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Sensors and Actuators A 187 (2012) 43–49

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

urface treatment of polymers for the fabrication of all-polymer MEMS devices

iheng Zhao, Debra A. Sheadel, Wei Xue ∗

echanical Engineering, School of Engineering and Computer Science, 14204 NE Salmon Creek Avenue, Washington State University, Vancouver, WA 98686, USA

r t i c l e i n f o

rticle history:eceived 17 March 2012eceived in revised form 3 August 2012ccepted 9 August 2012vailable online 17 August 2012

eywords:ll-polymer MEMS devicesicrofluidics

urface treatmentomogeneous bondingeterogeneous bonding

a b s t r a c t

Polymer microelectromechanical systems (MEMS) have received considerable attention due to their lowcost, remarkable biocompatibility, and high flexibility when compared to glass and silicon devices. How-ever, the fabrication process of all-polymer MEMS devices can be complicated and often requires a specialgroup of techniques. In particular, different types of polymers possess different properties in termsof surface chemistry and hydrophilicity, making device assembly a challenging task. In this paper, wedemonstrate the fabrication of an all-polymer microfluidic device through the investigation of the essen-tial surface treatment methods. An SU-8–SU-8-polydimethylsiloxane (PDMS) sandwiched structure isused in this research because SU-8 enables alignment capabilities during the fabrication process. It canalso be easily extended to other lithography-compatible processes. Both untreated SU-8 and PDMS arehydrophobic and they have different surface chemistry properties, so surface modifications are necessary.Three critical surface treatment steps are used in our process. The first step is to treat the first SU-8 layerwith low-power (10 W) oxygen plasma, making its surface hydrophilic. This step enables the uniform

coating of the second SU-8 layer. The next surface treatment is on the second SU-8 layer. Both oxygenplasma (40 W) etching and diluted 3-aminopropyltriethoxysilane (APTES, a silane solution) coating areneeded. APTES introduces amine (Si NH2) groups on the surface. The last treatment step is to introducesilanol (Si OH) groups on PDMS using oxygen plasma. These surface treatment steps are critical in thefabrication process and can determine the quality of the final device. The knowledge obtained from this

evelo

research can enable the d

. Introduction

The research of microelectromechanical systems (MEMS) hasade tremendous progress in the past thirty years. For many MEMS

evices, especially high-precision MEMS devices with integratedlectronics, silicon is still the most common material. However,s the application areas are expanding, silicon is not always theest choice. This is particularly true because biocompatibility hasecome a critical factor in many areas, especially bioMEMS [1–3].ince the early 1990s, polymers have gained special interest in theEMS area as low-cost alternatives to silicon and glass. They offer a

umber of advantages such as low material cost, high transparency,nd ease of fabrication. More importantly, polymers provide aide range of mechanical properties and surface chemistry, making

hem excellent candidates for new application areas [4,5].Among these areas, microfluidics has attracted the most atten-

ion for research and development. Since the introduction ofhe �TAS (micro total analysis systems) concept in 1990 [6],

icrofluidic devices have become increasingly prevalent in biol-

∗ Corresponding author. Tel.: +1 360 546 9250; fax: +1 360 546 9438.E-mail address: wxue@wsu.edu (W. Xue).

924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.08.018

pment of novel all-polymer MEMS devices for biomedical applications.© 2012 Elsevier B.V. All rights reserved.

ogy, analytical chemistry, and medicine [7,8]. Recent activities andprogress of microfluidic devices have revolutionized the biomedi-cal research field. Many innovative devices have been successfullydeveloped and used in practical applications. At the same time,novel approaches have been developed for the fabrication ofsuch devices. A number of recent review articles have summa-rized the latest advances in the fabrication of microfluidic devices[9,10].

However, the fabrication of all-polymer devices, especially thekey microfluidic components, often brings complications into theprocess. The situation worsens when different types of polymersneed to be permanently bonded together (called heterogeneousbonding). These different polymers usually possess diverse prop-erties in terms of surface chemistry and hydrophilicity, making theheterogeneous bonding a highly challenging task. For example, themost common method used in microfluidics fabrication is to bondpolydimethylsiloxane (PDMS, a silicon-containing elastomer) toanother solid material (silicon, glass, or polymers). However, if thatmaterial does not contain silicon atoms, direct bonding becomesalmost impossible [11]. Although adhesives or clamps can be

used for non-permanent bonding, they also change the designparameters and affect the device performance. To overcome theselimitations, researchers have investigated alternative approachesto obtain permanent bonding between different types of polymers

44 J. Zhao et al. / Sensors and Actuators A 187 (2012) 43–49

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ig. 1. (a) Schematic of an all-polymer microfluidic device made of SU-8 and PDMS. (bandwiched structure under a microscope.

ncluding thermoplastics and epoxy polymers [11–15]. These pilottudies have demonstrated simple ways to achieve heterogeneousonding. At the same time, there has been extensive research

nvestigating how to tune the surface hydrophilicity of commonolymers including PDMS and photo-definable epoxy polymersuch as SU-8 photoresist [16–21]. Effective strategies have beenemonstrated and they can be extended to polymer bonding. How-ver, how to efficiently integrate surface treatment and polymeronding into the actual fabrication process to create high-uality devices remains underexplored, leaving room for further

mprovement.In this paper we report the design and fabrication of an all-

olymer microfluidic device using simple and low-temperaturepproaches. We investigate the essential surface treatment meth-ds to achieve both homogeneous and heterogeneous bondingf polymers. An SU-8–SU-8–PDMS sandwiched structure is usedecause SU-8 is a photo-definable material and it enables align-ent capabilities during the fabrication process. The bonding steps

re carried out without damaging the microfluidic components.hree bonding strategies are investigated for the homogeneousonding between two SU-8 layers while eight different strategiesre evaluated for the heterogeneous bonding of SU-8 and PDMS.he optimized treatment steps reported here are critical in the fab-ication process and can determine the quality of the final device.ecause the entire process is simple, low-temperature, and usesnly common chemicals and equipment in the laboratory, it cane easily extended to the development of other all-polymer MEMSevices.

. Materials

The two structural polymers used in this research are SU-8 andDMS. The high-viscosity negative photoresist SU-8 3050 and theU-8 developer were purchased from MicroChem Corp. (Newton,A). The elastomer PDMS Sylgard 184 was purchased from Dow

ptical image of a fabricated device. (c) Cross-sectional view of the SU-8–SU-8–PDMS

Corning (Midland, MI). It came as a two-part silicone elastomer kitwith a base polymer and a curing agent. The two components weremixed together with a weight ratio of 10:1 to form PDMS. The airbubbles generated during the mixing step were removed using acentrifuge system rotating at 2500 rpm for 5 min. The mixed PDMSwas poured over a flat silicon wafer and cured at 60 ◦C for 8 h in anoven. After curing, the PDMS was peeled off from the mold and cutinto small pieces. The thickness of the PDMS layer is controlled tobe 3–5 mm.

The silane solution, 3-aminopropyltriethoxysilane (APTES,(H2N(CH2)3Si(OC2H5)3, purity ≥ 98%), was purchased fromSigma–Aldrich Co. (St. Louis, MO). It was used as the surfacetreatment agent to introduce silicon-containing amine (Si NH2)groups on the surface of SU-8. The APTES solution was diluted totwo different concentrations, 1% and 5% in volume, with deionized(DI) water. OmniCoat was purchased from MicroChem Corp.(Newton, MA). It was used as a sacrificial material to release thefinal all-polymer microfluidic device from the silicon substrate.Four-inch silicon wafers were cut into smaller pieces (3 cm × 2 cm)and used as the mechanical support for the device during thefabrication process. They were also used as flat substrates to curethe PDMS films.

3. Device design and fabrication

Fig. 1(a) illustrates the schematic view of the all-polymermicrofluidic device. This device is made of three tightly bondedpolymer layers including two SU-8 layers and one PDMS cover. Thefirst SU-8 layer is used as a flat, solid substrate. It also functionsas the bottom seal of the device. The second SU-8 layer containsall the microfluidic components including a microchannel, an inlet,

and an outlet. The PDMS is used as the top seal of the microfluidiccomponents. Fig. 1(b) shows an optical image of a fabricated device.The cross-sectional SU-8–SU-8–PDMS sandwiched structure can beclearly observed under the optical microscope, as shown in Fig. 1(c).

J. Zhao et al. / Sensors and Actua

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Fig. 2. Fabrication process of the all-polymer microfluidic device.

SU-8 is a photo-definable epoxy and its fabrication relies onell-developed optical lithography-based microfabrication tech-iques, the process can therefore be easily extended to other

ithography-compatible processes. The SU-8–SU-8–PDMS sand-iched structure provides advantages such as high transparency,

ow cost, low weight, and high biocompatibility, making this devicepromising candidate for a wide variety of applications, especially

n the biomedical area.The fabrication process of the device is illustrated in Fig. 2. The

rocess starts with spin coating a thin layer of OmniCoat (3000 rpmor 30 s) on a silicon substrate (Fig. 2(a)). The OmniCoat is used as aacrificial material to release the device from the silicon substraten a later step. The first layer of SU-8 photoresist is spin coated onhe wafer surface at a rotation speed of 3000 rpm for 30 s, result-ng in a 50 �m thick film. This SU-8 layer is soft baked at 95 ◦Cor 13 min on a hot plate and then exposed under ultraviolet (UV)ight for 20 s in a hard contact aligner. A post exposure bake step at5 ◦C for 4 min is carried out to further harden the SU-8 layer. Theesulting thickness of the cross-linked SU-8 layer is approximately0 �m (Fig. 2(b). The surface of this SU-8 layer is treated with low-ower oxygen plasma to increase its hydrophilicity (Fig. 2(c)). The

econd layer of SU-8 is spin coated on the surface followed by softake, UV exposure, and post exposure bake using the same param-ters. The exposed device is soaked in the SU-8 developer for 8 minith steady hand agitation of the solution. The development step

tors A 187 (2012) 43–49 45

dissolves the protected SU-8 layer under the photomask, result-ing in microfluidic structures in this layer (Fig. 2(d)). A piece of flatPDMS is bonded to the patterned SU-8 structures as the top seal(Fig. 2(e)). This heterogeneous bonding process, described in detailin the next section, involves plasma and chemical modificationson SU-8 and plasma etching on PDMS. The final step is to removethe OmniCoat layer in a diluted Microposit 351 developer solution(with a 1:5 volume ratio in water) for 20–40 min. The all-polymerdevice is therefore released from the supporting silicon substrateand can now be used as a standalone microfluidic device.

4. Surface treatment and bonding results

The untreated SU-8 photoresist has a hydrophobic surface withlow surface energy. The contact angle of water on its surface is in therange of 85◦. Various methods have been successfully developed toincrease the hydrophilicity of the native SU-8 surface. For example,the contact angle of 85◦ can be reduced to 5◦, 7.9◦, and 23◦ usingoxygen plasma, argon plasma, and wet chemical treatment withethanolamine, respectively [16–18]. Although these methods wereoriginally developed to enhance the in-channel capillary flow, theycan also be used to facilitate the homogeneous and heterogeneousbonding of another polymer piece on a treated SU-8 surface.

In this research, there are a total of three surface treatment stepsinvolved in the fabrication process. The first step is to treat the firstSU-8 layer with oxygen plasma. Because the untreated SU-8 sur-face is hydrophobic, it is difficult to obtain a uniform coating on it,even though the coating is from the same material. Therefore, sur-face modification with oxygen plasma is needed. The parametersfor the plasma treatment are set as 10 W for 10 s with a chamberpressure of 120 mTorr in the plasma reactor. The power is keptlow to minimize the damage on the sacrificial OmniCoat layer. Thewater contact angle of the untreated SU-8 surface is reduced from85◦ to 17◦ after oxygen plasma treatment, as illustrated in Fig. 3.The water droplet stays as a semi-spherical bead on the untreatedsurface but spreads out quickly on the treated SU-8 surface.

The plasma treatment significantly enhances the surfacehydrophilicity of the SU-8 surface, enabling the homogeneousbonding between two SU-8 layers. In our experiments, the secondSU-8 layer is applied on the surface of the first one via spin coat-ing. Fig. 4 compares the results of the spin coated SU-8 layer underthree different conditions.

(i) With a primer: a thin layer of hexamethyldisilazane (HMDS) isspin coated on the first SU-8 layer as a primer followed by thespin coating of the second SU-8 layer. However, the second SU-8 layer shrinks to the center of the substrate right after the spincoating step, as shown in Fig. 4(a). The poor film uniformityprevents any realistic fabrication steps on the SU-8 layer.

(ii) Without any treatment: the second SU-8 layer is directlyapplied on the substrate without any previous treatment. Forthis untreated surface, the second SU-8 layer can form a rel-atively uniform film at first. But the corners of the film startto pull back once the device is placed on the hot plate. Thefilm eventually shrinks to the center of the substrate duringthe soft bake step, indicating poor adhesion between the twoSU-8 layers (Fig. 4(b)). The resulting film is also unusable in thefabrication process.

(iii) With oxygen plasma: the substrate is treated with low-poweroxygen plasma for 10 s followed by the spin coating of the

second SU-8 layer. For the plasma-treated SU-8 surface, thesecond SU-8 layer can form a uniform film and remain sta-ble during the soft bake step, as shown in Fig. 4(c). All thecommon microfabrication steps can therefore be carried out

46 J. Zhao et al. / Sensors and Actuators A 187 (2012) 43–49

Fig. 3. Contact angles of water on the first SU-8 layer surface before (left) and after (right) plasma treatment.

Table 1Heterogeneous SU-8 and PDMS bonding results under different surface treatment conditions.

SU-8PDMS

Bonding

StrengthStep 1 Step 2 Step 3

1 No plasma APTES 1% Plasma Plasma No

2 No plasma APTES 5% Plasma Plasma No

3 No plasma APTES 1% No plasma Plasma Weak

4 No plasma APTES 5% No plasma Plasma Medium

5 Plasma APTES 1% Plasma Plasma No

6 Plasma APTES 5% Plasma Plasma No

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7 Plasma APTES 1%

8 Plasma APTES 5%

on this sample. In our experiments, all the microfluidic com-ponents are patterned on the second SU-8 layer.

After the creation of the microfluidic components, further sur-ace treatment is conducted on the second SU-8 layer and PDMS.hese steps are critical for the sealing of the microfluidic compo-ents. Because the two materials have different surface chemistryroperties, additional modifications are needed. Fortunately, aumber of approaches have been previously developed for theonding between thermoplastics and PDMS [12,13]. Some of theman be easily extended to the heterogeneous bonding between SU-8nd PDMS.

Here we investigate eight different surface treatment strate-ies to evaluate the bonding strength between the two polymers.ecause SU-8 does not contain silicon atoms, it cannot be directlyonded to PDMS (a silicon-containing polymer) even after plasma

ig. 4. Coating results of SU-8 on an existing SU-8 layer under three conditions: (a) with H

No plasma Plasma Strong

No plasma Plasma Strong

treatment. Therefore, a chemical modification step is needed tointroduce silicon-containing groups on its surface.

The eight different surface treatment strategies and their corre-sponding results are summarized in Table 1. Three steps are used forthe SU-8 layer and only one step is needed for the PDMS layer. Theparameters for the plasma treatment are set as 40 W for 20 s witha chamber pressure of 120 mTorr. The APTES treatment is carriedout by soaking the polymer sample in the diluted APTES solution for20 min on a hot plate (80 ◦C). To remove the excess APTES coatingfrom the surface, the treated SU-8 sample is rinsed under DI waterfor 10 s and dried with compressed air.

The dashed box (No. 3 and No. 4 in Table 1) and solid box (No.

7 and No. 8) indicate the cases where SU-8 and PDMS can formpermanent bonding. However, when no plasma etching is appliedto SU-8, the APTES-treated SU-8 can only be loosely bonded tothe PDMS layer (No. 3 and No. 4). The PDMS cover can be easily

MDS primer coating; (b) without any treatment; (c) with oxygen plasma treatment.

J. Zhao et al. / Sensors and Actuators A 187 (2012) 43–49 47

urfac

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Fig. 5. Contact angles of water on (a) the second SU-8 layer s

eeled off from the substrate with tweezers using small forces. Thiss because the surface coating of APTES on SU-8 is limited due to

he high hydrophobicity of the SU-8 surface before plasma etch-ng. By comparison, when the plasma treatment is applied on theurface before APTES coating, the SU-8 and PDMS can be stronglyonded together (No. 7 and No. 8). Using tweezers to break the

Fig. 6. Essential SU-8 surface modifications with plasma and APTES and PDMS s

e and (b) the PDMS layer surface before and after treatment.

bonded structure can only damage the polymers without separat-ing them. Furthermore, an additional plasma etching step on the

APTES-treated SU-8 can damage its surface chemistry, leading tofailed bonding results (No. 5 and No. 6).

For the cases where strong bonding occurs, the surfacehydrophilicity of the SU-8 and PDMS samples are evaluated using

urface modification with plasma for the heterogeneous polymer bonding.

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he water contact angle method. For the SU-8 layer, the 40 W oxy-en plasma reduces its contact angle and enables a hydrophilicurface. The APTES, 1% and 5% solution, provides amine termi-al groups on the SU-8 surface without significantly changing itsydrophilicity and the contact angle remains approximately 19◦, ashown in Fig. 5(a). In addition, the treated surface is air stable andan remain hydrophilic for a relatively long time (3–5 h). This hightability provides the SU-8 surface an additional advantage for theeterogeneous bonding process.

Surface modifications and aging properties of PDMS have beenreviously investigated by a number of groups [19,20]. In partic-lar, using oxygen plasma to enhance the hydrophilicity of PDMSas proven to be highly effective; it can reduce the water contactngle from 105◦ to 15◦. However, previous research shows that thereated PDMS surface can only remain hydrophilic for 5–20 minefore it becomes hydrophobic again [21]. We have observed sim-

lar phenomena in our experiments. The PDMS surface becomesighly hydrophilic after 20 s of oxygen plasma etching with its con-act angle being reduced from 105◦ to 10◦, as shown in Fig. 5(b).

hen this treated PDMS is bonded to an APTES-treated SU-8 sam-le within 2–3 min, they can form strong bonding. However, if thereated PDMS sample is left in air longer than 5 min, its surfaceeeds to be treated with oxygen plasma again before bonding.

The bonding strength measurement is based on leaking testsy pumping fluids through the microchannel in the device. Fluidsith various flowing rates are introduced into the microchan-el controlled by a syringe pump. The device remains operationalnder certain flowing rates (∼2 ml/min). However, when a higherow rate (>5 ml/min) is used, the pressure generated inside theicrochannel can cause leakage at the interface of SU-8 and

DMS layers. Based on this bonding method, we fabricated ai–SU-8–PDMS lab-on-a-chip device used for glycerol concentra-ion detection [22]. That device showed long-term stability withoutny leaking problems in routine experiments.

The heterogeneous bonding of SU-8 and PDMS involves fun-amental changes of their surface chemistry properties. The basicrinciples for this bonding process are illustrated in Fig. 6. The SU-8urface is first treated with oxygen plasma to increase its surfaceydrophilicity. The following APTES solution soaking step intro-uces a silylated layer on the substrate, forming surface amineSi NH2) groups. The plasma treatment on PDMS introduces silanolSi OH) groups on its surface. Once the SU-8 and PDMS samplesre pushed together tightly, the amine and silanol groups can formtrong and permanent Si O Si covalent bonds between them.

To enhance the bonding quality, the process is carried out on aot plate heated at 70 ◦C for 10 min. A heavy stainless steel cylinder42 N) is placed on top of the device to ensure the conformal contactetween the SU-8 and PDMS layers.

The fabricated all-polymer device is transparent, low-cost,ight-weight, and biocompatible; it has great potential foriomedical-related applications (Fig. 1(b)). The entire device fabri-ation process relies only on common microfabrication equipmentuch as lithography aligners, plasma reactors, and hot plates. Moremportantly, it is a low-temperature process: the highest temper-tures used are 95 ◦C for the photoresist baking steps and 80 ◦Cor the surface modification steps. We believe that such a low-emperature process can provide unique advantages in fabricationnd can be easily extended to the development of other all-polymerevices.

. Conclusions

In this research, we have investigated the homogeneousnd heterogeneous bonding of polymers for the fabrication ofll-polymer MEMS devices. A microfluidic device has been

[

tors A 187 (2012) 43–49

developed by using SU-8 and PDMS. Because both materials arehydrophobic, surface treatment is needed before bonding. Threedifferent strategies are investigated for the homogeneous bondingbetween two SU-8 layers. A low-power oxygen plasma treatmentstep is used to enhance the hydrophilicity of the SU-8 surface,enabling a uniform coating of the second SU-8 layer. Eight differ-ent strategies are evaluated and compared for the heterogeneousbonding of SU-8 and PDMS. Both oxygen plasma treatment andAPTES coating are required to introduce silicon-containing amine(Si NH2) groups on the SU-8 surface. Oxygen plasma etchingcan introduce silanol (Si OH) groups on the PDMS surface. Thestrong bonding of SU-8 and PDMS is based on the permanentSi O Si covalent bonds. Because the presented process is a low-temperature approach and uses only common microfabricationequipment, it is well suited for the fabrication of other all-polymerMEMS devices. It is anticipated that such devices will have greatpotential for a wide range of modern sensing and analyzing appli-cations.

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iographies

iheng Zhao received his BE degree in energy and environment system engineer-

ng from Shandong University, Jinan, Shandong, China, in 2011. Currently he is araduate student of mechanical engineering at Washington State University, Van-ouver, WA. His research interest is in the design and development of polymer-basedicroelectromechanical systems (MEMS) and microfluidic devices for biological

pplications.

tors A 187 (2012) 43–49 49

Debra A. Sheadel received a BS degree (1994) in pulp and paper science, a BSdegree (1995) in chemical engineering, and a MS degree (2012) in medical deviceengineering from the University of Washington, Seattle, WA. She worked as aresearch assistant at Washington State University, Vancouver, WA in 2011. Cur-rently she is working as a quality engineer at TOK America, Hillsboro, OR. Herresearch interest is the integration of microfluidics and sensors for biomedicalapplications.

Wei Xue received his BS and MS degrees in electrical engineering from ShandongUniversity, Jinan, China, in 1997 and 2000, respectively, and a PhD degree in mechan-ical engineering from the University of Minnesota, Minneapolis, MN, in 2007.

He is currently an assistant professor of mechanical engineering at WashingtonState University, Vancouver, WA. His main research interest includes microfabri-cation, nanotechnology, polymer/silicon microelectromechanical systems (MEMS),micro/nano electronics, and chemical/biological sensors. He is a member of ACS,ASME, IEEE, and Sigma Xi.