Scanning Near-Field Photolithography Techniques
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Optical methods continue to surprise us. The Abbé limit, of approximately λ/2, was once thought to be an insurmountable barrier to their exploitation, but the ingenuity of physicists continues to confound this unduly pessimistic outlook. There are currently a number of tools, including near-field, two-photon, fluorescence-based, and plasmonic techniques that all offer subdiffraction limit spatial resolution. Moreover, while the end of photolithography has been forecast for about two decades, the semiconductor device industry remains firmly wedded to it as its primary manufacturing tool, and electronic engineers have been very creative in adapting it to facilitate the onward progress of their industry along the path charted by Moore's Law. However, at the very smallest length scale, electron beam lithography has continued to be seen as the gold standard fabrication tool. While electron beam methods are serial ones and do not readily translate into a manufacturing environment, they have been thought to offer a degree of resolution beyond the reach of photolithography.
Here we review new work that suggests this view is incorrect. By using scanning near-field photolithography (SNP), it is possible to generate structures in organic monolayers that are comparably small to anything fabricated by electron beam lithography. Importantly, however, these approaches, based on the excitation of photochemical reactions, are capable of being implemented under ambient conditions or even under fluid. Although such near-field lithographic tools are in their infancy, they offer exciting prospects for use in lithography at extremely small length scales, along with a capability to manipulate biomolecular and organic molecular systems that are inaccessible to electron beam methods. Combined with multiplexing, they may, given their lower cost, offer much greater potential for commercial exploitation than has ever been the case for electron beam lithography, and they may prove a valuable tool in the drive toward highly miniaturized molecular devices and materials.