Chain introduction means for a crystal growth pulling head

A crystal puller uses a chain and sprocket in the pulling head for vertical support of the growing crystal. Replacement of the chain is effectuated by providing an aperture in the top of the pulling head aligned with the sprocket. Replacement chain fed through the aperture can be directed to a storage area initially then by reverse sprocket motion lowered to commence a crystal growth cycle.

DESCRIPTION 
Field of the Invention 
The invention is in the field of apparatus for drawing a single crystal 
from a melt and more particularly relates to a pulling head for a 
Czochralski crystal growth apparatus wherein said crystal is supported by 
a chain. 
BACKGROUND OF THE INVENTION 
Apparatus for the growth of single crystals by the Czochralski method 
includes a crucible to contain a melt in a mechanism for concurrently 
drawing a crystal from the melt on a vertical axis at a substantially 
steady rate while providing relative rotation about this axis for the 
growing crystal in respect to the melt. Vertical lift of the growing 
crystal is effectuated from the "pulling head" portion of the apparatus 
which must maintain mechanical alignment of the axis of rotation with the 
vertical pulling axis while providing ease of access for maintenance and 
rapid turn around 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 prior art to employ a ball chain for supporting and 
vertically lifting the crystal as it is drawn from the melt together with 
a take-up drum for storage of the untensioned chain. In a known apparatus 
the alignment of the pulling axis, gravitationally defined by the ball 
chain with the rotation axis is maintained by causing the take-up drum to 
travel along a lead screw, transverse to the crystal pulling rotation 
axis. The known drum translation apparatus employs 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 requirements for the drum of this prior art mechanism 
further enlarges the volume of the pulling head and effects the balance, 
the mechanism which controls the rotation about the vertical axis. The 
winding on the chain of 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 thereby introduced. 
Crystal pulling apparatus of the form above-described utilizes a "ball 
chain" taking the form of a linkage of spherical beads. In common with the 
prior art apparatus above-described, copending application Ser. No. 
119,913 describes a compact pulling head which effectuates long pull 
lengths utilizing the ball-chain and a sprocket adapted to conform to the 
shape of the chain bead by formation of pockets in the lateral surface of 
the sprocket. The desired support and rotation axis is congruent with a 
tangent from the sprocket and untensioned chain is accommodated in a 
storage space proximate the sprocket. In this apparatus stabiity and 
control of the growing crystal are substantially improved while 
maintaining uniform rotational and vertical motions. This arrangement has 
a number of advantageous properties but in common with the afore-mentioned 
prior art mechanism, the chain or cable must be replaced at moderately 
frequent intervals of use. In the prior art apparatus the chain is secured 
to the drum directly by mechanical fasteners. Consequently, the pulling 
head must be opened to secure the fixed end of the replacement chain to 
the drum. In the apparatus of the copending application it is also 
necessary to open the pulling head to install a new chain although no drum 
must be aligned, nor must the chain be secured by a fastener to a take-up 
mechanism. Thus, both of the above described apparatus require disturbing 
large vacuum seals and further result in downtime of the order of hours. 
BRIEF DESCRIPTION OF THE INVENTION 
An object of the present invention is the improvement of a pulling head for 
a crystal growing furnace of the Czochralski type. 
In one feature of the invention the growing crystal is suspended by ball 
chain, pulling is accomplished with a sprocket adapted to such chain and 
said chain is introduced to said apparatus through an aperture above and 
aligned with said sprocket. 
In another feature of the invention, the chain is introduced to the 
aperture through a guide tube. 
In yet another feature of the invention, the guide tube is adapted to 
receive a sealing cap to hermetically seal an envelope enclosing the 
crystal growing furnace. 
This object and features are realized in apparatus which separates the 
vertical actuation of the chain and the chain take-up through provision of 
a sprocket for the vertical pulling function and a storage space for the 
storage of untensioned (taken-up) chain. The sprocket engaging the chain 
is characterized by concave pockets forming the sprocket teeth, such 
pockets conforming to the spherical beads of the ball-chain. As a result 
of this configuration the chain, introduced from above is easily engaged 
by the sprocket and permits simple replacement of the entire chain length 
without disassembly of the pulling head. 
Chain replacement is accomplished by discharge of the free end of the chain 
from the sprocket into the storage space.

DETAILED DESCRIPTION OF THE INVENTION 
A crystal growing system which defines the context of the present invention 
is outlined briefly in FIG. 1. A furnace 1 heats a melt 3 of the feedstock 
of the desired crystal material contained in crucible 2 of refractory 
material (ordinarily quartz for its infra-red transmission properties). 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 a hermetic envelope to 
support a vacuum or inert atmosphere. Motors 8 and 9 provide relative 
rotational and pulling motions of the growing crystal with respect to the 
melt. 
The invention is best described with the aid of FIGS. 2 and 3 which show 
the salient components of the pulling head and mechanism. A housing 10, 
preferably formed of a transparent material, is closed on both ends by a 
top plate 12 and by a bottom plate 14 to form the pulling head enclosure. 
Seals 12' and 14' are shown here as flat gaskets merely for convenience. 
The bottom plate 14 communicates through guide plug 15 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 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 dumbbell-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 or alternatively in an unconfined 
portion of the envelope adjacent sprocket 24. 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 the sprocket 24 to support the weight of the 
growing crystal, and idler pulley 28 constrains the free end of ball-chain 
22 to sprocket 24. Switch actuator 29 senses an imminent end-of-chain 
condition. This assures the desired feeding of the free end of ball-chain 
22 to the seedholder. 
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 a rotational speed in 
the desired range from about 0.1 to 1.5 revolutions per hour. Further 
adjustment of pulling rate is obtained by choice of the diameter for 
sprocket 24. This has been selected to produce a circumference of about 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' 
through intermediate reduction gearing 44. Vertical shaft 42 is built in 
two portions with couplings 45 provided to permit simple removal of top 
plate 12 for axis to the mechanisms when major maintenance is required. 
Right angle drive 46, for example, a worm in worm-wheel set, provides 
further reduction gearing and transmits power to sprocket shaft 40. 
Whereas the traveling drum pulling head of the prior art employed 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 approximate the crystal 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 
the 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 occurence resulting in loss of a crystal grown at 
great expense, and likely damage to the crucible and hot zone components 
as well as considerable amounts of time to restore the condition of the 
apparatus. 
A conscientious program of preventive maintenance is essential to assure 
trouble-free operation of the apparatus. A central aspect of this program 
is the replacement of chain at the appropriate intervals. To facilitate 
this replacement the present invention employs aperture 50 in cover plate 
12. This aperture is aligned with the plane of sprocket 24. A flange 52 is 
adapted to join tubular member 54 to cover plate 12 in a vacuum sealing 
relationship. Vacuum seals are not shown in these conceptual drawings but 
such seals are well-known and need not be further specified. A fixture 56 
is brazed or otherwise hermetically-joined to the end of tubular member 54 
to provide a sealing closure in conjunction with a sealing cap, not shown, 
when the crystal puller is evacuated. 
Chain reloading is simplified in several aspects. First, it is unnecessary 
to disturb the vacuum seal 12' by opening the pulling head cover plate 12. 
Instead new chain is directed through the tubular means 54 after removal 
of the sealing plug. The new chain is guided through aperture 50 until it 
engages sprocket 24. The sprocket is actuated to rotate in its "lift" mode 
discharging chain into the storage area 26, or equivalent. When the chain 
end rests on the sprocket 24, the sprocket drive is reversed to its "down" 
mode and the chain is guided through internal guide plug 15 for attachment 
to the seed holder to initiate new crystal growth. 
Since many changes can be made in the described construction and many 
apparently widely different embodiments of this invention can be made 
without departing from the scope thereof, it is intended that all matter 
contained in the above description and shown in the accompanying drawings 
shall be interpreted as illustrative and not in a limiting sense.