NANOFABRICATION AND SCANNING PROBE LITHOGRAPHY
“Nanofabrication” is the process of making functional structures with arbitrary patterns having minimum dimensions currently defined (more-or-less arbitrarily) to be e100 nm. Microelectronic devices and information technologies have improved and will continue to improve as a result of large-scale, commercial implementation of nanofabrication. The motivation for these improvements is to increase the density of components, lower their cost, and increase their performance per device and per integrated circuit. The smallest physical gate length of a microprocessor currently in production is 37 nm and current half-pitch, or periodicity, of manufactured dynamic random-access memory (DRAM) is 90 nm. The International Technology Roadmap for Semiconductors (ITRS), published by the Semiconductor Industry Association (SIA), projects reaching the 45-nm node in 2010 (corresponding to transistor gate lengths down to 18 nm and DRAM spacing of 45 nm).1 It is likely that a number of new technologies will evolve with further developments in nanofabrication. Many materials with minimum dimensions on the nanoscale have properties different than those observed for the bulk material. For example, quantum dots can exhibit single-electron tunneling carbon nanotubes can have high electrical conductivity and mechanical strength and thin polymer films can have glass-transition temperatures higher or lower than thick films. There is an expectation that new technologies will emerge from the fabrication of nanostructures and nanostructured materials and also that nanofabrication will have new applications beyond information processing and storage in areas such as optics, biomedicine, and materials science.
Different methods of nanofabrication:
Methods used to generate nanoscale structures and nanostructured materials are commonly characterized as “top-down” and “bottom-up”. The top-down approach uses various methods of lithography to pattern nanoscale structures. This approach includes serial and parallel techniques for patterning features - typically in two-dimensions (2D) - over length scales approximately 4 orders of magnitude larger (in linear dimension) than an individual structure. The bottom up approach uses interactions between molecules or colloidal particles to assemble discrete nanoscale structures in two and three dimensions. Furthermore, Nanofabrication techniques can be divided in two sections - 1.Conventional techniques 2. Unconventional techniques.
Conventional Techniques: These top-down techniques include photolithography and scanning beam (or maskless) lithography (e.g., electron beam and focused ion beam lithography). The limitations of these conventional approaches such as their high capital and operating costs, the difficulty in accessing the facilities necessary to use them, and their restricted applicability to many important classes of problemssmotivate the exploration and development of new, or “unconventional”, nanofabrication techniques. These unconventional approaches, of course, have limitations of their own. Unconventional Techniques: Unconventional nanofabrication includes both top-down and bottom-up approaches. Some of the areas of unconventional nanofabrication techniques are - molding, embossing, printing, scanning probe lithography (SPL), edge lithography and self assembly. The first three techniques are primarily top-down approaches to nanofabrication. Scanning probe lithography and self-assembly, however, bridge top-down and bottom-up strategies for nanofabrication; these two techniques often use templates fabricated by top-down methods to direct the bottom up assembly of components. “Conventional” and “unconventional” techniques are at different stages of development. Conventional techniques for nanofabrication are...
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