Imprinting, or embossing, is a well-known technique to generate microstructures in hard polymers by pressing a rigid master containing surface-relief features into a thin thermoplastic polymer film that is then heated close to or, more generally, above the Tg (see Figure 2). Nanoimprint lithography (NIL) has the potential of high-throughput due to the parallel processing, does not require sophisticated tools, and allows nanoscale replication for data storage. NIL is also compatible with conventional device processing techniques. The quality of the nanoimprinting process depends on a number of experimental parameters like T, viscosity in the melt, adhesion of the polymer to the mold, etc. PMMA has been most widely used as the imprintable material, but a range of thermoplastic and thermosetting polymers is under investigation to optimize the imprinting and subsequent etching steps.
Figure 2. Schematic overview of nanoimprint lithography.
The rigid master is usually prepared via e-beam lithography and has feature sizes in the 10–100 nm size range. After imprinting the polymer film, further etching can transfer the pattern into the underlying substrate. Alternatively, metal evaporation and lift-off of the polymer mask produces nanopattern metal features.
Soft Lithography
Nanoimprint lithography (NIL) has primarily been used to emboss hard thermoplastic polymers. The micromolding and embossing of elastomers has attracted considerable interest as these materials have found important applications in softlithographic techniques such as microcontact printing (mCP). In this technique, a monolayer of a material is printed off an elastomeric stamp [made of poly(dimethylsiloxane) (PDMS)] after forming conformal contact between stamp and substrate (Figure 3). Sub-micron surface relief structures can easily be introduced in PDMS by curing the polymers against a lithographically prepared master. The advantage of mCP is the ability to pattern surfaces chemically at the sub-micron level.
Figure 3. Schematic overview of microcontact printing (mCP). (Images courtesy of Hongwei Li, Wilhelm T. S. Huck; University of Cambridge , Department of Chemistry , Melville Laboratory for Polymer Synthesis.)
An elastomeric stamp is inked with small molecules (thiols or silanes) and pressed against a clean substrate (gold or silicon wafer). Where the stamp is in contact with the surface, a monolayer of material is transferred to the substrate. A second thiol or silane is used to fill in the background to provide a chemically patterned surface.
Photochemical Acid Generators
Photoacid generators (or PAGs) are cationic photoinitiators. A photoinitiator is a compound especially added to a formulation to convert absorbed light energy, UV or visible light, into chemical energy in the form of initiating species, viz., free radicals or cations. Cationic photoinitiators are used extensively in optical lithography. The ability of some types of cationic photoinitiators to serve as latent photochemical sources of very strong protonic or Lewis acids is the basis for their use in photoimaging applications. The continuing decrease in device dimensions in the microelectronics industry is being achieved by pushing the limits of optical lithography. In chemically amplified resist technology, the radiation-sensitive material (resist) in which patterns are delineated typically includes a matrix polymer and an onium salt photoacid generator (or PAG). There are several materials’ issues to be considered in the choice of the PAG: sufficient radiation sensitivity to ensure adequate acid generation for good resist sensitivity, absence of metallic elements, temperature stability, etc.
The usual photo-supplied catalyst has been strong acid. Triarylsulfonium and diaryliodonium salts have become the standard PAG ingredients in CA resist formulations, because of their generally easy synthesis, thermal stability, high quantum yield for acid (and also radical) generation, and the strength and nonvolatility of the acids they supply. Simple onium salts are directly sensitive to DUV, X-ray and electron radiations, and can be structurally tailored, or mixed with photosensitizers, to also perform well at mid-UV and longer wavelengths. However, onium salts are ionic and many will phaseseparate from some apolar polymers, or not dissolve completely in some casting solvents. Nonionic PAGs such as phloroglucinyl and o,o-dinitrobenzyl sulfonates, benzylsulfones and some 1,1,1-trihalides are more compatible with hydrophobic media in general, although their thermal stabilities and quantum yields for acid generation are often lower.
The phenomenal rate of increase in the integration density of silicon chips has been sustained in large part by advances in optical lithography – the process, as described above, that patterns and guides the fabrication of the component semiconductor devices and circuitry. Although the introduction of shorter-wavelength light sources and resolution enhancement techniques should help maintain the current rate of device miniaturization for several more years, a point will be reached where optical lithography can no longer attain the required feature sizes. Several alternative lithographic techniques under development have the capability to overcome these resolution limits – EUV, X-ray, electron beam and ion beam lithographies, but, at present, no obvious successor to optical lithography has emerged.
Creating patterns and structures at this scale (a nanometre is a billionth of a metre) is a delicate task which is only possible with special techniques and methods. Thanks to the NaPa (‘Emerging Nanopatterning Methods’) project, Europe’s capabilities in this exciting new field are now stronger than ever. The project brought together 36 research groups from 12 EU Member States plus Switzerland and Russia. The team, which included some 80 % of Europe’s key players in the field, contained an even mix of scientists from industry, research institutes and universities.
By working together, they created a vibrant, united nanopatterning research community in Europe. In addition to developing new materials and tools for nanopatterning, the project partners filed several patents, published hundreds of articles and founded three spin-off companies. The project partners are continuing to work together to bring their results closer to commercialisation.
The next 10 years promises to be an exciting period in the history of computers and networks as nanotechnology takes off to redefine a new level in the way computers are manufactured. It’s not entirely radical as the Lithographic principles behind the manufacturing process can be adopted for nanotech processes. What is revolutionary are the minute molecular-level sizes at which those circuit boards can now be made. This is the core of nanotech Continue reading »
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I would lie if I said that I know enough about nanotechnology, but from little I know, dangerous questions arise in my dangerous mind.
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