Microwave-Plasma Dry-Etch for Fabrication of Conducting Polymer

Jan 13, 2014 - An inexpensive dry etch technology based on a low-pressure microwave plasma generated in a countertop microwave oven is characterized ...
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Microwave-Plasma Dry-Etch for Fabrication of Conducting Polymer Microelectrodes Richard F. Vreeland, Nicholas D. Laude, Sean M. Lambert, and Michael L. Heien* Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, AZ 85721, United States S Supporting Information *

ABSTRACT: An inexpensive dry etch technology based on a low-pressure microwave plasma generated in a countertop microwave oven is characterized for the patterning of a conductive polymer microelectrode. The etch process is described, and the microwave-generated plasma is characterized by emission spectroscopy. The plasma is generated with an atmospheric mixture of mostly nitrogen and oxygen. A 10 μm wide band microelectrode composed of PEDOT:Tosylate, an optically transparent conductive polymer, is fabricated on a plastic substrate. Conductive polymer etch rates are approximately 280−300 nm/minute. A patterned microelectrode is characterized by atomic force microscopy. The horizontal distance of a 10−90% height of a plasmaetched 150 nm thick electrode was measured to be 360 ± 200 nm (n = 5). Electrodes are further characterized using steady-state cyclic voltammetry, and they have an electroactive area congruent with their geometric area. Finally, a complete device is assembled and used as a separation platform for biogenic amines. A microwave-etched 250 μm PEDOT:PSS electrode is employed for end-channel electrochemical detection on this microchip, where an electrophoretic separation of dopamine and catechol and a micellar electrokinetic chromatography separation of dopamine and serotonin are performed. Both mass and concentration LODs are comparable to other electrochemical detectors in an end-channel configuration. With the added advantages of easy processing, robustness, optical transparency, and low cost, we expect microwave-etched polymer films to be a viable alternative to traditional electrodes.

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cells,18 and electrochemical sensor platforms with both bare PEDOT19 and enzyme-modified PEDOT.20−25 Polymer films are commonly deposited on substrates by gravure printing,26 inkjet printing,27 vacuum/vapor deposition,28,29 or spin coating.1 Gravure and inkjet printing allow for the simultaneous deposition and patterning of polymer films. However, the technique also suffers from problems with film thickness control and droplet aggregation, though substantial effort has been put forth to minimize these issues.30−36 Film etching offers advantages over gravure and inkjet printing. When patterning vacuum or vapor-deposited films, the film is deposited on a substrate and then subsequently etched to define structures. Modern etch methods include laser ablation,11 wet etches (e.g., KOH, hydrofluoroether solvents),37 and dry etches. Common dry etches for polymers include sputter etching, vapor-phase etching, and reactive ion etching (RIE).38,39 The latter technique is well-established for defining polymer structures from PEDOT films in a predictable manner;1,19,39,40 however, RIE can be prohibitively expensive. Gas usage, turbomolecular pumps, and vacuum-control systems

onducting polymer electrodes have recently emerged as an attractive alternative to metal and carbon-based electrodes in a variety of applications. They are inexpensive, easy to process, and robust. Additionally, these electrodes have electron transfer kinetics appropriate to measure the redox properties of biologically relevant molecules.1 The poly(3, 4ethylenedioxythiophene), PEDOT family of conducting polymers offers the additional benefits of high optical transparency, electrochromism,2 thermal stability up to 260 °C, and demonstrated biocompatibility.3−7 The optical transparency and biocompatibility of PEDOT electrodes has prompted uses in optogenetic and electrophysiological measurements.8,9 PEDOT is most commonly dip-coated or electropolymerized onto metal or carbon electrodes before being employed for electrochemical measurements. PEDOT-only band electrodes (i.e., those without a conducting metal under-layer) have emerged only recently and seen use as an electrochemical sensor for biogenic amines.1 Other applications of patterned conducting polymer films include antistatic coatings,10 ITOfree, and ITO-containing photovoltaic cells,11−13 capacitive touch screens in mobile devices,14 capacitors, lead-free solder, electrochemical actuators,15 and flexible displays in organic light-emitting devices.16 Patterned PEDOT films are found in PEDOT:PSS-containing solar cell arrays, large-area organic diodes,17 PEDOT nanotube arrays in dye sensitized solar © 2014 American Chemical Society

Received: October 23, 2013 Accepted: December 25, 2013 Published: January 13, 2014 1385

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was performed in a microwave etch chamber and was custom fabricated at the University of Arizona glassblowing facility (Figure 2B). Electrophoresis. Electrophoretic separations were carried out in unmodified PDMS microchannels. Microchip fabrication is described in the Supporting Information. High voltage was applied using a 4-channel custom power supply (University of Arizona Electronics Facility). Voltage was applied through platinum wires placed in wells A, B, and C and controlled using in-house LabVIEW (National Instruments, Austin, TX) software. Two chloridized silver wires were used as the reference electrode and the ground in well D. For the separation of dopamine and catechol, the buffer system was 20 mM MES at pH 6.0. For the separation of dopamine and serotonin, micellar electrokinetic chromatography (MEKC) was used and the buffer was 50 mM borate at pH 9.6 with 20 mM sodium dodecyl sulfate added. Standard solutions (100 μM) were prepared daily from fresh 2.7 mM stock solutions in 0.1 M perchloric acid and diluted in the separation buffer. Separation was performed by placing the analyte in well A, buffer in wells B, C, and D, applying the separation voltage (750 V in MES or 500 V in borate/SDS) to well C, and grounding in well A and D, while B floated. Injection was performed by floating the potential of well C and applying the separation voltage at well A for the duration of the injection. Injection times were 3 s for the dopamine and catechol separation, and 5 s for the MEKC dopamine and serotonin separation. Plasma Characterization. Microwave plasma emission spectra were collected using an Ocean Optics USB4000 fiber optic spectrometer (Ocean Optics, Dunedin, FL). The fiber optic cable was held against the microwave window and all light sources in the microwave were removed. The apparatus was covered in a blackout curtain and UV−vis spectra were collected every 500 ms (20 spectra were averaged). Surface Characterization. Atomic force microscopy was performed using a Veeco Dimension 3100 (Veeco, Plainview, NY) in tapping mode using a Mikro Masch NSC-15 n-type silicon tip (Mikromasch USA, San Jose, CA). Measured topography was software low-pass filtered twice (Nanoscope V531r1). Optical micrographs were collected using a Nikon Ti2000 microscope (Nikon Corporation, Tokyo, Japan). Electrochemistry. Electrochemical data was collected using an Ensman EI-400 potentiostat (Ensman instruments, Bloomington, IN) and custom LabVIEW software (National Instruments, Austin, TX). Electrical contact to the electrode was established by placing a platinum wire in a solution of 3.0 M KCl above the contact pad. Custom LabVIEW software utilizing real-time oversampling filtering collected the data.44 Data was collected at 250 kHz (NI 6221) and stored at 100 Hz. A 50 point nearest-neighbor smooth was applied to the MEKC separation traces. Cyclic voltammograms were collected at 5 mV/s and swept from −0.6 to 0.4 V versus Ag QRE. All electrochemical data were hardware low-pass filtered at 500 Hz and software low-pass filtered at 2 Hz. For cyclic voltammetry experiments, electrode geometry was defined by placing a PDMS well on top of the electrode and filling it with solution. The wire provided electrical contact with the PEDOT:Tosylate electrode through this KCl solution.

drive the cost even higher. Additionally, many reactive ion etchers are made from largely proprietary parts, making them difficult or expensive to service. Here, we introduce an inexpensive, facile alternative to reactive ion etching for defining the geometry of thin-layer conducting polymer electrodes using a microwave plasma generated in a countertop microwave oven. Microwave plasmas have previously been established as a viable method for etching polymers.41,42 Additionally, microwave plasmas have been utilized to modify the surface of polydimethylsiloxane (PDMS) for permanent bonding of these devices to glass substrates.43 Microwave etching allows for the fabrication of simple or complex polymer patterns on common substrates and can facilitate many structures on one substrate. The other required components are a sealed glass chamber with a spigot and an inexpensive rotary vane pump. The possible feature sizes are amenable to fabricating microelectrodes as well as larger structures, with a theoretical lower limit of hundreds of nanometers to an upper limit of 15 cm. Additionally, microwave etching is less expensive than most other methods of dry etching and can be executed on a benchtop. It requires no exotic gases or regulators, no reactive ion etcher, and no turbomolecular pumps, lowering the cost to the user by several thousand dollars. Here, we use the etch process to fabricate PEDOT microelectrodes and characterize their geometry and electrochemical performance. We demonstrate the use of microwave-etched microelectrodes as an amperometric sensor for biological molecules in a microchip electrophoresis platform for the separation and detection of transmitter molecules.

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EXPERIMENTAL SECTION PEDOT:Tosylate Electrode Fabrication. TOPAS 5013 Cyclic Olefin Copolymer (COC, Topas Advanced Polymers Inc., Florence, KY) substrates (Ø 50 × 2 mm wafers) were generated via injection molding. PEDOT:Tosylate was deposited on these substrates via vapor deposition. Then, films were patterned using UV lithography (Figure 1); the method is detailed in the Supporting Information. Film etching

Figure 1. Schematic of the microwave-etch process. (A) PEDOT:Tosylate film is generated on a TOPAS cyclic olefin copolymer substrate either by spin-coating or vapor deposition. (B) AZ 3312 photoresist is deposited on the surface of the PEDOT:Tosylate, and UV photolithography is used to define the geometry of protective structures. (C) The wafer is rinsed and inserted in the microwave plasma chamber for etching. D: Once etched, the wafer is submerged in acetone for 5 min to dissolve the photoresist. We are left with PEDOT:Tosylate microstructures.

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RESULTS AND DISCUSSION PEDOT Electrode Patterning. The microelectrode fabrication process (Figure 1) started with the deposition of a 1386

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Figure 2. (A) UV−visible emission spectrum of the microwave-generated plasma. (B) Photograph of the glass chamber used to contain the plasma and samples. Chamber is 30 cm in length and 10 cm in diameter. More details on chamber fabrication are available in the Supporting Information.

conducting polymer film on a substrate (Figure 1A). Both PEDOT:PSS and PEDOT:Tosylate were used to make devices due to their ubiquity in organic electronics. The substrate material, TOPAS cyclic olefin copolymer (COC), has excellent optical properties, chemical inertness to ketones, and a relatively high glass transition temperature (Tg = 115 °C). Additionally, PEDOT:Tosylate films adhere strongly to TOPAS. Glass slides were used as substrates for PEDOT:PSS electrode patterning as PEDOT:PSS does not adhere well to TOPAS. After PEDOT films were generated on substrates, UV photolithography was used to define protective structures (Figure 1B) prior to etching. A diazonaphthoquinone and novolac (DQN)-based photoresist (AZ-3312) chosen for a high glass transition temperature (125 °C), fast processing, high resolution (i-line 0.4 μm, g-line 0.6 μm), and high solubility in acetone for easy removal. A PEDOT-coated substrate with patterned photoresist structures was then placed in the microwave etch chamber (vide infra, Figure 2B), sealed in open atmosphere, and pumped down to ∼1 Torr. The chamber was placed in the microwave and the plasma was sparked, etching the unprotected PEDOT off of the substrate (Figure 1C). The substrate was then removed and soaked in acetone to dissolve the photoresist (Figure 1D), revealing the patterned microstructures. Three alternative methods were also successfully used to mask the PEDOT and define structures: (1) glass coverslip placed up against a portion of the PEDOTcoated substrate, (2) a 15 nm thick sputtered gold coating, and (3) a vinyl sticker with fluoropolymer coating (Bytac) pressed against the PEDOT:Tosylate. The AZ-3312 photoresist was chosen for ease of use and excellent feature resolution. Plasma Characterization. A conventional countertop microwave oven was used to generate plasma for etching films. A sealed glass chamber with a valve was pumped down to ∼1 Torr with a rotary vane pump. The chamber was placed in the microwave oven and a plasma was sparked in the chamber. A small piece of metal (e.g., aluminum plates, foil, or small metal screwdrivers) was sufficient to initiate the plasma. At pressures higher than approximately 100 Torr, a plasma is still generated, but arcing and excessive chamber heating occurs, along with a characteristically white plasma that does not etch films successfully. At pressures substantially lower than 1 Torr, a plasma is not initiated. Figure 2A shows the emission spectrum of the microwave plasma during typical operation. The bright pink plasma (Figure 2B, inset) is characteristic of air plasmas. The microwave plasma shares common emission wavelength peaks

with other air plasmas reported in literature.45 Atomic hydrogen electronic emission lines are visible at 486.1 and 655.2 nm, while atomic oxygen emission lines are present at 777.2 and 844.7 nm. The atomic hydrogen emission lines are hypothesized to be due to the presence of water vapor in the chamber. Emission manifolds for N2+ and N22+ are present at 560−780 and 350−480 nm, respectively. While the N22+ emission manifold reaches down to ∼250 nm, it was not observed in the collected emission spectra. This is because UVabsorbing materials were present between the emission and the optical fiber probe for data collection. Etch Characterization. An etch process punctuated by short pauses in plasma exposure provided the most clearly defined structures when compared to long etch times (10 s plasma exposures separated by 2 min repeated three times for PEDOT:Tosylate films and 15 s plasma exposures separated by 2 min repeated six times for PEDOT:PSS films). The advantages of a time-modulated etch are described elsewhere,46 though here our etches are punctuated by longer times. During long plasma exposures (>20 s), the temperature of the chamber becomes sufficiently high to pass the glass transition temperature of the photoresist (125 °C), causing reflow in the structures. With the 2 min of cooling time between plasma exposures, several repetitions of short etch times allowed for more temperature control. The COC surface morphology was unchanged for plasma exposure times under 2 min, as measured by atomic force microscopy before and after microwave etches. The surface RMS roughness of the COC substrate increased from 3.87 ± 0.13 nm RMS (n = 3) to 3.99 ± 0.59 nm RMS (n = 3), but this difference was not statistically significant (Student’s t test, two tailed, P = 0.80, Figure S-1 of the Supporting Information). The temperature of the chamber and substrate was 60−70 °C after 10 s of etching (as measured by an infrared thermometer). Etch rates of ∼300 nm/minute are attained for PEDOT:Tosylate films, and ∼150 nm/minute are attained for PEDOT:PSS films. This is on the same order as etch rates of polymer films attained with other oxygen plasmas.47 High power microwave ovens (2−3 kW) should be explored further, though sample heating may be of concern. Microwave-etched 10 μm wide PEDOT:Tosylate electrodes were imaged using atomic force microscopy (Figure 3). The image shows clearly defined edges on an electrode cast out of a 150 nm thick vapor-generated PEDOT:Tosylate film on a TOPAS substrate. The horizontal distance of a 10−90% height step from substrate to the top of a vapor-generated film was measured to be 360 ± 200 nm (n = 5). The atomic force 1387

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Figure 3. Atomic force micrograph of a section (50 μm2) of a 10 μm wide microwave-etched PEDOT:Tosylate band microelectrode on a TOPAS substrate illustrating the edge fidelity of the etch. Electrode width is approximately 10 μm. Inset: Optical micrograph of a 2165 × 10 μm PEDOT:Tosylate band microelectrode extending into a PDMS well.

micrograph and horizontal 10−90% height step distance suggest that smaller feature sizes are possible. Electrodes were also characterized electrochemically, and the geometric area of the electrodes is in congruence with the electroactive area as predicted by quasi-steady state oxidation of varying concentrations of ferrocene carboxylic acid (Figure S-2 of the Supporting Information). Separation and Detection of Biogenic Amines with PEDOT-PDMS Microchips. Microwave-etched PEDOT:PSS microelectrodes (250 μm × 1 cm × 100 nm) were utilized as end-channel electrochemical detectors for chip electrophoresis experiments. Here, PEDOT:PSS was employed because it is more robust than PEDOT:Tosylate in the presence of strong electric fields. In the first application, dopamine and catechol were separated on a 5 × 5 cm microchip and detected amperometrically using a PEDOT:PSS electrode. In the second application, dopamine and serotonin were separated and detected on the same platform. The separation channel had dimensions of 2.9 cm (L) × 45 μm (H) × 30 μm (W), and separations were performed at a field strength of 262 V/cm for dopamine/catechol and 175 V/cm for dopamine/serotonin. The detection electrode was aligned to be approximately 10 μm outside the end of the separation channel. Additional details are in pages S-4 and S-5 of the Supporting Information. Representative amperometric traces of these separations are shown in Figure 4. A summary of separation figures-of-merit, including resolution (Rs), limit of detection (LOD, calculated from peak height), sensitivity, peak current, relative standard deviation (RSD), and separation efficiency (TP) are quantified in Table 1 for one device with 19 consecutive injections. Baseline resolution of the two analytes was achieved in both separations. Submicromolar limits of detection typical of an end-channel electrochemical detection scheme were achieved, with limits of detection for dopamine and catechol comparable to carbon fiber, carbon ink, or palladium electrodes.48 Given the height of the chip separation channel (45 μm) and the endchannel geometry of the detector, a low coulometric detection efficiency was expected. The devices separated analytes continuously for approximately 1 h without replenishment of separation buffer or additional operator intervention, with an average of 8 ± 5% decrease (for the dopamine and catechol separation, n = 19 injections and separations) in sensitivity between the first and last runs and a 22 ± 11% increase in RMS noise (from 2.7 to

Figure 4. (A) Schematic of the microfluidic device used to separate and detect dopamine, catechol, and serotonin. (B) Separation and detection of 100 μM dopamine and 100 μM catechol in 20 mM MES buffer at pH 6.0 with a field strength of 262 V/cm. (C) MEKC separation and detection of 100 μM dopamine and 100 μM serotonin in 50 mM borate buffer at pH 9.6 with 20 mM SDS with a field strength of 175 V/cm.

3.3 pA). At this point, devices were regenerated by pressureflushing with 0.1 M NaOH for 3 min, followed by 3 min of nanopure water, finally followed by 5 min of separation buffer. The decrease in electrode sensitivity was accompanied with a slight broadening of peaks, suggesting this is likely separationrelated and not detector related. Given these comparable detector figures of merit, along with the optical transparency, low cost, facile electron transfer kinetics, and easy fabrication, PEDOT-based detectors in microchip electrophoresis are a competitive material with pyrolized photoresist, palladium, and carbon ink electrodes.

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CONCLUSION The method presented is an alternative to reactive ion etching for patterning thin-layer polymer structures. The technique allows for the fabrication of simple or complex patterns and can facilitate many electrodes on one substrate. It is less expensive than most other methods of dry etching. Microwave etching requires no exotic gases or regulators, no reactive ion etcher, and no turbomolecular pumps. If desired, the chamber can be backfilled with SF6, CxFy, or other common etch gases after pumping down. With the use of a Bytac mask instead of lithographic masks, the patterning and etching process can be performed in any laboratory. The microwave oven generates an air plasma similar to others published in literature. The feature size of microwave-etched structures is defined by the resolution of the photoresist and the photomask. Common plasma etchants such as SiCl4, CF4, SF6, and BCl3 should be investigated for etching polymers as well as inorganic semiconductor materials (these etchants are toxic and should only be used with ventilation under strict safety precautions). The geometric area of electrodes fabricated via microwave 1388

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Table 1. Figures of Merit from Electrophoretic Separations with Detection at a 250 μm PEDOT:PSS Electrodea separation buffer MES pH = 6.0 (n = 19) borate MEKC pH = 9.6 (n = 3) a

analyte

Rs

dopamine catechol dopamine 5-HT

2.0 ± 0.2 1.8 ± 0.2

TP 810 1300 380 510

± ± ± ±

LOD (μM) 30 50 40 60

0.24 0.40 0.35 1.4

± ± ± ±

0.06 0.05 0.08 0.2

LOD (amol) 110 64 160 620

± ± ± ±

30 8 40 80

sensitivity (pA/μM)

Ip RSD (%)

± ± ± ±

6.4 6.9 1.3 3.2

24 14 30 8

1 1 1 1

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etching is congruent with the electroactive area, indicating that this method etches down to the substrate and defines features predictably. The optical transparency of the device could be used for simultaneous electrochemical and fluorescent detection. A microchip electrophoresis device with a polymer detection electrode was fabricated and characterized. Additionally, the first microchip electrophoresis separation of dopamine and serotonin with amperometric electrochemical detection has been shown. Polymer electrodes offer unique advantages, including optical transparency, low cost, and easy processability, in addition to performing similarly to traditional biosensor electrode materials.

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ASSOCIATED CONTENT

S Supporting Information *

Further details on the fabrication of electrodes, microfluidic devices, and the microwave etch chamber are available in the Supporting Information. Additionally, details of the electrochemical characterization are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 520-621-8407. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Brooke Beam Massani, Paul Lee, Omid Mahdavi, and Dr. Chris Baker for fruitful discussion, technical expertise, and lithography materials. Additionally, we thank the University of Arizona for funding this work.

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