CA1165212A - Crucible assembly - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A crucible assembly for growing crystalline bodies in accordance with a capillary die process is disclosed. The improvement comprises an integrally formed crucible cover and die top so as to provide a more uniform thermal mass and to achieve more uniform growing conditions.

Description

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 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 "` 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 -.i ... ,.. ~ .

. ~ 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 -- 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 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.

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 ~ 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 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-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 ,,, ,, ... ,.,, . --- - - ~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~

Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an apparatus for use in growing a crystalline body by pulling said body from a liquid melt in accordance with a capillary die process, said apparatus being of the type comprising a crucible assembly including (a) a crucible unit having a side wall and an opened end, (b) a crucible cover fixedly mounted with respect to said crucible unit for covering the top end of said crucible unit, (c) forming means provided on the top side of said cover for defining a die top and (d) feeding means for feeding at least in part by capillary action melt disposed in said crucible to said die top, said die top being sized and shaped so as to support a pool of liquid melt above said die top when said crystalline body is being grown so that said body can be grown by pulling said body from said pool sandwiched between the growing body and said die top with the pool being replenished from the liquid melt in the said crucible at least in part by capillary action through said feeding means as said body is pulled from said pool, the improvement comprising said cover and said die top being integrally formed as a single integral unit.

2. Apparatus according to claim 1, wherein said feeding means includes at least one opening through said cover so as to communicate from the bottom side of said cover to said die top.

3. Apparatus according to claim 2, wherein said feeding means further includes capillary means defining at least one feed capillary and capable of being carried by said cover so that one end of said capillary communicates with said opening in said cover and the other end extends into the melt provided in said crucible unit so as to feed melt from said unit to said opening in said cover.

4. Apparatus according to claim 3, wherein said opening in said cover is a bore, said feeding means further includes a counter bore of said bore from the bottom side of said cover and said capillary means includes at least one feed tube having one end disposable in said counter bore and the other end extend-able into said melt.

5. Apparatus according to claim 3, wherein said feeding means further includes a slot formed in the bottom of said cover and communicating with said opening and said capillary means includes at least two plates connectable together so as to form said feed capillary and having one end disposable in said slot.

6. Apparatus according to claim 3, wherein said crucible assembly further includes container means for containing said melt material, said container means including a bottom and side wall and being open at its top end, said container means being dimensioned so as to be disposable within said crucible unit with said capillary means extending through the top end of said con-tainer means and terminating at its other end adjacent the bottom wall of said container means when said cover covers the open end of said crucible unit.

7. Apparatus according to claim 6, wherein said crucible unit is made of select material so as to function as a susceptor and said container means is made of a select material so as to function as a crucible.

8. Apparatus according to claim 6, wherein said crucible unit is made of a select material so as to function as a crucible and said container means is made of a select material so as to function as a crucible liner.

9. Apparatus according to claim 4, wherein said other end of said tube is sloted.

10. Apparatus according to claim 2, wherein said die top includes two inclined tapered portions each terminating in a die tip, the die tips of said inclined portions being spaced from one another so as to form a slit therebetween.

11. Apparatus according to claim 10, wherein said top side of said cover is substantially planar and said tapered portions extend above the plane of said top side.

12. Apparatus according to claim 11, wherein said top side of said cover is substantially planar, said cover further including grooves formed in the top side of said cover below the plane of said top side on opposite sides of said tapered portions so that said tapered portions are substantially disposed below the plane of said top side and said die tips are disposed substantially in said plane of the top side of the cover.

13. Apparatus according to claim 10, wherein the thickness of said cover varies so as to provide substantially isothermic conditions across said pool when pulling said body from said pool.

14. Apparatus according to claim 13, wherein said cover increases in thickness from the approximate center of said cover to the outer periphery of said cover.

15. Apparatus according to claim 14, wherein the top side of said cover is substantially planar.

16. Apparatus according to claim 15, wherein said cover further includes grooves formed in the top side of said cover below the plane of said top side on opposite sides of said tapered portions so that said tapered portions are substantially disposed below the plane of said top side and said die tips are disposed substantially in said plane of the top side of said cover.

17. Apparatus according to claim 10, wherein said crucible unit includes a bottom wall integrally formed with the side wall of said crucible unit and said cover includes at least one opening for permitting material to be passed through said opening and placed in said crucible unit so that it can be melted into said melt.

18. Apparatus according to claim 17 wherein said cover is sized to rest on said open top of said crucible unit in snug fitting relation to said side wall of said crucible unit.

19. Apparatus according to claim 17, wherein said cover includes a tubular extension of said opening integrally formed with said cover and extending above said die tips so that the level of said melt material can be maintained above said die tips so as to create a positive pressure head with regard to said pool.

20. Apparatus according to claim 17, wherein said cover includes at least two openings diametrically opposite to one another and symmetrically disposed with respect to said die tips.

21. Apparatus according to claim 20, wherein said cover includes a tubular extension of each of said openings, each of said extensions being integrally formed with said cover and extending above said die tips so that the level of said melt material in said crucible unit can be maintained above said die tips so as to create a positive pressure head with respect to said pool.

22. Apparatus according to claim 21, wherein the top ends of said tubular extensions are tapered toward said die tips.

23. Apparatus according to claim 1, further including shielding means, mountable with respect to said crucible assembly for reducing heat loss from said crucible cover.

24. Apparatus according to claim 1, wherein said crucible unit is made of a material so as to function both as a susceptor and crucible.

25. Apparatus according to claim 1, wherein said assembly further includes means for maintaining the level of said melt material in said crucible unit above said die top so as to create a positive pressure head with respect to said pool.