A susceptor for facilitating induction heating of silicon melt is described. The susceptor comprises a pair of susceptor halves of a thickness less than two skin depths, each being the mirror image of the other, disposed in mutually opposed, electrically insulated relation. The crucible comprises a quartz body supported by the graphite susceptor, whereby the R-F coil is electrically coupled with the melt

Berkman, S.

Temple, H. E.

English

NASA Technical Reports Server

United States Patent [191 
Berkman et al. 
11 4,242,553 
r45i Dee. 30. 1980 
[54] APPARATUS FOR USE IN THE 
CRYSTALS FROM A SILICON MELT 
1761 Inventors: Robert A. Frosch, Administrator of 
PRODUCTION OF RIBBON-SHAPED 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Samuel Berkman, 
Florham Park; Harold E. Temple, 
Trenton, both of N.J. 
[21] Appl. No.: 938,299 
[22] Filed: Aug. 31, 1978 
[51] Int. Cl.3 ......................... H05B 6/30; C30B 15/16 
[52] U.S. C1. .......................... 219/1@.49 R 219/10.67; 
422/246; 422/249; 156/608; 156/DIG. 96; 
432/264 
Field of Search ............... 219A0.49, 10.43, 10.41, 
219/10.57, 10.67; 156/DIG. 96, DIG. 88, 615, 
608; 422/246,249, 156; 432/252,263,264,265; 
13/26, 27, DIG. 1 
[58] 
1561 References Cited 
US. PATENT DOCUMENTS 
2,537,289 1/1951 Van Embden .................... 219/10.49 
3,180,917 4/1965 Morrison et al. ................. 219/10.49 
3,351,686 11/1967 Gayet et al. ...................... 219/10.43 
3,954,551 VI976 Cecil et al. .................. 156/DIG. 88 
4,157,373 6/1979 Berkman et al. ..................... 422/246 
3,431,326 3/1969 Letter ................................ 219/10.41 
OTHER PUBLICATIONS 
“Basics of Induction Heating”, Tudbury, 1960, vol. 1, 
pp. 77-80, Hollow Workpieces. 
Primav Examiner-B. A. Reynolds 
Assistant Examiner-Philip H. Leung 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning 
1571 ABSTRACT 
An improved apparatus characterized by an R-F coil, a 
crucible for confining a silicon melt, and a susceptor for 
supporting the crucible facilitating an electrical cou- 
pling of the coil with the melt. The susceptor comprises 
a pair of susceptor halves of a thickness less than two 
skin depths, each being the mirror image of the other, 
disposed in mutually opposed, electrically insulated 
relation, while the crucible comprises a quartz body 
supported by the graphite susceptor, whereby the R-F 
coil is electrically coupled with the melt. 
4 Claims, 4 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19810010862 2020-03-21T09:26:33+00:00Z
U.S. Patent Dec. 30, 1980 Sheet 1 of 2 4,242,5 5 3 
U.S. Patent Dec. 30, 1980 
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Sheet 2 of 2 4,242,553 
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4,242,553 
1 
APPARATUS FOR USE IN THE PRODUCTION OF 
MELT 
RIBBON-SHAPED CRYSTALS FROM A SILICON 
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 U.S.C. 2457). 
BACKGROUND O F  THE INVENTION 
1. Field of the Invention 
The invention generally relates to an apparatus em- 
ployed in the production of ribbon-shaped crystals and 
more particularly to an improved susceptor for facilitat- 
ing induction heating of silicon melt. 
2. Description of the Prior Art 
The prior art is, of course, replete with devices 
adapted to be employed in the production of semi-con- 
ductor ribbon, such as silicon ribbon, suited for use in 
the production of solar cells. 
For example, one known process for producing sili- 
con ribbons employs a V-shaped longitudinal trough 
having an inner wall forming a die for confining silicon 
melt. The die is provided with a longitudinal slit at the 
lower end of the V-trough through which a slit of rib- 
bon is pulled. This device comprises a shaping guide 
including a pair of plates arranged in a V-trough config- 
uration and supported by a one-piece susceptor formed 
of graphite and heated by induction heating coils effi- 
ciently coupled with the susceptor to induce a heating 
of the susceptor. 
While a small heating current incidentally may be 
established in the melt, by the R-F coil aforementioned, 
it has been found that the susceptor wall is maintained at 
a temperature greater than the temperature of the melt. 
For example, it has been determined that for a silicon 
melt having a melting point of 1420" C., the average 
temperature of the susceptor wall is 50" C. to 200" C. 
hotter than the melt. This fact, it is believed, clearly 
supports the contention that the susceptor is heated by 
induced heating currents and that the melt is heated by 
the susceptor through conduction. 
Such, of course, requires that the wall of the sus- 
ceptor be heated to a temperature well above that at 
which the melt is required to be heated. As a conse- 
quence, excessive energy is consumed in the process. 
It is, therefore, the general purpose of the instant 
invention to provide an improved apparatus which 
facilitates a direct coupling of an R-F coil with a silicon 
melt, whereby simplicity in construction, enhanced 
control of thermal gradients, and a conservation of 
input energy are realized. 
OBJECTS A N D  SUMMARY OF THE 
INVENTION 
It is, therefore, an object of the invention to provide 
an improved device which facilitates an electrical cou- 
pling of an induction heating coil with the melt in a 
process for producing ribbon-shaped crystals. 
It is another object to provide in an apparatus for use 
in the pulling of ribbon-shaped crystals an improved 
susceptor including a quartz crucible which is easier to 
fabricate, economical, and simple to employ. 
These and other objects and advantages are achieved 
through the use of a graphite susceptor supporting 
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2 
quartz die plates forming a crucible of a V-trough con- 
figuration, including a pair of susceptor halves, each 
being the mirror image of the other, disposed in op- 
posed, electrically insulated relation as will hereinafter 
become more readily apparent by reference to the fol- 
lowing description and claims in light of the accompa- 
nying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a fragmented side etevational view of an 
apparatus which embodies the principles of the instant 
invention. 
FIG. 2 is a cross sectional view taken generally along 
line 2-2 of FIG. 1. 
FIG. 3 is a top plan view of the apparatus shown in 
FIGS. 1 and 2. 
FIG. 4 is a cross sectional view taken generally along 
line 4-4 of FIG. 3. 
DESCRIPTION O F  THE PREFERRED 
EMBODIMENT 
Referring now to the drawings wherein like refer- 
ence characters designate like or corresponding parts 
throughout the several views, there is shown in FIG.  1 
an apparatus, generally designated 10 which embodies 
the principles of the instant invention. The apparatus 10 
includes an improved susceptor 12 including a pair of 
susceptor halves, designated 12a and 126. 
As shown, the susceptor 12 is received within a cylin- 
drical envelope 14 formed of a suitable material. It will 
be appreciated, of course, that the configuration of the 
envelope 14 is varied as desired. Disposed in circum- 
scribing relation with the envelope 14 there is an R-F 
coil 16 coupled with a suitable source of power. Since 
R-F coils and their functions are well known, a detailed 
description of the coil 16 is omitted in the interest of 
brevity. However, it is to be understood that the coil 
serves to heat nominally electrical conductive material 
through currents induced by a varying electro magnetic 
field. These induced currents often are referred to as 
eddy currents. 
The susceptor 12, in practice, serves to support a 
crucible 18 for a body of melt 20, FIG. 2. The crucible 
18 in effect forms a lining for the susceptor. While the 
crucible preferably is formed of quartz having a plate 
glass finish, it is important to understand that other 
non-electrical conducting materials including silicon 
nitride, mulite and aluminum oxide will suffice. It is 
important to note that the crucible includes side plates 
22 supported in downwardly converging planes, prefer- 
ably at an angle of approximately 30" with respect to the 
vertical whereby the crucible is caused to assume a 
V-trough configuration. 
The lowermost edge or apex of the crucible 18 is 
provided with an elongated slot defining a die 24. The 
die 24, in practice, is formed by forming in each of the 
plates 22 a recess which extends along an edge portion 
thereof. Each recess is positioned to form a mirror 
image of the other. Since such dies are well known, a 
more detailed description of the die 24 is omitted in the 
interest of brevity. It suffices to understand that the die 
24 comprises an elongated slot extending along the 
lowermost edge or apex portion of the crucible 18 and 
that the die facilitates a discharge of the melt from the 
crucible as a ribbon-shaped crystal 25 is pulled down- 
wardly from the melt. 
4,242,5 5 3 
3 
As a practical matter, it is important to note that the 
crucible 18 also includes a pair of mutually spaced end 
plates, designated 26. The purpose of the end plates is to 
define opposite ends for the crucible 18. In practice, the 
plates 26 also are formed of quartz and are configured to 
assume an inverted triangular configuration. 
In view of the foregoing, it can now be appreciated 
that the crucible 18 is of a generally V-shaped configu- 
ration, preferably is formed of quartz, and serves to 
coniine therein an electrical conducting melt, such as a 
silicon melt, to be heated by the R-F coil 16. 
Some electrical conducting crucible side plates 22 
can be used with non-electrical end plates 26. For exam- 
ple, vitreous carbon crucible die plates 22 coated with 
C V D  silicon nitride have been successfully tested with 
end plates 26 constructed of quartz. The quartz end 
plates 26 provide electrical isolation of the two con- 
ducting side plates 22. In this manner, each side plate 22 
becomes an electrical part of 12a or 12b and is thus slab 
heated. The combination of slab heated parts is subject 
to  the same electrical design rules for inefficient slab 
heating as 12a and 126 alone. Hence by suitable choice 
of materials and dimensions, the melt 20 of an electrical 
conducting material like silicon can be efficiently 
heated directly by induction heating. 
It is here noted that each of the susceptor halves, 
designated 1% and 12&, includes a side plate 28 of a 
substantially planar configuration, and a pair of end 
plates 30. The end plates 30 are normally related to the 
plane of their respective side plates and are disposed at 
each of its opposite ends. It is important, also, to note 
that between the susceptor halves 12a and 12b there is 
provided a longitudinal air gap 32. The air gap is bi- 
sected by a bisecting plane passing vertically through 
the center of the die 24. The gap 32, as a practical mat- 
ter, serves to electrically insulate each of the susceptor 
halves from the other for thus interrupting paths for 
heating currents established therein by the R-F coil. 
Each of the end plates 26 is supported in a fixed rela- 
tionship with the side plates 22 of the crucible by a pair 
of graphite shims 33. These shims are provided for 
assuring that a snug fit is established between the plates 
22 and 26. The shims 33 of each pair also are separated 
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by an air-gap approximating the tiansverse dimension of 
the air gap 32. Thus, the shims 33, at each of the oppo- 45 
site ends of the susceptor also are supported in a mutu- 
ally insulated relationship. 
In practice, the susceptor halves 12a and 126 of the 
susceptor are interconnected by a pair of vertically 
spaced strap plates 34 and 36. These plates extend along 50 
the upper and lower edge surfaces of the end plates 30 
while a pair of vertically oriented pins 38, formed of any 
suitable material, preferably sapphire, and extend 
through the end plate 30 and pass through the strap 
plates 34 and 36 in place. While the pins 38 are, in prac- 55 
tice, fabricated from sapphire other materials such as 
graphite may be employed. Further, the strap plates are 
fabricated from suitable electrical insulated materials 
such as sapphire, quartz or other ceramic. 
so-called slab heating technique, the frequency at which 
an R-F coil is operated determines skin depth character- 
istics of the coil, or the depth of current penetration. 
For  an efficient heating of a work piece it is necessary 
for the thickness thereof to be sufficient to permit a 65 
doubling-over of the current path. In the event the 
thickness of the work piece is not at least two skin 
depths, a doubling-over of the current paths is pre- 
As will be appreciated by those familiar with the 60 
4 
cluded so that current paths through the work piece 
thus are interrupted and rendered incomplete, where- 
upon a current cancelling effect is experienced with an 
attendent inefficient coupling of the R-F coil and work 
piece. 
The particular frequency at which the R-F coil 16 is 
operated is determined by the thickness of the plate 
forming the susceptor 12. The frequency may be calcu- 
lated, in a known manner, o r  established imperically, if 
so desired, in order to achieve a desired inefficient cou- 
pling of the coil 16 with the susceptor, so that a more 
efficient coupling of the coil 16 with the melt is thus 
achieved whereby a direct heating of the melt by the 
coil 16 is facilitated. Finally, it is noted that the end 
plates 30 are provided with a plurality of slots 42 of a 
depth such that the effective thickness of the end plates 
is less than two skin depths whereby a doubling-over of 
the current paths therethrough is thus avoided. 
With the apparatus 10 assembled in the manner here- 
inbefore described, it is possible to electrically couple 
the coil 16 with the melt confined in the crucible 18 so 
that eddy currents are established in principally the melt 
as well as incidently in the susceptor. This, of course, 
constitutes a reversal of the phenomena experienced 
when induction heating techniques of the prior art are 
relied upon for heating silicon melt. As a consequence 
of the coupling of the coil 16 with the melt, it is possible 
to attain a higher average temperature for the melt than 
that attained for the susceptor walls. Hence, the molten 
silicon or melt 20 is caused to serve as a primary source 
of heat and, as a result, important thermal gradients are 
easier to control and the input energy to the system is 
conserved. 
It should readily be apparent that the improved appa- 
ratus embodying the principles of the instant invention 
provides a practical solution to the problem of how to 
maintain a susceptor at lower temperatures than the 
melt contained therein when utilizing R-F coils as a 
source of energy. 
What is claimed is: 
1. In an apparatus for use in the pulling of ribbon- 
shaped crystals downwardly through a die set defined 
between a pair of quartz plates disposed in downwardly 
converging planes for forming an elongated, V-shaped 
crucible containing an electrical conductive melt, 
means for coupling induction heating energy to the melt 
comprising: 
A. susceptor supporting said crucible including a pair 
of susceptor halves having a pair of side plates 
disposed in mutually spaced opposed relation, each 
of said side plates being substantially planar in 
shape and formed of an electrically conductive 
material, said pair of plates being downwardly 
inclined toward each other and having an electri- 
cally insulative air-gap defined therebetween 
throughout the length thereof for electrically iso- 
lating the side plates, each from the other, and a 
pair of end plates integrally related with each side 
plate and disposed in a pair of planes normally 
related to the plane of the side plate, each end plate 
being characterized by a plurality of slots extended 
across one face thereof, whereby the effective 
thickness thereof is reduced; and 
B. means for effecting an induction heating of the 
electrical conductive melt comprising an R-F coil 
concentrically related to the susceptor and electri- 
cally coupled with the melt, said R-F coil being 
characterized by an operational frequency such 
4,242,553 
5 6 
that the thickness of the side plates and the effec- plates formed of an electrically non-conductive material 
tive thickness of the end plates is less than two skin disposed in downwardly converging planes, each plate 
depths for the operational frequency. being provided with a recess comprising a mirror image 
2. In the apparatus of claim 1 wherein the crucible of the other for defining between the plates an inverted 
further includes a pair of mutually spaced end plates of 5 die, and heating means including an R-F coil concentri- 
inverted triangular configurations, formed of quartz and cally related to the crucible and characterized by a 
extended transversely between the side plates of said known operational frequency, the improvement com- 
susceptor halves, and wherein the improved susceptor prising: 
further comprises a pair of shims comprising graphite A. means for electrically coupling the R-E coil with 
plates interposed between each end plate of the crucible 10 the melt comprising a susceptor supporting said 
and an end plate of a susceptor half, the shims at each of crucible including a pair of electrically conductive 
the opposite ends of said crucible being mutually spaced susceptor halves disposed in mutually electrically 
for defining therebetween an electrically insulative air- insulated relation, whereby electrical paths be- 
gap. tween the halves is interrupted, each susceptor half 
comprising a side plate and a pair of end plates 
p r i s i n g : normally related to the side plates, each of said side 
plates being characterized by a thickness less than 
that required to sustain a doubling-over of current 
paths through the side plate at the operational fre- 
quency of said R-F coil, whereby current paths 
4. In an apparatus for use in the production of ribbon- established in said side plates are completed 
through the melt. 
3. The improved susceptor of claim 2 further com- 15 
a pair of strap plates formed of non-conductive mate- 
rial extended between adjacent ends of the sus- 
ceptor halves and affixed thereto for uniting said 
susceptor halves in an integrated configuration. 20 
shaped crystals from a silicon melt characterized by a 
crucible of a V-trough configuration including a pair of * * * * *  
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Apparatus for use in the production of ribbon-shaped crystals from a silicon melt

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