Note: Descriptions are shown in the official language in which they were submitted.
1165~12
This invention relates to apparatus for growing crystalline
bodies and more particularly to novel crucible assemblies for
growing crystalline bodies from a crystalline melt material.
Various apparatus for and methods of growing crystalline
bodies are now known. One such process, referred to herein-
after as the "capillary die process", generally utilizes a
capillary forming member (which for want of a better name is
called a capillary die) supported with respect io
a crucible containing melt ma~erial. The capillary die process
can be carried out in accordance with various ~echniques. By
way of example, one such technique, is shown and described in
U.S. Patent 3,471,266 issued to Harold E. Laselle, Jr. for Growth
of Inorganic Filaments, while another, known as the edge-defined
film-fed growth technique (commonly referred to as the EFG
process) is described in U.S. Patent No. 3,591,348 issued to
Harold E. LaBelle, Jr. for Method of Growing Crystalline
Materials, and U.S. Patent No. 3,687,633 issued August 29, 1972
to Harold E. LaBelle, Jr. et al, for Apparatus for Growing
Crystalline Bodies from the Melt. In the EFG process the
cross-sectional shape of the crystalline body is determined by
the external or edge configuration of the end of the forming
member and more particularly the top end of the capillary die.
This process involves growth of the crystal body from a liquid
film of feed material sandwiched becween the growing body and the
top end surface of the die, with the liquid in the film being
continuously replenished from a suitable melt reservoir in
the crucible, typically via one or more feed capillaries communi-
cating with the die. Among the materials that have been grown by
MTA-30 -2- ~ ~
1165~12
these capillary die processes as monocrystalline bodies are
alpha-alumina (sapphire), spinel, chrysoberyl, barrium titanate,
lithium niobate, yttrium aluminum garnet and most notably
silicon.
An advantage of the capillary die process is that bodies of
selected shapes such as flat ribbon or round tubes can be produced
commencing with the simplest of seed geometries. Typical, conven-
tional crucible die assemblies for growing bodies in accordance
with such processes are shown and described in U.S. Patent
3,687,633 issued August 29, 1972 to LaBelle et al. Briefly, these
conventional assemblies are mounted within a crucible having a
crucible liner containing the melt material from which the body is
to be grown. Typically, for induction furnaces the crucible is
fitted within a susceptor made for example of graphite or
molybdenum for heating the crucible. The die assemblies shown
in U.S. Patent 3,687,633 each comprise a cover plate and one or
more die members either supporting or supported by the cover
plate. The cover plate fits within and is positioned with
respect to the crucible so that the die members extend from within
the crucible liner through the supporting plate to form the die
top end surface for supporting a film of melt above the cover
plate. In this manner the die members function both as the feed
capillaries and forming members. The die members are described
as being made of two principal parts that cooperate to define the
capillary. For growing tubular bodies, the die members comprise
two cylinders sized so that one fits within the other with an
intervening capillary space therebetween and means formed inte-
grally or separately from the cylinders for maintaining the
cylinders in concentric relation to each other. The outer cylinder
is formed with an outer shoulder for supporting the crucible
cover plate. For
MTA-30 -3-
1165212
growing ribbon-like crystalline bodies, the die members comprise
two flat-like elements, secured together with rivets or pins
so as to define a capillary through which melt can be drawn from
the crucible liner to the die top.
The latter type die members for growing ribbon-like bodies
also include "D-rings" (essentially solid D-shaped wedges) each
wedged between a corresponding one of the two flat elements and
the supporting cover plate for supporting the two elements in
relationship thereto.
Although crystalline bodies of preselected shapes can be
sucessfully grown with these types of die assemblies their
structures provide several less than idealistic growing conditions
which conditions can create difficulties in the growing process
and imperfections in the end product. Generally, since the die
assemblies are each made of many parts they provide (1) tight
tolerance requirements for each of the parts, (2) alignment
problems with respect to the interfi~ting elements and (3) assem-
bly problem~ such as an increased probability of contamination of
the various parts, breakage, etc. Most importantly, since the
various parts operate at high temperatures, (for example between
about 1600C to 1800C at the die top for silicon) unpredictable
and nonrepeatable heat transfer characteristics exist between the
interfacing surfaces of the various parts. For example, a tem-
perature differential or gradient can be as great as 100C between
two tightly contacting surfaces due to the unpredictable thermal
MTA-30 -4-
"` 1165~12
reslstance created therebetween. These unpredlctable heat
transfer characterlstlcs make lt dlfflcult to ~1) predlct
thermal (wldth) proflles as well as create lsothermal
condltions ln the liquld pool above the top end of the dle
top from which the body is pulled thereby mak;ng it
difficult to provlde unlform and stable temperatures at the
growlnq lnterface between the crystalllne body and melt
provided across the die top, and (2) have the die top
temperature move more in unison with changes in the
temperature of the holding system. Further, as a result of
the temperature differentials created by these unpredictable
thermal resistances between the interfitting parts, there
ls a greater chance that one or more of the parts may warp.
Accordingly, it is a general object of the present
invention to reduce or overcome the above-noted problems
encountered in the prior art.
A more specific object of the present invention is
to provide an improved crucible assembly for growing
crystalline bodies having fewer tolerance, alignment and
assembly problem~ of the type encountered with the multi-
part configuration, such as those disclosed in U.S. Patent
3,687,633.
These and other ob;ects of the present invention
are achieved by an apparatus for use in growing a crystalline
body by pulling thé body from a liquid melt in accordance
with a capillary die process, the apparatus being of the
type comprising a crucible assembly including (a) a crucible
unlt having a side wall and an opened end, tb) a crucible
cover fixedly mounted with respect to the crucible unit
for covering the top end of the crucible unit, (c) formlng
. mean-q provided on the top side of the cover for deilning a
~ dms k(-l - 5 -
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. ~ V ... . .~ .
llfi5212
dle top and ~d~ feeding means for feeding at least ln part
by capillary action melt disposed ln the crucible to the
die top, the die top belng sized and shaped so as to
support a pool of liquld melt above the dle top when the
crystalllne body is being grown so that the body can be
grown by pulling the body from the pool sandwiched between
the growing body and the die top with the pool being
replenished from the liquid melt in the said crucible at
least in part by capillary action through the feeding means
10 as the body is pulled from the pool, wherein the cover
and the die top are integrally formed as a single integral
unit.
; In a preferred embodiment of the present invention
the lmproved crucible assembly comprising a forming member
for growing crystalline bodies has a die top exhibiting
thermal profiles which are of a greater predictability
than those typically provided by the devices described in
U.S. Patent3~687~633.
In another preferred embodiment of the present
20 lnventlon an improved crucible assembly has a forming member
ln whlch the die top temperatures created during growth
of a crystalline body are more uniform and isothermal, move
more closely in unison with temperature changes in the
holding system and are of greater thermal stability.
In still another preferred embodiment of the
present inventlon an improved crucible assembly has a forming
member ln whlch the unpredictable and nonrepeatable heat
transfer characteristics created by the interfaclng of the
parts of the prior art are substantially reduced.
In yet another preferred embodiment of the present
lnventlon a crucible assembly tl) has a dle top exhibiting
,
B dm :`Kd - 6
-- 1 1652 12
-- leqs dle warpage, ~2~ has more accurate dle top allgnment
and height posltlon relatlve to the cruclble and (3)
exhlblts more rellable performances relatlve to, for example,
repeatable dle top temperatures, than that typically
provlded in the assemblies of the type described in U.S.
Patent 3,687,633.
And ln still another preferred embodiment of the
present invention a crucible assembly is adapted for use
ln RF furnaces in which a single element forms both crucible
and susceptor.
And ln yet another preferred embodiment of the present
invention an improved crucible assembly is constructed so
that a positive pressure head can be created in the
crystalline melt to aid in the growth of the crystalline
body.
dm :kcJ - 6a -
` ~1 1165212
Other features and many of the attendant advantages of
this invention are set forth or rendered obvious by the
following detailed description which is to be considered
together with the accompanying drawings wherein:
Fig. 1 is a cross-sectional side view of one
embodiment of the present invention.
Fig. 2 is a cross-sectional side view of the Fig. 1
embodiment rotated 90 from the view shown in Fig. l;
Fig. 3 is a top view of a second embodiment of the present
invention;
Fig. 4 is a sectional view taken along line 4-4 of Fig. 3;
Fig. 5 is a sectional view taken along line 5-5 of Fig. 3;
Fig. 6 is a top view of a third embodiment of the
present invention;
Fig. 7 is a sectional view taken along line 7-7 in Fig. 6;
Fig. 8 is a sectional view taken along line 8-8 in Fig. 6;
and
Fig. 9 shows a modification to the embodiment shown in Figs.
1 and 2.
In the drawings like numerals are used to designate
similar parts.
Turning now to Figs. 1 and 2, one illustrated embodiment
of the present invention comprises a crucible assembly described
for use in an RF furnace comprising a susceptor or heating element
10, a crucible 12, a cover and die top assembly 14 and a radiation
shield assembly 16. The susceptor 10 comprises a bottom wall 18
and a cylindrical side wall 20 may be integrally formed as a
single unit or as shown in Figs. 1 and 2, in two separate pieces.
Susceptor 10 is open at its top end and its
MT~-30 ~ -7-
11 ( 1165212 . I
cylindrical wall 20 is undercut so as to provide an interior
annular shoulder 22 spaced a short distance from the upper
end of the susceptor. The crucible 12 comprises a bottom wall 24
and a side wall 26 and is also open at its top end. The
crucible 12 is sized so as to fit within the susceptor with the
bottom wall 24 of the crucible resting on the bottom wall 18
of the susceptor, the side wall 26 of the crucible nested within
the cylindrical side wall 20 of the susceptor, and the open
top end of the crucible disposed just below the annular shoulder
22 of the susceptor.
The cover and die assembly 14 shown in Figs. 1 and 2
is adapted for growing flat ribbon-shaped crystalline bodies from
melt disposed within the crucible 12. The die member assembly of
the embodiment shown comprises an integrally-formed die top mem-
ber 28 and at least one feed capillary tube 30 defining a feed
capillary. More specifically, the die top member 28 is formed as
a single integral piece sized and shaped so as to be supported by
and rest on the annular shoulder 22 of susceptor 10. As shown the
bottom surface 32 of the member 28 is formed as a substantially
flat planar surface although the surface can be contoured as
described more fully in relationship to Figs. 3-8. The top
surface 34 of die top member 28 is formed as a substantially flat
planar surface having an upwardly-extending die tip 36. As shown
in Figs. 1 and 2 tip 36 is formed substantially as two tapered
elongate portions 38 extending lengthwise at least partially
diametrically across the top surface 34 and terminating vertically
at their top edges as die flats ti.e. relatively small flat
surfaces for supporting the liquid melt pool) preferably at the
same relative height above the top surface 34 and are spaced from
one another so as to form the slit 40 therebetween. At least one
bore 42 extends through the member 28 from the
ITP~-30 ll -8-
- ~ ~
~ll
116S212
bottom surface 32 to and in communication with the slit 40.
Bore 42 is counter bored from the bottom surface 32 at 44 so as
to receive feed capillary tube 30 in a snug fitting relationship.
The length of tube 30 is such that when properly fitted into
counter bore 44 it extends down into the crucible 12 and terminate
at end 46 close to or in contact with the bottom wall 24 of the
crucible 12. End 46 is preferably provided with at least one and
preferably two slots 48 to allow melt to flow from the bottom of
crucible 12 into the tube 30. The internal diameter of tube 30 an
the diameter of bore 42 are substantially equal. The number of
bores 42 and corresponding tubes 30, the internal diameter and
length of each tube 30, the diameter of each bore 42, the width
and length of slit 40 are all in part determined by the
crystalline material to be grown from the die tip 36 and are in
particular dimensioned so that melt disposed in the crucible 12 at
a level above slots 48 of the tube end 46 can be drawn up each
tube 30 through the corresonding bore 42 to the slit 40 by
capillary action to provide sufficient melt material across the
die flats of tip 36 while a body is being pulled from the die
tip in accordance with principles well known in the art.
The radiation shield assembly 16, although not essential to
the invention, can be used to reduce heat loss by radiation and
is shaped to provide a more flat temperature profile horizontally
across the die tip 36 from which the body is grown. The shield
assembly 16 is shown in Figs. 1 and 2 as covering the die top
member 28 and including a slot 50 through which the die top 36
can extend upward and through which a body of crystallin material
can be pulled in an upward direction from the die tip. The slot
50 is rectangular in cross-section so as to be substantially
~IA-30 _9_
116~212
coe~tensive with the die tip 36 and in particular the cross-
section of the body being grown from the die tip.
The slot 50 is preferably beveled at least along its longi-
tudinal edges 52 extending parallel to and adjacent the respec-
tive tapered portions 38 of the die tip 36 so as to provide a more
even temperature profile across the die tip. The shield assembly
16 is provided with apertures 54 positioned in the shield so as to
be aligned with suitable apertures 56 in the die top member
for receiving registration pins (not shown) so that the slot
50 remains properly aligned with the die tip 36 when the
pins are positioned in the apertures 54 and 56.
Although the problems associated with thermal gradients
between interfitting parts of a die top assembly are greatly
reduced by utilizing the two-piece assembly comprising die top
member 28 and feed capillary tube 30, an even greater stable
temperature profile can be achieved by eliminating the capillary
feed tube 30, and utilizing a single integral die top member as
shown in Figs. 3-8.
More specifically, the two embodiments shown in Figs. 3-5
and Figs. 6-8, respectively, each include a crucible assembly com-
prising only two parts, i.e., (1) a crucible (which coincidentally
can also function as a susceptor) and (2) a crucible cover and
die top member so that the element 12 and capillary tube 30 of
the embodiment of Figs. 1 and 2 are not needed.
MTA-30 -10-
1165~12
More particularly, the crucible assembly of Figs. 3-5 comprise
a combined crucible and susceptor unit lOA having a bottom wall
18A integrally formed with the cylindrical side wall 2 OA. In
this embodiment the cylindrical side wall 20A need not be formed
with an annular shoulder such as shoulder 22 of the embodiment of
Figs. 1 and 2 since as described hereinafter the cover and die
top member 28A is adapted to fit over the open top end of unit lOA.
In particular the member 28A iS formed with an annular
shoulder 60 around the peripheral edge of the lower surface 62
of the member so as to form a reduced cylindrical portion 64 of
the member 28A sized so as to be received by the top open end
of unit lOA in a snug fitting relation with shoulder 60
of member 28A resting on the edge of the open end of the unit
lOA. The member 28A includes a substantially flat upper surface
66 having the die tip 36 integrally formed thereon in a similar
mannex as described with reference to Figs. 1 and 2. Specifically
the two elongated tapered portions 3 8 oriented lengthwise at least
partially diametrically across the top surface 66 of member 28A
and terminating vertically at their top edges as die flats above
upper surface 66 preferably at the same relative height above the
upper surface 66 and so as to form the slit 40 therebetween. The
member 28A is formed with one or more bores 68 which extend from
the lower surface 62 to and communicating with slit 40. The bores
function as feeding means for feeding melt disposed in unit lOA
to the die tip 36. The number and location of bores 68 is such tha
sufficient melt material will be provided along the entire width
MTA--30 --11--
~ C 116~212
of the die tip 36 during growth of the crystalline body therefrom.
The member 28A includes at least one, and preferably two apertures
70 at diametrically opposite sides of the die tip 36. Each aper-
ture 70 includes a tubular extension 72 formed integrally with
member 28A, extending above top surface 66 of member 28A and ter-
minating at an end 74 which tapers toward the center of the mem-
ber 28. Aperture 70 as well as extension 72 is large enough so
that melt material can be passed therethrough into crucible lOA
where it can be heated. The ends 74 are positioned well above
the top edges or die flats of the tapered portions 38 of the die
tip 36 so that by maintaining the level of the melt in the exten-
sions 74 above the pool of melt provided along the slit 40 above
the die tip 36 so as to provide zero ullage within unit when a
crystalline body is being pulled therefrom, a positive pressure
head can be created if desired with respect to this pool to
aid in the feèding of material through the bores 68 to the slit 40.
The embodiment shown in Figs. 3-5 can be further modified
as illustrated in Figs. 6-8. As shown the member 28A of Figs. 3-5
is modified as the member 28B, wherein the thickness of the member
varies so as to provide substantially isothermic conditions
across the die tip 36 when a body is being pulled therefrom. More
particularly, since the crucible and susceptor unit lOA shown in
Figs. 3-5 are typically heated by heating the cylindrical side wall
20A from outside the crucible assembly, the crucible melt material
adjacent the side wall 20A will generally be at a higher tempera-
ture than the melt material in the center of the unit. Accord-
ingly, since the member 28A
I~lTl~--3 0
1165212
offers thermal resistance which is a function of its thickness,
as shown in Figs. 6-8, by making the member thinnest at its
approxima~e center 76 and increasing its thickness from this
center to the outer periphery 78, the temperature profile along
the die tip 36 will be closer to ideal isothermal conditions or
even cooler at the die ends. Additionally, by forming the pair
of grooves 80, one on each side of die tip 36 so that the
die tip can be disposed substantially below the substantially
flat planar upper surface 66 of the member 28B with the die flats
or top edges of tapered portions 38 being substantially even with
the surface 66. As such the pool of melt provided in slit 40
above the die flats of the die tip during growth of the crystal-
line body can be maintained at a more even temperature since it
will be disposed closer to the thermal mass provided by member 28B.
In operation, the entire unit lOB can be filled with melt material
to a level within the tubular extensions 72 so that the
melt material contacts the entire bottom surface of member 28B
and i8 fed into bores 68 and slit 40 and even provide a positive
pressure head with respect to the pool above the dip tip 36 during
growth of the crystalline body if desired.
The particular material with which the foregoing crucible
assemblies described are made is such that their composition
will withstand the operating temperatures and will not react to
the melt. With the possible ecception of the crucible 12 all the
parts of each assembly should be manufactured with the same or
similar composition so as to exhibit the same thermal character-
istics. Because of the properties of the crystalline materials
that are
U ~-30 -13-
116S212
processed into diverse shapes by the capillary die process,
only a limited number of materials can be used to
fabricate each of the crucible assemblies. For example,
in growing alpha-alumina bodies assemblies made of either
molybdenum or tungsten can be used, while assemblies used
in growing silicon are generally made of graphite or quartz.
Each assembly made in accordance with the invention can
be easily manufactured by using a lathe and a mill working
machine to provide the particular contours desired.
Although specific embodiments of the invention have been
described various modifications can be made without departing
from the invention. For example, although the three embodiments
described are useful in growing ribbon-shaped bodies, each member
28, 28A or 28B can easily be modified to have any die tip config-
uration to grow bodies of other cross sections. For example, an
annular die tip can be formed on the upper surface of each of
the members 28, 28A and 28B for growing tubular bodies while die
tips each having a central aperture can be used in growing solid
tubes. Further, as shown in Fig. 9 a pair of capillary plates 84
can be substituted for the feed capillary tube 30. Such plates
are well known and may, for example be specifically configured
as that shown in Fig. 10 of said U.S. Patent 3,687,633. In such
a configuration the several bores 42, or alternatively as shown
in Fig. 9 a slot 86 is formed in member 28 communicating between
the bottom of member 28 to the slot 40. A groove 88 is formed in
the bottom of member 28 which communicates with slot 86 and is
sized so that the top of plates 84 will be snugly received.
Plates 84 when snugly received in groove 88 will form a feed
capillary 90 which extends down into crucible similar to tube
30 in Fig. 1.
MT~-3~ 14-
~ I 1 1165212 (
The particular crucible assembly of the present invention
thus made can be used in other types of furnaces besides RF
furnaces, such as those using resistance heaters. In such a
situation, in the embodiment of Figs. 1 and 2, the susceptor 10
can be thought of as a crucible and crucible 12 can be thought
of as a crucible liner. Examples of materials employed for such
an assembly are taught in U.S. Patent 3,687,633, or as that
mentioned above.
The invention thus described has several advantages.
By using a single cover and die top member 28, 28A or 28B fewer
tolerances, alignment and assembly problems are encountered
when compated to those encountered in the multi-part configuration
such as disclosed in U.S. Patent 3,687,633. Further, the die tip
36 exhlbits thermal profiles which are of a greater predicta-
bility than ~aid prior art multi-part configurations. Since
a more uniform thermal resistance and thermal mass is provided by
member 28, 28A and 28B, the die tip temperatures created during
growth of a crystalline body are more uniform and isothermal,
move more closely in unison with temperature changes in the
holding system and exhibit greater thermal stability. Addi-
tionally, the unpredictable and nonrepeatable heat transfer
characteristics created by the interfacing of the parts of said
prior art multipart configuration is substantially reduced with
the integrally formed cover and die tip member 28, 28A and 28B.
Since the die top is integrally formed with the cover, the die
top will exhibit less die warpage, will be properly aligned and
at the proper height with respect to the thermal mass of the
MTA-30 -15-
,,, ,, ... ,.,, . --- - - ~Y
1~ ( 1165Z12
crucible assembly, and generally will perform more reliably.
In addition, utilizing the modifications of Figs. 3-8, the
capillary tube can be eliminated and reliability can be even
greater. Finally, by bringing the liquid melt into contact
with and varying the thickness of the cover member 28 as shown
in Figs. 6-8, and placing the die top closer to the thermal mass
presented by the member even greater results are achieved.
Since certain changes may be made in the above apparatus
without departing from the scope of the invention herein involved,
it is intended that all matter contained in the above description
or shown in the accompanying drawing shall be interpreted in an
illustrative and not in a limiting sense.
~5A-3~
