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Rf-Mems Components In MobileCommunication Terminal

Antriksh Singhal, Gaurang GuptaAbstract - Micro-machined passive components open new

perspectives to the integration of multi-standard RF transceiversfor mobile communications. The different types of componentsare reviewed and their potential use in transceiver architecturesis outlined.

I. INTRODUCTION

Traditionally, RF receivers or transceivers have beendesigned for a specific application with well defined carrierfrequency, modulation scheme, bandwidth etc. In mobilecommunications however, a variety of standards co-exist,none of which provides a world-wide coverage. All thesestandards use different frequency bands, access methods,modulation schemes etc. The concept of mobility associatedto the cell phone creates a strong demand for compatibility ofthe phone with as many standards as possible to ensure a largegeographical coverage. Just looking at present day GSMphones, we can observe that single band phones have beenabandoned, new phones offering dual band or triple bandoperation. The future introduction ofUMTS will increase thisdemand on multi-standard phones.

Traditional radio-technology makes use of bulky passivecomponents such as SAW filters, and is not adequate for truemulti-standard phones because of multiplication of thesepassive components and the corresponding increase of powerconsumption, volume and weight and last not least theincreased cost. In the search for alternative solutions, RFtransceiver architectures such as Zero-IF or Low-IFarchitectures have been rediscovered. These architectureshave the potential of eliminating some of the passivecomponents like image reject filters and IF filters. Theconcept of the so-called "software defined radio" emerged,which moves as much of the processing as possible to thedigital domain making the radio entirely re-configurable.Present and near future technology does however not allow toput the A/D and D/A interfaces directly at the antenna. TheRF front-end will therefore remain and it will become themajor bottle-neck of the software defined radio concept. Amajor research objective is now a wide-band RF front-endcovering the frequency range from 800 MHz to 2.5GHz.

The potential of micro-machining technology to implementpassive components suitable for RF has been recognizedseveral years ago. A number of researchers have reportedwork on spiral inductors, variable capacitors, switches andfilters. While not yet in commercial use, some of thesecomponents are approaching an advanced development stage.We will discuss their basic principles, their potentialapplication in a re-configurable RF front-end and theimprovements still to be made before commercial use can beenvisaged.Antriksh Singhal, Gaurang Gupta - Bharati Vidyapeeth's College OfEngineeringNew Delhi, India. a

II. MEMS COMPONENTS

Micro-machining technology has been extensivelyresearched over the past 15 years, and it is now possible tofabricate various micro-electro-mechanical (MEMS) or micro-electro-opto-mechanical (MOEMS) devices with IC-liketechnologies. The application micromachining technology topassive devices useful in RF transceivers such as high Qinductors, variable capacitors, switches and filters has beenexplored for a few years.

Fig. 1: Micro-machined copper inductor fabricated in "above IC"technology (courtesy of MEMSCAP)

A. HIGH Q INDUCTORS

Spiral inductors on silicon chips have suffered from low Qdue to the losses induced by the silicon substrate. Micro-machining technology has been used very early to etch thesilicon away below the inductor to increase the Q factor [1].The etching can be done as a post processing step on standardIC technologies.

Inductors can also be electroplated above silicon ICs. Anexample is shown in Fig. 1. The copper inductor is fabricatedon a polyamide film to provide Q-factors as high as 50 up to 2GHz. Some researchers have tried to realize variable inductorswith mobile core material, but these attempts have not beensuccessful due to the lack of an RF compliant core material.

B. VARIABLE CAPACITORS

The main application of variable capacitors or varicaps in amobile phone is the tuning of the VCO. Tuning range,linearity and Q-factor are main specifications. MEM varicapshave the potential of offering larger tuning ranges than theirsemiconductor active counterparts. The simplest principle isthe tuning of a parallel plate capacitor by changing the size ofthe air-gap between the two plates.

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Fig. 2: Tunable parallel plate MEM capacitor.

The bottom plate of such an assembly is fixed, while thetop plate is suspended by mechanical springs. Applying a DCvoltage across the capacitor creates a force of attractionbetween the plates, thus reducing the size of the gap. RFsignals don't affect the tuning, since the RF excitation isfiltered by the large mechanical time constants of the device.

The practical tuning range is however limited by the pull-ineffect. When x reaches xo /3, the electrical spring constantbecomes larger than the mechanical one, and the top platesnaps down on the bottom plate. The thin oxide on the bottomplate (Fig. 2) prevents a short circuit in that case. Thetheoretical maximum tuning range of such a capacitor isconsequently from 100% to 150% of Co (capacitance withoutDC bias). Variable capacitors of this type have been reportedboth with poly silicon [2] and metal [3] as electrode material.Since metal has lower resistivity, they are the preferredmaterial for RF applications.

The device shown in Fig. 3 has been fabricated in aGold/Nickel micro-machining process in the IST MELODICTproject [4]. A micro-machined bridge is anchored on theground lines of the co-planar waveguide and suspends overthe central signal line. The process, performed on highresistivity 6" silicon substrates, uses thick photo-resists andelectroplating of Nickel. The electroplating processes can bepost-processed on IC-wafers.

Other concepts have been proposed or are currently studiedto increase the tuning range, as show the two following recentexamples. In [5] the gap for the actuation is different from theactual capacitor gap (Fig. 4). If the gap d2 is at least 3 timesthe gap dl, an infinite tuning range could in theory bereached. In [6] a moveable dielectric is inserted between theparallel plates which remain fixed (Fig. 5). Here the nitridedielectric is pulled into the capacitor structure with increasingDC bias voltage. Thanks to the fixed capacitor plates veryhigh Q values and self resonance have been obtained.

Fig. 3: Parallel plate capacitor (ISIT/FhG) fabricated in a Gold,Nickel metal micro-machining process

These examples are however quite challenging with respectto the processing technology.

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C. RF SWITCHES

RF switches are useful for switching the signal betweentransmit and receive paths (i.e. antenna switch in a GSMphone) or to route signals to different blocks (multi-standardphone). MEM RF switches can be of two types: eithercapacitive switches or ohmic switches. The former areactually a variable capacitor switched between to values, Coffand Con, while the latter relies on a contact between twometals. The main difficulties in the realization of RF switchesare the requirements in terms of reliability, insertion loss (on-state) and isolation (off-state). Lifetime requirements are 106switching cycles for a configuration switch and 109 cycles foran antenna switch.

Fig. 6 shows the conceptual principle of a capacitive switch[7] [8]. The electrostatic actuation of the switch is obtained byapplying a DC potential higher than the pulling voltagebetween the two electrodes of the switch. One of the seriouslimitations of such a switch is the fact that the signal adds tothe actuation voltage. As the signal voltages close to theantenna can become quite high (several tens of volts), a high

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actuation voltage have to be chosen. This is however notdesirable, since that voltage has to be obtained via a DC-DCconverter from the battery voltage.

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[11]. This gives much more design flexibility. A recentlypresented contour-mode disk resonator exhibits a resonancefrequency of 156 MHz with a Q of close to 10 000 [12]. Analternative way of reducing the electrode gap is the use ofmobile electrodes [13]. This technique is fully compatiblewith existing commercial micromachining processes.

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Thermally actuated relays don't have the drawback of highactuation voltages. An example of a thermally actuated relayof CRONOS [9] is shown in Fig. 7. Nickel beams are heated,and their thermal expansion closes the contact. This type ofswitch suffers however from a very high power consumption(200-300mW) and slow switching time (several ms). Newdevices are in development providing two stable states. Thisreduces the power consumption drastically, as power is onlyneeded to change the state.

Fig. 7: Principle of thermally actuated micro-relay

D. MICRO-ELECTRO-MECHANICAL FILTERS

Low-frequency mechanical resonators fabricated in poly-silicon have been demonstrated many years ago and are usedas sensing devices, i.e. for inertial sensors. More recently, thepossibility of making resonators with resonance frequenciesabove 100MHz have been explored. These resonators arebased on poly-silicon cantilevers with capacitiveelectromechanical transducers. These resonators have thepotential of replacing SAW IF filters in super-heterodyneradio architectures, thus reducing space and powerconsumption. The resonance frequency can be fine-tuned viathe DC bias applied to the capacitive transducers. Thesedevices have to operate in vacuum to avoid air damping.

A major challenge is the realization of an extremely smallgap between the transducer electrode and the resonator beam.The gap determines the electromechanical couplingcoefficient and therefore the impedance level and insertionloss of the filter structures. Initial realizations used verticalgaps determined by the thickness of a sacrificial oxide layer(Fig. 8) [10]. Recently introduced techniques allow thefabrication of lateral electrodes, either in poly-silicon or metal

Fig. 8: Principle of a clamped-clamped beam resonator with verticaltransducer gap

A major challenge remains the design of filters with suchMEM resonators. Typical demonstrators used the coupledresonator technique. Mechanical springs were used to couplethe resonators. Only all-pole filters can be designed that way.Recent efforts [14] are aimed at filter architectures usingelectrostatic coupling and introducing transmission-zeros inorder to increase the selectivity for a given filter order.

On-going research aims resonators with much higherresonance frequencies in the GHz range. Such resonatorscould be used as band or image-reject filters at RF, or even forchannel selection at RF.

E. BULKACOUSTIC WAVE RESONATORS

An alternative to mechanical resonators are bulk acousticresonators. Bulk acoustic resonators made by a sandwich ofmetal - aluminium nitride -metal on a silicon substrate havebeen demonstrated and allow the construction of filters forfrequencies above 1 GHz [15]. Micro-machining technologyis used to release these structures from the underlying silicon.The FBAR technology from Agilent is now becomingcommercially available allowing. First products are duplexersfor CDMA systems.

III. PACKAGING AND ASSEMBLY

Packaging of the MEM components and assembly with theactive circuits are a critical issue to success. Requirements forpackaging are low cost and high reliability. Wafer-levelencapsulation of the micromechanical devices are now beingdeveloped, protecting the devices completely before dicingthe silicon wafers. Through-wafer holes are also needed toallow electrical access with low parasitic for applications atRF frequencies.

Post processing has of standard IC wafers has been studiedextensively. Tungsten metallization is one approach to allowhigh-temperature process steps. Si-Ge poly-silicon is now

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explored as a new material which allows to reduce theprocessing temperature from 800°C to 4000C, which remainscompatible with standard Al or Cu metallization. Metal-micro-machining technologies do not use high temperaturesteps and are naturally compatible with IC technologies. AsIC technologies tend to be much more expensive than MEMStechnologies. Post-processing may not prove to be really costeffective, and assembly techniques such as Flip chip andMCM are more likely to gain industrial acceptance.

IV. RADIO ARCHITECTURES

Fig. 9 suggests some of the applications of MEMcomponents in a radio [16]. Interest in IF filters has decreasedthanks to the Zero-IF radio architecture. In the short termmain emphasis is put on wide-band VCOs, LNAs and PAs aswell as switching and filtering signals at RF. MEMcomponents are particularly attractive for the design oftunable impedance matching networks for wide-band poweramplifiers.

V. REFERENCES

[1] Chang, J. & al., "Large suspended inductors on silicon andtheir use in a 2pm CMOS RF Amplifier", IEEE ElectronDevice Letters vol. 14, 1993, pp. 246-248[2] A. Dec and K. Suyama, "Electro-Mechanical Properties ofa Micro-machined Varactor with a Wide Tuning Range",ISCAS 1998[3] D.J. Young and B.E. Boser, "A Micro-machine-Based RFLow-Noise Voltage-Controlled Oscillator", CICC 1997[4]H.J. Quenzer, B. Wagner, IST MELODICT, 1st annualreport, Jan. 2001[5] J. Zou & al, "Development of a wide tuning range MEMStunable capacitor for wireless communication systems",IEDM 2000, pp. 403-406[6] J.B. Yoon and C.T. Nguyen, "A high-Q tunablemicromechanical capacitor with movable dielectric for RFapplications", IEDM 2000, pp. 489-492[7] C. Goldsmith & al., "Characteristics of micro-machinedswitches at microwave frequencies", MTT-S'96, pp. 1141-1144[8] J.J. Yao and M. Chang, "A surface micro-machinedminiature switch for telecommunications applications with

signal frequencies from DC to 4GHz", Transducers'95, pp.384-387[9] Cronos corp. , http:\\cronos.memsrus.com[10] A.-C. Wong, H. Ding, and C. T.-C. Nguyen,"Micromechanical mixer filters," IEDM 1998, pp. 471-474.[11] C.T. Nguyen & al., MEMS'2001[12] J. Clark & al., "High-Q micromechanical contour-modedisk resonators", IEDM 2000, pp.493-496[13] D. Galayko & al., to be published[14] D. Galayko & A. Kaiser, "Design methods for micro-electromechanical band pass filters", DTIP'2001, pp. 194-200[15] R Ruby, P Bradley, J D Larson III and Y Oshmyansky "PCS 1900 MHz duplexers using thin film bulk acousticresonators (FBARs)," Electronic Letters 13 May 1999 Vol 35No 10

VI. CONCLUSION

Micro-machining technology makes it possible to fabricatenew types of components which could be an enablingtechnology for truly re-configurable RF interfaces for mobilephones. However, a number of design, fabrication andassembly issues have still to be solved.

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MEMSTECH'2006, May 24-27, 2006, Lviv-Polyana, UKRAINE