CA1144739A - Production of low-cost polycrystalline silicon powder - Google Patents

Abstract

PRODUCTION OF LOW-COST POLYCRYSTALLINE SILICON POWDER
Abstract of the Disclosure A silicon-containing feed gas, such as silane, or a halosilane, is decomposed in the free space zone of a reactor maintained at from about 390-1400°C, preferably at about 800-1000°C, to form a silicon powder and by-product gas. The silicon powder is separated from said gas in a settling chamber with additional silicon powder being recovered by the passage of the by-product gas through dust collection means.
By introducing the feed gas turbulently into the free space zone, decomposition at the reactor wall and a silicon wall deposit build-up can be minimized. The feed gas may be introduced into the reactor essentially without dilution or together with hydrogen or an inert carrier gas. Silane is the preferred feed gas, with the feed gas preferably being injected into the free space zone through injector means at the top of the reactor. By preheating the injected feed gas, increased production rates or a reduction in the reactor wall temperature may be acheived.

S P E C I F I C A T I O N

Description

~G~S~Oo~ OP r~ 3 Fleld of the Invention .. ... .
The invention relates to the production of high purity siliconO More particularly, it relates to the low-cost production of h~gh purity silicon on a semicontinuous or continuous basis.
escription o~ the Prior Art .
The current commercial technology f~r the production of high purity, polycrystalline silicon is a low volume, batch operation that produces hlgh-cost polycrystalline silicon rods~ This technology, generally referred to as the Siemens process, is carried out in the controlled atmosphere of a quar~z b~ll jar re~c~or that Gontains silicon rods electrically heated to about 1100C. Chlorosilanes, in concentrations of less than 10~ in hydrogen, are ~ed into the reactor under conditions of gas flow rate~ composition, sll~con rod temperature~ and bell ~ar temperature ad~u~ted so as to promote the heterogeneous decomposition o~ the chlorosilanes on the substrate rod surfaces~ A general descriptlon of the Siemens type process can be found ln the Dietze et al patent, US 3~97g;490, In this conventional process, the s~licon deposition rate is primarily limited by the diffusion of ~he chlorosilanes to the substrate rod surface or the difusion of the re~ction products away from the surace, As a result, the production rate lncreases as the

-2-, 1144 739 D~11856 su~strate diameters or surface areas increase. In operating ~hP bell jar reactor, it is necessary to endeavor to preven~ bo~h gas - phase pyrol~sis o~ the chlorosilanes and chemical vapor deposition of silicon on ~he bell jar~ If a signi~icant amount of silicon deposits on the quartz bell jar, the jar wil~ break on cooldown after termination of the deposition run.
A major factor in such conventional processing is the electri~al power consumption necessary to maintain the uninsulated substrate rods at 1100 5, The use of hydrogen as a diluent li~ewise contxibutes to the high cos~s associa~ed with the low volume conven~ional ba~ch processing. Additional costs are incurred since the chlorosilane dacomposition reaction is a reverslbl-e and incomplete reaction, resulting in the need or the continual removal and separation of the corrosive reac~ion produc~s ~rom the unreacted corrosive reactan s.
A need thus exlsts in the art for a process ~0 for the produ~tion of silicon having the high quality obt$i~able by the Siemens process, combined with a capability of achia~7ing high volume on a corl~lnuous or semicontinuous basis and an appreciable reduction in the cost of the product polycrystalline silicon.
The aYallablli~y of such low-cost, high purity polycrystalline silicon would facllitate the use of silicon in desirable semiconductor and solar cell applica~ions. ~or silicon solar ~ells employed in spac craft applications and for industrial and commerclal applica~ions in general, crystals of high puri~y 9 ~ D-11856 semiconductor grade silicon are prepared by convertlng metallurgical grade silicon to trichlorosilane, that is reduced, as by the Siemens process; to polycrystalline, semiconductor grade silicon from which single crystals can be grown by known techniques. The amount of useable ma~erial is grea~ly red~ced by the necessary processing of single crystal boules to produce polished silicon wafers. Such processing includes cutting o~f the ~mpure end portions of su~h boules, and the cuttin~, etching and polishing o~ wafers. With accompanying losses due to mechanical breakage and electronic ~mperfections, less than 20% of the original polycrystalline, semiconductor greade silicon is generally recovered in the form of useable wafers o~ single crystal material.
Because of the very high r~sultant overall cost of such useable materlal and the relatively large area require-me~ts involved in solar ccll applications~ material costs constitute a very signi~ican~ actor in the overall economics of such applica~ions.
The economic easibility o utilizing solar cell ~echnology ~or significant por~ions o the present and prospective ne~ds for replenishable, non-pollu~ing energy sources would be enhaneed, ~herefore, by the developmen~ of techniques to reduce the cost of high purity, high perfection single crystal silicon wafers.
One significan aspect of such development would reside in the reduction of the cost of producing high purity polycrystalline silicon ~rom which such single crystal material can be grownO Techni~ues heretoore generally disclosed for refining m~tallurgical silicon for other 73~3 purposes have not resulted in refined silicons suitable for solar cell applications although the electronic characteristics of solar cell materials are less stringent than that for silicons employed in the complex circuitry used in the electronics ind~stry.
The need for a low-cost alternative to the Siemens process exists, ~here~ore, with respec~
to silicon materials for both solar cell and semiconductor applications. One approach disclosed in the art for the relatively large scale production of pol~crystalline silicon involves the use of a fluidized bed reactor as disclosed in the Ling patent, US 3,012,861, and in the Bertrand et al patent, US 3,012,862. In this approach, a silicon-con~aining vapor 1s injected in~o a reaction chamber containing particles of elemental silicon small enough to be fluidized and maintained in ebullient motion by the v~porized silicon compound. The reaction chambPr and the fluidized bed of silicon particles are maintained at a tcmperature within the thermal decomposition range and below the melting point of silicon. By the heterogeneous decomposit~on of the vaporized sillcon compound, silicon produGt is deposi~ed on the 1uidized bed par~icles, which increase in size until r~moved from the reaction chamber as product.
Both patents disclose the forming o~ seed particles for the fluidiæed bed by the grinding of a portion of the product silicon particles.
The potential for particle contamination during the grinding or other ~smmi~ion of the hard product silicon particles b~ conventional means known ~4~739 in the art represents, however, a major operating factor involved in the consideration of the ~luidized bed approach. At the hlgh levels of purity required for semiconductor and solar cell applications, such contamination during the grindi~g procedure would be unaccap~able as it would effec~ively preclude the obtaining of the high purity levels required for such applications. In addition, the silicon-containing co~pound injected into the reaction chæmber, particularly silane, will be subject to homogeneous dec~mposition upon exposure to the reaction conditions within the chamber as well as ~he desired heterogeneous decomposition and deposition o product silico~ on the seed particles. As a result of the homoganeous decomposition, considerable ~uantities of silicon dust are ~ormed, with such dus~ being generally undesired in the fluid bed process and resulting in considerable loss of materlal and/or addi~ional processing expense. Such undesired dust ormation, ~ogether with the considerations of product purity in the regenera~ion o~ fluldized bed seed partioles reerred to a~ove, have here~o~ore deterred the development of the 1uid bed approach as a practical alternative to the conventional Siemens process. The need continues, there~ore, for tha development of technically ~nd economically feasible alte~natives to ~he Si~mens process for tha produc~ion o high purity silicon fo~ se~iconductor and solar eell applications.
It is an object of the invention, therefore, to provide a process for the production of low-cost, high purity polycrys~alline silicon.

D~11856 ~ ~ 4~3~

It is another object o~ the invention to provide a process for the production of high purity silicon on a contin~ous or semicontinuous basis.
It is another object of the invention to provide a process ~or the production of silicon capable of advantageously employing silane as the silicon-containing feed material, It is a further object of ~he inv~n~ion to provide for the production at relatively high production rates, of high purit~ polycrystalline silicon suitable for semiconductor and solar cell applications.
With these and other objects in mind, the invention is h~reina~ter described in detail, the novel features th~reo~ being par~cularly pointed out in the appendcd claims.

SummarY o~ the Invention High purity polycrystalline powder ls conveniently produced by introducing a sil~con-containing gas into the hot free space ~one o a decompo~itlon reactor maintained at a temperature within the decomposition range or said gas~ As a resul~ of the homogeneous dec~mposition o~ thP gas wi~hin the free spacé reactor, polycrystalline silicon powder ls formed toge~her wlth by-product gas. The silicon powder can be readily separated r~m the by-product gas or subsequent consolidation or o~her txea~m~n~
and use ~hereo~. In one ~mbodiment, silane is employed as the eed gas stre~m wi~h product silicon powder and by-product hydrogen being produced. The ~ ~ ~ 4~3~

eed gas is preferably introduced into the ~rae space zone turbulently, as by injector means positioned at the top of the reactor, wlth the turbulence tending to minimize heterogeneous decomposition a~
the reac~or wall and consequent silicon wall deposit build-up.
Brief Description of the Invention The invention is further described with reerence to the single figure drawing illustra~ing an embodiment o the ~ree space reac~or appara~us suitable for carrying out the process o~ the in~ention.
Detailed Description of the Invention The objects of the invention are accomplished by the production of silicon powder in a free space zone of a decomposition chamber and ~he separa~ion of such sil~con powder fr~m by-pr~duct gas~ While the homogeneous decomposition of a silicon-contalning gas under gas dec~mposi~ion condi~ons is known, ~he invention represents a significan~ ad~ance in the artg enabling high purity polycrystalline silicon to be produced a~ relatively high production rates and at low-cost, while avoiding obstacles tha~ hav~ heretofore deterred the de~elopment of a prac~ical alternati~e to th~ conventional~ high-cost, relati~ely low production Siemens processO The in~ention is particularly advantageous in that it is highly suitable or use with silane as the ~eed gas stre~m~ Despite the inhexent advantages realized by the use of silane, ne~ther the conventional Siemens process or the _~_ ~ 73~

fluidized bed approach re~erred to abov~ are suitable to capitalize on such advantages that are referred to below with respect to the sub~ect invention.
! The invention may be used with any suitable silicon-containing gas stream capable o~ being thermally pyrolyzed or reduced in gas phase. Illustrative of the gases that may be employed are silane and the halosilanes o~ chlorine, bromine and iod~ne. While the chlorosilanes, such as mono-, di-, tri-, and tetrachlorosilane; bromosilanes, such as mono-, tri-and dibromosilane and including silicon tetrabromide;
and iodosilanes such as SiHI3 and including iodides such as SiI4 and Si2I6, may thus be employed, particular advantages are realized through the use of silane 9 i. e.
SiH4, as the source of high purity silicon. ~The exothermic silane pyrolysis reaction goes to completion, is irreversible and starts at a somewhat lower ~emperature, i.e. about 390C~ than the chloros~lanes.
In addition, silane and its decompostlon products, ~.eO sllicon and hydrogen~ are noncorrosive and nonpolluting. The by-product hydrogen generated upon dacomposition of silane may be used as a carrier gas, re~irculated as a pr~heater gas~ or bottled and sold or otherwise employed in the overall process for producing high purity silicon from me~allurgical grade silicon. The chlorosilane decomposition, on the other hand, is a re~ersi~le and incomplete ~ 73~ D-ll8s6 reaction and both the chlorosilanes and their decomposition by-products are corrosive in nature.
The overall advantages of utilizing silane are accompanied by some disad~antages as will be appreciate~ by those skilled în the art, however, namely in the spontaneous combustion of 9a ~ 7 3 ~

silane with air and in the higher current pr~ce of silane c~mpared to that of the chlorosilanes, The silicon-containing gas can be introduced into the hot free space zone of the decomposition reactor as essentially 100% silicon-containing gas without dilution or said gas may be diluted with inert gases, such as argon, hel~m or the like, or wi~h hydrogen or other silicon-containing gases. For op~mum product and production control, it may be advantageous to dilute the silane or other s~licon-containing gas with a suitable carrier prior to injection into the free space reactor.
In tha decomposition of s~lane, by-product hydrogen can conveniently be recycled ~or use as a carrier gas for additional quantities of silane feed gas in the semicon~inuous or continuous operations convenien~ly carried out in the free space reactor.
Reerring to the drawing, the illustrated embod~ment of the decomposi~ion reac~or employed i~ the pxactice of the invention is represen~ed by the numeral 1, with the hot free space zone 2 thereof being positioned over set~ling cham~er 3 in which silicon product is separa~ed fr~m by-produc~ gases.
Gas injector means 4 is located at the tap of reactor 1 and is shown extending down~ard into free space zone 2 essentially to the upper-most end of the hea~ed section thereofO As shown, th~ heated section of zone 2 is wrapped with a suitable layer of insulation 5 and is inductively heated by means of induction heatlng ~ 73 ~

coil 6. Settling chamber 3 having support members 7 secured thereto contai~s a dust collector 8 and exhaust lin~ 9 through ~hich by-product gases are withdrawn. A preheater 10 may be employed, i~ desired, ~or prehea~ing the silane or other ~ilicon-containing ~eed gas supplied fr~m a source 11 thereof through l~ne 12. Settling chamber 3 has opening 13 at the bottom ~hereof for withdrawal of product silicon.
It will be understood that various modifications to decomposition reactor 1 can be made with the various embGdiments oE the reactor being suitable for, or even advantageous in, the practice o~ the process of the invention, For ex~mple, gas injector means 4 may be loca~ed so as to in;ect ~he silicon-containing gas upward fr~m the bott~m of zone 2, and the injector may be positioned so as to extend to various dlst~nces wi~hi~ zone 2, It may also be advantageous to employ several injectors or multiple orfice injectors of either water-cooled or uncooled designO Means for employing elevated reac~on pressures may also be provided al~hough the invention can readily be practiced at essentially atmospheric pressure conditionsO
In the practice of the invention, a silicon-con~aining gas stream from source 11 ls passed through line 12 a~d preheater 10, if desired, to injector means 4 through which said gas is introduced into hot free space zone 2 of dec~mposition reactor 1 ~ 3~

Said free space zone is maintained at the de~ired operating temperature by means of induction h~ating coil 6 and reactor insulation 5. Upon exposure to the rPaction conditions in free space zone 2, the silicon-containing feed gas s~ream will rapidly be converted b~ homogeneous decomposition to fine silicon powder and by-product gas. The thus - formed silicon particles constitute silicon nuclei that react heterogeneously with the silicon-containing gas and thereby grow to submicron or low micron size that can conveniently be separated from the by-product gas.
The decomposition products thus ormed are passed to settling chamber 3 in which the silicon powder product is found to be of sufficient size to conveniently separate from the by-product gas that is thereupon exhausted from settling chamber 3 through line 9 for further disposition as indicated above.
To assure that essentially all of the silicon powder produced in zone 2 is recovered and to minimize the entraiNment of silicon powder in the ~y-product gas, sa~d gas is advantageously passed through dus~ collec-tion means 8 so that essentially all of the silicon product may be contained in the settling chamber or the auxiliary dust collector. A discharge opening 13 in settling chamber 3 i5 used to remove and recover product silicon from decomposition reactor 1.
The sllicon containing gas in introduced into the hot free space zone o~ the reactor maintained at a temperature within the decomposition temperature ~ 3~

range of the particular silicon-containlng gas and below the melting point te~pera~ure of silicon, i.e. about 1420C. For efficient dec~mpos~tion o the feed gas and sllicon powder ormation, it has been found desirable to employ a tempera~ure within the range of from about 390C up to about 1400C
~lth preerable temp~xatures being within ~he range of from about 800C to about lOOO~C. The silicon containing gas may be passad into ~ee ~pace zone 2 without preheat, as by use o~ a wa~er-cooled injector means, or may be preheated to a temperature less than the decompos~tion temperature ~hereo~. Such preheat will ordinarily occur upon passage o the gas ~hrough the portion of injection means 4 extending into said zone 2 in the absence of cool-ing, or may be accomplished by the use of preheatar 10 ~o which ho~ by-produc~
gas may ~e passed if desiredO It should be noted that, by preheating the ln~ec~ed ~eed gas, it may be possible to maintain the reac~or walls a~ a somewhat cooler ~empera~ure th~n otherwise, thareby reducing heat losses during the silicon produc~ion operation. Alternately, the reac~or walls may be m~in~ained ho~er in co~junction with such prehea~ing so as to achieve an increase in productio~ rates under the particular reactox conditions employed.
The invention may be carried out at essentially atmospheric condi~ions or, al~erna~ely;
at elevated reaction pressures, e.g. up to 100 psi or above. The use of elevated reactor pressures may tend ~o favor higher s:ilicon production ra~es -13~

~'1473~

and he formation of larger particles of silicon.
The use of lower reactor pressures may tend to favor reduction in pxoduct particle size. As indicated above~ the product silicon is ob~ained as a powder of sufficient size to permit convenient separation ~r~m by~product gases, with the silicon powder size ranging from submicron to low micron size, e.g. 5 u. It will be und0rstood that, in any particular embodiment or example o~ the invention, particles of silicon having a range of particle size will be obtained. The particle sizes are not determined directly, but are calculated, ass~ming spherical particles, from suxface area values. In illustrative examples of the invention, the actual average surface areas determined were found to vary fr~m approx~mately 0.5m2/gm to about 32m2/gm.
Average particle diameters calculated ~rom the values range from about 800 ~ to about 5 microns.
The silicon product is obtained as a high purity, polycrystalline powder that, upon discharge from the decomposition reactor, may be consolidated or mel~ed for further processing by con~entional means to produce a low~cos , high pu~i~y single crystal material for solar cell and/or semi~onductor applica~ion p~rposes.
In a highly desir~ble embodiment of the invention, the silicon-containing feed gas is in~roduced turbulently~ tha~ is under condi~ions of turbulent gas flow, into the free space zone of the decomposition reactor. By means of such tur~ulence the dec~mposition o~ the sillcon-containing gas occurs rapidly in a diverging region near the ~4~39 injection nozzle xather than at further distances from the injector.
As a result, the heterogeneous decomposition of the silane or other silicon-containing feed gas at the reactor wall, with consequent silicon wall deposit build-up can be minimized. It will be apprecia~ed by ~hose skilled in the art that condltions of turbulent flow are obtained at a Reynolds No~ of above about 2,000, with the \~ :

14a

3~

Reynolds No. being a ~unction of the particular nozzle diameter, and the inje~tion gas velocity, density, viscosity and the temperature of the injected gas. While turbulent injection of the eed gas is not an essential feature of the inv~ntion, it is o~ particular advantage and is generally preferred in practical silicon production operations in accordance with the process of the inventionO
The benefits of the invention were illustrated in an ex~mple in which silane was fed into the top of a decomposition reactor at the rate of 4 l/min, employing an injec~or with a 2mm. diameter orifice. The calculated room temperature velocity of the gas passing through the injector was 21.2 m /sec.
With the reactor wall at the center of the ~xee space zone maintained at approximately 800C, the silane inside ~he inj ector reached a tempera~ure oE
approximately 350~C. By the time the gas was hea~ed to said 350Cg it would undergo, assuming ideal gas conditicns, a volume expansion of abou~ 2.1 ~imes, resulting in an orifice exit veloclty o~ æbout 44.6 m/sec~ and a Reynolds No~ of about 3,000.
In another run, the reactox wall temperature was approximately 800C and a water-cooled injector was employed to assure against heating the silane to above its decompositlon temperature while in the inj ector. Silane was -Eed at the rate of 2 l/min. to the injectox and through a 2mm. dlzmeter orlEice~
resulting in an injection velocity of about 10,6 m~sec.

~ 7 3 ~

and a Reynolds No. of about 2700. In both cases, the decomposition products were silicon powder and by-product hydrogen gas. In both runs9 which were carried ou~ at near atmospheric pressure, decomposition of the silane at the reactor wall, and consequent silicon wall build-up were minimized by injecting the silane turbulently into the hot reactor.
In other examples using a larger reactor, silane flow rates up to 61 l/min, were achieved, employing a 4 . 6 mm. orifice, with an inj ection gas velocity of 61.6 m/sec. and a Reynolds No. o 36,000.

15a ~ 4~ 3~ D~11856 Reactor wall deposit build-up was again minlmi~ed by injecting the silane ~urbulently into the reactor. At a silane flow r~te of 61 l/min., the reactor wall temperature at the center of the hot zone was controlled at between 995 and 1030C. A silane decomposltion efficiency o ~8% was obtained, wlth the average particle size of the high purity, polycrystalline silicon powder product being 0.30 um. ~o measurable impurities are detectable iR the product upon examination us~ng the cathode layer emission spectroscopic technique.
The invention represents a significant development in the silicon production field. In overcoming the disadvantages o~ the conventiona Siemens process and the difficulties encountered in other ef~orts to provide a process for the high volume production of high purity silicon, the invention achisves the production o~ high purity siliconj in a recoverable form, with minimum pyrolysis ef~iciencies of 80% readily ob~ained and exceeded, and with the injection o~ undiluted as well as diluted feed gas s~reams. The decomposition of the silicon-containing gas readil~ occuxs in the ~ree re ctor space and can be achieved without building up re~ctor wall deposit~
of silicon to the extent tha~ undesired blockage of the free space zone of the reac~or occ~rs. In ~ ~ ~ 4~3 ~

this regard, it should be noted that the mlnimal deposit o silicon likely ~o occur in ~he practice of the invention is of advantage as a very thin liner of high purity silicon on ~he surface of the quar~z, graphite, stai~le~s steel or other sui~able reactor wall material. While the silicon powd2r produced in the free space reactor is generally less desirable than the rod-shaped silicon produced in a Siemens bell jar, the homogeneous decomposition of the feed gas and the heterogeneous grow~h of the resultant silicon powders to a readily recoverable size enables ~he process of the inven~ion to provide highly important advan~ages over the currently practiced technology.
As previously noted, the stlicon production rate in the Siemens process ls limited by the available sur~ace area. No such restrlction exists in the ree space decomposition process o~
the in~ention. Upon reaching is decomposition tempera~ure, the silicon-contalning gas decomposes homogeneously and heterogeneously within the free space zone . The ~ree space reactor process has g as a resul~, a m~ch higher production ratP capability ~han ~he conventional process.
As also indica~ed above~ the concen~ration of the inj ec~ed silicon-containing feed gas in the Siemens process should be less than 10% to prevent the ~ormation o~ silicon particles in the gas phase and to prevent silicon from ~eposiking on the quartz bell jar. In the ~ree space reactor ~ 73~

procass of the invention, however~ there is no theoretical composition limit, and the silicsn-containing feed gas can be utilized in undiluted form or diluted with a carrier gas if desired.
The expenses involved in using and handling large volumes o~ a carrier gas are not necessar~, therefore, in the pra~tice of the in~ention.
The advantages of high production and reduced carrler gas costs are further enhanced by the fact that the i~ven~ion can readily be carried ou~ on a semicontinuous or continuous basis as in other free space reactor applications heretofore known, such as in the production of nickel powder fxom nickel carbonyl as shown in US 3,367,768.
An additional limitation in the conventional process rasults from the practical limit to the size o the bell jar employed in the Siemens process and the resulting inhibition on scaleup. There is no kno~ size or scaleup l~mi~ation applicable to the free space reactor process of the invention.
An important ~actor in the overall cost of silicon production is the energy consumption requlred. The conventional Siemens process is an energy-intensive process. Energy losses are kno~n to occur through radiation from the substrate rods and through gas convection. A large gas ~9f9L739 throughpu~ is malntained ~o prevent the gas phase from reaching decomposition temperatures and for cooling the quartz bell jar. By contrast, the ex~ernal surface of a ree space reactor may be insulated to prevent radiation and air convection heat lossasO The internal gas and particle con~ection mo~ion within the hot free space zone provides a desirable means for ~ransferring heat rom the in~ernal surface of the ~ree space zone to the gas phase ~o promote the decomposition reac~ion.
The simplicity of ~he free space reactor design coupled with its desirable scaleup potential reduces the capital equipment costs ~hereof compared to a conventional Siemens process plant~ Such factors also tend to reduce the manpower opera~ing requiremen~s ~or commercial embodiments o~ the invention as compared to such requirements in carrying out the conventional process.
In addition to all such production and cos~ advantages of the invention, the abîli~y of the in~ention to advantageously utili~e sîlane should again be emphasi~edO Specific advantages o~ sllane as the feed stock were indica~ed above, An integxa~ed overall operations or producIng high purity silicon from metallurgical grade material~ the by-product hydrogen rom silane decomposi~lon can be employedg no~ only for carrier gas purposes as indica~ed above~ bu~ as ~ source of ~ 14 ~ 73 9 hydrogen for use iQ ~he hydroge~a~ion of .~, CC~4~ Si/~of~ WG~Jro~
metallurgical silicon~to produce trichloros~lane from which silane can conveniently be produced ~s, for ex~mple, b~ the one-s.t~p p~ess ~l~closed in US patent No. 3,968,199.
In light of all of the advantages hereinabove recited, the inven~ion can readily be appreciated as a highly significant development~
enabling high purity silicon to be prepared at high production rates and at low-cost on a continuous or semicontinuous basis. The invention thus constitu~es a major advance in the development of low-cost silicon materials and the utilization of such materials in the deveLopmen~
o~ commercially feasible solar cell technology and in satisfying the requirements for high purity silicon for semiconductor applications,

Claims (37)

WHAT IS CLAIMED IS:

1. A process for the production of high purity, polycrystalline silicon comprising:
(a) introducing a silicon - containing gas capable of gas phase decomposition into the hot free space zone of a decomposition reactor maintained within the decomposition temperature range of said gas and below the melting point temperature of silicon, said decomposition temperature range being from about 390°C
up to about 1400°C, thereby causing said silicon -containing gas to decompose and form silicon powder and by-product gas;
(b) removing said silicon powder and by-product gas from said hot free space of the decomposition reactor;
(c) separating said silicon powder from by-product gas, by passing said powder and gas into a settling chamber located beneath said free space zone of the decomposition reactor;
(d) withdrawing by-product gas from said settling chamber;
(e) recovering silicon powder separately from said settling chamber; and (f) passing by-product gas through dust collection means to recover additional silicon powder entrained in said by-product gas, whereby said silicon powder is recoverable as a low-cost, high purity polycrystalline powder capable of being produced at relatively high production rates on a semi-continuous or continuous basis.

2. The process of Claim 1 in which said decomposition temperature is from about 800°C to about 1000 °C.

3. The process of Claim 1 in which said silicon containing gas is taken from the group consisting of silane, and the halosilanes of chlorine, bromine and iodine.

4. The process of Claim 3 in which said gas comprises silane, said by-product gas being hydrogen.

5. The process of Claim 3 in which said gas comprises tetrachlorosilane.

6. The process of Claim 3 in which said gas comprises trichlorosilane.

7. The process of Claim 3 in which said gas comprises dichlorosilane.

8. The process of Claim 1 in which said silicon-containing gas is diluted with an inert gas prior to being introduced into said free space zone.

9. The process of Claim 4 in which said silane is introduced into said free space zone essentially as 100% silane without dilution prior to being introduced into said zone.

10. The process of Claim 4 in which said silane is diluted with hydrogen or an inert carrier gas prior to being introduced into said free space zone.

11. The process of Claim 10 in which said carrier gas comprises hydrogen.

12. The process of Claim 11 and including recycling by-product hydrogen as said carrier gas.

13. The process of Claim 1 in which said silicon-containing gas is introduced into said free space reactor through injection means located at the top of said reactor.

14. The process of Claim 13 in which said silicon-containing gas comprises silane.

15. The process of Claim 1 in which said silicon-containing gas is introduced turbulently into said free space zone, the resultant turbulence tending to minimize decomposition of the silicon-containing gas at the reactor wall and consequent silicon wall deposit build-up.

16. The process of Claim 15 in which said silicon-containing gas comprises silane.

17. The process of Claim 10 in which said decomposition temperature is from about 800°C to about 1000°C.

18. The process of Claim 17 in which said carrier gas comprises hydrogen.

19. The process of Claim 14 in which said decomposition temperature is from about 800°C to about 1000°C.

20. The process of Claim 1 in which said free space zone is operated essentially at atmospheric pressure.

21. The process of Claim 1 in which said free space zone is operated at elevated reactor pressure, said elevated pressure tending to favor higher silicon production rates and the formation of relatively large particles of silicon.

22. The process of Claim 20 in which said silicon-containing gas comprises silane, said decomposition temperature being from about 800°C to about 1000°C.

23. The process of Claim 1 in which said silicon-containing gas is preheated prior to being introduced into said free space zone, the preheat temperature being less than the decomposition temperature of said gas.

24. The process of Claim 23 in which said gas comprises silane.

25. The process of Claim 23 in which said decomposition temperature is from about 800°C to about 1000 °C.

26. A process for the production of high purity, polycrystalline silicon comprising:
(a) introducing a silicon-containing gas capable of gas phase reduction into the hot free space zone of a decomposition reactor through injection means located at the top of said reactor, said free space zone being maintained at a temperature in excess of about 390°C and below the melting point temperature of silicon, thereby causing said silicon-containing gas to decompose to form silicon powder and by-product gas;
(b) passing said silicon powder and by-product gas into a settling chamber located beneath said free space zone of the decomposition reactor;
(c) withdrawing by-product gas through dust collection means; and (d) recovering product silicon powder from said settling chamber and from said dust collection means, whereby said silicon powder is readily recoverable as a low-cost, high purity polycrystalline product capable of being produced at relatively high production rates on a semi-continuous or continuous basis.

27. The process of Claim 26 in which said decomposition temperature is from about 800°C to about 1000°C.

28. The process of Claim 26 in which said silicon-containing gas comprises silane.

29. The process of Claim 27 in which said silicon-containing gas comprises silane.

30. The process of Claim 28 in which said silane is introduced turbulently into said free space zone.

31. The process of Claim 30 in which said silane is preheated to a temperature less than the decomposition temperature thereof prior to being introduced into said free space zone.

32. The process of Claim 29 in which said silane is introduced turbulently into said free space zone.

33. The process of Claim 28 and including recycling by-product hydrogen as a carrier gas for said silane introduced into said hot free space zone of the decomposition reactor.

34. The process of Claim 28 and including reacting by-product hydrogen with metallurgical grade silicon and silicon tetrachloride to form trichlorosilane.

35. The process of Claim 34 in which said decomposition temperature is from about 800°C to about 1000°C.

36. The process of Claim 34 and including disproportionating said trichlorosilane to form the silane introduced into said free space zone.

37. The process of Claim 36 in which said decomposition temperature is from about 800°C to about 1000°C.