperture phase array antennas.’’ These systemsconsist of groups of antennas phase-shifted from each otherto take advantage of constructive and destructiveinterference in order to achieve high directionality.Such systems allow for electronically steered radiatedand received beams, which have greater agility and will notinterfere with the satellite’s position. Opticalcommunications could also play an important role in low-power, low mass, and long-distance missions such as theRealistic Interstellar Explorer (RISE) mission, whichseeks to send an explorer beyond the solar system, whichrequires traveling a distance of 200 to 1000 AU from theSun within a timeframe of about 10 to 50 years. Theprimary downlink for such a satellite would need to beoptical because of the distances and weight limitsinvolved. It has been proposed that a MEMSimplementation of the beam-steering mechanism maybe necessary to achieve the desired directional accuracywith a sufficiently low mass. MEMS in spacecommunication may well fall under the trendy term“disruptive technology” for their potential to redefinewhole systems [5,6,7].

MEMS are unique from a radiation-effects point of viewbecause they contain electronic control circuits coupled with

2012 24th International Conference on Microelectronics (ICM)

mechanical structures, both of which are potentiallysensitive to radiation damage. The electronic circuits inMEMS are either CMOS or bipolar technologies that areknown potentially to exhibit great sensitivity to radiationdamage. It is not at all obvious that radiation doses thatproduce measurable changes in performance in electroniccircuits will have any effect on mechanical structures;however, they can. The performance of MEMS devices inspace depends, critically, on the characteristics of theradiation environment. Before a device can be used in spaceit must be qualified to ensure that it will survive the rigorsof the space environment. Radiation qualification is one ofmany different qualification procedures that must beperformed. Others include temperature, pressure, andvibration. In the absence of specific guidelines forqualifying MEMS devices for a radiation environment,radiation test engineers make use of standard radiationqualification procedures that have been developed formicroelectronics.

II. RADIO FREQUENCYMEMS SWITCH FOR SPACESYSTEMS

RF MEMS switch performances and reliability; havebeen largely improving during the last years. Many examplesof successful concepts for RF switches can be found inacademia and companies. Fig.1 reports a summary of state-of-the-art MEMS switches.

MEMS-based RF switches are an attractive option forspace-based applications. In comparison with mechanicalcoaxial or waveguide devices and PIN diodes, MEMSswitches provide excellent RF performance with lowinsertion loss, high isolation, and high linearity, while alsooffering low power consumption, small size and lightweight.Satellite payloads typically have hundreds of switches

integrated in the form of switch matrices to provide systemredundancy. In case of any failure in the amplifiers, theseswitch matrices reroute the signal to a spare amplifier andmaintain the full functionality of the system.

Figure 1: Pictures of state-of-the-art RF MEMS devices [8], [5], [4].

For the communications field MEMS currently offerssolutions to filtering, switching local oscillation [9], [10].Radios are currently adapting an increasingly integratedprocess as semiconductor materials are able to handle higherfrequencies. By using MEMS wherever possible not only isthere a weight and volume benefit from the device itself but

from the lack of associated connectors, cabling, caseworkand packaging.

The MEMS weight and volume remains constant becausefor MEMS RF switches the size is frequency independentand for mechanical filtering the carrier package dominatesthe weight and volume figure over the die mass [11, 12]. Forthe table 1 the numbers reflect a full duplex communicationslink.

TABLE I. TABLE 1: SUMMARY OFMEMS COMMUNICATIONCOMPONENTS ADVANTAGES.

III. MODELING OF RFMEMS SWITCHES

A. Electrostatic model

The movable part of an electrostatic RF MEMS switch,for both the cases of a clamped-clamped bridge or acantilever beam, can be described in a compact form byusing a 2-D model of the traditional parallel plates capacitor,as depicted in Fig. 2. In this case, the anchors of themembrane are modelled by a linear spring k, attached to thetop electrode of the capacitor, which is free to move alongthe z-direction.

A thin dielectric layer of thickness td and dielectricconstant εr lays over the bottom electrode, which is fixed to the substrate, whereas the rest of the capacitor is filled withair. Note that such a capacitor works as the series of twocapacitances, one filled only with air and one filled with thedielectric layer.

Figure 2: Spring-capacitance system modeling the electro-mechanicalcoupling in a MEMS switch.

The threshold voltage (or the actuation voltage) is animportant index of the RF MEMS switch’s function, inwireless communication system, the magnitude of RFMEMS switches’ actuation voltage will directly affect theelectric circuit design and the performance of the wholesystem such as the system size, consumption etc. In structuredesign, the basic mechanics model of different cantileverbeam MEMS switches looks like a cantilever beam with aforce in one end, as shown in Fig.3, L is the length of thecantilever beam; is the flexibility degree; F is a force; g0 isthe original space between the parallel plank capacitor.

Because the width of the cantilever beam is far larger thanthe thickness and the direction of both the force and themovement are in one plane as shown in Fig.3, the movementof the cantilever beam can be analyzed as a flat problem.

According to simple beam theory, we can get the relationbetween the voltage and the flexibility degree of thecantilever beam:

For a cantilever beam RF switch, the dynamic characteristicand frequency responsibility of the cantilever beam affectthe executive function of the switch greatly.

Coplanar of line Conductor

Figure 3 : Structure micromécanique à géométrie simple

The displacement characteristic of the top electrodedescribes a hysteresis as a function of bias voltage, asillustrated in Fig. 4. The pull-out voltage is always smallerthan the pull-in, since the electrostatic force, acting after theactuation and then depending on the down-state capacitance,is higher then the one acting before the snap-down.

Figure 4: Displacement along z-axis of a clamped-free bridge as a functionof bias voltage. Note the hysteresis loop that characterizes any kind of RF

MEMS switch.

B. Mechanical model

A MEMS switch is made of a suspended conductivemembrane constrained to a fixed support (substrate). Themembrane can be a fixed-fixed beam or a cantilever beamaccording to the type and the number of anchor points. Afixed-fixed beam or air bridge is usually anchored to thesubstrate by means of at least two anchor points or anchorsprings. A cantilever beam or free-clamped beam is anchoredat one or more points at the same side.

In electrostatic MEMS switches, the suspendedmembrane is flexible enough to be considered elastically

movable. The movement is driven by an electrostatic forceprovided by applying a difference of potential between themembrane and a fixed electrode. The reaction force dependson the mechanical behaviour of the membrane, which inmost of the cases can be easily modelled by a linear springconstant. The MEMS switch used in this paper is shown inthe figure 5.

Figure 5: MEMS switch geometry

The RF switch characteristics are given in table 3.

TABLE II. TABLE 2: THE RF SWITCH CHARACTERISTICS

cantilever beam Length l 235 µm

width W 100µm

Tope electrode length ω 70µm Thickness of the beam t 3µm

Gap of the capacitive system g0 1.8µm

Thickness of the dielectric one tdiel 0.2µm

Permittivity of the dielectricone

εr diel

9.6

IV. FINITE ELEMENTANALYSIS OF THE RFMEMSSWITCH

Specifically, the RF MEMS switches can be classifiedaccording to the following three factors: A. the structure ofthe radio frequency electric circuit (series and parallel etc.);B. the mechanical structure (cantilever beam and filmstructure etc.); C. the contact method (capacitance type andresistance type). Current research of the RF MEMS switch isconcentrated on cantilever beam structure and film structureswitches. Cantilever beam structure is generally the seriesresistance type. Then according to the signal connections, thecantilever beam MEMS switches can be classified into inlineMEMS-series switches and broadside MEMS-seriesswitches [13, 14, and 15]. The basic structure of the switch isshown in Fig.5.

The finite element model (FEM) of a cantilever beamswitch is given by figure 6.

The boundary condition in the FEM simulation concernsthe one edge of the short side which is constrained(displacement of x, y and z are zero, and rotation of x, y andz are zero) in the cantilevered MEMS switch.

Figure 6: FEM model

(1)

(2)

(3)

The mechanical properties of materials used in the switchare given in the table 3.

TABLE III. TABLE 3:MATERIALS MECHANICAL PROPERTIES

Young modulusE(Gpa)

Poissonration ν

Density ρ (kg/m3)

polysilicon 170 0.22 2300

Silicon 169 0.36 2392

SiN 1100 0.07 3500

Al2O3 125 0.26 3900

V. RESULTS AND DISCUSSION

Figure 7 shows the frequencies modes of a cantileverbeam switch obtained using Msc. Patran/Nastran software

Figure 7 shows the results of the structure modal forms,and which makes it possible to see where are made the mostdeformations and which elements.

The coloured fringes give the amplitude of thedisplacement vector describing the shape of each mode. Theblack colour corresponds to null displacement and the redone presents the maximum amplitude.

Figure 7 shows the displacement of the cantilever beamMEMS switch in Z direction, the maximum is about 5 mmwitch is enough for the MEMS switch.

The lowest frequency was in 1st mode (2460.7 Hz),which gives the better the vibration direction

The frequency was increasing with each subsequentmode of vibration.

Mode1, f1=2460.7 Hz Mode2, f2=12398 Hz

Mode3, f3=15392 Hz Mode4, f4=39492 Hz

Figure 7: Various modes of the cantilever beamMEMS switch

VI. CONCLUSION

This paper takes the cantilever beam MEMS switch as anexample to discuss mechanical principle and functionsimulation of the RF MEMS switch in satellitecommunication system, whose main structure is a cantileverbeam controlled by electrostatic force.

This paper discusses the significant structural elements inRF MEMS switches

The first mode of natural frequency of the switch, whichis about 2460.7 Hz, gives the better the vibration direction

With all these technologies there is no quick and easysolution. Thus, the challenges facing the MEMS communityare great when it comes to the development of an integratedMEMS spacecraft.

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