Microfluidic etching driven by capillary forces for rapid prototyping of gold structures

Microelectronic Engineering 67–68 (2003) 229–236www.elsevier.com/ locate/mee

M icrofluidic etching driven by capillary forces for rapidprototyping of gold structures

*R.W. Stark , M. Sakai Stalder, A. StemmerNanotechnology Group, Swiss Federal Institute of Technology Zurich, ETH Center CLA, CH-8092 Zurich, Switzerland

Abstract

Soft lithographic methods based on poly(dimethylsiloxane) (PDMS) for pattern transfer are establishedalternatives to conventional lithographic methods. For applications in the biological sciences functionalized andstructured noble metal surfaces are required. In order to allow for rapid prototyping of such gold structures amethod is needed that does not change the surface chemistry. This can be achieved by a microfluidic system ona gold substrate that is filled with an etchant. From theoretical considerations simple thumb rules for thegeometric and chemical design of such a microfluidic system were established. For experimental testing amicrofluidic system was realized by contacting a structured PDMS stamp with a substrate consisting of a50-nm-thick gold layer on a glass object slide. The etchant was applied to the entrance of the capillary and theacid was drawn into the fluidic system by capillary forces. Taking advantage of a PDMS stamp structured withwetting and non-wetting regions no additional self-assembled monolayer was needed for masking. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Microfluidic etching; Rapid prototyping; Soft lithography

1 . Introduction

Soft lithographic methods for pattern transfer based on polymeric stamps fabricated with theelastomer poly(dimethylsiloxane) (PDMS) are established alternatives to conventional lithographicmethods [1]. Most prominent techniques are micro contact printing [2,3] and micro moulding incapillaries [4–6]. In a similar approach PDMS stamps are used to create microfluidic networks bybringing the stamp in contact with a surface [7–10] or by creating three-dimensional structures[11,12]. Using a two-dimensional system, transport of a liquid over a distance of several millimetersfrom the original delivery point to the desired location was possible [7]. Patterned deposition ofproteins in microfluidic networks was demonstrated successfully [13,14]. The confinement of the

*Corresponding author.E-mail address: [email protected](R.W. Stark).

0167-9317/03/$ – see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0167-9317(03)00076-5

mailto:[email protected]

230 R.W. Stark et al. / Microelectronic Engineering 67–68 (2003) 229–236

liquid in microfluidic devices can be enhanced by structuring the surface with wetting and non-wetting regions [15,13]. Moreover, the material PDMS has proven its stability against etchant in thefabrication of microstructures [16].

For applications in the biological sciences functionalized and structured noble metal surfaces arerequired. For rapid prototyping of such gold structures a method is desirable that does not completelychange the surface chemistry as it is the case in conventional soft lithography, where the gold surfaceis masked by a covalently bound self-assembled monolayer of alkane thiols. As an alternative tolift-off techniques [17], a direct approach to etch gold structures by taking advantage of a microfluidicsystem and thus generating the structure directly with the PDMS stamp is investigated. In thefollowing the strategy for microfluidic etching with open capillaries is discussed.

2 . Design strategy for the microfluidic system

In order to achieve uniform etching it is most important to distribute the etchant efficiently in themicro-capillary system. The time needed to fill the capillary system should be smaller than the typicaltime-scale for the etching process. Fig. 1a shows the geometry of an open capillary with widthw andheighth that is filled with a volumeV of etchant L to the depthx. The liquid is wetting the capillarysurfaceA. Three surfaces S of the capillary consist of PDMS with a solid–liquid interfacial freeenergyg and a solid–vapour interfacial free energyg . The fourth surface S9 is the gold on glassSL SV

substrate withg andg . The viscosity of the liquid ish, and the liquid–vapour interfacial freeS9L S9V

energy isg .LV

Under the assumption that the liquid drop at the entrance is large as compared to the dimensions ofthe capillary (r . .w,h) the rate of liquid flow in an open capillary system is given by [4,8]:

Fig. 1. (a) Scheme of a partially filled PDMS capillary on a gold on glass substrate with a liquid droplet at the entrance. Thecapillary is open at both ends. (b) Three-dimensional representation of the normalized rate of capillary flow as a function ofthe geometric capillary aspect ratiow /h and the contact angleu of the etchant on the PDMS (u 950).

R.W. Stark et al. / Microelectronic Engineering 67–68 (2003) 229–236 231

dx V/A 2h 1w w] ]] F]]] ]]] G5 ? ? g 2g 1 ? g 2gs d s dSV SL S9V S9Ldt 4hx 2 h 1w 2 h 1ws d s d

2 22 g 2g g 2g 1 g 2g g1 hw hws d s d s dSV SL SV SL S9V S9L LV]] S]]D F]]]] ]]]]]]]]G ]] S]]D5 ? ? 1 ¯ ?8hx h 1w w h 8hx h 1w

2cosu cosu 1 cosu 9F]] ]]]]]G? 1 (1)w h

Here, the relationsg ¯g cos u 1g and g ¯g cos u 91g between the interfacial freeSV LV SV S9V LV S 9V

energies and the respective contact anglesu andu 9 were used. The parameterh in Eq. (1) is usuallyfixed in typical rapid prototyping processes because the capillary height is given by the thickness of

~ ~the photoresist. Thus, it is convenient to introduce the dimensionless rate of liquid flowj5[x /(g h /LV

8hx)] and the capillary aspect ratioa 5w /h leading to:

2dj a 21] ]] fs d g¯ ? 2a 1 1 cosu 1 cosu 9 (2)S Ddt 11a

Fig. 1b illustrates the dependence of the normalized rate of capillary flow on the geometricdimensions as well as on the contact angle between PDMS and the etchant under the assumption of aperfectly hydrophilic substrate (u 950). It is evident, that narrow capillary structures are filled atsmaller rates because the contribution of the wetting substrate surface is reduced as compared to widestructures.

At u5608, which is a typical contact angle for water on plasma-treated PDMS, the normalized rate~of liquid flow is j 51.5 for an infinitely wide structure. Already ata51.38 the flow rate drops to`

~1/2j . Thus, in the design of the microcapillary system a minimum aspect ratio parametera has to be`

ensured in order to avoid large differences in the flow velocity to achieve a homogenous distributionof the etchant.

The conformal contact between the gold substrate and the PDMS can be disturbed by the surfaceroughness which gives rise to unwanted leakage capillaries perpendicular to the microcapillary. The

~rate of liquid flow in these leakage capillariesy is given by Eq. (1) where nowh is the height ofleak

the leakage capillary and w the respective width. Thus, from Fig. 1b it is clear that two importantparameters define the quality of the sealing between the capillaries: (i) the contact angleu between

~the etchant and the PDMS should be as large as possible to reduce the leakage flow ratey , and (ii)leak

the contact between the gold and the PDMS structure has to be as tight as possible because the~leakage flow ratey linearly scales with the height capillary of the leakage capillary.leak

For a capillary with square cross section (h5w) Eq. (1) simplifies to:

hgdx LV] ]]¯ ? 3cosu 1 cosu 9 (3)f gdt 32hx

~With u 950 the flow ratex vanishes foru5109.58. This means for example, that native PDMScapillaries will show poor performance in the transport of water because the contact angle betweenwater and native PDMS isu51058.

The maximum size of the structure can be estimated from the time needed to fill the longestcapillary of the system. This time should be shorter as the timescale for the etching process.Integration of Eq. (3) yields the time needed to fill a capillary with lengthl:


216hl]]]]]]]t 5 (4)hg 3cosu 1 cosu 9s dLV

23 2 23For example, water (h51.0310 Ns/m ,g 572.8310 N/m) fills a typical plasma treatedLV

PDMS capillary on a glass substrate (u5608, u 9508, h55 mm) of l51 cm within t54.4 s.From these considerations it is clear that there are several parameters that can be manipulated in

order to achieve enhanced capillary flow. Most important for the design are the capillary aspect ratioa and the contact angleu between PDMS and etchant. This leads to the following rules for thedesign:

(i) The extension of the structure is limited to:1 / 2hg 3cosu 1 cosu 9 ts dLV etch

]]]]]]]]l # (5)F Gmax 16h

where t is the time scale of the etching process.etch~(ii) The minimum aspect ratioa should be selected to ensure that capillary flow ratej(a ) is ofmin min

~the same order of magnitude as the maximum flow ratej .`

(iii)The capillary system must be chemically structured with wetting channels and non-wettingsealings in order to avoid unwanted etching.

3 . Experimental

3 .1. Materials and methods

3 .1.1. Stamp fabricationA silicon structure or a structured photoresist served as a master for the production of the stamp

(Fig. 2a). The PDMS (Sylgard 184, Dow Corning) prepolymer components were mixed following theinstructions by the manufacturer. After moderately evacuating the liquid PDMS to remove air bubblesit was poured onto the master (Fig. 2b) and cured at 508C for about 24 h to allow for cross linking.To make the material hydrophilic, the PDMS stamp was exposed to oxygen plasma for about 1 min(Fig. 2c). After contacting the plasma treated stamp with a piece of native PDMS to stimulatehydrophobic recovery of the surface (Fig. 2d) the stamp was ready to use. A simple model for theeffect of plasma treatment and the hydrophobicity recovery is displayed in Fig. 3 (For details cf. Refs.[18,19]).

3 .1.2. SubstrateAfter deposition of a 1-nm-thick chromium layer a 50-nm-thick gold layer was evaporated onto a

cleaned glass slide.

3 .1.3. EtchingThe microfluidic system was established by contacting a structured PDMS stamp with the

gold-coated glass substrate (Fig. 2e). The quality of the contact between the gold surface and thePDMS stamp was estimated visually: the force onto the stamp was increased carefully until the total


Fig. 2. Fabrication process of gold structures by microfluidic etching. (a) A structured surface serves as a master for a PDMSstamp (b). (c) The stamp is made hydrophillic by O plasma treatment. (d) To generate a hydrophobic sealing the stamp is2

brought into contact with an untreated PDMS surface. (e) A droplet of etchant is placed at the entrance of the fluidic system.(f) After removing the PDMS stamp a gold structure remains where the stamp contacted the surface.

internal reflection due to the remaining air gap at the interface between the PDMS and the goldsurface vanished. Then, the etchant (aqua regia) was delivered to the entrance of the capillary and theacid was drawn into the fluidic system by capillary forces. After several seconds of etching, thePDMS stamp and the glass substrate were separated under deionised water to immediately stop theetching process. The gold structure (Fig. 2f) was then carefully rinsed with deionised water and blowndry with compressed air.

3 .1.4. Contact angle measurementsThe contact angle was determined by the sessile drop method (NRL Contact Angle Goniometer

´Model 100-00, Rame-Hart, Inc., Mountain Lakes, NJ, USA).

4 . Experimental results and discussion

In order to determine the effect of the duration of plasma treatment contact angle measurementswere carried out on treated PDMS surfaces (Fig. 4a). Already a 1-min exposition to the oxygenplasma was sufficient to reduce the contact angle between the PDMS surface and a water droplet to avalue below 608. For longer plasma treatment times there is a strong variation in the contact angle. Itis evident that a defined contact angle of about 508 can easily be achieved by oxygen plasmatreatment, whereas a further reduction of the contact angle is difficult to control.

As a first proof of concept a linear microcapillary structure was composed (h50.8 mm, w¯100mm, i.e., a¯125) and a 1-ml droplet of etchant (aqua regia) was deposited at the entrance of thecapillaries. After 4.5 min the PDMS structure was removed and the substrate was cleaned with purewater and blown dry with compressed air. The resulting gold structures were investigated bytransmission light microscopy (Fig. 4b). The dark linear structures in the image represent the


Fig. 3. Creation of a hydrophillic channel with hydrophobic sealing. (a) The native PDMS surface is treated with aO –plasma (b) leading to a hydrophilic surface with exposed OH-groups. (c) Hydrophobic recovery of the sealings is2

induced by mechanical contact with a native PDMS surface, which leads to (d) a rearrangement of the polymer. (e) As resulthydrophillic channels with a hydrophobic sealing are obtained.

Fig. 4. (a) Contact angle of the PDMS surface after air–plasma treatment. (b) Light microscopic image (transmission) ofgold contacts produced by microfluidic etching.


remaining gold on the glass substrate (bright). This clearly demonstrates that the concept ofhydrophilic channels in combination with a hydrophobic sealing allows one to protect the gold outsidethe channels from the aggressive etchant on the micrometer scale.

5 . Conclusions

A straightforward approach to create gold structures with the aid of a microfluidic system has beeninvestigated. Taking advantage of a hydrophilic /hydrophobic structured PDMS stamp, no additionalself-assembled monolayer protecting the surface is needed for masking. Employing a stamp withhydrophilic capillaries and a hydrophobic sealing, gold structures on glass could be obtained. Simplerules limiting the design of the structure were developed from theoretical considerations. However,these limitations arise from the fact that the etching agent is only passively drawn into the system bycapillary forces. With the help of active devices like micropumps the performance of the system canbe enhanced. Promising fields of application for rapid prototyping of gold structures by microfluidicetching are the development of biosensors based on immobilized thiolated molecules and prototypingof electrode structures for dielectrophoresis.

A cknowledgements

¨ ¨We thank Dr. M. Muller (ETH, Zurich, Switzerland) for his support with the evaporation system,¨Prof. N. Spencer (ETH Zurich) for his help with the contact angle measurements and G. Schitter

¨(ETH Zurich) for fruitful discussions.

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Microfluidic etching driven by capillary forces for rapid prototyping of gold structures