characterization and reduction of mems sidewall friction using novel microtribometer and localized...

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 20, NO. 4, AUGUST 2011 991

Characterization and Reduction of MEMS SidewallFriction Using Novel Microtribometer and

Localized Lubrication MethodHongbin Yu, Guangya Zhou, Sujeet K. Sinha, Jonathan Y. Leong, and Fook Siong Chau

Abstract—A novel microtribometer is developed to characterizemicroelectromechanical systems (MEMS) sidewall friction withhigh resolution. The design is based on a rotational grating dis-placement sensing mechanism, with which 1.2-nm sensing sensi-tivity can be achieved. Employing it, the adhesion force (1.85 μN)and the coefficients of static (0.801) and kinetic (0.363) frictionson the sidewall of an as-fabricated MEMS device have beenmeasured. Besides these, the whole process of stick–slip associ-ated with the movement under friction, including the transitionbetween static and kinetic frictional states, has also been clearlyrevealed. To reduce friction, a localized lubrication method isdeveloped, with which liquid lubricant can be applied directlyonto the desired region without affecting other components onthe same device. From the experimental results, reduced valuesin adhesion force (1.23 μN) and coefficients of friction (0.262 forstatic and 0.183 for kinetic) are obtained in the same MEMS deviceafter lubrication treatment, demonstrating improved frictionalperformance. [2010-0234]

Index Terms—Adhesion force, coefficient of friction, microtri-bometer, position-sensitive device (PSD), rotational grating, side-wall friction.

I. INTRODUCTION

M ICROELECTROMECHANICAL systems (MEMS) isa technology that simultaneously integrates mechani-

cal and electronic components over a small region to realizeparticular functions such as parameter sensing and active per-formance control. Using well-developed fabrication processesand facilities arising from the microelectronics industry, MEMSdevices can be mass produced, thus demonstrating low-costpotential (the same as IC). At the same time, the compact devicevolume, low power consumption, and performances that arecomparable to conventional counterparts make MEMS technol-ogy very attractive for many applications [1]. A large numberof devices have been successfully developed using MEMS tech-

Manuscript received August 2, 2010; revised April 14, 2011; acceptedMay 6, 2011. Date of publication July 7, 2011; date of current versionAugust 3, 2011. This work was supported by the National Research Foun-dation (NRF), Singapore, under Award NRF-CRP 2-2007-04. Subject EditorD.-I. Cho.

H. Yu, G. Zhou, and F. S. Chau are with the Micro/Nano Systems Initiative,Department of Mechanical Engineering, National University of Singapore,Singapore 117576.

S. K. Sinha and J. Y. Leong are with the Material Laboratory, Department ofMechanical Engineering, National University of Singapore, Singapore 117576(e-mail: [email protected])

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

Digital Object Identifier 10.1109/JMEMS.2011.2159094

nology, e.g., a digital micromirror device invented by Texas In-struments has achieved huge commercial success and is widelyused in applications such as projector, HDTV, and masklesslithography [2], [3]. Meanwhile, MEMS-based accelerometershave been commonly adopted in airbag system by the automo-tive industry [4]. Other MEMS devices include pressure sensors[5], RF switches [6], optical attenuators [7], and gyroscopes [8],and all have also obtained widespread applications.

Since the structural dimensions involved in MEMS devicesfall into the micro or even smaller region, the associatedsurface-to-volume ratio becomes large, resulting in a well-known phenomenon called as the scaling effect [9]. As a result,MEMS devices behave differently from those of their macrocounterparts in many aspects. For example, gravity, which playsan important role in the macroworld, is overwhelmed by surfaceadhesion, stiction, and capillary forces in the microscale. Atthe same time, material properties such as Young’s modulusand fracture strength also exhibit different values. Againstthis background, more works have already been performed instudying adhesion/stiction, as well as friction at microscale. Thelatter has attracted more interest in recent years due to the factthat it is one of the most important factors that can cause failureof MEMS devices [10]–[16]. Besides the effort in character-izing the adhesion/stiction and friction themselves, in order toimprove the reliability of MEMS device, studies on reducingthese values have also obtained much interest. Until now, someuseful methods and results have been successfully reported.Ashurst et al. [17]–[19] from UC Berkley and Man et al.[20] focused on using antistiction monolayer coatings to im-prove MEMS reliability. Yee et al. [21] and Alley et al. [22]developed a surface roughness modification technique to re-duce sticking of microstructures. Meanwhile, researchers fromSandia National Laboratories have also performed extensivetheoretical and experimental works on relative area [23]–[27].

In this paper, a novel microtribometer using a new displace-ment detection method with high sensitivity is first presented.Different from the widely reported AFM-based method, itsworking principle is based on an integrated rotational gratingmechanism, with which the lateral displacement of interest canbe transformed into in-plane grating rotation. During the test,a laser beam is made incident onto the grating, and one ofthe diffraction orders is collected by a position-sensitive device(PSD). Any rotation of the grating causes the transmissiondirection of diffraction to change accordingly, moving its lightspot on the PSD plane. Considering the fact that the grating

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992 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 20, NO. 4, AUGUST 2011

Fig. 1. Schematic of the microtribometer.

rotation is directly dependent on the lateral displacement, thevariation of displacement under the effect of sidewall frictionappearing in different working conditions can be measuredin real time from reading the output of the PSD. Therefore,information about friction on the sidewall of MEMS devices,such as friction transition between the static and kinetic statesas well as the coefficients of static and kinetic frictions, canbe obtained eventually. Besides successful characterization ofsidewall friction, this paper also presents the development ofa localized lubrication method, through which a widely usedlubricant can be directly applied onto the desired sidewallregions of a MEMS device while leaving other regions (suchas pad for wiring) on the same device chip area unaffected bythe lubricant. At the same time, the effect of the lubricant layeron the friction performance is evaluated using the currentlydesigned microtribometer.

II. DEVICE STRUCTURE DESIGN

The proposed microtribometer consists of two components,namely, sliding and contact components, as shown in Fig. 1.During operation, the sliding component is dynamically actu-ated to move back and forth in-plane, providing the requiredrelative movement for the friction test, while the other, which isarranged in the orthogonal direction, is used to achieve sidewallcontact. At the same time, it controls the contact force as wellas the resultant friction. In the current design, all of the movablecomponents are actuated by electrostatic comb-drive actuators.It is well known that, given the same actuation condition of thesliding component, its displacement amplitude will be affectedby the level of friction—the larger the friction is, the smalleris the amplitude. As a result, the friction can be measured bydetecting the variation of amplitude.

It is obvious that the most important factor to consider is thein-plane displacement detection since its resolution will directlydetermine the measurement accuracy. There are several typesof detection methods that have been reported previously. Withrespect to the sensing medium involved, they can be groupedinto two main categories—electrical and optical detection. Inelectrical configuration, piezoresistive [28], [29] and capaci-tive sensing mechanisms [30], [31] are the two most widelyadopted methods. Piezoresistivity works based on the change of

resistance caused by displacement, and its measuring sensitivityis directly determined by the piezoresistive coefficient of thematerial constituting the resistor. For the capacitive type, acapacitor consisting of two or two sets of parallel plates iscommonly adopted. One plate is fixed to a substrate, and theother is constituted of the movable structure under test itself.Any deflection of the structure will change the distance oroverlapping area between these two plates, thus varying thecapacitance value. By detecting this variation, the deflectioncan be measured. Despite good and extensive results reported,there are several intrinsic problems associated with this typeof measuring method. For example, sophisticated wiring lay-out, signal sampling, and processing circuits are indispens-able, and the influence of the exterior environment, such aselectromagnetic interference and parasitic effects, needs alsoto be carefully treated. Compared with it, optical measuringmethods demonstrate advantages such as relatively simple con-figurations, easy treatment, and high detection sensitivity. Mostoptical measuring methods are based on laser interference [32]in configuration such as Michelson and Mirau interferometersintegrated into an optical system of microscopy. These havebeen widely adopted in commercialized profilers (e.g., Zygoand Wyko). This method is especially suitable for static mea-surements, and it can easily achieve subnanometer measuringresolution in the vertical (out-of-plane) direction, whereas theresolution for in-plane movement is mainly determined bythe optical system’s resolution. In order to achieve dynamicmeasurement, strobed illumination has been introduced intointerferometry [33], such as the DMEMS Dynamic MEMSMeasurement option of Veeco, USA. Although moving MEMSdevices have been successfully characterized, this method isappropriate for the case exhibiting good moving repeatabilitysince the measurement for movement in one period usuallyrequires the object under test to be moved several periods.Therefore, cases involving time-dependent and nonreversiblemovement, such as in friction and wear tests, and real-timemovement monitoring are beyond the capability of this method.

In the current design described here, a novel in-plane dis-placement detection mechanism is proposed. Its working prin-ciple is based on laser beam scanning caused by in-planegrating rotation, which was first developed by Zhou et al. [34]–[38] for high-speed scanning applications. In this configuration,a grating is suspended on two beams, as schematically shown inFig. 2, with one beam directly connected to the substrate, actingas anchor, and the other one fixed to the sliding componentunder test. As the sliding component moves, the grating willbe forced to rotate accordingly. Fig. 3 shows a simplified modelof this suspension structure, and its mechanical deflection canbe expressed by

EId2y(x)dx2

= Fx − M (1)

where y(x) is the deflection and F and M are the force andmoment applied at the tip by the sliding component, respec-tively. E is the Young’s modulus of the beam structure material,and I = hb3/12 is the moment of inertia in the displacementdirection, determined by the structural thickness h and width b.

YU et al.: CHARACTERIZATION AND REDUCTION OF MEMS SIDEWALL FRICTION 993

Fig. 2. Schematic of the rotational-grating-based displacement sensingmechanism.

Fig. 3. Model of the suspension structure during operation.

TABLE ISTRUCTURAL DIMENSIONS OF THE ROTATIONAL

GRATING SENSING MECHANISM

By considering the ends connected to the substrate, movableplatform, and rigid grating plate as fixed (due to the much largergrating diameter compared to the beam width), the boundaryconditions are

y4 = 0dy1

dx=

dy4

dx= 0

dy2

dx=

dy3

dxy2 = y3 +

dy2

dx· d(2)

where the number of the subscript denotes the value at thepoints, as shown in Fig. 3.

Using the structure design parameters given in Table I, themechanical relationship between the platform displacement(y1) under test and the resultant grating rotation (dy2/dx)can be obtained by solving (1). It can be seen (Fig. 4) that,within the measurement range of interest, the rotation angleof the grating linearly increases to 0.79◦ under a platformdisplacement of 10 μm.

During the test, a laser beam, acting as the sensing signal, isfirst made incident onto the grating, and one of its diffractionorders is then chosen and collected by a PSD. If gratingrotation, i.e., displacement of the sliding component, occurs,the transmission direction of diffraction will be changed, thuscausing spot movement on the PSD, as shown in Fig. 5. Thedisplacement can be eventually measured by monitoring theoutput of the PSD.

Fig. 4. Simulation results about the grating rotation angle as a function of theplatform displacement.

Fig. 5. Schematic of rotational grating during operation.

In theoretical analysis, when the laser beam is normallyincident onto the grating platform, the resultant spot movementof the mth order diffraction on the PSD arranged at distant aunder the effect of grating rotation can be described by

mY =a·tg(φ) mZ =a

tg(θm)·(

1cos(φ)

−1)

sin(θm)=mλ/d

(3)

where mY and mZ are the spot movements along the Y - andZ-axes, respectively. Φ is the grating rotation angle about theZ-axis, θm is the diffraction angle of the mth order diffraction,λ is the laser wavelength, and d is the grating period.

Considering the design parameters adopted (a = 1 m, d =4 μm, λ = 632.8 nm, and m = 5), the spot movement withrespect to the grating rotation angle can be obtained, as shownin Fig. 6. It is clear that, compared with the spot movementalong the Z-axis (mZ), mY exhibits much larger amplitudesunder the same grating rotation and has a better linear rela-tionship (variation slope is 17.493 mm/◦) with the change inrotation angle. Since the PSD possesses linear output charac-teristics with respect to the spot movement on it, the resul-tant larger spot movement will induce a larger PSD output,demonstrating higher measurement signal-to-noise ratio as wellas sensitivity. Since a better linear relationship between theparameter under test and system output is desired, the PSD


Fig. 6. Simulation results of the spot movement caused by grating rotation.

Fig. 7. Fabrication process flow. (a)–(b) First lithography. (c) First DRIE.(d) Photoresist removal. (e)–(f) Second lithography. (g) Second DRIE.(h) Photoresist and buried oxide removal. (i) Gold deposition.

output—corresponding to mY —is selected as the sensing sig-nal in measurement. By combining the results shown in Fig. 4,it can be deduced that a 1-μm platform displacement willeventually cause 1.382-mm spot movement (mY ) (1 μm ×0.079◦/μm × 17.493 mm/◦ = 1.382 mm) with the currentdesign, thus demonstrating a displacement magnification of1382×. Considering a 1.5-μm positional resolution associatedwith the PSD used in the experiment (S1880, with 12 mm ×12 mm effective area; Hamamatsu, Japan), a theoretical mea-surement sensitivity of 1.085 nm (1.5 μm/1328) of the platformdisplacement can be achieved with this proposed detectionmethod.

III. DEVICE FABRICATION

In the current experiment, all of the structures are fabricatedusing a commercial foundry process (SOIMUMPs) provided byMEMSCAP, USA. It begins with a blank double-side polishedSOI wafer [Fig. 7(a)], in which the thicknesses of the devicelayer, buried oxide, and substrate are 25, 1, and 400 μm,respectively. Standard lithography steps, including photoresistspin coating, exposure, and development, followed by deepreactive ion etching (DRIE) [Fig. 7(b) and (c)], are performed

Fig. 8. Pictures of the as-fabricated device. (a) Microscopy picture of thewhole device. (b) Enlarged image of the grating region. (c) Device afterpackage.

to fabricate all of the structures, including the sliding andcontact components and the rotational-grating-based displace-ment detection mechanism, into the device layer. Due to thelimitation of the fabrication capability, the minimum dimensioninvolved should not be smaller than 2 μm. Other lithographyand DRIE steps are used in sequence to remove the substrateunder the movable structure region, as shown in Fig. 7(e)–(g),and the exposed buried oxide layer is subsequently etchedaway to totally release the whole device [Fig. 7(h)]. Finally,a gold layer is selectively deposited onto the top surface of thedevice to simultaneously achieve an electrical connection padin the actuator region and high reflectivity in the grating region[Fig. 7(i)]. Fig. 8 shows images of the as-fabricated wholedevice and the enlarged grating region taken under microscopy,together with that of the device after packaging.

IV. EXPERIMENT AND RESULTS

Prior to the friction test, the displacement detection per-formance provided by the rotational-grating-based detectionmethod is first studied, during which several dc voltages areapplied onto the sliding component and the reconstructed spotmovements from the PSD output recorded, as shown in Fig. 9.It can be seen that there is a good linearity (0.00394-mm/V2

variation slope) between the spot movement and the square ofthe applied voltage, which agrees well with the characteristicassociated with the electrostatic comb-drive actuator. At thesame time, the real platform displacement on the same device is


Fig. 9. Reconstructed light spot movement as a function of the square of theapplied voltage obtained from the experiment.

Fig. 10. Measured platform deflections as a function of the square of theapplied voltage using Zygo profiler.

measured using a commercial profiler (Zygo, USA). From theresults shown in Fig. 10, a similar linear relationship is found,but the variation slope is now changed to 0.00317 μm/V2.Thus, it can be deduced that, with the current sensing mech-anism, the platform movement can be finally translated intospot movement on the PSD with 1243× (0.00394 mm/V2/0.00317 μm/V2) magnification as estimated. Considering the1.5-μm position resolution of PSD, the theoretical movementresolution provided by the current grating-based sensing mech-anism can reach 1.2 nm.

To further characterize the proposed detection method, thetransient response of the sliding component when subjected toa 1-Hz square wave actuation signal of 2-V Vp−p amplitudeand 5-V dc bias is also measured, as shown in Fig. 11. It canbe seen that a response characteristic typically associated withan under-damped second-order system can be found, which ismainly caused by the air damping effect as widely reported in

Fig. 11. Measured transient response of the system.

the operation of MEMS devices. Through standard analysis, itcan be seen that the raise time of the current system under test isaround 208 μs, and the overshoot and settling times are 78.6%and 80 ms, respectively. The period of oscillation is 0.8 ms;therefore, the damped natural frequency of the current systemcan be deduced to be 1250 Hz.

During the friction test, a similar method as reported in[14] is first used to measure the adhesion force. In the currentexperiment, the contact between these two components (contactand sliding components) was observed under microscope with1000× magnification. During the measurement, a dc voltagesource was directly connected to the comb-drive actuator of thecontact component. Then, this dc voltage was increased step bystep (in initial status, the voltage increasing amplitude can be alittle large (such as 5 V) to speed up the test, and when the twocomponents approach contact, fine adjustment with 0.01-V ad-justment resolution is adopted), and the corresponding imagesof the contact region at each voltage value were captured usingthe camera mounted on the microscope. These images werethen analyzed using a commercial image processing software,and the gap variation between them can be measured withnearly 10-nm resolution after performing calibration. From themeasurement, it can be found that, with the increase of the dcvoltage, the gap decreases continuously, and these two compo-nents finally get contact when a 44.9-V voltage is applied. Aftercontact, the dc voltage was gradually decreased (also 0.01-Vadjustment resolution), and sudden separation occurred whenthe voltage was reduced to 44.6 V. This represents an instabilitypoint for the system, encountered at the instant that the contactload decreases to zero. At this juncture, force balance dictatesthat the adhesion force Fadhesion is given by

Fcontact = Fadhesion + Fpush − Frestore = 0⇒ Fadhesion + Fpush = Frestore

⇒ Fadhesion = Frestore − Fpush

⇒ Fadhesion =Ncε0εrh

g· 44.92

− Ncε0εrh

g· 44.62 = 1.85 μN (4)


Fig. 12. Schematic of the driving signal adopted in the friction test.

where Nc (= 621) is the number of comb-drive fingers ofthe contact component, ε0 is the permittivity in vacuum(= 8.854 × 10−12 P/m), ε is the relative permittivity (= 1 forair), h (= 25 μm) is the height of the fingers, and g (= 2 μm)is the gap between fingers.

Considering the sampling rate of the current system and thefact that the movement speed will affect the kinetic frictionstatus, in order to clearly reveal the transition between static andkinetic frictions and to accurately characterize kinetic friction,a slow and linear displacement response to the driving signal,namely, constant relative movement speed, is much more de-sired during the test. At the same time, since the coefficient ofkinetic friction is obtained by comparing the variation of max-imum displacement of the sliding component under differentoperation conditions as described in the following, in order toobtain accurate measurement result, it is better to leave enoughtime for stable data sampling at this status. Consequently, in thecurrent experiment, a signal V (t) with a trapezoidal waveform,as shown in Fig. 12, is used to actuate the sliding component.Its period and amplitude are 5 s and 20 V, respectively, and thespace ratio between rising, top platform, falling, and bottomplatform is 2 : 1 : 2 : 1. In the experiment, two channels of signal,namely, V (t) and −V (t), are, respectively, applied to twosets of comb-drive fingers arranged at each side of the slidingcomponent, while a 2-V dc voltage is directly applied onto thesliding component. Therefore, the driving force exerted onto thesliding component is

F (t) =Nsε0εrh

g·{

[2 + V (t)]2 − [2 − V (t)]2}

= 8 · Nsε0εrh

gV (t) (5)

where Ns (= 590) is the number of comb-drive fingers of thesliding component.

Considering the linear deformation characteristic with re-spect to the force within the elastic region, it can be seenthat the resultant displacement will also be linearly dependenton V (t).

From a theoretical analysis, it can be seen that, when nocontact occurs, the sliding component should exhibit displace-ment (x(t) = F (t)/kSK) with a trapezoidal waveform, whichfollows well with the driving signal, as expected from (5). Ifthere exists friction, however, the waveform of displacementwill undergo a distinct change. Initially, due to the stiction effectand larger static friction that exists at the interface between the

two contact surfaces, the sliding and contact components willbe deflected together as a whole under the effect of drivingforce, i.e., no relative movement between them can be observed.Thus, the resultant displacement can be described by

x(t) =F (t)

kcx + ksx(6)

where kcx and ksx are the equivalent spring constants along thex-axis of the suspension structures of the contact and slidingcomponents, respectively.

With increasing driving force—as well as resultantdeflection—the restoring force (Fr(t) = kCX · x(t)) appliedon the contact component provided by its suspension structurewill be increased accordingly. When its value reaches that ofthe static friction, the codeflection of the contact and slidingcomponents cannot be supported anymore, and as a result,relative slipping between these two components will occur,indicating the transition from static to kinetic friction states. Itis well known that the coefficient of static friction μs as wellas the resultant force Fs is larger than those of kinetic friction(μk and Fk). Hence, given the same driving force, there will bea sudden increase of the sliding component displacement Δx

Δx =FS − FK

kSX=

(μS − μK)kSX

· FN (7)

where FN is the normal force at the contact interface.At the same time, the voltage at this transition moment can

be determined by

μSFN = Fr(ttransition) = kCX · x(ttransition)

= kCX · F (ttransition)kcx + ksx

⇒ μSFN · (kcx + ksx)kcx

= F (ttransition)

⇒ V (ttransition) ∝ FN . (8)

It is obvious that, under the effect of a larger contact force, thetransition will appear at a higher voltage and will result in alarger increment in displacement amplitude.

After this transition point, kinetic friction will continue toact, and the deflection of the sliding component will increaselinearly with increasing driving voltage, i.e.,

x(t) =F (t) − FK

ksx. (9)

When the driving signal falls into the top platform region(Fig. 12), the displacement of the sliding component reaches amaximum and keeps constant. At this point, there is no relativemovement between the two components, and the friction is nowchanged from kinetic status to static once again.

Subsequently, with the decrease of the driving voltage(falling region in Fig. 12), the restoring force provided by thesuspension structure will begin to move the sliding compo-nent back toward its original position, and the same stick–slip process mentioned previously, namely, static friction →


Fig. 13. Schematic of the sliding component displacement during the frictiontest.

transition from static to kinetic friction → kinetic friction, willrepeat until the voltage is reduced to zero.

From the aforementioned analysis, the waveform of thedisplacement of the sliding component under friction can beschematically represented as in Fig. 13, in which each status isrepresented by a different color.

In the actual test, the driving signal to the sliding componentand the output of the PSD are simultaneously captured by anoscilloscope. Therefore, any variation of the sliding componentdisplacement as well as the corresponding driving voltage atany instant can be clearly observed. First, no voltage is appliedto the contact component, and the displacement status of thesliding component under the driving condition mentioned pre-viously is recorded. Then, two different dc voltages, namely,45.5 V (condition 1) and 45.8 V (condition 2), are applied con-secutively to the contact component—thus applying differentcontact forces as well as friction onto the sliding component—while keeping the driving condition of the sliding componentconstant. Similarly, the displacement of the sliding componentis also recorded, as shown in Fig. 14. It is obvious that thereexist some fluctuations in the results especially in the plateauregion. These fluctuations are mainly caused by the noise fromthe detector itself as well as its corresponding electrical con-nection to the oscilloscope. At the same time, the effect fromenvironmental vibration (mainly contributed by the air condi-tioner in the room) should also be taken into consideration. Byperforming some signal processing strategies during posttestdata processing, such as filtering and averaging, this effectcan be minimized, and the eventual displacement measuringresolution about 5 nm can be achieved with the current system.From the experimental results shown in Fig. 14, it can be seenthat, under the original condition (no friction), the displacementof the sliding component follows closely that of the drivingsignal as expected, and a maximum of 0.5-μm displacementcan be achieved when a 20-V driving signal is applied. Oncefriction appears, a displacement waveform similar to that shownin Fig. 13 can be seen. Within the voltage rising region, ini-tially, the displacement increases with a small slope due to thecodeflection of the sliding and contact components as describedby (6) and the relatively large spring constant along the x-axis

Fig. 14. Measured sliding component displacement variation with time underdifferent working conditions.

of the suspension structure of the contact component (kcx).With increasing voltage, a sudden increase of the displacementcan be clearly seen in both working conditions (1 and 2). Thevoltage and sudden jump in amplitude at this transition momentare measured to be 12.3 V and 0.13 μm, respectively, forcondition 1, while larger values of 14.2 V and 0.16 μm areobtained for condition 2. This finding agrees well with thetheoretical analysis given by (7) and (8). With further voltageincrement, a linear displacement characteristic with a largerslope can be observed again, and the maximum displacementsare reduced to 0.40 and 0.37 μm for conditions 1 and 2, respec-tively, due to the onset of kinetic friction. When the voltage issubsequently decreased, a phenomenon similar to that observedbefore occurs. The jump amplitude of displacement at this tran-sition point is measured to be 0.129 μm for condition 1, whilethat for condition 2 is found to be 0.159 μm; both of whichagree well with the values obtained in the voltage rising region.

By substituting the experimental results obtained into (7)and (9), the coefficients of static and kinetic frictions can becalculated to be 0.801 and 0.363, respectively.

Since friction can cause wear as well as potential failure ofthe device, in many applications, it is not desired. The mostdirect solution to alleviate this problem is through lubrication.Considering the fact that several functional components (suchas sensing, wiring, and actuating parts) are commonly inte-grated into a MEMS device, traditional lubrication methodssuch as immersion in liquid or vapor, which interact withthe exposed areas, are normally not appropriate. In order tominimize the effects of lubricant acting on undesired regions,a localized lubrication method is developed, in which theliquid lubricant is directly introduced into the contact regionusing a syringe-based dispensing system. During operation, thesyringe, which is equipped with a 60-μm-diameter needle, isfirst filled with 4.0 wt% perfluoropolyether (PFPE) solution(one of the most commonly used lubricants) and is connectedto a dispensing controller via a plastic tube. The syringe is thenfixed onto a stage that is capable of multidegrees of freedomadjustment, as shown in Fig. 15. Subsequently, the needle tip ofthe syringe is positioned onto the desired device region with


Fig. 15. Experimental setup of the localized lubrication method.

Fig. 16. Measured sliding component displacement after lubrication.

the assistance of a microscope. By activating the dispensingcontroller, lubricant of a certain volume can be applied. Dueto capillary effect, the liquid lubricant can be finally deliveredonto the sidewall region. The whole device is then kept in air for24 h to completely evaporate the solvent, leaving a solid PFPElubricant layer of a few nanometers thick on the sidewall. Thesame adhesion force and friction tests as mentioned previouslyare performed. It is found that, in this case, the separation aftercontact can be achieved when the voltage applied onto thecontact component is reduced to 44.7 V. From (4), a smalleradhesion force of 1.23 μN can be deduced. The results of thefriction test (Fig. 16) show that, under the contact conditionof 48.8 V applied to the contact component, the maximumdisplacement of the sliding component of 0.262 μm can beobtained, and the voltage value and the jump amplitude atthe transition moment between static and kinetic frictions aremeasured to be 14.8 V and 0.103 μm, respectively. Using thesame data treatment as before, the coefficient of static frictionis calculated to be 0.263, while that under kinetic condition isaround 0.183. It is obvious that the localized lubrication methodreduces not only the adhesion but also the coefficient of friction(both static and kinetic). For better demonstration, all of themeasurement results are summarized in Table II.

TABLE IIMEASURED RESULTS ABOUT THE ADHESION FORCE AND THE

COEFFICIENT OF FRICTION (BOTH STATIC AND KINETIC)BEFORE AND AFTER LUBRICATION

V. CONCLUSION

In this paper, a microtribometer which mainly focuses oncharacterizing the sidewall friction in MEMS devices in realtime has been presented. The instrument employs a newly de-veloped rotational-grating-based displacement sensing mecha-nism. By translating the lateral displacement of platform undertest into in-plane grating rotation, a light spot movement with1243× magnification for the displacement amplitude under testcan be experimentally obtained. Depending on the sensitivityof the PSD adopted, displacements as small as 1.2 nm canbe resolved with the developed microtribometer. Using thismicrotribometer, the adhesion force and the coefficients ofstatic and kinetic frictions are characterized to be 1.85 μN,0.801, and 0.363, respectively, for a test device. Also presentedis a localized lubrication method that has been developedto improve the friction performance of MEMS devices. Themethod involves the application of liquid lubricant directly ontothe desired region while leaving other components on the sameMEMS chip unaffected. This method is demonstrated to beeffective as evidenced by the reduction of the values obtained(namely, 1.23-μN adhesion force and 0.262 (static) and 0.183(kinetic) coefficients of friction).

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Hongbin Yu received the B.S. degree in mechanicalengineering, the M.S. degree in electrical engineer-ing, and the Ph.D. degree in optical engineeringfrom Huazhong University of Science and Tech-nology, Wuhan, China, in 1999, 2002, and 2005,respectively.

He is currently a Senior Research Fellow withthe Micro/Nano Systems Initiative, Department ofMechanical Engineering, National University ofSingapore, Singapore. His research interests involvethe design, simulation, and fabrication technology of

MEMS/NEMS devices and optofluidics.

Guangya Zhou received the B.Eng. and Ph.D. de-grees in optical engineering from Zhejiang Uni-versity, Hangzhou, China, in 1992 and 1997,respectively.

He joined the National University of Singapore,Singapore, in 2001 as a Research Fellow, where heis currently an Assistant Professor in the Departmentof Mechanical Engineering. His main research inter-ests include microoptics, diffractive optics, MEMSdevices for optical applications, and nanophotonics.

Sujeet K. Sinha received the Ph.D. degree fromImperial College London, London, U.K., in 1994.

He is currently an Associate Professor in the De-partment of Mechanical Engineering, National Uni-versity of Singapore, Singapore. He has worked asa Postdoctoral Researcher at the National Institute ofStandards and Technology, Gaithersburg, MD, and atthe National Institute of Advanced Industrial Scienceand Technology (AIST), Tsukuba, Japan. His re-search interests have been in the areas of lubricationof microdevices and polymer tribology.


Jonathan Y. Leong received the B.S. degree in me-chanical engineering from the National University ofSingapore (NUS), Singapore, where he is currentlyworking toward the Ph.D. degree, investigating thelubrication and tribology of microelectromechanicaldevices.

Having completed the undergraduate degree inmechanical engineering at NUS, he pursued an in-terest in this area as a Research Engineer underA/Prof. S. S. Kumar, and since then, having his thirstfor knowledge, he joined the NUS–Imperial College

Joint Ph.D. program. His research interests lie mainly in the area of tribology,particularly at the microscale, and in polymer lubrication as a durable andecofriendly alternative to current forms of lubrication.

Fook Siong Chau received the B.Sc. (Eng.) andPh.D. degrees from the University of Nottingham,Nottingham, U.K., in 1974 and 1978, respectively.

He is currently an Associate Professor in theDepartment of Mechanical Engineering, NationalUniversity of Singapore, Singapore, where he headsthe Applied Mechanics Academic Group. His mainresearch interests are in the development and appli-cations of optical techniques for nondestructive eval-uation of components and the modeling, simulation,and characterization of microsystems, particularly

bio-MEMS and MOEMS.

characterization and reduction of mems sidewall friction using novel microtribometer and localized...

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