Abstract:
An improved mechanical arrangement controls the introduction of silicon particles into an EFG (Edge-defined Film-fed Growth) crucible/die unit for melt replenishment during a crystal growth run. A feeder unit injects silicon particles upwardly through a center hub of the crucible/die unit and the mechanical arrangement intercepts the injected particles and directs them so that they drop into the melt in a selected region of the crucible and at velocity which reduces splashing, whereby to reduce the likelihood of interruption of the growth process due to formation of a solid mass of silicon on the center hub and adjoining components. The invention also comprises use of a Faraday ring to alter the ratio of the electrical currents flowing through primary and secondary induction heating coils that heat the crucible die unit and the mechanical arrangement.

Description:
[0001] This invention was made under DOE Subcontract No. ZAX-8-17647-10. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to growing crystalline bodies from a melt by the Edge-defined Film-fed Growth (“EFG”) process, and more particularly to improvements in apparatus for growing hollow crystalline bodies by the EFG process.  
         BACKGROUND OF THE INVENTION  
         [0003]    The EFG process is well known, as evidenced by the following U.S. Pat. Nos. 4,230,674; 4,661,324; 4,647,437; 4,968,380; 5,037,622; 5,098,229; 5,106,763; 5,156,978; and 5,558,712. In the EFG process crystalline bodies having a predetermined cross-sectional shape are grown on a seed from a liquid film of a selected feed material which is transported by capillary action from a melt contained in a crucible through one or more capillaries in an EFG die to the top end surface of the die. The shape of the crystalline body is determined by the external or edge configuration of the top end surface of the die. A major use of the EFG process is to grow polygonally-shaped hollow bodies of silicon, e.g., “nonagons” or “octagons”. These shaped hollow bodies are subdivided at their corners into a plurality of flat substrates that are used to form photovoltaic solar cells.  
           [0004]    The preferred form of apparatus for growing hollow bodies by the EFG process comprises a capillary die/crucible assembly having a center hub which provides a passageway through which silicon particles are introduced to replenish the melt in the surrounding crucible during the growth process. In growing silicon bodies, the silicon particles are typically in the form of substantially spherical pellets having as size in the order of  2  mm. The particles are injected through he center hub into the space above the crucible, where they are deflected back down into the crucible. The common practice is to deliver the particles in predetermined quantities on an intermittent basis according to the rate of consumption of the melt, so as to maintain the level of the melt in the crucible within predetermined limits.  
           [0005]    Growth of large thin-walled hollow bodies by EFG (e.g., silicon octagons in which each side or facet is 10 cm. wide) necessitates precise control of the heat input, since it is essential to maintain the temperature at the growth interface, i.e., in the meniscus region between the top end face of the die and the seed or the body grown on the seed, substantially constant at a level that allows growth to occur at a selected rate. In the EFG apparatus commonly used to grow hollow silicon bodies, heating is provided by induction heating coils that surround the furnace enclosure in which the crucible/die assembly is mounted. Thermal control of the growing crystalline body is achieved by controlling the heating power and also, inter alia, by use of concentric inner and outer after heaters between which the growing body is pulled away from the die. The afterheaters are, in effect, susceptors and are heated by electromagnetic induction. The inner and outer afterheaters help to control the thermal gradient lengthwise of the growing crystal and also affect the thermal gradient of the die and crucible in a radial direction, i.e., normal to the pulling axis.  
           [0006]    Successful growth using the EFG process is complicated by the fact that variations in temperature tend to exist around the circumference of an EFG die and also radially of the die and crucible. Variations in thermal symmetry around the circumference of the die can cause local changes in thickness of the growing crystalline body. Such variations also make it difficult to sustain growth, often resulting in rupturing of the liquid menisci that extend between the die and the growing crystal body. When the menisci are ruptured, the growth stops.  
           [0007]    Improvements in die design have reduced variations in thermal symmetry around the circumference of the die, thereby improving the quality of the grown bodies and reducing the rate of occurrence of rupturing of the menisci. However, even with improved die designs, EFG crystal growth apparatus of the type using crucibles with a center hub have been handicapped by a tendency for solid silicon to become attached to or grow on the center hub region of the crucible during crystal growth in response to disturbances in the growth zone. In this connection it should be noted that the thermal gradient in a radial direction is such that center hub of the crucible tends to be colder than the outer perimeter of the crucible.  
           [0008]    It has been determined that prior EFG crystal growth apparatus lacks adequate means for controlling the path and speed of the particles as they travel out of the center hub and into the melt in the crucible, with the result that (a) sometimes the particles falling into the crucible cause splashing of the melt, with the result that liquid silicon impinges upon the upper portion of the center hub and (b) sometimes some of the solid particles come into direct contact with the upper end of the center hub. When this occurs, depending on the temperature of the center hub, the liquid silicon will solidify on and the silicon particles will become attached to and grow outward from the center hub, ultimately forming a mushroom-shaped solid mass that may be large enough to impede replenishment of the melt. Such solidification near the center hub region also affects the uniformity of the growing crystalline body and disrupts growth. Also fluctuations in temperature can result in pieces of the mushroom-shaped piece breaking off and failing into the melt, causing the crucible to overfill and flood the die.  
           [0009]    Prior to this invention a common heater arrangement has comprised coaxial primary and secondary induction heating coils connected in series with a suitable, medium-frequency, power supply, with the primary coil having three turns and the secondary coil having a single turn and located above and spaced from the primary coil. The heater arrangement has also included a saturable reactor connected in parallel with the primary coil for the purpose of controlling the ratio of the currents flowing through the two coils. The saturable reactor allows the ratio of the currents to be adjusted, thereby modifying the temperature distribution along the axis of the furnace. However, saturable reactors suffer from the fact that they are costly, noisy and electrically inefficient.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0010]    A primary object of this invention is to provide an improved feed distributor/EFG crucible/die unit arrangement that controls the path of silicon particles as they move under gravity from a central feed tube to the crucible.  
           [0011]    Another primary object is to provide an improved means for controlling the electromagnetic energy field used to heat an EFG crystal growth furnace.  
           [0012]    A further object of this invention is to provide in an EFG crystal growth apparatus a particle feed distributor of novel design to control delivery of silicon particles into the melt-containing crucible so as to effectively eliminate or substantially reduce the occurrence of crystal growth on the center hub of the crucible.  
           [0013]    Another object of the present invention is to provide in an EFG crystal growth apparatus for growing hollow bodies a combination particle distributor/inner afterheater assembly that substantially prevents or minimizes undesired solidification of melt material near the center region of the EFG die.  
           [0014]    Still another object is to provide an improved method of delivering silicon particles into a crucible in an EFG crystal growth apparatus.  
           [0015]    A further object is to provide an improved method of controlling the flow of electrical current in a pair of induction heating coils used to heat an EFG crystal growth apparatus.  
           [0016]    Another object of the invention is to avoid solidification of silicon onto members associated with the EFG die, whereby to avoid premature termination of a growth run.  
           [0017]    Still another object of the invention is to provide an improved method of growing a tubular crystalline body from a pool of melt.  
           [0018]    The foregoing and other objects of this invention are achieved by modifying the particle distributor/inner afterheater structure associated with an EFG crucible/capillary die assembly, as typified by the apparatus disclosed in U.S. Pat. Nos. 4,661,324, 4,968,380, 5,037,622; and 5,098,229, so as provide particle distribution and flow control means that reduce the likelihood of silicon particles and splashed molten silicon contacting and adhering to relatively cold portions of the center hub section of the crucible and serving as the nucleus or site for further accretions caused by solidification of molten silicon from the melt, whereby to avoid interruption of the growth process and/or production of tubular bodies of poor quality. Control of the rate of heating is improved by using a Faraday ring to adjust the ratio of power supplied by the primary and secondary induction heating coils.  
           [0019]    Other objects and features of the invention are set forth or described in the following detailed specification which is to be considered together with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings wherein:  
         [0021]    [0021]FIG. 1 is front elevation view, partly in axial cross-section, of an EFG crystal growth furnace apparatus that includes a silicon feed distributor assembly of the type used prior to the present invention;  
         [0022]    [0022]FIG. 2 is a view similar to FIG. 1 of a crystal growth furnace with an improved silicon feed distributor embodying the present invention;  
         [0023]    [0023]FIG. 3A is an enlarged exploded view of the lower portion of the improved feed distributor in relation to an EFG crucible/die assembly;  
         [0024]    [0024]FIG. 3B is an enlarged exploded view of the upper portion of the improved feed distributor;  
         [0025]    [0025]FIG. 4 is an enlarged sectional view of the discharge orifice section of the improved feed distributor; and  
         [0026]    [0026]FIGS. 5A and 5B are an enlarged fragmentary sectional views illustrating an alternative embodiment of the invention. 
     
    
       [0027]    In the several figures, identical numbers refer to identical elements.  
       DETAILED DESCRIPTION  
       [0028]    [0028]FIG. 1 illustrates a preferred form of EFG crystal growth apparatus in use prior to the present invention for growing hollow silicon bodies. This prior art apparatus comprises a crucible/capillary die unit  20 , made in accordance with the teachings of U.S. Pat. No. 5,037,622, installed in a furnace that comprises an enclosure  24  (only part of which is shown for simplicity of illustration), and a heating coil  26  surrounding enclosure  24 . The crucible/die unit  20  is made of graphite and includes a short, open-topped crucible comprising an upstanding outer side wall  32 , a bottom wall  34  and an annular inner side wall  36  which forms an annular hub. Outer side wall  32 , bottom wall  34  and inner side wall  36  together define an annular space in which a melt  38  is contained. Although not visible in FIG. 1, the outer side wall  32  includes a capillary die portion that has substantially the same construction as the one illustrated in FIGS.  3 - 7  of said U.S. Pat. No. 5,037,622, and the teachings of that patent are incorporated herein by reference. The apparatus of FIG. 1 further includes a heat susceptor  40  which is positioned directly below and serves as a support for crucible/capillary die unit  20 . Although not shown, it is to be understood that a pedestal mounted in furnace enclosure  24  acts as a support for susceptor  40  and crucible/die unit  20 . Susceptor  40  is made of graphite or other suitable material and is heated by suscepting electromagnetic energy generated by heating coil  26 . Susceptor  40  transmits its heat to the crucible/die unit  20  so as to maintain the silicon in the crucible in a molten state.  
         [0029]    The central hub  36  of the die/crucible unit  20  defines a center hole which is counterbored so as to form a shoulder  44 . Susceptor  40  has a center hole  46  which is counterbored so as to form a shoulder  48 . Susceptor  40  also has an annular projection or hub  50  on its upper side concentric with its center hole  46 . Hub  50  extends into and makes a close fit with the center hole of hub  36 , with shoulder  44  overlapping susceptor hub  50 . One or more graphite locator pins  52  disposed in holes in the under side of the crucible/die unit and the upper side of the susceptor serve to properly orient those members relative to one another. In this connection, it is to be noted that in growing a hollow body of polygon cross-sectional configuration, the susceptor, like the die, is shaped to conform generally to that cross-sectional configuration.  
         [0030]    The center hole  46  of the susceptor is filled with a multiple layers  53  of graphite felt insulation which surround a feed tube  54 . The latter is connected to a melt replenishment system represented schematically at  56  that is adapted to deliver solid particles of silicon feed stock through into the region above the crucible, from which region the particles fall down into melt  38 .  
         [0031]    Mounted on the center hub of the crucible/die unit  20  is a standoff ring  60  made of quartz or graphite, and overlying hub  50  of the susceptor and the graphite felt insulation is an annular graphite plate or disk  62  that has an annular hub  64  at its inner edge. A silicon feed director member or umbrella  66  having a conically-shaped upper surface  68  is supported near its outer edge by ring  60 . Feed director member  66  also is made of graphite and has a center hole that accommodates the top end of delivery tube  54  and a depending tubular extension  70  that sits on hub  64  of plate  62 . Member  66  functions like an umbrella in that it intercepts falling silicon particles which then slide down its sloped upper surface  68  and spill into the crucible. Member  66  directs the falling particles into the melt  38  in the crucible.  
         [0032]    The apparatus of FIG. 1 also includes a hat-shaped member  72  that encloses distributor member  66 . Member  72  comprises a side wall  74 , a top wall  76 , and a peripheral flange  78  at the bottom edge of the side wall. Member  72  and flange  78  are made of graphite, and flange  78  sits on a plurality of standoff pins  80  that that are mounted in blind holes in bottom wall  34  of the crucible/die unit. The center of member  72  is provided with a replaceable graphite insert  82  that acts as a deflector of silicon particles delivered via feed tube  54 .  
         [0033]    Member  72  has three functions. It acts as a plenum to contain and distribute the gas used to transport silicon particles up feed tube  54 . That gas passes between standoff pins  80  and also between flange  78  and the crucible/die unit  20  into the space inside of the growing hollow crystalline body  88 . Member  72  also acts as an inner afterheater for the growing body  88 . In this connection, it is to be noted that in growing a hollow body having a predetermined polygon cross-sectional configuration, member  72  (like the susceptor and die) is shaped in cross-section to conform generally to that predetermined cross-sectional configuration. A third function, produced primarily by deflector  82 , is to deflect the silicon particles down onto feed director  66 , whereby the particles can roll or fall down along conical surface  68  into the crucible to replenish the melt therein.  
         [0034]    A second outer afterheater  90 , like the one shown at  28  in FIG. 1 of said U.S. Pat. No. 5,037,622, surrounds the bottom end of the growing crystalline body  88 , and cooperates with the inner afterheater  72  to provide control of the temperature of that body in the region just above the growth interface. The outer afterheater is made of graphite and is supported by a plurality of graphite standoff pins  92  that are mounted in blind holes in the upper end of the crucible outer side wall  32 .  
         [0035]    The apparatus of FIG. 1 additionally comprises a seed holder schematically illustrated at  94  for holding a seed (not shown) onto which the crystalline body  88  is grown. The seed usually takes the form of a short section of a previously grown crystalline body so as to facilitate startup of the growth process. Seed holder  94  is attached to a pulling mechanism  96  which is adapted to move seed holder  94  axially toward and away from capillary die/crucible unit  20 . Seed holder  94  has a plurality of vent holes (not shown) to exhaust the gas from the plenum.  
         [0036]    Growth of hollow bodies of silicon, e.g., octagons, using the apparatus of FIG. 1 is straightforward. With growth initiated and maintained in the manner described in said U.S. Pat. No. 5,037,622, silicon particles are injected from the melt replenishment system  56  through feed pipe  54  by a jet of inert gas. Necessarily the particles are injected at a substantial velocity in order to make certain that they enter the space between members  66  and  72 . Typically the particles are injected by a gas stream having a velocity in the order of 1 meter/second. The particles discharged from pipe  54  are contained by graphite member  72  functioning as a plenum. The discharged silicon particles impinge upon deflector  82  which deflects them back down toward the conical upper surface  68  of umbrella-shaped feed director member  66 . Most of the particles fall directly onto surface  68 , and then slide down that surface and fall into the melt  38 . Other particles may ricochet off of surface  68  into actual or near contact with top wall  76  of member  72  before falling into the melt. As a consequence, the particles falling into the crucible cause splashing of the melt  38 , and some of the splashed liquid silicon may contact and solidify on the upper end of the crucible&#39;s center hub  36 . Additionally some of the ricocheting particles may come into direct contact with the center hub and become attached thereto. As more silicon particles are fed into the crucible via feed tube  54 , the solidified silicon on the upper end of hub  36  will tend to grow outward and form a mushroom-like solid piece as described above. The mushroom may even grow outwardly far enough to contact and become attached to the peripheral portion of umbrella-shaped member  66 , in which event it cause severe non-uniformity in the circumferential temperature distribution at the growth interface and also may grow large enough to prevent any significant amount of silicon from falling into the crucible, resulting in premature termination of growth due to depletion of the supply of melt  38  in the crucible. It has been observed also that occasionally chunks of silicon may break from the mushroom and fall into the melt, causing the crucible to overfill and flood, a condition that usually results in termination of growth. Even without such premature termination of growth, the non-uniform temperature distribution at the growth interface leads to an undesirable large variation in the wall thickness, which in turn reduces the total yield of acceptable silicon wafers cut from the grown hollow body.  
         [0037]    [0037]FIG. 2 illustrates a furnace which comprises an improved form of particle distributor/inner afterheater structure embodying the present invention which is preferred for growing cylindrical hollow thin-wall silicon bodies having a diameter of approximately  20  inches. The apparatus shown in FIG. 2 comprises a quartz furnace enclosure  100  which is closed off at its bottom end by a base  102 . A hollow pedestal  104  supported by base  102  carries a support plate  106 . A graphite susceptor  108  is supported by the pedestal  104  by means of several graphite stand-off pins  110 . The susceptor  108  is spaced from the support plate  106  and the intervening space is filled by multiple layers of graphite felt  112  which serves as a heat insulator (for convenience the several layers of graphite felt are represented as a single component in FIG. 2). Supported on the susceptor  108  is a crucible/die assembly  20 A which, like the crucible unit  20  disclosed in FIG. 1, is made of graphite and in accordance with the teachings of U.S. Pat. No. 5,037,622, except that it is configured to grow a cylindrical rather than a polygonal body. The teachings of that patent are incorporated herein by reference thereto. The susceptor  108  has an upstanding flange or hub  114  that surrounds the inner surface of the crucible/die unit  20 A. Additional graphite insulation  122  also surrounds the susceptor  108  and the crucible die assembly  20 A.  
         [0038]    The furnace base plate  102  has a center opening through which extends a graphite feed pipe  124 . The feed pipe extends up through a lower support tube  126  made of graphite which protrudes through a hole in support plate  106 . The upper end of support tube  126  carries a graphite radiation shield  130  which overlaps the inner portion of crucible/die unit  20 A.  
         [0039]    Mounted inside the support tube  126  and surrounding the feed tube  124  is an upper support tube  134 . The upper end of lower support tube  126  has an enlarged inner diameter to accommodate the upper support tube  134 . As seen in FIG. 1, the tube  134  rests on a shoulder  136  formed on the inner surface of tube  126 . The upper end of tube  134  extends into and supports a conical particle feed director  138  (FIGS. 2 and 3A) made of graphite. The center of the conical member  138  has an axial bore  140  which is occupied by the upper end of feed pipe  124 . The underside of conical member  138  is provided with a flange section  142  and is closed off by graphite plates  144 ,  145  which act collectively as a heat shield. The upper surface  139  of conical member  138  preferably extends at an angle in the range of 25 to 40 degrees from the horizontal, which is sufficient to assure that particles falling thereon will roll down to the bottom edge of the conical member. However, larger or smaller angles may also be employed. The bottom edge of the conical member  138 , as seen best in FIG. 4, terminates in a cylindrical outer edge surface  146 .  
         [0040]    Overlying the conical member  138  is a conically-shaped graphite particle deflector member  148  having a cylindrical tubular hub  150  at its apex. As seen best in FIG. 3B, the underside or bottom surface  149  of the upper conical member  148  is flat and extends at the same angle as the upper surface  139  of the lower conical member  138 . The upper surface  151  of conical member  148  is formed with a series of concentric circumferentially-extending grooves  152 . The grooves  152  are L-shaped in cross-section, as seen best in FIG. 3B, and are sized to receive baffles in the form of flat annular graphite plates  154 . In this case there are six plates  154 A- 154 F with identical size outer diameters. The inner diameters of the plates are progressively larger from plate  154 A to  154 F, with the inner edges  155  of the plates nesting in grooves  152  of the upper conical member  148 .  
         [0041]    Still referring to FIGS. 2 and 3B, conical member  148  has a peripheral rib  158  which acts as a shoulder to support a cylindrical inner afterheater member  160  made of graphite. The latter surrounds and is close to the outer edges of the plates  154 A- 154 F, and its upper end edge is seated in a groove  162  in a top plate  164 . The latter has a center hole  166  that is sized to accommodate a threaded graphite plug  168 . The latter is screwed into the hub  150  of conical member  148 , the latter being internally threaded as shown at  170  in FIG. 3B. The engagement of plug  168  with hub  150  serves to keep cylinder  160  tied to conical member  148  so as to form a unitary structure.  
         [0042]    Referring again to FIGS. 2 and 3A, the particle deflector member  148  is supported by a plurality of graphite standoff pins  174  which extend through holes  176  in conical member  138  and other aligned holes in plates  144 ,  145  and shield  130  and are received in blind holes in hub  114  of susceptor  108 . The standoff pins  174  are spaced circumferentially around the hub portion  114  of susceptor  108  and have a length such as to provide a narrow gap between the under surface  149  of the upper conical member  148  and the parallel upper surface  139  of the lower conical member  138 . Preferably, but not necessarily, that gap is in the range of 0.12 to 0.20 inch.  
         [0043]    Referring now to FIGS. 3B and 4, the upper conical member  148  is formed with a reverse or inturned lip  180  at the bottom end. Preferably the lip  180  has an inclined (conical) upper surface  181  joined to a cylindrical inner edge surface  182 , whereby the gap between the surfaces  139  and  149  is increased at its bottom end in the region of inclined surface  181  and then is decreased in the region of the inner edge surface  182  of the upper cone member  148 .  
         [0044]    The illustrated apparatus also includes a graphite outer afterheater  190  (FIGS. 2 and 3A) which surrounds and is spaced from the inner afterheater cylinder  160 . The afterheater  190  is supported by a plurality of graphite standoff pins  192  which extend through holes in the crucible die assembly and are supported by the susceptor  108 . The outer afterheater  190  and the afterheater member  160  define an annular axially-extending channel through which a crystalline tubular body  200  may be grown and pulled away from the crucible die unit  20 A. Preferably, the outer afterheater is surrounded by an insulating medium  210  in the form of graphite felt. The illustrated apparatus also includes a graphite seed holder  204  which is attached to a pulling mechanism  206 . The seed holder  204  holds a seed on which the crystalline body  200  is grown. According to customary practice, the seed is typically a section of a previously grown tubular body of like cross-sectional configuration. The seed holder is provided with vent holes  208  to vent gas from inside the growing crystalline body and thereby avoid pressure buildup that may adversely affect the growth process.  
         [0045]    The feed tube  124  extends through a cap  210  affixed to the base  102  and is coupled to a melt replenishment system  212  which is adapted to inject silicon particles on command so as to maintain the level of the melt in the crucible within predetermined limits. Suitable melt replenishment systems are described in U.S. Pat. No. 4,968,380 issued to G. M. Freedman et al.; U.S. Pat. No. 5,085,728 issued to B. H. Mackintosh et al.; and U.S. Pat. No. 5,098,229 issued to F. U. Meier et al.  
         [0046]    In the foregoing structure, the plates  154 A-F and cover plate  164  act to promote an approximately constant temperature gradient along the length of inner afterheater  160 , parallel to the pulling axis. Also, the two conical members  138  and  148  function as a distributor for silicon particles which are introduced from the melt replenishment system via the feed tube  124 . Conical member  148  serves as a particle deflector and conical member  138  acts as a particle director, with the two of them cooperating to channel the flow of silicon particles into the crucible. The particles are injected via feed tube  124  at a substantial velocity. The particles impinge on the plug  168  and are deflected downwardly into the conical gap between the surfaces  139  and  149 . The particles fall down along the gap and impinge on the inclined surface  181  of rib  180 . The particles tend to ricochet off of inclined surface  181 , with some striking the outer edge surface  146  of the inner cone  138 , and others tending to fall immediately through the gap between surface  146  and surface  182 . Essentially, the inclined surface  181  interrupts the fall of the particles, so that they thereafter fall into the crucible at a reduced velocity. Referring again to FIG. 2, the conical members  138  and  148  are sized so that the discharge orifice formed by the surfaces of lip  180  and the adjacent surface  146  of the bottom cone  138  is substantially centered between the inner and outer diameters of the portion of the crucible containing the melt. This assures that the particles falling into the melt cannot impinge on the hub portion of the crucible. Additionally, the reduction in velocity of the particles produced by engagement with the lip  180  assures that the particles will cause little or no splashing of the melt. As a consequence of the foregoing construction, the mushroom problem is eliminated or substantially reduced.  
         [0047]    It should be observed that the velocity at which the particles drop into the melt is a function of the angle of the surfaces  139  and  149 , and that having those surfaces extend at a shallower angle will help reduce the velocity at which the particles drop into the crucible. Accordingly, it is contemplated that the lip  180  may be omitted, as in the alternative embodiment described below.  
         [0048]    [0048]FIGS. 5A and 5B illustrate a modification of the invention which is preferred for growing hollow bodies of polygonal cross-sectional shape, e.g., “octagons”. FIG. 5A shows the lower portion and FIG. 5B shows the upper portion of a common structure. In this connection it should be noted that FIGS. 5A and 5B constitute a fragmentary illustration, with the furnace enclosure, outer afterheater and outer portion of the crucible and susceptor being omitted. However, it is to be understood that the embodiment of FIG. 5 is used in conjunction with a furnace as shown in FIG. 2.  
         [0049]    The apparatus shown in FIGS. 5A and 5B comprises susceptor  108  supporting the crucible/die assembly  20 A. The capillary die portion of crucible/die unit  20 A, and preferably also the outer wall of the crucible portion of the same unit, and also the outer afterheater (not shown) are shaped so as to conform in plan view to the polygonal cross-sectional configuration of the hollow body which is to be grown. In this case the center hub  36  of the crucible has an interior lip or flange  230  which overlies susceptor hub  114  and is spaced therefrom by a graphite spacer member  232  which is mounted on and surrounds a lower support tube  126 A. The latter surrounds and supports an upper support tube  134 A which serves as a guide for feed tube  124 . Seated on crucible hub  36  is a graphite standoff ring  236 . Seated on the standoff ring is a tapered particle director member  240  made of graphite that acts as an umbrella and is the functional equivalent of member  138 . The upper surface  242  of member  240  is essentially flat and is inclined at a selected angle to the horizontal so as to provide a conical profile in cross-section. In the instant case, surface  242  is shown as extending at a shallower angle than the surface  139  of the corresponding conical member  138  of FIG. 2. At its periphery, surface  242  is joined to an outer edge surface  244 . The latter is cylindrical and upper surface  242  is conical. Director member  240  has a center hole which is counterbored to receive the upper end of upper support tube  134 A. The upper end of feed tube  124  extends through tube  134 A to the apex of member  242 .  
         [0050]    The apparatus of FIGS. 5A and 5B also includes a tapered particle deflector member  250  which is the functional equivalent of member  148  of FIG. 2. Particle deflector member  250  has a central threaded opening  252  with a top flange  253 . A plug  254  is set into opening  252 . Member  250  also has a depending side edge wall  258 . The latter has a round configuration in plan view and its inner surface  260  extends parallel to the axis of feed tube  124 . The inner (bottom side) conical surface  262  of the upper conical member  250  is flat and extends at substantially the same angle as the upper surface  242  of the lower conical member  240 . The side wall  258  has a peripheral flange  268  at its bottom end which serves as a support for an annular plate  270 . The plate  270  supports a graphite inner afterheater  272  that has the same polygonal cross-sectional shape as side wall  258 . Although not shown, it is preferred that inner afterheater  272  consist of a plurality of flat graphite plates, one for each side of the polygonal body to be grown, with the plates sitting in grooves in plate  270  as shown.  
         [0051]    Deflector member  250  is formed with a continuous circular boss  274  that has threaded holes for receiving threaded tie rods  276  that are used to connect a plurality of flat graphite plates  278  that serve to assure a thermal gradient parallel to the axis of afterheater  272 . Spacer tubes  282  on the tie rods serve to keep a selected spacing between plates  278 . The top plate  284  is grooved to interlock with the upper ends of the plates that make up outer afterheater  272 . Nuts  286  on the upper end of tie rods  276  act to press a support plate  285  against top plate  284 , thereby causing plate  278 , afterheater  272  and plate  270  to be locked to member  250 , so as to form a unitary structure. That structure is supported by a plurality of standoff pins  288  which are received in blind holes in plate  270  and the crucible/die unit  20 A. The pins  288  have a length such as to maintain plate  270  in close proximity to the crucible/die unit as shown, with the spacing between surfaces  242  and  262  being large enough to assure free flow of particles down along surface  242  but small enough to control the trajectory of the particles as they are deflected by plug  254 .  
         [0052]    In this arrangement, particles introduced via the feed tube  124  impinge upon the deflector plug  254 . The particles are then deflected by the plug  254  so as to fall into the gap  290  formed between the two members  240  and  250 . The particles travel downwardly along the gap  290  into the annular space between the surfaces  244  and  260 . As the particles travel down along the gap  290 , they tend to strike the surface  260  at different points spaced vertically along the height of the surface  260 . Some of the particles ricochet off of the surface  260  against the surface  244 , while others fall from the surface  260  directly into the melt. The discharge orifice formed between surfaces  244  and  260  is located so as to discharge particles at a point which is intermediate the die portion of the crucible and the crucible hub  36 . As a consequence, the silicon particles dropping into the melt do not come into contact with the hub  36 . Also, because their velocity is impeded as a result of impinging on the surfaces  260  and  244 , the particles tend to drop into the melt without creating any substantial disturbance or splashing, thereby avoiding accretions of liquid silicon on the higher portions of the hub  36  which tend to be cooler.  
         [0053]    Referring again to FIG. 2, another aspect of the invention relates to the induction heating means associated with the crystal growth apparatus. FIG. 2 schematically illustrates two heating coils  296  and  298  surrounding the furnace enclosure, with coil  296  having two turns and coil  298  having a single turn. However, the number of turns characterizing each coil may be varied. Mechanical means (not shown) support coils  296  and  298  in concentric relation with the furnace enclosure. Preferably the coil  296  is disposed at the level of the susceptor  108  so as to impart most of its energy to that component, which in turn provides heat to the crucible. The coil  298  is preferably disposed so that it surrounds the bottom end the outer afterheater  190 , whereby to impart heat to both afterheaters. Disposed above coil  298  in surrounding relation to the furnace enclosure is a Faraday ring  300 . Mechanical means (not shown) support the Faraday ring so that it can be moved vertically toward or away from coil  298 . Although not shown, the coils  296  and  298  are connected in series with a suitable electrical power supply, whereby they may be energized to inductively heat the elements that they surround. The Faraday ring, also known as a “shorted turn” or “shorted ring”, interacts with the magnetic field of the secondary heating coil  298 , resulting in that ring providing a magnetic field that distorts and opposes the magnetic field created by energizing coil  298 . Moving ring  300  closer to coil  298  increases that opposing magnetic field. Changing the magnetic field of coil  298  affects the ratio of the current flowing in that coil to the current flowing in coil  296 . The net effect resulting from the opposing magnetic field is that it reduces the effective heat output of coil  298 , thereby modifying the ratio of its heat input to that of coil  296 . In practice, the Faraday ring position is adjusted until the operator is satisfied that the thermal input to the crucible and the outer afterheater is such as to optimize the growth process.  
         [0054]    The apparatus improvements described above offer the advantage of eliminating or substantially reducing the “mushroom” problem and also providing an impressive method of controlling the heating effect of two adjacent heating coils. A further advantage is that installing the improved particle distributor/inner afterheater structure in crystal growth apparatus as shown in FIG. 1 does not require any substantial changes to that apparatus or to the method of growing hollow bodies. In this connection it should be noted that the present invention retains all of the advantages of the invention disclosed and claimed in said U.S. Pat. No. 5,037,622 while providing better particle delivery to the crucible.  
         [0055]    Certain changes and modifications may be made in the above device without departing from the scope of the invention herein involved. Thus, for example, the top end surface of the capillary crucible/die unit may be designed to produce hollow bodies with circular, elliptical, triangular, rectangular, or other cross-sectional configurations. Also, the relative dimensions of different portions of the structure shown in the drawings may be varied. For example, the angles of the confronting surfaces of the deflector and director members may be varied to adjust the velocity at which particles are discharged into the melt and in accordance with changes in the dimension of the crucible/die unit as required to grow different size hollow bodies.  
         [0056]    Other changes in the design of the crucible/die assembly also may be made without departing from the principles of the present invention. Thus, for example, the crucible and EFG die may be formed as two separate and distinct members that are assembled to one another so as to form the functional equivalent of the integral crucible/die unit  20 A shown in the drawings. Also the particle distributor/inner afterheater structure of FIGS. 5A and 5B may be modified for use in growing large cylindrical bodies, and the corresponding structure of FIGS.  2 - 4  may be modified for use in growing octagons or other bodies with a polygonal or other cross-sectional configuration. In this connection, it is to be appreciated that the surface  244  of director member  240  and depending wall  258  of deflector member  250  could be shaped so as to be polygonal in plan view, e.g., octagonal, in the case of growing hollow bodies of octagonal cross-section. Although the invention is directed to an improved apparatus and method for growing tubular bodies of silicon, persons skilled in the art will appreciate that the invention may be used to grow shaped bodies of other crystalline materials, as described in other issued patents pertaining to EFG methods and apparatus. In growing bodies of a material other than silicon, the several components of the EFG growth zone, e.g., the crucible/die unit, afterheaters, radiation shields, etc. may need to be made of a material other than graphite in order to obtain bodies having suitable composition, purity and strength.  
         [0057]    Still other possible modifications will be obvious to persons skilled in the art. Therefore, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense.