Abstract:
A pulling head for a Czochralski type crystal growing furnance includes a ball linked chain for suspending the growing crystal ingot, a sprocket for engaging such chain to pull the ingot from a melt, motor means for driving the sprocket and a chain storage means to receive untensioned chain discharged from the sprocket.

Description:
This is a continuation of application Ser. No. 119,913, filed Feb. 8, 1980 (now abandoned). 
    
    
     FIELD OF THE INVENTION 
     The invention is in the field of apparatus for drawing single crystal ingots from a melt and more particularly relates to a pulling head for a Czochralski crystal growth apparatus. 
     The government has rights in this invention pursuant to Contract No. JPL 954884 awarded by the U.S. Department of Energy. 
     BACKGROUND OF THE INVENTION 
     Apparatus for the growth of single crystal ingots by the Czochralski method includes a crucible to contain a melt and a mechanism for concurrently drawing a crystal from the melt along a vertical axis at a steady rate while providing relative rotation about this axis for the growing crystal with respect to the melt. Vertical lift of the crystal ingot is effectuated from the &#34;pulling head&#34; portion of the apparatus which must maintain mechanical alignment of the axis of rotation with the vertical pulling axis while providing case of access for maintainence and rapid turnaround time in a production context. Moreover, in one class of apparatus it is desirable to carry out the process in a vacuum tight housing in order to obtain improved freedom from contamination. 
     It is known in the prior art to employ a ball chain and takeup drum for vertically lifting the crystal as it is drawn from the melt. In the known apparatus the alignment of the pulling axis, gravitationally defined by the ball chain, with the rotation axis is maintained by causing the takeup drum to travel along a lead screw, transverse to the crystal pulling rotation axis. 
     The known drum translation apparatus employes a keyed or splined mechanism, or linear ball bushing to constrain the drum translation to its rotational axis during rotation of the drum. A fixed nut and lead screw on the axis of the drum have such a thread pitch to translate the drum by an amount equal to the width of the chain. Thus, the chain is maintained tangent with the drum circumference and in constant alignment with the desired crystal rotation axis. The lubrication requirements for the lead screw and spline of the known chain pulling mechanism is clearly ill-suited to contain a contamination-free environment desired for crystal growth with the present invention. 
     The transverse travel requirement for the drum of the prior art mechanism further enlarges the volume of the pulling head and affects the balance of the mechanism which controls rotation about the vertical axis. The winding of the chain on the drum is a source of uneven lift as the chain increments relax against the drum surface while under tension. Such relaxation occurs in an uncontrolled fashion and small irregularities in vertical pulling are introduced thereby. 
     Crystal pulling apparatus of the form above described utilizes a chain taking the form of a linkage of spherical beads. This arrangement has a number of advantageous properties but the chain must be replaced at moderately frequent intervals of use. In the prior art one end of the chain is secured to the drum directly via mechanical fastener. Consequently the entire chain must be replaced as a routine preventative maintenance measure requiring that the pulling head chamber be opened for access. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An object of the present invention is the provision, in a crystal growing furnace of the Czochralski type, for a pulling head whih is compact while effectuating long pull lengths. 
     Another object of the present invention is the provision of a pulling head for a Czochralski type crystal growth apparatus which exhibits superior properties of stability and control for maintaining uniform rotational and vertical motions of the growing crystal ingot. 
     In one feature of the invention, the growing crystal is suspended by beaded chain and pulling is accomplished with a sprocket adapted to such chain, said sprocket discharging chain into a bin. 
     In another feature of the invention the lateral surfaces of the sprocket are adapted to conform to the shape of the chain beads by the formation of pockets in such lateral surface. 
     In yet another feature of the invention, excess chain is accommodated which permits routine discard of a portion of thermally exposed chain in preparation for new crystal growth. 
     These objects and features are realized by separating the vertical actuation of the chain and chain take-up through provision of a sprocket for the vertical pulling function and a simple container for the storage of untensioned chain. The sprocket engaging the chain is characterized by concave pockets forming the sprocket teeth, such pockets conforming to the spherical shaped beads of the ball chain. As a result of this configuration, the pulling function is executed more smoothly. 
     Chain takeup is accomplished by discharge of the free end of the chain from the sprocket into a container thereby eliminating the need for a traveling alignment mechanism synchronized with a rotating takeup means. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows schematically a crystal pulling system incorporating the present invention. 
     FIG. 2 is a partially schematic view of the pulling head of the present invention. 
     FIG. 3 is another view of a part of the mechanism of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A crystal growing system incorporating the present invention is outlined briefly in FIG. 1. A furnace 1 heats a crucible 2 of refractory material (ordinarily quartz) to maintain a melt 3 of the feedstock of the desired crystal material. A crystal 4 is pulled from the melt 3 by pulling head 5 through linkage 6. A housing 7 surrounds the entire apparatus to maintain freedom from contaminants and in some instances to provide hermetic envelope to support a vacuum or inert atmosphere within. Motors 8 and 9 provide relative rotational and pulling motions of the ingot with respect to the melt. 
     The invention is best described with the aid of FIG. 2 which shows the salient components of the pulling head. A housing 10, preferably formed of a transparent material, is closed on both ends by top plate 12 and bottom plate 14 to form the pulling head enclosure. The bottom plate 14 communicates with the vertical column 16 of the full crystal growing system. A growing crystal 18 drawn from a melt, not shown, is supported from a seed holder 20 supported by the beaded chain or ball chain 22. Ball chain 22 may be formed of a plurality of hollow spherical members, loosely linked to adjacent like members by rod or tubular dumbell shaped segments. The ball chain 22 engages sprocket 24 and the untensioned or free end of ball chain 22 is discharged from sprocket 24 to collect in container 26. Sprocket 24 is formed from a cylindrical section, the curved surface of which has pockets formed in such surface to accommodate the links of the ball chain 22. In order to assure enough wrap around sprocket 24 to support the weight of the growing crystal, an idler pulley 28 constrains the free end of ball chain 22 to sprocket 24. Switch actuator 29 senses an end-of-chain condition. This assures the desired feeding of the free end of ball chain 22 to feed the seed into the melt. It is apparent that a portion of the interior volume of the pulling head chamber may serve as a collecting volume in lieu of container 26. 
     The entire pulling head is adapted for rotation about vertical axis 30 through bearing assembly 32. Fixed motor 34 drives the entire pulling head through motor pulleys 35, belts 36 and head pulleys 37. 
     Vertical pulling power is provided by motor 38 through a transmission train schematically illustrated in FIG. 3. This transmission is conventional and provides a reduction appropriate to turn sprocket shaft 40 at rotational speeds in the range of 0.1 to 1.5 revolutions per hour. Further adjustment of pulling rate is obtained by choice diameter for sprocket 24. This has been selected to produce a circumference of 10 inches; the reduction ratios in cooperation with such sprocket diameter yields vertical pull rates in the range 1-15 inches per hour. Accordingly, rotational power is provided to shaft 42 on vertical transmission axis 42&#39; through intermediate reduction gearing 44. Vertical shaft 42 is built in two portions with coupling 45 provided to permit simple removal of top plate 12 for access to the mechanism. Right angle drive 46, for example, a worm and worm-wheel set, provides further reduction gearing and transmits power to the sprocket shaft 40. 
     Whereas traveling drum pulling heads of the prior art employ a fixed length chain, the present apparatus uses variable length chain. the excess chain stored in container 26 permits frequent removal of a portion of chain proximate the seed holder. Because the strength of the chain is degraded by continuous exposure to high temperatures in this region, the likelihood of chain failure is minimized by routine removal of a portion of chain so exposed before initiating new crystal growth. The excess chain permits this routine practice. Chain failure, it will be noted, is a disasterous occurrence resulting in loss of an ingot grown at great expense, likely damage to the crucible and hot zone components considerable amount of time to restore the condition of the apparatus. This excess chain feature in unavailable on known traveling drum pulling heads because the upper end of the chain in such apparatus is secured to the drum to sustain a correlation between the rotation of the drum pulling axis and ingot altitude on such axis. Another simple embodiment of the principle here described would utilize a drum or other spooling means in lieu of the sprocket with sufficient wraps of suspending medium to obtain a frictional stabilization of the load. The untensioned length of suspending medium, whether chain or cable, would be discharged into a portion of the apparatus envelope. 
     Since many changes could be made in the described construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.