U.S. patent number 3,798,007 [Application Number 04/882,571] was granted by the patent office on 1974-03-19 for method and apparatus for producing large diameter monocrystals.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Raymond A. Bochman, Ralph G. Dessauer, Dian P. Jen.
United States Patent |
3,798,007 |
Bochman , et al. |
March 19, 1974 |
METHOD AND APPARATUS FOR PRODUCING LARGE DIAMETER MONOCRYSTALS
Abstract
An improved method and apparatus for producing large diameter
semiconductor crystals by the Czochralski process wherein a
relatively flat temperature profile is maintained within the melt
by adding heat to the sides and top of the melt while
simultaneously removing heat from the melt through the crystal
being pulled and the lower portion of the melt. In the apparatus,
the temperature profile is maintained with a deflector to direct
heat energy to the top surface of the melt about the crystal being
pulled, a heat exchange element to facilitate removal of heat
through the crystal being pulled, and means to remove heat through
the lower portion of the crucible containing the melt.
Inventors: |
Bochman; Raymond A.
(Poughkeepsie, NY), Dessauer; Ralph G. (Poughkeepsie,
NY), Jen; Dian P. (Hopewell Junction, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25380873 |
Appl.
No.: |
04/882,571 |
Filed: |
December 5, 1969 |
Current U.S.
Class: |
117/30; 117/217;
117/932; 117/900 |
Current CPC
Class: |
C30B
15/22 (20130101); C30B 15/14 (20130101); Y10S
117/90 (20130101); Y10T 117/1068 (20150115) |
Current International
Class: |
C30B
15/14 (20060101); C30B 15/20 (20060101); C30B
15/22 (20060101); B01j 017/18 () |
Field of
Search: |
;23/31SP,273SP |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Foster; R. T.
Attorney, Agent or Firm: Stoffel; Wolmar J.
Claims
We claim:
1. A method for producing a large diameter single semiconductor
crystal having a low crystallographic defect density by the
Czochralski process, comprising;
pulling a monocrystal from a molten metal of the semiconductor
material, and
controlling the thermal conditions within the melt to establish and
maintain a relatively flat temperature profile, said profile
attained and maintained by directing with an annular baffle element
radiant heat energy emanating from the heater surrounding the melt
to the surface of the melt and away from the upper surface of the
crystal being grown,
simultaneously removing heat from the bottom of the melt,
the profile thereby maintained by proportionally increasing the
input of heat energy at the surface of the melt, removing heat
energy from the crystallization zone of the melt through the
crystal being grown, and through the bottom of the melt,
the total heat removed from the melt being equal to the heat added
plus the heat of formation of the crystal.
2. The method of claim 1 wherein heat is removed from the bottom of
the melt through a heat exchange means.
3. The method of claim 1 wherein a proportionately larger amount of
heat is directed to the top of the melt by providing a more
efficient insulation about the heating element in the region of the
top surface of the melt.
4. The method of claim 3 wherein relative rotation is provided
between the melt and the crystal being grown.
5. The method of claim 3 wherein heat is removed from the center
portion of the melt through the crystal.
6. The method of claim 5 wherein a heat exchange means is provided
to conduct heat away from the crystal being grown.
7. The method of claim 5 wherein a lower hot zone over the melt,
and an upper overlying cooler zone is maintained, and the crystal
is withdrawn upwardly through said zones.
8. The method of claim 7 wherein said hot zone is formed at least
in part by directing heat from a heating element inwardly and
downwardly toward the melt, and shielding the cooler zone from the
heating element.
9. The method of claim 8 wherein the heater temperature is
gradually increased as the length of the crystal increases to
compensate for increasing heat transfer through the crystal.
10. In an apparatus for producing a large diameter substantially
defect-free single semiconductor crystal by the Czochralski process
having a container for the melt, a heater disposed about the side
of said container, and a mechanism for supporting and lifting the
crystal formed in the melt, the improvement comprising;
a means to direct a greater porportion of heat energy to the top
region of the melt than to the bottom, including an annular shield
positioned above the heater to direct radiant heat energy from said
heater downwardly and inwardly toward the top surface of the melt,
and preventing at least in part radiant heat energy from impinging
on the upper portion of the crystal,
a heat exchange means to remove heat from the lower portion of the
melt,
said annular shield and heat exchange means adapted to establish
and maintain a relatively flat temperature profile in the melt in
the region slightly below the top surface of the melt.
11. The apparatus of claim 10 wherein said means to remove heat
from the lower portion of the container includes a support shaft
and a heat exchange means to cool said shaft.
12. The apparatus of claim 10 wherein said means to remove heat
from the upper portion of the melt includes a heat exchange means
adapted to effectively cool the crystal as it is withdrawn from the
melt.
13. The apparatus of claim 10 wherein said annular shield a ring of
anisotropic material which directs heat inwardly and restricts
upward heat transfer.
14. The apparatus of claim 13 wherein said annular shield is formed
of pyrolytic graphite.
15. The apparatus of claim 10 wherein said means to direct heat
energy to the top region of the melt includes insulation means
surrounding the upper portion of the crucible and shaped to deflect
a proportionately greater amount of heat toward the upper portion
of the crucible than the bottom portion.
16. The apparatus of claim 15 wherein said insulation means
includes a layer of pyrolytic graphite tape surrounding
substantially the entire outside area of the heater, and a layer of
insulation covering the upper portion of the heater, in the region
of the melt surface.
17. The apparatus of claim 16 wherein said layer of insulation is a
layer of graphite felt.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to crystal growing method and
apparatus, and more particularly to a crystal growing method and
apparatus suitable for growing large diameter single crystals of
material, such as silicon.
2. Description of the Prior Art
The rapidly expanding semiconductor industry is built around the
unique electrical properties and characteristics of monocrystalline
semiconductor material. Consequently, a great deal of development
has been done to produce in quantity the semiconductor material
having the desired quality. This has resulted over the years in a
number of techniques to produce monocrystalline semiconductor
material. These techniques include the zone recrystallization
method, the Czochralski method, and the web growth method. Each
method has inherent advantages as well as disadvantages which make
it attractive for one use and unattractive for another.
In the zone recrystallization method an elongated bar or rod of
polycrystalline material is joined to a relatively short length of
monocrystalline material which serves as a "feed" and subsequently
a relatively narrow zone is heated and passed through the bar to
cause recrystallization and to extend the crystal lattice of the
seed throughout the bar. Various techniques have been perfected to
suspend the resultant molten zone during the process. In this
method there is no significant crucible contamination but it does
have the disadvantage that thermal stresses appear in the crystal
which can result in a high density of crystalline defects.
The most commonly and most widely used method of producing single
crystals for use in a semiconductor device at the present time is
the Czochralski method. Basically the growing technique consists of
dipping a "seed" crystal in a melt of semiconductor material at a
precise temperature at or below the melting point and then
withdrawing the seed under controlled conditions. The semiconductor
material will freeze on the end of the seed in the same basic
crystalline formation resulting in a single crystal. Normally a
suitable dopant material is incorporated in the molten mass in the
crucible and is subsequently recrystallized along with the
semiconductor into the rod.
In the web growth technique the crystal is grown in the form of a
thin web. Basically two dendrites are formed beneath the surface of
the liquid melt which grow in long continuous lengths. The liquid
film, which is drawn up by surface tension between the dendrites,
solidifies to form the web. This is a relatively new development
which has not been fully perfected at present.
The usual method in the industry in fabricating integrated circuit
devices is to grow a monocrystalline silicon ingot by the
Czochralski process, slice the ingot into thin wafers, and produce
diffused regions, deposit passivating layers and metallurgy on the
surface to produce the desired integrated circuit device
configuration. In order to make the fabrication process more
efficient, it would be desirable to increase the diameter of the
semiconductor wafers in order to increase the number of devices per
wafer. The work involved in processing the wafer is not
significantly different in the case of small and large wafers. The
usual standard diameter of the wafer in industry is an inch and
one-quarter although efforts have been made to increase this
diameter. While crystals having a larger diameter can be grown by
the Czochralski process, it has been found that the resultant
wafers grown in accordance with known techniques have a very high
density of crystallographic imperfections. As the size of the
active devices in integrated circuit devices is decreased, which is
the trend in modern semiconductor fabrication, the disruptive
effect of crystallographic defects becomes more serious. A
crystallographic defect such as a dislocation can render a device
inoperative, or cause a leak which will destroy the utility of the
entire integrated circuit device, which may include up to several
hundred active devices. This very significantly reduces the yield
of a production line.
There is a great present need for a process and an apparatus which
will produce significantly larger diameter monocrystalline wafers
having a zero or very low crystallographic defect density. The
present interest in forming of devices on the <100>
crystalline orientation plane accentuates the problems with growing
large diameter crystals since growth in this general direction is
more difficult than grown in the <111> plane which was
previously universally utilized.
SUMMARY OF THE INVENTION
An object of this invention is to provide a method whereby large
diameter monocrystalline semiconductor boules can be grown by the
Czochralski process.
Another object of this invention is to provide an apparatus which
will maintain desirable thermal conditions in the melt whereby
large diameter crystals can be produced, which crystals have a zero
or low crystallographic defect density.
Another object of this invention is to provide a method and an
apparatus for growing monocrystalline semiconductor material in
which the thermal conditions in the melt is conducive to the
formation of large diameter crystals having a zero or low
crystallographic defect density.
Another object of this invention is to provide a method and
apparatus adapted to produce large diameter monocrystalline
semiconductor wafers of high quality suitable for use in
fabricating microminiaturized integrated circuit devices.
In the method of the invention for producing large diameter
substantially defect-free single semiconductor crystals by the
Czochralski process wherein a monocrystal is pulled from a molten
melt of semiconductor material the improvement resides in
maintaining a relatively flat temperature profile within the melt
by adding heat to the sides and top of the melt while
simultaneously removing heat from the melt through the crystal
being pulled and the bottom of the melt.
In the apparatus of the invention for producing large diameter
substantially defect-free semiconductor crystals by the Czochralski
process, which includes a heater or RF coil disposed about the side
of the container, a mechanism for supporting and lifting the
crystal formed in the melt, the improvement is providing a means to
direct more intense heat energy to the top region of the melt about
the crystal being pulled, a means to effectively remove heat from
the upper central portion of the melt through the crystal being
pulled, and a means to remove heat from the lower portion of the
container. The objective is to avoid a heat build-up under the
crystal being pulled and thereby cause crystallization to occur in
substantially a planar front. Further, by proper control of heating
the crystallization front can be placed very near the top of hot
zone, i.e. keeping only a small portion of the grown crystal in the
hot zone during growth, while the major portion is being pulled
through the cold zone, i.e. the region above the heater. This
provides a greater surface area for heat dissipation the space
enclosed by the heater, thereby promoting more effective heat
transfer and a more rapid rate of crystallization.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of a preferred embodiment of the invention as
illustrated in the accompanying drawing.
FIG. 1A is a schematic elevational view in cross-section of the
apparatus of the invention illustrating the general structure and
heat flow.
FIG. 1B is an idealized temperature profile which exists in the
melt of the apparatus and process of the invention.
FIG. 1C is an elevational view in broken section of a crystal
produced by the invention illustrating the planar surface shape of
the interface between the crystal and the melt.
FIG. 2A is a schematic elevational view in cross-section of a
crystal puller apparatus typical of the prior art illustrating the
heat flow into and out of the crucible.
FIG. 2B is a typical temperature profile which exists in a melt of
the type apparatus illustrated in FIG. 2A.
FIG. 2C is an elevational view in cross-section of a representative
crystal produced in the apparatus illustrated in FIG. 2A which
illustrates the surface shape of the interface between the crystal
and the melt.
FIG. 3 is an elevational view in cross-section of a preferred
specific embodiment of the crystal pulling apparatus of the
invention.
FIG. 4 is a graph of heater temperature vs percent of crystal
length from seed dip to withdrawal to be followed when practicing
the method of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The Czochralski process as practiced by the prior art is capable of
growing usable silicon crystals of up to two inches in diameter up
to generally a length of 5 to 6 inches. When crystals are longer in
length, the diameter must be significantly less than 2 inches. When
silicon crystals having a diameter significantly greater than two
inches are grown, the crystallographic defect density is so high
that use of the wafers for producing integrated circuit devices and
obtaining respectable yields are not economically feasible. FIG. 2A
illustrates a typical prior art apparatus for growing semiconductor
crystals by the Czochralski process. Apparatus 10 has a crucible 12
containing a melt 14 of silicon. Heater 16 is provided to maintain
the temperature of melt 14 near the freezing point. A uniform layer
of insulation 18 surrounds heater 16 to improve the efficiency of
the apparatus. The semiconductor crystal 20 is pulled from melt 14
by a suitable lift mechanism (not shown) controlled by a suitable
sensor (not shown) which is responsive to the diameter of the
crystal. A suitable mechanism (not shown) may be provided for
raising or lowering the crucible 12 to maintain the level of the
melt 14 in the same position relative to heater 16. The crystal
growing process is performed in an atmosphere of inert gas
typically argon, which is contained in a suitable chamber
associated with apparatus 10. Element 19 is shown as closing the
chamber below crucible 12 thus indicating schematically that
structure exists which impedes heat transfer.
Heat energy is added to melt 14 from two sources, namely from
heater 16 and from the crystallization process occurring at the
interface between the bottom of the crystal 20 and melt 14. The
heat energy rate added by crystallization process can be calculated
from the rate of growth of the crystal, the diameter of the
crystal, and the heat of crystallization of the silicon, or other
semiconductor material being grown. FIG. 2B indicates temperatures
in the melt in a plane slightly below the surface of the melt along
a diameter line of the crucible. As the profile depicted in FIG. 2B
indicates the temperature of the melt 14 is relatively high about
the outside wall of the crucible and in the center directly under
the semiconductor crystal 20. There is a temperature drop in the
melt in the area between the crystal and the exterior wall of the
crucible. When a crystal is pulled abruptly from the melt 14 thus
halting the crystallization process the bottom surface 21 as
indicated in 2C has a concave configuration. Surface 21 represents
the shape of the growing interface between the melt and the crystal
as it is being produced in the apparatus as it is known to the
prior art. It is theorized that the concave surface is caused by
the build-up of heat due to the heat of crystallization and
non-uniform heat loss across the crystal diameter. This results in
a nonplanar crystallization growth front which results in built-in
stresses in the crystal. Dislocations and other crystallographic
defects thus occur during the actual growth of the crystal, and
also result later due to the stresses being equalized when the
crystal 20 is cooled.
During operation a significant amount of heat is radiated from the
surface of the melt. Consequently, heat must be added by the heater
to maintain this portion of the melt near the melting point or the
surface will freeze and completely disrupt the operation. The
crystallization in an orderly operation occurs because heat energy
is removed from the melt through the crystal as it is being pulled.
It has been observed that in known apparatus the crystallization
actually occurs at or slightly below the surface of the melt. To
prevent excessive heat loss and resulting freezing, the
crystallization front must be kept well within the heat zone. This
significantly impedes heat transfer through the crystal and
consequently reduces the growth rate. The heat in this environment
must pass through the immersed portion of crystal, which is slow
because there is a small temperature differential, and subsequently
longitudinally through the crystal to a region of the crystal where
it can be dissipated by conduction or convection.
The arrows in FIG. 2A attempt to explain the thermal conditions
within the melt which result in the temperature profile depicted in
FIG. 2B. Arrow 22 represents the vector component of the heat
transferred from point 23 in the heater 16 in the general plane of
the melt surface that is lost from the heat emission from the top
of the heater. Arrow 24 depicts the amount of heat which emanates
outwardly from heater 16 and which is lost through insulation 18.
Arrow 26 represents the vector component of heat energy which is
transferred inwardly to the crucible 12 which in turn is
transferred to the melt 14. In the set of arrows below point 23 at
second point 28, arrow 29 depicts the amount of heat loss outwardly
from the heater 16 through insulation 18. Arrow 30 depicts the
amount of heat energy directed inwardly toward crucible 12 from
point 28. Note that the length of arrow 30 is somewhat longer than
corresponding arrow 26. However, the proportion of heat directed
inwardly and outwardly is generally similar to that at point 23.
The magnitude of the heat is different. This condition exists
because at point 28 there is a minimal amount of heat directed
upwardly or downwardly unlike point 23 where there is an upward
loss as indicated by arrow 22. This has the effect of increasing
the temperature of the melt in the lower portion of the
crucible.
Vector arrow 32 represents the amount of heat conducted upwardly
from point 31 located in the center of crystal 20 at the
crystallization front. Arrows 33 indicate the heat energy conducted
upwardly from the outside perimeter of the crystal front. This heat
is subsequently dissipated radially in the cold zone. As arrows 32
and 33 indicate, heat is conducted away from the crystallization
front faster at the periphery of the crystal than at the center.
This results in a concave crystallization front of the type shown
in FIG. 2C. Vector arrow 34 represents the heat lost through the
bottom of the crucible. Heat is radiated from the top surface of
the melt resulting in a lower surface temperature. The temperature
gradient of the melt produced by the conditions described results
in a relatively hot spot beneath the crystal downward wherein
dissipation of the heat of crystallization cannot proceed
efficiently because a greater heat is being applied to the bottom
portion of the crucible and no means is provided to promote
conduction of the heat away from the bottom of the crucible.
FIG. 1A is a schematic drawing of an apparatus which embodies the
inventive concept of the invention. As in FIG. 2A crucible 12
contains melt 14 from which crystal 20 is pulled therefrom. Heating
element 16 surrounds crucible 12 which is in turn enclosed in an
insulating member 38 having a relatively thick upper portion 40 and
a relatively thinner lower portion 42. The reason for the
insulation configuration will become apparent in the following
description of the process. Disposed above heater 16 resting on
insulation member 38 is a cover element 44 which minimizes upward
radiation of heat from heater 16 and directs the heat inwardly. The
space below crucible 12 is shown open to indicate generally that
the structure is such to promote efficient transfer of heat from
the bottom portion of the crucible.
FIG. 1C depicts the end portion of a crystal 20 which has been
abruptly pulled from the melt as it is being grown by the process
of this invention. The lower surface portion 48 is relatively
planar indicating that the interface between the melt and the
crystal during crystallization is planar. This results in the
formation of fewer crystallographic defects and stresses set up
within the crystal. FIG. 1B indicates the general shape of the
temperature profile of melt 14 along a center line slightly below
the surface of the melt during the crystallization process. As
indicated the temperature across the melt is relatively uniform
which is consistent with the formation of a flat surface 48 on
crystal 20. The arrows in FIG. 1A again depict the vector
quantities of heat radiating from various points in the apparatus.
Considering point 50, arrow 51 represents the upwardly directed
heat loss. Arrow 51 is smaller than corresponding arrow 22 in FIG.
2A since this component is minimized by shield 44. Arrow 52
represents the amount of heat radiated outwardly through insulation
40. This arrow is smaller in magnitude than corresponding arrow 24
in FIG. 2A since the insulation layer 40 is more effective to
reduce heat losses. Arrow 53 depicts the heat transferred inwardly
toward the crucible at the general region of the melt surface. Note
that this is larger than corresponding arrow 26 in FIG. 2A. There
is less heat loss upwardly and outwardly and thus the major portion
is directed inwardly toward the crucible. Considering point 54,
arrow 55 indicates a greater heat loss outwardly through the
portion of insulation 42. This is larger than corresponding vector
arrow 52 in FIG. 1A since the insulation 42 is substantially less
effective than insulation 40. Arrow 56 indicates the amount of heat
directed inwardly towards the crucible. The arrow 56 is smaller
than arrow 53. A greater proportion of the heat is directed
outwardly leaving a smaller portion to be directed inwardly. The
end result considering the heat distribution from points 50 and 54
is that a greater amount of heat is directed inwardly toward the
crucible in the region of the melt surface and less at the bottom
portion of the melt. Considering now point 58 located in the center
of crystal 20 at the crystal front, arrow 59 indicates in vector
form the amount of heat transferred upwardly through the central
portion of the crystal 20. Arrows 61 indicate the upward and
outward heat transferred from the peripheral region of the crystal
front. This heat is dissipated transversely from the surface of
crystal 20 at a region above the hot zone as indicated by the
broken arrows 61. Note that the magnitude of arrows 59 and 61 is
generally similar. This would promote a flat crystallization front
on crystal 20 which will minimize crystallographic defects. A
cooling coil (not illustrated) can be disposed about the upper
portion of the crystal to provide a more effective conduction of
heat. Further shield 44 maintains a cooler temperature in the
region above the crystal, i.e. the cold zone, thus providing a
greater temperature differential and more efficient cooling.
Further, the chamber about the crystal can be water cooled. Arrow
60 indicates the quantity of heat removed downwardly through the
bottom of the crucible 12. This quantity is relatively large when
compared to 34 because the bottom portion of the apparatus is
designed to more effectively conduct heat away from the bottom of
the crucible. This can be done by water cooling the support for the
crucible or otherwise providing means to more effectively conduct
heat away from the crucible bottom. The objective of the apparatus
is to inject heat near the top of the melt from the outside, and
confine the vertical distances or thickness of the hot zone, and to
more effectively conduct heat from the center of the melt which
would reduce the temperature difference illustrated in the profile
shown in FIG. 2B.
The crystallization front can be placed very near the top of the
hot zone by the action of the shield 44 and the insulation
configuration 40 which direct radiant heat to the surface of the
melt to compensate at least in part for the heat normally radiated
upwardly from the melt. Heating the surface reduces the amount of
heat that must be directed to the melt through the crucible.
Ordinarily the temperature of the surface must be maintained near
the freezing point to prevent freeze-up over the surface. The heat
in prior art apparatus comes from within the melt which must be at
a higher temperature. The surface temperature of the melt of the
subject invention is maintained more nearly at the temperature of
the lower melt. In operation, because the length of the crystal
that the heat of crystallization must pass through is shorter, a
more uniform heat transfer gradient across crystal front is
obtained, and also the crystallization rate can be significantly
increased.
In FIG. 3 is depicted a specific embodiment of the apparatus of the
invention adapted to carry out the method of the invention. The
apparatus illustrated has structure for controlling the thermal
conditions in the melt to promote crystal growth on the bottom of
the crystal being grown in a generally planar face. This objective
is accomplished by adding a proportionately greater amount of heat
to the top region of the melt than to the bottom from the outside,
and reducing the heat build-up at the center of the melt due to the
heat of crystallization.
Apparatus 70 has a crucible 72 supported by crucible support 74,
resting on support ring 76, in turn supported on support plate 78.
Plate 78 is mounted on the upper end of support rod 80 which
extends through base plate 82 and a water cooled seal assembly 84.
Base plate 82 may also be provided with a fluid cooling coil 85.
Support rod 80 is actuated by a suitable lift mechanism adapted to
maintain the upper surface of melt 73 in the same position relative
to the heating element 86 and which also rotates the crucible.
Heating element 86 is the "picket type" made of graphite having
electrodes 88 and 90 which are water cooled. Annular support ring
92 rests on base plate 82 and supports the cylindrical carbon
insulator support 94. The upper portion of 94 is provided with a
relatively thick insulation, typically graphite felt 95 and
pyrolytic graphite tape 97. At a point below the top surface of
melt 73, the thickness of the insulation is materially decreased.
Positioned about the entire assembly mounted on base 82 is a water
cooled jacket 96 provided with a top flat ledge 98. Within 98
resting on cylindrical support 94 is an annular molybdenum shield
100 disposed over an annular pyrolytic graphite ring 101. Pyrolytic
graphite is an anisotropic material which conducts heat quite
efficiently in one direction but is an effective insulation which
restricts heat transfer in the transverse direction. Ring 101 is
formed so that heat from the heating element is reflected and
directed inwardly toward the melt 73 and lower end of the crystal
75. This provides localized heating of the top surface of the melt
and maintains the region above the melt at a relatively cooler
temperature. This permits more efficient cooling of the crystal,
and growing of the crystal higher in the hot zone as discussed
previously. Frusto-conical, water cooled chamber 102 is mounted on
ledge 98. This chamber supports a mechanism to lift and rotate the
crystal 75 as it is grown from melt 73. The water cooled chamber
102 in combination with shield 100 and ring 101 serve to control
the temperature above the crucible and about the crystal. It is
normal practice to grow the crystal 75 in an atmosphere of inert
gas such as argon.
The objective of the aforedescribed preferred embodiment of the
invention is to achieve a flat temperature profile in crucible 72
by directing a proportionately greater amount of heat from the
heater 86 to the top portion of the crucible in the vicinity of the
melt surface, while decreasing the amount of heat directed to the
lower portion of the crucible. This objective is achieved by
providing more insulation near the top of the crucible, and
providing of shield 100 and ring 101 which direct the heat
downwardly into the melt maintaining a lower temperature in the
cold zone above the crucible. A portion of the heat in the center
of the crucible generated by the heat of crystallization is removed
through the lower portion of the crucible. Apertures 79 in support
plate 78, the water cooled jacket surrounding the support rod 80
and cooling coils 85 on base 82 all serve to aid in the transfer of
heat away from the melt. Thus, heat is removed from the lower
portion of the crucible by radiation and by conduction. Heat is
also removed from the center portion of the crucible through
crystal 75. Heat is generally radiated from the crystal 75
outwardly particularly above shield 100 and dissipated through the
upper water cooled jacket 102. Shield 100 and ring 101 by directing
the heat from the heating element 86 downwardly and inwardly
maintains essentially a relatively shallow hot zone immediately
over the melt and a cool zone above the hot zone. The resulting
temperature differential through which the crystal extends
increases the heat transfer. This arrangement makes possible a
crystal growth rate of 5 - 8 inches per hour for a crystal diameter
of 2 1/2 inches and greater. Prior art techniques permit generally
a rate of 2 to 3 inches per hour for a crystal less than 2 inches
in diameter.
The heat distribution to the crucible form the heating element can
also be achieved by designing the heat element to generate more
heat at the top portion than at the bottom in lieu of shaping the
insulation, such as an R.F. coil with many turns at top and fewer
below. Further, the objective might also be achieved by the
combination of heater and insulation design. Heat dissipation
through the crystal could be enhanced by providing a heat exchanger
to remove heat directly from the crystal. Heat removal through the
lower portion of the crucible could be enhanced by cooling the
crucible support by any suitable heat exchanger.
FIG. 4 depicts a heater temperature program for growing a crystal
by the method of the invention. Curve 110 depicts a program for
growing a large diameter crystal, while curve 112 is for growing a
smaller diameter crystal. In the initial growth portion both curves
indicate a sharp temperature drop of the heater temperature. This
is done to expand the diameter of the original seed crystal to the
desired crystal diameter. The heater temperature is subsequently
slowly increased to compensate for a larger amount of heat
conducted away from the melt through the crystal, as the length of
the crystal is increased. The increased length generates more
surface area for heat transfer within the cool zone above the melt.
The slope of curve 112 is steeper than the slope of curve 110.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood
by those skilled in the art of various changes and form and detail
may be made therein without departing from the spirit and scope of
the invention.
* * * * *