Abstract:
A Czochralski (“CZ”) single-crystal growth process system continuously grows crystal boules in a chamber furnace during a single thermal cycle. Finished boules are transferred from the furnace chamber, without need to cool the furnace, to an adjoining cooling chamber for controlled cooling. Controlled cooling is preferably accomplished by transporting boules along a path having an incrementally decreasing temperature. In order to maximize crystal boule yield in a single furnace thermal cycle, the crucible assembly may be recharged with crystal growth aggregate and/or slag may be discharged during the crystal boule growth process without opening the furnace.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to a method and apparatus to grow crystals for electronics and photonics applications. More particularly the present invention relates to a method and apparatus employing the Czochralski (“CZ”) single-crystal growth process, wherein multiple crystal boules may be grown and cooled during a single heating cycle of the crystal growth furnace. 
     2. Description of the Prior Art 
     In the known Czochralski (“CZ”) single-crystal growth process a sealed furnace structure having a noble metal crucible containing a crystal forming granular aggregate is heated in an inert atmosphere, forming a melt. A crystal growth seed rod is placed in contact with the melt and withdrawn at a speed that promotes growth of a single crystal boule. After the crystal boule is grown the furnace is cooled slowly in order to minimize risk of boule cracking or creation of other cooling induced imperfections. The cooled furnace chamber is opened, so that the boule may be removed for further processing. After cooling and boule removal, the furnace chamber and growth components are readied for another crystal growth cycle. Due to cooling constraints, considerable time (often many days) is required to complete a complete crystal growth cycle and ready the growth furnace for the next growth cycle. 
     In order to reduce cycle time between single crystal boule growth cycles, in the past it has been suggested to grow multiple crystal boules in a single crystal production cycle. One suggested solution has been to grow single boules serially in a single crucible and transferring grown crystal boules to a holding area in the furnace. Another suggested serial processing solution was to create a two-part furnace having the crucible in the first part and a removable growth rod/boule extraction tower removable from the first part. Upon completion of a boule growth, the tower section would be removed (potentially wasting heat as the furnace is opened and discharging inert gas normally occupying the furnace chamber when practicing the CZ process) and replenished with a new tower section. In either of these serial processing solutions, when the serial growth run was completed the furnace was cooled and multiple completed boules extracted. The furnace would then be prepared for another growth cycle. 
     Another suggested batch processing solution has been to grow simultaneously multiple crystals in parallel with multiple crucibles in a single furnace. Again, upon completion of the parallel growth cycle the furnace would be cooled, the multiple boules extracted and the furnace serviced for commencement of another production cycle. 
     In preparation for a subsequent growth cycle the furnace and components are serviced and repaired, as is often necessary, due to the high-temperature thermal stresses on the components. Individual thermal stress events which are exacerbated by thermal cycling from initial cold state to heated state and back to cold state. Crucible assembly repair and servicing is critical because it is subject to very high thermal stress, and thus is prone to warping and cracking. Additionally after a crystal growth cycle a cooled crucible contains re-solidified residual melt and slag that is difficult and time consuming to remove without damaging the crucible. When the noble metal crucible can no longer be repaired due to cracks and warpage, it must be scrapped and recycled due to the value of its material. 
     The same equipment servicing challenges exist for a serial or parallel multiple boule processing production cycle as does for a single boule processing cycle furnace. Additional new challenges for multiple boule processing cycles include: re-charging spent melt, if multiple boules are to be extracted from a single crucible; and waste slag removal from the crucible as more melt is added to a growth crucible. As slag builds in a growth crucible, less crucible volume is available for new melt. 
     A past solution for providing recharge melt for crystal formation crucibles has been to melt solid aggregate in a first melting crucible and then feed the melt to a downstream crystal formation crucible. Two common structural geometries for the dual melting/crystal formation crucibles have included coaxially nested crucibles or inclusion of siphon/gravity feed tubes from the melt crucible to the crystal formation crucible. 
     While past continuous crystal growth systems addressed crystal melt replenishment, they did not propose solutions for crucible slag accumulation resulting from melt replenishment. As is known by those skilled in the art, variations in slag concentration can negatively impact uniformity of dopant distribution within a melt at the crystal-melt interface. Deviations in dopant distribution in a single boule (e.g. variations at the top of the boule vs. the bottom of the boule) or in a series of boules will negatively impact uniformity of boule optical and scintillation properties. 
     There are needs in the crystal growth field to: (i) reduce boule fabrication cycle time; (ii) reduce heating energy costs associated with operation of a CZ crystal formation furnace; (iii) reduce service and maintenance costs associated with operation of CZ crystal formation furnaces and (iv) achieve boule uniformity of optical and scintillation properties within a boule or series of fabricated boules. 
     SUMMARY 
     Accordingly, an object of the present invention is to create and operate a CZ process crystal growth furnace that operates continuously to fabricate a plurality of crystal boules in a single thermal cycle without shutting down and cooling the furnace in order to extract completed boules, so that time is not wasted waiting for the furnace to cool to a desired temperature. 
     It is another object of the present invention to create and operate a CZ process crystal growth furnace that does not waste energy by engaging in repetitive furnace heating/cooling cycles during boule formation and removal. 
     It is yet another object of the present invention to create and operate a CZ process crystal growth furnace that reduces service costs associated with repetitive furnace heating/cooling cycles: e.g. that unduly warp or crack crucibles or require removal of hardened slag and melt remnants from crucibles between thermal cycles. 
     Lastly, it is an object of the present invention to create and operate a CZ process crystal growth furnace continuously for multiple boule fabrication that recharges depleted crystal growth melt and removes slag during a single furnace operational thermal cycle. 
     These and other objects are achieved in accordance with the present invention by the CZ furnace apparatus and methods of operation of embodiments of the present invention. 
     An embodiment of the present invention features a Czochralski (“CZ”) single-crystal growth process system continuously grows crystal boules in a chamber furnace during a single thermal cycle. Finished boules are transferred from the furnace chamber, without need to cool the furnace, to an adjoining cooling chamber for controlled cooling. Controlled cooling is preferably accomplished by transporting boules along a path having an incrementally decreasing temperature. In order to maximize crystal boule yield in a single furnace thermal cycle, the crucible assembly may be recharged with crystal growth aggregate and/or slag may be discharged during the crystal boule growth process without opening the furnace. 
     More particularly, an embodiment of the present invention features a crystal fabrication system for continuously growing multiple crystal boules utilizing the Czochralski (“CZ”) crystal growth process. The system has a furnace chamber capable of growing a plurality of crystal boules in a single thermal cycle of the furnace chamber, and includes a crucible assembly retaining a heated crystal melt. A crystal growth rod assembly is in communication with the crystal melt, capable of forming and drawing a crystal boule from the melt. A cooling chamber is directly coupled to the furnace chamber, capable of receiving and storing a plurality of crystal boules from the furnace chamber during a single thermal cycle of the furnace chamber. A boule transfer mechanism transfers boules from the furnace chamber to the cooling chamber without the need to cool the furnace chamber prior to transfer. 
     An embodiment of the present invention is also directed to a method for continuously growing multiple crystal boules utilizing the Czochralski (“CZ”) crystal growth process by growing a plurality of crystal boules in a furnace chamber during a single thermal cycle. The chamber includes a crucible assembly retaining a heated crystal melt and a crystal growth rod assembly in communication with the crystal melt, capable of forming and drawing a crystal boule from the melt. The method is further practiced by providing a cooling chamber directly coupled to the furnace chamber, capable of receiving and storing a plurality of crystal boules from the furnace chamber during a single thermal cycle of the furnace chamber; and transferring grown boules from the furnace chamber to the cooling chamber with a transfer mechanism without the need to cool the furnace chamber prior to transfer. 
     Additionally the cooling chamber may have a temperature regulation system. The temperature regulation system may decrease cooling chamber temperature from proximal to distal the furnace chamber. In order to enhance continuous crystal growth the system may further include a crystal aggregate supply coupled to the crucible assembly, enabling the crucible assembly to replenish heated crystal melt. The crucible assembly may have an aggregate melt crucible for receiving aggregate supply and pre-melting same, that is in fluid communication with a crystal growth crucible. A slag discharge mechanism may be operatively coupled to the crucible assembly. The crucible assembly may have trunions coupled to the furnace chamber for tilting a portion thereof, so that slag may be discharged to the slag discharge mechanism. 
     The objects and features of embodiments of the present invention can be utilized by one skilled in the art of fabrication and operation of CZ crystal growth furnaces jointly and severally in any desired combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of exemplary embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view of a crystal fabrication furnace of an embodiment of the present invention, having a furnace chamber and a crystal cooling chamber; 
         FIG. 2  is a perspective view similar to that of  FIG. 1  showing crystal boule transport from the furnace chamber to the cooling chamber; 
         FIG. 3  is an schematic elevational view of a first crucible embodiment of the present invention; 
         FIG. 4  is a schematic perspective view of a crucible slag dumping operation of an embodiment of the present invention; 
         FIG. 5  is an schematic elevational view of a second crucible embodiment of the present invention; and 
         FIG. 6  is a schematic perspective view of an alternative embodiment of a crystal cooling chamber of the present invention, having a generally helical crystal boule transport path. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     After considering the following description, those skilled in the art will clearly realize that the exemplary teachings of embodiments of the invention can be readily utilized in continuous crystal growth fabrication furnace. 
     Continuous Crystal Fabrication System Overview 
       FIG. 1  shows generally a perspective view of the crystal fabrication system  10  in accordance with an embodiment of the present invention. Furnace chamber  20  grows crystal boules  36  from melted aggregate using the well known Czochralski (“CZ”) crystal growth process. The fabrication system  10  in accordance with an embodiment of the present invention is intended to grow crystal boules  36  continuously in a single thermal cycle, rather than grow only a single boule in a thermal cycle. To this end, the system  10  is capable of recharging crystal growing aggregate via an aggregate supply  22 , having a supply valve  23 , and extract melt slag via a slag collector  24  chute on an ongoing basis. The furnace  20  has a furnace heater  26  that may be a radio frequency (RF) heater. Furnace door  28  provides an exit portal for completed crystal boules  36 , while maintaining furnace heat and retaining inert gas normally resident in the furnace  20  when practicing the CZ crystal formation process. 
     In the crystal growth apparatus, seed growth rod  30 , seed rod driver  32  and seed growth tip  34  are of known construction, and facilitate growth and extraction of a crystal boule  36  from a crystal formation melt  35 . The crucible assembly  40 , to be described in greater detail, is mounted on a swiveling trunion  42 , that enables slag pour-off to the slag collector  24 , as shown in  FIG. 4 . A plurality of crucibles, each having its own crystal growth apparatus, may be included in a single furnace  20 , in order to facilitate parallel boule  36  generation during a single furnace thermal cycle. 
     Crystal cooling chamber  50  is coupled to the furnace chamber  20 , and is capable of receiving cooling multiple crystal boules  36  in a furnace single thermal cycle through the furnace door  28 . A double door furnace airlock (not shown) may be incorporated in the system to minimize loss of inert gas from the furnace chamber  20  during boule transfer to the cooling chamber  50 . Coupling of chamber  50  directly to the furnace  20  minimizes heat loss from the furnace during boule  36  transfer and also inhibits loss of inert gas normally occupying the furnace chamber during boule growth in the CZ process. Cooling chamber  50  may include, but is not required to include, a cooling chamber heater  51  that can be configured to provide a continuous decreasing temperature gradient ΔT along the length of the cooling chamber. Alternatively, the cooling chamber may be configured to receive serially a plurality of boules  36  from the furnace chamber  20 , maintain all received boules at a designated temperature with the cooling chamber heater  51 , and then cool all boules simultaneously by reducing the heater temperature. As another alternative, boules  36  may be received serially in the cooling chamber  50  and allowed to cool to the chamber&#39;s ambient temperature without the assistance of a cooling chamber heater  51 . 
     In the cooling chamber  50  embodiments shown in  FIGS. 1 ,  2  and  6 , the cooling boules  36  preferably are transported by track  52 , driven by track drive  54 . Boule transport arm  56  removes completed boules  36  from the furnace chamber  20  via furnace door  28  and deposits them on track  52 . Boules  36  exit the cooling chamber  50  via cooling chamber exit door  58 . Controller  60  controls operation of the boule track driver  54  and the boule transport arm  56  by way of communications pathways  62 . The communication pathways  62  may be hard wired, a computer bus or a wireless communication system. An exemplary controller may be a programmable logic controller (“PLC”) executing software commands, a “soft” PLC that emulates PLC functions on a personal computer, or a personal computer. Alternatively, boules  36  may be removed from the furnace chamber  20  and stored in the cooling chamber  50  without a driven rack  52 . 
     The cooling chamber  50  is shown schematically in  FIGS. 1 ,  2  and  4  as having a linear planform, but any shape planform may be selected by those skilled in the art. The helical planform cooling chamber  50  shown in  FIG. 6  has additional advantages of smaller foot print for a given length of boule track  52  and efficient heat retention by minimizing sidewall surface area exposure. As previously referenced, if it is desired to minimize inert gas loss from the furnace chamber  20  during boule  36  transfer to the cooling chamber  50 , a double furnace door  28  and airlock (not shown) may be constructed, with possible need to relocate the boule transfer arm  56  to the airlock. Alternatively, the cooling chamber  50  may also be filled with the same inert gas as the furnace chamber  20 , so that no inert gas escapes to atmosphere during boule  36  transfer. If the cooling chamber  50  is also filled with inert gas a double cooling chamber exit door  58  with airlock may be constructed (not shown). 
     Continuous Crystal Formation Crucible 
     Two embodiments of crucible assemblies  40  are shown in  FIGS. 3 and 5 . Both embodiments provide for a crucible swiveling trunion  42 , for cleaning unwanted slag from a crucible assembly  40 , as shown in  FIG. 4 . An aggregate melt crucible  44  converts recycled and new melt aggregate into a molten mass necessary for fabrication of semiconductor devices. The melted aggregate is routed to a crystal growth crucible  46 . Two distinct embodiments of crucible assemblies  40  are shown respectively in  FIGS. 3 and 5 . The first embodiment of the crucible assembly  40  is shown in  FIG. 3 , wherein the aggregate melt crucible  44  retains the crystal growth crucible  46  in generally concentric fashion. Crystal growth crucible ports  47  enable the melt to flow into the growth crucible and refresh its contents as boules  36  are formed. In an alternative embodiment of  FIG. 5 , the respective aggregate melt crucible  44  and crystal growth crucible  46  are arrayed in tandem. An interconnecting siphon tube  48  enables melt to flow from the aggregate melt crucible  44  to the crystal growth crucible  46 . 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.