Process and apparatus for growing a crystal ribbon

A process and apparatus is disclosed for growing a crystal ribbon (42) of a substance of theoretically infinite length from a melt (32) of the substance. A pair of fixedly positioned edge defining members (44) are partially submerged into the melt (32) so as to break the surface (34) of the melt (32) at a predetermined distance from one another. The edge defining members (44) are wettable by the melt and the predetermined distance substantially corresponds to the width of the crystal ribbon (42) to be grown. The crystal ribbon (42) is grown by contacting the surface (34) of the melt (32) with a seed ribbon (38) between the edge defining members (44) whereby a meniscus (48) of the melt (32) is established on the seed ribbon (38). The meniscus (48) is stabilized by the meniscus (50) of the melt (32) on the edge defining members (44). Pulling the seed crystal ribbon (38) away from the melt (32) results in continuous growth of the crystal ribbon (42).

2. Background of the Invention 
The present invention is directed to an apparatus and process for growing a 
crystal ribbon of indefinite length from the melt of a substance. More 
particularly, the present invention is directed to an apparatus and 
process for growing a crystalline ribbon of silicon from a mass of molten 
silicon for semiconductor device purposes. 
3. Brief Description of the Prior Art 
Several methods are known in the prior art for growing crystals of a 
substance from a melt of the substance. Many processes of the prior art 
are particularly adapted for growing ingots or ribbons of crystalline 
silicon from a silicon melt. However, relatively recent advances in 
semiconductor, and related technology, and particularly the recently 
increasing demand for photovoltaic cells capable of directly converting 
sunlight into electric energy, have greatly increased the qualitative and 
quantitative requirements for crystalline ribbons of silicon. 
As is well known in the art, crystalline silicon used in photovoltaic cells 
must particularly be formed in thin ribbons or "wafers," ideally having 
large surface areas. Furthermore, the semiconductor material used in 
photovoltaic cells ideally should be entirely monocrystalline, although 
this requirement can be relaxed by accepting poorer cell performance. 
Practically, the silicon ribbons or "wafers" used in photovoltaic cells 
should have as high a degree of crystal perfection as possible. 
In order to illuminate the background of the present invention, the prior 
art processes and apparatus for growing silicon ingots and ribbons for 
semiconductor and photovoltaic purposes are briefly described as follows. 
A melt of silicon is maintained in a suitable container, such as a quartz 
crucible, under a suitable inert gas atmosphere. A properly oriented seed 
crystal of silicon is dipped into the melt. The temperature of the melt is 
controlled and adjusted so that the silicon tends to crystallize on the 
seed, because of a thermal gradient in the direction of the cooler seed. 
Thereafter the seed crystal is pulled upward from the melt in a direction 
substantially normal to the melt surface, causing molten silicon to 
crystallize onto the seed crystal at the liquid solid interface. The 
resulting crystalline body continuously grows substantially until the 
process is deliberately interrupted, or the supply of molten silicon is 
exhausted. 
While the above-noted crystal growing process is in progress, the melt must 
be carefully maintained at the proper temperature so that molten silicon 
material solidifies at the solid liquid interface rather than melts the 
solid crystal. Furthermore, in industrial processes, the melt may be 
continuously or intermittently replenished by addition of silicon (usually 
in the form of solid granules) and the velocity of pulling the solid 
crystal away from the melt must be carefully controlled. In order to 
satisfy the above-noted requirements, the prior art provided relatively 
reliable, sophisticated apparatus and instrumentation which, by-and-large, 
function well. An apparatus adapted for accomplishing the pulling function 
is generally referred to in the art as Czochralski type apparatus. Certain 
embodiments of the above-noted apparatus and instrumentation for 
monitoring and maintaining the temperature of the melt, sensing the level 
of the melt and automatically replenishing the same, and the mechanism for 
controlled pulling of the solid crystal away from the melt, are presently 
available in the United States on a commercial basis. 
The above-summarized process works well for growing silicon ingots of 
substantially circular cross-section. However, the ingots must be sliced 
into thin sections, or "wafers" to be useful in semiconductor device 
technology. On the other hand, the growing of silicon crystals of another 
cross-sectional configuration such as, for example, silicon ribbons, 
presents additional problems. In order to grow a crystalline ribbon, it is 
necessary to provide foreign objects or members in the melt to define, or 
at least partly define a meniscus of liquid silicon of the desired 
configuration on a continuously shifting liquid-crystal interface. 
One of the specific techniques of the prior art of the above-noted nature 
for growing crystalline silicon ribbons, utilizes a die positioned in the 
molten silicon. The die characteristically protrudes to a slight extent 
above the level of liquid silicon in the crucible, and incorporates an 
opening which is narrow enough for the liquid silicon to rise to the top 
surface of the die by capillary action. The seed crystal ribbon is then 
brought into contact with and is subsequently pulled away from the top 
surface of the die. 
The techniques for growing crystalline silicon with the assistance of a die 
wettable by liquid silicon are generally termed in the art "capillary die" 
techniques. Examples of this technique and apparatus for performing the 
same may be found in U.S. Pat. Nos. 3,650,703; 4,075,055; 4,090,851; 
4,099,924; 4,116,641; 4,121,965, and United States Published Patent 
Application No. B 584,997. 
Another example of a technique which utilizes a die having an opening is 
found in U.S. Pat. No. 4,211,600. The die of this patent is, however, not 
wettable by liquid silicon and the liquid silicon is caused to rise in the 
opening of the die by application of pressure. 
Another category of specific techniques used in the art for growing silicon 
crystals is termed the "dendritic web" technique. It involves the use of 
silicon dendrites, i.e., two spaced-apart silicon crystals which are 
dipped into the melt, and are allowed to propagate downwardly into the 
melt while being slowly pulled in an upwardly direction. The growth 
between the dendrites provides a crystalline web or ribbon of silicon. 
Yet a third category of silicon crystal growing technique is termed "edge 
supported pulling technique." In accordance with this technique, two 
spaced-apart filaments of a suitable material, such as quartz or graphite, 
are dipped into a melt of silicon, a ribbon-shaped seed crystal is touched 
to the surface of the melt between the two filaments, and thereafter the 
seed crystal is pulled away from the melt together with the filaments. In 
this technique, the meniscus of liquid silicon at the interface with the 
end of the ribbon-like seed crystal is stabilized on the edges of the seed 
crystal by the respective filaments. A detailed description of the 
edge-supported pulling technique may be found in a paper prepared by T. F. 
Ciszek and J. L. Hurd of the Solar Energy Research Institute for the U.S. 
Department of Energy, titled "Melt Growth of Silicon Sheets by 
Edge-Supported Pulling." 
Other patents and publications of general relevance to silicon crystal 
growing techniques and to the subject matter of the present application 
for patent are U.S. Pat. No. 4,242,553, an article by John Lenzing, 
"Survey of Semiconductor Crystal-Growing Processes and Equipment," Solid 
State Technology, February, 1975, pages 34-43, and an article by C. P. 
Chartier and C. B. Sibley, "Czochralski Silicon Crystal Growth at Reduced 
Pressures," Solid State Technology, February, 1975, pages 31-33. 
The above-noted prior art methods suffer from certain disadvantages, 
particularly when the methods are considered for producing large 
quantities of silicon ribbons for photovoltaic applications. More 
specifically, the die utilizing or "capillary die" techniques are 
inherently limited in the dimensions of the crystals which may be grown by 
the technique, because the opening in the die must be narrow enough to 
permit rise of the melt in the opening by capillary action. Furthermore, 
it is difficult to maintain adequate thermal control in the "capillary 
die" processes, and the foreign matter of the die tends to promote 
polycrystalline growth. As a result, silicon ribbons prepared by the 
capillary die technique are polycrystalline and therefore provide 
photovoltaic material that cannot attain as good a performance as single 
crystal material. 
A disadvantage of the "dendritic web" technique for growing silicon ribbons 
is that thermal control of the process is somewhat difficult, and the 
crystal growth is difficult to initiate, although the resulting single 
crystal ribbon serves very well for relatively high efficiency 
photovoltaic applications. 
Thermal control of the "edge supported pulling technique" is relatively 
easy, and this technique also gives rise to polycrystalline material with 
large crystallites conducive to relatively high efficiency photovoltaic 
performance. However, the length of a ribbon grown by the edge supported 
pulling technique is necessarily limited, because the two edge supporting 
filaments of finite length are pulled out of the crucible together with 
the growing ribbon. Although this disadvantage of the edge supported 
pulling technique might theoretically be overcome by continuously 
supplying two filaments of indefinite length to be pulled from the silicon 
melt, no acceptable, practical apparatus of this type has been devised in 
the prior art. 
An additional disadvantage of both the "dendritic web" and of the "edge 
supported pulling" processes is that the crystal ribbon obtained in both 
processes is bounded on its elongate edges by the dendrites or by the 
filaments, respectively. The dendrites or filaments must be removed from 
the crystal ribbon before the ribbon is cut and incorporated in 
photovoltaic devices. 
Other processes of the prior art which give rise to silicon ingots rather 
than ribbons, suffer from the serious disadvantage that for photovoltaic 
and most other applications the ingots must be sliced into thin sections 
or "wafers" at relatively great cost and waste. 
In light of the foregoing it is readily apparent that a need exists in the 
prior art for a technique for directly and relatively inexpensively 
producing crystalline ribbons of silicon which are highly adapted for use 
in photovoltaic cells and other semiconductor devices. The present 
invention serves to satisfy this need. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus and 
process for continuously growing a crystalline ribbon of silicon or other 
material, from a melt of the material. 
It is another object of the present invention to provide an apparatus and 
process for continuously growing a crystalline ribbon of silicon, or other 
material, from a melt of the material wherein the edges of the ribbon are 
free of concomitantly grown dendrites or foreign substance edge defining 
members. 
It is still another object of the present invention to provide an apparatus 
and process for continuously growing a crystalline ribbon of silicon, or 
other material, from the melt of the material, wherein the control of the 
process is relatively easily maintained. 
It is yet another object of the present invention to provide an apparatus 
and process for continuously growing a crystalline ribbon of silicon from 
molten silicon, wherein the resulting ribbon is highly suitable for use in 
photovoltaic cells. 
These and other objects and advantages are attained by a technique used in 
association with substantially conventional crystal pulling equipment, 
wherein a pair of edge defining members are fixedly positioned and 
stationarily mounted relative to a container. The container holds a melt 
of silicon or other material from which a crystal ribbon is to be pulled. 
The edge defining members are partially submerged in the melt at two 
spaced-apart locations which substantially correspond to the width of the 
desired crystal ribbon. The edge defining members are wettable by the 
melt, and partially protrude above the melt surface. Consequently, when a 
seed crystal ribbon is contacted with the melt and with the edge defining 
members, the edge defining members help to stabilize a meniscus of molten 
silicon around the periphery of the seed ribbon. Proper adjustment of the 
temperature and temperature gradient will allow subsequent pulling of the 
seed ribbon in a direction substantially normal to the surface of the melt 
to stabilize the meniscus defined by the edge defining members and results 
in the growth of a crystalline ribbon of indefinite length and of high 
photovoltaic quality. 
The features of the present invention can be best understood, together with 
further objects and advantages, by reference to the following description 
taken in connection with the accompanying drawings, wherein like numerals 
indicate like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following specification taken in conjunction with the drawings sets 
forth the preferred embodiments of the present invention in such a manner 
that any person skilled in the crystal growing arts can use the invention. 
The embodiments of the invention disclosed herein are the best modes 
contemplated by the inventor for carrying out his invention, although it 
should be understood that various modifications can be accomplished within 
the parameters of the present invention. 
Referring now to FIGS. 3-9, the crystal growing apparatus and process of 
the present invention are disclosed. It should be noted at the outset, 
that the crystal growing apparatus and process of the present invention 
are primarily adapted for growing crystalline ribbons of silicon from a 
melt of silicon. However, the hereinafter disclosed generic principles, 
process, and apparatus may also be utilized for growing crystalline 
ribbons of other materials, such as tungsten or molybdenium. As is well 
known in the art, the latter two materials are hard to roll and therefore 
hard to obtain in a ribbon-like configuration. However, in the ensuing 
description the crystalline ribbon grown in accordance with the present 
invention is referred to as a silicon ribbon. This is for the sake of 
simplicity and clarity of expression, and because the present invention is 
primarily directed to growing crystalline silicon ribbons. Furthermore, as 
it was pointed out in the introductory section of the present application 
for patent, long, thin, ideally monocrystalline, or at least large grain 
polycrystalline ribbons of silicon are the essential components of 
photovoltaic solar cells, and as such have tremendous scientific, 
industrial and commercial importance. 
It should be further noted, that many components of the overall crystal 
pulling equipment, wherein the present invention is utilized, are 
constructed in accordance with the state-of-the-art. The overall equipment 
incorporating a second preferred embodiment of the apparatus of the 
present invention, is schematically shown on FIG. 9, and is described here 
only to the extent necessary to understand the present invention. For a 
more detailed description of the state-of-the-art components of a crystal 
pulling apparatus utilized in conjunction with the present invention, 
reference is made to the article "Survey of Semiconductor Crystal-Growing 
Processes and Equipment," by John Lenzing, Solid State Technology, 
February 1975, pages 34-43, which is expressly incorporated herein by 
reference. 
Thus, briefly and with principal reference to the schematic view of FIG. 9, 
the crystal pulling equipment (Czochralski type equipment) includes a 
crucible or container 20 which is mounted within an enveloping water 
cooled chamber 22. An appropriate inert gas, such as argon or helium, is 
continuously passed through the chamber 22 through a gas inlet 24 and 
outlet 26. Usually a resistance heater 28 is utilized to surround and heat 
the crucible or container 20, although the heater 28 is often a susceptor 
that may be heated by RF induction. A shield 30 of insulating material 
surrounds the heater 28 and inhibits loss of heat outwardly in the 
direction of the chamber 22. 
The crucible 20 is made of high purity quartz or graphite, because these 
materials are capable of withstanding the temperatures necessary to melt 
silicon, and are relatively stable when in contact with molten silicon 32. 
The crucible 20 may be of various capacity; in present-day industrial 
applications the crucible 20 may contain tens of kilograms of molten 
silicon. 
In large scale industrial use, the level 34 of molten silicon 32 is usually 
maintained in the crucible 20 at a predetermined height, or in a 
predetermined range of height. An apparatus adapted for automatically 
maintaining the level 34 of molten silicon 32 in the crucible 20 by 
addition of silicon granules is not shown on FIG. 9. 
A pull rod 36 is disposed above the crucible 20. It is connected to a 
pulling mechanism (not shown) which permits precise controlled movement of 
the rod 36 in a direction substantially normal to the level 34 of liquid 
silicon 32 in the crucible 20. The pulling mechanism (not shown) may be 
designed to pull and take up crystal rods, ingots or ribbons of up to 
several meters in length. 
A lower part of the pull rod 36 is adapted to clamp a seed crystal 38, or 
crystal dendrites (not shown) or a seed crystal together with quartz or 
graphite filaments, as it is explained in the introductory section of the 
present application for patent in connection with the description of the 
prior art "edge supported pulling technique." A detector system, usually 
utilizing a narrow band infrared detector, may continually observe a 
meniscus at a liquid-crystal interface and control the replenishment of 
silicon 32 in the crucible 20. The infrared detector is schematically 
shown on FIG. 9 and bears the reference numeral 40. 
The principal function of the above-summarized crystal growing equipment is 
to lower the seed crystal 38 into the melt 32, and thereafter gradually 
pull the seed crystal 38 away from the melt under precisely controlled 
conditions of temperature and pulling speed so that molten silicon is 
gradually supplied to the crystal liquid interface by capillary action, 
and gradually crystallizes to give rise to a crystal. It should be 
emphasized at this point that the present invention principally lies in 
the manner of utilizing and stabilizing a properly shaped meniscus on the 
liquid-crystal interface so as to provide for growth of a ribbon-shaped 
silicon crystal of, at least in principle, infinite length. Although, when 
the crystal pulling process is in progress, the seed crystal 38 may not be 
visually distinguishable from the growing crystal 42, on FIG. 9 the seed 
crystal 38 is separately indicated by dotted lines. 
Thus referring now to FIGS. 3, 4 and 5, a pair of edge defining members 44 
are shown partially submerged in a melt of silicon 32 contained in a 
crucible 20. Although this is not shown on FIGS. 3, 4 and 5, the crucible 
20 comprises a part of the Czochralski type crystal pulling equipment 
briefly described above with reference to FIG. 9. 
The edge defining members 44 are made of a material which is wettable by 
molten silicon and chemically compatible therewith. Thus, the edge 
defining members 44 are preferably made of quartz or graphite, although 
carbon, silicon carbide, and silicon nitride are also suitable for the 
construction of the edge defining members 44. The cross-sectional shape of 
the edge defining members 44 is not critical, neither is their thickness. 
Quartz or graphite filaments of round cross-section and having a diameter 
of approximately 1-3 mm serve well in the present invention. 
It is a critical feature of the present invention that the edge defining 
members 44 are stationarily mounted relative to the crucible 20 and to the 
liquid silicon 32 contained therein, so as to break the surface or level 
34 of the melt 32 at a fixed, predetermined distance from one another. The 
edge defining members 44 may be fixedly mounted to a rigid structure 
disposed substantially above the crucible 20, or in any other manner, the 
exact mode of stationarily mounting of the edge defining members 44 not 
being critical. What is critical is that the edge defining members 44 are 
fixed and do not move during the entire crystal pulling process practiced 
in accordance with the present invention. This is in sharp contrast with 
the prior art edge supported pulling technique, shown in FIGS. 1 and 2, 
wherein the meniscus stabilizing filaments 44 are withdrawn from the melt 
32 together with the growing crystal ribbon 42. 
Because the edge defining members 44 are submerged into the melt, and are 
wettable thereby, the molten silicon 32 rises on the periphery of the edge 
defining members 44 due to surface tension. This is shown on FIG. 3. 
Furthermore, in the first preferred embodiment of the apparatus and 
process of the present invention, shown in FIGS. 3, 4 and 5, the edge 
defining members 44 are disposed in an angular relationship relative to 
the level or surface 34 of the melt 32. More accurately stated, in the 
first preferred embodiment wherein the edge defining members 44 are 
fixedly mounted substantially above the crucible 20, the edge defining 
members 44 are disposed tapered relative to one another so that a distance 
between the two edge defining members 44 increases with increasing 
distance from the surface 34 of the melt 32. This is amply illustrated in 
FIGS. 3, 4 and 5, and serves the purpose of permitting a seed crystal 
ribbon 38 to contact the surface 34 of the melt 32 between the edge 
defining members 44 and to be pulled from the surface 34 without 
interference from the edge defining members 44. 
Another important feature of the present invention is that the edge 
defining members 44 are disposed at a distance from one another which 
corresponds to the width of the crystal ribbon 42 desired to be grown. 
This distance may vary up to approximately 5-10 centimeters. 
FIG. 3 illustrates the disposition of the edge defining members 44 in the 
crucible 20 as the seed silicon crystal ribbon 38 is lowered to contact 
the surface 34 of the melt 32. The seed crystal ribbon 38, in accordance 
with the present invention, is substantially as wide as the distance 
between the edge defining members 44 on the surface 34 of the melt 32. 
Thus, as it was noted above, the seed silicon crystal ribbon 38 may be up 
to approximately 5-10 centimeters wide. The thickness of the seed crystal 
ribbon 38 may vary approximately between 200 microns to 3 millimeters. As 
it will become apparent from the ensuing description, the above-noted 
cross-sectional dimensions of the seed crystal ribbon 38 are also 
substantially the cross-sectional dimensions of the silicon crystal ribbon 
42 which is produced in accordance with the present invention. 
Furthermore, the above-noted dimensions or limits of the crystal ribbon 42 
are principally determined by the requirement that the crystal ribbon 42 
grown in accordance with the present invention be uniform in 
cross-section. Experience has shown that if the ribbon is substantially 
wider or thicker than the above-noted limits, then uneven cooling occurs 
during the crystallization process with resultant buckling of the ribbon 
42. 
FIG. 4 shows the seed crystal ribbon 38 in contact with the surface 34 of 
the melt 32, and also in contact with the edge defining members 44 at its 
lower lateral edges 46. As is shown in FIG. 4, the seed crystal ribbon 38 
and the edge defining members 44 cause the liquid silicon 32 to rise to a 
certain height on the seed crystal ribbon 38, and to establish a meniscus 
48 thereon. The meniscus 48 is stabilized by the edge defining members 44. 
Theoretical calculations and experience show that the meniscus 48 on the 
substantially flat silicon seed crystal ribbon 38 is approximately 6 to 7 
millimeters above the level of the melt 32 in the crucible 20. A meniscus 
50 of the liquid silicon 32 on the edge defining members 44 is 
substantially lower, as is shown on FIG. 4. This is highly advantageous 
because as the seed crystal ribbon 38 is gradually pulled away from the 
surface 34 of the melt 32, liquid silicon crystallizes in a liquid-crystal 
interface area 51 which is not in immediate contact with the foreign 
substance of the edge defining members 44. Consequently, the silicon 
ribbon 42 obtained in accordance with the present invention is highly 
monocrystalline, i.e., contains relatively few, but large crystals of 
silicon. Therefore, the silicon ribbon 42 obtained in accordance with the 
present invention is very well suited for use in photovoltaic cells (not 
shown). 
An additional advantage of the present invention is that more freedom for 
selecting the proper seed ribbon crystal orientation is possible than in 
the prior art edge supported pulling technique. This is because of the 
substantial absence of foreign substance in the liquid crystal interface. 
As is known in the prior art edge supported pulling technique, certain 
seed ribbon crystal orientations are utilized to overcome the undesirable 
effect of the foreign substance of the edge defining filaments on the 
crystal growth. 
FIG. 5 shows a step in the process of the present invention wherein the 
growing silicon ribbon 42 is pulled away from the melt 32. During this 
step, the liquid-crystal interface 51 is continuously maintained by 
continuous rise of the molten silicon 32 by capillary action. Throughout 
this entire crystal growing process the edge defining members 44 maintain 
and stabilize the ends of the meniscus 48 on the growing crystal 42. 
Pulling speeds in the process of the present invention, as in the prior 
art, vary according to the dimensions of the crystal ribbon 42, and are, 
generally speaking, approximately in the 1 to 2.5 cm/min range. 
As it should be apparent from the above description, and in contrast with 
the prior art, the length of the crystalline ribbon 42 which is grown in 
accordance with the present invention is not limited by the length of the 
edge defining members 44. In a crystal growing apparatus wherein the 
silicon melt 32 is automatically replenished in the crucible 20, and 
therefore the level 34 of the melt 32 is substantially maintained in a 
predetermined range, the length of the crystal ribbon 42 is limited 
substantially only by the capacity of a conventional take-up apparatus 
(not shown). A silicon crystal ribbon 42 grown in accordance with the 
present invention is schematically in FIG. 8. 
Referring now to FIGS. 6 and 7, a second preferred embodiment of the 
crystal pulling apparatus of the present invention is shown. The second 
preferred embodiment differs principally from the first preferred 
embodiment only in that in the second preferred embodiment the edge 
defining members 44 are fixedly mounted to the crucible 20 rather than to 
some other part of a rigid structure. The mode of fastening the edge 
defining members 44 to the crucible 20 is not critical; quartz filaments 
acting as the edge defining members 44 may, for example, be fixed to a 
bottom 52 of the crucible 20. It is critical, however, to position the 
edge defining members 44 at a distance from one another, which, at least 
on the surface 34 of the melt 32, corresponds to the width of the crystal 
ribbon 42 to be pulled. 
The edge defining members 44 of the second preferred embodiment protrude 
slightly above the surface 34 of the melt 32 so that a meniscus 50 is 
established on the edge defining members 44. However, as is shown on FIG. 
6, it is advantageous for the edge defining members 44 to protrude to a 
lesser distance from the melt 32 than the expected height of the meniscus 
48 on the seed crystal ribbon 38 or on the growing crystal ribbon 42. This 
is because the liquid-crystal interface 51 is then established 
substantially above the edge defining members 44, and therefore 
crystallization occurs in substantial absence of foreign material. As it 
was explained above, this promotes substantially monocrystalline growth. 
In all other substantial aspects, the second preferred embodiment of the 
present invention operates in the same manner as the first preferred 
embodiment. Thus, the seed crystal ribbon 38 is lowered to the surface 34 
of the melt 32, and is thereafter pulled in an upwardly direction away 
from the melt 32 to give rise to a crystal ribbon 42 of indefinite length 
and of high degree of crystal perfection. 
The crystal ribbon 42 grown in accordance with the present invention has no 
dendrite rails or foreign substance filaments attached to its longitudinal 
edges. This represents an additional advantage over the prior art 
"dendritic" or "edge supported pulling" techniques, because in the prior 
art the dendritic rails or filaments, as applicable, must be removed from 
the crystal ribbon. 
Several modifications of the hereinabovedescribed apparatus and process may 
become readily apparent to those skilled in the art in light of the above 
disclosure. Therefore, the scope of the present invention should be 
interpreted solely from the following claims.