JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL…nelsons/Papers/Cabrera et al JMEMS 2013.pdf · JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 1, FEBRUARY 2014 243 Performance

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 1, FEBRUARY 2014 243

Performance of Electro-Thermally DrivenVO2-Based MEMS Actuators

Rafmag Cabrera, Emmanuelle Merced, and Nelson Sepúlveda, Senior Member, IEEE

Abstract— The integration of VO2 thin films in a MEMSactuator device is presented. The structural phase transition ofVO2 was induced electro-thermally by resistive heaters monolith-ically integrated in the MEMS actuator. The drastic mechanicaldisplacements generated by the large stress induced duringthe VO2 thin film phase transition have been characterizedfor static and time-dependent current pulses to the resistiveheater, for air and vacuum environments. A comprehensive andsimplified finite element model is developed and validated withexperimental data. It was found that the cut-off frequencyof the 300 µm-long VO2-based MEMS actuator operated invacuum ( f3dB = 29 Hz) was mostly limited by conductive heatloss through the anchor, whereas convection losses were moredominant in air ( f3dB = 541 Hz). The cut-off frequency is foundto be strongly dependent on the dimensions of the cantileverwhen operated in air but far less dependent when operated invacuum. Total deflections of 68.7 and 28.5 µm were observedfor 300 and 200 µm-long MEMS cantilevers, respectively. Fullactuation in air required ∼16 times more power than invacuum. [2013-0042]

Index Terms— MEMS actuators, vanadium dioxide, phasetransition, actuator dynamics.

I. INTRODUCTION

IN 2010, a new mechanism for micrometer-sized actuationwas introduced [1]. The mechanism consisted on the use

of the crystallographic changes of vanadium dioxide (VO2)thin films across its structural phase transition (SPT), whichoccurs at ∼68 °C. When highly oriented VO2 thin film coat-ings are deposited on suspended micromechanical structures,the SPT of the VO2 induces strains up to −0.32% in thebimorph microactuator, which have been used to tune theresonant frequency of simple VO2-coated SiO2 micro-bridgesup to −23% [2], and to generate curvatures in VO2-coatedsingle crystal silicon (SCS) cantilevers over 2,000 m−1 [1].Larger curvatures changes were obtained on initially curvedsingle crystal VO2 microcantilevers (20,000 m−1) [3] and50 nm thick Cr cantilevers coated with polycrystalline VO2(14,000 m−1) [4]; which were straightened during the phasechange. This paper presents the first monolithically integrated

Manuscript received February 5, 2013; revised June 9, 2013; acceptedJune 25, 2013. Date of publication July 16, 2013; date of current versionJanuary 30, 2014. This work was supported in part by the National ScienceFoundation under Grants ECCS-1139773 (CAREER Program, administeredby Anupama Kaul) and DGE-0802267 (GRFP Program). Subject EditorS. M. Spearing.

The authors are with the Department of Electrical and Computer Engi-neering, Michigan State University, East Lansing, MI 48824 USA (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2013.2271774

VO2-based device (a micro-actuator) driven by electric sig-nals (i.e. joule heating). The paper is focused on describingthe dynamics of this VO2-based MEMS device, its designparameters, and the validation of a Finite Element Methodmodel that integrates the SPT effects into a lumped model,thus facilitating future designs. The presented device can bereadily used for micro-manipulation, and its applications couldbe extended to micro–robotics and micro-surgery fields.

A. Actuation Mechanisms

In 2005, a comprehensive review of the state of the micro-actuator field was published [5]. Each transducing mechanismcan have advantages and disadvantages in terms of design andfabrication simplicity, resolution, efficiency, practical limitsto motion range, generated force, and speed. Hence, becausethere is such a wide range of applications for electromechan-ical actuators there is no single type which is best suited forall applications.

Perhaps the most common actuation mechanism in MEMSdevices is electrostatic actuation, where an electric fieldcreated between the micro-actuator and a second electrodegenerates the driving force. This type of actuation usuallyproduces relatively low “out-of-plane” (i.e. perpendicular tothe substrate) displacements and forces since the resultingelectrostatic forces decrease rapidly with decreasing size.In addition, to avoid the instability and/or device failure thatcomes with the pull-in effect, the actuator deflection rangeis limited to 33% of the initial gap between the electrodes.Although this limitation has been lifted to 60% by [6] andto 90% by [7], the total displacements are still very small(below 2 μm) when compared with other actuation mecha-nisms. Other types of electrostatically-actuated actuators withdimensions in the millimeter range include coupled comb-drive devices – which have produced “in-plane” (i.e. parallelto the substrate) displacements up to 150 μm for a drivingvoltage of 145 V [8]– and mirrors, – which have producedtilting angles up to 8° for a driving voltage of 85 V [9].

Another actuation mechanism that has been used is based ona bilayer cantilever consisting of two materials with very dif-ferent thermal expansion coefficients [10]. In this approach, thetotal actuation (which typically relies on the displacement of asuspended micromechanical structure) is limited by the largesttemperature difference that the structure can sustain and by thelargest difference in thermal expansion coefficients betweenthe materials composing the bimorph structure. For most pairof solids, the difference in thermal expansion coefficients (�α)is less than 10−5 K−1, but the use of polymers (which typically

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244 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 1, FEBRUARY 2014

have very large thermal expansion coefficients) has resulted in�α in the range of 10−4 K−1 for cantilever bimorphs [11].

Electromagnetic and magnetostatic actuators with dimen-sions in the millimeter range typically have output forces up to1.24 mN [12] and generate displacements up to 50 μm [13].The forces generated by microactuators that rely on forcesinduced by magnetic fields decrease rapidly with size; andtheir monolithic integration into a single chip is not as simpleas other type of actuators. Piezoelectric-based actuators requirerelatively high operation voltages, involve complex compounds(and sometimes toxic; e.g. lead) which complicates their depo-sition as thin films and limits their applicability in MEMS andNEMS (especially for bio-related applications). Nevertheless,they have a clear advantage in terms of speed, with frequencyresponses near 10 kHz, which could be increased by up totwo orders of magnitude in piezoelectric bimorph geometriesat the cost of a substantial reduction in force capabilities [5].Millimeter-sized piezoelectric-based actuators can generateforces up to ∼1 mN, and displacements up to ∼200 μm [14].

A very important figure of merit in microactuators is thework per unit volume or strain energy density parameter,which normalizes the work that can be generated by themicroactuator per unit size. In terms of strain energy density,electroactive polymers have demonstrated strain energy den-sities of 5 × 104 J/m3, and thermal expansion mechanismshave shown values in the range of 105 J/m3. Shape memoryalloys (SMAs) have shown the largest strain energy densities(∼3 × 106 J/m3) [15], while VO2-based actuators follow withreported values of 8.1 × 105 J/m3 [16] (6.3 × 105 J/m3

for initially curved actuators [4]). SMAs are made of ductilemetals (typically titanium, nickel, and copper) and are thusmore susceptible to plastic deformation, creep, oxidation, andfatigue. In contrast, the bonding in monoclinic VO2 is ionic,and the compound has little or no ductility. Also, typical SMAsneed a wider temperature variation for inducing the phase-change, lose their memory capability, and are currently limitedto relatively low operating frequencies [4], [15].

B. Vanadium Dioxide (VO2)

From the multiple vanadium oxides, vanadium dioxide(VO2) has drawn particular interest since 1959, when Morinfirst observed an Insulator to Metal Transition (IMT) in VO2bulk crystals, which occurs upon heating at ∼68 °C [17].About 9 years later, it was reported that the optical proper-ties of the VO2 also change abruptly during the IMT [18],which demonstrated the material’s enhanced functionality.Although other vanadium oxides also exhibit phase transitions,their transition temperatures (TTr) are much further fromroom temperature than VO2 (e.g. V6O13 and V2O3 showphase transitions at TTr∼−123 °C, and V2O5 at TTr∼280 °C).The proximity of VO2’s TTr to room temperature has giventhis multifunctional material the upper hand when it comes tomost practical applications.

Although the electrical and optical changes of VO2 acrossthe phase transition have been studied for over half a cen-tury, studies on the material’s mechanical properties acrossthe phase transition were briefly studied in 1972 (with a

microindentation study on VO2 crystals at two temperatures:below and above TTr) [19], and were not revisited until thelast decade [20], [21]. Studies of the IMT in VO2 nanobeamswere done recently [21], [22], and in 2010 it was observedthat a VO2-coated 350 μm-long single crystal silicon (SCS)cantilevers experienced curvatures over 2000 m−1 when heatedacross VO2’s TTr [1]. Such phenomenon was not a result ofthe difference in thermal expansion coefficients (�α) betweenthe SCS and VO2, since the observed strain was much largerthan what could be achieved by this mechanism. Instead,the underlying mechanism was related to the crystallographicchanges that the VO2 experiences during its solid-solid phasetransition. The IMT in VO2 is immediately followed by amore energetically demanding SPT from a low temperaturemonoclinic phase to a high temperature rutile phase [23].During this SPT, the VO2 crystalline structure shrinks in thecr direction while it expands in the ar and br directions [24](ar, br, and cr represent the three lattice vectors in the crystal).When polycrystalline VO2 films are deposited by pulsed-laser-deposition (PLD) over amorphous SiO2, the material tendsto orient itself with its cr axis parallel to the substrate. Thecontraction of the VO2 crystal in the cr direction causesoverall contraction of the film in the direction parallel to thesubstrate as it undergoes the SPT. This contraction can beused to create large bending in bimorph structures, hundreds oftimes larger than the bending achievable by thermal expansionalone [1], [4].

Subsequent relevant work was mainly focused on the useof VO2’s SPT for micro-actuation purposes, which includedthe use of single crystal VO2 nanobeams [3], the photothermalinduction of the SPT in air and aqueous media [25] and theeffects of Cr doping concentrations and different monoclinicphases of VO2 [26]. In order to transform the SPT of VO2 intoa viable MEMS technology, it is necessary to comprehensivelycharacterize the dynamics of a completely monolithic VO2MEMS device. To this end, the present work describes thebehavior of VO2-coated SiO2 microcantilevers with integratedresistive heaters. This facilitates the direct control of thecantilever’s deflection using electrical signals, while avoidingthe nonlinearities involved in self heating of the VO2 filmitself. Different cantilever geometries are tested while varyingthe operating ambient conditions. A FEM model is developedand validated with experimental data in order to facilitatefuture designs.

II. DESIGN AND MODELING

A. Cantilever Design

The cantilevers are composed of SiO2 with integratedtitanium/platinum (Ti/Pt) metal layers, where the Ti layeris only used for adhesion purposes. The first layer of Ptforms the contact pads, traces, and a bottom electrode thatcan be used for capacitive sensing or electrostatic actuation.The second Pt layer forms the integrated heater and provideda reflecting surface, necessary for optical detection (the SiO2cantilever is transparent to the wavelength used for opticaldetection), which could also be used for electrostatic actuation.This second layer of Pt is buried inside the SiO2 cantilever,

CABRERA et al.: PERFORMANCE OF ELECTRO-THERMALLY DRIVEN VO2-BASED MEMS ACTUATORS 245

Fig. 1. General fabrication process flow. (A) Bottom electrode deposition and patterning by lift-off, (B) Sacrificial layer (amorphous silicon) deposition andpatterning by RIE, (C) First SiO2 deposition and patterning, (D) Top electrode deposition (Heater), (E) Second SiO2 deposition, (F) Patterning of the secondSiO2 layer, (G) Relase of the MEMS actuator, and (H) VO2-deposition by pulsed-lased deposition using in-situ shadow mask.

Fig. 2. Resistance as a function of temperature for the VO2 film used. TheVO2 film was a circular pattern of 6 mm in diamter, and the probes were2 mm apart.

electrically isolating the heater from the VO2 film, since theVO2 becomes conductive in the metallic phase.

B. Device Fabrication

Fig. 1 shows a general overview of the fabrication process.The starting substrate consisted on a silicon wafer with a1.2 μm thick layer of silicon dioxide (SiO2) oxide depositedby PECVD. A Ti (500 Å)/Pt(1500 Å) layer was depositedby evaporation and patterned by lift-off. The sacrificial layer(500 nm thick a-Si film) was deposited by LPCVD andpatterned by RIE. A second thin layer of SiO2 (500 nm)was deposited by LPCVD (in order to minimize voids in thecantilever’s structural material) and patterned using plasmaetching. A second metallization Ti(400 Å)/Pt(1500 Å) fol-lowed, this time by sputtering – to increase the film’s confor-mality. This second metal layer was also patterned by usinglift-off, and it formed the integrated resistive heater. The finallayer of SiO2 (500 nm) was deposited again by LPCVD andpatterned using plasma etching. The wafer was then diced andthe structures were released using XeF2.

The final step in the device fabrication (Fig. 1-H) is thedeposition of the VO2 thin film (200 nm thick), which

Fig. 3. SEM image of cantilevers with dimensions (top view).

was done using a shadow mask to guarantee that VO2 wasonly deposited on the area of the chip that contains thesuspended cantilever beams and away from the contact pads.The VO2 thin films were deposited by pulsed laser deposition(PLD) using a KrF excimer laser (LambdaPhysik LPX 200,λ=248 nm) at 10 Hz repetition rate and 350 mJ pulse energyincident on a rotating vanadium target for 30 min. The samplewas kept at 470 °C in a 15 mTorr Oxygen atmosphere. Afterdeposition the sample was annealed for 30 min under the samedeposition conditions. Film quality was verified by measuringthe change in resistance of the VO2 film across the IMT, whichshowed the typical hysteretic behavior of (see Fig. 2).

The finalized device was wire bonded to an IC package fortesting. Fig. 3 shows a scanning electron microscope (SEM)image of the two tested cantilevers with their dimensions.The final thickness of the cantilever is approximately 1 μm.

C. FEM Model

Although the simulation of the structural changes in aVO2 crystal could be addressed by modeling of the latticechanges of VO2 single crystals during the transition, the casefor polycrystalline VO2 is more complicated. PolycrystallineVO2 consists of multiple layers of VO2 crystals of differentsizes, which do not experience perfectly synchronized struc-tural changes with temperature, and which are not perfectly


TABLE I

MATERIAL PROPERTIES USED IN THE FEM MODEL

oriented – the induced strain during the phase change is notperfectly uniaxial. Additionally, the modeling of the shift inVO2’s TTr with stress is no longer possible by using theuniaxial stress obtained from the Clausius–Clapeyron equation– as it was demonstrated for single-crystal VO2 [27].

Therefore, the present work used experimental data togenerate a comprehensive lumped model for VO2-based actu-ators, by fitting a temperature dependent thermal expansioncoefficient for VO2 that emulated the stain induced in themicro-actuator during the phase change. The FEM model wasalso used to simulate the heat losses during actuation pulses,the design of the resistive heater, and the power dissipated inthe heater.

COMSOL Multiphysics was used to create such FEMmodel. The model includes two coupled physics: solidmechanics with thermal expansion, and heat transfer withjoule heating. The dimensions of the simulated structures arethose of the tested devices. There are two main heat lossmechanisms in these cantilevers. The first one is the heat lossdue to thermal conduction to the substrate through the anchor,and the other through convective heat transfer between thestructure and its surrounding. To simulate the anchor losses,the face of the cantilever that is in contact with the anchor wasgiven a constant temperature of 30 °C. The natural convectionboundary condition was approximated by estimating a heattransfer coefficient. This is determined by empirical handbookcorrelations [28], which the software calculates automati-cally. Table I shows the properties of Pt and VO2 used inthe model.

The FEM model simulated the contraction of VO2 duringthe transition and was used to calculate the electric powerdissipated at the Pt heater as a function of temperature.Given that the thermal expansion coefficient of Pt varies lessthan 2% in the temperature range that includes the phasechange in VO2 [29], this value was assumed to be constantin the analysis. The contraction of the VO2 thin film, and thepower dissipated at the Pt heater as a function of temperaturewere modeled by a temperature dependent thermal expansioncoefficient of VO2 (α(T )) and conductivity of Pt (σ(T )),respectively. The following expressions were used:

α (T ) = 6 × 10−6

1 + e−0.3(T−341.15)

−3 × 10−8 (T − 273.15) + 6.9 × 10−6, (1)

Fig. 4. Temperature dependent thermal expansion coefficient of VO2 (plotof (T) in Eq. 1).

σ (T ) = 8.9 × 106

1 + 0.003729(T − 293.15), (2)

where T is temperature in Kelvin. Equation (1) was obtainedfrom a curve fit to experimental measurements, while thenumerator of (2) is the conductivity for Pt at room temperature,and the expression in itself is commonly used for modelingchanges in conductivity as a function of temperature. A plotof equation 1 is shown in Fig. 4. This model was used toestimate the operating current, and the dynamic response ofthe system under various conditions.

D. Heater Design

The heater was designed to localize the power dissipationas much as possible along the cantilever’s length and tomaintain a uniform temperature across its width, to avoidtwisting when heating. A single thin Pt line was chosen asthe heater element, due to its simplicity and conventionality inMEMS processes. The width of this line was 2 μm, which alsowas the minimum feature size of the fabrication process. Theheater was connected to larger electrode pads through widertraces (8.5 μm) in order to concentrate most of the powerdissipation at the heating element region (i.e. at the 2 μm widePt resistor). Another design parameter was the location of theheater that would maximize the deflection of the cantileverwhile minimizing power consumption. As the heater locationapproaches the anchor, the conductive heat loss through theanchor increases, which increases the power necessary to reacha specific temperature. However, the closer to the anchor theVO2 film contracts, the greater the deflection of the cantilever.This indicates that the location of the heater involves a trade-off between power dissipation and cantilever deflection, whichin the present work was optimized. The thickness of the Ptheater also affects the electrical resistance of the heater, andconsequently the power dissipated in the heating element.For the particular device presented here, it was found that aTi/Pt thickness of 200 nm was large enough to avoid filmdiscontinuities and thin enough to maintain the operatingcurrents within the mA range. The final design was obtainedfrom finite element simulations that will be described later.

Fig. 5 shows the final heater design as well as the FEM sim-ulations results for power dissipation and optimum location of


b)

Fig. 5. (a) FEM simulation of the power dissipation in the heater. Scale isin W/m3 (b) simulation of the deflection of the cantilever as the position ofthe heater is varied. Optimum position is near 24%.

the heater. The optimal location of the heater was determinedto be at around 24% of the cantilever length from the anchor.This result also agrees with previous work, where the optimumposition of a heating laser spot was determined, for maximumamplitude [30]. The resistance of the heater was found to varybetween 16.5 � and 20.5 � as the current is increased from0 to 9 mA.

III. RESULTS AND DISCUSSION

A. Static Analysis

Fig. 6 shows the simulated temperature profiles for the300 μm long cantilever under vacuum and air. A more uniformtemperature over the surface of the cantilever is obtainedfor vacuum environment. This means that the difference intemperature between the heater and the rest of the cantilever,in vacuum is smaller than in air, and therefore the heater onlyneeds to reach about half the temperature needed in air toobtain the same deflection. Also, the resistance of the heateris smaller at lower temperatures, which translates to less powerconsumption in vacuum. This was confirmed experimentally asshown below. Fig. 7 shows a SEM picture of both cantileversbefore and after actuation under vacuum. There is an initialdeflection at room temperature, before actuation, due to theresidual stress from the deposition of VO2 which occursat nearly 470 °C. The total deflection for the 300 μm and200 μm cantilevers upon actuation was 68.7 μm and 28.5 μm,respectively.

Fig. 8 shows the deflection as a function of input currentfor both cantilevers. The deflection measurements were takenfrom SEM images as the current to the resistive heater was

Fig. 6. FEM simulation results of the temperature (°C) distribution for the300 μm cantilever in air and vacuum. The anchor is at the left.

increased in increments of 0.4 mA. The electrical signalsfor this experiment were provided by a voltage-to-currentconverter connected to the chip inside the SEM by elec-trical feedthroughs. For better visualization of the dynamicbehavior of the actuators presented here, the authors haveincluded a supplementary color avi file, which contains avideo made from consecutive SEM images of the 200 μm-long VO2-based MEMS actuator reported here during electro-thermal actuation. The video is available to download athttp://ieeexplore.ieee.org.

It can be seen that the simulation correctly predicts thedeflection and general shape of the response, but it is shiftedtowards larger current values, which represents an overestimateof the actual heat loss in the system. The experiment in air iscloser to the simulation results, which shows that the overes-timate is more likely due to the modeling of the anchor lossesthan to the modeling of the heat convection. The FEM modelassumes perfect anchor geometry, which does not exactlyrepresent the real system, since the metal and SiO2 depositionsare not perfectly conformal. This nonconformality results in acantilever structure with a smaller cross-section at the anchor,making the actual cantilever more thermally insulated from thesubstrate than the FEM model. It can also be noticed that, inair, the difference in operating current between the simulationand the real system is less than 10% of the full range for the300 μm-long cantilever; while this difference is about 20% forthe 200 μm-long cantilever (see Fig. 8(a–b)). This indicatesthat the error in FEM model predictions becomes larger asthe heat losses through the anchor become more dominant(the 200 μm-long cantilever has a lower surface-to-anchor arearatio). It is also noted that the displacement curves across thephase-change are steeper for the device operated in vacuummedia. This is because a given increase in current will producea larger increase in temperature for the device in vacuum, sinceconvection losses are almost non-existent.

Another observation can be made in terms of power con-sumption. Taking the average of the simulated change in resis-


Fig. 7. SEM images of (a) the 200 μm long cantilever (highlighted in green)cantilevers and (b) the 300 μm long cantilever (highlighted in red). The topand bottom images show the actuator before and after actuation, respectively.

tance for the Pt resistive heater mentioned earlier (18.5 �), andthe experimental measurements shown in Fig. 8, an estimateof the required power to achieve full-actuation can obtained.These results are summarized in Table II. It is noted thatthe necessary current to achieve full-actuation in air wasapproximately 3 times larger than in vacuum, which meansthat full actuation in air requires approximately 16 times morepower than in vacuum.

B. Frequency Response

The schematic of the set-up used for the frequency responsemeasurements is shown in Fig. 9. The set-up is based on laserdeflection (or light scattering) techniques, similar to those usedin the past [2]. A network analyzer (Agilent 3589A), was con-nected to an adjustable voltage to current converter. A HeNelaser (CW 632 nm) was focused on the cantilever inside a vac-uum chamber. The reflected beam was detected using a siliconphotodiode (thorlabs PDA10A) and a DC-300 MHz amplifier

Fig. 8. Static response and FEM simulation results for the 200 (a) and300 μm-long (b) cantilevers in vacuum and in air.

Fig. 9. Frequency measurement set-up system used for frequencymeasurements.

(Stanford Research Systesm model SR445). The vibration ofthe cantilever generates oscillatory movements of the reflectedlaser beam, which the photodiode converts to an oscillatoryvoltage signal detected by the network analyzer for determinngthe frequency. A CCD camera was used for facilitiating laseralignment prior to the measurements. The cantilevers wereoperated using a positive sinusoidal current with adjustedamplitude and offset values to guarantee that the cantileversremained inside the SPT. This was monitored through anoscilloscope (Tektronix TDS2004C) connected directly to thephotodiode.

During the frequency response measurements, it was foundthat the He-Ne laser used for detection heated the cantilever


TABLE II

ESTIMATED POWER CONSUMPTION

structure due to optical absorption. Since the cantilevers arethermally isolated from the substrate, this power absorptioninduced an increase in temperature that was added to thedesired change in temperature due to the resistive heater.Therefore, care was taken to ensure that the laser spot wasalways placed close to the resistive heater, and a neutraldensity filter was placed between the He-Ne laser and thefirst beam splitter, so that the minimum amount of intensitynecessary that allowed vibration detection from the reflectedlaser beam was used. It is reasonable to assume that the delaybetween the mechanical response and the thermal responseis negligible since the air damping effects don’t becomesignificant until the cantilever is actuated at higher frequenciesand the SPT of VO2 is much faster [31]. Therefore, the mostimportant effect to consider in electro-thermally driven VO2actuators is the dynamic response of the heat transfer in thecantilever. The model that was used to analyze the frequencyresponse of the cantilevers includes heat transfer and jouleheating mechanisms.

Fig. 10 shows the frequency response of the cantilevers.These results show the cut-off frequency as defined by a 3 dBdrop in amplitude (half power) with respect to the maximumamplitude at low frequencies. The frequency range was chosento include the first resonant mode of the cantilever, which isshown as a peak in the response close to 10 kHz. A lowerquality factor is observed for the measurements done in air,which suggests that air damping dominates the energy losses,as expected – air damping.

The cut-off frequency increases when the cantilever isexposed to air, which suggests that the limiting factor whenit comes to half-power frequency response is dominated byheat convection losses. For the two cantilevers in vacuum,the difference in heat dissipation by conduction through thecantilever’s anchor is minimal, since the anchor area is thesame for both cantilevers. This is shown by very similarcut-off frequencies in vacuum where the difference is only9%. In air, however, there is a larger difference in cut-off frequency (about 25%), which is also supportive of thesuggestion that heat convection losses dominate the cut-offfrequency, since both cantilevers have different surface areas.This indicates that when operating in vacuum or very ther-mally isolated conditions, the size of the cantilever affectsvery little the cut-off frequency of the cantilevers as longas you are operating the cantilever far below the resonantfrequency.

IV. CONCLUSION

VO2 thin films have been successfully integrated in anintegrated MEMS device. The effects of the SPT in VO2 thin

Fig. 10. Frequency response curves for the 200 μm (a) and 300 μm (b)cantilevers in vacuum and in air.

films when induced by joule heating on coated suspendedmicromechanical actuators has been studied and characterized.The necessary current for maximum deflection was found to bevery dependent on the ambient conditions particularly externalatmospheric pressure. Power consumption needed for inducingthe SPT in VO2 and causing complete actuation droppedsignificantly when operated in vacuum. A finite element modelfor the system was developed and validated. The error ofthe model was found to increase as the heat losses throughthe anchor became more dominant. The frequency responseof the cantilevers was also characterized. It was found thatthe cut-off frequency was very dependent on the surface areaof the cantilever when operated in air, but far less dependentwhen operated in vacuum.

ACKNOWLEDGMENT

The authors would like to thank Prof. T. Hogan for facilitat-ing the pulsed laser deposition system used for the VO2 thinfilm deposition used in this work. This work was performedin part at the Lurie Nanofabrication Facility, a member ofthe National Nanotechnology Infrastructure Network, which


is supported in part by the National Science Foundation.The SEM images were taken at the Composite Materials andStructures Center at Michigan State University.

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Rafmag Cabrera received the B.S. and M.S.degrees in Electrical and Computer Engineeringfrom the University of Puerto Rico at Mayaguezin 2009 and 2011, respectively, and is expectedto receive the Ph.D. degree in Electrical Engineer-ing from Michigan State University, East Lans-ing, MI, USA, in May 2014. He has participatedin a number of summer research programs withvarious institutions including; Cornell Center forMaterial Research and the Joint Institute for Labo-ratory Astrophysics, Boulder, CO, USA. His current

research interests include smart materials and the integration of such in micro-electromechanical systems (MEMS) with particular emphasis on vanadiumdioxide (VO2) thin films and the use of the structural phase transition for thedevelopment of MEMS-Memories.


Emmanuelle Merced received the B.Sc. and M.Sc.degrees in electrical engineering from the Universityof Puerto Rico, Mayaguez, Puerto Rico, in 2009 and2011, respectively. He is currently working towardthe Ph.D. degree in the Department of Electricaland Computer Engineering, Michigan State Univer-sity, East Lansing, MI, USA. His current researchinterests include design, fabrication, and implemen-tation of microelectromechanical actuators, smartmaterials-based microtransducers, control of hys-teretic systems, and tunable microresonators. Mr.

Merced was awarded the National Science Foundation Graduate ResearchFellowship in 2011. He was also named Outstanding Graduate Student for2012–2013.

Nelson Sepúlveda (S’05-M’06-SM’11) received theB.S. degree in Electrical and Computer Engineer-ing from the University of Puerto Rico, Mayaguez,Puerto Rico, in 2001 and the M.S. and Ph.D. degreesin Electrical and Computer Engineering from Michi-gan State University, East Lansing, MI, USA, in2002 and 2005, respectively.

During the last year of Graduate School, heattended Sandia National Laboratories as part of afellowship from the Microsystems and EngineeringSciences Applications program. In January 2006, he

joined the Electrical and Computer Engineering faculty at the Universityof Puerto Rico, Mayaguez, Puerto Rico. He has been a Visiting FacultyResearcher at the Air Force Research Laboratories since 2006, 2007, and2013, the National Nanotechnology Infrastructure Network, in 2008, and theCornell Center for Materials Research, in 2009, the last two being NSF-fundedcenters at Cornell University, Ithaca, NY, USA. In 2011, Nelson joined thefaculty of the Department of Electrical and Computer Engineering at MichiganState University (MSU), where he is currently an Assistant Professor. Hiscurrent research interests include smart materials and the integration of suchin microelectromechanical systems (MEMS), with particular emphasis onvanadium dioxide (VO2) thin films, and the use of the structural phasetransition for the development of smart microactuators.

Nelson received the NSF CAREER Award in 2010.

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