Fluorinated Silane Self-Assembled Monolayers as Resists for

May 29, 2003 - of densely packed defect-free SAMs on ITO is difficult due to the high surface roughness .... A FEI200 workstation was used for the FIB...
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Langmuir 2003, 19, 5273-5278

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Fluorinated Silane Self-Assembled Monolayers as Resists for Patterning Indium Tin Oxide Christine K. Luscombe, Hong-Wei Li, Wilhelm T. S. Huck,* and Andrew B. Holmes* Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Received February 7, 2003. In Final Form: April 15, 2003 We report the formation of self-assembled monolayers (SAMs) on indium tin oxide (ITO) substrates with perfluoroorganosilanes in liquid and supercritical carbon dioxide and a method of patterning the monolayer that does not use any organic solvents. The monolayers formed were used as an etch resistant during the formation of patterns as small as 300 nm. The monolayers were characterized using wettability experiments, surface FT-IR, cyclic voltammetry, and AFM. The effects of temperature and adsorption time on the formation of SAMs were explored. Advancing contact angles as high as 105° and fractional surface coverages up to 0.96 were achieved by exposing the ITO surfaces to silanes in scCO2 for 15 h. Surface FT-IR results show peaks at 1212 and 1152 cm-1, typical for disordered monolayers. Yet, these SAMs are resistant to wet etching for over 10 h, indicating that dense carbon dioxide is a superior solvent for SAM formation of perfluorosilanes on ITO.

Introduction SAMs have generated widespread research interest because of their potential applications related to control over wettability, biocompatibility, and corrosion resistance of surfaces.1-5 Many studies have been directed toward the development of microengineering tools for industrial application such as biomedical devices,6 optical devices,7 liquid crystals,8 and microlithography9 and as models of surface lubricants in microtribology.10 It is possible to form monolayers on a wide range of materials by selecting appropriately functionalized surfactant molecules.3 SAMs on indium tin oxide (ITO) are of particular interest owing to the widespread use of this material in the electronics industry. Studies have been performed on the formation (1) Yan, L.; Huck, W. T. S.; Whitesides, G. M. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; p 435. (2) Ulman, A. Introduction to Thin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic: Boston, MA, 1991. (3) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55-78. (4) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (5) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P.; Silvera, I. F. Rev. Sci. Instrum. 1986, 57, 1381-1383. (6) (a) Ha¨ussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569-572. (b) Ha¨ussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837-1840. (c) Spinke, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821-1825. (d) Muller, W.; Ringsdorf, H.; Rump, E.; Wildburg, G.; Zhang, X.; Angermaier, L.; Knoll, W.; Liley, M.; Spinke, J. Science 1993, 262, 1706-1708. (e) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (7) (a) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102-7107. (b) Caldwell, W. B.; Campbell, D. J.; Chen, K. M.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071-6082. (c) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.; Lin, W. P.; Wong, G. K.; Dutta, P. Langmuir 1996, 12, 4218-4223. (d) Kim, E.; Whitesides, G. M.; Lee, L. K.; Smith, S. P.; Prentiss, M. Adv. Mater. 1996, 8, 139-142. (8) (a) Drawhorn, R. A.; Abbott, N. L. J. Phys. Chem. 1995, 99, 1651116515. (b) Gupta, V. K.; Abbott, N. L. Phys. Rev. 1996, 54, 4540-4543. (9) (a) Sondag-Huethorst, J. A. M.; van Helloputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285-287. (b) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342-3343. (c) Rogers, J. A.; Jackman, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, 475-477. (10) Green, J.-B. D.; MacDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960-10965.

of SAMs on ITO using phosphonic acids,11,12 carboxylic acids,13-17 thiols,18,19 and amines.20 The formation of stable SAMs via carboxylic acid, thiol, and amine headgroups is difficult, as these moieties do not bind strongly to ITO. Phosphonic acid terminated monolayers have been used as etch resists.21 However, they show numerous defects when used as resists on ITO. Only a small number of studies have been reported which aim at attaching alkyl chains onto ITO surfaces via silane chemistry.22-24 The general conclusion of these studies is that the formation of densely packed defect-free SAMs on ITO is difficult due to the high surface roughness and low hydroxyl coverage (vide infra). However, silanes do bind strongly to the ITO surface and they have been used successfully as low energy self-developing e-beam resists.25 ITO is widely used as the transparent electrode for light emitting devices. Patterning this electrode is conventionally achieved via (11) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927-6933. (12) Appleyard, S. F. J.; Day, S. R.; Pickford, R. D.; Willis, M. R. J. Mater. Chem. 2000, 10, 169-173. (13) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X. H.; Peek, B. M.; Wall, C. G.; Oh, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952-3964. (14) Tanaka, T.; Honda, Y.; Sugi, M. Jpn. J. Appl. Phys. 1995, 34, 3250-3254. (15) Napier, M. E.; Thorp, H. H. Langmuir 1997, 13, 6342-6344. (16) Berlin, A.; Zotti, G.; Schiavon, G.; Zecchin, S. J. Am. Chem. Soc. 1998, 120, 13453-13460. (17) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G. Langmuir 1998, 14, 1728-1733. (18) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203-209. (19) Yan, C.; Zharnikov, M.; Golzhauser, A.; Grunze, M. Langmuir 2000, 16, 6208-6215. (20) Oh, S.-Y.; Yun, Y.-J.; Kim, D.-Y.; Han, S.-H. Langmuir 1999, 15, 4690-4692. (21) Breen, T. L.; Fryer, P. M.; Nunes, R. W.; Rothwell, M. E. Langmuir 2002, 18, 194-197. (22) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193-1194. (23) Hillebrandt, H.; Tanaka, M. J. Phys. Chem. B 2001, 105, 42704276. (24) Markovich, I.; Mandler, D. J. Electroanal. Chem. 2001, 500, 453-460. (25) St. John, P. M.; Craighead, H. G. J. Vac. Sci. Technol., B 1996, 14, 69-74.

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photolithography. This process involves many steps, wastes large percentages of the expensive photoresist material, and requires the use of organic solvents, many of which are toxic and/or expensive and/or have an adverse effect on the earth’s atmospheric chemistry. It is therefore desirable to try to find an alternative process for the patterning of substrates. Other methods of patterning ITO have been explored as cost-efficient alternatives. For example, Koide et al.26 have developed a method of patterning which involves “hot microcontact printing”. Breen et al.21 have also recently developed microcontact printing on ITO and IZO with alkanephosphonic acids and used the SAM as an etch resistant layer. Compressed CO2, which is an environmentally benign solvent, is potentially a superior solvent for SAM formation on ITO. First, the self-diffusion coefficient of CO2 is 1-3 orders of magnitude greater27 and the viscosity is 0.5-1 orders lower than that of traditional solvents,28 which might decrease the reactions times necessary for complete monolayer formation. Second, the solvent properties can be altered very easily by controlling the pressure and temperature. For reference, the critical point of CO2 is 31 °C and 73 atm. Recently, Cao and co-workers30 reported that CO2 weakly physisorbs to silica, thereby extracting the water which is adsorbed onto the silica surface. This displacement allows fast reactions of organosilanes onto these surfaces without the need to dry the silica surfaces beforehand. As it is known that trichlorosilanes polymerize in the presence of water, giving rise to a number of possible surface structures,31 it is to be expected that the amount of water present at the surface would greatly influence the quality of the SAMs. We report here that, by using CO2 as a solvent, the amount of polymerization is reduced and that the monolayers are formed on ITO almost as well as on silica. We have characterized our monolayers using contact angle measurements, cyclic voltammetry, surface-IR, and AFM. We also report a patterning technique which does not use any organic solvents, whereby the monolayers have been used as etch resists in the generation of submicron features in ITO, as a proof of the high quality of the silane SAMs. Experimental Section Chemicals. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane was purchased from Fluorochem and used as received. All other chemicals were purchased from Aldrich and used as received. Depositions in Carbon Dioxide. All depositions in supercritical carbon dioxide were conducted in a 10 mL stainless steel cell. Carbon dioxide, BOC 99.9995%, was delivered to the reaction cell using a Pickel PM 101 air driven pump at the desired pressure. Heating of the cell was achieved by the use of a heating tape. The system pressure was measured by a pressure transducer (A105m RDO Electronics). The internal temperature was monitored by an Industrial Mineral Insulated thermocouple (Type K, RS Electronics) and displayed on a temperature indicator (T200, RS Electronics). ITO Electrodes. ITO on glass sheets was purchased from IVC Technologies and had a surface conductivity of 40 Ω sq-1. The sheets were cut into 8 × 40 mm2 slides and cleaned by sonication first in acetone (10 min), then in dichloromethane (10 (26) Koide, Y.; Wang, Q. W.; Cui, J.; Benson, D. D.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 11266-11267. (27) (a) Gross, T.; Buchhauser, J.; Ludemann, H.-D. J. Chem. Phys. 1998, 109, 4518-4522. (b) Etesse, P.; Zega, J. A.; Kobayashi, R. J. Chem. Phys. 1992, 97, 2022-2029. (28) Cooper, A. I. J. Mater. Chem. 2000, 10, 207-234 and references contained therein. (29) Holmes, A. B.; Huck, W. T. S.; Luscombe, C. K.; Fukushima, H.; Ishida, M.; Miyashita, S. U. S. Patent 2 197 879, 2002. (30) Cao, C.; Fadeev, A. Y.; McCarthy, T. J. Langmuir 2001, 17, 757-761. (31) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274.

Luscombe et al. min), and finally in deionized water (2 min, five times). Subsequently, each slide was immersed in a solution of 1:1:5 H2O2 (aqueous 30%)/NH4OH (aqueous 30%)/H2O for 1 h at 70 °C with stirring. The slides were washed with a large amount of water and dried in an oven at 100 °C for 4 h. The clean, hydrophilic slides were then stored in a desiccator for a maximum of 24 h until further use. Silanization. The ITO slides were placed in the 10 mL stainless steel high-pressure vessel. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (2 µL) was flushed into the vessel with liquid CO2 via an injection loop. The vessel was filled with CO2 (23 °C, 1000 psi) and heated to the desired temperature. After the desired reaction time, the vessel was cooled, and the substrate in the cell was rinsed by filling the cell five more times with liquid CO2. The slides were stored in a desiccator until further use. Etching Experiments. Etching was performed by first treating the sample for 0.1 s with a focus ion beam (FIB) to form patterns which consisted of lines ranging from width 300 to 500 nm and periodicity 400 to 1000 nm. A FEI200 workstation was used for the FIB experiments. The energy used was 30 kV using beams of Ga+ with an ion flux of 70 pA. Lines were milled with line width/interline distances of 0.3/0.4 (with ion dose of 8 × 1014 ion/cm2), 0.5/0.5 (with ion dose of 14.6 × 1014 ion/cm2), and 0.5/ 1.0 (with ion dose of 14.6 × 1014 ion/cm2) µm. The milling time varies from 0.1 to 0.5 s for each line. The samples were then etched in an aqueous oxalic acid solution (0.05 M), with mild agitation for 5 min. Characterization Methods. Contact angle goniometry was performed using a homemade stage comprising a computercontrolled microsyringe and digital camera. The cyclic voltammograms of the ITO electrodes were measured with a Solartron 1287 potentiostat in 1 mM Ru(NH3)6Cl3 in 0.1 M Na2SO4 before and after the deposition with perfluoroalkylsilane. The voltage was applied versus a Ag/AgCl reference electrode with varying scan rates (25, 50, 100, and 150 mV/s). FTIR spectra were taken with a Biorad FTS 6000 spectrometer. Optical microscope images were taken using a Nikon Eclipse ME600. SEM images were acquired using a JEOL JSM-5510LV scanning electron microscope with thermoemission. The accelerating voltage was set at 15 kV.

Results and Discussions Film Formation. The formation of well-controlled and reproducible SAMs on ITO is not straightforward for two major reasons: (i) the high surface roughness of the ITO and (ii) the low hydroxy group coverage, which limits the number of groups available for chemisorption. The latter problem may be overcome by employing a number of pretreatments. These include treatment of ITO using acid followed by base32-34 and treatment by piranha solution.35 However, both treatments are quite harsh and can corrode the surface. We obtained the best results by applying a mixture of 5:1:1 H2O + H2O2 (aqueous 30%) + NH3 (aqueous 25%) for 1 h at 70 °C.24 After this pretreatment, the samples were placed in an oven at 100 °C for 4 h. Other parameters that have an effect on SAM formation include the reaction time, the temperature, and the amount of water present in the silanization mixture. The time of silanization that has been reported using silanes spans a wide range from less than 1 h to several days.35-39 It has been reported that, in order to obtain reproducible (32) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (33) Tabushi, I.; Kurihara, K.; Naka, K.; Yamamura, K.; Hatakeyama, H. Tetrahedron Lett. 1987, 28, 4299-4302. (34) Yamamura, K.; Hatakeyama, H.; Naka, K.; Tabushi, I.; Kurihara, K. Chem. Commun. 1988, 79-81. (35) Wilson, R.; Schiffrin, D. J. Analyst 1995, 120, 175-178. (36) White, H. S.; Murray, R. W. Anal. Chem. 1979, 51, 236-239. (37) Wier, L. M.; Murray, R. W. J. Electrochem. Soc. 1979, 126, 617623. (38) Moses, P. R.; Wier, L.; Murray, R. W. Anal. Chem. 1975, 47, 1882-1886. (39) Asanov, A. N.; Wilson, W. W.; Oldham, P. B. Anal. Chem. 1998, 70, 1156-1163.

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Figure 1. Graph to show the variation in contact angle with time using 1H,1H,2H,2H-perfluorodecyltrichlorosilane to form SAMs in scCO2.

results on ITO, a longer time is required compared to that for SAM formation on silica (a few days as opposed to a few hours).24 We also know from our own study and from Cao et al.29,30 that the formation of SAMs in scCO2 on silica is considerably faster than that in organic solvents (1 h compared with 24 h). With this in mind, a time range of 1-15 h was chosen (a time longer than that required for SAM formation on silica in scCO2 but considerably shorter than that required for SAM formation on ITO in organic solvents). It has been reported that temperatures around 70 °C are required for formation of reproducible SAMs on ITO.24 We carried out experiments to form monolayers at four different temperatures within the range 20-80 °C and found that 40 °C was the optimum temperature for SAM formation in scCO2. Film Characterization. When measuring a 5 × 5 µm2 area, the AFM software gave a surface roughness of 1213 nm. The high surface roughness of ITO and optical transparency make it impossible to use ellipsometry to measure the thickness of the films. The high surface roughness also meant that the AFM images of the bare ITO surface and monolayer/ITO surface did not show a significant difference. The presence as well as the organization of an alkyl monolayer can be estimated from contact angle measurements. The contact angle measurements are shown in Figure 1. The bare ITO water contact angle depends significantly on the pretreatment and can vary between 20° and 80°.40,41 In our case, a value of 36° was obtained, which is comparable to those obtained on oxidized surfaces, for example, through oxygen plasma or RCA treatment (5:1:1 H2O + H2O2 (aqueous 30%) + NH3 (aqueous 25%), followed by 50:1 H2O + HF (aqueous) and finally 5:1:1 H2O + H2O2 (30%) + HCl (aqueous)). (40) Kim, J. S.; Friend, R. H.; Cacialli, F. Synth. Met. 2000, 111, 369-372. (41) Kim, J. S.; Friend, R. H.; Cacialli, F. J. Appl. Phys. 1999, 86, 2774-2778.

As one can see from Figure 1, the hysteresis between advancing and receding water contact angles converges to about 110/100° after 15 h. This would imply that a minimum of 15 h is required for the best monolayer formation. The contact angle of water for a smooth silica surface covered by a densely packed 1H,1H,2H,2Hperfluorodecylsilane monolayer has been measured to be 105/91°.29,30 This indicates that we have achieved very good monolayer formation on ITO using scCO2. The higher contact angles on ITO are due to the increased roughness of the surface as compared with Si/SiO2. The effect of the surface roughness on the contact angle is given by Wenzel’s equation:42

cos θr ) r cos θtrue

(1)

where r is the roughness factor and θr and θtrue are the contact angles of the rough and the flat surfaces, respectively. Substituting our measured contact angle for θr and the highest reported contact angle measurement for θtrue, this would give r ) 1.26. This is in good agreement with the contact angle hysteresis, which indicates a surface of moderate roughness.24 IR Spectrum. Two regions of the IR spectrum show characteristic bands for perfluoroalkylsilanes. One is the C-H stretching region ranging from 2800 to 3000 cm-1 (Figure 2a), and the other is the C-F stretching region ranging from 800 to 1500 cm-1 (Figure 2b). SAMs derived from perfluoroalkylsilanes exhibit characteristic C-H stretching bands attributable to methylene groups. On hydroxylated surfaces with well-organized monolayers, asymmetric and symmetric bands are expected at 2918 and 2849 cm-1, respectively.43-46 For the perfluoroalkyl(42) Johnson, R. E.; Dettre, R. H. Contact angle, Wettability, and Adhesion; In ACS Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43. (43) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (44) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496.

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Figure 2. (a) IR spectrum (2970-2920 cm-1) of a perfluoroalklysilane monolayer. (b) IR spectrum (1400-1100 cm-1) of a perfluoroalklysilane monolayer formed in scCO2.

silane monolayer on ITO, bands were observed at 2932 and 2857 cm-1. These values are shifted significantly to higher wavenumbers, which indicate that the CH2 part of the monolayers is disorganized. This is in accordance with the low hydroxy group coverage on ITO and is in good agreement with those reported by Markovich and Mandler.24 The strong IR bands at 1212 and 1152 cm-1 have been assigned to both the asymmetric and symmetric stretching vibrations of the CF2 groups with the change in dipole moment perpendicular to the fluorocarbon chain. The CF2 stretching vibrations of 1H,1H,2H,2H-perfluorodecyltrichlorosilane on silica have been reported to be at 1210 and 1148 cm-1.47Although we observe a small shift in wavenumbers, the overall conclusion seems that the CF2 parts of the SAMs in ITO are quite dense and ordered with a helical structure. The weaker bands at 1375 and 1345 cm-1 were attributed to the axial CF2 stretching (45) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532-538. (46) Harder, P.; Bierbaum, K.; Woell, C.; Granze, M.; Heid, S.; Effenberger, F. Langmuir 1997, 13, 445-454. (47) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 46104617.

vibrations with a corresponding change in dipole moment parallel with the fluorocarbon chain. Electrochemical Characterization. The cyclic voltammograms of an ITO electrode in 1 mM Ru(NH3)6Cl3 in 0.1 M Na2SO4 before and after the deposition with perfluoroalkylsilane are presented in Figure 3. The voltage was applied versus a Ag/AgCl reference electrode with varying scan rates (25, 50, 100, and 150 mV/s). In the presence of a 1 mM Ru2+/Ru3+ redox couple, the voltammogram of the bare ITO electrode exhibited the typical shape for diffusion-limited electron-transfer processes.48 The reduction and the oxidation peak currents were about |Imax| ) 90 ( 10 µA cm-2 with a peak separation of 100 mV. After the ITO electrode was coated with a monolayer, the cathodic and anodic peak currents decreased by a factor of 2-3 and the potential peak separation increased by a few millivolts. In contrast to the above, C8H17SH monolayers on gold completely eliminate the redox peaks.49 The changes observed indicate that the monolayer is not (48) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 1980. (49) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

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Figure 3. Cyclic voltammograms of ITO electrodes at a scan rate of 150 mV/s of bare ITO and ITO-SAM treated for 0.5, 1, 2, 5, 10, and 15 h. The voltage was applied versus a Ag/AgCl reference electrode.

Figure 4. Fractional coverage of perfluorinated SAMs deposited in scCO2 after various reaction times.

as well packed as SAMs on gold and allows electron transfer at the electrode surface. K3Fe(CN6), an electroactive species with a significantly lower heterogeneous electron-transfer rate constant than Ru(NH3)6Cl3, was used for preliminary characterization of the monolayer. No redox peak was observed in this case. The absence of a redox peak indicates that no pinholes greater than the order of a micrometer are present.50 We did not observe irreversible electron transfer for the Ru(NH3)63+ redox couple. Amatore et al.51 showed that, with a partially blocked electrode involving numerous active sites with an average size of the active sites and the average distance between them small compared to the total diffusion layer, quasi-reversible behavior should be observed as long as the surface coverage is not too close to unity. This would (50) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 52, 946-950. (51) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51.

indicate that we have a brush-type monolayer with homogeneously distributed defects (placed at distances greater than the monolayer thickness) and that these defects are typically on the order of a nanometer. Many other systems using different probes and media were attempted for the electrochemical study, but they failed to give a redox peak. However, Koide et al.52 have reported that the monolayers fabricated using hot microcontact printing on ITO give rise to a redox peak using a 10 mM 1,1′-ferrocenedimethanol solution in water. This would seem to imply that the monolayers formed using our technique give rise to a more consistent SAM on ITO. The fractional coverage can be estimated using

ks ) ks,o(1 - f)

(2)

where ks and ks,o are the standard rate constants for a (52) Koide, Y.; Such, M. W.; Basu, R.; Evmenko, G.; Cui, J.; Dutta, P.; Hersam, M. C.; Marks, T. J. Langmuir 2003, 19, 86-93.

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potential separations with scan rates and introducing a diffusion coefficient of 6.7 × 10-6 cm2 s-1 (ref 54) for Ru(NH3)63+, we obtained values for ks and ks,o. Using eq 2, the fractional coverages of the monolayers over time were obtained, and this is shown in Figure 4. The maximum coverage obtained after 15 h was 0.96. From the CV data, we can conclude that this coverage is the result of a pinhole free, homogeneous, but not extremely dense SAM. It has been shown by Markovich and Mandler24 that treating the ITO/glass in the presence of perfluoroalklysilanes for 7 days is required to obtain a similar coverage using toluene. Etching Experiments. The ITO covered with perfluoroalkylsilane monolayers was patterned with a focused ion beam (FIB) to form patterns which consisted of lines of width 300-500 nm and periodicity 400-1000 nm. The ions in the FIB damage the SAMs, locally erasing this protective coating.25 The milling time of the focus ion beam was sufficiently short that it only just destroyed the perfluoroalkyl silane monolayer, and the patterns were not visible under the optical microscope after this initial treatment. The samples were then etched at 25 °C in an aqueous oxalic acid solution (0.05 M), with mild agitation for 5 min (following the conditions of Breen and co-workers for wet etching).21 The results are shown in Figure 5a and b. The patterns are clearly visible as dark areas where the ITO has been removed. The areas protected by the perfluoroalkylsilane SAMs are untouched, and we did not observe any defects in the area studied. The white dots shown in the SEM image are intrinsic to the material we used and are not due to the monolayer deposition process. Etching experiments were performed using homogeneous bare ITO/glass and homogeneous perfluoroalkylsilane SAM ITO/glass as well, and these results are shown in Figure 5c-g. It was found that, for bare ITO, only 5 min were required to etch away the ITO, as reported by Breen.21 However, for ITO covered with a fluorinated SAM, the ITO survived for more than 10 h exposure of to oxalic acid, showing that fluorinated SAMs are indeed excellent etch resists. The optical microscope images showed no change during the 10 h, and the ITO remained conductive with a resistance of 52 Ω before treatment and 57 Ω after treatment. Conclusion

Figure 5. (a) Optical micrographs of an ITO film patterned by a focus ion beam followed by wet etching. (b) SEM image of an ITO film patterned by a focus ion beam followed by wet etching. Optical microscope images (×100) during etching experiments: (c) bare ITO; (d) bare ITO after 2 min of etching; (e) bare ITO after 5 min of etching (The ITO is completely removed.); (f) ITO covered with a perfluoroalkylsilane SAM; (g) ITO covered with a perfluoroalkylsilane SAM after 10 h of etching (No change is observed.).

monolayer-covered electrode and a bare electrode, respectively. The rate constants were obtained using the method of Nicholson and Shain for a quasi-reversible system.53,54 By determining the variation of peak-to-peak (53) Nicholson, R. S. Anal. Chem. 1965, 37, 1351-1355. (54) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-723. (55) Wipf, D. O.; Kristensen, E. W.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1988, 60, 306-310.

We have demonstrated an easy and robust method for patterning ITO. This method relies on our procedure to form very good perfluorinated monolayers on ITO via deposition from compressed CO2. The formation of the monolayers is significantly faster than that of those from organic solvents, and the quality of the silane SAMs is comparable to that of those formed on Si/SiO2. The etch resistance of these silanes is remarkable, with no detectable deterioration of the surface after 10 h in the etching solution. The ease of preparation of the monolayers, combined with the ease of patterning and their high etch resistance, will open the way to their use as commercially important etch resists. Acknowledgment. We thank Seiko-Epson Corporation for generous financial support (studentship to C.K.L.), and we thank Mr. M. Ishida, Dr. H. Fukushima, and Mr. S. Nebashi (SE Corporation) for their interest in this work. W.T.S.H. is supported by ICI and the Isaac Newton Trust. LA0342114