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Lithographic Applications of Redox Probe Microscopy Diego J. Dı´az,† Jamie E. Hudson,† Gregory D. Storrier,† He´ctor D. Abrun˜a,*,† Narayanan Sundararajan,‡ and Christopher K. Ober‡ Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, and Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, New York 14853-1501 Received April 17, 2001 The molecular-scale lithographic applications of a technique which we have developed and termed redox probe microscopy, RPM, in which an AFM tip is modified with redox-active materials, are presented. In this technique, the interactions between the tip and an adsorbate or between the tip and a surface are modulated by means of the electrode potential, allowing for the generation of desired surface structures and patterns. Two cases are presented: (a) the use of an RPM tip to deliberately manipulate and position objects (“microtweezers”) and (b) the utilization of an RPM tip to “pattern” a pH-sensitive block copolymer using what we term the “pH-stylus”. In the first case, the AFM tip is modified with a film of poly(vinylferrocene) and the adhesive interactions of the modified tip with sulfonated chromatography beads are deliberately controlled to be either strong or weak depending on the redox state of the ferrocene centers, allowing for the deliberate positioning of the beads to generate a pattern. In the pH-stylus, an RPM tip is modified with a hydroquinone self-assembling monolayer which upon oxidation releases protons. By a scan of such a modified tip over a pH-sensitive block copolymer and control of the electrode potential, the polymer film can be exposed to generate patterns.
Introduction Identifying means to surpass the 0.1 µm limit of conventional lithographic techniques has generated a great deal of interest, especially within the semiconductor industry, as alternative technologies are sought that might be able to provide higher resolution without compromising speed.1-6 Not only have scanned probe microscopies (SPMs) been found to be invaluable tools for highresolution imaging, but the interactions of the scanning probe with a surface can also be exploited/tailored to make them useful as lithographic tools.7-10 There have been a number of approaches geared to the use of SPM-based techniques in lithographic applications. One of the earlier examples was based on the use of an SPM tip for “scratching” a surface thereby generating a pattern.11-15 This was done initially on bare surfaces and † ‡
Department of Chemistry and Chemical Biology. Department of Materials Science and Engineering.
(1) Moore. G. E. Electronics 1965, 38, 8. (2) Hamers, R. J. J. Phys. Chem. 1996, 100, 13103-13120. (3) Stix, G. Sci. Am. 1996, 4, 94-99. (4) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (5) Snow, E. S.; Campbell, P. M.; Perkins, F. K. Appl. Phys. Lett. 1999, 75, 1476-1478. Wang, D.; Thomas, S. G.; Wang, K. L. Appl. Phys. Lett. 1997, 70, 1593-1595. (6) Minne, S. C.; Adams, J. D.; Yaralioglu, G.; Manalis, S. R.; Atalar, A.; Quate, C. F. Appl. Phys. Lett. 1998, 73, 1742-1744. (7) Zhang, H.; Hordon, L. S.; Kuan, S. W. J.; Maccagno, P.; Pease, R. F. W. J. Vac. Sci. Technol., B 1989, 7, 1717-1722. (8) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632-636. (9) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 1195-1230. (10) Lillehei, P. T.; Bottomley, L. A. Anal. Chem. 2000, 72, 189R196R (e). Bottomley, L. A. Anal. Chem. 1998, 70, 425R-475R. (11) Schmitt, C. C.; Eilings, J. R. Nanoindenting, Scratching and Wear Testing Using Scanning Probe Microscopy; Digital Instruments App Note Nanoindentation. (12) Sumomogi, T.; Endo, T.; Kuwahara, K.; Kaneko, R. J. J. Vac. Sci. Technol., B 1994, 12, 1876-1880. (13) Silva, L. A.; Laitenberger, P.; Palmer, R. E. J. Vac. Sci. Technol., B 1993, 11, 1992-1999. (14) Wendel, M.; Kuhn, S.; Lorenz, H.; Kotthaus, J. P.; Holland, M. Appl. Phys. Lett. 1994, 65, 1775-1777. (15) Gobel, H.; van Blackenhagen, P. J. Vac. Sci. Technol., B 1995, 13, 1247-1251.
afterward on surfaces modified with organic layers.16-18 Although the principle was clearly demonstrated, these techniques suffered from numerous drawbacks including wide variability and the fact that the tip could be easily damaged and/or modified giving rise to unpredictable and uncontrolled variations in the patterns generated.9,19 Avouris and co-workers also demonstrated the ability to use AFM to induce the oxidation of Si wafers, thus generating patterns by the selective etching of the oxide.20,21 The use of electrodeposition with SPM probes for lithographic applications has also been demonstrated.22 For example, Kolb and co-workers have shown that patterns can be “written” with copper electrodeposited on gold substrates.23-25 This approach offers great resolution and control, and in addition, both the tip and the substrate can be active elements in the process. Because the approach depends on current flow, it is limited to conducting tips and substrates. Moreover, in the absence of an applied potential, that is, at open circuit, the deposited structures tend to dissolve. Mirkin and coworkers recently developed and demonstrated a technique which they termed “dip-pen” nanolithography where an AFM tip (the pen) is immersed in a solution of an alkane thiol (the ink) and the liquid is transferred, via capillary (16) Mamin, H. J.; Rugar, D. Appl. Phys. Lett. 1992, 61, 1003-1005. (17) Anoikin, E. V.; Yang, M. M.; Chao, J. L.; Eilings, J. R.; Brown, D. W. J. Vac. Sci. Technol., A 1998, 16, 1741-1744. (18) Yanchun, H.; Schmitt, S.; Friedrich, K. Appl. Compos. Mater. 1999, 6, 1-18. (19) Rui, Y.; Evans, D. F.; Hendrickson, W. A. Langmuir 1995, 11, 211-213. (20) Avouris, Ph.; Martel, R.; Hertel, T.; Sandstrom, R. Appl. Phys. A 1998, 66, S659-S667. (21) Avouris, Ph.; Hertel, T.; Martel, R. Appl. Phys. Lett. 1997, 71, 285-287. (22) Schneir, J.; Hansma, P. K.; Eilings, V.; Gurley, J.; Wickramasinghe, K.; Sonnenfeld, R. Proc. SPIEsInt. Soc. Opt. Eng. 1998, 16-19. (23) Randler, R. J.; Kolb, D. M.; Ocko, B. M.; Robinson, I. K. Surf. Sci. 2000, 447, 187-200. (24) Kolb, D. M.; Engelman, G. E.; Ziegler, J. C. Angew. Chem., Int. Ed. 2000, 39, 1123-1125. (25) Kolb, D. M.; Engelmann, G. E.; Ziegler, J. C. Solid State Ionics 2000, 131, 69-78.
10.1021/la010561j CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
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action, to a gold surface.26 They have achieved line widths of the order of 15 nm and have developed a number of strategies for employing different alkanethiols (inks) to provide contrast as well as multiple pen devices.27,28 Perhaps the ultimate lithographic method is based on the use of atoms as ink; the best examples to date being “atom-man” and the “Big Blue” (IBM) logo.29-31 However, the experimental requirements are extraordinarily stringent; UHV, very low temperatures (ca. 4 K), and very long times are needed to generate even the simplest patterns. It, however, stands as an example of the ultimate lithographic approach. Our group has developed a technique that we have termed “redox probe microscopy” (RPM) in which a metalcoated SPM probe is modified with a redox-active film so that tip-sample interactions, such as adhesion, can be modulated by the electrochemical potential.32 We have previously shown that tips modified with ferrocenyl groups (either poly(vinylferrocene) or a ferrocenyl-terminated thiol) exhibit electrochemically controlled/modulated adhesion toward surfaces modified with ferrocenyl, methyl, and carboxylate groups.32 Such potential-dependent interactions can be used to deliberately move adsorbates on a surface through a process that we have termed “microtweezers” and which we demonstrate here. Following a similar approach, we have developed an SPM-based “pH-stylus” by the functionalization of an AFM tip with a quinone/hydroquinone (Q/QH2)-containing monolayer. It is well-known that the quinone/hydroquinone redox reaction involves the transfer not only of electrons but also of protons and that such reactions are reversible.33-35 Having an AFM tip modified with such a (Q/QH2)-containing monolayer allows for the generation of protons with very high spatial resolution. Block copolymers can be synthesized with groups that are pH sensitive, and in fact, such materials have found lithographic applications through the use of so-called photoacid generators to achieve “exposure” of the resist.36,37 We have combined the use of (Q/QH2)-modified tips with pHsensitive block copolymers to develop a new lithographic technique which we have termed the pH-stylus. In essence, by cycling the electrochemical potential (over the (Q/QH2) couple) of such a modified tip while scanning over a surface modified with a film of one such pH-sensitive block copolymer, we can “write” on the polymer film by exposing areas of the surface to local changes in pH, hence the term pH-stylus. This can provide a lithographic alternative of very high resolution, limited only by the quality of the polymer film and the size of the tip. We present here lithographic applications of RPM based on electrochemically controlled adhesion (microtweezers) for the deliberate movement and placement of adsorbates as well as on local changes in pH (pH-stylus) when in combination with pH-sensitive block copolymers. (26) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661-663. (27) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523-525. (28) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808-1810. (29) Zeppenfeld, P.; Lutz, C. P.; Eigler, D. M. Ultramicroscopy 1992, 42, 128-133. (30) Eigler, D. M.; Schweizer, E. K. Nature 1990, 344, 524-526. (31) Stroscio, J. A.; Eigler, D. M. Science 1991, 254, 1319-1326. (32) Hudson, J. E.; Abrun˜a, H. D. J. Am. Chem. Soc. 1996, 118, 63036304. (33) Sasaki, T.; Bae, I. T.; Scherson, D. A. Langmuir 1990, 12341237. (34) Mo, Y.; Sandifer, M.; Sukenik, C.; Barriga, R. J.; Soriaga, M. P.; Scherson, D. Langmuir 1995, 11, 4626-4628. (35) Tae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 4508-4513. (36) Sundararajan, N.; Yang, S.; Ogino, K.; Valiyaveetil, S.; Jianguo, W.; Zhou, X.; Ober, C. K. Chem. Mater. 2000, 12, 41-48.
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Experimental Section 1. Synthesis. Poly(vinylferrocene) (PVF) was synthesized via free radical polymerization of vinylferrocene according to a previously published procedure.38,39 The polymer was recrystallized from ether three times prior to use. 6-(Ferrocenylcarbonyl)hexanethiol (thioFc) was prepared by Diels-Alder acylation of ferrocene with 6-bromohexanoyl chloride followed by conversion of the bromine to a thiol by refluxing with thioacetic acid and sodium metal in methylene chloride.40,41 The product was purified by column chromatography over alumina using 9:1 methylene chloride/hexane as eluent. ThioFc eluted as the second of three orange bands. Thiomethyl-2,5-hydroquinone (thioQH2) was prepared following published procedures.42 Methyl methacrylate, tert-butyl methyl methacylate, methyl acrylic acid (MMA-TBMA-MAA) block copolymers were synthesized via published procedures.43 2. Reagents. Water was purified by a Hydro purification train and a Millipore Milli-Q system. Methylene chloride (Burdick and Jackson) for electrochemical measurements was dried over activated 4 Å molecular sieves. Methylene chloride for synthesis was further dried by refluxing over sodium metal. Tetra-nbutylammonium perchlorate (TBAP) (G.F. Smith) was recrystallized three times from ethyl acetate and dried under vacuum for 72 h. All other reagents were of at least reagent grade quality and were used without further purification. Sulfonated chromatography beads were obtained from Pharmacia Biotech and used as received. 3. Procedures. Thin gold films, used as substrates in the microtweezers studies, were prepared by thermal evaporation onto green muscovite mica (ASTM, Grade V-2 or better; AshevilleSchoonmaker Mica Co.) in vacuum (typically 5 × 10-7 Torr). Atomically smooth films could be routinely made by heating the mica to approximately 350 °C prior to deposition, evaporating 150 nm of high-purity gold (99.99995%, Johnson-Matthey) at 1-3 Å/s, annealing the films at 350 °C for at least 2 h, and then cooling in vacuum. Commercially available silicon nitride (Si3N4) AFM tips (Digital Instruments, Santa Barbara, CA) were coated by thermal evaporation of a 25 Å chromium adhesion layer followed by a 500 Å layer of gold in a vacuum (5 × 10-7 Torr). Tips were then chemically modified with PVF, 6-(ferrocenylcarbonyl)-hexanethiol (thioFc), or thiomethyl-2,5-hydroquinone (thioQH2). PVF-modified tips were prepared by electrochemical deposition from methylene chloride solution according to previously described procedures.14 A film thickness of approximately 35 nm was estimated by integrating the charge under the cyclic voltammetric wave corresponding to the oxidation of the ferrocene centers of a PVF film deposited onto a planar electrode of known area for the same time and conditions as those of the tip and substrate. PVF-modified tips were characterized by scanning electron microscopy (SEM). Comparison of SEM images of goldcoated tips before and after modification with PVF showed that the polymer film was evenly spread across the electroactive regions of the tip. However, it should be noted that due to the difficulty of imaging PVF by SEM, the radius of curvature at the contact region of the tip is unknown. ThioFc-modified tips were prepared by exposure of gold-coated AFM tips to a 5 mM ethanolic solution of thioFc for at least 30 min. The spontaneous adsorption of alkane thiols onto gold surfaces to produce well-ordered monolayer structures is wellknown.44,45 The presence of a well-defined, reversible voltammetric surface wave corresponding to the oxidation of the (37) Sundararajan, N.; Keimel, C. F.; Bhargava, N.; Ober, C. K.; Opitz, J.; Allen, R. D.; Barclay, G.; Guangyu, X. J. Photopolym. Sci. Technol. 1999, 12, 457-468. (38) Roullier, L.; Waldner, E. J. Electroanal. Chem. 1985, 187, 97107. (39) Merz, A.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 3222. (40) Rosenblum, M. Chemistry of the Iron Group Metallocenes: Ferrocene, Ruthenocene, Osmocene, Part One; John Wiley & Sons: New York, 1965. (41) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (42) Alcalay, W. Helv. Chim. Acta 1947, 30, 578-584. (43) Ober, C. K.; Sundarajan, N.; Shu, Y.; Robert, D.; Ogino, K.; Kameyama, A.; Mates, T. Polym. Mater. Sci. Eng. 1999, 81, 49-50.
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ferrocene moiety was taken as evidence of a high-quality tip, and from the charge under the voltammetric wave, the coverage was estimated to be typically in the monolayer regime as is often observed for these types of systems. Thio-Q/QH2 modified tips were prepared following the same procedure employed for the thioFc-modified tips. For the microtweezers studies, a dilute aqueous dispersion of 15 µm sulfonated chromatography beads was spread onto gold films on mica or onto freshly cleaved highly oriented pyrolytic graphite, and the solvent was allowed to evaporate. Films of MMA-TBMA-MAA block copolymers were prepared by spin coating at 1500 rpm for 60 s onto a clean Si(100) wafer. The thickness of the polymer films was between 150 and 200 nm. The silicon wafers were cut into small pieces and mounted on a glass slide as a solid support. The samples were electrically grounded to avoid electrostatic buildup on the polymer sample. 4. Instrumentation. For the microtweezers studies, scanning probe microscopy measurements were made with a Nanoscope III multimode microscope (Digital Instruments, Santa Barbara, CA). For the in situ measurements, the Nanoscope III’s electrochemical microscope’s built-in potentiostat was found to be unacceptably noisy, so electrochemical measurements were performed using a BAS CV-27 potentiostat (BioAnalytical Systems, Inc.) operated independently of the microscope. A platinum coil and silver wire served as counter and reference electrodes, respectively, while the tip was the working electrode. In the case of pH-stylus studies, SPM experiments were carried out using a Digital Instruments Nanoscope E controller and Molecular Imaging 8 µm AFM scanner. AFM setpoints were kept between 0 and 1.0 V in order to minimize interactions with the surface. In addition, the setpoint was kept constant for any specific experiment in order to keep the pressure exerted by the tip constant. Scan rates used were between 4 and 6 Hz, and all images were acquired at the highest possible resolution of 512 samples. In situ electrochemical experiments were carried out using a Molecular Imaging PicoStat potentiostat, externally controlled through a DAQ card (National Instruments) in a pentium class computer running Labview (National Instruments). Positioning and controlled movement of the piezo scanner were achieved by scanning certain areas of the polymer sample while the electrode potential was controlled (cycled over the Q/QH2 redox couple in areas where we intended to write and kept at a potential where the hydroquinone was reduced so that no protons were generated while positioning the tip). More control of the movement of the tip was obtained by writing lithography scripts using the Digital Instruments scripting tool. The electrochemical experiments were carried out in pH 7.4 phosphate buffer. A silver wire was used as a QRE electrode, a Pt wire was used as the counter electrode, and the modified AFM tip was used as the working electrode. To use the tip as the working electrode, ohmic contacts need to be made between the external connections to the potentiostat and the cantilever holder (in a conventional AFM experiment, the working electrode would be the surface being scanned instead of the AFM tip as in the present case). The temperature of the solution and the polymer film was kept constant at 25.0 °C using a Lakeshore model 321 temperature controller/programmer (Lakeshore Cryotonics, Inc). Ex situ experiments were carried out on a conventional electrochemical cell in a typical three-electrode arrangement, where Ag/AgCl was used as a reference, a Pt wire served as the counter electrode, and a polycrystalline Au electrode was the working electrode. Scanning electron microscopy measurements were made using a Leica 440 microscope operated at 20 kV and 1-2 nA. Thin metal films were deposited using a Varian electron beam turbo pumped evaporating system with an Inficon thickness monitor.
Results 1. Surface Manipulation of Adsorbates through Electrochemically Controlled/Modulated Adhesion (Microtweezers). As mentioned before, this approach is (44) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Isreaelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (45) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368.
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Figure 1. Schematic depiction of the manipulation of a molecular adsorbate with an RPM tip which can be switched between strongly binding, dark gray, and weakly binding, light gray. By control of the movement and redox state of the tip, it can be used to move and reposition adsorbates. The adsorbate is assumed to bind with moderate strength to the substrate.
based on the use of an RPM probe modified with a redoxactive species so that the tip’s interaction (through adhesive effects) with an adsorbate can be controlled to be either strong or weak depending on the oxidation state of the redox center and the nature of the adsorbate. The adsorbate, on the other hand, is assumed to bind with moderate (intermediate) strength to the substrate surface. At a potential where the redox center is in the oxidation state where its interaction with the adsorbate is weak, the tip cannot overcome the adsorbate/substrate interaction and thus cannot displace/move the adsorbate. On the other hand, at a potential where the redox center is in the oxidation state that interacts strongly with the adsorbate, it should be possible to controllably bind and deliberately move and place the adsorbate in any arbitrarily desired location by a series of potential steps. This is schematically depicted in Figure 1, where the redox state of the tip can be modulated to one where it strongly binds the adsorbate (Figure 1A-D) in order to reposition the adsorbate and then switched to a redox state where interactions are weak in order to release the adsorbate (Figure 1E,F). As an example, we have employed a PVF-modified probe to reproducibly bind and reposition 15 µm, sulfonic acid terminated chromatography beads. Whereas the beads interact with moderate strength with the substrate, their interaction with the tip can be strongly modulated by the redox state of the ferrocene groups contained in the PVF film. At 0.0 V, the PVF film is neutral so that the tip interacts weakly with the beads. On the other hand, upon oxidation of the PVF to the ferricenium state, the film is positively charged so that the tip’s interaction with the beads is greatly enhanced. Thus, by using a programmed sequence of potential steps the beads can be moved and placed in any arbitrary location. Note that since the beads exhibit some interaction with the substrate, once moved and placed in a desired position, they remain at that location. Thus, experiments can be carried out at room temperature and the generated structure remains indefinitely stable. Figure 2 shows an example of the deliberate positioning of sulfonated chromatography beads using the abovementioned procedures. In this example, the beads were initially on a gold film substrate and in contact with a supporting electrolyte (pH 7 phosphate buffer) solution. The AFM tip, initially held at a potential where the PVF is neutral (e.g., 0.0 V), is positioned atop a bead (Figure
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Figure 3. (A) Molecular structure of thiomethyl-2,5-hydroquinone (thioQH2). (B) Molecular structure of a MMA-TBMAMAA block copolymer.
Figure 2. Manipulation of 15 µm chromatography beads using a PVF-modified redox probe microscopy tip. (A) The tip, held at a potential where the PVF film is neutral (reduced), is positioned atop a bead. The PVF film is then (B) oxidized (increasing the electrostatic interactions between the tip and the bead), (C) repositioned on the sample, and (D) released from the tip by reducing the PVF film. (E) This allows the preparation of deliberate structures, such as the smiley face.
2A). The potential is then adjusted to a value where the PVF is oxidized (e.g., +1.0 V) making it positively charged. This results in the binding of the bead to the tip (Figure 2B). While bound to the tip, the bead can then be moved to a new region/location on the sample (Figure 2C). Once the desired new location is reached, the potential of the tip is returned to a value where the PVF film is once again neutral resulting in the “release” of the bead from the tip’s surface (Figure 2D). In this way, beads can be deliberately moved to generate novel structures such as the “smiley face” shown in Figure 2E. To demonstrate that the ability to bind and reposition the beads was based on the potential-dependent adhesion, we have carried out similar experiments with tips modified with 6-(ferrocenylcarbonyl)hexanethiol (thioFc) as well as with octyl-thiol (C8-SH). Thiols are known to form strongly bound and ordered layers on gold substrates,44,45 and the above-mentioned materials were chosen so as to exemplify the cases of the presence (similar to PVF) and absence of a ferrocenyl group. Tips modified with thioFc exhibited behavior quite similar to that of those modified with PVF; that is, the sulfonated beads could be deliberately bound and repositioned. On the other hand, with tips modified with C8-SH, the beads could not be bound or repositioned neither at 0.0 V nor at +1.0 V. These studies clearly demonstrate that the ability to deliberately move and reposition the beads arose solely as a result of the modulation and control of the strength of adhesion. It is worth emphasizing the fact that these experiments were carried out at ambient temperature and pressure. It is also notable that this example represents the extreme upper size limit of the technology and the beads were employed so as to demonstrate the large interaction strengths (and their modulation) that can be achieved. Clearly, if objects as “massive” as the chromatography
Figure 4. Schematic depiction of the procedure for lithography based on the pH-stylus.
beads can be deliberately moved and repositioned, it should be possible to manipulate much smaller objects. In principle, the lower limit should be restricted only by the resolution of the microscope which should be in the nanometer regime. 2. The pH-Stylus. As mentioned earlier, in this application we have combined the use of RPM tips, modified with a thioQH2 monolayer (Figure 3A) which upon oxidation liberates protons, with pH-sensitive block copolymers (Figure 3B). The general process is depicted in Figure 4. An RPM probe modified with a thioQH2 monolayer is scanned over a surface coated with a pHsensitive block copolymer. By application of a potential where the hydroquinone (QH2) is oxidized to the quinone, two protons (per molecule) are generated and are used to “expose” the copolymer film (Figure 4). In essence, the
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protons are used as “ink” in this case to generate a desired pattern. The “pen” can be recharged by simply applying a potential where the hydroquinone (QH2) is regenerated. Since tip interactions with “soft” surfaces such as polymers can be quite significant (in fact, and as mentioned before, such interactions have been used in the past to generate patterns), a number of control studies were carried out to demonstrate that the writing was due to the local pH changes arising from the Q/QH2 reaction at the surface of the tip. To estimate the roughness of the copolymer films, we initially attempted to image the films in air; however, this generally gave rise to a high degree of tip-induced surface damage. On the other hand, carrying out the imaging in phosphate buffer or distilled water greatly reduced such damage and allowed us to identify which copolymers gave rise to the smoothest films. Such differences in the degree of damage to surfaces imaged in air versus under solvent are now well documented in the literature and are believed to arise from the lubricating effect that the liquid has on the tip-sample interactions. On the basis of these preliminary studies, we chose films derived from MMA-TBMA-MAA (Figure 3B) for all further studies. To assess the extent of tip-induced damage on the films, the tip was left scanning over an area of 500 nm × 500 nm of the block copolymer for 30 min, at 1.0 V setpoint and 6 Hz scan rate. (Under these conditions, the film on the tip was maintained in the quinone form so that no protons were generated.) The image scan area was then increased to 1500 nm × 1500 nm to determine the extent of the surface damage. A representative image obtained under such conditions is presented in Figure 5A where some damage to the surface is evident. The rectangular shape of the damaged area (instead of the squared 500 nm × 500 nm area scanned) is due to some drifting of the piezo upon prolonged scanning. When the scanning time was reduced to 10 min, however, the damage was greatly diminished and virtually imperceptible. Thus, although there can be tip-induced damage to the surface, it can be virtually eliminated by limiting the scanning time to 10 min for a 500 nm × 500 nm area. A different area on the sample, 500 nm × 500 nm, was subsequently scanned for 10 min but under conditions where the potential applied to the tip was scanned between -0.15 and 1.00 V at 150 mV/s versus Ag QRE, so that protons were generated (with all other imaging conditions being identical to those above) during the scanning. As in the previous case, the image scan area was then increased to 1500 nm × 1500 nm to determine the extent of surface patterning. As shown in Figure 5B, a very well defined 500 × 500 nm pattern is evident on the surface which unambiguously arises as a result of the interaction of the copolymer film with the tip-generated protons. Moreover, reducing the scanning time to 5 min did not result in any degradation of the generated pattern (scanning a comparable area for 5 min in the absence of tip-generated protons did not result in any discernible pattern (tipinduced damage)). By translating the scanner over the surface and scanning various 500 nm × 500 nm areas for 5 min while generating protons at the tip, we generated the smiley face shown in Figure 6. Clearly, this represents a proof-of-concept demonstration of the pH-stylus for lithographic applications. The scanning time appeared to affect the depth of the generated pattern. For a sample where the potential was scanned for 10 min in a 500 nm × 500 nm area, a section analysis showed a depth of 10 ( 2 nm, whereas for a sample
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Figure 5. 1500 nm × 1500 nm ECAFM images of block copolymer MMA-TBMA-MAA (A) showing the effects of tipinduced damage by scanning over an area of 500 nm × 500 nm on the surface for 30 min and then imaging that area. (B) Similar approach, but cycling the potential between -0.15 and +1.0 V vs Ag QRE for 10 min. Note the great enhancement in the pattern generated.
scanned for 5 min, a similar analysis revealed that the depth was 8 ( 1 nm. Similar results were observed at different times, where by increasing the scanning time up to 25 min, the depth increased to 30 ( 5 nm (although in this case it is hard to separate tip-induced damage from the exposed pattern). The section analyses for scans of 10 and 25 min are presented in parts A and B of Figure 7, respectively, where such differences are evident. To obtain a qualitative estimate of the resolution of the technique, several square patterns, of progressively smaller size, were generated (Figure 8). In these cases, the scanning time (during which protons were generated) was concomitantly reduced so as to be comparable to a 5 min scan for a 500 × 500 nm area. From these studies, it was found that the resolution of the generated patterns was largely controlled by the tip’s size and geometry. The patterns began to blur and became irregular when the size approached about 100 nm. This resolution limit was evident at different scanning times and was attributed to the size of the scanning tip. Figure 8A shows the patterns obtained from the different scan sizes where the abovementioned effects are apparent. Standard Si3N4 tips from DI are known to have radii of curvature between 30 and 50 nm, consistent with the above arguments and our
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Figure 6. Application of the pH-stylus for pattern generation on a pH-sensitive block copolymer. By a scan of different areas (as in Figure 5B) and translation of the stage, a smiley face, as shown, is generated. Imaging conditions were kept the same as in Figure 5B.
Figure 7. Section analyses of line patterns generated by scanning over the polymer while cycling the potential for (A) 10 min and (B) 25 min.
assertion that the resolution of the technique was limited by the tip’s size and geometry. This is evidenced further by using a lithography script to control the movement of the scanner and generate a linear pattern while protons
were generated. The width of such a line was approximately 60 nm as shown in Figure 8B. In an effort to increase the resolution of the technique, we are currently working on the further modification of the Si3N4 tips. In general, it would be desirable to have only the very apex of the tip covered by the Q/QH2containing film, while the rest of the tip is insulated. To achieve this, we are exploring the utility of using ophenylene diamine (OPD) to deliberately mask parts of the tip. OPD is known to electropolymerize rapidly on an electrode upon oxidation giving rise to a highly insulating layer.46,47 By softly bringing the tip into contact (“crashing”) with a soft and smooth surface (e.g., gold on mica), one could insulate the tip with an electropolymerized layer of OPD while protecting the tip’s apex which, after retraction, would be subsequently modified with the Q/QH2 layer as described previously. We are currently carrying out such studies, and we believe that such tips could increase the resolution of the technique by about 1 order of magnitude or perhaps even more. Another important aspect that can affect resolution is the fact that the tip-generated protons can diffuse away from the scan area. Upon repetitive scanning, it was found that pits (ostensibly originating from proton-induced degradation of the copolymer film) started to form on different areas of the surface that had not been previously scanned. As an example of this, Figure 9 shows an area where the presence of such pits is clearly evident. Some of such pits can also be observed in Figure 5B, indicating (46) Lee, H. Y.; Adams, R. N. Anal. Chem. 1962, 34, 1587. (47) Yacynych, A. M.; Mark, H. B. J. Electrochem. Soc. 1976, 123, 1346.
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that this “ink leakage” could be significantly reduced by working in buffer solutions of higher pH and by the tip modification with OPD. Modifying the tip with OPD would limit the number of protons coming from parts of the lever that are Au coated but are too far from the polymer surface and tend to diffuse more into the film (in our approach the entire lever is gold coated and thus modified with the quinone, generating protons at distances relatively far from the scanning area). A final limitation of the technique was that after a few hours of continuous scanning (writing), the coating on the tip seemed to wear out, requiring remodification of the tip or a change of tip. An alternative would be to use quinonecontaining polymers which we believe will have higher stability in addition to providing tips with a higher capacity (more ink). We have recently synthesized such polymers, and we are currently assessing their utility in these applications. We have also carried out some preliminary studies on the effects of temperature on the rate of pattern generation. One would anticipate that the kinetics of the exposure reaction (hydrolysis) would be enhanced by raising the temperature thereby requiring shorter times for pattern generation. However, and for similar reasons, the diffusion of protons would also be enhanced giving rise to a faster rate of pit formation. It was found that the polymer tended to expand upon heating, making it difficult to keep a constant tip pressure on the sample. In addition, higher temperatures also mean faster rates of solvent evaporation as well as tip bending due to thermal expansion. Moreover, it was also found that for temperatures above 75 °C, the films tended to peel away from the Si substrate. Clearly, there are a multitude of temperature-dependent effects which need careful assessment and which we are currently studying. Conclusions Figure 8. (A) Testing the resolution of the technique by generating square patterns of different sizes. The scanned areas for the generation of the patterns were 500 × 500 nm, 250 × 250 nm, and 100 × 100 nm. The scanning conditions were kept the same as in Figure 5B. (B) A 640 nm × 640 nm image showing a 50 nm wide strip.
Figure 9. ECAFM image showing the pits surrounding a pattern due to the diffusion of protons from the tip. Imaging conditions were kept the same as in Figure 5B.
that the process also occurs (although to a smaller extent) on shorter scanning times. The pits tended to decrease in number and size as areas farther away from those in which protons were generated were scanned. We believe
We have presented lithographic applications of the redox probe microscopy technique developed by our group. In RPM, a tip modified with a redox-active film allows for modulating molecular interactions via control of the electrode potential. This technique has proven very useful for surface modification as the interactions between the tip and a surface or the tip and an adsorbate can be easily and deliberately controlled through the electrode potential. Two approaches have been presented. First, an AFM tip is modified with PVF and used to reposition sulfonated chromatography beads on a substrate by controlling the electrostatic interactions of the beads to a charged tip. In the second case, an AFM tip is modified with a hydroquinone self-assembled monolayer and used as a proton generator so when scanned over a pH-sensitive block copolymer protons can be generated, exposing the polymer to generate a pattern. These two examples point to the possible applications of RPM not only for imaging surface structures but also for the creation of desired structures with higher resolution than conventional lithographic techniques. Further applications and refinements are being explored and will be presented elsewhere. Acknowledgment. This work was supported by the Office of Naval Research and the Cornell Center for Materials Research (CCMR). D.J.D. acknowledges CCMR support by the R. L. Sproull Fellowship and the Ford Foundation for the Ford Fellowship. LA010561J
