pubs.acs.org/NanoLett
Parallel Scanning Near-Field Photolithography: The Snomipede Ehtsham ul Haq,† Zhuming Liu,‡ Yuan Zhang,§ Shahrul A. Alang Ahmad,|,# Lu-Shin Wong,⊥ Steven P. Armes,| Jamie K. Hobbs,†,| Graham J. Leggett,*,| Jason Micklefield,⊥ Clive J. Roberts,‡ and John M. R. Weaver§ †
Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, U.K., School of Pharmacy, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., § Department of Electronics and Electrical Engineering, Rankine Building, University of Glasgow, Glasgow G12 8LT, U.K., | Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., and ⊥ School of Chemistry and Manchester Interdisciplinary Biocentre, The University of Manchester, 131 Princess Street, Manchester M1 7DN, U.K. ‡
ABSTRACT The “Millipede”, developed by Binnig and co-workers (Bining, G. K.; et al. IBM J. Res. Devel. 2000, 44, 323.), elegantly solves the problem of the serial nature of scanning probe lithography processes, by deploying massive parallelism. Here we fuse the “Millipede” concept with scanning near-field photolithography to yield a “Snomipede” that is capable of executing parallel chemical transformations at high resolution over macroscopic areas. Our prototype has sixteen probes that are separately controllable using a methodology that is, in principle, scalable to much larger arrays. Light beams generated by a spatial modulator or a zone plate array are coupled to arrays of cantilever probes with hollow, pyramidal tips. We demonstrate selective photodeprotection of nitrophenylpropyloxycarbonyl-protected aminosiloxane monolayers on silicon dioxide and subsequent growth of nanostructured polymer brushes by atom-transfer radical polymerization, and the fabrication of 70 nm structures in photoresist by a Snomipede probe array immersed under water. Such approaches offer a powerful means of integrating the top-down and bottom-up fabrication paradigms, facilitating the reactive processing of materials at nanometer resolution over macroscopic areas. KEYWORDS Nanolithography, near-field lithography, scanning probe array, parallel probe lithography, near-field optics, spatial light modulator
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Ideally we require a method for the selective initiation of chemical reactivity in nanometer regions of space, under fluids (to enable the supply of reagents to build multicomponent structures) and with full control over macroscopic distances. Photochemical methods are attractive in this context.22-25 Using a scanning near-field optical microscope (SNOM)26-28 coupled to a UV laser, we have previously written structures as small as 9 nm (λ/30, far beyond the conventional Rayleigh limit) in self-assembled monolayers (SAMs) of alkanethiols on gold surfaces,29 confirming that near-field methods are powerful tools for molecular lithography, and have additionally been demonstrated to be compatible with fluid phase operation.30 They provide a means to draw upon a large body of literature on bottomup photochemical methods for organic synthesis and integrate them with top-down methods, to yield nanoscale control of chemical reactivity. Of particular relevance is the development of methods for light-directed chemical synthesis;31 near-field methods enable the translation of such approaches to the nanometer scale. However, using a commercial SNOM system, it is only possible to write structures sequentially over a microscopic area, so that the patterning of macroscopic areas is extremely difficult. The “Millipede”, developed by Binnig and co-workers,1 is an elegant solution to the criticism that SPM-based lithographic techniques enable only serial fabrication. Our goal
he manipulation of molecular structure and organization at nanometer length scales (i.e., e100 nm) still presents substantial challenges. Indeed, the integration of top-down (lithographic) and bottom-up (synthetic) fabrication techniques remains one of the greatest challenges in molecular nanoscience. Despite much effort, there are few techniques that provide control of chemical reactivity with nanometer spatial resolution, and few that also do so over macroscopic length scales. Electron beam lithography provides the gold standard in spatial resolution for inorganic materials,1,2 but in molecular systems, the requirement to work under vacuum is problematic, and except in a small number of special cases,3-6 it provides no direct means to initiate chemical reactions. Scanning probe microscopy (SPM) techniques7-18 have been more widely used for molecular patterning, but rely upon sequential deposition or removal of materials (with the exception of dip-pen nanolithography (DPN), for which parallel deposition has been reported19-21).
* To whom correspondence should be addressed. E-mail: Graham.Leggett@ shef.ac.uk. # Current address: Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. Received for review: 05/27/2010 Published on Web: 10/14/2010
© 2010 American Chemical Society
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DOI: 10.1021/nl1018782 | Nano Lett. 2010, 10, 4375–4380
has been to fuse the capacity of SNOM-based methods to control chemical reactivity at high resolution with the elegance of Millipede-style massively parallel actuation: a “Snomipede”. In the present study, we describe a prototype instrument that facilitates parallel near-field lithography over distances of greater than 1 mm by deploying a parallel array of sixteen near-field optical probes (we demonstrate writing from nine probes in parallel in the present work). In principle, all of the design features are extendable to larger arrays of probes. The instrument incorporates the capacity for separate feedback measurement for each probe. To enable the parallel excitation of probes at microscopic spacings, we have developed two new approaches to optical coupling with the near-field aperture. We demonstrate the potential of such approaches for the integration of synthetic chemical methodology by utilizing simple protecting group strategies from organic synthesis, and applying them to the fabrication of polymer nanostructures. We also demonstrate the potential for parallel operation under fluid. A shear-force SNOM configuration,32-34 although widely used for microscopy, is unsuitable for parallelization because of the large dimensions of the probes typically used. Any approach in which arrays of apertures are formed in a fixed screen or in a bundle of fibers is precluded by the fact that precise tolerances must be maintained (the apertures should be
