rf/microwave mems devices and fabrication...

International Journal of Mechanical Engineering • July-December 2011 • Volume 4 • Issue 2

RF/Microwave MEMS Devices and Fabrication Technologies for Enhanced-Functionality Wireless Communications 101

RF/Microwave MEMS Devices and Fabrication Technologies forEnhanced-Functionality Wireless Communications

Vinod Kumar KhannaMEMS & Microsensors, Solid-State Devices Division, CSIR-Central Electronics

Engineering Research Institute, Pilani-333031 (Rajasthan), IndiaE-mail: [email protected]

ABSTRACT: Applications of microelectromechanical systems (MEMS) in RF (radio-frequency) and microwave electronicsare poised to revolutionize wireless communications. RF/microwave MEMS encompass switches, tunable capacitors,inductors, resonators, filters, phase shifters, couplers, antennas, oscillators, transceivers, etc. Unlike microelectronictechnology-based planar and stationary components, MEMS switches have mobile parts; varactor capacitances are controlledby altering the gaps between electrodes and on-chip spiral inductors stand off the substrates, reducing parasitic effects.RF MEMS, also called ‘Microwave MEMS’ or ‘Wireless MEMS’ has bestowed all types of communication systems: radar,handsets, base stations, satellites, radio, instrumentation and test equipment, the key properties of low weight, volume,power consumption, form factor and cost, together with increased frequency of operation, component integration andreconfigurability, enabling overall superior functionality and performance. This paper surveys the state of the art in thisfield, discusses the present scenario and highlights the challenges ahead. Attention is also drawn towards the shortfalls ofRF MEMS components. Packaging and reliability issues are addressed.Keywords: RF/Microwave devices, MEMS devices, wireless communications, tunable filters, phase shifters, antennas.

1. INTRODUCTIONThe ever-expanding demand for high-efficiencymicrowave communication systems has boosted theresearch and development endeavours for multi-functional, adaptive, low-power systems, bringing in itswake the challenges of reconfigurability, spectrumefficiency, device size miniaturization and cost minimi-zation. MEMS technology for fabricating miniaturedevices by combining mechanical parts and electroniccircuits, typically on a semiconductor chip, with dimen-sions varying from around 1 µm to several millimeters,represents the foremost enabling route for realization ofsuch microwave systems [1]. RF MEMS incorporatemechanical devices such as diaphragms, cantileverbeams, gears, springs, etc., with integrated circuits, fordifferent types of functions, as comprehensively re-viewed in [2-4]. The objective of this review article is toprovide perspectives of microwave MEMS devices,particularly in switching and tuning functions and inthe light of recent advancements, to researchers in thefield. Further, it aims to acquaint end-users with anoverall device picture and current status, enabling themto envision their performance capabilities and under-stand their limitations. Present problem areas in this fieldare also identified.

In this paper, Sec. 2 briefly introduces the vitalperformance parameters of RF/microwave devices andessential manufacturing processes used in MEMSrealization. A top-down treatment of the subject is

followed, in which Sec. 3 describes the applications ofRF/microwave MEMS, which have boosted researchefforts; these are MEMS technology-based tunable filters,phase shifters, couplers, antennas, voltage-controlledoscillators, etc. Subsequently from Sec. 4, elaboration ofthe building units of RF/microwave MEMS commenceswith transmission lines and membrane switches. Necessityof MEMS switches is emphasized. Different types ofMEMS switches are defined. The equations and methodsfor MEMS switch design are outlined, followed by theirfabrication steps and packaging techniques. Problemsfaced with MEMS switches are indicated. Reliabilityissues are dealt with in Sec. 5. From here onwards, focusshifts from switches towards other RF MEMS components,tracking the metamorphosis to tunable capacitors(Sec. 6). Inductors are cursorily dealt with in Sec. 7. Theensuing section 8 deals with MEMS resonators. Sec. 9touches upon integration of MEMS devices withmicroelectronics. The paper concludes with summarizingremarks, bottlenecks and hurdles faced, and futuretrends in Sec 10.

2. ORGANIZATION AND TERMINOLOGY OFRF/MICROWAVE MEMS, AND KEY

MEMS PROCESSESFig. 1 shows how the RF/microwave MEMS has evolvedfrom basic building blocks to application-oriented units.The common terminology of RF MEMS devices [2] isenlisted in Table 1.

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102 Vinod Kumar Khanna

Table 1RF MEMS Device Parameters

Sl. No. Parameter Related Device Definition

1. Insertion loss Switch RF power lost in the device.2. Return loss Switch RF power reflected back by the device.3. Isolation Switch RF isolation between the input and output.4. Linearity Switch Independence of device impedance from the input RF signal power.5. Series resistance RF component Equivalent resistance, R, present in a reactive component that is

being modeled as a simple series configuration between R andpure imaginary impedance, Z im, where Zim = 1/(jωC) for acapacitor, and Zim = jωL for an inductor.

6. Quality factor Electrical or mechanical Ratio of the energy stored in a device to the energy dissipated percomponent cycle of resonance.

7. Mechanical resonant Any device Particular frequency at which the stored kinetic and potentialfrequency energy resonates.

Commonly used processes in MEMS fabricationinclude surface and bulk micromachining (see Fig. 2),micromoulding, wafer-to-wafer/wafer-to-glass bondingand LIGA [5]. Structurally, the RF MEMS devices aredivided into surface- and bulk-micromachined categories. Onthe basis of functionalities, two types of devices are disting-

uished, namely, active and passive components. In themicrowave area, the term “active device” is usuallyreserved to devices giving net RF/microwave power tothe circuit, such as amplifiers. In this sense, a MEMSdevice is not truly active.

Figure 1: RF/Microwave MEMS: Structural Components and Applications.

Figure 2: Silicon Micromachining Processes.



2.1. Surface MicromachiningIt is the removal of thin skinny layers of material onsilicon by etching. In surface-micromachined MEMSdevices, 3-D structures are constructed by the controlledaddition and removal of a sequence of thin-film layersto/from the wafer surface called structural and sacrificiallayers, respectively. The substrate is used as a mechanicalsupport upon which the micromechanical elements arefabricated. PolySi, SiO2, Si3N4, metallization layers orphotoresist coatings at/near the surfaces of devices, aredeposited and etched away to release overlyingmechanical structures. Sometimes the resulting free-standing structures are anchored at one or morelocations. In other cases, they are free to rotate about pinjoints. Fig. 2(a) illustrates the use of surface micro-machining technique for the fabrication of a polysiliconcantilever using silicon dioxide as the sacrificial material.It may be noted that RF-MEMS discussed in this paperemploy metal surface micromachining, not polysilicon.

2.2. Bulk MicromachiningIt is the removal of large, deep sections of material. Bulkmicromachining is a method for sculpting 3-D structuresin a wafer by exploiting the anisotropic etching rates ofthe different atomic crystallographic planes in the wafer.It is performed isotropically or anisotropically (preferen-tially in certain directions). In bulk-micromachineddevices, the mechanical structures are formed by etchingthe supporting substrates. Fabrication of micromechanicaldevices by bulk micromachining through etching deeplyinto the silicon wafer is shown in Fig. 2(b).

2.3. MicromouldingHere, the structures are fabricated using moulds to definethe deposition of the structural layer only in those areasthat constitute the micro-device structure, in contrast tothe previous approaches. Afterwards, the mould isdissolved using an etchant that obviously does not attackthe structural material.

2.4. Bonding TechniquesAnodic bonding is a method of hermetically andpermanently joining a glass substrate with a silicon wafer

without any adhesives (Fig. 3). Depending on the glasstype, silicon and glass wafers are heated to temperaturesin the range of 300-500ºC. The components are broughtinto contact and a high voltage ~ 1000 V, is applied acrossthe combination, whereby glass becomes bonded tosilicon with a permanent chemical bond. Eutectic Bondingexploits the property of the eutectic temperature of Si-Au, which is around 368 °C. The melting temperature ofthe two surfaces in contact is lowered appreciably whenthey are in touch. Thermo-compression bonding involvesplacing the two mirror-finished surfaces of the siliconin intimate contact at a high temperature.

Figure 3: Glass-Si Anodic Bonding.

2.5. LIGA ProcessThe term ‘LIGA’ is an acronym representing the mainprocess steps involved, i.e., deep X-ray lithography,electroforming, and plastic moulding (in German, Litho-graphie, Galvanoformung, Abformung). In this process,structures of lateral design with high aspect ratios areproduced, i.e., with heights up to 1000 µm and lateralresolution down to 0.2µm, by using deep X-ray lithography.

3. RF/MICROWAVE MEMS IN WIRELESSCOMMUNICATIONS

3.1. MEMS Tunable FiltersA critical element enabled by MEMS technology istunable filter, of which several examples are given inTable 2 [6-12]. The Table includes several differenttechniques (acoustic, printed, etc.). Poor and good examplesare mixed to provide a perspective of the true scenario.

Contd. Table 2

Table 2Tunable Filters

Sl. No. Name of the filter Operational Frequency Insertion lossand reference mechanism (GHz) (dB)

1. Two-pole monolithic Surface-micromachined capacitors Switching filter centre < 2switched filter [6] provided a variable capacitance to frequency 37% between

a coupled coplanar strip filter. 10.7 GHz and 15.5 GHz

2. Three-pole tunable Distributed loading structures were 6 –10 3.3 – 3.8end-coupled filter [7] switched with MEMS capacitive switches.

3. Wide-band tunable Coplanar-waveguide filter was fabricated 12–18 4.5 and 6.8 dB at 17.8filter [8] on a glass substrate. Used loaded resonators and 12.2 GHz for the



with MEMS capacitive switches, resulting bias line resistancein a tuning range of 40% with ultrafine of 20 kΩ/sq.resolution.

4. Tunable stop-band Coplanar waveguide structure was loaded 39-48 Very small insertionfilter [9] in its central strip with complementary (Q-band) loss in allowed band;

split-ring resonators (CSRRs) entrenching rejection in stop bandMEMS variable capacitors. < – 40 dB

5. Three-pole combline Used MEMS capacitive switches on the 8.2 – 11.3 4.4 to 6 dBfilter [10] end of each resonator to choose fixed

metal-insulator-metal (MIM) capacitors.6. Single-chip Contour-mode aluminum nitride Up to 4 dB at 93 MHz

multiple-frequency piezoelectric micromechanical resonators 236 MHzAlN filter [11] were electrically cascaded in a ladder

structure to form low insertionloss, bandpass filter.

7. High-Q Evanescent-mode cavity was utilized Centre frequency 2.46 – 1.28evanescent-mode as a high-Q resonator and a high-Q = 4.19 – 6.59 GHz,tunable filter [12] cantilever-switch capacitance network Q = 535 – 845

was introduced as a tuning network.

Contd. Table

A glance at the table reveals the different directionspursued by researchers. For MEMS-based filters, twotypes of frequency-tuning methods are distinguished,viz., analog and digital tuning. Analog tuning iscomparatively easy with MEMS variable capacitors.Although it provides continuous frequency variation, itsuffers from the shortcoming that the tuning range islimited between 5–15%. On the other hand, in digital-type tuning, a capacitor is switched in and out of thecircuit, resulting in discrete centre frequencies andprovision of broader tuning ranges (20% – 60%). Severaldesigns focus on 0.1 – 10 GHz frequency range. The vitalbenefit of digital designs is their lower sensitivity to biasand Brownian noise. Also, the center frequency isaccurately known, and susceptible marginally to driftingwith thermal variations.

One filter example (Fig. 4), (Nordquist et al.) [6],consisted of a low-loss RF MEMS switched capacitor,optimized for low loss and high-Q at microwavefrequencies.

Figure 4: RF-MEMS Switched Filter [6].

A stop-band filter (Gil et al.) [9] based on meta-material transmission lines (artificial lines consisting ofa host line loaded with reactive elements) is shown inFig. 5. Resonant type metamaterial transmission linesemploy two principal strategies, namely, split-ring

resonators (SRRs) or complementary split-ring resonators(CSRRs) as loading elements. Stop-band behaviour isobserved for transmission lines loaded solely by theseelectrically small resonators. The explanation for itsorigination in circuit theory is the presence of a trans-mission zero. The reason in terms of continuous media,is the extreme value of the effective permeability (forSRR-loaded lines) or permittivity (for CSRR-loadedlines) in the neighbourhood of the resonance frequency(highly positive and negative).

Figure 5: Stop-Band Metamaterial-Based Filter [9].

Here rectangular-shaped complementary split-ringresonators (CSRRs) were etched in the central signal stripof a 50 Ω coplanar waveguide (CPW) transmission linewith RF-MEMS bridges on top of them [9]. These RF-MEMS bridges provided tunability to the structure,yielding a tunable stop-band filter with 20% tuning rangeoperating at Q-band. The structure consisted of a four-stage periodic device wherein the distance betweenadjoining CSRRs was kept as 220 µm. This example hasfairly high insertion loss (5 dB) out of band, which isdisadvantageous as a tunable RF-MEMS component.

For realizing bandpass filters using electrome-chanical resonators, there are two modalities. The firststyle uses electrically coupled filters. Here an array ofresonators is coupled together exclusively by electricalsignals. The second style uses mechanically coupled filters,wherein an array of resonators is coupled together by



purely mechanical linkages; it deals with mechanicalMEMS resonators, operating on a completely differentmode from the other ones. This is the work of G.L. Piazzaet al. [11] that is completely different from others. Acomparison of the two modalities is interesting. It revealsthat electrical coupling of resonators offers implemen-tational simplicity. Furthermore, external mechanicallinks, which generally pose manufacturing problems,particularly at high operational frequencies, are notnecessary. Also, it may be remarked that the link size isgenerally a fractional part of the resonator size, around0.1 times. At 1 GHz frequency, it is a critical dimension< 0.5 micron. Additionally, rapid realization of compli-cated designs is easily achieved through electricalcascading of resonators with zeros (termed as attenuationpoles) in the filter transfer function. On the contrary, thebeneficial aspects of mechanically coupled resonatorsinclude the capability to adjust the bandwidth of thefilter, quite independently of material properties. Thisis supplemented with the ability to improve out-of-bandrejection as an additional advantage. In the work ofPiazza et al. [11], electrically coupled resonators werechosen for filter construction, primarily because of theirvirtues of ease of design and fabrication.

Although micro resonator devices have a highQ-factor, they have the disadvantage of high motionalresistance. A large motional resistance value translateseither into a bulky matching element necessity orenormous insertion losses. Then it is not possible tointegrate these resonators with existing 50-ohm systems.Piezoelectric materials such as aluminum nitride orquartz offer larger electromechanical coupling coefficientsthat substantially reduce the motional resistance of the

resonators to a few ohms and provide the mandatorybroad bandwidths for filtering applications. Piazza etal. [11] realized monolithically integrated multiple-frequency aluminium nitride (AlN) band-pass filters,which represented a major breakthrough towards thegoal of highly integrated, single-chip, multi-bandsolutions. Based on a new offshoot of MEMS resonatortechnology supported on the excitation of contour modesof vibration in AlN microstructures, bandpass filters at93 and 236 MHz were demonstrated by these workersby electrically cascading up to eight resonators in aladder topology. These filters displayed very promisingperformances, because they produced low insertionlosses (4 dB at 93 MHz), gave large close-in and out-of-band rejection (40 and 27dB, respectively, for a 93-MHzfilter) and fairly sharp roll-off with a 20-dB shape factorof 2.2. Further, the filters were about 20 times smallerthan existing surface acoustic wave (SAW) device-basedfilters. This novel technology can have a revolutionaryimpact on wireless communication systems by allowingsimultaneous fabrication of multiple frequency filters onthe same chip, which will in turn, lower the form factorsand manufacturing costs.

3.2. MEMS Phase ShiftersThey employ switched line and distributed transmissionline approaches. Various topologies are available forshifting phase. Switched-line and loaded-line designsrequire distributed line lengths and tend to be large atX-band. Alternatively, phase shift is accomplished byswitching low-pass and high-pass filter elements, whichtypically requires large monolithic inductors. Table 3presents some examples from the literature [13-16].

Table 3MEMS Phase Shifters

Sl. No. Name of the phase Principle of Bits Frequency Phase Insertionshifter and reference operation (GHz) shift loss (dB)

1. Resonant switched Capacitive switches were 4-bit 34 GHz 0º – 337.5º 2.25 dB fortransmission line employed to perform two with 22.5º 4-bit and 1.7 dBfilter [13] quarter-wave transformations steps for 3-bit phase

enabling switching between shifterdifferent delay paths,thereby shifting the phase.

2. Microstrip distributed Was a distributed MEMS trans- 2 and DC to 262º (maximum) –2.8 dB for 2- bitphase shifter [14] mission line (DMTL) phase 4-bit 18 GHz at 16 GHz for

shifter comprising a high 2-bitimpedance line (> 50 Ω) capaciti-vely loaded by MEMS bridges 333° (maximum) –3.0 dB for 4-bit and microstrip radial stubs. at 16 GHz for 4-bit

3. Time delay Used metal contacting MEMS 6-bit DC to 393.75-ps (Total 1.8 ± 0.6 dBcircuit [15] switches to obtain series-shunt 10-GHz time delay) of loss at 10 GHz

SP4T switching networks.4. DTML phase shifter Was a DMTL phase shifter - 24 GHz 5.4°/mm 0.1 dB/mm

with non-galvanic fabricated in Si-bulk micro- differentialelectromagnetic machined technology, enabling phase shiftcoupling [16] to commonly suspend all capacitive at 24 GHz

loads on one movable plate andallowing full-range analog andhomogeneous gap variation.



A distributed MEMS transmission line (DMTL)phase shifter (Fig. 6), Voigt et al. [16], consists of a highimpedance transmission line, periodically loaded withlumped adjustable capacitors, which influence the lineimpedance and phase velocity. DMTL phase shiftersshow superior performance characteristics than theirsemiconductor competitors in terms of loss and powerconsumption. However, capability of analog phase shiftby semiconductor devices is difficult to realize withindividual MEMS bridges. Digital phase shifterscontaining sections of capacitive loads serve to achievedifferent states of phase shift.

Figure 6: (a) Top View and (b) Cross-Section of PhaseShifter [16].

A conventional figure of merit for phase shifters isthe degrees of phase shift per dB of loss, along with theoperating frequency. For instance, example 2 in the Tableshifts signals by 262/2.8 = 93.57° for one dB of loss at 16GHz, which is comparable to good MMIC phase shifters,and not very impressive. For example 4, from the datagiven in the table, the phase shift is = 5.4/0.1 = 54° perdB, at 24 GHz, which is not very good for a phase shifter,MEMS or non-MEMS. Further, RF-MEMS phase shiftersare not competitive for low-power applications, and low-power RF-MEMS phase shifters cannot be consideredas an active area of research.

3.3. Micromachined CouplersThese devices couple a secondary system only to a wavetravelling in a particular direction in a primary trans-mission system.

Sung-Chan et al. [17] fabricated a hybrid ring coupleron a GaAs substrate using surface micromachiningtechniques. It had dielectric-supported air-gappedmicrostrip line (DAML) structure. Coupling loss was 3.57± 0.22 dB and the transmission loss was 3.80 ± 0.08 dBacross the frequency range of 85 to 105 GHz. Theisolation characteristics and output phase differenceswere -34 dB and 180 ± 1°, at 94 GHz, respectively. Yusukeet al. [18] demonstrated a micromachined coplanarwaveguide (CPW) 3-dB hybrid coupler. They employeda micromachined enhanced coupling structure at themiddle of coupled transmission lines to obtain high direc-tivity. The coupling structure was composed of alternatelyoverlapping plated conductors with micro-machined air-gap structures. They fabricated the CPW hybrid couplerwith the enhanced coupling structure on a siliconsubstrate. The return loss was 28 dB, the insertion losswas 0.7 dB, and the isolation was 25 dB at 13 GHz.

3.4. Reconfigurable MEMS AntennasLately, tunable antennas have aroused considerableinterest because of the large number of internationalwireless standards in vicinity to one another. A singletunable antenna caters to various frequency bands,getting rid of multiple antennas. A MEMS-switchedreconfigurable multi-band antenna is dynamicallyreconfigured within a few microseconds to serve differentapplications at drastically different frequency bands,such as communications at L-band (1-2 GHz) and X-band(8–12.5 GHz). Case studies are cited in Table 4 [19-22].

Table 4MEMS Antennas

Sl. No. Name of the antenna Underlying Frequency Return lossand reference principle

1. Coplanar patch Operated by electrostatically tuning 5.185 to 5.545 GHz Better than –40 dBantenna [19] the resonant frequency of the antenna

by applying a DC bias voltage betweenthe MEMS varactor and the actuation padon the antenna.

2. Sierpinski Employed three sets of RF MEMS switches Between 2.4 and Maximum lossantenna [20] with different actuation voltages to sequentially 18 GHz < –35 dB

activate/deactivate parts of a multibandSierpinski fractal antenna.

3. Microstrip Worked by electronically changing the 1.16 to Better than –60 dBrectangular loop loop physical perimeter using RF MEMS 2.08 GHz (simulated)antenna [21] switches.

4. Planar inverted-F Had a tuning line with electrical path to 842 to 762 MHz –22 dB at theantenna (PIFA) [22] the ground plate through a MEMS switch resonant

and loading capacitor. By altering the switch frequencystatus, the resonant frequency of the PIFA wastuned with the loading capacitance value.



Figure 7: Coplanar Patch Antenna [19].

Fig. 7 shows the layout of the coplanar patch antennaof Maddela et al. [19]. This antenna had a coplanarwaveguide (CPW) feed at the bottom radiating edge anda MEMS varactor at the top radiating edge. The threeconstructional layers constituting the tunable coplanarpatch antenna were substrate, spacer and a polyimidefilm. The spacer layer provided the required spacingbetween the substrate and the polyimide layer. A flexibleKapton E polyimide film (capable of withstandingmillions of mechanical flexing cycles) with 100-150 A°NiCr seed layer and 3 µm thick Cu cladding was usedby these investigators.

3.5. Mixer-Filters, Amplifiers and Voltage-Controlled Oscillators (VCOs)

In the MEMS mixer-filter of Chen et al. [23], the nonlin-earity of the electrostatic force with drive voltage on theMEMS resonators was utilized, downconverting GHzRF input signals to excite MHz mechanical resonancefor intermediate frequency (IF) filtering, which wascapacitively transduced into an electrical IF output. Thusmixing and filtering functions were performed con-currently as the RF signals were traversing the resonators.

Figure 8: Schematic Diagram of MEMS VCO [25].

Parametric devices work on transference of powerfrom the pump frequency to the signal frequency, incontrast to standard amplifiers, which convey power

from the DC source to the signal frequency. The maincomponent of the MEMS parametric up-converter [24]was a time-varying capacitor, which consisted of a thindiaphragm-type top electrode suspended above abottom electrode. The magnitude of the pump signalvoltage was decreased by making the structure resonantat the pump frequency; this helped in minimizing thegap between the diaphragm and the bottom electrode,and evacuating the backside air between electrodes foravoiding damping.

Variable MEMS capacitors with high-Q MEMSinductors were used in the VCO circuit architecture(Fig. 8) of Ramachandran et al. [25] for switching betweenfrequencies > 400 MHz apart. The CMOS and MEMSinductor, both fabricated with a 5.8 GHz VCO [26],showed that in comparison to the CMOS inductor, theCMOS MEMS inductor succeeded in achieving a 5 dBlower phase noise improvement at 1 MHz offset in this5.8 GHz VCO.

4. UBIQUITOUS STRUCTURAL COMPONENTSOF RF MEMS

From here onwards, the building blocks of RF MEMSare pursued, beginning with two pervasive components,viz., transmission lines and switches.

4.1. Micromachined Transmission LinesGenesis of most of the transmission line limitations, suchas frequency dispersion and, to a certain degree, insertionloss, lies in the properties of the substrate or media wherethey are defined. MEMS technology has been success-fully exploited to lessen the influence of the substrate.

Figure 9: (a) A Membrane-Supported Microstrip Line, and(b) Microshield Transmission Line.

In the membrane-supported microstrip (Fig. 9a) [5], thetransmission line is defined on a thin membrane, withdielectric constant close to unity, by bulk-etching thesubstrate underneath the trace through processing frombackside. However, a drawback of the membrane-supported microstrip line is the absence of an intrinsicground plane. This requires that the structure be placedon top of another metallized substrate by soldering orfusion bonding. An alternative is the microshieldtransmission line (Fig.9b) [27] in which a central conductor



along with the ground planes is placed on the membrane.The purpose of the metallized cavity is to avoid cross-talk between adjoining lines and radiation into parasiticsubstrate modes. Other noteworthy transmission linestructures include top-etched coplanar waveguide, LIGAmicromachined coplanar transmission line, etc.

4.2. The Need of RF MEMS Switch TechnologyIn RF MEMS, the workhorse component is the MEMSswitch. To appreciate the need of MEMS technologies inRF/microwave applications, Table 5 makes a comparativepros and cons analysis of MEMS switches (resistive andcapacitive) with conventional switches [2, 28-30].

Table 5Pros and Cons of MEMS Switches Over Existing Switches

(a) Relative Pros of MEMS Switches

Parameter (Unit) MEMS switch GaAs MESFET P-I-N diode EM relay Remarks

Size (mm2) < 0.1 1-5 0.1 Medium-Large Consumes smaller space.

Series resistance (Ω) 0.5 3-5 1 0.1 – 0.5 ohm Lower than FET and P-I-Ndiode but comparable to EMrelay.

Insertion loss at 0.1 – 0.2 0.5 – 1 0.5 – 1 0.1 – 0.4 Lower than other switches.1 GHz (dB)

Isolation at 1 GHz (dB) > 40 20-40 40 15-25 Higher than other switches.

Linearity (dBm) 35 35 30-45 50 Comparable to otherswitches (~30 dBm like P-I-N diode or FET switches)but lower than relays.

Third order harmonics Very good Poor Poor Good-Very good Superior/comparable toother switches.

Integration compatibility Excellent Excellent Excellent Average-Difficult Excellent.

Power consumption 1 µW 1 – 5 mW 1 – 5 mW Medium-High Lowest amongst allswitches (near zero).

Control current < 10 µA < 10 µA 10 mA 3-5 mA Lowest amongst all switches.Upper frequency limit (Hz) 70 × 109 4 × 109 20 × 109 5-40 × 109 Highest of all the switches.

(b) Relative Cons of MEMS Switches

Parameter MEMS switch GaAs MESFET P-I-N diode EM relay Remarks

Actuation 20-80 1-10 1-10 5-12 Very high as compared tovoltage (Volts) other switches.

Switching power (W) 10 0.5 5 10-35 Lower than other switches(Continuous) (Continuous) (Continuous) (Continuous) except FET.

Breakdown voltage Low Low High High Lower than other switches.

Switching time (µs) 0.3< t < 1 µs 2-10 ns < 1 µs 250 µs- 1ms Larger than FET and diodebut smaller than EM relayswitches.

Life cycle (million) Up to 105 106 106 1 Smaller than FET and diodebut larger than EM relay.

Cost ratio 5.6 x x 1.8 x 2.6 Costliest of the swiztches,mainly because of packagingand the high-voltage drivechip costs.

The relative superiority of MEMS switches in termsof isolation and upper frequency limit over otherswitches is brought out in Fig.10 (a) and that of insertionloss in Fig.10 (b). The switching speed of one type ofcommercially available MEMS switches is between 300

nsec and 1 µsec. These MEMS switches have ademonstrated life of 100 billion switching cycles.Linearity of mechanical relays is comparable or betterthan MEMS relays. Evidently, cycles make no sense forGaAs or PIN switches.



Figure 10: Comparison of Important Parameters of DifferentSwitches: (a) Isolation, Linearity and Upper

Frequency; and (b) Insertion Loss.

4.3. Types of RF MEMS SwitchesRF-MEMS switches [31-38] are categorized by the circuitconfiguration or the type of switching contacts (series,resistive or ohmic and shunt), the positions of the armature(inline or broadside), lateral or vertical structures, and theactuation mechanisms. Typically the series and shunt(or parallel) is referred to the topology, not the type ofcontact. A series switch could be capacitive. As opposed tothis, shunt switches are not necessarily capacitiveswitches since shunt DC-contact switches with isolatedactuation electrodes are feasible. Distinction must bemade as series versus shunt; metal contact versus capacitive.

4.3.1 Series and Shunt SwitchesAs the name implies, a series switch is connected in serieswith the power line (Fig. 11). It either opens or closesthe line, turning it OFF or ON. The contacting surface isusually located at the end of a singly supported cantileverbeam. For switching ON or OFF, there is a control

electrode under the beam. By applying a voltage to thecontrol electrode, the beam is pulled down or up to comp-lete or discontinue the connection between two conductors.

Figure 11: Series Switch: (a) Connection, (b) 3-D view;(c) OFF Sate, and (d) ON State.

In a shunt switch, the power line is sandwichedbetween two ground lines (Fig.12a). The switch turnson to short the power on the signal line to the ground.This prevents the power from going past the switch(Fig. 12a). In the shunt switch (Figs.12b and c), the beamis clamped at both the ends and the control plate pullsdown the beam when a potential is applied to it. Thisensures that the signal traverses the shorter path to theground and is not transmitted to the ensuing circuit.

Figure 12: Shunt Switch: (a) Connection, (b) Up (ON) Stateand (c) Down (OFF) State.

Series and shunt switch structures are compared inTable 6.

Table 6Series Versus Shunt MEMS Switch

Sl. No. Series MEMS switch Shunt MEMS switch

1. Very low ON-state insertion loss. Higher insertion loss but independent of the contact force,relaxing the requirements of the actuation mechanism.

2. Very high OFF state isolation A trade-off between insertion loss and isolation exists.3. Very susceptible to stiction, corrosion Not prone to such effects.

and microscopic bonding of the metalsurfaces of contact electrodes.

4. Usually requires considerable force to Less force needed. It has a longer lifetime.create a good metal-to-metal contact.

5. Suitable for low and medium frequencies. Unsuitable for low frequencies but suitable for very high frequencies.

(b)

(a)



Table 7Frequency Ranges of Applications of MEMS Switches

DC LF HF UHF Microwave

Series switchShunt switchConsumer electronics (Mobile telephones, GPS, etc.)Automotives (Low power relays, navigation systems, radars)Instrumentation and measurements (Test equipment)Telecommunications Network (Satellites)

Homeland security (Satellites)

Various applications of MEMS switches aredescribed in Table 7.

4.3.2 Inline and Broadside Series/Shunt SwitchesSub-classification of both the above varieties of switchesis based on the actuation plane (Fig.13). In the inlinestructure (Fig.13a), the actuation plane is co-linear withthe transmission line whereas in the broadside structure,the actuation plane is orthogonal to it. The maindissimilarity between the two designs is that the RFsignal passes through the entire inline switch. As a result,inline switches must be fabricated using a thick metallayer (Au, Al, Pt, etc.). On the other hand, only the contactportion of the broadside switch (Fig.13b), needs to befabricated using a metal layer, and the actuation portionis composed of either a dielectric or a dielectric/metalcantilever.

Figure 13: (a) Inline Series Switch and (b) BroadsideSeries Switch.

4.3.3 Lateral and Vertical Switch StructuresThe two categories of switches are compared in Table 8[39-40].

Table 8Lateral and Vertical Switches

Sl. No. Lateral switch Vertical switch

1. The actuator, conductor pads, support These require several lithographic operations.structures and contacts are made in a singlestep of photolithography.

2. Dynamic behaviour is superior to many of the Inferior dynamic behaviour of many switches.vertical contacting switches.

3. Consume larger area. Require smaller area.4. Difficulty in metal deposition over a vertical side wall. Easy metal deposition on a horizontal surface.5. Contact mechanisms are very inferior to that of a vertical Provide superior contact mechanisms.

contacting switch, because of roughness in etchedside-surfaces and contact materials.

4.3.4 Electrostatically-, Electromagnetically-,Piezoelectrically- and Thermally - ActuatedSwitches

Electrostatically-actuated switches work on the Coulombicforce of attraction between two oppositely-chargedplates. They are the simplest of all the switches becausethey do not require any special processing steps, whichare not supported by normal CMOS technology. Inpiezoelectrically-actuated switches [41-44], a piezoelectricactuator attached to the switch membrane provides thenecessary force. Thermal actuation entails the usage of twomaterials with different expansion coefficients. On

heating the materials, the beam bends away from thematerial with the higher thermal expansion coefficient.Another method in this class employs shape memoryalloys. Clearly, these thermal methods have not beenpopular and commonplace due to their high powerconsumption. Electromagnetic methods of actuation workon alignment of a magnetic material in a magnetic field[45]. By changing the alignment direction, the switch isturned ON or OFF. This is a novel method and isadvantageous compared to other methods but requiresspecial processing steps involving magnetic materials.See Table 9 for a comparison of switch actuationmechanisms at a glance.



Table 9Comparison of RF MEMS Switches Actuated by

Different Mechanisms

Sl. No. Actuation Actuation Actuation Power Structuralmechanism voltage speed requirement features

1. Electrostatic High High Low Simple and robust.2. Piezoelectric Lower than High Low Prone to parasitic thermal

electrostatic expansion of layers.3. Electrothermal Low Low High Bulky.4. Electromagnetic Low Low High Provides high contact force.

4.4. Design Parameters and Equations of MEMSSwitches

The electrical model of a MEMS series switch is a seriescapacitance in the up-state position and a small resis-tance in the downstate position [29, 33, 46-49]. Theisolation of a series switch in the up-state position iswritten as [29, 33]

|S21|2 = 4ω2 Cu

2 Z20 for 2ω CuZ0 << 1 (1)

where Cu is the up-state capacitance and Z0 is thetransmission-line impedance. Isolation depends on thespacing between the input and output ports or the OFFstate capacitance of the switch.

Factors contributing to loss in the switch calledinsertion loss are contact resistance and resistance of thewave guide [29, 33]:

|S21| =0

1– sRZ (2)

where Rs is the contact resistance of the switch.

The principal factors influencing the magnitude ofthe actuation voltage are electrode spacing, area ofactuation electrode and dielectric material separating thetwo electrodes. Pull-down voltage is given by [29, 33].

Vp =3

0

827

kdAε

(3)

where the spring constant

k =3

3

0.25EWtl

(4)

d is the air gap, ε0 is permittivity of free space, A isthe area of membrane, and E is Young’s modulus of thematerial. W, t and l are the width, thickness and lengthof the beam respectively.

The power handling capacity of the switch is mainlydetermined by the properties of the transmission line.

The switching time [33]

t =0

3.67 p

s

VV ω

(5)

where Vs is the applied voltage. Resonant frequencyis [29, 33]

ω0 =km (6)

where m is the mass of the cantilever/bridge. For ashunt-capacitive switch, the up-state reflection coefficientis obtained from the equation [29, 33]

|S11|2 = 0.25ω2 Cu

2 Z02 for

12

ω CuZ0 << 1 (7)

The downstate isolation is [29, 33]

|S21|2 ≅

2 2 20

4dC Zω

, f<<f0 ;

=2

20

4 sRZ

, f ≅ f0 ; (8)

≅ 2 2

20

4 LZω , f>>f0

where L is the inductance in the lumped CLR modelof the switch, Cd is the down-state capacitance of theswitch and f0 is the down-state resonant frequency [29,33]

f0 =0.5

dLCπ (9)

CON/COFF ratio is a key parameter for the capacitivecoupling shunt switch as it is a determining factor forboth insertion loss and isolation. A large CON, requiredto maintain high isolation, requires an intimate contactbetween the membrane and the dielectric film over thebottom electrode in blocking or down state of the switch.A small COFF, required for maintaining low insertion loss,calls for a large gap between the membrane and thebottom electrode, which is a tradeoff with achieving alow pull-down voltage.

Given the main parameters of interest to the circuitdesigner, a typical design procedure for a MEMS switch,e.g. a capacitive switch, provides the circuit designerwith a simple set of parameters, along with an accurateRF model of the device. Design procedure starts with



identification of the type of implementation. Ideally, thedevices will be processed on the die containing the activedevices after fabrication of these devices. This may notbe often practicable due to die handling limitations,particularly for prototype designs. For concept demon-stration, the MEMS switches may be located on aseparate die, and later wire bonded to the active devicedie. However, the effect of the bond wire inductancemust be included, and this may constrain circuittopologies. The size ratios between the two capacitancesmust be determined in conjunction with the circuitdesign. Although the typical capacitance ratio for aMEMS switch is in the 30-40: 1 range, this may be toohigh. In cases where a smaller ratio is adequate, theswitch dimensions are adjusted or a fixed capacitance isadded in parallel with the switch. Fixation of capacitorsize and chip area allocation are done. The circuitdesigner must determine how much chip area can bedevoted to the switch.

It must be clearly pointed out here [50] that theRF-power handling capability is a fundamental perfor-mance parameter of MEMS switching devices. Besidesexcessive heat dissipation, the power handling capabilityis constrained by the self-biasing and/or RF-latchingphenomena consequential upon the induction of a non-zero electrostatic pulling force on the suspendedstructure by the available RF power from the source.Therefore, the hitherto implicitly postulated perfectmatching of the device to the network in the ON-state(i.e. absence of reflection) and thus a fixed DC-equivalentRMS voltage on the capacitor, in the study of self-biasingof RF-MEMS switches, becomes untenable. For RF powervalues exceeding a critical value, pull-in or self-biasingtakes place. Practically, however, the assumption of theperfect match is invalidated because of the switch capaci-tance increase with rising RF power, thereby causing avariation in the reflected signal and thus a fall in theDC-equivalent voltage source. In practice, an RF-MEMSshunt switch ideally matches for one and only oneaccurate gap height, in case of MEMS capacitancecompensation by a local appropriate design of the CPWline. No sooner than the electrostatic force begins to comeinto picture, bridge movement occurs and the matchingalters.

4.5. MEMS Switch FabricationFig. 14 shows the process flow diagram for series switchrealization, and Fig. 15 the same for shunt switch [51].In Fig. 14 for a series switch, starting from a Si wafer,first lowermost metal layer (e.g., Au) is deposited andits pattern defined by photolithography and etching (Fig.14a). Then the sacrificial layer [Plasma-EnhancedChemical Vapour Deposition (PECVD) silicon dioxide]is deposited and patterned by etching (Fig. 14b) followedby deposition (Fig. 14c) and selective etching of dielectric

layer (silicon nitride) (Fig. 14d). A contact dimple isformed. Finally, the top metal film (Au) is deposited andpatterned (Fig. 14e). The top metal is released by etchingthe sacrificial layer (Fig. 14f). Sometimes PECVD oxideis used as a structural material with polyimide as asacrificial layer. There are several variations of this basicprocess.

Figure 14: (a)-(f) Process Sequence for Series SwitchFabrication. (g) Actuation.

In Fig. 15 for a shunt switch, the first step (Fig. 15a)involves deposition and definition of bottom film tomake actuation electrodes and RF lines. The next step isdielectric film deposition and definition (Fig. 15b). Thenthe sacrificial film is deposited and delineated (Fig. 15c).



Subsequently, the top metal film is deposited and defined(Fig. 15d). The final step is releasing the top electrodeby etching the sacrificial film (Fig. 15e).

Figure 15: (a)-(e) Fabrication Steps of Shunt Switch.(f) Actuated Device.

4.6. Single-Pole Multiple Throw (SPMT) SwitchesSPMT switches containing thin metallic films are notmechanically reliable due to film deformation causedby heat or film stress effects during fabrication processsteps [52-54]. Even if best possible conditions are main-tained, insertion loss is high. The reasons are substrateloss and open-stub effects from multi-path fading. More-over, high drive voltage is necessary to create the largecontact force crucial for low insertion loss. Single-polefour-throw (SP4T) RF MEMS switch for band selectionin a multi-band, multi-mode front-end module of awireless transceiver system [55], was driven by a doublestop comb drive, with a lateral resistive contact, andcomposed of monocrystalline silicon on glass. A largecontact force at a low-drive voltage was obtained fromelectrostatic actuation of the double-stop comb drive.

5. RELIABILITY OF RF MEMS SWITCHES

5.1. Packaging IssuesA critical step in MEMS switch manufacturing is thepackaging technique [2]. In conventional practice, theMEMS switches are packaged with extreme precautionin a clean-room environment using established hermeticpackaging methods, but this is an exorbitantly expensiveapproach, representing the costliest process in the switchproduction chain and constituting a significant chunkof the final switch price. There is currently intensificationof research to develop wafer-scale packaging processes,which are compatible with aforesaid MEMS switchfabrication sequences. These are based on processes suchas low-temperature hermetic glass bonding, minimal outgassing, Au-to-Au bonding, etc. Contemporarily, thereliability and packaging of MEMS switches are areasdemanding frantic research efforts globally.

One approach for the zero-level package [2] is to bonda recessed capping chip on the MEMS device wafer.Needless to say that this bonding operation is performedat temperatures below 400 °C so that the metallizationand other materials of the RF-MEMS switching devicedo not undergo any degradation, resulting in impairmentof device functionality.

In the wafer level micro-encapsulation (WlµE)technique (Fig. 16) [56-58], instead of releasing themembrane at the time of etching the sacrificial layer of aMEMS switch, an additional cage sacrificial layer wasapplied over the unreleased switch membrane, followedby the dielectric cage deposition. The intent of this cagesacrificial layer was to produce the required gap betweenthe membrane and packaging cage. Holes were etchedinto the cage and the sacrificial layers were etched byplasma process, resulting in a released switch with apackaging superstructure stationed upon it. Afterrelease, a liquid encapsulant, such as spin-on-glass (SOG)or Cyclotene series 4000 benzocyclobutene (BCB), wasapplied over the wafer in a dry N2 ambient, jacketingthe cage structure but unable to drip through the cageholes due to surface tension. Curing of the SOG or BCBwas done at 250°C, forming a closed seal over the switch.The packages were reported to provide <0.1 dB packageinsertion losses up to 110 GHz.

Figure 16: Cross-Sectional View of Packaged MEMSSwitch [56-58].



5.2. Problems with MEMS SwitchesMain mechanisms responsible for malfunctioning ofMEMS switching devices are [59-62]: (i) Materialcracking, creep and fatigue, chiefly in the armature andthe hinged supports; (ii) Deterioration of ohmic contactscaused by cratering or wear, breakdown of the dielectriclayer(s), and other reasons; and (iii) Stiction (or adherence)of the switching contacts or of the actuation electrodes.The underlying causative factors are electrostatic forcesand interfacial forces, like capillary forces and Van derWaals forces, at the dimensional scale of microstructures.Currently, there are several problems that need to beresolved, notably lack of high-power RF MEMS switches,long-term reliability, etc. Packaging issues, high fabricationcost, high actuation voltage levels and phenomena likestiction, need to be seriously addressed in the near future[63-64]. These factors make RF MEMS switches aformidably challenging research field.

5.3. Novel Approaches for RF MEMS SwitchImprovement

Gold has been the material of choice for both the fixedelectrodes and the suspended microbridges in MEMSswitches. But Au microbridges are deficient in the highspring constant (linked to Young‘s modulus) needed tosurmount the stiction and electrostatic charging effects.For MEMS switches, SiC is appealing due to its chemicalinertness, anti-stiction properties and mechanicalstiffness [63, 65]. Crystalline SiC has a large Young'smodulus (>350 GPa) yielding a high spring constant.Also, SiC surfaces are hydrophobic, making it less proneto stiction. But the high thermal processing temperaturesrequired to manufacture single crystalline and poly-crystalline-SiC growth (>900 °C) are unfavourable forSiC microbridge-based RF MEMS switches, primarilybecause in such structures, the microbridge spans an Auelement which cannot forebear such high processingtemperatures. Plasma-enhanced chemical vapordeposition (PECVD) is used to produce SiC at muchlower deposition temperatures than conventional CVD,typically below 400 °C but SiC films deposited byPECVD are typically amorphous in microstructure. Theamorphous microstructure generally corresponds to areduced Young's modulus than found in crystalline SiC

but much higher than Au. Scardelletti et al. [59] and Parroet al. [65] fabricated PECVD-based amorphous-siliconcarbide switches. They observed that the Young'smodulus of the a-SiC films was insensitive to filmthickness. Upon metallization, it decreased slightly. Incontrast, metallizing the 300 nm-thick a-SiC film resultedin a significant increase in residual stress. Their findingssuggest that the residual stress of the metal layer mustbe considered when designing microbridge-basedswitches using submicron-thick structural layers.

Novel approaches have been explored to improvethe reliability of RF MEMS switches. Ke et al. [66] haveproposed a wafer-level packaged switch with acorrugated diaphragm for residual stress reduction. Theynoticed that pull-in voltage of the switch was drasticallyreduced from 105 V for a flat diaphragm to 51 V fortwo-corrugation diaphragm and to 42 V for four-corrugation diaphragm. Kim et al. [67] reported atechnique for decreasing the bending of the membranein a switch caused by internal stress gradient. Theyfabricated a thick and stiff membrane switch in whichthe membrane consisted of a flexible spring allowing anUP-DOWN actuation mode at low voltage and a pivotunder the membrane itself facilitating a seesaw modeON-OFF operation. The minimum actuation voltage was~ 10–12 V.

6. MEMS TUNABLE CAPACITORS

6.1. Figures of Merit of MEMS CapacitorsThese are unbiased base capacitance (values rangingtypically from tens of picoFarads for very-high-frequency(VHF) applications to about 0.1pF for applicationsapproaching the X-band); tuning ratio (varying fromabout tens of per cent to excess of 2:1); equivalent seriesresistance or quality factor, Q; associated inductance; anddevice linearity in response to RF power.

6.2. Comparison with Capacitive SemiconductorCounterparts

In Table 10 and Fig. 17, comparisons of MEMS tunablecapacitors with their traditional semiconductor varactorcounterparts are drawn out [2, 4, 5, 68-75].

Table 10Semiconductor Varactors Versus Tunable MEMS Capacitors

Semiconductor varactors RF-MEMS capacitors

Semiconductor on-chip varactors suffer from excessive RF-MEMS tunable capacitors offer substantial improvementseries resistive losses with associated low unloaded over on-chip varactors and comparable with off-chip varactorsQ-factors ~5-20 [76-77]. Off-chip discrete varactors have ~ 20-60 at 1 GHz and 100 at 0.4 GHz [78-81]. Also, MEMSa higher unloaded Q-factor of at least 40. But the most capacitors offer excellent linearity. These capacitors furtherimportant shortcoming of a semiconductor varactor is promise low noise and the ability to keep the signal circuit separate

the inherent dependence of the capacitance on the RF from the control circuit, simplifying the bias circuitry.signal power, making the component behave highly They provide integration capability with high-Q inductors,nonlinearly. generally not achievable in semiconductor technology.



Figure 17: Q-Factors for Various Capacitor Approaches.

MEMS capacitors have been fabricated in variable-area and variable-gap structural configurations. Fig. 18(a) and (b) show a tunable capacitor in which plates moveto readjust overlapping area for capacitance changes.Fig. 18 (c) illustrates the operation of a variable gap RFMEMS capacitor.

(a)

(b)

(c)

Figure 18: (a) Fixed and Movable Plate Arrangement in anInterdigitated-Plate Variable-Area Tunable Capacitor.(b) Two Pairs of Plates in a Practical Implementation.

(c) Principle of Parallel Plate Gap-Tunable MEMS Capacitor.

7. MEMS HIGH-Q INDUCTORSTable 11 and Fig. 19 portray the Q-factor advantage ofMEMS tunable inductors [2, 4, 5, 82-93] over other typesof inductors. Two types of inductor structures areillustrated in Fig. 20: planar and solenoidal. Solenoid-likeinductors, raised above the substrate, aimed at decoup-ling the inductor properties from those of the substrate,are fabricated by surface micromachining. Here, thedesigner has a greater flexibility to increase the numberof turns for inductance maximization or use a largerconductor track width for series resistance minimization.

Figure 19: Q-Factor Chart of Inductors.

Figure 20: (a) Suspended Planar Spiral InductorOver Cavity, and (b) 3D Inductor.

It is observed that amputation of the silicon substrateor construction of the inductor resembling a coil in airprovides the advantage that the parasitic capacitancesare annihilated, increasing the Q-factor. Induced eddycurrents in the substrate and accompanying energylosses due to Joule heating effect are also brought down.Traditional integrated circuit planar spiral inductorsreside on their host low-resistivity substrate, andconsequently, are afflicted with undesirable effects suchas low self-resonant frequency, low Q and limitedoperating bandwidth.



Table 11Q-Factors of Inductors in Si(Bi) CMOS, GaAs MMIC and MEMS Technologies

Si (Bi)CMOS or bipolar technologies GaAs MMIC technology MEMS technology

In standard 10 ohm-cm silicon wafer The maximum attainable Q-factors In membrane-supported spiral inductorstechnologies, the Q-factor of the on- are on the order of tens. Improved formed by etching away the siliconchip (spiral) inductors is < 10 and techniques, such as Au, Cu or thick substrate, Q values range from 6 to 28 atself-resonance frequencies are 5-20 (3-5 µm) Al, high-resistivity Si 6-18 GHz [103]. For the inductors suspen-GHz [94-95]. The low Q-factor substrates, copper damascene ded or levitated in air, the Q-factor is 17 atevolves from losses in the conductive interconnection technology, thick 3.5 GHz for a 1.5 nH inductor fabricated onsubstrate combined with resistive passivation layers below the inductor a 1-10 ohm-cm Si substrate [101]. Forlosses in the Al (alloy) metallization. or special 3D designs assist in raising integrated spiral inductors of 9 µm thick

the Q-factor up to 20-30 for Si-based Cu suspended over a glass substrate andtechnologies [96-102]. a low-resistivity Si substrate, inductances

vary from 15-40 nH and Q-factorsfrom 40 to 50 at frequencies of 0.9 – 2.5GHz [104-106].

8. MEMS RESONATORSBased upon their operating principle, resonators [4, 107-115] are broadly classified into two groups: (a)Electromagnetic (EM) wave resonators, and (b) Electrome-chanical or (Electro) acoustic wave (AW) resonators. Notableexamples of EM wave resonators are the lumped elementLC-type resonators, transmission line resonators, cavityresonators and dielectric resonators. Among acousticwave (AW) resonators, mention may be made of mechani-cal resonators, bulk acoustic wave (BAW) resonators andsurface acoustic wave (SAW) resonators [4]; the lattertwo classes are distinguished from the first on the basisof their fabrication method. Some important resonatorsare described below:

(i) LC resonators: A MEMS capacitor is imple-mented to tune the resonant frequency of the LC

tank, given by1LC

.

(ii) T(Transmission)-line resonators: Here, the MEMStechnology is utilized to remove the substrateunderneath the microstrip line, thus diminishingthe losses incurred due to the substrate. Fortuning T-line resonators, they are loaded withRF-MEMS tunable capacitors.

Figure 21: A Cavity Resonator.(iii) Cavity resonators: Micromachining techniques

are here used to make minuscule cavities(Fig. 21) and dielectric resonators offering the

advantage of easy combination with monolithicintegrated circuits [107]. A disadvantage of anair-filled cavity resonator is its excessively largesize (of the order of wavelength), even at mm-wave frequencies.

(iv) Dielectric Resonators: By using a filler materialwith a high relative dielectric constant εr, thewavelength and hence the dimensions of thecavity are lowered by a factor rε . This fact isexploited in a dielectric resonator (DR), whichessentially consists of a piece of a high relativepermittivity (εr) insulating material.

(v) Mechanical Resonators: rely on the resonance of astructural member, e.g., a beam or a disk. Veryhigh Q-factors ~ 1000–10,000 with peak valuesas high as 105, have been obtained for vacuum-encapsulated micromachined mechanical reso-nators. The piezoelectric layer is made of thinfilms of aluminum nitride (AlN) or zinc oxide(ZnO). The resonator sizes are as small as a fewtens to a few hundreds of µm on a side, represen-ting typically a MEMS design.

(vi) BAW and SAW Resonators: differ in theirfabrication approach from the above resonators.Fig. 22(a) shows the cross-section of a solidly-mounted thin film BAW resonator consisting of apiezoelectric AlN film sandwiched between Moelectrodes. (100) Si substrates covered with apolysilicon layer (in which an air gap was etchedapproximately 8 µm under the surface) wereused [115]. Aluminium nitride is the favouritepiezoelectric material for high-frequency piezo-acoustic devices because of its high acousticwave velocity and its large electromechanicalcoupling coefficient, besides its high thermalconductivity and the possibility to grow highlyc-axis oriented AlN films by room temperaturetechniques such as sputtering, therefore assuring



full compatibility with standard IC technology[115]. In Fig. 22(b), a reflector-on-membrane-typeBAW resonator is depicted in which the membraneas well as reflector materials are conductive. Theair gap is realized in a polysilicon supportinglayer; the reflector consists of a single pair of Ti/W. Mo was chosen as electrode material since itallows large effective coupling coefficient at highfrequencies. Additionally, it is characterized bysmall acoustic wave attenuation, low electricalresistivity and is selectively etched with respectto AlN. Also, it promotes the growth of highquality AlN layers. Membrane-type resonatorwas realized by directly depositing AlNpiezoelectric layer embedded into Mo electrodeson the poly-Si/Si substrate. Membrane-typeresonators generally present the advantages ofhigher fabrication yield and, in some cases,easier temperature coefficient of frequency(TCF) compensation, as compared to solidly-mounted resonators (SMRs); on the other hand,they suffer from lower mechanical stability andpower handling capabilities in view of the factthat membrane thickness needs to be reducedto 1 µm or less to avoid the presence of manyundesired resonances, at the expense of robustnessand heat conductivity.

Fig. 22 (c) shows a SAW resonator consisting of aninput interdigitated electrode (IGT) pair and an outputIGT pair on a piezoelectric quartz substrate.

Figure 22: (a) Solidly-Mounted BAW Resonator,(b) Membrane-Type BAW Resonator and (c) SAW Resonator.

9. INTEGRATION OF MEMS COMPONENTSIN CIRCUITS AND SYSTEMS

The future of RF-MEMS is deep-rooted, not so much inindividual components as in integrated RF microsystems.Understandably, monolithic integration of two or more

switches to yield multiple switch contacts is a viable nextstep. An example is single-pole double-throw (SPDT)switch. Although the integration of MEMS devices withactive circuitry has been demonstrated, integrated RFMEMS are still in infancy and are gradually developing.

Morris et al. [116] developed a flexible general-purpose RF MEMS manufacturing process based on amulti-function process stack. Their process flow consistsof three parts: (i) The substrate connect layer used toseparate the upper layers from the substrate, providesconnections to underlying circuits and offers aplanarized surface on which subsequent processing isdone. (ii) The thick metal layer, used for passive devicesand interconnections, is made of Cu embedded in silica.(iii) The thin metal layer comprises three layers of Au alloyof 0.5 micron thickness, two layers of sacrificial materialand a silica mechanical layer. The above process has beenimplemented in several foundries and different deviceshave been fabricated. It thus paves the way to theformation of a diversity of tunable and reconfigurableRF passive circuits.

Kuwabara et al. [117] reported a novel structure andfabrication process for integrated RF-MEMS technology.For integration, an adaptable multilayer structure andits fabrication process enabled realization of variousMEMS devices on the same substrate, whereas forprotection, a wafer-level encapsulation process createdsmall thin capsules to safeguard these devices. Eachcapsule had walls, a roof, and a sealing film. While thewalls and roof were formed simultaneously as thedevices, the etching holes in the roof were sealed withthin film by a selective sealing method. EncapsulatedMEMS devices, such as switches and varactors, were co-fabricated on the same substrate, showing promisingresults specially for fabricating RF MEMS transceivers.Present RF transceivers contain many LSI chips andabundant off-chip passive components. The escalatingnumber of available communication bands will multiplythe number of components and make RF transceiversbulky. This will also increase power consumption andcurtail battery life. Therefore, reconfigurable RF MEMStransceiver circuits, with alterable circuit configurationsaccording to the wireless standards used, will enablemultiband operation without enlarging the size andraising power consumption.

Integration of capacitively-transduced MEMSresonators with characteristic frequencies in the HF andVHF bands has evolved as one of the futuristic keyoptions to overcome the limitation presented by discretepassive elements in the downscaling to the chip levelfor RF communications systems. Lopez et al. [118] havedescribed a strategy to design and fabricate a MEMSresonator using a CMOS standard technology choosingthe optimal structural and sacrificial layers for theresonators by means of a defined figure of merit.



10. CONCLUSIONS AND OUTLOOKMEMS technology has offered exciting possibilities forrealizing a new generation of high-performance, small,low-power consuming, high-frequency componentssuch as switches, variable capacitors, tunable oscillatorsand filters. Functional principles and technologicalconsiderations of these components were discussed.With evolving development trends, it is expected thatthese RF-MEMS components will supplant their preva-lent discrete counterparts in a wide range of RF,microwave and millimeter-wave applications [119-122].But this can be accomplished only if major technicalobstacles have been removed, more specifically, throughthe development of an appropriate cost-effectivepackaging technology and by solution of reliabilityissues. Further, this must be achieved in large volumeproduction at a competitively low price. Although RFMEMS devices offer attractive performance features,practical problems on packaging, long-term reliabilityand integration with microelectronics need to beaddressed. Hot topics on this technology include tunersfor cell phones, switches for AlN-based acoustic filters, etc.Research on RF-MEMS is progressing towards perfectionof these technologies. The question “Are RF MEMSdevices commercially viable, or will they become so soon ?”was addressed at a 2011 International Microwave Symposiumpanel discussion titled “Commercial viability of RF MEMS-a reality or a dream?”, and the petite answer was “Yes”,RF MEMS devices are commercially viable [123].

ACKNOWLEDGEMENTThe author wishes to thank the Director, CSIR-CEERI,Pilani for encouragement and guidance.

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