Atomic Force Microscope Nanolithography on Organized Molecular Films
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The ongoing miniaturization in size and integrity of electronic and mechanic devices has led to an interest in fabrication of nanometer-sized uniform structures on surfaces. With the device downscaling dimensions to the nanometer range, current interest is focused on the state-of-the-art lithographic development such as electron beam lithography, extreme UV lithography, imprint lithography, and scanning probe microscope (SPM) lithography. Binnig et al. first developed the scanning probe microscope in 1982, and since then, SPM have been primarily used to obtain topographic and electronic surface maps. Nowadays, SPM is also being used to fabricate patterns and structures in nanometer scale, as well as to directly modify chemical or physical structure of surfaces.
Among several nanolithographic methods, scanning probe lithography (SPL) is one of the best lithographic techniques for forming nanostructures. Scanning probe lithography using a sharpened tip of an atomic force microscope (AFM) or a scanning tunneling microscope (STM) with the highest spatial resolution has demonstrated outstanding capabilities for atomic-level manipulation, and also wide varieties of potential applications such as electronic devices, biochips, and biosensors using different approaches—a direct method of writing patterns consisting of relatively small molecules on solid state surface, an exposure of organic thin films such as self-assembled monolayers (SAMs) and a selective anodic oxidation of various substrates. In particular, AFM lithography is the most promising method for fabricating organic thin films or the substrate itself in nanometer scale. Whereas STM-based lithography is mainly carried out in ultrahigh vacuum (UHV), fabrication using AFM lithography can be performed in liquid state as well as in ambient environment. Also, several novel methods using AFM as a lithographic tool have been developed in the past 10 years. This is a very active research area because of the flexibility and simplicity of the technique. In this technique, an AFM tip may be used to carry catalysts to selectively induce surface reactions, to attach molecules on surfaces as a pen, and to direct local oxidation on surfaces as an electrode.
Subsequently, several principal AFM nanolithography techniques on organized molecular films such as self-assembled monolayers, Langmuir–Blodgett (LB) monolayer, and spin-coated polymer films will be illustrated.