Nanofabrication For Nanoelectronics

The semiconductor industry has seen a remarkable miniaturization trend, driven by many scientific and technological innovations. However, if the trend is to continue and provide even faster and cheaper computers, the size of microelectronic circuit components will soon need to reach the scale of atoms or molecules [205].

Photolithography, the technology used to manufacture computer chips and virtually all other microelectronic systems, can be refined to make structures smaller than 100 nanometers, but doing so is very difficult, expensive, and inconvenient.

The electronics industry is deeply interested in developing new methods for nanofabrication so that it can continue its long-term trend of building even smaller, faster, and less expensive devices.

Despite enormous recent progress in the fabrication and demonstration of nanoelectronic devices, many challenges remain. For solid-state electronics, one of the most important challenges is to be able to produce reliable and uniformly in silicon the characteristic nanometer-scale features required for nanoelectronics: nanometer-scale islands, barriers, and "heterojunctions" between islands and barriers.

One leading contender is electron-beam lithography [206]. In this method, the circuitry pattern is written on a thin polymer with a beam of electrons. An electron beam does not diffract at atomic scales, so it does not cause blurring of the edges of features. Researchers have used this technique to write lines with widths of only a few nanometers in a layer of photoresist on a silicon substrate. The electron-beam instruments currently available, however, are very expensive and impractical for large-scale manufacturing.

Another contender is lithography using X-rays with wavelengths between 0.1 and 10 nanometers or extreme ultraviolet light with wavelengths between 10 and 70 nanometers. Because these forms of radiation have much shorter wavelengths than the ultraviolet light currently used in photolithography, they minimize the blurring caused by diffraction [206]. But these technologies have their own problems: conventional lenses are not transparent to extreme ultraviolet light and do not focus X-rays.

For top-down lithography, they begin with a pattern generated on a larger scale and reduce its lateral dimensions. But no top-down method is ideal; none can conveniently, cheaply, and quickly make nanostructures of any materials. Thus researchers have shown a growing interest in bottom-up methods, which start with atoms, molecules, and nanoparticles to build up nanostructures with dimensions between 2 and 10 nanometers and do so inexpensively [207].

Bottom-up approaches to nanoelectronics [208], where the functional electronic structures are assembled from well-defined nanoscale building blocks, such as carbon nanotubes [203-208], molecules [215-217], and/or semiconductor nano-wires [218-220], inorganic nanoparticles have the potential to go far beyond the limits of top-down manufacturing.

The formation of carbon nanotubes could be traced back to the discovery of the fullerence structure C60 (buckball) in 1985 [221]. The structure of the buckball, comprised of 60 carbon atoms arranged by 20 hexagonal and 12 pentagonal faces to form a sphere, is enlongated to form a long and narrow tube with a diameter of approximately 1 nm, which is the basic form of a carbon nanotube [222]. Nanotubes have extraordinary mechanical, electrical, and thermal properties while providing strong, light, and high toughness characteristics. Since the discovery of nanotubes, extensive research in carbon, fullenrence, nanosensor, nano-indentator, and nanomachine has blossomed in many different directions, and has attracted a great deal of attention to advanced composite society. It is well understood that carbon fibers or nanotubes could not be utilized alone without any supporting medium, such as a matrix to form structural components. Whereas, the investigation on the interfacial bonding properties between the nanotube and matrix system is of great interest, it ensures a good stress transfer between the nanotube and matrix of nanocomposite structures in the development of nanotube composites.

Carbon nanotubes exhibit several technologically important characteristics. Metallic(m) nanotubes can carry extremely large current densities [223, 224]. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs) [225, 226]. The two types may be joined covalently [227, 228].

Carbon nanotubes [222, 229-234] are very attractive because they are capable of emitting high currents (up to 1 A/cm2) at low fields (~5 V//m), and their potential as emitters in various devices have been amply demonstrated [235]. Furthermore, the controlled deposition of nanotubes on a substrate has become possible through the combined use of chemical vapor deposition (CVD) methods and catalyst patterning techniques.

The study of nanometer-sized semiconductor crystals has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical and chemical properties can be systematically tuned by variation of the size, according to increasingly well-established scaling laws. In the semiconductor quantum dot (QD), the electrons and holes are three dimensionally confined inside a nanometer scale "quantum box." The confinement causes a discrete atomic-like, energy-level structure for the carriers and a variety of new physical phenomenon are observed [236, 237]. Researchers have been exploring the possibility of combining quantum confinement with the introduction of extra carriers by means of doping and charging to obtain a new set of properties in these materials [238-244]. The impurity doping which is routine in bulk semiconductors is now extended to semiconductor nanocrystals. Many efforts that try to attack these problems have focused on doping II-VI semiconductor nanocrystals with metals such as Mn, 10, 11, or Cu12 and rare-earth elements such as Tb.13. These impurities do not affect the adsorption spectrum, but strongly modify the luminescence properties [245].

A PBG crystal (or photonic crystal) is a spatially periodic structure fabricated from materials having different dielectric material constants [246]. It can influence the propagation of electromagnetic waves in a similar way as a semiconductor does for electrons. The concept of this new class of material was first proposed independently by Yablonovich [247] and John [248] in 1987, and since then a wide variety of applications has been envisioned or demonstrated for this new class of materials. In silicon and other semiconductors, adjacent atoms are separated by about a quarter of a nanometer. Photonic bandgap materials involve similar structures but on a larger scale. A photonic crystal, for example, provides a convenient and powerful tool to confine, control, and manipulate photos in all three dimensions of space, for example, to block the propagation of photons irrespective of their polarization or direction; to localize photons to a specific area at restricted frequencies; to inhibit the spontaneous emission of an excited chro-mophore; to modulate or control simulated emission; and to serve as a lossless waveguide to direct the propagation of photons along a specific direction [249-254]. All of these photonic properties are technologically important because they can be exploited, in principle, to produce light-emitting diodes (LEDs) that display coherence properties, fabricate thresholdless semiconductor diode lasers, and significantly enhance the performance of many other types of optical, electro-optical, and quantum electronic devices. Patterning photonic crystal thin films into optical circuits would represent the ultimate limit of optoelectronic miniaturization.

Ultimately, it will be desirable to build ultra-dense, low-power nanoelectronics circuits made from purely nanometer-scale switching devices and wires. Molecules electronics, using individual covalently bonded molecules to act as wires and switching devices, is another longer-term alternative to build ultra-dense, low-power nanoelectronics circuits. Individual molecular switching devices could be as small as 1.5 nanometers across, with densities of approximately 1012 devices per sq cm. A primary advantage of molecular electronics is that molecules are natural nanometer-scale structures that can be made absolutely identical in vast quantities. Also, synthetic organic chemistry (the chemistry of carbon-based compounds) offers more options for designing and fabricating these intrinsically nanometer-scale devices than the technology presently available for producing solid-state chips with nanometer-scale features. It has been shown that small organic molecules can conduct and control electricity [255260] and large organic biomolecules are conductive [261].

There has been a great deal of interest in the use of both supermolecular assemblies and nanoparticle structures as components of electronic circuits [262, 263]. A machine based on molecules acting as components also requires a high level of complexity in the actual chemistry required. Switches are key components of any electronic device to be used for computational purposes, and for these uses, quantum states appear as the obvious choice to achieve this functionality. In order to construct useful switching structures based on individual molecules, the transfer function of a two-contact switching unit must involve a range of applied potentials in which the device exhibits a negative resistance.

Another approach in nanoscale electronics is based on the use of nanoparticles for the construction of electronic components of nanometer dimensions. Well-defined rules of how these materials can be organized have emerged. The vigorous recent efforts in synthesizing MPCs have resulted in the availability of functionalized nanomaterials that while retaining the properties of a metallic core can be handled like simple, stable chemical compounds. The question that needs to be addressed is would it be possible to synthesize chemically functionalized nanoscale objects that would spontaneously organize into superstructures with desirable functionalities?

Many ways have been developed for preparing nano-structures, ranging from single-atom manipulation to organic synthesis. The tremendous strength of the colloidal preparation routes is that they yield large amounts of monodisperse nanocrystals with easy size tuning. Furthermore, the presence of organic molecules on the surface of the nanocrystals enables extensive chemical manipulation after the synthesis.

The development of advanced materials from inorganic nanoparticles (NPs) is currently one of the most dynamic areas of science. Nanoparticles are attracting significantly fundamental and commercial interest, with a wide range of applications including the next generation of optics, electronics, and sensors [264]. In optical, electrical, and magnetic devices, NPs will be mostly used in the form of thin films. Currently, such films are typically made by spin-coating, spraying, or sometimes simple painting of nanoparticle-matrix mixtures.

There has recently been an intense effort to produce metal nanoparticles capped with a variety of functional groups [265]. The rapid development of this field is resulting in the availability of many different functionalized nano-particles that can be used in much the same way as organic chemical reagents. The main synthetic strategies that have been developed are based on either the direct one-step, single-phase reduction (gas-phase synthesis) or on the two-phase reduction (sol-gel processing), and also on the use of place-exchange reactions with a target ligand. Sol-gel processing is a wet chemical synthesis approach that can be used to generate nanoparticles by gelation, precipitation, and hydrothermal treatment (Fig. 13). Size distribution of semiconductor, metal, and metal oxide nanoparticles can be manipulated by either dopant introduction or heat treatment. Better size and stability control of quantum-confined semiconductor nanoparticles can be achieved through the use of inverted micelles, polymer matrix architecture based on block copolymers or polymer blends, porous glasses,

Metal Alkoxide Solution

Sol-Gel Technologies

Metal Alkoxide Solution

Sol-Gel Technologies

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Figure 13. Sol-gel technology in nanoparticle synthesis. Reprinted with permission from [267], http://www.chemat.com/html/solgel.html. © Chemat Technology, Inc.

and ex-situ particle-capping techniques. Supercritical solvents represent convenient media for synthesis because their solvation properties can be easily controlled by varying the pressure. Supercritical fluids exhibit gas-like mass transfer properties and yet have liquid-like solvation capability. Carbon dioxide is one of the mostly widely used gases for supercritical fluid applications because of its moderate critical constants, nontoxic nature, low cost, and availability in pure form. Metal ions are insoluble in supercritical CO2 because of the charge neutralization requirements and the weak solute-solvent interactions. However, when metal ions are bound to organic ligands, they may become quite soluble in supercritical CO2 [266].

Microemulsions have been extensively employed for nanoparticle synthesis because the reaction can be confined in a well-defined environment. This approach has been successfully developed for the synthesis of Ag nanoparticles in supercritical carbon dioxide (sc-CO2) by the reduction of Ag+ incorporated in a water-in CO2 microemulsion. In addition, phase-extraction methods, in which the aqueous colloidal metal solution is extracted into an immiscible organic phase containing the capping reagent, have been employed. Microemulsion copolymerization is shown to be a very useful method to produce stable aqueous suspensions of ultra-fine, selective metal-complexing nanoparticles with controlled size and adjustable metal-binding capacity [268].

The ability to assemble nanoparticles into highly ordered 2D or 3D arrays is very important in fabricating novel devices utilizing quantum dots and inorganic (or organic) colloidal particles. Several techniques have been demonstrated for forming such arrays: for example, 2D arrays have been fabricated using methods based on chemical vapor deposition, Langmuir-Blodgett films, molecular beam epitaxy, pulsed electrochemical deposition, and microcontact printing [269-275]; 3D arrays have been realized through self-assembly approaches that usually involve electrostatic interaction [276-281], sedimentation [282-287], and solvent evaporation [288-289].

Layer-by-layer assembly is one of the new methods of thin-film deposition, often realized as a sequentially absorbed monolayer of oppositely charged polyelectrolytes [290]. Layer-by-layer assembly has also been successfully applied to thin films of NPs and nanocolloids, that is, dispersed species with only one or two spatial dimensions in the nanoscale regime. A large number of studies on the assembly of semiconductor, metallic, magnetic, insulating, composite, and other materials have been undertaken [291-299]. The main advantages are the simplicity, universicality, and high quality of the resulting coating. Based on this, LBL assembly can be a convenient method for processing NPs into thin films, both for advanced materials research and industrial production.

Soft lithography generates micropatterns of SAMs [300305] by contact printing, and also forms microstructures in materials by embossing (imprinting) [306-315] and replica molding [316-318]. The strength of soft lithography is in replicating rather than fabricating the master, but rapid prototyping and the ability to deform the elastomeric stamp or mold gives it unique capabilities even in fabricating master patterns. Soft lithography is not subject to the limitation set by optical diffraction and optical transparency and can be usually carried out in the laboratory environment.

Researchers have recognized the utility of DNA in nanofabrication [319-323]. DNA oligonucleotides contain hybridization information encoded in their sequences. In exploiting this phenomenon of DNA hybridization, interactions can be made specific by the DNA sequence, while interparticle distance can be controlled through the selection of the oligonucleotide length. Consequently, a nano-structure much more complex than that which is possible using SAMs might be achieved by using DNA as a "linker" of the nanoparticles.

Template-directed synthesis is a convenient and versatile method for generating porous material. It is also a cost-effective and high-throughput procedure that allows the complex topology present on the surface of a template to be duplicated in a single step. In this technique, the template simply serves as a scaffold around which other kinds of materials are synthesized. By templating against super-molecular assemblies self-organized from small molecules, surfactants, and block copolymers, it has been possible to prepare various types of porous materials with pore sizes in the range of 0.3-10 nm [324-328]. With the use of mesoscale objects such as templates, the dimension of these pores can be significantly extended to cover a wide range that spans from ~10 nm to 10 /m [329-332]. In particular, templating against opaline arrays of colloidal spheres offers a generic route to macroporous materials which exhibit precisely controlled pore size and highly ordered 3D porous materials [333-339]. The fidelity of this procedure is mainly determined by van der Waals interactions, the wetting of the template surface, a kinetic factor such as the filling of the void spaces in the template, and the volume shinkage of precursors during the solidification. Templating against opaline arrays of colloidal spheres has been successfully applied to the fabrication of 3D microporous structures from a wide variety of materials, including organic polymers, ceramic materials, inorganic semiconductors, and metals [333-350].

Fabrication based on this approach is remarkable for its simplicity and its fidelity in transferring the structure from the template to the replica. The size of the pores and the periodicity of the porous structures can be precisely controlled and readily tuned by changing the size of the colloidal spheres.

Although this templating procedure may lack the characteristics required for high-volume production, it does provide a simple and effective route to 3D porous materials having tightly controlled pore sizes and well-defined periodic structures. These porous structures can serve as a unique model system to study many interesting subjects such as adsorption, condensation, transport (diffusion and flow), and separation of molecules in macroporous materials, as well as mechanical, thermal, and optical properties of 3D porous materials.

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