Method of obtaining polycrystalline silicon

Polycrystalline silicon is obtained by providing a silicon wafer having disposed over at least one face thereof a base coating of oxide, nitride or oxynitride composition, forming a substantially pinhole-free and scratch-free layer of carbon on said base coating over at least the face, forming on the face of the carbon layer a layer of polycrystalline silicon, and removing the silicon layer from the protective coating. Any of the carbon layer adhering to the silicon layer is easily removable to provide the silicon layer separate from the substrate. The wafer/coating unit is reusable in the procedure. The wafer/coating/carbon layer unit comprises a workpiece useful in the practice of the invention.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of obtaining polycrystalline 
silicon, and more particularly to a method of separating polycrystalline 
silicon from the substrate on which it is grown, and a workpiece useful in 
the practice of the method. 
Sheets of polycrystalline silicon are used for many applications, one of 
the applications being in solar cells or photovoltaics. A major factor 
limiting the use of such sheets in solar cells is the high cost of 
obtaining a sheet separated from the substrate upon which it is formed. 
For example, formation of a sheet directly on an expensive highly polished 
silicon wafer substrate makes it impossible to separate the sheet from the 
substrate. At the very least, the substrate must be cleaned and repolished 
after each use. 
In an attempt to ameloriate this problem, the wafer has been provided with 
an oxide, nitride or oxynitride coating and then the sheet formed directly 
on top of the coating. This has not proven satisfactory as the sheet 
adheres tightly to the coating and it is almost impossible to remove the 
sheet from the coating without damaging both the sheet and the coating. 
It is known that germanium grown on a thick layer of carbon over a 
substrate is easily separable from the quartz substrate. In an attempt to 
ameloriate the problem described above by use of this approach, a highly 
polished silicon wafer has been coated with carbon and then a sheet of 
polycrystalline silicon formed directly on top of the carbon layer. While 
this approach enabled easy separation of the sheet from the carbon layer 
when the carbon layer was essentially free from pinholes, an essentially 
pinhole-free carbon layer could be obtained only when the carbon layer was 
so thick that it was difficult to obtain the necessary flatness in the 
upper surface thereof and the process was economically unattractive due to 
the increased power requirements for heating the wafer/carbon unit during 
the growth step. Furthermore, it was difficult to remove the carbon 
adhering to the wafer after sheet separation, possibly due to the 
formation of silicon carbide compounds. In brief, this approach was not 
suitable for mass production techniques. 
Accordingly, it is an object of the present invention to provide a method 
of obtaining a sheet of polycrystalline silicon which enables easy 
separation of the sheet from the substrate wafer. 
Another object is to provide such method in which the substrate, or at 
least a major portion thereof, is reusable. 
A further object is to provide such a method which is economical and 
adapted to mass production techniques. 
A final object is to provide a workpiece which is useful in the practice of 
such a method and enables easy separation of the sheet from the substrate 
wafer. 
SUMMARY OF THE INVENTION 
It has now been found that the above and related objects of the present 
invention are obtained by providing a substrate comprising a wafer having 
a base coating of oxide, nitride or oxynitride on top thereof, and forming 
a layer of carbon on top of the coating and then a layer of 
polycrystalline silicon on top of the carbon layer. The silicon layer is 
then easily separable from the wafer and base coating--for example, by 
wedging one or more thin objects substantially intermediate the silicon 
layer and the base coating. 
While it has been learned that neither the base coating nor the carbon 
layer by itself enables easy removal of the silicon layer from the wafer, 
and that the combination of the two does enable such easy removal, it is 
not fully understood why the combination of the two is operable and each 
individually is not. 
More particularly, the method of obtaining polycrystalline silicon 
comprises the steps of providing a substrate body having a substantially 
planar face and a sidewall. A base coating of a composition selected from 
the group consisting of oxide, nitride and oxynitride compositions is 
disposed over at least the entire substrate body face, and preferably also 
the sidewall thereof. A substantially pinhole-free and scratch-free layer 
of carbon is formed on the base coating over at least the entire face 
thereof, and preferably also the sidewall thereof. A layer of 
polycrystalline silicon is then formed on the face of the carbon layer. 
Finally, the silicon layer is removed from the protective coating. 
Any of the carbon layer adhering to the base coating face is removed and 
the procedure repeated, starting with reconstitution of the carbon layer. 
Any of the carbon layer adhering to the silicon layer is removed in order 
to provide a silicon layer or sheet free from its substrate. 
The method described above may be repeated a plurality of times until the 
base coating deteriorates, at which point the base coating is 
reconstituted and the procedure repeated. 
In a preferred embodiment, in order to separate the silicon layer and the 
base coating, a thin object is wedged between the silicon layer and the 
base coating, preferably immediately below the portion of the silicon 
layer abutting the carbon layer face. 
Preferably the carbon layer is formed by exposing the base coating face and 
sidewall to the fumes of ignited xylene, or, alternatively, by heating and 
exposing to xylene the base coating face and sidewall. 
Preferably the substrate body is formed of silicon, the substrate body face 
is highly polished, the base coating is substantially pinhole-free and 
scratch-free, and the carbon layer is of uniform thickness. 
The workpiece of the present invention comprises a substrate body having a 
substantially planar upper face and a sidewall, a base coating of the 
aforementioned composition disposed over at least the entire body face 
(and preferably also the sidewall thereof), and a substantially 
pinhole-free and scratch-free layer of carbon disposed over at least the 
entire exposed base coating face (and preferably also the sidewall 
thereof). In a later stage of the method of the present invention the 
workpiece further includes a layer of polycrystalline silicon disposed 
over at least the exposed face of the carbon layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing, and in particular to FIG. 1 thereof, therein 
illustrated is a substrate body in the form of a silicon wafer, generally 
designated 10. Preferably the wafer 10 has the substantially planar upper 
face 12 thereof, and typically the sidewall 14, highly polished to provide 
a flat mirror-like finish. The backside 16 thereof may also be highly 
polished. The wafer 10 will generally be to uniform thickness (about 
250-1000 microns) and may be formed with any suitable diameter (for 
example, about 15 centimeters). 
A base coating, generally designated 20, is disposed at least over the 
entire wafer face 12 and preferably also over the sidewall 14 thereof. In 
order to complete sealing of the wafer 10, a useful procedure where the 
wafer 10 contains dopants, the base coating may also be disposed over the 
entire wafer backside 16, thereby to encapsulate the wafer 10. The coating 
20 is desirably substantially pinhole-free and scratch-free as well as of 
uniform thickness (generally about 2000-4000 Angstroms). The composition 
of the coating may be oxide, nitride, or oxynitride. The techniques for 
depositing such a coating on a wafer are well known in the art and need 
not be described in detail herein. See, for example, "Silicon Nitride 
Chemical Vapor Depositions in a Hot Wall Diffusion System", J. 
Electrochem. Soc., Volume 125, No. 9, pages 1557-1559 (September 1978); 
"Preparation and Some Properties of Chemically Vapor-Deposited Si-rich 
SiO.sub.2 and Si.sub.3 N.sub.4 Films", J. Electrochem. Soc., Volume 125, 
No. 5, pages 819-822 (May 1978); "Composition, Chemical Bonding, and 
Contamination of Low Temperature SiO.sub.x N.sub.y Insulating Films", J. 
Electrochem. Soc., Volume 125, No. 3, pages 424-430 (March 1978); 
"Improved Theoretical Predictions For the Steam Oxidation of Silicon at 
any Elevation", J. Electrochem. Soc., Volume 125, No. 9, pages 1514-1517 
(September 1978); and "Chemical Vapor Deposition of Silicon Nitride", J. 
Electrochem. Soc., Volume 125, No. 9, pages 1525-1529 (September 1978). 
The coating 20 not only provides a top face 22 and sidewall 24 from which 
the carbon layer later applied thereto will be easily removable, but it 
also aids in sealing the wafer 10 to protect the polycrystalline silicon 
sheet later grown thereon from the deleterious effects of pinholes in the 
carbon layer and outgassing of the wafer 10 under growth conditions. 
Referring now in particular to FIG. 2, a substantially pinhole-free and 
scratch-free layer of carbon, generally designated 30, is then formed over 
at least the entire upper face 22 of the coating 20, and preferably also 
the sidewall 24 thereof. The carbon layer 30 is preferably of uniform 
thickness, thereby to provide a flat upper face 32 on which the 
polycrystalline silicon sheet can later be grown as well as a sidewall 34 
on which some of the polycrystalline silicon may also form. The carbon 
layer 30 is extremely thin, preferably 12-380 microns in thickness. If the 
carbon layer is too thick, too much power is required to bring it up to 
the temperature required for chemical vapor deposition of the 
polycrystalline silicon sheet and it is difficult to insure uniform 
flatness of the upper surface 32 on which the sheet is grown. If the 
carbon layer 30 is too thin, it tends not to be substantially 
pinhole-free, rendering it difficult to separate the polycrystalline 
silicon sheet later grown thereon from the coating 20 as described 
hereinbelow. The carbon layer 30 may also be applied over the base coating 
on the wafer backside 16, but this is neither necessary nor useful as 
ordinarily the polycrystalline silicon will not form on the wafer 
backside. 
A variety of different techniques useful in forming the carbon layer 30 
will be readily apparent to those skilled in the chemical vapor deposition 
art. For example, analytical pyrolizing reagent-grade xylene may be 
ignited under carbonizing conditions. The wafer/protective coating unit 
may then be held on the wafer backside 16 (for example, by a vacuum chuck) 
and the front face 22 of the wafer/coating unit passed evenly over the 
flame to provide a thin uniform carbon layer 30. If necessary, the wafer 
should be tilted slightly from one side to another to insure that carbon 
layer 30 is also formed on the coating sidewall 24. Alternatively, the 
wafer/coating unit may be either rested with its backside 16 lying on a 
susceptor or suspended with its backside 16 held by a vacuum chuck, and 
the unit then exposed to a stream of an inert carrier gas which has been 
bubbled through xylene. In this instance the wafer/coating unit should be 
heated by conventional means as necessary to maintain the unit at the 
proper temperature for pyrolitic carbon formation. The carbon layer 30 may 
also be formed by other techniques such as dipping the appropriate 
surfaces into carbon powder (graphite) or spraying the appropriate 
surfaces with an emulsified carbon bath or applying a graphite solution to 
the appropriate surfaces with a spinner, provided in all instances that 
the carbon layer 30 thus formed is substantially pinhole-free and 
scratch-free, and not deleteriously contaminated (e.g., by emulsifiers, 
solvents and the like), and sufficiently adherent to the coating 20 so 
that it is not entirely blown away by the gases passing thereby during the 
later sheet formation step. The wafer/coating/carbon layer unit 
constitutes the basic workpiece useful in the practice of the method of 
the present invention. 
Referring now to FIG. 3 in particular, a layer or sheet 40 of 
polycrystalline silicon is then formed on the upper face 32 of the carbon 
layer 30, for example, by conventional techniques well known to those 
skilled in the epitaxy and chemical vapor deposition arts. See, for 
example, "The Fundamentals of Chemical Vapour Deposition", Journal of 
Material Sciences, 12 (Chapman & Hall Ltd. 1977), pp. 1285-1306. The sheet 
40 will typically extend downwardly over the carbon layer sidewall 34, but 
in a thinner layer. The sheet is generally 250-750 microns thick atop the 
carbon layer face 32. Thinner sheets have a tendency to warp or break as 
the sheet is being removed from the substrate as described hereinbelow, 
while thicker sheets are not economical and are difficult to form with the 
desired degree of uniform thickness. 
Referring now to FIG. 4 in particular, the polycrystalline silicon sheet 40 
is then removed from the wafer/coating unit. As shown, a plurality of 
substantially uniformly spaced thin objects 50 are wedged intermediate the 
sheet backside 52 and the coating upper surface 22. The objects 50 may be 
razor blades or the like having a thickness on the order of about 12 
microns. The objects 50 are preferably inserted immediately below the 
silicon sheet backside 52 abutting the carbon layer face 32. The task of 
knowing where to position the objects 50 is simplified by the fact that 
the polycrystalline silicon sheet 40 is gray whereas the carbon layer 30 
is black. The actual separation task is simplified by the fact that the 
polycrystalline silicon layer 40 tends to be rather thin along the carbon 
layer sidewall 34. 
Other techniques presently contemplated for use in separating the sheet 40 
from the protective coating 20 include the use of ultrasonics and thermal 
shock, for example, by rapid chilling of the wafer/coating unit with 
liquid nitrogen or rapid heating of the wafer/coating unit. The efficacy 
of these techniques, of course, depends upon the low level of adhesion of 
the sheet 40 to the coating 20 due to the presence of the intermediate 
carbon layer 30. 
Regardless of the specific technique utilized to separate the sheet 40 from 
the coating 20, there is likely to be a certain amount 30a of the now 
destroyed carbon layer 30 adhering to the sheet backside 52 and a certain 
amount 30b adhering to the coating upper face 22 and sidewall 24. The 
carbon 30a adhering to the sheet backside 52 may be removed by 
sandblasting, an acid dip (e.g., using acetic, nitric and hydrofluoric 
acids), grinding, ultrasonics, a combination thereof, or by other 
techniques well recognized in the art for removing carbon from a silicon 
sheet. The silicon sheet is then available for use, separate from its 
former substrate, as desired. 
The carbon 30b adhering to the coating upper face 22 is exposed, but the 
carbon 30b adhering to the coating sidewall 24 is covered by a thin layer 
of polycrystalline silicon 40. The thin layer of silicon is removed first, 
for example, by careful scraping to insure that the coating sidewall 24 is 
not damaged (although it is immaterial whether or not the intermediate 
carbon layer sidewall 34 is damaged). Then the carbon 30b is easily 
removed by simple soft nylon brushing and/or deionized water washing, care 
being taken to make sure that the carbon removal procedure does not injure 
the underlying coating 20. The wafer/coating unit is then available for 
reuse, and the procedure may be repeated starting with formation of the 
carbon layer 30 on the coating 20, as described hereinabove, to 
reconstitute the basic workpiece of the present invention. It has been 
found that the procedure may be repeated many times using the same 
wafer/coating unit so that the cost of the unit is amortizable over the 
many silicon sheets obtained by use thereof, this rendering the process 
economical. If the coating 20 of the wafer/coating unit becomes scratched 
or otherwise damaged, it is a simple and relatively inexpensive procedure 
to remove the damaged coating 20 from the wafer 10 and then to apply a 
fresh coating 20 to the wafer 10, thereby to reconstitute the coating 20 
and enable the procedure to restart. 
To summarize, the present invention provides an economical process for 
obtaining a sheet of polycrystalline silicon which enables easy separation 
of the sheet from the substrate wafer, the substrate wafer being undamaged 
and reusable in the process, thereby providing an economical mass 
production technique for obtaining polycrystalline silicon sheet separated 
from the substrate on which it is grown. The presence of both a base 
coating and a carbon layer intermediate the substrate wafer and the grown 
polycrystalline silicon facilitates the separation process. 
Now that the preferred embodiments have been shown and described in detail, 
various modifications and improvements thereon will become readily 
apparent to those skilled in the art. For example, while the preferred 
embodiments have been described in terms of a silicon substrate body 
because only silicon is currently known to be suitable for use as the 
substrate body in a chemical vapor deposition system for the growth of 
polycrystalline silicon, the principles of the present invention are 
equally applicable to substrate bodies formed of materials other than 
silicon which also meet the requirements for a substrate body useful in a 
chemical vapor deposition system for the growth of polycrystalline 
silicon. Accordingly, the spirit and scope of the present invention is to 
be limited only by the appended claims, and not by the foregoing 
disclosure.