2LAAS-CNRS,University of Toulouse,Toulouse, France
Received May 8, 2007Accepted July 17, 2007
Review
SU-8 as a structural material for labs-on-chipsand microelectromechanical systems
Since its introduction in the nineties, the negative resist SU-8 has been increasingly used inmicro- and nanotechnologies. SU-8 has made the fabrication of high-aspect ratio structuresaccessible to labs with no high-end facilities such as X-ray lithography systems or deepreactive ion etching systems. These low-cost techniques have been applied not only in thefabrication of metallic parts or molds, but also in numerous other micromachining pro-cesses. Its ease of use has made SU-8 to be used in many applications, even when high-aspect ratios are not required. Beyond these pattern transfer applications, SU-8 has beenused directly as a structural material for microelectromechanical systems and microfluidicsdue to its properties such as its excellent chemical resistance or the low Young modulus. Incontrast to conventional resists, which are used temporally, SU-8 has been used as a per-manent building material to fabricate microcomponents such as cantilevers, membranes,and microchannels. SU-8-based techniques have led to new low-temperature processessuitable for the fabrication of a wide range of objects, from the single component to thecomplete lab-on-chip. First, this article aims to review the different techniques and providesguidelines to the use of SU-8 as a structural material. Second, practical examples from ourrespective labs are presented.
Keywords:
Lab-on-chip / Microelectromechanical systems / Microtechnologies / SU-8DOI 10.1002/elps.200700333
Electrophoresis 2007, 28, 4539–4551 4539
1 Introduction
The introduction of polymeric material in micro- and nano-technologies has proven to be one of the recent key develop-ments of this field. Molding techniques for thermoplasticssuch as injection molding, hot embossing, and thermo-forming have been successfully adapted to the low-cost massproduction of microcomponents [1]. The availability of awide range of materials with tailored properties is crucial forapplications in research fields such as microfluidics, wherethe control of the material properties, e.g., wettability, auto-fluorescence, transparency, dielectric strength, and bio-
compatibility is required. The possibilities of bulk [2] andsurface modification [3] promise further control over thepolymeric material.
Recently, techniques using thermoset materials havebeen widely used in micromachining. A well-known exampleis the set of PDMS-based techniques which has applicationsin micro- and nanopatterning (i.e., “soft-lithography”) [4–9]and in the fabrication of microfluidic devices [10, 11]. One ofthe key advantages of this elastomer is its elasticity thatmakes demolding and bonding more effective. On the onehand, the elasticity is an advantage in microfluidics regard-ing the integration of actuators [12]. On the other hand, itpresents a drawback for the structural integrity of the micro-channels [13, 14]. PDMS is not compatible with manyorganic solvents and can not be used in some applications[15]. Recently, fluoroelastomers have been investigated andrepresent an attractive alternative to PDMS [16].
Photoresists have been traditionally used to transfer apattern in a thin film during photolithography, but have alsobeen used recently as structural materials in micro- andnanotechnologies. A typical example is SU-8 which, a nega-tive thick-film photoresist patented by IBM in 1989 (US1989). The resist is an EPON SU-8 resin (from Shell Chem-icals) photosensitized with triaryl sulfonium salts. SU-8 isparticularly well suited for thick-film applications since it can
Correspondence: Dr. Patrick Abgrall, Thermal and Fluids Re-search Lab #N3-B2C-06, Singapore-MIT Alliance/School ofMechanical and Aerospace Engineering, Nanyang TechnologicalUniversity, Nanyang Avenue, Singapore 639798, SingaporeE-mail: [email protected]:165-6791-1859
4540 P. Abgrall et al. Electrophoresis 2007, 28, 4539–4551
be dissolved at high concentrations and demonstrates a lowabsorbance in the near-UV range [17]. Consequently, layersof a few hundred of microns can be easily spin-coated andpatterned with conventional UV exposure systems.
In microtechnologies, this unique feature was firstexploited by replacing the expensive X-ray lithography pro-cess in the LIGA technique (German acronym for Litho-graphie, Galvanoformung und Abformung) by a conven-tional UV lithography process [18]. This process, also called“UV-LIGA” or “poor man’s LIGA” allows the low-cost massfabrication of metallic microparts and can be further exten-ded to the fabrication of molds for replication in thermo-plastics. Stripping of SU-8 has been known to be difficult,but recently efficient strippers have been reported [19].Alternative solutions for removing SU-8 include the use of ahigh-pressure water jet (MIMOTEC process) [20] or the pre-vious deposition of a sacrificial layer (e.g., Omnicoat™ fromMicrochem) (Microchem Corporation http://www.micro-chem.com/). Realization of multilevel open structures asdefined in Fig. 1 by successive spin-coating and photo-lithography is straightforward [21–24]. Planarity can beimproved by developing the different levels in a single finalprocess. The fabrication of closed geometry is more challen-ging and requires additional technological tricks as dis-cussed later in this paper.
The good chemical compatibility and biocompatibility[25] makes SU-8 a material of choice for microfluidic devices[26]. The fabrication of free-standing structures such as can-tilevers or bridges in polymers also present some advantagesover silicon. For instance, the actuation voltage needed for aSU-8 electrostatic actuator can be reduced due to the lowerYoung’s modulus. Electrical [27], magnetic [28], optical [29,30], or mechanical properties [31, 32] of the free-standingpart can be tuned by mixing the resist with the correspond-ing functional materials.
The use of a photoresists as a structural material alsorepresents new opportunities in system integration. As latershown in this paper, the ability of a resist to planarize previouslayers and to be aligned with an excellent accuracy allows both3-D stacking and hybrid integration with silicon-based de-vices. These characteristics, unique to photoresists, open anew path to a complete integration of polymeric micro-channels and silicon-sensors in a complex lab-on-chip (LOC).
Figure 1. Definition of an opened- and a closed multilevel ge-ometry.
This paper aims to review the state-of-the-art SU-8-basedtechnologies and to recommend guidelines for the use ofSU-8 as a structural material for microelectromechanicalsystems (MEMS) and microfluidic devices. In the first part,the paper presents a review on SU-8-based technologies andbasic guidelines for choosing the right technology accordingto the expected device geometry. The first part also reviewsdifferent techniques for reducing stress, releasing SU-8parts, and modification. The second part discusses the tech-nologies developed in our labs and the several applicationexamples for microfluidics and optical MEMS. Finally, thepaper concludes on what in the near future MEMS andmicrofluidic applications can expect from technologies basedon thick-film photoresists.
2 Review
Since SU-8 is a negative photoresist, direct fabrication ofclosed microstructures by multilevel photolithography is notachievable. Two basic techniques to realize closed micro-structures are: (i) using sacrificial layers, (ii) sealing an openstructure by bonding/transferring to another layer.
A first criterion of choice between these two techniquesis the geometry of the desired components. Figure 2 showsthree basic closed geometries: a free standing structure, afreely moving structure, and a microchannel. If we refer tothe definition given in the Fig. 1, a free-standing structure ispurely closed while (Fig. 2a) while freely moving structuresand microchannels are a combination of open and closedgeometries (Fig. 2b and c).
Figure 2. Three different types of closed structures (typicaldimensions are given but could vary from an application toanother): (a) free-standing structure; (b) freely moving structure;(c) microchannel.
2.1 Release techniques
A first and simple way to fabricate purely closed micro-structures in SU-8 is to construct an opened microstructuresusing multilevel photolithography on a sacrificial layer, andthen to release the structure by removing the sacrificialmaterial and reverse it (Fig. 3). The release process is usuallyperformed in liquid phase. Thin films of titanium [33], sili-con dioxide [28], copper[34, 35], chromium [36], and trilayers
Figure 3. Purely closed SU-8 microstructure made by fabricatinga purely opened SU-8 structure (a) and reversing it (b).
chromium/gold/chromium [37] were used as sacrificialmaterials for releasing areas with lateral dimensions smallerthan 1 mm. The film material is selected according to thecompatibility of the chemical etchant with the other ele-ments, such as electrodes that are to be integrated with theSU-8 component.
Larger parts can be released using thicker electroplatedmetal films [34] or polymer films (Microchem Corporationhttp://www.microchem.com/)[38]. The latter materials aresimply spin-coated and have also the advantage of a lowerroughness compared to metallic counterparts. As it wasdemonstrated in [38], polymeric surfaces as large as 50 cm2
can be released in only 60 min. Another option for releasinga SU-8 part is to etch the silicon substrate in a solution ofpotassium hydroxide [39, 40]. Anodic dissolution of alumi-num in a neutral solution of sodium chloride has resulted inthe release of SU-8 structures as large as 15 mm615 mmwithin 16 h [41]. Dry etching of a polysilicon layer has alsobeen demonstrated [42]. Convenient mechanical releasewithout aggressive chemical steps can be achieved by con-structing the device on a surface with low adhesion to SU-8but sufficient to carry the different steps of the process. Self-assembled monolayers of dodecyltrichlorosilane [43], Teflon-like [44, 45], polyimide (PI) [46], and polyethylene ter-ephthalate (PET) [26] films have been used for this purpose.
A further simple release method consists in the localrelease of the SU-8 part by back-etching techniques [47], asillustrated in Fig. 4. Deep reactive ion etching (DRIE) of sili-con is suitable for this purpose. This process is useful for thefabrication of hybrid silicon/SU-8 microsystems.
2.2 Sacrificial layers
Polymeric surface micromachining can be used for morecomplex microstructures, which are not purely closed andrequire a higher level of system integration. Sacrificial tech-niques are then better adapted. A typical process is shown inFig. 5. An important parameter is then the size of the air gapunder the free-standing structure. Components such as the
Figure 4. Free-standing microstructure made by back-etching: (a)patterning; (b) back etching.
Figure 5. Surface-micromachining of a free-standing micro-structure: (a) deposition and patterning; (b) releasing the sacrifi-cial material.
free-standing cantilever or the freely moving part in Figs. 2aand b have an accessible gap with a small surface (i.e., a fewmm2 up to a few 104 mm2). This configuration allows thesacrificial layer to be easily accessed and removed in a liquidphase.
In contrast to the sacrificial layers presented in the pre-vious section and used only to release the whole microparts,the sacrificial layer for polymeric surface micromachininghas to be patterned and its thickness defines the depth of thegap between the component and the substrate surface. For arelatively high gap (more than 1 mm), electroplated metalfilms have been used as a sacrificial layer [34, 48]. Polymersrepresent an interesting alternative. Polymeric sacrificialmaterial can often be structured using photolithography anddry etching with an oxygen plasma [49]. The use of photo-sensitive polymers further simplifies this process [33, 50].
Especially for analytical applications, microfluidic de-vices often have long microchannels with lengths on theorder of centimeters. In this particular case, the long releasetime by etching of is to be avoided. An original solution is theuse of thermodegradable polymers [51–53]. Polymers such aspolycarbonates (PCs) or polynorbonenes can be decomposedhomogeneously along the channel in a few hours without
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leaving residues by heating in temperature ranges of 200–300 and 400–4507C, respectively. Photosensitive polymers[54] have been used for this purpose as well, but workingwith a wavelength of 240 nm which is not commonly avail-able in a standard clean room. The degradation temperaturewas lowered to 1807C, and the process time was reduced to1 h [55]. These sacrificial materials can also be processedusing e-beam lithography (EBL) [53] or nanoimprint litho-graphy (NIL) [56]. SU-8 has been successfully used as astructural material using these sacrificial techniques [55, 57].
Another solution for the fabrication of microchannels,which has not been used with SU-8, could be the electro-chemical etching of a metal sacrificial layer. Impressiveresults have been obtained in a 4 cm long silica nanochannelpresenting a cross-section of 100 nm by 300 mm which wasreleased in 4 min [58]. Fast-etching materials such as poroussilica can also be envisaged but would lead to a high rough-ness of the cover layer.
For some applications necessitating a high gap, thicksacrificial layer, i.e., more than 10 mm, are required. Com-pared to the electroplating and polishing of thick metal films,the deposition of smooth and thick films by spin-coating ofpolymers is much cheaper and simpler. Accurate patterningof a thick sacrificial polymer layer is however challenging. Areported technique consisted in the filling of pre-existingthick structures with a polymer [21]. In fact, SU-8 itself maybe the only commercial polymer which can be so easily pat-terned with such a high-aspect ratio on large surfaces. That iswhy a solution with uncross-linked SU-8 as a sacrificial layernaturally emerged. Using this method, a way had to be foundto keep the underlying layer of SU-8 uncross-linked duringthe exposure of the top layers. A solution is the integration ofa UV-blocking layer between the sacrificial layer and thecover layer (Fig. 6). A few studies demonstrated that metalsuch as aluminum or magnesium can be used for this pur-pose [21, 59–62]. The following principles are used to mini-mize the exposure to UV during the process:
(i) deposition by evaporation of metals presenting a suf-ficient vapor pressure rate at a temperature low enough toprevent significant UV radiation is preferred,
(ii) the amount of heating of the substrate has to be keptsmall,
(iii) metal must be removed by wet etching to avoid anirradiation during a RIE.
An alternative solution is printing the metal layer using asoft PDMS stamp [45]. Using SU-8 as a sacrificial layer hasproven to be useful for structures such as micromirrors [59]or cantilevers [45, 62], but the long diluting time of uncros-slinked SU-8 prevented its use for the fabrication of longmicrochannels [21, 60].
2.3 Partial exposure and direct writing
Partial exposure of SU-8 was demonstrated using an antire-flection coating to remove the reflective component of theexposure and consequently increase the processing window
Figure 6. Fabrication of open- and closed structures usinguncross-linked SU-8 as a sacrificial layer and an UV-blockinglayer: (a) spin-coating of SU-8, (b) deposition and patterning ofan UV-blocking layer, (c) blanket exposure to UV, (d) spin-coatingof SU-8 and exposure to UV, and (e) development and etching ofthe UV-blocking layer.
[63]. The same process without reflection coating was alsorecently reported [64]. Another alternative was adding a toplayer of a positive resist which can partially absorbs the UVlight [30]. The process is similar to the polyMUMPS com-mercialized by MEMSCAPor the SUMMiTof Sandia NationalLaboratories. Planarization is intrinsically performed duringthe spin-coating step. In a recent publication, Ceyssens andPuers [62] made use of the higher absorption of UV light inSU-8 below a wavelength of 350 nm. Partial exposure wassuccessfully implemented using a wavelength of 313 nm.
Direct writing techniques using a laser [65], an electronbeam [66], or an ion beam [67] also allows partial or 3-Dexposure. Partial exposure techniques suits well to the fabri-cation of micromechanical parts. Direct writing methodsrepresent convenient ways for rapid prototyping. Despitethese advantages, the problem of removing uncross-linkedSU-8 in long microchannels is still present.
2.4 Bonding
Closed microstructures can also be formed by assemblingtwo wafers. These bonding techniques are particularly wellsuited to microfluidics, but also to completely closed struc-tures (e.g., a buried well filled with a liquid), where sacrificialmethods are not applicable. A typical process is depicted inFig. 7. Different approaches have been attempted.
The first possibility is to use cross-linked SU-8 as a bond-ing layer. After patterning of the photoresist, both wafers arealigned, and bonded in vacuum under high pressure and ele-vated temperature using a dedicated equipment [68].
Figure 7. Fabrication of closed microstructures by bonding: (a)alignment of two substrates with open SU-8 structures and (b)bonding.
Typical pressure, temperature, and bonding time are 3 bars,1007C, and 20 min, respectively [69]. The fabrication of 3-Dmicrofluidic structures have been demonstrated by succes-sive bonding and release steps [70].
Adhesive bonding of cross-linked structures, i.e.,introducing a glue, has also proven to be efficient [36, 71–73].poly(methyl methacrylate) (PMMA) was used as an inter-mediary layer in a few works [74, 75]. Channels as thin as300 nm have been created using this method [76]. Theintroduction of a second material in the structure leads tohomogeneities which can be useful by example for lightguiding applications but may be undesirable in applicationssuch as electroosmotic pumping, where surface propertiesare critical.
To overcome this issue, uncross-linked SU-8 itself can beused as a glue [22, 77–79]. In contrast to the method usingcrosslinked SU-8, this technique only requires simpleequipments such as an UV aligner and a hotplate. Typicalprocesses involve bonding temperatures between 50 and707C. Very thin layers of uncross-linked SU-8 were used asadhesive to avoid a filling of the gap [80–82]. A challenge inthis technique is the patterning of the uncross-linked layer.In many reported works with a blanket exposure through aglass substrate, patterning was not possible because the SU-8layer is not accessible by the developer after exposure. Thesubstrate has to be removed first. Tuomikoski and Franssila[79] etched the whole glass substrate in hydrofluoric acid torelease the device. But this method can not be used to accessthe SU-8 layer before development, since SU-8 is crosslinkedin a strong acid. Even using low-adhesion surfaces, releasingof a hard substrate is challenging.
2.5 Transfer techniques
Uncross-linked SU-8 can be transferred from a flexible car-rier substrate to another substrate. In opposition to the
bonding techniques presented in the previous paragraph,this flexible carrier substrate can be simply peeled off the SU-8 after bonding. This convenient release step allows to pat-tern the transferred SU-8 layers and avoids a subsequentdrilling step of the substrate. Layer-by-layer assembly allowsthe fabrication of complex 3-D structures. The flexibility ofthe carrier substrate also ensures a conformal contact andenhances the bonding quality. This process is schematicallydepicted in Fig. 8. This technique makes the deposition of anuncrosslinked SU-8 layer possible, without filling the under-lying open structures. After lamination, the layer is exposedto UV using a conventional aligner. A commercial film (Ris-ton®, Dupont Printed Circuit Materials) was used for thispurpose [21, 83]. A simple method for the fabrication of filmson a flexible substrate from virtually any liquid resist hasbeen developed at LAAS-CNRS. It has been used to makeand laminate uncross-linked SU-8 films with thicknessesfrom 10 to 120 mm [84]. A similar approach has been devel-oped at the same time at the Louisiana State UniversityCenter for Advanced Microstructures and Devices [85].
Figure 8. Fabrication of closed microstructures by lamination ortransfer of an uncross-linked SU-8 layer (PET stands for poly-ethylene terephthalate, PI stands for polyimide): (a) crosslinkedSU-8 structures, (b) lamination of a film of uncross-linked SU-8,(c) exposure to UV and PEB, (d) peeling of the release liner anddevelopment, and (e) repletion of step (b) and (c).
2.6 Stress considerations
The factor of ten between the coefficients of thermal expan-sion (CTE) of SU-8 (52 ppm/K) [86] and silicon (2.5 ppm/K)[87] results in a high level of residual stress, which leads tobending of the wafers. The stress also causes many problemsduring the subsequent process steps such as photo-lithography, bonding, or any operations requiring a handlingof the wafers with vacuum chucks. The stress will also even-
4544 P. Abgrall et al. Electrophoresis 2007, 28, 4539–4551
tually lead to cracks in the structure and is an importantsource of failures which reduce the lifetime of the devices.Different complementary strategies may be used to reducethese effects.
First, the surface of contact between SU-8 and siliconshould be minimized during the design. This is a criticalpoint in microfluidics where chips with large surfaces areoften required. A common trick is to design channels withthin walls on the order of few hundreds microns [80]. Pillarsare then necessary to support the capping layer.
The second strategy is to use a substrate material with aCTE close to that of SU-8 such as PMMA or polyether etherketone (PEEK) [46]. We have found that the adhesive PETlayer in our lamination process lower the stress by more than65% for a 100 mm thick layer of SU-8 [88]. This could be at-tributed to the closer CTE but also to a reflow of the adhesiveduring the baking steps. A metallic layer on top of SU-8 canbe used to compensate the compressive stress caused by thesilicon wafer. We calibrated the tensile stress of a metalliclayer as a function of the thickness and used it to reduceresidual stress in SU-8 [106]. Lowering the temperature ofthe bakes, especially the postexposure bake (PEB), also hasbeen demonstrated to reduce the stress by more than 70%compared to standard procedure [89]. We routinely per-formed soft and PEBs at a temperature as low a 507C. Bakingtime is increased but the standard cooling time from 95 to257C is reduced leading to time sensibly equivalent to that ofa standard process. Finally, SU-8 itself can be improved toresult in a lower stress. As an example, addition of silicananoparticles has been demonstrated to lower the stress inSU-8 [32].
2.7 Adhesion
Adhesion between SU-8 and the substrate has also beenidentified as a problem in many reported works. The lack ofadhesion combined with a high residual stress may cause thestructures to peel off from the surface. Solutions for thisproblem include the spin-coating of a thin SU-8 layer of fewmicrometers using a low-viscosity SU-8 resist [80] and sur-face-modification using a silane with an epoxy group at theother end of the molecule [90]. A similar strategy has beendemonstrated with an epoxy resist using a silane with anamine group at the end [91]. Adhesion of SU-8 on metalsurfaces (e.g., used as seed layer for a subsequent metal elec-trodeposition or as a sacrificial layer) or metal on SU-8 layers(e.g., electrodes for MEMS or microfluidic systems) may alsobe a problem [92]. Adhesion quality can be improved withoxygen plasma treatment or coating of an ultra-thin titaniumadhesion layer.
2.8 Surface modification
The control of surface properties is fundamental in MEMS(e.g., stiction and friction issues) as well as in microfluidics(e.g., hydrophilicity/hydrophobicity, density of surface char-
ges available for a given electrolyte, antifouling character).SU-8 is hydrophobic. A first technique to render it temporaryhydrophilic is exposing the surface to a plasma [93–95].Chang-Yen and Gale [96] simply dip the surface in sulfuricacid to induce surface charges and to make the adhesion of afluorescent dye possible. Other methods for surface mod-ification are opening the epoxy rings using acetic acid ornitric acid with a catalyst (ceric ammonium nitrate or CAN)and reaction with ethanolamine [94, 97].
The adsorption of protein was modified by using a copo-lymer PLL-g-PEG on surfaces oxidized by an oxygen plasma[98]. In collaboration with the Laboratory of Organic andMacromolecular Chemistry in Lille, we demonstrated twogeneric methods to modify SU-8 [88, 99]. The first method isbased on degrading the surface using a mixture of acetic acid,nitric acid and CAN and grafting an organosilane. The secondmethod is based on grafting a molecule comprising twoamine groups at each end on the residual epoxy rings andgrafting and hydrolyzing of a copolymer comprising maleicanhydride functions. After these two steps, a high density ofcarboxylic groups was available for further modifications.
2.9 Bulk modification
Mixing of SU-8 with other compounds before processing isnecessary to tune its bulk properties (e.g., mechanical, optical,electrical, and magnetic). The surface properties can also beaddressed by a bulk modification. Nanosilica particle havebeen added resulting in a higher sensitivity, a lower friction,and a smaller CTE, so a lower residual stress [31, 32]. The photocuring kinetics of SU-8 with and without silica nanoparticleshave been studied [100]. The SU-8 was made conductive byaddition of silver nanoparticles [101] and ferromagnetic byaddition of nickel nanospheres [28]. Refractive index waschanged by adding a liquid aliphatic resin [29]. These tech-niques were used in the fabrication of an all-SU-8 microsystemwith embedded microchannels and optical fibers.
3 Examples of applications
3.1 Single components using the release technique
This first example show how simple SU-8 microelementscan be made and assembled using the release techniquedescribed earlier, Fig. 3. Microvalves and piezoelectricmicropumps have been realized and characterized. Theassembly procedure and a scanning electron micrograph of acheck valve before the release step are shown in Figs. 9b andc, respectively [36, 39, 102].
The fabrication of fully polymeric microfluidic units wasalso demonstrated. Adhesive bonding has been used tocombine PMMA sheets machined with a CO2 laser and theSU-8 elements. The functionality of these functions has beeninvestigated. This process can easily be implemented for themass-fabrication of microfluidic cartridges.
Figure 9. (a) Flow rate of thepiezoelectric micropump as afunction of the voltage, (b) lami-nation assembly concept of amicropump and (c) a scanningelectron micrograph of a check-valve before release.
A similar process has been used for the fabrication of amicrogripper shown in Fig. 10. The low Young’s modulusand the high CTE compared to silicon make SU-8 an attrac-tive material for thermal actuators. In the specific case of amicrogripper, SU-8 offers a more gentle mechanical contactthan its silicon counterparts. The heaters have been
Figure 10. Scanning electron micrograph of a thermally-actuatedmicrogripper: (a) the body and (b) the tip.
created on the SU-8 by evaporating titanium (as an adhesionlayer) and platinum through a silicon stencil etched by DRIE.A silver-loaded epoxy glue was used to contact the electricalpads to the external wires. The tip had a gap of 30 mm. Adisplacement of 110 mm was measured for an actuation volt-age of 10 V.
3.2 Hybrid silicon-SU-8 structures using sacrificial
techniques
As highlighted in the previous part, sacrificial techniques areespecially well adapted to the integration of MEMS polymerstructures above integrated circuits (ICs). The relatively low-temperature process and the ability of SU-8 to planarizepreexisting structures make it suitable for this purpose. Inthe framework of a collaboration between the LAAS-CNRSand the Astrophysics Laboratory of Marseille, arrays of microdeformable mirrors (MDMs) have been fabricated for next-
4546 P. Abgrall et al. Electrophoresis 2007, 28, 4539–4551
generation adaptive optics system (e.g., next generationgiant telescopes) [59, 60, 106]. The structure consists of twoSU-8 levels with metallization (i.e., an electrostatic actuatorand a mirror), Fig. 11a. The actuator consists of a 10 mmthick plate with four springs attached to the substrate, andwith an air gap of 10 mm. The choice of SU-8 allows reduc-ing the actuation voltage due to its low Young’s moduluscompared to silicon. A photosensitive sol–gel material, acombination of SiO2 and the same sol–gel material anduncrosslinked SU-8 have been investigated as a sacrificiallayer [106].
Ten microliters thick layer of the silica-like sol–gelmaterial can be easily processed at temperature as low as857C by conventional photolithography with a resolution of1 mm. Its removal was done in a mixture of hydrofluoricacid with and without ethanol. For the same concentrationof hydrofluoric acid, the etching rate of the solutionincluding ethanol was much higher but led to a swelling ofthe structure because of the geometry of the MDM. Indeedthe second SU-8 structural layer covers the whole waferleading to a long release time.
A combination of SiO2 deposited by plasma-enhancedchemical vapor deposition (PECVD) at 3007C and sol–gelmaterial was also evaluated and the liberation was donein a solution of HF without ethanol resulting in a longerrelease time but without swelling. The fist sacrificiallayer in SiO2 resulted in a lower total release time of thestructure (see Fig. 11 and refs. [103] and [106]). The sec-ond layer was still deposited by sol–gel to avoid a toohigh temperature. An image of the results before thefinal metallization of the mirror is shown in Fig. 11b.The actuators can be seen through the second SU-8structural layer.
Figure 11. MDM: (a) principle, (b) SU-8 MDM without the finalmetallization: the actuators and the related electrical network arevisible through the top SU-8 layer, and (c) results of the actuationcharacterization by optical interferometry.
Uncross-linked SU-8 has also been investigated as asacrificial layer (see the process in Fig. 6) [60, 106]. Magne-sium was chosen as a UV-blocking layer due to its low tem-perature of evaporation and its fast etching in hydrochloricacid. The tensile residual stress generated during thedeposition of the magnesium was calibrated as a function ofthe thickness and used to compensate the compressive stressdue to SU-8. This solution, as the solution sol–gel material/hydrofluoric acid with ethanol, was effective for simple one-level structures such as a cantilever but also led to swellingdue to the longer release time (i.e., in our case with the twolevel structure). Finally, the chosen technology was the com-bination SiO2 by PECVD and sol–gel materials.
The actuators have been characterized and exhibited amotion of 2 mm for an applied voltage of 30 V. Figure 11cshows the results of the characterization of a mirror using anoptical interferometer and the corresponding results. Finiteelement method models was applied to this structure andshowed excellent agreement with the experiments. Using adedicated electronics, the actual location of the actuator ver-sus the expected location of the actuator is obtained with a SDof 21 nm [104].
3.3 SU-8 structures using lamination techniques
As stated previously, standard sacrificial techniques are notwell adapted for microfluidics where many centimeters longmicrochannels are often required. For this special applica-tion, we have developed at LAAS-CNRS a technique totransfer uncross-linked SU-8 films.[84] The objective was toclose without filling the preexisting open channels. For thispurpose, the lamination uncross-linked SU-8 films from aflexible substrate, presents many advantages. At first theflexibility of the layer and its substrate during the transferallow a more conformal contact and results in a better adhe-sion. The process is less sensitive to the thickness inhomo-geneities compared to bonding processes using a hard sub-strate to support the SU-8 layer. The second advantage ofusing a flexible substrate is the easy release by mechanicalpeeling after lamination. This allows a subsequent develop-ment of the structure (e.g., for the access holes) and to stackmany layers. Finally, the use of uncross-linked SU-8 allows agood adhesion and excellent layer-to-layer alignment using astandard photolithography aligner.
The flexible substrate carrying the uncrosslinked SU-8had to fulfill a few conditions. First the adhesion with SU-8had to be low enough to easily peel off the substrate afterlamination. But the adhesion had to be high enough for thespin coating or dip coating step. Finally, the material had tobe UV transparent because of the subsequent photo-lithography process. We have tried different materials andtechniques (PVDF, PI, PC, PET) and had the best resultswith the process described in the Fig. 12a. The process star-ted (step 1) with the lamination of a 50 mm thick adhesive
Figure 12. (a) Fabrication of an uncross-linked SU-8 film on a PETrelease liner: (1) lamination of an adhesive, (2) lamination of astack PET/adhesive/PET release liner, (3) spin-coating and soft-bake of SU-8 and (4) peeling of the stack SU-8/release liner, (b)scanning electron micrograph of a CE chip made by lamination offive SU-8 layers. (c) Picture of the flow of a fluorescent tracerinside a fluidic vortex microdiode made in SU-8. The liquidentered by the top level, escaped by the central via and flew in thelower channel.
(Adhesives Research ARClear 8932). Next an adhesive PET(Adhesives Research ARClear DEV-8796) is laminated with itsrelease liner (step 2). The stack comprised a 75 mm thick layerof PET, a 25 mm thick layer of adhesive and a 50 mm thick layerof PET which acted as a release liner. The side of the releaseliner towards the adhesive had a surface treatment whichallowed an easy peeling. Finally, a layer of SU-8 was spin-coated, soft-baked and the stack release liner/SU-8 was peeledoff from the hard substrate. The uncross-linked SU-8 film wasthen ready to be laminated on the existing microstructures, asit was explained previously and shown in Fig. 8.
We have been able to laminate up to five levels of SU-8with a resolution down to 10 mm. An oxygen plasma treat-ment on the pre-existing structure was necessary to improve
the adhesion before the lamination. The different technolo-gical parameters and their effects on the crosssection of thechannels have been investigated and will be published in acoming paper [88]. Compared to the parameters recom-mended by the provider Microchem, the soft-bake was three-times longer to avoid a clogging of the channel (MicrochemCorporation http://www.microchem.com/). The laminationpressure was around 2 bar. A higher lamination pressure (3–4 bar) results in partial filling of the structures. The temper-ature of lamination was 45 and 657C. A lower temperatureleads to bad adhesion, while a higher temperature leads topartial clogging. The cross-linking rate of a SU-8 is anincreasing function of the exposure dose. We have observed aslight collapse inside the channels for a too low exposuredose, due to the longer time available for the resist to flowbefore solidification during the PEB. We also observed that atoo high PEB temperature (i.e., 1157C led to a round-shapedroof of the microchannel that we attributed to the suddenexpansion of gas inside the microchannel while the materialis still soft.
Channels with a height of 35 mm and widths from 10 mmto 1 mm were realized. SU-8 films with thicknesses from 10to 120 mm were fabricated and transferred. The example of aCE chip made with five levels of SU-8 is shown in Fig. 12 (b).The total length of the separation channel was 1.35 cm on asurface of 1 mm2. The width and the height were 50 and40 mm, respectively. The different levels were aligned andinterconnected, as it is demonstrated with the example of afluidic vortex microdiode in the Fig. 12 (c). In this picture,the liquid entered by the top level, escaped by the central viaand flew in the lower channel. The top and bottom channelsonly communicate through the central fluidic via.
Capping a structure larger than 2 mm (e.g., a reservoir)led to the collapse of the film in the central part which wasnot a problem in our case since this part was an access holeand had to be removed by photolithography. For films thickerthan 100 mm, we observed a flow of uncrosslinked resistfrom the reservoir during the PEB. This resist may be diffi-cult to remove from the channel during the developmentstep. This could be a limitation when wide structures andthick films are required. We have calculated and measuredthat only small deformations occurred even for a thin cap-ping layer over a wide channel (i.e., a deflection of 8 mm inthe center of a 300 mm wide channel with a 10 mm thickcapping layer and a pressure of 2 bar while no sensitivedeformation was measured for a 40 mm layer). This indicatesthat thin capping layers can be used without affecting theflow.
We were also able to release purely SU-8 microfluidicchips by laminating a PET layer on the silicon substrate. Theadhesion was high enough to carry the whole process, andlow enough to easily peel off the structure at the end. Asmentioned above, we have also observed a decrease of theresidual stress by more than 65% for a 100 mm thick layer ofSU-8 using this first PET layer that we attributed to a closerCTE and a reflow of the adhesive layer during the baking
4548 P. Abgrall et al. Electrophoresis 2007, 28, 4539–4551
steps. The integration of electrodes inside the channels hasbeen demonstrated. In a multilevel framework, liquid pho-toresists would fill the closed channels after the spin-coating,so they can not be used for patterning a metal layer. Solutionsinclude the lamination of dry photoresist or the use of astencil. Different strategies of surface treatment have alsobeen developed in order to make this technology more ver-satile and were briefly described in the second part of thisarticle [88, 99]. MEMS structures such as cantilevers, bridges,or membranes have also been fabricated by lamination, as itis can be seen in Fig. 13. The above examples indicate thatfully polymer microfluidic systems integrating SU-8mechanical structures can be realized.
Figure 13. Examples of mechanical microstructures made bymultilevel lamination of SU-8 (a) cantilevers and bridges and (b)the gap of a test structure cut by a diamond saw.
3.4 Hybrid integration of silicon-chips and SU-8
structures using lamination techniques
Integration of microfluidic channels, sensors, actuators, andIC in a complete system is a real challenge. Though manyfunctions such as valving, pumping, or mixing can now beperformed by example using surface chemistry and electro-kinetic or capillary effects, i.e., without the need for siliconparts, there is always the need of integrating sensors or IC forthe concept of LOC to reach its full potential. In most of thecases, a microfluidic chip has a surface of many square cen-timeters what is, for the actual applications, much largerthan the surface required by the other functions. This obser-vation prevents the use of monolithic silicon technologies inan industrial context. Silicon technologies are cost effectivefor the fabrication of a high number of chips having a smallsurface. This problem leads to the necessity of hybrid inte-gration. The lamination technology described above wasdeveloped at LAAS-CNRS for this purpose. The idea was tocombine the photosensitive properties, the planarizingproperty of the spin-coating, and the “nonfilling” property ofthe lamination to lead to a hybrid silicon-polymer micro-fluidic system.
The complete integration process is schematized inFig. 14a ([88, 105] (Charlot, S., Gue, A. M., Tasseli, J., Marty, A.et al., J. Micromech. Microeng. 2007, submitted). The substrate
Figure 14. (a) Fabrication process of an hybrid silicon-polymerSU-8 microsystem: (1) PCB, (2) transfer of a silicon chip, (3)screen-printing of the conductive vias, (4) planarization of thechip by sreen-printing, (5) metallization through a stencil, (6)multilayer photolithography of SU-8 deposited by spin-coatingor lamination, and (7) bonding on electronic components. (b)Silicon pressure sensor after planarization and metallization(step 5). (c) Flow of a fluorescent tracer above the sensor inte-grated with a SU-8 microchannel using the same technology. Noleakage was observed at the interface Si/SU-8 and at the interfaceinsulating paste/SU-8.
was a printed circuit board (PCB) with some electrical tracks(step 1). It could also have been other materials such as glass,plastic, or ceramic depending on the applications. First asilicon chip was glued using a flip-chip equipment (step 2).Next, electrical vias were created by depositing 750 mm highmetallic islands by screen-printing (step 3). The purpose ofthese vias is to connect the PCB to the silicon chips andeventually to electrodes inside the microfluidic network. An
alternative would have been to fill with a conductive pastesome holes created in the insulating layer. The silicon chip isthen planarized using an insulating paste (step 4). A screen-printing equipment was specially designed for this applica-tion and is described elsewhere (Charlot, S., Gue, A. M.,Tasseli, J., Marty, A. et al., J Micromech Microeng 2007, sub-mitted). Different types of mask and insulating layers havebeen evaluated. The best results in term of smoothness andplanarization were obtained with a silicone Sylgard 186 hav-ing a higher viscosity compared to the Sylgard 184 widelyused in microfluidics (i.e., 80—120 Pa?s and 4 Pa?s, respec-tively). Electrical contacts were created between the metallicislands and the silicon chip using stenciling techniques (step5). A picture of the device at this step is shown in Fig. 14b.The stencil was simply realized using SU-8 technologiespeeled off a PET layer as described in the previous part.Finally the microfluidic network was fabricated layer by layerabove this system by spin-coating and lamination of SU-8(step 6). The spin-coating of a first SU-8 layer was necessaryto improve the adhesion between SU-8 and silicon, andcomplete the planarization at a lower scale. The compatibilitybetween materials in this process is challenging, and othercombinations have still to be investigated to improve the re-liability. Fluids with fluorescent tracers were pumpedthrough the microchannels for several hours, without obser-ving any leakage or short-circuits. Figure 14c represents theflow of the tracer above the silicon sensor. Pressure sensorswith a broken membrane were used for test purposes. Theblue square correspond to the fluorescent light emitted bythe printed-circuit board.
4 Conclusions
This paper aims to present the new opportunities offered bySU-8 as a structural material. The use of a photoresist as astructural material is a new tool for technologists who candevelop new, simpler, and cheaper processes. The propertiesof SU-8 (e.g., chemical resistance, optical, mechanical) com-pared to silicon and the possibility to tune them by bulk orsurface modification make it promising for many applica-tions such as LOCs or optical MEMS. In the first part wereviewed the different techniques to make closed micro-structures with SU-8. We demonstrated that simple (purelyclosed or purely opened) polymer microparts can be fabri-cated using multilevel photolithography and release tech-niques. Sacrificial techniques are well suited for the realiza-tion of free-standing or freely-moving micromechanicalparts on silicon or glass substrates, while lamination tech-niques suit better for the fabrication of long microchannelsand MEMS structures. Problems associated with residualstress and adhesion were also discussed. Differentapproaches of surface and bulk modification were presented.In the second part, examples illustrated this technologicalreview have been described, from the simplest componentsto the full hybrid microsystem.
Through this review, one can conclude that it is oftennecessary to modify SU-8 for technological reasons or be-cause of the applications. As an example, the UV adsorptionwas modified for a sacrificial process [30], films were fabri-cated from the liquid resist [84], refractive index was changedfor the fabrication of optical waveguides. SU-8 is clearly notthe right product for each application, and the availability ofnew products may have an important impact on the MEMSand microfluidic community. The techniques we havereviewed can be applied to any photoresist with the desiredstructural properties.
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