U.S. patent number RE31,473 [Application Number 06/354,015] was granted by the patent office on 1983-12-27 for system for fabrication of semiconductor bodies.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Jack S. Kilby, William R. McKee, Wilbur A. Porter.
United States Patent |
RE31,473 |
Kilby , et al. |
December 27, 1983 |
System for fabrication of semiconductor bodies
Abstract
A system and method is provided for forming semiconductor
tear-drop shaped bodies having minimal grain boundaries.
Semiconductor material is melted in a capillary tube at the top of
a tower, and forced under gas pressure through a nozzle. Separate
semiconductor bodies are formed. They are passed through a free
fall path over which a predetermined temperature gradient controls
solidification of the bodies. The resultant bodies are tear-drop
semiconductor bodies of near uniform size with minimal grain
boundaries.
Inventors: |
Kilby; Jack S. (Dallas, TX),
McKee; William R. (Plano, TX), Porter; Wilbur A.
(College Station, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
26998193 |
Appl.
No.: |
06/354,015 |
Filed: |
March 2, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
766223 |
Feb 7, 1977 |
04188177 |
Feb 12, 1980 |
|
|
Current U.S.
Class: |
425/6; 264/13;
425/DIG.13 |
Current CPC
Class: |
B22F
9/08 (20130101); C30B 29/60 (20130101); C30B
29/06 (20130101); C30B 11/00 (20130101); C30B
29/08 (20130101); C30B 11/00 (20130101); C30B
29/08 (20130101); C30B 11/00 (20130101); C30B
29/60 (20130101) |
Current International
Class: |
B22F
9/08 (20060101); C30B 11/00 (20060101); B29C
023/00 (); B29D 031/00 () |
Field of
Search: |
;425/5,6,143,404,445,446,DIG.13 ;264/5,13,26,12
;156/604,617R,617H |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hoag; Willard E.
Attorney, Agent or Firm: Richards, Harris & Medlock
Claims
What is claimed is: .[.1. A system for fabricating small
semiconductor bodies, which comprises:
(a) a capillary tube having heat transmissive properties for
receiving a solid charge of semiconductor material and having a
nozzle for discharging the semiconductor material;
(b) an outer tube enclosing said capillary tube;
(c) a heat susceptor internal to said outer tube and encircling
said capillary tube;
(d) heating means for energizing said heat susceptor to render said
charge molten; and
(e) means to pressurize said capillary to force said molten charge
through said nozzle..]. .[.2. The combination set forth in claim 1,
wherein: a removable end seal is connected to the upper end of said
capillary tube..]. .[.3. The combination set forth in claim 2,
wherein said outer tube has a gas port for flow of purge gas
through said system..]. .[.4. A system for forming small
semiconductor bodies of near uniform size with minimal grain
boundaries from bodies of semiconductor material, which
comprises:
(a) a feed tube for receiving said bodies;
(b) a heat susceptor encircling said feed tube and encompassing a
free fall path below said feed tube;
(c) a tube enclosing said susceptor; and
(d) heating means encircling said tube for establishing a
temperature level near said feed tube above the melting point of
said semiconductor material and establishing a predetermined
temperature gradient over the next lower portions of said free fall
path to a temperature below said melting point..]. .[.5. The
combination set forth in claim 4, wherein.]. .Iadd.A system for
forming small semiconductor bodies of near uniform size with
minimal grain boundaries from bodies of semiconductor material,
which comprises:
(a) a feed tube for receiving said bodies;
(b) a heat susceptor encircling said feed tube and encompassing a
free fall path below said feed tube;
(c) a tube enclosing said susceptor; and
(d) heating means encircling said tube for establishing a
temperature level near said feed tube above the melting point of
said semiconductor material and establishing a predetermined
temperature gradient over the next lower portions of said free fall
path to a temperature below said melting point, .Iaddend.said
heating means .[.is.]. .Iadd.being .Iaddend.an induction heating
coil of uneven diameter and turn density along the length thereof
for maintaining said temperature level and gradient over said free
fall path. .[.6. A system for forming small semiconductor bodies
having near uniform size and minimal grain boundaries from a charge
of semicondcutor material, which comprises:
(a) a capillary tube converging at the lower end to form a
nozzle;
(b) a cylindrical heat susceptor encircling the lower portion of
said capillary tube;
(c) an elongated tube encircling said heat susceptor and said
capillary and encompassing a free fall path below said nozzle of a
length to accommodate the solidification of molten semiconductor
bodies issuing from said nozzle;
(d) further heat susceptor structure axially aligned with said tube
along said free fall path;
(e) a pair of split heat shields concentric to each other and
encircling said further heat susceptor structure; and
(f) heating structure encircling said elongated tube in the area of
said heat susceptor for melting said charge and extending along
said further heat susceptor structure for establishing an
atmosphere of controlled temperature gradient in the region of said
further heat susceptor structure where the temperature decreases to
a point below the melting point of said semiconductor..]. .[.7. The
Combination set forth in claim 6, wherein.]. .Iadd.A system for
forming small semiconductor bodies having near uniform size and
minimal grain boundaries from a charge of semiconductor material,
which comprises:
(a) a capillary tube converging at the lower end to form a
nozzle;
(b) a cylindrical heat susceptor encircling the lower portion of
said capillary tube;
(c) an elongated tube encircling said heat susceptor and said
capillary and encompassing a free fall path below said nozzle of a
length to accommodate the solidification of molten semiconductor
bodies issuing from said nozzle;
(d) further heat susceptor structure axially aligned with said tube
along said free fall path;
(e) a pair of split heat shields concentric to each other and
encircling said further heat susceptor structure; and
(f) heating structure encircling said elongated tube in the area of
said heat susceptor for melting said charge and extending along
said further heat susceptor structure for establishing an
atmosphere of controlled temperature gradient in the region of said
further heat susceptor structure where the temperature decreases to
a point below the melting point of said semiconductor,
.Iaddend.said heating means .[.is.]. .Iadd.being .Iaddend.an
induction heater work coil of uniform diameter at
the top and hour-glass shape therebelow. 8. The combination set
forth in claim 7, wherein said free fall path is of a length and
the temperature is at levels therealong to slowly solidify said
bodies. .[.9. The combination set forth in claim 6 said capillary
tube has a gas port and the upper end of said tube has a gas
conduit leading thereto for selective flow of inert gas into said
capillary tube and said elongated tube..].
Description
FIELD OF THE INVENTION
The invention relates to forming small semiconductor bodies of
simple grain structure.
PRIOR ART
In U.S. Pat. No. 4,021,323, assigned to the assignee of the present
invention, an energy conversion system is disclosed which is
comprised of a sheet of separate photovoltaic cells having
tear-drop shaped semiconductor cores of one conductivity type, and
outer diffusion layers of a second conductivity type. With such
cells, much of the semiconductor material is within a diffusion
length of the junction and thus within the active portion of the
cell.
The efficiency of units employing tear-drop shaped photovoltaic
cells is substantially increased when the semiconductor bodies are
formed with minimal grain boundaries. The present invention
provides sphere-like semiconductor bodies in which the number of
grain boundaries is minimized.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention a quartz
capillary tube having a nozzle at the lower end is mounted with its
axis vertical in a tower with a controlled atmosphere. A
cylindrical graphite susceptor encircles the lower portion of the
capillary tube to a point below the nozzle. Both the capillary tube
and the graphite susceptor are enclosed in a quartz envelope which
is mounted with its axis vertical. The upper end of the quartz
envelope has a gas conduit connected thereto, and the lower end of
the envelope converges to a collector cup structure having a gas
purge conduit. An induction heater work coil encircles the glass
envelope in the area of the graphite susceptor.
A semiconductor charge placed within the capillary tube is heated
through electrical power applied to the work coil. An inert gas
such as helium, is applied to the capillary tube and the glass
envelope to purge the system of air. When the semiconductor
material is in a molten state, the inert gas pressure is increased
on the capillary tube to force the semiconductor material through
the nozzle of the tube. Molten semiconductor bodies are formed
thereby which fall the length of the glass envelope, and solidify
before reaching the collector cup.
Control of particle shape and grain structure is achieved by use of
either of two embodiments. In one embodiment the semiconductor
bodies gathered in the collector cup are passed through a remelt
tower to recrystallize the bodies and reduce the number of grain
boundaries. More particularly, a quartz feed tube having a nozzle
at the lower end is mounted vertically with its axis in the remelt
tower. A cylindrical graphite feed tube encircles the quartz feed
tube inside a hollow cylindrical graphite susceptor. The susceptor
is supported with its axis vertical in the remelt tower. An inner
cylindrical heat shield encompasses both the graphite susceptor and
a lower portion of the graphite feed tube. An outer cylindrical
heat shield encompasses the inner heat shield. A tubular fused
quartz envelope encompasses the heat shields. An induction heater
work coil preferably of hour glass shape encircles the quartz
envelope in the area of the graphite susceptor.
Semiconductor particles enter into a feed tube and pass through the
nozzle of the feed tube for free-fall through the graphite
susceptor. Power applied to the work coil develops a temperature
gradient along the susceptor to raise the temperature of the
particles as they fall to a point above the melting point of the
semiconductor material. The temperature then gradually decreases
along the fall path until the temperature of the body is well below
the freezing point. Semiconductor bodies formed in this manner have
few grain boundaries and are quite suitable for a variety of uses
such as in solar cell fabrication.
In a second embodiment of the invention, the shot forming tower and
the remelt tower are unitary for both forming small semiconductor
bodies and controlling the free-fall environment such that as they
gradually cool, the particles achieve near uniform shape and
minimal grain boundaries.
DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself, however, as
well as further objects and advantages thereof, will best be
understood by reference to the following detailed description of an
illustrative embodiment taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a cross-sectional view of a semiconductor particle
forming tower in accordance with the invention;
FIG. 2 is a cross-sectional view of selected sections of the tower
of FIG. 1;
FIG. 3 is a cross-sectional view of selected sections of a remelt
tower in accordance with the invention;
FIG. 4 is a cross-sectional view of the remelt tower of FIG. 3
taken along lines 4--4;
FIG. 5 is a temperature gradient graph taken along the length of
the remelt tower heater coil of FIG. 3; and
FIG. 6 is a second embodiment of the invention wherein the forming
tower of FIG. 1 and the remelt tower of FIG. 3 are combined into a
unitary tower structure.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIGS. 1 and 2
Referring to FIG. 1, a rectangular tower 10 has four corner posts
11 with frame members 12a-12c interconnecting the posts at three
levels above a base 13. Floor plates 12d, 12e and 12f rest on frame
members 12a, 12b and 12c, respectively. Plates 12d, 12e and 12f
each have a central circular hole therethrough. A quartz tube 14
extends downward from plate 12d. The lower end of the tube is fused
to a larger quartz tube 15. The lower end of tube 15 in turn
extends into the upper end of a slightly larger quartz tube 16. The
lower end of tube 16 is connected by means of a union 17 to a
quartz collector cup 18.
A first gas feed tube 19 leads through a capillary end seal 20. A
second gas feed tube 21 leads through a conduit 30 into the
interior of tube 14.
An induction heating coil 22 encircles the lower end of tube 14.
Heating coil 22 is connected to an induction heater (not shown),
preferably of a type manufactured and sold by the Lapel High
Frequency Laboratories, Inc. of New York, New York. In one
embodiment, the induction heater had a power rating of 71/2
kilowatts and was operated at 450 kilocycles.
Referring now to FIG. 2, capillary end seal 20 closes the upper end
of a quartz capillary tube 23. Tube 23 extends through a capillary
clamp 24 into the interior of tube 14. Clamp 24 is secured to a top
cover plate 25. Cover plate 25 and a gasket 27 are secured to the
top surface of floor plate 12d. The upper end of tube 14 is secured
in a top clamp plate 28, which in turn is secured to the lower
surface of floor plate 12d along with a gasket 29. Plates 25 and
28, and gaskets 27 and 29 may be secured to the top floor plate 12d
by well known means such as bolt and nut combinations, not
shown.
As above described, the feed tube 19 is flow connected to a conduit
depending from the end seal 20. Feed tube 21 is flow connected to a
conduit 30 passing through plates 25, 12d and 28 to the interior of
tube 14. Both tubes 19 and 21 supply an inert gas such as helium at
controlled pressures.
Capillary tube 23 converges to a nozzle 23a formed at the lower end
thereof. A charge 23b of semiconductor material such as silicon is
shown resting in the bottom of tube 23. In the preferred embodiment
described herein, the nozzle has an inside diameter of about 5
mils. A cylindrical graphite susceptor 31 encircles a lower portion
of tube 23, extending to a point slightly below nozzle 23a.
Susceptor 31 is positioned inside the tube 14 and is heated by
inductive coupling from coil 22.
Tube 15 extends downward through a center floor plate 12e and into
the interior of tube 16. Tube 15 is secured to the upper surface of
plate 12e by a clamp plate 33 and a gasket 34. Tube 16 is secured
to the lower surface of plate 12e by a clamp plate 35 and a gasket
36. Plates 33 and 35, and gaskets 34 and 36 are secured to plate
12e by well known and suitable means.
Tube 16 extends through a clamp plate 37, a gasket 38 and a lower
floor plate 12f. Plate 37 and gasket 38 are secured to plate 12f.
The lower end of tube 16 converges to a union 17 which
interconnects the tube 16 with collector cup 18. A gas purge tube
40 extends laterally from the union 17.
In operation, the capillary end seal 20 is removed from the end of
tube 23, and one or more solid semiconductor bodies forming a
charge are placed in the tube. The semiconductor charge is
preferably a rod of solid silicon doped to provide particles of
desired conductivity type and resistivity. The semiconductor charge
rests at the bottom of tube 23, and is encircled by the graphite
susceptor 31 and the work coil 22. The end seal 20 is replaced, and
an inert gas, such as helium, under a pressure of a few pounds per
square inch is then applied through the feed tubes 19 and 21 to
quartz tubes 23 and 14, respectively. Initial flow of helium purges
the system of air. The helium flows downward through both tubes,
and exits at the conduit 40 which vents union 17. Electrical power
applied to the heating coil 22 heats the graphite susceptor 31
which causes the charges to be heated. When the semiconductor
charge is fully molten, helium gas again is applied through the
feed tube 19 at a pressure of about 10 psig to force the molten
material through the nozzle 23a. Flow of helium gas also is
maintained through conduit 21 and tubes 14, 15 and 16 at a rate of
about 10 cubic feet per hour to provide a controlled atmosphere in
tubes 14-16. As the semiconductor material is discharged from
nozzle 23a, it dissociates into small droplets which fall under the
force of gravity along the length of tubes 15 and 16 to be
collected in the collector cup 18.
With the nozzle 23a having an inside diameter of about 3-4 mils,
the semiconductor bodies formed in tower 10 have a nominal diameter
in the range of about 10 to 15 mils. It has been found that the
droplets are generally characterized by an orange peel surface and
a tail protruding from one end, and that numerous grain boundaries
exist.
A remelt operation is then performed to improve the shape and grain
structure as will now be described.
FIGS. 3 and 4
In FIG. 3, a remelt tower 50 includes a frame comprised of vertical
posts 51 and cross frames 52a and 52b which support plates 52c and
52d. The tower 50 extends downward below cross frame 52b to a base
such as shown in FIGS. 1 and 2. A quartz feed tube 53 is connected
to a quartz vial 53a with a short length of flexible plastic tubing
53b. A nozzle 53c is at the lower end of feed tube 53. Tube 53
extends through top plate 54a, and a gasket 55a both supported on
top plate 52c. Feed tube 53 extends axially within a quartz tube
57.
Tube 57 is secured to the top plate 52c by a clamp plate 54b and a
gasket 55b. Plates 54a and 54b, and gaskets 55a and 55b are secured
to plate 52c. Inside tube 57, a graphite tube 58 encircles a lower
portion of tube 53 and extends to a point below nozzle 53c. The
lower end of tube 58 is flanged and is seated within a recess in
the upper end of a cylindrical graphite susceptor 59. Susceptor 59
is supported at the lower end thereof by a cylindrical graphite
pedestal 60. An inner molybdenum shield 61 encircles a lower
portion of the tube 58, the susceptor 59 and an upper portion of
the pedestal 60. The inner shield consists of two half cylinders
located on opposite sides of susceptor 59 with the edges thereof
parallel and spaced apart leaving gaps 61a, as is best seen in FIG.
4. Both halves are supported by an upper annular flange 60a on the
outer surface of the pedestal 60. The inner shield 61 in turn is
encircled by a second molybdenum shield 62 which is intermediate to
tube 57 and shield 61. Shield 62 is supported by a lower annular
flange 60b on the outer surface of the pedestal 60. The shield 62
also consists of two halves with the gaps therebetween rotated 90
degrees from the gaps 61a in the inner shield 61 as shown in FIG.
4.
A graphite centering ring 63 encircles the tube 53, and is
supported by vertical members of shields 61 and 62.
The pedestal 60 extends through a lower plate 52d into the interior
of a lower quartz tube 65. The flange 60b abuts the top surface of
plate 52d to support the pedestal 60. The tube 57 is laterally
supported at plate 52d by a clamp 66 and a gasket 67. Plate 66 and
gasket 67 are secured to plate 52d. Tube 65 is laterally supported
at plate 52d by a clamp 68 and a gasket 69 contiguous to the under
surface of plate 52d. The clamp 68 and gasket 69 also are secured
to plate 52d. The structure of the tower 50 below cross member 52b
is similar to that of tower 10 below cross member 12b, and thus
need not be further described.
Below nozzle 53c and above the graphite pedestal 60, an induction
heater work coil 70 of about 20 turns encircles the tube 57. It has
been found that with an hour glass shaped coil, the occurrence of
electrical arcs between the quartz tube 65 and the outer shields 62
is substantially decreased.
By way of example, in the embodiment shown in FIGS. 3 and 4, feed
tube 53 was approximately 8 inches in length and 0.25 inches in
diameter. The graphite feed tube 58 is approximately 8 in length,
and has an inner diameter of about 1 inch. Further, the graphite
susceptor 59 has a length of about 40 inches and an inner diameter
of about 2.5 inches. The shields 61 and 62 have lengths of
approximately 50 and 54 inches, respectively, and inner diameters
of about 3 and 4 inches, respectively. The quartz tube 57 is
approxmately 60 inches in length and 5 inches in diameter. The work
coil 70 may be formed of 3/8" copper tubing, and is driven by a 60
Kw, 450 Kc induction heater (not shown) preferably of a type
manufactured and sold by Taylor-Winfield, Warren, Ohio, Model
S-6000LF.
An inert gas, such as helium or argon, is introduced into the tower
through a fitting (not shown). When all of the air has been purged,
as power is applied to the work coil 70 the graphite susceptor 59
is heated. The semiconductor bodies formed by the shot forming
tower 10 of FIGS. 1 and 2 flow from the vial 53a into feed tube 53.
The semiconductor bodies fall to the lower portion of tube 53 where
they pile up and slowly feed through the exit at the nozzle 53c.
The bodies then free-fall, passing through susceptor 59. Coil 70
controls the temperature gradient of susceptor 59 to maintain the
temperature above the melting point of the semiconductor material
over a path length which will ensure that each body is fully melted
before leaving the hot zone within susceptor 59. The semiconductor
bodies thus are melted as they travel the length of coil 70, and
are resolidified as they fall through a controlled temperature
gradient section at the lower end of susceptor 59. The process of
melting and resolidifying in a controlled thermal gradient
transforms the polycrystalline bodies produced in the process of
FIGS. 1 and 2 into tear-drop shaped bodies of near uniform shape in
which the number of grain boundaries is substantially decreased. An
0.010 inch diameter body treated in the tower of FIGS. 3 and 4
typically has five or less grains.
Referring to FIG. 4 and looking down onto the upper face of plate
52d, the work coil 70 encircles and is concentric to quartz tube
57. The tube 57 in turn encircles the molybdenum shield segments
62a and the shield segments 61 which are concentric to the graphite
susceptor 59.
A suitable coil configuration involves approximately 20 turns
wherein the outer diameter of the 3 upper turns is approximately 10
inches, followed by two turns having an outer diameter of
approximately 81/2 inches and 10 turns having an outer diameter of
about 71/2 inches and two turns having an outer diameter of
approximately 81/2 inches. The lower 4 turns of the coil have an
outer diameter of about 9 inches.
FIG. 5
FIG. 5 illustrates graphically the temperature gradient along the
free fall path through the work coil 70 of FIG. 3.
Referring to FIG. 5, the temperature profile is illustrated on a
scale identified in terms of reference points along the length of
coil 70.
Reference point 1, at the upper end of the coil, is approximately
51/2 inches below the nozzle 53b.
Reference point 2 is approximately 11/2 inches below the reference
point 1.
Reference point 3 is approximately 31/2 inches below the reference
point 2.
Reference point 4 is approximately 31/2 inches below the reference
point 3 and reference point 5 is about 7 inches below the reference
point 4.
Reference point 6 is about 7 inches below the reference point 5,
and reference point 7 is about 31/2 inches below reference point
6.
Reference point 8 is approximately 31/2 inches below the reference
point 7, and reference point 9 is about 2 inches below the
reference point 8. Reference point 9 is at the lower end of the
coil 70. Reference points 1, 2, 3, 8 and 9 are shown at
representative levels in FIG. 4.
A horizontal dotted line 80 of FIG. 5 indicates the melting point
of silicon, i.e., 1410 degrees C. Thus it will be appreciated that
the system operates to apply heat to the bodies as they traverse
the free-fall path in a controlled way to make more uniform the
internal structure.
In FIG. 3 the turns of work coil 70 are more closely spaced near
the top relative to those near the bottom of the coil in order to
control the temperature profile in the susceptor 59 in the manner
indicated by curve 81 of FIG. 5. The temperature profile may be
measured by inserting a tungsten-rhenium thermocouple in place of
feed tube 53. By use of a 60 Kw induction heater of the type
manufactured and sold by Taylor-Winfield, the desired temperature
profile 81 was reached with the 60 Kw heater set for full output.
In preferred form, the temperature gradient of curve 81 over the
portion of negative slope is about 35 to 30 degrees per inch.
This profile provides fast heating of the falling bodies. It is
also shaped to assure that the bodies are cooled slowly as they
pass through the silicon melting point.
Final adjustment of the operating conditions may be established by
examination of bodies which have passed through the tower. Bodies
which have been properly remelted and resolidified have a tear drop
shape and a very smooth surface. If the bodies are not completely
melted, the peak temperature should be increased. Bodies subjected
to temperatures which are too high will have a rough surface. Small
bodies may require lower temperatures than largsr ones.
Semiconductor materials such as silicon and germanium expand upon
cooling. When they cool rapidly, as in the tower of FIGS. 1 and 2,
it is believed that an outer skin is formed over a molten core. As
the core solidifies, the skin is ruptured and the excess material
is expelled to form an attached secondary protrusion. Although the
change in shape is of little significance, bodies of this type are
usually highly polycrystalline, sometimes containing as many as
10,000 crystalletes in an 0.010 inch diameter sphere.
It is believed that the beneficial results of the remelting
operation performed in the tower of FIG. 3 occur because the
cooling is directional. That is, the freezing is initiated at one
point at or near the surface of the body and progresses until all
of the material is solidified. Because the semiconductor material
expands upon freezing, material is pushed ahead of the solid-liquid
interface, resulting in a particle with a "raindrop" or "tear drop"
shape. "Tear-drop" shaped crystals frequently are single crystals,
usually have less than five crystalletes in an 0.010 inch body. It
has been found that "tear-drop" shaped crystals make excellent
solar cells, with air mass 1 efficiencies greater than 10%.
Although the embodiments described are for silicon, other
semiconducting materials may be formed in the same manner.
Germanium solidifies at about 950 degrees C., so the temperatures
indicated should be reduced accordingly. Materials like gallium
arsenide may require constant pressurization due to the volatility
of arsenic. Such procedures are well known to those skilled in the
art.
FIG. 6
FIG. 6 illustrates a unitary structure combining the function of
the tower of FIGS. 1 and 2 and the remelt tower of FIG. 3.
Referring to FIG. 6, a quartz tube 90 is shown. An insulating ring
91, preferably of alumina, is centered in tube 90. Insulating ring
91 supports and locates a susceptor of graphite 92, and two heat
shields 93 and 94, segmented and oriented like shields 61a and 62a,
FIG. 4. A quartz capillary 95, which may be identical to capillary
23 of FIG. 2, is centered in the upper end of susceptor 92.
The nozzle 95a of the capillary extends into an open area 96 of
susceptor 92. The lower end of the susceptor 92 is supported by
means including an upfacing insulating ring not shown but similar
to ring 91.
Work coil 97 surrounds tube 90, and extends the full length of the
susceptor 92. The turns at the top are closer together than
therebelow, so that the temperature at the top, surrounding the
capillary 95, is higher than that of the bottom end of capillary 95
by approximately 100 degrees C. Work coil 97 will be shaped as a
combination of coils 22 and 70 (FIGS. 1 and 4) and may be driven by
a 25 Kw 450 Kc induction heat control 97a. A suitable unit is made
by Taylor-Winfield.
In operation, a semiconductor charge 98 is placed in capillary 95.
Thereafter, tubes 95 and 90 are purged with inert gas such as
helium or argon. Electrical power is then applied to heater coil
97. When the charge 98 is molten, inert gas at a pressure of 10 psi
is applied to capillary 95. The semiconductor bodies formed at
nozzle 95a fall freely through region 96 of susceptor 92, and
solidify near the lower end to the susceptor. The solidified bodies
continue to fall, and are collected in a cup similar to cup 18 of
FIG. 2.
The temperature gradient in zone 96 is of the character shown by
the negative slope portion of the curve 81 of FIG. 5.
It is to be understood that the embodiments herein described are
illustrative of the invention. Other arrangements may be devised by
those skilled in the art without departing from the spirit and
scope of the invention as defined by the appended claims.
* * * * *