Fabrication of a normally-closed microvalve utilizing lithographically defined silicone micro O-

rings

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J. Micromech. Microeng. 21 (2011) 025011 (11pp) doi:10.1088/0960-1317/21/2/025011

Fabrication of a normally-closedmicrovalve utilizing lithographicallydefined silicone micro O-ringsT Lemke, J Kloeker, G Biancuzzi, T Huesgen, F Goldschmidtboeingand P Woias

Laboratory for Design of Microsystems, Department of Microsystems Engineering-IMTEK,University of Freiburg, Georges-Koehler-Allee 102, D-79110 Freiburg, Germany

E-mail: thomas.lemke@imtek.de

Received 16 September 2010, in final form 26 October 2010Published 11 January 2011Online at stacks.iop.org/JMM/21/025011

AbstractThe focus of this work is on the development of a simple and variable process chain for theintegration of flexible silicone material into silicon-based microfluidic devices. Anormally-closed microvalve is chosen as a demonstrator device, as it combines features thatare not easily obtained from silicon devices alone, especially, a high leak tightness of up to1 bar pressure difference in the closed state and a high forward flow of several mL min−1 inthe open state. For this purpose, a photopatternable silicone is used as a deformable circularvalve lip between a piezoelectrically actuated membrane and a valve seat, similar to a microO-ring with a width of 50 μm. The microvalve is piezo actuated by monolayer piezo actuatorswith a peak-to-peak driving voltage of Vp–p = 200 V. The micro O-ring is pre-deformed by2.8 μm during the valve fabrication process to yield the normally-closed behavior. A dry filmresist lamination technology is developed for this critical process step to mate the two siliconwafers with the actuation membrane, the valve seat and the silicone O-ring in between at awell-defined distance. The dry film resist is used in a multifunctional way, not only topre-deform the valve lip, but also to define the geometry of the valve chamber and to ensure aleak-tight connection of both wafers. Altogether, a peak value for the on- to off-ratio of thenormally-closed microvalve higher than 30 000 is measured. This opens a wide range ofpotential applications, e.g. in micro-dosing, drug delivery, μ-TAS and microfluidics forbiological or chemical applications in general.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Over the last two decades, in the field of microfluidics alarge number of different types of microvalves have emerged[1]. As a general classification, microvalves can be dividedinto active and passive ones. Furthermore, these types ofmicrovalves can be categorized concerning their actuationprinciple. We focus here on active, mechanical, piezo actuatedand normally-closed microvalves which are fabricated ina combination of silicon bulk and surface micromachiningprocesses [2]. Compared to existing valve types, reviewedin [1] and [3], most of the MEMS-based silicon microvalvespresent valve seats are directly structured from silicon [1].

These types of microvalves often comprise bare silicon-on-silicon contact sealings. In this context, especially to realizethe hard contact sealing, full-wafer bonding technologies, e.g.silicon direct, anodic or eutectic bonding steps, are of primeimportance. Substrate parameters like bow, warp or substrateflatness and material compositions become critical issues inthe manufacturing process. Special substrate pre-treatmentsto modify the substrate’s surface before bonding, for instanceoxidation, metal deposition steps, special process conditionsat high temperature, high voltage or high pressure are neededand can become ‘mission-critical’ in MEMS processing. Inthis field, wafer-level packaging technologies like adhesive fullwafer bonding, in-depth reviewed by Niklaus et al [4], have

0960-1317/11/025011+11$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA

J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

been developed, which are less sensitive to the used materialcomposition and substrate parameters. If only the hardsilicon contact sealing is considered, the main disadvantage,especially for pure silicon microfluidic devices, is the lossof particle tolerance due to imperfectness of the sealingmechanism, which yields an increased leakage rate. Finally,the sealing structures, e.g. valve lips, gaskets or flanges, haveto be machined directly out of silicon, using processes likedeep reactive ion etching (DRIE). These technologies are mostsuitable, but they are time consuming and often complex. Herewe demonstrate the possibility of rearranging and changingconventional manufacturing processes to produce microfluidicdevices in a much simpler and highly flexible way. Themain element is a dry film resist technology that uses aphotosensitive polymer to realize the wafer-level packagingof MEMS microvalves. This cost-effective technology offersa large freedom of design. It allows us to apply conformaladhesive bond layers in a full wafer format using a lowtemperature lamination step, which is applicable to siliconwafers with large surface topologies or through-silicon vias(TSVs). Simultaneously, by performing only one lithographystep, this adhesive bond layer is used to define a microfluidicchannel layer between two silicon wafers. In the valvedesign presented here, this channel layer is used for the valvechamber. To overcome the disadvantages of the conventionalsilicon based sealing technologies, a silicone-based sealingconcept, similar to a micro O-ring structure, is introduced.Our microvalve is based on a piezoelectric unimorph actuationprinciple utilizing a monolayer piezo actuator. Due to itsnormally-closed characteristic, energy is only required to openthe microvalve. This paper is divided into four sections. Thefirst section gives the reader an introduction to the designof the fabricated microvalve followed by the second section,where the important material properties of the used siliconematerial are presented, and the micro-fabrication process ofthe microvalve is shown. In the third section, measurementresults of the fabricated microvalves are shown. Conclusionsand outlook are given at the end.

2. Microvalve design and working principle

As mentioned above, in contrast to silicon-based sealingmechanisms, here the integration of a low stress, highlyflexible and deformable photopatternable silicone material(Dow Corning WL5150 spin-on silicone) is shown to fabricatethe valve lips directly with a single photolithography step. Thissilicone material is well known from microelectronics andpackaging applications [5, 6]. The flexible sealing materialmakes the microvalve more particle tolerant and provides avery leak-tight sealing. Also conformity to the unevennessand roughness of the flange surfaces can be achieved withthis flexible sealing material [7]. Shinohara et al [8] havealready successfully demonstrated the usage of a siliconerubber material in a normally-closed micropump. Theymanually injected the gasket material into the microvalve afteranodic full-wafer bonding. In contrast to this technique, herewe present a process for precisely structuring and aligningthe micro O-ring by photolithography and bond-alignment.

To realize the normally-closed character of the microvalve,the silicone material has to be pre-deformed (compressed).Miserendino et al [9] have recently shown an approach tointegrate and pre-deform lithographically structured micro O-rings and gaskets made from WL5150 spin-on silicone. Toyield the pre-deformation of the micro O-ring and obtain aleak-tight sealing for a microfluidic packaging application,they mechanically clamped a chip-to-chip arrangement withthe silicone material in between. A defined torque has tobe applied via conventional screws to deform and pre-stressthe micro O-ring to the gasket. The aim of our concept isto pre-deform the micro O-ring directly within a full wafer-bonding step without structuring a gasket or flange. Therefore,a dry film resist lamination technology, developed by Vultoet al [10] and Huesgen et al [11], is used and optimized. Byan accurate definition of the different layer thicknesses of thespin-on silicone and the dry film resist layer, a pre-deformationcan be applied to the O-ring structure within the full wafer-bonding step. The designed microvalve (see figure 1) consistsof two substrates. The top substrate (see more detail infigure 7(a)), which is bulk micromachined from the frontside, forms a 60 μm membrane where the piezo actuator ismounted on top to drive the microvalve. On the backside of thetop substrate, a silicone ring from a photopatternable siliconeWL5150 (Dow Corning) is structured lithographically to forma micro O-ring to seal the inlet. The bottom substrate containsthe fluidic inlet and outlet ports and is bulk micromachined aswell. Both substrates are bonded together using a permanentdry film resist (Ordyl SY300, Elga Europe [12]), which islaminated over the whole bottom substrate and lithographicallystructured as well. The resulting dry film resist structure servesboth as a bonding layer and defines the fluidic valve chamber.There is no need for additional microstructuring steps likeDRIE. To seal the microvalve safely, the rubber material is pre-deformed during the wafer bonding step due to the differentthicknesses of the bonding layer (here 17.6 μm) and of themicro O-ring (see for detail figure 1(c)). An average O-ringthickness of 20.4 μm has been fabricated to ensure the pre-deformation of the silicone structure. There is no structuredvalve seat or flange, as shown in figure 1(c). A pressedbulk monolayer piezo actuator (material: Stelco PKG21,thickness = 200 μm, d33 = 400 × 10−12 C N−1, d31 =−200 × 10−12 C N−1) was chosen for the membraneactuator. After application of a negative driving voltage V − =−100 V, the piezo material compresses due to the d31-effect,and it applies a bending moment to the membrane–actuatorstack. As a result, the whole membrane with the O-ring islifted upward and opens the inlet (cf figure 1). By applicationof a short positive driving pulse of V + = +100 V to thepiezo actuator, the valve membrane with the structured O-ring is pushed downward onto the inlet. Hereby, the O-ring is deformed again and will immediately block the inletflow. After releasing the driving voltage to zero and removingleftover charges from the actuator electrodes, the actuator–membrane stack remains in a downward bent position dueto the hysteretic behavior of the piezo actuator. Once themembrane remains bent, the leak tightness of the valve isensured by the deformation of the flexible silicone material.

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

(a)

(b)

TopSubstrate

BottomSubstrate

Adhesive Bondlayer

FluidicIn-/Outlets

Piezo actuator

undeformedO-ring

Silicon membrane

deformed O-ring

(c)

Micro O-ring

fluid

1mm

500µm

Outlet

Silicone O-ringInlet

(d )

htg

nel

tel

nI

Chip length

Silicon membrane

Top view

Mask Opening

Piezo actuator

Figure 1. Rendered CAD picture of the microvalve design. In (a) the open and in (b) the closed state are depicted. In (c) both states of themicrovalve can be seen more in detail. In (d) a top view schematic of the microvalve with the relevant elements and geometric dimensions isshown.

Figure 2. Measured valve membrane hysteresis for an 8 × 8 mm2

60 μm thick silicon membrane actuated by a monolayer piezoactuator. The position where the valve is open and also the closedposition, where the flexible silicone O-ring is touching the base ofthe valve and is then further deformed, can be seen in the plot.

The inlet flow is blocked as long as the membrane togetherwith the deformed O-ring stays in the downward bent positionand until the inlet pressure exceeds a threshold value whichdepends on the bending stiffness of the valve membrane andthe degree of compression of the O-ring. Figure 2 symbolizesthe nonlinear hysteretic valve membrane motion for an 8 ×8 mm2 valve membrane driven with 200 Vp–p.

Table 1 summarizes the varied fabrication parametersand dimensions of the microvalve. These parameters havebeen determined by calculating the deformation of the valvemembrane for different actuation voltages, O-ring diametersand inlet pressures. Models from hydraulic theory have beenapplied to calculate and minimize the fluidic resistance fordifferent load cases (inlet pressure and actuation voltage) in

Table 1. Main geometry parameters for the fabricated microvalves.

Parameter Value Unit

Chip size (square) 14 × 14 mm2

Mask opening (square) 8 × 8, 10 × 10, 12 × 12 mm2

O-ring outer radius ro 468, 751, 1034 μmO-ring inner radius ri ro−50 μmO-ring width 50 μmMembrane thickness 60 μmGlue layer thickness 10 μm

the open state and also in the closed state. A maximumbackpressure of 500 mbar was preset for all the calculations.The cover area of the actuator on the valve membrane hasalso been optimized to yield a maximum displacement ofthe driving membrane and the O-ring. The optimal ratio ofcoverage has been determined by applying Kirchhoff‘s platetheory to be 0.85. All optimizations have been modeled inMathematica and will be detailed by means of a separatepublication.

3. Photopatternable silicone: important materialproperties and processing details

To understand the material behavior and technological featuresof the elastic silicone material, a separate experimental studyhas been performed. The measured spin-on curve for theWL5150 is shown in figure 3. First the substrates havebeen cleaned in Piranha solution (H2SO4+H2O2; 4:1 ratio) for10 min at 70 ◦C to remove organic contaminations. Residualmoisture is baked out in an oven at 120 ◦C for 120 min.The photopatternable silicone (5–6 ml) is dispensed in staticmode, spread with 350 rpm for 15 s and finally spun onwith varying rotation speeds. No edge bead removal wasdone. After spin coating, the resist is pre-baked at 110 ◦C for120 s on a hotplate and then exposed to UV-light. Thereafter,

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Figure 3. Resulting spin-on curve for WL5150, measured with along distance profiler (P11) from Tencor.

a mask aligner (MA-6, Suss Microtec) with a broadbandUV-light source (intensity = 9 mW cm−2) in soft-contactmode with a 300 μm alignment gap has been used to exposedifferent patterns of freestanding test structures (e.g. rings, barsand square pillars). As the coated and unexposed WL5150layers are tacky, this large alignment gap has been used toprevent sticking of the coated substrate to the photomaskand contact with the exposure machine in general due tothe material tackiness. A broadband UV-intensity of 800–900 mJ cm−2 is recommended to obtain optimal exposureresults [6]. In our experiments, best lithographic structuringresults have been achieved at an average exposure dose of828 mJ cm−2, which corresponds to a mean exposure time oftexp = 92 s. Longer exposure times, e.g. 95, 105 or 110 s,resulted in undesired substrate reflections and overexposureeffects. The UV-light exposure causes the activation ofa photo-sensitive compound in the resist formulation thatperforms a selective cross-linking of the exposed regionsduring a post exposure bake (PEB). This is done at 135 ◦Cfor 150 s again on a hotplate, followed by a puddle

(a) (b)

Figure 4. (a) Measured freestanding structures (rings) of different structure widths for dfilm= 37.5 μm and dfilm= 14.7 μm (averagethickness). (b) Measured sidewall angle. All structures were measured using a long distance profiler (P11) from Tencor.

development step using Dow Corning Semiconductor GradeSiloxane Solvent (DCSGSS) developer [6]. Finally, thesubstrates are cleaned in isopropanol for 30 s and rinsedwith DI-water (60 s). A hardbake has been done for60 min at 180 ◦C on a hotplate. The measured spin-oncurve is comparable to published data from [5], [13] and[6]. We have noticed differing top surface profiles for thetest patterns, which resulted from internal film stress during thebake-out process and higher exposure dosages due to substratereflections especially at the edges of the exposed patterns[14]. Smaller structures (wstructure � 50 μm) show slightlyconvex tops while larger ones are more concavely shaped (seefigure 4(a)). This pillow-like shape has also been reportedfrom [15].

Due to the fact that we have used a large (300 μm)exposure gap and a foilmask, the minimum feature size ofthe spin-on silicone structures has been determined to be28 μm, which has been measured for different test patterns(rings and bars). This is twice the value (15 μm) whichis expected by Dow Corning [6]. The maximum aspectratio has been measured to be 1.48, which is comparable tothe published data [5, 6]. The angle of the sidewall slopefor the fabricated structures has been measured to be 56◦

(see figure 4(b)). Additionally, the material hardness andYoung’s modulus of some structured square patterns havebeen studied by nanoindentation using a Veeco Multimodeatomic force microscope (AFM). A cubic corner indentertip from monocrystalline diamond has been used (E =1140 GPa, ν = 0.07). The maximum loading force was16 μN. The indentation modulus of the samples has beencalculated using the Oliver–Pharr method [16]. From theslope of curvature of the unloading curve, a reduced modulusEr can be measured, which accounts for the elastic recoveryof the indented sample material and the diamond indentermaterial (1):

Er = ((1 − ν2) · E−1

︸ ︷︷ ︸

sample

+ (1 − ν2) · E−1

︸ ︷︷ ︸

indenter

)−1, (1)

Ei = E · (1 − ν2)−1

︸ ︷︷ ︸

sample

. (2)

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Table 2. Summarized experimental results and material properties compared to published results and the supplied material data.

Parameter Measured value Dow Corning [6] Zhang et al [13]

Nanoindentation modulus (MPa) 243.76 ± 71.535a 301b 146.3Youngs’s modulus (MPa) 187.59 ± 55.053 160 112.6Indentation hardness (MPa) 9.64 9.5 9.8Min feature size (μm) 28 15 40c

Aspect ratio 1.48 �1.3 n/aAngle of sidewall slope (deg) 56 60 n/a

a The tensile modulus and average reduced modulus differ due to the influence of both thesample and indenter response. Due to the high difference of the indenter material hardnessand Young’s modulus compared to the sample data, the influence is negligible.b Derived data from a continuous stiffness measurement (CSM) of a 20 μm thick spin coatedfilm [6].c Line width.

Figure 5. Loading and unloading curve for a spin-coated 14.6 μmthick film, fabricated according to the mentioned process conditions,resulting from nanoindentation using a Veeco Multimode AFM atroom temperature.

The resulting loading and unloading curve can be seen infigure 5. We have measured an average reduced modulus Er

of 243.76 ± 71.535 MPa. According to (2) Young’s modulusE is 187.59 ± 55.053 MPa, assuming a Poisson’s ratio of 0.48[17]. According to [16] the nanoindentation hardness Hsample

can be calculated at the point of peak indentation load Fmax

and the corresponding projected contact area Acontact:

Hsample = Fmax · A−1contact. (3)

From (3) an indentation hardness of 9.64 MPa has beenmeasured which is in good agreement with the material datasupplied from Dow Corning [6].

All measured and derived parameters are shown in table 2and compared to published results. It can be seen that the foundmaterial properties are in good agreement with the suppliedmaterial data from Dow Corning.

4. Microvalve fabrication process

In this section the fabrication process of the microvalve isshown (cf figure 7). Starting point for both substrates are 4′′-silicon wafers (525 μm average thickness, n-doped, (1 0 0)-

oriented). First a 300 nm wet oxide SiO2 has been grown at950 ◦C followed by a 100 nm LPCVD Si3N4 deposition step at760 ◦C to form an etch-resistant hardmask for the subsequentpotassium hydroxide (KOH) etch process. The front sides ofthe top and bottom substrates have then been lithographicallystructured using a standard positive photoresist of 1.8 μmthickness (AZ1518, Microresist Germany) on a Karl Sussmask aligner (MA-6). After opening the hardmask withreactive ion etching (RIE, STS-Multiplex System), squaremembranes and square vias of different sizes (cf table 1 andfigure 7(a), (b)) have been etched at 60 ◦C in 30 w% KOHto prevent membrane notching effects. Time control has beenapplied to set the desired etch depth. Afterward, the substrateshave been stripped with 10% HF acid to remove the leftoverSiO2 and Si3N4 hardmask layers. On the top wafer’s frontside, 20 nm chromium and 200 nm gold have been evaporatedas a conductive ground electrode for the piezo actuators. Asdescribed above, the backside of the top wafer is then spin-coated with spin-on silicone and different sizes of sealing rings(cf figure 6(a)–(c) and table 1) are lithographically structured.Following the results of the lithographic performance study,we have chosen an O-ring width of 50 μm to yield a safelysealing micro O-ring. The thickness of the silicone layer hasbeen adjusted to 21.6 ± 0.54 μm (estimated thickness changeover a 4′′-silicon substrate) to generate a pre-deformation of theO-ring in the closed microvalve. A magnified SEM picture ofthe resulting micro O-ring can be seen in figure 6(a)–(c). Afterthe O-ring structuring step, there is still a thin residual siliconescum layer present on the substrate surface. This residual layercould prevent a good bond interface to the adhesive Ordylbondlayer and has therefore been removed by applying a RIE-etch process (STS-Multiplex System, parallel plate reactortype, f radio = 13.56 MHz) to the substrate. A mixture of SF6

and O2 in a ratio of 28 to 50 sccm (SF6:O2) at 100 mTorrpressure with 200 W of plasma power has been found as anoptimal gas composition to etch back the residual layer in ashort time (tRIE = 30...60 s). After the plasma treatment, thesubstrates have been rinsed in DI-water to remove all leftoverand re-deposited silicone from the surface. The bottomsubstrate is treated with Piranha solution (H2SO4 + H2O2) for10 min at 70 ◦C to clean all organic contaminations followedby a bake-out process to remove residual moisture at 120 ◦C for1 h in an oven. Afterward, the backside of the bottom substrate

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

(a)

(b)

(c)

Figure 6. (a) SEM picture of a lithographically structured 35 μm width micro O-ring. In (b) the micro O-ring can be seen magnified,whereas in (c) the smooth surface of the O-ring can be seen more in detail.

Si

SiO2Si N3 4

PhotoresistSilicone

CrAu

(b)(a)

T=95°C, F=60N

(c)

Piezo actuatorEpoxy glue

Ordyl SY320

Figure 7. Process chart for (a) the top substrate and (b) the bottom substrate. (c) The schematic bonding procedure.

has been laminated with a permanent dry film resist of 20 μmthickness at 95 ◦C with a lamination speed of v = 0.9 cm s−1 ina Mylam-12 office lamination system. After lamination of the

dry film resist, the substrates have been exposed to broadbandUV-light for an exposure time texp = 15 s (exposure dose =135 mJ cm−2). Front side to backside alignment has been

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Figure 8. Top- and bottom-chuck temperature as well as bond-toolpressure can be seen in this graph.

done again in an MA-6 mask aligner system. A PEB has beencarried out afterward at 85 ◦C for t = 15 s on a hotplate. Thenthe dry film resist has been developed in a sequence of xylene-based developer supported with ultrasonics, isopropanol andDI-water (each step 1 min), which results in a 17.6 μm (averagethickness) thick resist layer. The whole process of lamination,exposure and development is described in depth by Vultoet al [10] and Huesgen et al [11] and will not be detailedhere.

After preparing both substrates, bond-alignment is donein a bond aligner (BA-6) to ensure the correct micro O-ringplacement. Full wafer bonding is then carried out in an SB-6bond tool also from Karl Suss. Both substrates are bondedtogether without using vacuum. The bonding temperature isT = 95 ◦C with a bond force Fb of 60 N cm−2 (see figure 8).The bond-tool pressure ptool has been determined accordingto (4):

ptool = Fb · AOrdyl, (4)

Silicone

Ordyl

(a)

10 mm(c)

A A`

Sectional View A-A`

(b)

M5-Screw

Silicon

Figure 9. Schematic layout of the pull-test structure (a) with integrated silicone drums in a cross-sectional view (b). In (c) a pulled teststructure can be seen in the photograph.

with AOrdyl being the lithographically defined bond area. Adetailed output of the process conditions (top- and bottom-chuck temperature, bond-tool pressure) of the wafer-bondingprocess can be seen in figure 8.

After bonding both substrates together, the wafer stackhas been cured in air at 150 ◦C for 2 h in an oven to ensurefull bond strength. Huesgen et al [11] reported a tensilebond strength of 13.3 ± 2.36 MPa for the adhesive Ordylbondlayer. Here we monitored the tensile bond strength withfour test structures of size 10 × 10 mm2, which have beenintegrated into the microvalve production process. Two of thefour test structures included both Ordyl and silicone drums ofdiameters DOrdyl= 1000 μm, Dsilicone= 300 μm and the othertwo only included structured drums from Ordyl. The tensilebond strength has been measured using a self-developed tensiletesting machine. Therefore the specimens have been glued to aspecimen holder with an epoxy glue (UHU Endfest 300, two-component epoxy glue) and afterward pulled to the load limit(cf figure 9). The maximum pull force has been measured to bethe fracture strength of the Ordyl bondlayer material and hasbeen determined with a load cell (KD9363, ME-Messsysteme,Germany) [11]. A maximum tensile strength of 12.92 ±2.52 MPa has been determined for the specimens withintegrated silicone drums, which is slightly (below 5% ) lowerthan the reported values from [11]. Finally the substrates arediced. With the help of an epoxy die-bonder in pick and placemode (T-3000-M, Dr.Tresky) piezo actuators have been gluedonto the membranes. A 10 μm thick conductive epoxy gluelayer [18] has been precisely dispensed using a pipette. Infigure 10 a fabricated microvalve can be seen in a photograph.

5. Measurement results

The schematic measurement setup is depicted in figures 11(a)–(b). All experiments have been performed with de-ionizedwater at room temperature. For the tests, the specimens

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Figure 10. Photograph of a fabricated microvalve compared to a 1cent coin.

reservoir

pressurecontrol

microvalve flowmeter/scale

drain

piezo control

+U0

V

valve opening

t

valve closing

0

V

t

tpulse

(a)

(b)

-U

+U

-U

Figure 11. (a) Schematic of the measurement setup where theforward and backward flow was determined. (b) Voltage level of thepiezo actuator symbolized for the open and closed microvalve states.

have been mounted and clamped into a PMMA-housingwith the inlet port connected to a reservoir that has beenpressurized with nitrogen gas from a pressure controller (DPI-520, Druck Limited). The outlet port has been connectedto a scale (Sartorius, ED822) and the fluid flow measuredusing gravimetric techniques. Additionally the backward-flow has been measured with a liquid flow sensor (ASL-1600-20, Sensirion AG). The piezo actuator has been connectedto a high-voltage amplifier (SVR 350-3-bip, PiezomechanikGmbH, Germany) which supplied the driving voltage ofV − = −100 V to open and V + = +100 V to close themicrovalve, respectively. The backward or leakage-flow hasbeen measured by first applying a short positive driving pulseof V + = +100 V to the piezo actuator to push down the valvemembrane to the inlet and actively close the microvalve. Afterthat, the electrodes of the piezo actuator have been short-circuited to remove leftover charges from the piezo actuatorand move the microvalve into the closed state (cf againfigure 11(b)). According to [19] the leakage ratio Lvalve, which

Figure 12. Measured forward flow for different 8 × 8 mm2

microvalve designs.

Figure 13. Measured forward flow for a 10 × 10 mm2 microvalvedesign.

is defined as the ratio of the forward-flow Q̇forward and leakage-flow Q̇leak by the following equation (5):

Lvalve = Q̇forward · Q̇−1leak , (5)

has been determined. Lvalve should be as high as possible inthe active closed state as well as in the passive closed state toobtain normally-closed behavior of the microvalve.

5.1. Forward flow

To compare all different fabricated microvalve designs, themaximum pressure value for the forward flow measurementshas been varied up to 500 mbar. Results fromdifferent forward flow measurements can be seen fromfigures 12–14.

It can be clearly seen from figure 12 that for increasingO-ring radii a lower forward flowrate has been measured. Themaximum flowrates were measured to be 1.865 and 2.5 mLmin−1 for a 1034 and 751 μm O-ring design, respectively. The

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Figure 14. Measured forward flow for different 12 × 12 mm2

microvalves.

measured fluidic resistance for all measured devices exhibiteda nonlinear behavior. Due to the increased valve-membranestiffness for the 8 × 8 mm2 membrane size designs, the overallvalve membrane deflection and further the gapflow at theposition of the structured micro O-ring is lowered at higherO-ring radii which resulted in a higher fluidic resistance. Fora larger (10 × 10 mm2) microvalve design, from figure 13 itcan be seen that the maximum flowrate has been measuredto be 3.92 mL min−1 for a 468 μm O-ring design, whichis nearly twice as large compared to the 8 × 8 mm2 for a1034 μm design. Maximum flowrates of 11.34, 9.92 and9.86 mL min−1 have been measured for the 12 × 12 mm2

microvalve designs with O-ring radii ranging from 468, 751to 1034 μm, respectively. The flowrate difference betweenthe smallest radius (468 μm) and nearly doubled radius(751 μm) design at the same pressure (500 mbar) is in therange of 0.6%. Again, nonlinear flow behavior is noticed forthe large valve membrane designs. Due to the reduced stiffnessof the large membrane designs (12 × 12 mm2), compared to thesmaller valve membrane designs (8 × 8 mm2), which has beenrevealed by simulations and measurements [20], and the highermembrane displacement induced by the piezo actuator also atthe position of the micro O-rings at 468, 751 and 1034 μm, themeasured flow was measured to be in the same range. In thecase of larger membrane sizes also the pressurized area, wherethe pressure directly acts, depends on the O-ring diameter.The larger the O-ring diameter, the larger the force that actson the membrane will be. This backpressure-induced forceadds up to the actuation-force induced by the piezoelectricunimorph principle and lifts up the membrane additionally.As a result, the overall flow resistance of the valve reduces.This effect, depicted in figure 15, was already described byGoldschmidtboing et al [21] in a bidirectional micropump withactive silicon microvalves in terms of a pressure-dependentflow resistance, while the mechanical stiffness of the valveswas expressed by a mechanical constant fv with units of lengthper pressure [21].

p(x) hv

fluid

h +v hv

fv

q

x

silicon membrane

silicone valve lip

inlet

Figure 15. Schematic illustration of the pressure-inducedvalve-membrane bending.

Figure 16. Results of the calibration measurement. The backwardflow versus pressure difference with only a dead-plug connected tothe measurement setup is depicted, including the standard deviation.

5.2. Backward flow

The backward flow in the closed state versus applied pressureexemplary for different microvalve designs can be seen fromfigure 17. Before the measurements have been made, ashort positive driving pulse of V + = +100 V has beenapplied to the piezo actuator and leftover charges from theelectrodes were removed by short-circuiting them to bring themicrovalve into the closed state, as depicted in figure 11(b).In order to guarantee the measurement quality and to detectas small as possible backward flows, the whole measurementsetup has been calibrated in the range of zero up to 1.2 barwithout connecting the microvalve but only a dead-plug. Ascan be seen from the calibration measurement depicted infigure 16, an average offset leakage value of qoffset = 22.65 ±0.559 μL min−1 has been measured, which resulted from theused flowmeter sensor and the measurement setup. This valuewas constant over the whole measurement range, indicatinga stable measurement setup. For an 8 x 8 mm2 (751 μm)design, an average backward flow of 22.42 ± 0.586 μL min−1

has been measured with an applied pressure of up to 1 bar(cf figure 17). This value has been then adjusted to thepreviously done calibration measurement by subtracting theoffset flow of the calibration measurement qoffset from theaverage measured backward flow, indicating a mean backward

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Figure 17. Measured backward flow for an 8 × 8 mm2 microvalvedesign with 751 μm micro O-ring. The values were measured usinga Sensirion flowmeter sensor.

Figure 18. Endurance measurement of the leakage-flow of an 8 ×8 mm2 (751 μm) and a 12 × 12 mm2 (1034 μm) microvalve designtogether with a calibration measurement with a dead-plug connectedto the setup. A backpressure of 600 mbar has been applied over theentire measurement period of t >10 h.

flowrate of qleak = 230 nL min−1. This value is close to theminimum measurement limit of the used flowmeter sensor(ASL1600, Sensirion) with a minimum calibrated flowratedetection threshold of 200 nL min−1 [22].

5.3. Endurance measurement

In order to ensure the long-term stability of the fabricatedmicrovalves, endurance measurements over an increasedperiod of time (>10 h) have been completed. As is shownin figure 18 for an applied pressure of 600 mbar, the 8 ×8 mm2 microvalve remains closed with an average leakageflow of qleak = 360 nL min−1. For the 12 × 12 mm2

microvalve, an average leakage flow of qleak = 315 nL min−1

is measured. Two different regions are identified. In thefirst region (0 < t < 22 000 s), the leakage flow increasesslightly, both for the performed measurement with a dead plug

connected to the measurement system and with the microvalvedevice. This increase results from the fact that the wholemeasurement setup was filled with a very small amount of airbubbles trapped in the used connectors, pipes and pipe elbows.Also the flexibility (compliance) of the used pipes could resultin a virtual leakage flow. In the second region, after t =22 000 s depicted in figure 18, the leakage flow stabilizes toa constant value. From the measured leakage-flow values,Lvalve is calculated to be 6944 for the 8 × 8 mm2 (751 μm)microvalve design. For the 12 × 12 mm2 (rout = 1034 μm)microvalve, a maximum Lvalve of 36 288 is calculated.

6. Conclusion and outlook

This paper presents the design and application of a simpleand robust process platform for microfluidic devices. Anormally-closed silicon microvalve is chosen as a practicalapplication for this fabrication concept. The main featureof the process platform, the integration of a photopatternablesilicone as a micro O-ring material into a fully compatibleMEMS process is demonstrated. For this purpose, the mostimportant silicone material and processing parameters aredetermined first. The derived material parameters are ingood agreement with the supplied data from Dow Corning.A minimum O-ring width of 50 μm is chosen to obtain a leaktight, reproducible and photolithographically definable seal.The O-ring surface is observed to be smooth and thereforeadaptable to substrate unevenness and particles. A variation ofseveral O-ring and membrane dimensions is shown in differentmicrovalve designs. As a second element of the processplatform the usage of an adhesive polymer bonding technologywith a dry film resist, which simultaneously defines the fluidicvalve chamber and the bond interface, is demonstrated. Thisbonding technology is robust, cost effective and, comparedto other full wafer bonding technologies, adaptable to highlystructured substrate topographies. Adhesive wafer bondingwith dry film resist also makes it possible to full-wafer bondsubstrates with lower substrate requirements, e.g. with bowand warp specifications beyond 10 μm. The microvalvesthat are developed with this process platform are workingwith actuation voltages in the range of 200 Vp–p. Maximumflowrates in the open state have been measured to be11.34 mL min−1 at a maximum pressure of 500 mbar. Thelarger the valve membrane, the higher the nonlinearity betweenthe measured flow and pressure drop becomes. This behavioris explained by a mechanical stiffness difference for differentsized valve-membranes, which results in an additional forceacting on the membrane lifting it upward and increasing the gapunderneath the valve lip. The highest measured backpressurewith zero back-flow is above 1 bar for all fabricated microvalvedesigns. A pre-deformation of 2.8 μm, which is 13.73% of thefinal spun on silicone thickness, is found to comply with thenormally-closed requirements. The microvalve will be furtheroptimized in the near future. The monolayer piezo actuatorwill be replaced by a multilayer piezo actuator, which willresult in a dramatically reduced peak-to-peak actuation voltageof e.g. 20 Vp–p. Also, the integration of this normally-closedvalve concept in a mechanical reciprocating bidirectionalworking micropump is planned.

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J. Micromech. Microeng. 21 (2011) 025011 T Lemke et al

Acknowledgments

The support of the Federal Ministry of Education and Research(BMBF) and the Clean Room Service Team of the IMTEK(RSC) is gratefully acknowledged. Mr Herman Meynenfrom Dow Corning is acknowledged for the contributedsupport regarding processing issues. Dr Yi Thomann fromthe Freiburg Materials Research Center (FMF) is gratefullyacknowledged for performing the AFM measurements. MrHagen Feth is acknowledged for the support during the fluidiccharacterizations.

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