440 IEEE SENSORS JOURNAL, VOL. 13, NO. 2, FEBRUARY 2013

Cantilever Resonator With Integrated Actuation and SensingFabricated Using a Single Step Lithography

S. M. Mohanasundaram, Rudra Pratap, and Arindam Ghosh

Abstract— Micro- and nano-mechanical resonators have beenproposed for a variety of applications ranging from mass sensingto signal processing. Often their actuation and/or detectioninvolve external subsystems that are much larger than theresonator itself. We have designed a simple microcantileverresonator with integrated sensor and actuator, facilitating theintegration of large arrays of resonators. This unique design canbe manufactured with a low-cost fabrication process, involvingjust a single step of lithography. The bilayer cantilever of goldand silicon dioxide is used as piezoresistive sensor as well asthermal bimorph actuator. The ac current used for actuation andthe dc current used for piezoresistive detection are separated inthe frequency-domain using a bias-tee circuit configuration. Theresonant response is measured by detecting the second harmonicof the actuation current using a lock-in amplifier.

Index Terms— Microelectromechanical devices, microres-onators, nanotechnology, piezoresistance.

I. INTRODUCTION

CANTILEVERS are demonstrably one of the mostversatile sensing elements in nanotechnology. Cantilever

based sensors are being developed for myriad of applications,including scanning probe microscopy, biomolecular sensing,high density data storage, mass spectroscopy, etc. In many ofthese applications, the cantilevers are operated in resonanceto exploit the high sensitivity of these oscillators to externalstimuli. Nanomechanical resonators operating at very highfrequencies have spawned a new class of high performancesensors [1].

Traditionally, cantilever motion has been detected usingoptical techniques such as laser beam deflection and interfer-ometry. Since these off-chip readout methods require precisealignment with rather bulky instrumentation, self-sensing can-tilevers gained importance from a technological standpoint.Self-sensing cantilevers mainly use piezoresistive or piezo-electric transduction. Such resonators are typically actuatedusing external electric or magnetic fields apart from beingmounted on a platform that can be vibrated at the requiredfrequencies. It is highly desirable to have both actuation and

Manuscript received June 6, 2012; revised October 9, 2012; acceptedOctober 11, 2012. Date of publication October 16, 2012; date of currentversion January 11, 2013. This work was supported by the Departmentof Science and Technology, Government of India. The associate editorcoordinating the review of this letter and approving it for publication wasProf. Sang-Seok Lee.

S. M. Mohanasundaram and A. Ghosh are with the Department of Physics,Indian Institute of Science, Bangalore 560012, India (e-mail: contactmohan@gmail.com; arindam@physics.iisc.ernet.in).

R. Pratap is with the Center for Nano Science and Engineering,Indian Institute of Science, Bangalore 560012, India (e-mail: pratap@mecheng.iisc.ernet.in).

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

Digital Object Identifier 10.1109/JSEN.2012.2225045

sensing mechanism integrated into the cantilever, because thatwill open up a whole new range of applications, where no con-straints are placed on the environment in which the sensor canoperate. Self-sensing and self-actuating cantilevers also enablethe possibility of integrating a large array of these sensors inmany applications that can benefit from multiplexing.

Cantilevers with integrated actuation and sensing have beenrealized using piezoelectric materials such as PZT (LeadZirconate Titanate) [2]. But such functional materials are notcompatible with standard MEMS (Micro Electro MechanicalSystem) or CMOS (Complementary Metal Oxide Semiconduc-tor) foundry. An alternative is to integrate thermal actuatorsonto piezoresistive cantilevers [3]. Silicon microcantileverswith thermal actuators and doped-silicon piezoresistors havebeen successfully demonstrated in applications [4], [5]. Butthese devices are fabricated using a long sequence of sophisti-cated processing steps such as ion implantation and involvingup to six lithography steps [6], which makes these devicesquite expensive to manufacture. Pourkamali, et. al. have devel-oped a silicon resonator [7] that is fabricated with a single steplithography. The realized device is an in-plane resonator witha single structure that is both an actuator and a sensor.

In this letter, we present a cantilever design that vibratesout-of-plane in the flexural mode and can be fabricated with asingle step lithography. Unlike the cantilever designs reportedso far, where the actuator and sensor are electrically isolatedby physical separation, the actuator and sensor in our designare electrically connected, yet are operated independently.We achieve this by separating the electrical signals used foractuation and sensing in the frequency domain with the helpof the measurement circuit that we have developed to detectthe cantilever oscillation.

The simplicity of fabrication originates from our choiceto use a simple metal (gold) as the active material insteadof the commonly used doped silicon. A thin layer of goldfilm deposited on to an insulating structural layer can actas a piezoresistive sensor as well as a thermal bimorphactuator. In fact, a metal film piezoresistor can provide muchbetter signal-to-noise ratio than semiconductor piezoresistor atnanoscale [1] due to low resistivity and high carrier densityof the former. Taking full advantage of this fact, our design,which includes an actuator as well, is scalable to much lowerdimensions than what we present here.

II. EXPERIMENTA. Device Fabrication

We start with a silicon (100) wafer and grow the silicondioxide structural layer of required thickness (300 nm for thedevice shown) using thermal wet oxidation method at 1100 °C.The cantilever is patterned using electron-beam lithography

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MOHANASUNDARAM et al.: CANTILEVER RESONATOR WITH INTEGRATED ACTUATOR AND SENSOR 441

Fig. 1. Fabrication process flow.

and SiO2 etching in buffered hydrofluoric acid. The SiO2structure is released by silicon bulk micromachining usingtetramethyl ammonium hydroxide. Subsequently, a 50 nm goldfilm with a 15 nm titanium adhesion layer is deposited bythermal evaporation. The fabrication process steps are picto-rially represented in Fig. 1. Notice that there is always someundercut after bulk micromachining which ensures the goldfilm in the trench is disconnected from the device and contacts.

B. Cantilever Design

The cantilever consists of a suspended body and three legseach of which is anchored to the substrate. The middle leg ismeant to be the sensing leg with a piezoresistive strain gauge.Of the outer legs, one is used for sourcing the actuation currentand the other is a common ground for both actuation andsensing currents. There is a notch in the middle leg, that servestwo purposes. First, it serves as a stress concentrator when thecantilever oscillates. Second, it increases the resistance of themiddle leg relative to the other legs, thus increasing the voltagedrop across the same due to the bias current. It is fair to saythat it the notch region that primarily acts as the strain gauge.An SEM (Scanning Electron Microscope) image of one of thecantilevers just after fabrication is shown in Fig. 2(b).

C. Measurement

A schematic diagram of the measurement setup is shown inFig. 2(a). The circuit primarily comprises a bias-tee configu-ration in which an inductor is used to block ac current and acapacitor is used to block dc current. We use an ac voltagesource at frequency f/2 to oscillate the cantilever at f asthermal actuation (due to Joule heating) doubles the frequency.This ac actuation current is prevented from flowing into themiddle leg (strain gauge) by the inductor L. So the actuationcurrent is restricted to flow along the outer legs as representedby a dashed arrow in Fig. 2(a).

The cantilever oscillations generate a sinusoidal strain, thusresulting in a sinusoidal change in the resistance of the straingauge. To measure this resistance change a dc bias currentis passed through the inductor to the common ground asrepresented by a solid arrow in Fig. 2(a). The dc bias current isprevented from flowing into the actuation source by the capaci-tor C . Due to the bias current a sinusoidal voltage at frequencyf develops at the contact of the middle leg. This signal is mea-sured by phase sensitive detection using a lock-in amplifier.

The actuation frequency is varied across the fundamentalresonance frequency to obtain the frequency response shownin Fig. 2(c). The circuit parameters used in the measurement

(a)

(b)

(c) (d)

Fig. 2. (a) Schematic diagram of the device structure along with themeasurement setup. (b) SEM image of a fabricated device. (c) Piezoresistivelydetected frequency response of the cantilever. (d) Frequency response aftersubtracting the background.

are: C = 86 nF, Ra = Rb = 1 k�, L = 0.93 mH,Vac = 1 Vrms , Vbias = 2 V and the strain gauge resistance is17 �. This measurement is done under ambient conditionsand quality factor (Q) of the resonator is ∼ 22. Q canbe improved by appropriate engineering including vacuumpackaging, increasing the SiO2 thickness or by using a betterstructural material such as silicon nitride, silicon carbide, etc.

The change in output voltage �V0 plotted as a function ofactuation frequency [Fig. 2(d)] is obtained after subtracting apolynomial-fit background curve [dashed curve in Fig. 2(c)].The background voltage is due to interference that originatesmainly from the thermal cross-talk. Thermal cross-talk isthe signal generated due to the temperature oscillations inthe grounded outer leg which is in series with the straingauge. Harmonic distortion from the actuation source can alsocontribute to the background voltage. The background voltagecan be easily distinguished and eliminated (as we have done)because it varies much more slowly with frequency. Thus, theresonator fabricated using the simple process described in thisletter incorporates both actuation and sensing successfully.

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