Building Materials Molecule-by-Molecule
Scanning Near-field Photolithography
Photolithographic methods have previously been successfully used to create patterned self-assembled monolayers (SAMs), but have been regarded until now as being limited to the micron scale because of the well-known diffraction limit: passing light through an aperture smaller than half its wavelength results in diffraction. However, by making the novel step of utilising a scanning near-field optical microscope (SNOM) in place of the conventional mask and lamp combination, we have shown that it is possible to use photochemical processes to routinely fabricate features with linewidths of 40 nm and smaller. The process, called scanning near-field photolithography (SNP), is illustrated schematically in figure 1.
A frequency-doubled argon ion laser (wavelength 244 nm) was coupled via a fused silica optical fibre to a SNOM. Lines were written in a SAM formed from a first thiol, leading to the photo-oxidation of adsorbed thiols to soluble alkyl sulfonates where the surface was exposed to UV light from the SNOM. In the second step, the sample was immersed in a solution containing a thiol molecule with a contrasting terminal group functionality; this second thiol displaced the oxidation products of the lithographic step leading to the formation of a chemical pattern. The resulting patterns were imaged using friction force microscopy (FFM). Figure 2 shows results for a monolayer of mercaptoundecanoic acid into which lines have been written. The sample has subsequently been immersed in a solution of hexadecanethiol. Regions functionalised with the carboxylic acid terminated adsorbate exhibit brighter contrast in the FFM image than those occupied by the non-polar adsorbate. Dark lines are observed where the acid-terminated thiol has been photo-oxidised by the SNOM and displaced by the methyl-terminated thiol. The topographical image shows no contrast, confirming that the contrast in the FFM image is due to spatial variations in chemical composition.
The lines in figure 2 are only 40 nm wide. This kind of resolution is achievable quite routinely. However, we have recently found that significantly better resolution is achievable. Fifure 3 shows a high magnification image of several lines written in a similar way using SNP. The line width is only 20 nm, comparable with the resolution achievable by electron beam lithography for these materials, but with the lines being, in this case, fabricated under ambient conditions.
These lines are much smaller than the aperture in the NSOM fibre, which is a surprising result, because a priori, one might have expected the best resultion achievable to be comparable to the diameter of the aperture. An explanation may lie in the morphology of the substrate. These samples were prepared on polycrystalline gold films, and the line width has been found to vary with the grain size of the underlying substrate. Small grain sizes yield narrow lines, while atomically flat ones lead to lines with widths comparable to the diameter of the NSOM aperture (ca 50 - 55 nm). We believe that the grains in the metal film function as an array of antennae, focussing the electric field associated with the near-field excitation in a small region. The phenomenon may be regarded as an inversion of the so-called "lightning rod effect" that forms the basis of apertureless NSOM. The effect is not restricted to gold, for a number of materials are capable of acting as antennae in this way.
S. Sun and G. J. Leggett, "Matching the resolution of Electron Beam Lithography using Scanning Near-field Photolithography", Nano Lett.4 (2004) 1381-1384
S. Sun and G. J. Leggett, "Generation of Nanostructures by Scanning Near-field Photolithogrophy of Self-Assembled Monolayers and Wet Chemical Etching", Nano Letters 2 (2002) 1223-1227.
S. Sun, K.S. L. Chong and G. J. Leggett, "Scanning Near-field Optical Lithography of Self-assembled Monolayers", J. Am. Chem. Soc. 124 (2002) 2414-2415.
G. J. Leggett, "Biological Nanostructures: Platforms for Analytical Chemistry at the sub-Zeptomolar Level", invited article, The Analyst, 130 (2005) 159-264.
Material Copyright © 2008 Graham Leggett