A method in which a quartz tube is charged with chunks of metallurgical grade silicon and/or a mixture of such chunks and high purity quartz sand, and impurities from a class including aluminum, boron, as well as certain transition metals including nickel, iron, and manganese is described. The tube is then evacuated and heated to a temperature within a range of 800 C to 1400 C. A stream of gas comprising a reactant, such as silicon tetrafluoride, is continuously delivered at low pressures through the charge for causing a metathetical reaction of impurities of the silicon and the reactant to occur for forming a volatile halide and leaving a residue of silicon of an improved purity. The reactant which included carbon monoxide gas and impurities such as iron and nickel react to form volatile carbonyls

Chaney, R. E.

Ingle, W. M.

Thompson, S. W.

English

NASA Technical Reports Server

United States Patent [191 
Ingle et al. 
[111 4,172,883 
[45] Oct. 30, 1979 
[54] METHOD OF PURIFYING 
METALLURGICAL GRADE SILICON FOREIGN .PATENT DOCUMENTS 
EMPLOYING REDUCED PRESURE 625027 8/1961 Canada ..................................... 423/348 
ATMOSPHERIC CONTROL 1039752 9/1958 Fed. Rep. of Germany ........... 423/348 
[76] Inventors: 
[21] Appl. No.: 
[22] Filed: 
Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention by, William M. Ingle; 
Stephen W. Thompson, both of 
Phoenix; Robert E. Chaney, 
Scottsdale, all of Ariz. 
918,537 
Jun. 23,1978 
[51] Int. c1.2 .............................................. COlB 33/02 
[52] U.S. Cl. .................................... 423/348, 423/149; 
423/293; 423/417; 423/625 
[58] Field of Search ............... 423/348, 149, 293, 625, 
423/417 
PI References Cited 
U.S. PATENT DOCUMENTS 
1,103,747 7/1914 Fierz .................................... 423/149 
2,989,376 6/1961 Schaefer ............................... 423/349 
3,034,886 5/1962 Chambery et al. .............. 423/348 X 
3,097,068 7/1963 Litz et al. ............................. 423/348 
4,045,541 8/1977 Mercer ............................. 423/149 X 
4,070,444 1/1978 Ingle ..................................... 423/349 
Primary Examiner-Jack Cooper 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Wilfred Grifka 
[571 ABSTRACT 
A method wherein a quartz tube is charged with chunks 
of metallurgical grade silicon and/or a mixture of such 
chunks and high purity quartz sand, and impurities from 
a class including aluminum, boron, and the like, as well 
as certain transition metals including nickel, iron, man- 
ganese .and the like. The tube is then evacuated and 
heated to a temperature within a range of 800" C. to 
1400" C., whereupon a stream of gas comprising a reac- 
tant, such as silicon tetrafluoride, continuously is deliv- 
ered at low pressures through the charge for causing a 
metathetical reaction of impurities of the silicon and the 
reactant to occur for forming a volatile halide and leav- 
ing a residue of silicon of an improved purity. Addition- 
ally, the reactant may include carbon monoxide gas, 
whereby impurites such as iron and nickel react there- 
with to form volatile carbonyls. 
4 Claims, 1 Drawing Figure 
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Oct. 30, 1979 
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4,192,883 e 1 
METHOD OF PURIFYING METALLURGICAL 
GRADE SILICON EMPLQYING REDUCED 
PRESUFtE ATMOSPHERIC CON'EWOH. 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention generally relates to a process for use in 
purifying metallurgical grade silicon, and more particu- 
larly to a method of prepurifying metallurgical grade 
silicon through atmospheric control to remove a major 
portion of the impurities in producing silicon of a purity 
high enough for direct solar cell fabrication employing 
conventional Czochralski crystal growth processes. 
2. Description of the Prior Art 
Silicon solar cells, of course, are well known and 
have long been employed in celestial space environ- 
ments. However, as is discussed in the instant inventor's 
prior U.S. Pat. No. 4,070,444, there recently has devel- 
oped an .increased interest in the use of photovoltaic 
silicon solar energy cells in the production of electrical 
energy for terrestrial uses. Unfortunately, the processes 
currently employed in obtaining silicon of a purity suffi- 
cient to fabricate such cells are relatively expensive, 
particularly in terms of the energy required in the pro- 
duction thereof. 
Various attempts have been made to purify metallur- 
gical grade silicon through chemical processes in which 
silicon halides are formed. For example, see U.S. Pat. 
Nos. 2,989,376 to Schaefer; 3,034,886 to Pruvot et al; 
and 4,070,444 to Ingle. However, the common ap- 
proach has been simply to remove silicon from impuri- 
ties. 
b 
for liberating the reactant, as will become more readily 
apparent by reference to the following description and 
claims in light of the accompanying drawing. 
DESCRIPTION OF THE DRAWING 
The FIGURE is a diagrammatic view illustrating a 
system typifying a system employed in performing a 
method which embodies the principles of the instant 
invention. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
With reference to the FIGURE, it is noted that the 
system includes a quartz tube, generally designated 10, 
l5 having an inside diameter of one to four inches and an 
overall length of twenty-four to thirty-six inches. While 
the tube is illustrated in a horizontal disposition, it often 
is found convenient to support the tube PO in a vertical 
orientation whereby the obvious advantages of gravita- 
2o tion is realized. Suitable heating means, generally desig- 
nated 12, are provided for heating the quartz tube to a 
suitable temperature, preferably within a range of 800" 
C. to 1400" C. As illustrated, the heating means 12 com- 
prises a resistence heating coil 14 connected to a suitable 
25 power circuit 16. However, it is to be understood that, 
where desired, the heating means 12 includes an RF 
coil. Of course, when employing an RF coil a graphite 
susceptor is provided for reasons well understood by 
The tube 18 is suitably capped at each of its opposite 
ends, designated 18 and is connected in communication 
with a source of vacuum 19. Suitable caps, not shown, 
provided for purposes of capping the tube include ori- 
35 fices coupled in communicating relation with the inte- 
rior of the tube. A nipple 20 is employed at each end of 
the quartz tube 10 for connecting caps to a circuit for a 
reactant gas. The circuit includes a supply system gen- 
erally designated 22, and a recycling system, generally 
designated 24. The purpose of the supply system 22 is to 
5 
10 
3o those familiar with such devices. 
." 
It has been found possible to acquire a greater yield of ProGide a reactant gas to the tube while the purpose of 
silicon from metallurgical grade silicon, two or three the recycling system 24 is to reclaim and return reactant 
times better than that obtainable when employing pro- gas to the supply system 22. In practice, the system 24 
ceSSeS of the prior ad, by removing impurities from the includes a bed of Amberlite which serves to absorb 
metallurgical grade silicon. 45 boron. However, where so desired, the system includes 
It is, therefore, the general purpose of the instant a distillation purification unit, not shown. 
invention to provide an improved method for use in As shown, the SUPPlY system 22 includes a Source of 
removing impurities from metallurgical grade silicon. silicon tetrafluoride 26 connected to the quartz tube 10 
via a flow control circuit 28. The flow control circuit 
OBJECTS AND SUMMARY OF THE 50 28, where SO desired, also serves to connect a source of 
carbon monoxide to the quartz tube. Since the details of INVENTION 
It is an object of the instant invention to provide an the Supply system form no specific part of the instant 
improved method for purifying metallurgical grade invention and is of a convenient design, a detailed de- 
silicon. scription thereof is omitted in the interest of brevity. 
It is another object to provide a method wherein 55 However, it is to be understood that the system 22, in 
metallurgical grade silicon is purified, employing re- effect, is adapted to supply to the tube 10 a reactant gas 
duced pressure atmospheric control, in removing impu- and that the gas preferably comprises silicon tetrafluo- 
rities therefrom. ride, or where so preferred, a mixture of silicon tetraflu- 
These and other objects and advantages are achieved oride and carbon monoxide. 
through a method wherein a quartz tube is charged with 60 For purposes of providing for a complete understand- 
chunks of metallurgical grade silicon and/or Si02 sand, ing of the instant invention, the following example is 
heated to a temperature within a range of 800" C. to provided. 
1400" C., preferably 1300" C. to 1350" C., and treated In a series of experiments, a quartz tube having an 
with a stream of gas comprising silicon tetrafluoride inside diameter of one to four inches was charged with 
and/or carbon monoxide for thus causing to occur a 65 one to five kilograms of metallurgical grade silicon. The 
metathetical reaction of impurities with the gas leaving metallurgical grade silicon was provided in the form of 
a residue of silicon of improved purity, and thereafter chunks of 0.1 to ten millimeters. The tube 10 was then 
reacting the resultant compound with silicon monoxide evacuated and heated employing a resistence heating 
3 
4,172,883 
coil 14 to temperatures within a range of 8W C. to 
1350" C. Thereafter, silicon tetrafluoride was passed 
through the charge for periods of four to twenty hours. 
In one such experiment, the quartz tube was sec- 
'ioned in four places and metallurgical grade silicon 
samples removed for analysis. The analysis revealed 
that the aluminum concentration of the metallurgical 
grade silicon had been reduced from 0.5% (five thou- 
sand ppm) to 2-3 ppm, or purification of greater than 
three orders of magnitude. Importantly, boron was 
reduced from 50 ppm to 3-5 ppm. Also, it was noted 
that a removal of titanium, barium, copper, magnesium 
and calcium occurred, although not extensive, it was 
apparent that such purification did occur. 
A number of factors apparently affect atmospheric- 
control purification of the metallurgical grade silicon. 
The first step in the purification process appears to be a 
metathetical reaction of the impurity with the silicon 
tetrafluoride. In cases of aluminum and boron the fol- 
lowing reactions occur: 
4A1+3SiF&4AlF3+Si 
4B +3SiF454BF3+Si 
It is believed that the 12 AI-F or B-F bonds are 
formed at the expense of 12 Si-F bonds. In as much as 
the AI-F bond in AlF3 is 17K cal/mole and the B-F 
bond in BF3 is 38K cal/mole stronger than the corre- 
sponding silicon fluoride bond in silicon tetrafluoride, 
the reaction would be expected to proceed rapidly. 
Once aluminum trifluoride is formed it contacts a hot 
Si02 surface, the aluminum trifluoride forms silicon 
tetrafluoride and aluminum oxide. The aluminum oxide 
is deposited as a coating on the quartz, the major por- 
tion thereof being deposited within the tube. It also has 
been found, upon analysis, that boron, copper, calcium 
and magnesium are volatized and concentrated in the 
cooler zones outside the furnace. 
It should be apparent that improved purification of 
metallurigical grade silicon through the use of silicon 
tetrafluoride, occurs for many elements. Even though 
little or no purification for the transition metals, such as 
iron, nickel, maganese, colbalt and so forth was found to 
occur, the process is viable when used in conjunction 
with a further purification process such as Czochralski 
4 
crystal growth. Further, where so desired, a mixing of 
SiF4 with CO gas permits the transition metals to form 
carbonyls which are volatile and thus may be removed 
from the tube 10 simultaneously with the halides. For 
5 example, it is known that nickel reacts with CO above 
200" C. to form Ni (CO)4 a volatile gas. Similarly, Fe, 
Mn and CO carbonyls may be formed and removed 
from the tube. 
In view of the foregoing, it is believed to be apparent 
10 that the method of the instant invention provides a 
practical technique for removing main group elements 
from metallurgical grade silicon at lower costs and 
higher rates, whereby production of solar grade silicon 
is enhanced at greatly reduced costs. 
l5 What is claimed is: 
1. In a method of purifying metallurgical grade sili- 
A. charging a quartz tube with chunks of metallurgi- 
cal grade silicon containing impurities consisting 
essentially of aluminum and boron; 
B. heating the silicon to a temperature within a range 
of 800" C. to 1350" C. in vacuo; 
C. continuously passing through said chunks a stream I 
of reactant gas at 0.1 torr to one atmosphere con- 
sisting essentially of silicon tetrafluoride to effect a 
metathetical reaction of aluminum and boron impu- 
rities of the silicon with the silicon tetrafluoride 
with AlF3 and BF3 being formed and leaving a 
residue of silicon of improved impurity; and 
wherein the A1F3 formed reacts 
with the surface of the quartz tube to form silicon 
tetrafluoride and aluminum oxide which deposits as 
a coating on said surface. 
2. The method of claim 1 wherein the chunks are of a 
35 size within a range of 0.1-10 mm, and the reactant gas is 
passed through said chunks for a period of four to 
twenty hours. 
3. The method of claim 2 wherein the quartz tube is 
4o characterized by an inside diameter of 1-4 inches and a 
length of 24-36 inches. 
4. The method of claim 2 wherein the impurities 
further contain nickel and the reactant gas further con- 
tains carbon monoxide and wherein the carbon monox- 
con, the steps comprising: 
2o 
25 
30 
45 ide reacts with the nickel to form volatile Ni(C0)4. * * * * *  
50 
55 
60 
65 


Method of purifying metallurgical grade silicon employing reduced pressure atmospheric control

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