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Tunable MEMS capacitors using vertical carbon nanotube arrays grown on metal lines

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2011 Nanotechnology 22 025203

(http://iopscience.iop.org/0957-4484/22/2/025203)

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 22 (2011) 025203 (9pp) doi:10.1088/0957-4484/22/2/025203

Tunable MEMS capacitors using verticalcarbon nanotube arrays grown on metallinesAnupama Arun1, Helene Le Poche2, Tonio Idda3,Donatello Acquaviva1, Montserrat Fernandez-Bolanos Badia1,Philippe Pantigny2, Paul Salet1 and Adrian Mihai Ionescu1

1 Nanolab, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland2 CEA-LITEN, DTNM, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France3 LAAS–CNRS, MINC (G37), 7 Avenue du Colonel Roche, 31077 Toulouse Cedex, France

E-mail: adrian.ionescu@epfl.ch

Received 10 July 2010, in final form 17 November 2010Published 7 December 2010Online at stacks.iop.org/Nano/22/025203

AbstractIn this work, tunable MEMS capacitors are realized using a vertically grown carbon nanotubearray. The vertical CNT array forms an effective CNT membrane, which can be electrostaticallyactuated like the conventional metal plates used in MEMS capacitors. The CNT membrane isgrown on titanium nitride metal lines, with a Al/Fe bi-layer as buffer layer and catalyst materialrespectively, using chemical vapor deposition process. Two different anchor configurations areinvestigated. A maximum capacitance of 400 fF and maximum tunability of 5.8% is extractedfrom the S-parameter measurements. Using the tunable MEMS vertical array capacitor avoltage controlled oscillator (VCO) is demonstrated showing promise for integrating CNTs forcommunications applications.

S Online supplementary data available from stacks.iop.org/Nano/22/025203/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Radio frequency (RF) microelectromechanical systems(MEMS) have emerged as a new class of devices holdinghigh promise for communications applications due to theirlow power consumption, tunable properties and especially highperformance at GHz frequencies [1]. RF MEMS capacitors,also referred to as varactors are key elements needed in manyRFICs such as oscillators, filters etc. Integrating a highQ tunable MEM capacitor into silicon IC technology hasbeen previously addressed by using various conductive layers.Carbon nanotubes (CNTs) possess excellent electrical andmechanical properties. They are stiff, lightweight, and havevery high electrical conductivity without electron migrationissues [2], which is prevalent with metals. Thus, since thediscovery of CNTs in 1991 by Iijima [3] they have beenextensively studied for application in electronic devices. Thereis a great interest in integrating them to fabricate electro-mechanical devices [4–7] and for their subsequent application

in RF MEMS. CNTs are 1D materials with a high aspectratio. To realize devices with any prospect of applicationit is necessary to operate many of them in parallel. Thisdemands a CNT growth technology to be able to preciselygrow many CNTs in parallel in an array configuration. Withprogress in CNT growth technology using chemical vapordeposition, it is possible to grow vertical arrays of CNTsat predefined locations on a metal substrate [8]. It is alsopossible to grow them in different shapes, controlled bypatterning the catalyst using photo-lithography [9]. It isof interest to characterize these CNT arrays, which form amembrane-like structure and check their viability for electro-mechanical applications. This could provide an alternateimplementation method to fabricate novel electro-mechanicaldevices in vertical configurations [10–12]. With scaling ofCMOS transistors on the horizon, the possibility arises toconsider novel device architectures.

In this work we report tunable varactors using verticalarrays of CNTs behaving like a conducting membrane.

0957-4484/11/025203+09$33.00 © 2011 IOP Publishing Ltd Printed in the UK & the USA1

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References [13, 14] have demonstrated the potential offabricating high aspect ratio vertical CNT membranes whichcan be electrostatically actuated. They also extracted thecapacitance of the varactor formed by two CNT membranesat high frequency and estimated the Young’s modulus ofthe entire CNT membrane. However, the high aspect ratiodemonstrated a poor induced membrane alignment which wasnot compatible with the fabrication of higher capacitance up-scaled devices. In our work we investigated different anchordesigns and designed high capacitance tunable varactorsmade of several metal fingers with an interleaved electrodeconfiguration separated by smaller spacing. The aspectratio of the membrane is accurately optimized to ensurea good vertical alignment, thereby avoiding short-circuitsresulting from membrane collapsing. In this work we reportvertical CNT arrays fabricated on an interleaved electrodestructure. All CNT membranes across the two electrodes canbe simultaneously actuated, resulting in higher capacitancethan reported in [13, 14]. The capacitance value is scalableby increasing the number of fingers within the interleavedstructure. Compared to our previous works [15, 16], we reportcapacitance extraction on two different anchor configurations,larger capacitance than reported earlier using a differentcatalyst system. We also report the implementation of aVCO using tunable vertical array capacitors that is amongthe very few circuits implemented to date using CNT MEMdevices [17]. Overall, this work demonstrates that CNT-basedMEMS can be realized based on substrates and techniquescompatible with silicon IC fabrication and their properties areof interest for high frequency applications.

2. Design and fabrication

The array of CNTs which forms a membrane-like structure isgrown on adjacent metal lines. The membranes can be actuatedby applying a bias voltage across the two electrodes. Figure 1shows the schematic of the two proposed devices. Figure 1(a)shows a simple CNT array varactor design where the verticalmembranes are anchored only to the substrate without lateralanchors, called design A. Figure 1(b) shows design B, in whichthe membrane is grown with additional lateral anchoring toprovide stability. The anchors are thicker than the membraneitself, thus providing a stable structure. CNTs are grown ina concentric fashion to give it a robust structure. Figure 1represents the proposed device in its simplest form. It canbe considered as a two-finger structure. It is possible to scalethe capacitance by inserting many fingers in parallel with aninterleaved electrode configuration (figures 2(a) and (b)).

It is also possible to conceive devices with a thin layerof dielectric and metal deposited on the CNT membrane [10].This increases the capacitance; the CNT-insulator–metal formsa composite structure with improved mechanical properties.In [18] the authors embedded a vertically grown CNT arraygrown by PECVD with a height of 3–10 μm within silicondioxide (SiO2). The CNTs were found to preserve theirintegrity and alignment after the SiO2 deposition and polishingsteps. It is also possible to design MEMS switches and phaseshifters in the proposed configuration.

Figure 1. Schematic of vertical CNT array varactor. (a) Design A,anchored only to the metal substrate without lateral anchoringand (b) design B with additional lateral anchoring to provide a stablestructure.

Different designs of varactor corresponding to differentdimensions were investigated. The number of metal fingers(N) varied from 2 to 23. Two thicknesses of catalyst lineswere studied, 2.5 and 5 μm, as defined on the mask. Thespacing between the CNT membranes ranged from 3 to 8 μm,as defined on the mask (referred to as ‘g’). The minimumspacing was limited to 3 μm due to the limits of optical photo-lithography and the precision of the mask aligner.

High resistivity silicon (5 k� cm) was used as thesubstrate. The SiO2 layer is necessary to isolate the devicefrom the substrate; if grown directly on high resistivity silicon,a depletion and/or inversion layer can be formed at the Si–SiO2

interface. This induced charge layer is voltage-dependent,degrading the device properties when high DC biases areapplied. To alleviate this problem, amorphous silicon (a-Si) was deposited on the Si surface as a passivation layer,preventing the accumulation of electrons [19]. 300 nm LPCVDa-Si was deposited as a passivation layer and 500 nm of SiO2

insulator was sputtered. Titanium nitride (TiN) metal lines100 nm thick were deposited by sputtering. The metal lineswere patterned by optical photo-lithography and inductivelycoupled plasma etching. The 7 nm thick buffer layer (Al)and the 1 nm catalyst layer (Fe) necessary for the CNTgrowth above TiN was patterned by optical lithography andlift-off technique. The challenge for the CNT growth was theoptimization of the CNT density in the array in order to obtainvertical free-standing CNT membranes with maximal aspectratios, thereby avoiding any short-circuits caused by collapsingof the membrane. A dedicated CVD process was developedin an industrial vertical hot-wall Plassys reactor, previouslydescribed in the framework of other growth processes [20].The equipment was fitted with a 13.56 MHz radio frequency(RF) plasma power supply. Here, it allowed a preliminaryplasma pre-treatment of the catalyst to be performed at roomtemperature under an oxidizing atmosphere. This step iscrucial for the reproducibility of the following CNT growthrealized via a pure thermal CVD process. The CNT growthwas carried out at 590 ◦C and 0.4 mbar from a C2H2/H2/Hegas mixture, where acetylene was used as the carbon source.

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Figure 2. SEM images of the fabricated structures. (a) Design A: no lateral anchor configuration, (b) design B: concentric structure withlateral anchoring, (c) and (d) zoomed in SEM images, (e) and (f) high-resolution TEM images of CNTs extracted from the devices anddispersed in an isopropyl alcohol solution.

The height of the resulting CNT membranes was tuned bycontrolling the growth duration.

3. Morphological and structural characterizations

The morphology and structure of the vertical array grownwere studied using TEM and SEM. Figures 2(a)–(d) shows theSEM images of the fabricated devices. Figures 2(a) and (b)corresponds to design A, with no lateral anchors, and designB, with lateral anchors having a concentric-like configuration,respectively. Two different heights of CNT membranes, 64 μmand 74 μm, were fabricated, corresponding respectively to agrowth step performed for 60 min and 75 min. The thicknessesof the CNT membranes (t) were greater than the thicknessesof the catalyst lines seeding them defined on the mask. Twomembrane thicknesses of 2.5 and 5 μm, as defined on themask, were fabricated. The thicknesses of the fabricatedmembranes corresponds to 3.5 μm and 6 μm, corresponding tothickness of 2.5 μm and 5 μm respectively as measured on theSEM. As determined by high-resolution transmission electronmicroscopy, the CNT membranes were made of small diametermulti-walled CNTs (figures 2(e) and (f)). The average diameterwas about 5 nm, corresponding to CNTs typically made of 3or 4 walls. Actually, the distribution was rather wide, withCNT diameter values ranging from 3 to 9 nm, related to anumber of walls lying between 2 and 8. By considering 5 nmdiameter CNTs, the density was approximated at about 8 ±2 ×1011 CNT cm−2. This value was extrapolated from a lineardensity estimated by counting on SEM images (figure 2(d)).

As illustrated in figure 2, the relatively high CNT densityobtained enabled the CNT membranes with a height of64 μm to stand vertically, thereby ensuring a good electricalinsulation of the adjacent membranes for all the varactordesigns investigated. Even for the smallest gaps definedbetween the catalyst lines (g = 3 μm) the adjacent membraneswere not short-circuited. However, the CNT membraneswith a height of 74 μm exhibited poor alignment comparedto the membrane with a height of 65 μm (due to higheraspect ratio) resulting in membrane collapse and electricalshort-circuits for many devices. For this membrane height,only devices with two electrodes were reliable for electro-mechanical characterizations. Whatever the membrane height(L), the membrane alignment was never perfect. Accordingly,the real effective gap obtained after growth between themembranes was difficult to determine: it was not necessarilyconstant on a same device and could be higher or lower thanthe spacing defined between the catalyst lines on the mask,depending on the thickness of the membrane and the anchorconfiguration used.

4. Electro-mechanical characterization

4.1. DC characterization

DC characterization was carried out using an HP 4156Csemiconductor analyzer. A DC voltage bias was applied acrossthe two CNT membranes and the current flow between thetwo electrodes was monitored when the voltage was increased,

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Figure 3. DC characterization illustrating pull-in behavior on deviceswith the same spacing gap and different membrane thickness.

in order to detect the electrostatic actuation. Figure 3 showsthe DC characterization on two devices with two fingersof different membrane thickness corresponding to membraneheights of 64 μm. Before the pull-in state the air-gap betweenthe two electrodes provides a very good electrical isolationand the current is of the order of fA. With an increased biasvoltage, we were able to detect pull-in or snapping of the CNTmembranes. CNTs were typically burnt upon pull-in due to thelarge current flow (∼1 mA) upon contact. To avoid this effect,we limited the current flow to 100 nA during measurements.As expected, the device with thicker membrane thicknesshas a higher pull-in voltage. Typically the CNT membranesremained stuck to each other due to the strong van der Waal’sinteraction occurring at the tips and it was not possible toachieve pull-out. In some devices even pull-out behavior wasobserved (not shown here) but was not reproducible. We alsoobserved the mechanical movement of the tubes under theoptical microscope.

4.1.1. Extraction of Young’s modulus. If we assume that theentire membrane as an equivalent material, having an effectiveYoung’s modulus (Eeff−CNT), the pull-in voltage is then definedas [12],

Vpull−in ≈ 0.39

√Eeff−CNT

ε0

t3gap3

L4

where Eeff is the effective Young’s modulus, t is the thicknessof the membrane, gap is the spacing between the membranes,and L is the height of the CNT membrane. The gap acrossthe membranes is not constant. Depending on the gap betweenthe membranes, membrane thickness and the anchor design,the CNT membranes tend to bend. To determine the pull-in voltage an effective gap has been assumed. From themeasured pull-in voltages the Young’s modulus of fabricatedCNT membranes reported in this work was extracted to be inthe range of 30–100 MPa. The variation is due to the errorin the approximation of the gap between the two electrodes.This value compares with the value estimated by [14]. Theyestimated the Young’s modulus of 1–10 MPa, by measuringthe deflection of the membrane under an optical microscope

and correlating it to the applied voltage and membrane physicalproperties such as the gap, thickness and length. In anotherstudy, [21] investigated the properties of compressive films ofmulti-walled nanotubes, the Young’s modulus was found tobe 6 MPa. In contrast with [21], our CNT membranes showYoung’s modulus in the range of tens of MPa, probably dueto the higher density of tubes. Note that density could befurther improved by solvent treatment. Reference [22] reportedan effective Young’s modulus of approximately 10 GPa fordensified single walled CNT films.

4.2. RF characterization

The capacitance of the designed capacitors was in the range oftens of fF up to 1 pF. Electrical probing of the fabricated CNTMEM capacitor needs metal pads with associated capacitancesof around 100 fF. Moreover, the applications foreseen forour capacitors are circuits operating in the GHz frequencyrange. For all these reasons the most appropriate technique toaccurately extract their capacitance versus voltage behavior isbased on S-parameters and de-embedding of the parasitics. S-parameter measurements were performed using an HP 8753Dnetwork analyzer and an external DC voltage source. Thedevice was tested in a Suss Microtec PMC 150 vacuumchamber at a pressure below 10−5 mbar at room temperature.All measurements were performed with 0 dB m (1 mW)RF power applied at the input. The measurement systemwas calibrated up to the probe tips using an external SOLTcalibration kit from Suss Microtech. The S-parametersbetween port 1 and port 2 for different DC bias were measuredfor the CNT capacitor and the open structure (reference devicewith only the metal electrodes and no CNTs referred to asopen from here on). The isolation response between the twovaractor ports decreased with increased DC bias voltage. Dueto the applied DC bias, the CNT membranes deflect, resultingin an increased coupling between the two ports (shown insupplementary information available at stacks.iop.org/Nano/22/025203/mmedia). In other words the isolation (S21)between the two ports decreases. When the bias voltage isremoved the isolation increases almost to the initial value witha small hysteretic effect. There was no influence of the appliedDC bias on the isolation between the two ports (shown insupplementary information available at stacks.iop.org/Nano/22/025203/mmedia). This indicates that the change in isolationin the case of the CNT varactor device is due to the mechanicaldeflection of the CNT membrane.

By using an equivalent electrical circuit, the CNT brushvaractor capacitance was then extracted after de-embeddingthe parasitic components (shown in figure 4(c)). Cp, CPAR,CCNT and Rs represent the substrate capacitance, parasiticcapacitance due to the metal electrodes, CNT varactorcapacitance and series resistance respectively. The seriesresistance is the combination of the resistance of the metallines and the contact resistance between the CNT and the metallines. Figure 4(a) shows the extracted capacitance of the CNTvaractor with two fingers, height of 74 μm, thickness of 6 μm,spacing (g) ∼4 μm, as a function of the applied bias voltage forboth the case of charging and discharging the capacitance. The

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Figure 4. (a) Capacitance extracted from S-parameter measurements from the model shown in (c). Figure clearly shows the charging anddischarging of the capacitor. (b) Capacitance variation of the reference or open structure with voltage. The capacitance is independent of theapplied bias voltage. (c) Electrical equivalent model. (d) Comparison of measurements and electrical circuit simulations validates of theelectrical model shown in (c). ADS circuit simulation and measurements show a good fit confirming the validity of the model.

capacitance voltage curve follows the trend of a typical MEMScapacitor due to a nonlinear electrostatic force (inverselyproportional to the square of the spacing). Since no dielectricis used, there is no effect of hysteresis. Figure 4(b) showsthe capacitance variation of the reference or open structurewith voltage. In the case of the open device the capacitanceis independent of the applied bias voltage. Circuit simulationwas performed using software from Advanced Design Systems(ADS) to validate the electrical model used in figure 4(c) andthe measurements. Figure 4(d) shows a good fit, confirming thevalidity of the model used. The electrical model parametersCs, Rs, Cp and CPAR were 17.9 fF, 95.46 �, 37.46 fF and7.39 fF respectively for this particular device. The capacitanceof this structure, estimated from the formula C = εo A/d ,where εo is the permittivity of free space, A is the area of themembrane and d is the spacing between the two membranes,is 16.6 fF. This value is close to the extracted value of 17.9 fF.This confirms the design with the measurement and extractiontechnique used to extract the capacitance.

The influence of the two anchor designs on the fabricatedstructure was studied under the SEM and its impact wasalso seen on electrical measurements. The main differencebetween the devices with and without lateral anchors wasin the vertical alignment of the CNT membrane. Withoutlateral anchoring, as the spacing between the CNT membraneswas reduced (∼4 μm) the CNT membranes across the twoelectrodes were pushed away from each other (shown insupplementary information available at stacks.iop.org/Nano/22/025203/mmedia). When the separation between the

membranes was 10 μm the influence of the gap was reduced,meaning that the CNT membranes were vertically aligned. Forthe lateral anchor configuration the anchoring effect dominatesthe spacing across the two electrodes. To form a stablestructure a concentric elliptical structure was formed. CNTmembranes with thickness of 3.5 μm were influenced moreby the gap than membranes with thickness 6 μm. Since theCNT membranes of thickness 3.5 μm were softer than theCNT membranes of thickness 6 μm, they form an ellipticalpattern (shown in supplementary information available atstacks.iop.org/Nano/22/025203/mmedia), whereas the 6 μmthick membranes form a more robust structure as definedduring the design. Compared to the spacing defined on themask, without lateral anchoring the effective gap across the twomembranes was found to be larger compared to the case withconcentric lateral anchoring. This is also seen in the electricalcharacterization. Figure 5 shows the capacitance extractionof the two devices with two fingers of same dimensions,with and without anchors. Due the reduced effective gap,the capacitance was larger for the design with lateral anchorscompared to the design with no lateral anchors. The tuningranges for the two configurations with and without anchorsfor this particular device were 3.20% and 2.06% respectively.The design with lateral anchors is a more robust configurationcompared to the no anchor configuration in designing stablevertical membranes.

Figure 6(a) shows the capacitance extraction for twodevices with different CNT membrane thicknesses. Theother parameters, such as the height of the membranes (L),

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Table 1. Capacitance, tuning range and quality factor extracted for different designs.

# offingers

Gap(μm)

Width of themembrane (μm)

Thickness ofCNT membrane(μm) Anchoring

CNTheight(μm)

Capacitance(fF)

Tuningrange (%) Q@100 MHz Q@2.5 GHz

2 4 100 6 No anchors 75 17.9 2@30 V 659.5 26.32 4 100 6 Concentric 75 20.2 3.2@30 V 505.9 20.23

10 4 200 6 No anchors 65 248 2.2@30 V 126.1 510 4 200 3.5 Concentric 65 298 5.8@15 V 111 4.4423 6 200 3.5 No anchors 65 404 1.9@20 V 39.6 1.5823 6 200 6 No anchors 65 362.1 0.4@30 V 34.5 1.3823 6 200 6 Concentric 65 373.5 0.5@30 V 67.9 1.35

Figure 5. Capacitance extraction for devices of design A with nolateral anchoring and design B with lateral anchoring for the samedevice dimensions. The effective gap across the membrane with nolateral anchors is less than in the case with anchors. The capacitanceand tunability were found to be higher for the design with lateralanchors due to the smaller effective gap.

number of fingers (N), width of the membrane (W ), anchorconfiguration (concentric structure with lateral anchors in thiscase) and spacing across the electrodes (g) (L = 64 μm,N = 10, W = 200 μm, g = 6 μm) are the same for the twodevices. The stiffness of the CNT membrane increases withthe thickness. As expected, the tuning range for a given biasvoltage is lower for the thicker CNT membrane structure. Atuning range of 3.3% at 20 V and 0.66% at 30 V was obtainedfor the device with membrane thicknesses of 3.5 μm and 6 μmrespectively. A similar trend was seen for the device with nolateral anchoring. The capacitance is scalable by increasingthe number of fingers. Figure 6(b) shows the capacitanceon two devices with 10 and 23 fingers. As expected thecapacitance increases with an increase in the number of fingers.A maximum capacitance of ∼404 fF with a tuning range of1.9% at 20 V was obtained for the device with 23 fingers(L = 64 μm, t = 6 μm, g = 6 μm).

For a series capacitor measurement configuration, thequality factor can be extracted from the circuit model usingthe relation, Q = 1/(2π f RsCs). Table 1 summarizes theextracted capacitance, tuning range and quality factor (Q) forCNT varactors of different designs. The quality factor could beimproved by reducing the series resistance. In case of the CNTvaractor, the series resistance is a combination of the resistanceof the transmission line and the contact resistance between the

metal line and the CNTs. Olofsson et al [14] obtained a seriesresistance of 380 � using sputtered molybdenum electrodes.In our devices the series resistance was found to be 60–160 �.This variation could be attributed to the measurement errordue to calibration and noise. Using aluminum (1 μm thick)as metal lines instead of titanium nitride (100 nm) showedapproximately 4–5 times improvement in quality factor. Theparasitic capacitance due to the metal electrodes can bereduced by using semi-suspended CPW lines [19], which willalso improve the quality factor. The quality factor can befurther improved by reducing the contact resistance betweenthe CNT and the metal substrate. Reference [23] measuredthe contact resistance between the vertically grown CNT arrayand the metallic alloy on which it was grown using a micro-manipulator. They found the resistance to be of the orderof 500 �. This also includes the contact resistance fromthe measurement probe. Reference [24] using four-proberesistance measurements reported the CNT resistivity of asgrown membrane by CVD before densification to be 0.08 � cmand after densification in a solvent to be 0.0077 � cm [25]. Theresistivity of metals used in MEMS is of the order of tens ofμ� cm, which is three or four orders of magnitude lower thanthat of densified and non-densified CNT membrane. We do nothave an estimate of the contact resistance between the CNTmembrane and the metal electrode for our device. Increasingthe density will increase the conductivity of the membrane.It certainly is a parameter to be considered for study andoptimization. The extracted Young’s modulus and qualityfactor indicates the fabricated CNT array forms a metamaterialwhere the volume of CNT filled-in is very low and most of thevolume is air. It has been observed that most of the volume ofthe as-grown CNT array is air (up to 97%) [26]. There has beenclear evidence of improved densification when the as-grownCNT membrane is introduced in a solvent [27], indicatingmost of the volume is actually air. Even though significantprogress has been made since their discovery in terms ofcontrol of position on the wafer and alignment of the CNTson a substrate, CNT growth technology is relatively immatureand needs further breakthroughs to be able to fabricate denselypacked, uniformly spaced, perfectly aligned CNT arrays.

5. Discussion

The main advantage of the vertical architecture is that itreduces the footprint of the device. This is again feasible onlyif the technology enables very high aspect ratio membranes

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Figure 6. (a) Capacitance extraction on devices with two different membrane thicknesses. (b) Capacitance extraction on devices withdifferent numbers of fingers.

Figure 7. (a) Schematic diagram of the resonator implementation. (b) PCB board of the realized voltage controlled oscillator based on thetunable vertical CNT array varactor.

which can be spaced close to each other with spacings ofthe order of a few tens of nm across the two membranes.In our devices the capacitance footprint is estimated to be0.005–0.01 fF μm−2, which is comparable to that of theexisting MEMS devices in the OFF state. If we can scale thethickness of the membrane and the gap across the membraneby 0.5 we will already increase the capacitance by four times.If we can scale the gap and the thickness of the membranesaggressively to a few tens of nm, we can reduce the capacitancefootprint by 2–3 orders of magnitude. It is possible to achievethis without compromising on the key performance parameters,such as quality factor, provided the density of the CNT array,resistivity and contact resistance can be optimized.

6. Voltage controlled oscillator (VCO) with CNTarray capacitor

A voltage controlled oscillator (VCO) has been chosen as ademonstrator for CNT varactor applications. In this type ofdevice the application of a bias voltage to the varactor changesits capacitance and hence the overall resonance frequencyof circuit. Therefore, the capacitance variation of the CNTvaractor is translated into a shift of the VCO output frequency.The high voltages needed for tuning the capacitance valuesof the CNT MEM varactor reported in this work madethem incompatible for an oscillator demonstration using anintegrated CMOS circuit. Therefore a discrete implementationof VCO has been chosen. The configuration of VCO is basedon a ‘negative resistance’ topology that uses the IC’s internalparasitic elements of the commercial oscillator to create anegative resistance at the base-emitter port. A resonant circuit,

tuned to the appropriate frequency, cancels the real part of thenegative resistance and causes oscillations. The varactor isused in the resonant circuit to tune the resonance frequency andcontrol the overall frequency of oscillation. The VCO’s activecircuit includes a commercially available Colpitts oscillatorchip mounted on a home-made PCB circuit. The oscillator iscoupled to the discrete LC resonator on the same circuit board.The connection of varactors to the VCO circuit has been doneby means of wire bonding.

The simplified schematic diagram of the resonatorimplementation is reported in figure 7(a). The resonantfrequency is determined by a ceramic dielectric resonatorcoupled to a 1 pF parallel capacitor. The CNT varactor isplaced in a shunt configuration and is DC isolated from theactive circuit by means of a DC blocking capacitor. In thismanner the oscillator is protected against the high voltage levelapplied to tune the varactor. The biasing of the CNT varactor isprovided through a high-value resistor by means of an externalDC voltage source (a SMA connector on the PCB has beenused for this purpose).

The VCO has been biased with a Vcc supply voltage of3.5 V with a stable DC voltage source. The CNT varactorwas biased via an Agilent 6626A DC power supply so thatthe tuning voltage can be varied from 0 to 100 V. Figures 8(a)and (b) show the VCO output response at different biasvoltages. The response is centered at a frequency of 918 MHz,with peak amplitude of −1.5 dB m. The frequency tuningcan be seen in figure 8, with the observed frequency tunabilityprovided by the electrostatic actuation of the CNT varactor.The oscillator fundamental frequency shift is less than 100 kHzfor a bias voltage variation from 30 to 40 V. When the CNT

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Nanotechnology 22 (2011) 025203 A Arun et al

Figure 8. VCO output signal for different DC bias. The device has been tested with a Vcc of 3.5 V. The varactor has been biased with (a) 30,35 and 40 V. (b) 30 and 50 V.

varactor is biased with 30 and 50 V, the tuning of the centralfrequency is about 250 kHz (figure 8(b)). The observedsensitivity of the device is in the range of 10–20 kHz V−1. Thisvariation is nonlinear due to the nonlinearity of the MEMSelectrostatic actuation force. Higher tuning sensitivity isobserved when the applied voltage is closer to 50 V. In order toincrease the tuning bandwidth it was necessary to connect fourvaractors in parallel (three devices having 23 fingers and 6 μmgap and one having 23 fingers and 4 μm gap). The overall OFFstate capacitance was around 1.5 pF. The limit in the actuationvoltage of varactors limits the available control voltage thatcan be applied to VCO, thereby reducing the available tuningrange. Also, the long bond wire used to connect the varactorto the VCO adds high parasitic inductances that contribute toperformance degradation of the VCO.

7. Conclusion and perspectives

Tunable vertical CNT array MEMS capacitors with maximumcapacitance of 400 fF and maximum tunability of 5.8%have been demonstrated. The influence of different designparameters, namely, the anchor design, the spacing acrossthe electrodes, the thickness of the membrane and thenumber of fingers has been studied. DC and RF electricalcharacterizations were performed. DC characterizationshowed pull-in behavior like conventional MEMS devices.The Young’s modulus of the as-grown vertical CNT arraywas extracted from the measured pull-in voltage, and aneffective Young’s modulus of 30–100 MPa was obtained.An equivalent electrical model was developed to extract thecapacitance from the S-parameters measurements. The modelwas validated by ADS circuit simulations. Based on thefabricated tunable MEMS capacitor a discrete componentVCO has been implemented.

This work is a contribution to the evaluation of thepotential of CNT arrays to serve as membranes with lowmass, high conductivity and metal-like stiffness in order tofabricate MEMS capacitors. With further development inCNT growth technology, if CNTs can be grown at lowertemperatures (<450 ◦C) we could integrate the CNT growthprocess with standard IC processes. To improve the CNT

capacitor performance, that is to increase the capacitance,reduce the actuation voltage, improve the quality factor,tunability and reduce the capacitance footprint it is necessaryto grow very dense, uniformly spaced and perfectly alignedCNT array. To reduce the actuation voltage it is necessaryto reduce the membrane thickness while maintaining a highaspect ratio and reduce the gap across the two membranes. Thiswill also improve the capacitance density. The quality factorcan be improved by reducing the resistivity of the membraneand the series resistance between the CNT membrane and themetal electrode. Further breakthroughs in the field of CNTtechnology are needed to be able to fabricate devices with thehigh performance necessary for practical applications.

Acknowledgment

This project is funded by NANO-RF, an FP6 European project.

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