Abstract: about the various fabrication processes utilized

Abstract:  Due to the high energy demands which arefollowed by the crisis of petroleum, the desire for the future lies in therenewable energy resources such as solar energy.

In Photovoltaic cells, the mainlyused material is Silicon in both crystalline and also amorphous form for the fabricationand also used in manufacturing industries. This research papergives the overall overview about the materials and also the processes used forfabricating a solar cell. The aim of this paper  is to study the solar cell fabricationtechnology and also the fabrication of the solar cells. However, there are alot of challenges involved such as high manufacturing costs, energy conversionefficiency, uniformity, easy handling and storage etc. In response, solutionshave been suggested in terms of both alternatives, manufacturing methods andmaterials used in the photovoltaic cells. The paper further explains in detailabout the various fabrication processes utilized in the modern era. The paperends with contrasting the various techniques and pushes the idea of using themost efficient solar fabrication processes.

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 INTRODUCTION: Solar energy isthe energy generated from the atomic combination in a star, i.e. the sun. Theenergy is released when the fusion process takes place. That energy goesthrough the layers of the sun until the point when it achieves the surface ofthe sun, where the light is emitted. Of the transmitted energy that reaches theatmosphere is known as the solar constant.

Solar panels are made up of solarcells which converts light, energy, into electric or electricity form. The earthreceives more energy from the sun for just one hour than the world which usesin a whole year. This shows a very high sun based radiation that should beutilized; as a substitute strategy for the non-renewable energy sourcesutilized today. Solar panelboards which consist of cells situated together in modules which mean the solarcells hold a noteworthy part in the panel’s last execution. In solar markettoday we use commonly three sorts of photo voltaic cells; single and, poly-crystallinecells, thin film.

Without any difficulties the cells are separated by theirappearance, thin film cells are sometimes black and even in their shades,single-crystal cells have thick blue color and poly-crystalline cells have variousshades of blue. SOLARCELL CONCEPTS:The most common semi-conductive materialused in solar cells is silicon where it is important to separate amorphous(un-structured) and crystalline (ordered) silicon. Monocrystallinecells: Crystallinesilicon solar cells represent about 90% of the PV market today. Bothcrystalline cells have similar performances; they have high durability and ahigh expected lifetime of about 25 years. Of the two types of crystalline solarcells, the mono-crystalline cells tend to be a bit smaller in size per gainedwatt but also a bit more expensive than the polycrystalline cells.Single-crystalline solar cells are cut from pieces of unbroken siliconcrystals.

The crystals are shaped as cylinders and sliced into circular disksof about 1mm. An advantageous property of the single silicon crystal cell isthat they are not known to ever wear out. Polycrystallinecells: are alsoordinarily made from silicon however the manufacturing process is somewhatdifferent.

Instead for the material to be grown into a single crystal it ismelted and poured into a mould. The mould forms a squared shape and the blockis then cut into thin slices. Since the discs are squared already less or nomaterial has to be cut off and go to waste.

When the material cools down itcrystallizes in an imperfect manner which gives the polycrystalline cells asomewhat lower energy conversion efficiency compared to the single crystallinecells. In consequence the polycrystalline cells are slightly larger in size pergained watt than the single crystal cells are. After the disks of crystallinecells (mono and poly) have been made, they are carefully polished and treatedto repair any damage the slicing might have caused. ThinFilms: A more recentlydeveloped concept is the thin film solar cell. In principle it is amicroscopically thin piece of amorphous (non-crystalline) silicon, as analternative to the millimeter-thick disk, which leads to less used material.Instead of the cell being a component in itself, the thin film cells are placeddirectly on a sheet of glass or metal. Therefore, the cutting and slicing stepsof the production process are removed completely.

Furthermore, instead ofmechanically assembling the cells next to each other they are simply depositedas such on the material sheet. Silicon is the material most for the thin filmcells but some other materials such as cadmium telluride may be used. Becausethe cells are so thin, the panels can be made very flexible entirely dependenton how the flexible the material is that the cells are placed onto. Advantages can be won from thin filmmodules compared to the traditional crystalline ones in both flexibility andweight. They are also known to perform better in poor light conditions.

However, thin film technology offers lower efficiency which means that for thesame amount of output energy a larger area would be needed. Despite the thinfilms lower efficiencies, the price per unit of capacity is lower than forcrystalline sells. They also tend to degrade over time because of instabilityin the material structure, making the durability of the panels less certain.Fromcells to modules Solar cells arebuilt into modules or panels because the output from a single cell is smallwhile the combination of many cells can provide a useful amount of energy. Designof solar panels is reliant of the type of solar cell that is used. Thecrystalline cells build stiff modules that can be integrated between somelayers of material-sheets and then cut in different shapes whereas thin filmpanels are very flexible, making them applicable in other areas. Often solarpanels are located on rooftops or in separate constructions where the optimalsolar angle is received. To make sure the cell loose as little light aspossible in reflection, the incident angle is kept at a minimum.

The bestalternative for the panels would be perpendicular to the incoming sunlight;which is made complicated by the earth moving. Sometimes constructionalternatives on roofs are not available or simply undesirable due to glassroofs, flat roofs, small gardens etc. In such cases a more flexible alternativeof solar panels is found from the ones made of thin film cell modules.                 FABRICATION TECHNIQUESPhysicalVapor Deposition: PVD comprises of Evaporation andSputtering Mechanisms. Evaporation:Used to depositthin layers (thin films) of metal on a substrate. Some metals films that areeasily deposited by evaporation: aluminum, chrome gold, silver, titanium.

Electron BeamEvaporation (commonly referred to as E-beam Evaporation) is a process inwhich a target material is bombarded with an electron beam given off by atungsten filament under high vacuum. The electron beam causes atoms from thesource material to evaporate into the gaseous phase. These atoms thenprecipitate into solid form, coating everything in the vacuum chamber (withinline of sight) with a thin layer of the anode material. A clear advantage ofthis process is it permits direct transfer of energy to source during heatingand very efficient in depositing pure evaporated material to substrate. Also,deposition rate in this process can be as low as 1 nm per minute to as high asfew micrometers per minute. The material utilization efficiency is highrelative to other methods and the process offers structural and morphologicalcontrol of films. Due to the very high deposition rate, this process haspotential industrial application for wear resistant and thermal barriercoatings aerospace industries, hard coatings for cutting and tool industries,and electronic and optical films for semiconductor industries.

SPUTTERING: Sputteringprocess involves ejecting material from a “target” that is a source onto a”substrate” (such as a silicon wafer) in a vacuum chamber. This effect iscaused by the bombardment of the target by ionized gas which often is an inertgas such as argon. Sputtering is used extensively in the semiconductor industryto deposit thin films of various materials in integrated circuit processing.Anti-reflection coatings on glass for optical applications are also depositedby sputtering.

Because of the low substrate temperatures used, sputtering is anideal method to deposit metals for thin-film transistors. Perhaps the mostfamiliar products of sputtering are low-emissivity coatings on glass, used indouble-pane window assemblies. An important advantage of sputtering is thateven materials with very high melting points are easily sputtered whileevaporation of these materials in a resistance evaporator or Knudsen cell isdifficult and problematic. CHEMICAL MECHANICALPOLISHING: Chemical mechanical planarization or chemicalmechanical polishing CMP is a process that can remove topography from siliconoxide, poly silicon and metal surfaces. It is the preferred planarization techniqueutilized in deep sub-micron IC manufacturing.The smaller the requested resolution of the structure, the higher is therequest for planarity of the surface.

There is a local height variation betweenchip areas of different pattern densities. CMP is the only technique thatperforms global planarization of the wafer.                                           Oxide planarizationOriginally CMP wasused mainly to planarize silicon dioxide interlevel dielectricsSilicon dioxide deposit thicker than the final thickness requested and thematerial is then polished back until the step heights are removed. This resultsin a good flat surface for the next level. The process can be repeated forevery level of wiring that is added.Poly-silicon planarization Poly-silicon can be polished easily with almost the sametypes of polishers, similar pads and slurries as they are used for theplanarization of silicon oxide. Applications are typically the polishing ofpoly silicon plugs, removing the poly silicon from the inter level dielectric andleaving only the plug filled with polysilicon.

Poly-silicon planarization canalso be used for the end phase of wafer thinning or just for silicon waferpolishing. Typical Process Conditions Pressure: 2 to 7 psi Temperature: 10 C to 70 C Slurry flow rate: 100 to 200 mL/min Typical removal rates: Oxide CMP ~2800Å/min                     ADVANTAGES: Good selectivity (No lapping). Reduced resist thickness variation. Better resolution of photolithographic process by reducing depth of focus. Multilevel structures. Improved step coverage of subsequent layer deposition      PHOSPHOROUSDIFFUSION: Phosphorus (P) diffusion is currently theprimary method for emitter fabrication in silicon (Si) solar cell processing.

The diffusion depends on various factors of which temperature and gaseousenvironment is most important. P-type silicon wafers are widely used in solarindustries and therefore diffusion technologies have been developed to depositn-type doping elements to create the p-n junction. Due to its low boilingtemperature (105.

8 ?C), at temperatures between 850-900 ?C in the diffusionchamber, POCl3 is decomposed into simple phosphoruscompounds like P4, P8, P2O5, etc. The phosphorus diffusion fabricationof crystalline silicon solar cell with emitter diffusion, surface passivationand screen printing of electrode leads to formation of n+ type emitter at thetop surface of the wafer. Phosphorus oxychloride (POCl3) is a liquid source which vaporizes at room temperatureitself hence it should be kept in cool place. For the diffusion process, thevapors are carried out by the carrier nitrogen and oxygen is passed throughanother valve.

The reaction takes place, the phosphorus oxychloride reacts withoxygen forms phosphorus pentoxide and then the phosphorus pen oxide reacts withthe silicon to give the silicon dioxide and the phosphorus. Pre-depositioninvolves the formation of phosphorous-rich oxide films on the siliconsubstrate. During drive-in, the phosphorous-rich oxide film acts as an infinitesource for phosphorous diffusion into the Si substrate. During pre-deposition,phosphorus pentoxide (P2O5) forms on thesurface of the wafers by the reaction of phosphorous with oxygen. The P2O5 immediately reacts with the silicon, byresulting in diffusion of phosphorus and formation of the phosphosilicateglass(PSG). The phosphorus atoms formed at the PSG-Si interface penetratethrough the silicon wafer.

  POCl3(liquid) + N2(bubble) ? POCl3(vapor)(predeposition) 4 POCl3+ 3O2?2P2O55 + 6Cl2 2P2O5+ 5Si ?4P + SiO2 (drive-in)  P + 3Si ? n-type doped Si Advantages: No Damage to Process. Cost Associated is Low Disadvantages: Can’t be carried out at room temperature Shallow Junctions are difficult to fabricate.          FLOWCHART Cleaning & Texturing Edge Isolation (Screen Printing) POCl3 diffusion Back metallization Front metallization Drying Rapid Thermal Annealing LIV Testing         IONIMPLANTATION:The alternative to deposition diffusion is Ion Implantationand is utilized to produce a region of dopant atoms deposited into a siliconwafer of shallow surface. In this process a light emission particles ofimpurity ions is accelerated to kinetic energies in the range of tens of kV andis also coordinated to the surface of the silicon. As the impurity atoms enterthe crystal, when it is collided they passes their energy to the lattice and finallyit reaches to rest at some average penetration depth, called the projectedrange expressed in terms of micro meters (um).

Depending on the impurity andits implantation energy, the range in a given semiconductor may vary from a few100angstroms to about 1 um (micro meter). Typical distribution of impurityalong the projected range is approximately Gaussian. By performing fewimplantations at various energies, it is possible to synthesize a desiredimpurity distribution, example:  auniformly doped region.  A gas containingthe coveted debasement is ionized inside the particle source. The ions are producedand repulsed from their source in a wandering bar that is engagedearlier if goes through a mass separator that coordinates just the particles ofthe desired species through a narrow aperture. A second lens focuses thisresolved light emission which then passes through an accelerator that bringsthe particles to their required energy before they strike the objective andbecome embedded in the exposed areas of the silicon wafers.

The voltages areaccelerated from 20 kV to as much as 250 kV. In some ion implanters, the separationof mass occurs after the ions are accelerated with the very high energy.Because the ion light emission is quite small, which means they are providedfor scanning it uniform across the wafers.

For this purpose, the focused ion lightemission is scanned electro statically over the surface of the wafer in the objectivechamber. The depth of the penetration of any particular type of ion willincrease with increasing accelerating voltage. The penetration depth willgenerally be in the range of 0.1 to 1.0 micro meters (um).


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