research paper

RF-MEMS switches with AlN dielectricand their applications

montserrat fernarsquo ndez-bolan~os badirsquoa1

pierre nicole2

and adrian mihai ionescu1

This paper reports on the potential of RF-MEMS technology based on aluminum nitride capacitive dielectric and nickel-suspended membranes to provide RF circuit functions in reconfigurable front-end radios The RF performance of capacitiveswitches distributed MEMS transmission lines (DMTLs) phase shifters for beam steering and tunable filters including centerfrequency and bandwidth tuning of bandpass and band-stop filters are presented Detailed characterization based onS-parameter data demonstrates very promising figures of merit of all fabricated demonstrators from 5 to 40 GHz

Keywords RF MEMS switch phase shifter tunable filter modelling simulation and characterization of devices and systems

Received 1 March 2011 Revised 30 April 2011 first published online 15 June 2011

I I N T R O D U C T I O N

In the last half-century RF-MEMS switches have been exten-sively studied as they accomplish superior RF performanceand higher linearity than semiconductor junction switchessuch as Positive-Intrinsic-Negative (PIN) diodes or Field-effect transistor (FETs) [1] Despite their higher control voltagescompared to some solid-state switches electrostatic-actuatedMicro-electro-mechanical-systems (MEMS) offer significantpower savings as well as potential lower cost and the capabilityof miniaturization which result in attractiveness for their use inwireless applications

In reconfigurable front-end RF ICs functions like RFswitching filtering and phase shifting in reflector array anten-nas for beam steeringforming networks (as illustrated inFig 1) can be realized with RF-MEMS components andsystems using the same technology in a single fabricationprocess RF MEMS offer new possibilities for improved per-formance and allow higher density and capacity of theoverall systems

In this paper we present capacitive shunt switches in termsof performance as well as their applications as phase shiftersand tunable filters In Section II the state of the art of capaci-tive switches is studied and compared with different fabricatedswitch designs including those based on analog and digitalcapacitor concepts and switches with highly inductive com-ponents that were specifically designed as core devices forthe applications presented in the subsequent sections Thenin Section III analog and 2-bit digital true time delay (TTD)phase shifters are described as well as the co-integration of

phase shifters and the dipole antennas for beam steering appli-cations Finally tunable filters with different functionalities asbandpass and band-stop filter configurations are shown inSection IV

I I R F - M E M S C A P A C I T I V ES W I T C H rsaquo S W I T C H I N G

A) ClassificationCapacitive switches are mainly divided into two sub-categories depending on their different functionalities theRF-MEMS capacitive switch or analog MEMS capacitorwith high isolation and an increased capacitance ratio Cr(30) and the MEMS switch capacitor or digital MEMScapacitor with a moderate and usually defined Cr 5Analog MEMS capacitors can be used for signal routing pur-poses in reconfigurable front-ends digitized capacitorbanks and reconfigurable antenna orientation or aperturesIn order to achieve high isolation many different configur-ations have been reported based on shunt and seriesswitches [1ndash8] low spring constant anchoring [3] inductivebehavior [8] andor using high dielectric constant materials[4 6 7]

On the other hand the digital MEMS capacitor is generallyused as part of tunable systems or reconfigurable networks forexample filters phase shifters or impedance matching withtwo known capacitive states The concept of the digital capaci-tor was first introduced by Hayden and Rebeiz [9] and isdefined as the standard MEMS capacitive switch in serieswith a fixed small capacitance possibly realized with metalndashinsulatorndashmetal (MIM) capacitor [10] metalndashairndashmetal(MAM) capacitor [11] or interdigitated capacitors Figure 2illustrates for comparison purposes the capacitance versusvoltage curve (CV-curve) of a standard analog capacitor and

Corresponding authorM Fernandez-Bolanos BadıaEmail montserratfernandez-bolanosepflch

1Ecole Polytechnique Federale de Lausanne (EPFL) Nanolab Switzerland Phone+41 21 693 39732THALES Airborne Systems Elancourt France

509

International Journal of Microwave and Wireless Technologies 2011 3(5) 509ndash520 Cambridge University Press and the European Microwave Association 2011doi101017S175907871100064X

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a digital capacitor made with MIM fix series capacitances Thehigh capacitance ratio of the fabricated analog capacitor (Cr frac1420ndash40) and the MIM digital capacitor designed can beobserved to have a Cr frac14 3

B) State of the artCapacitive shunt switches have been realized with a largevariety of dielectrics Silicon nitride (Si3N4) with a dielectricconstant of 1r frac14 75 is the most popular material for capacitiveMEMS [2 3 8] however other high dielectric constantmaterials such as strontium titanate oxide (1r frac14 120 [4])piezoelectric lead zirconate titanate (1r frac14 190 [6]) aluminumnitride (AlN 1r frac14 98 [7]) and even dielectric-less structures[5] present excellent RF performance up to 40 GHz All ofthem have demonstrated to achieve very low insertion losses(02 dB at 40 GHz) and high isolation (30 dB) with anelevated capacitance ratio (Cr 30) and acceptable switching

time (3ndash20 ms) at a relatively low actuation voltage (30 V)for billions of cycles [6 7 12 13]

The RF-MEMS capacitive shunt switches as well as theother MEMS presented in this article use a common fabrica-tion process developed at the Fraunhofer Institute of SiliconTechnology (ISiT) The original process includes a combi-nation of thin and thick nickel (Ni) for the suspended mem-brane and the anchors (andor stiffening bars) respectivelyIn addition AlN is used as a high-lifetime dielectric Thefabricated capacitive switches have performances comparableto state of the art of the technology

Figure 3 compares the most representative RF capacitiveswitches regarding the area and the isolation with respect tothe applied actuation voltage In the figure the selected darksquares represent the desired performances of a switch Thefabricated capacitive switch is in general more compact andachieves higher isolation than their counterpart MEMSswitches [1ndash6] for similar actuation voltages (Fig 3(a)) Inaddition Fig 3(b) depicts the capacitance ratio (Cr) for eachswitch which is in principle a good figure of merit of theRF capacitive switch performance However a device with avery high capacitance ratio (Cr 600 [4]) could result inpoor isolation (20 dB) due to bad dielectric RF properties

It is noteworthy to remark in Fig 4 that a trade-off existsbetween low spring constants (for low actuation voltagespurpose [3]) and the switching time or speed of the switch fol-lowing a linear dependency A known way of reducing theswitching time is by device miniaturization [13] The priceto pay is a decrease in the RF performance

The superior characteristics of the fabricated RF-MEMSswitch can be attributed to excellent mechanical behavior ofNi membranes good anchoring designs that reduce theeffect of induced stress and good AlN dielectric properties

C) Design and fabricationThe RF-MEMS capacitive shunt switches have been developedat ISiT as part of a multi-project wafer consisting of an eight-mask process fabricated on highly resistive Si substrate (3 V

cm) presented in detail elsewhere [14 15] The devices areloaded on a 78 V high-impedance line consisting of 2 mminsulated thermal oxide and 3 mm gold (Au) CPW Thetotal CPW width is 300 mm with SWS of 12550125 mmThe process combines to achieve a good mechanical andreliability behavior a thin 1 mm evaporated AuNiAu multi-metal layer for the movable membrane a thick 14 mm electro-plated Ni for the anchors and stiffening bars and 300 nmmedium-dielectric constant amorphous AlN dielectric Therelease structure presents an approximate air gap of 2 mmFigure 5 illustrates a cross-sectional view of a capacitiveMEMS switch with a fixedndashfixed beam configurationshowing the main layers of the process and a detailed SEM(scanning electron microscopy) image of a fabricated device

Different analog and digital RF-MEMS capacitive shuntswitches have been specifically designed using mainly threekinds of low spring constant anchoring (spring typemeander type and crabbed-meander type) to achieve differentactuation voltages as well as specific inductive componentsEach of them was conceived to be used as core devices forapplications in phase shifters and tunable filters and theirdesign was optimized for their work at specific frequenciesFigure 6 depicts SEM images of the main switches withFig 2 CV-curves of an analog capacitor and an MIM digital MEMS capacitor

Fig 1 Reconfigurable front-end RF IC typical simplified architecture

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different anchoring indicating the application for which theywere designed

D) Characterization and modelingThe switch is electrostatically actuated directly from thebottom electrode by applying a DC voltage inline with theRF signal of the CPW central conductor The suspendedbridge (top electrode of the capacitor) is anchored to theground plane of the CPW (Fig 7) and is actuated thanks tothe resulting electrostatic force

Device characterization from 6 to 40 GHz is carried outon E8361A Vector Network Analyzer and the calibrationis performed using an external short-open-line-thru com-mercial calibration kit De-embedding corrections are per-formed by measuring a thru line structure in the wafer toremove the losses and the parasitic pad capacitances of theunloaded line

A 12 V low actuation RF-MEMS shunt capacitive switch isdesigned using crabbed-meander-type anchoring (shown inFig 6(b)) with an approximate spring constant (KS) of 9 NmThe achieved capacitance ratio Cr is 27 at pull-in voltage(VPI) while it increases up to 40 with a higher actuationvoltage of 40 V Figure 8 presents the S-parameter measure-ments of the shunt switch The device provides good RF per-formances with isolation better than 37 dB an insertion loss of02 dB and a return loss of 12 dB at 40 GHz In additionreliability tests have been performed in hot switching con-ditions (+5 dBm RF power) over 2 times 109 cycles without per-manent failure or stiction [7] and an excellent thermalbehavior has been measured from 100 to 21308C witha VPI shift less than 2 V for the entire temperature rangeDT frac14 2308C [10]

The shunt switch is modeled by an accurate equivalentcircuit T-model as shown in Fig 9 Where Ct Lt and Rt cor-respond to the unloaded CPW and their values for a 460 mm

Fig 3 Comparison of the actuation voltage of state-of-the-art switches versus (a) the total switch area and (b) the isolation S21 at 40 GHz as well as the Cr

Fig 4 Comparison of the switching time of the state-of-the-art switches versus the actuation voltage

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length (dref) are Ct frac14 35 fF Lt frac14 143 pH and Rt frac14 01 V

(extracted from the thru de-embedded line) CMEMS Lp andRp are the components of the parallel shunt impedance (Zp)and correspond to the MEMS membrane When the switchis not actuated CMEMS Up is very small and Lp and Rp canbe neglected Finally DLs and DRs are corrective serieselements used to keep the length of the discontinuity closeto zero when modeling the device As a result DLs can benegative while DRs is expected to be positive [16] MEMSswitch circuit parameters are shown in Table 1 and havebeen extracted from the S-parameter measurements of thecrabbed-meander-type switch and using the equivalentT-circuit model (Fig 9)

Agilent-Advanced Device System (ADS) software has beenused to perform equivalent circuit simulations of the extractedparameters and results are plotted in Fig 10(a) Moreover 3Dfinite-element electromagnetic (EM) full-wave simulations ofthe crabbed-meander-type capacitive switch are performedby means of Ansoftrsquos HFSS software and depicted inFig 10(b) Both simulations are in good agreement with themeasurements validating the extracting parameter methodand the HFSS 3D EM model

E) Capacitive switches with a significantinductive componentIn order to achieve higher isolation at specific desired frequen-cies one possible solution is to design a switch with a highlydominant inductance component A highly inductive design

can be achieved by varying the anchoring or the switchdesign over the slots of the CPW as in the case of the meander-type switch (shown in Fig 6(c)) However in this kind ofdesign high isolation is only obtained in general in a relativelynarrow bandwidth

It can be observed in Fig 11 that a minimum of 5 dBimprovement in isolation can be achieved compared to thestandard crabbed-meander-type anchoring capacitive switchpreviously presented (Fig 8) The membrane bridge induc-tance has been varied from 10 to 324 pH in five different con-figurations They result in down-state natural resonantfrequencies f0 (or isolation region) at 78 133 178 and275 GHz and higher than 40 GHz for the standard switchA lower than 02 dB insertion loss is achieved for all devices

Fig 5 (a) Cross-sectional view and (b) SEM image of a detail in a fabricated device showing the different layers of the process

Fig 6 Three different capacitive switches designs used as core devices for the following applications (a) spring type and (b) crabbed meander type used incombination for analog and digital phase shifter and bandpass filters and (c) standard meander-type for stop band filter applications

Fig 7 Schematic view of a general RF-MEMS capacitive shunt switchelectrostatically actuated

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with a minimum isolation of 12 dB in the lower frequencyband (at 78 GHz) Insertion loss and isolation measurements(S21) of a set of inductive switches in the up and down staterespectively are depicted in Fig 11

F) Digital MEMS capacitor designThe series fixed capacitance CFIX is designed with a valuemuch smaller than CMEMS DOWN to limit the value of thetotal down-state digital MEMS capacitance The idea consistsin creating two well-defined capacitive states less dependenton the technology This device eliminates the uncertainty ofthe MEMS down-state capacitance which is much dependenton the fabrication process (due to the roughness and theplanarization process of the top and bottom electrodes) andit is very difficult to accurately estimate its value in advance(50 standard deviation) The total contribution of theMEMS and fixed capacitances placed in series configurationresults to be similar to the smallest capacitance value CFIXConsequently small or medium changes in the value of the

CMEMS DOWN (1ndash15 pF see Table 1) can be neglected forthe device performance

Among different CFIX approaches considered to designdigital MEMS capacitors interdigitated capacitors seem tobe the least dependent on the technology The capacitance isrealized in a single metal photolithography step and the accu-racy of its value depends only on the lateral resolution of theequipment However it is not a miniaturized solution overallif the interdigitated metal layer (in our case 400 nm TiPtAuPt underpath metal) is relatively thin MAM capacitorsmainly depend on the sacrificial layer (which will define theair gap thickness) and on their uniformity around the waferUnfortunately the air gap is highly dependent on the technol-ogy and usually has a minimum of 25 uncertainty FinallyMIM capacitors depend only on the lateral resolution ofphotolithography and on the dielectric thickness The dielec-tric layer can be deposited with high precision and its thick-ness can differ by a very few percentage of error Inaddition MIM capacitors are very compact and result in themost interesting approach [10]

Figure 2 illustrates the capacitance versus voltage of a stan-dard analog capacitor and an MIM digital capacitor It can beappreciated how MIM digital capacitors are almost indepen-dent on the technology The total contribution of the up-down-state capacitances are CTotal UP frac14 335 fF and CTotal

DOWN frac14 99 fF which corresponds almost exactly to thedesigned Cr of 3 [10] The designed CFIX results in a valueof 90 fF Unfortunately due to this added series fixed capaci-tance the actuation voltage needed to snap down the MEMSmembrane will substantially increase by an approximatefactor of 13ndash15

Figure 12 depicts the S21 phase or argument of the indi-vidual digital switch (of 05 mm cell length) The figureshows a phase shift between both states of 158 at 17 GHzThese digital capacitors will be used as the core device forphase-shifting applications in the next section For a phaseshift of 1808 at 17 GHz approximately 13 switches period-ically loaded in a distributed MEMS transmission line(DMTL) CPW of 6 mm total length would be needed

I I I R F - M E M S T R U E - T I M E D E L A Y( T T D ) L I N E rsaquo P H A S E S H I F T I N G

RF-MEMS capacitors have been widely investigated for theirapplication in DTMLs phase shifters both in analog [17]and digital modes [1 9ndash11] The idea is to periodically loada coplanar waveguide (CPW) with voltage-controlled varac-tors (or two-state digital capacitors [10]) to tune the distribu-ted capacitance the phase velocity and the propagation delayin the line By actuating the loaded MEMS capacitors a differ-ent phase state can be achieved The main advantage of DMTLphase shifter is that it behaves as a real TTD line with a con-stant time delay over a wide-band of frequencies Electronicallybeam-steered antenna consists of dynamically switching thebeam configurations of an antenna array by controllingthe relative phases at the input of each element to change theorientation of the whole antenna beam [17]

Figure 13 depicts the typical CndashV curve of both a singledigital capacitor and a full array of capacitors loaded on aCPW namely the DMTL phase shifter which are simul-taneously operated One can observe a quasi-identical pull-involtage (VPI) for all the switches (22 V) and a pull-out

Fig 8 S-parameter measurements of crabbed-meander-type anchoringcapacitive shunt switch (shown in Fig 6(b)) for different applied voltagesThe pull-in voltage VPI occurs at 12 V

Fig 9 Equivalent T-model used to extract the circuit parameters of the switchfrom RF measurements

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voltage (VPO) slightly higher than 10 V The polarity of theapplied voltage should not have any influence on the actuationsince the electrostatic force is proportional to the voltagesquared A symmetrical curve around V frac14 0 is achievedwhich suggests either that no mobile residual charges existor that they are compensated

A) Analog DMTL phase shifterDistributed MEMS TTD phase shifters with an analog control[18] are limited by the MEMS intrinsic instability appearing ata third of the original air gap This corresponds to a voltage-controllable capacitance ratio (Cr) that is lower than 15during the stable up-state Analog DMTL phase shifters areactuated below the pull-in voltage of the MEMS switchesachieving a continuous and quasi-linear phase shiftingOnce the membranes snap down the Cr can rise up to 20ndash100 Such high Cr is usually advantageous for switching appli-cation where high isolation is needed However for otherapplications such as TTD phase shifters even if it mightseem advantageous for miniaturization purposes the mis-match produced by such an enormous Cr is prohibitive

The actuation is similar to that of the individual switchesDC voltage is applied through the central conductor of theCPW inline with the RF signal Figure 14 illustrates the fabri-cated analog DMTL phase shifter using crabbed-meander-typeswitches

The main advantage of analog DMTL is its continuousphase shifting with a single voltage command However this

phase shifter approach is not very compact and themaximum phase shift is limited to Cr 15 The analogphase shift achieves a maximum continuous shifting of 436 pstime delay (DD) in a wide frequency band (from 5 to25 GHz) The phase shifter includes 13 crabbed-meander-typeanchoring analog capacitors periodically loaded in a CPWtransmission line and separated by 400 mm from center tocenter of each individual MEM cell The total size of thephase shifter is 54 times 06 mm2

The insertion loss is on average 239 dB at 17 GHz and thereturn loss is better than 15 dB for all the states over the entirefrequency range as shown in Fig 15

The analog DMTL phase shifter results in a moderatephase shift of 538cm and 68dB at 17 GHz The linearity ofthe differential phase shift with respect to frequency can beobserved up to 25 GHz

B) Digital DMTL phase shifterIn order to achieve more controllable and miniaturized sol-utions digital phase shifters are adopted using periodicallyloaded digital capacitors with MIM fixed capacitors and aredesigned to have a higher capacitive ratio (Cr 3) thantheir analog counterparts Digital DMTLs result in a largeramount of degrees per centimeter performance than theanalog phase shifters thanks to its greater design compactnessrelated to the higher Cr However they are limited to discretestates

Fig 10 (a) ADS circuit T-model simulations for all the actuation voltages shown in Table 1 (b) S-parametersrsquo HFSS Ansoft simulation of the 3D EM model of thecrabbed-meander-type anchoring capacitive switch in the up and down states

Table 1 T-model circuit parameter extraction from S-parameter measurements

Parameter at 6ndash40 GHz Meander-type capacitive shunt switch

Up Down

Voltage 0 V 8 V 10 V 12 V 13 V 20 V 40 VParallel Zp

Cp 40 fF 445 fF 485 fF 59 fF 111 pF 128 pF 155 pFRp ndash ndash ndash ndash 025 V

Lp ndash ndash ndash ndash 12 pHSeries Zs

DLs 210 pH 210 pHDRs 264 V 054 V

Natural frequency ndash ndash ndash ndash 436 GHz 406 GHz 369 GHz

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In principle with a single control voltage just two phasestates are possible in 1-bit DMTL line [7 10 11] In orderto achieve multi-state phase shifters the standard solution isto add multi-bit control commands which substantiallyincrease the complexity of the device [9 19] Howeverother approaches could be envisioned For instance athree-state digital phase shifter consisting in switches withdifferent spring constant devices would result in switcheswith different pull-in voltages Such a phase shifter couldachieve at least three phase shifters states with a single actua-tion command This seems to be a superior solution in termsof simplicity and miniaturization for phase shifters when onlythree discrete states are needed

However multi-bit actuation phase shifters remain thestandard design method by cascading a number N ofDMTLs (cascaded-bit CB-DMTL) Each section provides adelay corresponding to a bit weight (related to the numberof switches and corresponding length) [19] and are indepen-dently actuated by N voltage controls This kind of phaseshifter suffers from poor matching in the intermediate

states On the other hand interlaced-bit configuration(IB-DMTL) has demonstrated theoretically to alleviate thisproblem [20] The price to pay is a higher complexity ofthe optimization design overall if more than 2 bit arerequired

A two-bit DMTL phase shifter has been designed withIB-DMTL configuration and optimized to attain minimummismatch at 17 GHz (frequency of interest for the beam steer-ing application) in all its phase states An SEM image of thefabricated phase shifter is shown in Fig 16 Eighteen spring-type anchoring bridges (with 8 and 10 bridges for bit A andbit B respectively) are loaded interlaced and spaced by344 mm The total size of the 2-bit IB-DMTL phase shifter is61 times 09 mm2 The bits are individually actuated using high-resistivity bias lines made of 25 nm of tantalum (100 Vsq)to prevent the coupling between the RF signals and the biascircuitry

The fabricated 2-bit IB-DMTL phase shifter achieved ade-embedded insertion loss of 19 dB on average and an excel-lent optimized matching better than 21 dB at 17 GHz for allthe phase shifter states (Fig 17(a)) In addition an acceptablereturn loss (better than 12 dB) is obtained from 5 to 25 GHz

Fig 11 Insertion loss and isolation measurements (S21) of thecrabbed-meander-type anchoring switch (black line) and highly inductivecapacitive shunt switches made specifically to resonate at a certain frequency

Fig 12 Measurements of the S21 argument for the updown states of thedigital MEMS capacitor It represents the phase shift achieved for anindividual cell when switching from both states

Fig 13 Measured CndashV curves of a single digital capacitor and the array ofparallel-loaded 13 MEMS capacitors forming the phase shifter withapproximately 22 V pull-in for all the 13 MEMS capacitors simultaneouslyoperated

Fig 14 SEM image of the fabricated distributed MEMS analog phase shifterusing 13-loaded RF-MEMS analog capacitive switches which corresponds tothe one measured in Fig 8 (only four switches are shown in this figure)

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Constant and equidistant weight-bit time delays of 62 119and 18 ps which correspond to 01 10 and 11 states respect-ively were measured (Fig 17(b))

The loaded characteristic impedance of the phase shifter inthe different states is shown in Fig 18(a) up to 40 GHz Thereal impedance corresponds to 54 509 481 and 454 V

from 00 to 11 phase states These values demonstrate and suc-cessfully validate the IB-DMTL configuration proposed in[20] as well as the frequency matching optimizationmethod The figures of merit of the phase shifter result in1808cm and 208dB (without de-embedding the structure)at 17 GHz as shown in Fig 18(b)

C) 17 GHz fully integrated antenna beamsteering conceptA full integration of an antenna beam steering concept hasbeen performed in a single chip for wireless sensor nodesThe system includes a dipole antenna a balun and a 2-bitdigital DMTL phase shifter An SEM of the entire fabricatedsystem is depicted in Fig 19(a) The dipole antenna is pat-terned in a single thick metal as the CPW lines as well as

the balun but with two fixed air bridges to connect bothground planes of the CPW

The antenna and the phase shifter designs were optimizedto have good matching and return loss (40 dB) at thedesired RF frequency (172 GHz) using a surface-passivatedHR-Si substrate [21] which was thinned to 300 mm toimprove the radiation pattern performance The balun pro-vides a right-angle transition from the CPW of the phaseshifter to the coplanar strip feeding line of the dipoleantenna The Si chip consisting in an antenna array of twoelements in an area of 6 times 7 mm2 was successfully fabricatedand assembled to a customized PCB with an SMA connectorfor testing (Fig 19(b))

The first measurements of the beam steering conceptdemonstrate good return loss performance with small devi-ation versus simulations [22 23] Typical antenna radiationpattern behavior was measured and active beam steeringmeasurements are currently ongoing

I V T U N A B L E R F - M E M S F I L T E R S rsaquoF I L T E R I N G

A) Bandpass filterA compact and simple controllable tunable bandpass filter hasbeen fabricated based on a lumpedndashdistributed transmissionline filter concept [24] to select discretely the specific centerfrequency (from 20 to 23 GHz) with an independent control-lable channel bandwidth (of approximately 05ndash1 GHz)

The electrical length of the filter corresponds approxi-mately to l2 at the maximum center frequency of thetuning range in a slow-wave structure (meaning the CPWincluding the MEMS loaded capacitors in the up-state) Thefilter combines two kinds of MEMS devices shown inthe SEM image of the filter partly represented in Fig 20The first kind of devices the shunt digital capacitors areused to digitally tune from one frequency band implementedby using the concept of DMTL lines [10] Changing the stateof these capacitors discretely modify the phase velocity andelectrical length of the resonator and thus the resonant fre-quency The second part of the system is implemented withMEMS interdigitated capacitors in series configuration

Fig 15 (a) S-parameter measurements and (b) continuous TTD of the analog DMTL phase shifter based on analog MEM capacitors

Fig 16 SEM image of 2-bit DMTL phase shifters in interlaced (IB) andcascaded (CB) bit configuration loaded in a CPW and actuated throughhigh-resistivity bias lines

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(series-analog capacitor) They work in analog continuousmode and its main function is to continuously tune the band-width of the bandpass filter by modifying the value of thecoupling series-analog capacitor placed at both the inputand the output of the filter In addition they can be used tofine de-tune the filter center frequency by an extra correcting

element originally placed at the center of the filter and firstintroduced by the authors in [24]

S-parameter measurements were taken from 15 to 25 GHzwith a center frequency in f0 frac14 215 GHz Actuation of theanalog and digital series and shunt MEMS capacitors wasaccomplished with voltages from 15 to 40 V

Fig 18 Two-bit IB-DMTL frequency optimized phase shifter (a) the real and imaginary impedance and (b) constant time delay states at different actuation bits

Fig 17 Two-bit IB-DMTL phase shifter with an enhanced optimization method to improve the matching at 17 GHz (a) S-parameter measurements and (b)constant time delay states at different actuation bits

Fig 19 (a) SEM image of the fabricated entire system for applications in antenna beam steering and (b) optical image of the antenna array integration with thePCB showing the wire bonding connections

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The center frequency of the bandpass filter shifts from2045 to 226 GHz which corresponds to a 953 f0 tuningwith relative stable fractional bandwidth (151 + 19)and minimum variation of the insertion loss (405 +025 dB) across the complete tuning range (Fig 21(a))Tuning of the bandwidth is also demonstrated in a constantcenter frequency thanks to action of the correcting element[24] A 136 of bandwidth tuning is achieved at 205 GHzcenter frequency with a 1 dB fractional bandwidth varyingfrom 17 to 20 It is the first time that such independentcenter frequency and bandwidth tuning is demonstratedfor filters working in the K-band of frequencies Similar per-formance can be found in the literature but for filtersworking in the lower UHF frequency bands (from300 MHz to 14 GHz) [25]

B) Band-stop filterA wideband high-frequency tunable band-stop filter workingin the Ku- to Ka-band frequencies was originally devised[14 26] The filter is based on a miniature single-MEMdevice combining a central capacitive MEM switch loadedin a CPW line and suspension arms designed as multiple

inductive meanders with voltage-controlled coupling to thesubstrate (Fig 22)

The meander-type anchoring of the suspended structurehas been specifically designed to modify the inductance ofthe LC tank to resonate at the frequency of interest and pro-ducing a considerable notch response The main tuning capa-bility is achieved thanks to the variable capacitance betweenthe suspended metal membrane and the RF signal bottomelectrode via an electrostatic actuation The tuning rangecan be increased by reducing the stiffness of the anchorsthus creating a zipping effect after the pull-in avoiding aperfect contact of the membrane at pull-in Due to thefabrication-induced stress and the low stiffness of the bridgeat zero bias the central part of the bridge slightly touchesthe bottom electrode enabling an enlarged stable tuningregion before pull-in (capacitance ratio in the stable regionCr frac14 15) and a zipping effect after the pull-in The tuning be-havior is well controlled and the capacitance smoothlychanges in a repeatable way and in a continuous and quasi-linear fashion until an improved full contact of bothelectrodes

In addition the intrinsic inductance of the meandersover the slot (between the RF signal and the groundplanes of the CPW) plays a significant role in the deviceoperation as their coupling to the substrate additionallytune the capacitive component of the device due to thebending of the metal meander anchors during electrostaticactuation (Fig 22(b))

The fabricated device has an air gap of 3 mm The widthand length of the meanders are 10 and 135 mm respectivelyand the capacitive actuation area is 135 mm times 80 mm Thetunable band-stop filter has been successfully characterizedat room temperature and under high vacuum (1025 mbar)High tuning range of the filter center frequency of over 55is achieved The center frequency tunes from 167 to37 GHz (Fig 23) A linear capacitive tuning is achievedfrom 0 to 275 V which corresponds to a continuous linear fre-quency tuning of 20 The two center frequency tuningmechanisms are visible in Fig 23 First the MEMS capacitivetuning and second the zipping effect of the inductive armcoupling capacitance can be seen in this figure

The insertion loss in the lower pass-band is better than025 dB and the rejection level is around 20 dB for all the

Fig 20 SEM images of the fabricated tunable bandpass filter with parallel andseries MEMS devices

Fig 21 S-parameter measurements of a distributed-lumped bandpass filter with independent (a) center frequency tuning from 205 to 225 GHz (95) and (b) arelative bandwidth tuning of 136

518 montserrat fernarsquo ndez-bolan~ os badirsquoa pierre nicole and adrian mihai ionescu

httpswwwcambridgeorgcoreterms httpsdoiorg101017S175907871100064XDownloaded from httpswwwcambridgeorgcore University of Basel Library on 30 May 2017 at 162416 subject to the Cambridge Core terms of use available at

rejection bands A constant fractional bandwidth of 33 and57 GHz has been measured at 50 and 75 of the rejectionlevel of the filter (23 and 26 dB) which corresponds to a rela-tive bandwidth of 10 The temperature dependence is

45 MHz8C for the temperature range from 20 to 1008Cand gets around the double for the extended range (2100 to1008C) The fabricated device is very robust and showhighly reproducible characteristics at wafer level the filtercharacteristics in air and vacuum are quasi-identical andstay stable over 3 month of wafer storage in ambientconditions

V C O N C L U S I O N S

This paper presents a detailed investigation of RF-MEMScapacitive switches using AlN as dielectric and Ni as the struc-tural material in phase shifting and filtering functionsMaterials and design optimization have allowed the reductionof insertion losses and dissipation in miniaturized systemswith novel functionalities The presented RF-MEMS technol-ogy is not only a solution for improved RF performance in awideband (5ndash40 GHz) but also shows high robustness andreliability

A C K N O W L E D G E M E N T

The authors would like to thank T Lisec from IsiTFraunhofer Germany for the fabrication support andE Buitrago for language revision This work was partially sup-ported by the FP7 Integrated project e-CUBES e-BRAINSand by the Thales Company France

R E F E R E N C E S

[1] Rebeiz GM RF MEMS Theory Design and TechnologyWiley-Interscience New York 2003

[2] Goldsmith CL Zhimin Y Eshelman S Denniston DPerformance of low-loss RF MEMS capacitive switches IEEEMicrow Guided Wave Lett 8 (8) (1998) 269ndash271

[3] Peroulis D Pacheco SP Sarabandi K Katehi LPBElectromechanical considerations in developing low-voltage RFMEMS switches IEEE Trans Microw Theory Tech 51 (1)(2003) 259ndash270

[4] Park JY Kim GH Chung KW Bu JU Fully integrated micro-machined capacitive switches for RF applications in Proc IEEEMTT-S Int Microw Symp Dig vol 1 June 2000 283ndash286

[5] Blondy P et al Dielectric less capacitive MEMS switches inProc IEEE MTT-S Int Microw Symp Dig vol 2 June 2004573ndash576

[6] Ziaei A Dean T Mancuso Y Lifetime characterization of capaci-tive power RF MEMS switches in Proc 35th Eur Microw Conf vol3 October 2005 4

[7] Fernandez-Bolanos M Tsamados D Dainesi P Ionescu AMReliability of RF MEMS capacitive switches and distributed MEMSphase shifters using AlN dielectric in Proc IEEE 22nd Int ConfMicro Electro Mech Syst January 2009 638ndash641

[8] Muldavin JB Rebeiz GM High-isolation inductively-tunedX-band MEMS shunt switches in Proc IEEE MTT-S Int MicrowSymp Dig vol 1 June 2000 169ndash172

[9] Hayden JS Rebeiz GM Low-loss cascadable MEMS distributedX-band phase shifters IEEE Microw Guided Wave Lett 10 (4)(2000) 142ndash144

Fig 22 SEM images of (a) the fabricated band-stop filter device and (b) themeandering arm from the filter showing the zipping effect of the anchor

Fig 23 S-parameters of the band-stop filter showing an outstanding tuningrange of over 55

rf-mems switches with aln dielectric 519

httpswwwcambridgeorgcoreterms httpsdoiorg101017S175907871100064XDownloaded from httpswwwcambridgeorgcore University of Basel Library on 30 May 2017 at 162416 subject to the Cambridge Core terms of use available at

[10] Fernandez-Bolanos M Lisec T Dainesi P Ionescu AMThermally stable distributed MEMS phase shifter for airborne andspace applications in Proc 38th Eur Microw Conf October 2008100ndash103

[11] Perruisseau-Carrier J Fritschi R Crespo-Valero P SkrivervikAK Modeling of periodic distributed MEMS-application to thedesign of variable true time delay lines IEEE Trans MicrowTheory Tech 54 (1) (2006) 383ndash392

[12] Goldsmith C Maciel J McKillop J Demonstrating reliabilityIEEE Microw Mag 8 (2007) 56ndash60

[13] Lakshminarayanan B Mercier D Rebeiz GM High-reliabilityminiature RF-MEMS switched capacitors IEEE Trans MicrowTheory Tech 56 (4) (2008) 971ndash981

[14] Fernandez-Bolanos M Lisec T Dehollain C Tsamados DNicole P Ionescu AM Highly tunable band-stop filters basedon AlN RF MEMS capacitive switches with inductive arms andzipping capacitive coupling in Int Electron Device Meeting IEDM2009 December 2009 805ndash808

[15] Fernandez-Bolanos M Tsamados D Ionescu AM RFperformance and lifetime study of MEMS capacitive switches usingAlN as dielectric compared to Si3N4 J Microelectromech Syst(2010)

[16] Simons R Coplanar Waveguide Circuits Components andSystems John Wiley and Sons Inc New York 2001

[17] Vinoy KJ Varadan VK Design of reconfigurable fractal antennasand RF-MEMS for space-based systems Smart Mater Struct 10(2001) 1211ndash1223

[18] Barker S Rebeiz GM Distributed MEMS true-time delay phaseshifter and wide-band switches IEEE Trans Microw TheoryTech 46 (11) (1998) 1881ndash1890

[19] Hung J-J Dussopt L Rebeiz GM Distributed 2- and 3-bitW-band MEMS phase shifters on glass substrates IEEE TransMicrow Theory Tech 52 (2) (2004) 600ndash606

[20] Perruisseau-Carrier J Fritschi R Skrivervik AK Design ofenhanced multi-bit distributed MEMS variable true-time delaylines in Research in Microelectron Electron PhD vol 1 July2005 189ndash192

[21] Fernandez-Bolanos M Perruisseau-Carrier J Dainesi P IonescuAM RF MEMS capacitive switch on semi-suspended CPW usinglow-loss high-resistivity silicon substrate J Microelectron Eng 85(5ndash6) (2008) 1039ndash1042

[22] Fernandez-Bolanos M et al Dipole antenna and distributedMEMS phase shifter fully integrated in a single wafer process forbeam steering applications J Microelectron Eng 87 (2010)1290ndash1293

[23] Vasylchenko A Fernandez-Bolanos M Brebels S De Raedt WVandenbosch GAE Conformal phased array for a miniature wire-less sensor node in IEEE 20th Int Conf on Applied Electromagneticsand Comm September 2010 (invited)

[24] Fernanez-Bolanos M Dehollain C Ayoz S Nicole P IonescuAM Center frequency and bandwidth tunable bandpass filterbased on RF MEMS (10 GHz- 14 GHz) in Proc 39th Eur MicrowConf September 2009 1708ndash1711

[25] Carey-Smith BE Warr PA Beach MA Nesimoglu T Widetuning-range planar filters using lumped-distributed coupledresonators IEEE Trans Microw Theory Tech 53 (2) (2005)777ndash785

[26] Fernandez-Bolanos M Dehollain C Nicole P Ionescu AMTunable band-stop filter based on single RF MEMS capacitiveshunt switch with meander arm inductance J Solid-StateElectronic 54 (9) (2010) 1033ndash1040

Montserrat Fernandez-Bolanos Badıareceived her MSc degree in Telecom-munication Engineering from the Uni-versity of Seville Spain in 2005 Shereceived her PhD degree in Microsys-tems and Microelectronic from EcolePolytechnique Federale de Lausanne(EPFL) in 2010 Since August 2010 shehas been working as a scientific collabor-

ator in the Nanoelectronic device laboratory (NANOLAB) atEPFL The focus of her research is in the field of RF-MEMSswitches phase shifter and tunable filters for airborne andspace applications Her present research interests include theopen challenges of RF-MEMS devices such as the reliabilitythe packaging and their 3D integration with RF ICs

Pierre Nicole is the RampD project man-ager at THALES Airborne Systems Hehas more than 30 years of experience inthe fields of Optoelectronics and Micro-wave Functions and Systems RampD Hereceived his eng degree from INPG(ENSERG) in 1980 and joined SAT toparticipate in the first projects inFrance on optical links for telecommuni-

cations In 1984 he then joined the Technical Directorate ofDassault Electronique where he continued works on digitaland microwave optical links for harsh aeronautical systemsHe then contributed to the development of the new field ofmicrowave wireless sensor networks in 1994 where he devel-oped demonstrators of wireless sensors embedded in concreteand patented different issues about ldquoAbandoned Sensorsrdquo re-motely or self-energized through microwave optical andacoustical waves He holds various patents in each field Hiscurrent RampD activities are related to millimeter-wave opticallinks microwave MEMS functions and applications wirelesssensors smart antennas and autonomous microsystems

Adrian Mihai Ionescu is an AssociateProfessor at the Swiss Federal Instituteof Technology Lausanne SwitzerlandHe received the BSMS and PhD de-grees from the Polytechnic Institute ofBucharest Romania and the NationalPolytechnic Institute of GrenobleFrance in 1989 and 1997 respectivelyHe has held staff andor visiting pos-

itions at LETI-CEA Grenoble France LPCS-ENSERG Gre-noble France and Stanford University USA during 1998and 1999 His research interests focus on micro- andnano-electronic devices aimed at Integrated Circuit design ndashespecially process development modeling and electricalcharacterization Dr Ionescu has published more than 250articles in international journals and conferences He receivedthree best paper awards in international conferences and theAnnual Award of the Technical Section of the RomanianAcademy (of Sciences) in 1994 He is director of the Labora-tory of MicroNanoelectronic Devices (NANOLAB) andhead of the Doctoral School in Microsystems and Microelec-tronics of EPFL

520 montserrat fernarsquo ndez-bolan~ os badirsquoa pierre nicole and adrian mihai ionescu

httpswwwcambridgeorgcoreterms httpsdoiorg101017S175907871100064XDownloaded from httpswwwcambridgeorgcore University of Basel Library on 30 May 2017 at 162416 subject to the Cambridge Core terms of use available at