Proceedings of the 1st IEEE InternationalConference on Nano/Micro Engineered and Molecular Systems

January 18 - 21, 2006, Zhuhai, China

Design of a Hybrid Micro/Nano Cantilever-based ResonantSensor and its Fabrication Method

Rong Zhu'*, Dingqu Wang', Zhaoying Zhoul, Xiongying Ye''State Key Laboratory ofPrecision Measurement Technology and Instruments, Department ofPrecision Instruments and

Mechanology, Tsinghua University, Beijing, China

Abstract-Due to novel mechanical and electrical properties ofnanomaterials, such as carbon nanotubes and semiconductingoxide nanobelts, utilizing nanomaterials in microelectromechanical structure to improve performances of sensorshas attracted many interests. In this paper, we report a novelmicro/nano cantilever structure applying to sense accelerationsbased on resonant detection. The structure is comprised of aseismic micromachined inertial mass supported by two parallelZnO nanobelts as doubly-clamped nano-cantilevers suspendingbetween two metal electrodes located over a micromachinedtrench. Another bottom electrode located underneath the mass isutilized to apply electrostatic force on the mass. The accelerationnormal to the mass plane induces electrostatic stiffness changeand, in turn, a change in the resonant frequency. We proposedesign and an effective fabrication for this sensor structure.

Keywords-nanoelectromechanical systems(NEMS);resonantSensor; acceleration sensor

I. INTRODUCTION

Nanoelectromechanical systems (NEMS) are characterizedby small dimensions, where the dimensions are relevant for thefunction of the sensors. Critical feature sizes ofNEMS are fromhundreds to a few nanometers. New physical properties,resulting from the small dimensions, may dominate theoperation of the sensors, and new fabrication approaches maybe required to make them.

Cantilever based sensors have become a standard on micro-electromechanical systems (MEMS) for detection of a widerange of magnitudes with resolution in the pico-scale. It hasbeen also demonstrated that an improvement on sensitivity,power consumption and spatial resolution can be achieved byreducing the cantilever dimensions to the nanometer range [1].Accordingly Nano-cantilever-based NEMS have attracted greatinterests for high-precision measurements of forces anddisplacements [2]. In this scale, electrostatic excitation andcapacitive detection arises as the most simple and easilyscalable combination. This combination is based on excitingelectrostatically the oscillation of the cantilever by means of aclosely placed driver electrode and detecting the displacementresonant current through the cantilever.

This paper reports on a new hybrid micro/nanoelectromechanical sensor. The sensor is used for accelerationmeasurements based on resonant principle, which incorporates asuspended micromachined proof mass with doubly-clampednano-cantilevers that serve as key motion enabling anddetecting elements. The nano-cantilevers are nanomaterialssynthesized by such as chemical vapor deposition. Novel

performances are expected from this hybrid structure due to theexcellent mechanical and electrical properties of appliednanomaterials. The paper presents the design and fabricationprocess of this sensor.

II. DESCRIPTION AND DESIGN OF SENSORA resonant sensor has many advantages over conventional

type; especially, the wide dynamic range, high sensitivity aswell as the quasi-digital nature of the output signal and theinherent continuous self-test capability. For accelerometer,conventional resonant sensor utilizes an individual resonator(usually a thin double ended tuning fork structure) connectedto an inertial mass vibrates laterally at its resonant frequency.This conventional resonant sensor has feature of force sensing,so it is prone to be affected by an internal residual stress,especially for the case of the polycrystalline silicon vibratingstructure. Recently, a new resonant accelerometer, whichutilizes the electrostatic stiffness changing effect of a parallelplate capacitive actuator, has been developed using the surfacemicromachining process [3]. This accelerometer hasadvantage of electrical tunability, so enhanced sensitivity canbe obtained simply by tuning the bias voltage at the final stageand this is a very useful method for removing the effect of theresidual error.

On the other hand, many one dimensional nanomaterials,such as nanotubes, nanowires, and nanobelts, have attractedextensive interest. These nanomaterials have multiple novelmechanical and electrical properties so that can act as goodcandidates for cantilevers. Amongest them, semiconductingoxide nanobelts, such as ZnO and SnO2, are ultralong beltlike(or ribbonlike) nanostructures (so-called nanobelts)synthesized by vapor phase evaporation. Compared withcarbon nanotubes, as-synthesized oxide nanobelts are pure,structurally uniform, and single crystalline, and most of themare free from defects and dislocations [4]. They have arectanglelike cross section with typical widths of 30 to 300nanometers, width-to-thickness ratios of 5 to 10, and lengthsof up to a few millimeters. The beltlike morphology appears tobe a distinctive and common structural characteristic for thefamily of semiconducting oxides with cations of differentvalence states and materials of distinct crystallographicstructures. Equally important, nanobelts can act as a transistoror piezoelectricity transducer and thus can be used to sensemotion [5][6]. In this paper, we will use ZnO nanobelts assupporting cantilevers of the resonant accelerometer.

A schematic view of the sensor structure is shown in Fig.1. The working principle of this sensor is based on the

The work wasfunded by National Natural Science Foundation ofChina(Project code. 50575113)

*Contact author. Rong Zhu is with Department ofPrecision Instrumentsand Mechanology, Tsinghua University, Beijing 100084, China (phone. +8610 62793166; fax. +86 10 62771478; email.zrwzwyj@sh163.net).

1-4244-0140-2/06/$20.00 C)2006 IEEE 766

electrostatic stiffness changing effect. Two ZnO nanobelts(typically 100-300nm wide, 10-50nm thick) acting as doublyclamped cantilevers support a suspended micromachinedpolysilicon proof mass (8ptm wide, 30pm long, 2pm thick).The nanobelts reside on two metal (Cr/Au) electrodes overtwo sides of a micromachined trench (16ptm wide, 4ptm deep).The vibration of the mass is excited and controlledelectrostatically by applying a large dc voltage bias (tuningvoltage) and a small ac voltage from the gate electrodeunderneath the mass and detected by resonant current inducedfrom the transistor property of semiconducting nanobelts. Theacceleration change acting on the proof mass will induce theoscillation frequency shift at resonance of suspensionnanobelts, in such a way the value of the acceleration can bedetected from the shift in the resonant frequency of thenanobelts.

Fig. 2 shows the working principle of the sensor. Thesensor shown in Fig. 1 and 2 can be characterized bygeometrical, material and electrical parameters. Geometricalfactors are cantilever length (21), width (nw), thickness (nh),volume of the inertial mass (v) and initial gap distancebetween mass and gate ( do ). Material properties aredetermined by cantilever Young modulus (E) and massdensity (p). Electrical parameters are the driving ac voltage Va,the driving dc voltage bias Vd6 the permittivity constant & of themedium (air, vacuum), and the driving electrode area A. Thedynamic model of the sensor can be expressed as:

my+cy+ky = ma+FI (1)

where a is the acceleration applied on the mass, m is weight ofthe polysilicon inertial mass: m = v* p, the parameter c is thedamping coefficient related with environmental pressure andgap distance, k is the equivalent spring stiffness of supporting

cantilevers: k =4

, and I is the cantilever momentum of

inertia: I n (nh)= . The electrostatic force is calculated as:12

C'V2 E*V*(Va)22 2(do - y)2

(2)

Assuming V/ << Vd and y << do . Equation (1) can berewritten as:

j )j '(A2=AVd Vi (3)

where Yi is the alternating displacement of inertial massaround the equilibrium position, Ya is the variation of theequilibrium gap from the initial position do induced by theinertial force ma and the constant electrostatic force.Neglecting the higher-order perturbation term, Ya can becalculated as:

Ya=[ma+ 2d,2 ]I[k- d (4)

rl Feedback

rlMton detection Mto rv

Electrode Proof mass Electrode\ / ~~~~~~~~El(ctrode= =N=. ==../..............................:.

f

Poly Si

P-doped Si

InsulatinglayerPSG

Metal Cr/Au

- Nanobelts

Figure 1. A schematic view of the sensor structure

From (3) and (4), we can see that the external accelerationa changes the equilibrium gap Ya between the inertial massand the bottom gate electrode. This gap variation Ya changes

the total effective stiffness keff = [k- '*A2] of the

(do Ya)3resonator, so does the resultant resonant frequency. Theresonant frequency is modeled as:

f I2 [k-A

]/m (5)

It indicates that the bias voltage Vd is correlative with thefrequency shift, thus can be used to tune the sensitivity of thesensor at the final stage.

The resonant amplitude of the mass oscillation can becalculated by:

z= c,.A.Vd -Yap2)zcf.c.(d0 Ya)2

And the quality factor of the sensor can be evaluated by thefollowing formula:

i ' ~~~~~~Vd+Vado±Y Y

Figure 2. A simplified sensor structure and the working principle illustration

Q =m.[k- d l]C

According to above modeling and analysis, we supposeddual 100-nm-wide, 40-nm-thick, and 20-ptm-long nanobelts as

767

(6)

(7)

supporting cantilevers (effective cantilever length is 8pim) ofthe sensor. Preliminary calculations were executed forevaluating the sensor performances. The volume of thepolysilicon proof mass is calculated to be 4.8 X 10-16 m3 andthe weight is 1.2 X 10-6 micrograms. The initial gap betweenthe mass and bottom electrode is 2ptm. The driving dc voltagebias is set to be 5.2Volt and the driving electrode area is2.4 X 10-" M2. We simulated the dependence of resonantfrequency shifts on applied accelerations, shown in Fig. 3. Theacceleration sensitivity of the sensor was estimated to be about100 Hz/g. The quality factor Q was estimated to be about2 X 104 at a pressure of 75 mtorr.

III. FABRICATION OF SENSOR STRUCTURENanoscale fabrication mainly consists of two major

approaches: top-down high-resolution and bottom-up directedassembling processes. The top-down approach evolved fromclassical lithography and fabrication techniques. The approachtakes a bulk material and modifies or breaks it into smallerdesired shapes. The approach becomes possible through theincreasing precision of tools that have been largely developedwith the progress of microelectronics and enable to fabricateartificial functional objects with sizes down to the scale ofnanometers. On the other hand, the bottom-up approachinvolves synthetic methods of chemistry and biochemistry,such as molecular recognition and self assembly. Amongstseveral bottom-up approaches for nanoscale fabrication andcomponent integration, the force field directed self-assemblyare under investigation for site-specific localization,positioning and integration of nano-scale inorganic materials.The externally controlled force fields include fluidic, magneticor electric. In commercialization, the top-down approaches arerather mature and will continue to encounter many thechallenges as they approach their fundamental size limits,while the bottom-up approaches are in their infant stage and"there's plenty ofroom at the bottom" as mentioned by RichardP. Feyman in 1959. In this paper, we present a hybridmicro/nano cantilever structure fabricated by combination ofthe top-down micromaching techniques and the bottom-upforce field directed assembly using synthesized nanomaterials.

4.06

4.04

Ijc 4.02

x 3 980

q 3 96

3 94

>U

a 3.92

0

-it 3.9

3.88

5 0 5Applied Acceleration (mI/2)

10

Figure 3. Simulated resonant frequency of the structure versus appliedacceleration

In practical fabrication, it is necessary to take account ofdecreasing technical complexity and try to use standardmicromachining techniques. Consequently the fabricationprocess for the sensor structure was designed to separate intotwo stages. The first stage is to fabricate the microelectromechanical structure including proof mass andindividual electrodes using the standard micromachiningprocess. The second stage is used for assemblingnanomaterials with the pre-fabricated structure to form thefinal sensor structure. The entire fabrication process is shownin Fig. 4. Fig.4 (a) - (e) illustrate the first stage, and Fig. 4 (f)and (g) exhibit the second stage.

The first stage process begins with phosphorus diffusioninto an n-type silicon wafer. This P-doped silicon substrate isused for the bottom electrode of the structure. A 40nm thermaloxide layer and a 150 nm thick LPCVD silicon nitride (Si3N4)layer are deposited and patterned on the substrate (Fig. 4(a)).Then a 2ptm thick PSG sacrificial layer is deposited andpatterned (Fig. 4(b)). A 2ptm thick polysilicon layer issubsequently deposited on the wafer and heavily phosphorus-doped. Then a 4/80 nm thick Cr/Au film used for electricalcontacts and electrodes is sputtered (Fig. 4(c)). The Cr/Au filmis patterned by photolithography and wet-etching, and then theexposed polysilicon is etched by ICP to define the proof mass(Fig. 4(d)). Finally the sacrificial layer is etched to releases theproof mass (Fig. 4(e)). At this moment, the microelectromechanical structure has been constructed as shown inFig. 4(h).

In the second fabrication stage, an assembling process isperformed to integrate the above micro structure with pre-synthesized nanobelts. The nanobelts are firstly dispersed inethanol solution and ultrasonicated. The nanobelt suspension isthen dripped directly on the wafer with the above microstructure. An electric field-directed assembly methoddielectrophoresis [7] is subsequently applied to align and trapthe nanobelts onto electrodes on both sides of the trench (Fig.4(f) and (i)). In assembling experiments, we observed thatincreasing the applied voltage on electrode pair could help toincrease the quantity of the assembled nanobelts, whileincreasing the frequency of ac applied to electrodes couldimprove orientational uniformity of assembled nanobelts. Wealso found that the desirable frequency and peak-to-peakamplitude of the ac voltage applied to the electrodes fortrapping the nanobelts are 10-50 MHz and 5-20 V,respectively. Focus ion beam (FIB) is subsequently applied todeposit thin Pt coating on the contacts between the nanobeltsand electrodes as well as the contacts between the nanobeltsand mass for enhancing the contact adhesion (Fig. 4(g) and (j)).Finally the polysilicon beams, which previously support themass shown in Fig. 4(h), are etched off by using FIB so as tofree the proof mass (Fig. 4(k)). The final structure is supposedthat the inertial mass is supported by doubly clamped nanobelts.SEM imagines of a preliminary fabricated sensor structure areshown in Fig. 5.

IV. CONCLUSIONSA novel hybrid micro/nano cantilever-based resonant

acceleration sensor is introduced in the paper. The workingprinciple of the sensor is based on the electrostatic stiffness

768

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shift of supporting cantilevers induced from the accelerationapplied on a micromachined inertial mass. This accelerometerhas advantage of electrical tunability, specifically enhancedsensitivity can be obtained simply by tuning the bias voltage atthe final stage and this is a very useful method for removingthe effect of the residual error. Semiconducting oxide nanobeltsare used as the cantilevers in the structure, which serve as keymotion enabling and detecting elements. The design andfabrication of this sensor are presented in the paper and thepreliminary results of the sensor's performance are given aswell.

ACKNOWLEDGMENT

The authors thank to Prof. Zhonglin Wang in GeorgiaInstitute of Technology of USA providing samples of ZnOnanobelts for this work.

(a) (b)

ZnONanobelt Mass

Z Si3N4\ SiO2

P-doped Si

(c)-

Cr/Au

-PolySi Figure 5. SEM images ofthe sensor structure. (a) SEM image of fabricatedMEMS structure. (b) SEM image ofZnO nanobelts. (c) SEM image of

assembling nanobelts with MEMS structure

AssembledZnO nanobelt

- FIB| deposited Pt

REFERENCES

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[4] Z. W. Pan, Z. R. Dai, and Z. L. Wang, "Nanobelts of SemiconductingOxides", Science, 291(2001), pp. 1947-1949.

[5] M. S. Arnold, P. Avouris, Z. W. Pan, and Z. L. Wang, "Field-EffectTransistors Based on Single Semiconducting Oxide Nanobelts", J. Phys.Chem. B, 107(2003), pp. 659-663.

[6] M. H. Zhao, Z. L. Wang, and S. X. Mao, "Piezoelectric Characterizationof Individual Zinc Oxide Nanobelt Probed by Piezoresponse ForceMicroscope", Nano Letters, 4(2004), pp. 587-590.

[7] C. Yu, Q. Hao, and S. Saha, "Integration of Metal Oxide Nanobelts withMicrosystems for Sensor Applications", Applied Physics Letters, 86(2005), pp. 063101-063103.

Figure 4. The sensor fabrication process flow.

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