Nanopatterning

Techniques for fabricating on sub-micron length scales span a wide range, from sophisticated lithographic methods that have their origins in the semiconductor industry to more recent materials and chemical advances that rely on self-organization. For delineating patterns below 100 nm, several approaches have been proposed (and indeed demonstrated). These include nano-imprint lithography (including micro-contact printing, mold-assisted lithography, and hot embossing lithography), near-field optical lithography, direct patterning on a nanometer scale with scanning-probe microscopes, self-assembly of monolayers, pattern formation based on phase separation of polymers, etc. The search is on for nonphotolithographic methods that could provide technologically simpler and cheaper nanofabrication strategies. Some of these approaches are better suited for producing individual nano-structures for the investigation of nanometer-scale devices; the throughput is likely to always remain impracticably low for commercial application. Others such as nanoimprint lithography have the potential of high throughput due to parallel processing, do not require sophisticated tools, and allow nanoscale replication for data storage.

The natural length scales of polymer chains and their morphologies in the bulk, which lie in the nanometer domain, make polymers ideal building blocks for nanopatterning. Recent developments in the use of polymers for the fabrication of nanostructures via lithographic and self-assembling strategies have been reviewed.

Nanopatterning via Phase Separation of Polymers

Block copolymers of flexible, chemically incompatible, and dissimilar blocks can microphase separate into a variety of morphologies with nanometer scale dimensions. This self-assembly process is driven by an unfavorable mixing enthalpy and a small mixing entropy, while the covalent bond between the two blocks prevents macrophase separation. When the microphase separated morphology can be controlled and turned into a useful structure, phase separation of block copolymers can be a powerful tool for fabricating nanostructures without additional lithography and processing steps. In addition, to block copolymers copolymers, random copolymers comprising sufficiently dissimilar components based on size and chemical nature, for e.g., SSQ-MMA copolymer, SSQ/Polypropylene blend, or physical blends of hydrophobic and hydrophilic polymers driven by an underlying pattern of incompatibility have been shown to yield functional and arbitrary patterns. The use of surfactants in such blends may be used to accentuate the areas of different surface tension.

Soft lithographic approaches have been combined with surfactant and particulate templating procedures to create to create hierarchically ordered oxides. A recent report combines molecular scale, evaporation-induced self-assembly (EISA) of organically modified mesophases with macroscopic, evaporative printing procedures. This allows the rapid fabrication of hierarchical structures exhibiting form and function on multiple length scales and at multiple locations. The formulation of the “ink” in such hierarchial assembly strategies is complex and may use a variety of surfactants.

Self-Assembled Monolayer Systems (SAMS)

Polyelectrolytes are defined as materials for which the solution properties in solvents of high dielectric constant are governed by electrostatic interactions over distances larger than typical molecular dimensions. These materials are widely used in industrial applications as dispersants in aqueous media, flocculating agents to coagulate slurries and industrial wastes, for sizing in textile and paper manufacture, and as conditioning additives to drilling muds and soil to prevent abrasive damage. More recently, they have been applied in molecular self-assembly techniques for thin film deposition of electrically conducting polymers, conjugated polymers for light emitting devices, nanoparticles,and noncentrosymmetric-ordered second order nonlinear optical (NLO) devices.

The technique of Self-Assembled Monolayers or SAMs is an ingeniously simple, yet powerful nanoscale approach for the fabrication of functional supramolecular assemblies for various device applications. It involves the alternate adsorption of anionic and cationic polyelectrolytes onto a suitable substrate. Typically, only one of these is the active layer, while the other enables the composite multilayered film to be bound by electrostatic attraction. Alternatively, the oppositely charged polyelectrolytes may serve as a barrier for the sustained release of an active core. Controlled formation of highly ordered, three-dimensional, multifunctional, reactive, thin films containing biological molecules is seeing widespread application in the areas of biotechnology and biomaterials science.

Of the potential polyanionic candidates, poly(styrenesulfonic acid) (PSSA) and its salts have been used extensively. Why? Excellent adsorption properties, water solubility, smooth films with easily controllable thickness, controlled level of loading/penetration of active component—all properties essential in realizing high performance devices based on supramolecular structures in terms of selectivity, sensitivity, response time, and stability. In addition, PSSA has been used to dope thiophene-based conjugated polymers to make them conducting, e.g., PEDOT/PSS Aldrich products. Post treatment of polyaniline (emeraldine salt), grafted to lignin (see Aldrich products , with PSSA may also be used to enhance electrical conductivity.

Norbornadiene

A scan of the scientific and patent literature reveals that this versatile monomer has been applied over a vast spectrum of high technology applications, in fields ranging from materials science, ag-related, and pharmaceutical, to being used as a model system in fundamental research activity (for example, in testing new nanocatalysts – in single-step hydrogenations, tandem cycloaddition reactions, Pauson-Khand annelation as well as in comprehensive theoretical studies. The unique set of properties offered by Norbornadiene along with its transformations driving some of the reported applications are highlighted in the following table.

Scheme 1. Conversion between norbornadiene (NBD) and quadricylane (QC).

Table 1. Properties of norbornadiene.

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.

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