Introduction

NanotechnologyNano is the design and manufacture of devices with dimensions measured in nanometers. One nanometer is 10 -9 meter, or a millionth of a millimeter.

Nanofabrication is of interest to computer engineers because it opens the door to super-high density 

microprocessor   s and memory   chip   s . It has been suggested that each data bit could be stored in a single   atom   .

Carrying this further, a single atom might even be able to represent a byte or word of data.

,So we need to talk about :Nanofabrication has also caught the attention of the medical industry, the military, and the aerospace industry.

There are several ways that nanofabrication might be done. One method involves scaling down integrated-circuit ( IC   ) fabrication that has been standard since the 1970s, removing one atom at a time until the desired structure emerges. A more sophisticated hypothetical scheme involves the assembly of a chip atom-by-atom; this would resemble bricklaying. An extension of this is the notion that a chip might assemble itself atom-by-atom using programmable nanomachines. Finally, it has been suggested that a so-called biochip might be grown like a plant from a seed; the components would form by a process resembling cell division in living things.

Current researchNanomaterialsBottom-up approachesTop-down approachesFunctional approachesBiomimetic approachesSpeculativeNanotechnology as defined by size is naturally very broad, including fields of science as diverse as   surface science ,   organic chemistry ,   molecular biology ,   semiconductor physics ,   microfabrication ,   molecular engineering , etc .

The associated research and applications are equally diverse, ranging from extensions of conventional device physics to completely new approaches based upon molecular self-assembly, from developing new materials with dimensions on the nanoscale to direct control of matter on the atomic scale.

Applications  MOSFET's being made of small nanowires ~10 nm in length. Here is a simulation of such

a nanowire. carbon nano tube cmos

bacteriostatic   silver nanoparticles nanofabrication techniques:

1. Top Down.

A. scanning probe lithography

atomic force microscopes (AFMs)

Scanning probe microscopes(SPMs)

. Scanning tunneling microscopes (STMs)

B. soft lithography,

C. focused beam lithography

D. nanoimprint lithography

2. bottom up.

There are two pathways to approach nanoscale. One can start from bigger material and machine it down all the way down to the nanoscale. This is similar to carving a small sculpture out of big rock of stone. The craftmen has to chisel down the stone until the perfect shape is emerged. However, this method may require a large amount of material and lot of it will go to waste. The other nanofabrication pathway is bottom up approach. Here, nanotechnologist would start with much smaller materials such as atoms and molecules and build a nanostructure using them as a building block. Although, this method would provide great flexibility and low material wastage, controlling matter in this scale with such a precision is still a challenging task. Bottom up method primarily rely on the concept of “self-assembly” where molecules are encouraged to assemble themselves in to

more ordered structures.

Top down methodsThe most top down fabrication technique is nanolithography. In this process, required material is protected by a mask and the exposed material is etched away. Depending upon the level of resolution required for features in the final product, etching of the base material can be done chemically using acids or mechanically using ultraviolet light, x-rays or electron beams. This is the technique applied to the manufacture of computer chipsTop down approaches are primarily used for surface patterning of micro/nanoscale features in a desired substrate. The well-developed pattering techniques include, scanning probe lithography, soft lithography, focused beam lithography and nanoimprint lithography scanning probe lithographyScanning probe microscopes(SPMs)provides a great way to probe in to the nanoscale world. Similarly one can use SPMs to manipulate things in the nanoscale too. SPMs

or more specifically in an atomic force microscopes (AFMs) physically make contact with the specimen during the scanning. Same

strategy can be used to scratch, dimple or score a soft surface as the tip is dragged along the surface. They have been used to create wonderful creations

and art at the nanoscale. Scanning tunneling microscopes (STMs) have been used by scientist to create atomic level manipulations either by pushing individual atoms or picking up the individual atoms through making a temporary bond between the tip and the atom and then place it at a desired location.

Although, SPMs can be very precise and can manage matter at the smallest scale possible this technique suffer from limited scalability and slowness. Although, the rate of production can be increased with operating number of tips at a single time, its output cannot compete with the other techniques. Hence, this technique is appreciated as suitable to create works of art and wonderful structures, it may not be able to satisfy demand of the mass.

Focused beam lithographyAs the name suggests, focused beam lithography take use of electron or ion beam focused in to a resist surface to generate patterns by exposing the surface followed by removing the resist through a chemical process.

If electron beams are used, lithography can be used on an electron-sensitive resist film. Most widely use material for this is polymethylmethacrylate. Electron beam can induce very precise patterns to the resist surface in the scale of 3-4 nm if required. Electron beam lithography can be carried out in a modified electron microscope.

Focused ion beam lithography uses an ion beam to induce high-resolution patterns in a resist. This technique has found applications in semiconductor industry for

1.mask repairing and

2.device modification.

Unlike electron beam, ion beam has comparatively low backscattering and higher exposure sensitivity. It can also be used as a precise milling tool.

Soft   lithography Soft lithography is the technique of transfer or fabricate a structure by stams and molds made of elastomeric materials. The most widely used elastomeric material used in this technique is polydimethylsiloxane(PDMS) due to its favorable properties. This technique has found much interest due to its simplicity and low cost. In fact, it’s possible to make submicron length scale dimensions with relative ease with ordinary chemical apparatus.

The complete lithographic process consist of two main steps. First, precisely fabricated masks made by AFM, electron or ion beam lithography are used to transfer the pattern in to the elastomeric element or stamp. Then the elastomeric elements are used to pattern features back in to a desired surface using an ink or a polymer. Both molecular level and solid inks are used in the stamping.

Nanoimprint lithography

Nanoimprint lithography (NIL) is widely used technique to imprint micro/nanoscale patterns with low cost, high throughput and high resolution. Unlike soft lithography, NIL uses a hard mold to create patterns on a resist through direct mechanical deformation and therefore achieve limitations set by light diffraction or beam scattering in conventional lithographic processes. The resolution of the NIL primarily limited by the resolution of the template feature. The masks can be made of number of different substrates such as glass, silicone, polymers, etc.

Nanoimprint lithography has found applications in organic light emitting diode fabrication and sensor fabrication.

Bottom up approachesBottom up approaches rely on self assembly to build nanostructures with chemical or physical guidance. Here we discuss few of mostly used bottom up approaches in the field of nanofabrication.

Chemical and physical vapor depositionVapor deposition techniques are the most widely used bottom up nanofabrication approach in nanotechnology research and industry alike. There are two types of vapor deposition techniques: chemical and physical.

In chemical vapor deposition (CVD), coating chamber is filled with a precursor gas or gasses that can take a reaction with one or more heated objects in the chamber that needed to be coated. The chemical reactions occur at the interface of the gas and the solid producing a thin sold film on the surface. There are number of CVD variants developed for specific applications such as spray assisted vapor deposition, aerosol assisted chemical vapor deposition, metalorganic chemical vapor deposition, etc. This technique is quite versatile and can be used to apply wide range of materials in to a substrate. This technique is widely used in semiconductor and ceramic industry.

In the physical vapor deposition(PVD) involves vaporization of a material through heating, electron beam, ion beam, plasma or laser followed by solidification of the material back at the surface of the substrate that needed to be coated. There are no chemical interactions occur in surface of the material. PVD is usually carried out in high vacuum environment to enhance the affectivity of the coating by reducing interactions with other atoms. This technique also has found applications in metal coating, glass and semiconductor industry.

Dip pen lithographyDip pen lithography (DPN) is a nanofabrication technique that is based on scanning probe microscopy technique. In contrast to the scanning probe lithography, which induce structural deformation in to a substrate, it uses an ink to write a pattern on a surface. This is exactly similar to using a stylus that needed to be dipped in ink before writing, but at nanoscale. There are instruments developed with more than 50,000 DPN tips in parallel to increase the fabrication throughput. This technique has been used to transfer nanoscale structures with organic or biological molecules.

Self assemblyTheirs is a fundamental limitation with all the assembling techniques that we have discussed so far. These techniques requires too much work to precise handling and keeping track of everything every time. We always take control where should things be and where they shouldn’t. Self-assembly relies on the technique that you just mix the chemicals together and it’s us to the molecules themselves to sort it out to make a nanostructure. But a question arises, that is what would drive them to make these structures.

The main driving force behind this technique is common to everything in this universe. No matter how big or small, everything attempts to lower the energy level that is associate with them. When molecules are mixed together, they sort in such a way that their energy is at a minimum level. In fact self assembly is the pathway nature has taken to build amazing nanostructures. Mastering this technique would allow us to build everything from basic molecules. It’s hypothesized that on day we will grow our own food, cloths, computers and just about everything we need in a molecular factory that we may have in our houses. Although seem strange and far stretched, this is theoretically possible.

The driving forces of self-assembly are forces like hydrogen and van der Waal forces which are much weaker than the covalent bonds that hold molecules together. These weaker coulombic forces are found in many places in the world from water to DNA molecules.

There are other interactions such as hydrophobic interactions that allow hydrophobic materials to bind together in aqueous medium. The particles may not have a total charge yet they bind together to reduce the surface area that expose to the aqueous surrounding.

Molecules also can be self-assembled in to a reconstructed nanostructure driven by favorable attraction between particular atoms and molecules sometimes even making strong covalent bonds. The perfect example is assembly of fatty thiol molecules on a silver or gold surface.

Self assembly can also be driven by templates. Molecular level templates such as spheres, tubes and bi-planes can be made with self-assembly of surfactant molecules that can be solidified with different means. These template shapes can be changed by varying concentration, pH, temperature, etc.

Nano crystal growth

Over the years nanotechnologies have achieved great control over crystal growth at nanoscale. This paves the way to grow crystals with desired shape and sizes of various metallic, metal oxide and semiconductor materials. Crystals can be grown from precursor solutions containing a seed crystal. Growth of the crystal can be changed by controlling growth parameters such as surfactants and capping agents, temperature, pH, various ions present. Applications for these custom made nanocrystals in the rise especially in photovoltaic, photo catalytic, composites and optical applications.

A major study published more recently in Nature Nanotechnology suggests some forms of carbon nanotubes – a poster child for the "nanotechnology revolution" – could be as harmful as asbestos if inhaled in sufficient quantities. Anthony Seaton of the Institute of Occupational Medicine in Edinburgh, Scotland, who contributed to the article on carbon nanotubes said "We know that some of them probably have the potential to cause mesothelioma. So those sorts of materials need to be handled very

carefully."[75] In the absence of specific regulation forthcoming from governments, Paull and Lyons (2008) have called for an exclusion of engineered nanoparticles in food.[76] A newspaper article reports that workers in a paint factory developed serious lung disease and nanoparticles were found in their lungs

ImplicationsAn area of concern is the effect that industrial-scale manufacturing and use of nanomaterials would have on human health and the environment, as suggested by nanotoxicology research. ;For these reasons, some groups advocate that nanotechnology be regulated by governments. Others counter that overregulation would stifle scientific research and the development of beneficial innovations. Public health research agencies, such as the National Institute for Occupational Safety and Health are actively conducting research on potential health effects stemming from exposures to nanoparticles

Some nanoparticle products may have   unintended consequences. Researchers have discovered that  bacteriostatic   silver nanoparticles used in socks to reduce foot odor are being released in the wash. These particles are then flushed into the waste water stream and may destroy bacteria which are critical components of natural ecosystems, farms, and waste treatment processes. Public deliberations on risk perception in the US and UK carried out by the Center for Nanotechnology in Society found that participants were more positive about nanotechnologies for energy applications than for health applications, with health applications raising moral and ethical dilemmas such as cost and availability

.

Health and environmental concernsNanofibers are used in several areas and in different products, in everything from aircraft wings to tennis rackets. Inhaling airborne nanoparticles and nanofibers may lead to a number of pulmonary diseases, e.g. fibrosis.] Researchers have found that when rats breathed in nanoparticles, the particles settled in the brain and lungs, which led to significant increases in biomarkers for inflammation and stress response] and that nanoparticles induce skin aging through oxidative stress in hairless mice. A two-year study at UCLA's School of Public Health found lab mice consuming nano-titanium dioxide showed DNA and

chromosome damage to a degree "linked to all the big killers of man, namely cancer, heart disease, neurological disease and aging"