It’s great to make elaborate nanomaterials in a beaker and nanostructures in a small glass plate and write and read about it. But real benefits of nanotechnology will be available to most if we can make them in a scalable, reliable and cost-effective means. This however, demands lot of advances in technology in terms of precise manufacturing, material science and quality control. This is because the products that are going to be built-in the nanoscale cannot be seen without high-end equipment, cannot be worked by simple techniques and it’s simply not possible to test each of them as thousands of them can sit on a single pinhead. Still, some great advances has been made in the field of nanofabrication, especially in the area of electronics. The Apple mobile phone 5S, has approximately ten times more processing power and 16 million times more memory power than the computer that landed us on the moon: Apollo Guidance Computer(AGC). And the most amazing thing is it can fit in your palm(AGC is as big as an old CRT moniter) also it’s only 399 US dollars, much smaller compared to estimated 150,000 USD to make one AGC. This is a perfect example of benefits that nanofabrication could bring us. We will be able make much faster, better performing devices in smaller packages and at much lower costs.
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.
Here we will look at few major nanofabrication strategies that are frequently followed by nanotechnologists to create nanoscale structures.
Top down methods
Top 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 and discussed in this page.
Scanning probe lithography
Scanning 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 lithography
As 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 mask repairing and 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 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 (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 approaches
Bottom 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 deposition
Vapor 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 lithography
Dip 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.
Theirs 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.