research paper

RF MEMS ohmic switches for matrixconfigurations

giorgio de angelis1

andrea lucibello1

emanuela proietti1

romolo marcelli1

giancarlo bartolucci17

federico casini2

paola farinelli2

giovanni mannocchi3

sergio di nardo3

daniele pochesci3

benno margesin4

flavio giacomozzi4

olivier vendier5

taeyoung kim6

and larissa vietzorreck6

Two different topologies of radio frequency micro-electro-mechanical system (RF MEMS) series ohmic switches (cantileverand clampedndashclamped beams) in coplanar waveguide (CPW) configuration have been characterized by means of DCenvironmental and RF measurements In particular on-wafer checks have been followed by RF test after vibrationthermal shocks and temperature cycles The devices have been manufactured on high resistivity silicon substrates as buildingblocks to be implemented in different single-pole 4-throw (SP4 T) double-pole double-throw (DPDT) configurations and thenintegrated in Low Temperature Co-fired Ceramics (LTCC) technology for the realization of large-order Clos 3D networks

Keywords Modeling Simulation and characterizations of devices and circuits Passive components and circuits RF-MEMS andMOEMS Si-based devices and IC technologies

Received 1 March 2011 Revised 5 January 2012

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

Switch matrices have broad applications in redundant andbroadcasting systems Microwave and millimeter-waveswitch matrices are essential components in telecommunica-tion systems since they enhance satellite capacity by providingfull and flexible interconnectivity between the received andtransmitted signals The demand for advanced switchingsystems has increased due to the recent growth in multimediamobile and internet applications

In satellite payload systems mechanical switches are stillused they exhibit low loss but are bulky Micro-electro-mechanical system (MEMS) technology has attracted tremen-dous interest across the world and research efforts are constantlygrowing for reliability and integration purposes MEMS fabri-cation techniques take advantage from the maturity of the fab-rication technology commonly utilized in the production ofintegrated circuits (ICs) Several advances have been recently

performed in the realization of radio frequency (RF) MEMSThe integration of MEMS into traditional RF circuits resultedin systems with superior performances and lower manufactur-ing costs The incorporation of MEMS-based fabrication tech-nologies into microwave and millimeter-wave systems offersviable routes toward ICs with MEMS actuators antennasswitches and transmission lines [1ndash7] The resulting devicesoperate with an increased bandwidth and with an increasedradiation efficiency and they are very promising for theimplementation of aerospace and defense systems A keyrole is played by the RF MEMS switches These devices areable to process RF signals via a transmission line changingthe state from on to off by means of an electrostatic actuation(but it can also be magnetic piezoelectric thermal and so on)due to a voltage applied between a suspended metal mem-brane and an electrode They pass from an up state to adown state generating an open or a short circuit on the TXline [8ndash11] This paper presents the tests performed on single-pole single-throw (SPST) ohmic series switches to beimplemented in different topologies of single-pole 4-throw(SP4 T) double-pole double-throw (DPDT) and then inte-grated in LTCC technology for the realization of large-orderClos 3D networks Two different SPST switches have beentested based on cantilevers and clampedndashclamped beamsboth in coplanar waveguide (CPW) technology The charac-terization was performed to select the best SPST configurationin terms of the RF and mechanical response of the switchSpecifically the recorded data have been vital for simulationson a 16 times 16 matrix in order to decide the matrix topologyand the expected fulfillment of the electrical performancesfor the overall system From preliminary simulations on

Corresponding authorR MarcelliEmail RomoloMarcelliimmcnrit

1CNR-IMM Roma Via Fosso del cavaliere 100 00133 Roma Italy Phone +39 064993 45362Department of Electronic Engineering University of Perugia Via G Duranti 9306125 Perugia Italy3THALES ALENIA SPACE Italia Via Saccomuro 24 00131 Roma Italy4FBK-Center for Materials and Microsystems ndash Irst Via Sommarive 18 Povo 38100Trento Italy5THALES ALENIA SPACE France Toulouse France6Technische Universitat Munchen Arcisstrasse 21 80333 Munchen Germany7Department of Electronic Engineering University of Roma ldquoTor Vergatardquo Via delPolitecnico 1 00133 Roma Italy

421

International Journal of Microwave and Wireless Technologies 2012 4(4) 421ndash433 Cambridge University Press and the European Microwave Association 2012doi101017S1759078712000074

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such a system based on single switch data coming from pre-vious technological runs large-order Clos 3D networksembedded in a LTCC structure would be the best candidatesfor the final matrix The aim of this paper is to present thecurrent state-of-art for an on-going European Space Agency(ESA) supported project For this reason we are presentingnot only the building blocks characterization (single switchesSP4 T and DPDT) but also the general aims for this activityincluding the matrix simulations because this will be the finaloutput for the contract In this way the reader can have abetter feeling of the full story of the general requirementsand the currently available results We have performed a com-parison between two possible building blocks looking mainlyto their electrical performances leaving as a second future stepthe improvement in the reliability by using billion cycleswhich is by the way not a critical issue for this applicationFor this reason the single switches have not been stressedso much because the main aspect is to evaluate highly reliableswitches with a limited number of actuations Specifically theESA program supporting this activity requires a maximumnumber of actuations for re-configuration purposes in theorder of 1000

I I D E S I G N A N D T E C H N O L O G Y

RF MEMS switches are subjected to various failure mechan-isms eventually leading to stiction during several or long-termactuations [12] Charging effects due to dielectric layers used inthe actuation pads are presently one of the most limiting factorsin the device reliability [13] Further to the optimization of thedielectric properties of the deposited oxides in recent yearssome mechanical solutions have been developed to overcomesuch a problem A solution consists in the realization ofdielectric-free pads in order to avoid the contact of the bridgewith the actuation pad [14] In this section and in the followingones the terms ldquopillarsrdquo ldquostoppersrdquo and ldquostopping pillarsrdquo areused as synonyms This non-contact configuration is possibleby either realizing mechanical stoppers on the bottom surfaceof the bridge layer or building stopping pillars on the actuationpad The latter solution has been adopted because it does notrequire any modification of the MEMS process previouslydeveloped for double-clamped beams within the consortiumand it is shown in Figs 1 and 2 [7]

All SPST switches are monolithically manufactured onp-type 200 mm thick highly resistive silicon wafers by usingthe eight-mask MEMS process developed at FBK-irst [7]

The developed and tested series ohmic winged switches aresummarized in Table 1 and the general structure is shownin Fig 3 The dimensions of the bridge for this kind ofswitch are length frac14 500 mm and width (in winged area) frac14150 mm The cantilever switches are summarized in Table 2and the structure is shown in Fig 3 A high resistivitysilicon substrate with r 5000 V cm has been used Thepeculiar characteristics of the manufactured devices havebeen chosen to check the improvement due to the presenceof both (i) bumps under the wings and (ii) pillars for provid-ing the full actuation but avoiding or minimizing the contactbetween the metal beam and the actuation pads The first sol-ution is used to improve the ohmic resistance of the actuatedswitch because of the increasing number of contact points ofthe metal beam onto the substrate balancing possible shapeirregularities at the end of the beam (cantilever) or on thewings (double clamped structure) The second solutionalready studied by other groups provides an almost contact-less actuation thus preventing or decreasing the chargingeffects The manufacturing process includes a shaping alongthe double-clamped configuration and selective gold electro-plating Au is thicker in the center with respect to the sides toobtain a quite robust structure in the middle but maintaininga good flexibility on the lateral sides (close to the actuationpads) thus facilitating the full collapse and a good electricalcontact The same conditions have been utilized for the canti-lever to obtain a robust but easy to be actuated single-clamped configuration The pillars as it will be presented in

Fig 1 Multi-layer technology to realize (a) the contact bumps (b) the stoppers and (c) the electrodes for (d) the final device LTO refers to low thermal oxidedeposition by silane SiH4 TEOS to the higher temperature tetraethylorthosilicate deposition (Si(OC2H5)4) ldquoMetalrdquo means a multi-layer deposition comprising TiTiNAlTiTiN The partial thicknesses used for the metal multilayer are starting from the substrate toward the top surface are in nm Ti 30 TiN 50 Al 410 Ti 60TiN 80 arriving to a metal having a total thickness of 630 nm as it is shown in the figure In particular Ti 30 nm and TiN 50 nm are diffusion barriers toward thepoly whereas Ti 60 nm and TiN 80 nm on the surface are used for both diffusion barrier and to hindrance the hillocks formation during the LTO deposition

Fig 2 SEM picture of the double-clamped SPST series ohmic switchbelonging to a SPDT structure Two lateral wings are used for improving theelectrical contact at the IO ports Details of the switch are given in thefigure ldquoCrdquo means the place where the metal beam is clamped

422 giorgio de angelis et al

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the measurement section are a valid technological solution toalmost suppress the charging onto the actuation pads On theother hand some criticalities have still to be solved because ofthe consumption of the pillars after a number of actuationswhich can cause a premature failure of the switch whenminded for billion of cycles Technology is in progress toimprove this manufacturing detail

The exploited configurations differ between them for thenumber of pillars eventually using one stopping stripe onlyand for the number of bumps (from 3 to 11) as expressedin Tables 1 and 2 Additional splits for the same basic con-figurations are the length of the wings and the presence orabsence of dielectric films onto the actuation pads Squareor circular bumps do not make an evidence for a shape depen-dence of the electrical response Polysilicon has been used forthe feeding lines whereas TetraEthyl OrthoSilicate (TEOS) orLaw Temperature Oxide (LTO) films have been deposited forthe dielectric actuation pads (Fig 4)

I I I T E S T P R O C E D U R E S

The SPST devices have been tested by DC and RF measure-ments with the goal to select the most promising configurationIn particular five samples belonging to the same wafer ran-domly chosen have been characterized It has to be stressedthat because of the peculiar application ie signal routing forreconfiguration of the satellite no failure is allowed for theSPST used as building blocks or at least the limiting factors

for their utilization have to be properly addressed In particularbecause the technology for implementing the presented con-figurations is still in progress for optimizing their electrical per-formances we stopped the measurements when a significantchange in the S-parameter response was recorded The acceptedchanges in the S-parameters response have been defined at thebeginning of the activity according to ESA requirements onthe final matrix structure Actually 210 dB should be themaximum acceptable loss for the 16 times 16 matrix and215 dB the return loss As it will be demonstrated in the lastsection of this paper the expected performances for the buildingblocks are tight and they have to be compliant with the entirematrix requirements Then the SPST has to be very performanttoo For this reason an ohmic series switch has been evaluatedto be a good candidate if its loss is in the order of 203 +01 dB with a return loss not exceeding 230 dB up to 20 GHz

The procedures and the obtained results are summarized inthe following sub-sections

The tests have been done under nitrogen flux with a temp-erature ranging between 23 and 248C and relative humidity(RH) le30

A) DC testThe contact resistance versus the voltage Ramp signal (1 Vs)has been measured by using the experimental setup

Table 1 Series ohmic winged switches description

Name Description

SW Stopping pillars no dielectric above the polysilicon pad fivecircular bumps

SW2 Stopping strips instead of pillars no dielectric above thepolysilicon pad five circular bumps

SW3 Like SW three square bumps longer and shaped wingslarger pad no dielectric above the polysilicon padminimum overlap area

SW4 Like SW no dielectric above the polysilicon pad 11rectangular bumps

SWdiel Like SW but no stopping pillars TEOS and LTO abovethe pad five circular bumps

Table 2 Cantilever description

Name Description

CW Stopping pillars no dielectric above the polysilicon padseven circular bumps dimensions 110 times 170 mm2

CW2 Stopping pillars no dielectric above the polysilicon padseven circular bumps dimensions 110 times 145 mm2

CW3 Stopping pillars no dielectric above the polysilicon padthree circular bumps shaped cantilever tip

CW4 Stopping pillars no dielectric above the polysilicon pad13 rectangular bumps dimensions 110 times 170 mm2

CW5 Like CW thicker cantilever no dielectric above thepolysilicon pad larger pads no wing mechanism twocircular bumps

CWdiel Like CW no pillars TeOS and LTO above the polysiliconpad

Fig 3 Series ohmic winged switch Lateral wings have been included forimproving the electrical contact When the switch is actuated the bridgeisolated with respect to the ground closes the central conductor of the CPWwith a metal-to-metal contact and the device is in the ON state Fig 4 Top view of the cantilever MEMS switch

rf mems ohmic switches for matrix configurations 423

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diagrammed in Fig 5 The contact resistance versus thevoltage Ramp signal (1 Vs) has been evaluated The typicalbehavior for SW and CW switches is shown where the resist-ance change due to actuation and de-actuation is given It isworth noting that in the studied configurations charging pre-vention is obtained by means of pillars avoiding the contactonto the actuation pads In fact the measured actuationvoltage Vact has been found to range between ca 40 and 50V for both configurations also after 105 cycles and withunchanged RF performances at least up to 104 cycles for theCW device

The typical behavior for the CW and SW switches is shownin Figs 6(a) and 6(b) where the resistance change due to actua-tion (voltage required to actuate the bridge) and de-actuation(voltage required to de-actuate the bridge) is given togetherwith a preliminary reliability test including cycles up to 105In spite of a decrease in the performances due to the contactdegradation because of the current flow data belonging to theconfiguration labeled as CW are considered more promisingfor the future device implementations

Time offset is noticed in the pull-down response of thetested devices It is not clear which is the cause for such aneffect The authors believe that the story of the device is

affected by the number of cycles experienced but no evidencefrom the recorded data is obtained on a reliable measurementof the offset

1) rf test

On-wafer S-parameters of the switches in the ON and OFFstate have been measured by means of a HP VNA 8510Cfrom 45 MHz to 40 GHz with commercialshort-open-line-throw (SOLT) and in-house developed TRLcalibrations The experimental setup used for the RF charac-terization is shown in Fig 7

The environmental test conditions with for the measure-ment setup in Fig 7 are temperature T frac14 238C and relativehumidity RH asymp 30 For the RF test the following conditionshave been imposed SOLT calibration from 45 MHz to40 GHz power frac14 3 dBm number of points frac14 801 multi-meter resolution current 1 mA voltage 1 mV A voltmeter(Hp3478 Digital Multimeter2) is in parallel between the actua-tion pads of the switch and the ground pad to check theactuation voltage as close as possible to the device and acurrent meter (Hp3478 Digital Multimeter1) is in line tocheck the amount of current flowing in the switch

Fig 5 Experimental setup for the characterization of the contact resistance of the ohmic switches DUT is the device under test

424 giorgio de angelis et al

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Scattering parameters have been recorded before andduring the reliability test (up to 105 cycles) and the resultsare shown in Fig 8 (isolation) and in Fig 9 (return loss andtransmission) for the SW switch in the OFF and ON configur-ations respectively whereas in Figs 10 and 11 the same find-ings are shown for the CW one In Fig 9 the insertion loss ismore detailed with respect to the scale of the previous Fig 8In the captions of the following figures an extended notationof the tested device has been used to indentify its positionwithin the wafer In particular ldquoLrdquo and ldquoCrdquo mean ldquoLinerdquoand ldquoColumnrdquo respectively for a mask with several linesand columns containing repetitions of the same devicestructure

B) Environmental tests

1) thermal tests

Thermal tests are important verifications for determiningsome of the reliability aspects in RF MEMS devicesSpecifically temperature can change metal shape and proper-ties of the bridge because stress can be induced in the clamped

structure with more evidence in a double-clamped one wheretwo mechanical constrains are present Moderate thermaltreatments contribute in the elongation of the metal with sig-nificant variations of the actuation voltage because of the dis-tance change between the metal beam and the wafer surface(gap) Such a voltage will decrease or increase depending onthe direction of the residual stress characterizing the manufac-tured device Higher temperatures can modify permanentlythe shape of the beam and the metal will result in adamaged structure

For testing the exploited devices three kinds of experimen-tal procedures have been followed

(1) Gluing test The switches have been glued on a gold platedKovar carrier using different temperatures 30 80 and1258C

(2) Thermal storage The switches glued at 308C have beensubmitted to a thermal storage test which has been per-formed with the following settings 1000 h at 1258C innitrogen ambient according to ESA PSS-01-612MilSTD 883 ndash Method 1010 ldquoBrdquo

(3) Thermal shockscycles The switches subjected to thethermal storage test have been then measured under thefollowing thermal shockscycles 50 thermal shocks per-formed in nitrogen ambient using a dual chamberfrom 255 to +1258C (ESA PSS-01-612Mil STD 883 ndashMethod 1010 ldquoBrdquo) Ten thermal cycles performed againin nitrogen ambient with Tmin frac14 2358C andTmax frac14 +758C The temperature variation was imposedby means of a ramp R frac14 1 min8C with minimum andmaximum temperature hold time of 1 h

DC parameters (actuation voltage contact resistancede-actuation voltage) and RF parameters (S-parameters inup and down state) have been measured before and after allthe thermal tests

2) vibration test (or mechanical test)

Vibration test is a standard characterization in space appli-cations and generally speaking in harsh environments Forspace it means to check the response of the device or sub-system to be launched for determining its sensitivity tospecific steps of a mission with special care for the launchof a satellite Actually the sizing of a spacecraft design islargely based on the launcher vibration loads As everythingthat will go to space will have to be launched vibration testshave to be scheduled when validating a technology forspace Moreover during the launch phase the activity of theelectronic and electromagnetic instruments on board islimited Then vibration environment of the launcher has tobe taken into account together with the electromagnetic com-patibility of every instrument belonging to the specificlauncher In our case after the stress the RF and any otherdesigned specification have to be maintained

The standard vibration test imposed for ground determi-nation of the flight conditions during the launch consists ofa sequence encompassing Z-axis vibration X-axis vibrationand Y-axis vibration In turn each axis vibration includesthe following steps the search for the resonance frequencythe sine vibration and the random vibration respectivelyThe DC parameters including the following (i) actuationvoltage (ii) contact resistance and (iii) de-actuation voltageand the RF parameters including S-parameters in up and

Fig 6 Contact resistance of the CW (a) and SW (b) switches as a function oftime Reliability by means of cycles up to 105 actuations has been alsomeasured Better contact resistances have been measured for the CWconfiguration with respect to the SW one De-actuation refers to the voltageat which the bridge is re-stored to the initial position after being actuated

rf mems ohmic switches for matrix configurations 425

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down state have been measured before and after the vibrationtest

The results coming out from the above outlined thermaland mechanical tests are summarized in Tables 3 and 4

From the analysis of the results presented in the previoustables it turns out that presently thermal cycling can be akilling factor for some of our devices On the other handmechanical test performed on the same device after thethermal characterization is less critical and not dependent ofthe structure Specifically the cantilever configurationshaving one constrain only from a mechanical stand pointare affected as expected by a change in the actuationvoltage but failure (F) after thermal cycle is less probable

with respect to the double-clamped beams Generally speak-ing the SW devices (double-clamped) exhibit a trend in thedecrease of the actuation voltage probably due to a downwardreshaping of the bridge up to possible sticking (micro-welding) for high temperatures whereas the CW devices (can-tilevers single clamped) should exhibit an upward reshapingof the beam with a resultant increase in the actuation voltage

3) endurance test

The endurance test has been performed with the aim to checkthe device response after voltage stresses including cyclingand continuous DC biasing

It has been divided in two tests

(1) Cycling stress test Uni-polar biasing condition with a dutycycle of 50 and a frequency of 100 Hz has been appliedto the device under test

(2) DC stress test A continuous uni-polar biasing signal hasbeen applied for 10 min to the device under test TheS21(V ) test (S21 parameters Vs voltage from 2100 to100 V) has been used as parameter to check the perform-ance of the SPST before during and after the tests asshown in Fig 12 The amplitude of the signal voltageused for the previous stress test is adapted to thepull-in and pull-out voltage obtained in the S21(V ) testIt is usually set to 11ndash12 time the pull-in voltage

Looking at the results presented in the previous plots themeasured RF performances are in agreement withthe expected ones and with the required specifications forthe building block of the matrix For the series ohmic

Fig 7 The test bench is composed by the following instruments one dual power supply for the devices actuation one PC one VNA one waveform generator forthe pulse train definition two digital multimeters for controlling both actuation voltage and current flow (if any) one temperature-relative humidity sensor (TRH in the figure) The DUT is kept under nitrogen flow

Fig 8 Isolation performances of the SW switch Curves are practicallysuper-imposed

426 giorgio de angelis et al

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winged switches in the off state isolation is well under 220 dBup to 30 GHz Concerning the on state a return loss betterthan 220 dB has been measured for all devices in the fre-quency band ranging from 45 MHz to 40 GHz Insertionloss values vary a bit depending on the type of calibrationroughly ranging from 202 dB (TRL calibration) and

205 dB (SOLT calibration) for frequency values betweenDC and 20 GHz giving evidence for an equivalent contactresistance of about 18 V

All CW-type devices show an insertion loss better than203 dB and return losses better than 230 dB from DC to20 GHz Isolation is better than 220 dB up to 13 GHz andbetter than 210 dB up to 40 GHz All CW-type switchesshow on-state characteristics identical between them asexpected from simulations Identical off-state isolation per-formances have been also measured with the only exceptionof CW3 device which shows an isolation which is about2ndash3 dB better than the others This is perfectly in agreementwith what expected since the switch has been designed witha reduced overlap area between the cantilever and the signalline The CW-type switches are more robust than theSW-type switches and they are characterized by a higherpull-in voltage Owing to the results coming from the endur-ance tests more attention will be focused on CW2 and CW4test structures which have exhibited the most promisingresults

I V S W I T C H M A T R I X D E S I G N

With the proposed switches a 16 times 16 switch matrix will bebuilt From the simulations a 3D Clos network embeddedinto an LTCC surrounding seems to be the best candidateThe general topology is depicted in Fig 13 It consists offour stacked 4 times 4 blocks repeated three times with arotated orientation by 908 The result is a very compact struc-ture each of the 12 blocks can easily be realized in LTCC Anextension to a bigger matrix can easily be done just by stack-ing more sub-blocks with more in- and outputs For examplewith 48 planar 16 times 16 sub-blocks a 256 times 256 matrix can bebuilt by the same principle

Clos networks are a subclass of the re-arrangeable networks[16] Such a network consists of two symmetrical outsidestages of rectangular sub-blocks with an inner stage ofsquare sub-blocks Here all sub-blocks are of 4 times 4 typeEach sub-block can be realized utilizing six DPDT switchesin Benes topology [17] as shown in Fig 13 An even morecompact approach is based on SP4 T switches [15] whereonly eight SP4 T switches need to employed for one 4 times 4sub-block

The layouts of the SP4 T and DPDT based on the describedohmic switches in coplanar technology to be used in theswitch matrix are shown in Fig 14

The utilized SP4 T switch consists of a cascade of two SPDTswitches realized with cantilever ohmic switches In order toimprove the isolation an additional shunt switch is added inevery output path The DPDT switch in Fig 14(b) consistsof four SPDTs employing ohmic series switches of bridgetype The realization based on Benes block with DPDTs exhi-bits more switches in one connection path on the other handthe connection circuitry is less complicated with a low numberof crossings which can be realized easily with only two layersof LTCC The cross-section of on 4 times 4 block with the siliconchip embedded into a multilayer LTCC with metal packageand connector pins is shown in Fig 15

To evaluate the RF transition performances a full-wave simu-lation is performed part by part and their results are combinedusing circuit simulators for the feasible signal path [18 19]This is because the entire structure has lots of signal pathsFig 10 Measured isolation performances for the CW switch up to 105 cycles

Fig 9 Return loss (a) and transmission (b) for the SW switch

rf mems ohmic switches for matrix configurations 427

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and their configuration is too complicated to perform a full-wave simulation The simulation parts are carefully chosenso that the coupling with other parts is negligible for theycannot be taken into account during result combining by

the circuit simulator Every transition parts ndash layer to layerbond wire bond ribbon RF pin connectors etc ndash are opti-mized to get feasibly best performances [20] According tothe simulation the best results for the entire matrix wereobtained by the 4 times 4 block utilizing SP4 T switches ofwhich performances are depicted in Fig 16

In the shown configuration a high isolation can beexpected as the coupling of signals in the SP4 T and especiallybetween the isolated building blocks is low The insertion lossis well above 210 dB up to 15 GHz due to the low number of

Fig 11 Insertion loss and return loss for the CW switch up to 105 cycles

Table 3 Actuation voltage for different SW switches measured on thesame wafer after thermal and mechanical tests

Actuation voltage [V ]

Initial After T cycle After Mech test

SW 90 64 78 F F F 75 F FSW2 80 70 65 F F 60 F F 74SW3 55 60 70 55 F F 55 70 FSW4 88 90 75 74 F F 80 F FSWdiel 50 50 60 F 28 F F F FCW 50 55 62 F 65 F 90 69 40CW2 50 52 60 F 55 F F 60 100CW3 50 70 70 F 57 35 65 100 40CW4 52 90 65 F 55 55 100 50 40CW5 35 40 42 45 42 42 F 45 45CWdiel 55 55 63 74 55 55 85 60 65

SW SW2 etc refer to devices taken from different positions within thewafer Some failures have been recorded mainly for the SW configurationThe same happens for the CW devices Structures manufactured on theedge of the wafer usually present higher actuation voltages due to athicker electroplated bridge This effect appears to be less critical for theCW devices mostly having Vact in the order of 50ndash55 V

Table 4 Scattering parameters of the SW and CW devices after temperature and mechanical stress

S21 (dB)

Initial After T cycle After Mech test

6 GHz 12 GHz 18 GHz 6 GHz 12 GHz 18 GHz 6 GHz 12 GHz 18 GHz

SW 222 225 227 F F F F F FSW2 209 211 213 211 213 219 211 213 215SW3 217 219 221 F F F F F FSW4 216 218 221 F F F F F FSWDiel 213 215 217 F F F F F FCW 205 206 208 F F F 204 205 207CW2 206 207 21 F F F 211 213 214CW3 206 208 211 203 204 208 204 205 207CW4 214 215 217 203 204 209 203 204 209CW5 206 207 209 205 206 211 206 207 2092CWDiel 205 207 209 205 206 211 204 205 207

F means failure

Fig 12 S21(V ) test for the CW switch at 8 GHz The S21 change due to theactuation of the SW switch before and after the DC stress is plotted withfull overlap between the initial and final curves It is an evidence that theendurance test was passed without modifications in the electrical response ofthe device

428 giorgio de angelis et al

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employed switches in one pass The only critical parameter isthe return loss which is comparably high because of thehigher number of vias and connection pins Here somemore optimization of transitions and connections could leadto better results With the Benes network we obtained worseisolation as more coupling within one DPDT occurs Theinsertion loss is higher due to the higher number of employedswitches Only the return loss is comparable here the connec-tion circuitry is more simple however the number of tran-sitions from the silicon blocks to the LTCC substrate is higher

V C O N C L U S I O N

Cantilever and series ohmic winged switches for satelliteapplications have been fully tested Cantilever switchesexhibit better performances (lower contact resistance andinsertion loss) with respect to clampedndashclamped beam onesand superior characteristics on all reliability aspects (includingthermo-mechanical robustness) Moreover they generallyshow a better agreement with respect to preliminary simu-lations and are less sensible to the stopping pillars manufac-turing process As a consequence cantilever switches arecurrently preferred with respect to clampedndashclamped beamsDielectric layers on actuation pads prevent a reliable operationof the devices due to relevant charging effects and this tech-nological solution shall therefore be abandoned The efficiencyof the alternative solution based on the utilization of stoppingpillars is confirmed by testing and corrective actions in termsof geometry and dimensions shall be implemented on asecond run in order to ease their manufacturing Prototypesof SP4 T and DPDT with the switches shown in this paperhave been already measured as reported in [15] and theywill be implemented as soon as the corrective actions will beperformed on the SPST devices Matrix configurations usingthe proposed building blocks have been simulated for practicalrealization in an LTCC environment Currently sub-matrixelements have been manufactured with LTCC packagingThey are under test for final decisions about the assemblytechniques to be followed for the full matrix in order to

Fig 13 3D Clos configuration of the final matrix

Fig 14 (a) SP4 T switch based on SPDT switches with an additional shuntswitch in the second stage in order to improve isolation and (b) DPDTswitch based on ohmic switches

rf mems ohmic switches for matrix configurations 429

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fulfill the system requirements outlined at the beginning ofSection III

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

This work has been carried out in the frame of the ESA ProjectNo AO1-513606NLGLC The authors would like to thankFrancois Deborgies from ESAESTEC for his support andsuggestions

R E F E R E N C E S

[1] Kang S Kim HC Chun K Single pole four throw RF MEMSswitch with double stop comb drive In IEEE 21st Int Conf on

Micro Electro Mechanical Systems 2008 MEMS 2008 13ndash17January 2008 1036ndash1039

[2] Lucibello A De Angelis G Maiani M Proietti E Marcelli RSU-8 based processes for the realization of RF MEMS structuresin 10th Int Symp on RF MEMS and RF MicrosystemsMEMSWAVE 2009 Povo Trento Italy 6ndash8 July 2009 93ndash96

[3] Tan GL et al Low loss 2- and 4-bit TTD MEMS phase shiftersbased on SP4 T switches IEEE Trans Microw Theory Tech 51(2001) 297ndash304

[4] Muldavin J Bozler C Keast C Wafer-scale packaged RF-MEMSswitches In IEEE MTT-S Int Microwave Symp Digest 2006 11ndash16June 2006 267ndash270

[5] Lee J Je cH Kang S Choi C-A A low-loss single-pole six-throw switch based on compact RF MEMS switches IEEE TransMicrow Theory Tech 53 (11) (2005) 3335ndash3344

Fig 15 Cross-section of 4 times 4 sub-block

Fig 16 (a) Simulated return loss (b) simulated insertion loss and (c) simulated isolation of the entire matrix based on 3D Clos network with SP4 Ts

430 giorgio de angelis et al

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[6] Liu AQ Palei W Tang M Alphones A Single-pole-four-throwswitch using high-aspect-ratio lateral switches Electron Lett 40(18) (2004) 1125ndash1126

[7] Di Nardo S et al Broadband RF-MEMS based SPDT in 36thEuMC European Microwave Conf Manchester UK 10ndash15September 2006

[8] Yao JJ Chang MF A surface micromachined miniature switchfor telecommunications applications with single frequencies formDC up to 4 GHz In Dig of Technical Papers Transducersrsquo95Stockholm Sweden 25ndash29 June 1995 vol 2 384ndash387

[9] Cho I Song T Baek S A low-voltage and low-power RF MEMSswitch actuated by combination of electromagnetic and electrostaticforces In Proc 34th EuMC 2004 Amsterdam 11ndash15 October 2004vol 3 1445ndash1448

[10] Hyman D et al Surface micromachined RF MEMS switches onGaAs substrates Int J RF Microw Comput-Aided Eng 9 (4)(1999) 348ndash361

[11] Rebeiz GM RF MEMS Theory Design and Technology JohnWiley and Sons Hoboken 2003

[12] Stark B MEMS Reliability Assurance Guidelines for SpaceApplications Editor Jet Propulsion Laboratory Pasadena CAJanuary 1999

[13] Marcelli R et al Dielectric charging in microwavemicro-electro-mechanical ohmic series and capacitive shuntswitches J Appl Phys 105 (11) 114514-1ndash114514-10

[14] Blondy P et al Dielectric less capacitive MEMS switches in IEEEMTT-S Int Microwave Symp Digest 6ndash11 June 2004 vol 2 573ndash576

[15] Casini F High performance RF-MEMS SP4 T switches in CPW andmicrostrip technology in 10th Int Symp on RF MEMS and RFMicrosystems MEMSWAVE 2009 Povo Trento Italy 6ndash8 July2009 49ndash52

[16] Clos C A study of non-blocking switching networks Bell SystTech J 32 (5) (1953) 406ndash424

[17] Benes V E Mathematical Theory of Connecting Networks andTelephone Traffic Academic Press 1965 New York and London

[18] CST GmbH Germany CST STUDIO SUITE

[19] AWR Corporation USA Microwave office

[20] Kim T Faz MU Vietzorreck L Investigation and optimization oftransitions in an LTCC based RF MEMS switching matrix for spaceapplications Microwave Conf 2009 APMC 2009 Asia Pacific 7ndash10December 2009 988ndash991

Giorgio De Angelis was born in RomeItaly in 1980 He received the Laureadegree in Electronic Engineering fromthe University of Rome ldquoTor VergatardquoRome in 2006 where he is currentlyworking toward the PhD degree in in-tegrated circuits and telecommunica-tions He has been the recipient of aresearch fellowship on design test and

reliability of frequency tunable RF microdevices at the Insti-tute for Microelectronics and Microsystems National Re-search Council (IMM-CNR) Rome since 2006 His researchinterests include the design and test of microwave circuitsand microelectromechanical systems devices for Space andGround Applications

Andrea Lucibello was born in RomeItaly in 1980 He received the Laureadegree in Electronic Engineering at theUniversity of Roma ldquoTor Vergatardquo in2006 He is currently PhD student atthe same University and works since2006 in collaboration with the CNR In-stitute for Microelectronics and Micro-systems involved in Technology and

Test of RF MEMS for Space and Ground Applications

Emanuela Proietti received the degreein Electronic Engineering in 1996Hired by the Texas Instruments ItaliaAvezzano plant as Process EquipmentEngineer in the CVD (ChemicalVapour Deposition) group as responsibleof the oxide deposition Participated inthe technology transfer of Avezzanowafer fab from Texas Instruments pro-

duction baseline of 021 mm to Micron Technology productionbaseline of 018 mm CMOS dynamic memory devices In 1998she became Lead Engineer of CVD group responsible for wholeCVD group processes and maintenances In 2000 hired by theALCATEL Italia Rieti plant as Maintenance Director forSMT assembly production equipments In 2001 she joinedIMM-Rome as Research Engineer From 2005 she is theClean Room Area Responsible She works in the high-frequency microsystems activity and she is involved in theexperimental research of devices and microsystems

Romolo Marcelli was born in RomaItaly on February 26 1958 He obtainedthe degree in Physics at the University ofRoma ldquoLa Sapienzardquo in 1983 From 1987he is with the National Research Council(CNR) of Italy and from 1998 with theInstitute for Microelectronics and Mi-crosystems (IMM) of CNR Presentlyhe is a Senior Researcher and respon-

sible of the Research Line on High Frequency MicrosystemsTechnologies and Reliability for Ground and Space Appli-cations at CNR-IMM Roma where he is also responsible del-egate of the IMM Director His past and current interestsinclude technologies design and test activities in microwavemagnetics RF MEMS and metamaterials

Giancarlo Bartolucci graduated fromthe University of Roma La Sapienza in1982 with a thesis on integrated opticsIn 1982 he was with the ldquoFondazioneUgo Bordonirdquo Roma In 1983 he servedas officer in the Technical Corps of theArmy From 1984 to 1992 he was a Re-searcher at the Department of ElectronicEngineering of the University of Roma

ldquoTor Vergatardquo Since 1992 he is an Associate Professor of micro-wave integrated circuits in the same Department He wasworking on the development of equivalent circuits for microstripand coplanar discontinuities His current research interests arein the fields of the modeling of microwave and millimeterpassive components in particular for MEMS application and

rf mems ohmic switches for matrix configurations 431

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in the study of new topology of phase shifters and filteringstructures

Federico Casini was born in PerugiaItaly on December 7 1981 He receivedthe Laurea degree in Electronic Engin-eering from the Universita degli Studidi Perugia Perugia Italy in 2006 In2006 he joined the Department of Elec-tronic and Information Engineering(DIEI) of the University of Perugia(Italy) as a PhD student under the

advice of Prof Roberto Sorrentino His research interest in-clude design of RF beamforming networks design of MEMSdevices design of dual-band antennas and antennas forWLAN applications In February 2010 he received the PhDdegree from the University of Perugia Italy

Paola Farinelli was born in Assisi Italyon September 19 1978 She received theLaurea degree (with distinction) in elec-tronic engineering from the Universityof Perugia Italy in 2002 In 2002 shejoined the Department of Electronicand Information Engineering of theUniversity of Perugia as a PhD studentIn 2003 she conducted research work

at the ldquoInstitut de Recherche en Communications Optiquesat Microondes (IRCOM)rdquo Limoges University (France)From November 2004 to May 2005 she has been a visiting re-searcher at VTT Millilab (Finland) In January 2006 she re-ceived the PhD degree and she is currently working at theUniversity of Perugia as a contract researcher Her researchactivity includes the electromagnetic modelling and designof reconfigurable RF MEMS devices such as SPnT switchesmatrices phase shifters power dividers filters

Giovanni Mannocchi received the Degree in ElectronicEngineering at the University of Perugia Italy He is currentlyHead of RF Products for Active Arrays and Project Managerat THALES ALENIA SPACE involved in several activities forthe qualification of Space Components and Sub-Systems in-cluding RF MEMS applications He is also Scientific Assistantat Technische Universitat Munchen

Sergio Di Nardo After his PhD in Physics at the Universityof LrsquoAquila he was research fellow for some years in the fieldof material physics In 1998 he was recruited as TechnologyEngineer at Thales Alenia Space Italia (formerly AleniaSpazio)

Presently he is responsible of the Microelectronic Technol-ogies ndash Techno Projects Unit at the Competence Center Elec-tronics of Thales Alenia Space Italia

He is responsible of the substrates technologies processesfor microelectronic equipment (LTCC Thick Film ThinFilm technologies) He is responsible of RampD activities onnew materials and processes for substrates to be used inmicroelectronic equipment (LTCC MCM-D poly basedMEMS advanced packaging )

He participated to the definition and realization of severalRampD projects in the field of microelectronic hardware forspace applications (ESA ASI EU-FP7 )

He was Technical Manager of several RampD projectsHis team supervises the technological aspects of the Thales

Alenia Space Italia activities on RF-MEMS switches

Daniele Pochesci received the Degree inElectronic Engineering from the Univer-sity of Roma ldquoTor Vergatardquo Italy in2005 He was a Research Assistant atthe Institute for Microelectronics andMicrosystems (IMM) of the ItalianNational Research Council (CNR) inRoma from January 2006 to June 2007

He joined Thales Alenia Space inJune 2007 as microwave circuits designer and now he is in-volved in ESA research programs Currently he is pursuinga PhD in Electronic Engineering at the University of RomaldquoTor Vergatardquo

Benno Margesin received his doctordegree in Physics from the Universityof Bologna Italy in 1980 From 1982to 1985 he was involved in the develop-ment of heavy ion sources and heavy ionimplanters at the ITC-irst MaterialScience Division In 1986 he joined theIntegrated Circuit Laboratory at formerITC-irst where he worked on the devel-

opment of low-temperature microcalorimeters and mixedISFETCMOS chemical sensors Since 1997 he heads the Mi-cromechanics Group of FBK-irst and since the beginning of2008 he is responsible for the MEMS Research Unit of FBKCurrently he is involved in the development of RF-MEMswitches for space applications microwave filters capacitivemicrophones and bolometer arrays

Flavio Giacomozzi graduated in Mech-anical Engineering at the University ofPadova in 1982 Since 1983 he is work-ing at Fondazione Bruno Kessler FBK(Former ITC- IRST) Trento ItalyFrom 1983 to 1988 he worked on theimprovements of surfaces properties ofmaterials Since 1988 he has been withthe Materials and Microsystems Div-

ision where he was in charge of the development of fabrica-tion processes technological steps Since 1996 he is workingon MEMS technologies and the realization of prototypes ofseveral devices as sensors and microphones Currently he isinvolved mainly in the development and the fabrication ofRF MEMS devices

Olivier Vendier earned his engineeringdegree in applied physics and a postgraduate degree in microelectronicsfrom the Institut National des SciencesApplique es in Toulouse in 1993 From1993 to 1997 he worked as RampD engin-eer at the Georgia Institute of Technol-ogy college of electrical engineering inAtlanta Ga USA where he got a PhD

degree in electrical engineering Starting in summer 1997 hewas part of the technical staff from the Technology

432 giorgio de angelis et al

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Department at Thales Alenia Space where he was involved invarious advanced packaging based projects He is now in themicrowave product line department (advanced microwavestudy group) at Thales Alenia Space where he is heading theMNT activities for new generation of microwave productsfor space satellites He is now responsible for the technologysupport to the Solid State Power Amplifier product line Heis currently project leader of EC funded project and for severalESA and CNES funded project in advanced packaging He isalso expert for the European commission in the NOETARGET from the EC focusing on power amplifier technol-ogies Dr Vendier has published over 50 technical papers invarious journals and conferences and holds six patents on ad-vanced packaging for microwave applications

Taeyoung Kim received the BS degreein Mechanical Engineering and Elec-tronic amp Electrical Engineering fromPohang University of Science and Tech-nology (POSTECH) Pohang Republicof Korea in 2003 and the MS degreein high-frequency engineering from theTechnische Universitat Munchen(TUM) Munich Germany in 2008

Currently he works as Research Assistant at the Lehrstuhls

fur Hochfrequenztechnik TUM His current research inter-ests include design of RF MEMS switching matrices forspace applications and microwave applications onmetamaterials

Larissa Vietzorreck was born in Dus-seldorf Germany She received hermasterrsquos degree in Electrical Engineer-ing (Dipl-Ing) from theRuhr-Universitat Bochum Germanyin 1992 From 1992 to 1997 she wasa Research Assistant in the Departmentof Electrical Engineering of the FernUniversitat in Hagen where she was

working toward her doctoral degree In 1998 she joinedthe Lehrstuhl fur Hochfrequenztechnik at the TechnischeUniversitat Munchen Germany as an Assistant ProfessorIn 1999 she was Conference Secretary of the EuropeanMicrowave Conference in Munich She acts as reviewerfor several journals and conferences and has publishedmore than 80 contributions in journals and conference pro-ceedings Her current research interests are design andsimulation of microstructured components and developmentof numerical software tools

rf mems ohmic switches for matrix configurations 433

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