Cost Reduction in Polysilicon Manufacturing for Photovoltaics

8/3/2019 Cost Reduction in Polysilicon Manufacturing for Photovoltaics

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Cost Reduction in Polysilicon

manufacturing for Photovoltaics.

By: H.S.Gopala Krishna Murthy, Ph.D.,Director, ShanGo Technologies Private Limited,Bengaluru 560085, India.Mobile: 091 9449850875.

e-mail: [email protected], [email protected].

Today, polysilicon price is drastically coming down. What was being sold at more thanUS $ 400 per kg is today readily available at less than US $ 60 per kg. There is everypossibility of the price still coming down to US $ 35 to 50 per kg in the near future. Thesteep fall in price of polysilicon is due to the market supply-demand. A year ago, it wasin short supply and hence, the price was very high. Customers had to pay hefty advances

to manufacturers and wait for supply of the material. However, the recent recession,reduction in governmental programs and subsidies and almost complete collapse of thebanking systems in America and Europe resulting in no finances for new projects, regularmanufacturers and new entrants coming with new and higher capacities turned the tableand led to the collapse of the price.

Though fall in the price of polysilicon and its ready availability is good for the PVIndustry, it is a serious problem for the manufacturers particularly those who haveentered in to the manufacturing recently. Most of the old manufacturers have wellestablished processes and plants, fully integrated facilities where the by-products ofPolysilicon manufacture like silicon tetrachloride could be profitably used for makingother products and disposal of wastes generated is well established. Further, they haveincreased their capacities by using funds advanced by customers and hence, the highcapital cost of polysilicon manufacturing does not really affect them financially.However, the new entrants do not have these advantages. For most of them, the cost ofmanufacture of polysilicon is high of the order of about US $ 60 to 100. With the currentlow pricing for polysilicon they are facing serious financial problems and unless theyevolve cost cutting methods for silicon manufacture, they may become bankrupt.

There are many reasons why polysilicon is expensive to make.

1. The technology for manufacture of polysilicon is not readily available.2. The capital cost for a polysilicon plant is very high.3. The capacity of the plant is very small compared to the capital required.4. Production is basically batch process and hence labour intensive.5. Large quantity of electricity is required for manufacture.6. The purity of the material produced is more than required for solar grade.7. Considerable quantities of by-products and wastes are generated


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We shall discuss these points and suggest ways of reducing the costs.

Polysilicon manufacture.

The process of polysilicon manufacture by the Siemens process is now well known and

will not be described in detail here. Briefly, this consists of producing trichlorosilane(TCS) in a fluidized bed reactor using metallurgical silicon and hydrogen chloride gas.TCS is purified by fractional distillation. It is vapourised and fed along with hydrogengas in to

a special chemical vapour deposition (CVD) reactor in which hot inverted U shapedfilaments are kept at about 1100 C by resistive heating with electrical energy. Silicondeposits on the hot filaments which grow from a small diameter of about 8 mm to largersizes of 125 mm and higher. When sufficient deposition has taken place, the process isstopped, and the material deposited is taken out. The effluents from the reactorconsisting of unreacted TCS and hydrogen along with by-products silicon tetrachloride(STC), dichlorosilane (DCS), HCl and small quantities of polysilanes is subjected to

POLYSILICON PROCESS

Met. Si + HCl TCS (SiHCl3) +STC (SiCl4)

DISTILLATION PURIFICATION OFTCS

POLYSILICON REACTOR

ELECTRICITY

REACTOR EFFLUENT RECYCLING

WASTE HCl / SiO2 DISPOSAL

STC HYDRO

GENATION

H2RECYCLE

FRESH H2


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recovery and recycle of the feeds and byproducts. One would notice that manufacturingof polysilicon is a chemical process and involves many of the unit operations of thechemical industry. The only unique feature of the process is the CVD reactor with itsspecial design. Sometimes, a Converter is also used to convert STC produced duringdeposition to TCS. While these two are unique to polysilicon manufacture all the others

are regular equipment used in chemical processes. Fluidized bed reactor fortrichlorosilane manufacture cannot be considered to be unique because fluidized bedtechnology is well known and is being extensively used in the chemical industry. Hence,if the capital cost of polysilicon plant is high it should be mostly because of these twoequipment. Hence, one needs to look in to why these two capital equipment areexpensive and how their costs could be reduced.

One other reason why silicon from the Siemens Polysilicon process is expensive ishistorical. Polysilicon was originally developed for the semiconductor industry whichneeded extremely high purity silicon with impurities present at sub part per billion levels.Moreover, the cost of silicon in the final product namely the electronic device like a chip

is very smallless than 1% of the overall cost. Hence, the prime goal of the polysiliconmanufacture was purity rather than the cost. Therefore, processes and equipment to meetthis important criterion were developed. Further, because of the limited market forpolysilicon and because of the peculiar market situation, there were only a handful ofmanufacturers who were more concerned with purity than cost. Also, there was no realcompetition in marketing the product. In the same way, equipment fabricators were alsolimited and were able to get high prices. Hence, from the beginning till recently therehave been no serious attempts to reduce the cost of manufacture or cost of equipment.Since for photovoltaics, rejects from semiconductors were available at low prices, therewas no pressure on the cost of silicon from this sector. Though many new entries havebeen made in the last four years in technology licensing, capital equipment manufacture,and production facilities, because of the great demand for silicon and the high priceprevailing, there was a rush in to the arena by many new entrants some of whom knewvery little of the business or technology and were ready to invest huge capital for newplants as the cost of the final product was very lucrative. The order booking with theequipment supplier was high and delivery times were extended. Hence, the equipmentsuppliers could negotiate high prices for capital equipment. Now with the Photovoltaicindustry blooming, alternatives for silicon showing their teeth and market transformedfrom sellers to buyers leading to the price tumbling down like nine pins an urgent need tolook in to all aspects of the cost of polysilicon manufacture has come up. The samesituation prevails in the down stream operations of ingot casting and wafering. Thesetwo sectors will be dealt with separately elsewhere.

The quality of silicon required for PV is now clearly known. The high purity materialmade using the Siemens process is not really required for PV. Particularly, the levels ofboron and phosphorus, the two crucial impurities can be at about 0.1 to 1 ppm which isabout three to four orders higher than needed for semiconductor applications. Hence,there are many possibilities of reduction in the cost of manufacture and also new ways inwhich silicon could be made. Realisation that silicon required for PV could have puritieslevels of six 9s particularly with boron and phosphorus being at less than 1 ppm and


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metal impurities at even higher levels a detailed review of the Siemens process andpossible ways to reduce the cost of manufacture is essential. We shall study thesepossibilities further.

Basic features of the Siemens Polysilicon reactor.

In order to understand the possible means of reducing the costs of the Siemens reactor, itis essential to know the basic features of the polysilicon reactor. Further, as mentionedearlier, the polysilicon reactor is unique to the process and all the factors contributing tothe high cost of polysilicon could be attributed to the reactor design and operation. A

Figure 1

schematic of the Siemens reactor is provided in Figure 1. The Siemens reactor consistsof a pressure vessel made with a base plate and a bell jar. These two are hermiticallysealed and can operate at high pressures generally about 6 bar. The base plate is provided


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with ports for feed gas and exhaust gas and electrical feed-throughs. Inverted U shapedhairpins of silicon slim rods are mounted on the electrical feed-throughs their numberdepending on the size of the reactor. Electrical power source is provided to heat the slimrods to the working temperature of about 1100 C by resistive heating. The bell jar isprovided with view ports for measuring the temperature of the hairpins using non-contact

temperature gauges. Both the base plate and the bell jar are provided with jacketsthrough which cold water is circulated to remove the radiant heat from the hairpins. Thebase plate is placed on a firm pedestal. After assembling the hairpins, the bell jar iscovered over them and using suitable gaskets the bell jar is sealed on the base plate. Thehairpins are brought to the working temperature using complicated and sophisticatedelectrical power supply. A mixture of trichlorosilane and hydrogen gas is fed through theinlet feed nozzle. The exhaust gases are removed through the exhaust port. Typicalpolysilicon reactors have diameters of about 0.8 to 1.5 meter and heights of about 1 to 2.5meter and accommodate 9, 18, 24 or higher number of hairpins. Production capacities ofthe reactors could vary from 25 to 250 tons of polysilicon per annum.

During deposition, the reactor will have high temperature environment with corrosivespecies like HCl, TCS and STC. The materials of construction of the various parts of thereactor are selected such that they are kept cool by water-cooling or even if they areheated, they do not impart any impurities in to the deposited silicon. As highly hazardouschemicals like hydrogen and trichlorosilane are present inside the reactor at high pressureand temperature, the design of the reactor should take in to consideration, the safetyaspects of the process. The reduction of TCS takes place by a complexthermodynamically and kinetically controlled reaction and silicon deposits on the hothairpins which grow in diameter. The design of the reactor should be such that thedeposition is smooth and uniform over the entire length of the hairpins and is free ofcracks, pores, popcorn structures and other defects.

The polysilicon reactor can be subdivided broadly in to three parts, namely,

The reactor;

Power supply; and

Process control.

Let us study each of these and examine the scope of reduction in cost.

Polysilicon Bell Jar Reactor.

As stated earlier, the Siemens reactor is a pressure vessel operating at high pressures ofup to 30 bars typical operating pressure being about 6 bar. As the base plate is flat andthe reactor diameter exceeds one meter, to withstand the operating pressure, the baseplate has to be very thick which could be more than 100 mm for a diameter of one meter.As the inner surface of the base plate is exposed to direct radiation from the hairpins, itwill have a high skin temperature. This will lead to several problems. The first one ispossible reaction of the base plate material with the reactive environment of HCl andchlorosilane leading to incorporation of impurities in to the deposited silicon. For the


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same reason, the surface reacts with the chlorosilane to produce silicides of the metals inthe base plate. On opening the reactor, the silicides layers will peal off due to differentialthermal expansion as the silicides are brittle. Thus, the base plate is corroded in each runand may have to be replaced soon. To avoid this, extensive cooling paths have to beprovided in the base plate which makes it mechanically weak. Even then since there is a

thermal gradient from the top of the base plate to the bottom, thermal stresses aregenerated which lead to deformation of the base plate. To overcome these problemsspecial grades of steel have to be used for the fabrication of the base plate. The cost ofthe material and the cost of fabrication are both high leading to high cost of the baseplate. Even then, the base plate has to be replaced sooner or later. This problem can besolved in a simple manner. Any one who has done mechanical design of pressure vesselwill use a dish end to seal a cylindrical portion of a vessel. In fact in the bell jar, the topend is a dish closure. If the same dish could be used in the base plate, then the thicknessrequired for withstanding the operating pressure will drastically come down. Indeed, thethickness required for operating a one meter diameter reactor at 6 bar will be hardly 6mm! For safety reasons, a 10 or 12 mm thick dish could be used. Forming such dish ends

is routine in pressure vessel fabrication. Moreover since thin plates are needed forfabrication, better material of construction can be used. We had made such dish bottomsfor the polysilicon reactors in the early nineties using Nickel 201 as the material. This isa much better material compared to say stainless steel of 304 or 316 grades. Portsrequired for the inlets and outlets and electrode feed-through ports can be easily made inthe dish using fabrication standards like ASTM or others. Figure 2 depicts a reactor with

Figure 2

a dish bottom. Suitable modifications will have to be made to fabricate electrode holdersand graphite holders taking in to account the dish shape of the base plate. We haveoperated such dish base plates for more than a decade and the performance has beenmuch superior compared to the conventional flat base plates. The cost of manufacturecomes down drastically which is the icing on the cake! The top bell jar is fabricated likea jacketed vessel and does not need any special features other than the normal fabricatingprocedures. With this knowledge, a polysilicon manufacturer can get a polysilicon


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reactor from a capable pressure vessel fabricator at a price which would be considerablylower than from the present vendors.

Reactor Power Supply

In the Siemens reactor, silicon slim rods are generally used for making hair-pinassemblies. While the material produced using these slim rods is of excellent quality, thecost of manufacture of the slim rods, the cost of slim rod pullers/sawing equipment forthis purpose will add to the cost of capital and manufacturing. Further assembling theseslim rods in to hairpins is cumbersome considering that the slim rods would be long andare very fragile and they have to be bridged at the top using special silicon strips withsuitable jointing means.

Apart from these mechanical difficulties in using silicon slim rods, there is a greaterdisadvantage. Silicon is a bad conductor at room temperature and will not pass electricalcurrent easily. To bring the slim rod to the working temperature it is to be heated by

means of electrical energy. For this purpose, there are two possibilities: one is to applyvery high voltage so that the high resistance of the slim rods assembly is overcome andthe rods are heated. Since silicon has a high negative coefficient or resistance, as thetemperature of the rods increases, the resistance of the slim rod assembly comes downdrastically. The voltage applied should be reduced very rapidly to ensure that the rodtemperature does not go above the melting point of silicon and the slim rod melts. At theworking temperature, the resistance of the assembly will be low and hence low voltagepower supply would be sufficient. Further, as deposition takes place, the resistancecomes down further but as the surface area from which heat is radiated also increases,increased current has to be supplied to sustain the working temperature. Thus, the powersupply needed would have to provide initially very high voltage of the order of a few

thousand volts, which should be quickly reduced to a low voltage and then the current isincreased from less than 100 amps to more than 2000 to 3000 amps depending on thediameter reached. This cannot be achieved by a single power supply; there will be twosupplies, one for initial heating and the other for the growth phase of the silicondeposited. The electrical feed-throughs should be designed to withstand the high voltageapplied and also to ensure that there is no arcing between two slim rods or between theslim rods and the base plate. With the limitation in availability of materials ofconstruction, this will pose serious challenges for the designer of the reactor powersupply.

The general practice therefore is to reduce the initial voltage required for heating by

preheating the slim rods by suitable means to about 400 to 600 C so that the voltagerequired for forcing the current is brought down considerably. However, the slim rodassembly is inside a water cooled bell jar reactor, heating from external heaters is notpossible. Different techniques have been developed for this purpose. In one such, aquartz enveloped heater is introduced from the top of the bell jar through an appropriateflanged opening. The heater is switched on with an inert gas atmosphere in side the belljar. When the slim rods are heated to reasonable temperature, the initiation of heating isstarted with the high voltage power supply. When the slim rods attain the working


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temperature, the heater is removed, the flange closed and the inert gas replaced byhydrogen and polysilicon deposition is started. This step is obviously cumbersome. In adifferent technique, heaters of graphite are provided at suitable positions in the base plateand these are heated by a simple power supply and the slim rods are heated by receivingradiant heat from the graphite heaters. In yet another technique, plasma heating is done

by using suitable plasma electrodes located in side the bell jar.

The power supply will therefore have three parts; one is the supply required for theexternal heater or plasma torch, the second one is a high voltage supply for triggering theslim rod; and the third one is for regular operations. As the resistance, voltage andcurrent of the slim rod assembly are highly sensitive to the temperature of the slim rods,the controls should be capable of fast switching from one to another and controlling thetemperature has to be very carefully monitored. Hence, the power supply would behighly sophisticated and hence, very expensive.

For producing semiconductor grade silicon, necessarily silicon slim rods have to be used

to ensure the purity of the produced silicon. However, as we have seen earlier, this purityis not required for photovoltaic silicon. Hence, there is possibility of using other slimrods for deposition which will very much simplify the starting process of the reactor andthe power supply to the reactor will also be come much simpler and of much lower cost.This is not a new concept. In fact, Theurer of Bell Telephone Labs had deposited siliconusing a tantalum strip. Rogers had taken patents on using tungsten rods as slim rods.Many polysilicon manufacturers used to deposit polysilicon on graphite tubes and othershapes to produce silicon ware for diffusion furnaces. However, such efforts were notexploited because silicon at that time was being produced for semiconductors and thepurity of silicon deposited with tungsten was not really good enough. Moreover, whentungsten or other material is used, the produced silicon has to be necessarily broken tosmaller pieces to remove the core of the metal which means long rods of silicon requiredfor the float zone crystal growth could not be made. Further, removal of the core neededlabour and hence, this was not favoured.

In todays changed circumstances, this procedure would be acceptable for reducing thecapital cost. In countries like China and India where labour is cheap, the cost of removalof the core would not be an important issue. Moreover, for directional solidification evenif some core material is present in the silicon, this will not matter as the metal impuritywill be thrown away in to the discarded portion. The present author has operated suchpolysilicon reactors for more than a decade and has produced polysilicon which has beenregularly used for growing single crystal of silicon and further wafered for making solarcells. The efficiency of solar cells made with such material has been found to be nodifferent from silicon produced in the conventional way. An extensive study on life timeof minority carriers in the wafers has proved that the quality of the wafers is excellent.

Several advantages of the metal hair pins become apparent:

The power supply will become very simple. As the metals are good conductors,they can be heated with low voltage supply. Hence, the complications of pre\-


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heating, high voltage triggering, changeover to regular power supply will not berequired. The cost of the power supply will thus be much smaller.

Making of the hairpins and assembly will be easy. As tungsten is sufficientlystrong, there is no fear of breakage.

The number of electrical feed-throughs could be drastically reduced by

interlinking many hairpins inside the reactor itself thus improving the integrity ofthe base plate. Since high voltage is not required, insulation of the feed-throughcan also be simple.

If there is a power failure in the middle of a process, it is possible to restart thereactor because of the conducting core. Such a possibility cannot be thought inthe case of silicon slim rod assemblies as bringing the grown rods to workingtemperature again would be almost impossible.

No investments are needed for slim rod pullers or diamond wire saws for cuttingslim rods. Thus, considerable savings in capital costs as well as labour can beachieved.

Process control: Thermodynamic Factors

The CVD process taking place in the Siemens reactor is complicated. A large number ofspecies are available in the high temperaturehigh pressure process even thoughhydrogen and trichlorosilane are the only two feeds to the reactor. The prominent amongthem are TCS, STC, DCS, HCl, H2 and SiCl2 at high temperature. Extensive studieshave been made on the thermodynamic equilibrium in the Si-H-Cl system by manygroups. Though such studies are theoretical and are based on minimization of the freeenergy of the system under the given conditions of pressure, temperature andcomposition, they throw valuable light on the actual process. From such studies, theoptimum temperature, pressure and feed composition at which the maximum deposition

of silicon can take place can be found out. In Figure 3 Si deposited is plotted againsttemperature for 3 different mole ratios of H2:TCS. It will be noticed that the maximumdeposition takes place at about 1160 C when the mole ratio of H2:TCS is 1:1. Thisincreases to 1180 C when the mole ratio is 1:0.5. However when the mole ratio is 1:0.1,the deposited quantity increases with temperature. Further, the graph shows that even atas low a temperature as 400 C some silicon is deposited. This is not observed in practicebecause of other factors.


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Si Deposited Vs Temperature C

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400 600 800 1000 1200 1400 1600

Temperature C

SiDeposited

kg

mol

H2: TCS 1:1 kg mol

H2: TCS1:0.5 kg mol

H2: TCS1:0.1 kg mol

1160 C

1180 C

Figure 3

In practical deposition situations, the optimum operating temperature range has beenfixed between 1100 and 1150 C.

In Figure 4 the effect of increase in pressure against efficiency of deposition is depicted.It may be noted that as the pressure increases, conversion efficiency decreases. However,for safe operation of the reactors and also facilitate treatment of effluents from thereactors, generally, the reactors are operated at about 5 to 6 bar. One important effect ofpressure which is not clear from the above figure is that with increased pressure, thenutrient present near the filaments increases linearly with pressure so that though there isa marginal decrease in efficiency of deposition, since the amount of TCS present ishigher, the deposition rate increases with pressure. It may be noted that when theSiemensC process was developed, the reactor made with fused quartz was operating atatmospheric pressure. When the size of the reactors had to be increased, the metal bell

jar concept was introduced. As the metal bell jar could be operated at high pressure, itwas noticed that the deposition rate increased tremendously reducing the energy requirefor deposition and increasing the capacities of the running plants several folds.

Effect of Pressure on Si Deposition Efficiency

SiHCl3:H2=1:1, 1100 C

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0 10 20 30 40 50

Pressure bar

Specieskgmol

SiCl4 SiHCl3

Efficiency

HCl

SiH2Cl2

SiCl2

Figure 4


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Conversion efficiency above 6 bar pressure will not increase much but the pressure ratingof the reactor and the downstream recovery equipment will have to be increased from 150ASA to 300 ASA or higher rating which leads to increased cost of equipment notcommensurate with the marginally increased deposition rate.

In addition to predicting the conversion efficiency of the process for a given feedcondition thermodynamic equilibrium studies will also throw light on how much of TCSis converted to STC which is a by-product of the process. Though in principle TCS fedalone to the reactor can produce silicon as shown in the Figure 5 below, this is not donebecause feeding vapourised TCS in to the reactor is not easy.

Equilibrium Concentrations for decomposition of

TCS Alone at 6 bar pressure

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400 600 800 1000 1200 1400 1600

Temperature C

kgmolsofspecies.

SiCl4

H2SiHCl3

Si

SiCl2

HCl

TCS/STC

SiH2Cl2

Figure 5

Further, extensive formation of STC and dangerous polysilanes by polymerization of

SiCl2 will result. Thus, always TCS is fed along with hydrogen. How much of each isfed depends on the site conditions. A composition of 1:1 TCS: H2 will produce moreSTC as shown in Figure. 6

TCS:H2=1:1, Pressure 6 bar

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400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Temperature

SpeciesKgmol SiHCl3

SiCl4

Si

HClSiCl2

SiH2Cl2

Figure 6


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However, recovery efficiency in the down stream operations will be better as will beexplained further. Higher hydrogen content increases conversion efficiency, reducesformation of STC and ensures smooth deposition as shown in Figure 7.

Effect of increased H2 on Equilibrium andefficiency

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0 10 20 30 40 50

Hydrogen Kg mol

Specieskgmol

HCl

EfficiencySiHCl3

SiCl4SiCl2 SiH2Cl2

Figure 7

However, the rate of deposition will decrease, the sizing of the down stream recoverysection will increase and the recovery efficiency reduces. This is for three reasons. Thefirst is the active silicon precursor namely TCS concentration decreases in the feed. Thefeed gas is now richer in H2 and the actual quantity of TCS available for deposition atany given time and space is small. Secondly space velocity in the reactor increases for agiven feed rate of TCS. This reduces conversion efficiency though the quality of thedeposit improves. Lastly, as hydrogen is a non condensable gas, when the effluent gas iscompressed and cooled to low temperature, it does not condense while the chlorosilanescondense to the extent of their equilibrium vapour pressures at the process condition.The remaining un-condensed chlorosilanes escape along with hydrogen. Depending onthe subsequent treatment of this stream, the chlorosilanes may end up as silica gel orfumed silica. Thus, the overall requirement of TCS for silicon deposition increases withincrease in the hydrogen concentration. If STC finds use for producing pyrogenic fumedsilica, then the formation of STC may not be an issue. However if STC has to bedisposed of or has to be converted back to TCS by hydrogenation (which is an expensiveprocess) then one has to carefully optimise process parameters depending on the siteconditions.

Process Control: Kinetic FactorsThough thermodynamic studies can predict the efficiency of conversion of trichlorosilaneto silicon, this is what obtains under equilibrium condition. However, the reactor isoperating in a dynamic condition of flow of species and the rate of deposition will dependon the kinetics of the process. Since CVD of silicon is a heterogeneous process involvingdeposition of solid silicon from a gaseous phase, the rate of deposition depends verymuch on the fluid dynamics in the reactor for a given composition of the feed. Silicon


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atoms to be deposited have to move from the gas phase to the solid surface. The reactiontakes place on the surface of the solid. Gas will be moving over the surface of thestationary solid surface. The layer of gas in immediate contact with the surface will alsobe stationary or moves at very low velocity compared to the bulk gas. This layer of gason the surface is called the boundary layer. Silicon molecules from the bulk will move

through this layer by diffusion and react on the surface. The gaseous products of thereaction also have to move from the surface through the boundary layer to the bulk.Hence, the rate of deposition will depend on the thickness of this boundary layer. Theboundary layer thickness can be reduced by increasing the turbulence in the gas phase.

Turbulence is dependent on many factors like:

Size of the reactor.

Feed rate which depends on the total design of the plant, number of reactors, thecapacity of the down stream exhaust handling system, etc.

Design, numbers and location of the feed and exhaust nozzles.

Temperature gradient between the wall of the reactor and the slim rods. The numbers and relative location of the hairpins in the reactor.

The size of the deposited rods.

Final Diameter of the rods.

Let us consider the influence of these factors on the performance of the reactor.

Size of the reactor: As the size of the reactor increases, the diameter and heightincrease. Thus the cross section through which the gas flows increases. To keep acertain minimum space velocity which will generate sufficient turbulence, the feedrate has to be increased. This is dealt with in the next paragraph.

Feed rate: Since the reactor is of a finite size, the space velocity in the reactor isdependent on the feed rate. Increased feed rate increases the space velocity. Thus,at any given point, higher feed rate ensures that higher concentration of TCSmolecules are available at the surface for reaction leading to higher deposition rates.However the feed rate cannot be increased to any extent because of physicallimitations in the plant. Basically, the plant should be capable of handling increasedthroughputs from the polysilicon reactors which means the auxiliary systems of thepoly-reactors (which include the feed system consisting of H2 feed, liquid TCSpumps and vapourisers and the mixing chambers for H2 and TCS) and the down-stream reactor effluent handling system (which comprises of the exhaust cooler,

compressors, deep refrigeration units for condensing the chlorosilanes, scrubbersand dryers for hydrogen gas, distillation and hydrogenation reactors) should be ableto handle higher feed throughputs. Another aspect of the feed rate increase is thatthe conversion efficiency decreases with increased feed as there is less time for thegas to take part in the chemical reaction and bulk of the gas comes out without anyreaction taking place. This leads to increased load on the downstream recoveryunits and distillation columns which separate TCS and purify it. Generally, at thebeginning of the cycle, feed rate will be low, but is gradually increased as the


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diameter of the slim rods increases because as the diameter increases, surface areaavailable for reaction increases in geometric proportions. Hence, feed rate can beprogrammed to satisfy this condition.

Feed and exhaust nozzle: As the reactor is a cylindrical body in order to keep

the feed uniformity, generally the feed nozzle is kept at the center of the base plateor placed symmetrically around. If more than one nozzle is provided, it isimportant to ensure that all the nozzles have equal feed rates. If this is notachieved, then the gas dynamics in the reactor will be affected. Exhaust ports in thereactor will not drastically affect the turbulence of the system. However, theyshould be located such that the symmetry of the gas flow is not affected. Theexhaust gas will be at considerably high temperatures. It may therefore beeconomical to tap this to heat the feed gas by means of a heat exchanger whichshould be of proper material of construction such that impurities are not drawn in tothe feed gas.

Temperature gradient: Generally the slim rods are maintained at about 1000to 1180C. The bell-jar and the base plate are water cooled. Hence, the bell surfacewould be at about 50 to 90C depending on the temperature of the cooling water andits flow rate. Thus, there is a steep temperature gradient between the slim rods andthe reactor walls. Hence, thermal convection develops in the reactor. This isincreased by the central feed nozzle from which gas would be jetting out at highvelocity. Because of this, the turbulence in side the reactor would be automaticallyhigh and the deposition would be uniform. However, if the reactor walls are kept athigher temperatures by circulating thermic fluids for tapping the heat for downstream processes, then this gradient decreases. In such a situation, specialarrangements have to be made to increase the turbulence.

Number and location of hairpins: With increased demand for polysilicon,the size of the poly reactors is increasing. Hence, these days, reactors with 24 ormore hairpins are being made though reactors with 9 and 24 hairpins are also beingoperated. One important benefit of increasing the number of hairpins in the reactoris reduction in electrical energy consumption. When the number of hairpins ismore, the hot filaments will see more number of equally hot rods around themwhich reduces the loss of energy by radiation. The hairpins are located insymmetrical fashion so that the gas dynamics is uniform around them and feed gasconcentration is uniform throughout. Hence, there is limited scope for deciding thelocation of the hairpins in the reactors.

Size of the hairpins: The size of the hair pins should be as high as to occupythe maximum space in the bell jar reactor. This will not only ensure that the reactoris used to the optimum, but also, the conversion efficiency in the reactor is alsohigh. The gap between two rods of the hair-pin is decided by the final size of therods that are grown in the reactor. At the final stage of growth the gap between tworod surfaces decreases which results in higher temperature in these regions. Thiswill lead to poor quality of the deposit in these regions. Hence, the hair pin widths


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should be sufficiently large to get uniform deposition through the circumference ofthe rods.

Final Diameter of the rods: In the beginning of deposition the diameter ofthe rods will be smallabout 3 to 10 mm depending on the type of slim rods used.

Hence, the surface area available for reaction will be small. The deposition ratewill be high at this stage as the concentration of the nutrients in the reactor wouldbe much higher than what is required at this diameter. However, as the diameterincreases, the rate of deposition comes down. In order to keep higher depositionrates, the feed rate has to be increased in geometric proportion of the diameter asexplained above. As the weight of silicon deposited depends on the square ofdiameter, it is desirable to increase the diameter of the slim rods to as high a valueas possible. However, limitations in the power supply and the distance between toadjacent rods limits this.

Thus, one can notice that there are various process parameters which process engineers

can change to optimise the deposition process. However, since electrical energyconsumption is the most important parameter in the manufacture of polysilicon, utmostattention should be paid in reducing this. Thus, other parameters like per pass conversionefficiency and down-stream recovery processes could be compromised to achieve thistarget.

How to reduce cost of manufacture:

The most important cost factor in silicon manufacture is electrical energy. Differentreactor designs lead to different energy consumptions starting from 60 units to more than200 units for producing 1 kg of polysilicon.

Various ways in which the energy consumption in silicon production in the Siemensreactor can be reduced are considered now.

We have already discussed the effect of the size of the reactor on energy consumption.Larger the reactor less is the energy consumption. We have also considered ways andmeans of increasing the rate of deposition of silicon which has a direct impact on energyconsumption. Faster the deposition rate, shorter the time required for completion of a runand hence, net energy consumption would be lower.

Both can be optimised for lowering the energy consumption. There are limitations on

both the above techniques. Hence, other ways of energy reduction should be explored.

As we have seen, the high energy consumption in the Siemens reactor is because the rodsat about 1100 C radiate out heat to the cooled metal bell jar wall. Hence, if we cansomehow reduce this energy loss by radiation, then we can conserve energy.

One way is to provide good reflecting surface on the metal bell jar. This can be achievedby electro polishing the inside of the bell jar which makes the surface smooth and highly


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reflective. However, the reflectivity of the bell jar wall which in most cases is stainlesssteel is low and hence, one can expect only a modest reduction in energy consumption.Further, the surface will become tarnished with use because of deposition of thin films ofsilicon oxide and /or because of corrosion of the surface by hydrochloric acid liberated byslight traces of chlorosilanes/polysilanes present at the end of the process. Thus,

provision should be made to regularly re-polish the inner surface of the bell jar.

One important way of increasing the reflectivity of the wall of the bell jar is to use silver.As is well known, silver has the highest reflectivity among all materials for radiation.Hence, silver lining on the inside surface of the water cooled metal bell jar would greatlyreduce energy consumption. The present author had used a small bell jar made withsilver and found that the energy consumption was reduced by almost half compared tostainless steel bell jar. However, for large size reactors, using silver is not possible. Oneway of providing silver lining is to produce explosion-claded sheets of steel with thinsilver layer and fabricate the bell jar inside wall with such explosion cladded sheets.Wacker had patented an atomic hydrogen torch technique for brazing silver on steel

surface. One other way is to electroplate silver on the steel surface. However, sincesilver plating baths contain phosphorus bearing chemicals, there is a possibility ofphosphorus contamination in the produced silicon. Further, the integrity of the platedfilm is also doubtful. Hence, best results could be expected from mechanically coatedsilver on steel.

The other obvious way of reducing the radiation loss is to increase the temperature of thebell jar wall. This can be done by circulating water at higher temperature say 150 to 200C. This calls for high pressure circulation of water to keep it liquid at these temperatures.This could actually allow tapping of some heat from the polysilicon reactor forgeneration of steam from the hot water flowing through the jacket. This means the shellhas to be designed to take care of the higher operating pressure of the jacket and not justthe pressure inside the bell jar. The outer jacket also has to be designed for the higherpressure. Water pumping has to be done at high pressures. While all these are possible,being carried out regularly in steam boilers, the amount of saving in energy may not becommensurate with the efforts needed. Alternatively, a thermic fluid could be circulatedthrough the jacket. By choosing appropriate thermic fluid, the whole operation could beconducted at atmospheric pressure of the jacket. The hot thermic fluid could be used forboiling chlorosilanes during distillation purification. We should note that thermic fluidscan be used up to about 300 C with acceptable degradation of the fluid. However, theenergy saved by increasing the wall temperature is depicted in Figure 8. In this graph, an


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Saving in energy w ith increase in bell jar wall

temperature

0

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0.8

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Temperature C

RelativeEnergySaved

Figure 8

approximate reduction in energy consumption with respect to energy consumption whenthe wall temperature is 25 C has been made without going in to details of the radiationconditions. The graph can be taken for guidance only. One notes that energy reduction is

very small at less than 10% below wall temperatures of 600 C. Further, the value of theenergy thus recovered is much smaller than the value of the electrical energy put in to thereactor. Thus, reducing energy by increasing the wall temperature is not really worth theefforts and capital required.

From the graph, one finds that at about a wall temperature of 850 C, the energy reductionis about 30 to 35 % which is substantial. The present author achieved this by introducinga fused silica bell jar between the hair-pins and the metal bell jar. Figure 9 shows aschematic of such a reactor. As the deposition proceeds, the fused silica wall temperature

Figure 9

rises and thus energy consumption reduces. About 30 % reduction in energy wasachieved. Further, since fused silica is fairly pure, it contributes little impurity to thedeposited silicon. It is interesting to note that the original Siemens C reactor had quartzenvelop and worked at atmospheric pressure. Radiation shield outside the quartz bell jarwas provided to reduce energy consumption. Unfortunately, fused silica bell jars havelimited sizes and therefore, cannot be made to suit the big polysilicon reactors presently


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being operated. Moreover, they are expensive and will crack due to thermal stresses andneed frequent replacements.

Other materials with have better mechanical strength than fused silica can be consideredfor this purpose. One should bear in mind the following points: (a) the material used

should be sufficiently pure and should also be inert to the gases present in the reactor atthe high temperatures. (b). There should not be any deposition of silicon on suchinsulating material. If silicon deposition takes place, there will be damage to the insulatorand also wastage of valuable silicon. Fortunately, suitable materials are available and theprocess conditions can be so adjusted to prevent deposition of silicon. A polysiliconreactor with such insulation incorporated is shown in Figure 10. Here, a suitableinsulator covers the water cooled metal bell jar. A sacrificial inner liner has beensuggested to take care of any deposition of polysilicon so that only this sacrificial linercan be replaced instead of the entire insulation.

Figure 10

One can immediately see that depending on how good the insulation is, energy loss isreduced. Thus, if the insulation is sufficiently thick and good, considerable energy losscan be avoided. This can be done with the existing operating reactors and benefits oflower energy consumption could be achieved.


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In principle, if the surface temperature of the insulation material approaches thedeposition temperature, there could be deposition of silicon on the surface. In this case,the heat is coming from the silicon hairpins. One can think of a situation when heatersare provided from the insulation surface to heat a tubular body to the working

temperature of about 1100 C. When TCS/H2 mixture is passed through such a reactor,silicon will deposit on the inside of the tubular body and in course of time, sufficientbuild-up of silicon can take place. This is the general principle of tubular reactors forproducing polysilicon. Deposition of silicon inside a tube is well-known. In the initialstages of development of polysilicon production processes, many workers had usedexternally heated tubes for deposition of silicon. However these did not come toprominence at that time as there were no suitable materials for the tubes. Moreover, thepurity of the deposited silicon was affected by the material of the tube. The tube was alsofound to be either used up in each run or was breaking after deposition. Such tubularreactor designs have been proposed from almost the beginning of polysilicon productionduring the early 60s. Several organizations have obtained patents on such tubular

reactors. We give a schematic of a tubular polysilicon reactor in Figure 11.

Figure 11

Basically the tube reactor resembles a Siemens reactor in that it has a bell jar and baseplate. The main difference is instead of numerous hairpins which are fixed to electrodesat the base plate, a single tube is placed in the bell jar on the base plate. The tube issurrounded by an electrically heated heater which is well insulated from the water cooledbell jar by suitable insulation. The tube is brought to the working temperature ofdeposition by means of the external heater. A mixture of trichlorosilane and hydrogen isfed to the reactor in the usual manner. Silicon deposition takes place on the inside of thetube. With time, the thickness of the tube increases. However, there is no need to change

the power supply to the heater as heat is conserved and the heat loss that normally takesplace and heat removed by the gas is to be made up at all the times. Thus, the tubularreactor needs a very simple power supply. Further, as the heater is insulated the heat lossto the water cooled bell jar is very much reduced. Thus, the tube reactor will enableproduction of silicon with low electrical energy. The estimated energy requirement forpolysilicon deposition in the tubular reactor could be as low as 30 % of the Siemensreactor. Thus, the tubular reactor would lead to low cost production of silicon. The


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advantages of the tubular reactor over the Siemens reactor are given in the table below. Italso states the limitations of the two.

Features Tubular Reactor Siemens Reactor

Power supply Fairly simple. If the heaterelement is made of materiallike molybdenum orKanthal, the voltage andcurrent are small. Powercontrol is also very simpleand mostly constant powerwould be needed with verylittle variation

Power supply would becomplex. In order tomaintain the temperature ofthe surface high currentwould be needed. Powercontrol becomes complex aspower has to be graduallyincreased from the initial tofinal stage.

Energy consumption Will be small since theentire reactor would be

thermally insulated andhence heat loss by radiationwould be small. Heat lossdue to heating the input gasis only to be supplied plusthe normal losses that takeplace in a tubular resistancefurnace (for example adiffusion furnace).

Energy requirement wouldbe This is because the

heated hairpins are facing awater cooled bell jar andhence, the radiation loss hasto be compensated.

Mechanical design Similar to a Siemens reactorbut does not requirenumerous feed-throughs forelectrode connection.

Need numerous electricalfeed-throughs

High pressure operation Can be carried out Can be carried out

Gas feed and outlet Concentric at the centre ofthe base. Heat exchangebetween the two is easilyachieved by a double pipearrangement.

Feed and outlets will beseparate and have to bepiped to a double pipe heatexchanger.

Uniformity of deposition For obtaining good surface

smoothness, the design ofthe inlet nozzle has to becarefully made as there is nothermal convection insideand turbulence has to becreated by the energy of thefeed gas.

Deposition would be better

and smoother as there wouldbe considerable thermalconvection as a result of thetemperature differencebetween the hairpins and thewater cooled bell jar surface.However, as the feed andoutlets are at different


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Features Tubular Reactor Siemens Reactor

locations, in order to achieveuniformity in deposition,feed may have to be locatedin more than one place.

Assembly of the reactor Fairly easy as the tube has tobe fixed to the centrallocation. The electricalheater need not bedismantled and may be apart of the bell jar assembly.Simple external electricalconnections need to bemade.

More complicated assemblyprocedure as the hairpinshave to be carefullyconnected and electricalcontacts with buss bars haveto be made.

Rate of deposition Depends on the extent ofturbulence created inside thetube.

Can be better because ofnatural thermal convection.

Conversion of an existingSiemens reactor to the newdesign

Possible. High Power supplyhas to be replaced with asimple power supply

Not applicable

Quantity of silicon deposited Depends on the initialdiameter of the tube. Largediameter tube is required tostart with for getting more

deposition per cycle as theinner diameter decreaseswith time.

Depends on the finaldiameter of the depositedrods which is determined bythe design and power

supply.

Cost of the reactor Low High

Operation Easy Needs more attention

Maintenance Simple More complicated.

Thus, the tubular reactor has several important advantages over the conventional Siemensreactor. Particular note should be made of the low cost of the reactor and ease ofoperation and low energy consumption. Hence, it is desirable to use a tubular reactor.However, till now we are not aware of any body using tubular reactors for actualproduction. The main reasons for this are:

1. Suitable tube materials have not been available. Materials like fused quartz whilebeing good, is not acceptable because the quartz tube will shatter at the end of therun and hence, for each run, a new fused quartz tube has to be used. Even


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However, marketing fumed silica is not easy. It is used in small quantities of about 1 to2 % in various formulations. However, even at this small level, its role is important inthe formulations. Generally users do not want to change the source of supply for fear ofthe quality of the final product being affected by this small component. Hence, sellingfumed silica is not easy. All the big names in polysilicon have facilities for manufacture

of fumed silica and have good market share. They also have facilities for manufactureand formulation of silicones which use considerable quantity of fumed silica. Newentrants find it almost impossible to penetrate this market. Hence, they are forced todifferent ways of disposing silicon tetrachloride which will not add value in themanufacture and thereby increase the cost of silicon produced. The most important wayof use for silicon tetrachloride is to convert it to trichlorosilane by reaction with hydrogengas at high pressures and temperatures in a hydrogenation reactor also called a converter.Here, a equi-molar mixture of silicon tetrachloride and hydrogen is fed in to the converterwhich consists of hot graphite heaters kept at about 1200 C by means of electrical energy.About 12 to 18 % of silicon tetrachloride is converted to trichlorosilane according to thefollowing equation:

SiCl4 + H2 SiHCl3 + HCl

The hot exhaust gas from the converter is quenched to low temperatures to reduce the re-conversion of TCS to STC. The chlorosilanes are condensed and sent to distillationcolumns for separation of TCS from STC. HCl produced is sent to TCS production unit.This process consumes electrical energy to the extent of 10 to 40 units. This has to beadded to the total energy consumption in the manufacture of silicon. A better process isto pass the equimolar mixture through a bed of metallurgical silicon which results inhigher conversion rate for trichlorosilane and HCl produced is also converted totrichlorosilane. The chemical reaction can be described as follows:

Si + 3SiCl4 + 2H2 4 SiHCl3

This process was developed by Union Carbide and is presently owned by REC. Theadditional advantage of this process is that the gases can be heated using hydrocarbonfuels thus avoiding use of expensive electrical energy though exotic materials ofconstruction are required in making the reactor.

Both the above reactions are endothermic and need heat input for the reaction to proceed.

The present hydrogenation converters that are available in the market have their ownshortcomings. They use graphite heaters which are not only fragile but will not ensureuniform heating of the input gases to the required temperatures. Moreover, the heatersneed to be replaced at regular intervals adding to maintenance costs. The present authorhas developed a much more rugged reactor design with heated beds of silicon or graphitewhich ensure uniform heating and avoid failure of heaters. Such reactors are simple toconstruct, need no sophisticated controls and can be used with metallurgical silicon bedto avoid formation of HCl.


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Avoiding formation of silicon tetrachloride during polysilicon

production

As we have seen silicon tetrachloride is produced during manufacture of trichlorosilane

as well as during deposition of polysilicon in the Siemens reactor. We have seen howthis silicon tetrachloride is handled in the plant. Cost of production of polysilicon isincreased because of this handling of silicon tetrachloride. Therefore, if by some meanssilicon tetrachloride formation is suppressed, there could be great reduction in the cost ofmanufacture of polysilicon.

We have seen that during silicon deposition in the Siemens reactor, formation of silicontetrachloride is inevitable because of thermodynamic consideration. Attempts have beenmade to suppress this by adding the equivalent quantity of silicon tetrachloride to thestarting feed. Though, this suppresses silicon tetrachloride formation, the rate ofdeposition of silicon is greatly reduced thereby increasing energy requirement for

production and reducing the overall plant capacity. Hence, some different way ofhandling should be thought of.

As is well known, production of trichlorosilane by reaction of anhydrous hydrogenchloride gas with metallurgical silicon is a highly exothermic reaction. Elaborate stepsare used to remove the heat of reaction and to keep the fluidized bed at around 300 C tooptimise the production of trichlorosilane. This is achieved by providing heat-exchangersinside the FBR. However, this is easily said than done since the heat exchanger insidethe corrosive as well as abrasive fluid bed environment will fail. Attempts have beenmade to remove the heat of reaction by diluting the feed HCl with gases like hydrogen orinert gases. However, this will adversely affect the capacity of the FBR. Also, presence

of an inert will lead to high refrigeration load for recovery of the chlorosilane. Byinjecting liquid silicon tetrachloride in to the fluid bed, this problem can be very elegantlytackled. Liquid silicon tetrachloride will extract heat for its own evaporation and to attainthe reaction temperature of the fluid bed. Control of the temperature would be simple.Another advantage is that since silicon tetrachloride is condensable, the refrigeration loadwould be greatly reduced. One has to however, consider the requirement of energy forvapourising the condensed liquid now rich in silicon tetrachloride. However, this wouldbe much smaller compared to when an inert gas is used for cooling.

One important effect of using silicon tetrachloride for removal of heat of reaction needsgreater recognition than has been hitherto given. Presence of silicon tetrachloride will

not just suppress formation of silicon tetrachloride. More than this, some silicontetrachloride is converted to trichlorosilane. This is seen in the Figure12 below. Here,effect of addition of equi-molar quantity of silicon tetrachloride to the feed HCl onequilibrium composition calculated is compared with when only HCl is fed. One canfind that the amount of STC in the equilibrium composition is always less than 1 even athigher temperatures indicating that some STC has got converted to TCS. Hence, it ispossible for the TCS production FBR to act as a hydrogenation reactor. No additionalheat is required like in hydrogenation thus reducing the energy required.


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Hydrochlorination Effect of STC in Feed

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ol

STC with dilution

TCS with dilution

TCS without dilution

STC without dilution

Figure 12

Thus, addition of STC to HCl in the FBR for TCS manufacture leads to dramatic changesin the process and capital and manufacturing cost. One word of caution is that the

thermodynamic calculations should be taken for broad guidance as the actualcomposition may be different from thermodynamic predictions due to kinetic factors.One notices that in the above figure, at 300 C, the composition of the reactor exhaustwithout STC dilution shows only about 2/3rd of TCS formation. However, it is wellknown that the composition of TCS could be more than 90% under actual reactoroperations. This is because of the kinetic factors in the FBR. Therefore, one has to useexperience to optimise the process conditions.

CONCLUSIONS;

Attempt has been made to critically examine the limitations of the Siemens reactor for the

production of polysilicon. Why production cost of the Siemens process is high has beenexplained. Several novel ways in which the limitations of the Siemens process could beovercome have been described. Emphasis has been made on using out-of the waysolutions for cost reduction.

Cost Reduction in Polysilicon Manufacturing for Photovoltaics

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