NANO LETTERS
Matching the Resolution of Electron Beam Lithography by Scanning Near-Field Photolithography
2004 Vol. 4, No. 8 1381-1384
Shuqing Sun and Graham J. Leggett* Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, UK Received March 24, 2004
ABSTRACT Molecular features with widths of only 20 nm have been fabricated in self-assembled monolayers of alkanethiols on gold using a new lithographic tool, scanning near-field photolithography, based upon the use of a near-field scanning optical microscope (NSOM) coupled to a UV laser. Quite unexpectedly it has proved possible to routinely fabricate structures significantly smaller than the aperture in the NSOM probe. This exceptional performance is strongly correlated with the morphology of the gold film. In particular, the best results are achieved on films with comparatively small grain sizes. In contrast, the use of atomically flat, epitaxially deposited gold films leads to a minimum feature size comparable to the aperture diameter (ca 50 nm). It is concluded that nonradiative interactions (possibly the excitation of surface plasmons) between the gold substrate and the fiber lead to a pronounced focusing of the electric field beneath the aperture.
The manipulation of molecular structure on nanometer length scales remains central to the development of nanotechnology.1 Applications as diverse as the immobilization of electrically active molecules,2,3 for incorporation into novel molecular electronic devices, and the fabrication of nanostructured arrays of biological molecules, for ultrahigh sensitivity high-throughput screening in proteomics, metabolomics, and other branches of bioanalytical science, require the ability to position molecules on length scales down to tens of nanometers. For single-molecule immobilization and detection, sub-50 nm patterning of organic molecules is critical. However, very few methods offer the capability for control of surface chemistry on such small length scales. The most successful approaches to date have been based upon scanning probe microscopy, including dip pen nanolithography6,7 and nanoshaving.8 In the past, the diffraction limit has always represented a fundamental barrier to the exploitation of optical lithography.9,10 In semiconductor device fabrication, electron beam lithography has thus become established as the technique of choice for the fabrication of small structures. Recently some new approaches (such as phase mask photolithography11) have emerged that provide optical lithography on length scales beyond the diffraction limit.12-14 However, as yet they have not matched the performance of electron beam lithography (i.e. they have not repeatably yielded feature sizes smaller than 50 nm). Recently we reported a new approach, which we called scanning near-field photolithography (SNP),15 in which a * Corresponding author. Tel: +44 114 222 9556. Fax: +44 114 222 9346. E-mail:
[email protected] 10.1021/nl049540a CCC: $27.50 Published on Web 06/30/2004
© 2004 American Chemical Society
near-field scanning optical microscope (NSOM) coupled to a UV laser is used to selectively oxidize strongly bound alkanethiolate adsorbates in a self-assembled monolayer to weakly bound alkylsulfonates. The latter may readily be displaced, either with a contrasting, solution-phase thiol, to yield a chemical pattern, or with a solution-phase etchant, leading to selective removal of gold and the creation of threedimensional nanostructures.16 Light is a highly attractive tool for surface functionalization, because of the broad range of strategies that are available for the attachment of molecules to surfaces using photochemical reactions.17-19 However, a key question concerns the resolution limit of such approaches. Here we demonstrate that the feature sizes accessible using SNP are comparable to those achievable by electron beam lithography and that they are strongly dependent upon the morphology of the gold substrate. Exploitation of the phenomena reported here may provide a ready means to the photochemical patterning of surfaces on length scales down to 20 nm under ambient conditions, providing a versatile tool for surface functionalization on small length scales. For the lithography we have used a commercial optical fiber-based NSOM system (Aurora III, Veeco, Cambridge, UK). The probe-sample distance is controlled by using a tuning fork feedback mechanism. Polycrystalline gold films were prepared by evaporating 25 nm of Au (Goodfellow, Cambridge, UK) onto Cr-primed glass microscope slides (no. 2 thickness, Chance Proper, UK). For easier location of the nanopatterns generated by SNP using lateral force microscopy (LFM) on a separate atomic force microscope (AFM, ThermMicroscopes Explorer, Veeco, Cambridge, UK), poly-
Figure 1. (a) Friction image, (b) topographical image acquired simultaneously, and (c) high-resolution friction image, showing lines of C15CH3 written into a monolayer of C2COOH. The line width is 30 nm.
Figure 2. Influence of gold morphology on line width. Friction images showing lines of C15CH3 written into C2COOH SAMs on (a) epitaxially deposited Au; (b) polycrystalline Au with a grain size of ca. 40 nm; (c) polycrystalline Au with a grain size of ca. 20 nm. The line widths are: (a) 60 nm; (b) 40 nm; (c) 20 nm.
crystalline gold was evaporated through a copper grid to form microscopic relief features that could be visualized through an optical microscope. Epitaxially deposited gold films were purchased from Georg Albert PVD-Beschichtungen (Heidelberg, Germany) and were briefly flame-annealed prior to use. To enable precise location of regions on the surface, micronscale relief features in Cr were deposited by evaporation through a mask. Alkanethiols were purchased from Fluka and were used as received. SAMs were formed by immersion of the substrate in a dilute solution of the appropriate thiol in ethanol for 18 h. After SNP of SAMs of mercaptopropanoic acid (C2COOH), the sample was immersed in a solution of hexadecanethiol (C15CH3) for 2 h to displace the oxidized species. Figure 1 shows parallel lines of C15CH3 written into C2COOH. C15CH3 is observed with dark contrast in the lateral force image (Figure 1a) because it exhibits a smaller coefficient of friction than the highly polar C2COOH. The contrast in the LFM image is extremely clear and indicates the efficacy of the displacement of the alkylsulfonate species. No tendency to erosion of the short-chain adsorbate is observed, however, because it gains significant stabilization through hydrogen bonding between terminal carboxylic acid groups. The C15CH3 features exhibit brighter contrast in the topographical AFM image acquired simultaneously with the LFM image, because the adsorbates are longer. Examination of a high magnification LFM image (Figure 1c allows the accurate measurement of the line width, which is about 30 nm. This level of performance is highly repeatable. Impor1382
tantly, it represents a resolution significantly smaller than the diameter of the aperture in the NSOM probe, which SEM investigations show is typically not less than 50 nm and often larger. The resolution achieved by SNP is thus unexpected, because theoretical studies indicate that the electrostatic field diverges beneath a near-field aperture.20 Thus one would expect, a priori, that the best possible performance would be a resolution equal to the aperture diameter. Careful examination of high resolution images suggests that while the lines written by SNP are continuous, and sharply defined, they are also influenced by the grain structure of the underlying gold. One explanation for the very high resolution achieved here is that the morphology of the gold grains plays a regulating role on the near-field optical processes leading to photooxidation. To test this hypothesis, we carried out SNP under the same conditions using an atomically flat, epitaxially deposited gold film as the substrate. Large flat terraces were observed by AFM prior to patterning. When SNP was performed, the best line width achieved was ca. 50 nm, although typically it was slightly larger, being in the range 60-75 nm, i.e., slightly larger than the aperture size of the NSOM probe. A typical friction force image (Figure 2a) shows lines of C15CH3 written into a monolayer of C2COOH. It may also be observed that the lines in Figure 2a exhibit straighter edges than those fabricated on polycrystalline gold films. The effect of the size of the morphology of the polycrystalline film on the line width was investigated systematically. Gold films with different grain sizes were prepared by Nano Lett., Vol. 4, No. 8, 2004
Figure 3. Friction images showing (a) an array of dots and (b) a single dot of C15CH3 created in a monolayer of C2COOH using SNP.
varying the evaporation rate and film thickness during deposition. When a substrate composed of larger gold grains was used, under similar conditions, the line width was found to be increased, as can be seen in the friction force image in Figure 2b, where a width of 40 nm was measured for lines of C15CH3 written into C2COOH. The use of substrates composed of smaller gold grains gives rise to a decrease in the patterned line width. In Figure 2c, parallel lines as small only 20 nm wide with a separation of about 20 nm are imaged. The contrast between regions of different terminal group functionality is very clear, and lines written close together can be differentiated clearly. The resolution achieved here (λ/12) is substantially beyond the diffraction limit and rivals the performance of electron beam lithography for materials of this type. In contrast to electron beam lithography, however, SNP is an ambient technique compatible, in principle, with operation in a fluid medium, clearly indicating its potential for high resolution combined with flexibility in fabrication chemistry. As noted above, the apertures of our probes are typically not less than 50 nm, so the performance levels reported here are not the result of using of an unusual probe with a very small aperture. Moreover, if the aperture is as narrow as 20 nm, the power leaving the aperture would be greatly reduced, leading to a very slow writing rate. In our experience, there is no reduction in the writing rate required to fabricate the smallest structures. Finally, using the exactly the same probe, arrays of dots with diameters of ca. 50 nm have been generated on same batch of substrate (see Figure 3) with an irradiation time of 30 s for each dot, where the dosage is much higher than during line generation (0.5 s). This suggests that the aperture size of the probe is about 50 nm but the distribution of the light from the probe aperture has been perturbed by the gold grains in some way. Finally, a quantitative comparison of different substrates is shown in Figure 4. The line width is shown as a function of the mean grain size of a selection of samples for lithography carried out with new fibers. Old fibers sometimes have broadened apertures, which leads to a degradation of resolution not incorporated here. Samples were binned into groups with mean grain sizes of 20, 30, 40, and 50 ( 10 nm, and the mean line width determined. The error bars here represent the outer extremes of each group. It is clear that Nano Lett., Vol. 4, No. 8, 2004
Figure 4. Variation in the widths of lines written with new fibers as a function of the grain size of the underlying substrate. The error bars represent, for each point, the maximum and minimum values measured.
there is a correlation between the grain size and the resolution achieved. Although the data in Figure 4 appear to fit a straight line, the magnitudes of the error bars are large enough that other functions may be fitted to the data too. Rigorous fitting was not felt to be appropriate because of the difficulty associated with adequately controlling all of the experimental variables (grain sizes, fiber apertures, etc.). However, the general nature of the influence is clearly shown. The epitaxial gold film, with a mean line width of 60 nm, clearly represents the limit of infinite grain size. The observations that we have reported here are unexpected and no precedent exists in the literature for this kind of behavior. The following tentative explanation is consistent with the data, although it requires significant testing. Exposure of gold surfaces to visible light leads to the excitation of surface plasmons.21 These are known to be highly localized in the near-field at gold asperities,22 and this phenomenon is the basis for apertureless near-field scanning optical microscopy, which is capable of yielding resolution superior to fiber-based techniques.23-26 Much of the literature based upon apertureless,27,28 or scattering,29 NSOM focuses on the visible region of the spectrum, because 1383
of the interest in optical spectroscopic measurements with high spatial resolution. Less is known about the UV irradiation of gold surfaces.21 Excitation of surface plasmon (scattering) modes is known to occur, even where the incident light has a wavelength different from the maximum in the plasmon absorption spectrum, although the wavelength used here (244 nm) is some way from the maximum of the gold plasmon absorption. Nevertheless, the behavior exhibited here would be consistent with the excitation of highly localized surface plasmon modes on the gold grains in the polycrystalline gold film. Effectively, the grains in the film would function as an array of asperities, each of which would be capable of interacting nonradiatively with the electrostatic near field associated with the optical fiber as it traverses the sample surface. The result is a sharper spatial confinement of the field than would be the case were the sample ideally flat. In the case of SNP of SAMs on flat gold substrate, no such interaction may occur, and the electrostatic field behaves exactly as expected on the basis of theoretical predictions, diverging beneath the fiber so that the best resolution is similar to, or slightly larger than, the fiber aperture. If this explanation proves correct, it may have important ramifications for the characterization of materials by NSOM, because it suggests that deposition of sample molecules onto a gold surface may potentially yield spatial resolution superior to that by deposition onto a perfectly planar substrate. Our data imply that optical lithography is not only possible significantly beyond the diffraction limit but also that it is routinely possible with a resolution better than the NSOM fiber diameter. The development of a full understanding of the underlying physical process is expected to yield further improvements in resolution. It is also important that the optimal resolution was achieved here for polycrystalline substrates that are very readily prepared, and under ambient conditions. It is possible to carry out NSOM in a fluid environment; in principle SNP offers the prospect of transferability to a liquid medium with substantial further benefit compared to traditional approaches such as electron beam lithography. In summary, we have demonstrated that SNP can routinely rival the resolution of electron beam lithography utilizing photochemical processes. The smallest lines created have width of 20 nm with the same distance of separation and show good contrast. With this capability, SNP appears poised to fulfill its promise by combining the power of photolithography with nanometric spatial resolution for single molecular immobilization and detection.
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Acknowledgment. We thank the Engineering and Physical Sciences Research Council (EPSRC) for financial support (grant GR/N82197/01). G.J.L. thanks the EPSRC and the Royal Society of Chemistry Analytical Chemistry Trust Fund for their support. References (1) Kraemer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (2) Kaholek, M.; Lee, W.-K.; LaMattina, B.; Caster, K. C.; Zauscher, S. Nano Lett. 2004, 4, 373. (3) Liu, S.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 1055. (4) Yamato, M.; Konno, C.; Koike, S.; Isoi, Y.; Shimizu, T.; Kikuchi, A.; Makino, K.; Okano, T. J. Biomed. Mater. Res. 2003, 67, 1065. (5) Ying, L.; Bruckbauer, A.; Rothery, A. M.; Korchev, Y. E.; Klenerman, D. Anal. Chem. 2002, 74, 1380. (6) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science, 1999, 283, 661. (7) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523. (8) Liu, G. Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (9) Smith, H. I. ReV. Sci. Instrum. 1969, 40, 729. (10) Lin, B. J. Fine Line Lithography, Materials Processing-Theory and Practices Series; Newman, R., Ed.; North-Holland: New York, 1980; Vol. 1, pp. 105-150. (11) Rogers, J. A.; Paul, K. E.; Jackman, R. J.; Whitesides, G. M. J. Vac. Sci. Technol. B 1998, 16, 59. (12) Odom, T. W.; Thalladi, V. R.; Love, J. C.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 12112. (13) Ghislain, L. P.; Elings, V. B.; Crozier, K. B.; Manalis, S. R.; Minne, S. C.; Wilder, K.; Kino, G. S.; Quate, C. F. Appl. Phys. Lett. 1999, 74, 501. (14) Tarun, A.; Dazs, M. R. H.; Hayazawa, N.; Intuye, Y.; Kawata, S. Appl. Phys. Lett. 2002, 80, 3400. (15) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414. (16) Sun, S.; Leggett, G. J. Nano Lett. 2002, 2, 1223. (17) Yang, Z.; Frey, W.; Oliver, T.; Chilkoti, A. Langmuir 2000, 16, 1751. (18) Blawas, A. S.; Reichert, W. M. Biomaterials 1998, 19, 595. (19) Gao, H.; Saenfer, M.; Luginbuehl, R.; Sigrist, H. Biosens. Bioelectron. 1995, 10, 317. (20) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824. (21) Riehn, R.; Charas, A.; Morgado, J.; Cacialli, F. Appl. Phys. Lett. 2003, 82, 526. (22) Novotny, L.; Pohl, D. W.; Hecht, B. Ultramicrosc. 1995, 61, 1. (23) Larsen, R. E.; Metiu, H. J. Chem. Phys. 2001, 114, 6851. (24) Levy, J.; Hubert, C.; Trivelli, A. J. Chem. Phys. 2000, 112, 7848. (25) Hartschuh, A.; Pedrosa, H. N.; Novotny, L.; Krauss, T. D. Science 2003, 301, 1354. (26) Bouhelier, A.; Beversluis, M.; Hartschuh, A.; Novotny, L. Phys. ReV. Lett. 2003, 90, 013903. (27) Yin, X.; Fang, N.; Zhang, Z.; Martini, I. B.; Schwartz, B. J. Appl. Phys. Lett. 2002, 81, 3663. (28) Anderson, M. S. Appl. Phys. Lett. 2000, 76, 3130. (29) Raschke, G.; Kowarik, S.; Franzl, T.; Soennichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Urzinger, K.; Nano Lett. 2003, 3, 935.
NL049540A
Nano Lett., Vol. 4, No. 8, 2004
