A process for purifying solids, especially silicon, by melting and subsequent resolidification, is described. Silicon used in solar cell manufacturing is processed more efficiently and cost effectively

Herzer, H.

Rath, H. J.

Schmidt, D.

English

NASA Technical Reports Server

  
 
 
N O T I C E 
 
THIS DOCUMENT HAS BEEN REPRODUCED FROM 
MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 
CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 
IN THE INTEREST OF MAKING AVAILABLE AS MUCH 
INFORMATION AS POSSIBLE 
https://ntrs.nasa.gov/search.jsp?R=19810010819 2020-03-21T13:34:53+00:00Z
DNASA TECHNICAL MEMORANDUM
	
NASA TM-76551
PROCESS FOR PURIFICATION OF SOLIDS
(NASA-VI-75551) PROCESi FOR PURIFICATION OF	 N81-19346
SOLIDS (Rational Aeronautics and Space
Adainistration) 1 3 p HC A02/MF A01 CSCL 13H
Unclas
63/31 41716
H. Herzer, H. J. Rath.and D. Schmidt
Translation of "Verfahren zum Reinigen von Feststoffen",
Federal Republic of Germany, German Patent Office, Patent Applica-
tion (OFFENLEGUNGSSCHRIFT) 27 22 . 784, November 30, 1978, pp.. 1-10'
Aft r^^
^C
too
	:al
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, DC 20546	 FEBRUARY 1981
r
R$1ANIJARD TITLE PAGE
I Ra*ort No
	 v	 i hGevornment Accos ts*" No.
NASA TH-76551
4. Title and Su ► trHe
PROCESS FOR PURIFICATION OF SOLIDS
' 
 Auehor(sl	
_._.^_^
H. Herzer, H. J. Rath and D. Schmidt
9. PerMrnuny Organisation Name end Address
SCITRAN
P.O. BOX 5456
SANTA BARBARA. CA
 93108
12. Sponsoring Agency Nome and Address
15 Supplomen • o t y Norms
J. Recipient's Catalog No
S. Report Date
February 198E
6. Performing Orgonteot-on Code
4. Performing Otgenuathen Report No,
10. Work Unit No,
11. Contract or Grant No.!_^
NASW-3198
13, Type of Report end Period Coverall
Translation
14, Sponsoring Agency Cods
Translation of "Verfahren zum Erinigen von Feststoffen," Federal
Republic of Germany, German Patent Office, Patent Application
(OFFENLEGUNGSSCHRIFT) 27 22 784, November 30, 1978, pp. 1-10
16	 Abstract
17. Key Wmds (Seletaed by Author(s))
	 ^T	 16. Distribution Statement	 ^
Unclassified - Unlimited
t o
 Secu " ' _faint. tot th ' s report)	 20. Secujoy Cleuif. (of this Page)	 21. No. of Pages	 22. Paco
L Unclassified 	 Unclass ified	 10
NASA HO
ask I
E\
Int. Cl. 3
	
B 01 J 17/18
Federal Republic of Germany
German Patent Office
O"ZNLEGUNGSSCHRI" 27 22 784
Riterence:	 P 27 22 784.5
Application Date;	 May 20, 1977
Open to Public Dates . November 30, 1978
Union Priority:
Title:	 PROCESS FOR PURIFICATION OF SOLIDS
Applicant: Wacker-Chemitronic Company for Basic Electronic
Materials, irbli,	 8263 Burghausen
Inventor: Herzer, Heinz, graduate physicist, Dr.
Rath, Heinz-Jorg, graduate chemist, Dr.
Schmidt, Dietrich, graduate physicist, Dr.
8263 Burghausen.
1
I
F-19.
.Z
b5
i
15
, 15
4 1' .12	
Fi g. I
^^^ AR
4
29	 8
i
1 11 10
27
	3d	
-26
.	 2a	 -28
	. 64
	
JI°
kWACKER-CHEMITRONIC
Company for Basic Electronic
Materials, mbH
Munich, 13 May 1977
L-Pat/Dr. F/we
}
^^ F
3
a
Wa-Ch 7705
PROCESS FOR PURIFICATION OF SOLIDS
Patent Claims:
1. Process for purification of solids by fusion and subsequent
solidification, characterized by the fact that a rotating
roller made of material resistant to the fused solid and
cooled below the melting point of the solid to be purified
is immersed in the melted solid, and the fact that the
resolidified solid adhering to the roller passes through
a subsequent zone of higher temperature, above the melting
point of the solid to be purified, in which the solid
adhering to the roller is remelted and collected in a
suitable device.
2. Process as in Claim 1, characteri-ed by the fact that the
depth of immersion of the roller is adjusted to 0.1 to 0.2
tines the roller diameter.
3. Process as in Claims 1 and 2, characterized by the fact
that the resid--nce time of the roller surface immersed in
the melt of the solid to be purified, expressed in ^anutes,
is 0.005 to 0.25 times the numerical value of the roller
diameter.
4. Process_ as in Claims 1 to 3, characterized by the fact that
the solid resolidified on the roller is remelted before
it is reimmersed, at a distance from the melt surface of
0.1 to 4.5 —, d. measured on the roller circumference
;,here d is the roller diameter, and is collected in a
suitable device.
f
a 5.	 Process as in Claims 1 to 4,	 characterized by the fact that
E the roller is internally cooled.
.6.. Process as in Claims 1 to 5,	 characterized by the fact that
the roller is cooled by a cooling gas flowing-through the
interior of the roller.
5 7.	 Process as in one or more of Claims 1 to 6,	 characterized
by the fact that semiconductor materials are purified.
8. Process as in Claim 7, characterized by the fact that
silicon is purified.
i
9. Process as in Claim 8, characterized by the fact that the
silicon melt is held at a temperature of 1430 0 to 1500°C.
10. Process as in Claim 7 and one or more of Claims 1 to 9,
characterized by the fact that the roller is cooled internally
to a temperature of 800° to 1200°C.
1
	
	
The object of the invention is a process for purifyint
solids, especially silicon, by melting and subsequent
resolidification.
In discussions of energy in recent years, generation
of electrical power by direct conversion of solar er".ergy into
electrical energy has become increasingly important. But, in
order to make silicon solar cells, for instance, economically
competitive, it is essential to make a substantial reduction
in the cost of the raw material of required purity, which exists
on the earth in essentially unexhaustable quantity. Gas phase
purification, such as is standard today in production of silicon
for the electronic industry, is immediately ruled out because
such a process is too energy-intensive and, therefore, too
a
4
Vexpensive by a considerable factor. In addition, the purity
requirements for silicon to be used in solar cells are not as
great as those for 'wafer silicon' to be used in production of
expensive components.
To be sure, it is known from Swiss Patent No. 567 435 that
technical grade silicon can be purified by leaching with hydro-
chloric acid; but this process is extremely slow and gives as
the result a silicon which without further purification is useful
at best as an alloying component, and is not useful as a raw
material for production of electronic components or solar cells.
Purer silicon is also not obtainable from the known process of
gas blowing. With this process, as described in German Patent
10 39 752, silicon is purified by blowing chlorine into the
melt.
The object of the invention, then, was to find a process
of purifying silicon which gives a purer product than the
processes discussed above, and, at the sime time, considerably
lower costs than the purification process based on decomposing
gaseous silicon compounds.
This problem was solved by a process which is not limited
only to silicon, but is generally applicable in purification
of solids, and which is characterized by the fact that a roller
of material inert to the solid to be purified is immersed and
rotated in the fused solid; the roller is cooled below the
melting point of the solid to be purified, and material which
adheres to the roller and solidifies on it passes through a
subsequent zone of higher temperature, above the melting point
of the solid being purified, so that the material solidified
on the roller is again melted and collected in a suitable
device.
The parameters influencing crystal growth are adjusted
so as to yield a crystallization velocity perpendicular to the
W.
5
surface of 0.01 to'2 cm per minute, and preferably 0.2 to 1 cm
per minute. These parameters include the t depth of roller
immersion; the rate at which a point on the roller surface moves
through the melt; the temperature of the melt of the solid
being purified, and the temperature of the roller.
One attempts to get a layer thickness for material adhering to
the roller surface of 0.005 to 0.05 times the roller diameter,
and preferably 0.01 to 0.02 times the roller diameter.
The required immersion depth for the roller in the melt
is adjusted to 0.01 to 0.3 times the roller diameter, and
preferably to 0.1 to 0.2 times the roller diameter.
The late of roller rotation is established so that the
residence time of the roller surface in the melt of the
material to be purified. measured in minutes, is 0.0025
to 0.5 times the roller diameter, and preferably 0.005 to
0.25 times the numerical value of the roller diameter. The
temperature of the melt should not be more than 400° above
the melting point of the solid to be purified.
As a rule, good results are achieved, especially for
low-melting materials, with melt temperatures which are 20°
to 100° above the melting point. For the preferred raw
material, silicon, for instance, a silicon melt temperature
of 1430° to 1500° has proved good.
The melt can be heated by the usual methods, for instance,
by resistance heating, radiant heating, or, in case of
silicon, by suitable induction heating.
The temperature of the roller turning through the melt is,
of course, dependent on the established melt temperature.
If the melt temperature is very high Above the melting point
of the solid to be purified, then in general the temperature
of the roller must be very low; that is, far below the
melting point of the solid being purified, so that crystallization
6
will occur on the roller as it passes through the melt. That is,
if the melt temperature is, for instance, 400° above the melting
point of the solid to be purified, then the roller must be
at a temperature at least 400° to 800 0
 below the melting point
of the solid. With the preferred embodiment of the process,
as already mentioned, one can conveniently set the temperature
of the melt relatively close above the melting point of the
solid, so that the roller temperature need not be so far below
the melting point of the solid. In the purification of silicon,
the silicon melt is, as already stated, held at a temperature
of 1430° to 1500°, so that the recommended roller temperature
1-- from 800° to 1200 0 9 measured at the internal surface of the
roller with a cooling agent flowing through it. The stated
temperature ranges should be understood as average temperatures,
as the part of the roller immersed in the melt will certainly
be hotter than the part above it, which has already been out of
the melt for some time.
For cooling, a
of the roller. The
desired temperature
low melting points,
must be maintained,
could be used. For
gas could be used.
to silicon, it has
through the cooling
cooling medium flows through the interior
cooling medium is adjusted to give the.
Thus, if one must purify solids with
so that cooling temperatures below 1000
then, for instance, a liquid coolant
still lower temperatures, a liquefied
For the preferred application of the process
proved good to use saseous coolants flowing
roll, especially nitrogen or argon.
In general, the material in which the solid to be purified
is melted must be inert to that solid; that is, it must not
caused added contamination of the solid. In the case of
silicon, for instance quartz and especially vitreous carbon
are suitable. The requirements for the crucible material also
apply for the roller to be used. "here is the added requirement
for the rollers that the roller must be wetted b y the material
being purified; otherwise, no material would adhere to the
7
I
-roller. For purification of semiconductors, especially silicon,
vitreous carbon has proved to be a very good roller material.
The effective segregation coefficients of the impurities
considered are decisive for the purification action. After the
roller has continuously rer ved purified solid, such as
purified silicon, the remaining impurities are enriched in the
melt. The purification effect can be increased, for instance, if
the crucible is rotated, or a stirrer is used to move the melt,
or, for instance, even a gas is irrroduced, causing turbulence
and better mixing in the melt. If one wishes to run the process
continuously, the crucible can be repeatedly recharged with
silicon while the melt, enriched with contaminants, is
continuously or at intervals removed from below. The highly
impure melt can then be prepurified by known methods, outside
the containers, for instance, by leaching with acids, before
it is recharged into the crucible.
The resolidified coating of material adhering to the roller
(the crystalline silicon film, for instance) is removed in this
manner: The adhering solid material is remelted before it is
reimmersed, at a distance above the melt surface which is
preferably* 0.2 to 0.5 fi d, measured along the roller circum-
ference, where d is the roller diameter. The adhering
solid, crystallized silicon for example, is melted again by
a water-cooled high-frequency cutting bar running the length of
the roller, such as a single-turn copper or silver coil touching
the silicon adhering to the roller. Other possibilities for
melting the silicon could be attained using suitable laser
radiation focused by a concave mirror or, if the process is
carried out in high vacuum, with an oscillating electron beam.
The solid, melted in one way or another, drops under the
influence of gravity over a scraper, away from the roller
and is collected in a suitable device, e. g., a melt trough,
and is lead out of the reactor. The melt of purified solid
w:uch is drakn off can then, for instance, be taken directly
s	 ^
to a second purificeR.tion system. By setting up several such
systems in series, the purification action can be increased
considerably.
The efficiency of the purification process depends generally
on the effective segregation coefficients (distribution
coefficients). The effective segregation coefficient differs
from the theoretical segregation coefficient because it takes
into consideration factors such as the mixing of the melt, the
crystalization velocity, and the crystallite size.
The process is generally applicable at pressures from 10-9
to 2 bar. Operation under vacuum is preferred because of the
purification effect due to volatilization of impurities, depending
on their particular volatilization coefficients.
The process is particularly suited for purifying metals and
especially semiconductor materials such as germanium and silicon.
It is no doubt of greatest importance for purification of silicon,
which will certainly have to be availabl? in large quantities in
the near future for production of solar cells. As the purifying
action of the process described depends on the segregation
coefficients of the impurities, a contaminant like boron,
with a distribution coefficient of about 0.8 can practically not
be removed from silicon. This is not important because silicon
has to be p-dosed for production of solar cells anyway, so that
boron-dosed silicon is used.
The process of the invention will be explained using the
a figures: Figure 1 shows a system schematically from the side,
in section.. Such a system would be suitable for carrying out
the process of the invention. Figure 2 shows the same system
in section from the front.
The steel reactor 1 has a double-walled jacket through
which water can flow from connections 2 and 3. It has an
internal radiation reflector 4 of molybdenum plate.
9
'The reactor is charged through the closable tube 5 with
1.20 kg silicon having as iron content of 10 ppm. The charge
is placed into a vitreous carbon crucible, 6, having a volume
of 0.04 m3 , which is gated with a resin arne heater 7, under
argon. Before melting, the air in the reactor was flushed out
with argon flowing in from inlet 8, with outlet 9 open.
Then the fused silicon is poured, by tilting the crucible n,
into the vitreous carbon crucible 11 which is underneath.
The crucible 6 is recharged with silicon tfsough the filling
tube 5. Cruetble 11 has a length of 70 cm, a -iidth of 50 cm,
and a height of 20 cm. Then a hollow vitreous carbon roller 12
is immersed 4 cm into the melt surface. The roller, with an
outside diameter of 30 cm and a length of 50 cm with a wall
thickness of 0.4 cm, is mounted by the hollow shaft 13 at
both ends of the roller long axis. The shaft is held in
graphite-lubricated bearings in mounts i4 and 15. The distance
of the mounts 14 and 15 from the floor, 16 and 17, can be
adjusted by adjusting screws 18 and 19 on the stand, 20, 21.
This makes it possible to lower the roller 12 into the melt,
along with its mounts, 14 and 15, by creans of the gas-tight
stainless steel bellows 22 and 23. The roller 12 was then
driven through a gear drive, by the electric motor 24, at
9.5 rotations per minute, corresponding to a peripheral speed
of 14.25 cm/minute for any point on the surface. During this,
argon at about 25°C is passed through the hollow shaft at a
rate of 2 standard cvbic meters per hour, giving a mean
temperature of about 1000°C for the inside surface of the
roller. For a crystallization velocity of about 0.4 cm per
minute perpendicular to the surface, the silicon layer at
the surface of the roller had grown to about 0.6 cm thick when
it emerged from the melt. After moving about 220° after
emerging from the melt, at about 15 cm before reimmersion
(measured along the roller surface), the adhering silicon
layer was remelted by the one-turr water-cooled copper induction
coil 25, so that liquid silicon ..copped off. The liquid silicon
was passed out of the reactor through the outlet trough 27, which
was heated above the melting point of silicon by the resistance
10
heater 26. The sidearm 28 which takes up the silicon flow
could, if desired, be connected to an argon-covered silicon
collector vessel; or it could be connected to the filler tube, 5,
of a second similar purification system. Only one purification
step was carried out with the e.% mple depicted here, though.
That is, the silicon was collected and its purity was determined.
For a take-off of 59 kg purified material per hour, the process
was terminated after 48 hours. At hourly irLek As during this
period, 4 kg of melt enriched with impurities was drained off
through the outlet pipe 29 in the bottom of crucible 11, by
actuating lever 30. This was drained into the vessel 31,
of vitreous carbon, which could be unscrewed. The 190 kg of
residual melt collected during 48 hours was pre-purified by
leaching with hydrochloric acid and added to the silicon of
a new charge. The 2,832 kg purified silicon obtained in the
same period he an iron content of only 10 ppb by weight. The
decrease in the other impurities was correspondingly extenEive
except for Iron and, to a limited extent, phosphorus. The
p-conducting silicon thus obtained could immediately be cast
into appropriate plates for production of solar cells.
1?.


Process for purification of solids

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server