CA1135941A - Refined metallurgical silicon - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Silicon platelets essentially free of iron are precipitated from a solution of metallurgical grade silicon in molten aluminum and are melted in contact with silica slag to produce a refined metallurgical silicon (RMS). Multigrained bouler of said RMS may be pulled by the Czochralski method on a rotating silicon seed rod from a melt of said RMS. Alternatively, the silicon-slag melt can be directionally solidified to produce a directionally solidified refined metallurgical silicon (DS/RMS) separated from a region of solidified melt having a high concentra-tion of impurities rejected by the silicon as it solidifies, Multigrained DS/RMS boules may be pulled from a melt of said DS/RMS. Alternatively, the DS/RMS is remelted and directionally solidified a second time with single crystal DS/RMS boules being pulled from a melt of the twice directionally solidified material. The thus-pulled, jul multigrained RMS and DS/RMS and single crystal DS/RMS
materials are low-cost products having a substantially higher impurity content than in conventional high purity semiconductor grade silicon while, at the same time, having desirable solar cell properties.

Description

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Background of the Invention Field of the Invention - This invention relates :.., to low-cost refined, metallurgical silicon materials. More particularly, it relates to the production of such silicon materials having desirable properties for solar cell ; applications.
Descrip ion_of the Prior_Art -The development of new techniques and products for the low-cost utilization of non-polluting sources of energy is of paramount national and world-wide interest. Solar energy is among the energy sources of greatest interest because of its non-polluting nature and of its abundant, . . , non-diminishing availability. Two separate approaches - have been utilized in eforts to develop solar energy as a suitable energy source or satisfying significant portions of the ever-increasing energy requiremen~s of modern, 1~ ~ industrial societies. In one approach, solar energy is ,j converted into thermal energy, while the second approach involves the conversion of solar energy into electricity ~- ;
i 20 by means`of the photovoltaic effect upon the absorption of sunlight by so-called solar cells. The present invention -relates to the second approach and to the development of low-cos~ silicon materials for use in such solar cells.
SiLicon solar cells 9 the most commonly employed devices based on said photovoltaic effect, have been employed reliably in space craft applications for many years. For ~uch applications and for industrial and :

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11 ,9~0 commercial applications in general, crystals of high purity9 semiconductor grade silicon are co~monly utilized.
Such high purity, high perfection silicon is prepared by rather costly procedures involving converting metallurgical grade silicon to trichlorosilane, which is then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high. For example, polycrystalline ~emiconductor grade silicon made from metallurgical grade silicon costing about ~0.50/lb. will presently cost on the order of about $30/lb. and up. A single crystal is grown from this semiconductor grade material, the ends of the single crystal ingot or boule are cut off, and the boule is sawedi etched and polished to produce polished wafers for solar cell application, with mechanical breakage , and electronic imperfection reducing the amount o useable material obtained. As a result of such processing, less than 20% of the original polycrystalline, semiconductor grade silicon will generally be recovered in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a signiicant factor in the overall ~;, economics of such applications.
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The economic feasibility of utilizing solar cell

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technology for significant portions of the present and ~ ~ prospective needs for replenishable, non-polluting energy ; ~ sources would be enhanced, therefore, if the utilization .,.~ .
~ of high cost, high purity, high perfection single crystal ;t wafers could be avoided. Previous efforts to refine metallurgical silicon for other applications, however, have not resulted in the production of materials tha, can be utilized in solar cells although the electronic charac-teristics of various grades of silicons for solar cells are less stringent than, for example~ such silicons as employed for complex circuitry in the electronics industry. !:
Metallurgical silicon has heretofore been refined ` by slag oxidation to obtain a grade of metallurgical silicon or ferrosilicon advantageous as an alloying additive in the manufacturing of steels. As indicated in the ¦~
Barber et al patent, U. S. 2,797,988, and elsewhere, the slag oxidation approach has been utilized to remove im-purities and thus to purify and refine silicon h~ving an iron content therein such that the refined product consti~utes a ferrosilicon in which iron is considered an integral part l of the final refined product. For use of silicon as a ,,, ~, 1'-:
substrate in planar diodes and solar cells made therefrom, however, it is necessary that the refined silicon have as low an iron content as possible~as such iron is a deleterious impurity in a solar cell material. As indicated above, this ., , .,, ;~
circumstance is in contradistinction to the benign nature of the iron content of the refined silicon product as .. ' :

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utili~ed in the steel industry.
Silicon has also been purified heretofore by the dissolution and subsequent precipitation of silicon from a liquid metal system. Such purification, taught by Litz, . .
U. S. 3,097,068 and Wakefield, U. S. 3,933,981, involves the retention of silicon impurities by the liquid metal solvent when dissolution takes place at a higher tempera-ture and, subsequently, the temperature is lowered to precipitate a relatively pure silicon Such a silicon product is not suitable for use as a substrate in solar cell applications, however, since ~he liquid me.al of the solvent phase is present as an impurity in the product silicon so obtained in this processing technique.
While Litz states that the product of such re-~ .
fining by liquid metal solvent is too im~ure for use in transistors, use for rectifiers and solar batteries is indicated. The product, however, is said to contain 200 ; to 700 ppm of aluminum, with no mention of its iron content.
Such a level of aluminum would render ~he product purified by ehe Litz technique unsuitable ~or solar cell application.
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Litz also discloses a further lengthy and expensive procedure using silicon tetrachloride to reduce the aluminum content to 9 ppm.
Other efforts to develop acceptable solar cell materiaIs have likewise resulted either in high cost, or ,. . . .
high impurity levels such that acceptable efficiencies can not be obtained, or a combination of these disadvantages.

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~ An accep~able efficiency for a low cost, relatively impure . ~'Y.
silicon w ould, of course, ~ery likely represent some loss , ..:, from the high purity, single crystal material made from semiconduc~or grade silicon. Such high-cost material is capable of providing efficiencies of about 13-14%. The ., :.
~- lower cost of relatively impure silico~ material, particu-larly multigrained material, might well constitute an ad-vantageous trade-off enhancing the overall technical-economic feasibility of utilizing silicon solar cell technology in significant commercial operations.
The practical lower limit for economic solar -~ cell efficiencies is about 7-8%. One attempt to produce an acceptable low-cost ma~erial has involved the pulling of ribbons from a melt of metallurgical grade silicon by -~; known techniques. While such ribbons pulled from semi-conductor grade material have obtained efficiencies of up to 10%, the ribbon pulled from metallurgical silicon is relatively dirty and impure, with solar cell efficiencies s obtained fr~m such ribbon being limited to about 5%.
~i 20 Another approach has involved the pulling of ~i boules, in a multiple series of refining steps, from ,, j ,. -metallurgical grade silicon as a starting material by the known Czochralski-pulling technique. By remelting and re-pulling refined, multigrained silicon in a several stage process, a single crystal refined silicon is ultimately obtained that is capable of achieving efficiencies of about 8%. The cost of such material, however, is relatively high

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Chu, U. S. 3,961,997, discloses the fabrication of low-cost solar cell substrates from metallurgical grade, polycrystalline silicon. In this approach, successive layers of polycrystalline silicon containing appropriate dopants are deposited over substrates of metallurgical grade silicon, graphite or steel coated with particular diffusion barrier materials. The resulting products contain a high level of impurities such that efficiencies obtainable by modifications of this approach have not exceeded about 3-5%.
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- A genuine need thus exists for low-cost, relatively impure silicon products suitable for use in solar cells of acceptable efficiency. The resulting low-cost solar cells ' should preferably have efficiencies in excess of about 10%, with 7-8% representing a practical lower limit of efficiency as indicated above.
It is an object of this invention, therefore, to provide low-cost refined metallurgical silicon produc~s 20- suitable for solar cell applications.
It is another object of the invention to provide refined metallurgical silicon of a relatively impure nature as compared with semiconductor grade material while having useful properties for said solar cell applications.
It is a further object of the invention to provide mNltigrained refined metallurgical silicon useful for solar cell applicati.ons or readily convertible to such useful ; 11,980 3~
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silicon material.
With these and other objects in mind~ the invention is hereinafter described, the novel eatures thereof being particularly pointed Ollt in the appended claims.
SUMM~RY OF THE INVENTION
The objects of the invention are accomplis~ d prefer-ably by the slag oxidation of partially purified silicon precipitated in an essentially iron-free form from a solu~ion ~- of metallurgical grade silicon in a molten liquid solvent and treated to remove adherent impurities, thereby producing a ` refined metallurgical silicon (RMS), and desirably pulling, by the Czochralski method, multigrained boules of said RMS
from a melt thereof. By solidifying the silicon-slag melt from the slag oxidation operation in a unidirectional manner, a dixectionally solidified refined metallur~ical silicon (DS/RMS) is obtained, with multigrained DS/RM~ boules pulled fro~ a melt of said DS/RMS having particularly advantageous ~` solar cell properties. By remelting and solidifying said DS/RMS in a unidirectional manner a second time, single ;1 20 crystal DS/RMS boules can be obtained having further ad-vantages upon incorporation in planar diodes and solar cells made there~rom. Said single crystal DS/RMS, and said multigrained RMS and DS/RMS, are obtainPd at a substantially higher impurity cont~nt level than in conven~ional high purity semiconductor grade material while such RMS and DS/RMS products are, nevertheless9 o~ sufficient purity to permit such products to be used advantageously as low-cost ., .

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substrates in planar diodes and solar cells made therefrom.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a low-cost alter-native to the utilization of semiconductor grade material in the development and application of solar cell technology.
~ While high-cost, high-purity, high-perfection single crystal :~ silicon wafers are undoubtedly of value for specialized solar cell applications, the necessity for achieving a ~; more c~mmercially feasible cost structure is a major factor in the growth and development of solar cell technology for general industrial and commercial applications. As noted ; above, the large area requirements of solar cells, which are, in effect, simple large area planar diodes, render ,.
; material costs a significant factor in the economic feasi-:' bili~y of solar cells vis-a-vis other presently available energy sourcesO By means o~ the presen~ invention, practical, large-scale solar cells can be made from grades of silicon . .:
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having substantially higher impurity levels than in the extremely high purity semiconductor grade silicon heretofor employed for solar cell purposes~

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; Se~iconductor grade silicon from which high cost, . .
high purity single crystals are grown generally has a re-~ sistivity, measured as a manifestation of uncompensated charge carriersl of from 1 to 10 ohm~cm or higher, with the resulting single crystal wafers prior to junction formation having resistivities of on the order of 50 to 200 ohm-cm~
The present invention9 on the other hand, utilizes low-cost , 9 ;'.

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refined metallurgical products having resistivities in the range of from about 0004 to about 0.2 ohm-cm. While high cost, high purity single crystal wafers have very low impurity levels for each impur:ity element that are beyond the limits of detection by no~oal chemical analytical pro-cedures and are manifest by resistivity measurements, the low-cost refined metallurgical products of the present invention can have significantly higher impurity levels for various of said significant impurity elements while, nevertheless, having sufficient overall purity to serve as a practical solar cell substrate having a p-n junction grown on, or diffused therein, by known epitaxial or diffusion methods.
In the p-actice of the present invention, metallurgical grade silicon is refined but without con-version to semiconductor grade materialO Metallurgical grade silicon, as referred to herein, is a grade of silicon having a resistivity on the order of 0.005 ohm-cm and up to 3% iron, up to 0.75% aluminum~ up to 0.5% calcium and other impurities normally found in silicon produced by the carbothermic reduction of silica. It is also within the scope of the invention to employ a ferrosilicon material containing at least about 90% Si and up to 10% or more of iron. Such metallurgical grade silicon or ferrosilicon is processed, for purposes o the present invention, by solvent refining to reduce the iron content and other impurities after which the partially purified silicon is `~;

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melted in contact with acid silica slag with the resultant slag oxidation producing a reined metallurgical silicon (RMS) from which other products useful in solar cell applications are produced as described herein.
In the initial iron removal step, silicon plate-lets essentially free of iron are removed from a solution of metallurgical grade silicon in a liquid metal solvent.
While such metals as tin, zinc and silver may be employed, . ., - aluminum is the preerred solvent for use in the practice of the present invention. It will be understood that the term "aluminum" is intended to cover aluminum and alloys of aluminumi. Primary ingots of 99.5% to 99.9% purity and ! :-above may be used, with 99.9% nominal purity primary alumi- I
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- num ingot being preferred. Aluminum alloys are also gen-I erally suitable for purposes of the invention. One such I' alloy that has been used contains l~/o silicon and iron, 0.2% copper, 0.05% manganese and 0.1% zinc. Another such ~;
~- alloy contains 0.21% iron, 3% copper, 0.025% manganese, O~ 88% magnesium and 0.2% chromium. The solvent purity decreases, of course, with increasing cycles of fresh silicon charge dissolution ~herein.
;~ The temperature and aluminum (or other me~al)/
silicon ratio limits used in dissolving the metallurgical grade silicon are determined by the metal-silicon binary phase diagram. Employing aluminum as the metal solvent, - silicon must be removed by cooling the liquid solution I of silicon in molten aluminum to a temperature noc below .
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3~ 11,980 the eutectic temperature of about 577C, thus formlng a hypereutectic aluminum-silicon solution and causing silicon platelets to precipitat:e therefrom. At the eutectic temperature, ll.7% silicon is dissolved in aluminum. A homogeneous solution is formed at the high temperature dissolution point with up to 80% silicon in aluminum, preferably between 55% silicon at llOO~C and 20% silicon at about 700C, although temperatures closer to 600C may be feasible depending on the precision of , the silicon-aluminum separation procedures employed.
~' While silicon will commonly be added to molten aluminum or other liquid metal solvent, it is within the scope of the invention to add aluminum or other metal to molten silicon.
In cooling ~he solution, rapid cooling rates produce smaller precipitated silicon platelets than more . .
costly, slower cooling rates. The solution should preferably be cooled at the fastest rate that will produce platelets of acceptable purity and size. Platelets of 1/4"
or larger, suitable for ready separation from the silicon-aluminum melt by filtration, have been obtained at a cooling rate of 60C/hour, with no advantage having been obtained at slower cooling rates down to 20C/hour. The upper range of such cooling rates will likely depend on the geometry of the particular apparatus employed and will likely be in the order of about 200C/hour~ with heat ~`
transfer limitations possibly dictating the fastest cooling rates possible for large melts.
The solvent refining step of the invention may be carried out in any crucible that does not react with -12.
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, and/or contaminate the silicon-solvent metal alloy melt.
Carbon or graphite crucibles are preferred but other non-reactive materials, such as refractory oxides, can also be employed. A non oxidizing atmosphere, such as nitrogen, argon, helium or hydrogen-carbon monoxide mixtures can be employed to maximize the life of the crucible and related heating elements. While mechanical or other stirring is optional after the refined metallurgical silicon and metal solvent have been mixed, it is desirable to continue stirring during silicon dissolution, but stirring may be undesirable during silicon precipitation if it creates small platelet sizes such as to make platelet separation fro~ the melt difficult.
While the invention is herein described parti-cularly with reference to aluminum as the liquid metal solvent, it will be appreciated that the processing is applicable to embodiments in which other suitable sol-vents, such as those disclosed above, are employed, particular operating conditions, such as temperatures, metal/silicon ratios, cooling rates and the like being varied depending on the particular characteristics of any specific metal-silicon system.
For practical, commercial operations, the platelets will range generally from about 1/10 to 1 inch in size and will be removed from the solution of metal~
lurgical grade silicon in the aluminum or other liquid metal solvent while it is still molten. For this purpose, . , . 13- ~

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the entire melt can be filtered through a suitable filter, such as perforated carbon plates, quartz wool and stain-less steel screens. The partially purified, essentially , iron-free silicon platelets float on the melt and, in an alternate removal technique, can be scooped from the solu~ion melt surface as a semi-solid mass comprising said platelets and excess aluminum-silicon eutectic. In a generally preferred approach, this mass is subjected to centrifugal filtration to separate the partially purified silicon platelets from said excess eutectic material. The recovered essentially iron-free platelets will neverthe-less have some adherent impurities derived from the metal solvent associated therewith. For example, using a centrifugal filter operating at 400 rpm, silicon platelets having only about 9% aluminum can be obtained from a semi-solid mass of crystallized platelets and eutectic fed to said filtration step at an overall gross composition of approximately 50% aluminum/50% silicon. ;
The adherent material, e.g. aluminum-silicon eutectic, is advantageously removed from the filtered ;~ platelets before the partially purified platelets are contacted with silica slag to produce a refined metal-- lurgical silicon (RMS). In one embodiment of the inven-tion, such adherent impurities can be removed from ~he platelets by acid leaching, with any suitable aqueous ~ inorganic acid solution being employed in a wide range ` of suitable concentrations~ Aqueous HCl solutions in concentrations of about 4 to about 37% by weight are generally preferred. To assure complete washing, the ~; .
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' ~3S~ ,980 platelets are desirably crushecl and passe~ for example, through a 6 mesh screen, boilecl in concentrated aqueous `~;HCl for up to an hour, and finally washed in boiling water.
In an alternate embodiment, the adherent eutec-tic may be removed from the filtered platelets by washing the platele~s with anhydrous alcohols, e.g. isopropyl alcohol or amyl alcohol3 said alcohols reacting wlth the aluminum in said eutectic to form aluminum alkoxides, i.e Al(OR)3. The alkoxide has a resale value and, in a further embodiment, can be hydrolyzed to form hydrous aluminum oxide and the alcohol, which can be recycled for the washing of additional platelets. This embodiment thus represents a clean closed cycle process that may be less expensive than the acid leach referred to above even if a final HCl or other acid leach step were employed for the partially purified silicon material thus washed with said anhydrous alcohol.
Following the removal of adherent eutectic, the partially purified silicon will be essentially iron-free, having an iron content generally within the range of from about 3 to about 90 ppm, more typically, not exceeding about 20 ppm, i.e. parts per million parts by weight of said purified material. ~he aluminum content of said material will generally be in the range of from about 0.1 -to bout 0.5% by weight of said material, typically about 0.2% by weight. It is believed that the acid leaching or 15- ~
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, ~3~ 11,980 alcohol washing may effect some impurity removal beyond the removal of said eutectic. Iron removal in the eutectic removal operation would, of course, represent a fur~her ,; advantage - compensating in part for the cost of said step and its related waste acid environmental considerations The partially purified silicon platelets, essentially free of iron, are! thereafter melted in contact with a silica slag in a melting zone to remove residual and adherent impurities from the pla~elets. While the solvent refining step described above results in a drastic reduction in the iron and titanium content of metallurgical silicon, microscopic inclusions of aluminum eutectic remains within the platelets. The slag oxidation occurring upon contact of the partially purified plate-lets with said silica slag results in the removing of ..
residual and adherent impurities, including said aluminum : eutectic, that are detrimental to the production of an acceptable solar cell substrate material capable of achieving a satisfactory level of solar cell efficiency.
~0 It will be understood that the platelets thus exposed to slag oxidation include such platelets as treated by means such as those above, including crushing, for the removal ; of adherent impurities from the partially purified platelets.
The partially purified silicon is contacted - with said slag at temperatures above the melting point of silicon, e,g. at temperatures of from about 1410C to ." ~

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about 2000C. Lower temperatures, from about 1410C to about 1600C, are more efficient for oxidative removal of aluminum from silicon, wi~h temperatures of from about 1410C to about 1500C being generally preferred. Lower ; temperatures, e.g. down to about 750C, are less preferred.
In general, the slag mel~ing point should be less than the highest silicon melt temperature during slagging for rapid ; reaction kinetics.
~ In the practice of the slag oxidation step of : 10 the invention, silicon metal platelets are mixed with ~ high purity slag powder and the mixture is heated to the .~ i contact temperature range indicated above. Alternatively, high purity slag powder components, preferably at a temperature of from about 1,000-1200C, can be added directly to a melt of said silicon in a suitable crucible.
,~ Since the crucible must not contaminate the melt, most ~ metal and metal carbides cannot be used therein. Carbon, `~ silicon carbide, silicon nitride or silicon dioxide cru-; cibles or crucible coatings are generally preferred. Any atmosphere that protects the crucible and melt from oxidation, such as argon, helium or nitrogen, is desirable with nitrogen being preferred for economic reasons.
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The slag removes aluminum from the silicon, by ~ -oxidation, and dissolves the product A1203 with low resultant activity as is generally known in the art.
- For this purpose, high silica acid slags, i.e. slags in ! ~hich the Si/0 ratio lies between about 1/4 and about 1/2, J
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:', ' are employed. Since silicon reduces FeO and TiO2 impurities in the slag and introduces them, as Fe and Ti, into the silicon, it is necessary that the highest purity slag components be employed for advantageous results.
The amount of slag employed will generally be on the order of from about 8% to about 12% by weight based on the weight of silicon although larger amounts up to 20% or more may also be employed. A slag content of about 10% is ; preferred, although it will be understood that commercially available slags may exist and have utility for purposes of the invention but at concentrations varying somewhat from - those indicated herein. It is also within the scope of ... .
`~ the invention to employ multiple slag steps, e.g. at said ` 10% slag concentration, to progressively lower the impurity level of the silicon being treated.
The slag should desirably contain less than about 10 ppm of each impurity reducible by molten silicon at the slag-silicon contacting temperature. Such impurities include FeO, TiO2, Cr23~ B2O3 and V25 Any commercially available, high silica acid slag composition can be employed for the aluminum and other impurity removal purposes of the present invention.
Illustrative examples of such available slag compositions are those having 65-100% SiO2, 0-20~o MgO and 0-35% CaO, preferably 65% SiOz, 10% MgO and 25% CaO; ~hose having 45-55% SiO~ and 55-45% BaO, with 45% BaO/55% of SiO2 being preferred; ancl those having 50-85% SiO2, and 15-50~/~ Na2O, with 75% SiO2/25% Na2O being preferred.
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Enhanced slag-silicon contact speeds the i~ purification reactions, and, thus stirring is preferred although the process is operable in the absence of stirring.
Operable stirring rates are those that minimize melt splashing, such as rates of about 1-100 rpm.
After the slag oxidation reaction in the melting zone has removed residual and adherent impurities, par~icularly including aluminum, from the silicon~ the thus-slagged silicon is removed from the melting zone and cooled, as by chill casting. The resulting solidified product is a low-cost, multigrained refined metallurgical silicon (RMS) ., having a substantially higher impurity content level than ; in conventional, high purity semi-conductor grade silicon ~ but nevertheless constituting a valuable intermediate in .
the production of solar cell substrates.
In another embodiment of the invention, the silicon-slag melt is removed from the melting zone, following slag oxid~tion or without slag purifica~ion, in a unidirectional manner so as to directionally solidify the silicon. For this purpose, the silicon-slag melt is slowly withdrawn, ;
in its refining crucible, from the melting zone, as by slowly lowering the refining crucible from said zone.
For example, the invention has been practiced in operations in which the refining crucible has been slowly withdrawn from the melting zone at a rate of approximately one inch per hour. The slow withdrawal of the crucible results in the unidirectional solidification, or freezing, of the charge that initially is entirely molten. Such directional solidification results in the obtaining of solidified refined metallurgical siliron and a region of solidified melt having a high ooncentration of impurities rejected by the refined silicon a~ it solidifies. Thus, there is no ~ - .
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build-up of impurities at the solidification interface.
The impurities remain uniformlg distributed in the melt as silicon slowly solidifies in a unidirectional manner therefrom. The directionally solidified refined ., metallurgical silicon (DS/RMS) thus obtained is a low-cost multigrained silicon having a substantially higher impurity level than in conventional high purity semi-conductor grade silicon while, at the same time~ having suitable properties for desirable solar cell applications It will be appreciated that it is also within the scope of the invention to re-melt said DS/RMS and to direction-ally solidfy the silicon again to obtain further separation of refined silicon from a solidified melt of high impurity concentration The DS/RM~ can be mechanically separated from the solidified melt of the high impurity concentration, as by sawing, when the mass is cold. The solidified melt is waste material, the amount of which, in the embodiment in which slag oxidation of the partially purified platelets is omitted, considered in light of the simplified processing of this embodiment of the invention, provides an attractive trade-off in the development of low-cost solar cell materials.
Upon solidification of a silicon-slag melt, the interface between the slag and the silicon is not always well deined, resulting in the possibility of some material wastage. Upon directional solidification, the separation of silicon and slag is good. It has generally been found that when a quartz container is used to hold the reaction mixture, the slag is on the bottom and the silicon on the top, whereas the positioning appears to be reversed when a graphite crucible is employed. The ' : . .

l~L3~a3~1L 11, 980 DS/RMS having large columnar grains can readily be mechanically separated from the solidified melt having small grains of high impurity concentration when the mass is cold.
The RMS and DS/RMS multigrained products produced in accordance with the present invention have i:
resistivity values, using a 4-point probe method, within the range of from about 0.04 to about 0.2 ohm-cm, whereas semiconductor grade silicon has a resistivity of greater than lO ohm-cm. The resistivity is a manifestation only of uncompensated charge carriers. The resistivity is determined by the number of uncompensated impurity atoms present, and is thus a measure of the utility of the silicon that is different than the range of chemical composition of the silicon. The chemical composition i per se can be misleading since impurities of Group III
i' and Group V elements compensate each other electronically.
It will also be appreciated that the precise composition ~;; of any particular RMS or DS/RMS product will vary depending on a variety of the processing conditions set forth above, and on ~he composition of the metallurgic~l grade silicon starting material, the liquid metal solvent, the silica slag and various other factors. Because of all or a combination of these factors, the concentration of individual impurites in the product is subject to con-, siderable variation, the overall impurity level being such '~! that the low-cost, multigrained products of the invention , have suitable properties for desirable solar cell and other $ ::
silicon applications although said impurity level is '~ 21.

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substantially higher than in conventional high purity semiconduct~r grade material. Thus, the RMS product, ;~ having a resistivity of from about 0.04 to about 0.2 ohm-cm, typically about 0 1 to about 0.2 ohm-cm~ may have impurity concentrations of up to, but not generally exceeding9 about 15 ppm of aluminum, about 80 ppm of iron, about 15 ppm of titanium, about 2 ppm of boron, about 2 ppm of phosphorus~ about 10 ppm of chromium, about 100 ppm of calcium and about 65 ppm of magnesium. In parti-cular embodiments of the invention, various impurities ~ may have a concentration considerably below these limits ; and, it will be appreciated, concentrations of individual impurities may somewhat exceed the limits specifically set forth without significant deviation from the scope of the invention. Thus, aluminum and iron contents of on the order of 10 ppm have been obtained particularly with a calcium-magnesium type slag. While the use of a sodium- .
type slag has resulted in an aluminum content of about 5 ppm, the corresponding iron level was about 80 ppm.
Likewise, calcium and magnesium levels of about 100 ppm ~;~ and 63 ppm, respectively, were obtained when a calcium-magnesium slag was employed, with such levels being about 1 ppm and 4 ppm, respectively, when a sodium type slag was employed. These examples will serve to illustrate the innumerab:Le variations in composition obtainable for ~i ~ the RMS products of the invention within the overall `I composition levels and rèsistivity values indicated above.

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It will be appreciated that the term "ppm" as used herein shall mean parts per million by weight unless otherwise noted.
Likewise, the DS/RMS products of the invention have been found to have resistivity of from about 0.04 to about 0.2 ohm-cm, typically from about 0.l to about 0.2 ohm-cm. Such products may have impurity concentrations of up to, but not generally exceeding, about l ppm of aluminum, about 1 ppm of iron, about 2 ppm of ti~anium, about 2 ppm of ~e*n, about 2 ppm of phosphorous, about l ppm chromium, about l ppm of calcium and about l ppm of magnesiZ~m. Once again, it should be noted that various individual impurities may depart somewhat from the specific lLmits indicated without depar~ure from the scope of the invention. It should also be noted that, although the ;;11 impurity levels from DS/RMS are lower than for RMSJ the!1 resistivity values are within the same general range, ,~lZ~ denoting that the number of uncompensated impurity atoms 1~ was approximately the same at the different impurity "J 20 concentration levels. As indicated above, semiconductor ~ i , grade material will have resistivities of up to l0 ohm-cm `~ or more, together with very low concentration levels beyond the limits of detection for each impurity. The RMS and ,~ ~ DS/RMS of the present invention, therefore, represent relatively Lmpure grades of silicon having, nevertheless, !'~
-~ desirable utility in the production of solar cell substrates and for other known silicon applications, such as in 23.

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rectifiers, where the purity requirements are less restrictive than in transistor uses and specialized solar cell applications in which high purity silicon made from semiconductor grade material is required regardless of the cost thereof.
.
The DS/RM~ pro~ucts with4ut alagging have been found to have a resistivity of from about O.G* to about 0,2 ohm-cm. Such products may have impurity concentra-tions of up to, but not generally exceeding, about 15 ppm of aluminum, about 40 ppm of iron, about 1 pp~ of titanium, about 2 ppm of boron, about 2 ppm of phos-~; phorus~ about 0.5 ppm chromium, about 0.5 ppm of calcium and about 0.5 ppm of magnesium. It should be noted that various individual impurities may depart somewhat from the specific limits indicated without departure of the product from the scope of the invention. In addition, it will be appreciated that various impurities may be present in concentrations considerably below the indicated limits, as innumerable variations in composition are ` 20 obtainable within the overall composition levels and resistivity values indicated herein. As indicated - above, semiconductor grade material will have resistivi-,~, ties of greater than 10 ohm-cm together with concen-tration levels of less than 1 ppm for each impurity.
The DS/RM~ of the present invention, therefore, repre-sent relatively impure grades of silicon having, never-~heless, desirable utility in the production of solar cell substrates ~nd for other known silicon applications, such as in rectifiers, where the purity requirements are less restrictive than in transistor uses and specialized soIar cell applications in which high purity silicon made from semiconductor grade material is required regardless of the cost thereof.

-2~-11j980 11~5~?~
:' The RMS and DS/RMS materials as described above ., .
can be further refined by the well known Czochralski pulling technique to produce desirable solar cell substrate materials. According to this technique, boules of further refined metallurgical silicon can be pulled, on a rotating silicon seed rod, from a melt of said RMS and DS/RMS materials For this purpose, the seed rod is slowly moved, i.e~ lif~ed, while maintaining an interface between the rod with said boule being grown t~ reon and the molten silicon~ i.e. RMS or DS/RMS, from which said boules are pulled. In the application of this known tech-nique for purposes of the present invention, boules of refined crystals have been grown employing a seed rod movement of about 3.6 inches per hour, employing temperature ."
;j gradients and seed rotation rates in combination therewith, consistent with the features of the particular boule pullin : .
; appara~us employed The resulting boules pulled from an ';'I
RMS melt, i.e. a Cz-RMS matarial, are low-cost, multigrained materials of sufficient purity so that wafers cut therefrom :i ,~~ 20 achieve an acceptable level of solar cell afficiency.
Multigrained Cz-DS/RMS material can be obtained by the pulling of boules from a DS/RMS material prepared as indicated above. Somewhat :`

'1 . .................................................................... ~ , : : -. ~
. :
24-a-~ .

~ 11,9~0 ~.
',~
higher solar cell efficiencies are obtainable with this -~ material than with corresponding Cz-RMS material. It is also within the scope of the inventi.on to separate the DS/RMS
material, as originally prepared, from the region of solidified melt, to then re-melt said DS/RMS and to remove the resulting melt slowly from the melting zone, in a uni-directional manner, so as to directionally solidify the silicon a second time. A DS/RMS of enhanced purity is thus obtained and may be separated from a region of solidified melt having a high concentration of ~he remain-ing impurities therein. By pulling boules from this DS/RMS
material, low-cost single crystal silicon is obtained.
- While said material, and the multigrained Cz-RMS and Cz-DS/RMS materials indicated above, has a substantially higher impurity level than conventional, high purity semiconductor grade material, it nevertheless has suffi-!
t~~ cient purity to achieve very acceptable solar cell efficiencies when employed, in wafer form, as the solar cell substrate material. It will be appreciated that various processing techniques will be employed by those performing the Czochralski pulling form of crystal growth on a commercial basis to enhance the efficiency of said technique, Such techniques are not within the scope of the present invention since the pulling of boules as provided herein is a conventional step apart from its use in combination with the other features of the invention for ~he production of low-cost refined metallurgical silicon products. The Czochralski technique, which was 25.

, ~

~ 3 ~ , 11,980 ..~

-~ first developed in about 1917 with regard to the with-; drawal of seeds of Pb, Sn and Zn from the melt, is .~ described in "Silicon Semiconductor Technology" by W.R.
~; Runyan (McGraw-Hill), pp. 34-39 and in "The Growth of ,..
~; Single Crystals" by R.A. Laudise (Prentice-Hall) (1970), .- pp. 174-176.
.
~i~ The multigrained Cz-RMS product of the invention ,.,. i will have a resistivity value of from about 0.1 to about 0.2 ohm-cm, with impurity contents of up to, but not 10 generally exceeding about 1 ppm of aluminum, about 1 ppm of iron, about 0.5 ppm of titanium, about 2 ppm of boron, .... ~ about 2 ppm of phosphorous, about 0.5 ppm of chromium, ... ;, .
about 0.5 ppm of calcium, and about 0.5 ppm of magnesium, The multi~rained Cz-DStRMS product will have a resistivity value of about 0.04 to about 0.2 ohm-cm, with impurity concentrations of up to, but not generally excee~ing, about 002 ppm of aluminum, about 0.15 ppm of iron, about 0.03 ppm of titanium, about 1.5 ppm of boron, about 0.2 ppm of phosphorus, about 0.02 ppm of chromium, about 0.7 ppm of calcium and about 0.05 ppm of magnesium. The single crystal Cz-DS/RMS will have a resistivity value of from about 0.05 to about 0.2 ohm-cm, with impurity concentrations of from about 0.2 ppm of aluminum, about ~ ,,;j .
~ 0.15 ppm of iron, about 0.02 ppm of titanium, about 1.5 ~.
:;1 ^' ppm of boron, about 0.15 ppm of phosphorus, about 0.01 ;. ppm of chromium, about 0.7 ppm of calcium and about 0.17 ppm of magnesium. It should be noted that~ although the , 26. ~ -~

~ ,~

: ; ~

~S~ 98o crystal pulling step is a further refinement of the RMS
and DS/RMS materials, the resistivity levels obtained are not significantly differen~ and may appear to represent a more Lmpure product despite the reduc~ion in impurity concentrations achieved by said Czochralski-pulling technlque. As indicated above~ this circumstance results from the fact that the resistivity is a manifestation only of uncompensated charge carriers, with a decrease in the impurity levels of the various impurities actually resulting, in some instances, in an increase in the number of such ... .
uncom~ensated charge carriers.
The invention is further illustrated 9 but not limited, by the following specific examples:
Example 1 Four hundred grams of silicon metal containing 1.25% Fe and 0.48% aluminum were dissolved in 865 grams of molten aluminum alloy solvent, said aluminum containing 0.2% Cr. 0.05% Mn and 0.1% Zn, at 1050~C under a protective nitrogen atmosphere in a graphite crucible. The homogeneous Al/Si solution was cooled from 1050C to 690C at the rate of 60C/hr., thereby precipitating partially purified silicon platelets. The platelets were separated from the molten aluminum by filtration through a quar~z wool filter medium. The recovered platelets were wash~d in an aqueous HCl ~olution to remove adherent Al/Si eutectic msterial and crushed through a 6 mesh screen. The 6-20 mesh fraction was boiled with concentrated HCl and washed ~`

27.

., ~ 11,980 ~. .
with boiling water. Emission spectrographic analysis showed an aluminum content of 1,000-1500 ppm and an iron .
content of 20-50 ppm in the recovered platelets. One thousand grams of silicon pla~elets from several such preparations were combined and melted in a graphite crucible at 1500C. An acid silic slag having 25% CaO, 10% MgO and 65% SiO2 in an amount of about: 20% by weight was added to the silicon melt, and the mixture was periodically hand stirred using a graphite rod over a total contact time of 40 minutes. The purified silicon melt was chill case in a graphite crucible in air, yielding 850 grams of multi-, grained refined metallurgical Silicon (RMS). Elemental analysis showed that said RMS contained about 9 ppm of aluminum, about 13 ppm of iron, about 2 ppm of titanium, about 3 ppm of chromium, about 100 ppm of calcium and about 63 ppm of magnesium. Boron and phosphorus values were not obtained. The resistivity of the RM~ was from about 0.1 to about 0.2 ohm-cm The RMS was employed as a useful, low-cost intermediate from a melt of which boules of further , ., ` refined material was pulled on a rotating silicon seed ~ ~ .
rod utilizing the well-known Czochralski pulling technique.
According to this known technique, the seed rod was slowly moved while maintaining an interface between the rod with said boule being grown thereon and the molten silicon rom which the boules were pulled~ Movements of the seed rod was at about 3.5 inches per hour, with the diameter of the 28.
~:, ~ .

,~

~ 11,980 ,' .
boules being about 2 inches. The resulting Czochralski-pulled mat~rials, i.e. Cz-RMS 5 was a multigrained silicon having a resistivity of from about 0.1 to about 0.2 ohm-cm, having impurity contents of a~out 1 ppm of aluminu~, about 1.1 ppm of iron, less than about 0.5 ppm of titanium, about 0.5 ppm of chromium, less than 0.5 ppm of calcium and less than about 0.5 ppm of magnesium.
The Cz-RMS materials thus produced was used as active and passive solar cell substrate material in the production of planar diodes and related solar cell struc-tures. Thus, multigrained p-and n-type epitaxial layers were grown on wafers of said Cz-RMS as a passive silicon substrate in the formation of n-on-p-on-p substrate planar diodes and related solar cells by conventional techniques known in the art. Similarly, the Cz-RMS material was employed, in the production of planar diodes and solar cells 9 as a p-type multigrained silicon substrate into one side of which pentavalent n-type impurities, e.g.
phosphene, was diffused to form a p-n junction by known techniques.
Solar cell efficiencies, measured under Air Mass One conditions, i.e. at 97 milliwatts per centimeter squared, for the passive substrate cell havin~ said epitaxial layers grown thereon, of over 9% were obtained, e~gO 9.2% and 9~4%. Efficiencies of greater than 8% were likewise obtained with solar cells in which said Cz-RMS serves as an active substrate having a p-n junction diffused thereinO

29.

~ 11,980 '~

:' Example 2 One thousand grams of metallurgical grade silicon containing 0.32% Fe and O.21V/o aluminum was dissolved in 3,000 grams of the molten aluminum solvent used in Example 1 at 950C. in a graphite crucible having a protective nitrogen atmosphere. The resulting melt was cooled at 60C/hour to 710C, thereby precipitating relatively pure silicon platelets. The partially purified product was scooped from the melt sur~ace using a perfor-ated graphite bucket and filtered through a quartz wool filter medium. Following washing in aqueous HCl to remove adhering eutectic material as in Example 1, the resulting 6-20 mesh platelet fraction was shown, by emission spectro-graphic analysis, to contain more than 2,000 ppm of alumi-num and less than 30 ppm `of iron. A melt of said platelets was contacted, at 1500C, with an acid silica slag, containing 45~/O BaO and 55% SiO2, amounting to 20% by weight of the silicon melt. Contact time was about 30 minutes in a quartz container. The resulting multigrained RMS was found, upon elemental analysis, to contain about 15 ppm of aluminum, about 46 ppm of iron, about 14 ppm ~f titanium, -about 1.6 ppm of chromium, about L5ppm of calcium and about 108 ppm of magnesium. Boron and phosphorus were not measured. ~ ~
The resistivity of the material was about 0.1-to about 0.2 ~ `
ohm-cm. The RMS can be effectively employed as a low-cost intermediate in the production of C~-RMS as indicated above, with s~id Cz-RMS being useful as a substrate for the production of epitaxial and/or diffusion type planar diodes 30.

:::

~3~ 11,980 ,., and corresponding solar cells having adequate solar cell efficiencies to merit consideration for practical commercial solar cell applications Exam~ 3 ~` By means of processing similar to that describedin the examples above, RMS has been made from acid silica slag containing 25~/~ Na20 and 75% SiO2. The resultant RMS
product is a low cost, multigrained silicon having a resistivity of from about 0.1 ~o about 0,2 ohm-cm, with impurity concentrations of about 5.4 ppm of aluminum, about 80 ppm of iron, about 7 ppm of titanium, abou~ 8.8 ppm of chromium, about 1.3 ppm of calcium and about 3.8 ppm of magnesium. Boron and phosphorus contents were not ;~
measured. The product R*LS can likewise be used in the production of Cz-RMS suitable for use as low-cost substra~e material in epitaxial and diffusion t~pe solar cell structures capable of achieving acceptable solar cell - efficiencies.
Example 4 Metallurgical grade silicon containing 1.25% iron :
and 0.48% aluminum is dissolved in aluminum alloy solvent containing 0.~% Cr, 0.05% Mn and 0.1% Zn at 1050C. The solution, containing 35% Si and 65% Al, is cooled to 710C -at the rate of 60C/hour, precipitating partially purified silicon platelets. Following washing with an aqueous HCl i, -.
solution, said platelets are crushed to a 6-20 mesh size and melted in contact with a 45% BaOi55% SiO2 slag 31.

~ ' :` :

~3~ 11,980 ., employed in an amount of about 10% by weight of silicon.
Upon contact in the melting zone for about 45 minutes at 1450C, the melt is slowly removed from the melting zone in a unidirectional manner at ~ rate of about 6 inches in a six hour time period, causing the directional solidifi-ca~ion of the refined silicon material and with a separate region of solidified melt having a high concentration of impurities r~jected by the refined metallurgical silicon as it solidifies. After cropping the top containing said region of solidified melt, the resulting product is a low-cost, multigrained directionally solidified refined metal-lurgical silicon (DS/~MS) having suitable properties for solar cell applications. Such DS/RMS has a resistivity level of from about 0.04 to about 0.2~ typically about 0.1 to about 0.2, ohm-cm and impurity concentrations of less than 1 ppm aluminum and iron, less than 2 ppm of titanium, less than 5, typically about 3, ppm of phosphorus, less than 1 ppm of chromium, less than 1 ppm of calcium and less than 1 ppm of magnesi~m. The boron content was not measured.

In the practice of the invention, metal- I
lurgical grade silicon containing 1.25% iron and 0.48%
aluminum is dissolved in alumLnum alloy solvent con-taining 0.2% Cr~ 0.05% Mn and 0.1% Zn at 1050C. The solution, containing 35% Si and 65% Al, is cooled to 710C at the rate of 60C/hour, precipitating partially purified sil:Lcon platelets. Following washing with an aqueous HCl solution, said platelets are crushed to a 6-20 mesh sixe and melted in a melti~g zinc. The melt ,:

11,980 ~3~
..~
is slowly removed from the melting zone in a unidirec-tional manner by lowering the melting crucible at a ra~e of about ~ i~ches in a six hour time period, causing the direc~ional solidification of the refined silicon mater-ial and the forming of a separate region of solidified melt having a high concentration of impurities rejected by the refined metallurgical silicon as i~ solidifies.
After cropping the top containing said region of solidi-fied melt, the product obtained is a low-cost, multigrained directionally solidified refined metallurgical silicon (DS/RMS) having suitable properties for solar cell applications. Such DS/RMS has a resistivity level of from about 0.05 ohm-cm and impurity conce~trations of about 12.1 ppm of aluminum, about 34 ppm or iron, less thsn 1 ppm of titanium~ less than about 2 ppm of phos-phorus, less than 0.5 ppm of chromium, less ~han 0.5 ppm of calcium and less than 0.3 pp~ of magr.esium.
The boron content of this sample was not measured.
The DS/RMS product can be amployed as a solar cell substrate mater;al directly, or can be further refined as by the above-indicated Czochralski-type crystal growth to produce both multigrained and single crystal Cz-DS/RMS
material, All such materials can be employed as passive substrate wafers upon which epitaxial p-and n-type layers can be grown to form n-on-p-on-p substrate planar diodes and solar cells by conventional techniques. Said material '' ~
. .

~ -32-a-:;

~ ~ 3 ~ 11,980 i .
~.!, can also be employed as active p-type substrates upon ` one side of which pentavalent n-type impurities, such as .. phosphene can be diffused to form a p-n junction therein by conventional means known in the art. The single crystal Cz-DS/RMS material can readily be formed by re-melting and .. re-directionally solidifying F;aid DS/RMS before the pulling. of boules therefrom or, less clesirably, by re-melting and re-pulling boules from a melt of the boules of multigrained DS/RMS.
. .
. 10 DS/RM~ materials prepared as indicated above ^
were melted and boules of further refined material were pulled from the melt by the Czochralski technique, using a seed pull rate of about 3.5 inches/hour. A multigrained, Cz-DS/RMS prepared in this manner had a resistivity of on the order of 0.05 ohm-cm, with impurity concentrations of :
. about 0.19 ppm of aluminum, about O.ll ppm of iron, about :- :
.` 0.03 ppm of titanium, about 1.1 ppm of boron, about 0.16 - ~:
ppm of phosphorus., about 0.02 ppm of chromium, about 0.64 .. i ppm of calcium and about 0.05 ppm of magnesium. Multigrained : 20 DS/RMS was remelted for a second generation Cz-pull, with ;;
boules being pulled from the remelt under the same conditions as in the first boule pulling step. A single crystal, Cz- :
: DS/RMS was obtained in this manner, said material having a resistivity of about 0.05 ohm-cm, with impurity concentra- :
tions of about O.l9 ppm of aluminum, about 0.11 ppm of iron, -~ ~ ~ about 0.02 ppm of titanium~ about 1.1 ppm of boron~ about :
!:~ 0.12 ppm of phosphorus, about 0.01 ppm of chromium, about 0.64 ppm of calcium and about 0.16 ppm o magnesium. Similar ~: ~ 33, :
. ~ , ', .. , ,' ~3~ 980 impurity levels are obtainable by the more commercially preferred technique in which the RMS is directionally solidified a second time prior to the pulling of single crystal boules from a melt of said twice directionally solidified RMS.
Solar cell efficiencies of about 8~9~/o have been obtained using the directionally solidified refined, metallurgical silicon (DS/RMS) of the present invention as a passive subscrate having epitaxial p-and n-type layers grown thereon by conventional means. Efficiencies of 10.6% for a passive substrate and 9.6% for an active substrate have been obtained utilizing a single crystal, pulled DS/RMS material. Using a multigrained, pulled DS/RMS as a passive substrate, solar cell efficiencies of up to 9. 6~/o have been obtained.
It will be understood that all solar efficiencies are on said Air Mass One basis. It will also be appreciated that the claimed melting of partially purified platelets in contact with said silica slag includes also the contacting of a melt of said platelets with the slag to effect the desired removal of aluminum and other impurities as indi-cated in the examples. It is also pointed out that the silicon products will likely include impurities other than those specifically recited, the latter group being selected on the basis of their general significance to the functioning of the materials for solar cell applications and as distin-guishing the products of the invention from those of the prior art. While boron and, in some specific examples, phosphorus were not measured, these impurities were ,:

" , , . ,: : , , - , 11,980 ~3~

., included as being impurity components having some measure of efect on the performance of the products of the inven-tion. The boron and phosphorus levels recited for said impurities have been determirled on the basis of the anal-ysis of certain specific samples of the silicon products of the invention and related general information concerning the concent$ation of such si].icons in silicon m~terials, In a further embodirnent of the invention, it has been found that the refined metallur&ical silicon of the invention can be employed to produce a low-cost, multigrained ribbon having suitable properties for desirable solar cell and other silicon ribbon applica-tions. Such r~bbon is advantageously pulled from the directionally solidified, multigrained refined metal-lurgical silicon, i.eO DS/RMS, produced as indicated above. Such rlbbon, when employed as a passive solar cell substrate, with p-and n-type epitaxial layers grown thereon by known techniques to form an n-on-p-on-p substrate structure has provided solar cell efficiencies on the order of 8%. By contrast~ ribbons pulled from high cost~ high purity semiconductor material have achieved about l~/o efficiency, while ribbons pulled from metallurgical silicon have not achieved more than about 5% efficiency. The ribbon formed from DS/RMS
:i produced by the novel process of the invention thus offers an attractive, alternative form of solar cell substrate ~aterial9 fl~rther enhancing the feasi~ility of low-cost, practical solar cell applications establi~hed hereinabove with respect to the indicated RM~ and DS/RMS
products of the invention , .
., 11,980 3L~3~
.
The multigrained silicon ribbon of the invention is pulled from a melt of DS/RMLS which is prepared by the novel process of the inventi~n in which iron is first removed from metallurgical grade silicon by a solvent refining technique, aluminum and other impurities are removed from the essentially lron-free intermediate product by a slag oxidation method, and the silicon is slowly solidifed in a unidirectional manner to form said DS/RM~ and a region of solidified melt having a high concentration of the impurities rejected by the refined silicon as it solidifies in a unidirectional manner.

The ribbon can be pulled from the melt of DS/RMS by any known, commercially available ribbon pulling technique. The Edge-Defined, Film-Fed Growth (EFG) process is a convenient, commercially established technique suitable for the ribbon pulling step of the present invention. The process consists of growing .~
crystals of predetermined shape and dimensions from a film of the melt whose shape is determined by a "die"
through the apertures of which the melt is fed by capillary from a reservoir surrounding the lower part ~; of the die. Details of this well-known ribbon pulling technique are outside the scope of the present invention which is related to the unique properties of the ribbon -~
as derived from the novel DS/RMS starting material employed in the production of said ribbon. The EFG
process is further described in a paper entitled "The ~- S~licon Ribbon Solar Cell - A Way To Harness Solar Energy" - K.V. Ravi and A.I 9 Mlavsky, of Mobil Tyco Solar Energy Corporation, presented in November, 1975 at the COMPLE~S International Meeting in Dhahran, Saudi Arabia. Other ribbon-forming techniques are also known and available in the art.
.:
-35-a-' , - , . . . . . . .

S~

; The multigrained ribbon of the invention was prepar~d by the EFG process from multigrained DS/RMS
having a resistivity of about 0,04-0.05, and having an impurity content of about 0.19 ppm of aluminum, about 0,11 ppm of iron, about 0.03 ppm of tîtanium, about 1,1 ppm of boron, about 0.16 ppm of phosphorus about 0.02 ppm of chromium, about 0.64 ppm of calcium and about 0.05 ppm of magnesium. By kno~l techniques, epitaxial p-and n-type layers were gro~l on said rlbbon having a thi.ckness of on the order of 0.004" to fonm a n-on-p-on-p substrate structure. The p-layer was about 25 microns thick and had a resistivity of about 0~2 ohm-cm. The n-layer had a thickness of about 0,5 microns and had a resistivity of about 0.015 ohm-cm. An AMI efficiency of about 7.9% was obtained, said ribbon having an area of about 0.6cm2. It will be understood that said ribbon can be made from any such DS/RMS material prsduced as disclosed above, ~ith said ribbon being pulled by the EFG process or any other known, commercially available ribbon pulling process suitable for the preparation of silicon ribbon.
It will be appreciated also that the concen-trations of impurities in the DS/~MS are subject to wide fluctuation depending upon the precise materials and conditions emp}oyed ln the processing steps by w~iich the DS/RMS is prepared. The DS/RMS material and the resul-tant ribbon will be understood to include such mater~al hsving ~ res~stivity of from about 0,04 to ab~ut 0.2 ohm-cm, and b~aving impurity concentrations of up to, ; but not generally exceeding, about 1 ppm of aluminum, about 1 ppm of ~ron, about 2 ppm of titanium, about 2 ppm of boron, about 2 ppm of phosphorus, about 1 ppm of chromium, about 1 ppm of calcium and ab~ut 1 ppm of magnesium. It will also be appreciated th~t specific -35-~-.

11,980 3 ~P
. .

ribbon compositions may have individual impurity com=
ponent levels somewhat above the general limits set forth without departing from the scope of the novel ribbon materials disclosed and claimed herein The present invention provides a novel process for the production of refined metallurgical silicons having a desirable combination of 1O1w-cost and a purity level such that adequate solar efficiencies can be obtained thereby.
While the products of the invention are relatively impure as compared to semiconductor grade material, they provide an attractive economic-technical alternative to the use of such high cost, high purity material. Thus, single crystal Cz-DS/RMS material was shown to achieve 10/6% efficiency as a passive substrate. Significantly, multigrained Cz-DS/RMS
achieved over 9.0% efficiency, e.g 9.2 and 9.4%, and a DS/RMS, without crystal pulling achieved an afficiency of -~ 8.9%. Adequate solar cell eficiencies can also be achieva with low-cost active substrates made from the DS/RMS product ; of the present invention and from Cz-modiications of said ; 20 DS/RMS and the RMS of the present invention.
The present invention thus constitute~ a sig-nificant advance in the field of solar energy technology.
It represents, therefore, a major step in the urgently needed development of solar energy as a commercially feasible, non-polluting e~ergy source for satisfying sig-! _ nificant portions of the energy needs of industrial societies throughout the world.

35-c .~ ~
, . ~ : .

Claims (24)

No. 11980-C

WHAT IS CLAIMED IS:

1. A process for the production of low-cost multi-grained refined metallurgical silicon from metallurgical grade silicon comprising:
a. dissolving metallurgical grade silicon in a liquid metal solvent to form a metal solvent-silicon solution;
b. cooling said solution to a temperature at which silicon platelets form;
c. removing said silicon platelets from said solution of metallurgical grade silicon in a liquid metal solvent, thereby recovering partially purified, essentially iron-free silicon platelets with adherent impurities derived from said metal solvent;
d. melting said partially purified silicon platelets in contact with acid silica slag in a melting zone, the resulting slag oxidation removing residual and adherent impurities from said silicon; and e. pulling refined metallurgical silicon boules from a melt of the thus-refined silicon on a rotating silicon seed rod by the Czochralski pulling technique, the resulting pulled, refined metallurgical silicon comprising a multigrained silicon having a resistivity of from about 0.1 to about 0.2 ohm-cm, said silicon having impurity concentrations of up to, but not exceeding about 1 ppm of aluminum, about 1 ppm of iron, about 0.5 ppm of titanium, about 2 ppm of boron, about 2 ppm of phosphorus, about 0.5 ppm of chromium, about 0.5 ppm of calcium and about 0.5 ppm of magnesium, whereby said multigrained silicon is obtained at a substantially higher impurity 36 ::

No. 11980-C

level than in a conventional high purity semiconductor grade silicon and has suitable properties for solar cell applications.

2. The process of claim 1 in which said liquid metal solvent is aluminum or an aluminum alloy and said seed rod is slowly moved while maintaining an interface between the rod with said boule being grown thereon and the molten silicon from which said boules are pulled.

3. The process of claim 2 in which said cooling and removing of silicon platelets from solution comprises cool-ing the liquid solution of silicon in molten aluminum to a temperature not below the eutectic temperature of about 577° C., thus forming a supersaturated hypereutectic aluminum-silicon solution and causing silicon platelets to precipitate therefrom.

4. The process of claim 3 and including filtering said platelets from said aluminum-silicon solution.

5. The process of claim 3 and including (a) separat-ing a semi-solid mass comprising said platelets and excess aluminum-silicon eutectic from the solution melt, and (b) subjecting said mass to centrifugal filtration to separate the silicon platelets from said excess eutectic.

6. The process of claim 5 and including removing adherent aluminum-silicon eutectic from the filtered platelets.

7. The process of claim 6 in which said partially purified platelets are contacted with said silica slag at a temperature of from about 1410° C. to about 2000° C., said slag having an Si/O ratio of between about 1/4 to about 1/2, said slag being employed in an amount within the range No. 11980-C

of from about 8% to about 12% by weight based on the weight of silicon platelets melted in contact therewith.

8. The process of claim 7 in which said slag is of high purity, the concentration of each impurity in the slag reducible by molten silica at the silicon platelet-silica slag contacting temperature being less than about 10 ppm.

9. A process for the production of low-cost, multi-grained refined metallurgical silicon from metallurgical grade silicon comprising:
a. dissolving metallurgical grade silicon in a liquid metal solvent to form a metal solvent-silicon solution;
b. cooling said solution to a temperature at which silicon platelets form;
c. removing said silicon platelets from said solution of metallurgical grade silicon in a liquid metal solvent, thereby recovering partially purified, essentially iron-free silicon platelets with adherent impurities derived from said metal solvent;
d. melting said partially purified silicon platelets in contact with acid silica slag in a melting zone, the resulting slag oxidation removing residual and adherent impurities from said silicon;
e. removing the said silicon-slag melt in its refin-ing crucible from said melting zone so as to directionally solidify said silicon, said directional solidification resulting in the obtaining of solidified refined metallur-gical silicon and a region of solidified melt having a high concentration of impurities rejected by said silicon;
f. melting said directionally solidified refined metallurgical silicon;

No. 11980-C

g. pulling refined metallurgical silicon boules from a melt of said directionally solidified refined metallurgical silicon on a rotating silicon seed rod by the Czochralski pulling technique, the resulting pulled refined metallurgical silicon comprising a multigrained silicon having a resistivity of from about 0.04 to about 0.02 ohm-cm, said silicon having impurity concentrations of up to, but not exceeding, about 0.2 ppm of aluminum, about 0.15 ppm of iron, about 0.03 ppm of titanium, about 1.5 ppm of boron, about 0.2 ppm of phosphorus, about 0.02 ppm of chromium, about 0.7 ppm of calcium, and about 0.05 ppm of magnesium, whereby said multigrained refined metallurgical silicon is obtained at a substantially higher impurity level than in a conventional high purity semiconductor grade silicon and has suitable proper-ties for solar cell applications.

10. The process of claim 9 in which said liquid metal solvent is aluminum, and the cooling and removing of silicon platelets from solution comprises cooling the liquid solution of silicon in molten aluminum to a tem-perature not below the eutectic temperature of about 577° C., thus forming a supersaturated hypereutectic aluminum-silicon solution and causing silicon platelets to precipitate therefrom.

11. The process of claim 10 and including filtering said platelets from said aluminum-silicon solution.

12. The process of claim 10 and including separating a semi-solid mass comprising said platelets and excess aluminum-silicon eutectic from said solution melt, and (b) subjecting said mass to centrifugal filtration to separate the silicon platelets from said excess eutectic.

13. The process of claim 12 and including removing adherent eutectic from the filtered platelets.

14. The process of claim 13 in which said partially purified platelets are contacted with said silica slag at a temperature of about 1410° C. to about 2000° C., said slag having an Si/O ratio of between about 1/2, and about 1/2, said slag being employed in an amount within the range of from about 8% to about 12% by weight based on the weight of silicon platelets melted in contact therewith.

15. The process of claim 14 in which said slag is of high purity, the concentration of each impurity in the slag reducible by molten silicon at the silicon platelet-silica slag contacting temperature being less than about 10 ppm.

16. A process for the production of low-cost, refined metallurgical silicon from metallurgical grade silicon comprising:
a. dissolving metallurgical grade silicon in a liquid metal solvent to form a metal solvent-silicon solution;
b. cooling said solution to a temperature at which silicon platelets form;
c. removing said silicon platelets from said solution of metallurgical grade silicon in a liquid metal solvent, thereby recovering partially purified, essentially iron-free silicon platelets with adherent impurities derived from said metal solvent;
d. melting said partially purified silicon platelets No. 11980-C

in contact with acid silica slag in a melting zone, the resulting slag oxidation recovering residual and adherent impurities from said silicon;
e. removing the silicon-slag melt in its refining available from said melting zone so as to directionally solidify said silicon, said directional solidification resulting in the obtaining of solidified refined metallur-gical silicon and a region of solidified melt having a high concentration of impurities;
f. re-melting said directionally solidified refined metallurgical silicon after the separation thereof from said region of solidified melt;
g. removing the resulting silicon melt from said melting zone so as to directionally solidify said silicon a second time, said directional solidification resulting in the obtainment of a solidified refined metallurgical silicon of enhanced purity and a region of solidified melt having a high concentration of the remaining impuri-ties.
h. melting said directionally solidified refined metallurgical silicon of enhanced purity; and i. pulling silicon boules from said melt on a rotating seed rod by the Czochralski pulling technique, the resulting pulled refined metallurgical silicon com-prising a single crystal silicon having a resistivity of from about 0.05 to about 0.2 ohm-cm, said silicon having impurity concentrations of up to, but not exceeding, about 0.2 ppm of aluminum, about 0.15 ppm of iron, about 0.02 ppm of titanium, about 1.5 ppm of boron, about 0.15 of phosphorus, about 0.01 ppm of chromium, about 0.7 No. 11980-C

ppm of calcium and about 0.17 ppm of magnesium, whereby single crystal silicon is obtained at a substan-tially higher impurity level than in a conventional high purity semiconductor grade silicon and has suitable properties for solar cell applications.

17. The process of claim 16 in which said liquid metal solvent is an aluminum alloy.

18. The process of claim 17 in which said cooling and removing of silicon platelets from solution comprises cooling the liquid solution of silicon in molten alumi-num to a temperature not below the eutectic temperature of about 577° C., thus forming a supersaturated hypereu-tectic aluminum-silicon solution and causing silicon platelets to precipitate therefrom.

19. The process of claim 18 and including filtering said platelets from said aluminum-silicon solution.

20. The process of claim 18 and including (a) separating a semi-solid mass comprising said platelets and excess aluminum-silicon eutectic from the solution melt, and (b) subjecting said mass to centrifugal fil-tration to separate the silicon platelets from said excess eutectic.

21. The process of claim 20 and including removing adherent aluminum-silicon eutectic from the filtered platelets.

22. The process of claim 21 in which said partially purified platelets are contacted with said silica slag at a temperature of from about 1410° C. to about 2000° C., said slag having an Si/O ratio of from about 1/4 to about 1/2, said slag being employed in an amount within the No. 11980-C

range of from about 8% to about 12% by weight based on the weight of silicon platelets melted in contact therewith.

23. The process of claim 22 in which said slag is of high purity, the concentration of each impurity in the slag reducible by molten silicon at the silicon platelet-silica slag contacting temperature being less than about 10 ppm.

24. A process for the production of low-cost, refined metallurgical grade silicon comprising:
a. dissolving metallurgical grade silicon in a liquid metal solvent to form a metal solvent-silicon solution;
b. cooling said solution to a temperature at which silicon platelets form;
c. removing said silicon platelets from said solution of metallurgical grade silicon in a liquid metal solvent, thereby recovering partially purified, essentially iron-free silicon platelets with adherent impurities derived from said metal solvent;
d. melting said partially purified silicon platelets in contact with acid silica slag in a melting zone, the resulting slag oxidation recovering residual and adherent impurities from said silicon;
e. removing the silicon-slag melt in its refining available from said melting zone so as to directionally solidify said silicon, said directional solidification resulting in the obtaining of solidified refined metallur-gical silicon and a region of solidified melt having a high concentration of impurities;
f. re-melting said directionally solidified refined No 11980-C

metallurgical silicon after the separation thereof from said region of solidified melt;
g. pulling a silicon boule from said melt on a rotat-ing seed rod by the Czochralski pulling technique;
h. re-melting said silicon boule thus-pulled; and i. pulling a silicon boule from said re-melt on a rotating seed rod by the Czochralski pulling technique, the resulting pulled refined metallurgical silicon compris-ing a single crystal silicon having a resistivity of from about 0.05 to about 0.2 phm-cm, said silicon having impurity concentrations of up to, but not exceeding, about 0.2 ppm of aluminum, about 0.15 ppm of iron, about 0.02 ppm of titanium, about 1.5 ppm of boron, about 0.15 ppm of phos-phorus, about 0.01 ppm of chromium, about 0.7 ppm of calcium and about 0.17 ppm of magnesium, whereby single crystal silicon is obtained at a substantially higher impurity content level than in a conventional high purity semiconductor grade silicon and has suitable prop-erties for solar cell applications.