44 | Photolithography | Nanotechnology

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  Ece_naidu433@yahoo.co.in & ece_sravan440@yahoo.co.in  NANOTECHNOLOGY ABSTRACT:A nanometer is a billionth of a meter. As the name suggests, Nanotechnology symbolises atechnology in which the elemental entity is of the order of nano scale i.e. 10 A -9 meters. Thecentral thesis of nanotechnology is that almost any chemically stable structure that is notspecifically disallowed by the laws of physics can in fact be built. It would be interesting toknow that one of the driving factors of this technology has been device scaling i.e. making thingson a smaller scale so that they are more efficient, cheaper and consume less power. But an evenmore important driving force has been to acquire 'precision' i.e. schematics will be detailed andthere would be no unnecessary parts anywhere in the design.   Precise atomic level fabricationhas previously only been seen in the growth of crystals or in biological molecular machinery,like the ribosome, which assembles all proteins in living creatures or DNA, which carries theinstruction  for  creating  a  living  being.  If  we  incorporate  similar  processes  during  our development of nanotechnology, we will begin to gain a degree of complexity and control over systems that previously only nature and evolution have had.The  following  paper  cites  the  need  for  nanotechnology  today  and  how  the  so-callednanoproducts built under this technology are synthesized, i.e. their basic modes of synthesis andprocesses under each approach. The paper also attempts to throw a light on the variousnanostructures that are being synthesized and how these structures are being implemented inmedical and electronics applications. Nanotechnology has opened new doors in the field of medicine and electronics and this paper attempts to highlight those developments.Finally the paper talks about the pros and cons of this technology and it's future, i.e. where weare heading towards and the potential areas of development in nanotechnology.  9>0 TECHNOLOGY  INTRODUCTION:A nanometer is a billionth of a meter. As the name suggests, nanotechnology symbolises atechnology in which the elemental entity is of the order of nano scale i.e. 10*-° meters. Thecentral thesis of nanotechnology is that almost any chemically stable structure that is notspecifically disallowed by the laws of physics can in fact be built. Theoretical and computationalmodels indicate that molecular manufacturing systems are possible-that they do not violateexisting physical law. Molecular nanotechnology will be achieved when we are able to buildthings from the atom up and we will be able to rearrange matter with atomic precision. Todayscientists are devising numerous tools and techniques that would be needed to transformnanotechnology from computer models into reality.WHY DO WE NEED NANOTECHNOLOGY?One of the key trends that is driving the entire silicon revolution is making things smaller or inengineer speak-device scaling. In the early days of transistor, it was observed that when a devicewas made smaller, it's performance improved in terms of speed, power consumption, efficiencyand price. Therefore, once we are working on atomic scale, we can create machines that will goplaces about which we could once only dream. More information will be packed into smaller andsmaller spaces and we will be able to do much more with much less. But device-scaling is notthe only driving force behind nanotechnology, instead one of the more important motivators is'precision'.   In this application, precision means that there is space for every atom and every atomis in it's pllsce Le. schematics will be detailed and there would be no unnecessary parts anywherein the design. With this precision, we should be able to recycle all of the waste products producedby the manufacturing processes and put them into good use elsewhere. On the contrary, all our technologies today are bulk technologies. We take a lump of something and add or remove piecesuntil we are left with whatever object we are trying to create. We assemble our objects from partswithout regard to structure at molecular level, precise atomic level fabrication has previously onlybeen seen in the growth of crystals or in biological molecular machinery, like the ribosome, whichassembles all proteins in living creatures or DNA ,which carries the instruction for creating aliving being. If we incorporate similar processes during our development of nanotechnology, wewill begin to gain a degree of complexity and control over systems that previously only nature andevolution have had.SYNTHESIS :As in nanotechnology, building such small things from scratch requires extremely sensitiveequipment and many dangerous chemicals. Considering the many challenges in laboratoryenvironments, sample sensitivity and so on, the current synthesis of nanostructured materials is atime consuming task with extremely low yields.There are two broad methods of synthesising nanostructured materials-ã Top-downã Bottom-up  Top-down is an already established method whereby we start with a larger material and slowlyprocess it by removing matter and leaving behind nanoscale features. On the other hand it was onlywith nanotechnology that the whole concept of bottom-up approach was even possible.♦  TOP-DOWN :This method says that we begin with a bulk material and slowly remove bits of it to form thingsthat we ultimately desire. One of the methods under this paradigm is lithography . It is the onlyprocess that can mass produce microchips and other complex semiconductor devices.Lithography :The technique of lithography is all about patterning a substrate (wafer) with a desired layout. Tobegin with, we coat the substrate with photoresist. Next, the substrate is carefully aligned with it'smask which holds the key to patterning process. By selectively allowing the light to pass through,only certain layers are exposed. The light will either breakdown the layer of photoresist or hardenit. Any photoresist that remains will act as a barrier for any subsequent process like epitaxy,doping or etching.Other portions that are weakened can be removed with acid or volatilesubstances. It is with these layers of photoresist layering, masking, exposure, photoresist removaland semiconductor processing that makes up the whole semiconductor fabrication process.Lithography with respect to nanotechnology is very useful in the sense that it can be used to grownanostructures like nanowires and quantum dots only in the selected areas. It can also be used for a number of other structures. The fundamental restriction on the light that is used to pass throughthe mask is the 'Diffraction limit of light'. Using extreme UV rays, it is possible to have featuresizes of the order 90nm.While it is possible to create lithographic nanomachines that can gosmaller, prohibitive costs are retarding the growth. Nanotechnology is therefore the only way thatfuture miniaturization of electronics can proceed further beyond fundamental limits of lithography.♦  BOTTOM-UP:Instead of starting with large materials and clipping away to reveal small materials, the bottom-up approach starts with atoms and molecules and creates larger nanostructures. Two of the mostimportant techniques in this approach are- S  Electroplating S  Chemical vapour deposition (CVD). Electroplating :This technique has been used to create nanostructures. An applied electric field draws precursor ions towards the substrate's surface. Once the ion reaches the surface, it chemically bonds withcertain bonding sites and stays there. This technique is used to create mono layers and thin filmsto substrates that are conductive. One of the most common uses of electroplating comes in theform of nanocrystalline metals. In a solid-state, many identical crystals lie side-by-side inrandom orientations. These patches are called grains. We know by Hall-petch relationship thatmetal's hardness varies inversely with the root of it's grain's diameter. With electroplating, thisminimization of growth can be achieved by pulse plating and dopants. Dopants arecontaminating atoms that can be added through a variety of processes. Pulse plating is the one inwhich electrode deposition happens only in short bursts by pulsing the electric field. Thecombination of the above two has yielded many nanocrystalline metals. Chemical vapour deposition :  The basic operation of this method involves a gaseous precursor material that enters a sealedchamber. By maintaining a cooler temperature at the other end, the gas will move in thatdirection. Once there, it wifl chemically bond into an inert substrate and form a thin film.In most cases, the target substrate need only be coated with a catalytic layer before it is placed inthe chamber. The catalyst will then melt and form clusters on the surface of the substrate. It isfrom these clusters that important structures like wires, tubes etc. can be created on a nanoscale.Other methods like molecular beam epitaxy are also used to manufacture crystallinenanostructures.STRUCTURES:Nanotechnology has yielded a number of unique structures that are not found anywhere innature. These structures are very important to nanotech research. Buckvballs :It is officially called buckminsterrullerene. A buckyball is an entire class of spheres madeentirely from carbon. It's basic planar treatment is like a sheet of graphite. Buckyballs are notvery reactive, therefore they are highly stable. They are also insoluble in most solvents. For thesetwo reasons they have become the center of attention in the bio-nanotech research front. Sincethey are hollow, they could be a potentially hardy delivery method for drugs inside the body.Quantum dots:Quantum dots reign as the most functional and reproducible nanostructures available toresearchers. It's structure is much like a small dot. They are important because they confineelectrons in 3 directions. This means that electrons within a dot cannot move freely in anydirection. The only thing that behaves like this is an atom. But a quantum dot is 10 times bigger than an atom. This has lot of scientific importance because of the following reasons--> Theyexhibit quantized energy levels like an atom.For a given input energy, a quantum dot will emit specific spectra of light. With decreasingdiameters of quantum dots, there will be a corresponding increase in the energy of emitted light.This element of control has huge implications for both lasers and medical tags. Quantum dots arealready in practice a tags can be inserted into patients. They can help pinpoint biologicalprocesses as they occur. They can be manufactured by both top-down and bottom-up approach.Nanopores :A nanoporous material has many deep pores with diameters in the nanoscale. They can bemanufactured by both top-down and bottom-up approach of synthesis. They act as ideal storagesources for fuels and batteries. In medical sciences, a single nanopore could be tuned to thediameter of a DNA strand. This would act as a filter for DNA and would also allow researchersto analyse the entire length of DNA strand in a sequence. A more advanced application would bean oxygen-nitrogen separator, but the amount of precision required is so high since the differencebetween the diameters of oxygen and nitrogen is only 0.2 angstroms. Similarly nanorubes andnanowires which are primarily built using the chemical vapour deposition (CVD) process can beused to build p-n junctions, logic gates and host of other applications as discussed below.APPLICATIONS OF NANQTECHNOLOGY :Medical applications:One of the most important fields where nanotechnology has a great potential is medicine. Allbiological processes happen at cellular or molecular level and this is roughly the same as
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