Wafer-Scale Fabrication of Polymer-Based Microdevices via Injection

Jun 29, 2005 - ... most of these devices still consist of simple channels and chambers, which ...... J. D. Jeyaprakash S. Samuel , Thilo Brenner , Osw...
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Anal. Chem. 2005, 77, 5414-5420

Wafer-Scale Fabrication of Polymer-Based Microdevices via Injection Molding and Photolithographic Micropatterning Protocols Dae-Sik Lee,*,† Haesik Yang,*,‡ Kwang-Hyo Chung,† and Hyeon-Bong Pyo†

BioMEMS Group, ETRI, P.O. Box 106, Yuseong-Gu, Daejeon, 305-350, Korea, and Department of Chemistry and Center for Innovative BioPhysio Sensor Technology, Pusan National University, Busan 609-735, Korea

Because of their broad applications in biomedical analysis, integrated, polymer-based microdevices incorporating micropatterned metallic and insulating layers are significant in contemporary research. In this study, micropatterns for temperature sensing and microelectrode sets for electroanalysis have been implemented on an injectionmolded thin polymer membrane by employing conventional semiconductor processing techniques (i.e., standard photolithographic methods). Cyclic olefin copolymer (COC) is chosen as the polymer substrate because of its high chemical and thermal stability. A COC 5-in. wafer (1-mm thickness) is manufactured using an injection molding method, in which polymer membranes (∼130 µm thick and 3 mm × 6 mm in area) are implemented simultaneously in order to reduce local thermal mass around micropatterned heaters and temperature sensors. The highly polished surface (∼4 nm within 40 µm × 40 µm area) of the fabricated COC wafer as well as its good resistance to typical process chemicals makes it possible to use the standard photolithographic and etching protocols on the COC wafer. Gold micropatterns with a minimum 5-µm line width are fabricated for making microheaters, temperature sensors, and microelectrodes. An insulating layer of aluminum oxide (Al2O3) is prepared at a COC-endurable low temperature (∼120 °C) by using atomic layer deposition and micropatterning for the electrode contacts. The fabricated microdevice for heating and temperature sensing shows improved performance of thermal isolation, and microelectrodes display good electrochemical performances for electrochemical sensors. Thus, this novel 5-in. wafer-level microfabrication method is a simple and cost-effective protocol to prepare polymer substrate and demonstrates good potential for application to highly integrated and miniaturized biomedical devices. Polymer-based microdevices are indispensable to recent advances in biomedical analysis.1-5 In particular, they have been * Corresponding authors. E-mail: [email protected]; [email protected]. † BioMEMS Group. ‡ Pusan National Universit. (1) Vilkner, T.; Janasek, D.; Manz, A. Anal. Chem. 2004, 76, 3373-3386. (2) Mastrangelo, C. H.; Burns, M. A.; Burke, D. T. Proc. IEEE 1998, 86, 17691787.

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adapted in many microfluidic platforms owing to their low cost in fabrication and versatility in preparing complex microsacle structures.5-8 Nevertheless, most of these devices still consist of simple channels and chambers, which limits progress in the miniaturization for fast and parallel processing. For the purpose of making devices even smaller and for practical applications, many functions such as temperature control and signal detection should be realized within these polymer devices. The other challenge is to integrate micropatterns of metallic and insulating layers into polymer substrates.4,9 Finally, the fabrication cost should be minimized as low as possible for disposable use.7 To date, there have been many reports on fabricating biomedical microdevices on solid substrates such as silicon and glass by employing semiconductor processes including standard photolithography, thin-film deposition, and subsequent etching protocols.10-15 However, the cost of these fabrication techniques is relatively higher than that of polymer-based devices; while using standard photolithography has been just recently applied to manipulation of the polymer substrate.16 Although there are several reports on polymer microstructures for fluidic applications,8,9,16-18 very few papers have talked about the micropatterning (3) Huang, Y.; Mather, E. L.; Bell, J. L.; Madou, M. Anal. Bioanal. Chem. 2002, 372, 49-65. (4) Tu ¨ do ¨s, A. J.; Besselink, A. J.; Schasfoort, B. M. Lab Chip 2001, 1, 83-95. (5) Quake, S. R.; Scherer, A. Science 2000, 290, 1536-1540. (6) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153-184. (7) Madou. M.; Florkey, J. Chem. Rev. 2000, 100, 2679-2692. (8) Becker, H.; Locascio, L. E. Talanta 2002, 56, 267-287. (9) Boone, T. D.; Ricco, A. J.; Fan, Z. H.; Tan, H.; Hooper, H. H.; Williams, S. J. Anal. Chem. 2002, 74, 78A-86A. (10) Burns, M. A.; Johnson, B. N.; Brahmasandra, S. N.; Handique, K.; Webster, J. R.; Krishnan, M.; Sammarco, T. S.; Man, P. M.; Jones, D.; Heldsinger, D.; Mastrangelo, C. H.; Burke, D. T. Science 1998, 282, 484-487. (11) Yang, H.; Choi, C. A.; Chung, K. H.; Jun, C. H.; Kim, Y. T. Anal. Chem. 2004, 76, 1537-1543. (12) Lee, D. S.; Park, S. H.; Yang, H.; Chung, K. H.; Yoon, T. H.; Kim, S. J.; Kim, K.; Kim, Y. T. Lab Chip 2004, 4, 401-407. (13) Mourlas, N. J.; Gilchrist, K. H.; Maluf, N. I.; Kovacs, G. T. A. Sens. Actuators, B 2002, 64, 41-47. (14) Lagally, E. T.; Scherer, J. R.; Blazej, R. G.; Toriello, N. M.; Diep, B. A.; Ramchandani, M.; Sensabaugh, G. F.; Riley, L. W.; Mathies, R. A. Anal. Chem. 2004, 76, 3162-3170. (15) Paik, P.; Pamula, V. K.; Pollack, M. G.; Fair, R. B. Lab Chip 2003, 3, 2833. (16) Ahn, C. H.; Choi, J. W.; Beaucage, G.; Nevin, J. H.; Lee, J. B.; Puntambekar, A.; Lee, J. Y. Proc. IEEE 2004, 92, 154-173. (17) Rossier, J.; Reymond, F.; Michel, P. E. Electrophoresis 2002, 23, 858-867. (18) Becker, H.; Ga¨rtner, C. Electrophoresis 2000, 21, 12-26. 10.1021/ac050286w CCC: $30.25

© 2005 American Chemical Society Published on Web 06/29/2005

of metallic structures on polymer substrates.16,19-22 This is mainly due to the incompatibility of many polymers with most semiconductor process chemicals (e.g., solvents used in photolithography) and weak thermal endurances at the photoresist baking temperature (e.g., high baking temperature).8 Alternative techniques such as shadow masks,19 screen printing,23 liquid-phase deposition with self-assembled monolayers,24 or microcontact printing6,25,26 have been employed to prepare micropatterns on polymer substrate. Nevertheless, it is difficult to obtain process compatibility (with film deposition, photolithography, or chemical etching) and to obtain patterns of narrow line width.19,23,24 For example, as a photoresist-free patterning method, a shadow masking method in which the microelectrodes have 100µm line width on a poly(methyl methacrylate) (PMMA) substrate has been reported by Ko et al.19 An electroless deposition method with 300-µm line width on a PMMA or polycarbonate (PC) substrate has been performed by McCarley et al.22 Buch et al. reported on a modified photolithographic method with large line width on a PC substrate.20 Another modified photolithographic method with 50-µm line width on a PMMA substrate has been reported by Grass et al.21 Their approaches involve a laboratorymodified photoresist formulation based on solvents that are inert to PC or PMMA substrates, owing to swelling of the polymer by semiconductor process solvents. To obtain compatibility with standard photolithography,8 a process-compatible polymer material is the prerequisite. The other requirement is the surface morphology; a polymer substrate with highly polished surface is necessary to obtain wafer-level uniform micropatterns of the photoresist. An addition challenge is to prepare micropatterns of insulating materials on polymer substrates for electrical or electrochemical isolation from a solution. Compared to other polymer fabrication methods including hot embossing, casting, soft lithography, and laser photoablation, the injection molding has a unique advantage over other methods in that it can create polymer substrates with wide areas such as CD plates as well as three-dimensional structures via a simple and fast process.8,9 For example, an injection-molded wafer with a high aspect ratio microstructure was designed and fabricated by Ahn’s group.16,27 Temperature modulation with integrated microcomponents for heating and sensing is of practical importance as it offers precise control of reactions or fluidics without mechanical manipulation. It has been applied in the modulation of biomolecule binding,28-30 (19) Ko, J. S.; Yoon, H. C.; Yang, H.; Pyo, H. B.; Chung, K. H.; Kim, S. J.; Kim, Y. T. Lab Chip 2003, 3, 106-113. (20) Buch, J. S.; Kimball, C.; Rosenberger, F.; Highsmith, W. E.; DeVoe, D. L.; Lee, C. S. Anal. Chem. 2004, 76, 874-881. (21) Grass, B.; Neyer, A.; Jo¨hnck, M.; Siepe, D.; Eisenbeiss, F.; Weber, G.; Hergenro ¨der, R. Sens. Actuators, B 2001, 72, 249-258. (22) McCarley, R. L.; Vaidya, B.; Alison, S. W.; Patel, A. B.; Feng, J.; Murphy, M. C.; Soper, S. A. J. Am. Chem. Soc. 2005, 127, 842-843. (23) Koh, C. G.; Tan, W.; Zhao, M.; Ricco, A. J.; Fan, Z. H. Anal. Chem. 2003, 75, 4591-4598. (24) Xiang, J. X.; Zhu, P.; Masuda, Y.; Koumoto, K. Langmuir 2004, 20, 32783283. (25) Wang, Z.; Zhang, J.; Xing, R.; Yuan, J.; Yan, D.; Han, Y. J. Am. Chem. Soc. 2003, 125, 15278-15279. (26) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601-603. (27) Do, J.; Choi, J.; Ahn, C. H. IEEE T. Magn. 2004, 40, 3009-3011. (28) Schienle, M.; Frey, A.; Hofmann, F.; Holzapfl, B.; Paulus, C.; SchindlerBauer, P.; Thewes, R. IEEE Technol. Dig. ISSCC. 2004, 1, 220-221.

cell manipulation,13 microreactors,31-35 microfluidics,35-38 and other biochemical processes on surfaces.39-41 On the other hand, integrated microelectrodes have been used for electrochemical detection,41 for immobilization of biomolecules,26 and for microfluidic control.15,36 Therefore, preparation and integration of micropatterns for heating and temperature sensing as well as microelectrodes for electrochemical reactions on a polymer substrate is critical to the development of the next generation of biomedical microdevices. To date, the integrated components on polymer substrates include microelectrodes on PMMA for immunosensing,19 resistive thermal devices on PC for DNA mutation detection,20 microelectrodes on PMMA for electroporation,42 and conductivity detector on PMMA for istachophoresis.21,43 To obtain a high ramping rate and low power consumption in a microheater, micromachined dielectric membranes have been proposed, particularly in conjunction with silicon-based microdevices.2,11-13 In the case of polymers, there have been no reports on the use of a membrane for enhancement of the thermal response and power efficiency, due to the difficulties of thin membrane formation processes. The electrical and electrochemical insulation of the microcomponents from the test solution is very important in biomedical microdevices. As such, deposition techniques of high-quality insulating layers on a polymer below a polymer-endurable temperature, i.e., the glass transition temperature (Tg), and their micropatterning techniques, which have not been reported thus far, should be developed. Moreover, the bonding techniques between the polymer devices with the insulating layer and the polymer fluidic components should also be considered. In this study, we use a standard photolithography-compatible polymer material, cyclic olefin copolymer (COC), as the polymer substrate for the preparation of microelectrodes and micropatterns of insulating materials via an improved injection molding protocol. The thermal characteristics of thus prepared microdevices, including the heating and cooling rates and power consumption as a function of the membrane thickness, will be examined by direct measurements of the surface temperatures using the resistive (29) Huber, D. L.; Manginell, R. P.; Smara, M. A.; Kim, B. I.; Bunker, B. C. Science 2003, 301, 352-354. (30) Ding, Z.; Fong, R. B.; Long, C. J.; Stayton, P. S.; Hoffman, A. S. Nature 2001, 411, 59-62. (31) Kopp, M. U.; De Mello, A. J.; Manz, A. Science 1998, 280, 1046-1048. (32) Hu ¨ hmer, A. F. R.; Landers, J. P. Anal. Chem. 2000, 72, 5507-5512. (33) Okano, K.; Yasuda, K.; Ishiwata, S. Sens. Actuators, B 2000, 64, 88-94. (34) Chen, H. J. H.; Chen, T. F.; Huang, S. R. S.; Gong, J.; Li, J. C.; Chen, W. C.; Hseu, T. H.; Hsu, I. C. Sens. Actuators, A 2003, 108, 193-200. (35) Liu, R. H.; Yang, J.; Lenigk, R.; Bonanno, J.; Grodzinski, P. Anal. Chem. 2004, 76, 1824-1831. (36) Buchholz, B. A.; Doherty, E. A. S.; Albarghouthi, M. N.; Bogdan, F. M.; Zahn, J. M.; Barron, A. E. Anal. Chem. 2001, 73, 157-164. (37) Selvaganapathy, P.; Carlen, E. T.; Mastrangelo, C. H. Sens. Actuators, A 2003, 104, 275-282. (38) Darhuber, A. A.; Valentino, J. P.; Troian, S. M.; Wagner, S. IEEE J. Microelectromech. Syst. 2003, 12, 873-879. (39) Kajiyama, T.; Miyahara, Y.; Kricka, L. J.; Wilding, P.; Graes, D. J.; Surrey, S.; Fortina, P. Genome Res. 2003, 13, 467-475. (40) Kim, K.; Yang, H.; Park, S. H.; Lee, D. S.; Kim, S. J.; Lim, Y. T.; Kim, Y. T. Chem. Commun. 2004, 1466-1467. (41) Simon, I.; Baˆran, N.; Bauer, M.; Weimar, U. Sens. Actuators, B 2001, 73, 1-26. (42) Lin, Y. C.; Jen, C. M.; Huang, M. Y.; Wu, C. Y.; Lin, X. Z. Sens. Actuators, B 2001, 79, 137-143. (43) Grass, B.; Weber G.; Neyer, A.; Schilling, M.; D.; Eisenbeiss, G.; Hergenro ¨der, R. Spectrochim. Acta, Part B 2002, 57, 1575-1583.

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Table 1. Results of Chemical Resistance Tests of Transparent Polymers to Some Chemicals Used in Semiconductor Processes material (heat stability, Tg)b

acetone ethyl alcohol developer (NMD-3) PR stripper (ACT1) PR (GA2, AZ5214E) Pt etchant (Aqua regia) Au etchant (KI+I2+H2O) BHF (6:1) H3PO4 aRT,

temp,a °C

PC (130 °C)

PMMA (80 °C)

COC (140 °C)

PES (180 °C)

PS (90 °C)

RT RT RT 80 RT RT RT RT 40

× O O × × O O O O

× O O × × 4 O O O

O O O O O O O O O

× O O × O O O O O

× O O × × 4 O O O

room temperature. b Key: O, good; ×, bad; 4, good at 10 min but bad at 60 min.

thermal devices (RTD). In addition, electrochemical performance of the gold microelectrodes will be also tested. EXPERIMENTAL SECTION Polymer Substrate. We have been searching for a polymer material applicable to biomedical microdevices from several transparent thermoplastic materials (i.e., PC (LG-DOW, 201-22), PMMA (LG, IG840), COC (Topas, 6015), polyethylene sulfone (PES) (Sumitomo), and polystyrene (PS) (LG, 25SP)) by checking their chemical resistances to typical process solutions for 10 min and their thermal stabilities (e.g., hard baking temperature).8,18 To evaluate the chemical resistance, we chose typical process chemicals employed in standard photolithography44 (acetone, ethyl alcohol, developer, photoresist, and its stripper) and wet etching solutions for noble metals and insulating layers (aqua regia and aqueous KI3 solution). Some polymers turned to be unstable, i.e., showing melting, swelling, or bleaching. However, COC showed a good chemical resistance to all test chemicals. Also, the thermal endurance of the test polymers under harsh process temperature conditions was considered through an evaluation of whether Tg of the test polymers was well above the hard baking temperature of ∼120 °C. PMMA and PS displayed poor durability under the process condition of hard baking. The results are summarized in Table 1. From these results, COC was chosen as a substrate material for microdevices including photolithographic micropatterning. Fabrication of Micropatterns on Polymer Substrate. Here, we present the fabrication techniques of integrated micropatterns on a COC substrate, including adhesion of thin metal film, photolithographic micropatterning of the metal, deposition of an insulating layer, and photolithographic micropatterning of the insulating layer. The fabrication process of microheaters, RTDs, and microelectrodes was designed to be simple and suitable for polymer-based mass production. The process began with the creation of a 5-in. polymer wafer (1-mm thickness) with a highly polished surface for photolithographic micropatterning and thin polymer membranes for enhancing heating rates. In preparing the polymer wafer, a novel injection molding method employing both a new rapid thermal process (RTP) and a chemo-mechanical polishing (CMP) process was utilized. The implemented mold (44) Sze, S. M. In Semiconductor Devices: physics and technology; John Wiley & Sons: New York, 1985; pp 364-380, 428-467.

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Figure 1. Cross-sectional view showing structures and relative positions of a COC wafer and molds for injection molding. For fabricating the topside of polymer wafer, the mold surface was polished by chemomechanical polishing process. For fabricating the thin membrane, the gap between the upper mold and the lower mold was locally controlled.

structure for the designed COC wafer is shown in Figure 1. For fabricating the topside of polymer wafer, the mold surface was polished by a CMP process. For fabricating a thin membrane for low thermal mass, the gap between the upper mold and the lower mold was locally controlled. An injection molding method using a unique rapid thermal process was employed to heat the mold surface for a moment above the melting temperature of the polymer with flame torches before polymer injection. The process was utilized to fill the molten polymer in the very narrow gap between the upper and lower molds in order to form the thin isolation membrane. The microfabrication process is shown schematically in the Supporting Information. First, COC wafers were cleaned in a bubbling piranha solution (1:3 hydrogen peroxide (45%)/sulfuric acid (96%)) for 10 min. Caution is needed in this step since the mixing process is strongly exothermic and releases hot vapor, and the mixture is highly reactive. Personal protective ware including a face shield, apron, and gloves should be used. The cleaned wafers were rinsed in deionized water and dried in a wafer spin-dryer for 10 min. For the metal micropatterning on the injection-molded polymer substrate, a thin film of gold (1000 Å) was deposited without any adhesion layer at room temperature in an electron beam evaporator (Ferrofluidics Co.). In the system, an electron beam (8-10 keV, 100-200 mA) is scanned over a metal target.44 While it is generally known that it is necessary to deposit a thin adhesion layer such as Ti or Cr beneath a nonreactive metal such as Au or Pt, in our case, the adhesion strength of the gold layer deposited on the COC polymer is superior to that of a gold layer with a thin Cr adhesion layer (30nm thickness). The strength was simply tested with Scotch tape.

Standard photolithography techniques were used to pattern the resist layer. In detail, the surface of COC wafers was coated with a positive photoresist (GA2, Tokyo Ohka Kogyo Co.). For this, the wafer is held on a vacuum chuck and then spun at a speed of 4000 rpm for 30 s to produce a thin uniform layer with 1.5-µm thickness. This is followed by soft baking for 5 min in an oven at 90 °C in air atmosphere to improve adhesion and remove solvent from the photoresist. After the soft baking, the photoresist is exposed through a mask with high-intensity UV light for 4.5 s, using a bottom side alignment mask aligner (MA6, SussMicroTec Co.). The photoresist is developed by dipping the wafers for 2 min at 20 °C using a developer (NMD-3, Tokyo Ohka Kogyo Co.). Following development, hard baking in an oven for 20 min at 120 °C was employed to harden the photoresist and improve adhesion to the substrate. For gold wet etching, aqueous KI3 solution (4 g of KI/1 g of I2 in 100 mL of H2O) was utilized. Aqua regia, which is a 3:1 volumetric ratio of HCl (38%)/HNO3 (70%), was used to etch patterns in platinum layers at 85 °C. Caution is needed here as the mixtures are highly reactive. Personal protective ware including a face shield, apron, and gloves should be used. The etch rate in aqueous KI3 solution is ∼0.1 µm/min at 25 °C, depending on the density of the metal films. After patterning of metal layers, the photoresist was stripped from the surface, leaving patterns in the gold layer. Photoresist removal was performed by dipping the wafers for 5 min at 80 °C in a proprietary liquid resist stripper (ACT-1, Air Products & Chemicals Inc.), which causes the resist to swell and lose adhesion to the substrate. The wafers were subsequently rinsed in deionized water and spin-dried. To insulate micropatterns on the polymer wafer, an insulating film deposition technique at a temperature lower than Tg (140 °C) should be introduced. Recently, aluminum oxide (Al2O3) has been an alternative to SiO2, owing to its excellent electrical properties such as low leakage current and high dielectric constant. Al2O3 can even be grown at a temperature lower than 200 °C.45 Furthermore, Al2O3 deposited by atomic layer deposition (ALD) offers conformal deposition, high film density, and precise control of film thickness.25 Thus, Al2O3 has been considered promising for a gate electric material of thin-film transistors in a flexible polymer display. We employed a plasma-enhanced ALD (PEALD) Al2O3 as an electric insulating layer on a COC substrate, because this offers high film density and good electrical properties (current density, ∼10-8 A/cm2) even at a low deposition temperature.46 For etching of the aluminum oxide insulating layer to open electrode contacts, a liftoff process was used. First, the COC wafers were coated with an image reversal resist (AZ 5214E, Clariant Corp.). Standard photolithography techniques were also used to pattern the resist layer. For this, the wafer was spun at a speed of 4000 rpm for 30 s to produce a thin 1.5-µm-thick layer. After soft baking for 60 s in an oven at 105 °C in an air atmosphere, the photoresist is exposed through a second photo mask with UV light for 4 s followed by postexposure baking in an oven at 110 °C for 65 s in an air atmosphere and a flood exposure with UV light for 20 s. The photoresist was then developed via a process of dipping the wafers for 2 min at 20 °C using a developer (NMD(45) Ritala, M.; Reskela, M. Nanotechnology 1999, 10, 19-24. (46) Lim, J. W.; Yun, S. J. Electrochem. Solid-State Lett. 2004, 7, F45-F48.

3, Tokyo Ohka Kogyo Co.). Following development, the wafers were hard baked in an oven for 20 min at 120 °C. A thin (1000 Å) Al2O3 insulating film was then deposited on the polymer wafer by PEALD using a trimethylaluminum (TMA) precursor as a source of Al and O2 gas mixed with N2 precursor as the oxidant at 100 °C. In one cycle, TMA was cooled at 15 °C and introduced into reaction chamber with Ar carrier gas for 0.5 s. O2 gas mixed with N2 gas for 1.5 s is also introduced into the reaction chamber with the carrier gas, in sequence. The growth rate at 120 °C is 0.12 nm/cycle. Plasma generation was used to make use of oxygen as the reactive species for the formation of alumina oxide films and enhancing the reactivity of the reactive gas, using a radio frequency (rf) power source (300 W).46 The insulating layer pattern to open electrode contacts was formed via a liftoff process whereby the insulating layer on the photoresist was removed along with the resist in solvent (ACT-1), by a process of dipping the wafers for 5 min at 80 °C. The wafers were rinsed in deionized water and dried in a wafer spin-dryer for 10 min. These full fabrication processes of COC microdevices are much simpler and less expensive than those of Si microdevices. A comparison of the fabrication process for the silicon device12 and the polymer device with the same shape is provided in the Supporting Information. Instrumentation for Thermal Response Measurement and Electrochemistry. To evaluate the thermal ramping rate (°C/s) and the power efficiency (°C/mW) of the microdevices, a heatcontrolling system was implemented. For direct control of the temperature on the polymer surface, thin-film gold resistors were patterned on the topside of the membrane with a 3 mm × 6 mm area. One gold resistor with 30-µm line width acts as a resistive heater (100 Ω), while the other gold resistor with 5-µm line width acts as an RTD (1 k). The RTD was calibrated after placing the device with a contact manifold in an electric oven and increasing the temperature from room temperature to 100 °C. The data were fit to the Calendar-Van Dusen equation, RT ) R0 (1 + AT), where T is the temperature, RT is the resistance at temperature T, and R0 is the resistance at 0 °C. The A constants were then determined from the fitted data. Afterward, the chip was put into a pressure-fit electrical contact holder. Temperature control was performed using the LabVIEW program without the proportional, integral, or derivative control. The resistance across an RTD was collected using an NI DAQ board (NI 6034), and an NI analog-out board controlled by a personal computer with LabVIEW software was used to control the heater voltage applied through a homemade dc power source. The RTD resistance was tested in the ambient air. Electrochemical experiments were performed using a CHI model 660A potentiostat/galvanostat (CH Instruments, Austin, TX) interfaced with a PC and a three-electrode cell consisting of a microfabricated microelectrode, a Ag/AgCl (3 M KCl) reference electrode, and a platinum wire counter electrode. The microelectrodes, excluding the pads, were dipped in a test solution. The chip has large pads, which were connected to an external power supply and a potentiostat. Gold microelectrodes were cleaned by oxygen plasma-ashing. Potassium nitrate (KNO3), 1-dodecanethiol, and hexaamineruthenium(III) chloride (Ru(NH3)6Cl3) were purchased from Aldrich. Self-assembled monolayers of dodecanethiol Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

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Figure 2. Photographs of the microdevices fabricated using the standard photolithographic techniques: (a) AFM surface morphology of the topside, (b) a 5-in. polymer wafer with micropatterns, (c) a microheater and a resistance temperature detector, and (d) microelectrodes patterned on the injection-molded COC wafer.

were prepared by immersion of the gold microelectrode in a 1 mM ethanol solution of dodecanethiol for 12 h. RESULTS AND DISCUSSION The surface flatness of the injection-molded polymer substrate was evaluated by measuring the average surface roughness at five spots (center, top, bottom, right, and left) on the topside of a COC wafer with an atomic force microscope (AFM). An AFM 5418

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image is shown in Figure 2a, and the average surface roughness is ∼4 nm within 40 µm × 40 µm area. Through wafer-level sequential processes of electron beam gold deposition, standard photolithography, and metal wet etching, micropatterns are prepared on the polymer wafer. Then, through additional wafer-level sequential processes of standard photolithography with an image reversal resist, PEALD-based Al2O3 insulating layer deposition and liftoff are performed and open

Figure 4. Thermal response comparisons between the device with thin membrane and the device with thick membrane at a given electric power (62 mW).

Figure 3. Projection view of (a) a microfabricated polymer device with a thin membrane and (b) cross-sectional view.

microelectrodes and pads are prepared. Photographs of a 5-in. COC polymer wafer with micropatterns are shown in Figure 2b. An expanded image showing the microheater and RTD (5-µm line width) is given in Figure 2c, and the implemented microelectrodes (85 µm × 625 µm area) are displayed in Figure 2d. If larger line width patterns were used, it would be difficult to implement these micro RTDs and microheaters in a small area. Thus, using standard photolithography on the highly polished COC wafer, 5-in. wafer-level patterning with micrometer line widths could be easily implemented. When the temperature of a heater is elevated by Joule heating, the heating power increases with its thermal mass whereas the ramping rate of the heater decreases with its thermal mass. When the micropatterned heating area is defined, the thermal mass is highly related to the substrate thickness. Thus, thinning of the heating membrane on the polymer substrate is carried out by employing the injection molding technique. The projection view of a microdevice with gold micropatterns and COC membrane is shown in Figure 3a. This 130-µm-thick membrane is very thin, considering the large membrane area of 3 mm × 6 mm. For the smaller surface area of 1 mm × 2 mm, a thinner membrane (∼70 µm thick) can be obtained. A cross-sectional view of the membrane is shown in Figure 3b. To investigate the thermal characteristics of the micropatterned device, it is necessary to measure the RTD temperature on a thin membrane during heating and cooling of the microheater. For the temperature measurement, the resistance-temperature calibration curve obtained from a RTD is utilized. RTDs are designed to enable fast thermal control and accurate temperature measurements. The location of a RTD near a microheater (5 µm away) allows for direct thermal measurement at the point of interest. This close location increases the accuracy of measurement. The

resistance response as a function of temperature is extremely linear over the test temperature range from 20 to 100 °C. The temperature coefficient of resistance in RTD is ∼2900 ppm/°C in this experiment. Detailed information is shown in Supporting Information. RTDs are located near microheaters; thus, membrane surface temperature is proportional to the power of the microheater, as shown in Supporting Information. Thus, the effect of membrane thickness with a 3 mm × 6 mm area on the thermal response of the microheater is investigated by supplying electric power (∼62 mW) to the microheater (Figure 4). The coefficient of an RTD on the heating power of the microheater can be defined, because the line is linear. The coefficient obtained with a thick membrane (1-mm thickness) is 0.65 ( 0.05 °C/mW, whereas the coefficient obtained with a thin membrane (130-µm thickness) is 0.92 ( 0.05 °C/mW. A large increase of 41% in power efficiency is obtained due to the decrease of the thermal mass obtained by utilizing a thinner membrane. Furthermore, it is evident that the thin membrane beneath the microheater plays a significant role in the thermal isolation of the microheater. Thermal ramping speed and power consumption are important factors that need to be considered in the design of microfabricated heating systems. The thermal ramping rates are shown in Figure 4. The temperature reaches 95% of the steady-state value within 46 s during heating, and 60 s during cooling. However, by reducing membrane thickness from 1000 to 130 µm, the ramping rates increase by 50% for heating and 30% for cooling. Thus, the formation of a thin membrane on the polymer substrate enables rapid thermal ramping rates. Electrochemical characteristics of microelectrodes on COC substrate are shown in Figure 5. Cyclic voltammograms are obtained on a microfabricated gold microelectrode in a solution containing Ru(NH3)6Cl3 before and after the formation of selfassembled monolayers of dodecanethiol. The reduction current of Ru(NH3)63+ slightly displays the limiting current behavior that occurs at typical microelectrodes. After the formation of selfassembled monolayers, there is no significant oxidation current of Ru(NH3)63+ complex during cyclic scans. The current is also smaller than the background current of a bare gold microelectrode Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

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Figure 5. Cyclic voltammograms (scan rate 2 mV/s) obtained (a) at a bare gold microelectrode in a 0.5 M KNO3 solution containing 5 mM Ru(NH3)6Cl3 and (b) (dashed line) at a bare gold electrode in a 0.5 M KNO3 solution and (solid line) at a dodecanethiol-modified gold microelectrode in a 0.5 M KNO3 solution containing 5 mM Ru(NH3)6Cl3.

in a KNO3 solution. Moreover, the background current itself is not large. This indicates that the self-assembled monolayers are formed uniformly well and that there are few pinholes into which Ru(NH3)63+ can penetrate. Thus, the gold microelectrode on COC works well electrochemically and self-assembled monolayers, which are useful for the fabrication of biosensors and biochips, can be easily formed on the microelectrodes on the COC substrate. CONCLUSIONS We have presented wafer-scale fabrication techniques of polymer microdevices by employing injection molding and stan-

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dard photolithography. A highly polished COC wafer with a thin membrane has been injection-molded with the help of CMP and RTP processes. The fabrication of micropatterns of metal and insulating layers on the COC wafer using standard semiconductor processes makes it possible to integrate microheaters, micro RTDs and microelectrodes on the polymer substrate. Al2O3 deposited at COC-endurable low temperature using PEALD is utilized as an insulating layer and micropatterned by a liftoff process using a photoresist masking layer. The formation of a thin membrane beneath the microheater and RTD offers increased thermal isolation and reduced thermal mass, resulting in reduced power consumption and improved heating and cooling rates. Moreover, gold microelectrodes show good electrochemical performance for electrochemical sensors. Finally, by reducing the size of the unit components (microheater, temperature sensor, and microelectrode) and integrating them on the polymer membrane, a low-cost wafer-scale microfabrication method using standard photolithographic techniques could be implemented for polymer microdevices for biochemical analysis and diagnostics. With further enhancements to the microfabrication process including reduction of the membrane thickness, the rapid and precise temperature control needed for biochemical reactions such as polymerase chain reaction of DNA could be obtained easily. ACKNOWLEDGMENT The COC device fabrication was performed at the ETRI 0.5µm CMOS microfabrication fab. and MEMS Laboratory. The present work was financially supported by a Grant-in-Aid for Scientific Research from the Korean Ministry of Information and Communication. The authors thank Mr. Myung-Ho Kang at NADA Innovation Co. for his help with the injection molding process. The authors also thank Dr. Sang-Hee Ko Park and Dr. Jung Wook Lim at ETRI for their help with the ALD process. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review February 16, 2005. Accepted June 4, 2005. AC050286W