JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 20, NO. 5, OCTOBER 2011 1065

JMEMS LettersSilica-Encapsulated Nanoparticle Films as Surface

Modifications for MEMS

Kendall M. Hurst, Naveed Ansari, Christopher B. Roberts, andW. Robert Ashurst

Abstract—In an effort to improve the reliability of microelectromechan-ical systems (MEMS), silica thin films deposited by chemical vapordeposition were used to encapsulate gold nanoparticle coatings. Thesecomposite coatings were shown to provide extremely durable films thatsignificantly reduce the adhesion energy of silicon-based microcantileverbeams. The results discussed suggest that encapsulating nanoparticlefilms with a durable silica thin film may lead to improved MEMSreliability. [2011-0152]

Index Terms—Microelectromechanical systems (MEMS), nanoparticles,stiction, surface roughness, thin films.

I. INTRODUCTION

One great challenge hindering the commercialization of highly com-plex micromechanical and microelectromechanical systems (MEMS)is overcoming excessively strong interfacial interactions such as ad-hesion, friction, and wear [1]–[6]. Such interactions arise due to thevery large surface-area-to-volume ratios that are characteristic to mostmicromechanisms. Moreover, at the microscale, both friction and wearare highly dependent on the adhesion energy between structures [2],[3], [5], [7], [8]. In order to realize the potential of several usefulcomplex MEMS, the issues of adhesion and wear at MEMS interfacesmust be addressed.

In recent years, significant progress has been made in the devel-opment of antiadhesive surface coatings for MEMS [9], [10]. Morerecent studies have turned toward examining the roughening of siliconsurfaces to reduce adhesion. Our previous studies have shown thatmetallic nanoparticles, approximately 5 nm in diameter, deposited ontomicrostructures by means of a unique gas-expanded liquid (GXL)technique, can significantly reduce microstructure adhesion [11], [12].A limitation of the nanoparticle coating, however, is that mechanicalcontact between microstructures causes the particles to move upon thecoated surfaces, leaving behind particle-free regions which can exhibitstrong interfacial forces that can cause adhesion [11]. Therefore, meth-ods of developing robust nanoparticle-based surface modifications arerequired in order to achieve antistiction properties over high cycle-number actuations.

In this paper, we present a method that involves coupling of GXL-deposited nanoparticles with a vapor-phase-deposited thin silica layer.The silica layer, which has an approximate thickness of 2 nm, serves asan immobilizing layer by completely encapsulating nanoparticles andpreventing their movement during microstructure contact. Meanwhile,

Manuscript received May 16, 2011; revised June 24, 2011; accepted June 28,2011. Date of publication August 18, 2011; date of current versionSeptember 30, 2011. Subject Editor C. Mastrangelo.

K. M. Hurst was with the Department of Chemical Engineering, Auburn Uni-versity, Auburn, AL 36849-5127 USA. He is now with Chevron Phillips Chem-ical Company, Bartlesville, OK 74004 USA (e-mail: hurstkm@cpchem.com).

N. Ansari, C. B. Roberts, and W. R. Ashurst are with the Depart-ment of Chemical Engineering, Auburn University, Auburn, AL 36849-5127USA (e-mail: ansarna@tigermail.auburn.edu; robercr@auburn.edu; ashurst@auburn.edu).

Digital Object Identifier 10.1109/JMEMS.2011.2162494

the underlying nanoparticles induce significant surface roughness inthe silica thin film, which cannot be achieved otherwise. The durabilityof this coupled film and its effectiveness in reducing in-use in-planestiction between microstructure surfaces are explored.

II. MATERIALS AND METHODS

A. Cantilever Beam Fabrication and Actuation

Cantilever beams, which were used for the quantification of in-usein-plane stiction, were fabricated in house using the standard surfacemicromachining techniques and a silicon-on-insulator (SOI) wafer.The device layer used to fabricate the cantilever beams was a 2-μm-thick Si(100) film, and the sacrificial layer was a 2-μm-thick buriedoxide layer. Following fabrication, the cantilever beams were releasedby etching away the sacrificial layer from underneath them in 49-wt%HF using a timed etch. Further details on the fabrication and actuationof the cantilever beams can be found elsewhere [12], [15].

B. Nanoparticle Synthesis and Deposition

Gold nanoparticles (AuNPs) were synthesized following a doubleliquid-phase arrested precipitation method [11]–[13]. Briefly, an aque-ous solution of hydrogen tetrachloroaurate and an organic solution of aphase transfer catalyst, tetraoctylammonium bromide, were combinedand stirred for 1 h. During this time, gold ions were transferred into theorganic phase. A reducing agent was then added to the combined solu-tion to reduce the gold ions to ground-state gold atoms. After allowingthe nanoparticles to grow from these ground-state atoms for approxi-mately 8 h, 1-dodecanethiol was added to cap the grown nanoparticles,cease growth, and stabilize the nanoparticles in an organic solution.Following a cleaning procedure [11], the nanoparticles were analyzedusing transmission electron microscopy and were found to have anaverage diameter of 4.5 ± 1.2 nm. The AuNPs were deposited ontothe SOI cantilever beams using a CO2-expanded hexane and criticalpoint drying process, described in detail elsewhere [11], [12], [14].In short, device chips were submerged within a dispersion of AuNPswhich was then exposed to gaseous CO2. As CO2 dissociated withinthe dispersion, the AuNPs precipitate out of the solution and deposituniformly on the surfaces of the device chip. The AuNP-coated chip isthen supercritically dried with CO2.

C. Silica Film Deposition

The rough AuNP films deposited onto the surfaces of the micro-cantilever beams were then encapsulated by a smooth and thin silica(SiO2) layer deposited using chemical vapor deposition (CVD). Thegas phase reaction involved is the very well controlled hydrolysis oftetrachlorosilane (SiCl4), which follows [16]

SiCl4(v) + 2H2O(v) → SiO2(s) + 4HCl(v). (1)

The reaction was carried out at room temperature in a vacuumsystem with a base pressure of 20 mtorr. Approximately 20 torr ofSiCl4 was reacted with 20 torr of H2O for 10 min. This recipehas been shown to produce conformal and uniform SiO2 thin films,even at reduced temperatures [16]. Ellipsometry and contact angleanalysis were used to confirm the deposition of a complete SiO2 thinfilm. Atomic force microscopy (AFM) was performed to image the

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Fig. 1. AFM scan of a SiO2 thin film deposited using CVD on an atomicallysmooth (rms roughness = 0.20 nm) silica substrate.

Fig. 2. Line scan across the AFM image of a CVD SiO2 film deposited onan atomically smooth silica surface indicating that all the features are less than3 nm tall.

topography of the resulting coupled film and determine the surfaceroughness. Details on the experimental apparatus used for CVD canbe found elsewhere [16].

III. RESULTS AND DISCUSSION

A. Characterization of the Deposited SiO2 Thin Film

Fig. 1 shows an AFM image of a SiO2 film deposited on an atomi-cally smooth silica substrate. The root-mean-square (rms) roughnessof the smooth substrate was about 0.20 nm. Following SiO2 filmdeposition, the rms roughness increased to 0.92 nm, which is stillindicative of a smooth surface. Fig. 2 shows a line scan taken acrossthe AFM image in Fig. 1. The line scan indicates that all the featureson the CVD SiO2 thin film are less than 3 nm in height.

Using ellipsometry, the average thickness of the CVD-depositedSiO2 film was measured to be 4.3 ± 0.1 nm. The contact angle of adroplet of water on the deposited SiO2 film, as well as the atomicallysmooth SiO2 samples, was, on average, less than 10◦, indicating ahydrophilic surface as expected. The deposition of the CVD SiO2 layerhad no effect on the surface chemistry. When CVD SiO2 was depositedover a preexisting coating of AuNPs to form a SiO2-encapsulatedAuNP film, the rms surface roughness increased from 0.90 to about1.7 nm, indicating a change in topography. Surprisingly, the watercontact angle on this surface increased to about 40◦. However, thischange is due to the topographical changes to the surface and notchanges to the surface chemistry since it still falls in the range of ahydrophilic surface.

Fig. 3. Contact angle with water-droplet erosion time for various surfacecoatings, illustrating film durability.

In addition, the durability of films was investigated using a water-droplet technique. During this testing, the thin-film-coated sampleswere placed 18 in beneath a funnel flask of water. Water drops, fallingdue to gravity, impinged on the coated surfaces at a rate of 2/s. Atvarious points in time, the water contact angle was measured onthe tested surfaces to investigate the durability of the films againstthe impinging water drops. The results are shown in Fig. 3, whichcompares the durability of loose AuNPs deposited on silica, AuNPsimmobilized by attachment to an organic monolayer film [12], andAuNPs encapsulated by a CVD SiO2 film. In order to achieve amore desirable initial contact angle, a monolayer of perfluorooctyl-trichlorosilane was deposited using CVD over the CVD SiO2 layer.The results show that, while loose AuNPs and AuNPs immobilized byattachment to mercaptopropyltrimethoxysilane were easily removedfrom the surface, indicated by a decrease in water contact angle,the encapsulated AuNP films were extremely durable and showed nodecrease in water contact angle until after well over 10 h of water-droplet erosion.

B. Effect of SiO2-Encapsulated AuNPs on Adhesion

CVD SiO2 and SiO2-encapsulated AuNP films were conformallydeposited on the microcantilever beams to investigate their effect onin-use in-plane stiction. Fig. 4 shows the interferograms collected afteractuation of 500-μm-long cantilever beams coated with (a) nativeSiO2, (b) CVD SiO2, and (c) CVD SiO2-encapsulated AuNPs. Theinterferograms were collected using a long-working-distance interfer-ence microscope with a green LED light source that is monochromatedby a bandpass filter to 532 nm. The characteristic crack lengths(s in Fig. 4) from the beam anchor points to the point of completecontact with the underlying substrate are indicated by the dashed lines.The crack length is a qualitative measure of the apparent adhesionenergy of the MEMS surface—the smaller the crack length, the higherthe apparent adhesion energy. The images in Fig. 4 indicate thatthere is virtually no difference between the apparent adhesion energyexhibited by native-SiO2-coated in-plane surfaces and that exhibitedby CVD-SiO2-coated in-plane surfaces, even though the deposition ofthe CVD SiO2 film causes a slight increase in the surface roughnessas shown in Fig. 1. However, when MEMS surfaces are coated withthe coupled AuNP–CVD-SiO2 film, the additional surface roughnessinduced by the AuNPs (about 1.7 nm rms) results in an increased cracklength, as shown in Fig. 4(c). Table I compares the rms roughnesses,water contact angles, crack lengths, and apparent adhesion energiesassociated with the three samples shown in Fig. 4.

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Fig. 4. Interferograms collected after actuation of the microcantilever beamscoated with (a) native SiO2, (b) CVD SiO2, and (c) CVD SiO2-encapsulatedAuNPs.

TABLE ITRIBOLOGICAL COMPARISON OF THE THREE SURFACE COATINGS

IV. CONCLUSION

The results discussed in this paper suggest that the deposition ofa CVD SiO2 thin film on top of a nanoparticle coating results in anextremely durable composite coating, which is also effective in reduc-ing the adhesion energy of microstructured surfaces. CVD SiO2 filmsalone are not effective in reducing adhesion due to the high surfaceenergy of the silica surface. However, the additional surface roughnessinduced by the underlying AuNP coating reduces the surface energyas exhibited by water contact angles. This reduced surface energydirectly results in reduced adhesion energy. AuNPs, although easy tosynthesize and deposit onto such surfaces, are expensive. In theory,any nanomaterials in the same size range should have a similar effect.

Such surface modifications show promise for improving the reliabil-ity of complex MEMS and devices, which is limited by the adhesionof contacting surfaces.

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