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“Existence of every thing is felt with new innovations and inventions”. Thus to replace silicon a alternate technology to meet our present day needs so that the production cost could reduce to handsome amount, consume less power and overall make thing easy is this technology “POLYTRONICS”.
POLYTRONICS is an emerging advancement in the materialistic world which has enormous applications to change the existing conditions to a dramatic extent. The researches brought about innovative ideas on integrate plastics into mainstream electronics. Viewing of world could change through flat panel displays. Transistor could be made just like ink-jet printing on papers. Batteries could be made using plastics. Very soon we could see e-papers which could be continuously updated via the internet. The age of polymer electronics has begun and could revolutionize this world.
INTRODUCTION
Silicon considered as the best semiconductor and known to be “kith and kin “ of electronics has largely influenced the electronic industry and would continue to do so over a period of time. However we are looking for a replacement or an alternative that could meet our needs. We know pretty well the proverb “Necessity is the mother of invention”; exactly the same is this case too.
Today most of the electronic circuits are integrated circuits/semiconductor chips fabricated out of silicon. Producing these circuits involves huge investments known to be in millions of dollars. So we are coming out with a new technology known as POLYTRONICS which would be able to produce these circuits on plastics which are flexible enough to be easily rolled up have display screens that can be continuously updated with sharp images consume less power and above all can be manufactured at a fraction of the cost involved in making semiconductor chips.
Thus Polymer Electronics abbreviated as POLYTRONICS is a combination of two different terminologies meaning electronics using polymers or simply plastics.
Here’s a look into how plastics could revolutionize the world of electronics, what changes on existing things it could make, what new things it could bring about in detail in our paper.
What is a POLYMER?
Polymers are nothing but macromolecules built by repeated chain of monomers by the process of polymerization. These polymers are formed because of double and triple bonds between monomer to form a rigid structure and unique chemical and physical characteristics. There are many such polymers like polyethylene (polethene), polyactelyene, polyvinylchloride (PVC) and so on.
In case of polyactelyene, which possesses conjugated double bonds is as shown in fig.
Based on their ultimate form and use a polymer can be classified as plastics, elastomer, fibre or resin. When a polymer is shaped into hard and tough utility they are termed as plastics. As we know polymers or simply plastics are the extensively used in this materialistic world. Their uses and applications range right from your tooth brush till your clothing and containers. They are used to coat metal wires to prevent electric shocks. Such is the usage of polymers in this day today life.
HOW DOES A POLYMER CONDUCT?
Simply Ohms Law can define conductivity V= IR. Thus from this relationship conductivity is found. The conductivity depends on the number of charge carriers (number of electrons) in the material and their mobility. For example in a metal it is assumed that all the outer electrons are free to carry charge and the impedance to flow of charge is mainly due to the electrons "bumping" in to each other. Thus for metals as temperature is increased the resistance in the material increases as the electrons bump in to each other more as they are moving faster.
Insulators however have tightly bound electrons so that nearly no electron flow occurs so they offer high resistance to charge flow. So for conductance free electrons are needed. The diagram below shows how the conductivity of conjugated polymers like polyactelyene can vary from being an insulator to a conductor.
We think of Polymers as good insulators. However it is now recognized that there are some polymers which have typical conducting and light emitting properties. The chemical composition of these polymers is changed by doping (adding impurities) to make them conducting. Pentacene, oligothiopenes, polyacetelyene,etc,. are found to the best examples.
It is well known that graphite is a good conductor, previously it was thought that polymers which substitute a carbon (e.g. adding hydrogen's to make hydrocarbons) for another atom could not conduct, however our greater knowledge of conjugated systems has enabled the discovery of conducting polymers. As in a conjugated system the electrons are only loosely bound, electron flow may be possible. However as the polymers are covalently bonded the material needs to be doped for electron flow to occur. Doping is either the addition of electrons (reduction reaction) or the removal of electrons (oxidation reaction) from the polymer. Once doping has occurred, the electrons in the pi-bonds are able to "jump" around the polymer chain. As the electrons are moving along the molecule a electric current occurs.
However the conductivity of the material is limited, as the electrons have to "jump" across molecules so for better conductivity the molecules must be well ordered and closely packed to limit the distance "jumped" by the electrons. By doping, the conductivity increases from 10-3 S m-1 to 3000 S m-1.This is seen very well in trans undoped polyacetelyene An oxidation doping (removal of electrons) can be done using iodine. The iodine attracts an electron from the polymer from one of the pi bonds. Thus the remaining electron can move along the chain.
APPLICATIONS
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RECENT TRENDS
As we know the area of applications and usages of polymers are wide spread in electronics. Most importantly the replacement of silica to fabricate a microchip could bring about dramatic changes as because the cost will reduce to hand some amount and production too becomes easier.
Conducting polymers have many uses. The most documented are as follows:
• Corrosion Inhibitors
• Compact Capacitors
• Anti Static Coating
• Electromagnetic shielding for computers
A second generation of conducting polymers have been developed these have industrial uses like,
• The recent development allows thin-film transistors and even microchips to be made entirely of organic conductors.
• Display Technology to develop light-emitting polymers for use in the flat panel displays.
• Light Emitting Diodes (POLY LED’s)
• An inkjet-like technique for producing plastic transistors.
• Solar cells.
FLEXONICS
Although current silicon chip technology has had a huge impact on western lifestyles and the performance of chips is continually being improved, the silicon electronics industry is an industry facing limitations. Current methods of silicon chip production are very capital intensive, requiring huge plants and large numbers of chips produced at any one time to give small returns on large investments. In addition, the turnaround times are lengthy and mistakes are hugely expensive. Furthermore, supply and demand are far from stable and the process of producing chips is energy intensive, requires high temperatures and vacuum processing, and in the case of photolithographic methods, a lot of pure water. There is, however, a process that promises cheap circuits, tailor made for individual applications produced locally as they are needed.
Fabrication of microelectronic components plastic substrate instead of silicon would allow manufacturing of complete gadgets through just printing process in the near future. This technology would focus on building any electronic device from bottom up gradually, so instead of building a device by adding new components through the regular ‘assemble and build’ technique, the entire product would come out of the printer complete with electronic circuitry embedded in the product. The technology of producing such embedded electronic circuitry of plastic wafers is “FLEXONICS”.
The principles that apply to printing on polymer are similar to those used in industrial ink jet printing. Although plastic semiconductors are not yet kings of performance (plastic inhibits electron mobility), the technology could drastically reduce production costs, because it is much less volatile than silicon. It could help usher in low-cost smart appliances. But it is found a way to print clever materials in such a way that we can make practical.
The new technology will have the most immediate impact on various types of displays, including mobile-phone screens, flat-screen computer monitors, and televisions. The inclusion of plastic chips could mean that manufacturers of TFT (thin-film transistor) flat-panel screens and televisions, which currently use a traditional silicon-based transistor for each pixel, would be able to switch to much cheaper chips. The manufacturing process is simpler, because it doesn’t require vacuum processing or high temperatures. So facilities will cost a fraction of that price.
The huge cost of mass-manufacturing silicon microchips is due largely to the complex processes involved. Photolithographic techniques are used to pattern wafers with micro circuitry, which is grown in powerful vacuums while the wafers are baked at temperatures of several hundred degrees Celsius. Silicon foundries typically use wafers of only one size, each fabricated as a discrete unit in facilities that cost billions of dollars to design and build. There could be a continuous production line of plastic circuits printed on a plastic substrate and then cut into individual units. The substrate may perhaps be made of the same acetate material as transparent Vugraph sheets.
The whole thing works at ambient pressure, doing away with many of the costly vacuum steps needed for silicon. The printing of circuits on a scale far larger than is possible with silicon is also in view and of great importance for the development of large flat-screen displays. The Plastic Logic printer resembles any home office inkjet printer. A piezoelectric material expands when a voltage is passed across it, pressing on a reservoir of fluid and sending droplets flying out onto the substrate.
The water-based droplets contain an organic conductor--poly (3,4-ethylenedioxythiophene) doped with solution of polystyrene sulfonic acid, otherwise known as PEDOT/PSS. As the droplets dry, they become a conducting layer and form the source and drain of a transistor. These are then coated with a layer of a semi-conducting polymer (9,9-dioctylfluorene-co-bithiophene), followed by a dielectric layer of polyvinyl phenol. Finally, the gate is printed, creating a so-called top gate transistor.
The net result is plastic circuits whose advantages over their silicon counterparts include low capital investment, a large area capability, the ability to be printed on flexible substrates, an environmentally friendly production process, transparency, ease of customisation, quick cycle and turnaround times, robustness, light weight, and thinness.
The molecular chains must line up in a way that makes it easy for electrons to hop from one chain to another. But polymers tend to form into disordered microstructures that reduce electron-charge--the blight of earlier attempts to produce organic transistors efficiently.
However it is discovered that a careful choice of polymers would yield self-organized chains that achieved charge mobilities of up to 0.1 cm2/V/s. All of a sudden, thin film transistors could match at least some of the properties of their silicon cousins.
We have also had to overcome some inkjet printing limitations, notably a maximum resolution of around 600 dots per square inch (90/cm2) arising from natural variations in the droplets' flight paths. This translates into a feature size of around 50 µm. Now the smaller the transistor, the shorter the distances electrons must travel within it, and the faster the device can be switched on and off. Unfortunately, this 50-µm limit falls short of the 10-µm sizes needed for fast circuits.
So the resolution has to be increased. For now, they do it photo lithographically by coating the glass substrate with a hydrophobic film of polyimide in a pattern that defines transistor dimensions. When the water-based droplets fall on the surface, they are forced away from the hydrophobic regions in the required pattern. So far, single transistors and simple logic circuits have been produced with a feature length of as little as 5 µm. This should lead to circuits with the switching speeds of a few tens of kilohertz needed for display applications and smart tags.
It is believed that photolithography can be replaced by other techniques, such as photo patterning, in which having ultraviolet light shone on it patterns a single hydrophilic layer. Thus the circuit could still be fabricated in successive steps of coating and printing. Using photolithography now is an obvious shortcoming of initial demonstrations, but it won’t be a problem in the long term. To overcome this problem via a process of substrate surface energy patterning, this directs the flow of the water-based conducting polymer inkjet droplets. This in turn enables high-resolution definition of channel lengths, down to lengths of five microns and below.
More difficult will be making devices of greater complexity. Making a single transistor is in some sense trivial. Scaling up the technology is the difficult thing. It is planned to build a more complex prototype chip.
THIN FILM TRANSISTORS
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FLAT PANEL DISPLAY
The technology developed here enables the formation of a range of devices required in complex integrated circuits. A patented technology that allows manufacturers to print plastic onto a polymer substrate. The result is a plastic-based transistor that is inexpensive and flexible. Particular expertise has been developed in the creation of thin film transistors (TFTs), the key component of digital circuits. Techniques have also been created to enable the construction of other circuit elements including interconnects, resistors, capacitors, diodes and via-holes.
Thin Film Transistor acts as a switch that can be controlled by the voltage put on the three contacts. These three contacts are called the source, drain and the gate. The transistor consists of four layers. The thickness of each of the layers is less than 100 nm: