US3694165A

US3694165A - Crucible apparatus for a semiconductor crystal puller

Info

Publication number: US3694165A
Application number: US62938A
Authority: US
Inventors: Horst G Kramer
Original assignee: Monsanto Co
Current assignee: SunEdison Inc
Priority date: 1970-08-11
Filing date: 1970-08-11
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26
Also published as: BE771188A; DE2140070A1

Abstract

CRUCIBLE APPARATUS FOR A SEMICONDUCTOR CRYSTAL PULLER AND INCLUDING A MAIN CRUCIBLE FOR HOUSING A MOLTEN CHARGE OF SEMICONDUCTOR MATERIAL TO BE PULLED. A HOLLOW CRUCIBLE BASE OR SUPPORT MEMBER IS MOUNTED BETWEEN THE MAIN CRUCIBLE AND AN ELONGATED CRUCIBLE SUPPORT PEDESTAL, AND A PORTION OF THE MAIN CRUCIBLE BOTTOM IS SUSPENDED IN THE CRUCIBLE BASE MEMBER TO REDUCE CONDUCTIVE HEAT LOSSES THEREFROM. THE CRUCIBLE BASE MEMBER SHILEDS HEAT RADIATED FROM THE MAIN CRUCIBLE AND COUPLES INTO THE RF HEATER COIL THEREOF TO EVEN FURTHER REDUCE HEAT LOSSES FROM THE MAIN CRUCIBLE. THE CRUCIBLE BASE MEMBER THUS MINIMIZES TEMPERATURE GRADIENTS IN THE MAIN CRUCIBLE AND THEREBY ELIMINATES AN UNDERSIRABLE COOLING OF THE MOLTEN SEMICONDUCTOR MATERIAL DURING A CRYSTAL PULLING OPERATION.

Description

H G. KRAMER Sept. 26, 1972 CRUCIBLE APPARATUS FOR A SEMICONDUCTOR CRYSTAL FULLER Filed Aug. 11, 1970 INVENTOR HORST G. KRAMER ATTO RN EY United States Patent O US. Cl. 23-273 SP 12 Claims ABSTRACT OF THE DISCLOSURE Crucible apparatus for a semiconductor crystal puller and including a main crucible for housing a molten charge of semiconductor material to be pulled. A hollow crucible base or support member is mounted between the main crucible and an elongated crucible support pedestal, and a portion of the main crucible bottom is suspended in the crucible base member to reduce conductive heat losses therefrom. The crucible base member shields heat radiated from the main crucible and couples into the RF heater coil therefor to even further reduce heat losses from the main crucible. The crucible base member thus minimizes temperature gradients in the main crucible and thereby eliminates an undesirable cooling of the molten semiconductor material during a crystal pulling operation.

FIELD OF THE INVENTION This invention relates generally to crucibles used in a semiconductor crystal pulling operation. More particularly, this invention is directed to a novel crucible apparatus for housing a molten semiconductor material and having improved heat insulation characteristics.

BACKGROUND OF THE INVENTION Refractory crucibles which form an integral part of contemporary semiconductor crystal pulling apparatus are well known in the art (see prior art FIG. 1 of the accompanying drawing). These crucibles house and sup port a charge of molten semiconductor material from which semiconductor rods are pulled and are used, for example, in Czochralski crystal pullers which are generally well known in the semiconductor processing art. These crucibles are commonly fabricated of a suitable refractory material such as graphite in order to withstand elevated crystal growing temperatures produced by either resistance or RF inductive heating during a crystal pulling operation. Because graphite adversely reacts with some molten semiconductor materials (e.g. silicon) being pulled, an inner quartz liner for the crucible is usually placed inside the crucible chamber to contact and hold the molten semiconductor material during the crystal pulling operation.

It is desirable to maintain the temperature of the crucible at a predetermined and required high level during the crystal pulling operation so that a sufiicient amount of heat can be conductively transferred from the crucible and through the inner quartz liner thereof to the molten semiconductor material within this liner. The temperature of this molten semiconductor material should be maintained relatively uniform throughout and just above the melting point of the material for proper crystal pulling operation. Therefore, it is desirable to minimize the conductive heat losses or transfer in a direction away from the crucible and away from the molten semi-conductor material therein during the crystal pulling operation. As the heat flow away from the crucible chamber and the molten semiconductor material therein is reduced, the operating efliciency of the crystal pulling 3,694,165 Patented Sept. 26, 1972 operation is increased and undesirable temperature gradients within the crucible are reduced.

DESCRIPTION OF THE PRIOR ART A common prior art structure for minimizing conductive heat losses from the crucible of a semiconductor crystal puller includes an elongated pedestal which supports and positions the crucible at a selected location within a crystal growing chamber. (See pedestal 28 in FIG. 1.). This pedestal is normally constructed of a refractory material to withstand high temperatures Within the crystal growing chamber, and this free space type of crucible mounting limits conductive heat losses from the crucible to the heat transfer through the mounting pedestal. The conductive heat transfer away from the crucible and through the mounting pedestal therefor will depend upon the size (cross-section area) and heat conductivity characteristics of the pedestal as well as the location on the bottom of the crucible at which the pedestal is affixed.

Although the above prior art crucible apparatus serves its intended purpose, a substantial and undesirable conductive heat loss from the crucible occurs as a result of the geometry and location of the mounting pedestal. That is, with the elongated crucible support pedestal mounted in intimate contact with the geometrical center of the bottom wall of the crucible, thereis a substantial amount of conductive heat transfer from the molten semiconductor material within the crucible chamber, through the quartz crucible liner and crucible bottom wall to the cylindrical mounting pedestal. As shown in FIG. 1, the cylindrical mounting pedestal 28 is afiixed to a central location on the bottom surface of the crucible. At this location, the mounting pedestal will receive and conduct more heat from the crucible chamber than it would at any other decentralized location on the bottom surface of the crucible. At the same time, however, it is desirable to maintain the crucible support pedestal at or near the geometrical center of the crucible for the best support and balance of the crucible using this type of elongated pedestal mounting.

When excessive conductive heat flows from the crucible to the support pedestal therefor, the bottom of the crucible becomes over-cooled and an undesirable temperature gradient is established between the heated side walls of the crucible and the crucible bottom wall beneath the chamber of the crucible. This overcooling of the bottom of the crucible frequently leaves the temperature of the semiconductor melt in the crucible to a level that will cause dendrites to form on the semiconductor crystal being pulled from the melt. To avoid the formation of dendrites, an abrupt withdrawal of the semiconductor crystal from the melt is required. However, such abrupt withdrawal typically leaves about 10% of the semiconductor charge or melt in the crucible, resulting in waste and inefliciency of the crystal pulling operation. More importantly, the thermal shock experienced by the semiconductor crystal during such abrupt withdrawal of the crystal from the melt destroys the good crystal structure at particular locations in the pulled crystal; and this results in an additional waste of the semiconductor charge being pulled. When the prior art crucible apparatus described above and shown in FIG. 1 is used to pull antimony-doped silicon crystals, it has been found that, as a result of excessive conductive heat losses in the silicon melt, antimony exceeds its solid solubility in silicon when approximately 55% of the melt has been pulled. At this point, the antimony in the melt will begin to precipitate and render the remaining 45% of the melt unusable for the crystal pulling operation.

3 SUMMARY OF THE INVENTION The general purpose of this invention is to provide a new and improved crucible apparatus which possesses all of the advantages of similarly employed crucible apparatus and yet exhibits none of the disadvantages of the aforedescribed prior art crucible apparatus. To attain this, a novel heat retarding crucible base or support means is mounted between the crucible and an elongated crucible support pedestal. This heat retarding crucible base means is coupled into an RF field which also heats the crucible, and this inductive heating of the crucible base means retards the heat losses by conduction from the crucible. Simultaneously, the crucible base means surrounds the bottom of the crucible and minimizes radiative heat losses from the crucible. Accordingly, it is an object of the present invention to provide a new and improved crucible apparatus for a semiconductor crystal puller. This crucible apparatus has improved heat transfer characteristics, operates at a high efiiciency and aids in maintaining a crucible and molten semiconductor charge therein at a desired high temperature level.

Another object of this invention is to provide a novel crucible apparatus which may be utilized in a crystal pulling operation to eliminate the formation of dendrites on the semiconductor being pulled.

Another object of this invention is to provide a crucible apparatus of the type described which may be utilized in pulling substantially all of a semiconductor melt from the crucible, thereby eliminating waste re sulting from the unused or unpulled charge Within the crucible.

Another object of this invention is to provide a new and improved crucible apparatus of the type described which may be utilized in a crystal pulling operation to eliminate thermal shock and the resulting structural damage of the semiconductor crystal being pulled.

A further object of this invention is to provide a new and improved crucible apparatus of the type described which serves to delay the onset of precipitation when silicon crystals doped heavily with other substances are being pulled.

Briefly described, the present invention features a crucible apparatus for a semiconductor crystal puller and includes a main crucible having a chamber therein for housing a molten semiconductor material to be pulled. A crucible base member engages the main crucible at a selected location on the bottom of the main crucible and has a wall portion defining an opening beneath the main crucible. This crucible base member minimizes the conductive heat transfer from the main crucible, and further serves to shield heat radiataion from the chamber of the main crucible.

A feature of the present invention is the provision of a plurality of elongated openings in a wall of the crucible base member. These openings substantially reduce the size of the conductive heat path afforded by the crucible base member and thereby reduce the conductive heat losses from the main crucible.

Another feature of this invention is the RF coupling of an adjacent inductive heating coil to both the main crucible and the crucible base member. The simultaneous RF heating of these two members further minimizes conductive heat losses from the main crucible.

The above brief description, objects and features of the present invention will become more fully apparent in the following description of the accompanying drawing.

DRAWING FIG. 1 illustrates a typical prior art type crystal puller and is shown partially in isometric and partially in cross section view;

FIG. 2 is an exploded isometric view of a preferred embodiment of the crucible apparatus according to the present invention; and

FIG. 3 is a cross-sectional view of the crucible apparatus in FIG. 2 taken along line 3-3 of FIG. 2 when the three members in FIG. 2 are fitted together for proper cooperation.

Referring now to the drawing, there is shown in FIG. 1 a typical crystal puller apparatus designated generally 7 and having a semiconductor crystal growing chamber 8 in which a main graphite crucible 10 is mounted. In the semiconductor crystal pulling art, the member 10 is normally referred to as a susceptor when the crucible is heated by RF induction heating. Therefore, in the embodiment of the invention to be described with reference to FIGS. 2 and 3 and in the prior art crystal puller apparatus shown in FIG. 1, the crucible therein is more specifically identified as a susceptor. The phrase crucible apparatus as used herein includes, but is not limited to, (1) the main crucible, (2) all members supporting the main crucible, and (3) the associated RF induction heating means for the main crucible. A replaceable quartz inner crucible liner 12 may be insertesd into the chamber of the graphite crucible 10, and the liner 12 does not adversely react chemically with a molten silicon semiconductor charge 14 which is pulled from the chamber at relatively high temperatures.

The monocrystalline semiconductor rod being pulled from a molten polycrystalline semiconductor charge 14 forms a neck 16 which is pulled in a well-known manner from the charge 14 and which adheres to the monocrystalline seed crystal 17. The seed crystal 17 is held in place by a seed chuck 18 which in turn is secured to a rotatable pull shaft 20. An inductive RF heating or Work coil 22 for heating the semiconductor charge 14 may be conveniently mounted in a suitable coolant 24 which is confined to a cooling chamber 26, as shown, and the cooling chamber 26 is substantially concentric with the main semiconductor growing chamber 8.

A suitable ground frame or base 30 is provided to support both the cooling chamber 26 and the semiconductor growing chamber 8, and an elongated pedestal 28 eX- tending from the base 30 positions the crucible 10 in a preselected vertical location within the chamber 8 and with respect to the adjacent RF heating coil 22. When this coil is energized by radio frequency power, eddy currents are induced in the graphite crucible 10 to heat up and maintain the molten silicon charge 14 at an elevated temperature level.

The elongated cylindrical support pedestal 28 engages the graphite crucible 10 at a centralized location 32 in the bottom portion thereof. The integral of the lengths of all heat conductive paths between the locaion 32 and each particle of molten semiconductor charge 14 is a minimum value relative to similar conductive path length integrals for other pedestal locations on the bottom of crucible 10. This structural geometry for the prior art crucible apparatus in FIG. 1 results in substantial conductive heat losses from the crucible 10 and longitudinally through the pedestal 28 in a direction away from the crucible 10. This undesirable heat loss by conduction from the crucible 10 lowers the temperatures in the molten semiconductor charge 14 and produces deleterious effects on the crystal pulling operation in the following manner to be described.

In order to more completely understand and appreciate the significant advantages over the prior art provided by the present invention, consider the RF induction heating of the graphite crucible 10 in FIG. 1. Almost all of the RF induction heating provided by the coil 25 occurs on the outer vertical cylindrical surface or skin 29 of the crucible 10. This heating is due to induced surface eddy currents which are substantially confined to the vertical surface regions of the crucible 10. This surface heat generated is transmitted by conduction to other portions of the crucible 10, and the center of the bottom of the crucible 10 in the vicinity of the cavity 32 is almost completely removed and insulated from any RF surface induc tion heating of the crucible 10. Therefore, neglecting for the present the conductive heat losses through the pedestal 28, there is a temperature gradient between the vertical side wall 29 of the crucible and the bottom 31 thereof due to the fact that the RF inductive heating of the crucible 10 takes place primarily at the surface regions of the vertical wall 29. Therefore, even without the pedestal 28 in FIG. 1 the bottom wall 31 of the crucible 10 would be somewhat lower in temperature than the vertical side wall 29, and this temperature gradient is reflected to the semiconductor melt 14 from which the semiconductor rods are pulled. Since the melt 14 should be maintained slightly above the melting point of the semiconductor material being pulled, there is an upper limit to which the melt 14 can be heated. Therefore, overcooling of the bottom 31 of the crucible cannot be overcome by merely increasing the energy applied to RF heater coil 25, since this would tend to overheat the upper portion of the melt 14.

Now consider the heat conductive contract of the cylindrical pedestal 2'8 with the bottom cavity 32 of the crucible 10. The graphite pedestal 28 will, by virtue of its conductive heat transfer characteristic, further increase the temperature gradient between the vertical side wall 29 of the crucible 10 and the bottom wall 31 thereof, and this temperature gradient is also transferred to the semiconductor melt 14. As the melt 14 shrinks during crystal growth, it recedes further and further into the hemispherical bottom of the crucible liner 12, tending to further cool the lower portion of the melt 14. As a result of the latter and the conductive heat losses by the crucible 10 through the pedestal 28, the temperature of the bottom of the melt 14 as viewed in FIG. 1 will become supercooled during the latter stages of crystal growth and produce spurious nucleation in the melt 14. Thus, the pedestal 28 produces a relatively large temperature gradient between the surface of the melt 14 and the bottom thereof and this gradient results in dendritic growth at the latter stages of the crystal pulling operation. The inefliciency and waste caused by such dendritic growth have been previously described above.

Referring now to the preferred embodiment of the present invention which is illustrated in FIGS. 2 and 3, there is shown a refractory graphite crucible 34 having a chamber 36 therein which is defined by a cylindrical side wall 39 which is contoured to join a concave hemispherical cavity 43 in the bottom portion thereof as shown in FIG. 3. The outer cylindrical wall 38 of the crucible 34 may be inductively heated as previously described with reference to FIG. 1 by a RF heater coil (not shown). At the lower periphery of the graphite crucible 34, there is a recessed offset portion 40 which is adapted to frictionally engage the crucible base member 42 to be described.

The crucible base support member 42 has a cylindrical wall 44 defining an opening or passage 46 into which the offset recessed portion 40 of the crucible 34 fits. The crucible base member 42 includes a plurality of

elongated openings

50, 52 therein between which extend very narrow heat

conductive paths

54, 56 and 58. These paths have a very small cross-sectional area relative to the length of the

elongated openings

50 and 52, and the conductive heat flowing in the cylindrical wall 44 and away from the main graphite crucible 34 is confined to these narrow paths. The crucible base member 42 has a flat bottom 50 which is integrally joined to the lower periphery of the cylindrical wall 44, and the cylindrical wall 44 and bottom 60 of the crucible base member 44 provide a heat shield against radiative heat losses from the crucible 34.

As illustrated in FIG. 3, an elongated hollow cylindrical support pedestal 62 fits snugly into an opening 37 in the flat bottom 60 of the crucible base member 42. The bottom 60 of the crucible base member 42 rests on the shoulder position 66 of outer cylindrical wall 64 of the support pedestal 62.

In accordance with the present invention, the heat losses by conduction away from the graphite crucible 34 now result from the conductive heat transfer through a path which includes a lateral segment 41 of the bottom portion of the crucible 34 and the cylindrical wall 44 of the crucible base member 42. The heat flowing through the cylindrical wall 44 of the crucible base support member 42 is forced to pass through the very

narrow passages

54, 56 and 58 between the

elongated openings

50, 52 as previously mentioned. Thus, the conductive heat flowing away from the crucible 34 is retarded by the novel geometry of the cylindrical wall 44 before being transferred by conduction to the upper cylindrical section 68 of the cylindrical support pedestal 62.

A most significant feature of the present invention residues in the crucible base member 42 and its reduction of the temperature gradient between the vertical side wall 38 of the crucible 34 and the bottom portion 40 thereof. The center of the bottom 40 of the main graphite crucible 34 is no longer drained by heat conducted directly to a supporting pedestal and now is heated to approximately the same temperature as the vertical crucible side wall 38 as will be seen below. The conductive heat losses from all portions of the crucible 34 now flow toward the vertical cylindrical wall 44 of the crucible base member 42 and laterally from the lowermost regions of the crucible 34, with the effect that the total conductive heat losses from the crucible 34 are relatively uniform throughout all segments of this member. Therefore, any prior art temperature gradient between the top and bottom of the semiconductor melt 47 has been substantially reduced or eliminated. However, since the inner cylindrical surface of the wall 44 of the crucible base member 42 reflects or reradiates the heat radiated thereto from the bottom of the crucible 34 back to the crucible 34, such radiation by the base member 42 even further reduces any temperature gradient that may exist between the bottom wall 40 and vertical side wall 38 of the crucible 34. As a result of the latter novel feature of the crucible base member 42, a relatively uniform temperature distribution is provided in all portions of the crucible 34 and the semiconductor melt 45 therein.

The cylindrical graphite support pedestal 62 is smaller in diameter than the crucible base member 42, and the cross sectional area of the pedestal wall 64 is substantially less than the total cross-sectional area of the upper portion of the cylindrical wall 44 of the crucible base member 42. The support pedestal 62 is coaxial with both the crucible 34 and the crucible base member 42 and provides the necessary support for the latter two members at a position adjacent the RF heater coil 45 as shown in FIG. 3. At the same time, the relatively thin cylindrical wall of the pedestal 62 tends to further minimize the conductive heat' flow from the crucible base member 42 and through the pedestal 62.

The length of the conductive heat path from the crucible 34, through the crucible base member 42 and through the cylindrical wall 64 of the pedestal 62 is substantially greater than the length of the conductive heat path from the location 32 and through the pedestal 28 to the base 30 of the prior 'art crucible apparatus shown in FIG. 1. Additionally, the

elongated openings

50 and 52 in the crucible base member 42 further reduce the heat losses by conduction as described above, so that the combination of these two novel features render the present invention a much more efiicient crucible apparatus than the prior art crucible apparatus illustrated in FIG. 1. Furthermore, the RF electromagnetic field generated by radio frequency currents flowing in the induction heater coil 45 couples into the crucible base member 42 as well as the main crucible 34, so that the improved RF heating feature of the invention further minimizes the heat losses from the main crucible 34 by conduction through the crucible base member 42.

Another significant feature of the present invention resides in the construction of the bottom of the crucible base member 42 which facilitates its use with existing crucibles 34 and existing pedestals 62. For example, consider the prior art crucible apparatus in FIG. 1 wherein a pedestal 28 is directly inserted into the cylindrical cavity or receptacle 32 on the underside or bottom of the crucible 34. Thus, in constructing the present invention, it is not necessary to fabricate a new crucible 34 and a new pedestal 62, but instead, existing crucibles 34 and pedestals 62 may be reclaimed as shown in FIG. 3. Thus, the preferred embodiment of this invention is simple and economical in its construction and yet exhibits greatly improved heat insulative characteristics as described above.

Advantageously, the crucible 34, the crucible base member 42 and the pedestal 62 may be fabricated of a refractory material, such as graphite, which is adapted to withstand typical elevated crystal pulling temperatures in the order of 1600 C. Alternatively, refractory metals such as molybdenum, tantalum and tungsten and refractory ceramics such as alumina (A1 may be utilized instead of graphite, if desired, without departing from the scope of this invention.

The particular geometry of the crucible apparatus described above may be modified without departing from the scope of the present invention. For example, the cylindrical shape of all of the members of the crucible apparatus shown in FIGS. 2 and 3 may be changed to some other geometrical configuration having equivalent openings or passages therein if desired which serve the purpose of particular structural geometry described above. Similarly, the

elongated openings

50 and 52 are not restricted to the particular number, size and configuration shown in FIG. 2, but may be substantially modified and still reduce the conductive heat path cross section within a segment or segments of the wall 44 of the crucible base member 42.

The novel crucible apparatus according to the present invention is not limited to RF induction heating, and this crucible apparatus may be utilized, if desired, with resistance heating, using available carbon resistance heaters which are well known in the art. These resistance heaters are normally placed closely adjacent to the vertical side walls of the main crucible and heat the crucible by radiation. Therefore, it will be apparent that the present invention will be most useful with resistance heaters since these heaters tend to produce a temperature gradient similar to the temperature gradient described above with reference to FIG. 1.

The present invention is not limited to the crystal growing of silicon but may also be used for growing single crystals of germanium, as well as certain laser, garnet and ruby materials. In the process of growing or pulling germanium single crystals, an inner quartz crucible liner, e.g. liner 12 in FIG. 1, is not required since the graphite crucible 34 does not adversely react chemically with molten germanium. For this reason, an inner quartz crucible liner, such as the liner 12 shown in FIG. 1, has not been illustrated in FIGS. 2 and 3. However, it will be understood by those skilled in the art that a quartz crucible liner may be inserted into the changer of the crucible 34 in FIGS. 2 and 3 if such apparatus is utilized in the growth of monocrystalline silicon.

I claim:

1. Crucible apparatus for a crystal puller including, in combination:

(a) crucible means having a chamber therein for receiving a molten material,

(b) crucible base means engaging said crucible means at a point thereon removed from the center of'said crucible means, said crucible base means having an annular wall portion and a bottom wall defining a cavity in which said crucible means is suspended, said annular wall portion further having a plurality of spaced openings therein which serve to minimize the total wall cross sectional area of said annular wall portion and thereby substantially reduce the total conductive heat transfer capability of said crucible base means, whereby heat conductively transferred from said crucible means and through said annular wall portion of said crucible base means is minimized due to both the suspension of said crucible means in said crucible base means and the length of the conductive heat path from said crucible means and through said crucible base means; and said wall portion of said crucible base means further serving as a heat shield against radiative heat losses from said crucible means, and

(c) pedestal means engaging said bottom wall of said crucible base means and holding same and said crucible means at a desired location during the pulling of crystal from said crucible means, said pedestal means further extending the conductive heat path from said crucible means.

2. Apparatus for reducing conductive heat losses and heat transfer from a molten material and minimizing temperature gradients therein including, in combination:

(a) a crucible including a chamber therein for housing said molten material,

(b) crucible base means including an annular wall portion thereof engaging said crucible at a selected location thereon remote from the geometrical center of said chamber, said annular wall portion and a bottom wall defining a cavity beneath said crucible in which the bottom portion of the crucible is suspended, said annular wall portion of said crucible base means having a plurality of spaced openings therein for minimizing the total wall cross sectional area of said annular wall portion and thereby substantially reducing the total conductive heat transfer capability of said crucible base means, whereby the conductive heat transfer from said chamber through said annular wall portion of said crucible base means is minimized, and

(c) pedestal means engaging said bottom wall of said crucible base means and holding same and said crucible at a desired location during the pulling of crystals from said crucible.

3. The apparatus in claim 2 wherein said pedestal means engages said crucible base means in a central portion thereof and includes a wall portion defining a passage smaller than said cavity within said crucible base means, whereby said wall portion of said pedestal means further extends the conductive path for heat transfer from said chamber and also positions said crucible at a predetermined location for a crystal pulling operation.

4. The apparatus defined in claim 2 which further includes inductive heating means adjacent said crucible base means and adapted to be energized with radio frequency currents to thereby inductively couple to and heat both said crucible and said crucible base means, whereby the inductive coupling to and heating of said crucible base means further reduces the conductive flow of heat through said wall portion of said crucible base means.

5. The apparatus defined in claim 2 wherein:

(a) said crucible has an offset portion in the outer periphery thereof, and

(b) said wall portion of said crucible base means engages said offset portion of said crucible, whereby both said crucible and said crucible base means are in mutual intimate contact and adapted to be electrically coupled to a radio frequency inductive heating coil for the purpose of heating a molten material within said chamber of said crucible and maintaining heat transfer therefrom at a minimum.

6. The apparatus defined in claim 2 wherein said crucible base means has a bottom portion with an opening therein centrally located with respect to said wall portion, said opening adapted to engage a supporting pedestal which serves to properly position said crucible and said crucible base means with respect to an adjacent inductive heating coil.

7. The apparatus defined in claim 6 wherein said pedestal is an elongated hollow cylindrical member engaging an opening of said crucible base means and removed from any physical contact with said crucible.

8. The apparatus defined in claim 7 which further includes inductive heating means adjacent and inductively coupled to both said crucible and to said crucible base means to simultaneously heat said crucible and said crucible base means and therefore retard the conductive heat transfer from said crucible through said crucible base means.

9. In a susceptor apparatus for a crystal puller having a main crucible with a heating chamber therein and an elongated pedestal for positioning said crucible with respect to an inductive heating coil, the improvement comprising; crucible base means interposed between and engaging said crucible and said elongated pedestal, said crucible base means having an annular wall portion and a bottom wall engaging said crucible at a selected location thereon remote from the center and deepest part of said heating chamber, said annular wall portion and bottom wall of said crucible base means partially defining a cavity beneath said crucible and in which the bottom of said crucible is suspended, said annular wall portion having a plurality of openings therein selectively spaced so as to reduce the total cross sectional area of said crucible base means and thereby reduce its conductive heat transfer capability, whereby the conductive heat transferred from said main crucible is forced to flow through small cross sections of said annular wall portion between said openings therein, minimizing conductive heat losses from said heating chamber, whereby said crucible base means substantially reduces temperature gradients in said crucible by making a substantial portion of said bottom of said crucible free from any direct conductive heat losses.

10. The apparatus defined in claim 9 wherein:

(a) said crucible base means has a bottom wall thereof extending to said wall portion and an opening in said bottom wall for engaging said pedestal, and

(b) said crucible, said crucible base means and said pedestal being of a refractory material capable of withstanding elevated semiconductor crystal growing temperatures.

11. Supporting apparatus for a crucible adapted for housing a molten material, including in combination:

(a) an annular hollow crucible base member adapted for receiving therein the bottom wall portion of said crucible and having a side wall with a plurality of openings therein, whereby conductive heat transfer through said side wall occurs in narrow passages between said openings and thereby limits the heat transfer capability of said crucible base member, said crucible base member further having a bottom wall therein extending from said side wall and having an opening therein, and

(b) an elongated hollow pedestal mounted in said opening and operative for holding said crucible base mem her at a desired vertical location in a heating chamber, whereby said pedestal and said crucible base member serve to reduce the radiative and conductive heat losses from the bottom of said crucible to the surrounding ambient, thereby reducing temperature gradients in the molten material within said crucible.

12. Apparatus defined in claim 11 wherein:

(a) said crucible base member is a hollow cylindrical member having a plurality of elongated openings in the side walls thereof, and

(b) said pedestal is an elongated hollow cylindrical member having a shoulder at one end thereof for re ceiving said bottom wall of said pedestal at said openin g therein.

References Cited UNITED STATES PATENTS 3,093,456 6/1963 Runyan 23301 SP 3,268,297 8/ 1966 Fischer 23273 SP 3,291,650 12/1966 Dohnen 23273 SP 3,493,770 2/1970 Dessauer 23273 SP 3,556,732 1/1971 Chi Chang et al. 23301 SP 2,979,386 4/1961 Shockley 23273 SP OTHER REFERENCES Journal of Scientific Instruments, vol, 35, April 1958, pp. 121-125.

NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner