Process for the production of silicon of high purity

A process is provided for the low cost, high volume production of polycrystalline high purity silicon by a vapor phase reduction of a halosilane, with hydrogen, the resulting polycrystalline silicon being particularly suited for use in the production of single crystal silicon for the manufacture of semiconductor devices, solar cells, and the like. The process of the invention involves the reaction of metallurgical grade silicon (of a purity of about 98%) with a halogen or hydrogen halide to form a halosilane intermediate; the purification of the halosilane and of hydrogen; the separate pre-heating of the purified halosilane and of the purified hydrogen to a temperature range above the chemical reaction temperature of the halosilane and the hydrogen; injection of the halosilane and the hydrogen into a continuous flow reduction tubular reactor wherein the feed materials are instantaneously mixed in a manner which causes chemical reaction to be initiated followed by nucleation and growth of solid high purity silicon particles as the reaction mass flows through the tubular reactor; introduction of the solid-gas reaction mass stream into a cyclone type separator wherein the high purity silicon particles are collected and separated from the gas stream and ejected from the bottom of pg,2 the separator; emitting the gas stream from the top of the separator and conducting the gas stream to a condenser-scrubber system wherein unreacted hydrogen is separated and then recycled to the hydrogen pre-heater for re-use, unreacted silicon halosilane is separated and recycled to the intermediate pre-heater for re-use, and reaction product hydrogen halide is separated and recycled to the silicon halosilane generator for re-use.

BACKGROUND OF THE INVENTION 
The term halosilane as used in the present specification includes any one 
or more of the following: SiX.sub.4, HSiX.sub.3, H.sub.2 SiX.sub.2 and 
H.sub.3 Six, and is represented by the general chemical formula H.sub.n 
SiX.sub.(4-n) where X represents Cl, Br or I. 
Recent developments in the semiconductor industry have created a growing 
demand for low cost single crystal silicon of extremely high purity, which 
is known as semiconductor grade silicon. Semiconductor grade silicon is 
used in the manufacture of semiconductor devices, such as transistors, 
rectifiers, solar cells and the like. Processes are in use in the prior 
art for the production of polycrystalline semiconductor grade silicon, 
which can be converted into single crystal semiconductor grade silicon by 
means of special techniques, such as by the well known Czochralski method. 
In one such prior art process, for example, silicon tetraiodide is purified 
by crystallization, and vaporized, the vapor being subsequently caused to 
deposit silicon on a hot wire of a relative inert metal, such as tungsten. 
In such a prior art process, because of the difference in volatility of 
silicon and iodine, the reaction product iodine vapor diffuses away from 
the space near the heated wire, and the silicon is deposited on the heated 
wire and grows to form a substantial silicon crystalline mass. When the 
growth reaches a certain stage, the cooled mass of crystalline silicon is 
cut in layers from the wire substrate. 
Another prior art process for the preparation of polycrystalline 
semiconductor grade silicon includes the reaction of super-heated silicon 
tetrachloride of high purity with highly heated vapor of zinc, causing an 
interaction of the zinc vapor and the silicon tetrachloride. A heated 
silicon substrate is provided, and the zinc vapor and silicon 
tetrachloride cause elemental silicon to grow on the heated silicon 
substrate to provide polycrystalline elemental silicon which, under 
suitable conditions, is at least partially of simiconductor grade. 
Semiconductor grade polycrystalline silicon has also been produced in the 
prior art by the reduction of silicon halides with hydrogen in a furnace, 
the mixture being passed slowly through a heated tube of fused quartz 
located within the furnace. The silicon is deposited on the inner surface 
of the heated tube, and the tube is removed from the furnace from 
time-to-time to recover the silicon. 
Semiconductor grade polycrystalline silicon is presently being produced by 
a chemical vapor deposition process by which trichlorosilane 
(SiHCl.sub.2), or silicon tetrachloride (SiCl.sub.4), is reduced with 
hydrogen on a hot silicon substrate at approximately 1200.degree. C, 
according to the teachings of U.S. Pat. Nos. 3,053,638 and 3,240,623. The 
trichlorosilane and silicon tetrachloride are prepared in the processes 
from commercial or metallurgical grade silicon of the order of 98% purity, 
and they are purified by fractional distillation. 
The prior art processes have demonstrated the technical and economic 
feasibility of producing high purity polycrystalline silicon of 
semiconductor quality by hydrogen reduction of and halosilanes. All 
commercial semiconductor grade polycrystalline silicon is presently being 
manufactured in accordance with the aforesaid chemical vapor deposition 
process, which employs hydrogen reduction of dichlorosilane or 
trichlorosilane and the deposition of silicon on an electrically heated 
silicon filament substrate. The silicon filament substrate is maintained 
at temperatures above 1000.degree. C by electrical resistance heating, and 
the walls of the chamber enclosing the filament and reacting gases are 
maintained at temperatures of the order of 300.degree. C to avoid the 
deposition of silicon thereon. The heated substrate increases in diameter 
as the process proceeds until it reaches a diameter of the order of 3 
inches to 4 inches. The process is then discontinued until the substrate, 
which can be up to 4 feet in length, is removed from the chamber and 
replaced with a new starting rod which, for example, may be of 1/8 - 1/2 
inch in diameter. Generally, the continuous vapor deposition reactor 
effluent gases are not recycled in the prior art process but are disposed 
by appropriate means. 
Large amounts of electrical energy are required to operate the prior art 
continuous vapor deposition process of the order of 800-1000 kilowatt 
hours per kilogram of silicon produced. Capital and labor costs are also 
high due to the multiplicity of reaction chambers and silicon substrates 
required. The cost of production in a plant producing at a rate of 300-500 
metric tons per year is in the range of $25.00-$30.00 per kilogram at the 
present time. The present-day market price is about $65.00 per kilogram. 
The supply and demand for and of the semiconductor industry are in balance 
at the present time. There have been periods of severe shortages in recent 
years, and a potentially large new demand which could exceed the present 
semiconductor industry demand many times over is developing. The new 
demand is being created by the use of silicon solar cells for the 
photovoltaic conversion of solar energy into electrical energy. In order 
to realize this potentially new demand and to supply the demand, it will 
be necessary to reduce the manufacturing costs of semiconductor grade 
silicon to substantially less than $10.00 per kilogram, and to maintain 
silicon quality which will provide high efficiency of conversion of solar 
energy into electrical energy. 
It is among the objects of the present invention to provide a process and 
apparatus to meet the aforesaid demands and criteria. The present 
invention provides a process which operates continuously; in which energy 
requirements are greatly reduced; and in which reactor effluent gases are 
recovered, separated and recycled. In particular, the hydrogen halide 
by-product of the process of the invention is recycled to generate 
purified halosilane feed stock for the reduction reactor, and hydrogen is 
also recovered and recycled. The only raw material consumed in the process 
of the invention is low cost metallurgical grade silicon. 
The invention also provides a simple continuous flow reduction reactor in 
which the reactants have a short residence time of the order of 0.01-0.1 
seconds. The reactants and nucleants are separately pre-heated in 
efficient radiant gas fired heat exchangers. The product is granular in 
the range of 10-100 mesh. The net result is a small tubular reactor with 
high volume capacity, continuous operation and continuous removal of the 
product, and representing much lower capital and operating costs than for 
the prior art continuous vapor deposition processes. 
The continuous flow reduction process of the present invention represents a 
relatively low cost, high volume means for the continuous production of 
semiconductor grade silicon. In an embodiment to be described, impure 
(metallurgical grade) silicon is converted into a volatile halosilane 
intermediate compound according to the following chemical reactions (1) or 
(3): 
##STR1## 
Impurities in the form of halides are separated from the halosilane 
intermediate in the process of the invention by fractional distillation 
and rejected. The purified volatile halosilane is then pre-heated and 
reduced in accordance with the chemical reactions (2) and (4) with 
purified pre-heated hydrogen in a continuous flow tubular reactor. 
Purified semiconductor grade polycrystalline silicon is separated and 
recovered from the reaction gas-solid stream. Reaction product hydrogen 
halide is separated from the reaction gas stream and recycled to convert 
more metallurgical grade silicon into crude intermediate silicon compounds 
and hydrogen. Thus the process of the invention consumes impure silicon, 
and it produces pure silicon, rejecting impurities as liquid or solid 
halides.

DETAILED DESCRIPTION OF THE PROCESS OF THE INVENTION 
The invention provides a novel process and reactor for the continuous, high 
volume, low cost production of polycrystalline silicon of ultra-high 
purity by vapor phase reduction of a halosilane [SiH.sub.n X.sub.4-n ] 
with hydrogen [H.sub.2 ]. 
The silicon produced by the process of the invention, because of its 
uniform granular size and ultra-high purity is particularly suited for 
melting and drawing single crystal silicon for use in the manufacture of 
semiconductor devices, including integrated circuits, solar cells and the 
like. 
In the practice of the invention, a halogen [X.sub.2 ] or hydrogen halide 
[HX] is refined in a refiner 3, and is reacted with silicon containing 
small amounts of impurities in a silicon halide generator 5 at high 
temperature in the range of 350.degree. C-800.degree. C to produce crude 
halosilane [SiH.sub.n X.sub.4-n ] intermediate such as silicon halide 
[SiX.sub.4 ]. The silicon introduced to the generator 5 is, for example, 
metallurgical grade silicon. 
Either commercial grade (98.5%) silicon, or higher purity (99.9%) silicon 
generated by the generator 2 may be used. The generator 2 may be an 
electrothermic silicon generator, which is known to the art. The higher 
purity hydrogen halide [HX] is obtained from the commercial product in 
refiner 3 by fractional distillation. The silicon halide from generator 5 
is purified by fractional distillation in a refiner 6 in which impurities 
introduced into the process in the impure silicon are converted to metal 
halides and are eliminated as such by means of the fractional 
distillation. Refiner 6 may be of the type described in detail in U.S. 
Pat. No. 3,020,128 -- Adcock et al. 
The refined silicon intermediate compound [SiX.sub.4 ] or [SiH.sub.n 
X.sub.4-n ] is then pre-heated in pre-heater 8 to a high temperature in 
the range of 900.degree. C-1100.degree. C and injected into a continuous 
flow reduction reactor 10, of the type illustrated in FIG. 2, and which 
will be described in more detail subsequently. The pre-heater 8 may be a 
gas-fired type. 
Hydrogen [H.sub.2 [ from commercial sources and recycled hydrogen from the 
halosilane generator 5 are refined to high purity in a conventional manner 
in hydrogen refiner 1 which likewise may be of the type described in the 
Adcock et al. U.S. Pat. No. 3,020,128. The purified hydrogen is then 
pre-heated to high temperature in the range of 900.degree. C - 
1500.degree. C in a pre-heater 9, which may be similar to pre-heater 8, 
and the pre-heater hydrogen is injected into the continuous flow reduction 
reactor 10 where it is thoroughly mixed with the refined and separately 
pre-heated silicon intermediate compound in a manner which causes 
instantaneous chemical reaction. Pre-heated silicon particles of selected 
size and high purity derived, for example, from the output of cyclone 
solid-gas separator 11 are injected into the continuous flow reduction 
reactor 10 by means of the stream of hydrogen in a manner to be described 
subsequently. 
The silicon particles may catalyze the vapor phase hydrogen reduction of 
the silicon intermediate but primarily are intended to promote nucleation 
and growth of the silicon while the reaction mass flows through the 
continuous flow reduction reactor 10. This phenomenon is due to the 
extensive high temperature surface presented by the silicon particles and 
the favorable Gibbs free-energy of the reacting species created in the 
continuous flow reduction reactor. 
The solid-gas reaction mass stream after suitable retention time in the 
continuous flow reduction reactor 10 is conducted directly into a cyclone 
type solid-gas separator 11 wherein the silicon particles are collected 
and separated from the gas stream and then discharged from the bottom of 
the separator. The gas stream is discharged from the top of the cyclone 
separator 11 in a conventional manner and is then conducted to a gas 
cooler and condenser-scrubber system. Condensibles in the gas stream from 
the separator 11 consisting of unreacted [SiH.sub.n X.sub.4-n ] and by 
product hydrogen halide are separated in system 14 by condensation, 
absorption and fractional distillation. [SiH.sub.n X.sub.4-n ] so 
separated is then recycled to the pre-heater 8. The hydrogen halide [HX] 
so separated is conducted to the generator 5 where it is caused to react 
with more impure silicon to produce more intermediate and hydrogen. The 
[H.sub.2 ] produced in the generator 5 is recycled to the hydrogen refiner 
1, as mentioned above. The non-condensible gas in the gas stream issuing 
from the separator 11 is unreacted hydrogen [H.sub.2 ] which, after 
scrubbing in cold [HX] liquid is recycled to the hydrogen refiner 1. 
A small fraction, of the order of 10% of the hot silicon particles 
discharged from the cyclone separator 11, after suitable reduction in 
particle size, is recycled to pre-heater 15 and is then injected into the 
continuous flow reduction reaction by means of the pre-heated hydrogen 
stream. 
The remaining hot silicon particles from the separator 11 are gradually 
added to a molten pool of silicon metal located below the separator and 
from which single crystal silicon is continuously withdrawn. An alternate 
method is to cool the silicon particles emerging from the separator 11 and 
accumulate them for future use. 
Impurities introduced into the process in the raw materials employed being 
those principally in the impure silicon are as previously described 
converted principally into metal halides and are then separated from the 
crude halosilane by means of fractional distillation. The waste stream 
containing the metal halides can be discarded as such, or it can be 
treated in a conventional manner to recover the halides and the 
impurities. Final disposition of the impurities in whatever form depends 
largely on the quantities involved and the contained commercial values. 
The continuous flow reduction reactor 10 and the cyclone solid gas 
separator 11 are shown schematically in FIGS. 2 and 3. The SiX.sub.4 from 
the silicon halide pre-heater 8 is introduced to an injection mixer 20 
through inlets 21. The hydrogen stream from the hydrogen pre-heater 9 is 
introduced to the mixer 20 through an inlet 22. The silicon particles from 
separator 11 are fed into a hopper 23, and the contents of the hopper are 
injected into the hydrogen stream through a rotary valve 24. The mixer 20 
is coupled to a tubular reactor 25 which may, for example, be 4 inches to 
6 inches in diameter and 20 feet to 40 feet in length. The remote end of 
the tubular reactor 25 is coupled to a cyclone separator 26 of known 
construction. 
The apparatus of FIG. 2 provides for the continuous production of 
polycrystalline silicon by the high temperature vapor phase, reduction of 
the halosilane with hydrogen. The apparatus comprises means for separately 
pre-heating the halosilane vapors (pre-heater 8 of FIG. 1), of the 
hydrogen gas (hydrogen pre-heater 9 of FIG. 1) and of the silicon 
particles (pre-heater 15) to temperatures above 900.degree. C. The 
apparatus also includes the tubular reactor 25 including mixer 20 which 
may be an injection type nozzle mixer in which the reactants and nucleant 
are injected and intimately mixed in a manner which causes instantaneous 
chemical reaction. The mixer is directly connected to the tubular reactor 
25 in which nucleation and growth of silicon particles occurs. The 
apparatus also includes the cyclone type gas-solid separator 26 into which 
the reaction gases and silicon particles are conducted, and wherein the 
silicon particles are collected and separated from the gas stream. 
The principal features of the continuous flow reduction reactor 
advantageously employed in the process of the invention are the separate 
pre-heating of the reactants and nucleants to high temperatures in the 
range of 900.degree. C-1200.degree. C, and the high velocity injection and 
mixing of the reactants and nucleating silicon particles. It is important 
that the design of the reactor, the temperatures and the reactant flow 
rate be such that the reactants, nucleant and products of reaction remain 
within the reaction zone for a period of time that is long enough to 
insure substantially the theoretical maximum conversion and to insure 
maximum growth of silicon particle size. Retention times of the order of 
0.01 seconds to 0.1 seconds are generally sufficient. 
It is also important to pre-heat the silicon particles to the highest 
serviceable temperature of about 1200.degree. to promote nucleation and 
growth of silicon from the vapor phase onto the surface of the nucleant 
silicon particles. The high velocity turbulent flow of the reaction mass 
through the injection mixing nozzle 20 and tubular reactor 25 provides 
high volume flow in a small diameter tubular reactor. High velocity of the 
gas stream is necessary to maintain the silicon particles in a uniform 
state of suspension particularly when the tubular reactor 25 is disposed 
on a horizontal axis. In addition to performing an extensive nucleating 
surface, the silicon particles serve to scour the inner wall of the 
reactor and thus prevent deposition of silicon particles thereon. Silicon 
particle sizes in the range of -10 to +100 mesh are advantageously 
employed for the dual purpose of nucleant and scouring agent. 
EXAMPLE 1 
High purity silicon tetrabromide vapor from a continuous source of supply 
is fed at a rate of 0.49 liters per second into a pre-heater where it is 
heated to a temperature of 1090.degree. C, and then into a 25 mm I.D. 
horizontal tubular reactor 6 meters long fitted with a mixing nozzle head 
and a cyclone type gas-solids separator, substantially as shown in FIG. 2. 
High purity hydrogen from a continuous source of supply is fed at a rate 
of 8.67 liters per second into a pre-heater where it is heated to a 
temperature of 1090.degree. C, and then into the tubular reactor head 
where it intimately mixes and reacts with the pre-heated silicon 
tetrabromide vapor to form silicon and hydrogen bromide. Pure silicon 
particles in the range of -10 + 40 mesh (B.S.S.) from a continuous source 
of supply, and pre-heated to a temperature of 1100.degree. C, are fed into 
the heated hydrogen stream at a rate of 0.3 grams per second as it enters 
the tubular reactor mixing head. 
The entire gas-solids reduction mass continuously flows through the tubular 
reactor at high velocity of the order of 100 meters per second into the 
cyclone separator. Average residence time in the tubular reactor is about 
0.06 seconds. High purity silicon particles are separated from the gas 
stream and discharged from the separator at a rate of 0.68 grams per 
second. This presents a net yield of 0.38 grams per second. 
SUMMARY 
______________________________________ 
SiBr.sub.4 0.49 liters/sec 
H.sub.2 8.67 liters/sec 
nucleant Si 0.3 grams/sec 
product Si 0.68 grams/sec 
net Si 0.38 grams/sec 
______________________________________ 
Pre-heat temperatures: 
SiBr.sub.4 -- 1090.degree. C 
H.sub.2 -- 1090.degree. c 
si -- 1100.degree. C 
Velocity through reactor 100 meters/sec. 
EXAMPLE 2 
Using the same type of apparatus described in Example 1, high purity 
tribromosilane vapor from a continuous source of supply is fed into the 
pre-heater where it is heated to 955.degree. C, and then into the tubular 
reactor at a rate of 0.49 liters per second. High purity hydrogen from a 
continuous source of supply is fed into the pre-heater where it is heated 
to 955.degree. C, and then into the tubular reactor at a rate of 8.67 
liters per second. Pure silicon particles in the range of -10 + 'mesh 
(B.S.S.) from a continuous source of supply, and pre-heated to a 
temperature of 1100.degree. C, are fed into the heated hydrogen stream at 
a rate of 0.1 gram per second as it enters the tubular reactor mixing 
head. The entire gas-solids reaction mass flows through the tubular 
reactor at a rate of the order of 100 meters per second into the cyclone 
separator. High purity silicon particles are separated from the gas stream 
and discharged from the cyclone collector at a rate of 0.48 grams per 
second. This represents a net yield of 0.38 grams per second. 
SUMMARY 
______________________________________ 
HSiBr.sub.3 0.49 liters/sec 
H.sub.2 8.67 liters/sec 
nucleant Si 0.1 grams/sec 
product Si 0.48 grams/sec 
net Si 0.38 grams/sec 
______________________________________ 
EXAMPLE 3 
Temperatures same as for Example 2. Velocity through reactor 100 
meters/sec. 
______________________________________ 
HSiCl.sub.3 
29.4 liters/min 
H.sub.2 520 liters/min 
nucleant Si 60 grams/min 
product Si 87.6 grams/min 
net Si 27.6 grams/min 
______________________________________ 
EXAMPLE 4 
Temperatures same as Example 1. Velocity through reactor 200 m/sec. 
______________________________________ 
HSiCl.sub.3 
58.8 liters/min 
H.sub.2 1040 liters/min 
nucleant Si 120 grams/min 
product Si 147.8 grams/min 
net Si 27.8 grams/min 
______________________________________ 
While particular embodiments of the process of the invention have been 
shown and described, modifications may be made. It is intended in the 
claims to cover the modifications which come within the true spirit and 
scope of the invention.